Downsizing is the buzzword within the conventional powertrain industry, but now it is also being applied to electric drive systems for EVs. Engineers at U.K. transmission specialist Zeroshift believe that a route toward achieving this is to use a multispeed gearbox instead of the single-speed types now in general use.
The potential problem with multispeeds for EVs is the effect of torque interruption between shifts. But Zeroshift, which specializes in the development and application of power transmission and electronic control solutions, is developing a clutchless, multispeed transmission application of its proprietary gear shifting system that is claimed to meet the challenge.
Instead of a clutch, the system has a damper within the gear hubs together with electronic control of the motor. The configuration delivers shift quality that company engineers consider to be virtually seamless.
Managing Director Bill Martin believes the application of the technology could enable manufacturers to downsize EV electric motors and run them in the strata of medium speeds and loads, at which around 95% peak efficiency can be achieved.
The Zeroshift technology replaces the synchromesh of a conventional gearbox with paired, interlocking rings that change the ratios without causing torque interruption. Electronic control of the motor (or motors, depending on application) matches the shaft speed. An integrated passive damper system within the drive hub is used to isolate any vibration.
Martin explained that by varying parameters such as fluid properties, peak pressures, and end-of-travel speeds, it is possible to tune the damper’s performance to suit various applications. Zeroshift also uses sealed pockets of silicone fluid and mechanical compression springs to achieve shifts that Martin describes as “virtually unnoticeable” to the vehicle’s occupants.
He also stated that the company’s studies suggest that using a compact, multispeed transmission and a smaller electric motor improves operating efficiency by 10%. That gain "can be used to improve an EV’s range or to reduce the size, weight, and cost of battery packs,” Martin added.
Further benefits of the system would include an extension of in-service battery life. At present, range limitations may result in EV drivers depleting their vehicles' battery cells to maximum allowable discharge level. Such deep cycling can diminish battery life.
Improving motor efficiency uses less of the energy stored for a given journey, thus easing discharge/recharge cycles and potentially reducing recharge time.
As well as pure-EV applications, Zeroshift is working with manufacturers on its integration into the next generation of seamless layshaft automatic transmissions for hybrid vehicles, both passenger and commercial.
“Until now, none of the transmission alternatives have provided a satisfactory option for EVs," explained Martin.
He noted that conventional manual transmissions interrupt the drive to the wheels during gear shifting and require a clutch, thus adding cost and bulk to the tightly packaged EV powertrain. And conventional automatic transmissions also add bulk and introduce a 10% efficient loss, wiping out the potential motor efficiency benefits. Zeroshift may offer an alternative to both, as the industry's EV development continues to expand.