Lighting the Way: Toward a Sustainable Energy Future

  • AuthorInterAcademies Council
  • Release Date1 October 2007
  • Copyright2007
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2.5 Transportation energy efficiency

The transportation sector accounts for 22 percent of global energy use and 27 percent of global carbon emissions. In the major energy-using industrialized countries (specifically the 11 highest energy using IEA countries), nearly all (96 percent) of transportation energy comes from petroleum fuels, such as gasoline (47 percent) and diesel (31 percent). Road vehicles account for about three-quarters of all transportation energy use; roughly two-thirds of transport energy is used for passenger mobility while one-third is used to move freight (Price and others, 2006).

Trends in transportation-sector energy consumption
Transportation energy use has grown considerably faster in developing countries than in industrialized countries over the last three decades—the average annual rate of growth over the period from 1971 to 2002 was 4.8 percent for developing countries and 2 percent for industrialized countries. In absolute terms, however, industrialized countries still consume about twice as much energy (56 exajoules) for transportation as do developing countries (26 exajoules).

Transportation energy consumption in a specific country or region is driven by the amount of passenger and freight travel, the distribution of travel among various transportation modes, and the energy efficiency of individual vehicles or modes of transport. Figure 2.5 shows the distribution of energy use by mode of transport in the United States and illustrates the dominance of light-duty road vehicles (including automobiles, sport utility vehicles, pickups, minivans, and full-size vans) in terms of overall energy consumption. Similar patterns obtain in other countries, although a greater number of light-duty vehicles in Europe operate on diesel fuel.













Figure 2.5 U.S. transportation energy consumption by mode, 2005

Note: Total U.S. transportation energy consumption in 2005 was 27,385 trillion British thermal units.

Source: Davis and Diegel, 2006.

Energy-efficiency potential in the transportation sector
Overall demand for transportation services generally and personal vehicle travel specifically can be influenced by patterns of development and land-use planning, as well as by the availability of public transportation, fuel costs, government policies (including congestion, parking, and roadway fees), and other factors. Different modes of transport also have very different energy and emissions characteristics—as a means of moving freight, for example, rail transport is as much as ten times more energy-efficient per kilometer as road transport. Some of the policy options available for advancing sustainability objectives in the transportation sector are politically difficult to enact while others (notably land-use planning) are difficult to affect except over long periods of time—although substantial opportunities may exist in developing countries where new development is occurring at a rapid clip and land-use patterns are not already heavily determined by existing infrastructure. Several strategies for reducing travel demand are discussed in general terms in the next section.

At the level of individual vehicles, three types of approaches can be used to reduce energy consumption.24 The first is to reduce the load on the engine, thereby reducing the amount of energy required to move the vehicle. The second is to increase drive-train efficiency and capture energy losses (especially in braking). A third is to increase the engine load factor—that is, the amount of time the engine operates near its rated or maximum power output for a given speed. If the primary objective is to reduce greenhouse gas emissions, then a fourth approach (beyond improving efficiency) is to switch to a less carbon-intensive fuel. (Alternative-fuel options could include electricity or biofuels; the latter is discussed in a later section of this report).

For road vehicles, load on the engine can be minimized by reducing vehicle mass, aerodynamic drag, and tire-rolling resistance. Mass reductions can be achieved by replacing conventional steel in the bodies and engines of vehicles with materials that are equally strong, but significantly lighter in weight. A 10 percent reduction in vehicle weight can improve fuel economy by 4–8 percent. Increased use of lightweight but very strong materials, such as high-strength steel, aluminum, magnesium, and fiber-reinforced plastics, can produce substantial weight reductions without compromising vehicle safety. Such advanced materials are already being used in road vehicles; their use is growing, but they generally cost more than conventional materials. Smaller engines, capable of operating at high revolutions per minute or with turbo-charge for additional power, can also be used, as can smaller and lighter transmissions. Aerodynamic drag can be reduced through more streamlined body design but may also introduce trade-offs in terms of stability in crosswinds. Technologies that turn the engine off when idling can also produce energy savings.

