Drive

BMW’s electric car made of stronger, lighter carbon fibre

The more something weighs, the more energy it takes to move it. That’s a major issue for automakers, who need to reduce the weight of vehicles to improve fuel economy while still making cars that can pass crash standards.

It’s even more of an issue for electric cars, because extra weight reduces how far they can go on a charge. BMW recently unveiled its upcoming i3 electric vehicle, which is made entirely of lightweight materials, including carbon fibre reinforced plastic, or CFRP.

“Carbon fibre is based on acrylic fibre,” says Jörg Pohlman, managing director of SGL Automotive Carbon Fibers, a joint venture with BMW. “We apply heat, which turns it into 95 per cent carbon, and four to five per cent nitrogen. The first process is oxidization, at 230 to 250 C, and then carbonization, at 800 to 1,400 C.”

Carbon fibre is stronger than steel, and also much lighter. It’s made up of thin strands that are usually woven into sheets, in much the same way that cotton is woven into fabric. However, BMW uses a method that “sews” the strands in parallel. “If it’s woven, (the fibres are) bent a bit and we lose material properties,” Pohlman says. “This allows the engineers to put the layers in a form for the specific component.”

From there, the carbon fibre is impregnated with resin and cured with heat and pressure. BMW uses a special resin that cures faster than usual, taking minutes instead of hours, and allowing the pieces to be mass-produced.

Although carbon fibre components are ultra-light, they’re also very costly. Up until now, the material has been primarily used in expensive sports cars, where its rigidity and weight improves the driving performance.

The i3 will start at $44,950 when it goes on sale in Canada next year. It uses CFRP for its modular passenger cell and roof, along with aluminum, which costs more than steel, for its chassis. But the trick to its mid-range price is the lithium-ion battery that powers its electric motor. Because the car weighs so little, the battery can be smaller and correspondingly cheaper. Since the battery is typically the most expensive component in an electric car, its lower price helps offset the cost of the car’s high-tech construction.

More on the i3

  • No rust. The i3’s outer panels are made of thermal plastic, and since none of its construction materials are corrosive, the car won’t rust
  • The roof. The car’s roof is made with carbon fibre off-cuts left over from making the passenger cell
  • In case of accident. If the car crashes, damaged areas can be cut out and new carbon fibre plastic pieces glued

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