2010 BMW Group Innovation Days Mobility of the Future - Lightweight Design and the LifeDrive Concept.

SEE ALSO: 2010 BMW Group Innovation Days Mobility of the Future
Complete Report:
Chapter 1. Why Electromobility?
Chapter 2. Project i.
Chapter 3. The Electric Drivetrain.
Chapter 4. Lightweight design and the LifeDrive concept.
Chapter 5. CFRP - A Material for the Future.

Chapter 4. Lightweight Design and the LifeDrive Concept

New body concepts to meet the challenges of a new mobility.
Powering a vehicle electrically means more than just replacing the combustion engine with an electric drive system. The electrification of a vehicle involves far-reaching revisions to the entire body, as the electric drive system components place very different demands on the packaging space in a vehicle. The development work on the MINI E and BMW ActiveE concept projects quickly showed that “conversion cars” – i.e. vehicles designed to be powered by combustion engines and subsequently converted to run on electric power – do not represent an optimum long-term solution when it comes to meeting the demands of e-mobility. As important as these vehicles have been in amassing knowledge on the usage and operation of EVs, the integration of an electric drive system into a “foreign” vehicle environment is not the best way of exploiting the potential of e-mobility. Conversion cars are comparatively heavy. Added to which, accommodating the big and heavy battery modules and special drive electronics is a complex job, as the structural underpinnings of the vehicles are based on a very different set of requirements.

A new body concept therefore had to be developed which carefully addressed the full gamut of technical peculiarities of an electric drive system and provided the ideal response to all safety-related considerations. So how does a functional and effective body construction for an electric vehicle shape up?

Lightweight design for electric vehicles.
A modern vehicle body has to be not only strong but, above all, light as well. When you’re dealing with a vehicle powered by an electric drive system, lightweight design is particularly important because, alongside battery capacity, weight is the key limiting factor when it comes to the vehicle’s range. The lighter a vehicle, the longer the distance it will be able to travel – simply because the electric drive system will have less mass to move. Under acceleration, in particular, every kilogram of extra weight makes itself clearly felt in the form of reduced range. And in the city – the main hunting ground for an electric vehicle – the driver has to accelerate frequently due to the volume of traffic.

As well as a longer range, lower vehicle weight also makes for noticeably better performance. After all, a lightweight vehicle accelerates faster, is more nimble through corners and brakes to a standstill more quickly. Lightweight design therefore paves the way for greater driving pleasure, agility and safety. In addition, lower accelerated mass means that energy-absorbing crash structures can be scaled back, which in turn saves weight.

And so the task for the engineers is to keep the overall weight of an electric vehicle as low as possible from the outset. However, the fundamental aspects of an electric car’s construction are anything but helpful in this regard. The drivetrain of an EV is far heavier than that of a vehicle with a combustion engine, full tank of fuel included; an electric drive system (including battery) weighs around 100 kg more. The battery is the chief culprit here. To cancel out the extra weight it brings to the vehicle, the BMW Group is working rigorously on the application of lightweight design principles and the use of innovative materials. By using the optimum material for each component, depending on the requirements and area of usage, the BMW Group engineers have succeeded in ensuring that the heavy battery barely carries any weight, so to speak.

“Lightweight materials are an important enabler in the drive towards electromobility, as they can even out the extra weight added by the energy storage system.” (Bernhard Dressler)

Purpose design – the LifeDrive concept.
Lightweight design, however, is just one facet, albeit a very important one, of the development work which goes into modern body construction. The full electrification of a vehicle gave the BMW Group engineers the opportunity to completely rethink the vehicle architecture and to adapt it to the demands and realities of future mobility. With the LifeDrive concept they used purpose design to create a revolutionary body concept which is geared squarely to the vehicle’s purpose and area of usage in the future and offers an innovative use of materials.

Similarly to vehicles built around a frame, the LifeDrive concept consists of two horizontally separated, independent modules. The Drive module – the aluminium chassis – forms the solid foundation of the vehicle and integrates the battery, drive system and structural and basic crash functions into a single construction. Its partner, the Life module, consists primarily of a high-strength and extremely lightweight passenger cell made from carbon fibre-reinforced plastic, or CFRP for short. With this innovative concept the BMW Group adds a totally new dimension to the areas of lightweight design, vehicle architecture and crash safety.

