Fuel evaporative emission control device

When fuel tank internal pressure is at a first predetermined pressure P1 or above over a first predetermined time length t1, a fuel tank shutoff valve is opened and a vapor solenoid valve is closed to make piping internal pressure equal to the fuel tank internal pressure. Then, a purge control valve is opened to emit fuel evaporative gas from the fuel tank into an intake passage. When the fuel tank internal pressure is continuously at a second predetermined pressure P2 or below over the first predetermined time length t1, the fuel tank shutoff valve is closed, and when accumulated volume in high-pressure purge finishing phase reaches a second predetermined volume iv2 or above, the vapor solenoid valve is opened. When the accumulated volume in high-pressure purge finishing phase reaches a first predetermined volume iv1 or above, the purge control valve is opened and the engine is stopped.

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.

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. 1is 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 inFIG. 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 unit20for holding fuel and a fuel evaporative gas management unit30for managing fuel evaporative gas produced in the fuel storage unit20, all mounted on the vehicle, comprises an electronic control unit (hereinafter referred to as “ECU”)50including 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 switch61for opening and closing a fuel filler lid23of the vehicle, and a fuel filler lid sensor62for detecting position of the fuel filler lid23.

The engine10is a multi-point injection (MPI) four-cycle inline four-cylinder gasoline engine. The engine10has an intake passage11through which air is drawn into combustion chambers of the engine10. An intake pressure sensor14is fitted to the intake passage11to detect internal pressure in the intake passage11. Downstream of the intake passage11, fuel injection valves12are provided to inject fuel to intake ports of the engine10. The fuel injection valves12are connected to fuel piping13, through which fuel is sent to them.

The fuel storage unit20comprises a fuel tank21to hold fuel, a fuel filler opening22through which fuel is put into the fuel tank21, a fuel filler lid23fitted to the vehicle body to close the fuel filler opening22, a fuel pump24to send fuel from the fuel tank21to the fuel injection valves12through the fuel piping13, a pressure sensor25for detecting pressure in the fuel tank21, a fuel cut-off valve26for preventing fuel from flowing from the fuel tank21to the fuel evaporative gas management unit30by action of a float valve incorporated therein, not shown, and a leveling valve27to control liquid surface in the fuel tank21when filling the fuel tank. Fuel evaporative gas, produced within the fuel tank21, is emitted from the fuel tank21via the fuel cut-off valve26and the leveling valve27.

The fuel evaporative gas management unit30comprises a canister31, a vapor solenoid valve (canister opening/closing unit)32, a fuel tank shutoff valve (tank opening/closing unit)33, a safety valve34, an air filter35, a purge control valve (connecting passage opening/closing unit)37, vapor piping (connecting passage)38, and purge piping (connecting passage)39.

The canister31holds activated carbon inside. The canister31has a vapor port31athrough which fuel evaporative gas from the fuel tank21can flow in and fuel evaporative gas, adsorbed on the activated carbon, can flow out. The canister31also has an ambient air inlet31bto draw in ambient air to cause fuel evaporative gas to be released from the activated carbon and emitted from the canister31. Upstream of the ambient air inlet31b, an air filter35is arranged with its contaminants-entry prevention side directed to the atmosphere and the opposite side directed to the ambient air inlet31b.

The vapor solenoid valve32has a canister-connected port32aconnected to the vapor port31aof the canister31. The vapor solenoid valve32further has a vapor piping-connected port32bconnected to the vapor piping38, and a purge piping-connected port32cconnected to the purge piping39. The vapor piping38is connected to the leveling valve27of the fuel tank21, and the purge piping39is connected to the intake passage11of the engine10. The vapor solenoid valve32is 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 valve32in the open position keeps the canister-connected port32a, the vapor piping-connected port32band the purge piping-connected port32copen, so that fuel evaporative gas can flow in and out the canister31, and ambient air, drawn in through the air filter35, can flow in the vapor piping32and the purge piping39. While the solenoid is not activated, the vapor solenoid valve32in the closed position keeps only the vapor piping-connected port32band the purge piping-connected port32copen, and blocks the canister-connected port32a, thereby inhibiting fuel evaporative gas from flowing in and out the canister31and inhibiting ambient air from flowing in the vapor piping38and purge piping39via the air filter35. In other words, while in the closed position, the vapor solenoid valve32seals the canister31, and while in the open position, it keeps the canister31open.

