JP2015222052A - Internal combustion engine control unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To keep small the sulfur component occlusion quantity of an exhaust purification catalyst.SOLUTION: An internal combustion engine includes: an exhaust purification catalyst 20; a downstream air-fuel ratio sensor 41 disposed downstream of the exhaust purification catalyst 20; and temperature detection means 46 detecting a temperature of the exhaust purification catalyst 20. A control unit executes a feedback control so that an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst 20 is equal to a target air-fuel ratio. The target air-fuel ratio is alternately set to a rich air-fuel ratio and a lean air-fuel ratio, and a rich degree and a lean degree of the target air-fuel ratio while the target air-fuel ratio is set to the rich air-fuel ratio and to the lean air-fuel ratio, respectively are reduced. A variation difference obtained by subtracting a difference between the rich air-fuel ratio after a rich degree changing period and a stoichiometric air-fuel ratio from a difference between the lean air-fuel ratio after lean degree changing period and the stoichiometric air-fuel ratio if the temperature of the exhaust purification catalyst 20 detected by the temperature detection means 46 is equal to or lower than a preset upper limit temperature is set greater than a variation difference if the temperature is higher than the upper limit temperature.

Description

  The present invention relates to a control device for an internal combustion engine.

  2. Description of the Related Art Conventionally, a control device for an internal combustion engine in which an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine and the amount of fuel supplied to the internal combustion engine is controlled based on the output of the air-fuel ratio sensor is widely known. As such a control device, an air-fuel ratio sensor provided on the upstream side of an exhaust purification catalyst provided in an engine exhaust passage and an oxygen sensor provided on the downstream side are known (for example, Patent Documents 1 to 4). ).

  In particular, in the control device described in Patent Document 1, the amount of fuel supplied to the internal combustion engine is controlled according to the air-fuel ratio detected by the upstream air-fuel ratio sensor so that the air-fuel ratio becomes the target air-fuel ratio. I am doing so. In addition, the target air-fuel ratio is corrected according to the oxygen concentration detected by the downstream oxygen sensor. According to Patent Document 1, this enables the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to be matched with the target value even if the upstream air-fuel ratio sensor or the like has aged deterioration or solid variation. It is supposed to be.

JP-A-8-232723 JP 2005-163614 A JP 2006-183636 A JP-A-6-307271 Japanese Patent Laid-Open No. 62-126234

  By the way, according to the inventors of the present application, a control device that performs control different from the control device described in Patent Document 1 has been proposed. In this control apparatus, when the air-fuel ratio detected by the downstream side air-fuel ratio sensor becomes equal to or less than the rich determination air-fuel ratio (the air-fuel ratio slightly richer than the stoichiometric air-fuel ratio), the target air-fuel ratio is leaner than the stoichiometric air-fuel ratio. The air / fuel ratio is set to a low air / fuel ratio (hereinafter referred to as “lean air / fuel ratio”). In addition, the lean degree is once reduced while the target air-fuel ratio is set to the lean air-fuel ratio. On the other hand, when the air-fuel ratio detected by the downstream air-fuel ratio sensor becomes equal to or higher than the lean determination air-fuel ratio (an air-fuel ratio slightly leaner than the stoichiometric air-fuel ratio), the target air-fuel ratio is richer than the stoichiometric air-fuel ratio. (Hereinafter referred to as “rich air-fuel ratio”). In addition, the rich degree is once reduced while the target air-fuel ratio is set to the rich air-fuel ratio. That is, in this control device, the target air-fuel ratio is switched alternately between the rich air-fuel ratio and the lean air-fuel ratio.

  As described above, when the control for alternately switching the target air-fuel ratio between the rich air-fuel ratio and the lean air-fuel ratio is performed, oxygen is absorbed and released in the exhaust purification catalyst. Here, when the oxygen storage amount of the exhaust purification catalyst reaches the maximum storable oxygen amount, the exhaust purification catalyst can no longer store oxygen. For this reason, oxygen and NOx flow out from the exhaust purification catalyst. Further, when the oxygen storage amount of the exhaust purification catalyst reaches zero, the exhaust purification catalyst can no longer purify unburned gas. For this reason, unburned gas flows out from the exhaust purification catalyst. Therefore, in order to suppress the outflow of NOx and unburned gas from the exhaust purification catalyst, the maximum amount of storable oxygen of the exhaust purification catalyst is maintained, and the oxygen storage amount of the exhaust purification catalyst is reduced to the maximum storable oxygen amount or zero. It is necessary to reduce the frequency of reaching.

  By the way, the exhaust gas discharged from the engine body contains sulfur components such as SOx. When such a sulfur component is stored in the exhaust purification catalyst, the maximum storable oxygen amount of the exhaust purification catalyst is reduced by that amount. Therefore, from the viewpoint of keeping the maximum storable oxygen amount of the exhaust purification catalyst high, it is necessary to keep the sulfur component storage amount of the exhaust purification catalyst low.

  Therefore, in view of the above problems, an object of the present invention is to provide a sulfur component occlusion amount of an exhaust purification catalyst in a control device for an internal combustion engine that performs control to alternately switch a target air-fuel ratio between a rich air-fuel ratio and a lean air-fuel ratio. Is to keep it low.

  In order to solve the above-described problem, in the first invention, an exhaust purification catalyst that is disposed in an exhaust passage of an internal combustion engine and that can store oxygen, a downstream of the exhaust purification catalyst in the exhaust flow direction, and the above-described In the control device for an internal combustion engine, comprising: a downstream air-fuel ratio sensor that detects an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst; and a temperature detection means that detects or estimates the temperature of the exhaust purification catalyst. The feedback control is performed so that the air-fuel ratio of the exhaust gas flowing into the exhaust gas becomes the target air-fuel ratio, and the exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor has become less than the rich determination air-fuel ratio that is richer than the stoichiometric air-fuel ratio. Sometimes, the target air-fuel ratio is switched to a lean set air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, and after the target air-fuel ratio is set to the lean set air-fuel ratio, The exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor is lean-determined from the lean degree change timing before the exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor becomes more than the lean determination air-fuel ratio leaner than the stoichiometric air-fuel ratio. The target air-fuel ratio is set to a lean air-fuel ratio with a lean degree smaller than the lean set air-fuel ratio until the air-fuel ratio becomes equal to or higher than the air-fuel ratio, and the exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor is equal to or higher than the lean determination air-fuel ratio. The target air-fuel ratio is switched to a rich set air-fuel ratio richer than the stoichiometric air-fuel ratio, and after the target air-fuel ratio is set to the rich set air-fuel ratio, the downstream air-fuel ratio sensor Exhaust air detected by the downstream air-fuel ratio sensor from the rich degree change timing before the detected exhaust air-fuel ratio becomes equal to or less than the rich determination air-fuel ratio. The target air-fuel ratio is set to a rich air-fuel ratio that is less rich than the rich set air-fuel ratio until the ratio becomes equal to or less than the rich determination air-fuel ratio, and the temperature of the exhaust purification catalyst detected or estimated by the temperature detecting means Is less than or equal to a predetermined upper limit temperature, compared to when it is higher than the upper limit temperature, the difference between the average value of the lean air-fuel ratio after the lean degree change timing and the stoichiometric air-fuel ratio, and after the rich degree change timing A control device for an internal combustion engine is provided in which the difference in fluctuation obtained by subtracting the difference between the average value of the rich air-fuel ratio and the stoichiometric air-fuel ratio is increased.

  According to the present invention, the sulfur component storage amount of the exhaust purification catalyst can be kept low.

