Commentary on “Correction to: On the calculation of fuel savings through lightweight design in automotive life cycle assessments” by Koffler and Rohde-Brandeburger (2018)
In their recent article “Correction to: On the calculation of fuel savings through lightweight design in automotive life cycle assessments” ( https://doi.org/10.1007/s11367-018-1474-4), Koffler and Rohde-Brandenburger (2018) present a method to determine the “percentage fuel savings” from lightweighting a vehicle component. The original work by Koffler and Rohde-Brandenburger (2010) introduced a novel method of estimating generic fuel reduction values (FRVs) to assess the benefits of vehicle lightweighting. Koffler and Rohde-Brandenburger (2010) also described how FRVs could be used to assess the absolute fuel savings from component lightweighting. However, Koffler and Rohde-Brandenburger (2010) noted that for the case of powertrain resizing (changing engine displacement or gear ratios to match vehicle performance), it was not possible to assess relative fuel savings from component lightweighting because this would involve division by zero in their method. To address this limitation, Koffler and Rohde-Brandenburger (2018) suggest defining the use-phase fuel consumption of the original component as FRV multiplied by the baseline mass. The use-phase fuel consumption of a lightweighted component is then determined from saved mass and FRVs for the cases without and with powertrain resizing.
We comment that the method of Koffler and Rohde-Brandenburger (2018) overestimates the relative fuel savings from component lightweighting and we illustrate that excessively high or infeasible fuel-saving percentages are calculated using their method. This is particularly evident when substantial (> 40%) mass reductions are considered. We discuss the key issues in their method and offer an alternative approach based on our previous work (Kim and Wallington 2013, 2016; Kim et al. 2015) in which engine friction losses are included in the fuel consumption assessed for the baseline component.