Abstract
Keywords - CVT, efficiency, electromechanics, ratio control, clamping force
Abstract - In a car, a transmission is used to match engine characteristics to vehicle load. Most often manually shifted transmissions or stepped automatic transmissions are used. To reach a better performance, fuel economy and driveability, use of a continuously variable transmission (CVT) is a possibility. A few different CVT principles have arisen that are subject of ongoing research. The two most important groups are toroidal CVTs and belt- or chain type CVTs. In this article, the focus is on belt type CVTs.
With a CVT, continuous shifting under load is possible. It also offers the ability to choose engine rotational speed independently of vehicle speed, albeit between boundaries. This offers a potential efficiency benefit, because the engine can operate more often in fuel-efficient operating points. However, there are considerable power losses within the CVT itself. Consequently, many CVT equipped cars are less efficient than cars with a manual transmission. The internal losses are caused for a large part by the hydraulics that actuate the CVT.
An electromechanical way of CVT pulley actuation is proposed as a means to improve CVT efficiency. Spindles are driven from the fixed world through an epicyclic gearing by electric motors that are placed outside the transmission housing in a cool environment. A mechanical link between the adjustment mechanisms on the two shafts provides energy exchange, thus lowering shifting power demand and actuator size. Because ratio is now position controlled with a stiff servo, the bandwidth of the new design is expected to be considerably higher. Belt clamping force can be controlled independently by a second servo. When not changing ratio and clamping force, the actuators may be fixed by a passive braking system, so that they do not consume power. Efficiency gain is mainly expected in situations where the CVT is not changing ratio, for instance when driving at constant speed.
A prototype was designed as a modification on a commercially available belt type CVT. Pulley shafts are modified, as well as the outer cover of the transmission, which will contain the actuator mechanisms. Currently, a prototype is in the realisation phase, after which it will be evaluated. After trials on a test rig, the transmission will be built in to a vehicle to evaluate driveability and fuel consumption of this new concept.