Deterioration diagnostic device for an exhaust gas control catalytic converter

Abstract

Deterioration diagnostic device for an exhaust gas control catalyst, wherein the deterioration diagnostic device is configured to diagnose deterioration of the exhaust gas control catalyst which is provided in an exhaust port of an internal combustion engine and is configured to store oxygen, and wherein the deterioration diagnostic device comprises: a downstream air-fuel ratio sensor (43) configured to detect an air-fuel ratio of an exhaust gas that has flowed out of the exhaust control catalyst; and a control device (31) configured to control an air-fuel ratio of an exhaust gas flowing into the exhaust control catalyst and to diagnose deterioration of the exhaust control catalyst based on an output from the downstream air-fuel ratio sensor (43), wherein the control device (31) is configured in a deterioration diagnostic process to diagnose deterioration of the exhaust control catalyst to alternately and repeatedly perform a rich process and a lean process, wherein the rich process is a process in which the air-fuel ratio of the exhaust gas flowing into the exhaust control catalyst is controlled to a rich air-fuel ratio that is richer than a stoichiometric air-fuel ratio, and wherein the lean process is a process in which the air-fuel ratio of the exhaust gas flowing into the exhaust control catalyst is controlled to a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. to switch from the rich process to the lean process when the amount of oxygen released by the exhaust gas control catalyst since the start of the rich process equals the first amount of oxygen, and to switch from the lean process to the rich process when the amount of oxygen stored in the exhaust gas control catalyst since the start of the lean process equals the second amount of oxygen, which is lower than the first amount of oxygen, and to determine that the exhaust gas control catalyst has deteriorated when the lean process is carried out and a frequency at which an output air-fuel ratio of the downstream air-fuel ratio sensor (43) is equal to the lean air-fuel ratio is equal to or greater than a predetermined frequency.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The present disclosure relates to a deterioration diagnostic device for an exhaust gas control catalyst. 2. Description of the state of the art

[0002] The provision of an exhaust gas control catalyst capable of storing oxygen in an exhaust port of an internal combustion engine is known (e.g., from JP 2010-180 717 A and JP 2005-299 587 A). The exhaust gas control catalyst capable of storing oxygen stores oxygen from an exhaust gas when the air-fuel ratio of the incoming exhaust gas is leaner than the stoichiometric air-fuel ratio (hereinafter referred to as the "lean air-fuel ratio"), and releases the stored oxygen when the air-fuel ratio of the incoming exhaust gas is richer than the stoichiometric air-fuel ratio (hereinafter referred to as the "rich air-fuel ratio").

[0003] If such an exhaust gas control catalyst deteriorates, for example through sintering, its oxygen storage capacity is reduced. The device described in JP 2010-180 717 A performs active air-fuel ratio control by alternately and repeatedly running a rich cycle (where the air-fuel ratio of the exhaust gas entering the exhaust gas control catalyst is adjusted to a rich air-fuel ratio) and a lean cycle (where the air-fuel ratio of the exhaust gas entering the exhaust gas control catalyst is adjusted to a lean air-fuel ratio) to diagnose / detect deterioration of the exhaust gas control catalyst.In the active air-fuel ratio control described in JP 2010-180 717 A, the rich process is carried out until the output air-fuel ratio of a downstream air-fuel ratio sensor, provided downstream of the exhaust control catalyst, is equal to a rich air-fuel ratio, and the lean process is carried out until the output air-fuel ratio of the downstream air-fuel ratio sensor is equal to a lean air-fuel ratio.In the device described in JP 2010-180 717 A, the oxygen storage quantity during the lean process and the oxygen release quantity during the rich process are measured multiple times, the average value of the measured values ​​and fluctuations in the measured values ​​are calculated, a maximum storable oxygen quantity is estimated based on the average value and the fluctuations, and deterioration of the exhaust gas control catalyst is diagnosed based on the maximum storable oxygen quantity.

[0004] Furthermore, DE 696 13 430 T2 discloses a deterioration diagnostic device for an exhaust gas control catalyst, in which the amount of oxygen released from a three-way catalyst is maintained at a value suitable for determining the deterioration of the catalyst. The catalyst absorbs oxygen from the exhaust gas when the air-fuel ratio of the exhaust gas is lean and releases the absorbed oxygen when the air-fuel ratio of the exhaust gas is rich. The amount of oxygen released from the catalyst during the period with a rich air-fuel ratio of the exhaust gas is regulated to a predetermined value that is greater than the maximum oxygen absorption capacity of a worn catalyst and less than the maximum oxygen absorption capacity of a normal catalyst.

[0005] Furthermore, DE 691 06 247 T2 discloses a method for on-board detection of the deterioration of an automotive catalyst. The method comprises: measuring the air-fuel ratio by an independent sensor immediately downstream of the catalyst at certain events and determining whether there is no significant change in the air-fuel ratio between the events.

[0006] Furthermore, DE 10 2007 025 377 A1 describes an air / fuel ratio control device for an internal combustion engine for catalyst deterioration diagnosis. The device adjusts the air / fuel ratio of a mixture supplied to an engine according to an output from an upstream air / fuel ratio sensor and a predetermined control constant, thereby causing the air / fuel ratio to oscillate periodically between rich and lean mixtures. An average air / fuel ratio, obtained by averaging the periodically oscillating air / fuel ratio, is also oscillated between rich and lean mixtures by the device. Catalyst deterioration is diagnosed based on a correlation between the oscillation of the average air / fuel ratio and an output from the downstream oxygen sensor. SUMMARY OF THE INVENTION

[0007] In the active air-fuel ratio control for deterioration diagnosis described in JP 2010-180 717 A, the lean process is maintained until the air-fuel ratio output from the downstream air-fuel ratio sensor is equal to a lean air-fuel ratio. When the air-fuel ratio output from the downstream air-fuel ratio sensor is thus equal to a lean air-fuel ratio, it indicates that oxygen is flowing from the exhaust control catalyst, i.e., that nitrogen oxides (NOx) are flowing from the exhaust control catalyst. Therefore, there is a possibility that NOx is flowing from the exhaust control catalyst when the device described in JP 2010-180 717 A diagnoses deterioration of the exhaust control catalyst.

[0008] The present disclosure provides a deterioration diagnostic device for an exhaust gas control catalyst, wherein the deterioration diagnostic device is able to suppress the outflow of NOx from the exhaust gas control catalyst when deterioration of the exhaust gas control catalyst is diagnosed.

[0009] One aspect of the present disclosure provides for a deterioration diagnostic device for an exhaust gas control catalyst, wherein the deterioration diagnostic device is configured to diagnose deterioration of the exhaust gas control catalyst, which is provided in an exhaust port of an internal combustion engine and is configured to store oxygen. The deterioration diagnostic device for the exhaust gas control catalyst comprises a downstream air-fuel ratio sensor and a control device. The downstream air-fuel ratio sensor is configured to detect the air-fuel ratio of an exhaust gas that has flowed out of the exhaust gas control catalyst.The control device is configured to control the air-fuel ratio of an exhaust gas entering the exhaust control catalyst and to diagnose deterioration of the exhaust control catalyst based on an output from the downstream air-fuel ratio sensor.The control device is configured to alternately and repeatedly perform a rich process and a lean process in a deterioration diagnostic process for diagnosing the deterioration of the exhaust gas control catalyst, wherein the rich process is a process in which the air-fuel ratio of the exhaust gas flowing into the exhaust gas control catalyst is controlled to a rich air-fuel ratio that is richer than a stoichiometric air-fuel ratio, and wherein the lean process is a process in which the air-fuel ratio of the exhaust gas flowing into the exhaust gas control catalyst is controlled to a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio.The control device is configured to switch from the rich to the lean process during deterioration diagnostics when the amount of oxygen released by the exhaust gas recirculation catalyst since the start of the rich process equals the initial oxygen quantity, and to switch from the lean to the rich process when the amount of oxygen stored in the exhaust gas recirculation catalyst since the start of the lean process equals the second oxygen quantity, which is lower than the initial oxygen quantity. The control device is configured to determine, during deterioration diagnostics, that the exhaust gas recirculation catalyst has deteriorated when the lean process is running and the frequency at which an air-fuel ratio output from the downstream air-fuel ratio sensor is equal to or greater than the lean air-fuel ratio is equal to or greater than a predetermined frequency.

[0010] In the deterioration diagnostic device according to the aspect of the present disclosure, the control device can be configured to switch from the rich process to the lean process when the output air-fuel ratio of the downstream air-fuel ratio sensor is equal to the rich air-fuel ratio, even before the amount of oxygen released by the exhaust control catalyst since the start of the rich process is equal to the first amount of oxygen.In the deterioration diagnostic device according to the aspect of this disclosure, the control device can be configured to switch from the lean process to the rich process when the output air-fuel ratio of the downstream air-fuel ratio sensor equals the lean air-fuel ratio, even before the amount of oxygen stored in the exhaust control catalyst since the start of the lean process equals the second amount of oxygen. In the deterioration diagnostic device according to the aspect of this disclosure, the control device can be configured to execute the rich process first when the deterioration diagnostic process is started. In the deterioration diagnostic device according to the aspect of this disclosure, the control device can be configured to execute the rich process last when the deterioration diagnostic process is terminated.

