Titanium MATERIALS You will find the figures mentioned in this article in the German issue of ATZ 10/2004 beginning on page 928. Bewertung von Modularisierungsstrategien für unterschiedliche Fahrzeugkonzepte am Beispiel des Vorderwagens

Evaluation of Modularisation Strategies for Different Vehicle Concepts Demonstrated on a Vehicle Front Structure

Modularisation strategies nowadays play an important role for the realisation of motor vehicles. Aims of modularisation for vehicles are to lower the costs as well as to shorten the time to market. The following article describes the methodology and some results of a projekt done by Volkswagen AG and Forschungsgesellschaft Kraftfahrwesen mbH Aachen (fka) of the evaluation of modularisation concepts in body construction.

1 Introduction

By Thomas Krusche, Jörg Leyers, Torsten Oehmke and Thorsten Parr

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The customer preference for increasingly differentiated and individually designed vehicles already leads to a continuously growing number of vehicle variants for OEMs. The fulfillment of these customer preferences possesses risks concerning longer development times and reduced economies of scale. Modularisation strate-

gies can offer advantages concerning these issues compared to conventional vehicle design. The volume of a module strongly influences the choice of materials, construction and joining techniques. Therefore to implement a successful modularisation strategy in a defined derivative portfolio, the determination of the individual module volumes is aspired at an early stage of the vehicle development.

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RESEARCH

Calculation and Simulation

2 Problem Definition and Target Formulation

The described project focuses on the development and the evaluation of modularisation strategies. The main boundary conditions are summarised by the following characteristics: ■ development of modular vehicle concepts with regard to body structure and the total vehicle requirements ■ evaluation of modular body concepts of different vehicles for the realisation of derivatives with low R&D- and manufacturing expenditure ■ focus on front-wheel-driven vehicles in the European market and all-wheel-driven vehicles derived from these ■ evaluation of resulting possibilities for short-term realisation of niche vehicles with modules of existing "vehicle families“. The approach of the examination was classified into four phases: The determination of the customers’ demands, the evaluation of derivatives, the development of modular concepts and the evaluation of the modular concepts. The contents and results of these project phases are described in the following chapters. 2.1 Determination of the Customers' Demands

In this field of activity, the target consisted in the identification of current and future customer demands as well as the requirements of selected vehicle derivatives. In order to identify the interrelationship between customer demands and technical solution concepts of body design, a priorisation of the requirements and an overview of the resulting relevant boundary conditions under application of „Quality Function Deployment“ (QFD) was performed. The relevant body requirements were weighted regarding their importance. Afterwards the relevance of the technical realisation possibilities regarding the fulfilment of the individual customer demands was evaluated. As a result of this procedure the rate of importance of the various technical realisation possibilities was obtained, which among other things include the following selected main aspects: Flexibility of vehicle development by usage of modular concepts: ■ realisation of various vehicle derivatives ■ material application and use of uniform construction concepts ■ utilisation of manufacturing and joining techniques.

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Safety-related and environment specific requirements: ■ high energy absorption performance of the body structure and rigid passenger compartment ■ durability dimensioning of the components ■ compliance to present recycling requirements. Consideration of general market tendencies: ■ demand for energy-saving vehicles ■ gasoline and diesel engines as predominant propulsion systems ■ increasing use of hybrid cars, long-term application of fuel cell vehicles. Here it has to be noticed that the number of vehicle demands is driven by the number of derivatives which are considered concerning modularisation. Because of the various customer and manufacturer demands as well as the legislative specifications, the essential challenge of vehicle design lies in fulfilling this variety of demands and identifying required trade-offs. 2.2 Evaluation of Derivatives