FIG. 1 is a diagram schematically showing an internal combustion engine in which a control device of the present invention is used. FIG. 2 is a graph showing the relationship between the oxygen storage amount of the exhaust purification catalyst and the NOx concentration or HC, CO concentration in the exhaust gas flowing out from the exhaust purification catalyst. FIG. 3 is a diagram showing the relationship between the sensor applied voltage and the output current at each exhaust air-fuel ratio. FIG. 4 is a diagram showing the relationship between the exhaust air-fuel ratio and the output current when the sensor applied voltage is made constant. FIG. 5 is a time chart of the target air-fuel ratio and the like when basic air-fuel ratio control is performed by the control device for an internal combustion engine according to the present embodiment. FIG. 6 is a time chart of the target air-fuel ratio and the like when the weak lean set air-fuel ratio change control is performed in the present embodiment. FIG. 7 is a flowchart showing a control routine in the target air-fuel ratio setting control. FIG. 8 is a flowchart showing a control routine for change control of the weak rich set air-fuel ratio. FIG. 9 is a graph showing the Cmax ratio with respect to the rich time ratio in one cycle. FIG. 10 is a time chart of the target air-fuel ratio and the like when changing control of the lean set air-fuel ratio and the weak lean set air-fuel ratio is performed. FIG. 11 is a time chart of the target air-fuel ratio and the like when changing control of the weak rich set air-fuel ratio and the weak lean set air-fuel ratio is performed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals are assigned to similar components.

<Description of the internal combustion engine as a whole>
FIG. 1 is a diagram schematically showing an internal combustion engine in which a control device of the present invention is used. In FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a piston that reciprocates in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, and 5 is between the piston 3 and the cylinder head 4. , 6 is an intake valve, 7 is an intake port, 8 is an exhaust valve, and 9 is an exhaust port. The intake valve 6 opens and closes the intake port 7, and the exhaust valve 8 opens and closes the exhaust port 9.

  As shown in FIG. 1, a spark plug 10 is disposed at the center of the inner wall surface of the cylinder head 4, and a fuel injection valve 11 is disposed around the inner wall surface of the cylinder head 4. The spark plug 10 is configured to generate a spark in response to the ignition signal. The fuel injection valve 11 injects a predetermined amount of fuel into the combustion chamber 5 according to the injection signal. The fuel injection valve 11 may be arranged so as to inject fuel into the intake port 7. In this embodiment, gasoline having a theoretical air-fuel ratio of 14.6 is used as the fuel. However, the internal combustion engine of the present embodiment may use other fuels.

  The intake port 7 of each cylinder is connected to a surge tank 14 via a corresponding intake branch pipe 13, and the surge tank 14 is connected to an air cleaner 16 via an intake pipe 15. The intake port 7, the intake branch pipe 13, the surge tank 14, and the intake pipe 15 form an intake passage. A throttle valve 18 driven by a throttle valve drive actuator 17 is disposed in the intake pipe 15. The throttle valve 18 is rotated by a throttle valve drive actuator 17 so that the opening area of the intake passage can be changed.

  On the other hand, the exhaust port 9 of each cylinder is connected to an exhaust manifold 19. The exhaust manifold 19 has a plurality of branches connected to the exhaust ports 9 and a collective part in which these branches are assembled. A collecting portion of the exhaust manifold 19 is connected to an upstream casing 21 containing an upstream exhaust purification catalyst 20. The upstream casing 21 is connected to a downstream casing 23 containing a downstream exhaust purification catalyst 24 via an exhaust pipe 22. The exhaust port 9, the exhaust manifold 19, the upstream casing 21, the exhaust pipe 22, and the downstream casing 23 form an exhaust passage.

  An electronic control unit (ECU) 31 comprises a digital computer, and is connected to each other via a bidirectional bus 32, a RAM (Random Access Memory) 33, a ROM (Read Only Memory) 34, a CPU (Microprocessor) 35, and an input. A port 36 and an output port 37 are provided. An air flow meter 39 for detecting the flow rate of air flowing through the intake pipe 15 is disposed in the intake pipe 15, and the output of the air flow meter 39 is input to the input port 36 via the corresponding AD converter 38. Further, an upstream air-fuel ratio sensor 40 that detects the air-fuel ratio of the exhaust gas flowing through the exhaust manifold 19 (that is, the exhaust gas flowing into the upstream exhaust purification catalyst 20) is disposed at the collecting portion of the exhaust manifold 19. In addition, in the exhaust pipe 22, the downstream side that detects the air-fuel ratio of the exhaust gas that flows in the exhaust pipe 22 (that is, the exhaust gas that flows out of the upstream side exhaust purification catalyst 20 and flows into the downstream side exhaust purification catalyst 24). An air-fuel ratio sensor 41 is arranged. The outputs of these air-fuel ratio sensors 40 and 41 are also input to the input port 36 via the corresponding AD converter 38. Further, an upstream temperature sensor 46 that detects the temperature of the upstream side exhaust purification catalyst 20 is disposed in the upstream side exhaust purification catalyst 20, and the temperature of the downstream side exhaust purification catalyst 24 is detected in the downstream side exhaust purification catalyst 24. A downstream temperature sensor 47 is disposed. The outputs of these temperature sensors 46 and 47 are also input to the input port 36 via the corresponding AD converter 38.

  A load sensor 43 that generates an output voltage proportional to the amount of depression of the accelerator pedal 42 is connected to the accelerator pedal 42, and the output voltage of the load sensor 43 is input to the input port 36 via the corresponding AD converter 38. The For example, the crank angle sensor 44 generates an output pulse every time the crankshaft rotates 15 degrees, and this output pulse is input to the input port 36. The CPU 35 calculates the engine speed from the output pulse of the crank angle sensor 44. On the other hand, the output port 37 is connected to the spark plug 10, the fuel injection valve 11, and the throttle valve drive actuator 17 via the corresponding drive circuit 45. The ECU 31 functions as a control device that controls the internal combustion engine.

  The internal combustion engine according to this embodiment is a non-supercharged internal combustion engine using gasoline as fuel, but the configuration of the internal combustion engine according to the present invention is not limited to the above configuration. For example, an internal combustion engine according to the present invention is different from the above internal combustion engine in terms of cylinder arrangement, fuel injection mode, intake / exhaust system configuration, valve mechanism configuration, presence / absence of a supercharger, and supercharging mode. There may be.

<Description of exhaust purification catalyst>
Both the upstream side exhaust purification catalyst 20 and the downstream side exhaust purification catalyst 24 have the same configuration. The exhaust purification catalysts 20 and 24 are three-way catalysts having an oxygen storage capacity. Specifically, the exhaust purification catalysts 20 and 24 are made of a noble metal having a catalytic action (for example, platinum (Pt)) and a substance having an oxygen storage capacity (for example, ceria (CeO ₂ )) on a base material made of ceramic. It is supported. When the exhaust purification catalysts 20 and 24 reach a predetermined activation temperature, the exhaust purification catalysts 20 and 24 exhibit an oxygen storage capability in addition to the catalytic action of simultaneously purifying unburned gas (HC, CO, etc.) and nitrogen oxides (NOx).

  According to the oxygen storage capacity of the exhaust purification catalysts 20, 24, the exhaust purification catalysts 20, 24 are such that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 is leaner than the stoichiometric air-fuel ratio (lean air-fuel ratio). Sometimes it stores oxygen in the exhaust gas. On the other hand, the exhaust purification catalysts 20, 24 release the oxygen stored in the exhaust purification catalysts 20, 24 when the air-fuel ratio of the inflowing exhaust gas is richer than the stoichiometric air-fuel ratio (rich air-fuel ratio).

  The exhaust purification catalysts 20 and 24 have a catalytic action and an oxygen storage capacity, and thus have a NOx and unburned gas purification action according to the oxygen storage amount. That is, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 is a lean air-fuel ratio, as shown in FIG. 2A, the exhaust gas is exhausted by the exhaust purification catalysts 20, 24 when the oxygen storage amount is small. The oxygen inside is occluded. Along with this, NOx in the exhaust gas is reduced and purified. On the other hand, when the oxygen storage amount increases, the oxygen in the exhaust gas flowing out from the exhaust purification catalysts 20, 24 borders on a certain storage amount (Cuplim in the figure) near the maximum storable oxygen amount (upper limit storage amount) Cmax, and The concentration of NOx increases rapidly.

