外部废气对汽油机HCCI燃烧调控作用的数值模拟

CFD Study on Effects of External Exhaust Gas Recirculation on Gasoline HCCI Combustion

基于三维仿真平台,研究了不同外部废气率时汽油机均质充量压燃(homogeneous charge compression ignition,HCCI)燃烧着火前缸内平均热状态、温度和废气分布特征及燃烧过程的变化,重点分析了外部废气造成的温度分布变化对自燃过程的影响。模拟结果表明:保持内部废气率不变,外部废气率从0增至15%,缸内总工质质量基本不变,但混合气比热容增加,使得着火前平均温度下降了约30K,平均氧浓度降低了约2%;同时,缸内温度分布更均匀,温度不均匀度降低了38%,而废气不均匀度降幅不到4%。研究结果表明:缸内温度分布变均匀,加快了自燃点扩散速度,同时也改变了初始自燃点的产生位置,但这种分布状态的变化并不能在宏观上影响HCCI着火时刻和燃烧持续期;缸内平均温度和氧浓度降低是外部废气推迟HCCI着火、抑制过快燃烧的主要原因。

In order to explore the effects of external exhaust gas recirculation （eEGR） on HCCI combustion, three-dimensional numerical simulations of in-cylinder temperature, exhaust gas distribution and combustion process change before auto-ignition, especially of the effects of eEGR on HCCI spontaneous combustion process were performed with different eEGR rates. The results indicate that keeping the same internal EGR rate and increasing external EGR from 0 to 15% result in the same total working fluid, but increased heat capacity of gas mixture, making the average temperature before ignition decrease about 30 K and average oxygen concentration decrease about 2%, and at the same time making the temperature distribution in cylinder become more even by 38% but exhaust gas only by less than 4%. The increased evenness of temperature distribution leads to a faster development of auto-ignition sites and changing the locations of the zones where auto-ignition firstly occur. However the temperature distribution change due to different eEGR rate is not enough to control HCCI ignition timing and combustion duration. The suppression effect of eEGR on fast HCCI combustion mainly results from the lower average temperature and oxygen concentration.