Abstract:
A fuel cell system is provided which includes an off-gas recirculating mechanism working to mix an off-gas discharged from a fuel cell with a hydrogen gas supplied to the fuel cell. The off-gas recirculating mechanism is implemented by an ejector vacuum pump which is controllable of an area of an outlet thereof to bring an output pressure into agreement with a target one. Use of such a type of ejector vacuum pump ensures desired accuracy of recirculation of the off-gas and regulation of the pressure of fuel supplied to the fuel cell.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates generally to a fuel cell system suitable for vehicles, boats, ships, or portable generators which is equipped with a fuel cell working to convert the energy produced by electrochemical reaction of oxygen and hydrogen into electric power and designed to control a supply pressure of fuel with high accuracy. 
   2. Background Art 
   There are known fuel cell systems designed to suck an off-gas discharged from a fuel electrode of a fuel cell using a pump and mix it with a fuel supplied to the fuel cell. The pump is usually implemented by an ejector vacuum pump equipped with an ejector nozzle since it is capable of employing fluid energy of the supplied fuel for power saving. 
   Usually, the fuel cell systems are required to keep a supplied pressure of fuel at a given value in order to decrease a pressure difference between an oxygen electrode and a fuel electrode, stabilize output of the fuel cell, and purge moisture away from the fuel electrode. The ejector vacuum pump is, however, subjected to variation in pressure of fuel at an outlet thereof (i.e., a supplied pressure of the fuel) and flow rate of recirculated off-gas due to variations in pressure and flow rate of the fuel supplied to the fuel cell. The ejector vacuum pump also suffers from a drawback in that a controllable range of the flow rate of the off-gas is narrow. 
   Japanese Patent First Publication No. 2001-266922 discloses a fuel cell system which has pressure controlling lines and a plurality of pressure control valves capable of being controlled as a function of pressure in an oxidizing agent supplier. 
   The above system, however, has drawbacks in that the structure made up of the pressure control valves and the bypass lines is complex, control of the pressure of fuel supplied to the fuel cell depends upon the supplied pressure of the oxidizing agent, thereby making it difficult to meet a high-accuracy fuel supply control requirement, and a variation in supplied pressure of the oxidizing agent may result in a variation in supplied pressure of the fuel (i.e., hunting), which leads to instability of operation of the fuel cell. 
   Further, the moisture contained in the off-gas may freeze near the ejector nozzle in low-temperature environments, thereby resulting in changes in area of an outlet of the nozzle and state of a wall surface of the nozzle, which may cause a disturbance of control of flow rate of the off-gas. 
   SUMMARY OF THE INVENTION 
   It is therefore a principal object of the invention to avoid the disadvantages of the prior art. 
   It is another object of the invention to provide a fuel cell system which works to recirculate an off-gas and regulate pressure of fuel supplied to a fuel cell with high accuracy. 
   According to one aspect of the invention, there is provided a fuel cell system which may be employed in automotive vehicles. The fuel cell system comprises: (a) a fuel cell working to produce an electrical energy arising from chemical reaction of hydrogen with oxygen; (b) a hydrogen supply line supplying a hydrogen gas from a hydrogen supply device to the fuel cell; (c) an off-gas recirculating line extending from the fuel cell to the hydrogen supply line; (d) an off-gas recirculating mechanism designed to recirculate an off-gas, which is discharged from the fuel cell and includes hydrogen having unreacted with the oxygen in the chemical reaction, to the fuel cell through the off-gas recirculating line, the off-gas recirculating mechanism being designed to be controllable of an amount of the off-gas recirculated, working to mix the off-gas flowing through the off-gas recirculating line with the hydrogen gas flowing through the hydrogen supply line to output a mixture gas to the fuel cell; (e) an output demand determining circuit working to determine a demand for output of the electrical energy from the fuel cell; and (f) a controller working to control the amount of the off-gas recirculated through the off-gas recirculating mechanism as a function of the demand for output of the electrical energy determined by the output demand determining circuit, thereby controlling an output pressure of the off-gas recirculating mechanism. 
   Use of the off-gas recirculating mechanism designed to be controllable of the amount of the off-gas recirculated results in a simplified structure of the system and increased accuracy of controlling the output pressure of the off-gas recirculating mechanism, that is, the pressure of the hydrogen gas inputted to the fuel cell. 
