Reducing Aerodynamic Drag

Matt Timion · 09-21-2005, 12:51 PM

Author:Timion, MattPublication:www.gassavers.orgDate:09/12/2005In city driving, as much as 30% of your engine's power is used to overcome aerodynamic drag. In highway driving, this number increases to 60-70%. As a car moves, it pushes air out of it's way. If your vehicle is flat (much like Hummers and Jeeps wranglers) the air becomes more difficult to push out of the way. The opposite of this is an airplane. As you can see an airplane is shaped in a way to reduce aerodynamic drag.
The Rocky Mountain Institute has this to say about Aerodynamic Drag: 
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<td>taken from <a href="http://www.rmi.org/sitepages/pid431.php">http://www.rmi.org/sitepages/pid431.php</a></td>
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Aerodynamic Drag 
 
In conventional cars, the aerodynamic form of the visible upper body gets quite a lot of attention (mainly for looks and to reduce wind noise). Sleek styling tends to ignore some key aspects of low-drag design and may leave details—outside mirrors, for instance—that cause turbulence and thus increase aerodynamic drag. 
 
Typically, little or no attention is paid to the rough underbody, which creates significant energy-consuming turbulence. Measures such as a "chin spoiler" under the front bumper divert airflow away from the underbody to reduce this effect, but typically at the expense of increasing frontal area. (No matter how streamlined the shape, making it wider or taller increases drag.) In addition, large air intakes for the engine-cooling system impair the car's ability to part the air smoothly. At the rear, ignorantly designed body contours may induce a turbulent partial vacuum behind the car instead of laying the displaced air smoothly back in place. 
 
<img src="/files/gassavers/aerodynamics/1.gif" alt="Hypercar" height="146" width="425">
 
Regardless of its slipperiness, an American car the size of a Ford Taurus moves about 6 tons of air out of its way per mile. It takes power to displace that air. Air drag consumes about 30 percent of the engine's power in the city, rising to 60–70 percent on the highway (as air resistance rises and other losses, such as braking, are reduced). 
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The first thing you can do to reduce your aerodynamic drag is to drive the speed limit. As speed increases, so does rolling resistance on the tires and drag. The faster you go the more your engine must work to push the air around you out of the way.
 The major problem with vehicle design is the flat surface at the back of the car. Flat surfaces on the front or the back of the car affect your drag. On the front of the car a flat surface will make it more difficult to push the air out of the way. On the back of a car a flat surface will create a vacuum and will essentially prevent the vehicle from properly moving through the air. A hobbyist can add simple modifications to their vehicle to reduce aerodynamic drag.
Below is an example of a hobbyist who has reduced their aerodynamic drag by modifying the shape of their vehicle.
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<td>taken from <a href="http://www.max-mpg.com">www.max-mpg.com</a></td>
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<td>The Drag-Reducing "Wing" on the VW Beetle—How It Works 
 Most cars have very low aerodynamic drag. But some cars, and ALL trucks have high drag. We apply airfoil principles to improve drag for cars and trucks. An airfoil (like an aircraft wing) has very low drag. It looks like this: 
<div align="center"><img src="/files/gassavers/aerodynamics/2.gif"> 
Cd = 0.02 
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 Automobile drag occurs mostly at the rear. The rear part of a car should have a shape similar to an airfoil for low drag: 
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<img src="/files/gassavers/aerodynamics/3.gif"> 
Low-drag car     Cd = 0.30 
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 A well-designed car has a drag coefficient, Cd, between 0.25 and 0.30. By comparison, the regular VW Beetle has unusually high drag, Cd = 0.38. We added a shelf to the back of the Beetle like this: 
<div align="center"><img src="/files/gassavers/aerodynamics/4.gif">
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 And, finally, winglets were added on the ends of the shelf to give it strength and to further lower drag in a way reminiscent of airplane wings, like this: 
<div align="center"><img src="/files/gassavers/aerodynamics/5.jpg">
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 The drag reducer lowers the Beetle's drag to about Cd = 0.28. Handling at high speed and in windy conditions is greatly improved (as you would expect with a wing or spoiler). Fuel economy is increased by 5% to 8% depending on speed. 
 Our modified car is a 2003 Beetle TDI diesel. On the trip to Alaska, the mileage averaged 57 miles per gallon. On our test track* we get 60 mpg at 65 miles per hour. 
 * Our test track is the straight portion of I-80 from west of Salt Lake City to the Nevada border. If you drop by for a visit, you can come along on one our test runs. </td>
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Your drag coefficient can most likely be found in your owners manual. The lower the drag coefficient the better.
Some say that adding a front lip (usually a 3 inch piece of plastic that sits below the front bumper) will help keep air out from under car and preventing the wind from meeting rough surfaces underneath the car. Others say that cutting holes in your rear bumper will allow wind to flow through the bumper and not cause the bumper to act as a sail.
Removing the rear view mirrors has been a popular method of reducing aerodynamic drag. Unfortunately, such a modification is illegal in most states, so it is not recommended. 
This article is incomplete awaiting test results for simple modifications to reduce aerodynamic drag. Tests will be conducted using the modifications listed above, so please check back for updates as testing begins. Results will be linked to these pages.

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