It generates no emissions while running, is whisper-quiet and conserves resources: the new Citaro FuelCELL-Hybrid bus from Mercedes-Benz opens up a whole new era in drive technology for urban buses. The newcomer had its world premiere in June 2009 at the UITP Congress (the World Congress of the Inter­national Association of Public Transport) in Vienna and could be seen in action for the first time on the occasion of the final conference of the HyFLEET:CUTE project in Hamburg.

Citaro FuelCELL-Hybrid

The advent of the Citaro FuelCELL-Hybrid bus means that the future really has arrived: the launch of the prototype will shortly be followed by small series production while next year will see the start of a large-scale test of the Citaro FuelCELL-Hybrid bus on the basis of ten customer vehicles in service in Hamburg. The objective is for further in-service testing in customer fleets to be conducted at European level.

Innovative concept with proven components
The first fuel-cell hybrid bus from Mercedes-Benz is an innovative vehicle concept which combines elements of already proven Mercedes-Benz fuel-cell buses, the diesel-electric Citaro G BlueTec Hybrid bus and technical enhancements to form a new, trend-setting drive concept. The platform is provided by the successful Mercedes-Benz Citaro urban bus. Developed within the context of Daimler’s global „Shaping Future Transportation“ initiative, the Citaro FuelCELL-Hybrid bus represents another major step on the way towards emission-free travel.

Growing calls for environment-friendly drive systems
A rapidly increasing number of official low emission zones, discussions (occasionally heated) about particulate emissions from vehicles and particulate con­centrations in urban areas, the introduction next year of immission limits for nitrogen oxides in the European Union and last summer’s oil price shock – these are just a few of the many factors behind the growing calls for a drive system which is emission-free and helps conserve resources at the same time.

The answer to these calls is the Mercedes-Benz Citaro FuelCELL-Hybrid bus. Its outstanding characteristic is the comprehensive environmental friendliness with which it operates. It emits no pollutants whatsoever while running and is also virtually silent. It is therefore extremely well suited to operation in heavily polluted city centres and in metropolitan areas. The Citaro FuelCELL-Hybrid is the next logical step on the path to zero-emission public transport, which Daimler had already announced it would take, and thus represents an important element in the development of the mobility solutions of the future.

Reduced fuel consumption, extended service life
What’s more, the commitment with which the development of the new technology of fuel-cell drive systems is being pursued means that its economic efficiency is increasing. In consequence, the developers of the Citaro FuelCELL-Hybrid antici­pate that its hydrogen consumption will be reduced drastically – by almost half, in fact – compared with the first generation of fuel-cell buses.

At the same time, the technology has been enhanced and matured in all respects to such an extent that it is now significantly closer to series-production standard. In addition to the considerably increased service life of the fuel cells, which has now risen to at least six years or 12,000 hours of operation, the service require­ments have been reduced drastically: not only the fuel cells, but also the batteries and electric motors are practically maintenance-free for life. 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 which operate the buses.

New combination of fuel-cell and hybrid bus technology
The new Mercedes-Benz Citaro FuelCELL-Hybrid embodies the genes of not only the proven fuel-cell-drive Citaro, but also the latter’s pioneering ancestor, the NEBUS, and the current Citaro G BlueTec Hybrid. It brings them together to form a new whole and sets standards for the development of drive systems for urban buses.

Hydrogen storage capacity reduced despite increased operating range
As in the first-generation fuel-cell-drive Citaro, most of the technical components of the Citaro FuelCELL-Hybrid are accommodated out of the way under stylish panelling on the reinforced roof of the bus. The pressurized hydrogen tanks are situated at the front end. These are the only elements of the fuel-cell system to have been taken over directly from the predecessor model, all the other components having been newly developed.

As the drive technology of the Citaro FuelCELL-Hybrid cuts fuel consumption considerably, it has been possible to reduce the number of tanks from nine to seven with a total capacity of 35 kg of hydrogen.

Lithium-ion batteries for energy storage
In a first for fuel-cell buses, the traction batteries situated immediately behind the hydrogen tanks use lithium-ion technology. Their capacity of 27 kWh allows them to supply the electric motors with a constant 120 kW and is sufficient for the Citaro FuelCELL-Hybrid bus to run for several kilometres on battery power alone. In order to maximise performance and efficiency, the batteries are water-cooled as the optimum temperature range for lithium-ion batteries lies between 15 and 55 degrees Celsius.

Two highly efficient fuel-cell stacks
The heart of the drive system of the Mercedes-Benz Citaro FuelCELL-Hybrid is to be found in the form of the two fuel-cell stacks to the rear of the passenger-compartment air-conditioning system which is situated in the centre of the roof. Although adjacent to each other, the stacks function independently. They, too, deliver sufficient power for continuous operation of the electric motors at 120 kW. Each stack contains 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 by some 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 whatsoever; the only exhaust emission is harmless water vapour.

Electronic and electrical modules at rear of vehicle
A twelve-metre-long solo urban bus with three doors provides the platform for the Citaro FuelCELL-Hybrid. Whereas the rear of the diesel-powered 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 of the Citaro FuelCELL-Hybrid are all driven electrically on an on-demand basis for optimum efficiency. 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 which would normally be occupied by the automatic transmission in the diesel bus now accommodates two DC/AC converters on the left-hand side. 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 120 kW and peak (starting) output of 160 kW. The ample output of the motors means that they are also able to cope with very demanding topography. The wheel hub motors have also been taken over unchanged from the Citaro G BlueTec Hybrid, as have the DC/AC converters and batteries.