   In the preferred mode of the invention, the system further comprises a pressure sensor working to measure the output pressure of the off-gas recirculating mechanism. The control circuit controls the amount of the off-gas recirculated as a function of the output pressure measured by the pressure sensor. 
   The controller may monitor the output pressure of the off-gas recirculating mechanism to control the amount of the off-gas recirculated through the off-gas recirculating mechanism so as to bring the output pressure of the off-gas recirculating mechanism into agreement with a target one under feedback control. 
   The controller may alternatively monitor the output pressure of the off-gas recirculating mechanism to control the amount of the off-gas recirculated through the off-gas recirculating mechanism so as to have the output pressure of the off-gas recirculating mechanism fall within a target range under feedback control. When the output pressure of the off-gas recirculating mechanism lies within the target range, and an actual amount of the electrical energy produced by the fuel cell is smaller than the demand for output of the electrical energy from the fuel cell, the controller drains the off-gas from the off-gas recirculating line. 
   The off-gas recirculating mechanism may be implemented by an ejector vacuum pump which includes a nozzle having an outlet from which the hydrogen gas is discharged and is so designed as to be controllable of an area of the outlet of the nozzle. 
   The ejector vacuum pump has a tapered needle disposed within the nozzle coaxially therewith to be movable selectively in a first direction in which the tapered needle approaches the outlet of the nozzle and in a second direction in which the tapered needle moves away from the outlet of the nozzle, thereby changing the area of the outlet of the nozzle. 
   The system further includes an actuator which is electrically operable to move the tapered needle in a selected one of the first and second directions. 
   The system may also include a heater working to heat the off-gas recirculating mechanism. The heater is so installed as to extend from the outlet of the nozzle ranging downwardly of a flow of the hydrogen gas. The heater may be implemented by a PTC heater. 
   The system may also include a hydrogen supply pressure regulating mechanism working to regulate a pressure of the hydrogen gas outputted from the hydrogen supply device. 
   According to the second aspect of the invention, there is provided a fuel cell system comprising: (a) a fuel cell working to produce an electrical energy arising from chemical reaction of hydrogen with oxygen; (b) a hydrogen supply line supplying a hydrogen gas from a hydrogen supply device to the fuel cell; (c) an off-gas recirculating line extending from the fuel cell to the hydrogen supply line; and (d) an off-gas recirculating mechanism designed to recirculate an off-gas, which is discharged from the fuel cell and includes hydrogen having unreacted with the oxygen in the chemical reaction, to the fuel cell through the off-gas recirculating line. The off-gas recirculating mechanism works to mix the off-gas flowing through the off-gas recirculating line with the hydrogen gas flowing through the hydrogen supply line to output a mixture gas to the fuel cell. The off-gas recirculating mechanism is responsive to a pressure of the mixture gas outputted from the off-gas recirculating mechanism to bring the pressure of the mixture gas into agreement with a target one. 
   In the preferred mode of the invention, the off-gas recirculating mechanism is implemented by an ejector vacuum pump which includes a nozzle having an outlet from which the hydrogen gas is discharged and is so designed as to be variable of an area of the outlet of the nozzle in response to the pressure of the mixture. 
   The ejector vacuum pump may have a tapered needle disposed within the nozzle coaxially therewith to be movable selectively in a first direction in which the tapered needle approaches the outlet of the nozzle and in a second direction in which the tapered needle moves away from the outlet of the nozzle, thereby changing the area of the outlet of the nozzle. 
   The ejector vacuum pump has an elastic actuator which is elastically responsive to the pressure of the mixture to move the tapered needle in a selected one of the first and second directions. 
