Aerodynamics in Electric Cars: More Cost-Effective Than Every Kilowatt-Hour of Capacity

After the Quattro in 1980, Audi ignited the next technological stage with the new 100 Type 44 (internally known as C3) in 1982: With a cW value of 0.30, it had the best drag coefficient of all production vehicles and set a new benchmark. This sparked a true aerodynamics hype, which then subsided in the 1990s. With electromobility, aerodynamics is again coming sharply into focus. We are now on the threshold between cW 0.19 to 0.20! The challenge is complex and also influences acoustics and thermal management.

Why Aerodynamicists are Fondly Called the "Rose Technicians"

At some car manufacturers, aerodynamicists were often mocked as "Rose Technicians." Instead of squeezing more power from engines, the wind tunnel specialists focus on slicing through the air. With electric vehicles, priorities are changing as well. In the WLTP cycle, air resistance accounts for 40 percent(!) of consumption, and 34 percent of the battery’s energy is used to overcome air resistance. For comparison: with combustion engines, it is only ten percent. The electric drive is very efficient, so a larger proportion is used for propulsion. This shows that with electromobility, the cards are being reshuffled. Behind the tinkering in the wind tunnel are also hard economic interests. "Aerodynamics is cheaper than a big battery," clarifies Dr. Moni Islam, Head of Aerodynamics and Aeroacoustic Development at Audi. Which brings us back to the former aerodynamics champion, who is now presenting the virtual electric successor of the Type 44 with the A6 e-tron.

One might think that an electric car is naturally designed for aerodynamics. While the underbody of a conventional engine vehicle is disrupted by the engine, exhaust system, and possibly a driveshaft, a BEV typically has a smooth underbody due to the battery, which is part of the body structure. The fact that this is conducive to a low cW value is obvious.

EVs Also Need Cooling, Which Is Much More Complex Than for Combustion Engines

But it's not that simple. Electromobility faces different challenges in the wind tunnel. "BEVs have larger tires to carry the weight. This is an important issue for us aerodynamicists," explains Moni Islam. Additionally, the cooling measures are more complex than with combustion engines. Due to the higher weight of electric vehicles, the brakes have to do more work and must be accordingly cooled. To solve this complex task, wind artists rely on controllable active cooling systems, such as the opening and closing slats of the front bumper.

Efficient cooling also allows the use of aerodynamic wheels, which are generally more closed than those on vehicles with combustion engines. The software that manages thermal control for the cooling of brakes and the drive system is very complex, as it must ensure adequate cooling at all operating points. Only if this is ensured, can the aerodynamic air curtains, which guide the air "cleanly" around the front wheels, be fully utilized. "Thermal management and aerodynamics must already be considered during the platform development," says Moni Islam. On the other hand, an efficient thermal management system gives designers more flexibility.

Disadvantage of Height: Every Extra Centimeter Costs Around Three Kilometers of Range!

For the popular SUVs, additional factors come into play. Every centimeter the car stands higher reduces the range by about three kilometers. "And that's just the trim level," Islam clarifies. In addition, there are aerodynamically unfavorable factors such as large tires, bulky wheel arches, and especially the boxy design of these elevated EVs. Even a brief glance reveals that this silhouette is far from the streamlined shape of a fish. Hence, it's crucial for aerodynamicists to pull out all the stops to push the drag coefficient (cd value) below the magical threshold of 0.30 for such vehicles. The Porsche Macan E, for instance, has a cd value of 0.25.

Why the Camera Side Mirrors of the A6 e-tron Have a Small Step

To achieve a cd value of 0.21 for the Audi A6 e-tron Sportback with a maximum range of over 750 kilometers according to WLTP, numerous measures are necessary. The digital side mirrors alone add seven kilometers of range net (the power consumption of the displays has been accounted for). This feature sparked a dispute between aerodynamicists and designers. The best solution would be to slant the camera mirror backward, which is an optical disaster. The compromise is a small step on the underside of the housing.

The Rear End Is Always the Critical Point

The fact that wind tunnel specialists don't always get their way is evident from the small rear spoiler on the S6 e-tron Sportback, which provides the necessary downforce without significantly worsening the cd value. The rear end is the critical point of any car. "The key to aerodynamics is the flow detachment," says Moni Islam. The more compact or narrower the air streams converge from above and below, the more aerodynamic the vehicle is. Comparing the A5 e-tron Sportback with its sharply sloping roofline to the A6 e-tron Avant wagon reveals how much the airflow lines and their paths differ.

New: Flow-Directing Bodies in Front of the Front Wheels

The range of techniques used by aerodynamicists is extensive. What makes the task so challenging is the fact that every measure affects other areas of the car, especially the underbody, which is increasingly becoming a crucial playground for aerodynamicists filled with interesting aerodynamic tricks. In front of the front wheels, there is a flow-directing body that effectively elongates the wheel by calming the airflow so that the tires are more smoothly enveloped by the air.

We move on to Amsterdam, where the new Skoda Elroq is being presented to us. Karl Neuhold, Head of Exterior Design at Skoda, points out the front wheel arches that have been extended towards the wheel: "This reduces the air gap between the wheel and the wheel arch," explains the Austrian, who is not entirely happy with the solution aesthetically but acknowledges that it can regain range this way. "It sounds odd, but the wheel is one of the least aerodynamic parts of the car," Neuhold adds: Wheels are always in motion, and the tire contour is constantly at work, which is why tire manufacturers have long opted for extremely smooth sidewalls. The Elroq also features a flat underbody and a front as closed as possible.

The worst-case scenario is always when air can enter somewhere and then suddenly get slowed down by a resistance—usually a radiator. The art of engineering is to ensure the best possible airflow.

In the Rear, Several Airflows Need to Be Aligned

The tricks continue at the rear: In front of the rear wheels, there is also a small spoiler, and on the Sportback, a four-millimeter-tall Gurney flap calms the air in front of the rear axle. The Avant does not have this component because it would disrupt the flow to the small spoiler at the end of the diffuser. This diffuser spoiler is crucial as it ensures that the airflow matches that of the roof. The sensitivity of the aero system and the significant impact of individual parts are evident from the fact that the front flow-directing body of the Avant is shaped differently from that of the Sportback.

The rear of the Elroq has also been designed to be as aerodynamic as possible: "We don't have the large roof spoiler because it looks cool—though it does—but because it directs the airflow as long as possible," Neuhold explains to us. Wherever the air detaches, vortices form, deteriorating the aerodynamics. The worst case here is a semi-trailer that ends abruptly at the rear... even here, experiments are being conducted with sloped aero roofs and side flaps: But the former reduces storage space, and the latter are often damaged during maneuvers... but we digress...

An important aspect that the wind masters also have to consider is acoustics. With the noise of the combustion engine gone, wind noises become much more noticeable. An electric car should be quiet, especially inside the cabin.