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Title: Aviation Engines: Design—Construction—Operation and Repair

Author: Victor Wilfred Pagé

Release date: December 2, 2011 [eBook #38187]

Language: English

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JUST PUBLISHED
AVIATION ENGINES. Their Design, Construction, Operation
and Repair.
By Lieut. Victor W. Pagé, Aviation Section, S.C.U.S.R.
A practical work containing valuable instructions for
aviation students, mechanicians, squadron engineering
officers and all interested in the construction and upkeep of
airplane power plants. 576 octavo pages. 250 illustrations.
Price $3.00.
AVIATION CHART, or the Location of Airplane Power Plant
Troubles Made Easy.
By Lieut. Victor W. Pagé, A.S., S.C.U.S.R.
A large chart outlining all parts of a typical airplane power
plant, showing the points where trouble is apt to occur and
suggesting remedies for the common defects. Intended
especially for aviators and aviation mechanics on school and
field duty. Price 50 cents.
GLOSSARY OF AVIATION TERMS.
Compiled by Lieuts. Victor W. Pagé, A.S., S.C.U.S.R. and
Paul Montariol of the French Flying Corps on duty at
Signal Corps Aviation School, Mineola, L. I.
A complete glossary of practically all terms used in aviation,
having lists in both French and English, with equivalents in
either language. A very valuable book for all who are about
to leave for duty overseas. Price, cloth, $1.00.

THE NORMAN W. HENLEY PUBLISHING COMPANY
2 WEST 45th ST., NEW YORK

Large
image
(105 kB).

Part Sectional View of Hall-Scott Airplane Motor,
Showing Principal Parts.

CENSORED
This Book Entitled

AVIATION ENGINES
By LIEUT. VICTOR W. PAGÉ
has been censored by the United States Government, and pages and
parts of pages have been omitted by special instructions from
Washington.

The book has been passed by THE COMMITTEE ON PUBLIC
INFORMATION and is as complete as we can furnish it, and we so
advise the purchaser of it.
THE NORMAN W. HENLEY PUBLISHING COMPANY

AVIATION ENGINES
Design—Construction—Operation and Repair

A COMPLETE, PRACTICAL TREATISE OUTLINING CLEARLY THE
ELEMENTS OF INTERNAL COMBUSTION ENGINEERING WITH SPECIAL
REFERENCE TO THE DESIGN, CONSTRUCTION, OPERATION AND REPAIR
OF AIRPLANE POWER PLANTS; ALSO THE AUXILIARY ENGINE SYSTEMS,
SUCH AS LUBRICATION, CARBURETION, IGNITION AND COOLING.
IT INCLUDES COMPLETE INSTRUCTIONS FOR ENGINE REPAIRING AND
SYSTEMATIC LOCATION OF TROUBLES, TOOL EQUIPMENT AND USE OF
TOOLS, ALSO OUTLINES THE LATEST MECHANICAL PROCESSES.
BY

First Lieut. VICTOR W. PAGÉ, A. S. S. C., U. S. R.

Assistant Engineering Officer, Signal Corps Aviation School, Mineola, L. I.
Author of “The Modern Gasoline Automobile,” Etc.

CONTAINS VALUABLE INSTRUCTIONS FOR ALL AVIATION STUDENTS, MECHANICIANS,
SQUADRON ENGINEERING OFFICERS AND ALL INTERESTED IN THE CONSTRUCTION AND
UPKEEP OF AIRPLANE POWER PLANTS.

NEW YORK
THE NORMAN W. HENLEY PUBLISHING COMPANY

2 West 45th Street
1917

Copyrighted, 1917
by
The Norman W. Henley Publishing Co.

PRINTED IN U. S. A.

ALL ILLUSTRATIONS IN THIS BOOK HAVE BEEN SPECIALLY MADE BY THE PUBLISHERS,
AND THEIR USE, WITHOUT PERMISSION, IS STRICTLY PROHIBITED

COMPOSITION, ELECTROTYPING AND PRESSWORK
BY THE PUBLISHERS PRINTING CO., NEW YORK

PREFACE
In presenting this treatise on “Aviation Engines,” the writer realizes that the rapidly
developing art makes it difficult to outline all latest forms or describe all current engineering
practice. This exposition has been prepared primarily for instruction purposes and is adapted
for men in the Aviation Section, Signal Corps, and students who wish to become aviators or
aviation mechanicians. Every effort has been made to have the engineering information
accurate, but owing to the diversity of authorities consulted and use of data translated from
foreign language periodicals, it is expected that some slight errors will be present. The writer
wishes to acknowledge his indebtedness to such firms as the Curtiss Aeroplane and Motor
Co., Hall-Scott Company, Thomas-Morse Aircraft Corporation and General Vehicle
Company for photographs and helpful descriptive matter. Special attention has been paid to
instructions on tool equipment, use of tools, trouble “shooting” and engine repairs, as it is on
these points that the average aviation student is weakest. Only such theoretical consideration
of thermo-dynamics as was deemed absolutely necessary to secure a proper understanding
of engine action after consulting several instructors is included, the writer’s efforts having
been confined to the preparation of a practical series of instructions that would be of the
greatest value to those who need a diversified knowledge of internal-combustion engine
operation and repair, and who must acquire it quickly. The engines described and illustrated
are all practical forms that have been fitted to airplanes capable of making flights and may
be considered fairly representative of the present state of the art.
Victor W. Pagé,
1st Lieut. A. S. S. C., U. S. R.
Mineola, L. I.,
October, 1917.

CONTENTS
CHAPTER I

PAGES
Brief Consideration of Aircraft Types—Essential Requirements
of Aerial Motors—Aviation Engines Must Be Light—Factors
Influencing Power Needed—Why Explosive Motors Are Best
—Historical—Main Types of Internal Combustion Engines 17-36

CHAPTER II

Operating Principles of Two- and Four-Stroke Engines—Four-
cycle Action—Two-cycle Action—Comparing Two- and Four-
cycle Types—Theory of Gas and Gasoline Engine—Early Gas-
Engine Forms—Isothermal Law—Adiabatic Law—
Temperature Computations—Heat and Its Work—Conversion
of Heat to Power—Requisites for Best Power Effect 37-59

CHAPTER III

Efficiency of Internal Combustion Engines—Various Measures
of Efficiency—Temperatures and Pressures—Factors
Governing Economy—Losses in Wall Cooling—Value of
Indicator Cards—Compression in Explosive Motors—Factors
Limiting Compression—Causes of Heat Losses and
Inefficiency—Heat Losses to Cooling Water 60-79

CHAPTER IV

Engine Parts and Functions—Why Multiple Cylinder Engines
Are Best—Describing Sequence of Operations—Simple
Engines—Four and Six Cylinder Vertical Tandem Engines—
Eight and Twelve Cylinder V Engines—Radial Cylinder
Arrangement—Rotary Cylinder Forms 80-109

CHAPTER V

Properties of Liquid Fuels—Distillates of Crude Petroleum— 110-154
Principles of Carburetion Outlined—Air Needed to Burn

Gasoline—What a Carburetor Should Do—Liquid Fuel Storage
and Supply—Vacuum Fuel Feed—Early Vaporizer Forms—
Development of Float Feed Carburetor—Maybach’s Early
Design—Concentric Float and Jet Type—Schebler Carburetor
—Claudel Carburetor—Stewart Metering Pin Type—Multiple
Nozzle Vaporizers—Two-Stage Carburetor—Master Multiple
Jet Type—Compound Nozzle Zenith Carburetor—Utility of
Gasoline Strainers—Intake Manifold Design and Construction
—Compensating for Various Atmospheric Conditions—How
High Altitude Affects Power—The Diesel System—Notes on
Carburetor Installation—Notes on Carburetor Adjustment

CHAPTER VI

Early Ignition Systems—Electrical Ignition Best—
Fundamentals of Magnetism Outlined—Forms of Magneto—
Zones of Magnetic Influence—How Magnets are Made—
Electricity and Magnetism Related—Basic Principles of
Magneto Action—Essential Parts of Magneto and Functions—
Transformer Coil Systems—True High Tension Type—The
Berling Magneto—Timing and Care—The Dixie Magneto—
Spark-Plug Design and Application—Two-Spark Ignition—
Special Airplane Plug 155-200

CHAPTER VII

Why Lubrication Is Necessary—Friction Defined—Theory of
Lubrication—Derivation of Lubricants—Properties of Cylinder
Oils—Factors Influencing Lubrication System Selection—
Gnome Type Engines Use Castor Oil—Hall-Scott Lubrication
System—Oil Supply by Constant Level Splash System—Dry
Crank-Case System Best for Airplane Engines—Why Cooling
Systems Are Necessary—Cooling Systems Generally Applied
—Cooling by Positive Pump Circulation—Thermo-Syphon
System—Direct Air-Cooling Methods—Air-Cooled Engine
Design Considerations 201-232

CHAPTER VIII

Methods of Cylinder Construction—Block Castings—Influence 233-286
on Crank-Shaft Design—Combustion Chamber Design—Bore
and Stroke Ratio—Meaning of Piston Speed—Advantage of
Off-Set Cylinders—Valve Location of Vital Import—Valve
Installation Practice—Valve Design and Construction—Valve

Operation—Methods of Driving Cam-Shaft—Valve Springs—
Valve Timing—Blowing Back—Lead Given Exhaust Valve—
Exhaust Closing, Inlet Opening—Closing the Inlet Valve—
Time of Ignition—How an Engine is Timed—Gnome
“Monosoupape” Valve Timing—Springless Valves—Four
Valves per Cylinder

CHAPTER IX

Constructional Details of Pistons—Aluminum Cylinders and
Pistons—Piston Ring Construction—Leak Proof Piston Rings
—Keeping Oil Out of Combustion Chamber—Connecting Rod
Forms—Connecting Rods for Vee Engines—Cam-Shaft and
Crank-Shaft Designs—Ball Bearing Crank-Shafts—Engine
Base Construction 287-323

CHAPTER X

Power Plant Installation—Curtiss OX-2 Engine Mounting and
Operating Rules—Standard S. A. E. Engine Bed Dimensions—
Hall-Scott Engine Installation and Operation—Fuel System
Rules—Ignition System—Water System—Preparations to Start
Engine—Mounting Radial and Rotary Engines—Practical Hints
to Locate Engine Troubles—All Engine Troubles Summarized
—Location of Engine Troubles Made Easy 324-375

CHAPTER XI

Tools for Adjusting and Erecting—Forms of Wrenches—Use
and Care of Files—Split Pin Removal and Installation—
Complete Chisel Set—Drilling Machines—Drills, Reamers,
Taps and Dies—Measuring Tools—Micrometer Calipers and
Their Use—Typical Tool Outfits—Special Hall-Scott Tools—
Overhauling Airplane Engines—Taking Engine Down—Defects
in Cylinders—Carbon Deposits, Cause and Prevention—Use of
Carbon Scrapers—Burning Out Carbon with Oxygen—
Repairing Scored Cylinders—Valve Removal and Inspection—
Reseating and Truing Valves—Valve Grinding Processes—
Depreciation in Valve Operating System—Piston Troubles—
Piston Ring Manipulation—Fitting Piston Rings—Wrist-Pin
Wear—Inspection and Refitting of Engine Bearings—Scraping
Brasses to Fit—Fitting Connecting Rods—Testing for Bearing
Parallelism—Cam-Shafts and Timing Gears—Precautions in
Reassembling Parts 376-456

CHAPTER XII

Aviation Engine Types—Division in Classes—Anzani Engines
—Canton and Unné Engine—Construction of Gnome Engines
—“Monosoupape” Gnome—German “Gnome” Type—Le
Rhone Engine—Renault Air-Cooled Engine—Simplex Model
“A” Hispano-Suiza—Curtiss Aviation Motors—Thomas-Morse
Model 88 Engine—Duesenberg Engine—Aeromarine Six-
Cylinder—Wisconsin Aviation Engines—Hall-Scott Engines—
Mercedes Motor—Benz Motor—Austro-Daimler Engine—
Sunbeam-Coatalen—Indicating and Measuring Instruments—
Air Starting Systems—Electric Starting—Battery Ignition 457-571

INDEX 573

LIST OF ILLUSTRATIONS

AVIATION ENGINES
DESIGN—CONSTRUCTION—REPAIR

CHAPTER I
Brief Consideration of Aircraft Types—Essential Requirements of Aerial Motors—
Aviation Engines Must Be Light—Factors Influencing Power Needed—Why
Explosive Motors Are Best—Historical—Main Types of Internal Combustion
Engines.

BRIEF CONSIDERATION OF AIRCRAFT TYPES

The conquest of the air is one of the most stupendous achievements of the ages. Human
flight opens the sky to man as a new road, and because it is a road free of all obstructions
and leads everywhere, affording the shortest distance to any place, it offers to man the
prospect of unlimited freedom. The aircraft promises to span continents like railroads, to
bridge seas like ships, to go over mountains and forests like birds, and to quicken and
simplify the problems of transportation. While the actual conquest of the air is an
accomplishment just being realized in our days, the idea and yearning to conquer the air are
old, possibly as old as intellect itself. The myths of different races tell of winged gods and
flying men, and show that for ages to fly was the highest conception of the sublime. No
other agent is more responsible for sustained flight than the internal combustion motor, and
it was only when this form of prime mover had been fully developed that it was possible for
man to leave the ground and alight at will, not depending upon the caprices of the winds or
lifting power of gases as with the balloon. It is safe to say that the solution of the problem of
flight would have been attained many years ago if the proper source of power had been
available as all the essential elements of the modern aeroplane and dirigible balloon, other
than the power plant, were known to early philosophers and scientists.
Aeronautics is divided into two fundamentally different branches—aviatics and aerostatics.
The first comprises all types of aeroplanes and heavier than air flying machines such as the
helicopters, kites, etc.; the second includes dirigible balloons, passive balloons and all craft
which rise in the air by utilizing the lifting force of gases. Aeroplanes are the only practical
form of heavier-than-air machines, as the helicopters (machines intended to be lifted directly
into the air by propellers, without the sustaining effect of planes), and ornithopters, or
flapping wing types, have not been thoroughly developed, and in fact, there are so many
serious mechanical problems to be solved before either of these types of air craft will
function properly that experts express grave doubts regarding the practicability of either.
Aeroplanes are divided into two main types—monoplanes or single surface forms, and bi-
planes or machines having two sets of lifting surfaces, one suspended over the other. A third
type, the triplane, is not very widely used.
Dirigible balloons are divided into three classes: the rigid, the semi-rigid, and the non-rigid.
The rigid has a frame or skeleton of either wood or metal inside of the bag, to stiffen it; the
semi-rigid is reinforced by a wire net and metal attachments; while the non-rigid is just a

bag filled with gas. The aeroplane, more than the dirigible and balloon, stands as the
emblem of the conquest of the air. Two reasons for this are that power flight is a real
conquest of the air, a real victory over the battling elements; secondly, because the
aeroplane, or any flying machine that may follow, brings air travel within the reach of
everybody. In practical development, the dirigible may be the steamship of the air, which
will render invaluable services of a certain kind, and the aeroplane will be the automobile of
the air, to be used by the multitude, perhaps for as many purposes as the automobile is now
being used.

ESSENTIAL REQUIREMENTS OF AERIAL MOTORS

One of the marked features of aircraft development has been the effect it has had upon the
refinement and perfection of the internal combustion motor. Without question gasoline-
motors intended for aircraft are the nearest to perfection of any other type yet evolved.
Because of the peculiar demands imposed upon the aeronautical motor it must possess all
the features of reliability, economy and efficiency now present with automobile or marine
engines and then must have distinctive points of its own. Owing to the unstable nature of the
medium through which it is operated and the fact that heavier-than-air machines can
maintain flight only as long as the power plant is functioning properly, an airship motor
must be more reliable than any used on either land or water. While a few pounds of metal
more or less makes practically no difference in a marine motor and has very little effect
upon the speed or hill-climbing ability of an automobile, an airship motor must be as light as
it is possible to make it because every pound counts, whether the motor is to be fitted into an
aeroplane or in a dirigible balloon.
Airship motors, as a rule, must operate constantly at high speeds in order to obtain a
maximum power delivery with a minimum piston displacement. In automobiles, or motor
boats, motors are not required to run constantly at their maximum speed. Most aircraft
motors must function for extended periods at speed as nearly the maximum as possible.
Another thing that militates against the aircraft motor is the more or less unsteady
foundation to which it is attached. The necessarily light framework of the aeroplane makes it
hard for a motor to perform at maximum efficiency on account of the vibration of its
foundation while the craft is in flight. Marine and motor car engines, while not placed on
foundations as firm as those provided for stationary power plants, are installed on bases of
much more stability than the light structure of an aeroplane. The aircraft motor, therefore,
must be balanced to a nicety and must run steadily under the most unfavorable conditions.

AERIAL MOTORS MUST BE LIGHT

The capacity of light motors designed for aerial work per unit of mass is surprising to those
not fully conversant with the possibilities that a thorough knowledge of proportions of parts
and the use of special metals developed by the automobile industry make possible. Activity
in the development of light motors has been more pronounced in France than in any other
country. Some of these motors have been complicated types made light by the skillful

proportioning of parts, others are of the refined simpler form modified from current
automobile practice. There is a tendency to depart from the freakish or unconventional
construction and to adhere more closely to standard forms because it is necessary to have the
parts of such size that every quality making for reliability, efficiency and endurance are
incorporated in the design. Aeroplane motors range from two cylinders to forms having
fourteen and sixteen cylinders and the arrangement of these members varies from the
conventional vertical tandem and opposed placing to the V form or the more unusual radial
motors having either fixed or rotary cylinders. The weight has been reduced so it is possible
to obtain a complete power plant of the revolving cylinder air-cooled type that will not
weigh more than three pounds per actual horse-power and in some cases less than this.
If we give brief consideration to the requirements of the aviator it will be evident that one of
the most important is securing maximum power with minimum mass, and it is desirable to
conserve all of the good qualities existing in standard automobile motors. These are
certainty of operation, good mechanical balance and uniform delivery of power—
fundamental conditions which must be attained before a power plant can be considered
practical. There are in addition, secondary considerations, none the less desirable, if not
absolutely essential. These are minimum consumption of fuel and lubricating oil, which is
really a factor of import, for upon the economy depends the capacity and flying radius. As
the amount of liquid fuel must be limited the most suitable motor will be that which is
powerful and at the same time economical. Another important feature is to secure
accessibility of components in order to make easy repair or adjustment of parts possible. It is
possible to obtain sufficiently light-weight motors without radical departure from
established practice. Water-cooled power plants have been designed that will weigh but four
or five pounds per horse-power and in these forms we have a practical power plant capable
of extended operation.

FACTORS INFLUENCING POWER NEEDED

Work is performed whenever an object is moved against a resistance, and the amount of
work performed depends not only on the amount of resistance overcome but also upon the
amount of time utilized in accomplishing a given task. Work is measured in horse-power for
convenience. It will take one horse-power to move 33,000 pounds one foot in one minute or
550 pounds one foot in one second. The same work would be done if 330 pounds were
moved 100 feet in one minute. It requires a definite amount of power to move a vehicle over
the ground at a certain speed, so it must take power to overcome resistance of an airplane in
the air. Disregarding the factor of air density, it will take more power as the speed increases
if the weight or resistance remains constant, or more power if the speed remains constant
and the resistance increases. The airplane is supported by air reaction under the planes or
lifting surfaces and the value of this reaction depends upon the shape of the aerofoil, the
amount it is tilted and the speed at which it is drawn through the air. The angle of incidence
or degree of wing tilt regulates the power required to a certain degree as this affects the
speed of horizontal flight as well as the resistance. Resistance may be of two kinds, one that
is necessary and the other that it is desirable to reduce to the lowest point possible. There is
the wing resistance and the sum of the resistances of the rest of the machine such as

fuselage, struts, wires, landing gear, etc. If we assume that a certain airplane offered a total
resistance of 300 pounds and we wished to drive it through the air at a speed of sixty miles
per hour, we can find the horse-power needed by a very simple computation as follows:
The product of 300 pounds resistance times speed of 88 feet
per second times 60 seconds in a minute
= H.P. needed.
divided by 33,000 foot pounds per minute
in one horse-power
The result is the horse-power needed, or
300 × 88 × 60
= 48 H.P.
33,000
Just as it takes more power to climb a hill than it does to run a car on the level, it takes more
power to climb in the air with an airplane than it does to fly on the level. The more rapid the
climb, the more power it will take. If the resistance remains 300 pounds and it is necessary
to drive the plane at 90 miles per hour, we merely substitute proper values in the above
formula and we have
300 pounds times 132 feet per second times 60
seconds in a minute
= 72 H.P.
33,000 foot pounds per minute in one
horse-power
The same results can be obtained by dividing the product of the resistance in pounds times
speed in feet per second by 550, which is the foot-pounds of work done in one second to
equal one horse-power. Naturally, the amount of propeller thrust measured in pounds
necessary to drive an airplane must be greater than the resistance by a substantial margin if
the plane is to fly and climb as well. The following formulæ were given in “The Aeroplane”
of London and can be used to advantage by those desiring to make computations to ascertain
power requirements:

Fig. 1.—Diagrams Illustrating Computations for
Horse-Power Required for Airplane Flight.

The thrust of the propeller depends on the power of the motor, and on the diameter and pitch
of the propeller. If the required thrust to a certain machine is known, the calculation for the
horse-power of the motor should be an easy matter.
The required thrust is the sum of three different “resistances.” The first is the “drift”
(dynamical head resistance of the aerofoils), i.e., tan α × lift (L), lift being equal to the total
weight of machine (W) for horizontal flight and α equal to the angle of incidence. Certainly
we must take the tan α at the maximum Ky value for minimum speed, as then the drift is the
greatest (Fig. 1, A).
Another method for finding the drift is D = K × AV2, when we take the drift again so as to be
greatest.
The second “resistance” is the total head resistance of the machine, at its maximum velocity.
And the third is the thrust for climbing. The horse-power for climbing can be found out in
two different ways. I first propose to deal with the method, where we find out the actual
horse-power wanted for a certain climbing speed to our machine, where
climbing speed/sec. × W
H.P. =
550

In this case we know already the horse-power for climbing, and we can proceed with our
calculation.
With the other method we shall find out the “thrust” in pounds or kilograms wanted for
climbing and add it to drift and total head resistance, and we shall have the total “thrust” of
our machine and we shall denote it with T, while thrust for climbing shall be Tc.
The following calculation is at our service to find out this thrust for climbing
Vc × W
= H.P.,
550
thence
H.P. × 550
Vc = (1)
W
Tc × V
H.P. = ,
550
then from (1)

Tc × V
× 550
550 Tc × V
Vc = = ,
W W
thence,
Vc × W
Tc = .
V
Whether T means drifts, head resistance and thrust for climbing, or drift and head resistance
only, the following calculation is the same, only in the latter case, of course, we must add the
horse-power required for climbing to the result to obtain the total horse-power.
Now, when we know the total thrust, we shall find the horse-power in the following manner:
We know that the
P r 2π R
H.P. =
75 × 60
in kilograms, or in English measure,
P r 2π R
H.P. = (Fig. 1, B)
33,000
where
P = pressure in klgs. or lbs.

r = radius on which P is acting.
R = Revolution/min.
When P × r = M, then
M.R.2π
H.P. = ,
4,500
thence,
H.P. × 4,500 716.2 H.P.
M= = in meter kilograms,
R2π R
or in English system
H.P. 33,000 5253.1 H.P.
M= = in foot pounds.
R2π R
Now the power on the circumference of the propeller will be reduced by its radius, so it will
be M/r = p. A part of p will be used for counteracting the air and bearing friction, so that the
total power on the circumference of the propeller will be (M/r) × η = p where η is the
mechanical efficiency of the propeller. Now η/tan α = T, where α is taken on the tip of the
propeller.
I take α at the tip, but it can be taken, of course, at any point, but then in equation p = M/r, r
must be taken only up to this point, and not the whole radius; but it is more comfortable to
take it at the tip, as tan α = Pitch/r2π (Fig. 1, C).
Now we can write up the equation of the thrust:
716.2 H.P. η 5253.1 H.P. η
T= , or in English measure ,
R r tan α R r tan α
thence
T × R × r tan α T × R × r tan α
H.P. = , or in English measure .
716.2 η 5253.1 η
The computations and formulæ given are of most value to the student engineer rather than
matters of general interest, but are given so that a general idea may be secured of how
airplane design influences power needed to secure sustained flight. It will be apparent that
the resistance of an airplane depends upon numerous considerations of design which require
considerable research in aerodynamics to determine accurately. It is obvious that the more
resistance there is, the more power needed to fly at a given speed. Light monoplanes have
been flown with as little as 15 horse-power for short distances, but most planes now built
use engines of 100 horse-power or more. Giant airplanes have been constructed having
2,000 horse-power distributed in four power units. The amount of power provided for an
airplane of given design varies widely as many conditions govern this, but it will range from

approximately one horse-power to each 8 pounds weight in the case of very light, fast
machines to one horse-power to 15 or 18 pounds of the total weight in the case of medium
speed machines. The development in airplane and power plant design is so rapid, however,
that the figures given can be considered only in the light of general averages rather than
being typical of current practice.

WHY EXPLOSIVE MOTORS ARE BEST

Internal combustion engines are best for airplanes and all types of aircraft for the same
reasons that they are universally used as a source of power for automobiles. The gasoline
engine is the lightest known form of prime mover and a more efficient one than a steam
engine, especially in the small powers used for airplane propulsion. It has been stated that by
very careful designing a steam plant an engine could be made that would be practical for
airplane propulsion, but even with the latest development it is doubtful if steam power can
be utilized in aircraft to as good advantage as modern gasoline-engines are. While the steam-
engine is considered very much simpler than a gas-motor, the latter is much more easily
mastered by the non-technical aviator and certainly requires less attention. A weight of 10
pounds per horse-power is possible in a condensing steam plant but this figure is nearly
double or triple what is easily secured with a gas-motor which may weigh but 5 pounds per
horse-power in the water cooled forms and but 2 or 3 pounds in the air-cooled types. The
fuel consumption is twice as great in a steam-power plant (owing to heat losses) as would be
the case in a gasoline engine of equal power and much less weight.
The internal-combustion engine has come seemingly like an avalanche of a decade; but it
has come to stay, to take its well-deserved position among the powers for aiding labor. Its
ready adaptation to road, aerial and marine service has made it a wonder of the age in the
development of speed not before dreamed of as a possibility; yet in so short a time, its power
for speed has taken rank on the common road against the locomotive on the rail with its
century’s progress. It has made aerial navigation possible and practical, it furnishes power
for all marine craft from the light canoe to the transatlantic liner. It operates the machine
tools of the mechanic, tills the soil for the farmer and provides healthful recreation for
thousands by furnishing an economical means of transport by land and sea. It has been a
universal mechanical education for the masses, and in its present forms represents the great
refinement and development made possible by the concentration of the world’s master
minds on the problems incidental to internal combustion engineering.

HISTORICAL

Although the ideal principle of explosive power was conceived some two hundred years
ago, at which time experiments were made with gunpowder as the explosive element, it was
not until the last years of the eighteenth century that the idea took a patentable shape, and
not until about 1826 (Brown’s gas-vacuum engine) that a further progress was made in
England by condensing the products of combustion by a jet of water, thus creating a partial
vacuum.

Brown’s was probably the first explosive engine that did real work. It was clumsy and
unwieldy and was soon relegated to its place among the failures of previous experiments. No
approach to active explosive effect in a cylinder was reached in practice, although many
ingenious designs were described, until about 1838 and the following years. Barnett’s engine
in England was the first attempt to compress the charge before exploding. From this time on
to about 1860 many patents were issued in Europe and a few in the United States for gas-
engines, but the progress was slow, and its practical introduction for power came with
spasmodic effect and low efficiency. From 1860 on, practical improvement seems to have
been made, and the Lenoir motor was produced in France and brought to the United States.
It failed to meet expectations, and was soon followed by further improvements in the Hugon
motor in France (1862), followed by Beau de Rocha’s four-cycle idea, which has been
slowly developed through a long series of experimental trials by different inventors. In the
hands of Otto and Langdon a further progress was made, and numerous patents were issued
in England, France, and Germany, and followed up by an increasing interest in the United
States, with a few patents.
From 1870 improvements seem to have advanced at a steady rate, and largely in the valve-
gear and precision of governing for variable load. The early idea of the necessity of slow
combustion was a great drawback in the advancement of efficiency, and the suggestion of de
Rocha in 1862 did not take root as a prophetic truth until many failures and years of
experience had taught the fundamental axiom that rapidity of action in both combustion and
expansion was the basis of success in explosive motors.
With this truth and the demand for small and safe prime movers, the manufacture of gas-
engines increased in Europe and America at a more rapid rate, and improvements in
perfecting the details of this cheap and efficient prime mover have finally raised it to the
dignity of a standard motor and a dangerous rival of the steam-engine for small and
intermediate powers, with a prospect of largely increasing its individual units to many
hundred, if not to the thousand horse-power in a single cylinder. The unit size in a single
cylinder has now reached to about 700 horse-power and by combining cylinders in the same
machine, powers of from 1,500 to 2,000 horse-power are now available for large power-
plants.

MAIN TYPES OF INTERNAL-COMBUSTION ENGINES

This form of prime mover has been built in so many different types, all of which have
operated with some degree of success that the diversity in form will not be generally
appreciated unless some attempt is made to classify the various designs that have received
practical application. Obviously the same type of engine is not universally applicable,
because each class of work has individual peculiarities which can best be met by an engine
designed with the peculiar conditions present in view. The following tabular synopsis will
enable the reader to judge the extent of the development of what is now the most popular
prime mover for all purposes.

A. Internal Combustion (Standard Type)

1. Single Acting (Standard Type)
2. Double Acting (For Large Power Only)
3. Simple (Universal Form)
4. Compound (Rarely Used)
5. Reciprocating Piston (Standard Type)
6. Turbine (Revolving Rotor, not fully developed)

A1. Two-Stroke Cycle
a. Two Port
b. Three Port
c. Combined Two and Three Port
d. Fourth Port Accelerator
e. Differential Piston Type
f. Distributor Valve System

A2. Four-Stroke Cycle
a. Automatic Inlet Valve
b. Mechanical Inlet Valve
c. Poppet or Mushroom Valve
d. Slide Valve
d 1. Sleeve Valve
d 2. Reciprocating Ring Valve
d 3. Piston Valve
e. Rotary Valves
e 1. Disc
e 2. Cylinder or Barrel
e 3. Single Cone
e 4. Double Cone
f. Two Piston (Balanced Explosion)
g. Rotary Cylinder, Fixed Crank (Aerial)
h. Fixed Cylinder, Rotary Crank (Standard Type)

A3. Six-Stroke Cycle

B. External Combustion (Practically Obsolete)
a. Turbine, Revolving Rotor
b. Reciprocating Piston

CLASSIFICATION BY CYLINDER ARRANGEMENT

Single Cylinder
a. Vertical
b. Horizontal
c. Inverted Vertical

Double Cylinder
a. Vertical
b. Horizontal (Side by Side)
c. Horizontal (Opposed)
d. 45 to 90 Degrees V (Angularly Disposed)
e. Horizontal Tandem (Double Acting)

Three Cylinder
a. Vertical
b. Horizontal
c. Rotary (Cylinders Spaced at 120 Degrees)
d. Radially Placed (Stationary Cylinders)
e. One Vertical, One Each Side at an Angle
f. Compound (Two High Pressure, One Low Pressure)

Four Cylinder
a. Vertical
b. Horizontal (Side by Side)
c. Horizontal (Two Pairs Opposed)
d. 45 to 90 Degrees V
e. Twin Tandem (Double Acting)

Five Cylinder
a. Vertical (Five Throw Crankshaft)
b. Radially Spaced at 72 Degrees (Stationary)
c. Radially Placed Above Crankshaft (Stationary)
d. Placed Around Rotary Crankcase (72 Degrees Spacing)

Six Cylinder
a. Vertical
b. Horizontal (Three Pairs Opposed)
c. 45 to 90 Degrees V

Seven Cylinder
a. Equally Spaced (Rotary)

Eight Cylinder
a. Vertical
b. Horizontal (Four Pairs Opposed)
c. 45 to 90 Degrees V

Nine Cylinder
a. Equally Spaced (Rotary)

Twelve Cylinder
a. Vertical
b. Horizontal (Six Pairs Opposed)
c. 45 to 90 Degrees V

Fourteen Cylinder
a. Rotary

Sixteen Cylinder
a. 45 to 90 Degrees V
b. Horizontal (Eight Pairs Opposed)

Eighteen Cylinder
a. Rotary Cylinder

Two-Cylinder, Double Acting, Four Cycle Engine for Blast Furnace Gas Fuel
Weight 600 Pounds per Horsepower
Very slow speed, made in sizes up to 2000 Horsepower. 60 to 100 R.P.M.

Two Cylinder Opposed Gas Engine—150 to 650 Horsepower Sizes.
500 to 600 Pounds per Horsepower. 90 to 100 R.P.M.

Stationary Gas Engine
Stationary Diesel Engine Speed Approximately
Four Cycle—Two Cylinder 300
450 to 500 Pounds per Horsepower 200 R.P.M.
Pounds per Horsepower

Fig. 2.—Plate Showing Heavy, Slow Speed Internal Combustion Engines Used Only for
Stationary Power in Large Installations Giving Weight to Horse-Power Ratio.

Four Cylinder Diesel Engine for Marine Use
250 Pounds per Horsepower

Two Cycle Marine Engine
50-100 Pounds per Horsepower
600-800 R.P.M.

Single Cylinder Vertical Farm Engine
150 Pounds per Horsepower—Speed 400 R.P.M.

Two Cylinder Four Cycle Tractor Engine
75 Pounds per Horsepower
800 to 1000 R.P.M.
Four Cylinder Four Cycle Automobile Power Plant
Weighs about 25 Pounds per Horsepower
1200 to 2000 R.P.M.

Fig. 3.—Various Forms of Internal Combustion Engines Showing Decrease in Weight to
Horse-Power Ratio with Augmenting Speed of Rotation.

Eight Cylinder “Vee” Automobile Engine Two Cylinder Air Cooled Motorcycle
15 to 18 Pounds per Horsepower Engine weights 8-10 Pounds Horsepower
Speeds 1500 to 2000 R.P.M Speed 3000 R.P.M.

Six, Eight or Twelve Cylinder Water Cooled Aviation Engine, Tandem or V Form
4 to 6 Pounds per Horsepower
Speed 1500 R.P.M. Direct Coupled—2000 R.P.M. Geared Drive

Fourteen or Eighteen Cylinder
Seven or Nine Cylinder Revolving
Revolving Air Cooled Aviation Engine
Air Cooled
Speed 1200 R.P.M.
Speed 1200 R.P.M. 2.8 Pounds per Horsepower
2 Pounds per Horsepower

Fig. 4.—Internal Combustion Engine Types of Extremely Fine Construction and Refined
Design, Showing Great Power Outputs for Very Small Weight, a Feature Very Much Desired
in Airplane Power Plants.

Of all the types enumerated above engines having less than eight cylinders are the most
popular in everything but aircraft work. The four-cylinder vertical is without doubt the most
widely used of all types owing to the large number employed as automobile power plants.
Stationary engines in small and medium powers are invariably of the single or double form.
Three-cylinder engines are seldom used at the present time, except in marine work and in
some stationary forms. Eight- and twelve-cylinder motors have received but limited
application and practically always in automobiles, racing motor boats or in aircraft. The only
example of a fourteen-cylinder motor to be used to any extent is incorporated in aeroplane
construction. This is also true of the sixteen- and eighteen-cylinder forms and of twenty-
four-cylinder engines now in process of development.
The duty an engine is designed for determines the weight per horse-power. High powered
engines intended for steady service are always of the slow speed type and consequently are
of very massive construction. Various forms of heavy duty type stationary engines are
shown at Fig. 2. Some of these engines may weigh as much as 600 pounds per horse-power.
A further study is possible by consulting data given on Figs. 3 and 4. As the crank-shaft
speed increases and cylinders are multiplied the engines become lighter. While the big
stationary power plants may run for years without attention, airplane engines require
rebuilding after about 60 to 80 hours air service for the fixed cylinder types and 40 hours or
less for the rotary cylinder air-cooled forms. There is evidently a decrease in durability and
reliability as the weight is lessened. These illustrations also permit of obtaining a good idea
of the variety of forms internal combustion engines are made in.

CHAPTER II
Operating Principles of Two- and Four-Stroke Engines—Four-cycle Action—Two-
cycle Action—Comparing Two- and Four-cycle Types—Theory of Gas and
Gasoline Engine—Early Gas-Engine Forms—Isothermal Law—Adiabatic Law—
Temperature Computations—Heat and Its Work—Conversion of Heat to Power—
Requisites for Best Power Effect.

OPERATING PRINCIPLES OF TWO- AND FOUR-STROKE CYCLE ENGINES

Before discussing the construction of the various forms of internal combustion engines it
may be well to describe the operating cycle of the types most generally used. The two-cycle
engine is the simplest because there are no valves in connection with the cylinder, as the gas
is introduced into that member and expelled from it through ports cored into the cylinder
walls. These are covered by the piston at a certain portion of its travel and uncovered at
other parts of its stroke. In the four-cycle engine the explosive gas is admitted to the cylinder
through a port at the head end closed by a valve, while the exhaust gas is expelled through
another port controlled in a similar manner. These valves are operated by mechanism
distinct from the piston.

Fig. 5.—Outlining First Two Strokes of Piston in Four-Cycle Engine.

Fig. 6.—Outlining Second Two Strokes of Piston in Four-Cycle
Engine.

The action of the four-cycle type may be easily understood if one refers to illustrations at
Figs. 5 and 6. It is called the “four-stroke engine” because the piston must make four strokes
in the cylinder for each explosion or power impulse obtained. The principle of the gas-
engine of the internal combustion type is similar to that of a gun, i.e., power is obtained by
the rapid combustion of some explosive or other quick burning substance. The bullet is
driven out of the gun barrel by the pressure of the gas evolved when the charge of powder is
ignited. The piston or movable element of the gas-engine is driven from the closed or head
end to the crank end of the cylinder by a similar expansion of gases resulting from
combustion. The first operation in firing a gun or securing an explosion in the cylinder of the
gas-engine is to fill the combustion space with combustible material. This is done by a down
stroke of the piston during which time the inlet valve opens to admit the gaseous charge to
the cylinder interior. This operation is shown at Fig. 5, A. The second operation is to
compress this gas which is done by an upward stroke of the piston as shown at Fig. 5, B.
When the top of the compression stroke is reached, the gas is ignited and the piston is driven
down toward the open end of the cylinder, as indicated at Fig. 6, C. The fourth operation or
exhaust stroke is performed by the return upward movement of the piston as shown at Fig. 6,
D during which time the exhaust valve is opened to permit the burnt gases to leave the
cylinder. As soon as the piston reaches the top of its exhaust stroke, the energy stored in the
fly-wheel rim during the power stroke causes that member to continue revolving and as the

piston again travels on its down stroke the inlet valve opens and admits a charge of fresh gas
and the cycle of operations is repeated.

Fig. 7.—Sectional View of L Head Gasoline Engine Cylinder
Showing Piston Movements During Four-Stroke Cycle.

The illustrations at Fig. 7 show how the various cycle functions take place in an L head type
water cooled cylinder engine. The sections at A and C are taken through the inlet valve,
those at B and D are taken through the exhaust valve.
The two-cycle engine works on a different principle, as while only the combustion chamber
end of the piston is employed to do useful work in the four-cycle engine, both upper and
lower portions are called upon to perform the functions necessary to two-cycle engine
operation. Instead of the gas being admitted into the cylinder as is the case with the four-
stroke engine, it is first drawn into the engine base where it receives a preliminary
compression prior to its transfer to the working end of the cylinder. The views at Fig. 8
should indicate clearly the operation of the two-port two-cycle engine. At A the piston is
seen reaching the top of its stroke and the gas above the piston is being compressed ready
for ignition, while the suction in the engine base causes the automatic valve to open and
admits mixture from the carburetor to the crank case. When the piston reaches the top of its
stroke, the compressed gas is ignited and the piston is driven down on the power stroke,
compressing the gas in the engine base.

Fig. 8.—Showing Two-port, Two-cycle Engine Operation.

When the top of the piston uncovers the exhaust port the flaming gas escapes because of its
pressure. A downward movement of the piston uncovers the inlet port opposite the exhaust
and permits the fresh gas to bypass through the transfer passage from the engine base to the
cylinder. The conditions with the intake and exhaust port fully opened are clearly shown at
Fig. 8, C. The deflector plate on the top of the piston directs the entering fresh gas to the top
of the cylinder and prevents the main portion of the gas stream from flowing out through the
open exhaust port. On the next upstroke of the piston the gas in the cylinder is compressed
and the inlet valve opened, as shown at A to permit a fresh charge to enter the engine base.

Fig. 9.—Defining Three-port, Two-cycle Engine Action.

The operating principle of the three-port, two-cycle engine is practically the same as that
previously described with the exception that the gas is admitted to the crank-case through a
third port in the cylinder wall, which is uncovered by the piston when that member reaches
the end of its upstroke. The action of the three-port form can be readily ascertained by
studying the diagrams given at Fig. 9. Combination two- and three-port engines have been
evolved and other modifications made to improve the action.

THE TWO-CYCLE AND FOUR-CYCLE TYPES

In the earlier years of explosive-motor progress was evolved the two types of motors in
regard to the cycles of their operation. The early attempts to perfect the two-cycle principle
were for many years held in abeyance from the pressure of interests in the four-cycle type,
until its simplicity and power possibilities were demonstrated by Mr. Dugald Clerk in
England, who gave the principles of the two-cycle motor a broad bearing leading to
immediate improvements in design, which has made further progress in the United States,
until at the present time it has an equal standard value as a motor-power in some
applications as its ancient rival the four-cycle or Otto type, as demonstrated by Beau de
Rocha in 1862.
Thermodynamically, the methods of the two types are equal as far as combustion is
concerned, and compression may favor in a small degree the four-cycle type as well as the
purity of the charge. The cylinder volume of the two-cycle motor is much smaller per unit of

power, and the enveloping cylinder surface is therefore greater per unit of volume. Hence
more heat is carried off by the jacket water during compression, and the higher compression
available from this tends to increase the economy during compression which is lost during
expansion.
From the above considerations it may be safely stated that a lower temperature and higher
pressure of charge at the beginning of compression is obtained in the two-cycle motor,
greater weight of charge and greater specific power of higher compression resulting in
higher thermal efficiency. The smaller cylinder for the same power of the two-cycle motor
gives less friction surface per impulse than of the other type; although the crank-chamber
pressure may, in a measure, balance the friction of the four-cycle type. Probably the
strongest points in favor of the two-cycle type are the lighter fly-wheel and the absence of
valves and valve gear, making this type the most simple in construction and the lightest in
weight for its developed power. Yet, for the larger power units, the four-cycle type will no
doubt always maintain the standard for efficiency and durability of action.
The distribution of the charge and its degree of mixture with the remains of the previous
explosion in the clearance space, has been a matter of discussion for both types of explosive
motors, with doubtful results. In Fig. 10, A we illustrate what theory suggests as to the
distribution of the fresh charge in a two-cycle motor, and in Fig. 10, B what is the probable
distribution of the mixture when the piston starts on its compressive stroke. The arrows
show the probable direction of flow of the fresh charge and burnt gases at the crucial
moment.
In Fig. 10, C is shown the complete out-sweep of the products of combustion for the full
extent of the piston stroke of a four-cycle motor, leaving only the volume of the clearance to
mix with the new charge and at D the manner by which the new charge sweeps by the
ignition device, keeping it cool and avoiding possibilities of pre-ignition by undue heating of
the terminals of the sparking device. Thus, by enveloping the sparking device with the pure
mixture, ignition spreads through the charge with its greatest possible velocity, a most
desirable condition in high-speed motors with side-valve chambers and igniters within the
valve chamber.

Fig. 10.—Diagrams Contrasting Action of Two- and Four-Cycle
Cylinders on Exhaust and Intake Stroke.

THEORY OF THE GAS AND GASOLINE ENGINE

The laws controlling the elements that create a power by their expansion by heat due to
combustion, when properly understood, become a matter of computation in regard to their
value as an agent for generating power in the various kinds of explosive engines. The
method of heating the elements of power in explosive engines greatly widens the limits of
temperature as available in other types of heat-engines. It disposes of many of the practical
troubles of hot-air, and even of steam-engines, in the simplicity and directness of application
of the elements of power. In the explosive engine the difficulty of conveying heat for

producing expansive effect by convection is displaced by the generation of the required heat
within the expansive element and at the instant of its useful work. The low conductivity of
heat to and from air has been the great obstacle in the practical development of the hot-air
engine; while, on the contrary, it has become the source of economy and practicability in the
development of the internal-combustion engine.
The action of air, gas, and the vapors of gasoline and petroleum oil, whether singly or
mixed, is affected by changes of temperature practically in nearly the same ratio; but when
the elements that produce combustion are interchanged in confined spaces, there is a marked
difference of effect. The oxygen of the air, the hydrogen and carbon of a gas, or vapor of
gasoline or petroleum oil are the elements that by combustion produce heat to expand the
nitrogen of the air and the watery vapor produced by the union of the oxygen in the air and
the hydrogen in the gas, as well as also the monoxide and carbonic-acid gas that may be
formed by the union of the carbon of gas or vapor with part of the oxygen of the air. The
various mixtures as between air and gas, or air and vapor, with the proportion of the
products of combustion left in the cylinder from a previous combustion, form the elements
to be considered in estimating the amount of pressure that may be obtained by their
combustion and expansive force.

EARLY GAS ENGINE FORMS

The working process of the explosive motor may be divided into three principal types: 1.
Motors with charges igniting at constant volume without compression, such as the Lenoir,
Hugon, and other similar types now abandoned as wasteful in fuel and effect. 2. Motors with
charges igniting at constant pressure with compression, in which a receiver is charged by a
pump and the gases burned while being admitted to the motor cylinder, such as types of the
Simon and Brayton engine. 3. Motors with charges igniting at constant volume with variable
compression, such as the later two- and four-cycle motors with compression of the indrawn
charge; limited in the two-cycle type and variable in the four-cycle type with the ratios of the
clearance space in the cylinder. This principle produces the explosive motor of greatest
efficiency.
The phenomena of the brilliant light and its accompanying heat at the moment of explosion
have been witnessed in the experiments of Dugald Clerk in England, the illumination lasting
throughout the stroke; but in regard to time in a four-cycle engine, the incandescent state
exists only one-quarter of the running time. Thus the time interval, together with the non-
conductibility of the gases, makes the phenomena of a high-temperature combustion within
the comparatively cool walls of a cylinder a practical possibility.

THE ISOTHERMAL LAW

The natural laws, long since promulgated by Boyle, Gay Lussac, and others, on the subject
of the expansion and compression of gases by force and by heat, and their variable pressures
and temperatures when confined, are conceded to be practically true and applicable to all
gases, whether single, mixed, or combined.

The law formulated by Boyle only relates to the compression and expansion of gases
without a change of temperature, and is stated in these words:
If the temperature of a gas be kept constant, its pressure or elastic force will vary inversely
as the volume it occupies.
It is expressed in the formula P × V = C, or pressure × volume = constant. Hence, C/P = V
and C/V = P.
Thus the curve formed by increments of pressure during the expansion or compression of a
given volume of gas without change of temperature is designated as the isothermal curve in
which the volume multiplied by the pressure is a constant value in expansion, and inversely
the pressure divided by the volume is a constant value in compressing a gas.
But as compression and expansion of gases require force for their accomplishment
mechanically, or by the application or abstraction of heat chemically, or by convection, a
second condition becomes involved, which was formulated into a law of thermodynamics by
Gay Lussac under the following conditions: A given volume of gas under a free piston
expands by heat and contracts by the loss of heat, its volume causing a proportional
movement of a free piston equal to 1⁄273 part of the cylinder volume for each degree
Centigrade difference in temperature, or 1⁄492 part of its volume for each degree Fahrenheit.
With a fixed piston (constant volume), the pressure is increased or decreased by an increase
or decrease of heat in the same proportion of 1⁄273 part of its pressure for each degree
Centigrade, or 1⁄492 part of its pressure for each degree Fahrenheit change in temperature.
This is the natural sequence of the law of mechanical equivalent, which is a necessary
deduction from the principle that nothing in nature can be lost or wasted, for all the heat that
is imparted to or abstracted from a gaseous body must be accounted for, either as heat or its
equivalent transformed into some other form of energy. In the case of a piston moving in a
cylinder by the expansive force of heat in a gaseous body, all the heat expended in expansion
of the gas is turned into work; the balance must be accounted for in absorption by the
cylinder or radiation.

THE ADIABATIC LAW

This theory is equally applicable to the cooling of gases by abstraction of heat or by cooling
due to expansion by the motion of a piston. The denominators of these heat fractions of
expansion or contraction represent the absolute zero of cold below the freezing-point of
water, and read -273° C. or -492.66° = -460.66° F. below zero; and these are the starting-
points of reference in computing the heat expansion in gas-engines. According to Boyle’s
law, called the first law of gases, there are but two characteristics of a gas and their
variations to be considered, viz., volume and pressure: while by the law of Gay Lussac,
called the second law of gases, a third is added, consisting of the value of the absolute
temperature, counting from absolute zero to the temperatures at which the operations take
place. This is the Adiabatic law.

