Mercedes-Benz Aerodynamics: Emotion meets efficiency
For almost three decades, aerodynamic specialists at Mercedes-Benz have been breaking one record after another. "At present the brand's models occupy the top position for aerodynamics in practically every vehicle segment", says Prof. Dr. Thomas Weber, who is responsible for Group Research and Mercedes-Benz Cars Development. In its BlueEFFICIENCY Edition, the new CLA-Class has even established a new world record with a Cd figure of 0.22 and wind resistance x A of 0.49 sq. m. Weber: "This means that the CLA-Class is more streamlined than any other vehicle. Maintaining aerodynamic leadership is a major component of our MBC 2020 product strategy, which we are following systematically for all new vehicle models."
When it comes to compact cars like the new A-Class (2012, Cd = 0.26), coupés such as the E-Class Coupé (2010, Cd = 0.24), saloons such as the E-Class (2009, Cd = 0.25), sports cars like the SL (2012, Cd = 0.27) and SUVs like the M-Class (2011, Cd = 0.32), vehicles in these segments have never before achieved such low Cd figures. The new S-Class is set to continue this trend.
"Highly sophisticated improvements to both the overall vehicle and detailed features contribute to this good performance," says Dr. Teddy Woll, Head of aerodynamics/wind tunnels. Emotion meets efficiency: the more aerodynamically efficient a vehicle is, the lower its fuel consumption. Woll: "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." Moreover, safety, comfort and the environment also benefit from the elimination of air turbulence. Because low levels of lift ensure good roadholding, while low wind noise is welcome to both passengers and pedestrians.
Models from Mercedes-Benz also take the lead in almost all segments when it comes to aeroacoustics. The new CLA will have the lowest level of wind noise in its segment.
New aeroacoustics wind tunnel: measurements up to 265 km/h
With the "large wind tunnel" in Stuttgart-Untertürkheim, Mercedes-Benz was the first automobile manufacturer to possess a wind tunnel – the first documented measurement was carried out there exactly 70 years ago, on 5 February 1943. With the new aeroacoustic wind tunnel at the development centre in Sindelfingen, the company has once again placed itself at the forefront of aerodynamic testing. The new wind tunnel, which will be taken into operation in mid-2013, follows the Göttingen design. This means that after the measuring stretch the air is directed back to the blower and again accelerated to up to 265 km/h. Before the air accelerated by the blower reaches the measuring stretch via a nozzle system covering 28 sq. m., it must be directed and smoothed to eliminate unwanted turbulence and eddies. This done using rectifiers and sieves. Extensive noise insulation measures are integrated to allow use as an acoustic tunnel where interior and exterior wind noise can be measured for the relevant test vehicle. Even at 140 km/h the air flowing through the measuring stretch is therefore as quiet as a whisper.
The centrepiece of the 19-metre long measuring stretch in the wind tunnel is the roughly 90-tonne conveyor belt/balance system with a turntable. The new wind tunnel has a 5-belt system to simulate the road. The conveyor belt/balance system is integrated into a turntable with a diameter of twelve metres. This means that the vehicles to be measured can also be subjected to an angled airstream to simulate cross-winds. The traversing system enables the engineers to position various aerodynamic sensors and microphones around the test vehicle with very high precision.
II. Much ado about the wind: Aerodynamics world champion in almost all vehicle classes
For almost three decades, aerodynamic specialists at Mercedes-Benz have been breaking one record after another. At present the brand's models occupy the top position for aerodynamics in practically every vehicle segment. In its BlueEFFICIENCY Edition, the new CLA-Class has even established a new world record with a Cd figure of 0.22 and wind resistance x A of 0.49 sq. m. Highly sophisticated improvements to both the overall vehicle and detailed features contribute to this good performance. The coming new S-Class will likewise be at the top of its segment for aerodynamic efficiency. Mercedes-Benz models also take the lead in almost all vehicle classes where aeroacoustics are concerned.
1984 saw the debut of cable TV in Germany, the West runway at Frankfurt airport was taken into operation and the Olympic Games were held in Los Angeles. In the same year the W124-series E-Class scored an aerodynamic triumph with a Cd figure of 0.29. It was the first series production saloon to achieve a Cd figure below 0.30, setting a standard by which all saloon cars are judged to this day – and which by no means all can equal.
