Patent Publication Number: US-9850855-B2

Title: Fuel evaporative gas emission control apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a fuel evaporative gas emission control apparatus, and particularly to an abnormality detection technique of the fuel evaporative gas emission control apparatus. 
     Description of the Related Art 
     Conventionally, in order to prevent release of fuel evaporative gas that is evaporated in a fuel tank into the atmosphere, there has been proposed a fuel evaporative gas emission control apparatus that is configured by a canister that is interposed in a communication path that provides communication between a fuel tank and an intake passage of an internal combustion engine, a sealing valve that provides communication or blockage between the fuel tank and the canister, and a purge valve that provides communication and blockage of the communication path between the intake passage and the canister (refer to Japanese Patent No. 4107053, for example). The fuel evaporative gas emission control apparatus causes the fuel evaporative gas in the fuel tank to flow out to the canister by opening the sealing valve and closing the purge valve at a time of refueling, and causes the fuel evaporative gas to adsorb to the activated carbon that is placed in the canister. Subsequently, the fuel evaporative gas emission control apparatus opens the purge valve to discharge the fuel evaporative gas, which is caused to adsorb to the activated carbon in the canister, to the intake passage of the internal combustion engine and treats the fuel evaporative gas, at an operation time of the internal combustion engine. 
     Incidentally, in the fuel evaporative gas emission control apparatuses including canisters as above, there is also a fuel evaporative gas emission control apparatus that further includes a canister opening and closing valve that opens and closes the communication path and the canister. The canister opening and closing valve is used in leak monitoring of the fuel tank, the canister, the communication path and the like that configure the fuel evaporative gas emission control apparatus, for example, measures the state of change in the internal pressure of the communication path by closing the canister opening and closing valve, thereafter measures the state of change in the canister internal pressure by opening the canister opening and closing valve, and from the measurement results, determines presence or absence of leak in the communication path. 
     However, the leak monitoring described above cannot be carried out normally in some cases when the pressure in the fuel tank is high. For example, when the temperature of the fuel tank increases due to the high-temperature outside air during soak in which a vehicle is parked, the tank internal pressure increases and acts on the canister opening and closing valve which is being closed. Since in the state where the high tank internal pressure acts like this, a valve opening delay occurs to the canister opening and closing valve, and the canister opening and closing valve cannot be opened at a proper timing, there arises the problem that leak monitoring cannot be carried out normally. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the problem as above, and an object of the present invention is to provide a fuel evaporative gas emission control apparatus that can prevent a valve opening delay of a canister opening and closing valve due to high tank internal pressure, and can carry out leak monitoring by opening the canister opening and closing valve at a proper timing. 
     In order to achieve the above described object, a fuel evaporative gas emission control apparatus of the present invention includes a communication path that provides communication between an intake passage of an internal combustion engine and a fuel tank, a canister that is connected to the communication path and adsorbs fuel evaporative gas in the communication path, a canister opening and closing valve that opens and closes communication between the communication path and the canister, a purge valve that opens and closes the communication path between the intake passage and the canister, a sealing valve that opens and closes communication between the fuel tank and the communication path, a leak test execution unit that executes a leak test by opening the sealing valve and closing the canister opening and closing valve, and thereafter opening the canister opening and closing valve, in a state where the purge valve is closed, and a pressure regulation control unit that prior to the leak test by the leak test execution unit, reduces an internal pressure of the fuel tank to a valve opening guarantee determination value that is set in advance by opening the sealing valve which is being closed while keeping the canister opening and closing valve in an open state, and thereafter closes the canister opening and closing valve. 
     According to the fuel evaporative gas emission control apparatus which is configured as above, a valve opening delay of the canister opening and closing valve due to a high internal pressure of the tank can be prevented, and leak monitoring can be carried out by opening the canister opening and closing valve at a suitable timing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic configuration diagram of a fuel evaporative gas emission control apparatus according to one embodiment of the present invention. 
         FIG. 2  is a diagram showing an operation of internal components at a non-operation time of a changeover valve of an evaporative leak check module. 
         FIG. 3  is a diagram showing an operation of the internal components at an operation time of the changeover valve of the evaporative leak check module. 
         FIG. 4  is a flowchart showing a control procedure of leak monitoring that is executed by an ECU of the present embodiment. 
         FIG. 5  is a time chart showing a control situation of leak monitoring in a case where an entire system is normal. 
         FIG. 6  is a time chart showing a control situation of leak monitoring in a case where a bypass valve is stuck closed. 
         FIG. 7  is a time chart showing a control situation of leak monitoring in a case where a sealing valve is stuck closed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment of the present invention will be described based on the drawings. 
       FIG. 1  is a schematic configuration diagram of a fuel evaporative gas emission control apparatus  1  according to one embodiment of the present invention. Further,  FIG. 2  is a diagram showing an operation of internal components at a non-operation time of a changeover valve  34   e  of an evaporative leak check module  34 , and  FIG. 3  is a diagram showing an operation of the internal components at an operation time of the changeover valve  34   e  of the evaporative leak check module  34 . Arrows in  FIGS. 2 and 3  show directions of air flows in a case where a negative pressure pump  34   c  in the evaporative leak check module  34  is operated in states of the drawings. The changeover valve  34  is in an open state at the non-operation time in  FIG. 2 , and is in a closed state at the operation time in  FIG. 3 . Hereinafter, a configuration of the fuel evaporative gas emission control apparatus will be described. 
