Bosch is introducing a continuously variable transmission (CVT) with Bosch push belt, specially designed for electric vehicles: the CVT4EV. CVT4EV is a compact variable ratio transmission that supports the performance demands of electric vehicles for a wider audience. The system improves the economy and performance of the electric powertrain, while maintaining the smooth and comfortable character of the electric motor.
This is how a CVT works for an electric car. As the speed of car (x axis) increases, traction (y axis) decreases. The CVT ensures a smooth, stepless transition. Source: TU/e.
The variable gear ratio of the CVT4EV delivers an optimal trade-off between efficiency and performance. The system controls the speed and torque of the electric motor and improves the vehicle’s performance. This results in faster acceleration, higher top speeds and more torque at the wheels for towing and off-road performance.
CVT4EV electric drivetrain
Continuous shifting allows maximum power to be applied continuously, allowing the vehicle to accelerate better and drive faster up the mountain. Power also remains more available across the vehicle speed, leading to higher top speeds and lower energy consumption.
Volkswagen e-Golf demonstrator equipped with CVT4EV by Bosch
By running the electric motor at its most efficient operating point, the CVT4EV also enables lower energy consumption. In the combined efficiency of the inverter, electric motor and CVT, the system finds the optimum operating point of the electric powertrain by controlling the transmission ratio of the CVT. That is the point where the power and torque are delivered in the most efficient way.
Due to the reduced torque and speed requirement of the electric motor, the CVT4EV makes a more compact electric motor possible. Alternatively, a larger driving range with the same motor can be selected.
Bosch says that CVT4EV provides an additional cost advantage for car manufacturers due to its versatility. CVT4EV makes a single drivetrain suitable for a wide variety of applications, be it a mid-size car, sports car or light commercial vehicle.
This solution consists of one platform with a CVT4EV module, an inverter, an electric motor and a final drive with a ratio adapted to the vehicle. This creates a major cost advantage for car manufacturers, with less cost and complexity in the development, production and aftermarket of their applications and vehicles.
This variability also offers an improved user experience through the availability of different driving modes (eg uphill, highway or individual mode). The variability of the CVT4EV also offers car manufacturers a way to differentiate themselves with their vehicles by offering typical driving behavior related to their brand in all segments. As a result, the driving experience by the end user is still linked to the car make.
Unlike alternative multi-stage solutions for electric vehicles, the CVT4EV introduces fewer restrictions on torque and revs. In addition, it offers completely smooth shifting without shifting shocks. Extra measures to mask switching shocks are therefore not necessary; in addition, the potentially larger control range of the CVT reduces the revs of the electric motor, resulting in an even smoother and quieter ride.
The CVT4EV concept with numerous Bosch components was developed by Bosch Transmission Technology BV. Bosch worked with Eindhoven University of Technology (TU/e) PhD candidate Caiyang Wei on design ands optimization of the electric CVT.
Wei and Bosch used an optimization method that takes into account not only the control of the car’s speed, energy consumption and heat production, but also the physical design of the car’s electric motor and transmission system.
With this method—a combination of machine learning and a thorough analysis of the measurement data—the researchers were able to design a system that is optimal in terms of both performance and efficiency, while at the same time reducing the total costs for both the manufacturer and the user.
Caiyang Wei (2020) “Optimal design and control of electrified powertrains”