Patent Publication Number: US-8967122-B2

Title: Fuel evaporative emission control device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fuel evaporative emission control device, specifically control of operation of the fuel evaporative emission control device. 
     2. Description of the Related Art 
     In a prior-art technique to prevent fuel evaporative gas, produced within a fuel tank, from being emitted to the atmosphere, a fuel tank shutoff valve (sealing valve) is fitted to a passage connecting a fuel tank to a canister to seal the fuel tank, and at the time of filling the fuel tank, the sealing valve is opened to allow fuel evaporative gas to flow from the fuel tank into the canister and become adsorbed within the canister. 
     When the fuel tank is sealed by the sealing valve as in the aforementioned system, an increase in ambient air temperature may lead to a high pressure in the fuel tank because of more fuel evaporating within the fuel tank, which may lead to fuel evaporative gas being emitted to the atmosphere at the time of filling the fuel tank. 
     To prevent fuel evaporative gas from being emitted to the atmosphere at the time of filling the fuel tank, the sealing valve is opened upon detecting filling operations, and opening the fuel tank is inhibited until the pressure in the fuel tank decreases to a sufficiently low level. 
     However, it takes long for the pressure in the fuel tank to decrease to a desired level, and thus, it takes long before filling can be started. 
     To cope with this problem, a technique has been developed in which when the pressure in the fuel tank increases, if the engine is running and purge is being conducted, the sealing valve is opened to emit high-pressure fuel evaporative gas from the fuel tank into the intake passage of the engine, without letting them be adsorbed in the canister, thereby reducing the pressure in the fuel tank (JP 4110932 B2). 
     In the fuel evaporative gas management device in the aforementioned publication, if the pressure in the fuel tank increases to a high level while the engine is running, the sealing valve is opened and high-pressure fuel evaporative gas are directed from the fuel tank to the intake passage, and when the engine stops, the sealing valve is closed and purge is stopped. The manipulations of the sealing valve and the purge actions are thus synchronized. 
     When the manipulations of the sealing valve and the purge actions are synchronized, and thus, the purge is stopped at the same time that the sealing valve is closed, it follows that highly-concentrated fuel evaporative gas remain in the passage between the sealing valve and a purge control valve provided for control of purge. 
     If the engine is started and purge is resumed in this situation, the highly-concentrated fuel evaporative gas remaining in the passage is emitted into the intake passage. This is undesirable because it causes variations in air-fuel ratio of the intake air-fuel mixture drawn into the engine, which lead to variations in engine output and worse emissions. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a fuel evaporative emission control device capable of suppressing variations in air-fuel ratio of the mixture drawn into the internal combustion engine, caused by fuel evaporative gas. 
     To achieve the above object, the present invention provides a fuel evaporative emission control device, comprising a connecting passage connecting an intake passage of an internal combustion engine and a fuel tank, a canister for adsorbing fuel evaporative gas incoming through the connecting passage, a connecting passage opening/closing unit switchable between an open and a closed positions to allow or block flow from the connecting passage to the intake passage, a canister opening/closing unit switchable between an open and a closed positions to allow or block flow between the canister and the connecting passage, and a tank opening/closing unit switchable between an open and a closed positions to allow or block flow from the fuel tank to the connecting passage, wherein the fuel evaporative emission control device conducts conducting connecting-passage purge to purge the connecting passage by putting the connecting passage opening/closing unit in the open position, the canister opening/closing unit in the closed position and the tank opening/closing unit in the closed position, conducts canister purge to purge the canister by putting the connecting passage opening/closing unit in the open position, the canister opening/closing unit in the open position and the tank opening/closing unit in the closed position, and conducts fuel-tank purge to purge the fuel tank by putting the connecting passage opening/closing unit in the open position, the canister opening/closing unit in the closed position and the tank opening/closing unit in the open position, wherein after finishing the fuel-tank purge, the evaporative emission control device conducts the connecting-passage purge for a first predetermined time and then conducts the canister purge for a second predetermined time. 
