Catalyst deterioration diagnostic device for internal combustion engine

Abstract

The invention provides a catalyst deterioration diagnosis device for an internal combustion engine, which can generate an expected upstream A / F change state and can diagnose the deterioration of a catalyst in an appropriate diagnosis environment. The present invention is provided with: an air-fuel ratio detection unit for detecting the downstream air-fuel ratio of a catalyst; an A / F vibration correction value calculation unit that calculates an A / F vibration correction value for causing the upstream air-fuel ratio of the catalyst to vibrate at a predetermined period and amplitude; a downstream O2 feedback correction value calculation unit that calculates a downstream O2 feedback correction value for varying the upstream air-fuel ratio on the basis of the downstream air-fuel ratio; a diagnostic correction value calculation unit that adds the A / F vibration correction value and the downstream O2 feedback correction value to calculate a diagnostic correction value; a fuel injection control unit that injects fuel on the basis of the diagnostic correction value; an inversion counting unit that counts the number of inversions in which the downstream air-fuel ratio is between rich and lean; and a deterioration diagnosis unit that diagnoses the degree of deterioration of the catalyst on the basis of the number of vibrations of the A / F vibration correction value and the number of invertions of the inversion counting unit.

Description

Technical Field

[0001] The present invention relates to a catalyst degradation diagnostic device for an internal combustion engine. Background Art

[0002] Exhaust gas purification components such as three-way catalysts (hereinafter referred to as catalysts) installed in the exhaust passage of an internal combustion engine have a so-called O2 storage capacity, which allows them to store oxygen. Utilizing this property, the catalyst takes in excess oxygen when the incoming exhaust gas becomes lean and stores it, and releases this stored oxygen when the incoming exhaust gas becomes rich to purify the exhaust gas. There is an upper limit to the amount of oxygen that can be stored in the catalyst. If the OSC (oxygen storage capacity) specified as this upper limit is exceeded, the catalyst passes the exhaust gas through without storing oxygen. The exhaust gas purification performance of the catalyst gradually decreases with operation, and there is a high correlation between the degree of deterioration and the OSC. As purification performance decreases, the OSC also decreases. Therefore, methods for diagnosing catalyst degradation using OSC have been put into practical use.

[0003] For example, the degradation diagnosis device described in Patent Document 1 performs air-fuel ratio feedback (FB) based on the output of an O2 sensor located upstream of the catalyst (hereinafter referred to as the upstream O2 sensor). This causes the air-fuel ratio (A / F) of the exhaust gas flowing into the catalyst (hereinafter referred to as the upstream A / F) to oscillate between rich and lean. The device then counts the number of times the A / F of the exhaust gas flowing out of the catalyst (hereinafter referred to as the downstream A / F), detected by an O2 sensor located downstream of the catalyst (hereinafter referred to as the downstream O2 sensor), reverses between rich and lean. Catalyst degradation is diagnosed based on the ratio of the number of upstream A / F oscillations to the number of downstream A / F reversals.

[0004] When the catalyst is not deteriorating and has sufficient OSC, its O2 storage capacity causes the downstream A / F ratio to fluctuate smoothly, suppressing the number of reversals and thus preventing a determination of catalyst degradation. In contrast, as catalyst degradation progresses and OSC decreases, the downstream A / F ratio's fluctuations gradually resemble those of the downstream A / F ratio, leading to a frequent count of reversals and a determination of degradation at some point. Prior art literature Patent Literature

[0005] Patent Document 1: Japanese Patent No. 2722767 Summary of the Invention Technical problem to be solved by the invention

[0006] In diagnosing catalyst degradation as described above, it is crucial to ensure that the number of downstream A / F reversals changes clearly as OSC decreases. To create this diagnostic environment, it is necessary to appropriately set the period and amplitude of upstream A / F vibration, and to achieve the desired upstream A / F vibration state based on these period and amplitude. However, the technology of Patent Document 1 cannot meet this requirement because the detection information from the upstream O2 sensor, which serves as an indicator of the air-fuel ratio (FB), is unreliable for the following reasons.

[0007] The first reason is that the upstream O2 sensor's detection target, the exhaust gas, is non-uniform. Exhaust gas discharged from the internal combustion engine into the exhaust passage flows into the catalyst, where it is agitated as it passes through the catalyst's multiple structural units, and then flows out of the catalyst in a more uniform state. The upstream O2 sensor detects exhaust gas flowing into the catalyst, which has a non-uniform oxygen concentration. This leads to a discrepancy between its detected value and the actual upstream air-fuel ratio.

[0008] The second reason is significant degradation of the upstream O2 sensor. Exhaust gas from the internal combustion engine gradually cools as it flows through the exhaust passage, and harmful components are removed as it passes through the catalyst. However, since the upstream O2 sensor is continuously exposed to high-temperature exhaust gas containing harmful components, it degrades more significantly than, for example, the downstream O2 sensor, inevitably including significant errors in its detection information.

[0009] As a result, the technique of Patent Document 1 fails to produce the desired upstream A / F vibration state due to the air-fuel ratio FB based on the less reliable detection information from the upstream O2 sensor. Consequently, there is a problem of performing degradation diagnosis under inappropriate diagnostic conditions, resulting in reduced diagnostic accuracy.

[0010] The present invention has been made to solve such problems, and its object is to provide a catalyst degradation diagnosis device for an internal combustion engine that can generate a desired upstream A / F fluctuation state, thereby enabling catalyst degradation diagnosis to be performed under an appropriate diagnostic environment and improving diagnostic accuracy. Technical means for solving technical problems

[0011] In order to achieve the above-mentioned object, the catalyst degradation diagnostic device of the internal combustion engine of the present invention is characterized in that it includes: an air-fuel ratio detection unit, which is arranged on the downstream side of a catalyst inserted in the exhaust passage of the internal combustion engine, and detects the air-fuel ratio of the exhaust gas flowing out of the catalyst; a vibration waveform storage unit, which pre-stores the period and amplitude of the A / F vibration correction value for vibrating the air-fuel ratio of the exhaust gas flowing into the catalyst; an A / F vibration correction value calculation unit, which calculates the A / F vibration correction value based on the period and amplitude stored in the vibration waveform storage unit; a downstream O2 feedback correction value calculation unit, which calculates the A / F vibration correction value based on the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detection unit. a downstream O2 feedback correction value in which the air-fuel ratio of the exhaust gas of the catalyst varies with a period longer than the A / F vibration correction value and centered on the ideal air-fuel ratio; a diagnostic correction value calculation unit that calculates the diagnostic correction value by adding the A / F vibration correction value to the downstream O2 feedback correction value; a fuel injection control unit that performs internal combustion engine fuel injection control based on the diagnostic correction value; a reversal counting unit that counts the number of times the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detection unit reverses between rich and lean; and a degradation diagnosis unit that diagnoses the degree of degradation of the catalyst based on the number of vibrations of the air-fuel ratio of the exhaust gas based on the A / F vibration correction value and the number of reversals counted by the reversal counting unit.

[0012] As another method, the vibration waveform storage unit can also store the period and amplitude of the A / F vibration correction value for each operating area of the internal combustion engine, and the A / F vibration correction value calculation unit reads the period and amplitude corresponding to the operating area of the internal combustion engine from the vibration waveform storage unit and applies them to the calculation processing of the A / F vibration correction value.

