Power transmission fluids with enhanced lifetime characteristics

Abstract

A power transmission fluid having improved characteristics, a method of lubricating a transmission, and a vehicle containing the transmission fluid is disclosed. The transmission fluid includes an amount of silicon-containing antifoam agent having a kinematic viscosity of greater than about 20,000 centistokes at 25° C. that is effective to provide gear shifting times of no more than about 0.75 seconds over a useful lifetime of the transmission fluid.

Description

FIELD [0001] The present disclosure relates to power transmission fluids having improved characteristics particularly for aged fluids. The power transmission fluids disclosed herein may include fluids suitable for use in an automatic transmission (ATF) and / or a manual transmission. By “automatic transmissions” herein is meant both continuously variable transmissions, known as CVT, and conventional automatic transmissions. BACKGROUND AND SUMMARY [0002] Automatic transmission fluids (ATF) are designed to meet certain industry standards. A current standard for ATF is known as DEXRON® III-G specifications. However, a new specification was announced in June 2003. The new specification is known under the trade name DEXRON® III-H, which is an upgrade from the DEXRON® III-G specification. The two significant changes from III-G to III-H are: 1) an increase in the GM oxidation test (GMOT) length from 300 hours to 450 hours and 2) an increase the duration of the GM cycling test (GMCT) from 20,...

AI Technical Summary

Benefits of technology

[0004] A factor that may contribute to an increase in air dispersed in the oils as they age may be a decrease in surface tension of the oils. The reduction in surface tension is most likely caused by oil oxidation creating soluble and insoluble hydrophilic compounds, since such oxidized products are known to reduce surface tension. A reduction in surface tension also increases the amount of foaming of the oil. As foaming increases, greater amounts of air bubbles are present in the oil, increasing the compressibility of the oil. When a pressure is applied to the band and drum to activate the 1-2 shift, a longer shift time is required because of a greater fluid compressibility due to the presence of the air bubbles or foam. Accordingly, there continues to be a need for improved power transmission fluids that are less susceptible to foam formation as oils are aged during use.

[0007] In yet another exemplary embodiment there is provided a method of decreasing gear shifting time in an automatic transmission containing an aged transmission fluid. The method includes lubricating the transmission with a fluid comprising an oil having kinematic viscosity at 100° C. of 2-10 centistokes and an effective amount of a silicon-containing antifoam agent having a kinematic viscosity at 25° C. of greater than about 20,000 centistokes, wherein the gear shifting time for the aged fluid is reduced relative to the gear shifting time for an aged fluid in a transmission containing an antifoam agent having a kinematic viscosity at 25° C. of less than about 20,000 centistokes.

[0008] Conventionally, antifoam agents are used to decrease the surface tension of a fluid so that air is released thereby reducing the foaming tendency of the fluid. Adding more conventional antifoam agent may increase the ability of a fluid to release air therefrom. However, as the fluid ages, as in the GMCT described above, the effectiveness of conventional antifoam agents decreases. As the effectiveness decreases, the 1-2 shift time increases.

[0009] An advantage of embodiments of the disclosure is that certain high viscosity silicon-containing antifoam agents may be used in power transmission fluid applications to maintain a lower increase in the 1-2 shift times in the GMCT for aged fluids. Without desiring to be bound by theory, it is believed that higher viscosity silicon-containing antifoam agents may increase the surface tension of the fluid and maintain such increased surface tension even as the fluid ages. Accordingly, the high viscosity silicon-containing antifoam agents may reduce foaming by preventing air from dissolving or dispersing in the fluid initially and as the fluid ages.

Problems solved by technology

The overall required performance changes impose a severe challenge to automatic transmission fluid (ATF) formulators.

However, as the fluids age there is a tendency for the shifting times to increase.

As foaming increases, greater amounts of air bubbles are present in the oil, increasing the compressibility of the oil.

However, as the fluid ages, as in the GMCT described above, the effectiveness of conventional antifoam agents decreases.

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