Mercedes-Benz Citaro FuelCell-Hybrid bus

By: Allen Matzell

The first fuel-cell hybrid bus from Mercedes-Benz combines elements of its already-proven fuel-cell buses and technical enhancements into something entirely new.

Mercedes-Benz Citaro FuelCell-Hybrid bus
Its hydrogen fuel cells won’t spew out emissions while running and its operation is super quiet to boot. The Citaro FuelCell-Hybrid bus from Mercedes-Benz is very much from the future… right now.

It generates no emissions while running, it's whisper-quiet and it conserves resources: the new Citaro FuelCell-Hybrid bus from Mercedes-Benz heralds a new era in drive technology for urban buses.

The newcomer had its world premiere in June 2009 at the UITP Congress in Vienna and could be seen in action for the during the final conference of the HyFleet:Cute project in Hamburg. A 10-vehicle customer test around Hamburg and small series production will follow the launch.


Mercedes-Benz's first fuel-cell hybrid bus combines and builds on elements of the company's already-proven fuel-cell and diesel-electric buses. The engineering platform comes from the Citaro urban bus. Developed within the context of Daimler's global 'Shaping Future Transportation' initiative, the fuel-cell hybrid is a major step toward emissionless travel.

The developers of the Citaro FuelCell-Hybrid anticipate its hydrogen consumption will reduce by almost half compared to first generation fuel-cell buses. At the same time, the technology has been improved to near series-production standard.

In addition to the considerably increased service life of the fuel cells, now at least six years or 12,000 hours, the service requirements have been reduced drastically: the fuel cells, batteries and electric motors require practically no maintenance.

Furthermore, in a departure from the previous arrangement for fuel-cell buses, service and maintenance tasks will in future be performed by specially trained employees of the transport authorities that operate the buses.

As in the non-hybrid fuel-cell Citaros of the first generation, most of the technical components of the fuel-cell hybrid are out of the way, under panelling in the bus's reinforced roof. The pressurised hydrogen tanks are at the front end. These are the only elements of the fuel-cell system to have been taken over directly from the non-hybrid predecessor; all the other components have been newly developed.

As the bus's drive cuts fuel consumption considerably, it has been possible to reduce the number of tanks from nine to seven, the total capacity of hydrogen is 35 kg.

In a first for fuel-cell buses, the traction batteries (immediately behind the hydrogen tanks) use lithium-ion technology. The batteries' capacity of 27kWh means they supply the electric motors with a constant 120kW (162hp), this means the bus can run for several kilometres on battery power alone. As the optimal temperature range for lithium-ion batteries is between 15 and 55 degrees Celsius the batteries are water-cooled to maximise performance and efficiency.

The heart of the drive system are the two fuel-cell stacks to the rear of the passenger-compartment air-conditioner. Although adjacent to each other, the stacks function independently. They, too, deliver sufficient power for continuous operation of the electric motors at 120kW (162hp). Each stack has 396 individual fuel cells. Situated between the stacks are the metering systems for the hydrogen as well as the air feeds to the fuel cells.

The fuel-cell stacks of the Citaro FuelCell-Hybrid differ significantly from those of the previous bus generation. Their service life has been extended 50 percent to at least six years. Their efficiency has also been increased considerably: it now attains an impressive 51 to 58 percent compared with 38 to 43 percent for the first generation.

Heat exchangers to the rear of the stacks use the waste heat from the fuel cells to heat the passenger compartment. If no heating is needed or if the available heat is surplus to requirements, four fans evacuate the hot air. The exhaust at the rear of the bus emits no pollutants at all; the only exhaust is water vapour.

A 12-metre-long solo urban bus with three doors provides the platform for the Citaro FuelCell-Hybrid. The rear of the diesel version of this model accommodates the vertically-mounted engine followed by the automatic transmission and, in turn, the drive axle, the new bus has a different architecture. Here, the rear is home to the electronic and electrical modules which drive the ancillary components.

Whether power-steering pump, air-conditioning or air compressor - the ancillary components are all driven electrically and on-demand. As the components themselves are the same as those used in the Citaro G BlueTec Hybrid, the new model is able to benefit from mature technology and a proven operating strategy.