Some technologies, both commercially available and under development, can be used to increase the drive-train efficiency of road vehicles. Examples include multi-valve overhead camshafts, variable valve lift and timing, electromechanical valve throttling, camless-valve actuation, cylinder deactivation, variable compression ratio engines, continuously variable transmissions, and low-friction lubricants. In addition, new types of highly efficient drive-trains—such as direct injection gasoline and diesel engines, and hybrid electric vehicles—are now in production.

Several studies have estimated the overall potential increase in fuel economy that could be achieved through the use of multiple technologies in light-duty vehicles. These estimates range from a 25–33 percent increase in fuel economy at no incremental cost (NRC, 2002) to a 61 percent increase in fuel economy using parallel hybrid technology at an incremental vehicle cost of 20 percent (Owen and Gordon, 2003).

Hybrid-electric vehicles, which utilize both a conventional internal combustion engine and an electric motor in the drive-train, have immediate potential to reduce transportation energy use, mainly from shutting down the engine when stopped, recovering braking losses to recharge the battery, and allowing for the engine to be downsized by supplementing with electric power during acceleration. In the United States, the market for hybrid vehicles has grown rapidly in the last few years: the number of hybrid vehicles sold more than doubled between 2004 and 2005 and grew a further 28 percent between 2005 and 2006.25

In current production hybrids, the batteries are charged directly from the onboard engine and from regenerative braking. ‘Plug-in’ hybrids could also be charged from the electricity grid thereby further reducing petroleum use (especially if the vehicles are primarily used for short commutes). Such vehicles would require a larger battery and longer recharge times. Pairing this technology with clean, low-carbon means of producing electricity could also produce substantial environmental benefits. Widespread commercialization of plug-in hybrids would depend on the development of economical batteries that can sustain thousands of deep discharges without appreciable loss of energy storage capacity. It could also depend on whether on-grid, battery-charging patterns would require a substantial expansion of available electric-generating capacity.

Over a longer timeframe, substantial reductions in oil consumption and conventional pollutant emissions, along with near-zero carbon emissions, could potentially be achieved using hydrogen fuel-cell vehicles. In general, the specific environmental benefits of this technology will depend on how the hydrogen is produced: if a large part of the objective is to help address climate change risks, the hydrogen will have to be produced using low-carbon resources, or—if fossil sources are used—in combination with carbon capture and sequestration. Meanwhile, recent studies conclude that several significant technical barriers will need to be surmounted before hydrogen fuel-cell vehicles can be viable in large numbers. Chief among these barriers are the durability and cost of the fuel cell, the cost of producing hydrogen, the cost and difficulty of developing a new distribution infrastructure to handle a gaseous transportation fuel, and the challenge of developing on-board storage systems for hydrogen (NRC/NAE, 2004; TMC/MIRI, 2004). In one effort to begin demonstrating hydrogen technology, Daimler Chrysler has developed a fleet of hydrogen fuel-cell buses that are now in use in several cities around the world.

Motorcycles and two- and three-wheel scooters are already relatively efficient compared to cars, but in urban areas where two-stroke engines are heavily used they make a substantial contribution to air pollution. Conventional pollutant emissions from this category of transport vehicles can be reduced substantially, and efficiency can be further improved using some of the engine technologies developed for light vehicles Honda estimates that a prototype hybrid-electric scooter could reduce energy use by roughly 30 percent in stop-and-go driving, while producing even larger reductions in conventional pollutant emissions (Honda, 2004).

The main opportunity for reducing energy consumption in heavy-duty diesel trucks is through body improvements to reduce aerodynamic drag. Electric or hybrid-electric drive-train technologies are not considered practical for heavy-duty vehicle applications, although fuel cells may well be. However, hybrid-electric systems are well-suited for stop-and-go driving by buses and delivery vehicles in urban areas; studies have found that fuel economy improvements ranging from 10 percent (Foyt, 2005) to 57 percent (Chandler and others, 2006) could be achieved using hybrid technology in these applications.

