Abstract:
The present invention relates to a fuel cell system for vehicles and a method for controlling the same which stably maintains an output of a fuel cell by precisely estimating a recirculated hydrogen amount to a stack. A fuel cell system according to the present invention may include: a stack comprising a plurality of unit cells for generating electrical energy by electrochemical reaction of a fuel and an oxidizing agent; a blower for recirculating a gas exhausted from the stack so as to supply the gas back to the stack; an ejector for recirculating the gas exhausted from the stack, receiving hydrogen so as to mix the hydrogen to the recirculated gas, and supplying the mixture to the stack; a sensor module for detecting a driving condition of the vehicle; and a control portion for controlling operations of the blower and the ejector by using the driving condition of the vehicle and performance maps of the blower and the ejector.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application from U.S. patent application Ser. No. 12/960,812 filed on Dec. 6, 2010, issued as U.S. Pat. No. 8,709,669 and claims priority to and the benefit of Korean Patent Application No. 10-2010-0074234 filed in the Korean Intellectual Property Office on Jul. 30, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention generally relates to a fuel cell system for vehicles and a method for controlling the same. More particularly, the present invention relates to a fuel cell system for vehicles and a method for controlling the same which stably maintains an output of a fuel cell by precisely estimating a recirculated hydrogen amount to a fuel cell stack. 
     (b) Description of the Related Art 
     A fuel cell system is a well known type of electric generator system which converts chemical energy of a fuel directly into electrical energy. 
     The fuel cell system includes a fuel cell stack for generating electrical energy, a fuel supply for supplying a fuel (that is, hydrogen) to the fuel cell stack, an air supply for supplying oxygen in air (which is an oxidizing agent for the electrochemical reaction) to the fuel cell stack, and a device for managing heat and water for radiating reaction heat of the fuel cell stack to an exterior of the system and controlling operating temperature of the fuel cell stack. 
     Therefore, the fuel cell system generates electricity by an electrochemical reaction of the hydrogen, which is the fuel, and the oxygen in the air, and exhausts heat and water which are by-product of the reaction. 
     The fuel cell stack as applied to a fuel cell vehicle includes a plurality of unit batteries arranged sequentially. Each unit battery includes a membrane-electrode assembly (MEA) disposed at the innermost part thereof, and the membrane-electrode assembly includes an electrolyte membrane for transferring hydrogen ions, and catalytic layer. In particular, a cathode and an anode are spread at both sides of the electrolyte membrane so as to react the hydrogen with the oxygen. In addition, a gas diffusion layer (GDL) is positioned at an exterior portion of the membrane-electrode assembly (MEA). In particular, a GDL Is positioned at the exterior portion in which the cathode and the anode are positioned. Further, a separator is positioned at an exterior of the gas diffusion layer. The separator is formed of a flow field for supplying the fuel and the air to the cathode and the anode and for exhausting water generated by the reaction. 
     Thus, in a fuel cell, the hydrogen and the oxygen are ionized by a chemical reaction at each catalytic layer such that the hydrogen undergoes an oxidation reaction so as to generate hydrogen ions and electrons, and oxygen ions undergo a reduction reaction with the hydrogen ions so as to generate water. In particular, since the hydrogen is supplied to the anode (which is also referred to as an “oxidation electrode”) and the oxygen (or air) is supplied to the cathode (which is also referred to as a “reduction electrode”), the hydrogen supplied to the anode is ionized into hydrogen ions (H+) and electrons (e−) by the catalyst of the electrode layer formed at both sides of the electrolyte membrane. After that, only the hydrogen ions selectively pass through the electrolyte membrane, which is a cation-exchange membrane, and is transferred to the cathode. Simultaneously, the electrons (e−) are transferred to the cathode through the gas diffusion layer and the separator, which are conductors. 
     Therefore, the hydrogen ions supplied to the cathode through the electrolyte membrane and the electrons supplied to the cathode by the separator react with the oxygen in the air supplied to the cathode by an air supply so as to generate water. 
     At this time, movement of the hydrogen ions causes electrons to flow through an exterior conducting wire thereby generating current. When the water is generated by the reaction, heat is also generated. 
     In order to apply such a fuel cell system to the vehicle, it is important to maintain an output of the stack stably, and for this purpose the amount of recirculated hydrogen provided to the stack should be precisely detected or estimated. 
