Relatively simple modifications to heavy truck aerodynamics could provide a welcome decrease in fuel costs for the average over-the-road truck, according to research by Volvo Trucks North America.

Volvo showed a 2.3 percent improvement in fuel economy through the use of four modifications to a typical highway tractor and trailer. Three of the modifications are prototype designs, while the fourth is already available on Volvo trucks. Volvo displayed the modifications at a public event Tuesday outside the headquarters of the U.S. Department of Energy.
Volvo addressed improving airflow under the vehicle and reducing the interaction between the underside and the gap between the back of the tractor and the trailer. Aerodynamics are extremely important to achieve good fuel economy with modern trucks. Controlling airflow on the underside of tractor-trailer combinations can reduce the amount of turbulence and aerodynamic drag produced by a moving vehicle when air rushes by axles, wheels and other objects. Reducing the trailer gap also improves aerodynamics and is an area where Volvo has done previous work.
The results were part of a two-year cooperative research effort between DOE and the Truck Manufacturers Association, and four of TMA’s member companies. Volvo’s efforts to demonstrate the performance of all four of these devices in combination were funded by DOE.
Three devices were used for underside airflow management, said Michael Sorrells, Volvo’s lead design engineer for aerodynamic development. The first was a smooth underside area on the front of its test truck, essentially an extension of the bumper made from composite material. The device did not need to extend back very far and would have little impact on the truck’s operation or maintenance.
“We began the project expecting to find that a completely smooth underside on a truck would significantly reduce drag,” Sorrells said. “Instead, we found that if you address airflow correctly at the front of the truck, you can prevent a lot of airflow interactions you don’t want and thus reduce drag without the need for a completely smooth underside. This modification takes a step further what we already do in production of Volvo trucks, which have a very aerodynamic bumper design to begin with.”
The second modification was a plate between the chassis rails on the space between the back of the cab and the fifth wheel, called a deck closure. According to Sorrells, this prevents airflow under the tractor from rushing up into the trailer gap, which creates strong turbulence and aerodynamic drag.
The most noticeable modification was on the trailer, where Volvo installed an air deflector that wrapped around the front and sides of the trailer bogie. Sorrells said this reduces drag by smoothly moving air around trailer wheels and axles, which otherwise act as a dam to the airflow. While not as effective as a full aerodynamic skirt on a trailer, the bogie air deflector is smaller, lighter and less susceptible to damage.
The final modification involved the use of roof and side fairing extenders already available to customers from Volvo. These extenders reduce the gap between the rear of the tractor and the front of the trailer. A smaller gap improves airflow, whereas a larger gap creates drag as the air gets trapped against the front wall of the trailer. The roof extender uses a Volvo-patented design so it can be adjusted to not only reduce the trailer gap, but to optimize aerodynamics when using trailers of different heights or with different kingpin settings.