The fuel tank shutoff valve33is fitted to the vapor piping38. The fuel tank shutoff valve33is 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 valve33in the closed position blocks the vapor piping38. While the solenoid is activated externally by drive signal, the fuel tank shutoff valve33in the open position allows flow in the vapor piping38. In other words, while in the closed position, the fuel tank shutoff valve33seals the fuel tank21so that fuel evaporative gas, produced in the fuel tank21, cannot flow out the fuel tank21, and while in the open position, it allows fuel evaporative gas to flow from the fuel tank21to the canister31.

The safety valve34is fitted to the vapor piping38, in parallel with the fuel tank shutoff valve33. The safety valve34opens when the pressure in the fuel tank21increases to a preset level or higher, thereby allowing fuel evaporative gas to flow to the canister31to prevent explosion of the fuel tank21.

The purge control valve37is fitted to the purge piping39, between the intake passage11of the engine10and the vapor solenoid valve32. The purge control valve37is 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 valve37in the closed position blocks the purge piping39. While the solenoid is activated externally by drive signal, the purge control valve37in the open position allows flow in the purge piping39. In other words, while in the closed position, the purge control valve37inhibits fuel evaporative gas from flowing from the fuel evaporative gas management unit30to the engine10, and while in the open position, it allows fuel evaporative gas to flow from the fuel evaporative gas management unit30to the engine10.

The ECU50is 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 ECU50are connected the intake pressure sensor14, the pressure sensor25, the fuel filler lid opening/closing switch61for opening and closing the fuel filler lid23fitted to the vehicle, and the fuel filler lid sensor62for detecting position of the fuel filler lid23. The ECU50thus receives information from these sensors.

To the output of the ECU50are connected the fuel injection valves12, the fuel pump24, the vapor solenoid valve32, the fuel tank shutoff valve33and the purge control valve37.

On the basis of information from the sensors, the ECU50controls operation of the vapor solenoid valve32, the fuel tank shutoff valve33and the purge control valve37; pressure in the fuel tank21, pressure in the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37; and flow of fuel evaporative gas, including adsorption within the canister31and emission from the canister31into the intake passage11of the engine10.

Next, high-pressure purge control performed by the ECU50of the present invention described above to cause fuel evaporative gas to flow from the fuel tank21to the intake passage11of the engine10when internal pressure in the fuel tank21reaches a high level, thereby reducing the internal pressure in the fuel tank21will be described.

FIG. 2shows the sequence of high-pressure purge control actions of the fuel evaporative emission control device according to the present invention.FIG. 2shows, from the top downward, control modes, pressures, a high-pressure determination timer TM1, a fuel tank high-pressure flag FL1, a normal control flag FL2, a high-pressure purge start control flag FL3, a high-pressure control flag FL4, a high-pressure purge finish control flag FL5, a high-pressure start timer TM2, accumulated volume in high-pressure purge finishing phase, fuel tank shutoff valve33operating position, vapor solenoid valve32operating position, an engine operation demand flag FL6, a purge inhibition flag FL7, a purge control flag FL8, engine rotating speed, and purge flow rate. The control modes inFIG. 2are modes of the high-pressure purge control. The pressures shown inFIG. 2are fuel tank21internal pressure and piping internal pressure, or pressure in the vapor piping38and purge piping39. P1is a first predetermined pressure and P2a second predetermined pressure. The purge inhibition flag FL7inFIG. 2indicates whether to activate the purge control valve37. The purge inhibition flag FL7being “ON” indicates that the purge control valve37should be closed, and its being “OFF” indicates that the purge control valve37should be open. Also the purge control flag FL8inFIG. 2indicates whether to activate the purge control valve37. The purge control flag FL8being “ON” indicates that the purge control valve37should be open, and its being “OFF” indicates that the purge control valve37should be closed. Between the purge inhibition flag FL7and the purge control flag FL8, preference is given to the former. InFIG. 2, t1indicates a first predetermined time length, t2a second predetermined time length, iv1a first predetermined volume, iv2a second predetermined volume, and Ne1a predetermined speed.FIGS. 3 to 7are schematic diagrams showing what operating position each valve is in, at times (a) to (h) inFIG. 2, respectively.

