Much ado for a low drag coefficient

Oct 28, 2020

With a cd figure from 0.22,[1] the S-Class is one of the world's most aerodynamic cars, and especially so in the luxury saloon segment. Aerodynamic measures affecting the body, underbody and detachable parts allow a good showing in the wind tunnel and in real operation. Extensive airflow simulations were already carried out using high-performance computer clusters during an early development phase. A great deal of fine-tuning also went into the aeroacoustics. The previous generation of the S-Class already excelled with a very high level of interior noise comfort. The new model is even quieter.

Although the frontal area (A) of the new S-Class has increased slightly to 2.5 sq. m., the drag coefficient has been reduced even further compared to the preceding model. The product of cd and A is 0.56 sq. m., which is 200 sq. cm less than for the previous model after its last facelift.

One major focus of the aerodynamic measures was on the airflow in and around the front bumper. The space between the front wheel arch and the bumper is used as an air chamber. There are slits in the side walls of the recesses in the outer area of the bumper. These force air into the chambers and avoid interruption of the airflow ahead of the front wheels. At the same time some of the hot exhaust air from the engine compartment is directed into the air chamber. This is assisted by more effective sealing between the engine compartment and wheel arches. Each of the inlets ahead of the front wheels has a large exhaust air aperture preceded by a 3D suction spoiler. The mixture of cold and warm air is fed through these exhaust air apertures and ventilation slits to the insides of the front wheel arches. The resulting airflow ensures the best possible airflow along the underside of the vehicle and along the sides of the front wheel arches, and this also benefits thermal management in the engine compartment. In addition, the engine compartment panelling between the front wheels has been widened to reinforce this effect. Thanks to this improved thermal management, it has become possible to extend the rear engine compartment panelling even further and further reduce aerodynamic drag in comparison with a preceding model with a comparable engine.

Detailed improvements to the underbody and mounted parts such as the exterior mirrors and wheels also contribute to the very good aerodynamic performance of the vehicle.

  • The new S-Class has two-section rear light clusters. Owing to their lower height compared to the single-section version, it was necessary to dispense with spoiler lips in the lenses. This meant that particular attention was given to e.g. improving the airflow to the rear diffuser.
  • S-Class models with the panoramic glass sunroof have diagonal rear struts in the underbody to rigidify the bodyshell. To improve aerodynamic drag, these have been removed from the diffuser airflow and relocated above the exhaust system.
  • The position and airflow angles of the aerodynamically shaped cladding on the rear spring control arms and the fuel tank guard were reconfigured with further CFD-DOE improvements (see next section) and individual calculations, and later fine-tuned in the wind tunnel.
  • The exterior mirrors feature so-called aerostripes. These fine edges on the insides of the mirrors create turbulence and improve the airflow. The aerostripes also benefit the aeroacoustics.
  • Other measures include aero-wheels in almost every size, a cooling airflow control system behind the radiator grille and in the bumper, and improved sealing of the cooling airflow.

The three-dimensional airflow pattern around the vehicle was already calculated in high-performance simulation clusters using CFD (Computational Fluid Dynamics) in an early development phase. Shortly after this project started, during the dimensional drawing phase, several extensive DOE (Design of Experiments) studies were carried out on the basis of the preceding model, with up to 250 calculations per body area. In this process the aerodynamic engineers specify the parameters for certain components, e.g. the possible height of the boot lid.

Several hundred simulations were carried out over several days, fully covering the scope of the prescribed parameters. These simulations can be used to calculate a global or local optimum or, far more importantly in this phase, establish the influence of the individual parameters on the drag coefficient. Using the DOE method, specific aerodynamic requirements were reported to and discussed with personnel working on the dimensional concept and design in a very early phase.

In recent years such automated calculation processes including DOE have been intensively developed further by Mercedes-Benz aerodynamic engineers. As a result, digital development with DOE not only saves time, but is also cost-efficient: unlike for the preceding model, which required up to six hardware models, the new S-Class required only two 1:1 design models for wind tunnel improvements thanks to digital frontloading, i.e. advance selection of variants.

Aeroacoustics and NVH: even quieter interior than the previous model

The aim of sound insulation was to further improve the discreet noise level in the interior. The high rigidity of the bodyshell provides the basis for outstanding noise and vibration comfort, and this is enhanced with fine-tuning. For example, the apertures for the cable grommets in the firewall have double seals. To achieve an engine sound that is perceived as refined and unobtrusive in the interior, the firewall insulation has been extended into the side areas of the A-pillars and the floor area.

Mercedes-Benz is also using acoustic foam in certain bodyshell sections for the first time in series production. In this process a special foam is packed into the bodyshell while these areas are still accessible. These foam sections then expand in the oven during cathodic dip painting (CDP). This process has considerably improved sound insulation in the body structure – e.g. sound transmission through the C-pillar.

The S-Class was developed in the in-house aeroacoustic wind tunnel, which went into operation in 2013. To reduce wind noise, the bodyshell and sealing concept were improved as well as optimising the vehicle shape. Some of the aeroacoustic measures in detail:

  • The exterior mirrors feature so-called aerostripes. These fine edges on the insides of the mirrors create turbulence and ensure that the airflow is better able to follow the mirror contours, while reducing cutoff noise. The interior features were also fine-tuned: during the development phase, computer tomography was used for the mirror triangle. In this way the installed position of rails and multi-foam seals was verified.
  • The new, flush-fitted door handles (optional) are better sealed on the inside than conventional designs.
  • The seals in the window areas were improved, and the gap reduced.
  • Extensive airflow measures for the panoramic sliding sunroof ensure a high level of noise comfort.

The S-Class has green-tinted thermal insulation glass as standard. The windscreen is made of laminated safety glass with an integral acoustic membrane, while the side windows and rear window are of single-pane safety glass. Laminated safety glass all-round is available as an option. This insulates against heat and noise, and reflects infrared radiation.

[1] Best performance in SPORT+ driving mode