NAS Report on Truck Fuels: Use Biodiesel, Renewable Diesel

“Industry, NHTSA, EPA, and other relevant agencies should promote the use of biodiesel and renewable diesel in MHDV engines, prioritize the development and application of additive manufacturing and other promising manufacturing innovations, improve freight movement efficiency, and collect real-world fuel consumption and GHG emissions data to establish a regulatory baseline,” says a report issued by The National Academies of Sciences, Engineering, Medicine. Such a baseline “is essential for evaluating the effectiveness of regulations and identifying future priorities,” the report says. MHDV stands for medium- and heavy-duty vehicles.

Tractor-trailers, buses, commercial trucks, and other heavy vehicles are significant contributors in the transportation sector to energy consumption and greenhouse gas (GHG) emissions, accounting for approximately 22 percent of energy use by U.S. transportation.

An excerpt from the report is reprinted here. The excerpt addresses the technological and regulatory issues related to future regulations for MHDV fuel efficiency and GHG emissions. It recommends the National Highway Traffic Safety Administration (NHTSA) and U.S. Environmental Protection Agency (EPA) conduct an interim evaluation of MHDV regulations in 2021-2022 to ensure and improve their effectiveness and value. This evaluation should address vehicle technologies, fuel mix, efficiency of the freight system, and future regulatory frameworks.

The fuel consumption of MHDVs has only recently begun to be regulated at the federal level. To provide periodic advice on establishing fuel economy metrics and standards that are appropriate, cost-effective, and technologically feasible for commercial MHDVs, the Energy Independence and Security Act of 2007 mandated a series of studies from the National Academies.

(Note: A PDF of the complete 439-page report, published by National Academies Press, is available free at http://nap.edu/25542.)

VEHICLE TECHNOLOGY PROGRESS

The energy efficiency of the two widely used conventional engine platforms—compression ignition (i.e., diesel) and spark ignition—can and is still being improved. Diesel engines using the well-known diffusion-dominated compression-ignition combustion are presently the most efficient engines for MHDVs, with progress evident for further reductions in CO2 and fuel consumption, and NOx emissions.

Supported by the U.S. Department of Energy’s SuperTruck initiative, demonstration of 55 percent peak engine efficiency in a research vehicle environment may occur in the next few years. Improvements in waste-heat recovery will be a necessary precursor to reaching such efficiencies with current production engine architectures.

Spark-ignition (SI) engines continue to evolve and improve, with potential to reach more than 40 percent peak brake thermal efficiency while still achieving stringent criteria emissions with relatively low-cost aftertreatment. Even higher efficiency would be achievable in technology pathways where a high fuel octane number is used. Such pathways are well suited to renewable fuels such as ethanol or fuels with low carbon, like natural gas or propane. Based on emerging experimental and modeling results, the expected efficiency of SI engines could exceed the Phase II regulatory requirements, suggesting such engines could attain more stringent regulations depending on the extent that advanced technologies for passenger vehicle engines are adapted to MHDV gasoline engines and based on duty cycle considerations.

The current regulatory structure, which focuses on engine and truck fuel consumption and GHG emission standards, does not appear to have regulatory flexibility to address advanced fuels and methods of improving the efficiency of goods (i.e., freight) movement. For example, increases in heavy-duty vehicle size and weight limits could reduce total system fuel consumption; however, the authority to make such changes lies with Congress.

For Class 8 combination tractor-trailers, a total weight reduction of at least 4,000 pounds appears achievable, which would produce approximately 3 to 4 percent reduction in fuel consumption compared to trucks without light-weighting features, depending on duty cycle. Since most trucks operate at less than maximum weight limit, reduced weight benefits fuel consumption as well as load-specific fuel consumption (LSFC), with the impact of weight reduction greatest in the smaller truck classes.

The committee’s technology review, projections, and simulations indicate that, depending on vehicle class and the progress in engine efficiency within each, roughly 12 to 19 percent reduction in LSFC is technologically feasible for MHDVs in the 2030 time frame compared to the committee’s 2019 baseline. This expected feasible reduction pertains especially to Class 8 combination vehicles and their drive cycles that consume most of the fuel in the MHDV sector (whereas hybrid vehicles may achieve even greater reductions in fuel consumption in certain duties). Economic feasibility of such an efficiency improvement requires further analyses. This reduction is without the effects of significant changes in fuels or overall operational efficiencies. The committee also conducted simulations under an aggressive set of assumptions for engine efficiency (e.g., 55 percent peak thermal for a combination tractor) and key vehicle parameters, post-2027, and found roughly a 30 percent reduction over the 2019 baseline for the vehicle classes larger than a Class 2b pickup truck. These ambitious CO2 reductions at the vehicle level will need to be evaluated for sufficiency relative to meeting any national goals and international agreements to which the United States is a party. Substantial reductions greater than 30 percent in CO2 compared to a 2019 baseline may necessitate approaches beyond the vehicle such as efficiency gains from fuels, operations, and so forth.

