Fuel vapor treatment system

A fuel vapor treatment system is mounted on a hybrid vehicle having an internal combustion engine and an electric motor. Even when an internal combustion engine is stopped, a discharge of fuel vapor from a first canister to atmosphere can be detected. When the discharge of the fuel vapor from the first canister is detected, the internal combustion engine is started to perform a purge process. When it is detected that the purge process in the first canister is finished, the internal combustion engine is stopped to terminate the purge process.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2007-303067 filed on Nov. 22, 2007, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a fuel vapor treatment system for a hybrid vehicle which has an internal combustion engine and an electric motor for running.

BACKGROUND OF THE INVENTION

Conventionally, JP-5-18326A and JP-6-101534A show a fuel vapor treatment system in which fuel vapor generated in a fuel tank is temporarily adsorbed by a canister and a desorbed fuel vapor is purged into an intake passage of an internal combustion engine with air.

Recently, a plug-in hybrid vehicle has been developed. In the plug-in hybrid vehicle, a battery is charged by an external power source while the vehicle is being parked, and the vehicle is driven by almost the electric motor.

In a case that the plug-in hybrid vehicle is driven by only the electric motor, the internal combustion engine seldom works, so that a purge process in which the desorbed fuel vapor is combusted in the internal combustion engine is hardly performed. If a fuel vapor quantity exceeds a fuel vapor adsorbing capacity of the canister, the fuel vapor may be discharged into the atmosphere to cause air pollution. The situation where the fuel vapor quantity exceeds a fuel vapor adsorbing capacity of the canister is referred to as a breakthrough. Besides, if the internal combustion engine is operated frequently to perform the purge processing, the fuel economy will deteriorate.

SUMMARY OF THE INVENTION

The present invention is made in view of the above matters, and it is an object of the present invention to provide a fuel vapor treatment system for a hybrid vehicle having internal combustion engine and an electric motor, which is able to reduce a driving frequency of the internal combustion engine and to prevent the fuel vapor from being discharged from the canister into the atmosphere.

According to the present invention, a fuel vapor treatment system including:

a first canister temporarily adsorbing a fuel vapor generated in a fuel tank;

an open-passage connecting the first canister with atmosphere;

a purge passage for introducing an air-fuel mixture including the fuel vapor into an intake passage of the internal combustion engine and purging the fuel vapor into the intake passage;

a first detection passage provided with a restrictor therein;

a passage switching valve selectively connecting the first detection passage with one of the open-passage and the purge passage;

a second canister connecting with the first detection passage at an opposite end relative to the passage switching valve across the restrictor in order to adsorb the fuel vapor in the air-fuel mixture which flows therein from the first detection passage;

a second detection passage connecting with the second canister;

a gas-flow producing means connecting with the second detection passage to generate a gas-flow therein;

a pressure detecting means for detecting a pressure determined by the restrictor and the gas-flow producing means;

a fuel vapor discharge detecting means for detecting a discharge of the fuel vapor from the first canister into the atmosphere based on a pressure detected by the pressure detecting means;

a purge completion detecting means for detecting a purge completion of the first canister based on the pressure detected by the pressure detecting means;

a purge starting means for starting the internal combustion engine and starting a purge of the fuel vapor when the fuel vapor discharge detecting means detects the discharge of the fuel vapor from the first canister while the internal combustion engine is stopped; and

a purge stopping means for stopping the internal combustion engine and stopping the purge of the fuel vapor when the purge completion detecting means detects the purge completion of the first canister.

According to the present embodiment, when the breakthrough is detected in the first canister, the internal combustion engine is driven to perform the purge process. Thus, a driving frequency of the internal combustion engine is reduced, and it can be avoided to discharge the fuel vapor from the first canister to the atmosphere. Moreover, when the purge completion of the first canister is detected, the internal combustion engine is stopped. Thus, the driving frequency of the internal combustion engine becomes minimum value to avoid the deterioration in fuel economy.

According to another aspect of the present invention, the fuel vapor discharge detecting means detects the discharge of the fuel vapor from the first canister to the atmosphere based on a pressure detected by the pressure detecting means when the passage switching means connects the first detection passage with the open-passage.

