NASHVILLE, Tenn. — In the search for better fuel economy, a trailer can be a drag. Its tires add to rolling resistance, any uneven surfaces contribute unwanted friction, and traditional shapes hardly slice through the air with ease.
“Basically it’s a brick,” says Rod Ehrlich, senior vice-president and chief technology officer at Wabash International, and the holder of many trailer-related patents. The aerodynamic challenges are not limited to a box-like form, either. The gap between a tractor and trailer alters air pressures, while wheels and bogies create their own disturbances.
These are important challenges to overcome as the US Environmental Protection Agency (EPA) looks to limit greenhouse gas emissions – a target that can only be met by burning less fuel.
Trailer designs can make a difference, maintenance managers were told during the recent annual meeting of the Technology & Maintenance Council of the American Trucking Associations. The EPA’s SmartWay program already offers its stamp of approval to units which improve typical fuel economy by 5% through the help of side skirts, weight savings, and gap reducers or boat tails. A new SmartWay Elite designation is being created to identify trailers that offer gains of 9% or more.
“We’re trying to come up with ways we can make the ideal streamlined vehicle,” Ehrlich says.
There have been streamlined shapes before. He refers to models from decades ago which included rounded noses and roof lines, but those were abandoned. “They were expensive to build when you take metals and try to make compound curves,” Ehrlich says. Then there was the matter of fitting square boxes and skids into curved spaces; the classic challenge of trying to fit a square peg in a round hole. “Anything that we do to interfere with that primary reason for it to exist is not a good compromise,” he says.
Still, there are many gains to be had.
Trailer side skirts, now fitted on about half the trailers produced by Wabash, promise fuel savings of 4-7%. Trailer-mounted gap reducers offer their own gains of 1-2%. At the rear of the trailers, aerodynamic wings known as boat tails can boost savings by 1-6%, while underbody systems promise improvements of 1-2%.
About one in four 53-ft. van trailers are fitted with at least one aerodynamic device, says Mike Roeth, executive director of the North American Council for Freight Efficiencies. Side skirts are the most popular of all, and they present few maintenance costs.
The potential improvements are not limited to aerodynamic gains alone. Low rolling resistance (LRR) tires can reduce diesel demands by 3%, and wide-base tires offer savings of 3-5%. By automatically maintaining tire pressures, tire inflation systems contribute another 1%.
Then there are the opportunities to shed wasteful weight. A 1-1/8” composite floor can be 265 lbs lighter than a traditional wood floor, and another 100 lbs can be saved with a composite nose. “We can lower the weight of a trailer by 2,000 lbs pretty conveniently,” Ehrlich says. But there’s a catch: “It costs money to take weight out.”
As promising as any gains may sound, Ehrlich says fleets need to consider the total cost of ownership behind any trailer changes. This means considering up-front costs, safety, service support, durability and warranties.
“These are new devices,” he says. “How well are they going to be able to stand behind the product?”
Expected resale values and maintenance costs both need to be considered, agrees Roeth, whose group has studied changes made by “leadership fleets” including Canada’s Challenger Motor Freight and Bison Transport.
Claims also need to be proven. The tools for that can include everything from computational fluid dynamics (computer models showing how a trailer flows through the air), to wind tunnels, test trucks and road tests. Universities offer a valuable resource when analyzing test results, Ehrlich adds.
C.R. England, which has 4,300 tractors and 6,800 trailers, now devotes a pair of its own Class 8 tractors and 53-ft. trailers to nothing but tests of potential fuel-saving devices. Two full-time drivers are dedicated to the work, while one-third of another employee’s time is used to coordinate the tests and analyze the data. Even though it is running two tests per week, at a cost of $2,500 each, the queue to review new components is now six months long.
“It does take commitment,” says Ron Hall, the fleet’s senior director of equipment and fuel.
But the results have led to proven changes. C.R. England tractors and trailers are equipped with 44-inch fifth wheel gaps, perforated mud flaps, wheel covers, single-piece composite side skirts, boat tails, low rolling resistance tires, and tire inflation systems.
Some fuel-saving tools were still rejected because of costs. Inventories of wide-base tires were thought to be too pricey to manage, and the fleet is “neutral” in its view of aerodynamic changes on the trailer’s surface, behind the tandem, and under the vehicle. Still, Hall stresses that the devices can still have a role to play in other fleets. “It may not be neutral for everyone,” he says.
“The more data the better,” Roeth says. But he still believes that smaller fleets can conduct meaningful tests of their own. An operation with 15 trucks travelling the same route, for example, could simply add devices on a few of the trailers and measure the differences. “None of these tests are perfect,” he says. “None of these tests are bad.”
Potential fuel savings may even be hiding inside the trailer. One option being explored by C.R. England, which specializes in temperature-controlled freight, includes extra floor insulation.
“To us, fuel use on the trailer isn’t just what we save off the tractor. It’s also what we put into the reefer,” Hall says. This means tests also explore BTUs lost per hour, and use thermal imaging to identify where insulation might need to be improved.
The focus on fuel economy continues when the fleet’s equipment is on the road, tracking fuel purchases by truck, cross-referencing the numbers to data from electronic control modules, and comparing fuel economy by specification. “If the technology requires some kind of behavioural change by the driver, the fuel test is not going to measure the management of that behaviour,” Hall explains. Trailer boat tails, for example, still need to be opened.
And the ongoing costs of trailer enhancements are hardly limited to fuel. Hall adds between 25% and 50% to the cost of a new fuel-saving device to account for installation and maintenance. Even something that looks like it simply bolts into place can create secondary issues. C.R. England found that it had to mount side skirts aft of the landing gear, to make it easier to reach fuel tanks. Tire carriers also had to be remounted so they could still be reached.
Targets for the second phase of the EPA’s limits on greenhouse gas emissions will be published by March 31, 2016, and will focus on vehicles, engines and trailers alike. “We want real-world fuel savings,” says Dennis Johnson of the US Environmental Protection Agency’s technology assessment centre. “Not just data in a test cell.”