JP2005171982A - Air-fuel ratio control system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize air-fuel ratio feedback control which reduces unburned components or NOx by appropriately setting the target value of the output of an oxygen concentration sensor disposed downstream of a catalyst. <P>SOLUTION: An air-fuel ratio control system performs air-fuel ratio feedback control so that the target value will correspond with the output Voxs of the oxygen concentration sensor disposed downstream of a catalyst and in the exhaust passage of an internal combustion engine. The air-fuel ratio control system sets the target value at a substantially middle value (central value) between a maximum output value and a minimum output value, when the output Voxs is the maximum output value max or the minimum output value min. Further, when the output Voxs, after showing the minimum output value, is a value between a lower side threshold value Lohan slightly larger than the minimum output value and a higher side threshold value Hihan slightly smaller than the maximum output value, the target value is set at a lower side target value Lotgt which is smaller than the central value; and when the output Voxs, after showing the maximum output value, is a value between the higher side threshold value and the lower side threshold value, the target value is set at a higher side target value Hitgt which is larger than the central value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an air-fuel ratio control device that controls an air-fuel ratio of gas flowing into a catalyst based on an output of an oxygen concentration sensor provided downstream of the catalyst disposed in an exhaust passage of an internal combustion engine.

Conventionally, an oxygen concentration sensor has been provided downstream of a catalyst (three-way catalyst) disposed in the exhaust passage of the internal combustion engine, and the internal combustion engine has a value corresponding to the stoichiometric air-fuel ratio. An air-fuel ratio control apparatus that feedback-controls the air-fuel ratio of an air-fuel mixture to be supplied (thus, the air-fuel ratio of gas flowing into the catalyst) is widely known (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 9-60544 (page 5, FIG. 3)

  The oxygen concentration sensor used in such an air-fuel ratio control device is generally a sensor whose output changes based on the difference in oxygen concentration (oxygen partial pressure) between the detection electrode and the reference electrode. It is exposed to the atmosphere. Therefore, as shown in FIG. 1, if the air-fuel ratio of the gas that has reached the detection electrode of the oxygen concentration sensor (hereinafter simply referred to as “oxygen concentration sensor reaching gas”) is slightly richer than the stoichiometric air-fuel ratio. Since the difference in oxygen concentration between the detection electrode and the reference electrode becomes large, the output Voxs becomes the maximum output value max.

  On the other hand, if the air-fuel ratio of the gas reaching the oxygen concentration sensor is slightly leaner than the stoichiometric air-fuel ratio, the difference in oxygen concentration between the detection electrode and the reference electrode is reduced, so that the output Voxs is the minimum output value min. Become. Therefore, when the output of the oxygen concentration sensor is approximately halfway between the maximum output value and the minimum output value (hereinafter referred to as “median value a”), the air-fuel ratio of the gas reaching the oxygen concentration sensor is the stoichiometric air-fuel ratio. It is thought that there is. For this reason, the conventional air-fuel ratio control apparatus performs feedback control of the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine so that the output Voxs of the oxygen concentration sensor on the downstream side of the catalyst becomes the median value a.

On the other hand, when the catalyst contains excessive oxygen in the gas flowing into the catalyst (hereinafter also simply referred to as “catalyst inflow gas”) (that is, when the air-fuel ratio of the catalyst inflow gas is lean). When the oxygen in the gas is occluded in the interior and the catalyst inflow gas contains a large amount of unburned components and oxygen for oxidizing the unburned components is insufficient (that is, the catalyst inflow gas) When the air-fuel ratio of the gas is rich), it has a function of oxidizing the unburned components with oxygen stored therein. The function of this catalyst is referred to as “oxygen storage function” or “O ₂ storage effect”.