According to another aspect of the present invention, the purge completion detecting means detects the purge completion of the first canister based on a pressure detected by the pressure detecting means when the passage switching means connects the first detection passage with the purge passage.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereafter, an embodiment of the present invention is described.FIG. 1is a schematic view of the hybrid vehicle on which a fuel vapor treatment system of the present invention is mounted.

As shown inFIG. 1, the hybrid vehicle is provided with an internal combustion engine100and an electric motor200for driving the vehicle. The driving force is transmitted to drive wheels400through a transmission300. The electric motor200receives electricity from a secondary battery500through an inverter600. The inverter600converts direct-current voltage into alternating-current voltage and varies frequency of the alternating-current voltage so that the rotational speed of the motor200is controlled.

An alternator700driven by the engine100generates electricity when the amount of charge of the battery500is lowered than a specified value. The electricity generated by the alternator700is supplied to the battery500through the inverter600so that the battery is charged. Moreover, the secondary battery500can be charged by an external power source while the vehicle is being parked.

Furthermore, the hybrid vehicle is provided with an electronic control unit (ECU)800which controls the engine100, the transmission300, the inverter600, the alternator700, and a fuel vapor treatment system. The ECU800is mainly constructed of a microcomputer having a CPU, a ROM and a RAM.

The hybrid vehicle is driven in a plurality of driving modes. That is, the hybrid vehicle is driven in an engine driving mode where only the engine100is a driving source, a motor driving mode where only the motor200is the driving source, and a hybrid driving mode where both of the engine100and the motor200are the driving source.

FIG. 2shows an internal combustion engine100and a fuel vapor treatment system. The engine100is a gasoline engine that develops power by use of gasoline fuel received in a fuel tank2. The intake passage3of the engine100is provided with, for example, a fuel injection device4for controlling a fuel injection quantity, a throttle device5for controlling an intake air flow rate, an air flow sensor6for detecting the intake air flow rate, an intake pressure sensor7for detecting an intake pressure, and the like. Moreover, the discharge passage8of the engine100is provided with an air-fuel ratio sensor9for detecting an air-fuel ratio.

The fuel vapor treatment system treats fuel vapor produced in the fuel tank2and supplies the fuel vapor to the engine100. The fuel vapor treatment system is provided with a first canister12, a second canister13, a pump14, a differential pressure sensor16, a plurality of valves19to22, a plurality of passages27to35and the ECU800.

In the first canister12, a case42is partitioned by a partition wall43to form two adsorption parts44,45. The adsorption parts44,45are respectively packed with adsorptive agents46made of activated carbon or the like. The main adsorption part44is provided with an introduction passage27connecting with the inside of the fuel tank2. Hence, fuel vapor produced in the fuel tank2flows into the main adsorption part44through the introduction passage27and is adsorbed by the adsorptive agent46in the main adsorption part44. The main adsorption part44is further provided with a purge passage28connecting with the intake passage3.

A purge valve19, which is an electromagnetically driven two-way valve, is provided in the purge passage28. The purge valve19is opened/closed to control the connection between the first canister12and the intake passage3. With this, in a state where the purge valve19is opened, negative pressure developed downstream of the throttle device5of the intake passage3is applied to the main adsorption part44through the purge passage28. Therefore, when the negative pressure is applied to the main adsorption part44, fuel vapor is desorbed from the adsorptive agent46in the main adsorption part44and the desorbed fuel vapor is mixed with air and is introduced into the purge passage28, whereby fuel vapor in the air-fuel mixture is purged to the intake passage3. The fuel vapor purged into the intake passage3through the purge passage28is combusted in the engine100along with fuel injected from the fuel injection device4.

The main adsorption part44connects with a subordinate adsorption part45via a space at the inside bottom of the case42. When the purge valve19is opened, negative pressure generated in the intake pipe3is introduced into the subordinate adsorption part45through the purge passage28and the main adsorption part44. An open-passage35is connected to the subordinate adsorption part45. The open-passage35is provided with a canister-close valve22which is an electromagnetic valve. The open-passage35communicates to the atmosphere at the other end thereof. Therefore, in a state where the canister-closing valve22is opened, the subordinate adsorption part45is open to the atmosphere through the open-passage35. The open-passage35is provided with a filter51between the canister-close valve35and its opening end.

