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ANAC GMP Theoretical Exam (Powerplant Group) study materials
ANAC GMP Theoretical Exam (Powerplant Group) study resources

ANAC GMP Theoretical Exam (Powerplant Group) practice questions

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Question 1Aircraft Powerplant Fundamentals and Theory

In a four-stroke reciprocating aircraft engine, during which stroke is the intake valve normally open?

  1. Power stroke
  2. Compression stroke
  3. Exhaust stroke
  4. Intake stroke Correct answer.

Explanation

The intake stroke draws the fuel-air mixture into the cylinder, so the intake valve must be open while the piston moves down. During the other three strokes the intake valve is essentially closed, although slight overlap with adjacent strokes occurs at high power settings.

Question 2Aircraft Powerplant Fundamentals and Theory

What is the primary purpose of the compressor in a turbojet engine?

  1. To cool the turbine blades before combustion
  2. To supply high-pressure air to the combustion chamber Correct answer.
  3. To convert exhaust heat into shaft power
  4. To increase the velocity of the exhaust jet directly

Explanation

The compressor raises the pressure of incoming air so that combustion occurs at high pressure, which greatly increases the work available from the expanding gases. The turbine then extracts energy from those gases to drive the compressor and accessories.

Question 3Aircraft Powerplant Fundamentals and Theory

In a gas turbine engine, which component drives the compressor?

  1. The turbine Correct answer.
  2. The ignition exciter
  3. The nozzle
  4. The accessory gearbox

Explanation

The turbine extracts energy from the hot expanding combustion gases and converts it into rotational shaft work. A shaft connects the turbine to the compressor, so the turbine supplies the power needed to compress the incoming air.

Question 4Aircraft Powerplant Fundamentals and Theory

Which type of engine produces thrust mainly by accelerating a large mass of air with a small velocity increase?

  1. Turbojet with afterburner
  2. High-bypass turbofan Correct answer.
  3. Pure turbojet
  4. Single-spool turboprop with no propeller

Explanation

A high-bypass turbofan moves a large mass of air through the fan at a modest velocity increase, which is efficient at typical airliner speeds. Thrust equals mass flow multiplied by velocity change, so moving more air slowly is more efficient than moving less air very fast.

Question 5Aircraft Powerplant Fundamentals and Theory

What is the function of the exhaust nozzle in a turbojet engine?

  1. To compress incoming air before combustion
  2. To expand hot gases and increase exhaust velocity Correct answer.
  3. To mix fuel with air before ignition
  4. To reduce turbine inlet temperature

Explanation

The nozzle converts remaining pressure energy in the exhaust into kinetic energy, accelerating the gases rearward. By Newton's third law, this rearward acceleration produces forward thrust on the engine.

Question 6Aircraft Powerplant Fundamentals and Theory

What does the term 'compression ratio' describe in a reciprocating engine?

  1. Power output divided by engine displacement
  2. Cylinder volume at bottom dead center divided by volume at top dead center Correct answer.
  3. Fuel flow divided by airflow
  4. Manifold pressure divided by ambient pressure

Explanation

Compression ratio compares the total cylinder volume when the piston is at the bottom of its stroke with the clearance volume remaining at the top. A higher ratio generally raises thermal efficiency but increases detonation risk.

Question 7Aircraft Powerplant Fundamentals and Theory

Which of the following best describes the Brayton cycle?

  1. A refrigeration cycle using a compressor and expansion valve
  2. A steam cycle using condensation and boiling
  3. Constant-volume heat addition cycle used by reciprocating engines
  4. Constant-pressure heat addition cycle used by gas turbines Correct answer.

Explanation

The Brayton cycle models the gas turbine with compression, constant-pressure heat addition, expansion, and heat rejection. The Otto cycle, by contrast, adds heat at constant volume and models the spark-ignition piston engine.

Question 8Aircraft Powerplant Fundamentals and Theory

What is the purpose of the ignition system in a turbine engine during starting?

