Suction (charge exchange stroke)
When the piston moves down, air (diesel engine or direct injection petrol engine) or a fuel/air mixture (petrol engine) is drawn through the intake valve.
Compression (power stroke)
The cylinder volume is compressed.
Expansion (power stroke)
In the petrol engine, the fuel/air mixture is ignited by a spark plug, whereas in the diesel engine fuel is injected under high pressure and the mixture ignites spontaneously.
Exhaust (charge exchange stroke)
The exhaust gas is expelled when the piston moves up.