Lightweight materials are increasingly being used to improve light-duty vehicle fuel economy and reduce tailpipe greenhouse gas (GHG) emissions. However, the production of lightweight materials, including aluminum, can be more... [ view full abstract ]
Lightweight materials are increasingly being used to improve light-duty vehicle fuel economy and reduce tailpipe greenhouse gas (GHG) emissions. However, the production of lightweight materials, including aluminum, can be more GHG-intensive than the production of conventional materials, including steel. Therefore, comparisons of life cycle GHG emissions of the vehicles they are employed in are critical to understand the environmental merits of lightweighting. The literature is based on point estimates and limited sensitivity analyses. This uncertainty analysis provides a more comprehensive understanding of the life cycle GHG emission mitigation potential of vehicle lightweighting.
Life cycle assessments were completed to compare conventional and lightweight versions of internal combustion engine, hybrid electric and battery electric vehicles. The lightweight glider (vehicle without powertrain) is from the Multi-Material Lightweight Vehicle (MMLV), which is an aluminum-intensive concept vehicle funded by the US Department of Energy (2014) that is otherwise similar to the conventional Ford Fusion (Ford, 2013). Assumptions from GREET (Argonne National Laboratory, 2016) were used to estimate base case vehicle cycle (production, maintenance and disposal) and fuel cycle (production and use) GHG emissions. A Monte Carlo analysis quantified uncertainty by collectively examining key assumptions, which includes sources of variability (Steinmann et al., 2014).
Preliminary results show the probability that the MMLV glider can reduce life cycle GHG emissions is high (over 98%), regardless of powertrain type. This is despite a 97% probability of an increase in GHG emissions associated with glider production (depending on electricity GHG intensity and recycled material content) because the increase is minor (0.8 t CO2eq base case) compared to life cycle emissions (50-70 t CO2eq base case among vehicles with conventional gliders). This increase can thus be more than offset by with gasoline or electricity fuel savings. However, emissions reductions associated with electricity savings vary widely across the US.
The magnitudes of potential life cycle GHG reductions depend on powertrain type. Base case GHG mitigation is highest (10 t CO2eq over the vehicle lifetime) with the internal combustion engine vehicle because it is relatively inefficient and typically fuelled by carbon-intensive gasoline, and thus has high fuel cycle GHG emissions, which can be reduced by lightweighting. However, results are highly sensitive to lifetime driving distance, drive cycle, and whether the internal combustion engine is downsized to maintain performance. GHG mitigation can be similar (6-7 t CO2eq base case) with the hybrid and battery electric vehicles. Although battery electric vehicles are more efficient, they have heavy batteries that may be able to be made lighter without sacrificing driving range, if the MMLV glider is used, and thus provide additional mass reduction and fuel savings.
This study finds the use of the MMLV glider will likely mitigate life cycle GHG emissions. This study examined currently utilized technologies and results were found to be sensitive to powertrain efficiency and battery chemistry, among other variables that can change. Therefore, technological developments would prompt a need to reassess the ability for lightweighting to reduce life cycle GHG emissions.
