Engine exhaust purification system

Abstract

In an engine exhaust gas purification system that controls the air-fuel ratio of the exhaust gas to set it to a target air-fuel ratio, the amount of ammonia produced by the catalytic converter is effectively suppressed. [Solution] The engine exhaust gas purification device 100 includes a catalytic converter 51 containing palladium and rhodium as precious metals to purify exhaust gas, a linear A / F sensor SW7 provided on the exhaust passage 50 downstream of the catalytic converter 51, and a control device 60 that controls the fuel injection valve 18 to set the air-fuel ratio of the exhaust gas to a target air-fuel ratio based on the air-fuel ratio detected by the sensor SW7. The control device 60 determines whether the catalytic converter 51 is active and whether the catalytic converter 51 is deteriorated. If it is determined that the catalytic converter 51 is not active and is deteriorated, the control device 60 sets the target air-fuel ratio to a leaner first air-fuel ratio than when it is determined that the catalytic converter 51 is not deteriorated.

Description

Technical Field

[0001] The present invention relates to an exhaust gas purification device for an engine that has a catalyst device provided in an exhaust passage and performs control so as to set the air-fuel ratio of exhaust gas to a target air-fuel ratio.

Background Art

[0002] Conventionally, in order to ensure the exhaust gas purification performance of a catalyst device, a technique of performing feedback control (so-called air-fuel ratio control) on the fuel injection amount so as to set the air-fuel ratio of exhaust gas to a predetermined target air-fuel ratio (typically the stoichiometric air-fuel ratio) is known. This type of technique is described in, for example, Patent Document 1.

[0003] In particular, Patent Document 1 focuses on the generation of ammonia in a catalyst device including a three-way catalyst, and describes a technique for improving the purification efficiency of this ammonia. Specifically, in the technique described in Patent Document 1, the amount of ammonia generated by the catalyst device is calculated, and when this amount of ammonia generated is large, the target air-fuel ratio is set on the lean side. Thereby, ammonia is purified by using the oxygen increased by setting the air-fuel ratio on the lean side. Further, Patent Document 1 describes that a catalyst device with advanced deterioration over time has a lower activity of the three-way catalyst than a catalyst device immediately after manufacture, so that the amount of ammonia generated decreases.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As a result of diligent research, the inventors of this application have found that when the catalyst is inactive, if the catalyst is degraded, the amount of ammonia produced by the catalyst is greater than when the catalyst is not degraded (i.e., in a normal state). This result differs from the content described in Patent Document 1 above.

[0006] Thus, the reason why ammonia production is higher when the catalyst is degraded than when it is functioning normally, when the catalyst is inactive, is thought to be due to the influence of palladium (Pd) contained in the precious metals of the catalyst. In other words, it is thought that the degradation of palladium changes the degree of influence of N2O production associated with the NO-H2 reaction. When the catalyst is functioning normally (for example, immediately after manufacturing), when the catalyst is inactive, the NO decomposition reaction of palladium proceeds, selectively producing N2O. This is thought to be because, when the catalyst is inactive, HC and CO are hardly purified, so the contribution of the NO-H2 reaction is large. Therefore, when the catalyst is functioning normally, it can be said that almost no ammonia (NH3) is produced. In contrast, when the catalyst is degraded, the temperature range in which NO is purified becomes higher (because the precious metals (Pd, etc.) of the catalyst condense due to thermal stress, and the active surface shrinks), and the contribution of rhodium (Rh) becomes larger in the inactive temperature range of the catalyst, so N2O production is suppressed. Therefore, when the catalyst is degraded, it can be said that the amount of ammonia produced is relatively higher.

[0007] The present invention is based on the above findings and aims to effectively suppress the amount of ammonia produced by a catalytic converter in an engine exhaust purification device that has a catalytic converter installed in the exhaust passage and controls the air-fuel ratio of the exhaust gas to set it to a target air-fuel ratio. [Means for solving the problem]

[0008] To achieve the above objective, the present invention provides an engine exhaust gas purification device comprising: a fuel injection valve for supplying fuel to the combustion chamber of an engine; an exhaust passage for discharging exhaust gas from the combustion chamber; a catalytic converter provided on the exhaust passage and containing palladium and rhodium as at least precious metals and configured to purify the exhaust gas; a linear A / F sensor provided on the exhaust passage downstream of the catalytic converter and capable of detecting the air-fuel ratio of the exhaust gas; and a control device configured to control the fuel injection valve based on the air-fuel ratio detected by the linear A / F sensor in order to set the air-fuel ratio of the exhaust gas to a target air-fuel ratio, wherein the control device is configured to determine whether the catalytic converter is active and whether the catalytic converter is deteriorated, and if it is determined that the catalytic converter is not active and is deteriorated, the target air-fuel ratio is set to a leaner first air-fuel ratio than when it is determined that the catalytic converter is not deteriorated.

[0009] When a catalytic converter is inactive and deteriorated, the amount of ammonia produced from the catalytic converter tends to be higher than when the catalytic converter is not deteriorated (i.e., in a normal state). Therefore, in this invention, the control device corrects the target air-fuel ratio to a lean first air-fuel ratio when the catalytic converter is inactive and deteriorated. This makes it possible to effectively suppress ammonia production by utilizing the increased oxygen produced by setting the target air-fuel ratio to a leaner side in situations where the amount of ammonia produced by the catalytic converter is high.

[0010] In the present invention, preferably, the control device is configured to set the target air-fuel ratio to a second air-fuel ratio that is richer than the first air-fuel ratio when it determines that the catalytic converter is active. In the present invention configured as described above, when the catalytic converter is active, the amount of ammonia produced tends to decrease. Therefore, the control device sets the target air-fuel ratio to be richer than the first air-fuel ratio within the lean range. This makes it possible to suppress ammonia emissions while ensuring NOx purification performance.

[0011] In the present invention, preferably, the control device is configured to set the target air-fuel ratio to a second air-fuel ratio that is richer than the first air-fuel ratio when it determines that the catalytic converter has not deteriorated. In the present invention configured as described above, when the catalytic converter is not degraded (i.e., in a normal state), the amount of ammonia produced tends to be low. Therefore, the control device sets the target air-fuel ratio to be richer than the first air-fuel ratio within the lean range. This makes it possible to suppress ammonia emissions while ensuring NOx purification performance.

[0012] In the present invention, preferably, if the catalytic converter is a first catalytic converter, the engine exhaust gas purification device further includes a second catalytic converter provided in the exhaust passage downstream of the first catalytic converter and configured to purify the exhaust gas, and the control device is configured to determine whether or not the second catalytic converter is active, and if it is determined that the second catalytic converter is active, to set the target air-fuel ratio to a third air-fuel ratio that is richer than the second air-fuel ratio. When the downstream catalytic converter (second catalytic converter) is active, even if ammonia is generated from the upstream catalytic converter (first catalytic converter), this ammonia can be purified by the downstream catalytic converter. Therefore, in this case, the control device sets the target air-fuel ratio to a third air-fuel ratio, which is even richer than the second air-fuel ratio. This effectively ensures NOx purification performance while suppressing ammonia emissions.

