Engine exhaust purification system

Abstract

In an engine exhaust purification system 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, the system accurately learns the target air-fuel ratio. [Solution] The engine exhaust gas purification device 100 includes a fuel injection valve 18 for supplying fuel to a combustion chamber 17, an exhaust passage 50 for discharging exhaust gas from the combustion chamber 17, a catalytic converter 51 provided on the exhaust passage 50 for purifying 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 that controls 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 calculates the range of fluctuation of the air-fuel ratio detected by the linear A / F sensor SW7 within a predetermined time and corrects the target air-fuel ratio based on this range of fluctuation.

Description

Technical Field

[0001] The present invention relates to an exhaust gas purification device for an engine that has a catalytic 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] This type of technology is described in, for example, Patent Document 1. Patent Document 1 describes a technique in which a linear A / F sensor provided upstream of a catalytic device (catalytic converter) and a lambda O2 sensor provided downstream of the catalytic device are used, and based on the air-fuel ratio detected by these sensors, the fuel injection amount is feedback-controlled so that the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio. By suppressing the deviation between the target air-fuel ratio and the actual air-fuel ratio, the exhaust gas purification performance by the catalytic device is ensured.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Typically, the target air-fuel ratio used in the control (air-fuel ratio control) described above is set to the air-fuel ratio at which the catalytic converter's exhaust gas purification performance (purification rate) is maximized (hereinafter referred to as the "purification peak point" as appropriate. Note that this purification peak point does not represent a single point in the air-fuel ratio, but can have a certain range). Basically, the stoichiometric air-fuel ratio (or a value close to the stoichiometric air-fuel ratio) is applied as the purification peak point. However, this purification peak point tends to shift due to various disturbances, detection errors of sensors (such as linear A / F sensors and lambda O2 sensors), and variations in fuel injection valves. Therefore, in order to ensure sufficient exhaust gas purification performance by the catalytic converter, it is desirable to learn (correct) the target air-fuel ratio while taking into account the shift in the purification peak point.

[0005] Here, the inventors of the present invention, after diligent research, found a correlation between the air-fuel ratio of the exhaust gas downstream of the catalytic converter and the purification peak point. Specifically, they found that at the purification peak point, the oxygen consumption in the catalytic converter is maximized (because the oxidation-reduction efficiency of the catalytic converter for the exhaust gas is maximized), and the fluctuation range of the air-fuel ratio downstream of the catalytic converter becomes very small. From this, it can be said that the air-fuel ratio at which the fluctuation range of the air-fuel ratio downstream of the catalytic converter becomes very small corresponds to the purification peak point, and this air-fuel ratio should be applied to the target air-fuel ratio. Therefore, the inventors of the present invention considered learning (correcting) the target air-fuel ratio based on the fluctuation range of the air-fuel ratio downstream of the catalytic converter.

[0006] However, in the technology described in Patent Document 1, the air-fuel ratio downstream of the catalytic converter is detected by a lambda O2 sensor, but this lambda O2 sensor has a narrow detection range for the air-fuel ratio. Therefore, it is not possible to accurately detect the air-fuel ratio downstream of the catalytic converter. Consequently, in the technology described in Patent Document 1, it is not possible to accurately determine the fluctuation range of the air-fuel ratio downstream of the catalytic converter, and it is difficult to accurately learn (correct) the target air-fuel ratio.

[0007] It should be noted that lambda O2 sensors can basically only detect air-fuel ratios near the stoichiometric air-fuel ratio; in other words, they can only detect whether or not the exhaust gas air-fuel ratio deviates from the stoichiometric air-fuel ratio. In contrast, linear A / F sensors output a signal (voltage or current signal) corresponding to the magnitude of the air-fuel ratio, so their detection range for air-fuel ratios is significantly wider than that of lambda O2 sensors. Therefore, the inventors of this application considered using a linear A / F sensor to detect the air-fuel ratio downstream of the catalytic converter.

[0008] The present invention is based on the above findings and aims to accurately learn (correct) the target air-fuel ratio 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]

[0009] 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 for purifying 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 calculate the range of fluctuation of the air-fuel ratio detected by the linear A / F sensor within a predetermined time and to correct the target air-fuel ratio based on the range of fluctuation.

