喷油开始时刻对缸内直喷汽油机性能的影响

Start of Injection Impacts on Performance of a Direct-injection Spark-ignition Engine

通过对某涡轮增压缸内直喷汽油机缸内流动、混合气形成及燃烧过程的数值模拟,并借助发动机台架试验中获得的油耗、排放及燃烧数据,研究了喷油开始时刻对发动机性能的影响.结果显示,对于5 000r.min-1工况,喷油开始时刻为400°曲轴转角是混合气雾化混合的最佳方案,此时混合气分布比较均匀且点火时刻火花塞附近具有较高的湍动能,10%～90%燃烧持续期最短且HC排放较低,从而使其具有最佳的燃油经济性与燃烧稳定性.而2 000r.min-1工况的最佳喷油开始时刻推迟了30°曲轴转角.喷油提前,壁面油膜量增加,混合气当量比下降及火花塞附近较低的湍动能使得燃烧速率降低,因此HC排放、燃油经济性及燃烧稳定性均会变差.同样的情况也出现在推迟喷油中,由于混合不充分使得点火时刻混合气均匀度下降,从而使发动机性能恶化.

A numerical simulation was carried out on cylinder flow, mixture formation and combustion process of a turbo- charged gasoline direct injection engine, and then on the basis of the test data about the break specific fuel consumption, emission and combustion, an analysis was made of the best time to start the injection on engine performance. The results show in the case of 5 000 r ·min^-1 , the injection started at 400°crank angle obtains an optimal mixture formation, when it presents the best fuel economy and combustion stability with distribution, the higher turbulence kinetic energy around the spark plug at spark time and the shortest 10% - 90% burning duration as well as the lower HC emission. While the best time to start the injection is retarded at 30°crank angle with 2 000 r ·min^-1; When the injection starts in advance, the wall film increases, and the decreased equivalence ratio and the lower turbulence kinetic energy around spark lead to a lower burning rate, so He emissions, fuel economy and combustion stability may go bad. The same situation occurs in a delayed injection, the mixture uniformity worsens at spark time because of insufficient mix, consequently engine performance is deteriorated.