Abstract: This study tested a Ricardo E6 engine operating on simulated biogas formed from mixtures of natural gas and carbon dioxide. The tests covered a range of air fuel ratios from rich to the lean limit at several speeds and a number of compression ratios. Measured results were given for power, exhaust temperature, thermal efficiency, and the CO, NOx and THC emissions. Experimental results indicate that carbon dioxide can improve NOx emissions, but lower the cylinder pressure, reduce engine power and thermal efficiency and increase the THC. Measured data also suggest that increasing the compression ratio is an effective means of improving biogas fuelled engine performance, particularly, in terms of power and thermal efficiency, but raise the emissions of both NOx and THC .