Method for controlling internal combustion engine and device for controlling internal combustion engine

Abstract

An internal combustion engine (1) is feedback-controlled using an air-fuel ratio feedback correction coefficient so that the air-fuel ratio becomes a theoretical air-fuel ratio. When the air-fuel ratio feedback correction coefficient (α) is a value that has deviated from a prescribed reference value due to purge gas that contains evaporated fuel from a fuel tank (20), the internal combustion engine (1) implements a correction coefficient initialization process for returning the air-fuel ratio feedback correction coefficient toward the reference value when a step-wise decrease in purge rate occurs. Thus, the internal combustion engine (1) can bring the air-fuel ratio feedback correction coefficient (α) close to a value corresponding to the actual air-fuel ratio and can suppress fluctuation of the air-fuel ratio.

Description

Control method and control device for internal combustion engine

[0001] The present invention relates to a control method for an internal combustion engine and a control device for an internal combustion engine.

[0002] For example, Patent Document 1 discloses an evaporated fuel treatment device that temporarily stops purging when supercharging decreases in order to prevent the air-fuel ratio from becoming excessively rich due to evaporated fuel purging when supercharging decreases.

[0003] However, in Patent Document 1, if the purge gas containing evaporated fuel suddenly decreases, the fuel injection amount correction that has been deviated by the purge gas may not be able to keep up, and the air-fuel ratio may deteriorate (become lean).

[0004] Japanese Patent Application Publication No. 11-303692

[0005] The internal combustion engine of the present invention uses an air-fuel ratio feedback correction coefficient to perform feedback control so that the air-fuel ratio becomes the stoichiometric air-fuel ratio, and if the air-fuel ratio feedback correction coefficient has deviated from a predetermined reference value due to purge gas containing evaporated fuel from the fuel tank, when a step-like decrease in the purge rate occurs, correction coefficient initialization processing is performed to return the air-fuel ratio feedback correction coefficient toward the reference value.

[0006] According to the present invention, the internal combustion engine can bring the air-fuel ratio feedback correction coefficient closer to a value that corresponds to the actual air-fuel ratio, thereby suppressing fluctuations (deterioration) of the air-fuel ratio.

[0007] An explanatory diagram showing a schematic overview of the system configuration of an internal combustion engine to which the present invention is applied. A timing chart showing an example of the behavior of various parameters when purge is cut in an internal combustion engine of a first embodiment. A flowchart showing the control flow of the internal combustion engine of the first embodiment. A timing chart showing an example of the behavior of various parameters when purge is cut in an internal combustion engine of a second embodiment. A flowchart showing the control flow of the internal combustion engine of the second embodiment.

[0008] An embodiment of the present invention will be described in detail below with reference to the drawings.

[0009] FIG. 1 is an explanatory diagram that schematically shows the system configuration of an internal combustion engine 1 to which the present invention is applied.

[0010] The internal combustion engine 1 is, for example, a multi-cylinder spark-ignition gasoline engine, and is installed as a drive source in a vehicle such as an automobile. An intake passage 2 of the internal combustion engine 1 is provided with an air cleaner 3 that collects foreign matter in the intake air, an air flow meter 4 that detects the amount of intake air, an electric throttle valve 5, and an intake collector 6.

[0011] The air flow meter 4 is disposed downstream of the air cleaner 3 in the intake air flow direction.

[0012] The throttle valve 5 controls the amount of intake air into the internal combustion engine 1 in accordance with the load, and is disposed downstream of the air flow meter 4 in the intake air flow direction.

[0013] The intake collector 6 is located downstream of the throttle valve 5 in the intake air flow direction, and is connected to an intake port 8 of each cylinder of the internal combustion engine 1 via an intake manifold 7 .

[0014] The internal combustion engine 1 also has a turbocharger 9. The turbocharger 9 has a compressor 10 provided in the intake passage 2 and a turbine 11 provided in the exhaust passage 15. The compressor 10 and turbine 11 are arranged coaxially and rotate integrally. The compressor 10 is arranged upstream of the throttle valve 5 in the intake air flow direction and downstream of the air flow meter 4.

