CARB releases feasibility assessment of low NOx standards for HD diesel engines

23 April 2019

The California Air Resources Board (CARB) staff released a white paper that discusses staff’s assessment of the technical feasibility of lower NOx standards and associated test procedures for 2022 and subsequent model year medium- and heavy-duty diesel engines.

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CARB’s overall low-NOx emission standard package is focused on controlling actual NOx emission rates in the field over the life of the vehicle. The Heavy-duty Low NOx Omnibus Rulemaking—tentatively scheduled for CARB consideration in first quarter of 2020—will include a number of measures, including:

• a lower NOx emission standard on existing test cycles—the Federal Test Procedure (FTP) and the Supplemental Emission Test Ramped Modal Cycle (RMC-SET),
• the development of a new certification low load cycle (LLC) and associated NOx emission standard,
• revisions to the Not-to-Exceed (NTE) Heavy-Duty In-Use Testing (HDIUT) program,
• lengthening the useful life and warranty periods,
• clarifications to warranty corrective action provisions, and
• revisions to the durability demonstration procedures.

To support the development of these new requirements, CARB, in partnership with the South Coast Air Quality Management District (SCAQMD), US EPA, and the Manufacturers of Emission Controls Association (MECA) has been funding several research programs with Southwest Research Institute (SwRI) to demonstrate the feasibility of lower NOx emissions from on-road heavy-duty engines. The results from Stage 1 of the SwRI program were published in April 2017 and helped inform staff’s feasibility assessment for model year (MY) 2024. The final results from Stages 2 and 3 of the SwRI research program are expected to become available during the third or fourth quarter of 2019.

The CARB white paper introduces a three-step phase-in for the low NOx program: Step 1 in MY 2022-2023; Step 2 in MY 2024-2026, and Step 3 in MY 2027 and later. These three steps are summarized in the following tables. The program would be applicable to medium- and heavy-duty engines in vehicles above 10,000 lbs GVWR.

CARB staff believes a NOx standard of 0.05 to 0.08 g/bhp-hr on the FTP and the RMC-SET is feasible for the 2024 through 2026 MY, Table 2, and could be achieved without significant hardware architecture changes. Step 2 of the considered low NOx package would also tighten NOx emission requirements at low exhaust temperatures by the introduction of the Low Load Cycle. CARB believes a NOx standard of 1 to 3 times the proposed FTP standard is feasible on the LLC in 2024.

In Table 2, the current NTE in-use testing is replaced by a Euro VI-style real driving emissions test. CARB assessment of the current HDIUT program using the NTE methodology shows that the vast majority of driving conditions is not evaluated and goes unchecked for in-use compliance. This is due to the numerous exclusions incorporated in the NTE procedures, including those for intake manifold temperature and aftertreatment exhaust temperature, the NTE control area, and the requirement for a continuous 30-second operation for a valid NTE event. These limitations and inadequacies of the current NTE methodology have compelled CARB staff to pursue a MAW approach similar to the method used currently in Europe (Euro VI(D)). Unlike the Euro VI(D) that specifies the mix of route operation (rural, urban, highway), CARB plans to propose that the vehicle be driven on its regular fleet route. CARB is also considering an alternative compliance path using NOx sensor data collected using the On-Board Diagnostics (OBD) REAL monitoring system.

The purpose of the 0.005 g/bhp-hr PM standard, Table 2, is to maintain current levels of PM emission control performance in the future. Most engines currently have PM certification levels well below the current 0.01 g/bhp-hr standard and certify close to 0.001 g/bhp-hr. However, over the last few model years some manufacturers have elected to certify some of their engine families to higher PM emission levels as a result of changes to the diesel particulate filter (DPF) substrate. Some engine manufacturers are selecting more porous DPFs to reduce engine backpressure at the expense of higher PM emission rates, albeit still compliant with the current PM standard. Thus, to maintain current robust PM emission control performance at 0.001 g/bhp-hr levels, CARB staff is considering a lower PM standard of 0.005 g/bhp-hr.

Step 3, Table 3, would further tighten NOx standards and testing requirements, and would also lengthen the engine useful life and warranty periods.

Because about 60% of total heavy-duty vehicle miles traveled in the South Coast is accrued by trucks that were newly purchased outside of California, US EPA action to establish a new national low-NOx standard for heavy-duty trucks is critical, CARB said. In response to petitions for a low-NOx rulemaking from organizations from across the country, in November 2018 the US EPA announced the Cleaner Truck Initiative to develop regulations to further reduce NOx emissions from on-road heavy-duty trucks and engines. US EPA intends to publish a proposed rule in 2020. CARB intends to coordinate its regulatory efforts with US EPA.

Source: CARB