Improving engine efficiency via polymer shear thinning

Abstract

This thesis investigates the extent to which viscosity modifier additive shear thinning can be used to reduce hydrodynamic friction and thus improve engine efficiency while not impacting too greatly on hydrodynamic film thickness. Novel experimental techniques are developed to measure shear-dependent viscosity over a large shear rate range. A model hydrodynamic lubricated contact is established using a steel ball rolling/sliding against a polymer disc. This enables low contact pressures, thus avoiding significant piezoviscous effects. Novel methods are developed to measure film thickness and friction in such contacts. Film thickness measurements on Newtonian oils in soft contacts are used to test experimentally existing soft EHL theoretical equations and to revise these. Polymer-containing oils give thinner than predicted film thickness due to shear thinning in the contact inlet induced by lubricant backflow and by the sliding speed between the rubbing surfaces. Measurements have been compared with predictions implementing existing inlet strain rate equations combined with experimental flow curves. Theoretical equations under-predict the impact of inlet shear thinning on film thickness. Hydrodynamic friction measurements with Newtonian oils show the expected linear dependence against sliding speed. Unfortunately, the shear stress determined from such measurements is greater than expected from theory. Despite considerable efforts, the origins of this discrepancy have not been established, which necessitates a correction factor when determining the shear stress of polymer-containing oils. Using this factor the expected reduction in hydrodynamic friction of polymer-containing oils is observed. Overall it is found that shear thinning allows polymer-containing lubricants to produce smaller rates of increase in film thickness and friction with sliding speed in hydrodynamic lubrication than seen for Newtonian oils. Thus the slope of the Stribeck curve in the hydrodynamic regime is reduced. Hence, an oil may be designed to shear thin, adapt to the variable engine conditions and thus improve engine efficiency.