Electric vehicle transmission uprates performance and range 06 September 2011

Prototypes of a multi-speed transmission for electric vehicles (EVs) have begun on-road testing with an unnamed European vehicle manufacturer.

Designed and manufactured by Italian drivetrain specialist Oerlikon Graziano, with British control systems specialist Vocis, the transmission is aimed at increasing vehicle range or allowing reduced battery pack size – as well as providing improved low-speed pull away and higher-speed cruising.

Vocis technical director Richard Taylor explains that the wide operating range of a typical electric motor – which provides maximum torque from zero rpm – has led EV producers to use a single transmission gear ratio.

While reducing cost and packaging volume, the compromise is reduced efficiency and performance, he confirms.

"Electric motor efficiency drops off at low load levels and towards the extremes of speed," says Taylor. "Multiple gear ratios with electronic control allow the motor to be kept in the region of greatest efficiency for a much higher proportion of the time, allowing significant range extension."

The extra ratios also remove the compromise between top speed and hill-climbing ability. "An electric vehicle will typically have a transmission ratio that is higher than the ideal, simply to give it enough top speed," says Taylor.

"With multiple ratios, we can provide much better laden pull-away, as well as improved top speed without increasing the powertrain size," he adds.

The heart of the new transmission is a novel gearshifting concept based on principles similar to those used in a DCT – a transmission type with which Vocis has considerable expertise.

Unlike most EV powertrains, which use a single e-machine (motor/generator), the Vocis multi-speed transmission works with two small e-machines, each on an independently-controlled shaft.

Taylor says that, because the transmission technology is readily scalable – including in terms of the number of ratios – it is easily applied to a diverse range of vehicles.

"For example, the priority for a delivery van would be to improve fully laden performance during pull away or hill climbing. [It will also] benefit from the significant improvement in range," he says.

Author
John Challen

Related Companies
Oerlikon Graziano SpA
Vocis Ltd

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