1

eDCT: 4 Speed Seamless-Shift Technology For Electric Vehicles

B Stubbs, P M Fracchia CEng

Vocis Ltd, UK, enquiries@vocis.co.uk

Keywords: Seamless-shift, Multispeed-Electric-Vehicles, Demonstrator-Vehicles, Transmission-Control, Innovative-Design.

Abstract

For electric vehicles there is a fundamental shift in the philosophy of the electric powertrain. This manifests itself as a move away from single speed transmissions to multiple speed transmissions. For the comfort to the driver, the multi-speed transmission must be achieved with seamless-shift technology.

The eDCT is a novel design concept for electric vehicles that takes inspiration from DCT (Dual Clutch Transmission) technology, from which the eDCT takes all the benefits but with a reduction in terms of weight, size, power loss and cost. The solution is an ingenious mechanical design that is controlled by a reduced set of transmission control software, when compared to alternative automated transmission technology, giving performance and efficiency improvements that can translate into a greater vehicle range or a smaller battery pack.

The latest generation of eDCT design has allowed the opportunity to introduce flexibility into the design in terms of motor location, through modularity of design, allowing opposing motor or parallel motor installation, further increasing the desirability of the design for different applications. The opposing motor design has been integrated into a demonstrator vehicle, a Mercedes Vito Minibus. The cost and efficiency benefits of the eDCT in this application are detailed together with the first results of the testing on this vehicle.

1 Multi-Speed Electric Vehicle Transmissions

The original assumption for electric vehicle and electric 4WD drivelines was to use a single-speed transmission, relying on the torque spread of the electric motor to provide adequate performance under all operating conditions. Though simple to execute, this arrangement cannot achieve the best vehicle efficiency because, for much of the time, the traction motor is operating at loads and speeds that do not yield optimum efficiency. Also, single speed transmissions give rise to a

significant compromise in acceleration versus maximum speed, or the additional requirement of a disengage device forthrough-the-road 4WD systems.

As a result of the limitations of single speed transmissions there have been developments in many types of multi-speed transmissions for the electric powertrain. This increase in transmission technology can reduce energy consumption, improve performance and marketability, and for the case of the multi-speed concepts described here, can even maintain or indeed reduce the overall electric powertrain cost and weight.

The overall electric powertrain cost is a key driver in the development of electric vehicles and hybrid systems. There are currently multi-speed transmission concepts in development that need hydraulic actuation, multiple clutches or synchronisers. An alternative approach is to fully utilise the advantage of the electric motor functions, using electric power for actuation and developing novel seamless-shift technologies to minimise additional cost and complexity. This was the philosophy in the design of the 2SED (2 Speed Electric Drive) and eDCT (electric Dual Clutchless Transmission). This paper and presentation focuses on eDCT concept.

2 eDCT Overview

The eDCT has 4 gear ratios and utilises two traction motors, achieving powershifting by supporting gear shifts on the ‘non-shifting’ motor, keeping the mechanical design as simple and elegant as possible. This gives rise to 7 gear combinations (4 single gears and 3 dual gears), and is a cost effective solution for the automotive market with the omission of clutches and synchronisers because the motors can be used for launch manoeuvres and gear-shifts in the eDCT configuration.

The design is also modular, achieving not only scalability of the design, but also the ability to have the motors located at opposite ends of the gearbox, the layout shown in Figure 1, or parallel to each other, Figure 2, giving ultimate flexibility for vehicle installation. The design has been configured such that the opposing motor design can be split down the centre of the transmission and the two halves used in the adjacent motor

2

configuration with identical gears, shaft geometries and bearings. Having the motors on the same side of the gearbox also makes this configuration advantageous in terms of a very low axial length.

Figure 1: eDCT with the motor mounted at opposing ends of the transmission.

Figure 2: eDCT with both motors mounted at the same end of the transmission.

Figure 3: Example powerflows through the eDCT, including barrel cam position for gear actuation, showing driving in 1st and 2nd gear together.

