In one memorable scene from the classic movie “Breaking Away,” the bicycle-riding hero from Bloomington, Ind., drafts behind a tractor-trailer on a long training ride. While there has been considerable speculation since the film’s release in 1979 as to whether or not a bicyclist could ever maintain a 55-MPH draft, one of the movie’s technical experts confirms that the rider actually achieved 58 MPH while filming the scene.
Aerodynamics as a movie special effect is one thing—as a part of today’s emphasis on fleet efficiency, it’s an entirely essential business tool. Understanding what aerodynamics is—and isn’t—may seem like just a bunch of hot air, or it may seem like rocket science. As it turns out, the latter is the more appropriate choice.
Much of what we understand about the aerodynamics of trucks and trailers traversing America’s highways today comes from the research conducted more than 35 years ago by another bicyclist, who happened to be looking toward space, rather than down the highway.
In 1973, Edwin J. Saltzman, a bicyclist and aerospace engineer at NASA’s Dryden Flight Research Center in Edwards, Calif., felt the push and pull of large trucks at highway speeds while riding to work. “As a tractor trailer overtook him, he first felt the bow wave of air pushing him slightly away from the road and toward the sagebrush; as the truck swept past, its wake had the opposite effect, drawing him toward the road and even causing both rider and bicycle to lean toward the lane,” a NASA research paper explains. Saltzman realized that the larger a vehicle is and the faster it moves, the more air it pushes ahead. “For a large truck, this can mean a particularly large surface moving a large quantity of air at a high velocity—its blunt face acting like a fast-moving bulldozer, creating a zone of high pressure. The displaced air must go somewhere, spilling around the cab into swirling vortices. The air traveling along the side moves unevenly, adhering and breaking away, and sometimes dissipating into the surrounding air. At the end of the cab or trailer, the opposite effect of the high-pressure zone at the front develops; the airflow is confronted with an abrupt turn that it cannot negotiate, and a low-pressure zone develops,” stated Saltzman’s research.
A study published in Automotive Engineering in August 1975 found that a tractor trailer unit moving at 55 MPH displaced as much as 18 tons of air for every mile traveled. In such cases, roughly half of the truck’s horsepower is needed just to overcome aerodynamic drag. Saltzman and his NASA colleagues turned their work on aero and fluid dynamics to research into how trucks might slip through the air instead of just pushing through it, and at the same time decrease drag and increase fuel efficiency.
The results of their efforts have been obvious. “You don’t see very many long-haul tractors that aren’t pretty aerodynamic these days,” says Charlie Fetz, vice president of R&D with Great Dane Trailers. “Even modern day cabs have a decent amount of aerodynamics built in. People have realized the importance of aerodynamics for some time now, and the concept of ‘pushing a brick through the wind’ isn’t necessarily a bad thing anymore.”
Fetz explains that while sleek and sexy tractor front ends may seem to offer the ultimate aero efficiency, even flat-front cabovers can still slice through the wind. “They’re low to the ground, they keep the air out from under them, they close-couple the tractor and trailer. The combination of features results in some pretty low drag coeffiecients. And if you look at a lot of the big motorcoaches and buses it’s clear there are things that can help even the flattest-face vehicle.”
Go With The Flow
According to aerodynamics experts, it’s all about airflow. From a tractor and trailer standpoint, the main thing a fleet manager should pay attention to is the turbulence of air swirling under the trailer. Additionally, the air getting caught in the gap between the tractor and trailer, or the multiple gaps if multiple trailers are being run, and air clinging to the sides needs to be addressed.
“Air flow underneath the trailer is a major concern due to the significant turbulent flow patterns and their interaction with the rotating tires and the suspension assembly,” explains David Pickup, manager of product engineering development for Wabash National. “The rear of the trailer also acts to create a low-pressure, drag-inducing vortex which is a further and, depending upon speed, perhaps the second-most significant aerodynamic factor. The gap at the front of a trailer between the tractor is also clearly a non-aerodynamic feature and thus another fuel-sapping parasite.”
Pickup says the sides and top of the trailer likely have the least impact on aerodynamics. “Losses here are primarily a result of boundary layer drag and countermeasures would be difficult to apply due to vehicle width and/or height restrictions. Tractor configurations do impact overall aerodynamic performance and cab-mounted air deflectors are commonly used as a performance-enhancing measure and are to be encouraged.”
The impact on a fleet’s fuel efficiency by attention to aerodynamics can be dramatic. “At speeds greater than 50 MPH, aerodynamics is the most important factor in tractor-trailer fuel economy," explains Ralph Bott, of Carrier Transicold. “At 65 MPH, more than 50% of a truck’s fuel is used to overcome drag. Improved aerodynamics saves fuel as well as power, thereby decreasing demand on the engine and decreasing greenhouse gas emissions. This results in significant cost savings and a longer engine life, and it reduces the environmental impact.”
When a fleet manager makes a decision with his power equipment vendor, he needs to be looking at where the engine is running in its sweetspot and gearing it to run within an optimum range of RPM. They need to pair the aerodynamic performance along with the performance of the engine and transmission.
Actual fuel economy improvements range from 2% to 7 %, say experts, with 15% improvements seen in certain test applications.
