PURPOSE: To reduce the resistance during exhaust cycle so as to aim at enhancing the output power by forming an automatic discharge port adapted to be opened to a combustion chamber in the vicinity of the bottom dead center of a piston, in the side wall of an engine cylinder, and by disposing a slide valve adapted to be opened and closed by a cam, in an exhaust passage communicated to the discharge port.

CONSTITUTION: An automatic exhaust gas discharge port 11 adapted to be opened when a piston 2 reaches its bottom dead center is formed in the side wall of a cylinder in an engine. A slide valve 2 adapted to be opened and closed by a convex cam 15 which is rotated in association with the rotation of the engine is disposed in an exhaust passage connected to the discharge port 11. Further, the automatic exhaust gas discharge port 11 is controlled so as to be closed at the termination of intake cycle and at the initiation of compress on cycle but opened at the termination of exhaust cycle. With this arrangement, it is possible to remove the resistance during exhaust cycle, thereby it is possible to enhance the output power of the engine.