[0010] According to the present invention configured in this way, the target air-fuel ratio used in air-fuel ratio control is corrected (learned) based on the fluctuation range of the air-fuel ratio of the exhaust gas downstream of the catalytic converter, which is correlated with the air-fuel ratio at which the catalytic converter's exhaust gas purification performance is maximized (purification peak point). This makes it possible to determine the purification peak point based on the fluctuation range of the air-fuel ratio and correct the target air-fuel ratio toward this purification peak point. Therefore, according to the present invention, even if the purification peak point is shifted, it is possible to apply that purification peak point to the target air-fuel ratio through learning, thereby effectively ensuring the exhaust gas purification performance of the catalytic converter.

[0011] In the present invention, preferably, the control device is configured to correct the target air-fuel ratio so that the range of fluctuation is reduced. Since the fluctuation range becomes smaller at the purification peak point, by correcting the target air-fuel ratio to reduce the fluctuation range, as in the present invention described above, it becomes possible to accurately set the target air-fuel ratio to the purification peak point.

[0012] In the present invention, preferably, the control device is configured to correct the target air-fuel ratio to the rich side. According to the present invention configured in this manner, it is possible to prioritize the NOx purification performance of the catalytic device.

[0013] In the present invention, preferably, the linear A / F sensor is a second linear A / F sensor, the air-fuel ratio detected by the second linear A / F sensor is defined as the second air-fuel ratio, and the fluctuation range of the second air-fuel ratio detected by the second linear A / F sensor within a predetermined time is defined as the second fluctuation range. The engine exhaust gas purification device further includes a first linear A / F sensor provided in the exhaust passage upstream of the catalytic converter and capable of detecting the first air-fuel ratio of the exhaust gas. The control device is configured to calculate the first fluctuation range of the first air-fuel ratio detected by the first linear A / F sensor within a predetermined time, along with the second fluctuation range, calculate 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 correct the target air-fuel ratio based on the fluctuation range ratio. Since the first fluctuation range upstream of the catalytic converter is stable because it is not affected by the catalytic converter's exhaust gas purification performance, this invention uses this first fluctuation range as a reference to evaluate the magnitude of the second fluctuation range downstream of the catalytic converter. This ensures the learning accuracy of the target air-fuel ratio.

[0014] In the present invention, preferably, the control device is configured to correct the target air-fuel ratio when the fluctuation range ratio is less than a predetermined threshold, while not correcting the target air-fuel ratio when the fluctuation range ratio is equal to or greater than the threshold. With the present invention configured in this way, if the fluctuation range ratio (first fluctuation range / second fluctuation range) is less than the judgment threshold, the second fluctuation range downstream of the catalytic converter is large, so it can be determined that the currently applied target air-fuel ratio is deviating from the purification peak point, and the target air-fuel ratio can be corrected. On the other hand, if the fluctuation range ratio is equal to or greater than the judgment threshold, the second fluctuation range downstream of the catalytic converter is small, so it can be determined that the currently applied target air-fuel ratio is at the purification peak point, and the target air-fuel ratio can be left uncorrected.

[0015] In the present invention, preferably, the control device is configured to acquire the temperature of the catalyst device and to set the determination threshold to a smaller value as the temperature of the catalyst device decreases. In this configuration, when the catalytic converter temperature is low, the judgment threshold is set to a small value to make it difficult for the target air-fuel ratio to be corrected. This is because when the catalytic converter temperature is low, the exhaust gas purification performance of the catalytic converter is relatively low, so it is better not to correct the target air-fuel ratio. According to this invention, the learning accuracy of the target air-fuel ratio can be ensured.

[0016] In the present invention, preferably, the control device is configured such that in a first region where the temperature of the catalyst device is below a predetermined temperature, the rate of change of the judgment threshold for changes in the temperature of the catalyst device is greater than in a second region where the temperature of the catalyst device is above a predetermined temperature. In the present invention configured as described above, as the temperature of the catalyst device decreases, the determination threshold value is set to a smaller value. As a result, it becomes possible to effectively ensure the learning accuracy of the target air-fuel ratio.

[0017] In the present invention, preferably, when the engine is in steady operation, the control device corrects the target air-fuel ratio based on the fluctuation range, while when the engine is not in steady operation, the control device is configured not to correct the target air-fuel ratio based on the fluctuation range. In the present invention configured as described above, by correcting the target air-fuel ratio only when the engine is in steady operation, it is possible to ensure the learning accuracy of the target air-fuel ratio.