[0011] In the deterioration diagnostic device according to the aspect of the present disclosure, the control device can be configured to control the air-fuel ratio of an exhaust gas discharged from an engine body such that, during normal air-fuel ratio control, which differs from the deterioration diagnostic process, it alternately switches between a rich air-fuel ratio and a lean air-fuel ratio. The air-fuel ratio of the exhaust gas discharged from the engine body during the rich process can be richer than at a time when the air-fuel ratio of the exhaust gas discharged from the engine body is set to a rich air-fuel ratio during normal air-fuel ratio control.In the deterioration diagnostic device according to the aspect of the present disclosure, the control device can be configured such that, during normal air-fuel ratio control, which differs from the deterioration diagnostic process, the air-fuel ratio of the exhaust gas discharged from the engine body alternates between a rich and a lean air-fuel ratio. During the lean process, the air-fuel ratio of the exhaust gas discharged from the engine body can be leaner than when, during normal air-fuel ratio control, the air-fuel ratio of the exhaust gas discharged from the engine body is set to a lean air-fuel ratio.In the deterioration diagnostic device according to the aspect of the present disclosure, the first oxygen quantity can be adjusted to increase as the temperature of the exhaust gas control catalyst rises. In the deterioration diagnostic device according to the aspect of the present disclosure, the second oxygen quantity can be adjusted to increase as the temperature of the exhaust gas control catalyst rises. In the deterioration diagnostic device according to the aspect of the present disclosure, the exhaust gas control catalyst can function as a particulate filter that captures fine dust from the exhaust gas.

[0012] The deterioration diagnostic device according to the aspect of the present disclosure may further comprise a first air-fuel ratio sensor and a second air-fuel ratio sensor. A first catalyst and a second catalyst may be provided in an exhaust port of the internal combustion engine. The second catalyst may serve as an exhaust control catalyst and be provided downstream of the first catalyst. The first air-fuel ratio sensor may be arranged upstream of the first catalyst. The second air-fuel ratio sensor may be provided between the first catalyst and the second catalyst. A third air-fuel ratio sensor may be arranged downstream of the second catalyst. The third air-fuel ratio sensor may serve as the downstream air-fuel ratio sensor.In the deterioration diagnostic device according to the aspect of the present disclosure, the control device can be configured to perform a second deterioration diagnostic process when diagnosing the deterioration of the first catalyst. The second deterioration diagnostic process can differ from the first deterioration diagnostic process. The control device can be configured to perform the fat process and the lean process alternately and repeatedly in the second deterioration diagnostic process as well.The control device can be configured to initiate the lean process during the second deterioration diagnostic process by switching the air-fuel ratio of the exhaust gas flowing into the first catalyst from a rich to a lean air-fuel ratio when the output air-fuel ratio from the second air-fuel ratio sensor changes to a rich air-fuel ratio. The control device can also be configured to initiate the rich process during the second deterioration diagnostic process by switching the air-fuel ratio of the exhaust gas flowing into the first catalyst from a lean to a rich air-fuel ratio when the output air-fuel ratio from the second air-fuel ratio sensor changes to a lean air-fuel ratio.The control device can be configured to estimate, during the second deterioration diagnostic process, the amount of oxygen stored in the first catalyst in a lean process or the amount of oxygen released by the first catalyst in a rich process. Based on this estimated oxygen amount, the control device can then determine whether the first catalyst has deteriorated.

[0013] The deterioration diagnostic device according to the aspect of the present disclosure provides a deterioration diagnostic device for an exhaust gas control catalyst, wherein the deterioration diagnostic device is able to suppress the outflow of NOx from the exhaust gas control catalyst when deterioration of the exhaust gas control catalyst is diagnosed. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Features, advantages and technical and industrial significance of exemplary embodiments of the invention are described below with reference to the accompanying drawings, in which the same reference numerals denote the same elements, and wherein: Fig. Figure 1 schematically shows an internal combustion engine in which a deterioration diagnostic device according to one embodiment is used; Fig. 2 represents the relationship between the air-fuel ratio of an exhaust gas around an air-fuel ratio sensor and an output stream of the air-fuel ratio sensor; Fig. 3 a time diagram for a target air-fuel ratio, etc., for a case where normal air-fuel ratio control is performed; Fig. 4 a time diagram for the target air-fuel ratio etc. for a case in which deterioration of an upstream catalyst is diagnosed; Fig. 5 is a flowchart that represents a control sequence of a deterioration diagnostic process for the upstream catalyst; Fig. 6 a time diagram for the target air-fuel ratio etc. for a case in which deterioration of a downstream catalyst is diagnosed; Fig. 7 is a time diagram, similar to Fig. 6 is for a case in which deterioration of the downstream catalyst is diagnosed; Fig. 8 is a time diagram, similar to Fig. 6 is for a case in which deterioration of the downstream catalyst is diagnosed; Fig. 9 is a flowchart that represents a control sequence of an adjustment process for the target air-fuel ratio for a case in which a deterioration diagnostic process is carried out for the downstream catalyst; and Fig. 10 illustrates the relationship between the temperature of the downstream catalyst and the target oxygen storage quantity and the relationship between the temperature of the downstream catalyst and the target oxygen release quantity. DETAILED DESCRIPTION OF THE EXECUTION FORMS

[0015] One embodiment is described below with reference to the drawings. In the following description, similar components are identified by identical reference numerals. Complete internal combustion engine

[0016] Fig. Figure 1 schematically shows an internal combustion engine in which a deterioration diagnostic device according to one embodiment is used. With reference to Fig. Reference numeral 1 denotes an engine body, 2 a cylinder block, 3 a piston that moves back and forth within the cylinder block 2, 4 a cylinder head that is fixed on top of the cylinder block 2, 5 a combustion chamber formed between the piston 3 and the cylinder head 4, 6 an intake valve, 7 an intake port, 8 an exhaust valve, and 9 an exhaust port. The intake valve 6 opens and closes the intake port 7. The exhaust valve 8 opens and closes the exhaust port 9. In the present embodiment, a plurality of cylinders are formed within the cylinder block 2, and a piston 3 moves back and forth within each of the cylinders.

[0017] As in Fig. As shown in Figure 1, a spark plug 10 is arranged in the center of the inner wall surface of the cylinder head 4, and a fuel injector 11 is arranged on a circumferential section of the inner wall surface of the cylinder head 4. The spark plug 10 is configured to generate a spark in response to an ignition signal. The fuel injector 11 injects a predetermined quantity of fuel into the combustion chamber 5 in response to an injection signal. The fuel injector 11 can be arranged to inject fuel into the intake port 7. In the present embodiment, gasoline with a stoichiometric air-fuel ratio of 14.6 is used as the fuel. However, the internal combustion engine can also use a fuel other than gasoline or a fuel mixture containing gasoline.

[0018] The inlet port 7 of each cylinder is connected to a compensation tank 14 via a corresponding inlet branch pipe 13. The compensation tank 14 is connected to an air cleaning device 16 via an inlet pipe 15. The inlet port 7, the inlet branch pipe 13, the compensation tank 14, and the inlet pipe 15 form an inlet channel. A throttle valve 18 is arranged in the inlet pipe 15 and is actuated by a throttle valve actuator 17. The throttle valve 18 can be rotated by the throttle valve actuator 17 to change the opening range of the inlet channel.

[0019] The exhaust port 9 of each cylinder is coupled to an exhaust manifold 19. The exhaust manifold 19 has a plurality of branch sections, each coupled to the exhaust port 9, and a merging section in which the branch sections merge. The merging section of the exhaust manifold 19 is coupled to an upstream housing 21 in which an upstream exhaust control catalyst (hereinafter referred to as the "upstream catalyst") 20 is arranged. The upstream housing 21 is coupled via a first exhaust pipe 22 to a downstream housing 23 in which a downstream exhaust control catalyst (hereinafter referred to as the "downstream catalyst") 24 is arranged. The downstream housing 23 is coupled to a second exhaust pipe 25. The second exhaust pipe 25 is located at the B. via an exhaust trim (not shown) in contact with the environment.The outlet opening 9, the exhaust manifold 19, the downstream housing 21, the first outlet pipe 22, the downstream housing 23 and the second outlet pipe 25 form an outlet channel.

[0020] An electronic control unit (ECU) 31 is designed as a digital computer and comprises a random access memory (RAM) 33, a read-only memory (ROM) 34, a central processing unit (CPU) (microprocessor) 35, an input port 36, and an output port 37, which are interconnected via a bidirectional bus 32. An airflow meter 39 is arranged in the inlet pipe 15 to measure the flow rate of the air flowing in the inlet pipe 15. An output from the airflow meter 39 is fed into the input port 36 via a corresponding analog-to-digital (AD) converter 38.

[0021] In the junction section of the exhaust manifold 19, a first air-fuel ratio sensor 41 is arranged, which detects the air-fuel ratio of exhaust gas flowing in the exhaust manifold 19 (i.e., exhaust gas flowing into the upstream catalyst 20). Furthermore, in the first exhaust pipe 22, a second air-fuel ratio sensor 42 is arranged, which detects the air-fuel ratio of exhaust gas flowing in the first exhaust pipe 22 (i.e., exhaust gas flowing out of the upstream catalyst 20 and into the downstream catalyst 24). Finally, in the second exhaust pipe 25, a third air-fuel ratio sensor 43 is arranged, which detects the air-fuel ratio of exhaust gas flowing in the second exhaust pipe 25 (i.e., exhaust gas flowing out of the downstream catalyst 24).The outputs of the air-fuel ratio sensors 41, 42 and 43 are also fed to the input port 36 via corresponding AD converters 38.

