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CAA NZ AME Piston Engines study materials
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CAA NZ AME Piston Engines practice questions

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Question 1Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

In the theoretical Otto cycle, which pair of processes represents the addition and rejection of heat?

  1. Constant-volume heat addition and constant-pressure heat rejection
  2. Constant-pressure heat addition and constant-pressure heat rejection
  3. Isothermal heat addition and isothermal heat rejection
  4. Constant-volume heat addition and constant-volume heat rejection Correct answer.

Explanation

The ideal Otto cycle models the spark-ignition engine with two isentropic processes (compression and expansion) and two constant-volume processes for heat addition at top dead centre and heat rejection at bottom dead centre. Heat transfer at constant volume reflects the near-instantaneous combustion and the exhaust/blowdown phase.

Question 2Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

The thermal efficiency of the ideal Otto cycle with a fixed compression ratio depends primarily on which quantity?

  1. The exhaust back pressure
  2. The mass of fuel delivered per cycle
  3. The engine rotational speed
  4. The ratio of specific heats of the working fluid Correct answer.

Explanation

For the ideal Otto cycle, efficiency is 1 minus one over compression ratio raised to the power (gamma minus 1), where gamma is the ratio of specific heats. Efficiency therefore rises with compression ratio and with a higher gamma, and is independent of fuel mass, speed and exhaust pressure in the ideal model.

Question 3Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

A four-stroke piston engine running at 2400 rpm produces one power stroke per cylinder every how many revolutions, and therefore how many power strokes per second per cylinder?

  1. Every 2 revolutions, giving 40 power strokes per second
  2. Every 1 revolution, giving 40 power strokes per second
  3. Every 2 revolutions, giving 20 power strokes per second Correct answer.
  4. Every 4 revolutions, giving 10 power strokes per second

Explanation

In a four-stroke cycle the crankshaft turns twice per complete cycle, so each cylinder fires once every two revolutions. At 2400 rpm that is 1200 cycles per minute, or 20 power strokes per second.

Question 4Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Indicated mean effective pressure (IMEP) multiplied by engine displacement and divided by the number of revolutions per power stroke gives which quantity?

  1. Friction power
  2. Indicated power Correct answer.
  3. Brake power
  4. Volumetric efficiency

Explanation

Indicated power is calculated from IMEP, swept volume, speed and the number of strokes per cycle. Brake power would require brake mean effective pressure instead, and friction power is the difference between the two.

Question 5Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

What does brake mean effective pressure (BMEP) represent in a piston engine?

  1. A fictitious constant pressure that would produce the measured brake power over the working stroke Correct answer.
  2. The pressure of the lubricating oil at the main gallery
  3. The pressure drop across the air filter at full throttle
  4. The peak cylinder pressure recorded during combustion

Explanation

BMEP is a calculated average pressure acting on the piston throughout the power stroke that would yield the measured brake output. It allows engines of different displacement and speed to be compared for work output per cycle. It is not a measured instantaneous cylinder pressure.

Question 6Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Which statement correctly distinguishes indicated thermal efficiency from brake thermal efficiency?

  1. Indicated efficiency accounts for propeller losses, while brake efficiency accounts for friction losses
  2. The two are identical for all engines because friction is constant
  3. Indicated efficiency uses crankshaft work, while brake efficiency uses work at the piston
  4. Indicated efficiency uses indicated work per cycle, while brake efficiency uses work delivered at the crankshaft Correct answer.

Explanation

Indicated thermal efficiency compares the heat supplied with the work done on the piston, whereas brake thermal efficiency compares heat supplied with useful shaft work. The difference between them is the mechanical efficiency, which accounts for friction and accessory drives.

Question 7Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

An engine has a compression ratio of 8:1 and a swept volume of 600 cm3 per cylinder. What is the clearance volume per cylinder?

  1. Approximately 75 cm3
  2. Approximately 100 cm3
  3. Approximately 120 cm3
  4. Approximately 86 cm3 Correct answer.

Explanation

Compression ratio equals swept volume plus clearance volume, all divided by clearance volume. Rearranging gives clearance volume equal to swept volume divided by (compression ratio minus one), so 600 divided by 7 is about 86 cm3.

Question 8Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Why does increasing the compression ratio of a spark-ignition engine raise the risk of detonation?

