R E S E A R C H A N D A N A LY S I SGreenhouse Gas EmissionsPayback for LightweightedVehicles Using Aluminumand High-Strength SteelHyung-Ju Kim, Colin McMillan, Gregory A. Keoleian,and Steven J. SkerlosKeywords:automobileindustrial ecologylife cycle assessment (LCA)light metalmaterial substitutionrecyclingSupporting information is availableon the JIE Web siteAddress correspondence to:Dr. Hyung-Ju KimDepartment of Mechanical EngineeringCollege of EngineeringUniversity of Michigan2250 GG Brown Building2350 Hayward StreetAnn Arbor, MI 48109, /SummaryIn this article we consider interactions between life cycle emissions and materials flows associated with lightweighting (LW)automobiles. Both aluminum and high-strength steel (HSS)lightweighting are considered, with LW ranging from 6% to23% on the basis of literature references and input from industry experts. We compare the increase in greenhouse gas(GHG) emissions associated with producing lightweight vehicles with the saved emissions during vehicle use. This yields acalculation of how many years of vehicle use are required tooffset the added GHG emissions from the production stage.Payback periods for HSS are shorter than for aluminum. Nevertheless, achieving significant LW with HSS comparable toaluminum-intensive vehicles requires not only material substitution but also the achievement of secondary LW by downsizing of other vehicle components in addition to the vehiclestructure. GHG savings for aluminum LW varies strongly withlocation where the aluminum is produced and whether secondary aluminum can be utilized instead of primary. HSS isless sensitive to these parameters. In principle, payback timesfor vehicles lightweighted with aluminum can be shortenedby closed-loop recycling of wrought aluminum (i.e., use ofsecondary wrought aluminum). Over a 15-year time horizon, however, it is unlikely that this could significantly reduceemissions from the automotive industry, given the challengesinvolved with enabling a closed-loop aluminum infrastructurewithout downcycling automotive body structures. c 2010 by Yale UniversityDOI: 10.1111/j.1530-9290.2010.00283.xVolume 14, Number of Industrial Ecology929

R E S E A R C H A N D A N A LY S I SIntroductionEmissions of greenhouse gases (GHGs) fromhuman activities have led to a marked increasein atmospheric GHG concentrations. Global anthropogenic emissions grew 70% between 1970and 2004 (IPCC 2007). Even with climatechange mitigation policies and related sustainable development practices, global GHG emissions are expected to continue growing over thenext few decades (IPCC 2007). The Intergovernmental Panel on Climate Change (IPCC) indicates that there is substantial economic potentialfor the mitigation of GHG emissions over thecoming decades that could offset the projectedgrowth of global emissions or reduce emissionsbelow current levels (IPCC 2007). If the concentration of GHGs in the atmosphere is to stabilize,emissions need to peak and then decline. Thelower the stabilization level to be achieved is inGHG concentration, the more quickly this peakand decline need to occur. Mitigation efforts overthe next 2 or 3 decades will have a large impacton opportunities to achieve lower stabilizationlevels (IPCC 2007).Recent policy proposals aimed at reducingGHG emissions from automobiles have accelerated efforts to significantly improve vehicle fuelefficiency. One option is to reduce the overallmass of vehicles. Lightweighting (LW) of vehicles presents an opportunity for simultaneouslycutting petroleum consumption and GHG emissions. Among the potential LW materials, highstrength steel (HSS) and aluminum have beenproven to achieve weight reduction while meeting vehicle safety and performance requirements.Despite the potential LW advantages of thesemetals, their displacement of traditional materials, such as mild steels,1 has been slow due totheir higher costs, manufacturing challenges, andinstitutional as well as technical barriers.Available studies of lightweight vehicles fromthe literature are summarized in table 1. Someof the first popular aluminum-intensive vehicles (AIVs) were produced by Audi (models A8and A2) and Jaguar (model