Evaluation of On-Board Sensor-Based NOx Emissions from the Heavy-Duty Vehicles in an Inspection and Maintenance Program

Heavy-duty diesel vehicles (HDDVs) are important sources of urban nitrogen oxides (NOx) in an actual application for environmental compliance. The remote on-board sensing (OBS) is a cost-effective emission reduction approach for HDDVs. NOx emissions from fifty-four sediment hauling trucks were evaluated in a construction area, and this information was utilized to make changes to the hauling trucks being used on the project. Approximately 1/3 of the trucks had emissions that were comparable to or below 0.2 g/bhp-hr level. The fleet average NOx emissions were 0.38 g/bhp-hr for non-credit engines and 0.77 g/bhp-hr for credit engines, with 2013 and newer credit engines averaging 0.29 g/bhp-hr. The overall NOx conversion efficiencies of the selective catalytic reduction (SCR) system on many vehicles were in 80% to 90%, and SCR efficiencies for some high emitter vehicles were down to approximately 60%. This is due to high engine operation fractions at the lowest SCR inlet temperature zone (> 200℃). This study also found that the NOx emissions for non-2010–2012 Family Emission Limit (FEL) trucks were well below the California Clean Idle certification of 30 g/hour. Recommendation was provided to efficiently maintain the emission levels in the reservoir.