PURPOSE: To attain smooth dishcarging of combustion gas from subsidiary chamber, by constituting a subsidiary chamber by forming a recess in the inner surface of cylinder head at a portion thereof near the exhaust valve or in the upper surface of piston at a portion thereof accessible to the exhaust valve, and disposing an ignition plug in this subsidiary chamber.

CONSTITUTION: A subsidiary chamber 12 is constituted by forming a recess at a portion of cylinder head 2 located near exhaust value 11, in the manner that a portion of the wall defining the subsidiary chamber 12 is constisuted by exhaust valve 11. Shaft of intake valve 8 is fixed slantwise to the axis of piston 4, and the inner surface of intake valve 8 constitutes a slant upper surface of chamber 12. An ignition plug 13 is disposed in the subsidiary chamber 12, and fuel injection valve 14 is disposed in intake pipe 6 at a position space from intake valve 8 so that fuel injected from valve 14 is fed directly into chamber 12 when intake valve 8 is opened in suction stroke of piston 4. With such an arrangement, combustion gas in subsidiary chamber 12 can be discharged positively and smoothly after combustion of mixture, so that engine poerformance can be improved considerably.