In a four-stroke-cycle reciprocating engine, how many times does the crankshaft rotate to complete one full cycle of operation in a single cylinder?
ACS code: AM.III.A
Correct answer: Two complete revolutions (720 degrees).
Rationale: The four-stroke Otto cycle consists of the intake, compression, power, and exhaust strokes. Each stroke moves the piston the length of the cylinder, which corresponds to one-half crankshaft revolution. Four strokes therefore require two full crankshaft revolutions, or 720 degrees, to complete one cycle in a given cylinder.
What is the correct order of the four strokes in the Otto-cycle reciprocating engine?
ACS code: AM.III.A
Correct answer: Intake, compression, power, exhaust.
Rationale: The Otto cycle proceeds in a fixed sequence: the intake stroke draws in the fuel/air charge, the compression stroke compresses it, the power stroke delivers usable work after ignition, and the exhaust stroke expels the burned gases. This intake-compression-power-exhaust order is fundamental to four-stroke engine operation.
The compression ratio of a reciprocating engine is defined as the ratio of the cylinder volume with the piston at:
ACS code: AM.III.A
Correct answer: Bottom center compared to the volume at top center.
Rationale: Compression ratio is the total cylinder volume (piston at bottom center) divided by the clearance volume (piston at top center). It expresses how much the fuel/air charge is compressed during the compression stroke. A higher compression ratio generally increases power and efficiency but raises the tendency toward detonation.
Detonation in a reciprocating engine is best described as:
ACS code: AM.III.A
Correct answer: An uncontrolled, explosive burning of the fuel/air charge after normal ignition.
Rationale: Detonation is the spontaneous, almost instantaneous combustion of the remaining unburned charge after the normal flame front has been started by the spark plug. The resulting sudden rise in pressure and temperature can damage pistons, rings, and cylinders. It is distinct from preignition, which occurs before the spark fires.
On a horizontally opposed reciprocating engine, valve overlap refers to the period during which:
ACS code: AM.III.A
Correct answer: Both the intake and exhaust valves are open at the same time near top center.
Rationale: Valve overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke, when the intake valve opens before the exhaust valve closes. With both valves open near top center, the outrushing exhaust gases help scavenge the cylinder and draw in the fresh charge, improving volumetric efficiency and aiding cylinder cooling.
In a typical radial engine, the piston connected directly to the crankpin by a one-piece connecting rod that carries the other rods is called the:
ACS code: AM.III.A
Correct answer: Master rod piston.
Rationale: In a radial engine, one cylinder uses a master connecting rod that attaches directly to the single crankpin. The remaining cylinders use articulated (link) rods that pin to flanges on the master rod, not to the crankpin itself. The cylinder served by the master rod is the master rod cylinder, and its piston is the master rod piston.
Which type of reciprocating engine valve is frequently filled partially with metallic sodium to improve heat dissipation?
ACS code: AM.III.A
Correct answer: The exhaust valve.
Rationale: Exhaust valves operate in extremely high-temperature exhaust gas and require enhanced cooling. Many have hollow stems partially filled with metallic sodium, which melts in operation and sloshes back and forth, carrying heat from the hot valve head to the cooler stem where it transfers to the valve guide. Intake valves run cooler and generally do not require sodium cooling.
The primary purpose of the dry-sump lubrication system used on many reciprocating engines is to:
ACS code: AM.III.A
Correct answer: Store and supply oil from a separate tank rather than the crankcase sump.
Rationale: In a dry-sump system, the main oil supply is held in a separate external tank rather than in the engine crankcase. A pressure pump draws oil from the tank to lubricate the engine, while one or more scavenge pumps return oil from the sump back to the tank. This keeps the crankcase relatively free of accumulated oil and supports operation in various attitudes.
The firing order of a reciprocating engine is designed primarily to:
ACS code: AM.III.A
Correct answer: Distribute the power impulses to minimize vibration and balance loads.
