James L. Linden

Physical and Chemical Properties of a Typical Automatic Transmission Fluid

Twenty-two physical and chemical properties of a typical automatic transmission fluid were determined. In most cases the properties were determined over a range of temperatures. In general, air solubility, volumetric thermal expansion, and specific heat increase with increasing temperature; whereas, surface tension, specific gravity, viscosity, bulk modulus, density, thermal conductivity, and electrical resistivity decrease with increasing temperature.

Synthesis and Characterization of Alkyl-Bridged Bicycloheptanes as Traction Fluids

The traction fluid is a critical component of a toroidal-continuously variable transmission (T-CVT). As the medium that transmits power through the toroids, the traction fluid needs to provide a high traction coefficient and retain low dynamic viscosity at cold temperatures; this is a challenging combination of properties. A comparison of a variety of fluids shows a broad correlation between the traction coefficient and the fluids low temperature viscosity, or pour point. This work investigated a series of novel compounds as traction fluids through chemical synthesis and the measurement of their relevant physical properties. Specifically, four new alkyl-bridged bicycloheptane fluids have been synthesized and refined to high purity. Their traction coefficients, measured with a ball-on-disc traction apparatus, are comparable to those of commercial fluids over the relevant range of temperature and pressure. Their dynamic viscosities at low temperature, however, are higher than the viscosity of commercial fluids and exceed the value of 3 × 10 4 cP at −40°C. These bridged bicycloheptanes also exhibit a correlation between their low-temperature viscosity and traction coefficient. The reasons for this correlation are discussed, and the effect of the molecular structure on viscosity and traction coefficient is investigated. This analysis finds semi-quantitative relationships between fluid properties and the molecules volume, stiffness, and ring structure.

Estimating the Useful Life of an ATF Using an Integrated Bulk Oxidation and Friction Degradation Model

An integrated approach to modeling end of useful life of an automatic transmission fluid (ATF) has been developed. The flexible fluid life model allows either predictive or real-time calculations of the end of useful fluid life under different transmission design strategies and customer driver behaviors as reflected in operating temperatures, shift characteristics, fluid volume and fluid distribution throughout the transmission. An estimation of remaining useful fluid life is monitored using two metrics, namely bulk oxidation, as a general indicator of fluid quality, and frictional degradation, as an indicator of shift quality. As operating conditions increase in severity, ATF is subjected to conditions that may shorten its life. Using the developed technique, ATF useful life can be better predicted.

Determining Frictional Performance of Automatic Transmission Fluids in a Band Clutch

This paper reports on frictional characteristics of several automatic transmission fluids (ATFs) that were evaluated in an SAE No. 2 Friction Machine adapted to the band clutch from a front-wheel-drive, three-speed automatic transaxle. The bench test apparatus subjects the bank clutch to repeated engagements (24 000 in a 100-hour test period) at elevated temperature with representative clutch energy dissipation and lockup time, and with forced aeration of the fluid. Fluid frictional performance is judged from the level and stability of friction torque during the 24 000 clutch engagements. The test is repeatable and readily distinguishes fluids having different frictional characteristics. Fluid frictional behavior in the band machine is shown to reflect fluid effects on shift performance in an engine dynamometer, transaxle cycling test. Also, fluid effects determined in the cycling test are shown to match those obtained in a vehicle. Both shift quality and durability can be inferred from the band friction machine results.

MODELING TORQUE CONVERTER CLUTCH VISCOUS DAMPER PERFORMANCE

A viscous damper is used as part of the automatic transaxle torque converter in some new passenger cars for improved driveability and fuel economy. A viscous silicone fluid is a critical component of the damper. Fluid viscosity-shear rate characteristics were determined in the laboratory for temperatures of 25 to 150 deg C at shear rates from 400 to over 6000 reciprocal seconds. Using these data, and the dimensions of the viscous damper, a mathematical model was developed which predicts viscous damper output torque. Correlation between model predictions and actual measured torques from dynamometer tests was excellent. As a result, the mathematical model is useful for predicting viscous damper performance from measured fluid viscosity-shear rate characteristics. Also, since the model includes the critical viscous damper dimensions, it can be used for determining effects of damper design modifications on damper performance.

Factors Affecting Axle Lubricant Stability

There are many factors that can affect the service lifetime of a lubricant. In automobiles, one lubricant that has been heavily tested in recent years for fuel efficiency improvement and durability is axle lubricant. While a substantial amount of testing has been performed toward developing new axle lubricants to aid original equipment manufacturers to reduce warranty costs, improve Corporate Average Fuel Economy, and provide extended drain intervals, not as much testing has been documented to show some of the effects that different operating conditions have on these lubricants. The scope of this work is to bring to light some of the different parameters that affect axle lubricant.

Theoretical Advantages of Shear Thickening Behavior in Automatic Transmission Fluids

The rheological behavior of automatic transmission fluids (ATF) ranges from moderately shear-thinning to Newtonian. However, no commercially available ATFs are known to display shear-thickening behavior. A theoretical investigation was performed to determine if any advantages could be derived from the use of shear-thickening ATF in automatic transmission components and subsystems. A series of theoretical shear-thickening, shear-thinning, and Newtonian fluids were modeled by a power law function and compared to a reference shear-thinning ATF in simplified representations of transmission components and subsystem geometries. The results indicate that a shear-thickening ATF with zero shear viscosity, infinite shear viscosity, and power of 4.17 mPa-s, 6.23 mPa-s, and 1.03(dimensionless), respectively, displays optimized behavior with respect to the reference shear-thinning ATF.

Development of Revised DEXRON®-III Elastomers Test

General Motors (GM) currently uses about 1000 different seals for manufacturing all of its automatic transmissions worldwide. In order to assure that these seals function correctly in service, a method of measuring seal performance with service fill automatic transmission fluids (ATFs) has to be specified. Along with this measure, a pass/fail criterion for the evaluation of seal performance is implemented. Due to the large number of seals that are utilized, it would be impractical to test each one with every fluid that is submitted for GM DEXRON®-III and/or Allison C4 certification. It is also very difficult to use production seals in testing, due to the irregular shapes and material combinations, which make measurement of the seal material properties difficult. Therefore, a revised test will be included in the DEXRON®-III and Allison C4 service fill specifications to evaluate the compatibility of service fill ATFs with a representative sample of seal materials used in production. This work describes how a group of ten representative materials was selected and referenced in order to define the GM service fill elastomers test for current and future DEXRON® and C4 specifications.

Improving Transaxle Performance at Low Temperature with Reduced-Viscosity Automatic Transmission Fluids

The effects of automatic transmission fluid viscosity on the low-temperature performance of a front-wheel-drive transaxle were determined in a cold room maintained at a temperature of -20/sup 0/F (-28.9/sup 0/C), using both a cranking apparatus and a vehicle. Cranking and vehicle tests were conducted to determine the effects of fluid viscosity on the power required to crack a transaxle and on transaxle performance under low-temperature transient operation, respectively. Four automatic transmission fluids were tested, ranging in viscosity from 2 600 to 16 000 cP at - 20/sup 0/F. All test fluids contained the same additive package and were blended using the same types of base oils.