BMW i3, the inside story: what it’s made of, how it’s made

  • 09-May-2013 02:24 EDT
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BMW i3 unclothed: its 125-kW electric motor is positioned at the rear. Composite panels provide an outer skin.

The new i models coming from BMW do not represent the reinvention of the car, but they do mark something of a revolution in terms of materials, production, and vehicle integration.

“BMW i is about so much more than just electric cars,” said Project Director Dr. Carsten Breitfeld. There is no doubt that it would not exist but for the need to develop a convincing series production electromobility solution, and one that could confidently carry a “premium” label.

Unlike some companies that feel an electric motor can be installed in a regular model as an alternative powertrain, much as a choice can be made between gasoline and diesel, BMW believes that its entry into relatively high-volume electric car production necessitates a totally new design and engineering approach, with the extensive application of industrialized CFRP (carbon fiber reinforced plastic) in a totally new vehicle identity. (CFRP was first used for the BMW CSL coupe’s roof 10 years ago and later for the M3 roof and the M6’s roof and bumper supports.)

The result is the i3 city car and i8 sports car. “The only way we could achieve our weight targets was by using carbon fiber,” said Breitfeld. “We have optimized the process, achieved a shorter manufacturing time, and succeeded in taking a lot of the cost out.”

Creating such radically different production vehicles called for new thinking, a fresh approach to manufacturing and materials use, and a very clear business plan. “Just coming up with the idea was not enough,” he added.

The i cars will not be technologically isolated; BMW is convinced that many of the advances including wider use of CFRP, demonstrated by its new sub-brand (establishment of a separate, stand-alone brand was discussed at length by the BMW Board but rejected), will cascade into conventional vehicles.

Some details of the i products have been released, but as pilot production gets under way, BMW invited AEI to an extensive technology briefing and to view the i3 production line in Leipzig, Germany, where a carbon pressing plant, plastic parts manufacturing facility, body construction facility, assembly hall, and logistics hall have been built for an investment of some €400 million.

REX at the ready

The first production i3’s this fall will be all-electric, the more sophisticated i8 following in 2014 and built in the same hall as the i3. Electric range of the i3 is established at 130-160 km (81-99 mi). Early next year, a range extender (REX) using a two-cylinder 650-cm³ BMW motorcycle engine will be offered to allay “range anxiety.” Positioned in the rear of the car alongside the 125-kW, 250 N·m electric motor, it acts as a battery charger for completion of a journey, but a full charge would still take six hours at the end of that journey. Range will be effectively doubled.

Initially, REX take-up could be as much as 50%, although the purists among BMW engineers are hoping this will reduce to 25% or less as confidence grows in the capability of an electric-only drive. The company believes that about 90% of daily city electric car journeys would be less than 50 km (31 mi). Only if driven extensively on a motorway in temperatures below zero and with the car fully loaded would range drop to 70-80 km (43-50 mi), claims BMW. In good ambient conditions, motorway range would still be 130-160 km. But long distance journeys are not the raison d’être for the i3.

Fully charging the battery takes six hours, but an 80% charge could be achieved in 30 minutes from a public-charging installation.

BMW says the i3 demonstrates “at least” one-third less global warming potential (CO2e) than a similar high-efficiency vehicle with an ICE. But this could be reduced by about 50% if the power to drive the car came from a renewable source.

The company’s Mini E and Active E cars have provided some 20 million km (12 million mi) test experience from which the i cars will benefit.

The salient points of the four-seat i3 include a CFRP passenger cell (Life Module) clad with detachable plastic panels, extensive use of aluminum structures, a rear-mounted electric motor driving the rear wheels (aluminum Drive Module), and lithium-ion batteries (supplied by Samsung) positioned beneath the floor to achieve a low center of gravity and help imbue the car with the sort of dynamics expected of a regular BMW.

Investment to date is around €600 million in facilities in the U.S. (with partner SGL for carbon fiber production at Moses Lake, WA, using hydroelectric power) and Germany, plus development costs spread over several years. Breitfeld says BMW is confident that it will earn money on every i3 from Job One.

The core of the i3—quite literally—is its CFRP Life Module. Having a mass of 150 kg (330 lb), it is the result of a determined BMW and SGL campaign to cut the costs of the material’s manufacture, and of its application in automotive production, with reduced cycle times but without quality degradation. Although no precise details are given by BMW, production cycle times for CFRP components have been cut by a further 30% for the i3 compared to the M3 roof application. Breitfeld sees process times becoming still shorter, with no sign yet of a plateau. Production costs have been halved.

