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 Since the early 1960s no new trolleybuses had been manufactured, and Esslingen’s fleet was hopelessly out-of-date.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.
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.
Parts of this vision, from mechanical track guidance to tunnel operation, were realized by Daimler-Benz in Essen beginning in 1980, and later on under the keyword O-Bahn – though with diesel-powered buses – also in Adelaide, Australia. In 1984 the company also tested an electronic track guidance system in Fürth, Germany. Of course, the abundance of innovations also gave rise to a large number of unforeseen problems in detail. Today bus lanes and guidance systems have long since arrived on the regular bus service scene. The idea of a universal public transport conveyance could not gain general acceptance, however.
The testing of the combination of diesel-electric and battery-electric drive did not come to an end with the Esslingen dual-powered bus. In 1978, a five-year practical test commenced with 20 OE 305 hybrid electric buses in daily scheduled service in Stuttgart and Wesel. The partner in the tests was Gesellschaft für elektrischen Strassenverkehr (Association for Electric Road Transportation); the project was sponsored by the German Federal Ministry of Transportation and the states of Baden-Württemberg and North Rhine-Westphalia.
In the main the buses were identical with the predecessor, the OE 302. Fitted with a two-ton battery, they had a GVW of 19 tons so that they could carry up to 100 passengers. With heavier batteries, weighing 3.5 tons, the passenger capacity was reduced accordingly. The diesel engine generating the electricity was the 100 hp six-cylinder OM 352.
In Esslingen meanwhile, beginning in 1979 four dual-mode buses saw operation, two of which operated outside the overhead cable network on battery, as usual, while the two others were equipped with a diesel engine. They were joined by an articulated bus with current collector and diesel engine. This happened to be the converted world-first articulated pusher bus which the technical director of Hamburger Hochbahn, O.W.O. Schultz, had developed in Falkenried in 1976 and introduced in Hamburg in the following year.
The aim of this small-scale trial operation with dual-mode buses was to compare battery and diesel drive as supplements to trolley wire network operation. The final report of the Esslingen local public transport authority came to a sobering conclusion in 1993: “The approach to use purely electric vehicles, favored for reasons of environmental protection and the possibility of entirely substituting for petroleum as primary energy source, has proven unsuitable in practice. The two network/battery vehicles did not live up to expectations either from an economic or a technical viewpoint.”
The traction battery proved to be the main weak point,” the report continued: “The end of its useful life was already reached after 15 months of operation. Considering the high investment costs in the network/battery buses (90% higher than the cost of comparable diesel buses) and maintenance expenses which are 1.5 times higher than for diesel buses, we cannot recommend either continuing the operation of the two existing vehicles or following up any further on the vehicle concept.
On the way to an integrated transport system: The double-articulated Mercedes-Benz O 305 GG trolley bus (1981) remained a one-off unit for test purposes.
“Duo-Bus” in Esslingen in 1975: When it reached the end of the trolley wires, the OE 302 packed away its pantograph and drew electricity from the battery.