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NCAA AME Powerplant Exam Practice Test & Question Bank study materials
NCAA AME Powerplant Exam Practice Test & Question Bank study resources

NCAA AME Powerplant Exam Practice Test & Question Bank practice questions

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Question 1Fundamentals of Powerplants and Engine Theory

In a four-stroke reciprocating aircraft engine, during which stroke does the piston move from bottom dead center to top dead center with both valves closed, compressing the fuel-air charge?

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

Explanation

The compression stroke begins at bottom dead center with both valves seated, and the piston travels upward to top dead center, reducing cylinder volume and raising the pressure and temperature of the trapped charge. The intake stroke instead fills the cylinder, and the power stroke drives the piston downward after ignition.

Question 2Fundamentals of Powerplants and Engine Theory

Which statement correctly describes the valve action of a typical four-stroke aircraft reciprocating engine?

  1. Both valves are open together throughout the power stroke
  2. The intake valve opens once every two crankshaft revolutions Correct answer.
  3. The exhaust valve opens during the intake stroke
  4. Each valve opens and closes once per crankshaft revolution

Explanation

A four-stroke cycle requires two complete crankshaft revolutions, so each valve operates once per cycle, meaning once every two revolutions. The power stroke occurs with both valves closed, and the exhaust valve opens only near the end of the power stroke.

Question 3Fundamentals of Powerplants and Engine Theory

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

  1. To regulate oil pressure to the bearings
  2. To atomize fuel as it enters the intake port
  3. To advance the propeller blade angle
  4. To deliver a high-voltage spark across the spark plug gap at the correct time to ignite the compressed charge Correct answer.

Explanation

The magneto and harness produce a high-voltage pulse that jumps the spark plug gap, igniting the compressed fuel-air mixture near the end of the compression stroke. Fuel atomization is handled by the carburetor or fuel injection system, not the ignition system.

Question 4Fundamentals of Powerplants and Engine Theory

In a normally aspirated reciprocating engine, what does the term volumetric efficiency describe?

  1. The ratio of fuel flow to air flow by weight
  2. The ratio of actual air volume drawn into the cylinder to the theoretical volume the cylinder can hold Correct answer.
  3. The ratio of brake horsepower to indicated horsepower
  4. The ratio of exhaust gas temperature to cylinder head temperature

Explanation

Volumetric efficiency compares the air actually trapped in the cylinder with the ideal swept volume, and it falls off at high RPM because of flow losses and short valve-open duration. It is a breathing measure, distinct from mechanical or thermal efficiency.

Question 5Fundamentals of Powerplants and Engine Theory

Which type of engine is classified as a reaction engine?

  1. Radial reciprocating engine
  2. Turbojet engine Correct answer.
  3. Rotary piston engine
  4. Horizontally opposed reciprocating engine

Explanation

A reaction engine produces thrust by accelerating a mass of fluid rearward, and the turbojet accomplishes this by expelling high-velocity exhaust gases. Reciprocating and rotary piston engines are classified as reciprocating engines that drive a propeller to produce thrust.

Question 6Fundamentals of Powerplants and Engine Theory

In a gas turbine engine, what is the function of the diffuser section of the compressor?

  1. To increase air velocity while decreasing pressure
  2. To convert air velocity into pressure rise with minimal loss Correct answer.
  3. To drive the accessory gearbox
  4. To mix fuel with air before combustion

Explanation

The diffuser slows the incoming air so that its kinetic energy is converted into a static pressure rise before the air reaches the combustion chamber. Decelerating the flow reduces velocity and increases pressure, which supports stable combustion.

Question 7Fundamentals of Powerplants and Engine Theory

A reciprocating engine has a bore of 5.0 inches and a stroke of 4.5 inches. Using the swept-volume formula for one cylinder, what is the piston displacement of that cylinder?

  1. 110.4 cubic inches
  2. 88.4 cubic inches Correct answer.
  3. 70.7 cubic inches
  4. 176.7 cubic inches

Explanation

Displacement equals bore area times stroke: area is pi times radius squared, or 3.1416 times 2.5 squared, which is 19.635 square inches. Multiplying by the 4.5-inch stroke gives about 88.4 cubic inches for one cylinder.

