The California Air Resources Board (CARB) is working on its proposed Tier 5 rule, which aims to drastically reduce Nitrogen Oxides (NO_x) and Particulate Matter (PM) emissions from off-road diesel engines in the state of California. Plus, for the first time, CARB has devised standards to reduce Carbon Dioxide (CO₂) emissions and other greenhouse gas emissions (GHG).

While Tier 5 is an in-the-works proposed rule applicable to new off-road equipment intended for sale in the California market, under the Federal Clean Air Act CARB is preempted from regulating agricultural and construction equipment under 175 hp and will need to look to the U.S. EPA to further regulate emission standards for these lower hp machines. However, for larger equipment and equipment with non-preempted engines, CARB is well on its way to a significant tightening of emission requirements, leading the way for other states to potentially follow suit.

CARB’s goal is to finalize its proposal in 2025, with implementation of Tier 5 standards beginning in 2029.

Doosan Bobcat North America Director of International Standards and Regulations Steve Neva recently walked through the key provisions of CARB’s Tier 5 proposal during an AEM member education webinar in January.

Continue reading to see what could be in store for equipment manufacturers in coming years.

Emission Standards

According to Neva, CARB’s proposed emission limits for criteria pollutants are even more onerous than the European Union’s (EU) Stage V requirements. As compared to Tier 4 Final and EU Stage V, CARB wants to see NO_x emissions reduced by another 90% in the 56-560 kW power category, and PM reduced by another 50 to 75% in all power categories except for engines greater than 560 kW used in mobile machinery. In order to comply, Neva said it’s possible that equipment manufacturers will need to redesign equipment to accommodate either additional or larger aftertreatment devices while also providing for additional cooling.

CARB is also looking to introduce GHG standards for power categories of 19 kW and higher. CARB’s approach will vary by power category:

• 19-56 kW – CO₂ capped at 80th percentile of CO₂ emissions at Tier 4 Final levels
• 56-560 kW – CO₂ to be reduced by 6% from the baseline
• 560+ kW – CO₂ capped at 80th percentile of CO₂ emissions at Tier 4 Final levels

CARB is also looking to apply capping standards for nitrous oxide (N₂0) and methane (CH₄) on all power categories above 19 kW. An allowance program is also being developed that would enable manufacturers to offset these emissions by using C0₂ credits.

Aftertreatment Systems

CARB intends to take several steps to ensure that aftertreatment systems perform as intended in the field.

Aftertreatment aging. CARB has developed what it refers to as the DAAAC, the Diesel Aftertreatment Accelerated Aging Cycle. It’s a new protocol designed to better estimate the aging of aftertreatment devices in off-road equipment. “CARB has recognized that off-road equipment has a greater variety of operational application than on-road, and that the off-road environment is also typically harsher,” Neva said.

Selective Catalytic Reduction (SCR) inducements. Thus far, the EPA has only provided “guidance” pertaining to SCR inducements. Now CARB is looking to codify that guidance into its proposed Tier 5 rule. This could force engine and equipment manufacturers to make additional changes. For instance, both a warning lamp and audible alarm will be required when various system sensors detect low diesel exhaust fluid (DEF) or DEF quality tampering, culminating with idle or engine shutdown if the fault mode isn’t corrected within a certain timeframe.

Low load cycle. This proposed standard impacts engines in the 56-560 kW range. A new test cycle has been developed to include an extended idle period. This is to account for the impact idle time has on emissions, because CARB says its data suggests that off-road engines idle anywhere from 30-50% of the time. To address this new standard, Neva said engine manufacturers may need to incorporate more complex aftertreatment systems.

Idle NO_x requirement. This proposed standard impacts all power categories. Engines in the 56-560 kW range will satisfy this requirement by satisfying the low load cycle requirement mentioned above. All other power categories have their own idle requirements. To comply with the NO_x idle test, engines need to reach a pre-determined temperature at 3.5% rated power for 30 minutes. If the engine can’t meet the idle NO_x requirement specified for that power category, an engine shutdown system will be required.

More Reliance on Engine Data

A new CARB provision will require all Tier 5 engines that have an electronic control unit (ECU) to meet certain diagnostic requirements. Engines that incorporate an SCR must have an onboard monitoring & diagnostics (OBMD) system, while engines without a SCR can utilize an on-board diagnostic (OBD) system. The implementation phase includes several years of data gathering (2029-2034) which varies by power category. Enforcement would start in 2036 and 2037, varying by power category.

CARB has also devised new Off-Road In-Use Testing (ORIUT) for engines in the 56-560 kW range. The purpose to demonstrate real-world in-use emission control of NO_x and PM.

“This involves a collection of big data,” Neva said. Manufacturers can leverage the onboard sensors that are part of the aftertreatment system to monitor NO_x and PM using Off-Road Real Emissions Assessment Logging (OR-REAL). Mandatory reporting is also part of this requirement. Annually, engine manufacturers must submit a defined set of data on all engine families with at least 75% of the engine population reported. On a positive note, for engine families that are determined “clean” during the ORIUT, this new testing would exempt these engine families from a more burdensome compliance testing system known Portable Emission Measurement System (PEMS).

Warranties and Recalls

CARB has proposed to increase an engine’s useful life and warranty timeframe by roughly 50%, though engine hours will remain the same. For example, variable-speed engines of 37 kW and above will see their useful life increase from 10 to 15 years, and their warranty period extend from five to eight years.

Another new proposal relates to the Exhaust Gas Recirculation (EGR) cooler. The minimum maintenance interval has been 1,500 hours. CARB is looking to extend that to 3,000 hours for engines under 130 kW, and 4,500 hours for engines 130 kW and above.

Additional warranty-related changes are also being proposed. First, any emission-related component with a minimum maintenance interval that is shorter than the warranty period will need to be covered through the entire warranty period. CARB has also proposed that there will be recalls when aftertreatment devices fail, including onboard computers, urea dozers and hydrocarbon injectors.

CARB is also looking to decrease the threshold for when defects need to be addressed for large-volume products. For example, the Tier 4 threshold to take corrective action for machines under 560 kW was the greater of 50 engines or 4%. CARB’s proposal changes that to 12 engines or 4%.

Proposed Implementation

With respect to implementation of the proposed Tier 5 standards, engine manufacturers will be allowed to choose from four options:

• Direct compliance with Tier 5 Interim standards
• Phase-in/out compliance with 50% of engine sales at Tier 5 Final level and 50% Tier 4 Final, but meeting the Tier 5 Interim timeframe
• Direct compliance with Tier 5 Interim using California Average, Banking, and Trading (CA-ABT) credits
• Longer continuance of Tier 4 Final engines in exchange for an earlier introduction of Tier 5 Final engines

Which option a manufacturer chooses will impact the timing of their implementation schedule, as will the power category of the engine being certified. Generally speaking, smaller engines under 130 kW are looking at a 2031-2034 timeframe, 2029-2033 for 130-560 kW engines, and 2030-2034 for engines over 560 kW.

Again, it must be noted that CARB Tier 5 is currently in its proposal stages. CARB is continuing to seek comments as it works through the process. AEM’s Engine Emissions Technical Committee has submitted several comments, including concerns it has regarding the implementation period and the proposed Transition Program for Equipment Manufacturers (TPEM) that provides some implementation flexibility to equipment manufacturers.

For more information on CARB’s Tier 5 proposal, contact AEM’s Travis Webb at twebb@aem.org.

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