  1. To drive the starter motor
  2. To keep the engine running continuously after light-off
  3. To ignite the fuel-air mixture in the combustion chamber Correct answer.
  4. To pressurize the fuel manifold

Explanation

Igniters provide a high-energy spark that initiates combustion of the fuel-air mixture during start. Once combustion is self-sustaining, the flame remains lit without continuous sparking on most engines.

Question 9Aircraft Powerplant Fundamentals and Theory

A turbofan engine has a bypass ratio of 5. What does this mean?

  1. The exhaust velocity is five times the inlet velocity
  2. The engine has five compressor stages
  3. Five times as much air passes through the bypass duct as through the core Correct answer.
  4. The fan produces five times the thrust of the core

Explanation

Bypass ratio is the mass flow through the bypass duct divided by the mass flow through the engine core. A ratio of 5 means five units of air bypass the core for every unit that passes through it.

Question 10Aircraft Powerplant Fundamentals and Theory

As altitude increases while indicated airspeed is held constant, what happens to true airspeed?

  1. It remains constant
  2. It increases Correct answer.
  3. It decreases
  4. It decreases only above the tropopause

Explanation

True airspeed depends on air density, which falls with altitude. For a given indicated airspeed, the lower density means the aircraft must move faster through the air to produce the same dynamic pressure, so true airspeed rises.

Question 11Aircraft Powerplant Fundamentals and Theory

In a turboprop engine, what component converts turbine shaft power into thrust?

  1. The combustion chamber
  2. The diffuser
  3. The exhaust nozzle
  4. The propeller Correct answer.

Explanation

A turboprop uses the turbine to drive a propeller through a reduction gearbox, and the propeller accelerates a large mass of air to produce thrust. A small amount of residual jet thrust may also come from the exhaust.

Question 12Aircraft Powerplant Fundamentals and Theory

What is the primary function of the diffuser section in a gas turbine engine?

  1. To cool the combustion gases
  2. To mix fuel with air
  3. To reduce air velocity and increase static pressure Correct answer.
  4. To increase air velocity before the compressor

Explanation

The diffuser converts kinetic energy of the incoming air into static pressure by slowing the flow. This prepares the air for the compressor and, in some engines, provides a suitable velocity for combustion.

Question 13Aircraft Powerplant Fundamentals and Theory

Why are turbine blades often cooled with compressor bleed air?

  1. To allow operation at gas temperatures above the blade material's melting point Correct answer.
  2. To prevent the turbine from rotating too fast
  3. To reduce the compressor workload
  4. To increase the mass flow through the turbine

Explanation

Turbine inlet temperatures can exceed the melting point of the blade alloy, so internal cooling passages and film cooling keep the metal within safe limits. This allows higher temperatures and therefore greater efficiency and power.

Question 14Aircraft Powerplant Fundamentals and Theory

Which factor most directly determines the detonation margin of a reciprocating engine at a given power setting?

  1. Cylinder head temperature and fuel octane rating Correct answer.
  2. Oil pressure and oil temperature
  3. Propeller pitch and aircraft weight
  4. Alternator output and battery charge

Explanation

Detonation occurs when the end gas ignites spontaneously under high pressure and temperature. High cylinder head temperature and low fuel octane rating both reduce the margin before detonation begins.

Question 15Aircraft Powerplant Fundamentals and Theory

What happens to thrust as an aircraft accelerates at constant engine power setting?

  1. Thrust increases proportionally with speed
  2. Thrust generally decreases as forward speed increases Correct answer.
  3. Thrust becomes zero at cruise speed
  4. Thrust remains exactly constant

Explanation

Thrust is roughly proportional to the difference between exhaust velocity and aircraft velocity. As forward speed rises, this difference shrinks, so net thrust falls even though the engine setting is unchanged.

Question 16Aircraft Powerplant Fundamentals and Theory

What is the role of the accessory gearbox on a turbine engine?

  1. To drive pumps, generators, and the starter through engine rotation Correct answer.
  2. To compress air for combustion
  3. To reduce exhaust noise
  4. To control fuel flow electronically

Explanation

The accessory gearbox is driven by a shaft from the engine and provides mounting and drive for accessories such as fuel pumps, oil pumps, generators, and the starter. It transmits engine power to these components rather than producing thrust.

