Exhaust purification apparatus and exhaust purification method for internal combustion engine

Abstract

The present application is an exhaust purification device and an exhaust purification method for an internal combustion engine. The exhaust purification device for an internal combustion engine is provided with a catalyst, an air-fuel ratio sensor, and an air-fuel ratio control device, the catalyst is disposed in an exhaust passage of the internal combustion engine and is capable of storing oxygen, the air-fuel ratio sensor detects an air-fuel ratio of effluent exhaust gas flowing out from the catalyst, and the air-fuel ratio control device controls an air-fuel ratio of inflow exhaust gas flowing into the catalyst. The air-fuel ratio control device starts slight enrichment control when the air-fuel ratio of the effluent exhaust gas decreases to a concentration-side switching air-fuel ratio or less, and in the slight enrichment control, the air-fuel ratio of the inflow exhaust gas is controlled so that the air-fuel ratio of the effluent exhaust gas detected by the air-fuel ratio sensor is maintained at a slight enrichment set air-fuel ratio which is richer than the theoretical air-fuel ratio.

Description

Technical Field

[0001] This invention relates to an exhaust purification device and method for internal combustion engines. Background Technology

[0002] Previously, it was known to place oxygen-absorbing catalysts in the exhaust passage of internal combustion engines to purify HC, CO, NOx, and other substances in the exhaust gas. In the internal combustion engines described in Japanese Patent Application Publication No. 2008-128110 and Japanese Patent Application Publication No. 09-126012, in order to improve the exhaust purification performance of the catalyst, the air-fuel ratio of the exhaust gas is controlled based on the output of an air-fuel ratio sensor located downstream of the catalyst.

[0003] However, when oxygen is depleted in the catalyst, water-gas shift reaction and steam reforming reaction occur, and the hydrogen generated by these reactions flows out of the catalyst. As a result, the output of the air-fuel ratio sensor located downstream of the catalyst becomes erroneous. In contrast, Japanese Patent Application Laid-Open No. 2008-128110 describes a method for calculating the output error of the air-fuel ratio sensor caused by the hydrogen generated in the catalyst, and setting a target air-fuel ratio in a way that cancels out the output error. Summary of the Invention

[0004] However, the method described in Japanese Patent Application Publication No. 2008-128110 assumes that hydrogen is always produced in the catalyst, and does not implement air-fuel ratio control corresponding to the state of the catalyst. Therefore, when the state of the catalyst changes according to the operating conditions of the internal combustion engine, there is a risk of worsening exhaust emissions.

[0005] Therefore, the present invention provides a technique for suppressing the deterioration of exhaust emissions by implementing air-fuel ratio control corresponding to the hydrogen production status in the catalyst, when the exhaust air-fuel ratio is controlled based on the output of an air-fuel ratio sensor disposed downstream of the catalyst.

[0006] The first aspect of the present invention relates to an exhaust gas purification device for an internal combustion engine, comprising a catalyst, an air-fuel ratio sensor, and an air-fuel ratio control device. The catalyst is disposed in the exhaust passage of the internal combustion engine and is configured to absorb oxygen. The air-fuel ratio sensor is configured to detect the air-fuel ratio of the exhaust gas flowing out from the catalyst. The air-fuel ratio control device is configured to control the air-fuel ratio of the exhaust gas flowing into the catalyst. Furthermore, the air-fuel ratio control device is configured to initiate a slightly rich control when the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor decreases to below a rich-side switching air-fuel ratio richer than the stoichiometric air-fuel ratio. In this slightly rich control, the air-fuel ratio of the exhaust gas is controlled such that the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor is maintained at a slightly rich set air-fuel ratio richer than the stoichiometric air-fuel ratio.

[0007] In the exhaust purification device for an internal combustion engine according to the first embodiment described above, the air-fuel ratio control device may be configured to: during the period when the air-fuel ratio of the inflow exhaust is controlled such that the air-fuel ratio of the outflow exhaust detected by the air-fuel ratio sensor is maintained at or above the stoichiometric air-fuel ratio, the micro-enrichment control is initiated when the air-fuel ratio of the outflow exhaust detected by the air-fuel ratio sensor decreases to or below the rich-side switching air-fuel ratio.

[0008] In the exhaust purification device for an internal combustion engine according to the first embodiment described above, the air-fuel ratio control device can be configured to perform stoichiometric air-fuel ratio control, wherein the air-fuel ratio of the inflow exhaust is controlled such that the air-fuel ratio of the outflow exhaust detected by the air-fuel ratio sensor is maintained at the stoichiometric air-fuel ratio. Furthermore, the air-fuel ratio control device can be configured to initiate micro-enrichment control when the air-fuel ratio of the outflow exhaust detected by the air-fuel ratio sensor decreases below the rich-side switching air-fuel ratio during the stoichiometric air-fuel ratio control.

[0009] In the exhaust purification device of the internal combustion engine of the first embodiment described above, the air-fuel ratio control device can be configured such that, in the enrichment control, when the air-fuel ratio of the outflowing exhaust detected by the air-fuel ratio sensor rises to or exceeds the lean-side switching air-fuel ratio above the stoichiometric air-fuel ratio, the enrichment control is terminated.

[0010] In the exhaust purification device of the internal combustion engine configured as described above, the air-fuel ratio control device can be configured such that: in the enrichment control, when the air-fuel ratio of the outflowing exhaust detected by the air-fuel ratio sensor rises above the lean-side switching air-fuel ratio, stoichiometric air-fuel ratio control is started; in the stoichiometric air-fuel ratio control, the air-fuel ratio of the inflowing exhaust is controlled so that the air-fuel ratio of the outflowing exhaust detected by the air-fuel ratio sensor is maintained at the stoichiometric air-fuel ratio.

[0011] In the exhaust purification device of the internal combustion engine of the first embodiment described above, the air-fuel ratio control device may be configured to determine the richness of the slightly rich set air-fuel ratio based on the minimum air-fuel ratio when the air-fuel ratio of the outflowing exhaust gas detected by the air-fuel ratio sensor drops below the rich-side switching air-fuel ratio.

[0012] In the exhaust purification device of the internal combustion engine of the first embodiment described above, the air-fuel ratio control device may be configured to: estimate the hydrogen concentration in the outflowing exhaust gas, and determine the degree of enrichment of the slightly enriched set air-fuel ratio based on the hydrogen concentration.

[0013] The second aspect of the present invention relates to an exhaust gas purification method for an internal combustion engine comprising a catalyst, an air-fuel ratio sensor, and an air-fuel ratio control device. The catalyst is disposed in the exhaust passage of the internal combustion engine and is configured to absorb oxygen. The air-fuel ratio sensor is configured to detect the air-fuel ratio of the exhaust gas flowing out from the catalyst. The air-fuel ratio control device is configured to control the air-fuel ratio of the exhaust gas flowing into the catalyst to a target air-fuel ratio. In the exhaust gas purification method, when the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor decreases to below a rich-side switching air-fuel ratio richer than the stoichiometric air-fuel ratio, micro-enrichment control is initiated. In this micro-enrichment control, the air-fuel ratio of the exhaust gas is controlled such that the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor is maintained at a micro-enrichment set air-fuel ratio richer than the stoichiometric air-fuel ratio.

[0014] According to the exhaust purification device and method for an internal combustion engine of the present invention, when the air-fuel ratio of the exhaust is controlled based on the output of an air-fuel ratio sensor disposed downstream of the catalyst, the deterioration of exhaust emissions can be suppressed by implementing air-fuel ratio control corresponding to the hydrogen production status in the catalyst. Attached Figure Description

[0015] The features, advantages, and technical and industrial significance of typical embodiments of the present invention will be described below with reference to the accompanying drawings, wherein like reference numerals denote like elements, wherein:

[0016] Figure 1 This is a schematic diagram of an internal combustion engine to which the exhaust purification device of the internal combustion engine according to the first embodiment of the present invention is applied.

[0017] Figure 2 It is shown Figure 1 A diagram illustrating an example of the purification characteristics of the catalyst (three-way catalyst) shown.

[0018] Figure 3 yes Figure 1 The image shows a partial cross-sectional view of the downstream air-fuel ratio sensor.

[0019] Figure 4 This is a graph showing the relationship between the air-fuel ratio of the exhaust gas in the downstream air-fuel ratio sensor and the output current of the sensing element.

[0020] Figure 5A It is a time chart of various parameters as the air-fuel ratio of the exhaust gas flowing into the catalyst alternates between an air-fuel ratio richer than the stoichiometric air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio.

[0021] Figure 5B It is shown in a general way. Figure 5AThe graph shows the oxygen uptake state of the catalyst at various time points.

[0022] Figure 6 This is a time graph of various parameters when performing air-fuel ratio control in the first embodiment of the present invention.

[0023] Figure 7A This is a flowchart illustrating the control routine for air-fuel ratio control in the first embodiment.

[0024] Figure 7B This is a flowchart illustrating the control procedure for air-fuel ratio control in the first embodiment.

