Control method of flow regulating valve of oxidizing gas and flow regulation device

CROSS-REFERENCE TO RELATED APPLICATIONS

0001The present application claims priority from Japanese patent application P2014-232252 filed on Nov. 15, 2014, the content of which is hereby incorporated by reference into this application.

BACKGROUND

00021. Field

0003The present invention relates to a control method of a flow regulating valve of an oxidizing gas and a flow regulation device.

00042. Related Art

0005In a fuel cell system including a fuel cell that generates electric power with supplies of a fuel gas and an oxidizing gas, various flow regulating valves are provided to adjust the valve-opening position of each conduit and thereby regulate the flow rate in a fuel gas supply system, as well as in an oxidizing gas supply system and an exhaust gas discharge system. A failure of the flow regulating valve affects the operation of the fuel cell. A proposed technique detects a failure of a flow regulating valve by taking advantage of the behavior of a voltage generated in a fuel cell that is related to the operation of the flow regulating valve (for example, JP 2006-269128A). This proposed technique obtains a behavior of voltage of the fuel cell in the normal valve-opening state of a purge valve, which is one type of the flow regulating valve, in an exhaust system and detects a failure of the purge valve when the actual behavior of voltage at the time of outputting a valve-opening instruction of the purge valve is different from the obtained behavior of voltage.

SUMMARY

0006The failure detection technique that utilizes the behavior of voltage of the fuel cell is applicable to flow regulating valves other than the purge valve. With progress of research for maintaining the performance of the fuel cell, there is a need for fine adjustment of a low flow rate in the flow regulating valve used to regulate the flow rate of the oxidizing gas.

0007At stop of power generation of the fuel cell, when hydrogen remains in an anode-side flow path of the fuel cell and oxygen remains in a cathode-side flow path of the fuel cell, the fuel cell has an extremely high open circuit voltage (OCV). The excessively high open circuit voltage of the fuel cell leads to an excessively high electrode potential at an electrode (cathode) of the fuel cell. Thus, elution (deterioration) of the catalyst accordingly proceeds at the electrode. This results in deteriorating the power generation performance and the durability of the fuel cell.

0008After stop of power generation of the fuel cell, the fuel gas is generally circulated at a low flow rate, so that hydrogen remains in the anode-side flow path. The remaining hydrogen is transmitted across an electrolyte membrane of the fuel cell to the cathode-side flow path, and reaction of oxidizing the transmitted hydrogen with the remaining oxygen proceeds at the cathode. As a result, some time after stop of power generation of the fuel cell, the open circuit voltage (cathode potential) is decreased by consumption of oxygen remaining in the cathode-side flow path. In this case, the cathode catalyst is reduced and is more likely to be eluted at the time of a subsequent increase of the cathode potential. At stop of power generation of the fuel cell, there is a need to adjust the flow rate of the oxidizing gas such as to keep the open circuit voltage of the fuel cell at a low voltage level. There is accordingly a need for a control method of the flow regulating valve that allows for fine adjustment of the flow rate of the oxidizing gas by taking advantage of the behavior of voltage of the fuel cell.

0009In order to solve at least part of the above problems, the invention may be implemented by any of the following aspects.

0010(1) According to one aspect of the invention, there is provided a control method of a flow regulating valve of an oxidizing gas. The control method of the flow regulating valve of the oxidizing gas controls the flow regulating valve that is configured to regulate a flow rate of the oxidizing gas supplied to a cathode of a fuel cell. The control method of the flow regulating valve of the oxidizing gas may comprise a first process of continuously performing either a gradual valve-opening operation that opens the flow regulating valve by a predetermined valve opening each time from a full-close position or a gradual valve-closing operation that closes the flow regulating valve by a predetermined valve opening each time from a full-open position in a load disconnected state that the fuel cell is electrically disconnected from a load which receives supply of electric power from the fuel cell, so as to gradually change a supply amount of the oxidizing gas that is introduced to the cathode and cause hydrogen transmitted from an anode across an electrolyte membrane to the cathode of the fuel cell to be oxidized with oxygen included in the introduced oxidizing gas; a second process of measuring an open circuit voltage of the fuel cell accompanied with oxidation of the hydrogen after each gradual valve-opening operation or each gradual valve-closing operation; and a third process of storing either at least one valve-opening position among valve-opening positions of the flow regulating valve by the gradual valve-opening operation in the first process at a predetermined number of timings including a timing when the measured voltage shifts to an increase during continuation of the gradual valve-opening operation or at least one valve-opening position among valve-opening positions of the flow regulating valve by the gradual valve-closing operation in the first process at predetermined number of timings including a timing when the measured voltage shifts to a decrease during continuation of the gradual valve-closing operation, as a regulation reference valve-opening position

0011The control method of the flow regulating valve of the oxidizing gas according to the above aspect has the following advantages. The timing when the measured voltage shifts to an increase during continuation of the gradual valve-opening operation is a timing when the oxidizing gas is increased at the cathode during continuation of the gradual valve-opening operation after oxidation of entire hydrogen that is transmitted from the anode to the cathode across the electrolyte membrane prior to the gradual valve-opening operation (hereinafter referred to as transmitted hydrogen) with oxygen included in the remaining oxidizing gas at the cathode. In the state that the transmitted hydrogen is fully oxidized, oxygen included in the oxidizing gas is introduced to the cathode at the equivalent ratio of 1 to the transmitted hydrogen. The gradual valve-opening operation is performed in the load disconnected state. There is accordingly a small amount of the transmitted hydrogen, or in other words, a small amount of hydrogen remains at or is supplied to the anode, so that there is a low flow rate of the oxidizing gas containing oxygen that is introduced to the cathode at the equivalent ratio of 1 to the transmitted hydrogen. This low flow rate of the oxidizing gas is defined by at least one valve-opening position among valve-opening positions of the flow regulating valve by the gradual valve-opening operation at the timing when the measured voltage shifts to an increase during continuation of the gradual valve-opening operation and at a predetermined number of timings before and after the timing of this increasing voltage shift. This at least one valve-opening position is stored as the regulation reference valve-opening position. The valve-opening positions of the flow regulating valve by the gradual valve-opening operation at the predetermined number of timings are slightly different from the valve-opening position of the flow regulating valve at the timing when the measured voltage shifts to an increase during continuation of the gradual valve-opening operation, but these differences are relatively small. The valve-opening positions of the flow regulating valve by the gradual valve-opening operation at the timing of the increasing voltage shift and at the predetermined number of timings before and after this timing are regarded as substantially equivalent, and at least one of the valve-opening positions is stored as the regulation reference valve-opening position.

0012A timing when the measured voltage shifts to a decrease during continuation of the valve-closing operation is a timing when oxygen in the oxidizing gas introduced to the cathode prior to the gradual valve-closing operation is gradually consumed for oxidation of the transmitted hydrogen and decreases at the cathode to the equivalent ratio of to the transmitted hydrogen. The gradual valve-closing operation is also performed in the load disconnected state. There is accordingly a small amount of the transmitted hydrogen, or in other words, a small amount of hydrogen remains at or is supplied to the anode, so that there is a low flow rate of the oxidizing gas containing oxygen that remains at the cathode at the equivalent ratio of 1 to the transmitted hydrogen. This low flow rate of the oxidizing gas is defined by at least one valve-opening position among valve-opening positions of the flow regulating valve by the gradual valve-closing operation at the timing when the measured voltage shifts to a decrease during continuation of the gradual valve-closing operation and at a predetermined number of timings before and after the timing of this decreasing voltage shift. This at least one valve-opening position is stored as the regulation reference valve-opening position. The valve-opening positions of the flow regulating valve by the gradual valve-closing operation at the predetermined number of timings are slightly different from the valve-opening position of the flow regulating valve at the timing when the measured voltage shifts to a decrease during continuation of the gradual valve-closing operation, but these differences are relatively small. The valve-opening positions of the flow regulating valve by the gradual valve-closing operation at the timing of the decreasing voltage shift and at the predetermined number of timings before and after this timing are regarded as substantially equivalent, and at least one of the valve-opening positions is stored as the regulation reference valve-opening position. The control method of the flow regulating valve of the oxidizing gas according to the above aspect controls the flow regulating valve with setting the stored regulation reference valve-opening position as one target of regulation of the valve-opening position. This allows for fine adjustment of the flow rate of the oxidizing gas by taking advantage of the behavior of the measured voltage of the fuel cell.

0013(2) In the control method of the flow regulating valve of the oxidizing gas according to the above aspect, the first process may regulate either a valve-opening amount of the predetermined valve by each time of the gradual valve-opening operation or a valve-opening interval, such as to prevent the measured voltage from decreasing to a predetermined low voltage. This configuration has the following advantages. The gradual valve-opening operation prior to the timing when the measured voltage shifts to an increase during continuation of the valve-opening operation is performed while the measured voltage is still decreasing toward the shift point to an increase. Performing the gradual valve-opening operation without taking into account the valve-opening amount or the valve-opening interval is likely to decrease the open circuit voltage to a low voltage (for example, zero voltage) undesirable for the fuel cell. The control method of the flow regulating valve of the oxidizing gas according to this aspect suppresses the measured voltage that is the open circuit voltage from decreasing to the undesirable low voltage.

0014(3) In the control method of the flow regulating valve of the oxidizing gas according to the above aspect, the third process may control the flow regulating valve, such that the valve-opening position of the flow regulating valve stored as the regulation reference valve opening position is maintained over a predetermined time duration. This configuration suppresses elution and reduction of the cathode catalyst in the non-power generation state that the fuel cell is electrically disconnected from the load and improves the stability of the catalyst, thus allowing subsequent power generating operation of the fuel cell to be performed without difficulty.

0015(4) The control method of the flow regulating valve of the oxidizing gas according to the above aspect may further comprise a fourth process of updating and storing the regulation reference valve-opening position stored. in the third process. The third process may store an open circuit voltage of the fuel cell accompanied with oxidation of the hydrogen at the stored regulation reference valve-opening position as a comparison reference open circuit voltage, together with the regulation reference valve-opening position. The fourth process may comprise a comparison process of setting a valve-opening position of the flow regulating valve to the regulation reference valve-opening position stored in the third process in the load disconnected state of the fuel cell, measuring an open circuit voltage of the fuel cell accompanied with oxidation of the hydrogen as a comparative open circuit voltage, and comparing the comparative open circuit voltage with the comparison reference open circuit voltage; and a correction process of correcting the stored regulation reference valve-opening position to decrease a difference voltage between the comparative open circuit voltage and the comparison reference open circuit voltage obtained as a result of the comparison in the comparison process, and updating and storing the corrected regulation reference valve-opening position as a new regulation reference valve-opening position. Even when the opening and closing functions of the flow regulating valve is varied with time or is otherwise varied, the regulation reference valve-opening position is newly stored to compensate for the variation of the opening and closing functions This configuration controls the flow regulating valve with setting the new regulation reference valve-opening position as one target of regulation of the valve-opening position. This accordingly allows for continuous fine adjustment of the flow rate of the oxidizing gas by taking advantage of the behavior of the measured voltage of the fuel cell.

