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How does a four-stroke gasoline engine work?

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Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

The four-stroke cycle is the principle by which most modern automobile engines function. Each cycle requires four strokes of the piston and two revolutions of the crankshaft.12

The process follows these stages:

Intake: With the inlet valve open, the piston descends, drawing a mixture of gasoline vapour and air into the cylinder.12
Compression: The piston ascends with both valves closed, compressing the mixture.12
Power: Near the end of the compression stroke, an electric spark ignites the charge. The resulting expansion of burned gas presses on the piston head, driving it downward with both valves closed.12
Exhaust: The piston ascends again, forcing the spent combustion products out through the open exhaust valve.12

While the four-stroke cycle produces fewer power strokes than a two-stroke engine, it offers more efficient scavenging of exhaust gases and better cylinder reloading. The concept was originated by Alphonse Beau de Rochas and later implemented by Nikolaus August Otto.12