A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cyclewith two strokes of the piston (one up and one down movement) in one revolution of the crankshaft. (A four-stroke engine requires four strokes of the piston to complete a power cycle, in two crankshaft revolutions.) In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust (or scavenging) functions occurring at the same time.