0016(5) According to another aspect of the invention, there is provided a flow regulation device. The flow regulation device comprises a flow regulating valve that is configured to regulate a flow rate of an oxidizing gas supplied to a cathode of a fuel cell; a disconnector that is configured to electrically disconnect the fuel cell from a load which receives supply of electric power from the fuel cell and thereby set the fuel cell in a load disconnected state; a voltage meter that is configured to measure an open circuit voltage of the fuel cell; and a valve controller that controls opening and closing of the flow regulating valve in the load disconnected state. The valve controller comprises a first controller that is configured to continuously perform either a gradual valve-opening operation that opens the flow regulating valve by a predetermined valve opening each time from a full-close position or a gradual valve-closing operation that closes the flow regulating valve by a predetermined valve opening each time from a full-open position, so as to gradually change a supply amount of the oxidizing gas that is introduced to the cathode and cause hydrogen transmitted from an anode across an electrolyte membrane to the cathode of the fuel cell to be oxidized with oxygen included in the introduced oxidizing gas; a second controller that is configured to obtain the measured open circuit voltage of the fuel cell accompanied with oxidation of the hydrogen from the voltage meter after each gradual valve-opening operation or each gradual valve-closing operation; and a third controller that is configured to store either at least one valve-opening position among valve-opening positions of the flow regulating valve by the gradual valve-opening operation performed. by the first controller at a predetermined number of timings including a timing when the measured voltage shifts to an increase during continuation of the gradual valve-opening operation or at least one valve-opening position among valve-opening positions of the flow regulating valve by the gradual valve-closing operation performed by the first controller at predetermined number of timings including a timing when the measured voltage shifts to a decrease during continuation of the gradual valve-closing operation, as a regulation reference valve-opening position.

0017The flow regulation device of the above aspect controls the flow regulating valve with setting the stored regulation reference valve-opening position as one target of regulation of the valve-opening position. This allows for fine adjustment of the flow rate of the oxidizing gas by taking advantage of the behavior of the measured voltage of the fuel cell.

0018The invention may be implemented by any of various aspects other than the control method of the flow regulating valve of the oxidizing gas and the flow regulation device of the above aspects, for example, an oxidizing gas supply device that is configured to supply an oxidizing gas to a cathode of a fuel cell, a fuel cell system including the oxidizing gas supply device as well as a fuel cell and a method of supplying an oxidizing gas to a fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

0019<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram illustrating the schematic configuration of a fuel cell vehicle according to one embodiment of the invention;

0020<figref idref="DRAWINGS">FIG. 2</figref> is a diagram schematically illustrating a relationship between output current and output voltage and a relationship between output current and output power of a fuel cell;

0021<figref idref="DRAWINGS">FIG. 3</figref> is a diagram schematically illustrating a relationship between amount of oxygen supplied to a cathode-side flow path and OCV of the fuel cell;

0022<figref idref="DRAWINGS">FIG. 4</figref> is a flowchart showing an intermittent operation control routine performed by a controller as a series of operations at the time of selection of an intermittent operation mode;

0023<figref idref="DRAWINGS">FIG. 5</figref> is a flowchart showing a process of setting a reaction-field full-close position;

0024<figref idref="DRAWINGS">FIG. 6</figref> is a diagram showing a variation in relationship between back pressure valve opening position Xb corresponding to the equivalent ratio of 1 and number of steps by gradual valve-opening operations with an individual variation of a back pressure valve;

0025<figref idref="DRAWINGS">FIG. 7</figref> is a diagram schematically illustrating a behavior of voltage V_MEiby gradual valve-opening operations of the back pressure valve; and

0026<figref idref="DRAWINGS">FIG. 8</figref> is a flowchart showing a process of learning the reaction-field full-close position.

DESCRIPTION OF THE EMBODIMENTS

A. General Configuration of Fuel Cell System

0027<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram illustrating the schematic configuration of a fuel cell vehicle <b>20</b> according to one embodiment of the invention. The fuel cell vehicle <b>20</b> has a fuel cell system <b>30</b> mounted on a vehicle body <b>22</b>. The fuel cell system <b>30</b> is connected with a motor <b>170</b> for driving the fuel cell vehicle <b>20</b> by a wiring <b>178</b>, such that electric power generated by the fuel cell system <b>30</b> is supplied to the motor <b>10</b> via the wiring <b>178</b>.

0028The fuel cell system <b>30</b> includes a fuel cell <b>100</b>, a hydrogen gas supply system <b>120</b> including a hydrogen tank <b>110</b>, an air supply system <b>140</b> including a compressor <b>130</b>, a secondary battery <b>172</b>, a DC/DC converter <b>104</b>, a DC/DC converter <b>174</b> and a controller <b>200</b>. The fuel cell system <b>30</b> further includes a cooling medium circulation system (not shown) that is configured to make a flow of and circulate a cooling medium in the fuel cell <b>100</b> for cooling down the fuel cell <b>100</b>, in order to keep the temperature of the fuel cell <b>100</b> in a predetermined range. The fuel cell system <b>30</b> is configured such that electric power is suppliable to a load including the motor <b>170</b> from either one of the fuel cell <b>100</b> and the secondary battery <b>172</b> alone or from both the fuel cell <b>100</b> and the secondary battery <b>172</b> simultaneously.

0029The fuel cell <b>100</b> has stacked structure by stacking a plurality of unit cells. The fuel cell <b>100</b> of this embodiment is a polymer electrolyte fuel cell. An anode-side flow path configured to make a flow of hydrogen to an anode side and a cathode-side flow path configured to make a flow of oxygen to a cathode side are formed across an electrolyte membrane in each of the unit cells constituting the fuel cell <b>100</b>. The fuel cell <b>100</b> is connected with the load including the motor <b>170</b> via the DC/DC converter <b>104</b> and the wiring <b>178</b>. A voltage sensor <b>102</b> is provided to detect the voltage of the fuel cell <b>100</b>. The detection signal of the voltage sensor <b>102</b> is output to the controller <b>200</b>.

0030The DC/DC converter <b>104</b> serves to change the output condition of the fuel cell <b>100</b> in response to a control signal from the controller <b>200</b>. More specifically, the DC/DC converter <b>104</b> serves to set an output voltage in the course of power generation by the fuel cell <b>100</b>. The DC/DC converter <b>104</b> also serves to boost up the output voltage to a desired voltage in the course of supplying electric power generated by the fuel cell <b>100</b> to the load. The DC/DC converter <b>104</b> includes a diode (not shown). Providing the diode in the DC/DC converter <b>104</b> electrically disconnects the fuel cell <b>100</b> from the load when the output current of the fuel cell <b>100</b> decreases to or below a predetermined level. According to this embodiment, the diode included in the DC/DC converter <b>104</b> works to provide a load disconnected state in which the fuel cell <b>100</b> is electrically disconnected from the load.

0031The hydrogen tank <b>110</b> included in the hydrogen gas supply system <b>120</b> may be, for example, a hydrogen tank that stores high-pressure hydrogen gas or a tank filled with a hydrogen storage alloy that serves to absorb hydrogen and thereby store hydrogen. The hydrogen gas supply system system <b>120</b> includes a hydrogen supply path <b>121</b> that is arranged to connect the hydrogen tank <b>110</b> with the fuel cell <b>100</b>, a circulation path <b>122</b> that is arranged to circulate unconsumed hydrogen gas (anode off-gas) to the hydrogen supply path <b>121</b>, and a hydrogen release path <b>123</b> that is arranged to release the anode off-gas to the atmosphere. In the hydrogen gas supply system <b>120</b>, hydrogen gas stored in the hydrogen tank <b>110</b> flows through the hydrogen supply path <b>121</b> that is opened and closed by an on-off valve <b>124</b>, is subjected to pressure reduction by a pressure reducing valve <b>125</b> and is supplied from a hydrogen supply device <b>126</b> such as injector located downstream of the pressure reducing valve <b>125</b> to the anode-side flow path formed in each unit cell of the fuel cell <b>100</b>. The flow rate of hydrogen circulated through the circulation path <b>122</b> is regulated by a circulation pump <b>127</b> provided in the circulation path <b>122</b>. The controller <b>200</b> regulates the driving amounts of the hydrogen supply device <b>126</b> and the circulation pump <b>127</b> in response to a load request by referring to the pressure of the circulated flow of hydrogen detected by a pressure sensor <b>128</b>.

0032Part of the hydrogen gas flowing through the circulation path <b>122</b> flows through the hydrogen release path <b>123</b> that is branched off from the circulation path <b>122</b> and is opened and closed by an on-off valve <b>129</b> and is released to the atmosphere at a predetermined timing. This causes impurities (for example, water vapor and nitrogen) other than hydrogen included in the hydrogen gas circulated through the circulation path <b>122</b> to be discharged out of the flow path and thereby suppresses an increase in concentration of the impurities included in the hydrogen gas supplied to the fuel cell <b>100</b>. The controller <b>200</b> controls the open and close timings of the on-off valve <b>129</b>.

0033The air supply system <b>140</b> includes a first air flow path <b>141</b>, a second air flow path <b>145</b>, a third air flow path <b>146</b>, a distribution valve <b>144</b>, an air release path <b>142</b>, a back pressure valve <b>143</b> and a flow rate sensor <b>147</b>, in addition to the compressor <b>130</b>. The first air flow path <b>141</b> is configured to make a flow of the intake air taken in by the compressor <b>130</b>. The second air flow path <b>145</b> and the third air flow path <b>146</b> are branched off from the first air flow path <b>141</b>. The distribution valve <b>144</b> is placed at a branching position where the first air flow path <b>141</b> branches off to the second air flow path <b>145</b> and the third air flow path <b>146</b>. The distribution ratio of the air flowing from the first air flow path <b>141</b> into the second air flow path <b>145</b> or the third air flow path <b>146</b> is regulated under control of the controller <b>200</b>. Part of the second air flow path <b>145</b> forms the cathode-side flow path formed in each of the unit cells of the fuel cell <b>100</b>. The third air flow path <b>146</b> serves as a bypass flow path configured to introduce the air to the air release path <b>142</b> with bypassing the fuel cell <b>100</b>. The second air flow path <b>145</b> and the third air flow path <b>146</b> branched at the position of the distribution valve, <b>144</b> are both joined to the air release path <b>142</b>. The back pressure valve <b>143</b> is a throttle valve that is provided in the second air flow path <b>145</b> to be located upstream of the connecting point with the third air flow path <b>146</b>. The air release path <b>142</b> is configured to release the air passing through the third air flew path <b>146</b> along with the air or the cathode off-gas passing through the second air flow path <b>145</b> to the atmosphere. The hydrogen release path <b>123</b> described above is connected with the air release path <b>142</b>, such that hydrogen flowing through the hydrogen release path <b>123</b> is diluted with the air flowing through the air release path <b>142</b>, before being released to the atmosphere. The flow rate sensor <b>147</b> is provided in the first air flow path <b>141</b> to detect the total flow rate of the air taken in through the first air flow path <b>141</b>.