[0013] In the present invention, preferably, the difference between the first air-fuel ratio and the second air-fuel ratio is greater than the difference between the second air-fuel ratio and the third air-fuel ratio. According to the present invention configured in this manner, when the catalytic converter is inactive and deteriorated, a target air-fuel ratio with a high degree of leanness (first air-fuel ratio) can be used, thereby prioritizing the suppression of ammonia. On the other hand, when the catalytic converter is active or functioning normally, as described above, a target air-fuel ratio with a low degree of leanness (second air-fuel ratio) can be used, thereby effectively ensuring NOx purification performance while suppressing ammonia emissions.

[0014] In the present invention, preferably, the third air-fuel ratio is a normal air-fuel ratio that is set as the target air-fuel ratio when the control device determines that the catalytic converter is active and that the catalytic converter is not degraded. According to the present invention configured in this manner, when the catalytic converter is active and functioning correctly, the target air-fuel ratio can be returned to the normal air-fuel ratio, and the normal exhaust gas purification performance by the catalytic converter can be accurately ensured.

[0015] In the present invention, preferably, the control device is configured to determine the activity level of the catalytic converter based on the fluctuation range of the air-fuel ratio detected within a predetermined time by a linear A / F sensor, obtain the temperature of the catalytic converter when it reaches a predetermined activity level based on the determination result of the activity level, and determine whether or not the catalytic converter is degraded based on that temperature. In the present invention configured as described above, the control device uses the catalyst temperature to diagnose the catalytic converter, as the temperature at which the catalytic converter becomes active (catalyst temperature) changes depending on the degree of deterioration of the catalytic converter. Furthermore, the control device uses fluctuations in the air-fuel ratio downstream of the catalytic converter to determine the active state of the catalytic converter. This is because when the catalytic converter is not active, the air-fuel ratio downstream of the catalytic converter fluctuates relatively large, but as the catalytic converter becomes active, oxygen consumption in the catalytic converter increases, and fluctuations in the air-fuel ratio downstream of the catalytic converter become smaller. Therefore, the active state of the catalytic converter can be accurately determined based on these fluctuations in the air-fuel ratio. Based on the above, in the present invention, the control device determines the activity level of the catalytic converter based on the fluctuation range of the air-fuel ratio downstream of the catalytic converter, and determines the deterioration of the catalytic converter based on the catalytic converter temperature when the catalytic converter reaches a predetermined activity level. This significantly improves the accuracy of catalytic converter deterioration determination.

[0016] In the present invention, preferably, the control device determines that the catalyst device is degraded if the temperature of the catalyst device when the catalyst device reaches a predetermined level of activity is above a predetermined temperature. In this case, since the rise of the exhaust gas purification performance of the catalyst device is slow, the control device determines that the catalyst device is deteriorated. Thereby, it is possible to accurately determine the deterioration of the catalyst device.

Advantages of the Invention

[0017] According to the present invention, in an exhaust gas purification device for an engine that has a catalyst device provided in an exhaust passage and performs control to set the air-fuel ratio of exhaust gas to a target air-fuel ratio, the amount of ammonia generated by the catalyst device can be effectively suppressed.

Brief Description of the Drawings

[0018] [Figure 1] It is a schematic configuration diagram of an exhaust gas purification device for an engine according to an embodiment of the present invention. [Figure 2] It is a block diagram showing the electrical configuration of an exhaust gas purification device for an engine according to an embodiment of the present invention. [Figure 3] It is a flowchart showing air-fuel ratio control according to an embodiment of the present invention. [Figure 4] It is an explanatory diagram of the basic concept of air-fuel ratio correction processing according to an embodiment of the present invention. [Figure 5] It is a flowchart showing air-fuel ratio correction processing according to an embodiment of the present invention. [Figure 6] It is a time chart showing air-fuel ratio correction processing according to an embodiment of the present invention. [Figure 7] It is an explanatory diagram of catalyst deterioration determination processing according to an embodiment of the present invention. [Figure 8] It is a further explanatory diagram of catalyst deterioration determination processing according to an embodiment of the present invention. [Figure 9] It is a flowchart showing catalyst deterioration determination processing according to an embodiment of the present invention.

Modes for Carrying Out the Invention

[0019] Hereinafter, an exhaust gas purification device for an engine according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0020] [Device configuration] First, with reference to Figure 1, the overall configuration of the engine exhaust purification device according to this embodiment will be described. Figure 1 is a schematic diagram of the engine exhaust purification device according to this embodiment.

[0021] The engine exhaust gas purification device 100 is mounted on a vehicle (not shown) and, as shown in Figure 1, mainly comprises an engine 1 as an internal combustion engine that generates power (propulsion) for the vehicle, an intake passage 40 that supplies air (intake) to the engine 1, and an exhaust passage 50 that discharges exhaust gas from the engine 1.

[0022] Engine 1 is a four-stroke engine that performs intake, compression, expansion, and exhaust strokes. Engine 1 is a gasoline engine that uses gasoline as fuel. This fuel may be any liquid fuel containing at least gasoline, and may be gasoline containing, for example, bioethanol.

[0023] Specifically, engine 1 mainly comprises a cylinder block 11, a cylinder head 12 mounted on the cylinder block 11 and forming a cylinder 13 together with the cylinder block 11, a piston 14 that reciprocates within the cylinder 13, a connecting rod 15 connected to the piston 14, and a crankshaft 16 connected to the connecting rod 15. Engine 1 is, for example, a multi-cylinder engine containing multiple cylinders 13 (only one cylinder 13 is shown in Figure 1). The cylinder block 11, cylinder head 12, and piston 14 form the combustion chamber 17 of engine 1.

[0024] Furthermore, the engine 1 has fuel injectors 18 and spark plugs 19 provided in the cylinder head 12. The fuel injectors 18 inject fuel into the cylinder 13 (combustion chamber 17), and the spark plugs 19 ignite the fuel-air mixture in the cylinder 13. A fuel supply system (not shown) is connected to the fuel injectors 18, and fuel is supplied from this system. In the engine 1 shown in Figure 1, the fuel injectors 18 are shown positioned to inject fuel from above the combustion chamber 17, but the fuel injectors 18 may also be positioned to inject fuel from the side of the combustion chamber 17. In the latter case, the fuel injectors 18 can be provided in the cylinder block 11.

[0025] On the other hand, the intake passage 40 is provided with an air cleaner 41 and a throttle valve 43. The throttle valve 43 adjusts the amount of air introduced into the cylinder 13 according to its opening. An intake valve 21 is also provided between the intake passage 40 and the cylinder 13. The intake valve 21 is opened and closed at predetermined timings by a valve train. Typically, the valve train is an electrically or hydraulically operated variable valve train that varies the valve timing and / or valve lift. For example, the valve train is an intake S-VT (Sequential-Valve Timing) that can continuously change the rotational phase of the intake camshaft relative to the crankshaft 16 within a predetermined angular range.

[0026] Next, an exhaust valve 22 is provided in the exhaust passage 50. Specifically, the exhaust valve 22 is located between the cylinder 13 and the exhaust passage 50. The exhaust valve 22 is opened and closed at predetermined timings by a valve train. Typically, the valve train is an electrically or hydraulically operated variable valve train that varies the valve timing and / or valve lift. For example, the valve train is an exhaust S-VT that continuously changes the rotational phase of the exhaust camshaft relative to the crankshaft 16 within a predetermined angular range.

