Abstract
Reducing CO2 emissions, which cause global warming, is one of the most important challenges facing the automobile industry today. As a result, the demand for high-powered continuously variable transmissions (CVT) has been steadily increasing. The traction-drive type half-toroidal CVT meets this demand. One concept for getting higher capacity is that the contact angle to the power roller reduces spin loss at the traction contact point. This increases the transfer of power at the traction contact point and opens the way to development of a high-power toroidal CVT. In 1999, the first mass-produced model of a traction-drive CVT for large-size passenger vehicles entered the Japanese automobile market. In this mass production model, there were two power rollers per cavity. In the other hand, the demand for a more compact CVT with higher torque capacity is ever increasing especially in large size vehicles.
There are three methods for increasing transmitting torque T. The first method is to increase the traction coefficient µ. The traction coefficient is based on traction fluid properties. Regarding µ, the development of a superior traction fluid with higher traction coefficient is expected. The second method is to increase the contact force Fc. Increasing contact force Fc influences rolling fatigue life of rolling elements. One of the authors applied longer bearing life technology, which includes CVT casehardened steel with extra -pure specifications, and carbonitriding of the rolling elements. And, this approach has succeeded in increasing torque capacity. The third method is to increase the number of power rollers n to allow a more compact size and higher power transmission. A 3-roller system has one and half times the torque capacity of a conventional two-roller system. There are some issues regarding the 3-roller system that need to be considered, however. One issue is the development of a ratio control mechanism. Hydraulic pistons, which are attached to each trunnion, control transmission ratio. In a 3-roller system, these hydraulic pistons impede the design of a compact CVT, which is a major requirement for automobile application.
The concepts of a high power-density half-toroidal dual-cavity CVT are discussed in this paper. Increasing the number of power rollers, n, is effective for attaining higher power-density. However, there are limitations of n because of structure of the variator. The number of power rollers, n=3, is most effective for high power automotive applications. New designs of control piston arrangements, estimated performance of efficiency and some test results are also discussed. These show the possibilities that await CVT technology, which can be incorporated into large-size luxury vehicle while maintaining a compact size.