0034The back pressure valve <b>143</b> includes a stepping motor (not shown). The valve-opening position of the back pressure valve <b>143</b> is adjustable to any desired position with high accuracy by controlling the number of steps of the stepping motor. Adjusting the valve-opening position of the back pressure valve <b>143</b> changes the back pressure in the cathode-side flow path of the fuel cell <b>100</b> and results in controlling the flow rate of the air flowing through the second air flow path <b>145</b> into the cathode-side flow path of the fuel cell <b>100</b>. The back pressure valve <b>143</b> works as a flow regulation valve. The controller <b>200</b> regulates the driving amount of the compressor <b>130</b>, the distribution ratio of the air by the distribution valve <b>144</b> and the valve-opening position of the back pressure valve <b>143</b>.

0035The secondary battery <b>172</b> is connected with the wiring <b>178</b> via the DC/DC converter <b>174</b>. The DC/DC converter <b>174</b> and the DC/DC converter <b>104</b> are connected in parallel by the wiring <b>178</b>. For example, a lead acid battery, a nickel hydride battery or a lithium ion battery may be employed for the secondary battery <b>172</b>. The secondary battery <b>172</b> is provided with a state-of-charge monitor (not shown) that is configured to detect the state of charge (SOC) or remaining capacity of the secondary battery <b>172</b>. The state of charge detected by the state-of-charge monitor is output to the controller <b>200</b>. The state-of-charge monitor may be configured as an SOC meter that integrates the current value and the time duration of charging and discharging of the secondary battery <b>172</b>. Alternatively the state-of-charge monitor may be configured as a voltage sensor that detects the voltage of the secondary battery <b>172</b>, in place of the SOC meter.

0036The DC/DC converter <b>174</b> has charge and discharge control functions to control charging and discharging of the secondary battery <b>172</b>, and controls charging and discharging of the secondary battery <b>172</b> in response to a control signal from the controller <b>200</b>. The DC/DC converter <b>174</b> sets a target voltage on the output side under control of the controller <b>200</b> and variably regulates the electric power charged into or discharged from the secondary battery <b>172</b> and the voltage level applied to the motor <b>170</b>. When there is no need to charge or discharge the secondary battery <b>172</b>, the DC/DC converter <b>174</b> disconnects the secondary battery <b>172</b> from the wiring <b>178</b>.

0037The controller <b>200</b> is implemented by a microcomputer including a CPU that performs logical operations, a ROM and a RAM. The controller <b>200</b> inputs detection signals from various sensors, for example, an accelerator position sensor <b>180</b>, a shift position sensor, a vehicle speed sensor and an ambient temperature sensor (not shown), in addition to the sensors included in the hydrogen gas supply system <b>120</b> and the air supply system <b>140</b> described above and performs various controls of the fuel cell vehicle <b>20</b> based on the input detection signals. For example, the controller <b>200</b> determines the magnitude of a load request based on, for example, the detection signal of the accelerator position sensor <b>180</b> and outputs drive signals to the relevant parts of the fuel cell system <b>30</b> to obtain an electric power corresponding to the load request from at least one of the fuel cell <b>100</b> and the secondary battery <b>172</b>. More specifically in the case of obtaining electric power from the fuel cell <b>100</b>, the controller <b>200</b> regulates the supply amounts of the respective gases from the hydrogen gas supply system <b>120</b> and the air supply system <b>140</b>, in order to obtain a desired electric power from the fuel cell <b>100</b>. The controller <b>200</b> also controls the DC/DC converters <b>104</b> and <b>174</b> to supply a desired mount of electric power to the motor <b>170</b>. The controller <b>200</b> additionally includes a timer that is provided to measure a time elapsed since input of any of various signals or since execution of any of various processes.

B. Intermittent Operation Mode and Target Voltage

0038The fuel cell vehicle <b>20</b> of the embodiment changes the operation mode among a plurality of operation modes including an ordinary operation mode and an intermittent operation mode during operation of the fuel cell system <b>30</b>. The ordinary operation mode is an operation mode that is selected when a load request to the fuel cell system <b>30</b> exceeds a predefined reference value and causes at least part of the load request including a required electric power of the motor <b>170</b> to be covered by electric power generated by the fuel cell <b>100</b>. The intermittent operation mode is an operation mode that is selected when a load request to the fuel cell system <b>30</b> is equal to or lower than the predefined reference value and stops power generation of the fuel cell <b>100</b>.

0039The load that receives the supply of electric power from the fuel cell system <b>30</b> includes vehicle auxiliary machinery and fuel cell auxiliary machinery, in addition to the motor <b>170</b> for driving the fuel cell vehicle <b>20</b>. In the fuel cell system <b>30</b> of the embodiment, the load request includes a required electric power of the motor <b>170</b>, a required electric power of the vehicle auxiliary machinery and a required electric power of the fuel cell auxiliary machinery. The vehicle auxiliary machinery includes, for example, air conditioning equipment, lighting devices, hazard lamps and direction indicators. The fuel cell auxiliary machinery includes, for example, the compressor <b>130</b>, the circulation pump <b>127</b>, various valves including the distribution valve <b>144</b> and the back pressure valve <b>143</b>, a cooling medium pump configured to circulate the cooling medium and a radiator fan configured to cool down the cooling medium. In the case where the secondary battery <b>172</b> has a low state of charge (SOC), the secondary battery <b>172</b> may be part of the load. The fuel cell vehicle <b>20</b> of this embodiment determines a load request as the total of the required electric powers of the respective loads described above and selects the intermittent operation mode when the load request is equal to or lower than the predefined reference value. At the time of selection of the intermittent operation mode, the voltage of the fuel cell <b>100</b> during stop of power generation is controlled to a predetermined range.

0040<figref idref="DRAWINGS">FIG. 2</figref> is a diagram schematically illustrating a relationship between output current and output voltage and a relationship between output current and output power of the fuel cell <b>100</b>. The following briefly describes control at the time of selection of the ordinary operation mode and subsequently describes maintenance control in a non-power generation state.

0041According to this embodiment, the amount of electric power generated by the fuel cell <b>100</b> in the ordinary operation mode is controlled by setting the output voltage of the fuel cell <b>100</b>. As understood from the relationship between the output current and the output power shown in <figref idref="DRAWINGS">FIG. 2</figref>, determining an electric power P_FCto be output from the fuel cell <b>100</b> results in determining an output current I_FCof the fuel cell <b>100</b>. As shown by a current-voltage characteristic (I-V characteristic) in <figref idref="DRAWINGS">FIG. 2</figref>, determining the output current I_FCof the fuel cell <b>100</b> results in determining an output voltage V_FCof the fuel cell <b>100</b>. At the time of selection of the ordinary operation mode, the controller <b>200</b> gives an instruction to set the determined output voltage V_FCas a target voltage to the DC/DC converter <b>174</b>, so as to control the amount of electric power generated by the fuel cell <b>100</b> to a desired level.

0042In the case where power generation of the fuel cell <b>100</b> is stopped at the time of selection of the intermittent operation mode, the output current of the fuel cell <b>100</b> is equal to 0. When power generation of the fuel cell <b>100</b> is stopped or more specifically when the fuel cell <b>100</b> is disconnected from the load to have the output current equal to 0 while sufficient amounts of hydrogen and oxygen for power generation are supplied to the fuel cell <b>100</b>, the fuel cell <b>100</b> has an extremely high open-circuit voltage (hereinafter may be referred to as OCV) as shown in <figref idref="DRAWINGS">FIG. 2</figref>. This indicates a significant increase in electrode potential at the cathode of the fuel cell <b>100</b>. As is known, the high electrode potential of the fuel cell <b>100</b> causes elution of a catalyst metal such as platinum included in the electrode and deteriorates the performance of the fuel cell <b>100</b>. It is accordingly desirable to suppress an excessive increase in electrode potential of the fuel cell <b>100</b>, in order to suppress deterioration of the performance of the fuel cell <b>100</b>. The fuel cell system <b>30</b> of this embodiment regulates the amount of oxygen supplied to the cathode-side flow path during stop of power generation of the fuel cell <b>100</b> and thereby suppresses an excessive increase in electrode potential at the cathode.

0043<figref idref="DRAWINGS">FIG. 3</figref> is a diagram schematically illustrating a relationship between amount of oxygen supplied to the cathode-side flow path and OCV of the fuel. cell <b>100</b>. The voltage profile of <figref idref="DRAWINGS">FIG. 3</figref> schematically shows the relationship between the supply amount of oxygen and the open circuit voltage (OCV) of the fuel cell <b>100</b> when the amount of oxygen supplied to the cathode-side flow path is changed while a sufficient amount of hydrogen for power generation in the ordinary operation mode is supplied to the anode-side flow path during stop of power generation of the fuel cell <b>100</b>. In the case where an extremely small amount of oxygen is supplied to the cathode-side flow path, the OCV is substantially kept at extremely low level and is not significantly changed with a change in supply amount of oxygen. In the diagram of <figref idref="DRAWINGS">FIG. 3</figref>, this range of the supply amount of oxygen is shown by an arrow A as an oxygen deficiency area A. When the supply amount of oxygen is increased, the OCV drastically increases with an increase in supply amount of oxygen. In the diagram of <figref idref="DRAWINGS">FIG. 3</figref>, this range of the supply amount of oxygen is shown by an arrow B as an equivalent ratio-of-approximately-1 area B. When the supply amount of oxygen is further increased, the OCV is substantially kept at extremely high level and is not significantly changed with a change in supply amount of oxygen. In the diagram of <figref idref="DRAWINGS">FIG. 3</figref>, this range of the supply amount of oxygen is shown by an arrow C as an excess oxygen area C. According to this embodiment, at the time of selection of the intermittent operation mode, the supply amount of oxygen is controlled, such that the OCV is equal to a specified voltage in the equivalent ratio-of-approximately-1 area B. In other words, according to this embodiment, at the time of selection of the intermittent operation mode, voltage including a specified voltage in the equivalent ratio-of-approximately-1 area B is set in advance as a target voltage Vmark of OCV, and the amount of oxygen supplied to the fuel cell <b>100</b> is regulated to make the OCV of the fuel cell <b>100</b> equal to the target voltage Vmark.

0044In the air supply system <b>140</b> of the embodiment, the amount of the air, i.e., the amount of oxygen, supplied to the cathode-side flow path of the fuel cell <b>100</b> is determined according to the driving amount of the compressor <b>130</b>, the distribution ratio of the air by the distribution valve <b>144</b> and the valve-opening position of the back pressure valve <b>143</b> as described above. According to this embodiment, at the time of selection of the intermittent operation mode, the OCV of the fuel cell <b>100</b> is controlled to the target voltage Vmark by changing the valve-opening position of the back pressure valve <b>143</b> while fixing the driving amount of the compressor <b>130</b> and the distribution ratio of the air by the distribution valve <b>144</b> among these parameters. According to this embodiment, the target voltage Vmark and an initial value of the valve-opening position of the back pressure valve <b>143</b> (i.e., the driving amount of the back pressure valve <b>143</b>) for supplying an amount of oxygen that obtains the target voltage Vmark to the fuel cell <b>100</b> are stored in advance in a memory of the controller <b>200</b>. In the fuel cell system <b>30</b> of the embodiment, the valve-opening position of the back pressure valve <b>143</b> to obtain the target voltage Vmark is set or updated as appropriate by control in the intermittent operation mode described later.

