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.
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.
[Illustration: Fig. 75.--Showing Use of Magnifying Glass to Demonstrate
that Apparently Smooth Metal Surfaces May Have Minute Irregularities
which Produce Friction.]
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 deg. and 1,400 deg.
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.
[Illustration: 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
[Illustration: Fig. 77.--Diagram of Oiling System, Hall-Scott Type A 125
Horse-Power Engine.]
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.
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 deg. F.; viscosity of not less than 75
to 85 taken at 21 deg. 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.
DRY CRANK-CASE SYSTEM BEST FOR AIRPLANE ENGINES
[Illustration: 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.]
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.
[Illustration: 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.
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.
[Illustration: Fig. 80.--Why Pressure Feed System is Best for
Eight-Cylinder Vee Airplane Engines.]
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 33-1/3 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.
[Illustration: 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
[Illustration: 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.
[Illustration: 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.
[Illustration: 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
[Illustration: Fig. 85.--Anzani Testing His Five-Cylinder Air Cooled
Aviation Motor Installed in Bleriot Monoplane. Note Exposure of Flanged
Cylinders to Propeller Slip Stream.]
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.
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
4-1/2 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.