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English   Mercedes-Benz Citaro FuelCELL-Hybrid bus
01.08.2009 von admin

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.

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