   The elastic actuator may be implemented by a spring such as a bellows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
       FIG. 1  is a circuit diagram which shows a fuel cell system according to the first embodiment of the invention; 
       FIG. 2  is a longitudinal sectional view which shows an ejector vacuum pump used in the fuel cell system of  FIG. 1 ; 
       FIG. 3  is a flowchart of a program performed by controllers of the fuel cell system of  FIG. 1 ; 
       FIG. 4  is a longitudinal sectional view which shows an ejector vacuum pump according to the second embodiment of the invention; and 
       FIG. 5  is a longitudinal sectional view which shows an ejector vacuum pump according to the third embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to  FIG. 1 , there is shown a fuel cell system according to the first embodiment of the invention which consists essentially of a fuel cell stack  10 , an air supply device  21 , a fuel supply device  31 , an ejector vacuum pump  38 , and controllers  40  and  41 . 
   The fuel cell stack  10  works to convert the energy produced by electrochemical reaction of hydrogen, i.e., fuel and oxygen, i.e., emulsifying agent into electric power. The fuel cell stack  10  is made up of a plurality of solid polyelectrolyte fuel cells. Each cell is made of a pair of electrodes (will also called an oxygen and a hydrogen electrode below) and an electrolyte film disposed between the electrodes. The fuel cell stack  10  is used to supply the power to an electrical device such as a drive motor or a storage battery. The fuel cell stack  10  is supplied with hydrogen and air (oxygen) and induces electrochemical reactions thereof at the electrodes which are of the forms:
 
Hydrogen electrode H 2 →2H + +2e − 
 
Oxygen electrode 2H + +1/2O 2 +2e − →H 2 O
 
   The above electrochemical reactions produce water. Additionally, humidified hydrogen and air gasses are supplied into the fuel cell stack  10 , which will cause condensate water to be produced therein. The moisture is, thus, produced within the fuel cell stack  10 . The fuel cell stack  10  has disposed thereon a voltage sensor  11  which works to measure an output voltage of the fuel cell stack  10  and outputs a signal indicative thereof to the controller  40 . 
   The fuel cell system also has an air supply line  20  for supplying oxygen-contained air to the oxygen electrodes (i.e., positive electrodes) of the fuel cell stack  10  and a hydrogen supply line  30  for supplying hydrogen gas to the hydrogen electrodes (i.e., negative electrodes) of the fuel cell stack  10 . The fuel cell system further includes an air supply pressure sensor  22  and a hydrogen supply pressure sensor  33 . The air supply pressure sensor  22  is installed in the air supply line  20  near an air inlet of the fuel cell stack  10  and works to measure the pressure of air supplied to the fuel cell stack  10 . The hydrogen supply pressure sensor  33  is installed in the hydrogen supply line  30  near a hydrogen gas inlet of the fuel cell stack  10  and works to measure the pressure of hydrogen gas supplied to the fuel cell stack  10 . The pressure of hydrogen gas supplied to the fuel cell stack  10  is substantially equal to an output pressure of the ejector vacuum pump  38 . 
   An off-gas recirculating line  34  is disposed between a hydrogen outlet of the fuel cell stack  10  and a portion of the hydrogen supply line  30  located downstream of a regulator  32 . The off-gas recirculating line  34  works to combine an off-gas containing unreacted hydrogen gas discharged from the fuel cell stack  10  with a main flow of the hydrogen gas to the fuel cell stack  10 . The off-gas recirculating line  34  has disposed therein an gas-liquid separator  35 , a drain valve  36 , and a check valve  37 . The gas-liquid separator  35  works to separate moisture from the off-gas. The drain valve  36  works to discharge the off-gas outside the fuel cell system. The check valve  37  works to avoid a backflow of the off-gas when discharged outside the fuel cell system. The moisture separated by the gas-liquid separator  35  is drained by opening a drain valve installed beneath the gas-liquid separator  35 , as viewed in the drawing. 
   The ejector vacuum pump  38  is installed at a junction of the off-gas recirculating line  34  and the hydrogen supply line  30 . The ejector vacuum pump  38 , as will be described later in detail, works to suck therein the off-gas using fluid energy developed by a flow of the hydrogen gas outputted from the fuel supply device  31  and recirculate it to the fuel cell stack  10 . 
   The fuel cell system, as described above, has the two controllers  40  and  41  implemented by electronic control units. The first controller  40  receives an output signal of an accelerator position sensor  43  indicating a position of an accelerator pedal  42  of an automotive vehicle, for example, and calculates a required amount of electricity to be generated by the fuel cell stack  10  based on the position of the accelerator pedal  42 . The first controller  40  also works to calculate amounts of hydrogen gas and off-gas and a supply pressure of the hydrogen gas (i.e., the output pressure of the ejector vacuum pump  38 ) needed for the fuel cell stack  10  to generate the required amount of electricity and outputs a command signal to the second controller  41 . 
