Slippery Slope: Production Cars Making Jaw-Dropping Aerodynamic Gains

Mercedes-Benz surprised the automotive world earlier this year by boasting its new CLA sedan is the most aerodynamically efficient production car in the world, better than even the sleekest hybrid or electric vehicle.

That claim raised eyebrows from Toyota City to Detroit, in part because the stylish, highly sculpted entry-level luxury car does not feature the wind-swept, jellybean silhouette of well-known low drag-coefficient cars such as the Toyota Prius or General Motors’ discontinued EV1 electric car.

Another aero champ (sort of) is lurking on the global stage: the Volkswagen XL1 diesel-electric plug-in hybrid electric vehicle. Equipped with every wind-cheating trick in the book, the 2-seat car features a 0.189 Cd and claims fuel economy of 261 mpg (0.9 L/100 km). Technically, the XL1 is a production car. VW says it will produce about 250 units, priced at more than E100,000 ($130,000).

While interesting, the XL1’s fender skirts and boat-tail rear end has a retro, been-there-done-that aspect. And its coefficient of drag is about the same as the EV1, introduced in 1996.

While not quite as slippery as the XL1, the CLA is part of a new trend sweeping the industry: Aerodynamic vehicles that look fashionable, rather than like science projects. In addition to the Mercedes, mainstream models including the Chevrolet Malibu and Toyota Avalon are sporting shockingly low Cds without looking like Sci-Fi props.

Teardrop-shaped bodies, fender skirts and other typical aero gimmicks may work miracles in the wind tunnel, but when they hit the street, consumers reject them. GM’s EV1 and first-generation Honda Insight HEV were the most aerodynamic vehicles of their day, sporting Cds of 0.19 and 0.25, respectively, more than 10 years ago. Despite innovative design and breathtaking efficiency, both flopped.

There are many reasons why the cars failed, but nerdy fender skirts and pinched hindquarters did not help.

“Customers don’t want to buy the most aerodynamic car. They want the prettiest car with the best performance at the best price point,” says Stefan Young, manager of the thermal systems and aerodynamics groups at Toyota Motor Engineering and Manufacturing North America.

“Cars don’t have to look that aero,” adds GM Aerodynamics Performance Engineer John Bednarchik. “That’s the fun part of working in aerodynamics these days, balancing the aerodynamic shape with a great-looking design.”

Bednarchik points to both the ’13 Chevy Malibu and new C7 Corvette as examples of cars that are beautiful and aerodynamically efficient.

The CLA achieves much of its slipperiness like most new vehicles: Designers carefully manage how air streams around the car’s A-pillars and side mirrors, and how it flows through the engine compartment and around the wheels. Special underbody panels allow air to pass more freely underneath.

However, the CLA180 BlueEfficiency model achieves a world-leading 0.22 Cd with an additional list of features, including a sport chassis and suspension for a lower ride height; smaller 15-in. wheels made of a light alloy and designed to be highly aerodynamic; a radiator partially covered by a blind for active regulation of cooling air and additional belly pan and under floor treatments.

These efforts are topped off by a special rear bumper that hides the exhaust pipes and plays a significant role in lowering aerodynamic drag, a Mercedes spokesman says.

The BlueEfficiency version will be sold only in Europe. CLA models available in the U.S. will have less-spectacular, but still impressive, Cds of 0.28 and 0.29, a Mercedes spokesman says.

That means the new CLA 250 and 250 4Matic that go on sale in the U.S. in September will be aero equals to cars such as the Chevy Volt extended-range electric vehicle and the eco version of the ’13 Malibu as well as the new Toyota Avalon sedan. In the U.S. at least, the ’13 Prius HEV and Tesla Model S EV still can claim aero superiority with Cds of 0.25 and 0.24, respectively.

But it’s unlikely even these impressive numbers will hold for long. Engineers say there is plenty of room for improvement in areas consumers will not even notice.

The vehicle underbody is responsible for about 30% of overall aerodynamic drag and the engine compartment cooling system another 10%, says Toyota’s Young. More underbody sheathing will continue to yield benefits in the future, plus under floor panels offer the additional benefits of reducing road noise and increasing high-speed driving stability.

So-called “cooling drag” will be reduced by the swiftly growing adoption of grille shutters and other technologies related to the engine compartment. “Underhood air flow is a huge contributor to aerodynamic drag,” says GM’s Bednarchik. “On the new Corvette, we did a lot with the cooling system, the way the air flows through the engine compartment and around the wheels.”

A growing number of other active and passive countermeasures also are being employed to improve aerodynamics that are invisible to consumers.

Some vehicles feature suspensions that lower body height several inches at highway speeds to reduce drag. Others use special ducts around the wheels and elsewhere to re-route air. Coming next are “active airflow” ducts and baffles that strategically direct airflow to improve aerodynamics in varying situations.

Even so, 60% of drag still is related to the upper body of the vehicle, and tapering the roofline and rear end is on the mind of every auto maker in an effort to reduce a vehicle’s wake area. Because this can negatively impact rear-seat hip space and luggage volume, as well as aesthetics, designers are proceeding cautiously.   

For aerodynamicists, there is one Holy Grail in this area.

“One thing I personally hope to see in my career is the elimination of side mirrors,” says Young. “We have the technology. Removing mirrors would be a pretty big advantage, probably more than fender skirts.”

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