PURPOSE: To further clean an exhaust gas and to extract a high output according to operating conditions by allowing six-cycle operation having two strokes, intake and exhaust for air only, added to a four-cycle engine.

CONSTITUTION: In a four-cycle engine, the operating mechanism of an intake and exhaust valve is remodeled. Two operating modes are switched according to operating conditions, namely, four-cycle operation plus six-cycle operation having additional air intake and exhaust strokes for exhausting a residual combustion gas in a cylinder 7. For example, the rotary signal 2 of a crank shaft is once converted into an electrical signal to input to an electranagnetic valve control unit 3. The electrical signal is divided into a four-cycle driving signal by rotating the crank shaft twice and a six-cycle driving signal by rotating the crank shaft three times. Thus a comparatively high output is developed in low-cycle operation, resulting in an engine having a clean exhaust gas in high-cycle operation.