ORIGINAL PAPER

Super Light Car—lightweight construction thanksto a multi-material design and function integration

Martin Goede & Marc Stehlin & Lukas Rafflenbeul &Gundolf Kopp & Elmar Beeh

Received: 2 October 2008 /Accepted: 5 November 2008 / Published online: 26 November 2008# European Conference of Transport Research Institutes (ECTRI) 2008

Abstract The Super Light Car (SLC) project is one of themost important research projects in the European Commu-nity for automotive lightweight construction with a multi-material approach. The paper shows the motivation andobjectives for a front structure designed with a light multi-material-mix.

Keywords Super Light Car . Automotive lightweightconstruction .Multi-material structures

1 Motivation and challenge

Global warming is a fact. One of the main reasons are theCO2 emissions (23%) being caused by the vehicle traffic. In2005, this corresponded to approx. 6 billion metric tons ofCO2 [1]. Without conducting an evaluation showing indetail how these emissions affect global warming, it can besaid that the political pressure to reduce these emissionsfrom vehicle traffic is increasing.

As a result, several approaches have already beenaccomplished by the automotive industry. The reductionin consumption and emissions remains the greatest techno-logical challenge for the automotive industry [2, 3]. The

reduction in consumption required cannot be achievedthrough continual improvement of the drive chain and thusan increase in energy efficiency alone. The path of reducingdriving resistances, especially vehicle weight, must also befollowed. Vehicle mass affects the energy consumption ofthe vehicle through mass-related driving resistances.

Reducing weight by 100 kg leads to a fuel savings of0.35 l/100 km and 8.4 g CO2/km with gasoline engines inthe NEDC if taking into account an adjustment of the gearshifting without a change in elasticity and accelerationvalues due to the lower weight [4].

2 Super Light Car (SLC) EU project

The “Super Light Car” (SLC) EU-project started at thebeginning of 2005 with the motivation to reduce weight invehicle bodies through the economically feasible produc-tion of multi-material structures (Figs. 1 and 2) whichwould contribute to a reduction of consumption and CO2 inthe automobile. One of the objectives in the project is toreduce the weight in the body in white (B.I.W.) by at least30% (reference: A-class segment unibody structure)through a lightweight construction for volume productionof 1,000 vehicles per day. This should be achieved underconsideration of various vehicle-specific requirements, e.g.high crash safety and stiffness.

A series of goal conflicts which need to be analysed andassessed results from these goals and requirements. Thetask of the 37 project partners was to find the best possiblesolution with regard to lightweight construction and thespecified requirements. In addition to lightweight construc-tion, issues such as economic viability and reliablemanufacturing processes for individual structures, assem-blies and the body are examined here. Today, lightweight

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M. Goede :M. Stehlin (*) : L. RafflenbeulVolkswagen AG,Konzernforschung,Wolfsburg, Germanye-mail: marc.stehlin@volkswagen.de

G. Kopp : E. BeehInstitut für Fahrzeugkonzepte,Deutsches Zentrum für Luft- und Raumfahrt e.V.,Stuttgart, Germany

construction can not longer be achieved through materialsalone. Appropriate construction principles, manufacturingand joining technologies must also be considered as focalpoints in multi-material design. Approaches to a solutionshow positive trends toward lighter vehicle concepts, butmust be continually scrutinised with regard to the recyclingrequirements. These problems were examined in the SLCproject by various project partners from the industry andresearch fields.

The Institute of Vehicle Concepts (DLR) designed,constructed and simulated a front end module withmagnesium and aluminium structural components in theEU project. This concept was created building upon amethodical analysis of the reference structure and resultedin a weight reduction of 24 kg through justifiable additionalcosts and fulfilment of the requirements, e.g. crash andstiffness. These technological results lead the SLC consor-tium to select this concept as the prototype.

Before this decision could be made, a host of variousdesign concepts were developed in the “Design Concepts”subproject. This work included the construction, simula-

tion, evaluation and optimisation of the individual struc-tures in the front end. Design of the front end, floor andstructural modules was carried out under the direction ofVolkswagen AG, which was in charge of overall co-ordination of the SLC project. These modules wereoptimised in three different development threads withrespect to the overall SLC body design. The three threads,“steel intensive”, “Multi material, economic” and “Multimaterial, advanced”, had different aims during developmentwith regard to weight reduction and the additional costsinvolved in lightweight construction in comparison to thebenchmark vehicle. The specifications for weight achieveda reduction from 10 to 25 to 45%.

Concept development was supported and supplementedby the “Materials & Manufacturing Technologies” and“Simulation & Testing” subprojects, in which the requiredtechnologies for economic multi-material lightweight con-struction were developed. In addition, the costs and LCA(Life Cycle Assessment) were taken into account duringconcept development. The SLC body concept will finallybe presented as a prototype for physical testing andsubsequently validated in the fourth subproject at thebeginning of 2009.

3 Front end concept

With the front end concept, the challenge was to reduce theweight of a 76 kg module using a variety of lightweightmaterials in the optimum places. To proceed methodically,the front end area of the mostly-steel benchmark structurewas analysed with regard to the materials used, productionmethods and component costs and their requirements whenthe work was begun. Preliminary conceptual considerationsfor lighter front end structures were made while taking intoaccount aluminium alloy casting, the assembly of parts andthe integration of functions.

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Fig. 1 Multi-Material-Design Research Approach

Fig. 2 Approach and scheduleof SLC

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Using the parts list of the reference structure, Fig. 3,components suitable for integration into a single cast partdue to their arrangement, function and specific costs wereidentified.

