Gaseous fuels produced, for example, by waste or agricultural by-products fermentation (biogas) can be burned in-situ by cogeneration systems like spark-ignition internal combustion engines. However, the more and more stringent legislation for exhaust gas emissions requires improvement of the combustion process particularly when catalytic after treatment is not reliable as in the case of sewage or landfill biogas. The system proposed in this paper is the use of an unscavenged combustion prechamber instead of direct ignition on a turbocharged 6 cylinder 150 kW gas engine. This prechamber is used for operation with a simulated biogas (40% CO2 in natural gas). The results show that, compared to natural gas operation for the same rated power output of 150 kW and the same NOx emissions, the CO emissions are reduced by 15% and the HC emissions at least by 8%. Efficiencies higher than 36% are achieved which is very promising and the lower CO emissions give a margin to consider an increase of compression ratio.