Review of the Fuel Saving, Life Cycle GHG Emission, and Ownership

Critical Review pubs.acs.org/est

Review of the Fuel Saving, Life Cycle GHG Emission, and Ownership Cost Impacts of Lightweighting Vehicles with Different Powertrains Jason M. Luk,*,† Hyung Chul Kim,‡ Robert De Kleine,‡ Timothy J. Wallington,‡ and Heather L. MacLean† †

Department of Civil Engineering, University of Toronto, 35 St. George Street, Toronto, Ontario M5S 1A4 Canada Materials & Manufacturing R&A Department, Ford Motor Company, Dearborn, Michigan 48121-2053, United States

‡

S Supporting Information *

ABSTRACT: The literature analyzing the fuel saving, life cycle greenhouse gas (GHG) emission, and ownership cost impacts of lightweighting vehicles with different powertrains is reviewed. Vehicles with lower powertrain efficiencies have higher fuel consumption. Thus, fuel savings from lightweighting internal combustion engine vehicles can be higher than those of hybrid electric and battery electric vehicles. However, the impact of fuel savings on life cycle costs and GHG emissions depends on fuel prices, fuel carbon intensities and fuel storage requirements. Battery electric vehicle fuel savings enable reduction of battery size without sacrificing driving range. This reduces the battery production cost and mass, the latter results in further fuel savings. The carbon intensity of electricity varies widely and is a major source of uncertainty when evaluating the benefits of fuel savings. Hybrid electric vehicles use gasoline more efficiently than internal combustion engine vehicles and do not require large plug-in batteries. Therefore, the benefits of lightweighting depend on the vehicle powertrain. We discuss the value proposition of the use of lightweight materials and alternative powertrains. Future assessments of the benefits of vehicle lightweighting should capture the unique characteristics of emerging vehicle powertrains.

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carbon fiber) and powertrains (internal combustion engine vehicles (ICEV), hybrid electric vehicles (HEV), fuel cell electric vehicles (FCEV) and battery electric vehicles) but did not mention any interactions. Kim and Wallington58 conducted a review of the impact of lightweighting on life cycle GHG emissions but explicitly excluded analyses of different powertrains due to the dominance of internal combustion engine vehicle analyses in the literature. Sah et al.59 provided a review that examined the properties of lightweight materials and presented an ICEV case study, but did not mention the significance of other powertrains. Conversely, reviews of the environmental impacts of electric and other alternative vehicles by MacLean and Lave60 Poullikkas,61 Nordelof et al.62 and Hawkins et al.63 did not discuss the impacts of lightweighting. Our objective in this review is to improve the understanding of the value proposition of lightweighting and alternative powertrain use from both environmental and financial perspectives. We start with background information on lightweight materials and alternative powertrains. Then, we discuss the role of powertrain type on fuel savings, GHG emissions, and ownership costs of vehicle lightweighting. Finally, opportunities

INTRODUCTION U.S. light-duty vehicles have an average 2025 model year laboratory fuel economy target of 54.5 mpg1 and a tailpipe greenhouse gas (GHG) emissions target of 163 g CO2e/mile.2 Vehicle lightweighting and the use of alternative powertrains are key approaches to deliver increased fuel economy. Many analyses that examine the use of these technologies report that their potential costs and benefits differ depending on whether they are used independently or in combination.3−36 Vehicle lightweighting decreases fuel consumption, the use of alternative powertrains affects potential fuel savings4,5,7,8,10,11,22 and the associated reductions in GHG emissions6,14,16,20,21,24,25,27−32,34,36−49 and financial costs.3,9,12,13,15,17−19,21,23,25,26,29,32,38,43,46−48,50−53 These studies are further analyzed below. The complex relationship between vehicle lightweighting and alternative powertrain use has been largely overlooked in literature reviews. In a review of sustainable automotive designs, Mayyas et al.54 only briefly acknowledged the combined use of lightweight materials and different powertrains with a single citation to the steel industry. Friedrich55 provided a review discussing the importance of material selection in general, but only qualitatively mentioned different powertrains. The review of lightweighting by Lutsey56 ignored negative aspects of lightweighting electric vehicles. Gearhart57 provided a review of vehicle emission reduction technologies including different automotive materials (high strength steel, aluminum, magnesium, and © XXXX American Chemical Society

Received: February 18, 2017 Revised: June 19, 2017 Accepted: June 28, 2017

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DOI: 10.1021/acs.est.7b00909 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 1. Range of material substitution ratios of different lightweight materials reported by the U.S. Department of Energy64 and error bars representing theoretical limits as calculated by Kelly et al.65

Figure 2. Number of studies of different aspects of the lightweight alternative powertrain vehicles.

The use of lightweight materials could result in more energy being required to produce the vehicle.68 On a mass equivalent basis, the primary energy demand to produce lightweight materials can be higher than for steel. This could result in additional GHG emissions from the production of lightweight vehicles (depending on the carbon intensity of the primary energy source,69 recycling rates and other factors).58 These emissions may be offset by fuel savings; however, these savings depend on the powertrain.

for further research are highlighted. This work informs policymakers regulating vehicle fuel economy and GHG emissions, as well as industry stakeholders developing and implementing these technologies.

