Exhaust purification apparatus for internal combustion engine and exhaust purification method thereof

Abstract

The present application is an exhaust purification device for an internal combustion engine and an exhaust purification method thereof. The exhaust purification device for an internal combustion engine includes a catalyst arranged in an exhaust passage; an upstream side air-fuel ratio sensor configured to detect an air-fuel ratio of inflow exhaust gas flowing into the catalyst; a downstream side air-fuel ratio sensor configured to detect an air-fuel ratio of outflow exhaust gas flowing out from the catalyst; and an electronic control unit configured to control the air-fuel ratio of the inflow exhaust gas. The electronic control unit is configured to control the air-fuel ratio of the inflow exhaust gas based on an output of the downstream side air-fuel ratio sensor without using an output of the upstream side air-fuel ratio sensor when a prescribed condition is satisfied, and control the air-fuel ratio of the inflow exhaust gas based on the output of the upstream side air-fuel ratio sensor when the prescribed condition is not satisfied.

Description

Technical Field

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

[0002] It is known that in internal combustion engines, a catalyst capable of absorbing oxygen is placed in the exhaust passage to purify HC, CO, NOx, and other pollutants in the exhaust gas. Japanese Patent Application Publication Nos. 2020-067071, 2010-159672, 2007-218096, and 2006-022755 describe methods for controlling the air-fuel ratio of the exhaust gas flowing into the catalyst based on the outputs of an upstream air-fuel ratio sensor located upstream of the catalyst and a downstream air-fuel ratio sensor located downstream of the catalyst in order to effectively purify the exhaust gas using a catalyst. Summary of the Invention

[0003] However, when the combustion state of the air-fuel mixture is unstable, such as during a cold start in an internal combustion engine, exhaust containing a large amount of unburned high-molecular-weight HC is discharged into the exhaust passage. At this time, due to the low diffusion coefficient of high-molecular-weight HC, the air-fuel ratio detected by the upstream air-fuel ratio sensor deviates from the lean side compared to the actual value. Therefore, if feedback control of the air-fuel ratio based on the output of the upstream air-fuel ratio sensor is implemented, the actual air-fuel ratio deviates from the target value from the rich side, potentially worsening exhaust emissions.

[0004] Therefore, it is necessary to suppress the deterioration of exhaust emissions due to the output deviation of the air-fuel ratio sensor located upstream of the catalyst.

[0005] A first aspect of the present invention relates to an exhaust gas purification device for an internal combustion engine, comprising a catalyst, an upstream air-fuel ratio sensor, a downstream air-fuel ratio sensor, and an electronic control unit. The catalyst is disposed in an exhaust passage. The upstream air-fuel ratio sensor is configured to detect the air-fuel ratio of inflow exhaust gas flowing into the catalyst. The downstream air-fuel ratio sensor is configured to detect the air-fuel ratio of outflow exhaust gas flowing out of the catalyst. The electronic control unit is configured to control the air-fuel ratio of the inflow exhaust gas. Furthermore, the electronic control unit is configured to control the air-fuel ratio of the inflow exhaust gas based on the output of the downstream air-fuel ratio sensor, without using the output of the upstream air-fuel ratio sensor, when a predetermined condition is met. The electronic control unit is configured to control the air-fuel ratio of the inflow exhaust gas based on the output of the upstream air-fuel ratio sensor when the predetermined condition is not met.

[0006] In the exhaust purification device of the internal combustion engine in the first embodiment described above, the electronic control unit can be configured to: when the specified conditions are met, not use the output of the upstream air-fuel ratio sensor, but control the air-fuel ratio of the inflow exhaust in such a way that the air-fuel ratio detected by the downstream air-fuel ratio sensor becomes the theoretical air-fuel ratio.

[0007] In the exhaust purification device for the internal combustion engine in the first embodiment described above, the specified condition may be that the preheating of the internal combustion engine has not been completed.

[0008] In the exhaust purification device of the internal combustion engine configured as described above, the electronic control unit can determine that the preheating of the internal combustion engine is complete when the temperature of the cooling water of the internal combustion engine rises to a specified temperature.

[0009] In the exhaust purification device of the internal combustion engine in the first embodiment described above, the specified condition may be that the intake air volume is below a specified value.

