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

Abstract

The objective of this research project was to compare the emissions of oxides of nitrogen (NOx) from transit buses on as many as five different fuels and three standard transit duty cycles to establish if there is a real-world biodiesel NOx increase for transit bus duty cycles and engine calibrations. Prior studies have shown that B20 can cause a small but significant increase in NOx emissions for some engines and duty cycles. Six buses spanning engine build years 1998 to 2011 were tested on the National Renewable Energy Laboratory's Renewable Fuels and Lubricants 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 UDDS test cycles. The buses selected represented the majority of the current national transit fleet as well as including hybrid and selective catalyst reduction (SCR) systems that are increasing penetration in the fleet.The engine emissions certification level had the dominant effect on NOx, with the kinetic intensity of the tested duty cycle being the secondary driving factor. The biodiesel effect on NOx emissions was not statistically significant for most buses and duty cycles for blends with certification diesel, with the exception being a 2008 model year bus. CARB fuel had many more instances of a statistically significant effect of biodiesel increasing NOx. SCR systems proved effective at reducing NOx 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. As all but one test bus were equipped with diesel particulate filter aftertreatment PM emissions were negligible and trends could not be drawn. On the oldest sample bus without aftertreatment PM emissions were reduced with B20 blends. Fuel economy was not significantly changed by 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.

Impact of biodiesel on regulated and unregulated emissions, and redox and proinflammatory properties of PM emitted from heavy-duty vehicles

“…The results reported here also showed a rather strong fuel effect with biodiesel blends compared to CARB ULSD for the SCR-equipped vehicle. This is in contrast with previous studies showing that the effect of SCR negates the effect of biodiesel blends and pure biodiesel on NO x emissions (Lammert et al, 2012), but in agreement with other studies reporting increases in NO x emissions with biodiesel from vehicles equipped with SCR catalysts (Borillo et al, 2015;Kawano et al, 2010).…”

The GREET Model Expansion for Well-to-Wheels Analysis of Heavy-Duty Vehicles

“…We adopted the MOVES-based emission factors for the aggregated Class 6 and Class 7 school buses, Class 8 transit buses, and Class 8 intercity buses, as shown in Tables 23-25, to represent the emission profiles of the dominant Type C Class 6 school buses, Class 8a transit buses, and Class 8a intercity buses, respectively (U.S. Environmental Protection Agency, 2013a). The NO x emission factors for MY 2010 and later transit buses agree with the chassis dynamometer testing results for a 2010 Cummins ISL transit bus equipped with SCR driving under the Orange County Transit Authority (OCTA) cycle, and were lower than those measured during the more stop-and-go MAN cycle with probably lower SCR efficiency (Lammert et al, 2012).…”

“…For MY 2010 and MY 2011 buses equipped with SCR, Lammert et al (2012) found that the biodiesel effect on NO x emissions was not statistically significant, as the SCR aftertreatment system was so effective that emissions were reduced to near the detection limit for both ULSDand B20-fueled buses. Nylund and Koponen (2012) found that NO x emissions of BD20-fueled MY 2010 buses varied and no clear trend of increase or decrease was observed compared to those of the petroleum diesel counterparts.…”

“…As a result, no conclusive relationship between the PM reduction and biodiesel fuel content could be derived for these buses. In addition, Lammert et al (2012) found that most tests on post-MY 2007 buses equipped with DPF showed negligible PM emissions and that no clear trends of PM emissions from ULSD-or B20-fueled buses could be drawn. In GREET, we assumed that B20 has no impact on PM emissions for MY 2007 and later heavy-duty trucks, as the literature suggests no consensus on this issue.…”

“…Chassis emissions tests on the urban driving cycle showed that B20 reduced all measured pollutants, including NO x , when compared to conventional diesel (McCormick et al, 2006;Proc et al, 2006). Six buses spanning engine MYs from 1998 to 2011 were tested on a heavy-duty chassis dynamometer with California certification diesel, certification B20 blend, low aromatic diesel, low aromatic B20, and B100 fuels over the Manhattan, Orange County, and Urban Dynamometer Driving Schedule (UDDS) test cycles (Lammert et al, 2012). The study found that the biodiesel effect on NO x emissions was not statistically significant for most buses and duty cycles for the biodiesel blends.…”