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BACKGROUND ON LIGHTWEIGHT MATERIALS Vehicles can be produced from a variety of different materials. Figure 1 shows the material substitution ratio, which is the mass of a lightweight material required to replace 1 kg of conventional steel or iron casting, for various lightweight materials. The material substitution ratio quantifies the degree to which lightweight materials can reduce vehicle mass. The average mass of new U.S. light-duty vehicles has increased by 26% between model years 1980 and 2015.66 Some of this increase occurred as consumers shifted from cars (1620 kg average 2015 mass) to light-duty trucks (2180 kg average 2015 mass), which include sport utility vehicles.66 The increased use of lightweight materials has helped limit the increase in vehicle mass. Medium and high strength steels are being used in place of conventional steels to provide structural integrity.67 Aluminum is increasingly being used in place of iron castings in engine blocks and to replace steel wheels.67 Plastics are being used in place of steel for interior components.67 The total mass of these specific materials has remained relatively consistent since 1980 despite the shift from cars to light-duty trucks. However, overall vehicle mass has increased because of the increased use of other materials, which include fluids, rubber, glass, and textiles. Despite these shifts, conventional steel continues to comprise approximately one-third of average light-duty vehicle mass.67 Therefore, there remains ample opportunity for further use of lightweight materials.

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OVERVIEW OF THE LITERATURE The literature analyzing both vehicle lightweighting and alternative powertrain use examines several different metrics. This is shown in Figure 2 and detailed with a table in the Supporting Information (SI). Vehicle fuel savings,4,5,7,8,10,11,22 GHG emissions,6,14,16,20,21,24,25,27−32,34,36−49 and ownership costs (vehicle price and/or fuel costs)3,9,12,13,15,17−19,21,23,25,26,29,32,38,43,46−48,50−53 are the most common metrics examined (though not always on a life cycle basis, as shown in Tables 1 and 2), and are thus the focus of this review. Less common metrics include air pollution,24,38 life cycle energy use20,24,25,29,33 or energy security,38 solid waste,70 and structural integrity.71−74 Several articles are also limited to being a description of the design of a particular vehicle24,75−80 or component.81−83 The focus of this review is on studies that explicitly examine the relationship between vehicle lightweighting and alternative powertrain use. This requires an examination of these technologies as independent options (i.e., comparing lightweight and conventional weight versions of alternative powertrain vehicles, and/or alternative and conventional powertrain versions of lightweight vehicles). However, some studies treat these B


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TurboFuel charged ConsumpFuel Vehicle WorldAu- European PHEV tion Production Cycle GREET toSteel Aluminum Other US Europe Other

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Note: ICEV = internal combustion engine vehicle, HEV = hybrid electric vehicle, PHEV = plug-in hybrid electric vehicle, BEV = battery electric vehicle, FCEV = fuel cell electric vehicle.

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Luk et al.36

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Egede49

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Egede et al.34 2015 X

Muttana and Sardar21

Ricardo

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Lewis et al.27 2014

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Lewis et al.28 2014

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Schuh et al.24 2013

Faßbender et al.15

Lewis et al.16 2012

WorldAutoS- 2011 X teel14

Moon et al.6

lightweight material

TurboAlumi- Carbon Glass MultiDiesel charged Year Steel num Fiber Fiber Material ICEV HEV PHEV BEV FCEV ICEV ICEV

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Table 1. Summary of the Literature Analysing the Life Cycle GHG Emissions Impact of Lightweighting Alternative Powertrain Vehiclesa

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Note: ICEV = internal combustion engine vehicle, HEV = hybrid electric vehicle, PHEV = plug-in hybrid electric vehicle, BEV = battery electric vehicle, FCEV = fuel cell electric vehicle.

Muttana and Sardar21

WorldAutoSteel14 2011 X

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Brooker et al.23

Faßbender et al.15 2012

Hofer et al.

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quant. point quant. qual. extended estimate quant. cost cost carbon multinot turbocharged diesel diesel methanol Range powertrain/ (s) range Curve Curve year steel aluminum fiber material specified ICEV HEV PHEV BEV FCEV ICEV ICEV HEV FCEV BEV glider battery Fuel

Frangi3

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Table 2. Summary of the Literature Analyzing the Impact on Ownership Cost of Lightweighting Alternative Powertrain Vehiclesa

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of fuel per 100 km driven. Fuel-mass correlations are the percentage reduction in fuel consumption, or increase in fuel economy (mpg), per 10% reduction in vehicle mass. Reductions in fuel consumption are less than 10% because there are nonmass related loads (e.g., aerodynamic drag and auxiliary loads),31 which can differ among vehicles (e.g., sport utility vehicles typically have more aerodynamic drag than cars). Thus, we use fuel reduction values in Figure 3 to avoid conflating assumptions related to nonmass related loads. Figure 3a illustrates the range of estimated fuel savings from lightweighting found in or calculated from (e.g., if fuel consumption and mass of both conventional and lightweight vehicles are presented in a reference) the literature for ICEVs, HEVs, and BEVs. Despite the wide ranges, it is clear from the literature that the marginal fuel savings from lightweighting are lower for BEVs than ICEVs. This is because vehicles with greater reliance on electric motors have higher powertrain efficiencies (e.g., Kim and Wallington31 estimated an electric motor and battery discharge efficiency of 85% versus an internal

technologies as mutually exclusive options, by excluding their combined use.43,50,76 Conversely, others analyze these technologies as mutually inclusive options by only examining their combined use, which obscures the impact of each technology.37,38,44,46−48,51−53,71−75,77−80,82 Similarly, others obscure the impact of each technology by analyzing vehicle fleets comprised of a range of different vehicles and technologies.20,25,29,33,39−42,45