     As stated above, after the fuel-tank purge is finished, the connecting-passage purge is conducted for the first predetermined time and then the canister purge is conducted for the second predetermined time. 
     In the fuel-tank purge, fuel evaporative gas is emitted from the fuel tank into the intake passage of the internal combustion engine via the connecting passage. At the time that the fuel-tank purge is finished, fuel evaporative gas not reaching the intake passage but remaining in the connecting passage may form a pressure higher than the atmospheric pressure. Thus, by conducting the connecting-passage purge for the first predetermined time, fuel evaporative gas remaining in the connecting passage is emitted into the intake passage, preliminarily, to stabilize the pressure in the connecting passage at the atmospheric pressure. After the pressure in the connecting passage is reduced to the atmospheric pressure, the canister purge is conducted for the second predetermined time so that not only fuel evaporative gas remaining in the connecting passage but also fuel evaporative gas present in the canister in the form of being adsorbed on an adsorbent can be emitted into the intake passage. 
     Fuel evaporative gas is thus prevented from remaining in the connecting passage and the canister. As a result, in the next purging of the canister, emission of highly-concentrated fuel evaporative gas into the intake passage is prevented, and thus, abrupt change in air-fuel ratio of the mixture drawn into the internal combustion engine is prevented. 
    
    
     
       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 diagram schematically showing the configuration of a fuel evaporative emission control device according to the present invention; 
         FIG. 2  is a diagram showing a sequence of high-pressure purge control actions of the fuel evaporative emission control device according to the present invention; 
         FIG. 3  is a diagram schematically showing operating positions of valves at times (a), (b) and (h) in  FIG. 2 ; 
         FIG. 4  is a diagram schematically showing operating positions of valves at time (c) in  FIG. 2 ; 
         FIG. 5  is a diagram schematically showing operating positions of valves at times (d) and (e) in  FIG. 2 ; 
         FIG. 6  is a diagram schematically showing operating positions of valves at time (f) in  FIG. 2 ; and 
         FIG. 7  is a diagram schematically showing operating positions of valves at time (g) in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings attached, a fuel evaporative emission control device according to the present invention will be described below. 
       FIG. 1  is a diagram schematically showing the configuration of a fuel evaporative emission control device according to the present invention. Now the configuration of the fuel evaporative emission control device according to the present invention will be described. 
     As seen in  FIG. 1 , the fuel evaporative emission control device according to the present invention, which performs general control of the vehicle by controlling, roughly speaking, an engine (internal combustions engine)  10 , a fuel storage unit  20  for holding fuel and a fuel evaporative gas management unit  30  for managing fuel evaporative gas produced in the fuel storage unit  20 , all mounted on the vehicle, comprises an electronic control unit (hereinafter referred to as “ECU”)  50  including an input-output device, memory (including ROM, RAM and non-volatile RAM), a central processing unit (CPU) and others, a fuel filler lid opening/closing switch  61  for opening and closing a fuel filler lid  23  of the vehicle, and a fuel filler lid sensor  62  for detecting position of the fuel filler lid  23 . 
     The engine  10  is a multi-point injection (MPI) four-cycle inline four-cylinder gasoline engine. The engine  10  has an intake passage  11  through which air is drawn into combustion chambers of the engine  10 . An intake pressure sensor  14  is fitted to the intake passage  11  to detect internal pressure in the intake passage  11 . Downstream of the intake passage  11 , fuel injection valves  12  are provided to inject fuel to intake ports of the engine  10 . The fuel injection valves  12  are connected to fuel piping  13 , through which fuel is sent to them. 
