To enable stable premixed compression self-ignition combustion in a wide load range by using internal EGR for making part of exhaust gas flow back from an exhaust port to inside of a combustion chamber.

The second exhaust port 22 for making part of exhaust gas flow back to a central part of the combustion chamber 16 in the middle of an intake stroke is provided as a helical port. A first intake port 19 supplies intake air to a wall face side of the combustion chamber 16 so as to make the intake air flow in the same direction as that of a flow of exhaust gas flowing from the second exhaust port 22 into inside of the combustion chamber 16. A second intake port 20 supplies air fuel mixture to the combustion chamber 16 so as to make the air fuel mixture flow in the same direction as that of a flow of exhaust gas flowing into inside of the combustion chamber 16 and between air supplied from the first intake port 19 and the exhaust gas.

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