To prevent a ratio of specific heat from being decreased by heat dissociation so as to improve mechanical antiknock property within a combustion chamber by returning into the combustion chamber burned gas once discharged to an exhaust passage and reduced in temperature in a cooling stroke, and to increase combustion pressure in a combustion process by improving the filling efficiency of intake air so as to heighten the combustion efficiency while preventing occurrence of knocking in an intake stroke.

The six-stroke gasoline engine is provided with a piston 14 slidably inserted in a cylinder hole 15A, and includes four strokes of intake, compression, expansion, and exhaust performed corresponding to upward and downward movements of the piston, and two strokes of cooling and discharge performed after the four strokes, wherein the piston is moved upward and downward under such a condition that an inlet valve is closed and an exhaust valve is opened.

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