[0013] As another embodiment, the reverse counting unit may start counting when a predetermined waiting time has elapsed after the fuel injection control unit starts the fuel injection control based on the diagnostic correction value.

[0014] As another embodiment, the degradation diagnosis unit can also diagnose the degree of degradation of the catalyst when the number of vibrations of the air-fuel ratio of the exhaust gas based on the A / F vibration correction value reaches a predetermined first judgment number, and when the number of reversals counted by the reversal counting unit reaches a predetermined second judgment number that is smaller than the first judgment number.

[0015] As another method, the downstream O2 feedback correction value calculation unit may make a rich judgment and make the downstream O2 feedback correction value lean when the state in which the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detection unit exceeds the ideal air-fuel ratio and changes toward the rich side continues for a predetermined control judgment time, and may make a lean judgment and make the downstream O2 feedback correction value rich when the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detection unit exceeds the ideal air-fuel ratio and changes toward the lean side continues for a predetermined control judgment time.

[0016] As another aspect, the control determination time may be set to a value exceeding a half period of the vibration waveform based on the A / F vibration correction value.

[0017] As another embodiment, the control determination time may be composed of a rich determination time for rich determination and a lean determination time for lean determination, and the rich determination time and the lean determination time may be set to different values. Effects of the Invention

[0018] According to the catalyst degradation diagnosis device for an internal combustion engine of the present invention, it is possible to generate an expected upstream A / F fluctuation state, thereby enabling catalyst degradation diagnosis to be performed under an appropriate diagnosis environment, thereby improving the diagnosis accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 1 is a system configuration diagram showing a catalyst degradation diagnostic device for an internal combustion engine according to an embodiment. Figure 2 This is a control block diagram showing the configuration of an ECU for diagnosing catalyst degradation. Figure 3 This is an explanatory diagram showing the relationship between the A / F vibration correction value, the downstream O2FB correction value, and the diagnostic correction value. Figure 4 is a flow chart showing the waveform generation and injection control routine executed by the ECU. Figure 5 is a flowchart representing the downstream O2FB correction value calculation routine executed by the ECU. Figure 6 This is a time chart showing the control state of the degradation diagnosis mode when the catalyst is normal. Figure 7 : is a flowchart showing a reverse number counting routine executed by the ECU. Figure 8 is a flowchart showing a degradation diagnosis routine executed by the ECU. Figure 9 This is a time chart showing the control state of the degradation diagnosis mode when catalyst degradation progresses. DETAILED DESCRIPTION

[0020] Hereinafter, an embodiment in which the present invention is embodied as a catalyst degradation diagnostic device for an internal combustion engine will be described. Figure 1 1 is a system configuration diagram showing a catalyst degradation diagnostic device 1 for an internal combustion engine according to the present embodiment. In this diagram, a cross section of a specific cylinder of the internal combustion engine is shown, but the other cylinders have the same configuration.

[0021] A piston 3 is slidably inserted into a cylinder 2a formed in a cylinder block 2 of the internal combustion engine and is connected to a crankshaft (not shown) via a connecting rod 4. A cylinder head 5 is fixed to the cylinder block 2, and a combustion chamber 6 for each cylinder is defined between the lower surface of the cylinder head 5 and the upper surface of the piston 3. The tip of a spark plug 8 driven by an ignition coil 7 is inserted into the combustion chamber 6 of each cylinder. The combustion chamber 6 of each cylinder communicates with a common intake passage 10 via an intake manifold 9 and with a common exhaust passage 12 via an exhaust manifold 11.

[0022] The cylinder head 5 is provided with an intake camshaft 13 and an exhaust camshaft 14, each of which is driven to rotate in conjunction with the crankshaft. The intake camshaft 13 opens and closes an intake valve 15 at a predetermined crank angle, thereby connecting or blocking the combustion chamber 6 and the intake passage 10. Furthermore, the exhaust camshaft 14 opens and closes an exhaust valve 16 at a predetermined crank angle, thereby connecting or blocking the combustion chamber 6 and the exhaust passage 12.

[0023] An air cleaner 17 and a throttle device 18 are provided in the intake passage from the upstream side, and a fuel injection valve 19 is provided corresponding to each cylinder in the intake manifold 9. In addition, a catalyst (three-way catalyst) 20 is inserted in the exhaust passage 12.

[0024] During engine operation, intake air filtered by air cleaner 17 has its flow rate adjusted based on the opening of throttle valve 18a built into throttle device 18. It is then distributed to each cylinder by intake manifold 9, injected with fuel from fuel injection valve 19, and supplied as a mixture into combustion chamber 6 when intake valve 15 opens. When spark plug 8 ignites at a predetermined crank angle near compression top dead center, the mixture combusts, depressing piston 3 and applying torque to the crankshaft via connecting rod 4. Exhaust gas after combustion is discharged into exhaust passage 12 when exhaust valve 16 opens, where it is purified as it passes through catalyst 20 before being discharged to the outside.

[0025] The ECU 30 (engine control unit) includes an input / output device, a storage device (ROM, RAM, non-volatile RAM, etc.), a central processing unit (CPU), a timer counter, and the like. Connected to the input side of the ECU 30 are various sensors, including a throttle position sensor 31, an intake air temperature sensor 32, and an intake air pressure sensor 33, all built into the throttle device 18. Furthermore, connected to the ECU 30 are various sensors, including a water temperature sensor 34, a crank angle sensor 35, an upstream O2 sensor 36 located upstream of the catalyst 20, and a downstream O2 sensor 37 located downstream of the catalyst 20. The downstream O2 sensor 37 corresponds to the "air-fuel ratio detection unit" of the present invention.

[0026] Detection information from various sensors, such as the throttle opening, intake air temperature, intake air pressure, engine coolant temperature, engine speed, the air / fuel ratio (A / F) of the exhaust gas flowing into the catalyst 20 (upstream A / F), and the air / fuel ratio (A / F) of the exhaust gas flowing out of the catalyst 20 (downstream A / F), is input to the ECU 30. Furthermore, various devices, such as an electric motor (not shown) for driving the throttle valve 18a of the throttle device 18 and the fuel injection valve 19 for each cylinder, are connected to the output side of the ECU 30. Based on the detection information from the various sensors, the ECU 30 controls the throttle opening, the fuel injection amount, and the like, thereby operating the internal combustion engine.

[0027] Since the exhaust gas purification performance of the catalyst 20 gradually deteriorates during operation, the ECU 30 of the present embodiment has a function for diagnosing degradation of the catalyst 20 as a countermeasure. However, in a configuration where such a degradation diagnosis device performs air-fuel ratio (FB) adjustment based on detection information from an upstream O2 sensor with low reliability, the expected upstream A / F vibration state cannot be generated, resulting in a problem of reduced accuracy in degradation diagnosis.