A passage switching valve20is an electromagnetic valve that performs a two-position action. The passage switching valve20can be mechanically connected to one end of a first detection passage29, and can be mechanically connected to one end of an atmosphere passage30. The other end of the atmosphere passage30is connected to the open-passage35between the canister-close valve22and the filter35. Thereby, the atmosphere passage30communicates to the atmosphere through the open-passage35. Moreover, the passage switching valve20is mechanically connected to a branch passage31branched from the purge passage28between the main adsorption part44and the purge valve19. The passage switching valve20selectively changes a passage connecting with the first detection passage29between the atmosphere passage30and the branch passage31. Therefore, in a first position where the atmosphere passage30connects with the first detection passage29, the air in the atmosphere passage30can flow into the first detection passage29. Moreover, in a second position where the branch passage31connects with the first detection passage29, the air-fuel mixture containing the fuel vapor in the purge passage28can flow into the first detection passage29.

The pump14, which is a gas flow generating means, is constructed of, for example, an electrically driven vane pump. The suction port of the pump14connects with one end of a second detection passage32, and the discharge port of the pump14connects with one end of a discharge passage34. The other end of the discharge passage34connects with the atmosphere passage30. The discharge port of the pump14connects with the atmosphere through the discharge passage34, the atmosphere passage30and the open-passage35. When energized, the pump14decompresses the second detection passage32to generate the gas flow in the second detection passage32. The generated gas flow is discharged into the discharge passage34. When the pump14is stopped, the second detection passage32and the discharge passage34are communicated with each other through an interior of the pump14.

A second canister13has an adsorption part41in a case40packed with an adsorptive agent39made of activated carbon or the like. The total capacity of the adsorptive agent39in the second canister13is established smaller than the total capacity of the adsorptive agent46in the first canister12. The first detection passage29connects with the second detection passage32through the adsorption part41. Hence, when the pump14is operated in a state where the air-fuel mixture exists in the first detection passage29, the negative pressure generated in the second detection passage32is introduced into the first detection passage29through the second canister13, so that the air-fuel mixture in the first detection passage29flows into the adsorption part41and fuel vapor in the air-fuel mixture is adsorbed by the adsorptive agent39in the adsorption part41. In a case that the purge valve19is opened and the passage switching valve20is positioned in the second position, when the negative pressure in the intake passage3is introduced into the first detection passage29through the purge passage28and the branch passage31, the air is introduced from the atmosphere passage30toward the pump14. Thus, the fuel vapor adsorbed in the adsorptive agent39is desorbed. The desorbed fuel vapor is purged into the intake passage3through the first detection passage29and the purge passage28.

A restrictor50which restricts a passage area is provided in the first detection passage29. Moreover, a passage opening/closing valve21made of an electromagnetically driven two-way valve is provided in the middle portion of the first detection passage29between the second canister13and the restrictor50. The passage opening/closing valve21opens or closes the first detection passage29. That is, when the passage opening/closing valve21is closed, the first detection passage29is closed between the restrictor50and the second canister13. When the passage opening/closing valve21is opened, the first detection passage29is opened.

The differential pressure sensor16connects with a pressure introducing passage33branched from the second detection passage32between the second canister13and the pump14. The differential pressure sensor16detects a pressure difference between pressure in the second detection passage32and the atmospheric pressure. Therefore, a differential pressure detected by the differential pressure sensor16when the pump14is operated is substantially equal to the pressure difference between both ends of the restrictor50in a state where the passage opening/closing valve21is opened. Moreover, in a state where the passage opening/closing valve21is closed, the first detection passage29is closed on the suction side of the pump14. Hence, a pressure difference detected by the differential pressure sensor16when the pump14is operated is substantially equal to the shutoff pressure of the pump14. As described above, the differential pressure sensor16can detect pressure which is determined based on the restrictor50and the pump14.

