four-stroke engine

Wiktionary

1. n. An engine in which the pistons perform four strokes per engine cycle (i.e. intake, compression, power, exhaust).

WordNet 3.0

1. n. an internal-combustion engine in which an explosive mixture is drawn into the cylinder on the first stroke and is compressed and ignited on the second stroke; work is done on the third stroke and the products of combustion are exhausted on the fourth stroke