     The fuel storage unit  20  comprises a fuel tank  21  to hold fuel, a fuel filler opening  22  through which fuel is put into the fuel tank  21 , a fuel filler lid  23  fitted to the vehicle body to close the fuel filler opening  22 , a fuel pump  24  to send fuel from the fuel tank  21  to the fuel injection valves  12  through the fuel piping  13 , a pressure sensor  25  for detecting pressure in the fuel tank  21 , a fuel cut-off valve  26  for preventing fuel from flowing from the fuel tank  21  to the fuel evaporative gas management unit  30  by action of a float valve incorporated therein, not shown, and a leveling valve  27  to control liquid surface in the fuel tank  21  when filling the fuel tank. Fuel evaporative gas, produced within the fuel tank  21 , is emitted from the fuel tank  21  via the fuel cut-off valve  26  and the leveling valve  27 . 
     The fuel evaporative gas management unit  30  comprises a canister  31 , a vapor solenoid valve (canister opening/closing unit)  32 , a fuel tank shutoff valve (tank opening/closing unit)  33 , a safety valve  34 , an air filter  35 , a purge control valve (connecting passage opening/closing unit)  37 , vapor piping (connecting passage)  38 , and purge piping (connecting passage)  39 . 
     The canister  31  holds activated carbon inside. The canister  31  has a vapor port  31   a  through which fuel evaporative gas from the fuel tank  21  can flow in and fuel evaporative gas, adsorbed on the activated carbon, can flow out. The canister  31  also has an ambient air inlet  31   b  to draw in ambient air to cause fuel evaporative gas to be released from the activated carbon and emitted from the canister  31 . Upstream of the ambient air inlet  31   b , an air filter  35  is arranged with its contaminants-entry prevention side directed to the atmosphere and the opposite side directed to the ambient air inlet  31   b.    
     The vapor solenoid valve  32  has a canister-connected port  32   a  connected to the vapor port  31   a  of the canister  31 . The vapor solenoid valve  32  further has a vapor piping-connected port  32   b  connected to the vapor piping  38 , and a purge piping-connected port  32   c  connected to the purge piping  39 . The vapor piping  38  is connected to the leveling valve  27  of the fuel tank  21 , and the purge piping  39  is connected to the intake passage  11  of the engine  10 . The vapor solenoid valve  32  is a normally-closed solenoid valve which is closed while a solenoid is not activated, and open while the solenoid is activated externally by drive signal. While the solenoid is activated externally by drive signal, the vapor solenoid valve  32  in the open position keeps the canister-connected port  32   a , the vapor piping-connected port  32   b  and the purge piping-connected port  32   c  open, so that fuel evaporative gas can flow in and out the canister  31 , and ambient air, drawn in through the air filter  35 , can flow in the vapor piping  32  and the purge piping  39 . While the solenoid is not activated, the vapor solenoid valve  32  in the closed position keeps only the vapor piping-connected port  32   b  and the purge piping-connected port  32   c  open, and blocks the canister-connected port  32   a , thereby inhibiting fuel evaporative gas from flowing in and out the canister  31  and inhibiting ambient air from flowing in the vapor piping  38  and purge piping  39  via the air filter  35 . In other words, while in the closed position, the vapor solenoid valve  32  seals the canister  31 , and while in the open position, it keeps the canister  31  open. 
     The fuel tank shutoff valve  33  is fitted to the vapor piping  38 . The fuel tank shutoff valve  33  is a normally-closed solenoid valve which is closed while a solenoid is not activated, and open while the solenoid is activated externally by drive signal. While the solenoid is not activated, the fuel tank shutoff valve  33  in the closed position blocks the vapor piping  38 . While the solenoid is activated externally by drive signal, the fuel tank shutoff valve  33  in the open position allows flow in the vapor piping  38 . In other words, while in the closed position, the fuel tank shutoff valve  33  seals the fuel tank  21  so that fuel evaporative gas, produced in the fuel tank  21 , cannot flow out the fuel tank  21 , and while in the open position, it allows fuel evaporative gas to flow from the fuel tank  21  to the canister  31 . 