XJ; Scamans 2005;Henn and Leyers 2006). In one case, a Ford AIVproject achieved a body-in-white (BIW) mass of205 kilograms (kg),2 which was 136 kg lighterthan its baseline vehicle (Ford Taurus). BIW is930Journal of Industrial Ecologythe stage in which the car body sheet metal (including doors, hoods, and deck lids) has been assembled or designed but before the components(chassis, motor) and trim (windshields, seats,upholstery, electronics, etc.) have been added(Babylon 2009). Taken together, the examplesin table 1 reveal the technical possibility of reducing the curb weight (i.e., the weight of a motor vehicle with standard equipment, maximumcapacity of liquids [Babylon 2009]) of vehiclesby 11% to 25% through aluminum substitutionsalone. Primary mass reduction makes it possibleto further reduce vehicle weight by downsizingother components (e.g., engine, fuel tank), giventhe inherently higher efficiency of lighter vehicles. This subsequent weight reduction is definedas secondary mass reduction. In table 1, secondarylightweighting effects by the AIVs were notreported.HSS provides an alternative to aluminumlightweighting. An ongoing detailed analysis ofHSS has been performed through the UltralightSteel Auto Body (ULSAB) project, a consortium of 35 steel manufacturers from 18 countries active in the period 1994–1998 (Obenchainet al. 2002). One of its first achievements was todemonstrate that HSS can both reduce body massand improve stiffness simultaneously. In the original ULSAB project, a BIW weight reduction of68 kg (from 271 kg to 203 kg) was achieved frombenchmarked vehicles in the concept phase ofthe study (Obenchain et al. 2002; Wallentowitzet al. 2006). Because HSS has the same density as steel, the increased strength of loadbearing components means less steel is requiredfor an equivalent function when compared toaluminum. Nonetheless, using HSS as a substitute for nonloading applications has no advantage relative to aluminum, because the option to use less material for a given function isnot likely to exist. Substituting aluminum forsteel in nonload-bearing applications does provide an advantage due to the lower density ofaluminum relative to steel. Given the similar densities of HSS and mild steel, achieving intensivelightweighting through HSS requires significantreliance on secondary lightweighting enabled byprimary mass reductions. For instance, in theULSAB Advanced Vehicle Concepts (ULSABAVC) project, based on a compact-class vehicle,3

CCCS2004200420032005ULSAB-AVCC classULSAB-AVCPNGV classNSB fromThyssenKruppBMW 3 series(E90)CSSCC19972002200219981999Audi A8JaguarFord AIVFord P2000SULSAB1999YearAudi A2Vehicle orproject ryLuxuryMid-sizedMid-sizedSubcompactClassOpel Zafira1999BMW 3 series1998 (E46)Ford Taurus1994C classvehiclePNGV ClassAudi A8Jaguar XJFord TaurusFord TaurusAudi W forreferencevehicle (kg)267240218202203222250205232153BIW forlight-weightedvehicle (kg)1777606068N/A10013615580Weightsaving inBIW (kg)N/A98473217N/A500200N/AN/A135N/A18.8 (incl.downsizing)32.1 htsaving insaving incurb weight curb weight(kg)(%)Sources: Obenchain et al. 2002; Scamans 2005; Wallentowitz et al. 2006.Note: One kilogram (kg, SI) 2.204 pounds (lb). N/A not available. BIW body in white, the stage in which the car body sheet metal has been assembled or designed but beforethe components and trim have been added (Babylon 2009). Curb weight (CW) is the weight of a motor vehicle with standard equipment, maximum capacity of liquids (Babylon 2009).ULSAB Ultralight Steel Auto Body; ULSAB-AVC ULSAB Advanced Vehicle Concepts; PNGV partnership for a new generation of vehicles; incl. including.HSS-intensive vehicleAluminum-intensivevehicleLightweighted vehicleConcept car(C) or vehiclein massproduction (S)Table 1 List of reference projects on vehicle lightweightingR E S E A R C H A N D A N A LY S I SKim et al., Greenhouse Gas Emissions Payback for Lightweighted Vehicles931