[0015] An intercooler 12 is provided in the intake passage 2 upstream of the throttle valve 5. The intercooler 12 is disposed downstream of the compressor 10 in the intake air flow direction. The intercooler 12 is provided to cool the intake air compressed (pressurized) by the compressor 10 to improve charging efficiency.

[0016] An intake bypass passage 13 is connected to the intake passage 2. The intake bypass passage 13 is formed to bypass the compressor 10 and connect the upstream side and downstream side of the compressor 10.

[0017] An electric recirculation valve 14 is provided in the intake bypass passage 13. The recirculation valve 14 is normally closed, but opens when, for example, the pressure downstream of the compressor 10 becomes high. Opening the recirculation valve 14 allows the high-pressure intake air downstream of the compressor 10 to be returned to the upstream side of the compressor 10 via the intake bypass passage 13. Note that the recirculation valve 14 may also be a so-called check valve that opens only when the pressure downstream of the compressor 10 reaches or exceeds a predetermined pressure.

[0018] An exhaust bypass passage 16 is connected to the exhaust passage 15, bypassing the turbine 11 and connecting the upstream side and downstream side of the turbine 11. An electric wastegate valve 17 is arranged in the exhaust bypass passage 16 to control the exhaust flow rate in the exhaust bypass passage 16.

[0019] An exhaust gas purification catalyst 18 is disposed in the exhaust passage 15 downstream of the turbine 11. An A / F sensor 19 is disposed upstream (at the inlet) of the catalyst 18. The A / F sensor 19 is a so-called wide-range air-fuel ratio sensor that has an output characteristic that is approximately linear in response to the air-fuel ratio.

[0020] Further, an evaporated fuel treatment system 21 that treats evaporated fuel in a fuel tank 20 is connected to the intake passage 2 .

[0021] The evaporative fuel treatment system 21 has a purge passage 22, a canister 23 capable of adsorbing and desorbing evaporative fuel, and an electric purge control valve 24 located between the canister 23 and the intake passage 2, and is capable of introducing purge gas containing evaporative fuel from the fuel tank 20 into the intake passage 2.

[0022] The purge passage 22 introduces fuel vapor into the intake passage 2 , and is connected to the intake passage 2 at a position downstream of the throttle valve 5 and upstream of the intake collector 6 .

[0023] The purge passage 22 is provided with a purge control valve 24 and a canister 23 capable of adsorbing and desorbing evaporated fuel. The canister 23 adsorbs evaporated fuel generated in the fuel tank 20. The purge control valve 24 is disposed between the canister 23 and the intake passage 2.

[0024] The control unit 25, which serves as a control section, is a well-known digital computer equipped with a CPU, ROM, RAM, and an input / output interface. The control unit 25 receives detection signals from various sensors, such as the air flow meter 4, the A / F sensor 19, a crank angle sensor 26, a pressure sensor 27 that detects the intake pressure (intake manifold pressure) in the intake manifold 7, and an atmospheric pressure sensor 28.

[0025] The crank angle sensor 26 detects the crank angle of the crankshaft of the internal combustion engine 1 and is capable of detecting the engine speed of the internal combustion engine 1 .

[0026] The pressure sensor 27 is capable of detecting the intake pressure downstream of the throttle valve 5 and is attached to the intake collector 6 or the intake manifold 7, for example.

[0027] The control unit 25 controls the opening and closing of the throttle valve 5, the recirculation valve 14, the wastegate valve 17, and the purge control valve 24 based on detection signals from various sensors.

[0028] The control unit 25 is also capable of controlling the internal combustion engine 1 so that the air-fuel ratio becomes the stoichiometric air-fuel ratio by known air-fuel ratio feedback control based on the detection signal of the A / F sensor 19. The control unit 25 calculates an air-fuel ratio feedback correction coefficient α (ALPHA) based on the detection signal of the A / F sensor 19, and multiplies the basic fuel injection amount by this air-fuel ratio feedback correction coefficient α, thereby controlling the internal combustion engine 1 so that the air-fuel ratio becomes the stoichiometric air-fuel ratio. The basic fuel injection amount is calculated using the intake air amount and the engine speed of the internal combustion engine 1. If purging is being performed while purge gas is being introduced into the intake passage 2, the basic fuel injection amount is corrected based on the estimated purge gas concentration value.

