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Transmission specialist Zeroshift has devised a multi-speed gearbox for electric vehicles (EVs) that does not require a clutch.

A damper inside the gear hubs and the electronic control of the motor enable seamless ratio changes, according to the company.

Having more than one gear ratio and the ability to shift without interrupting torque mean the motor runs at higher efficiency, extending the range and battery life expectancy by up to 10 per cent.

The development of the concept is under way using Zeroshift’s proprietary gear-shifting system.

The concept could enable manufacturers to downsize the electric motors of EVs, keeping them running for longer at the medium loads and speeds where peak efficiency of more than 95 per cent is possible.

Bill Martin, Zeroshift’s managing director, 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 per cent.

‘You can use that 10 per cent to improve EVs’ range or reduce the size, weight and cost of battery packs,’ he added.

The new 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; however, the motor then spends more of its time outside of its optimum efficiency range.

Martin continued: ‘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,’ he said.

The improved efficiency offered by Zeroshift’s concept could also extend the in-service battery life of EVs.

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 the life expectancy of batteries.

Improving motor efficiency uses less of the energy stored for a given journey, easing the discharge-recharge cycles and reducing recharge time.

The company is developing its compact transmission with several vehicle manufacturers and 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 the 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 makes shifts virtually unnoticeable to occupants, according to the company.

Zeroshift has carried out extensive damper optimisation using simulation tools correlated against development test results.

Martin said: ‘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 power train of an EV.

‘Conventional automatic transmissions also add bulk and introduce a 10 per cent efficiency loss, wiping out the potential motor efficiency benefits,’ he added.

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