Strategies to improve the performance of a spark ignition engine using fuel blends of biogas with natural gas, propane and hydrogen

This work presents the strategies applied to improve the performance of a spark ignition (SI) biogas engine. A diesel engine with a high compression ratio (CR) was converted to SI to be fueled with gaseous fuels. Biogas was used as the main fuel to increase knocking resistance of the blends. Biogas was blended with natural gas, propane, and hydrogen to improve fuel combustion properties. The spark timing (ST) was adjusted for optimum generating efficiencies close to the knocking threshold. The engine was operated on each blend at the maximum output power under stable combustion conditions. The maximum output powerwas measured at partial throttle limited by engine knocking threshold. The use of biogas in the engine resulted in a power derating of 6.25% compared with the original diesel engine (8 kW @ 1800 rpm). 50% biogas + 50% natural gas was the blend with the highest output power (8.66 kW @1800 rpm) and the highest generating efficiency (29.8%); this blend indeed got better results than the blends enriched with propane and hydrogen. Tests conditions were selected to achieve an average knocking peak pressure between 0.3 and 0.5 bar and COV of IMEP lower than 4% using 200 consecutive cycles as reference. With the blends of biogas, propane, and hydrogen, the output power obtained was just over 8 kW whereas the blends of biogas, natural gas, and hydrogen the output power were close to 8.6 kW. Moreover, a new approach to evaluate the maximum output power in gas engines is proposed, which does not depend on the engine % throttle but on the limit defined by the knocking threshold and cyclic variations.