     However, since a gas recirculated in a fuel cell vehicle has a high water content, and since the gas passes through a short recirculation passage, it is difficult to directly detect the recirculated hydrogen amount by means of a flowmeter. 
     Therefore, the recirculated hydrogen amount has been typically estimated by using a thermal equilibrium equation. According to the thermal equilibrium equation, heat acquired from the supplied hydrogen is the same as heat lost by the recirculated gas. According to this thermal equilibrium equation, if the detected values (e.g., temperature, pressure, hydrogen concentration, and so on) are precise, a reliable recirculated hydrogen amount may be calculated. 
     However, the prediction of recirculated hydrogen amount using the thermal equilibrium equation depends greatly on temperatures. In particular, the prediction of the recirculated hydrogen amount using the thermal equilibrium equation is precise when equilibrium temperature is reached. However, the time required to reach equilibrium temperature is very long in an actual system. Therefore, a method for predicting the recirculated hydrogen amount by using the thermal equilibrium equation cannot be applied to an actual system. 
     In addition, since various heat generations/losses, such as heat generated by a blower, expansion heat of the hydrogen, condensation heat of mixed gas, and heat loss in the line, exist in an actual system, taking these effects into consideration makes the calculation very complex. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fuel cell system for vehicles and a method for controlling the same having advantages of maintaining a constant output of a stack by precisely predicting a recirculated hydrogen amount and maintaining the recirculated hydrogen amount to be larger than a predetermined hydrogen amount. 
     A fuel cell system for vehicles according to one embodiment of the present invention may include: a stack made up of a plurality of unit cells for generating electrical energy by electrochemical reaction of a fuel and an oxidizing agent; a blower for recirculating a gas exhausted from the stack so as to supply the gas back to the stack; an ejector for recirculating the gas exhausted from the stack, receiving hydrogen so as to mix the hydrogen with the recirculated gas, and supplying the mixture to the stack; a sensor module for detecting a driving condition of the vehicle; and a control portion for controlling operations of the blower and the ejector by using the driving condition of the vehicle and performance maps of the blower and the ejector. 
     According to various embodiments, a recirculated hydrogen amount according to a pressure difference between an inlet and an outlet of the stack and a blower RPM may be stored in the performance map of the blower. Further, a recirculated hydrogen amount according to the pressure difference between the inlet and the outlet of the stack and a hydrogen amount supplied to the ejector may be stored in the performance map of the ejector. 
     In various embodiments, the fuel cell system may further include a purge valve for discharging impurities in the stack. The control portion may be further configured to calculate hydrogen concentration according to the driving condition of the vehicle, and to control the opening and closing of the purge valve according to the calculated hydrogen concentration. 
     In various embodiments, the fuel cell system may further include a condensed water discharge valve for discharging condensed water in the stack. The control portion may further be configured to calculate a vapor amount according to the driving condition of the vehicle, and to control the opening and closing of the condensed water discharge valve according to the calculated vapor amount. 
     In some embodiments, the sensor module may include: a first temperature sensor for detecting an inlet temperature of the stack; a second temperature sensor for detecting an outlet temperature of the stack; a first pressure sensor for detecting an inlet pressure of the stack; a second pressure sensor for detecting an outlet pressure of the stack; an RPM sensor for detecting the blower RPM; and an ammeter for detecting a current output from the stack. 
     According to another embodiment of the present invention, a method is provided for controlling a fuel cell system for vehicles. In particular, the method may include: detecting a driving condition of the vehicle; calculating a recirculated hydrogen amount by using the driving condition of the vehicle and performance maps of the blower and the ejector; calculating a stoichiometric ratio (SR) of the hydrogen in the stack; comparing the SR of the hydrogen in the stack with a predetermined value; and increasing the recirculated hydrogen amount if the SR of the hydrogen in the stack is smaller than the predetermined value. 
     In some embodiments, the SR of the hydrogen in the stack may be calculated by dividing a sum of the hydrogen amount directly supplied to the stack and the recirculated hydrogen amount supplied to the stack by the hydrogen amount directly supplied to the stack. 
     The recirculated hydrogen amount may be increased if desired by increasing the hydrogen amount supplied to the ejector and/or by increasing the blower RPM. 