Sophisticated energy management, reduced fuel consumption
The entire drive system is designed for the greatest possible efficiency. Like the diesel-powered hybrid bus, the fuel economy of the Citaro FuelCELL-Hybrid bus benefits from regenerative braking – that is to say, the recovery of braking energy. Thanks to this technology, the Citaro FuelCELL-Hybrid is able to achieve hydrogen savings of between about 10 and 25 percent, depending on the traffic conditions and topography.
The particularly sophisticated energy management system for the serial hybrid drive is another notable feature of the Citaro FuelCELL-Hybrid bus. Depending on the topography, the bus can cover a distance of some 2 to 3 km on battery power alone. When operating in this mode, it 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 some 11 to 13 kg of hydrogen per 100 km. This compares with a consumption of some 22 kg of hydrogen per 100 km which was achieved by earlier buses with a fuel-cell drive. In addition to the economic benefit, the reduced consumption of the Citaro FuelCELL-Hybrid helps reduce the demand on the resources required for hydrogen production.

Weight reduced considerably (now approx. 1 tonne lighter)
The reduced weight also contributes to the lower fuel consumption: despite its additional batteries, the Mercedes-Benz Citaro FuelCELL-Hybrid is about one tonne lighter – at approximately 13.2 t (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 significant role. Nevertheless, the operating range is increased – depending on the topography – from about 200 km to some 250 km. 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 life, both of which have been increased once again. For example, the valves which meter the hydrogen are now engineered for the special requirements associated with use in a road vehicle, as are the power electronics. As well as being operated on an on-demand basis, all the ancillary components are actuated and loaded in accordance with individually defined parameters in order to maximise their service life. The availability of two power sources also minimises the load on the drive system.

Fuel gauge: hydrogen instead of diesel
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. This sense of familiarity applies not only to the actual driving experience, but also to the look and feel of the cockpit – right down to the fuel gauge: instead of indicating the amount of diesel in the tank, this now shows how much hydrogen is left.
In comparison with the first generation of fuel-cell buses, the Citaro FuelCELL-Hybrid offers the driver and passengers considerably improved ride comfort. This is thanks to the exemplary response and handling provided by the Citaro’s independent front suspension. As in the first Citaro fuel-cell bus, a pitch/roll control system prevents uncomfortable body roll when cornering.

The interior layout corresponds to that of a diesel-powered Citaro. The sense of spaciousness is enhanced by the absence of the diesel tank. Furthermore, passengers aboard the Citaro FuelCELL-Hybrid prototype will notice that it differs from conventional urban buses in a number of ways: with its stainless steel handrails and ship’s-deck-effect flooring, the passenger cabin reflects the exclusivity and sustainability which characterise the drive system.

Win-win for buses and passenger cars
The Citaro FuelCELL-Hybrid bus underlines the leadership of Mercedes-Benz and Daimler Buses in drive system technologies. The Group’s engineers at the Fuel-Cell Competence Centre in Kirchheim-Nabern benefit from their cross-divisional role within the Group: they develop fuel-cell drive technology centrally for all brands and divisions.

This structure makes for a highly effective development function which is geared to getting results, not least through the exchange of findings and test results for the Citaro FuelCELL-Hybrid bus and passenger cars with fuel-cell drives. As this technology is used in several model series with correspondingly high production volumes, it is possible to develop components which would never be viable for a manufacturer who produces buses or commercial vehicles only. For example, the fuel-cell stacks of the new Citaro FuelCELL-Hybrid bus are identical to those of the Mercedes-Benz B-Class with fuel-cell drive, which will enter series production in the course of next year.

Linear development from NEBUS to Citaro FuelCELL-Hybrid
The urban bus has always been the priority for Daimler Buses in the course of its rigorous and single-minded pursuit of the goal of emission-free driving. It started in 1997 with the NEBUS research vehicle – the world’s first bus to be equipped with a fuel-cell drive system. There then followed the fuel-cell Citaro and the Citaro G BlueTec Hybrid with a diesel-electric hybrid drive system and now the new Citaro FuelCELL-Hybrid of 2009.

Fleet test with ten buses in Hamburg
The Citaro FuellCell Hybrid isn’t a futuristic study – it is already a reality: indeed, from next year, Mercedes-Benz Buses will be monitoring its performance in normal line-service operations very closely in the course of a fleet test conducted in conjunction with Hamburg public transport operator Hamburger Hochbahn. A small series of ten buses will be built for this fleet test which is being supported by the German Federal Ministry of Transport, Building and Urban Affairs.

A further goal is a Europe-wide large-scale test in several cities. It is intended that this should proceed along the lines of the successful 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 twelve public transport operators (located as far apart as Iceland and Australia) within the context of the CUTE test and its HyFLEET CUTE follow-on project as well as other related testing programmes. To date, the buses have been driven a combined total of more than two million kilometres in some 135,000 hours of operation. Their availability – between 90 and 95 percent – has provided an impressive demonstration of the environmentally friendly fuel-cell drive’s suitability for everyday use in urban regular-service buses. The new Mercedes-Benz Citaro FuelCELL-Hybrid bus will continue this successful development.

Source: Daimler AG