Recommendation: NHTSA, in coordination with EPA, should evaluate and quantify the lifecycle GHG emissions and fuel consumption of all fuels and technologies whose use could contribute to meeting a third phase of standards, and take them into consideration in developing a third phase of regulation, in order to best accomplish overall goals. It will be critically important to incorporate a lifecycle perspective in those instances where some fuel-technology pathways life-cycle emissions may lead to an increase, rather than a decrease, in emissions. However, given the resources needed for detailed lifecycle analysis and uncertainty characterization, and given the resource constraints from the agencies, we would recommend assessing carefully which fuel-technology pathways may deserve specific attention in a life-cycle perspective.

Recommendation: NHTSA, in cooperation with EPA, should establish what MHDV GHG and fuel consumption reductions need to be achieved in the 2030 to 2050 time frame, consistent with national goals and international agreements to which the United States is a party.

Recommendation: Engine manufacturers, with appropriate engagement of NHTSA and EPA, should allow and promote use of biodiesel and renewable diesel in their engines. NHTSA, in concert with EPA, is encouraged to continue some types of incentives for use of biomass-derived diesel fuel, provided that analyses continue to show overall life-cycle GHG reduction. The agencies are encouraged to seek harmony and synergy between their separate rules for renewable fuels and tailpipe regulations to enable further GHG reduction and petroleum displacement.

Recommendation: NHTSA and EPA, through their representation in or coordination with the relevant entities within the U.S. Department of Commerce, the U.S. Department of Energy, and the Office of Science and Technology Policy, should prioritize development and application of additive manufacturing, materials joining processes, nanostructured materials, and other yet-to-be-identified promising manufacturing innovations on those parts and systems that offer the greatest potential for fuel consumption and GHG reduction. The progress of these technologies, their prioritization status, and prognosis for effective commercialization within the Phase III regulatory period should be included in the interim evaluation proposed by this report.

Recommendation: MHDV fuel consumption standards should include expectations for weight reduction adoption, with the likely potential for 3 to 4 percent reduction in load-specific fuel consumption at market-acceptable return on investment.

Recommendation: Government and industry should continue the development of higher-efficiency SI gasoline and natural gas engines for vocational vehicles and should continue to ensure substantial CO2 and fuel consumption reductions are realized, because (1) higher efficiency appears feasible and (2) market forces may cause a shift toward the SI engines.

ALTERNATIVE TECHNOLOGY AND APPROACHES

Implementation of new propulsion technologies, new low-carbon fuels, and more efficient freight operations and logistics offers the opportunity to reduce fuel consumption and GHG emissions beyond what is achievable from improving the efficiency of conventional MHDVs—which may be necessary to meet future climate goals. There are numerous alternative-configuration combustion engines. Many of these are merely alternative mechanisms to carry out thermodynamic processes very similar to conventional engines, but there are a few that provide clear enhancements via changing the basic cycles or reducing a prominent loss mechanism (such as heat transfer or friction). Few alternative concepts have reached a scale of development where they could be tested in a MHDV system for performance and emission requirements. With limited data found on alternative configurations, the magnitude of improvement over advanced conventional engines is not readily predictable and will depend on prevailing emission requirements.

For example, progress in understanding and demonstrating the benefits and challenges for kinetics-dominated combustion has continued, but there are still many recipes of the phenomena in flux, depending on fuel properties and stratification modes. Full-scale heavy-duty engines have been successfully demonstrated on test stands, and light-duty vehicle demonstrations have been conducted. A full commercial application is still pending.

Further, waste-heat recovery (WHR) used in Class 8 over-the-road vehicles potentially offers significant cost-effective fuel savings. Expected progress in WHR technology suggests this technology will see increasing penetration in Class 8 combination tractors. There are many approaches to waste-heat conversion to power that can provide up to 4 percent efficiency improvements in modern truck engines.

This effect is comparable to other engine improvements in combustion, air handling, friction, etc., and even comparable to many vehicle technology improvements. The WHR systems have proven technically feasible in a research vehicle, yet carry challenges in their weight, packaging, thermal response, and cost.

Progress in addressing the challenges is evident. The Regulatory Impact Analysis noted a potential 25 percent penetration in some Class 8 segments by the end of the Phase II rule period.

With respect to hybridization of commercial vehicles, several international light-duty vehicle manufacturers and Tier 1 battery suppliers are projecting costs for plug-in hybrid-electric or battery electric vehicle battery packs that will achieve $120/kWh by 2020 and $100/kWh or less by 2025. At these cost levels, significant market growth in electrified light-duty vehicles, worldwide, can be expected by 2030. However, due to the duty cycle, service life, and other boundary conditions for commercial vehicles, the battery-system costs are expected to reduce significantly by 2030 but will remain somewhat higher than light-duty battery systems.

