weight-reducing carbon composite structural materials and less energy-intensive electric systems.
Because the DOE expects air travel to grow about 3 percent per year over the next several decades, efforts to reduce aviation-fuel consumption face a stiff challenge. The expected efficiency improvement of 1–2 percent per year (Lee et al., 2004) will not be enough to offset the expected growth in demand.
The movement of freight represents about 6–7 percent of the U.S. GDP. Given that the EIA expects freight transport to continue to increase by 2 percent per year over the next two to three decades, energy use in the freight sector could grow by more than 40 percent by 2030.
Pressure to reduce fuel costs has led truck manufacturers to make continuous progress in raising engine efficiency. Technological improvements have included more sophisticated fuel-injection systems, enhanced combustion, higher cylinder pressure (due to increased turbocharging), and automated manual-transmission systems. Technologies on the horizon include continuously variable transmissions, power-shift transmissions, and hybrid-electric systems that could be used to modulate auxiliaries (pumping, fans, compressors, air-conditioning, and power steering) and reduce idling. Auxiliary-power units with greater efficiency could increase fuel economy, as could use of utility-supplied electricity when parked at truck stops. Reduced idling is especially desirable in urban-duty cycles and in sleeper cabs, where idling alone can account for 10 percent of the vehicle’s fuel use.
A small proportion (less than 1 percent by weight) of total freight shipments is transported by air. Potential gains in efficiency would stem from the same improvements made to passenger aircraft.
Rail accounts for 2 percent of transportation energy use in the United States but about 10 percent of all freight shipments by weight. Freight railroads are nearly all diesel powered, unlike the mostly electrified rail systems of Europe and Japan. Per ton-mile, rail is 10 times more efficient than trucking is. Still, improvements in railroad technology would offer modest opportunities for gains in U.S.