For rail engines, advances have been made in reducing aerodynamic drag and weight, and in developing regenerative brakes (at railside or onboard) and higher efficiency motors. A 1993 Japanese report illustrates how a train with a stainless-steel car body, inverter control, and regenerative braking system could cut electricity use in half over a conventional train (JREast Group, 2003). Alternative power plants are also a possibility for rail travel.

Today’s aircraft are 70 percent more fuel-efficient per passenger-kilometer than the aircraft of 40 years ago; most of this improvement has come from increasing passenger capacity but gains have also been achieved by reducing weight and improving engine technology. Options for further reducing energy use in aviation include laminar flow technology and blended wing bodies,26 both of which reduce air drag, and further engine improvements and weight reductions. Airplane manufacturer Boeing claims that its new 787 family of aircraft will achieve a 20 percent improvement in fuel economy, in part through the extensive use of composite materials (Boeing, 2007). Other, longer-term options include larger aircraft, use of unconventional fuels or blends, and new engines using liquid hydrogen fuel.

Obviously, the overall efficiency of road, air, and rail transport also depends to a considerable extent on utilization: higher occupancy ratios on buses, trains, and airplanes will result in lower energy consumption or emissions per passenger-mile.

Technology options for reducing energy use in the shipping industry include hydrodynamic improvements and machinery; these technologies could reduce energy use by 5–30 percent on new ships and 4–20 percent when retrofitted on old ships. Since ship engines have a typical lifetime of 30 years or more, the introduction of new engine technologies will occur gradually. A combination of fleet optimization and routing changes could produce energy savings in the short term; reducing ship speed would also have this effect but may not be a realistic option given other considerations. It has been estimated that the average energy intensity of shipping could be reduced by 18 percent in 2010, and by 28 percent in 2020, primarily via reduced speed and eventually new technology. This improvement would not, however, be enough to overcome additional energy use from projected demand growth (shipping is estimated to grow 72 percent by 2020). Inland ferries and offshore supply ships in Norway are using natural gas in diesel ship engines and achieving a 20 percent reduction in energy use, but this option is limited by access to liquefied natural gas and cost. Where natural gas is available and especially where the gas would otherwise be flared, use of liquefied natural gas as a ship fuel could produce significant emissions reductions. Large sails, solar panels, and hydrogen fuel cells are potential long-term (2050) options for reducing ship-related energy use and carbon emissions.

Policies to promote transportation-sector energy efficiency
The primary policy mechanisms available to promote energy efficiency in transportation include new vehicle standards, fuel taxes and economic incentives, operational restrictions, and land-use planning.















Figure 2.6 Comparison of auto fuel efficiency by auto fuel economy standards among countries, normalized to U.S. test procedure

Note: Y-axis shows miles per gallon (mpg) according to Corporate Average Fuel Economy (CAFE) standards [1 mpg equals 0.425 kilometers per liter]. Dotted lines denote proposed standards. Japan has recently announced that it wants to implement even tougher standards, which would put it on par with the EU beyond 2014 (An and others, 2007).

Source: An and Sauer, 2004.

Many countries now have efficiency standards for new light-duty vehicles, typically in the form of performance standards that are applied to the average efficiency (or fuel economy) of a manufacturer’s fleet (Figure 2.6). This flexibility allows manufacturers to offer models with a range of fuel- economy characteristics. The introduction of fuel-economy standards in the late 1970s led to substantial efficiency gains in the U.S. automobile fleet throughout the 1980s, but it has proved politically difficult to increase the standards over time to reflect advances in vehicle technology. In fact, fuel economy standards in the United States have remained largely unchanged for the last two decades. Meanwhile, the growing market share of minivans, sport utility vehicles, and pickup trucks—which are designated as ‘light trucks’ and are therefore subject to a considerably lower fleet-average standard—has actually produced a decline in the effective fuel economy standard for passenger vehicles in the United States since the 1980s.27 Finally, because such standards generally apply to new vehicles only and because the average life of a passenger vehicle is 13 years (the average life of large diesel engines is even longer), there is a substantial lag time between the adoption of standards and appreciable improvements in fleet-wide efficiency.