[0010] In the exhaust purification device for the internal combustion engine of the first embodiment described above, the specified condition may be that the internal combustion engine is idling.

[0011] A second aspect of the present invention relates to an exhaust purification method for an exhaust purification device for an internal combustion engine, the exhaust purification device comprising a catalyst, an upstream air-fuel ratio sensor, a downstream air-fuel ratio sensor, and an electronic control unit. Here, the catalyst is disposed in an exhaust passage. The upstream air-fuel ratio sensor is configured to detect the air-fuel ratio of the inflow exhaust flowing into the catalyst. The downstream air-fuel ratio sensor is configured to detect the air-fuel ratio of the outflow exhaust flowing out of the catalyst. The electronic control unit is configured to control the air-fuel ratio of the inflow exhaust. The exhaust purification method, (i) when a predetermined condition is met, controls the air-fuel ratio of the inflow exhaust based on the output of the downstream air-fuel ratio sensor instead of using the output of the upstream air-fuel ratio sensor; (ii) when the predetermined condition is not met, controls the air-fuel ratio of the inflow exhaust based on the output of the upstream air-fuel ratio sensor.

[0012] The exhaust purification device and method for an internal combustion engine according to the present invention can suppress the deterioration of exhaust emissions due to the output deviation of the air-fuel ratio sensor disposed upstream of the catalyst. Attached Figure Description

[0013] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, wherein the same reference numerals denote the same elements, wherein:

[0014] Figure 1 This is a schematic diagram of an internal combustion engine to which an exhaust purification device for an internal combustion engine according to an embodiment of the present invention is applied.

[0015] Figure 2 This is a diagram illustrating an example of the purification characteristics of a three-way catalyst.

[0016] Figure 3 yes Figure 1 The image shows a partial cross-sectional view of the upstream air-fuel ratio sensor.

[0017] Figure 4 This is a graph showing the voltage-current characteristics of the upstream air-fuel ratio sensor.

[0018] Figure 5 This is a graph showing the relationship between the air-fuel ratio of the exhaust gas in the upstream air-fuel ratio sensor and the output current when a constant voltage is applied.

[0019] Figure 6 It is a time chart showing the various parameters during the preheating of the internal combustion engine.

[0020] Figure 7 This is a time diagram of various parameters when the air-fuel ratio control according to the embodiment of the present invention is implemented during the cold start of the internal combustion engine.

[0021] Figure 8 This is a flowchart illustrating the control procedure for air-fuel ratio control in this embodiment. Detailed Implementation

[0022] 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.

[0023] 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 an exhaust purification device for an internal combustion engine according to an embodiment of the present invention is applied. Figure 1 The internal combustion engine shown is a spark-ignition internal combustion engine. Internal combustion engines are installed in vehicles to function as the vehicle's power source.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] Additionally, 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 varied according to the degree of its opening.

[0029] 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.

[0030] In addition, vehicles equipped with internal combustion engines are equipped with an electronic control unit (ECU) 31. The electronic control unit (ECU) 31 functions as an air-fuel ratio control device. Figure 1 As shown, the ECU 31 is a digital computer, equipped with RAM (Random Access Memory) 33, ROM (Read Only Memory) 34, CPU (Central Processing Unit) 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.

[0031] 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 coolant temperature sensor 43, the load sensor 45, and the crankshaft angle sensor 46 are sent to the ECU 31.

[0032] 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 analog-to-digital (AD) converter 38.

[0033] 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.

[0034] 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.

[0035] The water temperature sensor 43 is configured in the cooling water circuit of the internal combustion engine to detect the temperature of the cooling water (internal combustion engine water temperature). The water temperature sensor 43 is electrically connected to the ECU 31, and the output of the water temperature sensor 43 is input to the input port 36 via the corresponding AD converter 38.

[0036] A load sensor 45 is connected to the accelerator pedal 44 of a vehicle equipped with an internal combustion engine to detect the amount of time the accelerator pedal 44 is depressed (throttle opening). The load sensor 45 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 45.

[0037] The crankshaft angle sensor 46 generates an output pulse whenever the crankshaft of the internal combustion engine rotates by a specified angle (e.g., 10 degrees). The crankshaft angle sensor 46 is electrically connected to the ECU 31, and its output is input to input port 36. The ECU 31 calculates the internal combustion engine speed based on the output of the crankshaft angle sensor 46.