Since then Mercedes-Benz has established one aerodynamic world record after another – across all vehicle classes and body configurations. The following factors are the secret behind this continued leadership:
- Good basic dimensional concept and optimised external shape
- Aerodynamically optimised underbody with extensive engine compartment and underbody panelling
- Large area of cladding on the rear axle
- Aerodynamically optimised rear silencer followed by a diffuser
- Wheel arches with optimised spoilers and other features such as slits
- Large front wheel overhangs with optimised airflow onto and over the front wheel arches
- Aerodynamically efficient design of the front and rear aprons and the rear roof edge
- Hub covers or specially developed aero light-alloy wheels with low air resistance
- Aerodynamically optimised exterior mirror housings
- Low A-pillar shoulder with adapted A-pillar geometry
- Spoiler lip on each tail light
- Improved sealing of the radiator section and airflow guidance to make efficient use of the available cooling air
- Use of a cooling air metering system (adjustable louvre)
- Sealed headlamp surrounds
One very recent underbody feature is the patented wheel spoiler already mentioned, which was developed with the help of airflow analysis. This seemingly innocuous component reduces undesirable air turbulence in the wheel arches in no less than three ways, by lowering linear turbulence, deflecting the airflow from the wheels and stabilising shear waves with its serrated edge. The wheel spoiler remains effective whatever the suspension level and wheel dimensions.
It was first employed for the B-Class (2011), which with its help achieved a new top Cd figure of 0.26 for series production cars with an enclosed rear. Tail-end turbulence is also considerably reduced. With a basic shape defined according to aerodynamic aspects, the large roof spoiler acts as an airflow breakaway, as do the tail lights. A total of 278,000 CPU-hours of computer time and 1100 hours in the wind tunnel went into the aerodynamic optimisation of the B-Class.
The patented wheel spoiler is also used in the CLA-Class (2013), which 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, which is due to be launched in June, will even better this benchmark with a Cd figure of 0.22.
When it comes to compact cars like the new A-Class (2012, Cd = 0.26), coupés such as the E-Class Coupé (2010, Cd = 0.24), saloons such as the E-Class (2009, Cd = 0.25), sports cars like the SL (2012, Cd = 0.27) and SUVs like the M-Class (2011, Cd = 0.32), vehicles in these segments have never before achieved such low Cd figures. The new S-Class is set to continue this trend.
An overview of the key aerodynamic figures for the entire Mercedes-Benz passenger car range:
More driving comfort thanks to less wind noise: aeroacoustics
Models from Mercedes-Benz also take the lead in almost all segments when it comes to aeroacoustics. The new CLA will have the lowest level of wind noise in its segment. Numerous measures contribute to this peak position:
- Frameless doors with a multi-stage sealing concept
- The high insulating effect of the side windows
- Stemmed exterior mirrors with an optimised shapefamiliar from the C, E and S-Class
Exterior noise is also minimised by the reduced height of the A-pillar shoulder.
Thanks to its acoustic soft top fitted as standard, when closed the E-Class Cabriolet has one of the quietest interiors in the segment for four-seater premium convertibles with a fabric roof. The soft top's exceptionally high-quality insulation brings about a clearly noticeable reduction in the interior noise level compared to conventional fabric soft tops. Exterior noise from other vehicles, as well as wind noise, is more efficiently absorbed.
Contribution to active safety: anti-soiling measures
Anti-soiling measures are another aerodynamic discipline – and another area in which aerodynamic specialists at Mercedes-Benz have led the field for many years. Soiling can arise from rain, vehicles travelling ahead and spray thrown up by the vehicle's own wheels. Keeping windows and exterior mirror lenses as clean as possible, and therefore having the best possible visibility under all conditions, makes a contribution to active safety.
In the wind tunnel the aerodynamic specialists optimise components with the help of a fluorescent liquid which makes the soiling clearly visible. The aim is to direct water away so that the side windows and exterior mirror lenses remain clean. This is influenced by the geometry of the A-pillar with its integral components and the geometry of the exterior mirrors and window frames, or trim strips in the case of frameless doors.