[0025] Figure 7C This is a flowchart illustrating the control procedure for air-fuel ratio control in the first embodiment.

[0026] Figure 8 It is a graph showing the minimum air-fuel ratio when the output air-fuel ratio of the downstream air-fuel ratio sensor in the internal combustion engine drops below the rich-side switching air-fuel ratio.

[0027] Figure 9 This is a flowchart illustrating the control procedure for air-fuel ratio control in the second embodiment of the present invention.

[0028] Figure 10 This is a diagram illustrating an example of a map in the second embodiment used to determine the values ​​of the slightly enriched set air-fuel ratio, the first upper-side determined air-fuel ratio, and the first lower-side determined air-fuel ratio based on the minimum air-fuel ratio.

[0029] Figure 11 This is a diagram schematically illustrating a portion of an internal combustion engine to which the exhaust purification device of the internal combustion engine according to the third embodiment of the present invention is applied.

[0030] Figure 12 This is a flowchart illustrating the control procedure for air-fuel ratio control in the third embodiment. Detailed Implementation

[0031] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Furthermore, in the following description, the same reference numerals will be used to denote the same constituent elements.

[0032] First, refer to Figures 1 to 7C The first embodiment of the present invention will be described.

[0033] First, let's give an overall explanation of the internal combustion engine. Figure 1 This is a schematic diagram of an internal combustion engine to which the exhaust purification device of the internal combustion engine according to the first embodiment of the present invention is applied. Figure 1The internal combustion engine shown is a spark-ignition internal combustion engine. Internal combustion engines are installed in vehicles and used as the vehicle's power source.

[0034] The internal combustion engine has an engine body 1 comprising a cylinder block 2 and a cylinder head 4. Multiple (e.g., four) cylinders are formed inside the cylinder block 2. A piston 3 is disposed in each cylinder, reciprocating along the cylinder's axial direction. A combustion chamber 5 is formed between the piston 3 and the cylinder head 4.

[0035] An intake port 7 and an exhaust port 9 are formed in the cylinder head 4. The intake port 7 and the exhaust port 9 are respectively connected to the combustion chamber 5.

[0036] In addition, the internal combustion engine has an intake valve 6 and an exhaust valve 8 located in the cylinder head 4. The intake valve 6 opens and closes the intake port 7, and the exhaust valve 8 opens and closes the exhaust port 9.

[0037] In addition, the internal combustion engine includes a spark plug 10 and a fuel injection valve 11. The spark plug 10 is disposed in the center of the inner wall of the cylinder head 4 and generates a spark according to an ignition signal. The fuel injection valve 11 is disposed in the periphery of the inner wall of the cylinder head 4 and injects fuel into the combustion chamber 5 according to an injection signal. In this embodiment, gasoline with a stoichiometric air-fuel ratio of 14.6 is used as the fuel supplied to the fuel injection valve 11.

[0038] In addition, the internal combustion engine includes an intake manifold 13, a surge tank 14, an intake pipe 15, an air filter 16, and a throttle valve 18. The intake ports 7 of each cylinder are connected to the surge tank 14 via their respective intake manifolds 13, and the surge tank 14 is connected to the air filter 16 via the intake pipe 15. The intake ports 7, intake manifold 13, surge tank 14, and intake pipe 15 form an intake passage for introducing air into the combustion chamber 5. The throttle valve 18 is disposed within the intake pipe 15 between the surge tank 14 and the air filter 16, and is driven by a throttle valve actuator 17 (e.g., a DC motor). By rotating the throttle valve actuator 17, the opening area of ​​the intake passage can be changed according to its opening degree.

[0039] In addition, the internal combustion engine includes an exhaust manifold 19, a catalyst 20, a casing 21, and an exhaust pipe 22. The exhaust ports 9 of each cylinder are connected to the exhaust manifold 19. The exhaust manifold 19 has multiple branches connected to each exhaust port 9 and a collection section formed by these branches. The collection section of the exhaust manifold 19 is connected to the casing 21, which houses the catalyst 20. The casing 21 is connected to the exhaust pipe 22. The exhaust ports 9, exhaust manifold 19, casing 21, and exhaust pipe 22 form an exhaust passage for discharging exhaust gases produced by the combustion of the air-fuel mixture in the combustion chamber 5.

[0040] In addition, vehicles equipped with internal combustion engines are equipped with an electronic control unit (ECU) 31. Figure 1 As shown, the ECU 31 is a digital computer, equipped with RAM (Random Access Memory) 33, ROM (Read Only Memory) 34, CPU (Microprocessor) 35, input port 36, and output port 37, all interconnected via a bidirectional bus 32. Furthermore, in this embodiment, only one ECU 31 is provided, but multiple ECUs may be provided according to their respective functions.

[0041] The ECU 31 performs various controls on the internal combustion engine based on the outputs of various sensors installed in the vehicle or internal combustion engine. Therefore, the outputs of various sensors are sent to the ECU 31. In this embodiment, the outputs of the air flow meter 40, the upstream air-fuel ratio sensor 41, the downstream air-fuel ratio sensor 42, the load sensor 44, and the crankshaft angle sensor 45 are sent to the ECU 31.

[0042] Air flow meter 40 is disposed in the intake passage of the internal combustion engine, specifically in the intake pipe 15 upstream of the throttle valve 18. Air flow meter 40 detects the flow rate of air flowing in the intake passage. Air flow meter 40 is electrically connected to ECU 31, and the output of air flow meter 40 is input to input port 36 via a corresponding AD converter 38.

[0043] An upstream air-fuel ratio sensor 41 is disposed in the exhaust passage upstream of the catalyst 20, specifically in the collection section of the exhaust manifold 19. The upstream air-fuel ratio sensor 41 detects the air-fuel ratio of the exhaust flowing within the exhaust manifold 19, i.e., the exhaust discharged from the cylinders of the internal combustion engine and flowing into the catalyst 20. The upstream air-fuel ratio sensor 41 is electrically connected to the ECU 31, and its output is input to the input port 36 via a corresponding AD converter 38.

[0044] The downstream air-fuel ratio sensor 42 is disposed in the exhaust passage downstream of the catalyst 20, specifically in the exhaust pipe 22. The downstream air-fuel ratio sensor 42 detects the air-fuel ratio of the exhaust gas flowing in the exhaust pipe 22, i.e., the exhaust gas flowing out of the catalyst 20. The downstream air-fuel ratio sensor 42 is electrically connected to the ECU 31, and the output of the downstream air-fuel ratio sensor 42 is input to the input port 36 via the corresponding AD converter 38.

[0045] A load sensor 44 is connected to the accelerator pedal 43 of a vehicle equipped with an internal combustion engine to detect the amount of time the accelerator pedal 43 is depressed. The load sensor 44 is electrically connected to the ECU 31, and its output is input to the input port 36 via a corresponding AD converter 38. The ECU 31 calculates the internal combustion engine load based on the output of the load sensor 44.

[0046] Each time the crankshaft of the internal combustion engine rotates by a specified angle (e.g., 10 degrees), the crankshaft angle sensor 45 generates an output pulse. The crankshaft angle sensor 45 is electrically connected to the ECU 31, and its output is input to the input port 36. The ECU 31 calculates the internal combustion engine speed based on the output of the crankshaft angle sensor 45.

[0047] On the other hand, the output port 37 of ECU31 is connected to the spark plug 10, the fuel injection valve 11, and the throttle valve actuator 17 via the corresponding drive circuit 39, and ECU31 controls them. Specifically, ECU31 controls the ignition timing of spark plug 10, the injection timing and injection quantity of fuel injected from fuel injection valve 11, and the opening degree of throttle valve 18.

[0048] Furthermore, while the aforementioned internal combustion engine is a gasoline-powered, turbocharged internal combustion engine, the structure of an internal combustion engine is not limited to this. Therefore, specific aspects of an internal combustion engine's structure, such as cylinder arrangement, fuel injection method, intake and exhaust system configuration, valve mechanism configuration, and the presence or absence of a turbocharger, can also be related to... Figure 1 The configurations shown are different. For example, the fuel injection valve 11 can also be configured to inject fuel into the intake port 7. Alternatively, a structure for recirculating EGR gas from the exhaust passage to the intake passage can also be provided.

[0049] The exhaust purification device for an internal combustion engine (hereinafter referred to as "exhaust purification device") according to the first embodiment of the present invention will be described below. The exhaust purification device includes a catalyst 20, an upstream air-fuel ratio sensor 41, a downstream air-fuel ratio sensor 42, and an air-fuel ratio control device. In this embodiment, the ECU 31 functions as the air-fuel ratio control device.

[0050] Catalyst 20 is disposed in the exhaust passage of an internal combustion engine and is configured to purify the exhaust gas flowing in the exhaust passage. In this embodiment, catalyst 20 is capable of oxygen storage, for example, it is a three-way catalyst capable of simultaneously purifying hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Catalyst 20 comprises: a support (substrate) made of ceramic or metal, a noble metal with catalytic activity (e.g., platinum (Pt), palladium (Pd), rhodium (Rh), etc.), and a co-catalyst with oxygen storage capacity (e.g., cerium dioxide (CeO2), etc.). The noble metal and the co-catalyst are supported on the support.