[0027] Furthermore, the exhaust passage 50 is provided with two catalytic converters 51 and 52, each containing a three-way catalytic converter. Catalytic converter 51 is located upstream of catalytic converter 52, and catalytic converter 52 is located downstream of catalytic converter 51. The three-way catalytic converter contains platinum group elements (PGMs) such as platinum (Pt), palladium (Pd), and rhodium (Rh), and purifies HC, CO, NOx, etc., in the exhaust gas. Basically, the three-way catalytic converter purifies (oxidizes) HC and CO when the air-fuel ratio of the exhaust gas is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio (lean), and purifies (reduces) NOx when the air-fuel ratio of the exhaust gas is near the stoichiometric air-fuel ratio or lower than the stoichiometric air-fuel ratio (rich). Catalytic converters 51 and 52 correspond to the "first catalytic converter" and "second catalytic converter" in this invention, respectively.

[0028] Furthermore, as shown in Figure 1, the engine exhaust purification device 100 includes an airflow sensor SW1, an intake air temperature sensor SW2, a water temperature sensor SW3, a crank angle sensor SW4, an accelerator opening sensor SW5, and linear A / F sensors SW6 and SW7.

[0029] The airflow sensor SW1 is located on the intake passage 40 downstream of the air cleaner 41 and detects the flow rate of air flowing through the intake passage 40. The intake air temperature sensor SW2 is located on the intake passage 40 downstream of the air cleaner 41 and detects the temperature of the air flowing through the intake passage 40. The water temperature sensor SW3 is located on the engine 1 and detects the temperature of the coolant in the engine 1. The crank angle sensor SW4 is located on the engine 1 and detects the rotation angle of the crankshaft 16. The accelerator opening sensor SW5 is located on the accelerator pedal mechanism 30 and detects the accelerator opening corresponding to the amount of accelerator pedal operation. The linear A / F sensor SW6 is located on the exhaust passage 50 upstream of the catalytic converter 51 and detects the air-fuel ratio of the exhaust gas flowing into the catalytic converter 51 (hereinafter referred to as the "first detected air-fuel ratio" as appropriate). The linear A / F sensor SW7 is installed in the exhaust passage 50 downstream of the catalytic converter 51 and detects the air-fuel ratio of the exhaust gas flowing out of the catalytic converter 51 (hereinafter referred to as the "second detected air-fuel ratio" as appropriate). The linear A / F sensors SW6 and SW7 output a signal (voltage or current signal) corresponding to the magnitude of the air-fuel ratio.

[0030] Next, with reference to Figure 2, the electrical configuration of the engine exhaust purification device 100 according to this embodiment will be described. Figure 2 is a block diagram showing the electrical configuration of the engine exhaust purification device 100 according to this embodiment.

[0031] As shown in Figure 2, the engine exhaust purification device 100 has a control device 60 configured to perform various controls on the device 100. The control device 60 is composed of a computer comprising one or more processors 60a (typically a CPU) and memory 60b such as ROM or RAM that stores various programs (including basic control programs such as an OS and application programs launched on the OS to realize specific functions) and various data interpreted and executed on the processors 60a. For example, the control device 60 is an ECU (Electronic Control Unit).

[0032] In addition to the sensors SW1 to SW7 mentioned above, the control device 60 is supplied with detection signals (output signals) from the ambient temperature sensor SW8, which detects the ambient temperature. Based on these detection signals, the control device 60 controls the fuel injector 18, spark plug 19, throttle valve 43, and other components of the engine 1. The control device 60 also controls the warning lights 70 to notify the vehicle occupants of any abnormalities.

[0033] In particular, in this embodiment, the control device 60 performs control (hereinafter referred to as "air-fuel ratio control") to set the air-fuel ratio of the exhaust gas to a target air-fuel ratio based on the air-fuel ratio detected by the linear A / F sensors SW6 and SW7. In this air-fuel ratio control, the control device 60 performs feedback control to the fuel injection valve 18. In addition, in this embodiment, the control device 60 performs processing to correct the target air-fuel ratio used in the air-fuel ratio control (hereinafter referred to as "air-fuel ratio correction processing"), and also performs processing (hereinafter referred to as "catalyst deterioration determination processing") necessary for this air-fuel ratio correction processing to determine the deterioration of the catalyst device 51. Details of these controls and processes will be described later.

[0034] [Air-fuel ratio control] Next, the air-fuel ratio control according to this embodiment will be described with reference to Figure 3. Figure 3 is a flowchart showing the air-fuel ratio control according to this embodiment. This flow is repeatedly executed by the control device 60 at a predetermined period. More specifically, the processor 60a in the control device 60 reads a program stored in the memory 60b and executes the program, thereby realizing the control related to this flow.

[0035] First, in step S10, the control device 60 acquires various information, including the detected values ​​from the sensors SW1 to SW8 (Figure 2) described above. The control device 60 also acquires the target air-fuel ratio for which the exhaust gas air-fuel ratio should be set in the air-fuel ratio control. This target air-fuel ratio is corrected in the air-fuel ratio correction process described later.

[0036] Next, in step S11, the control device 60 calculates the amount of air in the combustion chamber 17 based on the intake air flow rate detected by the airflow sensor SW1. Then, in step S12, the control device 60 calculates the amount of fuel that will result in the target air-fuel ratio at this amount of air, based on the target air-fuel ratio obtained in step S10 and the amount of air calculated in step S11, as the basic injection amount. Then, in step S13, the control device 60 calculates the target exhaust air-fuel ratio, which is the exhaust air-fuel ratio corresponding to the target air-fuel ratio (i.e., the exhaust air-fuel ratio at the position where the linear A / F sensor SW6 is installed).

[0037] Next, in step S14, the control device 60 calculates the deviation (first deviation) between the first detected air-fuel ratio detected by the linear A / F sensor SW6 and the target exhaust air-fuel ratio calculated in step S13. Then, in step S15, the control device 60 calculates the adjustment amount for the basic injection amount calculated in step S12 based on the first deviation calculated in step S14.

[0038] Next, in step S16, the control device 60 calculates the deviation (second deviation) between the second detected air-fuel ratio detected by the linear A / F sensor SW7 and the target air-fuel ratio obtained in step S10. Then, in step S17, the control device 60 corrects the adjustment amount of the basic injection amount calculated in step S15 based on the second deviation calculated in step S16. Then, in step S18, the control device 60 calculates the final injection amount by applying the adjustment amount corrected in step S17 to the basic injection amount.

[0039] Next, in step S19, the control device 60 controls the fuel injector 18 (fuel injection control) so that the final injection amount calculated in step S18 is injected from the fuel injector 18.

[0040] [Air-fuel ratio correction process] Next, in this embodiment, an air-fuel ratio correction process for correcting the target air-fuel ratio used in the air-fuel ratio control described above will be explained.

[0041] In this embodiment, if the catalytic converter 51 is not activated and is degraded, the control device 60 corrects the target air-fuel ratio to a leaner side than when the catalytic converter 51 is not degraded (i.e., when the catalytic converter 51 is functioning normally). In this case, the control device 60 sets the target air-fuel ratio to a leaner air-fuel ratio than the normal air-fuel ratio used when the catalytic converter 51 is activated and functioning normally (corresponding to the "first air-fuel ratio" in this invention, and hereinafter referred to as the "first lean air-fuel ratio"). The normal air-fuel ratio corresponds to the "third air-fuel ratio" in this invention and is set to the stoichiometric air-fuel ratio or a value close to the stoichiometric air-fuel ratio (for example, around 14.5).