[0018] In the present invention, preferably, the control device acquires the temperature of the catalyst device, and when the temperature of the catalyst device is equal to or higher than a predetermined temperature, the control device corrects the target air-fuel ratio based on the fluctuation range, while when the temperature of the catalyst device is lower than the predetermined temperature, the control device is configured not to correct the target air-fuel ratio based on the fluctuation range. In the present invention configured as described above, by correcting the target air-fuel ratio only when the catalyst temperature is equal to or higher than a predetermined temperature (for example, when the catalyst device is active), it is possible to ensure the learning accuracy of the target air-fuel ratio.

Advantages of the Invention

[0019] 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 controls the air-fuel ratio of exhaust gas to be set to a target air-fuel ratio, the target air-fuel ratio can be accurately learned (corrected).

Brief Description of the Drawings

[0020] [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 learning control according to an embodiment of the present invention. [Figure 5] It is a flowchart showing air-fuel ratio learning control according to an embodiment of the present invention. [Figure 6] It is an explanatory diagram of a method for setting a determination threshold value used in air-fuel ratio learning control according to an embodiment of the present invention. [Figure 7] It is a time chart showing air-fuel ratio learning control according to an embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0021] Hereinafter, an engine exhaust purification device according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0022] [Device Configuration] First, referring to FIG. 1, the overall configuration of the engine exhaust purification device according to the present embodiment will be described. FIG. 1 is a schematic configuration diagram of the engine exhaust purification device according to the present embodiment.

[0023] The engine exhaust purification device 100 is mounted on a vehicle (not shown). As shown in FIG. 1, it mainly includes an engine 1 as an internal combustion engine that generates power (propulsion force) of the vehicle, an intake passage 40 that supplies air (intake air) to the engine 1, and an exhaust passage 50 that discharges exhaust gas from the engine 1.

[0024] The engine 1 is a four-stroke engine that performs an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke. The engine is a gasoline engine that uses gasoline as fuel. This fuel may be a liquid fuel containing at least gasoline, for example, gasoline containing bioethanol or the like.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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). Note that it is not limited to using two catalytic converters 51 and 52; at least catalytic converter 51 is sufficient, and catalytic converter 52 does not need to be located downstream of catalytic converter 51.

[0030] 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.

[0031] 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.

[0032] Linear A / F sensors SW6 and SW7 correspond to the "first linear A / F sensor" and "second linear A / F sensor" in the present invention, respectively, and the first and second detected air-fuel ratios correspond to the "first air-fuel ratio" and "second air-fuel ratio" in the present invention, respectively.

[0033] 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.

[0034] 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).

[0035] The control device 60 is supplied with detection signals (output signals) from at least the sensors SW1 to SW7 described above. Based on these detection signals, the control device 60 controls the fuel injector 18, spark plug 19, throttle valve 43, etc. of the engine 1. 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 injector 18. In addition, in this embodiment, the control device 60 performs control (hereinafter referred to as "air-fuel ratio learning control") to learn (correct) the target air-fuel ratio used in the air-fuel ratio control. Details of these controls will be described later.

[0036] [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.

[0037] First, in step S10, the control device 60 acquires various information, including the detected values ​​from the sensors SW1 to SW7 (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 learned (corrected) by the air-fuel ratio learning control described later.

[0038] 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).

[0039] 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.

[0040] 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.

[0041] 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.

[0042] [Air-fuel ratio learning control] Next, the air-fuel ratio learning control according to this embodiment will be described. First, the basic concept of the air-fuel ratio learning control according to this embodiment will be explained.

[0043] As described above, the target air-fuel ratio used in air-fuel ratio control is set to the air-fuel ratio (purification peak point) at which the exhaust gas purification performance of the catalytic converter 51 is maximized, in order to ensure the exhaust gas purification performance (purification rate) of the catalytic converter 51. Basically, the purification peak point corresponds to the stoichiometric air-fuel ratio (or a value close to the stoichiometric air-fuel ratio). However, this purification peak point tends to shift due to various disturbances, sensor detection errors, and variations in the fuel injection valve 18. Therefore, in order to ensure sufficient exhaust gas purification performance by the catalytic converter 51, it is desirable to learn (correct) the target air-fuel ratio while taking into account the shift in the purification peak point.

