Chassis Dynamometer Emissions Characterization of a Urea-SCR Transit Bus

West Virginia University characterized the emissions and fuel economy performance of a 30-foot 2010 transit bus equipped with urea selective catalytic reduction (u-SCR) exhaust aftertreatment. The bus was exercised over speed-time driving schedules representative of both urban and on-highway activity using a chassis dynamometer while the exhaust was routed to a full-scale dilution tunnel with research grade emissions analyzers. The Paris speed-time driving schedule was used to represent slow urban transit bus activity while the Cruise driving schedule was used to represent on-highway activity. Vehicle weights representative of both one-half and empty passenger loading were evaluated. Fuel economy observed during testing with the urban driving schedule was significantly lower (55%) than testing performed with the on-highway driving schedule. Fuel economy during evaluations representing half passenger load, which represented a 10% higher vehicle weight than empty weight, were 6% over the Paris driving schedule and 3% lower over the Cruise schedule than that observed during evaluations representative of empty/curb weight. The u-SCR exhaust aftertreatment system presented emissions and performance testing challenges which had not previously been encountered with earlier model year diesel vehicles. These issues resulted from the temperature dependence of the u-SCR in reducing oxides of nitrogen and emissions system control algorithms used to raise catalyst temperature after startup and maintain catalyst temperature to provide for oxides of nitrogen reduction. Oxides of nitrogen emissions levels in the exhaust were highly dependent on the temperature of the aftertreatment system at the beginning of each test and the rate at which exhaust energy heated the aftertreatment system during post-startup operation. Cold-start distance-specific oxides of nitrogen emissions integrated over the entire drive schedule using the Paris driving schedule were 8.7 times higher than for hot-start tests and 2.7 times higher than for hot-start emissions runs over the Cruise driving schedule. A significant portion of the difference in cold and hot start oxides of nitrogen emissions were the result of lower u-SCR system temperatures during cold-start operation. Run-to-run variability in u-SCR temperature resulted in run-to-run NOx repeatability issues. Clearly the task of gathering chassis dynamometer data for inventory or model development use is made more difficult by the complexity of NOx reduction aftertreatment.