     In some embodiments, the method for controlling a fuel cell system for vehicles may further include: calculating a hydrogen concentration according to the driving condition of the vehicle; comparing the calculated hydrogen concentration with a predetermined hydrogen concentration; and opening the purge valve if the calculated hydrogen concentration is lower than the predetermined hydrogen concentration. 
     In some embodiments, the hydrogen concentration according to the driving condition of the vehicle may be predetermined as a function of a purge cycle and a current output. 
     In some embodiments, the method for controlling a fuel cell system for vehicles may further include: calculating a vapor amount according to the driving condition of the vehicle; comparing the calculated vapor amount with a predetermined vapor amount; and opening the condensed water discharge valve if the calculated vapor amount is larger than the predetermined vapor amount. 
     In some embodiments, the vapor amount according to the driving condition of the vehicle may be predetermined as a function of an inlet temperature of the stack, an outlet temperature of the stack, an inlet pressure of the stack, and an outlet pressure of the stack. 
     It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a fuel cell system for vehicles according to an exemplary embodiment of the present invention. 
         FIG. 2  is a block diagram of a fuel cell system for vehicles according to an exemplary embodiment of the present invention. 
         FIG. 3  is a flowchart of a method for controlling a fuel cell system for vehicles according to an exemplary embodiment of the present invention. 
         FIG. 4  shows one example of a blower performance map used in a method for controlling a fuel cell system for vehicles according to an exemplary embodiment of the present invention. 
         FIG. 5  shows one example of an ejector performance map used in a method for controlling a fuel cell system for vehicles according to an exemplary embodiment of the present invention. 
         FIG. 6  is a graph showing a recirculated hydrogen amount, an anode SR, and a current when a method for controlling a fuel cell system for vehicles according to an exemplary embodiment of the present invention is used. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An exemplary embodiment of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
     A fuel cell system according to an exemplary embodiment of the present invention is provided in a fuel cell vehicle and is operated as an electric generator system which generates electrical energy by an electrochemical reaction of a fuel and an oxidizing agent. 
     If the fuel cell system is a direct oxidation fuel cell, the fuel can include, for example, alcoholic liquid fuel such as methanol and ethanol and hydrocarbon family liquefied gas fuel, the chief ingredients of which are methane, ethane, propane, and butane. 
     On the other hand, if the fuel cell system is a polymer electrolyte membrane fuel cell, the fuel can include, for example, reforming gas generated from the liquid fuel or liquefied gas fuel by a reformer. 
     For convenience of explanation, the fuel will be referred to as hydrogen in this specification. Or course, it is understood that the present invention is not limited only to hydrogen fuel, and other known fuels are encompassed by the present invention. 
     In addition, according to the exemplary embodiment of the invention, the oxidizing agent may be oxygen gas stored in a special tank or may be natural air. However, as described herein, the oxidizing agent will be referred to as air. Or course, it is understood that the present invention is not limited only to air, and other known oxygen agents are encompassed by the present invention 
       FIG. 1  is a schematic diagram of a fuel cell system for vehicles according to an exemplary embodiment of the present invention. As shown in  FIG. 1 , the fuel cell system for vehicles according to this embodiment includes a compressor  10 , a hydrogen storage  20 , a stack  30 , an outlet manifold  40 , a blower  50 , an ejector  60 , and a mixing chamber  70 . 
     As shown, the compressor  10  is in connection with the stack  30  through an air supply line  12 . The compressor  10  pressurizes air and supplies the air to the stack  30 . In addition, in some embodiments, a valve (not shown) for controlling air supply may be provided in the air supply line  12 . 
     Hydrogen is stored in the hydrogen storage  20 , and the stored hydrogen can be supplied to the ejector  60  and/or the mixing chamber  70 . For example, the hydrogen storage  20  can be in connection with the mixing chamber  70  through a first hydrogen supply line  22 , and can also be in connection with the ejector  60  through a second hydrogen supply line  24 . Valves (not shown) for controlling hydrogen supply may be further provided in the first and second hydrogen supply lines  22  and  24 . In some embodiments, if desired, the hydrogen may be supplied only through one of the supply lines (e.g. only the second hydrogen supply line  24 ) by removing the other supply line (e.g. by removing the first hydrogen supply line  22 ), or for example by closing a valve positioned in one of the supply lines. 
