BMW Outlines LifeDrive Architecture for Upcoming Megacity Vehicle; A Focus on CFRP-Enabled Lightweight Design and Safety

BMW Group Megacity Vehicle Design Sketch. Click to enlarge.

At BMW Group’s Innovation Days: Mobility of the Future briefing in Munich, the company outlined its plans for the upcoming electric Megacity Vehicle (MCV), due onto the market in 2013 (earlier post), as well as the new LifeDrive architecture upon which the MCV is based.

The LifeDrive concept consists of two horizontally separated, independent modules. The Drive module—the aluminum chassis—integrates the battery, drive system and structural and 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 fiber reinforced plastic (CFRP). The new vehicle architecture enables new production processes which are both simpler and more flexible, and use less energy, BMW said.

The Megacity Vehicle is a revolutionary automobile. It will be the world’s first volume-produced vehicle with a passenger cell made from carbon. Our LifeDrive architecture is helping us to open a new chapter in automotive lightweight design. Indeed, this concept allows us to practically offset the extra 250 to 350 kilograms of weight typically found in electrically powered vehicles.

The drive system remains the heartbeat of a car, and that also applies to electric vehicles. Powertrains also remain a core area of expertise of Bayerische Motoren Werke. Electromobility and the hallmark BMW driving pleasure make an excellent match, if you go about things the right way. For this reason we are developing the powertrain for the Megacity Vehicle in-house—that includes the electric motor, the power electronics and the battery system.

—Klaus Draeger, Member of the Board of Management for Development

The importance of weight. Powering a vehicle electrically means more than just replacing the combustion engine with an electric drive system, BMW said. The electrification of a vehicle involves extensive revisions to the entire body, as the electric drive system components place very different demands on the packaging space in a vehicle. Based on its development work on the MINI E and BMW ActiveE concept projects, BMW concluded 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 company said, 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. In addition, 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.

BMW therefore set out to develop a new body concept which carefully addressed the full gamut of technical peculiarities of an electric drive system. Lightweight design is particularly important for electric vehicles because, alongside battery capacity, weight is the key limiting factor when it comes to the vehicle’s range. 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 area for an electric vehicle—the driver has to accelerate frequently due to the volume of traffic.

However, 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 on the application of lightweight design principles and the use of innovative materials.

Drive Module. 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 with 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 also houses 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. The Life module is a passenger cell mounted on the load-bearing structure of the Drive module. The primary characteristic of the Life module is its construction mainly out of carbon fiber-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.

CFRP offers many advantages over steel; while it is at least as strong as steel, it is also around 50% lighter. Aluminium, by contrast, would save 30% in weight terms over steel. This makes CFRP the lightest material that can be used in body construction without compromising safety.

BMW believes that, with more than ten years of intensive research work and experience with a program of process optimization, it is currently the only carmaker with the manufacturing experience necessary to use CFRP in volume production.

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. It also delivers handling benefits—the stiffness of the material makes the driving experience more direct. 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.

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 channelled to the rear wheels but which took up a lot of room in the interior. The Megacity Vehicle (MCV) therefore offers more room for its occupants within the same wheelbase. This new structure also enables the integration of new functionalities, allowing a new degree of freedom in the design of the vehicle architecture.

CFRP in body construction. CFRP offers a number 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.

Carbon fibers are exceptionally tear-resistant longitudinally. The fibers are woven into lattice structures and embedded in a plastic matrix to create the carbon fiber/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 fibers also allows CFRP components to be given a high-strength design by following their direction of loading. To this end, the fibers are arranged within the component according to their load characteristics. By overlaying the fiber 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.

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.

Its 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.

The ability of CFRP to absorb energy is truly extraordinary, BMW said. Pole impacts and side-on collisions highlight the safety-enhancing properties of CFRP. Despite the heavy, and in some cases, concentrated forces, the material barely sustains a dent. This makes CFRP perfectly suited for use in a vehicle's flanks.

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

In April, SGL Automotive Carbon Fibers LLC, the joint venture between SGL Group and BMW Group announced that it will build a carbon fiber manufacturing plant in Moses Lake, WA. The fibers manufactured at Moses Lake will be used exclusively for the MCV. (Earlier post.)

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