As seen fromFIG. 2, the high-pressure purge control, provided to reduce the internal pressure in the fuel tank21when 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 canister31, from the canister31into the intake passage11, are performed depending on the vehicle operating state. In the start control mode, the piping internal pressure, or internal pressure in the vapor piping38and purge piping39between the fuel tank21and the purge control valve37is regulated in order to perform high-pressure purge because of high internal pressure in the fuel tank21. In the high-pressure purge control mode, the internal pressure in the fuel tank21is reduced by emitting fuel evaporative gas from the fuel tank21into the intake passage11via the vapor piping38and purge piping39(fuel-tank purge). In the finish control mode, fuel evaporative gas remaining in the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37are emitted into the intake passage11(connecting-passage purge), and in addition to this connecting passage purge, fuel evaporative gas present in the canister31in the form of being adsorbed on the activated carbon are emitted into the intake passage11(canister purge). Next, with reference toFIG. 2, control actions will be described in chronological order.

As seen at time (a) inFIG. 2, normally the normal control flag FL2is “ON” and normal purge actions are performed depending on the vehicle operating state. In the case ofFIG. 2given by way of example, at time (a), the engine10is at rest, the fuel tank shutoff valve33and the purge control valve37are closed, and the vapor solenoid valve32is open, as seen inFIG. 3. When the internal pressure in the fuel tank21, detected by the pressure sensor25, increases to the first predetermined pressure P1or above as a result of more fuel evaporating within the fuel tank21, the high-pressure determination timer TM1is started to count up. If the internal pressure in the fuel tank21decreases below the first predetermined pressure P1, the high-pressure determination timer TM1is reset to “0”.

If the internal pressure in the fuel tank21is continuously at or above the first predetermined pressure P1so that the value in the high-pressure determination timer TM1reaches the first predetermined time length t1as seen at time (b) inFIG. 2, it is determined that the internal pressure in the fuel tank21is high, and the fuel tank high-pressure flag FL1is set to “ON”. In addition, the normal control flag FL2is set to “OFF” and the high-pressure purge start control flag FL3is 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 FL6is set to “ON” and the engine10is started if it is at rest, and at the same time, the purge inhibition flag FL7is set to “ON” and the purge control valve37is closed if it is open.

Then, when the engine rotating speed increases to the predetermined speed Ne1or above as seen at time (c) inFIG. 2, the fuel tank shutoff valve33is opened, and at the same time, the vapor solenoid valve32is closed, as seen inFIG. 4. As a result, high-pressure fuel evaporative gas is emitted from the fuel tank21into the vapor piping38and purge piping39and spread up to the purge control valve37. At the same time, the high-pressure start timer TM2is started to count up. The vapor solenoid valve32is closed so that the fuel evaporative gas emitted will not become adsorbed on the activated carbon in the canister31.

When the value in the high-pressure start timer TM2reaches the second predetermined time length t2or above as seen at time (d) inFIG. 2, the high-pressure purge start control flag FL3is set to “OFF”, the high-pressure control flag FL4is 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 FL7is set to “OFF”, the purge control flag FL8is set to “ON”, and the purge control valve37is opened to allow flow from the fuel tank21to the intake passage11as seen inFIG. 5. As a result, high-pressure fuel evaporative gas is emitted from the fuel tank21into the intake passage11. The second predetermined time length t2is the time taken for the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37to reach the same internal pressure as the fuel tank21, which is obtained in advance experimentally or otherwise. Thus, now that the piping internal pressure, or internal pressure in the vapor piping38and purge piping39is equal to the internal pressure in the fuel tank21, the purge flow rate, or flow rate of fuel evaporative gas emitted into the intake passage11is calculated from the internal pressure in the fuel tank21, detected by the pressure sensor25, the pressure in the intake passage11, detected by the intake pressure sensor14, and how much the purge control valve37is open.