[0022] In the present embodiment, air-fuel ratio sensors 41, 42, and 43 of a limiting flow type are used. Thus, the air-fuel ratio sensors 41, 42, and 43 are configured such that an output flow from the air-fuel ratio sensors 41, 42, and 43 increases when the air-fuel ratio of an exhaust gas around the air-fuel ratio sensors 41, 42, and 43 increases (i.e., becomes leaner), as shown in Fig. Figure 2 shows. In particular, the air-fuel ratio sensors 41, 42, and 43 are configured according to the present embodiment such that the output flow changes linearly with respect to the air-fuel ratio of an exhaust gas around the air-fuel ratio sensors 41, 42, and 43. While limit-flow air-fuel ratio sensors 41, 42, and 43 are used as air-fuel ratio sensors in the present embodiment, other air-fuel ratio sensors than limit-flow air-fuel ratio sensors can also be used if the output flows of the sensors change in relation to the air-fuel ratio of an exhaust gas. Examples of such air-fuel ratio sensors include an oxygen sensor, etc.with an output that changes abruptly near the stoichiometric air-fuel ratio, without any voltage being applied between the electrodes that form the sensor.

[0023] A load sensor 45, which generates an output voltage proportional to the amount of pressure applied to the accelerator pedal 44, is connected to the accelerator pedal 44. The output voltage of the load sensor 45 is fed to the input terminal 36 via a corresponding analog-to-digital converter 38. A crankshaft angle sensor 46 generates an output pulse each time a crankshaft is rotated by, for example, 15 degrees. The output pulse is fed into the input terminal 36. The CPU 35 calculates an engine speed from the output pulse of the crankshaft angle sensor 46. The output terminal 37, in turn, is connected via corresponding drive circuits 47 to the spark plug 10, the fuel injector 11, and the throttle actuator 17. The ECU 31 controls the air-fuel ratio of the exhaust gas emitted from the engine block 1 by controlling the opening degree of the throttle valve 18 and the amount of fuel injected by the fuel injector 11.Additionally, the ECU 31 diagnoses deterioration of the downstream catalyst 24 based on an output from the third air-fuel ratio sensor 43, as described later. Thus, the ECU 31 acts as a control device that regulates the air-fuel ratio of exhaust gas emitted from the engine body 1 and diagnoses deterioration of the downstream catalyst 24 based on an output from the third air-fuel ratio sensor 43.

[0024] The exhaust gas control catalysts (upstream catalyst 20 and downstream catalyst 24) are each a three-way catalyst with an oxygen storage capacity. Specifically, exhaust gas control catalysts 20 and 24 are each a three-way catalyst in which a catalytically active noble metal (e.g., platinum (Pt)) and a substance (e.g., cerium oxide (CeO2)) with an oxygen storage capacity are supported by a ceramic substrate. The three-way catalyst functions to simultaneously reduce unburned hydrocarbons (HC), carbon monoxide (CO), and NOx when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is maintained at the stoichiometric air-fuel ratio.When a certain amount of oxygen is stored in the exhaust control catalysts 20 and 24, unburned HC, CO and NOx are also reduced simultaneously, even if the air-fuel ratio of an exhaust gas flowing into the exhaust control catalysts 20 and 24 deviates slightly from the stoichiometric air-fuel ratio towards the rich or lean side.

[0025] This means that if the exhaust gas control catalysts 20 and 24 can store oxygen, that is, if the amount of oxygen stored in the exhaust gas control catalysts 20 and 24 is less than the maximum amount of oxygen that can be stored, excess oxygen contained in an exhaust gas flowing into the exhaust gas control catalysts 20 and 24 will be stored in the exhaust gas control catalysts 20 and 24 if the air-fuel ratio of the exhaust gas is slightly leaner than the stoichiometric air-fuel ratio. Therefore, the air-fuel ratio at the surfaces of the exhaust gas control catalysts 20 and 24 is maintained at the stoichiometric air-fuel ratio.As a result, unburned HC, CO and NOx are simultaneously reduced on the surfaces of the exhaust control catalysts 20 and 24, and the air-fuel ratio of an exhaust gas flowing out of the exhaust control catalysts 20 and 24 at this time is the stoichiometric air-fuel ratio.

[0026] If the exhaust gas control catalysts 20 and 24 are capable of releasing oxygen, i.e., if the amount of oxygen stored in the exhaust gas control catalysts 20 and 24 is greater than zero, then, conversely, oxygen that is needed for the reduction of unburned HC and CO contained in the exhaust gas flowing into the exhaust gas control catalysts 20 and 24 is released by the exhaust gas control catalysts 20 and 24 if the air-fuel ratio of the exhaust gas is slightly richer than the stoichiometric air-fuel ratio. Therefore, in this case as well, the air-fuel ratio at the surfaces of the exhaust gas control catalysts 20 and 24 is maintained at the stoichiometric air-fuel ratio.Consequently, unburned HC, CO and NOx are reduced simultaneously on the surfaces of the exhaust gas control catalysts 20 and 24, and the air-fuel ratio of an exhaust gas flowing out of the exhaust gas control catalysts 20 and 24 at this time is the stoichiometric air-fuel ratio.

[0027] In this way, when a certain amount of oxygen is stored in the exhaust control catalysts 20 and 24, unburned HC, CO and NOx are reduced simultaneously, even if the air-fuel ratio of an exhaust gas flowing into the exhaust control catalysts 20 and 24 deviates slightly from the stoichiometric air-fuel ratio towards the rich or lean side, and the air-fuel ratio of an exhaust gas flowing out of the exhaust control catalysts 20 and 24 is equal to the stoichiometric air-fuel ratio.

[0028] In the present embodiment, the downstream catalyst 24 is designed as a particulate filter for trapping fine dust. Thus, the downstream catalyst 24 is designed such that exhaust gas flows through a porous partition of the filter. The downstream catalyst 24 must not be designed as a particulate filter. Normal air-fuel ratio control

[0029] Next, an overview of the normal air-fuel ratio control, which is normally performed by the control device for the internal combustion engine according to the present embodiment, is described. In the normal air-fuel ratio control according to the present embodiment, a feedback control is performed in which the amount of fuel injected by the fuel injector 11 is controlled such that the output air-fuel ratio of the first air-fuel ratio sensor 41 is equal to the set air-fuel ratio. The term "output air-fuel ratio" means an air-fuel ratio that corresponds to an output value of an air-fuel ratio sensor.

[0030] In the normal air-fuel ratio control according to the present embodiment, the target air-fuel ratio is set based on the air-fuel ratio output of the second air-fuel ratio sensor 42, etc. A process for setting the target air-fuel ratio in the normal air-fuel ratio control is described below with reference to Fig. 3 described. Fig. Figure 3 is a time diagram of a target air-fuel ratio AFT, an output air-fuel ratio AF1 of the first air-fuel ratio sensor 41, an oxygen storage quantity OSAup of the upstream catalyst 20, an integral oxygen storage / release quantity ΣOSRup of the upstream catalyst 20 and an output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 for a case in which normal air-fuel ratio control is carried out according to the present embodiment.

[0031] When the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 is equal to an air-fuel ratio (hereinafter referred to as the "rich air-fuel ratio") that is richer than the stoichiometric air-fuel ratio (at times t1, t3, and t5 in the drawing), the oxygen storage quantity OSAup of the upstream catalyst 20 is essentially zero. In the present embodiment, a lean process is initiated at such times, in which the target air-fuel ratio AFT is controlled to an air-fuel ratio (hereinafter referred to as the "lean air-fuel ratio") that is leaner than the stoichiometric air-fuel ratio. Consequently, the air-fuel ratio of an exhaust gas discharged from the engine body 1 is equal to a lean air-fuel ratio.In the lean-burn process of the normal air-fuel ratio control according to the present embodiment, the target air-fuel ratio AFT is specifically set to a first lean-burn air-fuel ratio AFTlean1, which is a pre-defined air-fuel ratio (e.g., approximately 14.65 to 16) that is more or less leaner than the stoichiometric air-fuel ratio. In the present embodiment, it is determined that the output air-fuel ratio of an air-fuel ratio sensor has become a rich air-fuel ratio when the output air-fuel ratio of the air-fuel ratio sensor has become equal to or less than a rich-burn air-fuel ratio AFrich (e.g., 14.55), which is slightly richer than the stoichiometric air-fuel ratio.

[0032] The integration of an oxygen storage / release quantity OSRup of the upstream catalyst 20 is initiated simultaneously with the start of the lean-burn process at times t1, t3, and t5. The oxygen storage / release quantity OSRup of the upstream catalyst 20 is defined as the amount of oxygen stored in the upstream catalyst 20 from an exhaust gas flowing into the upstream catalyst 20, or the amount of oxygen released from the upstream catalyst 20 into such an exhaust gas. In other words, the oxygen storage / release quantity OSRup of the upstream catalyst 20 represents the amount of oxygen that is excess or the amount of oxygen that is insufficient (the amount of excess unburned HC, CO, etc.).(hereinafter referred to as "unburned gas"), when attempting to make the air-fuel ratio of an exhaust gas entering the upstream catalyst 20 equal to the stoichiometric air-fuel ratio. In particular, the oxygen in an exhaust gas entering the upstream catalyst 20 is excess during the lean process, and the excess oxygen is stored in the upstream catalyst 20. Thus, an integral value (hereinafter referred to as the "integral oxygen storage quantity") ΣOSRup of the oxygen storage / release quantity is considered an estimate of the oxygen storage quantity of the upstream catalyst 20. When the lean process is initiated, the oxygen storage quantity OSAup of the upstream catalyst 20 is gradually increased, and therefore the integral oxygen storage / release quantity ΣOSRup is also gradually increased.

