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It is almost certain that your training aircraft will be fitted with a four cylinder Lycoming or Continental engine. These areair-cooled flat fours, that is four horizontally opposed cylinders. The old VW Beatle car had an engine of the sameconfiguration and indeed some homebuilt aircraft have used a modified version of this powerplant with some success.
Aircraft engines are built to very different criteria to those in automobiles. As you cannot pull over to the side of the sky inthe event of breakdown, reliability has been the first and foremost priority. These designs have been around for decades, and
just about every possible bug has been designed out. They are also designed to give maximum power output continuously for2000 hours. If you tried that with an automobile engine, it would be in the scrap yard very fast indeed! As a student pilot,
you will be expected to understand the basic principles of operation, and you will be taught how to check oil levels andcheck for integrity of the alternator belt.
E n g i n e O p e r a t i o n
Flat four aero engine - The cylinders are arranged in two banks on opposite sides of the engine. This arrangement is by far the mostcommon.
T h e I n t a k e S t r o k e
During the intake or admission stroke, the piston moves downward as a charge of combustible fuel and air isadmitted into the cylinder through the open intake valve. At the completion of this stroke the intake valvecloses.
C o m p r e s s i o n S t r o k e
During the compression stroke, the crankshaft continues to rotate, the piston is forced upward in the cylinder,and both intake and exhaust valves are closed. The movement of the piston upward compresses the fuel-air
mixture.
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P o w e r S t r o k e
As the piston approaches the top of its stroke within the cylinder, an electric spark jumps across the points ofthe spark plugs and ignites the compressed fuel-air mixture. This is the ignition event. The intake and exhaustvalves are closed.
Having been ignited, the fuel-air mixture burns. It expands as it burns and drives the piston downward. Thiscauses the crankshaft to revolve. Since it is the only stroke and event that furnishes power to the crankshaft,it is usually called the power stroke, although it is sometimes called the expansion stroke for purposes of
instruction. This is event power stroke. The intake and exhaust valves are closed.
E x h a u s t S t r o k e
During the power or expansion stroke, the hot gases obtained by combustion exert tremendous pressure on thepiston to force it to move downward, but near the end of the stroke this pressure is greatly reduced becauseof the expansion of the gases. At this stage, the exhaust valve opens as the crankshaft continues to revolveand the piston is again moved upward in the cylinder by the connecting rod. The burning gases remaining inthe cylinder are forced out through the exhaust valve, hence this stroke is usually called the exhaust stroke,
although it may be called the scavenging stroke for purposes of instruction. One engine cycle has beencompleted.
R e c i p r o c a t i n g - E n g i n e H o r s e p o w e r
According to the most common definition of horsepower, one horsepower is defined as exactly:
1 hp = 745.69987158227022 W
Most persons are acquainted with the term horsepower as applied to automobile and aircraft reciprocatingengines. The horsepower was first used by James Watt during a business venture where his steam enginessubstituted horses. It was defined that a horse can lift 33,000 pounds force (the weight of a 15,000 pound masson Earth) with a speed of 1 foot per minute: 33,000 ftlbfmin-1. This is roughly equivalent to lifting 147,000
Newtons (the weight of a 15,000 kg mass on Earth) at a speed of 0.005 metre per second.
If an aircraft reciprocating engine is rated at 200 horsepower, it means the engine is capable of producing thismuch power. However, the engine has to be running at a certain speed before that much power is produced.The same is true for all other types of reciprocating engines. Unlike car engines, aircraft engines are designedto operate at their full rated power output for a considerable time (usually 2000 hours)
E n g i n e p o w e r o u t p u t
Engine power is the product of force by time: torque is the force and engine speed, measured in crankshaftrevolutions per minute [rpm], is the time. Torque is the rotational force produced by a force acting about theengine crankshaft i.e. it is the product of the firing stroke in the cylinder and the radius of the crank to whichthe connecting rod is attached; and the bigger the cylinder the bigger the force the bang.
N o r m a l l y a s p i r a t e d a e r o e n g i n e s
The maximum power which can be developed, in the cylinders of a particular piston engine, increases ordecreases directly with the intake manifold air density, and air density decreases as altitude increases ortemperature increases. Thus the full throttle power output of a normally aspirated engine ( i.e. one thatrelies solely on the ambient atmospheric density) decreases as operating altitude increases. The diagram inbelow shows how maximum brake horse power, delivered at full throttle in a normally aspirated engine,decreases with altitude. A 100 hp engine operating at 65% power will be delivering 65 hp.
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Power produced is proportional to the air density at the air intake manifold, the cylinder displacement andcompression ratio, the number of cylinders and the rpm. Of those items only the air density at the air intakemanifold and the engine rpm alter, or are alterable, during flight. A traditional four stroke light aircraft
engine, such as the Lycoming O-235, has an individual cylinder displacement of 950 cc, a compression ratio of7:1 and a maximum design speed of 2600 rpm at which its rated 110 brake horsepower [bhp] is produced in sealevel ISA conditions. The Rotax 912, the most common light weight four cylinder aero engine, utilises anindividual cylinder displacement of only 300 cc, a compression ratio of 9:1 but doubles the maximum designspeed to 5500 rpm to achieve its rated 100 bhp. The very light weight Jabiru 2200 has an individual cylinderdisplacement of 550 cc, a compression ratio around 8:1 and a maximum design speed of 3300 rpm to achieveits rated 80 hp.
The three engines mentioned are all horizontally opposed, four stroke and four cylinder; a popularconfiguration providing a fully balanced engine that doesn't require crankshaft balance weights. Engines areoften described in terms of 'total capacity' [cylinder displacement by number of cylinders] in litres or cubiccentimetres. Thus the Lycoming O-235 is 3800 cc or 3.8 litres [235 cubic inches], the Rotax 912 is 1200 cc or1.2 litres and the Jabiru 2200 is 2.2 litres. Most engines used in ultralights tend to be around 30% lighter (interms of weight per rated hp) than the ubiquitous Lycoming and Continental piston engines used in generalaviation aircraft: thus they are cheaper to manufacture but less robust, with a consequent shorter timebetween overhaul [TBO].
