Automotive Mass Benchmarking – The Advantages of Steel Over Aluminum

The WorldAutoSteel report ‘A new paradigm for automotive mass benchmarking’ examines critically the claims made by both the steel and aluminum industries regarding mass reduction in the manufacture of modern vehicles.

The vehicle weight has a direct effect on the powertrain required by the car, and consequently, its fuel efficiency and ‘green credentials’. Advocates of aluminum have traditionally claimed weight savings of up to 50% when compared to conventional mild steel construction.

However, the steel industry have countered that by applying modern high-strength and advanced high-strength steels (AHSS) to car construction, and claim they can make weight savings of up to 25% and often provide more mass reduction than aluminum.

New Statistical Benchmarking System

Car manufacturers are often faced with the dilemma of understanding the light-weighting potential of materials for new vehicle production. The most usual method of obtaining data about light-weight components is by benchmarking. In this method, a suitable reference vehicle is selected (usually similar in size to the new car and the most mass efficient).

The teardown mass data of the reference vehicle is then used to set targets for the new vehicle design. But how can we know if every subsystem is truly mass efficient? Without data on the most mass efficient components in every part of the vehicle, informed and aggressive decisions cannot be made to produce mass efficient designs.

A new statistical approach, developed by Dr Don Malen at the University of Michigan and Jason Hughes of ‘A2MAC1 Automotive Benchmarking’, applies benchmarking to the light-weighting process during the primary planning phase for new vehicles. This allows the most light-weight efficient design for each component to be accessed and analyzed prior to making new design decisions.

By using a large vehicle database for benchmarking, e.g. the A2MAC1 database with 240 vehicles, systems can be compared across the industry and the best and most mass efficient designs identified.

Steel vs. Aluminum

The new benchmarking study ‘A new paradigm for automotive mass benchmarking’ was commissioned by WorldAutoSteel and the Steel Market Development Institute (SMDI) and conducted by EDAG Int’l, Inc. using the powerful new statistics-based benchmarking methodology to benchmark vehicle closures and body structures.

The study showed that aluminum body structures deliver mass savings of between 16-40% relative to older and outdated conventional mild-steel designs. The new AHSS optimized designs provided a favorable comparison for steel with mass savings of between 21-25% over older designs.

This verifies that newer advanced steel automotive designs compare very favorably to their aluminum counterparts in reducing the mass of modern vehicles. Comparing efficient steel designs to aluminum designs in the study, the mass savings achieved were significantly decreased, and sometimes resulted in a mass increase for aluminum compared to steel.

Further benefits have also been established by the UltraLight Steel Auto Body (ULSAB) consortium and the majority of major auto-manufacturers who have adopted new optimized AHSS designs with significant improvements in both crash safety and performance. Steel has even more potential with state-of-the-art GigaPascal strength steels with their increased ductility and flexibility to provide potential design and weight improvements across the automotive sector.

Mass Savings are Not Always Realized at the System Level

An important finding of the benchmarking study was that mass savings that can be realized at the component level are not always coming through to the system level. The reasons for this are not clear at the current time and further study will be necessary.  This is a focus of a 2016 study update.

However, speculation blames over-engineering at the design stage where elements of a system are not downsized to take into account parity with new rationalized design constituents. It has been mooted that mass savings are made to allow the inclusion of other heavier parts of the design.

Curb Weight is an Issue

Curb weight is a crucial factor in automotive design, and the EDAG study demonstrated a narrow margin between vehicles using efficient steel structures and aluminum structures. Data indicated that aluminum efficient structures reduce total system vehicle curb weight by 9.3% compared to average older design steel structures.

However, newer efficient steel structures give a total system reduction in vehicle curb weight of 6.5% compared to older steel structures. This means that aluminum has only a 2.8% advantage over high efficiency steel.

Factors in Fuel Economy

A significant factor in influencing fuel economy is the type and size of powertrain in the vehicle. The figures most often quoted are that a 10% reduction in mass can result in a 6-8% reduction in fuel consumption.

A previous study by FKA looked at the influence of weight elasticity on fuel economy for a range of vehicles in different driving cycles and with different powertrains. The study concluded that weight elasticity values can vary from 1.9-8.2% depending upon driving cycle, vehicle size, powertrain selection, and whether the powertrain is adjusted for equivalent acceleration for the reduced weight vehicle.

In theory, 6-8% reductions in fuel consumption are possible with powertrain resizing for equivalent acceleration using conventional gasoline powertrains. However, in practice this is not possible because manufacturers do not have enough powertrain systems to apply to changes in vehicle weight, and more importantly, the market demands more emphasis on acceleration than fuel economy in engine technology.

More realistic figures are 2-4% for weight/fuel consumption elasticity values. FKA have also reported that gasoline powertrains have more scope for weight/fuel consumption improvements than advanced powertrain systems such as hybrids, electric and fuel cell.

Conclusion

Older steel designs may be inefficient from a mass point of view, and thus lose out to the lower weight of aluminum. Newer highly efficient steels can provide a lighter and stronger design, and can in some cases have a lower weight than the equivalent aluminum design.

Modern lighter steel designs have a lot of potential in the automotive industry. The industry should adopt the new statistical regression analysis benchmarking system and identify where mass reduction savings can be made.

Automotive design has the potential to be leaner, lighter and more fuel efficient, and a comprehensive statistical analysis of whole vehicle subsystems provides an excellent place for the designer to begin.

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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