Green Principles for Vehicle Lightweighting - American Chemical

Mar 20, 2019 - Jarod C. Kelly,. ‡. Sujit Das ... States. ⊥. Lightweight Innovations For Tomorrow, 1400 Rosa Parks Boulevard, Detroit, Michigan 482...
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Green Principles for Vehicle Lightweighting Geoffrey M. Lewis, Cailin A. Buchanan, Krutarth D. Jhaveri, John L. Sullivan, Jarod C Kelly, Sujit Das, Alan I. Taub, and Gregory A. Keoleian Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05897 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 24, 2019

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Environmental Science & Technology

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Green Principles for Vehicle Lightweighting

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Geoffrey M. Lewis a, Cailin A. Buchanan a, d, Krutarth D. Jhaveri a, d, John L. Sullivan a, Jarod C. Kelly b, Sujit Das c, Alan I. Taub d, e, Gregory A. Keoleian* a, e for Sustainable Systems, School for Environment & Sustainability, University of Michigan; 440 Church St., Ann Arbor, MI 48109 b Energy Systems Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439 c Energy and Transportation Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 d College of Engineering, University of Michigan, 2098 HH Dow, Ann Arbor, MI 48019 e Lightweight Innovations For Tomorrow, 1400 Rosa Parks Blvd., Detroit. MI 48216 *corresponding author [email protected] (734) 764-3194

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A large portion of life cycle transportation impacts occur during vehicle operation, and key

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improvement strategies include increasing powertrain efficiency, vehicle electrification and

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lightweighting vehicles by reducing their mass. The potential energy benefits of vehicle

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lightweighting are large, given that 29.5 EJ was used in all modes of U.S. transportation in 2016

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and roughly half of the energy spent in wheeled transportation and the majority of energy spent

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in aircraft is used to move vehicle mass. We collect and review previous work on lightweighting,

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identify key parameters affecting vehicle environmental performance (e.g., vehicle mode, fuel

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type, material type and recyclability), and propose a set of ten principles, with examples, to guide

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environmental improvement of vehicle systems through lightweighting. These principles, based

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on a life cycle perspective and taken as a set, allow a wide range of stakeholders (designers,

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policy-makers, and vehicle manufacturers and their material and component suppliers) to

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evaluate the tradeoffs inherent in these complex systems. This set of principles can be used to

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evaluate tradeoffs between impact categories and to help avoid shifting of burdens to other life

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cycle phases in the process of improving use phase environmental performance.

a Center

Abstract

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Introduction

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Studies on transportation vehicles (cars, trucks, buses, trains, airplanes, ships) have found that

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the bulk of the impacts over a vehicle’s life cycle occur in the use phase (vehicle operation), so

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improving vehicle life cycle environmental performance should focus first on reducing use-phase

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burdens 1. Fuel economy regulations in the U.S. have helped drive technological advancements

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to reduce use-phase impacts of individual vehicles 2. These advancements include improved

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powertrains (high-efficiency internal combustion, electric, and hybrid) 3–7, and vehicle mass

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reduction (lightweighting) 8–12. Vehicle fuel consumption comprises two components, one

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associated with moving the mass of the vehicle, and one that accounts for other losses (e.g.,

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aerodynamic drag, accessories, engine and powertrain friction) 10,13–16. Up to 50% of a wheeled

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vehicle’s fuel consumption can be mass-dependent, depending on the vehicle type and duty

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cycle, so lightweighting strategies are of high value in reducing use-phase fuel consumption.

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Vehicle lightweighting provides the opportunity to reduce use-phase fuel consumption further

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through secondary mass reduction and powertrain adaptation and resizing.

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Lightweighting can be achieved in several ways, with material substitution and changes in design

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and construction being the most common 4,5,8,17–20. Substitution ratios that indicate the amount of

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lightweight (LW) material used to replace an existing material (e.g., aluminum for steel) have

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been developed for a range of applications and materials 12,21,22. An alternative approach

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advocates making vehicle parts using near-net-shape manufacturing pathways to reduce waste of

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existing materials 23–27. The Appendix to the Midterm Evaluation of Light-Duty Vehicle

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Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards for Model

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Years 2022-2025 lists a number of mass reduction technologies for materials such as steel and

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aluminum, as well as lightweighting implementation examples like Ford Motor Company’s

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aluminum-intensive F150 28. While material substitution and other lightweighting approaches

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can reduce use phase energy consumption and GHG emissions, they often result in higher

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burdens in other life cycle phases. For example, an aluminum part reduces use-phase fuel

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consumption compared to a functionally equivalent steel part, but it takes significantly more

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energy to produce virgin aluminum (135.3 MJ/kg) than virgin steel (35.3 MJ/kg) 29.

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Manufacturing burdens differ between materials 30, but it is also critical to understand how

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mechanical and structural properties of materials influence factors such as durability and part

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replacement frequency 31–34. The recoverability of a material at the end of a vehicle’s lifetime is

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another important criterion, as it is desirable to keep materials in functional uses once they have

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been produced and to reduce waste. Recyclability depends on material properties, ability to

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separate materials at end-of-life, and the amount and type of contamination present after

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separation. Lightweight material selection and design to reduce life cycle environmental impact

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is complicated and involves evaluating tradeoffs.

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One of the first comprehensive methods for quantifying the environmental performance of

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products across their entire life cycles (i.e., from cradle to grave) was developed by a panel of

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industry, regulatory, and academic stakeholders 35. This method – life cycle assessment (LCA) –

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was standardized as ISO 14040 in 1998 and 2000, and updated in 2006 36. The LCA method is

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used to develop environmental product declarations (EPDs) according to product category rules

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(PCRs) under ISO 14025 37. Partly through the application of LCA, it became evident that the

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process of improving environmental performance of a product across its life cycle often involved

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evaluation of material production burdens, tradeoffs between life cycle phases, and

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determination of how to account for recycled material. LCA provides guidance for navigating the

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complex product environmental improvement process, but itself involves significant data

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collection and analysis effort. The objective of this paper is to establish a set of principles based

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on a life cycle framework to guide lightweighting technology development and application in

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material selection, vehicle design, and policy.

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Principles Background

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Sets of principles, based on LCA concepts, data, methods, and applied experience, are intended

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to simplify and guide the environmental improvement process, and have been developed in

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several fields. Anastas and Warner developed a set of principles tailored specifically for green

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chemistry 38, and Anastas and Zimmerman continued with 12 Principles for Green Engineering

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39.

