Certification and In-Use Compliance Testing for Heavy-Duty Diesel Engines to Understand High In-Use NOx Emissions

This page last reviewed April 3, 2015


Considerable reduction in nitrogen oxides (NOx) emissions is needed for the State of California to meet ambient air quality standards for ozone and particulate matter. To achieve some of the reduction, the Board adopted a NOx emission standard for model year 2010 and later heavy-duty on-road engines of 0.20 g/bhp-hr. This represents a 90 percent reduction compared to the pre-2010 standard. To meet the standard, most diesel engine manufacturers are using selective catalytic reduction (SCR) aftertreatment systems. SCR requires adequate temperatures in the exhaust to function effectively. It is known that engine emissions will exceed certification emissions when the engines are cold. However, it has been observed during many in-use test programs that emissions from in-use vehicles exceed emission levels expected based on the engine certification standards, even when the engines are fully warmed up. This discrepancy affects the accuracy of SIP planning and affects the effectiveness of potential rules to further lower emission standards. The cause of this discrepancy is unknown.

The objective of this research is to understand why in-use NOx emission rates from warmed-up, SCR-equipped vehicles in normal use exceed the emission rates engine-certification emission rates. Possible sources of the discrepancy include: certifications engines are pampered special engines compared to in-use engines; engines respond differently to certification test equipment than to in-use test equipment; and, certification test cycles are not representative of in-use test cycles.


Conference Presentations

  • Seunju Yoon et al. (2015). Comparison of NOx Emissions from In-Use 2010 Technology Heavy-Duty Engines to Their Certification Standards. 25th CRC On-road Emissions Workshop, March 23-25, 2015, Long Beach, CA. PDF (1.85 MB)
  • Seunju Yoon et al. (2015). Impacts of Modifying Exhaust Temperature and Event Duration Limits on Not-To-Exceed Compliance for In-Use NOx Emissions from 2010-Technology Heavy-Duty Diesel Vehicles. 5th UCR Portable Emission Measurement System Conference, March 26-27, 2015, Riverside, CA. PDF (2.48 MB)