The ratio of the variation of the three conditions—volume, pressure, and heat—from the
absolute zero temperature has a certain rate, in which the volume multiplied by the pressure
and the product divided by the absolute temperature equals the ratio of expansion for each
degree. If a volume of air is contained in a cylinder having a piston and fitted with an
indicator, the piston, if moved to and fro slowly, will alternately compress and expand the
air, and the indicator pencil will trace a line or lines upon the card, which lines register the
change of pressure and volume occurring in the cylinder. If the piston is perfectly free from
leakage, and it be supposed that the temperature of the air is kept quite constant, then the
line so traced is called an Isothermal line, and the pressure at any point when multiplied by
the volume is a constant, according to Boyle’s law,
pv = a constant.
If, however, the piston is moved very rapidly, the air will not remain at constant temperature,
but the temperature will increase because work has been done upon the air, and the heat has
no time to escape by conduction. If no heat whatever is lost by any cause, the line will be
traced over and over again by the indicator pencil, the cooling by expansion doing work
precisely equalling the heating by compression. This is the line of no transmission of heat,
therefore known as Adiabatic.

Fig. 11.—Diagram Isothermal and Adiabatic Lines.

The expansion of a gas 1⁄273 of its volume for every degree Centigrade, added to its
temperature, is equal to the decimal .00366, the coefficient of expansion for Centigrade
units. To any given volume of a gas, its expansion may be computed by multiplying the
coefficient by the number of degrees, and by reversing the process the degree of acquired
heat may be obtained approximately. These methods are not strictly in conformity with the
absolute mathematical formula, because there is a small increase in the increment of
expansion of a dry gas, and there is also a slight difference in the increment of expansion

due to moisture in the atmosphere and to the vapor of water formed by the union of the
hydrogen and oxygen in the combustion chamber of explosive engines.

TEMPERATURE COMPUTATIONS

The ratio of expansion on the Fahrenheit scale is derived from the absolute temperature
below the freezing-point of water (32°) to correspond with the Centigrade scale; therefore
1⁄492.66 = .0020297, the ratio of expansion from 32° for each degree rise in temperature on the

Fahrenheit scale. As an example, if the temperature of any volume of air or gas at constant
volume is raised, say from 60° to 2000° F., the increase in temperature will be 1940°. The
ratio will be 1⁄520.66 = .0019206. Then by the formula:
Ratio × acquired temp. × initial pressure = the gauge pressure; and .0019206 × 1940° × 14.7
= 54.77 lbs.
By another formula, a convenient ratio is obtained by (absolute pressure)/(absolute temp.) or
14.7⁄520.66 = .028233; then, using the difference of temperature as before, .028233 × 1940° =

54.77 lbs. pressure.
By another formula, leaving out a small increment due to specific heat at high temperatures:
Atmospheric pressure × absolute temp. + acquired temp.
I. =
Absolute temp. + initial temp.
absolute pressure due to the acquired temperature, from which the atmospheric pressure is
deducted for the gauge pressure. Using the foregoing example, we have
14.7 × 460.66° + 2000°
= 69.47 - 14.7 = 54.77, the gauge pressure,
460.66 + 60°
460.66 being the absolute temperature for zero Fahrenheit.
For obtaining the volume of expansion of a gas from a given increment of heat, we have the
approximate formula:
Volume × absolute temp. + acquired temp.
II. = heated volume.
Absolute temp. + initial temp.
In applying this formula to the foregoing example, the figures become:
460.66° + 2000°
I. × = 4.72604 volumes.
460.66 + 60°
From this last term the gauge pressure may be obtained as follows:
III. 4.72604 × 14.7 = 69.47 lbs. absolute - 14.7 lbs. atmospheric pressure = 54.77 lbs. gauge
pressure; which is the theoretical pressure due to heating air in a confined space, or at
constant volume from 60° to 2000° F.

By inversion of the heat formula for absolute pressure we have the formula for the acquired
heat, derived from combustion at constant volume from atmospheric pressure to gauge
pressure plus atmospheric pressure as derived from Example I., by which the expression
absolute pressure × absolute temp. + initial temp.
initial absolute pressure
= absolute temperature + temperature of combustion, from which the acquired temperature
is obtained by subtracting the absolute temperature.
Then, for example,
69.47 × 460.66 + 60
= 2460.66, and 2460.66 - 460.66 = 2000°,
14.7
the theoretical heat of combustion. The dropping of terminal decimals makes a small
decimal difference in the result in the different formulas.

HEAT AND ITS WORK

By Joule’s law of the mechanical equivalent of heat, whenever heat is imparted to an elastic
body, as air or gas, energy is generated and mechanical work produced by the expansion of
the air or gas. When the heat is imparted by combustion within a cylinder containing a
movable piston, the mechanical work becomes an amount measurable by the observed
pressure and movement of the piston. The heat generated by the explosive elements and the
expansion of the non-combining elements of nitrogen and water vapor that may have been
injected into the cylinder as moisture in the air, and the water vapor formed by the union of
the oxygen of the air with the hydrogen of the gas, all add to the energy of the work from
their expansion by the heat of internal combustion. As against this, the absorption of heat by
the walls of the cylinder, the piston, and cylinder-head or clearance walls, becomes a
modifying condition in the force imparted to the moving piston.
It is found that when any explosive mixture of air and gas or hydrocarbon vapor is fired, the
pressure falls far short of the pressure computed from the theoretical effect of the heat
produced, and from gauging the expansion of the contents of a cylinder. It is now well
known that in practice the high efficiency which is promised by theoretical calculation is
never realized; but it must always be remembered that the heat of combustion is the real
agent, and that the gases and vapors are but the medium for the conversion of inert elements
of power into the activity of energy by their chemical union. The theory of combustion has
been the leading stimulus to large expectations with inventors and constructors of explosive
motors; its entanglement with the modifying elements in practice has delayed the best
development in construction, and as yet no really positive design of best form or action
seems to have been accomplished, although great progress has been made during the past
decade in the development of speed, reliability, economy, and power output of the individual
units of this comparatively new power.

One of the most serious difficulties in the practical development of pressure, due to the
theoretical computations of the pressure value of the full heat, is probably caused by
imparting the heat of the fresh charge to the balance of the previous charge that has been
cooled by expansion from the maximum pressure to near the atmospheric pressure of the
exhaust. The retardation in the velocity of combustion of perfectly mixed elements is now
well known from experimental trials with measured quantities; but the principal difficulty in
applying these conditions to the practical work of an explosive engine where a necessity for
a large clearance space cannot be obviated, is in the inability to obtain a maximum effect
from the imperfect mixture and the mingling of the products of the last explosion with the
new mixture, which produces a clouded condition that makes the ignition of the mass
irregular or chattering, as observed in the expansion lines of indicator cards; but this must
not be confounded with the reaction of the spring in the indicator.
Stratification of the mixture has been claimed as taking place in the clearance chamber of
the cylinder; but this is not a satisfactory explanation in view of the vortical effect of the
violent injection of the air and gas or vapor mixture. It certainly cannot become a perfect
mixture in the time of a stroke of a high-speed motor of the two-cycle class. In a four-cycle
engine, making 1,500 revolutions per minute, the injection and compression in any one
cylinder take place in one twenty-fifth of a second—formerly considered far too short a time
for a perfect infusion of the elements of combustion but now very easily taken care of
despite the extremely high speed of numerous aviation and automobile power-plants.

Table I.—Explosion at Constant Volume in a Closed Chamber.

Observed
Diagram Temp. of Time of Computed
Gauge
Curve Mixture Injected. Injection Explosion. Temp.
Pressure.
Fig. 8. Fahr. Second. Fahr.
Pounds.
a 1 volume gas to 14 volumes air. 64° 0.45 40. 1,483°
b 1 „ „ „ 13 „ „ 51° 0.31 51.5 1,859°
c 1 „ „ „ 12 „ „ 51° 0.24 60. 2,195°
d 1 „ „ „ 11 „ „ 51° 0.17 61. 2,228°
e 1 „ „ „ 9 „ „ 62° 0.08 78. 2,835°
f 1 „ „ „ 7 „ „ 62° 0.06 87. 3,151°
g 1 „ „ „ 6 „ „ 51° 0.04 90. 3,257°
h 1 „ „ „ 5 „ „ 51° 0.055 91. 3,293°
i 1 „ „ „ 4 „ „ 66° 0.16 80. 2,871°

In an examination of the times of explosion and the corresponding pressures in both tables,
it will be seen that a mixture of 1 part gas to 6 parts air is the most effective and will give the
highest mean pressure in a gas-engine. There is a limit to the relative proportions of
illuminating gas and air mixture that is explosive, somewhat variable, depending upon the
proportion of hydrogen in the gas. With ordinary coal-gas, 1 of gas to 15 parts of air; and on
the lower end of the scale, 1 volume of gas to 2 parts air, are non-explosive. With gasoline
vapor the explosive effect ceases at 1 to 16, and a saturated mixture of equal volumes of
vapor and air will not explode, while the most intense explosive effect is from a mixture of 1
part vapor to 9 parts air. In the use of gasoline and air mixtures from a carburetor, the best
effect is from 1 part saturated air to 8 parts free air.

Table II.—Properties and Explosive Temperature of a Mixture of One Part
of Illuminating Gas of 660 Thermal Units per Cubic Foot with Various
Proportions of Air without Mixture of Charge with the Products of a
Previous Explosion.

Usual
Specific Heat. Heat to Rise of
Propor- Pounds Heat Units Required Usual
Raise One Heat Units Temperature
tion, in One to Raise 1 Lb. 1 Deg. Ratio Combus-
Cubic Foot Evolved by due to
Air to Cubic Fahrenheit. Col. tion
of Mixture Combus- Explosion
Gas by Foot of 6/5 Efficien-
1 Deg. tion. at
Volumes. Mixture. Constant Constant cy.
Fahr. Constant
Pressure. Volume. Volume.
6 to 1 .074195 .2668 .1913 .014189 94.28 6644.6 .465 3090
7 to 1 .075012 .2628 .1882 .014116 82. 5844.4 .518 3027
8 to 1 .075647 .2598 .1858 .014059 73.33 5216.1 .543 2832
9 to 1 .076155 .2575 .1846 .014013 66. 4709.9 .56 2637
10 to 1 .076571 .2555 .1825 .013976 60. 4293. .575 2468
11 to 1 .076917 .2540 .1813 .013945 55. 3944. .585 2307
12 to 1 .077211 .2526 .1803 .013922 50.77 3646.7 .58 2115

The weight of a cubic foot of gas and air mixture as given in Col. 2 is found by adding the
number of volumes of air multiplied by its weight, .0807, to one volume of gas of weight
.035 pound per cubic foot and dividing by the total number of volumes; for example, as in
the table, 6 × .0807 = .5192⁄7 = .074195 as in the first line, and so on for any mixture or for
other gases of different specific weight per cubic foot. The heat units evolved by combustion
of the mixture (Col. 6) are obtained by dividing the total heat units in a cubic foot of gas by
the total proportion of the mixture, 660⁄7 = 94.28 as in the first line of the table. Col. 5 is
obtained by multiplying the weight of a cubic foot of the mixture in Col. 2 by the specific
heat at a constant volume (Col. 4), Col. 6/Col. 5 = Col. 7 the total heat ratio, of which Col. 8
gives the usual combustion efficiency—Col. 7 × Col. 8 gives the absolute rise in
temperature of a pure mixture, as given in Col. 9.
The many recorded experiments made to solve the discrepancy between the theoretical and
the actual heat development and resulting pressures in the cylinder of an explosive motor, to
which much discussion has been given as to the possibilities of dissociation and the
increased specific heat of the elements of combustion and non-combustion, as well, also, of
absorption and radiation of heat, have as yet furnished no satisfactory conclusion as to what
really takes place within the cylinder walls. There seems to be very little known about
dissociation, and somewhat vague theories have been advanced to explain the phenomenon.
The fact is, nevertheless, apparent as shown in the production of water and other producer
gases by the use of steam in contact with highly incandescent fuel. It is known that a
maximum explosive mixture of pure gases, as hydrogen and oxygen or carbonic oxide and
oxygen, suffers a contraction of one-third their volume by combustion to their compounds,
steam or carbonic acid. In the explosive mixtures in the cylinder of a motor, however, the
combining elements form so small a proportion of the contents of the cylinder that the
shrinkage of their volume amounts to no more than 3 per cent. of the cylinder volume. This
by no means accounts for the great heat and pressure differences between the theoretical and
actual effects.

CONVERSION OF HEAT TO POWER

The utilization of heat in any heat-engine has long been a theme of inquiry and experiment
with scientists and engineers, for the purpose of obtaining the best practical conditions and
construction of heat-engines that would represent the highest efficiency or the nearest
approach to the theoretical value of heat, as measured by empirical laws that have been
derived from experimental researches relating to its ultimate volume. It is well known that
the steam-engine returns only from 12 to 18 per cent. of the power due to the heat generated
by the fuel, about 25 per cent. of the total heat being lost in the chimney, the only use of
which is to create a draught for the fire; the balance, some 60 per cent., is lost in the exhaust
and by radiation. The problem of utmost utilization of force in steam has nearly reached its
limit.
The internal-combustion system of creating power is comparatively new in practice, and is
but just settling into definite shape by repeated trials and modification of details, so as to
give somewhat reliable data as to what may be expected from the rival of the steam-engine
as a prime mover. For small powers, the gas, gasoline, and petroleum-oil engines are forging
ahead at a rapid rate, filling the thousand wants of manufacture and business for a power that
does not require expensive care, that is perfectly safe at all times, that can be used in any
place in the wide world to which its concentrated fuel can be conveyed, and that has
eliminated the constant handling of crude fuel and water.

REQUISITES FOR BEST POWER EFFECT

The utilization of heat in a gas-engine is mainly due to the manner in which the products
entering into combustion are distributed in relation to the movement of the piston. The
investigation of the foremost exponent of the theory of the explosive motor was prophetic in
consideration of the later realization of the best conditions under which these motors can be
made to meet the requirements of economy and practicability. As early as 1862, Beau de
Rocha announced, in regard to the coming power, that four requisites were the basis of
operation for economy and best effect. 1. The greatest possible cylinder volume with the
least possible cooling surface. 2. The greatest possible rapidity of expansion. Hence, high
speed. 3. The greatest possible expansion. Long stroke. 4. The greatest possible pressure at
the commencement of expansion. High compression.

CHAPTER III
Efficiency of Internal Combustion Engines—Various Measures of Efficiency—
Temperatures and Pressures—Factors Governing Economy—Losses in Wall
Cooling—Value of Indicator Cards—Compression in Explosive Motors—Factors
Limiting Compression—Causes of Heat Losses and Inefficiency—Heat Losses to
Cooling Water.

EFFICIENCY OF INTERNAL COMBUSTION ENGINES

Efficiencies are worked out through intricate formulas for a variety of theoretical and
unknown conditions of combustion in the cylinder: ratios of clearance and cylinder volume,
and the uncertain condition of the products of combustion left from the last impulse and the
wall temperature. But they are of but little value, except as a mathematical inquiry as to
possibilities. The real commercial efficiency of a gas or gasoline-engine depends upon the
volume of gas or liquid at some assigned cost, required per actual brake horse-power per
hour, in which an indicator card should show that the mechanical action of the valve gear
and ignition was as perfect as practicable, and that the ratio of clearance, space, and cylinder
volume gave a satisfactory terminal pressure and compression: i.e., the difference between
the power figured from the indicator card and the brake power being the friction loss of the
engine.
In four-cycle motors of the compression type, the efficiencies are greatly advanced by
compression, producing a more complete infusion of the mixture of gas or vapor and air,
quicker firing, and far greater pressure than is possible with the two-cycle type previously
described. In the practical operation of the gas-engine during the past twenty years, the gas-
consumption efficiencies per indicated horse-power have gradually risen from 17 per cent.
to a maximum of 40 per cent. of the theoretical heat, and this has been done chiefly through
a decreased combustion chamber and increased compression—the compression having
gradually increased in practice from 30 lbs. per square inch to above 100; but there seems to
be a limit to compression, as the efficiency ratio decreases with greater increase in
compression. It has been shown that an ideal efficiency of 33 per cent. for 38 lbs.,
compression will increase to 40 per cent. for 66 lbs., and 43 per cent. for 88 lbs.
compression. On the other hand, greater compression means greater explosive pressure and
greater strain on the engine structure, which will probably retain in future practice the
compression between the limits of 40 and 90 lbs. except in super-compression engines
intended for high altitude work where compression pressures as high as 125 pounds have
been used.
In experiments made by Dugald Clerk, in England, with a combustion chamber equal to 0.6
of the space swept by the piston, with a compression of 38 lbs., the consumption of gas was
24 cubic feet per indicated horse-power per hour. With 0.4 compression space and 61 lbs.

compression, the consumption of gas was 20 cubic feet per indicated horse-power per hour;
and with 0.34 compression space and 87 lbs. compression, the consumption of gas fell to
14.8 cubic feet per indicated horse-power per hour—the actual efficiencies being
respectively 17, 21, and 25 per cent. This was with a Crossley four-cycle engine.

VARIOUS MEASURES OF EFFICIENCY

The efficiencies in regard to power in a heat-engine may be divided into four kinds, as
follows: I. The first is known as the maximum theoretical efficiency of a perfect engine
(represented by the lines in the indicator diagram). It is expressed by the formula
T 1 - T0
T1
and shows the work of a perfect cycle in an engine working between the received
temperature + absolute temperature (T1) and the initial atmospheric temperature + absolute
temperature (T0). II. The second is the actual heat efficiency, or the ratio of the heat turned
into work to the total heat received by the engine. It expresses the indicated horse-power.
III. The third is the ratio between the second or actual heat efficiency and the first or
maximum theoretical efficiency of a perfect cycle. It represents the greatest possible
utilization of the power of heat in an internal-combustion engine. IV. The fourth is the
mechanical efficiency. This is the ratio between the actual horse-power delivered by the
engine through a dynamometer or measured by a brake (brake horse-power), and the
indicated horse-power. The difference between the two is the power lost by engine friction.
In regard to the general heat efficiency of the materials of power in explosive engines, we
find that with good illuminating gas the practical efficiency varies from 25 to 40 per cent.;
kerosene-motors, 20 to 30; gasoline-motors, 20 to 32; acetylene, 25 to 35; alcohol, 20 to 30
per cent. of their heat value. The great variation is no doubt due to imperfect mixtures and
variable conditions of the old and new charge in the cylinder; uncertainty as to leakage and
the perfection of combustion. In the Diesel motors operating under high pressure, up to
nearly 500 pounds, an efficiency of 36 per cent. is claimed.

Fig. 12.—Graphic Diagram Showing Approximate Utilization of
Fuel Burned in Internal-Combustion Engine.

The graphic diagram at Fig. 12 is of special value as it shows clearly how the heat produced by charge
combustion is expended in an engine of average design.
On general principles the greater difference between the heat of combustion and the heat at exhaust is
the relative measure of the heat turned into work, which represents the degree of efficiency without
loss during expansion. The mathematical formulas appertaining to the computation of the element of
heat and its work in an explosive engine are in a large measure dependent upon assumed values, as the
conditions of the heat of combustion are made uncertain by the mixing of the fresh charge with the
products of a previous combustion, and by absorption, radiation, and leakage. The computation of the
temperature from the observed pressure may be made as before explained, but for compression-
engines the needed starting-points for computation are very uncertain, and can only be approximated
from the exact measure and value of the elements of combustion in a cylinder charge.

TEMPERATURES AND PRESSURES

Owing to the decrease from atmospheric pressure in the indrawing charge of the cylinder, caused by
valve and frictional obstruction, the compression seldom starts above 13 lbs. absolute, especially in
high-speed engines. Col. 3 in the following table represents the approximate absolute compression
pressure for the clearance percentage and ratio in Cols. 1 and 2, while Col. 4 indicates the gauge
pressure from the atmospheric line. The temperatures in Col. 5 are due to the compression in Col. 3
from an assumed temperature of 560° F. in the mixture of the fresh charge of 6 air to 1 gas with the
products of combustion left in the clearance chamber from the exhaust stroke of a medium-speed
motor. This temperature is subject to considerable variation from the difference in the heat-unit power
of the gases and vapors used for explosive power, as also of the cylinder-cooling effect. In Col. 6 is
given the approximate temperatures of explosion for a mixture of air 6 to gas 1 of 660 heat units per
cubic foot, for the relative values of the clearance ratio in Col. 2 at constant volume.

Table III.—Gas-Engine Clearance Ratios, Approximate Compression,
Temperatures of Explosion and Explosive Pressures with a Mixture
of Gas of 660 Heat Units per Cubic Foot and Mixture of Gas
1 to 6 of Air.

Clearance Ratio Approximate Approximate Absolute Absolute Approximate Approximate Approximate
Per Cent. V Compression Gauge Temperature Temperature Explosion Gauge Temperature
of = from Pressure. of of Explosion. Pressure Pressure. of
Vc
Piston 13 Pounds Compression Gas, 1 part; Absolute. Explosion,
Volume. P + C Vol. Absolute. from 560 Deg. Air, 6 parts. Fahrenheit.

Clearance Fahrenheit
in Cylinder.
1 2 3 4 5 6 7 8 9
Lbs. Deg. Deg. Lbs. Lbs. Deg.
.50 3. 57. 42. 822. 2488 169 144 2027
.444 3.25 65. 50. 846. 2568 197 182 2107
.40 3.50 70. 55. 868. 2638 212 197 2177
.363 3.75 77. 62. 889. 2701 234 219 2240
.333 4. 84. 69. 910. 2751 254 239 2290
.285 4.50 102. 88. 955. 2842 303 288 2381
.25 5. 114. 99. 983. 2901 336 321 2440

FACTORS GOVERNING ECONOMY

In view of the experiments in this direction, it clearly shows that in practical work, to obtain the
greatest economy per effective brake horse-power, it is necessary: 1st. To transform the heat into work
with the greatest rapidity mechanically allowable. This means high piston speed. 2d. To have high
initial compression. 3d. To reduce the duration of contact between the hot gases and the cylinder walls
to the smallest amount possible; which means short stroke and quick speed, with a spherical cylinder
head. 4th. To adjust the temperature of the jacket water to obtain the most economical output of actual
power. This means water-tanks or water-coils, with air-cooling surfaces suitable and adjustable to the
most economical requirement of the engine, which by late trials requires the jacket water to be
discharged at about 200° F. 5th. To reduce the wall surface of the clearance space or combustion
chamber to the smallest possible area, in proportion to its required volume. This lessens the loss of the
heat of combustion by exposure to a large surface, and allows of a higher mean wall temperature to
facilitate the heat of compression.

LOSSES IN WALL COOLING

In an experimental investigation of the efficiency of a gas-engine under variable piston speeds made in
France, it was found that the useful effect increases with the velocity of the piston—that is, with the
rate of expansion of the burning gases with mixtures of uniform volumes: so that the variations of time
of complete combustion at constant pressure, and the variations due to speed, in a way compensate in
their efficiencies. The dilute mixture, being slow burning, will have its time and pressure quickened by
increasing the speed.
Careful trials give unmistakable evidence that the useful effect increases with the velocity of the piston
—that is, with the rate of expansion of the burning gases. The time necessary for the explosion to
become complete and to attain its maximum pressure depends not only on the composition of the
mixture, but also upon the rate of expansion. This has been verified in experiments with a high-speed
motor, at speeds from 500 to 2,000 revolutions per minute, or piston speeds of from 16 to 64 feet per
second. The increased speed of combustion due to increased piston speed is a matter of great
importance to builders of gas-engines, as well as to the users, as indicating the mechanical direction of
improvements to lessen the wearing strain due to high speed and to lighten the vibrating parts with
increased strength, in order that the balancing of high-speed engines may be accomplished with the
least weight.
From many experiments made in Europe and in the United States, it has been conclusively proved that
excessive cylinder cooling by the water-jacket results in a marked loss of efficiency. In a series of
experiments with a simplex engine in France, it was found that a saving of 7 per cent. in gas
consumption per brake horse-power was made by raising the temperature of the jacket water from

141° to 165° F. A still greater saving was made in a trial with an Otto engine by raising the temperature
of the jacket water from 61° to 140° F.—it being 9.5 per cent. less gas per brake horse-power.
It has been stated that volumes of similar cylinders increase as the cube of their diameters, while the
surface of their cold walls varies as the square of their diameters; so that for large cylinders the ratio of
surface to volume is less than for small ones. This points to greater economy in the larger engines. The
study of many experiments goes to prove that combustion takes place gradually in the gas-engine
cylinder, and that the rate of increase of pressure or rapidity of firing is controlled by dilution and
compression of the mixture, as well as by the rate of expansion or piston speed. The rate of combustion
also depends on the size and shape of the explosion chamber, and is increased by the mechanical
agitation of the mixture during combustion, and still more by the mode of firing.

VALUE OF INDICATOR CARDS

Fig. 13.—Otto Four-Cycle Card.

To the uninitiated, indicator cards are considerable of a mystery; to those capable of reading them they
form an index relative to the action of any engine. An indicator card, such as shown at Fig. 13, is
merely a graphical representation of the various pressures existing in the cylinder for different
positions of the piston. The length is to some scale that represents the stroke of the piston. During the
intake stroke, the pressure falls below the atmospheric line. During compression, the curve gradually
becomes higher owing to increasing pressure as the volume is reduced. After ignition the pressure line
moves upward almost straight, then as the piston goes down on the explosion stroke, the pressure falls
gradually to the point of exhaust valve, opening when the sudden release of the imprisoned gas causes
a reduction in pressure to nearly atmospheric. An indicator card, or a series of them, will always show
by its lines the normal or defective condition of the inlet valve and passages; the actual line of
compression; the firing moment; the pressure of explosion; the velocity of combustion; the normal or
defective line of expansion, as measured by the adiabatic curve, and the normal or defective operation
of the exhaust valve, exhaust passages, and exhaust pipe. In fact, all the cycles of an explosive motor
may be made a practical study from a close investigation of the lines of an indicator card.

Fig. 14.—Diesel Motor Card.

A most unique card is that of the Diesel motor (Fig. 14), which involves a distinct principle in the
design and operation of internal-combustion motors, in that instead of taking a mixed charge for
instantaneous explosion, its charge primarily is of air and its compression to a pressure at which a
temperature is attained above the igniting point of the fuel, then injecting the fuel under a still higher
pressure by which spontaneous combustion takes place gradually with increasing volume over the
compression for part of the stroke or until the fuel charge is consumed. The motor thus operating
between the pressures of 500 and 35 lbs. per square inch, with a clearance of about 7 per cent., has
given an efficiency of 36 per cent. of the total heat value of kerosene oil.

COMPRESSION IN EXPLOSIVE MOTORS

That the compression in a gas, gasoline, or oil-engine has a direct relation to the power obtained, has
been long known to experienced builders, having been suggested by M. Beau de Rocha, in 1862, and
afterward brought into practical use in the four-cycle or Otto type about 1880. The degree of
compression has had a growth from zero, in the early engines, to the highest available due to the
varying ignition temperatures of the different gases and vapors used for explosive fuel, in order to
avoid premature explosion from the heat of compression. Much of the increased power for equal-
cylinder capacity is due to compression of the charge from the fact that the most powerful explosion of
gases, or of any form of explosive material, takes place when the particles are in the closest contact or
cohesion with one another, less energy in this form being consumed by the ingredients themselves to
bring about their chemical combination, and consequently more energy is given out in useful or
available work. This is best shown by the ignition of gunpowder, which, when ignited in the open air,
burns rapidly, but without explosion, an explosion only taking place if the powder be confined or
compressed into a small space.

Fig. 15.—Diagram of Heat in the Gas Engine
Cylinder.

In a gas or gasoline-motor with a small clearance or compression space—with high compression—the
surface with which the burning gases come into contact is much smaller in comparison with the
compression space in a low-compression motor. Another advantage of a high-compression motor is
that on account of the smaller clearance of combustion space less cooling water is required than with a
low-compression motor, as the temperature, and consequently the pressure, falls more rapidly. The loss
of heat through the water-jacket is thus less in the case of a high-compression than in that of a low-
compression motor. In the non-compression type of motor the best results were obtained with a charge
of 16 to 18 parts of gas and 100 parts of air, while in the compression type the best results are obtained
with an explosive mixture of 7 to 10 parts of gas and 100 parts of air, thus showing that by the
utilization of compression a weaker charge with a greater thermal efficiency is permissible.
It has been found that the explosive pressure resulting from the ignition of the charge of gas or
gasoline-vapor and air in the gas-engine cylinder is about 41⁄2 times the pressure prior to ignition. The
difficulty about getting high compression is that if the pressure is too high the charge is likely to ignite
prematurely, as compression always results in increased temperature. The cylinder may become too
hot, a deposit of carbon, a projecting electrode or plug body in the cylinder may become incandescent
and ignite the charge which has been excessively heated by the high compression and mixture of the
hot gases of the previous explosion.

FACTORS LIMITING COMPRESSION

With gasoline-vapor and air the compression should not be raised above about 90 to 95 pounds to the
square inch, many manufacturers not going above 65 or 70 pounds. For natural gas the compression
pressure may easily be raised to from 85 to 100 pounds per square inch. For gases of low calorific
value, such as blast-furnace or producer-gas, the compression may be increased to from 140 to 190
pounds. In fact the ability to raise the compression to a high point with these gases is one of the
principal reasons for their successful adoption for gas-engine use. In kerosene injection engines the

compression of 250 pounds per square inch has been used with marked economy. Many troubles in
regard to loss of power and increase of fuel have occurred and will no doubt continue, owing to the
wear of valves, piston, and cylinder, which produces a loss in compression and explosive pressure and
a waste of fuel by leakage. Faulty adjustment of valve movement is also a cause of loss of power;
which may be from tardy closing of the inlet-valve or a too early opening of the exhaust-valve.
The explosive pressure varies to a considerable amount in proportion to the compression pressure by
the difference in fuel value and the proportions of air mixtures, so that for good illuminating gas the
explosive pressure may be from 2.5 to 4 times the compression pressure. For natural gas 3 to 4.5, for
gasoline 3 to 5, for producer-gas 2 to 3, and for kerosene by injection 3 to 6.
The compression temperatures, although well known and easily computed from a known normal
temperature of the explosive mixture, are subject to the effect of the uncertain temperature of the gases
of the previous explosion remaining in the cylinder, the temperature of its walls, and the relative
volume of the charge, whether full or scant; which are terms too variable to make any computations
reliable or available.
For the theoretical compression temperatures from a known normal temperature, we append a table of
the rise in temperature for the compression pressures in the following table:

Table IV.—Compression Temperatures
from a Normal Temperature of
60 Degrees Fahrenheit

100 lbs. gauge 484° 60 lbs. gauge 373°
90 lbs. gauge 459° 50 lbs. gauge 339°
80 lbs. gauge 433° 40 lbs. gauge 301°
70 lbs. gauge 404° 30 lbs. gauge 258°

CHART FOR DETERMINING COMPRESSION PRESSURES

Fig. 16.—Chart Showing Relation Between Compression Volume
and Pressure.

A very useful chart (Fig. 16) for determining compression pressures in gasoline-engine cylinders for
various ratios of compression space to total cylinder volume is given by P. S. Tice, and described in the
Chilton Automobile Directory by the originator as follows:
It is many times desirable to have at hand a convenient means for at once determining with accuracy
what the compression pressure will be in a gasoline-engine cylinder, the relationship between the
volume of the compression space and the total cylinder volume or that swept by the piston being
known. The curve at Fig. 16 is offered as such a means. It is based on empirical data gathered from
upward of two dozen modern automobile engines and represents what may be taken to be the results as
found in practice. It is usual for the designer to find compression pressure values, knowing the
volumes from the equation
V1 1.4
P2 = P 1 ( ) 1
V2
which is for adiabatic compression of air. Equation (1) is right enough in general form but gives results
which are entirely too high, as almost all designers know from experience. The trouble lies in the
interchange of heat between the compressed gases and the cylinder walls, in the diminution of the
exponent (1.4 in the above) due to the lesser ratio of specific heat of gasoline vapor and in the transfer
of heat from the gases which are being compressed to whatever fuel may enter the cylinder in an
unvaporized condition. Also, there is always some piston leakage, and, if the form of the equation (1)
is to be retained, this also tends to lower the value of the exponent. From experience with many
engines, it appears that compression reaches its highest value in the cylinder for but a short range of
motor speeds, usually during the mid-range. Also, it appears that, at those speeds at which compression
shows its highest values, the initial pressure at the start of the compression stroke is from .5 to .9 lb.
below atmospheric. Taking this latter loss value, which shows more often than those of lesser value,
the compression is seen to start from an initial pressure of 13.9 lbs. per sq. in. absolute.
Also, experiment shows that if the exponent be given the value 1.26, instead of 1.4, the equation will
embrace all heat losses in the compressed gas, and compensate for the changed ratio of specific heats
for the mixture and also for all piston leakage, in the average engine with rings in good condition and
tight. In the light of the foregoing, and in view of results obtained from its use, the above curve is
offered—values of P2 being found from the equation
V1 1.26
P2 = 13.8 ( )
V2
In using this curve it must be remembered that pressures are absolute. Thus: suppose it is desired to
know the volumetric relationships of the cylinder for a compression pressure of 75 lbs. gauge. Add
atmospheric pressure to the desired gauge pressure 14.7 + 75 = 89.7 lbs. absolute. Locate this pressure
on the scale of ordinates and follow horizontally across to the curve and then vertically downward to
the scale of abscissas, where the ratio of the combustion chamber volume to the total cylinder volume
is given, which latter is equal to the sum of the combustion chamber volume and that of the piston
sweep. In the above case it is found that the combustion space for a compression pressure of 75 lbs.
gauge will be .225 of the total cylinder volume, or .225 ÷ .775 = .2905 of the piston sweep volume.
Conversely, knowing the volumetric ratios, compression pressure can be read directly by proceeding
from the scale of abscissas vertically to the curve and thence horizontally to the scale of ordinates.

CAUSES OF HEAT LOSS AND INEFFICIENCY IN EXPLOSIVE MOTORS

The difference realized in the practical operation of an internal combustion heat engine from the
computed effect derived from the values of the explosive elements is probably the most serious
difficulty that engineers have encountered in their endeavors to arrive at a rational conclusion as to
where the losses were located, and the ways and means of design that would eliminate the causes of
loss and raise the efficiency step by step to a reasonable percentage of the total efficiency of a perfect
cycle.
An authority on the relative condition of the chemical elements under combustion in closed cylinders
attributes the variation of temperature shown in the fall of the expansion curve, and the suppression or
retarded evolution of heat, entirely to the cooling action of the cylinder walls, and to this nearly all the
phenomena hitherto obscure in the cylinder of a gas-engine. Others attribute the great difference
between the theoretical temperature of combustion and the actual temperature realized in the practical
operation of the gas-engine, a loss of more than one-half of the total heat energy of the combustibles,
partly to the dissociation of the elements of combustion at extremely high temperatures and their
reassociation by expansion in the cylinder, to account for the supposed continued combustion and extra
adiabatic curve of the expansion line on the indicator card.

Fig. 17.—The Thompson Indicator, an Instrument
for Determining Compressions and Explosion
Pressure Values and Recording Them on Chart.

The loss of heat to the walls of the cylinder, piston, and clearance space, as regards the proportion of
wall surface to the volume, has gradually brought this point to its smallest ratio in the concave piston-
head and globular cylinder-head, with the smallest possible space in the inlet and exhaust passage. The
wall surface of a cylindrical clearance space or combustion chamber of one-half its unit diameter in
length is equal to 3.1416 square units, its volume but 0.3927 of a cubic unit; while the same wall
surface in a spherical form has a volume of 0.5236 of a cubic unit. It will be readily seen that the
volume is increased 331⁄3 per cent. in a spherical over a cylindrical form for equal wall surfaces at the
moment of explosion, when it is desirable that the greatest amount of heat is generated, and carrying
with it the greatest possible pressure from which the expansion takes place by the movement of the
piston.

Fig. 18.—Spherical Combustion Chamber.

Fig. 19.—Enlarged Combustion Chamber.

The spherical form cannot continue during the stroke for mechanical reasons; therefore some
proportion of piston stroke of cylinder volume must be found to correspond with a spherical form of
the combustion chamber to produce the least loss of heat through the walls during the combustion and
expansion part of the stroke. This idea is illustrated in Figs. 18 and 19, showing how the relative
volumes of cylinder stroke and combustion chamber may be varied to suit the requirements due to the
quality of the elements of combustion.
Although the concave piston-head shows economy in regard to the relation of the clearance volume to
the wall area at the moment of explosive combustion, it may be clearly seen that its concavity
increases its surface area and its capacity for absorbing heat, for which there is no provision for
cooling the piston, save its contact with the walls of the cylinder and the slight air cooling of its back
by its reciprocal motion. For this reason the concave piston-head has not been generally adopted and
the concave cylinder-head, as shown in Fig. 19, with a flat piston-head is the latest and best practice in
airplane engine construction.

Fig. 20.—Mercedes Aviation Engine Cylinder Section
Showing Approximately Spherical Combustion
Chamber and Concave Piston Top.

The practical application of the principle just outlined to one of the most efficient airplane motors ever
designed, the Mercedes, is clearly outlined at Fig. 20.

HEAT LOSSES TO COOLING WATER

The mean temperature of the wall surface of the combustion chamber and cylinder, as indicated by the
temperatures of the circulating water, has been found to be an important item in the economy of the
gas-engine. Dugald Clerk, in England, a high authority in practical work with the gas-engine, found
that 10 per cent. of the gas for a stated amount of power was saved by using water at a temperature in
which the ejected water from the cylinder-jacket was near the boiling-point, and ventures the opinion
that a still higher temperature for the circulating water may be used as a source of economy. This could
be made practical in the case of aviation engines by adjusting the air-cooling surface of the radiator so
as to maintain the inlet water at just below the boiling point, and by the rapid circulation induced by
the pump pressure, to return the water from the cylinder-jacket a few degrees above the boiling point.
The thermal displacement systems of cooling employed in automobiles are working under more
favorable temperature conditions than those engines in which cooling is more energetic.
For a given amount of heat taken from the cylinder by the largest volume of circulating water, the
difference in temperature between inlet and outlet of the water-jacket should be the least possible, and
this condition of the water circulation gives a more even temperature to all parts of the cylinder; while,
on the contrary, a cold-water supply, say at 60° F., so slow as to allow the ejected water to flow off at a
temperature near the boiling-point, must make a great difference in temperature between the bottom

and top of the cylinder, with a loss in economy in gas and other fuels, as well as in water, if it is
obtained by measurement.
From the foregoing considerations of losses and inefficiencies, we find that the practice in motor
design and construction has not yet reached the desired perfection in its cycular operation. Step by step
improvements have been made with many changes in design though many have been without merit as
an improvement, farther than to gratify the longings of designers for something different from the
other thing, and to establish a special construction of their own. These efforts may in time produce a
motor of normal or standard design for each kind of fuel that will give the highest possible efficiency
for all conditions of service.

CHAPTER IV
Engine Parts and Functions—Why Multiple Cylinder Engines Are Best—Describing
Sequence of Operations—Simple Engines—Four and Six Cylinder Vertical Tandem Engines
—Eight and Twelve Cylinder V Engines—Radial Cylinder Arrangement—Rotary Cylinder
Forms.

ENGINE PARTS AND FUNCTIONS

The principal elements of a gas engine are not difficult to understand and their functions are easily
defined. In place of the barrel of the gun one has a smoothly machined cylinder in which a small
cylindrical or barrel-shaped element fitting the bore closely may be likened to a bullet or cannon ball.
It differs in this important respect, however, as while the shot is discharged from the mouth of the
cannon the piston member sliding inside of the main cylinder cannot leave it, as its movements back
and forth from the open to the closed end and back again are limited by simple mechanical connection
or linkage which comprises crank and connection rod. It is by this means that the reciprocating
movement of the piston is transformed into a rotary motion of the crank-shaft.
The fly-wheel is a heavy member attached to the crank-shaft of an automobile engine which has
energy stored in its rim as the member revolves, and the momentum of this revolving mass tends to
equalize the intermittent pushes on the piston head produced by the explosion of the gas in the
cylinder. In aviation engines, the weight of the propeller or that of rotating cylinders themselves
performs the duty of a fly-wheel, so no separate member is needed. If some explosive is placed in the
chamber formed by the piston and closed end of the cylinder and exploded, the piston would be the
only part that would yield to the pressure which would produce a downward movement. As this is
forced down the crank-shaft is turned by the connecting rod, and as this part is hinged at both ends it is
free to oscillate as the crank turns, and thus the piston may slide back and forth while the crank-shaft is
rotating or describing a curvilinear path.
Large
image
(135 kB).

Fig. 21.—Side Sectional View of Typical Airplane Engine, Showing Parts and Their Relation
to Each Other. This Engine is an Aeromarine Design and Utilizes a Distinctive Concentric
Valve Construction.

In addition to the simple elements described it is evident that a gasoline engine must have other parts.
The most important of these are the valves, of which there are generally two to each cylinder. One
closes the passage connecting to the gas supply and opens during one stroke of the piston in order to
let the explosive gas into the combustion chamber. The other member, or exhaust valve, serves as a
cover for the opening through which the burned gases can leave the cylinder after their work is done.
The spark plug is a simple device which may be compared to the fuse or percussion cap of the cannon.
It permits one to produce an electric spark in the cylinder when the piston is at the best point to utilize
the pressure which obtains when the compressed gas is fired. The valves are open one at a time, the
inlet valve being lifted from its seat while the cylinder is filling and the exhaust valve is opened when
the cylinder is being cleared. They are normally kept seated by means of compression springs. In the
simple motor shown at Fig. 5, the exhaust valve is operated by means of a pivoted bell crank rocked by
a cam which turns at half the speed of the crank-shaft. The inlet valve operates automatically, as will
be explained in proper sequence.
In order to obtain a perfectly tight combustion chamber, both intake and exhaust valves are closed
before the gas is ignited, because all of the pressure produced by the exploding gas is to be directed
against the top of the movable piston. When the piston reaches the bottom of its power stroke, the
exhaust valve is lifted by means of the bell crank which is rocked because of the point or lift on the
cam. The cam-shaft is driven by positive gearing and revolves at half the engine speed. The exhaust
valve remains open during the whole of the return stroke of the piston, and as this member moves
toward the closed end of the cylinder it forces out burned gases ahead of it, through the passage
controlled by the exhaust valve. The cam-shaft is revolved at half the engine speed because the exhaust
valve is raised from its seat during only one stroke out of four, or only once every two revolutions.
Obviously, if the cam was turned at the same speed as the crank-shaft it would remain open once every
revolution, whereas the burned gases are expelled from the individual cylinders only once in two turns
of the crank-shaft.

WHY MULTIPLE CYLINDER FORMS ARE BEST

Owing to the vibration which obtains from the heavy explosion in the large single-cylinder engines
used for stationary power other forms were evolved in which the cylinder was smaller and power
obtained by running the engine faster, but these are suitable only for very low powers.
When a single-cylinder engine is employed a very heavy fly-wheel is needed to carry the moving parts
through idle strokes necessary to obtain a power impulse. For this reason automobile and aircraft
designers must use more than one cylinder, and the tendency is to produce power by frequently
occurring light impulses rather than by a smaller number of explosions having greater force. When a
single-cylinder motor is employed the construction is heavier than is needed with a multiple-cylinder
form. Using two or more cylinders conduces to steady power generation and a lessening of vibration.
Most modern motor cars employ four-cylinder engines because a power impulse may be secured twice
every revolution of the crank-shaft, or a total of four power strokes during two revolutions. The parts
are so arranged that while the charge of gas in one cylinder is exploding, those which come next in
firing order are compressing, discharging the inert gases and drawing in a fresh charge respectively.
When the power stroke is completed in one cylinder, the piston in that member in which a charge of
gas has just been compressed has reached the top of its stroke and when the gas is exploded the piston
is reciprocated and keeps the crank-shaft turning. When a multiple-cylinder engine is used the fly-
wheel can be made much lighter than that of the simpler form and eliminated altogether in some

designs. In fact, many modern multiple-cylinder engines developing 300 horse-power weigh less than
the early single- and double-cylinder forms which developed but one-tenth or one-twentieth that
amount of energy.

DESCRIBING SEQUENCE OF OPERATIONS

Referring to Fig. 22, A, the sequence of operation in a single-cylinder motor can be easily understood.
Assuming that the crank-shaft is turning in the direction of the arrow, it will be seen that the intake
stroke comes first, then the compression, which is followed by the power impulse, and lastly the
exhaust stroke. If two cylinders are used, it is possible to balance the explosions in such a way that one
will occur each revolution. This is true with either one of two forms of four-cycle motors. At B, a two-
cylinder vertical engine using a crank-shaft in which the crank-pins are on the same plane is shown.
The two pistons move up and down simultaneously. Referring to the diagram describing the strokes,
and assuming that the outer circle represents the cycle of operations in one cylinder while the inner
circle represents the sequence of events in the other cylinder, while cylinder No. 1 is taking in a fresh
charge of gas, cylinder No. 2 is exploding. When cylinder No. 1 is compressing, cylinder No. 2 is
exhausting. During the time that the charge in cylinder No. 1 is exploded, cylinder No. 2 is being filled
with fresh gas. While the exhaust gases are being discharged from cylinder No. 1, cylinder No. 2 is
compressing the gas previously taken.

Fig. 22.—Diagrams Illustrating Sequence of
Cycles in One- and Two-Cylinder Engines
Showing More Uniform Turning Effort on
Crank-Shaft with Two-Cylinder Motors.

The same condition obtains when the crank-pins are arranged at one hundred and eighty degrees and
the cylinders are opposed, as shown at C. The reason that the two-cylinder opposed motor is more
popular than that having two vertical cylinders is that it is difficult to balance the construction shown at
B, so that the vibration will not be excessive. The two-cylinder opposed motor has much less vibration
than the other form, and as the explosions occur evenly and the motor is a simple one to construct, it
has been very popular in the past on light cars and has received limited application on some early, light
airplanes.

Fig. 23.—Diagrams Demonstrating Clearly
Advantages which Obtain when Multiple-
Cylinder Motors are Used as Power Plants.

To demonstrate very clearly the advantages of multiple-cylinder engines the diagrams at Fig. 23 have
been prepared. At A, a three-cylinder motor, having crank-pins at one hundred and twenty degrees,
which means that they are spaced at thirds of the circle, we have a form of construction that gives a
more even turning than that possible with a two-cylinder engine. Instead of one explosion per
revolution of the crank-shaft, one will obtain three explosions in two revolutions. The manner in which
the explosion strokes occur and the manner they overlap strokes in the other cylinder is shown at A.
Assuming that the cylinders fire in the following order, first No. 1, then No. 2, and last No. 3, we will
see that while cylinder No. 1, represented by the outer circle, is on the power stroke, cylinder No. 3 has
completed the last two-thirds of its exhaust stroke and has started on its intake stroke. Cylinder No. 2,
represented by the middle circle, during this same period has completed its intake stroke and two-
thirds of its compression stroke. A study of the diagram will show that there is an appreciable lapse of
time between each explosion.
Three-cylinder engines are not used on aircraft at the present time, though Bleriot’s flight across the
British Channel was made with a three-cylinder Anzani motor. It was not a conventional form,

however. The three-cylinder engine is practically obsolete at this time for any purpose except
“penguins” or school machines that are incapable of flight and which are used in some French training
schools for aviators.

FOUR- AND SIX-CYLINDER ENGINES

In the four-cylinder engine operation which is shown at Fig. 23, B, it will be seen that the power
strokes follow each other without loss of time, and one cylinder begins to fire and the piston moves
down just as soon as the member ahead of it has completed its power stroke. In a four-cylinder motor,
the crank-pins are placed at one hundred and eighty degrees, or on the halves of the crank circle. The
crank-pins for cylinders No. 1 and No. 4 are on the same plane, while those for cylinders No. 2 and
No. 3 also move in unison. The diagram describing sequence of operations in each cylinder is based on
a firing order of one, two, four, three. The outer circle, as in previous instances, represents the cycle of
operations in cylinder one. The next one toward the center, cylinder No. 2, the third circle represents
the sequence of events in cylinder No. 3, while the inner circle outlines the strokes in cylinder four.
The various cylinders are working as follows:
1. 2. 3. 4.
Explosion Compression Exhaust Intake
Exhaust Explosion Intake Compression
Intake Exhaust Compression Explosion
Compression Intake Explosion Exhaust

It will be obvious that regardless of the method of construction, or the number of cylinders employed,
exactly the same number of parts must be used in each cylinder assembly and one can conveniently
compare any multiple-cylinder power plant as a series of single-cylinder engines joined one behind the
other and so coupled that one will deliver power and produce useful energy at the crank-shaft where
the other leaves off. The same fundamental laws governing the action of a single cylinder obtain when
a number are employed, and the sequence of operation is the same in all members, except that the
necessary functions take place at different times. If, for instance, all the cylinders of a four-cylinder
motor were fired at the same time, one would obtain the same effect as though a one-piston engine was
used, which had a piston displacement equal to that of the four smaller members. As is the case with a
single-cylinder engine, the motor would be out of correct mechanical balance because all the
connecting rods would be placed on crank-pins that lie in the same plane. A very large fly-wheel would
be necessary to carry the piston through the idle strokes, and large balance weights would be fitted to
the crank-shaft in an effort to compensate for the weight of the four pistons, and thus reduce vibratory
stresses which obtain when parts are not in correct balance.
There would be no advantage gained by using four cylinders in this manner, and there would be more
loss of heat and more power consumed in friction than in a one-piston motor of the same capacity. This
is the reason that when four cylinders are used the arrangement of crank-pins is always as shown at
Fig. 23, B—i.e., two pistons are up, while the other two are at the bottom of the stroke. With this
construction, we have seen that it is possible to string out the explosions so that there will always be
one cylinder applying power to the crank-shaft. The explosions are spaced equally. The parts are in
correct mechanical balance because two pistons are on the upstroke while the other two are
descending. Care is taken to have one set of moving members weigh exactly the same as the other.
With a four-cylinder engine one has correct balance and continuous application of energy. This insures
a smoother running motor which has greater efficiency than the simpler one-, two-, and three-cylinder
forms previously described. Eliminating the stresses which would obtain if we had an unbalanced
mechanism and irregular power application makes for longer life. Obviously a large number of

relatively light explosions will produce less wear and strain than would a lesser number of powerful
ones. As the parts can be built lighter if the explosions are not heavy, the engine can be operated at
higher rotative speeds than when large and cumbersome members are utilized. Four-cylinder engines
intended for aviation work have been built according to the designs shown at Fig. 24, but these forms
are unconventional and seldom if ever used.