[0029] The introduction of purge gas via the purge control valve 24 is basically carried out when the conditions for air-fuel ratio feedback control of the internal combustion engine 1 using the A / F sensor 19 are met.

[0030] The control unit 25 estimates the concentration of the purge gas. The concentration of the purge gas is estimated, for example, from the amount of fluctuation of the air-fuel ratio feedback correction coefficient α relative to the amount of introduced purge gas. In this specification, the estimated value of the concentration of the purge gas is an estimated value of the air / fuel ratio in the purge gas.

[0031] Here, if the amount of purge gas introduced (supplied) into the intake passage 2 decreases stepwise during purging, the air-fuel ratio feedback correction coefficient α may not be able to keep up with the change in the amount of introduced purge gas, and the air-fuel ratio may fluctuate significantly toward the lean side. For example, when a purge cut occurs due to a stepwise decrease in the purge rate caused by supercharging, the internal combustion engine 1 performs a correction coefficient initialization process to quickly return the air-fuel ratio feedback correction coefficient α toward the reference value. The purge rate is the ratio of the amount of purge gas to the amount of intake air. Furthermore, purge cut means stopping the flow of purge gas into the intake passage 2.

[0032] FIG. 2 is a timing chart showing an example of the behavior of various parameters when purge is cut in the internal combustion engine 1 of the first embodiment.

[0033] Time t1 in Figure 2 is the timing when the accelerator pedal is depressed, the idling state of the internal combustion engine 1 ends, and purging is started. The estimated purge gas concentration value shown by the solid line in Figure 2 is an erroneously estimated value on the lean side relative to the actual purge gas concentration shown by the characteristic line Lc (dashed line) in Figure 2. Therefore, the air-fuel ratio and air-fuel ratio feedback correction coefficient α of the internal combustion engine 1 deviate toward the rich side when purging is started. If the estimated purge gas concentration value is erroneously estimated to be leaner than the actual value, the air-fuel ratio of the internal combustion engine 1 will deviate toward the rich side as the correction by the air-fuel ratio feedback correction coefficient α cannot keep up (is not in time) with the increase in purge rate after purging is started.

[0034] 2, the value of the air-fuel ratio feedback correction coefficient α when the air-fuel ratio is set to the stoichiometric air-fuel ratio without purge gas being introduced is a value that is larger by a predetermined amount than the reference value. This is due to product variations in the internal combustion engine 1. Therefore, the estimated value of the purge gas concentration is erroneously estimated to be lean, as described above.

[0035] 2, the air-fuel ratio feedback correction coefficient α reaches a predetermined lower threshold value. The air-fuel ratio feedback correction coefficient α is set so as not to be smaller than the lower threshold value, which is a predetermined amount smaller than the reference value.

[0036] The air-fuel ratio feedback correction coefficient α reaches the lower threshold value at time t2 in FIG. 2 and is limited so as not to become any smaller. Furthermore, since the air-fuel ratio feedback correction coefficient α is limited by the lower threshold value, the purge rate does not change from the value corresponding to the lower threshold value at time t2 in FIG. 2 . The purge rate reaches a value equal to or greater than a predetermined judgment threshold value prior to time t2. The judgment threshold value is an index for determining whether the air-fuel ratio feedback correction coefficient α has changed due to purging. In the first embodiment, if the purge rate exceeds the judgment threshold value, it is determined that the air-fuel ratio feedback correction coefficient α has changed due to purging. In other words, if the purge rate is equal to or greater than the judgment threshold value, it is determined that the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to purge gas.

[0037] Time t3 in FIG. 2 is the timing when the intake pressure (intake manifold pressure) in the intake manifold 7 becomes atmospheric pressure after the accelerator pedal is fully opened. When the intake pressure becomes equal to atmospheric pressure, the flow of purge gas into the intake passage 2 due to the negative pressure stops. The purge rate becomes "0" at time t3 and falls below a first purge threshold, which is smaller than the determination threshold. The first purge threshold is an index used to determine whether purge has been cut. In the first embodiment, when the purge rate falls below the first purge threshold, it is determined that purge has been cut. Note that the intake pressure in the intake manifold 7 is greater than a preset pressure threshold due to the full accelerator pedal opening prior to time t3. The pressure threshold is set to a value smaller than atmospheric pressure. In this embodiment, it is determined that purge has been cut due to supercharging when the intake pressure in the intake manifold 7 becomes greater than the pressure threshold.

