Abstract:
A fuel cell system and a control method therefor are capable of improving power generation efficiency of the fuel cell more reliably during a normal operation. A fuel cell system is arranged along a vehicle frame of a motorcycle. The fuel cell system includes a fuel cell having a cathode, an air pump which supplies the cathode with oxygen-containing air, and a CPU which controls operation of elements which constitute the fuel cell system. The CPU determines, depending on situations, whether or not to perform an oxygen-starving process which is a process of starving the cathode of the oxidizer during the normal operation, and stops the air pump when a determination is made to perform the oxygen-starving process.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a fuel cell system and a control method therefor, and more specifically to a fuel cell system which supplies oxidizer to a cathode in the fuel cell, and a control method therefor. 
         [0003]    2. Description of the Related Art 
         [0004]    Conventionally, in the field of fuel cell systems, an oxidizer-starving process (air starvation) is used, which is a process of temporarily stopping or reducing a supply of the oxidizer to the cathode in the fuel cell thereby starving the cathode of the oxidizer. 
         [0005]    It is generally known that the output of the fuel cell (electromotive force in particular) increases after an oxidizer-starving process in comparison to the output before the oxidizer-starving process. This can be utilized in different ways. JP-A 63-26961 discloses a technique of performing an oxidizer-starving process in a normal operation when constant power generation is underway, thereby restoring an output of the fuel cell which will otherwise decrease with time. 
         [0006]    Also, PCT(WO) 2003-504807 discloses a technique in which an oxidizer-starving process is performed based on the temperature of the fuel cell at start-up time for increased over-voltage (internal resistance) of the fuel cell which leads to an increased amount of heat generation. 
         [0007]    However, JP-A 63-26961 discloses nothing about determination criteria for determining whether or not to perform an oxidizer-starving process, nor does it disclose a timing when to perform the oxidizer-starving process. An output (electric power) from the fuel cell decreases during an oxidizer-starving process, which means that performing an oxidizer-starving process and restoring an output of the fuel cell system can lead to a situation where the amount of electric energy increased by the oxidizer-starving process is smaller than the amount of electric energy decreased by the oxidizer-starving process, depending on conditions of the fuel cell system (such as conditions of the electrolyte in the fuel cell). In other words, power generation efficiency of the fuel cell can be decreased by performing an oxidizer-starving process, depending on conditions of the electrolyte in the fuel cell. 
         [0008]    The technique disclosed in PCT(WO) 2003-504807 is to perform an oxidizer-starving process at a startup time when output of the fuel cell is not yet stable. This technique attempts to quickly increase the temperature of the fuel cell thereby bringing the fuel cell system quickly into a normal operation where the fuel cell is able to more stably generate power. In other words, the technique described in PCT(WO) 2003-504807 has nothing to do with improving power generation efficiency through restoration of the output of the fuel cell, which will otherwise decrease with time during a normal operation, by performing an oxidizer-starving process. 
       SUMMARY OF THE INVENTION 
       [0009]    In order to overcome the problems described above, preferred embodiments of the present invention provide a fuel cell system and a control method therefor that reliably improve power generation efficiency of the fuel cell during normal operation. 
         [0010]    One preferred embodiment of the present invention provides a fuel cell system which includes a fuel cell having a cathode supplied with oxidizer, an oxidizer supply arranged to supply the cathode with the oxidizer, a determination unit arranged to determine, depending on a situation, whether or not to perform an oxidizer-starving process during a normal operation, and a controller arranged to control operation of the oxidizer supply during the normal operation based on a result of the determination by the determination unit. 
         [0011]    Another preferred embodiment of the present invention provides a method of controlling a fuel cell system which supplies oxidizer to a cathode in a fuel cell. The method includes a determining step of determining, depending on a situation, whether or not to perform an oxidizer-starving process during a normal operation, and a controlling step of controlling an amount of supply of the oxidizer to the cathode, based on a result of the determination whether or not to perform the oxidizer-starving process during the normal operation. 
         [0012]    In a preferred embodiment of the present invention, during a normal operation, a determination is made whether or not to perform an oxidizer-starving process, depending on the situation. If the determination is for performing an oxidizer-starving process, the oxidizer supply to the cathode is temporarily stopped or reduced from the amount supplied up until then. As described, situations are checked to determine whether or not to perform an oxidizer-starving process, and then the oxidizer-starving process is performed depending on the necessity of the situation. By doing so, it becomes possible to more reliably improve power generation efficiency of the fuel cell during the normal operation. 
