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CARB provides an update on Tier 5 emission standards for off-road engines

4 November 2023 | Updated 17 November 2023

On October 30-31, 2023, the California Air Resources Board (CARB) held remotely a two-day workshop to provide an update on the development of Tier 5 emission standards for new off-road diesel engines. Compared to the potential Tier 5 requirements considered earlier, the emission limits and other provisions have been relaxed in an attempt to prevent engine manufacturers from abandoning the California market, as it happened with the 2024 onroad emission standards under the Omnibus regulation.

The Tier 5 regulation would be applicable to manufacturers of new off-road diesel engines. The emission standards would be based on diesel engine technology, with no zero emission vehicle (ZEV) requirements. The regulation would not be applicable to new engines below 175 hp that are used in construction or farm equipment (these engines are preempted from California regulations). Other excluded engine categories are engines used in transport refrigeration units (which would have more stringent zero-emission requirements) and new engines used in locomotives.

Some of the relaxed requirements and new flexibilities are:

Some emission limits have also been strengthened. These include the PM limit for genset engines above 560 kW, which has been lowered from 0.015 to 0.008 g/kWh. The stronger PM standards are intended to force diesel particulate filters on all affected engines. More than 50% of engine families certified to the current Tier 4 Final standards are not equipped with particulate filters.

The Tier 5 standards would require a 90% NOx emission reduction and a 75% PM emission reduction from the current US EPA Tier 4 levels. The Tier 5 emission limits currently considered for criteria pollutants over the NRTC and RMC test cycles are summarized below.

Proposed Tier 5 criteria standards, g/kWh
Nonroad Transient Test Cycle (NRTC) and Steady-State/Ramped Modal Cycles (RMC)
Power CategoryImplementation PeriodNOxPMNMHCCO
< 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 (Gen Sets)
(> 750 hp)
> 560 kW (Mobile)
(> 750 hp)
* NMHC + NOx
a The NMHC standard for lean-burn NG engine families remains at 0.19 g/kWh

In addition, Tier 5 Final emission limits for NOx, PM, and NMHC over the nonroad LLC test cycle would be applicable for engines in the 56-560 kW power category.

NOx idle emission standards, defined as a function of engine power, would be applicable for engines above 19 kW (except for gensets above 560 kW). If an engine meets the applicable LLC NOx emission standard, it is assumed it also meets the NOx idle standard. In lieu of meeting NOx idle standards, engines may be equipped with a 5 min engine shutdown system.

The current proposal maintains the current engine useful life periods and warranty periods expressed in hours, but extends the year limits by about 50%. For engines above 37 kW, the useful life period would be 8,000 hours/15 years (up from the current 10 years), while the warranty period would be 3,000 hours/8 years (up from the current 5 years).

The workshop also covered SCR inducement, onboard monitoring and diagnostics (OBMD and OBD), and other requirements of the proposed Tier 5 regulation.

Source: CARB