[0033] The oxygen storage / release quantity OSRup of the upstream catalyst 20 is calculated based on the output air-fuel ratio AF1 of the first air-fuel ratio sensor 41 and an estimated value of the amount of air drawn into the combustion chamber 5, calculated based on an output from the airflow meter 39, etc., or the amount of fuel supplied by the fuel injector 11, etc. Specifically, the oxygen storage / release quantity OSRup of the upstream catalyst 20 is calculated, for example, according to the following formula (1). OSRup=0.23×Qi×(AF1−AFR)

[0034] In the formula, 0.23 represents the oxygen concentration in the air, Qi the fuel injection quantity, AF1 the output air-fuel ratio of the first air-fuel ratio sensor 41, and AFR the stoichiometric air-fuel ratio.

[0035] In the present embodiment, when the integral oxygen storage / release quantity ΣOSRup of the upstream catalyst 20, calculated in this way, becomes equal to or greater than a switching reference value OSRref determined in advance (at times t2 and t4), a rich process is initiated in which the target air-fuel ratio AFT is set to a rich air-fuel ratio. This results in the air-fuel ratio of an exhaust gas emitted from the engine body 1 being equal to a rich air-fuel ratio. In the rich process of the normal air-fuel ratio control according to the present embodiment, the target air-fuel ratio AFT is specifically set to a rich air-fuel ratio AFTrich, which is a predetermined air-fuel ratio (e.g., approximately 14 to 14.55) that is richer or less rich than the stoichiometric air-fuel ratio.The switching reference value OSRref is set to a certain amount (e.g., half; corresponding to Cref in . Fig. 3) set to a value lower than the maximum storable oxygen quantity Cmax at a time when the upstream catalyst 20 is brand new. In the present embodiment, the rich combustion process is therefore started before the oxygen storage quantity of the upstream catalyst 20 approaches the maximum storable oxygen quantity Cmax. Therefore, the rich combustion process is started before oxygen or NOx escapes from the upstream catalyst 20.

[0036] Subsequently, when the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 becomes equal to or lower than the rich air-fuel ratio, the rich process is restarted, and similar processes are then repeated. In this way, in the normal air-fuel ratio control according to the present embodiment, the rich and lean processes are carried out alternately and repeatedly. In other words, in the normal air-fuel ratio control according to the present embodiment, the air-fuel ratio of an exhaust gas emitted from the engine body 1 alternates between a rich air-fuel ratio and a lean air-fuel ratio.

[0037] In the normal air-fuel ratio control described above, no NOx generally flows out of the upstream catalyst 20. However, at times t1, t3, and t5, unburned HC, CO, etc., temporarily flow out of the upstream catalyst 20. The unburned HC and CO that flowed out of the upstream catalyst 20 are reduced in the downstream catalyst 24. The oxygen storage capacity of the downstream catalyst 24 is increased to the maximum storable oxygen quantity Cmax during a fuel cut-off control, in which the combustion engine is caused to operate without fuel supply, and subsequently reduced once the unburned HC and CO have flowed out of the downstream catalyst 20 to be removed.

[0038] It is not always necessary to implement the control described above as normal air-fuel ratio control, which is implemented when fuel cut-off control, fuel boost control (where the fuel supply is temporarily increased), etc., are not used. Various types of control can be implemented as normal air-fuel ratio control, as long as the time-averaged air-fuel ratio of an exhaust gas entering the downstream catalyst 24 is controlled to a stoichiometric air-fuel ratio or a rich air-fuel ratio. Diagnosis of upstream catalyst deterioration

[0039] The exhaust gas control catalyst 20 gradually deteriorates with repeated use. In particular, when the temperature of the exhaust gas control catalysts 20 and 24 becomes high, the catalytic activity of the exhaust gas control catalysts 20 and 24 decreases, and the catalytic precious metal carried by the support sinters. When the exhaust gas control catalysts 20 and 24 deteriorate in this way, it is necessary to change a control mode of the normal air-fuel ratio control or to replace the exhaust gas control catalysts 20 and 24. Therefore, the deterioration diagnostic device according to the present embodiment diagnoses the deterioration of the exhaust gas control catalysts 20 and 24. First, the diagnosis of the deterioration of the upstream catalyst 20 (second deterioration diagnostic process) is carried out with reference to Fig. 4 described.

[0040] Fig. 4 is a time graph, similar to Fig. 3 is, via the target air-fuel ratio AFT etc. for a case in which a deterioration diagnostic process is carried out for the upstream catalyst 20. In which in Fig. In the example shown in section 4, the diagnosis of the deterioration of the upstream catalyst 20 is started at time t0.

[0041] When diagnosing deterioration of the upstream catalyst 20, the lean process is initiated if the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 is equal to a rich air-fuel ratio (at times t1, t3 and t5 in Fig. 4), and the target air-fuel ratio AFT is switched to a second lean-set air-fuel ratio AFTlean2. The second lean-set air-fuel ratio AFTlean2 is set to an air-fuel ratio that is higher (higher in leanness) than the first lean-set air-fuel ratio AFTlean1. As a result, the air-fuel ratio of an exhaust gas discharged from the engine body 1 to flow into the upstream catalyst 20 is equal to a lean air-fuel ratio, and the oxygen storage quantity OSAup of the upstream catalyst 20 is gradually increased.

[0042] In addition, the rich process is initiated when diagnosing deterioration of the upstream catalyst 20 if the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 is equal to a lean air-fuel ratio (at times t2 and t4 in Fig. 4), and the target air-fuel ratio AFT is changed to a second rich air-fuel ratio AFTrich2. The second rich air-fuel ratio AFTrich2 is set to an air-fuel ratio that is lower (higher in richness) than a first rich air-fuel ratio AFTrich1. As a result, the air-fuel ratio of an exhaust gas discharged from the engine body 1 to flow into the upstream catalyst 20 is equal to a rich air-fuel ratio, and the oxygen storage quantity OSAup of the upstream catalyst 20 is gradually reduced.

[0043] When diagnosing deterioration of the upstream catalyst 20, the target air-fuel ratio AFT is alternately set to a rich and a lean air-fuel ratio. If the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 is equal to a rich air-fuel ratio, this means that the oxygen storage quantity OSAup of the upstream catalyst 20 is essentially zero. On the other hand, if the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 is equal to a lean air-fuel ratio, this means that the oxygen storage quantity OSAup of the upstream catalyst 20 has reached the maximum storable oxygen quantity Cmax. Thus, the integral oxygen storage / release quantity ΣOSRup (Q1 in Fig. 4) during a period in which the target air-fuel ratio (AFT) is set to a lean air-fuel ratio, and the integral oxygen storage / release quantity (ΣOSRup) (Q2 in Fig. 4) during a period in which the target air-fuel ratio AFT is set to a rich air-fuel ratio, the maximum storable oxygen quantity Cmax of the upstream catalyst 20.

[0044] If an exhaust gas control catalyst deteriorates, the maximum oxygen storage quantity Cmax is reduced accordingly. Thus, deterioration of the exhaust gas control catalyst can be diagnosed based on the maximum oxygen storage quantity Cmax. In diagnosing the deterioration of the upstream catalyst 20 according to the present embodiment, it is determined that the upstream catalyst 20 has deteriorated when the integral oxygen storage / release quantity ΣOSRup (Q1 in Fig. 4) during a lean process and / or the integral oxygen storage / release quantity ΣOSRup (Q2 in Fig. 4) during a fat process are / is lower than a predetermined lower limit (e.g., about 30% of the maximum storable oxygen quantity Cmax at the time when the upstream catalyst 20 is brand new).

[0045] Fig. Figure 5 is a flowchart showing the control sequence of the deterioration diagnostic process for the upstream catalyst 20. The depicted control sequence is executed by the CPU 35 of the ECU 31 at specific time intervals.

[0046] First, as in Fig. Figure 5 shows that in step S11, it is determined whether an execution flag Fu for a deterioration diagnostic process for the upstream catalyst 20 is switched off. The execution flag Fu is a flag that is set to ON when the deterioration diagnostic process for the upstream catalyst 20 is being executed, and is otherwise set to OFF. If step S11 determines that the execution flag Fu is set to OFF, the control sequence continues with step S12.

[0047] In step S12, it is determined whether an execution condition for the deterioration diagnosis process for the upstream catalyst 20 is met. The execution condition for the deterioration diagnosis process for the upstream catalyst 20 is met, for example, if the elapsed time or the distance traveled since the previous deterioration diagnosis process is equal to or greater than a certain value and the combustion engine has finished warming up. If step S12 determines that the execution condition for the deterioration diagnosis process for the upstream catalyst 20 is not met, the control sequence is terminated. Thus, the deterioration diagnosis process for the upstream catalyst 20 is not executed, and therefore normal air-fuel ratio control, as used, for example, in [reference to relevant software / methodology], is not maintained. Fig. As shown in section 3, it is executed.

[0048] If, in step S12, it is determined that the execution condition for the deterioration diagnosis process for the upstream catalyst 20 is met, the control sequence continues with steps S14 to S16. In steps S14 to S16, the execution flag Fu for the deterioration diagnosis process for the upstream catalyst 20 is set to ON, a rich mixture process is initiated by setting the target air-fuel ratio AFT to the second rich air-fuel ratio AFTrich2, and a rich mixture flag Fr is set to ON. The rich mixture flag Fr is a flag that is set to ON when the target air-fuel ratio is set to a rich air-fuel ratio and is set to OFF otherwise.