Then, when the internal pressure in the fuel tank21decreases to the second predetermined pressure P2or below as a result of emitting fuel evaporative gas from the fuel tank21into the intake passage11, as seen at time (e) inFIG. 2, the high-pressure determination timer TM1is started to count down from the first predetermined time length t1.

Then, as seen at time (f) inFIG. 2, when the value in the high-pressure determination timer TM1reaches “0” while the internal pressure in the fuel tank21is continuously at or below the second predetermined pressure P2, it is determined that the internal pressure in the fuel tank21has decreased, and the fuel tank high-pressure flag FL1is set to “OFF”. In addition, the high-pressure control flag FL4is set to “OFF”, the high-pressure purge finish control flag FL5is set to “ON”, and the high-pressure purge control enters the finish control mode. In the finish control mode, first, the fuel tank shutoff valve33is closed as seen inFIG. 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 piping38and purge piping39after the fuel tank shutoff valve33is 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 piping38and purge piping39is equal to the internal pressure in the fuel tank21. The purge flow rate ΔQ is calculated at regular intervals from the internal pressure P(n) in the vapor piping38and purge piping39, and the pressure in the intake passage11, detected by the intake sensor14. 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 piping38and purge piping39into the intake passage11during 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 piping38and purge piping39and the pressure in the intake passage11, detected by the intake pressure sensor14) and time ΔT by expression (1) below:
ΔV=ΔQ×ΔT(1)

The volume V(n) of air in the vapor piping38and purge piping39after time ΔT of purging is calculated from the volume V(n−1) of air in the vapor piping38and purge piping39calculated last time (the initial volume of air in the vapor piping38and purge piping39is the inner volume V of the vapor piping38and purge piping39) and the volume ΔV of air purged during time ΔT, by expression (2) below:
V(n)=V(n−1)−ΔV(2)

The internal pressure P(n) in the vapor piping38and purge piping39after time ΔT of purging is calculated from the internal pressure P in the vapor piping38and purge piping39at the time that the high-pressure purge control enters the finish control mode, the inner volume V of the vapor piping38and purge piping39, and the volume of air V(n) in the vapor piping38and purge piping39after 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 iv2or above as seen at time (g) inFIG. 2(the time between time (f) and time (g) inFIG. 2is the “first predetermined time” in claims), the vapor solenoid valve32is opened as seen inFIG. 7. The second predetermined volume iv2is registered as the time taken for the internal pressure in the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37to decrease to the atmospheric pressure. The relation between approximate accumulated volume and time taken for the internal pressure in the vapor piping38and purge piping39to decrease to the atmospheric pressure is obtained in advance experimentally or otherwise, and stored in the form of a map in the ECU50. The time taken for the internal pressure in the vapor piping38and purge piping39to 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 piping38and purge piping39and the pressure in the intake passage11, detected by the intake pressure sensor14.