The ECU800is comprised of a microcomputer having a CPU and a memory, and is electrically connected to the pump14, the differential pressure sensor16, the valves19-22, and the elements4-7,9of the engine100. The ECU800controls the respective operations of the pump14and the valves19to22on the basis of the detection results of the respective sensors16,6,7,9, a temperature of cooling water of the engine100, a temperature of working oil of the vehicle, a rotational speed of the engine100, the accelerator position of the vehicle, the ON/OFF state of an ignition switch, and the like. Further, the ECU800controls a fuel injection quantity, an opening degree of a throttle valve5, an ignition timing of the engine100, and the like.

Referring toFIG. 3, a main operation of the fuel vapor treatment system will be described,FIG. 3is a flowchart which the ECU800executes. The main operation is started when the ignition switch is turned ON. When the ignition switch is ON, the engine100and/or the electric motor200can drive the vehicle. When the ignition switch is OFF, the operations of the engine100and the electric motor200are prohibited.

In step S101, the computer determines whether a breakthrough detecting condition for the first canister12is established. Specifically, when an elapsed time after the previous breakthrough detecting process is completed exceeds a first preset time, the breakthrough detecting condition is established.

When the answer is YES in step S101, the procedure proceeds to step S102in which the breakthrough detecting process is performed for determining whether the first canister12is in a situation of the breakthrough. Then, the procedure proceeds to step S103in which the computer determines whether a first canister breakthrough flag is set as ON, which indicates the first canister is in the situation of the breakthrough. When the answer is YES in step S103, the procedure proceeds to step S104in which a purge process is performed to combust the fuel vapor desorbed from the first canister12and the second canister13. Then, the procedure goes back to step S101. When the answer is NO in step S103, the procedure goes back to step S201.

When the answer is NO in step S101, the procedure proceeds to step S105. In step S105, the computer determines whether the key switch is OFF. When the key switch is ON, the procedure goes back to step S101. When the key switch is OFF, the procedure is terminated. In the fuel vapor treatment system, after the main operation is finished, a first canister opening operation that brings the respective valves19to22to the states shown inFIG. 4is performed to open the first canister12to the atmosphere.

The breakthrough detection process in step S102will be described in more detail. First, the measurement principle of the fuel vapor concentration “D” that is a parameter for the breakthrough detection will be described. For example, in a case of the pump14having internal leak such as a vane pump, the quantity of internal leak varies according to load. Hence, as shown inFIG. 5, the pressure (P)−flow rate (Q) characteristic curve Cpmpof the pump14is expressed by a following equation (1). In the equation (1), K1and K2are constants specific to the pump14.
Q=K1·P+K2  (1)

Assuming that the shutoff pressure of the pump14is Pt, the flow rate Q becomes zero and following equation (2) is obtained.
K2=−K1·Pt(2)

In the fuel vapor treatment system, the pressure loss of flowing gas is reduced to as small a quantity as can be neglected on a side closer to the second canister13and the second detection passage32than the restrictor50of the first detection passage29. In a state where the passage opening/closing valve21is opened, the pressure P of the pump14is thought to be substantially equal to a differential pressure ΔP between both ends of the restrictor50(hereinafter simply referred to as “differential pressure”). When the pressure loss of flowing gas cannot be neglected, it is preferable that the pressure loss is previously stored in the ECU800and the differential pressure ΔP is corrected as required.

When the passage opening/closing valve21is opened and only air passes through the restrictor50; the air passes through the second canister13to be suctioned by the pump14. Thus, the passing air flow rate QAiris substantially equal to the intake air flow rate Q. Therefore, the flow rate QAirand the differential pressure ΔPAirwhen air passes through the restrictor50satisfy the following relationship equation (3) obtained from the equations (1), (2).
QAir=K1·(ΔPAir−Pt)  (3)

Meanwhile, when the air-fuel mixture containing fuel vapor (hereinafter simply referred to as “air-fuel mixture”) passes through the restrictor50in a state where the passage opening/closing valve21is open, the second canister13passes only air and hence the passing air flow rate QAir′ in the air-fuel mixture is substantially equal to the suction airflow rate Q of the pump14. Therefore, when the air-fuel mixture passes through the restrictor50, the passing flow rate QAir′ and the differential pressure ΔPGassatisfy the following equation (4) obtained by the equations (1) and (2).
QAir′=K1·(ΔPGas−Pt)  (4)

