Abstract:
A fuel cell system includes, an oxidant feeder configured to supply an oxidant to a fuel cell, an oxidant passage which communicates with the fuel cell, a bypass passage which branches from the oxidant passage and along which part of the oxidant flows so as to bypass the fuel cell, a bypass valve which is provided in the bypass passage, an oxidant quantity-of-flow control unit which is configured to supply the quantity of flow of the oxidant corresponding to an amount of electricity required by the fuel cell, and an oxidant quantity-of-flow control unit for a sound vibration mode configured to supply a constant quantity of flow of the oxidant, and further includes a bypass valve control unit configured to control the bypass valve according to a requirement of the fuel cell when the oxidant quantity-of-flow control unit for the sound vibration mode controls the oxidant feeder.

Description:
TECHNICAL FIELD 
     The present invention relates to a device that controls a fuel cell system. 
     BACKGROUND ART 
     Although in a fuel cell system, it is necessary to supply air according to the amount of electricity generated by the fuel cell, an electric generation requirement is likely to vary by SOC control on a battery or the like, and periodic variation sound from a compressor supplying air to a fuel cell stack is likely to be generated. Such variation sound, in particular, is likely to be recognized by a passenger, for example, when a vehicle is at rest with background noise low. The passenger feels such variation sound harsh to have an uncomfortable feeling. 
     On the other hand, in JP2006-179331A, the variation sound is masked by irregular noise such as in a radiator fan, and thus it is unlikely to be recognized by a passenger. 
     Other related literatures includes JP3895263B. 
     SUMMARY OF INVENTION 
     However, it is possible to perform the method described above only when the radiator fan is driven. On the other hand, although one way to solve this problem is to make the quantity of flow of the compressor constant, this causes unnecessary air to be supplied to the electric generation of the fuel cell stack, with the result that there is a concern that, for example, over-drying is disadvantageously produced. 
     Hence, the present invention is made by focusing on such a problem. An object of the present invention is to provide a control device for a fuel cell system in which the operating sound of a compressor is unlikely to be recognized by a passenger. 
     A control device for a fuel cell system according to one aspect of the present invention supplies power to an external load with a storage cell and a fuel cell. The fuel cell system includes, an oxidant feeder configured to supply an oxidant to the fuel cell; an oxidant passage which communicates with the fuel cell and along which the oxidant supplied from the oxidant feeder flows, a bypass passage which branches from the oxidant passage upstream with respect to the fuel cell and along which part of the oxidant supplied by the oxidant feeder flows so as to bypass the fuel cell, a bypass valve which is provided in the bypass passage and which is configured to adjusts a quantity of flow of the oxidant flowing along the bypass passage; an oxidant quantity-of-flow control unit configured to supply, with the oxidant feeder, the quantity of flow of the oxidant corresponding to an amount of electricity required by the fuel cell, and an oxidant quantity-of-flow control unit for a sound vibration mode configured to supply, with the oxidant feeder, a constant quantity of flow of the oxidant. The control device further includes a bypass valve control unit configured to control the bypass valve according to a requirement of the fuel cell when the oxidant quantity-of-flow control unit for the sound vibration mode configured to control the oxidant feeder. 
     Embodiments of the present invention and advantages of the present invention will be described in detail below with reference to accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a basic system to which a control device for a fuel cell system according to the present invention is applied; 
         FIG. 2  is a correlation diagram between a charging rate SOC of a battery and the generated power of a fuel cell stack; 
         FIG. 3  is a diagram illustrating a problem to be solved in an embodiment; 
         FIG. 4  is a control flowchart that is performed by the controller of the control device for the fuel cell system according to the present invention; 
         FIG. 5  is a block diagram showing a function of computing the amount of air supplied to the fuel cell stack; 
         FIG. 6  is a diagram showing a subroutine that determines an operation mode; 
         FIG. 7  is a diagram showing a subroutine in a normal mode operation; 
         FIG. 8  is a block diagram showing a function of computing the amount of air bled in the normal mode operation; 
         FIG. 9  is a diagram showing a subroutine of a sound vibration mode operation; 
         FIG. 10  is a block diagram showing the function of computing the amount of air bled in the sound vibration mode operation; 
         FIG. 11  is a diagram illustrating an operation in the normal mode operation in a first embodiment; 
         FIG. 12  is a diagram illustrating an operation in the sound vibration mode operation in the first embodiment; 
         FIG. 13  is a time chart illustrating the operation in the sound vibration mode operation in the first embodiment; 
         FIG. 14  is a diagram showing an operation mode determination subroutine in a second embodiment of the fuel cell system according to the present invention; 
         FIG. 15  is a diagram showing a sound vibration mode operation subroutine in a third embodiment of the fuel cell system according to the present invention; 
         FIG. 16  is a diagram illustrating an operation method of the compressor in the normal mode operation; 
         FIG. 17  is a diagram illustrating a problem when in the sound vibration mode operation, a cathode pressure adjustment valve is not controlled; 
         FIG. 18  is a diagram illustrating the effect of action in the third embodiment in the sound vibration mode operation; 
         FIG. 19  is a diagram showing an operation mode determination subroutine in a fourth embodiment of the fuel cell system according to the present invention; 
         FIG. 20  is a diagram showing an operation mode determination subroutine in a fifth embodiment of the fuel cell system according to the present invention; 
         FIG. 21  is a diagram showing a sound vibration mode subroutine in a sixth embodiment of the fuel cell system according to the present invention; 
         FIG. 22  is a block diagram showing a function of computing surplus generated power; 
         FIG. 23  is a block diagram showing a function of computing an increased amount of air supplied by the compressor; and 
         FIG. 24  is a diagram illustrating the effect of action in the sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a diagram showing a basic system to which a control device for a fuel cell system according to the present invention is applied. 
