DETROIT -- The number of vehicles with diesel or hybrid powertrains will more than double over the next seven years, according to J.D. Power and Associates.

Alternative powertrain vehicles will make up a combined 11 percent of the U.S. market by 2012, a jump from the 4.8 percent slice today, Power says.

Hybrid vehicles, which combine an internal combustion engine with an electric motor, accounted for 0.5 percent of U.S. sales in 2004. But that could rise to 3.5 percent by 2012, the consulting firm forecasts.

Diesels could more than double their share of the market, from 3 percent in 2004 to 7.5 percent in 2012.

The growth will be spurred, in part, by an increase in the number of hybrid or diesel models available. The study predicts the number of hybrid models on sale will increase from 10 this year to 44 by 2012. The number of diesel offerings is expected to swell from 14 to 26 during the same period.

High gasoline prices will be a catalyst in the increased demand for alternative powertrain vehicles, said Anthony Pratt, senior manager of global powertrain forecasting at J.D. Power-LMC Automotive Forecasting Services.

"We anticipate this will lead to dramatic growth, particularly with diesels over the next several years," Pratt said.

He added that SUVs and mid-sized cars are the most likely candidates to get hybrid powertrains, while the growth in diesels will be concentrated on SUVs, pickups and luxury vehicles.

Hybrid and diesel powertrains do face challenges. Chief among them is extra cost. The price premium "continued to be a prohibiting factor that will limit the potential for both hybrid- and diesel-powered vehicles," Pratt said.

Automakers must find ways to cut the cost penalty to attract more buyers to the fuel-saving powertrains, he noted.

Diesels also face the challenge of meeting the EPA's stricter emissions standards scheduled to go into effect in 2007 and 2008.

Alternative powertrain vehicles will make up a combined 11 percent of the U.S. market by 2012, a jump from the 4.8 percent slice today, Power says.