Using the side window as an example, minor geometrical modifications to the mirror housing and detailed refinements using seals and a special water-retaining strip enable soiling to be considerably reduced. The standard at Mercedes-Benz is that in the so-called core visibility area, no spray, no trickles and only individual water drops must appear on the mirror lens.
III. Aerodynamics: The basis for efficiency and comfort
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. Because 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 (model series W124) 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 expresses it: "If we are able to reduce the Cd figure by ten thousandths, fuel consumption across the customer average falls by one tenth of a litre, and at very fast motorway speeds by up to 0.4 litres per 100 kilometres. To achieve the same saving with lightweight construction methods, we would have to reduce the weight of our cars 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 even betters this benchmark 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, its cross-sectional area facing the airflow. 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 depicted and used to calculate the individual segments of the entire area. Nowadays the frontal area is measured 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 sq. 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, and especially at higher speeds. This is because the air flowing around the vehicle body can have the undesirable ability 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 do not change 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 especially applies to cars with frameless side windows such as the CLA, but also to other Mercedes-Benz coupés.
Sealing is only the second step, however, The increasing focus of development work is on identifying and eliminating the sources of wind noise, e.g. at the exterior mirrors, or where the bonnet meets the windscreen, and the latter 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, sporty E-Class Coupé, 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 airflow so 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 to ascertain the path taken by the soiling at various speeds. To avoid inflicting soiling tests on the highly sensitive measuring technology 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 in 2004 was the debut of AIRSCARF in the SLK. This patented neck-level heating system causes warmed air to circulate 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 for the E-Class Cabriolet, which can be had with the automatic draught-stop AIRCAP. This can be activated at the push of a button in order to reduce interior turbulence significantly in the open-top four-seater. AIRCAP consists of two components: a wind deflector that can be extended by six centimetres with a net in the roof frame, and a likewise extending draught-stop between the rear seats.
IV. The new aeroacoustic wind tunnel: New tool in the battle against wind resistance and wind noise
With the more than 70 year-old wind tunnel in Stuttgart-Untertürkheim, Mercedes-Benz was the first automobile manufacturer to possess a wind tunnel. With the new aeroacoustic wind tunnel at the development centre in Sindelfingen, the company has once again placed itself at the forefront of aerodynamic testing.
As a measure of the company's confidence in the future, the go-ahead for the construction of the new Mercedes-Benz aeroacoustic wind tunnel was given in 2008 - in the middle of the most severe economic crisis for decades. This means that from mid-2013, Mercedes-Benz developers will for the first time have the right facilities directly at the passenger car development centre in Sindelfingen. Joining the climate tunnel taken into operation two years ago and the new driving simulator centre, this rounds off the new test facilities at the Mercedes-Benz Technology Centre and strengthens the development potential of Mercedes-Benz in Germany.
Blower: wind speed of 265 km/h, 5 MW output
The new wind tunnel follows the Göttingen design, i.e. the air is redirected to the blower after the measuring stretch and re-accelerated, which saves a great deal of energy. The blower has a diameter of nine metres and has 18 vanes which set the air into motion. At 202,150 Nm the maximum torque of the electric drive motor is approx. 1000 times that of a powerful automobile. At a wind speed of 250 km/h the power consumption is five megawatts. The blower is then rotating at 238 rpm, and the volume of air passing through it is 2000 cu. m. or around three family houses per second. The maximum wind speed is 265 km/h.
The air temperature in the wind tunnel is maintained at 23 to 24°C. To ensure that measurements are precise even in wintry outside temperatures, the concrete shell of the tunnel is surrounded by an outside wall and therefore insulated. During operation, the drive motor of the blower heats up the air circulating within the wind tunnel. In warm outside temperatures it is therefore cooled by a heat exchanger located downstream of the blower.
Before the air accelerated by the blower reaches the measuring stretch via a nozzle system covering 28 sq. m., it must be directed and smoothed to eliminate unwanted turbulence and eddies. This done using rectifiers and sieves.
