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.
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