It’s early March and bitterly cold. The snowfields all around glisten in the morning sun. There is a yellow, low-floor articulated bus on the extensive car park of the Carezza mountain hotel in the South Tyrolean Alps. Here, at an altitude of 1,750 metres above sea level, the regular-service bus seems out of place. A passer-by stops and looks in surprise at the small white cloud slowly curling out of the roof structures.

It is actually water vapour, which condenses in the cold mountain air – an unmistakable sign that the fuel cell on the roof of the bus is at work. Electricity is generated from hydrogen and atmospheric oxygen – with water vapour as the only emission.

The articulated bus is one of four prototypes of the eCitaro fuel cell. The first electric bus from Mercedes-Benz in which a fuel cell system extends the range is on a trial run through the Alps.

Omnibus Magazine is on the road with a team of test engineers from the Mercedes plant in Mannheim. “Here in the Alps, we want to test the cold starting of the fuel cell and the functionality of the completely redesigned thermal management system in the eCitaro fuel cell in winter temperatures,” says test manager Jonas Steinki. At the same time, driving to heights beyond 1700 metres above sea level should provide information about the high-altitude functionality of the fuel cell system. Furthermore, the new drive system can prove its performance on demanding mountain passes with uphill and downhill gradients of up to 15 percent.

“During the drive, test engineers constantly monitor countless measuring points and data,” explains Steinki. Besides the temperatures of the battery, fuel cell, motors and passenger compartment, these include the energy consumption of the drive and the heating, but also other auxiliary consumers, such as the charge level indicator of the batteries or the fill level indicator of the hydrogen tanks. Test engineers Rainer Bickel, Stephan Lutz and Hannes Mayer permanently monitor all the important parameters on their screens, look for anomalies and compare the data with the calculated setpoint values.

The test lasting several days begins with crossing the Alps from Neu-Ulm via Füssen, the Fern pass and the Reschen pass to Bolzano. The combination of four battery packs with 98 kWh each and 30 kilograms of hydrogen on board was expected to be more than sufficient for the 350 kilometre route. Nevertheless, it is difficult to estimate the energy consumption in advance on the long steep passes at temperatures around freezing. The team therefore decided not to take any risk and, as a precaution, to recharge the batteries to a SoC (State of Charge) of 93.5 percent at Allgäuer Tor service station.

On the way across the Fern pass, one thing already becomes clear: the eCitaro fuel cell performs superbly on the demanding road. “Despite the gradient, the fuel cell is operating in the most efficient power range of 20 to 30 kW,” explains Rainer Bickel, pointing to the corresponding value on the monitor. "In addition, the new thermal management reuses the waste heat from the fuel cell to control the interior temperature. Electric heating is therefore hardly used, and the average energy consumption of all auxiliary units such as heating, steering and compressor together is very low."

But not only uphill, but also downhill, the eCitaro fuel cell excels with good performance data in the winter test. When test driver Andreas Hoffmann decelerates before the bends, the value of the recuperation on the control monitor increases to up to 280 kW. This means that: the four motors of the two drive axles now work as generators and charge the batteries with up to 280 kW – almost twice as much as a fast charging station. “That’s all it takes,” says Rainer Bickel. “In order not to put too much strain on the batteries, we have limited the recuperation output to 280 kW.”

With such a high recuperation power, it is not surprising that, after the 22 kilometre downhill stretch to Imst, the batteries have almost 12 percent more SoC than on the Fern pass. 78.5 percent charge after more than half of the total distance to Bolzano: an encouraging energy balance.

Arriving in Bolzano – after 368 kilometres and intermediate charging with about 75 kWh – the battery charge indicator still shows 56 percent. The hydrogen tanks are also at a great 42 percent fill level so that the eCitaro fuel cell could still have easily covered the 100 kilometres to Lake Garda.

It is not without reason that the test team chose Bolzano as the destination of the winter test drive. Bolzano has one of the best H2 infrastructures in Europe. Furthermore, the Dolomites lying around Bolzano offer numerous demanding mountain passes and high spots, which allow high-altitude testing of the fuel cell system and further tests at temperatures way below freezing point.

“However, winter testing is just one of several tests that the eCitaro fuel cell has to undergo before it enters into series production starting in the summer,” stresses Project Manager Javed Shahrukh. “In particular, the hydrogen tanks and the fuel cell system have successfully completed extensive safety tests, some of which go far beyond the tests required by law.” These include impact and vibration tests as well as sled tests with the fastening system, which simulate an accident.

For testing under hot conditions, the eCitaro fuel cell had previously been taken to the truck climate chamber of their Truck colleagues in Wörth. This is equipped with a roller dynamometer and wind turbine so as to simulate realistic driving simulations and test stress situations of the thermal management under extreme conditions.

However, Shahrukh has no doubt that driving under real conditions is still necessary despite the climate chamber with its roller dynamometer: “the many different measurement results and discoveries that we were able to obtain thanks to the high altitudes, steep routes and low temperatures on this trip are invaluable.” Especially when the trial vehicle passes the tests as successfully as the eCitaro fuel cell did.