Question 8Fundamentals of Powerplants and Engine Theory

An engine develops 150 indicated horsepower and 120 brake horsepower on a test stand. What is its mechanical efficiency?

  1. 80 percent Correct answer.
  2. 85 percent
  3. 70 percent
  4. 125 percent

Explanation

Mechanical efficiency is brake horsepower divided by indicated horsepower, so 120 divided by 150 equals 0.80, or 80 percent. The remaining 30 horsepower represents friction and pumping losses within the engine.

Question 9Fundamentals of Powerplants and Engine Theory

A turbojet engine produces 4,500 pounds of thrust while burning 3,000 pounds of fuel per hour. What is its specific fuel consumption in pounds of fuel per pound of thrust per hour?

  1. 1.50
  2. 2.25
  3. 0.45
  4. 0.67 Correct answer.

Explanation

Specific fuel consumption equals fuel flow divided by thrust, so 3,000 divided by 4,500 equals about 0.67 pounds of fuel per pound of thrust per hour. Lower values indicate a more fuel-efficient engine at that operating condition.

Question 10Fundamentals of Powerplants and Engine Theory

Why does the compression ratio of a spark-ignition aircraft engine affect its thermal efficiency?

  1. Compression ratio determines the octane rating of the fuel used
  2. A higher compression ratio lowers the peak combustion temperature and reduces power
  3. Compression ratio affects only the weight of the engine, not efficiency
  4. A higher compression ratio increases the expansion ratio, converting a greater fraction of the heat added into useful work Correct answer.

Explanation

In the ideal Otto cycle, increasing the compression ratio also increases the expansion ratio, so a larger fraction of the heat added becomes net work. With constant specific heat, temperature rise per unit heat added is unchanged. In real engines, detonation limits the usable compression ratio.

Question 11Fundamentals of Powerplants and Engine Theory

What is the purpose of the bypass ratio in a turbofan engine?

  1. It defines the ratio of air passing through the fan duct to air passing through the core engine Correct answer.
  2. It defines the ratio of fuel flow to exhaust gas flow
  3. It defines the ratio of compressor stages to turbine stages
  4. It defines the ratio of thrust to weight for the whole engine

Explanation

Bypass ratio compares the mass flow through the fan bypass duct with the mass flow through the engine core. A high bypass ratio improves propulsive efficiency and reduces specific fuel consumption at typical cruise speeds.

Question 12Fundamentals of Powerplants and Engine Theory

During ground operation, a pilot advances the throttle on a turbojet engine and observes that thrust rises, but the turbine inlet temperature approaches its limit before full RPM is reached. Which explanation best fits this behavior?

  1. The engine has reached its maximum allowable RPM and cannot accelerate further
  2. The compressor is producing too much pressure for the turbine to handle
  3. The engine is operating near its temperature limit, so the fuel control schedules fuel to protect the turbine while limiting achievable thrust Correct answer.
  4. The exhaust nozzle area has increased automatically

Explanation

Turbine inlet temperature limits acceleration and maximum thrust, so the fuel control meters fuel to keep temperatures within limits even when the throttle is advanced. Thrust is therefore limited by temperature margin rather than by RPM alone.

Question 13Fundamentals of Powerplants and Engine Theory

In a four-stroke engine, what is valve overlap?

  1. The clearance between the valve stem and rocker arm
  2. The condition in which one valve is stuck open
  3. The period when both intake and exhaust valves are open near the end of the exhaust and start of the intake stroke Correct answer.
  4. The time required for the valve to seat after closing

Explanation

Valve overlap occurs around top dead center when the intake valve has opened slightly before the exhaust valve fully closes. This brief simultaneous opening uses exhaust gas inertia to help scavenge the cylinder and draw in a fresh charge.

Question 14Fundamentals of Powerplants and Engine Theory

Why is a rich mixture often used for takeoff power in a carbureted aircraft engine?