[0042] The reason for correcting the target air-fuel ratio in this way is that when the catalytic converter 51 is inactive, if the catalytic converter 51 is degraded, the amount of ammonia produced from the catalytic converter 51, including the three-way catalyst, will be greater than when the catalytic converter 51 is functioning normally. This is thought to be because the palladium (Pd) contained in the precious metals of the catalytic converter 51 has degraded, changing the degree of influence of N2O production associated with the NO-H2 reaction. When the catalytic converter 51 is functioning normally (for example, immediately after manufacturing), when the catalytic converter 51 is inactive, the NO decomposition reaction of palladium proceeds, selectively producing N2O. This is thought to be because, when the catalytic converter 51 is inactive, HC and CO are hardly purified, so the contribution of the NO-H2 reaction is large. Therefore, when the catalytic converter 51 is functioning normally, almost no ammonia (NH3) is produced. In contrast, when the catalyst device 51 deteriorates, the temperature range in which NO is purified becomes higher (because thermal stress causes the precious metals (such as Pd) in the catalyst device 51 to condense and the active surface to shrink), and the contribution of rhodium (Rh) becomes larger in the inactive temperature range of the catalyst device 51, thus suppressing N2O production. Therefore, it can be said that when the catalyst device 51 is deteriorated, the amount of ammonia produced will be relatively higher.

[0043] Therefore, in this embodiment, when the catalyst 51 is inactive, the control device 60 sets the target air-fuel ratio to the leaner first lean air-fuel ratio, which is higher than when the catalyst 51 is functioning normally, if the catalyst 51 is degraded. This allows the increased oxygen produced by the catalyst 51 to be used to purify the ammonia in situations where the amount of ammonia produced by the catalyst 51 is high, thereby effectively suppressing the amount of ammonia produced.

[0044] Furthermore, in this embodiment, the control device 60 sets the target air-fuel ratio to a richer air-fuel ratio than the first lean air-fuel ratio and leaner than the normal air-fuel ratio (corresponding to the "second air-fuel ratio" in this invention, and hereinafter referred to as the "second lean air-fuel ratio") when the catalyst device 51 is active (for example, when the catalyst device 51 is active but degraded) and when the catalyst device 51 is normal (for example, when the catalyst device 51 is normal but inactive). By setting the target air-fuel ratio to a richer side than the first lean air-fuel ratio, that is, by bringing the target air-fuel ratio closer to the normal air-fuel ratio within the lean range, the NOx purification performance of the catalyst device 51 is ensured.

[0045] Furthermore, in this embodiment, when the catalytic converter 52 downstream of the catalytic converter 51 is activated, the control device 60 sets the target air-fuel ratio to the normal air-fuel ratio, that is, it returns the target air-fuel ratio, which was set to the lean side, back to the original normal air-fuel ratio. In this case, even if ammonia is generated from the catalytic converter 51, this ammonia can be purified by the catalytic converter 52 downstream.

[0046] Here, with reference to Figure 4, the basic concept of the air-fuel ratio correction process according to this embodiment will be explained in detail. In Figure 4, the horizontal axis shows the temperature of the catalyst device 51 (catalyst temperature), and the vertical axis shows the target air-fuel ratio. Note that the catalyst temperature used is, for example, the temperature estimated by the control device 60.

[0047] In Figure 4, catalyst temperature T1 corresponds to the temperature at which catalyst device 51 is activated, and catalyst temperature T2 (> catalyst temperature T1) corresponds to the temperature of catalyst device 51 when catalyst device 52, located downstream of catalyst device 51, is activated. When catalyst device 51 reaches catalyst temperature T2, which is higher than catalyst temperature T1, the downstream catalyst device 52 also becomes activated. Furthermore, region R1 below catalyst temperature T1 is the temperature range in which catalyst device 51 is inactive, regions R2 and R3 above catalyst temperature T1 are the temperature range in which catalyst device 51 is activated, and region R3 above catalyst temperature T2 is the temperature range in which catalyst device 52 is activated.

[0048] Furthermore, in Figure 4, graph G1 shows the target air-fuel ratio set when the catalytic converter 51 is degraded, and graph G2 shows the target air-fuel ratio set when the catalytic converter 51 is functioning normally. First, when the catalytic converter 51 is degraded, as shown in graph G1, the control device 60 sets the target air-fuel ratio to the first lean air-fuel ratio AF1 when the catalytic converter 51 is inactive (region R1), sets the target air-fuel ratio to the second lean air-fuel ratio AF2, which is richer than the first lean air-fuel ratio AF1, when the catalytic converter 51 is active but the catalytic converter 52 is inactive (region R2), and sets the target air-fuel ratio to the normal air-fuel ratio AF0, which is richer than the second lean air-fuel ratio AF2, when the catalytic converter 52 is active (region R3). In contrast, when the catalytic converter 51 is functioning normally, as shown in graph G2, the control device 60 sets the target air-fuel ratio to the second lean air-fuel ratio AF2 when the catalytic converter 51 is inactive (region R1) and when the catalytic converter 52 is inactive (region R2), and sets the target air-fuel ratio to the normal air-fuel ratio AF0 when the catalytic converter 52 is active (region R3).

[0049] As shown in Figure 4, the difference between the first lean air-fuel ratio AF1 and the second lean air-fuel ratio AF2 is greater than the difference between the second lean air-fuel ratio AF2 and the normal air-fuel ratio AF0. This allows the use of a target air-fuel ratio (first lean air-fuel ratio AF1) that is set relatively far lean when the catalytic converter 51 is inactive and degraded, thereby prioritizing the suppression of ammonia. On the other hand, when the catalytic converter 51 is active or functioning normally, a target air-fuel ratio (second lean air-fuel ratio AF2) that is set relatively close to the normal air-fuel ratio AF0 can be used, enabling the suppression of ammonia as well as ensuring NOx purification performance.

[0050] Next, with reference to Figure 5, a flowchart illustrating the air-fuel ratio correction process according to this embodiment will be described. This flow is repeatedly executed by the control device 60 at predetermined intervals. Specifically, the processor 60a within the control device 60 reads a program stored in the memory 60b and executes the program, thereby realizing the control related to this flow.

[0051] First, in step S20, the control device 60 acquires various information, including detected values ​​from sensors SW1 to SW8 (Figure 2) as described above. The control device 60 also acquires the temperature of the catalytic converter 51. The control device 60 estimates the temperature of the catalytic converter 51 based on the heat balance, taking into account the heat generated by the engine 1 (determined from the engine speed and load of the engine 1), the heat consumed in the exhaust passage 50 up to the catalytic converter 51, and the reaction heat in the catalytic converter 51. Furthermore, the control device 60 also acquires the temperature of the catalytic converter 52. In this case as well, the control device 60 estimates the temperature of the catalytic converter 52 based on the heat balance, in the same manner as the temperature of the catalytic converter 51. Alternatively, instead of estimating the temperatures of the catalytic converters 51 and 52, temperature sensors may be provided on the catalytic converters 51 and 52 to directly detect their temperatures.

