Good things come to those who wait: as far back as the early 1990s, when developing the cabriolet variant of the E-Class of that time (W124), the Mercedes aerodynamicists were looking for ways to reduce turbulence in the interior in order to give all passengers a true open-air experience without the all-too-annoying draught. The project was called KOF4 – from the German for ‘comfortable open-top driving for 4’. They tinkered for some two years before the project was finally shelved in 1992.
People today who see the pictures of the variants tested in the 1990s quickly understand why the company management gave KOF4 the thumbs down: the various draught-stop combinations, which could be mounted on the windscreen and behind the rear seats, looked too clumsy. As ever, the engineers had been very thorough at the start of the project: the basic principles were tested on an A124 prototype in the wind tunnel. The result of these tests is still valid to this day: the only way to achieve the desired effects is to combine a wind deflector at the front, including controlled airflow to elevate the flow, with a draught-stop behind the rear seats. Showing typical Swabian thoroughness, the engineers then tested numerous profiles, louvres and flap solutions to optimise the effect and minimise the amount of installation space required. Their conclusions: satisfactory functioning, but inadequate aesthetics and controllability. The technical design was not yet advanced enough and the components not yet small enough for KOF4 to go into series production.
Shelved but not forgotten: ten years later, in July 2003, the KOF4 project was resurrected for the new E-Class Cabriolet. A specifications book was drawn up, and initial test drives took place as early as November of that year with a louvred variant. The designers tested an active solution, deciding on a pivotable variant. But they were quick to realise that the result did not meet the Mercedes requirements for quality and design.
But Mercedes engineers are nothing if not persistent, and the project was revived again as early as October 2004. This time, they looked at a “snap-on” solution; however, this solution did not deliver the desired success in terms of easy handling. Detailed aerodynamic investigations in this project phase revealed that the KOF4 objectives were achievable: significantly enhanced comfort in the rear seats and no adverse effects for the front passengers when compared with the conventional bracketed draught-stop.
The fourth attempt, which began in late 2005, finally brought the breakthrough: a wind deflector which is retracted into the windscreen frame when not in use, complemented by an adjustable draught-stop between the rear head restraints. A great many design challenges still had to be overcome but, a good four years later and almost 20 years after the first idea, and after non-stop hard work characterised by dogged determination, KOF4 is now going into series production in the new E-Class Cabriolet. Now the roof made of air has a new name – AIRCAP® – to tie in with the name AIRSCARF® used for the neck-level heating system. Now the ‘cap and scarf’ are available as an option for four-seater cabriolets made by Mercedes.
Under the microscope: component and full-vehicle testing
Two summers and two winters in one year
Always on the road all over the world in the very latest cars – this exciting job description applies to test drivers as well as motoring journalists. Jochen Prokein from the Mercedes department responsible for full-vehicle testing is one of the latter and had already clocked up tens of thousands of miles in the new Mercedes-Benz E-Class Cabriolet long before its launch.
But before a new car undergoes full-vehicle testing, the individual components have already come through an exhaustive programme of tests of their own.
Take AIRCAP®, for example: in a special climatic chamber at temperatures between minus 25 and plus 80 degrees Celsius, the front wind deflector was retracted and extended 50,000 times and, on top of this, blasted with dust and water aimed specifically at the bearing points. In addition to this, the component took part in an extreme wind-load test involving wind speeds of between 60 and 250 km/h, in which it had to be activated more than 10,000 times to prove its wind-load resistance. For this test, the “small wind tunnel” in Sindelfingen, usually employed for developing the 1:4 models, had to be put to a different use. In special climatic chambers in which huge amounts of chemicals are admitted, the wind deflector was tested in its opened and closed states, during which time it had to remain resistant to corrosion for an entire vehicle lifetime. Further methods of torture involved carwashes as well as steam-blasting and icing tests.
Only once the individual components have come through this sequence of suffering can full-vehicle testing begin. Then the key factor becomes the interaction of all the components in the vehicle. “We are in effect the first customers,” says Prokein, who organised this part of the testing for the E-Class Cabriolet. “The old rule about a new vehicle having to experience two summers and two winters during testing essentially still applies,” explains Prokein. “The only difference now is that all this has to happen within a single year if possible.” This is why the testers work in two shifts, testing the prototypes on the road day and night.
And they do this all over the world: in Texas, the cars undergo “hot-country testing” in temperatures of up to 45 degrees Celsius at the Daimler proving ground. “This time, we were particularly keen to see how the AIRCAP® innovation fared in a combination of heat, high humidity and dust,” states Prokein. Naturally, the new draught-stop system also had to withstand these punishing conditions.
Testing also focussed on the standard-fit acoustic soft top, which is stowed in the soft top compartment in a different manner to a conventional soft top on account of its special design. As one would expect, the standard Mercedes test programme for open-top cars also includes frequent activation of the soft top as well as driving on torture tracks to test torsional stiffness.
