The four-stroke cycle
A four-stroke engine is an internal combustion engine in which one complete operating cycle contains four piston strokes. A stroke is one complete movement of the piston up or down the cylinder.
One four-stroke cycle requires two revolutions of the crankshaft.
Induction
The piston moves down and the inlet valve is open. Air or a fuel-air mixture enters the cylinder, depending on the type of engine.
Compression
The inlet valve closes and the piston moves upwards. Work is done on the gas, reducing its volume and increasing its pressure.
Power
Combustion produces high-pressure gas which expands and pushes the piston down. Work is done by the gas on the piston.
Exhaust
The exhaust valve opens and the piston rises, forcing the burnt gases out of the cylinder.
Exam Tip: Keep stroke, cycle and revolution distinct. Four piston strokes make one complete cycle, while the crankshaft turns through two revolutions.