     The safety valve  34  is fitted to the vapor piping  38 , in parallel with the fuel tank shutoff valve  33 . The safety valve  34  opens when the pressure in the fuel tank  21  increases to a preset level or higher, thereby allowing fuel evaporative gas to flow to the canister  31  to prevent explosion of the fuel tank  21 . 
     The purge control valve  37  is fitted to the purge piping  39 , between the intake passage  11  of the engine  10  and the vapor solenoid valve  32 . The purge control valve  37  is a normally-closed solenoid valve which is closed while a solenoid is not activated, and open while the solenoid is activated externally by drive signal. While the solenoid is not activated, the purge control valve  37  in the closed position blocks the purge piping  39 . While the solenoid is activated externally by drive signal, the purge control valve  37  in the open position allows flow in the purge piping  39 . In other words, while in the closed position, the purge control valve  37  inhibits fuel evaporative gas from flowing from the fuel evaporative gas management unit  30  to the engine  10 , and while in the open position, it allows fuel evaporative gas to flow from the fuel evaporative gas management unit  30  to the engine  10 . 
     The ECU  50  is a control unit performing general control of the vehicle, and comprises an input-output device, memory (including ROM, RAM and non-volatile RAM), a central processing unit (CPU), a timer and others. 
     To the input of the ECU  50  are connected the intake pressure sensor  14 , the pressure sensor  25 , the fuel filler lid opening/closing switch  61  for opening and closing the fuel filler lid  23  fitted to the vehicle, and the fuel filler lid sensor  62  for detecting position of the fuel filler lid  23 . The ECU  50  thus receives information from these sensors. 
     To the output of the ECU  50  are connected the fuel injection valves  12 , the fuel pump  24 , the vapor solenoid valve  32 , the fuel tank shutoff valve  33  and the purge control valve  37 . 
     On the basis of information from the sensors, the ECU  50  controls operation of the vapor solenoid valve  32 , the fuel tank shutoff valve  33  and the purge control valve  37 ; pressure in the fuel tank  21 , pressure in the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37 ; and flow of fuel evaporative gas, including adsorption within the canister  31  and emission from the canister  31  into the intake passage  11  of the engine  10 . 
     Next, high-pressure purge control performed by the ECU  50  of the present invention described above to cause fuel evaporative gas to flow from the fuel tank  21  to the intake passage  11  of the engine  10  when internal pressure in the fuel tank  21  reaches a high level, thereby reducing the internal pressure in the fuel tank  21  will be described. 
       FIG. 2  shows the sequence of high-pressure purge control actions of the fuel evaporative emission control device according to the present invention.  FIG. 2  shows, from the top downward, control modes, pressures, a high-pressure determination timer TM 1 , a fuel tank high-pressure flag FL 1 , a normal control flag FL 2 , a high-pressure purge start control flag FL 3 , a high-pressure control flag FL 4 , a high-pressure purge finish control flag FL 5 , a high-pressure start timer TM 2 , accumulated volume in high-pressure purge finishing phase, fuel tank shutoff valve  33  operating position, vapor solenoid valve  32  operating position, an engine operation demand flag FL 6 , a purge inhibition flag FL 7 , a purge control flag FL 8 , engine rotating speed, and purge flow rate. The control modes in  FIG. 2  are modes of the high-pressure purge control. The pressures shown in  FIG. 2  are fuel tank  21  internal pressure and piping internal pressure, or pressure in the vapor piping  38  and purge piping  39 . P 1  is a first predetermined pressure and P 2  a second predetermined pressure. The purge inhibition flag FL 7  in  FIG. 2  indicates whether to activate the purge control valve  37 . The purge inhibition flag FL 7  being “ON” indicates that the purge control valve  37  should be closed, and its being “OFF” indicates that the purge control valve  37  should be open. Also the purge control flag FL 8  in  FIG. 2  indicates whether to activate the purge control valve  37 . The purge control flag FL 8  being “ON” indicates that the purge control valve  37  should be open, and its being “OFF” indicates that the purge control valve  37  should be closed. Between the purge inhibition flag FL 7  and the purge control flag FL 8 , preference is given to the former. In  FIG. 2 , t 1  indicates a first predetermined time length, t 2  a second predetermined time length, iv 1  a first predetermined volume, iv 2  a second predetermined volume, and Ne 1  a predetermined speed.  FIGS. 3 to 7  are schematic diagrams showing what operating position each valve is in, at times (a) to (h) in  FIG. 2 , respectively. 