  1. Higher compression lowers the temperature of the end gas before flame arrival
  2. Higher compression reduces the octane requirement of the fuel
  3. Higher compression reduces the mass trapped in the cylinder
  4. Higher compression raises the unburned charge temperature and pressure, shortening the ignition delay of the end gas Correct answer.

Explanation

Compressing the charge more raises both its pressure and temperature before the spark, so the unburned end gas reaches its autoignition conditions sooner. This shortens the delay period and increases the likelihood of spontaneous ignition ahead of the flame front, which is detonation.

Question 9Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

The Diesel cycle differs from the Otto cycle in that heat addition is modelled as occurring at which condition?

  1. Constant entropy
  2. Constant temperature
  3. Constant pressure Correct answer.
  4. Constant volume

Explanation

In the ideal Diesel cycle, heat is added at constant pressure during the initial part of the expansion, representing fuel injection and burning as the piston moves down. This follows the isentropic compression and precedes the isentropic expansion.

Question 10Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Which factor most directly governs the volumetric efficiency of a normally aspirated aircraft piston engine?

  1. The ratio of connecting rod length to crank throw
  2. The heat value of the fuel
  3. The compression ratio of the engine
  4. The pressure and temperature of the induction air entering the cylinder Correct answer.

Explanation

Volumetric efficiency compares the actual mass of air trapped with the theoretical mass that would fill the swept volume at inlet conditions. Induction air density, set by ambient pressure and temperature and by losses through the induction system, therefore governs it. Compression ratio and fuel energy content do not determine how much charge is trapped.

Question 11Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Which formula correctly expresses mechanical efficiency of a piston engine?

  1. Indicated power divided by brake power
  2. Brake power divided by fuel energy supplied per second
  3. Indicated power divided by fuel energy supplied per second
  4. Brake power divided by indicated power Correct answer.

Explanation

Mechanical efficiency is the ratio of useful power at the crankshaft to the total power developed on the piston. It is always less than one because friction and accessory drives consume part of the indicated power.

Question 12Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

In an air-standard analysis of a piston engine, what does the term 'air-standard' signify?

  1. The combustion process is assumed to occur at constant temperature
  2. The working fluid is treated as pure air with constant or temperature-dependent specific heats Correct answer.
  3. The engine is assumed to run on aviation gasoline of known octane rating
  4. The cycle is assumed to operate without any heat rejection

Explanation

Air-standard assumptions replace the fuel-air mixture and combustion products with air, simplifying analysis of the cycle. Specific heats may be treated as constant for simplicity or varied with temperature for closer approximation. These assumptions permit comparison of cycles independent of fuel chemistry.

Question 13Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

A normally aspirated engine develops 120 kW at sea level. Ignoring other effects, what power would be expected at an altitude where ambient pressure is 75 percent of sea-level pressure, if power varies directly with air density?

  1. Approximately 110 kW
  2. Approximately 90 kW Correct answer.
  3. Approximately 120 kW
  4. Approximately 100 kW

Explanation

For a normally aspirated engine, the trapped air mass per cycle scales roughly with ambient density, so power falls proportionally. Seventy-five percent of 120 kW is 90 kW. Temperature effects would modify this, but with density assumed proportional to pressure the answer is 90 kW.

Question 14Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Which change would increase the theoretical thermal efficiency of an ideal Otto cycle?

  1. Raising the compression ratio while keeping the working fluid the same Correct answer.
  2. Reducing the exhaust valve opening duration
  3. Increasing the fuel flow rate at constant compression ratio
  4. Lowering the compression ratio while keeping the working fluid the same

Explanation

Ideal Otto efficiency rises monotonically with compression ratio for a given ratio of specific heats. Fuel flow changes power but not the ideal efficiency, and valve timing does not enter the air-standard formula.

Question 15Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Why is the brake thermal efficiency of a piston engine always lower than its indicated thermal efficiency?

  1. Some exhaust heat is always recovered by the turbocharger
  2. The fuel is only partly vaporised before combustion
  3. Part of the indicated work is consumed by friction and by driving engine accessories Correct answer.
  4. The indicated efficiency ignores the heat value of the fuel

Explanation

Indicated work is what the gas does on the piston; brake work is what remains after friction in bearings, pistons and valve gear and after driving pumps and generators. Mechanical efficiency is the ratio of the two, so brake thermal efficiency must be lower.