Rationale: The firing order is arranged so that power impulses are evenly distributed around the crankshaft, reducing vibration and providing smooth operation while balancing the loads on the crankshaft and bearings. Cylinders do not fire simultaneously, and the sequence is specifically chosen so that consecutively firing cylinders are not physically adjacent.
When performing a differential compression check on a reciprocating engine cylinder, air leakage heard at the exhaust pipe (tailpipe) most likely indicates:
ACS code: AM.III.A
Correct answer: A leaking or poorly seated exhaust valve.
Rationale: During a differential compression check, the source of audible leakage identifies the leak path. Air escaping at the exhaust pipe indicates the exhaust valve is not sealing. Air heard at the carburetor or intake points to the intake valve, while air heard at the crankcase breather or oil filler indicates leakage past the piston rings.
During the intake stroke of a four-stroke reciprocating engine, the piston moves toward bottom center and the:
ACS code: AM.III.A
Correct answer: Intake valve is open and the exhaust valve is closed.
Rationale: On the intake stroke, the descending piston creates a low-pressure area in the cylinder, and the open intake valve allows the fuel/air charge to be drawn in while the exhaust valve is closed. (Brief valve overlap near top center is the only time both valves are open.) After the charge is admitted, the intake valve closes for the compression stroke.
To perform a static power check of a reciprocating engine driving a controllable-pitch propeller, the propeller should be set to
ACS code: AM.III.A
Correct answer: low pitch, and advance the throttle to the specified target rpm.
Rationale: A power check on a reciprocating engine is made with the propeller in the low (fine) pitch position so the engine is lightly loaded and can be run up to the specified target rpm, where the power indications are compared against the reference figures. Running the engine to its maximum rpm rather than the specified target value would not provide a valid, comparable check.
During engine warm-up of a piston engine driving a controllable-pitch propeller, the mixture and propeller should be set to
ACS code: AM.III.A
Correct answer: rich mixture with the blades in low pitch.
Rationale: During warm-up the engine runs at low power, so the mixture is kept rich for safe, cool running and the blades are set to low (fine) pitch to keep the load light and let the engine warm steadily. Setting the blades to feather would load or stall the warm-up, and leaning at this stage risks rough running and detonation.
The viscosity index of a lubricating oil is based on which quality of the oil?
ACS code: AM.III.A
Correct answer: Its rate of change in viscosity with changes in temperature.
Rationale: The viscosity index is a numerical measure of how much an oil's viscosity changes as temperature changes. It is based on this rate of change of viscosity with temperature, a high index meaning small change. It is not a single-temperature flow rate or a comparison of flow resistance at one temperature.
Assuming all other requirements can be met, what type of oil would provide theoretically perfect engine lubrication?
ACS code: AM.III.A
Correct answer: The thinnest oil that will stay in place and maintain a reasonable film strength.
Rationale: Ideal lubrication uses the thinnest oil that still maintains an adequate film between the moving surfaces, because a thinner oil minimizes fluid friction and churning losses while still preventing metal-to-metal contact. Excess viscosity only adds drag, so the goal is the lowest viscosity that keeps a reasonable film strength.
Which factor helps determine the proper grade of oil to use in a particular reciprocating engine?
ACS code: AM.III.A
Correct answer: The operating speeds of the bearings.
Rationale: Selecting the correct oil grade depends largely on the operating speed of the bearings: higher-speed bearings generally require a thinner oil to limit fluid friction and ensure the film is carried into the contact, whereas slower, heavily loaded bearings need a more viscous oil. The other options describe lubrication behavior, not grade selection.
Specific gravity is a comparison of the weight of a substance to the weight of an equal volume of
ACS code: AM.III.A
Correct answer: distilled water at a specified temperature.
Rationale: Specific gravity compares the weight (or density) of a substance with that of an equal volume of pure (distilled) water at a stated temperature, water being the standard reference with a specific gravity of 1. Mercury or oil is not the reference for specific gravity.
What advantage do mineral-base lubricants have over vegetable-oil-base lubricants when used in aircraft engines?
ACS code: AM.III.A
Correct answer: Greater chemical stability.