[0038] 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.

[0039] 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 can be provided for recirculating exhaust gas (EGR) from the exhaust passage to the intake passage.

[0040] The following describes an exhaust purification device for an internal combustion engine (hereinafter referred to as "exhaust purification device") according to an embodiment of the present invention. The exhaust purification device includes a catalyst 20, an upstream air-fuel ratio sensor 41, a downstream air-fuel ratio sensor 42, and an ECU 31. As described above, the ECU 31 functions as an air-fuel ratio control device.

[0041] 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.

[0042] Figure 2 This is a diagram illustrating an example of the purification characteristics of a three-way catalyst. (See diagram for example.) 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.

[0043] 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.

[0044] like Figure 1 As shown, the upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42 are disposed in the exhaust passage of the 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.

[0045] Figure 3 This is a partial cross-sectional view of the upstream air-fuel ratio sensor 41. The upstream air-fuel ratio sensor 41 has a known configuration; therefore, its configuration will be briefly described below. Furthermore, the downstream air-fuel ratio sensor 42 has the same configuration as the upstream air-fuel ratio sensor 41.

[0046] The upstream air-fuel ratio sensor 41 includes a sensing element 411 and a heater 420. In this embodiment, the upstream air-fuel ratio sensor 41 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 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 layer 413, and an atmospheric chamber 419 is formed between the solid electrolyte layer 412 and the first impermeable layer 414.

[0047] Exhaust gas, as the gas to be measured, is introduced into the gas chamber 418 via the diffusion velocity layer 413, while atmospheric air is introduced into the atmospheric chamber 419. When the air-fuel ratio sensor 41 detects the air-fuel ratio of the exhaust gas, a voltage is applied to the sensing element 411 in such a way that the potential of the atmospheric side electrode 417 becomes higher than the potential of the exhaust side electrode 416. 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 current flowing between the exhaust side electrode 416 and the atmospheric side electrode 417, i.e., the output current of the upstream air-fuel ratio sensor 41, varies according to the air-fuel ratio of the exhaust gas.

[0048] Figure 4 This is a graph showing the voltage-current (VI) characteristics of the upstream air-fuel ratio sensor 41. (See diagram below.) Figure 4 As shown, the higher the air-fuel ratio of the exhaust (the leaner it is), the larger the output current I. Furthermore, within the VI line corresponding to each air-fuel ratio, there exists a region roughly parallel to the V-axis—a region where the output current hardly changes even when the voltage applied to the sensor changes. This voltage region is called the limit current region, and the current at this point is called the limit current. Figure 4 In the middle, the boundary current region and boundary current when the exhaust air-fuel ratio is 18 are respectively represented by W. 18 I 18 As shown.

[0049] The limit current value IL of the air-fuel ratio sensor is generally expressed by the following formula (1).

[0050] IL=D×(4FP / RT)×(S / L)×ln(1-(P o2 / P))…(1)

[0051] Here, D is the diffusion coefficient, F is the Faraday constant, P is the total pressure of the exhaust gas, R is the gas constant, T is the absolute temperature, S is the electrode surface area, L is the diffusion distance, and P o2 It is the oxygen partial pressure in the exhaust gas.

[0052] Figure 5 This is a graph showing the relationship between the air-fuel ratio of the exhaust gas in the upstream air-fuel ratio sensor 41 and the output current I when the applied voltage is constant. Figure 5 In this example, a voltage of 0.45V is applied to the sensing element 411. (As per...) Figure 5As is known, 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.

[0053] Furthermore, in this embodiment, a limiting current type air-fuel ratio sensor is used as both the upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42. However, if the air-fuel ratio sensor outputs a linearly changing value relative to the exhaust air-fuel ratio, then non-limiting current type air-fuel ratio sensors can also be used as both 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.

[0054] ECU 31 controls the air-fuel ratio of the exhaust gas flowing into the catalyst 20 (hereinafter referred to as "inflow exhaust"). As described above, the air-fuel ratio of the inflow exhaust is detected by the upstream air-fuel ratio sensor 41. Therefore, ECU 31 controls the air-fuel ratio of the inflow exhaust based on the output of the upstream air-fuel ratio sensor 41. Specifically, feedback control is used 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 matches 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, i.e., the air-fuel ratio detected by the air-fuel ratio sensor.