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FUEL SAVINGS FROM LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS Vehicle lightweighting can reduce life cycle GHG emissions and ownership costs by decreasing vehicle fuel consumption. Two typical metrics used in the literature to measure fuel savings from lightweighting are fuel reduction values and fuel-mass correlations. Fuel reduction values are the reduction in fuel consumption (L/100 km) per 100 kg reduction in vehicle mass. For example, Figure 3a shows that a 100 kg mass reduction could result in an ICEV saving between 0.114,22,31 and 0.54,8 L

Figure 3. Ranges of fuel savings from lightweighting for ICEVs, HEVs, and BEVs (a) and impacts of powertrain resizing (b) (using data from Kim and Wallington31 based on unadjusted 2-cycle fuel economy testing, midsize ICEV and HEV and compact BEV), drive cycle (c) (data from Brooker et al.23 based on powertrain resizing and compact cars), and vehicle class (d) (data from WorldAutoSteel14 based on 5-cycle fuel economy testing and powertrain resizing). E


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software, which Wohlecker et al.8 and WorldAutoSteel14 also used. Vehicle design changes that are not accompanied by gear ratio adjustments can change the operating conditions of the ICE, which affect ICE efficiency. These findings help explain why the range of ICEV fuel savings in the literature and presented in Figure 3a is larger than the range of BEV fuel savings.

combustion engine thermodynamic efficiency of 41%), lower fuel consumption, and thus lower potential fuel savings from lightweighting. The wide ranges of results are due to the use of many different assumptions in the literature including those related to powertrain downsizing, drive cycle, and vehicle class. These three commonly examined variables, and their impacts on marginal fuel savings from lightweighting, are explained below. Outliers produced from suspect methods are excluded from Figure 3a. These include estimates derived from linear regression of the fuel consumption and mass of different vehicles7,42 because this method can conflate vehicle characteristics, such as vehicle size and aerodynamic drag coefficient, which may be correlated with vehicle mass (e.g., cars versus SUVs) but would not be reduced through the use of lightweight materials. The ranges in Figure 3a include data based on linear regression analyses that account for differences in vehicle size and aerodynamic drag.9 Excluded from the ranges is one set of results that is both physically implausible (i.e., where marginal fuel savings are not correlated with powertrain efficiency) and based on methods that are not transparent (i.e., unnamed software).3 Studies that assume the marginal fuel savings of a particular vehicle are applicable to other vehicles with different powertrains (a simplification in some studies focused on life cycle GHG emissions or ownership costs) are also excluded from Figure 3a but are discussed below. The ranges in Figure 3a capture the diversity of results in the literature but are not definitive because the variables examined in the literature are not exhaustive. For example, Egede et al.34 noted that climate control and hill climbing are sources of uncertainty that can affect fuel consumption, which could increase the range of potential fuel savings from lightweighting. Vehicle rolling resistance is a function of vehicle mass and a friction coefficient,31 which could also be reduced over time (e.g., higher efficiency tires) and thus affect potential fuel savings from lightweighting. These factors are not examined in the literature, which is largely based on hypothetical vehicles (with the exceptions being Brooker et al.,23 Carlson et al.22 and Kim and Wallington,31 which provided specific vehicle model names and model years, and are discussed below) without explicit temporal or performance assumptions.

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IMPACT OF DRIVE CYCLE ON FUEL SAVINGS FROM LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS The literature data show that fuel savings are sensitive to the drive cycles used to test vehicles.8,11,14,19,22,30 Driving conditions with more vehicle acceleration have higher fuel savings from lightweighting because fuel is required to accelerate vehicle mass. Therefore, Carlson et al.22 found particularly low potential fuel savings (based on experimental analysis) for real world vehicles being tested with highway driving conditions, as shown in Figure 3c (fuel savings 60−92% lower than in city driving conditions). The results from Carlson et al.22 also show that vehicles with regenerative braking have fuel savings from lightweighting that are less sensitive to changes in acceleration demands. This is because energy otherwise lost during braking is used to assist with reacceleration. Vehicles with more efficient powertrains (even if they have the same powertrain type) are less sensitive to changes in drive cycles in general, because they require less fuel to accelerate. While Carlson et al.22 did not examine powertrain resizing, Wohlecker et al.8 and WorldAutoSteel14 also had similar findings regarding drive cycles (as Carlson et al.22) based on their analyses with powertrain resizing. These findings help explain why the magnitude and range of ICEV fuel savings in Figure 3a are larger than those for BEVs.

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IMPACT OF VEHICLE CLASS ON FUEL SAVINGS FROM LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS Many studies have investigated the relationship between lightduty vehicle class (e.g., car vs SUV) and potential fuel savings.4,5,8,11,14,20 These studies have found that fuel savings per unit mass reduction are largely insensitive to changes in vehicle class (although total fuel savings may differ due to heavier vehicles having greater potential to use lightweight materials and reduce mass). Results from WorldAutoSteel14 are shown in Figure 3d as an example. Minor differences between the midsize car and SUV could be a result of SUVs having larger ICEs (higher ICE friction losses) than cars. However, as noted above, Pagerit et al.5 found minor differences in computer simulated fuel savings could be due to a lack of transmission gear ratio adjustments.

