The Evaluation of the Fuel-Economy Performance of Low-Viscosity Drive-Train Lubricants and the Development of Oils with Improved Fatigue Life

In recent years, progress has been made in reducing the viscosities of manual transmission fluids (MTFs) and automatic transmission fluids (ATFs). Lower viscosities of MTFs and ATFs are expected to improve the fuel economy of automobiles by reducing the viscous resistance. Examples of low-viscosity ATFs already commercially available include Toyota Auto Fluid WS and ZF Friedrichshafen AG's ZNF 13014. This paper first reports methods for measuring the torque transmission efficiency in manual and automatic transmissions. We explain a simple rig test that we developed using an IAE gear test machine, and we describe oil temperature increase tests and torque measurement tests using actual transmissions and fuel economy tests using actual vehicles. Next, we describe the effects of lower viscosities on the torque transfer efficiency as measured with these measurement methods. Because lower viscosities are likely to worsen the wear resistance, extreme-pressure performance, and fatigue resistance of MTFs and ATFs, we studied the effect of the base oil composition and the molecular structure of PMA-based viscosity index improvers (VIIs) on the oil film thickness. We were able to determine by three methods that the film thickness of low viscosity lubricant could be made greater than that of conventional lubricant. The oil film was made thicker in the first method by increasing the molecular weight distribution of the base oil, in the second method by reducing the molecular weight of the PMA-based VIIs, and in the third method by changing the alkyl groups of the polymethacrylate-based (PMA-based) VIIs. The results of our study showed that our optimized lower-viscosity MTF, when evaluated in terms of its wear prevention, extreme-pressure properties, and fatigue prevention, has equal or better performance in each area in comparison with MTFs with conventional viscosities.