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Fraunhofer Testing its Flywheel-Hybrid Tram

by Rafael Seidl

Energy and propulsion options for AutoTram. Click to enlarge.

Fraunhofer engineers are now testing a prototype of the AutoTram: a rubber-wheeled, trackless, flywheel-hybrid tram. (Earlier post.)

The basic version of the AutoTram is a three-axle diesel-electric hybrid. A 245 hp (183 kW) V8 Euro-3 diesel powering three 45kW asynchronous motors (one for each axle) and uses a 4 kWh flywheel energy storage system. Fraunhofer is also developing an 80kW fuel-cell variant.

The CCM flywheel.

A third option under consideration is the use of galvanic docking stations featuring grid-connected stationary ultracapacitor stores at bus stops to support “plug-in” (PHEV) operation.

Each of the three axles can be steered independently. The basic diesel hybrid AutoTram has a top speed of 70 km/h (43.5 mph) and has an all-electric, zero-emission range of up to 2 km (1.2 miles).

At this point, only a prototype exists. It is being put through its paces at the IVI test track. Now the researchers are hoping to build a roadworthy version but need federal funding to do so. It would take another two years before the AutoTram could be seen on city streets. Matthias Klinger [Frauenhofer Institute for Traffic Infrastructure Systems] is optimistic: “The AutoTram yields savings of 30 to 50% for cities because they do not need to put down any tracks. Leipzig and Dresden have already expressed interest.”

—(Die Welt)




I like the docking station at the stops. It would be an ideal way for a plugin bus to be able to use electricity from the grid (or other sources) >90% of the time, with an APU covering the rest (emergencies, or due to circumstances). The one problem may be that it is built into the curb, and can get clogged/coated with debris/deicer salt. Perhaps a lampost overhead outlet, above the street at the stop, may be the solution.
_The fuel cell is probably PEM. I would like to see SOFC for multifuel capability. A stirling waste heat scavenger, (further down the line thermoelectrics) could boost the efficiency.

Rafael Seidl

Allen -

the fuel cell option is based on a Ballard PEM stack rated at 80kW. At this point, I am not aware of anyone contemplating an SOFC in any road vehicle; they are bulky and heavy. Perhaps the long ceramic tubes cannot tolerate significant vibrations at high temperatures, either.

The docking stations are indeed an interesting idea. However, integrating the conductive couplers proposed by Fraunhofer in the curb could indeed prove problematic: in addition to salty slush in winter, there are juvenile idiots, suicidal maniacs and dogs to consider. A more reliable - and safer - alternative would be to mount the sockets high above street level, e.g. on top of the bus stop structure's roof or on a pole.

Alternatively, high-frequency inductive couplers embedded in the road could perhaps be used. Engineer-Poet recently pointed out that ferrous trash could pose a hazard (high-frequency magnetic fields would cause it to heat up). It's a valid concern, but perhaps a manageable one for isolated recharge points at bus stops.

The pros and cons of flywheel energy stores were recently discussed here as well. From a safety perspective, I would prefer to see ultracap banks (perhaps ultracap/battery combos) on-board and stationary flywheels near the docking stations, i.e. exactly the opposite of what Fraunhofer is proposing.


There was a Swedish proposal for a flywheel powered bus that recharged it power through a system of contacts mounted on an arm that stuck out over the street like a stop light pole. When the bus stopped under it, two arms would rise up and make contact to provide power to energize the flywheel. When the bus was ready to go, the arms would retract and the bus would be on its way for another 5 miles.


Conductive connections are the way to go; they eliminate the conversion losses and EM-field issues of magnetic systems, and they're vastly cheaper.  If the TGV can use overhead wires, there's no reason buses can't.


can someone explain to me what exactly makes this a tram, if it's rubber-tired, trackless, and has no overhead wires? come on, call things by what they's a bus.

Rafael Seidl

Technically, the light rail aspects are the availability of a driver's cabin fore and aft, the video/DGPS-assisted three-axle steering (virtual track) and the ability to couple up to three vehicles into a train with 300 seats.

The concept is somewhere in-between a regular articulated bus and a tram. Perhaps the most critical aspect is customer acceptance: light rail has traditionally been perceived as a higher quality service, chiefly because the tracks represent dedicated routes that are permanently segregated from the congested roads.

This applies especially to subways in large metropolitan ares. The AutoTram may be a better fit for smaller cities (<1 million inhabitants).

Rafael Seidl

Lensovet -

btw, at least one line on the Paris metro also features rolling stock with (solid) rubber tires running on concrete guide tracks, with a conductive third rail in-between. What makes it a light rail system is the fact that the track is dedicated and, the total length of the vehicle.

Tim Russell

The Montreal metro is a rubber tire system. Nice and smooth ride too. As with the Paris system it is a dedicated track so again it's light rail.

It seems that a pair of contacts on an arm that can rise up and connect to an overhead souce would work well. It would be automatic or at least automated and being high above the street level keeps people out of harms way.


It's a bus.


let's not confuse terminology here...the paris METRO is rubber-tired. but it's a metro, aka rapid transit, aka subway, aka underground.
that has nothing to do with a tram. location of the cab is also a remote feature of a tram and only a recent one - most trams only have a cabin in the "front."
the main reason why anything on rails is perceived as higher quality is because it is. a ride on rails is in most cases smoother, not really subject to potholes, and also generally faster.
p.s. light rail is just that - light. most subway systems are considered heavy rail.


