Cutting The Drag
With aerodynamics, its got to be the holistic approach – the whole vehicle – tractor and trailer.
By Oliver B. Patton, Washington Editor
Aerodynamics is a Greek word meaning there's a whole bunch of fuel and money to be saved.
Most everyone in trucking is familiar with the aerodynamic work that's been done on tractors. It's there in the shaped hoods, rounded corners and air deflection devices that characterize modern designs. And the dividends in fuel efficiency are well understood.
Turns out these gains are just the tip of the aero-iceberg of what can be obtained from applying such science to the tractor and trailer as a unit.
In fact, new research indicates that the holistic approach can reduce aerodynamic drag by as much as 23 percent, which translates to more than a 10 percent improvement in fuel efficiency for vehicles that spend most of their time on the highway. In a business where most gains are measured in the low single digits, that's pretty impressive. It could add up to almost a billion gallons of diesel a year for the national fleet, said Bob Clarke, president of the Truck Manufacturers Association.
"Clearly what we demonstrated in this project is that we can do lots of good things on tractors, but unless we address the whole vehicle – including the trailer – there are going to be limited amounts of overall fuel efficiency that we can gain from aerodynamic improvements," Clarke said. "You've got to do the trailer."
With funding from the U.S. Department of Energy, the association coordinated a two-year project in which four major truck manufacturers – International Truck and Engine, Freightliner LLC, Mack Trucks and Volvo Trucks North America – cooperated on designing and testing holistic aerodynamic technologies. DOE put up a little less than half of the $3.4 million spent on the project, Clarke said.
International worked with Great Dane and Wal-Mart on the tractor-trailer gap and other trailer devices. Freightliner worked on mirrors and mirror mounts. Mack also tested gap and trailer technologies. And Volvo focused on underside air flow devices.
Each worked separately on its share of the project, but they shared technical data and results for the sake of collective knowledge – a first for this highly competitive industry.
"We gained respect for what our competitors can do – and gained knowledge from what our competitors have done," said Don Smith, a senior development engineer for Freightliner.
Getting everybody involved, Clarke said, is as valuable as the technology work that gets done. "When they're all in it together, even though they compete and show different aspects of the same thing, there's great power in the collective weight of all of them doing something."
Power – and practicality. The fact is, the aerodynamic gains demonstrated in this project will not come to market unless carriers demand improvements and tractor and trailer builders work together.
"It is much harder to get (aerodynamic) technology on trailers than it is on tractors," Clarke said. "Carriers have to want to do this. The pull has always been the carriers – if the carriers demand something, the carriers get it."
One fleet that has taken the initiative is Wal-Mart. Its work with International and Great Dane is part of a companywide effort to achieve a 25 percent increase in logistics efficiency by 2008. Besides aerodynamics, Wal-Mart is working on improving fuel economy with engine developments, use of APUs, more efficient tires, hybrid drive systems, and systemic improvements such as minimizing empty miles and reducing packaging. The long-term goal is to move twice the tonnage with the same amount of fuel by 2020.
International used wind tunnel and road testing with Great Dane and Wal-Mart to demonstrate a 7 percent fuel economy improvement – that's 7.6 million gallons a year for Wal-Mart's fleet of 6,000 trucks, according to International.
Some of the devices they tested are obvious to the eye. Tractor side extenders, for example, to smooth the air flow between the tractor and trailer; trailer side skirts that descend automatically above 35 mph and stow at lower speed to improve ground clearance; and tapered extenders that reduce turbulence immediately behind the trailer. This "boat tail," as it is described, is a permanent fixture that does not restrict the trailer door opening yet is strong enough to withstand the wear and tear of backing up to a loading dock.
Not as obvious are some changes in the trailer shape. They sharpened the front end of the trailer slightly – its frontal area was shrunk by 8 percent – to reduce drag. The trailer floor was lowered to recoup some of the cargo space lost in the front-end reduction. An automatic suspension lift raises the lower trailer floor to standard dock heights.
The companies also tested a cone on the trailer to control air flow in the tractor-trailer gap, which produced a 2 percent economy gain, and automatic tractor side extenders that deploy at highway speed, which produced a 1 percent gain, International said.
Freightliner's focus was on mirrors, which produce a surprising amount of drag. Wind tunnel tests showed that the design of a mirror, its placement on the tractor and even the shape of the tractor near the mirror can create more than 6 percent of a vehicle's drag.
"Mirrors need to be integrated into the specific vehicle design, because what works on one make or model may not perform the same on another," Smith said. "The real benefit from our study is that we have produced design guidelines that can be incorporated on any heavy-duty vehicle design to improve airflow around mirrors."
Those who like the style of the classic West Coast mirror should be aware that there's a price to be paid for that look, Smith said. All that piping, and the shape of the mirror itself, produce excessive drag.
Mack reported an 8 percent improvement in fuel economy with tractor side extenders to enclose the tractor-trailer gap, plus trailer side skirts and a boat tail. Mark Kachmarsky, chief project manager, said Mack engineers are working on a production version of the gap enclosure device, and have passed their side skirt and boat tail data on to trailer manufacturers.
Volvo found 2.3 percent fuel savings in a quartet of devices designed to smooth air flow underneath and between the tractor and trailer.
The first step was to extend the bottom of the front bumper with a composite material to create a smooth air flow beneath the tractor. Next, a plate between the chassis rails behind the tractor cab – a "deck closure" – prevented air from rushing upwards into the space between the tractor and trailer. Then engineers installed a skirt in front of the trailer's rear bogie to smooth air flow around the axles and wheels. This innovation is not as effective as side skirts but is less expensive and less prone to damage, said Michael Sorrells, lead design engineer for aerodynamic development. Last – and more familiar – were tractor side and roof extenders to close the gap between tractor and trailer.
On hand at the announcement of these results in November was Rep. David Hobson, R-Ohio, who described the event as "a great day for taxpayers" because it showed how public funds can stimulate private investment.
"There's greater acceptance of what's done if the private sector is involved," he said. "We can't reduce the number of trucks on the road, but we can make them better – more efficient and cleaner. If we're going to survive in the world economy we are going to have to leave fossil fuels behind."
Clarke hopes DOE will continue to support this kind of cooperative research. He envisions a "Super Truck" program, in which the manufacturers would each build and test several "clean sheet" vehicles that incorporate all of the advancements of the past five to 10 years. "We hope there can be some funding for it," he said.
Meanwhile, there remains the challenge of bringing the technologies of this project to market – not an easy proposition. The economics of investing in aerodynamic devices gets more complicated when it includes trailers as well as tractors. Trailers are less expensive and less complex than tractors, so features such as side skirts and boat tails can be a more prominent expense than, say, a tractor side extender.
"We need sophisticated customers," Clarke said. "Somebody who wants it and will pay for it, too."
It is clear that, as Freightliner's Don Smith put it, trailers are where the aerodynamic fruit hangs low for the plucking.
Once carriers like Wal-Mart blaze the trail, others are likely to follow.
Emissions Authority continued...
