This paper establishes a dynamic mean value model for diesel engine based on effect of both scavenging coefficient and volumetric efficiency on excess air ratio. Variable gas constant and specific heat ratio are considered and these influential factors are usually neglected by other researches. 在综合考虑扫气系数和容积效率对过量空气系数影响的基础上,采用随过量空气系数变化的气体常数和比热比,建立了大型低速二冲程柴油机的平均值模型。