     The stack  30  is made up of a plurality of unit cells  11  sequentially arranged. Each unit cell generates electrical energy through electrochemical reaction of fuel and air. Each unit cell may be a polymer electrolyte membrane fuel cell or a direct oxidation fuel cell according to the fuel used therein. 
     The unit cell includes a membrane-electrode assembly (MEA)(not shown, and which can be in accordance with any known MEA) and a plurality of separators in contact with and disposed on both sides of the membrane-electrode assembly. In some embodiments, the separator is plate shaped and has conductivity, and is provided with a channel for flowing the fuel and the air to a surface in close contact with the membrane-electrode assembly. 
     Further, an anode electrode (hereinafter, referred to as an “anode”) is provided at one surface of the membrane-electrode assembly, and a cathode electrode (hereinafter, referred to as a “cathode”) is provided at the other surface of the membrane-electrode assembly. In addition, an electrolyte membrane is provided between the anode and the cathode. 
     The anode ionizes the fuel supplied through the channel of the separator into electrons and hydrogen ions by an oxidation reaction, and the electrolyte membrane transports the hydrogen ions to the cathode. The cathode generates water and heat though a reduction reaction between the electrons and the hydrogen ions supplied from the anode, and oxygen in the air supplied through the channel of the separator. 
     An air exhaust line  14  can further be in connection with the cathode of the stack  30  such that the air going through the chemical reaction is exhausted to the exterior of the vehicle through the air exhaust line  14 . 
     In addition, nitrogen, which is in the air supplied to the cathode of the stack  30 , penetrates the electrolyte membrane and passes to the anode, to thereby increase nitrogen concentration in the anode. If the nitrogen concentration in the anode increases to certain levels, diffusion of the hydrogen is hindered. Therefore, it is preferred that nitrogen in the anode be removed. 
     Further, a portion of the water generated in the cathode by the chemical reaction penetrates the electrolyte membrane and passes to the anode. If the water flowing to the anode remains in a catalytic layer, then the catalyst&#39;s reaction capacity is reduced. In addition, if the water flowing to the anode remains in the channel, the path for supplying the hydrogen becomes blocked. Therefore, water in the catalytic layer and the channel of the anode should be removed. In some embodiments, the anode of the stack  30  can be connected to a purge line  36 , and a purge valve  80  or the like can be mounted at the purge line  36 . The purge valve  80  can be configured to open at certain times, such as every predetermined purge cycle, or as necessary so as to exhaust impurities (i.e., nitrogen and water) in the anode. 
     In some embodiments, the anode of the stack  30  can be connected to a condensed water exhaust line  38 , and a condensed water discharge valve  82  or the like can be mounted at the condensed water exhaust line  38 . If the condensed water discharge valve  82  is opened, the water in the catalytic layer and/or the channel of the anode is exhausted. In some embodiments, instead of using an additional condensed water discharge valve  82 , the purge valve  80  can also be used as the condensed water discharge valve  82 . 
     As further shown in  FIG. 1 , the stack  30  is in connection with a recirculation inlet line  32  so as to receive a recirculated gas, and is in connection with a recirculation outlet line  34  so as to exhaust a gas generated by the reaction of the air and the hydrogen in the stack  30 . 
     As shown in  FIG. 1 , an outlet manifold  40  can be in connection with the stack  30  through a recirculation outlet line  34 . Therefore, gas generated at the stack  30  can be gathered in the outlet manifold  40  and recirculated through two paths. In particular, the outlet manifold  40  can be in connection with the blower  50  through a first recirculation line  42 , and can be in connection with the ejector  60  through a second recirculation line  44 . The outlet manifold  40  is not particularly limited and can be in accordance with any convention design, and is generally configured so as to provide a location at which the gas generated at the stack  30  can gather. 
     The blower  50 , as shown in  FIG. 1 , can be in connection with the outlet manifold  40  through the first recirculation line  42 , and can be in connection with the mixing chamber  70  through a first connecting line  46 . As such, the blower  50  can recirculate gas gathered at the outlet manifold  40  and supply the gas to the mixing chamber  70 . 
