瞬变工况下残余废气系数对车用汽油机热功转换过程的影响

Influences of Residual Gas Fraction on Heat-work Conversion Process of Vehicle Gasoline Engine Under Transient Conditions

基于动态压力实测与数值计算相结合的方法，对一台乘用车汽油机瞬变过程进行连续检测，得出车用汽油机瞬变工况下残余废气系数（RGF）的变化范围、变化规律和影响因素。通过分析RGF与汽油机运行参数的内在联系，揭示了RGF对汽油机瞬变状态下热功转换过程的影响。结果表明，在瞬变工况下，转速的增加引发新鲜充量的增加和RGF的下降。转速变化时，RGF在4％～30％范围内变化；在大多数工作循环内，RGF介于16％～20％之间。RGF主要受进气压力的影响，气门正时的调节作用相对较小。在低负荷时，RGF较大且点火提前角变化范围较大，引起较大的燃烧循环变动，从而导致指示平均有效压力和指示热效率波动较大。在点火提前角固定时（进气压力为0．05MPa），指示热效率随RGF的增大而稍有下降。本研究为改善车用汽油机瞬变工况性能提供依据。

Based on the method combining the measurement of dynamic pressure with numerical calcula- tion, a successive test of the transient process of a vehicle gasoline engine is conducted, and the changing range, changing law and influencing factors of the residual gas fraction （RGF） in the engine under transient conditions are obtained. Then, by analyzing the intrinsic relationship between RGF and engine operating parameters, the influ- ences of RGF on the heat-work conversion process of the engine under transient conditions are revealed. The results show that under transient conditions, the increase of engine speed results in the rise of fresh charge and the decline of RGF. The RGF changes in a range of 4% to 30 % with the change of engine speed. In most of the working cy- cles, RGF changes from 16% to 20%. RGF depends largely on the intake pressure, while the effect of valve timing is relatively small. Under low load, due to the high RGF and the wide variation of ignition advance angle, the cyclic variations of combustion are significant, leading to the apparent fluctuations of indicated mean effective pressure and indicated thermal efficiency. Under an intake pressure of 0.05 MPa with the ignition timing fixed, the indicated thermal efficiency lowers slightly with the increase of RGF. The findings of the study provide a basis for improving the performances of vehicle gasoline engine under transient conditions.