Patent Publication Number: US-8528528-B2

Title: Vaporized fuel processing device for internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2010-094294 filed on Apr. 15, 2010, which is incorporated herein by reference in its entirety including the specification, drawings and abstract. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a vaporized fuel processing device for an internal combustion engine, in which the fuel vapor produced in a fuel tank is introduced into an intake passage of the internal combustion engine and burned while the engine is in operation. 
     2. Description of the Related Art 
     In a vaporized fuel processing device for an internal combustion engine mounted on a vehicle or the like, the fuel vapor produced in a fuel tank is introduced into a canister and temporarily adsorbed by the adsorbing material in the canister. While the engine is in operation, a purge is performed, in which the air in the canister is sucked into the intake passage with the use of the negative pressure in the intake passage and air is introduced into the canister through an atmosphere introduction passage. By performing such a purge, the fuel adsorbed by the adsorbing material is desorbed and introduced into the intake passage along with the air, and the fuel desorbed from the adsorbing material is burned in the internal combustion engine. 
     In the above vaporized fuel processing device, by performing a purge to desorb the fuel adsorbed by the adsorbing material, the adsorbing capacity of the adsorbing material is recovered to suppress the occurrence of saturation of the adsorbing material. Thus, when the purge passage that allows the canister and the intake passage of the internal combustion engine to communicate with each other is clogged and closed, for example, and it therefore becomes impossible to normally perform the purge, the fuel cannot be desorbed from the adsorbing material and, as a result, the adsorbing material is easily saturated. 
     When the adsorbing material is saturated, the adsorbing material can no longer adsorb the fuel, so that the fuel vapor introduced into the canister passes through the canister and is discharged into the atmosphere through the atmosphere introduction passage. 
     Thus, in the vaporized fuel processing device for an internal combustion engine as described above, a purge flow detection process is performed, in which the pressure in the passage connected to the canister is detected to make sure that a flow of gas in the passage occurs as a purge is performed based on the pressure, while the purge is performed. In this way, by performing the purge flow detection process to make sure that a flow of gas in the passage connected to the canister occurs as the purge is performed and by making sure that the purge is being normally performed based on the result of the purge flow detection process, the reliability of the purge is ensured. 
     A vaporized fuel processing device is also available, in which a stop valve and a relief valve are provided between the canister and the fuel tank to hermetically close the fuel tank. In such a vaporized fuel processing device, in which the fuel tank is hermetically closed, when the stop valve or the relief valve is opened during a purge flow detection, the pressure in the passage connected to the canister varies as the valve is opened, so that it becomes impossible to accurately detect the flow of gas accompanying the purge based on the pressure in the passage. 
     Thus, in a vaporized fuel processing device for an internal combustion engine described in Japanese Patent Application Publication No. 2005-256624 (JP-A-2005-256624) that includes a relief valve that is opened when the pressure in a fuel tank becomes out of a predetermined first pressure range, it is inhibited to perform the purge flow detection process when the pressure in the fuel tank is out of a second pressure range included in the first pressure range and therefore there is a fear that the relief valve is opened. On the other hand, when the pressure in the fuel tank is within the second pressure range and therefore there is no fear that the relief valve is opened, the purge flow detection process is performed. 
     With such a configuration, it is possible to reduce the possibility that the relief valve is opened while the purge flow detection process is performed and the variation in pressure caused by the opening of the relief valve makes the result of the purge flow detection process incorrect. 
     However, when the fuel tank is hermetically closed, the decrease in the amount of fuel in the fuel tank due to the operation of the engine causes the pressure in the fuel tank to decrease because of the lowering of the liquid level. Thus, while the engine is in operation, the pressure in the fuel tank tends to be reduced to a level out of the second pressure range, at which level the relief valve can be opened, and therefore, when a configuration is employed, in which the purge flow detection process is performed when the pressure in the fuel tank is within the second pressure range, the number of times the purge flow detection process is performed becomes small. 
     Specifically, the number of times the purge flow detection process is performed to make sure that the purge is being normally performed becomes small and it therefore becomes difficult to secure the reliability of the purge, although the purge can be performed. 
     SUMMARY OF THE INVENTION 
     The invention provides a vaporized fuel processing device for an internal combustion engine, with which it is possible to prevent the frequency of performing a purge flow detection process from becoming small and at the same time, it is possible to reduce the possibility that a valve that hermetically closes the fuel tank is opened while the purge flow detection process is performed and the result of the purge flow detection process therefore becomes incorrect. 
     A first aspect of the invention is a vaporized fuel processing device for an internal combustion engine, including: a canister; a relief valve that, when a pressure in a fuel tank is out of a first pressure range, is opened to allow the fuel tank and the canister to communicate with each other; a stop valve that opens and closes a passage that bypasses the relief valve and allows the fuel tank and the canister to communicate with each other; and a control unit that performs a purge flow detection process when the pressure in the fuel tank is within a second pressure range included in the first pressure range, wherein the control unit opens the stop valve when a purge is being performed, a frequency of performing the purge flow detection process is small, and the pressure in the fuel tank is out of the second pressure range. 
     According to the first aspect of the invention, the stop valve is opened and the fuel tank and the canister are allowed to communicate with each other via the passage bypassing the relief valve when a purge is being performed, a frequency of performing the purge flow detection process is small, and the pressure in the fuel tank is out of the second pressure range, within which the purge flow detection process is performed. 