  On the other hand, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 is a rich air-fuel ratio, as shown in FIG. 2B, when the oxygen storage amount is large, the exhaust purification catalysts 20, 24 store the exhaust gas. The released oxygen is released and the unburned gas in the exhaust gas is oxidized and purified. On the other hand, when the oxygen storage amount decreases, the concentration of unburned gas in the exhaust gas flowing out from the exhaust purification catalysts 20 and 24 suddenly increases at a certain storage amount (Clowlim in the figure) near zero (lower limit storage amount). To rise.

  As described above, according to the exhaust purification catalysts 20 and 24 used in the present embodiment, NOx and unburned in the exhaust gas according to the air-fuel ratio and oxygen storage amount of the exhaust gas flowing into the exhaust purification catalysts 20 and 24. Gas purification characteristics change. The exhaust purification catalysts 20 and 24 may be different from the three-way catalyst as long as they have a catalytic action and an oxygen storage capacity.

<Output characteristics of air-fuel ratio sensor>
Next, output characteristics of the air-fuel ratio sensors 40 and 41 in the present embodiment will be described with reference to FIGS. FIG. 3 is a diagram showing the voltage-current (V-I) characteristics of the air-fuel ratio sensors 40 and 41 in the present embodiment, and FIG. 4 shows the air-fuel ratio sensors 40 and 41 when the applied voltage is kept constant. 2 is a diagram showing a relationship between an air-fuel ratio (hereinafter referred to as “exhaust air-fuel ratio”) of exhaust gas flowing around and an output current I. FIG. In the present embodiment, air-fuel ratio sensors having the same configuration are used as the air-fuel ratio sensors 40 and 41.

As can be seen from FIG. 3, in the air-fuel ratio sensors 40 and 41 of the present embodiment, the output current I increases as the exhaust air-fuel ratio increases (lean). The V-I line at each exhaust air-fuel ratio has a region substantially parallel to the V axis, that is, a region where the output current hardly changes even when the sensor applied voltage changes. This voltage region is referred to as a limiting current region, and the current at this time is referred to as a limiting current. In FIG. 3, the limit current region and limit current when the exhaust air-fuel ratio is 18 are indicated by W ₁₈ and I ₁₈ , respectively. Therefore, it can be said that the air-fuel ratio sensors 40 and 41 are limit current type air-fuel ratio sensors.

  FIG. 4 is a diagram showing the relationship between the exhaust air-fuel ratio and the output current I when the applied voltage is kept constant at about 0.45V. As can be seen from FIG. 4, in the air-fuel ratio sensors 40 and 41, the exhaust air-fuel ratio becomes higher so that the output current I from the air-fuel ratio sensors 40 and 41 becomes larger as the exhaust air-fuel ratio becomes higher (that is, the leaner the air-fuel ratio). On the other hand, the output current changes linearly (in proportion). In addition, the air-fuel ratio sensors 40 and 41 are configured such that the output current I becomes zero when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio. Further, when the exhaust air-fuel ratio becomes larger than a certain value or when it becomes smaller than a certain value, the ratio of the change in the output current to the change in the exhaust air-fuel ratio becomes smaller.

  In the above example, limit current type air-fuel ratio sensors are used as the air-fuel ratio sensors 40 and 41. However, as long as the output current changes linearly with respect to the exhaust air-fuel ratio, any air-fuel ratio sensor such as an air-fuel ratio sensor that is not a limit current type may be used as the air-fuel ratio sensors 40 and 41. Further, the air-fuel ratio sensors 40 and 41 may be air-fuel ratio sensors having different structures.

<Outline of basic air-fuel ratio control>
Next, an outline of air-fuel ratio control in the control apparatus for an internal combustion engine of the present invention will be described. In the present embodiment, feedback control for controlling the fuel injection amount from the fuel injection valve 11 based on the output air-fuel ratio of the upstream air-fuel ratio sensor 40 so that the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes the target air-fuel ratio. Is done. “Output air-fuel ratio” means an air-fuel ratio corresponding to the output value of the air-fuel ratio sensor.

  Further, in the air-fuel ratio control of the present embodiment, target air-fuel ratio setting control for setting the target air-fuel ratio based on the output air-fuel ratio of the downstream air-fuel ratio sensor 41 and the like is performed. In the target air-fuel ratio setting control, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes less than the rich air-fuel ratio (for example, 14.55) slightly richer than the theoretical air-fuel ratio, the downstream air-fuel ratio is set. It is determined that the air-fuel ratio of the exhaust gas detected by the sensor 41 has become a rich air-fuel ratio. At this time, the target air-fuel ratio is set to a lean set air-fuel ratio. Here, the lean set air-fuel ratio is a predetermined air-fuel ratio that is somewhat leaner than the stoichiometric air-fuel ratio, and is, for example, 14.65 to 20, preferably 14.65 to 18, and more preferably 14. It is set to about 65 to 16.

  Thereafter, in a state where the target air-fuel ratio is set to the lean set air-fuel ratio, the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 is leaner than the rich judged air-fuel ratio (the air closer to the stoichiometric air-fuel ratio than the rich judged air-fuel ratio). When it is determined that the air-fuel ratio of the exhaust gas detected by the downstream air-fuel ratio sensor 41 is almost the stoichiometric air-fuel ratio. At this time, the target air-fuel ratio is set to a weak lean set air-fuel ratio. Here, the weak lean set air-fuel ratio is a lean air-fuel ratio with a lean degree smaller than the lean set air-fuel ratio (small difference from the theoretical air-fuel ratio), for example, 14.62 to 15.7, preferably 14. 63 to 15.2, more preferably about 14.65 to 14.9.

  On the other hand, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes equal to or higher than the lean determination air-fuel ratio (for example, 14.65) that is slightly leaner than the theoretical air-fuel ratio, the downstream air-fuel ratio sensor 41 detects it. It is determined that the air-fuel ratio of the exhaust gas has become a lean air-fuel ratio. At this time, the target air-fuel ratio is set to the rich set air-fuel ratio. Here, the rich set air-fuel ratio is a predetermined air-fuel ratio that is somewhat richer than the stoichiometric air-fuel ratio, and is, for example, 10 to 14.55, preferably 12 to 14.52, more preferably 13 to It is about 14.5.

  Thereafter, with the target air-fuel ratio set to the rich set air-fuel ratio, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 is richer than the lean determined air-fuel ratio (the air closer to the stoichiometric air-fuel ratio than the lean determined air-fuel ratio). When it is determined that the air-fuel ratio of the exhaust gas detected by the downstream air-fuel ratio sensor 41 is almost the stoichiometric air-fuel ratio. At this time, the target air-fuel ratio is set to a slightly rich set air-fuel ratio. Here, the weak rich set air-fuel ratio is a rich air-fuel ratio that is less rich than the rich set air-fuel ratio (small difference from the theoretical air-fuel ratio), and is, for example, 13.5-14.58, preferably 14- It is set to about 14.57, more preferably about 14.3 to 14.55.

  As a result, in the present embodiment, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio, first, the target air-fuel ratio is set to the lean set air-fuel ratio. Thereafter, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes larger than the rich determination air-fuel ratio, the target air-fuel ratio is set to the weak lean set air-fuel ratio. On the other hand, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes equal to or higher than the lean determination air-fuel ratio, first, the target air-fuel ratio is set to the rich set air-fuel ratio. Thereafter, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes smaller than the lean determination air-fuel ratio, the target air-fuel ratio is set to the slightly rich set air-fuel ratio. Thereafter, similar control is repeated.

  Note that the rich determination air-fuel ratio and the lean determination air-fuel ratio are those within 1%, preferably within 0.5%, more preferably within 0.35% of the theoretical air-fuel ratio. Therefore, the difference between the rich determination air-fuel ratio and the lean determination air-fuel ratio from the stoichiometric air-fuel ratio is 0.15 or less, preferably 0.073 or less, more preferably 0.051 when the stoichiometric air-fuel ratio is 14.6. It is as follows. Further, the difference between the target air-fuel ratio (for example, the weak rich set air-fuel ratio and the lean set air-fuel ratio) from the theoretical air-fuel ratio is set to be larger than the above-described difference.

