Generally, gasoline engines produce significantly fewer pollutants than diesel engines. In the past, diesel engines have put out much more soot and nitrogen oxides than comparable gasoline engines, although new U.S. vehicle requirements for 2009 may eliminate some of the gap between the two fuel types.
According to the Union of Concerned Scientists, diesel engines have historically emitted 10 to 100 times more soot than comparable gasoline engines. Soot can cause lung cancer and also increases the risk of heart attack and other health problems.
Diesel engines typically emit twice the level of nitrogen oxides (NOx) compared with that of gasoline engines. Nitrogen oxides are a key factor in the development of warm-weather "smog" in urban areas.
A diesel engine burns fuel quite efficiently, so it generally produces somewhat less carbon monoxide than a similar gasoline engine.
Diesels get better per-gallon mileage than gasoline engines, as diesel has about 14 percent more energy per gallon than gas and diesel engines are also more efficient. However, it takes more petroleum to make diesel fuel, so there really isn't much of an overall advantage in lowering carbon dioxide (CO2) emissions when using regular diesel.
Biodiesel blends (2 to 20 percent biodiesel) significantly reduce levels of air hazards such as particulate matter (soot), air toxics such as polycyclic aromatic hydrocarbons (PAHs) and sulfur dioxide. The use of a B20 blend (20 percent biodiesel) significantly reduces the emission of the global warming gas carbon dioxide when compared with the use of regular diesel, giving an overall reduction advantage when compared with gasoline combustion.
