Abstract: In a spark-ignited gas engine with relatively high compression ratio of approximately .epsilon.=12, operation takes place with relatively poor mixtures with air/fuel ratios of about .lambda.=1.6 or over in order to reduce NO.sub.x -production. At loads about approximately 70% of the (normally aspirated) nominal power, the oxygen-ratio must be reduced in order to aspirate sufficient fuel for the power demand. NO.sub.x -production would steeply rise, but according to the invention exhaust gas (EGR) is recirculated at least up to nominal load, in order to maintain complete combustion at .lambda.=1.0 and meanwhile keep NO.sub.x -production low. A .lambda.-sensor in the exhaust gases may be used for that purpose. A new venturi-based mixing device for the air-fuel-EGR is disclosed, placed upstream of the main throttle valve. The system is also applicable to diesel-gas, and straight diesel engines, with or without pressure charging. Due to the high compression ratio, thermal efficiency remains high.