“The LifeDrive concept links all the systems required to drive the vehicle with the realities and requirements of electromobility, and puts them into practice with a new approach – yet still in trademark BMW Group style.” (Uwe Gaedicke)

Drive module – the basis and solid foundation.
The Drive module brings together several functions within a lightweight and high-strength aluminium structure. This is the basic body, complete with the suspension, crash element, energy storage device and drive unit. Weighing around 250 kg and with dimensions similar to those of a child’s mattress, the energy storage system is the driving element of the integrative and functional design of the Drive module. The initial priority in the conception of the Drive module was therefore to integrate the battery – the largest and heaviest factor in the electric vehicle in terms of construction – into the vehicle structure so that it would be operationally reliable and safe in a crash.

The Drive module is divided into three areas. The central section houses the battery and surrounds it securely with powerful aluminium profiles. The two crash-active structures in the front and rear end provide the necessary crumple zone in the event of a front or rear-end impact. The Drive module is also where you will find the components of the electric drive unit and numerous suspension components. The electric drive system is, as a whole, much more compact than a comparable combustion engine, cleverly accommodating the electric motor, gear assembly, power electronics and axles within a small space.

Life module – CFRP enters a new dimension.
The LifeDrive concept is rounded off by the Life module, a passenger cell mounted on the load-bearing structure of the Drive module. The stand-out characteristic of the Life module is its construction mainly out of carbon fibre-reinforced plastic (CFRP). The selection of this high-tech material – on this scale – for a volume-produced vehicle is unprecedented, as the extensive use of CFRP has previously been thought of as too expensive and still not sufficiently flexible to work with and produce. However, with more than ten years of intensive research work and a programme of process optimisation under its belt, the BMW Group is the only carmaker with the manufacturing experience necessary to use CFRP in volume production. CFRP offers many advantages over steel; it is extremely strong, yet at the same time very light. Indeed, while it is at least as strong as steel, it is also around 50% lighter. Aluminium, by contrast, would save “only” 30% in weight terms over steel. This makes CFRP the lightest material that can be used in body construction without compromising safety.

The extensive use of this high-tech material makes the Life module extremely light and gives the car both a longer range and improved performance. Added to which, it also has clear benefits in terms of the car’s handling; the stiffness of the material makes the driving experience more direct, with even rapid steering movements executed with flawless precision. At the same time, CFRP enables a higher level of ride comfort, as the stiff body dampens energy inputs extremely effectively. As a result, unwanted vibrations on the move are eliminated: there are no rattles or shakes.

As well as being extremely lightweight, the Life module also opens up a whole new perspective on how a vehicle interior can be perceived and designed. The integration of all the drive components into the Drive module allows the removal of the transmission tunnel – through which the engine’s power was previously channeled to the rear wheels but which took up a lot of room in the interior. The Megacity Vehicle (MCV) therefore offers significantly more room for its occupants within the same wheelbase. This new structure also enables the integration of new functionalities, allows a new degree of freedom in the design of the vehicle architecture and therefore clears the way for the interior to be optimally adapted to the demands of urban mobility.

CFRP in body construction.
CFRP has a wealth of benefits as a material for a vehicle body. It is extremely corrosion-resistant and does not rust, giving it a far longer lifespan than metal. Complex corrosion protection measures are unnecessary and CFRP retains its integrity under all climatic conditions.

The secret of this extremely high-strength material lies in the carbon fibres. They are exceptionally tear-resistant longitudinally. The fibres are woven into lattice structures and embedded in a plastic matrix to create the carbon fibre/plastic composite material CFRP. In its dry, resin-free state CFRP can be worked almost like a textile, and as such allows a high degree of flexibility in how it is shaped. The composite only gains its rigid, final form after the resin injected into the lattice has hardened. This makes it at least as durable as steel, but it is much more lightweight.

The high tear resistance along the length of the fibres also allows CFRP components to be given a high-strength design by following their direction of loading. To this end, the fibres are arranged within the component according to their load characteristics. By overlaying the fibre alignment, components can also be strengthened against load in several different directions. In this way, the components can be given a significantly more efficient and effective design than is possible with any other material that is equally durable in all directions – such as metal. This, in turn, allows further reductions in terms of both material use and weight, leading to another new wave of savings potential. The lower accelerated mass in the event of a crash means that energy-absorbing structures can be scaled back, cutting the weight of the vehicle.