[0044] Here, the inventors of this invention, after diligent research, found a correlation between the air-fuel ratio of the exhaust gas downstream of the catalytic converter 51 and the purification peak point. Specifically, they found that at the purification peak point, the oxygen consumption in the catalytic converter 51 is maximized (because the oxidation-reduction efficiency of the catalytic converter 51 with respect to the exhaust gas is maximized), and the fluctuation range of the air-fuel ratio downstream of the catalytic converter 51 becomes very small. From this, it can be said that the air-fuel ratio at which the fluctuation range of the air-fuel ratio downstream of the catalytic converter 51 becomes very small corresponds to the purification peak point, and this air-fuel ratio should be applied to the target air-fuel ratio.

[0045] Therefore, the inventors of the present invention considered learning (correcting) the target air-fuel ratio based on the fluctuation range of the air-fuel ratio downstream of the catalytic converter 51. Furthermore, since the linear A / F sensor has a significantly wider detection range than the lambda O2 sensor, the inventors considered using this linear A / F sensor (i.e., the linear A / F sensor SW7) to detect the air-fuel ratio downstream of the catalytic converter 51.

[0046] Based on the above, in this embodiment, the control device 60 calculates the fluctuation range of the second detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW7 provided downstream of the catalytic converter 51, and corrects (learns) the target air-fuel ratio based on this fluctuation range. In particular, the control device 60 corrects the target air-fuel ratio so that the fluctuation range of the second detected air-fuel ratio becomes smaller. This is because if the fluctuation range is large, the target air-fuel ratio is out of the purification peak point, so in order to learn the target air-fuel ratio toward the purification peak point, the target air-fuel ratio should be corrected so that the fluctuation range becomes smaller.

[0047] Furthermore, in this embodiment, the control device 60 corrects the target air-fuel ratio to the richer side. Setting the air-fuel ratio to the richer side improves the NOx purification performance. Generally, exhaust gas regulations are particularly strict regarding NOx emissions. For this reason, in this embodiment, the target air-fuel ratio is corrected to the richer side to prioritize ensuring the NOx purification performance of the catalytic converter 51.

[0048] 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 located 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 located upstream of the catalytic converter 51, 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 corrects the target air-fuel ratio 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.

[0049] Furthermore, in this embodiment, the control device 60 corrects the target air-fuel ratio if the above-mentioned fluctuation range ratio is less than a predetermined judgment threshold, while not correcting the target air-fuel ratio if the fluctuation range ratio is equal to or greater than the judgment threshold. In other words, if the fluctuation range ratio (first fluctuation range / second fluctuation range) is less than the judgment threshold, the control device 60 determines that the second fluctuation range downstream of the catalytic converter 51 is large (in this case, the currently applied target air-fuel ratio is deviating from the purification peak point), and corrects the target air-fuel ratio. Conversely, if the fluctuation range ratio (first fluctuation range / second fluctuation range) is equal to or greater than the judgment threshold, the control device 60 determines that the second fluctuation range downstream of the catalytic converter 51 is small (in this case, the currently applied target air-fuel ratio can be said to be at the purification peak point), and does not correct the target air-fuel ratio.

[0050] Here, with reference to Figure 4, the basic concept of air-fuel ratio learning control according to this embodiment will be explained in detail. The upper part of Figure 4 shows the target air-fuel ratio, and the lower part of Figure 4 shows the fluctuation range ratio (first fluctuation range / second fluctuation range) described above. In particular, in the lower part of Figure 4, graphs G1 and G show the standard deviation of the fluctuation range ratio.

[0051] First, graph G1 shows a case where the target air-fuel ratio (hereinafter referred to as the "normal target air-fuel ratio" as appropriate; for example, this air-fuel ratio is approximately 14.5) is set to approximately the stoichiometric air-fuel ratio, and the fluctuation range ratio at this normal target air-fuel ratio is greater than or equal to the judgment threshold. In other words, this is the case where the normal target air-fuel ratio is the purification peak point. In this case, there is no need to correct the target air-fuel ratio. In contrast, graph G2, as can be seen when compared to graph G1, shows a case where the air-fuel ratio at which the fluctuation range ratio is greater than or equal to the judgment threshold is deviated from the normal target air-fuel ratio. In other words, in this case, the purification peak point is deviated from the normal target air-fuel ratio. Therefore, the control device 60 corrects the target air-fuel ratio toward the rich side (arrows A1, A2) in order to set the target air-fuel ratio to the air-fuel ratio at which the fluctuation range ratio is greater than or equal to the judgment threshold (purification peak point). For example, the control device 60 corrects the target air-fuel ratio toward the rich side along the straight line shown by the dashed line in Figure 4.