     The fuel evaporative gas emission control apparatus  1  according to the present embodiment is used in a hybrid vehicle or a plug-in hybrid vehicle that includes a drive motor not illustrated and an engine  10  (an internal combustion engine), and travels by using either one or both of the drive motor and the engine  10 . 
     As shown in  FIG. 1 , the fuel evaporative gas emission control apparatus  1  is mainly configured by the engine  10  which is loaded on a vehicle, a fuel storage section  20  that stores fuel, a fuel evaporative gas treatment section  30  that treats evaporative gas of the fuel which is evaporated in the fuel storage section  20 , and an electronic control unit (hereinafter, referred to as ECU)  40  that is a control device for performing overall control of the vehicle. 
     The engine  10  is a multi point injection (Multi Point Injection: MPI) type gasoline engine. The engine  10  is provided with an intake passage  11  that takes air into a combustion chamber of the engine  10 . Further, a fuel injection valve  12  that injects fuel into an intake port of the engine  10  is provided downstream of the intake passage  11 . A fuel pipe  13  is connected to the fuel injection valve  12 , and is supplied with fuel from a fuel tank  21  that stores the fuel. 
     In the intake passage  11  of the engine  10 , an intake air temperature sensor  14  that detects a temperature of the air which is taken in is placed. Further, in the engine  10 , a water temperature sensor  15  that detects a temperature of cooling water that cools the engine  10  is placed. 
     The fuel storage section  20  is configured by the fuel tank  21 , a fuel supply port  22  that is a fuel filling port to the fuel tank  21 , a fuel pump  23  that supplies fuel to the fuel injection valve  12  via the fuel pipe  13  from the fuel tank  21 , a fuel cutoff valve  24  that prevents outflow of the fuel from the fuel tank  21  to a fuel evaporative gas treatment section  30 , and a leveling valve  25  that controls a liquid level in the fuel tank  21  at a time of refueling. Further, the fuel evaporative gas that is generated in the fuel tank  21  is discharged to the fuel evaporative gas treatment section  30  via the leveling valve  25  from the fuel cutoff valve  24 . 
     The fuel evaporative gas treatment section  30  is configured by a purge pipe  31  (a communication path), a vapor pipe  32  (a communication path), a canister  33 , the evaporative leak check module  34 , a sealing valve  35 , a purge valve  36  (a purge valve), a bypass valve  37  (a canister opening and closing valve) and a pressure sensor  38 . 
     The purge pipe  31  is provided to provides communication between the intake passage  11  of the engine  10  and the canister  33 . 
     The vapor pipe  32  is provided to provide communication between the leveling valve  25  of the fuel tank  21  and the purge pipe  31 . That is, the vapor pipe  32  is provided to provide communication between the fuel tank  21  and the purge pipe  31 . 
     The canister  33  has an activated carbon therein. Further, the purge pipe  31  is connected to the canister  33  so that the fuel evaporative gas generated in the fuel tank  21  or the fuel evaporative gas that is adsorbed by the activated carbon can flow through the purge pipe  31 . Further, the canister  33  is provided with an atmosphere hole  33   a  that takes in the outside air when the fuel evaporative gas that is adsorbed by the activated carbon is released to the intake passage  11  of the engine  10 . 
     As shown in  FIGS. 2 and 3 , the evaporative leak check module  34  is provided with a canister side passage  34   a  that leads to an atmosphere hole  33   a  of the canister  33 , and an atmosphere side passage  34   b  that leads to the atmosphere. A pump passage  34   d  that includes the negative pressure pump  34   c  communicates with the atmosphere side passage  34   b.  Further, the evaporative leak check module  34  is provided with the changeover valve  34   e  and a bypass passage  34   f.  The changeover valve  34   e  includes an electromagnetic solenoid, and is driven by the electromagnetic solenoid. When the electromagnetic solenoid is in a nonenergized state (OFF), the changeover valve  34   e  provides communication between the canister side passage  34   a  and the atmosphere side passage  34   b  (corresponding to an open state of the changeover valve  34   e ). Further, when the electromagnetic solenoid is supplied with a drive signal from outside and is in an energized state (ON), the changeover valve  34   e  provides communication between the canister side passage  34   a  and the pump passage  34   d  as shown in  FIG. 3  (corresponding to a closed state of the changeover valve  34   e ). 
     The bypass passage  34   f  is a passage that always causes the canister side passage  34   a  and the pump passage  34   d  to continue to each other, and is provided with a reference orifice  34   g  with a small diameter (for example, a diameter of 0.45 mm). Further, a pressure sensor  34   h  that detects a canister internal pressure Pcan is provided between the negative pressure pump  34   c  of the pump passage  34   d  and the reference orifice  34   g  of the bypass passage  34   f.  A detection target of the pressure sensor  34   h  is switched in accordance with opening and closing of the changeover valve  34   e,  and at an opening time of the changeover valve  34   e,  a pressure in the bypass passage  34   f  downstream of the reference orifice  34   g  is detected as the canister internal pressure Pcan, and at a closing time of the changeover valve  34   e,  a pressure inside the canister  33  is detected as the canister internal pressure Pcan via the canister side passage  34   a.    
