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The object of this work is to test the unscavenged prechamber ignition system on a 150 kW cogeneration engine feed with biogas. The engine performances in term of fuel conversion efficiency are observed and the reduction potential f the exhaust emissions below the Swiss limit are verified (NOx CO : 400 and 650 mg/Nm3, 5 % O2 and 0 % relative humidity) ; and the non-limited HC emissions too. The interesting variables are the air-fuel ratio, the fuel composition, the compression ratio and the spark-timing. The aim of this work is to reach at least 36 % in fuel conversion efficiency with exhaust gas emissions lower than the regulations limits without exhaust gas after-treatment. The engine used is a turbocharged spark ignited Liebherr G926 TI, 10 L, 6 cylinders in line engine fitted with combustion prechambers (with a volume of 3 % of the cylinder volume). As a first step, the engine fuel supply has been modified to allow the use of natural gas / CO2 mixtures to simulate biogas. Natural gas tests have been performed with direct ignition and prechamber ignition as references. The same experimental conditions have been applied for the simulated biogas tests. Different spark timing advances have been tested in the configuration with the initial compression ratio of 12,0. Afterwards new pistons have been developped to reach a compression ratio of 13,3. Two different CO2 proportions (20 % and 40 %) have been used to observe the influence of CO2 in fuel on the performances. With a compression ratio of 12,0, 40 % of CO2 inducee a CO reducion of 15 % and an HC reduction of 8 % for the same NOx emissions 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 caBTDC, of 1,63 and 39,5 % CO2 are a fuel conversion efficiency of 38 % and NOx and CO emissions below the swiss limit. Those performances and emissions are compared to the ones obtained for the natural gas with a compression ratio of 12,0 and prechamber ignition. The use of a compression ratio of 13,3 instead of 12,0 allows to balance the loss in fuel conversion efficiency due to the 40 % of CO2 . If the percentage of CO2 is lower then the efficiency is higher but the CO and HC emissions too. Moreover, it seems better to use a higher compression ratio than a sooner spark timing (see fig. 29, 30, 31). Those tests demonstate that the CO2 does not modify the combustion process nor the prechamber ignition mode. This study shows that gas engines with unscavenged prechambers could allow a significant boost in energy conversion from biogas while keeping the emissions within the tough Swiss limits.