Tractor/Trailer Aerodynamics

 

Aerodynamic drag (wind resistance) accounts for most truck energy losses at highway speeds.  Reducing drag improves fuel efficiency.

Manufacturers made significant progress over the past two decades in reducing the drag coefficient (a measure of wind resistance) of a typical freight truck about 20 percent.  Additional efforts to improve aerodynamics could result in a further reduction in the drag coefficient.  This could have a significant impact on fuel economy.  For example, cutting drag by 25 percent could raise fuel economy up to 15 percent at highway speed.  A number of options exist to improve aerodynamics and improve fuel efficiency. 

Tractor Aerodynamics

Truck tractor aerodynamic options include roof fairings (an integrated air deflector mounted on the top of the cab), cab extenders (known as gap seals, which reduce the gap between the tractor and the trailer), side fairings, and a front bumper air dam (to reduce air flow beneath the truck).  Truck manufacturers offer aerodynamic models that include a streamlined front profile, sloped hood, and a full package of add-on devices. 

Trailer Aerodynamics

Trailer aerodynamics can be improved minimizing the gap between the tractor and the trailer, to reduce air turbulence.  Beyond the cab extenders mentioned above, physically positioning the trailer as close to the rear of the tractor as possible can also reduce tractor-trailer gap.   Reducing trailer gap from 45 to 25 inches could improve fuel economy as much as 2 percent. 

Another trailer option is to use side skirts.  These panels hang down from the bottom of a trailer to enclose the open space between the rear wheels of the tractor and the rear wheels of the trailer.  Cargo “profile” is also important.  On flatbed trailers, drag is reduced by arranging cargo as low and smooth as possible.  Securing loose tarpaulins and closing the curtains on empty curtain-sided trailers will improve fuel economy by up to 2.5 percent and 4.5 percent, respectively.

Single Unit Truck Aerodynamics

Many new single-unit truck models incorporate a sloped hood and a more streamlined front profile as a standard feature in order to reduce drag.  Rounded air deflector bubbles can be added to single-unit trucks with van-style bodies to reduce drag by up to 5 percent.  This would yield annual fuel savings of up to 80 gallons.  Single unit trucks with higher annual mileage could realize even larger benefits.

Therefore, using a streamlined profile tractor and trailer combination with aerodynamic devices can reduce fuel costs and reduce GHG emissions.  In fact, the EPA estimates a switch from a classic non-aerodynamic long haul combination truck to an aerodynamic long haul combination truck can save approximately $6,200 in fuel cost and 16.7 metric tons of carbon dioxide.  When using a single unit truck with aerodynamic devices the cost saving may not be quite as high as the combination tractor trailer, however, they are still significant.[1]



[1] SmartWay, Improved Aerodynamics, http://www.epa.gov/smartway/documents/partnership/trucks/partnership/techsheets-truck/EPA-420-F00-036.pdf, (May 20, 2013).