Mercedes-Benz eCitaro: This is how the electric bus is created

Tradition and future: Mannheim plant is the perfect embodiment of both. Opened in 1908, in October 2019 it will be 111 years old. This is where modern engines for trucks and buses are created, and at the same time the bodyshells for all Mercedes-Benz and Setra buses. But above all, Mannheim is home to the large family of the Mercedes-Benz Citaro city bus - and thus also of the fully electric eCitaro.

The eCitaro fits seamlessly into the production and is created on the same lines as the Citaro with an internal combustion engine which has proved itself thousands of times. Around 3500 employees work on bus production in Mannheim – with the support of many machines and some robots they produce a city bus from up to 30,000 parts.

Bodyshell: the frame of the eCitaro is created from profiles and sheet metal panels

In Building 45 of the Mannheim plant some 1100 employees manufacture the frame of the monocoque city bus from supplied steel profiles in two shifts. This is where the profiles and sheet metal panels are firstly elongated and bevelled, and steel plates are cut out. Laser-cutting machines work with the utmost precision and a cutting capacity of up to 5000 W, which is precisely adapted to the respective sheet thickness. Bending robots get the sheets into shape, sometimes with several folded edges in differing degrees.

The devices in which entire side walls, roof frames and the substructure are manufactured are very impressive. Experienced employees place the fully prepared square tubes in them by hand, and some of the substructure segments come from the partner plant in Holysov. An even more impressive feature is the "dome" in Building 135 for assembling these components, including the front and rear section. In each case seven employees weld the substructure, side walls, roof, and front and rear section together to form a complete load-bearing bus frame. Special parts such as holders and sheet metal panels are then installed.

Cathodic dip painting: immersion bath as reliable protection against corrosion

The crucial factor in corrosion prevention, cathodic dip painting, follows. After a cleaning procedure and phosphating the entire bus frame is dipped into a 16-metre-long, four-metre-wide and six-metre-deep immersion tank. This is filled with 420,000 litres of liquid. The frame is swivelled around in it several times for nearly a quarter of an hour to enable the important protective coating to reach every part of the frame. To ensure that all the profiles on the inside are also optimally protected they have around 1000 long boreholes. A small proportion of these boreholes are later used in assembly as fastening points for detachable parts. The comprehensive cathodic dip coating is 22 thousandths of a millimetre, the specialist term for this is 22 mµ. After drying at 90 degrees Celsius the coating is encrusted at 220 degrees Celsius. Following application of the seam seal, underbody sealing and assembly of the first components the eCitaro goes on its first journey, as do all the other buses, by train for painting at the Neu-Ulm plant.

Cathodic dip priming

Installation: high-tech city bus from the tradition-steeped building of Benz & Cie.

On their return to Mannheim, having been panelled and painted, the body of the eCitaro is cleaned and dried. Then assembly of the eCitaro starts in Building 34 of the plant. Here, and in the other assembly and finishing shops, around 1000 employees work in mainly single-shift operation.

The eCitaro is now growing into a fully electrically driven city bus, station by station. At an elevated station, components such as the climate control system and the rear rack with the battery cooling system are installed on the roof. High-voltage cables which are orange-coloured or covered in protective orange sheathing are brought in and laid. The pipe work is then carried out on the roof panels. At the subsequent stations the eCitaro's side windows are then fitted. Following further pipe work in the rear area the floor is then fitted to the interior and the first components are fitted beneath the ceiling.

Main assembly: now the eCitaro is running on its own wheels

Building 30 right next door is where the main assembly work is done. Here the Citaro vehicles are carried on two parallel lines. Depending on the equipment, this enables different work scopes to be balanced out. Everyone is always hard at work: at the front, in the centre, and at the rear employees are carrying out their tasks simultaneously. Now compressed-air reservoirs are installed, as are the channels for the heating and climate control. The electrics and cables are then installed in the city bus. The central point here is the cross channel between the cockpit and passenger compartment. Whether it's the eCitaro or Citaro: in every city bus several 100 kilograms of cables are laid for the many different functions and lamps. One of the characteristic steps for the eCitaro is the installation of the cooler at the rear left in the former engine tower for the drive system and the auxiliary assemblies.

There then comes a decisive station: assembly of the e-drive axle with the wheel hub motors. A major advantage of the axle: the vehicle lift support points are identical to those of the usual hub reduction gear axle on the Citaro with a combustion engine. The front axle is also assembled. Whilst the combustion engine is fitted in the conventional Citaro, the eCitaro receives an assembly consisting of four battery packs in the identical place at the rear left.

The interior fittings for the city bus follow: the ceiling, air ducts, the side wall trim in the passenger compartment, the cockpit and many other components. The driver's seat and the passenger seats are installed, as are the grab bars and partitions. The doors and, finally, the windscreen are added.

Finishing shop: battery assembly and commissioning of high-voltage technology

Once again the eCitaro moves to a different hall, this time Building 32. In the meantime the eCitaro's roof-mounted battery modules – like the battery assembly in the tail end – have been preassembled within the plant site at the KEM, the Centre of Competence for Emission-Free Mobility. The roof batteries are fitted first in Building 32 and connected to the cooling system, which is filled directly after a leak test.

After commissioning of its 24 V system the end of production is in sight and for the first time the eCitaro deviates from the usual process. In a cordoned-off area the high-voltage system including insulation is checked and put into operation. It goes without saying that the employees are specially trained in dealing with high-voltage technology. As the batteries are delivered in a precharged state the vehicle is drivable straight away. The eCitaro is now also given its distinctive roof-edge ridge, and its batteries' charging is also checked at a rapid charging station.

Thorough test drive on the test track and on public roads

The eCitaro is then channelled back into the familiar process. Now is the time for minor work such as the application of pictograms or for adding final touches of trim. There are also intensive tests on every single bus. During production the quality is constantly inspected: to do this the fitters use tablets containing checklists of much more than the standard procedures; additional points of awareness can also be included at any time. At what are known as the quality gates, employees from Quality Control also scrutinise the work carried out. On every Citaro a drive on the brake test stand is obligatory, as is the rain test and the thorough final inspection.

Every eCitaro is driven both on a test track within the plant and also on an external stretch of about 50 km on public roads, where it is again thoroughly checked by independent Quality Assurance.

Only then is it ready for handover to the respective transport company. This takes place either directly on the company's site or at the nearest Bus World Home, on request also accompanied by driver training or training at the customer's company depot. Around six weeks have passed since its production began with processing of the square tubes for the bus frame.

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