R E S E A R C H A N D A N A LY S I Sa curb-weight lightweighting of 18.8% wasachieved when secondary weight reductions dueto component downsizing were included (Obenchain et al. 2002; Wallentowitz et al. 2006).From the emissions perspective, HSS has amuch lower GHG footprint per kilogram thanprimary aluminum (Tessieri and Ng 1995; Nget al. 1999; Zapp et al. 2003). Therefore, the literature has been more concerned about the environmental impacts of AIVs. A common conclusion in the literature is that AIVs should beaccompanied by the creation of a closed-loop recycling system for the resulting end-of-life (EoL)scrap (Tessieri and Ng 1995; Ng et al. 1999).Life cycle assessment (LCA) studies today forAIVs generally assume a closed-loop recyclingof the aluminum alloys used in the vehicle structure and powertrain (e.g., Tessieri and Ng 1995).Although this recycling is possible in theory, inpractice it is not possible to produce wroughtaluminum from recycled material in the automotive industry due to economics and a lack ofinfrastructure. For this reason, AIVs (and HSSvehicles) initially require the input of primarymetals because of a lack of available infrastructure for producing secondary wrought metals thatcan be used in the vehicle structure. In this article, we consider the life cycle emissions of AIVsboth with and without a closed-loop infrastructure for wrought aluminum materials, for comparative purposes.Although previous research has studied themaximum achievable magnitude of vehiclelightweighting using aluminum and HSS (Daset al. 1997; Ng et al. 1999; Das 2000a, 2000b;IAI 2000; Austin et al. 1999; Obenchain et al.2002; Wallentowitz et al. 2006), only a few studies have been performed to investigate the tradeoffs between aluminum and HSS. These articles(Dieffenbach and Mascarin 1993; Han and Clark1995; Han 1996; Kelkar et al. 2001; Geyer 2008)concentrated on economic and environmentalimpacts of steel and aluminum BIW. They didnot, however, cover the whole vehicle–only theBIW. Furthermore, these investigations did notconsider the context of a specific vehicle forwhich emissions calculations for the use phasefollowed from a detailed engineering model. Inthe present article, we develop the analysis in thecontext of a specific vehicle to provide a com932Journal of Industrial Ecologyplete picture of the magnitude of GHG trade-offsbetween aluminum and HSS for a real vehicle,absent the assumption of a closed-loop aluminuminfrastructure that is decades away. The articletherefore provides an indication of the importance of production versus use-phase trade-offs inthe context of meeting societal goals for reducingGHG emissions from the automotive sector, aswell as the time frames over which these benefitscan be achieved.The life cycle model developed in this articlebegins to answer the following questions: What are the life cycle emissions reductionsassociated with different levels of LW whenaluminum versus HSS is used for a specificcompact-sized vehicle? How does the comparison change with theintroduction of a closed-loop wrought material recycling system for aluminum, andwith attention to where the primary material was produced? Are the GHG payback periods significantrelative to the time scales over which emissions reductions are needed from the passenger vehicle industry?To answer these questions in a realistic scenario, we evaluate life cycle GHG emissionsfor a range of LW, production carbon intensity,and end-of-life vehicle management scenarios forboth aluminum and HSS. We begin the next section by describing the detailed LW modeling ofa specific compact vehicle and then offer a sensitivity analysis for key assumptions employed inthe system model.Methods and ModelsOur baseline vehicle is based on a compactsized Ford Focus ZX3 (Model 2000 2004 withinitial curb weight 1,159 kg). We assume thebaseline vehicle BIW subassembly has a primarylightweighting potential of up to 11% with aluminum, on the basis of analysis by Nathani andArnsberg (2002) and Neumann and Schindler(2002), and up to 6% with HSS, on the basis ofwork by Wallentowitz and colleagues (2006). Wealso assume that lightweighting with aluminumbeyond the BIW can achieve a total curb weight

