Alternative drive systems in Buses of Daimler AG – Part I
23.12.2008 - 00:00

German version

Alternative drive systems in Buses of Daimler AG – Part I
  • 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

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.

Propulsion energy at times of hardship, available ex factory: Mercedes-Benz 170 VG (1935) with wood gas burner.

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.”
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.”

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.

Electric hotel bus built by Motorfahrzeuge- und Motorenfabrik Berlin-Marienfelde (purchased by Daimler Motoren Gesellschaft in 1902) in Berlins Friedrichstrasse, 1899.

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.

Rapid start: In 1908, the Berlin fire brigade opted for the Mercedes Electrique with electric drive and purchased a fleet consisting of four vehicles. The wheel hub motors in the front wheels are clearly visible.

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.

Mercedes-Benz trolley bus for 33 people with 70 hp electric motor, 1930-1931.

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.”

Mercedes-Benz O 6600 T for Argentina - 1952

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.

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.

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.

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.

Mercedes-Benz O 305 TD DUO solo bus in regular service in Esslingen/Neckar, 1979.

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.

Mercedes-Benz O 305 GTD DUO articulated bus in regular service in Esslingen/Neckar, 1979.

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.”

Milestones O 305 GTD and O 405 GTD: The long road to large-scale production

Further development work therefore concentrated on the combination of overhead cable and diesel drive. Instead of solo buses, without exception articulated buses now were used because the additional cost of the electric drive was almost equal, but could be distributed over a larger number of passengers. However, in the case of the very first articulated bus put into service in 1979 the electric drive proved too weak with its 80 kilowatts continuous output and 180 kilowatts peak output: to simplify matters the same Bosch motor had been used that powered the solo buses.

For this reason the new dual-powered articulated buses of model O 305 GTD were equipped with a more powerful unit from AEG. This time the continuous output of the electric motor was 180 kilowatts, and the diesel engine developed 280 hp. Nevertheless, the electric drive system, 445 millimeters high and featuring controls with sophisticated chopper technology and gate-controlled switches, easily fit under the level bus floor so that – owing to the low weight as well – there was no need to sacrifice passenger capacity. Two prototypes saw service from 1982 in Esslingen, one in Essen, while another bus remained at the testing facility in Rastatt as a reference vehicle. The company also tested the O 305 GG double articulated trolleybus on an O-Bahn test track there.

Mercedes-Benz O-Bahn large capacity bus O 305 GG, suitable for automatic track guidance, e.g. on narrow city centre routes, 1981.

“In the course of almost two years of testing, it turned out that the principle of electric direct drive, which is used exclusively in rail operation, is also suitable for the dual-powered bus and affords great advantages,” the manufacturer’s final technical report summed up in 1986: “Following the anticipated teething problems and their elimination, the vehicles prove very reliable.” As sole drawback the report cites a 15 to 20 percent higher original price, which could not be reduced any further owing to the double drive.

Emission-free route: The series production duo bus, here's a copy from the year 1993, has a pure electric drive with dual energy source. The Power comes from either a sub-floor battery or arrived via trolley into the vehicle ( "O Bus"). This brings flexibility away from the cable network.

In the meantime, the O 405, the standard regular service bus of the second generation, had gone into large-scale production and now also constituted the basis for the production of the dual-powered buses. The O 405 GTD adopted the 250 or 300 hp six-cylinder OM 447 hA/hLA from the diesel production vehicles. Initially 18 went to Essen starting in 1986, where they operated on two O-Bahn track-guided busways at speeds of up to 75 km/h and partly used a route together with streetcars. Esslingen initially received seven O 405 GTD dual-powered buses in 1988; by 1995 the total came to 18.

At the same time the dual-powered articulated buses began to go underground in Essen, into the subway system. For this purpose, several units were even fitted with doors on both sides so that they could use intermediate and side platforms. This, of course, was not the only change required to take it up with subway trains in tunnels: in contrast to the rail-bound subway train the trolleybus needs a double overhead cable. The question of the switches had to be cleared up. The safety system had to be reassessed. And in the long run the gravel bed was not up to handling bus operation. Finally, after seven years Essen terminated tunnel operation in 1995.

In addition to the dual-powered articulated buses, Daimler-Benz also manufactured 100 pure trolleybuses of type O 405 GTZ for Switzerland. The “Z” refers to Zurich, where most of the vehicles saw use. By contrast, the O 405 GNTD which was manufactured in 1995 and also went to Zurich remained a unique specimen.

Mercedes-Benz O 405 GTZ

O 405 GNTD: The renaissance of wheel hub motors

With the O 405 GNTD Daimler-Benz again took up the idea of wheel hub motors, now almost a century old. Since the early days of the “Mixte” drive system enormous progress had been made, of course. In the earlier experimental electric vehicles of the 1970s, DC shunt motors had been used, whereas the four ZF wheel hub motors of the dual-powered articulated bus were asynchronous motors that worked with three-phase current. Their continuous output was 50 kilowatts each; the maximum output, 75 kilowatts. Outside the trolley wire network the energy was supplied by a 300 hp OM 447 hLA diesel engine.

Just one example survives of the O 405 GTND with wheel hub motors produced in 1996, which saw service in Zurich.

