Aerodynamics: The basis for efficiency and comfort

Sep 5, 2013
The more aerodynamically efficient a vehicle is, the lower its fuel consumption. Moreover, safety, comfort and the environment also benefit from the elimination of air turbulence, since low levels of lift ensure good roadholding, while low wind noise is welcome to both passengers and pedestrians. And open-top driving in comfort is a particular speciality of the aerodynamicists at Mercedes-Benz.
In 1984, the E-Class (W124 model series) achieved an aerodynamic landmark, posting a Cd figure of 0.29. It became, and remains, the benchmark against which all saloons must measure themselves – and one that very few manage to match. Design elements such as smooth surfaces, an inwards-drawn rear end and a clear spoiler lip on the boot lid remain at the heart of good aerodynamic design to this day.
Since then the aim has been to shave down the second number after the decimal point of the Cd figure. In the New European Driving Cycle (NEDC), improving the Cd figure by 0.01 already lowers CO2 emissions per km by one gram, by two grams as a function of mean on-the-road consumption, and at 150 km/h by no less than five grams of CO2 per kilometre. Or, as Dr Teddy Woll, Head of Aerodynamics at Mercedes-Benz, puts it: "If we succeed in lowering the Cd figure by ten thousandths, the fuel consumption drops by one tenth of a litre across the customer average, and by up to 0.4 litres per 100 kilometres at very high motorway speeds. To achieve the same savings effect with lightweight-construction measures, the weight of our cars would have to be reduced by at least 35 kilograms."
The focus is therefore on the dimensionless drag coefficient - the Cd figure: this is the measure of the aerodynamic efficiency of a solid body, and therefore of an automobile. The Mercedes-Benz CLA has the lowest of all Cd figures, at 0.23 – both within the Mercedes-Benz model portfolio and amongst all series production cars. The CLA 180 BlueEFFICIENCY Edition improves upon this benchmark even further, with a Cd figure of 0.22.
The Cd figure alone is not what decides the wind resistance. The second determining factor is the vehicle's frontal area, the cross-sectional area facing the air flow. In the past, the frontal area was calculated by projecting the shadow of the body onto a transparent screen using a lamp positioned some considerable distance away. The outline was then traced, and the overall area calculated on the basis of the individual segments. Nowadays the frontal area is scanned using laser light barriers.
But since cars are becoming increasingly wider and higher, e.g. for reasons of comfort, the hands of the aerodynamic specialists are more or less tied in this respect. The wind resistance as a measure of the efficiency with which a vehicle passes through the air is calculated as the product of the drag coefficient and the vehicle's frontal area. As the coefficient is dimensionless, the wind resistance is indicated in square metres.
In the case of the aerodynamics world champion, the CLA, this means that the drag area Cd x A is also the benchmark at 0.51 sq m, well below that of many subcompact cars. In its CLA 180 BlueEFFICIENCY Edition version it even betters this figure: at just 0.49 sq m the wind resistance of the four-door coupé breaks through a magic barrier.
No rising to the occasion: reducing aerodynamic lift
As all motor racing fans know, aerodynamics also have a major influence on driving characteristics, especially at higher speeds. This is because the air flowing around the vehicle body can have the undesirable tendency to generate lift. What makes aircraft able to rise into the air is understandably unwanted for automobiles. Aerodynamic optimisation therefore not only means reducing the wind resistance, but also generating as little lift as possible. The deciding factor is not only the absolute figures for the front and rear axles, but the achievement of the best possible harmonisation between these front and rear values. The driving characteristics at high speeds are not changed by addressing one factor alone.
Peace and quiet: acoustic optimisation right from the start
Wind noise is another discipline of aerodynamics. Key requirements for a low wind noise level in the interior include draughtproof door and window seals. This requirement applies especially to cars with frameless side windows.
But the seals are in fact only the second step. Increasing priority is given during the development phase to the search to identify and eliminate the sources of wind noise, for example around the exterior mirrors or at the transitional points where the bonnet meets the windscreen and the windscreen the roof.
Measuring tools such as dummy human heads and directional microphones enable even the slightest weak spots to be pinpointed. These can then be eliminated by implementing the best possible technical solutions. At a very early stage in the development of the new E-Class Coupé, for example, a three-metre concave acoustic mirror was used to optimise the exterior shape of the A-pillars and the shape of the exterior mirrors in the wind tunnel.
Maximum visibility even in the rain: aerodynamics keep the field of vision clear
The battle for clear visibility is another area of activity for the wind tunnel specialists. The aim is to direct the air flow in such a way that a clear view through the side windows and of the exterior mirrors is ensured even in murky weather conditions. During the relevant tests, fluorescent water droplets are made visible under UV lighting, making it possible to ascertain the path taken by the soiling at different speeds. To avoid inflicting soiling tests on the highly sensitive measuring equipment and moving belts in the new acoustic wind tunnel in Sindelfingen, these will continue to be conducted in the large wind tunnel at the plant in Stuttgart-Untertürkheim.
Special case: open-top driving in comfort
No manufacturer has such a long and unbroken tradition of vehicles with no fixed roof as Mercedes-Benz. Convertibles and roadsters have been part of the company's model range for very many years, and offer driving pleasure in its most emotionally appealing form. Nowadays however, customers want a choice of whether to feel the wind full in their faces or enjoy the fresh air with as little draught as possible. For this particular form of refined sportiness, Mercedes-Benz customers currently have a choice between four product lines: the SLK, SL, SLS AMG and E-Class Cabriolet.
With the introduction of the draught-stop as a world first in the SL model of 1989, Mercedes-Benz for the first time provided an aerodynamic remedy to the blast of air into the cockpit. The next step was the 2004 debut of AIRSCARF in the SLK. With this patented neck-level heating system, warmed air circulates around the head and neck areas of the occupants from the head restraints.
The most extensive package of comfort-enhancing aerodynamic measures has been available since 2010 with the E-Class Cabriolet, which is available with the automatic draught-stop AIRCAP. This can be activated at the push of a button in order to reduce significantly interior turbulence in the open-top four-seater. AIRCAP consists of two components: a wind deflector with a net, set into the roof frame, which can be extended by six centimetres, plus a similarly extendable draught-stop between the rear seats.