ARB researchers evaluate in-use heavy-duty NOx aftertreatment systems, find elevated levels during certain lower-temperature operations
A team from the California Air Resources Board (ARB) reports on their evaluation of the in-use emissions performance of four different heavy-duty diesel engines certified to the MY 2010 or interim MY 2010 NOx standards over a wide range of driving conditions in California in a paper published in the ACS journal Environmental Science & Technology.
One of the trucks was equipped with exhaust gas recirculation (EGR); three were equipped with EGR and a selective catalytic reduction (SCR) device. The results indicated that brake-specific NOx emissions for the truck equipped only with an EGR were independent of the driving conditions—and exceeded the certification value over each segment of the entire test route. Results also showed that for typical highway driving conditions, the SCR technology is proving to be effective in controlling NOx emissions. However, they also found that under operations where the SCR systems do not reach minimum operating temperature—e.g., cold starts and some low-load/slow-speed driving conditions—NOx emissions are still elevated.
The study indicated that strategies used to maintain exhaust temperature above a certain threshold, which are used in some of the newer SCRs, have the potential to control NOx emissions during certain low-load/slow-speed driving conditions.
Results from the study will be used to evaluate in-use compliance with emissions standards; update emissions inventories that inform air quality planning and regulatory policy development, and develop potential approaches for further NOx control.
To meet stringent NOx emission standards being implemented over the years, various aftertreatment devices such as exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) have been developed. EGRs were primarily introduced for on-road heavy-duty diesel engines over a decade ago to meet the tighter MY 2004 NOx standard (2 g/bhp-hr). EGR reduces NOx formation by introducing cooled exhaust gases into the combustion chamber. The reintroduced exhaust gas is lower in oxygen content than the ambient air, which limits the conversion of nitrogen to NOx at high temper- ature. It should be noted that using only an EGR to reduce NO emissions can lead to reduced engine efficiency and increased PM emissions. While EGR technology has advanced since it was originally introduced, it is still not effective in achieving the stringent MY 2010 NOx standard for which SCRs may be required.
Because MY 2010 and later engines use SCR systems to reduce NOx, engine manufacturers have chosen to use engine calibrations (which involves setting the engines engine control unit or ECU parameters to provide the engine with the correct fuel and ignition values to run as desired) that produce higher engine-out NOx and lower engine-out PM than was acceptable for MY 2007−2009 engines (which were able to meet less stringent NOx standards without using an SCR).
The higher NOx engine calibrations facilitate passive DPF regeneration by producing less engine-out soot, more NOx for soot oxidation, and higher engine operating temperatures. SCR technology is generally more effective in reducing these higher engine-out NOx emissions to meet lower emission standards and is being implemented by the major engine manufacturers. The functioning of an SCR is dependent on catalyst material, urea dosing strategy, temperature, and physical system layout. A certain minimum exhaust temperature is required in SCR to promote hydrolysis of urea into ammonia (NH3) which then reduces NOx into nitrogen (N2) and water (H2O). As a result, the functioning of SCR is dependent on driving conditions to the extent it affects the exhaust temperature and urea dosing strategy. Because real-world driving conditions are much more variable than certification cycles, it is important to understand how real-world driving conditions affect SCR NOx control effectiveness.—Misra et al.
In addition to evaluating the in-use emissions performance of the four different heavy-duty diesels, the ARB team also used the study to evaluate recent improvements in the use of SCR devices for mobile source applications. Some of the improvements—introduced by manufacturers after consulting with ARB—include:
- Thermal management strategies;
- Inducement strategies to guard against failure to refill diesel exhaust fluid (DEF-an aqueous solution of 32.5% high purity urea and 67.5% deionized water);
- poor DEF quality; and
- SCR system tampering.
The ARB team used a commercially available Portable Emissions Measurement System (PEMS) to collect emissions, global positioning system (GPS) location, and engine broadcast data.
The four trucks (MY 2010−2011) were of different makes and models and had displacement ranging from 12.4 to 14.9 L with power ratings from 405 to 450 hp. Odometer values ranged from 13,500 miles to 70,000 miles (21,726 to 112,654 km).
The engine of the one truck with only EGR was certified to meet the MY 2010 interim standard of 0.5 g NOx/bhp-hr). Of the SCR-equipped trucks, one was powered by an engine certified to meet the same standard, while the other two trucks used engines certified to meet the MY 2010 emissions standard (0.2 g NOx/bhp-hr).
Emissions testing was performed while driving the test vehicles over two prescribed routes in and around Sacramento, California, with different payloads.
Results from this study suggest that elevated NOx emissions may occur during slow-speed, low-load operations that are not covered by the FTP for the MY 2010 emissions standard, and by cold starts that are weighted into the MY 2010 emissions standard. As stricter air quality regulations encourage the adoption of SCR by the trucking fleets in the United States and worldwide, it is important that the certification tests reflect real- world driving conditions.
One way to do this would be to develop improved certification programs to assess SCR performance during the full range of real-world driving including cold start and urban driving conditions, either via engine dynamometer or in-use PEMS based testing. The World Harmonized Test Cycle (WHTC) is one example of real-world conditions being reflected in truck engine certification. This proposed cycle includes substantial amounts of engine operating time at low load/slow speed conditions to capture urban driving conditions.
A different approach would be to expand in-use requirements. These could be in the form of on board diagnostic (OBD) or broadcast data to remove the burden of PEMS field testing. For example, the OEM can establish a performance temperature specification based on bench testing, catalyst suppliers, or other tests. This could then be used with some type of fleet average SCR inlet telecommunications data to verify compliance. The fleet average temperature information and the OEM’s performance thresholds could establish compliance, and PEMS could be used to do field screening and confirmation.—Misra et al.
Chandan Misra, John F. Collins, Jorn D. Herner, Todd Sax, Mohan Krishnamurthy, Wayne Sobieralski, Mark Burntizki, and Don Chernich (2013) In-Use NOx Emissions from Model Year 2010 and 2011 Heavy-Duty Diesel Engines Equipped with Aftertreatment Devices. Environmental Science & Technology doi: 10.1021/es4006288