Real-world
nitrogen oxides (NOx) 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 NOx emissions,
with 12 HDDVs emitting more than three times the standard during hot-running
and idling operations in the real world. The overall NOx 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 NOx sensor data. Valid NTE events covered only 4.2–16.4%
of the engine operation and 6.6–34.6% of the estimated NOx emissions. This work shows that low cost
on-board NOx sensors are a convenient
tool to monitor in-use NOx emissions in
real-time, evaluate the SCR system performance, and identify vehicle
operating modes with high NOx emissions.
This information can inform certification and compliance programs
to ensure low in-use NOx emissions
