PURPOSE: To improve the output of an engine in the whole area of speed by connecting a blind auxiliary buffer pipe in branch near an engine exhaust hole and providing a switch valve near this branch point.

CONSTITUTION: When a piston 6 passes over an exhaust hole 15 at the end of an expansion stroke, the exhaust gas generated by the combustion of the air-fuel mixture in a combustion chamber S2 is exhausted into the open air through an exhaust pipe 16. When the piston 6 is brought down, a scavenging pipe 14 is connected with a crank chamber S1 through a scavenging passage 13, and the air-fuel mixture compressed in the crank chamber S1 is introduced into the combustion chamber S2 to push out and replace the exhaust gas in the combustion chamber S2. When the revolution speed of an engine is low, a switch valve 18 is opened, and when high, the same is closed.