40 years ago, many drivers got to know a new word: cw value When the new Audi 100 hit the market in 1982, the manufacturer presented it as “the world’s most streamlined production sedan”. The document, which was interesting at the time, was used as evidence cw value of 0.30. The fact that car towing suddenly became a selling point was due to the oil crises of 1973 and 1979, just a few years ago.
This also increased the importance of aerodynamics. Air resistance plays an important role in fuel consumption, especially at high speeds. “From about 80 kilometers per hour it becomes more important than the resistance of the tires,” explains Marcel Straub, specialist project manager for aerodynamics and thermal management at Porsche Engineering. “And because it increases with the square of the speed, aerodynamics is important for fuel consumption, especially when driving on the highway.”
The product of the frontal area and the vehicle determines the vehicle’s drag cThe w value at the end shows how the geometric shape is balanced. The following applies: the smaller, the better. Water drops come very close to ideal because they are round in the front and long in the back. His cthe value of w is only 0.05. However, it is difficult to carry the vehicle, passengers and payload in teardrop-shaped vehicles.
The classic wedge shape with a rounded front and angular rear has been prevalent since the 1980s. Its main purpose is to reduce drag at the rear of the car. Sharp edges allow the flow to break in a targeted manner and reduce negative pressure, which reduces air resistance. That’s what they were cw-values keep getting better: the Opel Calibra reached 0.26 in 1990, and the Audi A2 reached 0.25 ten years later. “Those were very big steps in aerodynamics,” remembers Prof. Andreas Wagner, Chair of Automotive Engineering at the University of Stuttgart.
The next step is currently underway, driven by the electric transition. “Electric drives are more efficient than combustion engines, so other influences on energy consumption are more important,” explains Dr. Thomas Wiegand, Head of Aerodynamics Development at Porsche AG. “In the WLTP driving cycle, aerodynamics is responsible for 30 to 40 percent of the losses in electric cars, as opposed to less than ten percent in a car with a diesel or gasoline engine. And because the average speed in the cycles close to the customer is much higher than that of WLTP, this value should be more than 50 percent in the real operation of electric vehicles.”
Accordingly, manufacturers place great importance on the improved aerodynamics of their electric vehicles. New drive technology suits them well: Cars with combustion engines have a tunnel in the middle of the bottom and an exhaust system that has to be cooled by ambient air. The reinforced surface causes air turbulence and increases rolling resistance. In e-cars, on the other hand, the battery is located between the front and rear axles. Its underside is quite smooth, which contributes to good aerodynamics.
intervention of active aerodynamics
Another advantage of e-mobility is that the motors generate less heat, meaning that less energy has to be transmitted through the cooler. Therefore, little or no air flow through the engine compartment is necessary, which reduces the air resistance of e-cars. In most electric vehicles, individually controllable cooling air curves in the air intakes ensure that only the required amount of air is passed through the radiator and brake discs. Because the technology actively intervenes depending on the driving situation, experts refer to such measures as “active aerodynamics”.
This also includes retractable and extendable spoilers and an air-sprung chassis that lowers the car at high speeds. “To implement these measures, we at Porsche Engineering are developing our expertise in the area of work and software development,” Straub says. “In this way we can safely bring an active step on the performance side to the maturity of the series.” Modern electric cars make the most of these technical possibilities: With drag coefficients of 0.22 and 0.20, the Porsche Taycan and Mercedes EQS are well ahead. terms of front aerodynamics.
Active aerodynamic measures may play a greater role in the future and significantly change the appearance of cars when driving. For example, Mercedes-Benz presented the Vision EQXX concept car with a drag coefficient of 0.17. Noticeable changes when driving include a diffuser on the lower edge of the rear: it directly extends back by 20 centimeters at speeds above 60 km/h. With a sharp spoiler lip on an unusually long tail, it ensures low air resistance.
