Patent Publication Number: US-2017373334-A1

Title: Fuel cell system

Description:
TECHNICAL FIELD 
     This invention relates to a fuel cell system. 
     BACKGROUND ART 
     In US 2012/0,156,575 A, there is discussed a fuel cell system, in which a gas amount discharged from the anode system is computed based on a pressure change inside the anode system in the downstream from the hydrogen supply valve while a supply of hydrogen to the fuel cell stops by closing the hydrogen supply valve. 
     SUMMARY OF INVENTION 
     While the purge valve is closed during the operation of the fuel cell system, nitrogen and the like transmitted from the cathode of the fuel cell to the anode through an electrolytic membrane are accumulated in the anode system, so that the hydrogen concentration inside the anode system gradually decreases. As the hydrogen concentration inside the anode system decreases, a voltage drop may occur during the operation of the fuel cell system. 
     In order to prevent such a voltage drop, the off-gas containing nitrogen or hydrogen is discharged from the inside of the anode system through the purge valve by opening the purge valve as necessary, so that the hydrogen concentration inside the anode system is maintained such that the voltage drop does not occur. In this case, whether or not the hydrogen concentration inside the anode system is appropriately managed is determined by estimating the amount of the off-gas discharged from the inside of the anode system using a purge valve. It is necessary to perform an additional purging operation when the purging amount is insufficient. This is because the hydrogen concentration inside the anode system changes depending on the purging amount, and the hydrogen concentration increases as the purging amount increases. 
     However, in the fuel cell system described above, the purge valve is opened arbitrarily regardless of the open/close state of the supply valve, and the purge valve is not opened necessarily in the hydrogen supply valve close state. For this reason, it is difficult to guarantee frequency of estimating the purge flow rate, and sometimes it takes time until the purge flow rate is estimated. Therefore, even when the hydrogen concentration inside the anode system decreases in the meantime, it is difficult to perform the additional purging operation and prevent the voltage drop. 
     This invention has been made to guarantee estimation frequency of the purge flow rate. 
     According to an aspect of this invention, there is provided a fuel cell system having a fuel cell supplied with an anode gas and a cathode gas to generate electricity from the fuel cell depending on a load. The fuel cell system comprises a supply valve configured to control a supply of the anode gas into an anode system of the fuel cell system, a purge valve configured to discharge an off-gas from the anode system, a pressure detecting portion configured to estimate or measures a pressure inside the anode system, a supply valve control portion configured to control an open/close operation of the supply valve based on a load of the fuel cell, a purge flow rate estimating portion configured to estimate a purge flow rate of the off-gas discharged from the anode system through the purge valve based on a pressure decrease of the anode system in a supply valve close state, and a purge valve control portion configured to open the purge valve in synchronization with the supply valve close state. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a fuel cell system according to an embodiment of this invention. 
         FIG. 2  is a diagram illustrating a method of estimating a purging amount according to an embodiment of this invention. 
         FIG. 3  is a diagram illustrating a gas input/output flow in the anode system in a hydrogen supply valve close state. 
         FIG. 4  is a flowchart illustrating a purge control operation according to an embodiment of this invention. 
         FIG. 5  is a map for computing a reference duty ratio based on a load of a fuel cell stack and a stack temperature. 
         FIG. 6  is a flowchart illustrating a purge valve open request signal generation process in detail. 
         FIG. 7  is a flowchart illustrating a purge valve open/close process in detail. 
         FIG. 8  is a flowchart illustrating a purge flow rate estimation process in detail. 
         FIG. 9  is a map for computing a gas amount per operation cycle flowing out from the inside of the anode system in a purge valve open state based on an anode pressure decrease amount. 
         FIG. 10  is a map for computing a hydrogen amount per operation cycle consumed in the fuel cell stack by generating electricity in a purge valve open state based on an output electric current. 
         FIG. 11  is a diagram illustrating a purge flow rate computation method. 
         FIG. 12  is a diagram illustrating a threshold value of the purge flow rate. 
         FIG. 13  is a flowchart illustrating a high-load purging process in detail. 
         FIG. 14A  is a flowchart illustrating a control of the hydrogen supply valve according to an embodiment of this invention. 
         FIG. 14B  is a map for computing pulsation upper/lower limit pressures based on a target output current. 
         FIG. 15  is a timing chart illustrating a purge control operation when the purge flow rate is equal to or higher than a threshold value in a typical operation region. 
         FIG. 16  is a timing chart illustrating a purge control operation when the purge flow rate is lower than the threshold value in a typical operation region. 
         FIG. 17  is a timing chart illustrating a purge control operation when the purge flow rate is equal to or higher than the threshold value in a high-load operation region. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of this invention will be described with reference to the accompanying drawings. 
     A fuel cell has an anode (fuel electrode), a cathode (oxidant electrode), and an electrolytic membrane interposed therebetween. The fuel cell generates electricity by receiving an anode gas (fuel gas) containing hydrogen supplied to the anode and a cathode gas (oxidant gas) containing oxygen supplied to the cathode. Electrode reactions generated in both the anode and the cathode are expressed as follows. 
       anode: 2H 2 →4H + +4 e   −   (1)
 
       cathode: 4H + +4 e   − +O 2 →2H 2 O   (2)
 
     Through the electrode reactions (1) and (2), the fuel cell generates an electromotive force of approximately 1 V. 
     In order to use such a fuel cell as a power source of a vehicle, a fuel cell stack obtained by stacking several hundreds of fuel cells is employed because high electric power is necessary. In addition, a fuel cell system is provided to supply the anode gas and the cathode gas to the fuel cell stack, so that electric power for driving a vehicle is extracted. 
       FIG. 1  is a schematic configuration diagram illustrating a fuel cell system  100  according to an embodiment of this invention. 
     The fuel cell system  100  comprises a fuel cell stack  1 , a cathode gas supply/discharge unit  2 , an anode gas supply/ discharge unit  3 , and a controller  4 . 
     The fuel cell stack  1  is obtained by stacking a plurality of fuel cells. The fuel cell stack  1  is supplied with an anode gas and a cathode gas to generate electric power necessary to drive a vehicle. 
     The cathode gas supply/discharge unit  2  comprises a cathode gas supply passage  21 , a cathode gas discharge passage  22 , a filter  23 , an air flow sensor  24 , a cathode compressor  25 , a cathode pressure sensor  26 , a water recovery device (hereinafter, referred to as a “WRD”)  27 , and a cathode pressure control valve  28 . The cathode gas supply/discharge unit  2  supplies the fuel cell stack  1  with the cathode gas and discharges the cathode off-gas discharged from the fuel cell stack  1  to the outside. 
     The cathode gas supply passage  21  is a passage for flowing the cathode gas supplied to the fuel cell stack  1 . One end of the cathode gas supply passage  21  is connected to the filter  23 , and the other end is connected to the cathode gas inlet hole of the fuel cell stack  1 . 
     The cathode gas discharge passage  22  is a passage for flowing the cathode off-gas discharged from the fuel cell stack  1 . One end of the cathode gas discharge passage  22  is connected to the cathode gas outlet hole of the fuel cell stack  1 , and the other end is opened. The cathode off-gas is a mixture gas between the cathode gas and water vapor generated through the electrode reactions. 
