Despite considerable progress towards clean air in previous decades, parts of the United States continue to struggle with the challenge of meeting the ambient air quality targets for smog-forming ozone mandated by the U.S. EPA, with some of the most significant challenges being seen in California. These continuing issues have highlighted the need for further reductions in emissions of NOX, which is a precursor for ozone formation, from a number of key sectors including the commercial vehicle sector. In response, the California Air Resources Board (CARB) embarked on a regulatory effort culminating in the adoption of the California Heavy-Duty Low NOX Omnibus regulation.[1] This regulatory effort was supported by a series of technical programs conducted at Southwest Research Institute (SwRI). These programs were aimed at demonstrating technologies that could enable heavy-duty on-highway engines to reach tailpipe NOX levels up to 90% below the current standards, which were implemented in 2010, while maintaining a path towards compliance with current heavy-duty Phase 2 GHG standards. These efforts culminated in the Stage 3 Low NOX program, the results of which have been documented in previous publications.
In parallel with the completion of the Stage 3 technical effort, EPA began an effort to promulgate a national heavy-duty low NOX regulation, with the goal of completing the regulation in 2022 to support a 2027 model year implementation.[2, 3] As part of that regulatory effort, EPA leveraged the test platform that was developed under the Stage 3 program to continue investigation of Low NOX technology capabilities and limitations. The emission control system was upgraded in several ways, and a number of topics were examined that expanded the scope of the evaluation. These included investigation of system performance under a variety of field duty cycles, examination of extended useful life out to 800,000 miles, the impact of low ambient temperatures on performance, and others. The performance of the updated system, and the results of the wider system investigations are summarized in this paper.