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Dec012014

Aero Imperative

Volkswagen XL1 Prototype (Source: Volkswagen)

Meeting the new CAFE standard of 54.5 mpg by 2025 will demand herculean efforts by designers and engineers to reduce all fuel-wasting sources of resistance to a car's motion. Reducing the mass of a car by replacing steel with lighter materials—like aluminum and carbon fiber reinforced plastics used in the Volkswagen's breakthrough XL1 prototype—is an obvious way to decrease its consumption of fuel or electricity in the case of an electric vehicle or hybrid. So "light-weighting" has become today's automotive buzzword.

The XL1 will only be available in limited numbers in Germany and Austria. Meanwhile, in America, aluminum has replaced much of the steel normally used for the body of the newest variant of the perennial sales leader, Ford's 2015 F-150 pickup. It consequently weighs some 450 pounds less than the previous model. Some of the additional cost of aluminum can be offset by a "weight-compounding effect." A lighter vehicle requires smaller, lighter and less expensive brakes, for example, and other suspension and driveline components. A smaller, lighter breed of three- and four-cylinder engines displacing as little as one liter, are coming onstream in place of larger engines without sacrificing performance.

Reducing air resistance, known simply as streamlining, delivers more bang for the buck than any other way of increasing fuel efficiency because it depends primarily on the car's shape. Although VW's designers and engineers might have spent more hours than usual in the wind tunnel tweaking the XL1's design, its shape wouldn't have made it inherently more expensive than any other.

Aerodynamic drag is especially pernicious at highway speeds. Rolling resistance, due largely to weight that continually flexes tires as they roll, essentially doubles when a car's speed increases from 30 to 60 mph. But aero drag multiplies as the square of the increase; it quadruples between 30 and 60. Even more astounding, the fuel consumed due to air drag increases as the cube of the increase; air drag consumes eight times the fuel at 60 mph as at 30. It turns out that most of the fuel burned by a typical car going faster than about 45 mph is spent overcoming aero drag. As engineers succeed in making cars ever lighter the crossover speed will continually drop below 45 mph. As engineers shave more weight, reducing air drag will increasingly become the most imperative way to increase efficiency.

So why wait? Since a car shaped for aerodynamic efficiency cost no more than one that struggles through the air, why not pull the stops and go all out for maximum aerodynamic efficiency right away? Because super slippery cars would differ so much from well-established norms that they would shock and alienate consumers who would regard them as strange, weird or downright ugly. We expect next year's models to be new. Otherwise, cars might still resemble Ford's Model T. But most of us have no taste for revolutionary design. So aesthetics will present as many obstacles as physics in the industry's quest for 54.5 mpg because, at best, we tolerate only evolutionary departures from designs we are already familiar with. If you were to introduce today's most beautiful car in 1930 it would have been a spectacular flop. Consider some recent and not so recent examples where consumer tastes have stymied aerodynamic progress: 

1934 Chrysler Airflow and Union Pacific Streamliner (Source: Chrysler)

Chrysler's infamous 1934 Airflow still sends chills down the spines of designers who would dare to pen radical aero designs. It was created during the so-called Streamlined Decade of the 1930s when cars, trains and busses weren't the only products displaying the so-called "streamlined" look inspired by a new class of sleek, all-metal airplanes like the Douglas DC-3. Vacuum cleaners, refrigerators and virtually every other class of product bore the streamlined look.

A car's coefficient of drag (CD) is the standard measure of a car's aerodynamic drag. It is derived from a comparison of the car's air drag to that of a disk of the same area as the car's frontal area—its profile as seen from head on, including all visible protrusions such as tires, door handles and mirrors. If its drag equalled that of the disk it would have a CD of 1.00—the worst possible case. The Chrysler Airflow's CD was reportedly around 0.50, corresponding to 50 percent of the draf of a disk with the same fontal area. Assuming the Airflow had a frontal area of 30 square feet, a wind tunnel test would have registered the same air drag as a 15 square-foot disk (0.50 X 30 sq. ft.).  That represented an improvement of better than 28 percent over the typical car of the period, which reputedly had a much worse CD of around 0.70. But that superior performance wasn't enough to counter the strangeness of its design.

Aero Citation Concept with 1980 Chevrolet Citation

When the oil crises of the 1970s led to escalating gasoline prices and long lines at gas stations the typical American car had a CD of between 0.55 and 0.6. Along with downsizing, automakers got more serious about improving aerodynamic efficiency. The 1980 Chevrolet Citation, for example, had a commendable CD of just 0.42.

But it could have been better. Using a method for predicting a design's CD developed by England's Motor Industry Research Association (MIRA) (which I used while working in Ford's Advanced Vehicle Concepts Dept. during the 1960s) I altered its design for a 1980 magazine article. I retained the aerodynamically desirable fastback form, smoothed and rounded the nose, gave the windshield more curvature and skirted the rear wheels.  The method yielded a design with a predicted CD of just 0.27. This would have amounted to a 37 percent decrease in the Citation's air drag that might have yielded a 10 percent improvement in fuel economy. Despite quite humdrum looks by today's standards, it looked odd the time. When the magazine's editor saw my illustration of it he called it "the ugliest car I have ever seen."

1986 Mercury Sable (Source: Ford)

Six years after my Citation redesign exercise appeared in the magazine the similarly rounded and smoothed 1986 Mercury Sable achieved a CD of 0.29. (Skirts might have brought it nearer 0.27, but might have made the Sable even harder to accept.) The unusual, nearly grille-less designs of it and its Ford Taurus sibling met with initial consumer resistance. But they went on to become award winning sales champs and design icons that forced competition back to the drawing boards

2010 Toyota Prius with 2004 Prius (background) (Source: Toyota) 

Toyota's 2004 Prius broke new ground with a CD of 0.26. But, to this day, many people have not warmed to its unusual design. Designers tried to fix the design by incorporating crisper, more fashionable creases and squared-off edges for the 2010 model. Toyota claims a slightly better CD of 0.25 for the revised design. But I think they must have burned a lot of midnight oil in the wind tunnel eking out that extra one percent because the previous model looks inherently sleeker to me.

Volkswagen's XL1 prototype has a CD of 0.19. It is a plug-in hybrid with an 800 cc engine delivering 261 miles per gallon of diesel fuel. I think it's beautiful and wish I could have one. But what do you think?

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