Extensive noise insulation measures are integrated to allow use as an acoustic tunnel where interior and exterior wind noise can be measured for the relevant test vehicle. Even at 140 km/h the air flowing through the measuring stretch is therefore as quiet as a whisper
Measuring stretch: five conveyor belts up to 265 km/h
The centrepiece of the 19-metre long measuring stretch in the wind tunnel is the roughly 90-tonne conveyor belt/balance system with a turntable. The new wind tunnel has a 5-belt system to simulate the road: a small conveyor belt runs under each wheel, and a central belt with a length of nine metres and a width of over one metre runs between the wheels. All five belts are synchronised with the wind, so that up to 265 km/h they exactly simulate the road conditions. The 24-tonne balance on which the vehicles are fixed in place is extremely sensitive, with a weighing precision of a few grams. Even the measuring cables must be routed so that they do not introduce extraneous forces into the system. The values obtained with the help of the aerodynamic balance are used as the basis for calculating the coefficients of wind resistance, lateral forces and lift at each axle, and pitching, rolling and yawing movements.
The conveyor belt/balance system is integrated into a turntable with a diameter of twelve metres. This also enables the test vehicles to be subjected to an angled airflow to simulate cross-winds. The turntable also allows vehicles in the test stretch to be exchanged rapidly. The test vehicles are prepared in workshops within the wind tunnel building, immediately adjacent to the measuring stretch. Great attention was paid to a rapid exchange of test vehicles, so that the wind tunnel can be used as effectively as possible.
Traversing system: precise measurements even at 265 km/h
The traversing system enables the engineers to position a variety of aerodynamic sensors and microphones around the test object with a high degree of precision, so that pressure, acoustic and speed measurements are exact. The system in the new Sindelfingen wind tunnel has seven axes (three translation [parallel movement] and four rotation axes), enabling it to cover a measuring volume of 19 x 14 x 5 metres. The weight of this system is 26 tonnes, as the measuring sensors must remain exactly in place with no vibrations even at the maximum wind speed.
In addition to advanced computer simulation programmes, Mercedes-Benz now has all the facilities required to further consolidate its leading position in aerodynamic efficiency and further improve the acoustic comfort of its vehicles. Dr. Teddy Woll, Head of the Aerodynamics/Wind Tunnels department at Daimler AG, explains: "Computers and wind tunnels are outstanding tools that complement each other perfectly: using numerical flow calculations we are able to examine very complex airflow phenomena and follow even the smallest turbulence back to its source. And in the wind tunnel we are able to test a large number of variations very rapidly – which means that one day in the wind tunnel can often lead to major improvements both outside and inside the vehicle."
V. Interview: Dr. Teddy Woll, head of aerodynamics - "We fight for every third decimal place"
Dr. Teddy Woll, 50, has headed the Aerodynamics/Wind Tunnels department at Daimler AG since April 1999. We spoke to this doctor of industrial engineering about efficiency improvements and airflow simulation.
Dr. Woll, for years Mercedes-Benz has been establishing new aerodynamic records with practically every new model series. Does further progress become increasingly difficult as Cd figures get smaller?
You are absolutely right. It is quite true that we are gradually approaching an asymptotic limit if we do not dramatically change the appearance of our cars, e.g. by making them much longer and smoother, and equipping them with slim rear ends and narrow wheels. Fortunately, however, we can still find details where improvements are possible - both in the wind tunnel and increasingly often by computer. Moreover, aerodynamic optimisation of our vehicles is one of the core strategies in the "Real Life Efficiency" philosophy of Mercedes-Benz. The importance attached to our work within the conflicting aims in car development is therefore correspondingly high throughout the company, and our proposals are correspondingly often incorporated into our vehicles. Nonetheless, we fight for every third decimal place in our daily work.
You have just mentioned a key term, 'Real Life Efficiency’. How do Mercedes drivers benefit from this battle for the third decimal place?
It's a very simple matter: if we are able to reduce the Cd figure by ten thousandths, fuel consumption across the customer average falls by one tenth of a litre, and at very fast motorway speeds by up to 0.4 litres per 100 kilometres. To achieve the same saving with lightweight construction methods, we would have to reduce the weight of our cars by at least 35 kilograms.