[0055] Furthermore, as described above, the air-fuel ratio of the exhaust gas flowing out of the catalyst 20 (hereinafter referred to as "outflow exhaust") is detected by the downstream air-fuel ratio sensor 42. The air-fuel ratio of the outflow exhaust indicates the purification status of the exhaust gas in the catalyst 20. When the exhaust gas is not properly purified in the catalyst 20, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 deviates from the stoichiometric air-fuel ratio. Therefore, the ECU 31 corrects the air-fuel ratio control based on the output of the downstream air-fuel ratio sensor 42. For example, the ECU 31 corrects the target air-fuel ratio of the inflow exhaust based on the output of the downstream air-fuel ratio sensor 42. Thus, the air-fuel ratio of the inflow exhaust can be controlled to an appropriate value, and the exhaust gas can be effectively purified in the catalyst 20.

[0056] However, when the combustion state of the air-fuel mixture is unstable, such as during a cold start of an internal combustion engine, exhaust containing a large amount of unburned high-molecular-weight HC is discharged into the exhaust passage, and its air-fuel ratio is detected by the upstream air-fuel ratio sensor 41. In the case where the exhaust contains a large amount of high-molecular-weight HC, the diffusion coefficient D in the aforementioned equation (1) for the limiting current value IL becomes smaller than a predetermined value based on the porosity of the diffusion rate layer 413. As a result, the output current of the sensing element 411 becomes larger than a value equivalent to the actual air-fuel ratio of the exhaust, and the output air-fuel ratio of the upstream air-fuel ratio sensor 41 deviates towards the lean side compared to the actual value. Therefore, if feedback control of the air-fuel ratio based on the output of the upstream air-fuel ratio sensor 41 is implemented, the actual air-fuel ratio deviates towards the rich side compared to the target value, potentially leading to worsened exhaust emissions.

[0057] Figure 6 It is a time graph showing various parameters during the preheating of an internal combustion engine. Figure 6 The table shows various parameters, including the temperature of the internal combustion engine's cooling water (internal combustion engine water temperature), the speed of the vehicle equipped with the internal combustion engine (vehicle speed), the air-fuel ratio of the inflow exhaust detected by the upstream air-fuel ratio sensor 41 (detected air-fuel ratio), and the calculated air-fuel ratio of the inflow exhaust (calculated air-fuel ratio). Figure 6 In the curve graph on the upper side, the detected air-fuel ratio is represented by a solid line, the calculated air-fuel ratio is represented by a dashed line, and the vehicle speed is represented by a single-dot dashed line.

[0058] exist Figure 6 In the example, after 100 seconds, the internal combustion engine coolant temperature is low, and preheating is not complete. At this point, although the detected air-fuel ratio is maintained near the stoichiometric air-fuel ratio, the calculated air-fuel ratio, which approximates the actual air-fuel ratio, becomes a richer value than the stoichiometric air-fuel ratio. That is, Figure 6 The results show that when the internal combustion engine is cold-started, and air-fuel ratio control is implemented to maintain the output air-fuel ratio of the upstream air-fuel ratio sensor 41 at the stoichiometric air-fuel ratio, the actual air-fuel ratio of the exhaust becomes richer than the stoichiometric air-fuel ratio due to the influence of high molecular weight HC in the exhaust.

[0059] On the other hand, even if exhaust containing a large amount of unburned high-molecular-weight HC is discharged into the exhaust passage, the high-molecular-weight HC in the exhaust is purified or decomposed into HC with smaller molecular weight in the catalyst 20. Therefore, it is difficult for the downstream air-fuel ratio sensor 42, which is located downstream of the catalyst 20, to produce an output deviation as seen in the upstream air-fuel ratio sensor 41.

[0060] Therefore, in this embodiment, the ECU 31 controls the air-fuel ratio flowing into the exhaust gas based on the output of the downstream air-fuel ratio sensor 42 instead of using the output of the upstream air-fuel ratio sensor 41 when the specified conditions are met, and controls the air-fuel ratio flowing into the exhaust gas based on the output of the upstream air-fuel ratio sensor 41 when the specified conditions are not met. This reduces the impact of the output deviation of the upstream air-fuel ratio sensor 41, thereby suppressing the deterioration of exhaust emissions due to the output deviation of the upstream air-fuel ratio sensor 41.