When it is assumed that the passing flow rate of the whole air-mixture at the restrictor50is QGasand the fuel vapor concentration is D (%), the passing air flow rate QAir′ satisfies the following equation (5). Hence, the following equation (6) can be obtained from this equation (5).
QAir′=QGas·(1−D/100)  (5)
D=100·(1−QAir′/QGas)  (6)

The differential pressure ΔP−flow rate Q characteristic curve at the restrictor50is expressed by the following equation (7) using the density p of the gas passing through the restrictor50. “K3” in the equation (7) is a constant specific to the restrictor50and is a value expressed by the following equation (8) when the diameter and the flow coefficient of the restrictor50are assumed to be “d” and “α”, respectively.
Q=K3·(ΔP/ρ)1/2(7)
K3=α·π·d2/4·21/2(8)

Therefore, the ΔP−Q characteristic curve CAirshown inFIG. 5is expressed by the following equation (9) using the density ρAirof air.
QAir=K3·(ΔPAir/ρAir)1/2(9)

Moreover, the ΔP−Q characteristic curve CGasof the air-fuel mixture shown inFIG. 5is expressed by the following equation (10) by the use of the density ρGasof the air-fuel mixture. When it is assumed that the density of hydrocarbon (HC) of the fuel vapor is ρHC, there is a relationship expressed by the following relationship equation (11) between the density ρGasof the air-fuel mixture and the fuel vapor concentration D (%) in the air-fuel mixture.
QGas=K3·(ΔPGas/ρGas)1/2(10)
D=100·(ρAir−ρGas)/(ρAir−ρHC)  (11)

From the above-mentioned equations, by eliminating K1from the equations (3) and (4), the following equation (12) is obtained. Moreover, by eliminating K3from the equations (9) and (10), the following equation (13) is obtained.
QAir/QAir′=(ΔPAir−Pt)/(ΔPGas−Pt)  (12)
QAir/QGas={(ΔPAir/ΔPGas)·(ρGas/ρAir)}1/2(13)

Furthermore, by eliminating QAirfrom the equations (12) and (13), the following equation (14) is obtained, and the following equation (15) is obtained from the equation (11). Hence, the following equation (16) is obtained from these equations (14), (15), and (6). P1, P2, and ρ in the equation (16) are expressed by the following equations (17), (18), and (19).
QAir′/QGas=(ΔPGas−Pt)/(ΔPAir−Pt)·{(ΔPAir/ΔPGas)·(ρGas/ρAir)}1/2(14)
ρGas=ρAir−(ρAir−ρHC)·D/100  (15)
D=100·[1−P1{P2·(1−ρ·D)}1/2]  (16)
P1=(ΔPGas−Pt)/(ΔPAir−Pt)  (17)
P2=ΔPAir/ΔPGas(18)
ρ=(ρAir−ρHC)/(100·ρAir)  (19)

When both sides of the equation (16) are squared and rearranged for D, the following quadratic equation (20) is obtained. When this quadratic equation (20) is solved for D, the following solution (21) is obtained. M1and M2in the solution (21) are expressed by the following equations (22) and (23).
D2+100·(100·P12·P2·ρ−2)·D+1002·(1−P12·2)  (20)
D=50−{−M1±(M12−4·M2)1/2}  (21)
M1=100·P12·P2·ρ−2  (22)
M2=1−P12·P2  (23)

Therefore, because a value beyond a range from 0 to 100 of the solutions (21) of the quadratic equation (20) does not hold as the concentration D of fuel vapor, a value within the range from 0 to 100 of the solutions (21) is obtained as the equation (24) of computing the concentration D of fuel vapor.
D=50·{−M1−(M12−4·M2)1/2}  (24)

In the equation (24) of computing the concentration D of fuel vapor obtained in this manner, among variables included in M1and M2, ρAirand ρHCare values determined as physical constants and are stored as parts of the equation (24) in the memory of the ECU800in this embodiment. Therefore, to compute the concentration D of fuel vapor by the use of the equation (24), among variables included in M1and M2, the differential pressure ΔPAir, ΔPGaswhen air and air-fuel mixture pass through the restrictor50and the shutoff pressure Pt of the pump14are necessary. Since each of the differential pressure ΔPAir, ΔPGasis substantially equal to the pressure detected by the pressure sensor16, in the breakthrough detection process in step S102, the pressure differences ΔPAir, ΔPGasand the shutoff pressure Pt are detected and the concentration D of fuel vapor is computed from these detected values.

