Valvetrain friction and wear: Influence of surface engineering and lubricants

Abstract

Customer demand and legislative pressure are continually challenging automotive manufacturers to improve the fuel economy of their vehicles. Several technologies are currently being evaluated to meet this challenge. Tribology can play a significant role in bringing some of these technologies to a more mature state and can also make enhancements to existing technologies to reduce frictional losses in the powertrain. Although the valvetrain contributes only 6-10 per cent of the total frictional loss in an engine, all opportunities are explored to reduce frictional losses further, provided they are cost effective. This paper describes attempts to reduce frictional losses in directacting mechanical bucket tappet type valvetrains through surface finish, surface texture and coatings. It was observed that surface finish and surface texture could reduce frictional losses, but engine oil formulation also plays a significant role. Improvements were observed with non-friction-modified oils, but with friction-modified oils the overall friction was lower but no additional friction benefit could be observed with surface finish or surface texture. This paper also addresses wear concern associated with implementation of proposed GF-4 low-phosphorus engine oils. The wear was measured by a radiotracer technique. It was observed that, if the engine oils are formulated properly, low-phosphorus engine oils provide similar or better wear protection compared with GF-3 oils containing 0.1 per cent phosphorus.