Soon after this stroke, the inlet valve is closed. Both inlet and exhaust valves are closed. The rising piston compresses the mixture in the combustion chamber and compression heat vaporises the mixture. The compressed gas is ignited by a spark from the spark-plug. Expansion of burning gas drives the piston down.
Exhaust valve opens.
The inlet valve is closed, and exhaust valve open. The piston rises to expel burnt gases, inlet valve opens, exhaust valve closes. Then the cycle restarts. Arrows indicate strokes of piston and related coloured lines show period valves are open. Pressure on the piston causes it and the connecting rod to move downwards, and rotate the crankshaft.