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less toxic materials; and design for durability. The American Institute of Chemical Engineers

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consolidated these principles into a set of nine focused on implementation 40, and continued the

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consolidation with a set of four principles. This would be an academic exercise if these principles

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had not been adopted and put into practice by practitioners who understood their value. Two

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examples of adoption are Dow Chemical’s corporate sustainability goals 41–43 and BASF’s

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selection of environmental categories for their Eco-efficiency analysis 44,45.

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Green principles have been developed for other engineering disciplines. The Hannover Principles

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focused on design for sustainability 46 and the Sandestin Conference principles were the product

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of an effort to provide a set of green engineering principles applicable to all engineering

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disciplines 40. A recent set of principles focuses on the development and deployment of energy

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storage on the electricity grid 47, and these principles have been recognized as an important

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contribution to the design of battery technologies 48,49.

Examples of principles from these sets include: use renewable materials; reduce waste; use

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Principles collect, distill, and organize information in a topic area to provide useful guidance to

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those who are making design and deployment decisions. Many economic aspects are currently

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internalized in decision-making processes, so principles are of the highest value when they focus

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on aspects, especially environmental and social, that are not yet internalized. Externalities such

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as greenhouse gas emissions are one example. Material selection 50 and lightweight (LW) design

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making that include cost, performance, and environmental factors. We present here a set of ten

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principles, and an organizing framework, focused on guiding the environmental improvement of

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transportation vehicle systems (including, but not limited to, passenger cars, trucks, trains, ships,

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and planes) through lightweighting.

have both been the subject of work to develop multi-objective frameworks for decision-

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Lightweighting Principles Framework

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The principles framework (Fig 1) illustrates the relationship between the ten principles and

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product life cycle phases (material production, manufacturing, use, and end-of-life). The

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principles are intended for multiple stakeholders, including designers, vehicle manufacturers

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(OEMs), the supply chain for these manufacturers (i.e., materials and component suppliers), and

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policy-makers.

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Figure 1: Vehicle Lightweighting Green Principles Framework

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Though these lightweighting principles are focused on the environmental dimension of

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sustainability, they have economic and social components too. For example, a principle focused

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on reducing energy use will also have an effect on the economic performance of a product or

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system. In addition to the tradeoff between material production and use phases, there are also

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potential tradeoffs between other life cycle phases, between stakeholders’ interests, and between

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part, vehicle, fleet, and transportation system scales. These principles, employed as a holistic set

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to evaluate performance and resolve tradeoffs, provide consistent and systematic guidance on

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lightweighting strategies to minimize life cycle environmental impacts of the vehicle system.

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Lightweighting Principles

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Each of the ten environmental principles for vehicle lightweighting are discussed below, in both

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short (bold) and more explanatory form (italics), with background and illustrative examples for

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each, as well as candidate tools and metrics to quantify improvement for that principle and to

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identify potential gaps in current practice.

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Principle 1: Resolve tradeoffs between technical, economic, and environmental performance

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Successful and high impact lightweighting strategies must resolve tradeoffs between technical,

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economic, and environmental performance while maintaining safety standards.

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Lightweighting initiatives to improve environmental performance can result in increased

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production costs and/or decreased technical performance, and can also affect cost of vehicle

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ownership52. For example, while magnesium is a candidate for LW designs because of its low

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density, its cost has traditionally been prohibitive in designs for automotive applications, and its

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corrosive nature has limited its use in the aerospace industry 53,54. Material cost issues can be

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addressed by designing specifically for LW applications, reducing the amount of material

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required by near-net-shape applications and keeping costs low, and by employing efficient, low

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cost manufacturing 53. In the case of magnesium, new cast and wrought alloys with better

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mechanical and corrosion properties are required for implementation to expand 54,55. The

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introduction of LW materials into vehicle designs also introduces technical issues related to LW

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material forming and joining. For example, while aluminum has ⅓ the density of steel, it also has

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⅓ the strength and tensile modulus 56. To counteract this lower tensile modulus, a larger hollow

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cross section must be formed or part thickness must be increased. Many LW alloys require

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harder tools and more rigid presses, and their lower ductility limits design options 56. Joining

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dissimilar materials, often necessary when designing with LW materials, can be challenging due

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to weaker connection points and dissimilar physical and chemical properties 57,58. Alternatives to

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traditional resistance and arc welding techniques include joining by forming, stir and inertia

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friction welding 56, and emerging reversible adhesive bonding technology. Joining is discussed

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further in Principle 6.

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Metrics, Tools, and Gaps in Practice

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Despite the challenges associated with LW materials, they offer a promising path to reduce

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environmental burdens in transportation vehicle designs. Ashby charts, which display the range

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of selected properties for a variety of materials, can be a helpful tool in comparing candidate

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materials 59. Figure 2 is a schematic adaptation of an Ashby chart, displaying ranges of

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production energy (per m3) and Young’s modulus for several broad material types 60, as well as

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material production energy data points from GREET for some selected materials 29. Ashby charts

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help designers select LW materials with the best environmental performance that also maintain

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specified material properties. Other metrics that could be displayed in an Ashby chart include

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cost, energy density, GHG emissions factor, and mass.

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Figure 2: Ashby chart (after 60) depicting production energy versus Young’s modulus for several material types, using material production energy data from 29.

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Vehicle designers need to have adequate information, experience, and tools (e.g. finite element

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models and testing facilities) to assess whether the part made from an alternative material will

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meet vehicle structural and safety requirements. Potential data gaps include material substitution

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factors 61, durability, and life cycle inventory data that are needed for the cradle-to-gate life cycle

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inventory (LCI). These data are also especially important for Principle 7. Exploring several

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alternative lower density materials for an application can help identify the “right” material when

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resolving tradeoffs (Principle 7).

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Principle 2: Source abundant and low environmental impact materials

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Choose to use materials that are abundant and use lower impact material production energy

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sources.

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The global abundance and geographic origin of materials influences the environmental

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sustainability of products and systems in which they are used. Steel and aluminum are currently

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two of the most common materials (by mass) in transportation vehicles, and both of them are

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abundant. Some predict that society’s access to aluminum will be constrained by energy

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limitations and price before scarcity 62, though many (non-lightweight) materials used in

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vehicles, such as copper, lead, and zinc, are far less abundant 63. Scarcity of cobalt, a critical

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material in magnets, aircraft engines, turbines, cutting tools, and batteries is a concern due to

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potentially widespread impacts 64,65. Lightweighting will become an even more important

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strategy as more electric vehicles are produced, since battery performance is directly related to

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the amount of mass being moved. Demand for battery materials (e.g., lithium) will also increase.

