E-Mobility in local transit SOFT-SHOE CITY SHUFFLE

Whenever people discuss electro mobility, they tend to talk about cars. But electrically powered buses can also solve the noise and pollution issues that plague inner cities, improving the quality of urban life. Buses with ZF technology on board are now running in many cities in Europe and Asia.

Energy management

The 79 bus to Stuttgart Airport sets off from Plieningen, Stuttgart’s southernmost suburb. As the bus passengers blink sleepily in the slanting spring sunshine, they occasionally glance at an information display inside the vehicle. It shows a kind of X-ray image of the bus lit up by various colored lines.

Subsidized by Germany’s Federal Ministry of Transport and Digital Infrastructure, the 79 bus is a hybrid fuel-cell vehicle. It runs entirely on electricity and emits no exhaust gases at all, apart from some water vapor. That’s because it’s a Citaro Fuel Cell Hybrid built by Mercedes-Benz. In this case, the term “hybrid” applies to a 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 display inside the bus shows the energy management systems at work, controlling the interaction between fuel cell, lithium-ion batteries and electric motors. The motors, an integral part of ZF’s all-in-one AVE 130 low-floor portal axle, are mounted right next to the rear wheels.

Pilot projects involving buses fitted with ZF’s electric axle drives are currently running on the streets of many Asian and European cities (see box); these test vehicles have already clocked up millions of miles. Like all electric motors, the AVE 130 can also work as a generator, feeding electric current back into the batteries. The process is known as regenerative braking, and bus drivers are given special training in how and when to use it. “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 SSB (Stuttgarter Strassenbahnen AG), Stuttgart’s regional transit operator.

This all-electric technology makes no difference to passengers. They do notice that the bus runs 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 field-testing four of the fuel-cell hybrids, at some extra cost. 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 accessing the vehicle roofs. “We’ve always been happy to partner with companies who want to test new drive systems for buses,” says Press Officer Birte Schaper. “This gives us useful early insights into technologies that may be suitable for future use once the time comes to replace diesel engines.”

Charging bus batteries by induction

Charging pad

The city of Mannheim is also testing possible electric bus solutions. Since June 2015, two all-electric prototypes have been operating in the city center, working the bus route 63 between Pfalzplatz, the university and the central railway station. ZF’s electric low-floor axle is not the only stand-out feature: current is passed to the buses by six charging stations buried beneath the road surface at bus stops, using the wireless principle of induction – just like an electric toothbrush.

With practiced precision, driver Jacqueline Limpio (30) positions her electric bus above the charging pad at the Pfalzplatz bus stop and presses a button that lowers the metal plate holding the bus battery down into the induction field. To ensure 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.

A conventional diesel bus helps out for a few circuits each day while the resting e-buses fully recharge their batteries. For continuous, detailed information about the buses’ state of charge and charging cycles, Rhein-Neckar-VerkehrGmbH (rnv) relies on Openmatics. Supplied by a ZF subsidiary, the telematics platform stores all relevant data, making it much easier to evaluate the project. “To date, the availability of the electric buses has exceeded 82 percent,” says Sebastian Menges, Project Manager of rnv. “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.

all-electric central drive

As the newest addition to its broad e-mobility solution portfolio, ZF has now developed an all-electric central drive that brings city buses and delivery trucks one step closer to zero-emission driving. The central drive is ideal for vehicles with driveline configurations similar to those currently in use in conventional vehicles, thus making it easy to integrate into existing vehicle concepts. The fact that the central drive can be combined with both single rear drive axles and conventional low-floor axles means it can be installed in low-entry buses and all kinds of low-floor buses.

Electro mobility in buses is an international market trend

The AVE 130 electric portal axle is fitted with two electric wheel-hub motors. Each has a peak output of 125 kilowatts.

The AVE 130 axle system has already proved just how practical it is in multiple field tests over the last few years. More than 300 electric portal axles have already covered over 8.5 million miles in cities throughout Europe and Asia. Much of that distance was covered in Germany, in serial hybrid configurations coupled with downsized diesel generators. The AVE axle can run on electricity from any source, which is why other European and Asian cities also selected it for use in all-electric applications, including battery-powered low-floor buses and trolleybuses. As well as versatility, the electric portal axle has other advantages – it produces less noise, can be installed in the same space as conventional low-floor axles, and has profoundly impressed users by its ability to climb even very steep or lengthy uphill inclines.

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