In most mass transit systems that I've seen, subways are considered "heavy" rail while streetcars and trolleys are considered "light" rail. By the way, streetcars (i.e. trolleys running in mixed traffic -- most trolleys were run this way in America before WWII) are rarely used in the U.S., in large part due to federal regulations aimed at discouraging them.

A different definition of "heavy" versus "light" might obtain if you were talking to a railroad engineer who worked with mainline freight systems -- then, only railcars that complied with federal crashworthiness standards (or their European equivalents) would be considered "heavy," and would be allowed to run on mainline trackage; subways cars (even if their wheels are built at standard gauge width) usually do not comply. "Tram" is a popular term in Europe to refer to what Americans would probably call a trolley, usually operated on a street level but separated right of way. We also have to note the existance of trolleybus systems (buses powered by overhead electric lines which run in mixed traffic) and Bus Rapid Transit systems, which run bus-like vehicles in dedicated rights of way for all or a substantial portion of their routes.

Sorting through this alphabet soup is by no means clear cut, and dickering about semantics can quickly lead to unproductive hair-splitting. The functional factors are what are important. These include:

1. Grade separation and/or dedicated right of way, versus operation in mixed traffic. Buses and old streetcar systems fall in the latter category, subways and newer light rail systems in the former category. BRT often attempts to use dedicated rights of way part of the time, with mixed traffic operation for the other part, with varying results. Operating in mixed traffic tends to slow you down a lot, but allows flexibility in designing routes and curb-to-curb service.

1a. All dedicated rights of way are not created equal. Designating a certain lane in a boulevard to be a "bus only" lane still makes it subject to cars which break the law, snow accumulation, adverse traffic lights, and intersections where space is constrained, causing the dedicated lane to disappear for a few critical yards, significantly slowing the BRT vehicle. This affects the Silver Line Dudley Square Branch in Boston. Even trams that run in dedicated median tracks (like the Green Line in Boston) tend to be subject to adverse traffic lights and crossing car traffic. Completely grade-separated tunnels or elevated tracks are the most immune from such interference, but are very costly to construct. Mainline rail trackage (think commuter trains that run over regular tracks) has grade crossings, but since trains are always given right of way, this does not affect operations much, save for when a car gets stuck on the tracks and a catastrophic crash results. Truly separated rights of way (for rails or rubber tire operation) also tend to have smoother rides, while mixed traffic operation has more potholes and jerky "start and stop" driving patterns (which tend to give me a headache). The latter can also affect a dedicated right of way subject to too many stops for crossing car traffic.

2. Onboard power plant versus externally provided power. Buses tend to have the former, streetcars, trolleybuses, grade-separated trams and subways the latter. If you are already using a fixed right of way, adding power lines usually only adds marginally to the cost, while saving on fuel, maintenance, air quality and the like. In tunnels, adding ventilation to allow for the use of combustion engines would add considerable cost, and adding wires or third rails makes a lot of sense. If you are operating in mixed traffic (or on unelectrified freight rail trackage) external power tends to constrain routes unduly. BRT systems which operate partly in tunnels and partly in mixed traffic (Boston's Silver Line Airport Branch and the bus routes that go through the transit tunnel in Seattle are examples) tend to be dual mode, allowing for electric operation while in the tunnel and combustion operation in mixed traffic.

3. Ability to string multiple cars together to form a "train." Buses and BRT vehicles typically cannot form trains, while subway cars and modern trolleys / trams can. Trains allow for high capacity and variable capacity operation, which is useful for rush-hour operations in big, dense cities.

4. Size of vehicles. Buses tend to stay within a more constrained size and ground clearance envelope (relating to the practical and regulatory needs of operating on public roads in mixed traffic), while subway cars, trams and the like tend to have larger sizes and shapes, and different ground clearances. These issues of form tend to relate to the functions noted previously -- higher bus ground clearance needed so as to clear potholes and uneven roads, subway vehicles sized to fit on whatever gauge track is being used, higher passenger capacity needs, etc.

The technology described in this posting seems to be useable in several contexts. It could be used in a mixed traffic vehicle akin to a standard bus, a BRT vehicle operating full time or part time in a dedicated right of way, or a tram system operating in a dedicated right of way and equipped to form trains, etc.

Calling this drivetrain a bus or a tram at this stage seems premature. Depending on the system into which this drivetrain is incorporated, it could be one of several different things.


Final note:

The 1.2 mile range periodic recharge system probably defies conventional categorization, in that you'd need a good number of remote charging points to make such a system useful. While you are not stringing up a set of continuous power wires, you are also not restricting the system to a few-times-a-day battery pack swap-out, or the like. Charging can only take place while the bus is stopped at a station, I imagine. How long does a top off providing 1.2 miles of range take? How much does it cost?

This is electricity, external power of a sort, stored in smaller quantities and for short periods aboard the vehicle, frequently yet not continuously supplied, and not the only source of power for the vehicle. We should probably invent a new name to cover this sort of power system, and I'll leave it to others to come up with clever suggestions.


Well, if the typical bus drivetrain is 25% efficient (guesstimate) and it gets 4 MPG on diesel (at 140,000 BTU/gallon), the energy to the wheels is 8,750 BTU/mile or about 2.6 kWh/mile.  If you can send juice to the bus at 480 volts and 1000 amps, that's 480 kW or 2.6 kWh in about 20 seconds.

Cheryl Ho

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