大缸径柴油机燃烧系统优化模拟

Simulation on the optimization of the combustion system ofa large-bore diesel engine

为提高某缸径200 mm船用发电柴油机的燃油经济性,本文设计了活塞燃烧室和燃油喷射系统的升级方案并进行了模拟优化。升级方案提高了压缩比和燃油喷射压力,采用大径深比浅ω燃烧室配合158°喷油夹角喷油嘴。对不同方案下发动机的缸内工作过程进行了计算流体力学模拟,计算了高压指示功和放热率相位,分析了缸内温度、反应过量空气系数和速度分布及演化。模拟结果表明:升级方案能够提高发动机热效率。增加喷孔数并减小孔径,可以在保持NOx排放基本不变的条件下提高高压指示功4.5%,降低碳烟排放约60%。采用“平顶”浅ω燃烧室与158°喷油夹角喷雾配合,油气混合气快速进入余隙并形成逆时针的漩涡流动,能够加速油气混合和燃烧过程,提高热效率。

To improve the fuel economy of a marine diesel engine with a cylindrical diameter of 200 mm,in this study,a scheme for upgrading the piston combustion chamber and the fuel injection system was designed,and a simulative optimization was conducted.The upgrade scheme raised the compression ratio and fuel injection pressure and employed a shallowωcombustion chamber with a large diameter-to-depth ratio and a fuel nozzle with a 158°spray-included angle.We performed the computational fluid dynamics simulation for the working process of the engine inside the cylinder,calculated the high-pressure indicated work and the heat release rate phases,and analyzed the temperature inside the cylinder,reaction excess air coefficient,speed distribution,and evolution.The simulation results showed that the upgraded scheme could improve the thermal efficiency of the engine,increase the number of nozzle orifices,and reduce the bore diameter;the high-pressure indicated work could be improved by 4.5%,and soot emission could be reduced by about 60%while keeping the NO x emission unchanged.By combining a“flattop”shallow combustion chamber and 158°fuel-spray-included angle,the fuel and air mixture flows into the piston’s top clearance quickly and forms a counterclockwise vortex that can quicken the mix of fuel and air and the combustion process,thereby improving the thermal efficiency.