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Topic Name: Researchers Found Conventional Aerodynamic Streamlining Could Improve Fuel Efficiency in Heavy Truck
Category: Fuel Technology
Research persons: Robert Englar
Location: Georgia Tech Research Institute (GTRI), United States
Details
Diesel fuel prices approaching $5 a gallon – and the resulting economic
impact on products transported by truck – have created renewed interest in
fuel-saving technologies developed during the past decade at the
Georgia Tech Research Institute
(GTRI).
Use of pressurized air "active flow control" techniques combined with
conventional aerodynamic streamlining could improve fuel efficiency by 8 to 12
percent in the heavy trucks used to transport a broad range of products. If
installed throughout the U.S. trucking fleet, these technologies for reducing
aerodynamic drag could save between 1.6 and 2.4 billion gallons of fuel per
year.
"The dramatic increase in diesel prices has led the trucking industry to
reconsider aerodynamic fuel efficiency improvements that might not have been
cost effective only a few years ago," said Robert Englar, a GTRI principal
research engineer and principal investigator for the project. "Though there are
technical challenges ahead, we believe our techniques for improving fuel
efficiency offer significant potential to reduce the impact of these fuel cost
increases. Beyond the trucking industry, that would help consumers who see the
effects of fuel costs in everything they buy."
Since diesel prices began their rapid increase, Englar has seen growing
interest in the GTRI low-drag active flow control aerodynamic technologies,
which were developed with support from the
U.S. Department of Energy
starting in the late 1990s. He has received numerous inquiries for information
from both large and small trucking companies, and also from railroads – whose
higher-speed western track runs could also benefit from aerodynamic drag
reduction.
Aerodynamic drag is the major component of heavy vehicle resistance at
typical highway speeds, and overcoming that resistance requires increased energy
use. Truck designers have reduced drag on the tractor portion of the vehicles by
applying such aerodynamic streamlining approaches as roof fairings, but those
have done little to address drag on the aft portion of the trailers.
Because only limited streamlining can be done for trailers due to their
length, the GTRI researchers added the active flow control techniques, which use
patented pneumatic devices to blow air from slots over small curved aerodynamic
surfaces at the rear of the trailers. These air jets smooth the flow of air over
the boxy trailers to eliminate air-flow separation, vorticity and suction on the
aft doors, which reduces aerodynamic drag at highway speeds.
The researchers also evaluated aerodynamic improvements that involved
rounding aft trailer corners, installing fairings around wheels and making other
changes designed to better streamline the trailers.
These active flow control techniques are based on aerodynamic research done
during the 1980s for applications on U.S. military aircraft. Beyond the fuel
savings, they have also been shown to enhance braking and directional control
for the heavy trucks without using any moving external parts, potentially
improving safety.
"Aerodynamically, we have resolved unknowns raised in earlier testing, and
the next step is to get this into a fleet of trucks for more extensive testing,"
Englar said. "At highway speeds, each one percent improvement in fuel economy
would result in saving about 200 million gallons of fuel for the U.S. heavy
truck fleet. We believe that is worth pursuing."
The fuel efficiency project began in the late 1990s with tests of simple
scale model tractor-trailers in GTRI's low-speed wind tunnel. The researchers
then applied those principles to a full-sized test truck, working with Volvo
Trucks of North America and Great Dane Trailers, manufacturers of the basic test
tractor and trailer respectively.
A series of Interstate-speed test runs at the Transportation Research
Center's Ohio fuel-economy test track have demonstrated substantial fuel
savings. The tests involved operating a test tractor-trailer for several
different 45-mile runs around a 7.5-mile oval track at highway speeds of 65 and
75 miles per hour. A control truck that did not have either the aerodynamic
improvements or pneumatic flow control system was operated under the same
conditions to provide a comparison. For additional comparisons, the test truck
was also run without the experimental blowing equipment.
The tests showed that the techniques could provide drag coefficient
reductions of up to 31 percent, which translates to a fuel efficiency increase
of 11 to 12 percent. When the energy required by the air compressor installed on
the truck to provide the compressed air for these prototype tests was subtracted
from those savings, those tests showed that the low-drag techniques could
produce an overall fuel efficiency increase of 8 to 9 percent.