     With reference to  FIG. 1 , a description will first be given of the basic system to which the control device for the fuel cell system according to the present invention is applied. 
     In a fuel cell stack  10 , while an electrolyte membrane is being kept in an appropriate humid state, reactive gases (oxygen O 2  and hydrogen H 2 ) are supplied to generate electricity. To achieve this, a cathode line  20 , an anode line  30  and a cooling water circulation line  40  are connected to the fuel cell stack  10 . A current generated by the fuel cell stack  10  is detected by a current sensor  101 . A voltage generated by the fuel cell stack  10  is detected by a voltage sensor  102 . 
     Air containing the oxygen O 2  supplied to the fuel cell stack  10  flows as a cathode gas along the cathode line  20 . In the cathode line  20 , a compressor  21 , a WRD (water recovery device)  22  and a cathode pressure adjustment valve  23  are provided. A bleed line  200  is also provided in parallel to the cathode line  20 . The bleed line  200  is branched from the cathode line  20  that is downstream with respect to the compressor  21  but is upstream with respect to the WRD  22 , and is combined with the cathode line  20  that is downstream with respect to the cathode pressure adjustment valve  23 . With this configuration, part of the air blown by the compressor  21  flows into the bleed line  200 , and bypasses the fuel cell stack  10 . In the bleed line  200 , a bleed valve  210  is provided. The bleed line  200  corresponds to a bypass passage in the scope of claims. The bleed valve  210  corresponds to a bypass valve in the scope of claims. 
     In the present embodiment, the compressor  21  is, for example, a centrifugal turbo compressor. The compressor  21  is arranged in the cathode line  20  that is upstream with respect to the fuel cell stack  10  and the WRD  22 . The compressor  21  is driven by a motor M. The compressor  21  adjusts the quantity of flow of the cathode gas flowing along the cathode line  20 . The quantity of flow of the cathode gas is adjusted by the rotation speed of the compressor  21 . 
     The WRD  22  humidifies the air introduced into the fuel cell stack  10 . The WRD  22  includes a humidified portion through which a gas that is a humidification target flows and a humidifying portion through which a water-containing gas that is a humidifying source flows. The air introduced by the compressor  21  flows through the humidified portion. The water-containing gas flows through the humidifying portion via the fuel cell stack  10 . 
     The cathode pressure adjustment valve  23  is provided in the cathode line  20  that is downstream with respect to the fuel cell stack  10 . The cathode pressure adjustment valve  23  adjusts the pressure of the cathode gas flowing along the cathode line  20 . The pressure of the cathode gas is adjusted by the degree of opening of the cathode pressure adjustment valve  23 . 
     A pressure P1 of the cathode gas flowing along the cathode line  20  that is upstream with respect to the compressor  21  is detected by a cathode pressure sensor  201 . The cathode pressure sensor  201  is provided upstream with respect to the compressor  21 . 
     The quantity of flow Q of the cathode gas flowing along the cathode line  20  is detected by a cathode quantity-of-flow sensor  202 . The cathode quantity-of-flow sensor  202  is provided downstream with respect to the compressor  21  but upstream with reference to the WRD  22 . 
     The pressure of the cathode gas flowing along the cathode line  20  is detected by a cathode pressure sensor  203 . The cathode pressure sensor  203  is provided downstream with respect to the compressor  21  but upstream with respect to the WRD  22 . Furthermore, in  FIG. 1 , the cathode pressure sensor  203  is located downstream with respect to the cathode quantity-of-flow sensor  202 . 
     The bleed valve  210  is provided in the bleed line  200 . The bleed valve  210  adjusts the quantity of flow of the cathode gas flowing along the bleed line  200 . The quantity of flow of the cathode gas is adjusted by the degree of opening of the bleed valve  210 . 
     The hydrogen H 2  supplied to the fuel cell stack  10  flows as an anode gas along the anode line  30 . In the anode line  30 , a cylinder  31 , an anode pressure adjustment valve  32  and a purge valve  33 . 
     In the cylinder  31 , the hydrogen H 2  is stored so as to be in a high pressure state. The cylinder  31  is provided most upstream of the anode line  30 . 
     The anode pressure adjustment valve  32  is provided downstream with respect to the cylinder  31 . The anode pressure adjustment valve  32  adjusts the pressure of the anode gas newly supplied from the cylinder  31  to the anode line  30 . The pressure of the anode gas is adjusted by the degree of opening of the anode pressure adjustment valve  32 . 
     A purge valve  34  is provided downstream with respect to the fuel cell stack  10 . When the purge valve  34  is opened, the anode gas is purged. 
     The pressure of the anode gas flowing along the anode line  30  is detected by an anode pressure sensor  301 . The anode pressure sensor  301  is provided downstream with respect to the adjustment valve  32  but upstream with respect to the fuel cell stack  10 . 
     Cooling water supplied to the fuel cell stack  10  flows along the cooling water circulation line  40 . In the cooling water circulation line  40 , a radiator  41 , a three-way valve  42  and a water pump  43  are provided. A bypass line  400  is provided in parallel to the cooling water circulation line  40 . The bypass line  400  is branched from the cooling water circulation line  40  that is upstream with respect to the radiator  41 , and is combined with the cooling water circulation line  40  that is downstream with respect to the radiator  41 . Hence, the cooling water flowing along the bypass line  400  bypasses the radiator  41 . 
     The radiator  41  cools the cooling water. In the radiator  41 , a cooling fan  410  is provided. 
     The three-way valve  42  is located in a combination portion between the bypass line  400  and the cooling water circulation line  40 . The three-way valve  42  adjusts, according to the degree of opening, the quantity of flow of the cooling water flowing along the line on the radiator side and the quantity of flow of the cooling water flowing along the bypass line. Thus, the temperature of the cooling water is adjusted. 
