A significant determinant of the fuel economy of a vehicle, in particular automobiles, is the gross vehicle mass (GVM). Reducing the GVM can offer substantial improvements in fuel economy, varying (depending on the estimate) between 2-8% improvements in fuel economy for each 10% savings in mass. Manufacturers have many methods to reduce the GVM, including material substitution (plastic for metal) and novel designs. Although these techniques can result in substantial primary mass savings, they actually represent an underestimate of the total possible mass that can be removed. This is because as the automobile is made lighter, structural components can also be made lighter, resulting in secondary mass savings (SMS). So, for instance, a lighter car would require a smaller and lighter transmission, or smaller and lighter brakes. A new paper from Alonso et al. discusses a new method for manufacturers to estimate SMS more accurately.
Estimating SMS can be a challenging task, because of the nature of the design of automobiles. Cars are typically designed over about 5 years, and the design of the systems proceeds in parallel, at least to start. As the design progresses, the design of certain systems is frozen, to allow progress on other systems. This means that the mass of that system can not be reduced any further, even if changes later in the design process could allow for potential mass savings in the frozen system. Manufacturers use estimates of SMS to determine whether going back to redesign a system will be worth it, based on mass savings and the cost associated with the redesign. Therefore, underestimating SMS may result in certain systems not receiving redesigns when they could actually offer substantial mass savings.
Alonso and coworkers, from MIT and the GM Research and Development Center, have developed a new method of determining SMS. Traditional estimates of SMS divided the auto up into several large systems made of many parts, but the new work suggests that estimating the mass savings from a finer-grained division down to the parts level is an important step. In addition, Alonso et al. consider the effect of mass decompounding. This is a somewhat more complicated concept, but simply, it is the opposite of compounding interest on a savings account. Saving mass in one part reduces the GVM, allowing SMS, which allows further SMS (because the first round of SMS has reduced the GVM as well), and so on ad infinitum. This mass decompounding greatly magnifies the effect of the SMS from each individual component.
Alonso et al. conclude by determining the average SMS for currently available vehicles, if mass savings were to be realized somewhere on the vehicle. According to their work, an average of 0.95 kg could be saved by SMS for every 1 kg of other mass reduction. This is a substantial savings, and could result in a nearly 10% increase in fuel economy! Even the most conservative estimate they make, of 0.7 kg SMS for every 1 kg of other mass reduced, would result in an approximately 8% increase in fuel economy. Should manufacturers implement this new SMS estimation strategy, the fuel economy improvements could save nearly 1 million barrels of oil per day, which translates to nearly 200 million metric tons of carbon dioxide equivalent per year, not to mention the substantial savings in cost for both the consumer (on gas) and the manufacturer (on materials). Encouraging the auto industry to improve their estimates of SMS could be an important step to improving climate change and energy security.