Diesel vs Gasoline Vehicles 

When shopping for a new vehicle, a question many are asking is "should I choose a diesel in order to get good fuel economy?" Based on the U.S. Environmental Protection Agency's 2005 Green Vehicle Guide 1, the short answer to that question is "no," if you aim to be environmentally conscious.

The EPA identifies two types of engine emissions. The first is greenhouse gas, which reflects the emissions of carbon dioxide. Carbon dioxide is released whenever fuel is burned and contributes to the trapping of heat in the atmosphere, a factor in climate change. The second type of emissions is air pollutants that can cause smog and health problems. The se pollutants consist of very fine particulate matter, nitrogen oxides, vapors and other gases. The EPA rates each vehicle with a greenhouse gas score and an air pollution score of 0 to 10, where 10 is best.

The only diesel-powered vehicles listed in the EPA guide are the Mercedes-Benz E-320 and the Volkswagen Golf, New Beetle, Jetta and Passat models. With only one exception (the manual VW New Beetle), all of the diesel models score 4 out of 10 on air pollution versus 7 to 10 out of 10 for the corresponding gasoline models. Because of their better fuel efficiency, the diesels do score somewhat higher than gasoline engines on greenhouse gas, 7 to 9 for diesels versus 6 to 7 for the gasoline engines. Overall, the current diesel engines have an environmental disadvantage to the corresponding gasoline engine models despite their higher fuel efficiency.

The re is a distinction between gasoline and diesel-powered vehicles in the nature of the pollutants in the exhaust. Although both types of engines emit toxic pollutants that can contribute to asthma, allergies and other lung diseases, the higher amounts of fine particulate matter from diesels are of concern. Unfortunately, little is known about the comparative health hazards of emissions from different types of vehicles, or the roles played by different components of emissions (particles, gases, and vapors). Many important studies of toxicity are underway, but the final conclusions are not yet available.

For the future, our choice of engines is not so clear, because fuels and engines are improving in response to upcoming new Environmental Protection Agency regulations. The se regulations are leading to the introduction of new technologies that provide significant decreases in pollutants from both gasoline and diesel engines. The new regulations may result in the pollutants in diesel exhausts becoming comparable to gasoline engines.

To sum up, today's diesel engines are still bigger polluters than their gasoline counterparts. However, the engines are improving rapidly, so it is important to keep track of the changes including what is learned about the toxicity of the pollutants over the next few years.

In order to buy a vehicle today that has excellent fuel economy and the least air pollution, the answer is one of the gasoline-electric hybrid vehicles, such as the Toyota Prius, Honda Civic hybrid or Ford Escape hybrid. The se vehicles all score either 9 or 10 on both air pollution and greenhouse gas ratings.

This article was written by volunteer Bob Levy.

1 U.S. Environmental Protection Agency Green Vehicle Guide, 2005, http://www.epa.gov/greenvehicles

Figure 1. Comparison of emissions from diesel vehicles and equivalent gasoline powered models, on a score of 1-10.