BMW recently unveiled some of the secrets of its battery-powered i3 hatchback, which is expected to go on sale late this year. BMW claims that the i3 will be the world's most sustainable car, and journalists were allowed access to the i3's remarkable wind-turbine-powered, carbon-neutral production plant near Leipzig in Germany. Andrew English of London's Daily Telegraph filed this story.
And carbon-fibre is the key to the i3's low (for a battery car) kerb weight of 1,250kg (2,755lb), but while carbon-fibre is commonly used in aerospace, motor racing and supercar manufacturing, it is produced in an artisanal process. BMW has applied systems analysis to batch produce the carbon-fibre panels required for the i3's passenger cell, a manufacturing first.
Carsten Breitfeld, head of the i project, says: "The key to reaching our targets for range and performance, as well as having a fast, exciting vehicle that is fun to drive, is to get low weight, because we had to compensate for the battery. So the question wasn't whether to use carbon-fibre or not, but how to optimise production."
BMW has closely streamlined every stage of the production of the Resin Transfer Moulded (RTM) carbon-fibre that makes up the inner body of this four-door family hatchback – the rolling chassis is made of aluminium alloy and the outer body panels are a polymer plastic.
It established a joint venture with carbon specialist SGL Automotive Carbon Fibres to weave carbon thread into dry mats at its hydro-electric-powered plant at Moses Lake in Washington State, USA. The mats contain a binder agent and when they arrive at the Leipzig factory (which is powered by four massive wind-turbines capable of generating 26,000 Megawatt hours – as long as the wind blows) they are pre-formed using ultrasound to set the binder agent and retain the shape.
Each body side comprises nine preformed panels and the excess is trimmed, recombed and used to make the i3's roof. The preformed mats are placed in moulds which are then injected with resin at 1,160psi and heated to 100ºC to set. Each panel is then trimmed, cleaned and the required holes are cut using water jets. The body is assembled by heavy duty robots to ensure accuracy, using 160 metres of two-pack polyurethane glue.
Cleanliness is vital so workers wear gloves, because the sweat off a finger can jeopardise a glued joint. The key processes are done by 160 robots in the assembly hall. It's slower than a conventional stamped-steel car plant, but not by much according to BMW. Eliminating the press shop and the paint shop saves time in the process. Using light and heat to set glues helps save time, too.
Elsewhere in Leipzig, BMW 1-series are built at a rate of 700 to 750 a day from two shifts. Uwe Taver, the i3 bodyshop manager, admits that i3 production is some way off that, "but i3 bodies take just a few minutes to set. Compared with classic body construction, we are getting closer."
Investment is less, too. There are no conveyors, no welding, the robots (apart from the glueing models) are lighter duty models as they don't have to carry the same weight and the bodyshop is almost ethereally peaceful, without the incessant thumps and crashes of a conventional metal-bashing shop. Paint is still involved though, and some of the environmental claims for the i3 plant depend on another factory spraying the plastic body panels.
BMW claims that in the 100 or so crash tests it has performed on the i3, its carbon-fibre construction is at least as safe, if not safer, in a crash. In an impact the main battery is isolated using an explosive fuse, the residual system charge is drained and the motor regeneration electronics are isolated. Carbon-fibre is also mainly cheaper to repair in a crash, according to BMW, with between 10 and 20 per cent reduced costs and 10 to 15 per cent reduced labour times. What's more, BMW claims that more than 95 per cent of typical accidents will only affect the outer skin panels which can be easily replaced, with just 2.5 per cent affecting the aluminium rolling chassis and another 2.5 per cent the inner carbon-fibre body.
If not exactly cheaper, carbon-fibre construction is certainly lighter. A conventional pressed-steel version would have weighed about 1,500kg (3,306lb) and an all-aluminium version about 1,425kg (3,141lb). It's that near quarter-ton weight saving that BMW claims will give the i3 an agility and range that will set it apart from its heavier battery-powered rivals such as the Nissan Leaf (1,565kg/3,450lb), Renault Fluence (1,605kg/3,538lb) and forthcoming battery-powered cars from Ford and others.
The i3 doesn't have a full carbon-fibre monocoque like supercars such as the Lamborghini Aventador or McLaren P1. In fact the running gear, suspension and steering are mounted on an aluminium rolling chassis that's reminiscent of GM's 2002 "AUTOnomy" fuel-cell concept. The 230kg, 22kWh square battery pack sits under the floor cooled by passing wind and the car's air conditioning, protected by stout chassis rails and honeycomb-section carbon fibre in the sills. The 168bhp AC motor will give a 0-62 mph acceleration in 7.9sec and top speed will be limited to 93mph. Range is quoted at 140 miles in the EU Combined cycle, but realistically between 80 and 100 miles, which is about 30 per cent more than the Nissan Leaf.
While the performance is impressive, the inevitable drawback of exotic materials is the price. Breitfeld says the i3 will "cost less than you think" – rumour has it that it could be about £35,000 in the UK. That would translate to $56,000 in the U.S., but cars are typically more expensive in the UK. ––Paul Duchene