[0038] Time t4 in FIG. 2 is the timing when a predetermined time has elapsed since time t3, and is the timing when the correction coefficient initialization process is performed. The predetermined time is set, for example, so that no purge gas remains in the intake system of the internal combustion engine 1. In the first embodiment, at time t4 in FIG. 2, the air-fuel ratio feedback correction coefficient α is suddenly returned to the reference value in a feedforward manner. The dashed characteristic line S1 in FIG. 2 shows how the air-fuel ratio feedback correction coefficient α changes when the air-fuel ratio feedback correction coefficient α is not suddenly returned to the reference value at time t4 (i.e., when feedback control is continued as is). The dashed characteristic line S2 in FIG. 2 shows how the air-fuel ratio of the internal combustion engine 1 changes when the air-fuel ratio feedback correction coefficient α changes as shown by the characteristic line S1.

[0039] As shown in FIG. 2, the air-fuel ratio of the internal combustion engine 1 is prevented from fluctuating toward the lean side by returning the air-fuel ratio feedback correction coefficient α to the reference value at the timing of time t4.

[0040] FIG. 3 is a flowchart showing the flow of control of the internal combustion engine 1 in the first embodiment described above.

[0041] In step S1, it is determined whether or not the purge has been cut off from a state in which the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to the purge gas.

[0042] That is, the purge rate and the air-fuel ratio feedback correction coefficient α are used to determine whether or not the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to the flow of purge gas into the intake passage 2. When the purge rate is equal to or greater than the determination threshold value and the air-fuel ratio feedback correction coefficient α is equal to or less than the correction coefficient threshold value, it is determined that the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to the purge gas.

[0043] Furthermore, when the intake pressure in the intake manifold 7 becomes equal to or greater than the pressure threshold, it is determined that purge has been cut due to supercharging.

[0044] In step S2, it is determined whether the purge rate is less than the first purge threshold. If the purge rate is less than the first purge threshold, the process proceeds to step S3, where a delay timer is started to count. The delay timer measures the predetermined time.

[0045] In step S4, it is determined whether or not the predetermined time has elapsed since the delay timer started counting, and if so, the process proceeds to step S5.

[0046] In step S5, a correction coefficient initialization process is carried out to return the air-fuel ratio feedback correction coefficient α to a reference value.

[0047] In the internal combustion engine 1 of the first embodiment, if a step-like decrease in the purge rate occurs due to supercharging when the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to purge gas, the correction coefficient initialization process is performed at a timing delayed by the predetermined time from the timing when the inflow of purge gas into the intake passage 2 is stopped due to supercharging.

[0048] This allows the internal combustion engine 1 to bring the air-fuel ratio feedback correction coefficient α closer to a value that corresponds to the actual air-fuel ratio, thereby suppressing fluctuations (deterioration) of the air-fuel ratio. In other words, the internal combustion engine 1 can suppress a large fluctuation in the air-fuel ratio to the lean side when a step-like decrease in the purge rate occurs due to the supply of purge gas being stopped.

[0049] Furthermore, the internal combustion engine 1 performs the correction coefficient initialization process at a timing delayed by the predetermined time from the timing at which the flow of purge gas into the intake passage 2 stops, thereby preventing the air-fuel ratio from becoming rich.

[0050] The timing at which the supply of purge gas stops is the timing at which the purge control valve 24 closes. Therefore, even if the purge control valve 24 is closed, there is purge gas in the intake passage 2 that has not reached (not been taken into) the combustion chambers of the internal combustion engine 1. The purge gas that is present in the intake passage 2 of the internal combustion engine 1 immediately after the purge control valve 24 closes decreases over time because it is gradually sent to the combustion chambers of the internal combustion engine 1. Therefore, the internal combustion engine 1 can prevent the air-fuel ratio from becoming rich by returning the air-fuel ratio feedback correction coefficient α to a reference value in consideration of the purge gas remaining in the intake passage 2 immediately after the purge control valve 24 closes.

