Tests of the unscavenged prechamber ignition system on a 150 kW cogeneration engine fed with biogas are presented. The objective was to reach at least 36% in fuel conversion efficiency with exhaust gas emissions lower than the Swiss regulations limits (NOx CO: 400 and 650 mg/Nm(3), 5% O-2 and 0% relative humidity) without exhaust gas after treatment. With a compression ratio of 12.0, it is shown that 40% of CO2 induces a CO reduction of 15% and an HC reduction of 8% for the same NOx emissions, but with a slight reduction of 1% in fuel conversion efficiency compared to pure natural gas. The results for the compression ratio of 13.3, a spark timing of 8 ca(BTDC), an air factor lambda of 1.57 and 39.5% CO2 are a fuel conversion efficiency of 37.7% while keeping the NOx and CO emissions below the Swiss limit. This study shows that using unscavenged prechambers in biogas engines is particularly beneficial to reach interesting conversion efficiencies with low emissions. It allows a significant boost in energy conversion from biogas compared to existing systems. (c) 2005 Elsevier Ltd. All rights reserved.