Testing: Endurance tests around the globe: on the road to series production

By the time of the market launch, prototypes and pre-series versions of the EQC (combined power consumption: 22.2 kWh/100 km; combined CO2 emissions: 0 g/km, provisional data)1 will have covered several million kilometres during testing on four continents (Europe, North America, Asia and Africa). The test program comprises more than 500 individual tests. Like all Mercedes‑Benz vehicles, the EQC must absolve the demanding standard test programme. There are also special tests for the electric powertrain, the battery, and the interplay of all powertrain components. In the process the test experts are able to build upon the extensive findings from their digital testing work, used to ensure both the buildability of the vehicle and for the simulation of e.g. crash behaviour, aerodynamics and NVH: noise, vibration, harshness. In addition there are intensive tests on numerous test rigs at the Mercedes-Benz Technology Centre in Sindelfingen. The ratio between digital and real testing is roughly 35 to 65 percent.

Testing of the EQC began long before the first prototypes were ready to drive – in the computer. One special feature affecting the handling characteristics is the low centre of gravity thanks to the location of the battery in the underfloor area. As early as the planning phase, and on the basis of available design and functional data, simulations were carried out to determine the vehicle dynamics and handling safety.

Stability at high speed, ride comfort, yaw and self-steering behaviour when cornering (understeer or oversteer) and braking stability were already predicted very precisely in this early development phase. In addition a real ESP® (Electronic Stability Program) system was connected to a simulation model. This allowed very precise predictions of behaviour with respect to vehicle dynamics and handling safety.

[1] Figures for power consumption and CO2 emissions are provisional and were determined by the German Technical Service corporation. The range figures are also provisional. EC type approval and conformity certification with official figures are not yet available. There may be differences between the stated figures and the official figures.

Digital testing in 3D

Assemblies from the future vehicle were three-dimensionally projected in the Virtual Reality Center (VRC) on the basis of design and surface data – even entire vehicles can be photo-realistically represented here. The projections are jointly viewed by the design and test engineers, and assessed as if in a real vehicle according to standardised test stages in what amounts to a “stationary test drive”. In this way it is possible to assess around 30 percent of the over 1000 standardised test stages in the digital vehicle as if a real road test was being conducted. The assessments include e.g. the opening angle of the tailgate, the swept area of the windscreen wipers or the controlled dripping of water when opening a door after a rain shower.

Many details relating to the acoustics can also be evaluated in advance. The test engineer views the dimensions, thickness, size, installation location and material of the respective insulation components, and approval is given after a positive assessment together with the design engineer. Another example is testing and assessment of potential chafing: in this case the test engineer views the routing of flexible components such as hoses and wiring harnesses at a very early stage to identify potentially critical areas. The crash behaviour, aerodynamics and even the handling dynamics are also verified at an early stage by simulations: Assessments can be made on the basis of the steering, the geometrical layout of axle components and the wheel control. The basic situation is this: the more detailed the data preparation, the more aspects can be assessed more precisely.

From computer screen to dynamometer ...

The next step was component testing on numerous test rigs and facilities. For example, the suspension engineers used the driving simulator at the Mercedes‑Benz Technology Center (MTC) in Sindelfingen to test and verify the comfort and handling characteristics of the EQC long before driveable prototypes became available.

The simulator for ride comfort (suspension comfort) was e.g. used to subjectively experience and assess the calculatory comfort performance of the EQC at an early stage. This means that the real testing commenced from a considerably better starting level. As a result, the engineers in the later development phase had more time for fine-tuning work.

More than 70 configurations possible: Crash-testing in the TFS

The safety of the EQC was validated at the Mercedes-Benz Technology Centre for Vehicle Safety (TFS), which opened in November 2016 as the world's most cutting-edge crash test centre. The special features of the centre include a crash facility measuring 90 x 90 m which has no supporting pillars. This obstacle-free area makes it possible to test e.g. pre-crash systems in the pre-accident phase, and to conduct vehicle-to-vehicle crash tests. The flexible and efficient crash track concept allows complex testing in around 70 different crash test configurations.

The new crash tests now possible include the following:

  • Accidents on road junctions (both vehicles are moving and collide at selectable angles and speeds, with one vehicle sustaining a side impact).
  • Car2Car compatibility crash tests.
  • Driving manoeuvres for realistic representation of the pre-accident phase enable new assistance and PRE-SAFE® systems to be tested.
  • Car2Car testing with angle of impact from 0°-180°.

In addition to the Mercedes-Benz specific roof drop test, new rollover tests are also possible in which the likewise new inclined ramp is used. In this test the vehicle is accelerated on a 2 m high sled and fed onto the inclined ramp (ramp length approx. 20 m). The ramp angle can be adjusted from 25° to 50°. The rollover tests are used e.g. to verify the airbag deployment logic and validate the automatic emergency call systems.

