What is Lightweighting and How Does it Improve Fuel Economy in Vehicles?

August 24, 2020 | 3:19 pm
Jeremy Lipshaw

Leading up to the 2020 decision to replace the 2012 Corporate Average Fuel Economy (CAFE) standards or clean car standards, there was a noticeable change-in-tune within the regulatory bodies governing automotive fuel regulations. In the EPA’s July 2016 midterm review of the standards, they professed that “automakers can build a single fleet of vehicles across the US that satisfies all GHG/CAFE requirements” and “a wider range of technologies exist for manufacturers to meet the [model year] 2022-2025 standards, and at costs that are similar or lower, than those projected in the 2012 rule”. In 2018, the Trump Administration’s EPA reversed their position: “[We are] proposing new CAFE standards… because they are no longer maximum feasible standards”.

All of a sudden, in the view of those regulating bodies, the technology needed to meet the required standards vanished. As a young engineer who has recently entered the workforce, I can confidently say that the technology needed to meet the Obama-era clean car standards still exists, and, that they are plentiful and accessible to manufacturers.

Lightweighting helps increase your MPG

A popular technique used to meet fuel efficiency standards is lightweighting automotive components. Classical physics tells us that with a decreased vehicle mass, less energy is needed to accelerate it, resulting in less Greenhouse Gases (GHG) emitted (and increased miles per gallon, or MPG, of the vehicle). Experts estimate that for every 220-pound decrease in weight of a vehicle, there will be an associated 3-5% decrease in carbon emissions, depending on total vehicle size and powertrain type.

Lightweighting comes in many forms. While strictly using less material is the obvious approach, lightweighting bares its strength when manufacturers take advantage of multiple methodologies such as shape optimization, part consolidation, material substitution, technology innovation, and others. Of course, all modifications must go through rigorous testing to ensure that all strength and engineering requirements are properly met.

Through my schooling and the beginning of my career, I have observed how utilizing the metalcasting process can be an effective lightweighting solution as it can combine shape optimization, part consolidation, and material substitution all in one process. For example, in my senior design project, we were tasked with decreasing the cost and weight of an in-use stamped steel control arm by 10%. In only three months and by converting to a heat-treated cast iron, we met the cost goal and virtually met the weight goal with a 9% reduction. Think of what could have been done with full-time, experienced engineers.

These sort of lightweighting opportunities are not limited to senior design teams. Successful casting conversion projects, where existing multi-part wrought assemblies are converted to one or two-piece castings, are constantly being developed and only exemplify one approach to lightweighting.

Life Cycle Assessments prevent accidentally increasing GHG emissions

However, if the objective is truly to limit total GHG emissions, then lightweighting does not tell the whole story. Less dense materials tend to take substantially more energy to produce and recycle. This can minimize the advantages from lightweighting and may go as far as inadvertently causing a net increase in GHG emissions over the life of the vehicle. In order to prevent that outcome, “Life Cycle Assessments” (LCA) can be employed. LCA is a relatively new discipline that has increasingly been embraced by global regulatory agencies, with some going as far as decrying it as “the best framework for assessing the potential environmental impacts of products, process, and systems”.

Automotive LCAs incorporate the three major phases in a vehicle’s life to calculate variables such as total embodied energy, pollutants, and GHG emissions. The three phases consist of production, use, and end-of-life/recycling. The Union of Concerned Scientists has used LCAs to compare battery-electric vehicles to similar gasoline vehicles in order to determine which vehicle type produces less lifetime global warming emissions (spoiler: it’s battery-electric).

LCAs are progressively being used to gauge whether a lightweighted design truly provides a net decrease in GHG emissions compared to its previous design. One published example of a lightweighting concept that successfully utilized LCA in order to decrease net GHG emissions was the “2017 ultralight automotive door”. Applying a multi-material design which included aluminum, plastic, glass fiber-reinforced plastic composite, and other polymers, the team was able to achieve a 40% mass reduction while simultaneously decreasing the lifecycle GHG emissions and primary energy demand per door by 1.5 metric tons of CO2-eq and 21 gigajoules, respectively.

Yet, cars and SUVs are getting heavier

While lightweighting opportunities resulting in a net decrease of GHG emissions over the life of the vehicle (assuming all other factors remain constant) are extensive, it is important to acknowledge whether they are being used responsibly. A recent paper suggests that, despite the vast popularity of lightweighting, the weight of cars and SUVs have actually increased over time due to a phenomenon dubbed “rebound of design”.

“Rebound of design” describes the added features and weight due to consumer demand for safety and comfort or due to other regulatory requirements. As an example, the BMW 7-series added improvements to handling and safety which increased the mass by 150 lbs. Ultimately, the manufacturers were able to remove enough mass through lighter components to reduce the weight compared to its predecessor; however, many manufacturers are not as lucky. (This is not to suggest that lightweighting compromises safety. A 2017 Yale study provides evidence that lowering the average weight of vehicles will save lives and offsets the negative effects of an increased weight differential between vehicles.)

Therefore, it could be said that, in a systems’ perspective, the overall benefits from lightweighting components have effectively been cancelled out Even worse, for vehicles that have increased weight and utilized material substitutions that switched to less dense, more energy-intensive materials, the net greenhouse gas impact of the vehicle has likely increased. Fortunately, this phenomenon comes with a bright side: manufacturers have been able to hit MPG targets purely through improvements in engine efficiency and other improvements within the powertrain. Consequently, it can also be concluded that the potential benefits of lightweighting have yet to be fully explored!

The 2012 standards are successful and attainable

The 2012 CAFE standards were predicted to have saved 190 million metric tons of GHG emissions. I can back what the 2016 midterm review proclaims: there is a wide range of technologies available to continue improving a vehicle’s MPG at its current rate. I am not alone recognizing that more can be done: BMW, Ford, Honda, Volvo, and Volkswagen are all in agreement.

The 2020 SAFE rule is a major step backward in the mitigation of climate change and not only should it be rescinded, but policymakers should aim for higher standards in 2026 and beyond. Since the previous 2012 standards only dealt with the GHG emissions generated from the tailpipe during the use-phase, policymakers and automakers should adopt Life Cycle Assessments as a tool and regulatory benchmark for future standards. By utilizing Life Cycle Assessments and responsible lightweighting, automakers can continue to meet the 2012 standards and guarantee that they will minimize GHG emissions throughout the lifecycle of their vehicles!

Jeremy Lipshaw graduated from the University of Michigan with a Bachelors in Materials Science and Engineering in 2017 and a Masters in Mechanical Engineering in 2018. In addition to his studies, Jeremy worked at a local foundry where he was exposed to countless facets of the industry. After graduation, he joined the heat treatment industry, focusing on characterizing new materials for heat treatment and assisting in the design of lightweight casting conversions. Jeremy is passionate about sustainability and foresees Life Cycle Assessment as a key tool for industry and regulators to help meet carbon targets.

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