  1. The excess fuel evaporates and cools the charge, helping to suppress detonation at high power settings Correct answer.
  2. The excess fuel raises the compression ratio temporarily
  3. The excess fuel increases the octane rating of the mixture
  4. The excess fuel improves the volumetric efficiency of the intake stroke

Explanation

At high power settings, extra fuel provides evaporative cooling of the intake charge, which lowers peak combustion temperatures and reduces the risk of detonation. The rich setting also protects valves and cylinder heads from excessive heat.

Question 15Fundamentals of Powerplants and Engine Theory

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

  1. To route burned gases away from the engine and, in some installations, to provide muffler heat for the cabin Correct answer.
  2. To reduce the compression ratio during the exhaust stroke
  3. To cool the engine cylinders by directing air across the fins
  4. To supply compressed air to the carburetor

Explanation

The exhaust system carries combustion products overboard and, through a shroud around the muffler, provides heated air for cabin heat and carburetor heat. Cylinder cooling is the job of the baffles and cowling, not the exhaust.

Question 16Fundamentals of Powerplants and Engine Theory

Which statement best describes why a turbocharged engine can maintain rated power at higher altitude than a normally aspirated engine?

  1. The turbocharger compresses intake air to a higher density, compensating for reduced atmospheric pressure Correct answer.
  2. The turbocharger cools the intake air without changing its pressure
  3. The turbocharger increases the fuel flow without changing air density
  4. The turbocharger increases the compression ratio of the pistons

Explanation

As altitude rises, ambient pressure falls and a normally aspirated engine loses manifold pressure. The turbocharger uses exhaust energy to compress intake air back to a higher density, restoring the mass of air available for combustion.

Question 17Fundamentals of Powerplants and Engine Theory

A four-cylinder engine is rated at 200 brake horsepower at 2,700 RPM. Using the torque relationship, what approximate torque is the engine producing at that condition?

  1. About 1,050 pound-feet
  2. About 525 pound-feet
  3. About 740 pound-feet
  4. About 389 pound-feet Correct answer.

Explanation

Brake horsepower equals torque times RPM divided by 5,252, so torque equals 200 times 5,252 divided by 2,700, which is about 389 pound-feet. The constant 5,252 converts rotational speed in revolutions per minute to radians per second.

Question 18Fundamentals of Powerplants and Engine Theory

During a turbine engine start, the starter motor disengages and the engine begins to accelerate on its own, but exhaust gas temperature rises rapidly. Which condition does this most likely indicate?

  1. A normal start sequence with expected temperature rise
  2. A compressor stall caused by too little fuel during acceleration
  3. A hot start, where excess fuel causes temperature to exceed limits before light-off is established Correct answer.
  4. A hung start, where the engine fails to accelerate to idle speed

Explanation

A hot start occurs when fuel is introduced but ignition or acceleration is delayed, so accumulated fuel burns with excessive temperature. The rapid EGT rise before the engine stabilizes at idle is the identifying signature, and the start must be aborted.

Question 19Fundamentals of Powerplants and Engine Theory

Why does a high-bypass turbofan generally achieve better propulsive efficiency than a pure turbojet at subsonic cruise speeds?

  1. It operates at a higher exhaust velocity than a turbojet
  2. It accelerates a larger mass of air to a lower velocity, reducing the kinetic energy wasted in the exhaust Correct answer.
  3. It burns fuel at a higher combustion temperature than a turbojet
  4. It eliminates the need for a compressor and turbine

Explanation

Propulsive efficiency improves when thrust is produced by moving a large mass of air at a modest velocity rather than a small mass at high velocity. The bypass fan accomplishes this, so less energy remains in the exhaust as wasted kinetic energy.

Question 20Fundamentals of Powerplants and Engine Theory

An engineer compares two engines of identical displacement and compression ratio, one operating at a higher RPM. Which factor most directly explains the higher power output of the faster engine?

  1. The compression ratio increases with RPM
  2. The fuel has a higher energy content at higher RPM
  3. The mechanical efficiency rises to 100 percent at high RPM
  4. More power strokes occur per unit time, so more work is delivered per minute Correct answer.

