Engine basics

Question 1

The pressure exerted on the piston during the power stroke increases as the mass of charge induced
[a] increases and combustion temperature increases
[b] decreases and combustion temperature increases
[c] increases and combustion temperature decreases
[d] decreases and combustion temperature decreases

Answer 1

A
Pressure in a gas depends upon the number of molecules present ie mass of charge,
and the speed at which each molecule moves ie temperature of the charge.

Question 2

As the throttle is moved towards the fully open position
[a] manifold pressure increases and mass flow decreases
[b] manifold pressure decreases and mass flow increases
[c] manifold pressure increases and mass flow increases
[d] manifold pressure decreases and mass flow decreases

Answer 2

C
Manifold pressure is the pressure outside the cylinder at the inlet port. The position of
the throttle decides how much gas flows through the inlet manifold to the port. The
higher the manifold pressure, the greater the mass flow of gas into the cylinder when
the inlet valve opens.

Question 3

The volumetric efficiency of an engine depends upon
[a] throttle position, ambient temperature, ambient pressure and RPM
[b] throttle position only
[c] throttle position, ambient temperature, ambient pressure but not RPM
[d] throttle position and mixture strength

Answer 3

A
Factors affecting volumetric efficiency: Anything that reduces the mass of gas that flows into
the cylinder will reduce volumetric efficiency. If we ignore engine design features which the pilot
can do nothing about, the factors affecting volumetric efficiency include:-
Ambient air density. Obviously the number of molecules that can be drawn into the cylinder will
be controlled to a great degree by the number of molecules available in the outside air in the first
place, ie ambient air density. Hot days and high altitudes reduce the engine’s volumetric
efficiency.
Throttle position. Volumetric efficiency is at its best when the engine is operating at full throttle.
This results in maximum flow into the cylinders. As the throttle is closed, the flow of gases into
the cylinders is restricted, reducing volumetric efficiency and decreasing power output.
Engine RPM. We have seen that valve and ignition timing are designed around one particular
RPM setting. At high RPM the velocity of the flow through the induction system increases. This
gives rise to increased friction with the tubes, ports and valves. Also at high RPM, the inlet and
exhaust valves are open for a shorter time, giving less opportunity for gas to flow into or out of
the cylinders.
The temperature of the incoming charge. Hot air expands and becomes less dense. If the air is
heated on its way to the cylinders volumetric efficiency will be reduced. This could be due to high
engine temperatures or the application of carburettor heat. You will hear more about carburettor
heat later.
Supercharging. A supercharger compresses the air before it enters the cylinders. This produces
a much higher mass flow, increasing volumetric efficiency.

Question 4

The best action to take at the onset of detonation in an engine is
[a] lean the mixture and reduce the power
[b] lean the mixture and increase the power
[c] decrease the indicated air speed and maintain the power
[d] select mixture fully rich and decrease the power

Answer 4

D
Anything that decreases the temperature of the charge will help minimise the risk of
detonation. The most immediate effect will always be achieved by placing the mixture
control into fully rich. This sends extra cooling fuel to where it is most needed - the
inside of the cylinder.

Question 5

The onset of detonation in an engine is indicated by
[a] vibration, rising temperatures and reduced indicated air speed
[b] vibration, falling temperatures and reduced indicated air speed
[c] vibration, rising temperatures and increased indicated air speed
[d] vibration, falling temperatures and increased indicated air speed

Answer 5

A
The explosion of the charge sends shocks through the engine which are felt as vibrations.
The sudden release of the heat of combustion while the piston is at or near TDC
concentrates the heat into the cylinder head, causing the temperature to rise and power
to drop.

Question 6

One of the limitations applying to increased RPM for increased power in a piston engine is
[a] high fuel consumption
[b] excessive propeller tip speed
[c] high oil pressure
[d] high cylinder head temperature

Answer 6

B
As propeller RPM increase, the propeller tip speed approaches and may exceed the
speed of sound. This degrades the propeller’s aerodynamic efficiency.

Question 7

The warm up period for an engine prior to take off provides
[a] proper oil viscosity and uniform heating of engine components
[c] higher oil pressure for take off
[c] a means of expelling moisture from the engine crank case
[d] adequate fuel pressure for take off

Answer 7

A
As the engine heats up, the viscosity of the oil is brought into the range required for
effective lubrication.

