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Most of us would not immediately associate the Chinese automotive industry with the price of baked beans in UK supermarkets. But ROBIN WAGER finds the connection as he investigates the ingenious quest for lighter, leaner cars...

QUOTE

Next to air and rolling resistance, vehicle weight decisively influences the need for mechanical driving energy. Present efforts should be strengthened, with the goal of halving vehicle weight.' - Energie Spiegel, journal of the Paul Scherre Institut, Switzerland.
 

QUOTE

'A range of modern, powerful engines and an advanced, lightweight construction is the perfect recipe for a luxury saloon that rewards its driver with outstanding levels of refinement, performance and fuel economy.' - David Scholes, chief programme engineer XJ range, Jaguar Cars.

 

EVEN less than 100 years ago, cars were still being built on wood frames. Not only were both this natural material and the necessary skilled joiners in plentiful supply, but it was strong and easily adapted to suit varying body styles. Crucially, it was also relatively light.

Traditionally, though, the major component material of any vehicle has been steel.

Through the earlier stages of its development, the motor vehicle tended to have an engine at the front, driving the rear wheels through a gearbox, torque shaft and rear-mounted final drive. This arrangement gave good balance of the weightiest components throughout the length of the vehicle.
As time went on, models like the VW Beetle, with rear engine and transaxle, were the exceptions whose success proved the rule; and even the earliest front-wheel-drive cars were seen as oddballs. But gradually, in the second half of the 20th Century, front-engine, front-driven cars began to predominate, thanks largely to advances in constant-velocity joints enabling the drive shafts to withstand the high stresses involved in combining driving and steering functions over considerable mileages.

The concentration of engine, transmission and all major components over the front wheels brought a fundamental change in the balance of the overall vehicle that was dealt with mainly through independent suspension, tyre pressures, spring ratings - and driving technique. For the driver was required to cope often subconsciously with the unfamiliar and generally pronounced concept of understeer in place of traditional rear-wheel drive oversteer. Still, at this point in the history of the car, vehicle kerb weight was not frequently a major factor in manufacturing decisions.

Come the 1970s and the first fuel crisis, and suddenly fuel consumption was paramount. First casualty was the almost universal carburettor, replaced by fuel injection systems on more up-market cars, and pretty soon on cheaper ones too, even if this was only the 'poor man's' single-point injection. That helped, and of course fuel injection has since been further refined beyond all recognition, integrated with sophisticated electronic engine management systems. Though mandatory fitment of catalytic converters has militated against ultimate fuel efficiency.

Weight had already become important as part of the drive towards such efficiency. Increasing use of plastics and composites was helping, but weight is not a function of the original design concept alone. Politicians can add weight - and we're not talking about the size of one or two well known political figures. Safety additions such as side-impact beams, mandated for compulsory crash resistance standards, add considerably to overall mass. And finally customer demands add weight. Market forces continue to introduce more and more equipment even to basic models to satisfy the expectations of buyers, which generally also include not wanting to pay a resultant performance/ economy penalty.
It's a constant battle - legal obligations and added value versus performance and fuel efficiency. Hence the ongoing search for weight savings in the form of materials that are both light and strong. Steel is a versatile and readily available medium but, except in a direct comparison with for example lead, nobody would describe it as particularly light.

Then there is cost. Even during wartime, the world market for steel has never been bigger than it is currently, thanks mainly to the huge demand coming from rapidly emerging economies like China, now getting involved big-time in the motor industry. Steel makers are finding it difficult to keep up - and an inevitable result of demand exceeding supply is a rise in prices. Baked bean suppliers recently warned that their product prices would have to increase, mainly because of the rising cost of steel for the cans.

Non-metallic composites can be both strong and versatile, but have their limitations, and buyers of prestige marques in particular are likely to balk at being offered what they see as a 'plastic car'. Hence the increasing use of light alloys, and particularly aluminium, a material pioneered in the aircraft industry many years ago.

One of the most familiar early examples of the use of light alloy in cars was for road wheels. Originally introduced to reduce unsprung weight for sporting use, the 'set of alloys' has become a must-have on even the most mundane of models, with economies of scale reducing them to an affordable price.
While aluminium has become familiar to most people in cast and machined form, the production and working of sheet aluminium had traditionally been a difficult and labour-intensive process, also presenting problems in welding, joining, repairing and painting.

