Super Light Car—lightweight construction thanks to a multi-material design and function integration
© European Conference of Transport Research Institutes (ECTRI) 2008
Received: 2 October 2008
Accepted: 5 November 2008
Published: 26 November 2008
The Super Light Car (SLC) project is one of the most important research projects in the European Community for automotive lightweight construction with a multi-material approach. The paper shows the motivation and objective for a front structure designed with a light multi-material-mix.
1 Motivation and challenge
Global warming is a fact. One of the main reasons are the CO2 emissions (23%) being caused by the vehicle traffic. In 2005, this corresponded to approx. 6 billion metric tons of CO2 . Without conducting an evaluation showing in detail how these emissions affect global warming, it can be said that the political pressure to reduce these emissions from vehicle traffic is increasing.
As a result, several approaches have already been accomplished by the automotive industry. The reduction in consumption and emissions remains the greatest technological challenge for the automotive industry [2, 3]. The reduction in consumption required cannot be achieved through continual improvement of the drive chain and thus an increase in energy efficiency alone. The path of reducing driving resistances, especially vehicle weight, must also be followed. Vehicle mass affects the energy consumption of the vehicle through mass-related driving resistances.
Reducing weight by 100 kg leads to a fuel savings of 0.35 l/100 km and 8.4 g CO2/km with gasoline engines in the NEDC if taking into account an adjustment of the gear shifting without a change in elasticity and acceleration values due to the lower weight .
2 Super Light Car (SLC) EU project
A series of goal conflicts which need to be analysed and assessed results from these goals and requirements. The task of the 37 project partners was to find the best possible solution with regard to lightweight construction and the specified requirements. In addition to lightweight construction, issues such as economic viability and reliable manufacturing processes for individual structures, assemblies and the body are examined here. Today, lightweight construction can not longer be achieved through materials alone. Appropriate construction principles, manufacturing and joining technologies must also be considered as focal points in multi-material design. Approaches to a solution show positive trends toward lighter vehicle concepts, but must be continually scrutinised with regard to the recycling requirements. These problems were examined in the SLC project by various project partners from the industry and research fields.
The Institute of Vehicle Concepts (DLR) designed, constructed and simulated a front end module with magnesium and aluminium structural components in the EU project. This concept was created building upon a methodical analysis of the reference structure and resulted in a weight reduction of 24 kg through justifiable additional costs and fulfilment of the requirements, e.g. crash and stiffness. These technological results lead the SLC consortium to select this concept as the prototype.
Before this decision could be made, a host of various design concepts were developed in the “Design Concepts” subproject. This work included the construction, simulation, evaluation and optimisation of the individual structures in the front end. Design of the front end, floor and structural modules was carried out under the direction of Volkswagen AG, which was in charge of overall co-ordination of the SLC project. These modules were optimised in three different development threads with respect to the overall SLC body design. The three threads, “steel intensive”, “Multi material, economic” and “Multi material, advanced”, had different aims during development with regard to weight reduction and the additional costs involved in lightweight construction in comparison to the benchmark vehicle. The specifications for weight achieved a reduction from 10 to 25 to 45%.
Concept development was supported and supplemented by the “Materials & Manufacturing Technologies” and “Simulation & Testing” subprojects, in which the required technologies for economic multi-material lightweight construction were developed. In addition, the costs and LCA (Life Cycle Assessment) were taken into account during concept development. The SLC body concept will finally be presented as a prototype for physical testing and subsequently validated in the fourth subproject at the beginning of 2009.
3 Front end concept
With the front end concept, the challenge was to reduce the weight of a 76 kg module using a variety of lightweight materials in the optimum places. To proceed methodically, the front end area of the mostly-steel benchmark structure was analysed with regard to the materials used, production methods and component costs and their requirements when the work was begun. Preliminary conceptual considerations for lighter front end structures were made while taking into account aluminium alloy casting, the assembly of parts and the integration of functions.
In taking these elements into account, the conceptual idea of combining the area of the top longitudinal rail, the strut tower and the engine and transmission mount into a single large aluminium alloy cast component was developed. This area has stringent requirements regarding crash safety of the vehicle, e.g. in case of a head-on collision, and the torsion and flexural stiffness of the entire vehicle structure.
Detail development took place over several development loops between construction (CATIA V5) and simulation (LS-Dyna). The optimisation of crash behaviour of the multi-material structure, in particular, required more than 80 overall vehicle crash calculations.
Despite a reduction in weight of 32% in the front end, excellent crash behaviour was achieved. In several areas, the crash behaviour was actually improved in comparison to the reference structure. For example, the footwell intrusion measurement of 51 mm of the DLR concept is clearly an improvement over the 100 mm of the reference structure.
By reducing the weight of the front end by 24 kg, the set project goal of more than 30% was reached. The highly-integrated magnesium cast component described above, which combines some 12 steel components into a single cast component and reduces the weight by more than 60%, was responsible for a considerable portion of this.
4 Conclusion and outlook
In addition to the vehicle body, systematic lightweight design concepts will integrate equipment, chassis, engine and electronics. Sustainable concepts which benefit from secondary lightweight effects will play a decisive role in future car design.
ODB—Offset Deformable Barrier
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