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Toward More Aerodynamic, Efficient Heavy Trucks

written by: Erik Hinrichsen • edited by: Lamar Stonecypher • updated: 9/6/2010

Exciting new technologies promise large improvements in the fuel efficiency of heavy trucks, which account for about 20 percent of traffic in the United States. This article reviews the technologies and explains how they work; it also assesses their viability.

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    Introduction

    As gas prices continue their inexorable climb year after year, and as more and more evidence suggests a link between emissions and climate change, engineers have finally started looking into improving the efficiency of heavy trucks: engineers all over the country are developing more aerodynamic truck concepts. The trucking industry constitutes 19 percent of all traffic in the United States, sucking up 39 billion gallons of diesel fuel each year. Heavy tractor trailer trucks have always been rather inefficient, though they have gotten better over the years. Today, heavy trucks average around 5.5 miles per gallon, whereas the 1982 average was only 4 mpg. Still, there is plenty of room for improvement. Even a ten percent improvement in fuel efficiency would save about 1 billion gallons of diesel fuel per year in the US alone, even though that would only bring efficiency to 6.05 mpg.

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    Making Trucks More Aerodynamic

    One of the biggest reasons for trucks' low efficiency is drag problems, which become worse with higher speed. Accordingly, researchers have focused on reducing drag effects on trucks, and in the last few years several have announced breakthroughs.

    The square back of trailers is a big contributor to inefficiency. At high speeds, the square edges of the truck begin to generate a vortex which acts opposite to the direction of motion. This vortex increases the energy required from the truck engine, and drags down fuel efficiency. Engineers at a Georgia Tech institute, who have been studying this problem since the early 1990's, are in the late stages of testing a retrofit design which they say will fix the vortex issue.

    Controlling Vortex Formation and Directing Airflow

    This design works to prevent drag in two ways. First, rounded panels are added to the back of the truck to prevent vortex formation. Secondly, a system blows compressed air over the edges of the truck. This step is necessary to redirect the airflow and prevent it from collapsing into a vortex. The compressed air may be provided from a variety of sources, such as exhaust gas, air off the truck's turbocharger, or an onboard air compressor.

    This gas could also be directed to slow the truck during braking, which would save energy and help make trucks safer.

    Real world tests demonstrate that this system reduced drag by 35%, which translates into a 12% increase in fuel efficiency. The technology is working, and a 12% fuel savings is more than enough to prompt the trucking industry to adapt the technology.

    The fact that the Georgia Tech design is a fairly simple retrofit bodes well for the technology's future adoption. It can be added to trucks quickly and cheaply, and won't necessitate buying a whole fleet of expensive trucks to replace today's models. Though researchers haven't provided a price point yet, installation of the system would likely pay for itself in fuel savings fairly quickly; this will be especially true if some sort of government subsidy or tax break is introduced.

    Pictured: Old NASA-designed fuel efficient truck.

    Image: NASA - E-38096

    NASA truck 

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    Side Skirts to improve Semi Truck Efficiency

    Along similar lines to the previous section, Dutch researchers have recently completed real-world tests demonstrating the effectiveness of side skirts, which are mounted along the sides of the truck's trailer. This new design is a more aerodynamic side skirt than has traditionally been used. It creates less drag and directs air flow away from the underside of the truck. Tests on the side skirts have shown a fuel efficiency increase of as much as 15%, making the technology a very promising one indeed. Additionally, the side skirts are easy to install, as they are a simple retrofit and may be customized themselves.

    Note that the efficiencies described here do not simply add on other fuel savings in a linear way. However, several technologies in conjunction should further improve fuel efficiency, provided that they do not interfere with each other’s operation.