A coherent life cycle assessment of a range of lightweighting strategies for compact vehicles

Abstract

A complete and fully consistent LCA-based comparison of a range of lightweighting options for compact passenger vehicles is presented and discussed, using advanced lightweight materials (Al, Mg and carbon fibre composites), and including all life cycle stages and a number of alternative end-of-life scenarios. Results underline the importance of expanding the analysis beyond the use phase, and point to maximum achievable reductions of environmental impact of approximately 7% in most impact categories. In particular, lightweighting strategies based on the use of aluminium were found to be the most robust and consistent in terms of reducing the environmental impacts (with the notable exception of a relatively high potential toxicity). The benefits of using magnesium instead appear to be less clear-cut, and strongly depend on achieving the complete phase-out of SF₆ in the metal production process, as well as the establishment of a separate closed-loop recycling scheme. Finally, the use of carbon fibre composites leads to similar environmental benefits to those achieved by using Al, albeit generally at a higher economic cost.

Introduction

The automotive industry's current position as a significant contributor to global environmental impact, and more specifically greenhouse gas emissions, is clearly unsustainable, and also less and less acceptable to modern environmentally aware societies. In the United Kingdom, for instance, transportation currently accounts for approximately a quarter of the total direct greenhouse gas emissions in the country (UK Government, 2014a). Cars have also been shown to be the most carbon-intensive means of personal transportation on land (Borken-Kleefeld et al., 2013). Besides turning to more efficient and environmentally friendly power train options, one of the most effective strategies to cut down on a whole range of environmental impacts associated to the use phase of a vehicle is, unquestionably, to reduce its kerb mass (US-DoE, 2015, Koffler and Rhode-Brandenburger, 2010), and indeed a range of lightweighting strategies are currently being considered and tested by many car manufacturers in Europe and elsewhere (e.g. Audi, 2013, BMW, 2015, Ford, 2014, JLR, 2015). However, the production and processing of lightweight material parts often entails higher specific environmental burdens compared to the predominantly steel parts that they replace (Liu and Müller, 2012, IMA, 2013, Das, 2011), and it is therefore essential to expand the scope of the analysis beyond the use phase and include all other stages of a vehicle's life cycle, lest the advantages afforded by vehicle lightweighting be overestimated. Life cycle assessment (LCA) is thus arguably the most appropriate approach for the evaluation of the overall environmental consequences of vehicle improvement strategies based on the use of advanced lightweight materials.

A large number of individual LCA studies are available in the literature in which specific lightweighting strategies have been discussed and analysed (EAA, 2013, Das, 2014, Tharumarajah and Koltum, 2007, Du et al., 2010, Duflou et al., 2009, Koffler, 2014, Schmidt et al., 2004, Mayyas et al., 2012, Kim et al., 2010). However, the lack of consistency in terms of scope, assumptions, boundary conditions and impact metrics render any comparison across different studies fraught with difficulty and potentially misleading. An ex-post harmonization effort is undoubtedly helpful in removing unnecessary inconsistencies and allowing some general inferences to be made (Kim and Wallington, 2013); yet, ultimately, complete and fully coherent comparisons of alternative lightweighting strategies are still few and far between. This paper aims to fill this void by discussing the results of a new comparative LCA of a range of vehicle lightweighting options using advanced lightweight materials (aluminium, magnesium and carbon fibre reinforced polymers), including all life cycle stages of the entire vehicle and considering alternative end-of-life (EoL) scenarios. All options and scenarios are based on the same fundamental model and share the same underlying assumptions and system boundary, thereby ensuring strict methodological consistency across the board and allowing robust and scientifically sound conclusions to be drawn.