R E S E A R C H A N D A N A LY S I Sreduction of up to 23% relative to the baselinevehicle (Wallentowitz et al. 2006).In addition to this primary weight reduction,secondary weight reductions are also considered.Secondary weight reductions can be achievedwhen reducing the weight of a given vehicle subsystem allows weight savings in supporting vehicle subsystems. Because of the dependency ofsecondary weight saving on the primary weightsaving, a weight-saving factor as the quotientof the two percentages can be calculated. Inthree literature sources, a 2:1 ratio for the primary to the secondary weight saving is claimed(Das 2000a, 2000b; Gaines and Cuenca 2002;Wallentowitz et al. 2002). Another literaturesource, however, claimed a 3.0% to 8.2% fuelsaving effect due to secondary weight reductionby powertrain redesigns (Wohlecker et al. 2007).In the present article, we applied a 3.0% to 8.2%fuel-saving effect, because of a lack of data tosupport the 2:1 assumption and according to theadvice and experience of a materials expert working at one of the major U.S. automakers.On the basis of a detailed survey of the components and material composition of a genericcompact-sized vehicle described in the literature(Ng et al. 1999), we created five LW optionswith aluminum and HSS. These LW options areas follows (percentages are with reference to curbweight): 6% LW with HSS6% LW with aluminum11% LW with aluminum19% LW with HSS (Obenchain et al. 2002;Wallentowitz et al. 2006), including secondary weight reductions from componentdownsizing 23% LW with aluminum.To understand the GHG emissions of theseLW vehicles relative to their baseline compactvehicle, we needed to create the analysis modelsthat are described in the following sections: component-level material model fuel economy performance and vehiclemiles traveled (VMT)4 models life cycle GHG emissions and EoL5 materials recovery models.Figure 1 Material composition for the baselinevehicle and five lightweighting options. kg kilogram; HSS high-strength steel; Al aluminum.Component-Level Material ModelWe began by evaluating the magnitude of vehicle lightweighting that is technologically feasible and then calculating the associated massof aluminum or HSS required. Starting with acurb weight of 1,159 kg for the baseline vehicle, we estimated that the baseline vehicle has asteel/iron mass of 737 kg, a light metals6 mass of115 kg, and an aluminum mass of 86 kg (Tessieriand Ng 1995; Nathani and Arnsberg 2002). Fromhere, we performed a most-likely material analysisat the component level for the baseline vehiclecomponents. For verification purposes, we confirmed that the sum of the component materialmasses was approximately equal to the knownmass of each material in the vehicle. Given arealistic component-level material model of thebaseline vehicle, it was possible to lightweightthe vehicle, given the opportunity to substituteeach major component of the vehicle with eitheraluminum (Tessieri and Ng 1995; Austin et al.2002; Neumann and Schindler 2002) or HSS(Das et al. 1997; Das 1999). Figure 1 providesthe detailed material compositions of the baselinevehicle and each LW scenario considered in thiswork.As mentioned above, we performedcomponent-level and subsystem-level analyses to ensure feasible weight reductions andinputs of LW materials. In other words, we verified every kilogram of additional lightweightingbeyond the baseline vehicle to ensure feasibility.Kim et al., Greenhouse Gas Emissions Payback for Lightweighted Vehicles933

R E S E A R C H A N D A N A LY S I STable 2 Substitution of materials for vehicle components with respect to lightweighting (LW) optionsComponent(A) Powertrain(1) EngineBalance shaftcarrierBelt tensionerBracketsCatalytic converterheat shieldChain caseCylinder headCylinder linerEngine blockFuel filling railIntake manifoldOil filter adapterOil panOil pumpbody/coverPistonsRocker/cam coversRocker armRocker assemblypedestalsStarter motor nosehousingSuper chargercover/bedringplateSuper chargerhousingSuper charger rotorThrottle bodyTiming chaincover/front coverTurbo intakeWater pumpassemblyQuantityTotalTotalLWLWLWLWLWperweight asweight as6%6%11% 19% 23%vehicle aluminum (kg) HSS (kg) Baseline HSS aluminum aluminum HSS 3.31113. Note: Check marks indicate substitution into the light metals. HSS high-strength steel.An example of how this was done is providedin table 2. Table 2 is an excerpt of a largertable that includes all major vehicle componentsand subsystems for which lightweighting is anoption in the baseline vehicle. For example,table 2 shows that the engine cylinder head isalready assumed to be aluminum in the baseline934Journal of Industrial Ecologyvehicle. Therefore, it is not a candidate foradditional lightweighting. Aluminum enginebrackets, however, are not assumed in thebaseline vehicle and are therefore applied in the11% LW and more intensive options. Similarly,the steel balance shaft carrier is replaced byaluminum in the 19% LW option. We prioritized

