The influence of piston ring geometry and topography on friction

This article provides solution for isothermal mixed hydrodynamic conjunction of the compression ring to cylinder liner. This is obtained using the average flow model representation of Reynolds equation based on pressure- and shear-induced flow factors. In particular, the effects of compression ring axial profile along its face-width and surface topography of contiguous solids are investigated. It is shown that ring geometry may be optimized to improve lubrication, whilst care should be taken in order to avoid oil loss or degradation resulting from any loss of sealing. In predicting friction, it is shown that appropriate surface parameters should be used in-line with the state of wear of the ring. For a new ring against a plateau honed liner, boundary friction contribution during the initial running-in wear phase should be predicted according to the average asperity peak heights protruding above the plateau, whilst the plateau height also takes into account the valleys within the surface roughness or grooves created by any cross-hatch honing would be the appropriate measure of topography for worn rings. The main contributions of the article are in providing an analytic solution as well investigation of ring face-width geometry and effect of wear upon friction. However, it is acknowledged that generated heat, inlet boundary starvation and circumferential non-conformity of ring to the bore surface would affect the film thickness and exacerbate generated friction accordingly. These further considerations would require a numerical solution, rather than an analytical one presented here

Elastodynamics of piston compression rings

The piston ring pack accounts for a disproportionate amount of the total engine frictional losses. The frictional behaviour of piston rings is significantly affected and governed by its flexible dynamics. The dynamically changing shape of the ring determines its contact geometry with the cylinder liner and hence affects the frictional losses. The compression ring undergoes a multitude of complex motions during the engine cycle prescribed by the gas pressure, contact reaction, ring tension, friction between the ring and its groove and inertial forces that excite a plethora of the ring’s modal responses. This adversely compromises the functionality of the ring through a number of undesired phenomena such as ring flutter, twist, rotation and jump. Therefore, a prerequisite for improving the prediction of tribological conditions is an accurate determination of the ring’s elastodynamic response. This paper presents a methodology to directly solve the governing differential equations of motion for different forms of beam cross-section, where the shear and mass centres are not coincident, typical of the complex cross-sections of a variety of different piston compression rings. Combined numerical and experimental investigations are undertaken to determine the dynamic behaviour of the compression ring