Current state of NOx emissions from in-use heavy-duty diesel vehicles in the United States
This report assesses the real-world nitrogen oxide (NOx) emissions behavior of heavy-duty diesel vehicles in the United States measured using portable emissions measurement systems (PEMS). This assessment is based on 160 PEMS tests from engines certified to 0.2 grams per brake horsepower-hour (g/bhp-hr) of NOx. The tests include data from eight manufacturers and 26 unique engine families certified between 2010 and 2016. Second-by-second data was analyzed to compare against the compliance evaluation results and to assess the impact of vehicle speed, vehicle type, and manufacturer on real-world NOx emissions.
- The Not-to-Exceed (NTE) protocol evaluates less than 10% of the total emissions data to determine compliance for heavy-duty in-use NOx emissions. The average emission value of 0.18 g/bhp-hr obtained from the NTE evaluation is significantly lower than the value of 0.42 g/bhp-hr obtained when evaluated on a total route basis.
- A disproportionate amount of NOx emissions from heavy-duty vehicles is emitted during the low-speed operation characteristic of urban driving. Vehicle operation at speeds of less than 25 mph results in NOx emissions of more than five times the certification limit for the average heavy-duty vehicle in the study.
- At mid-speed driving conditions, between 25 and 50 mph and characteristic of suburban driving, average NOx emissions from heavy-duty vehicles (HDVs) are 2.7 times the certification limit. Only at highway speeds, above 50 mph, do HDVs present average NOx emissions at the certification limit and below the in-use NTE emissions limit of 0.3 g/bhp-hr.
- Line-haul trucks have the highest average NOx emissions at less than highway speeds. Their average NOx emissions of 1.41 g/bhp-hr are more than 7 times the engine certification limit in urban driving and more than 3 times the limit in suburban driving. Only during high-speed operation do line-haul trucks emit NOx at engine certification limit levels.
- A single line-haul truck emits the NOx equivalent of 100 cars for each mile driven in urban driving. Under urban driving conditions, line-haul trucks are emitting on average 7.0 g/mi of NOx, compared with less than 0.07 g/mi for a gasoline car. The PEMS data shows that these trucks, which are optimized for highway driving, spend on average 43% of their time and emit 40% of the total mass of NOx in urban-like operation, including low-speed driving and idling.
The current NTE in-use testing protocol is inadequate to evaluate the in-use performance of HDVs in the United States, especially at low-speed conditions. The prospect of future NOx standards set over low-load cycle and idle tests, in addition to the traditional federal test procedure, demands the adoption of a different tool for proper in-use compliance evaluation. That tool should ensure that in-use compliance is evaluated not only with highway data but also includes low-speed, low-load, and idle data. This would ensure that engine dynamometer emission results obtained in the laboratory translate to real-world benefits.