[0051] Figure 2 This is a diagram illustrating an example of the purification characteristics of a three-way catalyst. (As shown...) Figure 2 As shown, the purification efficiency of the three-way catalytic converter for HC, CO, and NOx is achieved when the air-fuel ratio of the exhaust gas flowing into the three-way catalytic converter is near the stoichiometric air-fuel ratio. Figure 2The purification window (A) in the exhaust becomes very high. Therefore, catalyst 20 can effectively purify HC, CO, and NOx when the exhaust air-fuel ratio is maintained near the stoichiometric air-fuel ratio.

[0052] Furthermore, catalyst 20 utilizes a co-catalyst to absorb or release oxygen based on the exhaust air-fuel ratio. Specifically, catalyst 20 absorbs excess oxygen in the exhaust when the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio. On the other hand, catalyst 20 releases insufficient oxygen for the oxidation of HC and CO when the exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio. As a result, even when the exhaust air-fuel ratio deviates slightly from the stoichiometric air-fuel ratio, the air-fuel ratio on the surface of catalyst 20 is maintained near the stoichiometric air-fuel ratio, and HC, CO, and NOx are effectively purified in catalyst 20.

[0053] An upstream air-fuel ratio sensor 41 and a downstream air-fuel ratio sensor 42 are disposed in the exhaust passage of an internal combustion engine, with the downstream air-fuel ratio sensor 42 disposed downstream of the upstream air-fuel ratio sensor 41. The upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42 are respectively configured to detect the air-fuel ratio of the exhaust gas flowing in the exhaust passage.

[0054] Figure 3 This is a partial cross-sectional view of the downstream air-fuel ratio sensor 42. The downstream air-fuel ratio sensor 42 has a known configuration, and its configuration will be briefly described below. Furthermore, the upstream air-fuel ratio sensor 41 has the same configuration as the downstream air-fuel ratio sensor 42.

[0055] The downstream air-fuel ratio sensor 42 includes a sensing element 411 and a heater 420. In this embodiment, the downstream air-fuel ratio sensor 42 is a stacked air-fuel ratio sensor composed of multiple layers. Figure 3 As shown, the sensing element 411 includes a solid electrolyte layer 412, a diffusion rate limiting layer 413, a first impermeable layer 414, a second impermeable layer 415, an exhaust-side electrode 416, and an atmospheric-side electrode 417. A gas chamber 418 is formed between the solid electrolyte layer 412 and the diffusion rate limiting layer 413, and an atmospheric chamber 419 is formed between the solid electrolyte layer 412 and the first impermeable layer 414.

[0056] Exhaust gas, as the gas to be measured, is introduced into the gas chamber 418 via the diffusion velocity limiting layer 413, while atmospheric air is introduced into the atmospheric chamber 419. When a voltage is applied to the sensing element 411, oxide ions move between the exhaust-side electrode 416 and the atmospheric-side electrode 417 according to the air-fuel ratio of the exhaust gas on the exhaust-side electrode 416. As a result, the output current of the sensing element 411 varies according to the air-fuel ratio of the exhaust gas.

[0057] Figure 4This is a graph showing the relationship between the air-fuel ratio of the exhaust gas in the downstream air-fuel ratio sensor 42 and the output current I of the sensing element 411. Figure 4 In this example, a voltage of 0.45V is applied to the sensing element 411. Figure 4 It is known that when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio, the output current I becomes zero. Furthermore, in the downstream air-fuel ratio sensor 42, the higher the oxygen concentration in the exhaust, i.e., the leaner the exhaust air-fuel ratio, the larger the output current I. Therefore, both the downstream air-fuel ratio sensor 42 and the upstream air-fuel ratio sensor 41, which has the same configuration as the downstream air-fuel ratio sensor 42, can continuously (linearly) detect the exhaust air-fuel ratio.

[0058] Furthermore, in this embodiment, limiting current type air-fuel ratio sensors are used as the upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42. However, if the output current changes linearly with respect to the exhaust air-fuel ratio, non-limiting current type air-fuel ratio sensors can also be used as the upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42. Additionally, the upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42 can also be air-fuel ratio sensors with different structures.

[0059] The air-fuel ratio control device controls the air-fuel ratio of the exhaust gas flowing into the catalyst 20 (hereinafter referred to as "inflow exhaust gas"). In this embodiment, the air-fuel ratio control device controls the air-fuel ratio of the inflow exhaust gas based on the outputs of the upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42. Specifically, the air-fuel ratio control device sets a target air-fuel ratio for the inflow exhaust gas based on the output of the downstream air-fuel ratio sensor 42, and feeds back to control the amount of fuel supplied to the combustion chamber 5 so that the output air-fuel ratio of the upstream air-fuel ratio sensor 41 is consistent with the target air-fuel ratio. Here, "output air-fuel ratio" means the air-fuel ratio equivalent to the output value of the air-fuel ratio sensor, that is, the air-fuel ratio detected by the air-fuel ratio sensor.

[0060] Furthermore, the air-fuel ratio control device may also omit the upstream air-fuel ratio sensor 41 and control the amount of fuel supplied to the combustion chamber 5 to ensure that the air-fuel ratio flowing into the exhaust is consistent with the target air-fuel ratio. In this case, the upstream air-fuel ratio sensor 41 is omitted from the exhaust purification device, and the air-fuel ratio control device calculates the amount of fuel supplied to the combustion chamber 5 based on the intake air volume, the internal combustion engine speed, and the target air-fuel ratio to ensure that the ratio of fuel to air supplied to the combustion chamber 5 is consistent with the target air-fuel ratio.

[0061] In this embodiment, the air-fuel ratio of the inflow exhaust gas is basically controlled in a manner that maintains the catalyst 20 in a state suitable for exhaust gas purification. When the catalyst 20 is in a state suitable for exhaust gas purification, the exhaust gas is purified in the catalyst 20, and the air-fuel ratio of the exhaust gas flowing out of the catalyst 20 (hereinafter referred to as "outflow exhaust gas") becomes the stoichiometric air-fuel ratio. Therefore, it is possible to control the air-fuel ratio of the inflow exhaust gas such that the output air-fuel ratio of the downstream air-fuel ratio sensor 42 disposed downstream of the catalyst 20 becomes the stoichiometric air-fuel ratio.

[0062] However, when oxygen is depleted in catalyst 20, the following water-gas shift reaction (1) and steam reforming reaction (2) occur, generating hydrogen in catalyst 20.

[0063] CO + H₂O → H₂ + CO₂…(1)

[0064] HC + H₂O → CO + H₂…(2)

[0065] As a result, exhaust gas containing hydrogen flows out of catalyst 20 and into downstream air-fuel ratio sensor 42. Since the molecular weight of hydrogen is smaller than that of oxygen, the hydrogen in the exhaust gas passes through diffusion limiting layer 413 and reaches exhaust-side electrode 416 faster than the oxygen in the exhaust gas. Therefore, the oxygen concentration in the exhaust gas at exhaust-side electrode 416 is lower than the oxygen concentration in the exhaust gas passage. Consequently, the output of downstream air-fuel ratio sensor 42 deviates, becoming more rich than the actual value. Therefore, the reliability of the output of downstream air-fuel ratio sensor 42 decreases as hydrogen flows from catalyst 20 into downstream air-fuel ratio sensor 42.

[0066] Figure 5A It is a time graph of various parameters as the air-fuel ratio flowing into the exhaust alternates between an air-fuel ratio richer than the stoichiometric air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio. Figure 5A In the figure, various parameters are shown, including the output air-fuel ratio of the downstream air-fuel ratio sensor 42, the target air-fuel ratio of the inflow exhaust, the output air-fuel ratio of the upstream air-fuel ratio sensor 41, the hydrogen concentration in the outflow exhaust, the CO concentration in the outflow exhaust, and the NOx concentration in the outflow exhaust.

[0067] Figure 5B It is shown in a general way. Figure 5A The graph shows the oxygen uptake state of catalyst 20 at various time points (t0 to t5). Figure 5B The diagram shows the direction of exhaust flow relative to catalyst 20 and the oxygen uptake state of catalyst 20. The shaded areas of catalyst 20 represent oxygen-depleted regions, while the rest of catalyst 20 represents oxygen-filled regions.

[0068] In this example, at time t0, the target air-fuel ratio of the incoming exhaust is set to a rich set air-fuel ratio (TAFrich) that is richer than the stoichiometric air-fuel ratio. As the rich air-fuel ratio exhaust flows into the oxygen-filled catalyst 20, oxygen is gradually released from the upstream side of the catalyst 20. The result is as follows: Figure 5B As shown, at time t0, an oxygen depletion region is generated on the upstream side of catalyst 20. In this case, the hydrogen generated in the oxygen depletion region is oxidized on the downstream side of catalyst 20, so almost no hydrogen flows out of catalyst 20. In addition, since CO and NOx in the exhaust gas are effectively purified in catalyst 20, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is maintained at the stoichiometric air-fuel ratio.