Question 17Aircraft Powerplant Fundamentals and Theory

In a reciprocating engine, what is valve overlap?

  1. The clearance between valve stem and rocker arm
  2. The difference between intake and exhaust valve diameters
  3. The time during which both valves are closed
  4. A period when both intake and exhaust valves are open near top dead center Correct answer.

Explanation

Valve overlap occurs near the end of the exhaust stroke and start of the intake stroke, when both valves are briefly open. It helps scavenge exhaust gases and improve cylinder filling at higher engine speeds.

Question 18Aircraft Powerplant Fundamentals and Theory

Which of the following describes the function of the fuel control unit in a turbine engine?

  1. It meters fuel to match the selected power setting and protects against limits Correct answer.
  2. It cools the fuel before combustion
  3. It filters contaminants from the fuel
  4. It ignites the fuel-air mixture

Explanation

The fuel control unit schedules fuel flow according to throttle position, compressor speed, and ambient conditions while preventing overspeed and overtemperature. It is the heart of engine power management.

Question 19Aircraft Powerplant Fundamentals and Theory

Why does a gas turbine engine generally produce more thrust at lower ambient temperatures?

  1. Fuel burns more completely in cold air
  2. The turbine expands gases more efficiently at low temperature
  3. Compressor pressure ratio decreases in cold air
  4. Denser air increases mass flow through the engine Correct answer.

Explanation

Cold air is denser, so for a given volumetric flow the engine ingests more mass per second. Since thrust depends on mass flow, the engine produces more thrust on a cold day than on a hot day.

Question 20Aircraft Powerplant Fundamentals and Theory

What is the function of the inlet guide vanes ahead of the first compressor stage?

  1. To support the turbine blades
  2. To meter fuel into the airstream
  3. To increase the engine's bypass ratio
  4. To direct airflow into the compressor at an appropriate angle Correct answer.

Explanation

Inlet guide vanes turn the incoming air to the proper angle for the first rotor stage and, on some engines, modulate airflow to improve stall margin. They are stationary and do not produce thrust themselves.

Question 21Aircraft Powerplant Fundamentals and Theory

During a hung start in a turbine engine, what is the typical observed condition?

  1. Engine accelerates normally but oil pressure falls
  2. Engine starts but the generator fails to come online
  3. Engine speed stabilizes below idle and exhaust gas temperature rises Correct answer.
  4. Engine speed overshoots idle and fuel flow decreases

Explanation

A hung start occurs when the engine fails to accelerate to idle, often due to insufficient starter power or fuel scheduling. The starter cuts out at a low speed, and continued fuel flow can cause exhaust gas temperature to climb toward limits.

Question 22Aircraft Powerplant Fundamentals and Theory

A turbojet engine operating at high subsonic speed experiences a rise in compressor inlet temperature due to ram effect. What is the consequence for the compressor?

  1. The compressor efficiency rises sharply
  2. The pressure ratio available before exceeding temperature limits decreases Correct answer.
  3. The turbine inlet temperature falls automatically
  4. The mass flow increases without limit

Explanation

Ram rise raises compressor inlet temperature, so the same pressure ratio produces a higher compressor discharge temperature. This reduces the temperature margin available for combustion and lowers the usable pressure ratio.

Question 23Aircraft Powerplant Fundamentals and Theory

In a free-turbine turboshaft engine, why is the power turbine mechanically independent of the gas generator?

  1. To reduce the number of bearings required
  2. To prevent the compressor from stalling
  3. To eliminate the need for a reduction gearbox
  4. To allow the output shaft speed to vary independently of gas generator speed Correct answer.

Explanation

A free power turbine is driven only by the gas stream from the gas generator and is not connected by shaft to the compressor. This lets the rotor or output shaft slow or stop while the gas generator keeps running, which suits helicopters and variable-load applications.

Question 24Aircraft Powerplant Fundamentals and Theory

Why does a high-bypass turbofan generally have a lower specific fuel consumption than a low-bypass turbofan at the same cruise speed?