Example powerflows through the eDCT are shown in Figure 3, with the layout shown being for the motors in the parallel configuration.

The design with opposing motors or adjacent (parallel) motors have a high degree of commonality of parts; the primary physical difference being that two secondary shafts are required in place of one for the adjacent design. The gears on the shafts are the same meaning that these can be common between the two designs. The input shafts and selector mechanism are also common, and the two designs, highlighting the commonality of design internals are shown in Figure 4.

Figure 4: The eDCT design showing the assembly for opposing motor design on the left and the parallel design on the right, highlighting the commonality of parts between the two designs.

The gear actuation is via an electrically actuated barrel cam. This solution has the advantage over an electro-hydraulic solution because it removes the complexity and cost of the electro-hydraulic pack, but has high performance for the dog clutch engagements, with a target gear selection time (or neutral selection time) of 50ms.

3

3 Efficiency Benefits

The demand for more speeds comes in part from the efficiency variability of the electric drive-: Permanent Magnet Synchronous Motors operate with peak efficiency in excess of90% but this can fall to 60-70% depending on torque and speed. Multiple ratios can keep the traction motor at a higher efficiency for a wider range of operating torques and speeds. For the 2SED this translates to a predicted 6% improvement on the NEDC cycle, and for the eDCT, 10%. - Figures takenfrom simulations of the described Mercedes Vito demo vehicle, generated by The University of Surrey.

Figure 5: Torque-Speed envelope comparing 1-speed and 2-speed applications for a 70kW traction motor with the 4-speed application with 2x35kW traction motors.

Figure 5 shows the theoretical Torque-Speed envelope comparing 1-speed and 2-speed applications for a 70kW traction motor with the 4-speed application with 2x35kW traction motors. The electric powertrain specifications match those for the Mercedes Vito Minibus using the 2SED and eDCT. The 70kW single speed application is from an equivalent Mercedes Vito vehicle with a single speed Oerlikon Graziano / Vocis transmission. The area highlighted in red shows the area of operating efficiency above approximately 90%.

4 Powertrain Cost Comparisons

An evaluation of trend costs has been undertaken in partnership with the motor supplier Zytek Automotive. This has concentrated on the comparative cost of a single speed 70kW traction motor with single speed gearbox and a 40kWh battery, in relation to the 2SED with the same traction motor and the eDCT with 2x smaller 35kW traction motors (these indicative costs apply directly to the demonstrator vehicle highlighted above). The associated power electronics were also considered and the costs are presented below in Figure 6for an electric vehicle of comparable range when considering a reduced battery size equating to the predicted efficiency benefits highlighted in the previous section.

Figure 6: Evaluation of overall indicative electric powertrain cost, for electric vehicle with 70kW total traction input power.

5 Demonstrator Vehicle

The two multispeed concepts, the 2SED and eDCT, have been made as prototypes and demonstrated in-vehicle.

The 2SED transmission has been demonstrated in a Mercedes Vito Minibus coupled to a 70kW traction motor manufactured by Zytek Automotive. This vehicle has been used to prove the 2SED concept, illustrated in Figure 7, and operates with seamless shifting over the full operating vehicle range. This vehicle and powertrain configuration competed in the RAC

4

Future Car Challenge and won the Most Energy Efficient Light Commercial Vehicle Prototype class.

Figure 7: The 2SED transmission in the Mercede Vito Minibus Demonstrator Vehicle.

The eDCT has been successfully implemented in a customer vehicle and has proved the concept for this transmission type, with very positive feedback regarding the shift quality. To enable Oerlikon Graziano and Vocis to be able to demonstrate the eDCT technology the Mercedes Vito Minibus, above has been converted to accept the eDCT with 2x 25kW traction motors (with potential to upgrade to 2x 35Kw at a later stage).

In collaboration with Zytek and The University of Surrey, part funded by the Niche Vehicle Network, the demonstrator vehicle was completed and driving within the planned 6 month time scale. Developments have been made continuously since the project and the vehicle is available for demonstration. Thanks to the high ratio first and second gears a vehicle weight approaching 3 tonnes is moved comfortably using 2 motor that are not much bigger that alternators.