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IMPACT OF POWERTRAIN RESIZING ON FUEL SAVINGS FROM LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS Reducing vehicle mass while maintaining powertrain component (ICE and/or electric motor) size increases a vehicle’s power to weight ratio, which is an indicator of its acceleration performance.66 Powertrain downsizing can maintain acceleration performance, while increasing potential fuel savings. The increase in potential fuel savings from powertrain downsizing is higher for vehicles with greater reliance on an ICE. These increases (55% for ICEV vs 0% for BEV) are illustrated in Figure 3b with results from Kim and Wallington31 produced using physics-based equations and data from real world vehicles to quantify the high ICE friction losses, which can be reduced with powertrain downsizing. Pagerit et al.,5 Wohlecker et al.8 and WorldAutoSteel14 used PSAT84 vehicle simulation software and found that powertrain resizing has a substantial impact on the fuel savings of ICEVs (increase of up to 320%) and a negligible impact (increase as low as 0%) on the fuel savings of BEVs and FCEVs. However, Pagerit et al.5 noted that some changes in fuel savings can be a result of assumptions regarding transmission gear ratios within PSAT84 vehicle simulation

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ESTIMATES IN THE LIFE CYCLE LITERATURE OF FUEL SAVINGS FROM LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS There are complexities in estimating fuel savings, due in large part to the variables discussed above (particularly powertrain downsizing and drive cycles), variability among real world vehicles (even among those with common powertrain type),31 and inherent uncertainties in quantifying other variables (e.g., precise efficiency specifications). However, the literature is clear that the fuel savings from lightweighting are dependent on vehicle powertrain. Despite this, some studies disregard this by F


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savings and GHG emission reductions than in less efficient ICEVs. Schuh et al.24 found that GHG emissions also depend on driving conditions, which impact powertrain efficiency. The carbon intensity of fuel savings can also depend on powertrain type. The use of less/more carbon intensive fuels results in lower/higher GHG emissions to be mitigated by lightweighting. Thus, Kim and Wallington31 found the GHG reductions from the lightweighting of BEVs using U.S. gridelectricity (average generated from all grid-connected sources of electricity) could be similar to those from the lightweighting of less efficient HEVs. WorldAutoSteel14 commissioned a life cycle model that found certain means of lightweighting ICEVs that could decrease life cycle GHG emissions if gasoline is used, but not if a less carbon-intensive fuel such as biodiesel, hydrogen, or ethanol were used instead. However, carbon intensities can be highly uncertain, particularly for some alternative fuels. For example, Lewis et al.27 reported that there is greater uncertainty in the GHG emissions from ethanol than gasoline production. Similarly, Lewis et al.16 and Ricardo30 showed the GHG emissions for plug-in vehicles are highly sensitive to the fuel cycle emissions of electricity, which depend on the means of electricity production and can vary based on jurisdiction. Thus, different countries are analyzed by Egede et al.,34 whereas Egede49 presented results that are a function of the carbon intensity of the electricity. The diversity of alternative fuel GHG emissions is another reason why conclusions regarding lightweighting based on ICEVs cannot be assumed to be applicable to alternative powertrain vehicles.

assuming fuel savings are independent of powertrain type. These studies are focused on analyzing life cycle GHG emissions39−41 or ownership costs,9,46 which depend on estimates of fuel savings. Some studies rely on fuel-mass correlations to generalize fuel savings from lightweighting.39,40,46 In contrast to fuel reduction values (shown in Figure 3), relative metrics can be somewhat more consistent across vehicles with different powertrains. This is a result of this relative metric capturing, albeit imprecisely, both the higher mass and higher efficiency of electric vehicles compared to the ICEV. However, the mass of electric vehicles is highly sensitive to battery size, which is independent of powertrain efficiency. Additionally, the fraction of fuel consumption that can be reduced from lightweighting depends on nonmass related loads, which can vary among different vehicles. Therefore, relative metrics should not be assumed to be broadly applicable to all vehicles. Not all life cycle studies disregard differences among powertrains when estimating fuel savings from lightweighting. In particular, GREET,68 WorldAutoSteel,14 European Aluminum,30 and FASTsim85 life cycle models take into account powertrain type to some extent. These models are further analyzed in the sections below, which discusses life cycle GHG emissions and ownership costs.

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LIFE CYCLE GHG EMISSIONS The literature analyzing the impacts on life cycle GHG emissions of lightweighting vehicles with different powertrains is summarized in Table 1. Life cycle GHG emissions include those emitted during fuel production and use (which comprise the fuel cycle) in addition to those from vehicle production and disposal (which comprise the vehicle cycle). Results depend on many factors, including the lightweight material and powertrain type. Much of the literature is based on publicly available models including the Argonne National Laboratory’s GREET model,6,16,27,28,31 or models commissioned by WorldAutoSteel14 or European Aluminum,30 while others were developed with GaBi86 life cycle software,24 the EcoInvent87 database34,35 or based on literature citations.15,21 Default model results are typically based on U.S. or European characteristics (e.g., electricity grid and drive cycle). Other studies are based on conditions in India21 or focused on comparing different countries.34,49 The majority of studies6,14−16,21,24,27,28,30 compare life cycle emissions of theoretical generic vehicles that have different powertrains but are otherwise similar, while Kim and Wallington31 analyzed theoretical mass reductions in real world vehicles.

