The Why Files The Why Files -- whyfiles.org

Barreling down the highway
10 NOVEMBER 2005

Barreling down the highway
Ever wonder why an 18-wheeler needs such a big fuel tank? Because a loaded semi-trailer only manages to squeeze five or six miles from a gallon of diesel. As with all vehicles moving at highway speed, a gob of energy is spent pushing through the air. Truck with box on the back, slightly smaller than the trailer, and 4' deepBig trucks use about two-thirds of their fuel for this purpose at 70 mph, says Ken Visser, who teaches aircraft design at Clarkson University in upstate New York.

This truck ran cross-country in 2000, showing that attention to the tail can save energy. These clunky plates can fold up when it's time to open the doors. Photo: Courtesy Ken Visser, Clarkson University.

Soaring fuel prices during the 1970s sparked a major improvement in auto efficiency, and much of the benefit came from aerodynamic designs that slip through the air instead of crashing into it. Big trucks, however, focused on front-end aero design, starting with front-mounted fairings that gently push the air aside instead of smacking it with the flat face of the trailer. Few people thought much about the rear end, even though, according to Visser, it accounts for 20 to 25 percent of total aerodynamic resistance. Rear-end drag results from a low-pressure zone that, in effect, "pulls" the truck backward. "When the air flow reaches the rear of the truck, it can't stay attached to the vehicle," Visser says. "It separates, and you get a low pressure zone."

The tail end of a semi creates air drag, shown by ragged edges of air in simulation.Model shows how eddies form in a trailer's wake. This swirling pattern causes a low-pressure zone that "pulls" the truck backward, sapping fuel. Model: Lawrence Livermore National Laboratory

In 1999, Visser, who formerly worked at NASA and Boeing, enlisted students in an effort to streamline truck ends, working through about 100 designs in a wind tunnel, and eventually devising a rectangular box at the rear. "My students found this concept in German work from the early 1970s." By 2000, he staged a cross-country run that demonstrated a one-half mile-per-gallon reduction in fuel usage.

As the students came and went, Visser knew that having a brainstorm and making it work are two different things. Early designs, for example, did not account for side loads. "If the wind is gusting from the side, what is the impact on drag?" Visser says. It turned out that the plates caught side winds, increasing drag.

So another graduate student dove into another study, varying the position and angle of the plates to come up with the design that was tested on one truck last summer, showing a 10 percent saving in fuel. That would amount to $4,000 per year on the average semi-trailer, Visser says.

Unsightly flaps have been attached to the end of this semi trailer to improve aerodynamics. It may not be beautiful, but Visser's latest design saved 10 percent on the diesel bill during tests. Notice that the plates are now angled instead of straight. Courtesy Ken Visser

Practically speaking
Beyond design, there are plenty of practical obstacles to having a change in truck structure accepted. The trucking industry is conservative, Visser says, and any innovations must work reliably and simply, without requiring drastic changes in existing systems or procedures. Now that Visser has a working prototype, he is getting cooperation from the trailer manufacturer Great Dane Trailers, and from Volvo Trucks North America, to stage more precise tests of fuel economy. "I'm just a professor, I'm trying to get a little more data, preferably from independent sources." Visser has also applied for a patent, listing Composite Factory, Inc. of Plattsburgh, N.Y. as co-inventor. The company, he says, has been collaborating on design and fabrication for two years.

One final question: Aerodynamics usually involves curves, not planes. Would curved baffles work better? "Off the cuff, I'd say probably yes," Visser says. "But when you are manufacturing, you need something that will fold up and out of way [for loading and unloading the truck], and with curved panels it's harder to do that. I thought, let's see what we can do with the simplest thing. Straight is simpler. If it saves 10 percent, that would be good for the [Ford] Model T, we can work on the Model A later. My goal is to see the technology we develop here out on the road."

To an engineer, practicality is everything, he indicates. "I worked on the [Boeing] 767-400, had a hand in the design of parts that are flying today. I want this out on the road, not just sitting as a little report on a shelf. If we can save 10 percent on fuel, we are talking billions of dollars each year."

Not to mention a couple of barrels of imported oil, and even some global warming avoided...

'-- David Tenenbaum' gets hauled away by semi

Related Why Files
Fuel Cell Advance
Electric Cars
Positive Energy


©2017, University of Wisconsin, Board of Regents.