FIG. 6is a flowchart showing the breakthrough detection process which the ECU800executes. At a starting of the breakthrough detection process, as shown in a column of “FIRST CANISTER OPENING CONDITION” inFIG. 4, the purge valve19and the passage opening/closing valve21are closed, the passage switching valve20is in the first position, and the canister-close valve22is opened, so that the first canister12connects with the atmosphere.

In step S201, the pump14is driven to decompress the second detection passage32. At this time, each valve19-22is the same state as the first canister opening condition as shown in a column of “S201” inFIG. 4. Thereby, since the first detection passage29is closed as shown inFIG. 7, the pressure detected by the pressure sensor16is varies to the shutoff pressure Pt. Then, in this step S202, when the differential pressure detected by the pressure sensor16becomes stable, the stable value is stored as the shutoff pressure Pt of the pump14in the memory of the ECU800.

Then, the procedure proceeds to step S203in which the computer determines whether a difference between the shutoff pressure Pt and a reference shutoff pressure Pt0is smaller than a permissible value P3in order to determine whether the first canister12is in the situation of the breakthrough.

When the fuel vapor passes through the first canister12and flows into the pump14through the atmosphere passage30and the discharge passage34, a situation of an internal leak in the pump14will be varied due to a variation in viscosity of gas. As shown inFIG. 8, as the fuel vapor concentration in the open-passage35increases, the shutoff pressure Pt detected by the pressure sensor16increases. That is, the shutoff pressure Pt varies toward the atmospheric pressure. When the answer is NO in step S203, the computer determines that the first canister12is in a situation of the breakthrough. The procedure proceeds to step S204.

In step S204, while the pump14is driven, the passage opening/closing valve21is opened. Since the condition of each valves19-22will be in the condition shown in the column of “S204” inFIG. 4, the second detection passage32, which is decompressed by the pump14, is communicated with the first detection passage29, the atmosphere passage30and the open-passage35, so that the air passes through the restrictor50as shown inFIG. 9. Then, the procedure proceeds to step S205. In this step S205, when the differential pressure detected by the differential pressure sensor16becomes stable, the stable value is stored in the memory of the ECU800as a first differential pressure ΔPAir.

Then, the procedure proceeds to step S206in which the computer determines whether a difference between the first differential pressure ΔPAirand the first reference differential pressure ΔPAir0is less than a permissible value P4in order to determine whether the first canister12is in the situation of the breakthrough. Besides, the first reference differential pressure ΔPAir0corresponds to a pressure detected by the pressure sensor16when the air containing no fuel vapor passes through the restrictor50.

When the first canister12is in the situation of the breakthrough, the fuel vapor exists in the atmosphere passage30and the open-passage35. Thus, the density of the gas passing through the restrictor50is varied. As shown inFIG. 10, as the fuel vapor concentration in the open-passage35increases, the first differential pressure ΔPAirdetected by the pressure sensor16decreases. When the answer is NO in step S206, the computer determines that the first canister12is in the situation of the breakthrough.

Then, the procedure proceeds to step S207in which a fist canister breakthrough flag is tuned ON which indicates the first canister12is in the situation of the breakthrough. Since the computer determines whether the first canister12is in the situation of the breakthrough in both of steps S203, S206, an erroneous determination can be avoided.

Then, the procedure proceeds to step S208in which the passage switching valve20is switched to the second position. Thereby, since the condition of the valves19-22will be in the condition shown in the column of “S208” inFIG. 4, the air-fuel mixture containing fuel vapor flows into the first detection passage29from the branch passage31as shown inFIG. 11. Therefore, the pressure detected by the pressure sensor16is the differential pressure ΔPGasaccording to the fuel vapor concentration D. In step S209, when the differential pressure detected by the pressure sensor16becomes stable, the stable value is stored in the memory of the ECU800as a second differential pressure ΔPGas.