  1. It operates at a higher exhaust velocity
  2. It burns fuel at a higher combustion temperature
  3. It accelerates a larger mass of air to a lower velocity increase Correct answer.
  4. It uses a smaller fan diameter

Explanation

Propulsive efficiency improves when a large mass of air is accelerated only slightly, because less kinetic energy is left wasted in the jet. A high-bypass fan therefore delivers the same thrust with lower fuel burn at cruise speeds.

Question 25Aircraft Powerplant Fundamentals and Theory

A reciprocating engine shows a sudden rise in oil temperature and a drop in oil pressure in cruise. Which interpretation is most consistent with these indications?

  1. A faulty tachometer reading
  2. A rich mixture setting that needs leaning
  3. Normal behavior as the engine warms up
  4. Possible loss of oil supply or oil pump degradation requiring immediate attention Correct answer.

Explanation

Rising oil temperature combined with falling oil pressure suggests the oil system is losing its ability to circulate and cool, such as a pump problem or a leak. This is a serious condition that warrants reducing power and landing as soon as practicable.

Question 26Aircraft Powerplant Fundamentals and Theory

In a four-stroke reciprocating engine, during which stroke does the intake valve normally close?

  1. At the start of the intake stroke, before top dead center
  2. At the midpoint of the compression stroke
  3. At the end of the exhaust stroke, after top dead center
  4. At the end of the intake stroke, after bottom dead center Correct answer.

Explanation

The intake valve typically remains open past bottom dead center so that the incoming charge can continue to fill the cylinder while piston velocity is still low. It then closes early in the compression stroke. This late closing is a deliberate timing choice related to volumetric efficiency, not a fault.

Question 27Aircraft Powerplant Fundamentals and Theory

A reciprocating engine is running at a constant manifold pressure and rpm, and the mixture is leaned from very rich toward peak exhaust gas temperature. What happens to cylinder head temperature during this change?

  1. It drops sharply as soon as leaning begins
  2. It rises toward a maximum near peak EGT Correct answer.
  3. It falls steadily as the mixture is leaned
  4. It stays constant until the mixture becomes lean of peak

Explanation

Leaning from a very rich mixture moves combustion closer to stoichiometric, releasing more heat per unit of charge, so cylinder head temperature climbs. It typically peaks somewhat rich of peak EGT and then falls as the mixture goes lean. This is why leaning is done gradually with temperature monitoring.

Question 28Aircraft Powerplant Fundamentals and Theory

Which statement correctly describes the relationship between engine speed and power output for a reciprocating engine driving a fixed-pitch propeller?

  1. Power is independent of engine speed once the throttle is fully open
  2. Power is directly proportional to engine speed with no exponent
  3. Power varies with the cube of engine speed because the propeller load rises steeply Correct answer.
  4. Power varies with the square root of engine speed

Explanation

A fixed-pitch propeller absorbs power roughly in proportion to the cube of rotational speed, so the engine and propeller reach equilibrium at a speed set by throttle opening. This steep load curve is why small throttle changes produce noticeable rpm changes. It also explains why rpm is limited near full throttle.

Question 29Aircraft Powerplant Fundamentals and Theory

What is the primary purpose of the exhaust system in a reciprocating engine installation?

  1. To meter fuel into the intake manifold
  2. To carry combustion gases away and reduce noise Correct answer.
  3. To increase the compression ratio of the engine
  4. To cool the engine oil before it returns to the sump

Explanation

The exhaust system routes hot combustion products away from the engine and airframe and uses mufflers or other means to reduce noise. In some installations it also provides cabin heat or turbocharger drive, but its fundamental function is gas disposal. Compression ratio and fuel metering are set by mechanical design and the fuel system.

Question 30Aircraft Powerplant Fundamentals and Theory

A turbine engine is fitted with a can-annular combustor. Which characteristic is most accurate for this design compared with a purely annular combustor?

