The four-stroke engine
A piston moves up and down in a cylinder, and a crankshaft turns that into rotation. One full cycle takes four strokes, or two turns of the crankshaft.
1. Intake
The piston moves down while the intake valve is open. That pulls fresh fuel-air mixture into the cylinder.
2. Compression
Both valves are closed and the piston rises, squeezing the mixture into the small space under the head. Compressed mixture burns much more powerfully.
3. Power
Just before top dead centre the spark plug fires. The burning gas expands and pushes the piston down hard, and this is the only stroke that does work.
4. Exhaust
The exhaust valve opens and the rising piston pushes the burnt gas out. Then the cycle starts again with intake.
Crank and camshafts
The crankshaft turns twice per cycle, but the camshafts turn only once, so each valve opens once every four strokes. The flywheel carries the engine through the three strokes that make no power.