Study: Lightweight Materials In Vehicles Increases Vehicle-Cycle GHG Emissions But Improves Fuel Economy

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The use of lightweight materials (carbon-fiber reinforced plastic, wrought aluminum, etc.) in vehicle manufacture results in higher vehicle-cycle greenhouse gas emissions, but also in improved fuel-economy, which leads to a net benefit as far as total life-cycle greenhouse gas emissions go, according to a new study from Argonne National Laboratory.

The improved fuel economy of lighter-weight vehicles is primarily the result of a reduced weight. As far as the differences between the various lightweight materials used as a substitute for steel — the use of wrought aluminum led to lower total life-cycle greenhouse gas emissions in all assumed cases, whereas the use of carbon-fiber reinforced plastic (CFRP) or magnesium only led to a lower net total in most cases.


The researchers behind the work used The Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation Model (GREET) developed at Argonne for the energy and emissions data utilized.

The new study was published in the ACS journal Environmental Science & Technology. Here’s an overview of the findings (courtesy of Green Car Congress):

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.

…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.

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.

Most greenhouse gas emissions associated with conventional vehicles (80-90%) are the direct result of gas or diesel consumption, so reducing vehicle weight is obviously (generally) an easy way of reducing the total carbon footprint. Another means is of course the lowering of the drag coefficient, which is something that is a major factor in the determination of electric vehicle range as well. On that note, the aim for the Tesla Model 3 is for the drag coefficient to be below 0.20, reportedly — which will play into the achievement of presumably a rather low net life-cycle greenhouse gas emissions tally for the car.

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James Ayre

James Ayre's background is predominantly in geopolitics and history, but he has an obsessive interest in pretty much everything. After an early life spent in the Imperial Free City of Dortmund, James followed the river Ruhr to Cofbuokheim, where he attended the University of Astnide. And where he also briefly considered entering the coal mining business. He currently writes for a living, on a broad variety of subjects, ranging from science, to politics, to military history, to renewable energy.

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26 thoughts on “Study: Lightweight Materials In Vehicles Increases Vehicle-Cycle GHG Emissions But Improves Fuel Economy

  • The GHG to create a KG of Aluminum or Magnesium? I’m not an expert, but I think these are both made using electricty as the primary energy input, wheras steel uses coal. It should be easier to decarbonize the creation of these metals than to decarbonize steel.

    • The assumption in the article is that cast Al is recycled and that wrought Al is ‘virgin’, and will require more energy. I believe this assumption is wrong, as nearly all Al is recycled and the ‘purification’ step in the process is rather simple gravitic separation.

      • Seems that this is a “today” but probably not a “tomorrow” study.

        As grids use less fossil fuels and as more aluminum enters the ”aluminum cycle” the carbon output for aluminum should drop.

        Cleaning the grid is obvious.

        Aluminum is 100% recyclable. Currently about 70% of aluminum is recycled. And the rate should increase with cars as they won’t go to landfills as happens with a lot of our stuff.

        Bubba can put his aluminum bodied pickup up on blocks out back and the aluminum will still be recyclable after he kicks and the county comes to clean up the mess he left.

    • Iron (and thus steel) can be made electrically, just like aluminum. The coal process is easier and cheaper, but certainly not the only viable one.

      For steel recycling, electric arc furnaces are already the dominant mode of smelting.

      If, as some people predict, renewables will eventually reduce the cost of electricity to a level considerably below what it is today (or a significant carbon tax is levied globally), electrically produced steel might take off.

    • I think the point I was trying to make (and I think most cleantechnica commeneters implicitly get) is that the GHG content of material inputs is time dependent, and some of these materials GHG content may drop dramatically in the future.

  • It would be simpler to start with getting rid if wing mirrors. Webcams and head-up displays can do a better job.

    • Well, displays use energy too…

      • But for highway speeds, they save something like 5%, which is probably in the KW range. As compared to a few watts for displays, thats huge.

    • “There’s no silver bullet; it’s more like silver buckshot.” We can have lighter AND more aerodynamic cars!

