Scalability is now de rigueur for many aspects of vehicles, and transmissions are no exception. Oerlikon Graziano is looking well into the future as it reveals its second-generation multi-speed transmissions for electric vehicles (EVs).
Claudio Torrelli, the company’s head of product development, explains that the latest evolution features modular architecture and a scalable design, for greater compatibility with a wide range of vehicle sizes—from small city cars to commercial vehicles.
Oerlikon Graziano's previous-generation design, for a high-performance GT car, used two 240-N·m (177-lb·ft) electric motors and electrohydraulic actuators for the shift mechanism. But the second generation focuses on A- and B-segment cars. It continues to use a pair of motors but each with between 64 and 75 N·m (47-75 lb·ft) and peak power output between 50 and 70 kW (76-94 hp). Shift actuation is electromechanical via an electric motor and barrel cam.
Typical production EVs use a single-speed transmission, relying on the torque spread of the electric motor to provide adequate performance. But this means the motor spends much of its time operating at only 60-70% efficiency, Torrelli noted. The new transmission is a 4-speed unit, which allows two smaller motors to run close to their peak efficiency of around 90% for a greater proportion of time.
“Overall energy consumption is reduced by up to 15%, which translates into increased range or improved performance," he told AEI, adding that improving the cost, range and performance of an electric drivetrain is a continuing challenge.
“The multi-speed transmission provides better laden pull-away, improved top speed, and greater hill-climbing ability for a given motor size. Alternatively, a smaller motor can be specified, to give the same performance with less battery drain, providing greater range,” he explained.
With a modular architecture, the motors can be installed parallel to each other or on opposite sides of the transmission, to suit vehicle installation priorities, Torrelli said. Both parallel and opposing arrangements share high levels of component commonality, including input shafts, gear selection actuation, and differential assemblies. The gearbox is arranged in two-stages with helical gears and uses splash lubrication and “the minimal number of meshes and bearings for maximum efficiency," he revealed.
The scalability of the modular concept makes the design suitable for a wide range of vehicle types, Torrelli said. Compact, low-cost electric city cars can use an electric front-or-rear axle configuration in isolation. A desirable 4wd hybrid can be created by the addition of an electrically driven axle assembly to a performance car, luxury car, or supercar, while city bus or truck applications have also been catered for.
"Even a series/parallel hybrid arrangement with only two driven wheels can be accommodated by the inclusion of a single clutch within the combined powertrain,” he noted.
Torrelli describes the system’s configuration of the transmission as “unique.” It uses two input shafts, each driven by its own electric motor, following dual-clutch transmission (DCT) principles but with two motors instead of twin clutches. One motor drives a shaft that carries first and third gears; the other drives a shaft carrying second and fourth. This set-up allows pre-selection of the next gear before the previous one has been disengaged, using the two motors to synchronize shaft speeds so that no synchronizers are needed.
Following gear selection, the drive torque is transferred “seamlessly” between the motors to complete the shift. “Sharing torque between the motors offers shift opportunities beyond those achievable with a DCT because of the enhanced response, compared to an internal combustion engine,” explained Torrelli. “The control software is also simpler than for a DCT application.“
The shift control system was developed by Vocis Driveline Controls, part owned by Oerlikon Graziano. Vocis used a range of existing algorithms from their software library, covering driver strategy, shift sequencing, gear actuation, and safety functions complying with OBD II requirements.