[0051] When the purge rate is equal to or greater than the determination threshold value, the internal combustion engine 1 determines that the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to the purge gas. Therefore, when the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to a factor other than the purge gas, the internal combustion engine 1 can avoid performing the correction coefficient initialization process.

[0052] Furthermore, the internal combustion engine 1 performs the correction coefficient initialization process when the air-fuel ratio feedback correction coefficient α is equal to or smaller than a predetermined correction coefficient threshold value that is smaller than the reference value. Therefore, the internal combustion engine 1 can prevent the air-fuel ratio from being adversely affected by unnecessary execution of the correction coefficient initialization process.

[0053] The correction coefficient threshold value is an index of the air-fuel ratio feedback correction coefficient α that would cause the air-fuel ratio to exceed a predetermined lean limit value on the lean side that is set in advance if the correction coefficient initialization process is not performed when the purge rate decreases in a stepwise manner.

[0054] That is, when the air-fuel ratio feedback correction coefficient α is equal to or less than the correction coefficient threshold, if the correction coefficient initialization process is not performed when the purge rate decreases in a stepwise manner, the air-fuel ratio will exceed the lean limit value. On the other hand, when the air-fuel ratio feedback correction coefficient α is equal to or less than the reference value and greater than the correction coefficient threshold, the deviation of the air-fuel ratio feedback correction coefficient from the reference value is small, so that the air-fuel ratio will not exceed the lean limit value even if the correction coefficient initialization process is not performed when the purge rate decreases in a stepwise manner. In other words, the internal combustion engine 1 performs the correction coefficient initialization process when the air-fuel ratio of the internal combustion engine 1 will exceed the lean limit value as the purge rate decreases in a stepwise manner.

[0055] The internal combustion engine 1 performs the correction coefficient initialization process when the intake pressure in the intake manifold 7 is equal to or higher than the pressure threshold value. Therefore, the internal combustion engine 1 can avoid performing the correction coefficient initialization process due to factors other than supercharging and a decrease in the intake pressure (negative pressure).

[0056] The internal combustion engine 1 determines that the purge has been cut when the purge rate falls below the first purge threshold value, and therefore can avoid performing the correction coefficient initialization process until the amount of purge gas decreases to a level that does not affect the air-fuel ratio.

[0057] Another embodiment of the present invention will be described below, in which the same components as those in the above-described embodiment are designated by the same reference numerals and redundant description will be omitted.

[0058] In the second embodiment of the present invention, the correction coefficient initialization process described above is applied to an internal combustion engine that does not have a turbocharger 9 .

[0059] The internal combustion engine of the second embodiment has substantially the same configuration as the internal combustion engine 1 of the first embodiment, and when a stepwise decrease in the purge rate occurs due to an increase in intake pressure, the correction coefficient initialization process is performed at a timing delayed by the predetermined time from the timing when the inflow of purge gas into the intake passage 2 becomes less than the second purge threshold. The second purge threshold is a predetermined value that is set in advance and is an index for determining whether purge cut has been performed.

[0060] In the second embodiment, the correction coefficient initialization process is performed when the purge rate decreases at a rate equal to or greater than a predetermined rate threshold, the purge rate becomes less than the second purge threshold, and the supply amount of purge gas becomes a small amount less than a predetermined amount, where the rate of change is the amount of change in the purge rate per unit time.

[0061] The air-fuel ratio feedback correction coefficient α is a value that follows the change in the purge ratio when the change in the purge ratio is not rapid (gradual). Therefore, if the correction coefficient initialization process is performed when the purge ratio changes gradually and becomes less than the second purge threshold, there is a risk that the air-fuel ratio will fluctuate significantly with respect to the target air-fuel ratio (stoichiometric air-fuel ratio). Therefore, the internal combustion engine of the second embodiment performs the correction coefficient initialization process only when a step-like decrease in the purge ratio occurs.

[0062] FIG. 4 is a timing chart showing an example of the behavior of various parameters when purge is cut in the internal combustion engine of the second embodiment.