<Description of control using time chart>
With reference to FIG. 5, the operation as described above will be specifically described. FIG. 5 shows the target air-fuel ratio AFT, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40, and the upstream side exhaust purification catalyst 20 when basic air-fuel ratio control is performed by the control device for an internal combustion engine according to the present embodiment. Is a time chart of the oxygen storage amount OSA, the cumulative oxygen excess / deficiency ΣOED in the exhaust gas flowing into the upstream side exhaust purification catalyst 20, and the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41.

In the illustrated example, the target air-fuel ratio AFT is set to the weak rich set air-fuel ratio AFTsr before the time t ₁ . Accordingly, the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes a rich air-fuel ratio. Unburned gas contained in the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is purified by the upstream side exhaust purification catalyst 20, and accordingly, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually increases. It will decrease. On the other hand, since the exhaust gas flowing out of the upstream side exhaust purification catalyst 20 by purification in the upstream side exhaust purification catalyst 20 does not include unburned gas, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is almost theoretically empty. It becomes the fuel ratio.

When the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases, the oxygen storage amount OSA approaches zero at time t ₁ (for example, Clowlim in FIG. 2). Along with this, a part of the unburned gas flowing into the upstream side exhaust purification catalyst 20 starts to flow out without being purified by the upstream side exhaust purification catalyst 20. As a result, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 gradually decreases after time t ₁ . As a result, in the illustrated example, at time t _2, the conjunction will the oxygen storage amount OSA substantially zero, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich.

  In the present embodiment, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio AFrich, the target air-fuel ratio AFT is switched to the lean set air-fuel ratio AFTl in order to increase the oxygen storage amount OSA. Therefore, the target air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio.

  In the present embodiment, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is not immediately changed from the stoichiometric air-fuel ratio to the rich air-fuel ratio, but after reaching the rich determination air-fuel ratio AFrich, the target air-fuel ratio AFT Switching is performed. This is because even if the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is sufficient, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 may slightly deviate from the stoichiometric air-fuel ratio. It is. In other words, the rich determination air-fuel ratio is such that the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 does not reach when the oxygen storage amount of the upstream side exhaust purification catalyst 20 is sufficient. The fuel ratio is set. The same applies to the above-described lean determination air-fuel ratio.

In time t _2, the switch the target air-fuel ratio to the lean air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the upstream exhaust purification catalyst 20 is changed to a lean air-fuel ratio from the rich air-fuel ratio. Accordingly, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 becomes a lean air-fuel ratio (actually, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 after switching the target air-fuel ratio) However, in the example shown in the figure, it is assumed that it changes simultaneously for the sake of convenience). When the air-fuel ratio of the exhaust gas flowing into the upstream exhaust purification catalyst 20 is changed to the lean air-fuel ratio at time t _2, the oxygen storage amount of the upstream exhaust purification catalyst 20 OSA is gradually increased.

Thus, as the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 increases, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 changes toward the stoichiometric air-fuel ratio. In the example shown in FIG. 5, at time t ₃ , the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes a value larger than the rich determination air-fuel ratio AFrich. That is, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is substantially the stoichiometric air-fuel ratio. This means that the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is increased to some extent.

Therefore, in the present embodiment, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 changes to a value larger than the rich determination air-fuel ratio AFrich, the target air-fuel ratio AFT is switched to the weak lean set air-fuel ratio AFTsl. Thus, the lean degree of the target air-fuel ratio at time t ₃ is made to decrease. In the following, it referred to as the time t ₃ and the lean degree change time.

At time t ₃ is lean degree change timing, switching the target air-fuel ratio AFT to slightly lean set air-fuel ratio AFTsl, leanness of the exhaust gas flowing into the upstream exhaust purification catalyst 20 is also reduced. Along with this, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 becomes smaller, and the increase rate of the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 decreases.

After the time t ₃ , the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually increases although its increase rate is slow. When the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually increases, the oxygen storage amount OSA eventually approaches the maximum storable oxygen amount Cmax (for example, Cuplim in FIG. 2). When the oxygen storage amount OSA approaches the maximum storable oxygen amount Cmax at time t ₄ , part of the oxygen that has flowed into the upstream side exhaust purification catalyst 20 starts to flow out without being stored in the upstream side exhaust purification catalyst 20. As a result, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 gradually increases. As a result, in the illustrated example, at time t ₅ , the oxygen storage amount OSA reaches the maximum storable oxygen amount Cmax, and the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the lean determination air-fuel ratio AFlean.

  In the present embodiment, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or greater than the lean determination air-fuel ratio AFlean, the target air-fuel ratio AFT is switched to the rich set air-fuel ratio AFTr in order to decrease the oxygen storage amount OSA. Therefore, the target air-fuel ratio is switched from the lean air-fuel ratio to the rich air-fuel ratio.

At time t _5, when switching the target air-fuel ratio to a rich air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the upstream exhaust purification catalyst 20 changes from a lean air-fuel ratio to a rich air-fuel ratio. Accordingly, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 becomes a rich air-fuel ratio (actually, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 after switching the target air-fuel ratio) However, in the example shown in the figure, it is assumed that it changes simultaneously for the sake of convenience). When the air-fuel ratio of the exhaust gas flowing into the upstream exhaust purification catalyst 20 changes to a rich air-fuel ratio at time t _5, the oxygen storage amount of the upstream exhaust purification catalyst 20 OSA decreases.

Thus, when the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 decreases, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 changes toward the stoichiometric air-fuel ratio. In the example shown in FIG. 5, at time t _6, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes a value smaller than the lean determining the air-fuel ratio AFlean. That is, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is substantially the stoichiometric air-fuel ratio. This means that the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is somewhat reduced.

  Therefore, in the present embodiment, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 changes to a value smaller than the lean determination air-fuel ratio AFlean, the target air-fuel ratio AFT is changed from the rich set air-fuel ratio to the weak rich set air-fuel ratio AFTsr. Can be switched.

When the target air-fuel ratio AFT is switched to the slightly rich set air-fuel ratio AFTsr at time t ₆ , the richness of the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is also reduced. Along with this, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 increases, and the decrease rate of the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 decreases.

After time t ₆ , the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases, although the decrease rate is slow. When the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases, the oxygen storage amount OSA eventually approaches zero at time t ₇ and decreases to Cdwnlim in FIG. 2 as at time t ₁ . Thereafter, at time t ₈ , similarly to time t ₂ , the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich. Thereafter, an operation similar to the operation at times t _{1 to} t ₆ is repeated.

<Advantages in basic control>
According to the basic air-fuel ratio control described above, immediately after the target air-fuel ratio is changed from the rich air-fuel ratio to the lean air-fuel ratio at time t _2, the and the target air-fuel ratio from the lean air-fuel ratio at time t ₅ to a rich air-fuel ratio Immediately after the change, the difference between the target air-fuel ratio and the stoichiometric air-fuel ratio is made large (that is, the rich degree or lean degree is made large). Therefore, it is possible to reduce the NOx that has been flowing from the upstream exhaust purification catalyst 20 rapidly in unburned gas and the time t ₅ that was flowing out of the upstream exhaust purification catalyst 20 at time t _2. Therefore, the outflow of unburned gas and NOx from the upstream side exhaust purification catalyst 20 can be suppressed.

Further, according to the air-fuel ratio control of the present embodiment, after setting the target air-fuel ratio to a lean set air-fuel ratio at time t _2, the stops outflow of unburned gas from the upstream exhaust purification catalyst 20 and the oxygen storage amount from OSA is to some extent recovery, the target air-fuel ratio is switched to the weak lean set air-fuel ratio at time t _3. Thus, by reducing the rich degree of the target air-fuel ratio (difference from the theoretical air-fuel ratio), even if NOx flows out from the upstream side exhaust purification catalyst 20, the outflow amount per unit time can be reduced. it can. In particular, according to the air-fuel ratio control, but NOx from the upstream side exhaust purification catalyst 20 will flow out at the time t _5, it can be suppressed to be small outflow amount at this time.

