Simanaitis Says
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AERO, LONG-HAUL
“IF YOU got it, a truck brought it.” And, if it came any distance, there’s a good chance that a long-haul 18-wheeler was involved. These are already among the most fuel efficient of vehicles, and a lot of research is taking place to make them even more so.
Aerodynamic sleekness is particularly important, as is shown by a bit of analysis. Any vehicle’s fuel consumption is directly related to its total aerodynamic drag. Drag, in turn, is the product of its coefficient of drag—its sleekness—times its frontal area; briefly, CD x A.
Given constraints of carrying capacity and overall length, a truck’s frontal area is essentially fixed. But its CD is open to lots of tweaks—and good tweaks improve fuel consumption.
A familiar sight on highways: This Kenworth has a roof fairing that reduces fuel consumption by 7 percent.
In a 1999 report (http://goo.gl/vTlLl), NASA’s Dryden Flight Research Center discussed aerodynamics of a traditional cab-over-engine layout. Researchers devised a baseline rectangular box on wheels, which had a CD of 0.89, and refined its shape down to a CD range of around 0.24.
With this end-cap and other enhancements, this NASA Dryden shape had a 0.238 drag coefficient. Even truncated to the intermediate markings, it was still a respectable 0.242.
A key aspect of the NASA Dryden optimization was its rear end-cap, crucial in reducing the vehicle’s turbulent wake—and the energy wasted in its “drafting” pull.
There’s an inherent challenge in end-caps, though: Semi trailers often serve as shipping containers on ship and rail as well as on the highway, and changing their shape has implications throughout the cargo-hauling industry.
A company named ATDynamics has offered a potential solution. Its TrailerTail is a bolt-on set of collapsible panels tapering inward and extending about 4 ft. behind the trailer. The company claims fuel economy improvements of up to 6.6 percent at 65 mph with no essential modification of trailer shape.
The TrailerTail developed by ATDynamics is said to reduce trailer wake in an operationally efficient fashion.
Lawrence Livermore National Laboratory has studied the optimization of long-haul trucks, and its theoretical results concur with ATDynamics’ findings. Other aerodynamic benefits come from side skirts (as much as 10 percent), roof fairings (6.9 percent), even optimized mud guards (3.5 percent) and side mirrors (0.2 percent).
This Mercedes-Benz rig displays side skirts and a modest application of the trailer end-cap concept.
A study by the International Council on Clean Transportation in 2010 (http://goo.gl/J9nl0) stressed the importance of these aspects in reducing fuel consumption and CO2 emissions. It cited a U.S. Department of Energy estimate that Class 8 trucks (i.e., 18-wheelers) got around 6.0 mpg and were projected to improve only modestly (by about 13 percent) to 6.8 mpg by 2025.
By contrast, the ICCT identified technologies that could reduce fuel consumption of new rigs by as much as 50 percent beginning in 2017 (i.e., from the baseline 6.0 to around 12 mpg). These technologies include aerodynamics, engine and transmission improvements as well as changes in systems and logistics.
Not only do trucks bring us what we bought. They are bringing it with increasing efficiency. ds
© Dennis Simanaitis, SimanaitisSays.com, 2012
Simanaitis Says 
Roy Lunn, chief engineer on the original Ford GT40 who proposed a road-going version and developed the Mustang concept car (no, not the one that became the Mustang but the one Dan Gurney showcased at Watkins Glen), once worked on the Big Red project at Ford, an aerodynamic big rig designed for both safety and 15-percent fuel savings. At one time, Ford used the prototype to shuttle parts on a Detroit-LA route.
I always envisioned an inflatable, optimally contoured Kamm tail that could be deflated for loading / unloading (or could permanently enhance curtainside trailers, which are gaining use even for refrigerated foods). Other interesting work is being done under the trailer, some of which incorporate side underride guards. And across the pond, a deeper upper coupling position yields a larger swing radius, allowing a diminished tractor – trailer gap. Of interest, aerodynamic demand is a larger part of total power demand for partially loaded or empty trucks, and many “deadheading” miles are still part of the job.
Further to that last bit on powertrains, I have often wondered why trucks do not use diesel electric like train locomotives. With weight being a smaller consideration, they could also carry a fairly large battery and use regen braking instead of the wasteful and annoying jake brakes.