Abstract:
A fuel cell system comprises: a turbo type oxidizing agent pump, the rotary shaft of which is pivotally supported by an air bearing to take in and supply an oxidizing agent gas to a fuel cell by the rotary motion; an actual flow rate detection means for the oxidizing agent gas; a pressure adjustment means for the oxidizing agent gas; a rotary speed monitoring means for the oxidizing agent pump; and a control means which, when the rotary speed of the oxidizing agent pump is within the range of the minimum rotary speed that allows the rotary shaft to be pivotally supported by the air bearing, if the actual flow rate of the oxidizing agent gas is larger than a target flow rate, increases the pressure of the oxidizing agent gas via the pressure adjustment means.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a fuel cell system which generates electric power by use of fuel gas and oxidant gas. 
       BACKGROUND ART 
       [0002]    The fuel cell system is a power generating system in which a fuel cell (fuel-cell stack) electrochemically generates electric power by use of fuel gas supplied from a fuel-gas source and oxidant gas supplied from an oxidant-gas source. Usually, air is used as the oxidant gas, and pumped by a compressor to the fuel cell. 
         [0003]    According to Patent Literature 1, a controller calculates the target rotational speed of the compressor pumping the air to the fuel cell, on the basis of the accelerator position, the vehicle speed, and the air flow rate in a fuel-cell car, and controls the rotational speed of the compressor and the flow rate of the air fed to the fuel cell. For example, when the value of the flow rate detected by a flow-rate sensor is within a normal range calculated on the basis of the operational condition of the fuel cell, the air flow rate is feedback controlled by use of the detected value of the flow rate. When the detected value of the flow rate deviates from the normal range, the air flow rate (the rotational speed of the compressor) is feedforward controlled. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: Japanese Patent Laid-Open No. 2010-241384 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    Incidentally, a type of the above compressor is the turbo type air pump, the shaft of which is supported by an air bearing. In some cases where the air bearing is used, air cannot be fed to the fuel cell at a desired flow rate by the control of the rotational speed only. Specifically, in the case where an attempt to feed the air at a desired air flow rate is made by feedback control of the rotational speed using the detected value of the air flow rate, when a command value of the rotational speed is near the lowest rotational speed which is needed for flotation of the shaft by the air bearing, the air cannot be fed at the desired air flow rate by the control of the rotational speed only. For example, it is found that when the command value of the rotational speed of the air pump is fixed to the lower limit because of errors in the flow-rate sensor and variations in the intake air pressure and the temperature, the flow rate of the air pump greatly increases due to a characteristic specific to the air pump. (See  FIG. 4 .) 
         [0006]    It is also found that the above phenomenon becomes prominent particularly in the system in which a device (such as a humidifier bypass valve) which changes the magnitude of the pressure loss is arranged between the air pump and the fuel cell. 
         [0007]    In view of above, the object of the present invention is to solve the above problem, and provide a fuel cell system in which appropriate feeding at the target flow rate can be realized. 
       Solution to Problem 
       [0008]    (1) According to the present invention which accomplishes the above object, a fuel cell system is provided. The fuel cell system is characterized in including: a fuel cell to which fuel gas and oxidant gas are fed and which generates electric power; an oxidant feed path to the fuel cell; an oxidant pump of a turbo type in which a shaft is supported by an air bearing, and which takes in and sends out the oxidant gas by rotary motion; an actual-flow-rate detection means for the oxidant gas; a pressure control means for the oxidant gas; a rotational-speed confirmation means for the oxidant pump; and a control means which increases the pressure of the oxidant gas through the pressure control means when the actual flow rate of the oxidant gas is greater than a target flow rate while the rotational speed of the oxidant pump is within a lowest range of the rotational speed in which the air bearing can support the shaft. 
         [0009]    According to the present invention, when the actual flow rate of the oxidant gas is greater than the target flow rate, the pressure of the oxidant gas is increased, and the actual flow rate by the air pump is controlled. 
         [0010]    (2) An additional feature of the fuel cell system according to the present invention is that the control means sets the target flow rate and a target pressure on the basis of a value of current requested from the fuel cell, and when the actual flow rate is greater than the target flow rate the control means sets a sum of a first predetermined value and the target pressure as a new target pressure. 
         [0011]    According to the present invention described above, when the actual flow rate of the oxidant gas is greater than the target flow rate, the target pressure is increased (the oxidant pressure is raised), and therefore the actual flow rate by the air pump is controlled. 
