Study On Co-combustion Characteristics Of A Dual-fuel Engine Fueled With LNG And Diesel As Pilot Fuel

Under the double pressures of energy crisis and environmental pollution, developing clean and efficient vehicle alternative fuels and studying on new combustion modes of engine have become the inevitable choice for the sustainable development of automobile industry. As one of the most promising alternative fuels for internal combustion engine, natural gas has attracted more and more attentions because of its advantages of abundant reserves, low emissions as well as good economic performance. Natural gas-diesel dual-fuel engine has been considered as one of the most feasible approaches to apply natural gas in diesel engines with the features of easy modification and good flexibility to switch combustion modes. The dual-fuel engines can effectively reduce smoke emission which is the major pollutant of diesel engines. Meanwhile, the dynamic and economic performances of the dual-fuel engines maintain about the same level as those of diesel engines.Co-combustion ratio, which is the ratio of natural gas energy to total fuel energy, is an important parameter for natural gas-diesel dual-fuel engines. The performances of dual-fuel engines are greatly affected by co-combustion ratio. In this paper, the effects of co-combustion ratio on the economic performance, exhaust emissions and combustion characteristics of a dual-fuel engine fueled with LNG and diesel are studied by engine bench tests. The experiments are based on the premise that the power performance of dual-fuel mode is the same as that of pure diesel mode and the working process is stable and reliable. The LNG-diesel dual-fuel engine used in this paper is refitted by an YC6G270-30 diesel engine with electronically controlled common rail system. The influences of co-combustion ratio on the economic performance and exhaust emissions of the dual-fuel engine are investigated at speeds of 1400r/min and 1800r/min with loads of 15%, 20%, 30%, 60% and 100%. Moreover, the combustion characteristics of the dual-fuel engine under varied co-combustion ratios are compared and analyzed at the rated torque speed of 1400r/min with loads of 30% and 60%.The experimental results indicate that the maximum co-combustion ratio of LNG-diesel dual-fuel engine can reach above 80% and increases with the increase of loads. With the increase of co-combustion ratio in dual-fuel mode, the brake specific fuel consumption increases under low loads while decreases under high and full loads. Compared with pure diesel mode, CO and HC emissions in dual-fuel mode are both increased, CO₂ emission decreases slightly, while smoke emission reduces dramatically. With the increase of co-combustion ratio, CO emission first increases and then decreases, HC emission increases, and CO₂ and smoke emissions have the tendency of decreasing. Under low and middle loads, as co-combustion ratio increases, NOx emission first increases and then decreases, and NOx emission is lower than that of pure diesel mode when co-combustion ratio reach the maximum. Under high loads, NOx emission increases with the increase of co-combustion ratio. In-cylinder pressure, pressure rise rate, instantaneous heat release rate and in-cylinder temperature in dual-fuel mode are all higher than those in pure diesel mode and have the tendency of first increasing and then decreasing with the increase of co-combustion ratio. Compared with pure diesel mode, combustion cyclic variation in dual-fuel mode increase. With the increase of co-combustion ratio, the coefficient of variation of the maximum pressure rise rate first decreases and then increases and reaches the minimum at an intermediate co-combustion ratio.