Piston Engines

7/30/2019 Piston Engines

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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


17/21

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


18/21

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


19/21

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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20/21

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.


21/21

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.

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