Rationale: Mineral-base lubricants resist oxidation and chemical breakdown far better than vegetable-oil-base lubricants, which tend to gum, oxidize, and form acids in service. This greater chemical stability gives mineral oils a much longer useful life in aircraft engines.
High tooth pressures combined with high rubbing velocities, such as occur with spur-type gears, require the use of
ACS code: AM.III.A
Correct answer: an extreme-pressure (EP) lubricant.
Rationale: Where gear teeth combine high contact pressures with high sliding (rubbing) velocities, an ordinary oil film can be squeezed out, allowing metal-to-metal contact. An extreme-pressure (EP) lubricant contains additives that form a protective surface film under these severe conditions, preventing scuffing and welding of the teeth.
What does volumetric efficiency measure in a reciprocating engine?
ACS code: AM.III.A
Correct answer: The ratio of the actual volume of air drawn into the cylinder to the theoretical maximum volume
Rationale: Volumetric efficiency compares the actual volume of charge that enters the cylinder against the theoretical volume that would fill the swept volume at ambient conditions. A figure below 100% reflects intake restriction, valve timing, and heating losses. It is a measure of how well the cylinder breathes, not a pressure or compression-ratio comparison.
In a four-stroke reciprocating engine, how many crankshaft revolutions are needed to complete one full operating cycle?
ACS code: AM.III.A
Correct answer: Two
Rationale: A four-stroke engine completes intake, compression, power, and exhaust over four piston strokes. Since each crankshaft revolution produces two strokes, the full cycle takes two crankshaft revolutions to fire once. A two-stroke, by contrast, completes its cycle in a single revolution.
In a four-stroke reciprocating engine, which stroke compresses the fuel-air mixture?
ACS code: AM.III.A
Correct answer: Compression stroke
Rationale: During the compression stroke both valves are closed and the rising piston squeezes the fuel-air charge into the clearance volume, raising its pressure and temperature in preparation for ignition. This compression allows efficient, rapid combustion on the following power stroke.
What is the compression ratio of a reciprocating engine?
ACS code: AM.III.A
Correct answer: The ratio of total cylinder volume to clearance volume
Rationale: Compression ratio is the total cylinder volume with the piston at bottom dead center divided by the clearance volume remaining at top dead center. It expresses how much the charge is compressed before ignition; a higher ratio extracts more work from each combustion event.
In a two-stroke reciprocating engine, how many crankshaft revolutions complete one operating cycle?
ACS code: AM.III.A
Correct answer: One
Rationale: A two-stroke engine completes its full cycle in just two piston strokes, which equals a single crankshaft revolution, giving one power stroke per revolution. This is why a two-stroke produces a power event twice as often as a four-stroke running at the same speed.
What does thermal efficiency measure in a reciprocating engine?
ACS code: AM.III.A
Correct answer: The ratio of useful work output to the heat energy in the fuel
Rationale: Thermal efficiency measures how much of the chemical heat energy released by burning the fuel is converted into useful mechanical work. The remainder is lost as exhaust heat, cooling, and friction, so a higher thermal efficiency means more of the fuel's energy does useful work.
The Otto cycle, used by aircraft spark-ignition reciprocating engines, idealizes the combustion process as occurring at:
ACS code: AM.III.A
Correct answer: Constant volume
Rationale: The Otto cycle idealizes spark-ignition combustion as occurring at essentially constant volume, with the charge burning rapidly near top dead center while the piston is nearly stationary. This contrasts with the Diesel cycle, where heat is added at constant pressure.
The Diesel cycle idealizes the combustion process as occurring at:
ACS code: AM.III.A
Correct answer: Constant pressure
Rationale: The Diesel cycle models compression-ignition combustion as heat addition at constant pressure, with fuel injected and burned progressively as the piston begins to descend. This differs from the Otto cycle's near constant-volume burn around top dead center.