     The water pump  43  is located downstream with respect to the three-way valve  42 . The water pump  43  feeds the cooling water flowing through the three-way valve  42  to the fuel cell stack  10 . 
     The temperature of the cooling water flowing along the cooling water circulation line  40  is detected by a water temperature sensor  401 . The water temperature sensor  401  is provided upstream with respect to the portion where the bypass line  400  is branched. 
     A controller inputs the signals of the current sensor  101 , the voltage sensor  102 , the cathode pressure sensor  201 , the cathode quantity-of-flow sensor  202 , the cathode pressure sensor  203 , the anode pressure sensor  301  and the water temperature sensor  401 . Then, the controller outputs the signals to control the operations of the compressor  21 , the cathode pressure adjustment valve  23 , the bleed valve  210 , the anode pressure adjustment valve  32 , the purge valve  34 , the three-way valve  42  and the water pump  43 . 
     With this configuration, the fuel cell stack  10  is kept at an appropriate temperature, and while the electrolyte membrane is being kept in an appropriate humid state, the reactive gases (the oxygen O 2  and the hydrogen H 2 ) for generating electricity are supplied sufficiently, with the result that it is possible to perform stable electric generation. Power generated by the fuel cell stack  10  is supplied through a DC/DC converter  11  to a travel motor  12 , a battery  13  and a load  14 . 
       FIG. 2  is a correlation diagram between the charging rate SOC of the battery and the generated power of the fuel cell stack. 
     The inventors are in the process of developing a system that uses the generated power of the fuel cell stack  10  and/or the power of the battery  13  to drive the travel motor  12  and the load  14 . In order not to reduce the life of the battery  13 , it is preferable to manage the battery charging rate SOC to a predetermined value. However, since power is consumed significantly in a transient operating state, the battery charging rate SOC is more likely to be varied. Even in a steady operating state, the battery charging rate SOC is more likely to be varied by a variation in the generated power of the fuel cell stack or a variation in an auxiliary load. Although in the first place, the battery charging rate SOC is calculated by totaling current sensor values, resetting is performed as necessary to remove an error. In this way, an error is removed, and at the same time the battery charging rate SOC is likely to be varied. It is important to reduce the variation in the battery charging rate SOC described above and to manage the battery charging rate SOC to the predetermined value. 
     Hence, as shown in  FIG. 2 , when the battery charging rate SOC is higher than a management value, the generated power of the fuel cell stack  10  is reduced. In this way, the battery  13  is discharged. Consequently, the battery charging rate SOC approaches the management value. When the battery charging rate SOC is lower than the management value, the generated power of the fuel cell stack  10  is increased. In this way, the battery  13  is charged. Consequently, the battery charging rate SOC approaches the management value. 
       FIG. 3  is a diagram illustrating a problem to be solved in the present embodiment. 
     As described above, depending on whether the battery charging rate SOC is higher or lower than the predetermined value (management value), the generated power of the fuel cell stack  10  is adjusted, and thus the battery charging rate SOC is managed to the predetermined value (management value). 
     However, when the generated power of the fuel cell stack  10  is adjusted, conventionally, as shown in  FIG. 3 , the quantity of flow supplied from the compressor is controlled to vary the quantity of flow of the air flowing along the cathode line  20 . Here, the rotation speed of the compressor  21  is varied. 
     For example, when a vehicle is at rest, the sound of the compressor  21  is varied, and thus a passenger feels such sound harsh to have an uncomfortable feeling. 
     Hence, in the present embodiment, without the rotation speed of the compressor  21  being varied, the generated power of the fuel cell stack  10  is adjusted. 
     A specific manner will be described below. 
       FIG. 4  is a control flowchart that is performed by a controller of the control device for the fuel cell system according to the present invention. The controller repeatedly performs this flowchart every minute time (for example, 10 milliseconds). 
     In step S 1 , the controller computes the amount of air supplied to the fuel cell stack  10 . Specific details will be described later. 
     In step S 2 , the controller determines an operation mode. Specific details will be described later. 
     In step S 3 , the controller determines whether or not the operation mode is a sound vibration mode. If the operation mode is not the sound vibration mode, the controller transfers the processing to step S 4  whereas if the operation mode is the sound vibration mode, the controller transfers the processing to step S 5 . 
     In step S 4 , the controller performs a normal mode. Specific details will be described later. 
     In step S 5 , the controller performs the sound vibration mode. Specific details will be described later. 
       FIG. 5  is a block diagram showing the function of computing the amount of air supplied to the fuel cell stack. 
     The following individual blocks shown in the block diagram indicate the individual functions of the controller as imaginary units, and the blocks do not mean physical entities. 
     The block B 101  computes travel power based on a shift range, an accelerator pedal operation amount and a vehicle speed. Specifically, among a plurality of maps prepared previously, a map corresponding to the present shift range is selected. Then, the accelerator pedal operation amount and the vehicle speed are applied to the map, and the travel power (=power necessary for the motor to satisfy the requirement of a driver) for which the motor produces a torque necessary for travel is computed. 
     The block B 102  computes the generated power for managing the battery charging rate SOC to the management value. 
     The block B 103  adds the consumption power of the auxiliary load, the travel power computed by the block B 101  and the generated power computed by the block B 102  to determine target generated power to be generated by the fuel cell. In the generated power of the battery, when the battery is charged, a positive value is set whereas when the battery is discharged, a negative value is set. When the battery needs charging, in order to perform the charging by the generated power of the fuel cell, it is necessary for the fuel cell to increase the generated power with consideration given to the charging for the battery. On the other hand, when the battery needs discharging, in order for the battery to compensate for part of the power to the motor, it is necessary to reduce the generated power of the fuel cell. 
     The block B 104  computes the amount of air supplied to the fuel cell stack  10  based on the target generated power computed by the block B 103 . 
     As described above, the processing of step S 1  in the flowchart has been performed. 
