Thursday, May 14, 2009

Multi-Speed Transmission Improves Electric Cars

Vocis has developed a multi-speed transmission that will contribute to the acceptance of electric vehicles, and will display it at EVS, the Electric Vehicle Symposium (to be held 13-16 May in Norway). Simulations show that a two-speed configuration could reduce the battery energy consumed by five to 10 per cent over the industry-standard New European Drive Cycle, without increasing cost or packaging volume; it could also extend battery life and potentially help downsize the power-train and battery pack.

Andy Turner, EV transmissions specialist at Vocis, said: 'Electric motors have a very wide operating range, but that doesn't mean that they are equally efficient at every speed. '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 drive-cycle.' 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 allow a smaller and lighter cooling system.

Battery life is increased as there is less need for deep discharge to deliver the required range; this improves the financial viability of electric vehicles. The heart of the system is a novel gear-shifting concept, based on proven twin-shaft principles, that allows the ratio to be changed with no break in torque delivery. New high-precision electromechanical actuators are being developed, working at traction battery voltage (typically 300V) to improve efficiency.

Electronic control provides full driveline integration, which, combined with multiple ratios, will allow the implementation of alternative calibrations to tailor the feel and performance of the vehicle to the driver's personal preferences. Mike Everitt, managing director of Vocis, said: '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.

'Fundamental benefits of our technology are zero torque-interruption during shifts, very low additional losses compared with a single-speed transmission, scalability to any practical number of ratios, and mechanical robustness through the use of technologies that have all been proven in other applications.' An electric vehicle fitted with a Vocis two-speed transmission will be available for demonstrations before the end of 2009.

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