In a typical four-stroke engine, the crankshaft runs at half engine speed.

• A. The crankshaft is driven at half engine speed.
• B. The crankshaft runs at the same speed as the camshaft.
• C. The camshaft is driven at half engine speed.
• D. The crankshaft is driven at twice engine speed.

Answer: (A)

In a four-stroke engine, each cylinder completes its cycle over two crankshaft revolutions, while the valves must open and close once per cycle. The camshaft is responsible for valve timing, so it must complete its cycle once for every two crankshaft revolutions. That means the camshaft rotates at half the crankshaft speed, achieved by the 2:1 drive ratio between them. As a result, the component that ends up driven at half engine speed is the camshaft, not the crankshaft. The crankshaft runs at the engine speed, and statements suggesting the crankshaft runs at half or twice that speed don’t align with how valve timing is synchronized with piston motion.