A numerical investigation into the anomalous slight NO_x increase when burning biodiesel; A new (old) theory

Biodiesel is a notable alternative to diesel fuel because it comes from natural sources, is essentially CO₂ neutral, and it lowers an engine's emission of most pollutants as compared to No. 2 diesel. However, the use of biodiesel often slightly increases a diesel engine's emission of NO_x. Previously proposed theories for this slight NO_x increase are reviewed, including theories based biodiesel's cetane number, which leads to differing amounts of charge preheating, and theories based on the fuel's bulk modulus, which affects injection timing. A new theory explaining this NO_x increase was proposed; the increase in double bonds in biodiesel, compared to No. 2 diesel, increases its flame temperature, which in turn increases NO_x. NO_x was predominantly due to the Zeldovich mechanism. This is an abstract of a paper presented at the 231st ACS National Meeting (Atlanta, GA 3/26-30/2006).