     The sealing valve  35  is interposed in the vapor pipe  32  between the fuel tank  21  and the purge pipe  31 . The sealing valve  35  includes an electromagnetic solenoid, and is driven by the electromagnetic solenoid. The sealing valve  35  is an electromagnetic valve of a normally closed type that is brought into a closed state in a state where the electromagnetic solenoid is nonenergized (OFF), and is brought into an open state when the electromagnetic solenoid is supplied with a drive signal from outside and is brought into an energized state (ON). The sealing valve  35  closes the vapor pipe  32  when the electromagnetic solenoid is in a nonenergized state (OFF) and the sealing valve  35  is in a closed state, and opens the vapor pipe  32  when the electromagnetic solenoid is supplied with a drive signal from outside and is in an energized state (ON) and the sealing valve is in an open state. That is, the sealing valve  35  closes the fuel tank  21  into a sealed state when the sealing valve  35  is in the closed state, disables outflow of the fuel evaporative gas which is generated in the fuel tank  21  into the canister  33  or the intake passage  11  of the engine  10 , and enables outflow of the fuel evaporative gas into the canister  33  or the intake passage  11  of the engine  10  when the sealing valve  35  is in an open state. 
     The purge valve  36  is interposed in the purge pipe  31  between the intake passage  11  and a connection portion of the purge pipe  31  to the vapor pipe  32 . The purge valve  36  includes an electromagnetic solenoid, and is driven by the electromagnetic solenoid. The purge valve  36  is a normally closed type electromagnetic valve that is brought into a closed state when the electromagnetic solenoid is in a nonenergized state (OFF), and is brought into an open state when the electromagnetic solenoid is supplied with a drive signal from outside and is in an energized state (ON). The purge valve  36  closes the purge pipe  31  when the electromagnetic solenoid is in a nonenergized state (OFF) and in a closed state, and opens the purge pipe  31  when the electromagnetic solenoid is supplied with a drive signal from outside and is in an energized state (ON) and the purge valve  36  is in an open state. That is, the purge valve  36  disables outflow of the fuel evaporative gas to the intake passage  11  of the engine  10  from the canister  33  or the fuel tank  21  when the purge valve  36  is in a closed state, and enables outflow of the fuel evaporative gas into the intake passage  11  of the engine  10  from the canister  33  or the fuel tank  21  when the purge valve  36  is in an open state. 
     The bypass valve  37  is interposed in the purge pipe  31  between the connection portion of the vapor pipe  32  to the purge pipe  31  and the canister  33 . The bypass valve  37  includes an electromagnetic solenoid, and is driven by the electromagnetic solenoid. The bypass valve  37  is a normally open type electromagnetic valve that is brought into an open state when the electromagnetic solenoid is in a nonenergized state (OFF), and is brought into a closed state when the electromagnetic solenoid is supplied with a drive signal from outside and is brought into an energized state (ON). The bypass valve  37  opens the canister  33  to the purge pipe  31  when the electromagnetic solenoid is in the nonenergized state (OFF) and the bypass valve  37  is in an open state, and closes the canister  33  when the electromagnetic solenoid is supplied with a drive signal from outside and is in an energized state (ON) and the bypass valve  37  is in the closed state. That is, the bypass valve  37  seals the canister  33  when the bypass valve  37  is in a closed state, and disables inflow of the fuel evaporative gas to the canister  33  or outflow of the fuel evaporative gas from the canister  33 . When the bypass valve  37  is in the open state, the bypass valve  37  enables inflow of the fuel evaporative gas to the canister  33  or outflow of the fuel evaporative gas from the canister  33 . 
     The pressure sensor  38  is placed in the vapor pipe  32  between the fuel tank  21  and the sealing valve  35 . The pressure sensor  38  detects the tank internal pressure Ptan which is the internal pressure of the fuel tank  21 . The pressure sensor  38  can detect the internal pressure of only the fuel tank  21  only when the sealing valve  35  is in the closed state and the fuel tank  21  is sealed. 
     The ECU  40  is a control device for performing overall control of the vehicle, and is configured by including an input and output device, a storage device (a ROM, a RAM, a nonvolatile RAM and the like), a central processing unit (CPU), a timer and the like. 
     To an input side of the ECU  40 , the intake temperature sensor  14 , the water temperature sensor  15 , the pressure sensor  34   h  and the pressure sensor  38  that are described above are connected, and detection information from these sensors is inputted. 
     Meanwhile, to an output side of the ECU  40 , the fuel injection valve  12 , the fuel pump  23 , the negative pressure pump  34   c,  the changeover valve  34   e,  the sealing valve  35 , the purge valve  36  and the bypass valve  37  which are described above are connected. 
     The ECU  40  controls an operation of the negative pressure pump  34   c,  and opening and closing of the changeover valve  34   e,  the sealing valve  35 , the purge valve  36  and the bypass valve  37 , and enables adsorption of the fuel evaporative gas which is generated in the fuel tank  21  to the canister  33 , and purge treatment (canister purge, tank purge) that discharges the fuel evaporative gas that adsorbs to the canister  33  at the operation time of the engine  10 , and the fuel evaporative gas which is generated in the fuel tank  21  to the intake passage  11  of the engine  10 . 