It had to be CFRP

CFRP weighs half as much as steel and 30% less than aluminum, claims BMW. At no time did BMW seriously consider building the i3 and i8 using only aluminum, Breitfeld stressing that together with battery capacity, vehicle weight is the limiting factor for electric vehicle range; minimizing it was crucial. The i3 mass comes in at 1250 kg (2760 lb) and the i8 is expected to be less than 1500 kg (3310 lb).

Compared to a steel shell, CFRP also uses only about a third of the body parts and demands only half the production area; shaping is used instead of pressing, and bonding instead of welding. There is no press shop and no paint shop on the Leipzig production line, which is quiet and ultra clean.

Power for production of the i cars comes from four Nordex N100/2500 wind turbines within the Leipzig plant’s boundaries. Their output is 2.5 MW each, or about 26 GW·h a year for all four, sufficient for all power needs for building both i car models. Energy requirements are about 50% less and water consumption 70% less than in a regular BMW production facility.

The production process of the i3 begins with the fiber bundles from SGL being sent to the second site of the joint venture, at Wackersdorf Innovation Park (which has a capacity of “several thousand tons” per annum), where they are processed into light textile fabrics on an industrial scale, the fibers positioned side-by-side on one level, not interlaced. It is the fiber alignment in the fabric that provides required characteristics.

The Wackersdorf fabrics are then transported to pressing plants in Landshut (the innovation and product center for CFRP) and at the Leipzig vehicle production plant.

They undergo a preforming process, with a heating tool facilitating a stable, three-dimensional form for components to be assembled into a whole. This allows manufacture of large body components that would be very difficult to produce in aluminum or sheet steel. Indeed, the Life Module of the i3 in its CFRP production form would have been impossible to achieve in aluminum, stated BMW body specialists. It comprises some 150 parts.

After finishing and preforming comes resination under high pressure via RTM (resin transfer molding), with liquid resin injected into preformed work pieces under 4500-t (4960-ton) pressure—a common process within the aerospace industry—to bond the resin with a hardening agent and achieve curing. No autoclave is used; Breitfeld explained that BMW has developed a process that obviates the need.

The build process sees no need for paintshop or cathodic dip: “The production process is a very significant time saver and means that industrialization of large CFRP components is now realistic,” he said.

This means that a side-door frame can be completed with structural elements in place; really it is just a matter after that of cutting component contours and creating particular openings using waterjet cutting and sandblasting.

Sticking points

Individual components are assembled to a 1.5-mm (0.06-in) adhesive gap; all connecting components have the same gap to ensure no variation in adhesive quantity. Total bonded length is 160 m (525 ft). It takes about 30 minutes for the adhesive to harden, regarded by BMW as ten times faster than the norm. BMW has also developed a thermal process for some adhesion points, which “accelerate the curing process by a factor of 32.”

The highly robotically populated body assembly hall is said to produce minimal noise pollution. Thermoplastic injection molded plastic parts clad the CFRP and aluminum structure of the i3 and are screwed in place.

The i3’s Drive Module is provided by BMW’s Dingolfing plant and completed at Leipzig. It is screwed and glued to the Life Module. The underfloor lithium-ion battery has eight modules and 96 cells. Although the cells are supplied, the battery modules are a proprietary development of BMW and manufactured at Dingolfing. The individual battery blocks each have their own safety system.

The car’s air conditioning refrigerant cools the i3’s battery pack, obviating the need for fans or additional pumps. Heat for preconditioning in low temperatures comes from the charging electricity supply. The cabin is heated via a heat pump in urban traffic, a 30% saving against regular electric heating. Battery performance typically shows a 5% degradation at -15°C (+5°F), and testing has been completed down to -35°C (-31°F).

LEDs are used for interior and exterior lighting. The i8 will use a laser lighting system; full details will be released later. The i3 has a bipolar electrical system with the earth wired as a separate and fully insulated line.

As for crash performance (100 crash tests have been completed), the i3 meets or exceeds all test requirements including front offset, multi-rollover, and 32-km/h (20-mph) side impact—the last example seen by AEI demonstrated CFRP’s energy absorption and resistance to deformation. The high-voltage system has been designed to better legal crash requirements.

For lower energy crash damage, lengths of the CFRP door frame can be replaced using repair sections, such as a side sill, via use of a special kit including a patented milling tool. Lower energy accidents in the 15-20 km/h (9-12 mph) range would generally see the replaceable outer plastic panels absorbing impact. BMW quotes overall accident repair costs at about the same level as its 1 Series range.

Inductive charging potential is now an ongoing subject for BMW; the company wants to see an international standard established and has founded an official workgroup with German manufacturers under the aegis of the German Commission for Electrical, Electronic & Information Technologies. BMW says various manufacturers are also communicating on the issue at an international level.

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