Vocis launching demo EV with 2nd generation multi-speed transmission; projected 15% improvement in vehicle efficiency
|2nd generation eDCT prototype unit, with both motors mounted at the same end of the transmission. Source: Vocis. Click to enlarge.|
During the coming month, UK-based transmission engineering and control specialist Vocis will launch an electric demonstrator vehicle fitted with the second generation of its multi-speed eDCT transmission for electric vehicles. The demonstration vehicle, a Mercedes-Benz Vito minibus, will be made available for evaluation by interested parties from industry and the press.
The 4-speed dual-motor Vocis eDCT concept (earlier post) uses the principles of dual clutch transmissions (DCTs) to provide seamless shifting and up to 15% improvement in electric vehicle efficiency over the combined European cycle compared to a single speed gearbox Vocis transmission. 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 that use 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.
The simplicity of the eDCT is achieved because of the omission of the clutches and synchronizers—the motors are used for launch manoeuvres and gear-shifts in the eDCT configuration.
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.—Richard Taylor, technical director of Vocis Driveline Controls
|The Generation 2 design showing the assembly for opposing motor design on the left and the adjacent design on the right, highlighting the commonality of parts between the two designs. Source: Vocis. Click to enlarge.|
For this second generation design, the gears are selected by an electrically actuated barrel cam. This solution has the advantage over the electro-hydraulic solution because it removes the complexity and cost of the electro-hydraulic pack, Vocis says.
The 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 25 kW traction motors and the capability to accept two 35 kW 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, Vocis says. 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.
|Powertrain Specification figures for the Generation 2 Design eDCT (25kW and 35kW). Source: Vocis. Click to enlarge.|
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, according to the company.
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 prioritize 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 optimized at all times, enabling torque infill to be provided during each gear change, resulting in seamless shifting.