[0069] Subsequently, at time t1, most of the catalyst 20 becomes an oxygen-depleted region, and hydrogen and CO flow out of the catalyst 20. The air-fuel ratio output of the downstream air-fuel ratio sensor 42 begins to change towards the rich side. Figure 5A In the example, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reaches the rich set air-fuel ratio AFrich at time t2, the target air-fuel ratio flowing into the exhaust gas is switched from the rich set air-fuel ratio TAFrich to the lean set air-fuel ratio TAFlean, which is leaner than the stoichiometric air-fuel ratio. At time t2, as Figure 5B As shown, the entire region of catalyst 20 becomes an oxygen-depleted region.

[0070] When the lean air-fuel ratio exhaust flows into the oxygen-depleted catalyst 20, the catalyst 20 is gradually filled with oxygen from its upstream side. As a result, as... Figure 5B As shown, at time t3, the upstream side of catalyst 20 is filled with oxygen, while an oxygen-depleted region remains on the downstream side of catalyst 20. Under these conditions, CO and NOx in the exhaust gas are effectively purified in catalyst 20. However, because hydrogen generated in the oxygen-depleted region on the downstream side of catalyst 20 flows from catalyst 20 to the downstream air-fuel ratio sensor 42, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 shows a value richer than the stoichiometric air-fuel ratio.

[0071] Subsequently, at time t4, most of the catalyst 20 is filled with oxygen, and NOx begins to flow out of the catalyst 20. At this time, hydrogen generated in the oxygen-depleted region slightly remaining downstream of the catalyst 20 also flows out of the catalyst 20, and the output of the downstream air-fuel ratio sensor 42 is affected by the hydrogen. Figure 5A In the example, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reaches the lean set air-fuel ratio AFlean at time t5, the target air-fuel ratio flowing into the exhaust gas is switched from the lean set air-fuel ratio TAFlean to the rich set air-fuel ratio TAFrich. At time t5, as Figure 5BAs shown, the entire region of catalyst 20 is filled with oxygen. Therefore, at time t5, the outflow of hydrogen from catalyst 20 ends.

[0072] As by Figure 5A As is known, when hydrogen flows out of catalyst 20, catalyst 20 becomes suitable for exhaust gas purification when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is richer than the stoichiometric air-fuel ratio. Therefore, when the air-fuel ratio flowing into the exhaust gas is controlled regardless of the hydrogen production status in catalyst 20 so that the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is equal to the stoichiometric air-fuel ratio, the amount of NOx flowing out from catalyst 20 increases, posing a risk of worsening exhaust emissions.

[0073] On the other hand, when hydrogen does not flow out of catalyst 20, and the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is the stoichiometric air-fuel ratio, catalyst 20 becomes suitable for exhaust purification. Therefore, when air-fuel ratio control that takes into account the effect of hydrogen is always executed, there is a risk of worsening exhaust emissions when the state of catalyst 20 changes according to the operating state of the internal combustion engine.

[0074] Therefore, in this embodiment, the air-fuel ratio control device initiates micro-enrichment control when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 drops below the rich-side switching air-fuel ratio (which is richer than the stoichiometric air-fuel ratio). In this micro-enrichment control, the air-fuel ratio flowing into the exhaust gas is controlled so that the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is maintained at a micro-enrichment set air-fuel ratio (which is richer than the stoichiometric air-fuel ratio). Thus, when the possibility of hydrogen leakage from the catalyst 20 is high, air-fuel ratio control that takes into account the influence of hydrogen can be implemented. That is, in this embodiment, by implementing air-fuel ratio control corresponding to the hydrogen production status in the catalyst 20, the deterioration of exhaust emissions can be suppressed.

[0075] In a slightly rich air-fuel ratio control device, to maintain the output air-fuel ratio of the downstream air-fuel ratio sensor 42 at a slightly rich set air-fuel ratio, the air-fuel ratio flowing into the exhaust gas is controlled so that the output air-fuel ratio of the downstream air-fuel ratio sensor 42 varies within a specified range centered on the slightly rich set air-fuel ratio. For example, in a slightly rich air-fuel ratio control device, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 rises above the first upper-side determined air-fuel ratio, the target air-fuel ratio of the flowing into the exhaust gas is set to a rich set air-fuel ratio richer than the stoichiometric air-fuel ratio; when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 falls below the first lower-side determined air-fuel ratio, the target air-fuel ratio of the flowing into the exhaust gas is set to a lean set air-fuel ratio leaner than the stoichiometric air-fuel ratio. The first upper-side air-fuel ratio and the first lower-side air-fuel ratio are preset in such a way that the difference between the first upper-side air-fuel ratio and the slightly rich set air-fuel ratio is equal to the difference between the first lower-side air-fuel ratio and the slightly rich set air-fuel ratio, and the first upper-side air-fuel ratio is larger (lean) than the first lower-side air-fuel ratio.

[0076] In particular, in this embodiment, the air-fuel ratio control device, while controlling the air-fuel ratio of the inflow exhaust to maintain the output air-fuel ratio of the downstream air-fuel ratio sensor 42 at or above the stoichiometric air-fuel ratio—for example, while controlling the air-fuel ratio of the inflow exhaust to a value above the stoichiometric air-fuel ratio—initiates micro-enrichment control when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 drops below the rich-side switching air-fuel ratio. This allows for the suppression of exhaust emission deterioration should hydrogen unexpectedly flow out of the catalyst 20.

[0077] Furthermore, when the catalyst 20 becomes oxygen-saturated due to interference or other factors during enrichment control, the outflow of hydrogen from the catalyst 20 ceases. Therefore, in this embodiment, the air-fuel ratio control device terminates enrichment control when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 rises above the stoichiometric air-fuel ratio or the lean-side switching air-fuel ratio during enrichment control. This allows enrichment control to terminate at an appropriate timing when the outflow of hydrogen from the catalyst 20 ends.

[0078] When the flow of hydrogen from catalyst 20 ends, the output deviation of the downstream air-fuel ratio sensor 42 is eliminated. Therefore, the air-fuel ratio control device initiates stoichiometric air-fuel ratio control when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 rises above the lean-side switching air-fuel ratio. In this stoichiometric air-fuel ratio control, the air-fuel ratio flowing into the exhaust gas is controlled so that the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is maintained at the stoichiometric air-fuel ratio. This effectively suppresses the deterioration of exhaust emissions when hydrogen does not flow from catalyst 20.

[0079] In stoichiometric air-fuel ratio control, the air-fuel ratio control device controls the air-fuel ratio flowing into the exhaust gas to maintain the output air-fuel ratio of the downstream air-fuel ratio sensor 42 at the stoichiometric air-fuel ratio, so that the output air-fuel ratio of the downstream air-fuel ratio sensor 42 varies within a specified range centered on the stoichiometric air-fuel ratio. For example, in stoichiometric air-fuel ratio control, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 rises above the second upper-side determination air-fuel ratio, the target air-fuel ratio of the inflow exhaust gas is set to a rich set air-fuel ratio richer than the stoichiometric air-fuel ratio; when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 falls below the second lower-side determination air-fuel ratio, the target air-fuel ratio of the inflow exhaust gas is set to a lean set air-fuel ratio leaner than the stoichiometric air-fuel ratio. The second upper side air-fuel ratio and the second lower side air-fuel ratio are preset in such a way that the difference between the second upper side air-fuel ratio and the stoichiometric air-fuel ratio is equal to the difference between the second lower side air-fuel ratio and the stoichiometric air-fuel ratio, and the second upper side air-fuel ratio is larger (lean) than the second lower side air-fuel ratio.

[0080] Therefore, in this embodiment, the air-fuel ratio control device performs micro-enrichment control from the point where the output air-fuel ratio of the downstream air-fuel ratio sensor 42 decreases below the rich-side switching air-fuel ratio, until the output air-fuel ratio of the downstream air-fuel ratio sensor 42 rises above the lean-side switching air-fuel ratio. Furthermore, the air-fuel ratio control device performs stoichiometric air-fuel ratio control from the point where the output air-fuel ratio of the downstream air-fuel ratio sensor 42 rises above the lean-side switching air-fuel ratio, until the output air-fuel ratio of the downstream air-fuel ratio sensor 42 decreases below the rich-side switching air-fuel ratio. That is, in stoichiometric air-fuel ratio control, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 decreases below the rich-side switching air-fuel ratio, the air-fuel ratio control device starts micro-enrichment control; in micro-enrichment control, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 rises above the lean-side switching air-fuel ratio, stoichiometric air-fuel ratio control begins.