         [0013]    Preferably, the fuel cell system further includes a first memory arranged to store an output value of the fuel cell before a previous oxidizer-starving process and an output value of the fuel cell after the previous oxidizer-starving process. It is determined whether or not to perform the oxidizer-starving process, based on a result of comparison between the output value of the fuel cell before the previous oxidizer-starving process and the output value of the fuel cell after the previous oxidizer-starving process stored in the first memory. In this case, a determination is made to perform the oxidizer-starving process and the amount of oxidizer supplied to the cathode is controlled when the output of the fuel cell after the previous oxidizer-starving process, which is stored in the first memory, is greater than the output of the fuel cell before the previous oxidizer-starving process by a rate not smaller than a predetermined rate. In other words, a determination to perform an oxidizer-starving process is made when a result of the previous oxidizer-starving process indicates that the amount of increase in electric energy achievable after the oxidizer-starving process is greater than the amount of electric energy unavailable during the oxidizer-starving process. As described, by performing an oxidizer-starving process when there is a positive sign for improved power generation efficiency of the fuel cell, it becomes possible to more reliably improve power generation efficiency of the fuel cell during the normal operation. 
         [0014]    Preferably, the fuel cell system also includes a second memory arranged to store an anticipated output value of the fuel cell corresponding to a length of time passed, and it is determined whether or not to perform an oxidizer-starving process, based on a result of comparison between a current output value of the fuel cell and an anticipated output value of the fuel cell stored in the second memory. In this case, a determination is made for performing the oxidizer-starving process and the amount of oxidizer supplied to the cathode is controlled when the current value of the output of the fuel cell is smaller than the anticipated value of output of the fuel cell stored in the second memory. In other words, a determination for performing an oxidizer-starving process is made when a decrease in the output is quicker than a standard time-course output of the fuel cell. Therefore, it becomes possible to avoid unnecessary execution of the oxygen-starving process at a time when there is little need for restoring the output of the fuel cell, and to reduce an undesirable decrease in power generation efficiency of the fuel cell during the normal operation. 
         [0015]    Preferably, the fuel cell system further includes a secondary battery electrically connected with the fuel cell and an electric charge detector arranged to detect an amount of charge in the secondary battery. It is determined whether or not to perform an oxidizer-starving process based on the amount of charge in the secondary battery detected by the electric charge detector. The determination is made for performing the oxidizer-starving process and the amount of oxidizer supplied to the cathode is controlled when the amount of charge in the secondary battery is below a predetermined amount, i.e., when the secondary battery must be charged. There is no need for restoring the output of the fuel cell when the amount of charge in the secondary battery is sufficient. Thus, this arrangement avoids unnecessary execution of the oxidizer-starving process, and to reduce risk for undesirable decrease in power generation efficiency of the fuel cell. The arrangement also prevents such disadvantages as premature deterioration of the secondary battery caused by over-charging. 
         [0016]    Preferably, the fuel cell system further includes time measuring unit arranged to measure time. It is determined whether or not to perform an oxidizer-starving process, after a measurement by the time measuring unit, following an operation startup, of a predetermined amount of time necessary for transition from the operation startup to the normal operation. As described, by having time measuring unit automatically measure a predetermined amount of time necessary for transition from an operation startup to the normal operation following the operation startup, it becomes possible to eliminate the need for the operator to determine if the fuel cell system has entered its normal operation, thereby reducing the burden on the operator. 
         [0017]    In direct methanol fuel cell systems, methanol aqueous solution is directly supplied to the fuel cells so that direct methanol fuel cell systems do not require a reformer, and thus can have a simplified system configuration. For this and other reasons, direct methanol fuel cell systems are preferably used suitably in equipment in which portability is essential or in equipment in which a small size is desired. In order to operate a direct methanol fuel cell system and the equipment including it for a longer time, improvement in fuel cell power generation efficiency is essential. Preferred embodiments of the present invention are capable of improving power generation efficiency of the fuel cell, and therefore particularly effective in direct methanol fuel cell systems which are suitably used in equipment which requires portability as well as other fuel cell systems which supply aqueous solution fuel to fuel cells directly. 
         [0018]    When a fuel cell system is used in transportation equipment, the fuel cell system needs to be light and thus needs to be smaller than that used in stationary equipment. For this reason, power generation efficiency of the fuel cell needs to be improved in the application to transportation equipment. Especially when the fuel cell system includes a secondary battery, the secondary battery is usually small and light, and therefore tends to run short of the desired charge. For such a reason as this, it is essential to quickly increase the amount of charge in the secondary battery and thus it is important to improve power generation efficiency of the fuel cell. Therefore, a preferred embodiment of the present invention is suitably used in transportation equipment. 
         [0019]    It should be noted here that the term “normal operation” means a state of a fuel cell system in which the fuel cells can generate electricity constantly. 
         [0020]    Other features, elements, characteristics, and advantages will be apparent from the following detailed description of preferred embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is an illustrative drawing which shows a primary portion of a fuel cell system according to a preferred embodiment of the present invention. 
           [0022]      FIG. 2  is a perspective view which shows the fuel cell system mounted on a frame of a motorcycle. 
