Tractor Aerodynamics Continue to Improve
Comparatively speaking, today’s over-the-road tractors are slick machines.
Improved aerodynamics enables them to slide through the air better than any earlier generation while offering the potential for double-digit improvements in fuel efficiency. Aftermarket devices can improve tractor aerodynamics further, and truck makers are testing new concepts and engineering changes that could make the next generation tractor models slicker still.
The most practical challenge truck owners may face when considering adding an aerodynamic device or component is evaluating its efficacy. Is the product worth the cost? Will it deliver the advertised gains? In most cases, the answer is yes, but with caveats, industry executives said.
Tractor aerodynamics took a big leap forward a decade or so ago with the introduction of smaller, more efficient engines. Aside from the operating efficiencies these engines provided when matched with smarter gearing and new drivelines, they enabled original equipment manufacturers to reduce the truck’s frontal area significantly by sloping the hood and windshield and rounding out the sharp, wind-catching edges of bumpers, mirrors and other protuberances. The results were double-digit reductions in aerodynamic drag.
When it comes to adding aerodynamic devices on a tractor, Tracy Bogler, CEO of Woody Bogler Trucking Co. in Rosebud, Mo., has one piece of advice: spec wisely. Adding an aerodynamic device won’t help much unless it’s incorporated into a whole-vehicle spec.
“The biggest thing is [that] you have to do it all,” Bogler said. “Doing one thing helps, but everything feeds on each other. You can have all the aero devices, but if you don’t spec the truck right, you’re not going to get anything. And what do you do first? You do it all. Either you’re in or you’re out. You can’t half do it.”
Woody Bogler Trucking Co.
Woody Bogler Trucking’s fleet is averaging 8.5 mpg, with dry van combos averaging “a hair” over 9 mpg, dramatic improvements from the company’s fleetwide average of 5.5 mpg before it began adding aerodynamic packages, Bogler said.
Today, every OEM offers a range of standard and optional aero packages.
International Truck’s current LT model, for example, comes standard with a three-piece composite front bumper, sloped and curved front windshield and pedestal door mirrors. Adding optional roof fairings and side extenders on the Long Sleeper model could improve fuel economy by 12% when matched with a standard 53-foot skirted box trailer, said Jim Nachtman, director of on-highway product marketing. Other component savings include 6% for chassis skirts, 2% for an Aero bumper and 1% for a pedestal mirror.
Each new tractor model offers incremental gains over a previous version.
Freightliner’s 2018 Cascadia with the Aero package that includes 20-inch cab side extenders, full chassis fairing with a 4-inch ground clearance and rear wheel covers can add up to 1.1% improvement in fuel efficiency compared with the 2014 Cascadia with the Evolution aero package, said Kelly Gedert, director of product marketing. The AeroX package, with a low-clearance front bumper and air dam, 24-inch side extenders and drive axle wheel fairings will mean an additional 1.5% improvement, or a total of 2.6% compared with the 2014 model, Gedert said.
But the optional OEM aero offerings don’t always cover everything. Opportunities remain to reduce drag in and around the tractor, and the aftermarket is trying to fill those gaps.
The AeroKit from FlowBelow, for example, includes wheel covers and a pair of tandem fairings that reduce turbulence and drag around the tractor’s rear wheels. The kit is a factory option for several OEMs and can cost $1,000 to $1,500 with factory installation, according to company President Josh Butler. FlowBelow also offers AeroFender, an aerodynamic fairing in combination with a quarter fender.
Twist & Lock Aero Covers from RealWheels Corp. come in four styles and are made from aluminum, clear polycarbonate or ABS, an impact-resistant plastic. The suggested retail prices for a full tandem kit (four covers, brackets and hardware) range from about $320 for the ABS set to $509 for the stainless steel set with clear windows.
California-based xStream introduced TruckWings for tractors. Similar to cat tails on trailers, the system consists of a set of panels on the back of the tractor that automatically deploy at pre-set speeds to cover the sides and top of the tractor-trailer gap. This creates a continuous connection between the truck and trailer that allows the air to flow smoothly over the entire length of the truck, said Daniel Burrows, president and founder.
Similar to Woody Bogler Trucking Co., TransWay Inc., based in Holland, Mich., has been able to raise the average mileage stats for its units to “a little over 9 mpg” over the past seven years, Fleet Manager Randy McGregor said.
“I believe in technology and aerodynamics,” he said, noting the fleet’s tractors have all facets of aerodynamics, including roof fairings, sky skirts, cab extenders and FlowBelow kits. “We are very aerodynamically focused. Our cab extenders are 24 inches [and] from there we can do a gap reducer on the trailer.”
“We’ve even looked at the fifth wheel height, making sure that it isn’t so high it pushes the trailer roof above the tractor’s fairing height,” McGregor added.
Given the big improvements in tractor aerodynamics that already have been achieved, future efficiency improvements will be only marginally incremental, barring some radical new design, industry executives said.
“All the big fish are gone,” said Fritz Marinko, consultant, senior vice president and chief aerodynamicist at WEO. There really aren’t any big opportunities to improve tractor aerodynamics, “unless the shape of the truck changes dramatically.”
“It is fair to say that most of the improvements have been achieved,” said Patrick Dean, chief engineer for Kenworth Trucks. “At the same time, however, there have also been large strides in computational fluid analysis techniques, and that allows us to find opportunities that were previously hidden.
“Regulation changes, such as external mirror elimination by moving to camera-based solutions, also stand to provide measurable reductions in drag and associated fuel-economy improvement.”
Several design concepts would improve the air flow around the tractor but may not be practical for the market, Marinko noted. A single-person cab, for example, would be more aerodynamic, “but truck buyers, especially those in the secondary market, still want a two-person cab,” he said. “I don’t envision us going to single-driver position trucks en masse.”
One manufacturer, however, has relocated the seat. Tesla’s highly aerodynamic, all-electric Semi model, unveiled in November, places the driver’s seat in the center of the cab, along with a jump seat on the cab’s back wall. The truck also features a sloped cab design enabled by removing the internal combustion engine.
TransWay’s McGregor foresees more changes in materials. The bigger gains will be in lighter materials, such as carbon fiber and composites to lighten the components, he said.
There also may be more electrically assisted subsystems, such as electric drive assist axles to help on grades or during start up, McGregor said. “Maybe we’ll see more regenerative braking.”
Additional aerodynamic improvements will require “some out-of-the-box thinking on behalf of truck manufacturers and customers,” said Jason Spence, longhaul product marketing manager at Volvo Trucks. “Some designs that the SuperTruck programs have presented represent this type of approach.
“Consider raking the windshield so far that it forces the driver entry door rearward on the cab. How would this affect the overall proportions of the tractor along with the impact to daily operations? All the potential impacts need to be addressed before you see longhaul trucks start to resemble bullet trains.”
Marinko noted that other concepts to improve a tractor’s aerodynamics that have been suggested through the years include automatic shuttering of the grille, a “kneeling” truck that rides lower to the ground, and covering up everything under the tractor to smooth the air flow.