Detroit, MI--The next time you can't shake a driver who is riding your bumper, you might think twice about tapping the brake pedal. That's because you may actually be benefiting--in the way of reduced fuel consumption.
Contrary to conventional thinking, which says the trailing vehicle experiences a reduction in drag, researchers at the University of Southern California have determined that both vehicles experience a drag savings and, at sufficiently close spacing, the lead vehicle actually experiences a greater drag drop.
"We were surprised by the results, which were particularly clear for two close-following vehicles," says Fred Browand, a professor of Aerospace Engineering at USC. "For all the different geometries we tested in the lab, the result was always the same. At a gap length closer than about 0.3 to 0.4 vehicle lengths, the forward vehicle has the lower drag."
Confident that their windtunnel results were accurate, the researchers recently undertook full-scale tests at the El Mirage dry lakebed in Southern California. The road surface here is flat and smooth, with near-zero wind conditions early in the day. At a spacing of a fraction of a vehicle length, the leading van in the two-vehicle test experienced the greater savings in drag. Results were presented at the recent SAE technical meeting in Detroit.
Browand explains this phenomenon, called drag cross-over, in terms of the flow along streamlines that enter and leave the gap area between the vehicles. "For the flow along such a streamline, fluid pressure and airspeed are related such that high local airspeed corresponds to low pressure and vice versa. Since there is a much diminished airspeed in the gap, the pressure between the vehicles is higher than for each separately. This higher pressure produces an additional forward push on the lead vehicle (less drag), and a backward push on the trail vehicle (more drag). As the gap becomes smaller, these pressure drag increments become larger, resulting in less drag on the lead vehicle."
Drag reduction is important because of its impact on fuel savings. For steady travel on a level road, aerodynamic drag and rolling resistance are about equal at a speed of 55 to 60 mph. (Above this speed, the aerodynamic drag is a greater fraction of the total.) Thus, at 55 to 60 mph a 50% savings in drag would result in a 25% reduction in fuel expenditure. At higher speeds, the percentage saved by close-following would be even greater, but as Browand points out, there is a tradeoff: it would take more fuel to go that speed in the first place.
A gap of a mere fraction of a vehicle length wouldn't be practical under typical driving conditions today, of course. But this research has direct implications for work being done on the Automated Highway System, a concept researchers are studying whereby freeway traffic would be organized into platoons of close-following vehicles.