Question 16Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

In the ideal Diesel cycle, increasing the cutoff ratio while keeping compression ratio fixed has what effect on thermal efficiency?

  1. Efficiency becomes equal to the Otto value
  2. Efficiency increases
  3. Efficiency is unchanged
  4. Efficiency decreases Correct answer.

Explanation

The cutoff ratio reflects how long constant-pressure heat addition continues. A larger cutoff ratio lengthens the expansion at higher pressure and reduces the effective expansion ratio, lowering the ideal Diesel efficiency for a fixed compression ratio.

Question 17Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

An engine's indicated mean effective pressure is 900 kPa and its mechanical efficiency is 0.80. What is the brake mean effective pressure?

  1. 900 kPa
  2. 720 kPa Correct answer.
  3. 180 kPa
  4. 1125 kPa

Explanation

Mechanical efficiency equals BMEP divided by IMEP, so BMEP is IMEP multiplied by mechanical efficiency. Multiplying 900 kPa by 0.80 gives 720 kPa.

Question 18Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Which statement best explains why the constant-volume cycle is a closer model for a spark-ignition engine than for a compression-ignition engine?

  1. In the spark-ignition engine fuel is injected gradually throughout the expansion stroke
  2. In the spark-ignition engine the flame travels quickly through a premixed charge, so most heat release occurs near top dead centre Correct answer.
  3. In the spark-ignition engine combustion occurs at constant temperature
  4. In the spark-ignition engine the compression process is isothermal

Explanation

A premixed spark-ignition charge burns rapidly once ignited, so the piston moves little during heat release and constant-volume addition is a reasonable approximation. In the compression-ignition engine, fuel is injected over a period and burns as it enters, so constant-pressure addition models it better.

Question 19Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

A four-cylinder four-stroke engine has a total swept volume of 4000 cm3 and develops 80 kW of brake power at 2400 rpm. What is the approximate BMEP?

  1. About 2000 kPa
  2. About 250 kPa
  3. About 1000 kPa Correct answer.
  4. About 500 kPa

Explanation

Brake power equals BMEP times displacement times revolutions per second per power stroke. At 2400 rpm a four-stroke engine has 20 power strokes per second per cylinder, so BMEP equals 80000 divided by (0.004 times 20), giving about 1000000 Pa or 1000 kPa.

Question 20Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Which of the following best describes the effect of heat transfer to the cylinder walls on the real indicated cycle?

  1. It converts the cycle into a Carnot cycle
  2. It reduces the area enclosed by the indicator diagram, lowering indicated work compared with the ideal cycle Correct answer.
  3. It has no effect because the ideal cycle also includes wall heat transfer
  4. It increases the enclosed area by raising peak pressure

Explanation

Heat lost to the walls during compression and combustion reduces the pressure achieved and the work obtained during expansion. The real indicator diagram therefore encloses less area than the ideal air-standard diagram, which is one reason real efficiency is lower.

Question 21Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

The specific fuel consumption of a piston engine is expressed as which ratio?

  1. Fuel volume per unit of swept volume
  2. Fuel mass flow per unit of power produced Correct answer.
  3. Air mass flow per unit of fuel mass flow
  4. Power produced per unit of fuel mass flow

Explanation

Specific fuel consumption is the rate of fuel used per unit of power, commonly kilograms per kilowatt-hour. Its reciprocal expresses power per unit fuel flow, and air-fuel ratio is a separate combustion parameter.

Question 22Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Why does a rich mixture beyond the best-power setting reduce engine power in a spark-ignition engine?

  1. Excess fuel displaces air, reducing the oxygen available per cycle and cooling the charge excessively Correct answer.
  2. Excess fuel raises the compression ratio
  3. Excess fuel increases the heat value per kilogram of mixture
  4. Excess fuel shortens the exhaust stroke

Explanation

Beyond the best-power mixture, additional fuel displaces air in the trapped charge, so less oxygen is available for combustion and the energy released per cycle falls. The extra fuel also absorbs heat through vaporisation, further lowering cycle temperatures and pressure.

Question 23Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

Which quantity is held constant along the isentropic compression process of the ideal air-standard cycle?

