an additional 0.7 to 1.8 times the initial mass change. Comments by the Aluminum Transportation Group have estimated a factor of 64 percent for secondary mass reduction (NHTSA, 2010). The 2011 National Research Council (NRC) report Assessment of Fuel Economy Technologies for Light-Duty Vehicles pointed out the importance of secondary weight reduction “as the mass of a vehicle is reduced … other components of the vehicle can be reduced … for example brakes, fuel system, powertrain, and even crash management structures” (NRC, 2011, p. 113). It discussed a rule of thumb that for every pound saved in the design through material substitution or structural modifications, an additional 30 percent of the weight savings in secondary systems could be saved (NRC, 2011).
Potential Cost Impacts
Cost estimates for reducing vehicle mass have varied significantly. One difference is the cost savings from secondary weight reduction which can offset some of the costs related to lightweight materials and improved structural design. In this context, the net costs for mass reduction should include the secondary weight and drivetrain downsizing that are directly related to mass efficient vehicle designs. The impacts of weight reduction on drivetrain costs are discussed below.
NHTSA and EPA summarized three studies, which were first used in the 2012-2016 CAFE rulemaking, that concluded that weight could be reduced for approximately $1.50 per pound. Additionally, Sierra Research estimated a 10 percent reduction, with secondary weight reduction, could be accomplished for $1.01 per pound. The Massachusetts Institute of Technology (MIT) estimated that the weight of a vehicle could be reduced by 14 percent with no secondary weight reduction, for a cost of $1.36 per pound. The final NHTSA/EPA cost estimate for the 2012-2016 rulemaking was $1.32 per pound and was based on the average of the three referenced studies (NHTSA/EPA, 2010).
The 15 percent reduction in total vehicle weight estimated by IBIS for the Aluminum Transportation Group discussed above was estimated to cost $0.18 per pound. This cost was significantly less than the $1.32 per pound used in NHTSA/EPA’s rulemaking analysis—an estimate that did not account for secondary weight savings.
Downweighting is even more cost-effective for battery-powered vehicles (or other high-cost propulsion systems) because of the potential savings in battery/energy storage. The Aachen and IBIS reports produced detailed designs using aluminum intensive BIW and Closures with weight savings of 19 percent of total vehicle weight. The increased cost of aluminum was estimated at $630. Cost savings in the study were estimated at $450-$975 for the batteries (using $375/kWh).
The Lotus study estimated that a 21 percent mass reduction could be achieved by 2020 using high-strength steel with no cost impact. A 38 percent mass reduction could be achieved by 2020 with a moderate cost growth (e.g., a 3 percent increase in vehicle cost using aluminum, magnesium, and composites; Lotus, 2010).
For the 2017-2025 proposed rule, NHTSA and EPA updated their analysis of existing cost studies. Currently the government is proposing a formula that assumes mass reduction increases in cost as the absolute size of mass reduction increases, e.g., $4.32 × % weight reduction. Table F.1 shows the results over a range of mass reduction.
Down-weighting battery powered (or other high cost propulsion systems) vehicles is even more cost effective because of the potential savings in battery/energy storage (Ricardo, 2011).
Carbon fiber/plastics may also make a significant impact on mass reduction if costs are reduced: “Conservative estimates are that carbon fiber PMC can reduce the mass of a steel structure by 40 to 50 percent (Powers, 2000)” (NRC, 2011, p. 102). The 2011 NRC report states “that the price of carbon fiber has to fall to $5 to $7 per pound (about 50 percent) before it can be cost competitive for high-volume automobiles (Carpenter, 2008)” (NRC, 2011, p. 102). Research conducted at ORNL suggests that if a vehicle design with a weight reduction of 50 percent was achieved with a 50/50 mix of plastic resin (1.00 $/#) and carbon fiber (7.00 $/#), then an average cost for using carbon fiber/plastic would be $3 to $4 per pound at a high production volume (10 million pounds per year) (ORNL, 2008).