  With this oxygen storage function, even if a gas with a rich air-fuel ratio flows into the catalyst in which oxygen is stored, it will flow out of the catalyst until all of the oxygen stored in the catalyst is consumed. The air-fuel ratio of the gas (gas downstream of the catalyst) does not become rich but becomes the stoichiometric air-fuel ratio. On the other hand, even if a gas with a lean air-fuel ratio flows into the catalyst in a state where there is a surplus capacity capable of storing oxygen, the gas downstream of the catalyst is emptied until the catalyst can no longer store oxygen. The fuel ratio does not become lean but the stoichiometric air-fuel ratio.

  From the above, the air-fuel ratio of the air-fuel mixture supplied to the engine is feedback controlled by the conventional air-fuel ratio control device, and as a result, when the air-fuel ratio of the catalyst inflow gas changes between lean and rich, the catalyst downstream It is considered that the output Voxs of the oxygen concentration sensor on the side changes as shown by the broken line in FIG.

  That is, when the air-fuel ratio of the catalyst inflow gas is lean and oxygen cannot be stored in the catalyst, the output Voxs of the oxygen concentration sensor on the downstream side of the catalyst is the minimum output value as before time t1 in FIG. Value near min. Thereafter, when the air-fuel ratio of the gas flowing into the catalyst becomes rich by the feedback control described above, the oxygen concentration on the downstream side of the catalyst is consumed until the oxygen stored in the catalyst is consumed, as shown at times t1 to t2. The sensor output Voxs has a value in the vicinity of the median value a.

  Next, when all the oxygen stored in the catalyst is consumed, the output Voxs of the oxygen concentration sensor on the downstream side of the catalyst becomes a value near the maximum output value max as shown at times t2 to t3. Thereafter, when the air-fuel ratio of the gas flowing into the catalyst becomes lean due to the above-described air-fuel ratio feedback control, as shown at times t3 to t4, the oxygen concentration sensor on the downstream side of the catalyst until the oxygen cannot be stored in the catalyst. The output Voxs is a value near the median value a. When oxygen cannot be stored in the catalyst, the output Voxs of the oxygen concentration sensor on the downstream side of the catalyst again becomes a value near the minimum output value min as shown after time t4.

  However, the actual output Voxs of the oxygen concentration sensor changes as shown by the solid line in FIG. That is, after the air-fuel ratio of the gas flowing into the catalyst changes from lean to rich, the oxygen concentration on the downstream side of the catalyst during the period from time t1 to t2 until the oxygen stored in the catalyst is consumed. The sensor output Voxs is a value between the minimum output value min and the median value a. This phenomenon is caused by a large amount of oxygen molecules remaining in the previous period (before time t1) remaining around the detection electrode of the oxygen concentration sensor during the period until the oxygen stored in the catalyst is consumed. Therefore, it is considered that this occurs because the difference in oxygen concentration between the detection electrode and the reference electrode is small.

  Further, after the air-fuel ratio of the gas flowing into the catalyst changes from rich to lean, the output of the oxygen concentration sensor on the downstream side of the catalyst during the period from time t3 to t4 until oxygen cannot be stored in the catalyst. Voxs is a value between the maximum output value max and the median value a. In the period until oxygen can no longer be occluded in the catalyst, this phenomenon is almost completely caused by unburned components that have arrived around the detection electrode of the oxygen concentration sensor in the previous period (time t2 to t3). Since it disappears, it is considered that the oxygen concentration difference between the detection electrode and the reference electrode is increased.

  As understood from the above, even if the air-fuel ratio of the gas reaching the oxygen concentration sensor on the downstream side of the catalyst is the stoichiometric air-fuel ratio, the output of the oxygen concentration sensor is past (and / or present). In FIG. 4, the value varies depending on the air-fuel ratio (state) of the gas that has reached the oxygen concentration sensor.

