The four-stroke engine
The crankshaft turns, the connecting rod drives the piston up and down, and the valves let gas in and out. One full cycle takes four strokes, which is two turns of the crank.
1. Intake
The piston moves down while the intake valve is open. This draws air and sprayed fuel into the cylinder.
2. Compression
Both valves close and the piston moves back up. It squeezes the mixture into the small space under the head, which makes it hot and ready to burn.
3. Power
Near the top, the spark plug fires. The burning gas expands and drives the piston down hard. This is the only stroke that produces work.
4. Exhaust
The exhaust valve opens and the rising piston pushes the burned gas out. Then the cycle starts again.
Crank, rod and flywheel
The connecting rod turns the piston's straight push into rotation at the crank pin. Counterweights balance the crank, and the heavy flywheel carries it through the three strokes that produce no power.
Valve timing
A timing belt drives the camshafts from the crank. The cam sprockets are twice the size of the crank sprocket, so the cams turn once per cycle and each lobe opens its valve exactly once.