CARB releases proposed Tier 5 off-road engine emission standards

8 October 2024

The California Air Resources Board (CARB) held today another workshop meeting to discuss the proposed Tier 5 emission standards for off-road diesel engines. Before the workshop, the agency released the Draft Proposed Regulation Order, dated September 24^th.

The workshop focused on changes to the proposed Tier 5 provisions introduced since October 2023. Most of the changes and updates were related to the off-road onboard diagnostic system (OR OBD) and to several other compliance details—there were no changes to the emission limits and implementation dates considered at the previous Tier 5 workshop.

The Tier 5 regulation would be phased-in between 2029 and 2034. Engine manufacturers would choose one of four compliance options that allow phase-in flexibility and the use of emission credits. Three of these compliance options involve a set of Tier 5 interim standards, as shown below.

The Tier 5 interim (Tier 5i) compliance options are:

1. Direct compliance with Tier 5i standards without emission credits.
2. 50% of engine sales within a power category at the Tier 5 final (Tier 5f) level and 50% at the Tier 4 final (Tier 4f) level.
3. Direct compliance with Tier 5i standards using emission credits.
4. Longer continuance of Tier 4f compliant engines (up to 5% of manufacturer’s California production volume for up to five MYs) in exchange for earlier introduction of Tier 5f compliant engines.

A large part of the workshop was devoted to the updated OBD and OBMD (on-board monitoring and diagnostics) requirements. Some of the updates to the OBD proposal include:

• All engines fitted with SCR or tailpipe NOx sensors must to be equipped with OBMD system that requires NOx monitoring.
• Only the SAE J1939 communication protocol is allowed, J1979-2 in no longer acceptable.
• A list of specific parameters has been added to data stream requirements.
• More components are added to OBMD, including the DOC and the closed crankcase ventilation system.
• Changes are introduced in OBMD emission thresholds for NOx and PM.
• Changes are introduced in provisions for OBM NOx monitor demonstration testing and malfunction detection testing.

The Tier 5 proposal introduces a new certification pathway for hydrogen-fueled internal combustion engines (ICE). Off-road H₂-ICEs of MY 2029 and later would be required to certify as compression ignition (CI) engines if they employ boosted air induction or the H₂-ICE engine is designed to operate in equipment historically powered with an off-road CI engine.

This approach is designed to ensure that H₂-ICEs that replace diesel engines would be certified to Tier 5 standards, while allowing applications historically powered by naturally-aspirated large spark-ignited (LSI) engines to certify to the LSI standards.

H₂-ICEs would generate emission credits for criteria pollutants, but would not generate CO₂ credits, as the H₂ ICE does not meet CARB zero emission (ZE) criteria.

Several changes have been introduced to the California emissions averaging, banking, and trading (CA-ABT) system. One of the changes would limit the generation of Tier 5 NOx and PM emission credits to engines above 130 kW. This change is intended to promote faster transition to ZE (i.e., battery-electric) equipment in the power category below 130 kW.

Among other changes, the Tier 5 proposal aligns California regulations with the federal definition of an ‘engine’ [40 CFR 1068.30] to include both complete and partially complete engines.

Engine testing requirements have not been discussed—CARB plans to release the draft Tier 5 testing procedure in the 4^th quarter 2024.

The agency expects to release the Initial Statement of Reasons and the 45-day public notice for the rule in August 2025 and to consider the Tier 5 regulation at the Board hearing in October 2025.

Source: CARB