[0052] Next, in step S21, the control device 60 performs a catalyst degradation determination process to determine the degradation of the catalyst device 51. Details of this catalyst degradation determination process will be described later (Figure 9). Based on the catalyst degradation determination process, the control device 60 determines in step S22 whether or not the catalyst device 51 is degraded. If the control device 60 determines that the catalyst device 51 is degraded (step S22: Yes), it proceeds to step S23. In this case, in step S23, the control device 60 corrects the target air-fuel ratio to the lean side. Specifically, the control device 60 sets the target air-fuel ratio to the first lean air-fuel ratio AF1.

[0053] Next, in step S24, the control device 60 determines whether or not the catalytic converter 51 is active. In this case, the control device 60 determines whether or not the catalytic converter 51 is active based on the estimated temperature of the catalytic converter 51. If the control device 60 determines that the catalytic converter 51 is active (step S24: Yes), it proceeds to step S25. In this case, in step S25, the control device 60 corrects the target air-fuel ratio to be richer than the first lean air-fuel ratio AF1. Specifically, the control device 60 sets the target air-fuel ratio to the second lean air-fuel ratio AF2. Then, the control device 60 proceeds to step S26. On the other hand, if the control device 60 does not determine that the catalytic converter 51 is active (step S24: No), that is, if the catalytic converter 51 is inactive, it returns to step S23. In this case, the control device 60 continues to set the target air-fuel ratio to the first lean air-fuel ratio AF1 until the catalytic converter 51 is activated.

[0054] Next, in step S26, the control device 60 determines whether the catalytic converter 52 downstream of the catalytic converter 51 is active or not. In this case, the control device 60 determines whether the catalytic converter 52 is active or not based on the estimated temperature of the catalytic converter 51 and / or the estimated temperature of the catalytic converter 52. If the control device 60 determines that the catalytic converter 52 is active (step S26: Yes), it proceeds to step S27. In this case, in step S27, the control device 60 sets the target air-fuel ratio to the normal air-fuel ratio AF0. On the other hand, if the control device 60 does not determine that the catalytic converter 52 is active (step S26: No), that is, if the catalytic converter 52 is inactive, it returns to step S25. In this case, the control device 60 continues to set the target air-fuel ratio to the second lean air-fuel ratio AF2 until the catalytic converter 52 is activated.

[0055] On the other hand, if the control device 60 does not determine in step S22 that the catalytic converter 51 is degraded (step S22: No), that is, if the catalytic converter 51 is functioning normally, the control device 60 proceeds to step S25. In this case, in step S25, the control device 60 sets the target air-fuel ratio to the second lean air-fuel ratio AF2. After this, the control device 60 performs the processes from step S26 onwards as described above.

[0056] Next, with reference to Figure 6, the time chart of the air-fuel ratio correction process according to this embodiment will be described. Figure 6 shows, from top to bottom, the on / off status of the degradation determination flag set when it is determined that the catalyst device 51 is degraded, the catalyst temperature of catalyst device 51, the catalyst temperature of catalyst device 52, and the target air-fuel ratio over time. Note that the catalyst temperature of catalyst device 51 and the catalyst temperature of catalyst device 52 are temperatures estimated by, for example, the control device 60.

[0057] First, at time t1, the control device 60 determines that the catalytic converter 51 is degraded and turns on the degradation determination flag. In this case, the control device 60 determines that the catalytic converter 51 is degraded and inactive, and sets the target air-fuel ratio to the first lean air-fuel ratio AF1. Subsequently, at time t2, the temperature of the catalytic converter 51 rises above the activation temperature, meaning the catalytic converter 51 becomes active. Therefore, at time t2, the control device 60 sets the target air-fuel ratio to the second lean air-fuel ratio AF2, which is richer than the first lean air-fuel ratio AF1. Subsequently, at time t3, the temperature of the catalytic converter 52 rises above the activation temperature, meaning the catalytic converter 52 becomes active. Therefore, at time t3, the control device 60 sets the target air-fuel ratio to the normal air-fuel ratio AF0, which is richer than the second lean air-fuel ratio AF2. In other words, the control device 60 returns the target air-fuel ratio to the normal air-fuel ratio AF0.

[0058] [Catalyst degradation detection process] Next, the catalyst degradation determination process according to this embodiment will be described. This catalyst degradation determination is performed in step S21 of the air-fuel ratio correction process flow shown in Figure 5.

[0059] In this embodiment, the control device 60 typically performs catalyst degradation determination processing when the engine 1 is started after soaking, i.e., when it is cold (assuming the catalyst device 51 is not in an active state). This is because the catalyst device 51 can be diagnosed accurately when the engine 1 is cold. The reason why it can be diagnosed accurately when cold is as follows.

[0060] When the engine is cold, the catalytic converter 51 gradually becomes active as the temperature rises, meaning that the exhaust gas purification performance (purification rate) gradually increases (light-off performance). At this time, if the catalytic converter 51 is functioning normally, the activation is quick, but if the catalytic converter 51 is degraded, the activation tends to be slow. This is because when the catalytic converter 51 deteriorates, the precious metals condense on the catalyst surface due to thermal stress, reducing the active surface area and decreasing the exhaust gas purification performance. Therefore, when the engine is cold, the difference depending on the degree of deterioration of the catalytic converter 51 is clearly evident.

[0061] In particular, the temperature at which the catalyst 51 is activated varies depending on the degree of deterioration of the catalyst 51. Specifically, when the catalyst 51 is functioning normally, it is activated at a relatively low temperature (thus resulting in a faster rise in exhaust gas purification performance), while when the catalyst 51 is deteriorated, it is not activated until a relatively high temperature is reached (thus resulting in a slower rise in exhaust gas purification performance). Therefore, in this embodiment, the control device 60 diagnoses the catalyst 51 based on the temperature at which the catalyst 51 reaches a predetermined level of activation (catalyst temperature) when it is cold.

[0062] Furthermore, in this embodiment, the control device 60 uses the air-fuel ratio of the exhaust gas downstream of the catalytic converter 51 to determine the activity level of the catalytic converter 51. This is because, when the catalytic converter 51 is inactive, the air-fuel ratio downstream of the catalytic converter 51 fluctuates relatively large, but as the catalytic converter 51 becomes active, the oxygen consumption in the catalytic converter 51 increases (because the oxidation-reduction efficiency of the catalytic converter 51 with respect to the exhaust gas increases), and the fluctuation in the air-fuel ratio downstream of the catalytic converter 51 becomes smaller. Therefore, it can be said that the activity level of the catalytic converter 51 can be accurately determined based on the fluctuation in the air-fuel ratio of the exhaust gas downstream of the catalytic converter 51.

[0063] Based on the above, in this embodiment, the control device 60 acquires the temperature of the catalytic converter 51 (catalyst temperature) when the engine 1 is cold, calculates the fluctuation range of the second detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW7, and diagnoses the catalytic converter 51 based on the relationship between this fluctuation range and the catalyst temperature. Specifically, the control device 60 determines the degree of activity of the catalytic converter 51 based on the fluctuation range of the second detected air-fuel ratio, and diagnoses the catalytic converter 51 based on the catalyst temperature when the catalytic converter 51 reaches a predetermined degree of activity. If the acquired catalyst temperature is above a predetermined temperature, the control device 60 determines that the catalytic converter 51 is degraded. This is because the rise of the exhaust gas purification performance of the catalytic converter 51 is slow.

