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Please note: This summary is provided to help you understand the regulations. Consult the references provided for links to the full text of the regulations.

Zero Emission Vehicles

Road Rail Air Water

This page summarizes the key advances in technology that have made zero emission vehicles a viable option for a wide range of applications. Environmental regulations covering trucks of all sizes are likely to change as a result of these advances. The page includes a summary of the most recent changes to EPA’s greenhouse gas emissions standards for heavy-duty vehicles.

Who is covered by the regulations?

The vehicle and engine performance standards specified in the existing Phase 2 Greenhouse Gas Emissions regulations, published October 25, 2016, cover "model years 2021-2027 for semi-trucks, large pickup trucks, vans, and all types and sizes of buses and work trucks."

An updated set of greenhouse gas regulations (Phase 3) was made final by EPA on March 29, 2024. The scope of the Phase 3 regulations is described by EPA as follows: "The new standards will be applicable to HD vocational vehicles (such as delivery trucks, refuse haulers, public utility trucks, transit, shuttle, school buses, etc.) and tractors (such as day cabs and sleeper cabs on tractor-trailer trucks)."

What is the purpose of the regulations?

The ultimate purpose of the regulations, "to reduce greenhouse gas emissions (GHG) and increase fuel efficiency for on-road heavy-duty vehicles", has not changed between Phase 2 and Phase 3, but the world has. As outlined below, the technological and economic outlook for vehicles of all types has undergone some radical changes since 2016. The rules have been developed in response to those changes. According to an EPA announcement, "The ... standards align with and support the commitments and billions of dollars’ worth of investments from trucking fleets, vehicle manufacturers, and U.S. states as they plan to increase the use of zero-emissions technologies in heavy-duty fleets."

Summary of recent technology developments

The new factor in the calculation is the lithium-ion battery. The lithium-ion battery has changed the rules. This is not the result of a new or sudden development, but of a long period of steady improvement. What has happened recently is that the improvements have extended the capabilities of the battery to a threshhold beyond which it makes economic and practical sense to use batteries as the source of motive power to run light duty vehicles. In 2016, only a fraction of a percent of U. S. light duty vehicle sales were powered by batteries alone; by 2023, that had risen to 6.7%, on track to exceed one million vehicles in the U. S. alone in 2023.

These successes are fueling accelerating improvement. As larger battery pack sizes become available, manufacturers are beginning to offer battery powered heavy-duty equipment for an increasingly wide range of applications. Battery power is starting to become a popular choice for fixed-route vehicles, such as delivery trucks and school buses.

The use of battery powered vehicles for heavy-duty long-haul transport has been limited by the need to recharge batteries after a much shorter travel distance than Diesel-powered vehicles can travel before refueling, and by the fact that it takes significantly longer to recharge batteries than to refill a tank. Higher capacity batteries and faster recharge times are starting to close the gap. But the gap is wide: a Diesel-powered Class 8 vehicle averaging 7 miles per gallon and carrying a full 150 gallon tank can travel over a thousand miles without refueling. The best that currently available battery-powered Class 8 vehicles can offer is closer to 300 miles. That will doubtlessly improve in the future, but is likely to remain a fact of life for some time.

A variety of different technology approaches are being actively developed to address this performance gap:

  • One option is to continue to use an internal combustion engine, but to use something other than a fossil fuel to burn in it. Alternatives range from zero-carbon alternatives like hydrogen to fuels from "recent" carbon, like plant-derived alcohols or oils, food wastes, or landfill gas. This addresses the greenhouse gas problem, but still requires emission controls to deal with non-GHG pollutants like particles and nitrogen oxides.
  • Another approach is based on fuel cell technology, which can turn fuels like hydrogen directly into electric power. This can avoid any emissions problem, since the reactions involved are happening at a low enough temperature to avoid creating nitrogen oxides, so only water is emitted. The electric power can be used directly to turn the wheels or (more commonly) to keep an on-board battery pack recharged. (A fuel cell big enough to supply enough power for acceleration and hill-climbing at acceptable speeds would be unacceptably expensive).

This activity to find alternatives for Diesel power is not being driven by environmental considerations alone, but would arguably be happening even if greenhouse gases were not recognized as a problem. Even with limited range, battery-powered vehicles are beginning to demonstrate some inherent advantages over Diesel power. Once significant electric power storage is available on a vehicle, it becomes possible to recover some of the power which would otherwise be lost (dissipated by brake heating) when the vehicle is decelerating and going downhill, through regenerative braking. Stored power can be used to power auxiliary equipment (e.g. for refrigeration, or cab temperature control) or to extend the range of the vehicle (to a limited extent). Now that battery packs for vehicles have become powerful enough to serve as the sole source of motive power, other advantages are becoming apparent. Battery-powered vehicles need no transmissions, deliver instant power on demand, and are much easier to maintain than Diesels.

