In the service of the environment: Alternative drive systems in commercial vehicles of Daimler AG - PART III


Stuttgart, Germany, Jul 16, 2008

After the Second World War: Major order from Argentina

The decision to devote oneself to the trolleybus again after the war was connected with a call for tenders from Argentina. In August 1951 Daimler-Benz submitted a bid based on a post-war model introduced at the beginning of the year, the O 6600 H, an eleven meter long forward-control vehicle with rear-mounted engine, with seats for 38 passengers and standing room for another 52. At the time it was not yet decided whether BBC, Siemens-Schuckert or AEG would contribute the electrical system. Instead of contact-wire bus they now spoke of a trolleybus, despite the fact that the vehicles had long since ceased to pull a contact carriage behind them.

In February 1952 the decision was announced: exactly half of the offered volume of 700 buses went to Daimler-Benz; Henschel and MAN divided the other half between them. At a unit price of 26,300 dollars per vehicle this figured out to aggregate sales of more than ten million dollars. From the one moment to the next, Daimler-Benz became the biggest exporter of trolleybuses, and the O 6600 T became the best-selling German trolleybus of the 1950s. Delivery in 14 installments was agreed, the first of which already left the factory in May 1952, whereas the last was to arrive in Buenos Aires at the end of July 1953.

In the end, the contract for the electrical system was awarded to Kiepe. While the first 50 trolleys were on their way to Buenos Aires, the Gaggenau factory put the O 6600 T to a thorough test in the Baden-Baden trolleybus network from June 23 to September 10, 1952. After 7,626 contact-wire kilometers the head tester came to the reassuring conclusion: “The electric system generally performed to our satisfaction.”

But that was about it for the trolleybus business. Although Wiesbaden, Pforzheim, Heilbronn and Baden-Baden did take an interest in the trolley, except for eight units for Offenbach, only a handful of the German version of the O 6600 T2 went to other cities. By the mid-fifties the trolleybus euphoria had already passed its peak. Many of the lines in the 68 German cities that used trolleybuses in that period were too short to occupy an adequate market segment between streetcar lines and bus systems. Despite unquestionable advantages – low-noise, emission-free operation, better traction on hills – the trolleybus found itself at a competitive disadvantage versus the diesel engine and has not been able to make up for it since.
But while one manufacturer after the other gave up on the electric drive, and one city after the other discontinued its trolleybus lines, in the late 1960s Daimler-Benz began looking around for new solutions.

New beginning under ecological aspects: The “Electromobile” development project

Strictly confidential!” is stamped on the minutes of a technical meeting on November 13, 1967, in Stuttgart-Untertürkheim, in which along with Daimler-Benz Development chief Dr. Hans Scherenberg other members of the company’s Board of Management as well as representatives of Volkswagen took part. The subject of their meeting was the “Electromobile” development project. They talked about development contracts for the electric drive system of a
Mercedes-Benz urban bus and a VW van, about a hybrid drive using a gas turbine, a diesel or Wankel engine and battery, and about the development of titanium hydride storage systems for hydrogen by an institute in Geneva. The last topic on their agenda was the rigorous emission control regulations expected in the USA beginning in 1970.
Dr. Müller then reported on the further studies at DB in connection with the electric urban bus,” the minutes state. “The first candidate for testing is the model O 302. The nominal electric output of the urban bus is around 150 hp, and as drive engine for the generator the four-cylinder OM 314 diesel engine has been chosen, which delivers an output of around 50 hp if optimally adjusted in terms of service life, fuel consumption, noise behavior, etc. With this configuration, 24-hour operation without recharging the batteries in between appears possible. The necessary battery volume for this design is about the same as that for pure battery operation of such a vehicle with hourly recharging.

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 in the Automobiltechnische Zeitschrift article mentioned above. 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.
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.
How far ahead the engineers were looking is shown by a chart contained in the article by Müller-Berner and Strifler. In a comparison of the specific output and energy of various possible drive systems, along with lead storage batteries and internal combustion engines they included nickel-cadmium batteries – which were expensive, on the other hand, with an additional problem being the disposal of the highly toxic cadmium – and, even at that early point, the fuel cell.

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