BMW plans to develop carbon fibre further for EVs
The BMW i3 and i8’s use of carbon fibre is a massive step forward. Until now the material has been reserved for race and supercars: it’s just too expensive and difficult to produce in high volumes. BMW is intent on changing that as it develops its range of electric vehicles.
At the ESV vehicle safety conference in Seoul earlier this year, BMW carbon fibre expert Dr Dirk Lukaszewicz spoke in some detail about the company’s approach to carbon fibre design and the company’s plans for the material in safety structures. Lukaszewicz works in the Special Projects Passive Safety Group at BMW and is responsible for early design and concept development, including vehicles made from carbon fibre reinforced plastic (CFRP).
You can read the paper presented by Dr Lukaszewicz here: Design Drivers for Enhanced Crash Performance of Automotive CFRP Structures
“The car is the result of a long, on-going strategy at BMW,” said Lukaszewicz. “We have progressed from using the composite materials for secondary structural components, such as bumper beams, to primary structural parts, the load paths in a crash. At the same time BMW has progressed from low- to high-volume composite production.”
The i3 electric vehicle is the culmination of this process. It is the first large-volume production vehicle that uses composites for some of its crash load paths.
By using carbon fibre to make the car lighter, the batteries can be smaller and money is saved. For high-volume cars with conventional engines, the business case for carbon fibre isn’t there
How much of the BMW i3 is carbon fibre?
Not all of the vehicle’s structure is carbon fibre. The undercarriage of the i3 is in aluminium and extends from the front to the rear. In frontal and rear crashes, the load paths are aluminium, whereas the side crash load paths are made from composites: the side frame, the roof, the roof cross members and the bulkheads, are all carbon fibre.
What difference does that make? The i3 weighs about the same as the current Mini. If you look at the pole impact tests for both, you’ll see that both are good. For the Mini, the intrusions into the passenger compartment are quite low. The pole still imprints itself into the vehicle and the occupant is protected by the airbag, which comes between them and the pole.
“If you look at how the i3 handles the same crash, you will notice a significant improvement,” said Lukaszewicz. “The car basically rebounds. There’s almost no intrusion so the passenger is fully protected from the pole. Part of the reason for this is that the design not only needs to protect the passenger but it must also protect the batteries under the floor. We have to guarantee the car’s electro-safety.”
The i3’s aluminium front and rear impact structure functions well: it absorbs energy well and is relatively light. The obvious next step is to replace the metal with composites in the main load path.
“We’re going to try to do more energy absorption using CFRP,” said Lukaszewicz. “CFRP has significant potential to enhance energy absorption. The challenge is that the way that CFRP fails is completely different to aluminium or steel. An aluminium side rail concertinas in a plastic deformation. A CFRP structure undergoing failure suffers large structural disintegration.”
Carbon fibre’s microstructures the key
The challenge is to make the carbon fibre’s disintegration predictable and controllable. BMW has been working to understand how the carbon fibre’s microstructures influence the material’s failure modes.
At the conference, Lukaszewicz presented some of the company’s research. By changing the way that the carbon fibre’s microstructures are aligned, it is possible to increase the energy absorption by up to 30%, said Lukaszewicz. That’s done by braiding the material with different types of fibre, such as glass and carbon, as well as braiding at different angles.
“You can improve energy absorption to such an extent that it can absorb almost four times as much energy as is currently possible with steel or aluminium,” said Lukaszewicz. “However, you can get more variability in the structure’s response as well. If you don’t understand why your structure is performing a certain way, you won’t be able to control the failure behaviour. Then you get very dramatic, brittle fractures, which give you no energy absorption at all.”
The big concern with carbon fibre structures is that they will fail catastrophically if they are hit at an angle. That’s partly why the i3 uses them for the side structures – it’s much easier to predict the angle of impact. BMW’s research suggests that it should be possible to design different energy absorbing load paths into the structure: you arrange the microstructures in different layers work in various directions.
“If you can design energy absorbing load paths, you get very robust performance up to 20°,” said Lukaszewicz. “BMW hasn’t tested at greater angles because we don’t have the testing capability at present. We get stable energy absorption between 0°and 10° of oblique angle and we still get stable energy absorption and no catastrophic failure at 20° oblique angle.”
Carbon fibre only viable for EVs
That’s encouraging. It means that it is technically possible to include composites in main load paths without compromising safety levels, possibly even advancing them. It will be interesting to see what BMW does next with the materials.
The i3 has been designed around an electric drivetrain and carbon fibre reinforced plastic is one of the enablers for the design. Carbon fibre may be expensive and difficult to manufacture, but it is still cheaper than a big lithium ion battery. By using carbon fibre to make the car lighter, the batteries can be smaller and money is saved. Lukaszewicz indicated that, for the time being at least, If you’re designing a high-volume car with a conventional engine, the business case for carbon fibre isn’t there.