The decisive factor for the EQC was that all the safety precautions were taken in the Technology Centre to be able to test prototypes with large electric batteries under hard crash conditions at a very early development stage. This is because vehicle safety not only includes assessment of the effects on occupants during a crash, but also the crashworthiness of the battery. During the course of this development, a number of measures described in the section on “Passive safety” were taken.

The battery components were tested in parallel at the development centre of Deutsche ACCUMOTIVE, a wholly-owned Daimler subsidiary. Test criteria included the battery’s behaviour under impact and when penetrated by foreign bodies, with overheating and overloading also tested.

Details of the high-voltage safety concept can likewise be found in the section on “Passive safety”.

Thunder and lightning: The storm takes place in the laboratory

In the climatic wind tunnels in the immediate vicinity of the TFS, the EQC was not only aerodynamically fine-tuned - its body, drive system and air conditioning were also tested under laboratory conditions in the most extreme weather. 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. Is the body weatherproof? How long does it take iced-up windows to thaw? Does the air conditioning system also keep the windows clear in extreme air humidity? Can the doors also be opened in very low temperatures? These are just some of the many questions that are investigated.

… and from the dynamometer to the road

After the first prototypes were roadworthy, the summer and winter road testing of the EQC began. Test drives were carried out on numerous sites in Arjeplog (Sweden), near Barcelona (Spain), in Phoenix (USA), in Peking, Heihe and Chengdu (China) and in Boxberg and Papenburg (Germany). Even before its official opening, tests are also being carried out on the new Daimler test site in Immendingen (Germany) this year. The road tests took the test teams all over Europe, e.g. to the Sierra Nevada (Spain) and the low mountain ranges of the Swabian Alb and the Black Forest in Germany.

The test program comprised more than 500 individual tests, which in addition to the standard test regimen for all vehicles also included special tests for the electric powertrain, the battery and the interplay of all powertrain components. The special endurance tests also include the so-called taxi-test, which represents urban driving with frequent idle times, and the high-mileage driver cycle where the focus is on high daily mileages.

Hot on ice: Winter trials

The company has been testing each new model under extreme conditions near the Arctic Circle for decades – in icy temperatures down to minus 35 degrees Celsius, on snow-covered roads and on sheer ice on frozen lakes. To this end, Mercedes-Benz has set up a test centre in the small northern Swedish town of Arjeplog in Lapland. In addition to the road testing in the far north, tests take place on specially set-up testing tracks here. Demanding hill climbs with gradients of up to 20 percent, test tracks with varying coefficients of friction, handling courses, and skid pads on the almost sheer ice of the frozen lake pose the stiffest of challenges for the powertrain and control systems.

New challenges for an electric vehicle include the output of the electric motor during cold starting and with a cold battery, the vehicle’s range under the customer’s normal operating conditions, handling of charging cables in icy temperatures, pre-entry climate control - in this case heating the interior before starting off - and the operating strategy complete with recuperation. In view of the dynamic torque distribution from the two electric motors to the front and rear axles, another factor is special configuration of the vehicle dynamics and the ESP® systems. Is control of the two electric motors fast and precise enough? Does recuperation interfere with ESP® control? Does the brake pedal also give reliable feedback in combination with recuperation? Do the steering, brakes and all-wheel drive respond as intended on surfaces with different levels of friction on the left and right (µ-split)?

In the heat of the south: Summer trials

Some of the summer trials for the overall vehicle took place in Spain - in intense sunshine and at temperatures up to 50° Celsius.

The greatest challenge for an electric car is dry heat. Because while the battery of an electric car “only” delivers less power in the cold, it can possibly sustain damage in the heat. The extreme tests in Spain were designed to help prevent this.

The coolant circuit of the battery was therefore tested, for example: How does it cope with high power requirements? How does an almost fully charged battery respond to further charging? What influence does the heat have on operating range? Discharging tests – i.e. test runs in which the battery was completely emptied - were also part of the programme. Other aspects included cooling down the interior and the noise characteristics of individual components in the heat. Other questions such as “Is the time allowed for pre-entry climate control sufficient?”, “Does the air conditioner compressor drone under full load?” and “Is the estimate of range correct?” were also answered.

Sand and dust were the second major challenge during the trials in Spain. Because the test technicians wanted to know where this dust might be deposited in the components, and whether the sealing concept works in practice.

Another topic on the agenda during all driving trials in other countries was national differences: some details of the road infrastructure are also different in Spain. A few examples: Are the slightly differently designed traffic signs recognised? Are toll stations reliably recognised? Are speed limits correctly detected by the camera of the assistance systems? All of these questions were also examined by the developers during their busy test weeks in Spain.