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Lightweight Design worldwide

, Volume 11, Issue 1, pp 30–35 | Cite as

Concept of an L7e micro car with high passive safety

  • Rainer Wolsfeld
  • Fabian Klein
  • Kristian Seidel
  • Lukas Zehnpfennig
Construction Car Concept
  • 210 Downloads

In a collaboration the fka, the School of Design Pforzheim and the material producer Hydro have developed the concept of a L7e-class micro car. The legal safety requirements for this vehicle class are low. The newly developed car’s passive safety, however, exceeds these requirements, inter alia thanks to the use of aluminum.

High Safety at Low Weight

The individual mobility in Europe faces new challenges today. With an average degree of urbanization of 75 % in European countries[1] a large amount of the mobility takes place in metropolitan regions with limited space conditions. The current debate of emission reduction to increase the urban air quality resulted in the announcement of first cities to ban conventional driven cars in the city areas. New mobility and vehicle concepts are required to meet these challenges. Electrified micro cars of the L7e class could provide a possible solution for the future urban individual mobility. These vehicles can avoid local emissions and require only limited space due to their size. In comparison to conventional cars of the M1-class, the legal prescriptions with regard to passenger safety are very low. These contrast with the customer wishes, which demand the highest possible vehicle safety [2]. The scope of the current project is the development of a vehicle with a high passenger safety which does not exceed the permissible gross weight of 450 kg excluding the battery system as a boundary condition of the L7e classification. New solutions are to be developed to solve the conflict of objectives between guaranteeing the residual space at small outer vehicle dimensions and the prescribed weight limit. The car body material aluminum shows the potential to combine a low specific material weightwith good structural properties and a high weight-specific energy absorption. The project partner Hydro supports the implementation of suitable aluminum materials and products into the vehicle development.

Requirements

To develop the technical specifications, requirements were considered, which were prescribed by law, defined by OEMs or customers, and by consumer organizations. These points were complemented by recommendations and standards of the Society of Automotive Engineers (SAE) for the vehicle development. The legislative type-approval requirements of heavy quadri-mobiles for the passenger transport are defined in the L7e-CP subcategory of the EU regulation No. 168/2013:
  • ▸ permitted vehicle dimensions ≤ 3700 mm × 1500 mm × 2500 mm (length × width × height)

  • ▸ mass in running order ≤ 450 kg without battery system

  • ▸ maximum continuous rated power ≤ 15 kW

  • ▸ maximum design vehicle speed ≤ 90 km/h

  • ▸ enclosed driving and passenger compartment accessible via maximum three sides

  • ▸ maximum four non-straddle seats, including the seating position for the driver.

The deformation space with small exterior dimensions is a conflict of goals requiring new concepts for package and structure.

The additional legal guidelines UN- ECE-R125 and UN-ECE-R 46 include requirements regarding the direct and indirect driver’s field of view. Other safety related requirements like the availability of safety belts or the equipment of safety glass are of a rudimentary character. As a result, crash test methods from the consumer organization Euro NCAP for front and side impact were considered to evaluate the passive safety. Since Euro NCAP does not provide crash test methods for the rear impact, the safety standard from FMVSS, which is valid for the US, was considered. Especially the structural integrity of the battery system was evaluated.

Based on the defined requirements for the vehicle development, a weight balance of the main components was estimated, Table 1. In conclusion, a weight target of 184.7 kg for the body in white (BIW) was identified.
Table 1

Weight balance of main components (© fka)

Assembly

Description

Mass [kg]

Body

BIW, doors and closures, hang on parts

184.7

Chassis

McPherson front and rear axle, stabilizer, wheel brake, steering, wheels and tyres

107.3

Drive train

Electric motor with gearbox, further components

84

Interior

Pedals, steering wheel, dash board etc.

40

Thermal management

HVAC system, radiator

16

Safety systems

Restraint systems: safety belts and airbags

11.4

Others

Operating fluid, horn, windscreen wiper etc.

