Mercedes-Benz and Setra buses stand for the highest levels of safety, controllability and operation. Both Daimler Buses brands are long-standing pioneers in these areas. One important basis for this prominence is intensive and early testing of components, entire assemblies and complete buses under extreme conditions – in the annual winter tests in Arctic temperatures and on snow and ice in the Arctic Circle, for example.
The Arctic Circle: a Mecca for the Daimler Buses test team
Arvidsjaur and Arjeplog in Sweden, Rovaniemi in Finland – testing and development experts in the automotive industry are very familiar with these three towns in Lapland in the Arctic Circle. The area is a Mecca for extreme-testing teams. Every winter from January through to March, this particular area of the far north attracts engineers, mechanics and drivers from the testing department at Daimler Buses, which includes the Mercedes-Benz Bus and Coach unit and the Setra brand. The tortures that await in the bitter cold, ice and snow of Scandinavia’s cold season are just what the test team are looking for: Arvidsjaur, Arjeplog and Rovaniemi provide testing areas for the drive system, suspension, components and material like nowhere else.
Arvidsjaur and Arjeplog: bus tests on iced-over lakes
Winter actually starts in autumn at Arvidsjaur in northern Sweden and nearby Arjeplog: as early as October, the minimum daily temperature regularly drops below zero degrees Celsius, and this continues right through to May. On average, the weather station reports 186 frost days a year, with an annual average temperature of almost exactly zero degrees.
Heavy downfalls are likewise the norm – those looking for snow in the winter months have come to the right place. And once on the ground, the snow stays there: generally the daily highs do not start climbing above zero degrees again until April onwards.
The two towns on the 65th and 66th parallel are situated just below the Arctic Circle and have been important winter destinations for the Daimler Buses test team for many years. Here the main item on the agenda is the driving dynamics testing on the iced-over lakes at Arjeplog. The Electronic Stability Program (ESP), for example, has to prove itself under extreme conditions in skidpad tests on specially prepared icy surfaces.
Specialist local “icemakers” prepare bespoke driving surfaces for testing new buses and components: as soon as the lakes have frozen over, snow is regularly cleared from the ice surface to allow the ice to become extremely thick here. This clearing increases the ice’s load-bearing capacity so that vehicles can drive on it. There are roughened-up lanes, ice-slicked stretches and almost impassable icy tracks coated with a fine water mist – whatever the testers ask for and the test schedule demands. Perfect for test drives under constant conditions.
The water beneath the several-metre-thick ice layer on the lakes is over 200 metres deep. But there is no danger as the disciplined test drivers always stick to the prepared areas: the ice is approved for a weight of up to 40 tonnes.
Rovaniemi: tests at Santa Claus’ airport
Similar climatic conditions prevail some 500 km further to the east in the Finnish town of Rovaniemi. Located in Lapland, it is considered Santa Claus’ home town. There’s a Santa Claus Village here and even a Santa Claus Post Office. But this is not what regularly attracts the Daimler Buses test team to Rovaniemi every winter from January through to March: here on the 66th parallel, right on the edge of the Arctic Circle, the Daimler Buses testing department has found the ideal conditions to perform its gruelling winter programme.
At the airport complex, the test team uses its own test track with skid pan as well as brake manufacturer Wabco’s proving ground: an 800-metre-long straight braking track for tests on ice, snow and tarmac. It is also possible to combine different friction coefficients, such as a heated tarmac surface offering good grip on one side of the vehicle and sheer ice on the other.
Plus there is a circular “driving dynamics” skid pan with a diameter of 280 metres, not to mention a climbing hill and a handling course. In addition, the town of Rovaniemi, which has a population of around 60,000, gives the team the opportunity to test urban regular-service buses under extreme winter conditions in real traffic situations.
In-house target: perfect operation at temperatures down to minus 25 degrees Celsius
Mercedes-Benz and Setra buses are more capable than others. They offer greater safety, greater economy and greater comfort. Mercedes-Benz and Setra are not just pioneers when it comes to new safety and assistance systems, for example. These and other components are often able to far exceed the legal requirements – and far outstrip the performance of competitors. Active Brake Assist 3 and AEBS (Advanced Emergency Braking System) are already able to easily meet the legal requirements that come into force in November of this year. They even already comply with stricter regulations that are not due to become law until 2018.
Daimler Buses has ambitious in-house targets: correct functioning of all Mercedes-Benz or Setra bus components must be reliably guaranteed at outside temperatures down to minus 25 degrees Celsius. Some functions even have to work at temperatures down to minus 40 degrees Celsius. Accepted constraints include the fact that the central display in the cockpit is not ready to start immediately. Even the sequence of the essential functions is specified: first the engine, then all major components around the driver’s area, then the passenger compartment – safety and functional reliability are always top priorities in a Mercedes-Benz or Setra bus.