  1. Volume
  2. Temperature
  3. Pressure
  4. Entropy Correct answer.

Explanation

An isentropic process is reversible and adiabatic, so entropy remains constant while pressure rises and volume falls. Temperature rises as work is done on the gas.

Question 24Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)

A turbocharger fitted to a piston engine primarily improves which aspect of engine performance?

  1. The mass of air trapped per cycle, allowing more fuel to be burned Correct answer.
  2. The compression ratio of the engine
  3. The heat value of the fuel
  4. The mechanical efficiency of the bearings

Explanation

A turbocharger compresses the induction air, raising its density so that a greater mass of air is trapped each cycle. More fuel can then be burned to produce more power, and the higher manifold pressure also helps maintain power at altitude.

Question 25Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

In a horizontally opposed aircraft piston engine, how are the cylinders arranged relative to the crankshaft?

  1. In two banks on opposite sides of the crankcase, with pistons moving in opposite directions Correct answer.
  2. In two banks set at 60 degrees to each other above the crankshaft
  3. In three banks spaced 120 degrees apart around the crankshaft
  4. In a single bank all on the same side of the crankcase

Explanation

A horizontally opposed (flat) engine has two cylinder banks on opposite sides of a common crankcase, and the opposing pistons reach top dead centre at the same time but move in opposite directions. This arrangement gives a short, stiff crankshaft and a low frontal area. The 120-degree three-bank layout describes a radial engine, not a flat engine.

Question 26Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

What is the primary structural function of the crankcase in a piston aero engine?

  1. To provide the combustion chamber volume above each piston
  2. To carry the main bearing supports and keep the crankshaft and camshaft aligned Correct answer.
  3. To act as the mounting face for the propeller hub
  4. To store the lubricating oil supplied to the bearings

Explanation

The crankcase is the backbone of the engine: it houses and supports the main bearings, holds the cylinder banks and accessory housings in correct alignment, and carries the engine mounts. Oil storage is a separate sump or tank function, and combustion volume is formed by the cylinder and piston crown.

Question 27Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

Which component converts the reciprocating motion of the piston into rotary motion of the crankshaft?

  1. The camshaft acting through the tappets
  2. The connecting rod acting through the crank throw Correct answer.
  3. The piston pin acting directly on the cylinder wall
  4. The counterweight acting on the main bearing journal

Explanation

The connecting rod is pivoted at the piston pin and at the crankpin, so as the piston reciprocates the rod swings and forces the crank throw to rotate. This is the only linkage that transforms linear piston travel into crankshaft rotation. The camshaft and tappets operate the valves and play no part in this conversion.

Question 28Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

Why are plain shell bearings, rather than ball or roller bearings, normally used for the main and crankpin bearings of an aero engine?

  1. They allow the crankshaft to be dismantled without removing the crankcase
  2. They tolerate high steady loads and are fed pressurised oil to carry the load on a fluid film Correct answer.
  3. They require no lubrication and therefore simplify the oil system
  4. They generate less heat because there is metal-to-metal contact at all times

Explanation

Plain bearings present a large projected area to the connecting rod and main journal loads and rely on a pressurised oil film to keep the surfaces separated. Ball and roller bearings have line or point contact and cannot carry the very high, fluctuating cylinder pressures of an aero engine as well. Correct lubrication is essential; they are not lubrication-free.

Question 29Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

In a four-cylinder horizontally opposed engine, what is the firing interval between successive power strokes?

  1. 720 degrees of crankshaft rotation
  2. 90 degrees of crankshaft rotation
  3. 360 degrees of crankshaft rotation
  4. 180 degrees of crankshaft rotation Correct answer.

Explanation

A four-stroke cycle takes 720 degrees of crankshaft rotation, so with four cylinders evenly spaced each power stroke occurs every 720 divided by 4, which is 180 degrees. This even spacing gives smooth torque delivery. A 360-degree interval would suit a two-cylinder four-stroke engine.

Question 30Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

What is the purpose of the counterweights fitted to the crankshaft of a horizontally opposed engine?

  1. To provide a mounting surface for the propeller reduction gear
  2. To pump oil from the crankcase to the main galleries
  3. To increase the moment of inertia so the engine accelerates more slowly
  4. To balance the rotating masses and reduce bending loads on the main bearings Correct answer.

