To make active a catalyst at the time of low load and to improve an exhaust gas characteristic by setting a suction/exhaustion period during low load running such that time constant overlapping is provided and the overlapping of suction and exhaustion after a piston reaches its upper dead point is larger than the overlapping of suction and exhaustion before it reaches the upper dead point.
For cams for first and second intake valves 21 respectively provided in a first intake port 17 for swirl generation and a second intake port 18 having a port opening/closing adjust valve 23 and cams for the exhaust valves 22 of exhaust ports 19 and 20, shaped are set such that while the adjust valve 23 is closed, overlapping Rb after TDC is larger than overlapping Ra before an upper dead point and, when the adjust valve 23 is open, Rb is smaller than Ra. Moreover, the shape of each cam is set such that overlapping between the intake and exhaust valves 21 and 22 is -10° or higher represented by a crank angle from an exhaust valve opening time to its opening time defined by the 1mm lifting positions of the exhaust valves 21 and 22.
ISO NAOHIDE
ARAKI KEIJI
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