  However, in the conventional air-fuel ratio control apparatus, since the air-fuel ratio of the air-fuel mixture supplied to the engine is feedback-controlled so that the output of the oxygen concentration sensor on the downstream side of the catalyst always has a constant median value a. In the case where nitrogen oxides and unburned components are sufficiently purified in the catalyst and the air-fuel ratio of the gas flowing out from the catalyst is a gas equivalent to the theoretical air-fuel ratio (that is, the air-fuel ratio of the gas flowing into the catalyst is Even if it is an appropriate value), the air-fuel ratio of the air-fuel mixture supplied to the engine (and hence the air-fuel ratio of the gas flowing into the catalyst) is overcorrected to rich or lean, and as a result, Although there is a possibility that the amount of nitrogen oxides and unburned components discharged into the atmosphere can be further reduced, there is a problem that this possibility is lost.

The air-fuel ratio control apparatus of the present invention has been made to address the above problems,
A catalyst disposed in an exhaust passage of the internal combustion engine;
An oxygen concentration sensor disposed downstream of the catalyst in the exhaust passage and outputting a value corresponding to the oxygen concentration in the gas flowing out from the catalyst;
Air-fuel ratio control amount calculating means for calculating an air-fuel ratio control amount for making the current output of the oxygen concentration sensor coincide with a predetermined target value corresponding to the theoretical air-fuel ratio;
Air-fuel ratio control means for controlling the air-fuel ratio of the gas flowing into the catalyst based on the calculated air-fuel ratio control amount;
In an air-fuel ratio control device comprising:
The air-fuel ratio control amount calculating means includes
The air-fuel ratio control apparatus is configured to change the target value based on a state of gas that has reached the oxygen concentration sensor.

  According to this, the said target value is changed based on the state of the gas which reached | attained the oxygen concentration sensor which is arrange | positioned downstream of the catalyst and outputs the value according to the oxygen concentration in the gas which flows out from the catalyst. Therefore, when the stoichiometric air-fuel ratio gas reaches the oxygen concentration sensor, the value output from the oxygen concentration sensor and the target value can be substantially matched. It is possible to maintain the state of purification in the chamber as much as possible.

  In this case, the air-fuel ratio control amount calculating means can be configured to determine the state of the gas that has reached the oxygen concentration sensor based on the current and past outputs of the oxygen concentration sensor.

  More specifically, the air-fuel ratio control amount calculating means is configured to reduce the output of the oxygen concentration sensor to a value smaller than a median value that is a substantially intermediate value between the maximum output value and the minimum output value of the oxygen concentration sensor. The target value is set to the same median value when it is smaller than the lower threshold value, which is slightly larger than the output value, and then the output of the oxygen concentration sensor increases to be greater than the lower threshold value and the same median value. The target value can be set to a low-side target value between the low-side threshold value and the median value when the value is between a high-side threshold value slightly smaller than the output value.

  Further, the air-fuel ratio control amount calculating means is configured such that the output of the oxygen concentration sensor is larger than a median value that is a substantially intermediate value between the maximum output value and the minimum output value of the oxygen concentration sensor and slightly less than the maximum output value. The target value is set to the same median value when it is larger than the small high side threshold value, and then the output of the oxygen concentration sensor decreases to be smaller than the high side threshold value and the same median value and slightly less than the minimum output value. When the value is between a large low side threshold value, the target value can be set to a high side target value between the high side threshold value and the median value.

  According to these, the output of the oxygen concentration sensor with respect to the stoichiometric air-fuel ratio gas and the target value can be substantially matched, so that the air-fuel ratio of the gas flowing out from the catalyst can be the stoichiometric air-fuel ratio. As a result, nitrogen oxides and unburned components can be purified very effectively in the catalyst.

  Furthermore, it is also preferable to acquire the temperature of the catalyst and to fix the target value to the median value when the acquired temperature of the catalyst is lower than a threshold temperature.

  When the temperature of the catalyst is low (that is, when the temperature of the catalyst is lower than the activation temperature), since the catalyst can hardly perform the oxygen storage function, the oxygen concentration sensor includes either a large amount of oxygen or a large amount of unburned components. Will arrive. In such a case, the output of the oxygen concentration sensor with respect to the stoichiometric air-fuel ratio becomes the median value. Therefore, according to the above configuration, when the temperature of the catalyst is low, the target value can be made substantially coincident with the output of the oxygen concentration sensor for the stoichiometric air-fuel ratio gas.

