Real-World NO X Emissions of Transit Buses Equipped with Diesel Exhaust Aftertreatment Systems

Abstract

In this work, we compare nitrogen oxide (NOX) emissions from vehicles equipped with 2013 and 2015 model year (MY) diesel engines and exhaust aftertreatment, both certified to the same emissions standards, over a variety of real-world drive cycles. Our study concludes that 2013MY and 2015MY buses achieved real-world NOX conversion efficiencies of 75 and 95%, respectively. Engine-out NOX levels remained unchanged between the two busses when driven over the same transit bus routes. Therefore, emissions reductions are attributed to greater NOX conversion efficiency by the exhaust aftertreatment system, especially in low catalyst temperature and transient response scenarios. Although it is likely that other transit buses and vocational vehicles will express different characteristics based on their specific operating conditions and powertrain setup; our results show that improvements to aftertreatment systems, in the areas of catalyst activity, aftertreatment controls, and urea dosing strategy can significantly reduce NOX emissions over the investigated real-world drive cycles.