In step S210, the computer computes a fuel vapor concentration D when no purge is performed by use of Pt, ΔPAir, ΔPGasand the above equation (24). In step S211, the computed fuel vapor concentration D is stored in the memory of the ECU800.

In step S212, the condition of the valves19-22is switched to the condition shown in the column of “FIRST CANISTER OPENING CONDITION” inFIG. 4. In step S213, the pump14is stopped and the breakthrough detection process is terminated.

When the answer is YES in step S203, the procedure proceeds to step S213. When the answer is YES is step S206, the procedure proceeds to step S212.

The purge process which is performed in step S104will be described hereinafter.FIG. 12is a flowchart showing a purge process executed by the ECU800. At a starting of the purge process, the condition of the valves19-22is in the condition shown in the column of “FIRST CANISTER OPENING CONDITION” inFIG. 4. The first canister12is communicated with the atmosphere.

In step S301, the computer determines whether the internal combustion engine100is running, When the answer is NO in step S301, the procedure proceeds to step S302in which the engine100is started. Then the procedure proceeds to step S303. At this moment, the driving force of the internal combustion engine100is utilized to drive the alternator700. When the answer is YES in step S301, the procedure proceeds to step S303.

In S303, the computer determines whether a purge execution condition is established. The purge execution condition is established when the engine100is started and the engine speed reaches a predetermined value stored in the memory.

When the purge execution condition is not established, the process in S303is repeated until the purge execution condition is established. When the answer is YES in step S303, the procedure proceeds to step S304. In step S304, the computer reads out the fuel vapor concentration D stored in the memory in step S211. In step S305, the computer determines an opening degree of the purge valve19based on the fuel vapor concentration D and the vehicle driving quantity such as accelerator position.

In step S306, the purge valve19and the passage opening/closing valve21are opened, and the passage switching valve20is switched to the second position. Then, the purge process is started. Since the condition of the valves19-22is in the condition shown in the column of “S306”, the negative pressure in the intake passage3is applied to not only the first canister12but also the second canister13through the first detection passage29. Thus, the residual fuel vapor in the second canister13and the first detection passage29is introduced into the purge passage28, and is purged into the intake passage3with the fuel vapor desorbed from the first canister12.

In step S307, the computer determines whether a preset time T1has elapsed after the purge process is started. The preset time T1is required for the first canister12to be recovered from the breakthrough situation to the adsorbing situation. The preset time T1is previously stored in the memory.

When the answer is YES in step S307, the procedure proceeds to step S308in which a purge concentration measurement process is performed. In the purge concentration measurement process, the computer determines whether the purge of the first canister12is completed based on the fuel vapor concentration D of the purged air-fuel mixture. Then, the procedure proceeds to step S309in which the computer determines whether the fist canister breakthrough flag is OFF. When the answer is YES in step S309, the procedure proceeds to step S310in which the condition of the valves19-22is returned to the condition shown in the column of “FIRST CANISTER OPENING CONDITION” inFIG. 4.

In step S311, the computer determines whether a condition for continuing the driving of the engine100is established. Specifically, when the vehicle driving mode is an engine driving mode or a hybrid driving mode, or when the alternator700is needed to be driven by the engine100, the condition for continuing the driving of the engine100is established.

When the answer is NO in step S311, the procedure proceeds to step S312in which the engine100is stopped and the purge process is terminated. When the answer is YES in step S311, the purge process is terminated.

When the answer is NO in step S307, the procedure proceeds to step S313in which the computer determines whether the key switch is turned OFF. When the key switch is ON, the process in step307is repeatedly performed until an affirmative determination is made in step S307.

When the answer is NO in step S309, the procedure proceeds to step S313. When the key switch is ON, the processes in steps S307-S309are repeatedly performed until an affirmative determination is made in step S309.