In addition, according to the air-fuel ratio control of the present embodiment, after setting the target air-fuel ratio to a rich set air-fuel ratio at time t _5, it stops the outflow of NOx (oxygen) from the upstream exhaust purification catalyst 20 and the oxygen from storage amount OSA is to some extent reduced, the target air-fuel ratio is switched to the weak rich set air-fuel ratio at time t _6. Thus, by reducing the rich degree of the target air-fuel ratio (difference from the theoretical air-fuel ratio), even if unburned gas flows out from the upstream side exhaust purification catalyst 20, the outflow amount per unit time is reduced. be able to. In particular, according to the above air-fuel ratio control, unburned gas flows out from the upstream side exhaust purification catalyst 20 at times t ₂ and t ₈ , and at this time, the outflow amount can be reduced.

  Further, in the present embodiment, an air-fuel ratio sensor 41 is used as a sensor for detecting the air-fuel ratio of the exhaust gas on the downstream side. Unlike the oxygen sensor, the air-fuel ratio sensor 41 does not have hysteresis. Therefore, the air-fuel ratio sensor 41 has high responsiveness to the actual exhaust air-fuel ratio, and can quickly detect the outflow of unburned gas and oxygen (and NOx) from the upstream side exhaust purification catalyst 20. . Therefore, also according to this embodiment, the outflow of unburned gas and NOx (and oxygen) from the upstream side exhaust purification catalyst 20 can be suppressed.

  Further, in an exhaust purification catalyst capable of storing oxygen, maintaining its oxygen storage amount substantially constant leads to a decrease in its oxygen storage capacity. Therefore, in order to maintain the oxygen storage capacity as much as possible, it is necessary to change the oxygen storage amount up and down when the exhaust purification catalyst is used. According to the air-fuel ratio control according to the present embodiment, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 repeatedly changes up and down between near zero and near the maximum storable oxygen amount. For this reason, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 can be maintained as high as possible.

In the above embodiment, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes larger than the rich determination air-fuel ratio AFrich at time t ₃ , the target air-fuel ratio AFT becomes less than the lean set air-fuel ratio AFTl. It is switched to the weak lean set air-fuel ratio AFTsl. In the above embodiment, at time t _6, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes smaller than a lean determining air AFlean, the target air-fuel ratio AFT from the rich set air-fuel ratio AFTr It is switched to the weak rich set air-fuel ratio AFTsr. However, the timing for switching the target air-fuel ratio AFT does not necessarily need to be set based on the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41, and may be determined based on other parameters.

For example, the timing for switching these target air-fuel ratios AFT may be determined based on the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20. For example, as shown in FIG. 5, when the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 reaches a predetermined amount α after the target air-fuel ratio is switched to the lean air-fuel ratio at time t ₂ , The air-fuel ratio AFT is switched to the weak lean set air-fuel ratio AFTsl. Further, at time t ₅ , when the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is decreased by a predetermined amount α after switching the target air-fuel ratio to the rich air-fuel ratio, the target air-fuel ratio AFT is set to be slightly rich. The air-fuel ratio is switched to AFTsr.

  In this case, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is estimated based on the cumulative oxygen excess / deficiency of the exhaust gas flowing into the upstream side exhaust purification catalyst 20. The oxygen excess / deficiency means excess oxygen or insufficient oxygen (amount of excess unburned gas, etc.) when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is set to the stoichiometric air-fuel ratio. To do. In particular, when the target air-fuel ratio is the lean set air-fuel ratio, oxygen in the exhaust gas flowing into the upstream side exhaust purification catalyst 20 becomes excessive, and this excess oxygen is stored in the upstream side exhaust purification catalyst 20. Therefore, it can be said that the integrated value of oxygen excess / deficiency (hereinafter referred to as “accumulated oxygen excess / deficiency”) represents the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20. As shown in FIG. 5, in this embodiment, the cumulative oxygen excess / deficiency ΣOED is reset to zero when the target air-fuel ratio changes beyond the theoretical air-fuel ratio.

Note that the oxygen excess / deficiency is the estimated value of the intake air amount into the combustion chamber 5 calculated based on the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 and the air flow meter 39 or the like, or from the fuel injection valve 11. This is based on the amount of fuel supplied. Specifically, the oxygen excess / deficiency OED is calculated by, for example, the following formula (1).
OED = 0.23 · Qi / (AFup-14.6) (1)
Here, 0.23 represents the oxygen concentration in the air, Qi represents the fuel injection amount, and AFup represents the output air-fuel ratio of the upstream air-fuel ratio sensor 40.

Alternatively, when the target air-fuel ratio AFT is switched to the slightly lean set air-fuel ratio AFTsl (lean degree change timing), the elapsed time after switching the target air-fuel ratio to the lean air-fuel ratio (time t ₂ ) or the integrated value of the intake air amount Etc. may be determined based on the above. Similarly, when the target air-fuel ratio AFT is switched to the slightly rich set air-fuel ratio AFTsr (rich degree change timing), the elapsed time after switching the target air-fuel ratio to the rich air-fuel ratio (time t ₅ ) and the integration of the intake air amount It may be determined based on a value or the like.

  Thus, the rich degree change time and the lean degree change time are determined based on various parameters. In any case, the lean degree change timing is a timing after the target air-fuel ratio is set to the lean set air-fuel ratio and before the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes equal to or higher than the lean determination air-fuel ratio. The Similarly, the rich degree change timing is a timing after the target air-fuel ratio is set to the rich set air-fuel ratio and before the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio. .

In the above embodiment, at time t ₂ ~t _3, the target air-fuel ratio AFT is maintained constant at the lean set air-fuel ratio AFTl. However, during such a period, the target air-fuel ratio AFT does not necessarily have to be kept constant, and may change so as to gradually decrease (approach the theoretical air-fuel ratio). Similarly, in the above embodiment, the target air-fuel ratio AFT is kept constant at the weak lean set air-fuel ratio AFTsl at times t _{3 to} t ₅ . However, during such a period, the target air-fuel ratio AFT does not necessarily have to be kept constant, and may change so as to gradually decrease (approach the stoichiometric air-fuel ratio), for example. The same can be said for the times t _{5 to} t ₆ and the times t _{6 to} t ₈ .

<Characteristics concerning storage of sulfur component>
By the way, the maximum storable oxygen amount Cmax of the upstream side exhaust purification catalyst 20 is not always constant and decreases due to deterioration of the upstream side exhaust purification catalyst 20 or the like. One reason for reducing the maximum storable oxygen amount Cmax in this manner is the storage of the sulfur component in the upstream side exhaust purification catalyst 20.

  In general, the exhaust gas discharged from the combustion chamber 5 contains a small amount of sulfur components such as SOx, and therefore, the exhaust gas containing such sulfur components flows into the upstream side exhaust purification catalyst 20. Become. In the upstream side exhaust purification catalyst 20, if the inflowing exhaust gas contains a sulfur component, the sulfur component is occluded depending on conditions such as the temperature of the upstream side exhaust purification catalyst 20. As described above, when the upstream side exhaust purification catalyst 20 stores the sulfur component, the maximum storable oxygen amount Cmax of the upstream side exhaust purification catalyst 20 decreases accordingly. Therefore, in order to keep the maximum storable oxygen amount Cmax of the upstream side exhaust purification catalyst 20 high, it is necessary to keep the sulfur component storage amount of the upstream side exhaust purification catalyst 20 low.

  Here, the presence or absence of storage of the sulfur component by the upstream side exhaust purification catalyst 20 varies greatly depending on the temperature of the upstream side exhaust purification catalyst 20. When the temperature of the upstream side exhaust purification catalyst 20 is equal to or lower than a certain sulfur storage upper limit temperature (for example, 600 ° C.), the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a lean air-fuel ratio. Sulfur components in the inflowing exhaust gas are occluded in the upstream side exhaust purification catalyst 20. On the other hand, even at this time, if the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a rich air-fuel ratio, even if the inflowing exhaust gas contains a sulfur component, the upstream side exhaust purification catalyst 20 Almost no sulfur component is occluded. On the other hand, when the temperature of the upstream side exhaust purification catalyst 20 is equal to or higher than the sulfur storage upper limit temperature, no sulfur component is stored in the upstream side exhaust purification catalyst 20 regardless of the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20. .