     As seen from  FIG. 2 , the high-pressure purge control, provided to reduce the internal pressure in the fuel tank  21  when it reaches a high level, is broadly divided into four modes: a normal control mode, a start control mode, a high-pressure purge control mode and a finish control mode. In the normal control mode, normal purge actions, including emission of fuel evaporative gas, adsorbed within the canister  31 , from the canister  31  into the intake passage  11 , are performed depending on the vehicle operating state. In the start control mode, the piping internal pressure, or internal pressure in the vapor piping  38  and purge piping  39  between the fuel tank  21  and the purge control valve  37  is regulated in order to perform high-pressure purge because of high internal pressure in the fuel tank  21 . In the high-pressure purge control mode, the internal pressure in the fuel tank  21  is reduced by emitting fuel evaporative gas from the fuel tank  21  into the intake passage  11  via the vapor piping  38  and purge piping  39  (fuel-tank purge). In the finish control mode, fuel evaporative gas remaining in the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37  are emitted into the intake passage  11  (connecting-passage purge), and in addition to this connecting passage purge, fuel evaporative gas present in the canister  31  in the form of being adsorbed on the activated carbon are emitted into the intake passage  11  (canister purge). Next, with reference to  FIG. 2 , control actions will be described in chronological order. 
     As seen at time (a) in  FIG. 2 , normally the normal control flag FL 2  is “ON” and normal purge actions are performed depending on the vehicle operating state. In the case of  FIG. 2  given by way of example, at time (a), the engine  10  is at rest, the fuel tank shutoff valve  33  and the purge control valve  37  are closed, and the vapor solenoid valve  32  is open, as seen in  FIG. 3 . When the internal pressure in the fuel tank  21 , detected by the pressure sensor  25 , increases to the first predetermined pressure P 1  or above as a result of more fuel evaporating within the fuel tank  21 , the high-pressure determination timer TM 1  is started to count up. If the internal pressure in the fuel tank  21  decreases below the first predetermined pressure P 1 , the high-pressure determination timer TM 1  is reset to “0”. 
     If the internal pressure in the fuel tank  21  is continuously at or above the first predetermined pressure P 1  so that the value in the high-pressure determination timer TM 1  reaches the first predetermined time length t 1  as seen at time (b) in  FIG. 2 , it is determined that the internal pressure in the fuel tank  21  is high, and the fuel tank high-pressure flag FL 1  is set to “ON”. In addition, the normal control flag FL 2  is set to “OFF” and the high-pressure purge start control flag FL 3  is set to “ON”, and the high-pressure purge control enters the start control mode. In the start control mode, first, the engine operation demand flag FL 6  is set to “ON” and the engine  10  is started if it is at rest, and at the same time, the purge inhibition flag FL 7  is set to “ON” and the purge control valve  37  is closed if it is open. 
     Then, when the engine rotating speed increases to the predetermined speed Ne 1  or above as seen at time (c) in  FIG. 2 , the fuel tank shutoff valve  33  is opened, and at the same time, the vapor solenoid valve  32  is closed, as seen in  FIG. 4 . As a result, high-pressure fuel evaporative gas is emitted from the fuel tank  21  into the vapor piping  38  and purge piping  39  and spread up to the purge control valve  37 . At the same time, the high-pressure start timer TM 2  is started to count up. The vapor solenoid valve  32  is closed so that the fuel evaporative gas emitted will not become adsorbed on the activated carbon in the canister  31 . 
