Fuel Economy and Emissions


The National Highway Traffic and Safety Administration (NHTSA) finalized Corporate Average Fuel Economy (CAFE) standards for model years 2012-2016 and 2017-2021 and model years 2022-2025 under the Energy Policy and Conservation Act, as amended by the Energy Independence and Security Act. The new vehicle regulations include a “mid-term review” to be completed in 2018 which may impact 2022-2025 targets.

Figure 2 shows the new fuel economy standards increasing to 54.5 miles per gallon equivalent (mpge) fleet average between passenger cars and light trucks. The new CAFE standards are different than previous standards as they are coupled with greenhouse gas (GHG), or CO2e emission standards. For vehicles sold in the U.S., EPA finalized greenhouse gas emission standards for model years 2012-2016 and higher targets for the model years 2017-2025.

Fuel economy improvements are also linked to progressively more difficult new vehicle tailpipe emission standards across the globe. Those U.S. vehicle manufacturers selling models globally must consider these additional standards together with the various standards that exist in different countries.

Figure 3 shows the progressive reduction in vehicle emissions by year through 2025 for U.S., Canada, Australia, South Korea, China, the European Union (EU) and Japan. U.S., California and Canadian standards converge to the same value in 2016. In the 2020-to-2025, the standards fall in a small normalized range between 95 and 117 grams of CO2 per kilometer.

Figure 2. Increasing Vehicle Miles per Gallon – Reaching 54.5 MPGe in 2025

Figure 3. Increasingly Restrictive Global Vehicle Emissions Standards

Reducing Emissions through the Vehicle Life Cycle

The life cycle of a vehicle has three phases: production; use (driving); and end-of-life disposal (recycling), as illustrated in Figure 4. In considering the goal of emissions reduction with respect to vehicles, it makes sense to consider the entire life cycle of a vehicle.

Figure 4. The Life Cycle of Steel

The production phase accounts for up to about 30 percent of total GHG emissions for internal combustion engine and hybrid electric vehicles, and as much as 47 percent for battery electric vehicles (BEVs). As automotive fleet fuel economy increases and as the share of alternate power train vehicles, such as BEVs, also increases, production emissions will become a greater proportion of the total emissions – and thus more important to achieving the goal of reducing emissions.

Production emissions are an important environmental consideration because, in addition to the widespread adoption of AHSS, automakers are also considering aluminum, magnesium and carbon fiber. Producing primary aluminum ingot in North America currently generates at least four times the emissions of producing steel, (1.9 ton CO2e / ton for steel vs. 8.94 ton CO2e / ton aluminum).

These figures employ the aluminum industry’s assertion aluminum smelters in North America operate using 75 percent hydropower.  Power accounting methods using regional grid information and / or recognizing the use of imported aluminum ingots would skew production emissions even higher. Production of the other lightweighting materials (magnesium and carbon fiber reinforced composites) can generate 20 times the emissions of steel.

Thus, from a life cycle perspective, these production phase emissions from the manufacturing of materials used in vehicles result in a substantial environmental impact. However, they are not accounted for in current fuel economy regulations or factored into most automotive design practices. Once emitted, GHGs immediately begin absorbing energy from the sun leading to warming of the atmosphere. Major GHGs remain in the atmosphere for a range of seconds (ex. 10 to 100s methane) to years (ex. CO2) after being released. Therefore, the timing of GHG emissions is another important consideration.

It is anticipated the materials production phase, if not the entire life cycle, of the vehicle will be addressed through regulation at some point in the future.


Mass Reduction through AHSS and Optimized Vehicle Design

An assessment of the contributors of energy losses influencing fuel economy performance is shown in Figure 5. This figure shows the powertrain system made up of the engine and transmission, along with aerodynamics and tire rolling resistance have the most impact on vehicle fuel economy. While vehicle mass is responsible for only about 11 percent of the energy losses in driving a typical sedan. This, however, is the portion material selection can influence. Reduction in vehicle mass through the use of AHSS offers the highest value to the automaker. This savings can in turn be used to fund improvements in powertrain technologies.

(Source: 54.5 CAFE: Challenges and Opportunities, IABC 2013)

Figure 5. Distribution of Sedan Energy Losses

Figure 6 shows how the expected technology needs support fuel economy improvements with the main contributions coming from powertrain improvements, shown here as approximately 13 mpg and mass reduction, shown at about 6 mpg, with a gap for other technologies around 2.5 mpg. Steel’s opportunity to contribute to fuel economy improvements is through vehicle mass reduction mainly in the body, closures and chassis / suspension systems which make up more than 50 percent of the mass of the vehicle as shown in Figure 7.

(Source: ArcelorMittal)

Figure 6. Technology Needs for Fuel Economy Improvements

Figure 7. Vehicle Mass Breakdown by Major Component Areas

The “Volpe Model,” used by NHTSA to develop the current regulations, shows the fuel economy gap produced by the shortfall in powertrain improvements along with the relationship to mass reduction seen in Figure 8. The green squares indicate the fuel economy target is met and the red bars indicate there is a gap in achieving the fuel economy target. With average mass reductions of 25 percent obtained when applying AHSS to the average vehicle, fuel economy targets will be met unless the expected powertrain improvement shortfall is more than 10 percent.

(Source: ArcelorMittal)

Figure 8. The Interpreted Volpe Analysis – AHSS can close the weight reduction gap even with some limited powertrain improvement short fall.

There are several studies and examples later in this document offering weight reduction solutions for vehicles. Additionally, steel companies both collaboratively, under the Steel Market Development Institute (SMDI), and individually have successfully demonstrated mass reduction with individual automakers.

Life Cycle Assessment (LCA)

As explained earlier, vehicle mass reduction offers improvements in fuel economy but at a lower impact than other technologies such as engine and transmission or aerodynamic improvements.

Figure 9 shows the material production emissions of the common body and chassis materials along with their average mass savings potential. Even with more conservative CO2e values for material production, such as 8.97 kg CO2e per kg aluminum, lightweighting with AHSS offers significant emissions advantages over other automotive materials.

Figure 9.  LCA Emissions from Material Production

Table of Contents | Introduction | Fuel Economy and Emissions | Vehicle Strength | Additional Steel Industry Solutions | Future Steel Solutions | Primary Strategic Areas | Download the full PDF