Explanation

Power is work per unit time, so increasing RPM increases the number of power strokes completed each minute and therefore the power produced, provided volumetric efficiency is maintained. Friction and pumping losses also rise with RPM, which is why power eventually peaks and declines.

Question 21Reciprocating Engine Construction, Components, and Operation

In a horizontally opposed aircraft engine, what is the primary function of the counterweights attached to the crankshaft?

  1. They drive the oil pump through a geared interface on the crank cheek
  2. They act as flywheel mass to smooth power pulses between firing strokes
  3. They increase the compression ratio by raising piston height at top dead center
  4. They balance the rotating and reciprocating masses to reduce vibration and bearing loads Correct answer.

Explanation

Counterweights on the crankshaft offset the centrifugal forces produced by the crank throws, pistons, and connecting rods, thereby reducing vibration and main bearing loads. They do not change compression ratio, drive accessories, or serve as the flywheel. Their role is dynamic balance of rotating and reciprocating masses.

Question 22Reciprocating Engine Construction, Components, and Operation

A four-stroke-cycle reciprocating engine completes one full power cycle in how many degrees of crankshaft rotation?

  1. 180 degrees
  2. 360 degrees
  3. 720 degrees Correct answer.
  4. 540 degrees

Explanation

A four-stroke cycle requires intake, compression, power, and exhaust strokes, each occupying one half revolution of the crankshaft, for a total of two revolutions or 720 degrees. A 360-degree figure corresponds to a two-stroke cycle. The 540-degree figure does not match any complete cycle.

Question 23Reciprocating Engine Construction, Components, and Operation

Which component transfers the reciprocating motion of the piston to the rotating crankshaft?

  1. The connecting rod Correct answer.
  2. The pushrod
  3. The valve lifter
  4. The camshaft

Explanation

The connecting rod links the piston pin to the crankshaft throw, converting linear piston motion into rotary crankshaft motion. The camshaft operates the valves, and the lifter and pushrod are part of the valve train. None of those parts transmit piston force to the crank.

Question 24Reciprocating Engine Construction, Components, and Operation

In a typical air-cooled aircraft cylinder, what is the purpose of the deep fins machined into the barrel and head?

  1. They stiffen the barrel against combustion pressure loads
  2. They provide mounting points for the ignition harness
  3. They retain oil film to lubricate the piston skirt
  4. They increase the surface area for heat transfer to the cooling airflow Correct answer.

Explanation

Cooling fins multiply the exposed surface area so that heat conducted from the combustion chamber and barrel can be carried away by ram air. Structural stiffness, harness mounting, and oil retention are not their design purpose. Effective fin area and airflow are the key cooling variables.

Question 25Reciprocating Engine Construction, Components, and Operation

What is the function of the piston rings installed in the grooves of an aircraft piston?

  1. They lock the piston pin in place within the boss
  2. They set the piston-to-valve clearance at top dead center
  3. They seal combustion gases and control oil on the cylinder wall Correct answer.
  4. They transfer heat from the crown to the connecting rod

Explanation

Compression rings seal the combustion chamber against blowby, while oil control rings scrape excess oil from the cylinder wall and return it to the sump. Piston pin retention is handled by retainers or plugs, and valve clearance is set by the valve train. Rings are central to sealing and oil control.

Question 26Reciprocating Engine Construction, Components, and Operation

Which type of valve is most commonly used as the intake and exhaust valve in modern horizontally opposed aircraft engines?

  1. Poppet valve Correct answer.
  2. Sleeve valve
  3. Rotary valve
  4. Reed valve

Explanation

Poppet valves, mushroom-shaped and moved by the valve train, are the standard in aircraft reciprocating engines because they seal well and tolerate high combustion temperatures. Reed valves are used in two-stroke and some compressor applications, while rotary and sleeve valves are uncommon in modern aircraft powerplants.

Question 27Reciprocating Engine Construction, Components, and Operation

During which stroke of the four-stroke cycle is the intake valve open and the piston moving toward bottom dead center?

