Patent Publication Number: US-2021188102-A1

Title: Fuel cell vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This nonprovisional application is based on Japanese Patent Application No. 2019-231243 filed with the Japan Patent Office on Dec. 23, 2019, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Field 
     The present disclosure relates to a fuel cell vehicle. 
     Description of the Background Art 
     WO2011/004493 discloses a fuel cell vehicle on which fuel cells are mounted (a fuel cell being referred to as an “FC” and a fuel cell vehicle being referred to as an “FCV” below). The FCV includes an FC stack and a battery. The battery functions as a source of storage of excessive electric power, a source of storage of regenerative energy during regenerative braking, and an energy buffer in case of variation in load with acceleration or deceleration of a vehicle. 
     In an FCV including FCs and a power storage such as a battery, there is a great difference in energy amount between hydrogen serving as fuel for the FCs and electric power stored in the power storage. In general, energy of electric power stored in the power storage is lower than hydrogen energy. Therefore, when fuel (hydrogen) is finished up earlier than a state of charge (SOC) of the power storage, although a vehicle can thereafter travel with electric power stored in the power storage, traveling performance is suddenly significantly restricted at the time point when fuel runs out. 
     SUMMARY 
     The present disclosure was made to solve such a problem, and an object of the present disclosure is to avoid sudden significant restriction of traveling performance due to early run-out of fuel for FCs in an FCV including the FCs and a power storage. 
     An FCV according to the present disclosure includes an FC system output of which can be adjusted, a power storage, a driving device that receives electric power from at least one of the FC system and the power storage and generates travel power, and a controller that controls output from the FC system to adjust an SOC of the power storage to a target SOC while electric power is supplied from the FC system to the driving device. When a remaining amount of fuel in the FC system decreases, the controller lowers the target SOC. 
     As set forth above, by lowering the target SOC of the power storage when a remaining amount of fuel decreases, in general, electric power is provided also from the power storage to the driving device and hence preferential use of the FC over the power storage can be suppressed. Therefore, according to the FCV, sudden significant restriction of traveling performance due to early run-out of fuel for the FC can be avoided. 
     When the remaining amount of fuel in the FC becomes smaller than a threshold value, the controller may lower the target SOC with decrease in the remaining amount of fuel. 
     When the SOC lowers with lowering in target SOC, output from the power storage is restricted and hence traveling performance is restricted. Since the target SOC is lowered when the remaining amount of fuel becomes smaller than the threshold value in the FCV, output from the power storage is secured and sufficient traveling performance can be secured until the remaining amount of fuel decreases to the threshold value. 
     The controller may lower the target SOC with decrease in the remaining amount of fuel such that the target SOC attains to a lower limit value at the time when fuel in the FC system runs out. 
     Thus, such a situation that one of run-out of fuel and the SOC reaching the lower limit value precedes the other and one of the FC system and the power storage is unable to provide output can be avoided. Consequently, sudden significant restriction of traveling performance can be avoided. 
     The FC system may include a tank where fuel (hydrogen) is stored, an FC stack that generates electric power with fuel stored in the tank, and a converter that adjusts output from the FC stack. The power storage may electrically be connected to a power line between the converter and the driving device. 
     According to such a configuration, output from the FC stack and the power storage can be adjusted by controlling the converter. For example, by reducing output from the FC stack relative to travel power, output from the power storage can be increased. Thus, sudden significant restriction of traveling performance due to early run-out of fuel for the FC can be avoided. 
     The FCV may further include a charger that charges the power storage with a power supply outside the vehicle. 
     According to the FCV, travel over a long distance can be achieved with fuel (hydrogen) stored in the tank and electric power supplied from the outside of the vehicle and stored in the power storage. 
     The foregoing and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of the present disclosure when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an overall configuration of an FCV according to a first embodiment of the present disclosure. 
         FIG. 2  is a diagram showing a travel mode provided in the FCV. 
         FIG. 3  is a diagram showing a basic flow of electric power in an FC mode. 
         FIG. 4  is a diagram showing a basic flow of electric power in an FCEV mode. 
         FIG. 5  is a diagram showing comparison between energy of hydrogen and energy of electric power stored in a battery. 
