Curb weight reduction by material substitution is one of the many methods to reduce CO2 emissions in cars. Currently, the most common substitution is from ferrous components to light alloy components, namely, aluminium or magnesium alloys. Substitutions of aluminium components by magnesium components have also been considered. CO2 emissions are most commonly examined as the environmental inhibitor as it is the major contributor to the greenhouse effect and other emissions effects are usually accounted for as CO2-equivalents.
Recent work indicated that the cost and environmental viability of light alloys substitutions in automotive applications strongly depended on the mechanical function of the component (Caceres 2007). Mechanical function of the component refers to the structural performance of the component defined by its' shape. The effect of shape and structural performance of materials have been discussed by Ashby (2005). Mechanical functions relevant to cars include cast components at equal volume and panels and beams at constant stiffness.
Life cycle assessment or LCA has been one of the main analytical methods used in assessing environmental impacts from substitution of materials in cars. Initially, there were many difficulties inherent to this sort of analyses, however, by using the LCA method proposed by Field et al (2001) emissions were able to be assessed more naturally and consistently by considering time in the analysis. Conventionally light alloys release more CO2 when manufacturing the material, but because it is lighter, less fuel is used and hence less CO2 will be produced by each car. Therefore, the overall CO2 emissions produced by cars eventually offset the case where steel is used. This point in time is called the cross-over time and it is the essence of a viability of a material substitution.
This research found that the cross-over time was significantly longer for panels and beams compared to castings due to performance, energy usage, CO2 footprint and recycling. More importantly though, the results of each substitution was presented and this was shown below in Figure 1 and Table 1. Considering a lifetime of a car to be 20 years, the most viable substitutions were steel to aluminium castings and steel to electrolytic magnesium castings. However, it was also found that improved results could be obtained if the replacement rate of old cars were higher, the use of lighter alloys were maximised in cars, more electrolytic magnesium were introduced into the market and if recycling rates were improved. Consequently, these results should only be taken as a reference as many factors can change the results. Nevertheless, the priorities involved in reducing overall CO2 emissions from material substitution in cars should be clear after examining the results.