Mercedes Benz

Fine-tuning the dynamic handling control systems is one of the major challenges when developing an SUV. Compared with conventional passenger cars, this vehicle category has to reconcile an even wider range of requirements: active safety, ride comfort and driving pleasure not only need to be guaranteed on-road but also off-road, on rudimentary tracks, unsurfaced roads or in demanding terrain. The systems also need to work reliably under all possible (and impossible!) meteorological conditions experienced in the world's many different climate zones.

In view of this, the test engineers select the world's most gruelling onditions to test SUVs like the GLK up to their physical limits, thus ensuring that the subsequent series-production vehicles will operate reliably for many years, regardless of operating and environmental conditions.

Development testing therefore takes place under punishing desert conditions in the baking Namib Desert, while the sub-zero conditions of the Arctic Circle provide the opposite extreme to measure and test the GLK prototypes. An ongoing comparison between "Fire & Ice" is designed to ensure that the various findings do not have a reciprocal negative influence. The same principle of harmonisation is applied when resolving other conflicting aims during the GLK fine-tuning process, as illustrated by the "Climb & Speed" development programme: if the chassis tuning sessions on the racetrack show the GLK to be directionally stable and safe at high speed, while the vehicle delivers the hallmark Mercedes ride comfort on torturous bone-rattling stretches, the question is then whether the set-up can produce the desired results when clambering through off-road terrain. Overall the developers clocked up some 4.5 million test kilometres worldwide. The field tests using prototypes are supplemented by extensive measurement programmes on a wide range of test rigs and the detailed simulation programmes conducted as part of the Digital Prototype. In the course of the development phase, the experts tested around 1000 different vehicle variants in virtual simulations and conducted over 220,000 driving manoeuvres in the process.

While the fine-tuning of control systems such as ESP®, 4ETS, ASR, ABS is based on objective measurement criteria using state-of-the-art technology, the finesse and experience of the test engineers continue to play a decisive role. For instance, the developers in the MTC managed to optimise the control quality and interaction of the individual systems in such a way as to substantially improve powertrain performance. At the start of the development test drives, the individual control systems still had a reciprocal effect on one another in specific driving situations that resulted in part of the drive torque going to waste, for instance on demanding off-road terrain. After completing the tuning sessions, the systems were harmoniously networked to such an advanced degree that the wheels had another 40 Newton metres or so of traction-enhancing torque at their disposal.

During the "Fire & Ice" development testing – as the fine-tuning drives staged in desert conditions in southern Africa and in the wintry wastes of the Arctic Circle have come to be known – two interviews were conducted which give a special insight into the development work performed by the engineers.

First is Thomas Merker, Head of Development for the Mercedes-Benz M-, GL- and GLK-Class, speaking during the heat trials in the Namib Desert:

Within the context of our global development process, Namibia covers a portion of the challenges we wish to overcome with our GLK. And the prevailing conditions found in the Namib Desert especially are precisely what we want for our torture programme under extreme desert conditions. On the one hand, there are the extreme temperatures, with the air heating up to 50 degrees Celsius and surface temperatures that can even reach 80 degrees. On top of that you have extremely fine sand and the notorious corrugated tracks through hostile rocky terrain. A vehicle that can withstand the conditions here is ready for any desert in the world. Naturally, the findings made in the Namib are always considered in the context of our test results under other operating conditions. Any enhancements must also satisfy completely contradictory requirements, of course, meaning they must also work in the bitter cold of the Arctic, the humid heat of the tropics, as well as in routine day-to-day operation in Central Europe.

For one thing, we inspect the thermal stability of the vehicle as a whole, and check for instance whether the AGILITY CONTROL suspension or the electronic control systems continue to operate flawlessly when exposed to the extreme environmental and track conditions. We also conduct "sand testing", an endurance test specially designed for the GLK.

Compared to the G-, GL- and M-Class, the GLK is certainly the most road-oriented model series within the Mercedes-Benz SUV and off-roader line-up. Yet we still attach great importance to the vehicle's performance abilities off the beaten track, and even under the most extreme conditions. That's part of the SUV philosophy at Mercedes-Benz: regardless of the fact that only a few customers venture off-road, we must still prepare the vehicle for this eventuality and guarantee a certain level of off-road performance.

Plus, of course, we also need to think of the car markets in countries where conditions are no less out of the ordinary, such as Russia, for example, or the Middle East states. There are also the requirements of the world's developing automotive markets, such as India or China, to bear in mind. At the end of the day, our customers in Europe also stand to benefit from these developments, as there is no difference between the models delivered to a whole array of markets.