[0081] Next, the air-fuel ratio control using a time-map method will be explained. (Refer to...) Figure 6 The air-fuel ratio control described above will be explained in detail below. Figure 6 This is a time graph of various parameters during the air-fuel ratio control in the first embodiment of the present invention. Figure 6 In the figure, various parameters are shown, including the output air-fuel ratio of the downstream air-fuel ratio sensor 42, the target output value of the downstream air-fuel ratio sensor 42, the target air-fuel ratio of the inflow exhaust, the hydrogen concentration in the outflow exhaust, the CO concentration in the outflow exhaust, and the NOx concentration in the outflow exhaust.

[0082] exist Figure 6 In the example, at time t0, stoichiometric air-fuel ratio control is performed, and the target output value of the downstream air-fuel ratio sensor 42 is set to the stoichiometric air-fuel ratio (14.6). Additionally, at time t0, in the stoichiometric air-fuel ratio control, the target air-fuel ratio of the inflow exhaust is set to a rich set air-fuel ratio (TAFrich), which is richer than the stoichiometric air-fuel ratio. Therefore, after time t0, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 gradually decreases. When the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reaches the second lower-side determination air-fuel ratio (JAFdwn2) at time t1, the target air-fuel ratio of the inflow exhaust is set to a lean set air-fuel ratio (TAFlean), which is leaner than the stoichiometric air-fuel ratio.

[0083] exist Figure 6In the example, although the target air-fuel ratio flowing into the exhaust gas is set to a lean set air-fuel ratio (TAFlean) in stoichiometric air-fuel ratio control, at time t2, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reaches the rich-side switching air-fuel ratio (SWrich) due to interference and other factors. That is, in stoichiometric air-fuel ratio control, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 decreases from a value above the stoichiometric air-fuel ratio to the rich-side switching air-fuel ratio (SWrich). Therefore, at time t2, stoichiometric air-fuel ratio control ends, and slightly rich control begins. That is, the target output value of the downstream air-fuel ratio sensor 42 is switched from the stoichiometric air-fuel ratio to a slightly rich set air-fuel ratio (RAFTsrich) that is richer than the stoichiometric air-fuel ratio.

[0084] Furthermore, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 decreases towards the rich side (SWrich), the oxygen in the catalyst 20 is depleted, and hydrogen and CO flow out of the catalyst 20. As a result, exhaust gas containing hydrogen flows into the downstream air-fuel ratio sensor 42, causing a deviation in the output of the downstream air-fuel ratio sensor 42. However, by starting micro-enrichment control at time t2, the catalyst 20 can be made into a state suitable for exhaust gas purification, effectively suppressing the outflow of CO and NOx after time t2.

[0085] After time t2, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reaches the first upper-side determined air-fuel ratio JAFup1 at time t3, the target air-fuel ratio flowing into the exhaust gas in the enrichment control is switched from the lean set air-fuel ratio TAFlean to the rich set air-fuel ratio TAFrich. Furthermore, in Figure 6 In the example, the value of the first upper-side air-fuel ratio JAFup1 is equal to the value of the second lower-side air-fuel ratio JAFdwn2.

[0086] After time t3, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reaches the first lower-side determined air-fuel ratio JAFdwn1 at time t4, the target air-fuel ratio flowing into the exhaust gas in the enrichment control is switched from the rich set air-fuel ratio TAFrich to the lean set air-fuel ratio TAFlean. Subsequently, also in the enrichment control, the target air-fuel ratio flowing into the exhaust gas switches between the rich set air-fuel ratio TAFrich and the lean set air-fuel ratio TAFlean based on the output air-fuel ratio of the downstream air-fuel ratio sensor 42.

[0087] exist Figure 6 In the example, although the target air-fuel ratio flowing into the exhaust gas is set to the rich set air-fuel ratio TAFrich in the micro-rich control, at time t5, due to interference and other factors, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reaches the lean-side switching air-fuel ratio SWlean (in Figure 6In the example, it is 14.6). Therefore, at time t5, the enrichment control ends and the stoichiometric air-fuel ratio control begins. That is, the target output value of the downstream air-fuel ratio sensor 42 is switched from the enrichment set air-fuel ratio RAFTsrich to the stoichiometric air-fuel ratio.

[0088] Furthermore, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 increases towards the lean-side switching air-fuel ratio SWlean, the catalyst 20 is filled with oxygen, and NOx flows out from the catalyst 20. As a result, the outflow of hydrogen from the catalyst 20 ends, and the output deviation of the downstream air-fuel ratio sensor 42 is eliminated. However, by starting stoichiometric air-fuel ratio control at time t5, the catalyst 20 can be made into a state suitable for exhaust gas purification, effectively suppressing the outflow of CO and NOx after time t5.

[0089] After time t5, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reaches the second lower-side determination air-fuel ratio JAFdwn2 at time t6, the target air-fuel ratio flowing into the exhaust gas in the stoichiometric air-fuel ratio control is switched from the rich set air-fuel ratio TAFrich to the lean set air-fuel ratio TAFlean. After time t6, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reaches the second upper-side determination air-fuel ratio JAFup2 at time t7, the target air-fuel ratio flowing into the exhaust gas in the stoichiometric air-fuel ratio control is switched from the lean set air-fuel ratio TAFlean to the rich set air-fuel ratio TAFrich. Subsequently, in the stoichiometric air-fuel ratio control, the target air-fuel ratio flowing into the exhaust gas switches between the rich set air-fuel ratio TAFrich and the lean set air-fuel ratio TAFlean in the same way.

[0090] The following uses Figures 7A to 7C The flowchart above will be used to explain the air-fuel ratio control in detail. Figures 7A to 7C This is a flowchart illustrating the control procedure for air-fuel ratio control in the first embodiment. This control procedure is repeatedly executed by ECU 31, which functions as an air-fuel ratio control device, at predetermined execution intervals.

[0091] Initially, in step S101, the air-fuel ratio control device determines whether the execution conditions for air-fuel ratio control are met. The execution conditions for air-fuel ratio control are, for example, when the temperature of the catalyst 20 is above a predetermined activation temperature and the element temperatures of the upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42 are above a predetermined activation temperature. The temperature of the catalyst 20 is calculated, for example, based on the output of a temperature sensor located in or near the catalyst 20 in the exhaust passage, or based on predetermined state quantities of the internal combustion engine (e.g., engine coolant temperature, intake air volume, engine load, etc.). The element temperatures of the upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42 are calculated, for example, based on the impedance of the sensing element. Furthermore, the execution conditions for air-fuel ratio control may also include: a predetermined time has elapsed since the internal combustion engine started, and predetermined components of the internal combustion engine (fuel injection valve 11, catalyst 20, upstream air-fuel ratio sensor 41, downstream air-fuel ratio sensor 42, etc.) are functioning normally.

[0092] If the execution conditions for air-fuel ratio control are not met in step S101, the control program ends. On the other hand, if the execution conditions for air-fuel ratio control are met in step S101, the control program proceeds to step S102.

[0093] In step S102, the air-fuel ratio control device determines whether the richness flag Fr is 1. The richness flag Fr is set to 1 at the start of the micro-rich control and set to zero at the end of the micro-rich control. Furthermore, the initial value of the richness flag Fr when the internal combustion engine starts is zero. If it is determined in step S102 that the richness flag Fr is zero, the control program proceeds to step S103.

[0094] In step S103, the air-fuel ratio control device determines whether the stoichiometric flag Fs is 1. The stoichiometric flag Fs is set to 1 at the start of stoichiometric air-fuel ratio control and set to zero at the end of stoichiometric air-fuel ratio control. Furthermore, the initial value of the stoichiometric flag Fs when the internal combustion engine starts is zero. If it is determined in step S103 that the stoichiometric flag Fs is zero, the control program proceeds to step S104.

[0095] In step S104, the air-fuel ratio control device initiates a slightly rich air-fuel ratio control. That is, the air-fuel ratio control device sets the target output value of the downstream air-fuel ratio sensor 42 to a slightly rich set air-fuel ratio. This slightly rich set air-fuel ratio is preset to be slightly richer than the stoichiometric air-fuel ratio. For example, the slightly rich set air-fuel ratio is set to 14.50 to 14.58, preferably 14.58.

[0096] Next, in step S105, the air-fuel ratio control device sets the target air-fuel ratio (TAF) of the incoming exhaust gas to a lean set air-fuel ratio (TAFlean). That is, the air-fuel ratio control device uses the output of the upstream air-fuel ratio sensor 41 to feedback control the air-fuel ratio of the incoming exhaust gas to a lean set air-fuel ratio (TAFlean). The lean set air-fuel ratio (TAFlean) is preset and is set to an air-fuel ratio leaner than the stoichiometric air-fuel ratio (e.g., 14.7 to 15.7).

[0097] Next, in step S106, the air-fuel ratio control device sets the richness flag Fr to 1, and the control program proceeds to step S107. On the other hand, if the micro-rich control has already been executed at the start time of the control program, and it is determined in step S102 that the richness flag Fr is 1, the control program skips steps S103 to S106 and proceeds to step S107.

