i3 - Source: BMW Group

When will the carbon car come?

Four years ago, BMW launched the i3 electric car, the world’s first series-production vehicle with a carbon body. Nevertheless, fibre-reinforced plastic is still the exception in cars. Textile researcher Dr Christoph Greb explains why the super-lightweight material will not become the standard in automobile manufacturing – and why it is not a dead-end street for the Bavarian automobile company.

Dr Greb, as head of the fibre-reinforced plastics division at the RWTH Institute for Textile Technology in Aachen, you work together with automobile manufacturers and suppliers on the car of the future. Seriously, when will the first carbon car roll off the production line?

Dr.-Ing. Christoph Greb - Source: ITA

Dr.-Ing. Christoph Greb – Source: ITA

There will never be a car made completely of carbon. Not, at least, as a mass-produced product. The fibre material is very expensive to make, at present anyway. Today, carbon fibre costs between € 15 to 20 per kilo. In comparison, steel costs € 1 per kilo. Put it this way: there are challenges regarding carbon as a material and they cannot be simply explained away. All the same, I think that many people see the subject in excessively black and white terms.

In your opinion, what is the future for carbon in automobiles?

BMW has come up with a good strategy for using carbon purposefully in cars with great progress having been made on both the material and process sides. However, it will take time to catch up on the development advances made in the field of conventional materials. At present, the trend is towards hybrid structures, i.e., carbon in combination with aluminium or steel. Metals continue to be unbeatable for certain automobile parts while carbon is better suited for other components.

For example?

Together with the Institute for Automobiles of RWTH Aachen and partners such as Evonik, the carbon-fibre supplier Toho Tenax and automotive supplier Johnson Controls – now called Adient – we have developed a seat shell that unifies three materials: thermally workable carbon-fibre compositions – one reinforced with endless fibres, one using recycled carbon fibres – and connecting elements made of steel. One of the aims of the project was to save as much weight as possible.

Which brings us to lightweight construction: BMW aims to sell 100,000 electric cars this year. Can electric mobility become a driving force for the use of carbon?

Electric mobility needs lightweight constructions, certainly. But that need not necessarily come about through using carbon. In theory, carbon has an unattained potential in this respect. However, it is currently difficult and expensive to exploit this in practice.

 Why not make metal thinner and lighter – and simply do without carbon?

You shouldn’t reduce carbon to lightweight construction. Metals are isotropic materials, which means they have the same mechanical properties in all directions, in other words, also where no load is applied. Although metal can be optimised in this respect, carbon makes a quite different approach possible.

What do you mean?

Certain properties can be achieved depending on how a carbon part is positioned during production. This is because carbon placed in the direction of the fibres has different properties than when it is laid across them. Thus, by orienting the fibres, carbon components can be designed to meet specific requirements.

For example?

One from the premium segment: in the current Audi R8, the rear wall and middle tunnel are made of carbon because anisotropic loads, i.e., directional loads, occur there. A fibre material can cope with these loads much more effectively than metal. Naturally, a solution can be found with metal but this involves numerous parts and, therefore, more weight, which is not so good in the case of a sports car. That, by the way, is another advantage of carbon – fewer components.

Why?

Integral constructions are possible with carbon. In other words, a big or complex component can be made as one piece. In distinction to this, the metal equivalent must be made by fitting many separate components together. Formula 1 fans are likely to be familiar with the integral construction method: the body of the car consists of just one part, the ‘monocoque’.

Automatic manufacturing processes are also very important.

Yes, among other reasons, because carbon fibres are very pliable. By way of explanation: before being impregnated with plastic, carbon-fibre textiles are very pliable, i.e., much less able to keep their shape than metallic materials. This represents an enormous challenge for process automation, and something research institutes and the industry are working on flat out on. Generally speaking: to make cars efficiently with carbon components, we need to gather more material data and improve the processing methods. In the last analysis, the cost efficiency of the components will continue to improve as the production technology is optimised.

You mean that BMW should heed the saying, ‘more haste, less speed’?

I am certain that, as an established automobile manufacturer, BMW will benefit from having banked at an early stage on a material, the properties or which are still not yet fully known. And we still haven’t spoken about the integration of functions such as self-repairing capacity, sensors and light in fibre parts for cars …

Ronny Eckert

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