In what areas of the car have you achieved the greatest progress in recent years?
There are three areas: the airflow through the engine compartment, the airflow around and within the front wheels and the underbody. In the front wheel arches of the new compact models alone, for example, we have been able to improve by eleven thousandths thanks to the combined effect of serrated wheel spoilers, slits in the wheel arches and optimised aero-wheels. But we take a close look at every nook and cranny of the vehicle: from the front apron with all its edges and apertures to the small spoiler lips in the lenses of the tail lights, to perfect the breakaway of the airflow at the rear.
What technological developments help you with your aerodynamic improvements?
In recent years we have made great progress thanks to the digitisation of the development process, as calculations are very much faster and much more refined. Digital vehicle models nowadays have over 50 million cells, and if necessary they can already produce results overnight. Ten years ago, the same simulation would have taken 6 months and would not have been half as precise.
In the mid-term, will numerical airflow simulation replace the classic wind tunnel with its smoke trail?
No, computers and wind tunnels are outstanding tools that complement each other perfectly: using numerical flow calculations we are able to examine very complex airflow phenomena and follow even the smallest turbulence (e.g. in the wheel arch as mentioned above) back to its source, and try to eliminate it. In the wind tunnel we are then able to try out a large number of variations which are measured down to half a thousandth. Which means that one day in the wind tunnel can often lead to major improvements both outside and inside the vehicle.
What exactly is a numerical airflow simulation?
This has meanwhile become almost as precise as a wind tunnel. Where wind resistance is concerned, we have less than one percent error compared to the results from the wind tunnel, though the error is marginally greater for aerodynamic lift depending on vehicle type. We hope to improve our precision even further by the improved modelling of turbulence.
How far advanced is simulation for aeroacoustics?
Wind noises are a highly complex phenomenon: apart from the airflow, the noise source and noise transmission must be adequately represented, and this is a major challenge in view of the different levels and wavelengths of turbulence and sound. This is compounded by noise transmission to the driver's ears via a wide variety of materials such as sheet metal, glass, rubber and plastics, which makes it extremely difficult to model the entire frequency range. I expect that it will be years before we are able to simulate wind noises reasonably well by computer. We hope that the new aeroacoustic wind tunnel with its new high-tech equipment will enable us to understand such complex inter-relationships even better.
An aerodynamics engineer must nowadays have a perfect grasp of computer tools. Are there other core qualifications?
Yes. Naturally experience still plays an important role. In addition, "soft skills" such as a talent for communication and cooperation are very important indeed. Because when it comes to conflicting aims, e.g. with the designers or other functional areas, the best solution is reached by working together, not against each other.
VI. Under the microscope: development in aerodynamics - From the Kamm rear to the CLA
Aerodynamics first became a focus of scientific research almost 100 years ago – but it is only after the second oil crisis some 30 years ago that it was really given a high priority in vehicle development. Nowadays aerodynamics make a major contribution to the energy efficiency of passenger cars.
The first passenger cars were not only derived from the horse-drawn coach, they were not in the least concerned with wind resistance because of the low speeds which were possible. Even the first "real" cars marketed by Daimler under the Mercedes brand from 1901 presented all manner of resistance to the rush of oncoming wind. The Mercedes Simplex of 1902, for example, not only had a frontal area of around 3 sq. m. – its Cd figure of 1.05 also meant that the wind encountered almost ten times the resistance offered by a modern passenger car.
Shortly after the First World War, specialists inspired by advances in the world of aviation began to examine the aerodynamics of automobiles. In 1921 the aircraft designer Eduard Rumpler (1872 – 1940) presented his "teardrop car", whose slim bodywork not only addressed the problem of frontal area (2,4 sq. m.), but whose teardrop shape broke new ground in minimising air turbulence both at the front and especially behind the rear. The result looked unusual, but was a highly significant advance with its Cd figure of 0.28 and the resulting wind resistance of 0.67 sq. m.