0045After stop of power generation of the fuel cell <b>100</b>, hydrogen is transmitted from the anode-side flow path to the cathode-side flow path across the electrolyte membrane in each unit cell, and the oxidation reaction of the transmitted hydrogen proceeds at the cathode. As a result, oxygen in the cathode-side flow path is consumed by the oxidation reaction of the hydrogen transmitted through the electrolyte membrane. In order to obtain a desired open-circuit voltage in the equivalent ratio-of-approximately-1 area in the fuel cell <b>100</b> during stop of power generation, there is a need to supply the amount of oxygen that is to be consumed by the oxidation reaction of the transmitted hydrogen (amount of oxygen consumed by transmitted hydrogen) in addition to the amount of oxygen determined corresponding to the desired open-circuit voltage from <figref idref="DRAWINGS">FIG. 3</figref> (amount of oxygen required for electromotive force). Accordingly, the amount of oxygen that is to be supplied to the fuel cell <b>100</b> in order to obtain a desired open-circuit voltage at the time of selection of the intermittent operation mode (amount of oxygen for maintaining cell voltage) is expressed by Equation (1) given below:

0000<br />(amount of oxygen for maintaining cell voltage)=(amount of oxygen required for electromotive force)+(amount of oxygen consumed by transmitted hydrogen)  (1)

0046When the amount of oxygen supplied to the fuel cell <b>100</b> just satisfies Equation (1) given above in the case where the valve-opening position of the back pressure valve <b>143</b> is adjusted to the valve-opening position stored in the memory of the controller <b>200</b>, the open-circuit voltage of the fuel cell <b>100</b> becomes equal to the target voltage Vmark. The amount of hydrogen transmitted through the electrolyte membrane is, however, varied according to the pressure of hydrogen in the anode-side flow path, the internal temperature of the fuel cell <b>100</b> and the internal humidity of the fuel cell <b>100</b>. When the supply amount of oxygen is insufficient due to these factors, the open-circuit voltage of the fuel cell <b>100</b> becomes lower than the target voltage Vmark, When the supply amount of oxygen is excessive, on the other hand, the open-circuit voltage of the fuel cell <b>100</b> becomes higher than the target voltage Vmark. The fuel cell system <b>30</b> of this embodiment performs control to vary the supply amount of oxygen to the fuel cell <b>100</b> based on the result of comparison between the detection value of the open-circuit voltage of the fuel cell <b>100</b> and the target voltage Vmark, so as to make the open-circuit voltage of the fuel cell <b>100</b> approach the target voltage Vmark. The details of this control will be described later.

0047With regard to the target voltage Vmark of OCV of the fuel cell <b>100</b> (average cell voltage according to this embodiment as described later) set at the time of selection of the intermittent operation mode, the voltage of each unit cell is preferably not higher than 0.9 V is more preferably not higher than 0.85V and is furthermore preferably not higher than 0.8 V, in terms of suppressing deterioration (elution) of the electrode catalyst caused by the high potential.

0048The lower cell voltage at the cathode or, in other words, the lower oxygen partial pressure in the cathode-side flow path, is expected to facilitate reduction of the electrode catalyst at the cathode (i.e., is more likely to eliminate the oxide film on the surface of the catalyst). Reducing the electrode catalyst at the cathode may cause a problem that elution of the electrode catalyst at the cathode is more likely to proceed with a subsequent increase in potential at the cathode by a subsequent supply of oxygen into the cathode-side flow path. Accordingly, at the time of selection of the intermittent operation mode, it is desirable that the cell voltage does not decrease to 0 V in any of the unit cells of the fuel cell <b>100</b>. Accordingly, in terms of suppressing this problem caused by a decrease in cell voltage, the target voltage Vmark of OCV (average cell voltage) set at the time of selection of the intermittent operation mode is preferably not lower than 0.1 V and is more preferably not lower than 0.2 V.

0049At the time of selection of the intermittent operation mode, the load request is equal to or lower than the predefined reference value. The fuel cell system <b>30</b> is, however, in operation (with no input of the user's instruction to stop the system), so that the load request is likely to increase in a short time period. It is accordingly desirable not to excessively decrease the amount of oxygen in the cathode-side flow path, in terms of obtaining a desired electric power quickly in response to a subsequent increase in load request. In other words, in terms of ensuring the response to a subsequent increase in load request, it is desirable to set a higher value to the target voltage Vmark OCV (average cell voltage) at the time of selection of the intermittent operation mode. In order to ensure the response to a load request, for example, the target voltage Vmark of each cell is preferably not lower than 0.6 V and is more preferably not lower than 0.7 V.

0050Even at the time of selection of the intermittent operation mode, the anode-side flow path in the fuel cell <b>100</b> is maintained in the state that is in the presence of an amount of hydrogen which enables electric power exceeding the load request as the basis for selection of the intermittent operation mode to be generated immediately. In other words, even at the time of selection of the intermittent operation mode, the circulation pump <b>127</b> is continuously driven, while hydrogen is supplied from the hydrogen supply device <b>126</b> in order to compensate for the lost hydrogen that is transmitted across the electrolyte membrane to the cathode-side flow path.

0051At the time of selection of the intermittent operation mode, part of the hydrogen gas flowing through the circulation path <b>122</b> is released to outside of the fuel cell vehicle <b>20</b> by means of the on-off valve <b>129</b> provided in the hydrogen release path <b>123</b>. This reduces the concentration of impurities (concentration of nitrogen and water vapor) included in the hydrogen gas flowing through the circulation path <b>122</b>. At the time of selection of the intermittent operation mode, nitrogen in the air flows from the cathode-side flow path across the electrolyte membrane to the anode-side flow path. At the time of selection of the intermittent operation mode, hydrogen transmitted from the anode-side flow path to the cathode-side, flow path is oxidized at the cathode to produce water, and part of the produced water is transmitted in the form of water vapor to the anode-side flow path. Accordingly, upon determination that the concentration of impurities in the anode-side flow path exceeds a predetermined reference concentration, the controller <b>200</b> performs an operation of opening the on-off valve <b>129</b> for a predetermined time duration. The amounts of transmission of nitrogen and hydrogen depend on the time duration. The determination that the concentration of impurities in the anode-side flow path exceeds the reference concentration may thus be regarded as determination that an elapsed time since a previous operation of opening the on-off valve <b>129</b> exceeds a predetermined reference time. The operation of opening the on-off valve <b>129</b> may be performed at a timing based on this elapsed time. According to a modification, the concentration of impurities (concentration of nitrogen and/or concentration of water vapor) in the circulation path <b>122</b> may be measured directly, and the timing of the operation of opening the on-off valve <b>129</b> may be determined based on the measured concentration of impurities.

C. Control at Selection of Intermittent Operation Mode

0052<figref idref="DRAWINGS">FIG. 4</figref> is a flowchart showing an intermittent operation control routine performed by the controller <b>200</b> as a series of operations at the time of selection of the intermittent operation mode. This routine is triggered after start of the fuel cell system <b>30</b> and is repeatedly performed during operation of the fuel cell system <b>30</b> until input of the user's instruction to stop the system. The interval at which this routine is repeatedly performed is set to a longer time duration (for example, 20 to 30 seconds) than the time duration required between a change in valve-opening position of the back pressure valve <b>143</b> in this routine and a resulting actual change in amount of oxygen supplied to the cathode-side flow path.

0053On start of this routine, the controller <b>200</b> calculates a load request (step S<b>100</b>). The load request denotes the total amount of the required electric power of the motor <b>170</b> and the required electric power of the vehicle auxiliary machinery and the fuel cell auxiliary machinery as described above. The required electric power of the motor <b>170</b> may be determined, based on the detection signals of the accelerator position sensor <b>180</b> and the vehicle speed sensor. The required electric power of the vehicle auxiliary machinery and the fuel cell auxiliary machinery may be determined, based on the driving signals output to the respective auxiliary machines.

0054The controller <b>200</b> subsequently determines whether the calculated load request is equal to or lower than a predefined reference value (step S<b>110</b>). When it is determined that the load request is higher than the predefined reference value, the fuel cell system <b>30</b> does not select the intermittent operation mode, so that the controller <b>200</b> immediately terminates this routine.

0055When it is determined at step S<b>110</b> that the load request is equal to or lower than the predefined reference value, on the other hand, the controller <b>200</b> continues the series of processing in the intermittent operation mode. After it is determined at step S<b>110</b> that the load request is equal to or lower than the predefined reference value, the intermittent operation control routine of <figref idref="DRAWINGS">FIG. 4</figref> is repeatedly performed until it is determined at step S<b>110</b> that the calculated load request becomes higher than the predefined reference value. In this time duration, the operation mode of the fuel cell system <b>30</b> is maintained to the intermittent operation mode.

0056When it is determined at step S<b>110</b> that the load request is equal to or lower than the predefined reference value, the controller <b>200</b> subsequently determines whether a reaction-field full-close position has been stored in a specified memory area (step S<b>115</b>). The reaction-field full-close position herein denotes a valve-opening position of the back pressure valve <b>143</b> that supplies an amount of oxygen at a boundary between the oxygen deficiency area A and the equivalent ratio-of-approximately-1 area B shown in <figref idref="DRAWINGS">FIG. 3</figref>. In other words, the reaction-field full-close position denotes a valve-opening position of the back pressure valve <b>143</b> that supplies an amount of oxygen which is required for oxidation of hydrogen transmitted through the electrolyte membrane (transmitted hydrogen) during stop of power generation of the fuel cell <b>100</b>, to the fuel cell <b>100</b>. Setting the back pressure valve <b>143</b> to the reaction-field full-close position at step S<b>170</b> (described later) significantly decreases the amount of oxygen supplied to the fuel cell <b>100</b>. According to this embodiment, the reaction-field full-close position of the back pressure valve <b>143</b> is set (step S<b>200</b>) or is updated as appropriate (step S<b>300</b>) as described below and is stored in the memory of the controller <b>200</b>.