         [0012]    (3) An additional feature of the fuel cell system according to the present invention is that the fuel cell system further includes: a humidifier arranged between the oxidant pump and the fuel cell; a humidifier bypass which bypasses the humidifier; and an oxidant-flow regulating means which controls proportions of the oxidant gas between the humidifier and the humidifier bypass. In the fuel cell system, the control means sets the target pressure according to the proportions controlled by the oxidant-flow regulating means. 
         [0013]    According to the present invention described above, even in the case where the flow rate of the oxidant gas fed to the fuel cell is unchanged, the pressure loss in the oxidant flow in the section between the oxidant pump and the fuel cell differs according to the controlled proportion of the flow to the humidifier bypass. Even in this case, the target pressure is set according to the controlled proportion. 
         [0014]    (4) An additional feature of the fuel cell system according to the present invention is that a value of pressure loss in an oxidant flow is obtained on the basis of a relationship among the proportions controlled by the oxidant-flow regulating means, the target flow rate, and the pressure loss in the oxidant flow, and in the case where a sum of the target pressure and the value of the pressure loss in the oxidant flow is smaller than a predetermined value a sum of a second predetermined value and the target pressure is set as a new target pressure. 
         [0015]    According to the present invention described above, for example, the control operations in step S 22  to S 29  in the second embodiment explained later are performed, so that the flow rate through the oxidant pump can be controlled more appropriately. 
         [0016]    (5) An additional feature of the fuel cell system according to the present invention is that the oxidant-flow regulating means is a flow-rate regulator valve arranged in the humidifier bypass, and in the case where an aperture of the flow-rate regulator valve is set greater than a predetermined aperture the control means sets a sum of a predetermined value and the target pressure is set in advance as a new target pressure. 
         [0017]    Since the aperture of the flow-rate regulator valve is increased, the pressure loss in the section between the oxidant pump and the fuel cell is reduced, and the flow rate of the oxidant gas tends to increase. Therefore, according to the present invention described above, increase in the flow rate of the oxidant gas is suppressed by increasing the target pressure by adding the predetermined value to the target pressure. 
         [0018]    (6) In addition, according to the present invention, another fuel cell system is provided. The fuel cell system is characterized in including: a fuel cell to which fuel gas and oxidant gas are fed and which generates electric power; an oxidant feed path through which the oxidant gas to be fed to the fuel cell flows; an oxidant exhaust path through which fuel off-gas exhausted from the fuel cell flows; an oxidant pump of a rotary type which takes in and sends out the oxidant gas by rotary motion; an actual-flow-rate detection means which detects an actual flow rate of the oxidant gas; a rotational-speed confirmation means which confirms a rotational speed of the oxidant pump; a back pressure valve which is arranged in the oxidant exhaust path and controls a pressure of the oxidant gas fed to a cathode of the fuel cell; and a control means which controls the back pressure valve to decrease an aperture of the back pressure valve in steps smaller than steps in which the aperture of the back pressure valve is controlled to increase, in the case where the actual flow rate is greater than the target flow rate even when the rotational speed of the oxidant pump is lowered to a lowest range of the rotational speed after the back pressure valve is controlled to increase the aperture. 
         [0019]    According to the present invention described above, in the case where the actual flow rate is greater than the target flow rate even when the rotational speed of the oxidant pump is lowered to the lowest range of the rotational speed after the back pressure valve is controlled to increase the aperture, the back pressure valve is controlled to decrease the aperture in steps smaller than the steps in which the aperture of the back pressure valve is controlled to increase, so that the pressure of the oxidant gas rises and the actual flow rate can be lowered to the target flow rate. 
         [0020]    (7) An additional feature of the fuel cell system according to the present invention is that the fuel cell system further includes a pressure detection means which detects the pressure of the oxidant gas fed to the cathode, and the control means completes control of the aperture of the back pressure valve when the pressure rises to a predetermined pressure. 
         [0021]    According to the present invention described above, the control means determines the completion of the control of the aperture of the back pressure valve on the basis of the detected value of the pressure of the oxidant gas. Therefore, the timing of the completion of the control of the aperture of the back pressure valve can be appropriately controlled. 
         [0022]    (8) An additional feature of the fuel cell system according to the present invention is that the control means controls the aperture of the back pressure valve in the case where an output of the fuel cell is set in a predetermined low-output state. 
         [0023]    According to the present invention described above, the control means can control the actual flow rate of the oxidant gas to be the target flow rate by controlling the aperture of the back pressure valve, for example, even in the case where the vehicle (car) in which the fuel cell system is mounted decelerates and transitions to an idle state (a predetermined low-output state). 