     The filter  23  is used to remove a foreign object out of the cathode gas input to the cathode gas supply passage  21 . 
     The air flow sensor  24  is provided in the cathode gas supply passage  21  in the upstream from the cathode compressor  25 . The air flow sensor  24  detects a flow rate of the cathode gas supplied to the cathode compressor  25  and supplied finally to the fuel cell stack  1 . 
     The cathode compressor  25  is provided in the cathode gas supply passage  21 . The cathode compressor  25  inputs the air (the atmospheric air) as a cathode gas to the cathode gas supply passage  21  through the filter  23  and supplies it to the fuel cell stack  1 . 
     The cathode pressure sensor  26  is provided in the cathode gas supply passage  21  between the cathode compressor  25  and the WRD  27 . The cathode pressure sensor  26  detects a pressure of the cathode gas supplied to the fuel cell stack  1  (hereinafter, referred to as a “cathode pressure”). 
     The WRD  27  is connected to each of the cathode gas supply passage  21  and the cathode gas discharge passage  22  to recover moisture in the cathode off-gas flowing through the cathode gas discharge passage  22  and use the recovered moisture to humidify the cathode gas flowing through the cathode gas supply passage  21 . 
     The cathode pressure control valve  28  is provided in the cathode gas discharge passage  22  in the downstream from the WRD  27 . The open/close state of the cathode pressure control valve  28  is controlled by the controller  4  to control a pressure of the cathode gas supplied to the fuel cell stack  1 . It is noted that, according to this embodiment, the cathode pressure is controlled to a desired pressure (target cathode pressure) basically by adjusting a rotation speed of the cathode compressor  25  and an opening level of the cathode pressure control valve  28 . 
     The anode gas supply/discharge unit  3  supplies the fuel cell stack  1  with the anode gas and discharges the anode off-gas discharged from the fuel cell stack  1  into the cathode gas discharge passage  22 . The anode gas supply/discharge unit  3  comprises a high-pressure hydrogen tank  31 , an anode gas supply passage  32 , a hydrogen supply valve  33 , an anode pressure sensor  34 , an anode gas discharge passage  35 , and a purge valve  36 . 
     The high-pressure hydrogen tank  31  maintains and stores the anode gas supplied to the fuel cell stack  1  in a high pressure state. 
     The anode gas supply passage  32  is a passage for supplying the anode gas discharged from the high-pressure hydrogen tank  31  to the fuel cell stack  1 . One end of the anode gas supply passage  32  is connected to the high-pressure hydrogen tank  31 , and the other end is connected to the anode gas inlet hole of the fuel cell stack  1 . 
     The hydrogen supply valve  33  is provided in the anode gas supply passage  32 . The open/close operation of the hydrogen supply valve  33  is controlled by the controller  4  to adjust a pressure of the anode gas supplied to the fuel cell stack  1  to a desired pressure. In addition, by controlling the open/close operation of the hydrogen supply valve  33 , the flow rate of the anode gas supplied to the fuel cell stack  1  is also controlled. 
     The anode pressure sensor  34  is provided in the anode gas supply passage  32  in the downstream from the hydrogen supply valve  33 . The anode pressure sensor  34  is a pressure detecting portion that detects a pressure of the anode gas supply passage  32  in the downstream from the hydrogen supply valve  33 . According to this embodiment, the pressure detected by the anode pressure sensor  34  is used as a pressure inside the anode system (hereinafter, referred to as an “anode pressure”) from the hydrogen supply valve  33  to the purge valve  36 . 
     The anode gas discharge passage  35  is a passage for flowing the anode off-gas discharged from the fuel cell stack  1 . The anode off-gas is a mixture gas between the remaining hydrogen (anode gas) not used in the electrode reactions, and nitrogen or water vapor transmitted from the cathode side to the anode side through an electrolytic membrane. One end of the anode gas discharge passage  35  is connected to the anode gas outlet hole of the fuel cell stack  1 , and the other end is connected to the cathode gas discharge passage  22 . 
     The anode off-gas discharged to the cathode gas discharge passage  22  is mixed with the cathode off-gas in the cathode gas discharge passage  22  and is discharged to the outside of the fuel cell system  100 . Since the anode off-gas contains the remaining hydrogen not used in the electrode reactions, the anode off-gas is mixed with the cathode off-gas and is discharged to the outside of the fuel cell system  100  to maintain a hydrogen concentration in the discharged gas at a predetermined concentration or lower. 
     The purge valve  36  is provided in the anode gas discharge passage  35 . The open/close operation of the purge valve  36  is controlled by the controller  4  to control a flow rate of the anode off-gas (hereinafter, referred to as a “purge flow rate”) discharged from the inside of the anode system to the cathode gas discharge passage  22 . 
     The controller  4  is a microcomputer comprising a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), and an input/output interface (I/O interface). 
     The controller  4  receives signals from various sensors for detecting an operation state of the fuel cell system  100 , such as an accelerator stroke sensor  41  for detecting a depression level of an accelerator pedal (hereinafter, referred to as an “accelerator manipulation amount”), a temperature sensor  42  for detecting a temperature of the coolant (hereinafter, referred to as a “stack temperature”) for cooling the fuel cell stack  1 , and a current sensor  43  for detecting an output current of the fuel cell stack  1  in addition to the air flow sensor  24  described above. 
     The controller  4  computes a target output current of the fuel cell stack  1  based on an operation state of the fuel cell system  100 . Specifically, the controller  4  computes the target output current of the fuel cell stack  1  based on a request of power from a driving motor (not illustrated) for driving a vehicle, requests of power from accessories such as cathode compressor  25 , and a charge/discharge request from a battery (not illustrated). 
     The controller  4  performs pulsating operation for increasing/decreasing the anode pressure periodically based on an operation state of the fuel cell system  100 . In the pulsating operation, basically, the anode pressure is pulsated by periodically increasing or decreasing the anode pressure within a range of the pulsation upper limit pressure and the pulsation lower limit pressure set depending on the target output current of the fuel cell stack  1 . By performing this pulsating operation, it is possible to obtain water drainability by discharging liquid water inside the anode system to the outside of the anode system when the anode pressure increases. 
     Here, during electricity generation of the fuel cell stack  1 , nitrogen or water vapor is transmitted from the cathode side to the anode side through the electrolytic membrane. For this reason, if the purge valve  36  remains in a closed state, hydrogen is consumed in the fuel cell stack  1 , and the transmitted nitrogen and the like are accumulated in the anode system. As a result, even when a pressure inside the anode system (anode pressure) is controlled to the same pressure, the hydrogen concentration inside the anode system gradually decreases as much as the transmitted nitrogen and the like. In this manner, if electricity is generated while the hydrogen concentration inside the anode system decreases, hydrogen necessary to generate electricity inside the fuel cell stack  1  becomes short, and a voltage drop may occur even when the anode pressure is controlled to a target value. 
     Meanwhile, when the purge valve  36  is opened, nitrogen and the like accumulated in the anode system are discharged from the inside of the anode system as an anode off-gas. Therefore, the hydrogen concentration inside the anode system increases (is recovered). That is, the hydrogen concentration inside the anode system changes depending on the amount of the anode off-gas (hereinafter, referred to as a “purging amount”) discharged from the inside of the anode system through the purge valve  36 . Specifically, as the purging amount increases, the hydrogen concentration inside the anode system increases. 