Six weeks of testing plus the journey there and back
The testing phase in the Arctic winter lasts around six weeks – from the end of January until mid-March. A further three weeks of preparation are required prior to this: measuring instruments are installed in the buses, test parts prepared, spare parts packed. The buses are also filled with ballast, the amount of which depends on the test objectives and procedures.
Even the journey to the Arctic Circle is part of the testing: around 3000 km lie between the testing department in Neu-Ulm and the destination in Arjeplog or Rovaniemi.
With the exception of a ferry trip across the Baltic Sea, the vehicles complete the entire journey under their own steam. The trip takes four days, during which time the heating and climate control is tested to the limit due to the huge fluctuations in temperature and humidity.
For touring coaches and rural-service buses, the route to the Arctic Circle and back is an intensive test in winter traffic on motorways and trunk roads. The town of Rovaniemi has a defined urban route for road testing of urban regular-service buses, plus there are defined rural-service routes for touring coaches and rural-service buses.
All basic models undergo intensive testing
Given the wide range of Mercedes-Benz and Setra models available, it is not possible to scrutinise each individual length, height, engine and transmission variant. The engineers therefore define representative basic models for the tests. They cover all models, such as two- or three-axle vehicles, solo buses and articulated buses. Weight distribution and vehicle height are likewise taken into account – a Setra TopClass 500 model, for instance, is a super-high-deck touring coach and naturally has completely different handling characteristics to a low-floor urban bus such as the Mercedes-Benz Citaro.
Focus: correct functioning in the cold, driving on snow and ice
The focal points of the annual winter tests are correct functioning in a cold environment and driving on low-friction surfaces, i.e. ice and snow.
Winter tests obviously cover the heating, ventilation and climate control for the driver’s and tour guide’s areas as well as the passenger compartment. Defrosting of the windscreen is likewise important, as is the correct functioning of the windscreen wipers and the leak-tightness of doors and flaps at extreme sub-zero temperatures. How easy is it to fit snow chains, are the automatic snow chains guaranteed to work on the driven axle?
Are air-operated and electrically operated doors sure to open and close even if there is a severe frost? Do the air lines remain leak-tight at temperatures down to minus 30 degrees? Can very fine snow penetrate through the intake opening and clog the air filter? Does the AdBlue preheating work? The list of functional tests is a long one.
Some key aspects only become apparent at second glance: in the case of the panelling, for example, how do different materials such as sheet steel and plastic behave together, since each has a different expansion coefficient, depending on temperature?How do the flexible adhesives on a side panel react at extreme sub-zero temperatures, when the steel frame remains rigid whereas a plastic panel contracts?
Tests are performed on the engine, the exhaust gas aftertreatment system, the transmission and all the drive system peripherals. Prior to the cold-starting test, the test vehicles are left with the engine flap open over the weekend so that the components cool right down. Even in the oil sump, the temperature then drops to minus 25 degrees or lower. And even the most experienced engineers are pleased when, under such conditions and at an outside temperature of minus 30 degrees Celsius, a heavy-duty diesel engine starts reliably after just a few seconds and runs smoothly straight away.
In the case of the chassis and suspension, the primary concern is ESP and its numerous subfunctions, the brakes and the steering. How does the power-steering pump operate at a temperature of minus 25 degrees Celsius, for example? How early is ESP supposed to intervene?
On the move on snow and ice with Mercedes-Benz and Setra
The results of the extensive tuning test drives on snow and ice are impressive. This can literally be experienced in reality in the Arctic Circle based on elements of the Omniplus driving safety training. Be it full brake application on sheer ice, different friction coefficients on the left and right (µ-split), driving in a circle at different speeds on ice, spectacular evasive manoeuvres with single or even double lane changes based on the model of the VDA obstacle avoidance test – the Mercedes-Benz and Setra buses reliably cope with even the most demanding of tasks and precarious situations. And, above all, the intensive tuning work ensures that the vehicle always reacts in the way the driver expects. This applies to all vehicles, from minibuses right up to super-high-deck touring coaches.
As proven by the agile Mercedes-Benz Sprinter Travel 65 minibus. It features the Sprinter’s suspension system, which is tuned with the emphasis on both safety and comfort. The compact Mercedes-Benz Tourismo K is not only amazingly manoeuvrable thanks to its short wheelbase, it is also reassuringly reliable in all driving situations thanks to its cleverly thought-out axle load distribution and precisely tuned safety and assistance systems.
The Mercedes-Benz Travego fully lives up to its billing as a “Safety Coach”, even on snow and ice: aided by all currently available safety and assistance systems, the 12.18-metre-long premium high-decker is an astounding performer, even on glass-like road surfaces. The same applies to the alluring Setra TopClass S 516 HDH super-high-decker – a three-axle vehicle that combines luxury and safety at the highest level, even in highly adverse road conditions.
Bus-specific tuning takes priority
Here the focus is on bus-specific fine tuning alongside general functioning. Directly transferring systems from top-selling trucks would be far too easy for Mercedes-Benz and Setra. After all, a bus not only handles very differently to a truck because of its size, weight, axle configuration and speed profile, but also on account of its “cargo”: passengers.
