Recently, General Motors (GM) has faced criticism for shutting down production of its EV-1 electric vehicle. Yesterday at the North American International Auto Show, in Detroit, it unveiled the EV-1's successor, an electric-motor-driven concept car called the Chevrolet Volt. It's the first example of a new "E-Flex" vehicle platform that the company is moving toward production.

The concept car runs off electricity from the grid, stored on board in a battery, for the first 40 miles of a drive. For longer trips, a small gasoline-powered generator kicks on to recharge the battery pack, allowing a total range of 640 miles. In between trips, the battery can be recharged in six and a half hours at an ordinary wall outlet. For longer trips, the generator can provide power at the equivalent of 50 miles per gallon.

GM representatives say their reason for including the gasoline generator is to overcome one of the biggest limitations of the earlier electric vehicle: the short range. The original EV-1 had a range of only about 90 miles, and it required an eight-hour recharge at a dedicated 220-volt electrical outlet. The Volt uses lithium-ion batteries that take up one-third of the space of the EV-1's original lead-acid battery pack, while providing the same total energy storage.

The new vehicle is an example of what's known as a series hybrid, another in a growing type of hybrid-vehicle variant. In conventional hybrid vehicles, such as the Toyota Prius, the car can be propelled by a gasoline engine, a battery-powered motor, or both. In a series hybrid, the gasoline engine has no direct contact with the wheels. It serves only to recharge the batteries. One of the major advantages of such a system is that any source of electricity can be used to charge the battery. This includes electricity from a wall outlet, but also electricity generated on board by a gasoline-, ethanol-, or diesel-powered generator. Because these generators can run at a constant speed, they are more efficient than an engine that has to ramp up and down to meet demand.

GM is developing a combination electric motor, alternator-like generator, and battery pack that can be used with various power sources. For example, an engine that runs on diesel might be preferred in Europe. An engine that runs on ethanol might be favored in Brazil. In a hydrogen fuel-cell version, the battery pack would be smaller and primarily used to provide a boost of power and to recapture energy lost from braking.

Last year at the Los Angeles Auto Show, GM announced work on a plug-in hybrid. (See "GM's Plug-in Hybrid.") As with the Volt, the plug-in hybrid can recharge from a wall socket. But the electric motor will not be the sole source of propulsion: an internal combustion engine could provide bursts of acceleration or power for climbing hills. The plug-in hybrid would not switch among the different sources of energy as easily as the series-hybrid system could.

For both the plug-in and series hybrids, GM says the timeline for commercializing the vehicles will depend on the development of the battery systems. But such systems may not be far off. GM representatives say that they have already seen lithium-ion cells that have the performance required for both plug-in and series-hybrid applications. What remains to be done is to combine these cells into large, complex battery packs and make sure they work well together in an actual vehicle. Last week, GM announced that it has a contract with two sets of companies for building lithium-ion-based battery packs and control systems for plug-in hybrids.

Copyright Technology Review 2007.