Prior to the 2010 model year (MY), some studies showed contradictory results for emissions of nitrogen oxides with B20 compared to diesel fuel. With new emission control technologies, this is no longer an issue, because diesel fuel burns as cleanly as B20 in MY 2010 and newer engines. In older engines, biodiesel blends may offer some additional emissions reduction benefits, particulary for particulate matter, carbon monoxide, and unburned hydrocarbons. The amount of the benefit will depend on the engine's emission control technology, the age of the engine, the percent of biodiesel in the blend, and how the vehicle is operated. As Figure 1 illustrates, the oldest engines and technologies will reap the greatest emissions benefits from the use of biodiesel.
Biodiesel also reduces greenhouse gas emissions on a lifecycle basis. This is because the carbon dioxide released during combustion is offset by the carbon dioxide sequesterd while growing the feedstocks that are used to produce the fuel. Greenhouse gas emission reductions are an important component of being an advanced biofuel under the Renewable Fuel Standard. The U.S. Environmental Protection Agency does not differntiate among the various biodiesel feedstocks--vegetable oils and waster fats, greases, and oils --in defining biodiesl as an advanced fuel.