[0052] Furthermore, in this embodiment, as shown in Figure 4, when the control device 60 corrects the target air-fuel ratio, it corrects the target air-fuel ratio within the range of guard Gu, in other words, it prevents the target air-fuel ratio from being corrected beyond guard Gu. This suppresses the target air-fuel ratio from deviating significantly from the normal target air-fuel ratio (for example, deviating significantly towards the rich side).

[0053] Next, with reference to Figure 5, the air-fuel ratio learning control according to this embodiment will be described. Figure 5 is a flowchart showing the air-fuel ratio learning control according to this embodiment. This flow is also 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.

[0054] First, in step S20, the control device 60 acquires various information, including the detected values ​​detected by the sensors SW1 to SW7 (Figure 2) described above.

[0055] Next, in step S21, the control device 60 determines whether the conditions for performing air-fuel ratio learning have been met. This determination in step S21 is performed in order to perform air-fuel ratio learning with accuracy. Specifically, the control device 60 determines that the conditions for performing air-fuel ratio learning have been met when the engine 1 is operating in a steady state and the temperature of the catalytic converter 51 (hereinafter referred to as "catalyst temperature") is above a predetermined temperature. Specifically, the control device 60 determines that the engine 1 is operating in a steady state when the rotational speed and load of the engine 1 are approximately constant, or when the opening degree of the throttle valve 43 is approximately constant. The control device 60 also estimates the catalyst temperature and determines whether this catalyst temperature is above a predetermined temperature determined according to the activation temperature of the catalytic converter 51. For example, the control device 60 estimates the catalyst temperature based on the heat balance, taking into account the heat generated by the engine 1 (determined from the rotational 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. Alternatively, instead of estimating the catalyst temperature, a temperature sensor may be installed in the catalyst device 51 to directly detect the catalyst temperature.

[0056] If the control device 60 determines that the conditions for performing air-fuel ratio learning have been met (Step S21: Yes) as a result of step S21, it proceeds to step S22. If it does not determine that the conditions for performing air-fuel ratio learning have been met (Step S21: No), it terminates the air-fuel ratio learning control.

[0057] Next, in step S22, the control device 60 acquires the maximum and minimum values ​​of the first detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW6 provided upstream of the catalytic converter 51. Then, in step S23, the control device 60 calculates the first fluctuation range for the first detected air-fuel ratio by subtracting the maximum and minimum values ​​of the first detected air-fuel ratio acquired in step S22.

[0058] Next, in step S24, the control device 60 acquires 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, in step S25, the control device 60 calculates the second fluctuation range for the second detected air-fuel ratio by subtracting the maximum and minimum values ​​of the second detected air-fuel ratio acquired in step S24.

[0059] Next, in step S26, the control device 60 calculates a fluctuation range ratio (first fluctuation range / second fluctuation range), which is the ratio between the first fluctuation range calculated in step S23 and the second fluctuation range calculated in step S25.

[0060] Next, in step S27, the control device 60 sets a determination threshold for determining the fluctuation range ratio calculated in step S26. Specifically, the control device 60 sets the determination threshold based on the catalyst temperature estimated as described above.

[0061] Here, with reference to Figure 6, the method for setting the judgment threshold according to this embodiment will be explained. Figure 6 shows the catalyst temperature on the horizontal axis and the judgment threshold on the vertical axis. This Figure 6 shows a map (hereinafter referred to as the "judgment threshold map") which defines the judgment threshold to be set according to the catalyst temperature. As shown in Figure 6, the judgment threshold map is defined such that the judgment threshold becomes smaller as the catalyst temperature decreases. More specifically, the judgment threshold map is defined such that in the first region R1, where the catalyst temperature is below a predetermined temperature T1, the rate of change of the judgment threshold with respect to the change in catalyst temperature is larger than in the second region R2, where the catalyst temperature is above the predetermined temperature T1. Furthermore, the predetermined temperature T1 that defines these first and second regions R1 and R2 is set in advance according to the activation temperature of the catalyst device 51.

[0062] According to this type of judgment threshold map, the lower the catalyst temperature, the smaller the judgment threshold becomes, and the less likely the fluctuation range ratio is to fall below the judgment threshold (because the fluctuation range ratio is more likely to be above the judgment threshold), thus making it more difficult to learn the target air-fuel ratio. This is because, at low catalyst temperatures, the exhaust gas purification performance of the catalytic converter 51 decreases, making it desirable not to perform air-fuel ratio learning.