[0028] In this embodiment, air-fuel ratio feedback (hereinafter also referred to as air-fuel ratio FB) is performed based on the output of the downstream O2 sensor 37, rather than the output of the upstream O2 sensor 36. This air-fuel ratio FB causes the upstream air-fuel ratio to oscillate at a predetermined period and amplitude, switching the upstream air-fuel ratio between rich and lean, centered around the ideal air-fuel ratio. Air-fuel ratio FB based on the output of the downstream O2 sensor 37 is referred to as downstream O2 feedback control (hereinafter also referred to as downstream O2FB control). The following describes in detail the catalyst 20 degradation diagnosis including this downstream O2FB control.

[0029] In the normal operating mode, where catalyst 20 degradation diagnosis is not performed, the ECU 30 calculates a base injection quantity based on intake pressure and engine speed, for example, using a pre-set control map. The ECU 30 then performs fuel injection control based on the final injection quantity obtained by correcting this base injection quantity using various correction values derived from factors such as intake air temperature and upstream air / fuel ratio. During engine operation in this normal operating mode, when predetermined diagnostic start conditions are met, the ECU 30 initiates a diagnostic mode for diagnosing catalyst 20 degradation. For example, diagnostic start conditions include completion of engine warm-up, activation of the downstream O2 sensor 37, and stable engine operation within a predetermined range. When all of these conditions are met, the ECU 30 switches from the normal operating mode to the degradation diagnostic mode, initiating degradation diagnosis.

[0030] Figure 2 is a control block diagram showing the configuration of the ECU 30 for diagnosing degradation of the catalyst 20. Figure 3 This is an explanatory diagram showing the relationship between the A / F vibration correction value, the downstream O2FB correction value, and the diagnostic correction value. like Figure 2 As shown, the ECU 30 includes a vibration waveform storage unit 30a, an A / F vibration correction value calculation unit 30b, a downstream O2FB correction value calculation unit 30c, a diagnostic correction value calculation unit 30d, a fuel injection control unit 30e, a reversal counter unit 30f, and a degradation diagnosis unit 30g.

[0031] The vibration waveform storage unit 30a serves to store the Figure 3 The function of the period and amplitude of the A / F vibration correction value shown. As will be described in detail later, in the degradation diagnosis, while the upstream A / F is varied, the degree of degradation of the catalyst 20 is diagnosed based on the number of reversals of the downstream A / F between rich and lean. In order to obtain a highly accurate diagnostic result, it is necessary to create a diagnostic environment in which the number of reversals of the downstream A / F varies significantly depending on the progression of degradation of the catalyst 20 and the reduction in OSC. The A / F vibration correction value is one of the elements that varies the upstream A / F, and the A / F vibration correction value used to create a suitable diagnostic environment differs depending on the operating area of the internal combustion engine. Therefore, in the present embodiment, for example, the period and amplitude of the respectively preferred A / F vibration correction values are stored in the vibration waveform storage unit 30a according to the operating area of the internal combustion engine specified by the engine speed and the intake pressure.

[0032] The A / F vibration correction value calculation unit 30b reads the period and amplitude of the A / F vibration correction value corresponding to the current engine operating range from the vibration waveform storage unit 30a and, based on these period and amplitude, calculates the A / F vibration correction value used to vary the upstream A / F ratio. Because the period of variation of the A / F vibration correction value is shorter than the period of variation of the downstream O2FB correction value (described below), the variation behavior of the upstream A / F ratio based on the A / F vibration correction value will be referred to as "vibration" in the following description.

[0033] The downstream O2FB correction value calculation unit 30c performs the following functions: judging the rich / lean state of the downstream A / F based on the output of the downstream O2 sensor 37, and calculating the value for changing the upstream A / F based on the judgment result. Figure 3 Downstream O2FB correction values shown.

[0034] The diagnostic correction value calculation unit 30d adds the A / F vibration correction value and the downstream O2FB correction value to calculate Figure 3 Function of the diagnostic correction value shown. The fuel injection control unit 30 e has a function of adding a diagnostic correction value to a basic injection amount used in the normal operation mode to calculate a final injection amount, and driving the fuel injection valve 19 based on the final injection amount to perform fuel injection control.

[0035] The reversal counter 30f counts the number of times the downstream A / F reverses to the lean side. Specifically, in the degradation diagnosis mode, the reversal counter 30f constantly monitors whether the downstream A / F reverses from rich to lean. Each time a lean determination is made through reversal, the reversal counter N2, described below, is incremented. Incrementing the reversal counter N2 corresponds to "counting up" in the present invention.

[0036] The degradation diagnosis unit 30g functions to diagnose degradation of the catalyst 20 based on a reversal number ratio R, which is a ratio between a vibration counter N1 and a reversal counter N2 described later based on an A / F vibration correction value.

[0037] Next, the processing executed by ECU 30 in the degradation diagnosis mode will be described. When the diagnosis start condition is satisfied during the operation of the internal combustion engine, the ECU 30 executes the following control at a predetermined control interval: Figure 4 The waveform generation and injection control routine shown is executed by the vibration waveform storage unit 30a, the A / F vibration correction value calculation unit 30b, the downstream O2FB correction value calculation unit 30c, the diagnostic correction value calculation unit 30d, and the fuel injection control unit 30e.

[0038] First, in step S1, the period and amplitude of the A / F vibration correction value corresponding to the current operating range of the internal combustion engine are read from the vibration waveform storage unit 30a. In step 2, based on the read period and amplitude, the A / F vibration correction value used to vibrate the upstream A / F is calculated. Subsequently, in step 3, the calculation process of the downstream O2FB correction value is executed, and the details of this process will be described later. After the processing in step 3 is completed, the process transfers to step 4, where the A / F vibration correction value and the downstream O2FB correction value are added to calculate a diagnostic correction value. Next, in step 5, the diagnostic correction value is added to the basic injection amount to calculate the final injection amount. In step 6, the fuel injection valve 19 is driven according to the final injection amount to perform fuel injection control, and the routine is temporarily terminated.

[0039] Thus, whenever you execute Figure 4 During the routine, the A / F vibration correction value, downstream O2FB correction value, diagnostic correction value and final injection amount are calculated. Figure 3 The downstream O2FB correction value calculated successively is also changed according to the vibration waveform in Figure 3 As a result, in the degradation diagnosis mode, based on the Figure 3 Fuel injection control is performed based on the final injection amount obtained by the diagnostic correction value that changes according to the final waveform in the flow.

[0040] The A / F ratio of the exhaust gas flowing into the catalyst 20, or the upstream A / F ratio, is affected by increases and decreases in the base injection amount, but generally changes in accordance with increases and decreases in the diagnostic correction value. Therefore, the upstream A / F ratio generally changes in accordance with the increase and decrease cycle of the final waveform based on the diagnostic correction value. When the diagnostic correction value increases, the upstream A / F ratio shifts toward the rich side, and when the diagnostic correction value decreases, the upstream A / F ratio shifts toward the lean side.

[0041] In addition, it takes a certain amount of time before the change in the upstream A / F based on the setting of the downstream O2FB correction value is reflected in the downstream A / F. For example, when the upstream A / F is lean, the remaining oxygen in the exhaust gas is stored in the catalyst 20, and the downstream A / F will not change to the lean side until its oxygen storage reaches OSC. On the other hand, if the upstream A / F is rich, oxygen is released from the catalyst 20 to purify the exhaust gas, and the downstream A / F will not change to the rich side until its oxygen storage reaches 0. Therefore, the period of the FB waveform is longer than the period of the vibration waveform. As a result, the final waveform is as follows: Figure 3 As shown, a characteristic is obtained in which minute vibrations reflecting the short period and amplitude of the vibration waveform are repeated, while gentle fluctuations reflecting the longer period and amplitude of the FB waveform are repeated.