     The canister purge is performed for a predetermined time period directly after engine start, for example. 
     The ECU  40  opens the purge valve  36  and the bypass valve  37  during an engine operation, in the canister purge. At this time, the sealing valve  35  is in a closed state, and the changeover valve  34   e  is in an open state. Thereby, the purge pipe  31  and the canister  33  communicate with the intake passage  11  of the engine  10 , and therefore, atmosphere passes through the canister  33  and the purge pipe  31  to flow into the intake passage  11  which has a negative pressure by the operation of the engine  10  from an outside air inlet port of the canister  33 . Accordingly, the fuel evaporative gas which is adsorbed by the canister  33  is discharged to the intake passage  11  and is treated. 
     Tank purge is performed when the pressure in the fuel tank  21  becomes a high pressure of a predetermined pressure P 1  or, more until the pressure in the fuel tank  21  is reduced, during an operation of the engine  10 . The ECU  40  opens the sealing valve  35  and the purge valve  36 , and closes the bypass valve  37 , as the tank purge. Thereby, the fuel tank  21  communicates with the intake passage  11  of the engine  10  via the purge pipe  31  and the vapor pipe  32 , and therefore, the fuel evaporative gas passes through the vapor pipe  32  and the purge pipe  31  from the fuel tank  21  and flows into the intake passage  11  which has a negative pressure by the operation of the engine  10 . Accordingly, the fuel evaporative gas in the fuel tank  21  is discharged to the intake passage  11  and is treated, and the pressure in the fuel tank  21  is reduced. The tank purge is performed with higher priority than the canister purge. Accordingly, when the pressure in the fuel tank  21  is the predetermined pressure P 1  or more immediately after startup of the engine, the canister purge is performed after the tank purge is performed. 
     Further, the ECU  40  executes leak monitoring that determines presence or absence of leak of the fuel tank  21 , the canister  33 , the purge pipe  31  and the vapor pipe  32  (sticking of the sealing valve  35 , the changeover valve  34   c,  the bypass valve  37  and the purge valve  36  in addition) while the ignition switch is turned off (a leak test execution unit). 
       FIG. 4  is a flowchart showing a control procedure of leak monitoring that is executed by the ECU  40 . Further,  FIGS. 5 to 7  are time charts showing states of change of drive signals of the respective valves (the sealing valve  35 , the changeover valve  34   e,  the bypass valve  37  and the purge valve  36 ), a drive signal of the negative pressure pump  34   c,  the canister internal pressure Pcan and the tank internal pressure Ptan in leak monitoring.  FIG. 5  shows a control state (subdivided into time periods  1  to  6 ) in a case where the entire system of the fuel evaporative gas emission control apparatus is normal.  FIG. 6  shows a control state (time periods  1  to  5 A) in a case where the bypass valve  37  is stuck closed.  FIG. 7  shows a control state (time periods  1  to  5 A) in a case where the sealing valve  35  is stuck closed.  FIG. 4  shows processing that is executed by the ECU  40  (mainly processing of reducing the internal pressure of the fuel tank  21 ) in a time period  2  in each of the time charts. 
     As shown in  FIGS. 5 to 7 , in an initial time period  1  when leak monitoring is started, the sealing valve  35  is closed, the changeover valve  34   e  is opened, the bypass valve  37  is opened, and the purge valve  36  is closed. The state shifts to a time period  2 , and the processing in  FIG. 4  is started by the ECU  40 . First, in step S 2 , an operation of the negative pressure pump  34   c  is started for the purpose of warming up, determination of sticking of the changeover valve  34   e  and the negative pressure pump  34   c  is performed in subsequent step S 4 , and when it is determined that sticking is present in either one of them, the routine is ended. Accordingly, at this point of time, the leak monitoring is stopped. 
     Further, when it is determined that no sticking is present in both the changeover valve  34   e  and the negative pressure pump  34   c  in step S 4 , the sealing valve  35  is opened in step S 6 . In subsequent step S 8 , measurement of the tank internal pressure Ptan is started, and count of an operation continuation time period T of the negative pressure pump  34   c  is started. Thereafter, in step S 10 , it is determined whether or not the tank internal pressure Ptan is a valve opening guarantee determination value P 0  or smaller, and when the determination is No (negative), the flow shifts to step S 12  and it is determined whether or not the operation continuation time period T becomes a limit time period Tlmt or more. The valve opening guarantee determination value P 0  is a positive value with which a valve opening delay of the bypass valve  37  does not occur, and is set in advance as the minimum tank internal pressure Ptan that can ensure precision of leak determination based on a tank internal pressure decompression amount ΔP 1  and a canister internal pressure change amount ΔP 2  that will be described as follows. Further, the limit time period Tlmt is set in advance as a condition for forcefully shifting to next processing when the tank internal pressure Ptan does not reduce due to sticking to closure of the bypass valve  37  and the sealing valve  35  that will be described later. 