Paul Jaray (1889 – 1974), the other "father of streamlining", also came from the world of aviation. In 1921 he registered a patent which still reads like a list of instructions for building a modern passenger car body: "The lower half of the vehicle body takes the form of a streamlined construction covering the chassis with the wheels, engine compartment and passenger compartment. Its underside is smooth and runs parallel with the ground. This main structure carries a considerably slimmer streamlined construction supported by a lattice-like frame likewise mounted on the chassis." For the first time the wheels were no longer exposed, but were integrated into the bodywork, while the "fastback" rear end minimised turbulence in this area. As conventional drive technology could be incorporated into Jaray's body form, a number of car manufacturers built vehicles following the same principles – including Mercedes-Benz, which built a corresponding prototype in 1935. In series production Tatra had a measure of success with streamlining, starting with the Model 87 produced from 1936 to 1950. The VW Beetle also had a streamlined appearance, though with no effect: with a Cd figure of 0.49 it crawled into the wind rather laboriously.
The great disadvantage of Jaray’s streamlining was the long, tapering rear end – the longer, the more aerodynamically efficient. This "dead" space was an obstacle to practical implementation, which is why Tatra placed the engine there. In the 1930s the solution was found by Wunibald Kamm (1893 - 1966), the first professor of automobile engineering at the Technical University of Stuttgart and founder of the private, non profit-making research institute for automobile engineering and vehicle engines (FKFS) in Stuttgart in 1930. Kamm abruptly cut off the streamlined rear end and developed the "K-car" between 1938 and 1941, a prototype for an aerodynamically innovative passenger car. The term "Kamm rear" is still in use to denote an airflow breakaway edge at the rear. The K3 car was based on a Mercedes-Benz 170 V, and excelled with a frontal area of 2.1 sq. m. and a Cd figure of 0.23, as measured in the model wind tunnel at the time.
In the 1950s, increasing affluence and falling fuel prices caused efforts to reduce aerodynamic drag to fade into the background, with performance achieved by large-capacity engines. While the classic, large tailfin sedans of this era used a feature familiar from aircraft engineering, this was only for decoration: with Cd figures of around 0.60 and large frontal areas, they were roughly as streamlined as Elvis Presley's villa Graceland.
It was only the second oil crisis in 1980 that drew the industry's attention back to minimising fuel consumption, and to an effective means of achieving this: lowering the wind resistance. Audi achieved an initial success with the 100 in 1982 (Cd = 0.30), in 1984 Mercedes-Benz took the continuing lead for saloon cars with the W124-series E-Class (Cd = 0.29), and in 1991 Opel's Calibra showed what is possible for a coupé (Cd = 0.26).
Despite unfavourable parameters (e.g. increasing tyre widths and the greater cooling requirement of powerful engines) there has been a downward trend in wind resistance since then. Especially at Mercedes-Benz, which now leads the industry in practically all vehicle segments with respect to wind resistance and the other aerodynamic disciplines (see the chapter "Aerodynamics world champion in every vehicle class").
A milestone: the "large wind tunnel" in Stuttgart-Untertürkheim
The first documented measurement took place exactly 70 years ago, on 5 February 1943: Daimler AG's "large wind tunnel" at its parent plant in Stuttgart-Untertürkheim was the first in the world to be designed specifically for analysing the aerodynamic properties of motor vehicles. The building work began in 1939, inspired by the legendary aerodynamics pioneer Wunibald Kamm.
Because of the war, it was not until 1954 that the wind tunnel became the first in the world to be regularly used for measurements on full-size passenger cars. Since then it has played a key role in developing the aerodynamic efficiency of the car – especially models bearing the Mercedes star. But not exclusively: up until the 1970s, the wind tunnel was operated by FKFS, an independent institute, and was therefore available for general research as well as being open to other manufacturers. Daimler, the current owner, was one of the most frequent hirers – much as at the new FKFS wind tunnel on the university campus in Stuttgart-Vaihingen, which went into operation in 1988 and will be thoroughly overhauled in 2014.
Yet the wind tunnel in Untertürkheim, which has been repeatedly technically updated, is still indispensable for the Mercedes-Benz developers. Not just for optimising aerodynamic design, but also for soiling analyses or windscreen wiper testing. Furthermore, the "large wind tunnel" more than lives up to its name, for it is here that Mercedes-Benz commercial vehicles are also honed to perfection.