Further destinations for the team testing the full vehicle included Lower Saxony, Italy and, for winter testing, northern Sweden. The ageing factor in these tests was approximately ten. In other words, 5000 kilometres on the notorious Belgian Block route at the proving ground in Texas are equivalent to around 50,000 kilometres of normal everyday driving, for example.
“We are always on the road with at least four vehicles, and we try to cover as many models in the range as possible,” explains Prokein, who was a designer prior to working in full-vehicle testing. As many different engines, equipment items and appointments as possible – from seats to cover fabrics – are used, since this approach allows the team to assess the interaction between literally all of the components. The test vehicles used by Prokein and his team are always state-of-the-art: whether it be modified components or the latest software updates, whatever is devised during the process of making a new car ready for series production in Sindelfingen goes straight into the cars used for full-vehicle testing.
Under the microscope: flow simulation
Sea of air Sindelfingen
Neither wellingtons nor a compass were required for this expedition as the Mercedes engineers actually discovered a sea of warm air in the wind tunnel. Such volumes of air can also be discovered by means of computer-aided flow simulation, although this requires a great deal of calculation work and aerodynamic know-how. For numeric flow simulations, Mercedes-Benz uses a finite-volume process as a mathematical model. Using the CAD data for the new Mercedes-Benz E-Class Cabriolet, Alexander Wäschle and his team constructed a closed surface network.
By modelling the vehicle environment using around 30 million hexahedrons (small cubes), the engineers were able to simulate driving across an open area in a large, virtual wind tunnel. They allowed the air to flow at a realistic speed and with realistic turbulence.
As the flow conditions change very quickly over time whilst driving, the calculations had to be made in a stationary state using many small time steps. This meant that a high processing capacity was required: around 500 processors in parallel performed calculations for some two weeks, allowing the Sindelfingen airflow specialists to analyse the sea of warm air in a virtual environment.
“If you turn things around, the system functions in a similar way, of course,” says AIRCAP® developer Peter Dannhäuser: “If the E-Class Cabriolet driver wants the air conditioning to cool the interior in the summer, AIRCAP® can help again by providing what is essentially a sea of cool air. All at the push of a button.”
Under the microscope: Tanja the test dummy
Half a woman
Tanja shivers. She has just had the most tiring of afternoons involving hours of tough tests. We meet the young lady near where she works: in the wind tunnel at the University of Stuttgart in the suburb of Vaihingen. Mercedes-Benz is a long-term tenant here. And Tanja is a regular. For she is the test dummy the Mercedes developers use to test new cabriolet models in the wind tunnel. And this can take time – hour after hour in severe storms. So it’s hardly surprising that she looks a little dishevelled. But she kindly agreed to be interviewed nonetheless.
– may we call you Tanja…?
Tanja: …of course, that’s what everyone calls me…
So, how did you come by this interesting job?
Tanja: My current employer discovered me in the store room at a boutique and recruited me immediately, marking the end of my career in the shop window. But I was glad to be out of it and get the opportunity to do something really useful. I am a Swabian, after all.
What happened next?
Tanja: The engineers gave me a sleeveless little black number and then set about accessorising my head (minus the usual wig, unfortunately, without which I feel so naked), my neck and my arms with a total of 16 speed sensors to measure the wind flow speed in the interior of the car.
And what happens with these measurements?
Tanja: The measurements are subsequently weighted, since a draught on the neck is far more annoying than a draught on the top of the head. The weighted measured values are then added together to give what is known as a “wellbeing factor”. This figure indicates how effectively people sitting in the cabriolet are protected against draughts.
If we may be so indiscreet, Tanja, you appear to be a just half a woman. Why is this?
Tanja: Well, perhaps that’s because I actually work as a man. Or, to be more exact, as two men: a 75-percent man at the front and a 50-percent man‑in the rear of the car.
Amazing. What are we to understand by this?
Tanja: It’s quite simple. At the front, the way I’m sat means that I’m taller than 75 percent of all men. At the rear, I’m as tall as the average man. But I’m lucky really: my colleague Walter doesn’t have a body at all.
This is the first we’ve heard of Walter.
Tanja: “Heard” is an apt word to use. Walter is a dummy head. They use him to measure wind noise in the cabriolets.
Back to you, Tanja. What happened in the tests?
Tanja: It was tough at the start. AIRCAP® was not yet perfect. And then they started moving the thing in and out, over and over again. There was plenty of noise. At the end of my shift, I was in a bit of a tizzy.
And now?
Tanja: A salvation. You would not believe what it feels like after all that turbulence in the sea of warm air. Even on the rear seats. Even if it does blow around your head a bit more there. Nevertheless, AIRCAP® is top of the list for me when it
comes to quality at the workplace. And the best thing of all is that, if it sometimes gets too warm, you can get a blast of cool air at the push of a button.
One last question: what sort of car would you choose away from work?
Tanja: To be brutally honest, a coupé. I get enough cabriolet driving as part of my job.
