In the next few days, bus passengers in Bremen will have a new experience on the main regular service routes operated by Bremer Straßenbahn AG.
The Mercedes-Benz Citaro G BlueTec Hybrid arrives at bus-stops almost silently, and moves off again just as quietly. In the presence of Dr. Reinhard Loske, Senator for the environment, construction, transport and Europe, and Dr. Gerald Rausch, Project Manager for the furtherance of electromobility in Bremen/Oldenburg, the first two Mercedes-Benz hybrid buses for Bremen were handed over to Michael Hünig, the member of the Bremer Straßenbahn AG executive board responsible for HR and social affairs, by Thorsten Gallo, plant representative for Mercedes-Benz Buses.
Mercedes-Benz Citaro G BlueTec Hybrid
To date the Citaro G BlueTec Hybrid is the only hybrid bus able to cover sections of its route under purely electric power, without its diesel engine, and arrive at bus-stops almost silently. Without the usual engine noise, it then moves off just as quietly to cover several bus-stops on the route. The bus is powered by four electric wheel hub motors which obtain their power from one of the world’s largest lithium-ion batteries in automotive use. Electrical power is generated by braking recuperation, and when required it can also be obtained from a generator driven by a comparatively small diesel engine for this vehicle class. Specialists refer to this configuration as a serial hybrid drive system.
High-tech in a well-proven vehicle concept
This technologically sophisticated diesel-electric hybrid concept has been realised in a well-proven articulated variant of the world’s most successful city bus family, the Mercedes-Benz Citaro. Apart from the higher roof structure for the battery, there is no visual difference from the conventional diesel bus. The difference is only noticeable on the move, when the four electric motors smoothly and rapidly accelerate the 18-metre long city bus in almost complete silence. These are installed near the wheel hubs at the centre and rear axles, and deliver a total output of 320 kW which can be obtained from the battery and the diesel engine driven generator when required. The electric motors are also used for braking, however. In this case they act like a generator or bicycle dynamo, converting the braking energy into electric power for the battery. When the battery reports an additional energy requirement, the diesel engine cuts in automatically to generate more power. The internal combustion engine is only half the size of that in a conventional diesel bus, weighs only 450 kg rather than 1000 kg and has a displacement of only 4.8 litres rather than twelve litres. Its output is approx. one third lower at 160 kW.
One of the world’s largest lithium-ion batteries in automotive use
In the new Mercedes-Benz Citaro G BlueTec Hybrid, energy is stored in a lithium-ion battery whose performance is of a previously unprecedented order. The battery system has a maximum output of 240 kW, and is comparatively light in weight at under 350 kg. Major advantages over conventional battery systems include a higher energy density combined with a high storage capacity and low battery weight. By way of comparison, experimental hybrid-drive buses in Stuttgart carried lead/acid accumulators weighing 3000 kg thirty years ago.
Lithium-ion batteries first became commercially available in 1991. Today they are used practically wherever high capacity and low weight are important: mobile phones, digital cameras, camcorders, notebooks etc. They are still relatively recent in large vehicles such as passenger cars, trucks and buses. Daimler is already well experienced with lithium-ion batteries in buses. More than 1000 of the 3000 Orion hybrid buses on the roads – Orion is a Daimler brand in the USA, and the world market leader in hybrid buses – are equipped with lithium-ion batteries by the same manufacturer who supplies the battery system for the new Mercedes-Benz Citaro G BlueTec Hybrid. This delivers an output roughly appoximating to that of 9000 large laptop batteries.
Source: Daimler AG