Fig. 24.—Showing Three Possible Though Unconventional
Arrangements of Four-Cylinder Engines.

The six-cylinder type of motor, the action of which is shown at Fig. 23, C, is superior to the four-
cylinder, inasmuch as the power strokes overlap, and instead of having two explosions each revolution
we have three explosions. The conventional crank-shaft arrangement in a six-cylinder engine is just the
same as though one used two three-cylinder shafts fastened together, so pistons 1 and 6 are on the
same plane as are pistons 2 and 5. Pistons 3 and 4 also travel together. With the cranks arranged as
outlined at Fig. 23, C, the firing order is one, five, three, six, two, four. The manner in which the power
strokes overlap is clearly shown in the diagram. An interesting comparison is also made in the
diagrams at Fig. 25 and in the upper corner of Fig. 23, C.
Large
scale
image
(84 kB).

Fig. 25.—Diagrams Outlining Advantages of Multiple Cylinder Motors, and Why
They Deliver Power More Evenly Than Single Cylinder Types.

A rectangle is divided into four columns; each of these corresponds to one hundred and eighty degrees,
or half a revolution. Thus the first revolution of the crank-shaft is represented by the first two columns,
while the second revolution is represented by the last two. Taking the portion of the diagram which
shows the power impulse in a one-cylinder engine, we see that during the first revolution there has
been no power impulse. During the first half of the second revolution, however, an explosion takes
place and a power impulse is obtained. The last portion of the second revolution is devoted to
exhausting the burned gases, so that there are three idle strokes and but one power stroke. The effect
when two cylinders are employed is shown immediately below.
Large
scale
image
(161 kB).

Fig. 26.—Diagrams Showing Duration of Events for a Four-Stroke
Cycle, Six-Cylinder Engine.

Here we have one explosion during the first half of the first revolution in one cylinder and another
during the first half of the second revolution in the other cylinder. With a four-cylinder engine there is
an explosion each half revolution, while in a six-cylinder engine there is one and one-half explosions
during each half revolution. When six cylinders are used there is no lapse of time between power
impulses, as these overlap and a continuous and smooth-turning movement is imparted to the crank
shaft. The diagram shown at Fig. 26, prepared by E. P. Pulley, can be studied to advantage in securing
an idea of the coordination of effort that takes place in an engine of the six-cylinder type.

ACTUAL DURATION OF DIFFERENT STROKES

Fig. 27.—Diagram Showing Actual Duration of Different
Strokes in Degrees.

In the diagrams previously presented the writer has assumed, for the sake of simplicity, that each
stroke takes place during half of one revolution of the crank-shaft, which corresponds to a crank-pin
travel of one hundred and eighty degrees. The actual duration of these strokes is somewhat different.
For example, the inlet stroke is usually a trifle more than a half revolution, and the exhaust is always
considerably more. The diagram showing the comparative duration of the strokes is shown at Fig. 27.
The inlet valve opens ten degrees after the piston starts to go down and remains open thirty degrees
after the piston has reached the bottom of its stroke. This means that the suction stroke corresponds to
a crank-pin travel of two hundred degrees, while the compression stroke is measured by a movement
of but one hundred and fifty degrees. It is common practice to open the exhaust valve before the piston
reaches the end of the power stroke so that the actual duration of the power stroke is about one
hundred and forty degrees, while the exhaust stroke corresponds to a crank-pin travel of two hundred
and twenty-five degrees. In this diagram, which represents proper time for the valves to open and
close, the dimensions in inches given are measured on the fly-wheel and apply only to a certain
automobile motor. If the fly-wheel were smaller ten degrees would take up less than the dimensions
given, while if the fly-wheel was larger a greater space on its circumference would represent the same
crank-pin travel. Aviation engines are timed by using a timing disc attached to the crank-shaft as they
are not provided with fly-wheels. Obviously, the distance measured in inches will depend upon the
diameter of the disc, though the number of degrees interval would not change.

Fig. 28.—Another Diagram to Facilitate Understanding Sequence of Functions in
Six-Cylinder Engine.

EIGHT- AND TWELVE-CYLINDER V ENGINES

Those who have followed the development of the gasoline engine will recall the arguments that were
made when the six-cylinder motor was introduced at a time that the four-cylinder type was considered
standard. The arrival of the eight-cylinder has created similar futile discussion of its practicability as
this is so clearly established as to be accepted without question. It has been a standard power plant for
aeroplanes for many years, early exponents having been the Antoinette, the Woolsley, the Renault, the
E. N. V. in Europe and the Curtiss in the United States.

Fig. 29.—Types of Eight-Cylinder Engines Showing the Advantage
of the V Method of Cylinder Placing.

The reason the V type shown at Fig. 29, A is favored is that the “all-in-line form” which is shown at
Fig. 29, B is not practical for aircraft because of its length. Compared to the standard four-cylinder
engine it is nearly twice as long and it required a much stronger and longer crank-shaft. It will be
evident that it could not be located to advantage in the airplane fuselage. These undesirable factors are
eliminated in the V type eight-cylinder motor, as it consists of two blocks of four cylinders each, so
arranged that one set or block is at an angle of forty-five degrees from the vertical center line of the
motor, or at an angle of ninety degrees with the other set. This arrangement of cylinders produces a
motor that is no longer than a four-cylinder engine of half the power would be.

Fig. 30.—Curves Showing Torque of Various Engine Types Demonstrate
Graphically Marked Advantage of the Eight-Cylinder Type.

Apparently there is considerable misconception as to the advantage of the two extra cylinders of the
eight as compared with the six-cylinder. It should be borne in mind that the multiplication in the
number of cylinders noticed since the early days of automobile development has not been for solely
increasing the power of the engine, but to secure a more even turning movement, greater flexibility and
to eliminate destructive vibration. The ideal internal combustion motor, is the one having the most
uniform turning movement with the least mechanical friction loss. Study of the torque outlines or
plotted graphics shown at Figs. 25 and 30 will show how multiplication of cylinders will produce
steady power delivery due to overlapping impulses. The most practical form would be that which more
nearly conforms to the steady running produced by a steam turbine or electric motor. The advocates of
the eight-cylinder engine bring up the item of uniform torque as one of the most important advantages
of the eight-cylinder design. A number of torque diagrams are shown at Fig. 30. While these appear to
be deeply technical, they may be very easily followed when their purpose is explained. At the top is
shown the torque diagram of a single-cylinder motor of the four-cycle type. The high point in the line
represents the period of greatest torque or power generation, and it will be evident that this occurs
early in the first revolution of the crank-shaft. Below this diagram is shown a similar curve except that
it is produced by a four-cylinder engine. Inspection will show that the turning-moment is much more
uniform than in the single cylinder; similarly, the six-cylinder diagram is an improvement over the
four, and the eight-cylinder diagram is an improvement over the six-cylinder.

Fig. 31—Diagrams Showing How Increasing Number of Cylinders
Makes for More Uniform Power Application.

Fig. 32.—How the Angle Between the Cylinders of an Eight- and Twelve-Cylinder V
Motor Varies.

The reason that practically continuous torque is obtained in an eight-cylinder engine is that one
cylinder fires every ninety degrees of crank-shaft rotation, and as each impulse lasts nearly seventy-
five per cent. of the stroke, one can easily appreciate that an engine that will give four explosions per
revolution of the crank-shaft will run more uniformly than one that gives but three explosions per
revolution, as the six-cylinder does, and will be twice as smooth running as a four-cylinder, in which

but two explosions occur per revolution of the crank-shaft. The comparison is so clearly shown in
graphical diagrams and in Fig. 31 that further description is unnecessary.
Any eight-cylinder engine may be considered a “twin-four,” twelve-cylinder engines may be
considered “twin sixes.”

Fig. 33.—The Hall-Scott Four-Cylinder 100 Horse-Power Aviation
Motor.

Fig. 34.—Two Views of the Duesenberg Sixteen Valve
Four-Cylinder Aviation Motor.

The only points in which an eight-cylinder motor differs from a four-cylinder is in the arrangement of
the connecting rod, as in many designs it is necessary to have two rods working from the same crank-
pin. This difficulty is easily overcome in some designs by staggering the cylinders and having the two
connecting rod big ends of conventional form side by side on a common crank-pin. In other designs
one rod is a forked form and works on the outside of a rod of the regular pattern. Still another method
is to have a boss just above the main bearing on one connecting rod to which the lower portion of the
connecting rod in the opposite cylinder is hinged. As the eight-cylinder engine may actually be made
lighter than the six-cylinder of equal power, it is possible to use smaller reciprocating parts, such as
pistons, connecting rods and valve gear, and obtain higher engine speed with practically no vibration.
The firing order in nearly every case is the same as in a four-cylinder except that the explosions occur
alternately in each set of cylinders. The firing order of an eight-cylinder motor is apt to be confusing to
the motorist, especially if one considers that there are eight possible sequences. The majority of
engineers favor the alternate firing from side to side. Firing orders will be considered in proper
sequence.

Fig. 35.—The Hall-Scott Six-Cylinder Aviation Engine.

Fig. 36.—The Curtiss Eight-Cylinder, 200 Horse-Power Aviation
Engine.

The demand of aircraft designers for more power has stimulated designers to work out twelve-cylinder
motors. These are high-speed motors incorporating all recent features of design in securing light
reciprocating parts, large valve openings, etc. The twelve-cylinder motor incorporates the best features
of high-speed motor design and there is no need at this time to discuss further the pros and cons of the
twelve-cylinder versus the eight or six, because it is conceded by all that there is the same degree of

steady power application in the twelve over the eight as there would be in the eight over the six. The
question resolves itself into having a motor of high power that will run with minimum vibration and
that produces smooth action. This is well shown by diagrams at Fig. 31. It should be remembered that
if an eight-cylinder engine will give four explosions per revolution of the fly-wheel, a twelve-cylinder
type will give six explosions per revolution, and instead of the impulses coming 90 degrees crank
travel apart, as in the case of the eight-cylinder, these will come but 60 degrees of crank travel apart in
the case of the twelve-cylinder. For this reason, the cylinders of a twelve are usually separated by 60
degrees while the eight has the blocks spaced 90 degrees apart. The comparison can be easily made by
comparing the sectional views of Vee engines at Fig. 32. When one realizes that the actual duration of
the power stroke is considerably greater than 120 degrees crank travel, it will be apparent that the
overlapping of explosions must deliver a very uniform application of power. Vee engines have been
devised having the cylinders spaced but 45 degrees apart, but the explosions cannot be timed at equal
intervals as when 90 degrees separate the cylinder center lines.

Fig. 37.—The Sturtevant Eight-Cylinder, High Speed Aviation
Motor.

RADIAL CYLINDER ARRANGEMENTS

While the fixed cylinder forms of engines, having the cylinders in tandem in the four- and six-cylinder
models as shown at Figs. 33 to 35 inclusive and the eight-cylinder V types as outlined at Figs. 36 and
37 have been generally used and are most in favor at the present time, other forms of motors having
unconventional cylinder arrangements have been devised, though most of these are practically
obsolete. While many methods of decreasing weight and increasing mechanical efficiency of a motor
are known to designers, one of the first to be applied to the construction of aeronautical power plants
was an endeavor to group the components, which in themselves were not extremely light, into a form
that would be considerably lighter than the conventional design. As an example, we may consider
those multiple-cylinder forms in which the cylinders are disposed around a short crank-case, either
radiating from a common center as at Fig. 38 or of the fan shape shown at Fig. 39. This makes it
possible to use a crank-case but slightly larger than that needed for one or two cylinders and it also
permits of a corresponding decrease in length of the crank-shaft. The weight of the engine is lessened
because of the reduction in crank-shaft and crank-case weight and the elimination of a number of
intermediate bearings and their supporting webs which would be necessary with the usual tandem

construction. While there are six power impulses to every two revolutions of the crank-shaft, in the
six-cylinder engine, they are not evenly spaced as is possible with the conventional arrangement.

Fig. 38.—Anzani 40-50 Horse-Power Five-Cylinder Air Cooled
Engine.

In the Anzani form, which is shown at Fig. 38, the crank-case is stationary and a revolving crank-shaft
is employed as in conventional construction. The cylinders are five in number and the engine develops
40 to 50 H.P. with a weight of 72 kilograms or 158.4 lbs. The cylinders are of the usual air-cooled form
having cooling flanges only part of the way down the cylinder. By using five cylinders it is possible to
have the power impulses come regularly, they coming 145° crank-shaft travel apart, the crank-shaft
making two turns to every five explosions. The balance is good and power output regular. The valves
are placed directly in the cylinder head and are operated by a common pushrod. Attention is directed to
the novel method of installing the carburetor which supplies the mixture to the engine base from which
inlet pipes radiate to the various cylinders. This engine is used on French school machines.

Fig. 39.—Unconventional Six-Cylinder Aircraft Motor of Masson
Design.

In the form shown at Fig. 39 six cylinders are used, all being placed above the crank-shaft center line.
This engine is also of the air-cooled form and develops 50 H. P. and weighs 105 kilograms, or 231 lbs.
The carburetor is connected to a manifold casting attached to the engine base from which the induction
pipes radiate to the various cylinders. The propeller design and size relative to the engine is clearly
shown in this view. While flights have been made with both of the engines described, this method of
construction is not generally followed and has been almost entirely displaced abroad by the revolving
motors or by the more conventional eight-cylinder V engines. Both of the engines shown were
designed about eight years ago and would be entirely too small and weak for use in modern airplanes
intended for active duty.

ROTARY ENGINES

Fig. 40.—The Gnome Fourteen-Cylinder Revolving Motor.

Rotary engines such as shown at Fig. 40 are generally associated with the idea of light construction
and it is rather an interesting point that is often overlooked in connection with the application of this
idea to flight motors, that the reason why rotary engines are popularly supposed to be lighter than the
others is because they form their own fly-wheel, yet on aeroplanes, engines are seldom fitted with a
fly-wheel at all. As a matter of fact the Gnome engine is not so light because it is a rotary motor, and it
is a rotary motor because the design that has been adopted as that most conducive to lightness is also
most suited to an engine working in this way. The cylinders could be fixed and crank-shaft revolve
without increasing the weight to any extent. There are two prime factors governing the lightness of an
engine, one being the initial design, and the other the quality of the materials employed. The
consideration of reducing weight by cutting away metal is a subsidiary method that ought not to play a
part in standard practice, however useful it may be in special cases. In the Gnome rotary engine the
lightness is entirely due to the initial design and to the materials employed in manufacture. Thus, in the
first case, the engine is a radial engine, and has its seven or nine cylinders spaced equally around a
crank-chamber that is no wider or rather longer than would be required for any one of the cylinders.
This shortening of the crank-chamber not only effects a considerable saving of weight on its own
account, but there is a corresponding saving in the shafts and other members, the dimensions of which
are governed by the size of the crank-chamber. With regard to materials, nothing but steel is used
throughout, and most of the metal is forged chrome nickel steel. The beautifully steady running of the
engine is largely due to the fact that there are literally no reciprocating parts in the absolute sense, the
apparent reciprocation between the pistons and cylinders being solely a relative reciprocation since
both travel in circular paths, that of the pistons, however, being electric by one-half of the stroke length
to that of the cylinder.
While the Gnome engine has many advantages, on the other hand the head resistance offered by a
motor of this type is considerable; there is a large waste of lubricating oil due to the centrifugal force
which tends to throw the oil away from the cylinders; the gyroscopic effect of the rotary motor is

detrimental to the best working of the aeroplane, and moreover it requires about seven per cent. of the
total power developed by the motor to drive the revolving cylinders around the shaft. Of necessity, the
compression of this type of motor is rather low, and an additional disadvantage manifests itself in the
fact that there is as yet no satisfactory way of muffling the rotary type of motor. The modern Gnome
engine has been widely copied in various European countries, but its design was originated in
America, the early Adams-Farwell engine being the pioneer form. It has been made in seven- and nine-
cylinder types and forms of double these numbers. The engine illustrated at Fig. 40 is a fourteen-
cylinder form. The simple engines have an odd number of cylinders in order to secure evenly spaced
explosions. In the seven-cylinder, the impulses come 102.8° apart. In the nine-cylinder form, the power
strokes are spaced 80° apart. The fourteen-cylinder engine is virtually two seven-cylinder types
mounted together, the cranks being just the same as in a double cylinder opposed motor, the explosions
coming 51.4° apart; while in the eighteen-cylinder model the power impulses come every 40° cylinder
travel. Other rotary motors have been devised, such as the Le Rhone and the Clerget in France and
several German copies of these various types. The mechanical features of these motors will be fully
considered later.

CHAPTER V
Properties of Liquid Fuels—Distillates of Crude Petroleum—Principles of Carburetion
Outlined—Air Needed to Burn Gasoline—What a Carburetor Should Do—Liquid Fuel
Storage and Supply—Vacuum Fuel Feed—Early Vaporizer Forms—Development of Float
Feed Carburetor—Maybach’s Early Design—Concentric Float and Jet Type—Schebler
Carburetor—Claudel Carburetor—Stewart Metering Pin Type—Multiple Nozzle Vaporizers
—Two-Stage Carburetor—Master Multiple Jet Type—Compound Nozzle Zenith Carburetor
—Utility of Gasoline Strainers—Intake Manifold Design and Construction—Compensating
for Various Atmospheric Conditions—How High Altitude Affects Power—The Diesel
System—Notes on Carburetor Installation—Notes on Carburetor Adjustment.
There is no appliance that has more material value upon the efficiency of the internal combustion
motor than the carburetor or vaporizer which supplies the explosive gas to the cylinders. It is only in
recent years that engineers have realized the importance of using carburetors that are efficient and that
are so strongly and simply made that there will be little liability of derangement. As the power
obtained from the gas-engine depends upon the combustion of fuel in the cylinders, it is evident that if
the gas supplied does not have the proper proportions of elements to insure rapid combustion the
efficiency of the engine will be low. When a gas engine is used as a stationary installation it is possible
to use ordinary illuminating or natural gas for fuel, but when this prime mover is applied to
automobiles or airplanes it is evident that considerable difficulty would be experienced in carrying
enough compressed coal gas to supply the engine for even a very short trip. Fortunately, the
development of the internal-combustion motor was not delayed by the lack of suitable fuel.
Engineers were familiar with the properties of certain liquids which gave off vapors that could be
mixed with air to form an explosive gas which burned very well in the engine cylinders. A very small
quantity of such liquids would suffice for a very satisfactory period of operation. The problem to be
solved before these liquids could be applied in a practical manner was to evolve suitable apparatus for
vaporizing them without waste. Among the liquids that can be combined with air and burned, gasoline
is the most volatile and is the fuel utilized by internal-combustion engines.
The widely increasing scope of usefulness of the internal-combustion motor has made it imperative
that other fuels be applied in some instances because the supply of gasoline may in time become
inadequate to supply the demand. In fact, abroad this fuel sells for fifty to two hundred per cent. more
than it does in America because most of the gasoline used must be imported from this country or
Russia. Because of this foreign engineers have experimented widely with other substances, such as
alcohol, benzol, and kerosene, but more to determine if they can be used to advantage in motor cars
than in airplane engines.

DISTILLATES OF CRUDE PETROLEUM

Crude petroleum is found in small quantities in almost all parts of the world, but a large portion of that
produced commercially is derived from American wells. The petroleum obtained in this country yields
more of the volatile products than those of foreign production, and for that reason the demand for it is
greater. The oil fields of this country are found in Pennsylvania, Indiana, and Ohio, and the crude
petroleum is usually in association with natural gas. This mineral oil is an agent from which many
compounds and products are derived, and the products will vary from heavy sludges, such as asphalt,

to the lighter and more volatile components, some of which will evaporate very easily at ordinary
temperatures.
The compounds derived from crude petroleum are composed principally of hydrogen and carbon and
are termed “Hydrocarbons.” In the crude product one finds many impurities, such as free carbon,
sulphur, and various earthy elements. Before the oil can be utilized it must be subjected to a process of
purifying which is known as refining, and it is during this process, which is one of destructive
distillation, that the various liquids are separated. The oil was formerly broken up into three main
groups of products as follows: Highly volatile, naphtha, benzine, gasoline, eight to ten per cent. Light
oils, such as kerosene and light lubricating oils seventy to eighty per cent. Heavy oils or residuum five
to nine per cent. From the foregoing it will be seen that the available supply of gasoline is determined
largely by the demand existing for the light oils forming the larger part of the products derived from
crude petroleum. New processes have been recently discovered by which the lighter oils, such as
kerosene, are reduced in proportion and that of gasoline increased, though the resulting liquid is
neither the high grade, volatile gasoline known in the early days of motoring nor the low grade
kerosene.

PRINCIPLES OF CARBURETION OUTLINED

The process of carburetion is combining the volatile vapors which evaporate from the hydrocarbon
liquids with certain proportions of air to form an inflammable gas. The quantities of air needed vary
with different liquids and some mixtures burn quicker than do other combinations of air and vapor.
Combustion is simply burning and it may be rapid, moderate or slow. Mixtures of gasoline and air
burn quickly, in fact the combustion is so rapid that it is almost instantaneous and we obtain what is
commonly termed an “explosion.” Therefore the explosion of gas in the automobile engine cylinder
which produces the power is really a combination of chemical elements which produce heat and an
increase in the volume of the gas because of the increase in temperature.
If the gasoline mixture is not properly proportioned the rate of burning will vary, and if the mixture is
either too rich or too weak the power of the explosion is reduced and the amount of power applied to
the piston is decreased proportionately. In determining the proper proportions of gasoline and air, one
must take the chemical composition of gasoline into account. The ordinary liquid used for fuel is said
to contain about eight-four per cent. carbon and sixteen per cent. hydrogen. Air is composed of oxygen
and nitrogen and the former has a great affinity, or combining power, with the two constituents of
hydro-carbon liquids. Therefore, what we call an explosion is merely an indication that oxygen in the
air has combined with the carbon and hydrogen of the gasoline.

AIR NEEDED TO BURN GASOLINE

In figuring the proper volume of air to mix with a given quantity of fuel, one takes into account the
fact that one pound of hydrogen requires eight pounds of oxygen to burn it, and one pound of carbon
needs two and one-third pounds of oxygen to insure its combustion. Air is composed of one part of
oxygen to three and one-half portions of nitrogen by weight. Therefore for each pound of oxygen one
needs to burn hydrogen or carbon four and one-half pounds of air must be allowed. To insure
combustion of one pound of gasoline which is composed of hydrogen and carbon we must furnish
about ten pounds of air to burn the carbon and about six pounds of air to insure combustion of
hydrogen, the other component of gasoline. This means that to burn one pound of gasoline one must
provide about sixteen pounds of air.

While one does not usually consider air as having much weight, at a temperature of sixty-two degrees
Fahrenheit about fourteen cubic feet of air will weigh a pound, and to burn a pound of gasoline one
would require about two hundred cubic feet of air. This amount will provide for combustion
theoretically, but it is common practice to allow twice this amount because the element nitrogen, which
is the main constituent of air, is an inert gas and instead of aiding combustion it acts as a deterrent of
burning. In order to be explosive, gasoline vapor must be combined with definite quantities of air.
Mixtures that are rich in gasoline ignite quicker than those which have more air, but these are only
suitable when starting or when running slowly, as a rich mixture ignites much quicker than a weak
mixture. The richer mixture of gasoline and air not only burns quicker but produces the most heat and
the most effective pressure in pounds per square inch of piston top area.
The amount of compression of the charge before ignition also has material bearing on the force of the
explosion. The higher the degree of compression the greater the force exerted by the rapid combustion
of the gas. It may be stated that as a general thing the maximum explosive pressure is somewhat more
than four times the compression pressure prior to ignition. A charge compressed to sixty pounds will
have a maximum of approximately two hundred and forty pounds; compacted to eighty pounds it will
produce a pressure of about three hundred pounds on each square inch of piston area at the beginning
of the power stroke. Mixtures varying from one part of gasoline vapor to four of air to others having
one part of gasoline vapor to thirteen of air can be ignited, but the best results are obtained when the
proportions are one to five or one to seven, as this mixture is said to be the one that will produce the
highest temperature, the quickest explosion, and the most pressure.

WHAT A CARBURETOR SHOULD DO

While it is apparent that the chief function of a carbureting device is to mix hydrocarbon vapors with
air to secure mixtures that will burn, there are a number of factors which must be considered before
describing the principles of vaporizing devices. Almost any device which permits a current of air to
pass over or through a volatile liquid will produce a gas which will explode when compressed and
ignited in the motor cylinder. Modern carburetors are not only called upon to supply certain quantities
of gas, but these must deliver a mixture to the cylinders that is accurately proportioned and which will
be of proper composition at all engine speeds.
Flexible control of the engine is sought by varying the engine speed by regulating the supply of gas to
the cylinders. The power plant should run from its lowest to its highest speed without any irregularity
in torque, i.e., the acceleration should be gradual rather than spasmodic. As the degree of compression
will vary in value with the amount of throttle opening, the conditions necessary to obtain maximum
power differ with varying engine speeds. When the throttle is barely opened the engine speed is low
and the gas must be richer in fuel than when the throttle is wide open and the engine speed high.
When an engine is turning over slowly the compression has low value and the conditions are not so
favorable to rapid combustion as when the compression is high. At high engine speeds the gas velocity
through the intake piping is higher than at low speeds, and regular engine action is not so apt to be
disturbed by condensation of liquid fuel in the manifold due to excessively rich mixture or a
superabundance of liquid in the stream of carbureted air.

LIQUID FUEL STORAGE AND SUPPLY

The problem of gasoline storage and method of supplying the carburetor is one that is determined
solely by design of the airplane. While the object of designers should be to supply the fuel to the
carburetor by as simple means as possible the fuel supply system of some airplanes is quite complex.

The first point to consider is the location of the gasoline tank. This depends upon the amount of fuel
needed and the space available in the fuselage.
Large
image
(193 kB).

Fig. 41.—How Gravity Feed Fuel Tank May Be Mounted Back of
Engine and Secure Short Fuel Line.

A very simple and compact fuel supply system is shown at Fig. 41. In this instance the fuel container is
placed immediately back of the engine cylinder. The carburetor which is carried as indicated is joined
to the tank by a short piece of copper or flexible rubber tubing. This is the simplest possible form of
fuel supply system and one used on a number of excellent airplanes.
As the sizes of engines increase and the power plant fuel consumption augments it is necessary to use
more fuel, and to obtain a satisfactory flying radius without frequent landings for filling the fuel tank it
is necessary to supply large containers.
When a very powerful power plant is fitted, as on battle planes of high capacity, it is necessary to carry
large quantities of gasoline. In order to use a tank of sufficiently large capacity it may be necessary to
carry it lower than the carburetor. When installed in this manner it is necessary to force fuel out of the
tank by air pressure or to pump it with a vacuum tank because the gasoline tank is lower than the
carburetor it supplies and the gasoline cannot flow by gravity as in the simpler systems. While the
pressure and gravity feed systems are generally used in airplanes, it may be well to describe the
vacuum lift system which has been widely applied to motor cars and which may have some use in
connection with airplanes as these machines are developed.

STEWART VACUUM FUEL FEED

One of the marked tendencies has been the adoption of a vacuum fuel feed system to draw the gasoline
from tanks placed lower than the carburetor instead of using either exhaust gas or air pressure to
achieve this end. The device generally fitted is the Stewart vacuum feed tank which is clearly shown in
section at Fig. 42. In this system the suction of a motor is employed to draw gasoline from the main
fuel tank to the auxiliary tank incorporated in the device and from this tank the liquid flows to the
carburetor. It is claimed that all the advantages of the pressure system are obtained with very little
more complication than is found on the ordinary gravity feed. The mechanism is all contained in the
cylindrical tank shown, which may be mounted either on the front of the dash or on the side of the
engine as shown.

Fig. 42.—The Stewart Vacuum Fuel Feed Tank.

The tank is divided into two chambers, the upper one being the filling chamber and the lower one the
emptying chamber. The former, which is at the top of the device, contains the float valve, as well as the
pipes running to the main fuel container and to the intake manifold. The lower chamber is used to
supply the carburetor with gasoline and is under atmospheric pressure at all times, so the flow of fuel
from it is by means of gravity only. Since this chamber is located somewhat above the carburetor, there
must always be free flow of fuel. Atmospheric pressure is maintained by the pipes A and B, the latter
opening into the air. In order that the fuel will be sucked from a main tank to the upper chamber, the
suction valve must be opened and the atmospheric valve closed. Under these conditions the float is at
the bottom and the suction at the intake manifold produces a vacuum in the tank which draws the
gasoline from the main tank to the upper chamber. When the upper chamber is filled at the proper
height the float rises to the top, this closing the suction valve and opening the atmospheric valve. As
the suction is now cut off, the lower chamber is filled by gravity owing to there being atmospheric
pressure in both upper and lower chambers. A flap valve is provided between the two chambers to
prevent the gasoline in the lower one from being sucked back into the upper one. The atmospheric and
suction valves are controlled by the levers C and D, both of which are pivoted at E, their outer ends
being connected by two coil springs. It is seen that the arrangement of these two springs is such that
the float must be held at the extremity of its movement, and that it cannot assume an intermediate
position.
This intermittent action is required to insure that the upper part of the tank may be under atmospheric
pressure part of the time for the gasoline to flow to the lower chamber. When the level of gasoline
drops to a certain point, the float falls, thus opening the suction valve and closing the atmospheric
valve. The suction of the motor then causes a flow of fuel from the main container. As soon as the level
rises to the proper height the float returns to its upper position. It takes about two seconds for the
chamber to become full enough to raise the float, as but .05 gallon is transferred at a time. The pipe

running from the bottom of the lower chamber to the carburetor extends up a ways, so that there is but
little chance of dirt or water being carried to the float chamber.
If the engine is allowed to stand long enough so that the tank becomes empty, it will be replenished
after the motor has been cranked over four or five times with the throttle closed. The installation of the
Stewart Vacuum-Gravity System is very simple. The suction pipe is tapped into the manifold at a point
as near the cylinders as possible, while the fuel pipe is inserted into the gasoline tank and runs to the
bottom of that member. There is a screen at the end of the fuel pipe to prevent any trouble due to
deposits of sediment in the main container. As the fuel is sucked from the gasoline tank a small vent
must be made in the tank filler cap so that the pressure in the main tank will always be that of the
atmosphere.

EARLY VAPORIZER FORMS

The early types of carbureting devices were very crude and cumbersome, and the mixture of gasoline
vapor and air was accomplished in three ways. The air stream was passed over the surface of the liquid
itself, through loosely placed absorbent material saturated with liquid, or directly through the fuel. The
first type is known as the surface carburetor and is now practically obsolete. The second form is called
the “wick” carburetor because the air stream was passed over or through saturated wicking. The third
form was known as a “bubbling” carburetor. While these primitive forms gave fairly good results with
the early slow-speed engines and the high grade, or very volatile, gasoline which was first used for
fuel, they would be entirely unsuitable for present forms of engines because they would not carburate
the lower grades of gasoline which are used to-day, and would not supply the modern high-speed
engines with gas of the proper consistency fast enough even if they did not have to use very volatile
gasoline. The form of carburetor used at the present time operates on a different principle. These
devices are known as “spraying carburetors.” The fuel is reduced to a spray by the suction effect of the
entering air stream drawing it through a fine opening.
The advantage of this construction is that a more thorough amalgamation of the gasoline and air
particles is obtained. With the earlier types previously considered the air would combine with only the
more volatile elements, leaving the heavier constituents in the tank. As the fuel became stale it was
difficult to vaporize it, and it had to be drained off and fresh fuel provided before the proper mixture
would be produced. It will be evident that when the fuel is sprayed into the air stream, all the fuel will
be used up and the heavier portions of the gasoline will be taken into the cylinder and vaporized just as
well as the more volatile vapors.

Fig. 43.—Marine-Type Mixing Valve, by which Gasoline is
Sprayed into Air Stream Through Small Opening in Air-
Valve Seat.

The simplest form of spray carburetor is that shown at Fig. 43. In this the gasoline opening through
which the fuel is sprayed into the entering air stream is closed by the spring-controlled mushroom
valve which regulates the main air opening as well. When the engine draws in a charge of air it unseats
the valve and at the same time the air flowing around it is saturated with gasoline particles through the
gasoline opening. The mixture thus formed goes to the engine through the mixture passage. Two
methods of varying the fuel proportions are provided. One of these consists of a needle valve to
regulate the amount of gasoline, the other is a knurled screw which controls the amount of air by
limiting the lift of the jump valve.

DEVELOPMENT OF FLOAT-FEED CARBURETOR

The modern form of spraying carburetor is provided with two chambers, one a mixing chamber
through which the air stream passes and mixes with a gasoline spray, the other a float chamber in
which a constant level of fuel is maintained by simple mechanism. A jet or standpipe is used in the
mixing chamber to spray the fuel through and the object of the float is to maintain the fuel level to
such a point that it will not overflow the jet when the motor is not drawing in a charge of gas. With the
simple forms of generator valve in which the gasoline opening is controlled by the air valve, a leak
anywhere in either valve or valve seat will allow the gasoline to flow continuously whether the engine
is drawing in a charge or not. The liquid fuel collects around the air opening, and when the engine
inspires a charge it is saturated with gasoline globules and is excessively rich. With a float-feed
construction, which maintains a constant level of gasoline at the right height in the standpipe, liquid
fuel will only be supplied when drawn out of the jet by the suction effect of the entering air stream.

MAYBACH’S EARLY DESIGN

The first form of spraying carburetor ever applied successfully was evolved by Maybach for use on
one of the earliest Daimler engines. The general principles of operation of this pioneer float-feed
carburetor are shown at Fig. 44, A. The mixing chamber and valve chamber were one and the
standpipe or jet protruded into the mixing chamber. It was connected to the float compartment by a
pipe. The fuel from the tank entered the top of the float compartment and the opening was closed by a
needle valve carried on top of a hollow metal float. When the level of gasoline in the float chamber
was lowered the float would fall and the needle valve uncover the opening. This would permit the
gasoline from the tank to flow into the float chamber, and as the chamber filled the float would rise
until the proper level had been reached, under which conditions the float would shut off the gasoline
opening. On every suction stroke of the engine the inlet valve, which was an automatic type, would
leave its seat and a stream of air would be drawn through the air opening and around the standpipe or
jet. This would cause the gasoline to spray out of the tube and mix with the entering air stream.
Large
image
(101 kB).

Fig. 44.—Tracing Evolution of Modern Spray Carburetor. A—Early
Form Evolved by Maybach. B.—Phœnix-Daimler Modification of
Maybach’s Principle. C—Modern Concentric Float Automatic
Compensating Carburetor.

The form shown at B was a modification of Maybach’s simple device and was first used on the
Phœnix-Daimler engines. Several improvements are noted in this device. First, the carburetor was
made one unit by casting the float and mixing chambers together instead of making them separate and
joining them by a pipe, as shown at A. The float construction was improved and the gasoline shut-off
valve was operated through leverage instead of being directly fastened to the float. The spray nozzle
was surrounded by a choke tube which concentrated the air stream around it and made for more rapid
air flow at low engine speeds. A conical piece was placed over the jet to break up the entering spray
into a mist and insure more intimate admixture of air and gasoline. The air opening was provided with
an air cone which had a shutter controlling the opening so that the amount of air entering could be
regulated and thus vary the mixture proportions within certain limits.

CONCENTRIC FLOAT AND JET TYPE

The form shown at B has been further improved, and the type shown at C is representative of modern
single jet practice. In this the float chamber and mixing chamber are concentric. A balanced float
mechanism which insures steadiness of feed is used, the gasoline jet or standpipe is provided with a

needle valve to vary the amount of gasoline supplied the mixture and two air openings are provided.
The main air port is at the bottom of the vaporizer, while an auxiliary air inlet is provided at the side of
the mixing chamber. There are two methods of controlling the mixture proportions in this form of
carburetor. One may regulate the gasoline needle or adjust the auxiliary air valve.

SCHEBLER CARBURETOR

A Schebler carburetor, which has been used on some airplane engines, is shown in Fig. 45. It will be
noticed that a metering pin or needle valve opens the jet when the air valve opens. The long arm of a
leverage is connected to the air valve, while the short arm is connected to the needle, the reduction in
leverage being such that the needle valve is made to travel much less than the air valve. For setting the
amount of fuel passed or the size of the jet orifice when running with the air valve closed, there is a
screw which raises or lowers the fulcrum of the lever and there is also a dash control having the same
effect by pushing down the fulcrum against a small spring. A long extension is given to the venturi
tube which is very narrow around the jet orifices, which are horizontal and shown at A in the drawing.
Fuel enters the float chamber through the union M, and the spring P holds the metering pin upward
against the restraining action of the lever. The air valve may be set by an easily adjustable knurled
screw shown in the drawing, and fluttering of the valve is prevented by the piston dash pot carried in a
chamber above the valve into which the valve stem projects. The primary air enters beneath the jet
passage and there is a small throttle in the intake to increase the speed of air flow for starting purposes.
The carburetor is adapted for the use of a hot-air connection to the stove around the exhaust pipe and it
is recommended that such a fitting be supplied. The lever which controls the supply of air through the
primary air intake is so arranged that if desired it can be connected with a linkage on the dash or
control column by means of a flexible wire.

Fig. 45.—New Model of Schebler Carburetor With Metering Valve and
Extended Venturi. Note Mechanical Connection Between Air Valve and Fuel
Regulating Needle.

THE CLAUDEL (FRENCH) CARBURETOR

This carburetor is of extremely simple construction, because it has no supplementary or auxiliary air
valve and no moving parts except the throttle controlling the gas flow. The construction is already

shown in Fig. 46. The spray jet is eccentric with a surrounding sleeve or tube in which there are two
series of small orifices, one at the top and the other near the bottom. The former are about level with
the spray jet opening. The sleeve surrounding the nozzle is closed at the top. The air, passing the upper
holes in the sleeve, produces a vacuum in the sleeve, thereby drawing air in through the bottom holes.
It is this moving interior column of air that controls the flow of gasoline from the nozzle. Owing to the
friction of the small passages, the speed of air flow through the sleeve does not increase as fast as the
speed of air flow outside the sleeve, hence there is a tendency for the mixture to remain constant. The
throttle of this carburetor is of the barrel type, and the top of the spray nozzle and its surrounding
sleeve are located inside the throttle.

Fig. 46.—The Claudel Carburetor.

STEWART METERING PIN CARBURETOR

The carburetor shown at Fig. 47 is a metering type in which the vacuum at the jet is controlled by the
weight of the metering valve surrounding the upright metering pin. The only moving part is the
metering valve, which rises and falls with the changes in vacuum. The air chamber surrounds the
metering valve, and there is a mixing chamber above. As the valve is drawn up the gasoline passage is
enlarged on account of the predetermined taper on the metering pin, and the air passage also is
increased proportionately, giving the correct mixture. A dashpot at the bottom of the valve checks
flutter. In idling the valve rests on its seat, practically closing the air and giving the necessary idling
mixture. A passage through the valve acts as an aspirating tube. When the valve is closed altogether the
primary air passes through ducts in the valve itself, giving the proper amount for idling. The one
adjustment consists in raising or lowering the tapered metering pin, increasing or decreasing the supply
of gasoline. Dash control is supplied. This pulls down the metering pin, increasing the gasoline flow.
The duplex type for eight- and twelve-cylinder motors is the same in principle as model 25, but it is a
double carburetor synchronized as to throttle movements, adjustments, etc. The duplex for aeronautical
motors is made of cast aluminum alloy.

Fig. 47.—The Stewart Metering Pin Carburetor.

MULTIPLE NOZZLE VAPORIZERS

To secure properly proportioned mixtures some carburetor designers have evolved forms in which two
or more nozzles are used in a common mixing chamber. The usual construction is to use two, one
having a small opening and placed in a small air tube and used only for low speeds, the other being
placed in a larger air tube and having a slightly augmented bore so that it is employed on intermediate
speeds. At high speeds both jets would be used in series. Some multiple jet carburetors could be
considered as a series of these instruments, each one being designed for certain conditions of engine
action. They would vary from small size just sufficient to run the engine at low speed to others having
sufficient capacity to furnish gas for the highest possible engine speed when used in conjunction with
the smaller members which have been brought into service progressively as the engine speed has been
augmented. The multiple nozzle carburetor differs from that in which a single spray tube is used only
in the construction of the mixing chamber, as a common float bowl can be used to supply all spray
pipes. It is common practice to bring the jets into action progressively by some form of mechanical
connection with the throttle or by automatic valves.
The object of any multiple nozzle carburetor is to secure greater flexibility and endeavor to supply
mixtures of proper proportions at all speeds of the engine. It should be stated, however, that while
devices of this nature lend themselves readily to practical application it is more difficult to adjust them
than the simpler forms having but one nozzle. When a number of jets are used the liability of clogging
up the carburetor is increased, and if one or more of the nozzles is choked by a particle of dirt or water
the resulting mixture trouble is difficult to detect. One of the nozzles may supply enough gasoline to

permit the engine to run well at certain speeds and yet not be adequate to supply the proper amount of
gas under other conditions. In adjusting a multiple jet carburetor in which the jets are provided with
gasoline regulating needles, it is customary to consider each nozzle as a distinct carburetor and to
regulate it to secure the best motor action at that throttle position which corresponds to the conditions
under which the jet is brought into service. For instance, that supplied the primary mixing chamber
should be regulated with the throttle partly closed, while the auxiliary jet should be adjusted with the
throttle fully opened.

BALL AND BALL TWO-STAGE CARBURETOR

This is a two-stage vaporizing device, hot air being used in the primary or initial stage of vaporization
and cold air in the supplementary stage. Referring to the sectional illustration at Fig. 48, it will be seen
that there is a hot-air passage with a choke-valve; the primary venturi appears at B; J is its gasoline jet,
and V is a spring-loaded idling valve in a fixed air opening. These parts constitute the primary system.
In the secondary system A is a cold-air passage, T a butterfly valve and J a gasoline jet discharging into
the cold-air passage. This system is brought into operation by opening the butterfly T. A connection
between the butterfly T and the throttle, not shown, throws the butterfly wide open when the throttle is
not quite wide open; at all other times the butterfly is held closed by a spring. The cylindrical chamber
at the right of the mixing chamber has an extension E of reduced diameter connecting it with the intake
manifold through a passage D. A restricted opening connects the float chamber with the cylindrical
chamber so that the gasoline level is the same in both. A loosely fitting plunger P in the cylindrical
chamber has an upward extension into the small part of the chamber. O is a small air opening and M is
a passage from the cylindrical chamber to the mixing chamber. Air constantly passes through this when
the carburetor is in operation. The carburetor is really two in one. The primary carburetor is made up
of a central jet in a venturi passage. The float chamber is eccentric. In the air passage there is a fixed
opening, and additional air is taken in by the opening through suction of a spring-opposed air valve.
The second stage, which comes into play as soon as the carburetor is called upon for additional
mixture above low medium speeds, is made up of an independent air passage containing another air
valve. As the valve is opened this jet is uncovered, and air is led past it. For easy starting an extra
passage leads from the float bowl passage to a point above the throttle. All the suction falls upon this
passage when the throttle is closed. The passage contains a plunger and acts as a pick-up device. When
the vacuum increases the plunger rises and shuts off the flow of gasoline from the intake passage. As
the throttle is opened the vacuum in the intake passage is broken, and the plunger falls, causing
gasoline to gather above it. This is immediately drawn through the pick-up passage and gives the
desired mixture for acceleration.

Fig. 48.—The Ball and Ball Two-Stage Carburetor.

MASTER MULTIPLE-JET CARBURETOR

This carburetor, shown in detail in Figs. 49 and 50, has been very popular in racing cars and aviation
engines because of exceptionally good pick-up qualities and its thorough atomization of fuel. Its
principle of operation is the breaking up of the fuel by a series of jets, which vary in number from
fourteen to twenty-one, according to the size of the carburetor. These are uncovered by opening the
throttle, which is curved—a patented feature—to secure the correct progression of jets. The carburetor
has an eccentric float chamber, from which the gasoline is led to the jet piece from which the jets stand
up in a row. The tops of these jets are closed until the throttle is opened far enough to pass them, which
it does progressively. The air opening is at the bottom, and the throttle opening is such that a modified
venturi is formed. The throttle is carried in a cylindrical barrel with the jets placed below it, and the
passage from the barrel to the intake is arranged so that there is no interruption in the flow. For easy
starting a dash-controlled shutter closes off the air, throwing the suction on the jets, thus giving a rich
mixture.

Fig. 49.—The Master Carburetor.

Fig. 50.—Sectional View of Master Carburetor Showing
Parts.

The only adjustment is for idling, and once that is fixed it need never be touched. This is in the form of
a screw and regulates the position of the throttle when at idling position. The dash control has high-
speed, normal and rich-starting positions. In installing the Master carburetor the float chamber may be
turned either toward the radiator or driver’s seat. If the float is turned toward the radiator, however, a
forward lug plate should be ordered; otherwise it will be difficult to install the control. The throttle

lever must go all the way to the stop lug or maximum power will not be secured. In adjusting the idle
screw it is turned in for rich and out for lean.

COMPOUND NOZZLE ZENITH CARBURETOR

The Zenith carburetor, shown at Fig. 51, has become very popular for airplane engine use because of
its simplicity, as mixture compensation is secured by a compensating compound nozzle principle that
works very well in practice. To illustrate this principle briefly, let us consider the elementary type of
carburetor or mixing valve, as shown in Fig. 52, A. It consists of a single jet or spraying nozzle placed
in the path of the incoming air and fed from the usual float chamber. It is a natural inference to suppose
that as the speed of the motor increases, both the flow of air and of gasoline will increase in the same
proportion. Unhappily, such is not the case. There is a law of liquid bodies which states that the flow of
gasoline from the jet increases under suction faster than the flow of air, giving a mixture which grows
richer and richer—a mixture containing a much higher percentage of gasoline at high suction than at
low. The tendency is shown by the accompanying curve (Fig. 52, B), which gives the ratio of gasoline
to air at varying speeds from this type of jet. The mixture is practically constant only between narrow
limits and at very high speed. The most common method of correcting this defect is by putting various
auxiliary air valves which, adding air, tends to dilute this mixture as it gets too rich. It is difficult with
makeshift devices to gauge this dilution accurately for every motor speed.

Fig. 51.—Sectional View of Zenith Compound Nozzle
Compensating Carburetor.

Fig. 52.—Diagrams Explaining Action of Baverey Compound Nozzle Used in Zenith
Carburetor.

Now, if we have a jet which grows richer as the suction increases, the opposite type of jet is one which
would grow leaner under similar conditions. Baverey, the inventor of the Zenith, discovered the
principle of the constant flow device which is shown in Fig. 52, C. Here a certain fixed amount of
gasoline determined by the opening I is permitted to flow by gravity into the well J open to the air. The
suction at jet H has no effect upon the gravity compensator I because the suction is destroyed by the
open well J. The compensator, then, delivers a steady rate of flow per unit of time, and as the motor
suction increases more air is drawn up, while the amount of gasoline remains the same and the mixture
grows poorer and poorer. Fig. 52, D, shows this curve.
By combining these two types of rich and poor mixture carburetors the Zenith compound nozzle was
evolved. In Fig. 52, E, we have both the direct suction or richer type leading through pipe E and nozzle
G and the “constant flow” device of Baverey shown at J, I, K and nozzle H. One counteracts the
defects of the other, so that from the cranking of the motor to its highest speed there is a constant ratio
of air and gasoline to supply efficient combustion.
In addition to the compound nozzle the Zenith is equipped with a starting and idling well, shown in the
cut of Model L carburetor at P and J. This terminates in a priming hole at the edge of the butterfly
valve, where the suction is greatest when this valve is slightly open. The gasoline is drawn up by the
suction at the priming hole and, mixed with the air rushing by the butterfly, gives an ideal slow speed
mixture. At higher speeds with the butterfly valve opened further the priming well ceases to operate
and the compound nozzle drains the well and compensates correctly for any motor speed.

Fig. 53.—The Zenith Duplex Carburetor for Airplane Motors of the
V Type.

With the coming of the double motor containing eight or twelve cylinders arranged in two V blocks,
the question of good carburetion has been a problem requiring much study. The single carburetor has
given only indifferent results due to the strong cross suction in the inlet manifold from one set of
cylinders to the other. This naturally led to the adoption of two carburetors in which each set of
cylinders was independently fed by a separate carburetor. Results from this system were very good
when the two carburetors were working exactly in unison, but as it was extremely difficult to
accomplish this co-operation, especially where the adjustable type was employed, this system never
gained in favor. The next logical step was the Zenith Duplex, shown at Fig. 53. This consists of two
separate and distinct carburetors joined together so that a common gasoline float chamber and air inlet
could be used by both. It does away with cross suction in the manifold because each set of cylinders
has a separate intake of its own. It does away with two carburetors and makes for simplicity. The
practical application of the Zenith carburetor to the Curtiss 90 horse-power OX-2 motor used on the
JN-4 standard training machine is shown at Fig. 54, which outlines a rear view of the engine in
question. The carburetor is carried low to permit of fuel supply from a gravity tank carried back of the
motor.

Fig. 54.—Rear View of Curtiss OX-2 90 Horse-Power
Airplane Motor Showing Carburetor Location and Hot Air
Leads.

