When you step on the gas in an ordinary car, most of us have a pretty good idea what happens, having driven them since we were teenagers. But what happens when you depress the accelerator in a Volt? A lot more than you might expect.
The Volt has three power sources: a 111 kW electric motor, a 55 kW motor/generator, and a 62 kW gas engine. Driving conditions dictate when they work together; and the Volt’s unique electric drive system dictates how they work together. The drive unit consists of two motors and a planetary gear set to which the motors are connected by three clutches. Two of the clutches lock the ring gear of the planetary gear set when the Volt runs on just the motor, additionally connecting the motor/generator when more power is needed. When range extension is needed, the third clutch connects the gas engine to the motor/generator to provide additional torque.
The Volt has four driving modes:
- Single-Motor EV Driving: At low to moderate speeds the Volt is powered by its primary electric motor. The ring gear is locked and the motor/generator is decoupled from both the engine and the gear set. In this mode the Volt is a pure electric vehicle, providing high torque and snappy acceleration.
- *Two-Motor EV Driving: At high speed—typically around 70 mph—the main drive motor is approaching 6500 rpm and rapidly losing efficiency. At this point a clutch unlocks the ring gear and couples it to the motor/generator. Both motors now work in parallel and at a slower speed than would be required of a single motor.
- Single-Motor Extended-Range Driving: When the battery reaches its minimum stage of charge—around 30%–the gas engine starts up and the third clutch couples it to the motor/generator, which now functions strictly as a generator to recharge the batteries. The ring gear is locked and the electric motor alone powers the car, while the generator, via the inverter, keeps the batteries at their minimum state of charge.
- Two-Motor Extended-Range Driving: When the battery is low and you still want to run at high speed (>70 mph)—or you need a sudden burst of acceleration—all three clutches kick in, connecting the motor/generator to both the ring gear and the gas engine so that all three power sources are powering the vehicle.
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