PURPOSE:To improve the scavenging operation of an engine and reduce its fuel consumption, by forming a subscavenging port to direct toward the direction of diagonally crossing a combustion chamber upward and providing a valve for permitting only the flow motion of new gas into the combustion chamber in the subscavenging passage. CONSTITUTION:When a piston 6 begins to open subscavenging ports 40a, 40b, fresh gas, in a state of increasing its pressure in a crank chamber 16, flows into a combustion chamber 9 in accordance with a change of internal pressure of the combustion chamber 9, accordingly the fresh gas passes through a valve 43 to flow into the combustion chamber. The fresh gas flowing out from the subscavenging ports 40a, 40b acts to push burnt gas from behind to an exhaust port 30, to promote the exhaust operation. At the moment with a higher internal pressure in the combustion chamber 9 than the internal pressure in the crank chamber 16, the valve 43 acts to block a reverse flow of the burnt gas to the crank chamber 16.
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