An electrically driven unit

New automotive concepts call for new body structures. Many requirements must be met—both for electric and autonomous vehicles.

All is ready for electro mobility to take off. However, it will be some time before electric vehicles populate our roads in large numbers. To fully reinvent the vehicle, the additional time will be necessary—especially when you consider the individual components required. The bodyshell, for instance, constitutes a large portion of a vehicle's design, including significant functions such as safety, vibration characteristics and rigidity, as well as to connect various elements of the vehicle. The body is the largest component and functions as the backbone of the vehicle. It unites all the components to form a unit capable of movement.

‘Safety, lightweight construction, vibration characteristics, durability and the potential for large-scale production—these features remain just as important with the body construction of electric vehicles,’ says Stefan Hummel, Head of the Bodyshell Technical Centre and Manufacturing Technologies at the Development division of Mercedes-Benz Cars in Sindelfingen. ‘These factors must all be considered, as they span all requirements and requests—including those of our customers.’

The vehicle battery affects bodyshell construction

However there are differences to conventional cars. With an all-electric vehicle, the weight and size of the battery affects how the bodyshell is constructed. The battery comprises several cells, usually configured as one compact component, which is surrounded by a sturdy protective housing. The battery is generally mounted at a low position beneath the vehicle. The low centre of gravity is beneficial for driving dynamics.

Experts generally opt to install the battery at the centre of the vehicle, in order to keep it as far away from the outer skin as possible. This protects the battery from a direct force of impact in case of an accident. Protecting the battery from side impacts is another important factor. This ensures an ideal use of the available, but extremely limited, deformation space. Specific constructive measures and crash elements are utilised for side impact protection, ensuring the best possible protection for passengers, as well as for the battery.

The bodyshell of an electric vehicle, in its entirety, of course adheres to the highly complex crash safety requirements.

Stefan Hummel, Head of the Bodyshell Technical Centre and Manufacturing Technologies at the Development division of Mercedes-Benz Cars

‘The bodyshell of an electric vehicle, in its entirety, of course adheres to the highly complex crash safety requirements,’ Hummel explains. ‘This is achieved through specifically developed body concepts, the shape and design of the components, by using optimum materials, and through joining techniques which ensure that the individual body components connect to form a uniform whole.’ The goal is to considerably reduce—or if possible completely redirect—the acting forces arising from an accident.

These are not the only changes the electric car will bring; when internal combustion engines are replaced by smaller electric motors—which one may even place directly at the axle—the engine compartment, as it is currently built, will no longer be fully necessary. This will open new opportunities for design engineers and designers, such as with the installation of additional function assemblies and safety elements or the design of the interior. Of course, autonomous vehicles will present even more opportunities, as steering the vehicle will no longer be an issue.

An intelligent mix of different materials

‘In each case, we develop a custom body concept to meet the various requirements,’ says Michael Vogel, a member of the Bodyshell Technical Centre and Manufacturing Technologies team at the Development division of Mercedes-Benz Cars. ‘To do so, we employ an extensive construction kit, which is being continuously developed further.’ At Daimler, they use the term ‘intelligent mix’ to describe their materials. ‘Steel is still the dominant material,’ Vogel explains. The high-strength and thus very thin parts of sheet metal are often on par with light alloys and plastics. Furthermore, as these parts can be made to measure, they tend to have characteristics that only affect a certain local part of the body, but which are highly important at precisely this spot. Aluminium is used wherever a minimal amount of weight is needed.

Lightweight construction offers some important leverage in minimizing the energy consumption of a vehicle.

Thomas Schweiker, member of the Bodyshell Technical Centre and Manufacturing Technologies team at the Development division at Mercedes-Benz Cars

For some time now, fibre-reinforced plastics are part of the material mix of the Development division of Mercedes-Benz Cars, such as ones based on carbon or glass fibres. These plastics are lightweight but very robust. However, they are also expensive. ‘We only consider using them if there is a specific place in the bodyshell where they would be ideal based on cost and function,’ Thomas Schweiker says, also a team member at the Bodyshell Technical Centre and Manufacturing Technologies at the Development division at Mercedes-Benz Cars. ‘To leverage the maximum energy efficiency of a vehicle, lightweight construction is key. But it is not of utmost importance. Especially, if it gets in the way of other functions of the body, such as safety.’

Lastly, it is important that the bodyshell can be mass-produced in a cost-efficient manner, ideally using existing facilities. Design engineers take this into account. After all, an electric vehicle must also be affordable. However, depending on the cost of the vehicle, there is some leeway; the more exclusive and expensive a vehicle is, the more possibilities there are to use cost-intensive materials to meet particular requirements, such as to achieve an optimal weight for a sports car.

Noise, vibration, harshness

A body concept must also always meet specific requirements pertaining to the ‘noise, vibration, harshness’ (NVH) of the vehicle. ‘Noise, vibration, harshness’ cover only a portion of what is actually required. An example from vibration physics makes this clear: when a car is in motion, vibrations from the road are transferred into the body structure. The drive system of a vehicle also generates vibrations. Additionally, noises that occur while driving also create acoustic vibrations.

NVH ensures that none of these vibrations have an unintended effect, such as unpleasant noises in the vehicle interior, which could comprise the driving experience. NVH requirements are already considered when constructing the bodyshell of the vehicle body. Insulation materials are also playing a role here.

What will the future of body construction bring? ‘We expect that “purpose-built design” will become more important so that smaller-scale productions can be economically viable,’ Karl-Heinz Füller from the Daimler research centre in Ulm says. ‘The immense progress that has been made with manufacturing technologies will perhaps create new opportunities in this area. With 3D printing, a start has been made.’

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