[0042] On the other hand, when ECU 30 transfers to Figure 4 Step 3: Start Figure 5 The downstream O2FB correction value calculation routine shown in FIG. 1 is first performed in step S21 to determine whether the upstream A / F is currently being leaned, that is, whether the upstream A / F is changing to the lean side. If the determination is "yes" (affirmative), the routine proceeds to step S22 to determine whether the output of the downstream O2 sensor 37 has exceeded 0.55V, which corresponds to the ideal air-fuel ratio, and is decreasing. In the following step S23, it is determined whether the lean determination time Tfb, which is preset as the control determination time, has elapsed. -L When the determination in either step 22 or step 23 is "No", that is, when the downstream A / F has not exceeded the ideal air-fuel ratio and is changing to the lean side, or when the downstream A / F is changing to the lean side but the lean determination time Tfb has not elapsed, -L In the case of , the routine proceeds to step 24. In step 24, the downstream O2FB correction value is corrected to decrease in order to continue the leaning of the upstream A / F, and the routine is then temporarily terminated.

[0043] In addition, when the conditions of steps 22 and 23 are satisfied, that is, the downstream A / F exceeds the ideal air-fuel ratio and is changing toward the lean side for the lean determination time Tfb, -L In the case of , the downstream A / F is judged to be lean in step 25. Next, in step 26, the downstream O2FB correction value is increased stepwise to switch to rich upstream A / F, and the routine is temporarily terminated.

[0044] On the other hand, if the determination in step 21 is "No", that is, if the upstream A / F is currently being rich, the process proceeds to step 27 to determine whether the output of the downstream O2 sensor 37 exceeds 0.55 V and is increasing. Then, in step 28, it is determined whether the rich determination time Tfb, which is preset as the control determination time, has elapsed. -R When the determination in either step 27 or step 28 is "No", that is, when the downstream A / F has not exceeded the ideal air-fuel ratio and is changing to the rich side, or when the downstream A / F is changing to the rich side but the rich determination time Tfb has not elapsed, -R In the case of , the routine proceeds to step 29. In step 29, the downstream O2FB correction value is increased to continue enriching the upstream A / F, and the routine is temporarily terminated.

[0045] If the conditions of steps 27 and 28 are satisfied, that is, the downstream A / F exceeds the ideal air-fuel ratio and changes toward the rich side for the rich determination time Tfb, -R In this case, the downstream A / F is judged to be rich in step 30 , and in the next step 31 , the downstream O2FB correction value is reduced stepwise to switch to a lean upstream A / F, after which the routine is temporarily terminated.

[0046] In this embodiment, the lean determination time Tfb -L Rich judgment time Tfb -R Such setting takes into account the difference in response characteristics of the downstream A / F when the upstream A / F changes to the lean side and when it changes to the rich side, and thus accurately determines whether the downstream A / F is lean or rich.

[0047] In addition, the lean determination time Tfb -L Rich judgment time Tfb -R The period is set to exceed half the cycle of the A / F vibration correction value. This is mainly to prevent the downstream O2FB correction value from being erroneously switched due to the downstream A / F behavior when it is reversed between rich and lean. Details will be described later. -L Rich judgment time Tfb -R The length of will affect the cycle of the downstream O2FB correction value, but by setting it to a relatively long period as described above, a FB waveform with a longer cycle can be generated based on the downstream O2FB correction value.

[0048] Figure 6 This is a time chart showing the control state of the degradation diagnosis mode when the catalyst 20 is normal. Figure 5 In the downstream O2FB correction value calculation routine, the downstream O2FB correction value is set as follows.

[0049] exist Figure 6 At the beginning of the timing diagram, the downstream A / F performs leaning. Since the downstream A / F has a delay in following the change of the upstream A / F, the output of the downstream O2 sensor 37 is still kept at a high voltage exceeding 0.55V. Figure 5 When the process of step 21 is executed through step S22 to step 24, the downstream O2FB correction value is corrected to decrease. Figure 6 As shown in FIG. 1 , while gradually reducing the downstream O2FB correction value, the upstream A / F continues to be lean. Then, at a certain moment, the output of the downstream O2 sensor 37 exceeds 0.55V and continues to decrease ( Figure 6 Point a) in the figure, and then after the lean determination time Tfb -L ( Figure 6 When these conditions are satisfied, the ECU 30 makes a lean determination in step 25 and increases the downstream O2FB correction in a stepwise manner to make it rich in step 26.

[0050] exist Figure 6At point a' in FIG. 1 , the output of the downstream O2 sensor 37 also exceeds 0.55 V and is decreasing, but due to the passage of the lean determination time Tfb -L Previously, it turned to increase, so no lean judgment was made.

[0051] Thereafter, the ECU 30 transfers from step 21 to step 27, and performs an incremental correction on the downstream O2FB correction value each time the process of step 29 is executed. Figure 6 As shown in FIG. 1 , while gradually increasing the downstream O2FB correction value, the upstream A / F continues to be rich. Then, at a certain moment, the output of the downstream O2 sensor 37 exceeds 0.55 V and continues to increase ( Figure 6 point c) in the figure, and then after the rich judgment time Tfb -R ( Figure 6 When these conditions are satisfied, the ECU 30 makes a rich determination in step 30 and reduces the downstream O2FB correction value stepwise for leaning in step 31 . By repeating the above-mentioned processing, the downstream O2FB correction value is set based on the rich-lean determination so that the final waveform changes reflecting the period and amplitude of the FB waveform based on the determination.

[0052] On the other hand, the ECU 30 executes the control at predetermined control intervals in the degradation diagnosis mode. Figure 7 The reverse count number routine shown is executed by the reverse count unit 30f described above.

[0053] In step 41, ECU30 determines whether a preset waiting time Twait has passed since the start of the degradation diagnosis mode. If "No", the routine is temporarily terminated. If the waiting time Twait has passed, the determination of step 41 is "Yes", and the program is transferred to step 42. In step 42, it is determined whether the output of the downstream O2 sensor 37 exceeds 0.55V, which is equivalent to the ideal air-fuel ratio, and is decreasing. Then, in step 43, it is determined whether a preset counting determination time Tcount has passed. As the counting determination time Tcount, in order to prevent the occurrence of a lean determination due to the influence of noise contained in the output of the downstream O2 sensor 37, it is set to a period slightly longer than the general noise duration, for example, about 30 milliseconds (msec). Compared with the above-mentioned lean determination time Tfb -L Rich judgment time Tfb -R In contrast, the counting determination time Tcount is set to a shorter period.

[0054] If a "NO" determination is made in either step 42 or step 43, that is, if the downstream A / F has not exceeded the ideal air-fuel ratio and is changing toward the lean side, or if the counting determination time Tcount has not elapsed even if changing toward the lean side, the routine is temporarily terminated.

