Mixed thermo-elastohydrodynamic cam-tappet power loss in low-speed emission cycles

Abstract

The key driving forces in engine development are fuel efficiency and emission levels. These aspects are particularly poignant under vehicle idling or low crawling motions in typical city driving. Under these conditions, the parasitic frictional losses are exacerbated and the emission levels are especially high. A key engine sub-system is the valve-train system. Although it accounts for only 2–3% of the overall engine losses, it is the highest loaded conjunction in the engine, thus limiting the opportunity for lowering the lubricant bulk viscosity. The paper presents detailed tribology of the cam–tappet contact, subjected to a mixed thermo-elastohydrodynamic regime of lubrication. In particular, the frictional behaviour of the conjunction is investigated under the stringent North American emission testing city cycle. Such a comprehensive approach has not hitherto been reported in the literature. The predictions show good conformance with vehicle frictional assessments in industry. It further demonstrates that under the aforementioned cycle, the highest power losses occur mainly as the result of lubricant film viscous shear at low sliding speeds and below the lubricant limiting Eyring shear stress.