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Concerns have been raised regarding the abundance of lithium 66,67, but several studies indicate

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that the demand for lithium batteries can be supported for decades 68–70. LW materials are often

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more scarce than traditional metals. Magnesium’s price and availability has limited its use as a

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LW material 71,72, and there is also concern in the automotive industry regarding the scarcity of

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rare-earth elements 73.

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Some studies argue that resource scarcity is less of a problem than resource extraction 74,75,

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which has been shown to have the largest environmental impact in the product supply chain 76.

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Extraction and processing of scarce materials causes negative environmental impacts, such as

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high energy consumption, toxic emissions, and ecosystem degradation 30,77–79, and resource

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depletion exacerbates these impacts as it typically requires extraction from lower and lower ore

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grades.

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In addition to raw materials, sources of energy also need to be considered. Energy used in

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extraction and processing is a primary cause of environmental impact associated with materials,

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and fossil fuels are still the dominant energy source for this activity 80. Due to regional

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differences in the mix of fuels used to produce electricity in the U.S., GHG emissions per kg of

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aluminum ingot vary between 5 and 30 kg CO2e depending on production location 81. Energy

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and GHG emissions from producing virgin aluminum will also vary over time as the grid mix

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evolves 82. While many LCAs currently assume that aluminum is being produced in North

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America, where hydroelectric powered production is common and thus GHG emissions are

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lower, industry experts state that Chinese aluminum production, which is primarily coal-

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powered, is becoming increasingly relevant, accounting for 65% of global aluminum production

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capacity 61. This influences the production burdens of vehicles that use appreciable amounts of

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aluminum. Processing methods and associated energy and emissions burdens also vary with

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source ore type, which varies geographically. Variance in SOx emissions resulting from

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extraction and processing of different nickel ores in Canada and Russia is an example of this

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spatial variation 83.

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Metrics, Tools, and Gaps in Practice

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The reserves-to-production ratio, which indicates the anticipated lifetime of a non-renewable

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resource, characterizes material scarcity. Comparing the reserves-to-production ratio of bauxite

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(100 years) to copper (42 years), it is evident that aluminum is more abundant at current

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production rates 63. Material production energy (MJ/kg) is used to ensure the lowest impact

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energy sources are being chosen. While databases such as GREET (Greenhouse gases, Regulated

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Emissions, and Energy use in Transportation) 29 report energy and emissions factors for a range

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of materials, data are not always available for newer and less common materials. Resource

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depletion (RD) is an established life cycle impact category that has several issues, including

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significant variation in characterization methods (over 20 different methods) and the lack of an

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included recycling framework, both of which are gaps in practice. Alonso et al. describe material

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depletion metrics and potential impacts on economies, technologies, and geographies 65. Other

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tools include: Sankey diagrams, which highlight material flow pathways throughout their life

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cycle; Ashby charts, which demonstrate tradeoffs in a LW material such as abundance and

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density; and life cycle assessment databases and software, such as GREET, SimaPro or GaBi.

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Principle 3: Use secondary materials

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Secondary materials may reduce demand for virgin materials and can result in lower material

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production impacts (e.g., energy, GHG emissions).

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The use of secondary (i.e., scrap and recycled) materials reduces material production impacts

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compared with the generally higher energy, emissions, and cost for primary material production,

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as well as reducing the need for raw material production and processing 29. Figure 3 illustrates

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the difference in material production energy and GHGs between primary and secondary iron,

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steel, and aluminum.

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GHG (kg CO2e/kg)

. Al

.

nd ar y

Al Se co

ar y Pr im

nd ar y

Pr im

ar y

St

St

ee l

Ir on nd ar y

ar Se co

(P rim Ir on Pi g

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GHG

ee l

Energy

8 7 6 5 4 3 2 1 0

Se co

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y)

Energy (MJ/kg)

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Figure 3: Reduction in material production energy and GHGs substituting primary with secondary materials variability due to location, processing methods, etc. is not reported in GREET to ensure source confidentiality 29

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Secondary materials can be unsuitable for some applications. They often do not have the same

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material properties and composition as their primary counterparts due to alloying elements and

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contaminants remaining after the recycling process 61,84–88. Contamination by other elements

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reduces the quality of recovered materials, limiting the application of LW secondary materials,

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as they can often only be used in applications that accept lower quality material 89,90. For

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example, calcium, lithium, and zirconium lower the quality of magnesium 87, and aluminum is

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compromised by the presence of copper, iron, or zinc 90. Recycled plastics have not been widely

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used in transportation due to a lack of effective plastic recycling infrastructure and the inability

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to produce recycled plastic equivalent to primary material 91,92. These end-of-life issues, as well

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as market interactions between primary and secondary materials, will be discussed further in

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Principle 6.

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Metrics, Tools, and Gaps in Practice

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Metrics that can be used to assess performance under Principle 3 include secondary material

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content (%), material production energy (MJ/kg), and carbon intensity (CO2e/kg). Higher

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percentage of secondary material is desirable in terms of reducing resource depletion and

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material production energy. Tools that can be used to facilitate improvement on Principle 3 are

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Sankey diagrams, which highlight how much material is being recycled for secondary use at a

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regional, national or global scale, and GREET or other LCA software/material databases, which

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contain energy and emissions factors.

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A major gap in information necessary for assessing performance on this principle is the recycled

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content of a material. While the steel and aluminum industries have well-established recycling

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data for the automotive industry, other vehicle modes have far less information on primary and

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secondary material flows. For instance, ship breaking is the common end-of-life process, yet the

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flow of materials resulting from ship breaking is not well documented 93. Additionally, few

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technologies are capable of material recovery without compromising material quality. There is a

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need for recycling technologies that can accomplish material separation without detrimental

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contamination, as well as identifying appropriate uses for secondary materials with degraded

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properties. The Aircraft Fleet Recycling Association (AFRA), the global organization dedicated

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to airplane recycling, has noted that the quality of recycled composite materials needs to

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improve, and expanded markets need to be identified for aircraft recycling to improve 94. The

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cost and environmental burdens of future end-of-life infrastructure are unknown, as is how these

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will influence use of secondary materials in design. Issues with end-of-life modeling and

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recovery of materials will be discussed further in Principle 6.

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Principle 4: Maximize efficiency in manufacturing for lightweight technologies

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Develop and select environmentally preferable manufacturing technologies that have high

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material yields and match target production volumes.

