Perhaps you've recently been surprised to drive up behind an 18-wheeler on the Interstate whose trailer doors appeared to be ajar. No need to be alarmed: What you saw was probably one of several attempts at improving the fuel economy of long-haul trucks.
One such effort, the TrailerTail developed by ATDynamics, was designed to reduce the aerodynamic drag generated at the rear of trailers.
Devices to help cut the amount of energy needed to push large trucks through the air are already in wide use. Cowls and fairings designed to round off the front end, and filler panels that bridge the gaps between a truck's cabin and the trailer behind, are already common, as are side skirts that close off the lower portion of the trailer.
But no less important is the drag created by the turbulent wake behind a trailer's squared-off rear end, according to a report prepared by the Lawrence Livermore National Laboratory in Livermore, Calif. As air rushes into the low-pressure immediately behind the trailer -- an aerodynamic effect that can be felt as a distinct pull in a closely following car -- the swirling flow generates considerable drag.
Reducing this drag can raise fuel economy. Indeed, more than half of the energy needed to keep a truck moving at highway speeds is expended just to overcome aerodynamic drag, the laboratory's report says.
Coming up with a solution to reduce drag at the rear of the trailer faces some challenges, however. Trailers often serve as shipping containers, not only on the road but also on trains and container ships. Simply changing the shape of a trailer could have implications throughout the cargo-hauling industry.
ATDynamics' TrailerTail sidesteps this problem with a bolt-on set of collapsible panels that extend about 4 feet behind the trailer, tapering inward to the rear. The panels can be folded flat or opened easily by a driver without obstructing access to the cargo doors, says Andrew Smith, chief executive of the company.
The tail alone can improve fuel economy by up to 6.6 percent at 65 mph, Smith says. That number agrees with the upper end of the range of theoretical results projected in the national laboratory's report.
"There's no doubt that they work," says Steve Phillips, senior vice president for operations at Werner Enterprises, a transportation and shipping company based in Omaha, Neb.
Phillips says his company is still in the testing stage, however, with about 134 of the company's 23,000 trailers outfitted with TrailerTails. He estimates that the tails, which cost about $2,200 each, will pay for themselves in fuel savings within a year.
There are roughly 2 million tractor-trailers on American highways, says ATDynamics. Based on Energy Department figures, a 6 percent reduction in diesel fuel use would amount to savings of 1.6 billion gallons a year, a reduction of about 14 million tons of carbon dioxide emissions -- and save about $6.6 billion.