The major targets of transmission design today are higher efficiency, higher torque capacity and reduced size. Increasingly smaller transmissions with higher torque lead to increasing operating temperatures. This trend is further intensified by the use of noise abatement devices and improved aerodynamic body styling that reduces the airflow around transmissions. The friction in the transmission is responsible for temperature increase and efficiency losses, and thus the reduction of friction is the main measure in order to improve the efficiency and to keep the operating temperature low.
The lubricant influences and is subjected to all changes of operating conditions. Higher operating temperatures result in a higher consumption of friction modifiers, extreme pressure and anti wear additives, higher corrosion and oxidation rates, and a thinner oil film separating the various components. If the lubricant is not formulated to withstand these more severe conditions the transmission may be damaged. On the other hand, the torque transmission losses and, as a consequence, the operating temperature can be reduced by the right choice of base oil and additives.
In vehicle application today, most OEMs start the lubricant development in an early phase of the development of a new transmission in order to get the optimum contribution of the lubricant. These OEMs see the lubricant as a construction element needed for the development of performance optimized transmissions. In order to select the most effective combination of additives and base stocks offered by the lubricant industry, the OEMs have developed screening tests with good correlation to field performance.
Such tests range from a simple bearing test to OEM-specific transmission or axle tests. We have used two tests developed by VW, that correlate well with a car installed on a chassis-dynamometer and a transmission rig test, in order to identify and quantify the influences of fluid viscosity, base stock types, and VI Improver chemistry on efficiency and operating temperature.
Finally, examples of improved semi- and fully synthetic formulations with significantly reduced operating temperatures and optimum efficiency are presented.