柴油机分段喷射下混合气形成及其对燃烧排放的影响

Mixture Formation with Split Injection and Its Effect on Combustion and Emissions of Diesel Engines

基于数值模拟计算,研究了不同预喷策略下某非道路直喷柴油机混合气形成和燃烧过程,分析了挤流和涡流对主、预喷燃油当量比分布和燃烧的影响。结果表明:适当的主预喷间隔(20°)能够使预喷燃油在挤流带动下进入燃烧室凹坑;适当强度的进气涡流比(1.5~2.0)能使预喷油束偏转并在相邻两束主喷油束之间燃烧。二者都可以改善主喷和预喷油束的重叠,并且可以充分利用预喷燃油的放热量促进主喷燃油的混合和提升燃烧速率。优化喷油策略结合适度的进气涡流可以提高发动机功率,降低碳烟排放,但会使NOx排放增加。

The fuel-air mixture formation and combustion of an off-road direct-injection diesel engine with split injection were computationally investigated, and the effects of squish and swirl flows on the equivalence ratio distribution and combustion of the fuels from both pilot and main injections were analyzed. The results show that the squish flow tend to transport the pilot-injected fuel vapor into the piston bowl when properly increasing the interval between pilot and main injections(20° in crank angle) and a moderate swirl flow(from 1.5 to 2.0) can shift the pilot injection sprays towards the regions between two adjacent main-injection sprays. Remarkably, both of the two effects can reduce the overlap of the pilot and main injections, and the heat released from the pilot injection combustion can further accelerate the mixing and combustion processes of the main injection. An optimal pilot injection strategy together with a proper swirl ratio can increase engine power with reduced soot emissions, although the NO x emissions will be unfavorably increased.