多次喷油实现清洁高效柴油预混燃烧的机理

采用试验和数值模拟方法研究了多次喷油模式耦合高EGR率和进气增压实现高效清洁柴油机预混燃烧的潜力.结果表明,EGR可以推迟高温反应定时,降低化学反应速率,特别是在较高负荷时,EGR还明显降低NOx排放.平均指示压力(IMEP)为0.4,MPa左右时,较少的喷油量使得单次喷油和多次喷油模式着火前一时刻形成的混合气均在当量比φ<2之内,故排放差别不大.氧体积分数为10%左右时,提高喷油量,进入侧隙内的气相燃油量增加并形成φ>2的浓区,两模式的碳烟、CO和UHC排放均升高.但IMEP为0.7 MPa左右时,多次喷油更有利于生成稀且均匀的混合气,减少局部过浓区,故其碳烟、CO和UHC排放远低于单次喷油.高EGR率时,多次喷油耦合进气增压,混合气形成得到进一步改善,可在更高负荷下实现高效清洁燃烧.

Mechanism of Multiple Injections on Clean and High Efficiency Diesel PCCI Combustion

Comparing with single fuel injection,the superiority of multi-injection combined with high exhaust gas recirculation(EGR)rate and intake boost in reducing emissions and increasing indicated thermal efficiency of diesel premixed charge compression ignition(PCCI)combustion is studied experimentally and numerically.It is found that EGR can delay the start of hot temperature reactions,reduce the reaction speed at higher load.EGR can also reduce NOx emissions.There are no significant differences in emissions and indicated thermal efficiency using single and multi-injection at IMEP around 0.4 MPa due to the small quantity of the fuel injected,and the mixture formed just before ignition distributes within φ<2.At approximately 10% oxygen concentration,when increasing the fuel mass injected,more fuel will enter the side clearance,where rich mixture with φ>2 is formed.As a result,the soot,CO and UHC emissions are increased using both injection modes.However,at IMEP of 0.7 MPa around,CO and UHC emissions using multi-injection are much lower than those using single injection.The mechanism is that,more lean and homogenous mixture is formed and the amount of the local rich mixture is reduced using multi-injection.At high EGR rate,combined multi-injection with increasing boost pressure,the mixture formed is further improved,high efficiency and low emissions are simultaneously achieved at higher engine load.