     In this regard, according to this embodiment, a threshold value of the purge flow rate (or the purging amount) is obtained in advance through experiments or the like, at which the hydrogen concentration inside the anode system does not generate a voltage drop (target hydrogen concentration: for example, 60%) depending on a load of the fuel cell stack  1 . In addition, the purge flow rate in the purge valve  36  open state is estimated, and the estimated purge flow rate is compared with the threshold value. If the estimated purge flow rate is equal to or lower than the threshold value, it is determined that the purging amount necessary to manage the hydrogen concentration inside the anode system at the target hydrogen concentration is short, and an additional purging operation is performed. 
     Here, as a method of estimating the purging amount, for example, it is conceived that a gas amount flowing out from the anode system during a purge valve  36  open duration is estimated based on the anode pressure decrease amount during the purge valve  36  open duration in the hydrogen supply valve close state, and a value obtained by subtracting the hydrogen amount consumed for generating electricity during the purge valve  36  open duration from this gas amount is estimated as the amount of the anode off-gas discharged from the anode system through the purge valve  36 , that is, the purging amount. It is noted that the purge flow rate is obtained by dividing this purging amount by the corresponding duration. 
     However, the gas flowing out from the anode system in the hydrogen supply valve  33  close state contains components other than the gas flowing out through the purge valve  36  (hereinafter, referred to as a “purge gas”) or the hydrogen consumed by generating electricity. For example, the hydrogen transmitted from the anode side to the cathode side through the electrolytic membrane (hereinafter, referred to as a “transmitted hydrogen”) or the water vapor which is condensed to be liquid water in the anode system. The transmitted hydrogen contributes to a decrease of the hydrogen concentration in the anode system. In addition, since hydrogen has a small molecular weight, the amount of hydrogen transmitting through the electrolytic membrane is not negligible. 
     In this manner, while the purge gas contributes to an increase of the hydrogen concentration in the anode system, the transmitted hydrogen contributes to a decrease of the hydrogen concentration in the anode system. 
     Therefore, typically, if a ratio of the transmitted hydrogen amount increases out of the purging amount estimated according to the estimation method described above, the hydrogen concentration in the anode system is recovered as the purging amount increases; however, the recovery amount is reduced. Therefore, if the transmitted hydrogen amount is neglected and is included in the purging amount, the actual hydrogen concentration in the anode system is lowered than the assumed value even when the purge flow rate is equal to or higher than the threshold value. This may generate an unanticipated voltage drop. 
     In this regard, according to this embodiment, only the gas flowing out through the purge valve  36  (purge gas) is computed as the purge amount. Hereinafter, a purge amount computation method according to this embodiment will be described with reference to  FIGS. 2 and 3 . 
       FIG. 2  is a diagram illustrating a purge amount computation method according to this embodiment.  FIG. 3  is a diagram illustrating a gas input/output flow in the anode system in the hydrogen supply valve  33  close state. 
     According to this embodiment, only the gas (purge gas) flowing out through the purge valve  36  is computed as a purging amount based on the pressure decrease during the purge valve  36  open duration in the hydrogen supply valve  33  close state and a pressure decrease during the purge valve  36  close duration in the hydrogen supply valve  33  close state. 
     As illustrated in  FIG. 2 , as the hydrogen supply valve  33  is closed at the timing t 11  during electricity generation of the fuel cell stack  1 , so that the supply of the anode gas to the fuel cell stack  1  stops, the anode pressure gradually decreases. This change of the anode pressure is generated due to the following reasons. 
     Referring to  FIG. 2 , the purge valve  36  is opened from the timing t 11  to the timing t 13 , and is closed from the timing t 13  to the timing t 14 . A description will be made for factors of changing the anode pressure regardless of the open/close state of the purge valve  36 . 
     First, a first factor is hydrogen inside the anode system consumed by generating electricity while the hydrogen supply valve  33  is closed as illustrated in  FIGS. 2 and 3 . This hydrogen consumed for generating electricity lowers the anode pressure. A second factor is a fact that liquid water inside the anode system is vaporized and changes to water vapor, or reversely, water vapor is condensed and changes to liquid water. The anode pressure changes depending on the balance between the vaporization and condensation. A third factor is hydrogen (transmitted hydrogen) transmitted from the anode side to the cathode side through the electrolytic membrane, or reversely, nitrogen and oxygen transmitted from the cathode side to the anode side through the electrolytic membrane. The anode pressure changes depending on the balance of these transmitted gases. 
     Since the purge valve  36  is closed from the timing t 13  to the timing t 14 , the anode pressure decreases due to the three factors. 
     Meanwhile, since the purge valve  36  is opened from the timing t 11  to the timing t 13 , the anode pressure decreases due to the gas (purge gas) flowing out through the purge valve  36  in addition to the three factors. It is noted that, due to the structure of the purge valve  36 , as the purge valve  36  is opened, first, liquid water is discharged, and the anode off-gas is then discharged as the purge gas. Therefore, as illustrated in  FIG. 2 , the anode off-gas is discharged from the timing t 12 . 
     In this regard, according to this embodiment, the gas amount inside the anode system lost by the three factors during the purge valve close duration (timing t 13  to t 14 ) is obtained based on a pressure decrease during the purge valve close duration (timing t 13  to t 14 ) in which the purge valve  36  is closed in the hydrogen supply valve  33  close state. By dividing this gas amount by the purge valve close duration, it is possible to compute the gas amount inside the anode system per unit time lost by the three factors. 
     Similarly, the gas amount inside the anode system lost by the purging operation is obtained based on the pressure decrease of the purge valve open duration (timing t 11  to t 13 ) in addition to the three factors of the purge valve open duration. In addition, by dividing this gas amount by the purge valve open duration, the gas amount inside the anode system per unit time lost by the purging operation in addition to the three factors during the purge valve open duration is computed. 
     Here, it is conceived that the gas amount inside the anode system per unit time lost by the three factors is not different from that of the purge valve close duration even in the purge valve open duration from the timing t 11  to the timing t 13 . 
     Therefore, by subtracting the gas amount (the flow rate of the gas flowing out from the anode system during the purge valve close duration) inside the anode system per unit time lost by the three factors during the purge valve close duration from the gas amount (the flow rate of the gas flowing out from the anode system during the purge valve open duration) inside the anode system per unit time lost by the purging operation in addition to the three factors during the purge valve open duration, it is possible to compute the flow rate only of the gas (purge gas) flowing out through the purge valve  36  with high accuracy. 
     In this manner, according to this embodiment, in order to estimate the purging amount or the purge flow rate, it is necessary to open and close the purge valve  36  in the hydrogen supply valve  33  close state. Therefore, if the open/close operation of the purge valve  36  is arbitrarily performed regardless of the open/close state of the hydrogen supply valve  33 , it may be difficult to guarantee frequency of estimating the purging amount or the purge flow rate. 
     In this regard, according to this embodiment, the open/close operation of the purge valve  36  is controlled such that the purge valve  36  is opened in the hydrogen supply valve  33  close state. 
     Hereinafter, a purge control operation according to this embodiment will be described with reference to  FIGS. 4 to 13 . 
       FIG. 4  is a flowchart illustrating a purge control operation according to this embodiment. The controller  4  executes this routine repeatedly at a predetermined operation cycle. 