In the further project progress vehicle derivatives were inspected regarding the specific module demands. For this a portfolio with 37 different derivative types was chosen, which includes the most important ones in the current European automotive market. These vehicle types were separated into four vehicle classes: Sub-compact class/mini passenger cars, compact class, upper medium-/medium class and luxury/high class. The vehicles were compared with each other by means of various evaluation criteria (e.g. vehicle type, outer dimensions, wheelbase, total vehicle weight, seats, number of units, types of construction etc.). Afterwards a segmentation of the vehicle body into 41 components, respectively assembly groups (e.g. bumper cross beam, crashboxes etc.) was accomplished as a starting basis for the determination of the modular concepts for the chosen derivatives. Thereby the components are allocated to four different areas of the vehicle: front structure, passenger compartment, rear structure and doors/hoods. The 41 body components were examined regarding their “carry-over potential” for usage in various vehicle derivatives. The amount of the potential carry over parts for different vehicle types of the components hereby indicates their modularisation potential. This means, the feasibility of a component to be part of module rises with the number of derivatives in which a component can be used as a carry-over part.

2.3 Development of Modular Concepts

Using the derivatives evaluation as a basis, the next phase of the project included an analysis of the efficiency of individual modular concepts. In this project phase, based on the existing results from the carry-over part analysis, an evaluation of various possibilities to form vehicle modules was conducted. Thereby identification of feasible joining planes for the vehicle body played an important role. In a multi-level iterative process it was determined which body components can be selected for a module under consideration of the technical requirements on the body structure. A systematic procedure was established and applied for determination of the various body modules. The procedure considers the demands on the complete vehicle as well as the functional requirements of the single components, Figure 1. Starting from the vehicle requirements of the considered derivative portfolio, the body components investigated in terms of the carry-over part analysis were summarised to first “module proposals”. The attributes of the modules were evaluated in a multi-level verification process concerning the relevant module requirements (e.g. weight of the derivatives, package specifications and vehicle derivatives). In this process the number of derivatives covered by a single module has been reduced, until the requirements for a single module appeared to be technically feasible. The success of the modularisation strategy thereby basically depends on the selection of the individual joining planes for the modules. The requirements on these planes influence the available joining techniques, the number of components in a module and size of the module to a great extend. Important requirements consist in the functional body specifications and process specific manufacturing demands as well as in economic aims. Identifying areas of predominant modularisation potential is guided by two aspects. On the one hand derivates need to share an accordingly high number of carryover components and on the other hand derivates only need to have a low number of variants of one component to cover all versions. Especially the front structure and parts of the passenger compartment offers a high modularisation potential, while the rear structure offers a rather low modularisation potential. In contrast to this the roof as well as clesures and hang on parts can be hardly unitised for several derivatives at the same time. The front structure thereby offers a large range of components that can be integra-

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Titanium

ted into a module. The extend of a frontstructure module influences the number of covered derivatives to a great degree. The front module in Figure 2 features the highest modularisation potential concerning the joining-plane evaluation with regard to the identified boundary conditions. The joining planes of the front strcture module to the A-pillar hereby offer the possibility of creating an autonomous body structure regarding frontal crash requirements. Apart from the crash boxes, the front end and the bumper cross beam, all crash relevant components are part of this module. These are main structural parts of the vehicle front, which form a rigid frame from the front cross beam up to the A-pillar. The characteristics of the module are basically defined by the joining planes. Thus, the joining planes towards the firewall and towards the crossbeam determine the vehicle’s width. The frontal section plane at the crash box allows the mounting of the crash box and the front end by an “easy to maintain” connection of the components. With this component configuration of the module, principally 11 of the 37 derivatives can be modularised (e.g. limousine, short back, coupé, estate etc.). As shown in Figure 3, mainly vehicles of the compact class and upper middle class are coverd. Other modules with the same extent of components have much lower production numbers and cover less derivates. The weights of the vehicles that are covered by the already mentioned “high volume module” require the front structure to cover a relatively wide weight spectrum of 1100 to 1700 kg. The vehicles derivates of the other front modules are positioned in the same weight classes, facilitating the design of these modules. Thereby a pre-assembly of the modules with drive train aggregates is principally possible (e.g. engine, drive train components). For the “high volume module” mainly solutions out of steel with conventional joining techniques are suitable (e.g. spot welding, laser welding etc.). The predominant steel-monocoque design is not changed by this module type. Multi-material or aluminium constructions can offer alternatives in the luxury/upper class. Space frame concepts or ladder frames, the ladder frame mainly for off-road vehicles, are used with these materials. 2.4 Evaluation of the Modular Concepts

