Path of least resistance:
Aero trailers gaining traction
Aero trailers gaining traction
Jack Roberts|November
14, 2012
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| Old concepts of good truck and trailer design are being replaced |
Aerodynamic tractors have been a growing part
of the heavy-duty truck market for decades. With much of the tractor aero gains
already carved out from years of wind-tunnel testing and refined engineering,
attention now has swung to trailers. MinStar, a
long-haul dry van fleet running out of Eagan, Minn., began experimenting with
aerodynamic tractors when fuel was still less than $1 a gallon. It wasn’t until
last year that MinStar began working with trailer aerodynamics.
“We had some classic
models in our fleet a few years ago,” says Mitch Miller, MinStar president.
“But we stay away from them now entirely because of the cost of fuel.”
The carrier’s primary
focus now is on closing the gap between the cab and the trailer with adjustable
fifth wheels and cab extenders, while also using tank fairings and trailer
skirts.
“Based on our own
testing, a full aerodynamic tractor-trailer combination gets ¾ of a mile per
gallon better fuel economy than a non-aerodynamic rig,” Miller says. “We feel
that aerodynamics allow us to operate a fuel-efficient vehicle for $800 a month
less than for a non-aerodynamic model.”
“When diesel fuel
prices drop to less than $3 a gallon, there is not much interest in paying for
an aerodynamic improvement that will take six years to pay for itself,” says
Dave McKenna, director of powertrain sales and marketing at Mack Trucks. “But
at approximately $4 per gallon, there are a lot of ‘aero-religious’ converts.”
The other driving
force for more aerodynamic tractor-trailers is environmental concern for lower
emissions. That’s why California and the U.S. Environmental Protection Agency
have been pushing aero equipment aggressively.
“Already many millions
of dollars have been earmarked by the U.S. Department of Energy to help the
cause,” says Sean Graham, president of Freight Wing, an aerodynamic trailer
component designer and manufacturer involved in DOE’s effort. “Our goal is to
realize a 15 percent improvement in fuel economy over trailers without
fairings.”
That’s a 6 percent
improvement over what is commercially available now through Freight Wing’s side
skirt and gap fairing. “This would come from a combination of side skirts, gap
fairings that streamline turbulence between the tractor and trailer, and
fairings at the end of the trailer,” Graham says. “We’re well on course to make
this happen.”
Drag increases
exponentially with speed. “Even with all available aerodynamic gains, a sharp
increase in road speed can easily negate any net fuel economy gains,” McKenna says.
“Our research shows that an average road speed of 62 to 65 miles per hour with
a full array of aerodynamic components at work is the best of both worlds in
terms of productivity and fuel economy.”
One reason for the
attention on trailer aerodynamics is that all components of a vehicle’s design
interact with each other. Optimizing the tractor, trailer and driver as a
complete system of mutually supporting elements is essential, says Rick
Mihelic, manager of vehicle performance and engineering analysis for Peterbilt.
“Enhancing synergy
between leading-edge areas like the crown, windshield edges, mirrors and bumper
with downstream effects on the tractor and trailer will yield optimal
performance gains,” Mihelic says.
“We’re already seeing
this in current designs as OEMs and third parties work to provide components
that further integrate combination vehicles,” says Frank Bio, product manager
for Volvo Trucks. In redesigning a truck for optimum aerodynamics, Volvo looks at
all exterior components and how they interact together – all the way to the
trailer. “The entire truck works as a system, so a change to one component can
affect how air flows around another,” Bio says.
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| Trailer-tails are tough and designed to fold away quickly and easily before docking and can reduce drag by up to 5% on the highway. |
It’s hard to imagine a
less aerodynamic structure than a 53-foot long box with no rounded edges.
Making matters worse with a trailer’s aerodynamics is the gap between it and
the tractor, says T.J. Reed, director of product marketing for Freightliner
Trucks.
