BY: Tara Baukus Mello

You can’t see it, but every child feels its effect when they push their hand out the window of a moving car and into the onrushing wind. A carefully cupped hand can create the same upward or downward force used by aircraft and car designers to make planes fly and cars stay on the road.
At slow speeds the air creates little force, but as the speed increases the force rapidly grows stronger. At highway speeds, it becomes difficult to hold your hand in place against the onrushing wind. The effort required is the similar to the energy your car expends pushing itself through the air. This force is called drag, and it can sap your car’s energy as quickly as it steals it from your muscles.

More than anything else, drag puts the greatest demand on a car’s engine at highway speeds, even on level ground. Careful aerodynamic design can reduce the horsepower and, thus, the fuel needed to push a vehicle along at 70 miles per hour. While doubling a vehicle’s speed requires the engine to put out eight-times more power to overcome air resistance, small reductions in drag can greatly reduce the power, size, and, cost of the engine needed to push the car along.

In general, automobile engines are designed to provide adequate acceleration and are five to ten times more powerful than necessary to propel a car at highway speeds. This means when driving on the highway your engine is often operating far from its “sweet spot,” reducing fuel economy. But by combining careful aerodynamic design with hybrid-electric drivetrains, you can create a super-efficient car, such as Honda’s Insight, which achieves 70 miles per gallon fuel efficiency on the highway.

While today’s vehicles are far more aerodynamic than those built 20 years ago, there are still many ways aerodynamics can be improved. Surprisingly, the biggest area for improvement is the underside of the car. Air flows over all surfaces of a moving vehicle, including the bottom. If you get down on your knees and look at the underside of your car, chances are you will see parts —exhaust pipes, cables and other things—hanging out in the breeze. All this roughness creates places for air to get caught. Many of today’s higher-end luxury automobiles pay more attention to the aerodynamics of the underside of the car, making a smooth surface that gently slopes up toward the tail. Not only does this design decrease drag but it also reduces dangerous lift forces that can make a car unstable at high speed.

Even though the topside of most modern cars is smoother than the underbelly, a car’s body can be improved as well. Since every protrusion creates drag, auto designers are now looking carefully at seams between body panels, exposed windshield wipers and even side mirrors for opportunities to reduce air drag. Smoothing the bottom, tucking away windshield wipers when not in use, using flush mounted miniature video cameras to replace traditional mirrors (which has the added benefit of eliminating blind spots as well) and slightly tapering the entire body shape toward the tail can reduce drag forces. The Honda Insight’s aerodynamic shaping reduced the power required at highway speed by 30 percent compared to a similarly sized Honda Civic.

Auto designers compare different vehicles by a figure known as the Coefficient of Drag. Written as Cd, this figure is not dependent on how wide or tall vehicle is, only on how well designers smoothed and sculpted the body shape. The lower the Cd, the more slippery the vehicle. Twenty years ago an average new U.S. car had a 0.48 Cd; today that figure is 0.33, with the very best mass-produced vehicles achieving 0.28. Pickup trucks and SUVs, with their choppy shape and high ride typically have coefficients of drag that are over 0.44.

Automakers are in the process of developing cars of the future that will approach and eventually surpass the “holy grail” of Cd 0.2. Surprisingly, these new designs do not look or feel much different from cars we see on the road today. GM’s five-passenger Precept prototype has a Cd of 0.163 accounting for 15 percent of the vehicles exceptional 80 mile per gallon fuel economy. The Insight is a two-seat production car currently available to the public that sports a 0.25 Cd. Both the Toyota Prius, a five-seat hybrid-electric, and the Toyota Celica, a two-seat sports coupe, have a Cd of 0.29. These vehicle designs show that aerodynamic vehicles can be both practical and attractive—not the futuristic egg-shaped vehicles of science fiction.

To learn more about aerodynamics and other advanced technologies, visit www.rmi.org and select the Transportation link.

This article series is made possible by a grant through the non-profit Rocky Mountain Institute’s Hypercar Center® (www.hypercarcenter.org) whose mission is to educate and inform policymakers, industry and the public about advanced vehicle technology.