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

Explanation

The intake stroke draws the fuel-air charge into the cylinder as the piston descends with the intake valve open. Compression and power strokes occur with both valves closed, and the exhaust stroke opens the exhaust valve as the piston rises. Stroke identification follows piston direction and valve state.

Question 28Reciprocating Engine Construction, Components, and Operation

What is the primary purpose of the crankcase in a horizontally opposed aircraft engine?

  1. It mixes fuel and air before the intake port
  2. It stores compressed air for starting
  3. It supports the crankshaft and cylinders and encloses the lubricating oil Correct answer.
  4. It provides the combustion chamber volume

Explanation

The crankcase is the structural backbone that supports the crankshaft main bearings and cylinder assemblies and encloses the lubricating oil. Fuel-air mixing occurs in the carburetor or fuel injection system, and combustion occurs in the cylinder. It does not store starting air.

Question 29Reciprocating Engine Construction, Components, and Operation

An engine has a bore of 5.125 inches and a stroke of 4.375 inches with four cylinders. What is the approximate total displacement?

  1. About 360 cubic inches Correct answer.
  2. About 180 cubic inches
  3. About 720 cubic inches
  4. About 90 cubic inches

Explanation

Single-cylinder displacement equals pi/4 times bore squared times stroke, about 0.7854 times 26.27 times 4.375, or roughly 90 cubic inches. Multiplying by four cylinders gives approximately 360 cubic inches. Displacement scales linearly with the number of cylinders.

Question 30Reciprocating Engine Construction, Components, and Operation

A mechanic measures cylinder compression and finds one cylinder significantly lower than the others, but the reading rises noticeably after adding oil to the cylinder. What does this pattern most likely indicate?

  1. Worn piston rings or cylinder wall Correct answer.
  2. A leaking exhaust valve
  3. An intake valve not opening
  4. A faulty magneto timing

Explanation

When oil temporarily seals the piston ring-to-wall gap, compression rises, pointing to ring or cylinder wall wear. A leaking valve would not be corrected by adding oil, and valve opening faults or ignition timing would not produce this wet-versus-dry difference. The oil-sealing test distinguishes ring sealing from valve leakage.

Question 31Reciprocating Engine Construction, Components, and Operation

In a reciprocating engine, valve clearance (lash) is provided mainly to allow for which condition?

  1. Centrifugal stretching of the crankshaft at high rpm
  2. Compression of the piston rings during combustion
  3. Thermal expansion of the valve stem and cylinder components Correct answer.
  4. Oil film thickness on the camshaft bearing journals

Explanation

Valve clearance prevents the valve from being held off its seat as the stem and cylinder head expand with heat, which would cause leakage and burned valves. Crankshaft stretch, ring compression, and cam bearing oil film are not the reason clearance is specified. Correct lash ensures full seating when hot.

Question 32Reciprocating Engine Construction, Components, and Operation

What is valve overlap in a four-stroke aircraft engine?

  1. The amount the valve head protrudes into the combustion chamber
  2. The period when both intake and exhaust valves are open near top dead center Correct answer.
  3. The distance the valve spring is compressed at full lift
  4. The clearance between the valve stem tip and the rocker arm

Explanation

Valve overlap occurs when the intake valve opens slightly before the exhaust valve closes, using the inertia of the exhaust gases to help scavenge and draw in a fresh charge. Clearance, protrusion, and spring compression are different measurements. Overlap is a timing characteristic tied to camshaft design.

Question 33Reciprocating Engine Construction, Components, and Operation

A camshaft in a four-stroke engine rotates at what speed relative to the crankshaft, and why?

  1. Half crankshaft speed, because each valve must operate once per two crank revolutions Correct answer.
  2. One quarter crankshaft speed, to reduce valve train inertia
  3. Twice crankshaft speed, to open valves during every revolution
  4. The same speed as the crankshaft, because valves follow piston motion directly

Explanation

Each valve opens once per complete four-stroke cycle, which spans two crankshaft revolutions, so the camshaft turns at one-half crankshaft speed. This is achieved through the cam drive gear or belt ratio. Cam speed is a direct consequence of cycle timing.

