The Case for ULTRA LIGHT Fabricated Assemblies
There is a compelling case for Fabricated Lightweight Vehicle Assemblies made by joining high quality castings, extrusions & sheet products into a single ultra light assembly. Such fabrications could include combinations of aluminum alloy & magnesium alloy parts joined together by welding, riveting and/or industrial adhesives to satisfy the structural requirements at the lightest possible overall design weight.
Single part replacement which is often employed to reduce weight with magnesium die-castings has a somewhat marginal effect on reducing overall vehicle weight because it incorporates only a single lightweight component into an existing assembly. Such a practice ignores substantial secondary weight saving opportunities that arise from optimizing the overall design of the structure.
The importance of primary & secondary weight savings was demonstrated in the US Automotive Materials Partnership (“USAMP”) published report “Magnesium Vision 2020”. USAMP used a detailed component-by-component analysis to determine that a significant 227 kg (15%) weight saving for the average North American vehicle would result from the use of Fabricated Magnesium Assemblies. Study results show that primary savings of 132 kg can be obtained by replacing 227 kg of steel & 59 kg of aluminum with 154 kg of magnesium. Importantly, an additional secondary savings of 95 kg (42% of the total) could be realized by optimizing the design of the resulting lightweight structure.
Such large weight savings are only possible if the industry utilizes a strategy to incorporate optimized ultra-light assemblies fabricated from high quality weldable castings, extrusions & sheet products such as shown in the left hand figure.
Using a Cadillac unibody steel front end as a Baseline, USAMP designed and tested an optimized Ultra-light Magnesium Assembly prototype front end fabricated from high quality vacuum die-cast, extruded & sheet magnesium alloys.
In this case, the Baseline consisted of 110 individual steel component parts weighing a total of 99.6 kg.
The optimized Magnesium Fabricated Assembly yielded
- a ~60% reduction in the number of individual component parts
- a ~45% weight saving over the Steel Baseline, and,
- a ~25% weight saving over a fully Al design
The USAMP analysis confirmed that the Fabricated Magnesium Assembly satisfied the following critical design requirements
- Crashworthiness
- Noise, vibration & harshness
- Fatigue
- Durability & corrosion resistance
- Joining & fabrication
In this case, the USAMP prototype Fabricated Assembly used only Magnesium parts to reduce design & manufacturing complexity and to achieve a significantly lower overall front end weight. Similar opportunities exist with multi-material fabrications utilizing combinations of Aluminum & Magnesium alloy cast, extruded & sheet parts joined together to achieve optimized mechanical properties & weight savings.
CO2 & Fuel Savings for Internal Combustion Engine Vehicles
While reduced weight improves vehicle handling & braking distances, the main driver for Weight Saving in internal combustion engine (ICE) vehicles is the need to reduce fuel consumption & CO2 emissions (e.g USA government CAFE fuel standards).
A number of studies have reported that ICE vehicles achieve
- about a 7% reduction in fuel consumption from every 10% reduction in vehicle weight, and,
- a 10.84 kg reduction in tailpipe CO2 per 1,000 km driven from every 100 kg reduction in vehicle weight ( F.D’Errico et al. (JOM, Vol. 61, No.4, 2009)
As shown in the LCA below, Tailpipe Emission Reduction from Weight Savings can be calculated using the method described by D’Errico et al. which assumes the average vehicle travels 20,000 km per year over its lifespan of 10 years.
Electric Vehicles (EV) Weight
consumer obsession with large vehicles & extended range has driven up EV weight at a much faster rate than ICE vehicles. [2]
Electric Vehicles (EV) need electricity consumption regulati
Even though EVs do not have tailpipe emissions, the need for vehicle light-weighting is still compelling for the following reasons.
- High Vehicle Weight - the average EV currently weighs ~2133 kg with some being 3000 kg & more. By comparison, the average ICE vehicle weight currently ranges between ~365 kg lighter in the USA and up to more than 500 kg lighter in Europe [1]
- Increased EV Weight increases Energy Consumption - a recent study shown by the graph to the left confirmed an almost linear relationship exists between watts per mile and total EV weight. [3] The following equation was developed using data in the 627-1822 kg range. Assuming this linear relationship provides a good estimation of energy utilization for EV's outside the weight range, it can be used to predict the energy savings from EV lightweighting.
watts per mile = 0.1305 EV Weight (kg) + 64.15.
