E-Mobility in local transit Clean, quiet city travel with ZF technology

Whenever people discuss electromobility, they tend to talk about cars. But electrically powered buses could also solve the noise and pollution issues that plague inner cities, improving the quality of urban life. In Germany, experimental e-buses are now running in the cities of Stuttgart and Mannheim – fitted with ZF technology.

Mehmet Imreras is a passenger traveling on bus route 79 from Stuttgart-Plieningen to Stuttgart Airport. He gazes pensively out at the cornfields shining golden beneath the fall sunshine, and occasionally glances at a monitor inside the bus. The display shows a kind of X-ray image of the vehicle, on which various differently colored lines are flickering.

Trials to establish whether electric buses are also capable of withstanding the daily rigors of passenger transportation are also running in Mannheim.

Subsidized by the German Federal Ministry of Transport and Digital Infrastructure, Stuttgart’s 79 bus is a fuel-cell bus that runs entirely on electricity, emitting no exhaust gases at all apart from some water vapor. That’s because it’s a Citaro FuelCELL Hybrid built by Mercedes-Benz. In this case, the term “hybrid” applies to the combination of fuel cell and high-voltage battery. Both of them are mounted on the roof of the bus, along with seven high-pressure hydrogen tanks and their cooling units. The monitor inside the bus shows the energy management systems at work, controlling the interaction between the fuel cell, lithium-ion batteries and electric motors. The latter are part of ZF’s all-in-one AVE 130 low-floor portal axle, and are mounted right next to the wheels.

Like all electric motors, they can also run in generator mode, in which case they feed power back into the batteries. The process is known as regenerative braking, and bus drivers are given special training in how to use it effectively. “Unlike diesel vehicles, you should deliberately keep on braking for as long as possible, so plenty of current flows back into the batteries,” explains Markus Modlmeir, Director of the Bus Driving School run by Stuttgarter Strassenbahnen AG (SSB), Stuttgart’s regional transit operator.

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No difference for passengers

This all-electric technology doesn’t impose any changes on passengers. In fact, Mehmet Imreras only realizes that he’s traveling in an electric bus when he’s asked about it. Other passengers have noticed that the bus does run more quietly – especially when pulling away from bus stops – and unlike diesel buses, doesn’t change gear. As for boarding and alighting, the vehicle has all the usual advantages of a full low-floor bus. It also has just as much space inside for passengers.

SSB is testing four of the fuel-cell hybrid buses in operation. The trial is expected to last until May 2016, and has incurred some extra costs. A total of 200 drivers were given an induction course. And in order to maintain the roof-mounted structures, SSB had to train staff how to handle high-voltage systems. They also had to purchase special stands and mobile scaffolding for working on vehicle roofs. “Traditionally, we’re happy to partner with companies who want to test new drive systems for buses,” says Press Officer Birte Schaper. “This gives us an early insight into technologies that may be suitable for future use, once the time comes to replace diesel engines.”

For passengers, it’s important that buses should be reliable. For city authorities, cost efficiency is another key consideration. Meaning that electrically powered city buses could win a hands-down victory once their costs are on a par with diesel buses.

Inductive charging in Mannheim

Mannheim is also involved in testing electric solutions for bus services. Since June 2015, two prototype all-electric buses have been running on bus route 63 in the city center, between Pfalzplatz, the university and the central railway station. Once again, ZF’s electric low-floor axle is a key component. But the special thing about this system is that current is passed to the buses by six charging stations buried under the surface of the pavement at bus stops – using the wireless principle of induction, just like an electric toothbrush.

With expert precision, driver Jacqueline Limpio positions her electric bus above the charging pad at the Pfalzplatz bus stop, then presses a button to lower the metal plate holding the bus battery down into the induction field. To ensure that no foreign objects such as soda cans are sandwiched between charging pad and battery, the underside of the bus is fitted with a camera system. If she had to, Jacqueline Limpio (30) could reverse the bus and use a broom to sweep debris off the charging pad. “But that’s never happened yet,” she says. Like 40 other bus drivers employed by transit operator Rhein-Neckar-Verkehr GmbH (RNV), she has been given a special training course in inductive charging procedures.

Using Openmatics to monitor charging status

Because the electric buses have to fully recharge their batteries at each end of the line, they can’t make up for any delays by shortening the time they spend there. Even so, Project Manager Sebastian Menges is very pleased with the results. “The charging process is incorporated into the total trip time, which means the two electric buses can travel up and down route 63 all day long,” he explains. Without any recharging at all, the two Hess buses can complete two circuits of the bus route, a total of 12 miles. But the aim is to keep the battery charge high by frequent inductive charging – a process known as “opportunity charging”. At present, a conventional diesel bus helps out for a few circuits each day, enabling the resting e-bus to fully recharge its batteries. For continuous, detailed information about the vehicles’ charging status and charging stops, RNV relies on Openmatics. The telematics platform – supplied by ZF – stores all relevant data, making it easier to evaluate the project. “To date, the availability of the electric buses has exceeded 82 percent,” says Sebastian Menges. “That’s significantly higher than we were expecting.” This means that on bus route 63 alone, at least 150 metric tons of CO₂ are being saved each year. Inge Jekel feels the benefits of the new buses very directly.

Because she lives right next to the bus route and regularly travels on the buses, she has been following the project with great interest. “I think it’s great,” says the pensioner. “After all, everybody gets something out of it – the air is cleaner, and it’s much quieter every time a 63 bus drives down the street.”

The fuel-cell bus in Stuttgart connects the Plieningen district with Stuttgart Airport. Its fuel cells use the chemical reaction of hydrogen and oxygen to generate electricity. The energy is either used for driving, or temporarily stored in batteries.

Pictures: Matthias Schmiedel

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