The term 'brake horsepower' is a measure of the power delivered at the engine output shaft measured bymeans of a dynamometer or similar braking device. The term 'shaft horsepower' [shp] is a measure of theengine power available at the propeller shaft. Generally it is the same as bhp but if the coupling is not directdrive reduction gearing is interposed between the crankshaft output and the propeller shaft as in the Rotax912 the shp will be a little less than bhp because of the power loss in driving the belt, or gear, drivenreduction device.
Although aero engines can quite happily operate continually at their rated power, doing so is not goodpractise, because it is uneconomical in terms of fuel efficiency but, more importantly, it may shorten enginelife if engine operating temperatures and pressures are exceeded. Normally the maximum and optimum power setting for continuous cruise operation is 75% of rated power.
T u r b o c h a r g i n g
The volumetric efficiency (i.e. the cylinder filling capability) of an engine can be improved by increasing thedensity of the fuel/air charge delivered to the cylinders through compressing the air in the atmospheric airintake manifold; this process is supercharging and develops more torque at all engine speeds. The compressoris usually a lightweight centrifugal impeller driven by a gas turbine that utilises the otherwise wasted engineexhaust gas energy. Such a system is a turbine powered supercharger, otherwise known as a turbocharger. Anoil pressure driven butterfly valve or waste gate is incorporated within the exhaust manifold system,automatically adjusting according to the pressure within the intake manifold to allow all, or a portion, ofthe exhaust gases to bypass the turbine; thus continually maintaining the system within the designed operatinglimits. There is a slight penalty in that turbocharging also increases the temperature of the charge, andconsequently decreases the achievable density, unless a charge cooling device an intercooler is
incorporated between the compressor and the cylinders.
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Turbocharging may be used to increase the sea level rated power of the engine, but the use of that fullthrottle power at low altitudes would normally be limited to short periods because of engine temperaturelimitations. The big advantage is the increase in power available at altitude. The diagram plots the powerachieved (percentage of rated power) at full throttle in ISA standard conditions for a normally aspirated
engine and the turbocharged version. The turbocharged engine can maintain its rated power from sea level upto the 'critical altitude', probably around 6000 or 7000 feet, after which it will decrease. The waste gate wouldprobably be fully open at sea level and then start closing with altitude increase so that it would be fully closedat, and above, the critical altitude.
Turbocharging raises the service ceiling of the aircraft. The service ceiling is the ISA altitude at which theaircraft's best rate of climb (from an extended climb starting at MTOW and unassisted by any atmosphericphenomena) drops below 100 feet per minute which is recognised as the minimum useful climb rate. Thisshould be the aircraft's ceiling quoted by the manufacturer.
The Turbosupercharger and the Airplane Power Plant
General Electric
January, 1943
This pamphlet was published by General Electric, the builders of most U.S.
turbosuperchargers before and during World War II. This document was later published as
Technical Manual TM 1-404, dated 30 December, 1943. Translated to HTML format andproduction Copyright 1997 by Randy Wilson.
NOTE: To keep this document from loading too slowly, not all figures are displayed.
However, all figures are available to view by clicking on the linked Fig. number. To
return to this document, click on your browser's BACK button or arrow.
1. General
a. Rapid progress has been made in the last few years in the development and
production of military aircraft for effective high-altitude operation. High
altitude precision bombing and the advantage of full power at high altitude in
combat have proved their value as important elements of air supremacy.
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b. Airplane power plants of greater power output and reduced weight per unit of
power output are continually being produced.
c. To understand thoroughly the important part played by the turbosupercharger
in achieving the above objectives, it is necessary to consider the source of
power output of a gasoline engine.
2. Combustiona. The combustion, or burning, which occurs within a gasoline engine, is the
source of power which drives an aircraft. The carbon and hydrogen in the
gasoline combine with the oxygen in the air and the resultant vapor enters the
engine. When such a chemical combination takes place, the proportion by
weight of the elements entering into the combination is definitely fixed by
chemical laws. As the composition of gasoline is very uniformly 85 per cent
carbon and 15 per cent hydrogen by weight, and air is 21 per cent oxygen by
weight, the weight of gasoline which will completely burn in a pound of air
without waste is definitely fixed. Fourteen and one-half pounds of air will
support the complete combustion of one pound of gasoline or, as more
commonly expressed, 0.069 pounds of gasoline per pound of air. It isnecessary to have the right amounts of both fuel and air in order to develop
maximum power.
b. If less than this weight of
gasoline is supplied to the
engine for each pound of
air supplied, we say that he
mixture is lean. In this
case, the oxygen in the
excess air can find no
carbon or hydrogen with
which to combine and,
therefore, performs no
useful work in the engine.
The power generated by
the engine is accordingly
reduced below normal, since the available space in the cylinders is not fully
used in burning the fuel from which the power is derived. Reducing the rate at
which fuel is fed to the engine may be regarded as a form of throttling.
c. When more than 0.069 pounds of gasoline is supplied to an engine for each
pound of air supplied, we say the mixture is rich. Some carbon and hydrogen
in the excess gasoline will pass through the engine in a gaseous form withoutbeing burned. As these gases could be burned to perform work if sufficient
oxygen (air) were present, there is a resulting loss of combustion efficiency,
which means poor fuel economy.
The mixture condition, or pounds of fuel per pound of air supplied to an
aircraft engine is called the fuel-air ratio or F/A. Fig. 1 shows the range of
fuel-air ratios used with an aircraft engine. Note that an airplane engine is
usually operated on the rich side of the chemically correct fuel-air ratio
(0.069), and that a very considerable excess of fuel is used for take-off and full
military power. This excess fuel serves to cool the engine, and the engine
tends to run more smoothly with less danger of backfiring into the intake
manifold. There is also an increase in power with richer mixtures.
