Improving performances of a lean burn cogeneration biogas engine equipped with combustion prechambers

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 (NO_x, CO: 400 and 650 mg/Nm³, 5% O₂ 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 NO_x 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 λ of 1.57 and 39.5% CO₂ are a fuel conversion efficiency of 37.7% while keeping the NO_x 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.