         FIG. 6  is a diagram showing a basic flow of electric power in an EV mode. 
         FIG. 7  is a diagram showing a basic flow of electric power in a CHG mode. 
         FIG. 8  is a flowchart showing an exemplary procedure in processing performed by an FDC-ECU. 
         FIG. 9  is a flowchart showing an exemplary method of calculating a target SOC when the travel mode is set to the FCEV mode in step S 60  in  FIG. 8 . 
         FIG. 10  is a diagram showing transition of a remaining amount of hydrogen, a target SOC, and a system output upper limit when the travel mode is set to the FCEV mode. 
         FIG. 11  is a diagram showing relation of a remaining amount of energy in an FC system and a battery with a system output upper limit. 
         FIG. 12  is a flowchart showing an exemplary method of calculating a target SOC when the travel mode is set to the FCEV mode in a second embodiment. 
         FIG. 13  is a diagram showing transition of a remaining amount of hydrogen, a target SOC, and a system output upper limit when the travel mode is set to the FCEV mode in the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure will be described below in detail with reference to the drawings. The same or corresponding elements in the drawings have the same reference characters allotted and description thereof will not be repeated. 
     First Embodiment 
       FIG. 1  is a diagram showing an overall configuration of an FCV  1  according to a first embodiment of the present disclosure. Referring to  FIG. 1 , FCV  1  includes a motor generator (which is referred to as an “MG” below)  10 , an inverter  12 , an FC system  20 , a hydrogen tank  28 , a supply valve  30 , an air filter  32 , and a compressor  34 . 
     MG  10  is an alternating-current (AC) rotating electric machine, and it is, for example, a three-phase AC synchronous motor having a permanent magnet embedded in a rotor. MG  10  generates rotational driving force as it is driven by inverter  12 . Driving force generated by MG  10  is transmitted to a not-shown drive wheel. During braking of FCV  1 , MG  10  serves as a generator and generates electric power. Electric power generated by MG  10  is rectified by inverter  12  and rectified electric power can be stored in a battery  40 . 
     Inverter  12  is provided between a power line  70  and MG  10  and drives MG  10  based on a drive signal from an MG-ECU  66  (which will be described later). Inverter  12  is implemented, for example, by a bridge circuit including switching elements of three phases. 
     FC system  20  includes an FC stack  22 , a boost converter  24 , and a relay  26 . FC stack  22  is, for example, a structure in which a plurality of (for example, several ten to several hundred) cells of a solid polymer type are stacked in series. Each cell is made, for example, by joining a catalyst electrode to each of opposing surfaces of an electrolyte membrane and sandwiching the membrane between conductive separators. Each cell generates electric power as a result of electrochemical reaction between hydrogen supplied to an anode and oxygen (air) supplied to a cathode. 
     Boost converter  24  boosts electric power (for example, at several hundred V) generated by FC stack  22  based on a control signal from an FDC-ECU  60  (which will be described later) and outputs boosted electric power to power line  70 . Relay  26  is provided in an electric path between FC stack  22  and boost converter  24  and opened while a vehicle system remains stopped or while FC system  20  is not used. 
     Hydrogen tank  28  stores hydrogen as fuel supplied to FC stack  22 . Hydrogen tank  28  is a high pressure tank that is light in weight and high in strength and includes, for example, a carbon fiber reinforced plastic layer, and can store, for example, hydrogen at several ten MPa. Hydrogen is supplied from hydrogen tank  28  through supply valve  30  to FC stack  22 . 
     Compressor  34  is a device for supplying oxygen to FC stack  22 . Compressor  34  suctions oxygen (air) through air filter  32 , compresses oxygen (air), and supplies compressed oxygen (air) to FC stack  22 . 
     FCV  1  further includes battery  40 , a direct current (DC) inlet  44 , an AC inlet  48 , a charger  50 , and relays  42 ,  46 , and  52 . 
     Battery  40  is a chargeable and dischargeable power storage. Battery  40  includes a battery assembly constituted of a plurality of cells (for example, several hundred cells). Each cell is, for example, a secondary battery such as a lithium ion battery or a nickel metal hydride battery. A lithium ion secondary battery is a secondary battery containing lithium as a charge carrier, and may include not only a general lithium ion secondary battery containing a liquid electrolyte but also what is called an all-solid-state battery containing a solid electrolyte. A power storage element such as an electric double layer capacitor may be employed instead of battery  40 . 
