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Real Prospects for Energy Efficiency in the United States 3 Energy Efficiency in Transportation Energy efficiency in the U.S. transportation sector merits special attention from the standpoint of energy security and the environment because this sector is almost solely dependent on a single fuel—petroleum—about 60 percent of which is imported. Moreover, the transportation sector is responsible for about 30 percent of U.S. emissions of greenhouse gases. 3.1 SCOPE AND CONTENT OF THIS CHAPTER This chapter describes the U.S. transportation system and its energy consumption. It identifies near-term (through 2020) opportunities for energy efficiency and the technologies that could capitalize on them. (See Box 3.1 for definitions of fuel efficiency, fuel economy, and fuel consumption.) It considers technologies that could improve energy efficiency in the medium term (through 2030–2035), as well as longer-term opportunities that could stem from technologies that are now at an early stage of research and development (R&D). Finally, it touches on the possibilities for broader changes in transportation systems. Reflecting the charge to the Panel on Energy Efficiency Technologies, the transportation technologies covered here are described in terms of their performance (improvements in energy efficiency and fuel consumption), their costs, and their effects on the environment (mainly reductions in greenhouse gas emissions). This review is not an in-depth study of all the factors that could improve technology performance, cost, or deployment, or associated environmental effects. Hence, the potential improvements discussed here should be considered as first-step technology assessments rather than as forecasts.
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Real Prospects for Energy Efficiency in the United States BOX 3.1 Fuel Efficiency, Fuel Economy, and Fuel Consumption “Energy efficiency” in transportation is generally discussed using terminology specific to this sector of the economy, as defined below. The primary terms used to quantify the fuel consumed by a vehicle as it is driven are “fuel economy” (or “fuel efficiency”) and “fuel consumption.” Fuel efficiency is a relative term used to describe how effectively fuel is used to move a vehicle. Thus, a heavy and a light vehicle, using the same technology in the same ways, would have the same fuel efficiency but very different fuel economy. Note that fuel-efficiency improvements do not necessarily result in increased fuel economy, as they are often offset by the negative effects of increases in vehicle power and weight. Thus, fuel efficiency is related to the amount of useful work that is derived from the combustion of fuel. Whether that useful work is applied to increase the number of miles that can be traveled per gallon of fuel or to provide other amenities (such as size and power) is a separate question. Fuel economy is expressed as miles per gallon of fuel consumed; it is the term most commonly used in the United States in discussing vehicle fuel consumption. Fuel consumption is the inverse of fuel economy. It refers to the fuel consumed by the vehicle as it travels a given distance. Widely used in the Europe (expressed in liters per 100 km), this metric is a clearer measure of fuel use than is fuel economy. The amount of fuel consumed in driving from one place to another (say, New York City to Washington, D.C.) is what matters to consumers. In U.S. units, fuel consumption is usually expressed as gallons per 100 miles. To illustrate: A vehicle with fuel economy of 50 miles per gallon (mpg), which corresponds to fuel consumption of 2 gallons per 100 miles, is twice as fuelefficient as a vehicle of the same size, weight, and power that gets 25 mpg, corresponding to 4 gallons per 100 miles. Passengers and freight are transported by land vehicles, aircraft, and waterborne vessels through vast networks of land, air, and marine infrastructure. For this report, the panel partitioned this sector into passenger transport and freight transport and separated each of these into highway transportation and nonhighway transportation. Highway transportation is responsible for 75 percent of the energy used in transportation and has the greatest potential for energy efficiency; it is therefore the focus of this chapter. Nonetheless, nonhighway modes of transportation (aviation, railroad, and marine) together account for about 17 percent of the energy used in the sector and are an important potential source of energy savings collec-
