Investigation into the Impact of Weight on Diesel Trucks’ NOx Emissions: An Overweight Case Study

Abstract

To build an understanding of the emission contribution, especially nitrogen oxides (NOx) emissions, of many overweight/oversized trucks operating in the Dallas-Fort Worth area, U.S., the North Central Texas Council of Governments commissioned the Texas A&M Transportation Institute to conduct an emission testing study. The study team collected and analyzed data on these vehicles’ activities and emissions characteristics. Vehicle activity data collection was performed using portable activity measurement systems, and exhaust emissions data was measured using two portable emission measurement systems. Using methodologies developed by the Environmental Protection Agency for the MOtor Vehicle Emissions Simulator (MOVES) model, both datasets were processed and expressed as emission rates for three loading scenarios. Researchers observed that, because of the sensitivity of the performance of selective catalytic reduction (SCR) to exhaust temperature, NOx data observed during a test cycle may not result in emission rates that are representative of the real-world operations. The team developed a methodology to adjust the resulting NOx rates for SCR-equipped trucks. The findings of the analysis suggest that the current MOVES opMode equations do not appear to adequately capture the impact of weight on the emission rates of heavy-duty diesel vehicles. The team also found that SCR-equipped trucks can have lower emission rates under heavier loads, especially in higher-speed and higher-power opMode bins, which typically have higher exhaust temperatures. Under normal loads (approximately 55,000 lb), SCR does not seem to provide a NOx emission reduction benefit at speeds under 50 mph compared with exhaust gas recirculation-equipped trucks.