A complete 3-D description of the elastic behavior of a piston ring and its influence on the tribological behavior of the piston ring-cylinder liner interface

Advances in modern engine development are becoming more and more challenging. The intense increase of thermal and mechanical loads as a result of higher power density requires perfecting the function of all engine components especially with regard to emission and friction reduction. In particular, piston rings pack represent one of the most important cause of mechanical friction loss in internal combustion engine [1], and inadequate ring-liner lubrication leads to high fuel/oil consumption and increased engine emissions with dramatic impact over the entire system efficiency. A desirable piston ring-pack set has to provide efficient sealing performance with both the cylinder wall in a radial direction and the top or bottom sides of the piston ring housing in an axial direction, leading to minimal gas blow-by, oil consumption and friction loss. Moreover piston ring other requirement are low friction, low wear and good resistance against mechanical/thermal fatigue. This is a challenging task due to the nature of the phenomena and interactions associated with piston rings. For example, increasing the installed ring tension, which is a method to control oil consumption, also tends to increase ring friction. Hence, any attempt to optimize ring-pack performance output parameters requires a good understanding of the dynamics of all the involved components. Different key elements have to be considered: the ring shape in its free state (namely free shape), the ring crossing-section geometry and the contact surface profile, which play important roles in determining the ring behavior. In order to achieve sealing, the piston rings are first held against the cylinder liner in its front face by their tension after being installed into the cylinder bore. The contact pressure on the cylinder wall is achieved by the inherent spring force of the ring in conjunction with the gas pressure behind the ring. During different engine operating conditions, the piston rings experience dynamically changing forces and axial as well as radial movements of the rings can occur. The contact on the side of the piston housing is achieved by the axial forces acting on the ring. The axial forces are composed of the gas pressure above and under the ring, the mass forces (inertia), and the friction forces. These forces change their direction during the cycle, and, as a result, the piston ring moves from one side of the groove to the other during the engine cycle. This is known as ring fluttering when the axial movement becomes excessive. This behavior open an additional gas flow path: gas can flow around the inner diameter of the ring which results in very high engine blow-by loss. In addition the ring pack design should also consider other factors, including gas blowback and, as said before, ring pack friction. The blowback is the reverse process of blowby and is highly related to engine emission and oil consumption while ring friction can cause severe ring and cylinder wall wear, which results in the ring losing its sealing capability. These factors are related to the ring circumferential pressure distribution, which is defined by the ring free shape and the ring cross section geometry. The understanding of the piston ring behavior is an hard challenge for automotive engineer. Firstly, in 1936 Castleman [2] investigated and proposed the concept of hydrodynamic lubrication for the piston ring. Thereafter, more and more research has been done in this field. Dowson et al. [3] predicted the behavior of a piston ring using the EHL theory. Sun [4] conducted his study for ring-bore conformability, in which the ring was modeled as a curved beam under in-plane loads. Liu and Tian [5, 6] developed an FEA tool for piston ring design. Ejakov et al. [7] modeled ring twist behavior predicting ring axial, radial displacements, bending and twisting angles along the ring periphery over an engine cycle. In this contributions a complete 3D model of the piston ring is proposed, and the influence of an important parameter like the twist angle of the piston ring section on both ring-liner and ring-piston groove interaction is investigated.