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Multi-speed EV transmission is put through its paces

09 June 2013

Throughout May, transmission engineering and control specialist, Vocis made available to interested parties an electric demonstrator vehicle fitted with the latest iteration of its novel, multi-speed, seamless-shift transmission.

The innovative Vocis transmission uses the principles of dual clutch transmissions (DCTs) to provide seamless shifting and up to 15 percent improvement in electric vehicle efficiency. The second generation unit has been developed with partial funding from the Niche Vehicle Network and was shortlisted for an SMMT Automotive Innovation award in 2012.

Unlike many EV powertrains based on a single e-machine (motor/generator), the Vocis multi-speed transmission works with two smaller e-machines, each driving a separate input shaft. Described as an eDCT (electric DCT) the transmission resembles a DCT configuration but with two motors instead of twin clutches, as Richard Taylor, technical director of Vocis Driveline Controls, explains:

“We have used a modular architecture for the second-generation transmission, used in the demonstrator, which means the motors can be installed parallel to each other or on opposite sides of the unit. This flexibility of layout, plus the scalability of the modular concept, makes the design suitable for a wide range of vehicle types.”

The demonstration vehicle - a Mercedes-Benz Vito minibus - was previously fitted with the company’s 2SED two-speed transmission and competed successfully in the RAC Future Car Challenge, winning the Most Energy Efficient Light Commercial Vehicle Prototype class. The vehicle was subsequently modified, now accommodating a Vocis eDCT with two 25kW traction motors and the capability to accept two 35kW motors in a future upgrade.

Besides improving efficiency, a multi-speed transmission leads directly to faster acceleration and a higher top speed. By improving hill climbing and pull-away performance, a multi-speed transmission can increase the load carrying capacity of an electric delivery vehicle. The higher ratios in the transmission can then be specified to provide more efficient high speed cruising, enabling EVs to mix more easily with other vehicles on highways.

The improved powertrain efficiency of the multi-speed transmission can also help reduce the cost of the battery pack, usually the biggest single expense within an EV powertrain. A smaller battery pack, in combination with the multi-speed transmission, can provide the same vehicle range as a larger battery with only a single speed.

Electronic control enables full driveline integration which, combined with the multiple ratios, provides the facility for strategies such as automatic ratio optimisation to maximise range and extend battery life by making deep discharging less frequent. It also allows the implementation of alternative calibrations in order to tailor the feel and performance of the vehicle to the driver’s personal preferences.

Different strategies are used to prioritise economy or performance, or any transition between the two. In this respect, sharing torque between the motors offers shift opportunities beyond those achievable with a DCT. The design allows the distribution of drive and regeneration energy between the two e-machines to be optimised at all times, enabling torque infill to be provided during each gear change, resulting in seamless shifting.

As Richard Taylor recalls, the torque infill has made gearshifts so imperceptible that the first customer for the system asked to be shown that shifting was taking place because he couldn’t feel it when driving.

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