   In this embodiment a ratio of the amount of hydrogen gas supplied from the fuel supply device  31  to the amount of off-gas to be recirculated to the fuel cell stack  10  is a fixed value (e.g., 1:0.2). A determination of the recirculated amount of off-gas is, thus, made by determining the supplied amount of hydrogen gas. The first controller  40  has disposed therein a map listing a relation among the amount of hydrogen gas required to be supplied to the fuel cell stack  10 , the amount of off-gas required to be recirculated to the fuel cell stack  10 , and the pressure required to supply the hydrogen gas to the fuel cell stack  10 . 
   The first controller  40  also calculates the amount of air required for the fuel cell stack  10  to generate the required amount of electricity and control the speed of a compressor  21 . Specifically, the first controller  40  monitors an output of the air supply pressure sensor  22  to modify the speed of the compressor  21  under feedback control. The first controller  40  also controls the generation of electricity in the fuel cell stack  10  based on an output of the voltage sensor  11 . 
   The second controller  41  receives a control signal from the first controller  40  and an output of the hydrogen supply pressure sensor  33 . The second controller  41  calculates a target valve open position of a regulator  32  based on the required amount of hydrogen gas supplied and a nozzle open position of the ejector vacuum pump  34  based on the required amount of off-gas recirculated and outputs control signals to the regulator  32  and the ejector vacuum pump  38 . The second controller  41  also outputs control signals to the gas-liquid separator  35  and the drain valve  36 . 
   The ejector vacuum pump  38 , as clearly shown in  FIG. 2 , includes a hydrogen inlet port  381 , an off-gas inlet port  382 , a nozzle  383 , an output  384 , a movable needle  385 , a needle guide  386 , a worm gear  387 , and an electric motor  388 . 
   The hydrogen inlet port  381  leads to the hydrogen supply line  30 . The off-gas inlet port  382  connects with the off-gas recirculating line  34 . The nozzle  383  is made of a hollow cylinder which has an inner fluid path. The nozzle  383  is installed within a hollow cylindrical pump housing to define an outer fluid path between an outer all of the nozzle  383  and an inner wall of the pump housing. The inner fluid path communicates with the hydrogen inlet port  381 . The outer fluid path communicates with the off-gas inlet port  382 . The inner fluid path has a tapered outlet which works to output a high-speed flow of the hydrogen gas. 
   The high-speed flow of the hydrogen gas travels to the outlet  384  while drawing the off-gas thereinto. Specifically, the high-speed flow results in generation of a negative pressure or vacuum around the periphery of the nozzle  383 , thereby sucking the off-gas flowing through the off-gas recirculating line  34  into the flow of hydrogen gas, so that a mixture of the off-gas and the hydrogen gas is discharged from the outlet  384  and supplied to the fuel cell stack  10  through the hydrogen supply line  30 . 
   The ejector vacuum pump  38 , as described above, has the movable needle  385  disposed within the nozzle  383  coaxially therewith. The movable needle  385  is mechanically coupled with an output shaft of the motor  388  through the worm gear  387 , so that it may move in a lengthwise direction thereof within the nozzle  383  to change a sectional area of the outlet of the nozzle  383 . Specifically, the needle  385  has a tapered head which is moved by the motor  388  to adjust the sectional area of the outlet of the nozzle  383  to a desired one. When the needle  385  is moved inwardly of the nozzle  383  (i.e., the left direction as viewed in the drawing), the sectional area of the outlet of the nozzle  383  increases, while when the needle  385  is moved outwardly of the nozzle  383 , the sectional area of the outlet of the nozzle  383  decreases. 