Question 34Reciprocating Engine Construction, Components, and Operation

Which statement correctly describes the firing order of a typical four-cylinder horizontally opposed aircraft engine?

  1. Only two of the four cylinders fire, with the others acting as compressors
  2. Cylinders fire in a sequence that balances power pulses and avoids excessive crankshaft torsional stress Correct answer.
  3. Cylinders fire in numerical order regardless of crankshaft throw geometry
  4. All four cylinders fire simultaneously once per revolution

Explanation

The firing order is chosen to distribute power impulses evenly around the crankshaft and minimize torsional vibration and bearing loads. Simultaneous firing, arbitrary numerical order, or using cylinders as compressors would defeat smooth operation. Firing order is tied to crank throw design.

Question 35Reciprocating Engine Construction, Components, and Operation

What is the purpose of the valve spring in an aircraft engine valve train?

  1. To adjust valve clearance automatically during operation
  2. To close the valve and keep it seated against the seat when the lifter is not acting Correct answer.
  3. To open the valve against combustion pressure
  4. To retain the valve guide in the cylinder head

Explanation

The valve spring returns the valve to its seat and holds it closed against the pressure of the cam and combustion. The cam and lifter open the valve, and the guide is retained by the head itself. Hydraulic lash adjusters, not the spring, may compensate for clearance.

Question 36Reciprocating Engine Construction, Components, and Operation

A piston in an aircraft engine is described as having a domed crown. What effect does this design have?

  1. It eliminates the need for a head gasket
  2. It improves oil drainage from the cylinder wall
  3. It increases compression ratio and helps direct the fuel-air charge during combustion Correct answer.
  4. It reduces the piston weight to allow higher rpm

Explanation

A domed or contoured crown reduces combustion chamber volume, raising compression ratio, and can shape the charge for better combustion. It is not primarily a weight-reduction or oil-drainage feature, and it does not replace sealing components. Crown geometry is a compression and combustion design variable.

Question 37Reciprocating Engine Construction, Components, and Operation

What is the function of the crankshaft main bearings in a horizontally opposed engine?

  1. They seal the crankcase against oil leakage at the propeller shaft
  2. They support the crankshaft journals and absorb radial loads from combustion and inertia Correct answer.
  3. They connect the connecting rod to the piston pin
  4. They transmit camshaft rotation to the valve lifters

Explanation

Main bearings carry the crankshaft and absorb the radial loads created by combustion pressure and reciprocating mass forces. Connecting rod bearings link rod to crank, while piston pins link rod to piston. Camshaft drive and oil sealing are separate functions.

Question 38Reciprocating Engine Construction, Components, and Operation

A four-cylinder engine has a total displacement of 360 cubic inches and develops 180 horsepower at 2700 rpm. What is its approximate brake mean effective pressure indicator of power density per unit displacement?

  1. About 4.0 horsepower per cubic inch
  2. About 0.25 horsepower per cubic inch
  3. About 0.5 horsepower per cubic inch Correct answer.
  4. About 2.0 horsepower per cubic inch

Explanation

Dividing 180 horsepower by 360 cubic inches gives 0.5 horsepower per cubic inch, a typical specific output for a naturally aspirated aircraft engine. The other values would imply either much higher or much lower output per unit displacement. Specific output is a useful comparison metric.

Question 39Reciprocating Engine Construction, Components, and Operation

During a four-stroke cycle, at what point does the exhaust valve begin to open in most aircraft engines?

  1. Just before bottom dead center at the end of the power stroke Correct answer.
  2. Exactly at top dead center at the start of the intake stroke
  3. At the midpoint of the compression stroke
  4. Just after top dead center at the start of the power stroke

Explanation

The exhaust valve typically opens slightly before bottom dead center so that residual cylinder pressure helps blow down the burned gases and scavenge the cylinder. Opening at top dead center or during compression or power would lose energy or disrupt combustion. Exhaust valve lead is a cam timing feature.