However, the situation has changed quite rapidly in recent years as a result of intensive R&D by a number of leading motor manufacturers, notably Audi, which has been researching the use of aluminium for more than 20 years. The company employed the material in its original quattro rally car that revolutionised the sport back in the 1980s, and has been working on its greater incorporation into car manufacturing ever since.

Audi's all-aluminium quattro Spyder concept was a star exhibit at the 1991 Frankfurt Show. Just a month later in Tokyo, the Audi Avus quattro - presented in unpainted, polished aluminium, recalling the great Auto Union record breakers of the 1930s - made world headlines. Its importance has earned it a place in Audi's award-winning museum in Ingolstadt.

Originally it seemed that all this might be more about underpinning the Vorsprung durch Technik slogan of VW's most technically innovative subsidiary, than providing more tangible volume production advantages. Both points of view were bolstered at the turn of the century by the appearance of the unusually-styled A2 model, using aluminium body panels over an aluminium Audi Space Frame - ASF.
This small car, which was expensive to produce and has undoubtedly made little money for the company, for the first time embodied much newly evolved technology, requiring techniques, processes, tools and production methods to be radically modified or developed from scratch.

At that time, the company's resources in lightweight production were still fragmented, with the development, production planning and quality assurance departments all involved. So just over 10 years ago, Audi established the 'Aluminium Centre' to bring all the relevant expertise under one umbrella at its second Bavarian plant in Neckarsulm. Representing an initial investment of some €8 million, this was the catalyst for Audi's major progression to leadership in lightweight design.

By 2002 it was renamed the 'Neckarsulm Aluminium and Lightweight Design Centre', reflecting the fact that research and development activities have long since progressed beyond aluminium. High-strength steels, tailored blanks, reinforced plastics and magnesium are used increasingly by Audi. And once again the systematic implementation of lightweight design and materials in vehicle bodies is securing noticeable reductions in fuel consumption.

As research and development continues, the potential of the materials available for lightweight vehicles will be exploited even more effectively in future. Heinrich Timm, head of the Aluminium and Lightweight Design Centre and one of the pioneers in the development of aluminium bodies, says: 'My vision is of economical lightweight design for volume production. We need to exploit the potential of materials for lightweight vehicles as effectively as possible, and use the right materials in the right place. Aluminium of course remains the principal material in lightweight design, but we are increasingly looking into the potential of other materials.'

Engineers at the Centre developed ground-breaking models such as the Audi A2 and the A8, while Lamborghini, the supercar arm of the Volkswagen Group, also utilised its expertise in the development of the Gallardo high-performance sports car.

Audi's enthusiastic adoption of the aluminium vehicle body has already had an impact on the steel industry. In response to the development of the first line-production aluminium body for the low-volume Audi A8, steel manufacturers have stepped up their efforts to compete by developing high-strength and super-high-strength steels for body manufacture.

Less than a year after the Lightweight Centre was established, the Audi A8, first vehicle to feature the Audi Space Frame concept - ASF, was launched. Not only lighter but stronger than a conventional steel structure, and using similar techniques to those employed in aircraft, ASF embodies the principle that every panel surface contributes to load-bearing. Extruded aluminium sections are connected together by pressure-cast joint elements and large-area aluminium panels integrated into the resulting cellular structure.

Five years later came the smaller Audi A2, the first high-volume car with this technology. Audi estimated the A2 to be 40 per cent lighter than if it were built from steel.

After a further three years, the second-generation Audi A8 emerged at the Lightweight Design Centre, while production of the Lamborghini Gallardo began in Sant'Agata, Italy, one year later.

During this period, Audi continued to develop new production technology, enabling the level of automation in body manufacturing to reach its present level of 80 per cent - comparable to the degree of automation in the production of a conventional steel body. There is industry-wide acknowledgement of the successful volume production of lightweight bodies at Neckarsulm, where the workforce has already built around 320,000 aluminium-bodied vehicles.

expertise

Staff at the Aluminium and Lightweight Design Centre, currently numbering 110, continue their work to refine and extend Audi's lightweight design expertise, as it adopted increasingly in the flow of new - primarily FWD - models.

While its R&D work in this field has been of such historical importance that its establishment at Neckarsulm has become the unofficial world centre of such research, Audi is of course not the only manufacturer employing lightweight technology.