Piston displacement of a reciprocating engine cylinder is defined as the:
ACS code: AM.III.A
Correct answer: Volume swept by the piston moving from TDC to BDC
Rationale: Piston displacement is the swept volume the piston generates traveling from top dead center to bottom dead center. It does not include the clearance volume above the piston at TDC, which is counted separately when finding total cylinder volume.
The mechanical efficiency of a reciprocating engine is the ratio of:
ACS code: AM.III.A
Correct answer: Brake horsepower to indicated horsepower
Rationale: Mechanical efficiency is brake horsepower divided by indicated horsepower, showing how much of the power developed in the cylinders actually reaches the output shaft. The shortfall is friction horsepower lost to bearings, pistons, and driven accessories.
In a four-stroke reciprocating engine, during which stroke is the exhaust valve open?
ACS code: AM.III.A
Correct answer: Exhaust stroke
Rationale: The exhaust valve opens to let burned gases escape, and the upward exhaust stroke pushes them out of the cylinder. The intake stroke uses the intake valve, so the exhaust valve is the one open during the exhaust stroke.
The firing order of a reciprocating engine is selected primarily to control:
ACS code: AM.III.A
Correct answer: Engine vibration and smooth power delivery
Rationale: Firing order is chosen to space successive power strokes evenly around the crankshaft, which spreads the loads on the crank and bearings and minimizes torsional vibration. It is about balance and smooth running, not raising peak power or controlling individual cylinder temperatures directly.
In a four-stroke Otto cycle engine, useful work is produced during which stroke?
ACS code: AM.III.A
Correct answer: Power stroke
Rationale: Combustion and the resulting expansion drive the piston down on the power stroke, the only stroke that produces useful work in the Otto cycle. Ignition is timed just before top dead center so peak pressure acts as the piston starts this downward travel.
In reciprocating engine terminology, BDC stands for:
ACS code: AM.III.A
Correct answer: Bottom dead center
Rationale: BDC stands for bottom dead center, the lowest point of piston travel in the cylinder where the crank throw is farthest from the cylinder head. It marks the end of the intake and power strokes.
In reciprocating engine terminology, TDC stands for:
ACS code: AM.III.A
Correct answer: Top dead center
Rationale: TDC stands for top dead center, the highest point of piston travel where it is nearest the cylinder head. It defines the end of the compression and exhaust strokes and is the reference point for ignition and valve timing.
In a horizontally opposed reciprocating engine, the cylinders are arranged:
ACS code: AM.III.A
Correct answer: On opposite sides of the crankshaft in the same horizontal plane
Rationale: In a horizontally opposed (flat) engine the cylinders lie in two banks on opposite sides of the crankshaft in the same horizontal plane, with opposing pistons moving outward and inward together. This layout gives good balance and a low frontal profile favored in light aircraft.
A radial reciprocating engine has its cylinders arranged:
ACS code: AM.III.A
Correct answer: In a circular pattern around the crankshaft
Rationale: A radial engine arranges its cylinders in a circular pattern around a single central crankshaft, with all connecting rods driving one crank throw via a master rod. This gives a short, large-diameter engine once common on older aircraft.
Which type of aircraft reciprocating engine uses spark plugs for ignition?
ACS code: AM.III.A
Correct answer: Otto cycle (gasoline) engine
Rationale: The spark-ignition Otto cycle gasoline engine relies on a timed spark from the plug to ignite the compressed charge. Diesel and other compression-ignition engines instead ignite the fuel using the heat generated by compression alone, so they do not use spark plugs.
In a Diesel (compression-ignition) engine, ignition of the fuel is achieved by:
ACS code: AM.III.A
Correct answer: Heat of compression
Rationale: A Diesel engine is a compression-ignition engine: air is compressed to a high ratio so its temperature rises enough to ignite the fuel as it is injected, with no spark required. Glow plugs only assist cold starting and are not the normal ignition source.
What occurs during the intake stroke of a four-stroke reciprocating engine?
ACS code: AM.III.A
Correct answer: The piston moves down, drawing the fuel-air mixture into the cylinder
Rationale: On the intake stroke the intake valve is open and the descending piston creates a low pressure that draws the fuel-air charge into the cylinder. Pushing exhaust out describes the exhaust stroke, and ignition forcing the piston down describes the power stroke.
