USA: Heavy-Duty Vehicles: GHG Emissions & Fuel Economy

Background

Initially, US GHG emissions and fuel efficiency standards for heavy- and medium-duty vehicles were jointly developed by the Environmental Protection Agency (EPA) and the National Highway Traffic Safety Administration (NHTSA), DOT. The NHTSA developed fuel consumption standards under the authority of the 2007 Energy Independence and Security Act (EISA), while the EPA developed a GHG emissions program under the Clean Air Act. Later, the EPA developed GHG emission standards without accompanying NHTSA fuel consumption standards. The GHG program includes CO₂ emission standards, as well as emission standards for N₂O and CH₄, and provisions to control hydrofluorocarbon leaks from air conditioning systems.

The standards are applicable to all on-road vehicles rated at a GVW≥8,500 lbs, and the engines that power them, except those covered by the GHG emissions and Corporate Average Fuel Economy (CAFE) standards for MY 2012-2016 and MY 2017-2025 light-duty vehicles. The GHG/FE standards were adopted in three phases:

1. Phase 1 regulation—adopted on August 9, 2011 [2324]—covered model years (MY) 2014-2018, with NHTSA fuel economy standards being voluntary in MY 2014-2015 to satisfy EISA lead time requirements.
2. Phase 2 regulation—published on August 16, 2016 [2918]—applies to MY 2021-2027 vehicles. The Phase 2 rule also introduced new standards for trailers, a category not previously regulated. The EPA trailer standards began in MY 2018 (for certain trailers), while NHTSA’s standards took effect in MY 2021.

   Note: In November 2021, the trailer standards were blocked by a court, in a case brought by the Truck Trailer Manufacturers Association (TTMA). The court ruled that the EPA and NHTSA had exceeded their authority because trailers are not ‘motor vehicles’ and cannot be subject to fuel economy standards.

3. Phase 3 regulation—published April 22, 2024 [6327][6328]—established GHG limits for MY 2027-2032 heavy-duty vocational vehicles and tractors. The Phase 3 rule also revises GHG standards for MY 2027 Class 2b-7 vocational vehicles previously established under the Phase 2 rule. While the Phase 3 regulation does not apply to heavy-duty pickup trucks and vans, standards for these vehicles were included in the “Multi Pollutant Emissions Standards for Model Years 2027 and Later Light-Duty and Medium-Duty Vehicles” published April 18, 2024 [6330].

The Phase 1/2 standards are applicable federally, as well as in California. However, the California Air Resources Board (CARB) deemed the federal Phase 2 program not sufficiently strong to meet California GHG emission reduction goals. CARB started the development of California Phase 2 GHG emission standards for heavy-duty vehicles that would provide GHG emission reductions beyond those of the federal Phase 2 program but eventually California aligned with the federal Phase 2 standards in structure, timing, and stringency—with some minor differences in credit provisions, certification and reporting requirements and labeling. While California Phase 2 standards include GHG requirements applicable to 2020 and later trailers, enforcement was suspended [6332]. California also has zero-emission vehicle (ZEV) requirements applicable to MY 2024 and later heavy-duty vehicles.

Under the Phase 1 to Phase 3 regulations, different CO₂ and fuel consumption standards are applicable to different categories of vehicles, including combination tractors, trailers, vocational vehicles, and heavy-duty pickups and vans:

• Combination tractors (the semi-trucks that typically pull trailers): Phase 1 engine and vehicle standards begin in MY 2014 and achieve 7-20% reduction in CO₂ emissions and fuel consumption by MY 2017 over the 2010 baselines. Phase 2 standards begin in MY 2021 and achieve 15-27% reduction in CO₂ emissions by MY 2027 over the 2017 baselines. Phase 3 standards further reduce CO₂ emissions by 25-40% by MY 2032 relative to Phase 2 standards.
• Trailers: The standards start in MY 2018 and achieve 6-10% reduction in fuel consumption and CO₂ emissions by MY 2027 over the 2017 baselines (the standards have been blocked by a court).
• Vocational vehicles: Phase 1 engine and vehicle standards start in MY 2014 and achieve up to a 10% reduction in fuel consumption and CO₂ emissions by MY 2017 over the 2010 baselines. Phase 2 standards start in MY 2021 and require a 10-18% reduction in CO₂ emissions from gasoline vehicles and a 12-24% CO₂ emission reduction from diesel vehicles by MY 2027 over the 2017 baselines. Phase 3 standards further reduce CO₂ emissions by 30-60% by MY 2032 relative to Phase 2 standards.
• Heavy-duty pickup trucks and vans: Phase 1 standards phase-in starting in MY 2014 and achieve up to a 10% reduction in CO₂ emissions and fuel consumption for gasoline vehicles and 15% reduction for diesel vehicles by MY 2018. Phase 2 standards require a 16% CO₂ emissions reduction from MY 2021 to MY 2027. In a rulemaking activity separate from the Phase 3 regulation and covering MY 2027 to MY 2032 heavy-duty pickup trucks and vans, the EPA projects a 44% reduction in emissions by 2032 relative to the MY 2026 standards [6330]. Also in 2024, the NHTSA published fuel efficiency standards for heavy-duty (Class 2b/3) pickup trucks and vans, that will increase 10% per year for model years 2030-2032 and 8% per year for model years 2033-2035 and result in a fleetwide average of approximately 35 mpg by model year 2035 [6331].