Jaguar, for example, which has historical experience of working with aluminium, relies heavily on the material in its latest XJ range to, in the company's own words, 'create a car that blends the ultra-modern with traditional Jaguar values. Lightweight aluminium construction provides new levels of strength, robustness and dynamic ability'.

'We chose a lightweight aluminium vehicle architecture for the new XJ not because it was something new, but because it enabled us to deliver real and significant benefits to our customers,' said Jaguar's chief programme engineer David Scholes.

Despite being somewhat larger than its predecessor, the new XJ is as much as 200kg lighter overall, offering real performance and economy gains. Its BIW - body in white - is manufactured almost entirely from aluminium, and weighs some 40 per cent less than the equivalent steel structure while providing 60 per cent greater torsional rigidity.

Though this is primarily a conventional monocoque construction, structural castings and extrusions are also used locally to enhance the structural integrity and reduce the panel count. An industry 'first' was the use of rivet-bonded joining technology for the entire body, in which self-pierce rivets are used in combination with aerospace-sourced epoxy adhesive to join the aluminium pressings, castings and extrusions.

To further optimise weight savings, Jaguar also employs the more exotic magnesium - equal in strength to aluminium but 30 per cent lighter still.

A magnesium cross-beam supports the facia and instrument panel, while the steering column is a combination of the two materials, and magnesium castings are used in the seat frames. The double-wishbone suspension system is aluminium-intensive in construction, and the car also benefits from aluminium-intensive engines.

discipline

Weight-saving technology is not, of course, an isolated discipline. Of primary importance to body and chassis engineers, it nevertheless also has ramifications throughout the vehicle. So, while performance and economy advantages are a given, the effects on ride and handling must also be considered.
Despite favouring different driven wheels, the fact that both Audi and Jaguar, for example, have adopted air suspension on their larger aluminium-intensive models is no coincidence. For, as net vehicle weight is reduced, so the occupants represent a relatively higher proportion of GVW - gross vehicle weight.

Air suspension ensures that full suspension travel is always available by increasing spring stiffness relative to payload, and that ride height remains constant regardless of the load and its distribution. It can also be programmed to reduce ride height as speed increases, to improve stability and dynamic efficiency.

Over at BMW, where they have long boasted of their products' almost perfect front-rear weight distribution, air suspension also features on the new 5-Series. Here the latest acronym is LAFE - Lightweight Aluminium Front End, a combination of aluminium and steel that uses sophisticated and elaborate joining processes developed by the company's engineers to ensure optimum connections with supreme precision. Depending on material and application, the various components are bonded, riveted, welded or joined by the latest laser process.

Introduced on the 5-Series at Dingolfing, BMW's largest plant 100 km east of Munich, LAFE production on that model comprises a material mix of AlMgSi, Magsimal 59, AlMg3, 5Mg, TRIP 700 and composite panels. The 45 kg LAFE assembly utilises 300 of the total of 1000 robots used in 5-series production, to apply:

-         15.2 m of bonded flanges.
-         2.94 m of aluminium MIG-welding seams.
-         1.74 m of aluminium laser-welding seams.
-         48 aluminium bolts.
-         599 punch rivets.

strength

The different joining technologies are not confined to the LAFE. Taking approximate figures for the entire BIW, 60m of connection seams are bonded, 4,000 spot welds are applied and 70 connection points are bolted. MAG welding, MIG soldering, laser welding and beading, riveting, sub-coating, sealing, clinching and clipping are also employed, with more than 98 per cent of operations fully automated.

BMW reckons the 5-Series' all-aluminium chassis has reduced overall weight by up to 50 kg over the previous generation, despite extra equipment for the new car. While the entire body ahead of the A-pillar is of aluminium, the remainder of the BIW uses high quality steel. The company says this ensures superior strength and rigidity, but it also has much to do with its 'Holy Grail' of 50:50 weight distribution.

rigidity

'When it comes to reducing the weight of a BMW automobile, we must not at the same time reduce vehicle safety, comfort, or ultimative driving enjoyment,' says the company.
Using the mixture of metals was not without difficulty. As is well known, the different chemical structures of the two mean that welding them together leads to corrosion cells.
To overcome this, BMW developed a system of riveting and bonding, which also contributes to increased body rigidity.

