压缩比及燃料十六烷值对超高EGR低温燃烧的影响

Effects of compression ratio and fuel cetane number on ultra-high EGR low temperature diesel combustion

低温燃烧是目前大幅度降低柴油机排放的热点研究之一。该文试验研究了燃料十六烷值及压缩比对大量废气再循环(EGR)条件下无烟低温柴油机燃烧特性的影响。通过调整EGR率控制进气氧浓度实现超低氮氧化物(NOx)排放的同时,采用低压缩比及低十六烷值的组合可以确保燃料与空气充分混合,以实现上死点附近着火和无烟超低NOx燃烧。然而,进气氧浓度较高时,碳氢化合物(THC)及一氧化碳(CO)浓度较低而最大压升率较高;降低进气氧浓度有利于抑制最大压升率,但是存在失火的问题。比较而言,在较高氧浓度条件下,通过延迟喷油时刻优化燃烧相位可以在降低最大压升率的同时实现更低的THC及CO排放、以及更高的燃烧效率。最后,通过应用较低十六烷值及较低压缩比的组合并优化喷油时刻及EGR率,对扩展无烟超低(NOx)高效柴油机燃烧的可能运行范围进行了探讨。

To analyze the effects of lowered combustion temperature and improved fuel-air mixing with ultra high exhaust gas recirculation （EGR） on smoke reduction, fuels with five cetane numbers as well as three compression ratios were systematically examined. Smokeless combustion was realized 1） at the ＂premixed-combustion state＂ when the premixed duration defined as the period between the end of fuel injection and ignition is larger than 4~CA regardless of combustion temperature; and also 2） with sufficiently low combustion temperature at the ＂premixed- combustion transition state＂ when the premixed duration is between 0 and 4~CA. A combination of low compression ratio and low cetane number fuel is advantageous to realize smokeless combustion, but the issue with increased pressure rise rate should be addressed. Finally, with optimized intake oxygen concentration, injection timing, compression ratio and cetane number, the operation load range of clean diesel combustion is discussed.