           [0023]      FIG. 3  is an illustrative drawing which shows a primary portion of the fuel cell system. 
           [0024]      FIG. 4  is a block diagram which shows an electrical configuration of the fuel cell system. 
           [0025]      FIG. 5  is a flowchart showing an example of primary steps after power generation startup of the fuel cell system. 
           [0026]      FIG. 6  is a continuation of the flowchart in  FIG. 5 . 
           [0027]      FIG. 7  is a flowchart showing an example of an oxygen-starving process to be performed in the fuel cell system. 
           [0028]      FIG. 8  is an illustrative drawing which shows how an output of the fuel cell changes when the oxygen-starving process is performed. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. 
         [0030]    As shown in  FIG. 1  through  FIG. 4 , a fuel cell system  10  according to a preferred embodiment of the present invention is provided as a direct methanol fuel cell system. Direct methanol fuel cell systems do not require a reformer and therefore are used suitably in equipment in which portability is essential and/or a small size is desired. Here, description will be made in which the fuel cell system  10  is used in a motorcycle as an example of transportation equipment. As shown in  FIG. 2 , the motorcycle will be represented only by a vehicle frame  200 , with the left-hand side being the front side of the vehicle and the right-hand side being the rear side thereof in the figure. The fuel cell system  10  is disposed along the vehicle frame  200 . 
         [0031]    Referring mainly to  FIG. 1 , the fuel cell system  10  includes a fuel cell  12 . The fuel cell  12  is constructed as a fuel cell stack or a plurality of fuel cells connected (laminated) in series, each of which includes an electrolyte  12   a  provided by a solid polymer film, and an anode (fuel electrode)  12   b  and a cathode (air electrode)  12   c  which sandwich the electrolyte  12   a.    
         [0032]    The fuel cell system  10  includes a fuel tank  14  which holds highly concentrated methanol fuel (aqueous solution containing approximately 50 wt % of methanol) F. The fuel tank  14  is connected, via a fuel supply pipe  16 , with an aqueous solution tank  18  which stores methanol aqueous solution S. The fuel supply pipe  16  is provided with a fuel pump  20 . The fuel pump  20  supplies the aqueous solution tank  18  with the methanol fuel F from the fuel tank  14 . 
         [0033]    The fuel tank  14  is provided with a level sensor  15  for detecting the level of methanol fuel F in the fuel tank  14 . The aqueous solution tank  18  is provided with a level sensor  22  for detecting the level of methanol aqueous solution S in the aqueous solution tank  18 . The aqueous solution tank  18  is connected, via an aqueous solution pipe  24 , with the anode  12   b  of the fuel cell stack  12 . The aqueous solution pipe  24  is provided with an aqueous solution pump  26 , a radiator  28  serving as a heat exchanger, and an aqueous solution filter  30 , respectively from the upstream side. A cooling fan  32  is disposed near the radiator  28  for cooling the radiator  28 . The methanol aqueous solution S in the aqueous solution tank  18  is supplied by the aqueous solution pump  26  toward the anode  12   b , cooled by the radiator  28  as necessary, and then purified by the aqueous solution filter  30  before being supplied to the anode  12   b.    
         [0034]    On the other hand, the cathode  12   c  in the fuel cell  12  is connected with an air pump  34  via an air pipe  36 . The air pipe  36  is provided with an air filter  38 . Thus, air which contains oxygen (oxidizer) is sent from the air pump  34 , purified by the air filter  38  and then supplied to the cathode  12   c.    
         [0035]    The anode  12   b  and the aqueous solution tank  18  are connected with each other via a pipe  40 , so unused methanol aqueous solution and produced carbon dioxide discharged from the anode  12   b  are supplied to the aqueous solution tank  18 . 
         [0036]    Further, the cathode  12   c  is connected with the water tank  44  via a pipe  42 . The pipe  42  is provided with a radiator  46  serving as a gas-liquid separator, and near the radiator  46  is a cooling fan  48  disposed for cooling the radiator  46 . Exhaust gas which is discharged from the cathode  12   c  and contains moisture (water and water vapor) is moved to the water tank  44  via the pipe  42 . 
         [0037]    The aqueous solution tank  18  and the water tank  44  are connected with each other via the CO2 vent pipe  50 . The CO2 vent pipe  50  is provided with a methanol trap  52  which separates methanol aqueous solution S. The carbon dioxide discharged from the aqueous solution tank  18  is thus supplied to the water tank  44 . 
         [0038]    The water tank  44  is provided with a level sensor  54  which detects the level of water in the water tank  44 . The water tank  44  is provided with an exhaust gas pipe  56 . The exhaust gas pipe  56  discharges carbon dioxide and the exhaust gas from the cathode  12   c.    
         [0039]    The water tank  44  is connected with the aqueous solution tank  18  via the water recycling pipe  58 . The water recycling pipe  58  is provided with a water pump  60 . Water in the water tank  44  is recycled by the water pump  60  to the aqueous solution tank  18  as necessary depending on the situation of the aqueous solution tank  18 . 