UTILITY OF GASOLINE STRAINERS

Many carburetors include a filtering screen at the point where the liquid enters the float chamber in
order to keep dirt or any other foreign matter which may be present in the fuel from entering the float
chamber. This is not general practice, however, and the majority of vaporizers do not include a filter in
their construction. It is very desirable that the dirt should be kept out of the carburetor because it may
get under the float control fuel valve and cause flooding by keeping it raised from its seat. If it finds its
way into the spray nozzle it may block the opening so that no gasoline will issue or may so constrict
the passage that only very small quantities of fuel will be supplied the mixture. Where the carburetor
itself is not provided with a filtering screen a simple filter is usually installed in the pipe line between
the gasoline tank and the float chamber.

Fig. 55.—Types of Strainers Interposed Between Vaporizer
and Gasoline Tank to Prevent Water or Dirt Passing Into
Carbureting Device.

Some simple forms of filters and separators are shown at Fig. 55. That at A consists of a simple brass
casting having a readily detachable gauze screen and a settling chamber of sufficient capacity to allow
the foreign matter to settle to the bottom, from which it is drained out by a pet cock. Any water or dirt
in the gasoline will settle to the bottom of the chamber, and as all fuel delivered to the carburetor must
pass through the wire gauze screen it is not likely to contain impurities when it reaches the float
chamber. The heavier particles, such as scale from the tank or dirt and even water, all of which have
greater weight than the gasoline, will sink to the bottom of the chamber, whereas light particles, such
as lint, will be prevented from flowing into the carburetor by the filtering screen.
The filtering device shown at B is a larger appliance than that shown at A, and should be more efficient
as a separator because the gasoline is forced to pass through three filtering screens before it reaches the
carburetor. The gasoline enters the device shown at C through a bent pipe which leads directly to the
settling chamber and from thence through a wire gauze screen to the upper compartment which leads
to the carburetor. The device shown at D is a combination strainer, drain, and sediment cup. The
filtering screen is held in place by a spring and both are removed by taking out a plug at the bottom of
the device. The shut-off valve at the top of the device is interposed between the sediment cup and the
carburetor. This separating device is incorporated with the gasoline tank and forms an integral part of
the gasoline supply system. The other types shown are designed to be interposed between the gasoline
tank and the carburetor at any point in the pipe line where they may be conveniently placed.

INTAKE MANIFOLD DESIGN AND CONSTRUCTION

On four- and six-cylinder engines and in fact on all multiple-cylinder forms, it is important that the
piping leading from the carburetor to the cylinders be made in such a way that the various cylinders
will receive their full quota of gas and that each cylinder will receive its charge at about the same point

in the cycle of operations. In order to make the passages direct the bends should be as few as possible,
and when curves are necessary they should be of large radius because an abrupt corner will not only
impede gas flow but will tend to promote condensation of the fuel. Every precaution should be taken
with four- and six-cylinder engines to insure equitable gas distribution to the valve chambers if regular
action of the power plant is desired. If the gas pipe has many turns and angles it will be difficult to
charge all cylinders properly. On some six-cylinder aviation engines, two carburetors are used because
of trouble experienced with manifolds designed for one carburetor. Duplex carburetors are necessary to
secure the best results from eight- and twelve-cylinder V engines.
The problem of intake piping is simplified to some extent on block motors where the intake passage is
cored in the cylinder casting and where but one short pipe is needed to join this passage to the
carburetor. If the cylinders are cast in pairs a simple pipe of T or Y form can be used with success.
When the engine is of a type using individual cylinder castings, especially in the six-cylinder power
plants, the proper application and installation of suitable piping is a difficult problem. The reader is
referred to the various engine designs outlined to ascertain how the inlet piping has been arranged on
representative aviation engines. Intake piping is constructed in two ways, the most common method
being to cast the manifold of brass or aluminum. The other method, which is more costly, is to use a
built-up construction of copper or brass tubing with cast metal elbows and Y pieces. One of the
disadvantages advanced against the cast manifold is that blowholes may exist which produce imperfect
castings and which will cause mixture troubles because the entering gas from the carburetor, which
may be of proper proportions, is diluted by the excess air which leaks in through the porous casting.
Another factor of some moment is that the roughness of the walls has a certain amount of friction
which tends to reduce the velocity of the gases, and when projecting pieces are present, such as core
wire or other points of metal, these tend to collect the drops of liquid fuel and thus promote
condensation. The advantage of the built-up construction is that the walls of the tubing are very
smooth, and as the castings are small it is not difficult to clean them out thoroughly before they are
incorporated in the manifold. The tubing and castings are joined together by hard soldering, brazing or
autogenous welding.

COMPENSATING FOR VARYING ATMOSPHERIC CONDITIONS

The low-grade gasoline used at the present time makes it necessary to use vaporizers that are more
susceptible to atmospheric variations than when higher grade and more volatile liquids are vaporized.
Sudden temperature changes, sometimes being as much as forty degrees rise or fall in twelve hours,
affect the mixture proportions to some extent, and not only changes in temperature but variations in
altitude also have a bearing on mixture proportions by affecting both gasoline and air. As the
temperature falls the specific gravity of the gasoline increases and it becomes heavier, this producing
difficulty in vaporizing. The tendency of very cold air is to condense gasoline instead of vaporizing it
and therefore it is necessary to supply heated air to some carburetors to obtain proper mixtures during
cold weather. In order that the gas mixtures will ignite properly the fuel must be vaporized and
thoroughly mixed with the entering air either by heat or high velocity of the gases. The application of
air stoves to the Curtiss OX-2 motor is clearly shown at Fig. 54. It will be seen that flexible metal
pipes are used to convey the heated air to the air intakes of the duplex mixing chamber.

Fig. 56.—Chart Showing Diminution of Air Pressure as
Altitude Increases.

HOW HIGH ALTITUDE AFFECTS POWER

Any internal combustion engine will show less power at high altitudes than it
will deliver at sea level, and this has caused a great deal of questioning. “There
is a good reason for this,” says a writer in “Motor Age,” “and it is a physical
impossibility for the engine to do otherwise. The difference is due to the lower
atmospheric pressure the higher up we get. That is, at sea level the atmosphere
has a pressure of 14.7 pounds per square inch; at 5,000 feet above sea level the
pressure is approximately 12.13 pounds per square inch, and at 10,000 feet it is
10 pounds per square inch. From this it will be seen that the final pressure
attained after the piston has driven the gas into compressed condition ready for
firing is lower as the atmospheric pressure drops. This means that there is not so
much power in the compressed charge of gas the higher up you get above sea
level.
“For example, suppose the compression ratio to be 41⁄2 to 1; in other words,
suppose the air space above the piston to have 41⁄2 times the volume when the
piston is at the bottom of its stroke that it has when the piston is at the top of the
stroke. That is a common compression ratio for an average motor, and is chosen
because it is considered to be the best for maximum horse-power and in order

that the compression pressure will not be so high as to cause pre-ignition.
Knowing the compression ratio, we can determine the final pressure
immediately before ignition by substituting in the standard formula:
V 1.3
P1 = ( )
V1
in which P is the atmospheric pressure; P1 is the final pressure, and V/V1 is the
compression ratio, therefore P1 = 14.7 (4.5)1.3 = 104 pounds per square inch,
absolute.
“That is, 104 pounds per square inch is the most efficient final compression
pressure to have for this engine at sea level, since it comes directly from the
compression ratio.
“Now supposing we consider that the altitude is 7,000 feet above sea level. At
this height the atmospheric pressure is 11.25 pounds per square inch,
approximately. In this case we can again substitute in the formula, using the
new atmospheric pressure figure. The equation becomes:
P1 = 11.25 (4.5)1.3—79.4 pounds per square inch, absolute.
“Therefore we now have a final compression pressure of only 79.4 pounds per
square inch, which is considerably below the pressure we have just found to be
the most efficient for the motor. The resulting power drop is evident.
“It should be borne in mind that these final compression pressures are absolute
pressures—that is, they include the atmospheric pressure. In the first case, to get
the pressure above atmospheric you would subtract 14.7 and in the latter 11.25
would have to be deducted. In other words, where the sea level compression is
89.3 pounds per square inch above the atmosphere, the same motor will have
only a compression pressure of 68.15 pounds per square inch above the
atmosphere at 7,000 feet elevation.
“From the above it is evident that in order to bring the final compression
pressure up to the efficient figure we have determined, a different compression
ratio would have to be used. That is, the final volume would have to be less, and
as it is impossible to vary this to meet the conditions of altitude, the loss of
power cannot be helped except by the replacing of the standard pistons with
some that are longer above the wrist-pin so as to reduce the space above the
pistons when on top center. Then if the ratio is thereby raised to some such
figures as 5 to 1, the engine will again have its proper final pressure, but it will

still not have as much power as it would have at sea level, since the horse-
power varies directly with the atmospheric pressure, final compression being
kept constant. That is, at 7,000 feet the horse-power of an engine that had 40
horse-power at sea level would be equal to
11.25
= 30.6 horse-power.
14.7
“If the original compression ratio of 4.5 were retained, the drop in horse-power
would be even greater than this. These computations and remarks will make it
clear that the designer who contemplates building an airplane for high altitude
use should see to it that it is of sufficient power to compensate for the drop that
is inevitable when it is up in the air. This is often illustrated in stationary gas-
engine installations. An engine that had a sea-level rating amply sufficient for
the work required, might not be powerful enough when brought up several
thousand feet.” When one considers that airplanes attain heights of over 18,000
feet, it will be evident that an ample margin of engine power is necessary.

THE DIESEL SYSTEM

A system of fuel supply developed by the late Dr. Diesel, a German chemist and
engineer, is attracting considerable attention at the present time on account of
the ability of the Diesel engine to burn low-grade fuels, such as crude
petroleum. In this system the engines are built so that very high compressions
are used, and only pure air is taken into the cylinder on the induction stroke.
This is compressed to a pressure of about 500 pounds per square inch, and
sufficient heat is produced by this compression to explode a hydrocarbon
mixture. As the air which is compressed to this high point cannot burn, the fuel
is introduced into the cylinder combustion chamber under still higher
compression than that of the compressed air, and as it is injected in a fine stream
it is immediately vaporized because of the heat. Just as soon as the compressed
air becomes thoroughly saturated with the liquid fuel, it will explode on account
of the degree of heat present in the combustion chamber. Such motors have
been used in marine and stationary applications, but are not practical for
airplanes or motor cars because of lack of flexibility and great weight in
proportion to power developed. The Diesel engine is the standard power plant
used in submarine boats and motor ships, as its efficiency renders it particularly
well adapted for large units.

NOTES ON CARBURETOR INSTALLATION IN AIRPLANES

A writer in “The Aeroplane,” an English publication, discourses on some
features of carburetor installation that may be of interest to the aviation student,
so portions of the dissertation are reproduced herewith.
“Users of airplanes fitted with ordinary type carburetors will do well to note carefully the way
in which these are fitted, for several costly machines have been burnt lately through the sheer
carelessness of their users. These particular machines were fitted with a high powered V-type
engine, made by a firm which is famous as manufacturers of automobiles de luxe. In these
engines there are four carburetors, mounted in the V between the cylinders. When the engine is
fitted as a tractor, the float chambers are in front of the jet chambers. Consequently, when the
tail of the machine is resting on the ground, the jets are lower than the level of the gasoline in
the float chamber.
“Quite naturally, the gasoline runs out of the jet, if it is left turned on when the machine is
standing in its normal position, and trickles into the V at the top of the crank-case. Thence it
runs down to the tail of the engine, where the magnetos are fitted, and saturates them. If left
long enough, the gasoline manages to soak well into the fuselage before evaporating. And what
does evaporate makes an inflammable gas in the forward cockpit. Then some one comes along
and starts up the engine. The spark-gap of the magneto gives one flash, and the whole front of
the machine proceeds to give a Fourth of July performance forthwith. Naturally, one safeguard
is to turn the petrol off directly the machine lands. Another is never to turn it on till the engine is
actually being started up.
“One would be asking too much of the human boy—who is officially regarded as the only
person fit to fly an aeroplane—if one depended upon his memory of such a detail to save his
machine, though one might perhaps reasonably expect the older pilots to remember not to
forget. Even so, other means of prevention are preferable, for fire is quite as likely to occur from
just the same cause if the engine happens to be a trifle obstinate in starting, and so gives the
carburetors several minutes in which to drip—in which operation they would probably be
assisted by air-mechanics ‘tickling’ them.
“One way out of the trouble is to fit drip tins under the jet chamber to catch the gasoline as it
falls. This is all very well just to prevent fire while the machine is being started up, but it will
not save it if it is left standing with the tail on the ground and the petrol turned on, for the drip
tins will then fill up and run over. And if it catches then, the contents of the drip tins merely add
fuel to the fire.

Reversing Carburetors

“Yet another way is to turn the carburetors round, so that the float chambers are behind the jets,
and so come below them when the tail is on the ground, thus cutting off the gasoline low down
in the jets. There seems to be no particular mechanical difficulty about this, though I must
confess that I did not note very carefully whether the reversal of the float chambers would make
them foul any other fittings on the engine. It has been argued, however, that doing this would
starve the engine of gasoline when climbing at a steep angle, as the gasoline would then be
lowered in the jets and need more suction to get into the cylinders. This is rather a pretty point
of amateur motor mechanics to discuss, for, obviously, when the same engine is used as a
‘pusher’ instead of a tractor, the jets are in front of the floats, and there seems to be no falling
off in power.

Starvation of Mixture

“Moreover, the higher a machine goes the lower is the atmospheric pressure, and, consequently,
the less is the amount of air sucked in at each induction stroke. This means, of course, that with
the gasoline supply the mixture at high altitudes is too rich, so that, in order to get precisely the
right mixture when very high up, it is necessary to reduce the gasoline supply by screwing down
the needle valve between the tank and the carburetor—at least, that has been the experience of
various high-flying pilots. No doubt something might be done in the way of forced air feed to
compensate for reduced atmospheric pressure, but it remains to be proved whether the extra
weight of mechanism involved would pay for the extra power obtained. Variable compression
might do something, also, to even things up, but here, also, weight of mechanism has to be
considered.
“In any case, at present, the higher one goes the more the power of the engine is reduced, for
less air means a less volume of mixture per cylinder, and as the petrol feed has to be starved to
suit the smaller amount of air available, this means further loss of power. I do not know whether
anyone has evolved a carburetor which automatically starves the gasoline feed when high up,
but it seems possible that when an airplane is sagging about ‘up against the ceiling’—as a
French pilot described the absolute limit of climb for his particular machine—it might be a good
thing to have the jets in front of the float chamber, for then a certain amount of automatic
starvation would take place.
“When a machine is right up at its limiting height, and the pilot is doing his best to make it go
higher still, it is probably flying with its tail as low as the pilot dares to let it go, and the lateral
and longitudinal controls are on the verge of vanishing, so that if the carburetor jets are behind
the float chambers there is bound to be an over-rich mixture in any case. There is even a
possibility of a careless or ignorant pilot carrying on in this tail-down position till one set of
cylinders cuts out altogether, in which case the carburetor feeding that set may flood over, just
as if the machine were on the ground, and the whole thing may catch fire. Whereas, with the jets
in front of the floats, though the mixture may starve a trifle, there is, at any rate, no danger of
fire through climbing with the tail down.

A Diving Danger

“On the other hand, in a ‘pusher’ with this type of engine, if the jets are in their normal position
—which is in front of the floats—there is danger of fire in a dive. That is to say, if the pilot
throttles right down, or switches off and relies on air pressure on his propeller to start the engine
again, so that the gasoline is flooding over out of the jets instead of being sucked into the
engine, there may be flooding over the magnetos if the dive is very steep and prolonged. In any
case, a long dive will mean a certain amount of flooding, and, probably, a good deal of choking
and spitting by the engine before it gets rid of the over-rich mixture and picks up steady firing
again. Which may indicate to young pilots that it is not good to come down too low under such
circumstances, trusting entirely to their engines to pick up at once and get going before they hit
the ground.
“On the whole, it seems that it might be better practice to set the carburetors thwartwise of
engines, for then jets and floats would always be at approximately the same level, no matter
what the longitudinal position of the machine, and it is never long enough in one position at a
big lateral angle to raise any serious carburetor troubles. Car manufacturers who dive cheerfully
into the troubled waters of aero-engine designs are a trifle apt to forget that their engines are put
into positions on airplanes which would be positively indecent in a motor car. An angle of 1 in
10 is the exception on a car, but it is common on an airplane, and no one ever heard of a car
going down a hill of 10 to 1—which is not quite a vertical dive. Therefore, there is every excuse
for a well-designed and properly brought-up carburetor misbehaving itself in an aeroplane.

“It seems, then, that it is up to the manufacturers to produce better carburetors—say, with the jet
central with the float. But it also behooves the user to show ordinary common sense in handling
the material at present available, and not to make a practice of burning up $25,000 worth or so
of airplane just because he is too lazy to turn off his gasoline, or to have the tail of his machine
lifted up while he is tinkering with his engines.”

NOTES ON CARBURETOR ADJUSTMENT

The modern float feed carburetor is a delicate and nicely balanced appliance
that requires a certain amount of attention and care in order to obtain the best
results. The adjustments can only be made by one possessing an intelligent
knowledge of carburetor construction and must never be made unless the reason
for changing the old adjustment is understood. Before altering the adjustment of
the leading forms of carburetors, a few hints regarding the quality to be
obtained in the mixture should be given some consideration, as if these are
properly understood this knowledge will prove of great assistance in adjusting
the vaporizer to give a good working proportion of fuel and air. There is some
question regarding the best mixture proportions and it is estimated that gas will
be explosive in which the proportions of fuel vapor and air will vary from one
part of the former to a wide range included between four and eighteen parts of
the latter. A one to four mixture is much too rich, while the one in eighteen is
much too lean to provide positive ignition.
A rich mixture should be avoided because the excessive fuel used will deposit
carbon and will soot the cylinder walls, combustion chamber interior, piston top
and valves and also tend to overheat the motor. A rich mixture will also
seriously interfere with flexible control of the engine, as it will choke up on low
throttle and run well on open throttle when the full amount of gas is needed. A
rich mixture may be quickly discovered by black smoke issuing from the
muffler, the exhaust gas having a very pungent odor. If the mixture contains a
surplus of air there will be popping sounds in the carburetor, which is
commonly termed “blowing back.” To adjust a carburetor is not a difficult
matter when the purpose of the various control members is understood. The first
thing to do in adjusting a carburetor is to start the motor and to retard the
sparking lever so the motor will run slowly leaving the throttle about half open.
In order to ascertain if the mixture is too rich cut down the gasoline flow
gradually by screwing down the needle valve until the motor commences to run
irregularly or misfire. Close the needle valves as far as possible without having
the engine come to a stop, and after having found the minimum amount of fuel
gradually unscrew the adjusting valve until you arrive at the point where the

engine develops its highest speed. When this adjustment is secured the lock nut
is screwed in place so the needle valve will keep the adjustment. The next point
to look out for is regulation of the auxiliary air supply on those types of
carburetors where an adjustable air valve is provided. This is done by advancing
the spark lever and opening the throttle. The air valve is first opened or the
spring tension reduced to a point where the engine misfires or pops back in the
carburetor. When the point of maximum air supply the engine will run on is thus
determined, the air valve spring may be tightened by screwing in on the
regulating screw until the point is reached where an appreciable speeding up of
the engine is noticed. If both fuel and air valves are set right, it will be possible
to accelerate the engine speed uniformly without interfering with regularity of
engine operation by moving the throttle lever or accelerator pedal from its
closed to its wide open position, this being done with the spark lever advanced.
All types of carburetors do not have the same means of adjustment; in fact,
some adjust only with the gasoline regulating needle; others must have a
complete change of spray nozzles; while in others the mixture proportions may
be varied only by adjustment of the quantity of entering air. Changing the float
level is effective in some carburetors, but this should never be done unless it is
certain that the level is not correct. Full instructions for locating carburetion
troubles will be given in proper sequence.
It is a fact well known to experienced repairmen and motorists that atmospheric
conditions have much to do with carburetor action. It is often observed that a
motor seems to develop more power at night than during the day, a
circumstance which is attributed to the presence of more moisture in the cooler
night air. Likewise, taking a motor from sea level to an altitude of 10,000 feet
involves using rarefied air in the engine cylinders and atmospheric pressures
ranging from 14.7 pounds at sea level to 10.1 pounds per square inch at the high
altitude. All carburetors will require some adjustment in the course of any
material change from one level to another. Great changes of altitude also have a
marked effect on the cooling system of an airplane. Water boils at 212 degrees
F. only at sea level. At an altitude of 10,000 feet it will boil at a temperature
nineteen degrees lower, or 193 degrees F.
In high altitudes the reduced atmospheric pressure, for 5,000 feet or higher than
sea level, results in not enough air reaching the mixture, so that either the
auxiliary air opening has to be increased, or the gasoline in the mixture cut
down. If the user is to be continually at high altitudes he should immediately
purchase either a larger dome or a smaller strangling tube, mentioning the size
carburetor that is at present in use and the type of motor that it is on, including

details as to the bore and stroke. The smaller strangling tube makes an increased
suction at the spray nozzle; the air will have to be readjusted to meet it and you
can use more auxiliary air, which is necessary. The effect on the motor without a
smaller strangling tube is a perceptible sluggishness and failure to speed up to
its normal crank-shaft revolutions, as well as failure to give power. It means that
about one-third of the regular speed is cut out. The reduced atmospheric
pressure reduces the power of the explosion, in that there is not the same
quantity of oxygen in the combustion chamber as at sea level; to increase the
amount taken in, you must also increase the gasoline speed, which is done by an
increased suction through the smaller strangling aperture. Some forms of
carburetors are affected more than others by changes of altitude, which explains
why the Zenith is so widely employed for airplane engine use. The
compensating nozzle construction is not influenced as much by changes of
altitude as the simpler nozzle types are.

CHAPTER VI
Early Ignition Systems—Electrical Ignition Best—Fundamentals of
Magnetism Outlined—Forms of Magneto—Zones of Magnetic
Influence—How Magnets are Made—Electricity and Magnetism
Related—Basic Principles of Magneto Action—Essential Parts of
Magneto and Functions—Transformer Coil Systems—True High
Tension Type—The Berling Magneto—Timing and Care—The Dixie
Magneto—Spark Plug Design and Application—Two-Spark Ignition—
Special Airplane Plug.

EARLY IGNITION SYSTEMS

One of the most important auxiliary groups of the gasoline engine comprising
the airplane power plant and one absolutely necessary to insure engine action is
the ignition system or the method employed of kindling the compressed gas in
the cylinder to produce an explosion and useful power. The ignition system has
been fully as well developed as other parts of the engine, and at the present time
practically all ignition systems follow principles which have become standard
through wide acceptance.
During the early stages of development of the gasoline engine various methods
of exploding the charge of combustible gas in the cylinder were employed. On
some of the earliest engines a flame burned close to the cylinder head, and at
the proper time for ignition a slide or valve moved to provide an opening which
permitted the flame to ignite the gas back of the piston. This system was
practical only on the primitive form of gas engines in which the charge was not
compressed before ignition. Later, when it was found desirable to compress the
gas a certain degree before exploding it, an incandescent platinum tube in the
combustion chamber, which was kept in a heated condition by a flame burning
in it, exploded the gas. The naked flame was not suitable in this application
because when the slide was opened to provide communication between the
flame and the gas the compressed charge escaped from the cylinder with
enough pressure to blow out the flame at times and thus cause irregular ignition.
When the flame was housed in a platinum tube it was protected from the direct

action of the gas, and as long as the tube was maintained at the proper point of
incandescence regular ignition was obtained.
Some engineers utilized the property of gases firing themselves if compressed
to a sufficient degree, while others depended upon the heat stored in the
cylinder-head to fire the highly compressed gas. None of these methods were
practical in their application to motor car engines because they did not permit
flexible engine action which is so desirable. At the present time, electrical
ignition systems in which the compressed gas is exploded by the heating value
of the minute electric arc or spark in the cylinder are standard, and the general
practice seems to be toward the use of mechanical producers of electricity rather
than chemical batteries.

ELECTRICAL IGNITION BEST

Two general forms of electrical ignition systems may be used, the most popular
being that in which a current of electricity under high tension is made to leap a
gap or air space between the points of the sparking plug screwed into the
cylinder. The other form, which has been almost entirely abandoned in
automobile and which was never used with airplane engine practice, but which
is still used to some extent on marine engines, is called the low-tension system
because current of low voltage is used and the spark is produced by moving
electrodes in the combustion chamber.
The essential elements of any electrical ignition system, either high or low
tension, are: First, a simple and practical method of current production; second,
suitable timing apparatus to cause the spark to occur at the right point in the
cycle of engine action; third, suitable wiring and other apparatus to convey the
current produced by the generator to the sparking member in the cylinder.
The various appliances necessary to secure prompt ignition of the compressed
gases should be described in some detail because of the importance of the
ignition system. It is patent that the scope of a work of this character does not
permit one to go fully into the theory and principles of operation of all
appliances which may be used in connection with gasoline motor ignition, but
at the same time it is important that the elementary principles be considered to
some extent in order that the reader should have a proper understanding of the
very essential ignition apparatus. The first point considered will be the common
methods of generating the electricity, then the appliances to utilize it and
produce the required spark in the cylinder. Inasmuch as magneto ignition is

universally used in connection with airplane engine ignition it will not be
necessary to consider battery ignition systems.

FUNDAMENTALS OF MAGNETISM OUTLINED

To properly understand the phenomena and forces involved in the generation of
electrical energy by mechanical means it is necessary to become familiar with
some of the elementary principles of magnetism and its relation to electricity.
The following matter can be read with profit by those who are not familiar with
the subject. Most persons know that magnetism exists in certain substances, but
many are not able to grasp the terms used in describing the operation of various
electrical devices because of not possessing a knowledge of the basic facts upon
which the action of such apparatus is based.
Magnetism is a property possessed by certain substances and is manifested by
the ability to attract and repel other materials susceptible to its effects. When
this phenomenon is manifested by a conductor or wire through which a current
of electricity is flowing it is termed “electro-magnetism.” Magnetism and
electricity are closely related, each being capable of producing the other.
Practically all of the phenomena manifested by materials which possess
magnetic qualities naturally can be easily reproduced by passing a current of
electricity through a body which, when not under electrical influence, is not a
magnetic substance. Only certain substances show magnetic properties, these
being iron, nickel, cobalt and their alloys.
The earliest known substance possessing magnetic properties was a stone first
found in Asia Minor. It was called the lodestone or leading stone, because of its
tendency, if arranged so it could be moved freely, of pointing one particular
portion toward the north. The compass of the ancient Chinese mariners was a
piece of this material, now known to be iron ore, suspended by a light thread or
floated on a cork in some liquid so one end would point toward the north
magnetic pole of the earth. The reason that this stone was magnetic was hard to
define for a time, until it was learned that the earth was one huge magnet and
that the iron ore, being particularly susceptible, absorbed and retained some of
this magnetism.
Most of us are familiar with some of the properties of the magnet because of the
extensive sale and use of small horseshoe magnets as toys. As they only cost a
few pennies every one has owned one at some time or other and has
experimented with various materials to see if they would be attracted. Small

pieces of iron or steel were quickly attracted to the magnet and adhered to the
pole pieces when brought within the zone of magnetic influence. It was soon
learned that brass, copper, tin or zinc were not affected by the magnet. A simple
experiment that serves to illustrate magnetic attraction of several substances is
shown at A, Fig. 57. In this, several balls are hung from a standard or support,
one of these being of iron, another of steel. When a magnet is brought near
either of these they will be attracted toward it, while the others will remain
indifferent to the magnetic force. Experimenters soon learned that of the
common metals only iron or steel were magnetic.

Fig. 57.—Some Simple Experiments to Demonstrate
Various Magnetic Phenomena and Clearly Outline
Effects of Magnetism and Various Forms of
Magnets.

If the ordinary bar or horseshoe magnet be carefully examined, one end will be
found to be marked N. This indicates the north pole, while the other end is not

usually marked and is the south pole. If the north pole of one magnet is brought
near the south pole of another, a strong attraction will exist between them, this
depending upon the size of the magnets used and the air gap separating the
poles. If the south pole of one magnet is brought close to the end of the same
polarity of the other there will be a pronounced repulsion of like force. These
facts are easily proved by the simple experiment outlined at B, Fig. 57. A
magnet will only attract or influence a substance having similar qualities. The
like poles of magnets will repel each other because of the obvious impossibility
of uniting two influences or forces of practically equal strength but flowing in
opposite directions. The unlike poles of magnets attract each other because the
force is flowing in the same direction. The flow of magnetism is through the
magnet from south to north and the circuit is completed by the flow of magnetic
influence through the air gap or metal armature bridging it from the north to the
south pole.

FORMS OF MAGNETS AND ZONE OF MAGNETIC INFLUENCE
DEFINED

Magnets are commonly made in two forms, either in the shape of a bar or
horseshoe. These two forms are made in two types, simple or compound. The
latter are composed of a number of magnets of the same form united so the ends
of like polarity are laced together, and such a construction will be more efficient
and have more strength than a simple magnet of the same weight. The two
common forms of simple and compound magnets are shown at C, Fig. 57. The
zone in which a magnetic influence occurs is called the magnetic field, and this
force can be graphically shown by means of imaginary lines, which are termed
“lines of force.” As will be seen from the diagram at D, Fig. 57, the lines show
the direction of action of the magnetic force and also show its strength, as they
are closer together and more numerous when the intensity of the magnetic field
is at its maximum. A simple method of demonstrating the presence of the force
is to lay a piece of thin paper over the pole pieces of either a bar or horseshoe
magnet and sprinkle fine iron filings on it. The particles of metal arrange
themselves in very much the manner shown in the illustrations and prove that
the magnetic field actually exists.
The form of magnet used will materially affect the size and area of the magnetic
field. It will be noted that the field will be concentrated to a greater extent with
the horseshoe form because of the proximity of the poles. It should be
understood that these lines have no actual existence, but are imaginary and

assumed to exist only to show the way the magnetic field is distributed. The
magnetic influence is always greater at the poles than at the center, and that is
why a horseshoe or U-form magnet is used in practically all magnetos or
dynamos. This greater attraction at the poles can be clearly demonstrated by
sprinkling iron filings on bar and U magnets, as outlined at E, Fig. 57. A large
mass gathers at the pole pieces, gradually tapering down toward the point where
the attraction is least.
From the diagrams it will be seen that the flow of magnetism is from one pole
to the other by means of curved paths between them. This circuit is completed
by the magnetism flowing from one pole to the other through the magnet, and
as this flow is continued as long as the body remains magnetic it constitutes a
magnetic circuit. If this flow were temporarily interrupted by means of a
conductor of electricity moving through the field there would be a current of
electricity induced in the conductor every time it cut the lines of force. There
are three kinds of magnetic circuits. A non-magnetic circuit is one in which the
magnetic influence completes its circuit through some substance not susceptible
to the force. A closed magnetic circuit is one in which the influence completes
its circuit through some magnetic material which bridges the gap between the
poles. A compound circuit is that in which the magnetic influence passes
through magnetic substances and non-magnetic substances in order to complete
its circuit.

HOW IRON AND STEEL BARS ARE MADE MAGNETIC

Magnetism may be produced in two ways, by contact or induction. If a piece of
steel is rubbed on a magnet it will be found a magnet when removed, having a
north and south pole and all of the properties found in the energizing magnet.
This is magnetizing by contact. A piece of steel will retain the magnetism
imparted to it for a considerable length of time, and the influence that remains is
known as residual magnetism. This property may be increased by alloying the
steel with tungsten and hardening it before it is magnetized. Any material that
will retain its magnetic influence after removal from the source of magnetism is
known as a permanent magnet. If a piece of iron or steel is brought into the
magnetic field of a powerful magnet it becomes a magnet without actual contact
with the energizer. This is magnetizing by magnetic induction. If a powerful
electric current flows through an insulated conductor wound around a piece of
iron or steel it will make a magnet of it. This is magnetizing by electro-magnetic
induction. A magnet made in this manner is termed an electro-magnet and

usually the metal is of such a nature that it will not retain its magnetism when
the current ceases to flow around it. Steel is used in all cases where permanent
magnets are required, while soft iron is employed in all cases where an
intermittent magnetic action is desired. Magneto field magnets are always made
of tungsten steel alloy, so treated that it will retain its magnetism for lengthy
periods.

ELECTRICITY AND MAGNETISM CLOSELY RELATED

There are many points in which magnetism and electricity are alike. For
instance, air is a medium that offers considerable resistance to the passage of
both magnetic influence and electric energy, although it offers more resistance
to the passage of the latter. Minerals like iron or steel are very easily influenced
by magnetism and easily penetrated by it. When one of these is present in the
magnetic circuit the magnetism will flow through the metal. Any metal is a
good conductor for the passage of the electric current, but few metals are good
conductors of magnetic energy. A body of the proper metal will become a
magnet due to induction if placed in the magnetic field, having a south pole
where the lines of force enter it and a north pole where they pass out.
We have seen that a magnet is constantly surrounded by a magnetic field and
that an electrical conductor when carrying a current is also surrounded by a field
of magnetic influence. Now if the conductor carrying a current of electricity
will induce magnetism in a bar of iron or steel, by a reversal of this process, a
magnetized iron or steel bar will produce a current of electricity in a conductor.
It is upon this principle that the modern dynamo or magneto is constructed. If
an electro-motive force is induced in a conductor by moving it across a field of
magnetic influence, or by passing a magnetic field near a conductor, electricity
is said to be generated by magneto-electric induction. All mechanical generators
of the electric current using permanent steel magnets to produce a field of
magnetic influence are of this type.

BASIC PRINCIPLES OF MAGNETO OUTLINED

The accompanying diagram, Fig. 58, will show these principles very clearly. As
stated on an earlier page, if the lines of force in the magnetic field are cut by a
suitable conductor an electrical impulse will be produced in that conductor. In
this simple machine the lines of force exist between the poles of a horseshoe

magnet. The conductor, which in this case is a loop of copper wire, is mounted
upon a spindle in order that it may be rotated in the magnetic field to cut the
lines of magnetic influence present between the pole pieces. Both of the ends of
this loop are connected, one with the insulated drum shown upon the shaft, the
other to the shaft. Two metal brushes are employed to collect the current and
cause it to flow through the external circuit. It can be seen that when the shaft is
turned in the direction of the arrow the loop will cut through the lines of
magnetic influence and a current will be generated therein.

Fig. 58.—Elementary Form of Magneto Showing Principal
Parts Simplified to Make Method of Current Generation
Clear.

The pressure of the current and the amount produced vary in accordance to the
rapidity with which the lines of magnetic influence are cut. The armature of a
practical magneto, therefore, differs materially from that shown in the diagram.
A large number of loops of wire would be mounted upon this shaft in order that
the lines of magnetic influence would be cut a greater number of times in a
given period and a core of iron used as a backing for the wire. This would give

a more rapid alternating current and a higher electro-motive force than would be
the case with a smaller number of loops of wire.

Fig. 59.—Showing How Strength of Magnetic
Influence and of the Currents Induced in the
Windings of Armature Vary with the Rapidity of
Changes of Flow.

The illustrations at Fig. 59 show a conventional double winding armature and
field magnetic of a practical magneto in part section and will serve to more fully
emphasize the points previously made. If the armature or spindle were removed
from between the pole pieces there would exist a field of magnetic influence as
shown at Fig. 57, but the introduction of this component provides a conductor
(the iron core) for the magnetic energy, regardless of its position, though the
facility with which the influence will be transmitted depends entirely upon the
position of the core. As shown at A, the magnetic flow is through the main body

in a straight line, while at B, which position the armature has attained after one-
eighth revolution, or 45 degrees travel in the direction of the arrow, the
magnetism must pass through in the manner indicated. At C, which position is
attained every half revolution, the magnetic energy abandons the longer path
through the body of the core for the shorter passage offered by the side pieces,
and the field thrown out by the cross bar disappears. On further rotation of the
armature, as at D, the body of the core again becomes energized as the magnetic
influence resumes its flow through it. These changes in the strength of the
magnetic field when distorted by the armature core, as well as the intensity of
the energy existing in the field, affect the windings, and the electrical energy
induced therein corresponds in strength to the rapidity with which these changes
in magnetic flow occur. The most pronounced changes in the strength of the
field will occur as the armature passes from position B to D, because the
magnetic field existing around the core will be destroyed and again re-
established.
During the most of the armature rotation the changes in strength will be slight
and the currents induced in the wire correspondingly small; but at the instant the
core becomes remagnetized, as the armature leaves position C, the current
produced will be at its maximum, and it is necessary to so time the rotation of
the armature that at this instant one of the cylinders is in condition to be fired. It
is imperative that the armature be driven in such relation to the crank-shaft that
each production of maximum current coincides with the ignition point, this
condition existing twice during each revolution of the armature, or at every 180
degrees travel. Each position shown corresponds to 45 degrees travel of the
armature, or one-eighth of a turn, and it takes just three-eighths revolution to
change the position from A to that shown at D.

ESSENTIAL PARTS OF A MAGNETO AND THEIR FUNCTIONS

The magnets which produce the influence that in turn induces the electrical
energy in the winding or loops of wire on the armature, and which may have
any even number of opposed poles, are called field magnets. The loops of wire
which are mounted upon a suitable drum and rotate in the field of magnetic
influence in order to cut the lines of force is called an armature winding, while
the core is the metal portion. The entire assembly is called the armature. The
exposed ends of the magnets are called pole pieces and the arrangement used to
collect the current is either a commutator or a collector. The stationary pieces
which bear against the collector or commutator and act as terminals for the

outside circuit are called brushes. These brushes are often of copper, or some of
its alloys, because copper has a greater electrical conductivity than any other
metal.
These brushes are nearly always of carbon, which is sometimes electroplated
with copper to increase its electrical conductivity, though cylinders of copper
wire gauze impregnated with graphite are utilized at times. Carbon is used
because it is not so liable to cut the metal of the commutator as might be the
case if the contact was of the metal to metal type. The reason for this is that
carbon has the peculiar property in that it materially assists in the lubrication of
the commutator, and being of soft, unctuous composition, will wear and
conform to any irregularities on the surface of the metal collector rings.
The magneto in common use consists of a number of horseshoe magnets which
are compound in form and attached to suitable cast-iron pole pieces used to
collect and concentrate the magnetic influence of the various magnets. Between
these pole pieces an armature rotates. This is usually shaped like a shuttle,
around which are wound coils of insulated wire. These are composed of a large
number of turns and the current produced depends in great measure upon the
size of the wire and the number of turns per coil. An armature winding of large
wire will deliver a current of great amperage, but of small voltage. An armature
wound with very fine wire will deliver a current of high voltage but of low
amperage. In the ordinary form of magneto, such as used for ignition, the
current is alternating in character and the break in the circuit should be timed to
occur when the armature is at the point of its greatest potential or pressure.
Where such a generator is designed for direct current production the ends of the
winding are attached to the segments of a commutator, but where the instrument
is designed to deliver an alternating current one end of the winding is fastened
to an insulator ring on one end of the armature shaft and the other end is
grounded on the frame of the machine.
The quantity of the current depends upon the strength of the magnetic field and
the number of lines of magnetic influence acting through the armature. The
electro-motive force varies as to the length of the armature winding and the
number of revolutions at which the armature is rotated.

THE TRANSFORMER SYSTEM USES LOW VOLTAGE MAGNETO

The magneto in the various systems which employ a transformer coil is very
similar to a low-tension generator in general construction, and the current

delivered at the terminals seldom exceeds 100 volts. As it requires many times
that potential or pressure to leap the gap which exists between the points of the
conventional spark plug, a separate coil is placed in circuit to intensify the
current to one of greater capacity. The essential parts of such a system and their
relation to each other are shown in diagrammatic form at Fig. 60 and as a
complete system at Fig. 61. As is true of other systems the magnetic influence is
produced by permanent steel magnets clamped to the cast-iron pole pieces
between which the armature rotates. At the point of greatest potential in the
armature winding the current is broken by the contact breaker, which is actuated
by a cam, and a current of higher value is induced in the secondary winding of
the transformer coil when the low voltage current is passed through the primary
winding.

Fig. 60.—Diagrams Explaining Action of Low Tension Transformer
Coil and True High Tension Magneto Ignition Systems.

Large
image
(95 kB).

Fig. 60A.—Side Sectional View of Bosch High-Tension Magneto Shows
Disposition of Parts. End Elevation Depicts Arrangement of Interruptor and
Distributor Mechanism.

It will be noted that the points of the contact breaker are together except for the
brief instant when separated by the action of the point of the cam upon the lever.
It is obvious that the armature winding is short-circuited upon itself except
when the contact points are separated. While the armature winding is thus short-
circuited there will be practically no generation of current. When the points are
separated there is a sudden flow of current through the primary winding of the
transformer coil, inducing a secondary current in the other winding, which can
be varied in strength by certain considerations in the preliminary design of the
apparatus. This current of higher potential or voltage is conducted directly to the
plug if the device is fitted to a single-cylinder engine, or to the distributor arm if
fitted to a multiple-cylinder motor. The distributor consists of an insulator in
which is placed a number of segments, one for each cylinder to be fired, and so
spaced that the number of degrees between them correspond to the ignition
points of the motor. A two-cylinder motor would have two segments, a three-
cylinder, three segments, and so on within the capacity of the instrument. In the
illustration a four-cylinder distributor is fitted, and the distributing arm is in
contact with the segment corresponding to the cylinder about to be fired.

Fig. 61.—Berling Two-Spark Dual Ignition System.

TRUE HIGH-TENSION MAGNETOS ARE SELF-CONTAINED

The true high-tension magneto differs from the preceding inasmuch as the
current of high voltage is produced in the armature winding direct, without the
use of the separate coil. Instead of but one coil, the armature carries two, one of
comparatively coarse wire, the other of many turns of finer wire. The
arrangement of these windings can be readily ascertained by reference to the
diagram B, Fig. 60, which shows the principle of operation very clearly. The
simplicity of the ignition system is evident by inspection of Fig. 62. One end of
the primary winding (coarse wire) is coupled or grounded to the armature core,
and the other passes to the insulated part of the interrupter. While in some forms
the interrupter or contact breaker mechanism does not revolve, the desired
motion being imparted to the contact lever to separate the points of a revolving
cam, in this the cam or tripping mechanism is stationary and the contact breaker
revolves. This arrangement makes it possible to conduct the current from the
revolving primary coil to the interrupter by a direct connection, eliminating the
use of brushes, which would otherwise be necessary. In other forms of this
appliance where the winding is stationary, the interrupter may be operated by a

revolving cam, though, if desired, the used of a brush at this point will permit
this construction with a revolving winding.

Fig. 62.—Berling Double-Spark Independent System.

During the revolution of the armature the grounded lever makes and breaks
contact with the insulated point, short-circuiting the primary winding upon itself
until the armature reaches the proper position of maximum intensity of current
production, at which time the circuit is broken, as in the former instance. One
end of the secondary winding (fine wire) is grounded on the live end of the
primary, the other end being attached to the revolving arm of the distributor
mechanism. So long as a closed circuit is maintained feeble currents will pass
through the primary winding, and so long as the contact points are together this
condition will exist. When the current reaches its maximum value, because of
the armature being in the best position, the cam operates the interrupter and the
points are separated, breaking the short circuit which has existed in the primary
winding.
The secondary circuit has been open while the distributor arm has moved from
one contact to another and there has been no flow of energy through this
winding. While the electrical pressure will rise in this, even if the distributor
arm contacted with one of the segments, there would be no spark at the plug
until the contact points separated, because the current in the secondary winding
would not be of sufficient strength. When the interrupter operates, however, the
maximum primary current will be diverted from its short circuit and can flow to
the ground only through the secondary winding and spark-plug circuit. The high
pressure now existing in the secondary winding will be greatly increased by the

sudden flow of primary current, and energy of high enough potential to
successfully bridge the gap at the plug is thereby produced in the winding.

THE BERLING MAGNETO

The Berling magneto is a true high tension type delivering two impulses per
revolution, but it is made in a variety of forms, both single and double spark. Its
principle of action does not differ in essentials from the high tension type
previously described. This magneto is used on Curtiss aviation engines and will
deliver sparks in a positive manner sufficient to insure ignition of engines up to
200 horse-power and at rotative speeds of the magneto armature up to 4,000 r.
p. m. which is sufficient to take care of an eight-cylinder V engine running up to
2,000 r. p. m. The magneto is driven at crank-shaft speed on four-cylinder
engines, at 11⁄2 times crank-shaft speed on six-cylinder engines and at twice
crank-shaft speed on eight-cylinder V types. The types “D” and “DD”
BERLING Magnetos are interchangeable with corresponding magnetos of other
standard makes. The dimensions of the four-, six- and eight-cylinder types “D”
and “DD” are all the same.

Fig. 63.—Type DD Berling High Tension Magneto.

The ideal method of driving the magneto is by means of flexible direct
connecting coupling to a shaft intended for the purpose of driving the magneto.
As the magneto must be driven at a high speed, a coupling of some flexibility is
preferable. The employment of such a coupling will facilitate the mounting of
the magneto, because a small inaccuracy in the lining up of the magneto with
the driving shaft will be taken care of by the flexible coupling, whereas with a
perfectly rigid coupling the line-up of the magneto must be absolutely accurate.
Another advantage of the flexible coupling is that the vibration of the motor
will not be as fully transmitted to the armature shaft on the magneto as in case a
rigid coupling is used. This means prolonged life for the magneto.
The next best method of driving the magneto is by means of a gear keyed to the
armature shaft. When this method of driving is employed, great care must be
exercised in providing sufficient clearance between the gear on the magneto and
the driving gear. If there should be a tight spot between these two gears it will
react disadvantageously on the magneto. The third available method is to drive
the magneto by means of a chain. This is the least desirable of the three
methods and should be resorted to only in case of absolute necessity. It is
difficult to provide sufficient clearance when using a chain without rendering
the timing less accurate and positive.

Fig. 64.—Wiring Diagrams of Berling Magneto Ignition
Systems.

Fig. 64, A shows diagrammatically the circuit of the “D” type two-spark
independent magneto and the switch used with it. In position OFF the primary
winding of the magneto is short-circuited and in this position the switch serves
as an ordinary cut-out or grounding switch. In position “1” the switch connects
the magneto in such a way that it operates as an ordinary single-spark magneto.
In this position one end of the secondary winding is grounded to the body of the
motor. This is the starting position. In this position of the switch the entire
voltage generated in the magneto is concentrated at one spark-plug instead of
being divided in half. With the motor turning over very slowly, as is the case in
starting, the full voltage generated by the magneto will not in all cases be
sufficient to bridge simultaneously two spark gaps, but is amply sufficient to
bridge one. Also, this position of the switch tends to retard the ignition and
should be used in starting to prevent back-firing. With the switch in position “2”
the magneto applies ignition to both plugs in each cylinder simultaneously. This
is the normal running position.

Fig. 64, B shows diagrammatically the circuit of the type “DD” BERLING
high-tension two-spark dual magneto. This type is recommended for certain
types of heavy-duty airplane motors, which it is impossible to turn over fast
enough to give the magneto sufficient speed to generate even a single spark of
volume great enough to ignite the gas in the cylinder. The dual feature consists
of the addition to the magneto of a battery interrupter. The equipment consists
of the magneto, coil and special high-tension switch. The coil is intended to
operate on six volts. Either a storage battery or dry cells may be used.
With the switch in the OFF position, the magneto is grounded, and the battery
circuit is open. With the switch in the second or battery position marked “BAT,”
one end of the secondary winding of the magneto is grounded, and the magneto
operates as a single-spark magneto delivering high-tension current to the inside
distributor, and the battery circuit being closed the high-tension current from the
coil is delivered to the outside distributor. In this position the battery current is
supplied to one set of spark plugs, no matter how slowly the motor is turned
over, but as soon as the motor starts, the magneto supplies current as a single-
spark magneto to the other set of the spark-plugs. After the engine is running,
the switch should be thrown to the position marked “MAG.” The battery and
coil are then disconnected, and the magneto furnishes ignition to both plugs in
each cylinder. This is the normal running position. Either a non-vibrating coil
type “N-1” is furnished or a combined vibrating and non-vibrating coil type
“VN-1.”

SETTING BERLING MAGNETO

The magneto may be set according to one of two different methods, the
selection of which is, to some extent, governed by the characteristics of the
engine, but largely due to the personal preference on the part of the user. In the
first method described below, the most advantageous position of the piston for
fully advanced ignition is determined in relation to the extreme advanced
position of the magneto. In this case, the fully retarded ignition will not be a
matter of selection, but the timing range of the magneto is wide enough to bring
the fully retarded ignition after top-center position of the piston. The second
method for the setting of the magneto fixes the fully retarded position of the
magneto in relation to that position of the piston where fully retarded ignition is
desired. In this case, the extreme advance position of the magneto will not
always correspond with the best position of the piston for fully advanced

ignition, and the amount of advance the magneto should have to meet ideal
requirements in this respect must be determined by experiment.

First Method:

1. Designate one cylinder as cylinder No. 1.
2. Turn the crank-shaft until the piston in cylinder No. 1 is in the position where
the fully advanced spark is desired to occur.
3. Remove the cover from the distributor block and turn the armature shaft in
the direction of rotation of the magneto until the distributor finger-brush comes
into such a position that this brush makes contact with the segment which is
connected to the cable terminal marked “1.” This is either one of the two bottom
segments, depending upon the direction of rotation.
4. Place the cam housing in extreme advance, i.e., turn the cam housing until it
stops, in the direction opposite to the direction of rotation of the armature. With
the cam housing in this position, open the cover.
5. With the armature in the approximate position as described in “3,” turn the
armature slightly in either direction to such a point that the platinum points of
the magneto interrupter will just begin to open at the end of the cam, adjacent to
the fibre lever on the interrupter.
6. With this exact position of the armature, fix the magneto to the driving
member of the engine.

