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The paper highlights the use of a light duty axle efficiency test for evaluating the fuel economy performance of automotive gear lubricants. Both final peak axle temperatures and torque efficiencies are recorded for several multigrade automotive gear lubricants. The dependence of temperature on torque efficiencies for the gear lubricants tested are discussed for a variety of driving conditions: city, highway and severe service. Temperature and torque efficiency data show strong dependence on additive system and viscosity- temperature characteristics of the gear lubricants under different driving conditions. A discussion of lubricant rheology and its importance to maintaining film strength for adequate bearing and gear lubrication as related to optimum torque efficiency and axle temperature under varying loads and pinion speeds is also provided.

This study evaluates the shear-thinning effects of a commercial Viscosity Index (VI) improver in a variety of mineral oil base stocks to produce a series of viscosity grades from 0W-20 to 20W-50. All of these fluids also include a DI package in addition to an olefin copolymer VI improver and base oil. The Penn State high shear capillary viscometer was used to collect the primary data at shear rates of up to 106 s-1 and at 10 to 177°C. Data are fitted to a double truncated power law model to determine the incipient non-Newtonian shear rate Υ̇1, the power law index n, and the incipient second Newtonian shear rate Υ̇2. These parameters are found to be smooth functions of temperature and polymer concentration in the different base stocks. Using the data obtained from OW-20, 10W-30, and 20W-50 SAE grade formulations, these parameters (Υ̇1, Υ̇2 and n) are correlated as functions of temperature, polymer concentration, and a viscosity temperature property of the base oil.

The Cannon High Temperature High Shear (HTHS) capillary viscometer is currently used in the ASTM D4624 procedure to measure the viscosity of polymer containing lubricants at 150°C and shear rate of 106 sec-1. An expansion of the utility of this Cannon instrument is described in this paper to cover the temperature range of lubricants from 35 to 175°C and shear rates of up to 106 sec-1. A finite difference model is used to solve the transport equations for capillary flow at each temperature. The solution accounted for temperature, pressure, non-Newtonian and kinetic energy effects on viscosity. Fitting these data to a double truncated power law model provides the incipient non-Newtonian shear rate γ̇1, the power law index n, and the incipient second Newtonian shear rate γ̇2. All these parameters can be measured at temperatures of around 100°C or less. The n and γ̇2 were found to be regular functions of temperature while γ̇i is always in the measured range.

Capillary viscometers provide a convenient method of measuring the viscosity of polymer containing lubricants at 150°C and 106 sec-1 shear rate as specified in ASTM D4624 Procedure. The commercial Cannon HTHS viscometer and the Penn State HTHS viscometer were used in this study. Improved calibration of the capillaries in the commercial viscometer provided an order of magnitude improvement of the HTHS repeatable values from 4.47% to 0.39% for a typical polymer containing lubricant. The viscosities of seven polymer containing lubricants at 106 sec-1 and 150°C gave an absolute mean percentage difference of 0.62% from measurements made with both capillary viscometers used in this study. The data suggests that substantial improvements can be made in the repeatability of ASTM D4624 HTHS viscosity measurements without changing the design of the current commercial viscometer.

The Penn State high shear viscometer has been used to measure viscosities of polymer solutions at shear rates up to 106 sec-1 over a temperature range of 20°C to 175°C. A double truncated power law model has been developed for analysis of the primary data from the viscometer. This model can be used to determine the shear rate at which incipient non-Newtonian behavior occurs, the power law index, and the shear rate at which a second Newtonian region is found. These three parameters appear to be regular functions of temperature for a given polymer (Viscosity Index Improver) type. The correlation of these three functions with temperature provides a convenient method of predicting viscosity properties of polymer solutions at various temperatures and shear rates up to 107 sec-1. The Penn State high shear viscometer has been used to evaluate methacrylate and olefin copolymer VI improvers in mineral oil base stocks at shear rates of 106 sec-1 at various temperatures.