  1. It produces a perfectly uniform temperature profile with no hot spots
  2. It requires no fuel nozzles because fuel is vaporized by compressor heat alone
  3. It combines individual cans with a shared annular liner and offers easier maintenance access Correct answer.
  4. It eliminates the need for any dilution or cooling air

Explanation

A can-annular combustor places separate can liners inside a common annular casing, retaining the accessibility and modularity of can designs while shortening overall length. It still needs dilution and cooling air and multiple fuel nozzles. Temperature uniformity is improved over pure can designs but is not perfect.

Question 31Reciprocating Engine Construction and Operation

In a horizontally opposed four-cylinder aircraft engine, what does the term 'opposed' describe about the cylinder arrangement?

  1. Cylinders are arranged radially around a single crank throw
  2. Cylinders are arranged in a single row with axes parallel to the crankshaft
  3. Cylinders are arranged in two banks on opposite sides of a common crankcase Correct answer.
  4. Cylinders are arranged in two rows staggered along the crankshaft axis

Explanation

In an opposed engine the cylinders are mounted in two banks on opposite sides of a single crankcase, and opposing pistons share a common crank throw. This flat layout gives a short, stiff crankshaft and a low frontal area. Radial and inline arrangements describe different cylinder geometry.

Question 32Reciprocating Engine Construction and Operation

What is the primary function of the crankcase in a reciprocating aircraft engine?

  1. It converts reciprocating piston motion into rotary motion
  2. It mixes fuel with air before the mixture enters the cylinders
  3. It supports the main bearings and carries the cylinders and accessory drives Correct answer.
  4. It stores the lubricating oil that is circulated to the bearings

Explanation

The crankcase is the structural backbone of the engine: it supports the main bearing saddles, mounts the cylinders, and provides pads for accessory drives. It also encloses and protects the rotating assembly. Oil storage and mixture preparation are handled by other components.

Question 33Reciprocating Engine Construction and Operation

Which component connects the piston to the crankshaft and transmits the gas load?

  1. The pushrod
  2. The wrist pin
  3. The camshaft
  4. The connecting rod Correct answer.

Explanation

The connecting rod links the piston, through the wrist pin, to the crankshaft journal. It carries the combustion load in compression and the inertia load in tension. The wrist pin is the joint inside the piston, while pushrods and camshafts belong to the valve train.

Question 34Reciprocating Engine Construction and Operation

What is the purpose of the piston rings fitted to an aircraft engine piston?

  1. To transfer heat from the piston to the connecting rod
  2. To lock the wrist pin in place inside the piston
  3. To guide the valve stems during their travel
  4. To seal combustion gases and control oil on the cylinder wall Correct answer.

Explanation

Compression rings seal the combustion chamber against blow-by, and oil control rings scrape excess oil from the cylinder wall. The rings also conduct a large share of piston heat to the cylinder barrel. They do not retain the wrist pin or guide valves.

Question 35Reciprocating Engine Construction and Operation

In a four-stroke cycle engine, during which stroke does the intake valve close?

  1. At the start of the compression stroke, after bottom dead center Correct answer.
  2. Exactly at top dead center of the exhaust stroke
  3. Just before the piston reaches bottom dead center on the intake stroke
  4. During the middle of the power stroke

Explanation

The intake valve remains open past bottom dead center to take advantage of intake inertia, then closes shortly after the compression stroke begins. This late closing improves volumetric efficiency at cruise speeds. Closing exactly at bottom dead center would waste the ram effect of the incoming charge.

Question 36Reciprocating Engine Construction and Operation

What is valve overlap in a reciprocating engine?

  1. The clearance between the rocker arm and the valve stem tip
  2. The angular lag between the crankshaft and the camshaft
  3. A period when both intake and exhaust valves are open near top dead center Correct answer.
  4. The distance the valve travels from its seat to full open

Explanation

Overlap is the brief interval around top dead center when the intake valve has opened before the exhaust valve fully closes. It uses exhaust gas inertia to help scavenge the cylinder and draw in the fresh charge. Too much overlap causes rough idle and poor low-speed running.

Question 37Reciprocating Engine Construction and Operation

What is the function of the magneto in a spark-ignition aircraft engine?

