Transmissions for ElEcTric VEhiclEsA new family of multi-speed transmissions from Oerlikon Graziano, based on

the principles of dual clutch transmissions, provides an alternative approach

to increasing the efficiency, performance and range of electric vehicles.to increasing the efficiency, performance and range of electric vehicles.

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Cover Story TrAnsmissiOn And EnGinE

Motor effiCienCy

Conventional thinking on electric vehicle (EV) drivelines is to use a single-speed transmission, relying on the torque spread of the electric motor to provide adequate performance under all operating condi-tions. 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.

At the lowest and highest vehicle speeds, and under conditions of light loading, motor efficiency typically falls to between 60 and 70 %. Under optimum speed and load conditions, it increases to around 90 %, ❶. Detailed simulation has indicated that an overall improvement in vehicle efficiency and energy consump-tion of around 15 % can be achieved over the NEDC (New European Driving Cycle) by using a four-speed transmission with a pair of small electric motors. The eDCT (electric Dual Clutch Transmission) allows for a total of seven gear combina-tions from the four distinct gear ratios through a combination of single-motor driving (first, second, third and fourth gear) and dual-motor driving combina-tions (first and second, second and third, third and fourth gear).

Reducing the energy consumption of an electric vehicle helps to overcome a key issue for many users, range anxiety, by providing greater operational range from an existing battery pack. Alterna-tively, the vehicle can be provided with increased performance, or existing per-

formance can be maintained using smaller, lighter battery packs and motors.

replaCing ClutCheS with MotorS

The transmission concept of the eDCT from Oerlikon Graziano is based on the principles of a dual clutch transmission (DCT), providing seamless shifting between ratios without torque interruption yet using no clutches or synchronisers. The configuration is similar to a DCT but using two motors in place of the twin clutches. One motor drives a shaft carrying first and third gears (the “odd” shaft); the other drives a shaft carrying second and fourth (the “even” shaft). This allows pre-selection of the next gear before the previ-ous one has been disengaged, using the two motors to synchronise shaft speeds so that no synchronisers are needed.

The first generation design was pro-duced for a high-performance GT car, using two 240 Nm electric motors. Designed with splash lubrication and minimal numbers of meshes and bear-ings for maximum efficiency, the gearbox is arranged in two-stages with helical gears. A limited slip differential is incor-porated into the final drive and electrohy-draulic actuators for the shift mechanism are integrated into the assembly.

DevelopMent anD arrangeMent of the two generationS of tranSMiSSion

The first generation transmission was developed quickly, with detail design

AuThOr

Dr.-ing. ClauDio torrelli is head of Product development at

Oerlikon Graziano in rivoli (italy).

❶ Trajectory of motor efficiency

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completed within four weeks of start-up and prototype assembly achieved during week eleven. Simultaneous development of software enabled functional testing and vehicle commissioning to begin by week 13. Four months after project start the shift performance had been devel-oped sufficiently for demonstration purposes.

A second generation design, for A and B class cars, uses a pair of motors, each with between 64 and 75 Nm torque and peak power output between 50 and 70 kW. By using a modular architecture for the transmission, the motors can be installed parallel to each other or on opposite sides of the transmission, to suit vehicle installation priorities. Both paral-lel and opposing arrangements share high levels of component commonality, including input shafts, gear selection actuation and differential assemblies. ❷ shows the opposed motor arrange-

ment, ❸ the parallel arrangement. The A/B class car application uses electro-mechanical shift actuation, via an elec-tric motor and barrel cam, and an open differential.

variability thankS to MoDularity

Use of a modular concept also makes the design highly scalable to suit a wide range of other vehicle types. Compact low-cost electric city cars can use an electric front or rear axle configuration, potentially based on a 48 V architecture. The addition of an electrically driven axle assembly to performance cars, lux-ury cars and supercars can create a desirable four-wheel drive (4WD) hybrid, while city bus or truck applications have also been catered for. Even a series or parallel hybrid arrangement with only two driven wheels can be accommo-

dated by the inclusion of a single clutch within the combined powertrain.

The shift control system was devel-oped by UK controls specialists, Vocis Driveline Controls, itself part-owned by Oerlikon Graziano. Using expertise from previous DCT applications, Vocis used a range of existing algorithms from their software library to cover driver strategy, shift sequencing, gear actuation and safety functions complying with OBDII requirements. The software is simpler than for a DCT application because the engine interface is replaced by a motor interface while the clutch and synchron-iser control algorithms are unnecessary and are deleted entirely, ❹. The first gen-eration prototype used a Vocis TMS-20 controller that, in addition to direct inputs and outputs, allows robust CAN communication with other controllers on the vehicle to ensure seamless shifting and intuitive control.

❸ Transmission with parallel motor arrangement

❷ Transmission with opposed motor arrangement

❹ Software components of the shift control system compared to previous DCT applications

Cover Story TrAnsmissiOn And EnGinE

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analySiS of a Shift event

Although gear shifting appears seamless to the vehicle occupants, there is a defined sequence of individual events, each one scheduled and controlled by the software. ❺ shows the full sequence for a typical power-on upshift during dual-motor driving.

Initially, the vehicle is accelerating with first gear and second gear driving. The required torque is provided by motor 1 (blue line) and motor 2 (red line). As the torque from motor 1 rolls off, the torque from motor 2 increases until it provides all the driving torque, compensating for the loss in torque from motor 1 as the motor 1 torque reduces to zero. At this point, first gear is disen-gaged, neutral is selected on the odd shaft and the speed of motor 1 is com-manded to the synchronous speed asso-ciated with third gear.

The vehicle accelerates in second gear under the increased torque from motor 2, the speed of motor 2 increasing in line with vehicle speed. When the speed of

motor 1 matches the required speed to select third gear the odd shaft is shifted from neutral to third.

With third gear pre-selected, the torque output from motor 1 increases and that from motor 2 falls until driving torque is shared by motor 1 and motor 2 and the vehicle is now driving in second and third gear, achieving a synchronous shift without the need for clutch or syn-chronisers. A corresponding sequence is then used for the transition to third and fourth gear. The gear selector cam tracks correspond to the shift sequences described.

ConCluSion

A subset of the sequence described is also applicable for single-motor driving or transitions between single-motor and dual-motor driving, giving a full comple-ment of control strategies that can priori-tise economy or performance, and the transitions between the two, as appropri-ate to the particular application. Direct control of the input shafts using two

electric motors provides enhanced response, compared to an internal com-bustion engine driving through a DCT. Shift strategies have even been imple-mented that make some gearshifts im -perceptible to the test drivers, requiring close examination of logged data to ana-lyse the shift event.

❺ Full sequence of a shift event

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