R E S E A R C H A N D A N A LY S I Sapplications of LW by applying lowest-costmaterial substitutions first. The complete versionof table 2, found as Table S-3 in the SupportingInformation on the Web, also lists the mostlikely manufacturing process assumed (on thebasis of publications by Tessieri and Ng 1995) forproducing the aluminum components–amongcasting, flat rolling, extrusion, and forging.This was required to estimate manufacturingGHG emissions from the production of LWcomponents.In this study, mild steel 140/270 (materialprice:7 0.80/kg; scrap price: 0.10/kg; density:7.85 g/cc) and HSSs (material price: 0.85 to 0.95/kg; scrap price: 0.10/kg; density: 7.85g/cc), such as Brinell Hardness (BH) 260/370e,high strength low alloy (HSLA) 350/450, or dualphase (DP) 350/600e, are used as reference materials for the material modeling. These HSSs areassumed to have a 10% weight reduction potential in load-bearing applications over their conventional counterparts. The information was provided by a delegation from the International Ironand Steel Institute (IISI) (Opbroek 2007).In this research, we assumed a most probablealloy type for each component on the basis ofresearch by Tessieri and Ng (1995). We neededto do this to understand which components canbe sorted and recycled together while avoidingdowncycling. For example, the balance shaft carrier was assumed to be produced from 100% ofaluminum alloy 356. Aluminum engine bracketsare produced from 12% of aluminum alloy 319and 88% of aluminum alloy 380. This means thatdisassembly of the shaft carrier from the brackets,as well as the separation of brackets, would benecessary for a closed-loop material managementsystem to avoid alloy mixing and resultant downcycling. This has significant ramifications for theultimate cost, although not the technical feasibility, of a closed-loop aluminum infrastructure.Fuel Economy Performance of LWVehiclesOur estimates of GHG emissions from the usephase require two major elements: an estimateof miles per gallon (mpg 8 ; also known as fueleconomy) under various lightweighting strategies, and an assumption about VMT. Here weuse the VMT model by Das (2000a, 2000b), summarized in Supporting Information Part D, available on the Web. Fuel consumption per mile wasdetermined with the simulation software AVLCruise (AVL LIST GmbH) for each LW option.In the model, only vehicle weight affects the fueleconomy (material type used to achieve a givenweight does not matter). The model assumes thatVMT is the same for all vehicles (181,195 miles).As shown in figure 2, AVL Cruise calculatesthat the baseline vehicle has a fuel economyof 33.0 miles per gallon (mpg) for the FederalTest Procedure-75 (FTP75; the drive cycle thatis the most common fuel economy metric in theUnited States and used for regulation). The fueleconomy is increased to 34.2 mpg in the 6%LW option and further increased to 35.1 mpg,36.7 mpg, and 37.5 mpg in the 11%, 19%, and23% lightweighting options, respectively. Thesimulation results fall within the range of otherstudies (Wohlecker et al. 2007; Montalbo et al.2008). We observe from the simulation result thatthe secondary weight reduction effect increaseswith higher LW. The fuel economy differencebetween secondary LW and primary LW is 0.1to 0.3 mpg for the 6% LW option versus 1.1 to1.8 mpg for the 23% LW option.Life Cycle GHG Emissions and EoLMaterials RecoveryWe used previous LCA studies of aluminumand steel production (Ecobalance and NPPC1997; Das 2000a, 2000b; IAI 2000; Choate andGreen 2003; Schifo and Radia 2004; US EPA2004; KNCPC 2005; IPCC 2007; NREL 