     When the value in the high-pressure start timer TM 2  reaches the second predetermined time length t 2  or above as seen at time (d) in  FIG. 2 , the high-pressure purge start control flag FL 3  is set to “OFF”, the high-pressure control flag FL 4  is set to “ON”, and the high-pressure purge control enters the high-pressure purge control mode. In the high-pressure purge control mode, the purge inhibition flag FL 7  is set to “OFF”, the purge control flag FL 8  is set to “ON”, and the purge control valve  37  is opened to allow flow from the fuel tank  21  to the intake passage  11  as seen in  FIG. 5 . As a result, high-pressure fuel evaporative gas is emitted from the fuel tank  21  into the intake passage  11 . The second predetermined time length t 2  is the time taken for the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37  to reach the same internal pressure as the fuel tank  21 , which is obtained in advance experimentally or otherwise. Thus, now that the piping internal pressure, or internal pressure in the vapor piping  38  and purge piping  39  is equal to the internal pressure in the fuel tank  21 , the purge flow rate, or flow rate of fuel evaporative gas emitted into the intake passage  11  is calculated from the internal pressure in the fuel tank  21 , detected by the pressure sensor  25 , the pressure in the intake passage  11 , detected by the intake pressure sensor  14 , and how much the purge control valve  37  is open. 
     Then, when the internal pressure in the fuel tank  21  decreases to the second predetermined pressure P 2  or below as a result of emitting fuel evaporative gas from the fuel tank  21  into the intake passage  11 , as seen at time (e) in  FIG. 2 , the high-pressure determination timer TM 1  is started to count down from the first predetermined time length t 1 . 
     Then, as seen at time (f) in  FIG. 2 , when the value in the high-pressure determination timer TM 1  reaches “0” while the internal pressure in the fuel tank  21  is continuously at or below the second predetermined pressure P 2 , it is determined that the internal pressure in the fuel tank  21  has decreased, and the fuel tank high-pressure flag FL 1  is set to “OFF”. In addition, the high-pressure control flag FL 4  is set to “OFF”, the high-pressure purge finish control flag FL 5  is set to “ON”, and the high-pressure purge control enters the finish control mode. In the finish control mode, first, the fuel tank shutoff valve  33  is closed as seen in  FIG. 6 , and calculation of accumulated volume in high-pressure purge finishing phase, or accumulated volume of fuel evaporative gas, or air containing gaseous fuel purged via the vapor piping  38  and purge piping  39  after the fuel tank shutoff valve  33  is closed is started. 
     The way of calculating the accumulated volume in high-pressure purge finishing phase is as follows: at the time that the high-pressure purge control enters the finish control mode, the internal pressure P(n) in the vapor piping  38  and purge piping  39  is equal to the internal pressure in the fuel tank  21 . The purge flow rate ΔQ is calculated at regular intervals from the internal pressure P(n) in the vapor piping  38  and purge piping  39 , and the pressure in the intake passage  11 , detected by the intake sensor  14 . The accumulated volume in high-pressure purge finishing phase is calculated from the purge flow rate ΔQ calculated this way. More specifically, the volume ΔV of air purged, or drawn from the vapor piping  38  and purge piping  39  into the intake passage  11  during time ΔT is calculated from the purge flow rate ΔQ (the initial purge flow rate is calculated from the internal pressure P in the vapor piping  38  and purge piping  39  and the pressure in the intake passage  11 , detected by the intake pressure sensor  14 ) and time ΔT by expression (1) below:
 
Δ V=ΔQ×ΔT    (1)
 
     The volume V(n) of air in the vapor piping  38  and purge piping  39  after time ΔT of purging is calculated from the volume V(n−1) of air in the vapor piping  38  and purge piping  39  calculated last time (the initial volume of air in the vapor piping  38  and purge piping  39  is the inner volume V of the vapor piping  38  and purge piping  39 ) and the volume ΔV of air purged during time ΔT, by expression (2) below:
 
 V ( n )= V ( n− 1)−Δ V    (2)
 
     The internal pressure P(n) in the vapor piping  38  and purge piping  39  after time ΔT of purging is calculated from the internal pressure P in the vapor piping  38  and purge piping  39  at the time that the high-pressure purge control enters the finish control mode, the inner volume V of the vapor piping  38  and purge piping  39 , and the volume of air V(n) in the vapor piping  38  and purge piping  39  after time ΔT of purging, by expression (3) below:
 
 P ( n )= P×V/V ( n )   (3)
 
     The accumulated volume in high-pressure purge finishing phase is calculated by summing the volumes ΔV of air purged during each interval. 