         [0024]    (9) An additional feature of the fuel cell system according to the present invention is that the control means controls the aperture of the back pressure valve when an output of the fuel cell is maintained in a predetermined low-output state. 
         [0025]    According to the present invention described above, the actual flow rate can be lowered to the target flow rate by controlling the aperture of the back pressure valve, for example, even in the case where lowering of the actual flow rate of the oxidant gas to the target flow rate becomes impossible during an idle state (a predetermined low-output state) of a vehicle (car) in which the fuel cell system is mounted. 
       Effect of Invention 
       [0026]    According to the present invention, the fuel cell systems which enables appropriate feeding at a target flow rate. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0027]      FIG. 1  is a diagram illustrating a configuration common to fuel cell systems according to embodiments (first and second embodiments) of the present invention. 
           [0028]      FIG. 2  is a flow diagram of control operations according to the first embodiment of the present invention. 
           [0029]      FIG. 3  is a flow diagram of control operations according to the second embodiment of the present invention. 
           [0030]      FIG. 4  is a diagram indicating a characteristic of an air pump, where the abscissa corresponds to an intake volume flow rate, and the ordinate corresponds to a pressure rate. 
           [0031]      FIG. 5  is a flow diagram of control operations according to a third embodiment of the present invention. 
           [0032]      FIG. 6  is a timing diagram of the control operations according to the third embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0033]    An aspect (embodiment) of the present invention is explained in detail below with reference to accompanying drawings. 
         [0034]      FIG. 1  is a diagram schematically illustrating the entire configuration of a fuel cell system  1  according to an embodiment of the present invention. The fuel cell system is assumed to be mounted as a power supply on a fuel-cell vehicle, which runs on electric motors. 
         [0035]    As illustrated in  FIG. 1 , the fuel cell system  1  includes a fuel cell  10 , an air feed system  20 , a hydrogen feed system  30 , a control device  40 , and other components. The air feed system  20  feeds and exhausts air as the oxidant gas to and from the fuel cell  10 . The hydrogen feed system  30  feeds and exhausts hydrogen as fuel gas to and from the fuel cell  10 . The control device  40  controls the fuel cell system  1 . 
         [0036]    The fuel cell  10  is a known electric power generator which includes an anode (hydrogen electrode)  11  and a cathode (air electrode)  12 , and electrochemically generates electric power by use of hydrogen and air, which are respectively fed to the anode  11  and the cathode  12 . 
         [0037]    The air feed system  20  includes as main constituents an air pump  21 , a humidifier  22 , a humidifier bypass valve  23 , and a back pressure valve  24 . The air feed system  20  includes air feed piping  20   a,  bypass piping  20   b,  and air exhaust piping  20   c.  The air feed piping  20   a  feeds air by connecting the air pump  21  and an inlet side of the cathode  12  in the fuel cell  10  through the humidifier  22 . The bypass piping  20   b  bypasses the humidifier  22 . The air exhaust piping  20   c  exhausts air off-gas through the humidifier  20 , where the air off-gas is exhausted from the outlet side of the cathode  12  in the fuel cell  10 . The humidifier bypass valve  23  is provided in the bypass piping  20   b.  Since the above constituents are generally used types, explanations on the above constituents are not presented here. 
         [0038]    The air pump  21  is a turbo type supported by an air bearing as explained in the “Background Art”, and has functions of taking air in and sending air out by rotary motion. The air pump  21  has a characteristic feature that the air pump  21  cannot achieve a desired air flow rate when a command value of the rotational speed is near the lowest rotational speed (in the lowest range of the rotational speed) which is necessary for flotation of the shaft in the air bearing. (See  FIG. 4 .) 
         [0039]    The control device  40  is assumed to confirm the rotational speed of the air pump  21  by the command value of the rotational speed, which is generated by the control device  40 . Alternatively, the fuel cell system  1  may be provided with a rotational-speed sensor using a Hall device, and the control device  40  may confirm the rotational speed of the air pump  21  by the rotational-speed sensor. Further, a technique for controlling rotation in a sensorless manner is generally known. Further, although the humidifier bypass valve  23  is used as the “oxidant-flow regulating means”, alternatively, another valve, instead of or in combination with the humidifier bypass valve  23 , may be arranged immediately in front of the inlet of or immediately at the back of the outlet of the humidifier  22 . In this case, it is possible to consider that the above valve, instead of or in combination with the humidifier bypass valve  23 , realizes the “oxidant-flow regulating means”. 
