the apparatus and method convert the fuel system of an internal combustion engine in a pre - existing vehicle to operate on a mixture of fuels from a single fuel tank . the apparatus includes a fuel composition sensor installed in the fuel line and an electronic control unit with at least one fuel injector driver circuit for controlling an output signal to at least one fuel injector for controlling the air to fuel ratio of the engine . the electronic control unit controls both ignition timing and the air to fuel ratio of the engine based upon the percentage or ratio of the alternative fuel to gasoline from the fuel composition sensor . the electronic control unit receives both timing signals and fuel injector control signals from the engine &# 39 ; s original engine control module and the original emission control devices are not modified or replaced .