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d. It is evident from the above discussion that the performance of a gasoline
engine is as dependent upon the weight of air it receives as it is upon the
amount or fuel supplied to it. It is the intent of this manual to discuss the
problems, and the solutions to the problems, of supplying the aircraft power
plant with the combustion air it requires for maximum performance.
3. Supercharginga. The conventional
automobile engine is an
unsupercharged engine. In
this type of engine, air fills
the cylinder when the
piston moves down. Power
developed depends on
pounds of air in the
cylinders. Supercharging,
by in-creasing pressure,
puts more pounds of air inthe same cylinder volume
and, therefore, more power
is developed. Referring to
Fig. 2, the amount of
mixture charge which
flows into the cylinder during the intake stroke depends upon the difference
between the pressure in the manifold (Pm) and the pressure in the cylinder
(Pc). There must be a pressure difference between these two points to offset
the pressure losses caused by the flow of the mixture through the intake valve
and to overcome the inertia of the gases in the manifold. The pressure in the
manifold is always somewhat less than atmospheric pressure (Pa) because of
the resistance to flow set up by the carburetor. If the pressure of the mixture in
the manifold (Pm) is increased by mechanical means above the manifold
pressure round in the unsupercharged engine, more mixture charge will flow
into the cylinder, with resulting increase in horsepower. The process of
mechanically increasing the manifold pressure is called supercharging, and
where this is done the engine is a supercharged engine.
4. The Effect of Altitude on Engine Performance
a. The weight of the earth's atmosphere is sufficient to exert considerable
pressure on objects at sea level. At altitudes above sea level, the atmospheric
pressure will be lower. For example, the weight of air above the earth exerts apressure or 14.7 pounds per square inch on objects at sea level. The weight of
air above 25,000 feet exerts a pressure or only 5.45 pounds per square inch on
objects at that altitude. Fig. 3shows the variation of the pressure of standard
air with altitude.
b. The density of air is the weight of a cubic root of air. Density of air depends
upon its pressure and temperature. (The effect of temperature on density will
be discussed later.) The greater the pressure of the air, the greater is the weight
of a cubic foot of air. The less the pressure, the less the air weight per cubic
foot. At 25,000 feet, a cubic foot of air weighs only 45 per cent as much as a
cubic foot of air at sea level. It was stated that the engine required an adequate
weight of air, in order to develop its rate output. As the altitude at which anaircraft is flown is increased, the air pressure becomes less, and the density, or
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pressure (and, therefore, at the right density). Either a pump or a blower may
be designed to do this. When the proper condition of pressure is created in the
intake manifold, the weight flow of the mixture into the engine cylinder will
be adequate.
b. The engine manifold pressure, being a measure of the weight flow of the fuel-
air mixture to the engine cylinders, is also a measure of the power output ofthe engine running at a constant speed. Fig. 6 shows the relative effect of
manifold pressure on the power output of an engine. From this figure it is
evident that increasing the manifold pressure (supercharging) is a most
effective way of greatly increasing the horsepower output of an engine without
increasing its size and weight.
c. It is very important to note that, during warm-up or flight operation, the
maximum manifold pressure specified by the engine manufacturer must not be
exceeded. Excessive manifold pressure will force so great a weight charge of
mixture into the engine that the power output, internal heat, and mechanical
stress within the engine will be greater than that for which the engine was
designed. This overstressing of the engine would greatly reduce its life ofoperation and might result in immediate engine failure. This is a frequent
cause for burning up cylinders and blowing off cylinder heads.
d. Operation recommendations provide for use of high manifold pressure and
maximum power for short periods of time. This "military" or "take-off.' power
is used when high power is required for take-off or to meet a military
emergency in combat.
e. The characteristics of the centrifugal compressor make it the most effective
type for aircraft-engine supercharging. This type of compressor operates most
effectively at high speeds, and has the ability to compress a large volume of
air at low pressure. Because the centrifugal compressor runs at high speeds, its
size is relatively small and its weight is light. It also has minimum moving
parts, and the problems of lubrication and maintenance are thereby minimized.
The centrifugal compressor consists of three basic elements -- the impeller, the
diffuser, and the casing. Air enters the impeller at the center and is discharged
radially at the ends of the blades with high velocity energy. The diffuser
converts this energy to pressure energy. The casing collects the air under
pressure for delivery to the engine induction system. Fig. 7 shows a
centrifugal compressor with half of the casing removed to expose the impeller
and diffuser.
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7. Superchargersa. Internal Superchargers
1. A supercharger which is located between the carburetor outlet and the
intake manifold of the engine is called an internal supercharger. A
supercharger thus located serves to provide a uniform distribution of
the fuel-air charge to the various cylinders, as well as to increase the
density of the charge. Maintaining a high manifold pressure ahead of
the intake valves allows the use of "valve overlap" in the engine, so
that the intake valve opens just before the exhaust valve closes at the
end of the exhaust stroke. This allows the compressed mixture from the
intake manifold to scavenge the spent gases out of the clearance
volume of the cylinder and also tends to improve the cooling of the
exhaust valve. This cooling is of particular importance when operating
at "military" or "take-off" power. With no supercharging between the
carburetor and the intake valves, "valve overlapping" would permit
exhaust gases to flow back into the intake manifold. This would cause
backfiring and dilution of the next charge of mixture with burned
exhaust gases.
2. An internal supercharger is always used to obtain maximum
performance from modern high-power aircraft engines and high-grade
fuels. The internal supercharger is built into the airplane engine and is
called a geared supercharger.3. The power required to drive the geared supercharger is taken, through
a train of gears, from the engine crankshaft. Thus, the net power output
available to drive the propeller is decreased by the amount which is
taken from the crankshaft to drive the geared supercharger.
b. External Superchargers
1. A supercharger located ahead of the carburetor in the induction system
is called an external supercharger. An external supercharger is used
primarily to obtain full-power engine performance at high altitudes,
and is generally driven by an exhaust-gas turbine. A supercharger so
driven is called a turbosupercharger. In a turbosupercharged power
plant, the high-altitude air is compressed to approximately sea-levelpressure before delivery to the engine carburetor. The temperature of
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the air passing through the compressor of the turbosupercharger is
considerably increased as the result of the compression. This effect of
temperature increase is similar to the increase in the temperature of the
barrel of a tire pump during use.