     When determination of Yes (affirmative) is made in either step S 10  or step S 12 , the flow shifts to step S 14  and the bypass valve  37  is closed, and measurement of the tank internal pressure decompression amount ΔP 1  is started in step S 16 . The tank internal pressure decompression amount ΔP 1  is measured as a pressure reduction amount over time of the tank internal pressure Ptan after closing the bypass valve  37  (in other words, a pressure reduction amount over time in a site where the purge pipe  31  and the vapor pipe  32  communicate with the fuel tank  21 ). In subsequent step S 18 , it is determined whether or not the operation continuation time period T becomes a warm-up time period Twu (for example, 300 sec) that is set in advance or longer, and when determination of Yes is made, the flow shifts to step S 20 . In step S 20 , pulsation of the negative pressure pump  34   c  is detected. In subsequent step S 22 , presence or absence of a failure of the negative pressure pump  34   c  is determined based on the pulsation detected in step S 22 , and when a failure is present, the routine is ended. 
     Further, when it is determined that no failure is present in the negative pressure pump  34   c,  the flow shifts to step S 24 , and leveling processing of the pressure that is detected by the pressure sensor  34   h  is performed. Since the changeover valve  34   e  at this time is opened, air in the bypass passage  34   f  upstream of the reference orifice  34   g  is taken out to a downstream side via the reference orifice  34   g  by the negative pressure pump  34   c  to generate a negative pressure, and the negative pressure is detected by the pressure sensor  34   h  and is set as a leak determination reference value ΔP 3 ref as will be described later. In subsequent step S 26 , presence or absence of abnormality of the reference orifice  34   g  is determined based on the leak determination reference value ΔP 3 ref after leveling, and when abnormality is present, the routine is ended, whereas when abnormality is absent, the flow shifts to a time period  3  to continue the processing of leak monitoring in succession. 
     The processing of the time period  2  of the leak monitoring is executed by the ECU  40  as above. Next, the control state of the entire leak monitoring including the time period  2  will be described in sequence based on  FIGS. 5 to 7 . 
     First, when the entire system of the fuel evaporative gas emission control apparatus  1  is normal, that is, when all the valves (the sealing valve  35 , the changeover valve  34   e,  the bypass valve  37  and the purge valve  36 ) normally open and close without being stuck, and no leak is present in the fuel tank  21 , the canister  33 , the purge pipe  31  and the vapor pipe  32 , control advances as shown in  FIG. 5 . 
     In the time period  1  as described above, the sealing valve  35  is closed, the changeover valve  34   e  is opened, the bypass valve  37  is opened, the purge valve  36  is closed, and the canister internal pressure Pcan is kept at atmospheric pressure. Further, in the fuel tank  21 , the temperature increases by high-temperature outside air during soak when the vehicle is parked, and the tank internal pressure Ptan is increased to a pressure to such an extent that a valve opening delay of the bypass valve  37  occurs, as described in [Problem to be solved by the invention]. 
     When the state shifts from the time period  1  to the time period  2 , an operation of the negative pressure pump  34   c  is started to warm up first (step S 2  in  FIG. 4 ), the air in the bypass passage  34   f  upstream of the reference orifice  34   g  is taken out to the downstream side via the reference orifice  34   g,  whereby the canister internal pressure Pcan which is detected by the pressure sensor  34   h  is reduced stepwise to be lower than the atmospheric pressure. 
     In parallel with the warm-up of the negative pressure pump  34   c,  a leak test for the purge pipe  31  and the vapor pipe  32  as targets (corresponding to a leak test execution unit of the present invention) is executed, and prior to the leak test, an operation of reducing the internal pressure of the fuel tank  21  (corresponding to a pressure regulation control unit of the present invention) is performed. First, the sealing valve  35  is opened (step S 6  in  FIG. 4 ), fuel evaporative gas in the fuel tank  21  flows into and adsorbed by the canister  33 , and the air after adsorption is discharged to the atmosphere from the changeover valve  34   e,  whereby the tank internal pressure Ptan gradually reduces. When the tank internal pressure Ptan reduces to the valve opening guarantee determination value P 0  (Yes in step S 10  in  FIG. 4 ), the bypass valve  37  is closed (step S 14  in  FIG. 4 ). 
     Warm-up of the negative pressure pump  34   c  is continued for the warm-up time period Twu, and the tank internal pressure decompression amount ΔP 1  is measured in parallel in the warm-up time period Twu (steps S 16  and  18  in  FIG. 4 ). Since the entire system of the fuel evaporative gas emission control apparatus  1  is normal, leak does not occur to the purge pipe  31  and the vapor pipe  32 , the tank internal pressure Ptan continues to be kept in a vicinity of the valve opening guarantee determination value P 0 , and 0 or a very small value is measured as the tank internal pressure decompression amount ΔP 1 . 
     When the state shifts to the time period  3  from the time period  2 , the negative pressure pump  34   c  is stopped, the negative pressure by the reference orifice  34   g  disappears, and the canister internal pressure Pcan is returned to the atmospheric pressure. Thereafter, the changeover valve  34   e  is closed, the inside of the canister  33  is shut off from the atmosphere, and after the bypass valve  37  is opened, the change amount ΔP 2  of the canister internal pressure Pcan is measured. Valve opening of the bypass valve  37  is executed under the situation where the tank internal pressure Ptan is reduced to the valve opening guarantee determination value P 0 , and therefore the bypass valve  37  is opened quickly without causing a delay. Since the entire system of the fuel evaporative gas emission control apparatus  1  is normal, the fuel evaporative gas in the fuel tank  21  flows into the canister  33  through the vapor pipe  32  to increase the canister internal pressure Pcan rapidly, and a large value is measured as the canister internal pressure change amount ΔP 2 . The tank internal pressure Ptan reduces a little due to outflow of the fuel evaporative gas from the fuel tank  21 . 