[0055] In addition, if Figure 6 As shown, if the output of the downstream O2 sensor 37 exceeds 0.55V and continues to decrease ( Figure 6 Point a' in the middle), and after counting, the determination time Tcount ( Figure 6 If the downstream A / F exceeds the ideal air-fuel ratio and reverses to the lean side, the process proceeds to step 44, where the reverse counter N2 is incremented by (N2+1), and the routine ends. Therefore, the reverse counter N2 indicates the number of reverses of the downstream A / F.

[0056] In the degradation diagnosis mode, the ECU 30 executes the control at predetermined intervals. Figure 8 This routine is executed by the degradation diagnosis unit 30g. ECU 30 determines in step 51 whether one cycle of the A / F vibration correction value has elapsed. If the determination is "No," the process proceeds to step 52, where it determines whether the reversal counter N2 is equal to or greater than "25," which is pre-set as the second determination number. If the determination is "No," the process temporarily terminates the routine. If the determination in step 51 is "Yes," the process proceeds to step 53, where the vibration counter N1 is incremented by (N1+1). In the following step 54, the process determines whether the vibration counter N1 is equal to or greater than "50," which is pre-set as the first determination number. If the determination is "No," the process proceeds to step 52.

[0057] The vibration counter N1 gradually increases according to the cycle of the vibration waveform. If the determination in step 54 is "yes", the process proceeds to step 55. In addition, the reversal counter N2 gradually increases according to the number of reversals of the downstream A / F. If the determination in step 52 is "yes", the process proceeds to step 55.

[0058] In step 55 , the reversal number ratio R is calculated according to the following equation based on the reversal counter N2 and the vibration counter N1 . Reversal number ratio R = Reversal counter N2 / Vibration counter N1

[0059] When the process proceeds from step 54 to step 55, the vibration counter N1 indicates "50," and the reversal counter N2 indicates the number of reversals accumulated during the period when the vibration counter N1 increments from 0 to 50. Furthermore, when the process proceeds from step 52 to step 56, the reversal counter N2 indicates "25," and the vibration counter N1 indicates the number of oscillations of the A / F vibration correction value generated during the period when the reversal counter N2 increments from 0 to 25. That is, in either case, the reversal number ratio R is calculated as a value correlated with the degree of deterioration of the catalyst 20.

[0060] After the processing in step 55 is completed, the routine proceeds to step 56, where the vibration counter N1 and the reversal counter N2 are reset to 0. Next, in step 57, a determination is made as to whether the reversal count ratio R is greater than a preset degradation determination ratio R0. If the determination is "No," the catalyst 20 is deemed not to have deteriorated and operation can continue, and the routine ends immediately. Alternatively, if the determination in step 57 is "Yes," the catalyst 20 is deemed to have deteriorated further and operation should not be continued. A determination of catalyst 20 degradation is then made in step 58, and the routine ends. In this case, degradation of the catalyst 20 is notified by, for example, lighting of a warning light (not shown), and necessary measures, such as replacement of the catalyst 20, are implemented accordingly.

[0061] Next, the execution status of the degradation diagnosis of the catalyst 20 based on the above-mentioned processing of the ECU 30 will be described. like Figure 6 As shown, at the beginning of upstream A / F leaning, the oxygen storage in the catalyst 20 is reduced to approximately zero due to the previously implemented upstream A / F enrichment. The downstream A / F remains on the rich side, and the output of the downstream O2 sensor 37 remains at a high voltage corresponding to rich. As the upstream A / F leans, it repeats fluctuations reflecting the final waveform, gradually approaching the stoichiometric air-fuel ratio from the rich side. Initially, the lean-side peak, reflecting the vibration of the vibration waveform, crosses the stoichiometric air-fuel ratio and intrudes into the lean side, with oxygen corresponding to this intrusion being stored in the catalyst 20. With each cycle of the vibration waveform, the lean-side peak of the upstream A / F crosses the stoichiometric air-fuel ratio, and the amount of intrusion, and thus the amount of oxygen stored in the catalyst 20, gradually increases with each cycle. However, at this point, all the oxygen in the exhaust gas is stored in the catalyst 20, and the downstream A / F remains on the rich side, with the output of the downstream O2 sensor 37 remaining at a high voltage corresponding to rich.

[0062] Then, by repeatedly storing oxygen in the catalyst 20 in this intermittent manner, the oxygen storage amount of the catalyst 20 gradually increases while repeatedly increasing and decreasing, reaching the OSC at a certain point and continuously maintaining the OSC. During this process, the following phenomenon occurs: the oxygen storage amount of the catalyst 20 that has reached the OSC temporarily decreases as the upstream A / F changes to the rich side, and increases again as the upstream A / F changes to the lean side, thus reaching the OSC. Figure 6 In the example shown in FIG. 1 , this phenomenon occurs during a period A indicated by a blank square. During this period A, the catalyst 20 does not store oxygen in the exhaust gas but allows it to pass through directly. Therefore, the downstream A / F temporarily changes to the lean side, and the output of the downstream O2 sensor 37 decreases. During this period A, the output of the downstream O2 sensor 37 decreases ( Figure 6 Point a'-b') satisfies Figure 7 The conditions of steps 42 and 43 are as follows: Figure 6 As shown in , the inversion counter N2 is incremented.

[0063] Then, after period A, oxygen is released from the catalyst 20 in response to the change in the upstream A / F toward the rich side, which reflects the vibration of the vibration waveform. Therefore, the oxygen storage amount temporarily decreases from OSC, the downstream A / F changes toward the rich side, and the output of the downstream O2 sensor 37 increases. However, these changes are temporary. Since the upstream A / F changes toward the lean side in response to the change in the FB waveform, the output of the downstream O2 sensor 37 decreases again as the downstream A / F changes toward the rich side. At the same time, the oxygen storage amount of the catalyst 20 quickly increases to OSC and then remains at OSC. If the output of the downstream O2 sensor 37 decreases at this time ( Figure 6 Point ab) satisfies Figure 5 If the conditions of steps 22 and 23 are met, the lean determination is based on the downstream A / F, such as Figure 6 As shown in , the downstream O2FB correction value increases stepwise, and the upstream A / F starts to become rich.

[0064] As mentioned above, it takes a certain amount of time for the upstream A / F change to be reflected in the downstream A / F. Therefore, even if the upstream A / F starts to become rich, the downstream A / F will temporarily remain on the lean side, and the output of the downstream O2 sensor 37 will remain at a low voltage corresponding to the lean state. Figure 6 In the example shown, the oxygen storage capacity of the catalyst 20 remains at OSC during period B, indicated by the hatched box. The upstream A / F ratio changes from the lean side to the rich side, crossing the stoichiometric air-fuel ratio while repeatedly reflecting the fluctuations of the final waveform. At this point, the rich peak, reflecting the vibration of the vibration waveform, first intrudes into the rich side, crossing the stoichiometric air-fuel ratio. An amount of oxygen corresponding to this intrusion is released from the catalyst 20 to purify the exhaust gas. With each cycle of the vibration waveform, the rich peak of the upstream A / F ratio exceeds the stoichiometric air-fuel ratio, and the amount of intrusion into the rich side, and consequently the amount of oxygen released from the catalyst 20, gradually increases with each cycle.

