A new National Research Council report finds that by the year 2050, the U.S. may be able to reduce petroleum consumption and greenhouse gas emissions by 80 percent for light-duty vehicles -- cars and small trucks -- via a combination of more efficient vehicles; the use of alternative fuels like biofuels, electricity, and hydrogen; and strong government policies to overcome high costs and influence consumer choices. While achieving these goals will be difficult, improving technologies driven by strong and effective policies could make deep reductions possible. "To reach the 2050 goals for reducing petroleum use and greenhouse gases, vehicles must become dramatically more efficient, regardless of how they are powered," said Douglas M. Chapin, principal of MPR Associates, and chair of the committee that wrote the report."In addition, alternative fuels to petroleum must be readily available, cost-effective and produced with low emissions of greenhouse gases.Such a transition will be costly and require several decades.The committee's model calculations, while exploratory and highly uncertain, indicate that the benefits of making the transition, i.e. energy cost savings, improved vehicle technologies, and reductions in petroleum use and greenhouse gas emissions, exceed the additional costs of the transition（转型期） over and above what the market is willing to do voluntarily."

Improving the efficiency of conventional vehicles is, up to a point, the most economical and easiest-to-implement approach to saving fuel and lowering emissions, the report says.This approach includes reducing work the engine must perform -- reducing vehicle weight, aerodynamic（空气动力的） resistance, rolling resistance, and accessories -- plus improving the efficiency of the internal combustion engine powertrain.

Improved efficiency alone will not meet the 2050 goals, however.The average fuel economy of vehicles on the road would have to exceed 180 mpg, which, the report says, is extremely unlikely with current technologies.Therefore, the study committee also considered other alternatives for vehicles and fuels, including:

·hybrid electric vehicles, such as the Toyota Prius;

·plug-in hybrid electric vehicles, such as the Chevrolet Volt;

·battery electric vehicles, such as the Nissan Leaf;

·hydrogen fuel cell electric vehicles, such as the Mercedes F-Cell, scheduled to be introduced about 2014; and

·compressed natural gas vehicles, such as the Honda Civic Natural Gas.