       FIG. 6  is a diagram showing a subroutine that determines the operation mode. 
     In step S 21 , the controller determines whether or not the sound pressure level of the compressor  21  is higher than that of background noise. The sound pressure level is preferably detected by attaching, for example, a microphone. Preferably, the determination in the present step is performed, for example, when the vehicle is at rest or when the fuel cell is in an idle state (the state where the power to the motor is 0, and only the power supply to the auxiliary device and the battery is performed). If the determination result is negative, the controller transfers the processing to step S 22  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 22 , the controller determines the normal mode. 
     In step S 23 , the controller determines the sound vibration mode. 
       FIG. 7  is a diagram showing a subroutine in a normal mode operation. 
     In step S 41 , the controller computes the amount of air corresponding to the generated power required for the fuel cell. On the other hand, the amount of air is compared with the minimum quantity of flow that the compressor  21  achieves. As this quantity of flow, there is a quantity of flow for avoiding the surge of the compressor  21  (for example, when the quantity of flow in the compressor is low, if the pressure of the stack is high, a surge is produced, and hence, the minimum quantity of flow for preventing a surge is set). Then, the quantity of flow of air corresponding to the generated power is compared with the minimum quantity of flow that the compressor needs to achieve, and if the quantity of flow of air corresponding to the generated power is higher, the compressor  21  supplies the quantity of flow of air corresponding to the generated power. On the other hand, if the minimum quantity of flow that the compressor  21  needs to achieve is higher, the compressor  21  supplies this quantity of flow. 
     In step S 42 , the controller computes the amount of air flowing along the bleed line  200 . When the compressor  21  supplies the quantity of flow corresponding to the generated power of the fuel cell, the quantity of flow that is bled is 0. However, when the compressor  21  supplies the minimum quantity of flow that the compressor  21  needs to achieve, the quantity of flow higher than that needed by the fuel cell is supplied to the fuel cell. Since an unnecessary quantity of flow is supplied to the fuel cell stack, for example, the humid state of the fuel cell is disadvantageously dried. Since the electric generation is determined by the load of an external load, the unnecessary air does not affect the power balance. Hence, in order to prevent the unnecessary air from being supplied to the fuel cell, the bleed valve is opened such that only the amount of air necessary for the electric generation is supplied to the fuel cell stack. In the present step, the quantity of flow to the bleed valve is computed. Specific details will be described later. 
     In step S 43 , the controller adjusts the degree of opening of the bleed valve  210  such that the amount of air computed in step S 42  flows along the bleed line  200 . 
       FIG. 8  is a block diagram showing the function of computing the amount of air bled in the normal mode operation. 
     The block B 421  subtracts the amount of air supplied to the fuel cell stack  10  from the lower limit quantity of flow in the compressor  21 . When the quantity of flow supplied is excessively low, the compressor  21  may be subjected to a surge. The lower limit quantity of flow in the compressor  21  refers to the minimum quantity of flow for preventing such a problem. When the amount of air supplied to the fuel cell stack  10  is higher than the lower limit quantity of flow in the compressor  21 , the block B 421  outputs a negative value. When the amount of air supplied to the fuel cell stack  10  is lower than the lower limit quantity of flow in the compressor  21 , the block B 421  outputs a positive value. 
     When the output result of the block B 421  is a positive value, the block B 422  outputs such a positive value as it is whereas when the output result is a negative value, the block B 422  outputs zero. The case of a negative value is that under conditions in which the compressor  21  supplies the air necessary for the electric generation of the fuel cell stack, the bleed valve  210  is preferably all closed. Specifically, it is necessary to compute a value at which the bleed valve does not pass the air, and since such a value is 0, in the case of a negative value, the value is set at zero. On the other hand, the case of a positive value is the conditions under which the compressor  21  passes a higher quantity of flow than the quantity of flow that the fuel cell stack needs. Here, the bleed valve  210  passes the quantity of flow that is the positive value, as it is, to the bleed line  200 , and thus only the quantity of flow necessary for the electric generation is supplied to the fuel cell stack. 
     A description will now be given again of the blocks B 423 , B 424  and B 425 . 
     In the above discussion, the computation of the quantity of flow bled in a case where the minimum quantity of flow for avoiding the surge of the compressor  21  itself is set as the quantity of flow in the compressor  21  has been described. Here, a description will be given of the computation of the quantity of flow bled in a case where the quantity of flow for the dilution of the purge gas of the fuel cell is set in the compressor  21 . 
     A description will first be given of the dilution. 
     Nitrogen on the cathode side is permeated, through an ion exchange membrane, which is an electric generation area, to the anode side of the fuel cell stack. Since in order to stably generate electricity on the anode side, it is necessary to maintain highly concentrated hydrogen, the nitrogen is purged together with hydrogen gas either periodically or by detecting the concentration of the nitrogen. In this way, it is necessary to keep the concentration of the hydrogen within the anode high. 
     On the other hand, when the purge is performed, the hydrogen is discharged out of the fuel cell stack together with the nitrogen, and in the present embodiment, in order for the concentration of the hydrogen to be reduced to a predetermined concentration or less, the off-gas of the hydrogen is mixed with the off-gas of the oxygen to perform the dilution. 
     In general, when the generated power is high, since the quantity of flow of air necessary for the electric generation of the fuel cell stack is supplied, and thus the quantity of flow necessary for the dilution is supplied, the compressor  21  supplies the amount of air corresponding to the generated power. 
     However, when the generated power is lower, with respect to the hydrogen to be purged from the fuel cell stack, the desired concentration of the hydrogen or less may not be achieved only by the quantity of flow necessary for the electric generation. Hence, the quantity of flow necessary for the dilution is supplied by the compressor  21 . 