     In the step S 100 , the controller  4  computes a reference duty ratio based on a load (output current) of the fuel cell stack  1  and a stack temperature by referencing the map of  FIG. 5 . The reference duty ratio is a duty ratio of the purge valve  36  obtained in advance through experiments and the like, at which electricity can be stably generated from each load by fixing the purging period to a certain value (reference purging period: set to 5 seconds in this embodiment, but may be change appropriately). In other words, the reference duty ratio is a duty ratio of the purge valve  36  at which the hydrogen concentration inside the anode system can be managed to the target hydrogen concentration. It is noted that the reference duty ratio may be computed based on only the load (output current) of the fuel cell stack  1 . 
     In the step S 200 , the controller  4  performs a process of generating a valve open request signal of the purge valve  36  based on the reference duty ratio. The purge valve open request signal generation process will be described below in detail with reference to  FIG. 6 . 
     In the step S 300 , the controller  4  performs a process of opening/closing the purge valve  36  in practice in the hydrogen supply valve  33  close state (while the anode gas supply stops) based on the purge valve open request signal and the like. This purge valve  36  open/close process will be described below in detail with reference to  FIG. 7 . 
     In the step S 400 , the controller  4  performs a process of estimating the purge flow rate based on each of the pressure decrease levels when the purge valve  36  is opened and when the purge valve  36  is closed during the pulsated depressurization (while the anode gas supply stops). This purge flow rate estimation process will be described below in detail with reference to  FIG. 8 . 
       FIG. 6  is a flowchart illustrating a purge valve open request signal generation process in detail. 
     In the step S 201 , the controller  4  determines whether or not a value obtained by dividing an integrated time for which the purge valve open request signal described below is turned on (hereinafter, referred to as a “purge valve open request turn-on time”) by a predetermined reference purging period is greater than the reference duty ratio. That is, the controller  4  determines whether or not a proportion of the purge valve open request turn-on time occupied in the reference purging period is greater than the reference duty ratio. The controller  4  performs the process of the step S 202  if the division result is equal to or smaller than the reference duty ratio. If the division result is greater than the reference duty ratio, the controller  4  performs the process of the step S 203 . 
     In the step S 202 , the controller  4  turns on the purge valve open request signal. 
     In the step S 203 , the controller  4  turns off the purge valve open request signal. 
     In the step S 204 , the controller  4  computes the purge valve open request turn-on time by integrating the time for which the purge valve open request signal is turned on. 
     In the step S 205 , the controller  4  determines whether or not a single reference purging period elapses. That is, for example, when the reference purging period is set to 5 seconds, it is determined whether or not 5 seconds elapses after the count of the reference purging period starts. If it is determined that a single reference purging period does not elapse, the controller  4  terminates this process. If it is determined that a single reference purging period elapses, the controller  4  processes the step S 206 . 
     In the step S 206 , the controller  4  resets the purge valve open request turn-on time computed in the step S 204  to zero, and also resets the count of the reference purging period to zero. 
       FIG. 7  is a flowchart illustrating the purge valve open/close process in details. 
     In the step S 301 , the controller  4  computes the purge valve open request turn-on time separately from the step S 204  described above. 
     In the step S 302 , the controller  4  determines whether or not the hydrogen supply valve  33  is opened. The controller  4  processes the step S 302  when the hydrogen supply valve  33  is opened, that is, while the anode gas is supplied. Otherwise, when the hydrogen supply valve  33  is closed, that is, while the anode gas supply stops, the controller  4  processes the step S 309 . According to this embodiment, basically, the determination of the step S 302  is performed in order to open the purge valve  36  when the anode gas supply stops. 
     In step S 303 , the controller  4  determines whether or not the operation region of the fuel cell stack  1  is in a high load region. The controller  4  determines that the operation region is in the high load region, for example, when the output current is larger than a predetermined current. The controller  4  processes the step S 600  if the operation region of the fuel cell stack  1  is in the high load region. Otherwise, the controller  4  processes the step S 304 . 
     In the step S 600 , the controller  4  performs a purge valve  36  open/close process in the high load region. This high-load purging process will be described below in detail with reference to  FIG. 13 . In summary, the liquid water accumulated in the anode system in the high load region increases, compared to a typical region. Due to the structure of the purge valve  36 , as the purge valve  36  is opened, first, the liquid water is discharged from the inside of the anode system, and the anode off-gas is then discharged. For this reason, in the high load region, the purge valve  36  can be opened even during the pulsated pressurization, so that the liquid water inside the anode system is reliably discharged from the inside of the anode system, and the anode off-gas is then appropriately discharged from the inside of the anode system. 
     In the step S 304 , the controller  4  closes the purge valve  36 . 
     In the step S 305 , the controller  4  determines whether or not a purge valve open continuation flag is asserted. If the purge valve open continuation flag is asserted, the controller  4  processes the step S 306 . If the purge valve open continuation flag is not asserted, the controller  4  terminates this process. 
     The purge valve open continuation flag is a flag asserted until the purge valve open time while the anode gas supply stops exceeds a second predetermined value set in advance as a time for opening the purge valve  36  in practice when the anode gas supply stops. This purge valve open continuation flag is a flag set to continuously maintain the current purge valve open time without a change, for example, when the purge valve  36  is opened while the anode gas supply stops, and the anode gas supply starts before the second predetermined value elapses. As a result, the purge valve  36  is opened only for the remaining time when the next anode gas supply stops (split purging). 
     In the step S 306 , the controller  4  determines whether or not the purge valve open request turn-on time computed in the step S 301  is equal to or longer than the first predetermined value set in advance. The controller  4  processes the step S 307  if the purge valve open request turn-on time is equal to or longer than the first predetermined value. If the purge valve open request turn-on time is shorter than the first predetermined value, the controller  4  processes the step S 308 . 
     In the step S 307 , the controller  4  turns on the purge valve open command. The initial value of the purge valve open command is set to OFF. If the anode gas supply stop time elapses while the purge valve open command is turned on, the purge valve  36  is opened in practice. As a result, it is possible to open the purge valve  36  in synchronization with the water supply valve close state. 
     In this manner, according to this embodiment, after the purge valve open request turn-on time (=integration value of the reference duty ratio) becomes equal to or longer than the first predetermined value, the purge valve open command is turned on, and the purge valve open operation is allowed. In this embodiment, this enables the anode off-gas to be reliably discharged through the purge valve  36  in the purge valve open state by guaranteeing a certain time for opening the purge valve  36  to reliably perform water drainage in the hydrogen supply valve  33  close state. 
     In this manner, the reference duty ratio decreases as the output current decreases. Therefore, it is possible to increase the time elapsing until the purge valve open command is turned on as the output current decreases. That is, by changing the interval for outputting the purge valve open command based on the output current, an interval (purging interval) until the purge valve is opened after it is closed increases as the output current decreases. When the output current is low, the cathode pressure is also low. Therefore, the amount of nitrogen transmitted from the cathode side is also small. For this reason, by increasing the purging interval as the output current decreases, it is possible to guarantee the time for opening the purge valve  36  necessary to estimate the purge flow rate in the hydrogen supply valve  33  close state. 
     In the step S 308 , the controller  4  turns off the purge valve open command. 