[0063] Returning to Figure 5, the process from step S28 onward will be explained. In step S28, the control device 60 determines whether the fluctuation range ratio calculated in step S26 is less than the judgment threshold set in step S27.

[0064] If, as a result of step S28, the control device 60 determines that the fluctuation range ratio is less than the judgment threshold (step S28: Yes), it corrects the target air-fuel ratio because the second fluctuation range downstream of the catalytic converter 51 is large, meaning the currently applied target air-fuel ratio is deviating from the purification peak point. Conversely, if the control device 60 does not determine that the fluctuation range ratio is less than the judgment threshold (step S28: No), meaning that the fluctuation range ratio is greater than or equal to the judgment threshold, it terminates the air-fuel ratio learning control. In this case, typically, the second fluctuation range downstream of the catalytic converter 51 is small, so the currently applied target air-fuel ratio is at the purification peak point. Therefore, the control device 60 does not correct the target air-fuel ratio.

[0065] Next, after step S29, the control device 60 proceeds to step S30 to determine whether the fluctuation range ratio is greater than or equal to a determination threshold. Here, before making the determination in step S30, the control device 60 applies the target air-fuel ratio corrected in step S29 and performs air-fuel ratio control (Figure 3). Then, the control device 60 calculates a new fluctuation range ratio from the first detected air-fuel ratio and the second detected air-fuel ratio obtained during this air-fuel ratio control (steps S22 to S26 in Figure 5), and determines this fluctuation range ratio in step S30.

[0066] If, as a result of step S30, the control device 60 determines that the fluctuation range ratio is greater than or equal to the judgment threshold (step S30: Yes), then because the second fluctuation range downstream of the catalytic converter 51 is small, meaning that the currently applied target air-fuel ratio is at the purification peak point, the control device terminates the air-fuel ratio learning control without correcting the target air-fuel ratio.

[0067] In response to this, if the control device 60 does not determine that the fluctuation range ratio is greater than or equal to the judgment threshold (step S30: No), that is, if the fluctuation range ratio is less than the judgment threshold, it returns to step S29. In this case, the control device 60 corrects the target air-fuel ratio in step S29 because the second fluctuation range downstream of the catalytic converter 51 is still large, that is, the currently applied target air-fuel ratio is still deviating from the purification peak point. In this way, the control device 60 continues to correct the target air-fuel ratio by repeating steps S29 and S30 until the fluctuation range ratio falls below the judgment threshold.

[0068] Next, the flow of air-fuel ratio learning control according to this embodiment will be explained with reference to Figure 7. Figure 7 is an example of a time chart showing air-fuel ratio learning control according to this embodiment. From top to bottom, Figure 7 shows the time changes of the maximum and minimum values ​​of the first detected air-fuel ratio, the maximum and minimum values ​​of the second detected air-fuel ratio, the fluctuation range ratio, and the target air-fuel ratio.

[0069] First, at time t1, the above-described conditions for air-fuel ratio learning (including the condition that engine 1 is operating in a steady state and that the catalyst temperature is above a predetermined temperature) are met. Therefore, from time t1, the control device 60 acquires the maximum and minimum values ​​of the first detected air-fuel ratio detected by the linear A / F sensor SW6 located upstream of the catalytic converter 51, and also acquires the maximum and minimum values ​​of the second detected air-fuel ratio detected by the linear A / F sensor SW7 located downstream of the catalytic converter 51. Then, the control device 60 calculates the first fluctuation range from the maximum and minimum values ​​of the first detected air-fuel ratio, and calculates the second fluctuation range from the maximum and minimum values ​​of the second detected air-fuel ratio, and calculates the fluctuation range ratio (first fluctuation range / second fluctuation range) from these first and second fluctuation ranges.

[0070] Then, at time t2, the number of samples of the calculated fluctuation range ratio becomes greater than or equal to a predetermined value (a timer may be used instead of this number of samples), so the control device 60 starts comparing the fluctuation range ratio with the judgment threshold. As a result, at time t2 when the comparison process started, the control device 60 finds that the fluctuation range ratio is less than the judgment threshold, so it starts correcting the target air-fuel ratio. In this case, the control device 60 corrects the target air-fuel ratio to the richer side from the normal target air-fuel ratio.