     Thus, the stop valve is opened and the fuel tank and the canister are allowed to communicate with each other to recover the pressure in the fuel tank to a level within the second pressure range when it is determined that the frequency of performing the purge flow detection process is small while the pressure in the fuel tank varies below the second pressure range as a result of lowering of the liquid level of the fuel stored in the fuel tank and, although a purge is being performed, the purge flow detection process is not performed. As a result, the purge flow detection process is performed, so that it is possible to suppress the occurrence of the situation where the purge flow detection process is not performed for a long period of time. 
     In addition, the purge flow detection process is performed when the pressure in the fuel tank is within the second pressure range included in the first pressure range, within which the relief valve is not opened. Thus, it is also possible to reduce the possibility that the relief valve is opened while the purge flow detection process is performed and the result of the purge flow detection process therefore becomes incorrect. 
     Thus, according to the first aspect of the invention, it is possible to prevent the frequency of performing a purge flow detection process from becoming small and at the same time, it is possible to reduce the possibility that a valve that hermetically closes the fuel tank is opened while the purge flow detection process is performed and the result of the purge flow detection process therefore becomes incorrect. 
     A second aspect of the invention is a vaporized fuel processing device for an internal combustion engine, including: a canister; a relief valve that, when a pressure in a fuel tank is out of a first pressure range, is opened to allow the fuel tank and the canister to communicate with each other; a stop valve that opens and closes a passage that bypasses the relief valve and allows the fuel tank and the canister to communicate with each other; a control unit that performs a purge flow detection process when the pressure in the fuel tank is within a second pressure range included in the first pressure range; and a memory that stores information indicating that the purge flow detection process is completed, wherein the control unit opens the stop valve when a purge is being performed, the information indicating that the purge flow detection process is completed is not stored in the memory, and the pressure in the fuel tank is out of the second pressure range. 
     According to the second aspect of the invention, the stop valve is opened and the fuel tank and the canister are allowed to communicate with each other via the passage bypassing the relief valve when a purge is being performed, the information indicating that the purge flow detection process is completed is not stored in the memory, and the pressure in the fuel tank is out of the second pressure range, within which the purge flow detection process is performed. 
     Thus, the stop valve is opened and the fuel tank and the canister are allowed to communicate with each other to recover the pressure in the fuel tank to a level within the second pressure range when it is determined that the information indicating that the purge flow detection process is completed is not stored while the pressure in the fuel tank varies below the second pressure range as a result of lowering of the liquid level of the fuel stored in the fuel tank and, although a purge is being performed, the purge flow detection process is not performed. As a result, the purge flow detection process is performed, so that it is possible to suppress the occurrence of the situation where the purge flow detection process is not performed for a long period of time. 
     In addition, the purge flow detection process is performed when the pressure in the fuel tank is within the second pressure range included in the first pressure range, within which the relief valve is not opened. Thus, it is also possible to reduce the possibility that the relief valve is opened while the purge flow detection process is performed and the result of the purge flow detection process therefore becomes incorrect. 
     Thus, according to the second aspect of the invention, it is possible to prevent the frequency of performing a purge flow detection process from becoming small and at the same time, it is possible to reduce the possibility that a valve that hermetically closes the fuel tank is opened while the purge flow detection process is performed and the result of the purge flow detection process therefore becomes incorrect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is a schematic diagram showing a schematic configuration of a vaporized fuel processing device according to an embodiment of the invention; 
         FIG. 2  is a explanatory diagram for explaining the relation between the pressure range, within which it is allowed to perform a purge flow detection process, and the pressures, at which a relief valve is opened; 
         FIG. 3  is a flow chart showing a series of steps performed in a pressure recovery process according to the embodiment; and 
         FIG. 4  is a flow chart showing a series of steps performed in a detection completion flag resetting process. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A vaporized fuel processing device for an internal combustion engine according to the invention will be described below with reference to  FIGS. 1 to 3 , taking as an example an embodiment in the form of a vaporized fuel processing device for an internal combustion engine mounted on a vehicle.  FIG. 1  shows a schematic configuration of the vaporized fuel processing device  200  according to the embodiment. 
     As shown in a lower part of  FIG. 1 , a fuel tank  100  is provided with a fuel pump module  120  that sucks up the fuel stored in the fuel tank  100 . In an upper portion of the fuel tank  100 , a tank pressure sensor  513  that detects the pressure Pt in the fuel tank  100  is provided. 
     The fuel pump module  120  is connected to an injector  11  of the internal combustion engine  10  via a fuel supply pipe  121 . Thus, the fuel that is sucked up from the fuel tank  100  by the fuel pump module  120  is supplied to the injector  11  via the fuel supply pipe  121 . Note that the fuel pump module  120  is provided with a fuel sender gauge  514  for detecting the level of the fuel stored in the fuel tank  100  based on the position of a float  514   a  that floats on the fuel stored in the fuel tank  100 . 
     As shown in a right part of  FIG. 1 , a fuel inlet pipe  130  is fitted to the fuel tank  100 . A fuel inlet port  130   a  that is located at a tip of the fuel inlet pipe  130  is housed in a fuel inlet box  132  provided in a body of the vehicle. Note that the fuel inlet pipe  130  is provided with a circulation pipe  131  that connects between an upper part of the fuel tank  100  and an upstream part of the fuel inlet pipe  130 . 
