PURPOSE: To enhance the rate of fuel consumption and the output power by furnishing a main and a sub-cylinder, supplying the gas compressed in the bus- cylinder to the main cylinder during its suction stroke of combustion, and by reducing the loss owing to possible blow-through.

CONSTITUTION: In an engine in operation, the gass supplied from the fuel supply system is sent to a sub-cylinder 2 from a suction port 14 when a suction valve 16 is opened, and compressed by a piston rising 18. the main cylinder 1 is in expansion stroke at this time, to transfer to the exhaust stroke when the piston 8 turns to rising from its lower dead point. For the gas compressed in said sub- cylinder 2, an exhaust valve 17 and suction valve 6 are opened sequentially when the combustion gas is fully exhausted out of the main cylinder 1 and when the piston 8 begins sinking from its upper dead point. Thereby gas is sent into the main cylinder 1, and an ignition plug 3 makes ignition to provide combustive expansion. Now a rotational power is imparted to the crank shaft 10.