[0049] If the execution flag Fu is set to ON in step S14, the next control sequence continues from step S11 to step S17. In step S17, it is determined whether an end condition for the deterioration diagnostic process for the upstream catalyst 20 is met. The end condition for the deterioration diagnostic process for the upstream catalyst 20 is met, for example, if the rich and lean processes are executed multiple times. If step S17 determines that the end condition is not met, the control sequence continues to step S18.

[0050] In step S18, it is determined whether the rich flag Fr is set to ON. If step S18 determines that the rich flag Fr is set to ON, the control sequence continues with step S19. In step S19, it is determined whether the output air-fuel ratio AF2 from the second air-fuel ratio sensor 42 is equal to or lower than the rich-determined air-fuel ratio AFrich, i.e., whether the output air-fuel ratio AF2 from the second air-fuel ratio sensor 42 is a rich air-fuel ratio. If it is determined that the output air-fuel ratio AF2 from the second air-fuel ratio sensor 42 is greater than the rich-determined air-fuel ratio AFrich, the target air-fuel ratio AFT in step S20 is maintained at the second rich-determined air-fuel ratio AFTrich2, and thus the rich-fuel process continues.

[0051] If, subsequently, the oxygen storage quantity of the upstream catalyst 20 is reduced and the air-fuel ratio of an exhaust gas flowing out of the upstream catalyst 20 is determined, step S19 establishes that the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 is equal to or lower than the rich air-fuel ratio AFrich. In this case, the control sequence continues with step S21, and the target air-fuel ratio AFT is switched to the second lean air-fuel ratio AFTlean2, and the lean process is initiated. Then, in step S22, the rich flag Fr is set to OFF.

[0052] If the grease flag Fr is set to OFF, the next control sequence continues from step S18 to step S23. In step S23, it is determined whether the output air-fuel ratio AF2 from the second air-fuel ratio sensor 42 is equal to or greater than a lean air-fuel ratio AFlean, that is, whether the output air-fuel ratio AF2 from the second air-fuel ratio sensor 42 is a lean air-fuel ratio.

[0053] If it is determined that the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 is lower than the lean determined air-fuel ratio AFlean, the target air-fuel ratio AFT in step S24 is maintained at the second lean determined air-fuel ratio AFlean2 and thus the lean process continues.

[0054] If, subsequently, the oxygen storage capacity of the upstream catalyst 20 is increased and the air-fuel ratio of an exhaust gas flowing from the upstream catalyst 20 becomes greater, step S23 determines that the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 is equal to or greater than the lean-set air-fuel ratio AFlean. In this case, the control sequence continues with step S25, and the target air-fuel ratio AFT is switched to the second rich-set air-fuel ratio AFTrich2, and the rich-fuel process is initiated. Then, in step S26, the rich-fuel flag Fr is set to ON.

[0055] When the target air-fuel ratio (AFT) has been set to a rich air-fuel ratio and a lean air-fuel ratio a predetermined number of times, step S17 determines that the end condition for the deterioration diagnostic process for the upstream catalyst 20 is met, and the control sequence proceeds to step S27. In step S27, an average value ΣOSRave of the oxygen storage / release quantity during the rich and lean processes is calculated. Specifically, step S27 calculates the integral oxygen storage / release quantity ΣOSRup of the upstream catalyst 20 for a period during which each rich process is carried out (i.e., for the period from when the target air-fuel ratio (AFT) is set to a rich air-fuel ratio until when the target air-fuel ratio (AFT) is switched to a lean air-fuel ratio).This corresponds to an estimate of the amount of oxygen released by the upstream catalyst 20 during each rich cycle. In step S27, the integral oxygen storage / release quantity ΣOSRup of the upstream catalyst 20 is also calculated for a period during which each lean cycle is performed (i.e., for a period from when the target air-fuel ratio AFT was set to a lean air-fuel ratio until when the target air-fuel ratio AFT was changed to a rich air-fuel ratio). This corresponds to an estimate of the amount of oxygen stored in the upstream catalyst 20 during each lean cycle. In step S27, a value obtained by averaging the integral oxygen storage / release quantity ΣOSRup of the upstream catalyst 20 calculated in this way is calculated as the average value ΣOSRave of the oxygen storage / release quantity.This corresponds to an estimated value of the maximum amount of oxygen that can be stored, Cmax, of the upstream catalyst 20.

[0056] Subsequently, in step S28, deterioration of the upstream catalyst 20 is diagnosed based on the average OSRave value of the oxygen storage / release quantity calculated in step S27. Specifically, deterioration of the upstream catalyst 20 is determined if the average ΣOSRave value of the oxygen storage / release quantity of the upstream catalyst 20 is lower than a predefined lower limit. Then, in step S29, the execution flag Fu for the deterioration diagnosis process for the upstream catalyst 20 is set to OFF.

[0057] In the present embodiment, the lean process and the fat process are performed multiple times in the deterioration diagnosis process. However, it is possible that either the lean process or the fat process, or both, are performed only once. Diagnosis of downstream catalyst deterioration

[0058] As described above, exhaust gas control catalysts deteriorate due to sintering of the catalytic precious metal, etc., when the exhaust gas control catalyst temperature becomes high. Generally, the downstream catalyst 24 does not get very hot, since high-temperature exhaust gas does not typically flow into the downstream catalyst 24. Thus, it is not always necessary to diagnose deterioration of the downstream catalyst 24. However, in the present embodiment, the downstream catalyst 24 functions as a particulate filter. Therefore, it is necessary to increase the temperature of the downstream catalyst 24 to regularly burn off and remove the particulate matter deposited on the particulate filter.If the temperature of the downstream catalyst 24 is increased in this way, the downstream catalyst 24 can also deteriorate, and therefore it is necessary to diagnose deterioration of the downstream catalyst 24. Therefore, the deterioration diagnostic device according to the present embodiment diagnoses the deterioration of the downstream catalyst 24. The diagnosis of the deterioration of the downstream catalyst 24 (first deterioration diagnostic process) is described below with reference to... Fig. 6, Fig. 7, Fig. 8 to Fig. 9 described.

[0059] Fig. Figure 6 is a time graph for the target air-fuel ratio (AFT), etc., for a case where deterioration of the downstream catalyst 24 is diagnosed. In particular, it shows Fig. 6 a case in which there is no deterioration of the downstream catalyst 24.

[0060] In the Fig. In example 6, the normal air-fuel ratio control is carried out before time t1. In particular, in the example shown in Fig. In the example shown, the target air-fuel ratio AFT is set to the first lean air-fuel ratio AFTlean1 at this time. Since oxygen from an exhaust gas is stored in the upstream catalyst 20, the output air-fuel ratios of the second air-fuel ratio sensor 42 and the third air-fuel ratio sensor 43 are essentially the stoichiometric air-fuel ratios.

[0061] When the deterioration diagnostic process for the upstream catalyst 24 is started at time t1, a rich process is initially carried out. In the present embodiment, the target air-fuel ratio AFT is set at this time to a third rich air-fuel ratio AFTrich3, which is lower (higher in richness) than the first rich air-fuel ratio AFTrich1. As a result, the air-fuel ratio of an exhaust gas discharged from the engine body 1 is equal to a rich air-fuel ratio, and the oxygen storage quantity of the upstream catalyst 20 is gradually reduced, reaching essentially zero at time t2.

[0062] When the oxygen storage capacity of the upstream catalyst 20 thus reaches essentially zero, the upstream catalyst 20 can no longer reduce unburned HC or CO from exhaust gas flowing into it. Therefore, after time t2, exhaust gas containing unburned HC and CO, i.e., exhaust gas with a rich air-fuel ratio, flows out of the upstream catalyst 20. Consequently, the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 corresponds to a rich air-fuel ratio at time t2. An exhaust gas with a rich air-fuel ratio flows into the upstream catalyst 24 after time t2. This releases oxygen stored in the downstream catalyst 24 and reduces the oxygen storage quantity OSAdwn of the downstream catalyst 24 after time t2.

[0063] In the present embodiment, the integration of an oxygen storage / release quantity OSRdwn of the downstream catalyst 24 is started at time t2. The oxygen storage / release quantity OSRdwn of the downstream catalyst 24 is understood to be the quantity of oxygen stored in the downstream catalyst 24 from an exhaust gas flowing into the downstream catalyst 24, or the quantity of oxygen released from the downstream catalyst 24 into such an exhaust gas. Assuming that an exhaust gas with the stoichiometric air-fuel ratio flows into the downstream catalyst 24 during a period from time t1 to time t2, an integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 after the proximity to time t2 represents an estimate of the amount of oxygen released from the downstream catalyst 24 since the start of the rich process.

