Low-mass Zytek powertrain key to Murray's new 700-kg EV

  • 12-Jul-2011 01:08 EDT
Gordon Murray Des T27 Zytek 25kW motor+transmission.jpg

Heart of the T.27's powertrain is its air-cooled Zytek traction motor and Vocis single-speed transmission. 

The electric Gordon Murray Design T.27 city car was recently unveiled in near-production ready form. Based closely on the company's T.25 design, which Murray claims could establish a new “world car class” for efficient vehicles, the T.27 emphasizes light weight and what the company regards as a “unique, holistic” design manufacturing process (its patented iStream).

The T.27 has a claimed curb weight of less than 700 kg (1543 lb). Its electric powertrain was designed and built by Zytek Automotive, a key member of the consortium established to develop the project. Powered by a 25-kW (33.5-hp) electric motor via a 12.1-kW·h lithium-ion battery, the EV has performance targets that include a top speed of 105 km/h (65 mph) and 0-100 km/h (0-62 mph) acceleration in less than 15 s.

Claimed range between charges on the New European Drive Cycle (NEDC) is 100 mi (160 km). The claimed ECE 15 (European Drive Cycle) range is 130 mi (209 km).

Zytek developed the car’s powertrain to be the most compact, power-dense system available for use in lightweight EVs. With £1.5m funding from the U.K. Technology Strategy Board (the U.K. government-backed board has pumped £4.5m into the whole T.27 program), Zytek was responsible for all aspects of the design and manufacture of the complete drive system, including motor, inverter, and battery. It also worked closely with gearbox supplier Vocis on the car's lightweight single-speed drive unit.

Air-cooled battery for low mass

Creating a lightweight structure for T.27 allowed the electric motor to be small and light, too, although it needed to have a high maximum operating speed. Zytek settled on 14,500 rpm, which gave required torque of 64 N·m (47 lb·ft). This met the requirement for the vehicle to be sufficiently lively and responsive. The 25-kW peak power output is available for 30 s.

Although Zytek has 15 years’ experience in designing electric powertrains, the company opted to create the T.27’s propulsion system from a “clean sheet of paper,” said CEO Neil Heslington.

“A working prototype was completed within six months for dyno-based work and led to a second stage design for installation in the car,” he explained.

Exceptionally high powertrain efficiency with light weight were design priorities from the start of the program. So an air-cooled configuration for motor, inverter, and battery was applied. Combined weight of the motor, gearbox, and three-phase inverter is less than 50 kg (110 lb). The inverter combines battery-charging and high-voltage-switching functions; this helps reduce copper cabling, thus saving weight while avoiding any reliability issues regarding high voltage connectors.

Powertrain mass is approximately 45% of that of a comparable production liquid-cooled electric drivetrain with its associated radiator, coolant, pumps etc., Heslington noted.

The compact Zytek unit allows both luggage space enhancement and also a reduction in carbon footprint during component manufacture. The relationship with Vocis facilitated more effective communication between the two companies, resulting in significant systems optimization.

New battery pack design

The choice of lithium-ion battery design, package size, and weight were crucial aspects of this optimization. It took a year of CFD analyses of various cooling system possibilities to achieve a satisfactory design of a new pouch-cell frame incorporating cooling channels and a dedicated battery management system.

Heslington stated that the battery has been safety tested in prototype form to assess thermal characteristics, together with cell performance under extremes of electrical and mechanical abuse. Battery-pack weight is 129 kg (284 lb). Heslington describes the design as being a single structure instead of a traditional “box within a box” configuration. The battery is designated Zytek 25kW E-Drive.

Said former Brabham and McLaren Formula 1 designer Murray of the T.27: “Light weight is the most powerful tool we have in our armory in the fight against emissions and fuel consumption. A lightweight car means a lightweight battery, increasing the levels of safety and reducing the retail price dramatically.”

He claims the T.27 sets new standards in weight, footprint, small car dynamics, safety, packaging, and efficiency and addresses full life-cycle CO2 emissions, congestion, parking, and low-cost motoring issues.

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