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

Fully automatic contact wire hookup: Dual mode in Esslingen

Since 1969 two units of the OE 302, later on also an OE 305 standard regular service bus with 100 hp engine and a gross vehicle weight of 19 tons, had been operating in trial service in various cities including Hamburg, Baden-Baden, Wiesbaden and Fontainebleau. But at the same time the city of Esslingen, which was one of only three cities in the Federal Republic of Germany that still had a trolleybus network, was near despair in its search for new vehicles. Since the early 1960s no new trolleybuses had been manufactured, and Esslingen’s fleet was hopelessly out-of-date.

This offered Daimler-Benz an opportunity to gain more experience with the hybrid electric drive. The crucial drawback of the trolleybus system had always been that the buses were confined in their movement to the overhead cable network, which normally did not extend into the outlying areas of communities. This systemic disadvantage could be avoided by switching over to battery operation. So in the case of the electric dual-mode bus it was not a diesel engine which charged the batteries outside the city center; rather it was the overhead cable in the central area, whereas the battery was then used on the outlying sections of the line.

In the winter of 1974/75 one of the two OE 302 electric buses was converted along these lines, and on April 7, 1975, trial operation began on the Esslingen lines. In keeping with the local operating conditions the bus was now equipped with batteries weighing 2.75 tons and thus licensed to carry 82 passengers, 37 seated and 45 standing. A special feature was the current collector or trolley arm from Dornier, which was fitted with mechanical sensors which were supposed to enable fully automatic hookup at practically any point. This was entering unknown territory: experience had to be gathered over a lengthier period until a truly practical solution was found.

Mercedes-Benz OE 302 hybrid bus in Esslingen/Neckar, 1975.

From December 1976 to 1978 the OE 302, converted to a dual-powered bus for battery and trolley wire operation, saw regular scheduled service in Esslingen. But Daimler-Benz and the Esslingen local public transport authority already were looking ahead: “Preparations for the testing of a third dual-mode drive – again with the assistance of the Federal Ministry for Research and Technology – are underway in Esslingen. In this case, overhead cable operation will alternate with low-emission diesel operation (cooperation is again with Dornier and Bosch),” a 1975 press release stated.

The press release began by summing up the innovations: “Daimler-Benz has taken a further step towards development of a complete transportation system for our cities.” The text described the Esslingen project in detail and then explained further: “The aforesaid drive system developments of Daimler-Benz are just a few building blocks of future bus transit systems.”

The other building blocks which they were thinking of included central control systems and separate bus lanes. Articulated and double articulated buses were planned. Mechanical track guidance in special lanes was supposed to enable even longer vehicle combinations: thanks to serial coupling, track guidance and tunnels, in future the dual-mode bus would not only extend the radius of action of trolleybuses, but in the long term replace subways and streetcars.

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Alternative drive systems in Buses of Daimler AG – Part II
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