Glossary
compression stroke
The compression stroke is the second of the four strokes in a four-stroke engine: the piston rises with the valves closed and squeezes the air-fuel mixture into a smaller space.
The compression stroke is the second of the four strokes of a four-stroke engine. With both valves closed, the piston rises and squeezes the air-fuel mixture drawn in on the intake stroke into a much smaller space.
Squeezing is what gives combustion its punch. Compressing the mixture raises its temperature and makes it burn faster and more completely, and the ratio of the cylinder's largest volume to its smallest is the engine's compression ratio. A diesel engine relies on this: it compresses air alone so hard that the heat of compression ignites the fuel, which is why a diesel has no spark plugs.
The four strokes in order are intake, compression, power and exhaust. Only the power stroke produces power; the other three are carried out by the momentum of the crankshaft and the flywheel turning the engine over.
Taught in
Related terms
- piston — A piston is the close-fitting plug that slides up and down in an engine's cylinder. It receives the push of combustion and passes it to the crankshaft through the connecting rod.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)