Question 8

If an engine is overheating during a long climb, an appropriate pilot action would be
[a] raise the nose to reduce indicated air speed
[b] lean the mixture to best economy
[c] reduce power and indicated air speed
[d] increase indicated air speed, richen the mixture and if necessary, reduce power

Answer 8

D
Engine temperature depends upon the rate at which heat is being generated [power],
and the rate at which it is being carried away [IAS]. The extra fuel in a rich mixture
helps reduce the temperature of combustion.

Question 9

If cylinder head temperatures are becoming too low during a long descent, the pilot should
[a] reduce indicated air speed and power
[b] increase indicated air speed and reduce power
[c] increases indicated air speed and power
[d] reduce indicated air speed and increase power

Answer 9

D
This is really the opposite to question 8 above. Increase the rate at which heat is being
generated and decrease the rate at which it is being carried away.

Question 10

The octane rating of a fuel is a measure of
[a] its specific gravity
[b] its resistance to detonation
[c] its resistance to vaporisation
[d] its anti-misting properties in the event of fire

Answer 10

B
The higher the octane rating of a fuel, the greater its ability to withstand compression
and heat without detonating.

Question 11

A horizontally opposed engine should be held at about 1000 RPM after a cold start rather than idle to avoid
[a] damage due to vibration at low RPM
[b] excessive cylinder wear due to poor lubrication at low RPM
[c] damage due to low oil pressure at idle
[d] a large increase in the time required to raise engine temperatures

Answer 11

B
A horizontally opposed engine relies on oil being flung from the rotating crankshaft to
adequately lubricate the cylinder walls.

Question 12

A radial engine always has an uneven number of cylinders, commonly 5, 7 or 11. This is a necessary design feature
to ensure
[a] uniformly spaced power strokes during the cycle
[b] adequate engine cooling
[c] correct mass balancing during high power operation
[d] enough space is left between cylinders for proper air cooling during flight

Answer 12

A
The only way to fire every cylinder during two rotations of the crankshaft is to fire
every second cylinder in the direction of rotation.

Question 13

The function of oil in an engine is to
[a] clean
[b] lubricate
[c] cool
[d] all of the above

Answer 13

D
1 It lubricates by providing a boundary layer of oil between moving parts to
prevent metal to metal contact. This reduces friction and energy loss and
prevents excessive wear and damage to engine components.
2 It cools by carrying heat away to the oil cooler where it is dissipated to the air.
This is not unlike the action of water in a car engine, which carries engine heat to
the radiator.
3 It cleans by carrying away sludge and other residue from the moving parts of the
engine and depositing them in the engine oil filter.
4 It seals the spaces between the cylinder walls and the piston rings preventing
gases from leaking past during the compression and power strokes.
5 It protects the metal components of the engine from oxygen, water and other
corrosive agents. It forms a cushion between surfaces under high impact loads.

Question 14

The viscosity of an oil is a measure of
[a] the oil’s ability to flow
[b] the oil’s resistance to flow
[c] the temperature at which it will burn
[d] the oils detergent properties

Answer 14

B
The higher the viscosity, the ‘thicker’ the oil becomes. It resists flowing and spreading.

Question 15

The purpose of an oil cooler bypass is
[a] to prevent the oil from becoming too hot
[b] to return overheated oil to the cooler
[c] to prevent oil from passing through the cooler if it is already cold
[d] to allow oil to bypass the cooler if the cooler becomes blocked

Answer 15

C
Oil coolers are more correctly called oil temperature regulators. It is important to
warm the oil when it is cold, just as it is important to cool it when it is hot.

Question 16

If airflow to the oil cooler is interrupted by an obstruction in the duct
[a] oil temperature and oil pressure will rise
[b] oil temperature will drop and oil pressure will rise
[c] oil temperature will rise and oil pressure will fall
[d] oil temperature and oil pressure will both fall

Answer 16

C
Airflow through the cooler is required to carry the heat away. If the airflow is interrupted,
the oil temperature will rise. Hot oil flows too easily and eventually the
pressure will become lower.

