PURPOSE: To reduce the quantity of remaining combustion gas by reciprocating a piston so that the combustion chamber-volume at the time when the piston is at top dead center after the completion of exhaust stroke is constantly smaller than the combustion chamber-volume at the time when the piston is at top dead center after the completion of compression stroke.

CONSTITUTION: One end of a connecting rod 2 is connected to the off-centered position of a gear 3, and the other end is connected to a piston 1. A crank 4 is connected to the rotary shaft of the gear 3. The gear 3 is then brought into mesh with a gear 5 having a diameter 3.5 times that of the gear itself. Thus, the piston is reciprocated in a cylinder 7 so that the combustion chamber- volume at the time when the piston is at top dead center after the completion of exhaust stroke is constantly smaller than the combustion chamber-volume at the time when the piston is at top dead center after the completion of compression stroke. The combustion gas remaining in the combustion chamber-space can thus be exhausted much more by the piston 1.