Vocis' second-gen multi-speed transmission promises lower-cost EVs

UK-based transmission engineering and control specialist Vocis wants to take electric vehicles in another direction, one with dual clutch transmissions. Vocis says its dual clutch transmissions provide seamless shifting and up to 15 percent improvement in EV efficiency. Vocis' DCT is currently on display in a Mercedes-Benz Vito minibus demonstration vehicle.

Many EV powertrans use a single e-machine (motor/generator). Vocis uses two smaller e-machines, each driving a separate input shaft. It's described as an eDCT (electric dual clutch transmission) and resembles a DCT configuration – but with two motors instead of twin clutches.

A multi-speed transmission offers faster acceleration and a higher top speed than a single e-machine. This improves hill climbing and pull-away performance, and its higher ratios in the transmission can provide more efficient high-speed cruising.

The multi-speed transmission can also help reduce the cost, because a smaller battery battery pack combined with the multi-speed transmission can deliver the same traveling range as a larger battery on a single-speed EV. Smaller battery, lower cost.

The eDCT was designed with modular architecture to make it pliable for a wide range of vehicle types, said Richard Taylor, technical director of Vocis Driveline Controls. The motors can be installed parallel to each other or on opposite sides of the unit, providing layout flexibility. Vocis' second-generation unit has received partial funding from the Niche Vehicle Network, a specialist car and chassis manufacturer association. It also was nominated for an SMMT Automotive Innovation Award last year.

Vocis first showed off its EV two-speed transmission during the Low Carbon Vehicle Event 2011 at Rockingham Motor Speedway in the UK. It was fitted for yet another minibus – this one a prototype electric minibus developed with powertrain supplier Zytek.

New multi-speed EV transmission launched in demonstrator vehicle

On-road testing of new multi-speed transmission for EVs from Oerlikon Graziano/Vocis begins; potential for increased range or reduced pack size

The red region shows the most efficient speed range for a traction motor. By keeping the motor operating in, or close to, this region, the new multi-speed transmission developed by Vocis could increase the range of electric vehicles with very little impact. Click to enlarge.

Prototypes of a novel, multi-speed transmission for electric vehicles (EVs) have begun on-road testing with a European vehicle manufacturer. Designed and manufactured by Italian drivetrain specialist Oerlikon Graziano with UK control systems specialist Vocis (itself part-owned by Oerlikon Graziano), the concept is intended to increase vehicle range or to allow reduced battery pack size, as well as providing improved low-speed pull away and higher-speed cruising.

The wide operating range of a typical electric motor, which provides maximum torque from zero RPM, has led most EV producers to use a single transmission gear ratio. While reducing cost and packaging volume, the compromise is reduced efficiency and performance, according to the partners.

Electric motor efficiency drops off at low load levels and towards the extremes of speed. Multiple gear ratios with electronic control allow the motor to be kept in the region of greatest efficiency for a much higher proportion of the time, allowing significant range extension.

—Vocis technical director, Richard Taylor

The extra ratios also remove the compromise between top speed and hill-climbing ability.

An EV will typically have a transmission ratio that is higher than the ideal, simply to give it enough top speed. “With multiple ratios, we can provide much better laden pull-away as well as improved top speed without increasing the powertrain size.

—Vocis technical director, Richard Taylor

The heart of the new transmission is a novel gearshifting concept based on principles similar to those used in a DCT (dual clutch transmission), a transmission type in which Vocis has considerable expertise (earlier post). Unlike most EV powertrains, which use a single e-machine (motor/generator), the Vocis multi-speed transmission works with two small e-machines, each on an independently-controlled shaft.

The design allows the distribution of drive and recovered energy to be balanced between the two e-machines, with seamless changing provided by torque infill during each shift. The torque infill makes 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, according to Taylor.

Electronic control provides full driveline integration which, combined with multiple ratios, will also allow the implementation of alternative calibrations in order to tailor the feel and performance of the vehicle to the driver’s personal preferences or to match brand characteristics. It also provides the facility for strategies such as automatic ratio optimization to maximize range.

Because the technology is readily scalable, including the number of ratios, it is easily applied to a diverse range of vehicles.

The priority for a passenger car application is to increase the performance to that of a conventional car by improving acceleration and top speed. The priority for a delivery van would be to improve fully laden performance during pull away or hill climbing. And both will benefit from the significant improvement in range.

—Paul Taylor

Though more complex than a single-speed transmission, the cost and weight of the multi-speed system compares favorably to a conventional gearbox because it requires no clutch or synchronizers. This inherent simplicity makes the technology attractive to EV producers, the partners say.

Vocis will demonstrate this technology, along with its two-speed electric vehicle transmission (earlier post) at the upcoming LCV2011 (the Low Carbon Vehicle show) on 7-8 September at Rockingham Motor Speedway in the UK. The new twin-speed transmission will be demonstrated in a prototype electric minibus developed with powertrain supplier Zytek. (Earlier post.)

Our two-speed transmission demonstrated the benefits of EVs moving beyond a single speed. We are now extending those gains by using multiple ratios to make EVs more competitive in a variety of applications. This gives us single, twin and multi-speed EV transmissions, all of which are already in running vehicles. We believe this to be one of the most comprehensive ranges available anywhere.

—Vocis managing director Mike Everitt

Source: Green Car Congress