近后喷对高压共轨柴油机燃烧过程影响的数值计算

Numerical analysis about effect of close post-injection on combustion process of common-rail diesel engine

应用CFD模拟软件FIRE针对车用增压共轨柴油机在高负荷工况下采用近后喷策略的缸内工作过程进行了数值计算。分析了后喷量和主/后间隔角对燃烧过程的影响。研究结果表明,随着后喷量和主/后间隔角度的增加,缸内燃烧压力、放热规律及缸内温度曲线的峰值降低,燃烧重心推迟,燃烧噪音和NOx生成量减少,但经济性有所恶化。同时,增大后喷量一方面可使后喷燃油的贯穿距增大,在壁面附近高温缺氧区域内形成的燃油蒸汽增多,增加了后喷燃油的Soot的生成量,另一方面增强了对缸内流场的扰动效果,使得更多的氧气进入燃烧产物区,加速了Soot的氧化,因此选取适当的后喷量可以获得较低的Soot的生成量。后喷间隔过大时,Soot排放有明显增加的趋势。

The in-cylinder processes of an automotive turbocharged common-rail diesel engine using close post fuel injection strategy under high load conditions were numerically computed by means of the 3D CFD software FIRE. The effects of the close post-injection fuel amount and the interval between the main and the post fuel injections on the combustion process were analyzed. The computed results show that with the increase of the post-injection fuel amount and the injection interval, the in-cylinder maximum combustion pressure and temperature as well as the heat release rate reduce, the centroid of the heat release rate diagram shifts backward, and all these result in less NOx emissions and worse fuel economy. The increase of the post-injection, on one hand, enhances the penetration of fuel spray, increases the fuel vapor in the high-temperature oxygen-insufficient region near the combustion chamber wall, so enhances the soot generation, on the other hand, the post-injection strengthens the disturbance effect to the flow field, may entrain more oxygen into the combustion product zone and accelerates the oxidation of generated soot. So, appropriate selection of the post-injection amount may result in lower soot emission. When the interval between injections too large, there is an obvious rising trend of soot emission.