We must of course ensure that optimisation of a particular parameter to safeguard resilience to high ambient temperatures does not take place at the expense of the vehicle's performance capabilities when faced with Arctic conditions. To take an example: if we were to optimise the engine's cooling air supply solely with a view to operation in extreme heat, problems would occur during the warm-up phase at low outside temperatures, and vice versa. Painstakingly detailed work needs to be carried out in order to cover all bases. And this applies to virtually all areas of development. Does the heating respond fast at minus 30 degrees, does the THERMOTRONIC quickly cool down the interior rapidly when the sun is at its most intense and temperatures are around plus 45 degrees. Or the fuel supply to the engine: does diesel fuel continue to flow freely at extremely low temperatures, is the formation of vapour bubbles in petrol engines prevented at extremely high temperatures? Do the filter systems for the engine and passenger compartment operate reliably? Do all of the different sealing systems perform their tasks efficiently?

As incredible as it may sound, there are indeed parallels. The loads exerted on the vehicle when driving over dunes or through deep sand, for example, are certainly similar to the demands experienced when driving in deep snow or up Alpine passes. Powder snow and desert dust likewise behave in a similar fashion. We are experience this for ourselves when driving in a long line of traffic: in winter when the road is covered in snow, you find yourself driving through a fine cloud of snow when following a vehicle in front; in summer on a dried-out track the effect is the same, but the cloud is of dust instead. In both cases, we have to prevent the fine particles from being drawn into the vehicle, where they could clog up the filter systems of the engine or passenger compartment ventilation, for instance.

In the second interview, Wolfgang Keller, who is Head of Overall Vehicle Testing for the Mercedes-Benz M-, GL- and GLK-Class, gives his thoughts on the specific test demands experienced under Arctic conditions:

Just like the fine-tuning drives in the Namib, the test drives near the Arctic Circle form part of our approval procedure prior to the start-up of series production. In both instances, the test vehicles are the same, meaning we can be sure that the future customer vehicles will meet the stringent Mercedes requirements. It is true that very few of our customers will be driving their GLK both in the Namib and near the Arctic Circle; but anyone who makes extensive use of their GLK – for example, for skiing in the Alps in the winter then a beach holiday in Spain in the summer – will come close to experiencing the conditions we simulate.

Naturally we inspect all vehicle components back in the early stages of development, both in special test vehicles and on test rigs, as well as by conducting simulations as part of our digital prototyping. Yet, as the development process draws to an end and all assemblies are performing at their optimum, in line with our specifications, there comes a point at which the vehicle needs to be precision tuned. This brings the system as a whole into perfect harmony and ensures that it operates reliably and to the best of its abilities under all conditions. There is a clear comparison to be made with the making of a grand piano: here again, the individual components are developed with utmost care and manufactured with absolute precision. Following its assembly, however, minutely detailed work is required to tune the instrument and produce an optimum sound. And that is precisely what we seek to give our customers – the best possible "sound"!

In Namibia, there is a low but relatively constant level of grip, both on the sand tracks and in the dunes. The idea here is to distribute the drive or braking torque evenly between all four wheels, thereby achieving maximum traction combined with high handling stability. Things are far tougher at the Arctic Circle though. Here, we are faced with the problem of tremendous fluctuations in friction: you can have more powdery or more compacted snow, sheet ice and areas of clear tarmac, all within the space of just a few metres. Handling stability and, consequently, driving safety must be absolutely guaranteed, both when accelerating or braking and when cornering. Propulsion is of secondary importance in this case.

When driving in wintry road conditions, it is especially important that the ESP, ASR and 4ETS control systems act not just effectively, but smoothly too. Smooth, harmonious intervention is most important from the point of view of driver-fitness safety. Hectic, inharmonious interventions by the control systems may assist the driver, but they would also distract his attention from what's happening around and prove wearying in the long run. We signal critical driving states by means of the ESP warning lamp, which prompts the driver to drive more cautiously.

The latest-generation 4MATIC system boasts the ideal credentials for driving in wintry conditions. The friction-optimised construction means we have substantially less frictional losses in the powertrain than other systems where the engines are installed transversely, for instance. The basic split of the drive torque enhances traction and agility. The basic locking torque in the transfer case furthermore enables the GLK to pull away with ease, even on surfaces completely covered in ice.