The Auto Research Center (Arc) believes it has made major gains in aerodynamic research with what it says is the first rolling-road testing of trucks and trailers in North America.
Using a detailed semi-truck scale model in its wind tunnel in Indianapolis, ARC found that there was a 12.4% increase in aerodynamic drag with the rolling road, compared to a stationary road.
“The significance here is that we're not talking a percentage or two,” ARC business development manager Fritz Marinko says. “Just between turning the road on and off, we're measuring a minimum 12% difference in drag. And that's just in a simplistic truck model.”
Says operations manager Mike Camosy, “When you add complexity to the model and make it into a certain specific OEM's tractor or trailer, you will gain more drag because you're being more detailed. It's like when you put your hand out the car window and feel the air on it. You're sticking something else out and gaining more detail and it's adding more drag. This becomes more important with the rolling road, and the effects are exponential; they're not scalable. So it really gets down to changing the way the industry looks at developing trucks and trailers to be able to save a potential billions of dollars.”
Marinko says that the average truck runs 144,000 miles a year, so if drag can be reduced by even 10%, that results in a 5% reduction in fuel consumption.
“Without trying real hard, there's an awful lot of low-hanging fruit there,” he says. “There are huge opportunities out there. I know we can make an impact in reducing fuel consumption for trucks and trailers.”
Camosy says that since the first oil crisis in the 1970s, truck aerodynamic efficiency has been a focus of scientific investigation using generic truck models. A number of possible aerodynamic solutions have been suggested, he says, but many of these designs have failed to gain mass acceptance within the trucking industry because: overall tractor-trailer design is limited by federal regulations as well as the current infrastructure in which they have to operate (loading platforms, etc); and tractor shape is designed to meet the sometimes conflicting requirements of aerodynamic performance and styling that are driven by customer desires.
He says the rolling-road concept is nothing new. Previous research by ARC with vehicles ranging from passenger cars to open-wheel racecars has shown that rotating wheels play a key role in aerodynamic performance. In many tests at the ARC, it has been found that changes made to a vehicle may show a drag decrease in a fixed-floor tunnel test, yet show an increase when the wheels are rotated.
The opposite effect has also been witnessed. Camosy says this highly non-linear interaction of rotating wheels with the overall air flow around the vehicle must be considered carefully when designing aerodynamically optimal vehicles. He believes ARC's study confirms that this is also the case for tests conducted with rotating wheels on both standard production vehicles and Class 8 trucks.
“Each evaluation yields similar dependency trends, which emphasizes drag reductions found with fixed non-rotating wheels are at times drag increases once the wheels are rotated and vice versa,” he says.
Refined model
In investigating the influence of rotating wheels on semi-truck aerodynamics, the baseline configuration was based on the generic simplified truck previously studied by the National Aeronautics and Space Administration (NASA). The model was further refined to include rotating wheels as well as additional details to study flows in the engine compartment and the underbody of the tractor and trailer. Further comprehensive testing with rotating the wheels on semi-trucks while utilizing several underbody flow devices was conducted utilizing two base models, the NASA generic model and a more representative truck model. The testing included parts that are meant to highlight the interaction of the rotating wheels with the air flow and not necessarily practical for implementation on real-world trucks.
Camosy says that because the air flow around the truck has to be managed for the real road conditions of a real truck, one of the truck models is representative of a normal production truck with attention to detail given towards the undercarriage, or underbody, and engine compartment. Nevertheless, he says, the effects of rotating wheels can be studied and quantified more easily if the truck body is a streamlined shape. By omitting complex geometries, internal flows, interference effects of the cooling airflow as well as of the underbody flow spillage, he says correlation can be made between ARC tests and previous tests conducted by NASA. Therefore, the basic studies were conducted with a simplified truck model (Generic Conventional Model, or GCM) in a scale of 1:8. In addition, because the GCM has been tested by NASA, it has one more advantage: data is already available for the non-rotating wheels case, and validation of ARC experiments became simplified for the non-rotating wheels configuration.
He says there were some differences in the setup between the stationary-road GCM model tested in ARC's tunnel compared to the GCM tested in the NASA tunnels:
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Model mounting. Four posts were used to suspend the model by NASA. These were not required for ARC testing due to the overhead sting mounting method.
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Solid blockage. NASA tests were conducted in closed jet wind tunnels, which generated small variation in the streamlines around the same model configuration. The ARC wind tunnel was a closed-circuit, single-return, ¾-open jet type.
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Test section boundary layer. To compensate for the boundary-layer thickness, NASA had to mount the model above the wind-tunnel floor.
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Model geometry. The rotating-wheels requirement for the model run by ARC yield to an alteration of the initial GCM design. The use of flexures to connect the tractor and trailer by NASA lead to an inclination of the tractor.
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Body axis force and moment calculation. NASA measurements were made by the facility scale system in the wind-axis coordinate system, sensitive to errors in yaw angle.
He says the wind tunnel at ARC had the capability of running with the rolling road in motion or stationary, so it was simple to compare results to those previously published by NASA with the ARC GCM simply by running with the road stationary.
The ARC GCM model had two base configurations. The first utilized wheels that were not capable of rotating and matched the previous NASA GCM model tested in other tunnels. The second included a wheel system capable of rotating the wheels on the same GCM model. Ensuring consistency, the first baseline GCM (non-rotating wheels), was initially run through multiple yaw angles in one yawing direction (driver's left direction). This model was run through a beta sweep of 0o to -10o in 2o increments. Since the previous NASA work ran this condition with its model raised just above the boundary layer, the same was done by ARC.