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Material lost as scrap through the production process necessitates production of more input

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material than ends up in the finished product. Economics have been the primary motivation for

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increasing material efficiency. Barriers to implementing environmental material efficiency

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strategies include an industry perception that they raise costs and the lack of expert knowledge

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and resources 95,96. There have been efforts to address challenges in increasing material

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efficiency 97. Allwood et al. 74 discuss material efficiency as a pathway to reduce the

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environmental burdens of material production and processing, and identify increasing process

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yields as one strategy.

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Since manufacturing operations increase embodied energy, it is important to select

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manufacturing processes strategically based on target production volume to minimize burdens

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per part (not per kg). Fysikopoulos et al. estimated the production energy for an automotive

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Body-in-White (BIW) at varying production volume, and found up to 17% lower energy and cost

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per part at higher BIW production volume 98. Kim, Keoleian, and Skerlos demonstrate how, as

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production volume increases, the difference in BIW manufacturing cost between steel and

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aluminum reduces, with aluminum vehicles becoming more cost advantageous at higher volumes

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99.

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additive manufacturing or laser sintering), can be used to produce high-value products at low

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volumes with less material waste. While additive manufacturing may not be suitable for

Additive manufacturing, in which parts are built up layer-by-layer to near-final shape (e.g.,

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automotive applications because of its low volume characteristic, it is well suited to aerospace

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applications. Additive manufacturing has been adopted by aircraft component manufacturers due

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to its capability to produce LW and cost effective designs 100. Additive manufacturing of

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components requires as little as 33% of the energy used to produce conventionally manufactured

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components, and the LW components could result in up to a 6.4% reduction in fuel consumption

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100.

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determined that they used 10% of the material used in traditional (subtractive) manufacturing

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101,102.

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landing gear found that, at low production volumes, sintering is more economical than die-

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casting when producing functionally equivalent parts 103. Production volumes and part size vary

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based on vehicle mode (i.e., higher volumes for passenger cars, lower volumes for container

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ships), which will help determine the best manufacturing technology.

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Metrics, Tools, and Gaps in Practice

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Metrics to assess progress under Principle 4 include process material yield, product rejection

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rate, and energy intensity. Higher process material yield and lower product rejection or defect

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rate are both indicators of improved process efficiency. The production energy intensity of a part

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will help identify an appropriate production volume. GREET can be used for these efficiency

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and energy metrics. Manufacturing energy data are not always available, especially on a part or

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component basis or for new technologies or materials, and models to determine optimal

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manufacturing technology and production volumes are not available for most industries. The

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lack of environmental performance measurement systems (PMS) for manufacturing represents a

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gap in Principle 4 104.

An analysis of titanium aerospace components produced by additive manufacturing

A comparison between die-casting and laser sintering methods to produce an aircraft

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Principle 5: Don’t overdesign

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Evaluate the potential increase in production burdens of strategies to extend service life and

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durability against fuel savings in the use phase, design for reasonable part lifetimes, and use

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optimal replacement strategies to reduce environmental impacts.

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Avoiding overdesign of vehicle components is a well-established principle in both conventional

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and LW vehicle design, driven by economic incentives and supported by the adoption of

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computer-aided design and analysis to optimize vehicle parameters without the need for

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prototypes 61,105–110. When extending the service life of a LW part or vehicle, a reasonable

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product lifetime must be considered 111,112. For instance, a LW engine block that is designed to

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last 30 years is overdesigned for a vehicle that will be used for 15 years 113. Optimal replacement

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strategies are helpful in understanding the relationship between life cycle impact and product

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lifetime. While increasing vehicle lifetime does reduce the life cycle significance of production

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energy and emissions 61, obsolete products “can be responsible for higher impacts during the use

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phase if compared with newer and more efficient products” 114. Older products that are less

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efficient need to be replaced to reduce environmental burdens 115. Factors that influence a

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vehicle’s optimal lifetime include technology improvements, vehicle deterioration with age, and

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regulatory factors such as Corporate Average Fuel Economy (CAFE) standards.

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Extending the service life of a vehicle allows lightweighting benefits to accumulate longer.

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However, strategies aimed at increasing service life, like treatments to reduce rust or improve

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adhesion, introduce additional material production, manufacturing, and end-of-life burdens, such

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as the release of volatile organic compounds and increased energy demand 116,117. For example,

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aircraft and marine applications of aluminum are common, but higher mechanical strength

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requirements compared to automotive applications necessitate the use of copper alloys, which

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reduce corrosion resistance 118. Chromic acid anodizing of aluminum helps to reduce corrosion,

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but chromates are environmentally toxic, and this tradeoff between strength, corrosion resistance,

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and the use of toxins must be evaluated 118,119.

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Metrics, Tools, and Gaps in Practice

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Metrics that can be used to assess progress on Principle 5 include the design life of the part,

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durability, and remanufacturability. If design life of the part is greater than the design life of the

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vehicle, then designs with a shorter design life should be considered to reduce unnecessary part

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production burdens. While durability metrics are not well established, the European Commission

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Joint Research Centre (JRC) has developed a durability index 114. This extensive approach

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accounts for reusability / recyclability / recoverability, recycled content, use of relevant

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resources, use of hazardous substances, and durability. A tool to assess durability is the

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Goodman diagram, which plots alternating versus mean stress to indicate likelihood of failure

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34,120.

363

life is to compute changes in life cycle energy or GHG emissions of the vehicle due to one more

364

or one fewer replacement parts.

365

If specific components have longer lifespans than the vehicle, remanufacturing can be considered

366

to reduce resource depletion and extend service life 121. In principle, a remanufacturability credit

367

could be included in vehicle life cycle calculations. Tracking remanufactured parts in the used

368

part marketplace is not an established practice, and uncertainty remains in how credit is applied

369

to the first use of a material 122. This gap in data and analysis method is related to end-of-life

370

allocation issues discussed in Principle 6.

A simpler approach to evaluate the environmental benefit of a part with extended service

371

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Principle 6: Design for maximum material recovery

373

Select lightweight materials that are recoverable through existing and planned end-of-life

374

infrastructure and do not impede recoverability of other materials. Seek to use materials that

375

have the maximum potential to reduce demand for primary materials upon recovery and

376

minimize down-cycling.

