The ways the wheels turn
The Chevy Volt is a pure electric, a series hybrid, and a parallel hybrid all rolled into one. The heart of its many modes is an automatic transmission consisting of one planetary gearset and three electronically controlled, hydraulically activated multiplate clutches. This ingenious transmission efficiently blends engine and electric-motor torque to drive the wheels with utmost efficiency. All Volt powertrain components -- two electric motor/generators, the gasoline engine, the aforementioned transmission, and a final-drive differential -- are bolted together in a single unit to save space and weight and to optimize NVH characteristics. To provide 25 to 50 miles of pure-electric operation and 300 or so miles of additional range with the gasoline engine running, the Volt has five distinct operating modes:
1. Battery discharging, acceleration or low-speed cruising: With energy provided from the battery pack, the Volt uses only its larger 149-hp electric motor for cruising below 40 mph or accelerating at any speed. A clutch locks the ring gear of the planetary gearset for a 7.0:1 speed reduction and torque multiplication ratio.
2. Battery discharging, high-speed cruising: Like any energy-conversion device, electric motors are more efficient at certain operating speeds. When the Volt exceeds 40 mph, the clutch holding the ring gear is released. A second clutch engages to connect that ring gear to the smaller of the two electric motors. Now both electric motors draw electricity from the battery pack, and the effective gear ratio is numerically lower, reducing the rpm of the primary drive motor.
3. Battery depleted, acceleration or low-speed cruising: The Volt never empties its battery completely; the gas engine starts when the battery is drained to about thirty percent. A clutch connects the gas engine to the smaller electric generator, which provides electricity to the main electric-drive motor. The ring gear is again fixed for a low overall drive ratio between the motor and the half shafts powering the Volt's front wheels. This series-hybrid regime is used for speeds below 40 mph.
4. Battery depleted, high-speed cruising: The planetary ring gear is again released, shifting the drive motor to a higher gear ratio. The engine continues providing the power to spin the generator, which in turn supplies the drive motor with electric current. In addition, the engine supplies torque to the planetary ring gear through the smaller electric generator; hence, some of the gas/engine power goes to the wheels. Operating in this regime, the Volt is both a series hybrid and a parallel hybrid.
5. Braking: Like all hybrids and electric vehicles, the Volt uses regenerative braking to convert unwanted momentum to electrical energy. When accelerator-pedal pressure is reduced or the brake pedal is applied, the main drive motor temporarily operates as a generator and the electrical current produced restores a portion of the battery's charge. - Eric Tingwall