Effect of air-fuel ratio on the performance and emission characteristic of a biogas fuelled spark ignition engine

In the current investigation, a single cylinder, 4-stroke, water cooled, manifold injection, variable compression ratio (VCR), spark ignition (SI) engine of rated power of 4.5 kW was operated with raw biogas at three different compression ratios (CRs) ranging from 8 to 10. The fuel was tested in the VCR engine under various operating equivalence ratios (ER) to find out the leaner misfire and the richer knock limit as well. The comparative performance and emission characteristics of the engine with different compression ratios are reported in the paper. It has been found that, the lean misfire limit is considerably extended with increasing compression ratio of the engine. The lean limit indicated by misfire is at an ER of 0.673 with a CR of 10 as against 0.764 with that of a CR of 8. The peak brake power output and brake thermal efficiencies at CR 10, CR 09 and CR 08 were observed at ERs of 1.04, 0.967 and 1.03, respectively. The minimum value of HC and the maximum value of NOx concentration coincide with the position of the best thermal efficiency.