[0063] Time t1 in Fig. 4 is the timing when the accelerator pedal is depressed, the idling state of the internal combustion engine ends, and purging is started. The estimated purge gas concentration value shown by the solid line in Fig. 4 is an erroneous estimate on the lean side relative to the actual purge gas concentration shown by the dashed line Lc in Fig. 4. Therefore, the air-fuel ratio of the internal combustion engine and the air-fuel ratio feedback correction coefficient α shift to the rich side when purging is started.

[0064] In the example shown in Fig. 4, the value of the air-fuel ratio feedback correction coefficient α when the air-fuel ratio is set to the stoichiometric air-fuel ratio without purge gas being introduced is a value that is larger by a predetermined amount than the reference value. This is due to product variations in internal combustion engines. Therefore, the estimated value of the purge gas concentration is erroneously estimated to be lean, as described above.

[0065] 4, the air-fuel ratio feedback correction coefficient α reaches a predetermined lower threshold value. The air-fuel ratio feedback correction coefficient α is set so as not to be smaller than the lower threshold value, which is a predetermined amount smaller than the reference value.

[0066] The air-fuel ratio feedback correction coefficient α reaches the lower limit threshold value from time t2 in Figure 4 and is limited so as not to become a smaller value. Furthermore, since the air-fuel ratio feedback correction coefficient α is limited by the lower limit threshold value, the purge rate does not change from the value corresponding to the lower limit threshold value from time t2 in Figure 4. Note that the purge rate has reached a value equal to or greater than the determination threshold value prior to time t2. In the second embodiment, when the purge rate reaches or exceeds the determination threshold value, it is determined that the air-fuel ratio feedback correction coefficient α has changed due to purging.

[0067] Time t3 in Figure 4 is the timing when the accelerator pedal is fully opened and the intake pressure (intake manifold pressure) in the intake manifold 7 rises to a value close to atmospheric pressure, causing the purge rate to become smaller than the second purge threshold, which is smaller than the determination threshold. In the second embodiment, it is determined that purge cut has been performed when the purge rate falls below the second purge threshold. The second purge threshold is greater than the first purge threshold. Note that the intake pressure in the intake manifold 7 is greater than the pressure threshold due to the full accelerator pedal opening prior to time t3. In the second embodiment, it is determined that purge cut has been performed when the intake pressure in the intake manifold 7 becomes greater than the pressure threshold and the purge rate becomes less than the second purge value threshold.

[0068] Time t4 in Fig. 4 is the timing when the predetermined time has elapsed since time t3, and is the timing when the correction coefficient initialization process is performed. In the second embodiment, the air-fuel ratio feedback correction coefficient α is suddenly returned toward the reference value at time t4 in Fig. 4. The characteristic line S1 shown by a dashed line in Fig. 2 shows how the air-fuel ratio feedback correction coefficient α changes when the air-fuel ratio feedback correction coefficient α is not suddenly returned to the reference value at time t4. The characteristic line S2 shown by a dashed line in Fig. 2 shows how the air-fuel ratio of the internal combustion engine changes when the air-fuel ratio feedback correction coefficient α changes as shown by the characteristic line S1.

[0069] As shown in FIG. 4, the air-fuel ratio of the internal combustion engine is prevented from fluctuating toward the lean side by returning the air-fuel ratio feedback correction coefficient α to the reference value at a time t4.

[0070] FIG. 5 is a flowchart showing the flow of control of the internal combustion engine in the second embodiment.

[0071] In step S11, it is determined whether or not the purge has been cut off from a state in which the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to the purge gas.

[0072] That is, the purge rate and the air-fuel ratio feedback correction coefficient α are used to determine whether or not the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to the flow of purge gas into the intake passage 2. When the purge rate is equal to or greater than the determination threshold value and the air-fuel ratio feedback correction coefficient α is equal to or less than the correction coefficient threshold value, it is determined that the air-fuel ratio feedback correction coefficient α has deviated from the reference value due to the purge gas.

[0073] Also, when the intake pressure in the intake manifold 7 becomes equal to or greater than the pressure threshold, it is determined that purge has been cut due to the increase in intake pressure.

[0074] In step S12, it is determined whether or not the purge rate has suddenly decreased. That is, in step S12, if the purge rate has decreased at a rate of change equal to or greater than a predetermined threshold value, it is determined that a stepwise decrease in the purge rate has occurred.

