Super-light yet strong

Weight loss is the key to the future of automobiles. Lighter vehicles use less fuel, which in turn reduce CO2 emissions. The days when innovations improving comfort and safety made each new series heavier than its predecessor are over, as Gundolf Kopp of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt – DLR) in Stuttgart explains. In fact, the trend has been reversed. Models launched in the last two years have not got heavier and in future, steep decreases are planned in this department, taking as much as thirty per cent off the weight of the bodywork, compared with a medium-class steel car. That is a difference of 85 kilogrammes – or 6.8g CO2/km. Stricter EU directives on CO2 emissions and continually risingfuel prices motivate the search for ways of reducing fuel consumption, such as by using lighter materials. Developing new concepts for light construction has become a European challenge; Volkswagen AG is heading the project Super Light Car (SLC) with 37 partners from nine countries. Seven big car-producing companies have brought their component suppliers and renowned research institutions on board, including the DLR with its Institute of Vehicle Concepts in Stuttgart.

The partners in development of light vehicles started work in 2005 under the sixth EU Framework Programme and have a total budget of 19.2 million euros. The research motto is Multi Material Design. As the DLR project director for SLC Gundolf Kopp explains, it would be possible to build a car of super-light carbon-fibre reinforced plastic (CFP), but no one would be able to afford it. Instead, efforts are concentrated on using the right material in the right places. The scientists test each component to see if weight can be saved and which of the light materials – magnesium, aluminium, steel types, CFP – is most suitable.

It is not enough simply to consider the technical strengths of each material. Kopp emphasises that the project partners must consider the vehicle’s whole life-cycle, including recycling. The materials cannot be too expensive and the manufacturers must be able to work with them. The vehicle must also meet all the car industry’s demands regarding crash safety, corrosion protection and material fatigue, without the price spiralling out of reach. The DLR team in Stuttgart has contributed a 24 kg reduction to the front module of the car where the motor is located, for example, by designing a magnesium strut tower and a longitudinal rail and other parts of aluminium. Gundolf Kopp claims that the use of magnesium brings a double bonus: magnesium is lighter than steel or aluminium, and it can also be cast, so different elements can be united in one component. This reduces weight and makes assembly easier.

The disadvantage of magnesium is that it is less ductile than other light metals. This means that the engineers must test – initially using virtual crash testing methods - whether the components fail too soon, possibly penetrating the car interior in a serious accident. “It is an enormous challenge to unite all these sometimes contradictory qualities – light and safe, innovative, reliable and inexpensive,” says Kopp. This makes it all the more important to share the international partners’ expertise in various areas and to support each other in parts of the project, such as complex calculations or materials testing.

The Stuttgart car engineers contribute their expertise in design, construction and simulation and their experience with various materials, including magnesium as a light material. “The Italian car manufacturer Fiat is an important partner, especially on issues of combining different materials and components, such as strut towers and longitudinal rails,” says Kopp. They also collaborate closely with the Swedishengineers at Volvo, who specialise in optimising the wall thickness of structural elements. Finally, the British company ARUP provides a computer simulation of the whole vehicle. Various car manufacturers and suppliers aim to put the lightweight construction concept into practice by early 2009. They will produce several prototypes of the newly developed structures – from individual elements through the front end to the complete chassis. The prototypes will then be thoroughly tested to ensure that they fulfil all expectations, not just the anticipated weight reduction.