         [0040]    The hydrogen feed system  30  includes a hydrogen feed apparatus  31  as a main constituent. The hydrogen feed system  30  includes hydrogen feed piping  30   a,  hydrogen exhaust piping  30   b,  and hydrogen return piping  30   c.  The hydrogen feed piping  30   a  feeds hydrogen to the anode  11  in the fuel cell  10 , the hydrogen exhaust piping  30   b  exhausts hydrogen off-gas emitted from the anode  11  in the fuel cell  10 , and the hydrogen return piping  30   c  branches off from the hydrogen exhaust piping  30   b  and returns to the hydrogen feed piping. Although the hydrogen feed apparatus  31  is assumed to be provided with a hydrogen reservoir (not shown) reserving hydrogen at a very high pressure of, for example, 30 MPa or 70 MPa. Alternatively, a reforming apparatus which generates hydrogen by reforming or the like of liquid raw fuel such as methanol may be provided in the hydrogen feed apparatus  31 . 
         [0041]    Some types of constituents of the hydrogen feed system  30  such as ejectors and purge valves which are generally used are not shown in  FIG. 1 , and explanations on such constituents are not presented. 
         [0042]    The control device  40  includes a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), various interfaces, and other components. The control device  40  is connected to an atmospheric pressure sensor Pa, an intake-air temperature sensor T, a flow-rate sensor Q, and a pressure sensor Pb through the various interfaces so that the values detected by the above sensors are inputted into the control device  40 . The control device  40  is further connected to another control device (not shown) which determines a value of current requested from the fuel cell  10 , so that the value of the requested current is inputted to the control device  40 . (Hereinafter, the value of the requested current is referred to as “the FC request current”.) The FC request current is approximately the sum of a value of current obtained in proportion to the amount of depression of a throttle pedal and a value of current obtained in proportion to the amount of operation of an auxiliary machine or the like. 
         [0043]    In addition, the control device  40  is connected to the air pump  21  (and a drive circuit for the air pump  21 ), the humidifier bypass valve  23  (and a drive circuit for the humidifier bypass valve  23 ), and the back pressure valve  24  (and a drive circuit for the back pressure valve  24 ) through the various interfaces. The control device  40  is configured to generate a command value of the rotational speed of the air pump  21 , send the command value of the rotational speed to the air pump  21 , generate command values of the apertures of the humidifier bypass valve  23  and the back pressure valve  24 , and send the command values of the apertures to the humidifier bypass valve  23  and the back pressure valve  24 . When the command value of the rotational speed increases, the rotational speed of the air pump  21  increases, so that the flow rate of air fed to the cathode  12  in the fuel cell  10  increases (i.e., the flow rate detected by the flow-rate sensor Q increases). When the aperture of the humidifier bypass valve  23  increases, the flow rate of air flowing through the humidifier  22  decreases (i.e., the flow rate of air fed to the cathode  12  without passing the humidifier  22  increases). When the aperture of the back pressure valve  24  increases, the pressure at the cathode  12  decreases (i.e., the pressure detected by the pressure sensor Pb decreases). 
         [0044]    Although the control device  40  in the present embodiment actively controls the flow rate of the air fed by the air feed system  20  by controlling the rotational speed of the air pump  21 , the control device  40  does not specifically control the flow rate of hydrogen in the hydrogen feed system  30 . The control device  40  is configured such that the feed rate of hydrogen from the hydrogen feed apparatus  31  through a regulator (not shown) automatically (passively) increases when the amount of hydrogen consumed at the anode  11  increases. 
         [0045]    The operations of the fuel cell system  1  according to the first embodiment having the above configuration are explained below with reference to the flow diagram of  FIG. 2 . In addition, the operations of controlling the humidifier bypass valve  23  are explained with reference to the flow diagram of  FIG. 3 . 
         [0046]    First, the current which the fuel cell  10  is requested to generate (i.e., the FC request current) is set by the other control device on the basis of the amount of depression of the throttle pedal and the load of an air conditioner (the amount of operation of an auxiliary machine and the like) in consideration of charging and discharging of a high-voltage battery (not shown), and the FC request current is sent to the control device  40 . The control device  40  calculates (in step S 10 ) a target flow rate of the air to be fed to the fuel cell  10  on the basis of the FC request current by reference to a table, a map, or the like which is memorized in advance. Similarly, the control device  40  calculates (in step S 20 ) a target air pressure on the basis of the FC request current by reference to a table, a map, or the like. The target air pressure is for the pressure at the inlet to the cathode  12  (at the position of the pressure sensor Pb). 
