BMW’s first production EVs, the electric Mini and the 1 Series ActiveE, were launched five years ago in limited numbers, but each provided invaluable R&D and production experience for what would become the hugely ambitious BMW i-project. The second of two design study proposals for BMW’s first mass production plug-in car was revealed at the LA automotive show in November 2012. Then came the final version, the i3, assembly of which started recently at a heavily modified Leipzig plant in Germany.
The company is being careful not to make any public statements about how many plug-in cars it plans to build (see interview with BMW’s head of production, p18), lest it regret over-ambitious forecasts in the coming years. Given the adverse publicity that the increasingly popular Nissan Leaf and the Chevrolet Volt attracted in their early years, this is no doubt a good idea.
Where BMW has really learned from the mistakes of others is in spreading its risk. There has been no major spending on refitting Leipzig with new paintshops or bodyshops – these are among the operations which have been either outsourced or eliminated. Partners were brought in early during the design stages of what started out as the Megacity project – the concept of launching a premiumpriced, lightweight, composite architecture commuter car.
BMW’s thinking was that it would not matter if there was no battery technology breakthrough by the time of the main investment decisions for the i3. A lightweight body-onframe structure and an aluminium chassis, or ‘life module’ as the company calls it, would offset the heavy battery pack. Therefore, a Mini-sized car weighing just under 1,200kg could be spatious and have a good range, as its batteries would be relatively small and light.
Pure EV or Range Extender? – the customer decides
That BMW took the decision to factor in an optional, range-extending 25kW/34hp two-cylinder engine shows the thoroughness of its design process. At a stroke, this eliminates the issue of range anxiety which has dogged other EVs.
The engine, which can be mounted above the rear axle and adjacent to the electric motor, increases the car’s maximum range in day-to-day driving by around 100km to as much as 300km. The Taiwanese firm Kymco is said to be the supplier of this unit, which also powers the BMW C650 GT scooter. The fuel tank, which has a capacity of just nine litres, is front-mounted, meaning that a car with the 650cc engine has the same luggage capacity as a pure EV model. The partnership for the car’s lightweight panels and overall construction is one of the many fascinating aspects of the new i3, and of a further model to come in 2014: the plug-in hybrid i8 supercar. BMW created a joint venture with specialist firm SGL to make carbon fibre-reinforced plastics (CFRP) for the body panels at its own Landshut plant in Lower Bavaria.
All visible body parts are impregnated with colour, thus eliminating the need for a painting stage on the assembly line. The customer can choose from two standard colours and four metallics. The panels are then sent to BMW’s Leipzig plant for assembly, where 160 robots mount the panels on rolling aluminum chassis using polyurethane glue. There are no conveyors or welding, as use of carbon fibre eliminates both the press shop and the paintshop.
All power used to produce the i3 at Leipzig is windgenerated; this is the first vehicle plant in Germany to have on-site turbines installed. According to BMW, 25% of the materials used in the i3 interior and the same percentage of the thermoplastics are recycled or made from renewable resources. In addition, 80% of aluminium parts are produced using clean energy or are made from recycled aluminum.
CFRP production – the US connection
While most of the i3’s component manufacture and assembly takes place in BMW’s home market, there is a US link. A second plant, specifically built for the manufacture of carbon fibre components, was erected at Moses Lake, Washington. Like Leipzig, this facility is carbon neutral, although the energy used here is derived from renewable, locally generated hydro-electric power. The two production lines currently have a capacity of 1,500 tonnes a year, which means this facility already accounts for 10% of the global supply of CFRP.
The US plant produces carbon fibres from what is known as a polyacrylonitrile-based thermoplastic textile fibre precursor. The various constituents of the fibre are removed by gasification, eventually leaving a material which consists of virtually pure carbon with a stable graphite structure. The resulting fibres are just seven microns (0.007mm) thick. By comparison, human hair has a diameter of 50 microns. Approximately 50,000 of these individual filaments are then bundled into ‘rovings’ or heavy tows and wound on reels.
From Moses Lake, the rovings are sent to the Wackersdorf Innovation Park in Germany, the JV’s second site. Here, they undergo industrial processing to become lightweight carbon fibre laminates. In contrast to a woven fabric, in these laminates the fibres are not interlaced but all lie in the same plane. Weaving would kink the fibres and detract from their properties; orientation in the laminate is crucial to achieving optimal quality in a CFRP component.
Several thousand tonnes of carbon fibre laminates can be manufactured annually at Wackersdorf. These laminates form the raw material for the manufacture of CFRP components at the BMW plants in Leipzig and Landshut. The second of these facilities has many years’ experience of making the carbon roof for the current M6 and former M3 models.
A new press shop in Leipzig now produces its own carbon fibre composite materials. The formulation, strength and geometry of the parts can be adapted to suit design requirements. During an initial pre-forming stage, the precut carbon fibre laminate supplied by Wackersdorf begins to acquire a shape. During this process, heat is used to give a fabric stack a stable, three-dimensional form. Several of these pre-formed stacks, or pre-formed blanks, can be joined to form a larger part. As a result, CFRP is suitable for producing components with a large surface area that would be difficult to manufacture from either sheet steel or aluminium.
Pre-forming and pre-form joining are followed by highpressure resin injection using resin transfer moulding (RTM). This technique involves high-pressure injection of liquid resin into the pre-forms. As the fibres and the resin bond, and the hardening process occurs, the material acquires the necessary rigidity. The CFRP presses then apply a clamping force of up to 4,500 tonnes, until the resin and hardener are fully cross-linked and the resin has reached the required hardness. A manufacturing process developed by BMW eliminates the need for additional time-consuming hardening in a separate oven, which would normally be needed.