     Then, when the accumulated volume in high-pressure purge finishing phase reaches the second predetermined volume iv 2  or above as seen at time (g) in  FIG. 2  (the time between time (f) and time (g) in  FIG. 2  is the “first predetermined time” in claims), the vapor solenoid valve  32  is opened as seen in  FIG. 7 . The second predetermined volume iv 2  is registered as the time taken for the internal pressure in the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37  to decrease to the atmospheric pressure. The relation between approximate accumulated volume and time taken for the internal pressure in the vapor piping  38  and purge piping  39  to decrease to the atmospheric pressure is obtained in advance experimentally or otherwise, and stored in the form of a map in the ECU  50 . The time taken for the internal pressure in the vapor piping  38  and purge piping  39  to decrease to the atmospheric pressure in each situation is obtained from the map depending on the purge flow rate calculated from the internal pressure P(n) in the vapor piping  38  and purge piping  39  and the pressure in the intake passage  11 , detected by the intake pressure sensor  14 . 
     Then, when the accumulated volume in high-pressure purge finishing phase reaches the first predetermined volume iv 1  or above as seen at time (h) in  FIG. 2  (the time between time (g) and time (h) in  FIG. 2  is the “second predetermined time” in claims), the high-pressure purge finish control flag FL 5  is set to “OFF”, the normal control flag FL 2  is set to “ON” and the high-pressure purge control returns to the normal control mode. In the normal control mode, the purge control flag FL 8  is set to “OFF” and the purge control valve  37  is closed as seen in  FIG. 3 . In addition, the engine operation demand flag FL 6  is set to “OFF” and the engine  10  is stopped. The first predetermined volume iv 1  is at least the inner volume of the vapor piping  38  and purge piping  39  added to the second predetermined volume iv 2 . The first predetermined volume iv 1  may be the inner volume of the canister  31  further added to the above two volumes. 