<Control of weak lean air-fuel ratio>
Therefore, in the embodiment of the present invention, the difference (lean degree) of the weak lean set air-fuel ratio from the stoichiometric air-fuel ratio is changed according to the temperature of the upstream side exhaust purification catalyst 20.

  FIG. 6 is a time chart of the target air-fuel ratio AFT and the like when the lean set air-fuel ratio change control is performed in the present embodiment. In the example shown in FIG. 6 as well, basically the same air-fuel ratio control as in FIG. 5 is performed.

In the example shown in FIG. 6, previously at time t _7, the temperature CT of the upstream exhaust purification catalyst 20 becomes higher temperatures than the sulfur storage limit temperature CTLIM. The weak rich set air-fuel ratio AFTsr and the weak lean set air-fuel ratio AFTsl at this time are set to the first weak rich set air-fuel ratio AFTsr ₁ and the first weak lean set air-fuel ratio AFTsl ₁ , respectively. Here, the difference of the first weak rich set air-fuel ratio AFTsr _{1 from} the theoretical air-fuel ratio is the first rich degree ΔAFTsr ₁ . The difference between the first weak lean set air-fuel ratio AFTsl _{1 and} the stoichiometric air-fuel ratio is the first lean degree ΔAFTsl ₁ .

Therefore, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 changes from the rich determination air-fuel ratio AFrich or lower to an air-fuel ratio larger than the rich determination air-fuel ratio AFrich at time t ₂ or the like, the target air-fuel ratio AFT becomes the first weak lean air-fuel ratio. The set air-fuel ratio is switched to AFTsl ₁ . At time t _4, etc., the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 changes from the lean determination air AFlean more small air-fuel ratio than the lean determination air AFlean, the target air-fuel ratio AFT is first weak rich The set air-fuel ratio is switched to AFTsr ₁ . After that, until the time t ₇ is, such a cycle is repeated.

Then, at time t _7, when the temperature CT of the upstream exhaust purification catalyst 20 becomes less than the sulfur storage limit temperature CTLIM, the value of the weak lean set air-fuel ratio AFTsl is changed. In the example illustrated in FIG. 6, at time t ₇ , the weak lean set air-fuel ratio AFTsl is changed from the first weak lean set air-fuel ratio AFTsl ₁ to the second weak lean set air-fuel ratio AFTsl ₂ . The difference of the second weak lean set air-fuel ratio AFTsl ₂ from the stoichiometric air-fuel ratio is a second lean degree ΔAFTsl ₂ that is larger than the first lean degree ΔAFTsl ₁ . Therefore, the second weak lean set air-fuel ratio AFTsl ₂ is larger (lean side) than the first lean set air-fuel ratio AFTsl ₁ .

Here, a value obtained by subtracting the first richness DerutaAFTsr ₁ from the time t ₇ the first lean degree DerutaAFTsl ₁ before the first variation difference _{ΔLR 1 (ΔLR 1 = ΔAFTsl 1} -ΔAFTsr 1). Similarly, a value obtained by subtracting the first rich degree ΔAFTsr ₁ from the second lean degree ΔAFTsl ₂ after time t _{7 is} set as a second fluctuation difference ΔLR ₂ (ΔLR ₂ = ΔAFTsl ₂ −ΔAFTsr ₁ ). In this case, in the embodiment of the present invention, the second variation difference ΔLR ₂ is set to a value equal to or larger than the first variation difference ΔLR ₁ (ΔLR ₂ ≧ ΔLR ₁ ).

Thereafter, while the temperature CT of the upstream exhaust purification catalyst 20 is equal to or less than the sulfur storage limit temperature CTlim is slightly lean set air-fuel ratio AFTsl is maintained at the second weak lean set air-fuel ratio AFTsl _2. When the temperature CT of the upstream side exhaust purification catalyst 20 changes again to a temperature higher than the sulfur storage upper limit temperature CTlim, the weak lean set air-fuel ratio AFTsl is changed to the first weak lean set air-fuel ratio AFTsl ₁ .

In the above embodiment, the time until the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or higher than the rich determination air-fuel ratio AFrich and becomes larger than the rich determination air-fuel ratio AFrich (for example, from time t _{1 to} t ₂ , t _{8 to} t ₉ ), the target air-fuel ratio AFT is kept constant. That is, the lean set air-fuel ratio is kept constant. However, the lean set air-fuel ratio is not necessarily constant and may vary to some extent.

In the above-described embodiment, the time from when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is greater than the rich determination air-fuel ratio AFrich to the lean determination air-fuel ratio AFlean or more (for example, time t _{2 to} t ₃ , T _{9 to} t ₁₀ ), the target air-fuel ratio AFT is kept constant. That is, the weak lean set air-fuel ratio is maintained constant. However, the weak lean set air-fuel ratio is not necessarily constant and may vary to some extent. However, even in this case, the lean degree of the average value of the weak lean set air-fuel ratio at time t ₉ ~t ₁₀ is greater than the lean degree of the average value of the weak lean set air-fuel ratio at time t ₂ ~t ₃ It is supposed to be.

Similarly, in the above-described embodiment, the time from when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or higher than the lean determination air-fuel ratio AFleen until it becomes smaller than the lean determination air-fuel ratio AFlean (for example, from time t _{3 to} t ₄ , t _{10 to} t ₁₁ ), the target air-fuel ratio AFT is kept constant. That is, the rich set air-fuel ratio is kept constant. However, the rich set air-fuel ratio is not necessarily constant and may vary to some extent.

In the above embodiment, during the period from the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is smaller than the lean determining the air-fuel ratio AFlean until less rich determination air AFlean (e.g., time t ₄ ~t ₅ T _{11 to} t ₁₂ ), the target air-fuel ratio AFT is kept constant. That is, the weak rich set air-fuel ratio is maintained constant. However, the weak rich set air-fuel ratio is not necessarily constant and may vary to some extent.

  In the above embodiment, the rich set air-fuel ratio and the lean set air-fuel ratio are changed when the temperature CT of the upstream side exhaust purification catalyst 20 exceeds the sulfur storage upper limit temperature CTlim. However, the temperature for switching the set air-fuel ratio is not necessarily the sulfur storage upper limit temperature CTlim, and may be a temperature lower than this. Further, the temperature of the upstream side exhaust purification catalyst 20 is not necessarily detected by providing the upstream side temperature sensor 46, but other parameters related to the temperature of the upstream side exhaust purification catalyst 20 (for example, the engine operating state and the intake You may estimate based on air quantity etc.). In this case, for example, when the engine operation state is an idle operation state, or when the intake air amount is equal to or less than a predetermined air amount, the temperature CT of the upstream side exhaust purification catalyst 20 is equal to or less than the sulfur storage upper limit temperature CTlim. It is determined that there is.

<Flowchart>
FIG. 7 is a flowchart showing a control routine in target air-fuel ratio calculation control. The illustrated control routine is performed by interruption at regular time intervals.

  As shown in FIG. 7, first, in step S11, it is determined whether a calculation condition for the target air-fuel ratio AFT is satisfied. The case where the calculation condition of the target air-fuel ratio AFT is satisfied includes that normal control is being performed, for example, that fuel cut control is not being performed. If it is determined in step S11 that the target air-fuel ratio AFT calculation condition is satisfied, the process proceeds to step S12.

  In step S12, it is determined whether or not the lean setting flag Fl is set to OFF. The lean setting flag Fl is a flag that is turned on when the target air-fuel ratio is set to the lean air-fuel ratio, and is turned off otherwise. If it is determined in step S12 that the lean setting flag Fl is set to OFF, the process proceeds to step S13. In step S13, it is determined whether or not the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is equal to or less than the rich determination air-fuel ratio AFrich.

  If it is determined in step S13 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is greater than the rich determination air-fuel ratio AFrich, the process proceeds to step S14. In step S14, it is determined whether or not the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is smaller than the lean determination air-fuel ratio AFlean. If it is determined that the output air-fuel ratio AFdwn is greater than or equal to the lean determination air-fuel ratio AFlean, the process proceeds to step S15. In step S15, the target air-fuel ratio AFT is set to the rich set air-fuel ratio AFTr, and the control routine is ended.