[0061] The specified condition is a high concentration of high-molecular-weight HC in the exhaust gas discharged into the exhaust passage, such as when the internal combustion engine has not been fully preheated. In this case, the ECU 31 does not use the output of the upstream air-fuel ratio sensor 41 from the time the internal combustion engine is started until the preheating of the internal combustion engine is completed. Instead, it controls the air-fuel ratio flowing into the exhaust gas based on the output of the downstream air-fuel ratio sensor 42. Furthermore, even before the preheating of the internal combustion engine is complete, the downstream air-fuel ratio sensor 42 can be activated in advance by heating the sensing element with a heater.

[0062] In this embodiment, when specified conditions are met, the ECU 31 controls the air-fuel ratio flowing into the exhaust gas by not using the output of the upstream air-fuel ratio sensor 41, but in a manner that makes the output air-fuel ratio of the downstream air-fuel ratio sensor 42 equal to the stoichiometric air-fuel ratio. This allows the air-fuel ratio of the outflowing exhaust gas to be close to the stoichiometric air-fuel ratio, suppressing exhaust emission degradation. For example, when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is below a specified rich-to-rich threshold (more than the stoichiometric air-fuel ratio), the ECU 31 sets the target air-fuel ratio of the inflowing exhaust gas to a value leaner than the stoichiometric air-fuel ratio; when the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is above a specified lean-to-lean threshold (more than the stoichiometric air-fuel ratio), the ECU 31 sets the target air-fuel ratio of the inflowing exhaust gas to a value richer than the stoichiometric air-fuel ratio.

[0063] Next, the air-fuel ratio control using a time-map method will be explained. See below for reference. Figure 7 The air-fuel ratio control described above will be explained in detail below. Figure 7 This is a time diagram showing the various parameters used when implementing the air-fuel ratio control according to the embodiments of the present invention during the cold start of an internal combustion engine. Figure 7 The diagram displays various parameters, including the output air-fuel ratio of the downstream air-fuel ratio sensor 42 (output air-fuel ratio of the downstream sensor), the target air-fuel ratio of the inflow exhaust, the output air-fuel ratio of the upstream air-fuel ratio sensor 41 (output air-fuel ratio of the upstream sensor), the temperature of the engine coolant (engine water temperature), and a preheating completion indicator. The preheating completion indicator is set to zero when the engine is started and to 1 when the engine preheating is complete.

[0064] exist Figure 7In the example, at time t0, before the internal combustion engine has fully preheated, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is below the rich-determined air-fuel ratio JAFrich. Therefore, the target air-fuel ratio for the incoming exhaust is set to a lean set air-fuel ratio TAFlean, which is leaner than the stoichiometric air-fuel ratio. At this time, due to the influence of high-molecular-weight HC, the output of the upstream air-fuel ratio sensor 41 deviates, and the output air-fuel ratio of the upstream air-fuel ratio sensor 41 becomes a value leaner than the lean set air-fuel ratio TAFlean. As the internal combustion engine coolant temperature rises, the concentration of high-molecular-weight HC in the exhaust gradually decreases. Therefore, after time t0, the output air-fuel ratio of the upstream air-fuel ratio sensor 41 gradually approaches the target air-fuel ratio for the incoming exhaust.

[0065] After time t0, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 changes towards the stoichiometric air-fuel ratio, reaching the rich determination air-fuel ratio JAFrich at time t1. As a result, the target air-fuel ratio flowing into the exhaust gas is changed from the lean set air-fuel ratio TAFlean to the stoichiometric air-fuel ratio (14.6).

[0066] Subsequently, at time t2, due to interference and other factors, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 reached the lean-determined air-fuel ratio JAFlean. As a result, in order to make the air-fuel ratio of the outflowing exhaust closer to the stoichiometric air-fuel ratio, the target air-fuel ratio of the inflowing exhaust was changed from the stoichiometric air-fuel ratio to the rich-determined air-fuel ratio TAFrich.