The identified modules were afterwards examined and evaluated regarding their degree of performance with respect to the former identified requirements. Figure 4 de-

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picts the results of the modularisation of the total vehicle. Thus, the feasibility of the modules, the potentials for saving weight, joining techniques, the degree of coverage as well as the need for changes of the structural design due to usage of the modules form the evaluation criteria. A reduction of vehicle weight is among other things possible by the use of multimaterial constructions. The challenges of modularisation thereby especially lie in the application of these multi-material concepts and the necessary joining techniques. Aspects which deal with the economy of several construction types and joining techniques in dependence of the production volume play an important role. At high volume production there is low scope for alternative structural designs (e.g. space frame). Here mostly steel-unibodies are still to be used. Also multi-material constructions are able to be realised in individual vehicle classes (e.g. luxury class). Depending on the production number it has to be decided if mono- or multi-material concepts, this means modules with different designs and materials, are to be used for the complete vehicle. (Cost-) saving potentials of the modularisation can only be evaluated in single case. On the one hand an analysis of synergy effects for a defined derivative portfolio has to be performed, on the other hand the effects on all other affected derivatives have to be considered. To cope with this challenge, a process is to be applied, which includes a holistic consideration of the specific defined derivative portfolio. Modules based on this have to be examined regarding their possibilities of designing a uniform body structure and modular construction. Furthermore the degree of performance on functional demands as well as the economic consequences caused by the usage of different materials and joining techniques have to be analysed in great detail. The fulfilment of various customer, market and manufacturer demands can only be accomplished by a trade-off. Hereby on the one hand the desired vehicle functionalities are to be offered and on the other hand economic and legislative requirements have to be taken into consideration. Regarding identification and development of modular concepts it is especially important to account for the influence of the chosen derivative portfolio in consideration to the resulting vehicle requirements (e.g. vehicle classes, body forms, number of produced units etc.). A successful modularisation of vehicle areas is only effective, if in parallel all technical requirements of the

MATERIALS

derivatives to be considered (e.g. vehicle dimensions, wheel base, complete mass of the vehicle etc.) are included in the analyses very early. 3 Summary

The development and evaluation of the individual body modules clearly shows, that an entire modularisation of the complete portfolio (37 derivatives) is not possible. Depending on the derivatives and vehicle classes to be examined modularisation potential exists for selected segments of the body, Figure 5, Figure 6. As a result of the analysis the vehicle front structure and the vehicle underbody seem to offer the highest modularisation potential. Vehicles of compact or middlesize offer the highest possibilities for the usage of modules in these areas. In comparison the modularisation potential for these vehicle classes in the roof area exists only to a limited extend. Vehicles of the sub-compact class and small vehicles also offer possibilities for modularisation. Derivatives of the luxury and upper class consist of very unique body structures, that apart from super sport cars, are only suitable for modularisation in limited segments of the body like e.g. front structure and vehicle floor. The future challenges are posed by the application of multi-material modular concepts, this means usage of various materials within one body structure. Apart from realisation of functional and economic requirements the selection of adequate joining techniques plays a very important role. Besides the technical performance of a joining technique the economic influences must be considered. For the estimation of saving potentials in modular vehicle concepts among other things a vehicle specific analysis of costs is necessary. This analysis should contain the identification as well as the comparison of added modularisation costs for the single derivatives and the already existing cost accountings for individual vehicles of the existing portfolio. The modularisation is based on several requirements that have to be fulfilled in the future. With regard to these requirements it can be seen that the modularisation could be one step realising vehicle with reduced costs and decreased time for development. ■

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