Mode 1: Low-speed EV Propulsion (Engine Off). In this mode, the ring gear is held (locked) by clutch C1. With clutch C2 and C3 disengaged, the generator-motor is decoupled from the engine as well as the planetary gearset. As the traction motor is permanently coupled to the sun gear, the planetary carriers must rotate when the traction motor rotates. Since the planetary carriers are permanently coupled to the final drive, the traction motor propels the vehicle. The generator-motor and the engine are idle during this mode, although the engine is free to start if necessary (example: engine maintenance mode).
Virtually all of the vehicle’s motive power is therefore delivered by the traction motor in this mode, including hard accelerations, using power supplied by the battery pack. With this configuration, the traction motor can produce up to 111 kW (149 hp) and deliver up to 370 N·m (273 ft-lb) of torque.
Mode 2: High-Speed EV Propulsion (Engine Off). As vehicle speed increases, motor speed and losses also increase. To engage both motors and preserve motor efficiency, clutch C1 is disengaged, allowing the ring gear to rotate. At the same time, clutch C2 is engaged, connecting the ring gear to the generator-motor. The generator-motor is then fed current from the inverter, and runs as a motor. The engine remains disengaged from the generator-motor.
This mode allows the two electric machines to operate in tandem at a lower speed than if the traction motor alone was providing torque. The speed of the traction motor in this mode drops to about 3250 rpm from 6500 rpm in the 1 motor mode, according to Fletcher.
This strategy allows the Volt to wring out as much as two extra miles of all-electric operation out of its battery pack, depending on operating conditions. However, switching from low-speed to high-speed EV mode requires the simultaneous operation of two clutches. GM’s experience with simultaneous clutch operation in their two-mode transmissions and transaxles was key to the development of the Volt’s transaxle control strategy.