     When the quantity of flow necessary for the dilution is passed, with respect to the quantity of flow necessary for the electric generation of the fuel cell stack, the bleed valve  210  is used to pass the unnecessary quantity of flow to the bleed line  200 , and this point is the same as what has been described above. 
     The block B 423  first subtracts the quantity of flow consumed by the electric generation from the amount of air supplied to the fuel cell stack  10 . Thus, the amount of air that is discharged without being consumed in the fuel cell stack  10  is output. 
     The block B 424  subtracts the amount of air that is discharged without being consumed in the fuel cell stack  10  from a dilution requirement air amount. When the purge valve  34  is opened, the anode gas H 2  is discharged. The amount of air necessary to dilute the anode gas H 2  is the dilution requirement air amount. When the amount of air that is discharged without being consumed by the electric generation in the fuel cell stack  10  is higher than the dilution requirement air amount, the block B 424  outputs a negative value. When the amount of air that is discharged without being consumed by the electric generation in the fuel cell stack  10  is lower than the dilution requirement air amount, the block B 424  outputs a positive value. 
     When the output result of the block B 424  is the positive value, the block B 425  outputs it as it is whereas the output result of the block B 424  is the negative value, the block B 425  outputs zero. 
     The block B 426  compares the output of the block B 422  and the output of the block B 425 , and outputs the higher one as a bleed air amount. 
     In the present embodiment, the supply amount by the compressor  21  is varied according to the electric generation of the fuel cell. In particular, while the fuel cell is performing an idle operation, when the generated power required by the fuel cell is varied by SOC control, the rotation speed of the compressor  21  is varied according to such variation, with the result that there is a concern that under conditions in which background noise is low, an uncomfortable feeling is given to the driver. Even when the compressor  21  itself passes the minimum quantity of flow or the quantity of flow for the dilution, if the quantity of flow of air based on the electric generation requirement of the fuel cell is increased by the requirement of the SOC, there is a concern that the rotation speed of the compressor  21  is likewise varied. 
     Hence, a description will be given below of a control logic for preventing the rotation speed of the compressor  21  from being varied according to the requirement of the SOC when background noise is low. 
       FIG. 9  is a diagram showing a subroutine of a sound vibration mode operation. 
     In step S 51 , the controller supplies a sound vibration mode air amount (constant value) from the compressor  21 . This value is preferably a value that is, in the sound vibration mode, higher than the maximum amount of air set based on the electric generation requirement. This is because such a value is set and thus it is possible to prevent the rotation speed of the compressor  21  from being varied completely based on the SOC requirement. 
     Even in the sound vibration mode, since the compressor  21  supplies the quantity of flow higher than the quantity of flow based on the electric generation requirement of the fuel cell stack, the amount of air unnecessary for the electric generation of the fuel cell stack is passed to the bleed line  200  by the bleed valve  210 . 
     In step S 52 , the controller first computes the amount of air passed to the bleed line  200 . Specific details will be described later. 
     In step S 53 , the controller adjusts the degree of opening of the bleed valve  210  such that the amount of air computed in step S 52  is passed to the bleed line  200 . 
       FIG. 10  is a block diagram showing the function of computing the amount of air bled in the sound vibration mode operation. 
     A block B 521  subtracts the amount of air supplied to the fuel cell stack  10  from the lower limit quantity of flow in the compressor  21 . The block B 521  is basically the same as the block B 421 . 
     When the output result of the block B 521  is a positive value, the block B 522  outputs such a positive value as it is whereas when the output result is a negative value, the block B 522  outputs zero. The block B 522  is basically the same as the block B 422 . 
     A block B 523  subtracts the quantity of flow consumed by the electric generation from the amount of air supplied to the fuel cell stack  10 . The block B 523  is basically the same as the block B 423 . 
     A block B 524  subtracts the amount of air that is discharged without being consumed by the electric generation in the fuel cell stack  10  from the dilution requirement air amount. The block B 524  is basically the same as the block B 424 . 
     When the output result of the block B 524  is a positive value, the block B 525  outputs such a positive value as it is whereas when the output result is a negative value, the block B 525  outputs zero. The block B 525  is basically the same as the block B 425 . 
     A block B 526  subtracts the amount of air corresponding to the generated power required by the fuel cell stack  10  from the sound vibration mode air amount. Under conditions in which the compressor  21  supplies the amount of air in the sound vibration mode, it is desired to supply only the air necessary for the electric generation to the fuel cell stack. The amount of air supplied to the bleed line  200  to achieve this is computed as described above. In the sound vibration mode operation, since the sound vibration mode air amount is higher than the amount of air supplied to the fuel cell stack  10 , the block B 526  outputs a positive value. 
     Since the output result of the block B 526  is a positive value, a block B 527  generally outputs the positive value as it is. However, since it is assumed that a certain abnormality occurs in computation by a computer and thus the quantity of flow based on the electric generation requirement may become higher than the sound vibration mode air amount, the block B 526  deals with this case. If a negative value is output from the block B 526 , the block B 527  deals with this case by outputting zero. 
     A block B 528  compares the output of the block B 522 , the output of the block B 525  and the output of the block B 527 , and outputs the highest one as the amount of air bled. 
       FIG. 11  is a diagram illustrating an operation in the normal mode operation in a first embodiment. 
     In the normal mode operation, the amount of air corresponding to the electric generation requirement to the fuel cell is supplied from the compressor. However, if the quantity of flow supplied is excessively low, a surge is likely to occur. Hence, even when the quantity of flow supplied is low such as when the electric generation is low in  FIG. 11 , the air corresponding to the lower limit quantity of flow in the compressor  21  is supplied, and an extra amount of air exceeding the quantity of flow (FC stack supply air amount) necessary for generating the target generated power of the fuel cell stack is passed to the bleed line. 
     When the electric generation is increased, the amount of air higher than when the electric generation is low is supplied from the compressor. 
