THE PROBLEMAIICS OF ENSURING THE RELIABILITY OF THE SIDE SURFACE OF THE PISTONS OF BOOSTED ICES

Prospective directions and trends in the development of special-purpose engines are analyzed. An increase in swept volume power density of engines leads to a critical raise in thermal and mechanical stress in the engine structural elements and to a corresponding reduction in its resource. Pistons are among the elements of boosted engines that are most prone to failure. Among the characteristic critical zones of the piston design, in which losses of parametric and physical reliability were observed, a separate critical zone on the loaded and unloaded side of the piston skirt near the piston pin bores is singled out. An analysis of publications was carried out and it was shown that the occurrence of the phenomenon of wear or seizures of the side surface of the piston skirt, which is observed for the boosted designs in the specified zone, is not explained exhaustively in the existing system of scientific views. Understanding the causes of the described phenomenon of loss of reliability is necessary for diagnosing and developing measures to prevent it at the design stage. The paper puts forward a hypothesis that explains the occurrence or non-occurrence of the loss of reliability in the process of deformation of the local zone of the side surface of the piston skirt during uneven expansion of the material in presence of creep and simultaneous stress relaxation with possible strengthening of the material by the property of creep limit. The possible cases of deformation of a certain local zone of the side surface of the piston in an elastic setting and in the presence of creep and stress relaxation processes are considered. It is shown that the process of creep deformation in the studied zone can end before the occurrence of reliability loss even for the materials that are unable to be strengthened by the property of creep limit or could be strengthened only insignificantly. The general ways of increasing the reliability of the side surface of the piston are outlined, related to ensuring the level of local stresses being below the creep limit, ensuring the completion of the creep process before the clearance reaches the critical values, as well as achieving strengthening of the material by the property of creep limit, for materials capable of it, before the occurrence of creep in operation.