Evaluation of Field NO _x Performance of Diesel Vehicles using ECM - Provided OBD/SAEJ1979 Data 2015-01-1067

Investigations of on-road emissions performance of vehicles have been made using various methods and instrumentation, some of which are very complex and costly. For the particular case of NO_x emissions on Diesel road vehicles equipped with SCR catalysts (Selective Catalytic Reduction), many of these vehicles are equipped with NO_x sensor(s) for the purpose of OBD (On-Board Diagnostics), and the ECU (Engine Control Unit) makes this data available via the diagnostic connector under the SAEJ1979 protocol for light duty vehicles. Data for mass air flow and fuel flow are also available per J1979, so the ongoing NO_x mass flow can be estimated when the NO_x sensors are active with no additional instrumentation.

Heavy duty pickup trucks with SCR systems from 3 major US manufacturers, each certified to the optional chassis certification of 0.2 g/mi NO_x on the FTP75, were obtained to be evaluated for SCR system behavior under normal driving conditions. The vehicles were tested on a chassis dyno, and the NO_x emissions data resulting from the traditional emissions bench and as - calculated using the ECU SAEJ1979 available data were compared to validate the use of the ECU data for on - road evaluation. Note that the NO_x sensors have a certain warmup time, so data is not available immediately upon cold start, but in all cases the rear NO_x sensor is active within a few minutes under normal conditions. All of the vehicles were run on the road for a minimum of 30,000 miles with data collected continuously in this manner, under various types of operation and ambient temperatures. As expected, low tailpipe NO_x emissions where reached as required under regulatory drive cycles. However, it was observed that these vehicles also emit very low tailpipe NO_x under a broad range of real driving conditions.

The primary purpose of this paper is to explore the potential of using the onboard NO_x sensors for observing the NO_x trends on the road over a variety of conditions and vehicles at minimal cost and intrusion. It is not intended to provide detailed quantified data on the vehicles which would be better accomplished with more accurate instrumentation such as conventional emissions benches / FTIRs or on-road PEMS. For that reason, no attempt is made to quantify the differences between the observed NO_x via laboratory instrumentation and the sensor reading.