Citing forecasts compiled by the Energy Information Administration, government officials said they expect hybrid electric vehicles to seize almost 7 percent of the world market by 2020. The hybrids will claim a far larger share than purely electric models (less than 1 percent), natural gas-powered vehicles (1 percent to 2 percent) or any other advanced light-duty vehicles, including turbo direct injection and alcohol-burning models.

Even so, the number of hybrids on the road 20 years from now will pale in comparison to the number of vehicles powered exclusively by internal combustion engines, which will still claim more than 80 percent of the market, the study said. The statistics were presented here this week (Aug. 21-23) at the International Future Transportation Technology Conference, sponsored by the Society of Automotive Engineers.

Engineers attributed the relatively quick growth in popularity of hybrids to rapid advances in technology. New hybrids offer far better gas mileage than they did a few years ago, and they dispel the concerns over limited run time and range that have caused consumers to bypass purely electric vehicles.

General Motors' new Precept, for example, achieves a fuel efficiency rating of 108 miles per gallon. The hybrids introduced just two years ago, by contrast, were closer to 70 mpg.

One key advance in hybrid technology is the advent of parallel propulsion systems. Parallel hybrids simultaneously employ two forms of propulsion. The Precept, for example, uses a 35-kilowatt permanent-magnet alternating current electric motor to drive the vehicle's front wheels and a diesel engine to drive the rear wheels. The diesel engine also supplies up to 10 kW of power to the battery pack, which consists of 28 12-volt nickel-metal hydride batteries. The parallel propulsion scheme differs from the simpler series hybrids, which use the internal combustion engine exclusively to charge the batteries.

Engineers say that parallel hybrids are more efficient because they require fewer energy conversions. Whereas series hybrids convert mechanical energy to electrical and then back to mechanical, parallel hybrids apply mechanical energy directly to the drive train. In an analysis of today's hybrid vehicle market, a researcher at Argonne National Laboratory rated two parallel hybrids — the Precept and the Toyota Prius — as the top two hybrids in terms of gas mileage and driving range.

Many engineers at the conference said they expect gasoline-burning hybrids to give way eventually to fuel cell hybrids. "If you're looking out at the end of the rainbow, you're going to see a hybrid that uses a fuel cell to produce its electricity," said conference chairman William Guentzler of San Diego State University. "Fuel cells are cleaner and quieter than internal combustion engines."

Fuel cell hybrids would burn hydrogen or would use a reformer to extract hydrogen from conventional fuels, such as gasoline. During operation, the fuel cells would create electricity from the hydrogen, store it in a battery pack and use the current from the battery to turn an inverter motor, which would drive the wheels.

The Energy Information Administration study, however, predicts that fuel cells will only garner a sliver of the market — far less than 1 percent — by 2020. The primary reasons cited were high costs and the lack of existing infrastructure for hydrogen fuels.

One consensus was that the key element in favor of hybrid technology will continue to be its driving range. "Most consumers in the United States will never consider an electric vehicle, because they are afraid of 'running out of volts,' " Guentzler said. "With a hybrid you don't have those worries, because you always have the internal combustion engine.

"That's why the hybrid will continue to be so much more popular."