Explanation

Counterweights offset the mass of the crank throws and connecting rod big ends, reducing out-of-balance forces and the bending moments imposed on the crankshaft and main bearings. They are a balancing feature, not an oil pump or a gear mounting. Slower acceleration would be an undesirable side effect, not a design aim.

Question 31Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

Which statement correctly describes the big-end bearing of a connecting rod in a typical horizontally opposed aero engine?

  1. It is a split plain bearing clamped around the crankpin by the rod cap and cap bolts Correct answer.
  2. It is a one-piece bush pressed into the rod and slid over the crankpin
  3. It is a ball bearing housed in the piston skirt
  4. It is a white-metal pad that bears directly on the cylinder barrel

Explanation

The big end encircles the crankpin, so it must be split into a rod and cap assembly held by high-tensile bolts, with renewable shell bearings inside. A one-piece bush cannot be fitted over a crankpin that is integral with the crankshaft webs. The small end, by contrast, is a one-piece bush for the piston pin.

Question 32Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

A crankshaft is described as having a hollow bore through the crankpins and main journals. What is the main purpose of this drilling?

  1. To allow crankcase gases to vent to atmosphere
  2. To provide a path for cooling air through the crankshaft
  3. To supply pressurised oil from the main gallery to the crankpin and big-end bearings Correct answer.
  4. To reduce the mass of the crankshaft so it can spin faster

Explanation

Drillings through the main journals and crank throws form part of the pressure lubrication path, carrying filtered oil from the main bearings to the connecting rod big ends. Mass reduction may be a secondary consequence, but the design purpose is oil delivery. Crankcase breathing and cooling air are handled by separate systems.

Question 33Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

What is the function of the thrust bearing on an aero engine crankshaft?

  1. To limit the maximum engine rpm
  2. To resist axial loads and control crankshaft end float Correct answer.
  3. To carry the radial load of the propeller
  4. To absorb torsional vibration in the crankshaft

Explanation

The propeller produces a forward thrust that tries to pull the crankshaft forward, so one main bearing is arranged to take this axial load and set the end float. Radial loads are carried by the plain main bearings. Torsional vibration is addressed by damper arrangements, not by the thrust bearing.

Question 34Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

Why do some opposed aero engines use a one-piece crankcase split on the vertical centre plane rather than a split on the horizontal plane?

  1. It permits the use of roller bearings at the propeller end
  2. It allows the cylinders to be bolted to the crankshaft directly
  3. It allows the crankshaft and camshaft to be assembled and the main bearing line to be machined as a matched pair Correct answer.
  4. It removes the need for main bearings altogether

Explanation

A vertical split leaves the main bearing supports in the two halves, which are line-bored together so the bearing bores align accurately when the halves are bolted up. It also simplifies assembly of the crankshaft and camshaft into the case. Main bearings are still required, and cylinders mount to the crankcase deck, not to the crankshaft.

Question 35Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

What does the term 'stroke' mean in the context of piston engine construction?

  1. The ratio of cylinder volume to clearance volume
  2. The distance the piston travels from top dead centre to bottom dead centre Correct answer.
  3. The diameter of the cylinder bore
  4. The angle between the two cylinder banks

Explanation

Stroke is the linear distance travelled by the piston between the extreme positions of top dead centre and bottom dead centre. Bore is the cylinder diameter, and the ratio of total volume to clearance volume is the compression ratio. Stroke and bore together determine the swept volume of each cylinder.

Question 36Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

Why is the small-end bush of a connecting rod usually a bronze or white-metal bush rather than a rolling element bearing?

  1. It must allow the piston pin to rotate continuously at crankshaft speed
  2. It must oscillate through a small angle under high load with only splash or intermittent pressure lubrication Correct answer.
  3. It must seal combustion gases from the crankcase
  4. It must transmit torque from the piston to the crankshaft

Explanation

The small end oscillates rather than rotating continuously, so a plain bush suits the reversing motion and can survive on splash or intermittent oil feed. A rolling bearing would need continuous oil flow and would suffer from the small oscillating angle. The small end carries no sealing or torque-transmission duty of its own.

Question 37Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

In a six-cylinder horizontally opposed engine, what is the spacing between power strokes in degrees of crankshaft rotation?

