The REAL NOx regulation requires tracking and reporting of NOx emissions starting in 2022MY for both medium-duty and heavy-duty diesel vehicles with potential to be considered during the next light-duty rulemaking. The regulation includes minimum NOx mass measurement accuracy requirements of either +/−20 percent or +/− 0.1 g/bhp-hr. Existing NOx sensor technology may not be able to meet the regulated accuracy requirements especially when exposed to other sources of variation within the emissions control system. This paper provides an assessment of real-world NOx sensor accuracy and the impact of other sources of variation and noise factors on NOx measurement accuracy. Noise factors investigated include NOx sensor tolerance, exhaust flow rate estimation, NOx sensor ammonia (NH3) cross sensitivity, mass air flow (MAF) sensor accuracy, NOx sensor placement, and laboratory emissions measurement capability. NOx sensors were often not able to meet the +/−20 percent accuracy requirement under transient operating conditions, and the addition of noise factors further degraded NOx sensor accuracy. A more complete operational definition of accuracy and support of that definition with a statistical analysis of repeatability and reproducibility, or Gage R&R, is introduced. An experiment to define NOx measurement repeatability and reproducibility is proposed in order to determine a measurement system capability-based metric for NOx sensor accuracy.
