喷射时刻和掺氢比对直喷发动机燃烧特性的影响

在缸内直喷火花点火发动机上对天然气掺混氢气的体积分数为0％～18％的混合燃料不同喷射时刻下发动机的燃烧和排放特性进行了试验研究．研究结果表明：对于给定的喷射持续期和点火时刻，喷射时刻对发动机性能、燃烧和排放有较大影响，喷射太迟燃烧持续期长，放热速率慢，喷射过早会导致充量系数下降；对于给定转速，发动机存在一个最佳的喷射时刻，此时缸内最高压力升高率和最高燃气平均温度高，燃烧持续期短，燃烧过程定容度高，发动机热效率高，HC排放低；在同一喷射时刻下，当氢气的体积分数小于10％时，HC排放略有上升，当氢气的体积分数达到18％时，发动机HC排放与纯天然气排放水平相当；掺氢对NOx、CO和CO2排放影响不大．

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 NOx, CO and CO2.