Wind of Change - Aerodynamics & E-mobility
For decades, manufacturers have been successfully optimising the aerodynamics of their vehicles. However, by switching to e-mobility, the latest models are leaping at the Cd value. And the potential still needs to be exhausted: Porsche Engineering reports on active aerodynamic measures and new development methods that promise further improvements.
40 years ago, many drivers learned a new word: CW-value. When the new Audi 100 came onto the market in 1982, the manufacturer presented it as "the most aerodynamic production sedan in the world". The CW value of 0.30, which was impressive for the time, served as evidence. The fact that vehicle air resistance suddenly became a selling point was due to the oil crises of 1973 and 1979, which took place only a few years earlier. Fuel prices have risen sharply since then, and the efficiency of vehicles has become more and more the focus of attention.
This also increased the importance of aerodynamics. Especially at higher speeds, air resistance plays a vital role in fuel consumption. "From around 80 km/h, it becomes more important than the rolling resistance of the tyres," explains Marcel Straub, Project Manager of Aerodynamics and Thermal Management at Porsche Engineering. "And because it increases quadratically with speed, aerodynamics are crucial for fuel consumption, especially when driving on the motorway."
The extent of the air resistance of a vehicle is determined by the product of the frontal area and cw value. The latter indicates how streamlined a geometric shape is. The following applies the smaller, the better. Water droplets come pretty close to the ideal because they are round at the front and long at the back. Their CW value is only 0.05. However, teardrop-shaped vehicles cannot accommodate the driver, passengers and payload.
Since the 1980s, the typical wedge shape with a rounded front and an angular rear has prevailed. Its primary purpose is to minimise the suction at the back of the vehicle. Sharp edges allow the flow to break off in a targeted manner and reduce the negative pressure, which reduces air resistance. As a result, the OpelCalibra achieved 0.26 in 1990, and the Audi A2 reached 0.25 ten years later.
The next leap is currently taking place, driven by the transition to electromobility. "Electric drives have a much better efficiency than a combustion engine so that 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 are responsible for 30 to 40 % of losses in electric cars, as opposed to less than ten % in vehicles with diesel or gasoline engines. And because the average speed in customer-oriented cycles is even higher than in the WLTP, this value is likely to be even more than 50 % in the real driving operation of electric vehicles."
Accordingly, manufacturers attach great importance to the optimised aerodynamics of their electric vehicles. The new drive technology accommodates them: cars with combustion engines have a central tunnel in the underbody and an exhaust system that the ambient air must cool. As a result, the jagged surface leads to air vortices and increases driving resistance. In electric cars, on the other hand, the battery sits between the front and rear axles. As a result, their underside is entirely smooth, which contributes to favourable aerodynamics.
Another advantage of e-mobility is the lower heat generation of the motors, so less energy must be dissipated via the radiator. As a result, less or no airflow through the engine compartment is required, which reduces the air resistance of electric vehicles. In many electric cars, individually controllable cooling air flaps in the air intakes ensure that only the amount of air required is routed through the radiators and brake discs. Because the technology actively intervenes here depending on the driving situation, experts refer to such measures as "active aerodynamics".
This also includes retractable and retractable spoilers and air-sprung suspensions that lower the car at high speeds. "To implement these measures, we at Porsche Engineering are building on our expertise in functional and software development," says Straub. "This allows us to safely bring the active measures on the functional side to series maturity." Modern electric vehicles use many of these technical possibilities: With Cd values of 0.22 and 0.20, the Porsche Taycan and the Mercedes EQS are far ahead in terms of aerodynamics.
Active aerodynamic measures could play an even more significant role in the future and significantly change the appearance of vehicles while driving. Mercedes-Benz, for example, presented the Vision EQXX concept car with a drag coefficient of 0.17. One of the visible changes while driving is the diffuser at the lower edge of the rear: it automatically extends 20 centimetres to the bottom from 60 km/h. Together with the sharp spoiler lip at the exceptionally long rear, 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. As a result, 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 cars changing their shape while driving in the future: "The rear could, for example, become more angular at high speeds to form sharper spoiler edges. The basis for this could be new shape memory materials. They change their geometry depending on the temperature or applied stress."
At the University of Stuttgart, the researchers are pursuing a new approach: "We are investigating whether the CW value can be reduced with targeted vibrations at certain points of the body," explains Wagner. "If you introduce a defined pulse into the flow around the flow with the help of loudspeakers, their detachment behavior can be influenced." In the case of an SUV, it was possible to reduce the cw value by seven %. "But that's still a long way from the series," says Wagner. "For example, we need to ensure that passengers don't hear buzzing or humming."
Engineers and designers check the extent to which their ideas affect the aerodynamics of new vehicles in the wind tunnel and with CFD (computational fluid dynamics, german: numerical fluid mechanics) simulations. "CFD simulations have become enormously important in the last 20 years," reports Wagner. "The mathematical methods have been better understood, more precise tools have been developed and the performance of the computers has also been increased."
However, computer simulations still reach their limits today. For example, it is currently only possible to calculate the effects of rotating tires to a limited extent. Even their deformation under the weight of the vehicle cannot be simulated with sufficient accuracy today. This should be possible in the future and the computer-aided optimisation of the vehicle geometry. "Numerous parameters such as the course of the sideline, the A-pillar, the height of the rear lid or the diffuser angle play a role here," explains Wagner. "This results in many possible combinations that a person can no longer keep track of them." On the other hand, intelligent algorithms could move through the number of variants and specifically find those combinations that promise a low CW value. It would also be possible to keep a parameter – such as the height of the rear lid – constant for design reasons and run through the remaining geometric variants under this boundary condition.
In the future, artificial intelligence (AI) will contribute to more efficient processes. "At the end of development, we are obliged to specify individual consumption or range values for each vehicle variant, to which aerodynamics contribute in addition to weight and rolling resistance," explains Wiegand. "That's why we have to generate a lot of data for the aerodynamic component." However, many wind tunnel measurements and simulation results are already available from the previous development phases. In the future, these will be better structured and analysed using modern methods. "AI algorithms could generate new data from a stock of existing data through interpolation and extrapolation. This allows us to plan experiments in a targeted manner and reduce their number. And we would no longer have to measure all variants for typing."
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, in the future, a neural network will calculate the influence on the CW value much faster. "You change a shape with the mouse and immediately see what it means for aerodynamics," says Straub. "We have already used this AI-based process for the wing profile of a Porsche GT3." The new approach will be further developed with the AI experts from Porsche Engineering and method development at Porsche AG in Weissach.
It is not to be expected that the aerodynamically optimised vehicles will all look the same in the future. "A good CW value can be achieved in different ways," says Wagner. "For example, if you want to optimise the rear, you can change the height of the rear lid and the diffuser in the underbody. In cooperation with the design, an optimum must be found to fit the brand. In this way, comparable aerodynamics can be achieved with different shapes." Expert Straub also does not believe in a future uniform design: "There will be no risk of confusion in the future – even with the aerodynamically best vehicles."
By switching to e-mobility, vehicles are currently leaping aerodynamics. In the future, active measures such as changeable shapes at the rear or targeted vibrations will increasingly contribute to this. Significant progress has also been made in simulations and experimental optimisation with artificial intelligence.