Before the pneumatic control system can be widely used in trucks, however,
researchers will have to choose the best source of compressed air for the
blowing system, Englar notes. An air compressor more efficient than the one used
in the testing would provide higher overall fuel efficiency. Options include a
small diesel-powered compressor installed on or under the trailer like current
refrigeration units; bleeding pressurized air from the truck's
supercharger/turbocharger, or a simple chain drive from the trailer's wheels to
turn air blowers.
Other practical issues – such as protecting the pneumatic surfaces from
damage during docking – still must be resolved. A simple solution, Englar noted,
could be to use stiff rubber surfaces.
Beyond boosting fuel efficiency, the pneumatic system can also provide a form
of aerodynamic braking to assist the mechanical brakes. "The pneumatic systems
can turn a low-drag configuration into a high-drag configuration very rapidly,
providing a lot more braking power," Englar said. "By turning the trailer into a
non-moving pneumatic rudder, blowing can also restore directional stability
should the truck be operating in destabilizing high side winds."
Further energy savings could come using a pulsed pneumatic system, which
preliminary wind-tunnel studies show could produce the same aerodynamic
efficiency with 40 to 50 percent less energy consumed by the blowing system.
Englar hopes to receive additional funding to study how this might affect the
truck aerodynamics – as well as fuel consumption.
"The ultimate proof would be to apply this overall aerodynamic system to a
small fleet of heavy trucks and run them on their normal cross-country routes
for a month or so to measure the operational increases in fuel efficiency and
safety," Englar said.
Note for Aerodynamic Drag
Aerodynamic drag refers to the retarding force on moving aerodynamic bodies
acting in the direction of the freestream flow. The drag in the body perspective
(near-field approach) comes from forces due to pressure distributions over the
body surface, symbolized Dpr, and forces due to skin friction, which is a result
of viscosity, denoted Df. Alternatively, the drag force calculated in the
flowfield perspective (far-field approach) comes from three natural phenomena:
shock waves, vortex sheet and viscosity.
The pressure distribution over the body surface exert a normal forces which,
summed and projected into the freestream direction, represent the drag force due
to pressure Dpr. The nature of these normal forces combines shock wave effects,
vortex system generation effects and wake viscous mechanisms all together.
When the viscosity effect over the pressure distribution is considered
separately, the resultant drag force is namely pressure drag or, alternatively,
form drag. In the absence of viscosity, the pressure forces on the vehicle
cancel each other and, hence, the drag is zero. Pressure drag is the dominant
component in the case of vehicles with regions of separated flow, in which the
pressure recovery is fairly ineffective.
The friction drag force, which is a tangential force on the aircraft surface,
depends substantially on boundary layer configuration and viscosity. The
calculated friction drag Df utilizes the x-projection of the viscous stress
tensor evaluated on each discretized body surface.
The sum of friction drag and pressure (form) drag is called viscous drag. This
drag component takes into account the influence of viscosity. In a thermodynamic
perspective, viscous effects represent irreversible phenomena and, therefore,
they create entropy. The calculated viscous drag Dv use entropy changes to
accurately predict the drag force.
When the airplane produces lift, another drag component comes in. Induced drag,
symbolized Di, comes about due to a modification on the pressure distribution
due to the trailing vortex system that accompanies the lift production. Induced
drag tends to be the most important component for airplanes during take-off or
landing flight. Other drag component, namely wave drag, Dw, comes about from
shock waves in transonic and supersonic flight speeds.
In figure 1, Researcher Robert Englar (right) and co-op student Daniel
Hegeman adjust blowing slots on a model truck inverted in GTRI's low-speed wind
tunnel. The testing helped assess the value of fuel-saving improvements.
In figure 2, Aerodynamic improvements on truck trailers -- such as rounded
corners -- coupled with pneumatic controls for blowing air from slots, help
reduce drag and improve fuel economy for heavy trucks.
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