  Thereafter, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 approaches the stoichiometric air-fuel ratio and becomes smaller than the lean determination air-fuel ratio AFlean, the process proceeds from step S14 to step S16 in the next control routine. In step S16, the target air-fuel ratio AFT is set to the slightly rich set air-fuel ratio AFTsr, and the control routine is ended.

  Thereafter, when the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 becomes substantially zero and the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio AFrich, the next control routine starts from step S13 to step S13. Proceed to S17. In step S17, the target air-fuel ratio AFT is set to the lean set air-fuel ratio AFTl. Next, at step S18, the lean setting flag Fl is set to ON, and the control routine is ended.

  When the lean setting flag Fl is set to ON, in the next control routine, the process proceeds from step S12 to step S19. In step S19, it is determined whether or not the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is equal to or greater than the lean determination air-fuel ratio AFlean.

  If it is determined in step S19 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is smaller than the lean determination air-fuel ratio AFlean, the process proceeds to step S20. In step S20, it is determined whether or not the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is larger than the rich determination air-fuel ratio AFrich. If it is determined that the output air-fuel ratio AFdwn is equal to or less than the rich determination air-fuel ratio AFrich, the process proceeds to step S21. In step S21, the target air-fuel ratio AFT is continuously set to the lean set air-fuel ratio AFT1, and the control routine is ended.

  Thereafter, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 approaches the stoichiometric air-fuel ratio and becomes larger than the rich determination air-fuel ratio AFrich, the process proceeds from step S20 to step S22 in the next control routine. In step S22, the target air-fuel ratio AFT is set to the weak lean set air-fuel ratio AFTsl, and the control routine is ended.

  Thereafter, when the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 becomes substantially the maximum storable oxygen amount and the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes equal to or greater than the lean determination air-fuel ratio AFlean, the following control routine: The process proceeds from step S19 to step S23. In step S23, the target air-fuel ratio AFT is set to the rich set air-fuel ratio AFTr. Next, in step S24, the lean setting flag Fl is reset to OFF, and the control routine is ended.

  FIG. 8 is a flowchart showing a control routine in the setting control of the weak lean set air-fuel ratio. The illustrated control routine is performed by interruption at regular time intervals.

First, in step S31, it is determined whether or not the temperature CT of the upstream side exhaust purification catalyst 20 detected by the upstream side temperature sensor 46 is equal to or lower than the sulfur storage upper limit temperature CTlim. When it is determined that the temperature CT is higher than the sulfur storage upper limit temperature CTlim, the process proceeds to step S32. In step S32, the weak lean set air-fuel ratio AFTsl is set to the first weak lean set air-fuel ratio AFTsl ₁ , and the control routine is ended. On the other hand, if it is determined in step S31 that the temperature CT is equal to or lower than the sulfur storage upper limit temperature CTlim, the process proceeds to step S33. In step S33, the weak lean set air-fuel ratio AFTsl is set to the second weak lean set air-fuel ratio AFTsl _2, the control routine is ended.

<Effect of set air-fuel ratio control>
Thus, in the present embodiment, when the temperature CT of the upstream side exhaust purification catalyst 20 is equal to or lower than the sulfur storage upper limit temperature CTlim, compared to when the temperature is higher than the sulfur storage upper limit temperature CTlim, from the lean degree of the weak lean set air-fuel ratio. The fluctuation difference ΔLR obtained by subtracting the rich degree of the weak rich set air-fuel ratio is increased (particularly, in consideration of fluctuations in the weak lean set air fuel ratio and the weak rich set air fuel ratio, in this embodiment, after the lean degree change timing It can be said that the fluctuation difference obtained by subtracting the rich degree of the average value of the rich air-fuel ratio after the rich degree change timing is increased from the lean degree of the average value of the lean air-fuel ratio. Hereinafter, the effect of controlling the weak rich set air-fuel ratio and the weak lean set air-fuel ratio in this way will be described.

As shown in FIG. 6, when the temperature CT of the upstream side exhaust purification catalyst 20 is higher than the sulfur storage upper limit temperature CTlim, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes equal to or lower than the rich determination air-fuel ratio AFrich. the time until more lean determination air AFlean and T ₁ from (e.g., time t ₁ ~t _3). Similarly, the time from when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or higher than the lean determination air-fuel ratio AFlean to when it becomes equal to or lower than the rich determination air-fuel ratio AFrich is defined as T ₂ (for example, time t _{3 to} t ₅ ). Therefore, the time taken for one cycle from when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich to when it reaches the rich determination air-fuel ratio AFrich again can be expressed as T ₁ + T ₂ (for example, time t ₁ ~t _5).

On the other hand, when the temperature CT of the upstream side exhaust purification catalyst 20 is equal to or lower than the sulfur storage upper limit temperature CTlim, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or higher than the lean determined air-fuel ratio AFlean after the output air-fuel ratio AFdwn becomes lower than the rich determined air-fuel ratio AFrich. the time until the T ₃ (e.g., time t ₈ ~t _10). Similarly, the time from when the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes equal to or higher than the lean determination air-fuel ratio AFlean to when it becomes equal to or lower than the rich determination air-fuel ratio AFrich is defined as T ₄ (for example, time t _{10 to} t ₁₂ ). Therefore, the time required for one cycle from when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich to when it reaches the rich determination air-fuel ratio AFrich again can be expressed as T ₃ + T ₄ (for example, time t ₈ ~t _12).

As can be seen from FIG. 6, in the present embodiment, when the temperature of the upstream exhaust purification catalyst 20 is high (time t ₇ earlier in the figure), the cycle time (T ₁ + T ₂₎ at the time T ₁ The ratio is not so low. That is, the time during which the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is the lean air-fuel ratio (hereinafter referred to as “lean time”) in one cycle time is not so short. On the other hand, when the temperature of the upstream side exhaust purification catalyst 20 is low (after time t ₇ in the figure), the ratio of the time T _{3 to} the time of one cycle (T ₃ + T ₄ ) is low. That is, the lean time becomes shorter in one cycle time. This is because the variation difference ΔLR is increased when the temperature CT of the upstream side exhaust purification catalyst 20 is low.

  Here, as described above, in the upstream side exhaust purification catalyst 20, when the temperature CT becomes equal to or lower than the sulfur storage upper limit temperature CTlim, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is the lean air-fuel ratio. The sulfur component is occluded. In the present embodiment, when the temperature of the upstream side exhaust purification catalyst 20 is low, the time during which the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a lean air-fuel ratio is shortened. Occlusion of sulfur components is suppressed.

  On the other hand, in this embodiment, when the temperature of the upstream side exhaust purification catalyst 20 is high, the lean time in one cycle time is not so short. However, as described above, in the upstream side exhaust purification catalyst 20, when the temperature CT is higher than the sulfur storage upper limit temperature CTlim, the upstream side exhaust purification catalyst 20 even if the air-fuel ratio of the exhaust gas is the lean air-fuel ratio. Almost no sulfur component is occluded. Therefore, even if the time during which the air-fuel ratio of the exhaust gas is the lean air-fuel ratio is not so short, the upstream side exhaust purification catalyst 20 hardly stores the sulfur component. As described above, according to the present embodiment, the storage of the sulfur component in the upstream side exhaust purification catalyst 20 can be suppressed, and thus the sulfur component storage amount of the upstream side exhaust purification catalyst 20 can be kept low.

The experimental result regarding this is shown in FIG. FIG. 9 is a graph showing the relationship between the ratio of rich time in one cycle time (for example, T ₂ / (T ₁ + T ₂ ), T ₄ / (T ₃ + T ₄ )) and the Cmax ratio. The graph shown in FIG. 9 shows how the maximum storable oxygen amount Cmax is obtained by operating the internal combustion engine while maintaining the ratio of the rich time in one cycle constant using a new exhaust purification catalyst. It shows how it has changed. The Cmax ratio in the figure represents the ratio of the maximum storable oxygen amount Cmax when the maximum storable oxygen amount Cmax is 1 when the exhaust purification catalyst is new.