         [0047]    The back pressure valve  24  is feedback (F/B) controlled (in step S 30 ) to adjust the aperture such that the value of the air pressure detected by the pressure sensor Pb is equalized with the target air pressure calculated in step S 20 . In addition, the rotational speed of the air pump  21  is feedback controlled (in step S 40 ) such that the value of the air flow rate detected by the flow-rate sensor Q is equalized with the target air flow rate calculated in step S 10 . That is, the control device  40  feedback controls the air pump  21  and the back pressure valve  24  by setting (generating) the command value of the rotational speed and the command values of the apertures. At this time, the air flow rate is assumed to be corrected with the air temperature detected by the intake-air temperature sensor T and the air pressure (atmospheric pressure) detected by the atmospheric pressure sensor Pa. 
         [0048]    Incidentally, the shaft of the air pump  21  in the present embodiment is supported by the air bearing as explained before, and the rotational speed of the air pump  21  is controlled to be higher than the lowest (lower-limit) rotational speed needed for flotation of the shaft in the air bearing. For example, in the case where the amount of depression of the throttle pedal or the amount of operation of an auxiliary machine is small, or in the case where discharge operations from the high-voltage battery are mainly performed, power generation by the fuel cell  10  is not needed, so that the rotational speed of the air pump  21  can become close to the lower limit. However, the air pump  21  is configured such that the rotational speed is slightly higher than the lower limit even in the above cases. Nevertheless, in some cases, variations of the intake air pressure and the intake air temperature (i.e., the weather condition or environment) cause the rotational speed of the air pump  21  to be fixed to the lower limit. Conventionally, the air flow rate cannot be appropriately controlled at the lower limit of the rotational speed as above. For example, the air flow rate can unintendedly increase at the lower limit of the rotational speed as above. (That is, the actual flow rate does not achieve the target flow rate.) 
         [0049]    Therefore, according to the first embodiment, it is determined (in step S 50 ) whether or not the rotational speed of the air pump  21  is equal to the lowest rotational speed (the lower limit). When the rotational speed of the air pump  21  is not equal to the lower limit, the operation returns (Return) to step S 10 , and the operations beginning from step S 10  are repeated. The lower limit is appropriately set by experiment or simulation. 
         [0050]    When the rotational speed of the air pump  21  is equal to the lower limit, i.e., when YES is determined in step S 50 , it is determined (in step S 60 ) whether or not the air flow rate (the measured value) is greater than the target air flow rate. This is because according to the characteristic of the air pump  21  the shaft of which is supported by an air bearing, the air flow rate greatly increases (to become greater than the target flow rate) in some cases where the rotational speed of the air pump is equal to the lower limit. (That is, the actual flow rate can becomes greater than the target flow rate.) As mentioned before, the control device  40  confirms the rotational speed of the air pump  21  on the basis of the command value of the rotational speed, which is generated by the control device  40 . 
         [0051]    When the air flow rate is not greater than the target air flow rate, i.e., when NO is determined in step S 60 , this situation can be regarded to be normal. Therefore, the operation returns (Return) to step S 10 , and the operations beginning from step S 10  are repeated. On the other hand, when the air flow rate is greater than the target air flow rate, i.e., when YES is determined in step S 60 , the control device  40  sets (in step S 70 ) a new target air pressure by adding a first predetermined value (which is equal to or greater than zero) to the target air pressure which is set in step S 20 . Then, the control device  40  controls the back pressure valve  24  to decrease the aperture in order to equalize the air pressure with the target air pressure (in step S 80 ). That is, the control device  40  controls the air flow rate so as to suppress excessive air flow. This control may be one or both of feedforward control and feedback control. The feedforward control has a characteristic feature that the response is quick. The first predetermined value is set by experiment or simulation, and may either a fixed value or a variable value which varies with, for example, the deviation of the air flow rate from the target flow rate. 
         [0052]    After the operation in step S 80 , the operation goes to step S 60  to continue the operations of  FIG. 2 . 
         [0053]    According to the first embodiment, when the rotational speed of the air pump  21  reaches the lower limit and the air flow rate becomes greater than the target air flow rate, i.e., when YES is determined in both of steps S 50  and S 60 , the air flow rate becomes excessive and out of control. Therefore, the control device  40  sets a great value as the target air flow rate (in step S 70 ), and controls the back pressure valve  24  to control the aperture in the valve closing direction (in step S 80 ). Therefore, an appropriate air flow rate can be ensured. 
