By Paul Abelson, senior technical editor
Continuing our quest for the 10 mpg truck takes us to solutions available today, things with proven results and relatively quick payback. Aerodynamics, the study of airflow around a body, can be a major contributor to fuel use. By properly managing air flow with new devices, we can significantly reduce the energy a truck consumes.
Remember, anything that requires energy lowers fuel mileage, and anything that lessens the need for energy improves fuel economy.
While we can’t see or feel it, we know air has mass. We’ve experienced it as kids, sticking our hands out car windows. With fingers pointing into the wind, we could hold them there quite easily. But when perpendicular to the air flow, much more force was needed to hold them. The greater the speed, the more effort it took.
Methods for overcoming air resistance depend on the frontal area and the size of the trucks. It is what it is, usually 102 inches wide and 13 feet 6 inches tall.
It also depends on the speed of the truck through the air. That force varies with the square of the speed, not in direct proportion to it. Doubling from 30 mph to 60 mph requires four times the energy to overcome the resistance. Cruising at 75 mph is only 25 percent faster than 60 mph, but it takes 56 percent more energy.
Coefficient of drag is a mathematical value reflecting the shape of the truck. The smoother the body, the lower the coefficient of drag. While I love the look of a classic old long-nose truck with its “barn door” chrome bumper, high radiator and chromed air cleaners on the side of the engine, I wouldn’t want to buy the fuel for one. With all the airflow changes around the truck, the difference in fuel economy can be 1 mpg or more.
Since the late 1970s, accessory makers and truck builders have been working to improve tractor aerodynamics. The first major device to smooth air flow was called the Airshield. It helped smooth air flow from tractor to trailer, and yielded an astonishing 13 percent initial fuel mileage improvement in TMC Type II tests.
By the mid-’90s, improvements in tractor design included front air dams to route air smoothly around and under the truck; cab extenders to transition flow from tractor to trailer; and side fairings to reduce turbulence around tool and battery boxes, fuel tanks and generator sets.
For years, the major emphasis was on tractors. With the exception of the Nose Cone and Belly Cone, almost no attention was paid to the trailer. But studies reported at the SAE International meetings show trailers accounting for three-quarters of total drag.
With a typical van trailer, about 30 percent of the drag occurs at the nose and in the gap. Thirty-five percent is at the rear where air becomes turbulent. The remaining 35 percent is below the trailer, as air tries to find a path around landing gear, suspensions, wheel assemblies and tires. Even the cross members of the trailer frame create drag.
Trailer aerodynamics have become a hot topic today, largely because of the California Air Resources Board’s greenhouse gas regulations regulating trucks in the state and EPA SmartWay program’s acceptance of various devices to improve fuel economy. The most visible and common are trailer side skirts.
Laydon Composites, Fleet Engineers Inc. and Transtex Composite recently showed their products at the TMC Annual Meeting and Product Exhibition. These devices attach to the trailer chassis. Running from the landing gear to just before the trailer tandems, they guide outside air past the trailer, leaving air under the trailer to be moved along in the space between the skirts so it creates minimal turbulence.
Side skirt models differ in how they are supported, their resiliency and ability to absorb impacts and abrasion, as well as designed-in structure and reinforcement. Several are modular, allowing for less expensive repairs. Fuel economy is improved 6 percent to 7.4 percent in SAE/TMC testing at 60 mph, so it shouldn’t take long to return your investment.
Another method of managing air under the trailer is a system of fairings developed by SmartTruck. It consists of a front tray that channels air over axle fairings and on to a rear diffuser in front of the ICC bar. Side fairings and a rain gutter divert air flowing along the sides and top to help fill in the drag-causing low pressure zone behind the trailer. The entire system has tested to greater than 10 percent improved mpg at highway speeds. It is less susceptible to damage than skirts.
By creating tapering tail extensions and then cutting them off, drag is greatly reduced. This is known as a Kammback, developed by Professor Wunibald Kamm and other German aerodynamics researchers in the 1930s. Kammback designs appeared on race cars in the 1960s, and recently on van trailers.
ATDdynamics’ TrailerTail first appeared at the 2008 Mid-America Trucking Show. It creates a taper that extends four feet beyond the trailer rear. Early models had to be folded down before trailer doors could be opened, and set up again when the doors were closed. The most recent TrailerTail, introduced at this year’s TMC show, sets up and folds automatically. The operator need only open and close trailer doors manually.
In SAE/TMC tests TrailerTail delivered 6.6 percent fuel economy gains at 65 mph.
Another approach to creating a Kammback truck comes from StreamLine Aerodynamics. Their TailStreamer system is inflatable. Made of Duratex, a proprietary laminate of 1000 Denier polyester and heavy PVC, it takes only 1 psi above atmosphere to inflate and keep the TailStreamer in its aerodynamic shape. The built-in air pump shuts off when the trailer doors open, allowing the device to deflate for loading and unloading. Test results approach a 7 percent gain.
With major areas of drag being addressed, aerodynamicists are examining others, such as inside wheels and behind tires. The Deflecktor Aero Wheel Cover has a plastic frame that fits into an outer dual or wide single wheel and a cover that zips onto the frame. It creates a flat wheel surface that eliminates turbulence inside the wheel. SAE/TMC tests have shown a 1 percent fuel economy gain per tandem, or 2 percent on tractor and trailer. The low-cost device, adopted by Schneider, offers a quick payback.
Finally, we’ve all seen mudguards flapping behind tires. That flapping is resisting aerodynamic force, and that consumes energy. Eco-Flaps from Anderson Flaps Inc. and the Spraydown Aero Guard System available through Betts Spring both developed mud flaps that allow air to pass through while they trap spray and channel it to the ground. Fuel savings for tractor and trailer are in the 3.5 to 4.0 percent range at 60 to 65 mph, also providing a high rate of return for a small investment.
With more than half the energy consumed due to drag, it makes sense to improve aerodynamics. It’s one of the most effective routes to the 10 mpg truck, and the improvements are available now. LL