- Increased EV Weight Reduces Range & Requires Longer Charging Times - Heavier EVs have reduced Range for a given battery size because proportionally more battery energy is needed to propel the additional weight. To put this into perspective, the 500 kg EV weight increase between 2019 & 2021 shown in the figure above would have resulted in a decrease of vehicle Range by ~11%. To recover the lost Range, battery size is steadily increasing which in turn results in longer charging times.[4]
- Increased EV Weight's Impact on Electricity Generation - a complete transition to EVs in the USA would increase demand for electricity by around 30% (an extra 1,182 TWh) if the future fleet has the same average size, weight and horsepower as the current 2011-2021 EVs. [5] As noted above, EV weight has increased sharply by 33% (~500 kg) between 2019 & 2023 which would represent a 33% increase in electricity demand (~390 TWh). Continued weight increases will only exacerbate the increasing demand for electricity
- CO2 Emissions from Electricity - The 2019 IEA Global Energy & CO2 Status Report indicated that the world average carbon footprint for electricity generation was 0.475 kg CO2 per kWh [6], There is an obvious need & expense to install significant new renewable electricity generation capacity to mitigate the potentially huge quantity of additional CO2 generated by EV recharging.
In summary, transitioning the world's ~90 million fleet of new vehicles produced each year to EVs will create a huge government expense for additional electricity generation capacity. To avoid a significant increase in CO2 emissions, this additional capacity will have to transition to renewable energy generation. Given the expense & level of regulatory approval, such a transformation can not happen overnight. In the face of such enormous capital costs, governments need to begin to impose regulations on EV energy efficiency (similar to CAFE fuel regulations) which will no doubt bring accelerated demand for lighter weight EV.
References
[1] https://www.sustainabilitybynumbers.com/p/weighty-issue-of-electric-cars
[2] https://robbieandrew.github.io/EV/
[3] https://www-esv.nhtsa.dot.gov/Proceedings/22/files/22ESV-000346.pdf
[4] https://energy5.com/effect-of-vehicle-load-on-ev-battery-performance
[5] https://www.sciencedirect.com/science/article/abs/pii/S0301421521006121
[6] https://www.iea.org/reports/global-energy-co2-status-report-2019/emissions
References
[1] https://www.sustainabilitybynumbers.com/p/weighty-issue-of-electric-cars
[2] https://www.iea.org/reports/global-fuel-economy-initiative-2021/executive-summary
[3] https://www-esv.nhtsa.dot.gov/Proceedings/22/files/22ESV-000346.pdf
[4] https://www.utilitysmarts.com/automobile/gasoline/how-much-co2-does-1-litre-of-petrol-produce/
[5] https://www.iea.org/reports/global-energy-co2-status-report-2019/emissions
[6] calculated using method reported by F.D’Errico et al. JOM, Vol. 61, No.4, 2009
[7] calculated using the projected energy consumption for the ave EV weight from ref [3] & the average world carbon footprint of 0.475 kg CO2 per kwh from ref [5]
Conclusions
EV & ICE Vehicle Carbon Footprint
Assuming the current world mix of fossil fuel generated & renewable electricity, the Life Cycle Assessment (LCA) indicates adopting EVs worldwide will contribute just over half of the ICE emissions from electricity generation. The need for new renewable electricity is clearly evident but this transition will no doubt take time & will be very costly given the amount of electricity required.
Effect of Lightweight Vehicles on CO2 Emissions
Assuming the same principles demonstrated in the USAMP Study will apply to both EV & ICE vehicles, a 227 kg vehicle weight reduction would reduce ICE tailpipe CO2 emissions by almost 12% and EV CO2 emissions from electricity generation by almost 9%. This reduction in greenhouse gases represents an important contribution especially during the extended period needed for the world to transition away from fossil fuels to cleaner electricity generation.
Magnesium can play an important role
While there are several lightweight material options, the very low carbon footprint, competitive production cost, low CAPEX & high weight saving potential associated with magnesium produced with Tech Magnesium's ULTRA CLEAN Aluminothermic Process makes it an ideal choice. The life cycle assessment (LCA) confirms the cumulative CO2 accrued from vehicle weight reduction breaks even with the CO2 from ULTRA CLEAN magnesium production in only 1.02 years for ICE vehicles & in 5.26 years for EVs. Given aluminum's ~25% higher weight per part, replacing ULTRA CLEAN magnesium alloys with aluminum alloys (even from the cleanest Al plants) would extend the break even threshold. Once the break even point is reached, both EV & ICE cars will provide the full CO2 saving annually (454 kg for ICE & 174.9 kg CO2 per year per vehicle until the end of their lifespan. Considering that world car production is ~90 million units per year, the cumulative greenhouse gas saving potential from lightweight EV & ICE vehicles is huge.
Energy Saving
As shown by the LCA, lightweight vehicles will save substantial amounts of energy. For ICE vehicles, the LCA weight reduction is expected to save 0.9171 liters of gas per km. For every 1 million similarly lightweight cars driving 20,000 km per year, annual gasoline savings are estimated to be in excess of 18,340 million liters. For each lightweight EV travelling the same 20,000 km annually, the electricity saving is estimated as 368.2 kWh or 368.2 million kWh of electricity saved per million vehicles. Given the capital expense and complexity to add new renewable electricity generation worldwide, such electricity savings represent a huge cost saving opportunity for governments worldwide. The need for electricity consumption regulations on EVs similar to government fuel standards for ICE vehicles is evident.