Green car buying guide

Diesel cars may seem like an odd choice for a green car-buying guide, but the truth is that modern diesels run cleaner than equivalent gas engine cars. Because diesel engines get 20 percent to 30 percent better mileage than gas engines, they produce 20 percent to 30 percent less carbon dioxide, the major automotive emission classified as a greenhouse gas. Diesel engines produce almost no carbon monoxide, but in the past, nitrogen oxide and particulate emissions caused diesel passenger cars to virtually disappear from U.S. roads.

New emissions systems from both German and Asian automakers substantially clean up the worst components of diesel emissions and have lead to the reintroduction of this type of car to the United States. The new generation of diesels use varying methods to clean up nitrogen oxide emissions, letting them pass emissions tests in all states that follow California Air Resources Board regulations. Mercedes-Benz has been particularly aggressive about getting new diesels on the market, while Honda is also poised to offer diesel versions of its cars. Other automakers, such as Volkswagen, BMW, Mitsubishi, and Nissan also have diesel cars they would like to sell in the U.S.

Biodiesel

Biodiesel is available in blends, going all the way from B5, or five percent biodiesel and 95 percent petroleum diesel, up to B100, or 100 percent biodiesel. No major automaker sells a biodiesel car and most will void the warranty if any blend greater than B5 is used. However, many individuals have used biodiesel blends, and even B100, in standard diesel cars without problems. Biodiesel filling stations can be found on the Department of Energy's Alternative Fuel Locator Web site.

Using biodiesel in a diesel car reduces some emissions significantly. Although carbon dioxide production is the same as with petroleum diesel, proponents point out that this carbon dioxide would be released into the atmosphere anyway. If the organic material used to create biodiesel were allowed to decompose naturally or was used in some other fashion, it would still release carbon dioxide. Carbon dioxide released from organic materials is part of a natural and stable cycle that has been going on for millennia. Burning petroleum diesel, or gasoline, for that matter, releases carbon dioxide that had been trapped, leading to our current problem of global warming.

As for other emissions, biodiesel produces 50 percent fewer hydrocarbons than petroleum diesel, according to EPA certification tests. Nitrogen oxide emissions theoretically go up about 10 percent over petroleum diesel, although those emissions have been shown to clean up considerably when passed through emissions control systems such as a catalytic converter.

See the CNET Insider Secret on using biodiesel for more information.

Straight vegetable oil

Where biodiesel is refined from vegetable oil, some people have converted their cars to run on straight vegetable oil, because it can often be obtained for free, and emissions similar to biodiesel. Like biodiesel, SVO can be used to fuel diesel engine cars. However, because SVO has a tendency to gel, SVO-fueled cars need a separate tank for the SVO, and a preheater to keep it in liquid form. While SVO can often be obtained for free from restaurants that use it for frying foods, it must be filtered to remove any solids before it is put in the tank.