2. If the temperature of the air entering the engine exceeds certain limits,
detonation (knocking) will occur in the engine. This detonation wouldcause a sharp decrease in the power output of the engine, and would
greatly overstress parts of the engine. The temperature limit of the
carburetor-inlet air at which detonation in the engine will occur
depends upon the design of the particular engine and the temperature
rise in its internal supercharger.
3. Also, the density (weight per cubic foot) of the air charge again enters
the picture. As the temperature of the entering air is increased, its
density becomes less. To offset the temperature rise due to
compression, an intercooler is installed between the turbosupercharger
air discharge and the carburetor inlet.
4. This intercooler is similar in its operation to the conventionalautomobile radiator, except that the transfer of heat is from the
compressed air to the cooling air instead of from water to the cooling
air. The cooling air is taken from a "ramming" air intake on the aircraft
and is fed by means of ducts through one series of passages in the
intercooler. Hot air from the turbosupercharger is fed through another
series of passages, running in a cross-direction to the cooling air. These
two paths of air flow are separated by thin metal walls, and the heat
transfer takes place through these walls.
5. Cooling air leaving the intercooler is normally ducted to a discharge
opening in the aircraft structure. Shutters or doors are located in the
cooling-air circuit so that the pilot can control the temperature of the
air entering the carburetor by varying the amount of cooling air used in
the intercooler. In practice, the intercooler is so designed that the
maximum temperature of the air which enters the carburetor will not
exceed a relatively high sea- level temperature (90 F to 100 F).
6. The engine exhaust is connected directly to the nozzle box of the
turbosupercharger with a gastight stack. In order that the energy in the
hot exhaust gases may be used to furnish the power which is used by
the turbine to drive the turbosupercharger, it is necessary to build up
the pressure in the exhaust stack and nozzle box sufficiently, so that
the gases will acquire a high velocity when expanding through thenozzles of the turbine down to the pressure of the atmosphere in which
the airplane is flying. Tests have shown that the pressure built up in the
exhaust stack by the turbine is almost the same as the pressure built up
in the carburetor by the compressor, and is normally about sea-level
pressure, regardless of altitude at which the airplane is flying. Thus,
under normal conditions of operation, the engine would receive its
charge at sea-level density and would exhaust at sea-level pressure.
Therefore, the engine will develop sea-level horsepower up to the rated
altitude of the installation. In brief, the turbo-supercharger supplies an
artificial sea-level atmosphere to the engine.
7. The turbosupercharger may be used to increase rated engine power atsea level, or to furnish "ground boost", as well as to maintain rated
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power at high altitude. Although the turbosupercharger speed required
for this purpose is much less than the rated speed, it is necessary to
build up the engine exhaust pressure appreciably above the
atmospheric pressure at sea level, and other low altitudes, in order to
furnish turbine power. The amount by which this exhaust pressure can
be increased without unduly affecting engine operation will dependupon the particular engine in question. It is very important that the
instructions covering the specific aircraft involved be closely followed
in the operation of a turbosupercharged power plant.
c. Turbosuperchargers
1. At the present time, turbosuperchargers are used in series with geared
superchargers, the intercooler and carburetor being located between
them. In this way, maximum use can be made of the advantages of
each type.
2. A geared supercharger has one obvious advantages of compactness,
lightness, and ease of installation. The greatest disadvantage of the
geared supercharger is its application for high-altitude flight is itsinflexibility of speed. If it is designed to develop sea-level pressure at
20,000 feet, for example, it will deliver an excessively high pressure at
sea level with the carburetor throttle opened wide, so that the throttle
must always be partially closed for low-altitude operation. However,
since the speed is not reduced, the supercharger drive still subtracts
from the power available to the propeller, an amount approximately
equal to the power taken at rated altitude. In an effort to reduce these
difficulties, two-speed and two-stage geared superchargers have been
designed and built. These are better than the single-speed, single-stage
machine, but they necessitate an increase of size, weight, and
complexity. No matter how many stages or different gear ratios are
used, such a geared supercharger can never have the perfect flexibility
of speed control of a turbosupercharger, and must always involve some
waste of power when operating below the altitude for which it was
designed. The speed of the turbosupercharger can be controlled to
maintain desired conditions of carburetor-inlet pressure without regard
to the engine speed.
d. Operating Characteristics
1. The operating
characteristics of
engines equippedwith various types
of superchargers is
shown on the
general altitude-vs-
engine-horsepower
diagram ofFig. 8.
The line (A)
indicates the
variation in
horsepower of an
engine builtwithout an internal
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supercharger or a turbosupercharger. As illustrated, the power begins
to fall off as soon as altitude is increased. The plane can fly to
considerable altitudes, but the power is so reduced that finally a point
will be reached where there is insufficient power to maintain the plane
in level flight.
2. Line (B) shows the approximate characteristics of an engine equippedwith a single-stage geared supercharger. In this case, there is some
reduction in power at sea level because some of the engine power is
required to drive the supercharger and, hence, is not available to the
propeller. This deficiency in power over that of an unsupercharged
engine is overcome very shortly as altitude is increased, and nearly
constant power continues to the critical altitude of the geared
supercharged engine, which is in the order of 6000 feet to 7000 feet.
Beyond this altitude power diminishes as altitude is increased, but the
altitude at which the plane will continue in level flight is greater than
that of an unsupercharged engine.
3. Line (c) shows the characteristic of a two-stage geared superchargerand, here again, the altitude at which engine power begins to fall off
has been advanced, because of the higher pressure ratio obtainable
when two stages of compression are employed. Somewhat the same
effect can be obtained by single-stage, two-speed super chargers. In
either of these combinations, however, the critical altitude where the
power begins to diminish is at about 18,000 feet.