0057When it is determined at step S<b>115</b> that the reaction-field full-close position has not been stored, i.e., in the case of a negative answer, the controller <b>200</b> performs a process of setting the reaction-field full-close position (step S<b>200</b>). The process of setting the reaction-field full-close position is performed to set the valve-opening position of the back pressure valve <b>143</b> as one control target parameter for operation control of the fuel cell <b>100</b>. The negative answer that the reaction-field full-close position has not been stored may be given, for example, in the following manner. For example, during pre-shipment inspection, maintenance and inspection or operation for accident restoration, a power supply (for example, 12 V power supply, not shown) for supplying electric power to an ECU is detached for the purpose of inspection of various devices, so that the storage of the ECU is deleted. Control target parameters required for various controls of the fuel cell <b>100</b> are deleted in such power-off state. There is accordingly a need to set or reset these control target parameters. When the operator indicates deletion of the control target parameters by operation of, for example, dip switches (not shown) for the purpose of pre-shipment inspection, maintenance and inspection or operation for accident restoration, the controller <b>200</b> may read the indication and give a negative answer that the reaction-field full-close position has not been stored at step S<b>115</b>. In another example, the controller <b>200</b> may read data in a specified memory area and give a negative answer that the reaction-field full-close position has not been stored when the read data is zero (null).

0058<figref idref="DRAWINGS">FIG. 5</figref> is a flowchart showing the process of setting the reaction-field full-close position (step S<b>200</b>). In the process of setting the reaction-field full-close position, the controller <b>200</b> first controls the DC/DC converter <b>104</b> (shown in <figref idref="DRAWINGS">FIG. 1</figref>) to electrically disconnect the fuel cell <b>100</b> from various loads including the motor <b>170</b> and thereby sets the fuel cell <b>100</b> in the load disconnected state (step S<b>210</b>). The controller <b>200</b> subsequently sets a tentative target value of voltage generated in each unit cell (hereinafter referred to as tentative target voltage Vz) to about 0.8 V and regulates the supplies of the hydrogen gas and the air at flow rates corresponding to this tentative target voltage Vz (step S<b>215</b>) and waits for stabilization of the gas supplies (step S<b>220</b>). Setting the fuel cell <b>100</b> in the load disconnected state at step S<b>210</b> and regulating the gas supplies at step S<b>215</b> are performed in parallel simultaneously.

0059The controller <b>200</b> subsequently drives the back pressure valve <b>143</b> of the fuel cell <b>100</b> to the full-close position (step S<b>225</b>). The controller <b>200</b> stores the number of steps of the stepping motor to drive the back pressure valve <b>143</b>, in the memory of the controller <b>200</b>. Accordingly, at step S<b>225</b>, the controller <b>200</b> reversely drives the stepping motor to the full-close position by a number of steps that is identical with the number of steps by which the back pressure valve <b>143</b> is driven in the valve-opening direction from the full-close position to supply the air at the flow rate regulated at step S<b>215</b>. The valve-opening position of the back pressure valve <b>143</b> in this state is the design full-close position and is the mechanical full-close position at which the valve opening is mechanically equal to 0 degree. Subsequent to step S<b>225</b>, the controller <b>200</b> reads an open circuit voltage V_ME0of the fuel cell <b>100</b> after fully closing the back pressure valve <b>143</b>, i.e., after setting the back pressure valve <b>143</b> to the mechanical full-close position to stop supply of the air to the cathode of the fuel cell <b>100</b>, from the voltage sensor <b>102</b> (shown in <figref idref="DRAWINGS">FIG. 1</figref>) (step S<b>230</b>). In the following description, the number of steps means the number of steps of the stepping motor to be driven. The voltage V_ME0of the fuel cell <b>100</b> denotes a voltage when the back pressure valve <b>143</b> is set at the full-close position. The suffix “0” represents initial value.

0060The controller <b>200</b> subsequently drives the back pressure valve <b>143</b> in the valve-opening direction from the full-close position by one step (step S<b>235</b>) and reads a voltage V_MEiof the fuel cell <b>100</b> after driving the back pressure valve <b>143</b> in the valve-opening direction by one step, from the to voltage sensor <b>102</b> (shown in <figref idref="DRAWINGS">FIG. 1</figref>) (step S<b>240</b>). The suffix “i” represents the number of steps driven in the valve-opening direction at step S<b>235</b>. After reading the voltage V_MEifrom the voltage sensor <b>102</b>, the controller <b>200</b> determines whether the voltage V_MEiafter driving the back pressure valve <b>143</b> in the valve-opening direction by one step shifts to an increase (step S<b>245</b>). When it is determined at step S<b>245</b> that the voltage V_MEidoes not shift to an increase, i.e., in the case of a negative answer, the controller <b>200</b> repeats the processing of steps S<b>235</b> to S<b>245</b> to drive the back pressure valve <b>143</b> in the valve-opening direction by one step, read the voltage V_MEifrom the voltage sensor <b>102</b> and determine whether the voltage V_MEishifts to an increase. In the load disconnected state that the fuel cell <b>100</b> is electrically disconnected from the load including the motor <b>170</b>, the controller <b>200</b> gradually opens the back pressure valve <b>143</b> from the mechanical full-close position by a valve opening corresponding to one step each time (step S<b>235</b>) and measures the voltage V_MEiafter each gradual valve-opening operation (step S<b>240</b>). Since the fuel cell <b>100</b> is in the load disconnected state, the voltage V_MEidenotes a voltage generated in each unit cell, accompanied with oxidation of hydrogen transmitted to the cathode with oxygen at the cathode.

0061When it is determined at step S<b>245</b> that the voltage VMEi shifts to an increase accompanied by gradually opening the back pressure valve <b>143</b> by a valve opening corresponding to one step of the stepping motor each time, i.e., in the case of an affirmative answer, the controller <b>200</b> stores either a valve-opening position corresponding to the integrated number of valve-opening steps by a gradual valve-opening operation at step S<b>235</b> at the time of this affirmative answer or a valve-opening position corresponding to the integrated number of valve-opening steps by a gradual valve-opening operation at step S<b>235</b> prior to this affirmative answer, as a back pressure valve opening position Xb corresponding to the equivalent ratio of 1 (step S<b>250</b>). The controller <b>200</b> then continuously supplies the air to the fuel cell <b>100</b> for a predetermined time duration, while keeping the valve-opening position of the back pressure valve <b>143</b> to the back pressure valve opening position Xb (step S<b>255</b>), and terminates this routine. In the course of supplying the air at step S<b>255</b>, the controller <b>200</b> also continues supplying the hydrogen gas at the flow rate regulated at step S<b>215</b>. The following describes a phenomenon caused by the processing of steps S<b>235</b> to S<b>250</b> with reference to the drawings. <figref idref="DRAWINGS">FIG. 6</figref> is a diagram showing a variation in relationship between back pressure valve opening position Xb corresponding to the equivalent ratio of 1 and number of steps by gradual valve-opening operations with an individual variation of the back pressure valve <b>143</b>. <figref idref="DRAWINGS">FIG. 7</figref> is a diagram schematically illustrating a behavior of voltage V_MEiby gradual valve-opening operations of the back pressure valve <b>143</b>.

0062The following describes setting the back pressure valve <b>143</b> to the mechanical full-close position that is the design full-close position with reference to <figref idref="DRAWINGS">FIG. 6</figref>. Setting the valve-opening position of the back pressure valve <b>143</b> to the mechanical full-close position may be related to the number of steps of the stepping motor in the back pressure valve <b>143</b>. In this case, the mechanical full-close position is included in a range of the number of steps corresponding to a “range of valve-opening position of 0 degree” shown in <figref idref="DRAWINGS">FIG. 6</figref>. The valve-opening position in this range converted from the number of steps of the stepping motor is lower than a specified valve-opening position Xa. Even when the back pressure valve <b>143</b> is continuously driven by gradual valve-opening operations to be gradually opened by a valve opening corresponding to one step of the stepping motor each time (step S<b>235</b>) from the mechanical full-close position (step S<b>225</b>), the supply amount of oxygen is kept zero in the “range of valve-opening position of 0 degree” as shown in <figref idref="DRAWINGS">FIG. 6</figref>. In this range, even when the back pressure valve <b>143</b> is driven in the valve-opening direction from the mechanical full-close position, the gas passage effective sectional area of the back pressure valve <b>143</b> is kept zero. This is attributed to that a seal member included in the back pressure valve <b>143</b> is kept compressed to provide the gas passage effective sectional area substantially equal to zero. When the number of gradual valve-opening operations (number of steps Si) is incremented to increase the valve-opening position of the back pressure valve <b>143</b> to the specified valve-opening position Xa, the compression of the seal member is released in the back pressure valve <b>143</b> to increase the gas passage effective sectional area above zero and start supplying oxygen. The specified valve-opening position Xa denotes a valve-opening position that actually starts supplying oxygen to the cathode of the fuel cell <b>100</b> and substantially differs from the mechanical full-close position. The specified valve-opening position Xa, however, differs from the mechanical full-close position by only about one step of the stepping motor and is still relatively close to the mechanical full-close position. Increasing the number of gradual valve-opening operations (number of steps Si) to make the valve-opening position of the back pressure valve <b>143</b> equal to or higher than this valve-opening position Xa results in increasing the amount of oxygen supplied to the cathode. The supply amount of oxygen is increased from the state that the back pressure valve <b>143</b> is driven to the full-close position at step S<b>225</b>. The increased supply amount of oxygen, however, still belongs to the oxygen deficiency area A shown in <figref idref="DRAWINGS">FIG. 3</figref> and does not reach the amount of oxygen corresponding to the equivalent ratio of 1 that allows the transmitted hydrogen to be fully oxidized. Gradual but continuous supply of oxygen to the cathode causes the OCV of the fuel cell <b>100</b> to be substantially kept at the voltage V_ME0measured at step S<b>230</b> as shown in <figref idref="DRAWINGS">FIG. 7</figref>. With progress of oxidation of the remaining hydrogen, however, oxygen becomes deficient and the voltage V_ME0gradually decreases.

0063When the number of gradual valve-opening operations (number of steps Si) is increased to make the valve-opening position of the back pressure valve <b>143</b> equal to or higher than this valve-opening position Xa, the increased supply amount of oxygen reaches the amount of oxygen corresponding to the equivalent ratio of 1 that allows the transmitted hydrogen to be fully oxidized. The OCV of the fuel cell <b>100</b> then shifts to an increase with continuation of the gradual valve-opening operations as shown in <figref idref="DRAWINGS">FIG. 7</figref>. A valve-opening position Xb at the timing when the OCV shifts to an increase supplies an amount of oxygen corresponding to the equivalent ratio of 1 that allows the transmitted hydrogen to be fully oxidized and is called reaction-field full-close position. The valve-opening position Xb shown in <figref idref="DRAWINGS">FIGS. 6 and 7</figref> is equivalent to the back pressure valve opening position Xb corresponding to the equivalent ratio of 1 at step S<b>250</b> in the process of setting the reaction-field full-close position in <figref idref="DRAWINGS">FIG. 5</figref>. Each gradual valve-opening operation opens the back pressure valve <b>143</b> by a valve opening corresponding to one step. The valve-opening position Xb at the timing when the OCV shifts to an increase may be identical with a valve-opening position of the back pressure valve <b>143</b> after any number of gradual valve-opening operations, for example, after an Si-th gradual valve-opening operation or may be between a valve-opening position after an Si-th gradual valve-opening operation and a valve-opening position after an (Si+1)-th gradual valve-opening operation as schematically shown in <figref idref="DRAWINGS">FIG. 6</figref>. By taking into account this likelihood, the controller <b>200</b> stores either the valve-opening position corresponding to the integrated number of valve-opening steps (Si+1) which is integrated up to the timing of affirmative answer when it is determined that the OCV shifts to an increase or the valve-opening position corresponding to the integrated number of valve-opening steps (Si) by a gradual valve-opening operation prior to the affirmative answer, as the back pressure valve opening position Xb corresponding to the equivalent ratio of 1 at step S<b>250</b> as described above. The stored back pressure valve opening position Xb is used for control of the back pressure valve <b>143</b> as appropriate during control in the intermittent operation mode described later. At step S<b>250</b>, the controller <b>200</b> stores the voltage V_ME0that provides the back pressure valve opening position Xb, as a comparison reference open circuit voltage OCVs, in addition to the back pressure valve opening position Xb. The valve-opening position corresponding to the integrated number of valve-opening steps (Si+1) differs from the valve-opening position corresponding to the previous integrated number of valve-opening steps (Si) by only a valve opening corresponding to one step. This difference is significantly small, so that either of these valve-opening positions is stored as the back pressure valve opening position Xb. The stored back pressure valve opening position Xb corresponding to the equivalent ratio of 1 is equivalent to the “regulation reference valve-opening position” described in SUMMARY.

