Before production start-up: The Mercedes-Benz eCitaro fuel cell has undergone extensive testing and trials

• Safety and functionality on the test stand
• Sled test and vibration test
• Real driving tests and endurance tests in hot and cold temperatures
• Test drive in the Alps at temperatures far below zero

Daimler Buses leaves nothing to chance when it comes to safety, reliability and function. For this reason, not only do all new vehicle models and model generations undergo extensive test procedures and safety inspections, but so do new technologies and components. This is the only way to ensure that the bus is able to meet the high demands of everyday urban traffic over a long period of time – and in a wide variety of climatic conditions.

Safety and functionality on the test stand

Because the eCitaro fuel cell is largely based on the proven Citaro and the battery-electric eCitaro in terms of the chassis, steering, brakes and body, developers were able to dispense with the otherwise usual endurance testing as well as the rough road endurance test and a crash test before the start of series production in this context. However, the new components, such as the hydrogen system or the fuel cell, the entire thermal management and the energy control were subjected to even more intensive and extensive tests.

In particular, engineers not only applied the currently valid standards for the safety-relevant tests. Rather, for example, the certification of the hydrogen tanks already took into account the UN ECE-R 134 standard that will come into force as of 2024. Compliance with this standard requires a successfully completed stress test. This means that the hydrogen system of the eCitaro fuel cell is also well equipped for future requirements.

For example, the H₂ tanks are subjected to extensive tests in which the burst strength and the material strength are tested at changing and extreme temperatures or under the influence of fire. A drop test, the effect of chemical substances or the testing of surface damage are also part of the extensive test scope. In a sequence test, one and the same tank must pass all tests one after the other in a specific sequence.

Sled test and vibration test

In addition, a sled test was carried out with the H₂ fuel system. It simulates a major deceleration, such as in the event of an impact with the vehicle. The objective of the test is to verify that the hydrogen tanks are securely mounted on the vehicle roof and can withstand the acceleration forces. The test was passed successfully.

The entire fuel cell unit was also subjected to extensive testing on a fuel cell test bench. Both the fuel cell unit and the H₂ fuel system of the eCitaro fuel cell also passed demanding oscillation and vibration tests. These test procedures are used to examine the mechanical loads on components and complete systems. The systems are subjected to a bundle of different loads that are based on actual demanding customer applications. The fuel cell unit and the H₂ fuel system successfully passed all tests.

Real driving tests and endurance tests in hot and cold temperatures

Both the lithium-ion batteries of the new NMC 3 generation and the fuel cell require the ideal temperature for efficient operation. This is around 25 degrees Celsius for the battery modules and around 63 degrees Celsius for the fuel cell, so that the waste heat can also be used efficiently to heat the vehicle interior. This requires sophisticated thermal management. The redesigned thermal management of the eCitaro fuel cell has therefore undergone not only laboratory tests, but also real driving tests and endurance tests at hot and cold ambient temperatures.

Today, it is no longer necessary to travel far in order to test the vehicle in extremely hot ambient temperatures, such as in southern Spain. For this, Daimler Buses used the truck climate chamber provided by truck colleagues in Wörth, Germany, among other facilities, for the eCitaro fuel cell. The climate chamber is equipped with a dynamometer and wind turbine so that realistic driving simulations can be carried out and stress situations for thermal management tested under extreme conditions. Numerous pressure, temperature and flow sensors continuously transmit hundreds of data chunks to test engineers' computers, whose complex tables, diagrams and schematics continuously provide detailed information on the status of energy and thermal management. The complex procedure is intended to identify any weak points as well as optimization potential in order to be able to adapt the software or, if necessary, the components before the start of series production.

Test drive in the Alps at temperatures far below zero

The same applies to testing at ambient temperatures far below the freezing point. However, developers did not just using the climate chamber for these tests. Rather, a team of test and development engineers crossed the Alps in the winter with an eCitaro fuel cell articulated bus and tackled several challenging passes in order to test the performance of not only the drive, but also the thermal management and to take them to the limit as much as possible. For example, one of the endurance tests envisaged starting the bus well below freezing point after an extremely frosty night, bringing the battery and fuel cell systems up to the ideal operating temperature as quickly as possible and having the fuel cell bus ready to drive in the shortest possible time. In an additional test, the aim was to cover a mountain route with a length of almost 30 kilometers featuring an average gradient of 5.25 percent and a maximum gradient of 12.0 percent in order to take the batteries, fuel cell and thermal management to their limits with a constantly high drive power.

The eCitaro fuel cell performed very well in all these tests including endurance tests under practical conditions. Neither thermal management nor energy management reached their limits. Despite the high outputs demanded, the fuel cell barely moved out of the efficient power range. The batteries and fuel cells always ran at the ideal temperature. This could also be confirmed for the passenger compartment and driver cockpit.