  1. 60 degrees
  2. 120 degrees Correct answer.
  3. 180 degrees
  4. 240 degrees

Explanation

Each cylinder fires once per 720 degrees, so six cylinders give an interval of 720 divided by 6, which is 120 degrees. Even spacing is what smooths torque delivery in a multi-cylinder engine. A 180-degree interval would correspond to four cylinders.

Question 38Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

What is the purpose of the main bearing cap bolts being torque-tensioned to a specified figure during crankcase assembly?

  1. To allow the crankshaft to be removed without splitting the crankcase
  2. To compress the bearing shell so it grips the crankshaft journal
  3. To ensure the bearing shell is clamped with the designed preload so the oil clearance is correct Correct answer.
  4. To set the crankshaft end float to zero

Explanation

Correct bolt tension produces the designed clamping load, which holds the bearing shell firmly in its housing and maintains the specified running clearance between shell and journal. Overtightening crushes the shell and closes the clearance; undertightening allows movement and fretting. End float is set separately by shims or thrust faces.

Question 39Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

A crankshaft shows a circumferential crack initiating at a crankpin oil hole. Which explanation best fits this finding?

  1. Use of a white-metal bearing shell instead of bronze
  2. Stress concentration at the oil hole edge combined with fatigue from cyclic bending Correct answer.
  3. Incorrect valve clearance allowing the piston to strike the head
  4. Excessive counterweight mass causing torsional resonance

Explanation

Oil holes interrupt the smooth surface of the journal and raise local stress, so fatigue cracks commonly start at the hole edge where cyclic bending stresses are highest. The feature is a classic stress raiser and is why hole edges are carefully radiused and polished. Valve clearance and bearing material choice do not create this failure mode.

Question 40Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

What is meant by the 'firing order' of a piston engine?

  1. The sequence in which the cylinders are numbered from the propeller end
  2. The sequence in which the cylinders deliver their power strokes Correct answer.
  3. The order in which the valves are adjusted during servicing
  4. The order in which the spark plugs are replaced

Explanation

Firing order defines the sequence of power strokes around the engine, chosen to distribute loads evenly on the crankshaft and crankcase. Cylinder numbering and valve adjustment sequence are separate conventions. Correct firing order also helps avoid excessive torsional loading of the crankshaft.

Question 41Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

Which cylinder arrangement gives the highest power-to-weight ratio for a given displacement in classic air-cooled aero engines, and why?

  1. Inline, because a long crankshaft improves torsional stiffness
  2. Opposed, because it requires no cooling airflow
  3. Radial, because a short crankshaft and direct cooling airflow to all cylinders save weight Correct answer.
  4. Vee, because the cylinders can be made larger than in any other layout

Explanation

The radial layout concentrates the crank throws close together on a short crankshaft and exposes every cylinder to the slipstream, which keeps weight low for the power produced. Inline engines need a long crankshaft and a bulky liquid cooling system. Opposed engines still require cooling airflow over the cylinders.

Question 42Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

During a crankshaft inspection the main journal diameter is found to be below the minimum permitted size. What is the correct course of action?

  1. Polish the journal and return it to service without further action
  2. Fit thicker bearing shells to restore the clearance with the worn journal
  3. Fit a new connecting rod to compensate for the worn journal
  4. Reject the crankshaft unless an approved overhaul scheme permits grinding to an undersize and fitting matching oversize shells Correct answer.

Explanation

Journal dimensions have hard limits set by the manufacturer, and a journal below minimum has insufficient strength and oil clearance margin. Grinding to an approved undersize with matching oversize shells is only acceptable where the overhaul manual specifically permits it. Fitting shells or rods cannot restore the lost journal material.

Question 43Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

What is the function of the oil groove and oil hole in a main bearing shell?

  1. To distribute oil around the journal and feed the drilling that supplies the big-end bearing Correct answer.
  2. To allow dirt to escape from the bearing surface
  3. To lock the shell against rotation in its housing
  4. To provide a reservoir of oil for the whole engine

Explanation

The groove spreads oil circumferentially and the hole aligns with the crankcase drilling so oil can pass through the crankshaft to the big end. Shell retention is achieved by the tang, not by the oil hole. Dirt is removed by the filter, not by grooves.

Question 44Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

A horizontally opposed engine is described as having a 'flat-four' configuration. How many power strokes occur per revolution of the crankshaft?