0064As the premise of determining the back pressure valve opening position Xb corresponding to the equivalent ratio of 1, the fuel cell system <b>30</b> of the embodiment drives the back pressure valve <b>143</b> to the mechanical full-close position at step S<b>225</b> as described above. The back pressure valve opening position Xb that is the reaction-field full-close position may be estimated to some extent by design or by test. In this case, the back pressure valve <b>143</b> may be driven to a predetermined valve-opening position that is included in the range of valve-opening position of 0 degree in <figref idref="DRAWINGS">FIG. 6</figref> and is on the valve-closing side of the estimated reaction-field full-close position, instead of the mechanical full-close position. There is, however, a possibility that the actual reaction-field full-close position is significantly different from the estimated reaction-field full-close position. By taking into account such likelihood, it is desirable to set the predetermined valve-opening position having a sufficiently large difference on the valve-closing side from the estimated reaction-field full-close position. When this is described by the number of steps of the stepping motor of the back pressure valve <b>143</b>, the back pressure valve <b>143</b> is to be driven in the valve-closing direction by a number of steps that is greater than the number of steps S<b>0</b> to S<b>2</b> shown in <figref idref="DRAWINGS">FIG. 6</figref> and corresponds to a valve-opening position having a sufficiently large difference on the valve-closing side from the estimated reaction-field full-close position.

0065Referring back to <figref idref="DRAWINGS">FIG. 4</figref>, when it is determined at step S<b>115</b> that the reaction-field full-close position has been stored, i.e., in the case of an affirmative answer, the controller <b>200</b> performs a process of learning the reaction-field full-close position (step S<b>300</b>). <figref idref="DRAWINGS">FIG. 8</figref> is a flowchart showing the process of learning the reaction-field full-close position. In the process of learning the reaction-field full-close position, the controller <b>200</b> first increments a counter n for setting the learning span by value 1 (step S<b>302</b>) and determines whether the incremented counter n becomes greater than a learning span-specifying counter value Mn (step S<b>304</b>). In the case of a negative answer at step S<b>304</b>, the controller <b>200</b> determines that the current time is not a learning timing of the stored reactive-field full-close position and proceeds to step S<b>120</b> shown in <figref idref="DRAWINGS">FIG. 4</figref> without any further processing in the process of learning the reactive-field full-close position. In the case of an affirmative answer at step S<b>304</b>, on the other hand, in order to start learning the stored reaction-field full-close position, like step S<b>210</b> in <figref idref="DRAWINGS">FIG. 5</figref> described above, the controller <b>200</b> controls the DC/DC converter <b>104</b> (shown in <figref idref="DRAWINGS">FIG. 1</figref>) to electrically disconnect the fuel cell <b>100</b> from various loads including the motor <b>170</b> and thereby sets the fuel cell <b>100</b> in the load disconnected state (step S<b>305</b>). The controller <b>200</b> subsequently reads the back pressure valve opening position Xb and the comparison reference open circuit voltage OCVs that is the voltage V_ME0providing this back pressure valve opening position Xb, which are stored at step S<b>250</b> in the process of setting the reaction-field full-close position shown in <figref idref="DRAWINGS">FIG. 5</figref> (step S<b>310</b>), and sets the valve-opening position of the back pressure valve to the read back pressure valve opening position Xb (step S<b>320</b>). More specifically, at step S<b>320</b>, the controller <b>200</b> drives the stepping motor of the back pressure valve <b>143</b> to a step corresponding to the read back pressure valve opening position Xb.

0066The controller <b>200</b> subsequently reads an open circuit voltage OCV_Xbin each unit cell of the fuel cell <b>100</b> at the back pressure valve opening position Xb from the voltage sensor <b>102</b> (shown in <figref idref="DRAWINGS">FIG. 1</figref>) (step S<b>330</b>) and compares an absolute value of difference voltage between the open circuit voltage OCV_Xband the comparison reference open circuit voltage OCVs with a predetermined threshold voltage β (step S<b>340</b>). The threshold voltage β is set by taking into account a possible fluctuation in gas supply caused by a tolerance in driving condition of the back pressure valve <b>143</b> in a predetermined time period. When it is determined at step S<b>340</b> that the absolute value of the difference voltage is equal to or smaller than the threshold voltage β, i.e., in the case of an affirmative answer, it is determined that the back pressure valve opening position Xb has no significant variation and that there is no need to learn the back pressure valve opening position Xb. The controller <b>200</b> accordingly proceeds to step S<b>370</b> described later.

0067When it is determined at step S<b>340</b> that the open circuit voltage OCV_Xbis higher than the comparison reference open circuit voltage OCVs by more than the threshold voltage β, on the other hand, the controller <b>200</b> determines that the effective sectional area of the back pressure valve <b>143</b> is increased to an excessively opened position by some reason and thereby decreases the back pressure valve opening position Xb by one step and updates the storage (step S<b>350</b>). When it is determined at step S<b>340</b> that the open circuit voltage OCV_Xbis lower than the comparison reference open circuit voltage OCVs by more than the threshold voltage β, the controller <b>200</b> determines that the effective sectional area of the back pressure valve <b>143</b> is decreased to an excessively closed position by some reason and thereby increases the back pressure valve opening position Xb by one step and updates the storage (step S<b>360</b>). Accordingly the controller <b>200</b> performs either step S<b>350</b> or step S<b>360</b> to correct the stored back pressure valve opening position Xb such as to decrease the difference voltage between the open circuit voltage OCV_Xbat the stored back pressure valve opening position Xb and the comparison reference open circuit voltage OCVs and update the storage by storing the corrected back pressure valve opening position Xb as new back pressure valve opening position Xb, After the processing of step S<b>340</b>, step S<b>350</b> or step S<b>360</b>, the controller <b>200</b> sets the counter n to value 0 and clears the counter n (step S<b>370</b>) and proceeds to step S<b>120</b> described below.

0068Subsequent to step S<b>300</b>, the controller <b>200</b> proceeds to step S<b>120</b> shown in <figref idref="DRAWINGS">FIG. 4</figref> to determine whether the current cycle of the intermittent operation control routine is a first cycle of the intermittent operation control routine performed in the fuel cell system <b>30</b> after an operation of an ignition switch (not shown). When it is determined that the current cycle is the first cycle after a change to the intermittent operation mode this time, the controller <b>200</b> sets the valve-opening position of the back pressure valve <b>143</b> to the back pressure valve opening position Xb set at step S<b>250</b> (shown in <figref idref="DRAWINGS">FIG. 5</figref>) or the back pressure valve opening position Xb updated at step S<b>350</b> or step S<b>360</b> (shown in <figref idref="DRAWINGS">FIG. 8</figref>) and thereby controls the back pressure valve <b>143</b> to a reaction-field full-close state (step S<b>170</b>). The controller <b>200</b> subsequently obtains a voltage value Vme of the fuel cell <b>100</b> from the voltage sensor <b>102</b> (step S<b>175</b>).

0069The reaction-field full-close state herein means a valve-opening position of the back pressure valve <b>143</b> that supplies the amount of oxygen at a boundary between the oxygen deficiency area A and the equivalent ratio-of-approximately-1 area B shown in <figref idref="DRAWINGS">FIG. 3</figref>, In other words, the reaction-field full-close state denotes a valve-opening position of the back pressure valve <b>143</b> that supplies an amount of oxygen which is required for oxidation of hydrogen transmitted through the electrolyte membrane during stop of power generation of the fuel cell <b>100</b>, to the fuel cell <b>100</b>. Controlling the back pressure valve <b>143</b> to the reaction-field full-close state at step S<b>170</b> drastically decreases the amount of oxygen supplied to the fuel cell <b>100</b>. According to this embodiment, the valve-opening position of the back pressure valve <b>143</b> that provides the reaction-field full-close state is set in advance as described above or is further learned as appropriate as described above and is stored in the memory of the controller <b>200</b>.

0070When it is determined at step S<b>120</b> that the current cycle is the first cycle after a change to the intermittent operation mode this time, i.e., around the time that a change from the ordinary operation mode to the intermittent operation mode, an excess amount of oxygen is supplied to the fuel cell <b>100</b>. Stopping power generation of the fuel cell <b>100</b> immediately after step S<b>170</b> is thus likely to increase the OCV of the fuel cell <b>100</b> to an unacceptable level. Power generation of the fuel cell <b>100</b> may thus he continued even after step S<b>170</b>, for example, in the state that the output voltage of the fuel cell <b>100</b> is equal to or lower than an allowable upper limit. In the case where power generation of the fuel cell <b>100</b> is continued after step S<b>170</b>, the amount of oxygen in the cathode-side flow path of the fuel cell <b>100</b> is consumed by power generation and drastically decreases. This leads to a gradual decrease in output current of the fuel cell <b>100</b>. When the output current of the fuel cell <b>100</b> decreases to a certain level, the diode included in the DC/DC converter <b>104</b> works to shut off the power supply from the fuel cell <b>100</b> to the load and thereby stop power generation of the fuel cell <b>100</b>.

0071In the case where power generation of the fuel cell <b>100</b> is stopped after step S<b>170</b>, on the other hand, the amount of oxygen in the cathode-side flow path in the fuel cell <b>100</b> is rapidly decreased by oxidation of hydrogen transmitted through the electrolyte membrane on the cathode. Decreasing the amount of oxygen in the fuel cell <b>100</b> causes the OCV of the fuel cell <b>100</b> after stop of power generation to be decreased and approach the allowable upper limit. Further continuation of stop of power generation of the fuel cell <b>100</b> causes the OCV of the fuel cell <b>100</b> to be decreased to or below this allowable upper limit.

