Fuel cell system

A fuel cell system provided with a fuel cell that generates electric power by receiving supply of fuel gas and oxidant gas, the fuel cell system comprising:

• a fuel gas supply passage for supplying the fuel gas to the fuel cell;
• a fuel off-gas exhaust passage for exhausting fuel off-gas discharged from the fuel cell to an outside of the fuel cell system;
• a fuel off-gas circulation passage for circulating at least a part of the fuel off-gas from the fuel off-gas exhaust passage to the fuel gas supply passage;
• a fuel gas supply unit placed in the fuel gas supply passage and configured to supply the fuel gas;
• an ejector placed downstream of the fuel gas supply unit in the fuel gas supply passage and configured to discharge a mixture of the fuel gas supplied from the fuel gas supply unit and the fuel off-gas circulating through the fuel off-gas circulation passage;
• an exhaust-drain valve placed in the fuel off-gas exhaust passage and configured to exhaust the fuel off-gas to the outside;
• a pressure sensor placed in the fuel gas supply passage and used to measure a pressure of the fuel gas upstream of the ejector;
• and
• a controller configured to control the exhaust-drain valve,
• wherein the controller variably controls a time ratio between an opening time and a closing time of the exhaust-drain valve per one control cycle, and controls the number of opening-closing operations of the exhaust-drain valve per unit time by keeping the opening time constant and controlling the closing time variably, and
• the controller controls the number of opening-closing operations of the exhaust-drain valve according to a pressure measured value of the pressure sensor in order to adjust a concentration of the fuel gas in the fuel off-gas caused to circulate to the fuel gas supply passage via the fuel off-gas circulation passage.