Wet Clutch Performance in a Mineral-Based, and in a Partial-Synthetic-Based Automatic Transmission Fluid 970976

The physical and chemical properties of a mineral-based (Fluid-M) and a partial-synthetic-based (Fluid-PS) automatic transmission fluid were compared by the analyses of Gel Permeation Chromatography (GPC), Thermogravimetry (TG), Fourier Transform Infrared Spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR), Gas Chromatography (GC), viscometry, thermo-oxidative stability, torque response curve shape and metal-to-metal wear preventive characteristics. The effects of various properties of Fluid-M and Fluid-PS on wet friction material performance were investigated from the viewpoints of compressibility, durability, tensile strength, surface interactions and friction-pressure-speed-temperature characteristics. Friction material specifications for partial-synthetic fluid applications will be different from those for mineral-based fluid applications.

GPC showed that Fluid-PS has a higher concentration and a lower molecular weight of VI Improver than Fluid-M. Chemical analyses indicated major differences in the detergent, anti-wear and anti-oxidant additive packages of Fluid-M and Fluid-PS. Thermal oxidation tests indicated different degradation paths for the fluids. Both fluid formed acid, ketone, and ester. However, Fluid-M showed more tendency to polymerize to diester or polyester than Fluid-PS. Fluid-PS reduced friction and wear in metal-to-metal contacts in 4-Ball tests as compared to Fluid-M. The less metallic wear in Fluid-PS was explained with the superior viscometrics and anti-wear additive characteristics of the fluid. Fluid-M and Fluid-PS had similar torque response curve performance in SAE tests at low temperatures. However, Fluid-PS showed an increasing friction with decreasing speed (negative μ-v slope) whereas Fluid-M showed a positive μ-v slope at high temperatures and low speeds. Hence, Fluid-PS tended to cause more “shudder” at high temperatures, especially with rigid friction materials. The factors contributing to the negative μ -v slope in Fluid-PS are the narrow molecular weight distribution of the synthetic base oil, and the detergent free additive package of the fluid. In general, the friction materials wetted with Fluid-PS had less compressibility than the ones wetted with Fluid-M. The difference in compression modulus in the fluids were explained with the differences in base oil type and VI improver package of the fluids. Fluid-PS' tendency to increase the rigidity of a wet clutch system can be compensated by tailoring the compressibility of friction materials.