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FUEL AND VEHICLE CYCLE GHG EMISSIONS IMPACT OF SECONDARY MASS REDUCTIONS FROM LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS Substituting conventional materials with lightweight materials can reduce vehicle mass. This is referred to as primary mass reduction. However, the use of lightweight materials can also enable secondary sources of mass reduction. Lewis et al.28 analyzed the importance of downsizing powertrain components to maintain acceleration performance, as a means of reducing engine friction and component mass to increasing fuel savings. Additionally, Lewis et al.28 analyzed the potential to resize chassis components while maintaining structural integrity to further reduce mass. However, Joshi et al.12 cautioned that there may not be sufficient benefits to justify the complexities in redesigning multiple subsystems and the manufacturing and assembly processes. Fuel savings from lightweighting reduce the battery energy capacity/size required for a BEV to provide a particular driving range. Batteries can account for a substantial fraction of BEV mass and reduction in battery mass is a valuable secondary mass reduction that further increases fuel savings (beyond those from primary mass reduction alone). Additionally, reducing battery size also decreases vehicle cycle emissions. For example, Muttana and Sardar21 examined the impact of BEV lightweighting with aluminum in India. Their results show a reduction in the “breakeven driving distance” before the additional GHG emissions associated with aluminum production (further discussed below) are offset by fuel savings, if the battery size is reduced to maintain the original driving range.21 Additional analysis of breakeven parameters can be found in Kelly et al.,65 although it is limited to ICEV lightweighting. Lightweighting could also be used to extend driving range without increasing

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FUEL CYCLE GHG EMISSIONS IMPACT OF LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS Fuel savings reduce the quantity of fuel that must be produced and thus overall fuel cycle GHG emissions. The reduction in fuel cycle GHG emissions is a product of the quantity (on an energy basis) and carbon intensity (per unit energy) of the fuel savings. As analyzed above, the quantity of fuel savings from lightweighting depends on powertrain type. Higher efficiency powertrains have less fuel use and potential fuel cycle GHG emissions savings. Accordingly, Lewis et al.,28 WorldAutoSteel,14 Moon et al.,6 Lewis et al.27 and Kim and Wallington31 each found that the lightweighting of more efficient gasoline vehicles (HEVs or turbocharged ICEVs) results in less fuel G


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Figure 4. Life cycle GHG emissions from GREET,68 WorldAutoSteel14 and European Aluminum30 models based on 200 000 km driving, U.S. drive cycles, and U.S. grid-electricity.

may overestimate the GHG emissions associated with aluminum use. WorldAutoSteel14 and European Aluminum30 findings are also affected by their use of different recycling allocation methods, although neither method is incorrect. Regardless, the discrepancy between these results highlights the importance of broad and critical examination of the literature. It also reveals a limitation of the discussion of the impact of lightweighting alternative powertrain vehicles in a review of sustainable automotive designs,54 which only cited results published by WorldAutoSteel.14

battery size/energy capacity. Thus, battery size may not actually be reduced as modeled in the literature. However, lightweighting could still be viewed as a means of reducing the battery size that is required to provide the extended driving range.

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VEHICLE CYCLE GHG EMISSIONS IMPACT OF LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS The primary energy required to produce lightweight materials can be higher than that of conventional materials, on a per unit mass basis (see Figure 3).64 However, lightweight materials with higher primary energy use can also have lower material substitution ratios, which means less mass is required than for the conventional material (see Figure 1).58 In addition to the carbon intensity of the primary energy sources,69 these are factors that can result in the GHG emissions from the production of a lightweight material for an application being higher or lower than those from the production of the conventional material. Kim and Wallington58 showed that the assumed primary energy use and substitution ratios of lightweight material required to replace conventional steel vary widely in the literature. In particular, primary energy use is highly sensitive to assumptions regarding recycled material content. This variability explains some of the differences in the models commissioned by different industry groups. The model commissioned by WorldAutoSteel14 estimates that vehicle bodies produced from advanced high strength steel (AHSS) are able to reduce life cycle GHG emissions more than those produced from aluminum, while the model commissioned by European Aluminum30 concludes the opposite. Both models have similar assumptions regarding the substitution ratio and recycled content of AHSS; therefore, both conclude a similar reduction in life cycle GHG emissions from advanced high strength steel use. However, the WorldAutoSteel model14 uses much less favorable assumptions regarding the substitution ratio and recycling rates of aluminum than those in the European Aluminum model.30 The findings of Kim and Wallington58 suggest the assumptions used in the WorldAutoSteel14 model