     As a result, the tank internal pressure decompression amount ΔP 1  which is measured in the time period  2  becomes less than a decompression determination value ΔP 1 ref, and the canister internal pressure change amount ΔP 2  which is measured in the time period  3  exceeds a change amount determination value ΔP 2 ref. Therefore, it is determined that no leak is present in the purge pipe  31  and the vapor pipe  32 . As for the fuel tank  21 , absence of leak is determined at a time point at which a high pressure is kept during soak, and therefore, a detection method B that narrows down the target of the leak test to only the canister  33  (in more detail, also including the purge pipe  31  at the canister  33  side from the bypass valve  37 ) is selected. 
     When selection of the above detection method is ended, the state shifts to the time period  4 B from the time period  3 , and the leak test by the detection method B is started. First, the sealing valve  35  is closed, the changeover valve  34   e  is opened thereafter, and the purge valve  36  is opened. The canister  33  is opened to the atmosphere via the changeover valve  34   e  and is opened to the atmosphere via the bypass valve  37  and the purge valve  36 , and the canister internal pressure Pcan which is rapidly increased by inflow of the fuel evaporative gas from the fuel tank  21  is quickly returned to the atmospheric pressure. These operations are preliminary arrangements for the leak test by the detection method B. The purge valve  36  is closed after reduction in the canister internal pressure Pcan, an operation of the negative pressure pump  34   c  is started, and the canister internal pressure Pcan reduces stepwise again due to the negative pressure which occurs by the reference orifice  34   g.    
     Subsequently, when the state shifts to a time period  5 B from the time period  4 B, the changeover valve  34   e  is closed, and the bypass valve  37  is closed. As a result, the test condition by the detection method B in which all the valves (the sealing valve  35 , the changeover valve  34   e,  the bypass valve  37  and the purge valve  36 ) are closed is satisfied. By closing of the changeover valve  34   e,  the sensor  34   h  communicates with the inside of the canister  33 , and the canister internal pressure Pcan is temporarily returned to the atmospheric pressure, and thereafter gradually reduces as the air in the canister  33  is discharged into the atmosphere by the operation of the negative pressure pump  34   c.  The reduction amount of the canister internal pressure Pcan from the atmospheric pressure at the time of the canister  33  being decompressed like this is measured as a canister internal pressure reduction amount ΔP 3 . 
     The state shifts to a time period  6  from the time period  5 B thereafter, the changeover valve  34   e  is opened, and the bypass valve  37  is opened. The canister internal pressure Pcan reduces stepwise again due to the negative pressure which is generated by the reference orifice  34   g.  The canister internal pressure Pcan at this time corresponds to a detection value in a case where the negative pressure pump  34   c  is operated in a leak occurring state by a hole corresponding to the reference orifice  34   g,  and the value is set as the leak determination reference value ΔP 3 ref. 
     Subsequently, the canister internal pressure reduction amount ΔP 3  which is measured in the time period  5 B and the leak determination reference value ΔP 3 ref which is set in the time period  6  are compared, and when the canister internal pressure reduction amount ΔP 3  exceeds the leak determination reference value ΔP 3 ref (reduction of the canister internal pressure Pcan is rapid), it is determined that no leak is present in the canister  33 , whereas when the canister internal pressure reduction amount ΔP 3  is equal to or smaller than the leak determination reference value ΔP 3 ref (reduction of the canister internal pressure Pcan is slow), it is determined that leak is present in the canister  33 . In this case, the entire system of the fuel evaporative gas emission control apparatus  1  is normal, and therefore the former determination of no leak is made. 
     Next, a case where the bypass valve  37  is stuck closed will be described based on  FIG. 6 . 
     A state until the sealing valve  35  is opened after the state shifts to the time period  2  from the time period  1  and the operation of the negative pressure pump  34   c  is started is the same as the state at the system normal time described above. In this case, the air in the fuel tank  21  is not discharged to the atmosphere via the canister  33 , due to sticking to closure of the bypass valve  37 , and therefore, the tank internal pressure Ptan is kept at an initial value (a high-pressure value in soak) without reducing to the valve opening guarantee determination value P 0 . Therefore, at a time point at which the operation continuation time period T becomes the limit time period Tlmt or longer, a valve opening operation of the bypass valve  37  is performed irrespective of the tank internal pressure Ptan (actually already being stuck closed). Since the tank internal pressure Ptan which should be reduced by opening of the sealing valve  35  if the system is normal does not reduce like this, it can be assumed that a certain failure (specifically, sticking to closure of the bypass valve  37  or the sealing valve  35 ) occurs at this point of time, and determination of a failure spot is performed by subsequent processing. 
     The closing operation of the bypass valve  37  is performed based on the operation continuation time period T from the start of the operation of the negative pressure pump  34   c  like this, but instead of this, the opening operation of the bypass valve  37  may be performed at a time point at which an elapsed time from the sealing valve  35  being opened (that is, a time period in which air is actually discharged from the inside of the fuel tank  21 ) becomes the limit time period Tlmt or longer. 
