Effect of compression ratio on the performance and combustion of a biogas fuelled spark ignition engine

► Experiments were conducted on a biogas fuelled SI engine at various equivalence ratio and number of compression ratios. ► Improvement in brake power and brake thermal efficiency with increase in compression ratio. ► Lean misfire limit is extended with increase in compression ratio. ► Enhanced compression ratio decreases hydrocarbon and carbon monoxide emission level and increase in NO emission. ► The reduction in ignition delay and higher heat release rate with increase in compression ratio. A single cylinder diesel engine was modified to operate as a biogas operated spark ignition engine. The engine was operated at 1500rpm at throttle opening of 25% and 100% at various equivalence ratios. The tests were covered a range of equivalence ratios from rich to the lean operating limit and a number of compression ratios. The spark timing was set to MBT (Minimum advance for Best Torque). The performance, emission and combustion characteristics with different compression ratios are compared. It has been found from the results that the higher the compression ratio, the higher the brake thermal efficiency. When the compression ratio was above a critical value of 13:1, brake power and thermal efficiency increased little. At higher compression ratios above 13:1, increased NOx, HC, and CO emissions were measured. Power and thermal efficiency reached their highest values with the compression ratio between 13:1 and 15:1 and the equivalence ratio between 1.08 and 0.95. Under these conditions, HC and CO emissions were low but the NOx values were high. Power and thermal efficiency reduced for leaner mixtures. The MBT spark timing is retarded with increase in compression ratio. The peak pressure decreases, as the mixture becomes lean at all the compression ratios. The peak pressure is higher with higher compression ratio. Increase in compression ratio leads to high heat release rate.