  As can be seen from FIG. 9, when the temperature of the exhaust purification catalyst is low (400 ° C.), the Cmax ratio increases as the rich time ratio increases, that is, as the lean time ratio decreases. This confirms that the sulfur component is less likely to be stored in the exhaust purification catalyst as the lean time ratio decreases. On the other hand, when the temperature of the exhaust purification catalyst is high (700 ° C.), the Cmax ratio is higher than when the temperature of the exhaust purification catalyst is low, and is almost constant regardless of the ratio of the rich time. Therefore, the graph shown in FIG. 9 also supports that according to the present embodiment, the storage of the sulfur component in the upstream side exhaust purification catalyst 20 can be suppressed.

  Further, in the embodiment shown in FIG. 6, when the temperature of the upstream side exhaust purification catalyst 20 is low, only the weak lean set air-fuel ratio AFTsl is substantially changed. Therefore, even if the temperature of the exhaust purification catalyst is low, the lean set air-fuel ratio AFTl and the rich set air-fuel ratio AFTr are not changed.

Here, as described above, the rich set air-fuel ratio AFTr and the lean set air-fuel ratio AFTl have a relatively large richness and lean degree compared to the weak rich set air-fuel ratio AFTsr and the weak lean set air-fuel ratio AFTsl. Is done. This is because the unburned gas and NOx flowing out from the upstream side exhaust purification catalyst 20 at time t ₁ and time t ₃ in FIG. 6 are rapidly reduced as described above. For this reason, if the rich degree of the rich set air-fuel ratio AFTr and the lean degree of the lean set air-fuel ratio AFTl are reduced when the temperature of the upstream side exhaust purification catalyst 20 is lowered, It becomes impossible to reduce fuel gas and NOx quickly.

  On the other hand, according to the above embodiment, even when the temperature of the upstream side exhaust purification catalyst 20 is low, the lean set air-fuel ratio AFTl and the rich set air-fuel ratio AFTr are not changed. For this reason, according to this embodiment, the unburned gas and NOx which have flowed out of the upstream side exhaust purification catalyst 20 can be rapidly reduced.

Note that when the temperature of the upstream side exhaust purification catalyst 20 is low, the lean set air-fuel ratio AFTl and the rich set air-fuel ratio AFTr may be changed. In this case, for example, as shown in FIG. 10, when the temperature CT of the upstream side exhaust purification catalyst 20 is higher than the sulfur storage upper limit temperature CTlim, the lean set air-fuel ratio AFTl is set to the first lean set air-fuel ratio AFTl _1. The In addition, when the temperature CT of the upstream exhaust purification catalyst 20 is less than the sulfur storage limit temperature CTlim is lean set air-fuel ratio AFTl is set to the second lean set air-fuel ratio AFTl _2. Here, the difference (rich degree) of the second lean set air-fuel ratio AFTl ₂ from the stoichiometric air-fuel ratio is larger than the difference (rich degree) of the first lean set air-fuel ratio AFTl ₁ from the stoichiometric air-fuel ratio. Yes.

Thus, in the example shown in FIG. 10, when the minus the difference between the rich set air-fuel ratio AFTr and the theoretical air-fuel ratio from the difference between the lean set air-fuel ratio AFTl and the theoretical air-fuel ratio variation difference, time t ₇ the previously In comparison, it can be said that the fluctuation difference after time t ₇ is larger. When the temperature CT of the upstream side exhaust purification catalyst 20 is equal to or lower than the sulfur storage upper limit temperature CTlim, if such a difference in difference can be made larger than when it is higher than this, the rich set air-fuel ratio AFTr and the lean set air-fuel ratio AFTl may be changed in any way.

  Further, in the embodiment shown in FIG. 6, when the temperature of the upstream side exhaust purification catalyst 20 is low, the lean degree of the weak lean set air-fuel ratio AFTsl is made larger than when the temperature is high. However, the rich degree of the weak rich set air-fuel ratio AFTsr is maintained as it is even when the temperature CT of the upstream side exhaust purification catalyst 20 changes beyond the sulfur storage upper limit temperature CTlim.

Here, as described above, from the viewpoint of setting the second fluctuation difference ΔLR ₂ to a value equal to or larger than the first fluctuation difference ΔLR ₁ , the temperature of the upstream side exhaust purification catalyst 20 is lower than that when the temperature is high. It is conceivable to reduce the rich degree of the weak rich set air-fuel ratio AFTsr. However, for example, when there is a slight error in the output air-fuel ratio of the upstream air-fuel ratio sensor 40, if the rich degree of the weak rich set air-fuel ratio AFTsr is made smaller than necessary, the target air-fuel ratio is actually the rich air-fuel ratio. There is a possibility that the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 becomes the stoichiometric air-fuel ratio or the lean air-fuel ratio. Therefore, in the above embodiment, the rich degree of the weak rich set air-fuel ratio AFTsr is maintained as it is even if the temperature CT of the upstream side exhaust purification catalyst 20 changes beyond the sulfur storage upper limit temperature CTlim.

As shown in FIG. 11, when the rich degree of the weak rich set air-fuel ratio AFTsr ₁ when the temperature of the upstream side exhaust purification catalyst 20 is high, the temperature CT of the upstream side exhaust purification catalyst 20 is high. When the sulfur storage upper limit temperature CTlim is lower than or equal to the sulfur storage upper limit temperature CTlim, the degree of richness of the weak rich set air-fuel ratio AFTsr may be smaller than when it is higher than the sulfur storage upper limit temperature CTlim (weak rich set air-fuel ratio AFTsr _{2 in} FIG. 11). ).

DESCRIPTION OF SYMBOLS 1 Engine body 5 Combustion chamber 7 Intake port 9 Exhaust port 19 Exhaust manifold 20 Upstream exhaust purification catalyst 24 Downstream exhaust purification catalyst 31 ECU
40 upstream air-fuel ratio sensor 41 downstream air-fuel ratio sensor 46 temperature sensor 47 temperature sensor

Claims (1)

An exhaust purification catalyst that is disposed in the exhaust passage of the internal combustion engine and can store oxygen, and an air-fuel ratio of the exhaust gas that is disposed downstream of the exhaust purification catalyst in the exhaust flow direction and flows out of the exhaust purification catalyst. In a control device for an internal combustion engine comprising a downstream air-fuel ratio sensor and temperature detection means for detecting or estimating the temperature of the exhaust purification catalyst,
Perform feedback control so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes the target air-fuel ratio,
When the exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor becomes equal to or less than the rich determination air-fuel ratio richer than the stoichiometric air-fuel ratio, the target air-fuel ratio is switched to a lean set air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. And
The degree of lean after the target air-fuel ratio is set to the lean set air-fuel ratio and before the exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor becomes equal to or higher than the lean determination air-fuel ratio leaner than the stoichiometric air-fuel ratio The target air-fuel ratio is set to a lean air-fuel ratio that is smaller than the lean set air-fuel ratio until the exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor becomes equal to or higher than the lean determination air-fuel ratio from the change timing,
When the exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor becomes equal to or higher than the lean determination air-fuel ratio, the target air-fuel ratio is switched to a rich set air-fuel ratio that is richer than the stoichiometric air-fuel ratio,
After the target air-fuel ratio is set to the rich set air-fuel ratio, the downstream air-fuel ratio is changed from the rich degree change timing before the exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor becomes equal to or less than the rich determination air-fuel ratio. The target air-fuel ratio is set to a rich air-fuel ratio that is less rich than the rich set air-fuel ratio until the exhaust air-fuel ratio detected by the fuel ratio sensor becomes equal to or less than the rich determination air-fuel ratio,
When the temperature of the exhaust purification catalyst detected or estimated by the temperature detection means is equal to or lower than a predetermined upper limit temperature, the average of the lean air-fuel ratio after the lean degree change timing is higher than when the temperature is higher than the upper limit temperature. A control device for an internal combustion engine, wherein a fluctuation difference obtained by subtracting a difference between an average value of the rich air-fuel ratio after the rich degree change timing and the theoretical air-fuel ratio from the difference between the value and the theoretical air-fuel ratio is increased.

Images