Argonne study finds lightweight material substitution increases vehicle-cycle GHGs, but results in total life-cycle benefit
A team at Argonne National Laboratory has taken a closer look at vehicle-cycle (all processes related to vehicle manufacturing) and vehicle total life-cycle (vehicle-cycle plus fuel cycle—i.e., the use phase) impacts of substituting lightweight materials into vehicles.
In a study published in the ACS journal Environmental Science & Technology, they reported that while material substitution can reduce vehicle weight, it often increases vehicle-cycle greenhouse gas emissions GHGs—for example, replacing steel with wrought aluminum, carbon fiber reinforced plastic (CRFP), or magnesium increases the vehicle-cycle GHGs. However, lifetime fuel economy benefits often outweigh the vehicle-cycle, resulting in a net total life-cycle GHG benefit, they found. This is the case for steel replaced by wrought aluminum in all assumed cases, and for CFRP and magnesium except for high substitution ratio and low fuel reduction value.
The greatest potential for life-cycle savings arises from vehicle petroleum consumption reductions. Petroleum consumption contributes 80−90% of a conventional vehicle’s life-cycle GHG emissions, when considering the fuel’s well-to- pump and combustion phases (this may change with new technologies). However, it is important to consider the entire life-cycle of the vehicle, including the potential shifts in GHG burdens associated with lightweight materials, such as aluminum, magnesium, carbon fiber reinforced plastics (CFRP), and high-strength and advanced high-strength steels (HSS and AHSS). Many lightweight materials have increased GHG burdens compared to conventional vehicle materials. Further, a recent study highlights the regional variability of GHG emissions for electricity-intensive material production, such as aluminum. Of concern, then, is whether or not increases in vehicle-cycle burdens are outweighed by reductions in the fuel cycle.
This article examines the total life-cycle GHG emissions implications of vehicle weight reduction through the use of material substitution, and uses the fuel reduction value methodology to account for reduced fuel consumption from reduced weight. … Finally, this article extends current understanding by presenting a detailed mathematical examination of the underlying substitution equations, providing resolved breakeven equations for driven distance and material pair substitution ratio, along with detailed discussion of each variable’s influence.—Kelly et al.
The Argonne team used GREET (The Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation Model, developed at Argonne) for energy and emissions data and modified vehicle models within GREET to explore material substitution effects.
The study focused on primary weight reductions and did not consider secondary weight reductions.
The researchers examined the effects of material substitution on five parts (engine block, door frame, IP beam, rear k-frame, and front steering knuckles) from three different systems (powertrain, body, and chassis), as well as the impact of body lightweighting and chassis lightweighting.
The Argonne team observed several trends from this portion of the analysis:
Steel affords a large potential for weight reduction within the vehicle.
Replacing steel with HSS and AHSS yields both mass and vehicle-cycle GHG emissions reductions. This is due to essentially a source reduction, since HSS and AHSS GHG emissions intensities are equal to steel’s.
Cast aluminum enables reductions in vehicle weight and vehicle-cycle GHG emissions, while wrought aluminum reduces weight but increases vehicle-cycle GHG emissions. The difference in vehicle-cycle GHG emission changes between cast and wrought aluminum is due to the high amount of recycled content used within cast aluminum vs the lower recycled content of wrought aluminum.
As with wrought aluminum, replacing steel with CFRP and magnesium allows appreciable weight reductions, but at the cost of increased vehicle-cycle GHG emissions.
The researchers then examined fuel-cycle GHG reductions from lightweighting. In their study, FRV denotes the fuel reduction value for a weight change only and FRV* denotes a reduction value for weight change and powertrain adjustment to maintain the same performance.
They found that the breakeven substitution ratio (the mass ratio of the new material to the old material) increases as driven distance increases. This basically means that the further a vehicle is expected to be driven, the higher the substitution ratio can be in order to achieve a GHG payback. Also, as FRV increases the breakeven substitution ratio increases.
Increasing FRV increases the breakeven substitution ratio, below which a GHG benefit will be achieved over the course of a 260,000 km vehicle lifetime. Compared to the literature, many material pairs, such as cast iron to cast aluminum, indicate that the breakeven substitution ratios for d = 260k km is higher than, or within, the literature ratio ranges. However, we see that changing from cast aluminum to cast magnesium, for an FRV of 0.15 L/(100 km·100 kg), would require a substitution ratio below that which has thus been reported in the literature, indicating that the vehicle would need to be driven beyond the proposed 260,000 km to achieve breakeven GHGs for that FRV.—Kelly et al.
Jarod C. Kelly, John L. Sullivan, Andrew Burnham, and Amgad Elgowainy (2015) “Impacts of Vehicle Weight Reduction via Material Substitution on Life-Cycle Greenhouse Gas Emissions” Environmental Science & Technology doi: 10.1021/acs.est.5b03192