Effect of B20 and Low Aromatic Diesel on Transit Bus NOx Emissions Over Driving Cycles with a Range of Kinetic Intensity

Abstract

This research project compares the emissions of oxides of nitrogen (NOx) from transit buses on as many as five different fuels and three standard transit duty cycles. The objective of the project is to establish if there is a real-world biodiesel NOx increase for transit bus duty cycles and engine calibrations. Prior studies have shown that biodiesel, sometimes called B20, can cause a small but significant increase in NOx emissions for some engines and duty cycles. Six buses that span engine build years from 1998 to 2011 were tested on the National Renewable Energy Laboratory's (NREL) Renewable Fuels and Lubricants (ReFUEL) research laboratory's heavy-duty chassis dynamometer with certification diesel, certification B20 blend, low aromatic California Air Resources Board (CARB) diesel, low aromatic B20 blend, and B100 fuels over the Manhattan, Orange County and the Urban Dynamometer Driving Schedule (UDDS) test cycles. The buses that were selected did represent a majority of the current national transit fleet. The selected buses also included hybrid and selective catalyst reduction (SCR) systems that are increasing in numbers in current transit vehicle fleets. The kinetic intensity of the tested duty cycle being the secondary driving factor, the engine emissions certification level had the dominant effect on NOx. With the exception of the 2008 model year bus, the biodiesel effect on NOx emissions was not statistically significant for most buses and duty cycles for blends with certification diesel. CARB fuel had many more instances of a statistically significant effect of biodiesel by substantially increasing NOx. SCR systems proved effective at reducing NOx emissions to near the detection limit on all duty cycles and fuels, including B100. A hybrid system proved to significantly increase NOx emissions over a same model year bus with a conventional drivetrain and the same engine. Because all but one test bus were equipped with diesel particulate filter after-treatment, particulate matter (PM) emissions were negligible and trends could not be drawn. On the oldest sample bus without after-treatment, PM emissions were reduced with B20 blends. The project shows how fuel economy was not significantly changed by the engine certification level, except that the 2008 conventional bus had the best performance on all cycles while all other buses had very similar results on each cycle. All buses had lower fuel economy with increased kinetic intensity of the cycle.