The future of fuel economy may lie in trailer aerodynamics
By Paul Abelson
The price of diesel may have a new floor: $3 a gallon. That’s spending 50 cents or more a mile just for fuel. Something has to be done to improve fuel mileage — but what? Engine efficiency is holding or improving slightly despite all the new hardware needed to meet emissions regulations. Tractors have become more aerodynamic, helping bring us from less than four mpg to more than six. Every manufacturer has second generation aerodynamic tractors, so any further improvements will be incremental, perhaps just two percent or so. The days of huge improvement that were experienced with the first Airshield air deflectors are past.
Richard Wood, president of Solutions and Technologies LLC (SOLUS), a research and development company, believes there are major gains to be realized in aerodynamic efficiency, but not from the tractor. At the Commercial Vehicle Meeting of the Society of Automotive Engineers (SAE), Wood chaired and organized a session on aerodynamics and fuel economy. He predicted that improvements in trailers could yield as much as a 50 percent boost in aerodynamics, which translates to about 20 percent fuel savings. Where will they come from? The trailer. Since about 70 percent of tractors pull either reefers or dry van trailers, they are the focus of most research.
Four major areas for improvement will yield the expected 20 percent: the trailer gap area, the base vehicle, the undercarriage and the tractor as it interacts with the trailer. “Twenty percent is about the greatest improvement anyone will realize. That’s if the trailer is dirty now, and everything is done that can be. For example, a reefer will limit what can be done with the nose. Most people will realize 10 percent or more,” Wood says.
Let’s look at principles of aerodynamics that apply to our trucks. Do you remember, as a kid, sticking your arm out a car window and having your hand either slice through the wind or be moved about by it? The wind acted differently depending on whether your hand was cupped or straight or if your fingers were pointed into the wind or perpendicular to it. The effect also varied with speed. The faster the car, the more effort it took.
You experienced the factors that affect aerodynamics. Air has mass, and it takes force to move the air out of the way so a truck can slip through. The less the air has to move, the less force will be required. Every time the air changes direction, more force will be required. The larger the area presented to the wind, the more force will be required. The faster the speed, the more force — in geometric proportion. Drag is a function of shape or coefficient of drag (Cd), frontal area and velocity squared.
Let’s deal with velocity first. Without going into theoretical physics, it has been proven that if speed doubles, aerodynamic drag quadruples. At 60 mph, drag is four times what it was at 30, and nine times what it was at 20. Speed limiters reduce drag, but no driver wants to be governed.
Frontal area is a fact of life. As long as vans have to hold cargo, frontal area will remain unchanged. What can be changed is the shape of the trailer. Getting air to flow more smoothly from tractor to trailer and around, over and under the trailer, means the trailer’s Cd can be reduced. Reductions from the old tractors to modern ones have resulted in significant fuel economy improvements.
In fact, drive trains, engines, tires and tractors have all improved over the last generation, resulting in more than double the miles per gallon being realized. Everything has improved except trailers — and now it’s their turn.
A 5 percent maximum improvement in mpg can be achieved by closing the gap to the tractor, adding cab extenders, creatively directing airflow with air deflectors and placing vertical strips at the trailer’s nose to trap air vortexes that form in the gap. It takes energy to keep turbulent air moving, so the more we limit turbulence, the less energy will be needed. Bubble fairings, such as the Nose Cone, help air flow smoothly across the front of the trailer.
Another 5 percent or so can be gained from shape. Edge rounding helps air to flow more efficiently, and there are devices available that can make the air flow behave as if the trailer had a long, streamlined tail extending way behind it. Bobtail extenders, frame extensions, inflatable mock tails or wake boards let air begin to join together in the rear without creating the large volume of turbulent air as today’s trailers do. Vortex generators at the trailing edge do the same. Vortexes are spirals of air that flow off the tips of objects as they move. By carefully designing the objects, engineers closely control vortex shape, effectively creating a mass of air that serves to fool the overall flow into behaving like it was traveling over a perfectly streamlined shape.
About 5 percent improvement can be attained from the bottom. The suspension, axles and tires, and the reefer fuel tank (if there is one), are all drag creators. Air flowing over these complex shapes keeps changing direction, and every change consumes energy. Fairings keep air out from below the trailer, minimizing the turbulence at the undercarriage. Air dams deflect air from axles and suspensions, cleaning up air flow below the trailer.
The final 5 percent can be gained from the tractor where it interfaces with the trailer. There’s always the potential to improve tractor aerodynamics using air deflectors and other devices. Vortex generators at the tractor’s or sleeper cab’s trailing edges, top and sides create an air bridge to the trailer, so less energy is wasted with turbulence in the gap.
How real are these fuel economy improvements? One of the largest private fleets in the U.S., Wal-Mart, is about to find out. As part of their stated goal of doubling fuel economy to 13 mpg by 2015, the fleet is operating specially equipped trailers as well as new tractors. They are testing side fairings, wheel covers and rear extenders. As devices become available, more and more operations will be trying them, and trailer aerodynamics will become the new frontier of fuel economy.
Every Little Bit Counts
With many devices and designs making small contributions to improving fuel economy, how do you determine what is or is not worthwhile? One way is to ask for results of industry-approved testing. The Technology and Maintenance Council (TMC) and the Society of Automotive Engineers (SAE) jointly developed test procedures that eliminate outside influences and focus on the item being tested. Testing must be conducted by a reputable, independent testing facility.
A good example is the Ecoflap by Andersen Flaps Incorporated, a design with eight columns of adjacent louvers placed continuously across the mud flap surface. The claim is that instead of a solid obstruction to air flow with turbulence behind it, the louvers allow air to flow through the flap, diverting water downward so spray doesn’t obstruct others’ visibility, and turbulence is reduced. The theory sounds good, but does it work?
Andersen (www.ecoflaps.com, (866) 543-5277) had the Transportation Research Center conduct SAE J1321/TMC Type II tests that indicated a 1.7 percent mpg improvement. The margin of error is +/- 1 percent, so this device can be expected to offer improvement from 0.7 to 2.7 percent for this configuration of tractor-trailer. Conservatively, it could be one percent or less, which isn’t a bad return for a mud flap.
But this illustrates why it takes a number of devices and developments to achieve any significant mpg improvement.