Question 17

One cause of high oil temperature and low oil pressure could be
[a] very low oil level in the sump
[b] the oil’s viscosity being too high for the engine type
[c] the oil sump being overfilled
[d] the oil cooler bypass not working

Answer 17

A
The lower the quantity of oil in the sump, the more frequently it must circulate to carry
engine heat away. The oil that is present will be come hotter.

Question 18

The purpose of gearing a propeller in an aircraft engine is to permit the propeller to turn at
[a] higher RPM and lower torque than the engine
[b] lower RPM and torque than the engine
[c] lower RPM and higher torque than the engine
[d] higher RPM and lower torque than the engine

Answer 18

C
Higher RPM produce more engine power. However, high RPM reduces the propeller’s
efficiency. A gear box allows the engine’s power to be transmitted to the propeller in
the form of greater torque but lower RPM.

Question 19

Operating an engine with too low an oil quantity will produce
[a] rising oil temperature and pressure
[b] falling oil temperature and rising oil pressure
[c] falling oil pressure and falling oil temperature
[d] rising oil temperature and dropping oil pressure

Answer 19

D
The lower the quantity of oil in the sump, the more frequently it must circulate to carry
engine heat away. The oil that is present will be come hotter.

Question 20

During a long climb, the cylinder head temperature becomes too high. This can be rectified by
[a] closing the cowl flaps
[b] reducing the climbing indicated air speed
[c] leaning the mixture to best power
[d] richening the mixture to full rich and increasing the climbing indicated air speed

Answer 20

D
Engine temperature depends upon the rate at which heat is being generated [power],
and the rate at which it is being carried away [IAS]. The extra fuel in a rich mixture
helps reduce the temperature of combustion.

Question 21

If maximum power is applied for take-off while the oil temperature is too low
[a] the engine components could suffer stresses due to uneven heating
[b] take-off manifold pressure could be lower than normal
[c] cylinder head temperature would become too high during take-off
[d] take-off power would be severely reduced

Answer 21

A
Engine warm-up is essential prior to demanding take-off power. This not only ensures
that the oil is the correct viscosity to properly lubricate the various components, but
also lessens the ‘thermal shock’ of large and sudden temperature increase.

Question 22

If the oil pressure gauge begins to fluctuate during flight
[a] the oil temperature is too high
[b] the oil pressure gauge is unserviceable
[c] the oil temperature is too low
[d] the oil quantity is very low

Answer 22

D
If oil level is very low, as the oil moves about in flight eg in turbulence, the oil pump
pick-up screen becomes uncovered causing the pump to suck air at intervals. This
causes fluctuations in oil pressure. To cause this to happen, the oil level would have to
be critically low. Oil temperature would be very high.

Question 23

The cause of an abnormally high oil pressure indication could be
[a] oil quantity is too low
[b] oil temperature is too low
[c] oil temperature is too high
[d] the oil sump is overfilled

Answer 23

B
Oil has a high viscosity at low temperature. This offers a high resistance to the oil
pump causing oil pressure to rise.

Question 24

If the oil level in an operating engine is below the specified minimum
[a] the engine could overheat at high power settings
[b] oil temperature would be lower than normal
[c] engine power will be reduced
[d] there will be a large power loss due to increased engine friction

Answer 24

A
The lower the quantity of oil, the hotter that oil becomes. At high power settings, the
smaller quantity of oil must carry the extra heat away from the engine.

Question 25

To prevent excessive cooling of an engine during a long descent at a fixed throttle setting it is necessary to
[a] decrease indicated air speed and accept a reduced rate of descent
[b] increase indicated air speed and accept a reduced rate of descent
[c] decrease indicated air speed and increase rate of descent
[d] increase indicated air speed and increase rate of descent

Answer 25

A
[Assuming a cruise power-on descent at high IAS.] To reduce the rate at which heat is
being carried away from the engine, the IAS would have to be reduced. Power would
be kept constant, so the rate of descent would decrease.

Question 26

If oil temperature is rising to near the red line during a long climb a remedy would be
[a] decrease power and indicated air speed
[b] increase power and indicated air speed
[c] decrease power and increase indicated air speed
[d] increase power and decrease indicated air speed

Answer 26

C
To help cool the engine, the rate at which heat is being generated [power], must be
reduced. The rate at which heat is being carried away [IAS], should be increased. The
aircraft would suffer a decrease in the rate of climb.