Second Method:

1. Designate one cylinder as cylinder No. 1.
2. Turn the crank-shaft until the piston in cylinder No. 1 is in the position at
which the fully retarded spark is desired to occur.
3. Same as No. 3 under First Method.
4. Place the cam housing in extreme retard, i.e., turn the cam housing until it
stops, in the same direction as the direction of rotation of the armature. With the
cam housing in this position, open the cover.
5. Same as No. 5 under First Method.

6. Same as No. 6 under First Method.

WIRING THE MAGNETO

The wiring of the magneto is clearly shown by wiring diagram.
First determine the sequence of firing for the cylinders and then connect the
cables to the spark plug in the cylinders in proper sequence, beginning with
cylinder No. 1 marked on the distributor block.
The switch used with the independent type must be mounted in such a manner
that there will be a metallic connection between the frame of the magneto and
the metal portion of the switch.
It is advisable to use a separate battery, either storage or dry cells, as a source of
current for the dual equipment. Connecting to the same battery that is used with
the generator and other electrical equipment may cause trouble, as a “ground” in
this battery causes the coil to overheat.

CARE AND MAINTENANCE

Lubrication:

Use only the very best of oil for the oil cups.
Put five drops of oil in the oil cup at the driving end of the magneto for every
fifty hours of actual running.
Put five drops of oil in the oil cup at the interrupter end of the magneto, located
at one side of the cam housing, for every hundred hours of actual running.
Lubricate the embossed cams in the cam housing with a thin film of vaseline
every fifty hours of actual running. Wipe off all superfluous vaseline. Never use
oil in the interrupter. Do not lubricate any other part of the interrupter.

Adjusting the Interrupter:

With the fibre lever in the center of one of the embossed cams, as at Fig. 65, the
opening between the platinum contacts should be not less than .016′′ and not

more than .020′′. The gauge riveted to the adjusting wrench should barely be
able to pass between the contacts when fully open. The platinum contacts must
be smoothed off with a very fine file. When in closed position, the platinum
contacts should make contact with each other over their entire surfaces.

Fig. 65.—The Berling Magneto Breaker Box
Showing Contact Points Separated and Interruptor
Lever on Cam.

When inspecting the interrupter, make sure that the ground brush in the back of
the interrupter base is making good contact with the surface on which it rubs.

Cleaning the Distributor:

The distributor block cover should be removed for inspection every twenty-five
hours of actual running and the carbon deposit from the distributor finger-brush
wiped off the distributor block by rubbing with a rag or piece of waste dipped in
gasoline or kerosene. The high-tension terminal brush on the side of the
magneto should also be carefully inspected for proper tension.

LOCATING TROUBLE

Trouble in the ignition system is indicated by the motor “missing,” stopping
entirely, or by inability to start.
It is safe to assume that the trouble is not in the magneto, and the carburetor,
gasoline supply and spark-plugs should first be investigated.

If the magneto is suspected, the first thing to do is to determine if it will deliver
a spark. To determine this, disconnect one of the high-tension leads from the
spark-plug in one of the cylinders and place it so that there is approximately 1⁄16′′
between the terminal and the cylinder frame.
Open the pet cocks on the other cylinders to prevent the engine from firing and
turn over the engine until the piston is approaching the end of the compression
stroke in the cylinder from which the cable has been removed. Set the magneto
in the advance position and rapidly rock the engine over the top-center position,
observing closely if a spark occurs between the end of the high-tension cable
and the frame.
If the magneto is of the dual type, the trouble may be either in the magneto or in
the battery or coil system, therefore disconnect the battery and place the switch
in the position marked “MAG.” The magneto will then operate as an
independent magneto and should spark in the proper manner. After this the
battery system should be investigated. To test the operation of the battery and
coil, examine all connections, making sure that they are clean and tight, and
then with the switch, in the “BAT,” rock the piston slowly back and forth. If a
type “VN-1” coil is used, a shower of sparks should jump between the high-
tension cable terminal and the cylinder frame when the piston is in the correct
position for firing. If no spark occurs, remove the cover from the coil and see
that the vibrating tongue is free. If a type “N-1” coil is used, a single spark will
occur. The battery should furnish six volts when connected to the coil, and this
should also be verified.
If the coil still refuses to give a spark and all connections are correct, the coil
should be replaced and the defective coil returned to the manufacturer.
If both magneto and coil give a spark when tested as just described, the spark-
plugs should be investigated. To do this, disconnect the cables and remove the
spark-plugs. Then reconnect the cables to the plugs and place them so that the
frame portions of the plugs are in metallic connection with the frame of the
motor. Then turn over the motor, thus revolving the magneto armature, and see
if a spark is produced at the spark gaps of the plugs.
The most common defects in spark-plugs are breaking down of the insulation,
fouling due to carbon, or too large or small a spark gap. To clean the plugs a
stiff brush and gasoline should be used. The spark gap should be about 1⁄32′′ and
never less than 1⁄64′′. Too small a gap may have been caused by beads of metal

forming due to the heat of the spark. Too long a gap may have been caused by
the points burning off.
If the magneto and spark plugs are in good condition and the engine does not
run satisfactorily, the setting should be verified according to instructions
previously given, and, if necessary, readjusted.
Be careful to observe that both the type “VN-1” and type “N-1” coils are so
arranged that the spark occurs on the opening of the contacts of the timer. As
this is just the reverse of the usual operation, it should be carefully noted when
any change in the setting of the timer is made. The timer on the dual type
magneto is adjusted so that the battery spark occurs about 5° later than the
magneto spark. This provides an automatic advance as soon as the switch is
thrown to the magneto position “MAG.” This relative timing can be easily
adjusted by removing the interrupter and shifting the cam in the direction
desired.

Fig. 66.—The Dixie Model 60 for Six-Cylinder Airplane
Engine Ignition.

THE DIXIE MAGNETO

The Dixie magneto, shown at Fig. 66, operates on a different principle than the
rotary armature type. It is used on the Hall-Scott and other aviation engines. In
this magneto the rotating member consists of two pieces of magnetic material
separated by a non-magnetic center piece. This member constitutes true rotating
poles for the magnet and rotates in a field structure, composed of two laminated
field pieces, riveted between two non-magnetic rings. The bearings for the
rotating poles are mounted in steel plates, which lie against the poles of the
magnets. When the magnet poles rotate, the magnetic lines of force from each
magnet pole are carried directly to the field pieces and through the windings,
without reversal through the mass of the rotating member and with only a single
air gap. There are no losses by flux reversal in the rotating part, such as take
place in other machines, and this is said to account for the high efficiency of the
instrument.

Fig. 67.—Installation Dimensions of Dixie Model 60 Magneto.

Fig. 68.—The Rotating Elements of the Dixie
Magneto.

And this “Mason Principle” involved in the operation of the Dixie is simplified
by a glance at the field structure, consisting of the non-magnetic rings,
assembled to which are the field pieces between which the rotating poles
revolve (see Fig. 68). Rotating between the limbs of the magnets, these two
pieces of magnetic material form true extensions to the poles of the magnets,
and are, in consequence, always of the same polarity. It will be seen there is no
reversal of the magnetism through them, and consequently no eddy current or
hysteresis losses which are present in the usual rotor or inductor types. The
simplicity features of construction stand out prominently here, in that there are
no revolving windings, a detail entirely differing from the orthodox high-
tension instrument. This simplicity becomes instantly apparent when it is found
that the circuit breaker, instead of revolving as it does in other types, is
stationary and that the whole breaker mechanism is exposed by simply turning
the cover spring aside and removing cover. This makes inspection and
adjustment particularly simple, and the fact that no special tool is necessary for

adjustment of the platinum points—an ordinary small screw-driver is the whole
“kit of tools” needed in the work of disassembling or assembling—is a feature
of some value.

Fig. 69.—Suggestions for Adjusting and Dismantling Dixie Magneto.
A—Screw Driver Adjusts Contact Points. B—Distributor Block
Removed. C—Taking off Magnets. D—Showing How Easily
Condenser and High Tension Windings are Removed.

With dust- and water-protecting casing removed, and one of the magnets
withdrawn, as in Fig. 69, the winding can be seen with its core resting on the
field pole pieces and the primary lead attached to its side. An important feature
of the high-tension winding is that the heads are of insulating material, and
there is not the tendency for the high-tension current to jump to the side as in
the ordinary armature type magneto. The high-tension current is carried to the
distributor by means of an insulated block with a spindle, at one end of which is
a spring brush bearing directly on the winding, thus shortening the path of the
high-tension current and eliminating the use of rubber spools and insulating

parts. The moving parts of the magneto need never be disturbed if the high-
tension winding is to be removed. This winding constitutes all of the magneto
windings, no external spark coil being necessary. The condenser is placed
directly above the winding and is easily removable by taking out two screws,
instead of being placed in an armature where it is inaccessible except to an
expert, and where it cannot be replaced except at the factory whence it
emanated.

CARE OF THE DIXIE MAGNETO

The bearings of the magneto are provided with oil cups and a few drops of light
oil every 1,000 miles are sufficient. The breaker lever should be lubricated
every 1,000 miles with a drop of light oil, applied with a tooth-pick. The proper
distance between the platinum points when separated should not exceed .020 or
one-fiftieth of an inch. A gauge of the proper size is attached to the screwdriver
furnished with the magneto. The platinum contacts should be kept clean and
properly adjusted. Should the contacts become pitted, a fine file should be used
to smooth them in order to permit them to come into perfect contact. The
distributor block should be removed occasionally and inspected for an
accumulation of carbon dust. The inside of the distributor block should be
cleaned with a cloth moistened with gasoline and then wiped dry with a clean
cloth. When replacing the block, care must be exercised in pushing the carbon
brush into the socket. Do not pull out the carbon brushes in the distributor
because you think there is not enough tension on the small brass springs. In
order to obtain the most efficient results, the normal setting of the spark-plug
points should not exceed .025 of an inch, and it is advisable to have the gap just
right before a spark-plug is inserted.
The spark-plug electrodes may be easily set by means of the gauge attached to
the screwdriver. The setting of the spark-plug points is an important function
which is usually overlooked, with the result that the magneto is blamed when it
is not at fault.

TIMING OF THE DIXIE MAGNETO

In order to obtain the utmost efficiency from the engine, the magneto must be
correctly timed to it. This operation is usually performed when the magneto is
fitted to the engine at the factory. The correct setting may vary according to

individuality of the engine, and some engines may require an earlier setting in
order to obtain the best results. However, should the occasion arise to retime the
magneto, the procedure is as follows: Rotate the crank-shaft of the engine until
one of the pistons, preferably that of cylinder No. 1, is 1⁄16 of an inch ahead of
the end of the compression stroke. With the timing lever in full retard position,
the driving shaft of the magneto should be rotated in the direction in which it
will be driven. The circuit breaker should be closely observed and when the
platinum contact points are about to separate, the drive gear or coupling should
be secured to the drive shaft of the magneto. Care should be taken not to alter
the position of the magneto shaft when tightening the nut to secure the gear or
coupling, after which the magneto should be secured to its base. Remove the
distributor block and determine which terminal of the block is in contact with
the carbon brush of the distributor finger and connect with plug wire leading to
No. 1 cylinder to this terminal. Connect the remaining plug wires in turn
according to the proper sequence of firing of the cylinders. (See the wiring
diagram for a typical six-cylinder engine at Fig. 70.) A terminal on the end of
the cover spring of the magneto is provided for the purpose of connecting the
wire leading to a ground switch for stopping the engine.
Large
image
(108 kB).

Fig. 69A.—Sectional Views Outlining Construction of Dixie Magneto with
Compound Distributor for Eight-Cylinder Engine Ignition.

A special model or type of magneto is made for V engines which use a
compound distributor construction instead of the simple type on the model
illustrated and a different interior arrangement permits the production of four
sparks per revolution of the rotors. This makes it possible to run the magneto
slower than would be possible with the two-spark form. The application of two
compound distributor magnetos of this type to a Thomas-Morse 135 horse-
power motor of the eight-cylinder V pattern is clearly shown at Fig. 71.
Large
image
(51 kB).

Fig. 70.—Wiring Diagram of Dixie Magneto Installation on Hall-
Scott Six-Cylinder 125 Horse-Power Aeronautic Motor.

Fig. 71.—How Magneto Ignition is Installed on
Thomas-Morse 135 Horse-Power Motor.

SPARK-PLUG DESIGN AND APPLICATION

With the high-tension system of ignition the spark is produced by a current of
high voltage jumping between two points which break the complete circuit,
which would exist otherwise in the secondary coil and its external connections.
The spark-plug is a simple device which consists of two terminal electrodes
carried in a suitable shell member, which is screwed into the cylinder. Typical
spark-plugs are shown in section at Fig. 72 and the construction can be easily
understood. The secondary wire from the coil is attached to a terminal at the top
of a central electrode member, which is supported in a bushing of some form of
insulating material. The type shown at A employs a molded porcelain as an
insulator, while that depicted at B uses a bushing of mica. The insulating
bushing and electrode are housed in a steel body, which is provided with a

screw thread at the bottom, by which means it is screwed into the combustion
chamber.

Fig. 72.—Spark-Plug Types Showing Construction and Arrangement of Parts.

When porcelain is used as an insulating material it is kept from direct contact
with the metal portion by some form of yielding packing, usually asbestos. This
is necessary because the steel and porcelain have different coefficients of
expansion and some flexibility must be provided at the joints to permit the
materials to expand differently when heated. The steel body of the plug which is
screwed into the cylinder is in metallic contact with it and carries sparking
points which form one of the terminals of the air gap over which the spark
occurs. The current entering at the top of the plug cannot reach the ground,
which is represented by the metal portion of the engine, until it has traversed the
full length of the central electrode and overcome the resistance of the gap
between it and the terminal point on the shell. The porcelain bushing is firmly
seated against the asbestos packing by means of a brass screw gland which sets
against a flange formed on the porcelain, and which screws into a thread at the
upper portion of the plug body.
The mica plug shown at B is somewhat simpler in construction than that shown
at A. The mica core which keeps the central electrode separated from the steel
body is composed of several layers of pure sheet mica wound around the steel
rod longitudinally, and hundreds of stamped steel washers which are forced
over this member and compacted under high pressure with some form of a
binding material between them. Porcelain insulators are usually molded from

high-grade clay and are approximately of the shapes desired by the designers of
the plug. The central electrode may be held in place by mechanical means such
as nuts, packings, and a shoulder on the rod, as shown at A. Another method
sometimes used is to cement the electrode in place by means of some form of
fire-clay cement. Whatever method of fastening is used, it is imperative that the
joints be absolutely tight so that no gas can escape at the time of explosion.
Porcelain is the material most widely used because it can be glazed so that it
will not absorb oil, and it is subjected to such high temperature in baking that it
is not liable to crack when heated.
The spark-plugs may be screwed into any convenient part of the combustion
chamber, the general practice being to install them in the caps over the inlet
valves, or in the side of the combustion chamber, so the points will be directly
in the path of the entering fresh gases from the carburetor.
Other insulating materials sometimes used are glass, steatite (which is a form of
soapstone) and lava. Mica and porcelain are the two common materials used
because they give the best results. Glass is liable to crack, while lava or the
soapstone insulating bushings absorb oil. The spark gap of the average plug is
equal to about 1⁄32 of an inch for coil ignition and 1⁄40 of an inch when used in
magneto circuits. A simple gauge for determining the gap setting is the
thickness of an ordinary visiting card for magneto plugs, or a space equal to the
thickness of a worn dime for a coil plug. The insulating bushings are made in a
number of different ways, and while details of construction vary, spark-plugs do
not differ essentially in design. The dimensions of the standardized plug
recommended by the S. A. E. are shown at Fig. 73.

Fig. 73.—Standard Airplane Engine Plug Suggested by S. A. E.
Standards Committee.

It is often desirable to have a water-tight joint between the high-tension cable
and the terminal screw on top of the insulating bushing of the spark-plug,
especially in marine applications. The plug shown at C, Fig. 72, is provided
with an insulating member or hood of porcelain, which is secured by a clip in
such a manner that it makes a water-tight connection. Should the porcelain of a
conventional form of plug become covered with water or dirty oil, the high-
tension current is apt to run down this conducting material on the porcelain and
reach the ground without having to complete its circuit by jumping the air gap
and producing a spark. It will be evident that wherever a plug is exposed to the
elements, which is often the case in airplane service, that it should be protected
by an insulating hood which will keep the insulator dry and prevent short
circuiting of the spark. The same end can be attained by slipping an ordinary
rubber nipple over the porcelain insulator of any conventional plug and bringing
up one end over the cable.

TWO-SPARK IGNITION

On most aviation engines, especially those having large cylinders, it is
sometimes difficult to secure complete combustion by using a single-spark plug.
If the combustion is not rapid the efficiency of the engine will be reduced

proportionately. The compressed charge in the cylinder does not ignite all at
once or instantaneously, as many assume, but it is the strata of gas nearest the
plug which is ignited first. This in turn sets fire to consecutive layers of the
charge until the entire mass is aflame. One may compare the combustion of gas
in the gas-engine cylinder to the phenomenon which obtains when a heavy
object is thrown into a pool of still water. First a small circle is seen at the point
where the object has passed into the water, this circle in turn inducing other and
larger circles until the whole surface of the pool has been agitated from the one
central point. The method of igniting the gas is very similar, as the spark ignites
the circle of gas immediately adjacent to the sparking point, and this circle in
turn ignites a little larger one concentric with it. The second circle of flame sets
fire to more of the gas, and finally the entire contents of the combustion
chamber are burning.
While ordinarily combustion is sufficiently rapid with a single plug so that the
proper explosion is obtained at moderate engine speeds, if the engine is working
fast and the cylinders are of large capacity more power may be obtained by
setting fire to the mixture at two different points instead of but one. This may be
accomplished by using two sparking-plugs in the cylinder instead of one, and
experiments have shown that it is possible to gain from twenty-five to thirty per
cent. in motor power at high speed with two-spark plugs, because the
combustion of gas is accelerated by igniting the gas simultaneously in two
places. The double-plug system on airplane engines is also a safeguard, as in
event of failure of one plug in the cylinder the other would continue to fire the
gas, and the engine will continue to function properly.
In using magneto ignition some precautions are necessary relating to wiring and
also the character of the spark-plugs employed. The conductor should be of
good quality, have ample insulation, and be well protected from accumulations
of oil, which would tend to decompose rubber insulation. It is customary to
protect the wiring by running it through the conduits of fiber or metal tubing
lined with insulating material. Multiple strand cables should be used for both
primary and secondary wiring, and the insulation should be of rubber at least 3⁄16
inch thick.
The spark-plugs commonly used for battery and coil ignition cannot always be
employed when a magneto is fitted. The current produced by the mechanical
generator has a greater amperage and more heat value than that obtained from
transformer coils excited by battery current. The greater heat may burn or fuse
the slender points used on some battery plugs and heavier electrodes are needed

to resist the heating effect of the more intense arc. While the current has greater
amperage it is not of as high potential or voltage as that commonly produced by
the secondary winding of an induction coil, and it cannot overcome as much of
a gap. Manufacturers of magneto plugs usually set the spark points about 1⁄64 of
an inch apart. The most efficient magneto plug has a plurality of points so that
when the distance between one set becomes too great the spark will take place
between one of the other pairs of electrodes which are not separated by so great
an air space.

SPECIAL PLUGS FOR AIRPLANE WORK

Airplane work calls for special construction of spark-plugs, owing to the high
compression used in the engines and the fact that they are operated on open
throttle practically all the time, thus causing a great deal of heat to be
developed. The plug shown at Fig. 74 was recently described in “The
Automobile,” and has been devised especially for airplane engines and
automobile racing power plants. The core C is built up of mica washers, and has
square shoulders. As mica washers of different sizes may be used, and accurate
machining, such as is necessary with conical clamping surfaces, is not required,
the plug can be produced economically. The square shoulders of the core afford
two gasket seats, and when the core is clamped in the shell by means of check
nut E, it is accurately centered and a tight joint is formed. This construction also
makes a shorter plug than where conical fits are used, thus improving the heat
radiation through the stem. The lower end of the shell is provided with a baffle
plate O, which tends to keep the oil away from the mica. There are perforations
L in this baffle plate to prevent burnt gases being pocketed behind the baffle
plate and pre-igniting the new charge. This construction also brings the firing
point out into the firing chamber of the engine, and has all the other advantages
of a closed-end plug. The stem P is made of brass or copper, on account of their
superior heat conductivity, and the electrode J is swedged into the bottom of the
stem, as shown at K, in a secure manner.

Fig. 74.—Special Mica Plug for Aviation Engines.

The shell is finned, as shown at G, to provide greater heat radiating surface.
There is also a fin F at the top of the stem, to increase the radiation of heat from
the stem and electrode. The top of this finned portion is slightly countersunk,
and the stem is riveted into same, thereby reducing the possibility of leakage
past the threads on the stem. This finned portion is necked at A to take a slip
terminal.
In building up the core a small section of washers, I, is built up before the mica
insulating tube D is placed on. This construction gives a better support to
section I. Baffle plate O is bored out to allow the electrode J to pass through,
and the clearance between baffle plate and electrode is made larger than the
width of the gap between the firing points, so that there is no danger of the
spark jumping from the electrode to the baffle plate.
This plug will be furnished either with or without the finned portion, to meet
individual requirements. The manufacturers lay special stress upon the
simplicity of construction and upon the method of clamping, which is claimed
to make the plug absolutely gas-tight.

CHAPTER VII
Why Lubrication Is Necessary—Friction Defined—Theory of
Lubrication—Derivation of Lubricants—Properties of Cylinder Oils—
Factors Influencing Lubrication System Selection—Gnome Type
Engines Use Castor Oil—Hall-Scott Lubrication System—Oil Supply
by Constant Level Splash System—Dry Crank-Case System Best for
Airplane Engines—Why Cooling Systems Are Necessary—Cooling
Systems Generally Applied—Cooling by Positive Pump Circulation—
Thermo-Syphon System—Direct Air-Cooling Methods—Air-Cooled
Engine Design Considerations.

WHY LUBRICATION IS NECESSARY

The importance of minimizing friction at the various bearing surfaces of
machines to secure mechanical efficiency is fully recognized by all mechanics,
and proper lubricity of all parts of the mechanism is a very essential factor upon
which the durability and successful operation of the motor car power plant
depends. All of the moving members of the engine which are in contact with
other portions, whether the motion is continuous or intermittent, of high or low
velocity, or of rectilinear or continued rotary nature, should be provided with an
adequate supply of oil. No other assemblage of mechanism is operated under
conditions which are so much to its disadvantage as the motor car, and the
tendency is toward a simplification of oiling methods so that the supply will be
ample and automatically applied to the points needing it.
In all machinery in motion the members which are in contact have a tendency to
stick to each other, and the very minute projections which exist on even the
smoothest of surfaces would have a tendency to cling or adhere to each other if
the surfaces were not kept apart by some elastic and unctuous substance. This
will flow or spread out over the surfaces and smooth out the inequalities
existing which tend to produce heat and retard motion of the pieces relative to
each other.
A general impression which obtains is that well machined surfaces are smooth,
but while they are apparently free from roughness, and no projections are

visible to the naked eye, any smooth bearing surface, even if very carefully
ground, will have a rough appearance if examined with a magnifying glass. An
exaggerated condition to illustrate this point is shown at Fig. 75. The amount of
friction will vary in proportion to the pressure on the surfaces in contact and
will augment as the loads increase; the rougher surfaces will have more friction
than smoother ones and soft bodies will produce more friction than hard
substances.

Fig. 75.—Showing Use of Magnifying Glass to Demonstrate
that Apparently Smooth Metal Surfaces May Have Minute
Irregularities which Produce Friction.

FRICTION DEFINED

Friction is always present in any mechanism as a resisting force that tends to
retard motion and bring all moving parts to a state of rest. The absorption of
power by friction may be gauged by the amount of heat which exists at the
bearing points. Friction of solids may be divided into two classes: sliding
friction, such as exists between the piston and cylinder, or the bearings of a gas-
engine, and rolling friction, which is that present when the load is supported by
ball or roller bearings, or that which exists between the tires or the driving
wheels and the road. Engineers endeavor to keep friction losses as low as
possible, and much care is taken in all modern airplane engines to provide

adequate methods of lubrication, or anti-friction bearings at all points where
considerable friction exists.

THEORY OF LUBRICATION

The reason a lubricant is supplied to bearing points will be easily understood if
one considers that these elastic substances flow between the close fitting
surfaces, and by filling up the minute depressions in the surfaces and covering
the high spots act as a cushion which absorbs the heat generated and takes the
wear instead of the metallic bearing surface. The closer the parts fit together the
more fluid the lubricant must be to pass between their surfaces, and at the same
time it must possess sufficient body so that it will not be entirely forced out by
the pressure existing between the parts.
Oils should have good adhesive, as well as cohesive, qualities. The former are
necessary so that the oil film will cling well to the surfaces of the bearings; the
latter, so the oil particles will cling together and resist the tendency to separation
which exists all the time the bearings are in operation. When used for gas-
engine lubrication the oil should be capable of withstanding considerable heat in
order that it will not be vaporized by the hot portions of the cylinder. It should
have sufficient cold test so that it will remain fluid and flow readily at low
temperature. Lubricants should be free from acid, or alkalies, which tend to
produce a chemical action with metals and result in corrosion of the parts to
which they are applied. It is imperative that the oil be exactly the proper quality
and nature for the purpose intended and that it be applied in a positive manner.
The requirements may be briefly summarized as follows:
First—It must have sufficient body to prevent seizing of the parts to which it is
applied and between which it is depended upon to maintain an elastic film, and
yet it must not have too much viscosity, in order to minimize the internal or
fluid friction which exists between the particles of the lubricant itself.
Second—The lubricant must not coagulate or gum; must not injure the parts to
which it is applied, either by chemical action or by producing injurious deposits,
and it should not evaporate readily.
Third—The character of the work will demand that the oil should not vaporize
when heated or thicken to such a point that it will not flow readily when cold.

Fourth—The oil must be free from acid, alkalies, animal or vegetable fillers, or
other injurious agencies.
Fifth—It must be carefully selected for the work required and should be a good
conductor of heat.

DERIVATION OF LUBRICANTS

The first oils which were used for lubricating machinery were obtained from
animal and vegetable sources, though at the present time most unguents are of
mineral derivation. Lubricants may exist as fluids, semifluids, or solids. The
viscosity will vary from light spindle or dynamo oils, which have but little more
body than kerosene, to the heaviest greases and tallows. The most common
solid employed as a lubricant is graphite, sometimes termed “plumbago” or
“black lead.” This substance is of mineral derivation.
The disadvantage of oils of organic origin, such as those obtained from animal
fats or vegetable substances, is that they will absorb oxygen from the
atmosphere, which causes them to thicken or become rancid. Such oils have a
very poor cold test, as they solidify at comparatively high temperatures, and
their flashing point is so low that they cannot be used at points where much heat
exists. In most animal oils various acids are present in greater or less quantities,
and for this reason they are not well adapted for lubricating metallic surfaces
which may be raised high enough in temperature to cause decomposition of the
oils.
Lubricants derived from the crude petroleum are called “Oleonaphthas” and
they are a product of the process of refining petroleum through which gasoline
and kerosene are obtained. They are of lower cost than vegetable or animal oil,
and as they are of non-organic origin, they do not become rancid or gummy by
constant exposure to the air, and they will have no corrosive action on metals
because they contain no deleterious substances in chemical composition. By the
process of fractional distillation mineral oils of all grades can be obtained. They
have a lower cold and higher flash test and there is not the liability of
spontaneous combustion that exists with animal oils.
The organic oils are derived from fatty substances, which are present in the
bodies of all animals and in some portions of plants. The general method of
extracting oil from animal bodies is by a rendering process, which consists of
applying sufficient heat to liquefy the oil and then separating it from the tissue

with which it is combined by compression. The only oil which is used to any
extent in gas-engine lubrication that is not of mineral derivation is castor oil.
This substance has been used on high-speed racing automobile engines and on
airplane power plants. It is obtained from the seeds of the castor plant, which
contain a large percentage of oil.
Among the solid substances which may be used for lubricating purposes may be
mentioned tallow, which is obtained from the fat of animals, and graphite and
soapstone, which are of mineral derivation. Tallow is never used at points where
it will be exposed to much heat, though it is often employed as a filler for
greases used in transmission gearing of autos. Graphite is sometimes mixed
with oil and applied to cylinder lubrication, though it is most often used in
connection with greases in the landing gear parts and for coating wires and
cables of the airplane. Graphite is not affected by heat, cold, acids, or alkalies,
and has a strong attraction for metal surfaces. It mixes readily with oils and
greases and increases their efficiency in many applications. It is sometimes used
where it would not be possible to use other lubricants because of extremes of
temperature.
The oils used for cylinder lubrication are obtained almost exclusively from
crude petroleum derived from American wells. Special care must be taken in the
selection of crude material, as every variety will not yield oil of the proper
quality to be used as a cylinder lubricant. The crude petroleum is distilled as
rapidly as possible with fire heat to vaporize off the naphthas and the burning
oils. After these vapors have been given off superheated steam is provided to
assist in distilling. When enough of the light elements have been eliminated the
residue is drawn off, passed through a strainer to free it from grit and earthy
matters, and is afterwards cooled to separate the wax from it. This is the dark
cylinder oil and is the grade usually used for steam-engine cylinders.

PROPERTIES OF CYLINDER OILS

The oil that is to be used in the gasoline engine must be of high quality, and for
that reason the best grades are distilled in a vacuum that the light distillates may
be separated at much lower temperatures than ordinary conditions of distilling
permit. If the degree of heat is not high the product is not so apt to decompose
and deposit carbon. If it is desired to remove the color of the oil which is caused
by free carbon and other impurities it can be accomplished by filtering the oil
through charcoal. The greater the number of times the oil is filtered, the lighter

it will become in color. The best cylinder oils have flash points usually in excess
of 500 degrees F., and while they have a high degree of viscosity at 100 degrees
F. they become more fluid as the temperature increases.
The lubricating oils obtained by refining crude petroleum may be divided into
three classes:
First—The natural oils of great body which are prepared for use by allowing the
crude material to settle in tanks at high temperature and from which the
impurities are removed by natural filtration. These oils are given the necessary
body and are free from the volatile substances they contain by means of
superheated steam which provides a source of heat.
Second—Another grade of these natural oils which are filtered again at high
temperatures and under pressure through beds of animal charcoal to improve
their color.
Third—Pale, limpid oils, obtained by distillation and subsequent chemical
treatment from the residuum produced in refining petroleum to obtain the fuel
oils.
Authorities agree that any form of mixed oil in which animal and mineral
lubricants are combined should never be used in the cylinder of a gas engine as
the admixture of the lubricants does not prevent the decomposition of the
organic oil into the glycerides and fatty acids peculiar to the fat used. In a gas-
engine cylinder the flame tends to produce more or less charring. The deposits
of carbon will be much greater with animal oils than with those derived from
the petroleum base because the constituents of a fat or tallow are not of the
same volatile character as those which comprise the hydro-carbon oils which
will evaporate or volatilize before they char in most instances.

FACTORS INFLUENCING LUBRICATION SYSTEM SELECTION

The suitability of oil for the proper and efficient lubrication of all internal
combustion engines is determined chiefly by the following factors:
1. Type of cooling system (operating temperatures).
2. Type of lubricating system (method of applying oil to the moving parts).
3. Rubbing speeds of contact surfaces.

Were the operating temperatures, bearing surface speeds and lubrication
systems identical, a single oil could be used in all engines with equal
satisfaction. The only change then necessary in viscosity would be that due to
climatic conditions. As engines are now designed, only three grades of oil are
necessary for the lubrication of all types with the exception of Knight, air-
cooled and some engines which run continuously at full load. In the
specification of engine lubricants the feature of load carried by the engine
should be carefully considered.
Full Load Engines.

1. Marine.
2. Racing automobile.
3. Aviation.
4. Farm tractor.
5. Some stationary.

Variable Load Engines.

1. Pleasure automobile.
2. Commercial vehicle.
3. Motor cycle.
4. Some stationary.

Of the forms outlined, the only one we have any immediate concern about is the
airplane power plant. The Platt & Washburn Refining Company, who have
made a careful study of the lubrication problem as applied to all types of
engines, have found a peculiar set of conditions to apply to oiling high-speed
constant-duty or “full-load” engines. Modern airplane engines are designed to
operate continuously at a fairly uniform high rotative speed and at full load over
long periods of time. As a sequence to this heavy duty the operating
temperatures are elevated. For the sake of extreme lightness in weight of all
parts, very thin alloy steel aluminum or cast iron pistons are fitted and the
temperature of the thin piston heads at the center reaches anywhere between
600° and 1,400° Fahr., as in automobile racing engines. Freely exposed to such
intense heat hydro-carbon oils are partially “cracked” into light and heavy
products or polymerized into solid hydro-carbons. From these facts it follows
that only heavy mineral oils of low carbon residue and of the greatest chemical
purity and stability should be used to secure good lubrication. In all cases the oil
should be sufficiently heavy to assure the highest horse-power and fuel and oil

economy compatible with perfect lubrication, avoiding, at the same time,
carbonization and ignition failure. When aluminum pistons are used their
superior heat-conducting properties aid materially in reducing the rate of oil
destruction.
The extraordinary evolutions described by airplanes in flight make it a matter of
vital necessity to operate engines inclined at all angles to the vertical as well as
in an upside-down position. To meet this situation lubricating systems have
been elaborated so as to deliver an abundance of oil where needed and to
eliminate possible flooding of cylinders. This is done by applying a full force
feed system, distributing oil under considerable pressure to all working parts.
Discharged through the bearings, the oil drains down to the suction side of a
second pump located in the bottom of the base chamber. This pump being of
greater capacity than the first prevents the accumulation of oil in the crank-case,
and forces it to a separate oil reservoir-cooler, whence it flows back in rapid
circulation to the pump feeding the bearings. With this arrangement positive
lubrication is entirely independent of engine position. The lubricating system of
the Thomas-Morse aviation engines, which is shown at Fig. 76, is typical of
current practice.

Fig. 76.—Pressure Feed Oiling System of Thomas Aviation Engine Includes Oil
Cooling Means.

GNOME TYPE ENGINES USE CASTOR OIL

The construction and operation of rotative radial cylinder engines introduce
additional difficulties of lubrication to those already referred to and merit
especial attention. Owing to the peculiar alimentation systems of Gnome type
engines, atomized gasoline mixed with air is drawn through the hollow
stationary crank-shaft directly into the crank-case which it fills on the way to
the cylinders. Therein lies the trouble. Hydrocarbon oils are soon dissolved by
the gasoline and washed off, leaving the bearing surfaces without adequate
protection and exposed to instant wear and destruction. So castor oil is resorted
to as an indispensable but unfortunate compromise. Of vegetable origin, it
leaves a much more bulky carbon deposit in the explosion chambers than does
mineral oil and its great affinity for oxygen causes the formation of voluminous
gummy deposit in the crank-case. Engines employing it need to be dismounted
and thoroughly scraped out at frequent intervals. It is advisable to use only
unblended chemically pure castor oil in rotative engines, first by virtue of its
insolubility in gasoline and second because its extra heavy body can resist the
high temperature of air-cooled cylinders.

HALL-SCOTT LUBRICATION SYSTEM

The oiling system of the Hall-Scott type A-5 125 horse-power engine is clearly
shown at Fig. 77. It is completely described in the instruction book issued by
the company from which the following extracts are reproduced by permission.
Crank-shaft, connecting rods and all other parts within the crank-case and
cylinders are lubricated directly or indirectly by a force-feed oiling system. The
cylinder walls and wrist pins are lubricated by oil spray thrown from the lower
end of connecting rod bearings. This system is used only upon A-5 engines.
Upon A-7a and A-5a engines a small tube supplies oil from connecting rod
bearing directly upon the wrist pin. The oil is drawn from the strainer located at
the lowest portion of the lower crank-case, forced around the main intake
manifold oil jacket. From here it is circulated to the main distributing pipe
located along the lower left hand side of upper crank-case. The oil is then forced
directly to the lower side of crank-shaft, through holes drilled in each main
bearing cup. Leakage from these main bearings is caught in scuppers placed
upon the cheeks of the crank-shafts furnishing oil under pressure to the
connecting rod bearings. A-7a and A-5a engines have small tubes leading from
these bearings which convey the oil under pressure to the wrist pins.

Fig. 77.—Diagram of Oiling System, Hall-Scott Type A 125 Horse-Power Engine.

A bi-pass located at the front end of the distributing oil pipe can be regulated to
lessen or raise the pressure. By screwing the valve in, the pressure will raise and
more oil will be forced to the bearings. By unscrewing, pressure is reduced and
less oil is fed. A-7a and A-5a engines have oil relief valves located just off of
the main oil pump in the lower crank-case. This regulates the pressure at all
times so that in cold weather there will be no danger of bursting oil pipes due to
excessive pressure. If it is found the oil pressure is not maintained at a high
enough level, inspect this valve. A stronger spring will not allow the oil to bi-
pass so freely, and consequently the pressure will be raised; a weaker spring
will bi-pass more oil and reduce the oil pressure materially. Independent of the
above-mentioned system, a small, directly driven rotary oiler feeds oil to the
base of each individual cylinder. The supply of oil is furnished by the main oil
pump located in the lower crank-case. A small sight-feed regulator is furnished
to control the supply of oil from this oiler. This instrument should be placed
higher than the auxiliary oil distributor itself to enable the oil to drain by gravity
feed to the oiler. If there is no available place with the necessary height in the
front seat of plane, connect it directly to the intake L fitting on the oiler in an
upright position. It should be regulated with full open throttle to maintain an oil
level in the glass, approximately half way.

An oil pressure gauge is provided. This should be run to the pilot’s instrument
board. The gauge registers the oil pressure upon the bearings, also determining
its circulation. Strict watch should be maintained of this instrument by pilot, and
if for any reason its hand should drop to 0 the motor should be immediately
stopped and the trouble found before restarting engine. Care should be taken
that the oil does not work up into the gauge, as it will prevent the correct gauge
registering of oil pressure. The oil pressure will vary according to weather
conditions and viscosity of oil used. In normal weather, with the engine
properly warmed up, the pressure will register on the oil gauge from 5 to 10
pounds when the engine is turning from 1,275 to 1,300 r. p. m. This does not
apply to all aviation engines, however, as the proper pressure advised for the
Curtiss OX-2 motor is from 40 to 55 pounds at the gauge.
The oil sump plug is located at the lowest point of the lower crank-case. This is
a combination dirt, water and sediment trap. It is easily removed by unscrewing.
Oil is furnished mechanically to the cam-shaft housing under pressure through a
small tube leading from the main distributing pipe at the propeller end of engine
directly into the end of cam-shaft housing. The opposite end of this housing is
amply relieved to allow the oil to rapidly flow down upon cam-shaft, magneto,
pinion-shaft, and crank-shaft gears, after which it returns to lower crank-case.
An outside overflow pipe is also provided to carry away the surplus oil.

DRAINING OIL FROM CRANK-CASE

The oil strainer is placed at the lowest point of the lower crank-case. This
strainer should be removed after every five to eight hours running of the engine
and cleaned thoroughly with gasoline. It is also advisable to squirt distillate up
into the case through the opening where the strainer has been removed. Allow
this distillate to drain out thoroughly before replacing the plug with strainer
attached. Be sure gasket is in place on plug before replacing. Pour new oil in
through either of the two breather pipes on exhaust side of motor. Be sure to
replace strainer screens if removed. If, through oversight, the engine does not
receive sufficient lubrication and begins to heat or pound, it should be stopped
immediately. After allowing engine to cool pour at least three gallons of oil into
oil sump. Fill radiator with water after engine has cooled. Should there be
apparent damage, the engine should be thoroughly inspected immediately
without further running. If no obvious damage has been done, the engine should
be given a careful examination at the earliest opportunity to see that the running
without oil has not burned the bearings or caused other trouble.

Oils best adapted for Hall-Scott engines have the following properties: A flash
test of not less than 400° F.; viscosity of not less than 75 to 85 taken at 21° F.
with Saybolt’s Universal Viscosimeter.
Zeroline heavy duty oil, manufactured by the Standard Oil Company of
California; also,
Gargoyle mobile B oil, manufactured by the Vacuum Oil Company, both fulfill
the above specifications. One or the other of these oils can be obtained all over
the world.
Monogram extra heavy is also recommended.

OIL SUPPLY BY CONSTANT LEVEL SPLASH SYSTEM

The splash system of lubrication that depends on the connecting rod to
distribute the lubricant is one of the most successful and simplest forms for
simple four- and six-cylinder vertical automobile engines, but is not as well
adapted to the oiling of airplane power plants for reasons previously stated. If
too much oil is supplied the surplus will work past the piston rings and into the
combustion chamber, where it will burn and cause carbon deposits. Too much
oil will also cause an engine to smoke and an excess of lubricating oil is usually
manifested by a bluish-white smoke issuing from the exhaust.
A good method of maintaining a constant level of oil for the successful
application of the splash system is shown at Fig. 78. The engine base casting
includes a separate chamber which serves as an oil container and which is
below the level of oil in the crank-case. The lubricant is drawn from the sump
or oil container by means of a positive oil pump which discharges directly into
the engine case. The level is maintained by an overflow pipe which allows all
excess lubricant to flow back into the oil container at the bottom of the cylinder.
Before passing into the pump again the oil is strained or filtered by a screen of
wire gauze and all foreign matter removed. Owing to the rapid circulation of the
oil it may be used over and over again for quite a period of time. The oil is
introduced directly into the crank-case by a breather pipe and the level is
indicated by a rod carried by a float which rises when the container is
replenished and falls when the available supply diminishes. It will be noted that
with such system the only apparatus required besides the oil tank which is cast
integral with the bottom of the crank-case is a suitable pump to maintain
circulation of oil. This member is always positively driven, either by means of

shaft and universal coupling or direct gearing. As the system is entirely
automatic in action, it will furnish a positive supply of oil at all desired points,
and it cannot be tampered with by the inexpert because no adjustments are
provided or needed.

Fig. 78.—Sectional View of Typical Motor Showing
Parts Needing Lubrication and Method of Applying
Oil by Constant Level Splash System. Note also
Water Jacket and Spaces for Water Circulation.

DRY CRANK-CASE SYSTEM BEST FOR AIRPLANE ENGINES

In most airplane power plants it is considered desirable to supply the oil directly
to the parts needing it by suitable leads instead of depending solely upon the
distributing action of scoops on the connecting rod big ends. A system of this
nature is shown at Fig. 77. The oil is carried in the crank-case, as is common

practice, but the normal oil level is below the point where it will be reached by
the connecting rod. It is drawn from the crank-case by a plunger pump which
directs it to a manifold leading directly to conductors which supply the main
journals. After the oil has been used on these points it drains back into the
bottom of the crank-case. An excess is provided which is supplied to the
connecting rod ends by passages drilled into the webs of the crank-shaft and
part way into the crank-pins as shown by the dotted lines. The oil which is
present at the connecting rod crank-pins is thrown off by centrifugal force and
lubricates the cylinder walls and other internal parts. Regulating screws are
provided so that the amount of oil supplied the different points may be regulated
at will. A relief check valve is installed to take care of excess lubricant and to
allow any oil that does not pass back into the pipe line to overflow or bi-pass
into the main container.

Fig. 79.—Pressure Feed Oil-Supply System of Airplane
Power Plants has Many Good Features.

A simple system of this nature is shown graphically in a phantom view of the
crank-case at Fig. 79, in which the oil passages are made specially prominent.
The oil is taken from a reservoir at the bottom of the engine base by the usual
form of gear oil pump and is supplied to a main feed manifold which extends
the length of the crank-case. Individual conductors lead to the five main
bearings, which in turn supply the crank-pins by passages drilled through the

crank-shaft web. In this power plant the connecting rods are hollow section
bronze castings and the passage through the center of the connecting rod serves
to convey the lubricant from the crank-pins to the wrist-pins. The cylinder walls
are oiled by the spray of lubricant thrown off the revolving crank-shaft by
centrifugal force. Oil projection by the dippers on the connecting rod ends from
constant level troughs is unequal upon the cylinder walls of the two-cylinder
blocks of an eight- or twelve-cylinder V engine. This gives rise, on one side of
the engine, to under-lubrication, and, on the other side, to over-lubrication, as
shown at Fig. 80, A. This applies to all modifications of splash lubricating
systems.

Fig. 80.—Why Pressure Feed System is Best for
Eight-Cylinder Vee Airplane Engines.

When a force-feed lubricating system is used, the oil, escaping past the cheeks
of both ends of the crank-pin bearings, is thrown off at a tangent to the crank-
pin circle in all directions, supplying the cylinders on both sides with an equal
quantity of oil, as at Fig. 80, B.

WHY COOLING SYSTEMS ARE NECESSARY

The reader should understand from preceding chapters that the power of an
internal-combustion motor is obtained by the rapid combustion and consequent
expansion of some inflammable gas. The operation in brief is that when air or
any other gas or vapor is heated, it will expand and that if this gas is confined in
a space which will not permit expansion, pressure will be exerted against all
sides of the containing chamber. The more a gas is heated, the more pressure it
will exert upon the walls of the combustion chamber it confines. Pressure in a
gas may be created by increasing its temperature and inversely heat may be
created by pressure. When a gas is compressed its total volume is reduced and
the temperature is augmented.
The efficiency of any form of heat engine is determined by the power obtained
from a certain fuel consumption. A definite amount of energy will be liberated
in the form of heat when a pound of any fuel is burned. The efficiency of any
heat engine is proportional to the power developed from a definite quantity of
fuel with the least loss of thermal units. If the greater proportion of the heat
units derived by burning the explosive mixture could be utilized in doing useful
work, the efficiency of the gasoline engine would be greater than that of any
other form of energizing power. There is a great loss of heat from various
causes, among which can be cited the reduction of pressure through cooling the
motor and the loss of heat through the exhaust valves when the burned gases are
expelled from the cylinder.
The loss through the water jacket of the average automobile power plant is over
50 per cent. of the total fuel efficiency. This means that more than half of the
heat units available for power are absorbed and dissipated by the cooling water.
Another 16 per cent. is lost through the exhaust valve, and but 331⁄3 per cent. of
the heat units do useful work. The great loss of heat through the cooling systems
cannot be avoided, as some method must be provided to keep the temperature of
the engine within proper bounds. It is apparent that the rapid combustion and
continued series of explosions would soon heat the metal portions of the engine
to a red heat if some means were not taken to conduct much of this heat away.

The high temperature of the parts would burn the lubricating oil, even that of
the best quality, and the piston and rings would expand to such a degree,
especially when deprived of oil, that they would seize in the cylinder. This
would score the walls, and the friction which ensued would tend to bind the
parts so tightly that the piston would stick, bearings would be burned out, the
valves would warp, and the engine would soon become inoperative.

Fig. 81.—Operating Temperatures of Automobile Engine Parts Useful as a
Guide to Understand Airplane Power Plant Heat.

The best temperature to secure efficient operation is one on which
considerable difference of opinion exists among engineers. The fact that the
efficiency of an engine is dependent upon the ratio of heat converted into
useful work compared to that generated by the explosion of the gas is an
accepted fact. It is very important that the engine should not get too hot, and
on the other hand it is equally vital that the cylinders be not robbed of too
much heat. The object of cylinder cooling is to keep the temperature of the
cylinder below the danger point, but at the same time to have it as high as
possible to secure maximum power from the gas burned. The usual
operating temperatures of an automobile engine are shown at Fig. 81, and
this can be taken as an approximation of the temperatures apt to exist in an
airplane engine of conventional design as well when at ground level or not
very high in the air. The newer very high compression airplane engines in
which compressions of eight or nine atmospheres are used, or about 125

pounds per square inch, will run considerably hotter than the temperatures
indicated.

COOLING SYSTEMS GENERALLY APPLIED

There are two general systems of engine cooling in common use, that in
which water is heated by the absorption of heat from the engine and then
cooled by air, and the other method in which the air is directed onto the
cylinder and absorbs the heat directly instead of through the medium of
water. When the liquid is employed in cooling it is circulated through
jackets which surround the cylinder casting and the water may be kept in
motion by two methods. The one generally favored is to use a positive
circulating pump of some form which is driven by the engine to keep the
water in motion. The other system is to utilize a natural principle that heated
water is lighter than cold liquid and that it will tend to rise to the top of the
cylinder when it becomes heated to the proper temperature and cooled
water takes its place at the bottom of the water jacket.
Air-cooling methods may be by radiation or convection. In the former case
the effective outer surface of the cylinder is increased by the addition of
flanges machined or cast thereon, and the air is depended on to rise from the
cylinder as heated and be replaced by cooler air. This, of course, is found
only on stationary engines. When a positive air draught is directed against
the cylinder by means of the propeller slip stream in an airplane, cooling is
by convection and radiation both. Sometimes the air draught may be
directed against the cylinder walls by some form of jacket which confines it
to the heated portions of the cylinder.

COOLING BY POSITIVE WATER CIRCULATION

Fig. 82.—Water Cooling of Salmson Seven-Cylinder Radial Airplane
Engine.

A typical water-cooling system in which a pump is depended upon to
promote circulation of the cooling liquid is shown at Figs. 82 and 83. The
radiator is carried at the front end of the fuselage in most cases, and serves
as a combined water tank and cooler, but in some cases it is carried at the
side of the engine, as in Fig. 84, or attached to the central portion of the
aerofoil or wing structure. It is composed of an upper and lower portion
joined together by a series of pipes which may be round and provided with
a series of fins to radiate the heat, or which may be flat in order to have the
water pass through in thin sheets and cool it more easily. Cellular or
honeycomb coolers are composed of a large number of bent tubes which
will expose a large area of surface to the cooling influence of the air draught
forced through the radiator either by the forward movement of the vehicle
or by some type of fan. The cellular and flat tube types have almost entirely
displaced the flange tube radiators which were formerly popular because
they cool the water more effectively, and may be made lighter than the
tubular radiator could be for engines of the same capacity.