     Battery  40  is connected to a power line  72  with relay  42  being interposed, and power line  72  is connected to power line  70 . Battery  40  stores electric power for driving MG  10  and supplies electric power to inverter  12  through power lines  72  and  70 . Battery  40  is charged with electric power generated by MG  10  during braking of FCV  1 . Battery  40  can function as an energy buffer that accommodates variation in load caused by acceleration and deceleration of FCV  1  or stores electric power generated by MG  10  during braking of FCV  1 . 
     In the present embodiment, battery  40  can be charged with electric power supplied from a power supply (not shown) outside a vehicle through DC inlet  44  or AC inlet  48  (charging of battery  40  by a power supply outside the vehicle being also referred to as “external charging” below). 
     DC inlet  44  is connected to a power line  74  with relay  46  being interposed and power line  74  is connected to power line  72 . DC inlet  44  is constructed such that a connector of a DC charging cable that extends from a charging stand (not shown) outside the vehicle can be fitted thereto, and DC inlet  44  receives DC power at a high voltage supplied from the charging stand and outputs DC power to power line  74 . 
     AC inlet  48  is connected to charger  50  with relay  52  being interposed. AC inlet  48  is constructed such that a connector of an AC charging cable that extends from a charging stand outside the vehicle can be fitted thereto, and AC inlet  48  receives AC power (for example, system power) supplied from the charging stand and outputs AC power to charger  50 . Charger  50  is connected to power line  74  and converts AC power input from AC inlet  48  to a voltage level of battery  40  and outputs DC power to power line  74 . 
     Relay  42  is provided between battery  40  and power line  72  and closed while the system of FCV  1  is ON or while external charging is being carried out. Relay  46  is provided between DC inlet  44  and power line  74  and closed during external charging (DC charging) through DC inlet  44 . Relay  52  is provided between AC inlet  48  and charger  50  and closed during external charging (AC charging) through AC inlet  48  and charger  50 . 
     FCV  1  is thus a plug-in FCV in which battery  40  can be charged with a power supply outside the vehicle connected to DC inlet  44  or AC inlet  48 , and it can travel with electric power stored in battery  40  by external charging. 
     FCV  1  further includes FDC-electronic control unit (ECU)  60 , a mode switch (MD-SW)  62 , a battery ECU  64 , and MG-ECU  66 . Each of FDC-ECU  60 , battery ECU  64 , and MG-ECU  66  includes a central processing unit (CPU), a memory (a read only memory (ROM) and a random access memory (RAM)), and an input and output buffer (none of which is shown). The CPU develops a program stored in the ROM on the RAM and executes the program. Processing to be performed by a corresponding ECU is described in a program stored in the ROM. 
     FDC-ECU  60  calculates output requested of FC system  20  (output electric power from FC system  20 ) based on travel power requested of FCV  1  and a request for charging and discharging of battery  40 , and controls boost converter  24  such that FC system  20  outputs calculated electric power. Travel power requested of FCV  1  is calculated based on an amount of operation of an accelerator pedal and a vehicle speed. Though travel power is calculated by FDC-ECU  60  in the first embodiment, it may be calculated by another ECU (for example, a vehicle ECU (not shown) that controls the entire vehicle in a centralized manner). 
     FDC-ECU  60  switches a travel mode in accordance with setting made through mode switch  62 . FCV  1  incorporates FC system  20  and battery  40  as power supplies, and battery  40  can store electric power. In FCV  1  according to the present first embodiment, four travel modes in accordance with usage of FC system  20  and battery  40  are available, and a user can select a travel mode by operating mode switch  62 . The travel mode will be described in detail later. 
     Mode switch  62  is a switch for a user to set the travel mode. Mode switch  62  may be a dedicated switch or may be implemented on a touch panel display of a navigation apparatus or the like. 
