1. ABSTRACTThe Miller system was implemented to control the intake-air throttle valve for a stationary natural gas engine. The throttle system consists of an additional rotary valve provided with a variable timing device and a conventional butterfly valve. This throttle system can control not only the mass flow rate of the air-fuel mixture by changing the intake duration, but also the effective compression ratio at the same time. The experimental results showed an improvement in thermal efficiency at part load and a reduction of NOx emissions for the gas engine.