Effects of Methyl Ester Biodiesel Blends on NOx Emissions

Effects of methyl ester biodiesel fuel blends on NOx emissions are studied experimentally and analytically. A precisely controlled single cylinder diesel engine experiment was conducted to determine the impact of a 20% blend of soy methyl ester biodiesel (B20) on NOx emissions. The data were then used to calibrate KIVA chemical kinetics models which were used to determine how the biodiesel blend affects NOx production during the combustion process. In addition, the impact on the engine control system of the lower specific energy content of biodiesel was determined. Both factors, combustion and controls, must be taken into account when determining the net NOx effect of biodiesel compared to conventional diesel fuel. Because the magnitude and even direction of NOx effect changes with engine load, the NOx effect associated with burning biodiesel blends over a duty cycle depends on the duty cycle average power and fuel cetane number. Higher duty cycle average power corresponds to a larger NOx increase. Results also show that for biodiesel blends containing less than 20% biodiesel, the NOx impact over the FTP cycle is proportional to the blend percentage of biodiesel. At B20, the difference in NOx emissions between a biodiesel blend and its base diesel fuel is relatively small, and is less than the difference that can be seen between two commercial diesel fuels within the normal range of aromatics content available in the marketplace.