     As further shown, the ejector  60  is in connection with the outlet manifold  40  through the second recirculation line  44 , and is in connection with the mixing chamber  70  through a second connecting line  48 . In addition, the ejector  60  can be in connection with the hydrogen storage  20  through the second hydrogen supply line  24 . As such, the ejector  60  can recirculate gas gathered at the outlet manifold  40 , mix hydrogen with the recirculated gas, and supply the mixture to the mixing chamber  70 . 
     As further shown in  FIG. 1 , the mixing chamber  70  is in connection with the hydrogen storage  20  through the first hydrogen supply line  22 , is in connection with the blower  50  through the first connecting line  46 , is in connection with the ejector  60  through the second connecting line  48 , and is in connection with the stack  30  through the recirculation inlet line  32 . As such, the mixing chamber  70  can mix the hydrogen supplied from the hydrogen storage  20  with the gas recirculated through the blower  50  and the ejector  60 , and supply the mixture to the stack  30 . The mixing chamber  70  is not particularly limited, and can be in accordance with any conventional mixing chamber, and is generally configured so as to provide a place at which recirculated gas and hydrogen can be mixed. 
     As shown, according to the exemplary embodiment of the present invention shown in  FIG. 2 , the fuel cell system for vehicles further includes a sensor module and a control portion  170 . 
     The sensor module is configured to detect a driving condition of the vehicle, and can include, for example, first and second temperature sensors  110  and  120 , first and second pressure sensors  130  and  140 , an RPM sensor  150 , and an ammeter  160 . In addition, a plurality of sensors for detecting the driving condition of the vehicle may be included in the sensor module. However, in the following description, the sensors used in a method for controlling a fuel cell system for vehicles according to an exemplary embodiment of the present invention will be described in further detail. 
     According to this exemplary embodiment, the first temperature sensor  110  is mounted at the inlet of the stack  30  or the recirculation inlet line  32 , and is configured to detect an inlet temperature of the stack  30 , and to transmit a signal corresponding thereto to the control portion  170 . The second temperature sensor  120  is mounted at the outlet of the stack  30  or the recirculation outlet line  34 , and is configured to detect an outlet temperature of the stack  30 , and to transmit a signal corresponding thereto to the control portion  170 . 
     Further, according to this exemplary embodiment, the first pressure sensor  130  is mounted at the inlet of the stack  30  or the recirculation inlet line  32 , and is configured to detect an inlet pressure of the stack  30 , and to transmit a signal corresponding thereto to the control portion  170 . The second pressure sensor  140  is mounted at the outlet of the stack  30  or the recirculation outlet line  34 , and is configured to detect an outlet pressure of the stack  30 , and to transmit a signal corresponding thereto to the control portion  170 . 
     Further, the RPM sensor  150  is configured to detect a rotation speed of the blower  50  (blower RPM), and to transmit a signal corresponding thereto to the control portion  170 . 
     The ammeter  160  is configured to detect the current generated in the stack  30 , and to transmit a signal corresponding thereto to the control portion  170 . It is noted that the current generated in the stack  30  is almost proportional to an output of the stack  30 , and may be a basis for predicting the output of the stack  30  or a load of the vehicle. 
     As shown, the control portion  170  receives signals corresponding to the driving condition of the vehicle from the sensor modules and controls all of the valves (e.g., condensed water discharge valve  82 , purge valve  80 , and so on), the ejector  60 , and the blower  50 . As such, the control portion  170  is electrically in connection with to the sensor module, the valves, the blower  50 , and the ejector  60 . 
     Hereinafter, a method for controlling the fuel cell system for vehicles according to the exemplary embodiment of the present invention will be further described in detail in connection with  FIG. 3  which is a flowchart of the exemplary method. 
     As shown in  FIG. 3 , the method for controlling the fuel cell system for vehicles begins by detecting the driving condition of the vehicle at step S 200 . The driving condition of the vehicle includes, for example, the inlet and outlet temperatures of the stack  30 , the inlet and outlet pressures of the stack  30 , the rotation speed of the blower  50 , and the current output of the stack  30 . 
     The control portion  170  then calculates a vapor amount according to the driving condition of the vehicle at step S 210 . The vapor amount W[%] according to the driving condition of the vehicle can be calculated from Equation 1.
