Form follows function

It’s all about reducing the amount of resistance as the vehicle makes its way through the air. The slipperier the shape, the easier it makes its way and the less energy is required to propel it forward. It all sounds so simple, and yet getting the shape to the optimum is a painstaking art, where the smallest changes can make huge differences.

In the day-to-day driving of electric cars, aerodynamic drag is the chief factor in driving resistance. Even when the Audi e-tron Sportback, for example, is driven in line with a standard cycle as defined by the World Harmonised Light-Duty Vehicles Test Procedure (WLTP), driving resistance is responsible for just over 40 percent of consumption. Compare that to accelerating the vehicle’s weight, which accounts for slightly less than 20 percent of consumption in that cycle. In a car like the Audi e-tron Sportback that boasts a very good drag coefficient (cd value), aerodynamic drag is the prime factor in driving resistance from speeds as low as about 80 kilometres per hour. When traveling on a country road at a speed of 100 kilometres per hour, aerodynamic drag represents over 60 percent of the total running resistance. The upshot is that this gives us engineers an enormous incentive to design all Audi e-tron vehicles to be as aerodynamic as possible.

Of course when it comes to designing efficient aerodynamics, electric vehicles offer distinct advantages, says Dr Moni Islam, Head of Aerodynamics/Aeroacoustics Development at Audi. It’s only thanks to the battery and dispensing with an exhaust system that we were able to create an enclosed and very smooth underfloor from nose to tail. For aerodynamics, this is a tremendous plus. What’s more, an electric motor is much more efficient than a combustion engine and not only releases less heat into the environment but does not require as much cooling nearly as often. Consequently, we are able to develop thermal management concepts that are beneficial to aerodynamics. The active inlet louvers on our Audi e-tron models’ Singleframe grilles are a case in point – and a pivotal aerodynamic measure at the front end. An electronically operated louver system on each of the two air inlets can be automatically opened or closed, depending on the degree of cooling the vehicle requires. The closed position is of course the perfect one for aerodynamics, but the system allows for the louvers to open as required to ‘climates’ the interior’ and shut again for optimum aerodynamics.

We often think of the drag coefficient as defining how aerodynamic a shape is. Of course, we’re always chasing a better c_d value for our vehicles as we develop successive models. 

With its virtual exterior mirrors, our Audi e-tron has a c_d value of 0.27—one of the best on the market for the SUV segment. Many of the currently available SUVs have higher values. That’s why we’re particularly proud of having scored such an outstanding value on a full-blooded SUV. Thanks to its basic streamlined silhouette, the Audi e-tron Sportback even goes one better with a drag coefficient of just 0.25.

Just how low a drag coefficient could conceivably go is realistically governed by the practicalities of vehicle usage. Studies record penguins as having a c_d value of 0.07, which can at times drop even further thanks to special surface effects and dynamic modifications to their body form during movement. For our purposes in vehicle developmetn though, we can only harness these natural phenomena to a very limited extent in practice. After all, a car functions differently than a penguin. 

At the end of the day it is our customers’ needs that define function which means usable boot space and real headroom – even if lowering the roof and making the tail narrower would vastly improve the aerodynamics.

For aerodynamicists, the rear is the most important zone of the car, as it is behind the vehicle that an area of low pressure forms. This negative pressure sucks the car backward, thus creating resistance. Our job is to keep the area of negative pressure at the rear of the vehicle as small and compact as we can. 

So we try to design all the features at the rear to be as narrow and as small as possible: a narrow track width, recessed wheels, a slim luggage compartment. And we use rear or roof spoilers to deflect the airflow in such a way that they meet symmetrically and at the same height behind the vehicle after passing above and below it. That’s why even one spoiler with a relatively small lip can have a really positive effect on the drag coefficient. 

On the Audi e-tron, we even using a small spoiler on the spare wheel well underneath the car to ensure that the air is deflected to the exact same point behind the vehicle where the airflow comes down off the roof.

Another of our goals as aerodynamicists is to come up with innovative technical details that give the designers greater scope for creativity. Virtual mirrors which are available an option on our Audi e-tron models are a good point. The WLTP cycle shows that their influence on the drag coefficient improves the range by the equivalent of around 2.5 kilometres compared with conventional exterior mirrors – and this improvement is even more pronounced on country roads and highways.

Aside from the virtual mirrors, most of the aerodynamic improvements are not immediately apparent, the subtle design cues that change the flow of air may not always have a streamlined ‘look’ but their disruptive effect to the air around a vehicle is profound.