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LIFE CYCLE GHG EMISSIONS IMPACT OF LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS There are several publicly available models that can be used to estimate the impact of lightweighting on life cycle GHG emissions. GREET68 is most commonly used in the literature and includes lightweight vehicle models comprised of carbon fiber and other materials. WorldAutoSteel14 and European Aluminum30 each commissioned models that cite GREET68 for some data but are focused on comparing the use of advanced high strength steel with aluminum for lightweighting. Results for the impact of lightweighting on life cycle GHG emissions from the three models are shown in Figure 4. We selected consistent values for variables to facilitate comparing model results and the models were run utilizing these values. These included 200 000 km of lifetime driving, U.S. drive cycles and U.S. average grid-electricity. The results from all three models indicated that lightweighting can reduce life cycle GHG emissions (from 2% to 20%). The life cycle GHG emissions of the nonlightweight vehicles (referred to here as conventional) differ in each model, largely due to assumptions regarding fuel consumption of the vehicles. For a particular powertrain type, differences among the models in the magnitude of GHG emission reductions from lightweighting largely reflect differences in the types of lightweight materials and associated substitution ratios assumed in the different models. The multimaterial lightweight vehicles within GREET68 include carbon fiber composites, which slightly increase vehicle production GHG emissions, but these are more than offset by the reduced H


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Environmental Science & Technology fuel use of the vehicle. The WorldAutoSteel14 model results for advanced high strength steel shows a slight reduction in vehicle production GHG emissions, but also lower fuel savings compared to the use of aluminum and carbon fiber composites in the European Aluminum and GREET models, respectively. While the results from these models are informative, it is important to acknowledge their limitations. GREET68 fuel savings are based on quadratic equations (which are not transparently explained), the default lightweight body mass varies drastically (306−519 kg) among vehicles with different powertrains, and the benefits of BEV battery downsizing in response to lightweighting are overstated because the lightweight version has a shorter driving range than the conventional version. This is a result of the assumed 39% reduction in battery size (28 vs 17 kWh) exceeding the assumed 17% reduction in fuel consumption (21% fuel economy improvement). As discussed in the previous section, the WorldAutoSteel14 model overestimates the emissions from aluminum use. Finally, the European Aluminum30 model assumes fuel savings from lightweighting are determined only by fuel type but not powertrain type (e.g., assuming gasoline fuel savings are identical for an ICEV and the higher efficiency PHEV). Luk et al.36 used GREET68 while avoiding the aforementioned concerns associated with the use of its default vehicle models. They modified default parameters to compare the use of real world conventional and multimaterial lightweight gliders in combination with ICEV, HEV, and BEV powertrains. Their incremental results for lightweighting vehicles with different powertrains have similarities with the models discussed above. As with the default results from GREET,68 WorldAutoSteel14 and European Aluminum30 models, Luk et al.36 found the ICEV to have greater potential reductions in life cycle GHG emissions than those of vehicles with more efficient powertrains. Luk et al.36 results for the HEV and BEV lightweighting are similar, with the reductions in life cycle GHG emissions slightly higher for the BEV because of the potential for battery downsizing and differences in fuel carbon intensities. This is consistent with the findings from WorldAutoSteel14 but not GREET68 (due to issues discussed above) or European Aluminum30 (which exclude HEVs). As with the results in Figure 4, Luk et al.36 analyzed the use of US grid-electricity. The use of more carbon intensive electricity would increase the benefit of BEV lightweighting, conversely the use of less carbon intensive electricity would decrease the benefit of BEV lighweighting. Lewis et al.16 also used GREET68 while avoiding the use of the default vehicle models. Instead, they modified the PHEV material compositions within GREET68 to model the impact of lightweighting on GHG emissions as a function of mass reduction. Lewis et al.16 found that a PHEV with a 10% mass reduction (primarily via carbon fiber use) can result in slightly higher life cycle GHG emissions than a conventional PHEV. Further lightweighting was found to lower GHG emissions, though the reasons for this are not stated. The findings suggest that the vehicle designs analyzed by Lewis et al.16 were not optimized to reduce GHG emissions, as less GHG intensive means of lightweighting could be prioritized. Regardless, Lewis et al.16 found that life cycle GHG emissions are relatively insensitive to the use of lightweight materials as compared to the carbon intensity of electricity used to charge the vehicles.

in Table 2. Life cycle ownership cost components examined in the literature include the glider (vehicle without powertrain/ battery), powertrain/battery and fuel costs. Other ownership costs such as maintenance and insurance are not examined in these studies. Results depend on both the type of lightweight material and alternative powertrain. Although some studies provide point estimates, others present ranges of potential costs. These ranges are in the form of discrete values or cost curves, and include quantitative or qualitative results. Quantitative results are typically based on theoretical vehicles and costs from literature, while Brooker et al.23 used the publicly available FASTsim85 model to analyze theoretical modifications to the real world Nissan Leaf BEV.88 Joshi et al.12 qualitatively explained the trade-off between the costs and benefits of lightweighting and used the real world Chevy Volt PHEV89 as an example.

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OWNERSHIP COSTS IMPACT OF LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS The potential fuel savings from lightweighting depend on powertrain type, as previously established. In estimating the reduction in cost associated with fuel savings, the costs of the different fuels must be taken into consideration. Frangi,3 Bull et al.,9 Brooker et al.23 and Hofer et al.26 compared the impact of lightweighting on vehicles using different types of fuel, which can have different energy equivalent costs. In particular, the price of electricity combined with the efficiency of an electric motor led Brooker et al.,23 and Hofer et al.,26 to find that the potential BEV fuel cost savings associated with lightweighting can be relatively minor compared to those of ICEVs. Accordingly, Hofer et al.26 found the benefit of lightweighting vehicles that use gasoline, but not those that use electricity, to be highly sensitive to lifetime driving distance/fuel consumption.