[0064] In the present embodiment, the oxygen storage / release quantity OSRdwn of the downstream catalyst 24 after time t2 is calculated, for example, using the following formula (2), as is the case for the oxygen storage / release quantity of the upstream catalyst 20. In the following formula (2), AF2 is the output air-fuel ratio of the second air-fuel ratio sensor 42. OSRdwn=0.23×Qi×(AF2−AFR)

[0065] The absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24, calculated in this way, is gradually increased as time approaches t2 and reaches a predetermined target oxygen release quantity OSRtr at time t3. In the present embodiment, the lean process is started when the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 reaches the target oxygen release quantity (an example of the initial oxygen quantity) OSRtr after time t2. That is, in the present embodiment, the process switches from the rich process to the lean process when the amount of oxygen released by the downstream catalyst 24 since the start of the rich process equals the target oxygen release quantity OSRtr. The target oxygen release quantity OSRtr is set to a specific amount (e.g.,equal to or less than half) is set, which is lower than the maximum storable oxygen quantity Cmax at the time when the downstream catalyst 24 is brand new.

[0066] In the present embodiment, the target air-fuel ratio AFT is adjusted during the lean process to a third lean-tuned air-fuel ratio AFTlean3, which is higher (higher in leanness) than the first lean-tuned air-fuel ratio AFTlean1. In the Fig. In the example shown in Figure 6, even after time t3 at the start of the lean process, exhaust gas with a rich air-fuel ratio flows from the upstream catalyst 20. This takes into account that unburned HC and CO in an exhaust gas that flowed into the upstream catalyst 20 between time t1 and time t3 is adsorbed on the upstream catalyst 20 and the adsorbed, unburned HC and CO flows out after time t3.

[0067] When the lean process is initiated at time t3, the air-fuel ratio of an exhaust gas discharged from engine body 1 is lean, and the oxygen storage capacity of the upstream catalyst 20 gradually increases, essentially reaching its maximum storage capacity at time t4. When the oxygen storage capacity of the upstream catalyst 20 thus essentially reaches its maximum storage capacity, the upstream catalyst 20 can no longer store oxygen from an exhaust gas flowing into it. Therefore, after time t4, an oxygen-rich exhaust gas, i.e., an exhaust gas with a lean air-fuel ratio, flows out of the upstream catalyst 20. Consequently, the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42 is also lean at time t4.After time t4, exhaust gas with a lean air-fuel ratio flows into the downstream catalyst 24. This causes oxygen to be stored in the downstream catalyst 24, and the oxygen storage quantity OSAdwn of the downstream catalyst 24 increases after time t4.

[0068] In the present embodiment, the integration of the oxygen storage quantity OSRdwn of the downstream catalyst 24 is started at time t4. Assuming that exhaust gas with an air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio flows into the downstream catalyst 24 during a period from time t3 to time t4, the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 as it approaches time t4 represents an estimate of the amount of oxygen stored in the downstream catalyst 24 since the start of the lean process.

[0069] The absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24, calculated in this way, is gradually increased after approaching time t4 and reaches a predetermined target oxygen storage quantity OSRts at time t5. In the present embodiment, the target oxygen storage quantity OSRts is set to a value that is lower than the target oxygen release quantity OSRtr. In particular, the target oxygen storage quantity OSRts is set to an amount (e.g., about one-quarter of the maximum storable oxygen quantity at the time when the downstream catalyst 24 is brand new) such that it is determined that the downstream catalyst 24 has deteriorated if the maximum storable oxygen quantity falls below this amount.In the present embodiment, the rich process is restarted, as at time t1, when the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 reaches the target oxygen storage quantity (an example of the second oxygen quantity) OSRts after time t4. That is, in the present embodiment, the process switches from the lean to the rich process when the amount of oxygen stored in the downstream catalyst 24 since the start of the rich process equals the target oxygen storage quantity OSRts.

[0070] Subsequently, the rich process is carried out alternately and repeatedly during a period from time t1 to time t3, and the lean process during a period from time t3 to t5. Thus, in the deterioration diagnostic process according to the present embodiment, a rich process, in which the air-fuel ratio of an exhaust gas flowing into the downstream catalyst 24 is controlled to a rich air-fuel ratio, and a lean process, in which the air-fuel ratio of an exhaust gas flowing into the downstream catalyst 24 is controlled to a lean air-fuel ratio, are carried out alternately and repeatedly.

[0071] Fig. 7 is a time graph, similar to Fig. 6 is for a case in which deterioration of the downstream catalyst 24 is diagnosed. Fig. Figure 7 also shows a case in which the downstream catalyst 24 has not deteriorated. In particular, Fig. 7. A time diagram for the time until the end of the

[0072] Also in the Fig. The example shown in 7 illustrates how, in the Fig. In the example shown in Figure 6, when exhaust gas with a rich air-fuel ratio flows from the upstream catalyst 20 (at times t1 and t5), the oxygen storage quantity OSAdwn of the downstream catalyst 24 gradually decreases. As described above, the target oxygen storage quantity (an example of the second oxygen quantity) OSRts is lower than the target oxygen release quantity (an example of the first oxygen quantity) OSRtr. Therefore, at times t2 and t5, in Fig. 7. The oxygen storage quantity OSAdwn of the downstream catalyst 24 is essentially zero before the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 reaches the target oxygen release quantity OSRtr, and an exhaust gas with a rich air-fuel ratio flows out of the downstream catalyst 24. As a result, as in Fig. Figure 7 shows that the output air-fuel ratio of the third air-fuel ratio sensor 43 is equal to a rich air-fuel ratio (at times t2 and t6). In the present embodiment, the process switches from rich to lean when the output air-fuel ratio of the third air-fuel ratio sensor 43 is thus equal to a rich air-fuel ratio, even before the absolute value of the integral oxygen storage / release quantity OSRdwn reaches the target oxygen release quantity OSRtr. However, the process must not switch from rich to lean before the absolute value of the integral oxygen storage / release quantity ΣOSRdwn has reached the target oxygen release quantity OSRtr, even if the output air-fuel ratio of the third air-fuel ratio sensor 43 is thus equal to a rich air-fuel ratio.

[0073] In the present embodiment, the deterioration diagnosis process is terminated when the lean process has been performed a predetermined number of times since the deterioration diagnosis process was started. As described in Fig. As shown in Figure 7, the enrichment process is executed last when the deterioration diagnosis process is to be terminated. This results in the oxygen storage quantity OSAdwn of the downstream catalyst 24 being essentially zero after the deterioration diagnosis process has ended. Therefore, even if, during normal air-fuel ratio control, exhaust gas with a lean air-fuel ratio inadvertently enters the downstream catalyst 24, the outflow of exhaust gas with a lean air-fuel ratio from the downstream catalyst 24, and thus the outflow of NOx from the downstream catalyst 24, can be suppressed / prevented.

[0074] In the present embodiment, as described above, the lean process is initially carried out in the deterioration diagnostic process for the downstream catalyst 24. The amount of oxygen stored in the downstream catalyst 24 during the lean process (target oxygen storage amount OSRts) is lower than the amount of oxygen released from the downstream catalyst 24 during the lean process (target oxygen release amount OSRtr). Thus, as described in Fig. 6 and Fig. As indicated in Figure 7, no exhaust gas with a lean air-fuel ratio is emitted from the downstream catalyst 24, while the maximum storable oxygen quantity Cmax of the downstream catalyst 24 is greater than the target oxygen storage quantity OSRts. Thus, the output air-fuel ratio of the third air-fuel ratio sensor 43 during the deterioration diagnostic process is not equal to a lean air-fuel ratio, while the maximum storable oxygen quantity Cmax of the downstream catalyst 24 is greater than the target oxygen storage quantity OSRts.

[0075] Fig. 8 is a time graph, similar to Fig. 6 is for a case in which deterioration of the downstream catalyst 24 is diagnosed. In particular, in Fig. Figure 8 presents a case in which the downstream catalyst 24 has deteriorated. Thus, in the Fig. In example 8, the maximum amount of oxygen that can be stored, Cmax, of the downstream catalyst 24 is low.

[0076] Also in the Fig. The example shown in point 8 illustrates how, in the Fig. In the example shown, the normal air-fuel ratio control is performed before time t1. When the deterioration diagnostic process for the downstream catalyst 24 is started at time t1, a rich process is initially carried out, and the target air-fuel ratio AFT is set to the third rich air-fuel ratio AFTrich3. As a result, the oxygen storage quantity of the upstream catalyst 20 becomes essentially zero at time t2, and after time t2, exhaust gas with a rich air-fuel ratio flows into the downstream catalyst 24.

[0077] When exhaust gas with a rich air-fuel ratio flows into the downstream catalyst 24, the oxygen storage quantity OSAdwn of the downstream catalyst 24 is gradually reduced. Since the maximum storable oxygen quantity Cmax of the downstream catalyst 24 is low, the oxygen storage quantity OSAdwn of the downstream catalyst 24 becomes essentially zero before the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 reaches the target oxygen release quantity OSRtr after time t2. Consequently, the output air-fuel ratio of the third air-fuel ratio sensor 43 at time t3 is equal to a rich air-fuel ratio before the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 reaches the target oxygen release quantity OSRtr after time t2.Therefore, at time t3, the process switches from the fat process to the lean process.

[0078] When the lean process is started at time t3, the target air-fuel ratio AFT is set to the third lean-tuned air-fuel ratio AFTlean3. As a result, the oxygen storage capacity of the upstream catalyst 20 reaches its maximum storage capacity at time t4, and after time t4, exhaust gas with a lean air-fuel ratio flows into the downstream catalyst 24.

