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


Stuttgart, Germany, Jul 16, 2008

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.

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.
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.

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.
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.

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.
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.

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.

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