If battery electric vehicles continue to demonstrate both performance and maintainability advantages over Diesel, other alternatives to extending the range of the vehicles start to look feasible. For example, fleets maintaining long haul routes might find it advantageous to stage mobile battery packs on trailers, precharged along the route. Trucks could be recharged in the time it takes to unhitch and reattach a power module, and the spent module could be recharged while waiting for the next truck to arrive.

If it makes so much sense, why isn't it happening? In 2023, the economics would be prohibitive. Using figures from the regulatory impact analysis issued with the Phase 3 rules, a Class 8 combination, sleeper cab - heavy haul (to take a particularly demanding case) requires 1,261 kilowatt-hours (kWh) of stored energy to keep that size truck moving for a full 8 hour shift. For batteries available in 2023, $135 per kWh is a typical cost. On that scale, the cost for the batteries alone, just for a single staged module, would exceed $175,000, about the cost of a whole new Diesel vehicle.

Prices are forecast to decrease as the technology improves, and as manufacturing capacity scales up. The question is whether prices will decline all the way to the next significant threshhold: the point that the extra revenue to be gained by keeping the vehicle on the road for the additional hours would return the initial investment in a reasonable time. In that case, the added complexity of having an additional power source on board simply to keep the battery charged would no longer make sense.

Of course, there is no guarantee that battery costs per kWh will continue to fall as they have. The Phase 3 regulations make no assumptions concerning which technology will ultimately prevail. In the next section, the regulations will be summarized and compared to the existing rules, to provide some idea of EPA's expectations.


Greenhouse gas regulation differs in some key respects from toxic air emission regulation. One particularly clear example is the technological remedy for nitrogen oxide pollution, an emission control system. Installing such a system in the vehicle exhaust stream unavoidably involves extra cost due to lower fuel economy and the continual need for maintenance. In contrast, the only practical way to control carbon dioxide emissions is to burn less carbon, which equals better fuel economy, and lower costs. So while toxic emissions regulation can involve pushing against economic forces, greenhouse gas regulation is economically downhill, and the rules can lock in lower emissions by simply following the market.

In proposing the Phase 3 rules, EPA has sought to take advantage of the growing availability of low carbon options. EPA is able to avoid mandating any single technology by allowing vehicles powered by a broad range of different technologies to be sold during any model year, not specifying any particular choices, but requiring only that, for the mixture of technologies sold by any manufacturer, the overall rate of greenhouse gas emissions fall under the limit set for that model year. To justify the emission limits, EPA has to make the case that the alternatives are not only available, but will actually be adopted by a significant portion of the industry. That case is much easier to make if EPA can show clear cost advantages in adopting the technology.

Alongside the actual text of the Phase 2 regulations, as it appears in the Code of Federal Regulations (CFR), EPA issued a Regulatory Impact Analysis for Phase 2, which spells out the industry information and assumptions that EPA used to arrive at the limits specified in the regulations. The basis for the Phase 3 regulations, spelled out in a proposed rule published in the Federal Register, is also accompanied by a Draft Regulatory Impact Analysis for Phase 3. The text in the CFR should be consulted for the definitive statement of the rules as they currently stand, but it is the Impact Analyses that provide the best window into the development of EPA's understanding of how current developments will play out. Accordingly, this section will focus on how the Impact Analyses have changed between 2016 and 2023.

Both the 2016 and 2023 analyses identify a collection of technologies and improvements in operations that have demonstrated the ability to accomplish the same amount of work with lower greenhouse gas emissions than have been demonstrated with the existing technology. The differences between 2016 and 2023 stand out when the lists of potential improvements considered by each analysis are compared.

The types of potential improvements listed in the Phase 2 analysis included

  • vehicle-level technologies:
    • idle reduction
    • improved tire rolling resistance, transmissions, axles, and accessories
    • weight reduction
    • aerodynamic technologies
  • engine-level technologies:
    • friction reduction
    • variable valve timing
    • cylinder deactivation
    • turbocharging
    • downsizing
    • combustion optimization
    • aftertreatment optimization
    • waste heat recovery
In other words, the Phase 2 analysis took into account mainly incremental improvements in the existing technology, with little consideration for radically new possibilities.