[0064] Furthermore, in this embodiment, the control device 60 not only calculates the fluctuation range of the second detected air-fuel ratio (second fluctuation range) detected within a predetermined time by the linear A / F sensor SW7 provided downstream of the catalytic converter 51, but also calculates the fluctuation range of the first detected air-fuel ratio (first fluctuation range) detected within a predetermined time by the linear A / F sensor SW6 provided upstream of the catalytic converter 51, and calculates the fluctuation range ratio (first fluctuation range / second fluctuation range), which is the ratio of the first fluctuation range to the second fluctuation range, and determines the degree of activity of the catalytic converter 51 based on this fluctuation range ratio. Since the first fluctuation range upstream of the catalytic converter 51 is stable because it is not affected by the exhaust gas purification performance of the catalytic converter 51, in this embodiment, such a first fluctuation range is used as a reference to evaluate the magnitude of the second fluctuation range downstream of the catalytic converter 51.

[0065] In particular, in this embodiment, the control device 60 normalizes the above-mentioned fluctuation range ratio, and when the normalized fluctuation range ratio (hereinafter referred to as the "normalized fluctuation range ratio") reaches a predetermined value, it determines that the catalyst device 51 has reached a predetermined level of activity, and diagnoses the catalyst device 51 based on the catalyst temperature at that time. The fluctuation range ratio has no units and changes depending on various factors such as the amount of precious metal in the catalyst device 51, its durability, and its deterioration state, so in order to eliminate the influence of these factors and make it more versatile, the system uses a normalized value of the fluctuation range ratio (normalized fluctuation range ratio) for processing.

[0066] Here, with reference to Figures 7 and 8, the catalyst degradation determination process performed in this embodiment will be specifically described. Figure 7 is an explanatory diagram of the catalyst temperature obtained according to the normalized fluctuation range ratio in this embodiment, and Figure 8 is an explanatory diagram of the degradation determination based on the catalyst temperature obtained from Figure 7.

[0067] In Figure 7, the horizontal axis shows time, and the vertical axis shows the normalized fluctuation range ratio and catalyst temperature (the temperature of the catalyst device 51; the same applies hereinafter). Specifically, graph G11 shows an example of the time change of the normalized fluctuation range ratio when the catalyst device 51 is functioning normally, graph G12 shows an example of the time change of the normalized fluctuation range ratio when the catalyst device 51 is at a degradation level that does not require replacement (hereinafter referred to as the "first degradation level"), and graph G13 shows an example of the time change of the normalized fluctuation range ratio when the catalyst device 51 is at a degradation level that requires replacement (corresponding to a malfunction; hereinafter referred to as the "second degradation level"). The normalized fluctuation range ratio is expressed in the range of 0 to 1 by performing a normalization process in which the maximum value of the fluctuation range ratio obtained in a series of processes (corresponding to the value when the time change of the fluctuation range ratio becomes very small) is defined as 1. Also, graph G14 shows an example of the time change of catalyst temperature. For example, this catalyst temperature is the temperature estimated by the control device 60.

[0068] Graphs G11, G12, and G13 show that when the catalyst 51 is degraded, the rise in the normalized fluctuation range ratio is slower than when the catalyst 51 is functioning normally. This means that the rise in the exhaust gas purification performance of the catalyst 51 is slower, or in other words, the catalyst 51 is activated more slowly. The control device 60 uses the time change of catalyst temperature shown in graph G14 to obtain the catalyst temperature when the normalized fluctuation range ratio reaches a predetermined value. For example, in graphs G11, G12, and G13, the control device 60 obtains temperatures T11, T12, and T13 as the catalyst temperatures when the normalized fluctuation range ratio reaches a predetermined value (0.5). When the catalyst 51 is degraded, the obtained catalyst temperatures are higher than when the catalyst 51 is functioning normally (T13 > T12 > T11).

[0069] Next, in Figure 8, the horizontal axis shows the exhaust gas flow rate and the vertical axis shows the catalyst temperature. Specifically, graph G15 is a determination line defined by the exhaust gas flow rate and catalyst temperature that distinguishes between the normal state and the first degradation level of the catalyst device 51, and graph G16 is a determination line defined by the exhaust gas flow rate and catalyst temperature that distinguishes between the first degradation level and the second degradation level of the catalyst device 51. For example, when the control device 60 obtains a catalyst temperature T11, it determines that the catalyst device 51 is normal because T11 is below the determination line G15. When the catalyst temperature T12 is obtained, it determines that the catalyst device 51 is at the first degradation level because T12 is above the determination line G15 and below the determination line G16. When the catalyst temperature T13 is obtained, it determines that the catalyst device 51 is at the second degradation level because T13 is above the determination line G16.

[0070] Next, with reference to Figure 9, a flowchart illustrating the catalyst degradation determination process according to this embodiment will be described. This flowchart is repeatedly executed at predetermined intervals by the control device 60. Specifically, the processor 60a within the control device 60 reads a program stored in memory 60b and executes the program, thereby realizing the control related to this flowchart. Note that the flowchart shown in Figure 9 is assumed to be performed, for example, when the engine 1 is started.

[0071] First, in step S30, the control device 60 acquires various information, including detected values ​​from sensors SW1 to SW8 (Figure 2) as described above. Typically, the control device 60 acquires the first detected air-fuel ratio detected by the linear A / F sensor SW6, the second detected air-fuel ratio detected by the linear A / F sensor SW7, and the ambient temperature detected by the ambient temperature sensor SW8. The control device 60 also acquires the temperature of the catalytic converter 51 (catalyst temperature). For example, the control device 60 estimates the catalyst temperature. By comparing this catalyst temperature with the ambient temperature, it is possible to determine whether it is the time of starting the engine 1 after soaking. In other words, if the catalyst temperature and the ambient temperature are roughly the same at the start of the catalyst degradation determination process, it can be said that it is the time of starting the engine after soaking. Alternatively, instead of estimating the catalyst temperature, a temperature sensor may be provided on the catalytic converter 51 to directly detect the catalyst temperature.

[0072] Next, in step S31, the control device 60 determines whether the linear A / F sensor SW6 has been activated. For example, the control device 60 determines the activation of the linear A / F sensor SW6 based on the magnitude of its internal resistance (which indicates the temperature of the linear A / F sensor SW6). If, as a result of step S31, the control device 60 determines that the linear A / F sensor SW6 has been activated (step S31: Yes), it proceeds to step S32 and controls the fuel injector 18 to adjust the air-fuel ratio upstream of the catalytic converter 51 to the target air-fuel ratio. Conversely, if the control device 60 does not determine that the linear A / F sensor SW6 has been activated (step S31: No), it returns to step S31.

[0073] Next, in step S33, the control device 60 determines whether the linear A / F sensor SW7 has been activated. For example, the control device 60 determines the activation of the linear A / F sensor SW7 based on the magnitude of its internal resistance (which indicates the temperature of the linear A / F sensor SW7). If, as a result of step S33, the control device 60 determines that the linear A / F sensor SW7 has been activated (step S33: Yes), it proceeds to step S34 and controls the fuel injector 18 to adjust the air-fuel ratio downstream of the catalytic converter 51 to the target air-fuel ratio. Conversely, if the control device 60 does not determine that the linear A / F sensor SW7 has been activated (step S33: No), it returns to step S33.

