The exhaust system and muffler are simplified as a set of 1 - d pipes with non - isentropic flow and with variable sections . the partial differential equations of non - stationary flow are transferred into the stationary differential equations by means of the characteristic lines . these equations are solved by finite difference method 在排气管系的计算中将排气管简化为一组一维非等熵变截面管，并通过“特征线法”将描述其运动的非定常流的偏微分方程转化为常微分方程，再用差分数值方法依靠计算机求解。
