A two-stroke engine is an internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 that completes the process cycle in one revolution of the crankshaft (an up stroke and a down stroke of the piston

Piston

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

, compared to twice that number for a four-stroke engine). This is accomplished by using the end of the combustion stroke and the beginning of the compression stroke to perform simultaneously the intake and exhaust (or scavenging

Scavenging (automotive)

In automotive usage, scavenging is the process of pushing exhausted gas-charge out of the cylinder and drawing in a fresh draught of air ready for the next cycle....

) functions. In this way, two-stroke engines often provide high specific power

Power-to-weight ratio

Power-to-weight ratio is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources...

, at least in a narrow range of rotational speeds.