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Dual drive gives seamless shift

01 May 2012

Oerlikon Graziano reveals more about its innovative multi-speed transmission for electric vehicles (EVs), which uses the principles of dual clutch transmissions to provide seamless shifting and up to 15 percent improvement in vehicle efficiency.

Most current EVs use a single-speed transmission, relying on the electric motor’s torque spread to provide adequate performance. However, this means the motor spends much of its time operating at only 60 to 70 percent efficiency. Oerlikon Graziano’s new transmission uses instead two smaller motors to provide four speeds, running the motors closer to their peak efficiency of approximately 90 percent. The result is an overall efficiency improvement of up to 15 percent, increasing range or, alternatively, improving vehicle performance, as Oerlikon Graziano’s head of product development, Claudio Torrelli explains:

“Vehicle manufacturers need innovative solutions to improve the cost, range and performance of electric drivetrains. Our multi-speed transmission can improve acceleration, top-speed and hill-climbing ability for a given motor size. Alternatively EV makers can specify smaller motors to give the same performance with less battery drain, providing greater range.”

The transmission’s novel configuration uses two input shafts, each driven by its own electric motor. The concept is similar to a dual clutch transmission (DCT) but using two motors in place of the twin clutches; one motor drives a shaft that carries first and third gears, the other drives a shaft carrying second and fourth. This allows pre-selection of the next gear before the previous one has been disengaged, using the two motors to synchronise shaft speeds so that no synchronisers are needed.

Following gear selection, the drive torque transfers seamlessly between the motors to complete the shift. Different strategies prioritise economy or performance and the transitions between the two.

The shift control system was developed in the UK by Vocis Driveline Controls, itself part owned by Oerlikon Graziano. Basing the work on its previous experience with DCT applications, Vocis used existing algorithms from its software portfolio to cover ‘driver strategy’, ‘shift sequencing’, ‘gear actuation’ and necessary OBDII compliant safety functions.

The way that the transmission shares torque between the motors offers shift opportunities beyond what is achievable with a DCT, claims Vocis technical director, Richard Taylor. “The shift strategy is so successful that test drivers find some shifts imperceptible and must refer to data logs to analyse each event.”

The software is simpler than for a DCT application. The engine interface is replaced by a motor interface while the clutch and synchroniser control algorithms are unnecessary and are deleted entirely. The prototype uses a Vocis TMS-20 controller that, in addition to direct inputs and outputs, allows robust CAN communication with other controllers on the vehicle to ensure seamless shifting and intuitive control.

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