Multi-speed transmission boosts electric vehicle's drive train efficiency

September 10, 2012 // By Christoph Hammerschmidt
Will electrical vehicles turn mechanical transmissions into a relict of the past? No, say the transmission experts from Italian mechatronics expert company Oerlikon Graziano (Torino). At the CTI Symposium in Shanghai, the company unveiled a multi-speed transmission for deployment in e-cars. The promise: It can significantly improve the overall efficiency of the vehicle's drive train.

Most current EVs use a single-speed transmission, relying on the electric motor's torque spread to provide adequate performance. However this means the motor spends much of its time operating at only 60 to 70 percent efficiency. The transmission from the Swiss mechatronics company instead uses two smaller motors and provides four speeds to keep the motors running closer to their peak efficiency of approximately 90 percent. The result is an overall efficiency improvement of up to 15 percent, which translates into increased range or improved performance for the vehicle.

“Vehicle manufacturers need innovative solutions to improve the cost, range and performance of electric drivetrains,” said Claudio Torrelli who oversees the product development at Oerlikon Graziano. “The multi-speed transmission can improve acceleration, top-speed and hill-climbing ability for a given motor size. Alternatively EV makers can specify smaller motors to give the same performance with less battery drain, providing greater range.”

Oerlikon Graziano showcases its transmission technology at the CTI Shanghai event as part of an increasing range of electric vehicle transmissions. Beside the 4-speed seamless-shift transaxles with e-DCT technology, the company exhibits single- and dual-speed transaxles, providing solutions for a certain range of electric vehicle applications.

For high performance electric GT car applications, the company exhibits a rear-drive transaxle assembly with twin 125kW electric motors. For hybrids and passenger or light commercial vehicles, a similar arrangement using twin 35kW or 25kW motors is displayed.

How the eDCT works

The eDCT transmission uses two input shafts, each driven by its own electric motor. The concept is similar to a DCT but using two motors in place of the twin clutches: one motor drives a shaft that carries first and third gears, the other drives a shaft carrying second and fourth. This allows pre-selection of the next gear before the previous one has been disengaged, using the two motors to synchronise shaft speeds so that no synchronisers are needed.

Following gear selection, the