In most cars the piston repeats the same four-stroke cycle again and again. The name comes from the four piston strokes it takes:

1. Intake. The intake valve opens and the piston moves down, drawing fresh air in. In a port-injection petrol engine fuel is sprayed into the incoming air; in a direct-injection engine it is sprayed into the cylinder itself.
2. Compression. Both valves are closed. The piston moves up and squeezes the gas into a much smaller space. Squeezing makes it hot and sets it up to burn.
3. Power. Near the top of the compression stroke the spark plug fires. The mixture burns, the hot gas expands and pushes the piston down. This is the only stroke that delivers energy to the crankshaft. Both valves are still closed.
4. Exhaust. The exhaust valve opens and the piston moves up, pushing the burnt gas out. Then the cycle begins again.