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Real Prospects for Energy Efficiency in the United States tively. Efficiency improvements in these transport modes are also discussed. Transport modes such as mass transit and intercity rail have important roles in bringing about more energy-efficient passenger and freight transportation, particularly if they shift traffic to modes that can be more energy-efficient. However, they are not treated in detail here. Section 3.2 outlines energy use for U.S. transportation overall. Passenger and freight transport are covered in Sections 3.3 and 3.4, respectively. Section 3.5 briefly discusses the effects of alternative fuels on the efficiency of highway vehicles. Much of the discussion in this chapter is “vehicle-centric” in the sense that it focuses on opportunities for boosting energy efficiency through the engineering of highway vehicles (and aircraft) and their subsystems and equipment (e.g., engines, transmissions, body designs, and tires). Indeed, a great deal of R&D attention has been given to vehicle engineering for energy efficiency. The energy required for transportation, however, is greatly influenced by the performance of the systems in which these vehicles operate. “Systems” refer to the physical networks of infrastructure through which vehicles move, as well as the underlying logistic, institutional, commercial, and economic considerations that influence the mix of vehicles used, how they are used, and how the infrastructure itself performs. For example, congestion management that allows vehicles to operate at more constant speeds, with fewer starts and stops and less idling, could increase overall transportation efficiency. System energy efficiency can also be improved through the more direct routing of trucks and aircraft, the optimization of operating speeds, more intense use of infrastructure, and changes in land-use density and patterns. Similarly, energy use in air transportation is influenced by air-traffic-management requirements. The degree to which the underlying systems operate effectively, therefore, can foster—or in some cases, hinder—energy efficiency. Some of these system-level issues are discussed briefly in Section 3.6. Energy use in transportation is also influenced by factors that give rise to the demand for travel and that affect the amount or type of travel. Change in these areas, however, is a complex topic that can only be touched on in this chapter. The demand for transportation comes from individuals and businesses pursuing social and economic activities. Reducing these activities may save energy, but may or may not be otherwise desirable. The panel did not examine possibilities for saving energy by reducing the activities that spur demand for transport. It focused instead on the use of more energy-efficient modes of transportation as a means of achieving energy savings (for example, using mass transit or freight rail in place of
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Real Prospects for Energy Efficiency in the United States individual vehicles or trucks), although this chapter does at times point to some of the system-level requirements, such as changes in land-use patterns and density, that may be needed to support such improvements. In examining opportunities for energy efficiency in transportation, the panel considered the three time periods set out in the America’s Energy Future (AEF) project: Early deployment: through 2020; Medium-range deployment: 2020 through 2030–2035; Longer-range deployment: beyond 2030–2035. Current technologies offer many improvements in fuel economy that become increasingly competitive and attractive as fuel prices rise. For the early period of its assessment (through 2020), the panel focused primarily on opportunities to improve the energy efficiency of mainstream power trains and vehicles. Reductions in fleet fuel consumption through 2020 are likely to come primarily from improving today’s spark-ignition engine, compression-ignition (diesel) engine, and hybridelectric vehicles fueled with petroleum, biofuels, and other nonpetroleum hydrocarbon fuels.1 Annual, incremental improvements in engines and transmissions are expected to continue. When coupled with changes in the deployment fractions of these propulsion systems, as well as substantial vehicle weight reductions, these improvements could reduce average vehicle fuel consumption steadily over this time period. In the medium-term (2020 through 2030–2035), changes in power-train and vehicle technologies that go beyond incremental changes become feasible. Plugin hybrid-electric vehicles using electricity plus any of the above fuels may well become a significant fraction of new-vehicle sales. Their deployment may be followed by substantial numbers of (fully) battery-electric vehicles. Over the longer term (beyond 2030–2035), major sales of hydrogen fuel-cell vehicles and the necessary hydrogen supply and distribution infrastructure may develop. 1 Note that biofuels and other nonpetroleum hydrocarbon fuels are covered not in this report, but in Liquid Transportation Fuels: Technological Status, Costs, and Environmental Impacts—the report of the AEF Panel on Alternative Liquid Transportation Fuels (NAS-NAE-NRC, 2009b).