[0079] When exhaust gas with a lean air-fuel ratio enters the downstream catalyst 24, the oxygen storage quantity OSAdwn of the downstream catalyst 24 is gradually increased. Since the maximum storable oxygen quantity Cmax of the downstream catalyst 24 is low, the oxygen storage quantity OSAdwn of the downstream catalyst 24 essentially becomes the maximum storable oxygen quantity Cmax before the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 reaches the target oxygen storage quantity OSRts after time t4. As a result, the output air-fuel ratio of the third air-fuel ratio sensor 43 at time t5 is equal to a lean air-fuel ratio before the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 reaches the target oxygen storage quantity OSRts after time t4.In the present embodiment, the process switches from lean to rich when the output air-fuel ratio of the third air-fuel ratio sensor 43 is equal to a lean air-fuel ratio, even before the absolute value of the integral oxygen storage / release quantity ΣOSRdwn has reached the target oxygen storage quantity OSRts. However, the process must not switch from lean to rich before the absolute value of the integral oxygen storage / release quantity ΣOSRdwn reaches the target oxygen storage quantity OSRts, even if the output air-fuel ratio of the third air-fuel ratio sensor 43 is equal to a lean air-fuel ratio.

[0080] In this way, the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 during the lean process in the deterioration diagnostic process for the downstream catalyst 24 is equal to a lean air-fuel ratio if the downstream catalyst 24 has deteriorated and the maximum storable oxygen quantity Cmax is low. In the present embodiment, it is thus determined that the downstream catalyst 24 has deteriorated during the execution of the lean process for the downstream catalyst 24 and that the frequency at which the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is equal to a lean air-fuel ratio is equal to or greater than a predetermined frequency (e.g., 3 / 4).In the present embodiment, it is determined that the downstream catalyst 24 is normal and has not deteriorated when the lean process for the deterioration diagnostic process for the downstream catalyst 24 is performed and when the frequency at which the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is equal to a lean air-fuel ratio is lower than the predetermined frequency.

[0081] In the embodiment described above, the lean process is executed multiple times in the deterioration diagnostic process for the downstream catalyst 24. However, the lean process may only be performed once in the deterioration diagnostic process for the downstream catalyst 24. In this case, it is determined that the downstream catalyst 24 has deteriorated if the lean process is performed and the frequency at which the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is equal to a lean air-fuel ratio is 1 / 1.

[0082] Fig. Figure 9 is a flowchart illustrating the control sequence of an adjustment process for the target air-fuel ratio (AFT) in a case where the deterioration diagnostic process for the downstream catalyst 24 is performed. The depicted control sequence is executed by the CPU 35 of the ECU 31 at specific time intervals.

[0083] First, as in Fig. Figure 9 shows that in step S31 it is determined whether the execution flag Fd for the deterioration diagnostic process for the downstream catalyst 24 is OFF. The execution flag Fd is a flag that is set to ON when the deterioration diagnostic process for the downstream catalyst 24 is executed, and is otherwise set to OFF. If step S31 determines that the execution flag Fd is set to OFF, the control sequence continues with step S32.

[0084] In step S32, it is determined whether an execution condition for the deterioration diagnostic process for the downstream catalyst 24 is met. The execution condition for the deterioration diagnostic process for the downstream catalyst 24 is met, for example, if the elapsed time or the distance traveled since the previous deterioration diagnostic process is equal to or greater than a certain value and the combustion engine has finished warming up. If step S32 determines that the execution condition for the deterioration diagnostic process for the downstream catalyst 24 is not met, the control sequence is terminated. Thus, the deterioration diagnostic process for the downstream catalyst 24 is not executed, and normal air-fuel ratio control, as used, for example, in Fig. The action shown in section 3 will be carried out.

[0085] If, in step S32, it is determined that the execution condition for the deterioration diagnosis process for the downstream catalyst 24 is met, the control sequence continues with steps S34 to S36. In steps S34 to S36, the execution flag Fd for the deterioration diagnosis process for the downstream catalyst 24 is set to ON, a rich process is initiated by setting the target air-fuel ratio AFT to the third rich-set air-fuel ratio AFTrich3, and the rich flag Fr is set to ON.

[0086] If the execution flag Fd is set to ON in step S34, the next control sequence continues from step S31 to step S37. In step S37, it is determined whether the grease flag Fr is set to ON. If step S37 determines that the grease flag Fr is set to ON, the control sequence continues with steps S38 and 39.

[0087] In step S38, it is determined whether the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is equal to or lower than the rich-determined air-fuel ratio AFrich, i.e., whether the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is a rich air-fuel ratio. In step S39, it is determined whether the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24, indicated after the output air-fuel ratio AF2 of the second air-fuel ratio sensor 42, is equal to or lower than the rich-determined air-fuel ratio AFrich, i.e., whether the amount of oxygen released from the downstream catalyst 24 during the lean process is equal to or greater than the target oxygen release quantity OSRtr.

[0088] The control sequence continues with step S40 if, in step S38, it is determined that the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is higher than the rich-set air-fuel ratio AFrich, and in step S39, it is determined that the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 is lower than the target oxygen release quantity OSRtr. In step S40, the target air-fuel ratio AFT is maintained at the third rich-set air-fuel ratio AFTrich3, and thus the rich-run process continues.

[0089] On the other hand, the control sequence continues with step S41 if, in step S38, it is determined that the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is equal to or lower than the rich determined air-fuel ratio AFrich, or if, in step S39, it is determined that the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 is equal to or greater than the target oxygen release quantity OSRtr. Step S41 determines whether a final condition for the deterioration diagnostic process for the downstream catalyst 24 is met.The end condition for the deterioration diagnostic process for the upstream catalyst 20 is met, for example, when the lean process has been executed a predetermined number of times or when a lean counter, which will be discussed later, has counted to or beyond a predetermined reference value, which will be discussed later. If step S41 determines that the end condition is not met, the control sequence continues with step S42. In step S42, the target air-fuel ratio AFT is changed to the third lean-set air-fuel ratio AFTlean3, and the lean process is started. Then, in step S43, the rich flag Fr is set to OFF.

[0090] If the grease flag Fr is set to OFF, the next control sequence continues from step S37 with steps S44 and S45. In step S44, it is determined whether the output air-fuel ratio AF3 from the third air-fuel ratio sensor 43 is equal to or greater than the lean-determined air-fuel ratio AFlean, i.e., whether the output air-fuel ratio AF3 from the third air-fuel ratio sensor 43 is a lean air-fuel ratio. In step S45, it is determined whether the absolute value of the integral oxygen storage quantity ΣOSRdwn of the downstream catalyst 24, displayed after the air-fuel ratio AF2 output by the second air-fuel ratio sensor 42, is equal to or greater than the lean-running determined air-fuel ratio AFlean, i.e., whether the amount of oxygen stored in the downstream catalyst 24 during the lean process is equal to or greater than the target oxygen storage quantity OSRts.

[0091] The control sequence continues with step S46 if, in step S44, it is determined that the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is lower than the lean-set air-fuel ratio AFlean, and if, in step S45, it is determined that the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 is lower than the target oxygen storage quantity OSRts. In step S46, the target air-fuel ratio AFT is maintained at the third lean-set air-fuel ratio AFTlean3, and thus the lean-run process continues.

[0092] On the other hand, the control sequence continues with step S47 if, in step S44, it is determined that the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is equal to or greater than the lean-determined air-fuel ratio AFlean. In step S47, the lean counter is incremented by one, and the control sequence continues with step S48. The lean counter is a counter that counts how many times the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 is equal to a lean air-fuel ratio during the deterioration diagnostic process for the downstream catalyst 24.

[0093] If, in step S45, it is determined that the absolute value of the integral oxygen storage / release quantity ΣOSRdwn of the downstream catalyst 24 is equal to or greater than the target oxygen storage quantity OSRts, the control sequence continues with step S48. In step S48, the target air-fuel ratio AFT is changed to the third richly set air-fuel ratio AFTrich3, and the enrichment process is started. Then, in step S49, the richness flag Fr is set to ON.

[0094] For example, if the lean process has been executed a predetermined number of times, the subsequent control sequence determines that the end condition in step S41 is met, and the control sequence proceeds to step S50. In step S50, the deterioration of the downstream catalyst 24 is diagnosed based on the value of the lean counter. Specifically, it is determined that the downstream catalyst 24 has deteriorated if the value of the lean counter is equal to or greater than a reference value (i.e., if the frequency at which the output air-fuel ratio AF3 of the third air-fuel ratio sensor 43 corresponds to a lean air-fuel ratio is equal to or greater than a predetermined frequency during the lean process). Conversely, it is determined that the downstream catalyst 24 is normal if the value of the lean counter is lower than the reference value.Then, in step S51, the execution flag Fd for the deterioration diagnostic process for the downstream catalyst 24 is set to OFF. Effect

[0095] In the embodiment described above, the target oxygen storage quantity OSRts is lower than the target oxygen release quantity OSRtr. Therefore, in the present embodiment, during the deterioration diagnostic process for the downstream catalyst 24, no exhaust gas with a leaner air-fuel ratio flows from the downstream catalyst 24, unless the downstream catalyst 24 has deteriorated. Thus, with the present embodiment, it is possible to suppress the outflow of NOx from the downstream catalyst 24 as much as possible.

[0096] In the present embodiment, the fat-burning process is initially carried out during the deterioration diagnosis process for the downstream catalyst 24. This makes it possible to prevent the oxygen storage quantity OSAdwn of the downstream catalyst 24 from reaching the maximum storable oxygen quantity Cmax during the deterioration diagnosis process, regardless of how high the oxygen storage quantity OSAdwn of the downstream catalyst 24 may be at the beginning of the deterioration diagnosis process, and thus makes it possible to suppress the outflow of NOx from the downstream catalyst 24.

