Multi speed electric transmission improves performance

A multi speed electric vehicle transmission is being developed by Vocis.

Simulations show that a two speed configuration could reduce the battery energy consumed by 5 to 10 % over the New European Drive Cycle, without any significant increase in overall cost or packaging volume, as well as extending battery life and providing the potential to substantially downsize the powertrain and battery pack.

"Electric motors have a very wide operating range, but that doesn't mean that they are equally efficient at every speed," explains Vocis EV transmissions specialist Andy Turner. "The torque curve of a typical traction motor is well suited to vehicle propulsion, having maximum torque from zero speed and a wide constant power region. However, there is a 'sweet spot', typically at medium speed and medium to high loads, where the delivery of power is most efficient. A choice of gear ratios allows the motor to be kept in this operating region during more of the drivecycle."

There are many benefits to this strategy. Because operating efficiency is increased, range can be substantially improved or the battery capacity can be decreased, reducing weight, cost and recycling issues. The availability of a low gear ratio for pull-away and for climbing gradients (especially important for laden commercial vehicles) allows the size, cost and weight of the motor to be reduced and this and the increased motor efficiency allows a smaller, lighter cooling system. Battery life can also be improved because there will be less need for deep discharge to deliver the required range, so greatly improving the financial viability of electric vehicles.

The heart of the system is a novel gearshifting concept, based on proven twin-shaft principles, that allows the ratio to be changed with no break in torque delivery. New electromechanical actuators are being developed, working at traction battery voltage (typically 300V) to allow improved efficiency. Electronic control provides full driveline integration which, combined with multiple ratios, will also allow the implementation of alternative calibrations in order to tailor the feel and performance of the vehicle to the driver's personal preferences.

"This is not the first two speed EV transmission concept, but it is the first that overcomes the issues that have so far prevented their successful introduction," says Vocis managing director Mike Everitt. "Fundamental benefits of our technology are zero torque interruption during shifts, very low additional losses compared with a single speed transmission, scaleability to any practical number of ratios, and mechanical robustness through the use of technologies that have all been proven in other applications."

A development electric vehicle fitted with a Vocis two-speed transmission will be available for demonstrations before the end of 2009. "We are investing a lot of money in proving this concept, which we believe will significantly improve the viability of electric vehicles with minimal impact on their overall cost," concludes Everitt.

Tom Shelley

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