         [0054]    Thus, in a system using a turbo type air pump the shaft of which is supported by an air bearing like the air pump  21  according to the present embodiment, control of the command value of the pressure (i.e., the aperture of the back pressure valve  24 ) enables a feed at a desired air flow rate even when the command value of the rotational speed of the air pump  21  is near the lowest rotational speed (i.e., in the lowest range of the rotational speed) which is needed for flotation of the shaft in the air bearing. Therefore, it is possible to prevent excessive air feed to (the cathode  12  in) the fuel cell  10 , and efficiently generate electric power while preventing overdrying of the electrolytic membrane. Thus, the reliability of the system is greatly improved. When the air feed increases, the air is likely to be dried, and the dried air causes the electrolytic membrane in the fuel cell  10  to be dried. Consequently, the I-V (current-voltage) characteristic of the fuel cell  10  is degraded, and therefore the power generation efficiency of the fuel cell  1  is lowered. However, in the fuel cell system  1  according to the present embodiment, occurrence of the above phenomenon can be suppressed, and the lowering of the system efficiency can also be suppressed by the prevention of useless air feed. 
         [0055]    Further, in the low rotational speed range of the air pump  21 , the amount of electric power generated by the fuel cell  10  is small, and therefore the amount of water generated by the electrochemical reaction is small. Consequently, the fuel cell  10  can be regarded to be in a situation in which the electrolytic membrane is likely to be dried. If an excessive amount of air is fed to the fuel cell  10  in the above situation, the electrolytic membrane is likely to be overdried (and therefore lowering of the power generation efficiency and degradation of the electrolytic membrane are likely to occur). However, according to the present embodiment, occurrence of the above phenomenon can be suppressed. 
       Second Embodiment 
       [0056]    Next, the second embodiment is explained below with reference to the flow diagram of  FIG. 3 . In the explanations,  FIG. 1  is also referred to when necessary. In the second embodiment, control of the aperture of the humidifier bypass valve  23  is added to the configuration of the fuel cell system illustrated in  FIG. 1 . Therefore, in the control flow diagram of  FIG. 3 , the same step numbers as  FIG. 2  are assigned to the portions shared with the first embodiment, and explanations on the portions are not presented in this specification. 
         [0057]    As illustrated in  FIG. 3 , similarly to the first embodiment, the control device  40  performs step S 10  (calculation of the target air flow rate) and step S 20  (calculation of the target air pressure). Subsequently, the control device  40  calculates the aperture (the humidifier bypass-valve aperture) of the humidifier bypass valve  23  on the basis of the FC request current (in step S 22 ). Thereafter, according to the second embodiment, the control device  40  calculates the pressure loss in the humidifier  22  (the humidifier pressure loss) on the basis of the target air flow rate and the aperture of the humidifier bypass valve  23  (in step S 24 ). 
         [0058]    A map indicating a relationship among the target air flow rate, the humidifier pressure loss, and the humidifier bypass-valve aperture is conceptually indicated in  FIG. 3 . As indicated in the map, the humidifier pressure loss increases with the target air flow rate, and decreases with increase in the humidifier bypass-valve aperture even when the target air flow rate is unchanged. The above map indicates “the relationship among the proportions controlled by the oxidant-flow regulating means, the target flow rate, and the pressure loss in the oxidant flow”. The map indicating the relationship is only an example, and the relationship may be indicated by a function, a table, or other means. 
         [0059]    The control device  40  determines (in step S 26 ) whether or not the sum of the target air pressure and the humidifier pressure loss is smaller than a predetermined value, i.e., (the target air pressure)+(the humidifier pressure loss)&lt;(the predetermined value). The predetermined value is set by experiment or simulation. 
         [0060]    When (the target air pressure)+(the humidifier pressure loss)&lt;(the predetermined value), i.e., when YES is determined in step S 26 , the control device  40  adds a second predetermined value to the target air pressure to calculate a new target air pressure. Then, the operation goes to step S 30 . When No is determined in step S 26 , the operation also goes to step S 30 . The second predetermined value is a value greater than zero, i.e., (the second predetermined value)&gt;0. The second predetermined value is set by experiment or simulation. Since the operations performed in step S 30  and the following steps are the same as the first embodiment, the explanations on the steps are not repeated. 