     Battery ECU  64  monitors a voltage, a current, and a temperature of battery  40 . A voltage, a current, and a temperature of battery  40  are detected by not-shown various sensors. Battery ECU  64  calculates an SOC of battery  40  based on values of detected voltage, current, and temperature of battery  40 . The calculated SOC value is transmitted to FDC-ECU  60 . The SOC may be calculated by FDC-ECU  60  based on values of detected voltage, current, and temperature of battery  40 . 
     In FCV  1 , battery  40  is connected to power line  70  without a converter being interposed, and an amount of charging and discharging of battery  40  is basically determined by a difference between travel power requested by inverter  12  and MG  10  and output from FC system  20 . Therefore, charging and discharging and the SOC of battery  40  can be controlled by control of output from FC system  20  by FDC-ECU  60  based on travel power. 
     In FCV  1  according to the first embodiment, a target SOC representing a target of the SOC is calculated by FDC-ECU  60  in accordance with the travel mode. A requested amount of charging and discharging of battery  40  is then calculated based on a difference between the SOC and the target SOC such that the SOC of battery  40  is closer to the target SOC, and output from FC system  20  is controlled by FDC-ECU  60  based on the calculated requested amount of charging and discharging and travel power. 
     The target SOC will be described in detail later. Various known approaches such as an approach using an open circuit voltage (OCV)-SOC curve (a map) representing relation between the OCV and the SOC and an approach using an accumulated value of currents input to and output from battery  40  can be used as the method of calculating the SOC. 
     MG-ECU  66  receives a calculated value of travel power requested of FCV  1  from FDC-ECU  60 , generates a signal for driving MG  10  with inverter  12  based on travel power, and outputs the signal to inverter  12 . 
     &lt;Description of Travel Mode&gt; 
     As described above, FCV  1  includes FC system  20  and battery  40 . In FCV  1  according to the present first embodiment, four travel modes in accordance with usage of FC system  20  and battery  40  are provided. 
       FIG. 2  is a diagram showing travel modes provided in FCV  1 . Referring to  FIG. 2 , for FCV  1  according to the present first embodiment, four travel modes of an “FC mode,” an “FCEV mode,” an “EV mode,” and a “CHG mode” are provided. A user of FCV  1  can select a desired travel mode from among the travel modes by using mode switch  62 . 
       FIG. 3  is a diagram showing a basic flow of electric power in the FC mode. Referring to  FIG. 3 , the FC mode refers to a travel mode for travel basically only with output from FC system  20  until fuel (hydrogen) in FC system  20  runs out. After fuel runs out, FCV  1  travels only with output from battery  40 . 
     In the FC mode, in order to travel only with output from FC system  20 , FDC-ECU  60  controls FC system  20  (boost converter  24 ) based on travel power such that FC system  20  outputs power comparable to power required by inverter  12 , that is, travel power (a requested value). 
     Even in the FC mode, when high travel power is requested by strong pressing of the accelerator pedal and travel power exceeds an output upper limit Wfc of FC system  20 , insufficiency in power is compensated for by battery  40 . When regeneration by MG  10  is carried out as in braking of FCV  1 , electric power generated by MG  10  is supplied from inverter  12  to battery  40 . When the SOC of battery  40  has attained to the upper limit, regeneration by MG  10  is not carried out. 
       FIG. 4  is a diagram showing a basic flow of electric power in the FCEV mode. Referring to  FIG. 4 , the FCEV mode is a characteristic travel mode in FCV  1  according to the present first embodiment and it is a hybrid mode in which output from FC system  20  and output from battery  40  are used in a balanced manner. Specifically, in the FCEV mode, FCV  1  travels with both of output from FC system  20  and output from battery  40  such that timing of run-out of fuel (hydrogen) in FC system  20  is simultaneous with timing of the SOC of battery  40  reaching a lower limit value SL. 
     In FCV  1  including FC system  20  and battery  40 , hydrogen serving as fuel in FC system  20  and electric power stored in battery  40  are greatly different from each other in energy amount. Specifically, as shown in  FIG. 5 , energy of electric power stored in battery  40  is lower than hydrogen energy. Therefore, if fuel (hydrogen) is finished up earlier than the SOC of battery  40 , although FCV  1  can thereafter travel with electric power stored in battery  40 , traveling performance of FCV  1  is suddenly significantly restricted at the time point when fuel runs out. 