The majority of vehicles covered by the regulations carry payloads of goods or equipment, in addition to passengers. To account for this, the standards have been expressed using two types of metrics:

1. Gram CO₂ per ton-mile (and gallon of fuel per 1,000 ton-mile) standards for vocational vehicles and combination tractors; and
2. Payload-dependent gram CO₂ per mile (and gallon of fuel per 100-mile) standards for pickups and vans.

Zero-Emission Vehicles. The Phase 3 regulation represents an implicit zero emission vehicle mandate. As the Phase 3 CO₂ standards are too demanding for vehicles powered by internal combustion engines, manufacturers are forced to introduce zero tailpipe emission vehicles, such as battery electric trucks, to their vehicle mix.

The EPA created several scenarios for the expected market penetration of heavy-duty ZEVs. In one scenario, the agency projected that in model year 2032, the Phase 3 new vehicle technology mix will include 60% ZEVs in the light heavy-duty vocational category, and 25% ZEVs among long haul tractors.

In another example, the EPA projected the following 2032 vehicle technology mix:

• Light heavy-duty vocational vehicles: 1% ICEV, 5% natural gas, 40% HEV, 24% H2-ICE
• Long haul tractors: 64% ICEV, 5% natural gas (NG), 10% HEV, 17% H2-ICE

Testing & Compliance Determination. The requirements for tractors and vocational vehicles include both engine and vehicle standards. Engine manufacturers are subject to the engine standards. Testing is conducted over one test cycle:

• Tractor engines are tested over the steady-state SET test,
• Vocational engines are tested over the FTP transient test.

The Phase 2 regulation introduced a new set of SET weighting factors, applicable only to GHG measurements for Phase 2 vehicles (not applicable to pollutant emission measurements). The new weighting factors address the trend towards engine downspeeding.

Chassis manufacturers are subject to the vehicle standards. The regulations do not require chassis testing—vehicle standards compliance is typically determined based on a vehicle simulation model, called the Greenhouse gas Emission Model (GEM), developed by the EPA specifically for the GHG/FE regulations. The Phase 2 regulation introduced a number of changes to the GEM model.

Instead of using a chassis dynamometer as an indirect way to evaluate real-world operation and performance, various characteristics of the vehicle are measured and these measurements are used as inputs to the model. These characteristics relate to key technologies applicable to a given truck category—including aerodynamic features, weight reductions, tire rolling resistance, the presence of idle-reducing technology, vehicle speed limiters, and other factors.

CO₂ and Fuel Consumption Standards

Combination Tractors

Differentiated standards were adopted for several subcategories of combination tractors based on three attributes: weight class, cab type and roof height. The standards phase in to the 2017 (Phase 1), 2027 (Phase 2) and 2032 (Phase 3) levels shown in Table 1.

The regulations also define standards for tractors during the phase-in periods: for MY 2014-2017, MY 2021-2026, and MY 2028-2031.

In addition to vehicle standards, engine-based standards must be met by heavy-heavy-duty (HHD) and medium-heavy-duty (MHD) diesel engines used in tractors, Table 2. These standards were unaffected by the Phase 3 regulation.

CO₂ emissions are tested on the same engine that is tested for pollutant emissions—typically the highest rated engine within an engine family. While this is the “worst case” rating for meeting pollutant emission standards, it is typically the rating with the lowest specific CO₂ emissions within the engine family.

Trailers

The Phase 2 program introduced a new set of standards to promote the efficiency of commercial trailers. The first trailer standards become effective in MY 2018. The final 2027 standards are shown in Table 3.

Note: In 2021, the trailer standards have been blocked by a court.

The Phase 2 rule also includes CO₂/FE standards for partial-aero box vans, as well as design-based standards (tire rolling resistance level and pressure system) for non-box trailers and non-aero box vans. Both sets of standards are phased in over 2018-2021.

