1.  Field of the Invention
The present invention relates to a fuel cell system having a regulator for regulating a pressure supplied to an anode of a fuel cell, and relates to a method of operating the fuel cell system. 
Priority is claimed on Japanese Patent Application No.  2004-312298, filed Oct. 27, 2004, the content of which is incorporated herein by reference. 
2.  Description of the Related Art
In a known fuel cell system mounted in a fuel cell automobile or the like, an oxidant gas and a fuel gas are respectively supplied to a cathode and an anode of a fuel cell so that electric power is generated due to an electrochemical reaction between the gases. 
In power generation using a fuel cell, a pressure difference between the anode and the cathode (i.e., a pressure difference between electrodes) must be maintained within a specific range.  Therefore, in order to maintain the pressure difference between an inlet gas pressure at the cathode (i.e., a reference pressure) and a gas pressure supplied to the anode within a specific range, some systems employ a regulator for regulating the pressure of the gas supplied to the anode based on the pressure of the oxidant gas which is applied (to the regulator) as a signal pressure.  For example, Japanese Unexamined Patent Application, First Publication No.  2002-373682 discloses a technique for controlling the pressure applied to the cathode by using an air injector, so that the pressure applied to the anode can be automatically regulated via a regulator. 
Generally, outside air (i.e., the air outside the fuel cell system) is drawn into the oxidant gas which is used for applying a signal pressure to the regulator.  Therefore, depending on the operational time period or outside environment, condensation may be generated in a signal pressure line or an air injector through which the air for applying the signal pressure to the regulator flows.  If the operation of the fuel cell system is stopped while such condensation remains in the signal pressure line or the air injector, the condensation may freeze in a low-temperature environment such as that having a sub-zero (or below-freezing) temperature.  If the condensation freezes, fine adjustment of the signal pressure cannot be performed when the fuel cell system is started in a low-temperature environment.