In taking these elements into account, the conceptualidea of combining the area of the top longitudinal rail, thestrut tower and the engine and transmission mount into asingle large aluminium alloy cast component was devel-oped. This area has stringent requirements regarding crashsafety of the vehicle, e.g. in case of a head-on collision, andthe torsion and flexural stiffness of the entire vehiclestructure.

In previous vehicles, these requirements were fulfilledusing sheet metal or cast metal solutions made of steel oraluminum. As a result of the goals for weight reduction, theengineers aimed at a solution using magnesium castingdespite the great challenges involved. This would involveunique new technologies for the use of magnesium in load-bearing structures. Sheet aluminium was identified as anideal solution for the adjoining areas and the rest of thefront end, Fig. 4.

Detail development took place over several developmentloops between construction (CATIA V5) and simulation

(LS-Dyna). The optimisation of crash behaviour of themulti-material structure, in particular, required more than80 overall vehicle crash calculations.

First, the crash behaviour of the magnesium componentwas optimised for the OBD1 crash load case (Figs. 5 and 6).Through precise analysis of the calculated componentfailure, the component was improved geometrically andwith regard to wall thickness distribution. To reproduce thematerial properties of the selected magnesium alloy AM50as realistically as possible, material failures with a breakageelongation of 8% were computed in the crash simulation. Inaddition, great importance was attached from the verybeginning to the manufacturing capability using the die-castprocess. In the second stage of development, the surround-ing steel structure was replaced with a lighter aluminiumstructure. This structure was then also optimised for advan-tageous crash behaviour in the overall vehicle structure.The force-displacement diagrams, intrusions, compareFig. 7 and the simulation films of every design variantwere analysed. The geometry, wall thickness or materialchanges of every new variant was examined to determinewhat benefits they could offer. This allowed the longitudi-nal rail structure to be better utilised with regard to specificenergy absorption and thus lightened in comparison to therespective reference structure.

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 comparisonto the reference structure. For example, the footwellintrusion measurement of 51 mm of the DLR concept isclearly an improvement over the 100 mm of the referencestructure.

By reducing the weight of the front end by 24 kg, the setproject goal of more than 30% was reached. The highly-integrated magnesium cast component described above,which combines some 12 steel components into a singlecast component and reduces the weight by more than 60%,was responsible for a considerable portion of this.

Strut TowerMagnesium DG, AM50

(3kg) Longitudinal Rail

Aluminium, Tailored WeldedBlank (4.3kg)

Fig. 4 DLR front end concept with magnesium cast strut tower(yellow) and aluminium structures (green)

Fig. 3 Example of benchmarkstructure parts and part list

0 ODB—Offset Deformable Barrier

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Fig. 6 Analyse of the plasticstrain of the reference (right)and SLC concept (left)

Fig. 5 ODB crash of the SLC vehicle

Fig. 7 Intrusion analyses of theSLC concept

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4 Conclusion and outlook

The results for the strut mounting show a part weight of2.8 kg and thus a weight reduction of up to 4.9 kg (Fig. 8).The great reduction in weight results firstly from thelightweight construction concept, in which functions andparts were integrated into a highly-integrated cast compo-nent, secondly from the lightweight materials, i.e. the use ofmagnesium, and finally from the lightweight layout and theresulting geometric optimisation. It should be mentionedthat the concept could also be manufactured usingaluminium casting exhibiting similar wall strength, howev-er the reduction in weight would not be as great in com-parison to magnesium. Simulation results show that allspecified static and dynamic load cases are satisfied. Theseresults lead to the decision that this concept would beexamined further, and testing of the results of simulations inreal trials is made possible through the creation ofprototypes for everything from individual structures to theentire vehicle body.

Upon completion of this extensive 4-year project in-volving 37 partners, a multi-material lightweight construc-tion vehicle body with a 35% reduction in weight will bepresented in February 2009 (Fig. 9).

Material, design and manufacturing technologies remainkey technologies in vehicle development. Only integratedapproaches that work on these three key technologies willbe successful in the future. In addition to the developmentof metals and light metals, the research on fibre-reinforcedplastics will play a major role. In the area of joiningtechnologies mechanical joining, adhesive and hybridmethods will complement to approve welding technologies(Fig. 10).

In addition to the vehicle body, systematic lightweightdesign concepts will integrate equipment, chassis, engineand electronics. Sustainable concepts which benefit from

Fig. 8 Representation of an A-Pillar component in prototyping byVolkswagen AG

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Fig. 9 Result of SLC

Fig. 10 Main challenge in bodydesign: the multi materialconcepts [5]

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secondary lightweight effects will play a decisive role infuture car design.

References

1. Goede M (2006) Super light car: Sustainable production technologiesof emission reduced light weight car concepts (SLC). TransportResearch Arena Europe 2006. June 12th–15th 2006, Göteborg, Sweden

2. Goede M (2007) Karosserieleichtbau als Baustein einer CO2-Reduzierungsstrategie; Aachener Kolloquium, Fahrzeug- und Motor-entechnik; Aachen; 8.–11. Oktober

3. Seiffert U (2002) Die Bedeutung des Leichtbaus im Fahrzeugbau.Clausthal, 2002, Industrie—Kolloquium, 6./7.2

4. Winterkorn M., Ludanek H., Rohde-Brandenburger K (2008) CO2-Reduzierungspotential durch Leichtbau und der Automobilindus-trie, Dresdner Leichtbausymposium, Dresden, 12.6

5. World Business Council for Sustainable Development: TheSustainable Mobility Project—Full Report 2004, p. 37

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