Green Car Congress reports that engineers at Munich Technical University (TUM) in Germany have developed a torque vectoring transmission that is optimized for use in electric vehicles. The design is part of the Visio.M project, which focuses on long range, agile driving dynamics and excellent safety for EVs.
When a car goes around a corner, the drive wheel on the outside travels a greater distance than the inside wheel, so it has to rotate faster to keep up. In basic cars, a differential lets the driven wheels rotate independently of each other. But to accomplish this, it sends power to the wheel with the least amount of traction. That’s why your Mom’s ’68 Buick was always getting stuck in the snow.
In high performance cars, a more complex device known as a limited slip differential (LSD) is used to send power to the wheel with the most traction. Torque vectoring is just a high tech name for using a limited slip differential.
Electric vehicles have different technological challenges than conventional cars because the drive wheels not only move the car forward but also help recapture kinetic energy while the car is coasting or under braking. That energy is stored in the battery for use later in the journey. If an EV is turning while coasting or when the brakes are applied, harvesting energy from both wheels equally can make the car unstable.
The TUM torque vectoring device accounts for different traction levels during cornering to maximize energy harvesting and minimize driving instability. When the car is accelerating in a corner, more torque is sent to the outside wheel to help guide the vehicle through the turn, resulting in more stable handling. Instead of the standard bevel gears used in differential transmissions, the engineers developed a spur gear differential in which additional torque can be applied from outside via a superimposed planetary gearbox.
Weight and cost are critical for EVs because today’s batteries are heavy and expensive. So anything that can be done to reduce the weight of the rest of the vehicle means using a smaller, less expensive battery. TUM has worked hard to get the weight of its torque vectoring device as low as possible. It uses aluminum for the housing and is looking to save even more weight by employing an aluminum and carbon fiber composite housing in the future.
TUM Professor Karsten Stahl says “The elegant thing about the torque vectoring transmission we have developed is that it not only has a higher recuperation level, and, with that, an increased driving range, the transmission also improves road handling dynamics, driving pleasure and safety. The continuously improving optimization measures leave us optimistic that in the near future both the weight and cost will be able to compete with today’s standard differential transmissions.”
Affordable, light weight torque vectoring systems will be an important component for electric cars of the future.