The bumpers of a car are heavy; reducing their weight can save fuel. Here we explore the replacement of a low alloy steel bumper with one of equal
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performance made from an age-hardening aluminum alloy, on a gasoline – powered family car. The steel bumper weighs 14 kg; the aluminum substitute weighs 10 kg, a reduction in weight of 28%. But the embodied energy of aluminum is much higher than that of steel. Is there a net saving? As in the previous case study, we take a vehicle life of 10 years and a car driven 25,000 km per year. Table 7.8 lists the energies involved. We take the energy penalty of weight for a gasoline-powered car (Table 6.7) to be 2.06 MJ/tonne. km.
The bar charts of Figure 7.10 compare the material and use energy, assuming the use of virgin material for both the steel and the aluminum bumper. The substitution of steel by aluminum results in a large increase in material energy but a drop in use energy. The right-side columns of the table list values and the totals; the aluminum substitute wins (it has a lower total) but not by much—the breakeven comes at about 200,000 km. And it costs more.
But this is not quite fair. A product like this would, if possible, incorporate recycled as well as virgin material. It is possible, using the data given in
the data sheets of Chapter 12, to correct the material energies for the recycle content. This is left to the appendix, where we discuss eco-audit tools.