Yet, the specific energy, charge and discharge rates, vehicle duty cycles, service life, and reliability requirements for commercial vehicles will limit the potential for direct carryover of battery systems from light-duty vehicles. Additional research and development, above that being done for light-duty applications, is necessary and could delay the market penetration of electrified commercial vehicles.

However, international regulatory incentives to promote combustion-free powertrains could support earlier introduction of electrified commercial vehicles than would normally occur in the technology and manufacturing cost-development cycle.

In 2027 and beyond, stop-start technology applications are expected to have payback periods that would make them attractive in many applications to private firms. The greater fuel savings, however, that are possible with the application of mild and strong hybrids are expected to be less attractive to private firms, given their relatively long payback periods. However, technological developments and likely cost evolution and potential life-cycle benefits results suggest that hybridization is a technology that is “next up” for meeting more stringent post–Phase II standards.

Further, petroleum-derived diesel fuel will likely remain the dominant CI engine fuel through the time period of this study (approximately 2030). Regarding changes to diesel fuel to reduce GHG emissions, the most effective measure would be additional use of biomass-derived fuel components (in contrast to changing diesel fuel performance specifications). Renewable diesel fuel (hydrogenation-derived renewable diesel) and biodiesel are both well developed, commercially available, and suitable for this path. They offer overall GHG reduction of up to about 80 percent relative to petroleum-derived diesel.

Since SI engines fueled with gasoline-based fuels are economically attractive for some market segments, GHG reductions with these engines can be enhanced by improving the performance properties of the fuel, such as octane number and maintaining or increasing the content of renewable fuels. The effect of a moderate increase in octane number (research octane number) on efficiency is 5 to 10 percent improvement, whereas the GHG reduction is on the order of 30 percent if renewable ethanol is the high-octane-number blending material.

The relatively high cost and long payback periods of “next-up” technologies—in particular, mild and strong hybrids—which will be necessary to meet aggressive reductions in fuel consumption in many of the medium- and heavy-duty truck segments, suggest that future efforts to increase fuel efficiency should look beyond vehicle technology solutions alone. Consideration should be given to actions that increase the efficiency of the overall freight system, as is described in Chapter 2.

A more integrated approach to systemic improvement in fuel efficiency would, however, require closer cooperation among government agencies than takes place today. A range of opportunities exists to improve energy efficiency and reduce GHG emissions in freight transportation, as detailed in Chapters 9 and 10. For example, modifying truck size and weight standards, facilitating intermodal shipments, and truck platooning could improve the fuel efficiency of freight shipments. Taking advantage of these opportunities will require action and leadership by the federal government, state and local governments (who own and operate the highway system), and private industry.

In considering methods and technologies for improving the efficiency of freight operations, the committee found that higher weight limits and longer combination vehicles could significantly improve productivity and therefore reduce the overall distance traveled in the heavy-vehicle long-haul transportation sector. In addition, the development of freight transfer facilities near urban areas would increase the use of more agile, fuel efficient, and less polluting vehicles for “last-mile” freight movements and would facilitate the early adoption of autonomous vehicles. As such, automated operation that enables truck platooning has the opportunity to save fuel and GHG emissions achieved through decreases in the drag of both trucks.

The commercialization and deployment of advanced technologies, fuels, and freight movement methods that are significantly different from those currently in use may need to start as early as 2030 if ambitious national GHG emissions reduction and fuel-economy goals are established for the 2050 time frame. Because of the long lead time needed to develop such advanced technologies and strategies, planning and preparation for such measures must begin well before the target dates for commercialization and implementation.

Recommendation: NHTSA should coordinate with EPA to identify a portfolio of technologies and options that could achieve national GHG emission and fuel-economy goals for the 2030 to 2050 time frame and to identify the research, planning, and preparation needed to commercialize and implement such technologies and options in the relevant time frame.

Recommendation: As SI engines continue to be improved, NHTSA and EPA should reassess the future balance in MHDVs between SI and CI, and the reductions in GHG emissions and fuel consumption that might be achieved with a more challenging efficiency requirement for SI engines, including an optimized low-carbon or renewable fuel.

Recommendation: NHTSA, together with EPA, the Department of Energy (DOE), and the Department of Defense (DOD), should maintain a technology assessment of developments and progress in alternative-configuration engines which evaluates them, analytically and experimentally, against benchmarks and fundamental criteria.

Recommendation: Industry and government should continue to research, develop, and apply waste-heat recovery (WHR) systems where technical and economic considerations are reasonable to capture this opportunity to reduce fuel consumption.

