后喷策略对柴油机瞬变过程高负荷工况性能的影响

Influence of Post Injection Strategy on Diesel Engine Performance at Heavy Load and under Transient Conditions

为了改善柴油机瞬变高负荷工况下缸内燃烧恶化和烟度骤升的问题,以8.6L车用重型柴油机为对象,开展了燃油后喷策略对瞬变工况性能的影响研究。试验研究了在柴油机瞬变过程的高负荷工况段加入后喷时不同后喷参数对瞬变工况性能的影响,并利用模拟手段分析了后喷对缸内燃烧状况、缸内气体的流动和碳烟排放的影响和作用机理。为了更好地说明缸内气体混合情况,引入氧气驱动能量这一概念。研究结果表明:在主喷燃烧阶段,氧气驱动能量对混合气的形成起主导作用;随着后喷油量增多,缸内气体扰动增加,缸内稀混合气比例增加,放热率第二峰值和第三峰值均提高且相位前移;后喷油量对NO_x排放影响较小。综合考虑指示热效率和碳烟排放,对于瞬变过程90%负荷工况,主后喷间隔为4°曲轴转角且后喷量25%时排放效果整体最佳。

In order to solve the combustion deterioration and emissions increase of heavy duty diesel engines under transient conditions,the study of the influence of the fuel post injection on transient performance was carried out on a heavy duty vehicle diesel engine with 8.6 L displacement,and the influence and its mechanism of the fuel post injection on combustion process,air flow and soot emissions were analyzed by simulation.In order to better illustrate the mixing process of mixture in the cylinder,the concept of oxygen driven energy was introduced thereafter.Results show that in the main injection stage,the oxygen driven energy plays a leading role in the formation of in-cylinder mixture,With the increase of the amount of fuel post injection the in-cylinder disturbance increases,resulting in an increase in the ratio of lean mixture which makes the second and third peak of heat release rate enhance and the combustion phase advance,but the amount of fuel post injection has little influence on the NO x emissions.In view of the combined consideration of thermal efficiency and soot emissions,the use of an interval of 4°crank angle between the main and post injections and amount of 25%post injection has a best emission effect at 90%load and under transient conditions.