“Wind moving around
even the most aerodynamic tractor gets sucked into this gap and creates a
tremendous amount of turbulence and drag,” Reed says. “Even crosswinds can add
to the drag created in this area. That’s why you need to reduce as much
friction and turbulence as possible.” Peterbilt has added scoops on its raised
roofs to push air over the trailer “because that transition of air between
tractor and trailer is so critical to good aerodynamic performance,” he says.
Air also gets sucked
in and under the trailer, creating still more drag and turbulence. “You’ve no
doubt seen more and more trailers equipped with side skirts to improve
aerodynamics and fuel efficiency,” says Graham. Freight Wing has invested
heavily in third-party tests that show fuel improvements of up to 7 percent on
trailers equipped with side skirts versus trailers without.
“We’ve found that
fleets typically report 4 to 6 percent fuel economy improvements, depending
upon their application and driving environment,” Graham says. “Most fleets see
a return on investment for a skirt-equipped trailer at about 50,000 miles.”
The final challenge
with trailer aerodynamics is due to the vacuum created by its tall squared-off
rear as the trailer moves at high speed. Because nature abhors a vacuum, air
flowing on all four sides of the trailer immediately curls inward to fill this
void, creating more drag.
Trailer tail devices
can minimize that effect. Babur Ozden, chief marketing officer for ATDynamics,
says his company’s TrailerTail fins create a funnel effect, preventing air from
attempting to fill the low-pressure area, thereby decreasing drag.
TrailerTails deliver 6.6 percent fuel savings
at 65 mph, according to third-party SAE Type II J1321 testing, Ozden says. Improved
trailer aerodynamics also increases trailer stability due to reduced turbulence
at the vehicle’s rear – and thus reduces tire wear and driver fatigue – and
improves safety through reduced spray in wet weather, he says.
Peter’s
Piece
Truck and trailer designers should be talking with
aircraft designers to learn about real aerodynamics.
For many years there has been a belief that
streamlining the tractor unit is all important in reducing drag and fuel
consumption. But it is the back end shape that is most important and anyone who
doubts this statement should take a look at both ends of a jumbo jet.
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| The Boeing 747 has one of the best high lift/low drag ratios of any airliner and a 747 was able to stay airborne for almost 30 minutes after volcanic ash stopped all four engines. When partial power was regained the aircraft still had more than 10,000 feet of air under it. Boeing designers regarded the rear of the fuselage as more important than the front for reducing drag. |
On all sub-sonic aircraft the blunt end comes first
and the streamlined end follows behind. This is because the leading end of the
fuselage, or the leading edge of the wing, is able to send an advance signal of
its approach through the air and the airflow then divides with little
resistance. Only on supersonic aircraft is it necessary to have a needle-shaped
fuselage and knife-edged wings to cut through the air because the sound is no
longer able to travel in advance.
With so little resistance caused by the front of an
aircraft, or truck, that leaves the rear-end as the main cause of high fuel
consumption.
One of the basics of aerodynamics is that a square box
in a wind tunnel will produce 70% of its drag from the rear surface because the
airflow must curl around to compensate for the decreased air pressure on that
surface.
Drag created by the top, sides and underneath of a
tractor-trailer is also more important than the front. Having a fully enclosed
underside will not only reduce drag and fuel consumption, but will also lower
maintenance costs and noise levels. The undersides of tractors and trailers at
present seem to be a case of out of sight, out of mind.
The trailer-tails mentioned above while claiming to
reduce fuel consumption by up to 7% are a rather clumsy attempt to address a
major problem. A better back-end could result from tapering, inflatable anti-drag
bags, which could be quickly deflated and stowed prior to docking.
The fuel
savings from such devices could be many times the 7% already claimed.
Most drivers and fleet operators understand that a
small increase in speed will result in a large increase in fuel consumption.
This fact has little to do with engine efficiency and more to do with air
resistance. To understand just how critical air resistance is it must be
remembered that when velocity is doubled air resistance goes up four-fold.
Cars, buses and trucks would be much more efficient, aerodynamically,
if they were turned turned around and run backwards.
