The Fixed Ratio Traction Drive Speed Reducer—An Enabling Technology for Downsizing EV Motors

Abstract

Since the 1990’s most vehicle manufacturers began to ‘downsize’ internal combustion engines while maintaining power and torque through the use of technologies such as turbocharging, direct injection and variable valve timing. Reductions in engine capacity and weight of 50% proved be possible in addition to substantial improvements in fuel efficiency and reduced emission. It has recently become accepted that motors for EVs can be reduced in size and weight by a similar or greater amount, primarily by increasing their speed from the historical limit of around 7000–20,000 rpm and beyond. For the motor design itself, electronic and mechanical innovations to allow this increase in speed have been developing rapidly. There remains a challenge in the development of efficient high speed transmissions to bring the 20,000 rpm motor speed down to a typical maximum wheel speed of 1300 rpm for a vehicle travelling at 160 kph corresponding to a 15:1 overall transmission ratio. High speed transmissions for industrial applications have a long history but for passenger vehicle applications the technical and commercial challenges are very different and the need has only been recognised over the last three to five years. The France based RedHV+ project is one research effort addressing the question of ‘whether it is possible to manufacture, at car cost, a high-speed, high efficiency gearbox operating up to 40,000 rpm …. while at the same time guaranteeing the reliability of the system under automobile conditions’. This paper suggests that ‘toothless’ traction drives may be inherently superior to a more conventional transmission for this highly important EV application. It provides a broad discussion of traction drives and specific information regarding the ‘Silk Drive’ together with the efficiency results for the Silk Drive from testing recently undertaken at the Tsinghua Automotive Research Institute (Suzhou) under the direction of Ultimate Transmissions Pty.Ltd. who are the inventors, developers and owners of the intellectual property associated with the Silk Drive.