140 CHAPTER 3 Materials for consideration and use
4.7 Hypercars 187
Carbon fiber composites have become increasingly evident in performance cars although not yet in high-volume models, and are regularly mentioned in relation to the Rocky Mountain Institute’s (RMIs) ‘Hypercar’ design concept,12 which combines an ultra-light and ultra-low drag platform with a hybrid-electric drive system. Computer modeling performed at the Hypercar Center predicted that such vehicles, at the same size and performance as the four to five passenger cars of the FIGURE 4.22
Typical rally car modifications. Courtesy Steve Hill Motorsport
4.7
Hypercars
187mid-1990s, could achieve a three-times better fuel economy. Figure 4.23, repro- duced from an RMI paper,12illustrates how the synergies between 63% lower mass;
55% lower aerodynamic drag; 65% lower rolling resistance; 300% more efficient accessories (lighting, heating, ventilation, and air conditioning, audio system, etc.);
60%-efficient regenerative braking (i.e. braking energy recovered); and 29%-effi- cient hybrid drive could improve a 1990 production platform’s fuel economy during level in-city driving.
In addition to the above attributes hypercars would employ composites that embed reinforcing fibers (e.g. carbon, aramid, high-strength glass) in a polymeric matrix. As is now confirmed by a decade of use by specialist manufacturers, these have outstanding properties for autobody use, including fatigue and corrosion resistance, highly tailorable material properties, generally low coefficients of thermal expansion, good attenuation of noise, vibration and harshness, and precise formability into complex shapes. Materials experts from various carmakers estimate that an all-advanced composite autobody could be 50e67% lighter than a current similarly sized steel autobody, 40e55% lighter than an aluminum autobody and 25e30% lighter than an optimized steel autobody. Furthermore, secondary weight savings result from the better performance, allowing frugal use of materials combined with less capital intensive manufacturing and assembly. This helps to overcome the cost-per-kilogram premium of composites, compared to steel. For the environmental benefits, see Chapter 8.
One litre
‘Avcar’ fuel production platform
(US 1990 average)
0.29 l Near-term
hypercar (1999–2003)
5 ml accessories
In highway driving, efficiency falls because there is far more irrecoverable loss to air drag (which rises as v3) and less recoverable loss to braking.
Aero drag CDA = 0.76 m2 Rolling drag r0M = 139 N Braking M = 1579 kg 0% recovered
Aero drag CDA = 0.34 m2 Rolling drag r0M = 49 N Net braking M = 585 kg 60% recovered 84 ml
15%–20%
efficient engine
20 ml accessories
29% efficient complete hybrid
driveline 200 ml
FIGURE 4.23
Two ways to drive 12 km in the cityeaccording to RMI
While the concept of the hypercar is thought provoking (and many of the attributes of carbon fiber composites highlighted above have been proved in F1 and perfor- mance car competitions) criticisms were inevitable. Doubts were cast as to handling under adverse weather conditions, towing performance and the necessity for increased weight associated with complex features such as hybrid drive systems. The modeling of braking energies is also considered to be over-optimistic. The vital element missing is the achievement of production rates consistent with mass production, a problem that is being addressed by companies such as BMW with the i3 type program. Their pragmatic approach is still only achieving stroke rates of one panel in 10 minutes, compared with one per 4 seconds for steel. As discussed in Chapter 9, the high cost of carbon composite material is also being addressed, but even with significant progress the economics are still unacceptable for volume production, as are the recycling and end-of-life issues referred to in Chapter 8. Similar initiatives to the hypercar have already been undertaken within large automotive organizations but, as with ULSAB, ASVT and the other initiatives highlighted above, these external stimuli are essential to ensure designers and suppliers are aware of future possibilities and can respond with suitably modified designs and costs.
LEARNING POINTS
1. The ECV and ASVT technology has demonstrated that a pressed aluminum monocoque can provide a vehicle with vastly improved fuel economy. Important secondary savings can also accrue from downsizing of associated chassis and powertrain resulting from the lighter body.
2. Significant improvements can be made to structural stiffness through the use of adhesive bonding, requiring a minor number of spot welds to prevent peeling in impact situations. However, a continuous pretreatment film of proven formula- tion must be applied if bond durability is to be maintained during the lifetime of the vehicle. Rivbonding may be considered as a ‘peel stopper’ in place of spot welding.
3. The use of robotic application and automatic prelubrication is recommended for consistent structural performance.
4. The ULSAB program has confirmed that significant bodyweight savings can be obtained from steel structures but without major facility or process changes. The use of TWBs may require revisiting to confirm that functional benefits are cost effective. The role of steel in electrically driven cars is being explored in the FutureSteelVehicle, again a collaborative program involving most major steel producers worldwide.