     Thereafter, the state shifts to the time period  3  from the time period  2 , and stop of the negative pressure pump  34   c,  closing of the changeover valve  34   e,  and opening of the bypass valve  37  are sequentially performed. Since the bypass valve  37  is stuck closed, the fuel evaporative gas in the fuel tank  21  does not flow into the canister  33 , the canister internal pressure Pcan does not increase, and therefore the canister internal pressure change amount ΔP 2  is measured to 0. Since the canister internal pressure change amount ΔP 2  is the change amount determination value ΔP 2 ref or smaller, it is determined that presence or absence of leak in the fuel tank  21 , the purge pipe  31  and the vapor pipe  32  is unclear, and a detection method A that sets the target of the leak test as the entire system of the fuel evaporative gas emission control apparatus  1  is selected. 
     When the state shifts to a time period  4 A from the time period  3 , a leak test by the detection method A is started, the changeover valve  34   e  is opened first, and the purge valve  36  is opened. Unlike the normal time of the system described above, the canister internal pressure Pcan is originally at the atmospheric pressure due to sticking to closure of the bypass valve  37 , while the sealing valve  35  is opened by selection of the detection method A, and therefore, the tank internal pressure Ptan reduces with opening of the purge valve  36  (by a reduction amount corresponding to the valve opening time period of the purge valve  36 ). 
     Since reduction of the tank internal pressure Ptan at this time means that the sealing valve  35  is normally opened, a factor that does not reduce the tank internal pressure Ptan which should be reduced due to opening of the sealing valve  35  in the above described time period  2  can be assumed to be closure of the bypass valve  37  which should be operated to be opened, and at this point of time, it is determined that the bypass valve  37  is stuck closed (a leak test execution unit). Thereafter, the purge valve  36  is closed, reduction of the tank internal pressure Ptan is stopped, an operation of the negative pressure pump  34   c  is started, the canister internal pressure Pcan reduces stepwise again by the negative pressure which is generated by the reference orifice  34   g,  and the canister internal pressure Pcan at this time is set as the leak determination reference value ΔP 3 ref. 
     When the state shifts to a time period  5 A from the time period  4 A, the changeover valve  34   e  is closed. As a result, a condition by the detection method A in which the sealing valve  35  is opened, the changeover valve  34   e  is closed, the bypass valve  37  is opened, and the purge valve  36  is closed is satisfied. The canister internal pressure 
     Pcan is temporarily returned to the atmospheric pressure and thereafter gradually reduces by the operation of the negative pressure pump  34   c,  and the reduction amount ΔP 3  of the canister internal pressure Pcan at this time is measured. When the canister internal pressure reduction amount ΔP 3  exceeds the leak determination reference value ΔP 3 ref, it is determined that no leak is present in the fuel tank  21 , the canister  33 , the purge pipe  31  and the vapor pipe  32 , and when the canister internal pressure reduction amount ΔP 3  is the leak determination reference value ΔP 3 ref or smaller, it is determined that leak is present in any of the fuel tank  21 , the canister  33 , the purge pipe  31  and the vapor pipe  32 . 
     A behavior of the tank internal pressure Ptan in reduction of the canister internal pressure Pcan in the time period  5 A like this becomes responsive to presence or absence of sticking to closure of the bypass valve  37 . That is, when the bypass valve  37  is normally opened, the tank internal pressure Ptan also reduces with the canister internal pressure Pcan by communication between the fuel tank  21  and the canister  33  via the purge pipe  31  and the vapor pipe  32 . In contrast to this, when the bypass valve  37  is stuck closed, the fuel tank  21  and the canister  33  are shut off from each other. Therefore, only the canister internal pressure Pcan reduces, and the tank internal pressure Ptan is kept at a fixed value without reducing as shown in the drawing. 
     Accordingly, when the canister internal pressure Pcan rapidly reduces (ΔP 3 &gt;ΔP 3 ref), and the tank internal pressure Ptan also rapidly reduces with the canister internal pressure Pcan, the bypass valve  37  can be regarded as normal (no sticking to closure), whereas when the tank internal pressure Ptan does not reduce or reduction thereof is slow, although the canister internal pressure Pcan rapidly reduces, the bypass valve  37  can be regarded as being stuck closed, and the determination corresponds to the determination result of sticking to closure of the bypass valve  37  described above. 
     Next, a case where the sealing valve  35  is stuck closed will be described based on  FIG. 7 . 
     A state until the time point at which the state shifts to the time period  3  from the time period  2 , and stop of the negative pressure pump  34   c,  closing of the changeover valve  34   e  and opening of the bypass valve  37  are sequentially performed is similar to the state at the time of sticking to closure of the bypass valve  37  described above. Since the sealing valve  35  is stuck closed in this case, the fuel evaporative gas in the fuel tank  21  does not flow into the canister  33  even if the bypass valve  37  is opened, and therefore, canister internal pressure change amount ΔP 2  is measured to 0. Consequently, the canister internal pressure change amount ΔP 2  is regarded as the change amount determination value ΔP 2 ref or smaller, and the detection method A which makes the target of the leak test the entire system of the fuel evaporative gas emission control apparatus  1  is selected. 
