How many crankshaft revolutions does a four-stroke cycle take?

Two. The crankshaft turns 720 degrees to complete intake, compression, power, and exhaust in one cylinder, and the camshaft turns once in that same time, which is why the cam gear on a typical opposed engine has twice as many teeth as the crankshaft gear. Each cylinder therefore fires once every two crankshaft revolutions.

What is the difference between detonation and pre-ignition?

Detonation is abnormal combustion after the spark plug fires: the unburned portion of the charge reaches its critical pressure and temperature and explodes rather than burning smoothly, producing a sharp pressure spike. Pre-ignition is the mixture being ignited before the spark by a local hot spot such as glowing carbon or a cracked plug insulator. Detonation can create the hot spots that then cause pre-ignition.

Why are some exhaust valves filled with sodium?

Metallic sodium conducts heat very well once melted, and it melts well below exhaust valve operating temperature. Partially filling the hollow stem lets liquid sodium slosh as the valve moves, carrying heat from the hot valve head into the stem and out through the guide, which lowers head temperature. Handle these valves with care during disposal, since sodium reacts violently with water.

What is the advantage of a dry-sump oil system?

A dry sump keeps the oil supply in a separate tank instead of in the engine sump, so the engine can carry a larger quantity of oil, the oil gets more time and surface area to cool and release entrained air, and lubrication keeps working through unusual attitudes. Scavenge pumps return oil from the engine to the tank and have greater capacity than the pressure pump.

What is valve overlap for?

Valve overlap is the period near top dead center between the exhaust and intake strokes when both valves are open. Incoming charge helps push the last of the exhaust gas out, escaping exhaust helps draw fresh charge in, and cool intake air flowing across the valves aids cooling. The result is better volumetric efficiency, particularly at higher engine speeds.

Reciprocating Engines — FAA A&P Test Questions (ACS AM.III.A)

Reciprocating engines is the theory-of-operation subject area on the Powerplant written test. It is where the exam checks that you understand the Otto cycle before it asks you to touch anything: how many crankshaft revolutions a four-stroke cycle takes, what compression ratio actually compares, what detonation is and how it differs from pre-ignition, when valve overlap occurs, and why a radial engine needs a master rod. The questions reward precise definitions, so learn the exact wording of each term rather than a general sense of it.

What ACS AM.III.A covers

This area maps to the reciprocating engine material in FAA-H-8083-32. Start with engine cycle and nomenclature. A four-stroke Otto-cycle engine completes intake, compression, power, and exhaust in that order, which takes two crankshaft revolutions, or 720 degrees, per cylinder, while the camshaft turns once. Compression ratio is the ratio of the cylinder volume with the piston at bottom dead center to the volume with the piston at top dead center. Valve timing and valve construction follow. Valve overlap is the interval near the top of the exhaust stroke when the intake and exhaust valves are open together; it improves volumetric efficiency and helps cool the cylinder. Exhaust valves are often partially filled with metallic sodium, which liquefies at operating temperature and carries heat from the valve head down the stem, and they typically differ from intake valves in stem size and face angle. Engine configuration is next. Horizontally opposed engines dominate light aircraft, while radial engines bring their own vocabulary, including the master rod that connects directly to the crankpin, the articulated rods that attach to it, and the odd-then-even firing order. Abnormal combustion is a reliable source of questions. Detonation is the uncontrolled, explosive burning of the remaining charge after normal ignition, and pre-ignition is the charge being lit early by a hot spot such as a glowing carbon deposit or a damaged spark plug electrode. The area closes with lubrication and cooling: wet-sump versus dry-sump systems, where a dry sump holds the oil in a separate tank and scavenge pumps return it to that tank, plus oil pump types, filters and screens, cylinder cooling fins, baffles, and cowl flaps.

Where this sits on the test

ACS AM.III.A is tested on the FAA Powerplant written test, one of 1,355 ACS-tagged questions in the Powerplant bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-32

Three traps candidates fall into

Compression ratio gets inverted. It is the volume with the piston at bottom dead center divided by the volume at top dead center, so the answer is always greater than one, such as 8.5 to 1. Candidates who set it up the other way reach for a fractional answer and miss the question.
Detonation and pre-ignition are used interchangeably. Detonation happens after the spark, when the last of the charge explodes instead of burning progressively. Pre-ignition happens before the spark, when a hot spot lights the mixture early. Both damage the engine, but the test asks which one starts before ignition.
Candidates place valve overlap in the wrong part of the cycle. Overlap sits at the end of the exhaust stroke and the beginning of the intake stroke, near the top dead center between those two strokes, not near the top of the compression stroke, where both valves are firmly closed.

7 free sample questions from ACS AM.III.A

AM.III.ATap an answer
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?
AM.III.ATap an answer
What is the correct order of the four strokes in the Otto-cycle reciprocating engine?
AM.III.ATap an answer
The compression ratio of a reciprocating engine is defined as the ratio of the cylinder volume with the piston at:
AM.III.ATap an answer
Detonation in a reciprocating engine is best described as:
AM.III.ATap an answer
On a horizontally opposed reciprocating engine, valve overlap refers to the period during which:
AM.III.ATap an answer
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:
AM.III.ATap an answer
Which type of reciprocating engine valve is frequently filled partially with metallic sodium to improve heat dissipation?

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.

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Frequently asked questions

How many crankshaft revolutions does a four-stroke cycle take?: Two. The crankshaft turns 720 degrees to complete intake, compression, power, and exhaust in one cylinder, and the camshaft turns once in that same time, which is why the cam gear on a typical opposed engine has twice as many teeth as the crankshaft gear. Each cylinder therefore fires once every two crankshaft revolutions.
What is the difference between detonation and pre-ignition?: Detonation is abnormal combustion after the spark plug fires: the unburned portion of the charge reaches its critical pressure and temperature and explodes rather than burning smoothly, producing a sharp pressure spike. Pre-ignition is the mixture being ignited before the spark by a local hot spot such as glowing carbon or a cracked plug insulator. Detonation can create the hot spots that then cause pre-ignition.
Why are some exhaust valves filled with sodium?: Metallic sodium conducts heat very well once melted, and it melts well below exhaust valve operating temperature. Partially filling the hollow stem lets liquid sodium slosh as the valve moves, carrying heat from the hot valve head into the stem and out through the guide, which lowers head temperature. Handle these valves with care during disposal, since sodium reacts violently with water.
What is the advantage of a dry-sump oil system?: A dry sump keeps the oil supply in a separate tank instead of in the engine sump, so the engine can carry a larger quantity of oil, the oil gets more time and surface area to cool and release entrained air, and lubrication keeps working through unusual attitudes. Scavenge pumps return oil from the engine to the tank and have greater capacity than the pressure pump.
What is valve overlap for?: Valve overlap is the period near top dead center between the exhaust and intake strokes when both valves are open. Incoming charge helps push the last of the exhaust gas out, escaping exhaust helps draw fresh charge in, and cool intake air flowing across the valves aids cooling. The result is better volumetric efficiency, particularly at higher engine speeds.

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