喷油时刻对高速缸内直喷汽油机性能的影响

汽油缸内直喷（gasoline direct injection, GDI ）技术有利于发动机的进气过程，表现出较好的NOx 排放和燃油经济性，被广泛运用于乘用车领域。喷油起始时刻（start of injection, SOI ）严重影响着缸内油气混合的均匀性，进而对发动机的燃烧和排放性能产生影响。本文通过Converge软件对某款高速GDI发动机进行仿真分析，研究不同SOI条件下发动机的性能变化。研究结果表明：发动机高速运行时，在缸内气流和燃油束气流的共同作用下，缸内左侧形成强滚流团，混合油气向进气侧方向聚集。随SOI延后，喷雾撞壁时刻推迟，燃油在缸内的破碎效果得到加强，但不利于顶面油膜蒸发，火花塞处形成不同浓度的混合油气。对比发现400° CA为最佳喷油时刻，此时缸内油气混合均匀性最好，燃烧重心提前，缸内最大爆压达到10.5 MPa，动力性能和燃油经济性得到提升；燃油的充分燃烧以及缸内燃烧温度的增加使CO和THC排放减少，但NOx排放有所增加。

The Influence of Injection Timing on High-Speed Direct Injection Gasoline Engine Performance

Gasoline direct injection (GDI) technology is conducive to the intake process of the engine, showing good NOx emission and fuel economy, and it is widely used in the field of passenger cars. The start of injection (SOI) seriously affects the uniformity of oil and gas mixing, which in turn affects the combustion and emission performance of the engine. In this paper, a high-speed GDI engine is simulated and analyzed by Converge software, and the performance changes of the engine under different SOI conditions are studied. The results show that as the engine runs at high speed, under the joint action of in cylinder air flow and fuel beam air flow, a strong tumble flow is formed on the left side of the cylinder, and the mixed oil and gas gather in the direction of the intake side. With the delay of SOI, the time of spray-wall interaction is delayed, and the fuel breaking is strengthened. However, it is not conducive to the evaporation of the oil film on top surface, and different concentrations of mixed oil-gas are formed at the spark plug. The 400 °CA is the best injection time, in which the fuel gas mixing uniformity is the best. At this time, the combustion center of gravity is advanced, and the maximum explosion pressure reaches 10.5MPa, the power performance and fuel economy are improved. The full combustion of fuel and the increase of combustion temperature in cylinder reduce CO and THC emissions, but NOx emissions increase.