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OWNERSHIP COSTS IMPACT OF SECONDARY MASS REDUCTIONS FROM LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS Secondary mass reductions not only affect GHG emissions, as discussed earlier, but also ownership costs. In particular, the use of lightweight materials to reduce the vehicle energy storage requirements and hence size of the plug-in battery required to provide a particular driving range also reduces battery costs. Bull,9 Faßbender et al.,15 Redelbach et al.,17 Hofer et al.,19 Brooker et al.,23 Hofer et al.,26 and Shaw et al.13 all found substantial financial savings from downsizing BEV batteries. Hofer et al.26 analyzed BEVs with different driving ranges and found those with longer electric driving ranges have greater potential cost savings and thus can financially justify more extensive use of lightweight materials. Likewise, Brooker et al.23 found potential battery cost savings from lightweighting to be higher in a BEV than a PHEV with a shorter electric driving range. Joshi et al.12 and Hofer et al.26 highlighted concerns over the cost-effectiveness of lightweighting over time as battery prices decrease. Lightweighting can also reduce the size of powertrain components required to provide a particular acceleration performance. This can reduce the cost of powertrain components. However, Hofer et al.19 and Hofer et al.26 find only minor cost savings from downsizing BEV powertrains to maintain acceleration performance. Similarly, Frangi3 found that the potential cost savings from downsizing ICEV, HEV, and FCEV powertrains are relatively minor. Although Das51 disagreed with the

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LIFE CYCLE OWNERSHIP COSTS The literature analyzing the impacts on ownership costs of lightweighting alternative powertrain vehicles is summarized I


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Figure 5. Illustrative relationship between mass reduction options and their cost adapted from Joshi et al.12

Figure 6. Example default ownership costs from the FASTSim85 model with marginal costs of lightweighting from Brooker et al.,23 which use FASTSim85

findings of Frangi3 by concluding there are substantial cost savings from fuel cell downsizing, Das51 acknowledged that these potential savings may be short-term as technology prices will decrease.

lightweighting with aluminum reduces vehicle price10 and another study presents a single point estimate of life cycle ownership cost reduction without explanation.21 Neither of the studies include references for their cost findings.

VEHICLE PRODUCTION COSTS IMPACT ON LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS Estimating the costs associated with lightweight material use in vehicles is complex. Wilhelm et al.18 and Frangi3 warned of the lack of reliable data in this regard in their analyses. Wilhelm et al.,18 Bull et al.9 and Faßbender et al.15 analyzed costs that differ not only based on the type of lightweight material used but also the particular application of the material (e.g., aluminum in sheets or castings). Additionally, studies noted that the cost of lightweighting decreases with higher production volumes (Frangi3) and technology improvements over time (Hofer et al.19). Redelbach et al.,17 Shaw et al.13 and Brooker et al.23 illustrated vehicle ownership costs as a function of the cost of lightweighting, as opposed to assuming a specific value. Several studies note the uncertainties of estimating the costs of lightweighting, but establish a relationship between mass reduction and cost of lightweighting to facilitate a discussion of vehicle design implications. Figure 5 is adapted from Joshi et al.,12 and shows that the lowest cost means of lightweighting may slightly reduce cost but the ability for these options to reduce mass would be limited (and some of these options may already be utilized in many modern vehicles),67 while higher mass reductions would be possible but with higher marginal costs ($/kg mass reduction). Wilhelm et al.,18 Hofer et al.,19 and Hofer et al.26 modeled the relationship between mass reduction and cost as exponential. Wilhelm et al.18 provided examples of high strength steel and aluminum, with the former having lower marginal costs and potential mass reductions than the latter. Not all studies include the nuances discussed above. A study affiliated with The Aluminum Association simply concludes that

LIFE CYCLE OWNERSHIP COST IMPACT OF LIGHTWEIGHTING VEHICLES WITH DIFFERENT POWERTRAINS The impact of lightweighting on life cycle ownership costs is a function of the costs associated with lightweight material use, fuel savings, and secondary mass reductions. As discussed above, powertrain type affects the potential fuel savings and secondary mass reductions. Therefore, the value proposition of lightweight material use depends on powertrain type. The net present value vehicle cost and 200,000 km fuel costs for conventional and lightweight vehicles with different powertrains are shown in Figure 6. Each powertrain is modeled with a conventional and lightweight glider based on default results from the publicly available FASTsim85 model and marginal costs of lightweighting from Brooker et al.,23 who analyzed the model. The ICEV ownership costs are reduced with lightweighting because the ICEV has relatively high fuel costs and thus potential fuel cost savings that more than offset the increase in vehicle price. The BEV ownership costs are also reduced with lightweighting because it has a relatively high vehicle price from plug-in battery costs, which can be reduced while maintaining driving range. Conversely, the HEV ownership costs are not reduced with lightweighting because it has relatively low fuel costs (compared to ICEVs), and lacks a large plug-in battery. The cost-effectiveness of lightweighting HEVs may be less than in vehicles with other powertrains, but that are otherwise similar. Not all studies analyze the impact on life cycle ownership costs of lightweighting vehicles with different powertrains by comparing an identical means of lightweighting, as is the case in Figure 6. As illustrated in Figure 5, the marginal cost of lightweighting varies with the degree to which mass is reduced.