377 378

Lightweight materials often have a complicated end-of-life phase, with issues including

379

incomplete recovery, separation challenges, contamination, and lack of a closed-loop recycling

380

path. For example, in magnesium die-casting operations, typically 50% of the initial material

381

ends up in the part, with a portion of the scrap (dross, chips, and slurry) being too impure for re-

382

melting 123. At vehicle end-of-life, magnesium remains in the nonferrous mix of metals and is

383

often too impure to be recycled 123. Recycling of cast and wrought aluminum parts generally

384

yields a mix of alloys that are currently only suitable for lower performance castings, so primary

385

aluminum is needed for all wrought and sheet applications 90. The use of LW materials in

386

conjunction with more traditional ones in vehicles has also introduced joining and separation

387

issues. For instance, overcasting (or overmolding for composites), in which two materials are

388

brought into contact to form a reaction zone that creates a continuous bond between the two 124,

389

has enabled the use of mixed material construction to reduce the mass of automotive sub-systems

390

such as engine cradles and instrument panel beams 125. Overcasting/molding can present issues at

391

end-of-life, because separating the two materials is not always technically or economically

392

feasible. Adhesive bonding is an alternative method of joining dissimilar materials (i.e., LW

393

metals, composites, and polymers) that reduces corrosion issues and improves impact resistance

394

126,

in addition to increasing BIW torsional rigidity, which improves vehicle performance 127.

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Disassembly of composites and polymers from metals at end-of-life has been labor intensive and

396

expensive, but innovative disassembly techniques such as thermally or electrically reversible

397

adhesives or functional additives are potential solutions 126,128.

398

Principle 6 asserts that stakeholders should strive to select LW materials that limit these end-of-

399

life challenges as much as possible. Current solutions include designing for manufacturability,

400

recyclability, and durability, which emphasize reducing number of parts, ease of disassembly,

401

and increasing durability 129. Using fewer materials and reducing part count can also be effective

402

130,

403

International and Ford Motor Company, which achieved a 23.5% mass reduction using part

404

consolidation and aluminum substitution 131. Less labor-intensive dismantling can be achieved

405

through modifications to standard designs and several significant policies have promoted the idea

406

of designing for dismantling and reuse, including the European Parliament’s Directive

407

2000/53/EC, which raised the required reuse of vehicle materials to 85% 129,132. Development of

408

new aluminum alloys that better match metal streams and do not require additional processing

409

would help to address some of the current limitations with aluminum end-of-life 91.

410

Improvements in recovery processes and the ensuing improvements in recovered material quality

411

will affect markets for these materials. Ready availability of high-quality and lower cost

412

secondary material could stimulate competition with primary material and result in an overall

413

increase in demand for material.

414

Metrics, Tools, and Gaps in Practice

415

Metrics used to assess progress on Principle 6 include % recyclability and % recoverability.

416

Other possible metrics include: the recyclability benefit rate (RBR), a ratio of environmental

417

benefits to the environmental burdens 133; the RBR with an added quality factor, assessed

as demonstrated by the Multi Material Lightweight Vehicle (MMLV), developed by Magna

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through material properties or economic parameters 114,133; the circular economy performance

419

indicator (CPI), a ratio of the actual waste treatment option’s over the ideal’s environmental

420

benefit 133; remanufacturability 121; and resource longevity indicators, which measure the

421

contribution to material retention 134. Many of these environmental indicators have been

422

developed and used in academic settings to assess material reuse, but few of them are

423

operationalized or used regularly in most industry sectors. This is a gap in end-of-life materials

424

management. Recycling metrics have been used extensively in both the electronics and

425

automotive industries, including disassembly time per product, % recycled material per

426

product, % recycled or diverted from landfill 135–138.

427

Through the use of LCA, OEMs and supply chain component designers can assess the life cycle

428

benefits of LW materials, and weigh them against end-of-life burdens (energy consumed

429

separating LW material from other materials 91). Policies and regulations that advance designing

430

for recoverability as well as research in LW materials recycling should be promoted.

431

Key barriers to better LW material recovery include a lack of cost-effective technologies to

432

separate and sort materials, which leads to a greater reliance on human labor in many emerging

433

economies such as India and China, and the lack of markets for many recovered materials

434

137,139,140.

435

and reprocessing technologies, life cycle analysis tools, new materials, and coatings removal

436

technologies” 141. The Department of Energy (DoE) has recognized that as lightweight and

437

electric vehicles become more prevalent, recycling will become more challenging. DoE initiated

438

research into the effects of recycling regulations and reviewed state-of-the-art recycling

439

technologies, in addition to developing and testing new recycling technologies 137,139. DoE also

440

facilitated a 2008 workshop between automotive companies, metals suppliers and recyclers,

Reclamation of automobile shredder residue (ASR) requires identification of “sortation

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national laboratories, and academia to identify R&D needs for improving future vehicle

442

recycling, which established future priorities 137. These included developing technologies to

443

remove detrimental impurities, creating more recycling-compatible aluminum alloys, and

444

establishing a cost-effective recycling technology for ASR.

445

Other gaps include the lack of closed-loop recycling infrastructure for many LW materials such

446

as composites 142 and plastics 143, operational metrics for assessing end-of-life recovery, and a

447

standardized end-of-life allocation method for LCA work 61. End-of-life allocation methods tend

448

to either account for burdens when they occur (the recycled content (RC) or cutoff approach) or

449

to assume that recovered materials will offset primary material production and are credited with

450

that offset (the end-of-life recycling (EoLR) or avoided burden approach) 144,145. The RC

451

approach is the more conservative approach, as it captures burdens when they occur in the

452

material life cycle and does not assume recovered material will displace primary material

453

production in the future. Life cycle energy and emissions benefits can vary significantly

454

depending on which approach is used, and the metals industry has advocated for the EoLR

455

approach as it incentivizes recycling 61,146 and can result in lower burdens due to the EoL credit

456

147.

457

material end-of-life 148, and a standardized method would add clarity and simplify quantifying

458

progress on this principle 149. There is also a significant need for research clarifying the

459

economic link between recovered material and primary material, specifically connecting

460

displacement of primary production with recovery of material at end-of-life150 as well as the

461

degree to which recovered material can displace primary material151.

There are many misconceptions around the complex task of evaluating and modeling

462

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Principle 7: Evaluate tradeoffs between life cycle phases

464

To ensure reduction in overall life cycle impacts, use LCA to quantify changes across life cycle

465

phases and between impact categories due to lightweighting.