The new mix of materials has also required changes in the finishing procedure. The LAFE can only be painted in a rotation dipping paint process - RoDip, in which the aluminium/steel body is dipped in a paint bath for the first time. RoDip has been used since 2002 for pre-treatment of the BIW - cleaning, degreasing and applying a phosphate layer.

Continuing the theme, the chassis and suspension of the 5-Series comprise some 500 steel and aluminium component parts weighing just 350 kg in total. Apart from obvious wearing components like thrust rods, wheel bearings and pivot joints, the spring strut tiebar front axle is wholly of aluminium. A U-shaped front axle subframe, which accommodates the steering rack, anti-roll bar, track control arms and tiebars, is reinforced by a thrust plate to ensure transverse stiffness and aid precision of steering response.

ASF and LAFE are not the only acronyms in current vogue in Bavaria's automotive industry: CFRP is also destined to become a familiar term for BMW drivers, who in the future may very well find themselves sitting in a cabin made from Carbon Fibre Reinforced Polymer.

BMW reckons this currently offers the greatest weight saving potential in car body construction, and it is developing CFRP technology as part of its 'Design for the Environment' programme. Some 50 per cent lighter than steel and as much as 30 per cent lighter than aluminium, the material is corrosion resistant and performs extremely well in vehicle crash tests.

Early work on CFRP led to a technological breakthrough in lightweight automobile construction with the Z22 design study by BMW engineers at Technik GmbH in August, 2000.

economy

Its fuel economy was one of the characteristics that made the Z22 unique from an environmental point of view. Although - in terms of comfort, performance, and interior space the new car was comparable to a BMW 530i touring - it consumed only 6 litres of gasoline per 100 km.

The 35 per cent weight reduction represented a major step forward in lightweight construction technology.

Weighing just 1,100 kg (10kg/kW), the Z22 was also the world's first 'Mechatronic' automobile. New steer-by-wire and brake-by-wire technologies in the Z22, for example, replace the mechanical steering and hydraulic brake systems.

A team of engineering specialists developed the 70 innovations and new composite materials featured on this technological tour de force, which sparked a technology chain reaction at BMW. Based on Z22 project results, the company decided to work more extensively with CFRP.

Since then, under the management of the experts at the BMW Research and Innovation Centre in Munich, an international project team comprising members from all the car maker's divisions has been working for some time on the development of CFRP for lightweight, series-production auto bodies. However, the company says it will start using the material in series-production only when it becomes cost-neutral compared to aluminium. Along with environmental criteria, economy is an important aspect in the development of new lightweight materials.

In common with most major manufacturers, the BMW group strives to minimise the environmental impact of its operations, and in this regard has already identified one major drawback: CFRP structural parts from end-of-life vehicles are not suitable for mechanical recycling due to the high disassembly costs.

Consequently, they will be thermally recovered in the future as a component of automotive shredder residue. In some cases this could lead to a conflict with the objectives of the lifetime environmental effects, and compliance with material recycling quotas in the EU end-of-life vehicle directive. But BMW materials flow managers are optimistic in their approach to handling waste materials from production, and by the time series production with CFRP is ready for launch, they expect to have a closed material recycling loop in place.

environment

In a direct comparison of environmental factors, including ELVs - end of life vehicles - and lightweight construction, a minor impact on the environment must be regarded as acceptable as long as the primary objective is environmental compatibility. In order to examine these relationships, BMW specialists are conducting an environmental life cycle analysis on CFRP materials and components. This research has thus far shown positive results.

In a comparison with a BMW 5-series Touring side-frame made of steel, a lighter CFRP frame performed extremely well in terms of environmental compatibility - ie: energy consumption, potential greenhouse effects, and resource depletion - demonstrating that CFRP is a valuable material when it comes to environmental aspects, with outstanding potential for environmental compatibility throughout the total life cycle. BMW has conducted further tests on CFRP materials in series production, designed to benchmark CFRP processes along with the economics of its use in manufacturing.

In fact, preparing for series production is one of the main tasks of the BMW Innovation and Technology Centre where the CFRP pilot system is located. The centre focuses on developing basic concepts into innovative standards. Opened in December 1999, it is located at the BMW plant in Landshut and consists of two main departments.

feasibility

These focus on plastics and light alloys, the primary materials used by the group in advanced lightweight construction. Since its establishment, a major part of the operations at the Landshut plant is in testing lightweight materials and composites.

