喷射参数对柴油机预混燃烧和排放的影响

Effect of Injection Parameters on PCCI Combustion and Emissions in a Diesel Engine

采用试验和数值模拟方法研究了中、小负荷下,喷油模式和喷油定时对柴油机预混燃烧和排放的影响.结果表明:小负荷(平均指示压力约为0.45,MPa)、单次喷油模式及喷油定时为35°,CA BTDC时,油束的撞壁位置将形成的混合气分为燃烧室上方和活塞凹坑两部分,可充分利用整个气缸内的空气形成均质混合气,此时的NOx排放最低.平均指示压力(IMEP)约为0.7,MPa时,采用单次喷油模式,喷油量增多,喷油持续期延长,混合时间缩短,碳烟、CO和UHC排放急剧升高;在混合时间和混合空间的共同作用下,喷油定时为35°,CA BTDC时获得最佳折中排放.IMEP约为0.7,MPa时,与单次喷油模式相比,采用多次喷油模式,将喷油量分为4个脉冲喷入缸内,增加了每个脉冲的混合时间,并且改善了燃氧混合空间,形成更均质的混合气,大幅降低了碳烟、CO和UHC排放;喷射定时为80、65、50和35°,,CA BTDC时获得最佳排放.

Experiment and simulation were used to study effects of injection mode and timing on premixed charge compression ignition（PCCI）combustion and emissions in a diesel engine.Results show that,at low load with IMEP≈0.45,MPa,when injection timing of single injection is 35°,CA BTDC,due to the impinging position of the spray,the mixture is divided into two parts：space above the chamber and in the piston bowl,helping to fully utilize the in-cylinder air to form homogenous mixture and giving the lowest NOx emissions.At IMEP≈0.7,MPa,with single injection,the injection mass is increased,injection duration is prolonged and mixing time is reduced,leading to sharp increase in soot,CO and UHC emissions.The best trade-off of emissions is presented at injection timing of 35°,CA BTDC due to the combined effect of optimized mixing time and space.At IMEP≈0.7,MPa,compared with the single injection,injection mass is divided into 4,pulses with multiple injections,the mixing times are increased,and the mixing space of the fuel/oxygen mixture is also improved,giving more homogenous mixture,greatly decreasing soot,CO and UHC emissions.The best emissions are achieved at injection timings of 80,65,50,35 °CA BTDC for the multiple injections.