Critical factors affecting life cycle assessments of material choice for vehicle mass reduction

This review examines the use of life-cycle assessments (LCAs) to compare different lightweight materials being developed to improve light-duty vehicle fuel economy. Vehicle manufacturers are designing passenger cars and light-duty trucks using lighter weight materials and design optimization techniques to meet EPA’s greenhouse gas (GHG) emissions standards and the National Highway Traffic Safety Administration’s (NHTSA’s) Corporate Average Fuel Economy (CAFE) standards in the United States. Vehicle mass reduction, along with other efficiency improvements, have the potential to reduce tailpipe or use-phase emissions significantly. While tailpipe emissions are by far the primary contributor to life-cycle energy use and GHG impacts for most vehicles, the production and end-of-life (EOL) phases of lightweight materials can differ significantly from the conventional materials used in automobile manufacturing. Twenty-six LCAs conducted since 2010 were reviewed and compared. Aluminum was identified as the material most often specified by the LCAs for providing the highest benefits in terms of life cycle energy and GHG reductions. Advanced high-strength steels were also effective lightweighting materials for reducing energy and GHG impacts. Magnesium and carbon fiber-reinforced polymers were the lightest material options but incurred much higher production GHG impacts and were modeled with low rates of recycling. Major factors from the LCAs are highlighted and discussed; including: secondary mass reduction, recycling allocation, powertrain size reductions and fuel reduction values, material substitution rates, lifetime vehicle travel distance, and production location and grid mix. With fleet-wide adoption, there may be major shifts in material flows which must be accounted for in LCAs intended to guide material selection for lightweight vehicles in addition to other consequential factors including grid mixes and recycling technologies. As the existing standards guide the reduction of use-phase impacts over time, LCA can provide insights regarding the important role of material production and EOL in determining the overall impacts from the automotive sector.