[0098] In step S107, the air-fuel ratio control device determines whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is greater than or equal to the lean-side switching air-fuel ratio SWlean. The lean-side switching air-fuel ratio SWlean is preset to a value greater than or equal to the stoichiometric air-fuel ratio. For example, the lean-side switching air-fuel ratio SWlean is set to 14.60 to 14.65, preferably to the stoichiometric air-fuel ratio (14.60). If, in step S107, it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is less than the lean-side switching air-fuel ratio SWlean, the control program proceeds to step S108, and the enrichment control continues.

[0099] In step S108, the air-fuel ratio control device determines whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is greater than or equal to the first upper-side determined air-fuel ratio JAFup1. The first upper-side determined air-fuel ratio JAFup1 is preset to be an air-fuel ratio that is richer than the stoichiometric air-fuel ratio and slightly leaner than the slightly rich set air-fuel ratio. For example, the first upper-side determined air-fuel ratio JAFup1 is set to a value 0.01 greater than the slightly rich set air-fuel ratio, and is set to 14.59 when the slightly rich set air-fuel ratio is 14.58.

[0100] If, in step S108, it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is higher than the first upper-side determined air-fuel ratio JAFup1, the control program proceeds to step S109. In step S109, the air-fuel ratio control device sets the target air-fuel ratio TAF of the incoming exhaust to a rich set air-fuel ratio TAFrich. That is, the air-fuel ratio control device uses the output of the upstream air-fuel ratio sensor 41 to feedback control the air-fuel ratio of the incoming exhaust to a rich set air-fuel ratio TAFrich. The rich set air-fuel ratio TAFrich is preset and is set to an air-fuel ratio richer than the stoichiometric air-fuel ratio (e.g., 13.5 to 14.5). After step S109, the control program ends.

[0101] On the other hand, if it is determined in step S108 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is less than the first upper-side determined air-fuel ratio JAFup1, the control program proceeds to step S110. In step S110, the air-fuel ratio control device determines whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is less than the first lower-side determined air-fuel ratio JAFdwn1. The first lower-side determined air-fuel ratio JAFdwn1 is preset and is set to an air-fuel ratio slightly richer than the slightly rich set air-fuel ratio. For example, the first lower-side determined air-fuel ratio JAFdwn1 is set to a value 0.01 smaller than the slightly rich set air-fuel ratio, and is set to 14.57 when the slightly rich set air-fuel ratio is 14.58.

[0102] If, in step S110, it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is greater than the first lower-level determined air-fuel ratio JAFdwn1, this control program ends, and the target air-fuel ratio TAF flowing into the exhaust is maintained at the current set value. On the other hand, if, in step S110, it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is less than or equal to the first lower-level determined air-fuel ratio JAFdwn1, this control program proceeds to step S111.

[0103] In step S111, the air-fuel ratio control device sets the target air-fuel ratio (TAF) of the incoming exhaust gas to a lean set air-fuel ratio (TAFlean). That is, the air-fuel ratio control device uses the output of the upstream air-fuel ratio sensor 41 to feedback control the air-fuel ratio of the incoming exhaust gas to a lean set air-fuel ratio (TAFlean). After step S111, this control program ends.

[0104] On the other hand, if it is determined in step S107 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is higher than or equal to the lean-side switching air-fuel ratio SWlean, the control program proceeds to step S112. In step S112, the air-fuel ratio control device ends the rich-fuel control and begins the stoichiometric air-fuel ratio control. That is, the air-fuel ratio control device sets the target output value of the downstream air-fuel ratio sensor 42 to the stoichiometric air-fuel ratio (14.60).

[0105] Next, in step S113, the air-fuel ratio control device sets the stoichiometric ratio flag Fs to 1 and the richness flag Fr to zero. After step S113, this control program ends. In this case, in step S103 of the next control program, it is determined that the stoichiometric ratio flag Fs is 1, and this control program proceeds to step S114.

[0106] In step S114, the air-fuel ratio control device determines whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is below the rich-side switching air-fuel ratio SWrich. The rich-side switching air-fuel ratio SWrich is preset and set to a value richer than the stoichiometric air-fuel ratio. For example, the rich-side switching air-fuel ratio SWrich is set to 14.50 to 14.58, preferably set to the same value as the slightly rich set air-fuel ratio (e.g., 14.58). If it is determined in step S114 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is greater than the rich-side switching air-fuel ratio SWrich, the control program proceeds to step S115, and the stoichiometric air-fuel ratio control continues.

[0107] In step S115, the air-fuel ratio control device determines whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is greater than or equal to the second upper-side determined air-fuel ratio JAFup2. The second upper-side determined air-fuel ratio JAFup2 is preset to be a slightly leaner air-fuel ratio than the stoichiometric air-fuel ratio. For example, the second upper-side determined air-fuel ratio JAFup2 is set to a value 0.01 greater than the stoichiometric air-fuel ratio (14.61).

[0108] If, in step S115, it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is higher than the second upper-side determined air-fuel ratio JAFup2, the control program proceeds to step S116. In step S116, the air-fuel ratio control device sets the target air-fuel ratio TAF of the incoming exhaust to a rich set air-fuel ratio TAFrich. That is, the air-fuel ratio control device uses the output of the upstream air-fuel ratio sensor 41 to feedback control the air-fuel ratio of the incoming exhaust to a rich set air-fuel ratio TAFrich. After step S116, the control program ends.

[0109] On the other hand, if it is determined in step S115 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is less than the second upper-side determined air-fuel ratio JAFup2, the control program proceeds to step S117. In step S117, the air-fuel ratio control device determines whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is below the second lower-side determined air-fuel ratio JAFdwn2. The second lower-side determined air-fuel ratio JAFdwn2 is preset and is set to an air-fuel ratio slightly richer than the stoichiometric air-fuel ratio. For example, the second lower-side determined air-fuel ratio JAFdwn2 is set to a value 0.01 smaller than the stoichiometric air-fuel ratio (14.59).

[0110] If, in step S117, it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is greater than the second lower-level determined air-fuel ratio JAFdwn2, this control procedure ends, and the target air-fuel ratio TAF flowing into the exhaust is maintained at the current set value. On the other hand, if, in step S117, it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is less than the second lower-level determined air-fuel ratio JAFdwn2, this control procedure proceeds to step S118.

[0111] In step S118, the air-fuel ratio control device sets the target air-fuel ratio (TAF) of the incoming exhaust gas to a lean set air-fuel ratio (TAFlean). That is, the air-fuel ratio control device uses the output of the upstream air-fuel ratio sensor 41 to feedback control the air-fuel ratio of the incoming exhaust gas to a lean set air-fuel ratio (TAFlean). After step S118, this control program ends.

[0112] On the other hand, if it is determined in step S114 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is below the rich-side switching air-fuel ratio SWrich, the control program proceeds to step S119. In step S119, the air-fuel ratio control device ends the theoretical air-fuel ratio control and begins micro-rich control. That is, the air-fuel ratio control device sets the target output value of the downstream air-fuel ratio sensor 42 to the micro-rich set air-fuel ratio.

[0113] Next, in step S120, the air-fuel ratio control device sets the richness flag Fr to 1 and the stoichiometric ratio flag Fs to zero. After step S120, this control program ends.

[0114] Furthermore, in at least one of steps S108 and S115, the air-fuel ratio control device may also determine whether the elapsed time since the target air-fuel ratio TAF in the inflow exhaust was set to a lean air-fuel ratio TAFlean, the cumulative intake air volume, etc., have reached a predetermined threshold. That is, the air-fuel ratio control device may also switch the target air-fuel ratio TAF in the inflow exhaust from a lean air-fuel ratio TAFlean to a rich air-fuel ratio TAFrich when the elapsed time since the target air-fuel ratio TAF in the inflow exhaust was set to a lean air-fuel ratio TAFlean, the cumulative intake air volume, etc., have reached a predetermined threshold in at least one of rich control and stoichiometric air-fuel ratio control.

[0115] Furthermore, in at least one of steps S110 and S117, the air-fuel ratio control device may also determine whether the elapsed time since the target air-fuel ratio TAF in the inflow exhaust was set to a rich set air-fuel ratio TAFrich, the cumulative intake air volume, etc., have reached a predetermined threshold. That is, the air-fuel ratio control device may also switch the target air-fuel ratio TAF in the inflow exhaust from a rich set air-fuel ratio TAFrich to a lean set air-fuel ratio TAFlean when the elapsed time since the target air-fuel ratio TAF in the inflow exhaust was set to a rich set air-fuel ratio TAFrich, the cumulative intake air volume, etc., have reached a predetermined threshold in at least one of the lean control and stoichiometric air-fuel ratio control.