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Real Prospects for Energy Efficiency in the United States Only when a sizable fraction of in-use vehicles incorporates efficiency improvements and new power trains can the effect of these changes on the nation’s energy use and greenhouse gas emissions reach significant levels. The panel constructed some illustrative scenarios of vehicle and technology deployment as a means of estimating the potential overall effects of improved passenger vehicles and technologies on fuel consumption and the environment. The results are discussed in Section 3.3. Finally, Section 3.7 outlines the challenges that will have to be met and the impediments that will have to be overcome to improve energy efficiency in transportation, and Section 3.8 presents the panel’s findings for the sector. 3.2 ENERGY USE IN TRANSPORTATION Energy use for transportation in the United States has experienced tremendous growth over the past several decades, although the trend registered brief pauses during the economic recessions of 1974, 1979–1982, 1990–1991, and 2001. In 2007 the United States consumed 29 quads (quadrillion British thermal units, or Btu) of energy for transportation, or about 28 percent of total U.S. energy use. Moreover, the sector used more than 70 percent of the petroleum consumed in the United States. Energy use in each mode of transportation reflects its degree of use as well as its energy efficiency characteristics. Figure 3.1 breaks down total U.S. transportation energy use into components, by mode, for the year 2003. As shown, passenger travel is dominated by automobiles and by air transport for longer distances.2 Mass transit and scheduled intercity rail and bus services have important roles in some locations but account for only a small proportion of total passenger-miles.3 On the freight side, the major transport modes are by truck, rail, water, pipeline, and air. Trucking dominates in terms of tons and value of shipments. In 2006, petroleum accounted for 96 percent of the energy used for transportation; gasoline accounted for 62 percent of the energy used (EIA, 2006). 2 Bureau of Transportation Statistics, National Transportation Statistics, Transportation Energy Data Book, available at http://www.bts.gov/publications/national_transportation_statistics/. 3 One passenger carried for 1 mile is referred to as a “passenger-mile.” For example, an automobile carrying four people 8 miles is responsible for 32 passenger-miles of travel.
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Real Prospects for Energy Efficiency in the United States FIGURE 3.1 U.S. transportation energy consumption (quads) by mode and vehicle in 2003. Note: Total U.S. energy consumption = 98.2 quads. Light-duty vehicles (LDVs), defined as passenger cars and light trucks, are the primary users of gasoline. Heavy-duty vehicles (HDVs), defined as heavy trucks and buses and medium-duty trucks, accounted for most of the diesel fuel consumed, about 17 percent of the energy used. LDVs and HDVs accounted for about 60 percent and 20 percent of transportation sector carbon dioxide (CO2) emissions, respectively. 3.2.1 Public Transit Although public transit consumes a relatively small fraction of overall transportation energy, it serves several important roles in urban transportation systems.
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Real Prospects for Energy Efficiency in the United States Today, the energy required for mass transit in the United States is less than 2 percent of the total energy used for transport (Davis et al., 2008). However, transit buses on average consume about the same amount of energy per passenger-mile as that consumed by LDVs, largely because of low average ridership, especially during non–rush hours (Davis et al., 2008). Rail transit is somewhat better in terms of energy use per passenger-mile, but apart from New York City and a few other densely populated cities that have heavy ridership during both peak and nonpeak hours, transit rail is also characterized by light usage for much of the day and thus high average energy use per rider. These averages mask the specific times (rush hours) and corridors during which public transit uses less energy per passengermile than passenger cars do and where targeted promotion of transit use could contribute to a reduction in total energy use. It merits noting that the run-up in gasoline prices in 2007–2008 coincided with increases in public transit ridership and that public transit ridership has grown by one-third in the United States over the past 12 years (APTA, 2008). Although the panel did not analyze the potential for energy efficiency gains in public transit per se, it does consider in Section 3.6 how energy efficiency can be improved through system-level improvements in the provision, use, and operation of transportation systems. In so doing, the panel mentions how changes in the provision of transit services and in the operation of the highway and aviation infrastructure can boost energy efficiency. 3.2.2 Commercial Versus Private Transportation The drivers for energy efficiency in commercial transportation differ from those for private transportation. Lifetime operating costs, and thus energy efficiency, are important to companies supplying passenger and freight transportation. The commercial transportation sector is so highly competitive that even small cost differentials among firms can have a major influence on their relative profitability and growth. In contrast, fuel is a small fraction of the lifetime cost of owning a motor vehicle for private transportation. For many consumers, vehicle comfort, style, and operating performance are more important than fuel consumption. The time period over which the costs of driving are considered (relative to initial vehicle costs) also tends to be shorter for passenger transport than for freight transport.
