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 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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