     In the step S 309 , the controller  4  determines whether or not the purge valve open command is turned on. If the purge valve open command is turned off, the controller  4  processes the step S 310 . If the purge valve open command is turned on, the controller  4  processes the step S 311 . 
     In the step S 310 , the controller  4  closes the purge valve  36 . In this manner, the purge valve  36  is not opened until the purge valve open request turn-on time is equal to or longer than the first predetermined value even when the anode gas supply stops. 
     In the step S 311 , the controller  4  opens the purge valve  36 . 
     In the step S 312 , the controller  4  computes the purge valve open time by integrating the time for which the purge valve  36  is opened. 
     In the step S 313 , the controller  4  determines whether or not the purge valve open time computed in the step S 312  is equal to or longer than a second predetermined value set in advance as a time for opening the purge valve  36  in practice when the anode gas supply stops. In this manner, according to this embodiment, if the purge valve open request turn-on time is equal to or longer than the first predetermined value, the purge valve  36  is opened only for the second predetermined value. Although the first and second predetermined values are set to the same value in this embodiment, they may be set to different values. The first and second predetermined values may be obtained in advance through experiments and the like as a value (for example, 0.5 seconds) at which the hydrogen concentration inside the anode system can be maintained at the target hydrogen concentration, and the purge flow rate can be estimated with high accuracy. The controller  4  processes the step S 314  if the purge valve open time is shorter than the second predetermined value. If the purge valve open time is equal to or longer than the second predetermined value, the controller  4  processes the step S 316 . 
     In the step S 314 , the controller  4  asserts the purge valve open continuation flag. 
     In the step S 315 , the controller  4  de-asserts the pressurization flag. The pressurization flag is a flag used in the high-load purging process. 
     In the step S 316 , the controller  4  resets the purge valve open time computed in the purge valve open/close process to zero. 
     In the step S 317 , the controller  4  de-asserts the purge valve open continuation flag. 
     In the step S 318 , the controller  4  turns off the purge valve open command. 
       FIG. 8  is a flowchart illustrating a purge flow rate estimation process in detail. 
     In the step S 401 , the controller  4  determines whether or not the hydrogen supply valve  33  is closed. If it is determined that the hydrogen supply valve  33  is closed, the controller  4  processes step S 402  and the subsequent steps for obtaining data for estimating the purge flow rate. Otherwise, if it is determined that the hydrogen supply valve  33  is opened, the controller  4  processes the step S 409  and the subsequent steps for estimating the purge flow rate based on the obtained data. 
     In the step S 402 , the controller  4  determines whether or not the purge valve  36  is opened. If it is determined that the purge valve  36  is opened, the controller  4  processes the step S 403 . If it is determined that the purge valve  36  is closed, the controller  4  processes the step S 406 . 
     In the step S 403 , the controller  4  computes the gas amount per operation cycle flowing out from the anode system in the purge valve open state based on a decrease of the anode pressure and adds this gas amount to the previous value to compute the flow-out gas amount in the purge valve open state. 
     The gas amount per operation cycle flowing out from the anode system in the purge valve open state may be computed based on the anode pressure decrease amount, for example, by creating the map of  FIG. 9  in advance through experiments and the like by associating the anode pressure decrease amount (previous anode pressure value—current anode pressure value) and the gas amount flowing out from the inside of the anode system and referencing this map. In  FIG. 9 , although the gas amount flowing out from the inside of the anode system is corrected based on the stack temperature, the correction using the stack temperature may not necessarily be performed. In addition, this gas amount may be obtained by computing changes of a molar amount of the gas inside the anode system obtained by applying the previous anode pressure value and the like to an equation of state of the gas and a molar amount of the gas inside the anode system obtained by applying the current anode pressure value and the like to an equation of state of the gas. 
     In the step S 404 , the controller  4  computes a hydrogen amount per operation cycle consumed in the fuel cell stack  1  by generating electricity in the purge valve open state based on the detection value (output current) of the current sensor  43  and adds this hydrogen amount to the previous one to compute a electricity-generation consumption hydrogen amount of the purge valve open operation. 
     This hydrogen amount per operation cycle consumed in the fuel cell stack  1  by generating electricity in the purge valve open state may be computed, for example, based on the output current by creating, in advance, the map of  FIG. 10  obtained by associating the output current and the consumed hydrogen amount through experiments and the like and referencing this map. It may be also obtained by the computation in which, for example, a molar mass of the consumed hydrogen is computed by applying the output current, operation cycle, and the number of fuel cells to a Faraday constant-based formula. 
     In the step S 405 , the controller  4  computes the purge valve open time separately from the step S 312 . 
     In the step S 406 , the controller  4  computes a gas amount per operation cycle flowing out from the inside of the anode system during the purge valve close operation based on a decrease of the anode pressure and adds this gas amount to the previous one to compute a flow-out gas amount of the purge valve close operation. 
     In the step S 407 , the controller  4  computes a hydrogen amount per operation cycle consumed in the fuel cell stack  1  by generating electricity in the purge valve close state based on the detection value (output current) of the current sensor  43  and adds this hydrogen amount to the previous one to compute the electricity-generation consumption hydrogen amount in the purge valve close state. 
     In the step S 408 , the controller  4  integrates the time for which the purge valve  36  is closed to compute a purge valve close time. 
     In the step S 409 , the controller  4  determines whether or not the amount of data for computing the purge flow rate is sufficient. Specifically, it is determined whether or not the purge valve open time and the purge valve close time computed in the steps S 405  and S 408  are longer than a predetermined time (for example, 0.5 seconds) set in advance. If the amount of data is sufficient, the controller  4  processes the step S 410 . If the amount of data is not sufficient, the controller  4  terminates this process. Therefore, if the amount of data is not sufficient, the purge flow rate is estimated based on the pressure change data of a plurality of pulsation cycles as well as data of a single pulsation cycle. 
     In the step S 410 , the controller  4  computes the purge flow rate based on the data obtained in the steps S 403  to S 408 . Specifically, the controller  4  performs computation of  FIG. 11  to compute the purge flow rate. As a method of computing the purge flow rate, a value obtained by subtracting a value obtained by dividing the flow-out gas amount of the purge valve close operation by the purge valve close time (flow-out gas flow rate of the purge valve close operation) from a value obtained by dividing the flow-out gas amount of the purge valve open operation by the purge valve open time (the flow-out gas flow rate of the purge valve open operation) may be set to the purge flow rate as described above with reference to  FIG. 2 . However, if the electricity-generation consumption hydrogen amount of the purge valve open operation is subtracted from the flow-out gas amount of the purge valve open operation in advance, and the electricity-generation consumption hydrogen amount of the purge valve close operation is subtracted from the flow-out gas amount of the purge valve close operation similarly as illustrated in  FIG. 11 , it is possible to more improve the estimation accuracy of the purge flow rate. This is because the electricity-generation consumption hydrogen amount changes depending on a load change, so that it is difficult to say that the electricity-generation consumption hydrogen amount is constant during the open/ close operation of the purge valve  36  at all times. 
     In the step S 411 , the controller  4  determines whether or not the computed purge flow rate is equal to or greater than a predetermined threshold value by referencing the table of  FIG. 12 . In other words, it is determined whether or not the purging amount is sufficient. If the purge flow rate is equal to or greater than the threshold value, the controller  4  processes the step S 412 . If the purge flow rate is smaller than the threshold value, the controller  4  processes the step S 413 . 
