The study was independently reviewed by expert representatives from Harvard, Massachusetts Institute of Technology, Argonne National Lab, and thinkstep to establish conformance with International Organization for Standardization (ISO), the globally recognized benchmark for standard setting.

Material production GHG emissions for common body structure and closure materials accounting for estimated part mass reduction. Source: SRI. Click to enlarge.

The study further highlights that solely focusing on tailpipe emissions will likely produce unintended consequences of higher total greenhouse gas (GHG) emissions to the atmosphere when considering lightweighting of vehicle body and closures.

This increase is a result of the significant differences in emissions between AHSS and aluminum in the production phase of the materials. The increased production emissions for aluminum can outweigh emission reductions in both the driving and recycling phases, according to the SRI study.

As driving emissions decrease to meet regulations, production emissions become an even more significant component of a vehicle’s full environmental footprint. If material production phase emissions continue to be overlooked, negative effects on the environment will begin before the vehicle is ever driven. Steel offers the best solution for the environment, the best performance and cost effective solution for automakers, and ultimately the best value for consumers.

—Jody Hall, vice president, automotive market, SMDI

Comparison of life cycle GHG emissions based on the range of methodologies studied. Click to enlarge.

Key findings of the study include:

• AHSS-intensive vehicles had lower or equivalent total life cycle GHG emissions than aluminum-intensive vehicles for every class of vehicle tested. AHSS-intensive vehicles were shown to have lower life cycle GHG emissions vs. aluminum-intensive vehicles in more than 90% of the 5,000 cases studied for each vehicle type.

• The use of aluminum instead of AHSS to lightweight the vehicle body structure and closures resulted in a significant increase in materials production GHG emissions and energy consumption for every scenario. These emissions occur at the start of (and remain in the atmosphere throughout) the vehicle life cycle. The increase in production emissions ranged from 30 - 60 percent for sedans, pickups, SUVs and HEVs when using aluminum over AHSS for lightweighting.

• The increase in production emissions for vehicles light-weighted with aluminum over AHSS is not offset by emission reduction benefits during the use phase until at least the end of the vehicle’s useful lifetime, and in several cases they are never offset. In the SUV example, the difference in life cycle GHG emissions ranged from no difference between the two vehicles to 6 percent higher life cycle emissions for the aluminum vehicle.

• In some cases, the aluminum vehicles had higher life cycle GHG emissions than even the original (pre-light-weighted) baseline vehicles.

Primary aluminum sourcing has a significant effect on the production phase emissions of the aluminum-intensive vehicles. North American primary aluminum production results in four to five times greater GHG emissions than steel production by weight, and imported primary aluminum releases on average eight to nine times greater GHG emissions.

Most steel used in North American vehicle production is produced in North America. In contrast, over the last several years, imports of primary aluminum ingot into North America have increased significantly. As an example, lightweighting all of the vehicles considered in the study with aluminum would result in higher GHG emissions of 12 million tons or the equivalent of the emissions from the annual electricity use of 1.6 million US homes. (Based on EPA’s GHG equivalencies calculator and 6.2 million vehicles, or one-third of the 2016 fleet these vehicles represent.)

The peer-reviewed, publicly available University of California Santa Barbara Automotive Materials Comparison Model (UCSB Model v5) was used in this study to assess average 2016 model year vehicles in several size ranges and with different powertrain systems, including a mid-size sedan, SUV, truck, mid-size hybrid and compact battery electric car. 

The baseline vehicles were each redesigned separately with aluminum and AHSS to reduce the overall weight of the vehicle while achieving the same general performance. Life cycle GHG emissions total and by individual phase were determined for each vehicle.

To address uncertainty and values used in other studies or industries, Monte Carlo simulations were used to test an expanded range of inputs for several key variables. Even when inputs more favorable to the aluminum vehicles were included, the AHSS vehicles had lower life cycle GHG emissions than the aluminum vehicles in at least 90 percent of the cases.

In 2016 SMDI released a white paper on life cycle GHG emissions; this study includes a comprehensive ISO-conformant peer review of that white paper and incorporates updated baseline vehicle models and data.

The baseline vehicles were each redesigned with AHSS-intensive and aluminum-intensive bodies to reduce the overall weight of the vehicle. The life cycle GHG emissions of the redesigned vehicles were then compared.