Since hybrid technologies and WHR potentially could play increasing roles in achieving reductions in fuel consumption in the post-2027 period, developments in the cost and efficiency of these technologies should be monitored and be included in a formal interim review of fuel-consumption standards.

Recommendation: NHTSA should coordinate with EPA to engage other agencies in the rulemaking process, such as DOE, Federal Motor Carrier Safety Administration, and Federal Highway Administration, who have authorities that can facilitate commercialization of low-carbon fuels and more efficient freight movement methods. NHTSA, in cooperation with EPA, should evaluate how incentives or other regulatory provisions can be incorporated to facilitate implementation of fuels and freight movement approaches that lie outside of NHTSA and EPA authorities.

Recommendation: NHTSA and DOT should consider urging Congress to adopt size and weight regulations that are in line with those of other trade partners.

ECONOMIC ASSESSMENT AND CONSIDERATIONS

The costs and benefits of specific measures to reduce fuel consumption can be estimated accurately in the near term. The committee had to consider technologies and approaches for medium- and heavy-duty vehicles that could be introduced in model year 2028 or later. Their costs and benefits cannot be estimated with accuracy at this time.

The committee chose to use the NHTSA Lifetime Benefits and Costs Evaluations and the NHTSA and EPA Regulatory Impact Analysis as the foundation of its evaluations. The analysis done by these agencies provided insight into the “marginal cost” and “marginal benefits” of going beyond the rule.

The committee assumed that the Phase II rule would take effect and the agencies would be considering a rule to take effect in the 2027 period and beyond. Under this assumption, the required 2.5 percent per annum improvement (needed to comply with the 2027 step of the Phase II rule) would already have been met and further improvements are being considered. In order to meet the higher standard, additional technology will need to be applied to the new vehicle fleet.

In its review of the Regulatory Impact Analysis, the committee found greater reductions are achieved, according to the results shown in Table 10-15 of that document, by the more widespread use of electrification technologies—electronic power steering and accessories, 12-volt stop-start, and strong hybrids. Other technologies either plateau at the high rates, such as eight-speed automatic transmission or turbocharging. Stop-start, mild hybrids, and strong hybrids appear to be a key incremental technology necessary to move beyond the proposed rule and can be used to measure the marginal cost—that is, the incremental cost of increasing fuel economy beyond the proposed rule—as well as the incremental benefit of compliance. Mass reduction and aerodynamic improvements also play a role in moving from 2.5 to 4.0 percent per annum improvement.

The committee’s own analysis, referenced in Chapters 6 through 8, generally supports the conclusions of the analysis of NHTSA and EPA. In addition, it finds the following:

• WHR used in Class 8 over-the-road vehicles potentially offers significant cost-effective fuel savings in the post-2027 period. Expected progress in WHR technology suggests that in the post-2027 period this technology will see increasing penetration in Class 8 tractors.

• In 2027 and beyond, stop-start technology applications are expected to have payback periods that would make them attractive in many applications to private firms. The greater fuel savings, however, that are possible with the application of mild and strong hybrids are expected to be less attractive to private firms, given their relatively long payback periods.

• Considering 10-year fuel savings, however, breakeven fuel prices are either below or at the social cost of fuel in many hybrid applications. These results suggest that hybridization is a technology that is “next up” for meeting more stringent post–Phase II standards. Only in Class 2b does the break-even fuel price for full hybrids exceed the social cost of fuel.

• If the anticipated cost reductions and efficiency gains are in fact consistently achieved, in the post-2027 period, increases in fuel efficiency beyond the Phase II rule are likely to be met with increasing applications of hybrid technologies, including stop-start, mild, and strong hybrids. The technology penetration will vary depending on vehicle class and duty cycle.

• The cost of technology tends to fall over time as experience accumulates and firms invest in research and development. Projecting these cost developments is difficult because of the variability in experiences—in the characteristics of specific technologies and in the level of firm investments. In order to capture learning effects, NHTSA and EPA use a complicated algorithm to project how technology costs will evolve over time. While there is ample evidence of cost reduction through learning, the empirical evidence does not support these agencies’ complex procedure. Recommendation: NHTSA and EPA should employ a simpler and more transparent method and rationale on how the unit costs of technologies will evolve, either over time or as a function of the production capacity, in their future assessment. Two options the agencies should consider include the following: (1) assuming specific costs per unit over time for each of the technologies under study, informed by a manufacturers’ and experts’ elicitation of likely costs in the future, and (2) applying simple parametric one-factor learning curves, complemented with a sensitivity analysis, to assess how high or low these learning factors would need to be for a technology to be more economically appealing than another. The agencies also are encouraged to do ex post assessments of their forecasts of learning effects.

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