Clutchless multi-speed gearbox

Look, no clutch

Multi-speed transmission changes gear without interrupting torque

Who

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 seamless. Having more than one gear ratio and the ability to shift without interrupting torque means the motor runs at higher efficiency, extending range and batteries’ life expectancy by up to 10%.

Technology

Development of the concept is under way using Zeroshift’s proprietary gear-shifting system. The innovation could enable manufacturers to downsize EVs’ electric motors, keeping them running longer at the medium loads and speeds where peak efficiency of more than 95% is possible. For drivers, that could mean more performance, greater range and longer lasting batteries.

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 instead use a single-speed transmission, but the motor then spends more of its time outside of its optimum efficiency range.

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, easing the discharge-recharge cycles and reducing recharge time.

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 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 make shifts virtually unnoticeable to occupants.

Application

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.

Bill Martin, Zeroshift’s managing director, 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%. You can use that 10% to improve EVs’ range or reduce the size, weight and cost of battery packs.

“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.”