[0071] The control device 60 continues to gradually correct the target air-fuel ratio toward the rich side from time t2 to time t3 because the fluctuation range ratio is below the judgment threshold from time t2 to time t3. When the control device 60 performs such correction of the target air-fuel ratio, that is, when performing air-fuel ratio learning control, it applies the corrected target air-fuel ratio in parallel with this air-fuel ratio learning control to perform air-fuel ratio control. As a result, the target air-fuel ratio gradually approaches the purification peak point, and the calculated fluctuation range ratio gradually approaches the judgment threshold.

[0072] Then, at time t3, the fluctuation range ratio becomes greater than or equal to the judgment threshold, so the control device 60 terminates the correction of the target air-fuel ratio at this time t3. In this case, the control device 60 fixes the target air-fuel ratio applied in the air-fuel ratio control to the target air-fuel ratio that has been corrected so far, that is, the target air-fuel ratio that was applied at time t3.

[0073] [Mechanism of Action and Effects] Next, the operation and effects of the engine exhaust purification device 100 according to this embodiment will be described.

[0074] 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 for purifying the 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 calculate the range of fluctuation of the air-fuel ratio detected by the linear A / F sensor SW7 within a predetermined time and to correct the target air-fuel ratio based on this range of fluctuation.

[0075] In this embodiment, the target air-fuel ratio used in air-fuel ratio control is corrected (learned) based on the fluctuation range of the air-fuel ratio of the exhaust gas downstream of the catalytic converter 51, which is correlated with the air-fuel ratio at which the exhaust gas purification performance of the catalytic converter 51 is maximized (purification peak point). This makes it possible to determine the purification peak point based on the fluctuation range of the air-fuel ratio and correct the target air-fuel ratio toward this purification peak point. Therefore, according to this embodiment, even if the purification peak point is shifted, it is possible to apply that purification peak point to the target air-fuel ratio through learning, thereby ensuring the exhaust gas purification performance of the catalytic converter 51.

[0076] Furthermore, according to this embodiment, the control device 60 is configured to correct the target air-fuel ratio so that the fluctuation range is reduced. Since the fluctuation range is reduced at the purification peak point, correcting the target air-fuel ratio so that the fluctuation range is reduced makes it possible to accurately set the target air-fuel ratio to the purification peak point.

[0077] Furthermore, according to this embodiment, the control device 60 is configured to correct the target air-fuel ratio to the rich side. This makes it possible to prioritize the NOx purification performance of the catalytic converter 51.

[0078] Furthermore, according to this embodiment, the engine exhaust gas purification device 100 is further provided on the exhaust passage 50 upstream of the catalytic converter 51 and has a linear A / F sensor SW6 capable of detecting the air-fuel ratio of the exhaust gas (first detected air-fuel ratio). The control device 60 calculates the first fluctuation range of the first detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW6, along with the second fluctuation range of the second detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW7. The control device 60 calculates the fluctuation range ratio, which is the ratio of the first fluctuation range to the second fluctuation range, and corrects the target air-fuel ratio based on the 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. This ensures the learning accuracy of the target air-fuel ratio.

[0079] Furthermore, according to this embodiment, the control device 60 is configured to correct the target air-fuel ratio when the fluctuation range ratio is less than a predetermined threshold, while not correcting the target air-fuel ratio when the fluctuation range ratio is equal to or greater than the threshold. As a result, when the fluctuation range ratio is less than the threshold, the second fluctuation range downstream of the catalytic converter 51 is large, so it can be determined that the currently applied target air-fuel ratio is deviating from the purification peak point, and the target air-fuel ratio can be corrected. On the other hand, when the fluctuation range ratio is equal to or greater than the threshold, the second fluctuation range downstream of the catalytic converter 51 is small, so it can be determined that the currently applied target air-fuel ratio is at the purification peak point, and the target air-fuel ratio can be not corrected. In other words, unnecessary correction of the target air-fuel ratio can be suppressed.

[0080] Furthermore, according to this embodiment, the control device 60 acquires (for example, estimates) the catalyst temperature, and is configured to set the judgment threshold to a smaller value as the catalyst temperature decreases. In other words, when the catalyst temperature is low, the judgment threshold is set to a smaller value to make it less likely for the target air-fuel ratio to be corrected. This is because when the catalyst temperature is low, the exhaust gas purification performance of the catalytic converter is relatively low, so it is better not to correct the target air-fuel ratio. According to this embodiment, the learning accuracy of the target air-fuel ratio can be ensured.

