In this engine, in place of valves, ports are provided, through which fresh mixture comes at the time of suction and the burnt (Waste) gases goes out at the time of exhaust. These parts keep on opening and closing as the piston moves up and down. When the piston completes a stroke from B.D.C. to T.D.C. the functions of suction and compression stroke are performed. When the piston completes the second stroke from T.D.C. to B.D.C. the functions of power stroke and exhaust are done.