     When the state shifts to the time period  4 A from the time period  3 , the leak test by the detection method A is started, the changeover valve  34   e  is opened first, and the purge valve  36  is opened. An opening operation of the sealing valve  35  is performed by selection of the detection method A, but the sealing valve  35  is actually stuck closed. Therefore, unlike the time of sticking to closure of the bypass valve  37  described above, the tank internal pressure is kept at a fixed value without reducing. That is, on the ground that the tank internal pressure Ptan does not reduce, it is determined that the sealing valve  35  is stuck closed at this point of time (the leak test execution unit). 
     Thereafter, as in the above description, the purge valve  36  is closed, an operation of the negative pressure pump  34   c  is started, the canister internal pressure Pcan reduces stepwise again by the negative pressure which is generated by the reference orifice  34   g  and is set as the leak determination reference value ΔP 3 ref. When the state shifts to the time period  5 A from the time period  4 A, the changeover valve  34   e  is closed, the test condition by the detection method A is satisfied, and the reduction amount ΔP 3  of the canister internal pressure Pcan which gradually reduces by the negative pressure pump  34   c  is measured. It is similar to the above description that leak is determined as absent in the fuel tank  21 , the canister  33 , the purge pipe  31  and the vapor pipe  32  when the canister internal pressure reduction amount ΔP 3  exceeds the leak determination reference value ΔP 3 ref, and leak is determined as present in any of them, when the canister internal pressure reduction amount ΔP 3  is the leak determination reference value ΔP 3 ref or smaller. 
     As above, according to the fuel evaporative gas emission control apparatus  1  of the present embodiment, when the leak test for the purge pipe  31  and the vapor pipe  32  as the targets is executed, the sealing valve  35  which is being closed is opened while the valve open state of the bypass valve  37  is kept prior to the execution of the leak test, whereby the fuel evaporative gas in the fuel tank  21  is caused to flow out to the canister  33  side to reduce the tank internal pressure Ptan to the valve opening guarantee determination value P 0 , and after the bypass valve  37  is closed thereafter, the leak test is started. Consequently, during the leak test, opening of the bypass valve  37  is executed under the situation where the tank internal pressure Ptan is reduced to the valve opening guarantee determination value P 0 , and the bypass valve  37  can be opened at a suitable timing without causing a delay. 
     Describing more specifically, the leak test on the purge pipe  31  and the vapor pipe  32  is carried out in sequence of measurement of the tank internal pressure decompression amount ΔP 1  after closing the bypass valve  37 , closing of the changeover valve  34   e,  opening of the bypass valve  37 , and measurement of the canister internal pressure change amount ΔP 2 , and from these measurement results, presence or absence of leak is determined. When a delay occurs to opening of the bypass valve  37  at this time, the canister internal pressure Pcan does not rapidly increase although no leak is present in the purge pipe  31  and the vapor pipe  32 , and therefore it is erroneously determined that leak is present based on the canister internal pressure change amount ΔP 2 ≦the change amount determination value ΔP 2 ref. As a result, the detection method A which extends the target of the subsequent leak test to the entire system is erroneously selected, but a situation like this can be prevented by the processing of reducing the tank internal pressure Ptan described above. 
     Further, in the leak tests by the detection methods A and B, presence or absence of leak is determined based on the canister internal pressure reduction amount ΔP 3  at the time of the inside of the canister  33  being decompressed by operating the negative pressure pump  34   c,  and the leak test on the purge pipe  31  and the vapor pipe  32  is executed in parallel with a warm-up of the negative pressure pump  34   c  prior to this. Since two different kinds of processing are carried out in parallel like this, a required time period of the entire leak monitoring can be significantly reduced. 
     Further, when the tank internal pressure Ptan does not reduce to the valve opening guarantee determination value P 0  due to sticking to closure of the bypass valve  37  and the sealing valve  35 , the bypass valve  37  is closed irrespective of the tank internal pressure Ptan, at the time point at which the operation continuation time period T becomes the limit time period Tlmt or longer (steps S 12  and  14  in  FIG. 4 ). Consequently, in such a case, leak monitoring can be completed by executing the processing of subsequent step S 18  and the following steps, system abnormality can be grasped at this point of time, the suitable leak test of the detection method A corresponding to this is selected, and the cause of the failure (sticking to closure of the bypass valve  37  or the sealing valve  35 ) can be reliably determined. 
     In the leak test by the detection method A, it is determined whether the cause of the failure is in sticking to closure of the bypass valve  37  or sticking to closure of the sealing valve  35  based on the behavior (being reduced or not) of the tank internal pressure Ptan at the time of opening the purge valve  36 . Since the cause of the failure can be determined by a simple operation like this, this factor also contributes to reduction in the required time period of the entire leak monitoring. 
     The forgoing is the explanation of the embodiment, but the mode of the present invention is not limited to the embodiment. For example, in the above described embodiment, the invention is embodied as the fuel evaporative gas emission control apparatus  1  which is loaded on a hybrid vehicle as a vehicle, but the kind of the vehicle is not limited to this, and the present invention may be applied to a gasoline vehicle, for example. 
     Further, in the above described embodiment, as the leak test which the leak test execution unit executes, the leak test for the purge pipe  31  and the vapor pipe  32  as the targets is executed, but the content thereof is not limited to this, and is arbitrarily changeable as long as the canister opening and closing valve (the bypass valve  37 ) is switched to open from closing during the leak test. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.