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Real Prospects for Energy Efficiency in the United States 3.3 THE POTENTIAL FOR ENERGY EFFICIENCY IMPROVEMENT IN PASSENGER TRANSPORTATION Automobiles, light trucks, and aviation are the main modes of passenger transportation in the United States. Private automobiles account for the vast majority of local and medium-distance passenger-trips4 (those under 750 miles), and airlines account for longer trips (BTS, 2006). 3.3.1 Light-Duty Vehicles—Efficiency Trends The major factors driving vehicles to become more (or less) fuel-efficient are the price of fuel (including taxes), regulations, and consumer preferences for particular vehicle attributes. This section reviews experience with these factors. 220.127.116.11 International Experience Europe has historically had high fuel and vehicle taxes that raise owner and user costs. Moreover, diesel fuel has often been taxed at a rate lower than that for gasoline. High prices have pushed consumers to demand fuel-efficient vehicles, giving a larger market share to diesel engines. The average fuel economy of new lightduty vehicles in Europe today approaches 40 miles per gallon (mpg), 60 percent higher than in the United States.5 Vehicle fuel economy in Japan is similar to that in Europe. In 2006, Japan revised its fuel economy standard to 47 mpg, to be achieved by 2015 (ICCT, 2007b). 18.104.22.168 U.S. Experience In contrast to the trend in Europe, from 1980 until recently, real gasoline prices had been falling in the United States, encouraging consumers to buy larger, heavier, more powerful vehicles and to drive more, rather than to seek greater fuel economy. During periods of high fuel prices (such as those prevailing in mid-2008), U.S. consumers have demonstrated more interest in fuel economy. The average fuel economy of recently sold new vehicles in the United States is about 25 mpg. The U.S. Energy Independence and Security Act of 2007 (EISA; Public 4 One passenger taking one trip, regardless of trip length, is referred to as a passenger-trip. 5 Note that “real-world” fuel economy values are lower—in Europe real-world fuel economy is about 28 mpg; in the United States it is about 21 mpg. See Schipper (2006).
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Real Prospects for Energy Efficiency in the United States Law 110-140) requires that corporate average fuel economy (CAFE) standards be set for LDVs for model years 2011 through 2020 which will ensure that, by 2020, the industry-wide CAFE for all new passenger cars and light trucks, combined, is at least 35 mpg.6 This is a 40 percent increase over today’s average fuel economy. Although fuel economy has not improved, fuel efficiency has improved. Owing to relatively low fuel prices and static fuel-economy standards between the mid-1980s and the early 2000s, increases in vehicle size and performance have offset energy efficiency gains in vehicles (Lutsey and Sperling, 2005; An and DeCicco, 2007). As a result, average new-vehicle fuel-economy levels have stagnated for nearly two decades, and total vehicle fleet use and greenhouse gas emissions have increased steadily owing to the increasing fleet size and vehicle-miles traveled (VMT).7 Before discussing specific energy-saving technologies for LDVs, the panel notes that energy prices will have a significant impact on the pace of the development and introduction of these technologies. The effects of fuel costs on driving and demand for motor fuel have been studied extensively. Recent work by Small and Van Dender (2007) suggests that, as U.S. incomes have risen, the dominant effect of increases in fuel prices has been more demand for vehicle fuel economy rather than reduced driving. In other words, people are more likely to drive vehicles with higher fuel economy than to sacrifice making trips in the face of rising gasoline prices. Small and Van Dender (2007) estimate that, in the short term, each 10 percent increase in fuel costs results in a 0.1 percent reduction in VMT but a 0.3 percent increase in realized miles per gallon, often achieved through more intensive use of the vehicles with the highest fuel economy in the household.8 In the longer term, when consumers have time to choose among alternative vehicle technologies and types, realized fuel economy increases by a full 2 percent for each 10 percent increase in fuel prices, while travel falls by 0.5 percent. (Figure 3.2 illustrates how recent increases in fuel prices have increased the fraction of new vehicles sold that are automobiles rather than light trucks, since cars average 27.5 mpg compared with 22.3 mpg for light trucks, and how the subsequent fall in prices has reversed this trend.) The response in the direction of 6 The Obama administration has recently proposed that these requirements, specified by Subtitle A of EISA (Public Law 110-140), be accelerated. 7 One vehicle-mile is one vehicle traveling 1 mile (regardless of the number of passengers). 8 Hughes et al. (2008) drew the same conclusion.