  1. It steps down battery voltage to operate the starter motor
  2. It varies the ignition timing according to manifold pressure
  3. It generates a high-voltage spark at the correct point in the cycle Correct answer.
  4. It regulates the fuel pressure delivered to the injectors

Explanation

A magneto is a self-contained, engine-driven generator that produces high voltage for the spark plugs at the moment the piston approaches top dead center. Because it needs no external electrical source, the engine continues to run if the aircraft electrical system fails. Timing advance is fixed by the magneto-to-engine coupling.

Question 38Reciprocating Engine Construction and Operation

Which type of lubrication system uses oil stored in a separate tank and circulated under pressure to the bearings?

  1. A grease-packed bearing system
  2. A wet-sump splash system
  3. A dry-sump pressure system Correct answer.
  4. A total-loss oiling system

Explanation

In a dry-sump system the oil supply is carried in a separate tank, and a pressure pump feeds the bearings while a scavenge pump returns oil from the sump. This arrangement allows a smaller, lighter crankcase and permits oil cooling in the tank. Wet-sump systems keep the supply inside the engine sump.

Question 39Reciprocating Engine Construction and Operation

An engine has a bore of 120 mm and a stroke of 100 mm. Using the piston displacement formula, what is the displacement of one cylinder in cubic centimetres?

  1. 1440
  2. 452
  3. 1131 Correct answer.
  4. 905

Explanation

Displacement equals pi divided by four, multiplied by bore squared and by stroke. With bore 12 cm and stroke 10 cm, the result is 0.7854 times 144 times 10, which is about 1131 cubic centimetres. Stroke and bore must be expressed in the same units before calculating.

Question 40Reciprocating Engine Construction and Operation

How is compression ratio defined for a reciprocating engine cylinder?

  1. Total cylinder volume with the piston at bottom dead center divided by clearance volume Correct answer.
  2. Piston displacement divided by the number of cylinders
  3. Clearance volume divided by total cylinder volume with the piston at bottom dead center
  4. The ratio of manifold pressure to ambient pressure at wide-open throttle

Explanation

Compression ratio compares the largest cylinder volume, with the piston at bottom dead center, to the smallest volume, with the piston at top dead center. It is therefore total volume divided by clearance volume. A higher ratio raises thermal efficiency but increases detonation tendency.

Question 41Reciprocating Engine Construction and Operation

A four-cylinder four-stroke engine turns at 2400 rpm. How many power strokes occur per second across the whole engine?

  1. 40
  2. 160
  3. 80 Correct answer.
  4. 20

Explanation

Each cylinder produces one power stroke every two crankshaft revolutions, so at 2400 rpm each cylinder fires 1200 times per minute. Four cylinders give 4800 power strokes per minute, which is 80 per second. Doubling or halving the cylinder count changes the result proportionally.

Question 42Reciprocating Engine Construction and Operation

What is the purpose of the valve clearance specified between the rocker arm and the valve stem tip?

  1. To keep the valve slightly open for cooling at idle
  2. To increase valve lift at high engine speeds
  3. To reduce the spring pressure acting on the valve
  4. To allow for thermal expansion of the valve train components Correct answer.

Explanation

Valve train parts expand as the engine warms, so a specified clearance prevents the valve from being held off its seat when hot. If the valve did not seat fully, compression would be lost and the valve face could burn. Clearance is checked and set with the engine cold on most engines.

Question 43Reciprocating Engine Construction and Operation

Why are exhaust valves in air-cooled aircraft engines often filled with metallic sodium?

  1. To lubricate the valve guide during operation
  2. To reduce the reciprocating weight of the valve train
  3. To conduct heat from the valve head to the stem and guide Correct answer.
  4. To damp valve spring surge at high speed

Explanation

Sodium melts at operating temperature and, as the valve moves, splashes between head and stem, transferring heat to the guide and cylinder head. Exhaust valves run hotter than intake valves, so this cooling path is critical. The hollow stem is sealed to contain the sodium.

Question 44Reciprocating Engine Construction and Operation

What does the term 'firing order' describe in a multi-cylinder engine?

