Reducing NOx emissions from a common-rail engine fueled with soybean biodiesel

Performance and emissions of a common-rail production diesel engine fueled with soybean-derived biodiesel was investigated. The work was broken down into two categories. First, adjustment of injection timing and EGR ratio was investigated as a means to reduce NOx emissions to levels comparable with those obtained when using pure diesel fuel. Next, simultaneous reduction of NOx and soot emissions was investigated using high rates of EGR combined with late injection timings to approach the low-temperature combustion regime. Results from the first part of the study indicate that optimization of engine control parameters for use with biodiesel can be beneficial to performance and emissions. It was found that adjusting the engine's MAF setpoint table to reflect the difference in stoichiometric air-fuel ratio between diesel and biodiesel brought NOx emissions to comparable or lower levels. This was accomplished while maintaining a significant decrease in soot emissions and similar fuel consumption to the baseline settings. In certain cases, combining this change with an advance in injection timing allowed for lower fuel consumption as well. Results from the second part of the study indicate that very low levels of NOx and soot may be obtained through a moderate range of engine speed and load. High to moderate rates of EGR are used for low to moderate engine load, respectively. A small pre-injection was utilized in all cases. The main injection timing had to be retarded with increasing load to allow for injection into lower temperature, more mixing time, and the resulting decrease in soot emissions. Meanwhile, for higher engine speed at constant load, injection timings were advanced to provide better combustion phasing and lower fuel consumption.