On October 29, the Mineta National Transit Research Consortium, a U.S. Department of Transportation Center for Excellence, released a peer-reviewed study that examines the emissions profile of a bus fleet run on a biodiesel blend.  Their study, Combustion Chemistry of Biodiesel for Use in Urban Transport Buses: Experiment and Modeling, found a significant reduction in the mass of particulate matter emitted by buses running on biodiesel blends.  

Biodiesel is a renewable fuel sourced from waste fats, oils and grease. Previously, it has been assumed that the emissions profile of biodiesel is better than that of ultra-low sulfur diesel.  Regardless, differing fuel characteristics between biodiesel and regular diesel, such as differences in viscosity and composition can cause the fuels to behave differently and potentially cause different emissions. Researchers at the University of Toledo College of Engineering tested the emissions of ten transit buses both in the field and in the laboratory, using B20 (20 percent biodiesel, 80 percent ultra-low sulfur diesel). They found:

  • Particulate matter emissions from buses decreased by 17 percent, on average, when running on B20. 
  • In both hot and cold idle conditions (cold idle is the morning start-up), total particulate matter was greatly reduced.
  • Biodiesel was found to have higher oxygen levels than regular diesel, which causes combustion temperatures to increase. The researchers attributed the reduction of particulate matter to the biodiesel’s higher oxygen levels.
  • Biodiesel was found to have a higher flash point than diesel, which means it is a safer fuel to handle and store.

Exposure to particulate matter is recognized as a significant health risk. Children are especially vulnerable to air pollutants, since they breathe in 50 percent more air than adults for their body size. Although the United States has significantly cleaned up diesel fuel and vehicle fleets, particulate matter from both diesel vehicles and passenger cars remain a significant concern. Classes of extremely small particles, ultrafine particulates (UFPs) are particularly concerning. Not only does the burning of gasoline and diesel produce these UFPs, they also act as vectors for polycyclic aromatic hydrocarbons (PAHs) also produced from fuel combustion.  These particles are carried into soft body tissues and bloodstreams, which larger particulates can’t reach. These results make the case for increased use of renewable alternatives, such as biodiesel, to help clear the air.



For more information see: 

Combustion Chemistry of Biodiesel for Use in Urban Transport Buses: Experiment and Modeling, Mineta National Transit Research Consortium 

Study Shows Air Quality Benefits of Biodiesel in City Buses, BIodiesel Magazine