     As described above, in the normal mode operation, the quantity of flow in the compressor  21  is varied according to the state of the operation. 
       FIG. 12  is a diagram illustrating an operation in the sound vibration mode operation in the first embodiment. 
     In the sound vibration mode operation, the sound vibration mode air amount is supplied from the compressor  21 . This sound vibration mode air amount is a constant value regardless of when the electric generation is decreased or when the electric generation is increased. 
     The extra amount of air exceeding the quantity of flow (FC stack supply air amount) necessary for generating the target generated power of the fuel cell stack is passed to the bleed line. In other words, the amount of air supplied to the fuel cell stack is adjusted according to the amount of air bled. 
       FIG. 13  is a time chart illustrating the operation in the sound vibration mode operation in the first embodiment. 
     As described above, the fuel cell system adjusts the generated power of the fuel cell stack  10  according to whether the battery charging rate SOC is higher or lower than the predetermined value (management value), and thereby manages the battery charging rate SOC to the predetermined value (management value). 
     In the present embodiment, a constant sound vibration mode air amount is supplied from the compressor  21 . In other words, the rotation speed of the compressor  21  is kept constant. Then, by adjusting the amount of air bled, the amount of air supplied to the fuel cell stack is adjusted. 
     When the generated power of the fuel cell stack  10  is adjusted, the rotation speed of the compressor  21  is varied, and thus the passenger feels harsh sound to have an uncomfortable feeling. 
     By contrast, in the present embodiment, since the rotation speed of the compressor  21  is kept constant, the operating sound of the compressor  21  is not varied, and thus the passenger does not have an uncomfortable feeling. 
     In the present embodiment, only when the sound pressure level of the compressor  21  is higher than that of background noise, the sound vibration mode operation is performed to keep the rotation speed of the compressor  21  at a high constant speed. 
     When the sound pressure level of the compressor  21  is lower than that of background noise, even if the sound of the compressor  21  is varied, it is not recognized. However, when the sound pressure level of the compressor  21  is higher than that of background noise, the variation in the sound of the compressor  21  is recognized. 
     The rotation speed of the compressor  21  is kept at a high constant speed, and power is wastefully consumed accordingly. 
     Hence, in the present embodiment, the operation scene in the sound vibration mode is limited, and thus it is possible to avoid waste power consumption as much as possible. 
     Second Embodiment 
       FIG. 14  is a diagram showing an operation mode determination subroutine in a second embodiment of the fuel cell system according to the present invention. 
     In the following description, parts having the same functions as described above are identified with the same symbols, and their description will be omitted as necessary. 
     In the first embodiment, for example, whether or not the sound pressure level of the compressor  21  is higher than that of background noise is determined directly by a sound pressure level detected by the microphone. However, in such a method, the microphone is needed, and thus the cost is increased. In the present embodiment, whether or not the sound pressure level of the compressor  21  is higher than that of background noise is estimated (determined indirectly) based on the state of the operation. Specific details will be described later. 
     In step S 211 , the controller determines whether or not the vehicle is at rest. If the determination result is negative, the controller transfers the processing to step S 212  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 212 , the controller determines whether or not the shift range is a parking range (“P range”) or a neutral range (“N range”). If the determination result is negative, the controller transfers the processing to step S 213  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 213 , the controller determines whether or not the vehicle is travelling at a low speed. The low-speed travel refers to travel at a speed lower than a speed when the sound pressure level of background noise is lower than that of the compressor  21 . A threshold value for determining whether or not the vehicle is travelling at a low speed is set by previously performing an experiment or the like. If the determination result is negative, the controller transfers the processing to step S 214  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 214 , the controller determines whether or not the mode is a quiet mode. The quiet mode is set by the operation of a switch by the driver and the like. If the determination result is negative, the controller transfers the processing to step S 215  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 215 , the controller determines whether or not the fuel cell stack  10  is in a low electric generation state. The low electric generation state refers to a state where the amount of electricity is lower than an amount of electricity when the sound pressure level of background noise is lower than that of the compressor  21 . A threshold value for determining whether or not the fuel cell stack  10  is in the low electric generation state is set by previously performing an experiment or the like. If the determination result is negative, the controller transfers the processing to step S 22  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 22 , the controller determines the normal mode. 
     In step S 23 , the controller determines the sound vibration mode. 
     In the present embodiment, in the operation scene where the sound pressure level of background noise is lower than that of the compressor  21 , it is estimated that the sound pressure level of background noise is lower than that of the compressor  21 . In this way, without use of a microphone, it is possible to estimate (indirectly determine) whether or not the sound pressure level of the compressor  21  is higher than that of background noise. Thus, it is possible to reduce the cost to be inexpensive. 
     Third Embodiment 
       FIG. 15  is a diagram showing a sound vibration mode operation subroutine in a third embodiment of the fuel cell system according to the present invention. 
     Since steps S 51  to S 53  are the same as in the first embodiment, their description will be omitted. 
     In step S 54 , the controller determines whether or not a surge occurs in the compressor  21 . This is determined based on the amount of air supplied by the compressor  21  and a pressure ratio in the compressor  21  (a ratio (P2/P1) of an exit pressure P2 to an entrance pressure P1 of the compressor  21 ). If the determination result is negative, the controller transfers the processing to step S 55  whereas if the determination result is positive, the controller transfers the processing to step S 56 . 
     In step S 55 , the controller determines whether or not a differential pressure between the front and back surfaces of the electrolyte membrane of the fuel cell does not exceed a membrane resistance differential pressure. The pressure on the cathode side is computed by subtracting a loss caused by the passing of the cathode gas through the WRD  22  or the like from the detection value of the cathode pressure sensor  203 . The pressure on the anode side is detected by the anode pressure sensor  301 . As the case where the differential pressure between the front and back surfaces of the electrolyte membrane of the fuel cell exceeds the membrane resistance differential pressure, a case where the pressure on the cathode side is high and a case where the pressure on the anode side is high can be generally considered. However, in the present embodiment, since the mode is the sound vibration mode (the mode in which as compared with the normal mode, the amount of air supplied from the compressor  21  is increased), the case where the pressure on the cathode side is high applies. If the determination result is negative, the controller temporarily stops the processing whereas if the determination result is positive, the controller transfers the processing to step S 56 . 