[0116] Furthermore, since it can be assumed that the oxygen uptake of catalyst 20 has not reached its maximum value when the internal combustion engine starts, a slightly rich control is performed as the initial air-fuel ratio control after the internal combustion engine starts in the above control program. However, stoichiometric air-fuel ratio control can also be performed as the initial air-fuel ratio control after the internal combustion engine starts. In addition, as the initial air-fuel ratio control after the internal combustion engine starts, the air-fuel ratio control device can also control the air-fuel ratio flowing into the exhaust gas based on the output of the upstream air-fuel ratio sensor 41 to make the air-fuel ratio flowing into the exhaust gas consistent with a predetermined value (e.g., stoichiometric air-fuel ratio). In this case, in the initial air-fuel ratio control, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 decreases below the rich-side switching air-fuel ratio SWrich, slightly rich control is started; in the initial air-fuel ratio control, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 rises above the lean-side switching air-fuel ratio SWlean, stoichiometric air-fuel ratio control is started.

[0117] Next, the second embodiment of the present invention will be described. The configuration and control of the exhaust gas purification device in the second embodiment are basically the same as those in the first embodiment, except for the aspects described below. Therefore, the second embodiment of the present invention will be described below focusing on the parts that differ from the first embodiment.

[0118] As described above, in the enrichment control, the target output value of the downstream air-fuel ratio sensor 42 is set as the enrichment set air-fuel ratio. In the first embodiment, a preset fixed value is used as the enrichment set air-fuel ratio. However, depending on the air-fuel ratio flowing into the exhaust gas and the state of the catalyst 20, the amount of hydrogen generated in the catalyst 20 may vary. Basically, the more hydrogen flows out of the catalyst 20, the greater the output deviation of the downstream air-fuel ratio sensor 42, and the richer the output air-fuel ratio of the downstream air-fuel ratio sensor 42.

[0119] Therefore, in the second embodiment, the air-fuel ratio control device determines the enrichment level of the micro-enrichment setting air-fuel ratio based on the minimum air-fuel ratio at which the output air-fuel ratio of the downstream air-fuel ratio sensor 42 drops below the rich-side switching air-fuel ratio. This allows the target output value of the downstream air-fuel ratio sensor 42 in micro-enrichment control to be set to a value suitable for the amount of hydrogen flowing from the catalyst 20, thereby more effectively suppressing the deterioration of exhaust emissions. Furthermore, the enrichment level of the micro-enrichment setting air-fuel ratio refers to the difference between the micro-enrichment setting air-fuel ratio, which is set as a value richer than the stoichiometric air-fuel ratio, and the stoichiometric air-fuel ratio. The greater the enrichment level of the micro-enrichment setting air-fuel ratio, the richer the micro-enrichment setting air-fuel ratio.

[0120] Figure 8 This is a graph showing the minimum air-fuel ratio when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 drops below the rich-side switching air-fuel ratio. Figure 8 The diagram shows the time-varying output air-fuel ratio of the downstream air-fuel ratio sensor 42. At time t1, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 decreases to the rich-side switching air-fuel ratio SWrich. The output air-fuel ratio of the downstream air-fuel ratio sensor 42 continues to decrease after time t1, reaching its minimum at time t2. The output air-fuel ratio of the downstream air-fuel ratio sensor 42 at time t2 corresponds to the minimum air-fuel ratio (AFmin) when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 decreases below the rich-side switching air-fuel ratio SWrich.

[0121] In the first embodiment, the control program for air-fuel ratio control uses Figures 7A to 7C The flowchart, however, in the second embodiment, as the control program for air-fuel ratio control, uses Figure 7A , Figure 7B as well as Figure 9 The flowchart is as follows. That is, in the second embodiment, when it is determined in step S114 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is below the rich-side switching air-fuel ratio SWrich, step S201 is executed before step S119.

[0122] In step S201, the air-fuel ratio control device determines the richness level of the enrichment setting air-fuel ratio in the enrichment control based on the minimum air-fuel ratio (hereinafter referred to as "minimum air-fuel ratio") when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 drops below the rich-side switching air-fuel ratio SWrich. Specifically, the smaller the minimum air-fuel ratio (richer), the more the air-fuel ratio control device increases the richness level of the enrichment setting air-fuel ratio. Furthermore, the air-fuel ratio control device changes the values ​​of the first upper-side determined air-fuel ratio JAFup1 and the first lower-side determined air-fuel ratio JAFdwn1 according to the set value of the enrichment setting air-fuel ratio. The richer the enrichment setting air-fuel ratio, the richer the values ​​of the first upper-side determined air-fuel ratio JAFup1 and the first lower-side determined air-fuel ratio JAFdwn1 become.

[0123] For example, the air-fuel ratio control device uses a mapping chart or calculation formula to determine the values ​​of the slightly rich set air-fuel ratio, the first upper determined air-fuel ratio JAFup1, and the first lower determined air-fuel ratio JAFdwn1 based on the minimum air-fuel ratio. Figure 10 This is an example diagram showing a mapping graph for determining the values ​​of the slightly enriched set air-fuel ratio, the first upper-side determined air-fuel ratio JAFup1, and the first lower-side determined air-fuel ratio JAFdwn1 based on the minimum air-fuel ratio. Figure 10 In the mapping chart, the richer the minimum air-fuel ratio, the richer the slightly rich set air-fuel ratio becomes. In addition, the richer the minimum air-fuel ratio, the greater the difference between the slightly rich set air-fuel ratio and the first upper-side determined air-fuel ratio JAFup1, and the greater the difference between the slightly rich set air-fuel ratio and the first lower-side determined air-fuel ratio JAFdwn1.

[0124] After step S201, micro-concentration control begins in step S119, as... Figure 7B Step S108 first upper side determines the value of air-fuel ratio JAFup1 and Figure 7B The value of the air-fuel ratio JAFdwn1 is determined on the first lower side of S110, using the value determined in step S201.

[0125] Next, the third embodiment of the present invention will be described. The configuration and control of the exhaust gas purification device in the third embodiment are basically the same as those in the first embodiment, except for the aspects described below. Therefore, the following description focuses on the parts of the third embodiment that differ from the first embodiment.

[0126] Figure 11This is a schematic diagram of a portion of an internal combustion engine to which the exhaust gas purification device of the internal combustion engine according to the third embodiment of the present invention is applied. In the third embodiment, in addition to a downstream air-fuel ratio sensor 42, a hydrogen sensor 50 is also provided in the exhaust passage (specifically, exhaust pipe 22) downstream of the catalyst 20. The hydrogen sensor 50 detects the hydrogen concentration in the exhaust gas flowing in the exhaust pipe 22, i.e., the exhaust gas flowing out of the catalyst 20. The hydrogen sensor 50 and ECU 31 (see reference) Figure 1 The hydrogen sensor 50 is electrically connected, and its output is input to the input port 36 via the corresponding AD converter 38.

[0127] Regarding the second embodiment, as described above, the greater the amount of hydrogen flowing out of the catalyst 20, the greater the output deviation of the downstream air-fuel ratio sensor 42, and the richer the output air-fuel ratio of the downstream air-fuel ratio sensor 42. Therefore, in the third embodiment, the air-fuel ratio control device estimates the hydrogen concentration in the outgoing exhaust gas based on the output of the hydrogen sensor 50, and determines the richness of the slightly rich set air-fuel ratio based on the hydrogen concentration. Thus, the target output value of the downstream air-fuel ratio sensor 42 in the slightly rich control can be set to a value suitable for the amount of hydrogen flowing out of the catalyst 20, thereby more effectively suppressing the deterioration of exhaust emissions.

[0128] In the first embodiment, the control program for air-fuel ratio control uses Figures 7A to 7C The flowchart, however, in the third embodiment, the control program for air-fuel ratio control uses Figure 12 , Figure 7B as well as Figure 7C The flowchart. That is, in the third embodiment, when it is determined in step S102 that the concentration flag Fr is 1, in Figure 7B Steps S301 and S302 are executed before step S107.

[0129] In step S301, the air-fuel ratio control device estimates the hydrogen concentration in the outflowing exhaust gas based on the output of the hydrogen sensor 50.

[0130] Next, in step S302, the air-fuel ratio control device determines the enrichment level of the enrichment setting air-fuel ratio in the enrichment control based on the hydrogen concentration in the outgoing exhaust gas. Specifically, the higher the hydrogen concentration in the outgoing exhaust gas, the more the air-fuel ratio control device increases the enrichment level of the enrichment setting air-fuel ratio. Furthermore, the air-fuel ratio control device changes the values ​​of the first upper-side determined air-fuel ratio JAFup1 and the first lower-side determined air-fuel ratio JAFdwn1 according to the set value of the enrichment setting air-fuel ratio. The richer the enrichment setting air-fuel ratio, the richer the values ​​of the first upper-side determined air-fuel ratio JAFup1 and the first lower-side determined air-fuel ratio JAFdwn1. For example, the air-fuel ratio control device uses a mapping chart or calculation formula to determine the values ​​of the enrichment setting air-fuel ratio, the first upper-side determined air-fuel ratio JAFup1, and the first lower-side determined air-fuel ratio JAFdwn1 based on the hydrogen concentration in the outgoing exhaust gas.