     As stated above, in the fuel evaporative emission control device according to the present invention, if the internal pressure in the fuel tank  21  increases to a high level, specifically the first predetermined pressure P 1  or above (time (a) in  FIG. 2 ) and is continuously at such high level over the first predetermined time length t 1 , the high-pressure purge control enters the start control mode, so that the engine  10  is started and the purge control valve  37  is closed (time (b) in  FIG. 2 ). Then, when the rotating speed of the engine  10  reaches the predetermined speed Net, the fuel tank shutoff valve  33  is opened and the vapor solenoid valve  32  is closed, and at the same time, the high-pressure start timer TM 2  is started to count up (time (c) in  FIG. 2 ). Then, when the value in the high-pressure start timer TM 2  reaches the second predetermined time length t 2 , the high-pressure purge control enters the high-pressure purge control mode, so that the purge control valve  37  is opened (time (d) in  FIG. 2 ). The second predetermined time length t 2  is the time taken for the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37  to reach the same internal pressure as the fuel tank  21 , which is obtained in advance experimentally or otherwise. Then, when the internal pressure in the fuel tank  21  decreases to the second predetermined pressure P 2  or below, the high-pressure determination timer TM 1  is started to count down from the first predetermined time length t 1  (time (e) in  FIG. 2 ). Then, when the value in the high-pressure determination timer TM 1  reaches “0”, the high-pressure purge control enters the finish control mode, so that the fuel tank shutoff valve  33  is closed, and calculation of accumulated volume in high-pressure purge finishing phase, or accumulated volume of fuel evaporative gas purged after the fuel tank shutoff valve  33  is closed is started (time (f) in  FIG. 2 ). Then, when the accumulated volume in high-pressure purge finishing phase reaches the second predetermined volume iv 2  or above, the vapor solenoid valve  32  is opened (time (g) in  FIG. 2 ). The second predetermined volume iv 2  is the volume to be purged for the internal pressure in the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37  to decrease to the atmospheric pressure (101.3 kPa). Then, when the accumulated volume in high-pressure purge finishing phase reaches the first predetermined volume iv 1  or above, the high-pressure purge control returns to the normal control mode, so that the purge control valve  37  is closed and the engine  10  is stopped. The first predetermined volume iv 1  is at least the inner volume of the vapor piping  38  and purge piping  39  up to the purge control valve  37  added to the second predetermined volume iv 2 . 
     In the high-pressure purge control mode, fuel evaporative gas is emitted from the fuel tank  21  into the intake passage  11  of the engine  10  via the vapor piping  38  and purge piping  39 . If the fuel tank shutoff valve  33  and the purge control valve  37  are closed immediately after the high-pressure purge control mode, it may result in the piping internal pressure being higher than the atmospheric pressure, because of fuel evaporative gas not reaching the intake passage  10  of the engine  10  but remaining in the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37 . 
     Thus, after the high-pressure purge control mode, the purge control valve  37  is kept open until the accumulated volume of fuel evaporative gas passing through the purge control valve  37  reaches the second predetermined volume iv 2 . Then, with the purge control valve  37  kept open, the vapor solenoid valve  32  is opened. The purge control valve  37  and the vapor solenoid valve  32  are kept open until the accumulated volume of fuel evaporative gas passing through the purge control valve  37  reaches the first predetermined volume iv 1 . The second predetermined volume iv 2  is the volume to be purged for the pressure in the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37  to decrease to the atmospheric pressure (101.3 kPa), and the first predetermined volume iv 1  is at least the inner volume of the vapor piping  38  and purge piping up to the purge control valve  37  added to the second predetermined volume iv 2 . By manipulating the purge control valve  37  and the vapor solenoid valve  32  in this manner, it is ensured that not only fuel evaporative gas remaining in the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  33  but also fuel evaporative gas present in the canister  31  in the form of being adsorbed on the activated carbon are emitted into the intake passage  11 . As a result, in the next purging of the canister  31 , emission of highly-concentrated fuel evaporative gas from the canister  31  into the intake passage  11  is prevented, and thus, abrupt change in air-fuel ratio of the mixture drawn into the engine  10  is prevented. 
     By preliminary keeping the purge control valve  37  open until the accumulated volume of fuel evaporative gas passing through the purge control valve  37  reaches the second predetermined volume iv 2 , the internal pressure in the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37  decreases to the atmospheric pressure. 
     Then, with the purge control valve  37  kept open, the vapor solenoid valve  32  is opened. This ensures that in addition to fuel evaporative gas remaining in the vapor piping  38  and purge piping  39  between the fuel tank shutoff valve  33  and the purge control valve  37 , fuel evaporative gas existing in the canister  31  in the form of being adsorbed on the activated carbon are emitted into the intake passage  11  of the engine  10 . 
     Although in the above-described embodiment, the tank sealing valve  33  is opened at the same as the vapor solenoid valve  32  is closed, it may be arranged such that first the vapor solenoid valve  32  is closed and thereafter the tank sealing valve  33  is opened.