The space that in the diesel bus is occupied by the automatic transmission now accommodates two DC/AC converters. The central motor which featured in the first generation of fuel-cell buses has now given way to water-cooled asynchronous wheel hub motors.

Together, these attain a continuous output of 120kW and peak (starting) output of 160kW (216hp). This amount of power means the bus can cope with steep climbs. The wheel hub motors have also been taken over unchanged from the Citaro G BlueTec Hybrid, as have the DC/AC converters and batteries.

The entire drive system is designed for the greatest possible efficiency. Like the diesel-hybrid bus, the fuel cell -hybrid uses regenerative braking. This means the bus can achieve hydrogen savings of 10 to 25 percent, depending on the traffic conditions and topography.

The energy management system for the serial hybrid drive is another notable feature. Depending on the topography, the bus can cover 2 to 3km on battery power. When operating in this mode, the bus is even quieter than when running on the fuel cells. If the bus should require its full drive power, when accelerating or negotiating steep ascents, for example, the fuel-cell drive cuts in to support the traction batteries.

Highly-efficient fuel cells, the reduced hydrogen storage capacity with the associated reduction in weight and on-demand activation of electrically powered ancillary components also reduce fuel consumption. Overall, this results in consumption of only 11 to 13kg of hydrogen per 100km. This compares with a consumption of 22kg of hydrogen per 100km achieved by earlier buses with a fuel-cell drive.

In addition to the economic benefit, the reduced consumption helps reduce the demand on the resources required for hydrogen production.

The reduced weight also contributes to lower fuel consumption: despite its additional batteries, the bus is about 1,000kg - at around 13.2 tonne kerb weight - than its predecessor. The new model's passenger capacity is correspondingly greater.

Factors responsible for the lower weight include the absence of the automatic transmission, lighter fuel-cell stacks and a smaller cooling system. The reduced hydrogen storage capacity also plays a role. Nevertheless, the operating range is increased - depending on the topography - from about 200km to 250km. Like the vehicle's performance, this is on a par with the figures for a conventional diesel bus.

During the development of the first generation of fuel-cell buses, priority was given to minimising risks, maximising reliability and, above all, ensuring that the technology functioned correctly. Now that the long-term viability has been validated, the new phase has shifted the focus to the optimisation of fuel consumption and economic efficiency in general. These considerations include such aspects as reliability and length of service, both of which have been increased. For example, the valves which metre the hydrogen are now engineered for the special requirements associated with use in a road vehicle.

Although the Citaro FuelCell-Hybrid is an entirely new development, there is no change from the driver's point of view - even in comparison with the diesel Citaro. The familiarity applies right through from the actual driving to the look and feel of the cockpit. Ride comfort on the Citaro FuelCell-Hybrid has been considerably improved over the first generation of fuel-cell buses.



The Citaro FuellCell Hybrid isn't a futuristic study - it is already a reality. From next year Mercedes-Benz will be monitoring its normal line-service performance in a 10 bus test conducted with Hamburg public transport operator Hamburger Hochbahn and supported by the German Federal Ministry of Transport. A further goal is a large-scale test in several European cities.

It is intended that this wider test should proceed along the lines of the CUTE fleet test conducted by the European Union. Since 2003, a total of 36 Mercedes-Benz Citaro buses have performed outstandingly well in service across three continents with 12 public transport operators (even Australia) within the context of the CUTE test and its HyFleet Cute follow-on project.

To date, the buses have been driven a combined total of more than two million kilometres and have 135,000 hours of operation.

The new Mercedes-Benz Citaro FuelCell-Hybrid bus will continue this successful development.


  • Emission-free, whisper-quiet vehicle conserves resources
  • Innovative concept with proven components
  • Reduced fuel consumption
  • Extended service life
  • New combination of fuel-cell and hybrid bus technology
  • Lithium-ion batteries for energy storage
  • Two highly efficient fuel-cell stacks
  • Weight reduction
  • Linear development from NEBUS to Citaro FuelCell-Hybrid

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