The Effect of Lubricant Viscosity and Composition on Engine Friction and Bearing Wear

Abstract

This paper investigates the effect of lubricant composition on engine friction and connecting-rod bearing wear. Special attention has been given to polymer-thickened (VI improved) oils since these oils are characterized by shear-dependent viscosity and a simultaneous occurrence of viscous and elastic properties. The variables investigated in this study included lubricant viscosity, polymer type, and concentration. Two sets of engine studies were conducted, one to determine engine friction, the other to measure connecting-rod bearing wear, using irradiated bearings. For Newtonian fluids, the engine friction and wear response can be predicted from classical lubrication theory—that is, (a) friction decreases with increaing viscosity until a viscosity is reached where friction is a minimum; beyond this viscosity, further increases in viscosity result in increased friction. (b) Bearing wear decreases with increasing viscosity, but as a step function, not linearly, and the transition viscosity (of the step) corresponds to the viscosity which gives a minimum engine friction. The addition of polymeric VI improvers (non-Newtonian fluids) to mineral oil base stocks reduces engine friction and lowers bearing wear—the amount of friction and wear reduction depending on the polymer type and concentration. This paper demonstrates that polymer-thickened oils actually give better bearing wear performance than their comparable mineral oil counterparts despite the fact that they have a lower apparent viscosity at high rates of shear. In addition, it appears that temporary viscosity loss is not the sale cause of the reduced engine friction of polymer-thickened oils.