Find the green car that is just right

Looking to reduce your carbon footprint? Your car is an obvious place to start. While standard, gas-fueled cars dominate the roads, there are a few current alternatives and some promising future technologies.

GM made big news when it showed off the Chevrolet Volt concept car at the 2007 Detroit auto show. The Volt is an electric car with an onboard electricity generator designed to improve its driving range, otherwise known as a range extender vehicle. Ford has also developed an REV platform called Hy-Series, and has shown it off in concepts at auto shows. REVs have a full electric powertrain, with one or more electric motors driving the wheels and drawing power from a battery. An onboard generator, such as a small gas engine or a hydrogen fuel cell, generates electricity to recharge the battery as needed. REVs are also designed with plug-in capability to minimize the use of the onboard generator, thereby reducing emissions.

REVs are sometimes called series hybrids, as the various power sources run in a line from the electric motor to the range extending device. Current hybrids on the road today are referred to as parallel hybrids, as both the electric motor and gas engine can send power to the wheels. Public interest in the Volt led GM to promise a production model sometime about 2010.

Read about the Chevrolet Volt.

Fuel cell vehicles are another form of the electric car. Because standard electric cars have such short range, carmakers have researched using fuel cells to generate extra electricity from hydrogen. Fuel cells draw electricity from the reaction when hydrogen combines with oxygen to form water, making water the only emission from a fuel cell vehicle. Almost every major automaker has a fuel cell research program, which has led to good gasoline-equivalent performance from the prototypes, with ranges in excess of 300 miles.

But the hydrogen car is also imperfect. Although hydrogen is one of the most abundant elements, it's generally not found in pure form and it takes energy to separate it from other elements to which it has bonded. Currently, generating pure hydrogen uses more energy than that contained in the hydrogen, although there is a benefit to having a transportable fuel. Research into more efficient ways of generating hydrogen is ongoing. There is also very little hydrogen infrastructure, as filling stations exist in only a few locations to support research vehicles.

Read about the Honda Clarity.

Most automakers have put working fuel cell vehicles in the hands of various government or scholarly bodies so they can get some real-world testing. Both GM and Honda have gone even further, putting their fuel cell cars in the hands of consumers. GM has placed 100 Chevrolet Equinox fuel cell cars with consumers as part of its Project Driveway test, while Honda is leasing its Clarity fuel cell vehicle to a select set of consumers. Both of these programs are confined to specific geographic areas, and the automakers choose the participants.

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