Abstract: A fuel supply device includes a) at least one first tank for diesel fuel, b) at least one second tank for vegetable oil/fat c) at least one third tank for holding a mixture of the diesel fuel and the vegetable oil/fat, d) wherein each tank has at least one fuel outlet, e) a fuel line system including fuel lines and valves, e1) which fuel line system connects the fuel outlets of all the tanks to the fuel inflow of the injection pump, and e2) which fuel line system connects the fuel return of the injection pump to a fuel inlet of the first tank, and to a fuel inlet of the third tank, and f) a control unit for controlling the fuel flow through the fuel line system. Implementations of the present invention further include methods of operating the same.