Abstract: The invention relates to a 2 stroke single-cylinder or multi-cylinder engine which improves the scavenging system and its supply. It is formed by two cylinders (10) and (11) forming an annular chamber (5), by a crown piston (6), two fresh gas inlets (4) and (5) and a pure air inlet (2). The principle consists in scavenging the cylinder (10) with pure air in order to evacuate the exhaust gases, avoiding any contact with the fresh gases, whilst improving the supply. The invention is particularly intended for small cylinders. Its application to the 4 stroke engine requires use of a special piston (D3), moving in an adapted cylinder formed by two elements numbered (D8) and (D9), forming with the piston an annular chamber (D1). This type of piston allows better filling of the cylinder and better evacuation of the exhaust gases by propelling pure air into the cylinder. The invention is intended for large capacity diesel or petrol 2 or 4 stroke engines.