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Real Prospects for Energy Efficiency in the United States FIGURE 3.2 U.S. car and light truck percentage of new vehicle sales versus average price of gasoline (all grades). Source: Gasoline prices (2007$) for 1980–2007 are from able 5.24 in EIA (2008). For 2008, the monthly nominal gas prices are from EIA (2009). Light truck and car percentages from 1980 to 2006 are from Table 4.6 of the Transportation Energy Data Book, Edition 27 (Davis et al., 2008). Light truck and car percentages for 2007 and 2008 are from Ward’s Automotive Group, a division of Prism Business Media, Inc., Ward’s U.S. Light Vehicle Sales Summary, Ward’s AutoInfoBank, available at http://wardsauto.com. improved fuel economy suggests that as incomes and energy prices rise, they will spur demand for the kind of energy-saving technologies discussed next. 3.3.2 Light-Duty Vehicles—Technologies Long-standing concern with oil imports and greenhouse gas emissions has led to several studies by the National Research Council (NRC), examining ways to reduce both of these. In particular, three studies have had as their main focus technologies that could improve fuel efficiency in light-duty vehicles (NRC, 1992, 2002, 2008b). Two other studies on hydrogen technologies and the hydrogen economy (NRC, 2008c and 2004a, respectively) also considered conventional vehicle technologies that could have an effect by 2020. Moreover, energy centers at leading U.S. universities and federal laboratories, as well as private consultants,
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Real Prospects for Energy Efficiency in the United States have recently carried out in-depth studies of transportation energy efficiency technologies and their potential to reduce petroleum use and greenhouse gas emissions through about 2030 (e.g., Bandivadekar et al., 2008; Lutsey, 2008). The following review of vehicle efficiency technologies draws on these and other studies; their quantitative estimates are in general agreement. The review is organized as follows: engine improvements, transmission improvements, and other (nonpropulsion system) improvements. A brief overview of the possibilities for each of these to increase fuel efficiency in LDVs is given first. This is followed by a summary of the overall decrease in petroleum consumption that could result from these changes.9 Many of these technologies are already being used in a limited number of vehicles, but their use is expected to expand to satisfy the higher fuel-economy standards specified in EISA. 22.214.171.124 Engine Improvements Gasoline Spark-Ignition Engine The gasoline spark-ignition (SI) engine efficiency improvements that could be deployed in large volume in the next decade include but are not limited to the following: variable valve timing, two- and three-step variable valve lift, cylinder deactivation, direct injection turbocharging with engine downsizing, enginefriction reduction, and smart cooling systems. Many of these are already in lowvolume production. In the medium-term (15–20 years), technologies such as camless valve actuation, continuously variable valve lift, and homogeneous-charge compression ignition (HCCI)10 could be deployed in increasing numbers. A survey of recent technology assessments shows that the above technologies have the potential to reduce vehicle fuel consumption, on average, by approximately 10–15 percent in the new-vehicle sales mix11 in the nearer term (by 2020) and by 9 Note that this review does not cover the effects of biofuels on petroleum consumption. 10 HCCI combines features of spark-ignition and compression-ignition (diesel) engines by making it possible to ignite gasoline and other hydrocarbon fuels using compression. 11 These reduction percentages indicate the fuel consumption of a new, state-of-the-art vehicle on the date stated, relative to its current-technology-equivalent vehicle. These percentages are panel estimates of what can realistically be expected from engine, drivetrain, and vehicle improvements in the near- and midterm future, based on estimates in the references given. Vehicle performance levels are assumed to be comparable to today’s values. The economic and regulatory context is assumed to place greater but not extreme market emphasis on lowering vehicle fuel consumption.
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Real Prospects for Energy Efficiency in the United States vices, opposition from entrenched interests such as taxis and transit operators, and insurance costs. Companies are emerging to offer these services, but their market share is very small. Still more innovative are automated highway lanes for cars and trucks, using advanced control technologies, sensors, and wireless communication technologies. But these efforts have faltered in the face of litigation and safety concerns. And even as these automation technologies enter the market, initially as “smart” cruise control, automated vehicle parking, and automated emergency braking, they will have minimal effects on energy use. Furthermore, as they are fully implemented, they might even increase vehicle travel and energy use, as the ease of “driving” induces people to live farther from work and to drive longer distances. Bus rapid transit service, which makes use of dedicated lanes and fare collection before bus entry (Levinson et al., 2002), combines the speed of subways with the flexibility of buses. For more personal service, smart paratransit, real-time carpools, and car-sharing services could reduce VMT.19 ITS and other advanced technologies may be used to create broader system changes with potentially much larger energy and greenhouse gas emission benefits. When transportation and land use are considered together, it is possible to imagine how new transportation systems could be developed that bring about improvements in energy efficiency. While a shift toward dense urban corridors would be at odds with long-term trends, changes in individual preferences (e.g., interest in urban amenities) and values (e.g., environmental concerns) may foster such a movement. For such a diversified system to evolve, numerous changes would need to occur, not only in people’s preferences but also in policies and institutions that govern land-use management and the provision of transportation services. The panel cannot delve into these broader topics in this report. When taking a longer-range view of options, however, if the goal is to reduce overall energy use and greenhouse gas emissions, the interconnections among land use, transportation, and life styles should not be neglected. For instance, while some new transit services might by themselves consume more energy per passenger-mile traveled than single-occupant vehicles, the net effect of greater mobility and locational choices could be an overall reduction in energy use and greenhouse gas emissions. 19 See Chapter 2 of Sperling and Gordon (2009).
