For decades, manufacturers have been successfully optimising the aerodynamics of their vehicles. Due to the switch to e-mobility, the latest models are currently making a leap in the cw value. And the potential is not yet exhausted: Porsche Engineering reports on active aerodynamic measures and new development methods that promise further improvements.
A guest article by Christian Buck also in the current eMove360° magazine in german language(download PDF), first published in Porsche Engineering Magazine, issue 2/2022.
40 years ago, many car drivers got to know a new word: cw value. When the new Audi 100 was launched in 1982, the manufacturer presented it as “the most streamlined production saloon in the world”. The impressive cw value of 0.30 served as proof. The fact that the aerodynamic drag of vehicles had suddenly become a selling point was due to the oil crises of 1973 and 1979, which had occurred only a few years earlier. Fuel prices had risen sharply since then, and the efficiency of vehicles was becoming more and more the focus of interest.
This also increased the importance of aerodynamics. Because especially at higher speeds, air resistance plays an important role in fuel consumption. “From around 80 km/h, it becomes more important than the rolling resistance of the tyres,” explains Marcel Straub, specialist project manager for aerodynamics and thermal management at Porsche Engineering. “And because it increases quadratically with speed, aerodynamics is quite decisive for fuel consumption, especially when driving on the motorway.”
How great the aerodynamic drag of a vehicle is is determined by the product of the frontal area and the cw value. The latter indicates how streamlined a geometric shape is. The rule is: the smaller, the better. Water drops come quite close to the ideal because they are round at the front and have a long taper at the rear. Their cd value is only 0.05. However, it is difficult to accommodate the drive, passengers and payload in drop-shaped vehicles.
Since the 1980s, the typical wedge shape with a rounded front and an angular rear has prevailed. Its main purpose is to minimise suction at the rear of the vehicle. Sharp edges allow the flow to break away in a targeted manner and reduce the negative pressure, which in turn reduces air resistance. This is how the cw values got better and better: the Opel Calibra came to 0.26 in 1990, and the Audi A2 reached 0.25 ten years later. “Those were real leaps in aerodynamics,” recalls Prof. Andreas Wagner, holder of the Chair of Automotive Engineering at the University of Stuttgart.
The next leap is currently taking place, driven by the transition to electric mobility. “Electric drives have a much better efficiency than an internal combustion engine, so the other influences on energy consumption are much more significant,” explains Dr Thomas Wiegand, Head of Aerodynamics Development at Porsche AG. “In the WLTP driving cycle, aerodynamics is responsible for 30 to 40 per cent of the losses in e-cars, as opposed to less than ten per cent in a vehicle with a diesel or petrol engine. And because the average speed in cycles close to the customer is even higher than in the WLTP, this value is likely to be even higher than 50 per cent in real-life driving of e-vehicles.”
Accordingly, manufacturers attach great importance to optimising the aerodynamics of their e-vehicles. The new drive technology helps them in this respect: Vehicles with combustion engines have a centre tunnel in the underbody and an exhaust system that must be cooled by the ambient air. The fissured surface leads to air turbulence and increases driving resistance. In electric cars, on the other hand, the battery sits between the front and rear axles. Its underside is completely smooth, which contributes to favourable aerodynamics.
Intervention of active aerodynamics
Another advantage of e-mobility is that the motors generate less heat, so less energy has to be dissipated via the radiator. Therefore, less to no airflow through the engine compartment is necessary, which reduces the aerodynamic drag of e-vehicles. In many e-vehicles, individually controllable cooling air flaps in the air intakes ensure that only the amount of air that is really needed is directed over the radiators and the brake discs. Because the technology actively intervenes here depending on the driving situation, experts refer to such measures as “active aerodynamics”.
They also include retractable and extendable spoilers and air-sprung chassis that lower the car at high speeds. “To implement these measures, we at Porsche Engineering build on our expertise in function and software development,” says Straub. “This allows us to safely bring the active measures to production readiness on the functional side.” Modern e-vehicles make use of many of these technical possibilities: With cw values of 0.22 and 0.20, the Porsche Taycan and the Mercedes EQS, respectively, are far ahead in terms of aerodynamics.