[0065] Thus, by repeatedly releasing oxygen from the catalyst 20 intermittently, the oxygen storage amount of the catalyst 20 gradually decreases while repeatedly increasing and decreasing, and at a certain point decreases to 0 and remains at 0. In this process, the following phenomenon occurs: the oxygen storage amount of the catalyst 20 that has decreased from the OSC temporarily increases as the upstream A / F changes to the lean side, and then decreases again as the upstream A / F changes to the rich side. Figure 6 In the example of FIG. 1 , this phenomenon occurs during the three periods C indicated by the black boxes. During these periods C, the catalyst 20 does not store oxygen in the exhaust gas but allows it to pass through directly. Therefore, the downstream A / F temporarily changes to the lean side, and the output of the downstream O2 sensor 37 decreases. The decrease in the output of the downstream O2 sensor 37 during these periods C satisfies Figure 7 The conditions of steps 42 and 43 are as follows: Figure 6 As shown in , the inversion counter N2 is incremented one by one.

[0066] Then, when the output of the downstream O2 sensor 37 increases ( Figure 6 The point cd) in Figure 5 When the conditions of steps 27 and 28 are met, the downstream A / F rich judgment is made, such as Figure 6 As shown in , the downstream O2FB correction value decreases stepwise, and the upstream A / F starts to become lean. Simultaneously, the oxygen storage of the catalyst 20 falls below OSC, so that the downstream A / F remains on the rich side, and the output of the downstream O2 sensor 37 remains at a high voltage corresponding to rich.

[0067] As described above, every time the oxygen storage amount of the catalyst 20 reaches OSC, the reversal counter N2 is appropriately incremented. However, when the degradation of the catalyst 20 progresses, OSC, which is the upper limit of the oxygen storage amount, is increased. Figure 6 The downward displacement in the middle makes it easier for the oxygen storage to reach OSC. Figure 9 1 is a time chart showing the control state of the degradation diagnosis mode when such catalyst degradation progresses.

[0068] exist Figure 9 In, also according to Figure 4 、 5 , 7, 8 flowcharts execute control, so the A / F vibration correction value and downstream O2FB correction value are the same as Figure 6 The same is true for the case of . In addition, the downstream A / F temporarily changes to the lean side during period A, continues to remain on the lean side during period B, and temporarily changes to the lean side during period C, which occurs in sequence according to the change in the upstream A / F that reflects the final waveform.

[0069] However, since the OSC of the catalyst 20 is low, when the upstream A / F repeatedly changes from the rich side to the ideal air-fuel ratio while reflecting the final waveform, the timing at which the oxygen storage of the catalyst 20 reaches the OSC becomes earlier. As a result, the phenomenon of period A, in which the downstream A / F temporarily changes to the lean side before shifting to period B, occurs more frequently. Figure 6 For example, Figure 9 In the example, period A increases to 4.

[0070] Furthermore, as the upstream A / F changes from the lean side to the rich side through the ideal air-fuel ratio while repeatedly reflecting the final waveform, the timing at which the oxygen storage amount of the catalyst 20 falls below OSC becomes later. As a result, the phenomenon of period C, in which the downstream A / F temporarily changes to the lean side after period B, occurs more frequently. Figure 6 For example, Figure 9 In the example, period C is increased to 5.

[0071] Then, since the output of the downstream O2 sensor 37 of A and C decreases during these periods, Figure 7 In the inversion count routine, the inversion counter N2 is incremented one by one. Figure 6 Compared to the example in Figure 9 In the example, the reverse counter N2 increases more rapidly. Therefore, whether Figure 8 If the condition of step 54 is met and the process moves to step 55, Figure 8 If the condition of step 52 is satisfied and the process moves to step 55, Figure 6 Compared with the example of , a larger reversal number ratio R is calculated in both cases, and it is determined that the possibility of the catalyst 20 being deteriorated has increased.

[0072] More specifically, the difference in the number of reversals of the downstream A / F corresponding to the OSC of the catalyst 20 is obtained by the characteristics of the final waveform. Figure 6 、 9 As described above, the final waveform repeats subtle vibrations reflecting the short period and amplitude of the vibration waveform, while also repeating gentle fluctuations reflecting the longer period and amplitude of the FB waveform. The gentle fluctuations of the FB waveform, based on downstream O2FB control, serve to maintain the upstream A / F ratio near the stoichiometric air-fuel ratio while simultaneously fluctuating between rich and lean. Furthermore, the subtle fluctuations of the vibration waveform serve to cause the upstream A / F ratio to shift across the stoichiometric air-fuel ratio, intermittently storing oxygen in the catalyst 20 to increase the OSC when leaning, and intermittently releasing oxygen from the catalyst 20 to reduce the OSC when richening.

[0073] Then, when leaning, the lower the OSC of the catalyst 20, the earlier the oxygen storage in the catalyst 20 reaches the OSC. Therefore, the period A during which the output of the downstream O2 sensor 37 temporarily increases occurs more frequently. Conversely, when riching, the lower the OSC of the catalyst 20, the later the oxygen storage in the catalyst 20 falls below the OSC. Therefore, the period C during which the output of the downstream O2 sensor 37 temporarily increases occurs more frequently. As a result, the increment of the reversal counter N2 varies, making it possible to diagnose deterioration of the catalyst 20 as described above.

[0074] In order to produce a significant difference in the increment of the reversal counter N2 in response to a decrease in the OSC of the catalyst 20, it is important to produce a desired upstream A / F fluctuation state. In this embodiment, a final waveform that substantially changes the upstream A / F is generated based on the vibration waveform and the F / B waveform. Therefore, to meet this requirement, it is necessary to set an A / F vibration correction value and a downstream O2FB correction value based on the desired characteristics, and further set the vibration waveform and the F / B waveform.

[0075] For example, the following characteristics are required for the A / F vibration correction value. If the period or amplitude of the A / F vibration correction value is set too small, the exhaust gas flowing through the catalyst 20 will agitate and even out the A / F ratio. This will result in insignificant changes in the output of the downstream O2 sensor 37, making degradation diagnosis impossible or reducing diagnostic accuracy. Therefore, the period and amplitude of the A / F vibration correction value are preferably set so that the downstream A / F ratio fluctuates approximately as degradation of the catalyst 20 progresses.

[0076] In this embodiment, a unique AF vibration correction value is calculated based on a preset period and amplitude, and then reflected in the final waveform as a vibration waveform. By calculating the AF vibration correction value based on a fixed value, it is possible to reflect a vibration waveform with desired characteristics in the final waveform without being affected by less reliable detection information, as in the case of the upstream O2 sensor disclosed in Patent Document 1. Consequently, for the reasons discussed above, it is possible to set the desired A / F vibration correction value, thereby setting a vibration waveform and reflecting it in the final waveform.

