Last April Mercedes had managed to make itself heard again thanks to the futuristic EQXX, an all-electric prototype capable of covering a distance of 1000 kilometers with a single battery charge, expressing an average consumption of 8.7 kWh/1000 km. The intent of the Stella engineers was to create an efficient car, combining overall lightness with a high-performance powertrain. However, the final goal is also the result of the exceptional work carried out on aerodynamics, developing the EQXX in the name of penetration and combining function with an elegant design. Even more than the penetration coefficient Cd of just 0.17, the ability to obtain such a result without adopting harmful aerodynamic solutions on a stylistic level arouses wonder.
New details on the aerodynamic development of the EQXX have recently emerged thanks to the interview with Alexander Washlea former Mercedes aerodynamicist, published by the US magazine Road&Track: “If you look at other cars with a drag coefficient of 0.17, like the 1937 W125 Rekordwagen or the C 111-IV, they have one thing in common, which is the rear wheels that are faired to be more aerodynamically efficient. To be honest, however, this solution is not elegant and the design department was not happy with it”. The turbulence generated by the rolling of the wheels is in fact one of the main sources of aerodynamic resistance, which is why their fairing can help reduce its influence. However, not being able to go down this road for aesthetic reasons, Mercedes had to think of other solutions, making a virtue of necessity. The EQXX was thus set up with the typical lines of the seats, to such an extent as to minimize the height space for the rear passengers. The intention is to contain the rear section on which the low-pressure zone behind the car acts, the difference between which and the high-pressure zone at the front generates resistance to forward movement. The long, tapered rear underscores the attempt to get closer to that teardrop shape which, on a theoretical level, corresponds to the best possible aerodynamic penetration.
The front section of the car is also reduced to a minimum, with a projection extension of 2.12m2. The rear track is also 50 mm narrower than the front one, again for aerodynamic reasons, with the functional need prevailing over the aesthetic one in this aspect. On the other hand, the EQXX lacks the external fairing of the wheels, which would have led to a further lengthening of the rear axle: “Another important aspect is the roadway,” Waschle explains. “Designers love to create a nice line, which implies that the tail is wider than the front. As aerodynamicists, however, we prefer the opposite. The trade-off was that the design department accepted the 50mm narrower rear track and we sacrificed the wheel covers. To be honest, looking at the Vision EQXX it was the right choice, although the drag coefficient could have been even better with the wheel covers.” A further gem of the Stella prototype is the rear diffuser, which by extending by about 20 cm allows drag to be reduced by another ten points. The extractor extends automatically once the 60 km/h is exceeded, when the air flow has enough energy not to detach from the surfaces.
Although without fairings, the wheels have been the subject of important studies: “The wheels play an important role in generating aerodynamic drag. Therefore, there are numerous measures to guide the air around the wheels to reduce their drag. For example, let’s start from air curtains in front of the front wheel, which channels the air adhering to the bumper directly to the rims with the aerodynamic fairing. Behind the front wheel, theair breather collects the air coming from the wheel itself and accompanies it along the front door without detachment from the surface”. Bridgestone Turanza Eco tires also have a new technology to reduce tire shoulder deformation during rolling and corneringpartly due to increased pre-stressing of the structure. The stiffness of the shoulder is essential to minimize the turbulence generated by its deformation, well known in the motorsport field under the name of tire squirt and to which Mercedes itself in Formula 1 devotes enormous attention to limit its aerodynamic impact, an extremely complex task.
Mercedes has also paid great attention to the cooling system to optimize the cooling drag, i.e. the contribution to aerodynamic resistance given by the passage of flows inside the car. The EQXX is thus equipped with a passive cooling system. The cooling circuit of the electric powertrain flows through a metal plate positioned under the front axle and directly exposed to the flow of external air in the environment below the car, so as not to admit superfluous air into the internal compartment. Mercedes estimates that this solution alone extends the range by about 20 km. In conditions where more stringent cooling is required, the front grille opens and lets air inside, crossing the radiator cores. The air flow is guided by the difference between the high pressure in the area in front of the car and the low pressure above the bonnet, where the two outlet vents are located. In this way, the work required of the vehicle to push the internal air flow during motion is reduced. In fact, with the grille open, the increase in the coefficient Cd is just 0.007.
In closing, for the rear-view mirrors, the Stella technicians had thought of a camera system due to its compact size and minimal impact on aerodynamics. However, all this would have required internal displays with consequent absorption of energy from the battery, going against the objective of overall efficiency. The EQXX was thus equipped with two traditional mirrors but, as reported by Road&Track, their contribution to the total drag was less than 2%, while in other applications the side mirrors can even reach 8%. The Mercedes EQXX is thus confirmed to be a masterpiece of aerodynamics in every single aspect of its external appearance. Furthermore, the working method adopted, paying special attention to the design without resorting to extreme solutions however effective, places the final result not only as an exercise in itself, but as an anticipation of what aerodynamics could be. of the car of a not too distant future.
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