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English   Alternative drive systems in Buses of Daimler AG – Part I
23.12.2008 von admin

Daimler-Benz went a step further with the low-floor interurban hybrid bus O 405 NÜH. Unlike the O 405 GNDE the interurban bus could switch off the diesel engine and continue operating in town centers in pure battery mode, emission-free. For this purpose the manufacturer tested novel sodium-nickel chloride batteries, also called zebra batteries, for the first time. They were developed by the Daimler-Benz subsidiary AEG in Ulm. They were considerably lighter than the old lead batteries: a set of four batteries weighed 800 kilograms, was mounted on the roof, and worked at an operating temperature of 300°C.

The low-floor O 405 NÜH interurban hybrid bus of 1996 was powered by wheel hub motors. The latter were driven by a diesel engine and generator and recharged by the battery.

Four vehicles of this type went into operation in September 1996 between Oberstdorf, Sonthofen and Kempten, sponsored by the Bavarian State Ministry for Regional Development and Environmental Protection. Theoretically the low-floor buses could have gone some 30 to 40 kilometers in pure battery mode. But this was not the reasoning behind the test setup, for in this case the bus would then have needed a longer downtime to recharge the batteries. The distance which the bus covered in emission-free operation in the centers of the health resorts was only about ten kilometers. The aim of trial operation was to make optimal use of the power of the diesel engine through intelligent energy management and so simultaneously cut consumption and reduce the burden on the environment.

For the two-axle bus the 250 hp version of the OM 447 hLA was sufficient. The turbodiesel powered a generator which produced alternating current which had to be rectified for control purposes. An inverted rectifier then converted the direct current into three-phase current to drive two asynchronous wheel hub motors. On the other hand, owing to the high field-weakening ratio of the electric motors a transmission was not needed: between 0 and 85 km/h the diesel-electric drive operated infinitely variably. In addition, the constantly high torque of the asynchronous motors also came in handy on the hilly routes of the Allgäu region, ensuring startability on gradients of 18 percent.

The zebra batteries were charged with the excess energy from the generator while the bus was moving, but also through braking energy recuperation, for the wheel hub motors normally also functioned as service brakes and so acted as a kind of generator themselves. The bus had drum brakes on the front axle and disc brakes in the rear merely in the event they were needed for emergency braking. To store the braking energy in the batteries, however, the three-phase current first had to be reconverted to direct current.

Ideally the batteries should never entirely discharge, but also should never be fully charged, as otherwise the energy remains unused and has to be given off to the environment in the form of heat. Synchronizing the different components with each other and tuning them to actual route conditions called for complex electronic controls, likewise developed by ZF. Recent advances in semiconductor technology had made them possible. Optimum environmental performance could only be attained if the use of the diesel engine with its relatively higher emissions was restricted to what was absolutely necessary. The aim of practical testing also was to arrive at a state of equilibrium: starting from the concrete characteristics of the routes on which the buses regularly operated, the batteries were to be drained in the course of a day exactly as much as they were recharged from the electric grid during the course of the night.

In practical testing, however, the wheel hub motors still were unable to deliver really convincing performance: after one-and-a-half years of trial operation from October 1997 to March 1999 the Zurich bus came back to Stuttgart first and then went on to Friedrichshafen for further study. After teething problems the 17 Stuttgart vehicles managed to remain in operation until 2002. One of the O 405 NÜH interurban buses was taken over by Berlin’s local public transport company in 2003 for further testing.

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