That’s why the maximum braking pressure provided by Active Brake Assist in Mercedes-Benz and Setra buses is applied gradually rather than instantly – to take account of any standing passengers. Fine-tuning of this kind requires a great deal of work: the bus testing team needed two winters to perfect the new AEBS (Advanced Emergency Brake System), for example. Including the summer tests performed in the period from spring through to autumn between these two winters, this amounts to one-and-a-half years of intensive testing.
The long and intensive testing route leading to Euro VI
The extensive and intensive development and testing work Daimler Buses performs with Mercedes-Benz and Setra is apparent in the key development steps leading up to the introduction of the new vehicle generation with Euro VI engines.
Intensive tests in icy temperatures were also necessary for the early and comprehensive introduction of Euro VI. After all, it is not just a new and highly sophisticated engine generation that is being used in the Mercedes-Benz and Setra buses. To maximise economy, an all-new power train layout and package was developed for Euro VI, both for the touring coaches and rural-service buses with raised floor and for the low-floor urban and rural-service buses.
This was one of the focal points of advance bus development from as early as 2004, a full ten years before Euro VI became a legal requirement. Two years later, the first buses of the current generation with Euro VI engines were already on the road in “test mule” guise. All-new Euro VI prototypes were ready for the off in the first half of 2008. Mercedes-Benz and Setra were the first brands to put these test buses on the road. Between 2007 and 2013, the new vehicle generation completed seven years of extensive winter and summer testing.
The end products of these tests are exceptionally well-engineered and economical solutions for transport operators and private companies. The Mercedes-Benz Citaro and Travego models – the first urban bus and touring coach to feature completely newly developed Euro VI engines – went into production at the same time. A total of around 60 prototypes, including 26 buses for endurance testing, underwent road testing in preparation for the changeover to Euro VI. Together they covered more than five million kilometres.These trials involved 42 test engineers, 33 mechanics and a large team of drivers, notably for the endurance tests.
Winter tests played an important role in this context. The newly designed engine compartments with their precisely calculated airflow were put to the test in sub-zero temperatures in Lapland. It was possible to test exhaust gas emission control on real urban regular-service routes with the critical mix of low load and extremely low temperatures.
Tested on snow and ice: the unique ATC Articulated Turntable Controller
Two of the vehicles that have lately had to prove themselves in the Scandinavian winter are the just recently unveiled Mercedes-Benz CapaCity L and the Citaro G. Here the focus was on testing the new and unique ATC (Articulated Turntable Controller).
ATC sets a new standard for handling and safety in articulated buses: the ATC dynamic control system works quickly and above all to the precise extent needed to regulate the hydraulic damping of the articulated joint as a function of the steering angle, articulation angle, speed and load. If the articulated bus becomes unstable, damping of the articulated joint is controlled quickly and on demand. Within the limits of physics, the articulated bus can thus be very quickly stabilised, so avoiding any see-sawing of the rear section or, in the worst case, the dreaded jackknifing effect. The new ATC system is thus the only system of its type to achieve anything like the effect of an electronic stability control system (ESP). This represents an even higher level of safety for articulated buses.
Prior to being introduced, ATC underwent intensive testing in the Arctic winter to optimise all parameters. The standardised conditions on snow and ice in Scandinavia were ideal for the tuning stage. Consequently, ATC reacts sensitively, quickly, precisely and predictably – so there are no nasty surprises for the driver of a Mercedes-Benz articulated bus in the event of an emergency.
The innovative ATC Articulated Turntable Controller in the Citaro G
Some of the most spectacular manoeuvres involve fast lane changes on slippery surfaces in the Mercedes-Benz Citaro G articulated urban bus. Thanks to the new, unique, dynamic ATC (Articulated Turntable Controller) system, the 18.13-metre-long bus also reliably masters this discipline. The same applies to full brake application on slippery and µ-split surfaces when travelling at 80 km/h – proof of the almost unflappable handling characteristics. In conclusion, the demonstration shows that there is no safer articulated bus than the Citaro G.
And no other articulated bus with one driven axle is more effective at getting out of tricky situations. This is underlined by a typical winter situation at a slightly blocked bus stop or a bus stop with snow from the road pushed up to its edges: the articulated bus is slightly articulated with the first and third axles on a road surface offering good grip while the centre axle is on the slippery surface of the bus stop bay. With conventional turntable control, the rear section would push the articulated bus via the centre axle when bending at the articulation joint, causing the bus to bend even further – but the Citaro G with variable turntable damping controlled by the innovative ATC system pulls safely and reliably out of the bus stop bay.
The sheer effort that has gone into precisely configuring ATC is highlighted in a further test with the new Mercedes-Benz CapaCity L. The large and imposing four-axle articulated bus measures an impressive 21.5 metres in length. Tests with the steering robot – a computer-controlled fully automatic steering system – show what effects changing individual ATC parameters has on the vehicle’s handling characteristics.
Source: Daimler Buses