Driven by global demands for improved fuel economy and reduced emissions, significant improvements have been made to engine designs and control systems, vehicle aerodynamics, and fuel quality. Improvements, such as the continuously slipping torque converter, have also been made to automatic transmissions to increase vehicle efficiency. Recently, belt-continuously variable transmissions (b-CVTs) have been commercialized with the promise of significant fuel economy improvements over conventional automatic transmissions. Automotive traction drive transmissions may soon join belt-CVTs as alternative automatic transmission technology. Much of the information reported in technical and trade publications has been on the mechanics of these traction drive systems. As automotive traction drives move closer to commercial reality, more attention must be given to the performance requirements of the automotive traction fluid.
Studies of traction coefficient as a function of molecular structure are well documented in the academic literature. And, the Santotrac® line of traction fluids has been in commercial use in industrial applications since 1972. Last year the number of papers published on automotive traction fluid research increased with the key performance requirement of high traction coefficient at specific conditions often the main focus of these studies. Additional performance requirements which must be addressed by lubricant suppliers in the formulation of practical automotive traction fluids include: low temperature viscometrics, volatility, flash point, oxidation stability, elastomer compatibility, shear stability, clutch friction performance, and antiwear protection. Although not a performance requirement, cost will be an important consideration in fluid development as the toroidal traction transmission competes in the mass production market against conventional automatic transmissions and b-CVTs.
We herein examine the challenges that exist in developing automotive traction fluid technology by comparing the performance of an industrial traction fluid, two commercial automotive traction fluids, and four experimental fluids formulated to investigate the balance among all the performance criteria and cost.