The article presents the results of research which aims to develop a model of the friction units of the cylinder-piston group, i.e., the “piston ring — cylinder liner” and “piston trunk — cylinder liner” couples taking into account the change in the real state of the working surfaces of the mating parts and lubricating oil. We have examined the factors that have a progressive impact on the operation of the engine friction units during their wear life. The impact of these factors on the wear and forecasting it has been assessed. We propose the method for predicting the operation life and the real state of the engine. An experimental study to validate the developed method to the particular engine has been conducted.