6.6

Design Contest

To further specify the vehicle development and to define the available design space, a two-step contest was performed in cooperation with students from the School of Design Pforzheim. In the first stage, the students made rough draft designs with minor technical boundary conditions. In the second stage, additional boundary conditions were considered. Standardly existing car body structures like the longitudinal member in the front section or the estimated mannequin position were considered and included in the updated design concepts. Finally, a draft design was chosen by the project partners, which was the basis for the CAD exterior construction and the design space definition, Figure 1.
Figure 1

Final design draft and technical implementation in CAD (© fka)

Package Development

In the early stage of development, all the package components were positioned regarding the outer dimensions. This allowed safety, ergonomic and function relevant components to be visually integrated into the existing design space concept and the evaluation of the space requirements. Comparative vehicles from the L7e vehicle class (for example Renault Twizy) and the M1 vehicle class (for example Smart fortwo) were used to validate the interior and exterior dimensions. The seating position and thus the visual concept had a high influence on the interior design. Therefore, two 95 percentile men were integrated into the CAD model according to DIN 33402, so that a representative part of the averagehuman body dimensions was considered. This dimensional concept was completedwith the package components of the powertrain, battery system, chassis, thermal management and safety equipment. The result was a dimensional concept that described the interior and exterior dimensions, Figure 2.
Figure 2

Interior and exterior dimensions of L7e vehicle, values in mm or ° (© fka)

Existing serial solutions were used to a large extent for the selection and dimensioning of the package components, Figure 3. This ensured the functionality of the individual components and allowed focusing on the body development. The drive train consisted of a three-phase asynchronous motor with a continuous performance of 13 kW and a gear unit flanged directly to the motor with a constant reduction ratio. This arrangement provided a space-saving overall package that offered driving dynamics advantages due to the low center of gravity.
Figure 3

Overview of package components (© fka )

The battery capacity was calculated according to the WLTC (Worldwide Harmonized Light-Duty Vehicles Test Cycle). To calculate the battery capacity, key vehicle parameters were estimated using comparable data from similar vehicles and data from the literature. For this vehicle concept with a range of 150 km, a battery capacity of 15.2 kWh was required. This capacity could be achieved with 26 battery modules consisting of 18650s cells with the outer dimensions 192 mm × 89.5 mm × 66.7 mm.

For the chassis selection, an existing evaluation scheme of different axle concepts [3] was used and adapted to the evaluation criteria weight and space utilization. As a result of the evaluation, a McPherson front axle was selected. The use of identical components and therefore possible economies of scale led to the choice of a mirrored front axle variant with locked tie rods when selecting the driven rear axle. For the design of the thermal management, a distinction between interior space and battery conditioning was made. The drive motor was excluded from this consideration due to an existing air cooling. The ventilation of the windshield is prescribed by law for interior conditioning. This was implemented by a heating and ventilation system (HV system) with integrated air PTC elements. An air-conditioning system is not initially planned for this vehicle concept. The battery conditioning included the heating by foil PTC elements and a liquid-based cooling system for the battery modules. The regulations taken into account ensured a traffic-compatible vehicle and were visually integrated into the CAD model as interfaces. This included the field of vision according to ECE-R 125, which describes the unrestricted visibility of the driver and was implemented in the model as a viewing angle. In addition, the curbstone clearance, slope angles, and ramp angles were also included in the model. The positioning of the vehicle lightning was taken into account in the CAD model according to ECE-R 1 and ECE-R 48. Finally, the CAD model contained envelope surfaces that corresponded to the wheel covers according to Regulation (EU) No. 1009/2010. Through their kinematics, moving components were transformed into envelope surfaces at an early stage in order to identify possible penetrations.

Description of the BIW

The developed vehicle concept with its one-box-design enabled a good use of space while achieving minimal exterior dimensions. Considering a small to medium volume production quantity, an intensive use of closed section profiles was chosen for the BIW-design. The structure represented a compromise between moderate cost for investments in machine tools and acceptable assembly expenditure, Figure 4.
Figure 4

Parts overview BIW (© fka)

The available deformation space of the concept with small exterior dimensions and the pursued increase of passive safety described a conflict of goals, which is additionally pushed by the legislative weight limit of 450 kg. As a result, a consequent lightweight design approach was pursued which was significantly influenced by the choice of lightweight materials and the structural design. Aluminum offers a high energy absorption capability. Despite the small deformation zones, the passenger accelerations could be kept on a low level due to a good load distribution.

