On the calculation of fuel savings through lightweight design in automotive life cycle assessments

Lightweight design is a common means of reducing a passenger car's fuel consumption. In order to calculate the resulting fuel savings, one has to estimate the total energy that is needed to move a certain weight over a defined distance in a distinct way, and express this energy in liter of gasoline or diesel. This can be accomplished by the so-called fuel reduction value (FRV) and based on a standardized driving cycle, e.g., the New European Driving Cycle (NEDC). The aim of this paper is to explain the theoretical background of the calculation of fuel savings in automotive lightweight life cycle assessments (LCAs) of internal combustion engine (ICE) vehicles in greater detail than it has been done before, to describe the resulting factors and their different applications, and to point out some notable particularities that need to be taken into account when conducting this type of LCA study. The first part of the paper explains the theoretical background of the FRV based on physical correlations and simulations. Based on these findings, its application in the context of automotive LCAs is described. The respective characteristics and preconditions are explained in detail. It is shown that for LCAs that deal with automotive parts or assemblies, it is not permissible to multiply their respective net weight by the FRV under the assumption that the vehicle's performance remains unchanged. However, the consideration of secondary lightweight effects concerning engine displacement or gear ratio is only possible under this assumption. This entails a significantly higher FRV, but in turn only allows for the calculation of the net fuel reduction, which is zero for the reference part and carries a negative sign for all lightweight design options. In practice, both FRV (0.12 and 0.28 l/(100 km*100 kg) for diesel vehicles resp. 0.15 and 0.35 l/(100 km*100 kg) for gasoline vehicles) are equally likely in case that no comprehensive information is available about whether a weight-induced power train adaptation will take place or not. This approach stresses the decision makers' responsibility to ensure such measures. It appears to be indicated from a scientific point of view to include power train adaptations in automotive lightweight LCA studies in order to preserve functional equality in terms of the vehicle's driving performance. Yet, in practice this adaptation will most likely not take place if the weight difference is rather small. If there is no reliable information available that a weight-induced power train adaptation is guaranteed to take place, then a scenario without power train adaptation should be presented to the decision makers as well in form of an equally probable best and worst case. It has been shown that the fuel consumption in order to move a mass of 100 kg over 100 km can be obtained based on the NEDC driving cycle and the differential efficiency of gasoline and diesel engines. If possible secondary measures are taken into account (gear ratio and engine displacement) the resulting values can be augmented significantly. It has also been shown that it is advisable to utilize mass differences rather than mass ratios when calculating the lightweight effect on fuel consumption during the use stage. This implies that the resulting fuel saving of a lightweight component compared to the reference component carries a negative sign, while a lightweight vehicle's fuel consumption is positive. It is strongly recommended to follow the proposed calculation procedure in future automotive lightweight studies. The authors further recommend the use of both FRV (with and without secondary measures) and the appropriate and explicit communication of the resulting implications.