The energy substitution efficiency (ESE) was calculated for a number of the options investigated in this study. The calculations show the effect of allocation methods as well as fuel blends and vehicle/fuel performance on the WtW net energy use and the energy sub­stitution efficiency. The results are given in Table 6 and illustrated in Figure 6.

FIGURE 6 WtW net non-renewable primary energy use of ethanol according to selected options.

The results indicate that the choice of the allocation method has a significant impact on the WtW net energy use, with values ranging from -1.485 MJp/km (S-4, i. e., substitution with both straw and DDGS as fuel) to 1.810 MJp/km (S-1, i. e., substitution with both straw and DDGS as animal feed), that is, from -143% to -48% with respect to gasoline, with E5-1 as the option regarding fuels blend and vehicle/fuel performance. The effect of the fuel blend and vehi — cle/fuel performance is also significant, with net energy uses ranging from 0.471 MJp/km (E10-1, i. e., ethanol used as E10 based on actual test data) to 1.029 MJp/km (E-3, i. e., energy basis), that is, from -86% to -70% with respect to gasoline, respectively.

Both these methodological choices also significantly affect the energy substitution effi­ciency (ESE). For a given fuel blend and vehicle/fuel performance, the higher the nonrenew­able primary energy use, the lower the ESE. For a given allocation method and bioethanol production pathway, the ESE is best when bioethanol is used in the form of E10. This notion is particularly useful when considering a given volume of bioethanol (at the scale of a country or a region for example). The results show how much more efficient it is to use this volume of bioethanol as E10 than to use it as E85 or even E5, for a given service (i. e., a given overall dis­tance traveled). The situation is obviously different when considering a vehicle owner traveling a given distance every year. The best choice (in terms of both energy and GHG bal­ance) for a specific consumer is obviously to use E85 (with a maximum volume of gasoline displaced), as long as the net energy use or net GHG emissions of the biofuel are better than those of gasoline.