Assessment of In-Use NOx Emissions from Heavy-Duty Diesel Vehicles Equipped with Selective Catalytic Reduction Systems

This work evaluated the nitrogen oxide (NOx) emissions of 277 heavy-duty diesel vehicles (HDDVs) from three portable emission measurement system testing programs. HDDVs in these programs were properly maintained before emission testing, so the malfunction indicator lamp (MIL) was not illuminated. NOx emissions of some HDDVs were significantly higher than the certification standard even during hot operations where exhaust temperature was ideal for selective catalytic reduction to reduce NOx. For engines certified to the 0.20 g/bhp-hr NOx standard, hot operation NOx emissions increased with engine age at 0.081 ± 0.016 g/bhp-hr per year. The correlation between emissions and mileage was weak because six trucks showed extraordinarily high apparent emission increase rates reaching several multiples of the standard within the first 15,000 miles of operation. The overall annual increase in NOx emissions for the HDDVs in this study was two-thirds of what was observed in real-world emissions for HDDVs at the Caldecott Tunnel over the past decade. The vehicles at the Caldecott Tunnel would include those without proper maintenance, and the inclusion of these vehicles possibly explains the difference in the rate of emission increase. The results suggest that HDDVs need robust strategies to better control in-use NOx emissions.

On-Board Sensor-Based NO x Emissions from Heavy-Duty Diesel Vehicles

Real-world nitrogen oxides (NO _x) emissions were estimated using on-board sensor readings from 72 heavy-duty diesel vehicles (HDDVs) equipped with a Selective Catalytic Reduction (SCR) system in California. The results showed that there were large differences between in-use and certification NO _x emissions, with 12 HDDVs emitting more than three times the standard during hot-running and idling operations in the real world. The overall NO _x conversion efficiencies of the SCR system on many vehicles were well below the 90% threshold that is expected for an efficient SCR system, even when the SCR system was above the optimum operating temperature threshold of 250 °C. This could potentially be associated with SCR catalyst deterioration on some engines. The Not-to-Exceed (NTE) requirements currently used by the heavy-duty in-use compliance program were evaluated using on-board NO _x sensor data. Valid NTE events covered only 4.2-16.4% of the engine operation and 6.6-34.6% of the estimated NO _x emissions. This work shows that low cost on-board NO _x sensors are a convenient tool to monitor in-use NO _x emissions in real-time, evaluate the SCR system performance, and identify vehicle operating modes with high NO _x emissions. This information can inform certification and compliance programs to ensure low in-use NO _x emissions.