A Review of Engineered Surfaces for Valvetrain Friction Reduction and Wear

摘要

The friction between two lubricated sliding surfaces and the related wear depends primarily on the contacting materials, load, lubricant formulations, and the lubrication regimes. Under boundary and mixed lubrication conditions, where the asperities of both surfaces touch each other, friction and wear could be controlled by lubricant formulations and thin film coatings. In direct acting bucket tappet type application, where the cam and tappet contact operates under boundary and mixed lubrication regimes, friction reduction has been achieved through reduced reciprocating mass, spring load, and surface finish for a given engine oil formulation. The presence of textures on contacting surfaces allows one additional degree of freedom for friction reduction. Ryk et al. observed 30-40 % friction reduction in laboratory bench tests when a laser dimpled piston ring slid against a cylinder liner material (1). Wakuda et al. (2) also observed a substantial friction reduction when sliding a steel pin (line contact) against a dimpled ceramic disk. Borghi et al. showed similar improvement with textured surfaces (3). Golloch et al. (4) evaluated the friction and wear reduction potential of laser surface texturing in a fired engine using a floating liner technique. The surface of cylinder liners was modified near the top dead center region by a laser beam to create discontinuous lines of 3μrn long separated by 2μm horizontally and vertically. The lines were 40 -60mμ wide and 10-25 μm deep. Laser textured liners showed significant reduction of friction force early on the power stroke when the combustion pressure is the highest. They also observed reduced wear in a 250 hour test. Etsion and Sher (5) reported 4% improvement in fuel consumption with laser textured ring.