Plug in hybrids

In plug-in hybrids, a large battery pack that is recharged by plugging it in stores enough energy to power a car entirely, or almost entirely, with electricity for the first 40 miles or so of driving. For longer trips, the car reverts to conventional hybrid operation, relying largely on gasoline for power but improving efficiency: by storing energy from braking in the battery and using it for acceleration, for example.
The study shows that if plug-in hybrids are adopted widely in the United States, and if measures are taken to clean up power plants, by 2050, plug-in hybrids could reduce carbon-dioxide emissions by 612 million metric tons, or roughly 5 percent of the total U.S. emissions expected in that time frame, according to Marcus Sarofim, a researcher at MIT's Joint Program for the Science and Policy of Global Change. That's a significant amount, he says, considering that transportation accounts for only about a third of the total greenhouse-gas emissions.