Aerodynamics: Wind energy

Mar 13, 2014
Outstanding fuel consumption figures require great aerodynamic properties. The new C-Class has the lowest drag coefficient in its segment. To achieve this goal, the aerodynamics engineers have performed intensive detail work during computer-based air-flow simulations and optimisations in the wind tunnel. At the same time they have minimised wind noise.
From a speed of about 70 km/h, aerodynamic drag exceeds the sum total of all other driving resistance factors. Reducing aerodynamic drag pays dividends in the form of fuel savings and a reduction in CO2 emissions – especially at higher speeds such as on motorways. The metric for aerodynamics is the Cd value. A reduction of this value by 0.01 translates into one gramme less of CO2 per km in the driving cycle (NEDC), while in average real-world consumption conditions it can equate to two grammes, and at 150 km/h actually to as much as five grammes of CO2 per kilometre.
Mercedes-Benz is setting the pace in the field of aerodynamics and delivers cars with the best Cd values in almost all vehicle classes. The new C-Class also follows this tradition: the C 220 BlueTEC BlueEFFICIENCY Saloon has a drag coefficient of Cd = 0.24 and is thus the benchmark in the medium-size category.
Intensive detail work for Cd benchmark
The aerodynamics engineers achieved the top Cd value of the C-Class with an extensive bundle of measures. In concert with the designers, the intensive use of computer-based air-flow simulations allowed finding a basic aerodynamic shape already in the very early design phase, which was the prerequisite for further optimisation in the wind tunnel. The areas at the front of the vehicle where the air flow is redirected represent the biggest leverage for reducing drag. They were designed to prevent the air flow from separating from the vehicle. Especially important are the design of the sides of the front apron and a transition from the vehicle front to the bonnet without air-flow separation. The specific tuning of the form and position of the lower wing directs the air cleanly under the vehicle and has positive effects on the flow of cooling air and negative lift.
Only by successfully guiding the air around the vehicle front can the downstream measures unfold their full potential. The A-pillar was optimised in many details and ensures a flow around the vehicle with little turbulence. Together with the aerodynamically and aero-acoustically optimised mirrors it contributes to a reduction in drag and wind noise.
The wake behind the vehicle, which has a negative impact on total drag, was minimised with a uniform flow separation at the rear. Visible signs of the successful set-up are, in addition to the indentations of the rear, the spoiler modelled on the edge of the boot lid and the separation edges integrated into the tail lights. The multifunction recess is designed as a diffuser and its angle and position is precisely tailored to them. Unwanted air turbulence under the car is minimised by an almost completely smooth underbody with large panelling of the engine compartment and the main floor.
Specially shaped front and rear wheel spoilers help optimise the air flow around the wheels. To this end the front wheel arch lining has slits in its side, whose optimal effect was first determined on a computer and then tested in practice in the wind tunnel. Wheel arches sealed off from the engine compartment further enhance this effect on the diesel-powered models. The available aero wheels with diameters of up to 17 inches are an important component of the overall aerodynamics package.
The Mercedes aerodynamics engineers also paid great attention to sealing the front in order to minimise leak flow through the engine compartment which increases drag. Special sealing measures around the headlamps and the so-called aero lip, which seals the gap between bonnet and bumper, are invisible from the outside but have a positive effect on the drag coefficient. The radiator surround was also sealed and the air flow in this area was designed to make efficient use of the available cooling air.
Aerodynamics rendered active and visible - that applies to the AIRPANEL, which depending on the selected engine is standard equipment or optional extra on the EXCLUSIVE design and equipment line. It opens and closes the radiator grille depending on the cooling demand by means of moving louvres. On the BlueEFFICIENCY Edition models and the C 180 BlueTEC and C 200 BlueTEC models the flow of cooling air is controlled by the familiar annular shutters behind the radiator. With savings of 2 to 3 g of CO2/km both measures for controlling the cooling air are effective - and in the case of the AIRPANEL even visible - aerodynamics measures.
Clear visibility in any weather
Another important task of the aerodynamics engineers is to provide the driver with an unobstructed view of other road users and the surroundings in any weather. On the new C-Class Saloon aerodynamic fine tuning ensures that the exterior mirrors, side windows and rear windscreen remain largely unsoiled when driving in the rain. To this end the ingenious water deflector along the A‑pillar and the roof purposefully channels away the dirty water. Seals in the area of transition between the windscreen and the roof trimmed with millimetre precision prevent water from collecting and running onto the side window. This considerably reduces disturbing drops or rivulets when looking in the exterior mirror.
The exterior mirrors are also incorporated into the water management. The special shape of the mirror housing and the mirror base, the water drain channel and the spoiler edge at the bottom largely prevent troublesome dirty water from reaching the mirror glass and the side window. Another important result of the detail optimisations is that the rear window of the Saloon remains clean even at high speeds.
Wind noises are absorbed
Special attention during the aeroacoustic development was on further reducing wind noises caused by the air flowing around the vehicle and its detachable parts compared with the preceding model. This was so successful that the new C-Class occupies a leading position in its segment with regard to wind noise and even surpasses some of the premium saloons of higher vehicle classes in this discipline.
Many detail optimisations contribute to this result as well – such as on the main floor of the body shell and the new aluminium doors. They lead to a reduction in structurally induced, low-frequency wind noises. The high-frequency wind noise components are reduced by door and window seals whose principle was adopted from the E and S-Class.
Effective noise insulation also plays a major role in the extraordinarily quiet interior. Optionally available laminated safety glass for the front doors can lower wind noise even further.