2007;WSA 2008) to estimate added upstream emissions associated with LW. These emissions factors are provided in table 3. Here we considerthe primary and secondary production of steeland aluminum, as well as the manufacturing processes required to produce finished LW components. Table 3 also provides emission factors forcast and wrought alloys of aluminum (IAI 2000;IPCC 2007). For both primary aluminum andHSS, we consider a “typical” emission factor tobe the U.S. average emission factor (IAI 2000;Schifo and Radia 2004). For a high emission factor estimate, we used the emissions factors fromcountries that use GHG-intensive energy andKim et al., Greenhouse Gas Emissions Payback for Lightweighted Vehicles935

R E S E A R C H A N D A N A LY S I SFigure 2 Fuel economy for eachlightweighting (LW) scenariodeveloped via AVL CRUISE.Secondary LW scenarios are appliedin a range of 3.0% to 8.2%, with theresultant impact on fuel economy asshown in the figure.material production facilities. For aluminum, weconsider the emission factor “high” estimate to bethe average emission factor for aluminum madein China (IAI 2000). For steel, we assume thehigh estimate of the steel emission factor to bethe one referenced by the Fourth IPCC Assessment Report on Climate Change (IPCC 2007).9The aluminum manufacturing processes considered include casting, extruding, and rolling.For aluminum casting, an average of the lostfoam, die, and sand casting processes is calculated as 5.5 kilograms of carbon dioxide equivalent per kilogram (kg CO2 -eq/kg; Schifo andRadia 2004). Aluminum rolling and extrudingare assumed to have emission factors of 0.26 kgCO2 -eq/kg and 0.34 kg CO2 -eq/kg, respectively(Choate and Green 2003).For a sensitivity analysis on VMT, we considered a vehicle lifetime from 11 years (Choateand Green 2003) to 16 years (Das 2000a, 2000b).This yielded a range of VMT from 120,000 milesto 181,195 miles. Given the VMT and the modeled fuel economy of each vehicle, the total usephase gasoline consumption was calculated. Lifecycle carbon dioxide (CO2 ) emissions from gasoline consumption for the vehicles were calculatedwith an emission factor of 10.4 kg CO2 /gallongasoline. This includes emissions from both thecombustion of gasoline (8.79 kg CO2 /gallon [USEPA 2004]) and the production and delivery ofgasoline (1.58 kg CO2 -eq/gallon [NREL 2007]).The vehicle use-phase gasoline consumption andGHG emissions are summarized in table 4.A separate model was created for treating EoLvehicle (ELV) emissions and reuse of materials.We use the vehicle EoL rate and vehicle scrappage rate model from work by Schmoyer (2001).Steel and aluminum shredding efficienciesare estimated at 90% and 70%, respectively. Weconsidered both “closed-loop recycling” (whereall the iron, steel, and aluminum materials aresorted by alloy and the alloys are recycled withoutTable 3 Greenhouse gas emission factors for steel and aluminum productionExtent of emissionMetalsEmission factorBaseline estimatePrimary steelPrimary high-strength steelSecondary steelPrimary cast AlPrimary wrought AlSecondary cast AlSecondary wrought AlPrimary steelPrimary cast Al2.2 kg CO2 -eq/kg steel (Schifo and Radia 2004)2.8 kg CO2 -eq/kg steel (KNCPC 2005)0.7 kg CO2 -eq/kg steel (WSA 2008)9.72 kg CO2 -eq/kg Al (IAI 2000)9.45 kg CO2 -eq/kg Al (IAI 2000)1.18 kg CO2 -eq/kg Al (IAI 2000)0.90 kg CO2 -eq/kg Al (IAI 2000)3.8 kg CO2 -eq/kg steel (IPCC et al. 2007)26.6 kg CO2 -eq/kg Al (IAI 2000)High estimateNote: CO2 -eq.: Carbon dioxide equivalent is a measure for describing the climate-forcing strength of a quantity ofgreenhouse gases using the functionally equivalent amount of carbon dioxide (CO2 ) as the reference; Al aluminum.936Journal of Industrial Ecology