[0074] Next, in step S35, the control device 60 calculates the first fluctuation range for the first detected air-fuel ratio from the maximum and minimum values ​​of the first detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW6 located upstream of the catalytic converter 51. At the same time, the control device 60 calculates the second fluctuation range for the second detected air-fuel ratio from the maximum and minimum values ​​of the second detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW7 located downstream of the catalytic converter 51. Then, the control device 60 proceeds to step S36 and calculates the fluctuation range ratio (first fluctuation range / second fluctuation range), which is the ratio between the first and second fluctuation ranges calculated in step S35. Then, the control device 60 proceeds to step S37 and performs a moving average processing on the fluctuation range ratio calculated in step S36 (the fluctuation range ratio used in subsequent processing will refer to the value after the moving average processing).

[0075] Next, in step S38, the control device 60 determines whether the fluctuation range ratio is stable or not. Here, the control device 60 determines whether the catalyst device 51 is active or not by checking whether the fluctuation range ratio is stable or not. For example, the control device 60 determines whether the fluctuation value of the fluctuation range ratio is less than a predetermined threshold or not. As a result of step S38, if the control device 60 determines that the fluctuation range ratio is stable (step S38: Yes), it proceeds to step S39, and if it does not determine that the fluctuation range ratio is stable (step S38: No), it returns to step S35.

[0076] Next, in step S39, the control device 60 determines whether the current situation is one in which the engine 1 (vehicle) has been started after being soaked. In this case, the control device 60 reads the soak time and determines whether the engine 1 has been soaked for a predetermined time or longer. This ensures that the catalyst degradation determination process is performed when the engine 1 and the catalyst device 51 have been sufficiently cooled before starting. If the control device 60 determines in step S39 that it is a start after soaking (step S39: Yes), it proceeds to step S40. If it does not determine that it is a start after soaking (step S39: No), it terminates the catalyst degradation determination process.

[0077] Next, in step S40, the control device 60 determines whether the deviation between the second detected air-fuel ratio detected by the linear A / F sensor SW7 and the target value is less than a threshold. Here again, the control device 60 determines whether the catalytic converter 51 is active by checking whether the second detected air-fuel ratio is stable. As a result of step S40, if the control device 60 determines that the deviation of the second detected air-fuel ratio is less than a threshold (step S40: Yes), it proceeds to step S41. If it does not determine that the deviation of the second detected air-fuel ratio is less than a threshold (step S40: No), it terminates the catalytic converter degradation determination process.

[0078] Next, in step S41, the control device 60 normalizes the fluctuation range ratios calculated so far. Specifically, the control device 60 sets the maximum value of the fluctuation range ratios calculated continuously from the start of the catalyst degradation determination process (corresponding to the value when the time change of the fluctuation range ratio becomes very small) to 1, and thus expresses all the fluctuation range ratios calculated continuously in the range of 0 to 1.

[0079] Next, in step S42, the control device 60 obtains the catalyst temperature when the normalized fluctuation range ratio reaches a predetermined value (for example, 0.5), based on the normalized fluctuation range ratio calculated in step S41 and the catalyst temperature estimated so far.

[0080] Next, in step S43, the control device 60 determines the degradation of the catalyst device 51 based on the catalyst temperature obtained in step S42. Specifically, the control device 60 determines that the catalyst device 51 is normal if the catalyst temperature is below the temperature defined by the determination line G15, determines that the catalyst device 51 is at the first degradation level if the catalyst temperature is above the temperature defined by the determination line G15 and below the temperature defined by the determination line G16, and determines that the catalyst device 51 is at the second degradation level if the catalyst temperature is above the temperature defined by the determination line G16.

[0081] Next, in step S44, the control device 60 performs control according to the determination result of step S44. Specifically, if the control device 60 diagnoses that the catalytic converter 51 is at the second deterioration level, it illuminates the warning light 70. Also, if the control device 60 diagnoses that the catalytic converter 51 is at the first deterioration level, it controls the fuel injector 18 so that when it later cuts off the engine 1's fuel supply, the amount of fuel added when it returns from the fuel cut is less than when the catalytic converter 51 is functioning normally. The reason for performing this control is as follows.

[0082] During fuel cut-off, only air is supplied to the catalytic converter 51, increasing its oxygen storage capacity (OSC). This increase in the catalytic converter 51 reduces its NOx purification performance after the fuel cut-off is restored. Therefore, the control device 60 temporarily increases the fuel amount to reduce the oxygen storage capacity of the catalytic converter 51 when the fuel cut-off is restored. When the catalytic converter 51 is functioning normally, it has a high oxygen storage capacity, so a larger fuel amount is needed. However, when the catalytic converter 51 is degraded, it has a low oxygen storage capacity, so a larger fuel amount is not needed. For this reason, the amount of fuel increased when the engine 1 restores from fuel cut-off is less when the catalytic converter 51 is degraded than when it is functioning normally.

[0083] In the embodiments described above, a catalyst degradation determination process capable of determining degradation with high accuracy was shown. However, the present invention is not limited to applying this catalyst degradation determination process, and various general catalyst degradation determination processes (for example, a method of determining degradation based on the oxygen storage amount of the catalyst device 51) may be applied.

[0084] [Mechanism of Action and Effects] Next, the operation and effects of the engine exhaust purification device 100 according to this embodiment will be described.

[0085] In this embodiment, the engine exhaust gas purification device 100 includes a fuel injection valve 18 for supplying fuel to the combustion chamber 17 of the engine 1, an exhaust passage 50 for discharging exhaust gas from the combustion chamber 17, a catalytic converter 51 provided on the exhaust passage 50 and containing at least palladium (Pd) and rhodium (Rh) as precious metals and configured to purify exhaust gas, a linear A / F sensor SW7 provided on the exhaust passage 50 downstream of the catalytic converter 51 and capable of detecting the air-fuel ratio of the exhaust gas, and a control device 60 configured to control the fuel injection valve 18 based on the air-fuel ratio detected by the linear A / F sensor SW7 in order to set the air-fuel ratio of the exhaust gas to a target air-fuel ratio. The control device 60 is configured to determine whether the catalytic converter 51 is active and whether the catalytic converter 51 is deteriorated, and if it is determined that the catalytic converter 51 is not active and is deteriorated, the target air-fuel ratio is set to a leaner first lean air-fuel ratio AF1 than when it is determined that the catalytic converter 51 is not deteriorated.

[0086] When the catalytic converter 51 is inactive and deteriorated, the amount of ammonia produced from the catalytic converter 51 tends to be higher than when the catalytic converter 51 is not deteriorated (i.e., in a normal state). Therefore, in this embodiment, when the catalytic converter 51 is inactive and deteriorated, the control device 60 corrects the target air-fuel ratio to the lean side, the first lean air-fuel ratio AF1. This makes it possible to effectively suppress ammonia production by utilizing the increased oxygen produced by setting the target air-fuel ratio to the lean side in situations where the amount of ammonia produced by the catalytic converter 51 is high.

[0087] Furthermore, in this embodiment, the control device 60 is configured to set the target air-fuel ratio to a second lean air-fuel ratio AF2, which is richer than the first lean air-fuel ratio AF1, when it determines that the catalyst device 51 is active. When the catalyst device 51 is active, the amount of ammonia produced tends to decrease, so the control device 60 sets the target air-fuel ratio to a richer side than the first lean air-fuel ratio AF1 within the lean range. This makes it possible to suppress ammonia emissions while ensuring NOx purification performance.

