Effect of fuel injection timings and hydrogen fraction on combustion characteristics of direct-injection engine

An experimental study on the effect of fuel injection time and hydrogen fraction on combustion and emission characteristics of a natural gas direct-injection spark-ignited engine was carried out. The results show that the fuel injection time has a significant influence on engine performance, combustion and emissions. Late injection will lead to insufficient time for fuel-air mixing, bringing poor quality of mixture formation, and subsequently resulting in slow combustion rate and long combustion duration. Early injection will reduce the volumetric efficiency. For a given engine speed, the optimum fuel injection time exists, where the maximum rate of pressure rise and the maximum mean gas temperature exhibit their highest values, and the shortest combustion duration and highest thermal efficiency can be achieved while maintaining the low level of HC emission. Under the same injection time, the exhaust HC emission increases with the hydrogen addition when the hydrogen fraction in volume is small, and it decreases when hydrogen fraction in volume reaches up to 18%. However, the addition of hydrogen has a slight influence on the exhaust emissions of NO_x, CO and CO₂.