The press shop, which was specially designed for CFRP, has little in common with a conventional sheet-steel facility in that there is a leaner production structure. Construction costs have also been significantly reduced, BMW claims, due to the fact that a conventional paintshop and cataphoretic immersion priming are not required. BMW’s innovative production process is said to make it feasible to mass produce large CFRP composite components, with newly formed parts leaving the press in under ten minutes. Even a complex assembly such as an entire side frame for the i3’s life module passenger cell leaves the Leipzig facility with many structural elements already integrated.
Finishing work – such as precise contour cutting and the insertion of further openings – is all that remains to be done, and this is performed using a special waterjet cutting system. The bonding surfaces are then sandblasted and roughened before further processing. By contrast, a conventional sheet-steel side frame would have to be built up successively from several different inner and outer components. Such an architecture would also need significantly more body parts and so be heavier than the i3 life module.
CFRP composite components produced in the new press shop and the CFRP components supplied from the Landshut press shop make their way to the new bodyshop. The basic architecture of the life module comprises some 150 components, roughly o one third of the number required for a conventional sheet-metal structure. Fully automated bonding technology replaces welding. The components are positioned 1.5mm apart at the bond line gap in order to ensure optimal strength of the resulting joint. Such precision means perfect transmission of forces between the individual CFRP components and therefore the highest level of volume production quality. The total length of bonded joints per vehicle is set precisely at 160 metres (20mm across).
CFRP passenger cells have traditionally been created only for high-priced, limited production vehicles or Formula One racing cars, where production costs and manufacturing complexity are not a concern. In such cases, the hardening times for bonded joints can be more than a day. The newly developed adhesive used in CFRP production at Leipzig is workable only 90 seconds after being applied to a component, before adhesion begins. An hour-and-ahalf later it is hard, a tenfold acceleration of conventional adhesive hardening times. In order to further reduce the hardening time to less than ten minutes, BMW has developed a supplementary thermal process. This involves heating specific points on the CFRP parts to be bonded, thereby further accelerating the hardening process by a factor of 32.
The production version of the i8, a front- and rear-wheel drive supercar, had its world premiere at the Frankfurt Motor Show in September and will be launched worldwide during 2014. Unlike the smaller i3, it has a gasoline-electric hybrid powertrain and the standard transmission is a six-speed automatic. The battery pack is mounted low and in the middle of this two-seater car.
The power output of the 1.5-litre turbocharged engine is 170kW/231hp and drive is sent to the rear wheels. The front wheels can also be driven, but rather than by the combustion engine, power is sent to them from a 96kW/131hp electric motor. The claimed EV mode range is up to 35km (22 miles). The car’s combined maximum power output is 266kW/362hp plus 570Nm of torque.
The i8 shares its electric motor, power electronics and high-voltage lithium-ion batteries with the i3, and the car has an unladen weight of just under 1,500kg.
BMW says this is the world’s fi rst volume production car to be fi tted with chemically hardened glass, a technology mostly used in smartphones. The partition between the passenger compartment and the boot consists of two layers of this glass, each of which is just 0.7mm thick, plus intermediate acoustic sheeting. A weight saving of 50% is claimed, compared with conventional glass.
Finally, the CFRP life module passes from the bodyshop to the assembly shop for the marriage, where it is united with the aluminium drive module. This module, supplied by BMW’s Dingolfing plant and built up in Leipzig, is bolted and bonded to the life module. Following this, the CFRP life module is fitted with its outer plastic skin. The painted multi-piece skin is made up mainly of injectionmoulded thermoplastics such as those used in conventional vehicle manufacturing (eg, front/rear aprons, side sills). The coloured plastic mouldings are bolted to the inner cell of the life module at special mounting points which cannot be seen from the outside.
The experimental Mini-E was state-of-the-art for its time (2008), running on a bank of 5,088 cells similar to those which powered the laptops of the era. The high-voltage lithium-ion battery in the BMW i3, however, is made up of eight modules, each with 12 individual cells, which together produce 360 volts and generate 18.8kW/h of energy. The pack itself is integrated within the underfloor section of the chassis and weighs 230kg. The battery was developed by BMW and has an eight-year or 100,000km guarantee. It can be recharged from a domestic power socket, BMW i Wallbox or a public charging station.
All this densely packed energy gives the i3 a stated range of 130km-160km. Top speed is limited to 150km/h for ‘efficiency reasons’, while acceleration from 0-100km/h is claimed to be 7.2 seconds (7.9s for the Range Extender). The electric motor weighs 50kg and sends power to the rear wheels via an integrated single-speed differential gear. Power is 125kW (170hp) and torque is 250Nm.
The car is BMW’s first one-box design and looks not dissimilar to the Audi A2, the lightweight city model which went out of production back in 2005. The i3, however, has an interior which is every bit as radical as the outside, with all materials either recycled or recyclable. For example, the dashboard is made from kenaf, a fast-growing sturdy grass.
BMW has also eliminated the B-pillar by hinging the rear doors in the same way as the Rolls-Royce Phantom, thus making it easier for back seat passengers to enter and exit. Moreover, interior space is increased by the lack of a centre tunnel. Further weight- and energy-saving features include: a hinged black glass panel instead of a multi-panel, conventional tailgate; a magnesium supporting structure for the instrument panel; forged aluminium suspension links; a hollow driveshaft; 19-inch forged alloy wheels; honeycomb windscreen wipers; and aluminium screws and bolts.