[0088] Furthermore, in this embodiment, the control device 60 is configured to set the target air-fuel ratio to a second lean air-fuel ratio AF2, which is richer than the first lean air-fuel ratio AF1, even when it determines that the catalytic converter 51 is not degraded. When the catalytic converter 51 is functioning normally, the amount of ammonia produced tends to be low, so the control device 60 sets the target air-fuel ratio to a richer side than the first lean air-fuel ratio AF1 within the lean range. This makes it possible to suppress ammonia emissions while ensuring NOx purification performance.

[0089] Furthermore, in this embodiment, the control device 60 determines whether the catalyst 52 downstream of the catalyst 51 is active or not. If it determines that the catalyst 52 is active, it sets the target air-fuel ratio to the normal air-fuel ratio AF0, which is richer than the second lean air-fuel ratio AF2. In this case, even if ammonia is generated from the catalyst 51, the ammonia can be purified by the active downstream catalyst 52, so the control device 60 sets the target air-fuel ratio to the normal air-fuel ratio. This makes it possible to effectively ensure NOx purification performance while suppressing ammonia emissions.

[0090] Furthermore, in this embodiment, the difference between the first lean air-fuel ratio AF1 and the second lean air-fuel ratio AF2 is greater than the difference between the second lean air-fuel ratio AF2 and the normal air-fuel ratio AF0. This allows the use of a target air-fuel ratio (first lean air-fuel ratio AF1) that is set relatively far lean when the catalytic converter 51 is inactive and degraded, thereby prioritizing the suppression of ammonia. On the other hand, when the catalytic converter 51 is active or functioning normally, as described above, a target air-fuel ratio (second lean air-fuel ratio AF2) that is set relatively close to the normal air-fuel ratio AF0 can be used, thereby effectively ensuring NOx purification performance while suppressing ammonia emissions.

[0091] Furthermore, in this embodiment, the control device 60 sets the target air-fuel ratio to the normal air-fuel ratio AF0 when it determines that the catalytic converter 51 is active and has not deteriorated. This allows the target air-fuel ratio to be returned to the normal air-fuel ratio AF0 when the catalytic converter 51 is active and functioning correctly, thereby ensuring the normal exhaust gas purification performance of the catalytic converter 51.

[0092] Furthermore, in this embodiment, the control device 60 determines the activity level of the catalytic converter 51 based on the fluctuation range of the air-fuel ratio detected within a predetermined time by the linear A / F sensor SW6, obtains the catalyst temperature when the catalytic converter 51 reaches a predetermined activity level based on the activity level determination result, and determines whether or not the catalytic converter 51 has deteriorated based on that temperature.

[0093] In this embodiment, the control device 60 uses the catalyst temperature to diagnose the catalyst 51 because the temperature at which the catalyst 51 becomes active (catalyst temperature) changes depending on the degree of deterioration of the catalyst 51. Furthermore, the control device 60 uses the fluctuation of the air-fuel ratio downstream of the catalyst 51 to determine the activity state of the catalyst 51. This is because when the catalyst 51 is not active, the air-fuel ratio downstream of the catalyst 51 fluctuates relatively large, but as the catalyst 51 becomes active, the oxygen consumption in the catalyst 51 increases, and the fluctuation of the air-fuel ratio downstream of the catalyst 51 becomes smaller. Therefore, the activity state of the catalyst 51 can be accurately determined based on such fluctuations in the air-fuel ratio. Thus, in this embodiment, the control device 60 determines the degree of activity of the catalyst 51 based on the fluctuation range of the air-fuel ratio downstream of the catalyst 51, and determines the deterioration of the catalyst 51 based on the catalyst temperature at which the catalyst 51 reaches a predetermined degree of activity. This significantly improves the accuracy of determining the deterioration of the catalyst 51.

[0094] Furthermore, in this embodiment, the control device 60 determines that the catalytic converter 51 is degraded if the acquired catalyst temperature is above a predetermined temperature. In this case, the control device 60 determines that the catalytic converter 51 is degraded because the exhaust gas purification performance of the catalytic converter 51 starts up slowly. This makes it possible to accurately determine the degradation of the catalytic converter 51. [Explanation of Symbols]

[0095] 1 Engine 13 cylinders 17 Combustion chamber 18 Fuel Injector 19 Spark plugs 40 Intake passage 50 Exhaust passage 51, 52 Catalytic converter 60 Control device 100 Engine exhaust purification system SW6, SW7 Linear A / F Sensors

Claims

1. An engine exhaust purification device, A fuel injection valve for supplying fuel to the combustion chamber of the engine, An exhaust passage for discharging exhaust gas from the combustion chamber, A catalytic converter provided on the exhaust passage, containing at least palladium and rhodium as precious metals, and configured to purify the exhaust gas, A linear A / F sensor is provided in the exhaust passage downstream of the catalytic converter and capable of detecting the air-fuel ratio of the exhaust gas, A control device configured to control the fuel injection valve based on the air-fuel ratio detected by the linear A / F sensor in order to set the air-fuel ratio of the exhaust gas to a target air-fuel ratio, It has, The control device is The catalyst device is determined to be active or not, and the catalyst device is determined to be degraded or not. If it is determined that the catalytic converter is not active and is also determined to be deteriorated, the target air-fuel ratio is set to the leaner first air-fuel ratio than when it is determined that the catalytic converter is not deteriorated. An engine exhaust purification device characterized by being configured in such a way.

2. The exhaust gas purification device for an engine according to claim 1, wherein the control device is configured to set the target air-fuel ratio to a second air-fuel ratio that is richer than the first air-fuel ratio when it determines that the catalytic converter is active.

3. The exhaust gas purification device for an engine according to claim 1, wherein the control device is configured to set the target air-fuel ratio to a second air-fuel ratio that is richer than the first air-fuel ratio when it determines that the catalytic converter has not deteriorated.

4. If the catalyst device is designated as the first catalyst device, the exhaust gas purification device of the engine further includes a second catalyst device provided in the exhaust passage downstream of the first catalyst device and configured to purify the exhaust gas. The control device is Determine whether the second catalyst device is active or not. If it is determined that the second catalytic converter is active, the target air-fuel ratio is set to a third air-fuel ratio that is richer than the second air-fuel ratio. An engine exhaust purification device according to claim 2 or 3, configured as described above.

5. The exhaust gas purification device for an engine according to claim 4, wherein the difference between the first air-fuel ratio and the second air-fuel ratio is greater than the difference between the second air-fuel ratio and the third air-fuel ratio.

6. The exhaust gas purification device for an engine according to claim 4, wherein the third air-fuel ratio is a normal air-fuel ratio set as the target air-fuel ratio when the control device determines that the catalytic converter is active and that the catalytic converter is not deteriorated.

7. The control device is Based on the range of fluctuation in the air-fuel ratio detected within a predetermined time by the linear A / F sensor, the activity level of the catalyst is determined. Based on the determination result of the activity level, the temperature of the catalyst device when it reaches a predetermined activity level is obtained, and based on that temperature, it is determined whether or not the catalyst device has deteriorated. An exhaust gas purification device for an engine according to claim 1, configured as described above.

8. The exhaust gas purification device for an engine according to claim 7, wherein the control device determines that the catalytic device is degraded if the temperature of the catalytic device is above a predetermined temperature when the catalytic device reaches the predetermined level of activity.

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