A new belt-drive continuously variable Transmission (B-CVT) was introduced into the Japanese market in September 1997 by Nissan Motor Co., Ltd. It transmits a maximum torque of 196 Nm and represents a major breakthrough of the torque limit transmitted by B-CVTs, thus opening a new epoch for the automatic transmission. The major features of the CVT are transmission of high torque between a steel belt and pulleys, electronic control of high hydraulic-pressure to pulleys and a torque converter with an electronically controlled lockup clutch engaging at low vehicle speeds.
A CVT fluid formulated for this CVT was designed to optimize these features and this paper describes the performance of the CVT fluid in lab-scale tests and an endurance test of the CVT unit. In order to realize high torque transmission between a steel belt and pulleys, high friction between metal/metal contacts is required with normal wear. A block-on-ring tester was used to evaluate the torque transmitting capacity of several CVT fluids and ATFs. A CVT fluid was selected and further tests showed it to give high and stable coefficients of friction and low level of wear. These frictional characteristics are derived from ZnDDPs and Ca type detergents. These additives contribute to the formation of a boundary film, enhancing friction and preventing damage to metal surfaces.
The selected CVT fluid was bench tested using the CVT unit for 60,000km. Based on used oil analysis and inspection of the CVT unit at the end of the test, the fluid gave very satisfactory performance.
Finally, future trends in CVT fluids are discussed.