         [0040]    Further, in the aqueous solution pipe  24 , a bypass pipe  62  is provided between the radiator  28  and the aqueous solution filter  30 . 
         [0041]    Reference is now made also to  FIG. 4 . In the fuel cell system  10 , the bypass pipe  62  is provided with a concentration sensor  64  for detecting the concentration of methanol aqueous solution S. A cell temperature sensor  66  for detecting the temperature of the fuel cell  12  is attached to the fuel cell  12  whereas an ambient temperature sensor  68  for detecting the ambient temperature is provided near the air pump  34 . 
         [0042]    As shown in  FIG. 4 , the fuel cell system  10  includes a control circuit  70 . 
         [0043]    The control circuit  70  includes a CPU  72  serving as a controller which performs necessary calculations and controls operations of the fuel cell system  10 , a clock circuit  74  which gives clock signals to the CPU  72 , a volatile memory  75  (e.g., RAM, DRAM or any other suitable memory device) for storing data, such as time passed, based on the clock signals given to the CPU  72 , flag data, etc., a non-volatile memory  76  (e.g., EEPROM, CMOS or any other suitable memory device) which stores programs and data necessary for controlling the fuel cell system  10  as well as calculation data etc., a reset IC  78  which prevents malfunction of the fuel cell system  10 , an interface circuit  80  for making connections with external devices, a voltage detection circuit  84  which detects voltages in an electric circuit  82  to which the fuel cell  12  is connected to power a motor  202  to drive the motorcycle, an electric current detection circuit  86  which detects values of the electric current flowing in the electric circuit  82 , an ON/OFF circuit  88  which opens and closes the electric circuit  82 , a voltage protection circuit  90  which prevents over voltage in the electric circuit  82 , a diode  92  provided in the electric circuit  82 , and a power source circuit  94  which supplies a predetermined voltage to the electric circuit  82 . 
         [0044]    In the control circuit  70  described above, the CPU  72  is supplied with detection signals from the concentration sensor  64 , temperature sensor  66  and the ambient temperature sensor  68  as well as detection signals from the level sensors  15 ,  22  and  54 . Further, the CPU  72  is supplied with detection signals from a roll-over switch  96  which detects whether or not the vehicle has been rolled over, and other signals for making various settings and information entry from an input unit  98 . 
         [0045]    The CPU  72  controls such components as the fuel pump  20 , the aqueous solution pump  26 , the air pump  34 , the heat-exchanger cooling fan  32 , the gas-liquid separator cooling fan  48  and the water pump  60 . The CPU  72  also controls a display  100  which displays various information to the motorcycle rider. 
         [0046]    In the present preferred embodiment, the CPU  72  preferably serves as a determining unit and a controller. However, any other logic or control unit may serve as the determining unit and the controller. The volatile memory  75  serves as the first memory whereas the non-volatile memory  76  serves as the second memory. Also in the present preferred embodiment, the CPU  72 , the clock circuit  74  and the volatile memory  75  are included in the time measuring unit. The oxidizer supply preferably includes the air pump  34 , or any other suitable device for supplying air and/or oxygen to the fuel cell. 
         [0047]    In the present preferred embodiment, the CPU  72  stores time information based on the clock signals from the clock circuit  74  in the volatile memory  75 , whereby a length of time since a certain process has started is measured. The first memory, e.g., the volatile memory  75  stores a value of output from the fuel cell  12  before an oxidizer-starving process (to be described later) and a value of output from the fuel cell  12  after the oxidizer-starving process. In this particular case, the output values before and after an oxidizer-starving process are voltage values, and so the voltage values are stored in the volatile memory  75 . The second memory, e.g., the non-volatile memory  76  stores table data of anticipated voltage values and anticipated electrical current values as standard anticipation values for the length of time elapsed. Output (electrical energy) from the fuel cell  12  generally decreases from a value right after the normal operation has started (initial output) by about 10% in 1000 operating hours, for example. The non-volatile memory  76  stores table data of anticipated voltage values and anticipated electrical current values for such a standard time-course output, i.e., table data of anticipated output values showing the standard time-course output. 
         [0048]    The fuel cell  12  is connected with a secondary battery  102  and a charge detection device  103  for detecting the amount of electric charge of a secondary battery  102  (a rate of charge with respect to the capacity of secondary battery  102 ). The secondary battery  102  and the charge detection device  103  are also connected with the motor  202 . The secondary battery  102  supplements the output from the fuel cell  12 , is charged with electric energy from the fuel cell  12 , and discharges to provide the motor  202  and other components with electric energy. The secondary battery  102  can be a Ni—H battery, Lithium ion battery, Ni—Cd battery, etc. Detection signals from the charge detector, e.g., signals from the charge detection device  103 , enter the CPU  72 . 
