indicated that universal material substitution (such as a switch from steel to aluminum body in white (BIW)1 structures) would not be feasible across all body lines in the 2017-2025 timeframe. In the TAR covering 2017-2025 Model Years, the government stated that “the ability of the industry to reduce mass beyond 20% while maintaining vehicle size … is an open technical issue” (EPA-NHTSA-CARB, 2010, p. 3-8).
The Partnership for New Generation Vehicles research effort from 1994-2002 was an early effort to conceptualize and build highly fuel efficient vehicles. The mass reduction goal was 40 percent. Actual vehicles achieved a mass reduction of 20 to 30 percent (NRC, 2001).
A recent study by the University of Aachen, done for the European Aluminum Association, looked at weight reduction opportunities for aluminum versus steel for subcompact and medium-sized passenger vehicles, crossover vehicles, and small multi-purpose vehicles. The Aachen study looked at optimizing the BIW and closures with aluminum intensive designs and concluded that a 40 percent weight savings in these areas was possible. BIW and Closure Reductions of 40-45 percent translate to an incremental (taking into account aluminum content already in standard production vehicles) 10-11 percent total vehicle weight reduction and with secondary weight savings yield approximately a 15 percent reduction in total vehicle weight (Aachen, 2010).
The 15 percent weight reduction of the total vehicle was repeated in detailed design studies by IBIS Associates, Inc., although secondary weight savings and use of lightweight materials in the rest of the body would result in much greater overall weight savings (IBIS, 2008). An interesting aspect of the Aachen study is that it looked specifically at the use of the aluminum-intensive parts from the standpoint of vehicle stiffness (handling, comfort, noise) and strength needed for managing crash energies and constrained the proposed design to meet or exceed current vehicle BIW performance when it quantified weight reduction opportunities.
Lotus showed similar conclusions to the Aachen study regarding BIW weight savings (Lotus, 2010). The Lotus study evaluated the total vehicle design and hypothesized a “high development” vehicle using an aluminum/magnesium intensive design with an overall weight reduction of about 40 percent. The primary areas of mass reduction are:
The aluminum industry sponsored studies, which looked strictly at weight reduction for the BIW and closures with associated secondary weight reduction, are in agreement with the Lotus study for similar areas of the vehicle. Lotus also used increased aluminum as part of the suspension and chassis optimized design.
Polymer-matrix composites (PMC, e.g., carbon fiber) have the potential to make a significant further contribution to reducing mass if the production costs of such materials can be reduced with mass production. “Conservative estimates are that carbon fiber PMC can reduce the mass of a steel structure by 40-50 percent …” (NRC, 2011, p. 102). However, there are currently production concerns for using carbon fiber in mass-produced vehicles. Currently, there still is not a known substitute for the existing carbon fiber process, which is too expensive for high-volume applications. Because of this uncertainty, the committee has not included carbon fiber in the 2050 mass reduction scenarios.
A key factor when evaluating design strategies for reducing mass is the corresponding secondary weight savings from rationalizing chassis, suspension, and drivetrain performance for the reduced mass. Estimates of the synergistic effects of mass reduction and the compounding effect that occurs along with it can vary significantly. In comments to various U.S. Corporate Average Fuel Economy (CAFE) rulemaking proposals, the Auto-Steel Partnership estimates that these secondary mass changes can save
1Body in white is the term for the stage in vehicle manufacture when all the fixed sheet metal components are fastened together. It does not include moveable parts such as doors, hood, and trunk (closures).