Thanks to an effective network combining technology, product planning, and development, the centre's staff can react quickly and efficiently to changing conditions and respond with economical solutions. This makes it possible to test new developments promptly and under real conditions, with prototype construction in collaboration with the research and engineering centre in Munich, a high-speed milling machine, a die model shop and tool shop all united at one location.

While process feasibility is handled by the technical centre, functionality is determined in the testing labs where all new developments are checked to ensure they conform to specifications. This concentration of know-how and technology makes it possible to shorten the lead time from initial concept to market launch...

Weight reduction is not only relevant in the car body, and combination with other factors also plays an important role. A lightweight cockpit and an optimised rear seat back assembly are included in the Innovation Centre's development program.

BMW continues to explore new approaches to weight reduction and to examine each assembly unit to determine whether lightweight construction can be intelligently incorporated. Its engineers remain keenly interested in the progress made in the development of fuel-efficient engines, especially in small cars, with innovations such as Valvetronic and the further development of hydrogen power.

The ongoing changes adopted by vehicle manufacturers have, as always, required close co-operation with suppliers. One of the major metals companies working with the motor industry is Corus, the international materials solutions supplier with manufacturing facilities and sales offices throughout the world.

Just two years ago Corus commissioned its new Continuous Annealing and Pre-treatment - CAPL - line, designed specifically for the processing and production of high-quality aluminium coil for the motor industry, in Duffel, Belgium. The €55 million investment was made in response to the growing demand from vehicle manufacturers for aluminium rolled products for specific automotive applications. Corus supplies these to Audi, Jaguar and BMW, and also to DaimlerChrysler, Land Rover, Michelin, Peugeot, Volkswagen and Volvo.

The design of the CAPL, the result of several years' intensive research, allows the company to process more cost-effectively the latest generation of stronger and lighter aluminium which is also easier to recycle.

benefits

Key features include:
-           An initial degreasing station to treat incoming raw strip, thus avoiding contamination of          finished surfaces - an important consideration for vehicle exterior panels.
-           A 63-metre air floating furnace, the longest in Europe, which can treat the strip at 80m per minute within a temperature accuracy of 3 C either way, giving consistent material performance.
-           A sophisticated inspection room, which ensures a constant feed of high-quality strip.

The line can also undertake a number of surface treatments, such as pickling, passivation and lubrication, traditionally carried out by vehicle manufacturers in-house. This offers substantial cost benefits for customers; and the line has also been designed to allow for additional future treatments still being developed by Corus.

The Duffel plant was already supplying a range of cut-to length, special blank products increasingly used for the manufacture of bonnet panels, for example. For the Lamborghini Gallardo, already mentioned, Corus supplied the world's first production-ready, aluminium tailor-welded blank for use in the car's front wheel arches.

The steel tailor-welded blank has for some time been an established automotive practice, already accounting for more than 15 per cent of the average vehicle body structure on high-volume models. Comprising two or more sheets of different grades and/or thicknesses, joined by 'mash welding', it provides optimum strength at specific points on the vehicle while using the least possible amount of material.

Johan Ameel, director of Corus Aluminium Rolled Products, says his company has been co-operating with Audi for years in the development of improved aluminium qualities for vehicle bodies. 'Developing this further into offering tailor-welded blanks in aluminium was a challenging but logical step,' he adds.

The Corus aluminium tailor-welded blank applies the principles used in the steel blanks to the lighter material - which, however, is more difficult to weld. This is where Corus's laser welding centre in Ijmuiden, Netherlands, comes in. It studies and optimises laser welding techniques to develop a robust process with the consistently high quality weld seam required to move from development prototypes to series production.

The blanks are made from both 5000- and 6000-series alloys, both of which can be welded perfectly using mono-beam laser technology, which offers strong forming capabilities, making them suitable for vehicle body and chassis applications.

demand

Despite the tremendous strides made in developing techniques specifically for the processing of aluminium, Corus predicts that steel will continue to be the most widely used material for the body and chassis structures of mass-production vehicles for the foreseeable future.

Nevertheless, says the company, demand is growing from vehicle manufacturers for specific applications of aluminium in order to achieve weight savings, thereby reducing fuel consumption and emissions.

This overview of work by manufacturers with a particularly high profile in lightweight vehicle construction can do little more than scratch the surface. A more in-depth approach would require an entire issue of Vehicle Engineer. Other manufacturers are, of course, involved.