The Effect of Viscosity Index on the Efficiency of Transmission Lubricants

The world is firmly focused on reducing energy consumption and on increasingly stringent regulations on CO₂ emissions. Examples of regulatory changes include the new United States Environmental Protection Agency's (U.S. EPA) fuel economy test procedures which were required beginning with the 2008 model year for vehicles sold in the US market. These test procedures include testing at higher speeds, more aggressive acceleration and deceleration, and hot-weather and cold-temperature testing. These revised procedures are intended to provide an estimate that more accurately reflects what consumers will experience under real world driving conditions. The U.S. EPA has also proposed changes, which will extend at least to 2016, that will increase the fleet wide corporate average fuel economy (CAFE) standards step wise, year on year and would impose limits on the amount of greenhouse gas emissions.^3,8

Typical operating temperatures in automatic transmissions during cold start-up and light-duty operation are between -20°C and 80°C. Temperatures during severe-duty intervals, such as towing, can be higher than 100°C. Recently, there has been a trend to make automatic transmission fluids (ATFs) with lower kinematic viscosities (KV) in the interest of improving fuel economy by improving the mechanical efficiency of the transmission. Unfortunately, the use of lower viscosity fluids can lead to gear and bearing durability problems among myriad other problems. Without making significant hardware changes the lower practical viscosity limit has been estimated to be about 5.0 cSt.^4-7 If decreasing the 100°C KV isn't always practical or possible, what else can be done to improve efficiency? One option is to keep the fluid viscosity low as possible under normal operating conditions for improved efficiency, and under high-temperature conditions, keep the viscosity as high as possible to maintain hydraulic system, pump, seal and clutch performance. Fluids with very high viscosity indices (VI) approach these viscometric properties. Both an increase in mechanical efficiency and fuel economy will be demonstrated in a comparison between a very high VI fluid and a lower VI fluid, where both fluids have the same KV at 100°C.