Zeroshift’s multi-speed transmission concept changes gear without interrupting the torque, allowing EVs to run for longer at peak efficiency. Click to enlarge.

UK-based 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 enables the motor to run at higher efficiency, extending range and batteries’ life expectancy by up to 10%, according to the company.

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 a 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. Zeroshift has carried out extensive damper optimization 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.

—Bill Martin, Zeroshift’s managing director

Development of the concept is underway using Zeroshift’s proprietary gear-shifting system. The new gearbox 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.

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. However, with a single-speed transmission prevalent in EVs, the motor 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.

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 percent. You can use that 10 percent 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.

—Bill Martin

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.