The WtW net GHG emissions of ethanol are calculated as the product of the WtT net GHG emissions and the specific fuel consumption of ethanol in the fuel blend (as reported in Table 3). The WtW net GHG emissions of ethanol (expressed in kg CO2 eq./km) are then com­pared to those of unleaded gasoline.

1.1 Net Energy Use and Energy Substitution Efficiency

The net energy use of a fuel most often refers to the consumption of nonrenewable primary energy along the life cycle of a biofuel or fossil fuel. Although the energy balance is often lim­ited to the comparative energy efficiency of fuels production, the actual performance of fuel blends must be taken into account in order to obtain a global picture of the potential sub­stitution of nonrenewable energy associated with biofuels. In order to measure the efficiency of nonrenewable primary energy substitution over the life cycle of bioethanol, the concept of energy substitution efficiency is defined later, including both production and utilization of the biofuel.

According to the data in Table 3, the most efficient use of fuel-bioethanol is in the form of E10 (1.174 MJth/km compared to 1.413 MJth/km for bioethanol as E5 and 2.485 MJth/km for bioethanol as E85). The energy substitution efficiency is here defined as the ratio of the savings of nonrenewable primary energy of a given bioethanol system (incl. production and use) with respect to conventional gasoline to the savings of nonrenewable primary energy of an ideal bioethanol system (i. e., bioethanol with a zero nonrenewable primary energy consumption and utilization as E10).