- Intake stroke - intake valve opened; fresh air is taken in from the outside. Expansion.
- Compression stroke - the piston moves upward, compressing the air.
- Combustion - fuel is released into the air. In a petrol engine a spark plug causes combustion; in a diesel engine the temperature itself causes combustion. This leads to the power stroke - the air expands, doing work on the piston.
- Exhaust - the exhaust valve is opened to let air out. Compression.