“CFRP allows you to build an extremely lightweight plastic body without having to make compromises in comfort and safety.” (Bernhard Dressler)

Lightweight design and safety – with CFRP, lighter also means safer.
In addition to lightweight design, passenger safety also played a major role in the development of the LifeDrive concept. The current impact stipulations for a vehicle body are extremely stringent and a wide range of different crash scenarios have to be taken into account. Generally speaking, this presents development engineers with serious challenges, especially as far as the use of new materials is concerned. However, the combination of aluminium in the Drive module and the CFRP passenger cell in the Life module exceeded all expectations – even in the initial testing phase – and clearly showed that lightweight design and safety are not a contradiction in terms.

“Lightweight design does not automatically mean ‘unsafe’ – quite the contrary, in fact: in some respects, the LifeDrive concept outperformed existing constructions in crash testing.” (Nils Borchers)

Impressive rigidity, combined with its ability to absorb an enormous amount of energy, makes CFRP extremely damage-tolerant. Even at high impact speeds it displays barely any deformation. As in a Formula One cockpit, this exceptionally stiff material provides an extremely strong survival space. Furthermore, the body remains intact in a front or rear-on impact, and the doors still open without a problem after a crash.

Unbeatable protection in a side-on impact.
The ability of CFRP to absorb energy is truly extraordinary. Pole impacts and side-on collisions both highlight the impressive safety-enhancing properties of CFRP. Despite the heavy, in some cases concentrated forces, the material barely sustains a dent, and passengers enjoy unbeatable protection. All of which makes CFRP perfectly suited for use in a vehicle’s flanks, where every centimetre of undamaged interior is invaluable.

“To demolish CFRP you need to apply extremely heavy forces and/or extremely heavy acceleration – significantly more than you’d think at first glance.” (Bernhard Dressler)

However, there are limits to what CFRP can endure. If the forces applied go beyond the limits of the material’s strength, the composite of fibres breaks up into its individual components in a controlled process.

The best of both worlds – combining aluminium and CFRP.
The new Drive module has also been carefully designed and structured with these exacting crash requirements in mind. Crash-active aluminium structures in the front and rear sections of the vehicle provide additional safety. In a front or rear-on collision, these absorb a large proportion of the energy generated. The battery, meanwhile, is mounted in the underbody section of the car to give it the best possible degree of protection. Statistically, this is the area that absorbs the least energy in the event of a crash, and the vehicle shows barely any deformation here as a result. Moreover, positioning the battery in the underbody allows the BMW Group development engineers to give the vehicle an ideal low centre of gravity, which makes it extremely agile and unlikely to roll over.

In a side-on collision the battery also benefits from the crash properties of the Life module, as it absorbs all the impact energy and stops it from reaching the energy storage system. The mixture of aluminium in the Drive module and CFRP in the Life module ensures that the battery also enjoys the best possible protection through the body sills.

“The Drive module is the safest form a battery can take.” (Hans-J�rgen Branz)

All in all, the high-strength CFRP passenger cell teams up with the intelligent distribution of forces in the LifeDrive module to lay the foundations for optimum occupant protection. And this allows the combination of materials in the LifeDrive module to provide better safety levels than a steel monocoque. Testing has shown how much potential there still is in CFRP and its use in combination with other materials. Indeed, in what are still only relatively early days, CFRP already outperforms other materials at a much more advanced stage of development.

Advantages of LifeDrive.
Purpose design allows the LifeDrive concept to integrate all the key features of e-mobility – such as the large and bulky battery and compact drive elements – into an impact-resistant structure. However, the advantages of the LifeDrive concept lie not only in the weight savings it allows, the longer range and improved performance characteristics this results in, and enhanced safety. It becomes evident how much more lies behind the LifeDrive concept when you consider not only the product itself but also the production processes associated with it. The LifeDrive principle allows it to meet all the demands placed on a sustainable product within a sustainable production chain.

The vehicle’s frame construction is extremely practicable when it comes to the production of moderate unit figures, while the use of parallel working processes ensures a high level of flexibility. The vehicle’s new architecture opens the door to totally new production processes which are both simpler and use less energy. For example, the horizontal separation of the modules allows the two elements to be manufactured separately before being put together virtually anywhere in the world in a straightforward assembly process.

“Development work over recent years has made it clear to me that the LifeDrive concept is currently the solution when it comes to meeting the full spectrum of requirements presented by electromobility, while at the same time making the best possible use of its inherent potential.” (Uwe Gaedicke)