466 467

In many instances, tradeoffs between life cycle phases or metrics will occur when implementing

468

a LW component or design. These tradeoffs need to be carefully evaluated so that the option

469

with the lowest environmental impact is selected. Life cycle impact categories that might be of

470

interest or concern (other than energy and GHGs) include resource depletion, ozone depletion,

471

smog formation, and acidification 152, though availability and quality of data related to these

472

impact categories is often an issue. While weighting and normalization techniques exist to select

473

the “optimum” solution when comparing multiple impact categories, their use is controversial

474

and they are listed as optional techniques by ISO due to the subjectivity inherent in weighting

475

and normalization 153.

476

When comparing conventional and LW materials to produce a part, designers need information

477

to compute net life cycle impacts (e.g., energy or GHG emissions) associated with the weight

478

reduction to determine whether use phase fuel savings outweigh any increase in production

479

burdens 22. Data required include original and LW part masses, mass dependence of vehicle fuel

480

consumption both with and without powertrain adaptation (as measured by fuel reduction value –

481

FRV), life cycle impacts of processing original and LW materials and part manufacturing,

482

anticipated changes in part lifetime, difference in maintenance and repair schedules, and end-of-

483

life treatment employed (EoLR vs. RC). The MMLV study discussed in Principle 6 compared

484

life cycle total primary energy (TPE) and global warming potential (GWP) for the 2013 Ford

485

Fusion and MMLV, finding that the lightweight vehicle achieved 16% less TPE and GWP

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486

despite an increase in production burdens 154. The net result over all life cycle phases can

487

typically be approximated by the sum of differences in the material production, manufacturing,

488

and use phases of the life cycle. For example, a study evaluating three air intake manifolds

489

determined that the end-of-life burdens contributed very little, while the use phase accounted for

490

at least 60% of life cycle energy 155.

491

Tradeoffs between life cycle phases are influenced by differences in regional fuel mixes. Figure

492

4 compares material production and use GHG emissions for aluminum and high strength steel in

493

different regions of the U.S. 156. In the Midwest region, where coal is still a significant fuel for

494

electricity production, aluminum actually has increased lifetime GHG emissions, which would

495

not have been apparent without a life cycle approach. Variability in life cycle impacts due to

496

geography is also apparent at the global scale 82.

497 Difference in Lifetime GHG Emissions Aluminum vs. Conventional Steel Baseline Nested Average Electricity Allocation Protocol

30% Recycled Al

100 50 0

Combined

150 Material Producion Use Phase

Lifetime Difference in GHG Emissions (kg CO2eq)

200

-50 -100 Pacific Northwest Midwest Aluminum Aluminum

498 499 500

Mid-Atlantic Aluminum

Southeast Aluminum

High Strength Steel

Figure 4: Material production and use phase impacts for aluminum production in various regions of the United States 156

501

Metrics, Tools, and Gaps in Practice

502

LCI tools such as GREET, SimaPro, and GaBi are used to assess life cycle tradeoffs. These tools

503

list important metrics such as life cycle energy and GHG emissions, as well as other life cycle

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indicators. Note that life cycle databases can be incomplete, outdated, or regionally specific.

505

Data on every step in material and part production should ideally be available, but typically are

506

not. Generally, the material production and use phases of the vehicle life cycle are dominant and

507

can be used as a reasonable approximation in assessing tradeoffs. Tradeoffs in cost, performance,

508

and environmental aspects should all be considered, as discussed in Principle 1, and cost/benefit

509

analyses can be used to evaluate these tradeoffs.

510 511

Principle 8: Develop lightweighting applications to maximize benefits

512

Decision-makers seeking to improve environmental performance across the transportation

513

sector should prioritize lightweighting applications that achieve the greatest environmental

514

benefit. Powertrain, mode, fleet distribution of vehicle types, and use-phase energy mix in the

515

deployment market all influence environmental benefit.

516 517

The net benefit of lightweighting will differ based on a variety of factors that influence vehicle

518

fuel consumption (and associated emissions), including powertrain, vehicle mode and duty cycle,

519

the types of vehicles within the operational fleet, and the kind of fuel being used for vehicle

520

operation. The magnitude of savings from lightweighting depends on the vehicle’s powertrain

521

type. Studies of life cycle GHG emissions of conventional and LW versions of an internal

522

combustion engine vehicle (ICEV), a hybrid electric vehicle (HEV), and a battery electric

523

vehicle (BEV) found that the more efficient electrified powertrains (HEV and BEV) result in

524

smaller fuel savings and GHG reductions from lightweighting than ICEVs 157,158.

525

FRV measures the change in fuel consumption for a reduction in mass and is a function of

526

vehicle type and duty cycle 9,10,22,159. Higher FRV implies a larger reduction in fuel consumption

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527

for a given change in vehicle mass. Figure 5 below indicates a variety of modal FRVs 15. Class 8

528

trucks are more fuel intensive (i.e., use more fuel to move a kg of cargo a given distance) than

529

rail and ships 160–163, suggesting that trucks should be prioritized for lightweighting. For freight

530

modes, the mass reduced from the vehicle may be added back on as cargo if the vehicle’s mass

531

capacity has not been reached. Life cycle inventory tools can be utilized to understand the

532

implications of lightweighting different modes based on available information, such as modal

533

FRV, how much mass can be reduced from the vehicle, and whether that reduced mass will be

534

replaced with cargo 160,164. Since different modes vary in fleet size and vehicle lifetime, it is

535

important for OEMs to consider fleetwide lightweighting effects 165,166. Even though energy

536

savings from lightweighting a single car are small relative to larger vehicles such as airplanes

537

and ferries, they could collectively realize the greatest total energy savings due to the size of the

538

automobile fleet 164. 1.4 1.2

0.8 0.6 0.4 0.2

p hi ta i

sp l

on

-d i

lC

w ig h

-d

sp

Lo

H

ne rS

nk Ta

80 A

-3 8

0-

tT gh ei

er

0F

ra in

s Fr

vy

D

Tr an

sit

Bu

k ut y

y ut M

ed

.D

A

Tr uc

Tr uc

T ve

LD

Ca ve A

k

0.0

r

FRV (gal/100 ton-mi)

1.0

H

539 540

Figure 5: FRVs for transportation vehicles 15

541

Fuel type influences the life cycle impact of lightweighting. A comparison of total fuel cycle

542

burdens of petroleum, biofuel, and natural gas fuels for heavy duty trucks determined that

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biodiesel and compressed natural gas result in fewer emissions 167. Modes and regions that use

544

dirtier fuels (i.e., more emissions per unit of energy consumed) will experience a greater

545

reduction in emissions than those that use cleaner fuels and so lightweighting will result in the

546

greatest reduction in dirtier regions. Relying on electricity will generally reduce emissions from

547

vehicle operation 168–171, though the fuel mix (and emissions) for electricity generation varies

548

regionally and nationally 172,173. It is possible, if unlikely, that this regional variability could

549

result in higher emissions for EVs than for ICEVs 174,175. FRV, fleet size, fleet turnover, and fuel

550

type all need to be considered to ensure the most effective lightweighting strategy is deployed.

