It is barely audible as it approaches the bus stop. After stopping, it moves off again without the usual engine noise and continues along its route. Passengers in Hamburg who experience a bus like this in the near future are likely to be travelling in the Mercedes-Benz Citaro G BlueTEC® Hybrid. Two of these vehicles have been acquired for field testing by the public transport company Hamburger Hochbahn. In the presence of Parliamentary State Secretary, Enak Ferlemann, from the German Transport Ministry and Hamburg’s Economics Senator, Axel Gedaschko, the two articulated hybrid buses were handed over today to the Chairman of Hamburger Hochbahn, Günter Elste, by Michael Göpfarth from EvoBus GmbH. "We are delighted that our long-standing partner is implementing the pioneering technology of our articulated hybrid buses in Hamburg," said Göpfarth.

The Citaro G BlueTEC® Hybrid is currently the only hybrid bus available that can cover sections of its route using electric power alone, i.e. without its diesel engine. The new Mercedes-Benz BlueTEC® Hybrid is powered by electric wheel hub motors that source electricity from one of the largest lithium-ion batteries ever used in mobile applications. Electricity is generated through braking when the bus is driving downhill, for example. If required, it can also be produced using a generator driven by a diesel engine that is relatively small for the Citaro’s vehicle category. Experts call this type of system a series hybrid drive.

This technologically advanced diesel-electric hybrid concept has been implemented in the tried-and-tested articulated variant of the world’s most successful city bus series – the Mercedes-Benz Citaro. Externally, the hybrid Citaro looks just like a conventional diesel bus. The only difference is a higher roof to house the battery. Passengers will also find familiar surroundings in the interior. The hybrid Citaro accommodates around 125 passengers and is fitted with 46 seats with the same configuration as all other buses used by Hamburger Hochbahn. The difference becomes apparent once the bus starts moving. Operating smoothly and virtually silently, the four electric motors accelerate the 18 metre bus very quickly. Boasting a combined output of 320 kW, these motors are installed directly in the wheel hubs at the centre and rear axles. The motors are also used when the bus brakes, acting like generators or bicycle dynamos as they transform braking energy into the electricity used to charge the battery. Should the battery require more energy, the diesel engine is switched on automatically to produce additional electricity. This diesel engine is only half the size of the combustion engine generally used in diesel buses, and weighs only 450 kg (as opposed to 1000 kg). It also boasts a 4.8 litre engine capacity instead of the usual 12 litres. And its 160 kW output is approximately one-third lower than that of a conventional diesel bus engine.

As its energy storage device, the new Mercedes-Benz Citaro G BlueTEC® Hybrid uses a lithium-ion battery with an output unprecedented in this performance category. The battery system generates a maximum of 240 kW and, weighing less than 350 kg, is comparatively light. Key advantages over conventional battery systems include a higher energy density, high storage capacity, and low weight. By way of comparison, hybrid drive buses tested in Stuttgart 30 years ago used 3000 kg lead batteries.

Lithium-ion batteries first came on the market in 1991. Today they can be found in virtually all devices that combine high energy requirements with low weight, e.g. mobile phones, digital cameras, camcorders, and notebooks. It was only relatively recently, however, that they began to be used in large vehicles like passenger cars, trucks, and buses. The use of lithium-ion batteries in buses is nothing new for Daimler. More than 500 of the 2500 hybrid buses produced by Orion (a Daimler AG brand in USA), the global market leader for hybrid buses, are equipped with lithium-ion batteries manufactured by the company that supplied the battery system for the new Mercedes-Benz Citaro G BlueTEC® Hybrid. The output of this system corresponds to that of approximately 9000 large laptop batteries.

The two Citaro G BlueTEC® Hybrid buses in Hamburg are the first to be sold and implemented as part of customer trials in Germany. This year, Daimler will also test some 30 Citaro G BlueTEC® Hybrid buses in selected cities (the so-called Model Regions). The buses will demonstrate their suitability for daily operation by being driven in areas with different topographies and speed profiles. It was back in March 2009 that Daimler gave 250 experts from various European public transport companies the opportunity to see the Mercedes-Benz Citaro G BlueTEC® Hybrid articulated bus in action for the first time. The new bus was also subjected to extensive internal trials. One of these involved several weeks of winter testing under gruelling conditions at the Arctic Circle, which the Mercedes-Benz Hybrid bus passed successfully.