Question 40Reciprocating Engine Construction, Components, and Operation

In a horizontally opposed engine, what is the purpose of the cylinder barrel choke, or taper, toward the top of the bore?

  1. To increase the compression ratio at the top of the stroke
  2. To provide clearance for valve heads at top dead center
  3. To allow easier installation of the piston during assembly
  4. To compensate for greater thermal expansion at the hotter top of the cylinder and maintain ring contact Correct answer.

Explanation

The top of the cylinder runs hotter and expands more than the bottom, so a slight taper or choke keeps the bore round and the rings in contact as the engine reaches operating temperature. It is not a compression, assembly, or valve clearance feature. Choke is a thermal compensation design.

Question 41Reciprocating Engine Construction, Components, and Operation

An engine shows excessive blowby, oil consumption, and low compression on all cylinders that does not improve with oil addition. Which cause best fits this pattern?

  1. A clogged oil cooler
  2. A single leaking intake valve
  3. Incorrect magneto timing
  4. Worn or stuck piston rings across all cylinders Correct answer.

Explanation

Uniform low compression with high blowby and oil consumption that does not respond to oil addition points to widespread ring or cylinder wall wear. A single valve leak would affect one cylinder, and ignition timing or oil cooler restrictions do not cause blowby. The pattern is systemic rather than localized.

Question 42Reciprocating Engine Construction, Components, and Operation

A mechanic observes that one cylinder's exhaust valve shows a burned margin and recession into the head. Which condition most directly leads to this failure?

  1. Over-advanced ignition timing causing detonation
  2. Insufficient valve clearance or weak spring causing the valve to leak when hot Correct answer.
  3. Excessive valve clearance causing the valve to not open fully
  4. A restricted oil supply to the rocker arm

Explanation

A valve that does not fully seat because of insufficient lash or a weak spring leaks hot combustion gases past its margin, causing burning and recession. Excessive clearance tends to reduce lift and cause noise rather than burning. Ignition timing and rocker lubrication are not the direct cause of margin burning.

Question 43Reciprocating Engine Construction, Components, and Operation

A reciprocating engine is described as having a compression ratio of 8.5:1. What does this ratio express?

  1. The ratio of manifold pressure to ambient pressure at full throttle
  2. The ratio of cylinder volume at bottom dead center to volume at top dead center Correct answer.
  3. The ratio of horsepower to displacement
  4. The ratio of valve lift to valve diameter

Explanation

Compression ratio compares the total cylinder volume with the piston at bottom dead center to the clearance volume with the piston at top dead center. Manifold pressure ratio, power density, and valve geometry are separate concepts. It is a geometric measure of how much the charge is compressed.

Question 44Reciprocating Engine Construction, Components, and Operation

In an opposed engine, what is the function of the dynamic counterweight system sometimes used on the crankshaft?

  1. It provides the sole means of retaining the main bearings
  2. It absorbs torsional vibration by allowing counterweights to move relative to the crank throw Correct answer.
  3. It locks the crankshaft to the propeller flange during starting
  4. It drives the oil pressure pump through a splined connection

Explanation

Dynamic counterweights pivot slightly so that their centrifugal force opposes the varying loads that produce torsional vibration, damping crank windup. They are not a starting lock, an oil pump drive, or a bearing retention device. This is a vibration control feature of the crankshaft.

Question 45Reciprocating Engine Construction, Components, and Operation

During a powerplant run-up, a mechanic notes that cylinder head temperatures are normal but one cylinder's exhaust gas temperature is significantly higher than the others at the same power setting. Which interpretation is most consistent with this finding?

  1. That cylinder has a stuck exhaust valve that is not opening
  2. That cylinder's magneto is firing too early
  3. That cylinder is receiving too much cooling airflow
  4. That cylinder is running leaner or is producing less power than the others Correct answer.

Explanation

A higher exhaust gas temperature on one cylinder suggests a leaner mixture or reduced power contribution from that cylinder, since the unburned charge and lower power raise exhaust temperature. A stuck valve or ignition timing fault would typically show other symptoms, and cooling airflow affects head temperature more than EGT. EGT comparison is a mixture and balance indicator.