R E S E A R C H A N D A N A LY S I STable 4 Vehicle use-phase gasoline consumption and greenhouse gas (GHG) emissions for low (120,000miles) and high (181,195 miles) vehicle miles traveled (VMT; kg CO2 -eq)Lifetime gasolineconsumption (gallons)Use-phase total emissions(kg CO2 -eq)Lightweighting optionsLowHighLowHighBaseline vehicle6% LW11% LW19% LW23% 44,54442,197Note: kg CO2 -eq kilograms carbon dioxide equivalent; LW lightweighting.downcycling for use in new vehicles) and conventional recycling as practiced today. The allocation approach used is a slight modification ofthe approach of Werner and Richter (2000). Weassumed that 93% of all LW metals are collectedat the end of life (IISI 2006) and that 95% and89% of the steel and aluminum, respectively, arerecovered after disassembling, shredding, separating, and sorting, on the basis of research by Zappand colleagues (2003). Recovery of metal scrapafter melting was assumed to be 95% for steel and91% for aluminum. Wrought and cast aluminumseparating efficiency was estimated at 95% (Das1999; Nathani and Arnsberg 2002).GHG emissions were also calculated for theELV processes of disassembly, shredding, andnonferrous separation on the basis of energy consumption data and an assumed CO2 emission perunit energy delivered (assumed totally as electricity; Schifo and Radia 2004). Transportation emissions were also calculated for the movement ofmaterial between process stages, as well as for thetransport of automotive shredder residue (ASR)to landfill disposal (Schifo and Radia 2004). Thetransportation distance between EoL activitieswas assumed to be 100 miles, consistent withanalysis by Keoleian (1997). The transportationdistance between the EoL activities and landfillwas assumed to be 200 miles (Keoleian 1997).The transportation mode was assumed to be asingle-unit diesel truck with emission factor obtained from the U.S. life cycle inventory (LCI)database (NREL 2007). Table 5 summarizes theEoL GHG emissions assumed in the analysis. Thevalues assume that all energy values are electric-ity, with electricity having life cycle GHG emissions equal to the average U.S. grid as per theU.S. LCI database (NREL 2007).ResultsLife Cycle GHG EmissionsTable 6 summarizes the vehicle GHG emissions by life cycle phase. As expected, the usephase dominates the total GHG emissions foreach vehicle (87% to 95%). By percentage of thetotal life cycle emissions, use-phase emissions decrease in percentage, as expected with increasedLW, whereas material production and manufacturing emissions increase in percentage due tothe additional use of higher GHG-intensity metals relative to the mild steels being replaced.Even with the highest emission factors applied forthese materials, the GHG emissions for the production phase are only increased by about 15%Table 5 Greenhouse gas (GHG) emissions fromend-of-life (EoL) process (kg CO2 -eq10 )LW optionBaseline vehicle6% LW11% LW19% LW23% LWELVTotalprocesses Transport (kg CO2 -eq)474439353210094837368147138122108100Note: kg CO2 -eq kilograms carbon dioxide equivalent;LW lightweighting; ELV end-of-life vehicle.Kim et al., Greenhouse Gas Emissions Payback for Lightweighted Vehicles937

R E S E A R C H A N D A N A LY S I STable 6 Life cycle greenhouse gas (GHG) emissions for each lightweighting (LW) scenario (kg CO2 -eq)ProductionLW optionsBaseline vehicle6% LW HSS6% LW AL11% LW AL19% LW HSS23% LW 1658,66654,48349,47248,837UseTotalNote: EoL end of life; HSS high-strength steel; AL aluminum.above the baseline vehicle production emissions.Conversely, if (entirely) secondary materials wereused for LW, the GHG emissions of productionwould be decreased by 20% to 47% relative to thebaseline vehicle production scenario (given theproduction emission factor lower bound valuesfrom table 6). Therefore, the LW material GHGemission factors and the use of secondary materials play the key roles in determining the GHGemissions of the production phase. For broadranges of assumptions on these emissions, LW isan effective approach to achieve a total life cycleGHG reduction from the modeled baseline vehicle. It should also be noted, however, that thereare minimum travel threshold distances (less than9,479 miles, less than 67,150 miles) for HSS (6%LW, 19% LW) and (less than 54,590 miles, lessthan 106,270 miles) for aluminum (6% LW, 23%LW) for which LW

Audi A2 1999 S Subcompact Audi A2 233 153 80 135 22.6 Audi A8 2002 S Luxury Audi A8 N/A 222 N/A 500 24.5 Jaguar 2002 S Luxury Jaguar XJ 350 250 100 200 11.4 Ford AIV 1998 C Mid-sized Ford Taurus 341 205 136 N/A 21.1 Ford P2000 1999 C Mid-sized Ford Taurus 387 232 155 N/A 0.6 HSS-