A four-stroke piston engine completes one thermodynamic/mechanical cycle in four piston strokes and two crankshaft revolutions:

1. Intake — piston moves down; intake valve opens and fresh charge enters.
2. Compression — piston moves up with valves closed, compressing the charge.
3. Power — combustion/high cylinder pressure drives the piston down.
4. Exhaust — piston moves up; exhaust valve opens and spent gases leave.

That means a given cylinder gets only one power stroke every two crank revolutions. In a four-cylinder engine, the cylinders are phased so power events are distributed through the cycle rather than all occurring together.

The camshaft in a conventional four-stroke engine rotates at half crankshaft speed, because the valves need one full sequence for every two crank rotations.

Gasoline and diesel engines share the four-stroke mechanical sequence but ignite differently: gasoline engines normally use a spark to ignite a premixed/port- or direct-injected charge; diesels compress air strongly enough that injected fuel autoignites in the hot compressed gas.

Takeaway: The piston does not simply “explode downward.” Most of its motion is intake, compression, and exhaust machinery supporting the one pressure-producing power stroke.