Vocational Trucks

At Phase 1, this vehicle segment was divided into three regulatory subcategories—Light Heavy (Class 2b through 5), Medium Heavy (Class 6 and 7), and Heavy Heavy (Class 8)—which is consistent with engine classifications. At Phase 2, the standards were further differentiated depending on engine type (diesel, gasoline) and the duty cycle: urban, multi-purpose and regional.

The Phase 3 regulations tightened the 2027 Phase 2 CO₂ emission limits for Class 2b-5 CI vehicles by 17%, Class 2b-5 SI vehicles by 15%, Class 6-7 CI vehicles by 13% and Class 6-7 SI vehicles by 11%. Accompanying changes to the NHTSA fuel consumption limits were not provided.

The final Phase 1 (2017), Phase 2 (2027), and Phase 3 (2032) vehicle standards are depicted in Table 4, Table 5, and Table 6, respectively.

Engine standards for light heavy-duty (LHD), medium heavy-duty (MHD), heavy heavy-duty (HHD) diesel engines and for heavy-duty gasoline engines are shown in Table 7. These standards were unaffected by the Phase 3 regulation.

Heavy-Duty Pickup Trucks and Vans

The commercial pickup and van category includes those vehicles of GVWR from 8,501 to 14,000 lbs that are not regulated under the light-duty GHG/CAFE vehicle regulations. These vehicles must meet corporate average CO₂ and fuel economy standards, in an approach similar to that taken for light-duty vehicles, but with different standards for gasoline and diesel vehicles.

The EPA has established CO₂ standards in the form of a set of target standard curves, based on a “work factor” that combines a vehicle’s payload, towing capabilities, and whether or not it has 4-wheel drive. The standards phase in with increasing stringency in each model year. The EPA standards include a separate standard to control air conditioning system leakage. NHTSA has set corporate average standards for fuel consumption that are equivalent to EPA‘s standards (though not including the EPA’s air conditioning leakage standard).

At Phase 1, manufacturers were provided with two alternative phase-in approaches. One alternative phased in the final standards at 15-20-40-60-100 percent in model years 2014-2015-2016-2017-2018. The other phased in the final standards at 15-20-67-67-67-100 percent in model years 2014-2015-2016-2017-2018-2019. Phase 2 standards are phased in over model years 2021-2027.

While the Phase 3 regulation did apply to these vehicles, the EPA published the “Multi Pollutant Emissions Standards for Model Years 2027 and Later Light-Duty and Medium-Duty Vehicles” in 2024 that phased in requirements for these vehicles over model years 2027-2032 [6330]. In June 2024, NHTSA announced fuel efficiency for MY 2030 to 2035 heavy-duty pickup trucks and vans [6331].

N₂O and CH₄ Emission Standards

The regulations introduced emission standards for nitrous oxide and methane:

• Engine testing (tractors & vocational, both tested over the FTP cycle): N₂O = 0.10 g/bhp-hr; CH₄ = 0.10 g/bhp-hr
• Chassis testing (pick-ups and vans, 55% FTP-75 + 45% HFET): N₂O = 0.05 g/mi; CH₄ = 0.05 g/mi (unlike the CO₂ standards, averaging between vehicles is not allowed)

Testing requirements started in MY 2015, consistently with the N₂O/CH₄ requirements for light-duty vehicles. The standards were designed to cap emissions at the levels circa 2010 to prevent N₂O/CH₄ emission increases in future engines.

Other Provisions

Flexibilities. The regulations include certain flexibilities for vehicle manufacturers. The Phase 1 rule included optional compliance schedules, with more relaxed standards to be met from 2013, and with identical final standards. At Phase 2, optional sets of CO₂ and fuel economy standards are available for some vehicle categories, which use different GEM model inputs.

A/C Leakage. EPA has adopted standards to assure that low-leakage components are used in air conditioning systems designed for heavy-duty pickup trucks and vans, and semi trucks. The standard for larger A/C systems (capacity above 733 g) is measured in percent total refrigerant leakage per year, while the standard for smaller A/C systems (capacity of 733 g or less) is measured in grams of refrigerant leakage per year.

Useful Life. The EPA CO₂ emissions must be met over the engine’s and vehicle’s useful life. The useful life definitions for engines and for vehicles that use the respective engine categories are identical to those defined for criteria pollutant standards for MY 2004 and later heavy-duty engines and Tier 2 & Tier 3 standards for light-duty vehicles:

• LHDDE—Phase 1: 110,000 miles/10 years; Phase 2 and later: 150,000 miles/15 years
• MHDDE—185,000 miles/10 years
• HHDDE—435,000 miles/10 years