[0067] After time t2, at time t3, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 is reduced to the lean determination air-fuel ratio JAFlean, and the target air-fuel ratio flowing into the exhaust is changed from the rich set air-fuel ratio TAFrich to the stoichiometric air-fuel ratio.

[0068] After time t3, the preheating of the internal combustion engine continues, and at time t4, the engine coolant temperature reaches the specified temperature Tth. As a result, the preheating of the internal combustion engine is determined to be complete, and the preheating completion flag is set to 1. At time t4, the output deviation of the upstream air-fuel ratio sensor 41 is eliminated, and the output air-fuel ratio of the upstream air-fuel ratio sensor 41 becomes the same as the target air-fuel ratio (theoretical air-fuel ratio) of the inflow exhaust. After time t4, feedback control of the air-fuel ratio is implemented in a manner that ensures the output air-fuel ratio of the upstream air-fuel ratio sensor 41 matches the target air-fuel ratio of the inflow exhaust.

[0069] Next, the flowchart for air-fuel ratio control will be explained. The following will use... Figure 8 The flowchart above will be used to illustrate the air-fuel ratio control process. Figure 8 This is a flowchart illustrating the control procedure for air-fuel ratio control in this embodiment. This control procedure is repeatedly executed by ECU 31, which functions as an air-fuel ratio control device, at predetermined execution intervals.

[0070] Initially, in step S101, ECU31 determines whether the preheating of the internal combustion engine is complete. For example, ECU31 determines that the preheating of the internal combustion engine is complete when the engine coolant temperature rises to a specified temperature. The engine coolant temperature is detected by the coolant temperature sensor 43, and the specified temperature is set, for example, to be 40°C to 60°C.

[0071] Furthermore, ECU 31 can also determine that the preheating of the internal combustion engine is complete when the cumulative flow of exhaust gas discharged into the exhaust passage after the internal combustion engine starts reaches a predetermined value. In this case, the exhaust flow rate is calculated based on the output of the air flow meter 40 or detected by a flow sensor in the exhaust passage located upstream of the catalyst 20. Alternatively, ECU 31 can also determine that the preheating of the internal combustion engine is complete when the temperature (bed temperature) of the catalyst 20 rises to a predetermined temperature. In this case, the temperature of the catalyst 20 is calculated based on predetermined state parameters of the internal combustion engine (e.g., engine coolant temperature, intake air volume, engine load, etc.) or detected by a temperature sensor in the exhaust passage located near or around the catalyst 20. Additionally, ECU 31 can also determine that the preheating of the internal combustion engine is complete when a predetermined time has elapsed since the internal combustion engine started.

[0072] Furthermore, if the preheating of the internal combustion engine is completed, thereby reducing the concentration of high-molecular-weight HC flowing into the exhaust, the output deviation of the upstream air-fuel ratio sensor 41 is eliminated, and the output of the upstream air-fuel ratio sensor 41 stabilizes. Therefore, the ECU 31 can also determine that the preheating of the internal combustion engine is complete when the change in the output of the upstream air-fuel ratio sensor 41 within a specified time becomes below a specified value. The change in output is calculated, for example, as the difference between the maximum and minimum values ​​of the output within the specified time, or the dispersion of the output detected within the specified time (the square of the deviation), etc.

[0073] If it is determined in step S101 that the preheating of the internal combustion engine is not complete, the control program proceeds to step S102. In step S102, the ECU 31 determines whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is below the rich air-fuel ratio JAFrich. The rich air-fuel ratio JAFrich is predetermined as a value indicating that the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, and is set to a value slightly richer than the stoichiometric air-fuel ratio (e.g., 14.55 to 14.58).

[0074] If, in step S102, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is determined to be below the rich air-fuel ratio JAFrich, the control program proceeds to step S103. In step S103, the ECU 31 sets the target air-fuel ratio TAF flowing into the exhaust gas to a lean set air-fuel ratio TAFlean in order to make the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 closer to the stoichiometric air-fuel ratio. The lean set air-fuel ratio TAFlean is predetermined and set to an air-fuel ratio leaner than the stoichiometric air-fuel ratio (e.g., 14.7 to 15.7). After step S103, the control program ends.