         [0061]    In the fuel cell system  1  in which a device changing the air pressure loss (the humidifier bypass valve  23  in this example) is placed between the air pump  21  and the fuel cell  10 , the aforementioned phenomenon, i.e., the increase in the air flow rate in the lower-limit rotational-speed range of the air pump  21 , becomes prominent. Specifically, when the aperture of the humidifier bypass valve  23  is changed, the pressure loss in the section from the air pump  21  to the fuel cell  10  varies. Normally, the air feed is controlled such that the pressure at the inlet to (the cathode  12  of) the fuel cell  10  (which is detected by the pressure sensor Pb) is at a predetermined level. However, when the aperture of the humidifier bypass valve  23  changes, the pressure at the outlet of the air pump  21 (i.e., the air-pump compression ratio) changes, so that the extent of the ordinates of the “dispersion” range (illustrated in  FIG. 4 ) greatly varies. Thus, the aforementioned phenomenon becomes prominent. 
         [0062]    On the other hand, according to the second embodiment, the humidifier pressure loss is calculated (in step S 24 ), control operations in consideration of the pressure loss are performed (in steps S 26  and S 28 ), a new target air pressure is set in consideration of the humidifier pressure loss, and the operation of controlling the back pressure valve  24  is performed in step S 30  on the basis of the new target air pressure. Therefore, (unintended) increase in the air flow rate is suppressed, so that an appropriate air flow rate is ensured. 
         [0063]    Thus, according to the second embodiment, the appropriate air flow rate enables appropriate operation of the fuel cell system  1  similarly to the first embodiment. Further, since the device which changes the air pressure loss in the section between the air pump  21  and the cathode  12  is additionally provided, according to the second embodiment, it is possible to more appropriately operate the fuel cell system  1  than the first embodiment. 
       Other Matters 
       [0064]    In the second embodiment explained above, the map in  FIG. 3  is referred to in step S 24 . For example, in the case where the device which controls the proportions of flows (the oxidant-flow regulating means) is a flow-rate regulator valve arranged in the bypass piping  20   b  (the humidifier bypass valve  23 ), and the aperture of the flow-rate regulator valve (the humidifier bypass valve  23 ) is greater than a predetermined aperture, and the compression ratio of the air pump  21  is expected to become low, the control device  40  may cause the air pump  21  to achieve the target air flow rate by increasing the target pressure in advance, i.e., by setting in advance a sum of a predetermined value (the third predetermined value) and the target pressure as a new target pressure. Each of the above predetermined values are appropriately set so as to adjust the actual flow rate to the target flow rate and achieve the purpose of feeding air at an appropriate air flow rate (i.e., so as to suppress an excessive flow rate). 
       Third Embodiment 
       [0065]    Next, the third embodiment is explained below with reference to the flow diagrams of  FIGS. 5 and 6 . In the explanations,  FIG. 1  is also referred to when necessary. In the third embodiment, the aperture of the back pressure valve  24  is controlled to be increased when a fuel-cell vehicle (which is a four-wheeled vehicle, a two-wheeled vehicle, or a vehicle of another type) on which a fuel cell system, for example, having the configuration of  FIG. 1  is mounted decelerates and transitions from a running state to an idle state in which a minimum output of the fuel cell  10  is maintained (i.e., the electric power generated by the fuel cell  10  is set to a predetermined low-output state). That is, the aperture of the back pressure valve  24  is changed to be greater when the fuel-cell vehicle decelerates and transitions from a running state to an idle state than when the aperture of the back pressure valve  24  is controlled to be increased during a normal run. In the third embodiment, the same step numbers as  FIG. 2  are assigned to the portions shared with the first embodiment, and explanations on the portions are not presented in this specification. 
         [0066]    As illustrated in  FIG. 5 , when the control device  40  determines that the air flow rate (the actual flow rate) is greater than the target air flow rate, i.e., when YES is determined in step S 60 , the control device  40  changes the aperture of the back pressure valve  24  by a small amount (in step S 90 ). That is, the control device  40  controls the aperture of the back pressure valve  24  to change the aperture by a smaller amount than the amount by which the aperture of the back pressure valve  24  is controlled to be increased when the deceleration causes the transition from the running state to the idle state. 
         [0067]    Then, the control device  40  determines whether or not the air pressure is equal to or greater than a predetermined pressure (in step S 100 ), where the predetermined pressure is a threshold for determining whether to complete the aperture adjustment of the back pressure valve  24 . When the control device  40  determines that the air pressure is not equal to or greater than the predetermined pressure, i.e., when NO is determined in step S 100 , the operation goes back to step S 60 . When the control device  40  determines that the air pressure is equal to or greater than the predetermined pressure, i.e., when YES is determined in step S 100 , the control device  40  completes the aperture adjustment of the back pressure valve  24  (in step S 110 ). 
