The effects of surface texture in reciprocating bearings

Abstract

In recent years, interest in reducing friction in internal combustion engines has grown significantly. This has mainly been due to stricter regulatory standards, with emission targets set as low as 95g of CO2/km from 2020 onwards. As a result, motor vehicle manufacturers have been focusing investment on energy efficient technologies with the aim of reducing fuel consumption. As part of this drive, the reduction of friction between the piston rings and cylinder liners is gaining considerable attention. A potentially effective way of doing this is to apply laser texturing to piston liner surfaces. However, little is understood about the mechanisms by which pockets affect friction, primarily because of the lack of reliable experimental measurements. In this study, the influence of surface texture on film thickness and friction force was measured simultaneously in a convergent-divergent bearing under different lubrication regimes, closely replicating an automotive piston ring-bore conjunction. This was achieved using a reciprocating sliding test rig, designed to control all operating parameters with a high degree of accuracy. Various pocket shapes were assessed in order to understand the beneficial or detrimental effects of surface texture under various operating conditions. This showed unequivocally that pockets are able to reduce friction in the mixed regime and increase friction under full film conditions. After the optimum pocket shape had been determined, various spatial parameters (depth, breadth, separation) were tested, under different lubrication regimes, in order to characterise the influence of geometry. A key finding here is that optimum pocket geometry varies along the length of the stroke. Next, film thickness was measured using a modified version of the ultra-thin film optical interferometry approach. The results here agreed with the friction data by showing how pockets increase film thickness in the mixed and boundary regime and decrease film thickness in the full film regime. Furthermore, through transient measurements, a number of insights into mechanisms by which surface texture reduce or increase friction were obtained. In addition, cavitation in the ring-liner pairing was investigated, as well as various aspects of pocket behaviour, such as orientation, location relative to reversal and the influence of steady state versus transient sliding. Finally, wear tests were conducted under highly loaded conditions which led to an understanding of the interactions between laser surface texture and wear behaviour.