[0075] On the other hand, if it is determined in step S102 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is leaner than the rich-determined air-fuel ratio JAFrich, the control program proceeds to step S104. In step S104, the ECU 31 determines whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is leaner than or equal to the lean-determined air-fuel ratio JAFlean. The lean-determined air-fuel ratio JAFlean is predetermined as a value indicating that the air-fuel ratio of the outflowing exhaust has become leaner than the stoichiometric air-fuel ratio, and is set to a value slightly leaner than the stoichiometric air-fuel ratio (e.g., 14.62 to 14.65).

[0076] If, in step S104, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is determined to be above the lean air-fuel ratio JAFlean, the control program proceeds to step S105. In step S105, the ECU 31 sets the target air-fuel ratio TAF flowing into the exhaust gas to a rich set air-fuel ratio TAFrich in order to make the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 closer to the stoichiometric air-fuel ratio. The rich set air-fuel ratio TAFrich is predetermined and set to an air-fuel ratio richer than the stoichiometric air-fuel ratio (e.g., 13.5 to 14.5). After step S105, the control program ends.

[0077] On the other hand, if in step S104 it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 is leaner than the lean air-fuel ratio JAFlean, the control program proceeds to step S106. In step S106, in order to maintain the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 42 at the stoichiometric air-fuel ratio, the ECU 31 sets the target air-fuel ratio TAF flowing into the exhaust gas to the stoichiometric air-fuel ratio (14.6). After step S106, the control program ends.

[0078] Furthermore, if it is determined in step S101 that the preheating of the internal combustion engine is complete, the control program proceeds to step S107. In step S107, the ECU 31 controls the air-fuel ratio flowing into the exhaust gas based on the output of the upstream air-fuel ratio sensor 41. Specifically, the fuel supply to the combustion chamber 5 is controlled in such a way that the output air-fuel ratio of the upstream air-fuel ratio sensor 41 matches the target air-fuel ratio of the exhaust gas. The target air-fuel ratio of the exhaust gas is, for example, set to the stoichiometric air-fuel ratio. Furthermore, the target air-fuel ratio of the exhaust gas can also be corrected based on the output of the downstream air-fuel ratio sensor 42. In addition, the ECU 31 can also switch the target air-fuel ratio of the exhaust gas between a rich set air-fuel ratio (TAFrich) and a lean set air-fuel ratio (TAFlean) based on the output of the downstream air-fuel ratio sensor 42, so that the oxygen uptake of the catalyst 20 varies between zero and the maximum oxygen uptake. After step S107, the control program ends.

[0079] Furthermore, when the intake air volume is low, such as at low loads, the combustion state of the air-fuel mixture can easily become unstable. Therefore, the specified condition can also be that the intake air volume is below a specified value. In this case, in step S101, the ECU 31 determines whether the intake air volume is more than the specified value, and the intake air volume is calculated, for example, based on the output of the air flow meter 40. That is, the ECU 31 can also control the air-fuel ratio flowing into the exhaust gas based on the output of the downstream air-fuel ratio sensor 42 instead of using the output of the upstream air-fuel ratio sensor 41 when the intake air volume is below the specified value.

[0080] Furthermore, when the internal combustion engine is idling, the combustion state of the air-fuel mixture can easily become unstable. Therefore, the specified condition can also be that the internal combustion engine is idling. Moreover, idling means that when the throttle opening is zero, the internal combustion engine speed is maintained at a specified low speed (e.g., 400-800 rpm) through the combustion of the air-fuel mixture. In this case, in step S101, the ECU31 determines whether the internal combustion engine is idling. If idling is being performed, the control program proceeds to step S102. That is, the ECU31 can also control the air-fuel ratio flowing into the exhaust gas based on the output of the downstream air-fuel ratio sensor 42 instead of using the output of the upstream air-fuel ratio sensor 41 when the internal combustion engine is idling.

[0081] Other embodiments will be described. 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, the ECU 31 may also, when certain conditions are met, control the air-fuel ratio flowing into the exhaust gas based on the output of the downstream air-fuel ratio sensor 42 using proportional-integral-derivative (PID) control or the like, so that the output air-fuel ratio of the downstream air-fuel ratio sensor 42 matches the stoichiometric air-fuel ratio.