In case of a frontal collision, the short vehicle front provided a comparably small deformation space before the structure of the survival space for the occupants was reached. This requires a high force level and a homogeneous load distribution in the front section. As a result, an upper and a lower load path complemented the conventional main load path, which was positioned at the height of the crash management system. Characteristic for the developed front end was a staged force progression over the deformation space. For the upper load path, the windscreen frame absorbed the energy as a reinforced bending beam. Besides the connection of the chassis, the sub-frame provided load transmitting structures in a frontal impact. The connection of the main longitudinal member was not realized by a torque box via a so-called “S-shape”. It was connected with the passenger compartment by an additional vertical bending beam. Due to the position of the longitudinal member and originated from the principle of the lever, the loads were primary transmitted into the floor structure.

To be combined opposites of the vehicle concept are the maximum vehicle width of 1.5 m combined with two side-by-side positioned passengers while maximizing the crumple zones in case of a side impact. To expand the side deformation space, the seats were offset-positioned in the longitudinal axis of the vehicle. This enabled a consistent interior comfort while positioning the seats narrower to each other in the vehicle’s transversal axis. To guarantee the structural integrity of the battery cells in the side pole impact according to Euro NCAP due to the local load transmission was challenging.

The side pole impact represents a critical test scenario.

Since large loads were carried into the floor structure in the developed vehicle concept, load-bearing multi-chamber extrusion profiles were integrated in the battery housing.

Based on design-specific requirements, the side door formed the outer energy absorbing structure in case of a side impact. To optimally use the existing deformation space, the door was designed as a sheet metal shell construction supported by extrusion and bended profiles.

Further Analyzes Regarding Passive Safety

As part of the project, target values of the acceptable occupant acceleration and the intrusions into the passenger compartment are defined. These allow the assessment of the developed structure based on defined crash load cases. The considered testing protocols comply with the requirements for the European market. The iterative technical design process considers adaptions of the sheet thicknesses and the variation of the used materials.

Table 2 shows the considered load cases. Despite the early consideration in the concept phase (offset seating position, maximum vehicle width 1500 mm), the available deformation space is very short. Thus, the side pole impact represents a critical test scenario. Theoretically, a maximum acceleration of approximately 30 g is possible. To achieve this optimum value, a constant force level of 192.5 kN over the whole deformation space of 150 mm is necessary. A maximum deceleration of about 35 g in the vehicle center of gravity is considered to be feasible.

The present concept development shows the possibility to increase the passive safety of L7e vehicles, which can lead to a higher customer acceptance of light quadrimobiles.

References

  1. [1]
    World Bank. Europäische Union: Urbanisierungsgrad in den Mitgliedsstaaten im Jahr 2016. In: Statista — Das Statistik-Portal. Online: https://de.statista.com/statistik/daten/studie/249029/ umfrage/urbanisierung-in-den-eu-laendern/, access: 11/3/2017
  2. [2]
    VuMA (Arbeitsgemeinschaft Verbrauchs- und Medienanalyse). Wichtigste Kriterien beim Autokaufin Deutschland in den Jahren 2014 bis 2016. In: Statista — Das Statistik-Portal. Online: https://de.statista.com/statistik/daten/studie/171605/ umfrage/wichtige-kriterien-beim-autokauf/, access 11/3/2017
  3. [3]
    Ersoy, M.; Gies, S.: Fahrwerkhandbuch. ATZ/MTZ-Fachbuch, Wiesbaden: Springer Vieweg, 2017Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Rainer Wolsfeld
    • 1
  • Fabian Klein
    • 1
  • Kristian Seidel
    • 2
  • Lukas Zehnpfennig
    • 1
  1. 1.RWTH Aachen UniversityGermany
  2. 2.fka Forschungsgesellschaft Kraftfahrwesen mbH AachenGermany

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