Diesel vehicles are significant sources of NOx and PM emissions, and to a lesser extent, emissions of CO, HC, and toxic species. For many years, biodiesel fuel (and blends of biodiesel) has been promoted as a “clean fuel” alternative to conventional diesel. Based upon previous reviews by EPA, a common understanding has arisen that biodiesel usage reduces CO, HC, and PM emissions significantly, but increases NOx emissions slightly.
This paper discusses a recent review of 94 published reports, from the period of 2000-2008. Assessments were made of the emissions impacts of biodistillate fuels from various engine types, operating conditions, control technologies, and fuel type. In each situation, emissions from the biodistillate case were compared with emissions from a reference diesel fuel case. Graphical displays were developed to show the effects of biodistillate blend level upon 4 emissions species (NOx, CO, HC, PM) from 3 engine types [heavy-duty (HD), light-duty (LD), and single cylinder test engine (TE)].
Results showed that use of biodistillates, even at a 20% blend level, substantially decreased emissions of CO, HC, and PM - generally by 10-20%. Although results varied considerably from one study to the next, similar benefits were seen in both LD and HD engines, regardless of engine technology or test condition. While data were much more limited for renewable diesel cases, these hydroprocessed fuels appeared to provide similar emissions reduction benefits for CO, HC, and PM.
NOx emissions impacts were much smaller, and more difficult to discern. Though highly variable, most studies indicated a slight NOx increase when using B100 fuel. For HD engines, the authors’ best estimates are that NOx emissions increase 2-3% with B100, but are unchanged from conventional diesel fuel for B20 blends. Thus, this review indicates smaller NOx effects of biodistillates in HD engines than defined by EPA several years ago. In LD engines, NOx effects appear to be somewhat larger, with increases of 10-15% observed when using B20 and B100, respectively. More sophisticated statistical analyses are required to assess the significance of these small effects.