   The adjustment of output gas pressure of the ejector vacuum pump  38  and the amount of off-gas recirculated is achieved by changing the open area of the nozzle  383 . The output gas pressure of the ejector vacuum pump  38  is the pressure of the mixture of hydrogen gas and off-gas to be supplied from the ejector vacuum pump  38  to the fuel cell stack  10 . Specifically, when the open area of the nozzle  383  is decreased, it results in decreases in flow velocity of the hydrogen gas and amount of off-gas recirculated and an elevation in output gas pressure of the ejector vacuum pump  38 . Conversely, when the open area of the nozzle  383  is decreased, it results in increases in flow velocity of the hydrogen gas and amount of off-gas recirculated and reduction in output gas pressure of the ejector vacuum pump  38 . 
     FIG. 3  is a flowchart of a sequence of logical steps or program performed by the controllers  40  and  41 . 
   After entering the program, the routine proceeds to step  100  wherein the first controller  40  monitors an output of the accelerator position sensor  43 , i.e., the position of the accelerator pedal  42  and determines the amount of electricity Wo required for the fuel cell stack  10  to generate the electricity. 
   The routine proceeds to step  101  wherein the first controller  40  determines the required amount of hydrogen gas to be supplied to the fuel cell stack  10 , the required amount of off-gas to be recirculated to the fuel cell stack  10 , and the required pressure Po of the hydrogen gas as a function of the required amount of electricity Wo. This determination is made by look-up using the map, as described above, listing the relation among the required amount of hydrogen gas, the required amount of off-gas, and the required pressure of the hydrogen gas. 
   The routine proceeds to step  102  wherein the second controller  41  controls the valve open position of the regulator  32  and the nozzle open position of the ejector vacuum pump  38 . The routine proceeds to step  103  wherein an actual pressure Ps of the hydrogen gas supplied to the fuel cell stack  10  is monitored through an output of the hydrogen supply pressure sensor  33  to determine whether the actual pressure Ps is controlled to the required pressure Po or not. Specifically, it is determined whether the actual pressure Ps lies within a range of the required pressure Po±α or not (α is a given control tolerance). If a NO answer is obtained meaning that the actual pressure Ps lies out of the range of the required pressure Po±α, then the routine proceeds to step  104  wherein it is determined whether the actual pressure Ps is greater or smaller than the range of Po±α to determine the direction in which the nozzle open position of the ejector vacuum pump  38  is to be corrected. 
   Specifically, if the actual pressure Ps is greater than Po+α, then the routine proceeds to step  105  wherein the nozzle open position or open area of the nozzle  383  of the ejector vacuum pump  38  is decreased, thereby increasing the flow velocity of the hydrogen gas supplied to the fuel cell stack  10 . This results in an increase in amount of off-gas recirculated and a reduction in pressure of the hydrogen gas outputted from the ejector vacuum pump  38 . Alternatively, if the actual pressure Ps is smaller than Po−α, then the routine proceeds to step  106  wherein the nozzle open position or open area of the nozzle  383  of the ejector vacuum pump  38  is increased, thereby decreasing the flow velocity of the hydrogen gas supplied to the fuel cell stack  10 . This results in a decrease in amount of off-gas recirculated and an elevation in pressure of the hydrogen gas outputted from the ejector vacuum pump  38 . 
   If it is determined in step  104  that the actual pressure Ps falls within the range of Po±α, then the routine proceeds to step  107  wherein an actual amount Wn of electricity produced by the fuel cell stack  10  is greater than or equal to the required amount Wo of electricity or not. If a YES answer is obtained meaning that the actual amount Wn fulfils the demand of electricity, then the routine terminates. Alternatively, if a NO answer is obtained, it is concluded that the lack in the actual amount Wn of electricity produced by the fuel cell stack  10  is due to a decrease in concentration of hydrogen contained in the off-gas. This is because the nitrogen contained in the air supplied to the oxygen electrodes of the fuel cell stack  10  travels to the hydrogen electrodes within an electrolyte film, thus resulting in a rise in concentration of nitrogen in the off-gas. The routine, thus, proceeds to step  108  wherein the drain valve  36  is opened to discharge the off-gas whose concentration of hydrogen is decreased outside the off-gas recirculating line  34 , thereby recovering the lack in concentration of hydrogen within the off-gas. Afterward, the routine returns back to step  103  to control the pressure of hydrogen gas supplied to the fuel cell stack  10  again. 
