California Air Resources Board

Principal Investigator/Author(s): Cynthia C. Webb

Contractor: Southwest Research Institute

Contract Number: 13-312

Project Status: Completed

Relevant CARB Programs: Diesel Vehicles, Mobile Source Emissions Research Program, Heavy-Duty Low NOx

Topic Areas: Air Pollution, Reduced NOx Standards

Research Summary:

The 2010 emission standards for heavy-duty engines have established a limit for oxides of nitrogen (NOX) emissions of 0.20 g/bhp-hr, a 90 percent reduction from the previous emission standards. However, it is projected that even when the entire on road fleet of heavy-duty vehicles operating in California is compliant with the 2010 NOX emission standards, the upcoming National Ambient Air Quality Standards (NAAQS) requirements for ambient particulate matter (PM) and ozone will not be achieved in California without further significant reduction in NOX emissions from the heavy-duty vehicles. The main goal of this project was to demonstrate that modern heavy duty engines can achieve a target of 0.02 g/bhp-hr for tailpipe NOX emission, which represents a 90 percent reduction from the 2010 standard, with currently available control technology. An additional goal of the project was that the final configurations of engine and aftertreatment systems should be consistent with a path toward meeting current and future heavy-duty greenhouse gas (GHG) and fuel economy standards. The project team selected two different engine platforms: a compressed natural gas (CNG) based engine system and a diesel-based engine system. To identify advanced, production feasible, ultra-low NOX control solutions for both of these platforms, extensive work on calibrating engine control strategies, screening advanced aftertreatment technologies, selecting optimal combinations of engine and aftertreament technologies, and demonstrating ultra-low NOX emissions was performed in sequence. The final system selected for the CNG engine was a combination of advanced air-fuel control strategies, close-coupled three-way catalyst (TWC), and conventional under floor TWC. NOX emissions measured from the final CNG engine were 0.010 g/bhp-hr over the FTP certification cycle, well below the project target level of 0.02 g/bhp-hr. The final system selected for the diesel engine was a combination of cold-start engine calibration and an advanced aftertreatment system. NOX emissions from the final diesel system were measured with three different aftertreatment catalyst aging stages: degreened, thermal aging, and thermal and chemical aging. The NOX emissions over the FTP cycle were 0.008 g/bhp-hr, 0.012 g/bhp-hr, and 0.034 g/bhp-hr for the degreened, thermal aging only, and thermal and chemical aging, respectively. The project team suspects that the slightly high NOX with the thermal and chemical aged catalysts was due in part to an incident happened during the final aftertreatment aging, failure and crumbling of the matting material holding the Passive NOX Adsorber (PNA) in its metal canister. As a result of this incident, there was an abnormally large build-up of soot and hydrocarbon on the PNA that likely caused uneven flow distribution that resulted in localized soot build-up on the selective catalytic reduction filter (SCRF) system. This research project found multiple ultra-low NOX technology pathways applicable for many diesel engine and aftertreatment configurations. The results will be very important when developing air quality plans and regulatory priorities to achieve further NOX reductions from heavy-duty fleet in California.

Keywords: low NOx, heavy duty vehicles, NOx emissions, CNG, engine system, aftertreatment