[0081] Furthermore, according to this embodiment, the control device 60 is configured such that in the first region R1, where the catalyst temperature is below a predetermined temperature T1, the rate of change of the judgment threshold in response to changes in catalyst temperature is greater than in the second region R2, where the catalyst temperature is above a predetermined temperature T1. As a result, the judgment threshold is set to a smaller value as the catalyst temperature decreases. This makes it possible to effectively ensure the learning accuracy of the target air-fuel ratio.

[0082] Furthermore, according to this embodiment, the control device 60 is configured to correct the target air-fuel ratio based on the fluctuation range when the engine 1 is operating in a steady state, but not to correct the target air-fuel ratio based on the fluctuation range when the engine 1 is not operating in a steady state. As a result, by correcting the target air-fuel ratio only when the engine 1 is operating in a steady state, the learning accuracy of the target air-fuel ratio can be ensured.

[0083] Furthermore, according to this embodiment, the control device 60 is configured to correct the target air-fuel ratio based on the fluctuation range when the catalyst temperature is above a predetermined temperature, but not when the catalyst temperature is below the predetermined temperature. This ensures that the learning accuracy of the target air-fuel ratio is ensured by correcting the target air-fuel ratio only when the catalyst temperature is above a predetermined temperature (typically when the catalytic converter 51 is active). [Explanation of symbols]

[0084] 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 is provided on the exhaust passage for purifying 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 engine exhaust gas purification device is characterized in that the control device is configured to calculate the range of fluctuation of the air-fuel ratio detected by the linear A / F sensor within a predetermined time, and to correct the target air-fuel ratio based on the range of fluctuation.

2. The exhaust gas purification device for an engine according to claim 1, wherein the control device is configured to correct the target air-fuel ratio so that the fluctuation range is reduced.

3. The exhaust gas purification device for an engine according to claim 1, wherein the control device is configured to correct the target air-fuel ratio to the rich side.

4. If the linear A / F sensor is designated as a second linear A / F sensor, the air-fuel ratio detected by the second linear A / F sensor is designated as the second air-fuel ratio, and the fluctuation range of the second air-fuel ratio detected by the second linear A / F sensor within the predetermined time is designated as the second fluctuation range, then the exhaust gas purification device of the engine further includes a first linear A / F sensor provided on the exhaust passage upstream of the catalytic converter, which is capable of detecting the first air-fuel ratio of the exhaust gas. The control device is Along with the second fluctuation range, the first fluctuation range of the first air-fuel ratio detected by the first linear A / F sensor within the predetermined time is calculated. The variation range ratio, which is the ratio of the first variation range to the second variation range, is calculated. The target air-fuel ratio is corrected based on the aforementioned fluctuation range ratio. An exhaust gas purification device for an engine according to claim 1, configured as described above.

5. The exhaust gas purification device for an engine according to claim 4, wherein the control device is configured to correct the target air-fuel ratio when the fluctuation range ratio is less than a predetermined determination threshold, and not to correct the target air-fuel ratio when the fluctuation range ratio is equal to or greater than the determination threshold.

6. The exhaust gas purification device for an engine according to claim 5, wherein the control device is configured to acquire the temperature of the catalyst device and to set the determination threshold to a smaller value as the temperature of the catalyst device decreases.

7. The exhaust gas purification device for an engine according to claim 6, wherein the control device is configured such that in a first region where the temperature of the catalyst is below a predetermined temperature, the rate of change of the determination threshold with respect to a change in the temperature of the catalyst is greater than in a second region where the temperature of the catalyst is above the predetermined temperature.

8. The exhaust gas purification device for an engine according to claim 1, wherein the control device is configured to correct the target air-fuel ratio based on the fluctuation range when the engine is in steady operation, but does not correct the target air-fuel ratio based on the fluctuation range when the engine is not in steady operation.

9. The exhaust gas purification device for an engine according to claim 1, wherein the control device is configured to acquire the temperature of the catalytic converter and, if the temperature of the catalytic converter is above a predetermined temperature, to correct the target air-fuel ratio based on the fluctuation range, while not correcting the target air-fuel ratio based on the fluctuation range if the temperature of the catalytic converter is below the predetermined temperature.

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