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On-Board Sensor-Based NO Emissions from Heavy-Duty Diesel Vehicles Yi Tan, Paul Henderick, Seungju Yoon, Jorn Herner, Thomas Montes, Kanok Boriboonsomsin, Kent C. Johnson, George Scora, Daniel Sandez, and Thomas D. Durbin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07048 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 21, 2019
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Environmental Science & Technology
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On-Board Sensor-Based NOx Emissions from Heavy-
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Duty Diesel Vehicles
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Yi Tan1,*, Paul Henderick2, Seungju Yoon1, Jorn Herner1, Thomas Montes2, Kanok
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Boriboonsomsin3, Kent Johnson3, George Scora3, Daniel Sandez3, Thomas D. Durbin3
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1California
Air Resources Board, 1001 I St, Sacramento, CA, 95814
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2California
Air Resources Board, 9500 Telstar Avenue, Ste. #2 El Monte, CA 91731
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3University
of California at Riverside, College of Engineering - Center for Environmental
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Research and Technology, 1084 Columbia Ave Riverside, CA 92507, USA
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Correspondence to:
[email protected] 10
ABSTRACT. Real-world nitrogen oxides (NOx) emissions were estimated using on-board sensor
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readings from 72 heavy-duty diesel vehicles (HDDVs) equipped with Selective Catalytic
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Reduction (SCR) system in California. The results showed that there were large differences
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between in-use and certification NOx emissions, with 12 HDDVs emitted more than three times
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the standard during hot-running and idling operations in the real world. The overall NOx
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conversion efficiencies of the SCR system on many vehicles were well below the 90% threshold
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that is expected for an efficient SCR system, even when the SCR system was above the
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optimum operating temperature threshold of 250°C. This could potentially be associated with
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SCR catalyst deterioration on some engines. The Not-to-Exceed (NTE) requirements currently
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used by the heavy-duty in-use compliance program were evaluated using on-board NOx sensor
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data. Valid NTE events covered only 4.2 – 16.4% of the engine operation and 6.6 – 34.6% of
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the estimated NOx emissions. This work shows that low cost on-board NOx sensors are a
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convenient tool to monitor in-use NOx emissions in real-time, evaluate the SCR system
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performance, and identify vehicle operating modes with high NOx emissions. This information
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can inform certification and compliance programs to ensure low in-use NOx emissions.
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INTRODUCTION
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Nitrogen oxides (NOx) play an important role in the formation of atmospheric ozone and fine
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particulate matter (PM2.5). Controlling NOx emissions is thought to be critical for attaining the
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National Ambient Air Quality Standards (NAAQS) for these two pollutants in the next decade in
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California, especially in the South Coast and San Joaquin Valley air basins.1 The estimated
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contribution of on-road heavy-duty diesel vehicles (HDDVs) to the total NOx emission inventory
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in California was ~33% in 2014, which is considerably higher than the US average (~15%).2, 3 ACS Paragon Plus Environment
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Recent studies have shown that on-road diesel emissions could be underestimated by emission
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inventory models.4, 5 In order to reduce NOx emissions from these vehicles, 2010 and newer
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model year (MY) heavy-duty diesel engines must comply with a NOx emissions standard of 0.20
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g/bhp-hr or a Family Emission Limit (FEL) not exceeding 0.50 g/bhp-hr over engine-
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dynamometer test cycles, including the transient Federal Test Procedure (FTP) and the
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Supplemental Emissions Test (SET). These vehicles should also meet the not-to-exceed (NTE)
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emission standard to ensure that NOx emissions are controlled under conditions thought to be
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experienced in-use.
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To meet these NOx standards, heavy-duty diesel engine manufacturers commonly employ
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Selective Catalytic Reduction (SCR) together with Exhaust Gas Recirculation (EGR) and other
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in-cylinder NOx control strategies.6 The EGR system cools and recirculates a portion of the
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exhaust back to the engine, which lowers the oxygen concentration, flame temperatures, and
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thus engine-out NOx emissions. On the other hand, SCR uses diesel exhaust fluid (DEF) in
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conjunction with a catalyst to control NOx emissions. DEF is an aqueous urea solution that when
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thermalized supplies gaseous ammonia for the SCR reaction. Platinum, vanadium, and zeolite
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are materials presently used for SCR catalysts.7 Copper (Cu)- and Iron (Fe)-based zeolite
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catalysts are widely used in heavy-duty applications because of their high NOx conversion ACS Paragon Plus Environment
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efficiency over a wide range of operating temperatures (200C – 450C for Cu-zeolite, and >
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250C for Fe-zeolite), relatively low cost, and improved thermal durability.8 The application of
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platinum-based catalysts is very limited because of their narrow operating temperature range (