Active aerodynamic measures could play an even greater role in the future and significantly change the appearance of vehicles while driving. Mercedes-Benz, for example, has presented the Vision EQXX concept car with a cd value of 0.17. Among the visible changes while driving there is the diffuser at the lower edge of the rear: it automatically extends backwards by 20 centimetres from 60 km/h. The diffuser is also a part of the rear end of the car. Together with the sharp tear-off edge at the exceptionally long rear end, it ensures minimal air resistance.
“With the EQXX, the focus was on energy efficiency,” reports Dr Stefan Kröber, aerodynamics engineer at Mercedes-Benz and lecturer at the Karlsruhe Institute of Technology. “An important part of this is the optimised aerodynamics. The EQXX should consume less than 10 kWh per 100 km, while the current EQS is still at least 15 kWh.” Expert Straub can also imagine that cars will change their shape while driving in the future: “For example, the rear could become more angular at high speeds in order to form sharper breakaway edges. The basis for this could be new shape memory materials. They change their geometry depending on temperature or applied tension.”
At the University of Stuttgart, the researchers are pursuing a completely new approach: “We are investigating whether it is possible to reduce the cw value by deliberately introducing vibrations at certain points of the car body,” Wagner explains. “If you introduce a defined pulse into the flow around the car using loudspeakers, their detachment behaviour can be influenced.” In the case of an SUV, he says, it was possible to reduce the cw value by seven percent. “But that is still a long way from series production,” says Wagner. “We have to make sure, for example, that passengers don’t hear any buzzing or humming.”
Better and better simulations
Engineers and designers check how much their ideas affect the aerodynamics of new vehicles in the wind tunnel and with CFD simulations (Computational Fluid Dynamics). “CFD simulations have become enormously important in the last 20 years,” Wagner reports. “The mathematical methods have been better understood, more precise tools have been developed and the performance of the computers has increased.
However, computer simulations still have their limits today. For example, it is currently only possible to calculate the effects of rotating tyres to a limited extent. Even their deformation under the weight of the vehicle cannot be simulated with sufficient accuracy today. In the future, this should be possible as well as the computer-aided optimisation of the vehicle geometry. “Numerous parameters play a role here, such as the course of the side line, the A-pillar, the boot lid height or the diffuser angle,” Wagner explains. “This results in so many possible combinations that a human being can no longer keep track of them.” Intelligent algorithms, on the other hand, could navigate through the multitude of variants and specifically find those combinations that promise a low cw value. It would then also be possible to keep one parameter – such as the height of the rear lid – constant for design reasons and to play through the remaining geometric variants under this boundary condition.
In the future, artificial intelligence (AI) should contribute to more efficient processes. “At the end of the development, we are obliged to specify individual consumption or range values for each vehicle variant, to which aerodynamics contributes in addition to weight and rolling resistance,” Wiegand explains. “We therefore have to generate a lot of data for the aerodynamic part.” However, a large amount of wind tunnel measurements and simulation results are already available from the previous development phases. In future, these will be better structured and analysed using modern methods. “AI algorithms could generate new data from a stock of existing data by interpolation and extrapolation. This would allow us to plan experiments in a targeted manner and reduce their number. And we would no longer have to measure all variants for typing.”
Real-time optimisation with AI
Porsche Engineering is also working on the use of AI methods. The developers’ goal is to predict the effects of changes to the vehicle geometry in real time. While a time-consuming CFD simulation is still necessary for each variant today, a neural network will calculate the influence on the cw value much faster in the future. “You change a shape with the mouse and immediately see what that means for the aerodynamics,” says Straub. “We have already used this AI-based method for the wing profile of a Porsche GT3.” The new approach is being further developed together with the AI experts from Porsche Engineering and the method development department of Porsche AG in Weissach.
It is not to be expected that the aerodynamically optimised vehicles will all look the same in future. “A good cw value can be achieved in different ways,” says Wagner. “If you want to optimise the rear, for example, you can change the boot lid height and the diffuser in the underbody. In cooperation with the design, an optimum must then be found that fits the brand. In this way, comparable aerodynamics can be achieved with different shapes.” Expert Straub also does not believe in a future one-size-fits-all design: “There will be no danger of confusion in the future – even with the best vehicles in terms of aerodynamics.”
Due to the switch to e-mobility, vehicles are currently making a leap in aerodynamics. In the future, active measures such as changeable shapes at the rear or deliberately introduced vibrations will increasingly contribute to this. Great progress has also been made in simulations and test optimisation with artificial intelligence.
The text was first published in Porsche Engineering Magazine, issue 2/2022.