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CARB holds Tier 5 workgroup meeting on OBD, ABT, idle requirements

16 December 2022

On December 14th, the California Air Resources Board (CARB) held another public workgroup meeting to discus the development of Tier 5 emission standards for new off-road diesel engines. The meeting covered three main topics: (1) on-board diagnostics (OBD) concepts, (2) emission averaging, banking, and trading (ABT), and (3) idle requirements concepts.

The California Tier 5 program is intended to force the use of particulate filters on all off-road diesel engines (as of model year 2020, more than half of new off-road CI engine families continue to be certified in California to the Tier 4 final emission standards without DPFs), to reduce NOx emissions by 90% (56-560 kW engines), and to ensure emission reductions under a wide range of real world operating conditions. The CARB Tier 5 regulation would include more stringent emission limits compared to the federal Tier 4 nonroad emission standards, as well as a number of other changes such as enhanced in-use compliance testing, extended useful-life periods, and a low-load emission test cycle.

However, as the US EPA is not developing a corresponding federal-level Tier 5 emission regulation, the California-only Tier 5 emission standards would have a limited effect, as California is currently preempted from regulating emissions from new farm and construction equipment under 175 hp and current regulations do not prevent California residents from bringing off-road engines and equipment from other states.

Potential California Tier 5 emission standards (NRTC, g/kWh)
Power CategoryApplicationPMNOxNMHCNOx + NMHCCOImplementation Date
< 8 kW
(< 11 hp)
8 ≤ kW < 19
(11 ≤ hp < 25)
19 ≤ kW < 56
(25 ≤ hp < 75)
56 ≤ kW < 130
(75 ≤ hp < 175)
130 ≤ kW ≤ 560
(175 ≤ hp ≤ 750)
> 560 kW
(> 750 hp)

OBD Requirements. OBD is currently not required for off-road engines. CARB staff has introduced several potential OBD features from existing on-road heavy-duty diesel engines that could be applied to off-road engines. The off-road OBD would be applicable at a minimum to the 56-560 kW engine category.

Potential OBD standardization requirements include communication protocols (J1939 and J1979-2), OBD data link connectors, fault codes, fault code handling, and more. Potential diagnostic requirements include monitoring of major emission control components including PM sensor-based DPF filtration efficiency, EGR system (e.g., low flow), NOx conversion efficiency of the SCR system, SCR reductant dosing, and NOx sensor performance.

ABT Program. The current federal ABT program allows engine manufacturers flexibility in complying with the standards by averaging emission levels across power categories and product lines nationwide. CARB is concerned that significantly more stringent off-road diesel standards in California could result in excessive usage of credits earned from sales in all 50 states in just California, potentially delaying the introduction of Tier 5 engines indefinitely. To prevent this, CARB is considering adopting statewide averaging requirements to ensure that credits are used equitably within California and that Tier 5 compliant engines enter the California market in a timely manner.

Additionally, the federal ABT program does not include credits for tailpipe CO2, which may become a regulated pollutant under California Tier 5.

Idle NOx Emissions. As idling is common for off-road equipment (potentially 30-50% of time), it represents a significant contribution to the overall NOx emissions. CARB is considering two idle reduction concepts: (1) a non-programmable 5-minute engine shutdown system, and (2) potential idle NOx standards.

The idle NOx standard would be calculated based on the corresponding NRTC standard as a function of the maximum engine power. A potential two-mode idle test procedure would be used, including 30 minutes at the manufacturer-recommended minimum idle speed and 30 minutes at 1100 rpm.

Source: CARB