[0075] In step S13, it is determined whether the purge rate is less than the second purge threshold. If the purge rate is less than the second purge threshold, the process proceeds to step S14, where a delay timer is started to count. The delay timer measures the predetermined time.

[0076] In step S15, it is determined whether or not the predetermined time has elapsed since the delay timer started counting, and if so, the process proceeds to step S16.

[0077] In step S16, a correction coefficient initialization process is carried out to return the air-fuel ratio feedback correction coefficient α to a reference value.

[0078] In the internal combustion engine of the second embodiment, by carrying out the correction coefficient initialization process, the air-fuel ratio feedback correction coefficient α can be brought closer to a value that corresponds to the actual air-fuel ratio, and fluctuations (deterioration) of the air-fuel ratio can be suppressed, thereby suppressing the air-fuel ratio from fluctuating significantly toward the lean side.

[0079] Furthermore, in the internal combustion engine of the second embodiment, the correction coefficient initialization process is performed at a timing delayed by the predetermined time, so that the air-fuel ratio can be prevented from becoming rich.

[0080] In the internal combustion engine of the second embodiment, when the purge rate is equal to or greater than the determination threshold value, it is determined that the air-fuel ratio feedback correction coefficient has deviated from the reference value due to the purge gas. Therefore, when the air-fuel ratio feedback correction coefficient α deviates from the reference value due to a factor other than the purge gas, the correction coefficient initialization process can be prevented from being performed.

[0081] Furthermore, the internal combustion engine of the second embodiment performs the correction coefficient initialization process when the air-fuel ratio feedback correction coefficient α is equal to or less than the correction coefficient threshold value and the air-fuel ratio of the internal combustion engine exceeds the lean limit value due to a stepwise decrease in the purge rate.

[0082] Therefore, the internal combustion engine of the second embodiment can prevent the air-fuel ratio from being adversely affected by the unnecessary execution of the correction coefficient initialization process.

[0083] The internal combustion engine of the second embodiment performs the correction coefficient initialization process when the intake pressure in the intake manifold 7 is equal to or greater than the pressure threshold, and therefore can avoid the correction coefficient initialization process being performed due to factors other than a decrease in intake pressure (negative pressure).

[0084] The internal combustion engine of the second embodiment determines that the purge has been cut when the purge rate is less than the second purge threshold value, and therefore can avoid performing the correction coefficient initialization process until the amount of purge gas has decreased to a level that does not affect the air-fuel ratio.

[0085] The internal combustion engine of the second embodiment performs the correction coefficient initialization process when the purge rate decreases at a rate equal to or greater than the change rate threshold and the amount of purge gas supplied to the intake passage 2 becomes a small amount equal to or less than a predetermined amount. Whether the amount of purge gas supplied is a small amount equal to or less than the predetermined amount is determined by whether the purge rate is less than the second purge threshold. That is, the internal combustion engine of the second embodiment determines that the amount of purge gas supplied to the intake passage 2 is a small amount less than the predetermined amount when the purge rate becomes less than the second threshold.

[0086] In the internal combustion engine of the second embodiment, even if the supply of purge gas to the intake passage 2 is not stopped, if the amount of purge gas supplied is suddenly reduced, there is a risk that the air-fuel ratio will fluctuate significantly to the lean side.

[0087] Therefore, in the internal combustion engine of the second embodiment, even if the supply of purge gas to the intake passage 2 is not stopped, when a sudden decrease in the supplied purge gas causes a step-like decrease in the purge rate and the purge rate becomes less than the second purge threshold, the correction coefficient initialization process is performed to prevent the air-fuel ratio from fluctuating significantly to the lean side.

[0088] Furthermore, the internal combustion engine of the second embodiment can avoid performing the correction coefficient initialization process until the supply amount of purge gas becomes small.

[0089] While specific embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the invention.

