Bye-draulics

Energy and fuel-efficient electric power steering systems, once restricted to smaller, lighter cars, are now finding their way on to mid-range models.

An electric power-steering system developed by ZF Lenksysteme of Germany is enabling heavier cars to run more efficiently, helping to cut carbon dioxide emissions.

Under an EU directive, by 2008 each manufacturer’s carbon dioxide emissions, averaged across new car sales, must be no more than 140g/km – a target that will be hard to meet, given the burgeoning heavy car market.

Traditional hydraulic power steering assemblies exacerbate the emissions problem because they run continuously, requiring constant energy and fuel. They are also bulky – consisting of a steering pump and valve, oil reservoir and high-pressure hoses to transfer energy from the pump to steering gear – so add weight to vehicles. By contrast, electric power steering units, which use an auto-servo motor, are compact, simpler, more reliable and operate only when the steering is used to turn the wheels.

Until now, electric power steering has been the preserve of smaller, lighter, easier-to-steer vehicles such as the Honda Insight, Volkswagen Lupo (which uses Delphi’s E-Steer technology), the Vauxhall Corsa and the MGF. But the ZF system can be fitted to cars of a larger class and weight, such as the Volkswagen Golf, Tauron, Audi A3 and the BMW Z4.

ZF spokesman Freidman Steinke said the non-hydraulic system produces fuel savings of around 0.2 litres per 100km during straight-ahead driving, as the system is inactive so requires no power. To achieve similar results, a vehicle with a conventional steering system would have to be at least 100kg lighter.

Steinke added that as well as requiring less maintenance than hydraulic systems, which can be prone to leaks, electric systems can be tested as a complete module before being fitted to the vehicle. This modular nature has also allowed the ZF system to be adapted to different, and heavier, types of vehicle.

Steinke said ZF has developed four different electric power steering systems so far, two of which are already on the market.

As well as having low power requirements, ZF claims that its electric steering system also has handling advantages. ‘In the US, where the roads aren’t flat or tend to slope down at one side, the system helps tyres to remain central and not veer off to the downward slope,’ said Steinke.

With the ZF system, the steering response can be electronically programmed, creating different feel and handling characteristics by taking into account speed, steering angle and steering wheel rotation speed.

The system uses engineering plastic in the toothed wheel of the servo-motor’s gearset. The plastic, Ultramid A4H, further reduces the system’s weight while displaying mechanical properties on a par with a metal alternative.

With electric steering, the servo-motor that provides power assistance can be linked to the steering at various points; on Volkswagen cars it is linked to the steering rack, and on BMWs to the steering column.

Recent figures show that despite initiatives from the government and manufacturers, UK average CO2 emissions have actually risen, from 168g/km in 2001 to 169g/km in 2002.

Electronic power steering, with its simplicity, adaptability and reliability, could offer manufacturers’ their best opportunity to reverse this situation and meet emissions targets by 2008.

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