Climate Change and Greenhouse Gas Emissions

Globally, existing or proposed regulations regarding vehicle GHG emissions address only the use phase (driving) of a vehicle’s total life cycle. From this perspective it is easily understood that, assuming all other things are equal, a lighter weight vehicle results in reduced fuel consumption and consequently reduced use phase GHG emissions.

Material choices that result in the lowest mass vehicle may be preferred if one considers only a vehicle’s use phase (Figure 1).

Figure 1

However, to fully assess a vehicle’s environmental footprint, all vehicle life phases must be considered. This includes the GHG emissions resulting from materials production, the manufacturing of the vehicle, the use phase and the end-of-life phase.

This approach, which considers all aspects of vehicle life (see Figure 2), is called Life Cycle Assessment (LCA) and it is recommended for evaluating a product’s impact on climate change.

Figure 2

The global warming potential of any GHG is usually measured in kilograms of carbon dioxide equivalents (CO2eq) accounting for the GHGs shown in the following table:

  Table:  Global Warming Potential of Key Substances

 

Material selection is a critical determinant of GHG emissions during the material production phase. As shown in the next table, alternatives to steel in automotive structural applications produce 5 to 20 times as much GHG emissions during their production (see Table following).

Energy sources used in material production (e.g. coal, hydro, petroleum, etc.) significantly affect the amount of GHG emissions in LCA studies. Analysis of material selection decisions that require new material production capacity must incorporate the impact of new marginal production energy sources. For example, hydro power may be used for some current production, but new production likely requires another energy source such as coal.

  Table: GHG emissions of various materials

GHG emissions from steel production consist of only carbon dioxide, whereas GHG emissions from aluminium production consist of carbon dioxide and up to 20% perfluorocarbons (CF4 and C2F6), and magnesium production generates up to 20% Sulphur Hexafluoride (SF6).

Consequently, alternative material applications front-load the environment with more GHG emissions resulting from material production than the steel application they replace. In the case where the alternative material results in reduced mass and reduced fuel consumption, the GHG emission improvement achieved during the driving phase is unlikely to offset the upfront loading of the material production phase when compared to optimized designs with AHSS.

Typical vehicles built with alternative materials will often net more GHG emissions during their lives than AHSS-intensive vehicles.

An LCA approach is the correct approach for assessing a vehicle’s climate change footprint and requires vehicle manufactures to balance the possible driving phase improvements against the manufacturing phase disadvantages when considering GHG - intensive materials, such as aluminium, magnesium and plastics.

To investigate the aspects of material selection on automotive LCA GHG emissions, a study entitled The Impact of Material Choice in Vehicle Design on Life Cycle Greenhouse Gas emissions - The Case of HSS and AHSS versus Aluminium for BIW applications (see Related Documents) was conducted at the University of California, Santa Barbara (UCSB) Bren School of Environmental Science and a peer review model for material comparisons was developed.

Case Study Examples

Consider two case study examples, using the UCSB model, based on a C-Class vehicle with a gasoline internal combustion engine. The case studies focus on the body-in-white and assume 25% mass reduction from a conventional steel baseline for AHSS and 11% further mass reduction for aluminium, along with additional secondary weight savings in both cases. Fuel savings and driving cycles are based on the fka study.

The UCSB model calculated GHG reduction that is achieved by optimizing the design with AHSS compared to conventional mild steel (Figure 3a). This is the situation of ‘steel re-inventing itself’ and replacing former steel materials and design with new steel materials and design.

Figure 3b Life Cycle GHG Comparisons: AHSS vs. Aluminium

The effect of 25% mass reduction in the body-in-white (the equivalent of a 9% total vehicle mass reduction when secondary mass savings are also included) is to reduce CO2 equivalent emissions in both the material production and use phase so that the vehicle’s total life cycle emissions are reduced by 5.7%. This is accomplished at no additional cost.

The UCSB model also compared an optimized aluminium design with the AHSS design (Fig. 3b). Although, this scenario assumes some additional mass savings can be achieved with aluminium, the increase of CO2 equivalent emissions from the material production phase more than offsets the reductions generated in the use phase.

Figure 3a Life Cycle GHG Comparisons: Conventional Steel vs. AHSS

The vehicle’s total life cycle emissions are increased by 2.6%. Furthermore, this environmental burden also comes with a significant cost increase.

The AHSS design advantage over aluminium depicted in this case study represents a relatively small percentage of the total vehicle GHG emissions.

In fact, the preferred material depends on the assumptions and inputs for the specific application and manufacturing processes. So although the preponderance of reasonable inputs demonstrates AHSS to be the preferred material over aluminium, there are sets of assumptions where the conclusion could be reversed.

Regardless of all reasonable inputs, the impact of material production and recycling on LCA GHG emissions are relatively small compared to total emissions, and significant improvements in reducing automotive GHG emissions will not be made by material substitution alone.

Using the LCA approach, comparisons can be made among other advanced automotive capabilities, such as powertrain, fuel choices and driving scenarios that are emerging into mainstream automotive technologies.

Figure 9 compares Conventional steel and AHSS body structures to aluminium and sheet moulding compound (SMC) body structures along with the cumulative impact of powertrain and fuel technologies on the total life cycle GHG emissions. The comparison finds that use of these upcoming technologies can have a dramatic influence on the total LCA GHG emission of a vehicle. The use of advanced powertrains (such as hybrids), advanced fuels (such as grain and cellulose ethanols) can result in a dramatic reduction in the use phase GHG emissions.

Figure 9 Life cycle GHG comparisons – powertrains & fuels

A key point, demonstrated by this graph, is that although the material production phase GHG emissions remain the same, they become a much more significant percentage of the total LCA GHG emissions as use phase efficiencies are achieved.

It is concluded that as other green technologies that improve vehicle GHG emissions are implemented in mainstream vehicle designs, the emissions from material production will become more important, placing greater emphasis on selecting a low GHG-intensive material such as steel.