0072At step S<b>175</b>, the controller <b>200</b> obtains the output voltage of the fuel cell <b>100</b> during power generation of the fuel cell <b>100</b>, while obtaining the OCV of the fuel cell <b>100</b> during stop of power generation of the fuel cell <b>100</b>. According to this embodiment, the voltage value Vme denotes an average cell voltage calculated by dividing the voltage value of the entire cell stack detected by the voltage sensor <b>102</b> by the number of cells included in the cell stack.

0073After obtaining the voltage value Vme of the fuel cell <b>100</b> at step S<b>175</b>, the controller <b>200</b> compares the obtained voltage value Vme with a target voltage Vmark +α (step S<b>180</b>). The target voltage Vmark is stored in advance in the memory of the controller <b>200</b> as described above, and α denotes a positive value provided to suppress the OCV of the fuel cell <b>100</b> from decreasing to below the target voltage Vmark due to a delay in increase of the supply amount of oxygen to the cathode-side flow path. As described above, the voltage value Vme gradually decreases after stop of power generation of the fuel cell <b>100</b>. According to this embodiment, the controller <b>200</b> repeats the input of the voltage value Vme at step S<b>175</b> and the determination at step S<b>180</b> until the voltage value Vme becomes equal to or lower than the target voltage Vmark +α.

0074When it is determined at step S<b>180</b> that the voltage value Vme becomes equal to or lower than the target voltage Vmark +α, the controller <b>200</b> outputs a driving signal to the stepping motor of the back pressure valve <b>143</b> in order to control the valve-opening position of the back pressure valve <b>143</b> to a valve-opening position stored in advance to achieve the target. voltage Vmark (step S<b>190</b>) and terminates this routine. This increases the amount of oxygen supplied to the fuel cell <b>100</b> from the amount of oxygen corresponding to the reaction-field full-close state to the amount of oxygen corresponding to the valve-opening position of the back pressure valve <b>143</b> that achieves the target voltage Vmark.

0075When the supply amount of oxygen is increased after a drop of the voltage value Vme to the target voltage Vmark, the voltage of the fuel cell <b>100</b> is likely to further decrease to below the target voltage Vmark until a desired amount of oxygen reaches the cathode. According to this embodiment, the supply amount of oxygen is increased at the timing when the voltage value Vme that is the average cell voltage becomes equal to or lower than the target voltage Vmark +α. This suppresses the voltage of the fuel cell <b>100</b> from decreasing to below the target voltage Vmark. The value a may be set adequately by taking into account the response of driving the to back pressure valve <b>143</b> and the time duration when oxygen actually reaches the cathode after an increase in supply amount. The time duration when oxygen actually reaches the cathode may be affected by, for example, the flow path resistance and the flow path length of the cathode-side flow path.

0076When it is determined at step S<b>120</b> the current cycle is not the first cycle after a change to the intermittent operation mode this time, i.e., when it is determined that control of the valve-opening position of the back pressure valve <b>143</b> has already been started, the controller <b>200</b> obtains the voltage value Vme of the fuel cell <b>100</b> from the voltage sensor <b>102</b> (step S<b>130</b>), Since power generation of the fuel cell <b>100</b> has already been stopped, the controller <b>200</b> obtains the open circuit voltage OCV of the fuel cell <b>100</b> as the voltage value Vme at step S<b>130</b>.

0077After obtaining the voltage value Vme at step S<b>130</b>, the controller <b>200</b> compares the obtained voltage value Vme with the target voltage Vmark (step S<b>150</b>). When the result of comparison shows that the obtained voltage value Vme is higher than the target voltage Vmark by at least a first value and indicates a voltage increasing state, the controller <b>200</b> decreases the valve opening position of the back pressure valve <b>143</b> in the non-power generation state to decrease the flow rate of oxygen supplied to the cathode-side flow path (step S<b>160</b>) and then terminates this routine. When the result of comparison at step S<b>150</b> shows that the obtained voltage value Vme is lower than the target voltage Vmark by at least a second value and indicates a voltage decreasing state, the controller <b>200</b> increases the valve opening position of the back pressure valve <b>143</b> in the non-power generation state to increase the flow rate of oxygen supplied to the cathode-side flow path (step S<b>162</b>) and then terminates this routine. When the result of comparison at step S<b>150</b> indicates neither the voltage increasing state nor the voltage decreasing state but indicates a voltage maintaining state, the controller <b>200</b> maintains the current valve opening position of the back pressure valve <b>143</b> in the non-power generation state to keep the amount of oxygen supplied to the cathode-side flow path (step S<b>164</b>) and then terminates this routine.

0078According to this embodiment, in the case of decreasing the valve-opening position of the back pressure valve <b>143</b> at step S<b>160</b> or in the case of increasing the valve-opening position of the back pressure valve <b>143</b> at step S<b>162</b>, the driving amount of the stepping motor of the back pressure valve <b>143</b> is varied to change the valve-opening position by one step. In other words, the valve-opening position of the back pressure valve <b>143</b> is changed by the minimum unit of changing the valve-opening position. This suppresses a drastic voltage variation of the fuel cell <b>100</b>. The amount of each change in valve-opening position of the back pressure valve <b>143</b> may, however, be set to two or more steps.

0079The first value used for the determination to decrease the amount of oxygen at step S<b>160</b> may be different from or may be identical with the second value used for the determination to increase the amount of oxygen at step S<b>162</b>. The first value and the second value may be set arbitrarily to positive values by taking into account, for example, the response of changing the flow rate of oxygen to the driving signal input to the back pressure valve <b>143</b>.

D. Effects

0080As described above, during operation of the fuel cell <b>100</b> in the load disconnected state that the fuel cell <b>100</b> is electrically disconnected from the load such as the motor <b>170</b>, the fuel cell system <b>30</b> of the embodiment sets the back pressure valve <b>143</b> to the full-close position to stop the supply of the air (supply of oxygen) to the cathode and subsequently performs the gradual valve-opening operation to open the back pressure valve <b>143</b> by one step each time (step S<b>235</b>). As shown in <figref idref="DRAWINGS">FIG. 7</figref>, when the open circuit voltage V_MEiof the fuel cell <b>100</b> measured after each gradual valve-opening operation shifts to an increase during continuation of gradual valve-opening operations, the fuel cell system <b>30</b> of the embodiment stores either the valve-opening position corresponding to the integrated number of valve-opening steps (Si+1) by a gradual valve-opening operation at the timing of determination that the voltage V_MEishifts to an increase or the valve-opening position corresponding to the integrated number of valve-opening steps (Si) by a gradual valve-opening operation prior to this determination, as the back pressure valve opening position Xb corresponding to the equivalent ratio of 1 (step S<b>250</b>). This back pressure valve opening position Xb supplies a just enough amount of oxygen corresponding to the equivalent ratio of 1 that allows hydrogen transmitted from the anode to the cathode to be oxidized.

0081The fuel cell system <b>30</b> of the embodiment regulates the back pressure valve opening position Xb corresponding to the equivalent ratio of 1 to make the open circuit voltage OCV during operation of the fuel cell <b>100</b> in the non-power generation state equal to the target voltage Vmark, so as to regulate the amount of oxygen supplied to the fuel cell <b>100</b>. In the state that the flow rate of oxygen supplied to the cathode is regulated by controlling the back pressure valve <b>143</b> to the back pressure valve opening position Xb, a small amount of oxygen remains at the anode or is supplied to the anode in the non-power generation state of the fuel cell <b>100</b>, so that the air is supplied to the cathode at a low flow rate that introduces oxygen at an equivalent ratio of 1 to the transmitted hydrogen to the cathode. The fuel cell system <b>30</b> of the embodiment controls the back pressure valve <b>143</b> to the stored back pressure valve opening position Xb corresponding to the equivalent ratio of 1 by using the target voltage Vmark and thereby allows for fine adjustment of the flow rate of the oxidizing gas taking advantage of the behavior of the measured voltage of the fuel cell <b>100</b>.

0082The fuel cell system <b>30</b> of the embodiment gradually opens the back pressure valve <b>143</b> by each step of the stepping motor (step S<b>235</b>) in the process of setting the back pressure valve opening position Xb corresponding to the equivalent ratio of 1. The fuel cell system <b>30</b> of the embodiment suppresses the open circuit voltage V_MEiof the fuel cell <b>100</b> from decreasing to a low voltage such as 0 volt in the course of gradually opening the back pressure valve <b>143</b> so as to cause the open circuit voltage V_MEiof the fuel cell <b>100</b> to shift to an increase during continuation of gradual valve-opening operations, thus suppressing deterioration of the catalyst in the fuel cell <b>100</b>. The back pressure valve <b>143</b> may be gradually opened by two steps or by three steps each time. The gradual valve-opening operations by a plurality of steps each time suppresses an excessive decrease in open circuit voltage V_MEiof the fuel cell <b>100</b>. In the case of gradually opening the back pressure valve <b>143</b> by a plurality of steps each time, the valve-opening speed may be simultaneously regulated, in order to more effectively suppress an excessive decrease in open circuit voltage V_MEiof the fuel cell <b>100</b>.

0083The fuel cell system <b>30</b> of the embodiment controls the back pressure valve <b>143</b> to maintain the back pressure valve opening position Xb corresponding to the equivalent ratio of 1 for a predetermined time duration (step S<b>255</b>). The fuel cell system <b>30</b> of the embodiment accordingly suppresses elution and reduction of the cathode catalyst in the load disconnected state that the fuel cell <b>100</b> is electrically disconnected from the load such as the motor <b>170</b> and improves the stability of the catalyst, thus allowing subsequent power generating operation'of the fuel cell <b>100</b> to be performed without difficulty.

0084The fuel cell system <b>30</b> of the embodiment gradually opens the back pressure valve <b>143</b> by one step each time in the process of setting the back pressure valve opening position Xb corresponding to the equivalent ratio of 1. The interval of the gradual valve-opening operations by one step is set to a time period that allows a voltage change accompanied with oxidation of transmitted hydrogen to be detected by the voltage sensor <b>102</b>. The fuel cell system <b>30</b> of the embodiment accordingly prevents a subsequent gradual valve-opening operation of the back pressure valve <b>143</b> from being performed prior to measurement of a voltage change accompanied with a gradual valve-opening operation by the voltage sensor <b>120</b>. The fuel cell system <b>30</b> of the embodiment thus accurately obtains the behavior of the open circuit voltage V_MEiof the fuel cell <b>100</b> and enhances the accuracy of controlling the back pressure valve opening position Xb.

