Experimental study of the effects of spark timing and water injection on combustion and emissions of a heavy-duty natural gas engine

The growing concern over global warming and stricter emissions regulations (EURO 6 standards) have led to natural gas engines being operated in stoichiometric combustion conditions with three-way catalytic (TWC) and exhaust gas recirculation (EGR) equipment. However, natural gas engines suffer from high thermal load, knock onset, and high turbine inlet temperatures at stoichiometric combustion conditions. Water injection is a promising approach to reduce thermal load and NOx emissions and suppress knock onset. In this study, the effects of water injection on the combustion and performance characteristics of a heavy-duty natural gas engine were experimentally studied. The experiments were conducted with four water to natural gas mass ratios (WNMRs) and various spark timings. The results showed that water injection decreased the burning velocity of natural gas, leading to a decrease in peak in-cylinder pressure, peak heat release rate, and combustion temperature. The flame development period (CA(0-10)) and the flame propagation period (CA(10-90)) increased with an increase in WNMR. Compared to pure natural gas, the brake thermal efficiency at the optimum spark timing increased from 27.8% to 28.2% at WNMR = 0.35. BSNOx emissions consistently decreased with an increase in WNMR, while BSTHC emissions and BSCO emissions increased slightly.