[0077] Furthermore, the relationship between the period of the vibration waveform and the period of the FB waveform affects the number of periods A to C caused by the reversal of the downstream A / F. By setting the period with an appropriate relationship, the number of periods A to C can be clearly differentiated according to the reduction of the OSC of the catalyst 20. For example, if the FB waveform is changed by approximately one cycle (enrichment + leanness) within 10 cycles of the vibration waveform, when the catalyst 20 is normal, the phenomenon of periods A and C will hardly occur, so the reversal frequency ratio R is suppressed to about 20%. In contrast, when the degradation of the catalyst 20 is advanced, although the lean determination time Tfb of period B has passed, the period A to C will be significantly different. -L Rich judgment time Tfb -R During this process, the downstream A / F ratio does not reverse, but it frequently reverses in roughly sync with the vibration waveform cycle. Therefore, the phenomena of periods A and C occur frequently, increasing the reversal frequency ratio R to approximately 80%. As a result, the reversal frequency ratio R varies significantly depending on the OSC, enabling more accurate deterioration diagnosis.

[0078] In order to achieve such a preferred relationship between the period of the vibration waveform and the period of the FB waveform, it is necessary to switch the downstream O2FB correction value between rich and lean at an appropriate timing. Figure 6 、 9 The rich increase and decrease characteristics of the downstream O2FB correction value, or the lean determination time Tfb -L Rich judgment time Tfb -R The length of the downstream O2FB correction value needs to be switched between rich and lean using detection information with higher reliability. According to the downstream O2FB control of this embodiment, the downstream O2FB correction value is switched between rich and lean based on the detection information from the downstream O2 sensor 37. The detection information from the downstream O2 sensor 37 is more reliable than the upstream O2 sensor used in Patent Document 1 for the following reasons.

[0079] The first reason is that the exhaust gas detected by the downstream O2 sensor 37 is uniform. The exhaust gas is stirred while flowing through the catalyst 20 and reaches the downstream O2 sensor 37 in a uniform state. Therefore, the exhaust gas with a uniform oxygen concentration becomes the detection target of the downstream O2 sensor 37.

[0080] The second reason is that the downstream O2 sensor 37 is less susceptible to degradation. Exhaust gas from the internal combustion engine gradually cools in temperature as it flows through the exhaust passage 12, and harmful components are removed as it passes through the catalyst 20. Even with continued exposure to such exhaust gas, degradation of the downstream O2 sensor 37 is minimal, and detection errors due to degradation are negligible. Consequently, downstream O2FB control is executed based on detection information from the highly reliable downstream O2 sensor 37. As illustrated above, the relationship between the period of the vibration waveform and the period of the FB waveform can be optimized.

[0081] The third reason is that the downstream A / F reflects the FB waveform for a longer period than the upstream A / F. This downstream A / F fluctuation is caused by the following factors: first, it takes a certain amount of time for the upstream A / F change to be reflected in the downstream A / F due to the influence of oxygen storage and release of the catalyst 20; second, the lean determination time Tfb -L Rich judgment time Tfb -R The period is set to a relatively long one. As a result, the fluctuation of the downstream A / F, which reflects the FB waveform, becomes more gradual. Therefore, the downstream O2 sensor 37 can accurately detect the behavior of the downstream A / F corresponding to OSC compared to a situation where the fluctuation is more rapid. Furthermore, even if some responsiveness degradation occurs in the downstream O2 sensor 37, the impact of the responsiveness degradation is reduced, thereby suppressing the occurrence of detection errors caused by this.

[0082] As described above, according to this embodiment, the AF vibration correction value and the downstream O2FB correction value can be set according to the desired characteristics, thereby setting the vibration waveform and F / B waveform. Based on these final waveforms, the desired upstream A / F fluctuation state can be generated. Therefore, the increase in the reversal counter N2 can be clearly differentiated according to the decrease in the OSC of the catalyst 20. This allows catalyst 20 degradation diagnosis to be performed under appropriate diagnostic conditions, thereby improving diagnostic accuracy compared to the technique of Patent Document 1.

[0083] Furthermore, in this embodiment, the A / F vibration correction value for creating a suitable diagnostic environment differs depending on the operating range of the internal combustion engine. Based on this view, the period and amplitude of the A / F vibration correction value suitable for each operating range of the internal combustion engine are stored in advance in the vibration waveform storage unit 30a. Then, in the degradation diagnosis mode, the period and amplitude corresponding to the current operating range of the internal combustion engine are read from the vibration waveform storage unit 30a and used in the calculation process of the A / F vibration correction value ( Figure 4 Therefore, the upstream A / F can be varied based on the A / F vibration correction value that is suitable for the operating range of the internal combustion engine at all times, and this factor also contributes to improving the diagnostic accuracy.

[0084] In addition, after the start of the degradation diagnosis of the catalyst 20 and the lapse of the waiting time Twait, the increment of the reversal counter N2 corresponding to the reversal of the downstream A / F is started ( Figure 7 Step 41). Because the details of fuel injection control differ between normal operating mode and degradation diagnosis mode, there is a period of unstable control during the transition period after switching from normal operating mode to degradation diagnosis mode, which may prevent accurate determination of downstream A / F reversal. Therefore, if reversal counter N2 increments during this transitional state, this could potentially cause errors in the diagnostic results. By setting the wait time Twait slightly longer than the estimated duration of the transitional state, accurate determination of downstream A / F reversal is achieved, improving the reliability of reversal counter N2 and contributing to improved diagnostic accuracy.

[0085] In addition, in this embodiment, the deterioration diagnosis of the catalyst 20 is not performed based on the simple passage of time, but is performed based on the number of vibrations of the upstream A / F and the number of reversals of the downstream A / F. Specifically, when the vibration counter N1, which is incremented based on the number of vibrations of the upstream A / F, reaches "50" ( Figure 8 When the step 54 is "Yes"), and the reversal counter N2 increased according to the number of reversals of the downstream A / F reaches "25" ( Figure 8 Step 52 is "Yes"), the degradation diagnosis of the catalyst 20 is performed respectively ( Figure 8 Steps 55 to 57).

[0086] When the catalyst 20 deteriorates, necessary measures such as replacement of the catalyst 20 must be promptly implemented. However, when the catalyst 20 is still functioning normally, it is not necessary to frequently perform degradation diagnosis of the catalyst 20. In contrast, as degradation of the catalyst 20 becomes more severe, in other words, the likelihood that prompt measures such as replacement of the catalyst 20 will be required increases, and therefore, it is preferable to perform degradation diagnosis more frequently.

[0087] According to this embodiment, when the catalyst 20 is functioning normally, the reversal counter N2 does not increase significantly. Therefore, degradation diagnosis of the catalyst 20 is performed at regular intervals based on the vibration counter N1 ≥ 50. However, if degradation of the catalyst 20 worsens and the downstream air-conditioning system begins to reverse frequently, at some point, the reversal counter N2 ≥ 25 condition is satisfied before the vibration counter N1 ≥ 50 condition. Subsequently, as degradation of the catalyst 20 worsens, the time required for the reversal counter N2 ≥ 25 condition to be satisfied decreases, and the intervals between degradation diagnosis operations gradually decrease, leading to more frequent execution. Therefore, when the catalyst 20 is functioning normally, frequent degradation diagnosis can be prevented, reducing the computational load on the ECU 30. On the other hand, if degradation of the catalyst 20 worsens, degradation diagnosis can be quickly performed and appropriate measures implemented before the purification function is lost.

