The article presents the results of research, which aims to develop a model of the friction units of the cylinder group— couples «piston ring— cylinder liner» and «trunk piston— cylinder liner», which takes into account the change in the real state of the working surfaces of mating parts and lubricating oil. Examined are the factors that have a progressive impact on the operation of the engine friction units while their wearing out. Assessed is the impact of these factors on the wear and its forecasting. We propose a method for predicting the operation life and the real state of the engine.