   As apparent form the above discussion, the fuel cell system of this embodiment works to monitor an output of the hydrogen supply pressure sensor  33  to modify the open area of the outlet port of the ejector vacuum pump  38 , thereby controlling the pressure of the hydrogen gas supplied to the fuel cell stack  10  (i.e., the output pressure of the ejector vacuum pump  38 ) and the amount of off-gas recirculated to the fuel cell stack  10 . This eliminates the need for a plurality of pressure control valves employed in the conventional system as discussed in the introductory part of this application, thus resulting in a simplified structure and a decrease in manufacturing costs of the fuel cell system. Further, a lag in propagation of pressure within lines  30  and  34  is minimized. The pressure of the hydrogen gas supplied to the fuel cell stack  10  is insensitive to the pressure of air supplied to the fuel cell stack  10 . It, therefore, becomes possible to control the pressure of the hydrogen gas supplied to the fuel cell stack  10  precisely. The use of the regulator  32  to control the pressure of the hydrogen gas results in an increase in controllable range of the output pressure of the ejector vacuum pump  38 . 
     FIG. 4  shows the ejector vacuum pump  38  according to the second embodiment of the invention. The same reference numbers as employed in the first embodiment refer to the same parts, and explanation thereof in detail will be omitted here. 
   The ejector vacuum pump  38  has a bellows  389  disposed within a chamber of the pump housing. The bellows  389  works as a spring to expand or contract in response to the output or discharge pressure of the ejector vacuum pump  38 . The bellows  389  is secured to an end of the needle  385  to move the needle  385  in a lengthwise direction thereof. A discharge pressure transmitting line  390  extends from the outlet  384  of the ejector vacuum pump  38  inside the bellows  389  to apply the discharge pressure of the ejector vacuum pump  38  to the bellows  389 . 
   When the discharge pressure of the ejector vacuum pump  38  rises, it will cause the bellows  389  to expand. Alternatively, when the discharge pressure of the ejector vacuum pump  38  drops, it will cause the bellows  389  to contract through its own spring pressure. Specifically, when the discharge pressure of the ejector vacuum pump  38  drops below a target value, it will cause the needle  385  to move in the left direction, as viewed in the drawing, thereby increasing the open area of the nozzle  383 , which results in a decrease in amount of the off-gas recirculated and a rise in pressure of the hydrogen gas (i.e., the mixture of the off-gas and the hydrogen gas) supplied to the fuel cell stack  10 . Conversely, when the discharge pressure rises above the target value, it will cause the needle  385  to move in the right direction, as viewed in the drawing, thereby decreasing the open area of the nozzle  383 , which results in an increase in amount of the off-gas recirculated and a drop in pressure of the hydrogen gas supplied to the fuel cell stack  10 . 
   Use of this type of ejector vacuum pump  38  enables the open area of the nozzle  383  to be regulated automatically as a function of the discharge pressure of the ejector vacuum pump  38 , thus resulting in a further simplified structure of the fuel cell system. 
     FIG. 5  shows the ejector vacuum pump  38  according to the third embodiment of the invention. The same reference numbers as employed in the first embodiment refer to the same parts, and explanation thereof in detail will be omitted here. 
   The ejector vacuum pump  38  has disposed thereon a PTC (Positive Temperature Coefficient) heater  392  working to avoid condensation of moisture within the ejector vacuum pump  38  in low-temperature environments. The PTC heater  392  is installed on a portion of the pump housing within which the off-gas is mixed with the hydrogen gas. Specifically, the PTC heater  392  extends from the tip of the nozzle  383  to the downstream side of the outlet  384 . The PTC element used in the heater  392 , as is well known in the art, works as a constant-temperature heater which performs a temperature self-control function, thus eliminating the need for control of electrical energization thereof. This results in a simplified structure of the fuel cell system as compared with when a hot wire heater is employed. 
   While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims. 
   For example, the fuel cell system of the first embodiment works to control the open area of the nozzle  383  of the ejector vacuum pump  38  as a function of the discharge pressure thereof, but may use a flow rate of the off-gas as measured by an off-gas sensor instead of the discharge pressure of the ejector vacuum pump  38 . 
   The fuel cell system of the third embodiment uses the PTC heater  392 , but may alternatively employ another type of heater.