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J


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Environmental Science & Technology Thus, several studies18,19,26 attempt to minimize ownership costs of vehicles with different powertrains by optimizing mass reduction. These studies show that theoretically optimal lightweighting depends on powertrain type. Wilhelm et al.18 concluded that it may be more cost-effective for automakers to invest in improving the powertrain efficiency (i.e., conventional HEV) or reducing the mass (i.e., lightweight ICEV) of a particular vehicle, rather than both. Hofer et al.19 and Hofer et al.26 found that optimal BEV and ICEV lightweighting is highly sensitive to marginal battery cost and gasoline price, respectively. In addition to the issues discussed here that impact the costs of individual vehicles, there are some fleet-level cost savings from having different vehicles share components.90 Although optimization exercizes in the literature generally rely on oversimplifications, they highlight differences among the potential vehicle-level cost savings from lightweighting vehicles with different powertrains. Figure 5 illustrates the importance of prioritizing more attractive means of lightweighting. This is in contrast to the increase and subsequent decrease in both vehicle cycle and life cycle GHG emissions in response to increasing mass reduction modeled by Lewis et al.,16 which suggests the use of less GHG intensive means of lightweighting was not prioritized. However, it is not clear if the prioritization of costs and GHG emissions are complementary; lower cost means of lightweighting may not be less GHG-intensive, and vice versa.

studies reviewed here that additional technology costs will be passed on to the consumer.91 Ownership cost studies in the literature also exclude maintenance costs, which could increase with the collision repair costs of lightweight vehicles92 or decrease with electric vehicle fuel consumption, battery usage/ degradation and thus potential battery replacement costs.93 Additionally, temporal assumptions could be investigated by quantifying the impact of both ongoing improvements in acceleration performance and powertrain efficiency on potential fuel savings, and thus life cycle impacts of lightweighting. There is an opportunity for future research to examine the effect of public policies on the life cycle impacts of lightweighting vehicles with different powertrains. For example, U.S. federal tax credits available to those purchasing plug-in vehicles are scaled by battery size,94 which may be a financial disincentive to reduce battery size via lightweighting. Therefore, studies could investigate the role of subsidies on the impact of lightweighting, and resulting life cycle ownership costs. Lightduty vehicle GHG standards,1 Zero Emission Vehicle programs95 and low carbon fuel standards96,97 regulate emissions related to fuel use. In the near term, this may be justified because the fuel cycle comprises the vast majority of life cycle emissions.68 However, these policies could result in a reduction in fuel cycle emissions, at the expense of higher vehicle cycle emissions. This review synthesizes scientific knowledge of the role of powertrain type on the life cycle ownership cost and GHG emission impacts of vehicle lightweighting and facilitates an improved understanding of the value proposition of the use of lightweight materials and alternative powertrains. Future research should build upon the findings examined in this review and not rely on broad generalizations about lightweighting, which fail to capture the unique characteristics of emerging vehicle powertrains.

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DISCUSSION Vehicles with lower powertrain efficiencies have higher fuel consumption. Thus, potential fuel savings from lightweighting of ICEVs can be higher than those of HEVs and BEVs. However, the impact of fuel savings on life cycle costs and GHG emissions depends on fuel prices, fuel carbon intensities and fuel storage requirements. Fuel savings for BEVs enable reduction of battery size without sacrificing driving range. This reduces the battery production cost and mass, the latter results in further fuel savings. The carbon intensity of electricity varies widely and is a major source of variability and uncertainty when evaluating the benefits of fuel savings. HEVs use gasoline more efficiently than ICEVs and do not require large plug-in batteries. Therefore, the benefits of lightweighting depend on the vehicle powertrain. Several limitations in the literature are identified. Some studies overestimate potential fuel savings from lightweighting by relying on linear regression analyses that conflate other vehicle characteristics (e.g., vehicle size) or unnamed models that produce outlier results. Other studies overgeneralize fuel savings by not taking into account powertrain type, which can mischaracterize the life cycle impacts of lightweighting, particularly as alternative powertrains are increasingly used. Further, some studies provide only a single perspective that is unambiguously positive or based on a single industry citation. Future studies on lightweighting should capture the complexities of different powertrains (including powertrain type, whether or not resizing occurs to maintain acceleration performance, fuel consumption under different drive cycles and fuel source) without conflating other vehicle design attributes. The scope of future studies could also be expanded. This includes an examination of fleet-level costs, which may be reduced by having vehicles with different powertrains share lightweight components that may not be cost-effective if utilized only in one vehicle model. Economic modeling of consumer demand could examine the assumption in the ownership costs

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b00909. Background discussion on alternative powertrains, a table summarizing key attributes of the literature reviewed, a table of fuel reduction values used to produce Figure 4 and a table of assumptions for each fuel reduction value (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jason M. Luk: 0000-0002-1673-9139 Hyung Chul Kim: 0000-0002-0992-4547 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Ford Motor Company through Project Northern Star. While this article is believed to contain correct information, Ford Motor Company (Ford) does not expressly or impliedly warrant, nor assume any responsibility, for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor K


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represent that its use would not infringe the rights of third parties. Reference to any commercial product or process does not constitute its endorsement. This article does not provide financial, safety, medical, consumer product, or public policy advice or recommendation. Readers should independently replicate all experiments, calculations, and results. The views and opinions expressed are of the authors and do not necessarily reflect those of Ford. This disclaimer may not be removed, altered, superseded or modified without prior Ford permission.

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