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Real Prospects for Energy Efficiency in the United States It is thus important to explore, at this still early stage of ITS development and implementation, how ITS can be used to address the multiple needs and problems associated with surface transportation in a synergistic manner. Note that for any of the above-described changes to have a significant impact on U.S. transportation’s fuel consumption and greenhouse gas emissions, they would have to be implemented on a substantial scale. This brief overview identifies many opportunities for reducing vehicle energy use and greenhouse gas emissions through changes in the ways that the U.S. transportation infrastructure is managed and used. However, major insights and improvements will come from a broader, deeper understanding of transportation system issues for all transportation modes. Developing better data and tools that can be used to further that understanding is an important task. 3.7 CHALLENGES AND BARRIERS In the United States, many factors, including a century of falling energy prices and rising incomes, together with personal preferences and various government policies, have contributed to decentralized land-use patterns and a transportation-intensive economy. Low-priced energy led to consumer purchasing behavior, vehicle designs, and operating decisions that emphasized convenience, style, and speed over fuel economy in automobiles and light trucks, and with added emphasis on cost-effectiveness in medium- and heavy-duty trucks, ocean shipping, and the air transport of passengers and freight. The primary barriers to realizing greater energy efficiency in the transportation sector are the expectations of individuals and companies about future energy prices, fuel availability, and government policies. Although an extensive menu of technologies exists for saving energy in transportation, before decision makers choose to invest in these technologies, they must be convinced that energy price increases (or other factors that stimulate market demand) will persist. A barrier to rapid changes in the mix of LDV annual sales is the capacity of the automotive industry to change both power trains and platforms rapidly, across all models, and its ability to set up a high-volume supplier base in high-risk items such as high-energy-storage batteries.
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Real Prospects for Energy Efficiency in the United States The vehicle design cycle can be 3–5 years if the change involves major new technologies or materials. Even when new or improved vehicle technologies are available on the market, barriers to purchase include high initial cost, safety concerns, reliability and durability concerns, and lack of awareness. For new technologies to reach a substantial fraction of vehicle sales usually takes more than a decade unless mandated by law or consumers clearly demand the new or improved technology. 3.8 FINDINGS T.1 In the transportation sector, the potential for energy savings and petroleum displacement resides both in increasing the efficiency with which liquid fuels (especially petroleum) are used and in shifting some of the vehicle fleet’s energy demand to electricity (including hydrogen fuel-cell vehicles). The overall energy use and greenhouse gas emissions (and other environmental effects) associated with such a shift depend on how the electricity or hydrogen is generated. T.2 An extensive menu of technologies exists today for increasing energy efficiency in transportation. Achieving the average new-vehicle fuel economy targets for 2020 set by the Energy Independence and Security Act of 2007 (EISA; P.L. 110-140), which represent a 40 percent increase over today’s value (and a 30 percent reduction in average fuel consumption), is thus a feasible, although challenging, objective. Reaching the EISA targets, and continuing to decrease fuel consumption, will require a shift from the historic U.S. emphasis on ever-increasing vehicle power and size to an emphasis on using efficiency improvements to improve vehicle fuel consumption. T.3 In the near term, fuel-consumption reductions will come predominantly from improved gasoline and diesel engines, improved transmissions, and reduced vehicle weight and drag. Through at least 2020, evolutionary improvements in vehicles with gasoline internal-combustion engines are likely to prove the most cost-effective approach to reducing petroleum consumption. Gasoline-electric hybrids will likely play an increasingly important role as their production volume increases and their cost, relative to that of conventional vehicles, decreases. Meeting the EISA