     In FCV  1  according to the present first embodiment, the FCEV mode is provided as one of the travel modes, and the user can select the FCEV mode by using mode switch  62 . In the FCEV mode, FCV  1  travels with both of output from FC system  20  and output from battery  40  in order to avoid such a situation that fuel in FC system  20  runs out early and FCV  1  does remaining travel only with output from battery  40 . Specifically, output from FC system  20  and output from battery  40  are adjusted such that timing of run-out of fuel in FC system  20  is simultaneous with timing of the SOC of battery  40  reaching lower limit value SL as described above. FCV  1  can travel with both of output from FC system  20  and output from battery  40  until energy (fuel and the battery) in FCV  1  runs out, and such a situation that fuel in FC system  20  runs out early and traveling performance of FCV  1  is suddenly significantly restricted can be avoided. 
     In order for timing of run-out of fuel in FC system  20  and timing of the SOC of battery  40  reaching the lower limit value to simultaneously come in the FCEV mode, in FCV  1  according to the present first embodiment, the target SOC of battery  40  is lowered with decrease in remaining amount of hydrogen in hydrogen tank  28 . In other words, the target SOC is lowered with decrease in remaining amount of hydrogen such that the target SOC reaches the lower limit value at the time when hydrogen in hydrogen tank  28  runs out. Preferential use of FC system  20  over battery  40  can thus be suppressed. 
     In order to travel with both of output from FC system  20  and output from battery  40  while the SOC is controlled to follow the target SOC that lowers with decrease in remaining amount of hydrogen, control as below is carried out in FCV  1  while the FCEV mode is selected. Specifically, FDC-ECU  60  controls FC system  20  (boost converter  24 ) based on travel power and the SOC of battery  40  such that battery  40  outputs electric power to lower the SOC to the target SOC and FC system  20  outputs the balance of power required by inverter  12 , that is, travel power (a requested value). 
     Even in the FCEV mode, when high travel power is requested by strong pressing of the accelerator pedal and travel power exceeds the output upper limit of FC system  20 , electric power equal to or higher than output in accordance with a difference between the SOC and the target SOC is compensated for by battery  40 . When regeneration by MG  10  is carried out as in braking of FCV  1 , electric power generated by MG  10  is supplied from inverter  12  to battery  40 . 
       FIG. 6  is a diagram showing a basic flow of electric power in the EV mode. Referring to  FIG. 6 , the EV mode refers to a travel mode for travel basically only with output from battery  40  without using fuel (hydrogen) in FC system  20 . 
     Even in the EV mode, when high travel power is requested by strong pressing of the accelerator pedal and travel power exceeds an output upper limit Wout of battery  40 , power comparable to insufficiency in power may be output from FC system  20 . When regeneration by MG  10  is carried out as in braking of FCV  1 , electric power generated by MG  10  is supplied from inverter  12  to battery  40 . 
       FIG. 7  is a diagram showing a basic flow of electric power in the CHG mode. Referring to  FIG. 7 , the CHG mode refers to a mode in which the SOC is raised to a prescribed level by positively charging battery  40  with output from FC system  20  when the SOC of battery  40  has lowered. 
     Even in the CHG mode, when travel power is requested by pressing of the accelerator pedal, electric power is supplied from FC system  20  to inverter  12 . Furthermore, when high travel power is requested by strong pressing of the accelerator pedal, electric power is supplied also from battery  40  to inverter  12 . When regeneration by MG  10  is carried out as in braking of FCV  1 , electric power generated by MG  10  is supplied from inverter  12  to battery  40 . 
       FIG. 8  is a flowchart showing an exemplary procedure in processing performed by FDC-ECU  60 . Some of processing may be allocated to battery ECU  64  or MG-ECU  66  or may be performed by another not-shown ECU (a vehicle ECU that controls the entire vehicle in a centralized manner). A series of processing shown in this flowchart is repeatedly performed every prescribed period. 
     Referring to  FIG. 8 , FDC-ECU  60  obtains information on an accelerator position, a shift range that has been selected, and a vehicle speed (step S 10 ). The accelerator position is detected by an accelerator position sensor and the vehicle speed is detected by a vehicle speed sensor (neither of which is shown). Instead of a vehicle speed, the number of rotations of a driveshaft or a propeller shaft may be employed. 
