Abstract: The effects of two classic wear situations on the engine performance were studied by building a thermodynamic model of internal combustion engines The first situation was the variation of valve clearance resulting from the wear and distortion of the distribution mechanism The second situation was the variation of blow by clearance results from the wear of cylinder liner -piston assembly It was shown from the simulation that the engine performance declined markedly in the Two wear situations Therefore , proper tribological measurements should be adopted to prolong the no -failure lifetime of key parts at design stage