[0082] Alternatively, in an internal combustion engine, a downstream catalyst, identical to catalyst 20, can be configured in the exhaust passage downstream of catalyst 20. In this case, ECU 31 can also control the air-fuel ratio flowing into the exhaust gas by not using the output of upstream air-fuel ratio sensor 41 when specified conditions are met, in order to control the state of downstream catalyst (oxygen uptake, etc.). Instead, it can control the air-fuel ratio flowing into the exhaust gas in a manner that makes the output air-fuel ratio of downstream air-fuel ratio sensor 42 a specified air-fuel ratio other than the stoichiometric air-fuel ratio.

Claims

1. An exhaust purification device for an internal combustion engine, characterized in that, Include: Catalyst placed in the exhaust passage; An upstream air-fuel ratio sensor is configured to detect the air-fuel ratio of the inflow exhaust gas flowing into the catalyst. A downstream air-fuel ratio sensor is configured to detect the air-fuel ratio of the exhaust gas flowing out from the catalyst. and An electronic control unit configured to control the air-fuel ratio of the incoming exhaust gas. The upstream air-fuel ratio sensor and the downstream air-fuel ratio sensor are capable of continuously detecting the air-fuel ratio of the exhaust gas. The electronic control unit is configured to control the air-fuel ratio of the incoming exhaust gas based on the output of the downstream air-fuel ratio sensor, rather than using the output of the upstream air-fuel ratio sensor, when certain conditions are met. Furthermore, the electronic control unit is configured to control the air-fuel ratio of the incoming exhaust gas based on the output of the upstream air-fuel ratio sensor when the specified conditions are not met. The specified condition is that the preheating of the internal combustion engine is not complete. The electronic control unit is configured such that when the change in the output of the upstream air-fuel ratio sensor within a specified time becomes below a specified value, it is determined that the preheating of the internal combustion engine is completed, and the change in output is calculated as the difference between the maximum and minimum values ​​of the output within the specified time or the discreteness of the output detected within the specified time.

2. The exhaust purification device for an internal combustion engine according to claim 1, characterized in that, The electronic control unit is configured to, when the specified conditions are met, control the air-fuel ratio of the incoming exhaust gas in a manner that makes the air-fuel ratio detected by the downstream air-fuel ratio sensor equal to the stoichiometric air-fuel ratio, without using the output of the upstream air-fuel ratio sensor.

3. The exhaust purification device for an internal combustion engine according to claim 1 or 2, characterized in that, It also includes a downstream catalyst disposed in an exhaust passage downstream of the catalyst. The electronic control unit is configured to control the air-fuel ratio of the inflow exhaust gas in a manner that, when the specified conditions are met, does not use the output of the upstream air-fuel ratio sensor, but instead controls the air-fuel ratio of the downstream air-fuel ratio sensor in a manner that makes the output air-fuel ratio of the downstream air-fuel ratio sensor a specified air-fuel ratio other than the stoichiometric air-fuel ratio.

4. An exhaust purification method for an internal combustion engine exhaust purification device, wherein, The exhaust gas purification device includes: a catalyst disposed in an exhaust passage; an upstream air-fuel ratio sensor configured to detect the air-fuel ratio of the inflow exhaust gas flowing into the catalyst; a downstream air-fuel ratio sensor configured to detect the air-fuel ratio of the outflow exhaust gas flowing out of the catalyst; and an electronic control unit configured to control the air-fuel ratio of the inflow exhaust gas, wherein the upstream air-fuel ratio sensor and the downstream air-fuel ratio sensor are each capable of continuously detecting the air-fuel ratio of the exhaust gas. The exhaust gas purification method is characterized by comprising: When specified conditions are met, the air-fuel ratio of the incoming exhaust gas is controlled based on the output of the downstream air-fuel ratio sensor, instead of using the output of the upstream air-fuel ratio sensor. When the specified conditions are not met, the air-fuel ratio of the incoming exhaust gas is controlled based on the output of the upstream air-fuel ratio sensor. The specified condition is that the preheating of the internal combustion engine is not complete. When the change in the output of the upstream air-fuel ratio sensor within a specified time becomes below a specified value, it is determined that the preheating of the internal combustion engine is completed. The change in output is calculated as the difference between the maximum and minimum values ​​of the output within the specified time or the discreteness of the output detected within the specified time.

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