     The fuel inlet box  132  is provided with a fuel lid  133 . At the time of refueling, the fuel lid  133  is opened and a cap  130   b  attached to the fuel inlet port  130   a  is removed, whereby it is made possible to fill the fuel into the fuel tank  100  through the fuel inlet port  130   a.    
     As shown in an upper part of  FIG. 1 , an injector  11  that injects the fuel supplied from the fuel tank  100  is provided in an intake passage  20  of the internal combustion engine  10 . An air filter  21  that cleans fine particulates and the like contained in the intake air is provided at an entrance portion of the intake passage  20 . 
     A throttle valve  24 , the amount of opening of which is controlled by a motor  23 , and that controls the intake air amount, GA, that is the amount of air taken into the internal combustion engine  10  is provided in a portion of the intake passage  20  upstream of a surge tank  22 . In addition, an air flow meter  510  that detects the intake air amount GA is provided in a portion of the intake passage  20  upstream of the throttle valve  24 . 
     As shown in a center part of  FIG. 1 , the vaporized fuel processing device  200  that processes the fuel vapor produced in the fuel tank  100  is connected to the intake passage  20  of the internal combustion engine  10 . The vaporized fuel processing device  200  includes a canister  210  incorporating an adsorbing material  211  that adsorbs the fuel vapor. In this embodiment, the adsorbing material  211  is activated carbon that adsorbs fuel. 
     The canister  210  is connected to an upper portion of the fuel tank  100  via an discharging passage  220 . As shown in  FIG. 1 , a stop valve unit  221  is provided in the discharging passage  220 . The stop valve unit  221  includes a relief valve  221   a  that is opened when the pressure Pt in the fuel tank  100  becomes out of a predetermined, first pressure range and the difference in pressure between an upstream portion and a downstream portion of the discharging passage  220  with respect to the stop valve unit  221  becomes significantly large. The stop valve unit  221  includes the stop valve  221   b  that opens and closes the passage that bypasses the relief valve  221   a . Note that the relief valve  221   a  is provided to suppress the occurrence of a situation where the pressure Pt in the fuel tank  100  becomes excessively high or excessively low and an excessive load is thus applied to the fuel tank  100 . The first pressure range is set in consideration of the durability of the fuel tank  100 . The stop valve  221   b  is an electromagnetically driven valve that is switched between an open state and a closed state according to the control command output from the electronic control unit  500 . 
     Because the stop valve unit  221  is provided in the discharging passage  220 , the discharging passage  220  is closed by the relief valve  221   a  and the stop valve  221   b  when the stop valve  221   b  is closed and the pressure Pt in the fuel tank  100  is within the first pressure range. 
     As shown in a lower part of  FIG. 1 , an entrance portion of the discharging passage  220  in the fuel tank  100  is provided with an on-board refueling vapor recovery (ORVR) valve  222  and a roll over valve  223 . 
     The ORVR valve  222  is opened when the pressure Pt in the fuel tank  100  increases due to the rise of the fuel level caused by refueling. Thus, when the stop valve  221   b  is open, the increase in the pressure Pt in the fuel tank  100  due to the rise of the liquid level causes the fuel vapor in the fuel tank  100  to be introduced into the canister  210  through the discharging passage  220 . Thus, the increase in the pressure Pt due to the rise of the liquid level is suppressed, so that the discharge of the fuel vapor into the atmosphere through the fuel inlet pipe  130  and the circulation pipe  131  at the time of refueling is suppressed. 
     The roll over valve  223  is closed when the vehicle is significantly tilted, so that the leakage of liquid fuel from the fuel tank  100  to the outside is prevented. The fuel vapor in the fuel tank  100  is introduced into the canister  210  through the discharging passage  220  when at least one of the relief valve  221   a  and the stop valve  221   b  is open and at least one of the ORVR valve  222  and the roll over valve  223  is open. The fuel vapor introduced into the canister  210  is adsorbed by the adsorbing material  211 . 
     Connected to the canister  210  is an atmosphere introduction passage  230  that communicates with the fuel inlet box  132  provided in the body of the vehicle. An air filter  231  is provided in the atmosphere introduction passage  230 . Note that a negative pressure pump module  232  that has a function of switching between a state where the negative pressure pump module  232  closes the atmosphere introduction passage  230  and a state where the negative pressure pump module  232  does not close the atmosphere introduction passage  230  and therefore allows the canister  210  and the fuel inlet box  132  to communicate with each other, is provided in a portion of the atmosphere introduction passage  230  downstream of the air filter  231 . Note that a pump module pressure sensor  233  for detecting the pressure Pm of the air that flows through the negative pressure pump module  232  and is introduced into the canister  210 , is provided in the negative pressure pump module  232 . 
     In addition to the atmosphere introduction passage  230 , a purge passage  240  that communicates with the intake passage  20  is connected to the canister  210 . As shown in  FIG. 1 , a purge control valve  241  that is opened and closed according to the control commands output from the electronic control unit  500  is provided in the purge passage  240 . 
     The pump module pressure sensor  233 , the air flow meter  510 , the tank pressure sensor  513 , and the fuel sender gauge  514  are connected to the electronic control unit  500  that performs centralized control of the vehicle. In addition, various sensors, such as an accelerator position sensor  511  that detects the amount of driver&#39;s operation of the accelerator and a crank position sensor  512  that detects the engine speed NE, are connected to the electronic control unit  500 . 
