In the service of the environment: Alternative drive systems in commercial vehicles of Daimler ag

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Press Information

Josef Ernst

Telephone: +49 (0) 711-17-7 64 77

April 2008
In the service of the environment: Alternative drive systems in commercial vehicles of Daimler AG

  • More than 100 years of experience with electric and hybrid drives

  • Pioneer of diesel-electric drive, natural gas and hydrogen

  • New transportation and drive systems: From vision to practical testing

  • Most comprehensive practical trials to date with fuel-cell-powered buses

Stuttgart. Natural gas, hybrid drive, fuel cell: although the diesel engine is getting increasingly cleaner, declining petroleum supplies, growing environmental awareness, and the already perceptible effects of global warming give more cause than ever to pursue alternative drive concepts. Daimler AG has experience in this field which no other company can match.

The forerunners: Gas, steam and electricity

Alternative to gasoline and diesel engines have existed since the early days of the automotive era. Long before Gottlieb Daimler and Carl Benz invented the gasoline-powered automobile in 1886, steamers traveled the roads of England. Jean Joseph Étienne Lenoir developed a road vehicle with gas engine in 1863, which Nikolaus August Otto took as model when he invented the patented high-speed four-stroke gasoline engine in 1876. In 1882, in Berlin-Halensee, Siemens presented the Elektromote, the world’s first trolleybus.

Admittedly, in earlier times the motives often were anything but concern for a clean environment, as in the case of the wood carburetors of the 1930s. However, the past experience was the basis from which the search for environment-friendly drive systems had to start towards the end of the 1960s.

Electric drive and wheel hub motors: First buses back in 1899

When commercial vehicle development chief A. H. Müller-Berner and engineer P. Strifler presented the OE 302 diesel-electric hybrid urban bus in Automobiltechnische Zeitschrift in 1970 – marking the beginning of the development of alternative drive systems at Daimler-Benz – in their introduction they recalled a forerunner from the year 1899. The bus, lettered “Kaiser Hotel” and strongly reminiscent of a horse-drawn coach, came from Motorfahrzeug- and Motorenfabrik Berlin-Marienfelde, which merged with Daimler-Motoren-Gesellschaft (DMG) in 1902. A DMG sales brochure of 1900 describes the bus as follows:

Our electric hotel bus shown above is the carriage par excellence for any modern hotel! Its advantages include noiseless and odorless travel, excellent steering and control, and, in addition, the quality that it is always ready to go. During short stops, for instance while waiting at the train station or in front of the hotel, the driver can leave it entirely unsupervised, perfectly safely, since the motor is stopped and, by withdrawing a key from the emergency stop switch, the driver makes the braked vehicle inoperable for any unauthorized person who is not in possession of the key.

Having now had the electric hotel bus in operation for one year at our hotel, I absolutely must confirm to you that the bus has always functioned excellently and provided smooth service,” the director of Hotel Kaiser on Friedrichstraße stated, and Allgemeine Berliner Omnibus-Actien-Gesellschaft agreed: “In response to your inquiry of the 20th of this month, we can tell you that so far the buses with electric drive system for 17 and 18 passengers put into operation by our transportation service have run well, as far as we can judge.” And there were several such electric buses plying busy Friedrichstraße in those days, as an enclosed photograph shows. Under favorable conditions they attained a range of 40 kilometers and a top speed of 18 km/h and could be had for a price of about 12,500 marks.

Wheel hub motors for hybrid and trolley operation

At the same time, Ferdinand Porsche, then employed at Hof-Wagen- and Automobilfabrik Jacob Lohner in Vienna, invented the wheel hub motor. When the young engineer went to Austro-Daimler in 1905, DMG seized upon the patent and produced the electric motors, which were fitted into the front wheels, in large numbers so that the system soon simply was called “Mercedes Electrique” or “Elektro-Daimler”.

At Austro-Daimler, Porsche also sought to bring his invention together with the core competence of the gasoline engine manufacturer by replacing the heavy lead storage batteries of the electric vehicle by a gasoline engine and generator which produced the electricity for the wheel hub motors: the hybrid drive, called Daimler-Mixte, was born. The bigger range compared with the storage battery, but above all the vehicles’ problem-free, permanent readiness for use, made the Mixte drive system an attractive solution for fire departments which until then had turned out to the scene of a fire at worst with horse and cart and bicycles, at best with electric and steam-powered automobiles. The big professional fire departments in Berlin and Hamburg soon put Mixte vehicles into service. Of course, the twin drive was a little more expensive for the customer than pure electric or gasoline drive.