Some jurisdictions regulate emissions from heavy-truck engines, and some have prescriptive standards that require four-stroke engines in motorcycles, snowmobiles, or personal watercraft. However, these standards are aimed at conventional-pollutant emissions rather than at reducing fuel use or carbon emissions. No countries have fuel-economy standards for aircraft, shipping, or locomotives, although some are developing standards that limit the emissions of pollutants other than carbon. In some cases, significant reductions in emissions and energy consumption can be achieved simply through mode-shifting (e.g., transporting freight by rail rather than by heavy truck).

Fuel taxes give operators an additional economic incentive to reduce energy use. In many respects fuel taxes are preferable to efficiency standards. They apply immediately to both old and new vehicles, across all transportation modes. They also leave consumers with great flexibility in how to respond, either by opting for more efficient vehicles or by changing their travel patterns, or both. Several EU member states have imposed large gasoline taxes for decades while such taxes have been extremely difficult to implement in the United States. And although fuel taxes have many theoretical advantages from the standpoint of economic efficiency, experience to date suggests they need to be quite high (given the relative price inelasticity of travel demand and the fact that fuel costs are often a small fraction of transportation-related expenses) to produce significant changes in consumers’ transportation choices or fuel consumption patterns.

‘Feebates’ have been proposed in the United States (and to achieve other environmental goals in other countries) as an alternative policy to surmount the political obstacles associated with both fuel-economy standards and fuel taxes. Fees would be levied on sales of vehicles with relatively poor fuel economy, while rebates would be given for sales of vehicles with high fuel economy. Most of the proposals are revenue neutral (i.e., the total rebate outlay would cover the total fee revenue). Although feebates have been proposed in several U.S. jurisdictions, they have never been enacted.

Another proposal for promoting light-duty vehicle efficiency is to transfer fixed vehicle costs—such as liability insurance, registration fees, and emission inspection fees—into variable costs based on the number of miles driven per year. Such a policy would provide direct incentives to drivers to reduce their miles driven and should result in reductions in urban congestion and air pollution as well as energy use. As yet, however, no jurisdiction has adopted this strategy, although the Netherlands expects to introduce a system like this in 2007/2008.

A more severe approach to managing transportation demand is to impose restrictions on where and when vehicles can operate. A mild form of this approach involves restricting the use of certain highway lanes to vehicles with at least two or three occupants during peak commute times. Another option that may be feasible in some settings is ‘congestion pricing’ whereby differential tolls are charged for road use at different times of day. Revenues from congestion pricing can in turn be used to subsidize mass transit. Several cities have imposed more severe restrictions on downtown centers, mostly as a means of reducing congestion and emissions of smog-forming pollutants. Singapore was the first large city to impose limits on automobiles in its central business district, requiring cars to purchase and display special permits to enter the area during business hours. This program, combined with an excellent subway system, has been successful in reducing congestion. A more recent program has been implemented by the City of London. It is similar to the approach pioneered by Singapore and has proved quite successful: an estimated 18 percent reduction in traffic in the zone has produced a 30 percent reduction in congestion, a 20 percent reduction in carbon dioxide emissions, and 16 percent reductions in nitrogen-oxide and particulate matter emissions (Transport for London, 2005).

Changes in land-use planning represent a long-term policy option that nonetheless can have a significant impact on energy consumption. Zoning and development policies that encourage high-density housing and well-mixed residential, retail, and business areas can dramatically reduce the number and length of trips taken in private automobiles. Such policies can also help ensure that future development is amenable to more efficient or environmentally friendly transportation modes, such as public transit, bicycling, or even walking. Public transit can make a significant contribution to energy and environmental objectives (while also reducing congestion and urban air pollution and increasing mobility for low-income and elderly citizens) so long as ridership on buses and trains is consistently high. Again, dense and well-mixed development is critical.

Document Date: October 1, 2007
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