Deep Dive: The BMW i3 Electric Car's Carbon Fiber Body

By · July 31, 2013

BMW i3 cutaway view

On Monday, BMW released details about its first all electric car, the long-awaited BMW i3. BMW's management team calls the i3 “revolutionary,” based in no small part on the i3's leap forward with the architecture and composition of its body. What’s so special about the vehicle body of the i3?

While most cars are built with the body and frame as one unit, the BMW i3 is built in two modules: dubbed the “Life” module and the “Drive” module.

The aluminum Drive module is the chassis, and provides a sturdy foundation to contain the battery pack, the powertrain system, and the i3's basic structure. The Life module is where the passengers sit. It’s a high strength, ultra-lightweight CFRP (carbon fiber reinforced plastic) unit that bolts onto the Drive module.

Both of these modules are made from lightweight but strong materials—aluminum and carbon fiber—to decrease weight and increase the i3's efficiency. Large-scale use of aluminum is rare enough in mass produced cars, but carbon fiber is even more uncommon.

As a result, the BMW i3 weighs just 2,799 pounds—versus 3,362 pounds for the BMW 328i Coupe, and 3,208 pounds for the BMW 128i Coupe. The lighter weight reduces the energy required to move the vehicle down the road, and helps explain the 80-mile range (not yet certified by EPA) on the relatively modest 22 kilowatt-hour battery pack. Many other electric cars require 24 kilowatt-hours or more to deliver an 80-mile range.

The Basics

What is carbon fiber, and why is this important? First, carbon fiber materials are extremely strong and light—but more costly. They're widely used in vehicles where a high strength-to-weight ratio is desired, such as Formula One racecars. Carbon fiber is made of pure carbon atoms. Reinforcing it with plastic means the fibers are bonded with resins, and then formed into parts.

CFRP is not only light, it is stronger than steel, very rigid, and can absorb an enormous amount of impact energy. When subjected to high-speed impacts, the CFRP panels on the BMW i3 show barely any deformation. In the event of a crash, the ultra rigid carbon fiber structure creates a safe space for passengers.

The rigidity of the Life module also means the i3 doesn't require a B-Pillar. The front doors swing forward, and the rear doors swing rearward, giving a large opening for getting in and out of the car.

CFRP has historically been too expensive to manufacture in quantities required for a high-volume car. BMW developed new manufacturing techniques to reduce costs. The process begins at a factory in Moses Lake Washington that's run by a joint venture between BMW and the SGL Group (SGL Automotive Carbon Fibers). The capacity of the plant is 1,500 tonnes a year, which is about 10 percent of global CFRP production today.

Energy Intensive

Production of the raw fibers is energy intensive, so the venture required a source of cheap renewable energy. The Moses Lake facility is entirely powered by electricity from hydro-electric dams in the Columbia River Gorge.

At BMW's Wackersdorf plant, the carbon fiber is laminated to make the raw material for manufacturing CFRP parts and components. That manufacturing takes place at BMW plants in Landshut and Leipzig, where the laminates are processed using technology BMW spent more than 10 years to develop. (The Leipzig plant is entirely powered by wind turbines installed on-site at the factory.) A key step is a high-pressure injection of liquid resin (resin transfer molding) in which the carbon fibers and the resin bond together to form car parts.

The CFRP panels manufactured through this process form the Life module structure. Comparing it to traditional car architectures, it reduces the number of parts.

The lack of a B pillar makes for easy entry and exit of the small car.

Because the CFRP is made from carbon, and not metal, does that automatically mean the carbon comes from renewable resource? SGL does not claim its CFRP materials are renewable, just that they use renewable energy to power the factory. The source material for the fibers is polyacrylonitrile, which is an acrylic material and is therefore not directly derived from plant material.

Even if the carbon fibers aren't renewably sourced from plant material, they are readily recyclable. BMW's factories can separate the resins from the fibers, with no damage to the fibers, allowing them to be reused in manufacturing new components.

EV innovation usually focuses on batteries and motors. But BMW has developed new technologies, materials and processes that go beyond the powertrain to re-envision the basics of car design.

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