[0088] On the other hand, in the downstream O2FB control, the downstream A / F exceeds the ideal air-fuel ratio and changes to the lean side for the lean determination time Tfb. -L In the case of the downstream A / F being lean, the downstream O2FB correction value is switched to the rich side. In addition, the downstream A / F exceeds the ideal air-fuel ratio and is changing to the rich side for the rich judgment time Tfb. -R , a judgment is made that the downstream A / F is rich, and the downstream O2FB correction value is switched to the lean side. Specifically, when it is determined that the catalyst 20 has stored oxygen to the upper limit, the upstream A / F starts to become rich, and when it is determined that the catalyst 20 has released oxygen to the lower limit, the upstream A / F starts to become lean. This ensures that the oxygen storage level of the catalyst 20 reliably fluctuates between 0 and OSC, resulting in further improved diagnostic accuracy through degradation diagnosis based on the highly reliable reverse counter N2.

[0089] In addition, the lean determination time Tfb applied to the downstream O2FB control -L Rich judgment time Tfb -R The period is set to exceed the half cycle of the vibration waveform based on the A / F vibration correction value. Based on the output of the downstream O2 sensor 37, the reversal of the downstream A / F is determined ( Figure 7 In parallel, rich / lean determination is performed based on the output of the downstream O2 sensor 37 ( Figure 5 If the rich / lean determination is not met, the vibration of the downstream A / F toward the rich side or the lean side should converge and reverse within half a cycle of the vibration waveform.

[0090] However, assuming that the lean determination time Tfb -L Rich judgment time Tfb -RIf the lean determination time Tfb is set to be shorter than the half cycle of the A / F vibration correction value, not only will the reverse counter N2 be incremented, but an erroneous rich / lean determination may be made, causing the downstream O2FB correction value to be switched. -L Rich judgment time Tfb -R , which can prevent this situation before it happens, so this factor also helps to improve diagnostic accuracy.

[0091] In addition, considering the difference in the response characteristics of the downstream A / F, the downstream lean determination time Tfb is set to -L Rich judgment time Tfb -R This helps improve the accuracy of rich / lean judgment, thereby generating an appropriate FB waveform and thus helping to improve diagnostic accuracy.

[0092] For example, in the above embodiment, the reversal counter N2 is incremented when the upstream A / F turns lean, but the reversal counter N2 may be incremented when the downstream A / F turns rich instead.

[0093] In the above embodiment, the period and amplitude applied to the calculation process of the A / F vibration correction value are varied according to the operating range of the internal combustion engine. However, the present invention is not limited thereto. For example, the A / F vibration correction value may be calculated based on a common period and amplitude.

[0094] In the above embodiment, the lean determination time Tfb is set to -L Rich judgment time Tfb -R Different values are set, but the present invention is not limited thereto, and for example, a common determination time may be applied.

[0095] Furthermore, in the above embodiment, the degree of degradation of the catalyst 20 is diagnosed when the vibration counter N1 based on the period of the vibration waveform reaches "50" set as the first determination number, and when the reversal counter N2 based on the reversal of the upstream A / F reaches "25" set as the second determination number, respectively. However, the present invention is not limited to this. For example, degradation diagnosis may be performed only when the condition related to the vibration counter N1 is met. Description of labels

[0096] 1Catalyst degradation diagnosis device 12 exhaust passages 20 catalysts 30a vibration waveform storage unit 30b A / F vibration correction value calculation unit 30c Downstream O2FB correction value calculation unit 30d diagnostic correction value calculation unit 30e fuel injection control unit 30f reverse counting unit 30g degradation diagnosis unit 37 Downstream O2 sensor (air-fuel ratio detection unit).

Claims

1. A catalyst degradation diagnostic device for an internal combustion engine, characterized in that: include: an air-fuel ratio detecting unit provided downstream of a catalyst inserted in an exhaust passage of an internal combustion engine and detecting the air-fuel ratio of exhaust gas flowing out of the catalyst; a vibration waveform storage unit that pre-stores a period and an amplitude of an air-fuel ratio vibration correction value for vibrating the air-fuel ratio of the exhaust gas flowing into the catalyst; an air-fuel ratio vibration correction value calculation unit that calculates the air-fuel ratio vibration correction value based on the period and amplitude stored in the vibration waveform storage unit; a downstream O2 feedback correction value calculation unit that calculates, based on the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detection unit, a downstream O2 feedback correction value for causing the air-fuel ratio of the exhaust gas flowing into the catalyst to fluctuate with a period longer than the air-fuel ratio vibration correction value and centered on the ideal air-fuel ratio; a diagnostic correction value calculation unit configured to calculate a diagnostic correction value by adding the air-fuel ratio vibration correction value to the downstream O2 feedback correction value; a fuel injection control unit that performs fuel injection control of the internal combustion engine based on the diagnostic correction value; a reversal counting unit that counts the number of times the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detecting unit reverses between rich and lean; and A degradation diagnosis unit diagnoses a degree of degradation of the catalyst based on the number of vibrations of the air-fuel ratio of the exhaust gas based on the air-fuel ratio vibration correction value and the number of reversals counted by the reversal counting unit.

2. The catalyst degradation diagnostic device for an internal combustion engine according to claim 1, wherein: The vibration waveform storage unit stores the period and amplitude of the air-fuel ratio vibration correction value for each operating range of the internal combustion engine. The air-fuel ratio vibration correction value calculation unit reads out a period and an amplitude corresponding to an operating range of the internal combustion engine from the vibration waveform storage unit, and applies the period and the amplitude to a calculation process of the air-fuel ratio vibration correction value.

3. The catalyst degradation diagnostic device for an internal combustion engine according to claim 1, wherein: The reverse counting unit starts counting when a predetermined waiting time has elapsed since the fuel injection control unit started the fuel injection control based on the diagnostic correction value.

4. The catalyst degradation diagnostic device for an internal combustion engine according to claim 1, wherein: The degradation diagnosis unit diagnoses the degree of degradation of the catalyst when the number of vibrations of the air-fuel ratio of the exhaust gas based on the air-fuel ratio vibration correction value reaches a predetermined first judgment number, and when the number of reversals counted by the reversal counting unit reaches a predetermined second judgment number that is smaller than the first judgment number.

5. The catalyst degradation diagnostic device for an internal combustion engine according to claim 1, wherein: The downstream O2 feedback correction value calculation unit makes a rich judgment and makes the downstream O2 feedback correction value lean when the state in which the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detection unit exceeds the ideal air-fuel ratio and changes toward the rich side continues for a predetermined control judgment time. The downstream O2 feedback correction value calculation unit makes a lean judgment and makes the downstream O2 feedback correction value rich when the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detection unit exceeds the ideal air-fuel ratio and changes toward the lean side continues for the control judgment time.

6. The catalyst degradation diagnostic device for an internal combustion engine according to claim 5, wherein: The control determination time is set to a value exceeding a half period of a vibration waveform based on the air-fuel ratio vibration correction value.

7. The catalyst degradation diagnostic device for an internal combustion engine according to claim 5, wherein: The control determination time includes a rich determination time for the rich determination and a lean determination time for the lean determination. The rich determination time and the lean determination time are set to different values.

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