Comparing Lightweight Automotive Materials for a Car Body Panels – Life Cycle Assessment

At the Great Designs in Steel Conference (GDIS) in 2009, General Motors and the Massachusetts Institute of Technology (MIT) presented research that compared lightweight materials for a lift gate closure panel, studying functional performance, mass and cost.

In the study a mild steel liftgate from a compact two-door vehicle was compared with three alternative designs, fabricated from advanced high-strength steel (AHSS), aluminium and SMC composite. The AHSS design included hydroformed tubes, the aluminium design was completely stamped and the SMC design was molded. The performance criteria was passed by each design. The materials, equivalent mass savings and the associated cost for mass reduction are summarized in Table 1:

Figure 1.

Table 1. GM/MIT Lift gate study mass savings results and associated cost

Total Mass (kg) Mass Savings (kg) Cost Difference Δ $ / Δ kg
Baseline 12.32 --- --- ---
AHSS 7.81 4.51 ($3.15) **
Aluminum 6.77 5.55 $28.54 5.14
Composite 8.05 4.27 $3.15 0.74

Life Cycle Assessment Parameters

For each liftgate material or design, a vehicle life cycle assessment was conducted using the latest UCSB GHG materials comparison model, featuring updated LCI’s for steel and aluminium. The mass of each lift gate design was substituted for the mass of the mild steel liftgate (our baseline). Secondary mass reduction is considered at 30%. The vehicle power train for the liftgate LCA is an internal combustion engine with an estimated lifetime mileage of 200,000 km and fuel consumption ratings of 7.0 L/100km or 33.6 mpg, typical for a compact car. All evaluations included consequential system expansion, which includes optimized recycling rates (alpha = 0.1), and hence materials with high manufacturing emissions are favoured. Finally, a HYZEM driving cycle is used which is a composite of aggressive urban-rural driving behaviour.

The following graphs and tables show the LCA results.

In Table 2, the powertrain is not resized, to simulate reality involving small component weight reductions. In Table 3, the powertrain is re-sized for equivalent vehicle performance, favouring light weight materials.

Table 2. GM Liftgate Study: Compact Car, ICE-G, 7L/100km, VCW 1260 kg, Hyzem, 200,000 km, Recycling - alpha = 0.1, no powertrain adjustments.

In kg of CO2eq
Liftgate Material Mass Material Recycling Use Vehicle Life Cycle
Conv Steel 12.32 kg 2,713 -1,135 40,103 41,681
AHSS 7.81 kg 2,691 -1,125 40,053 41,619
Aluminium 6.77 kg 2,786 -1,193 40,041 41,634
SMC Comp 8.05 kg 2,778 -1,114 40,055 41,719

Table 3. GM Liftgate Study: Compact Car, ICE-G, 7L/100km, VCW 1260 kg, Hyzem, 200,000 km, Recycling – alpha = 0.1; Powertrain re-sized for equivalent performance.

In kg of CO2eq
Liftgate Material Mass Material Recycling Use Vehicle Life Cycle
Conv Steel 12.32 kg 2,713 -1,135 40,103 41,681
AHSS 7.81 kg 2,691 -1,125 40,009 41,575
Aluminium 6.77 kg 2,786 -1,193 39,987 41,581
SMC Comp 8.05 kg 2,778 -1,114 40,014 41,678

Figure 2.

Figure 3.

Explanation of Life Cycle Assessment

Life cycle assessment (LCA) is an approach that considers a vehicle’s entire life cycle, from material and vehicle production (manufacturing phase) through its lifetime on the road (use phase) to the end-of-life disposal (end-of-life phase), as well as the life cycle of its fuel sources. Present regulations are focused on measuring use phase or tail pipe emissions alone. However it is not just vehicle use that generates GHG emissions, but all of its life cycle stages.

Conclusions

AHSS is the only material and design option that resulted in mass, cost and emission savings in all life cycle phases, compared to the baseline design. Life cycle assessment is needed to evaluate fully the impact of materials substitution on vehicle emissions, to avoid unintended consequences. Based on this study, it was inferred that the lowest mass design (Aluminium) does not achieve the lowest vehicle lifetime emissions, however the net differences in this study suggest equivalent performance.

The AHSS liftgate design causes a lower mass and vehicle emissions, with projected costs savings of around US$3.15 per kg. However there is no environmental performance benefit to replace steel with the costlier aluminium solution, despite additional mass savings of 1 kg. In this specific study, the SMC system design is heavier than the AHSS system design, and reflects increased costs and emissions. By reducing weight on a small component, overall vehicle emissions are not impacted sufficiently. Investment in alternative powertrains may be a better long-term solution.

This information has been sourced, reviewed and adapted from materials provided by WorldAutoSteel (World Auto Steel).

For more information on this source, please visit WorldAutoSteel (World Auto Steel).

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