Zytek's 25kW E-Drive Powertrain Moves To Phase 2

July 29, 2011: Zytek completes Phase 2 Development of 25kW E-Drive Powertrain. Successful application for Gordon Murray T.27 electric vehicle includes Zytek’s first automotive High Voltage battery. In parallel with the announcement of the completion of the Gordon Murray Design T.27 electric vehicle program, Zytek has released more details regarding the electric drivetrain developed for use in this cutting edge lightweight vehicle.

Partially funded by a £1.5m investment by the Technology Strategy Board, Zytek set out to develop a new powertrain with the goal of being the most compact, power dense drivetrain available for use in lightweight, low cost electric vehicles. Key to meeting these ambitious challenges was that Zytek were responsible for all aspects of the design and manufacture of the complete drive system, including motor, inverter, battery, in addition to working very closely with the gearbox supplier Vocis.

Minimising the torque requirement of the motor (to allow it to be smaller, lighter and more efficient) whilst maintaining vehicle performance, requires a high motor speed. A wide variety of motor topologies and designs were analysed using 2D and 3D simulation tools, leading to a reliable maximum motor speed of 14,500rpm. This high operating speed allowed motor torque to be reduced to 64Nm whilst maintaining vehicle based performance targets, most importantly meaning the T.27 is fun to drive. The peak motor power of 25kW (available for 30 seconds) means vehicle performance is maintained in all expected usage conditions.

The second major powertrain component is the three phase inverter. Following the same design guidelines of examining every component for potential weight saving opportunities, it was concluded that the inverter should also combine battery charging and high voltage switching capabilities. This has the added benefit of reducing the weight of high current copper cable and also avoiding any reliability issues associated with use of high voltage connectors.

A clean sheet of paper design was progressed to produce the first working prototype systems after just six months. This design underwent a highly accelerated dyno-based development and durability exercise, feeding into a second stage design which has been installed on the launch version of the Gordon Murray Design T.27 electric vehicle. This approach, coupled with over 15 years experience in the design of electric powertrains specifically intended for automotive applications, has resulted in a system that contributes to the low weight of the T.27.

In addition to weight reduction, great emphasis was placed on powertrain system efficiency, with the goal of maintaining an air cooled topology for the motor, inverter and battery. This further reduces vehicle weight and brings with it added advantages in powertrain packaging flexibility.

The total weight of the motor, inverter and gearbox is less than 50kg, approximately 45% of that of a comparative, current production, water cooled drivetrain with its associated radiator, cooling fluid, pumps etc. This light weight contributes to the low overall vehicle weight whilst the highly compact size means that luggage space can be increased compared to the regular internal combustion engined vehicle.

Added benefits from the highly compact electric drivetrain mean that as less material is used in the manufacture of the electric machine, the carbon footprint associated with the manufacture of each drive is very low. The close relationship with Vocis, and the tight integration of their high-efficiency, light-weight EV gearbox allowed significant optimisation of both the motor and the transmission.

The second iteration of drivetrain design also saw the introduction of Zytek’s very first Li-Ion battery. Following 12 months intensive effort selecting a suitable Li-Ion cell design, CFD analysis of multiple cooling system options, design of a new pouch cell frame incorporating cooling channels and the design of a new Battery Management System, Zytek has manufactured a highly optimised, lightweight high voltage electric vehicle battery. This battery has undergone considerable testing to ensure its safety in this prototype application, including a variety of cell, module and battery tests. These tests have assessed thermal characteristics, plus cell performance under extremes of electrical and mechanical abuse.

With low vehicle weight being a primary project target, every effort was taken to design and integrate the battery into the vehicle, and as such a single structure has been used, improving on the more traditional “box within a box” approach. This has yielded a total battery weight of 129kg. The battery capacity of 12.1kWh, coupled with the lightweight vehicle and powertrain design, has exceeded range targets, giving more than 100 miles over the New European Drive Cycle (limited to 100kph), and up to 130 miles over the urban European Drive Cycle (ECE15).

“We believe that the Zytek powertrain and battery make a significant contribution to the Gordon Murray Design T.27’s justified claim to be the most efficient electric car in the world,” says Zytek Automotive managing director Neil Heslington. “The success of this TSB funded programme has led to further interest in the Zytek 25kW E-Drive by several major European vehicle manufacturers and we expect to continue the development of this powertrain through to production.”

World-leading expertise: Zytek has designed and integrated electric drive systems for a wide range of European and US vehicle manufacturers and is currently building high performance electric drivetrains up to 70kW and 300Nm for cars, buses and light commercial vehicles. Their UK facility can accommodate up to 6,000 E-Drive integrations a year in batches as low as 100, providing vehicle manufacturers and large fleets with a highly flexible specialist production resource for their low carbon programmes. Zytek can also manufacture up to around 18,000 high efficiency electric traction motors each year, with the option of integrated control systems to further improve overall system weight and packaging. Zytek