4. Line (D) indicates the characteristic of a plane equipped with single-
stage internal-supercharged engines and turbosuperchargers. The
power available to the propeller is still the same as obtained from a
single-stage supercharged engine, and this power continues to an
altitude of approximately 25,000 feet. Beyond the critical altitude,
there is sufficient power to maintain flight to altitudes of at least
35,000 feet.
8. The Turbosupercharged Power Plant
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a. It is well to consider the turbosupercharged power plant with reference to the
part played by each of the elements which go to make up a turbosupercharger
system.Fig. 9 is a schematic diagram of a complete turbosupercharger system.
b. Air, which will eventually be supplied to the engine to support combustion, is
taken into the system through what is called a "ramming" air intake. This
intake is usually located on the leading edge of the engine nacelle or wing, andis designed to take full advantage of the velocity of the plane through the air or
the force of the air velocity of the propeller wash, to obtain what is, in effect, a
small amount of supercharging in the intake itself. This air is directed to the
inlet of the compressor of the turbosupercharger where the first compression
of the air is made. The amount of the compression depends upon the speed at
which the turbosupercharger is operated, and the compression ratio may be as
high as 2.86 to 1. This means that atmosphere taken into the compressor at an
altitude of 25,000 feet with apressure of approximately 11 inches of mercury,
may be discharged from the compressor with a pressure of approximately 50
inches of mercury. As a result of this compression, the temperature of the air
leaving the compressor would be sufficiently high to cause detonation, if feddirectly to the carburetor. For example, if the entering air is -30 F at an altitude
of 25,000 feet, the temperature of the air leaving the compressor would be
approximately 150 F. To reduce this temperature, the air is passed through the
intercooler before going to the carburetor. As mentioned before, this cooling
also serves to increase the density of the air charge. The mixture of the air and
fuel takes place at the inlet to the geared supercharger. The temperature of the
mixture leaving the carburetor is somewhat less than the entering air
temperature, because of the removal from the air of the heat requiredd to
vaporize the fuel.
c. The mixture charge from the carburetor is then fed to the inlet of the gear-
driven internal supercharger. In this second stage of compression, a
compression ratio of approximately 1.5 to 1 at rated engine speeds is normally
used. With the moderately low compression ratio of this stage, the temperature
rise of the mixture is not excessive, but sufficient pressure is maintained to
assure uniform distribution of the mixture to the cylinders and to allow the use
of "valve overlap" for exhaust-gas scavenging and exhaust-valve cooling.
d. Hot exhaust gases from the cylinders are collected in manifold or collector
ring through the exhaust stack to the nozzle box of the turbosupercharger. The
nozzles are designed to allow the gases to expand, and, thereby, to reach high
velocities before striking the buckets of the turbine wheel. Exhaust gases
which are not required to drive the turbine are by-passed through a waste gateto atmosphere before reaching the turbine nozzles. This completes the cycle of
the air required by the engine or combustion as it is passed through a turbo-
supercharged power plant.
Fig. 10 shows pressures and temperatures encountered in a typical
turbosupercharger application operating at 25,000 feet.
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9. Description of Turbosupercharger
a. Fig. 11 shows a cutaway view of a typical turbosupercharger. This is simply a
high-speed centrifugal compressor driven by a turbine which derives its power
from the hot exhaust gases of the engine. The exhaust stack connects to the
nozzle box (A), which is directly above the turbine wheel (B). The hot gasesescape from the nozzle box through fixed nozzles (C). The nozzles permit an
expansion of the exhaust gases which increases velocity and directs it against
the buckets (D) on the turbine wheel. The high speed and power of the turbine
are the results of the flow of these high-velocity gases against the turbine
buckets. The speed of the turbosupercharger is controlled by allowing excess
gases, not required for turbine operation, to escape through the waste gate (B),
instead of through the turbine nozzles and turbine wheel. With the waste gate
closed, all the gases will go through the turbine, and it will revolve with
maximum speed and power. With the waste gate wide open, the turbine will
idle.
b. The power developed is transmitted through the shaft (F) to the impeller (G)of the centrifugal compressor, which is mounted on the opposite end of the
shaft. Engine air is ducted from a ramming air intake on the leading edge of
the wing or front of the nacelle to the inlet of the compressor. The impeller (O)
and diffuser (H) are enclosed in a suitable housing called the compressor
casing (J), which collects the compressed air from the diffuser. The rotating
assembly is supported by ball and roller bearings which carry the thrust and
static loads imposed. The ball bearing (K) is located on the impeller end of the
shaft, and takes the thrust load of the shaft, which is in the direction of the
impeller when the turbosupercharger is running. The roller bearing (L) allows
for expansion of the shaft.
c. The baffle ring (M) assures the proper distribution of the turbosupercharger
cooling air between the nozzle box and the compressor casing. The baffle ring
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also serves as a shield to prevent the transfer of heat from the nozzle box to the
compressor casing by radiation.
d. The turbosupercharger is lubricated by a built-in oil pump (N), which is driven
by a worm gear from a worm sleeve keyed to the shaft. The pump and
bearings are enclosed by the bearing-and-pump casing (P). The turbine wheel
is cooled by a cooling cap. The standard type of cooling cap is of theconvection type, and directs cooling air from the aircraft slipstream against the
turbine wheel. Fig. 12,13, and 14 show the completed turbosupercharger
assembly.
10.Lubrication of Turbosuperchargers
a. Lubrication pump
1. The turbosupercharger lubrication pump is really two separate
positive-displacement pumps on the same shaft. One of the elements of
the pump supplies oil to the gears and bearings. The other element is a
scavenging pump which removes oil from the housing and returns it to
the supply tank.