     The electronic control unit  500  includes a central processing unit (CPU) that performs the calculation related to control of the internal combustion engine  10  and the calculation related to control that is performed to drive respective parts of the vaporized fuel processing device  200 . The electronic control unit  500  includes, as a memory for storing various pieces of information, such as results of calculation, a volatile memory  501  that can store and hold information while it is energized. 
     The electronic control unit  500  performs various calculations based on the signals output from the various sensors, and outputs control commands to respective parts to perform centralized control of the respective parts of the vehicle that include the vaporized fuel processing device  200 . 
     For example, while the internal combustion engine is in operation, the electronic control unit  500  controls the motor  23  based on the engine speed NE that is detected by the crank position sensor  512  and the accelerator operation amount that is detected by the accelerator position sensor  511 , whereby the throttle valve  24  is driven to control the intake air amount GA. In addition, the electronic control unit  500  controls the fuel injection amount by controlling the period of time, during which the injector  11  is opened, according to the intake air amount GA. 
     In addition, while the engine is in operation, the electronic control unit  500  controls the vaporized fuel processing device  200  to desorb the fuel that has been adsorbed by the adsorbing material  211  of the canister  210 , thereby performing a purge to introduce the desorbed fuel into the intake passage  20  along with air. 
     Specifically, while the engine is in operation, the purge control valve  241  is opened to cause the air in the canister  210  to be sucked into the intake passage  20  through the purge passage  240  by the negative pressure in the intake passage  20 . During this, the negative pressure pump module  232  is switched to the state where the negative pressure pump module  232  does not close the atmosphere introduction passage  230  and therefore allows the canister  210  and the fuel inlet box  132  to communicate with each other, whereby air is introduced into the canister  210  through the atmosphere introduction passage  230 . In this way, the fuel that has been adsorbed by the adsorbing material  211  is desorbed and the desorbed fuel is introduced into the intake passage  20  through the purge passage  240  along with air. 
     When the purge is performed while the engine is in operation, the fuel adsorbed by the adsorbing material  211  is desorbed from the adsorbing material  211  and it is therefore possible to suppress the occurrence of saturation of the adsorbing material  211 . In addition, because the desorbed fuel is introduced into the intake passage  20  along with air and burned in the internal combustion engine  10 , the fuel vapor produced in the fuel tank  100  is prevented from being discharged into the atmosphere and is, instead, burned and removed. 
     Note that in the vaporized fuel processing device  200  of this embodiment, the stop valve  221   b  is closed to close the discharging passage  220  while the engine is stopped, except during refueling. Thus, the fuel tank  100  is hermetically closed in principle while the engine is stopped, so that the fuel vapor is not introduced into the canister  210  unless the pressure Pt in the fuel tank  100  becomes out of the first pressure range and the relief valve  221   a  is therefore opened. 
     Thus, the fuel vapor is prevented from being adsorbed by the adsorbing material  211  of the canister  210  during the stop of the internal combustion engine  10 , during which no purge is performed, and therefore, it is possible to suppress the occurrence of saturation of the adsorbing material  211 . In addition, when the fuel tank  100  is hermetically closed while the engine is stopped, it is possible to suppress the discharge of the fuel vapor that cannot be adsorbed by the adsorbing material  211 , into the atmosphere through the canister  210  while the engine is stopped. 
     while the engine is stopped and the fuel tank  100  is hermetically closed, there is no space, into which the fuel vapor produced in the fuel tank  100  escapes, and therefore, the pressure Pt in the fuel tank  100  increases as the fuel vapor is produced. 
     When the cap  130   b  is removed and the fuel inlet port  130   a  is opened while the pressure Pt in the fuel tank  100  is higher than the atmospheric pressure Patm, the fuel vapor in the fuel tank  100  is discharged into the atmosphere through the fuel inlet pipe  130 . 
     Thus, in the vaporized fuel processing device  200  of this embodiment, during refueling, the stop valve  221   b  is opened to introduce the fuel vapor in the fuel tank  100  into the canister  210  through the discharging passage  220 , thereby reducing the pressure Pt in the fuel tank  100 . After it is made sure that the pressure Pt in the fuel tank  100  has been sufficiently reduced based on the pressure Pt in the fuel tank  100  detected by the tank pressure sensor  513 , the fuel lid  133  is unlocked. 
     With this configuration, in which the fuel lid  133  is unlocked after it is made sure that the pressure Pt in the fuel tank  100  has been sufficiently reduced, it is possible to suppress the discharge of the fuel vapor in the fuel tank  100  into the atmosphere through the fuel inlet pipe  130  when the fuel inlet port  130   a  is opened. 
     Note that as described above, in the vaporized fuel processing device  200 , the fuel adsorbed by the adsorbing material  211  is desorbed by performing a purge and the desorbed fuel is introduced into the internal combustion engine  10  and burned therein, so that the adsorbing capacity of the adsorbing material  211  is recovered and the occurrence of saturation of the adsorbing material  211  is suppressed. Thus, when the purge passage  240  is clogged and closed, for example, the purge operation cannot be normally performed and the fuel cannot be desorbed from the adsorbing material  211 , and as a result, the adsorbing material  211  is easily saturated. 
     In other words, when an abnormality occurs that inhibits normal purge operation as described above, the purge performed by opening the purge control valve  241  to recover the adsorbing capacity of the adsorbing material  211  cannot recover the adsorbing capacity of the adsorbing material  211 , which results in the degradation of the reliability of the purge. 
