ZF Elextric Drive Axle for Low-floor Buses

A concept for a new serial hybrid drive for city buses 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. 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. 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.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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