What is the general relationship between compression ratio and the thermal efficiency of a reciprocating engine?
ACS code: AM.III.A
Correct answer: A higher compression ratio generally increases thermal efficiency
Rationale: Raising the compression ratio increases the expansion of the burned gases and the cycle's thermal efficiency, extracting more work from each charge. The practical limit is detonation, which is why higher compression ratios demand higher-octane fuel.
An inline reciprocating engine has its cylinders arranged:
ACS code: AM.III.A
Correct answer: In a single straight row along the crankshaft
Rationale: An inline engine has all of its cylinders arranged in a single straight row along the crankshaft. A V-type layout uses two angled banks and a radial uses a circular arrangement, so neither describes an inline engine.
Which of the following does NOT directly affect the volumetric efficiency of a reciprocating engine?
ACS code: AM.III.A
Correct answer: Engine oil viscosity
Rationale: Volumetric efficiency depends on how freely the cylinder can breathe, so engine speed, valve timing, induction system design, and charge temperature all affect it. Oil viscosity relates to lubrication and friction, not the induction process, so it does not directly change volumetric efficiency.
The clearance volume of a reciprocating engine cylinder is the:
ACS code: AM.III.A
Correct answer: Volume remaining above the piston at TDC
Rationale: Clearance volume is the small space left above the piston when it reaches top dead center, forming the combustion chamber. Together with the swept (displacement) volume it makes up total cylinder volume and sets the compression ratio.
In a two-stroke engine, the power and exhaust events occur:
ACS code: AM.III.A
Correct answer: During the same downward piston movement
Rationale: In a two-stroke engine the power and exhaust events share one downward piston travel: as the piston descends it uncovers the exhaust and transfer ports, so combustion gases leave and fresh charge enters during the same stroke. This is what lets the cycle complete in one crankshaft revolution.
What advantage does a four-stroke reciprocating engine generally have over a comparable two-stroke engine?
ACS code: AM.III.A
Correct answer: Better fuel efficiency and lower emissions
Rationale: A four-stroke engine has dedicated intake and exhaust strokes with valves rather than ports, giving better charge control, more complete combustion, and no fuel loss through scavenging. The result is improved fuel efficiency and lower emissions than a comparable two-stroke.
The bore of a cylinder refers to:
ACS code: AM.III.A
Correct answer: The internal diameter of the cylinder
Rationale: The bore is the internal diameter of the cylinder in which the piston travels. Together with the stroke it defines the swept volume, but by itself it is simply the cylinder's bore diameter, not its depth or its displacement.
The stroke of a reciprocating engine refers to:
ACS code: AM.III.A
Correct answer: The distance the piston travels from TDC to BDC
Rationale: The stroke is the linear distance the piston moves between top dead center and bottom dead center, set by the crank throw. Multiplied by the bore area it yields the cylinder's swept volume; it is not the connecting-rod length or the clearance volume.
Which engine configuration is most commonly used in modern light aircraft?
ACS code: AM.III.A
Correct answer: Horizontally opposed
Rationale: Most modern light aircraft use horizontally opposed (flat) engines because they are well balanced, low in height, and present a small frontal area for good cooling and streamlining. Radial and V-type layouts are heavier or bulkier and are now uncommon in light aircraft.
What is indicated horsepower?
ACS code: AM.III.A
Correct answer: The total power developed inside the cylinders
Rationale: Indicated horsepower is the total power developed by combustion inside the cylinders, calculated from cylinder pressure and engine geometry before any losses. Subtracting friction horsepower from it leaves the brake horsepower available at the output shaft.
Brake horsepower is:
ACS code: AM.III.A
Correct answer: The power available at the output shaft
Rationale: Brake horsepower is the usable power measured at the engine's output shaft after internal friction and accessory losses. It equals indicated horsepower minus friction horsepower and is the figure that actually drives the propeller.