     As illustrated in  FIG. 12 , the threshold value is corrected to decrease as an interval for outputting the purge valve open command (an interval after the purge valve open command is issued until the next purge valve open command is issued, hereinafter, referred to as a “purging interval”) increases. 
     As the purging interval increases, the time period elapsing after the purge valve  36  is opened until the purge valve  36  is opened again increases. Therefore, the amount of liquid water accumulated in the anode system increases. For this reason, as the purging interval increases, the purging amount discharged when the purge valve  36  is opened relatively decreases. According to this embodiment, the purge valve  36  is opened depending on an open/close state of the hydrogen supply valve  33 . Therefore, the interval until the purge valve  36  is opened changes. In this case, the purging amount decreases when the purging interval is long compared to a case where the purging interval is short. This is because the amount of liquid water inside the anode system increases as the purging interval increases. Therefore, frequency of determination of purging shortage is reduced by decreasing the threshold value as the interval for issuing the purge valve open command increases. 
     It is noted that, in order to manage the hydrogen concentration inside the anode system for stably generating electricity, basically, it is necessary to increase the purging amount as the load of the fuel cell stack  1  increases. Referring  FIG. 12 , it may be conceived that this tendency is reversed because the threshold value decreases as the load of the fuel cell stack  1  increases. However, this is because the purge flow rate is expressed in the abscissa. The purging amount itself obtained by multiplying the purge valve  36  open time corresponding to the reference duty ratio for each load by the purge flow rate increases as the load of the fuel cell stack  1  increases. 
     In the step S 412 , the controller  4  subtracts the purge valve open time from the purge valve open request turn-on time computed in the step S 301 . 
     In the step S 413 , the controller  4  maintains the purge valve open request turn-on time computed in the step S 301  as it is. This is to perform additional purging in the next process because it is necessary to further perform purging in order to stably generate electricity when it is determined that the purge flow rate is smaller than the threshold value. 
     In this manner, when the purge flow rate is smaller than the threshold value (when the purging amount is short), the purge valve open time is not subtracted from the purge valve open request turn-on time, so that the purge valve open request turn-on time increases, compared to a case where the purging interval is shorter than the typical value, and the purge flow rate is equal to or greater than the threshold value (when the purging amount is sufficient). As a result, the time for opening the purge valve  36  increases as much as the increase amount, that is, the subtraction cancellation amount. 
     In the step S 414 , the controller  4  resets the data of the steps S 403  to S 408  to zero. 
       FIG. 13  is a flowchart illustrating the high-load purge process in detail. 
     In the step S 601 , the controller  4  determines whether or not the purge valve open command is turned on. The controller  4  process the step S 602  if the purge valve open command is turned on. If the purge valve open command is turned off, the controller  4  processes the step S 605 . 
     In the step S 602 , the controller  4  determines whether or not the purge valve open time computed in the step S 312  is zero, or whether or not the under-pressurization valve open flag is asserted. If any one of the conditions is satisfied, the controller  4  processes the step S 603 . If both conditions are not satisfied, the controller  4  processes the step S 605 . 
     In the step S 603 , the controller  4  asserts the under-pressurization valve open flag. 
     In the step S 604 , the controller  4  opens the purge valve  36 . 
     In the step S 605 , the controller  4  computes the purge valve open time by integrating the time for which the purge valve  36  is opened. 
     In the step S 606 , the controller  4  de-asserts the under-pressurization valve open flag. 
     In the step S 607 , the controller  4  closes the purge valve  36 . 
     In this manner, in a high load condition, the purge valve  36  is opened even when the hydrogen supply valve  33  is opened. This is to reliably discharge the liquid water by opening the purge valve  36  from the opening of the hydrogen supply valve  33  because the liquid water inside the fuel cell stack  1  increases in the high load condition. In addition, during the high load condition, since the hydrogen amount consumed by generating electricity increase, an anode pressure decrease rate after closing the hydrogen supply valve  33  increases, and the depressurization time is also shortened. Therefore, if the liquid water discharge efficiency is improved by opening the purge valve  36  during the pressurization, it is possible to reliably discharge the purge gas through the purge valve  36  after the hydrogen supply valve is closed even when the depressurization time is shortened. For this reason, it is possible to improve estimation accuracy of the purge flow rate. 
       FIG. 14A  is a flowchart illustrating a control of the hydrogen supply valve  33  according to this embodiment. 
     In the step S 1 , the controller  4  sets the pulsation upper limit pressure and the pulsation lower limit pressure of the anode pressure based on a target output current of the fuel cell stack  1  by referencing the map of  FIG. 14B . 
     In the step S 2 , the controller  4  determines whether or not the anode pressure is equal to or higher than the pulsation upper limit pressure. If the anode pressure is equal to or higher than the pulsation upper limit pressure, the controller  4  processes the step S 3  in order to decrease the anode pressure. Otherwise, if the anode pressure is lower than the pulsation upper limit pressure, the controller  4  processes the step S 4 . 
     In the step S 3 , the controller  4  sets the target anode pressure to the pulsation lower limit pressure. 
     In the step S 4 , the controller  4  determines whether or not the anode pressure is equal to or lower than the pulsation lower limit pressure. If the anode pressure is equal to or lower than the pulsation lower limit pressure, the controller  4  processes the step S 5  in order to increase the anode pressure. Otherwise, if the anode pressure is higher than the pulsation lower limit pressure, the controller  4  processes the step S 6 . 
     In the step S 5 , the controller  4  sets the target anode pressure to the pulsation upper limit pressure. 
     In the step S 6 , the controller  4  sets the target anode pressure to the previous target anode pressure. 
     In the step S 7 , when the pulsation lower limit pressure is set as the target anode pressure, the controller  4  performs a feedback control for the hydrogen supply valve  33  such that the anode pressure is set to the pulsation lower limit pressure. As a result of this feedback control, typically, the opening level of the hydrogen supply valve  33  is fully closed, so that the anode gas supply from the high-pressure hydrogen tank  31  to the fuel cell stack  1  stops. As a result, the anode pressure decreases due to consumption of the anode gas inside the fuel cell stack  1  for generating electricity or other reasons. 
     Meanwhile, when the pulsation upper limit pressure is set as the target anode pressure, the controller  4  performs a feedback control for the hydrogen supply valve  33  such that the anode pressure increases to the pulsation upper limit pressure. As a result of this feedback control, the hydrogen supply valve  33  is opened to a desired opening level, and the anode gas is supplied from the high-pressure hydrogen tank  31  to the fuel cell stack  1 , so that the anode pressure increases. 
       FIGS. 15 and 16  are timing charts illustrating a purge control according to this embodiment.  FIG. 15  is a timing chart when the purge flow rate is equal to or higher than the threshold value in a normal operation region. Meanwhile,  FIG. 16  is a timing chart when the purge flow rate is lower than the threshold value in a normal operation region. 
     As illustrated in  FIG. 15D , through the purge valve open request signal generation process, the purge valve open request signal is generated such that the purge valve open request signal is turned on only for the reference duty ratio during the reference purging period. In addition, as illustrated in  FIG. 15E , the purge valve open request turn-on time is computed by integrating the time for which the purge valve open request signal is turned on. 