  1. The sequence in which the cylinders deliver their power strokes Correct answer.
  2. The order of cylinders along the crankshaft from front to rear
  3. The order in which the spark plugs are replaced during maintenance
  4. The sequence in which the valves open within a single cylinder

Explanation

Firing order is the scheduled sequence of power strokes that spreads the firing impulses evenly around the crankshaft. Even spacing reduces torsional vibration and smooths engine operation. It is determined by crankshaft throw layout and camshaft design.

Question 45Reciprocating Engine Construction and Operation

What is the primary reason for using two spark plugs per cylinder in an aircraft engine?

  1. To improve combustion efficiency and provide ignition redundancy Correct answer.
  2. To reduce the load carried by each magneto winding
  3. To double the available spark energy at high altitude
  4. To allow the engine to run on two different fuel grades

Explanation

Dual ignition shortens the flame travel path, giving more complete and faster combustion, and it provides a backup if one magneto or plug fails. The result is slightly more power, better economy, and improved tolerance to detonation. Redundancy and combustion quality are the governing reasons.

Question 46Reciprocating Engine Construction and Operation

What is the effect of the intake valve closing later than bottom dead center at high engine speed?

  1. Volumetric efficiency decreases because the charge is pushed back into the intake
  2. Detonation margin improves because the effective compression ratio is raised
  3. Compression pressure rises sharply because the stroke is effectively lengthened
  4. Volumetric efficiency improves because intake inertia continues to fill the cylinder Correct answer.

Explanation

At high speed the incoming charge has enough inertia to keep flowing into the cylinder after the piston has passed bottom dead center. Late intake closing captures this extra charge and raises volumetric efficiency. At low speed the same timing can cause reversion into the intake port.

Question 47Reciprocating Engine Construction and Operation

During a compression check, one cylinder reads noticeably lower than the others but the reading improves when oil is injected into the cylinder. What does this indicate?

  1. A cracked cylinder head
  2. Worn or broken piston rings Correct answer.
  3. A burned exhaust valve
  4. A leaking intake valve

Explanation

Oil temporarily seals the gap between rings and cylinder wall, so pressure rises if the rings are the leak path. Valve or head leaks are not sealed by oil because the leak is in the combustion chamber structure. This test distinguishes ring wear from valve leakage.

Question 48Reciprocating Engine Construction and Operation

Why is a high compression ratio limited in a large air-cooled aircraft engine?

  1. It prevents the use of dual ignition systems
  2. It raises peak combustion temperature and pressure, increasing detonation tendency Correct answer.
  3. It requires a heavier crankshaft to absorb torque pulses
  4. It reduces the volumetric efficiency of the intake stroke

Explanation

Raising the compression ratio increases both the pressure and temperature of the compressed charge, which brings the mixture closer to spontaneous ignition. Air-cooled cylinders run hotter than liquid-cooled ones, so the detonation margin is tighter. Fuel octane rating and cooling capacity therefore set the practical limit.

Question 49Reciprocating Engine Construction and Operation

What is the function of the camshaft in a four-stroke aircraft engine?

  1. It opens and closes the valves in the correct sequence and timing Correct answer.
  2. It converts the reciprocating motion of the pistons into rotation
  3. It drives the magneto and tachometer through a gear train
  4. It meters fuel into the intake manifold

Explanation

The camshaft carries lobes ground to the required valve timing and, turning at half crankshaft speed in a four-stroke engine, actuates the valves through lifters, pushrods, and rockers. Its profile determines lift, duration, and overlap. It does not produce the engine's output rotation.

Question 50Reciprocating Engine Construction and Operation

In a horizontally opposed engine, what is the purpose of the crankcase through bolts?

  1. To clamp the crankcase halves together and carry the main bearing loads Correct answer.
  2. To lock the crankshaft thrust bearing in position
  3. To retain the accessory case on the rear of the engine
  4. To secure the cylinders to the crankcase flange

Explanation

Through bolts pass completely through the crankcase and clamp the halves together, preloading the joint and carrying the firing loads into the main bearing supports. They are among the most highly stressed fasteners in the engine. Cylinder hold-down and accessory attachment use separate studs or bolts.