Axle, chassis concepts on tap for ZF at Hannover

  • 15-Aug-2008 01:08 EDT
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ZF’s low-floor city bus electric drive axle is designed for use in serial hybrid drive systems. Production is scheduled to begin in 2010.

A serial hybrid drive axle concept for city buses, an independently suspended heavy-truck chassis concept, and revised automated and automatic transmissions are among the exhibits that ZF will be displaying at September’s IAA Commercial Vehicle Show in Hannover, Germany.

While ZF expects that parallel hybrid drive systems will be the choice for most vehicle manufacturers, the company believes that the challenges of inner city driving faced by city buses will make serial hybrid drive systems attractive. The company will be showing an electrically driven low-floor city bus axle for use with serial hybrid drive systems at the IAA show.

Designed for a maximum axle loading of 13 t (14 ton), each wheel on the concept axle is driven by a high-speed asynchronous motor with a downstream two-stage ratio. The motor's fixed ratio would eliminate gear changes and the potential discomfort this could bring to passengers. Each motor delivers maximum short-term drive power of 240 kW (322 hp) and continuous power of 120 kW (161 hp).

The liquid-cooled motors can be powered from a variety of sources either individually or in combination. These sources could include an onboard diesel generator, overhead power lines, or fuel cells. The electrical drive system offers further economies by operating auxiliary systems such as the power steering pump and air-conditioning compressor on demand. The required installation space on a city bus is only marginally greater than for conventional portal axles for low-floor buses, ZF claims. It plans to put the electric drive axle into production from 2010.

While independent suspension is relatively commonplace on city buses and coaches, heavy trucks still rely on beam and live axles for their operation. Electronically controlled dampers and air suspension have improved handling and ride for heavy trucks. ZF will display a concept heavy-truck chassis equipped with independent suspension, a pneumatic spring mount module, and rack-and-pinion steering at IAA. Adopting independent suspension would help to reduce unsprung weight, and in turn, the lighter chassis would respond better to irregularities in road surfaces, improving ride comfort and reducing road surface damage.

Since such a system would make the axle bracket obsolete, it could also free up space on the chassis. Available chassis space, particularly on European trucks, has been reduced with the introduction of selective catalytic reduction (SCR) for Euro 4 emissions control, necessitating the addition of tanks to carry urea additive for exhaust aftertreatment. By adopting rack-and-pinion steering, the relay lever and coupling rods can be omitted, freeing up more space and offering further weight reductions.

Similarly, the rear suspension could be simplified because the axle-locating bellows support and the four-point link together would locate the axle both longitudinally and transversely and support the vertical axle load. At the same time, the four-point link performs the active roll stabilization function. Such an arrangement would integrate the functions carried out by longitudinal control arms, wishbones, and antiroll bars into a few components.

ZF has also given details of a development project currently underway to network the driveline, chassis, steering system, and brakes. The project’s three objectives are to provide compensation for load alteration, stabilize driving dynamics, and permit “autonomous driving,” which could permit the remote control maneuvering of a vehicle within a depot to manage collections and deliveries and reduce damage to vehicles.

ZF provides some examples of the advantages of networking. For example, networking the electronic damper control system for the chassis and cab with an automated transmission control system could reduce the cab pitching and rolling that can accompany gear changing. This could be achieved by sharing suspension travel data from axle sensors with the transmission system, so that gear changes could be modified accordingly.

Similarly, yaw rate sensors used in active steering systems could share data with electronic damper systems. By factoring in data on vertical wheel forces, steering correction to counter such hazards as side winds could be further enhanced to counter snaking in articulated vehicles.

Among the modifications to existing ZF products to be seen at the IAA show are revisions to automated and automatic transmission systems for trucks and buses, the addition of hybrid drive automatic gearboxes, a new gear-shifting control strategy to compensate for road topography, and revisions to ZF’s Intarder integrated retarder.

ZF’s AS Tronic automated transmission has seen widespread application in European trucks and buses. This includes AS Tronic lite, based on ZF’s six-speed manual Ecolite transmission, which has been available for lighter trucks but will now be available for light city and suburban buses. ZF will aim the transmission at price-sensitive emerging markets.

There will be two versions: a smaller one designed for buses with gross vehicle weights (GVW) between 6.0 and 12 t (6.6 and 13 ton) and between 6.0 and 9.0 m (19.7 and 29.5 ft) long, with a torque capacity up to 700 N·m (516 lb·ft). The larger variant will be suitable for 9.0 to 18-t (9.9 to 19.8-ton) GVW, lengths between 9.0 to 12 m (29.5 to 39.4 ft), and a torque capacity up to 1000 N·m (738 lb·ft).

ZF’s latest automatic transmission, the ZF-PowerLine, is a six-speed torque converter automatic with a torque capacity between 600 and 1000 N·m (443 and 738 lb·ft). ZF has designed the transmission for light trucks and pickups. Rapid lockup of the torque converter clutch should deliver better fuel consumption. ZF has designed the transmission for a working life of 700,000 km (435,000 mi), with oil changes needed every 120,000 km (74,500 mi). Oil filter changes are not needed. The ZF-PowerLine has a mass of 140 kg (309 lb) and is approximately 650 mm (25.6 in) long. The design includes an integrated hydraulic control unit and sensors.

ZF claims more braking torque, less weight, and a longer service life for the two latest versions of its Intarder integrated retarder. The Power variant can deliver 4000 N·m (2950 lb·ft) of braking torque, with 3300 N·m (2434 lb·ft) of torque for the Eco version. ZF quotes some 25% greater braking torque than the previous model. Both variants have a mass of 65 kg (143 lb), are said to be easier to install, and work more quietly. Design changes include the adoption of a vane cell pump in place of an accumulator, providing quicker charging.

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