ZF Electric Drive for low-floor buses

By: Chris Smith


At the IAA Commercial Vehicles 2008, ZF will present a concept for a new serial hybrid drive for city buses

At the IAA Commercial Vehicles 2008, ZF will present a concept for a new serial hybrid drive for city buses which is currently being developed by ZF Passau GmbH, the ZF Off-Road Driveline Technology and Axle Systems division.

Two electric motors are integrated into the axle and are also used for energy recuperation during braking.

The serial hybrid drive concept makes particularly economical and environmentally friendly operation possible, in particular for city buses with tough inner-city applications characterized by numerous starting and braking processes.

Suitably dimensioned electric motors allow for zero-emission driving in certain operating states, for example at traffic lights and bus stops or when accelerating out of the bus stop.

Attractive for bus manufacturers: The hybrid drive axle does not require more installation space than a low-floor portal axle, type AV 132.

Today's short-distance passenger transport by rail is not imaginable without electric motors; they are quiet and highly efficient.

Now, their advantages can also be made use of in buses – also independent of overhead lines. Together with modern battery technology, they can store braking energy.

Thus, the diesel engine is not the only energy source anymore and can be much smaller in size. Having no fixed ratio with the wheel – energy is supplied to the system via a generator – the combustion engine can be operated additionally and optimally within its map or switched off completely.

Also in this operating state, the serial hybrid drive still offers full driving performance powered by the battery. These advantages have particularly strong effects in city traffic.

City buses are comparatively slow-moving: traffic lights, high traffic volume, bus stops. All of these things leads to frequent starting and braking as well as standstill times. For this reason, hybrid drives are a particularly economical and resource-preserving alternative.

Electric motors with several power sources
The electrically driven low-floor axle has been designed for city buses with a maximum axle load of 13 tons.

In the ZF concept, each wheel is driven by a high-speed asynchronous motor with a downstream two-stage ratio. Due to the fixed ratio, no gear changes are performed which contributes to increasing driving comfort. Short-term, maximum drive power of 240 kW and continuous power of 120 kW is available per axle.

The liquid-cooled asynchronous motors can be powered by several energy sources – also in combination – which makes this drive solution especially flexible: Besides diesel-electric power and batteries, also overhead lines and fuel cells are possible. Also auxiliaries, such as power-steering pump, air-conditioning compressor, and compressed-air unit can be operated in the electric system and achieve the same advantages as the vehicle drive system: Depending on the demand, they are activated only occasionally and allow for further fuel savings.

The environment is protected not only by saving fuel. Thanks to the balanced operation of the combustion engine, excellent emission values are possible.

In especially critical ranges, even zero-emission driving is possible. Also the noise level is lowered, both by the smaller engine and its operating method. Starting at bus stops can even be purely electric which proves advantageous for both residents and passengers.

Despite the integration of powerful electric motors, the required installation space of the hybrid drive axle almost corresponds to that of the standard portal axle for low-floor buses, which enables its use in a vehicle family of standard diesel buses without having to do complex adjustments of the undercarriage.

Due to the low contour of the axle support, it is possible to install it as central axle or rear axle in the articulated buses. As the drive energy is fed in via power cables, vehicle manufacturers have a new flexibility in how to arrange their components.

Two driven axles are no problem, many electric parts can be arranged on the roof in a space-saving manner, and a compact motor-generator unit in the back makes interior designing easier.

Wherever maintenance is required – brakes, brake disks, wheel bearings, and their seals – standard components have been used in spite of the innovative axle concept. This means that also buses with hybrid drive axles can be integrated into a fleet in an easy-to-service manner. Before this is the case, ongoing tests have to be completed in Passau.

From 2010 on, the electric low-floor axle will be available as a standard equipment.

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