On-Road NOx Emission Rates from 1994-2003 Heavy-Duty Diesel Trucks

In-service 1994-2003 heavy-duty trucks were acquired by West Virginia University (WVU), equipped with the WVU Mobile Emissions Measurement System (MEMS) to measure on-road NOx, and driven on road routes near Sabraton, West Virginia, and extending up to Washington, PA to obtain real-world oxides of nitrogen (NOx) emissions data on highways and local roads. The MEMS measured 5Hz NOx, and load was obtained from the electronic control unit. Trucks were loaded to about 95% of their gross vehicle weights. Emissions in g/mi and g/bhp-hr were computed over the various road routes. In addition, some of the trucks were tested 1 to 2 years later to determine emission changes that may have occurred for these trucks. Emission results varied significantly over the different road routes due to different speeds, driving patterns, and road grades. Careful analysis of the available data indicates that both the brake-specific and distance-specific NOx emissions on later model year trucks were lower than the earlier model year trucks, and these average NOx reductions were approximately proportional to the reductions in the brake-specific emission standards. Furthermore, analysis of the vehicles that were re-tested after 1-2 years (with mileage above their emissions useful life) indicated a small reduction in NOx emissions with mileage or age, on average. Lastly, the data presented in this report suggests that both the California EMFAC2007 model and the EPA MOBILE6.2 model emission rates could benefit by incorporating in-use emissions data of large population studies.