Thrifty trucks go with the flow.

Forcing air through strategically placed slits on a tractor trailer results in a major boost in fuel economy; new road tests of diesel-fueled trucks show.

Typically, a boxy trailer moving at highway speeds generates a turbulent wake that exerts a retarding force, or drag. But on a trailer modified to have slits and rounded contours at its back edges, sheets of air pumped by fans rearward through those slits hug the curved contours and entrain air rushing over the trailer into an orderly current. That effect sharply cuts drag.

In wind tunnel tests on modified truck models a few years ago (SN: 10/28/00, p. 279), Robert J. Englar of the Georgia Institute of Technology, in Atlanta and his colleagues found evidence of potentially huge reductions in drag. However, road tests on a full-scale truck, whose contours were more complex than those of the wind tunnel model, proved disappointing. The researcher-s answered that setback with more streamlining structures, or fairings, this time attached to underside of a test truck.

The team has submitted a report to its government sponsor, the Department of Energy, describing road tests last fall. Combined, the slit system and extra fairings boosted fuel economy by 8 to 9 percent. If the entire U.S. heavy-trucking fleet were thus modified, up to 1.8 billion gallons of fuel would be saved annually, says Englar.