551

Metrics, Tools, and Gaps in Practice

552

Metrics that can be used to assess progress towards maximum lightweighting benefits include

553

life cycle indicators (energy, GHG emissions, etc.) and FRV. Another physics-based approach to

554

assessing the effect of mass reduction on fuel consumption is mass-induced fuel consumption

555

(MIF) 13. MIF allocates total vehicle fuel consumption to mass by calculating mass-related

556

energy use over total energy use 13,176. Useful tools include ANL’s GREET Fleet Footprint

557

Calculator, which estimates the petroleum consumed and greenhouse gases emitted by fleets of

558

medium- and heavy-duty vehicles 177. While calculators for other modes are not currently

559

available, GREET can be used to estimate fleet wide impacts by scaling up available individual

560

vehicle values 29. ANL’s AFLEET Tool, based on GREET data, allows a determination of the

561

environmental costs and benefits of alternative fuel at the fleet scale 178, and the GREET VISION

562

Model 179 incorporates market penetration of advanced vehicles and alternative fuels. ANL’s

563

POLARIS tool models large-scale transportation systems and can reflect lightweighting benefits

564

at a fleetwide scale 180. Fleet size data by mode are not always available, which is a gap in

565

assessing progress on Principle 8. Regional and national GHG reduction policy would likely

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promote lightweighting 160. Fuel economy regulations, such as CAFE standards in the U.S., do

567

promote lightweighting 181,182 but focus primarily on the use phase.

568 569

Principle 9: Identify broader system opportunities

570

Evaluate additional benefits resulting from component and vehicle lightweighting, including

571

secondary mass savings and resized and alternative powertrains. Identify and characterize

572

system-level benefits and costs of lighter vehicles (e.g., transportation infrastructure, vehicle

573

miles traveled (VMT) rebound effect) to provide guidance toward enhanced environmental

574

sustainability.

575 576

Vehicle lightweighting can result in additional benefits, such as secondary mass savings (SMS)

577

and powertrain adaptation and resizing 20,183–186. The size and mass of some vehicle components

578

are driven by the mass of others 183, so lightweighting one component can result in

579

lightweighting of other components. The total mass reduction in these other components is

580

known as secondary mass savings. Depending on when mass savings are identified and the

581

flexibility in altering vehicle design, it may be possible to achieve more than a kilogram of SMS

582

per kilogram of primary mass reduction 185,187, though there is a lack of agreement in the

583

literature related to the potential of SMS 188. Powertrain adaptation is another technique

584

employed in conjunction with vehicle lightweighting. Taking mass off a vehicle alters the

585

vehicle’s performance, and powertrains are often adjusted to restore the vehicle to original

586

performance, resulting in improved fuel efficiency 22,61,176,189. While powertrain adaptation is

587

promising for additional mass reduction and is recognized as significant in life cycle assessments

588

of lightweighting 61, guidelines for it do not currently exist. A manufacturer may decide to

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reduce the mass of a vehicle, but if powertrains are designed without considering lightweighting

590

decisions for the full range of vehicles that will use them, then the potential benefits of SMS will

591

not be realized.

592

VMT rebound is also a potential consequence of vehicle lightweighting. Increased fuel

593

efficiency enables an increase in vehicle miles driven due to reduced fuel cost per mile. It is also

594

important to identify the potential consequences of lightweighting at the transportation system

595

level. There is a direct relationship between vehicle weight, pavement design (thickness and

596

construction and maintenance cost), and road damage 190. Lightweighting vehicles could reduce

597

damage to roads, but only if the reduced mass is not added back as additional cargo, resulting in

598

the same gross vehicle weight.

599

Another potential consequence of widespread adoption of lightweighting is a shift in demand for

600

(and cost of) materials. For materials with many non-vehicular uses (e.g., steel and aluminum),

601

these shifts may not be catastrophic for producers, but the threats and opportunities presented by

602

such a shift could be large for manufacturers of materials without many other uses. These are not

603

strictly environmental issues, but environmental effects could be assessed with a consequential

604

life cycle analysis, which is intended to evaluate systemic changes like large-scale shifts in

605

material flows and VMT rebound mentioned above.

606

Metrics, Tools, and Gaps in Practice

607

Total vehicle mass reduction, as well as fuel economy improvements both with and without

608

powertrain adaptation, can be used to understand the overall effects of lightweighting measures

609

183.

610

adaptation on infrastructure and rebound effects. Quantified and credible fuel consumption

611

reductions resulting from lightweighting can inform policy makers concerned with these two

Gaps include challenges in isolating and quantifying the effects of SMS and powertrain

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Environmental Science & Technology

612

effects on environmental and cost drivers. There is a need to identify and understand the life

613

cycle consequences of vehicle mass on road infrastructure and maintenance and repair to fully

614

quantify lightweighting benefits 191.

615 616

Principle 10: Develop and implement policies to advance life cycle environmental

617

performance

618

Implement holistic policies that support the adoption of materials, technologies, and strategies

619

that lead to better environmental outcomes and avoid shifting burden across life cycle phases.

620 621

Legislation has been recognized as a useful strategy to influence the behavior of industry 96,192.

622

Rebitzer et al. 193 have advocated LCA in policymaking, though this would require

623

manufacturers to provide reliable certification data for compliance. The U.S. EPA light-duty

624

GHG regulation is an example of policy to improve use-phase environmental performance,

625

though it does not account for other life cycle phases. European vehicle recycling targets, while

626

focused on end-of-life, will require design, material, and recycling technology changes 194.

627

Adopting a life cycle approach helps stakeholders across the entire life cycle be better

628

environmental stewards. Legislation and funding can influence the technologies that are

629

researched, developed, and deployed. Policy is an important tool in steering attention to modes

630

and strategies that have the greatest aggregate impact on improving environmental performance.

