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25/08/2010
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Transmission specialist Zeroshift has devised a multi-speed gearbox for electric vehicles (EVs) that needs no clutch - a damper inside the gear hubs and electronic control of the motor make ratio changes.
Having more than one gear ratio and the ability to shift without interrupting torque means the motor runs at higher efficiency, according to Zeroshift, extending range and batteries' life expectancy by up to 10%.
Development of the concept is underway using Zeroshift's proprietary gear-shifting system. The company believes the innovation could enable manufacturers to downsize EVs' electric motors, keeping them running longer at the medium loads and speeds where peak efficiency of over 95% is possible.
Zeroshift's managing director Bill Martin said: "Our studies suggest that by using a compact, multi-speed transmission and a smaller electric motor, manufacturers can gain an operating efficiency of up to 10%. You can use that 10% to improve EVs' range or reduce the size, weight and cost of battery packs."
Zeroshift's concept prevents any torque interruptions during ratio changes and does not require a clutch, issues that have so far deterred EV manufacturers from fitting multi-speed gearboxes. Current EVs use a single-speed transmission, but the motor then spends more of its time outside of its optimum efficiency range.
"The benefits of Zeroshift's transmission would be greatest for electric delivery vehicles where the compromises in efficiency are greatest," said Martin. "To be able to pull away fully laden on hills requires low gearing, but the motor then runs too fast and too inefficiently on the highway."
Current battery range limitations often lead EV drivers to drain the cells to the maximum allowable level of discharge in order to reach their destination and the deeper cycling can affect batteries' life expectancy.
Improving the motor efficiency uses less of the energy stored for a given journey, easing the discharge-recharge cycles and reducing recharge time.
Zeroshift says it is developing its compact transmission with several vehicle manufacturers. The company is also working with consultancy firms and manufacturers on the integration of its technology into the next generation of seamless layshaft automatic transmissions for hybrid medium-duty trucks, buses and passenger cars.
The company's technology replaces the synchromesh in a conventional manual gearbox with paired interlocking rings that change ratios without interrupting the torque. It uses electronic control of the electric motor or motors to match the shaft speeds and an integrated passive damper system within the drive hub to isolate any vibrations. The combination of sealed pockets of silicone fluid and mechanical compression springs are said to make shifts virtually unnoticeable to occupants.
Zeroshift has carried out extensive damper optimisation using simulation tools correlated against development test results. "By varying parameters such as fluid properties, peak pressures and end-of-travel speeds, we can tune the damper's performance to suit different applications," added Martin. "Until now, none of the transmission alternatives provided a satisfactory option for EVs. Conventional manual transmissions interrupt the drive to the wheels during gear shifting and require a clutch, adding cost and bulk to the tightly packaged powertrain of an EV. Conventional automatic transmissions also add bulk and introduce a 10% efficiency loss, wiping out the potential motor efficiency benefits."
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Author
Chris Shaw
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