Question 46Reciprocating Engine Construction, Components, and Operation

On a horizontally opposed aircraft engine, what is the primary function of the connecting rod?

  1. It transmits the piston's linear force to the crankshaft and converts it into rotating motion. Correct answer.
  2. It transfers valve motion from the camshaft to the rocker arm.
  3. It supports the crankshaft between main bearing journals and carries the propeller load.
  4. It seals combustion gases against the cylinder wall while guiding the piston.

Explanation

The connecting rod links the piston to the crankshaft journal, so its upper end reciprocates with the piston while its lower end rotates with the crank throw. This geometry converts linear piston force into crankshaft torque during the power stroke and carries loads in both tension and compression. It also aligns the piston within the bore through the wrist pin.

Question 47Reciprocating Engine Construction, Components, and Operation

Which component drives the magnetos and the fuel pump on a typical horizontally opposed aircraft engine?

  1. The propeller governor drive pad
  2. The camshaft, through gears at the accessory end
  3. The starter ring gear assembly
  4. The crankshaft, through the accessory gear train Correct answer.

Explanation

Accessory drives on opposed engines are taken from the crankshaft through a gear train at the rear of the engine, which turns accessories such as magnetos and the fuel pump at crankshaft speed or a defined ratio. The camshaft itself is driven from the crankshaft at half speed and primarily operates the valve train. Accessory gearing must therefore be timed to the crankshaft, not the camshaft.

Question 48Reciprocating Engine Construction, Components, and Operation

A mechanic is assembling a cylinder and finds the piston ring end gaps aligned in a straight line. Why is this unacceptable?

  1. Aligned gaps reduce ring tension and cause the rings to flutter at high rpm.
  2. Aligned gaps prevent proper oil return and cause the rings to seize in their grooves.
  3. Aligned gaps change the effective compression ratio of that cylinder.
  4. Aligned gaps allow combustion gases to escape directly past all rings, causing blowby and low compression. Correct answer.

Explanation

Ring gaps are staggered during assembly so that no continuous leak path exists from the combustion chamber to the crankcase. If all gaps line up, hot combustion gases can blow straight through, producing excessive blowby, oil consumption, and reduced compression. Staggering forces gases to travel a tortuous path and improves sealing.

Question 49Reciprocating Engine Construction, Components, and Operation

During a cylinder removal, a mechanic notes the exhaust valve guide is worn beyond limits while the intake guide is within limits. Which explanation best accounts for this difference?

  1. The exhaust valve is larger and therefore receives less lubrication from the rocker box.
  2. The exhaust valve opens against higher cylinder pressure, loading the guide sideways.
  3. The exhaust valve guide is machined from a softer bronze alloy than the intake guide.
  4. The exhaust valve runs hotter and its stem expands more, accelerating guide wear. Correct answer.

Explanation

Exhaust valves and guides operate at much higher temperatures because hot exhaust gas flows past them continuously. Higher temperature reduces oil film strength on the stem and accelerates wear, which is why exhaust guides typically reach their wear limit first. Intake guides are cooled by the incoming fuel-air charge and normally wear more slowly.

Question 50Reciprocating Engine Construction, Components, and Operation

An opposed engine's crankshaft is being inspected and the mechanic measures runout at the main bearing journals after the shaft has been supported on V-blocks. Which condition does this check primarily detect?

  1. Bending or bowing of the shaft along its axis Correct answer.
  2. Twisting of the shaft between throws from torsional vibration
  3. Journal out-of-roundness from uneven bearing wear
  4. Wear of the crankpin bearing surfaces

Explanation

Runout measured with the shaft supported on V-blocks and rotated indicates how far the shaft deviates from a straight centerline, which reveals bending or bowing. Out-of-roundness and taper require micrometer measurement at several points around and along each journal, not a dial indicator runout check. Runout limits are published by the manufacturer and an exceeded shaft must be replaced.