The design of fuel efficient automotive hypoid gear lubricants

Synthesised lubricant base stocks and the performance additives used in hypoid gear lubricants can both be chemically designed to give improved hypoid gear efficiency and, therefore, better vehicle fuel economy than provided by conventional lubricants In this paper, the authors review, relative to a widely used SAE 90 grade mineral oil-based lubricant, the changes in gear efficiency which have been documented using certain classes of synthesised lubricants. These advantages are described in terms of (a) EPA City and Highway Cycle efficiencies, (b) absolute data over a wide temperature range under four selected speed/torque combinations, and (c) absolute changes in inefficiency compared to the mineral oil product. Newtonian and non-Newtonian fluids have been studied. The bulk oil temperature at which a specific lubricant gives maximum efficiency is dependent on both gear operating conditions and on the chemical composition of the lubricant.