         [0068]    Further explanations are presented below with reference to  FIG. 6 . In order to cause a transition to a predetermined low-output state (e.g., an idle state), the aperture of the back pressure valve  24  is controlled to be increased, and the rotational speed of the air pump  21  is lowered (in the period from time t 0  to t 1 ). Specifically, the aperture of the back pressure valve  24  is controlled to be increased by the operation in step S 30  in  FIG. 5 , and the rotational speed of the air pump  21  is controlled to be lowered by the operation in step S 40  in  FIG. 5 , so that the air pressure is greatly lowered. 
         [0069]    After the aperture of the back pressure valve  24  is controlled to be increased, at time t 1 , the rotational speed of the air pump  21  reaches the lower limit of the rotational speed, so that the rotational speed of the air pump  21  cannot be further lowered, i.e., YES is determined in step S 50 . In this case, when the air flow rate (the actual flow rate indicated by the solid line) does not yet reach the target air flow rate (indicated by the dashed line), i.e., when YES is determined in step S 60 , excessiveness of the air flow rate (the actual flow rate) is detected, so that increase of the air pressure is necessary. Therefore, in order to decrease the air flow rate (the actual flow rate) by increasing the air pressure, the aperture of the back pressure valve  24  is changed in the valve closing direction in small steps (in step S 90 ). Since the aperture of the back pressure valve  24  is changed in the valve closing direction in small steps, the air pressure increases. Thus, the change of the aperture of the back pressure valve  24  in small steps causes the air flow rate (the actual flow rate) to gradually decrease. 
         [0070]    Thereafter, when the aperture adjustment of the back pressure valve  24  causes the air pressure to reach the predetermined pressure, i.e., when YES is determined in step S 100 , the aperture adjustment of the back pressure valve  24  is completed (in step S 110 ). At this time, the air flow rate (the actual flow rate) is equalized with the target air flow rate. 
         [0071]    Thus, according to the third embodiment, in the case where the air flow rate (the actual flow rate) is still greater than the target air flow rate even when the rotational speed of the air pump  21  decreases to the lower limit of the rotational speed (the lower range of the rotational speed) after the aperture of the back pressure valve  24  is greatly changed in the valve opening direction during a transition from a running state to an idle state, the aperture of the back pressure valve  24  is changed in the valve closing direction in small steps, so that the air pressure (the pressure of the oxidant gas) can be increased and the air flow rate (the actual flow rate) can be lowered to the target flow rate. Since the aperture of the back pressure valve  24  is adjusted as above, (unintended) increase in the air flow rate is suppressed, and appropriate air flow rate can be ensured. 
         [0072]    In addition, according to the third embodiment, the timing of completion of the aperture adjustment of the back pressure valve  24  can be appropriately controlled by completing the aperture adjustment of the back pressure valve  24  when the air pressure rises to the predetermined pressure. 
         [0073]    Further, in the case where the output of the fuel cell  10  is set to a predetermined low-output state (an idle state), according to the third embodiment, increase in the air flow rate can be suppressed and appropriate air flow rate can be ensured. 
         [0074]    In the explanations of the third embodiment, the case in which the vehicle decelerates and transitions to the idle state is taken as an example of a case in which the aperture control of the back pressure valve  24  is performed. However, the third embodiment can be applied to not only the above case. For example, the third embodiment can also be applied to the case in which the actual flow rate cannot be lowered to the target air flow rate because of some fluctuation of the actual flow rate caused by temporary variations in the electric power fed to the air pump  21  due to variations in the total load on the vehicle which are caused by, for example, a start of use of a heater. 
       LIST OF REFERENCES 
       [0000]    
       
           1 : Fuel Cell System 
           10 : Fuel cell 
           11 : Anode 
           12 : Cathode 
           20 : Air Feed System 
           20   a:  Air Feed Piping (Oxidant Feed Path) 
           20   b:  Bypass Piping (Humidifier Bypass) 
           20   c:  Air Exhaust Piping (Oxidant Exhaust Path) 
           21 : Air Pump (Oxidant Pump) 
           22 : Humidifier 
           23 : Humidifier Bypass Valve (Oxidant-flow Regulating Means, Flow-rate Regulator Valve) 
           24 : Back Pressure Valve (Pressure Regulating Means) 
           30 : Hydrogen Feed System 
           31 : Hydrogen Feed Apparatus 
           40 : Control Device (Control Means, Rotational-speed Confirmation Means) 
         Pb: Pressure Sensor (Pressure Detection Means) 
         Q: Actual-flow-rate Detection Means (Flow-rate Sensor)