    • I think there are regulatory issues involved.

    • Yes. Those are all non material design issues. Its scandalous how little attention is paid to aerodynamic drag right up to the present. Mirrors are an anachronism.

      • No, they’re not. They’re an essential safety feature for emergency situations.

        • But can they be done better with HUD and cameras.

          Example. Cameras can be mounted anywhere. Already they are coming on Prius and SUVs. You cannot see behind you properly with mirrors.
          And mirrors have a serious drag penalty.
          Now mind you, I am not saying they couldn’t be used for backup.
          Try this. Suppose the mirrors are there but folded. If the camera fails, the mirrors can be popped out.
          Lots of possibilities.

    • Absolutely not. If your electrics go out, you want the following things on your car to work:
      (1) brakes
      (2) all visibility features (windows, mirrors, etc.)

      There’s a reason these are supposed to be failsafe, and work even if the car’s main electrical system dies.

      It’s not worth making the car less safe in order to make it “more efficient”.

  • Could please somebody explain better? I’m missing the definition of “vehicle-cycle”. Plus, I can’t understand the difference between “total” and “net total”. So is BMW on the wrong path with their CFRP i3?

    • 1) Manufacturing a car body from Mg, aluminium or CFRP takes a lot more energy than making it from steel, hence the higher emissions from the vehicle’s life cycle (manufacturing, maintenance and dismantling).

      2) Lighter cars use less fuel (duh).

      3) Aluminium is a better choice environmentally than steel due to (2) in all cases. CFRP and Mg also beat steel in most cases, but lose out for people when your car doesn’t see much use. You’d need to drive the car for well over 200k miles in order to clearly beat steel. A BMW should get to 200k miles easily though.

      4) There are two types of aluminium: cast and wrought. Cast is heavier but can be made from lower grade recycled aluminium, wrought is lighter but made from virgin metal. This study shows that the cast aluminium is the best choice overall.

      What is the main conclusion from this study? That it’s not always the best idea to choose the lightest possible material. All lighter-than-steel materials beat steel, but the heaviest of those lighter alternatives (cast aluminium) is apparently the best.

      • Thank you.

      • Haven’t read the study, just this synopsis, but it very likely does not take into account the use of recycled carbon fiber. First production CF vehicles used only virgin CF with scrap factors of 30-35%. Over last 1.5 years they’ve progressed rapidly and currently scrap is now reintroduced into manufacturing (and it has under 1/10 the embodied energy of virgin CF.)
        As with aluminum, process also matters for energy (thus GHG) and it depends on whether they used dry fiber/RTM (resin transfer molding) process (i3) or pre-preg/press molding (Plasan and supercar folks). I should probably read the full study to check this when I have time. The net GHG at 260,000 km (162,500 miles) and 80% substitution should be the comparison standard, even though that is far in the future, and there CF composites win hands down – it’s all about weight/fuel economy then. Cast aluminum is also not viable for complete substitution by itself unless hi-pressure casting is used and even then it is limited, so the correct aluminum case should be developed only with appropriate balance of cast and wrought aluminum. Agree whole-heartedly that aluminum recycle is way under where it will eventually by, and it’s the intermediate opportunity for us (thanks Tesla, Ford) until carbon fiber manufacturing capacity gets to a decent scale (all CF produced – aero, auto, wind, consumer – is only around 80,000 MT/year now. We’re not ready for prime time!)
        Was improper and a major dis-service NOT to take the secondary weight savings into account as stated! Lots of room to increase the study benefit.

    • I think they mean “production phase” emissions vs total product life cycle. The emissions are greater in production phase but reduced during “use phase”.

  • Wood composites are less energy intensive. The problem until now was that there where no simulation models available to simulate crash behavior.

    Here’s a technology model from Magna..’Cult’ (Cars Ultralight Technology) with wooden base and other parts.
    They have developed a simulation model for wood and can now simulate these materials to substitute for high emission aluminum and heavy steel.

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