Bob Englar would like to help semi-trailer trucks fly ... not literally, of course. With support from the US Department of Energy and American Trucking Associations, Englar is applying aerodynamic concepts and "Circulation Control" flight systems developed for aircraft to 18-wheelers. The goals include reducing fuel consumption and giving drivers better control of the big rigs under adverse conditions.
"We are taking flight-proven aircraft technology - that we've already shown works on streamlined cars - and applying it to trucks," says Englar, principal research engineer at the Georgia Tech Research Institute (GTRI). "Current trucks, primarily the trailers, are anything but streamlined, so whatever can be done to reduce the drag will help fuel economy."
Heavy trucks consume lots of fuel in overcoming aerodynamic drag and rolling resistance. For semi-trailers, drag and rolling resistance predominate on level roads; at highway speeds of 90 - 105 km/h the two are roughly equal. As the truck goes faster, the horsepower needed to overcome aerodynamic drag becomes paramount, increasing with the cube of velocity. Fuel consumption is proportional to required power, so efforts to overcome drag are important for boosting fuel economy. Previous efforts have mainly involved improved fairings for the prime mover cab. Yet it is the trailer that encounters by far the most aerodynamic drag. Now, a program is under way to reduce retarding forces on the semi-trailer unit by applying aerodynamic technologies already proven on jet aircraft.
Englar expects the work - done under contract with the Oak Ridge National Laboratory - to reduce aerodynamic drag by at least 35 percent, and perhaps by 50 percent or more. A 35 percent drag reduction translates to about a 12 percent drop in fuel consumption for semi-trailers. If applied to the entire US fleet, that would save an estimated 4.5 billion litres of fuel a year!
Beyond the fuel savings, however, the Circulation Control system being developed and evaluated in GTRI's wind tunnel could improve directional control for trailers, increase traction and augment braking. Also known as pneumatic control, this aerodynamic system could also create lift on the trailer, effectively reducing its weight. That would cut rolling resistance on tyres, reduce wear and also increase fuel economy.
Recent experimental results have shown:
- Drag reductions of up to 35% on a streamlined automobile wind-tunnel model with pneumatic blowing
- Lift or download increases of 100-150%
- Ability to control aerodynamic moments about all three directional axes with no moving control surfaces
One feasibility study has shown that with combined reductions in drag and rolling resistance, there can be a 32% reduction in the horsepower required to propel a truck at a speed of 110 km/h!
The pneumatic system works by blowing compressed air from slots located on different parts of the trailer. Air blown over curved surfaces on top of the trailer smooths airflow there, decreasing drag and making the entire trailer act like a wing to lift as much as 15 percent of the weight off the tyres. Blowing air from slots on the bottom of the truck would have the opposite effect, multiplying downward force on the tyres to improve traction and braking when needed. Combined blowing from all sides further reduces flow separation and drag.
Blowing slots on each side of the trailer could counter crosswinds, giving the driver a way to fight the effects of sway, as well as jack-knifing. By selecting the right slot combination and blowing rates, the pneumatics could be used to increase drag to augment aerodynamic braking.
"We can augment force and/or moment, either increasing it or decreasing it, and produce it in whatever direction is needed, all without any moving external surfaces," Englar explains. "Producing aerodynamic drag and increasing resistance on the tyres could really help truckers in difficult downhill areas or for emergency stopping. Beyond the fuel cost issue, there is a lot of interest in this from the standpoint of improving safety."
Controlled by internal fast-acting valves, the pneumatic system would respond quickly to driver actions. "It would only take a fraction of a second between when the driver initiated the pulse and when the system would cause the effect," Englar says. "Using an automated system, the driver wouldn't even have to think about how it works. Pressing the brakes or accelerator, or moving the steering wheel, would cause the proper blowing slots to come on and assist the conventional controls."
Compressed air for the system could come from exhaust gases, the turbocharger on the truck engine, storage tanks or an electrically powered compressor in the trailer.
Englar's research team at the GTRI Aerospace, Transportation & Advanced Systems Lab has begun testing small-scale truck models in GTRI's low-speed wind tunnel. With computational support from Assistant Professor Marilyn Smith and graduate students in Georgia Tech's School of Aerospace Engineering, this initial program will evaluate the effectiveness of the pneumatic system and pave the way for eventual testing on full-size vehicles.
"There would be both fuel savings and a safety payoff, so the synergies are really quite interesting," says Victor Suski, a senior automotive engineer with the American Trucking Associations. "Everybody who has heard about this intuitively feels there is great potential here."
Rising fuel costs have boosted interest in fuel economy improvements, but most efforts so far have focused on the prime movers that pull the large trailers. Suski believes future efforts should target reducing drag on the trailers.
"All of the benefits that are yet to be gained in fuel economy will probably come from refining the trailer aerodynamics," he says. "Until that is done, we will continue to see trucking companies going out of business because they cannot afford their fuel bills. Fuel is not going to get cheaper over the long term."
Suski sees the potential for improving the average fuel efficiency in heavy trucks to as little as 19.6 litres/100 km, down from an industry average of greater than 34.6 litres/100 km today. If testing on full-sized vehicles demonstrates the savings expected, truck purchasers will demand the new technology, and manufacturers will incorporate it, he predicts.
Because fuel savings can be easily quantified, they will drive advancement of Circulation Control technology. But if the expected safety improvements ultimately result in lower accident rates, reduced insurance premiums could also add to the benefit, he notes.
Circulation Control systems were developed and tested on fixed-wing and rotary-wing aircraft by Englar and his associates in the 1970s and 1980s as a simplified means of greatly increasing lift, improving control and reducing take-off and landing distances. During the 1990s, GTRI engineers applied the technique to cars, demonstrating significant savings in drag and energy use.
Protected by two patents, the automotive application - known as "GTRI FutureCar" - produced measured drag reductions of up to 35 percent by altering the flow separation and vortex formation around the rear of the vehicle. Englar believes even larger reductions may be possible on trucks, which will gain substantial drag reductions above the basic streamlining already used in passenger cars.
"Clever design can already take perhaps 10 to 15 percent off the drag on a semi-trailer just through good non-blown aerodynamics," he says. "The trucking industry is already making use of fairings and other devices to reduce drag, but much more can be done."
Rounding off corners on trailers, matching the fairing height on tractors to the height of trailers and reducing the gap between tractor and trailer are among the steps that GTRI tests have shown would reduce drag and improve fuel economy, he notes.
The Georgia Tech team has been working with major manufacturers of prime movers and trailers to ensure that recommended changes would be practical. Englar expects much of the technology could be retrofitted onto the existing truck fleet as it becomes standard equipment on new vehicles.
Once the Circulation Control techniques have been demonstrated in the wind tunnel, the researchers will work with the Department of Energy and the American Trucking Associations to scale them up for testing on real vehicles. Trucks equipped with the new devices would be tested against standard vehicles across a wide range of conditions.
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