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Real Prospects for Energy Efficiency in the United States standards is likely to require that, over the next decade or two, an everlarger fraction of the new vehicle fleet be hybrids or plug-in hybrids. T.4 Beyond 2020, continuing reductions in fuel consumption are possible. Plausible efficiency improvements in light-duty vehicles, alongside weight reduction and more extensive use of hybrid and plug-in hybrid (and possibly battery-electric) vehicles, could reduce transportation fuel consumption to below the levels implied by the higher 2020 fueleconomy standards mandated by the EISA. An especially important R&D focus is developing marketable vehicles that use electricity, which will require improving the performance and reducing the cost of highenergy-storage batteries. T.5 A parallel longer-term prospect is fuel cells with hydrogen as the energy carrier. To be attractive, major improvements, especially in reducing costs, are needed. Widespread implementation requires significant investment in efficient, low-greenhouse-gas-emissions hydrogen supply and distribution systems. Onboard hydrogen storage is a key R&D issue. Establishing a new propulsion system technology and new fuel infrastructure on a large scale is a formidable task, and significant deployment of fuel-cell vehicles is unlikely before 2035. T.6 There are opportunities to reduce energy use in freight transportation by improving both vehicle efficiency and freight system logistics and infrastructure. Reductions of 10–20 percent in the fuel economy of heavy- and medium-duty vehicles appear feasible over a decade or so. A broad examination is needed of the potential for improving the effectiveness of the freight system to reduce energy consumption further. T.7 Air transport and waterborne shipping have become more energyefficient in response to higher fuel prices. Jet engine and aircraft technology has the potential to improve the efficiency of new aircraft by up to 35 percent over the next two decades. However, improvements in aviation efficiency for passenger transport are unlikely to fully offset projected growth in air travel. Major additional issues are the full greenhouse gas and other environmental impacts of aviation fuel use at high altitude and of growing airline travel; the potential for using biomass-based fuels in jets; and whether the use of low-grade residual fuel in oceangoing vessels will continue. T.8 Most transportation efficiency studies and proposals have focused on the considerable energy efficiency gains that could be achieved with
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Real Prospects for Energy Efficiency in the United States improved vehicles rather than in the transportation system as a whole. This emphasis is appropriate given the potential for and impact of such gains. However, major insights and improvements can result from a broader and deeper understanding of transportation system issues. The potential overall impact of such broader, system-based changes, such as densifying and reorganizing land use and collective modes of travel, needs further exploration and quantification. Developing better data and tools that can be used to analyze and forecast how different policies and investments might affect vehicle use and travel is thus an important task. 3.9 REFERENCES Alamgir, M., and A.M. Sastry. 2008. Efficient batteries for transportation applications. SAE Paper 2008-21-0017. SAE Convergence, Detroit, Mich. October. An, F., and D. Santini. 2004. Mass impacts on fuel economies of conventional vs. hybrid electric vehicles. SAE Technical Paper 2004-01-0572. Warrendale, Pa.: SAE International. An, F., and J. DeCicco. 2007. Trends in technical efficiency trade-offs for the U.S. light vehicle fleet. Technical Paper Series No. 2007-01-1325. Warrendale, Pa.: SAE International. April. Anderman, M., F.R. Kalhammer, and D. MacArthur. 2000. Advanced Batteries for Electric Vehicles: An Assessment of Performance, Cost, and Availability. Sacramento, Calif.: California Air Resources Board, Year 2000 Battery Technology Advisory Panel. Anderman, M. 2007. Status and Prospects of Battery Technology for Hybrid Electric Vehicles, Including Plug-in Hybrid Electric Vehicles. Briefing to the U.S. Senate Committee on Energy and Natural Resources, January 26. APTA (American Public Transportation Association). 2008. 2008 Fact Book. Washington, D.C.: APTA. Bandivadekar, A.P. 2008. Evaluating the Impact of Advanced Vehicle and Fuel Technologies in the U.S. Light-Duty Vehicle Fleet. Ph.D. Thesis, MIT Engineering Systems Division. Cambridge, Mass.: Massachusetts Institute of Technology. Bandivadekar, A.P., K. Bodek, L. Cheah, C. Evans, T. Groode, J. Heywood, E. Kasseris, K. Kromer, and M. Weiss. 2008. On the Road in 2035: Reducing Transportation’s Petroleum Consumption and GHG Emissions. Laboratory for Energy and the Environment Report No. LFEE 2008-05 RP. Cambridge, Mass: Massachusetts Institute of Technology. July.
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