“For 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 improved aerodynamics. The EQXX should consume less than 10 kWh over 100 km, while the current EQS is still at least 15 kWh.” Expert Straub can also imagine that cars will change their shape when driving in the future: “For example, the rear can be more angular at high speeds to create sharper breaking edges. The basis of this can be new shape-memory materials .They change their geometry depending on the temperature or applied voltage.
At the University of Stuttgart, researchers are pursuing a completely new approach: “We are investigating whether targeted vibrations at certain points on the body can be used cit can reduce the value of w,” explains Wagner. “If you introduce defined pulses into the flow around you with the help of loudspeakers, you can influence their dissociative behavior.” cw-value at seven percent. “But that’s still a long way from the range,” says Wagner. “For example, we have to make sure that the passengers don’t hear any buzzing or banging.”
Better and better simulation
Engineers and designers use wind tunnels and CFD (computerized fluid dynamics) simulations to check the extent to which their ideas affect the aerodynamics of new cars. “CFD simulation has gained in importance in the last 20 years,” reports Wagner. “Mathematical methods have become better understood, more precise tools have been developed and computer performance has also increased.”
However, computer simulations still reach their limits. At present it is only possible to quantify the effects of rotating tires on a small scale. Even their deformation under the weight of the car cannot be simulated with sufficient accuracy today. This should be possible in the future, with computer-aided optimization of vehicle geometry. “Many parameters play a role here, such as the movement of the lateral line, the A-pillar, the height of the trunk lid or the angle of the diffuser,” explains Wagner. “This leads to so many combinations that one can no longer keep track of them.” Intelligent algorithms, on the other hand, can go through a wide range of variables and find those combinations that have the lowest rate. cpromise of w-value. Then it will also be possible to set a parameter – such as the length of the trunk cover – constant for design reasons and then go through the remaining geometric variants under these boundary conditions.
In the future, artificial intelligence (AI) should contribute to more efficient processes. “At the end of the development, we have to determine the individual use or range levels for each vehicle variant, which weight and rolling resistance as well as aerodynamics contribute,” explains Wiegand. “So we have to provide a lot of data for the aerodynamic part.” However, a large number of wind tunnel measurements and simulation results are already available from early development phases. These should be better organized in the future and analyzed using modern methods. “AI algorithms can extract new data from existing data through interpretation and presentation. This allows us to plan tests in a targeted way and reduce their number. And we no longer need to test all typing variants.”
Real-time optimization with AI
Porsche engineering is also working on the use of AI methods. The developers aim to predict the effects of changes in vehicle geometry in real time. Although time-consuming CFD simulations are still required for every variant today, in the future neural networks will have an influence on cCalculate the value of w more quickly. “You change the shape with the mouse and you immediately see what that means for aerodynamics,” says Straub. “We have already applied this AI-based process to the wing profile of the Porsche GT3.” The new method is being further developed together with AI experts from Porsche Engineering and the road development department at Porsche AG in Weissach.
It is not expected that all aerospace-engineered vehicles will look the same in the future. “Good cThe w-value can be obtained in different ways,” says Wagner. “If you want to improve the rear, for example, you can change the height of the boot lid and the diffuser on the bottom. In conjunction with the design, the ideal must be found that fits the brand. In this way, comparable aerodynamics can be achieved in different shapes.” Even expert Straub does not believe in a future standard design: “There will be no risk of confusion in the future either – even with the best aerodynamic cars.”
It has been shortened
With the transition to electronic mobility, cars are flying in aerodynamics. In the future, more active measures such as back-changing shapes or deliberately introduced vibrations should account for this. There have also been significant advances in simulation and experimental optimization using artificial intelligence. .
The text was first published in Porsche Engineering Magazine, issue 2/2022.
Author: Christian Buck
Copyright: All images, videos and audio files published in this article are subject to copyright. Reproduction or reproduction in whole or in part is prohibited without the written permission of Dr. Ing. hc F. Porsche AG is not allowed. Please contact firstname.lastname@example.org for more information.