 
 W [%]= f ( T 1 ,T 2 ,P 1 ,P 2)  (Equation 1)
 
     Herein, T1 denotes the inlet temperature of the stack  30 , T2 denotes the outlet temperature of the stack  30 , P1 denotes the inlet pressure of the stack  30 , and P2 denotes the outlet pressure of the stack  30 . As such, if the inlet and outlet temperatures and the inlet and outlet pressures of the stack  30  are known, the vapor amount can be provided by the ideal gas equation or by experimental values. 
     After the vapor amount is calculated, the control portion  170  determines whether the calculated vapor amount is smaller than or equal to a predetermined vapor amount at step S 220 . The predetermined vapor amount, for example, may be 15% or any other suitable amount which can be determined by one of skill in the art. 
     If the vapor amount in the stack  30  is higher than the predetermined vapor amount, then it is difficult for the oxidation-reduction reaction in the stack  30  to occur. Therefore, if the calculated vapor amount is higher than the predetermined vapor amount, then the control portion  170  opens the condensed water discharge valve  82  so as to discharge the condensed water in the stack  30  at step S 280 , thereafter, the control portion  170  returns to step S 210 . 
     At this time, if the vapor amount in the stack  30  is smaller than or equal to the predetermined vapor amount at step S 220 , then the control portion  170  calculates the hydrogen concentration according to the driving conditions of the vehicle at step S 230 . The hydrogen concentration H[%] according to the driving condition of the vehicle can be calculated from Equation 2.
 
 H [%]= f ( T purge,Σ C )  (Equation 2)
 
     wherein, Tpurge denotes the purge cycle, and C denotes the current output of the stack  30 . 
     As described above, if the nitrogen moves from the cathode to the anode, the hydrogen concentration is reduced. In addition, if the purge valve  80  opened, the nitrogen concentration is reduced and the hydrogen concentration increases. Further, the nitrogen concentration of the anode relates to the hydrogen used in the stack  30 . As such, the hydrogen concentration in the stack  30  is related to (as shown in Equation 2) the purge cycle and the current output of the stack  30 . Thus, Equation 2 may be obtained by experimentation. 
     If the hydrogen concentration is calculated at step S 230 , the control portion  170  determines whether the hydrogen concentration in the stack  30  is higher than or equal to a predetermined hydrogen concentration at a step S 240 . The predetermined hydrogen concentration may be 80%, or any other suitable amount which can be determined by one of skill in the art. 
     If the hydrogen concentration in the stack  30  is lower than the predetermined hydrogen concentration, the nitrogen concentration in the stack  30  is high and it is difficult for the oxidation-reduction reaction to occur. In this case, the control portion  170  can opens the purge valve  80  so as to exhaust the impurities (e.g., nitrogen and water) in the stack  30 , and then return to steps S 290  and S 230 . 
     If the hydrogen concentration in the stack  30  is higher than or equal to the predetermined hydrogen concentration at step S 240 , then the control portion  170  can calculate the recirculated hydrogen amount by using the performance maps of the blower  50  and the ejector  60  at step S 250 . Step S 250  will be described in further detail in connection with  FIG. 4 , which shows the recirculated hydrogen amount according to the rotation speed of the blower  50  and a pressure difference between the inlet and the outlet of the stack  30 , which is determined by running experiments and gathering data so as to generate a blower performance map. For example,  FIG. 4  shows one example of a blower performance map. Of course, this is only one example, and the blower performance map is not limited to the one shown in  FIG. 4 . In particular, the blower performance map may be produced according to the inlet and outlet temperatures of the stack  30 , a composition ratio of the gas (i.e., composition ratios of hydrogen/nitrogen/vapor) as well as the rotation speed of the blower  50  and the pressure difference between the inlet and the outlet of the stack  30 . 
     If the blower performance map is produced, the recirculated hydrogen amount by the blower  50  is functionalized as Equation 3.
 
 Q [rec, H 2 ,HRB]=f{H [%], T 2 ,P 1 ,P 2 ,RPM}   (Equation 3)
 
     wherein, Q[rec, H2, HRB] denotes the recirculated hydrogen amount by the blower  50 , and an RPM denotes the rotation speed of the blower  50 . 
     If the driving condition of the vehicle detected at step S 200  is input into Equation 3, then the recirculated hydrogen amount by the blower  50  can be calculated. 
     Similarly, the recirculated hydrogen amount by the ejector  60  can be calculated as follows. 