2. There is a tachometer connection on the end of the pump shaft, fromwhich the speed of
the
turbosupercharger
can be determined.
b. Fig. 15 is a schematic
diagram of a
turbosupercharger
lubrication system, using a
separate oil tank. The oil
for lubricating the
turbosupercharger from the
oil pump enters the inside
of the bearing and pump
housing through a shroud
(A) which lubricates the pump drive gear (B). The oil is transferred from the
shroud by the drive gear to the mesh of the drive gear and the worm thread
sleeve (C). The bearings are oiled by the splash from the drive gear, and by the
oil mist which exists inside of the bearing housing as a result of the high
rotation speeds and churning of the oil. This combination of splashing and oil
mist is ideal lubrication for the ball and roller bearings. Some
turbosuperchargers are designed with jets which deliver oil directly on the balland roller bearings. This provides no better lubrication than the oil mist, but it
does provide for more efficient cooling of the bearing.
c. The capacity of the scavenging element of the pump is about three times that
of the pressure pump at all times. Because of this, two thirds of the
scavenging-pump delivery is air. The pumping of this air causes a slight
vacuum in the bearing housing, which is necessary to prevent oil leakage
through the shaft oil seals. The two shaft oil seals (D), one on the turbine end
and the other on the compressor end of the bearing housing, are not rubbing
seals, but have a clearance from the shaft of 0.002 in. to 0.005 inches. These
seals are threaded to cause an inward flow, which tends to keep the oil inside
the pump and bearing casing. This action is assisted by the vacuum which iscreated inside the casing by the excess capacity of the scavenging pump.
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d. The dumbbell valve (E) operates by gravity. The intake of the scavenging
pump is through this valve, and the valve position will always be such that the
scavenging-pump intake will draw only from the bottom of the bearing and
pump housing, regardless of the position of the plane in flight.
e. Turbosuperchargers are usually installed with a separate oil-supply tank of
about one to two gallons capacity, which is normally about 75 per cent full ofoil. The excess volume of the tank is necessary to accommodate any foaming
of the oil which may be induced by the scavenging pump.
f. The rotor of the turbosupercharger operates at extremely high speeds
compared with speeds normally encountered in other equipment -- 21,300 rpm
for a rated altitude of 25,000 feet for one type. At this speed, the balls in the
ball bearing, for example, are rotating at approximately 60,000 rpm about their
own axis. Bearings which will stand up under these extreme conditions of
speed are of special design, and are manufactured with extra-fine precision. It
is obvious that special care must be used in the handling and fitting of these
bearings during overhaul operations. It is also of utmost importance that no
foreign matter be allowed to get into the lubricating-oil system, and thatrecommendations on the oil used and the method of operation of the
turbosupercharger be closely followed.
11.Turbosupercharger Coo1ing Requirements
a. Fig. 16 shows a schematic diagram of the various paths of cooling-air flow
required in a normal turbosupercharger installation. Cooling air is required for
the exhaust-stack shroud, the turbine-wheel cooling cap, the back of the
turbine nozzle box, the compressor casing, the bearings, and the intercooler.
b. The exhaust stack is that part of the exhaust system which conducts the
exhaust gas from the engine collector ring or exhaust manifold to the turbine
nozzle box. In the exhaust stack are one or more flexible joints to allow for
thermal expansion and engine vibration. The usual design of the exhaust stack
includes a ventilated shroud which is a concentric pipe surrounding the
exhaust stack, and which is ventilated by a rammed-cooling-air blast. This
exhaust-stack shroud serves the dual purpose of forming a fire wall around the
high-pressure exhaust stack, and providing a means for precooling the exhaust
gases before entering the turbine nozzle box.
c. The convection-type cooling cap delivers cooling air to the rim of the turbine
wheel at the point of attachment of the turbine buckets to the wheel blank. The
air is discharged on the trailing edge of the wheel, to avoid recirculation of the
air over the wheel.
d. The bearings, compressor casing, and back of the nozzle box are cooled by aduct which delivers air radially inward toward the center of the
turbosupercharger. This air stream is divided by the baffle ring, with
approximately 40 per cent of the air passing between the baffle ring and
nozzle box, and the remainder between the baffle ring and compressor casing.
e. The volume of intercooler cooling air required is normally about double the
amount of the engine air which is cooled. Flaps or shutters, located
downstream from the intercooler in the cooling-air stream, provide a control of
the air temperature entering the carburetor.
f. The turbosupercharger cooling air is taken aboard where full use can be made
of the propeller slipstream, or of the velocity of the plane, to assure adequate
air supply and distribution. Care is taken in locating the cooling-air ducts, sothat minimum heat will be picked up before the air reaches the
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turbosupercharger or intercooler. In particular, the cooling-duct intakes are
located so that no hot exhaust gas or discharged engine-cooling air will be
rammed into the cooling passages.
12.Turbosupercharger Regulation
a. One of the advantages of the turbosupercharger is flexibility of control. The
speed of the turbosupercharger rotor and, consequently, the pressure suppliedto the engine, is controlled by changing the amount of exhaust gases that pass
through the nozzles to drive the turbine wheel. Opening the waste gate allows
more exhaust gases to be bypassed, and the rotor speed is decreased.
Conversely, closing the waste gate will increase the rotor speed.
b. A hydraulic regulator automatically moves the waste gate to hold the nozzle-
box pressure constant at the value determined by the position of the boost-
control lever in the cockpit. By changing the setting of the regulator, the
proper exhaust pressure and corresponding manifold pressure can be obtained
for the desired engine power, such as cruising, normal or military power.
c. Fig. 17 is a diagram of a typical hydraulic regulator. There is a tube leading
from the nozzle box to the top bellows. The bottom bellows is evacuated andserves to prevent the top bellows from acting in response to atmospheric
changes in pressure. Inside the top bellows is mounted a spring, one end of
which is connected to the junction between the two bellows, and the other end
to the range-shifting control lever. This lever is connected by linkages to the
cockpit boost control. The purpose of the spring and control-lever assembly is
to allow the pilot to vary the pressure on the spring for different nozzle-box
pressure, corresponding to different engine powers. This spring tension just
balances the pressure in the top bellows to the point where the servo-valve
ports to the servo piston are closed.
d. As pressure changes occur in the top bellows, they act on the servo valve and
shift its position, thereby opening ports which direct oil under pressure into
one side of the piston. The piston then moves under the unbalanced oil
pressure, and moves the waste gate with it. The servo piston continues to
move the waste gate until the pressure in the nozzle box has been corrected.