[0131] After step S302, the same procedure as in the first embodiment is followed. Figure 7B In steps S107 to S111, the values ​​determined in step S302 are used as the values ​​of the first upper air-fuel ratio JAFup1 determined in step S108 and the first lower air-fuel ratio JAFdwn1 determined in step S110.

[0132] Furthermore, the air-fuel ratio control device can also replace the hydrogen sensor 50 by using a mapping chart or calculation formula to estimate the hydrogen concentration in the exhaust gas based on specified state quantities of the internal combustion engine. These specified state quantities include, for example, engine speed, intake air volume, air-fuel ratio in the exhaust gas, exhaust gas temperature, oxygen uptake capacity of catalyst 20, and EGR rate (in the case where the internal combustion engine is equipped with a structure for EGR gas recirculation). These specified state quantities are calculated using known methods based on the outputs of various sensors (crankshaft angle sensor 45, air flow meter 40, upstream air-fuel ratio sensor 41, exhaust temperature sensor (not shown), etc.).

[0133] Alternatively, the air-fuel ratio control device can use a pre-learned regression model to estimate the hydrogen concentration in the exhaust gas, so as to output the hydrogen concentration in the exhaust gas according to the specified state parameters of the internal combustion engine. Examples of such regression models include machine learning models such as neural networks, support vector machines, and random forests.

[0134] In addition, in the control procedure described above, steps S301 and S302 are executed between steps S102 and S107, but steps S301 and S302 can also be executed between steps S102 and S106 and steps S107.

[0135] Other embodiments will be described below. The above describes suitable embodiments of the present invention, but the invention is not limited to these embodiments, and various modifications and variations can be implemented within the scope of the claims. For example, in an internal combustion engine, a downstream catalyst identical to catalyst 20 may be configured in the exhaust passage downstream of catalyst 20.

[0136] Furthermore, in a slightly rich air-fuel ratio control scenario, instead of switching the target air-fuel ratio of the inflow exhaust between a rich and lean set air-fuel ratio, the air-fuel ratio can be controlled based on the output of the downstream air-fuel ratio sensor 42 to ensure that the output air-fuel ratio of the downstream air-fuel ratio sensor 42 matches the slightly rich set air-fuel ratio. Similarly, in a stoichiometric air-fuel ratio control scenario, instead of switching the target air-fuel ratio of the inflow exhaust between a rich and lean set air-fuel ratio, the air-fuel ratio can be controlled based on the output of the downstream air-fuel ratio sensor 42 to ensure that the output air-fuel ratio of the downstream air-fuel ratio sensor 42 matches the stoichiometric air-fuel ratio. Additionally, the air-fuel ratio control device can also perform air-fuel ratio control other than stoichiometric air-fuel ratio control when slightly rich air-fuel ratio control is not being performed.

Claims

1. An exhaust purification device for an internal combustion engine, characterized in that, The system includes a catalyst, an air-fuel ratio sensor, and an air-fuel ratio control device. The catalyst is disposed in the exhaust passage of an internal combustion engine and is configured to absorb oxygen. The air-fuel ratio sensor is configured to detect the air-fuel ratio of the exhaust gas flowing out of the catalyst. The air-fuel ratio control device is configured to control the air-fuel ratio of the exhaust gas flowing into the catalyst. When oxygen is depleted, the catalyst undergoes a water-gas shift reaction and a steam reforming reaction to produce hydrogen. This hydrogen causes a deviation in the output of the air-fuel ratio sensor. When the air-fuel ratio of the outflowing exhaust gas, as detected by the air-fuel ratio sensor, drops below the rich-side switching air-fuel ratio (which is richer than the stoichiometric air-fuel ratio), the air-fuel ratio control device determines that the catalyst is depleted of oxygen and producing hydrogen, causing a deviation in the output of the air-fuel ratio sensor. It then initiates micro-enrichment control, in which the air-fuel ratio of the inflowing exhaust gas is controlled to maintain the air-fuel ratio of the outflowing exhaust gas, as detected by the air-fuel ratio sensor, at a micro-enrichment setpoint (richer than the stoichiometric air-fuel ratio). This eliminates the output deviation of the air-fuel ratio sensor and suppresses the deterioration of exhaust emissions. The air-fuel ratio control device is configured such that, during the enrichment control, when the air-fuel ratio of the outflowing exhaust gas, detected by the air-fuel ratio sensor, rises above the lean-side switching air-fuel ratio (above the stoichiometric air-fuel ratio), the enrichment control is terminated. The air-fuel ratio control device is configured such that, in the enrichment control, when the air-fuel ratio of the outflowing exhaust gas detected by the air-fuel ratio sensor rises above the lean-side switching air-fuel ratio, stoichiometric air-fuel ratio control is initiated, and in the stoichiometric air-fuel ratio control, the air-fuel ratio of the inflowing exhaust gas is controlled such that the air-fuel ratio of the outflowing exhaust gas detected by the air-fuel ratio sensor is maintained at the stoichiometric air-fuel ratio.

2. The exhaust purification device for an internal combustion engine according to claim 1, characterized in that, The air-fuel ratio control device is configured to initiate micro-enrichment control when the air-fuel ratio of the inflow exhaust gas decreases to below the rich-side switching air-fuel ratio while the air-fuel ratio of the inflow exhaust gas is controlled such that the air-fuel ratio of the outflow exhaust gas detected by the air-fuel ratio sensor is maintained above the stoichiometric air-fuel ratio.

3. The exhaust purification device for an internal combustion engine according to claim 1, characterized in that, The air-fuel ratio control device is configured to perform stoichiometric air-fuel ratio control, wherein the air-fuel ratio of the inflow exhaust is controlled such that the air-fuel ratio of the outflow exhaust detected by the air-fuel ratio sensor is maintained at the stoichiometric air-fuel ratio, and... The air-fuel ratio control device is configured such that, in the theoretical air-fuel ratio control, when the air-fuel ratio of the outflowing exhaust gas detected by the air-fuel ratio sensor decreases below the rich-side switching air-fuel ratio, the micro-enrichment control is initiated.

4. The exhaust purification device for an internal combustion engine according to any one of claims 1 to 3, characterized in that, The air-fuel ratio control device is configured to determine the richness of the slightly rich set air-fuel ratio based on the minimum air-fuel ratio at which the air-fuel ratio of the outflowing exhaust gas, detected by the air-fuel ratio sensor, drops below the rich-side switching air-fuel ratio.

5. The exhaust purification device for an internal combustion engine according to any one of claims 1 to 3, characterized in that, The air-fuel ratio control device is configured to: estimate the hydrogen concentration in the outflowing exhaust gas, and determine the enrichment level of the slightly enriched set air-fuel ratio based on the hydrogen concentration.

6. A method for purifying exhaust gas from an internal combustion engine. The internal combustion engine includes a catalyst, an air-fuel ratio sensor, and an air-fuel ratio control device. The catalyst is disposed in the exhaust passage of the internal combustion engine and is configured to absorb oxygen. The air-fuel ratio sensor is configured to detect the air-fuel ratio of the exhaust gas flowing out from the catalyst. The air-fuel ratio control device is configured to control the air-fuel ratio of the exhaust gas flowing into the catalyst to a target air-fuel ratio. When oxygen is depleted, the catalyst undergoes a water-gas shift reaction and a steam reforming reaction to produce hydrogen. This hydrogen causes a deviation in the output of the air-fuel ratio sensor. The exhaust gas purification method is characterized by comprising the following steps: When the air-fuel ratio of the outflowing exhaust gas, as detected by the air-fuel ratio sensor, drops below the rich-side switching air-fuel ratio (which is richer than the stoichiometric air-fuel ratio), it is determined that the catalyst is depleted of oxygen and produces hydrogen, causing a deviation in the output of the air-fuel ratio sensor. Micro-enrichment control is then initiated. In this micro-enrichment control, the air-fuel ratio of the inflowing exhaust gas is controlled so that the air-fuel ratio of the outflowing exhaust gas, as detected by the air-fuel ratio sensor, is maintained at a slightly rich set air-fuel ratio (which is richer than the stoichiometric air-fuel ratio). This eliminates the output deviation of the air-fuel ratio sensor and suppresses the deterioration of exhaust emissions. The air-fuel ratio control device is configured such that, during the enrichment control, when the air-fuel ratio of the outflowing exhaust gas, detected by the air-fuel ratio sensor, rises above the lean-side switching air-fuel ratio (above the stoichiometric air-fuel ratio), the enrichment control is terminated. The air-fuel ratio control device is configured such that, in the enrichment control, when the air-fuel ratio of the outflowing exhaust gas detected by the air-fuel ratio sensor rises above the lean-side switching air-fuel ratio, stoichiometric air-fuel ratio control is initiated, and in the stoichiometric air-fuel ratio control, the air-fuel ratio of the inflowing exhaust gas is controlled such that the air-fuel ratio of the outflowing exhaust gas detected by the air-fuel ratio sensor is maintained at the stoichiometric air-fuel ratio.

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