Due to the low profile of the opposing motor layout it was possible to package the transmission motors and invertorsunder the rear floor area – space originally taken by the differential and exhaust silencer – see figure 8.

Figure 8: Installation of motors, transmission and invertors.

The quality of the eDCT seamless shift is clearly evident when driving the demonstration vehicle. Under moderate acceleration the torque is shared between each motor. At the predetermined shift point the torque is rolled off from one of the motors (the ‘Odd’ motor for example), at the same time the other motor (‘Even’) torque is boosted to provide uninterrupted vehicle acceleration. Freed from the traction task the ‘Odd’ motor is used to synchronise the next gear. Once the new ‘Odd’ gear is engaged the torque is handed back and the shift is complete. Figure 9 shows an overview of this process occurring on the demonstrator vehicle.

Figure9: The eDCT transmission in the Mercede Vito Minibus Demonstrator Vehicle.

6 Minimalist Actuation

When compared to other power shifting automatics the eDCT has very simple yet effective actuation. The losses are kept toa minimum – through the use of splash lubrication and on demand electrical actuation. In a conventional DCT the actuation system has to perform two main functions – clutch and gear selection. The clutch actuation typically has to provide fast response and high clamp load required to transmit the torque – resulting in a high power requirement for the actuation. The gear selection also has a high speed and

5

force requirement – to move out of gear quickly and to push on the synchroniser. In the eDCT concept the function of the clutches are replaced by the twin traction motors. The synchronization is handled also by the traction machines-There are no friction synchronisers in the transmission - only dog clutches, so ‘all’ that is left for the low voltage actuator isto move the dogs in and out of engagement. For the prototypetransmission this is handled using a barrel cam mechanism driven by a BLDC motor. See Figure 10.

Figure 10: BLDC motor driving dog clutch actuation.

The choice of BLDC motor was made mainly to ensure a fast response. – aided by the low rotor inertia. Using a BLDCmotor necessitated a B6 drive capable of driving >30A. While possible to source low cost BLDC drives, none were found to satisfy all of the requirements; CAN interface, current measurement, sensored and sensorless (for another application) operation and functional flexibility. This led to the development of the Vocis TMS6 motor drive – figure 11.

Figure 11: Vocis TMS6 BLDC motor drive – Gen 2

The BLDC motor offers the best system performance when compared to brushed motors. But BLDC motors currently have a cost premium – due in part to the use of rare earth materials. If required the TMS6 can also be used to drive brushed motors.

In its current form the TMS6 provides for very capable ‘smart actuation’ functionality. The IO provided on the TMS6 Gen 2 allows it to function in a number of modes, in particular for this demonstrator vehicle it functions as a fast servo controller. In this mode the TMS6 reads a PWM absolute position sensor to give closed loop control, and handles stall conditions without intervention from the supervisor controller. This and other feature flexibility is made possible by developing all software for the TMS6 in house.

Summary

Future electric vehicle growth is a certainty and the 2SED and eDCT exploit the benefits of electric machines in a new and novel way. These mechanical designs and transmission control software give a complete transmission solution for electric vehicles and through-the-road hybrid 4WD that give improved economy and performance over more conventional systems. Additionally, the efficiency benefits can translate into overall electric powertrain costs being comparable or reduced in relation to a single speed application, making the multi-speed concepts presented here an appealing solution for many automotive applications, and adds to the ever increasing innovative EV product range that Oerlikon Graziano and Vocis can offer to customers.

Acknowledgements

The eDCT demonstrator vehicle described in this paper is the result of a collaborative project between Vocis Driveline Controls, Zytek Automotive Limited and The University of Surrey. The project was part funded and coordinated through the Niche Vehicle Network. The project was delivered within the demanding 6 month time scale thanks to the commitment and dedication of all project partners. The collaborative arrangement made possible by the NVN has allowed the construction of a vehicle with a very innovating prototype power train – within a time frame and cost that would have not been possible by any of the partners in isolation.