Fig. 83.—How Water Cooling System of Thomas Airplane Engine is
Installed in Fuselage.

The water is drawn from the lower header of the radiator by the pump and
is forced through a manifold to the lower portion of the water jackets of the
cylinder. It becomes heated as it passes around the cylinder walls and
combustion chambers and the hot water passes out of the top of the water
jacket to the upper portion of the radiator. Here it is divided in thin streams
and directed against comparatively cool metal which abstracts the heat from
the water. As it becomes cooler it falls to the bottom of the radiator because
its weight increases as the temperature becomes lower. By the time it
reaches the lower tank of the radiator it has been cooled sufficiently so that
it may be again passed around the cylinders of the motor. The popular form
of circulating pump is known as the “centrifugal type” because a rotary
impeller of paddle-wheel form throws water which it receives at a central
point toward the outside and thus causes it to maintain a definite rate of
circulation. The pump is always a separate appliance attached to the engine
and driven by positive gearing or direct-shaft connection. The centrifugal

pump is not as positive as the gear form, and some manufacturers prefer the
latter because of the positive pumping features. They are very simple in
form, consisting of a suitable cast body in which a pair of spur pinions
having large teeth are carried. One of these gears is driven by suitable
means, and as it turns the other member they maintain a flow of water
around the pump body. The pump should always be installed in series with
the water pipe which conveys the cool liquid from the lower compartment
of the radiator to the coolest portion of the water jacket.

Fig. 84.—Finned Tube Radiators at the Side of Hall-Scott Airplane
Power Plant Installed in Standard Fuselage.

WATER CIRCULATION BY NATURAL SYSTEM

Some automobile engineers contend that the rapid water circulation
obtained by using a pump may cool the cylinders too much, and that the
temperature of the engine may be reduced so much that the efficiency will
be lessened. For this reason there is a growing tendency to use the natural
method of water circulation as the cooling liquid is supplied to the cylinder
jackets just below the boiling point and the water issues from the jacket at

the top of the cylinder after it has absorbed sufficient heat to raise it just
about to the boiling point.
As the water becomes heated by contact with the hot cylinder and
combustion-chamber walls it rises to the top of the water jacket, flows to
the cooler, where enough of the heat is absorbed to cause it to become
sensibly greater in weight. As the water becomes cooler, it falls to the
bottom of the radiator and it is again supplied to the water jacket. The
circulation is entirely automatic and continues as long as there is a
difference in temperature between the liquid in the water spaces of the
engine and that in the cooler. The circulation becomes brisker as the engine
becomes hotter and thus the temperature of the cylinders is kept more
nearly to a fixed point. With the thermosyphon system the cooling liquid is
nearly always at its boiling point, whereas if the circulation is maintained
by a pump the engine will become cooler at high speed and will heat up
more at low speed.
With the thermosyphon, or natural system of cooling, more water must be
carried than with the pump-maintained circulation methods. The water
spaces around the cylinders should be larger, the inlet and discharge water
manifolds should have greater capacity, and be free from sharp corners
which might impede the flow. The radiator must also carry more water than
the form used in connection with the pump because of the brisker pump
circulation which maintains the engine temperature at a lower point.
Consideration of the above will show why the pump system is almost
universally used in connection with airplane power plant cooling.

DIRECT AIR-COOLING METHODS

The earliest known method of cooling the cylinder of gas-engines was by
means of a current of air passed through a jacket which confined it close to
the cylinder walls and was used by Daimler on his first gas-engine. The
gasoline engine of that time was not as efficient as the later form, and other
conditions which materialized made it desirable to cool the engine by water.
Even as gasoline engines became more and more perfected there has always
existed a prejudice against air cooling, though many forms of engines have

been used, both in automobile and aircraft applications where the air-
cooling method has proven to be very practical.
The simplest system of air cooling is that in which the cylinders are
provided with a series of flanges which increase the effective radiating
surface of the cylinder and directing an air-current from a fan against the
flanges to absorb the heat. This increase in the available radiating surface of
an air-cooled cylinder is necessary because air does not absorb heat as
readily as water and therefore more surface must be provided that the
excess heat be absorbed sufficiently fast to prevent distortion of the
cylinders. Air-cooling systems are based on a law formulated by Newton,
which is: “The rate for cooling for a body in a uniform current of air is
directly proportional to the speed of the air current and the amount of
radiating surface exposed to the cooling effect.”

AIR-COOLED ENGINE DESIGN CONSIDERATIONS

There are certain considerations which must be taken into account in
designing an air-cooled engine, which are often overlooked in those forms
cooled by water. Large valves must be provided to insure rapid expulsion of
the flaming exhaust gas and also to admit promptly the fresh cool mixture
from the carburetor. The valves of air-cooled engines are usually placed in
the cylinder-head, in order to eliminate any pockets or sharp passages which
would impede the flow of gas or retain some of the products of combustion
and their heat. When high power is desired multiple-cylinder engines
should be used, as there is a certain limit to the size of a successful air-
cooled cylinder. Much better results are secured from those having small
cubical contents because the heat from small quantities of gas will be more
quickly carried off than from greater amounts. All successful engines of the
aviation type which have been air-cooled have been of the multiple-cylinder
type.

Fig. 85.—Anzani Testing His Five-Cylinder Air
Cooled Aviation Motor Installed in Bleriot
Monoplane. Note Exposure of Flanged Cylinders to
Propeller Slip Stream.

An air-cooled engine must be placed in the fuselage, as at Fig. 85, in such a
way that there will be a positive circulation of air around it all the time that
it is in operation. The air current may be produced by the tractor screw at
the front end of the motor, or by a suction or blower fan attached to the
crank-shaft as in the Renault engine or by rotating the cylinders as in the Le
Rhone and Gnome motors. Greater care is required in lubrication of the air-
cooled cylinders and only the best quality of oil should be used to insure
satisfactory oiling.
The combustion chambers must be proportioned so that distribution of
metal is as uniform as possible in order to prevent uneven expansion during
increase in temperature and uneven contraction when the cylinder is cooled.
It is essential that the inside walls of the combustion chamber be as smooth
as possible because any sharp angle or projection may absorb sufficient heat

to remain incandescent and cause trouble by igniting the mixture before the
proper time. The best grades of cast iron or steel should be used in the
cylinder and piston and the machine work must be done very accurately so
the piston will operate with minimum friction in the cylinder. The cylinder
bore should not exceed 41⁄2 or 5 inches and the compression pressure should
never exceed 75 pounds absolute, or about five atmospheres, or serious
overheating will result.
As an example of the care taken in disposing of the exhaust gases in order
to obtain practical air-cooling, some cylinders are provided with a series of
auxiliary exhaust ports uncovered by the piston when it reaches the end of
its power stroke. The auxiliary exhaust ports open just as soon as the full
force of the explosion has been spent and a portion of the flaming gases is
discharged through the ports in the bottom of the cylinder. Less of the
exhaust gases remains to be discharged through the regular exhaust member
in the cylinder-head and this will not heat the walls of the cylinder nearly as
much as the larger quantity of hot gas would. That the auxiliary exhaust
port is of considerable value is conceded by many designers of fixed and
fan-shaped air-cooled motors for airplanes.
Among the advantages stated for direct air cooling, the greatest is the
elimination of cooling water and its cooling auxiliaries, which is a factor of
some moment, as it permits considerable reduction in horse-power-weight
ratio of the engine, something very much to be desired. In the temperate
zone, where the majority of airplanes are used, the weather conditions
change in a very few months from the warm summer to the extreme cold
winter, and when water-cooled systems are employed it is necessary to add
some chemical substance to the water to prevent it from freezing. The
substances commonly employed are glycerine, wood alcohol, or a saturated
solution of calcium chloride. Alcohol has the disadvantage in that it
vaporizes readily and must be often renewed. Glycerine affects the rubber
hose, while the calcium chloride solution crystallizes and deposits salt in
the radiator and water pipes.
One of the disadvantages of an air-cooling method, as stated by those who
do not favor this system, is that engines cooled by air cannot be operated for
extended periods under constant load or at very high speed without heating
up to such a point that premature ignition of the charge may result. The

water-cooling systems, at the other hand, maintain the temperature of the
engine more nearly constant than is possible with an air-cooled motor, and
an engine cooled by water can be operated under conditions of inferior
lubrication or poor mixture adjustment that would seriously interfere with
proper and efficient cooling by air.
Air-cooled motors, as a rule, use less fuel than water-cooled engines,
because the higher temperature of the cylinder does not permit of a full
charge of gas being inspired on the intake stroke. As special care is needed
in operating an air-cooled engine to obtain satisfactory results and because
of the greater difficulty which obtains in providing proper lubrication and
fuel mixtures which will not produce undue heating, the air-cooled system
has but few adherents at the present time, and practically all airplanes, with
but very few exceptions, are provided with water-cooled power plants.
Those fitted with air-cooled engines are usually short-flight types where
maximum lightness is desired in order to obtain high speed and quick
climb. The water-cooled engines are best suited for airplanes intended for
long flights. The Gnome, Le Rhone and Clerget engines are thoroughly
practical and have been widely used in France and England. These are
rotary radial cylinder types. The Anzani is a fixed cylinder engine used on
training machines, while the Renault is a V-type engine made in eight- and
twelve-cylinder V forms that has been used on reconnaissance and bombing
airplanes with success. These types will be fully considered in proper
sequence.

CHAPTER VIII
Methods of Cylinder Construction—Block Castings—Influence on Crank-
Shaft Design—Combustion Chamber Design—Bore and Stroke Ratio—
Meaning of Piston Speed—Advantage of Off-Set Cylinders—Valve
Location of Vital Import—Valve Installation Practice—Valve Design and
Construction—Valve Operation—Methods of Driving Cam-Shaft—Valve
Springs—Valve Timing—Blowing Back—Lead Given Exhaust Valve—
Exhaust Closing, Inlet Opening—Closing the Inlet Valve—Time of Ignition
—How an Engine Is Timed—Gnome “Monosoupape” Valve Timing—
Springless Valves—Four Valves per Cylinder.
The improvements noted in the modern internal combustion motors have
been due to many conditions. The continual experimenting by leading
mechanical minds could have but one ultimate result. The parts of the
engines have been lightened and strengthened, and greater power has been
obtained without increasing piston displacement. A careful study has been
made of the many conditions which make for efficient motor action, and
that the main principles are well recognized by all engineers is well shown
by the standardization of design noted in modern power plants. There are
many different methods of applying the same principle, and it will be the
purpose of this chapter to define the ways in which the construction may be
changed and still achieve the same results. The various components may
exist in many different forms, and all have their advantages and
disadvantages. That all methods are practical is best shown by the large
number of successful engines which use radically different designs.

METHODS OF CYLINDER CONSTRUCTION

One of the most important parts of the gasoline engine and one that has
material bearing upon its efficiency is the cylinder unit. The cylinders may
be cast individually, or in pairs, and it is possible to make all cylinders a
unit or block casting. Some typical methods of cylinder construction are
shown in accompanying illustrations. The appearance of individual cylinder

castings may be ascertained by examination of the Hall-Scott airplane
engine. Air-cooled engine cylinders are always of the individual pattern.
Considered from a purely theoretical point of view, the individual cylinder
casting has much in its favor. It is advanced that more uniform cooling is
possible than where the cylinders are cast either in pairs or three or four in
one casting. More uniform cooling insures that the expansion or change of
form due to heating will be more equal. This is an important condition
because the cylinder bore must remain true under all conditions of
operation. If the heating effect is not uniform, which condition is liable to
obtain if metal is not evenly distributed, the cylinder may become distorted
by heat and the bore be out of truth. When separate cylinders are used it is
possible to make a uniform water space and have the cooling liquid evenly
distributed around the cylinder. In multiple cylinder castings this is not
always the rule, as in many instances, especially in four-cylinder block
motors where compactness is the main feature, there is but little space
between the cylinders for the passage of water. Under such circumstances
the cooling effect is not even, and the stresses which obtain because of
unequal expansion may distort the cylinder to some extent. When steel
cylinders are made from forgings, the water jackets are usually of copper or
sheet steel attached to the forging by autogenous welding; in the case of the
latter and, in some cases, the former may be electro-deposited on the
cylinders.

BLOCK CASTINGS

The advantage of casting the cylinders in blocks is that a motor may be
much shorter than it would be if individual castings were used. It is
admitted that when the cylinders are cast together a more compact, rigid,
and stronger power plant is obtained than when cast separately. There is a
disadvantage, however, in that if one cylinder becomes damaged it will be
necessary to replace the entire unit, which means scrapping three good
cylinders because one of the four has failed. When the cylinders are cast
separately one need only replace the one that has become damaged. The
casting of four cylinders in one unit is made possible by improved foundry
methods, and when proper provision is made for holding the cores when the
metal is poured and the cylinder casts are good, the construction is one of

distinct merit. It is sometimes the case that the proportion of sound castings
is less when cylinders are cast in block, but if the proper precautions are
observed in molding and the proper mixtures of cast iron used, the ratio of
defective castings is no more than when cylinders are molded individually.
As an example of the courage of engineers in departing from old-
established rules, the cylinder casting shown at Fig. 86 may be considered
typical. This is used on the Duesenberg four-cylinder sixteen-valve 43⁄4′′ ×
7′′ engine which has a piston displacement of 496 cu. in. At a speed of
2,000 r.p.m., corresponding to a piston speed of 2,325 ft. per min., the
engine is guaranteed to develop 125 horse-power. The weight of the model
engine without gear reduction is 436 lbs., but a number of refinements have
been made in the design whereby it is expected to get the weight down to
390 lbs. The four cylinders are cast from semi-steel in a single block, with
integral heads. The cylinder construction is the same as that which has
always been used by Mr. Duesenberg, inlet and exhaust valves being
arranged horizontally opposite each other in the head. There are large
openings in the water jacket at both sides and at the ends, which are closed
by means of aluminum covers, water-tightness being secured by the use of
gaskets. This results in a saving in weight because the aluminum covers can
be made considerably lighter than it would be possible to cast the jacket
walls, and, besides, it permits of obtaining a more nearly uniform thickness
of cylinder wall, as the cores can be much better supported. The cooling
water passes completely around each cylinder, and there is a very
considerable space between the two central cylinders, this being made
necessary in order to get the large bearing area desirable for the central
bearing.

Fig. 86.—Views of Four-Cylinder Duesenberg
Airplane Engine Cylinder Block.

It is common practice to cast the water jackets integral with the cylinders, if
cast iron or aluminum is used, and this is also the most economical method
of applying it because it gives good results in practice. An important detail
is that the water spaces must be proportioned so that they are equal around
the cylinders whether these members are cast individually, in pairs, threes
or fours. When cylinders are cast in block form it is good practice to leave a
large opening in the jacket wall which will assist in supporting the core and
make for uniform water space. It will be noticed that the casting shown at
Fig. 86 has a large opening in the side of the cylinder block. These openings
are closed after the interior of the casting is thoroughly cleaned of all sand,
core wire, etc., by brass, cast iron or aluminum plates. These also have
particular value in that they may be removed after the motor has been in
use, thus permitting one to clean out the interior of the water jacket and

dispose of the rust, sediment, and incrustation which are always present
after the engine has been in active service for a time.
Among the advantages claimed for the practice of casting cylinders in
blocks may be mentioned compactness, lightness, rigidity, simplicity of
water piping, as well as permitting the use of simple forms of inlet and
exhaust manifolds. The light weight is not only due to the reduction of the
cylinder mass but because the block construction permits one to lighten the
entire motor. The fact that all cylinders are cast together decreases
vibration, and as the construction is very rigid, disalignment of working
parts is practically eliminated. When inlet and exhaust manifolds are cored
in the block casting, as is sometimes the case, but one joint is needed on
each of these instead of the multiplicity of joints which obtain when the
cylinders are individual castings. The water piping is also simplified. In the
case of a four-cylinder block motor but two pipes are used; one for the
water to enter the cylinder jacket, the other for the cooling liquid to
discharge through.

INFLUENCE ON CRANK-SHAFT DESIGN

The method of casting the cylinders has a material influence on the design
of the crank-shaft as will be shown in proper sequence. When four cylinders
are combined in one block it is possible to use a two-bearing crank-shaft.
Where cylinders are cast in pairs a three-bearing crank-shaft is commonly
supplied, and when cylinders are cast as individual units it is thought
necessary to supply a five-bearing crank-shaft, though sometimes shafts
having but three journals are used successfully. Obviously the shafts must
be stronger and stiffer to withstand the stresses imposed if two supporting
bearings are used than if a larger number are employed. In this connection it
may be stated that there is less difficulty in securing alignment with a lesser
number of bearings and there is also less friction. On the other hand, the
greater the number of points of support a crank-shaft has the lighter the
webs can be made and still have requisite strength.

COMBUSTION CHAMBER DESIGN

Fig. 87.—Twin-Cylinder Block of Sturtevant Airplane Engine is Cast
of Aluminum, and Has Removable Cylinder Head.

Another point of importance in the design of the cylinder, and one which
has considerable influence upon the power developed, is the shape of the
combustion chamber. The endeavor of designers is to obtain maximum
power from a cylinder of certain proportions, and the greater energy
obtained without increasing piston displacement or fuel consumption the
higher the efficiency of the motor. To prevent troubles due to pre-ignition it
is necessary that the combustion chamber be made so that there will be no
roughness, sharp corners, or edges of metal which may remain incandescent
when heated or which will serve to collect carbon deposits by providing a
point of anchorage. With the object of providing an absolutely clean
combustion chamber some makers use a separable head unit to their twin
cylinder castings, such as shown at Fig. 87 and Fig. 88. These permit one to
machine the entire interior of the cylinder and combustion chamber. The
relation of valve location and combustion chamber design will be
considered in proper sequence. These cylinders are cast of aluminum,
instead of cast iron, as is customary, and are provided with steel or cast iron
cylinder liners forced in the soft metal casting bores.

Fig. 88.—Aluminum Cylinder Pair Casting of Thomas 150 Horse-Power
Airplane Engine is of the L Head Type.

BORE AND STROKE RATIO

A question that has been a vexed one and which has been the subject of
considerable controversy is the proper proportion of the bore to the stroke.
The early gas engines had a certain well-defined bore to stroke ratio, as it
was usual at that time to make the stroke twice as long as the bore was
wide, but this cannot be done when high speed is desired. With the
development of the present-day motor the stroke or piston travel has been
gradually shortened so that the relative proportions of bore and stroke have
become nearly equal. Of late there seems to be a tendency among designers
to return to the proportions which formerly obtained, and the stroke is
sometimes one and a half or one and three-quarter times the bore.
Engines designed for high speed should have the stroke not much longer
than the diameter of the bore. The disadvantage of short-stroke engines is
that they will not pull well at low speeds, though they run with great
regularity and smoothness at high velocity. The long-stroke engine is much
superior for slow speed work, and it will pull steadily and with increasing
power at low speed. It was formerly thought that such engines should never
turn more than a moderate number of revolutions, in order not to exceed the

safe piston speed of 1,000 feet per minute. This old theory or rule of
practice has been discarded in designing high efficiency automobile racing
and aviation engines, and piston speeds from 2,500 to 3,000 feet per minute
are sometimes used, though the average is around 2,000 feet per minute.
While both short- and long-stroke motors have their advantages, it would
seem desirable to average between the two. That is why a proportion of four
to five or six seems to be more general than that of four to seven or eight,
which would be a long-stroke ratio. Careful analysis of a number of foreign
aviation motors shows that the average stroke is about 1.2 times the bore
dimensions, though some instances were noted where it was as high as 1.7
times the bore.

MEANING OF PISTON SPEED

The factor which limits the stroke and makes the speed of rotation so
dependent upon the travel of the piston is piston speed. Lubrication is the
main factor which determines piston speed, and the higher the rate of piston
travel the greater care must be taken to insure proper oiling. Let us fully
consider what is meant by piston speed.
Assume that a motor has a piston travel or stroke of six inches, for the sake
of illustration. It would take two strokes of the piston to cover one foot, or
twelve inches, and as there are two strokes to a revolution it will be seen
that this permits of a normal speed of 1,000 revolutions per minute for an
engine with a six-inch stroke, if one does not exceed 1,000 feet per minute.
If the stroke was only four inches, a normal speed of 1,500 revolutions per
minute would be possible without exceeding the prescribed limit. The
crank-shaft of a small engine, having three-inch stroke, could turn at a
speed of 2,000 revolutions per minute without danger of exceeding the safe
speed limit. It will be seen that the longer the stroke the slower the speed of
the engine, if one desires to keep the piston speed within the bounds as
recommended, but modern practice allows of greatly exceeding the speeds
formerly thought best.

ADVANTAGES OF OFF-SET CYLINDERS

Another point upon which considerable difference of opinion exists relates
to the method of placing the cylinder upon the crank-case—i.e., whether its
center line should be placed directly over the center of the crank-shaft, or to
one side of center. The motor shown at Fig. 90 is an off-set type, in that the
center line of the cylinder is a little to one side of the center of the crank-
shaft. Diagrams are presented at Fig. 91 which show the advantages of off-
set crank-shaft construction. The view at A is a section through a simple
motor with the conventional cylinder placing, the center line of both crank-
shaft and cylinder coinciding. The view at B shows the cylinder placed to
one side of center so that its center line is distinct from that of the crank-
shaft and at some distance from it. The amount of off-set allowed is a point
of contention, the usual amount being from fifteen to twenty-five per cent.
of the stroke. The advantages of the off-set are shown at Fig. 91, C. If the
crank turns in direction of the arrow there is a certain resistance to motion
which is proportional to the amount of energy exerted by the engine and the
resistance offered by the load. There are two thrusts acting against the
cylinder wall to be considered, that due to explosion or expansion of the
gas, and that which resists the motion of the piston. These thrusts may be
represented by arrows, one which acts directly in a vertical direction on the
piston top, the other along a straight line through the center of the
connecting rod. Between these two thrusts one can draw a line representing
a resultant force which serves to bring the piston in forcible contact with
one side of the cylinder wall, this being known as side thrust. As shown at
C, the crank-shaft is at 90 degrees, or about one-half stroke, and the
connecting rod is at 20 degrees angle. The shorter connecting rod would
increase the diagonal resultant and side thrusts, while a longer one would
reduce the angle of the connecting rod and the side thrust of the piston
would be less. With the off-set construction, as shown at D, it will be
noticed that with the same connecting-rod length as shown at C and with
the crank-shaft at 90 degrees of the circle that the connecting-rod angle is
14 degrees and the side thrust is reduced proportionately.

Fig. 90.—Cross Section of Austro-Daimler Engine, Showing
Offset Cylinder Construction. Note Applied Water Jacket
and Peculiar Valve Action.

Another important advantage is that greater efficiency is obtained from the
explosion with an off-set crank-shaft, because the crank is already inclined
when the piston is at top center, and all the energy imparted to the piston by
the burning mixture can be exerted directly into producing a useful turning
effort. When a cylinder is placed directly on a line with the crank-shaft, as
shown at A, it will be evident that some of the force produced by the
expansion of the gas will be exerted in a direct line and until the crank
moves the crank throw and connecting rod are practically a solid member.
The pressure which might be employed in obtaining useful turning effort is

wasted by causing a direct pressure upon the lower half of the main bearing
and the upper half of the crank-pin bushing.

Fig. 91.—Diagrams Demonstrating Advantages of Offset Crank-Shaft
Construction.

Very good and easily understood illustrations showing advantages of the
off-set construction are shown at E and F. This is a bicycle crank-hanger. It
is advanced that the effort of the rider is not as well applied when the crank
is at position E as when it is at position F. Position E corresponds to the
position of the parts when the cylinder is placed directly over the crank-
shaft center. Position F may be compared to the condition which is present
when the off-set cylinder construction is used.

VALVE LOCATION OF VITAL IMPORT

It has often been said that a chain is no stronger than its weakest link, and
this is as true of the explosive motor as it is of any other piece of
mechanism. Many motors which appeared to be excellently designed and
which were well constructed did not prove satisfactory because some minor
detail or part had not been properly considered by the designer. A factor
having material bearing upon the efficiency of the internal combustion
motor is the location of the valves and the shape of the combustion chamber
which is largely influenced by their placing. The fundamental consideration
of valve design is that the gases be admitted and discharged from the
cylinder as quickly as possible in order that the speed of gas flow will not
be impeded and produce back pressure. This is imperative in obtaining
satisfactory operation in any form of motor. If the inlet passages are
constricted the cylinder will not fill with explosive mixture promptly,
whereas if the exhaust gases are not fully expelled the parts of the inert
products of combustion retained dilute the fresh charge, making it slow
burning and causing lost power and overheating. When an engine employs
water as a cooling medium this substance will absorb the surplus heat
readily, and the effects of overheating are not noticed as quickly as when
air-cooled cylinders are employed. Valve sizes have a decided bearing upon
the speed of motors and some valve locations permit the use of larger
members than do other positions.
While piston velocity is an important factor in determinations of power
output, it must be considered from the aspect of the wear produced upon the
various parts of the motor. It is evident that engines which run very fast,
especially of high power, must be under a greater strain than those operating
at lower speeds. The valve-operating mechanism is especially susceptible to
the influence of rapid movement, and the slower the engine the longer the
parts will wear and the more reliable the valve action.

Fig. 92.—Diagram Showing Forms of Cylinder Demanded
by Different Valve Placings. A—T Head Type, Valves on
Opposite Sides. B—L Head Cylinder, Valves Side by Side. C
—L Head Cylinder, One Valve in Head, Other in Pocket. D
—Inlet Valve Over Exhaust Member, Both in Side Pocket. E
—Valve-in-the-Head Type with Vertical Valves. F—Inclined
Valves Placed to Open Directly into Combustion Chamber.

As will be seen by reference to the accompanying illustration, Fig. 92, there
are many ways in which valves may be placed in the cylinder. Each method
outlined possesses some point of advantage, because all of the types
illustrated are used by reputable automobile manufacturers. The method
outlined at Fig. 92, A, is widely used, and because of its shape the cylinder
is known as the “T” form. It is approved for automobile use for several
reasons, the most important being that large valves can be employed and a
well-balanced and symmetrical cylinder casting obtained. Two independent

cam-shafts are needed, one operating the inlet valves, the other the exhaust
members. The valve-operating mechanism can be very simple in form,
consisting of a plunger actuated by the cam which transmits the cam motion
to the valve-stem, raising the valve as the cam follower rides on the point of
the cam. Piping may be placed without crowding, and larger manifolds can
be fitted than in some other constructions. This has special value, as it
permits the use of an adequate discharge pipe on the exhaust side with its
obvious advantages. This method of cylinder construction is never found on
airplane engines because it does not permit of maximum power output.
On the other hand, if considered from a viewpoint of actual heat efficiency,
it is theoretically the worst form of combustion chamber. This disadvantage
is probably compensated for by uniformity of expansion of the cylinder
because of balanced design. The ignition spark-plug may be located directly
over the inlet valve in the path of the incoming fresh gases, and both valves
may be easily removed and inspected by unscrewing the valve caps without
taking off the manifolds.
The valve installation shown at C is somewhat unusual, though it provides
for the use of valves of large diameter. Easy charging is insured because of
the large inlet valve directly in the top of the cylinder. Conditions may be
reversed if necessary, and the gases discharged through this large valve.
Both methods are used, though it would seem that the free exhaust provided
by allowing the gases to escape directly from the combustion chamber
through the overhead valve to the exhaust manifold would make for more
power. The method outlined at Fig. 92, F and at Fig. 90 is one that has been
widely employed on large automobile racing motors where extreme power
is required, as well as in engines constructed for aviation service. The
inclination of the valves permits the use of large valves, and these open
directly into the combustion chamber. There are no pockets to retain heat or
dead gas, and free intake and outlet of gas is obtained. This form is quite
satisfactory from a theoretical point of view because of the almost ideal
combustion chamber form. Some difficulty is experienced, however, in
properly water-jacketing the valve chamber which experience has shown to
be necessary if the engine is to have high power.
The motor shown at Fig. 92, B and Fig. 88 employs cylinders of the “L”
type. Both valves are placed in a common extension from the combustion

chamber, and being located side by side both are actuated from a common
cam-shaft. The inlet and exhaust pipes may be placed on the same side of
the engine and a very compact assemblage is obtained, though this is
optional if passages are cored in the cylinder pairs to lead the gases to
opposite sides. The valves may be easily removed if desired, and the
construction is fairly good from the viewpoint of both foundry man and
machinist. The chief disadvantage is the limited area of the valves and the
loss of heat efficiency due to the pocket. This form of combustion chamber,
however, is more efficient than the “T” head construction, though with the
latter the use of larger valves probably compensates for the greater heat
loss. It has been stated as an advantage of this construction that both
manifolds can be placed at the same side of the engine and a compact
assembly secured. On the other hand, the disadvantage may be cited that in
order to put both pipes on the same side they must be of smaller size than
can be used when the valves are oppositely placed. The “L” form cylinder is
sometimes made more efficient if but one valve is placed in the pocket
while the other is placed over it. This construction is well shown at Fig. 92,
D and is found on Anzani motors.

Fig. 93.—Sectional View of Engine Cylinder Showing Valve
and Cage Installation.

The method of valve application shown at Fig. 87 is an ingenious method of
overcoming some of the disadvantages inherent with valve-in-the-head
motors. In the first place it is possible to water-jacket the valves thoroughly,
which is difficult to accomplish when they are mounted in cages. The water
circulates directly around the walls of the valve chambers, which is superior
to a construction where separate cages are used, as there are two thicknesses
of metal with the latter, that of the valve-cage proper and the wall of the
cylinder. The cooling medium is in contact only with the outer wall, and as
there is always a loss of heat conductivity at a joint it is practically
impossible to keep the exhaust valves and their seats at a uniform
temperature. The valves may be of larger size without the use of pockets
when seating directly in the head. In fact, they could be equal in diameter to
almost half the bore of the cylinder, which provides an ideal condition of
charge placement and exhaust. When valve grinding is necessary the entire
head is easily removed by taking off six nuts and loosening inlet manifold

connections, which operation would be necessary even if cages were
employed, as in the engine shown at Fig. 93.

Fig. 94.—Diagrams Showing How Gas Enters Cylinder
Through Overhead Valves and Other Types. A—Tee Head
Cylinder. B—L Head Cylinder. C—Overhead Valve.

Fig. 95.—Conventional Methods of Operating
Internal Combustion Motor Valves.

At Fig. 94, A and B, a section through a typical “L”-shaped cylinder is
depicted. It will be evident that where a pocket construction is employed, in
addition to its faculty for absorbing heat, the passage of gas would be
impeded. For example, the inlet gas rushing in through the open valve
would impinge sharply upon the valve-cap or combustion head directly
over the valve and then must turn at a sharp angle to enter the combustion
chamber and then at another sharp angle to fill the cylinders. The same
conditions apply to the exhaust gases, though they are reversed. When the
valve-in-the-head type of cylinder is employed, as at C, the only resistance
offered the gas is in the manifold. As far as the passage of the gases in and
out of the cylinder is concerned, ideal conditions obtain. It is claimed that
valve-in-the-head motors are more flexible and responsive than other forms,

but the construction has the disadvantage in that the valves must be opened
through a rather complicated system of push rods and rocker arms instead
of the simpler and direct plunger which can be used with either the “T” or
“L” head cylinders. This is clearly outlined in the illustrations at Fig. 95,
where A shows the valve in the head-operating mechanism necessary if the
cam-shaft is carried at the cylinder base, while B shows the most direct
push-rod action obtained with “T” or “L” head cylinder placing.

Fig. 96.—Examples of Direct Valve Actuation by Overhead Cam-Shaft. A—
Mercedes. B—Hall-Scott. C—Wisconsin.

Fig. 97.

CENSORED

Fig. 98.

CENSORED

The objection can be easily met by carrying the cam-shaft above the
cylinders and driving it by means of gearing. The types of engine cylinders
using this construction are shown at Fig. 96, and it will be evident that a
positive and direct valve action is possible by following the construction
originated by the Mercedes (German) aviation engine designers and
outlined at A. The other forms at B and C are very clearly adaptations of
this design. The Hall-Scott engine at Fig. 97 is depicted in part section and
no trouble will be experienced in understanding the bevel pinion and gear
drive from the crank-shaft to the overhead cam-shaft through a vertical
counter-shaft. A very direct valve action is used in the Duesenberg engines,
one of which is shown in part section at Fig. 98. The valves are parallel
with the piston top and are actuated by rocker arms, one end of which bears
against the valve stem, and the other rides the cam-shaft.

Fig. 99.—Sectional Views Showing Arrangement of Novel
Concentric Valve Arrangement Devised by Panhard for Aerial
Engines.

The form shown at Fig. 99 shows an ingenious application of the valve-in-
the-head idea which permits one to obtain large valves. It has been used on

some of the Panhard aviation engines and on the American Aeromarine
power plants. The inlet passage is controlled by the sliding sleeve which is
hollow and slotted so as to permit the inlet gases to enter the cylinder
through the regular type poppet valve which seats in the exhaust sleeve.
When the exhaust valve is operated by the tappet rod and rocker arm the
intake valve is also carried down with it. The intake gas passage is closed,
however, and the burned gases are discharged through the large annular
passage surrounding the sleeve. When the inlet valve leaves its seat in the
sleeve the passage of cool gas around the sleeve keeps the temperature of
both valves to a low point and the danger of warping is minimized. A dome-
shaped combustion chamber may be used, which is an ideal form in
conserving heat efficiency, and as large valves may be installed the flow of
both fresh and exhaust gases may be obtained with minimum resistance.
The intake valve is opened by a small auxiliary rocker arm which is lifted
when the cam follower rides into the depression in the cam by the action of
the strong spring around the push rod. When the cam follower rides on the
high point the exhaust sleeve is depressed from its seat against the cylinder.
By using a cam having both positive and negative profiles, a single rod
suffices for both valves because of its push and pull action.

VALVE DESIGN AND CONSTRUCTION

Valve dimensions are an important detail to be considered and can be
determined by several conditions, among which may be cited method of
installation, operating mechanism, material employed, engine speed
desired, manner of cylinder cooling and degree of lift desired. A review of
various methods of valve location has shown that when the valves are
placed directly in the head we can obtain the ideal cylinder form, though
larger valves may be used if housed in a separate pocket, as afforded by the
“T” head construction. The method of operation has much to do with the
size of the valves. For example, if an automatic inlet valve is employed it is
good practice to limit the lift and obtain the required area of port opening
by augmenting the diameter. Because of this a valve of the automatic type is
usually made twenty per cent. larger than one mechanically operated. When
both are actuated by cam mechanism, as is now common practice, they are
usually made the same size and are interchangeable, which greatly

simplifies manufacture. The relation of valve diameter to cylinder bore is
one that has been discussed for some time by engineers. The writer’s
experience would indicate that they should be at least half the bore, if
possible. While the mushroom type or poppet valve has become standard
and is the most widely used form at the present time, there is some
difference of opinion among designers as to the materials employed and the
angle of the seat. Most valves have a bevel seat, though some have a flat
seating. The flat seat valve has the distinctive advantage of providing a
clear opening with lesser lift, this conducing to free gas flow. It also has
value because it is silent in operation, but the disadvantage is present that
best material and workmanship must be used in their construction to obtain
satisfactory results. As it can be made very light it is particularly well
adapted for use as an automatic inlet valve. Among other disadvantages
cited is the claim that it is more susceptible to derangement, owing to the
particles of foreign matter getting under the seat. With a bevel seat it is
argued that the foreign matter would be more easily dislodged by the gas
flow, and that the valve would close tighter because it is drawn positively
against the bevel seat.
Several methods of valve construction are the vogue, the most popular form
being the one-piece type; those which are composed of a head of one
material and stem of another are seldom used in airplane engines because
they are not reliable. In the built-up construction the head is usually of high
nickel steel or cast iron, which metals possess good heat-resisting qualities.
Heads made of these materials are not likely to warp, scale, or pit, as is
sometimes the case when ordinary grades of machinery steel are used. The
cast-iron head construction is not popular because it is often difficult to
keep the head tight on the stem. There is a slight difference in expansion
ratio between the head and the stem, and as the stem is either screwed or
riveted to the cast-iron head the constant hammering of the valve against its
seat may loosen the joint. As soon as the head is loose on the stem the
action of the valve becomes erratic. The best practice is to machine the
valves from tungsten steel forgings. This material has splendid heat-
resisting qualities and will not pit or become scored easily. Even the
electrically welded head to stem types which are used in automobile
engines are not looked upon with favor in the aviation engine. Valve stem

guides and valve stems must be machined very accurately to insure correct
action. The usual practice in automobile engines is shown at Fig. 100.

Fig. 100.—Showing Clearance Allowed
Between Valve Stem and Valve Stem Guide
to Secure Free Action.

VALVE OPERATION

The methods of valve operation commonly used vary according to the type
of cylinder construction employed. In all cases the valves are lifted from
their seats by cam-actuated mechanism. Various forms of valve-lifting cams
are shown at Fig. 101. As will be seen, a cam consists of a circle to which a
raised, approximately triangular member has been added at one point. When
the cam follower rides on the circle, as shown at Fig. 102, there is no
difference in height between the cam center and its periphery and there is
no movement of the plunger. As soon as the raised portion of the cam
strikes the plunger it will lift it, and this reciprocating movement is
transmitted to the valve stem by suitable mechanical connections.

Fig. 101.—Forms of Valve-Lifting Cams Generally Employed. A—Cam
Profile for Long Dwell and Quick Lift. B—Typical Inlet Cam Used with
Mushroom Type Follower. C—Average Form of Cam. D—Designed to Give
Quick Lift and Gradual Closing.

The cam forms outlined at Fig. 101 are those commonly used. That at A is
used on engines where it is desired to obtain a quick lift and to keep the
valve fully opened as long as possible. It is a noisy form, however, and is
not very widely employed. That at B is utilized more often as an inlet cam
while the profile shown at C is generally depended on to operate exhaust
valves. The cam shown at D is a composite form which has some of the
features of the other three types. It will give the quick opening of form A,
the gradual closing of form B, and the time of maximum valve opening
provided by cam profile C.

Fig. 102.—Showing Principal Types of Cam Followers which Have
Received General Application.

The various types of valve plungers used are shown at Fig. 102. That shown
at A is the simplest form, consisting of a simple cylindrical member having
a rounded end which follows the cam profile. These are sometimes made of
square stock or kept from rotating by means of a key or pin. A line contact
is possible when the plunger is kept from turning, whereas but a single
point bearing is obtained when the plunger is cylindrical and free to
revolve. The plunger shown at A will follow only cam profiles which have
gradual lifts. The plunger shown at B is left free to revolve in the guide
bushing and is provided with a flat mushroom head which serves as a cam
follower. The type shown at C carries a roller at its lower end and may
follow very irregular cam profiles if abrupt lifts are desired. While forms A
and B are the simplest, that outlined at C in its various forms is more widely
used. Compound plungers are used on the Curtiss OX-2 motors, one inside
the other. The small or inner one works on a cam of conventional design,
the outer plunger follows a profile having a flat spot to permit of a pull rod
action instead of a push rod action. All the methods in which levers are used
to operate valves are more or less noisy because clearance must be left
between the valve stem and the stop of the plunger. The space must be
taken up before the valve will leave its seat, and when the engine is
operated at high speeds the forcible contact between the plunger and valve
stem produces a rattling sound until the valves become heated and expand
and the stems lengthen out. Clearance must be left between the valve stems
and actuating means. This clearance is clearly shown in Fig. 103 and should
be .020′′ (twenty thousandths) when engine is cold. The amount of
clearance allowed depends entirely upon the design of the engine and length
of valve stem. On the Curtiss OX-2 engines the clearance is but .010′′ (ten
thousandths) because the valve stems are shorter. Too little clearance will
result in loss of power or misfiring when engine is hot. Too much clearance
will not allow the valve to open its full amount and will disturb the timing.

Fig. 103.—Diagram Showing Proper Clearance to Allow Between
Adjusting Screw and Valve Stems in Hall-Scott Aviation Engines.

METHODS OF DRIVING CAM-SHAFT

Two systems of cam-shaft operation are used. The most common of these is
by means of gearing of some form. If the cam-shaft is at right angles to the
crank-shaft it may be driven by worm, spiral, or bevel gearing. If the cam-
shaft is parallel to the crank-shaft, simple spur gear or chain connection
may be used to turn it. A typical cam-shaft for an eight-cylinder V engine is
shown at Fig. 104. It will be seen that the sixteen cams are forged integrally
with the shaft and that it is spur-gear driven. The cam-shaft drive of the
Hall-Scott motor is shown at Fig. 97.

Fig. 104.—Cam-Shaft of Thomas Airplane Motor Has Cams Forged
Integral. Note Split Cam-Shaft Bearings and Method of Gear Retention.

While gearing is more commonly used, considerable attention has been
directed of late to silent chains for cam-shaft operation. The ordinary forms
of block or roller chain have not proven successful in this application, but
the silent chain, which is in reality a link belt operating over toothed
pulleys, has demonstrated its worth. The tendency to its use is more noted
on foreign motors than those of American design. It first came to public
notice when employed on the Daimler-Knight engine for driving the small
auxiliary crank-shafts which reciprocated the sleeve valves. The advantages
cited for the application of chains are, first, silent operation, which obtains
even after the chains have worn considerably; second, in designing it is not
necessary to figure on maintaining certain absolute center distances
between the crank-shaft and cam-shaft sprockets, as would be the case if
conventional forms of gearing were used. On some forms of motor
employing gears, three and even four members are needed to turn the cam-
shaft. With a chain drive but two sprockets are necessary, the chain forming
a flexible connection which permits the driving and driven members to be
placed at any distance apart that the exigencies of the design demand. When
chains are used it is advised that some means for compensating chain slack
be provided, or the valve timing will lag when chains are worn. Many
combination drives may be worked out with chains that would not be
possible with other forms of gearing. Direct gear drive is favored at the
present time by airplane engine designers because they are the most certain
and positive means, even when a number of gears must be used as
intermediate drive members. With overhead cam-shafts, bevel gears work
out very well in practice, as in the Hall-Scott motors and others of that type.

VALVE SPRINGS

Another consideration of importance is the use of proper valve-springs, and
particular care should be taken with those, of automatic valves. The spring
must be weak enough to allow the valve to open when the suction is light,
and must be of sufficient strength to close it in time at high speeds. It should
be made as large as possible in diameter and with a large number of
convolutions, in order that fatigue of the metal be obviated, and it is

imperative that all springs be of the same strength when used on a multiple-
cylinder engine. Practically all valves used to control the gas flow in
airplane engines are mechanically operated. On the exhaust valve the spring
must be strong enough so that the valve will not be sucked in on the inlet
stroke. It should be borne in mind that if the spring is too strong a strain will
be imposed on the valve-operating mechanism, and a hammering action
produced which may cause deformation of the valve-seat. Only pressure
enough to insure that the operating mechanism will follow the cam is
required. It is common practice to make the inlet and exhaust valve springs
of the same tension when the valves are of the same size and both
mechanically operated. This is done merely to simplify manufacture and not
because it is necessary for the inlet valve-spring to be as strong as the other.
Valve springs of the helical coil type are generally used, though torsion or
“scissors” springs and laminated or single-leaf springs are also utilized in
special applications. Two springs are used on each valve in some valve-in-
the-head types; a spring of small pitch diameter inside the regular valve-
spring and concentric with it. Its function is to keep the valve from falling
into the cylinder in event of breakage of the main spring in some cases, and
to provide a stronger return action in others.

Fig. 105.—Section Through Cylinder of Knight Motor,
Showing Important Parts of Valve Motion.

Fig. 106.—Diagrams Showing Knight Sleeve Valve Action.

KNIGHT SLIDE VALVE MOTOR

The sectional view through the cylinder at Fig. 105 shows the Knight
sliding sleeves and their actuating means very clearly. The diagrams at Fig.
106 show graphically the sleeve movements and their relation to the crank-
shaft and piston travel. The action may be summed up as follows: The inlet
port begins to open when the lower edge of the opening of the outside
sleeve which is moving down passes the top of the slot in the inner member

also moving downwardly. The inlet port is closed when the lower edge of
the slot in the inner sleeve which is moving up passes the top edge of the
port in the outer sleeve which is also moving toward the top of the cylinder.
The inlet opening extends over two hundred degrees of crank motion. The
exhaust port is uncovered slightly when the lower edge of the port in the
inner sleeve which is moving down passes the lower edge of the portion of
the cylinder head which protrudes in the cylinder. When the top of the port
in the outer sleeve traveling toward the bottom of the cylinder passes the
lower edge of the slot in the cylinder wall the exhaust passage is closed.
The exhaust opening extends over a period corresponding to about two
hundred and forty degrees of crank motion. The Knight motor has not been
applied to aircraft to the writer’s knowledge, but an eight-cylinder Vee
design that might be useful in that connection if lightened is shown at Fig.
107. The main object is to show that the Knight valve action is the only
other besides the mushroom or poppet valve that has been applied
successfully to high speed gasoline engines.

Fig. 107.—Cross Sectional View of Knight Type Eight Cylinder V
Engine.

VALVE TIMING

It is in valve timing that the greatest difference of opinion prevails among
engineers, and it is rare that one will see the same formula in different
motors. It is true that the same timing could not be used with motors of
different construction, as there are many factors which determine the
amount of lead to be given to the valves. The most important of these is the
relative size of the valve to the cylinder bore, the speed of rotation it is
desired to obtain, the fuel efficiency, the location of the valves, and other
factors too numerous to mention.

Most of the readers should be familiar with the cycle of operation of the
internal combustion motor of the four-stroke type, and it seems unnecessary
to go into detail except to present a review. The first stroke of the piston is
one in which a charge of gas is taken into the motor; the second stroke,
which is in reverse direction to the first, is a compression stroke, at the end
of which the spark takes place, exploding the charge and driving the piston
down on the third or expansion stroke, which is in the same direction as the
intake stroke, and finally, after the piston has nearly reached the end of this
stroke, another valve opens to allow the burned gases to escape, and
remains open until the piston has reached the end of the fourth stroke and is
in a position to begin the series over again. The ends of the strokes are
reached when the piston comes to a stop at either top or bottom of the
cylinder and reverses its motion. That point is known as a center, and there
are two for each cylinder, top and bottom centers, respectively.
All circles may be divided into 360 parts, each of which is known as a
degree, and, in turn, each of these degrees may be again divided into
minutes and seconds, though we need not concern ourselves with anything
less than the degree. Each stroke of the piston represents 180 degrees travel
of the crank, because two strokes represent one complete revolution of three
hundred and sixty degrees. The top and bottom centers are therefore
separated by 180 degrees. Theoretically each phase of a four-cycle engine
begins and ends at a center, though in actual practice the inertia or
movement of the gases makes it necessary to allow a lead or lag to the
valve, as the case may be. If a valve opens before a center, the distance is
called “lead”; if it closes after a center, this distance is known as “lag.” The
profile of the cams ordinarily used to open or close the valves represents a
considerable time in relation to the 180 degrees of the crank-shaft travel,
and the area of the passages through which the gases are admitted or
exhausted is quite small owing to the necessity of having to open or close
the valves at stated times; therefore, to open an adequately large passage for
the gases it is necessary to open the valves earlier and close them later than
at centers.
That advancing the opening of the exhaust valve was of value was
discovered on the early motors and is explained by the necessity of
releasing a large amount of gas, the volume of which has been greatly
raised by the heat of combustion. When the inlet valves were mechanically

operated it was found that allowing them to lag at closing enabled the
inspiration of a greater volume of gas. Disregarding the inertia or flow of
the gases, opening the exhaust at center would enable one to obtain full
value of the expanding gases the entire length of the piston stroke, and it
would not be necessary to keep the valve open after the top center, as the
reverse stroke would produce a suction effect which might draw some of
the inert charge back into the cylinder. On the other hand, giving full
consideration to the inertia of the gas, opening the valve before center is
reached will provide for quick expulsion of the gases, which have sufficient
velocity at the end of the stroke, so that if the valve is allowed to remain
open a little longer, the amount of lag varying with the opinions of the
designer, the cylinder is cleared in a more thorough manner.

BLOWING BACK

When the factor of retarded opening is considered without reckoning the
inertia of the gases, it would appear that if the valve were allowed to remain
open after center had passed, say, on the closing of the inlet, the piston,
having reversed its motion, would have the effect of expelling part of the
fresh charge through the still open valve as it passed inward at its
compression stroke. This effect is called blowing back, and is often noted
with motors where the valve settings are not absolutely correct, or where
the valve-springs or seats are defective and prevent proper closing.
This factor is not of as much import as might appear, as on closer
consideration it will be seen that the movement of the piston as the crank
reaches either end of the stroke is less per degree of angular movement than
it is when the angle of the connecting rod is greater. Then, again, a certain
length of time is required for the reversal of motion of the piston, during
which time the crank is in motion but the piston practically at a standstill. If
the valves are allowed to remain open during this period, the passage of the
gas in or out of the cylinder will be by its own momentum.

LEAD GIVEN EXHAUST VALVE

The faster a motor turns, all other things being equal, the greater the amount
of lead or advance it is necessary to give the opening of the exhaust valve.
It is self-evident truth that if the speed of a motor is doubled it travels twice
as many degrees in the time necessary to lower the pressure. As most
designers are cognizant of this fact, the valves are proportioned accordingly.
It is well to consider in this respect that the cam profile has much to do with
the manner in which the valve is opened; that is, the lift may be abrupt and
the gas allowed to escape in a body, or the opening may be gradual, the gas
issuing from the cylinder in thin streams. An analogy may be made with the
opening of any bottle which contains liquid highly carbonated. If the cork is
removed suddenly the gas escapes with a loud pop, but, on the other hand,
if the bottle is uncorked gradually, the gas escapes from the receptacle in
thin streams around the cork, and passage of the gases to the air is
accomplished without noise. While the second plan is not harsh, it is slower
than the former, as must be evident.