Question 27

A high cylinder head temperature during cruise could be due to
[a] manifold pressure too low for the selected RPM
[b] mixture set too rich
[c] cowl flaps left open
[d] detonation or pre-ignition

Answer 27

D
Both detonation and preignition are accompanied by a marked increase in engine
temperature.

Question 28

Spark plug fouling would be most likely during
[a] long periods of ground operation at low power
[b] climbs at high power settings
[c] cruising flight in cold weather
[d] operation in conditions where carburettor ice is likely to form

Answer 28

A
Especially in horizontally opposed engines, the oil tends to ‘pool’ on the bottom of the
cylinder causing plug fouling at low temperatures. The bottom plugs are usually the
culprits.

Question 29

One consequence of operating an engine with excessively high oil temperature is
[a] Spark plug fouling
[b] inadequate lubrication of some engine parts
[c] a very high oil pressure
[d] sticking exhaust valves

Answer 29

B
Very high oil temperature reduces the viscosity of the oil to the point where it no
longer maintains an unbroken film over the surfaces. This increases the possibility of
metal to metal contact and rapid engine wear.

Question 30

Oil pressure is usually measured -
[a] immediately before the pump.
[b] immediately after the pump.
[c] immediately before the oil enters the engine.
[d] as the oil returns to the sump.

Answer 30

C
Oil pressure is usually measured just before the oil goes to do its vital work i.e. just
before it enters the engine oil gallery.

Question 31

The rate at which the engine converts heat energy to mechanical energy
[a] is the indicated horsepower
[b] is the friction horsepower
[c] is the indicated horsepower or torque
[d] is the torque

Answer 31

A
Indicated Horse Power is a measure of the power going to actually move the piston
[mechanical energy] after allowing for the heat energy lost through the exhaust and
engine components.

Question 32

On a 4-stroke engine, the ignition is timed so that the spark occurs
[a] after TDC on the power stroke
[b] before TDC on the compression stroke
[c] after TDC on the compression stroke
[d] before TDC on the power stroke

Answer 32

B
The spark occurs just before TDC [between 20° and 30° before the piston reaches the
top of its travel]. This allows time for the fuel to burn so that combustion is complete
at the point where maximum torque can be produced.

Question 33

What is the effect of increased humidity on engine power output?
[a] no effect at sea level, but reduced output at altitude
[b] power output will decrease at all altitudes because
of the decreased density of the fuel/air charge
[c] the increased density of the fuel/air will increase power output at all altitudes
[d] no change in power output

Answer 33

B
Water vapour is less dense than dry air - so a mixture of water vapour and air is less
dense than the same volume of dry air. Humid air is lighter than dry air.

Question 34

As the OAT increases, power output from the engine will
[a] increase
[b] decrease
[c] remain the same
[d] increase if the humidity also increases

Answer 34

B
Hot air is less dense than cold air - that’s what makes a hot air baloon rise. An increase
in temperature has the same effect on air density as an increase in height. Power will
be lost.

Question 35

On an in-line engine with a 4-stroke cycle, each stroke commences
[a] when the piston is at either BDC or TDC
[b] when the inlet valve is either at BDC or TDC
[c] when the inlet valve is open
[d] when the exhaust valve is closed

Answer 35

A
Each stroke starts at either TDC or BDC. Each stroke occupies 180° of crankshaft
rotation i.e. half a turn.

Question 36

The ratio of the power output of an engine to the power developed by the pistons is known as
[a] compression ratio
[b] mechanical efficiency
[c] volumetric efficiency
[d] thermal efficiency

Answer 36

B
This is really the ratio between Indicated Horse Power and Brake Horse Power.

Question 37

The compression ratio of an engine
[a] depends on the clearance volume
[b] depends on the clearance volume and the swept volume
[c] is the term given to the swept volume
[d] depends on the swept volume

Answer 37

B
Compression ratio compares the clearance [or unswept] volume with the total volume
above the piston when it is at BDC i.e. the swept volume + the clearance volume. A
change to either the swept or clearance volume will change the compression ratio.

p27

Question 38

The duration of each stroke of a 4-stroke cycle is equivalent to
[a] 360 degrees of crankshaft rotation
[b] 180 degrees of crankshaft rotation or half a revolution
[c] 720 degrees of crankshaft rotation
[d] half a revolution so long as there are only 4 cylinders

Answer 38

B
Each stroke occupies half a turn of the crankshaft.

