PURPOSE: To improve a charging efficiency of air-fuel mixture, by a method wherein a cross section of a cylinder bore and a piston is formed in an oval or an elipse, a pair of poppet type suction and discharge valves are arranged in the direction of a long axle of the cylinder bore cross section, and an ignition plug is positioned in the middle of a distance between the two valves.

CONSTITUTION: In a 4-cylinder, 4-cycle engine, a cross section of a cylinder 5 is formed in an oval or an elipse, and the cylinder bore is arranged so that an axle of the cross section is positioned in parallel with an axial line of a crank shaft 4. Corresponding to the shape of the cylinder bore, a cross section of a piston 6 is also formed in an oval or an elipse. Suction and discharge ports 10 and 11, which are open to their respective combustion chambers 9, are located at a cylinder head 3, and are opened and closed by means of suction and discharge valves 12 and 13, respectively. Said suction and discharge valves 12 and 13 are arranged in the direction of the long axle of the cross section of the cylinder bore 5. Axes I and E of the two valves tilt against a central axis C, and an ignition plug 14 is positioned in the middle of a distance between the two valves 12 and 13.