NOVELTY - Method for simultaneously controlling fuel concentration and temperature of liquid fuel cell involves obtaining database by measuring fuel consumption rate/speed, and preparing fuel consumption equation; determining fuel consumption rate/speed in given operating condition; setting a target temperature, and supplying undiluted fuel to fuel mixer, and then supplying diluted fuel in fuel mixer to start operation of fuel cell; fixing target-condition control mode; and increasing or decreasing the undiluted fuel flux or flow rate.    USE - For controlling fuel concentration and temperature of liquid fuel cell by using temperature-control based feed-back control without using concentration sensor (claimed); used as power generation system for generating electricity by electrochemically reacting oxygen and fuel.    ADVANTAGE - The present fuel cell apparatus is a sensor-less type not using a concentration sensor, thus the apparatus has a simple and small design, which reduces the power consumed for operation and lowers the apparatus price. The system improves efficiency and stability of the fuel cell operation and regulates the fuel concentration quickly and accurately, compared to a prior sensor-less concentration controller. The method has a great advantage in that the heat loss of the fuel cell stack, which can occur due to change of ambient temperature, and the loss of water circulating in fuel circulation system can be minimized. The heat exchange rate of heat exchanger is controlled by means of feed-back control by comparing a present temperature of the fuel cell stack with the target temperature, thus minimizing problems which may occur due to the change of the ambient temperature.    DETAILED DESCRIPTION - Method (M1) for simultaneously controlling fuel concentration and temperature of liquid fuel cell by using temperature-control based feed-back control without using concentration sensor involves obtaining database by measuring fuel consumption rate or speed according to operating condition of liquid fuel cell, and preparing fuel consumption equation according to operating conditions based on database; determining fuel consumption rate or speed in given operating condition based on the fuel consumption equation, and determining undiluted fuel feed amount or speed corresponding to determined fuel consumption rate or speed; setting a target temperature and selecting either a target-condition control mode or present-condition control mode, and supplying undiluted fuel to fuel mixer in undiluted fuel feed amount or speed determined according to the selected mode, and then supplying diluted fuel in fuel mixer to a stack of fuel cell to start operation of fuel cell; fixing the target-condition control mode so that the undiluted fuel is supplied accordingly if temperature of stack reaches a predetermined intermediate temperature lower than the target temperature, and performing feed-back control to heat removal rate of heat exchanger comparing the present temperature of fuel cell stack with target temperature so that the temperature of fuel cell stack is maintained in normal state where temperature of the fuel cell stack is within a preset error bound from the target temperature; and increasing or decreasing the undiluted fuel flux or flow rate fixed according to the target-condition control mode, if the fuel cell is in abnormal state where the temperature of fuel cell stack deviates from preset error bound from target temperature although the heat exchanger operates in maximum manner or in minimum manner, by feed-back control comparing present temperature of fuel cell stack with target temperature so that the temperature of fuel cell stack comes back to the normal state where temperature of fuel cell stack is within a preset error bound from target temperature. INDEPENDENT CLAIMS are included for the following:    (1) apparatus for simultaneously controlling a fuel concentration and temperature of liquid fuel cell by using temperature-control based feed-back control without using concentration sensor comprising microprocessor programmed to perform the method (M1); and    (2) liquid fuel cell apparatus comprising fuel cell stack; heat exchange unit connected to the stack; fuel supply unit supplying a diluted fuel to the stack; device for measuring the fuel cell stack temperature; and device for measuring at least one of output voltage and output current of fuel cell stack, and control device.    DESCRIPTION OF DRAWING(S) - The figure shows flowchart of method for controlling fuel concentration and stack temperature control of liquid fuel cell.