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“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.
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.
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.
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).
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