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Improving Fuel Economy Through Aerodynamics

WARREN, MI (AP) – Birds do it. Airplanes do it. Now cars are using aerodynamic design to get more efficient.

Drag caused by wind can dramatically affect a vehicle’s fuel economy. According to General Motors Corp., aerodynamic drag accounts for 23 percent of the energy consumed in the average vehicle. Only engine friction is a larger contributor, at 27 percent. Tire resistance, vehicle weight and energy used by the transmission are smaller factors.

As a result, automakers are placing a growing emphasis on aerodynamics as a way to improve fuel economy. They’re redesigning grilles and roof racks, angling side mirrors so air flows around them better and lifting underbody components such as exhaust pipes so they won’t produce drag.

GM says its 2007 Chevrolet Tahoe reduces aerodynamic drag by 8 percent, which improves its fuel economy by 3 percent. The new four-wheel-drive Tahoe will get 20.3 miles per gallon, compared to 18.2 with the 2004 model, GM said.

An air dam on the front of Ford Motor Co.’s new Lincoln Zephyr sedan saves about one-tenth of a mile per gallon of gas because it prevents air from going under the car, said Steve Wegryn, Ford’s manager of North American aerodynamics.

“ Aerodynamics is extremely important and becoming more important by the day with the price of fuel,” Wegryn said. “We are being pressed harder and harder to come up with new and better ways to address the coefficient of drag on our vehicles.”

Aerodynamics engineers often work side by side with vehicle designers at the beginning stages of a vehicle’s development. At GM, engineers will start aerodynamic testing on a clay model one-third the size of the actual vehicle, so changes are easier to make. Eventually, the full-size model will be tested in the company’s massive wind tunnel in the Detroit suburb of Warren.

GM’s 23-year-old wind tunnel is powered by a fan that is 43 feet in diameter. The fan has six blades made of polished spruce that weigh one ton each. They can rotate at a speed of 415 miles per hour.

The testing area, which is separate from the fan, is 18 feet high, 34 feet wide and 70 feet long, large enough that it was used to test the Stars & Stripes sailboat before the America’s Cup race. The maximum wind speed in the testing section is around 120 miles per hour.

Automakers say testing in a wind tunnel also allows them to figure out ways to reduce wind noise in the vehicle. Toyota Motor Corp. spokeswoman Cindy Mahalak said Toyota did noise-reduction tests for its 2005 Toyota Avalon sedan in a U.S. wind tunnel and conducted separate aerodynamic tests at a wind tunnel in Japan.

Sometimes, design can win out over fuel economy. Designers may insist that the vehicle has a high back or a trunk design that doesn’t optimize aerodynamics. Automakers say they have to make tradeoffs.

“ How do you maintain the style but also do it in a way that lets you cut through the wind?” Ford spokesman Said Deep said. “Everybody can make a jelly bean, but how do you make a car with personality?”

Drivers also can play a role in improving aerodynamics. Deep said many pickup drivers believe they get better fuel economy if they drive with their tailgates down, but that’s not true. The tailgate helps collect the air and pushes it back toward the cab, helping the vehicle accelerate.

Ford said pickup bed covers also help make those vehicles more aerodynamic. Wegryn estimated covers can improve pickups’ fuel economy by 7 to 11 percent.

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