     Thus, in the vaporized fuel processing device  200 , during a purge, the pressure Pm in the negative pressure pump module  232  is detected by the pump module pressure sensor  233  to perform a purge flow detection process, in which it is determined based on the pressure Pm whether a flow of gas occurs in the purge passage  240  and the atmosphere introduction passage  230  when the purge is performed. In this way, by performing the purge flow detection process to make sure that a flow of gas occurs in the purge passage  240  and the atmosphere introduction passage  230  that are connected to the canister  210  as the purge is performed and by making sure that the purge is being normally performed based on the result of the purge flow detection process, the reliability of the purge is ensured. 
     Specifically, when the pressure Pm detected by the pump module pressure sensor  233  is equal to or lower than a reference pressure Ps that is lower than the atmospheric pressure while a purge is performed, based on this, it is determined that the purge is being normally performed, and on the other hand, when the pressure Pm is higher than the reference pressure Ps, based on this, it is determined that the purge is not being normally performed. 
     The reason why it is possible to determine whether the purge is being normally performed based on the pressure Pm is because a negative pressure due to the passage resistance in the air filter  231  occurs in a portion of the atmosphere introduction passage  230  downstream of the air filter  231  when a purge is being normally performed and a flow of gas occurs in the purge passage  240  and the atmosphere introduction passage  230 . Specifically, when the purge is being normally performed, the air in the canister  210  is sucked into the intake passage  20  as the purge is performed, which produces a negative pressure in the portion of the atmosphere introduction passage  230  downstream of the air filter  231 . Thus, in this case, the pressure Pm detected by the pump module pressure sensor  233  is equal to or lower than the reference pressure Ps that is lower than the atmospheric pressure Patm. On the other hand, when the purge is not being normally performed, the negative pressure in the portion of the atmosphere introduction passage  230  downstream of the air filter  231  is difficult to occur and the pressure Pm detected by the pump module pressure sensor  233  becomes higher than the reference pressure Ps. 
     Note that when the stop valve  221   b  and the relief valve  221   a  are opened while the purge flow detection process is performed, the pressure Pm in the portion of the atmosphere introduction passage  230  downstream of the air filter  231  varies and it becomes impossible to accurately detect the flow of gas accompanying the purge based on the pressure Pm. 
     Thus, in the vaporized fuel processing device  200  of this embodiment, similarly to the vaporized fuel processing device for an internal combustion engine described in JP-A-2005-256624, when the pressure Pt in the fuel tank  100  is out of a second pressure range included in the first pressure range and therefore there is a fear that the relief valve  221   a  is opened, it is inhibited to perform the purge flow detection process. On the other hand, when the pressure in the fuel tank  100  is within the second pressure range and therefore there is no fear that the relief valve  221   a  is opened, the purge flow detection process is performed. 
     Specifically, it is allowed to perform the purge flow detection process when the pressure Pt in the fuel tank  100  is within the second pressure range included in the first pressure range between a first pressure P 1  and a second pressure P 2 , the first pressure P 1  being the lower limit value of the pressures, at which the relief valve  221   a  is kept closed, the second pressure P 2  being the upper limit value of the pressures, at which the relief valve  221   a  is kept closed, as shown in  FIG. 2 . 
     A lower limit pressure PL, which is the lower limit of the second pressure range, and an upper limit pressure PH, which is the upper limit of the second pressure range are set in consideration of the variations of the characteristics of the relief valve  221   a  that are caused by the manufacturing tolerances, aged deterioration, etc. 
     Specifically, the lower limit pressure PL is set to a value higher than the first pressure P 1  by a margin A so that the relief valve  221   a  is not opened as long as the pressure Pt is equal to or higher than the lower limit pressure PL even when there are variations in the characteristics of the relief valve  221   a  and the pressure, at which the relief valve  221   a  is opened, is therefore higher than the first pressure P 1 . 
     On the other hand, the upper limit pressure PH is set to a value lower than the second pressure P 2  by a margin B so that the relief valve  221   a  is not opened as long as the pressure Pt is equal to or lower than the upper limit pressure PH even when there are variations in the characteristics of the relief valve  221   a  and the pressure, at which the relief valve  221   a  is opened, is therefore lower than the second pressure P 2 . 
     By setting the upper limit pressure PH and the lower limit pressure PL to set the second pressure range in this way and performing the purge flow detection process when the pressure Pt in the fuel tank  100  is within the second pressure range, it is possible to prevent the relief valve  221   a  from being opened while the purge flow detection process is performed. 
     Thus, it is possible to reduce the possibility that the relief valve  221   a  is opened while the purge flow detection process is performed and the result of the purge flow detection process therefore becomes incorrect. 
     When the stop valve  221   b  is closed and the fuel tank  100  is hermetically closed, the decrease in the amount of fuel in the fuel tank  100  due to the operation of the engine causes the pressure Pt in the fuel tank  100  to decrease because of the lowering of the liquid level. Thus, while the engine is in operation, the pressure Pt in the fuel tank  100  tends to vary below the lower limit pressure PL, that is, within the region lower than the lower limit pressure PL in  FIG. 2 . Thus, when a configuration is employed, in which the purge flow detection process is performed when the pressure Pt in the fuel tank  100  is within the second pressure range, the number of times the purge flow detection process is performed becomes small. 
     Specifically, the number of times the purge flow detection process is performed to make sure that the purge is normally performed becomes small and it therefore becomes difficult to secure the reliability of the purge, although the purge can be performed. 
