Shifting from Automatic to Continuously Variable Transmissions: A Look at Fluid Technology Requirements

New technologies are being commercialized across the automotive industry to address demands for improved fuel economy, emissions reductions, and improved customer satisfaction. Push-belt continuously variable transmissions (b-CVTs) are beginning to command a significant percentage of the market now dominated by manual and conventional automatic transmissions. In addition, automobile manufacturers plan to introduce the first traction drive toroidal-CVTs to the market place within the next five years. A review of the relative benefits and limitations of each of these automatic transmissions exists in the literature. In this paper we consider how the performance requirements of each of these automatic transmission systems impact automatic transmission fluid technology.

The physical characteristics and screen test performance of two commercial ATFs, a b-CVTF, and two traction fluids were examined. The overall performance characteristics of the b-CVT fluid were comparable to that of a modern ATF illustrating the evolutionary relationship betweenATF and b-CVTF technology. Commercial traction fluids had poor wet clutch and low temperature performance while the commercial ATFs and b-CVTF had low traction coefficients. This illustrates the separate paths on which these fluid technologies were developed. Lastly, results from two experimental fluids suggest that it may be possible to formulate a traction fluid for automotive use that exhibits wet clutch friction stability without significantly compromising traction performance.