Then, when the accumulated volume in high-pressure purge finishing phase reaches the first predetermined volume iv1or above as seen at time (h) inFIG. 2(the time between time (g) and time (h) inFIG. 2is the “second predetermined time” in claims), the high-pressure purge finish control flag FL5is set to “OFF”, the normal control flag FL2is set to “ON” and the high-pressure purge control returns to the normal control mode. In the normal control mode, the purge control flag FL8is set to “OFF” and the purge control valve37is closed as seen inFIG. 3. In addition, the engine operation demand flag FL6is set to “OFF” and the engine10is stopped. The first predetermined volume iv1is at least the inner volume of the vapor piping38and purge piping39added to the second predetermined volume iv2. The first predetermined volume iv1may be the inner volume of the canister31further 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 tank21increases to a high level, specifically the first predetermined pressure P1or above (time (a) inFIG. 2) and is continuously at such high level over the first predetermined time length t1, the high-pressure purge control enters the start control mode, so that the engine10is started and the purge control valve37is closed (time (b) inFIG. 2). Then, when the rotating speed of the engine10reaches the predetermined speed Net, the fuel tank shutoff valve33is opened and the vapor solenoid valve32is closed, and at the same time, the high-pressure start timer TM2is started to count up (time (c) inFIG. 2). Then, when the value in the high-pressure start timer TM2reaches the second predetermined time length t2, the high-pressure purge control enters the high-pressure purge control mode, so that the purge control valve37is opened (time (d) inFIG. 2). The second predetermined time length t2is the time taken for the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37to reach the same internal pressure as the fuel tank21, which is obtained in advance experimentally or otherwise. Then, when the internal pressure in the fuel tank21decreases to the second predetermined pressure P2or below, the high-pressure determination timer TM1is started to count down from the first predetermined time length t1(time (e) inFIG. 2). Then, when the value in the high-pressure determination timer TM1reaches “0”, the high-pressure purge control enters the finish control mode, so that the fuel tank shutoff valve33is 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 valve33is closed is started (time (f) inFIG. 2). Then, when the accumulated volume in high-pressure purge finishing phase reaches the second predetermined volume iv2or above, the vapor solenoid valve32is opened (time (g) inFIG. 2). The second predetermined volume iv2is the volume to be purged for the internal pressure in the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37to decrease to the atmospheric pressure (101.3 kPa). Then, when the accumulated volume in high-pressure purge finishing phase reaches the first predetermined volume iv1or above, the high-pressure purge control returns to the normal control mode, so that the purge control valve37is closed and the engine10is stopped. The first predetermined volume iv1is at least the inner volume of the vapor piping38and purge piping39up to the purge control valve37added to the second predetermined volume iv2.

In the high-pressure purge control mode, fuel evaporative gas is emitted from the fuel tank21into the intake passage11of the engine10via the vapor piping38and purge piping39. If the fuel tank shutoff valve33and the purge control valve37are 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 passage10of the engine10but remaining in the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37.

Thus, after the high-pressure purge control mode, the purge control valve37is kept open until the accumulated volume of fuel evaporative gas passing through the purge control valve37reaches the second predetermined volume iv2. Then, with the purge control valve37kept open, the vapor solenoid valve32is opened. The purge control valve37and the vapor solenoid valve32are kept open until the accumulated volume of fuel evaporative gas passing through the purge control valve37reaches the first predetermined volume iv1. The second predetermined volume iv2is the volume to be purged for the pressure in the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37to decrease to the atmospheric pressure (101.3 kPa), and the first predetermined volume iv1is at least the inner volume of the vapor piping38and purge piping up to the purge control valve37added to the second predetermined volume iv2. By manipulating the purge control valve37and the vapor solenoid valve32in this manner, it is ensured that not only fuel evaporative gas remaining in the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve33but also fuel evaporative gas present in the canister31in the form of being adsorbed on the activated carbon are emitted into the intake passage11. As a result, in the next purging of the canister31, emission of highly-concentrated fuel evaporative gas from the canister31into the intake passage11is prevented, and thus, abrupt change in air-fuel ratio of the mixture drawn into the engine10is prevented.

By preliminary keeping the purge control valve37open until the accumulated volume of fuel evaporative gas passing through the purge control valve37reaches the second predetermined volume iv2, the internal pressure in the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37decreases to the atmospheric pressure.

Then, with the purge control valve37kept open, the vapor solenoid valve32is opened. This ensures that in addition to fuel evaporative gas remaining in the vapor piping38and purge piping39between the fuel tank shutoff valve33and the purge control valve37, fuel evaporative gas existing in the canister31in the form of being adsorbed on the activated carbon are emitted into the intake passage11of the engine10.

Although in the above-described embodiment, the tank sealing valve33is opened at the same as the vapor solenoid valve32is closed, it may be arranged such that first the vapor solenoid valve32is closed and thereafter the tank sealing valve33is opened.