the pressure increases in the crankcase chamber 26 as the piston 16 continues to descend the exhaust port 50 is opened first after the piston 16 descends a little further the outlet ports 133 of the transfer passages 111 are opened and the inlet ports 211 and 311 are already open at this time the air fuel mixture in the crankcase chamber 26 is pushed toward the combustion chamber 30 through the fully open inlet ports 211 and 311 as the piston continues to descend the lower edge 116 of the piston 16 begins to close the transfer passage inlet ports 211 and 311 as shown in fig3 and 4 however since the cut outs 216 are aligned with the transfer passages 111 the air fuel mixture continues to flow into the combustion chamber 30 but at a rate determined by the shape and size of the cut outs 216 on the piston skirt thus the discharge rate of the air fuel mixture is now regulated by the shape of the cut outs 216 the flow rate is reduced as the piston 16 approaches bdc and it is possible to completely cut off the flow reduction in flow of air fuel mixture thus minimizes the loss of fresh air fuel mixture through the exhaust port 50 thus a significant reduction in emission may be seen with the engine built according to this embodiment the.