Iconic U.S. car company General Motors turned a page in its history on a cold day this winter in suburban New Jersey. It sold the first production version of its Chevrolet Volt to a retail buyer—a retired pilot named Jeffrey Kaffee. With that sale comes the start of the first real test of consumer appetites for two concepts long in the making: hybrid cars whose battery you can recharge by plugging them in at home, and so-called series hybrid technology.
The Volt is the world's first production series hybrid-electric vehicle. Like a conventional hybrid, it has both an electric traction motor and a gasoline engine. Once its 16-kilowatt-hour battery pack is depleted, the 1.4-liter 4-cylinder engine switches on but does not drive the wheels mechanically. Instead, it turns a 55-kW generator that provides current to the 111-kW electric motor that powers the front wheels. It's not a new concept—minus the battery pack, that's the same way diesel locomotives work. But until now, all hybrid cars have used their gasoline engines in parallel with their electric motors, combining their torque to turn the wheels.
The Volt's series-hybrid credentials came into question briefly last fall when GM power-train engineers revealed that in one mode, its engine directly contributes torque to the final drive. In other words, a Volt is sometimes a parallel hybrid, too. Crucially, in GM's view, it does not offer direct mechanical drive to the wheels. Instead, engine torque is transmitted through the generator—locked by clutches on both ends—into a set of gears that work only if they simultaneously receive torque from the electric motor.
For North America, where daily commuting distances are higher than in Europe or Asia, GM chose a series hybrid because it felt the design offered the best combination of electric use and limitless range. As the company points out, 78 percent of U.S. vehicles travel less than 64 kilometers (about 40 miles) per day—the pure electric range that it attributes to the Volt. Beyond that, the gasoline tank and combustion engine act as a backup battery.
While the Volt may be the first series hybrid, more are coming. The next one will be the 2011 Fisker Karma, a luxury sports sedan from the venture-funded car company started by former BMW designer Henrik Fisker. The Karma uses a 2.0-L 4-cylinder engine to generate current that drives a pair of 150-kW motors to power the rear wheels. Other makers plan even more complex hybrid systems that can operate in series-hybrid mode at certain times, parallel at others. Most notable of these may be Audi's planned A1 e-Tron, a subcompact hatchback that uses a tiny Wankel engine as its range extender. Even more esoteric, Jaguar's C-X75 concept car uses a pair of microturbines as its range extender.