     Then, FDC-ECU  60  calculates requested driving force (torque) based on information obtained in step S 10 , by using a driving force map prepared for each shift range that shows relation among requested driving force, an accelerator position, and a vehicle speed (step S 20 ). Then, FDC-ECU  60  calculates travel power (a requested value) of FCV  1  by multiplying the calculated requested driving force by the vehicle speed and adding prescribed loss power (step S 30 ). 
     In succession, FDC-ECU  60  calculates torque of MG  10  based on the requested driving force calculated in step S 20  (step S 40 ). Calculated torque of MG  10  is transmitted to MG-ECU  66 , which controls inverter  12  such that MG  10  outputs that torque. 
     Then, FDC-ECU  60  obtains setting of the travel mode from mode switch  62  (step S 50 ). When the travel mode has been set to the “FCEV mode,” FDC-ECU  60  calculates the target SOC of battery  40  (step S 60 ). The method of calculating the target SOC in the FCEV mode will be described later. 
     When the travel mode has been set to the “FC mode” or the “EV mode,” the target SOC is basically not calculated. When the travel mode has been set to the “CHG mode,” a value set in advance or a value set by a user is set as the target SOC. 
     Then, FDC-ECU  60  calculates a request for charging and discharging (power) of battery  40  (step S 70 ). This requested amount of charging and discharging is calculated based on the SOC and the target SOC of battery  40 . Specifically, by using the prepared map that shows relation between an SOC difference from the target SOC and the requested amount of charging and discharging, when the SOC is higher than the target SOC, the requested amount of charging and discharging is calculated as a larger positive value (a discharging request) as the SOC is higher, and when the SOC is lower than the target SOC, the requested amount of charging and discharging is calculated as a larger negative value (a charging request) as the SOC is lower. 
     An upper limit and a lower limit are set for the requested amount of charging and discharging. In the “FC mode” and the “EV mode” in which the target SOC is not calculated, the charging and discharging request is not calculated either. 
     Then, FDC-ECU  60  calculates output from FC system  20  (step S 80 ). Specifically, when the travel mode has been set to the “FCEV mode,” output from FC system  20  is calculated by subtracting the requested amount of charging and discharging calculated in step S 70  from travel power calculated in step S 30 . 
     When the travel mode has been set to the “FC mode,” travel power calculated in step S 30  is set as output from FC system  20 . When the travel mode has been set to the “EV mode,” output from FC system  20  is set to 0, and when the travel mode has been set to the “CHG mode,” an absolute value of the requested amount of charging and discharging (a negative value because charging is carried out) calculated in step S 70  is set as output from FC system  20 . 
     Boost converter  24  of FC system  20  is controlled by FDC-ECU  60  such that output from FC system  20  is set to the output calculated in step S 80 . 
       FIG. 9  is a flowchart showing an exemplary method of calculating a target SOC when the travel mode is set to the FCEV mode in step S 60  in  FIG. 8 . Referring to  FIG. 9 , FDC-ECU  60  obtains a remaining amount of hydrogen in hydrogen tank  28  (step S 110 ). The remaining amount of hydrogen can be calculated, for example, based on a detection value from a pressure sensor that detects a pressure in hydrogen tank  28 . 
     Then, FDC-ECU  60  calculates the target SOC in accordance with the remaining amount of hydrogen, based on the remaining amount of hydrogen obtained in step S 110  (step S 120 ). Specifically, FDC-ECU  60  calculates the target SOC such that the target SOC lowers with decrease in remaining amount of hydrogen. Then, FDC-ECU  60  lowers the target SOC with decrease in remaining amount of hydrogen such that the target SOC attains to prescribed SOC lower limit value SL at the time when hydrogen in hydrogen tank  28  runs out. 
       FIG. 10  is a diagram showing transition of a remaining amount of hydrogen, a target SOC, and a system output upper limit when the travel mode is set to the FCEV mode. The system output upper limit refers to the sum of the output upper limit of FC system  20  and the output upper limit of battery  40 , which determines traveling performance (an output characteristic) of FCV  1 . 
