VVL耦合喷油策略对GDI汽油机混合气形成的影响

Effect of VVL Coupling Injection Strategy on Mixture Formation of GDI Engine

利用三维仿真软件Ansys Fluent建立了GDI汽油机的仿真计算模型,就变气门升程耦合不同喷油策略对缸内气流运动和混合气形成的影响进行了模拟计算。结果表明,与大气门升程工况相比,小气门升程工况的缸内湍流运动强度、燃油蒸发和湿壁情况以及点火时刻混合气质量都明显改善;在小气门升程工况,采用两段喷油会缩短油气混合时间,过度推迟二次喷油时刻会恶化混合气质量和燃油湿壁情况;在大气门升程工况,两段喷油会改善混合气均匀性,随着二次喷油时刻推迟,燃油蒸发量增加,湿壁情况加剧,混合气质量得到改善;小气门升程工况下采用二次喷油时刻为470°曲轴转角,前后两次喷油量比例为7∶3的两段喷油方案在燃油蒸发和湿壁以及点火时刻缸内混合气质量这几个方面的效果都很好,是最合理的方案。

A simulation computation model of GDI engine was established by using Ansys Fluent 3D Simulation software to calculate the effect of variable valve lift coupled with different injection strategies on in-cylinder flow and mixture formation. The results showed that the low valve lift condition improved in-cylinder turbulent motion intensity, fuel evaporation,wall wetting and mixture quality at ignition timing more than the high valve lift condition did. In the low valve lift condition , two stage injection shortened mixing time of fuel and air and the seriously-delayed second injection would worsen mixture quality and wall wetting. When the second injection delayed, wall wetting and mixture quality deteriorated gradually. In the high valve lift condition, two stage injection improved mixture homogeneity. When the second injection delayed. the mass of fuel evaporation increased , wall wetting became serious and so mixture quality improved. For the low valve lift condition , the two stage in-jection strategy in which the second injection timing was 470℃A and the injection mass ratio of two injection was 7 ： 3 had a good effect on fuel evaporation , wall wetting and in-cylinder mixture quality at ignition timing , and was the most reasonable so-lution.