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Lotus Engineering Phase 2 lightweighting study for ARB shows crashworthiness of low-mass body structures and potential for cost-effective mass production
6 December 2012
|Phase 2 body-in-white material usage front three-quarter view. Source: Lotus Engineering. Click to enlarge.|
Building on a Phase 1 Lotus study published in 2010 (earlier post), the Phase 2 study demonstrated the crashworthiness of a low mass body-in-white (BIW) using computer aided analysis and simulation. The study also illustrates how a holistic, total vehicle approach to system mass and cost reduction can help offset the additional cost of a 37% mass reduced body structure. This study’s findings also indicate that the 30% mass-reduced vehicle could be cost-effectively mass-produced in the 2020 timeframe with materials and techniques technically feasible by 2017.
The study is the most comprehensive study of its kind to date, said ARB, and stands as an important body of work on vehicle lightweighting. Included with the report are a US EPA-contracted peer review by SRA International and EPA’s responses, as well as Lotus’ responses to the peer review.
Background. ARB contracted Lotus Engineering Inc to design a low-mass body structure and to evaluate the performance for Federal Motor Vehicle Safety Standards (FMVSS) for light duty vehicles for front, side, and rear impacts, roof crush, occupant restraints and several Insurance Institute for Highway Safety requirements for a 2020 model year vehicle, which could be widely commercialized by 2025. The target was a mass reduction greater than 30% for the total vehicle.
In April 2010, Lotus Engineering concluded the Phase 1 study (published by the International Council on Clean Transportation) that developed a vehicle comparable to the 2009 Toyota Venza; this model had equivalent dimensions, utility objectives, and passenger and interior volume. The projected mass reduction for the 2020 model year (MY) was 38% less mass for all systems except powertrain; this configuration was used as a starting point for the Phase Two study.
The 2010 study substantiated that a reduction in vehicle mass could be achieved for medium-production volume vehicles (approximately 50,000 units per year) with a 23% reduction in fuel consumption.
In September 2010 the California Air Resources Board (ARB) commissioned Lotus Engineering to initiate Phase 2 of the study and take a deeper look into the future of lighter, more efficient vehicles manufactured using lighter yet stronger materials.
|Body-in-white (BIW) refers to the end-product of the weld shop—i.e., the final vehicle body or shell before it goes to the paint shop.|
For the Phase 2 study, Lotus designed a new body structure based on the Phase 1 BIW with identical exterior and interior dimensions to the original baseline vehicle, the 2009 Toyota Venza. The mass reduction target was 40%. The Phase 2 study involves the non-body components—such as interior, suspension, chassis—relating back to the Phase 1 work.
The Phase 2 study was completed in the third quarter of 2011; the peer review process was completed in the third quarter of 2012.
Phase 2 study. Lotus Engineering’s Phase 2 body structure design was based on the dimensions of a 2009 Toyota Venza CUV. The body mass was reduced by 37% (311 lbs. or 141.6 kg), which contributed to a total vehicle mass reduction of 31% (1,162 lbs. or 528.3 kg) including the mass savings of other vehicle systems (interior, suspension, chassis, closures, etc.) that had previously been identified in Phase One.
The engineering methodology consisted of:
Total Structure Approach, including the investigation of all areas simultaneously using topology and a holistic approach targeting best solutions for total body structure by transferring loads efficiently throughout the vehicle.
Optimizing the load paths within structures and minimizing torques to reduce stresses on components.
Maximizing structural sections.
Designing each part to be a structural element.
Using a high level of component integration and function.
Elimination of parts.
Optimizing design for each material class used: ferrous, non-ferrous, composites and natural materials.
The Phase 1 2020 MY BIW make up was 30.0% magnesium, 37.0% aluminum, 6.6% steel and 21.0% composites. The remaining 5.4% consisted of paint (1.8%) and NVH material (3.6%). The Phase 2 BIW contains 18% less magnesium, 38% more aluminum, 1.4% more steel and 16% less composites. These changes were driven primarily by structural requirements and impact performance, Lotus said. Aluminum replaced magnesium as the key energy absorbing material and also replaced composites in sections of the floor structure.
The detailed Computer Aided Engineering (CAE) analysis indicated that a 31% mass-reduced vehicle with a 37% lighter BIW structure has the potential to meet US Federal impact requirements. This includes side impact and door beam intrusion, seatbelt loading, child seat tether loadings, front and rear chassis frame load buckling stability, full frontal crash stiffness and body compatibility and frame performance under low-speed bumper impact loads as defined by the Insurance Institute for Highway Safety (IIHS). The result is a BIW design with a 20% increase in torsional stiffness over the class leading CUV.
The Phase 2 multi-material body structure utilized relatively large quantities of advanced materials (e.g. advanced high-strength steels, aluminum, magnesium, and composites) and advanced joining and bonding techniques to achieve a substantial vehicular mass reduction without degrading size, utility or performance. Overall, vehicle body mass was reduced by 37 percent (141 kg), which contributed to a total vehicle mass reduction of 31 percent (527 kg) including the mass of other vehicle systems (interior, suspension, closures, chassis, etc.) which were optimized in a holistic redesign as part of the Phase 1 study. Additionally, this mass reduction was achieved using a parallel-hybrid drivetrain. It may be possible to further reduce total vehicle mass by using a lighter non-hybrid powertrain. Combining a 30% lighter vehicle with a 150 HP 1.0L three cylinder engine (Lotus is currently developing 145 Hp/L engines for OEMs) and reducing the Cd would result in substantial fuel savings while improving the weight/HP ratio of the baseline car.—Lotus Phase 2 study
Although the significant mass savings in the BIW design results in an increased BIW cost of $723, the overall vehicle cost is reduced through savings of $239 identified across the whole vehicle and when manufacturing and assembly costs are included in the analysis. A significant reduction in the parts count from 269 to 169, achieved by an increased level of component integration, also helped offset the increased BIW piece cost.
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