基于多缸CFD方法的直喷增压汽油机爆震研究

Study on Knock of a Turbocharged Direct Injection Gasoline Engine Based on Multi-Cylinder CFD Simulation

随着汽油机工作负荷的增加,燃烧室末端混合气自燃概率升高,爆震倾向增大,提高燃烧室冷却能力成为抑制爆震的重要手段之一.笔者对一台4缸直喷增压汽油机进行多缸计算流体动力学(CFD)模拟,揭示了各缸的爆震机理,燃烧室壁面温度以三维有限元热传导方法赋值,评估了燃油的蒸发和壁面温度对汽油机爆震的耦合影响.结果表明:由于燃油的蒸发和壁面温度共同作用,第1缸的缸内温度较均匀,第2缸由于排气侧右区燃油浓度偏稀,壁面温度较高,导致缸内温度较高,末端混合气更容易发生自燃,因而第2缸比第1缸更容易发生爆震,仿真与试验结果一致.以第1缸爆震指数为基准点,通过优化燃烧室冷却性能可降低壁面温度,第2缸爆震指数下降可降低爆震风险.

Knocking combustion caused by auto-ignition of the end gas could happen with the increase of gasoline engine load.Enhancing cooling of the combustion chamber can help to reduce the knock intensity.An air-fuel mix-ture and knocking combustion computational fluid dynamics(CFD)simulation was conducted based on multicylinder of a four-cylinder turbocharged direct injection gasoline engine.The wall temperature of the combustion chamber was assigned by the three-dimensional finite element heat transfer method.The effect of fuel evaporation and wall temperature on knock was investigated.The results show that fuel evaporation and wall temperature affect in-cylinder charge temperature simultaneously.The in-cylinder charge temperature in the first cylinder is more ho-mogeneous.While leaner mixture and higher wall temperature at the right hand side of the exhaust region in the second cylinder lead to higher in-cylinder charge temperature and more auto-ignition trend of the end gas.An experiment of the knock index was also carried out.Test results show that the value of knock index on the second cylinder is higher than that on the first cylinder.This indicates that the simulation results are in good agreement with the experimental results.Based on the knock index of the first cylinder,a cooling enhancement of the combustion chamber was conducted and the knock intensity on the second cylinder was reduced due to the reduction of the wall temperature.