         [0049]    The motor  202  is provided with a meter  204  which makes measurements for various data concerning the motor  202 . These data and status information about the motor  202  measured by the meter  204  are provided to the CPU  72  via the interface circuit  104 . 
         [0050]    Now, a power generation operation of the fuel cell system will be described. When an unillustrated main switch is turned ON, the fuel cell system  10  begins to drive its components, such as the aqueous solution pump  26  and the air pump  34 , and begins power generation (operation). 
         [0051]    When power generation is started, methanol aqueous solution S of a desired concentration which is stored in the aqueous solution tank  18  is pumped by the aqueous solution pump  26  toward the fuel cell  12 . The solution is cooled as necessary by the radiator  28 , purified by the aqueous solution filter  30 , and then supplied to the anode  12   b . On the other hand, air which contains oxygen as an oxidizer is pumped by the air pump  34  toward the fuel cell  12 . The air is first purified by the air filter  38  and then supplied to the cathode  12   c.    
         [0052]    On the anode  12   b  in the fuel cell  12 , methanol and water in the methanol aqueous solution S react electro-chemically with each other to produce carbon dioxide and hydrogen ions. The hydrogen ions move through the electrolyte  12   a  to the cathode  12   c , where the hydrogen ions react electro-chemically with oxygen in the air which is supplied to the cathode  12   c , to produce water (water vapor) and electric energy. 
         [0053]    Carbon dioxide created on the anode  12   b  in the fuel cell  12  flows through the pipe  40 , the aqueous solution tank  18 , and the CO2 vent pipe  50  to reach the water tank  44 , and then it is discharged from the exhaust gas pipe  56 . 
         [0054]    On the other hand, most of the water vapor created on the cathode  12   c  in the fuel cell  12  is liquefied and discharged in the form of water, with saturated water vapor being discharged in the form of gas. Part of the water vapor which was discharged from the cathode  12   c  is cooled and liquefied by lowering the dew point in the radiator  46 . The radiator  46  liquefies the water vapor through operation of the cooling fan  48 . Moisture (water and water vapor) and unused air from the cathode  12   c  are supplied to the water tank  44  via the pipe  42 . Also, water which has moved to the cathode  12   c  due to the water crossover is discharged from the cathode  12   c  and supplied to the water tank  44 . Further, water and carbon dioxide which are present at the cathode  12   c  due to the methanol crossover are discharged from the cathode  12   c  and supplied to the water tank  44 . 
         [0055]    It should be noted here that the term water crossover is a phenomenon in which a few mols of water move to the cathode  12   c , accompanying the hydrogen ions which occur at the anode  12   b  and are moving to the cathode  12   c . The term methanol crossover is a phenomenon in which methanol moves to the cathode  12   c , accompanying the hydrogen ions which move to the cathode  12   c . At the cathode  12   c , the methanol reacts with air supplied from the air pump  34 , and is thereby decomposed into water and carbon dioxide. 
         [0056]    Water (fluid) which was collected in the water tank  44  is pumped by the water pump  60  and recycled to the aqueous solution tank  18  as appropriate via the water recycling pipe  58 , where it is reused as water in the methanol aqueous solution S. 
         [0057]    Generally, in a generating fuel cell, output (especially electromotive force) increases when the cathode is temporarily starved of the oxidizer through an oxidizer-starving process. In the fuel cell system  10 , during a normal operation, the oxidizer-starving process (an oxygen-starving process in the present preferred embodiment) is performed by temporarily stopping the supply of air to the cathode  12   c , or by temporarily decreasing the supply of air to the cathode  12   c  from the amount of supply up until then. Through this operation, the fuel cell system  10  restores the output of the fuel cell  12 . 
         [0058]    Next, description will be given for an example of primary steps after power generation startup of the fuel cell system  10 . 
         [0059]    Note that in the present preferred embodiment, when the main switch is turned ON, flags  1  through  3  in the volatile memory  75  are in an OFF state. The term “flag” is a piece of information representing ON or OFF, for example, for the CPU  72  to determine the current situation and to perform a step appropriate to the situation. In the present preferred embodiment, the flag  1  is a piece of information for determining if an oxygen-starving process was performed at a past point in time relatively close to the current point in time. The flag  2  is a piece of information for determining if a determination has been made as to the need to perform an oxygen-starving process. The flag  3  is a piece of information for determining if the previous oxygen-starving process helped restore the output. Hereinafter, the following expressions will be used: A “flag is raised” when the flag status is changed from OFF to ON, a “flag is UP” when the flag status is ON, a “flag is lowered” when the flag status is changed from ON to OFF, and a “flag is down” when the flag status is OFF. 
