不同喷油正时的柴油机PCCI燃烧过程数值模拟

Numerical simulation of PCCI combustion in diesel engine with different injection timing

应用CFD软件对采用预喷-预喷-主喷喷油策略的单缸增压柴油机燃烧过程进行数值模拟,分析不同喷油正时方案对燃烧过程及NO和soot排放物生成历程的影响.结果表明:随着喷油时刻推迟,缸内压力逐渐降低,主放热峰下降且不断远离上止点,预混燃烧比例增加,缸内低温PCCI燃烧模式更加明显;放热率10%(q10)时刻以稀预混燃烧为主,soot生成量减少,但因预混放热快而使NO生成量增多,缸内整体的低温效应使q90时刻NO生成量降低,soot生成量先增加后又因燃烧空间扩大及预混燃烧比例增加而下降;柴油机低温PCCI燃烧模式可实现NO与soot排放兼顾控制.

The combustion process in a single-cylinder turbocharged diesel engine with pilot-pilot-main injection strategy was simulated by the computational fluid dynamics(CFD) software. The effects of injection timing on combustion process,NO generation and soot were analyzed in detail. The results show that when the injection timing is retarded,the in-cylinder pressure is decreased gradually with reduced main heat release peak,and the position of heat release peak moves far away from the top dead center with increased premixed combustion fraction to show more apparent feature of low temperture PCCI combustion mode. For 10% heat release(q10) with retarding of fuel injection timing,the soot generation is decreased as the lean premixed combustion takes the major,while more NO is generated due to the rapid heat release of premixed mixture. For 90% heat release(q90),NO emission is decreased due to the total low temperature effect in cylinder,while the soot is increased with latter decreasing due to the extended combustion space and the high premixed combustion fraction. The favorable tradeoff between NO and PM emissions can be well compromised in a diesel engine with the low temperature PCCI mode.