     Thus, in the vehicle according to this embodiment, a pressure recovery process shown in  FIG. 3  is performed to increase the number of times the purge flow detection process is performed, by opening the stop valve  221   b  to recover the pressure Pt in the fuel tank  100  to a level within the second pressure range. 
     The pressure recovery process according to this embodiment will be described below with reference to  FIG. 3 .  FIG. 3  is a flow chart showing a series of steps of the pressure recovery process according to this embodiment. 
     The pressure recovery process shown in  FIG. 3  is repeatedly performed in a predetermined control cycle by the electronic control unit  500  while the engine is in operation and the electronic control unit  500  is therefore energized. When the pressure recovery process is started, in step S 100 , the electronic control unit  500  first determines whether a purge is being performed. Whether a purge is being performed is determined based on whether the purge control valve  241  is open. Specifically, when the purge control valve  241  is open, it is determined that a purge is being performed and, when the purge control valve  241  is closed, it is determined that no purge is being performed. 
     When it is determined in step S 100  that a purge is being performed (YES in step S 100 ), the process proceeds to step S 110 . On the other hand, when it is determined in step S 100  that no purge is being performed (NO in step S 100 ), the electronic control unit  500  temporarily exits this process. 
     In step S 110 , the electronic control unit  500  determines whether a purge flow detection completion flag is “0”. The purge flow detection completion flag is a flag that is stored in the volatile memory  501  of the electronic control unit  500  as the information indicating whether the purge flow detection process is completed. The purge flow detection completion flag is initially set to “0” that indicates that the purge flow detection process is not completed. When the purge flow detection process is completed, the purge flow detection completion flag is changed from “0” to “1” that indicates that the purge flow detection process is completed. 
     In this embodiment, the purge flow detection completion flag is stored in the volatile memory  501 , the information in which is lost when the energization is stopped. Thus, every time the engine is stopped and the energization of the volatile memory  501  is stopped, the purge flow detection completion flag is initialized. Thus, in the vaporized fuel processing device  200  of this embodiment, when the engine is started, the purge flow detection completion flag is always “0”, which is the initial value. 
     In step S 110 , when it is determined that the purge flow detection completion flag is “0” (YES in step S 110 ), the process proceeds to step S 120  and the electronic control unit  500  determines whether the pressure Pt in the fuel tank  100  is lower than the lower limit pressure PL. 
     On the other hand, when it is determined in step S 110  that the purge flow detection completion flag is not “0” (NO in step S 110 ), the electronic control unit  500  temporarily exits this process. 
     When it is determined in step S 120  that the pressure Pt in the fuel tank  100  is lower than the lower limit pressure PL (YES in step S 120 ), the process proceeds to step S 130  and the electronic control unit  500  opens the stop valve  221   b . When the stop valve  221   b  is opened in this way, the electronic control unit  500  temporarily exits this process. 
     On the other hand, when it is determined in step S 120  that the pressure Pt in the fuel tank  100  is equal to or higher than the lower limit pressure PL (NO in step S 120 ), the process proceeds to step S 140  and the electronic control unit  500  determines whether the stop valve  221   b  is closed, thereby making sure that the stop valve  221   b  is closed. 
     When it is determined in step S 140  that the stop valve  221   b  is closed (YES in step S 140 ), the process proceeds to step S 150 , it is allowed to perform the purge flow detection process, and the pressure recovery process is temporarily exited. 
     On the other hand, when it is determined in step S 140  that the stop valve  221   b  is open (NO in step S 140 ), the stop valve  221   b  is closed in step S 145 , the process proceeds to step S 150 , it is allowed to perform the purge flow detection process, and the pressure recovery process is temporarily exited. 
     When the stop valve  221   b  is opened in step S 130 , air is introduced into the fuel tank  100  from the canister  210  side through the discharging passage  220  and the pressure Pt in the fuel tank  100  is therefore recovered to the level near the atmospheric pressure Patm. 
     Thus, while this pressure recovery process is repeatedly performed, the stop valve  221   b  is opened and then it is determined in step S 120  that the pressure Pt in the fuel tank  100  is equal to or higher than the lower limit pressure PL, so that it is allowed to perform the purge flow detection process. 
     Specifically, when the above pressure recovery process is repeatedly performed, the stop valve  221   b  is opened when the results of all the determinations in steps S 100  to S 120  are affirmative, so that the pressure Pt in the fuel tank  100  is recovered to the level within the second pressure range. As a result, it is allowed to perform the purge flow detection process and the purge flow detection process is performed. 
     According to the above described embodiment, the following effects are brought about. 
     (1) As described above, when the pressure recovery process is performed, the stop valve  221   b  is opened when a purge is being performed, the information indicating that the purge flow detection process is completed is not stored in the volatile memory  501 , and the pressure Pt in the fuel tank  100  is lower than the lower limit pressure PL, that is, out of the second pressure range. 
     Thus, the stop valve  221   b  is opened when it is determined that the purge flow detection completion flag is “0” while the pressure Pt in the fuel tank  100  varies below the second pressure range as a result of lowering of the liquid level of the stored fuel and, although a purge is being performed, the purge flow detection process is not performed. Thus, the fuel tank  100  and the canister  210  are allowed to communicate with each other and the pressure Pt in the fuel tank  100  is recovered to the level within the second pressure range. As a result, it is allowed to perform the purge flow detection process and the purge flow detection process is performed, so that it is possible to suppress the occurrence of the situation where the purge flow detection process is not performed for a long period of time. 