631

Figure 6 plots total energy consumed by transportation mode in the U.S. in 2015, and shows that

632

light duty vehicles (i.e., cars and light duty trucks) present the greatest opportunity for system-

633

wide fuel consumption reduction due to the large number of these vehicles. The overall potential

634

for energy savings resulting from vehicle lightweighting is significant, given that roughly half

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the energy spent in wheeled transportation vehicles and the majority of energy used in aircraft is

636

used to move vehicle mass 15 and that 28 Quads (29.5 EJ) were used for U.S. transportation in

637

2016 195. A 20% reduction in vehicle mass is estimated to reduce U.S. fleet fuel consumption in

638

cars 0.52 EJ, light duty trucks 0.55 EJ, medium and heavy-duty trucks 0.28 EJ, airplanes 0.18 EJ,

639

and buses and rail 0.03 EJ - totaling 1.56 EJ, approximately a 5% use phase energy savings 15. Air Rail Buses HDT MDT LDT1 Cars 0

2

4

6

8

10

Energy (EJ)

640

Friction_Accessories

Drag_friction_Accessories

Due to current vehicle mass

Energy saved due to weight reduction

641

Figure 6: Energy use by mode upon 20% reduction of vehicle mass (based on FRVs from 15)

642

To capture this opportunity, policy focused on lightweighting cars and light duty trucks (beyond

643

CAFE) could be prioritized and expanded. Internal industry policies can also be effective. There

644

have been individual as well as joint initiatives amongst industry stakeholders to adopt LCA to

645

guide decision-making 138,189,196,197, including those highlighting the benefits of a LW aluminum

646

pick-up truck 198.

647

Metrics, Tools, and Gaps in Practice

648

It is a significant challenge to measure policy effectiveness directly. Metrics that can be used to

649

assess progress on Principle 10 include number of life cycle policies currently implemented and

650

metrics that measure improvement in life cycle indicators over time (e.g., fleet fuel economy,

651

mass fraction of vehicle recovered at end-of-life, amount of primary material production

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avoided, energy intensity of materials used). Tools that can be used to characterize impacts at the

653

fleet scale include the GREET VISION model and Autonomie’s POLARIS. While there has

654

been a recognition by industry and the EPA that burdens from production and end-of-life phases

655

will likely make up a more significant share of total burdens as use-phase regulations become

656

more stringent 199, there are currently no policies addressing this issue. Other gaps include lack

657

of modal life cycle data, policies that address life cycle burdens and material recovery, and

658

industry adoption of policies.

659 660

Discussion

661

Sets of principles focused on environmental performance (i.e., green principles39,46,47) vary in

662

their generality, and some sets include both general and specific principles. Green engineering

663

principles39 are more broadly applicable, and the vehicle lightweighting principles presented here

664

are tailored to this application. Some of the examples and applications presented above are quite

665

general (e.g., the material efficiency of additive manufacturing applies to all parts produced with

666

that technology and not only vehicle parts) and some are more specific to vehicle lightweighting

667

(e.g., the tradeoff between reducing use phase burdens through lightweighting and increasing

668

material production and manufacturing burdens).

669

Green lightweighting principles synthesize models, metrics, and frameworks used to quantify

670

environmental performance 61, and they provide guidance for stakeholders (including designers,

671

manufacturers, suppliers, policy-makers, and researchers) to evaluate tradeoffs and make

672

decisions. Tradeoffs can be resolved best where complete life cycle inventory data exist for

673

lightweight materials, but principles can also provide guidance in early, less well-defined, or

674

more prospective situations when full LCAs are more difficult to conduct reliably. Pursuing each

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of these principles individually can lead to enhanced environmental performance, but they are

676

intended to be a holistic set and should be used together to evaluate the inevitable tradeoffs,

677

between life cycle phases and between impact categories, that arise when pursuing a vehicle

678

lightweighting effort.

679

Current practice is to prioritize economic and vehicle performance requirements when

680

considering the design, manufacture, deployment, operation, and retirement of vehicle systems,

681

and the manufacturing and use phases dominate decision-making. These green lightweighting

682

principles raise awareness and promote the consideration of broader perspectives by those who

683

are seeking to improve life cycle environmental performance across life cycle phases, and across

684

scales from individual parts and materials to the larger transportation system.

685 686

Acknowledgments

687

This work was directly supported by LIFT, operated by ALMMII (American Lightweight

688

Materials Manufacturing Innovation Institute), which is sponsored by the U.S. Navy’s Office of

689

Naval Research through cooperative agreement number N00014-14-2-002 issued by the

690

Department of Defense. We thank Cheryl Caffrey and Kevin Bolon at the USEPA National

691

Vehicle and Fuel Emissions Laboratory for their valuable comments.

692

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(1)

IEA. Transport Energy Efficiency; 2010.

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(2)

EPA; NHTSA. 2017 and Later Model Year Light-Duty Vehicle Greenhouse Gas

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Emissions and Corporate Average Fuel Economy Standards. In Federal Register; 2012;

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Vol. 77.

697

(3)

Ford Motor Company. Sustainability Report 2016/2017; 2017.

698

(4)

Cheah, L.; Heywood, J. Meeting U.S. Passenger Vehicle Fuel Economy Standards in 2016

699

and Beyond. Energy Policy 2011, 39 (1), 454–466.

700

https://doi.org/10.1016/j.enpol.2010.10.027

701

(5)

Century. 2006, 31 (April), 336–343.

702 703

Taub, A. I. Automotive Materials : Technology Trends and Challenges in the 21st

(6)

Kasseris, E. P.; Heywood, J. B. Comparative Analysis of Automotive Powertrain Choices

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for the Next 25 Years. Soc. Automot. Eng. Int. 2007, No. 2007-01-1605, 776-0790.

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https://doi.org/10.4271/2007-01-1605

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(7)

https://doi.org/10.1007/978-1-4020-6979-6

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(8)

Mackenzie, D.; Heywood, J.; Zoepf, S. Determinants of US Passenger Car Weight. Int. J. Veh. Des. 2014, 65 (1), 73–93. https://doi.org/10.1504/IJVD.2014.060066

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Sperling, D.; Cannon, J. S. Reducing Climate Impacts in the Transportation Sector; 2009.

(9)

Kim, H. C.; Wallington, T. J. Life-Cycle Energy and Greenhouse Gas Emission Benefits

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of Lightweighting in Automobiles: Review and Harmonization. Environ. Sci. Technol.

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2013, 47 (12), 6089–6097. https://doi.org/10.1021/es3042115

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(10)

Koffler, C.; Rohde-Brandenburger, K. On the Calculation of Fuel Savings through

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Environmental Science & Technology

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