Britannica Sources used in this answer

Why does the camshaft rotate at half the speed of the crankshaft in a four-stroke engine?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

In a four-stroke engine, the camshaft must rotate at exactly half the speed of the crankshaft to maintain proper valve timing.12

This synchronization is necessary because a complete four-stroke cycle—consisting of the intake, compression, power, and exhaust strokes—requires two full revolutions of the crankshaft. Since the camshaft is responsible for opening and closing the intake and exhaust valves only once during this entire four-stroke sequence, it must complete only one full rotation for every two rotations of the crankshaft.3412

By mechanically linking the two shafts via gears, chains, or belts at a 2:1 ratio, the engine ensures that the valves actuate at the precise moments required for each phase of the combustion process.12