     Referring to  FIG. 10 , at time t 11 , travel is assumed to start from a state that hydrogen tank  28  is filled up with hydrogen. A target SOC at this time is denoted as S 1  (for example, 70%) and a difference between S 1  and SOC lower limit value SL (for example, 20%) is denoted as ΔSOC (50% in this example). 
     As the remaining amount of hydrogen decreases with travel, the target SOC is lowered with decrease in remaining amount of hydrogen as described above. Specifically, the target SOC is calculated in an expression below such that the target SOC lowers to SOC lower limit value SL at the time when hydrogen runs out (the remaining amount of hydrogen attains to 0) at time t 13 . 
       Target SOC=remaining amount of hydrogen/amount of fully filled hydrogen×ΔSOC+SOC lower limit value  SL   (1)
 
     For the system output upper limit, a value Ws which is the sum of the output upper limit of FC system  20  and the output upper limit of battery  40  is secured until time t 12  and maximum traveling performance is secured. Relation between the system output upper limit and the SOC of battery  40  will now be described. 
       FIG. 11  is a diagram showing relation of a remaining amount of energy in FC system  20  and battery  40  with a system output upper limit. In  FIG. 11 , the abscissa represents a remaining amount of energy (%) in each of FC system  20  and battery  40 , and the ordinate represents the system output upper limit (W) which is the sum of the output upper limit of FC system  20  and the output upper limit of battery  40 . The remaining amount of energy on the abscissa represents the remaining amount of hydrogen (100% representing a fully filled state) for FC system  20  and represents the SOC for battery  40 . 
     Referring to  FIG. 11 , output upper limit Wfc of FC system  20  is constant regardless of the remaining amount of hydrogen. Namely, FC system  20  can output electric power up to output upper limit Wfc regardless of the remaining amount of hydrogen until fuel runs out. On the other hand, when the SOC becomes lower than a prescribed value S 2 , output upper limit Wout of battery  40  decreases with lowering in SOC, and attains to 0 when lower limit value SL is reached. Thus, when the SOC of battery  40  becomes lower than prescribed value S 2 , the system output upper limit which is the sum of output upper limit Wfc of FC system  20  and output upper limit Wout of battery  40  decreases with lowering in SOC. 
     Referring again to  FIG. 10 , when the target SOC becomes lower than S 2  at time t 12  or later, output upper limit Wout of battery  40  decreases with lowering in target SOC (≈actual SOC) and hence the system output upper limit also decreases from value Ws. At time t 13 , hydrogen runs out (the remaining amount of hydrogen attains to 0) and the SOC of battery  40  also attains to lower limit value SL. 
     In the present first embodiment, at time t 12  or later, the system output upper limit gradually decreases, however, FCV  1  can travel with both of output from FC system  20  and output from battery  40  until energy (hydrogen and the battery) in FCV  1  runs out. 
     In contrast, when the travel mode has been set to the FC mode, FCV  1  travels basically only with output from FC system  20  until hydrogen runs out. Therefore, when hydrogen runs out (faster than in setting of the travel mode to the FCEV mode), the system output upper limit becomes equal to output upper limit Wout of battery  40 . Therefore, after hydrogen runs out, traveling performance is greatly restricted. 
     As set forth above, in the first embodiment, when the travel mode has been set to the FCEV mode, the target SOC of battery  40  is lowered with decrease in remaining amount of hydrogen. Since output from battery  40  is thus also basically used for generation of travel power, preferential use of the FC than the battery as in the FC mode can be suppressed. Therefore, according to the first embodiment, sudden significant restriction of traveling performance due to early run-out of fuel can be avoided. 
     Second Embodiment 
     In this second embodiment, the target SOC of battery  40  is not lowered until the remaining amount of hydrogen decreases to a prescribed value, and when the remaining amount of hydrogen becomes smaller than the prescribed value, the target SOC is lowered with decrease in remaining amount of hydrogen. A period for which a maximum system output upper limit is secured, that is, a period for which maximum traveling performance is secured, can thus be longer than in the first embodiment. 
     The FCV according to this second embodiment is identical in overall configuration and travel mode to FCV  1  according to the first embodiment. The flow as a whole of processing performed by FDC-ECU  60  in the FCV according to the second embodiment is also the same as in the flowchart shown in  FIG. 8 . 