     As shown in  FIG. 5 , the recirculated hydrogen amount according to the hydrogen amount supplied to the ejector  60  and the pressure difference between the inlet and the outlet of the stack  30  is detected by performing a number of experiments, gathering the experimental data, and producing an ejector performance map.  FIG. 5  shows one example of an ejector performance map. Of course, this is only one example, and the ejector performance map is not limited to the one shown in  FIG. 5 . The ejector performance map may be produced according to the inlet and outlet temperatures of the stack  30 , a composition ratio of the gas (i.e., composition ratios of hydrogen/nitrogen/vapor) as well as the hydrogen amount supplied to the ejector  60  and the pressure difference between the inlet and the outlet of the stack  30 . 
     If the ejector performance map is produced, the recirculated hydrogen amount by the ejector  60  is functionalized as Equation 4.
 
 Q [rec, H 2,Ejector]= f{Hs,H [%], T 2 ,P 1 ,P 2}  (Equation 4)
 
     wherein, Q[rec, H2, Ejector] denotes the recirculated hydrogen amount by the ejector  60 , and Hs denotes the hydrogen amount supplied to the ejector  60 . 
     If the driving condition of the vehicle detected at step S 200  is input into Equation 4, the recirculated hydrogen amount by the ejector  60  can be calculated. 
     If the recirculated hydrogen amount by the blower  50  and the ejector  60  is calculated, the control portion  170  can then calculate an SR (Stoichiometric Ratio) of the hydrogen in the stack  30  (particularly, anode) at step S 260 . Hereinafter, the SR of the hydrogen in the anode will be referred to as an anode SR. The anode SR can be calculated from Equation 5 and Equation 6.
 
 Q [rec, H 2 ]=Q [rec, H 2 ,HRB]+Q [rec, H 2,Ejector]  (Equation 5)
 
anode  SR={Q[H 2,Supply]+ Q [rec, H 2 ]}/Q[H 2,Supply]  (Equation 6)
 
     wherein, Q[rec, H2] denotes a total hydrogen amount recirculated to the stack  30 , and Q[H2, Supply] denotes the hydrogen amount supplied from the hydrogen storage  20 . 
     If the anode SR is calculated, the control portion  170  can determines whether the anode SR is bigger than or equal to a predetermined value at step S 270 . The predetermined value may be 1.5, or any other suitable value which can be determined by one of skill in the art. 
     If the anode SR is smaller than the predetermined value, then the control portion  170  controls the blower  50  and the ejector  60  to increase the recirculated hydrogen amount to the stack  30  because the recirculated hydrogen amount to the stack  30  is small. In particular, the control portion  170  can increase the rotation speed of the blower  50  at step S 310  and increase the hydrogen amount supplied to the ejector  60  at step S 300 . The control portion  170  may perform both step S 300  and step S 310 , or may perform any one of step S 300  and step S 310 . 
     If the anode SR is bigger than or equal to the predetermined value at step S 270 , then the control portion  170  repeats the method for controlling the fuel cell system for vehicles according to the exemplary embodiment of the present invention from the beginning. 
     It is noted that the present method is not limited to the exemplary embodiment of the present invention, for example, in the order as described above. 
       FIG. 6  is a graph showing a recirculated hydrogen amount, an anode SR, and a current in a case that a method for controlling a fuel cell system for vehicles according to an exemplary embodiment of the present invention is used. 
     As shown in  FIG. 6 , the recirculated hydrogen amount is reduced if a load (current) of the vehicle is reduced, and the recirculated hydrogen amount increases if the load (current) of the vehicle increases. This shows that the recirculated hydrogen amount is suitably managed according to a load change of the vehicle. 
     As described above, pressure values having quick responsiveness can be used for predicting a recirculated hydrogen amount, and, thus, prediction of the recirculated hydrogen amount has high reliability according to the present invention. 
     Further, according to the present invention, since a constant amount of hydrogen can be reliably recirculated by precisely predicting the recirculated hydrogen amount, output of a stack may be maintained to be constant. 
     Further, since a blower, an ejector, a condensed water discharge valve, and a purge valve can be suitably controlled so as to increase the recirculated hydrogen amount, performance of a fuel cell system may be improved in accordance with the present invention. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.