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Carbon composites for the city car

Carbon composites for the city car

Audi will show a lightweight electric city car concept at the Frankfurt IAA motor show. Called the Carbon fibre 1, the car has a carbon fibre reinforced plastic body. With a 1 seating layout, and a lithium ion battery powering two motors, the vehicle details the OEM’s thinking for an Electric Vehicle (EV) to fit below the range-extended A1 e-tron currently under development.

Sources at Audi are known to have expressed that this urban concept car has not descended from any previous model. The Carbon fibre 1’s development is based on the strict principles of lightweight construction and efficiency. Devoid of unnecessary weight, the seat bases have been integrated into the composite body to save weight and simplify assembly. The pedals adjust to make the seating position more ergonomic.

Audi formed a partnership this year with materials supplier Voith to develop composites, including carbon composites, and wants to use these, and aluminium, to replace some of the steel in its vehicles’ body-in-white structures. Another German automaker, BMW, recently unveiled the BMW i3 city car and the BMW i8 hybrid sports sedan each, with a lightweight aluminium chassis and a ‘reinforced carbon-fibre body’. According to BMW sources, the reinforced carbon-fibre body compensates for the weight of the batteries. The bodywork of the i3 and i8 is described to have been made by carbon, carbon fibre, and carbon-fibre-reinforced polymer (CFRP). The terms seem to describe the body work as made from carbon fibre after all. Made of CFRP (carbon-fibre in short), the body work looks good say industry experts.

Apart from Audi and BMW, Porsche too is working on composites. While Porsche and performance electric carmaker Tesla are working with composites, Land Rover’s new Evoque uses plastic composites for light weighting. Jaguar is said to be developing a car that would be one of the fastest in the world. It too will extensively use carbon-fibre to achieve light weight construction. The Jaguar C-X75 too will be fuel-efficient, courtesy the use of carbon-composite in the body construction. The other bits include a hybrid power plant among other things.

Remarkable is the new McLaren sports car, MP4-12C. It does 0 to 100kph in 3.3 seconds, and is expensive. Laced with extraordinary handling characteristics, the MP4-12C has found an innovative way of using carbon-fibre composites. For McLaren, carbon composites are not new. It was nearly thirty years ago that the UK-based company first introduced a Formula 1 car with a carbon monocoque. The car won the 1981 British Grand Prix at Silverstone. Later that year, it also illustrated itself in dramatic fashion, its ability to withstand crashes when the driver walked unharmed from a spectacular pile-up at Monza. Within a few years every Formula 1 team took to employing carbon-based cars. But even today it can take 3,000 man-hours to make a carbon monocoque for an F1 car. The reason is the complexity of the process. Each monocoque of the Formula 1 racing car is made by impregnating carbon-fibre cloth with a thermosetting plastic such as epoxy resin, pressing the cloth into shape in a mould and then cooking the whole thing, mould and all, in an autoclave. The cloth, in turn, is made by weaving yarn that has itself been spun from fibres of pure carbon that are made by baking strands of a polymer such as polyacrylonitrile.

The reason for this rigmarole is that carbon-fibre body parts are stronger than steel but weigh half as much. The strength comes from the powerful chemical bonds that form between carbon atoms (think diamond). For high-performance parts needed in small numbers, such as aircraft wings and Formula 1 racing cars, the price of such lightness has been worth paying. Mass production, however, has proved elusive.

Use of carbon fibre in autos for anything more than custom production has been perceived as costly. A fact that is claimed to defeat the light weight advantage offered by carbon fibre. The very high manufacturing costs of lightweight composite car structure is principally due to three factors: the high cost of raw materials, both in the use of pre-impregnated fabrics and the very high waste level in laminating complex shape components.

The high labour cost required for manufacturing weight-optimised components, which need careful draping and alignment of very thin (typically 0.4mm) layers with the thickness tailored to suit load distribution. Very high cycle times for both lay-up and resin curing, hence low production rates from each tool set. Things are changing. A production process pioneered with Carbo Tech, an Austrian firm that specialises in composites, takes McLaren just four hours to make the monocoque of the MP4-12C! The new process differs from the old process in two important ways. First, the carbon cloth goes into the mould dry, rather than pre-impregnated with resin. Layers of cloth are placed into a large steel tool, which provides the shape to be moulded. The tool presses the sheets together and only then injects epoxy resin into them under pressure. That is a lot less messy than handling sticky cloth.

Moreover, the tool then applies the curing heat directly, rather than using an autoclave. Once hardened, the monocoque is transferred to another device, where it is machined to provide the fixing points for the car’s other components. The process is precise, and consistent. Some of it is also automated.

The second difference, which does not speed up the process but does make for a better product, is the use of unidirectional cloth. Actually, this is not really cloth at all, since it is not woven. Instead, it is a layer of fibres all pointing in the same direction. That eliminates the strains found in woven cloth at every point where two fibres cross. It also results in stronger and more precise structures, capable of being built layer by layer, and with the exact alignment of each layer being appropriate to the strains and stresses that the body-part in question is likely to experience.

How much of what McLaren is doing with Carbo Tech, is also being done by Porsche and Tesla, or Audi and Voith, is not known yet. What is however known, is that the price of the EVs that Audi and BMW are looking at, would be less than that of the MP4-12C. Substantially less. The fact that Volkswagen recently bought a stake in BMW’s partner, SGL, a German carbon-specialist, should speak about the stress automakers are laying on engineered carbon-fibre bodies automobiles.

Clearly McLaren is not alone in working on resin-transfer and other ways of making carbon-composite components in large quantities. BMW is planning to use carbon in a new range of electric cars for city use. So are BMW and Porsche. There could also be many others who are not yet in a position to announce their carbon fibre plans. The age of steel seems to be fast fading under the growing importance and practicality of carbon fibre composites.