When corrected, the pressure in the bellows restores the servo valve to its
closed-port position, and stops the piston motion.
e. The regulating process, described above, occurs in a very short space of time,
a few seconds at most. Therefore, in actual operation, as soon as the exhaust
pressure starts to change, the hydraulic regulator starts to move the waste
gatem a direction to counteract this change. That is, for a given setting of the
cockpit boost lever, the regulator always acts to maintain a constant exhaustpressure, and the actual exhaust pressure varies from this constant value only
temporarily during those few seconds required by the hydraulic regulator to
move the waste gate in a direction which restores the pressure.
f. For some pursuit airplanes equipped with turbosuperchargers, the size of the
evacuated bellows in the regulator is reduced. This causes the atmospheric
pressure to have some effect on the upper bellows and results in a slightly
decreasing nozzle-box pressure with increasing altitude. This type of regulator
holds an approximately constant manifold pressure without changing the
setting of the boost control lever up to the airplane's rated altitude.
13.Installation Considerations
a. Induction System
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1. The design and construction of the induction system is of primary
importance in the application of a turbosupercharger system. The
induction system carries the required engine air from the slipstream to
and through the compressor element of the turbosupercharger, then
through the intercooler to the engine carburetor.
2. Mention has been made of the use of rammed-air intakes. Theseintakes are specifically designed to take the required weight flow of air
on board the airplane with the least disturbance of the slipstream over
the air foil section. The "rammed-air intakes" are preferably located on
the leading edge of the wing or engine cowl. When air scoops, which
project from the surface of the aircraft are used, the trailing surface of
the scoop is appropriately streamlined so that minimum drag is
introduced. Because of the forward motion of the plane, the "rammed-
air intakes" tend to increase the pressure at the inlet to the compressor.
This gain in pressure achieved by the ramming-engine-air intake, is
particularly important in the turbosupercharged power plant, because
this gain is multiplied by the compression ratios of the two stages ofcompression which follow.
3. The duct from the ramming-air intake usually consists of two or more
sections of pipe connected by flexible joints. These flexible joints are
installed between the compressor and the ramming-air intake to isolate
vibration of the ship structure from the high-speed turbosupercharger
compressor. These flexible connectors are also incorporated between
the turbosupercharger compressor and the intercooler, and between the
intercooler and the carburetor-air intake. In the case of the connecting
duct to the carburetor inlet, the motion of the engine in its dynamic
mounting must also be isolated by the flexible connectors. Such
flexible connectors usually consist of a neoprene or synthetic-rubber
sleeve which is band-clamped to the abutting end of the duct section.
4. Another factor in the design of the induction system is to minimize the
pressure losses resulting from sharp bends, rapid changes in cross
section, and use of undersize ducts. Usually the airplane design dictates
the path of the induction system. The best use must be made of the
space available to keep the induction-system losses low.
5. Since the internal pressure of the induction system at high altitudes is
as much as 10 to 11 pounds per sq in. greater than the external
atmospheric pressure, the design requires that the induction system
withstand a pressure differential of 20 pounds per sq in., withoutleakage. Any leakage which develops in the system represents a loss,
and detracts from the efficiency of the installation.
b. The installation of an inter-cooler of the proper size and design is important in
maintaining the efficiency of the turbosupercharged power plant. The
intercooler should provide adequate cooling of the air discharged from the
compressor, to assure the proper charge density. At the same time, the inter-
cooler should not be of excessive weight, and should not offer too great a
resistance to the engine air flow in the induction system.
c. The exhaust system of the turbo-supercharged power plant carries the exhaust
gas from the cylinders to the nozzle box of the turbosupercharger. This piping
must also contain flexible joints to isolate vibration, and also to allow forexpansion caused by heat. It is very important that the exhaust piping shall
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exert no stresses on the nozzle box. If stresses are allowed, the nozzle box will
be distorted. During high-altitude operation, the pressure inside the exhaust
manifold may be 10 to 11 pounds per sq in. greater than the atmospheric
pressure. Again care must be taken to prevent leakage. Leaks in the exhaust
system will always tend to enlarge because of the high temperatures of the
exhaust gases. In normal installations, the exhaust stack is encircled by anexhaust cooling shroud. Cooling air is forced through this annular passage.
Gases leaking from the exhaust pipe will leak into this space, and not into the
plane structure. Leakage of the exhaust gases will detract from the power
available to drive the turbosupercharger, and will thereby reduce the critical
altitude of the installation.
d. Fig. 18, 19, 20, 21, and 22 are schematic diagrams of typical
turbosupercharger applications.
14.Care and Maintenance of Turbosuperchargers
a. Turbosupercharger installations require very careful inspection and
maintenance in service. The turbosupercharger, although very simple in
construction, operates at very high speeds. Very great differences intemperature are prevalent in a relatively small piece of equipment.
The effective operation of the turbosupercharged power plant depends upon
the efficient operation of all its component parts.
b. Specific instructions for inspection and maintenance are covered by Technical
Orders covering this type of equipment. A brief summary of the items which
require daily inspection follows:
1. Check the turbosupercharger, exhaust system, induction system,
intercooler, and control system for security of mounting and evidence
of failure.
2. Inspect the turbine buckets for distortion or other evidences of failure.
3. Check clearance between turbine wheel and nozzles.
4. Inspect and lubricate the waste gate.
5. Check the oil supply for correct amount, and inspect the lubrication
system for leaks.