       FIG. 12  is a flowchart showing an exemplary method of calculating a target SOC when the travel mode is set to the FCEV mode in the second embodiment. This flowchart corresponds to the flowchart shown in  FIG. 9  in the first embodiment. 
     Referring to  FIG. 12 , FDC-ECU  60  obtains a remaining amount of hydrogen in hydrogen tank  28  (step S 210 ). Then, FDC-ECU  60  determines whether or not the remaining amount of hydrogen is larger than a threshold value H 1  (step S 220 ). This threshold value H 1  defines timing to start lowering the target SOC in accordance with the remaining amount of hydrogen, and can be designed as appropriate depending on at which level the maximum system output upper limit (traveling performance) should be maintained (beyond which level the maximum system output upper limit should not be lowered). 
     When the remaining amount of hydrogen is determined in step S 220  as being larger than threshold value H 1  (YES in step S 220 ), FDC-ECU  60  sets the target SOC of battery  40  to S 1  (S 1 &gt;S 2 ) (step S 230 ). S 2  represents a threshold value that defines timing to start lowering output upper limit Wout of battery  40  with lowering in SOC as shown in  FIG. 11  and S 1  is larger than threshold value S 2 . 
     When the remaining amount of hydrogen is determined in step S 220  as being equal to or smaller than threshold value H 1  (NO in step S 220 ), FDC-ECU  60  calculates the target SOC in accordance with the remaining amount of hydrogen, based on the remaining amount of hydrogen obtained in step S 210  (step S 240 ). Specifically, FDC-ECU  60  calculates the target SOC such that the target SOC lowers with decrease in remaining amount of hydrogen. Specifically, the target SOC is calculated in an expression below such that the target SOC lowers to SOC lower limit value SL at the time when hydrogen runs out (the remaining amount of hydrogen attains to 0) at time t 24 . 
       Target SOC=remaining amount of hydrogen/threshold value  H 1×ΔSOC+SOC lower limit value  SL   (2)
 
       FIG. 13  is a diagram showing transition of a remaining amount of hydrogen, a target SOC, and a system output upper limit when the travel mode is set to the FCEV mode in the second embodiment.  FIG. 13  corresponds to  FIG. 10  referred to in the first embodiment. 
     Referring to  FIG. 13 , also in this example, travel is assumed to start at time t 21  from the state that hydrogen tank  28  is filled up with hydrogen. A target SOC at this time is denoted as S 1  (for example, 70%) and a difference between S 1  and SOC lower limit value SL (for example, 20%) is denoted as ΔSOC (50% in this example). 
     Though the remaining amount of hydrogen decreases with travel, the target SOC is not lowered until the remaining amount of hydrogen decreases to threshold value H 1  at time t 22  in the second embodiment. When the remaining amount of hydrogen becomes smaller than threshold value H 1  at time t 22  or later, the target SOC is lowered with decrease in remaining amount of hydrogen. Specifically, the target SOC is calculated based on the expression (2) above such that the target SOC lowers to SOC lower limit value SL at the time when hydrogen runs out (the remaining amount of hydrogen attains to 0) at time t 24 . 
     When the target SOC becomes lower than S 2  at time t 23 , output upper limit Wout of battery  40  is lowered ( FIG. 11 ) with lowering in target SOC (≈actual SOC) and hence the system output upper limit also lowers from value Ws. At time t 24 , hydrogen runs out (the remaining amount of hydrogen attains to 0) and the SOC of battery  40  also attains to lower limit value SL. 
     As set forth above, when the SOC lowers with lowering in target SOC and the SOC becomes lower than prescribed value S 2 , output from battery  40  is restricted and traveling performance is restricted. In this second embodiment, however, the target SOC is lowered when a remaining amount of fuel becomes smaller than threshold value H 1 . In other words, the target SOC is not lowered until the remaining amount of fuel becomes smaller than threshold value H 1 . Therefore, according to this second embodiment, sufficient traveling performance can be secured until the remaining amount of fuel decreases to threshold value H 1 . 
     Though embodiments of the present disclosure have been described above, it should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present disclosure is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.