Fuel cell system

1. A fuel cell system comprising a cathode gas supply system of a cathode gas bypass type in which a part of a cathode gas ejected from a compressor installed in a cathode gas supply channel bypasses a fuel cell via a bypass channel, wherein the fuel cell system comprises:

a first flow rate sensor which is installed in an upstream of a part connected with the bypass channel in the cathode gas supply channel and detects a cathode gas flow rate to be supplied to the compressor;

a second flow rate sensor which is installed in a downstream of a part connected with the bypass channel in the cathode gas supply channel and detects a cathode gas flow rate to be supplied to the fuel cell;

a bypass valve which is installed in the bypass channel and controls a cathode gas flow rate flowed in the bypass channel; and

a controller programmed to

execute an open/shut-off control of the bypass valve in accordance with an operation state of the fuel cell system, and

detect a mismatch of detected values of the first flow rate sensor and the second flow rate sensor based on the detected values of the both sensors during total shut-off of the bypass valve.

2. The fuel cell system according to claim 1 , wherein the controller is further programmed to:

control the compressor based on a requirement of the fuel cell; and

increase a cathode gas flow rate supplied by the compressor when a mismatch that a detected value of the first flow rate sensor becomes larger than a detected value of the second flow rate sensor occurs.

3. The fuel cell system according to claim 1 , wherein the controller is further programmed to:

control the compressor in such a way that a requirement of the fuel cell and a requirement of the fuel cell system can be satisfied simultaneously; and

control the compressor by further taking a diagnosis result of the controller into consideration.

4. The fuel cell system according to claim 2 , wherein

the controller is further programmed to

calculate, based on the requirement of the fuel cell, a target cathode gas flow rate to be supplied to the fuel cell;

calculate, based on a requirement of the fuel cell system, a lower limit flow rate of a cathode gas supplied by the compressor; and

set a larger one of the target cathode gas flow rate and the lower limit flow rate to a target supply flow rate of the compressor, and control the compressor based on the target supply flow rate and the detected value of the first flow rate sensor, and

when the mismatch that the detected value of the first flow rate sensor becomes larger than the detected value of the second flow rate sensor occurs, the lower limit flow rate is corrected so as to be increased based on a deviation of the detected values of the both sensors.

5. The fuel cell system according to claim 2 , wherein the controller is further programmed to:

calculate, based on the requirement of the fuel cell, a target cathode gas flow rate to be supplied to the fuel cell;

calculate, based on a requirement of the fuel cell system, a lower limit flow rate of a cathode gas supplied by the compressor; and

set a larger one of the target cathode gas flow rate and the lower limit flow rate to a target supply flow rate of the compressor, and control the compressor based on this target supply flow rate and a detected value of the first flow rate sensor, and

when a mismatch that the detected value of the first flow rate sensor becomes larger than a detected value of the second flow rate sensor occurs, the compressor is controlled based on a detected value of the second flow rate sensor and the target supply flow rate in place of the detected value of the first flow rate sensor.

6. The fuel cell system according to claim 1 , wherein

the controller is further programmed to detect the mismatch of the detected values of the both sensors by forcibly and totally shutting the bypass valve off when an open time of the bypass valve becomes more than a first predetermined time.

7. The fuel cell system according to claim 6 , wherein the controller is further programmed to:

calculate, based on a load of the fuel cell, a target humidity of an electrolyte membrane of the fuel cell; and

correct the target humidity so as to be increased when an open time of the bypass valve becomes more than a second predetermined time which is shorter than the first predetermined time.

8. The fuel cell system according to claim 6 , wherein the fuel cell system comprises a pressure control valve to control a pressure of a cathode gas to be supplied to the fuel cell; and

when the bypass valve is forcibly and totally shut-off, based on an opening degree of the bypass valve before total shut-off of the bypass valve or on the flow rate of a cathode gas flowing through the bypass channel, an opening degree of the pressure control valve is made larger than the opening degree of the bypass valve before total shut-off.