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08/09/2011
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Prototypes of a novel, multi-speed transmission for electric vehicles (EVs) have begun on-road testing with a European vehicle manufacturer.
 Designed and manufactured by Italian drivetrain specialist Oerlikon Graziano with British control systems specialist Vocis, the simple, elegant concept will increase vehicle range or allow reduced battery pack size, as well as providing improved low-speed pull away and higher-speed cruising.
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. Whilst reducing cost and packaging volume, the compromise is reduced efficiency and performance. "Electric motor efficiency drops off at low load levels and towards the extremes of speed," explains Vocis technical director, Richard 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 EV 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."
The heart of the new transmission is a novel gearshifting concept based on principles similar to those used in a DCT, a transmission type in 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.
The design allows the distribution of drive and recovered energy to be balanced between the two e-machines, with seamless changing provided by torque infill during each shift. "The torque infill makes gearshifts so imperceptible that the first customer for the system asked to be shown that shifting was taking place because he couldn't feel it when driving," says Taylor.
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Author
John Challen
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