     If the purge valve open request turn-on time is equal to or longer than the first predetermined value at the timing t 1 , the purge valve open command is turned on ( FIG. 15F ) when the hydrogen supply valve  33  is opened ( FIG. 15B ) at the timing t 2  subsequent to the timing t 1 . 
     In addition, if the hydrogen supply valve  33  is closed while the purge valve open command is turned on at the timing t 3 , the purge valve  36  is opened ( FIG. 15C ). As the purge valve  36  is opened, the purge valve open time is computed by integrating the time for which the purge valve  36  is opened as illustrated in  FIG. 15G . If the purge valve open time is equal to or longer than the second predetermined value at the timing t 4 , the purge valve open command is turned off ( FIG. 15F ), and the purge valve  36  is closed ( FIG. 15C ). 
     During the purge valve open duration of the timing t 3  to t 4 , the data for estimating the purge flow rate, that is, the flow-out gas amount of the purge valve open operation or the electricity-generation consumption hydrogen amount is computed. 
     Then, during the purge valve close duration of the timing t 4  to t 5 , the data for estimating the purge flow rate, that is, the flow-out gas amount of the purge valve close operation and electricity-generation consumption hydrogen amount are computed. 
     If the amount of data is sufficient when the hydrogen supply valve  33  is opened at the timing t 5 , the purge flow rate is computed based on the obtained data ( FIG. 151 ). 
     If the computed purge flow rate is equal to or higher than the threshold value, the purge valve open request turn-on time is reduced only for the purge valve open time (=second predetermined value) at the timing t 6  ( FIG. 15E ). As a result, even when the purge valve open request turn-on time is shorter than the first predetermined value, and the hydrogen supply valve  33  is closed at the timing t 7 , the purge valve  36  is not opened. 
     Meanwhile, as illustrated in  FIG. 16 , if the purge flow rate is lower than the threshold value, the purge valve open request turn-on time is maintained as it is at the timing t 6 . For this reason, the purge valve open command is turned on at the timing t 6 , so that the purge valve  36  can be opened at the timing t 7 . In this manner, if the purge flow rate is lower than the threshold value, an additional purging operation is performed by reducing the purging interval to be shorter than a normal purging interval and opening the purge valve  36  again when the hydrogen supply valve  33  is closed after estimation of the purge flow rate. Therefore, it is possible to manage the hydrogen concentration inside the anode system at which electricity is stably generated. 
       FIG. 17  is also a timing chart illustrating a purge control according to this embodiment.  FIG. 17  is a timing chart when the purge flow rate is equal to or higher than the threshold value when the operation region is in the high-load region. 
     As illustrated in  FIG. 17 , when the purge valve open request turn-on time is equal to or longer than the first predetermined value at the timing t 21  ( FIG. 17D ), the purge valve open command is turned on ( FIG. 17E ). In this case, if the operation region is in the high-load region, it is determined whether or not the purge valve open time is set to zero, or whether or not the under-pressurization valve open flag is asserted through the high-load purging process. At the timing t 21 , the purge valve open time is set to zero, so that the under-pressurization flag is asserted ( FIG. 17H ), and the purge valve  36  is opened in the hydrogen supply valve  33  open state ( FIG. 17B ). 
     In this manner, by opening the purge valve  36  in the hydrogen supply valve  33  open state in the high load condition, it is possible to reliably discharge the liquid water and reliably discharge the purge gas through the purge valve  36  after the hydrogen supply valve is closed. As a result, it is possible to improve estimation accuracy of the purge flow rate. 
     As the hydrogen supply valve  33  is closed at the timing t 22 , the under-pressurization flag is de-asserted ( FIG. 17H ). In addition, although the hydrogen supply valve  33  is opened at the timing t 23 , the purge valve open time in this case does not yet reach the second predetermined value ( FIG. 17F ). For this reason, the purge valve  36  is opened continuously in the hydrogen supply valve  33  close state ( FIG. 17B ). In addition, the purge valve open time is not reset ( FIG. 17F ), and the purge valve open command is also maintained at the ON-state ( FIG. 17E ). 
     As a result, at the timing t 23 , it is determined again whether or not the purge valve open time is set to zero, or whether or not the under-pressurization valve open flag is turned on. At the timing t 23 , since both the conditions are not satisfied, the purge valve  36  is closed in the hydrogen supply valve  33  open state even in the high-load region ( FIG. 17B ). 
     As the hydrogen supply valve  33  is closed at the timing t 24 , the purge valve  36  is opened ( FIG. 17B ), and the purge valve open time increases again ( FIG. 17F ). 
     At the timing t 25 , the hydrogen supply valve  33  is opened, but since the purge valve open time does not yet reach the second predetermined value ( FIG. 17F ), the purge valve  36  is maintained in the open state from the timing t 24  ( FIG. 17B ). In addition, the purge valve open time is not reset ( FIG. 17F ), and the purge valve open command is maintained in the ON-state ( FIG. 17E ). 
     As the hydrogen supply valve  33  is closed at the timing t 26 , and the purge valve open time reaches the second predetermined value at the timing t 27  ( FIG. 17F ), the purge valve open command is turned off ( FIG. 17E ), and the purge valve  36  is closed ( FIG. 17B ). 
     In this manner, when the interval elapsing after the hydrogen supply valve  33  is closed until the hydrogen supply valve  33  is opened is shortened in the high load region, the purge valve open time does not reach the second predetermined value through a single pulsated depressurization try in some cases. In this case, the purge valve  36  open operation is performed in a split manner, and only the initial purging is performed in the hydrogen supply valve  33  open state. 
     The fuel cell system  100  described above according to this embodiment comprises the hydrogen supply valve  33  (supply valve) that controls a supply of the anode gas to the inside of the anode system, the purge valve  36  that discharges the off-gas from the inside of the anode system, the anode pressure sensor  34  (pressure detecting portion) that measures a pressure inside the anode system, the supply valve control portion (controller  4 ) that controls opening/ closing the hydrogen supply valve  33  based on the load (output current) of the fuel cell stack  1 , the purge flow rate estimating portion (controller  4 ) that estimates the purge flow rate of the off-gas discharged from the inside of the anode system through the purge valve  36  based on a pressure decrease inside the anode system in the hydrogen supply valve  33  close state, and the purge valve control portion (controller  4 ) that opens the purge valve  36  in synchronization with the hydrogen supply valve  33  close state. 
     In this manner, by opening the purge valve  36  in synchronization with the hydrogen supply valve  33  close state, it is possible to increase frequency of opening the purge valve  36  when the pressure inside the anode system decreases in the hydrogen supply valve  33  close state, compared to a case where the purge valve  36  is opened arbitrarily regardless of the open/ close state of the hydrogen supply valve  33 . For this reason, it is possible to guarantee frequency of estimating the purge flow rate of the off-gas discharged from the inside of the anode system through the purge valve  36  based on a pressure decrease inside the anode system in the hydrogen supply valve  33  close state. 
     In addition, the purge valve control portion closes the purge valve  36  as the purge valve  36  is opened for a predetermined valve open time (second predetermined value) in the hydrogen supply valve  33  close state (closes the purge valve  36  before the hydrogen supply valve  33  is opened), so that the purge valve  36  can be opened or closed in the hydrogen supply valve  33  close state. For this reason, it is possible to guarantee the estimation frequency even when the purge flow rate is estimated based on a pressure decrease in the purge valve open state and a pressure decrease in the purge valve close state while the hydrogen supply valve  33  is closed. 
