Inside Zytek’s new EV propulsion system

  • 24-Mar-2014 10:38 EDT
Zytek03-14 Neil Cheesman.jpg

Zytek focused hard on thermal management in the development of its new-generation EV power systems, said Neil Cheeseman. The new Yamaha Motiv.e concept is shown at extreme far right.

Battery technology and vehicle range are invariably two salient areas of focus when an EV's capability is discussed. The traction motor, though, is usually something of an automotive Cinderella—unless it's the motor in Yamaha’s Motiv.e city car.

The Motiv.e was created in collaboration with Gordon Murray Design and unveiled at last fall’s Tokyo Motor Show. Details of its powertrain are now emerging and reveal much about the state of advanced e-motor development—in this case at Zytek, a subsidiary of Continental.

The Zytek “ultra-high-speed” motor revs to 15,000 rpm and weighs only 13kg (28.6 lb), which means “You can pick it up with one hand," noted Neil Cheeseman, Zytek’s Engineering Program Manager. And the motor is complemented by a single-speed reduction gearbox weighing just 11 kg (24 lb)—all good for maximizing EV range potential.

The two-seat, rear engine, rear-drive Motiv.e uses Gordon Murray Design’s iStream manufacturing technology, designed to provide a purpose-created EV that is both highly efficient and fun to drive but “affordable” – thus addressing another typical EV issue: high price. Gordon Murray Design describes its philosophy as reversing the current industry trend for sub-contracting, by having a complete in-house capability for design, prototyping and development. The concept incorporates elements of “F1 heritage.”

The Motiv.e has a steel frame carrying bonded composite monocoque panels. Weight is understood to be about 730 kg (1609 lb) for a car 2.69 m long (8.8 ft) and 1.48 m (4.9 ft) high. Suspension is all-independent.

Torque-dense motor, novel design

Cheeseman describes the car's motor as “the first of a new range of modular, high efficiency engines." He said the core magnetic motor design can be optimized for use as either a traction machine (predominant feature torque) or in hybrid applications, where high power density is more likely to be required. This is achieved by adjusting the axial length and winding specification.

At present, design iterations exist for e-machines with overall axial lengths of 125 mm and 165 mm (4.9 and 6.5-in, respectively), covering the power range 25 kW to 75 kW (33.5-100 hp).

Efficiency has been optimized by analyzing the entire system as a single unit, including the motor, inverter, control system and the cooling system. Cheeseman explained that as well as reducing weight and packaging volume, this allows greater mechanical and electrical efficiency. The motor and Vocis gearbox were designed simultaneously to allow them to be optimized as a system.

"Having a high-ratio reduction system allows the motor to be designed to run at higher speeds (up to 15,000 rpm), to deliver the same wheel torque as a larger conventional design, but at lower cost,” he told Automotive Engineering

The constraints on this approach are largely mechanical and thermal, rather than electrical, he said. 

“Thermal management is crucial to running harder for longer; we paid a lot of attention to it. Extensive studies of the loss mechanisms within the machine have resulted in a novel design combining both high copper slot fill factors and minimization of AC losses that would be present in conventional hairpin windings when used with high speed, high frequency designs," Cheeseman explained.

Heat created in the copper is transferred to the water jacket, permitting high power densities to be achieved—similar to the importance of adequate cooling of internal combustion engines, Cheeseman noted.

The Vocis transmission is single speed, but the company is working with Zytek to demonstrate a 4-speed (4-SED—4-Speed Electric Drive) driven by two 25-kW motors. It has operational similarities to those of dual-clutch transmissions and can provide up to a 15% improvement in EV efficiency, claimed Cheeseman. It does not require clutches and is basically a simplified solution.

Taking a fresh approach to powertrain design was essential to meet the requirements for Motiv.e and it was equally important that it would fit Yamaha’s role as both developer of and customer for high-efficiency engines with “character” befitting the company’s motorcycle heritage.

To reduce costs, Zytek used high volume off-the-shelf power electronics manufactured by Continental. The motor’s ability to achieve 15,000 rpm is much higher than comparable units, said Cheeseman. This brings several major benefits, notably allowing the electric engine to be smaller, lighter and more cost-effective than previous-generation units, he said.

Peak output is 25 kW, with 15 kW (20 hp) constantly available. Claimed peak torque is 896 N·m (661 lb·ft) with 659 N·m (486 lb·ft) continuously available. Range is expected to be about 160 km (100 mi) and recharge time of its 8.8 kW·h lithium-ion battery is about three hours via a standard Level 1 electrical supply, or one hour on DC fast charge.

New-generation EVCM

Highly efficient, dedicated power electronics are necessary to complement the powertrain. Cheeseman claimed the power electronics set new standards for weight and packaging, noting that the inverter weighs only 7.5 kg (16.5 lb) and by manufacturing everything in-house, Continental has eliminated many of the compromises that stem from using outsourced components.

Their substantial investment in power electronics has delivered a scalable, power-dense and cost-effective product range that is already proven on everything from small city cars to hybrid commercial vehicles, according to Cheeseman. 

The Zytek EVCM (electric vehicle control module) is built on an electronics platform that duals as a development tool and a cost-effective production unit. It complies with all relevant automotive standards. Cheeseman said that unlike other dual-purpose systems that are suitable for production, Zytek's is "cost-competitive with bespoke production technologies.”

It also may be the only EVCM with such a significant level of powertrain-control integration, with thermal management part of the decision-making algorithms. It is a new generation of EVCM that "integrates torque arbitration, temperature control and voltage management to allow better decision making," Cheeseman explained.

The control unit optimizes the driver’s torque request based on a broad range of parameters including battery charge and temperature and available tire grip.

"By integrating these decisions, we can provide more with less to improve both the driving experience and the range while reducing the size, weight and cost of the power electronics and battery pack,” added Cheeseman.

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