Battling against resistance: for 100 years engineers and designers have been working to improve cars’ aerodynamics by reducing the air resistance, expressed as the drag coefficient (Cd). This is crucial, especially in electric mobility.
The drag coefficient (Cd) is a numerical expression of a car’s aerodynamic properties and enables direct comparison of different vehicle categories. The lower the Cd, the lower the air resistance presented by the vehicle body. One major contributing factor is the front profile of the car, which has to displace the air while the vehicle is in motion. The air density is another key factor. It is determined from the temperature and the air pressure. Last not but least, the speed of travel also has an impact, so it is logical that the air resistance is greatest on the freeway. When travelling flat out, a car expends almost 90 percent of its fuel on battling against the wind. If the Cd can be reduced by only one percent, the car’s range on the freeway will rise by two to three percent. Streamlining the aerodynamics is particularly important for electric cars. The vehicle’s weight is less significant because energy is recuperated, but losses due to air displacement are all the more noticeable.
With a Cd of 0.22, Mercedes-Benz’ new S-Class is among the most aerodynamic vehicles out. The nippy electric Porsche Taycan has the same value – which happens to be the lowest of any current Porsche model. Designers and engineers are searching for the best compromise between effectiveness, benefit and comfort. So optimization tends to come in small steps. Convex front windshields, concealed windshield wipers, flat underbodies, optimized rear shapes, covered wheelhouses, flattened A-pillars, the absence of antennas, and the air flow through the radiator and the engine compartment all offer potential. The virtual exterior mirror on the Audi e-tron brought a 5-point improvement, which extends the car’s range by around 2.5 kilometers. The electric competitor EQC from Mercedes-Benz extended its range with a rounded front and 3-D wheel spoilers. But extreme shapes are needed to achieve any significant further reduction in air resistance. And that in turn rapidly brings disadvantages in the interior, and high costs – as with Volkswagen’s XL1. The one-liter two-seater unveiled in 2013 left the competition way behind on efficiency and had a Cd of 0.189. However, the narrow car was only built as a small series of 200 vehicles and each one had a price tag of more than 100,000 euros.
The Mercedes-Benz “Concept Intelligent Aerodynamic Automobile” has a Cd that is only one hundredth more. The hybrid car was unveiled at the IAA in 2015. It changes its shape and appearance while it is in motion. Once it reaches a speed of 80 km/h, the car switches either manually (at the press of a button) or automatically from design mode to aerodynamic mode. This had never been done before. Until then, a car had been more of a rigid structure without parts that adjusted intuitively to the driving situation – apart from extendible rear spoilers. But this car adds 39 centimeters at the rear. Extendible and moveable louvres improve the air flow past the front and the underbody. The five-spoke wheels are transformed into flat discs to cut down air turbulence. And to achieve the best possible air flow, the IAA Concept hunkers down to ten centimeters from the road surface. It dispenses with door handles, and has replaced the exterior mirrors with cameras connected to electronic displays. Whereas this prototype had a Cd of 0.25 in normal operation, the transformation brings the figure down to 0.19. A year later, BMW went one better with the concept study “Vision Next 100.” Not only does it have a streamlined design, but the wheels are hidden by the “Alive Geometry” and a stretchy skin adapts to the steering movements. The result is a drag coefficient of 0.18.
When looking for technical innovations and solutions to problems, designers and engineers like to pinch ideas from nature – bionics has often been a source of inspiration. The penguin is seen as the epitome of aerodynamics: it only needs to beat its short wings two or three times and it glides effortlessly through the ocean depths. Scientists from the former Institute for Oceanography – today the Helmholtz Centre for Ocean Research – and the bionics expert Rudolf Bannasch discovered that the long-tailed gentoo penguin swims more than 100 kilometers a day even though it’s only about 50 cm in size. It can reach 25 km/h while sprinting. The researchers also investigated the animal’s feeding habits and made the following conclusions: if the animal were to fill up on gasoline, it could travel 2,500 kilometers on just one liter. Bannasch conducted further experiments with penguin models in water tank and wind tunnel experiments. The results were astounding: the Cd was minuscule, reaching only 0.03. Shipping, aerospace and the military still benefit from these findings even today. So far the penguin shape with its almost perfect streamlining has not been imitated in car bodies – but the water droplet has.
As early as the beginning of the 20th century, the droplet shape inspired German engineers to develop bizarre concept vehicles. The famous ones include the Rumpler Tropfenwagen (“Rumpler drop car”) in 1921 and the Schlörwagen in 1939. Measurements by VW in the 1970s revealed that the Rumpler car had a Cd of 0.28, and the Schlörwagen a Cd of only 0.15. So both the oldies still put current series vehicles in the shade, although it must be said that modern cars are not designed solely to achieve the best Cd, but also to satisfy other requirements such as easy access and crash behavior. On the other hand, Formula 1 cars, that look like perfect streamlining on wheels, are aerodynamic bricks. When it comes to air resistance, superfast racing cars turn out to have Cd values of 0.8 to 1.2, because their design focuses more on cornering speed and stability. Even the average truck has a Cd between 0.5 and 0.85. Krone’s “Aero Liner” trailer and MAN’s “Concept S” tractor dating from 2012 make a combination that gets down to only 0.3.
Computer-aided simulation methods such as numerical fluid mechanics, and production methods such as rapid prototyping, are required to create these complex geometries and aerodynamic vehicle properties. Development engineers also need testing apparatus like wind tunnels. Manufacturers and suppliers are working closely with scientists, for example at the German Aerospace Center (DLR), to design vehicle parts that are more streamlined. The DLR’s research site in Göttingen has 20 wind tunnels and large-scale facilities for developing passenger cars, trucks, jets and even space shuttles. Laser techniques are used to simulate and measure wind flow. Porsche, the sports car specialist, has several wind tunnels for vehicles on scales of 1:1 and 1:3 in Weissach, which can operate either with or without road simulation. The test pieces range from simple foam bodies to models covered with pressure sensors, all the way to complex flow-through bodies. Various components can be analyzed at wind speeds of up to 300 km/h and then optimized in an iterative process. The facility tests more than just vehicles – bicycle racing teams, umbrella manufacturers and tent makers also come here to test and optimize their products’ resistance to wind and weather.
(Stage photo: © AUDI AG)
The IAA MOBILITY is transforming itself from a pure car show to an international mobility platform with four pillars: the Summit, the Conference, the “Blue Lane” and the downtown Munich Open Space. Under the slogan of “What will move us next”, it stands for the digital and climate-neutral mobility of the future. From 7 to 12 September 2021, the car, bike and tech industries come together at IAA MOBILITY in Munich.