A plain explanation of the intake, compression, power and exhaust strokes of a petrol engine, and of compression ignition in diesel engines.
In this article MSG Equipment explains the basic operating cycle of a piston engine. In a four-stroke petrol engine, one complete cycle consists of four strokes over two revolutions of the crankshaft.
- Intake: the piston moves down, the intake valve opens and the air-fuel mixture enters the cylinder.
- Compression: the piston rises and compresses the mixture.
- Power: the spark plug ignites the mixture and the pressure drives the piston and crankshaft.
- Exhaust: the piston rises again and pushes the exhaust gases out.
A diesel engine draws in only air, compresses it until it becomes very hot, and the injected fuel then ignites on its own without a spark plug.