[0090] For example, the return amount of the air-fuel ratio feedback correction coefficient α in the correction coefficient initialization process may be corrected in accordance with the atmospheric pressure. The lower the atmospheric pressure, the greater the amount of evaporated fuel in the purge gas. Therefore, when performing the correction coefficient initialization process, instead of returning the air-fuel ratio feedback correction coefficient α to the reference value, the return amount toward the reference value may be corrected, for example, so that the higher the atmospheric pressure, the smaller the return amount. In other words, when performing the correction coefficient initialization process, the air-fuel ratio feedback correction coefficient α may be returned so as to be smaller than the reference value as the atmospheric pressure increases.

[0091] For example, the return amount of the air-fuel ratio feedback correction coefficient α in the correction coefficient initialization process may be corrected in accordance with the fuel temperature in the fuel tank 20. The higher the fuel temperature, the more evaporated fuel there is in the purge gas. Therefore, when performing the correction coefficient initialization process, instead of returning the air-fuel ratio feedback correction coefficient α to the reference value, the return amount toward the reference value may be reduced, for example, as the fuel temperature decreases. In other words, when performing the correction coefficient initialization process, the air-fuel ratio feedback correction coefficient α may be returned to a value smaller than the reference value as the fuel temperature decreases.

[0092] The above-described embodiments relate to a control method for an internal combustion engine and a control device for an internal combustion engine.

Claims

1. In a control method for an internal combustion engine that uses an air-fuel ratio feedback correction coefficient to perform feedback control so that the air-fuel ratio becomes the stoichiometric air-fuel ratio, If the air-fuel ratio feedback correction coefficient deviates from a predetermined reference value due to purge gas containing evaporated fuel from the fuel tank, a step-like decrease in the purge rate occurs, and a correction coefficient initialization process is performed to return the air-fuel ratio feedback correction coefficient to the reference value. A control method for an internal combustion engine that corrects the amount of return of the air-fuel ratio feedback correction coefficient in the above correction coefficient initialization process according to atmospheric pressure.

2. The control method for an internal combustion engine according to claim 1, wherein the stepwise reduction in the purge rate is caused by supercharging.

3. A control method for an internal combustion engine according to claim 2, wherein the correction coefficient initialization process is performed at a predetermined time delay from the timing at which the inflow of the purge gas into the intake passage stops due to supercharging.

4. A control method for an internal combustion engine according to claim 2, wherein, when the purge rate is above a predetermined threshold, it is determined that the air-fuel ratio feedback correction coefficient has deviated from the reference value due to the purge gas.

5. The control method for an internal combustion engine according to claim 2, wherein the correction coefficient initialization process is performed when the above air-fuel ratio feedback correction coefficient is less than or equal to a predetermined correction coefficient threshold.

6. The control method for an internal combustion engine according to claim 2, wherein the correction coefficient initialization process is performed when the intake pressure in the intake manifold is equal to or greater than a predetermined pressure threshold.

7. A control method for an internal combustion engine according to claim 3, wherein it is determined that the inflow of the purge gas into the intake passage has stopped when the purge rate falls below a predetermined first purge threshold.

8. A control method for an internal combustion engine according to claim 1, wherein the correction coefficient initialization process is performed when the purge rate decreases at a rate of change equal to or greater than a predetermined rate of change threshold, and the supply amount of the purge gas becomes a small supply amount less than a predetermined amount.

9. The above rate of change is the amount of fluctuation in the purge rate per unit time, as described in claim 8, for the control method of an internal combustion engine.

10. A control method for an internal combustion engine according to claim 8, wherein when the purge rate falls below a predetermined second purge threshold, it is determined that the supply amount of the purge gas has become the low supply amount.

11. (delete)

12. The control method for an internal combustion engine according to claim 1, wherein the amount of return of the air-fuel ratio feedback correction coefficient in the above correction coefficient initialization process is corrected according to the fuel temperature in the fuel tank.

13. It has a control unit that uses an air-fuel ratio feedback correction coefficient to perform feedback control so that the air-fuel ratio becomes the stoichiometric air-fuel ratio. The control unit described above, when the air-fuel ratio feedback correction coefficient deviates from a predetermined reference value due to purge gas containing evaporated fuel from the fuel tank, performs a correction coefficient initialization process to return the air-fuel ratio feedback correction coefficient toward the reference value when a stepwise decrease in the purge rate occurs, and corrects the amount of the air-fuel ratio feedback correction coefficient returned in the correction coefficient initialization process according to atmospheric pressure.