Multi-speed gearbox enhances performance of electric vehicles

Paul Boughton

Transmission specialist Zeroshift has devised a multi-speed gearbox for electric vehicles (EVs) that needs no clutch; it is claimed that a combination of a damper inside the gear hubs and electronic control of the motor make ratio changes seamless. Having more than one gear ratio and the ability to shift without interrupting torque means the motor runs at higher efficiency, thereby extending range and the batteries' life expectancy by up to 10 per cent.
Development of the concept is underway using Zeroshift's proprietary gear-shifting system. The innovation could enable manufacturers to downsize the motors in EVs, keeping them running longer at the loads and speeds where peak efficiency of over 95 per cent is possible. For drivers, that could mean more performance, extended range and longer battery life.
Zeroshift's managing director Bill Martin says: "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 per cent. You can use that 10 per cent 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, both of which are issues that have so far deterred EV manufacturers from fitting multi-speed gearboxes. Current EVs instead use a single-speed transmission, but the motor then spends more of its time outside its optimum efficiency range.
Martin adds: "The benefits of Zeroshift's transmission would be greatest for electric delivery vehicles where the compromises in efficiency are greatest. 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."
The improved efficiency offered by Zeroshift's concept could also extend EVs' in-service battery life. Current range limitations often lead EV drivers to drain the cells to the maximum allowable level of discharge in order to reach their destination; the deeper cycling affects batteries' life expectancy. Improving the motor efficiency uses less of the energy stored for a given journey, thereby easing the discharge-recharge cycles and reducing recharge time.
Zeroshift 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. To provide the required levels of shift refinement, Zeroshift's concept uses electronic control of the electric motor or motors to match the shaft speeds, together with 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 make shifts virtually unnoticeable to vehicle occupants.
Zeroshift has carried out extensive damper optimisation using simulation tools correlated against development test results. Martin states: "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.
"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 per cent efficiency loss, wiping out the potential motor efficiency benefits."
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