0085The fuel cell system <b>30</b> of the embodiment sets the back pressure valve opening position Xb corresponding to the equivalent ratio of 1 for the purpose of, for example, maintenance and inspection and learns and updates the back pressure valve opening position Xb during a run of the vehicle in the intermittent operation control. This provides the following advantages, Each mechanical component such as the back pressure valve <b>143</b> generally has an allowable individual difference in functions and is subjected to aged deterioration of the functions, With regard to the back pressure valve <b>143</b>, as shown in <figref idref="DRAWINGS">FIG. 6</figref>, the valve opening position Xa that is the mechanical full-close position and the back pressure valve opening position Xb corresponding to the equivalent ratio of 1 have individually differences in the illustrated ranges. One back pressure valve <b>143</b> is also likely to have a variation in the illustrated range by the aged deterioration. The fuel cell system <b>30</b> of the embodiment sets the back pressure valve opening position Xb corresponding to the equivalent ratio of 1 for the purpose of, for example, maintenance and inspection and learns and updates the back pressure valve opening position Xb during a run of the vehicle in the intermittent operation control (step S<b>300</b>). This allows for drive control of the back pressure valve <b>143</b> by taking into account the variation in control based on the individual difference and the effect of the aged deterioration.

0086The fuel cell system <b>30</b> of the embodiment learns and corrects the back pressure valve opening position Xb (steps S<b>350</b> to S<b>360</b> in <figref idref="DRAWINGS">FIG. 8</figref>) in the intermittent operation control of <figref idref="DRAWINGS">FIG. 4</figref>. This learning and correction of the back pressure valve opening position Xb by driving the back pressure valve <b>143</b> is performed at the timing when the incremented counter n reaches the learning span-specifying counter value Mn. This adequately controls the frequency of driving the back pressure valve <b>143</b> to the back pressure valve opening position Xb in the intermittent operation mode.

0087The fuel cell system <b>30</b> of the embodiment can avoid the high potential state of the fuel cell <b>100</b> without power generation of the fuel cell <b>100</b> in a low load state that the load request is equal to or lower than the predefined reference value. There is accordingly no need to start unrequired, excessive power generation for only the purpose of avoiding the high potential state. This also suppresses the energy efficiency of the fuel cell system <b>30</b> from being decreased due to, for example, storage of the generated electric power into the secondary battery <b>172</b>.

0088As understood from the IV characteristic shown in <figref idref="DRAWINGS">FIG. 2</figref>, an available method of preventing the fuel cell <b>100</b> from having a high open circuit voltage OCV by stopping power generation of the fuel cell <b>100</b> may perform low-output power generation of the fuel cell <b>100</b> to provide a low output current I<b>1</b> such as to control the output voltage of the fuel cell <b>100</b> to an allowable level of high voltage V<b>1</b>. The secondary battery <b>172</b> may be charged with excessive electric power thus generated. The configuration.

0089that electric power generated by the fuel cell <b>100</b> is once charged into the secondary battery <b>172</b> and is then output from the secondary battery <b>172</b> to the load has a lower energy efficiency than the configuration that electric power is directly supplied from the fuel cell <b>100</b> to the load. The secondary battery <b>172</b> has the limited capacity, so that incasing the amount of power generation to avoid the high potential state is likely to cause a failure in continuing the low-output power generation for the purpose of avoiding the high potential state or a failure in recovering the regenerative electric power during braking of the vehicle. This may lead to a difficulty in adequately maintaining the energy balance in the fuel cell system. The fuel cell system <b>30</b> of the embodiment can avoid the high potential state with stopping power generation of the fuel cell <b>100</b> in the predetermined low load state and thereby avoids the potential problems described above.

0090The fuel cell system <b>30</b> of the embodiment regulates the valve-opening position of the back pressure valve <b>143</b> based on the current open circuit voltage OCV of the fuel cell <b>100</b>, so as to vary the amount of oxygen that is supplied to the cathode-side flow path and is required to maintain the voltage. An available method of regulating the amount of the air or the amount of oxygen supplied to the fuel cell <b>100</b> may directly detect the supply amount of oxygen and change the supply amount of oxygen to a desired value. This method, however, requires a flowmeter to detect the amount of oxygen supplied to the cathode-side flow path, for example, an air flowmeter to detect the supply amount of the air. The fuel cell system <b>30</b> of the embodiment, however, does not require the flowmeter and thus simplifies the system configuration.

0091The fuel cell system <b>30</b> of the embodiment branches off the flow path of the air supplied to the cathode-side flow path and controls the supply amount of oxygen by regulating the valve-opening position of the back pressure valve <b>143</b>. This enables the supply amount of oxygen to be controlled with high accuracy. An available method of varying the supply amount of oxygen may vary the driving amount of the compressor <b>130</b>. In this case, a compressor having a wide flow range to satisfy a wide range of load request from a minimum load request to a maximum load request may be employed for the compressor <b>130</b>. An additional compressor for the low flow rate may be separately provided to regulate the flow rate of oxygen. The fuel cell system <b>30</b> of the embodiment allows for fine adjustment of the air flow rate with the high accuracy without separately providing the additional compressor. A different method, for example, the method of separately providing an additional compressor for the low flow rate, may be applied to regulate the supply amount of oxygen and control the voltage value Vme to approach the target voltage Vmark. Another method may perform the control described above with varying the distribution ratio of the air by the distribution valve <b>144</b>. This configuration also achieves the advantageous effect of avoiding the high potential state with stopping power generation, like the embodiment described above.

E. Modifications

0092The invention is not limited to the embodiment described above but may be implemented by a diversity of other aspects and configurations without departing from the scope of the invention. For example, the technical features of the embodiment corresponding to the technical features of each of the aspects described in SUMMARY may be replaced or combined appropriately, in order to solve part or all of the problems described above or in order to achieve part or all of the advantageous effects described above. Any of the technical features may be omitted appropriately unless the technical feature is described as essential herein.,

0093The fuel cell system <b>30</b> of the embodiment regulates the back pressure by driving the back pressure valve <b>143</b> in the valve-closing direction and in the valve-opening direction, so as to regulate the flow rate of the air supplied to the cathode of the fuel cell <b>100</b>. This configuration is, however, not restrictive. For example, the distribution ratio of the distribution valve <b>144</b> may be regulated, in combination with regulation of the back pressure valve <b>143</b>. In another example, a throttle valve may be provided as a flow regulating valve in the second air flow path <b>145</b> that is the inlet flow path to the fuel cell <b>100</b> or throttle valves may be provided both at an inlet and an outlet of the fuel cell <b>100</b>. Providing a flow regulating valve to regulate the amount of oxygen supplied to the cathode achieves the advantageous effect similar to that of the embodiment described above.

0094The fuel cell system <b>30</b> of the embodiment stores either the valve-opening position corresponding to the integrated number of valve-opening steps (Si+1) at the timing of determination that the voltage V_MEiof the fuel cell <b>100</b> shifts to an increase during continuation of gradual valve-opening operations or the valve-opening position corresponding to the integrated number of valve-opening steps (Si) prior to this determination, as the back pressure valve opening position Xb corresponding to the equivalent ratio of 1. This configuration is, however, not restrictive. For example, a valve-opening position corresponding to an integrated number of valve-opening steps (Si+2) immediately after the integrated number of valve-opening steps (Si+1) by a gradual valve-opening operation at the timing of determination that the voltage V_MEiof the fuel cell <b>100</b> shifts to an increase may be stored as the back pressure valve opening position Xb corresponding to the equivalent ratio of 1. Among valve-opening positions of the back pressure valve <b>143</b> by gradual valve-opening operations at a predetermined number of timings including and before and after the timing when it is determined that the voltage V_MEiof the fuel cell <b>100</b> shifts to an increase during continuation of gradual valve-opening operations, any valve-opening position of the back pressure valve <b>143</b> by a gradual valve-opening operation at any timing may be stored as the back pressure valve opening position Xb, as long as a difference from the valve-opening position of the back pressure valve <b>143</b> at the timing of determination that the voltage V_MEiof the fuel cell <b>100</b> shifts to an increase during continuation of gradual valve-opening operations is within a predetermined range.

0095The fuel cell system <b>30</b> of the embodiment drives the back pressure valve <b>143</b> to the full-close position in the process of setting the back pressure valve opening position Xb corresponding to the equivalent ratio of 1. This configuration is, however, not restrictive. According to a modification, the back pressure valve <b>143</b> may be driven in the valve-closing direction to a specified valve position before the full-close position and subsequently perform gradual valve-opening operations, as along as a shift of the OCV to an increase is detectable during continuation of gradual valve-opening operations.

0096The fuel cal system <b>30</b> of the embodiment gradually opens the back pressure valve <b>143</b> in the process of setting the back pressure valve opening position Xb corresponding to the equivalent ratio of 1. This configuration is, however, not restrictive. According to a modification, the back pressure valve <b>143</b> may be driven in the valve-opening position to the full-open position and subsequently perform gradual valve-closing operations by one step each time. In the configuration of performing the gradual valve-closing operations, a valve-opening position corresponding to an integrated number of valve-closing steps of the stepping motor at the timing when it is determined that the open circuit voltage V_MEiof the fuel cell <b>100</b> shifts to a decrease during continuation of valve-closing operations or an integrated number of valve-closing steps immediately before or immediately after the determination may be stored as the back pressure valve opening position Xb corresponding to the equivalent ratio of 1. In this modification, any valve-opening position of the back pressure valve <b>143</b> by a gradual valve-closing operation at any timing may be stored as the back pressure valve opening position Xb, as long as a difference from the valve-opening position of the back pressure valve <b>143</b> at the timing of determination that the voltage V_MEiof the fuel cell <b>100</b> shifts to a decrease during continuation of gradual valve-closing operations is within a predetermined range,

0097The fuel cell system <b>30</b> of the embodiment sets the back pressure valve opening position Xb (step S<b>200</b> in <figref idref="DRAWINGS">FIG. 5</figref>) upon satisfaction of the condition of the load request (affirmative answer at step S<b>110</b>) in the intermittent operation control of <figref idref="DRAWINGS">FIG. 4</figref>. This configuration is, however, not restrictive. According to a modification, when only a short time has elapsed since learning and correction of the back pressure valve opening position Xb in the intermittent operation control of <figref idref="DRAWINGS">FIG. 4</figref>, the back pressure valve opening position Xb may not be reset even upon satisfaction of the condition of the load request. The seal member of the back pressure valve <b>143</b> is unlikely to be cured or deteriorated with time in such a short elapsed time, so that the back pressure valve opening position Xb is unlikely to have a significant variation. Omitting the process of resetting the valve-opening position in such cases desirably reduces the load of operations of the controller <b>200</b>.

0098The fuel cell system <b>30</b> of the embodiment learns and updates the back pressure valve opening position Xb by the series of processing at step S<b>300</b>, when the condition of the load request is satisfied (affirmative answer at step S<b>110</b>) and the reaction-field full-close position has been stored in the intermittent operation control of <figref idref="DRAWINGS">FIG. 4</figref>. This configuration is, however, not restrictive. According to a modification, the back pressure valve <b>143</b> may be gradually opened or gradually closed according to the result of comparison between the voltage value Vme and the target voltage Vmark at step S<b>150</b> in the intermittent operation control of <figref idref="DRAWINGS">FIG. 4</figref>. The modification may detect a variation in voltage value Vme after each gradual valve-opening operation or gradual valve-closing operation at step S<b>150</b> and may learn and update the back pressure valve opening position Xb at subsequent step S<b>160</b> or step S<b>162</b>.