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A gasoline engine with the expansion ratio of 14:1 for better efficiency is presented. The engine has intake control rotary valves to reduce the effective compression ratio for knock avoidance and to cotrol power output. Computer simulation, based on the method of characteristics and quasi-steady model, indicated the advantage of the rotary valve system as well as the optimum design of the valve actuation. Experimental results showed a good agreement with the prediction, and BSFC of under 240 g/kWh was achieved at load higher than 50%. However, at very light load, the conventional throttle valve operation was inevitable, which somewhat worsened the advantage of the new system. The mileage for a car driving cycle and constant speeds were also estimated and it was forecast that optimized supercharging would significantly benefit the fuel economy.