Friction modelling in an engine valve train considering the sensitivity to lubricant formulation

Abstract

In the automotive industry, within the last two decades, designers have noted widely adopted mathematical models to evaluate the tribological impact on the efficiency of the engine design and lubricant system. This study attempts to evaluate frictional losses in a Direct Acting Tappet (DAT)-type valve train through the use of tribofilm friction performance model in boundary and mixed lubrication conditions. A typical valve train friction model based on the elastohydrodynamic lubrication theory, mixed lubrication concept based on a transition model and tribofilm friction equation is described to evaluate the frictional and power losses in a cam cycle and to understand the sensitivity of the model to lubricant behaviour in boundary and mixed lubrication conditions. The chemical sensitivity of the additives used in automotive lubricants is characterized in terms of a typical friction modifier, molybdenum dithiocarbamate, in a model automotive engine lubricant also containing zinc dialkyldithiophosphate in synthetic polyalphaolefin base oil at a range of operating temperatures, sliding speeds, loads, initial surface roughness, and the initial lambda ratios. The mean friction torques predicted from the valve train friction model show good agreement with the experimental data from the motored cylinder head of a typical DAT design.