  1. Four
  2. Eight
  3. Two Correct answer.
  4. One

Explanation

A four-stroke engine fires each cylinder once per two revolutions, so a flat-four produces four power strokes per 720 degrees, which is two per revolution. The figure doubles for a two-stroke cycle but aero engines are almost always four-stroke. Eight would require sixteen cylinders.

Question 45Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

Why is the crankshaft of a geared aero engine often fitted with a torsional vibration damper at the front end?

  1. To increase the engine's rotational inertia for smoother idling
  2. To absorb torsional oscillations set up by the firing pulses before they reach the propeller and reduction gear Correct answer.
  3. To take the propeller thrust load off the thrust bearing
  4. To drive the oil pump at a constant speed

Explanation

Firing pulses excite torsional oscillation along the crankshaft, and a damper absorbs energy at the resonant frequency to keep shaft stresses within limits. It does not carry propeller thrust, which is a separate axial load path. Increasing inertia is not the purpose, and the oil pump is driven separately.

Question 46Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

In a horizontally opposed engine, which feature of the crankcase construction allows the cylinder barrels to be bolted on with an effective gas seal?

  1. A shrunk-fit cylinder barrel retained by the cooling fins
  2. A single continuous cylinder head casting covering both banks
  3. A threaded collar on the connecting rod
  4. Machined cylinder mounting pads with studs and a gasket or O-ring at each barrel flange Correct answer.

Explanation

Each barrel seats on a machined pad on the crankcase and is held by studs and nuts, with a gasket or O-ring providing the seal at the barrel flange. The head may join the barrel as one unit, but the crankcase attachment is at the barrel flange. Cooling fins and connecting rods play no part in this joint.

Question 47Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

A connecting rod is found to have its big-end bore out of round beyond limits. Why is it rejected rather than simply fitted with a new bearing shell?

  1. The housing bore controls the crush and clearance of the shell, so an out-of-round housing cannot support a correct oil film Correct answer.
  2. The big-end bore has no effect on bearing life
  3. The rod can be straightened by hand to restore the bore
  4. The shell will absorb the distortion and run correctly regardless of the housing

Explanation

The bearing shell relies on the housing bore to give it the correct crush and to maintain concentricity, so an out-of-round housing distorts the shell and destroys the oil film. A new shell cannot compensate for a defective housing. Rod alignment is corrected only by approved reconditioning, not by hand straightening.

Question 48Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)

What advantage does a two-piece crankcase with a horizontal split offer during maintenance of a horizontally opposed engine?

  1. It allows the propeller to remain attached to the crankshaft
  2. The upper half can be lifted clear to expose the crankshaft and camshaft while the lower half remains as an assembly cradle Correct answer.
  3. It eliminates the need for a sump
  4. It removes the requirement for a thrust bearing

Explanation

A horizontal split lets the upper half be removed so the crankshaft, camshaft and bearings can be inspected while the lower half supports the remaining assembly. It does not remove the need for a sump or thrust bearing, and the propeller is normally removed for major work anyway. The choice of split is a maintenance-access trade-off.

Question 49Cylinders, pistons, rings, valves and valve train

On an air-cooled aircraft piston engine, what is the primary function of the cooling fins machined into the cylinder barrel and head?

  1. To stiffen the barrel against combustion pressure loads
  2. To increase the surface area available for heat transfer to the cooling airflow Correct answer.
  3. To provide a mounting surface for the rocker box covers
  4. To retain oil film for bore lubrication

Explanation

Fins multiply the external surface area of the barrel and head so that a greater quantity of heat can be passed to the cooling air stream. Their depth, spacing and the baffling around them govern how effectively that heat is carried away.

Question 50Cylinders, pistons, rings, valves and valve train

What is the purpose of the choke (restriction) in the barrel of a classic plain-steel cylinder assembly?

  1. To provide clearance for carbon build-up at the top of the stroke
  2. To reduce bore diameter slightly at the top so it matches the greater expansion of the hotter cylinder head end Correct answer.
  3. To create a reservoir that holds oil for the exhaust stroke
  4. To allow the piston rings to be fitted without a compressor

Explanation

The head end of the barrel runs much hotter than the skirt end, so it expands more; the barrel is therefore machined with a slight taper so the bore is parallel when the engine is at operating temperature. This avoids a tight spot at the top of the stroke when hot.