Historic Bus With Tomorrow’s Powertrain

In many metropolises, tourists use doubledeckers converted into an open-top bus for city sightseeing tours. Yet nearly all these historic vehicles are driven by outdated diesel engines which, with frequent starts, stops and idlling, create unwelcome noise, vibrations and emissions. In cooperation with the drive technology specialist Ziehl-Abegg and the vehicle manufacturer Tassima, IAV is equipping Berlin tourist buses with an environmentally friendly, innovative wheel-hub electric drive. automotion interviewed the project partners: Ralf Arnold is the Managing Director of Ziehl-Abegg Automotive based in Kupferzell, a manufacturer of drive systems such as wheel hub motors and entire axle drive modules for electric double-deckers. Utz-Jens Beister is the Managing Director of IAV Cars and is forging ahead on the electric bus project together with experts from Commercial Vehicles Powertrain and IAV’s e-drive specialists. Roland Prejawa is Chairman of the Supervisory Board of Tassima, a Berlin electric vehicle retrofitting specialist, whose good contacts with the city tour firms enable the electric double-deckers to be launched on to the roads.

When was the last time you travelled on a city tour bus and why?

Utz-Jens Beister: I’d already been on a city tour bus in New York, as such a bus allows you to visit many places in a very short period. It was just as exciting, though, to catch the impressions of the other bus passengers – either in direct conversation or watching how they perceive the city. Otherwise, I rather explore other cities as a tourist on foot.

Ralf Arnold: My first city tour using hop-on/hopoff buses was in Berlin. Our three children were young at the time, and I felt it was very practical for us to be able to leave the vehicle at a place of interest and hop on again later. That enables five people with different interests to be catered for quite well together. Even back then, I said to my wife: ”This loud diesel engine is really annoying and the simple earplugs don’t close your ears properly. This means the commentary on the individual sights was not always easy to understand.“

Roland Prejawa: In big cities I travel a lot on public transport. I also think bike rental is a good way of exploring a city. I like city tour buses to get a good overall picture of a city. It’s a good first step, as you’re driven around passively in a city, and then you can pinpoint places of particular interest to visit.

What made you think of electrifying doubledecker buses for tourists?

Prejawa: End-of-life double-decker buses from Berlin’s public transport company have been converted into tourist buses since the 1980s. In the early 1990s, an English company approached us with the idea of taking the roof off the old double-decker buses. That immediately produced roars of laughter, but after the first test run in the open-top bus we realized it was just right. In an open bus the tour offers a totally different experience from that in a closed passenger compartment. Nowadays every company has an “opendeck” bus. For me, electrification is the logical continuation: the historic vehicles are given a modern e-drive that allows them to carry on for a long time. For companies, that pays off as well.

Arnold: What sets our drive system apart is that the rear axle forms a complete, ready-togo power unit with the wheels. This enables a conventional diesel powertrain to be converted relatively easily into an electric drive: the old drive shaft is removed and replaced by the electric axle. We call this type of very simple conversion a retrofit. When we found out about the Berlin project, we offered our system to put it to good use.

Beister: At first sight, it seems quite easy to replace the conventional powertrain by an electric axle module and a battery system. There are also clearly defined interfaces. At the end of the day, though, it’s rather more complex. The buses are up to 35 years old, and some don’t have the latest electrical systems. The converted overall system not only has to work well, it also has to be safe and stand up to an approval procedure. That is why we came in as an engineering partner. I find it particularly fascinating that we tackle the important issue of electrifying commercial vehicles in this project – something that has tended to have been neglected so far, especially in Germany.

Modern technology in a historic bus – that actually sounds exciting. What are the challenges in integrating the systems?

Beister: They are omnipresent, actually. For instance, integrating a 700-volt battery and a 24-volt vehicle electrical system. Or in the human-machine interface as the old bus has no display system on the battery’s state of charge. Or in the compressed air for the brakes, or the pneumatic operation of the doors. This used to be done by a compressor driven by the diesel engine. We had to find a new solution for all theses challenges.

What are the electric drive’s benefits for the city, operators and tourists?

Beister: Firstly, there is political discussion of driving bans in city centers. One day there may be exceptions for scheduled services, but presumably not for city tour operators. For the latter it is, of course, vital that they have direct access to the attractions in the city centers. Secondly, when driving with many stops and restarts, the diesel engine produces particularly strong noise and vibrations. Mr. Arnold has already mentioned his experience. A silent, locally zero-emission bus is far more attractive for bystanders and passengers.

Prejawa: The market for city tours in Berlin is highly competitive. If you offer tours with electric buses, you can win over a few customers who perhaps deliberately opt for this alternative. And acceptance by the general public ought to increase if the buses are quiet and emission-free. On some streets, the burden for residents is quite huge. Another advantage is that the e-drive has fewer wear parts. A diesel engine is not designed for continuous operation at 10 or 20 km/h and constant stops. Sometimes the cylinder head gasket fails or the exhaust falls off, or both. What I expect from e-drive is for these high repair costs to plunge. What is more important, however, is that based on our calculations in a typical driving cycle the net energy costs for electricity halve those for diesel, in some cases they are down by 80 percent.

Technically speaking, the wheel hub motor sounds particularly exciting. Why did you opt for this type of drive? Does it provide it sufficient power output?

Arnold: We have been developing wheel hub motors for 18 years now. Our concern has always been to achieve the best efficiency possible. Our electric drive system for the Berlin tourist buses comprises two permanent magnet synchronous motors installed directly into the two wheel hubs on the rear axle. This type of motor is particularly efficient in part-load operation, when starting for example. The system also works without transmission: the rotor speed equates to the wheel speed. That creates more benefits in terms of efficiency, as well as noise production – by the way, not just in actual driving, but also in recuperation, i. e. recovering brake energy. You don’t have to worry about power output: the axle produces up to 8,750 Newton meters of torque per wheel on the road.

Where else do you see the system being used?

Beister: With our concept we have come up with a very good balance between replacing and retaining. It makes no sense to convert every sub-system in the bus – not just for cost reasons, but the whole thing has to be homologated again. I can well imagine our pragmatic approach to electrifying city tour buses being transferred to scheduled service buses or to commercial vehicles in distribution transport.

Thank you very much, gentlemen!