A four-stroke engine, or colloquially four-stroke, is a piston combustion engine working in four strokes. This means that the working cycle of the engine will take place during four strokes of the piston and two revolutions of the crankshaft.

The work cycles are slightly different for spark and diesel engines depending on how the mixture is ignited. So let's look at how a gasoline four-stroke engine and a diesel four-stroke engine work.