[0097] In the present embodiment, the third rich air-fuel ratio setting, AFTrich3, is additionally lower (higher in richness) during the deterioration diagnostic process for the downstream catalyst 24 than the first rich air-fuel ratio setting, AFTrich1, during normal air-fuel ratio control. This can lead to a water-gas displacement reaction in the upstream catalyst 20 and the downstream catalyst 24, generating hydrogen. Hydrogen diffuses faster in an air-fuel ratio sensor than unburned HC, CO, etc., and therefore the air-fuel ratio sensor is likely to detect a rich air-fuel ratio at an early stage.Thus, with the present embodiment it is possible to detect an exhaust gas with a rich air-fuel ratio from the downstream catalyst 24 at an early stage and thus to suppress the outflow of unburned HC, CO, etc. from the downstream catalyst 24 as much as possible.

[0098] In the present embodiment, the third lean-set air-fuel ratio, AFTlean3, during the deterioration diagnostic process for the downstream catalyst 24 is higher (higher in leanness) than the first lean-set air-fuel ratio, AFTlean1, during normal air-fuel ratio control. If the leanness of an exhaust gas flowing into the downstream catalyst 24 is low, the leanness of the exhaust gas flowing out of the downstream catalyst 24 will also be low. Thus, there is a possibility that the third air-fuel ratio sensor 43 may fail to detect exhaust gas flowing out of the downstream catalyst 24 with a lean air-fuel ratio.In the present embodiment, the third air-fuel ratio sensor 43 can easily detect an outgoing exhaust gas with a lean air-fuel ratio, since the leanness degree of the third lean air-fuel ratio AFTlean3 is high during the deterioration diagnostic process for the downstream catalyst 24. Modifications

[0099] While one embodiment of the present invention has been described above, the present invention is not limited to such an embodiment and can be modified and altered in various ways within the scope of the claims.

[0100] For example, the target oxygen storage quantity OSRts and the target oxygen release quantity OSRtr are each constant values ​​in the embodiment described above. However, the target oxygen storage quantity OSRts and the target oxygen release quantity OSRtr can each be values ​​that change in relation to a parameter relating to the maximum amount of oxygen that can be stored by an exhaust gas control catalyst.

[0101] For example, the maximum amount of oxygen that an exhaust gas recirculation (EGR) catalyst can store increases as the EGR catalyst's temperature rises. Therefore, the target oxygen storage quantity (OSRts) and the target oxygen release quantity (OSRtr) can be varied in relation to the EGR catalyst's temperature. Fig. Figure 10 shows the relationship between the temperature of the exhaust gas control catalyst 24 and the target oxygen storage quantity, as well as the relationship between the temperature of the exhaust gas control catalyst 24 and the target oxygen release quantity. As in Fig. As indicated in section 10, the target oxygen storage quantity is set to increase as the temperature of the downstream catalyst 24 rises. Similarly, the target oxygen release quantity is set to increase as the temperature of the downstream catalyst 24 rises. In this case, the temperature of the downstream catalyst 24 is detected, for example, by a temperature sensor (not shown) provided in the downstream catalyst 24.

[0102] The deterioration diagnosis process described above for the downstream catalyst 24 can be used to diagnose deterioration of the upstream catalyst 20. In this case, an output from the second air-fuel ratio sensor 42 is used instead of an output from the third air-fuel ratio sensor 43.

Claims

Deterioration diagnostic device for an exhaust gas control catalyst, wherein the deterioration diagnostic device is configured to diagnose deterioration of the exhaust gas control catalyst, which is provided in an exhaust port of an internal combustion engine and is configured to store oxygen, and wherein the deterioration diagnostic device comprises: a downstream air-fuel ratio sensor (43) configured to detect an air-fuel ratio of an exhaust gas flowing out of the exhaust gas control catalyst; and a control device (31) configured to control an air-fuel ratio of an exhaust gas flowing into the exhaust gas control catalyst and to diagnose deterioration of the exhaust gas control catalyst based on an output from the downstream air-fuel ratio sensor (43), wherein the control device (31) is configuredIn a deterioration diagnostic process for diagnosing deterioration of the exhaust gas control catalyst, alternately and repeatedly perform a rich process and a lean process, wherein the rich process is a process in which the air-fuel ratio of the exhaust gas flowing into the exhaust gas control catalyst is controlled to a rich air-fuel ratio that is richer than a stoichiometric air-fuel ratio, and wherein the lean process is a process in which the air-fuel ratio of the exhaust gas flowing into the exhaust gas control catalyst is controlled to a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, switching from the rich process to the lean process when an amount of oxygen released by the exhaust gas control catalyst since the start of the rich process equals an initial amount of oxygen, and switching from the lean process to the rich process.if an amount of oxygen stored in the exhaust control catalyst since the start of the lean process is equal to a second amount of oxygen that is lower than the first amount of oxygen, and to determine that the exhaust control catalyst has deteriorated when the lean process is carried out and a frequency at which an output air-fuel ratio of the downstream air-fuel ratio sensor (43) is equal to the lean air-fuel ratio is equal to or greater than a predetermined frequency. Deterioration diagnostic device according to claim 1, wherein the control device (31) is configured to switch from the rich process to the lean process when the output air-fuel ratio of the downstream air-fuel ratio sensor (43) is equal to the rich air-fuel ratio, even before the amount of oxygen released by the exhaust control catalyst since the start of the rich process is equal to the first amount of oxygen. Deterioration diagnostic device according to claim 1 or 2, wherein the control device (31) is configured to switch from the lean process to the rich process when the output air-fuel ratio of the downstream air-fuel ratio sensor (43) is equal to the lean air-fuel ratio, even before the amount of oxygen that has been stored in the exhaust control catalyst since the start of the lean process is equal to the second amount of oxygen. Deterioration diagnostic device according to one of claims 1 to 3, wherein the control device (31) is configured to first perform the grease process when the deterioration diagnostic process is started. Deterioration diagnostic device according to one of claims 1 to 4, wherein the control device (31) is configured to perform the fat removal process last when the deterioration diagnostic process is completed. Deterioration diagnostic device according to any one of claims 1 to 5, wherein the control device (31) is configured to control an air-fuel ratio of an exhaust gas emitted from an engine body such that, during normal air-fuel ratio control, which differs from the deterioration diagnostic process, it alternates between a rich air-fuel ratio and a lean air-fuel ratio; and the air-fuel ratio of the exhaust gas emitted from the engine body during the rich process is higher in richness than at a time when the air-fuel ratio of the exhaust gas emitted from the engine body is set to the rich air-fuel ratio during normal air-fuel ratio control. Deterioration diagnostic device according to any one of claims 1 to 6, wherein the control device (31) is configured to control an air-fuel ratio of an exhaust gas emitted from an engine body such that, during normal air-fuel ratio control, which differs from the deterioration diagnostic process, it alternates between a rich air-fuel ratio and a lean air-fuel ratio; and the air-fuel ratio of the exhaust gas emitted from the engine body during the lean process has a higher degree of leanness than at a time when the air-fuel ratio of the exhaust gas emitted from the engine body is set to the lean air-fuel ratio during normal air-fuel ratio control. Deterioration diagnostic device according to one of claims 1 to 7, wherein the first oxygen quantity is set such that it increases when the temperature of the exhaust gas control catalyst increases. Deterioration diagnostic device according to one of claims 1 to 8, wherein the second oxygen quantity is adjusted to increase when the temperature of the exhaust gas control catalyst increases. Deterioration diagnostic device according to one of claims 1 to 9, wherein the exhaust gas control catalyst acts as a particulate filter that traps fine dust from the exhaust gas. Deterioration diagnostic device according to any one of claims 1 to 10, further comprising a first air-fuel ratio sensor (41) and a second air-fuel ratio sensor (42), wherein a first catalyst and a second catalyst are provided in an exhaust channel of the internal combustion engine, the second catalyst serving as an exhaust control catalyst and being provided downstream of the first catalyst; the first air-fuel ratio sensor (41) being arranged upstream of the first catalyst; the second air-fuel ratio sensor (42) being arranged between the first catalyst and the second catalyst; and a third air-fuel ratio sensor (43) being arranged downstream of the second catalyst, the third air-fuel ratio sensor (43) serving as the downstream air-fuel ratio sensor (43). Deterioration diagnostic device according to claim 11, wherein the control device (31) is configured to perform a second deterioration diagnostic process when it diagnoses deterioration of the first catalyst, the second deterioration diagnostic process being different from the deterioration diagnostic process; the control device (31) being configured to alternately and repeatedly perform the rich process and the lean process also in the second deterioration diagnostic process; and the control device (31) being configured in the second deterioration diagnostic process (i) to start the lean process by changing the air-fuel ratio of an exhaust gas flowing into the first catalyst from the rich air-fuel ratio to the lean air-fuel ratio when an output air-fuel ratio from the second air-fuel ratio sensor changes to the rich air-fuel ratio.(ii) to initiate the rich process by changing the air-fuel ratio of the exhaust gas flowing into the first catalyst from the lean air-fuel ratio to the rich air-fuel ratio when the output air-fuel ratio of the second air-fuel ratio sensor changes to the lean air-fuel ratio, (iii) to estimate the amount of oxygen stored in the first catalyst in a lean process or the amount of oxygen released from the first catalyst in a rich process, and (iv) to determine, based on the estimated amount of oxygen, whether the first catalyst has deteriorated.

Images