     In addition, if the hydrogen supply valve  33  is opened before the purge valve open time (second predetermined value) elapses, the purge valve control portion closes the purge valve  36  in synchronization with the hydrogen supply valve  33  open timing, and opens the purge valve  36  again in the next hydrogen supply valve  33  open operation. For this reason, it is possible to guarantee estimation frequency of the purge flow rate by opening the purge valve  36  in the hydrogen supply valve  33  close state and reliably guarantee the purging amount necessary to maintain the hydrogen concentration inside the anode system at which electricity generation is stabilized. 
     The fuel cell system  100  according to this embodiment comprises the determining portion (controller  4 ) that determines whether or not the purging amount is short based on the purge flow rate estimated by the purge flow rate estimating portion. In addition, the purge valve control portion changes the interval (purging interval) elapsing until the purge valve  36  is opened based on the load of the fuel cell stack  1 , if it is determined that the purging amount is short, the purge valve control portion reduces an interval elapsing until the purge valve  36  is opened to be shorter than the interval set based on the load of the fuel cell stack  1 . 
     Specifically, the purge valve control portion computes the valve open request time (reference duty ratio) of the purge valve  36  based on the load of the fuel cell stack  1 , and issues the purge valve  36  open command when the integration value of the valve open request time (purge valve open request turn-on time) is equal to or longer than the first predetermined value. If it is determined that the purging amount is not short, the purge valve control portion subtracts the purge value  36  open time from the integration value of the valve open request time. If it is determined that the purging amount is short, the purge valve control portion maintains the integration value of the valve open request time. 
     As a result, when the purging amount is short, it is possible to perform additional purging separately from the purging performed base on the load. Therefore, it is possible to suppress a decrease of the hydrogen concentration inside the anode system and continuously and stably generate electricity. 
     In addition, the determining portion determines whether or not the purging amount is short based on the purge flow rate estimated by the purge flow rate estimating portion and the threshold value set to match the load of the fuel cell stack  1 , and the threshold value is corrected to decrease as the interval elapsing until the purge valve  36  is opened increases. 
     Since the time period elapsing until the purge valve  36  is opened in the next time after it is opened increases as the purging interval increases, the amount of liquid water accumulated in the anode system increases. For this reason, the purging amount discharged when the purge valve  36  is opened relatively decreases as the purging interval increases. According to this embodiment, since the purge valve  36  is opened to match the open/close state of the hydrogen supply valve  33 , the interval elapsing until the purge valve  36  is opened changes. In this case, the purging amount decreases as the purging interval increases, as compared to a case where the purging interval is short, because the amount of liquid water inside the anode system increases as the purging interval increases. Therefore, by reducing the threshold value as the purging interval increases, it is possible to reduce frequency of determining the purging shortage carelessly. As a result, it is possible to prevent an additional purging operation from being performed carelessly, it is possible to suppress degradation of the fuel efficiency. 
     When the load of the fuel cell stack  1  is higher than a predetermined load, the purge valve control portion opens the purge valve  36  in the hydrogen supply valve  33  open state. Therefore, it is possible to reliably discharge the liquid water accumulated in the anode system and guarantee the amount of the anode off-gas discharged when the purge valve  36  is opened. For this reason, it is possible to improve the estimation accuracy of the purge flow rate. 
     In addition, if the purge valve  36  is opened during the valve close state of the hydrogen supply valve  33 , and the closed hydrogen supply valve  33  is re-opened before the purge valve open time (second predetermined value) elapses, the purge valve control portion closes the purge valve  36  in synchronization with the valve re-open timing of the hydrogen supply valve  33  and opens the purge valve  36  again in the subsequent valve close operation of the hydrogen supply valve  33 . In this manner, when the purging is performed in a split manner, the purge valve  36  is opened in the hydrogen supply valve  33  open state only in the initial purging operation. As a result, it is possible to guarantee estimation frequency of the purge flow rate by reliably discharging liquid water in the initial purging operation and opening the purge valve  36  in synchronization with the hydrogen supply valve  33  close timing in the subsequent purging operation. 
     The purge flow rate estimating portion estimates the purge flow rate of the off-gas discharged from the inside of the anode system through the purge valve  36  based on a pressure decrease of the purge valve open operation and a pressure decreases of the purge valve close operation when the anode gas supply into the anode system stops. Specifically, the purge flow rate estimating portion comprises a first estimating portion that estimates a gas flow rate discharged from the anode system during the purge valve open duration based on a pressure decrease of the purge valve open operation, and a second estimating portion that estimates a gas flow rate discharged from the anode system regardless of the open/close state of the purge valve  36  based on a pressure decrease of the purge valve close operation. The purge flow rate estimating portion estimates the purge flow rate based on the gas flow rates estimated by the first and second estimating portions. 
     As a result, it is possible to estimate the gas flow rate discharged from the anode system based on the pressure decrease of the purge valve close operation regardless of the open/close state of the purge valve  36 . Therefore, it is possible to estimate the flow rate of the off-gas discharged from the anode system through the purge valve  36  with high accuracy based on this estimation result and the gas flow rate discharged from the anode system during the purge valve open duration. 
     While embodiments of this invention have been described hereinbefore, the embodiments are just for illustrative purposes for the invention, and they are not intended to limit the spirit and scope of this invention to such specific configurations of the embodiments. 
     For example, although the purge valve  36  is opened simultaneously when the hydrogen supply valve  33  is closed in the aforementioned embodiment, the purge valve  36  may be opened when a predetermined time (for example, 80 ms) elapses after the hydrogen supply valve  33  is closed. As a result, it is possible to reduce an influence on the estimation of the purge flow rate from a response delay of the hydrogen supply valve  33  or an overshoot of the detection value of the anode pressure sensor  34  and further improve the estimation accuracy of the purge flow rate. 
     Although the detection value of the anode pressure sensor  34  is used as the pressure inside the anode system in the aforementioned embodiment, the pressure inside the anode system may be estimated, for example, based on the opening level of the hydrogen supply valve  33  and the like. 
     Although the pulsating operation for pulsating the anode pressure is performed in the aforementioned embodiment, the fuel cell system may constantly maintain the anode pressure depending on the load of the fuel cell stack. In this case, the purge valve  36  may be opened/closed during a transient time for decreasing the load (when the anode pressure decreases). In addition, the anode pressure may be pulsated primarily. 
     In the aforementioned embodiment, until the purge valve open time reaches the second predetermined value, the purge valve  36  is opened continuously in the hydrogen supply valve  33  close state, so that the purging is performed in a split manner in a high load condition and the like. In comparison, the purging operation may be performed in a split manner until the purge valve open time reaches the second predetermined value in the purge valve  36  close state before the hydrogen supply valve  33  is opened. 
     In the aforementioned embodiment, the anode gas discharge passage  35  in the upstream from the purge valve  36  may be connected to the anode gas supply passage  32  to circulate the anode off-gas. 
     This application is based on and claims priority to Japanese Patent Application No. 2014-219712 (filed in Japan Patent Office on Oct. 28, 2014), the entire content of which is incorporated herein by reference.