Cleanse your mind of what you think you know about the Chevy Volt. It is not just an electric car. It does not operate as implied by GM for the past three years. Now that production has begun and customer deliveries are about to commence, the secrets are out. A true explanation of exactly what makes the wheels turn can finally be told.
According to Larry Nitz, GM’s executive director of electric and hybrid powertrain engineering, this bit of bait and switch was necessary to safeguard the intellectual properties necessary to make the Volt a true technological leap forward. During the past three-plus year of engineering and development, GM applied for several patents to protect many small and large inventions. Those patents have been issued and Nitz is finally comfortable speaking frankly about what’s under Volt’s hood.
The Volt concept car, which debuted at the 2007 North American International Auto Show in Detroit, had an AC motor connected to and permanently driving the front wheels. A turbocharged 1.0-liter 3-cylinder gasoline engine powered a generator which supplied current to the drive motor, but only after the onboard lithium ion battery pack was depleted. This system, which GM called E-Flex, provided an electrical connection between the two portions of the powertrain but no mechanical connection.
When the evolution of the promising concept into a practical production car began in earnest, GM engineers immediately ventured beyond E-Flex. Maintaining a strong emphasis on electric propulsion, they added parts and increased the role of the gasoline engine. The finished product-now called Voltec–is much closer to a Toyota Prius than a Nissan Leaf. Volt is the first extended-range electric car, as advertised, and more.
Secret Gears and Clutches!
1. The previously undisclosed heart of the matter is an automatic transmission consisting of one planetary gear set and three electronically controlled, hydraulically activated multi-plate clutches. This ingenious transmission efficiently blends electrical and mechanical attributes to drive the wheels with utmost efficiency. All Volt powertrain components-two electric motor-generators, the 1.4-liter 84-hp 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.
Volt has two of practically everything: Two forms of portable energy (gasoline and electricity). Two electric motor-generator machines. Two primary modes of operation-using electricity stored in the lithium-ion battery pack or the energy available from 9.3-gallons of premium gasoline carried in a sealed tank. And two distinct transmission ranges. That yields a Rubik’s cube of driving possibilities. To provide 40 or so miles of pure-electric operation and 300 or so miles of ‘extended’ range with the gasoline engine running, the Volt has five distinct operating routines.
1. Leaving home with a fully charged battery pack provides 40 or so miles of driving range with the engine and generator inoperative. The 149-hp permanent-magnet AC motor draws electricity from the 16kWh battery pack to drive the front wheels through a 7.0:1 speed reducer/torque multiplier.
2. Like any energy-conversion device, electric motors are more efficient at certain operating speeds. When the Volt exceeds fifty mph, a clutch holding the planetary gear set’s 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 drive ratio shifts to 2.16:1. Top speed is governed at 100mph.
Hybrid Gasoline and Electric Propulsion
3. When the Volt’s battery is depleted to a 30-percent or so state of charge, the gasoline engine is automatically started to give the battery a rest. The clutch connecting the gasoline engine to the smaller electric machine closes so that this device, now acting as a generator, can provide electricity to the main electric-drive motor. Another clutch holds the ring gear fixed, yielding a 7.0:1 overall drive ratio between the motor and the half shafts powering the Volt’s front wheels. This regime is used for speeds below 30 mph.
4. At higher cruising speeds, the planetary ring gear is again released, shifting the AC motor drive ratio to 2.16:1. 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 machine. (This is the engine driving-the-wheels scenario heretofore denied by GM.) Operating in this regime, the Volt is both a series hybrid and a parallel hybrid.
5. Like all hybrids and electric vehicles, the Volt uses regenerative braking to convert unwanted momentum to electrical energy. When accelerator pedal pressure is reduced and/or the brake pedal is applied, the main drive motor temporarily operates as a generator and the electrical current so produced partially restores the battery’s state of charge.
To trump both the Prius and the Leaf, Volt combines their merits in one handy advanced-technology sedan. It employs cheaper and cleaner electrical energy drawn from the grid. It provides efficient electric drive without the usual compromises. It uses gasoline intelligently in a supporting role. It is a pure electric, a series hybrid, and a parallel hybrid all rolled into one.
That begs a daunting question: is Volt the right car for the times or another technological flight to nowhere? The only answers that matter will come from consumers — both those who buy a Volt and report their level of satisfaction and also those who kick the tires but ultimately spend their dollars elsewhere.