As well as hosting these tests, the facility is also often used for completely unrelated activities: ZDF, Germany's second TV channel, has shot film sequences for a hurricane report here; bobsleighs are optimised here; and speed skaters perfect their technique here. In short, anyone who has to work with or against the wind is welcome in Untertürkheim. Another major challenge overcome here was the aerodynamic testing of the revolutionary roof for Munich's Olympic Stadium.
VII. Glossary: Terms and innovations from A to Z
Aeroacoustics: This science is concerned with the origins and transmission of aerodynamically generated sounds and their reduction.
Aerodynamics: this science deals with all processes that occur when air flows around and through a vehicle.
Aerodynamic efficiency: As speed increases, wind resistance becomes an increasing factor as a proportion of total resistances: if the Cd figure can be reduced by ten thousandths, fuel consumption across the customer average falls by one tenth of a litre, and at very fast motorway speeds by up to 0.4 litres per 100 kilometres.
Aeroacoustic wind tunnel: This not only allows the aerodynamic influences on driving dynamics to be measured (wind resistance and lift), but also wind noise in and around the vehicle.
AIRCAP: This automatic draught-stop can be activated at the touch of a button in order to reduce interior turbulence significantly in the interior of the Mercedes-Benz E-Class Cabriolet. AIRCAP consists of two components: a wind deflector that can be extended by six centimetres with a net in the roof frame, and a draught-stop between the rear seats.
AIRSCARF: This unique neck-level heating system is integrated into the backrests of the front seats and blows warm air from outlets in the head restraints. In the E-Class Cabriolet the outlet nozzle can be pivoted upwards and downwards by 36 degrees.
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 even betters this benchmark with a Cd figure of 0.22.
Finlets: These distinctive side spoilers next to the rear window of the A-Class help to create a run-on design which significantly reduces the disruptive longitudinal vortices generated at the D-pillars.
Serrated wheel spoilers: The A-Class, B-Class and new CLA-Class feature serrated wheel spoilers at the front and rear for which a patent has been registered. In combination with slits in the wheel arches and optimised hub caps, flow losses at the front wheel arches have been reduced substantially. This improves the airflow around the side of the wheel arch.
Climatic wind tunnel: 18 months ago Mercedes-Benz inaugurated two new climatic wind tunnels at its Sindelfingen site, designed to bring extreme weather conditions indoors. Temperatures ranging from minus 40 to plus 60 degrees Celsius, hurricanes with wind speeds of up to 265 km/h, tropical downpours and heavy snowstorms are all part of the standard repertoire available to the test engineers. If needs be, they can even provide a realistic simulation of a mercilessly hot sun. The new climatic wind tunnels make it possible for the engineers to optimise new vehicles or components for all possible weather conditions at a very early stage of the development process.
Wind resistance: Proportionally the product of the drag coefficient (Cd figure) and the frontal area of a vehicle, indicated in m² and increasing as the square of the road speed. Unit of measurement indicating how efficiently a vehicle passes through the air.
Frontal area: this cross-sectional area of a body in flow direction is a key factor when measuring a car's wind resistance, alongside the Cd figure. 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 depicted and used to calculate the individual segments of the entire area. Nowadays the frontal area is scanned using laser light barriers.
Airflow simulation (CFD, Computational Fluid Dynamics): Every component of a new car model is improved in several control loops using airflow simulation before it is allowed to verify its aerodynamic properties with measurements in the wind tunnel. The numerical flow simulation is based on so-called finite volumes as a mathematical model. A volume network is formed of around 60 million hexahedrons or cubes. This incorporates the CAD data of the new vehicle model. In this way, driving on the open road can be simulated in a large virtual wind tunnel.
Adjustable radiator shutter: This feature improves cooling air resistance by allowing infinite adjustment of the airflow through the engine's cooling module and the engine compartment by means of a circular louvred shutter installed in parallel with the fan. When there is no special need for cooling air, the shutter is closed. This improves the aerodynamics and reduces fuel consumption.
Credits: Daimler AG
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