Big rigs ease down a long and windy road.

Big rigs ease down a long and windy road

A gallon of fuel saved is roughly a dollar earned, and a good environmental and public-health deed to boot. A small New Jersey company has devised an aerodynamic-drag-reducing device for big-rig trucks that it says could save 1 billion of the estimated 16.5 billion gallons of fuel used annually by the nearly 1 million registered U.S. tractor-trailer combinations.

The device, called an aerodynamic boat tail, reduces fuel use by eliminating about 10 percent of the drag on a truck's forward motion caused by air flowing haphazardly around the trailer. Since road-tire friction also retards forward motion, engineers estimate the device reduces overall drag by 6 percent. The prototype device, a hollow and open rectangular box, fits just within the boundaries of the trailer's rear panel and juts out about 3 feet. Air flowing over the top and around the sides of the truck turns inward when it reaches the back, and combines into an aerodynamically smoother, boat-tail-shaped flow.

"It doesn't look like it should work," remarks James C. Ross, an aerospace engineer at NASA's Ames Research Center in Moffet Field, Calif. But wind-tunnel tests he supervised there indicate otherwise. Six 40-foot-diameter fans sent air careering at ticketable speeds over a stationary test truck in the wind tunnel. Sensors measured vertical, lateral and other forces on the vehicle. When fitted with an aerodynamic boat tail, a truck suffers 10 percent less drag than it would without the device.

Many new tractor designs involve drag-damping geometries formed with such things as roof and bumper fairings and skirting that smoothes contours on the sides of the cabs and over the gas tanks. But trailer designs haven't changed much in 20 years, notes engineer Alan Bilanin, senior associate of Continuum Dynamics, Inc., the Princeton, N.J., company that designed the new device.

Prior to this decade, federal laws regulating trailer length would have precluded a device like the aerodynamic boat tail, Bilanin says. But legislative action in 1982, together with deregulation of the trucking industry, created a competitive environment in which cost-cutting tactics grow in importance, he adds.

"I believe you can make these [aerodynamic boat tails], install them and recoup your costs in one year of use," Bilanin says. In its most likely commercial design, drivers will be able to collapse the device against the rear panel. But he and others caution that hurdles remain. Each trailer manufacturer has several models, each of which has a different rear-panel design. Aerodynamic boat tails may have to be customized for each design, they say.