Apart from the Zurich dual-powered articulated bus, 17 further articulated buses of type O 405 GNDE without current collectors but with a diesel-electric hybrid drive were completed, the first of which went into operation on April 28, 1997, on Line 42 of Stuttgart’s local public transport company. The articulated buses were preceded by the 1117 Hybrid, a medium-duty truck presented by Daimler-Benz in 1994 at the Frankfurt Motor Show, as test vehicle. Wheel hub motors promised advantages mainly in low-floor buses where there was no room for a transmission and other power-transmitting components. Installed transversely and horizontally in the rear of the bus, the 300 hp EURO II engine OM 447 hLA drove a generator that supplied current to the four wheel hub motors.

Mercedes-Benz articulated buses with diesel-electric drive, O 405 GNDE.

Daimler-Benz went a step further with the low-floor interurban hybrid bus O 405 NÜH. Unlike the O 405 GNDE the interurban bus could switch off the diesel engine and continue operating in town centers in pure battery mode, emission-free. For this purpose the manufacturer tested novel sodium-nickel chloride batteries, also called zebra batteries, for the first time. They were developed by the Daimler-Benz subsidiary AEG in Ulm. They were considerably lighter than the old lead batteries: a set of four batteries weighed 800 kilograms, was mounted on the roof, and worked at an operating temperature of 300°C.

The low-floor O 405 NÜH interurban hybrid bus of 1996 was powered by wheel hub motors. The latter were driven by a diesel engine and generator and recharged by the battery.

Four vehicles of this type went into operation in September 1996 between Oberstdorf, Sonthofen and Kempten, sponsored by the Bavarian State Ministry for Regional Development and Environmental Protection. Theoretically the low-floor buses could have gone some 30 to 40 kilometers in pure battery mode. But this was not the reasoning behind the test setup, for in this case the bus would then have needed a longer downtime to recharge the batteries. The distance which the bus covered in emission-free operation in the centers of the health resorts was only about ten kilometers. The aim of trial operation was to make optimal use of the power of the diesel engine through intelligent energy management and so simultaneously cut consumption and reduce the burden on the environment.

For the two-axle bus the 250 hp version of the OM 447 hLA was sufficient. The turbodiesel powered a generator which produced alternating current which had to be rectified for control purposes. An inverted rectifier then converted the direct current into three-phase current to drive two asynchronous wheel hub motors. On the other hand, owing to the high field-weakening ratio of the electric motors a transmission was not needed: between 0 and 85 km/h the diesel-electric drive operated infinitely variably. In addition, the constantly high torque of the asynchronous motors also came in handy on the hilly routes of the Allgäu region, ensuring startability on gradients of 18 percent.

The zebra batteries were charged with the excess energy from the generator while the bus was moving, but also through braking energy recuperation, for the wheel hub motors normally also functioned as service brakes and so acted as a kind of generator themselves. The bus had drum brakes on the front axle and disc brakes in the rear merely in the event they were needed for emergency braking. To store the braking energy in the batteries, however, the three-phase current first had to be reconverted to direct current.

Ideally the batteries should never entirely discharge, but also should never be fully charged, as otherwise the energy remains unused and has to be given off to the environment in the form of heat. Synchronizing the different components with each other and tuning them to actual route conditions called for complex electronic controls, likewise developed by ZF. Recent advances in semiconductor technology had made them possible. Optimum environmental performance could only be attained if the use of the diesel engine with its relatively higher emissions was restricted to what was absolutely necessary. The aim of practical testing also was to arrive at a state of equilibrium: starting from the concrete characteristics of the routes on which the buses regularly operated, the batteries were to be drained in the course of a day exactly as much as they were recharged from the electric grid during the course of the night.

In practical testing, however, the wheel hub motors still were unable to deliver really convincing performance: after one-and-a-half years of trial operation from October 1997 to March 1999 the Zurich bus came back to Stuttgart first and then went on to Friedrichshafen for further study. After teething problems the 17 Stuttgart vehicles managed to remain in operation until 2002. One of the O 405 NÜH interurban buses was taken over by Berlin’s local public transport company in 2003 for further testing.

Hybrid drive in a production vehicle: The Cito midibus

In 1999 Daimler-Benz introduced a diesel-electric production vehicle of original design which occupied a special market segment: available in three lengths from eight to 9.60 meters for 45 to 65 passengers, the 2.35 meter wide Cito was especially well suited for negotiating the winding lanes of historic old town centers, but also for shuttle operation at airports and trade shows like the International Commercial Vehicle Show (IAA) in Hanover. The 125 hp four-liter diesel engine OM 904 LA, a generator and the asynchronous drive motor were combined into a soundproofed power pack in the rear to save space.

Diesel-electric operation in cities: In the Mercedes-Benz Cito (1998), a four-cylinder engine powered a generator which produced electricity for the traction motor. Purely electric operation was possible over short distances.

Vehicle behavior was particularly quiet and comfortable also owing to the infinitely variable acceleration, entirely without a transmission. And the bus had a low floor throughout, an entry height of just 32 centimeters, outstanding visibility through large windows, and a highly effective heating and air conditioning system. Moreover, with a gross vehicle weight of 11.1 to 12.5 tons thanks to plastic and aluminum components, the Cito definitely was a lightweight. As might have been expected, one year after its launch the midibus won the coveted “Bus of the Year 2001” award. At the same time the Mannheim local public transport company put five 9.60 meter long vehicles into operation.

In the long term, however, the order inflow fell short of expectations so that EvoBus terminated the project in 2003. Exactly 643 units had been produced by then.

Photos and text:
Daimler AG


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