喷油时刻对缸内直喷汽油机性能的影响

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.

Effects of Fuel Injection Timing on the performance of a Direct-Injection Spark-Ignition Engine

Through the numerical simulation on cylinder flow, mixture formation and combustion process of a turbo-charged gasoline direct injection engine, meanwhile, with the help of break specific fuel consumption, emission and combustion data from engine test, the effect of start of injection timing on engine performance was studied. The results show, for 5000r/min condition, the case whose injection is started at 400ºCA obtains the optimal mixture formation. Because of the more uniform mixture distribution, higher turbulence kinetic energy around spark plug at spark time, shortest 10-90% burning duration and lower HC emission, it has the best fuel economy and combustion stability. While the best start of injection timing is retarded 30ºCA at 2000r/min. When injection is advanced, wall film increases, the decreased equivalence ratio and lower turbulence kinetic energy around spark lead to lower burning rate, so HC emissions, fuel economy and combustion stability may go bad. The same situation occurs in delayed injection, the mixture uniformity goes worsens at spark time because of insufficient mix, consequently engine performance is deteriorated.