While history has shown that slowing down saves fuel, obviously for many fleets that’s just not an option. Concurs Fetz, “Rolling resistance tends to dominate up to about 50 MPH. Above 50 MPH, the aerodynamic drag dominates, and it goes up faster than linear. If you drive 75 you’ll see more aerodynamic benefit than you would at 65, although you’ll still need to adjust for the higher fuel consumption at the higher speed.”
Determining which of the many aerodynamic accessories is right for your application may be harder than actually realizing their benefits. Fleet managers can select between low rolling resistance tires, fairings for reefers or dry box trailers and side skirts in a variety of sizes to minimize turbulence under the trailer.
How can you choose?
“One key consideration is fleet deployment,” explains Pickup. “If the fleet’s operations are primarily on highways and at highway speeds, then a fleet manager should positively consider the employment of aerodynamic devices. Aerodynamic drag, and the energy required to overcome it, is a function of the square of speed, so doubling the speed will typically require four times the energy. This is less true at low speeds but certainly the case at highway speeds and above.”
Now, fleet managers can easily compare the costs and benefits of various technologies using The SmartWay Technology package Savings Calculator for Fleets.
The benefits of aerodynamics have been demonstrated by NASA, by the U.S. Department of Energy, by Transport Canada, by numerous test fleets and by the U.S. Environmental Protection Agency’s SmartWay Transport Partnership. Now, The California Air Resources Board (CARB) requires attention to aerodynamic principles. According to Laydon Composites, CARB’s 2011 requirements mean attention to detail for new trailers operating in the Golden State.
The CARB requirements state that any 53-ft. box trailer have aero devices that cut fuel consumption by 5% or more and that 53-ft. refrigerated vans have aero devices that cut fuel consumption by 4% or more. The rule went into effect Jan. 1 and covers all trailers designated as a 2011 model year or later.
“I think there may be a common misconception that aerodynamics is smoke and mirrors, that it doesn’t really work. That’s absolutely not true,” explains Great Dane’s Fetz. “But the truth is that while it does work, there are also tradeoffs. Believing that it either doesn’t work or works absolutely is incorrect.”
So how should a fleet manager approach the topic of improving aerodynamics? “I normally advise people to find a combination of aerodynamic packages and treatments that will work in their particular operations,” says Fetz. “Chances are, the driver already has an aero package on his tractor, and adding fairings over the fuel tanks is a fairly easy fix and durable fix, as well. But then you can look underneath at adding a skirt—that’s probably the next big bang for the buck. “
Silver Eagle Manufacturing Company offers a fairly simple formula to help in determining your return on investment: Cost of product plus cost to install minus fuel savings plus maintenance. Is the product right for you?
“Fuel and installation costs are straight forward,” explains a Silver Eagle whitepaper. “As you do your homework, you’ll find that all aerodynamic skirt fairings deliver similar fuel savings, generally in the 4% to 6% range. The results are based on a variety of tests, including fleet, wind tunnel, over-the-road and track testing.
“Installation costs are also similar, since they generally utilize the same practices of bolting the skirt fairings to the trailer. Installation for most skirt fairings can be handled at your fleet’s maintenance facility or you may choose to have a manufacturer’s certified installation garage do the work.”
Silver Eagle recommends that fleet managers recognize how different construction materials can impact their payback schedule. Aerodynamic technology demands the use of lightweight materials to be effective. For that reason you’ll find a wide range of construction materials—everything from soft and rigid plastics to rubber, stainless steel and lightweight aluminum. When evaluating aerodynamic products, you’ll want to look closely at materials, because it can affect how well and how long your fairing lasts.
“Generally, employing an aerodynamic skirt fairing should achieve payback for itself in a single year. Your main consideration, however, is that the skirt should continue paying for itself as long as you own the trailer, approximately 15 to 20 years. For that payback to continue, you’ll definitely want to focus on maintenance costs prior to purchasing fairings.”
Fetz agrees that attention to maintenance costs is critical. “A skirt is a very good solution if you can tolerate it. If you can’t tolerate it, by all means pay very good attention to your side extensions and your roof fairings on your tractor and how closely you can couple your tractor and trailer.”
The practicality of the device has to be established based on each fleet’s particular set of circumstances, says Pickup. “Aerodynamic skirting is probably one of the simpler devices to apply and a SmartWay Certified Product, used in highway operations, should start to yield an immediate fuel saving of around 5%. Boat tails for reduction of rear vortex drag would be another worthwhile addition, but these devices can be mechanically complex when it comes to loading dock accommodation. Trailer nose devices and seals at the Tractor Trailer interface also provide a worthwhile contribution and, unlike skirts and boat-tails, are mounted out of harm’s way. But when used in isolation from skirts and boat-tails, these other devices are less likely to provide similar fuel savings.”
Pickup says collectively, it may be possible to achieve around 10% fuel savings by the use of aerodynamic devices, but the extra consequent maintenance and loss of load capacity would have to be factored in to obtain the overall true bottom line benefits. Selected and used properly, aerodynamic accessories can be a real breath of fresh air to a fleet’s bottom line. FE