Question 39

During the compression and power strokes of a 4-stroke cycle
[a] the exhaust valve remains closed
[b] the inlet valve closes during the compression stroke and the exhaust valve opens during the
power stroke
[c] the inlet valve closes during the powe stroke and the exhaust valve opens during the
compression stroke
[d] both valves remain open

Answer 39

B
The inlet valve is open during the induction stroke and closes just after the compres
sion stroke begins. This allows for gas to continue to flow through the inlet port due to
inertia. It is called ‘valve lag’. The exhaust valve opens before BDC.

Question 40

To find the firing interval of a 4-stroke engine
[a] divide 720 degrees by the number of cylinders
[b] divide 720 degrees by 4
[c] divide 260 degrees by 4
[d] divide the number of degrees of crankshaft rotation by the number of cylinders

Answer 40

A
Firing interval of any four stroke engine is 720° ÷ Number of cylinders.

Question 41

An engine with a combustion chamber of 13 cubic inches capacity and a swept volume of 78 cubic inches, has a
compression ratio of
[a] 5 to I
[b] 6 to I
[c] 7 to I
[d] 8 to I

Answer 41

C
The compression ratio compares the volume above the piston when it is at BDC
[swept + clearance] with the volume above the piston when it is at TDC [clearance or
unswept]. It compares 13 with 78+13. i.e. 13 with 91. 91 ÷ 13 = 7 So the volume
after compression is one seventh of the volume before compression. See figure below
right.

Question 42

When the inlet valve is closed after BDC in a 4-stroke engine, the maximum quantity of fuel/air charge entering
the cylinder is ensured. This modification to the valve timing is known as
[a] valve lead
[b] valve overlap
[c] inlet valve lead
[d] valve lag

Answer 42

D
The inlet valve is open during the induction stroke and closes just after the compres
sion stroke begins. This allows for gas to continue to flow through the inlet port due to
inertia. It is called ‘valve lag’. The exhaust valve opens before BDC.

Question 43

Friction horsepower is
[a] the difference between the brake horsepower and the rated horsepower
[b] the difference between the indicated horsepower and the brake horsepower
[c] brake horsepower divided by indicated power
[d] the difference between indicated horsepower and the rated power of the engine

Answer 43

B
Friction Horse Power represents the loss between the power delivered to the piston
[Indicated Horse Power] and the power that is available to drive the crankshaft [Brake
Horse Power].

Question 44

How many degrees of crankshaft rotation are required to fire all cylinders on an in-line engine using the 4-stroke
cycle?
[a] depends on the number of cylinders
[b] 360
[c] 720
[d] 760

Answer 44

C
The engine must rotate twice to allow each of the four strokes to occur.

Question 45

Question No 45
The position of the valves during the intake stroke of an actual four stroke cycle engine is
[a] inlet open but starting to close at start of stroke and exhaust closed throughout
[b] inlet opening at start of stroke and exhaust closed throughout
[c] inlet and exhaust open at beginning of stroke, exhaust open and inlet closed at end of stroke
[d] exhaust open but starting to close at start of stroke and inlet open throughout

Answer 45

D
The exhaust valve closes late [after BDC on the exhaust stroke]. That means that it is still closing at the beginning of the intake [also called induction] stroke.

Question 46

An engine with a combustion chamber of 9 cubic inches capacity and a swept volume of 45 cubic inches has a
compression ratio of
[a] 6 to 1
[b] 5 to 1
[c] 7 to 1
[d] 8 to 1

Answer 46

A
(Swept + Unswept) ÷ Unswept [or combustion chamber volume].
(45 + 9) ÷ 9 = 6. The compression ratio is 6 to 1.

Question 47

The purpose of a scavenge pump in an oil system is to
[a] to pump the oil from the sump to the gallery in a wet sump system
[b] to pump the oil from the gallery to the sump in a wet sump system
[c] to provide stand-by power if the main oil pump fails in a dry sump system
[d] to remove the oil from the engine and send it to the external oil tank in a dry sump system

Answer 47

D
The scavenge pump is found only in a dry sump oil system.
It removes oil from the engine and pumps it to the external tank.