WordNet (r) 3.0 (2006):

four-stroke engine
    n 1: 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 [syn: four-stroke engine,
         four-stroke internal-combustion engine]