Electric driving

By John Donovan, Low-Power Design, for Drive for Innovation

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.

Drive Architecture

The Volt 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. This results in increased efficiency and thus greater range.

–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. You can then fully charge them when you get home at night.

–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.

The engine is never ‘directly’ connected to the driveshaft, though that is splitting hairs a bit since in this mode, it’s directly connected to the motor/generator’s shaft. On the other hand the engine can never power the vehicle by itself because of how the gear set is configured. In order for the planetary gear set to deliver torque, two of its three components must be driven or one locked, with the third element propelling the vehicle. That third element is always the main traction motor.

Give Me A Brake

Braking a 4500 lb. vehicle from 60 mph to a dead stop using friction brakes represents a huge waste of energy. In addition to conventional disk brakes, the Volt uses regenerative braking to convert mechanical to electrical energy. When you let up on the accelerator or step on the brake, the main drive motor operates as a generator to help top up the charge in the battery bank.

At the same time it supplies a reverse torque force to the front wheels, slowing the vehicle down. If you’re doing a lot of mountain driving, regenerative braking can go a long way toward keeping your batteries charged. However the regenerative brakes don’t have much stopping power, so it’s still the disk brakes that actually bring you to a stop. It turns out that even the braking system is a hybrid—though once again, well integrated and thought out.

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