     In step S 56 , the controller increases the degree of opening of the cathode pressure adjustment valve  23 . 
       FIG. 16  is a diagram illustrating an operation method of the compressor  21  in the normal mode operation. In  FIG. 16 , the horizontal axis is the quantity of flow of air, and the vertical axis is the pressure ratio (the ratio (P2/P1) of the exit pressure P2 to the entrance pressure P1 of the compressor  21 ). 
     In the present embodiment, although when the sound pressure level of background noise is low such as in the idle operation, the sound vibration mode operation is performed, even if the idle operation is performed, a case where the normal mode operation is performed will be considered. 
     In the idle operation, an operation point of the compressor  21  required from the generated power of the fuel cell stack  10  is A. In other words, the quantity of flow in the compressor  21  is preferably Q A . However, since the operation point A is in a surge region above a surge line, a surge may occur in the compressor  21 . Hence, the quantity of flow is set at Q B , and the compressor  21  is operated at operation point B. However, in this state, an extra amount of air is inevitably supplied to the fuel cell stack  10 . Hence, (Q B -Q A ) is bled. 
     When the electric generation is increased, an operation point of the compressor  21  required from the generated power of the fuel cell stack  10  is C. In other words, the quantity of flow in the compressor  21  is preferably Q C . However, since the operation point C is in the surge region above the surge line, a surge may occur in the compressor  21 . Hence, the quantity of flow is set at Q D , and the compressor  21  is operated at operation point D. However, in this state, an extra amount of air is inevitably supplied to the fuel cell stack  10 . Hence, (Q D -Q C ) is bled. 
     As described above, in the normal mode, the quantity of flow in the compressor  21  is adjusted from Q B  to Q D , and the quantity of flow exceeding the quantity of flow necessary for the electric generation is bled. 
       FIG. 17  is a diagram illustrating a problem when in the sound vibration mode operation, the cathode pressure adjustment valve is not controlled. 
     Then, a case where the sound pressure level of background noise is low and the sound vibration mode operation is performed will be considered. 
     Although in the present embodiment, when a surge is likely to occur in the compressor  21  (yes in step S 54 ), the quantity of flow in the compressor  21  is increased (step S 56 ), and the degree of opening of the cathode pressure adjustment valve  23  is increased (step S 57 ), a case where the degree of opening of the cathode pressure adjustment valve  23  is not adjusted will be temporarily considered. 
     In the sound vibration mode, a sound vibration mode air amount Q E  is supplied from the compressor  21 . However, in the idle operation, the quantity of flow required from the generated power of the fuel cell stack  10  may be Q A . Hence, in the idle operation, the sound vibration mode air amount Q E  is supplied from the compressor  21 , and (Q E -Q A ) is bled. 
     Then, a case where the amount of electricity is increased and the quantity of flow Q C  is supplied to the fuel cell stack  10  will be considered, and Q C =Q A +ΔQ. Here, the amount of air supplied from the compressor  21  remains the sound vibration mode air amount Q E , and (Q E -Q C ) is bled. 
     Here, if the degree of opening of the cathode pressure adjustment valve  23  is not adjusted, the amount of air supplied to the fuel cell stack  10  is increased by ΔQ, and the exit pressure P2 of the compressor  21  is increased accordingly, with the result that there is a possibility that it is impossible to avoid a surge. As the exit pressure P2 of the compressor  21  is increased, the pressure on the cathode side of the electrolyte membrane of the fuel cell is increased. Consequently, the differential pressure of the front and back surfaces of the electrolyte membrane is likely to exceed the membrane resistance differential pressure. 
       FIG. 18  is a diagram illustrating the effect of action in a third embodiment in the sound vibration mode operation. 
     As described above, when the amount of electricity is increased and the quantity of flow Q C  is supplied to the fuel cell stack  10 , if no measure is taken, the amount of air supplied to the fuel cell stack  10  is increased by ΔQ, and the exit pressure P2 of the compressor  21  is increased accordingly, with the result that there is a possibility that it is impossible to avoid a surge. 
     Hence, in the present embodiment, the degree of opening of the cathode pressure adjustment valve  23  is increased, and thus the exit pressure P2 of the compressor  21  is prevented from being increased, with the result that the compressor  21  is operated at operation point E0. Then, the extra amount of air (Q E -Q C ) is bled. In this way, it is possible to prevent a surge from occurring in the compressor  21 . 
     When the degree of opening of the cathode pressure adjustment valve  23  is excessively increased, as indicated by C1, it is likely that the exit pressure P2 of the compressor  21  is excessively lowered. In this case, the pressure on the cathode side of the electrolyte membrane of the fuel cell is low. Consequently, the differential pressure of the front and back surfaces of the electrolyte membrane is likely to exceed the membrane resistance differential pressure. In order to avoid this problem, the degree of opening of the cathode pressure adjustment valve  23  is adjusted. In this way, it is possible to protect the electrolyte membrane. 
     Fourth Embodiment 
       FIG. 19  is a diagram showing an operation mode determination subroutine in a fourth embodiment of the fuel cell system according to the present invention. 
     In step S 221 , the controller determines whether or not the bleed valve  210  is currently open. If the determination result is negative, the controller transfers the processing to step S 22  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 22 , the controller determines the normal mode. 
     In step S 23 , the controller determines the sound vibration mode. 