5. The increased use of hydroform parts appears to be a logical advancement in the future, as structural evaluation has demonstrated gains in torsional stiffness. Tube hydroforms also offer savings through parts consolidation, which makes increased utilization in the body structure a realistic proposition for the future providing effective joining methods can be demonstrated.
Learning points
1896. Assumptions should not be made regarding strength levels achieved by cold working during hydroforming. Strength can vary locally around the tube circumference and there is some evidence that cyclic softening can occur. The forming limit diagrams (FLDs) derived for sheet are not necessarily valid for hydroforming and maybe stress is a better indicator of criticality than strain (see Chapter 5).
7. Concept cars have a role in gaining acceptance by the public for future design features, the themes of which may vary from safety or weight savings to alter- native propulsion methods.
8. Competition cars, demanding materials with exceptional properties, provide excellent feedback under extreme conditions, and this technology is often incorporated in future production designs. More exotic materials such as carbon fiber composites, although exceptionally well proven, deserve more competitive costs!
References
1. Kewley D.The BL Technology ECV 3 Energy Conservation Vehicle. SAE Paper 850103, Detroit 1985.
2. Kewley D, et al.Manufacturing Feasibility of Adhesively Bonded Aluminum for Volume Car Production. SAE Paper 870150, 1987.
3. Lees H. Light Fantastic.Autocar and MotorNov. 1988:30.
4. Selwood PG, et al.The Evaluation of an Aluminum Bonded Aluminum Structure in an Austin-Rover Metro Vehicle. SAE Paper 870149, Detroit 1987.
5. Ashley C. Steel Body Structures.Automotive EngineerDec. 1995:28e32.
6. ULSAB UK. Launch Presentation, Heritage Centre. Gaydon Mar. 1998;25.
7. Davies GM, Walia S, Austin M. The Application of Zinc Coated Steel in Future Automotive Body Structures. Birmingham: Fifth Int. Conf. on Zinc Coated Steel Sheet;
1997.
8. Eckstein L, et al. Lightweight Floor Structure with Reinforcements of CFRP and GFRP.
ATZ AutotechnologyApril 2011;Vol. 11(2).
9. Macknight N.Technology of the F1 Car. Hazelton Publishing; 1998.
10. O’Rourke B. Formula 1 Applications of Composite Materials, Comprehensive Composite Materials. Oxford: Elsevier Press; 1999:382e393.
11. www.formula1.comArticle 15.1 Permitted Materials.
12. Fox JW.Hypercars: A Market Oriented Approach to Meeting Life Cycle Environmental Goals. SAE Publication SP-1263, Feb. 1997.
13. Curtis A. Techno-Triumph,MotorOct. 22, 1983.
14. Wheeler MJ. Aluminum Structured Vehicle Technology e A Comprehensive Approach to Vehicle Design and Manufacturing in Aluminum. SAE Paper 870146, 1987.
15. Noakes K. Build to Win. Composite Materials Technology for Cars and Motorcycles.
London: Osprey Publishing Ltd.; 1988.
16. Nye D.History of the Grand Prix Car 1966e91. Richmond, UK: Hazelton Publishing;
1992.
17. Walia S, et al.The Engineering of a Body Structure with Hydroformed Components.
IBEC Paper 1999-01-3181, 1999.
18. Boyles MW, Davies GM.Through Process Characterization of Steel for Hydroformed Body Structure Components. IBEC Paper 1999-01-3205, 1999.
19. Zhao, et al. A Theoretical and Experimental Investigation of Forming Limit Strains in Sheet Metal Forming.Int. J. Mech. Sci.1996;Vol. 38(No. 12):1307e1317.
References
191OBJECTIVE
The purpose of this chapter is to introduce the key parameters influencing material performance on conversion to the component form, and to describe the main manufacturing processes involved. The main focus is on the primary shaping of materials, but the subsequent operations are discussed where relevant.
CONTENT
Modern high-production pressworking is introduced and the parameters influencing formability are defined; the derivation of test values are explained and the signifi- cance of forming limit diagrams and use is summarized; an explanation is given of the influence of different steel surface topographies; main form and cutting tool materials are introduced together with heat treatment and repair; the different technologies of tube and sheet hydroforming are described; differences in the manufacturing practices required for aluminum compared with sheet steel are highlighted; the scope for superplastic forming of metals is considered and reference made to techniques used with plastics.