When the answer is YES in step S312, that is, when the key switch is turned OFF, the condition of the valves19-22is returned to the condition shown in the column of “FIRST CANISTER OPENING CONDITION” inFIG. 4. Then, the procedure proceeds to step S312in which the engine100is stopped.

Referring toFIG. 12, the purge concentration measurement process in step S308will be described.FIG. 13is a flowchart showing a purge concentration measurement process executed by the ECU800.

In step S401, the condition of the valves19-22is switched to a condition shown in the column of “FIRST CANISTER OPENING CONDITION” inFIG. 4, whereby the first detection passage29is closed as shown inFIG. 7. In step S402, the pump12is driven to decompress the second detection passage32. In step S403, the shutoff pressure Pt is detected, and in step S404, the shutoff pressure Pt is stored in the memory of the ECU800.

In step S405, while the pump14is driven, the passage opening/closing valve21is opened. Since the condition of the valves19-22becomes the condition shown in the column of “S204” inFIG. 4, the air passes the restrictor as shown inFIG. 9. In step S406, the pressure sensor16detects the first differential pressure ΔPAir, and in step S407, the first differential pressure ΔPAiris stored in the memory of the ECU800.

Then, the procedure proceeds to step S408in which the passage switching valve20is switched to the second position. Thereby, since the condition of the valves19-22will be in the condition shown in the column of “S208” inFIG. 4, the air-fuel mixture which will be purged into the engine100flows into the first detection passage29from the branch passage31as shown inFIG. 11. Therefore, the pressure sensor16detects the differential pressure ΔPGasaccording to the fuel vapor concentration D of the air-fuel mixture which will be purged. In step S409, the pressure sensor16detects the second differential pressure ΔPGas, and in step S410, the second differential pressure ΔPGasis stored in the memory of the ECU800.

In step S411, the computer computes a fuel vapor concentration D by use of Pt, ΔPAir, ΔPGasstored in the memory in steps S403,407,410and the above equation (24). Since the density of the gas varies according to the fuel vapor concentration, as shown inFIG. 14, as the fuel vapor concentration D of the air-fuel mixture which will be purged increases, the second differential pressure ΔPGasdetected by the pressure sensor16decreases.

In step S412, the computer determines whether the fuel vapor concentration computed in step S411is less than a permissible concentration D0previously stored in the memory of the ECU800in order to determine whether the purge of the first canister12is completed. Specifically, when the fuel vapor concentration D is less than the permissible concentration D0, the computer determines that the pure process in the first canister12is completed.

When the answer is YES in step S412, the procedure proceeds to step S413in which the first canister breakthrough flag is turned OFF.

In step S414, the purge valve19and the passage opening/closing valve21are opened, and the passage switching valve20is switched to the second position. Since the condition of valves19-22is in the condition shown in the column of “S306” inFIG. 4, the residual fuel vapor in the second canister13and the first detection passage29is introduced into the purge passage28, and is purged into the intake passage3with the fuel vapor desorbed from the first canister12. After the process in step in S414, the purge concentration measurement process is terminated.

When the answer is NO in step S412, that is, when the purge of the first canister12is not completed, the procedure proceeds to step S414in which the purge condition is returned. Then, the procedure goes back to the purge process (refer toFIG. 12), the purge process is performed until an affirmative determination is made in step S307or step S313.

According to the present embodiment, when the breakthrough is detected in the first canister12, the internal combustion engine100is started to perform the purge process. Thus, a driving frequency of the internal combustion engine100is reduced, and it can be avoided to discharge the fuel vapor from the first canister12to the atmosphere. Moreover, when the purge completion of the first canister12is detected, the internal combustion engine100is stopped. Thus, the driving frequency of the internal combustion engine100becomes minimum value to avoid the deterioration in fuel economy.

Besides, in step S307ofFIG. 12, the computer determines whether the purge process has been completed based on the elapsed time after the purge process is started. Alternatively, the computer can determine whether the purge process has been completed based on an integrated quantity of the purged fuel vapor which has passed the purge valve19. This integrated quantity is required for the first canister12to be recovered from the breakthrough situation to the adsorbing situation. The integrated quantity is previously stored in the memory.