For a four-stroke engine, which component completes one revolution for every two revolutions of the crankshaft?

• A. Piston
• B. Propeller
• C. Camshaft
• D. Crankshaft

Answer: (C)

In a four-stroke engine, valve timing is managed by the camshaft, which must rotate at a slower speed than the crankshaft. The piston’s movement completes a full four-stroke cycle (intake, compression, power, exhaust) over two turns of the crankshaft. To open and close the intake and exhaust valves at the correct times for that same cycle, the camshaft turns once during the entire two-crankshaft-revolution cycle. This is achieved by a timing drive (belt or chain) that typically provides a 2:1 ratio, so the camshaft completes one revolution for every two revolutions of the crankshaft.

Piston movement is linear, not rotational, so it doesn’t complete a rotation. The propeller’s speed can be different due to reduction gearing, but the guaranteed 1:2 relationship in a typical four-stroke engine is the camshaft.