For the non-rotating wheel (NRW) configuration, rotating the road without the capacity of the wheels to rotate increased the drag measured by 4.6%. For the rotating wheel (RW) configuration, rotating the road with the wheels rotating on the road increased the drag measured by 12.4%. Therefore, Camosy says, the air entrainment caused by the belt alone is approximately 4.6% of the increase, while the additional wheel rotation effects are responsible for another 7.8%.
“These results show how improvement in the general underbody of the truck will show an improvement in general, but improvements around the wheels will give a much bigger benefit,” the report says.
Camosy says that fleets and truck and trailer manufacturers have known for years that they could reduce drag, “but now we're giving them a way to see more drag than they were able to see before. There's more there to reduce than they initially understood. We've unleashed a pot of gold on them.”
He says ARC is not saying it has a widget that can be designed into trucks/trailers to revolutionize the industry.
“We're saying we have a way of testing to further develop their overall vehicles that is far superior to current ways and methodologies out there,” he says. “The big story here is testing. It's like going from not using a wind tunnel to using a wind tunnel. Now, it's using the correct wind tunnel for what they're doing.
“Because of the fuel-price increases, you have a million people coming up with little gizmos they think will reduce drag that they developed in garages. Then they run to the patent office and then to the end user and start trying to sell these things. You have a huge influx of recommended parts to these end users and they have no way of understanding, ‘Is this really cost effective for me? Is this really working?’ We're looking at growing a program into the Department of Energy that would be almost like a Consumer Reports for the trucking industry, where DOE would select parts on a monthly basis to be tested by an independent third party with a test-type of simulation technology — ie rolling road — and we would come out with a monthly report on the selected items from DOE. That way would save fleets millions of dollars in making the correct choices.”
Fleet inquiries
Marinko says ARC has received inquiries from many of the major fleets, all of the truck OEMs, and some trailer manufacturers.
“They want to understand how they can do this type of testing,” he says. “They have models out there, but they are fixed floor, with less detail and no capacity for rotating wheels, so our models get to be more expensive,” he says. “We're looking at a partnership with the Indiana state government, which is interested in playing a major role in the trucking industry. We're looking at a potential grant on a project that will allow us to kick this off with the trucking industry and make the ease of entry better for large OEMs.”
Charlie Fetz, vice-president of research and development for Great Dane, says he met ARC officials in August at “The Aerodynamics of Heavy Vehicles II:Trucks, Buses and Trains,” an event staged by Engineering Conferences International. He says that although he was impressed with their aerodynamic knowledge, he came away unconvinced that a rolling-road wind tunnel is relevant to the truck and trailer industries.
“I know a lot of people in the aerodynamic community,” he says, “and some I know and respect told me, ‘Look, if you're really concerned about lift and down force, then a moving-floor tunnel is going to be important. But if not, it's not going to be that important.’ Trucks and trailers don't go fast enough to generate significant lift or down force. They're so heavy and move relatively slow for their size.”
Fetz says Great Dane is committed to producing a more aerodynamic trailer — it collaborated with International Truck and Engine and Wal-Mart last year on an experimental aerodynamic trailer that ultimately could be integrated into Wal-Mart's fleet — but he's unsure whether the company would be interested in using ARC's services.
“To me, the challenge is not building something that's aerodynamic — it's building something that's commercially viable and practical in the marketplace that has acceptable aerodynamics,” he says. “I think it's easy to build something that's aerodynamic that you can't sell or that won't survive in the marketplace. Do you really need to go and spend a ton of time in a wind tunnel or doing CFD (computational fluid dynamics) simulations? There is an awful lot of that out there in literature that will tell you an awful lot. And studying aerodynamics doesn't tell you how to design a trailer. The trailer guy who is knowledgeable about aerodynamics and can bring aerodynamics to bare on the problem, in concert with customers who are forward- thinking and can help define the environment that the trailer has to live in — that's the combination that will be successful in bringing more and more aerodynamics to bare.
“I don't think there is any resistance in our industry to improving aerodynamics. It's a tough industry. Everything is driven on economics. If aerodynamics can be made cost effective and survivable and help improve the bottom line, people will use it. People are not out there buying more fuel efficient and maintenance-free equipment because it's good PR. They're doing it because it helps them run their business.”
For those fleets or manufacturers that choose to utilize ARC's facility, Camosy says ARC would make a detailed scale model in its facility of the truck and trailer — “it's not a matchbox car and a hair dryer for the wind tunnel” — through reverse engineering, scanning and surfacing, design, and then the ultimate part manufacturing and assembly.
“We can get them true data in a very short period of time,” he says. “If you have a model and put it on a rolling road, you can make changes in a matter of hours. It's not like you have to go out and run it on the road for five months to validate. You can actually make that change and have the results back in hours and know whether what you're proposing works. You can determine whether this product that somebody invented works when you put it on a trailer or whether it's all smoke and mirrors. We can determine whether, if we put five skirts on a certain brand of trailer with a certain truck in front, it does work.”
Says Marinko, “It will be a staple way of developing semi-trucks in the future. It's what happened in racing and in the auto sector, and it's just starting to get into the trucking sector. It won't be able to be ignored, because it's real. It's more accurate and it works. Initially groups fear change because they think change costs money, but eventually they realize they can't afford not to change, because their competitors are changing.”