         [0060]    Referring to  FIG. 5  and  FIG. 6 , first, when the main switch is turned ON and power generation (operation) is started, clock signals (pulse signals) from the clock circuit  74  are counted to measure the length of time since the power generation startup. In other words, a measurement of time since the power generation startup is started. Also, detection of a voltage value and an electric current value of the fuel cell  12  is started, and detection by the charge detection device  103  of the amount of electric charge in the secondary battery  102  is started (Step S 1 ). 
         [0061]    The time elapsed since the power generation startup is stored in the volatile memory  75 . The voltage values detected by the voltage detection circuit  84  and the current values detected by the current detection circuit  86  are each related to the time elapsed since the power generation startup and are stored in the volatile memory  75 . Likewise, the amount of charge in the secondary battery  102  detected by the charge detection device  103  is related to the time elapsed since the power generation startup and is stored in the volatile memory  75 . 
         [0062]    With the above-described arrangement, a determination is made if a predetermined amount of time since the power generation startup (10 minutes, for example) has passed (Step S 3 ). The predetermined amount of time used in Step S 3  as a norm is set on the basis of a length of time for the fuel cell  12  from the power generation startup to become able to generate power at a constant and stable output, i.e., a length of time necessary to complete a transition from the power generation startup to the normal operation. This is because no comparison of the current output value to the anticipated output value corresponding to the time elapsed is possible, as will be described later, while the output from the fuel cell  12  is unstable. In other words, the state of the electrolyte  12   a  cannot be determined from the output of the fuel cell  12  until the normal operation begins since the output from the fuel cell  12  is changing and therefore it is impossible to determine if the oxygen-starving process should be performed or not. 
         [0063]    If the predetermined amount of time has not passed since the power generation startup (Step S 3 : NO), the system waits until the predetermined amount of time has passed. When the predetermined amount of time has passed since the power generation startup (Step S 3 : YES), and the system is in the normal operation, a determination is made if the current amount of charge in the secondary battery  102  is below a predetermined amount (a rate of 90% charge, for example) in Step S 5 . 
         [0064]    If the current amount of charge in the secondary battery  102  is not smaller than the predetermined amount (Step S 5 : NO), there is no need for charging the secondary battery  102 , or there is no need for restoring the output of the fuel cell  12 . Thus, the system waits, starting from the time point when determination was made in Step S 5 , until a predetermined amount of time (three minutes, for example) has passed until the program Step S 6  becomes (YES). Once the predetermined amount of time has passed (Step S 6 : YES), the program goes to Step S 5 , where the system detects the current amount of charge in the secondary battery  102  to see if it is below the predetermined amount. 
         [0065]    On the other hand, if the current amount of charge in the secondary battery  102  is below the predetermined amount (Step S 5 : YES), the program checks if an oxygen-starving process has been performed before, i.e., if the flag  1  is up (Step S 7 ). If the flag  1  is down (Step S 7 : NO), the current output which is based on the current voltage and electric current values is compared to an anticipated output value which is based on the anticipated voltage and current values corresponding to the time elapsed. The system checks if the current output value is below the anticipated output value corresponding to the time elapsed (Step S 9 ). 
         [0066]    If the current output value is below the anticipated output value corresponding to the time elapsed (Step S 9 : YES), i.e., if the current output value is smaller than the standard output value, a flag  2  is raised which indicates that a determination is made for performing an oxygen-starving process, and the measuring of time since the flag  2  has been raised is started (Step S 11 ). Then, the current output value is compared to the anticipated output value corresponding to the time elapsed, to see if the current output value is not lower than the anticipated output value corresponding to the time elapsed (Step S 13 ). 
         [0067]    If the current output value is lower than the anticipated output value corresponding to the time elapsed (Step S 13 : NO), then the system checks if a predetermined amount of time (ten minutes, for example) has passed since the flag  2  was raised (Step S 14 ). If the predetermined amount of time has not passed since the flag  2  was raised (Step S 14 : NO), the program goes to Step S 13 , to check again if the current output value is not lower than the anticipated output value corresponding to the time elapsed or not. In other words, as long as Step S 13  is (NO) since the flag  2  is raised until the predetermined amount of time has passed, a cycle of comparison between the current output value and the anticipated output value corresponding to the time elapsed is repeated, and a plurality of checks are performed. 
         [0068]    If the output value is below the anticipated output value for a predetermined amount of time since the flag  2  is raised (Step S 14 : YES), a current voltage value is stored in the volatile memory  75  as the voltage value of the fuel cell  12  before the oxygen-starving process (Step S 15 ). Then the oxygen-starving process is performed in order to restore the output of the fuel cell  12  (Step S 17 ). As described, by watching (monitoring) the value of output of the fuel cell and the anticipated value of output from the time when the flag  2  is raised to the time when a predetermined amount of time has passed, it becomes possible to make an accurate determination, which eliminates unnecessary execution of the oxygen-starving process at times when, for example, the voltage value of the fuel cell  12  drops only momentarily below the anticipated voltage value. 
