Effect of ignition timing and hydrogen fraction on combustion and emission characteristics of natural gas direct-injection engine

Abstract

An experimental study on the combustion and emission characteristics of a direct-injection spark-ignited engine fueled with natural gas/hydrogen blends under various ignition timings was conducted. The results show that ignition timing has a significant influence on engine performance, combustion and emissions. The interval between the end of fuel injection and ignition timing is a very important parameter for direct-injection natural gas engines. The turbulent flow in the combustion chamber generated by the fuel jet remains high and relative strong mixture stratification is introduced when decreasing the angle interval between the end of fuel injection and ignition timing giving fast burning rates and high thermal efficiencies. The maximum cylinder gas pressure, maximum mean gas temperature, maximum rate of pressure rise and maximum heat release rate increase with the advancing of ignition timing. However, these parameters do not vary much with hydrogen addition under specific ignition timing indicating that a small hydrogen fraction addition of less than 20% in the present experiment has little influence on combustion parameters under specific ignition timing. The exhaust HC emission decreases while the exhaust CO₂ concentration increases with the advancing of ignition timing. In the lean combustion condition, the exhaust CO does not vary much with ignition timing. At the same ignition timing, the exhaust HC decreases with hydrogen addition while the exhaust CO and CO₂ do not vary much with hydrogen addition. The exhaust NOx increases with the advancing of ignition timing and the behavior tends to be more obvious at large ignition advance angle. The brake mean effective pressure and the effective thermal efficiency of natural gas/hydrogen mixture combustion increase compared with those of natural gas combustion when the hydrogen fraction is over 10%.