EXHAUST CLOSING, INLET OPENING

A point which has been much discussed by engineers is the proper relation
of the closing of the exhaust valve and the opening of the inlet.
Theoretically they should succeed each other, the exhaust closing at upper
dead center and the inlet opening immediately afterward. The reason why a
certain amount of lag is given the exhaust closing in practice is that the
piston cannot drive the gases out of the cylinder unless they are compressed
to a degree in excess of that existing in the manifold or passages, and while
toward the end of the stroke this pressure may be feeble, it is nevertheless
indispensable. At the end of the piston’s stroke, as marked by the upper
dead center, this compression still exists, no matter how little it may be, so
that if the exhaust valve is closed and the inlet opened immediately
afterward, the pressure which exists in the cylinder may retard the entrance
of the fresh gas and a certain portion of the inert gas may penetrate into the
manifold. As the piston immediately begins to aspirate, this may not be
serious, but as these gases are drawn back into the cylinder the fresh charge
will be diluted and weakened in value. If the spark-plug is in a pocket, the
points may be surrounded by this weak gas, and the explosion will not be
nearly as energetic as when the ignition spark takes place in pure mixture.

It is a well-known fact that the exhaust valve should close after dead center
and that a certain amount of lag should be given to opening of the inlet. The
lag given the closing of the exhaust valve should not be as great as that
given the closing of the inlet valve. Assuming that the excess pressure of
the exhaust will equal the depression during aspiration, the time necessary
to complete the emptying of the cylinder will be proportional to the volume
of the gas within it. At the end of the suction stroke the volume of gas
contained in the cylinder is equal to the cylindrical volume plus the space of
the combustion chamber. At the end of the exhaust stroke the volume is but
that of the dead space, and from one-third to one-fifth its volume before
compression. While it is natural to assume that this excess of burned gas
will escape faster than the fresh gas will enter the cylinder, it will be seen
that if the inlet valve were allowed to lag twenty degrees, the exhaust valve
lag need not be more than five degrees, providing that the capacity of the
combustion chamber was such that the gases occupied one-quarter of their
former volume.
It is evident that no absolute rule can be given, as back pressure will vary
with the design of the valve passages, the manifolds, and the construction of
the muffler. The more direct the opening, the sooner the valve can be closed
and the better the cylinder cleared. Ten degrees represent an appreciable
angle of the crank, and the time required for the crank to cover this angular
motion is not inconsiderable and an important quantity of the exhaust may
escape, but the piston is very close to the dead center after the distance has
been covered.
Before the inlet valve opens there should be a certain depression in the
cylinder, and considerable lag may be allowed before the depression is
appreciable. So far as the volume of fresh gas introduced during the
admission stroke is concerned, this is determined by the displacement of the
piston between the point where the inlet valve opens and the point of
closing, assuming that sufficient gas has been inspired so that an
equilibrium of pressure has been established between the interior of the
cylinder and the outer air. The point of inlet opening varies with different
motors. It would appear that a fair amount of lag would be fifteen degrees
past top center for the inlet opening, as a certain depression will exist in the
cylinder, assuming that the exhaust valve has closed five or ten degrees
after center, and at the same time the piston has not gone down far enough

on its stroke to materially decrease the amount of gas which will be taken
into the cylinder.

CLOSING THE INLET VALVE

As in the case with the other points of opening and closing, there is a wide
diversity of practice as relates to closing the inlet valve. Some of the
designers close this exactly at bottom center, but this practice cannot be
commended, as there is a considerable portion of time, at least ten or fifteen
degrees angular motion of the crank, before the piston will commence to
travel to any extent on its compression stroke. The gases rushing into the
cylinder have considerable velocity, and unless an equilibrium is obtained
between the pressure inside and that of the atmosphere outside, they will
continue to rush into the cylinder even after the piston ceases to exert any
suction effect.
For this reason, if the valve is closed exactly on center, a full charge may
not be inspired into the cylinder, though if the time of closing is delayed,
this momentum or inertia of the gas will be enough to insure that a
maximum charge is taken into the cylinder. The writer considers that
nothing will be gained if the valve is allowed to remain open longer than
twenty degrees, and an analysis of practice in this respect would seem to
confirm this opinion. From that point in the crank movement the piston
travel increases and the compressive effect is appreciable, and it would
appear that a considerable proportion of the charge might be exhausted into
the manifold and carburetor if the valve were allowed to remain open
beyond a point corresponding to twenty degrees angular movement of the
crank.

TIME OF IGNITION

In this country engineers unite in providing a variable time of ignition,
though abroad some difference of opinion is noted on this point. The
practice of advancing the time of ignition, when affected electrically, was
severely condemned by early makers, these maintaining that it was
necessary because of insufficient heat and volume of the spark, and it was

thought that advancing ignition was injurious. The engineers of to-day
appreciate the fact that the heat of the electric spark, especially when from a
mechanical generator of electrical energy, is the only means by which we
can obtain practically instantaneous explosion, as required by the operation
of motors at high speeds, and for the combustion of large volumes of gas.

Fig. 108.—Diagrams Explaining Valve and Ignition Timing
of Hall-Scott Aviation Engine.

It is apparent that a motor with a fixed point of ignition is not as desirable,
in every way, as one in which the ignition can be advanced to best meet
different requirements, and the writer does not readily perceive any

advantage outside of simplicity of control in establishing a fixed point of
ignition. In fact, there seems to be some difference of opinion among those
designers who favor fixed ignition, and in one case this is located forty-
three degrees ahead of center, and in another motor the point is fixed at
twenty degrees, so that it may be said that this will vary as much as one
hundred per cent. in various forms. This point will vary with different
methods of ignition, as well as the location of the spark-plug or igniter. For
the sake of simplicity, most airplane engines use set spark; if an advancing
and retarding mechanism is fitted, it is only to facilitate starting, as the
spark is kept advanced while in flight, and control is by throttle alone.

Fig. 109.—Timing Diagram of Typical Six-Cylinder
Engine.

It is obvious by consideration of the foregoing that there can be no arbitrary
rules established for timing, because of the many conditions which
determine the best times for opening and closing the valves. It is customary
to try various settings when a new motor is designed until the most
satisfactory points are determined, and the setting which will be very
suitable for one motor is not always right for one of different design. The

timing diagram shown at Fig. 108 applies to the Hall-Scott engine, and may
be considered typical. It should be easily followed in view of the very
complete explanation given in preceding pages. Another six-cylinder engine
diagram is shown at Fig. 109, and an eight-cylinder timing diagram is
shown at Fig. 110. In timing automobile engines no trouble is experienced,
because timing marks are always indicated on the engine fly-wheel register
with an indicating trammel on the crank-case. To time an airplane engine
accurately, as is necessary to test for a suspected cam-shaft defect, a timing
disc of aluminum is attached to the crank-shaft which has the timing marks
indicated thereon. If the disc is made 10 or 12 inches in diameter, it may be
divided into degrees without difficulty.

Fig. 110.—Timing Diagram of Typical Eight-Cylinder V
Engine.

HOW AN ENGINE IS TIMED

In timing a motor from the marks on the timing disc rim it is necessary to
regulate the valves of but one cylinder at a time. Assuming that the disc is
revolving in the direction of engine rotation, and that the firing order of the
cylinders is 1-3-4-2, the operation of timing would be carried on as follows:
The crank-shaft would be revolved until the line marked “Exhaust opens 1
and 4” registered with the trammel on the motor bed. At this point the
exhaust-valve of either cylinder No. 1 or No. 4 should begin to open. This
can be easily determined by noting which of these cylinders holds the
compressed charge ready for ignition. Assuming that the spark has occurred
in cylinder No. 1, then when the fly-wheel is turned from the position to
that in which the line marked “Exhaust opens 1 and 4” coincides with the
trammel point, the valve-plunger under the exhaust-valve of cylinder No. 1
should be adjusted in such a way that there is no clearance between it and
the valve stem. Further movement of the wheel in the same direction should
produce a lift of the exhaust valve. The disc is turned about two hundred
and twenty-five degrees, or a little less than three-quarters of a revolution;
then the line marked “Exhaust closes 1 and 4” will register with the
trammel point. At this period the valve-plunger and the valve-stem should
separate and a certain amount of clearance obtain between them. The next
cylinder to time would be No. 3. The crank-shaft is rotated until mark
“Exhaust opens 2 and 3” comes in line with the trammel. At this point the
exhaust valve of cylinder No. 3 should be just about opening. The closing is
determined by rotating the shaft until the line “Exhaust closes 2 and 3”
comes under the trammel.
This operation is carried on with all the cylinders, it being well to remember
that but one cylinder is working at a time and that a half-revolution of the
fly-wheel corresponds to a full working stroke of all the cylinders, and that
while one is exhausting the others are respectively taking in a new charge,
compressing and exploding. For instance, if cylinder No. 1 has just
completed its power-stroke, the piston in cylinder No. 3 has reached the
point where the gas may be ignited to advantage. The piston of cylinder No.
4, which is next to fire, is at the bottom of its stroke and will have inspired a
charge, while cylinder No. 2, which is the last to fire, will have just finished
expelling a charge of burned gas, and will be starting the intake stroke. This
timing relates to a four-cylinder engine in order to simplify the explanation.
The timing instructions given apply only to the conventional motor types.

Rotary cylinder engines, especially the Gnome “monosoupape,” have a
distinctive valve timing on account of the peculiarities of design.

GNOME “MONOSOUPAPE” VALVE TIMING

In the present design of the Gnome motor, a cycle of operations somewhat
different from that employed in the ordinary four-cycle engine is made use
of, says a writer in “The Automobile,” in describing the action of this
power-plant. This cycle does away with the need for the usual inlet valve
and makes the engine operable with only a single valve, hence the name
monosoupape, or “single-valve.” The cycle is as follows: A charge being
compressed in the outer end of the cylinder or combustion chamber, it is
ignited by a spark produced by the spark-plug located in the side of this
chamber, and the burning charge expands as the piston moves down in the
cylinder while the latter revolves around the crank-shaft. When the piston is
about half-way down on the power stroke, the exhaust valve, which is
located in the center of the cylinder-head, is mechanically opened, and
during the following upstroke of the piston the burnt gases are expelled
from the cylinder through the exhaust valve directly into the atmosphere.
Instead of closing at the end of the exhaust stroke, or a few degrees
thereafter, the exhaust valve is held open for about two-thirds of the
following inlet stroke of the piston, with the result that fresh air is drawn
through the exhaust valve into the cylinder. When the cylinder is still 65
degrees from the end of the inlet half-revolution, the exhaust valve closes.
As no more air can get into the cylinder, and as the piston continues to
move inwardly, it is obvious that a partial vacuum is formed.
When the cylinder approaches within 20 degrees of the end of the inlet half-
revolution a series of small inlet ports all around the circumference of the
cylinder wall is uncovered by the top edge of the piston, whereby the
combustion chamber is placed in communication with the crank chamber.
As the pressure in the crank chamber is substantially atmospheric and that
in the combustion chamber is below atmospheric, there results a suction
effect which causes the air from the crank chamber to flow into the
combustion chamber. The air in the crank chamber is heavily charged with
gasoline vapor, which is due to the fact that a spray nozzle connected with

the gasoline supply tank is located inside the chamber. The proportion of
gasoline vapor in the air in the crank chamber is several times as great as in
the ordinary combustible mixture drawn from a carburetor into the cylinder.
This extra-rich mixture is diluted in the combustion chamber with the air
which entered it through the exhaust valve during the first part of the inlet
stroke, thus forming a mixture of the proper proportion for complete
combustion.
The inlet ports in the cylinder wall remain open until 20 degrees of the
compression half-revolution has been completed, and from that moment to
near the end of the compression stroke the gases are compressed in the
cylinder. Near the end of the stroke ignition takes place and this completes
the cycle.

Fig. 111.—Timing Diagram Showing Peculiar Valve
Timing of Gnome “Monosoupape” Rotary Motor.

The exact timing of the different phases of the cycle is shown in the
diagram at Fig. 111. It will be seen that ignition occurs substantially 20
degrees ahead of the outer dead center, and expansion of the burning gases

continues until 85 degrees past the outer dead center, when the piston is a
little past half-stroke. Then the exhaust-valve opens and remains open for
somewhat more than a complete revolution of the cylinders, or, to be exact,
for 390 degrees of cylinder travel, until 115 degrees past the top dead center
on the second revolution. Then for 45 degrees of travel the charge within
the cylinder is expanded, whereupon the inlet ports are uncovered and
remain open for 40 degrees of cylinder travel, 20 degrees on each side of
the inward dead center position.

SPRINGLESS VALVES

Springless valves are the latest development on French racing car engines,
and it is possible that the positively-operated types will be introduced on
aviation engines also. Two makes of positively-actuated valves are shown at
Fig. 112. The positive-valve motor differs from the conventional form by
having no necessity for valve-springs, as a cam not only assures the opening
of the valve, but also causes it to return to the valve-seat. In this respect it is
much like the sleeve-valve motor, where the uncovering of the ports is
absolutely positive. The cars equipped with these valves were a success in
long-distance auto races. Claims made for this type of valve mechanism
include the possibility of a higher number of revolutions and consequently
greater engine power. With the spring-controlled, single-cam operated valve
a point is reached where the spring is not capable of returning the valve to
its seat before the cam has again begun its opening movement. It is possible
to extend the limits considerably by using a light valve on a strong spring,
but the valve still remains a limiting factor in the speed of the motor.

Fig. 112.—Two Methods of Operating Valves by Positive Cam
Mechanism Which Closes as Well as Opens Them.

A part sectional view through a cylinder of an engine designed by G.
Michaux is shown at Fig. 112, A. There are two valves per cylinder,
inclined at about ten degrees from the vertical. The valve-stems are of large
diameter, as owing to positive control, there is no necessity of lightening
this part in an unusual degree. A single overhead cam-shaft has eight pairs
of cams, which are shown in detail at B. For each valve there is a three-
armed rocker, one arm of which is connected to the stem of the valve and
the two others are in contact respectively with the opening and closing
cams. The connection to the end of the valve-stem is made by a short
connecting link, which is screwed on to the end of the valve-stem and
locked in position. This allows some adjustment to be made between the
valves and the actuating rocker. It will be evident that one cam and one
rocker arm produce the opening of the valve and that the corresponding
rocker arm and cam result in the closing of the valve. If the opening cam
has the usual convex profile, the closing cam has a correspondingly
concave profile. It will be noticed that a light valve-spring is shown in

drawing. This is provided to give a final seating to its valve after it has been
closed by the cam. This is not absolutely necessary, as an engine has been
run successfully without these springs. The whole mechanism is contained
within an overhead aluminum cover.
The positive-valve system used on the De Lage motor is shown at D. In this
the valves are actuated as shown in sectional views D and E. The valve
system is unique in that four valves are provided per cylinder, two for
exhaust and two for intake. The valves are mounted side by side, as shown
at E, so the double actuator member may be operated by a single set of
cams. The valve-operating member consists of a yoke having guide bars at
the top and bottom. The actuating cam works inside of this yoke. The usual
form of cam acts on the lower portion of the yoke to open the valve, while
the concave cam acts on the upper part to close the valves. In this design
provision is made for expansion of the valve-stems due to heat, and these
are not positively connected to the actuating member. As shown at E, the
valves are held against the seat by short coil springs at the upper end of the
stem. These are very stiff and are only intended to provide for expansion. A
slight space is left between the top of the valve-stem and the portion of the
operating member that bears against them when the regular profile cam
exerts its pressure on the bottom of the valve-operating mechanism.
Another novelty in this motor design is that the cam-shafts and the valve-
operating members are carried in casing attached above the motor by
housing supports in the form of small steel pillars. The overhead cam-shafts
are operated by means of bevel gearing.

FOUR VALVES PER CYLINDER

Fig. 113.—Diagram Comparing Two Large Valves and Four
Small Ones of Practically the Same Area. Note How Easily
Small Valves are Installed to Open Directly Into the
Cylinder.

Mention has been previously made of the sixteen-valve four-cylinder
Duesenberg motor and its great power output for the piston displacement.
This is made possible by the superior volumetric efficiency of a motor
provided with four valves in each cylinder instead of but two. This principle
was thoroughly tried out in racing automobile motors, and is especially
valuable in permitting of greater speed and power output from simple four-
and six-cylinder engines. On eight- and twelve-cylinder types, it is doubtful
if the resulting complication due to using a very large number of valves
would be worth while. When extremely large valves are used, as shown in
diagram at Fig. 113, it is difficult to have them open directly into the
cylinder, and pockets are sometimes necessary. A large valve would weigh
more than two smaller valves having an area slightly larger in the
aggregate; it would require a stiffer valve spring on account of its greater
weight. A certain amount of metal in the valve-head is necessary to prevent
warping; therefore, the inertia forces will be greater in the large valve than
in the two smaller valves. As a greater port area is obtained by the use of
two valves, the gases will be drawn into the cylinder or expelled faster than

with a lesser area. Even if the areas are practically the same as in the
diagram at Fig. 113, the smaller valves may have a greater lift without
imposing greater stresses on the valve-operating mechanism and quicker
gas intake and exhaust obtained. The smaller valves are not affected by heat
as much as larger ones are. The quicker gas movements made possible, as
well as reduction of inertia forces, permits of higher rotative speed, and,
consequently, greater power output for a given piston displacement. The
drawings at Fig. 114 show a sixteen-valve motor of the four-cylinder type
that has been designed for automobile racing purposes, and it is apparent
that very slight modifications would make it suitable for aviation purposes.
Part of the efficiency is due to the reduction of bearing friction by the use of
ball bearings, but the multiple-valve feature is primarily responsible for the
excellent performance.

Fig. 114.—Sectional Views of Sixteen-Valve Four-Cylinder Automobile Racing
Engine That May Have Possibilities for Aviation Service.

Fig. 115.—Front View of Curtiss OX-3 Aviation Motor,
Showing Unconventional Valve Action by Concentric Push
Rod and Pull Tube.

CHAPTER IX
Constructional Details of Pistons—Aluminum Cylinders and
Pistons—Piston Ring Construction—Leak Proof Piston Rings—
Keeping Oil Out of Combustion Chamber—Connecting Rod Forms
—Connecting Rods for Vee Engines—Cam-Shaft and Crank-Shaft
Designs—Ball Bearing Crank-Shafts—Engine Base Construction.

CONSTRUCTIONAL DETAILS OF PISTONS

The piston is one of the most important parts of the gasoline motor
inasmuch as it is the reciprocating member that receives the impact of the
explosion and which transforms the power obtained by the combustion of
gas to mechanical motion by means of the connecting rod to which it is
attached. The piston is one of the simplest elements of the motor, and it is
one component which does not vary much in form in different types of
motors. The piston is a cylindrical member provided with a series of
grooves in which packing rings are placed on the outside and two bosses
which serve to hold the wrist pin in its interior. It is usually made of cast
iron or aluminum, though in some motors where extreme lightness is
desired, such as those used for aëronautic work, it may be made of steel.
The use of the more resisting material enables the engineer to use lighter
sections where it is important that the weight of this member be kept as low
as possible consistent with strength.

Fig. 116.—Forms of Pistons Commonly Employed in
Gasoline Engines. A—Dome Head Piston and Three Packing
Rings. B—Flat Top Form Almost Universally Used. C—
Concave Piston Utilized in Knight Motors and Some Having
Overhead Valves. D—Two-Cycle Engine Member with
Deflector Plate Cast Integrally. E—Differential of Two-
Diameter Piston Used in Some Engines Operating on Two-
Cycle Principle.

A number of piston types are shown at Fig. 116. That at A has a round top
and is provided with four split packing rings and two oil grooves. A piston
of this type is generally employed in motors where the combustion chamber
is large and where it is desired to obtain a higher degree of compression
than would be possible with a flat top piston. This construction is also
stronger because of the arched piston top. The most common form of piston
is that shown at B, and it differs from that previously described only in that
it has a flat top. The piston outlined in section at C is a type used on some
of the sleeve-valve motors of the Knight pattern, and has a concave head
instead of the convex form shown at A. The design shown at D in side and
plan views is the conventional form employed in two-cycle engines. The
deflector plate on the top of the cylinder is cast integral and is utilized to
prevent the incoming fresh gases from flowing directly over the piston top
and out of the exhaust port, which is usually opposite the inlet opening. On

these types of two-cycle engines where a two-diameter cylinder is
employed, the piston shown at E is used. This is known as a “differential
piston,” and has an enlarged portion at its lower end which fits the pumping
cylinder. The usual form of deflector plate is provided at the top of the
piston and one may consider it as two pistons in one.

Fig. 117.—Typical Methods of Piston Pin Retention Generally Used in Engines of
American Design. A—Single Set Screw and Lock Nut. B—Set Screw and Check Nut
Fitting Groove in Wrist Pin. C, D—Two Locking Screws Passing Into Interior of
Hollow Wrist Pin. E—Split Ring Holds Pin in Place. F—Use of Taper Expanding
Plugs Outlined. G—Spring Pressed Plunger Type. H—Piston Pin Pinned to
Connecting Rod. I—Wrist Pin Clamped in Connecting Rod Small End by Bolt.

Fig. 118.—Typical Piston and Connecting Rod Assembly.

Fig. 119.—Parts of Sturtevant Aviation Engine. A—
Cylinder Head Showing Valves. B—Connecting Rod.
C—Piston and Rings.

One of the important conditions in piston design is the method of securing
the wrist pin which is used to connect the piston to the upper end of the
connecting rod. Various methods have been devised to keep the pin in place,
the most common of these being shown at Fig. 117. The wrist pin should be
retained by some positive means which is not liable to become loose under
the vibratory stresses which obtain at this point. If the wrist pin was free to
move it would work out of the bosses enough so that the end would bear
against the cylinder wall. As it is usually made of steel, which is a harder
material than cast iron used in cylinder construction, the rubbing action
would tend to cut a groove in the cylinder wall which would make for loss
of power because it would permit escape of gas. The wrist pin member is a

simple cylindrical element that fits the bosses closely, and it may be either
hollow or solid stock. A typical piston and connecting rod assembly which
shows a piston in section also is given at Fig. 118. The piston of the
Sturtevant aëronautical motor is shown at Fig. 119, the aluminum piston of
the Thomas airplane motor with piston rings in place is shown at Fig. 120.
A good view of the wrist pin and connecting rod are also given. The iron
piston of the Gnome “Monosoupape” airplane engine and the
unconventional connecting rod assembly are clearly depicted at Fig 121.

Fig. 120.—Aluminum Piston and Light But Strong Steel Connecting Rod and Wrist
Pin of Thomas Aviation Engine.

The method of retention shown at A is the simplest and consists of a set
screw having a projecting portion passing into the wrist pin and holding it
in place. The screw is kept from turning or loosening by means of a check
nut. The method outlined at B is similar to that shown at A, except that the
wrist pin is solid and the point of the set screw engages an annular groove
turned in the pin for its reception. A very positive method is shown at C.
Here the retention screws pass into the wrist pin and are then locked by a
piece of steel wire which passes through suitable holes in the ends. The
method outlined at D is sometimes employed, and it varies from that shown

at C only in that the locking wire, which is made of spring steel, is passed
through the heads of the locking screws. Some designers machine a large
groove around the piston at such a point that when the wrist pin is put in
place a large packing ring may be sprung in the groove and utilized to hold
the wrist pin in place.

Fig. 121.—Cast Iron Piston of “Monosoupape” Gnome Engine Installed On One of
the Short Connecting Rods.

The system shown at F is not so widely used as the simpler methods,
because it is more costly and does not offer any greater security when the
parts are new than the simple lock shown at A. In this a hollow wrist pin is
used, having a tapered thread cut at each end. The wrist pin is slotted at
three or four points, for a distance equal to the length of the boss, and when
taper expansion plugs are screwed in place the ends of the wrist pin are
expanded against the bosses. This method has the advantage of providing a
certain degree of adjustment if the wrist pin should loosen up after it has
been in use for some time. The taper plugs would be screwed in deeper and
the ends of the wrist pin expanded proportionately to take up the loss
motion. The method shown at G is an ingenious one. One of the piston
bosses is provided with a projection which is drilled out to receive a
plunger. The wrist pin is provided with a hole of sufficient size to receive
the plunger, which is kept in place by means of a spring in back of it. This
makes a very positive lock and one that can be easily loosened when it is
desired to remove the wrist pin. To unlock, a piece of fine rod is thrust into

the hole at the bottom of the boss which pushes the plunger back against the
spring until the wrist pin can be pushed out of the piston.
Some engineers think it advisable to oscillate the wrist pin in the piston
bosses, instead of in the connecting rod small end. It is argued that this
construction gives more bearing surface at the wrist pin and also provides
for more strength because of the longer bosses that can be used. When this
system is followed the piston pin is held in place by locking it to the
connecting rod by some means. At H the simplest method is outlined. This
consisted of driving a taper pin through both rod and wrist pin and then
preventing it from backing out by putting a split cotter through the small
end of the tapered locking pin. Another method, which is depicted at I,
consists of clamping the wrist pin by means of a suitable bolt which brings
the slit connecting rod end together as shown.

ALUMINUM FOR CYLINDERS AND PISTONS

Aluminum pistons outlined at Fig. 122, have replaced cast iron members in
many airplane engines, as these weigh about one-third as much as the cast
iron forms of the same size, while the reduction in the inertia forces has
made it possible to increase the engine speed without correspondingly
stressing the connecting rods, crank-shaft and engine bearings.

Fig. 122.—Types of Aluminum Pistons Used In Aviation
Engines.

Aluminum has not only been used for pistons, but a number of motors will
be built for the coming season that will use aluminum cylinder block
castings as well. Of course, the aluminum alloy is too soft to be used as a
bearing for the piston, and it will not withstand the hammering action of the
valve. This makes the use of cast iron or steel imperative in all motors.
When used in connection with an aluminum cylinder block the cast iron
pieces are placed in the mould so that they act as cylinder liners and valve
seats, and the molten metal is poured around them when the cylinder is cast.
It is said that this construction results in an intimate bond between the cast
iron and the surrounding aluminum metal. Steel liners may also be pressed
into the aluminum cylinders after these are bored out to receive them.
Aluminum has for a number of years been used in many motor car parts.
Alloys have been developed that have greater strength than cast iron and
that are not so brittle. Its use for manifolds and engine crank and gear cases
has been general for a number of years.
At first thought it would seem as though aluminum would be entirely
unsuited for use in those portions of internal combustion engines exposed to
the heat of the explosion, on account of the low melting point of that metal
and its disadvantageous quality of suddenly “wilting” when a critical point

in the temperature is reached. Those who hesitated to use aluminum on
account of this defect lost sight of the great heat conductivity of that metal,
which is considerably more than that of cast iron. It was found in early
experiments with aluminum pistons that this quality of quick radiation
meant that aluminum pistons remained considerably cooler than cast iron
ones in service, which was attested to by the reduced formation of carbon
deposit thereon. The use of aluminum makes possible a marked reduction in
power plant weight. A small four-cylinder engine which was not
particularly heavy even with cast iron cylinders was found to weigh 100
pounds less when the cylinder block, pistons, and upper half of the crank-
case had been made of aluminum instead of cast iron. Aluminum motors are
no longer an experiment, as a considerable number of these have been in
use on cars during the past year without the owners of the cars being
apprised of the fact. Absolutely no complaint was made in any case of the
aluminum motor and it was demonstrated, in addition to the saving in
weight, that the motors cost no more to assemble and cooled much more
efficiently than the cast iron form. One of the drawbacks to the use of
aluminum is its growing scarcity, which results in making it a “near
precious” metal.

PISTON RING CONSTRUCTION

As all pistons must be free to move up and down in the cylinder with
minimum friction, they must be less in diameter than the bore of the
cylinder. The amount of freedom or clearance provided varies with the
construction of the engine and the material the piston is made of, as well as
its size, but it is usual to provide from .005 to .010 of an inch to compensate
for the expansion of the piston due to heat and also to leave sufficient
clearance for the introduction of lubricant between the working surfaces.
Obviously, if the piston were not provided with packing rings, this amount
of clearance would enable a portion of the gases evolved when the charge is
exploded to escape by it into the engine crank-case. The packing members
or piston rings, as they are called, are split rings of cast iron, which are
sprung into suitable grooves machined on the exterior of the piston, three or
four of these being the usual number supplied. These have sufficient
elasticity so that they bear tightly against the cylinder wall and thus make a

gas-tight joint. Owing to the limited amount of surface in contact with the
cylinder wall and the elasticity of the split rings the amount of friction
resulting from the contact of properly fitted rings and the cylinder is not of
enough moment to cause any damage and the piston is free to slide up and
down in the cylinder bore.

Fig. 123.—Types of Piston Rings and Ring Joints. A—
Concentric Ring. B—Eccentrically Machined Form. C—Lap
Joint Ring. D—Butt Joint, Seldom Used. E—Diagonal Cut
Member, a Popular Form.

These rings are made in two forms, as outlined at Fig. 123. The design
shown at A is termed a “concentric ring,” because the inner circle is
concentric with the outer one and the ring is of uniform thickness at all
points. The ring shown at B is called an “eccentric ring,” and it is thicker at
one part than at others. It has theoretical advantages in that it will make a
tighter joint than the other form, as it is claimed its expansion due to heat is
more uniform. The piston rings must be split in order that they may be
sprung in place in the grooves, and also to insure that they will have
sufficient elasticity to take the form of the cylinder at the different points in
their travel. If the cylinder bore varies by small amounts the rings will
spring out at the points where the bore is larger than standard, and spring in
at those portions where it is smaller than standard.
It is important that the joint should be as nearly gas-tight as possible,
because if it were not a portion of the gases would escape through the slots
in the piston rings. The joint shown at C is termed a “lap joint,” because the
ends of the ring are cut in such a manner that they overlap. This is the
approved joint. The butt joint shown at D is seldom used and is a very poor
form, the only advantage being its cheapness. The diagonal cut shown at E
is a compromise between the very good form shown at C and the poor joint

depicted at D. It is also widely used, though most constructors prefer the lap
joint, because it does not permit the leakage of gas as much as the other two
types.
There seems to be some difference of opinion relative to the best piston ring
type—some favoring the eccentric pattern, others the concentric form. The
concentric ring has advantages from the lubricating engineer’s point of
view; as stated by the Platt & Washburn Company in their text-book on
engine lubrication, the smaller clearance behind the ring possible with the
ring of uniform section is advantageous.
Fig. 124, A, shows a concentric piston ring in its groove. Since the ring
itself is concentric with the groove, very small clearance between the back
of the ring and the bottom of its groove may be allowed. Small clearance
leaves less space for the accumulation of oil and carbon deposits. The
gasket effect of this ring is uniform throughout the entire length of its edges,
which is its marked advantage over the eccentric ring. This type of piston
ring rarely burns fast in its groove. There are a large number of different
concentric rings manufactured of different designs and of different
efficiency.

Fig. 124.—Diagrams Showing Advantages of Concentric Piston
Rings.

Figs. 124, B and 124, C show eccentric rings assembled in the ring groove.
It will be noted that there is a large space between the thin ends of this ring
and the bottom of the groove. This empty space fills up with oil which in

the case of the upper ring frequently is carbonized, restricting the action of
the ring and nullifying its usefulness. The edges of the thin ends are not
sufficiently wide to prevent rapid escape of gases past them. In a practical
way this leakage means loss of compression and noticeable drop in power.
When new and properly fitted, very little difference can be noted between
the tightness of eccentric and concentric rings. Nevertheless, after several
months’ use, a more rapid leakage will always occur past the eccentric than
past the concentric. If continuous trouble with the carbonization of
cylinders, smoking and sooting of spark-plugs is experienced, it is a sure
indication that mechanical defects exist in the engine, assuming of course,
that a suitable oil has been used. Such trouble can be greatly lessened, if not
entirely eliminated, by the application of concentric rings (lap joint), of any
good make, properly fitted into the grooves of the piston. Too much
emphasis cannot be put upon this point. If the oil used in the engine is of the
correct viscosity, and serious carbon deposit, smoking, etc., still result, the
only certain remedy then is to have the cylinders rebored and fitted with
properly designed, oversized pistons and piston rings.

LEAK-PROOF PISTON RINGS

In order to reduce the compression loss and leakage of gas by the ordinary
simple form of diagonal or lap joint one-piece piston ring a number of
compound rings have been devised and are offered by their makers to use in
making replacements. The leading forms are shown at Fig. 125. That shown
at A is known as the “Statite” and consists of three rings, one carried inside
while the other two are carried on the outside. The ring shown at B is a
double ring and is known as the McCadden. This is composed of two thin
concentric lap joint rings so disposed relative to each other that the opening
in the inner ring comes opposite to the opening in the outer ring.

Fig. 125.—Leak-Proof and Other Compound Piston Rings.

The form shown at C is known as the “Leektite,” and is a single ring
provided with a peculiar form of lap and dove tail joint. The ring shown at
D is known as the “Dunham” and is of the double concentric type being
composed of two rings with lap joints which are welded together at a point
opposite the joint so that there is no passage by which the gas can escape.
The Burd high compression ring is shown at E. The joints of these rings are
sealed by means of an H-shaped coupler of bronze which closes the
opening. The ring ends are made with tongues which interlock with the
coupling. The ring shown at F is called the “Evertite” and is a three-piece
ring composed of three members as shown in the sectional view below the
ring. The main part or inner ring has a circumferential channel in which the
two outer rings lock, the resulting cross-section being rectangular just the
same as that of a regular pattern ring. All three rings are diagonally split and
the joints are spaced equally and the distances maintained by small pins.
This results in each joint being sealed by the solid portion of the other rings.
The use of a number of light steel rings instead of one wide ring in the
groove is found on a number of automobile power plants, but as far as
known, this construction is not used in airplane power plants. It is
contended that where a number of light rings is employed a more flexible

packing means is obtained and the possibility of leakage is reduced. Rings
of this design are made of square section steel wire and are given a spring
temper. Owing to the limited width the diagonal cut joint is generally
employed instead of the lap joint which is so popular on wider rings.

KEEPING OIL OUT OF COMBUSTION CHAMBERS

An examination of the engine design that is economical in oil consumption
discloses the use of tight piston rings, large centrifugal rings on the crank-
shaft where it passes through the case, ample cooling fins in the pistons,
vents between the crank-case chamber and the valve enclosures, etc. Briefly
put, cooling of the oil in this engine has been properly cared for and leakage
reduced to a minimum. To be specific regarding details of design: Oil
surplus can be kept out of the explosion chambers by leaving the lower
edge of the piston skirt sharp and by the use of a shallow groove (C), Fig.
126, just below the lower piston ring. Small holes are bored through the
piston walls at the base of this groove and communicate with the crank-
case. The similarity of the sharp edges of piston skirt (D) and piston ring to
a carpenter’s plane bit, makes their operation plain.

Fig. 126.—Sectional View of Engine Showing Means
of Preventing Oil Leakage By Piston Rings.

The cooling of oil in the sump (A) can be accomplished most effectively by
radiating fins on its outer surface. The lower crank-case should be fully
exposed to the outer air. A settling basin for sediment (B) should be
provided having a cubic content not less than one-tenth of the total oil
capacity as outlined at Fig. 126. The depth of this basin should be at least
21⁄2 inches, and its walls vertical, as shown, to reduce the mixing of
sediment with the oil in circulation. The inlet opening to the oil pump
should be near the top of the sediment basin in order to prevent the entrance
into the pump with the oil of any solid matter or water condensed from the
products of combustion. This sediment basin should be drained after every
five to seven hours air service of an airplane engine. Concerning filtering
screens there is little to be said, save that their areas should be ample and
the mesh coarse enough (one-sixteenth of an inch) to offer no serious
resistance to the free flow of cold or heavy oil through them; otherwise the

oil in the crank-case may build up above them to an undesirable level. The
necessary frequency of draining and flushing out the oil sump differs
greatly with the age (condition) of the engine and the suitability of the oil
used. In broad terms, the oil sump of a new engine should be thoroughly
drained and flushed with kerosene at the end of the first 200 miles, next at
the end of 500 miles and thereafter every 1,000 miles. While these
instructions apply specifically to automobile motors, it is very good practice
to change the oil in airplane engines frequently. In many cases, the best
results have been secured when the oil supply is completely replenished
every five hours that the engine is in operation.

CONNECTING ROD FORMS

The connecting rod is the simple member that joins the piston to the crank-
shaft and which transmits the power imparted to the piston by the explosion
so that it may be usefully applied. It transforms the reciprocating movement
of the piston to a rotary motion at the crank-shaft. A typical connecting rod
and its wrist pin are shown at Fig. 120. It will be seen that it has two
bearings, one at either end. The small end is bored out to receive the wrist
pin which joins it to the piston, while the large end has a hole of sufficient
size to go on the crank-pin. The airplane and automobile engine connecting
rod is invariably a steel forging, though in marine engines it is sometimes
made a steel or high tensile strength bronze casting. In all cases it is
desirable to have softer metals than the crank-shaft and wrist pin at the
bearing point, and for this reason the connecting rod is usually provided
with bushings of anti-friction or white metal at the lower end, and bronze at
the upper. The upper end of the connecting rod may be one piece, because
the wrist pin can be introduced after it is in place between the bosses of the
piston. The lower bearing must be made in two parts in most cases, because
the crank-shaft cannot be passed through the bearing owing to its irregular
form. The rods of the Gnome engine are all one piece types, as shown at
Fig. 127, owing to the construction of the “mother” rod which receives the
crank-pins. The complete connecting rod assembly is shown in Fig. 121,
also at A, Fig. 127. The “mother” rod, with one of the other rods in place
and one about to be inserted, is shown at Fig. 127, B. The built-up crank-
shaft which makes this construction feasible is shown at Fig. 127, C.

Fig. 127.—Connecting Rod and Crank-Shaft Construction
of Gnome “Monosoupape” Engine.

Some of the various designs of connecting rods that have been used are
shown at Fig. 128. That at A is a simple form often employed in single-
cylinder motors, having built-up crank-shafts. Both ends of the connecting
rod are bushed with a one-piece bearing, as it can be assembled in place
before the crank-shaft assembly is built up. A built-up crank-shaft such as
this type of connecting rod would be used with is shown at Fig. 106. The
pattern shown at B is one that has been used to some extent on heavy work,
and is known as the “marine type.” It is made in three pieces, the main
portion being a steel forging having a flanged lower end to which the
bronze boxes are secured by bolts. The modified marine type depicted at C
is the form that has received the widest application in automobile and
aviation engine construction. It consists of two pieces, the main member
being a steel drop forging having the wrist-pin bearing and the upper crank-
pin bearing formed integral, while the lower crank-pin bearing member is a
separate forging secured to the connecting rod by bolts. In this construction
bushings of anti-friction metal are used at the lower end, and a bronze
bushing is forced into the upper- or wrist-pin end. The rod shown at D has
also been widely used. It is similar in construction to the form shown at C,
except that the upper end is split in order to permit of a degree of
adjustment of the wrist-pin bushing, and the lower bearing cap is a hinged

member which is retained by one bolt instead of two. When it is desired to
assemble it on the crank-shaft the lower cap is swung to one side and
brought back into place when the connecting rod has been properly located.
Sometimes the lower bearing member is split diagonally instead of
horizontally, such a construction being outlined at E.

Fig. 128.—Connecting Rod Types Summarized. A—Single Connecting Rod
Made in One Piece, Usually Fitted in Small Single-Cylinder Engines Having
Built-Up Crank-Shafts. B—Marine Type, a Popular Form on Heavy
Engines. C—Conventional Automobile Type, a Modified Marine Form. D—
Type Having Hinged Lower Cap and Split Wrist Pin Bushing. E—
Connecting Rod Having Diagonally Divided Big End. F—Ball-Bearing Rod.
G—Sections Showing Structural Shapes Commonly Employed in
Connecting Rod Construction.

In a number of instances, instead of plain bushed bearings anti-friction
forms using ball or rollers have been used at the lower end. A ball-bearing
connecting rod is shown at F. The big end may be made in one piece,
because if it is possible to get the ball bearing on the crank-pins it will be
easy to put the connecting rod in place. Ball bearings are not used very
often on connecting rod big ends because of difficulty of installation,
though when applied properly they give satisfactory service and reduce
friction to a minimum. One of the advantages of the ball bearing is that it

requires no adjustment, whereas the plain bushings depicted in the other
connecting rods must be taken up from time to time to compensate for wear.
This can be done in forms shown at B, C, D, and E by bringing the lower
bearing caps closer to the upper one and scraping out the brasses to fit the
shaft. A number of liners or shims of thin brass or copper stock, varying
from .002 inch to .005 inch, are sometimes interposed between the halves of
the bearings when it is first fitted to the crank-pin. As the brasses wear the
shims may be removed and the portions of the bearings brought close
enough together to take up any lost motion that may exist, though in some
motors no shims are provided and depreciation can be remedied only by
installing new brasses and scraping to fit.

Fig. 129.—Double Connecting Rod Assembly For Use On
Single Crank-Pin of Vee Engine.

The various structural shapes in which connecting rods are formed are
shown in section at G. Of these the I section is most widely used in airplane
engines, because it is strong and a very easy shape to form by the drop-
forging process or to machine out of the solid bar when extra good steel is
used. Where extreme lightness is desired, as in small high-speed motors
used for cycle propulsion, the section shown at the extreme left is often
used. If the rod is a cast member as in some marine engines, the cross,

hollow cylinder, or U sections are sometimes used. If the sections shown at
the right are employed, advantage is often taken of the opportunity for
passing lubricant through the center of the hollow round section on vertical
motors or at the bottom of the U section, which would be used on a
horizontal cylinder power plant.

Fig. 130.—Another Type of Double Connecting Rod for Vee
Engines.

Connecting rods of Vee engines are made in two distinct styles. The forked
or “scissors” joint rod assembly is employed when the cylinders are placed
directly opposite each other. The “blade” rod, as shown at Fig. 129, fits
between the lower ends of the forked rod, which oscillate on the bearing
which encircles the crank-pin. The lower end of the “blade” rod is usually
attached to the bearing brasses, the ends of the “forked” rod move on the
outer surfaces of the brasses. Another form of rod devised for use under
these conditions is shown at Fig. 130 and installed in an aviation engine at
Fig. 132. In this construction the shorter rod is attached to a boss on the
master rod by a short pin to form a hinge and to permit the short rod to
oscillate as the conditions dictate. This form of rod can be easily adjusted
when the bearing depreciates, a procedure that is difficult with the forked
type rod. The best practice, in the writer’s opinion, is to stagger the
cylinders and use side-by-side rods as is done in the Curtiss engine. Each

rod may be fitted independently of the other and perfect compensation for
wear of the big ends is possible.

Fig. 131.—Part Sectional View of Wisconsin Aviation Engine, Showing Four-Bearing
Crank-Shaft, Overhead Cam-Shaft, and Method of Combining Cylinders in Pairs.

Fig. 132.—Part Sectional View of Renault Twelve-Cylinder
Water-Cooled Engine, Showing Connecting Rod
Construction and Other Important Internal Parts.

CAM-SHAFT AND CRANK-SHAFT DESIGN

Before going extensively into the subject of crank-shaft construction it will
be well to consider cam-shaft design, which is properly a part of the valve
system and which has been considered in connection with the other
elements which have to do directly with cylinder construction to some
extent. Cam-shafts are usually simple members carried at the base of the
cylinder in the engine case of Vee type motors by suitable bearings and
having the cams employed to lift the valves attached at intervals. A typical

cam-shaft design is shown at Fig. 133. Two main methods of cam-shaft
construction are followed—that in which the cams are separate members,
keyed and pinned to the shaft, and the other where the cams are formed
integral, the latter being the most suitable for airplane engine requirements.

Fig. 133.—Typical Cam-Shaft, with Valve Lifting Cams and Gears to Operate
Auxiliary Devices Forged Integrally.

The cam-shafts shown at Figs. 133 and 134, B, are of the latter type, as the
cams are machined integrally. In this case not only the cams but also the
gears used in driving the auxiliary shafts are forged integral. This is a more
expensive construction, because of the high initial cost of forging dies as
well as the greater expense of machining. It has the advantage over the
other form in which the cams are keyed in place in that it is stronger, and as
the cams are a part of the shaft they can never become loose, as might be
possible where they are separately formed and assembled on a simple shaft.

Fig. 134.—Important Parts of Duesenberg Aviation Engine.
A—Three Main Bearing Crank-Shaft. B—Cam-Shaft with
Integral Cams. C—Piston and Connecting Rod Assembly. D
—Valve Rocker Group. E—Piston. F—Main Bearing
Brasses.

The importance of the crank-shaft has been previously considered, and
some of its forms have been shown in views of the motors presented in
earlier portions of this work. The crank-shaft is one of the parts subjected to
the greatest strain and extreme care is needed in its construction and design,
because practically the entire duty of transmitting the power generated by
the motor to the gearset devolves upon it. Crank-shafts are usually made of
high tensile strength steel of special composition. They may be made in
four ways, the most common being from a drop or machine forging which
is formed approximately to the shape of the finished shaft and in rare
instances (experimental motors only) they may be steel castings. Sometimes
they are made from machine forgings, where considerably more machine

work is necessary than would be the case where the shaft is formed between
dies. Some engineers favor blocking the shaft out of a solid slab of metal
and then machining this rough blank to form. In some radial-cylinder
motors of the Gnome and Le Rhone type the crank-shafts are built up of
two pieces, held together by taper fastenings or bolts.

Fig. 135.—Showing Method of Making Crank-Shaft. A—The Rough Steel Forging
Before Machining. B—The Finished Six-Throw, Seven-Bearing Crank-Shaft.

The form of the shaft depends on the number of cylinders and the form has
material influence on the method of construction. For instance, a four-
cylinder crank-shaft could be made by either of the methods outlined. On
the other hand, a three- or six-cylinder shaft is best made by the machine
forging process, because if drop forged or cut from the blank it will have to
be heated and the crank throws bent around so that the pins will lie in three
planes one hundred and twenty degrees apart, while the other types
described need no further attention, as the crank-pins lie in planes one
hundred and eighty degrees apart. This can be better understood by
referring to Fig. 135, which shows a six-cylinder shaft in the rough and
finished stages. At A the appearance of the machine forging before any of
the material is removed is shown, while at B the appearance of the finished
crank-shaft is clearly depicted. The built-up crank-shaft is seldom used on
multiple-cylinder motors, except in some cases where the crank-shafts
revolve on ball bearings as in some automobile racing engines.

Fig. 136.—Showing Form of Crank-Shaft for Twin-Cylinder Opposed Power
Plant.

Fig. 137.—Crank-Shaft of Thomas-Morse Eight-Cylinder Vee Engine.

Crank-shaft form will vary with a number of cylinders and it is possible to
use a number of different arrangements of crank-pins and bearings for the
same number of cylinders. The simplest form of crank-shaft is that used on
simple radial cylinder motors as it would consist of but one crank-pin, two
webs, and the crank-shaft. As the number of cylinders increase in Vee
motors as a general rule more crank-pins are used. The crank-shaft that
would be used on a two-cylinder opposed motor is shown at Fig. 136. This
has two throws and the crank-pins are spaced 180 degrees apart. The
bearings are exceptionally long. Four-cylinder crank-shafts may have two,
three or five main bearings and three or four crank-pins. In some forms of
two-bearing crank-shafts, such as used when four-cylinders are cast in a
block, or unit casting, two of the pistons are attached to one common crank-
pin, so that in reality the crank-shaft has but three crank-pins. A typical

three bearing, four-cylinder crank-shaft is shown at Fig. 134, A. The same
type can be used for an eight-cylinder Vee engine, except for the greater
length of crank-pins to permit of side by side rods as shown at Fig. 137. Six
cylinder vertical tandem and twelve-cylinder Vee engine crank-shafts
usually have four or seven main bearings depending upon the disposition of
the crank-pins and arrangement of cylinders. At Fig. 138, A, the bottom
view of a twelve-cylinder engine with bottom half of crank case removed is
given. This illustrates clearly the arrangement of main bearings when the
crank-shaft is supported on four journals. The crank-shaft shown at Fig.
138, B, is a twelve-cylinder seven-bearing type.

Fig. 138.—Crank-Case and Crank-Shaft Construction for Twelve-
Cylinder Motors. A—Duesenberg. B—Curtiss.

Fig. 139.—Counterbalanced Crank-Shafts Reduce Engine Vibration
and Permit of Higher Rotative Speeds.

In some automobile engines, extremely good results have been secured in
obtaining steady running with minimum vibration by counterbalancing the
crank-shafts as outlined at Fig. 139. The shaft at A is a type suitable for a
high speed four-cylinder vertical or an eight-cylinder Vee type. That at B is
for a six-cylinder vertical or a twelve-cylinder V with scissors joint rods. If
counterbalancing crank-shafts helps in an automobile engine, it should have
advantages of some moment in airplane engines, even though the crank-
shaft weight is greater.