     In addition, the purge flow detection process is performed when the pressure Pt in the fuel tank  100  is within the second pressure range included in the first pressure range, within which the relief valve  221   a  is not opened. Thus, it is possible to reduce the possibility that the relief valve  221   a  is opened while the purge flow detection process is performed and the result of the purge flow detection process therefore becomes incorrect. 
     In summary, according to the above embodiment, it is possible to prevent the frequency of performing the purge flow detection process from becoming small and at the same time, it is possible to reduce the possibility that the relief valve  221   a  is opened while the purge flow detection process is performed and the result of the purge flow detection process therefore becomes incorrect. 
     (2) In the above embodiment, it is determined whether to open the stop valve  221   b , by referring to the purge flow detection completion flag stored in the volatile memory  501 , the information in which is lost every time the engine is stopped and the energization is therefore stopped. Thus, if no purge flow detection process has been performed since the engine was started, the stop valve  221   b  is opened and the pressure Pt in the fuel tank  100  is recovered to the level within the second pressure range. Thus, according to the vaporized fuel processing device  200  of this embodiment, the purge flow detection process is performed every time the internal combustion engine  10  is started. 
     Note that the above embodiment may be implemented in the following modes obtained by appropriately modifying the above embodiment. 
     In the above embodiment, in the pressure recovery process described with reference to  FIG. 3 , the stop valve  221   b  is opened to recover the pressure Pt, provided that it is determined that the purge flow detection completion flag is “0”. The configuration, in which the stop valve  221   b  is opened provided that the purge flow detection completion flag is “0”, is an example of the configurations designed to determine that no purge flow detection process is performed based on the fact that the purge flow detection completion flag is “0” and, based on this determination, determine whether the frequency of performing the purge flow detection process is small. 
     The configuration, in which it is determined whether the frequency of performing the purge flow detection process is small based on the purge flow detection completion flag stored in the volatile memory  501  may be modified as appropriate. For example, a configuration may be employed, in which a deleting device for deleting the information stored in the volatile memory  501  when a predetermined period of time has elapsed since a purge flow detection process is completed is additionally provided and the information stored in the volatile memory  501  is deleted every time the predetermined period of time has elapsed since a purge flow detection process is completed. 
     Specifically, in addition to repeatedly performing the pressure recovery process as described with reference to  FIG. 3 , a detection completion flag resetting process as shown in  FIG. 4  may be repeatedly performed as the function of the deleting device while the engine is in operation. 
       FIG. 4  is a flow chart showing a series of steps in the detection completion flag resetting process, which functions as the deleting device. The detection completion flag resetting process is repeatedly performed in a predetermined cycle in the electronic control unit  500  while the engine is in operation. 
     When the detection completion flag resetting process is started, as shown in  FIG. 4 , in step S 200 , the electronic control unit  500  determines whether the purge flow detection completion flag is “1”. When it is determined in step S 200  that the purge flow detection completion flag is “0” (NO in step S 200 ), the electronic control unit  500  temporarily exits this process. 
     On the other hand, when it is determined in step S 200  that the purge flow detection completion flag is “1” (YES in step S 200 ), the process proceeds to step S 210  and the electronic control unit  500  increments a count value CT by “1”. Specifically, “1” is added to the current count value CT and the count value CT, to which “1” has been added, is employed as the new count value CT, whereby the count value CT is incremented by “1”. 
     Note that the count value CT is a value that is used to measure the time that has elapsed since the purge flow detection process is completed and the count value CT is initially set to “0”. When the count value CT is incremented by “1” in step S 210 , the process proceeds to step S 220  and the electronic control unit  500  determines whether the count value CT is equal to or greater than a reference value. 
     When it is determined in step S 220  that the count value CT is equal to or greater than the reference value (YES in step S 220 ), the process proceeds to step S 230  and the electronic control unit  500  sets the purge flow detection completion flag to “0”. Specifically, the electronic control unit  500  deletes the value of the purge flow detection completion flag, which is stored in the volatile memory  501  and is the information indicating that the purge flow detection process is completed, whereby the electronic control unit  500  resets the value of the purge flow detection completion flag to “0”, which is the initial value. When the purge flow detection completion flag is reset to “0”, the process proceeds to step S 240  and the electronic control unit  500  resets the count value CT to “0” and the process is temporarily exited. 
     On the other hand, when it is determined in step S 220  that the count value CT is still smaller than the reference value (NO in step S 220 ), the electronic control unit  500  skips steps S 230  and S 240  and temporarily exits the process. 
     When such a detection completion flag resetting process is repeatedly performed, the information stored in the volatile memory  501  is deleted every time the predetermined period of time has elapsed since a purge flow detection process is completed. Thus, the stop valve  221   b  is opened by the pressure recovery process and the pressure Pt in the fuel tank  100  is recovered to the level within the second pressure range when a time period longer than the predetermined period of time has elapsed since a purge flow detection process is completed. 
     In summary, also when such a configuration is employed, it is possible to prevent the frequency of performing the purge flow detection process from becoming small and at the same time, it is possible to reduce the possibility that the relief valve  221   a  is opened while the purge flow detection process is performed and the result of the purge flow detection process therefore becomes incorrect. 
     By changing the reference value that is compared with the count value CT in step S 220 , the intervals, at which the purge flow detection completion flag is reset, are changed, which makes it possible to freely set the frequency of performing the purge flow detection process. 
