Mercedes-Benz eCitaro: Production of an all-electric city bus

Since the fall of 2018 the eCitaro has been rolling off the production line in Mannheim. Here you can find out how, with the help of a lot of machines and several robots, around 3,500 employees produce an all-electric city bus from up to 30,000 parts within five weeks.

When you find yourself in the 111-year-old bus production facility of Daimler AG in Mannheim, there are two things that will immediately catch your eye: the Mercedes-Benz Citaro and the oldest production hall, a historical monument where the Citaro has been manufactured for more than 22 years. If you take a close look, you will also discover a new family member at the venerable facility: the Mercedes-Benz eCitaro.

Since the fall of 2018, this all-electric city bus has been manufactured on the same assembly lines as the Citaro, which has already proven its worth thousands of times, thanks to its low-emission combustion, hybrid, and natural-gas-drive engines. As a result, the eCitaro is seamlessly integrated into the facility’s production operations. But what does that mean exactly?

The bodyshell: Processing with high-precision laser cutters

As you enter the brick hall that was built back in 1910, you will immediately notice several steel frames that employees are skillfully maneuvering through the building. With the help of high-precision laser cutters, around 1,100 employees have turned steel profiles that were delivered to the plant into the frame of the unitized body of a city bus. In the body shop, only experts can distinguish the various drive types from one another, laypeople simply see the skeletal frame of a bus.

Corrosion protection and coating for the eCitaro

However, before the bus is fitted with the various components, its frame is dunked into a 16-meter long, four-meter wide, and six-meter deep pool of anti-corrosion fluid. Over the next 15 minutes or so, the frame is turned about in this pool several times so that all areas get covered by this important protective layer. After getting dried at 90 degrees Celsius, the layer is “baked” into the frame at 220 degrees Celsius. The seams are then sealed and the floor installed and glued down.

After that, a floor coating is applied to the underbody. Unlike all other Citaro buses, the eCitaro is not sent to Ulm because it is produced in the offline process from the very start, which means that the vehicle stays in Mannheim throughout. An intermediate step is currently still needed so that the bodies are identical when they enter the next stage of the manufacturing process.

Immersion of the body in the immersion bath

High flexibility in assembly

The assembly process begins in the next section of the hall. In the brightly illuminated hall, the employees work with great concentration as they screw and glue the various parts together. A special feature here is that all of the Citaros, no matter whether they are equipped with fuel-efficient diesel, electric, hybrid or natural gas drives, move along the same assembly line and are put through the same production processes.

The manufacturing system has to be very flexible as a result. Because only the work steps differ, the employees’ tasks are diverse and complex and their qualifications are correspondingly high. To ensure that the workers have the parts for every bus at hand as quickly as possible, the components for each bus and work step are individually prepared in advance and put on “set wagons” ready for immediate use.

Mounting the roof unit carrier

As the eCitaro moves from station to station in the historic Building 34, it grows into an all-electric city bus. Components are constantly being fitted from all sides. While the air conditioning and the rear rack containing the cooling system for the batteries are being attached to the roof from above, orange high-voltage lines are brought into the bus and installed inside. At the following stations, the eCitaro is given its side windows, the floor is installed in the interior, and the first components are attached under the ceiling.

Main assembly: electrics, interior and batteries for the eCitaro

The production process continues in the main assembly stage, which takes place in the adjacent building. Here the Citaros travel on two parallel assembly lines. Like at the other stations, the employees work on top of, around, and inside the vehicle. Compressed-air reservoirs as well as ducts for the heating and air conditioning systems are installed.

During this stage, the city bus also receives its various cables and electrical systems, thus providing it, in a sense, with its “spinal chord” and intelligence. No matter whether a vehicle is an eCitaro or a Citaro, hundreds of kilograms of cables are rolled out in each of the city buses for a diverse range of functions such as USB ports and lamps.

Mounting the battery pack on the roof

As assembly continues, the tasks differ from those of the usual production process. In addition to installing the front axle, the workers attach the electric drive axle with the wheel hub motors. At one particular station where the normal Citaro is fitted with a combustion engine, the eCitaro receives an assembly of four battery packs that is installed at the back of the vehicle on the left-hand side. All of this work is performed on the same production line on which the same employees have just build a hybrid or a natural gas bus using the same machines.

The final step is the interior fitting of the city bus. During this stage, the vehicle is equipped with an interior ceiling, air shafts, side wall trim for the passenger compartment, the cockpit, and other components. The seats for the driver and passengers, which are manufactured a floor higher up, are also installed at this point, as are the grab bars and the partitions. The doors and the windshield are added in the final step.

Connecting the HV battery and high-voltage commissioning

Commissioning of the high-voltage system

The eCitaro is then rolled to the next hall. Here, where the engine of a Citaro is started for the first time, the eCitaro still needs help, because although the battery modules have already been installed on the roof, the high-voltage on-board electrical systems aren’t commissioned until later. These are assembled in advance at the Competence Center for Emission-free Mobility (CEM).

The CEM builds prototypes of low-emission and emission-free vehicles of all kinds. It then produces small batches of these vehicles and later prepares the launch of series production at the various plants. The roof-mounted batteries are fitted. Synergies are exploited here as well, because the machine that hoists up the batteries is also used to lift the elements for the articulated buses. The batteries are then connected to the cooling system, which is filled right after the seals are checked.

Commissioning of the high-voltage system

The end of the production process is now in sight. The high-voltage electrical system and its insulation are then tested and put into operation in a closed-off area that no other Citaro gets sight of. In these tests, the employees check every cable and its resistances. Only especially qualified personnel have access to this area when high-voltage systems are being commissioned.

It goes without saying that all of the assembly workers are trained to handle high-voltage systems, for which they have completed a basic high-voltage awareness course. The vehicle is immediately ready to drive because the batteries are delivered pre-charged. At this point, the eCitaro is also given its characteristic raised roof edges around the battery modules.

Final tests before the journey begins

The eCitaro is then brought back into the normal production process. The eCitaro also has to undergo extensive testing, ranging from test drives on the plant site and on public roads to rain tests. During the entire production process, employees at the quality gates meticulously inspect the quality of the work done up to that point. Although quality is checked on an ongoing basis throughout the manufacturing process, each vehicle is once again thoroughly tested in the finishing hall.

Under bright lights, the employees inspect the paint job to see if there are any scratches, irregularities or other faults. High quality standards have to be met before a bus is handed over to the customer, who will use it to transport passengers quietly, effectively, and without producing any local emissions. After the tests are completed, smaller tasks are performed, such as the application of pictograms, which every transportation company can individually specify.

Beginning with the production of the bus frame, it takes about five weeks to manufacture an eCitaro. Although the eCitaro is not in operation yet, it has already traveled several kilometers. Many more will follow — at least 150 kilometers after each charging process if it is equipped with 10 batteries. The eCitaro is certainly fit for the future because the roof-mounted battery modules are easy to replace so that they can be adapted to future technological developments.

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Johannes is an intern in corporate communications at Daimler Buses. Here he is involved in the complete range of topics - from social intranet communication and the website to storytelling projects and external press communication. He devotes most of his spare time to music - as a drummer.