6. Immediately, at beginning engine run-up, determine that the turbine
wheel is rotating freely. Check operation of the regulator to determine
if sluggish.
c. Turbosupercharger overhaul, as well as any other repairs involving
disassembly of the unit, should be made only at air depots. To avoid damage
to the unit, the instructions covering the removal of the turbosupercharger
from the airplane should be carefully followed.d. Bearing replacement is recommended procedure on each normal overhaul.
e. The supercharger parts having been reconditioned, the complete rotor
assembly, Fig. 23, must be dynamically balanced, in accordance with the
detailed instructions of the Army Air Forces Technical Orders covering this
subject. Fig. 24 shows the balancing machine used for this purpose. Balance
of the impeller end of the rotor is obtained by removing material at the
scallops of the impeller. The turbine-wheel end of the rotor is balanced by
removing material (by grinding) from the inner face of the turbine wheel.
The nozzle box is made of corrosion resistant steel, and repairs involving
welding are made with a type of welding electrode approved by the
manufacturer. This electrode is coated, so that the resulting weld haspractically the same properties as the welded material.
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15.Operation
a. A turbosupercharger aircraft must be operated in strict accordance with the
specific instruction provided in the Army Air Forces Technical Order for
operation of each type of airplane. Operation may be discussed in a general
manner by considering the characteristics of the turbosupercharger and the
effect of the turbosupercharger on the aircraft power plant under differentconditions of flight operation.
b. Starting
The engine should be warmed up in the usual manner. The turbosupercharger
regulator should be in the OFF position so that the waste gate is in the open
position. Particular care must be taken to avoid any backfiring which may
distort the waste gate or balloon the nozzle box.
c. Take-off
After warm-up of the engine, set the propeller at the rpm specified for take-
off, then open the throttle wide and bring up to the desired manifold pressure
as quickly as possible by use of the supercharger regulator control. Lock the
regulator control and close the throttle after the turbosupercharger has hadsufficient time to reach the proper speed, and the manifold pressure becomes
stabilized. This procedure should be followed for each engine in the case of
multiengine airplanes. The airplane may then be taxied out for takeoff, and
when all the throttles are opened wide, the turbosupercharger regulator will
advance the manifold pressure to the predetermined value.
d. Climbing
After take-off, the regulator should be adjusted to hold the pressure specified
for climb, which will usually be less than required for take-off. If the regulator
is actuated by the exhaust pressure at the nozzle box, as is customary at
present, and the setting is kept constant, the waste gate will gradually close as
the airplane ascends, and the nozzle-box pressure will remain approximately
constant. Under such conditions, the intake-manifold pressure will increase
with altitude, and it will be necessary for the pilot to reset the regulator at
intervals to prevent this pressure from becoming too high. The engine should
never be supercharged more than the specified manifold pressure
recommended for each particular installation. Serious damage to the engine is
certain to result if excessive supercharging is continued.
e. Cruising
When necessary to reduce engine power considerably below full power, such
as for cruising, the manifold pressure should be reduced with the supercharger
control. It is not good practice to obtain reduced power by maintaining a highcarburetor-inlet pressure and partially closing the carburetor throttle, since this
produces an unnecessarily high back pressure against which the engine must
exhaust. This is not only harmful to the engine, but also tends to develop
excessive operating speeds of the supercharger rotor. It is sometimes found
advantageous to keep a high carburetor-inlet pressure and reduce the engine
speed by changing the propeller setting.
f. Descent
In descending from high altitudes, sufficient power should be used to keep the
engine warm. It will be noticed that the response to sudden opening of the
throttle in turbosupercharged engines appears to be slow. This fact must be
kept in mind when maneuvering close to the ground and when landing.
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g. In some types of pursuit airplanes, the turbosupercharger boost control and the
throttle are interlocked, so that they can be operated by a single lever. In this
case, the actual operation of the turbosupercharged power plant must be
constantly kept in mind.
Turbosupercharger regulators are now in use, which automatically reduce the
exhaust pressure of the system, as the altitude at which the airplane is flown isincreased. This results in maintaining an essentially constant manifold
pressure of the engine to the rated altitude of the airplane. This type of
regulator reduces the amount of turbosupercharger boost adjusting which the
pilot must do during climb to rated altitude.
h. High altitudes
1. At high altitudes, in addition to the effect of low atmospheric
pressures, the effects of the low temperatures encountered are of great
importance. The turbosupercharger must always be operated at
sufficient speed to insure that the oil in its lubrication system will not
congeal because of the low temperatures encountered. If the oil is
allowed to congeal, the interruption of the lubrication will causeserious damage to the bearings and pump drive. In particular, high-
altitude cruising or prolonged gliding should never be done with the
boost control in the OFF position.
2. Carburetor de-icing may be effected with the turbosupercharger power
plant by reducing or cutting off the cooling air to the intercooler. This
will cause high-temperature air from the compressor of the
turbosupercharger to supply the carburetor.
3. It is particularly important that the manifold pressure of the engine be
reduced as the airplane climbs above its rated altitude. If this is not
done, the turbosupercharger will be seriously overspeeded while
supplying air in excess of its rated design.
4. Flight test work on airplanes equipped with turbosuperchargers and
automatic-pilot control has indicated that extreme caution should be
taken. Supercharged airplanes will fly at altitudes far above those at
which the human body can survive without the aid of special
equipment.
Even with an abundance of oxygen, crew members should preferably be so
located that no person is entirely alone in any one compartment. Furthermore,
it is dangerous to move from one compartment to another without oxygen.
Pilots, especially, should be cautious in the use of the automatic pilot at highaltitudes, both from the control standpoint and the possibility of gradually
falling asleep as the oxygen supply is diminished. In this case, the airplane
will continue to fly on the automatic pilot until the gas supply is exhausted.
16.Care in Operation and Maintenance
The turbosupercharger has proved very reliable in operation under all conditions of
high-altitude combat flying. It requires the same care in operation and maintenance
that is required by any piece of high-precision equipment. Occasionally, in situations
of great emergency, the turbosupercharger is called upon to perform far in excess of
its design rating. The ability of the turbosupercharger to so perform, and its ultimate
life under such conditions, depend largely on the care which was exercised in itsprevious operation and maintenance.