     Alternatively, the deleting device may be configured to reset the purge flow detection completion flag every time the predetermined period of time has elapsed regardless of whether a purge flow detection process is completed, by omitting the step S 200  in  FIG. 4 , for example. When such a configuration is employed, the value of the purge flow detection completion flag is deleted every time the predetermined period of time has elapsed. Thus, the stop valve  221   b  is periodically opened and the pressure Pt in the fuel tank  100  is periodically recovered to the level within the second pressure range, so that it is possible to suppress the occurrence of the situation where the purge flow detection process is not performed for a long period of time. 
     Note that when the deleting device is provided as described above, the memory for storing the purge flow detection completion flag may be a non-volatile memory that holds the stored memory even when energization is stopped. 
     In the above description of the embodiment, as a configuration for determining whether the frequency of performing a purge is small, a configuration is described, in which the purge flow detection completion flag, which is the information indicating whether the purge flow detection process is completed, is stored in the volatile memory  501  and, by referring to the purge flow detection completion flag, it is determined whether the frequency of performing a purge is small. However, as a configuration for determining whether the frequency of performing a purge is small, another configuration, in which a flag indicating whether a predetermined period of time has elapsed since a purge flow detection process is performed and, by referring to this flag, it is determined whether the frequency of performing a purge is small, may also be employed. 
     Specifically, a configuration may be employed, in which when the flag is set to a value indicating that the predetermined period of time has elapsed since the preceding purge flow detection process is performed, based on this, it is determined that the frequency of performing the purge flow detection is small and the stop valve  221   b  is opened. 
     Also when such a configuration is employed, by determining whether the frequency of performing the purge flow detection is small and, based on the result of this determination, opening the stop valve  221   b , it is possible to recover the pressure Pt in the fuel tank  100  to the level within the second pressure range, which makes it possible to perform the purge flow detection process. 
     In the above description of the embodiment, as the sensor for detecting the purge flow, the pump module pressure sensor  233  provided in the negative pressure pump module  232  is taken as an example. However, the sensor for detecting the purge flow is not limited as long as the sensor can detect the flow of gas accompanying the purge. Thus, instead of the pump module pressure sensor  233 , a pressure sensor may be provided at a position other than in the negative pressure pump module  232 . 
     Although a configuration is described, in which the stop valve  221   b  and the relief valve  221   a  are formed in the stop valve unit  221 , the relief valve  221   a  is not limited as long as the valve closes the discharging passage  220  that allows the fuel tank  100  and the canister  210  to communicate with each other and the valve is opened when the pressure Pt in the fuel tank  100  becomes out of the first pressure range. The stop valve  221   b  is not limited as long as the valve opens and closes the passage that bypasses the relief valve  221   a  and allows the fuel tank  100  and the canister  210  to communicate with each other. Specifically, instead of integrating the stop valve  221   b  and the relief valve  221   a  into the stop valve unit  221  as described in the above description of the embodiment, the stop valve  221   b  and the relief valve  221   a  may be provided separately. Alternatively, in addition to the discharging passage  220 , a passage that bypasses the relief valve  221   a  may be provided and the stop valve  221   b  may be provided in the latter passage. 
     The configuration of the fuel tank  100  and the vaporized fuel processing device  200  described in the above description of the embodiment is an example of the embodiments of the invention. The configuration of the fuel tank  100  and the vaporized fuel processing device  200  may be modified as appropriate. As long as the control unit performs the control characteristic of the invention, the control unit of the invention is not limited to a single device, such as the above electronic control unit  500 , but may be a collection of a plurality of controllers, for example. 
     As a specific configuration for determining that the frequency of performing the purge flow detection is small, a configuration may be employed, in which a memory that stores information indicating that the purge flow detection process is completed is provided, wherein, when the information indicating that the purge flow detection process is completed is not stored in the memory, it is determined that the frequency of performing the purge flow detection process is small. 
     When a configuration, in which the memory that stores the information indicating that the purge flow detection process is completed is a volatile memory, information in which is lost every time energization of the memory is stopped, is employed, the stop valve is opened and the pressure in the fuel tank is recovered to the level within the second pressure range when no purge flow detection process has been performed since the engine was started and energization was therefore started. 
     The vaporized fuel processing device may further include a deleting device that deletes the information stored in the memory every time a predetermined period of time has elapsed since the purge flow detection process is completed, wherein the information stored in the memory is deleted every time the predetermined period of time has elapsed since the purge flow detection process is completed. 
     According to the above configuration, the information stored in the memory is deleted every time the predetermined period of time has elapsed since the purge flow detection process is completed. Thus, the stop valve is opened and the pressure in the fuel tank is recovered to the level within the second pressure range when a time period longer than the predetermined period of time has elapsed since a purge flow detection process is completed. 
     A configuration may be employed, in which the information stored in the memory is deleted every time a predetermined period of time has elapsed. With such a configuration, the information stored in the memory is deleted every time the predetermined period of time has elapsed. Thus, it is possible to suppress the occurrence of the situation where the purge flow detection process is not performed for a long period of time. 
     The invention has been described with reference to example embodiments for illustrative purposes only. It should be understood that the description is not intended to be exhaustive or to limit form of the invention and that the invention may be adapted for use in other systems and applications. The scope of the invention embraces various modifications and equivalent arrangements that may be conceived by one skilled in the art.