Real-world emissions from modern heavy-duty vehicles: Sensitivity analysis of in-use emissions analysis methods

This study presents an analysis of real-world emissions from two heavy-duty trucks using the Not-To-Exceed (NTE) and the European based Work-Based-Window (WBW) methodologies. The study also presents a sensitivity analysis of the NTE and WBW approach to key parameters such as window duration, torque, power, and exhaust aftertreatment thresholds. The analysis is performed on data collected from two heavy-duty diesel vehicles operated under real-world conditions in California over various routes that include a combination of urban and freeway type operation. The results presented show that only 10-30% of the total data meets the required exclusion criteria prescribed by the current NTE regulation. Reducing the NTE window duration from the existing 30 to 10 s increases the number of windows available to perform NTE analysis. The study also shows that exempting data that correspond to exhaust aftertreatment system (EATS) temperature threshold of below 250 degrees C is the most dominant parameter influencing the number of available windows in an urban-type operation. The WBW approach produces a rich collection of data points that are representative of the in-use emissions characteristic of the vehicle. The results of NOx vs CO2 using a 4-quadrant analysis from the WBW approach is indicative of a difference in engine manufacturer design and emissions control strategy pertaining to real-world operation. The results of the study show that revisions to the boundaries and exclusions to the NTE zone would contribute to broadening the scope of in-use assessment of heavy-duty vehicle operation. However, even with such modifications, the NTE may not apply to highly transient urban vocations such as delivery, refuse and transit bus vocations. While comparing the moving average WBW approach to the NTE, the former approach produces a significantly larger number of data points for in-use assessment.