Daimler sets milestones on the road to exhaust-free
commercial vehicles

At the Mercedes-Benz production plant in Mannheim, Germany, in early February 2008, the thick steel doors of Building 55 are locked as always; only a few people have permission to enter the site. Most of them are employees of EvoBus, a subsidiary of Daimler AG. The company’s engineers and technicians are working at a roller test rig to prepare the Citaro G BlueTec Hybrid bus for its first trip on the road. The vehicle is a prototype whose diesel electric drive system enables it to consume 30 percent less fuel than a conventional diesel bus. It also emits a correspondingly lower level of exhaust gas. What’s more, the 28-ton articulated bus can run solely on battery power — and thus emission-free — on short trips.

"Please shut off Clamp 7,” says one of the four engineers sitting with laptops in the center segment of the 18-meter-long Citaro. Various wires lead from the computers to thick wiring harnesses, which are normally hidden from view behind side paneling. The transmitted data is used to generate the measurement curves displayed on the laptop screens. “Clamp 7 is off,” the technician sitting in the driver’s seat calls from the front. “Okay, turn 7 on again,” requests the engineer. “Right now, we’re checking how the diesel engine and the electrical generator interact,” explains Khanh Vi Duong, who is responsible for the Citaro G’s “commissioning.” Part of his job is to ensure the smoothest possible interaction of all components before the vehicle enters the test operation phase. The results of the tests will be used to optimize component interaction even further.

Series hybrid

Enlarge   The engineers responsible for “commissioning” the Citaro G BlueTEC Hybrid make sure that all the components, including ancillary devices on the roof, function together reliably. The prototype is practically indistinguishable from a conventional Citaro bus in terms of the appearance of the exterior and most of the passenger area. Only the roof design and rear platform arrangement display any recognizable differences. Nevertheless, beneath the interior and exterior paneling of the Citaro G BlueTec Hybrid lies an extensive range of outstanding high-tech equipment. The vehicle is the world’s first articulated bus to be fitted with a series hybrid drive and wheel hub motors. “We’ve also consistently implemented the hybrid concept in this bus, which is why all the ancillary components are also electrically powered,” says development engineer York Störmer, who manages the Citaro G project for EvoBus.

In a series hybrid, the combustion engine is not used to directly propel the vehicle but instead to drive a generator that produces electricity. As a result, the diesel engine in the hybrid Citaro doesn’t need to reach its peak output and can thus be operated in an economical and environmentally friendly manner within a limited engine-speed range. This, in turn, makes it possible for the engineers to build a smaller engine. Instead of the six-cylinder in-line engine with 12 liters of displacement normally used for a bus of this size, the hybrid Citaro has a compact four-cylinder model with 4.8 liters of displacement that has an output of 160 kilowatts (218 hp). “This downsizing has lowered the engine weight from approximately 1,000 kilograms to around 450,” says Störmer.

Fact sheet CITARO G BlueTEC Hybrid

The four-cylinder diesel engine in the series hybrid drive operates only for a short time to ensure optimal fuel efficiency. The BlueTEC exhaust gas aftertreatment system significantly reduces nitrogen oxide emissions. Four electric hub motors power the vehicle and generate electricity during braking. Fuel savings of up to 30 percent compared to a conventional bus. All ancillary units, such as the power steering pump and the air conditioning unit, are operated electrically, thereby saving energy and improving efficiency.