引燃策略对天然气直喷发动机射流燃烧及排放的影响

Influence of Ignition Strategy on Jet Combustion and Emissions of Natural Gas Direct Injection Engines

基于计算流体力学(computational fluid dynamics,CFD)软件CONVERGE研究了天然气直喷发动机的高压天然气射流混合、引燃燃烧及排放物生成等基础过程,详细分析了引燃方式和引燃距离对这些基础过程的影响。结果表明:高压直喷天然气射流能够明显促进引燃柴油喷雾发展与混合,但引燃柴油喷射对天然气射流发展影响不明显。高压直喷天然气射流火焰中心存在温度较低而天然气浓度较高的火焰内陷区,该内陷区是碳烟的主要生成区域,碳烟高浓度区位于火焰内陷区中后部。与引燃方式相比,引燃距离对火焰浮起长度、火焰内陷长度及NO_(x)和碳烟排放有更明显影响。随引燃距离增加,火焰浮起长度和NO_(x)排放增加,火焰内陷长度和碳烟排放减小。柴油喷入天然气中引燃在3种引燃方式中具有最高的放热率峰值和最短的燃烧持续期,其NO_(x)和碳烟排放最低,但甲烷逃逸量最高。

The jet mixing,pilot combustion and emission generation processes of high pressure natural gas used on natural gas direct injection engines were studied based on the computational fluid dynamics(CFD)software CONVERGE.The effects of ignition mode and ignition distance on these basic processes were analyzed in detail.The results show that the high pressure direct injection natural gas jet can obviously promote the development and mixing process of pilot diesel spray,but the effect of pilot diesel injection on the development of natural gas jet is not obvious.In the center of high pressure direct injection natural gas jet flame,there is a flame recessed area with lower temperature and higher natural gas concentration.This flame recessed area is the main generation area of soot,and the high concentration area of soot is located in the middle and rear of the flame recessed area.Compared with the ignition mode,the ignition distance has a more obvious influence on the lift-off length,the flame recession length,NO_(x) and soot emissions.As the ignition distance increases,the lift-off length and NO_(x) emissions increase,and the flame recession length and soot emissions decrease.The ignition mode of injecting pilot diesel into natural gas jet has the highest peak heat release rate and the shortest combustion duration among the three ignition modes,and its NO_(x) and soot emissions are the lowest while the methane escape amount is the highest.