Compare these topics with the corresponding scope of the Phase 3 analysis:

  • Internal combustion engines (ICE)
  • hybrid, and plug-in hybrid powertrains
  • hydrogen ICEs
  • battery electric vehicles (BEVs)
  • fuel cell electric vehicles (FCEVs)

With these new technologies factored into the mix, the same amount of work can be done in 2023 with substantially less carbon dioxide emission than was possible in 2016. Accordingly, in the Phase 3 standards, EPA has required a revised set of emission limits for heavy-duty vehicles for model years through 2032.

If market forces are likely to move the emission rate in the desired direction even with no regulatory intervention, why the need to establish limits and enforce them with penalties? One advantage to announcing limits several years in advance is that it gives manufacturers some certainty. Product design begins many years in advance of product availability. Designing products for optimal performance involves numerous tradeoffs. If manufacturers are guessing at what will be considered acceptable in the next decade, only a few will have guessed right. If they are all shooting at a known target, they have a better chance of converging on a range of vehicles that will meet both the emission requirements and the performance levels expected in the future. As a result, buyers of new heavy-duty vehicles are likely to have a better set of choices in 2032 than they would have in the absence of set limits.

Even granting that both manufacturers and the trucking industry will be better off in 2032 if fixed limits are establshed, EPA is still faced with the task of determining what those limits should be, and defending the choice. EPA does not have to demonstrate that it has made the best or optimal choice, but it does have to show that the target limits can be met in practice. It can't count on any one particular technology taking over the market, nor can it assume that current trends will continue.

The method used by EPA to derive a defensible set of target limits is spelled out in detail in the phase 3 impact analysis. The main steps in their method are:

  • For each type of equipment falling under the regulations, determine the energy and power needed to perform a standard unit of work (for example, the energy needed to move one ton of freight one mile, expressed in units such as kilowatt hours per ton-mile)
  • For each technology being considered, determine the amount of greenhouse gas emitted if that technology is used to perform the standard work unit (expressed in units such as grams of carbon dioxide emitted per ton-mile)
  • Assume that lower emission technologies will begin to capture an increasing share of the new vehicle market, and in consequence the rate of emissions per unit of work will decrease. EPA must make the case that the target limits are attainable, given plausible assumptions about what percentage of total sales will be replaced by lower emitting technologies.
A "manufacturer, seller, or importer" of each type of equipment must then determine whether the mix of new vehicles sold during a given model year complies with the limit set for that model year. Software provided by EPA includes the numerical value of the emission rate per work unit for each technology type. The manufacturer can supply data on how many vehicles of each technology type were sold that year. With that input, the software can calculate a weighted average over the manufacturer's total sales. That overall fleet average emissions rate is what the manufacturer must meet to be in compliance.

While EPA can provide a reasonably airtight justification for energy requirements and emission rates, the third leg of the method depends, not on physical variables, but on the degree to which purchasers are assumed to adopt new (and particularly zero emission) technologies. Using data on actual and projected costs, EPA's analysis shows that investing money and effort in switching to unfamiliar technologies will almost certainly make economic sense, in terms of the time it will take to repay the investment. Whether purchasers will take the rational course is another matter.

Consider that, even after the inherent advantages of battery electric vehicles have been demonstrated in practice, and are clearly understood by potential purchasers, some caution might arise from other considerations. For example, a purchaser contemplating a switch to electric may reasonably conclude that the century-old Diesel supply chain may be better established and more reliable than the much more recently established battery supply chain. If the vehicle is out of service for several weeks waiting for a replacement part, its economic and technical advantages could evaporate quickly.

At this point, the market for light vehicles is currently developing somewhat faster than the market for heavy vehicles. Data on the rate of technology adoption in the light vehicle market is becoming available as the phase 3 rule is being put into practice. In addition, as the light vehicle sector scales up, inevitable wrinkles in the supply chain will be encountered, and confidence in the new technology will grow (or not) depending on how successfully the challenges are met. Ideally, if confidence in the new technology does grow, and if the supply chain proves to be up to the challenges of scale, the goals of the regulations could be met via individual purchasers' choices, rather than being driven by regulatory pressure.

EPA Resources

  • Compliance software:
    • Greenhouse Gas Emissions Model (GEM) for Medium- and Heavy-Duty Vehicle Compliance
    • HD TRUCS (Excel file) A modeling tool supplied by EPA for "evaluating the design features needed to meet the energy and power demands of 101 representative HD vehicle types that cover the full range of weight classes within the scope of the proposed standards in EPA's Greenhouse Gas Emissions Standards for Heavy-Duty Vehicles - Phase 3 rulemaking (i.e., Class 2b through 8 vocational vehicles and tractors)".

More Resources

The Department of Energy provides an Alternative Fuel and Advanced Vehicle Search page, where users can find and compare alternative fuel vehicles, engines, and hybrid/conversion systems.