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

Diesel-electric: The OE 302 hybrid electric bus

“Electric traction with its freedom from emissions and noise has great prospects in the urban transportation system of the future,” was the opinion stated by engineers Müller-Berner and Strifler. However, the battery-electric drive system was subject to very tight limitations at the time: the problem was the high weight of the lead storage batteries, compounded by short range.

In the case of the OE 302 introduced in 1969, with the maximum GVW being 16 tons and the batteries weighing 3.5 tons, the number of passengers compared with a diesel-powered bus was reduced from 110 to 65. Under these conditions and depending on road conditions, the battery was good for a range of about 40 or 50 kilometers or an operating time of little more than two hours. With a two-ton battery the bus could accommodate 90 passengers, which was not bad. But then its operating time was reduced to 1.5 hours and the range to16 kilometers. Higher passenger numbers could only be attained by raising the gross vehicle weight.

In pure battery operation, long idle times for battery charging would have been added to the economic drawback of smaller passenger numbers. Or the batteries would have had to be replaced, meaning that there would have to be two sets of batteries for each bus. Daimler-Benz went this route later in the case of the LE 306 van.

For the OE 302 the company opted instead for a hybrid diesel-electric drive. In downtown areas the diesel engine remained switched off and operating energy came solely from the battery. The Varta ironclad traction batteries – 189 cells in all in five containers – were arranged underfloor between the axles, cooled by a fan. In addition, all high-voltage elements and the Bosch control electronics were located there – at the time they still required a relatively large amount of space. Also from Bosch was the DC shunt motor in the rear, which drove the rear axle via a reduction gear with a ratio of 1:2.14 and a propeller shaft.

Diesel-electric: The Mercedes-Benz OE 302 test city bus was powered by electricity, its batteries being charged by a diesel engine. This vehicle marked a new start in hybrid drive development in 1969.

The service power was 115 kW; the electric drive provided 150 kW short-term peak power. The OM 314 passenger car diesel engine, likewise fitted in the rear, generated 65 hp. However, it did not serve to drive the vehicle but was only engaged outside downtown areas. Via three-phase alternator with a downstream rectifier it supplied the power for the electric motor and simultaneously charged the storage batteries. The advantage of this was that the engine always operated under optimal conditions, at constant load and speed, and thus easily could be set for minimal emissions.

Moreover the engine was fully encapsulated, as one of the particular advantages of the electric motor was its low noise emissions. This soundproofing also benefited the conventional diesel-engined urban bus in the form of a “noise-encapsulated diesel” which Daimler-Benz used during the 1972 Munich Olympics for demonstration purposes.

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