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In order to obtain the limiting volumetric efficiency curve, the relationships between volumetric efficiency and engine speed were investigated on three engines by varying through wide limits the effective angle areas of the inlet valves. It was found that the mean inlet Mach number Mim introduced by the authors showed the characteristics of volumetric efficiency better than the inlet Mach index Ms widely used and when Mim approached Mim=0.5, the flow around the inlet valve became critical causing inlet choking which made the volumetric efficiency decrease hyperbolically with an increase in engine speed. These phenomena were also observed in the test results of a Wankel type rotary engine.