         [0069]    Here, reference will be made to  FIG. 7  to describe the oxygen-starving process (air-starving process) in Step  17 . 
         [0070]    First, the air pump  34  stops to cut the supply of air to the cathode  12   c . At the same time, a measurement is started for a downtime of the air pump  34  (Step S 101 ). Note that the anode  12   b  continues to be supplied with methanol solution S from the aqueous solution pump  26  even after the air pump  34  is stopped. 
         [0071]    Then, a comparison is made between a voltage value of the fuel cell  12  and a preset voltage value (such as 5%-60% of the voltage value before oxygen-starving process), to see if the voltage value has dropped down to the preset voltage value (Step S 103 ). If the voltage value has not dropped to the preset voltage value (Step S 103 : NO), the program checks if the downtime of the air pump  34  has reached a preset time (ten seconds, for example) or not (Step S 105 ). 
         [0072]    If the downtime of the air pump  34  has not reached the preset time (Step S 105 : NO) the program goes to Step S 103 . If the downtime of the air pump  34  has reached the preset time (Step S 105 : YES), then the air pump  34  is started to resume the supply of air to the cathode  12   c , the downtime of the air pump  34  is cleared (Step S 107 ), and the oxygen-starving process is finished. If the voltage value has dropped to the preset voltage value (Step S 103 : YES), the program jumps to Step S 107  where the oxygen-starving process is finished. 
         [0073]    By performing such an oxygen-starving process during normal operation, the output of the fuel cell increases as shown, for example, in  FIG. 8 .  FIG. 8  shows a case in which an amount of increase in electric energy obtained after the oxygen-starving process exceeds an amount of electric energy which cannot be generated during the oxygen-starving process (an amount of electric energy not available during the oxygen-starving process). In other words,  FIG. 8  shows a case where an oxygen-starving process improves power generation efficiency of the fuel cell  12 .  FIG. 8  also shows that the oxygen-starving process is performed when a decrease in output of the fuel cell  12  is quicker than the standard time-course output depicted in an alternate long and short dashed line and when this situation continues for a period of ten minutes. 
         [0074]    Returning to  FIG. 5  and  FIG. 6 , upon finishing the oxygen-starving process in Step S 17 , a flag  1  is raised to indicate that an oxygen-starving process was performed, and a measurement is started for a length of time since the flag  1  has been raised (Step S 19 ). Next, a highest (peak) voltage value detected after the oxygen-starving process is stored in the volatile memory  75  as a voltage value of the fuel cell  12  after the oxygen-starving process (Step S 21 ). 
         [0075]    Then, a comparison is made between the voltage value of the fuel cell  12  before the oxygen-starving process and the voltage value of the fuel cell  12  after the oxygen-starving process, to see if the voltage value of the fuel cell  12  increased by a rate not smaller than a predetermined rate (about 5%, for example) as a result of the oxygen-starving process (Step S 23 ). The rate of increase in the voltage value used as a norm in Step S 23  is set on the basis of an increase in the electric energy anticipated to be necessary after the oxygen-starving process. Since the increase which is made after the oxygen-starving process is primarily an electromotive force, it is possible to make a generally good estimate of the increased amount of electric energy by comparing the voltage value before the oxygen-starving process and the voltage value after the oxygen-starving process. If the voltage value after the oxygen-starving process has increased over the voltage value before the oxygen-starving process by a rate not smaller than about 5%, it is expected that the an increased amount of electric energy obtained after the oxygen-starving process will exceed the amount of electric energy not available during the oxygen-starving process, as compared to a case depicted in a long dashed double-short dashed line in  FIG. 8  which is an output pattern when the oxygen-starving process was not performed. In other words, if the voltage value after the oxygen-starving process has increased over the voltage value before the oxygen-starving process by a rate not smaller than about 5%, it is expected that the power generation efficiency of the fuel cell  12  will be improved. 
         [0076]    If the voltage value after the oxygen-starving process has not increased over the voltage value before the oxygen-starving process by not smaller than the predetermined rate (Step S 23 : NO), a flag  3  is raised to indicate that the next oxygen-starving process can decrease power generation efficiency of the fuel cell  12 , and a measurement is started for a length of time since the flag  3  has been raised (Step S 25 ). Then, the flag  2  is lowered, and the time passed since the flag  2  was raised is cleared (Step S 27 ). If the voltage value after the oxygen-starving process has increased over the voltage value before the oxygen-starving process by a rate not smaller than the predetermined rate (Step S 23 : YES), the program goes to Step S 27 . Similarly, the program goes to Step S 27  if Step S 13  finds that the current output value is not smaller than the anticipated output value (if the answer is (YES). 