But the wheel hub motors could be used for trolleybuses as well. Significant progress had been achieved in that area since the beginning of the century. Engineer Carl Stoll further developed the Lombard-Gérin system which won a gold medal at the 1900 Paris world exposition. In 1902 he commenced trial operation on the 5.2 kilometer “Haidebahn” between the Arsenal industrial estate and Klotzsche near Dresden. The bus pulled a four-wheeled contact carriage, the trolley, behind it on an overhead cable. During the one year of scheduled operation from March 24, 1903, to March 19, 1904, the Haidebahn line actually carried 133,000 passengers. The Stoll System thus had demonstrated its practical suitability.

After Porsche joined the company, Austro-Daimler was soon also offering trolleybuses according to the “Mercedes-Electrique-Stoll System.” “An up-to-date, first rate, cheap means of transportation!” a 1909 brochure published by DMG Berlin-Marienfelde states: “Is a full-fledged replacement for an electric streetcar for smaller enterprises. Costs only a third of the streetcar and has lower operating expenses.” Depending on the length of the route and the number of vehicles and their equipment, a transportation service had to reckon with investment costs between 50,000 and 200,000 marks.

The first town, Gmünd in Lower Austria, took up operation in July 1907 on a 3.3 kilometer line between rail station and city center. The two 18-seat vehicles still had the wheel hub motors in the front wheels. Later Daimler switched to rear-wheel drive. In 1909, 25 trolleybuses were already operating on six lines in Greater Vienna, in Pressburg (Hungary then, today’s Bratislava in Slovakia) and in Budweis (České Budejovice). Judenburg, in the Styria region, where the bus had to negotiate a 15 percent incline, followed in 1910. With a top speed of around 25 km/h even uphill, the trolleybuses were clearly superior to gasoline-engined vehicles in those days.

A trolleybus line took up service in Germany for the first time on January 16, 1911, in Heilbronn. The 5.5 kilometer distance to the blue-collar suburb Böckingen presented unusual challenges to the construction engineers of Ludwig Stoll’s project office. There was a level crossing with a railway line, and in another place the overhead cable had to be routed under a 3.77 meter high railway bridge. By 1912 further lines had been started up in Fribourg in Switzerland, in Berlin-Steglitz, and finally even in Paris. At that point, 40 vehicles were operating on eleven lines with a total length of around 50 kilometers, and all were still providing reliable service after almost two million vehicle-kilometers.

O 6000 and O 10000: Trolleybuses of the 1930s

Trolleybus operation was discontinued during the First World War, and after the war there were no funds for new investment at first. Daimler-Benz first presented a trolleybus again in 1936 at the International Automobile and Motorcycle Show (IAMA) in Berlin. Since 1930 trolleybuses had begun to see service again in Germany, but the concept really owed its renewed breakthrough to the policy of the National Socialists, who wanted to make themselves independent of imported oil.

The 9.375 meter long 32-seater, developed jointly by Daimler-Benz and Brown, Boveri & Cie. (BBC), appeared astonishingly up-to-date: It was a square-faced, all-steel, cab-over-engine vehicle built on a low-frame chassis with a floor level of 70 centimeters above the ground and with wide, double folding doors at the front and in the middle. The engine output was 73.5 horsepower; the top speed was 40 km/h. But what made it special was that the contact-wire bus, as it was called then, needed no gearshift.

We’ve trodden completely new paths with the electric controls. Starting from the realization that the driver of such a vehicle must direct all his attention to the road, the REGULATION of the motor has been AUTOMATED, i.e., the usual gradual flooring of, or repeated stepping on, the accelerator was eliminated. In this new design the ACCELERATOR ONLY HAS TO BE STEPPED ON ONCE to start off and accelerate; the further shifting of the drum of the drum starter from step to step is performed automatically by a rotary magnet. The rate of progress depends on the supply of power from the motor: the speed adapts to the topographic conditions and is slower on gradients than in the flat. This avoids excessive OVERLOADING of the motor.”

In 1937 the company then launched a complete series of trolleybuses starting with the O 4000 for 39 passengers. However, of the four models, which now came in rounder shapes, only two were ever produced, the O 6000 and the O 10 000. All in all, though, no more than 26 units were produced, as an internal investigation found in 1952. Actually, another 264 orders were received through 1942, but a decision by the Nazi rulers to take trolleybus production out of the war program brought the further completion of these orders to a halt on March 12, 1943.

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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