     When the bleed valve  210  is open, at present, the compressor  21  supplies the amount of air exceeding the amount of air necessary for the electric generation of the fuel cell stack  10 , and bleeds the extra amount of air. In this case, an amount of air exceeding the sound vibration mode air amount is likely to be supplied. Thus, it is possible to supply the sound vibration mode air amount without wastefully operating the compressor  21 . Even if the sound vibration mode air amount is not exceeded, since the amount of air bled is present, as compared with the case where the bleed valve  210  is closed, it is possible to supply the sound vibration mode air amount with a small amount. Hence, in the present embodiment, it is possible to prevent the compressor  21  from being wastefully operated to waste power. 
     Fifth Embodiment 
       FIG. 20  is a diagram showing an operation mode determination subroutine in a fifth embodiment of the fuel cell system according to the present invention. 
     In step S 231 , the controller determines whether or not a dilution air amount excessing the amount of air necessary for the electric generation of the fuel cell stack  10  is needed. If the determination result is negative, the controller transfers the processing to step S 232  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 232 , the controller determines whether or not an operation in which the amount bled is increased for a transient operation is being performed. In the transient operation, since it is likely that the electric generation of the fuel cell stack  10  is rapidly increased, it is likely that the compressor  21  is previously rotated beyond necessity and the amount bled is increased. The controller determines whether or not the state of the operation described above is present. If the determination result is negative, the controller transfers the processing to step S 233  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 233 , the controller determines whether or not a surge avoiding operation is being performed. The surge avoiding operation is an operation in which in order to avoid a surge, the compressor  21  is rotated beyond necessity and the amount bled is increased. If the determination result is negative, the controller transfers the processing to step S 233  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 234 , the controller determines whether or not the fuel cell stack  10  has just been started up. Immediately after the fuel cell stack  10  has been started up, the anode pressure is increased and purge is continuously performed. Hence, at that time, in order to dilute purged hydrogen, the amount bled is increased. In order to increase the amount bled, the compressor  21  supplies an amount of air exceeding the amount of air necessary for the electric generation of the fuel cell stack  10 . The controller determines whether or not the state of the operation described above is present. If the determination result is negative, the controller transfers the processing to step S 22  whereas if the determination result is positive, the controller transfers the processing to step S 23 . 
     In step S 22 , the controller determines the normal mode. 
     In step S 23 , the controller determines the sound vibration mode. 
     In the present embodiment, in the scene where the compressor  21  currently supplies an amount of air exceeding the amount of air necessary for the electric generation of the fuel cell stack  10  and an extra amount of air is bled, the sound vibration mode operation is performed. In such a scene, it is likely that an amount of air exceeding the sound vibration mode air amount is supplied. Thus, it is possible to supply the sound vibration mode air amount without wastefully operating the compressor  21 . Even if the sound vibration mode air amount is not exceeded, since the amount of air bled is present, it is possible to supply the sound vibration mode air amount by a small increase in amount. Hence, in the present embodiment, it is possible to prevent the compressor  21  from being wastefully operated to waste power. 
     Sixth Embodiment 
       FIG. 21  is a diagram showing a sound vibration mode subroutine in a sixth embodiment of the fuel cell system according to the present invention. 
     Since steps S 51  and S 52  are the same as in the first embodiment, their description will be omitted. 
     In step S 521 , the controller computes surplus generated power. The surplus generated power is power that is generated in the fuel cell stack  10  when the entire amount of air supplied from the compressor  21  is supplied to the fuel cell stack  10  without being bled. A specific computation method will be described later. 
     In step S 522 , the controller determines whether or not the surplus generated power is lower than the target generated power. The target generated power is computed, for example, as shown in  FIG. 5 . If the determination result is negative, the controller transfers the processing to step S 53  whereas if the determination result is positive, the controller transfers the processing to step S 523 . 
     In step S 523 , the controller increases the quantity of flow in the compressor  21  such that the surplus generated power exceeds the target generated power. A specific computation method will be described later. 
       FIG. 22  is a block diagram showing the function of computing the surplus generated power. 
     A block B 5211  adds together the amount of air actually supplied to the fuel cell stack and the amount of air bled. 
     A block B 5212  applies the amount of air to a preset map to determine the surplus generated power. 
       FIG. 23  is a block diagram showing the function of computing an increased amount of air supplied by the compressor. 
     A block B 5231  computes, based on the target generated power, the amount of air supplied to the fuel cell stack  10 . 
     A block B 5232  computes an increased amount of air by subtracting the amount of air actually supplied from the compressor  21  from the amount of air computed in the block B 5231 . 
       FIG. 24  is a diagram illustrating the effect of action in the sixth embodiment. 
     In the present embodiment, when the target generated power is increased, the amount of air supplied from the compressor is increased. In this way, for example, even if the target generated power is increased by a decrease in the battery charging rate SOC and the like, the fuel cell stack  10  can appropriately generate electricity. 
     Although the embodiments of the present invention have been described above, the embodiments described above simply indicate part of examples to which the present invention is applied, and it is not intended that the technical range of the present invention is limited to the specific configurations of the embodiments described above. 
     For example, in the sound vibration mode, a constant amount of air (sound vibration mode air amount) is supplied so that the operating sound of the compressor is not varied. However, if the constant amount of air (sound vibration mode air amount) is excessive, the passenger may feel harsh sound to have an uncomfortable feeling without the operating sound of the compressor being varied. Hence, the compressor is preferably operated in such a range that the passenger does not have such an uncomfortable feeling. Even when as in the sixth embodiment, the amount of air supplied from the compressor is increased halfway, an upper limit may be provided so that the passenger does not have an uncomfortable feeling. 
     The embodiments described above can be combined as necessary. 
     This application claims priority based on Japanese Patent Application No. 2011-219298 filed on Oct. 3, 2011 with Japan Patent office, the entire disclosure of which is incorporated herein by reference.