         [0077]    Thereafter, the system checks if a predetermined amount of time (approximately five minutes, for example) has passed since the determination in Step S 9  (Step S 28 ). If the predetermined amount of time has passed (Step S 28 : YES), the program goes to Step S 5 . If the predetermined amount of time has not passed (Step S 28 : NO), the system waits until the predetermined amount of time has passed. Also, the program goes to Step  28  if Step S 9  finds that the current output value is not smaller than the anticipated output value corresponding to the time elapsed (if the answer is NO). 
         [0078]    If Step S 7  finds that the flag  1  is up (if the answer is YES), the system checks if the flag  3  is up (Step S 29 ). If the flag  3  is down (Step S 29 : NO), the program checks if a predetermined amount of time (ten minutes, for example) has passed since the flag  1  was raised (Step S 31 ). If the predetermined amount of time has passed since the flag  1  was raised (Step S 31 : YES), the flag  1  is lowered, the time passed since the flag  1  was raised is cleared (Step S 33 ), and the program goes to Step S 9 . If the predetermined amount of time has not passed since the flag  1  was raised (Step S 31 : NO), then the program goes to Step S 6 . 
         [0079]    On the other hand, if the flag  3  is up (Step S 29 : YES), the system checks if a predetermined amount of time (thirty minutes, for example) has passed since the flag  3  was raised (Step S 35 ). If the predetermined amount of time has passed since the flag  3  was raised (Step S 35 : YES), the flag  3  is lowered, the time passed since the flag  3  was raised is cleared (Step S 37 ), and the program goes to Step S 33 . 
         [0080]    If the predetermined amount of time has not passed since the flag  3  was raised (Step S 35 : NO), the program goes to Step S 6 . In other words, if a result of the previous oxygen-starving process indicates that the next oxygen-starving process can decrease power generation efficiency of the fuel cell  12 , the program goes to Step S 6  and avoids performing the oxygen-starving process. 
         [0081]    According to the fuel cell system  10  according to a preferred embodiment of the present invention as described above, the decision whether or not to perform an oxygen-starving process is made on the basis of the amount of charge in the secondary battery  102  and a result of comparison between the current output value and an anticipated output value, and the oxygen-starving process is performed when each of these conditions is satisfied. Also, if an oxygen-starving process has been performed before, a result of the previous oxygen-starving process is also considered when determining if an oxygen-starving process should be performed or not, and the oxygen-starving process is performed when each of the conditions is satisfied. Therefore, it becomes possible to avoid unnecessary oxygen-starving processes and oxygen-starving processes which can decrease power generation efficiency of the fuel cell  12 , making it possible to improve power generation efficiency of the fuel cell  12  more reliably in normal operation. 
         [0082]    Further, an oxygen-starving process is not performed when there is a large amount of charge in the secondary battery  102  (when charge rate is high). This eliminates such problems as deterioration of the secondary battery  102  due to overcharging at an end stage of the charging cycle, and incorrect determination on the timing for termination of the charging cycle for the secondary battery  102 . 
         [0083]    According to the fuel cell system  10  as described above, since the power generation efficiency can be improved more reliably, it becomes possible to quickly increase the amount of charge in the secondary battery  102  of a motorcycle. This means that a secondary battery  102  of a motorcycle can have a small capacity, and it is possible to use a small and light weight secondary battery  102 . Further, a predetermined amount of time which is necessary for transition from power generation startup to normal operation is measured automatically after the power generation startup. This means that there is no need for the rider of a motorcycle to determine if the fuel cell system has entered its normal operation, and thus it is possible to reduce the burden on the rider. 
         [0084]    It should be noted that in the above-described preferred embodiments, description was made for a case that a determination whether or not to perform an oxygen-starving process is made on the basis of the amount of charge in the secondary battery  102 , a result of comparison between the current output value and an anticipated output value, and a result of previous oxygen-starving process if there has been an previous oxygen-starving process. However, the present invention is not limited to this. For example, the determination whether to perform an oxygen-starving process or not may be based on one of the amount of charge in the secondary battery  102 , a result of comparison between the current output value and an anticipated output value, and a result of previous oxygen-starving process, so that the oxygen-starving process is performed if the condition is satisfied. 
         [0085]    The fuel cell system  10  can be used not only in motorcycles but also in automobiles, marine vessels and any other transportation equipment or vehicles. 
         [0086]    The present invention is also applicable to fuel cell systems which make use of a reformer, or fuel cell systems in which hydrogen is supplied to the fuel cell. Further, the present invention is applicable to small-scale, stationary-type fuel cell systems. 
         [0087]    The fuel to be used is not limited to methanol. The present invention is applicable to fuel cell systems which use any alcohol fuel such as ethanol. 
         [0088]    The present invention being thus far described and illustrated in detail, it is obvious that the description and drawings only represent an example of the present invention, and should not be interpreted as limiting the invention. The spirit and scope of the present invention is only limited by words used in the accompanied claims. 
         [0089]    While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.