BALL-BEARING CRANK-SHAFTS

While crank-shafts are usually supported in plain journals there seems to be
a growing tendency of late to use anti-friction bearings of the ball type for
their support. This is especially noticeable on block motors where but two
main bearings are utilized. When ball bearings are selected with proper
relation to the load which obtains they will give very satisfactory service.
They permit the crank-shaft to turn with minimum friction, and if properly

selected will never need adjustment. The front end is supported by a bearing
which is clamped in such a manner that it will take a certain amount of load
in a direction parallel to the axis of the shaft, while the rear end is so
supported that the outer race of the bearing has a certain amount of axial
freedom or “float.” The inner race or cone of each bearing is firmly
clamped against shoulders on the crank-shaft. At the front end of the crank-
shaft timing gear and a suitable check nut are used, while at the back end
the bearing is clamped by a threaded retention member between the fly-
wheel and a shoulder on the crank-shaft. The fly-wheel is held in place by a
taper and key retention. The ball bearings are carried in a light housing of
bronze or malleable iron, which in turn are held in the crank-case by bolts.
The Renault engine uses ball bearings at front and rear ends of the crank-
shaft, but has plain bearings around intermediate crank-shaft journals. The
rotary engines of the Gnome, Le Rhone and Clerget forms would not be
practical if ball bearings were not used as the bearing friction and
consequent depreciation would be very high.

ENGINE-BASE CONSTRUCTION

One of the important parts of the power plant is the substantial casing or
bed member, which is employed to support the cylinders and crank-shaft
and which is attached directly to the fuselage engine supporting members.
This will vary widely in form, but as a general thing it is an approximately
cylindrical member which may be divided either vertically or horizontally
in two or more parts. Airplane engine crank-cases are usually made of
aluminum, a material which has about the same strength as cast iron, but
which only weighs a third as much. In rare cases cast iron is employed, but
is not favored by most engineers because of its brittle nature, great weight
and low resistance to tensile stresses. Where exceptional strength is needed
alloys of bronze may be used, and in some cases where engines are
produced in large quantities a portion of the crank-case may be a sheet steel
or aluminum stamping.

Fig. 140.—View of Thomas 135 Horse-Power Aeromotor, Model 8,
Showing Conventional Method of Crank-Case Construction.

Crank-cases are always large enough to permit the crank-shaft and parts
attached to it to turn inside and obviously its length is determined by the
number of cylinders and their disposition. The crank-case of the radial
cylinder or double-opposed cylinder engine would be substantially the same
in length. That of a four-cylinder will vary in length with the method of
casting the cylinder. When the four-cylinders are cast in one unit and a two-
bearing crank-shaft is used, the crank-case is a very compact and short
member. When a three-bearing crank-shaft is utilized and the cylinders are
cast in pairs, the engine base is longer than it would be to support a block
casting, but is shorter than one designed to sustain individual cylinder
castings and a five-bearing crank-shaft. It is now common construction to
cast an oil container integral with the bottom of the engine base and to draw
the lubricating oil from it by means of a pump, as shown at Fig. 140. The
arms by which the motor is supported in the fuselage are substantial-ribbed
members cast integrally with the upper half.

Fig. 141.—Views of Upper Half of Thomas Aeromotor
Crank-Case.

Fig. 142.—Method of Constructing Eight-Cylinder
Vee Engine, Possible if Aluminum Cylinder and
Crank-Case Castings are Used.

The approved method of crank-case construction favored by the majority of
engineers is shown at the top of Fig. 141, bottom side up. The upper half
not only forms a bed for the cylinder but is used to hold the crank-shaft as
well. In the illustration, the three-bearing boxes form part of the case, while
the lower brasses are in the form of separately cast caps retained by suitable
bolts. In the construction outlined the bottom part of the case serves merely
as an oil container and a protection for the interior mechanism of the motor.
The cylinders are held down by means of studs screwed into the crank-case
top, as shown at Fig. 141, lower view. If the aluminum cylinder motor has
any future, the method of construction outlined at Fig. 142, which has been

used in cast iron for an automobile motor, might be used for an eight-
cylinder Vee engine for airplane use. The simplicity of the crank-case
needed for a revolving cylinder motor and its small weight can be well
understood by examination of the illustration at Fig. 143, which shows the
engine crank-case for the nine-cylinder “Monosoupape” Gnome engine.
This consists of two accurately machined forgings held together by bolts as
clearly indicated.

Fig. 143.—Simple and Compact Crank-Case, Possible When Radial
Cylinder Engine Design is Followed.

CHAPTER X
Power Plant Installation—Curtiss OX-2 Engine Mounting and
Operating Rules—Standard S. A. E. Engine Bed Dimensions—Hall-
Scott Engine Installation and Operation—Fuel System Rules—
Ignition System—Water System—Preparations to Start Engine—
Mounting Radial and Rotary Engines—Practical Hints to Locate
Engine Troubles—All Engine Troubles Summarized—Location of
Engine Troubles Made Easy.
The proper installation of the airplane power plant is more important than is
generally supposed, as while these engines are usually well balanced and
run with little vibration, it is necessary that they be securely anchored and
that various connections to the auxiliary parts be carefully made in order to
prevent breakage from vibration and that attendant risk of motor stoppage
while in the air. The type of motor to be installed determines the method of
installation to be followed. As a general rule six-cylinder vertical engine
and eight-cylinder Vee type are mounted in substantially the same way. The
radial, fixed cylinder forms and the radial, rotary cylinder Gnome and Le
Rhone rotary types require an entirely different method of mounting. Some
unconventional mountings have been devised, notably that shown at Fig.
144, which is a six-cylinder German engine that is installed in just the
opposite way to that commonly followed. The inverted cylinder
construction is not generally followed because even with pressure feed, dry
crank-case type lubricating system there is considerable danger of over-
lubrication and of oil collecting and carbonizing in the combustion chamber
and gumming up the valve action much quicker than would be the case if
the engine was operated in the conventional upright position. The reason for
mounting an engine in this way is to obtain a lower center of gravity and
also to make for more perfect streamlining of the front end of the fuselage
in some cases. It is rather doubtful if this slight advantage will compensate
for the disadvantages introduced by this unusual construction. It is not used
to any extent now but is presented merely to show one of the possible
systems of installing an airplane engine.

Fig. 144.—Unconventional Mounting of German Inverted
Cylinder Motor.

Fig. 145.—How Curtiss Model OX-2 Motor is Installed in Fuselage of
Curtiss Tractor Biplane. Note Similarity of Mounting to Automobile Power
Plant.

In a number of airplanes of the tractor-biplane type the power plant
installation is not very much different than that which is found in
automobile practice. The illustration at Fig. 145 is a very clear
representation of the method of mounting the Curtiss eight-cylinder 90 H. P.
or model OX-2 engine in the fuselage of the Curtiss JN-4 tractor biplane
which is so generally used in the United States as a training machine. It will
be observed that the fuel tank is mounted under a cowl directly behind the
motor and that it feeds the carburetor by means of a flexible fuel pipe. As
the tank is mounted higher than the carburetor, it will feed that member by
gravity. The radiator is mounted at the front end of the fuselage and
connected to the water piping on the motor by the usual rubber hose
connections. An oil pan is placed under the engine and the top is covered
with a hood just as in motor car practice. The panels of aluminum are
attached to the sides of the fuselage and are supplied with doors which open
and provide access to the carburetor, oil-gauge and other parts of the motor
requiring inspection. The complete installation with the power plant
enclosed is given at Fig. 146, and in this it will be observed that the exhaust
pipes are connected to discharge members that lead the gases above the top
plane. In the engine shown at Fig. 145 the exhaust flows directly into the air
at the sides of the machine through short pipes bolted to the exhaust gas
outlet ports. The installation of the radiator just back of the tractor screw
insures that adequate cooling will be obtained because of the rapid air flow
due to the propeller slip stream.

Fig. 146.—Latest Model of Curtiss JN-4 Training Machine, Showing
Thorough Enclosure of Power Plant and Method of Disposing of the
Exhaust Gases.

Fig. 147.—Front View of L. W. F. Tractor Biplane Fuselage, Showing
Method of Installing Thomas Aeromotor and Method of Disposing of
Exhaust Gases.

INSTALLATION OF CURTISS OX-2 ENGINE

The following instructions are given in the Curtiss Instruction Book for
installing the OX-2 engine and preparing it for flights, and taken in
connection with the very clear illustration presented no difficulty should be
experienced in understanding the proper installation, and mounting of this
power plant. The bearers or beds should be 2 inches wide by 3 inches deep,
preferably of laminated hard wood, and placed 115⁄8 inches apart. They must
be well braced. The six arms of the base of the motor are drilled for 3⁄8-inch
bolts, and none but this size should he used.
1. Anchoring the Motor. Put the bolts in from the bottom, with a large
washer under the head of each so the head cannot cut into the wood. On
every bolt use a castellated nut and a cotter pin, or an ordinary nut and a
lock washer, so the bolt will not work loose. Always set motor in place and
fasten before attaching any auxiliary apparatus, such as carburetor, etc.
2. Inspecting the Ignition-Switch Wires. The wires leading from the ignition
switch must be properly connected—one end to the motor body for ground,
and the other end to the post on the breaker box of the magneto.
3. Filling the Radiator. Be sure that the water from the radiator fills the
cylinder jackets. Pockets of air may remain in the cylinder jackets even
though the radiator may appear full. Turn the motor over a few times by
hand after filling the radiator, and then add more water if the radiator will
take it. The air pockets, if allowed to remain, may cause overheating and
develop serious trouble when the motor is running.
4. Filling the Oil Reservoir. Oil is admitted into the crank-case through the
breather tube at the rear. It is well to strain all oil put into the crank-case. In
filling the oil reservoir be sure to turn the handle on the oil sight-gauge till it
is at right angles with the gauge. The oil sight-gauge is on the side of the
lower half of the crank-case. Put in about 3 gallons of the best obtainable

oil, Mobile B recommended. It is important to remember that the very best
oil is none too good.
5. Oiling Exposed Moving Parts. Oil all rocker-arm bearings before each
flight. A little oil should be applied where the push rods pass through the
stirrup straps.
6. Filling the Gasoline Tanks. Be certain that all connections in the gasoline
system are tight.
7. Turning on the Gasoline. Open the cock leading from the gasoline tank to
the carburetor.
8. Charging the Cylinders. With the ignition switch OFF, prime the motor
by squirting a little gasoline in each exhaust port and then turn the propeller
backward two revolutions. Never open the exhaust valve by operating the
rocker-arm by hand, as the push-rod is liable to come out of its socket in the
cam follower and bend the rocker-arm when the motor turns over.
9. Starting the Motor by Hand. Always retard the spark part way, to prevent
back-firing, by pulling forward the wire attached to the breaker box. Failure
to so retard the spark in starting may result in serious injury to the operator.
Turn on the ignition switch with throttle partly open; give a quick, strong
pull down and outward on the starting crank or propeller. As soon as the
motor is started advance the spark by releasing the retard wire.
10. Oil Circulation. Let the motor run at low speed for a few minutes in
order to establish oil circulation in all bearings. With all parts functioning
properly, the throttle may be opened gradually for warming up before flight.

STANDARD S.A.E. ENGINE BED DIMENSIONS

The Society of Automotive Engineers have made efforts to standardize
dimensions of bed timbers for supporting power plant in an aeroplane.
Owing to the great difference in length no standardization is thought
possible in this regard. The dimensions recommended are as follows:
Distance between timbers 12 in. 14 in. 16 in.
Width of bed timbers 11⁄2 in. 13⁄4 in. 2 in.

Distance between centers of bolts 131⁄2 in. 153⁄4 in. 18 in.

It will be evident that if any standard of this nature were adopted by engine
builders that the designers of fuselage could easily arrange their bed timbers
to conform to these dimensions, whereas it would be difficult to have them
adhere to any standard longitudinal dimensions which are much more easily
varied in fuselages than the transverse dimensions are. It, however, should
be possible to standardize the longitudinal positions of the holding down
bolts as the engine designer would still be able to allow himself
considerable space fore-and-aft of the bolts.

HALL-SCOTT ENGINE INSTALLATION
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Fig. 148.—End Elevation of Hall-Scott A-7 Four-Cylinder Motor, with
Installation Dimensions.

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(113 kB).

Fig. 149.—Plan and Side Elevation of Hall-Scott A-7 Four-
Cylinder Airplane Engine, with Installation Dimensions.

The very thorough manner in which installation diagrams are prepared by
the leading engine makers leaves nothing to the imagination. The
dimensions of the Hall-Scott four-cylinder airplane engine are given clearly
in our inch measurements with the metric equivalents at Figs. 148 and 149,
the former showing a vertical elevation while the latter has a plan view and
side elevation. The installation of this engine in airplanes is clearly shown
at Figs. 150 and 151, the former having the radiator installed at the front of
the motor and having all exhaust pipes joined to one common discharge
funnel, which deflects the gas over the top plane while the latter has the
radiator placed vertically above the motor at the back end and has a direct
exhaust gas discharge to the air.

Fig. 150.

CENSORED
Fig. 151.

CENSORED
The dimensions of the six-cylinder Hall-Scott motor which is known as the
type A-5 125 H. P. are given at Fig. 152, which is an end sectional
elevation, and at Fig. 153, which is a plan view. The dimensions are given
both in inch sizes and the metric equivalents. The appearance of a Hall-
Scott six-cylinder engine installed in a fuselage is given at Fig. 154, while a
diagram showing the location of the engine and the various pipes leading to
the auxiliary groups is outlined at Fig. 155. The following instructions for
installing the Hall-Scott power plant are reproduced from the instruction
book issued by the maker. Operating instructions which are given should
enable any good mechanic to make a proper installation and to keep the
engine in good running condition.

Fig. 152.

CENSORED
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Fig. 153.—Plan View of Hall-Scott Type A-5 125 Horse-Power Airplane Engine,
Showing Installation Dimensions.

Fig. 154.—Three-Quarter View of Hall-Scott Type A-
5 125 Horse-Power Six-Cylinder Engine, with One of
the Side Radiators Removed to Show Installation in
Standard Fuselage.

Fig. 155.—Diagram Showing Proper Installation of Hall-Scott Type A-5 125 Horse-
Power Engine with Pressure Feed Fuel Supply System.

FUEL SYSTEM INSTALLATION

Gasoline giving the best results with this equipment is as follows: Gravity
58-62 deg. Baume A. Initial boiling point—Richmond method—102° Fahr.
Sulphur .014. Calorimetric bomb test 20610 B. T. U. per pound. If the
gasoline tank is placed in the fuselage below the level of the carburetor, a
hand pump must be used to maintain air pressure in gas tank to force the
gasoline to the carburetor. After starting the engine the small auxiliary air
pump upon the engine will maintain sufficient pressure. A-7a and A-5a
engines are furnished with a new type auxiliary air pump. This should be
frequently oiled and care taken so no grit or sand will enter which might
lodge between the valve and its seat, which would make it fail to operate
properly. An air relief valve is furnished with each engine. It should be
screwed into the gas tank and properly regulated to maintain the pressure
required. This is done by screwing the ratchet on top either up or down. If
two tanks are used in a plane one should be installed in each tank. All air
pump lines should be carefully gone over quite frequently to ascertain if
they are tight. Check valves have to be placed in these lines. In some cases

the gasoline tank is placed above the engine, allowing it to drain by gravity
to the carburetor. When using this system there should be a drop of not less
than two feet from the lowest portion of the gasoline tank to the upper part
of the carburetor float chamber. Even this height might not be sufficient to
maintain the proper volume of gasoline to the carburetor at high speeds. Air
pressure is advised upon all tanks to insure the proper supply of gasoline.
When using gravity feed without air pressure be sure to vent the tank to
allow circulation of air. If gravity tank is used and the engine runs
satisfactorily at low speeds but cuts out at high speeds the trouble is
undoubtedly due to insufficient height of the tank above the carburetor. The
tank should be raised or air pressure system used.

IGNITION SWITCHES

Two “DIXIE” switches are furnished with each engine. Both of these
should be installed in the pilot’s seat, one controlling the R. H., and the
other the L. H. magneto. By shorting either one or the other it can be
quickly determined if both magnetos, with their respective spark-plugs, are
working correctly. Care should be taken not to use spark-plugs having
special extensions or long protruding points. Plugs giving best results are
extremely small with short points.

WATER SYSTEMS

A temperature gauge should be installed in the water pipe, coming directly
from the cylinder nearest the propeller (note illustration above). This
instrument installed in the radiator cap has not always given satisfactory
results. This is especially noticeable when the water in the radiator becomes
low, not allowing it to touch the bulb on the moto-meter. For ordinary
running, it should not indicate over 150 degrees Fahr. In climbing tests,
however, a temperature of 160 degrees Fahr. can be maintained without any
ill effects upon the engine. In case the engine becomes overheated, the
indicator will register above 180 degrees Fahr., in which case it should be
stopped immediately. Overheating is most generally caused by retarded
spark, excessive carbon in the cylinders, insufficient lubrication, improperly

timed valves, lack of water, clogging of water system in any way which
would obstruct the free circulation of the water.
Overheating will cause the engine to knock, with possible damaging results.
Suction pipes should be made out of thin tubing, and run within a quarter or
an eighth of an inch of each other, so that when a hose is placed over the
two, it will not be possible to suck together. This is often the case when a
long rubber hose is used, which causes overheating. Radiators should be
flushed out and cleaned thoroughly quite often. A dirty radiator may cause
overheating.
When filling the radiator it is very important to remove the plug on top of
the water pump until water appears. This is to avoid air pockets being
formed in the circulating system, which might not only heat up the engine,
but cause considerable damage. All water pump hoses and connections
should be tightly taped and shellacked after the engine is properly installed
in the plane. The greatest care should be taken when making engine
installation not to use smaller inside diameter hose connection than water
pump suction end casting. One inch and a quarter inside diameter should be
used on A-7 and A-5 motors, while nothing less than one inch and a half
inside diameter hose or tubing on all A-7a and A-5a engines. It is further
important to have light spun tubing, void of any sharp turns, leads from
pump to radiator and cylinder water outlet to radiator. In other words, the
water circulation through the engine must be as little restricted as possible.
Be sure no light hose is used, that will often suck together when engine is
started. To thoroughly drain the water from the entire system, open the drain
cock at the lowest side of the water pump.

PREPARATIONS TO START ENGINE

Always replenish gasoline tanks through a strainer which is clean. This
strainer must catch all water and other impurities in the gasoline. Pour at
least three gallons of fresh oil into the lower crank-case. Oil all rocker arms
through oilers upon rocker arm housing caps. Be sure radiators are filled
within one inch of the top.
After all the parts are oiled, and the tanks filled, the following must be
looked after before starting: See if crank-shaft flange is tight on shaft. See if

propeller bolts are tight and evenly drawn up. See if propeller bolts are
wired. See if propeller is trued up to within 1⁄8′′.
Every four days the magnetos should be oiled if the engine is in daily use.
Every month all cylinder hold-down nuts should be gone over to ascertain if
they are tight. (Be sure to recotter nuts.)
See if magnetos are bolted on tight and wired.
See if magneto cables are in good condition.
See if rocker arm tappets have a .020′′ clearance from valve stem when
valve is seated.
See if tappet clamp screws are tight and cottered.
See if all gasoline, oil, water pipes and connections are in perfect condition.
Air on gas line should be tested for leaks.
Pump at least three pounds air pressure into gasoline tank.
After making sure that above rules have been observed, test compression of
cylinders by turning propeller.
“DO NOT FORGET TO SHORT BOTH MAGNETOS”
Be sure all compression release and priming cocks do not leak compression.
If they do, replace same with a new one immediately, as this might cause
premature firing.
Open priming cocks and squirt some gasoline into each.
Close cocks.
Open compression release cocks.
Open throttle slightly.
If using Berling magnetos they should be three-quarters advanced.

If all the foregoing directions have been carefully followed, the engine is
ready for starting.
In cranking engine either by starting crank, or propeller, it is essential to
throw it over compression quickly.
Immediately upon starting, close compression release cocks.
When engine is running, advance magnetos.
After it has warmed up, short one magneto and then the other, to be sure
both magnetos and spark-plugs are firing properly. If there is a miss, the
fouled plug must be located and cleaned. There is a possibility that the jets
in the carburetor are stopped up. If this is the case, do not attempt to clean
same with any sharp instrument. If this is done, it might change the opening
in the jets, thus spoiling the adjustment. Jets and nozzles should be blown
out with air or steam.
An open intake or exhaust valve, which might have become sluggish or
stuck from carbon, might cause trouble. Be sure to remedy this at once by
using a little coal-oil or kerosene on same, working the valve by hand until
it becomes free. We recommend using graphite on valve stems mixed with
oil to guard against sticking or undue wear.

INSTALLING ROTARY AND RADIAL CYLINDER ENGINES

Fig. 156.—Diagram Defining Installation of Gnome “Monosoupape” Motor in
Tractor Biplane. Note Necessary Piping for Fuel, Oil, and Air Lines.

Fig. 157.—Showing Two Methods of Placing Propeller on Gnome
Rotary Motor.

When rotary engines are installed simple steel stamping or “spiders,” are
attached to the fuselage to hold the fixed crank-shaft. Inasmuch as the

motor projects clear of the fuselage proper there is plenty of room back of
the front spider plate to install the auxiliary parts such as the oil pump, air
pump and ignition magneto and also the fuel and oil containers. The
diagram given at Fig. 156 shows how a Gnome “monosoupape” engine is
installed on the anchorage plates and it also outlines clearly the piping
necessary to convey the oil and fuel and also the air-piping needed to put
pressure on both fuel and oil tanks to insure positive supply of these liquids
which may be carried in tanks placed lower than the motor in some
installations. The diagram given at Figs. 157 and 158 shows other
mountings of Gnome engines and are self-explanatory. The simple
mounting possible when the Anzani ten-cylinder radial fixed type engine is
used given at Fig. 159. The front end of the fuselage is provided with a
substantial pressed steel plate having members projecting from it which
may be bolted to the longerons. The bolts that hold the two halves of the
crank-case together project through the steel plate and hold the engine
securely to the front end of the fuselage.

Fig. 158.—How Gnome Rotary Motor May Be Attached to Airplane
Fuselage Members.

Fig. 159.—How Anzani Ten-Cylinder Radial
Engine is Installed to Plate Securely
Attached to Front End of Tractor Airplane
Fuselage.

PRACTICAL HINTS TO LOCATE ENGINE TROUBLES

One who is not thoroughly familiar with engine construction will seldom
locate troubles by haphazard experimenting and it is only by a systematic
search that the cause can be discovered and the defects eliminated. In this
chapter the writer proposes to outline some of the most common power-
plant troubles and to give sufficient advice to enable those who are not
thoroughly informed to locate them by a logical process of elimination. The
internal-combustion motor, which is the power plant of all gasoline
automobiles as well as airplanes, is composed of a number of distinct

groups, which in turn include distinct components. These various
appliances are so closely related to each other that defective action of any
one may interrupt the operation of the entire power plant. Some of the
auxiliary groups are more necessary than others and the power plant will
continue to operate for a time even after the failure of some important parts
of some of the auxiliary groups. The gasoline engine in itself is a complete
mechanism, but it is evident that it cannot deliver any power without some
means of supplying gas to the cylinders and igniting the compressed gas
charge after it has been compressed in the cylinders. From this it is patent
that the ignition and carburetion systems are just as essential parts of the
power plant as the piston, connecting rod, or cylinder of the motor. The
failure of either the carburetor or igniting means to function properly will
be immediately apparent by faulty action of the power plant.
To insure that the motor will continue to operate it is necessary to keep it
from overheating by some form of cooling system and to supply oil to the
moving parts to reduce friction. The cooling and lubrication groups are not
so important as carburetion and ignition, as the engine would run for a
limited period of time even should the cooling system fail or the oil supply
cease. It would only be a few moments, however, before the engine would
overheat if the cooling system was at fault, and the parts seize if the
lubricating system should fail. Any derangement in the carburetor or
ignition mechanism would manifest itself at once because the engine
operation would be affected, but a defect in the cooling or oiling system
would not be noticed so readily.
The careful aviator will always inspect the motor mechanism before starting
on a trip of any consequence, and if inspection is carefully carried out and
loose parts tightened it is seldom that irregular operation will be found due
to actual breakage of any of the components of the mechanism.
Deterioration due to natural causes matures slowly, and sufficient warning
is always given when parts begin to wear so satisfactory repairs may be
promptly made before serious derangement or failure is manifested.

A TYPICAL ENGINE STOPPAGE ANALYZED

Before describing the points that may fail in the various auxiliary systems it
will be well to assume a typical case of engine failure and show the process
of locating the trouble in a systematic manner by indicating the various
steps which are in logical order and which could reasonably be followed. In
any case of engine failure the ignition system, motor compression, and
carburetor should be tested first. If the ignition system is functioning
properly one should determine the amount of compression in all cylinders
and if this is satisfactory the carbureting group should be tested. If the
ignition system is working properly and there is a decided resistance in the
cylinders when the propeller is turned, proving that there is good
compression, one may suspect the carburetor.
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Fig. 160.—Side Elevation of Thomas 135 Horse-Power Airplane
Engine, Giving Important Dimensions.

If the carburetor appears to be in good condition, the trouble may be caused
by the ignition being out of time, which condition is possible when the
magneto timing gear or coupling is attached to the armature shaft by a taper
and nut retention instead of the more positive key or taper-pin fastening. It
is possible that the inlet manifold may be broken or perforated, that the

exhaust valve is stuck on its seat because of a broken or bent stem, broken
or loose cam, or failure of the cam-shaft drive because the teeth are stripped
from the engine shaft or cam-shaft gears; or because the key or other
fastening on either gear has failed, allowing that member to turn
independently of the shaft to which it normally is attached. The gasoline
feed pipe may be clogged or broken, the fuel supply may be depleted, or the
shut-off cock in the gasoline line may have jarred closed. The gasoline filter
may be filled with dirt or water which prevents passage of the fuel.
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Fig. 161.—Front Elevation of Thomas-Morse 135 Horse-
Power Aeromotor, Showing Main Dimensions.

The defects outlined above, except the failure of the gasoline supply, are
very rare, and if the container is found to contain fuel and the pipe line to be
clear to the carburetor, it is safe to assume the vaporizing device is at fault.
If fuel continually runs out of the mixing chamber the carburetor is said to
be flooded. This condition results from failure of the shut-off needle to seat
properly or from a punctured hollow metal float or a gasoline-soaked cork
float. It is possible that not enough gasoline is present in the float chamber.
If the passage controlled by the float-needle valve is clogged or if the float
was badly out of adjustment, this contingency would be probable. When the
carburetor is examined, if the gasoline level appears to be at the proper
height, one may suspect that a particle of lint, or dust, or fine scale, or rust
from the gasoline tank has clogged the bore of the jet in the mixing
chamber.
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Fig. 162.—Front and Side Elevations of Sturtevant Airplane Engine, Giving
Principal Dimensions to Facilitate Installation.

If the ignition system and carburetor appear to be in good working order,
and the hand crank shows that there is no compression in one or more of the
cylinders, it means some defect in the valve system. If the engine is a
multiple-cylinder type and one finds poor compression in all of the
cylinders it may be due to the rare defect of improper valve timing. This
may be caused by a gear having altered its position on the cam-shaft or

crank-shaft, because of a sheared key or pin having permitted the gear to
turn about half of a revolution and then having caught and held the gear in
place by a broken or jagged end so that cam-shaft would turn, but the valves
open at the wrong time. If but one of the cylinders is at fault and the rest
appear to have good compression the trouble may be due to a defective
condition either inside or outside of that cylinder. The external parts may be
inspected easily, so the following should be looked for: a broken valve, a
warped valve-head, broken valve-springs, sticking or bent valve-stems, dirt
under valve-seat, leak at valve-chamber cap or spark-plug gasket. Defective
priming cock, cracked cylinder head (rarely occurs), leak through cracked
spark-plug insulation, valve-plunger stuck in the guide, lack of clearance
between valve-stem end and top of plunger caused by loose adjusting screw
which has worked up and kept the valve from seating. The faulty
compression may be due to defects inside the motor. The piston-head may
be cracked (rarely occurs), piston rings may be broken, the slots in the
piston rings may be in line, the rings may have lost their elasticity or have
become gummed in the grooves of the piston, or the piston and cylinder
walls may be badly scored by a loose wrist pin or by defective lubrication.
If the motor is a type with a separate head it is possible the gasket or
packing between the cylinder and combustion chamber may leak, either
admitting water to the cylinder or allowing compression to escape.

CONDITIONS THAT CAUSE FAILURE OF IGNITION SYSTEM

If the first test of the motor had showed that the compression was as it
should be and that there were no serious mechanical defects and there was
plenty of gasoline at the carburetor, this would have demonstrated that the
ignition system was not functioning properly. If a battery is employed to
supply current the first step is to take the spark-plugs out of the cylinders
and test the system by turning over the engine by hand. If there is no spark
in any of the plugs, this may be considered a positive indication that there is
a broken main current lead from the battery, a defective ground connection,
a loose battery terminal, or a broken connector. If none of these conditions
are present, it is safe to say that the battery is no longer capable of
delivering current. While magneto ignition is generally used on airplane
engines, there is apt to be some development of battery ignition, especially

on engines equipped with electric self-starters which are now being
experimented with. The spark-plugs may be short circuited by cracked
insulation or carbon and oil deposits around the electrode. The secondary
wires may be broken or have defective insulation which permits the current
to ground to some metal part of the fuselage or motor. The electrodes of the
spark-plug may be too far apart to permit a spark to overcome the resistance
of the compressed gas, even if a spark jumps the air space, when the plug is
laid on the cylinder.
If magnetos are fitted as is usually the case at present and a spark is
obtained between the points of the plug and that device or the wire leading
to it from the magneto is in proper condition, the trouble is probably caused
by the magneto being out of time. This may result if the driving gear is
loose on the armature-shaft or crank-shaft, and is a rare occurrence. If no
spark is produced at the plugs the secondary wire may be broken, the
ground wire may make contact with some metallic portion of the chassis
before it reaches the switch, the carbon collecting brushes may be broken or
not making contact, the contact points of the make-and-break device may
be out of adjustment, the wiring may be attached to wrong terminals, the
distributor filled with metallic particles, carbon, dust or oil accumulations,
the distributor contacts may not be making proper connection because of
wear and there may be a more serious derangement, such as a burned out
secondary winding or a punctured condenser.
If the motor runs intermittently, i.e., starts and runs only a few revolutions,
aside from the conditions previously outlined, defective operation may be
due to seizing between parts because of insufficient oil or deficient cooling,
too much oil in the crank-case which fouls the cylinder after the crank-shaft
has revolved a few turns, and derangements in the ignition or carburetion
systems that may be easily remedied. There are a number of defective
conditions which may exist in the ignition group, that will result in
“skipping” or irregular operation and the following points should be
considered first: weak source of current due to worn out dry cells or
discharged storage batteries; weak magnets in magneto, or defective
contacts at magneto; dirt in magneto distributor or poor contact at collecting
brushes. Dirty or cracked insulator at spark-plug will cause short circuit and
can only be detected by careful examination. The following points should
also be checked over when the plug is inspected: Excessive space between

electrodes, points too close together, loose central electrodes, or loose point
on plug body, soot or oil particles between electrodes, or on the surface of
the insulator, cracked insulator, oil or water on outside of insulator. Short
circuits in the condenser or internal wiring of induction coils or magnetos,
which are fortunately not common, can seldom be remedied except at the
factory where these devices were made. If an engine stops suddenly and the
defect is in the ignition system the trouble is usually never more serious
than a broken or loose wire. This may be easily located by inspecting the
wiring at the terminals. Irregular operation or misfiring is harder to locate
because the trouble can only be found after the many possible defective
conditions have been checked over, one by one.

COMMON DEFECTS IN FUEL SYSTEMS

Defective carburetion often causes misfiring or irregular operation. The
common derangement of the components of the fuel system that are
common enough to warrant suspicion and the best methods for their
location follows: First, disconnect the feed pipe from the carburetor and see
if the gasoline flows freely from the tank. If the stream coming out of the
pipe is not the full size of the orifice it is an indication that the pipe is
clogged with dirt or that there is an accumulation of rust, scale, or lint in the
strainer screens of the filter. It is also possible that the fuel shut-off valve
may be wholly or partly closed. If the gasoline flows by gravity the liquid
may be air bound in the tank, while if a pressure-feed system is utilized the
tank may leak so that it does not retain pressure; the check valve retaining
the pressure may be defective or the pipe conveying the air or gas under
pressure to the tank may be clogged.
If the gasoline flows from the pipe in a steady stream the carburetor
demands examination. There may be dirt or water in the float chamber,
which will constrict the passage between the float chamber and the spray
nozzle, or a particle of foreign matter may have entered the nozzle and
stopped up the fine holes therein. The float may bind on its guide, the
needle valve regulating the gasoline-inlet opening in bowl may stick to its
seat. Any of the conditions mentioned would cut down the gasoline supply
and the engine would not receive sufficient quantities of gas. The air-valve
spring may be weak or the air valve broken. The gasoline-adjusting needle

may be loose and jar out of adjustment, or the air-valve spring-adjusting
nuts may be such a poor fit on the stem that adjustments will not be
retained. These instructions apply only to carburetors having air valves and
mixture regulating means which are used only in rare instances in airplane
work. Air may leak in through the manifold, due to a porous casting, or
leaky joints in a built up form and dilute the mixture. The air-intake dust
screen may be so clogged with dirt and lint that not enough air will pass
through the mesh. Water or sediment in the gasoline will cause misfiring
because the fuel feed varies when the water or dirt constricts the standpipe
bore.
It is possible that the carburetor may be out of adjustment. If clouds of
black smoke are emitted at the exhaust pipe it is positive indication that too
much gasoline is being supplied the mixture and the supply should be cut
down by screwing in the needle valve on types where this method of
regulation is provided, and by making sure that the fuel level is at the
proper height, or that the proper nozzle is used in those forms where the
spray nozzle has no means of adjustment. If the mixture contains too much
air there will be a pronounced popping back in the carburetor. This may be
overcome by screwing in the air-valve adjustment so the spring tension is
increased or by slightly opening up the gasoline-supply regulation needle.
When a carburetor is properly adjusted and the mixture delivered the
cylinder burns properly, the exhaust gas will be clean and free from the
objectionable odor present when gasoline is burned in excess.
The character of combustion may be judged by the color of the flame which
issues from it when the engine is running with an open throttle after
nightfall. If the flame is red, it indicates too much gasoline. If yellowish, it
shows an excess of air, while a properly proportioned mixture will be
evidenced by a pronounced blue flame, such as given by a gas-stove burner.
The Duplex Model O. D. Zenith carburetor used upon most of the six- and
eight-cylinder airplane engines consists of a single float chamber, and a
single air intake, joined to two separate and distinct spray nozzles, venturi
and idling adjustments. It is to be noted that as the carburetor barrels are
arranged side by side, both valves are mounted on the same shaft, and work
in unison through a single operating lever. It is not necessary to alter their
position. In order to make the engine idle well, it is essential that the

ignition, especially the spark-plugs, should be in good condition. The
gaskets between carburetor and manifold, and between manifold and
cylinders should be absolutely air-tight. The adjustment for low speed on
the carburetor is made by turning in or out the two knurled screws, placed
one on each side of the float chamber. After starting the engine and
allowing it to become thoroughly warmed, one side of the carburetor should
be adjusted so that the three cylinders it affects fire properly at low speed.
The other side should be adjusted in the same manner until all six cylinders
fire perfectly at low speed. As the adjustment is changed on the knurled
screw a difference in the idling of the engine should be noticed. If the
engine begins to run evenly or speeds up it shows that the mixture becomes
right in its proportion.
Be sure the butterfly throttle is closed as far as possible by screwing out the
stop screw which regulates the closed position for idling. Care should be
taken to have the butterfly held firmly against this stop screw at all times
while idling engine. If three cylinders seem to run irregularly after changing
the position of the butterfly, still another adjustment may have to be made
with the knurled screw. Unscrewing this makes the mixture leaner.
Screwing in closes off some of the air supply to the idling jet, making it
richer. After one side has been made to idle satisfactorily repeat the same
procedure with the opposite three cylinders. In other words, each side
should be idled independently to about the same speed.
Remember that the main jet and compensating jet have no appreciable
effect on the idling of the engine. The idling mixture is drawn directly
through the opening determined by the knurled screw and enters the
carburetor barrel through the small hole at the edge of each butterfly. This is
called the priming hole and is only effective during idling. Beyond that
point the suction is transferred to the main jet and compensator, which
controls the power of the engine beyond the idling position of the throttle.

DEFECTS IN OILING SYSTEMS

While troubles existing in the ignition or carburetion groups are usually
denoted by imperfect operation of the motor, such as lost power, and
misfiring, derangements of the lubrication or cooling systems are usually

evident by overheating, diminution in engine capacity, or noisy operation.
Overheating may be caused by poor carburetion as much as by deficient
cooling or insufficient oiling. When the oiling group is not functioning as it
should the friction between the motor parts produces heat. If the cooling
system is in proper condition, as will be evidenced by the condition of the
water in the radiator, and the carburetion group appears to be in good
condition, the overheating is probably caused by some defect in the oiling
system.
The conditions that most commonly result in poor lubrication are:
Insufficient oil in the engine crank-case or sump, broken or clogged oil
pipes, screen at filter filled with lint or dirt, broken oil pump, or defective
oil-pump drive. The supply of oil may be reduced by a defective inlet or
discharge-check valve at the mechanical oiler or worn pumps. A clogged oil
passage or pipe leading to an important bearing point will cause trouble
because the oil cannot get between the working surfaces. It is well to
remember that much of the trouble caused by defective oiling may be
prevented by using only the best grades of lubricant, and even if all parts of
the oil system are working properly, oils of poor quality will cause friction
and overheating.

DEFECTS IN COOLING SYSTEMS OUTLINED

Cooling systems are very simple and are not liable to give trouble as a rule
if the radiator is kept full of clean water and the circulation is not impeded.
When overheating is due to defective cooling the most common troubles are
those that impede water circulation. If the radiator is clogged or the piping
of water jackets filled with rust or sediment the speed of water circulation
will be slow, which will also be the case if the water pump or its driving
means fail. Any scale or sediment in the water jackets or in the piping or
radiator passages will reduce the heat conductivity of the metal exposed to
the air, and the water will not be cooled as quickly as though the scale was
not present.
The rubber hose often used in making the flexible connections demanded
between the radiator and water manifolds of the engine may deteriorate
inside and particles of rubber hang down that will reduce the area of the

passage. The grease from the grease cups mounted on the pump-shaft
bearing to lubricate that member often finds its way into the water system
and rots the inner walls of the rubber hose, this resulting in strips of the
partly decomposed rubber lining hanging down and restricting the passage.
The cooling system is prone to overheat after antifreezing solutions of
which calcium chloride forms a part have been used. This is due to the
formation of crystals of salt in the radiator passages or water jackets, and
these crystals can only be dissolved by suitable chemical means, or
removed by scraping when the construction permits.
Overheating is often caused by some condition in the fuel system that
produces too rich or too lean mixture. Excess gasoline may be supplied if
any of the following conditions are present: Bore of spray nozzle or
standpipe too large, auxiliary air-valve spring too tight, gasoline level too
high, loose regulating valve, fuel-soaked cork float, punctured sheet-metal
float, dirt under float control shut-off valve or insufficient air supply
because of a clogged air screen. If pressure feed is utilized there may be too
much pressure in the tank, or the float controlled mechanism operating the
shut-off in the float bowl of the carburetor may not act quickly enough.

SOME CAUSES OF NOISY OPERATION

There are a number of power-plant derangements which give positive
indication because of noisy operation. Any knocking or rattling sounds are
usually produced by wear in connecting rods or main bearings of the
engine, though sometimes a sharp metallic knock, which is very much the
same as that produced by a loose bearing, is due to carbon deposits in the
cylinder heads, or premature ignition due to advanced spark-time lever.
Squeaking sounds invariably indicate dry bearings, and whenever such a
sound is heard it should be immediately located and oil applied to the parts
thus denoting their dry condition. Whistling or blowing sounds are
produced by leaks, either in the engine itself or in the gas manifolds. A
sharp whistle denotes the escape of gas under pressure and is usually caused
by a defective packing or gasket that seals a portion of the combustion
chamber or that is used for a joint as the exhaust manifold. A blowing sound
indicates a leaky packing in crank-case. Grinding noises in the motor are
usually caused by the timing gears and will obtain if these gears are dry or

if they have become worn. Whenever a loud knocking sound is heard
careful inspection should be made to locate the cause of the trouble. Much
harm may be done in a few minutes if the engine is run with loose
connecting rod or bearings that would be prevented by taking up the wear
or looseness between the parts by some means of adjustment.

BRIEF SUMMARY OF HINTS FOR STARTING ENGINE

First make sure that all cylinders have compression. To ascertain this, open
pet cocks of all cylinders except the one to be tested, crank over motor and
see that a strong opposition to cranking is met with once in two revolutions.
If motor has no pet cocks, crank and notice that oppositions are met at equal
distances, two to every revolution of the starting crank in a four-cylinder
motor. If compression is lacking, examine the parts of the cylinder or
cylinders at fault in the following order, trying to start the motor whenever
any one fault is found and remedied. See that the valve push rods or rocker
arms do not touch valve stems for more than approximately 1⁄2 revolution in
every 2 revolutions, and that there is not more than .010 to .020 inch
clearance between them depending on the make of the motor. Make sure
that the exhaust valve seats. To determine this examine the spring and see
that it is connected to the valve stem properly. Take out valve and see that
there is no obstruction, such as carbon, on its seat. See that valve works
freely in its guide. Examine inlet valve in same manner. Listen for hissing
sound while cranking motor for leaks at other places.
Make sure that a spark occurs in each cylinder as follows: If magneto or
magneto and battery with non-vibrating coil is used: Disconnect wire from
spark-plug, hold end about 1⁄8 inch from cylinder or terminal of spark-plug.
Have motor cranked briskly and see if spark occurs. Examine adjustment of
interrupter points. See that wires are placed correctly and not short
circuited. Take out spark-plug and lay it on the cylinder, being careful that
base of plug only touches the cylinder and that ignition wire is connected.
Have motor cranked briskly and see if spark occurs. Check timing of
magneto and see that all brushes are making contact.
See if there is gasoline in the carburetor. See that there is gasoline in the
tank. Examine valve at tank. Prime carburetor and see that spray nozzle

passage is clear. Be sure throttle is open. Prime cylinders by putting about a
teaspoonful of gasoline in through pet cock or spark-plug opening. Adjust
carburetor if necessary.

LOCATION OF ENGINE TROUBLES MADE EASY

The following tabulation has been prepared and originated by the writer to
outline in a simple manner the various troubles and derangements that
interfere with efficient internal-combustion engine action. The parts and
their functions are practically the same in all gas or gasoline engines of the
four-cycle type, and the general instructions given apply just as well to all
hydro-carbon engines, even if the parts differ in form materially. The
essential components are clearly indicated in the many part sectional
drawings in this book so they may be easily recognized. The various defects
that may materialize are tabulated in a manner that makes for ready
reference, and the various defective conditions are found opposite the part
affected, and under a heading that denotes the main trouble to which the
others are contributing causes. The various symptoms denoting the
individual troubles outlined are given to facilitate their recognition in a
positive manner.
Brief note is also made of the remedies for the restoration of the defective
part or condition. It is apparent that a table of this character is intended
merely as a guide, and it is a compilation of practically all the known
troubles that may materialize in gas-engine operation. While most of the
defects outlined are common enough to warrant suspicion, they will never
exist in an engine all at the same time, and it will be necessary to make a
systematic search for such of those as exist.
To use the list advantageously, it is necessary to know one main trouble
easily recognized. For example, if the power plant is noisy, look for the
possible troubles under the head of Noisy Operation; if it lacks capacity, the
derangement will undoubtedly be found under the head of Lost Power. It is
assumed in all cases that the trouble exists in the power plant or its
components, and not in the auxiliary members of the ignition, carburetion,
lubrication, or cooling systems. The novice and student will readily
recognize the parts of the average aviation engine by referring to the very

complete and clearly lettered illustrations of mechanism given in many
parts of this treatise.
LOST POWER AND OVERHEATING

PART NATURE OF SYMPTOMS
REMEDY
AFFECTED TROUBLE AND EFFECTS
Water Pipe Loose. Loss of water, Tighten bolts,
Joint. heating. replace gaskets.
Spark Plug. Leakage in Loss of power. Replace
threads, Hissing caused insulation if
insulation, or by escaping gas. defective, screw
packing. down tighter.
Compression Leak in threads. Loss of power. Tighten if loose.
Release Cock. Leak in fitting. Whistling or Grind fitting to
hissing. new seating in
body.
Combustion Crack or Loss of Fill by welding.
Chamber. blowhole. compression. Smooth out
Roughness. Preignition. roughness.
Carbon Scrape out or
deposits. Sharp dissolve carbon.
edges.
Valve Chamber Leak in threads. Loss of Remove. Apply
Cap. Defective compression. pipe compound
gasket. Hissing. to threads and
replace. Use
new gasket or
packing.
Valve Head. Warped. Scored Loss of True up in lathe.
or pitted. compression. Grind to seat.
Carbonized. Scrape off.
Covered with Smooth with
scale. Loose on emery cloth.
stem (two-piece Tighten by
valves only). riveting.

Valve Seat. Warped or Loss of Use reseating
pitted. Covered compression. reamer. Clean
with carbon. off and grind
Foreign matter valve to seat.
between valve
and seat.
Valve Stem. Covered with Valve does not Clean with
scale. Bent. close. Loss of emery cloth;
Binding in compression. straighten. True
guide. Stuck in up and smooth
guide. off. free with
kerosene.
Valve Stem Burnt or rough. Valve may Clean out hole.
Guide. Loose in valve stick. Action Screw in tighter.
chamber. irregular.
Valve Spring. Weak or Valve does not
broken. close.
Valve Operating Loose in guide. Valve action Replace with
Plunger. Too much poor. Lift new. Adjust
clearance insufficient. screw closer.
between valve
stem.
Valve Lift Threads Poor valve Replace with
Adjusting stripped. Too action. new. Adjust
Screw. near valve. Too with proper
far from valve. reference to
valve stem.
Valve Lift Cam. Worn cam Not enough Replace with
contour. Loose valve lift. Will new. Replace
on shaft. Out of not lift valve. pins or keys.
time. Valve opens at Set to open
wrong time. properly.
Cam-shaft. Sprung or Valves out of Straighten.
twisted. time.
Cam-shaft Worn. Not enough Replace.
Bushing. valve lift.

Cam-shaft Loose on shaft. Irregular valve Fasten securely.
Drive Gear. Out of time. action. Time properly.
Worn or broken Replace with
teeth. new.
Cam Worn or broken. Valves out of Replace with
Fastenings. time. new.
Cylinder Wall. Scored, gas Poor Grind out bore.
leaks. Poor compression. Repair oiling
lubrication Overheating. system.
causes friction.
Piston. Binds in Overheating. Lap off excess
cylinder. Walls Poor metal. Replace
scored. Worn compression. with new.
out of round.
Piston Rings. Loss of spring. Loss of Peen ring or
Loose in compression. replace. Fit new
grooves. Gas blows by. rings. Grind
Scored. Worn or smooth.
broken. Slots in Replace. Turn
line. slots apart.
Carbon in Overheating Remove
grooves. because of deposits. File
Insufficient friction. slot. Grind or
opening. lap to fit
Binding on cylinder bore.
cylinder.
Wristpin. Loose, scores Loss of Fasten securely.
cylinder. compression. Replace
cylinder if
groove is deep.
Crank-shaft. Scored or rough Overheating Smooth up.
on journals. because of Straighten.
Sprung. friction.
Crank Bearings. Adjusted too Overheating Adjust freely,
Main Bearings. tight. Defective because of clean out oil
friction. holes and

oiling. Brasses enlarge oil
burned. grooves.
Oil Sump. Insufficient oil. Overheating. Replenish
Poor lubricant. supply. Use best
Dirty oil. oil. Wash out
with kerosene;
put in clean oil.
Water Space. Clogged with Overheating. Dissolve
Water Pipes. sediment or foreign matter
scale. and remove.
Piston Head. Cracked (rare). Loss of Weld by
Carbon compression. autogenous
deposits. Preignition. process. Scrape
off carbon
accumulations.

NOISY OPERATION OF POWER PLANT

PART NATURE OF CHARACTER
REMEDY
AFFECTED TROUBLE OF NOISE
Compression Leakage. Hissing. Previously
Release Cock. given.
Spark Plug. Leakage. Hissing. Previously
given.
Valve Chamber Leakage. Hiss or whistle. Previously
Cap. given.
Combustion Carbon Knocking. Previously
Chamber. deposits. given.
Inlet Valve Seat. Defects Popping in Previously
previously carburetor. given.
given.
Valve Head. Loose on stem. Clicking. Previously
given.
Valve Stem. Wear or Rattle or Previously
Valve Stem looseness. clicking. given.
Guide.

Inlet Valve. Closes too late. Blowback in Previously
Opens too early. carburetor. given.
Valve Spring. Weak or Blowback in Previously
broken. carburetor. given.
Cylinder Retaining bolts Sharp metallic Tighten bolts.
Casting. loose. Piston knock. Round edges of
strikes at upper piston top.
end.
Cylinder Wall. Scored. Hissing. Previously
given.
Valve Stem Too much. Too Clicking. Previously
Clearance. little (inlet Blowback in given.
valve). carburetor.
Valve Operating Looseness. Rattle or Previously
Plunger. clicking. given.
Plunger Guide.
Timing Gears. Loose on Metallic knock. Previously
fastenings. Rattle. given.
Worn teeth. Grinding.
Meshed too
deeply.
Cylinder or No oil, or poor Grinding. Repair oil
Piston. lubricant. system.
Cam. Loose on shaft. Metallic knock. Previously
Worn contour. given.
Cam-shaft Looseness or Slight knock. Previously
Bearing. wear. given.
Cam Fastening. Looseness. Clicking. Previously
given.
Piston. Binding in Grinding or dull Previously
cylinder. Worn squeak. Dull given.
oval, causes hammering.
side slap in
cylinder.
Piston Head. Carbon Knocking. Previously
deposits. given.