  Further, as the deterioration of the catalyst proceeds, the activation temperature of the catalyst increases. Therefore, the air-fuel ratio control amount calculation means acquires a value indicating the degree of deterioration of the catalyst and changes the threshold temperature based on the acquired value indicating the degree of deterioration (specifically, deterioration of the catalyst). It is also preferable that the threshold temperature is increased as the temperature increases. In this case, the value indicating the degree of deterioration of the catalyst includes, for example, the maximum oxygen storage amount of the catalyst, the travel distance of the vehicle on which the catalyst is mounted, the integrated value of the gas passage amount to the catalyst, and the oxygen concentration sensor provided upstream of the catalyst Or the locus ratio of the output of the air-fuel ratio sensor and the output of the oxygen concentration sensor provided downstream of the catalyst.

  Further, the air-fuel ratio control amount calculating means acquires the maximum oxygen storage amount of the catalyst and sets the target value when the acquired maximum oxygen storage amount is smaller than a predetermined oxygen storage amount. It is preferable that the output value and the minimum output value are fixed to a median value that is substantially intermediate.

  As the deterioration of the catalyst progresses, the maximum oxygen storage amount of the catalyst decreases. Since a catalyst having a small maximum oxygen storage amount does not sufficiently exhibit the oxygen storage function, either a large amount of oxygen or a large amount of unburned components reaches the oxygen concentration sensor. In such a case, the output of the oxygen concentration sensor with respect to the stoichiometric air-fuel ratio becomes the median value. Therefore, according to the above configuration, even when a deteriorated catalyst is mounted, the target value can be made substantially coincident with the output of the oxygen concentration sensor for the stoichiometric air-fuel ratio gas.

  Further, the air-fuel ratio control amount calculating means is configured to estimate an oxygen storage amount of the catalyst and determine a state of the gas that has reached the oxygen concentration sensor based on the estimated oxygen storage amount. Is also suitable.

  The oxygen storage amount of the catalyst is also an amount indicating the state of the gas that has reached the oxygen concentration sensor disposed downstream of the catalyst. That is, if the oxygen storage amount is not substantially a value near the maximum oxygen storage amount and the oxygen storage amount is not substantially “0”, the stoichiometric air-fuel ratio gas has reached the oxygen concentration sensor. In this case, the oxygen concentration sensor indicates an output value affected by the gas that has reached in the past. Therefore, if the target value is changed according to the oxygen storage amount as in the above-described configuration, the output of the oxygen concentration sensor for the stoichiometric air-fuel ratio and the target value can be substantially matched.

  In this case, more specifically, the air-fuel ratio control amount calculating means is configured such that the oxygen storage amount of the catalyst is larger than a half value that is approximately half the maximum oxygen storage amount of the catalyst and is greater than the maximum oxygen storage amount. The target value is set to a median value that is approximately half of the maximum output value and the minimum output value of the oxygen concentration sensor when the value is slightly higher than the high-side oxygen storage amount threshold, and then the oxygen storage The target value is smaller than the median value when the amount decreases and becomes a value between the high-side oxygen storage amount threshold value and a low-side oxygen storage amount threshold value that is smaller than the same half value and slightly larger than 0; The lower target value may be set larger than the minimum output value.

  Further, the air-fuel ratio control amount calculating means is configured such that the oxygen storage amount of the catalyst is smaller than a half value that is substantially half the maximum oxygen storage amount of the catalyst and smaller than a low-side oxygen storage amount threshold value slightly larger than 0. The target value is set to a median value that is approximately half of the maximum output value and the minimum output value of the oxygen concentration sensor, and then the oxygen storage amount increases and the low oxygen storage amount increases. The target value is larger than the median value and smaller than the maximum output value when the value is between the threshold value and the high oxygen storage amount threshold value that is greater than the same half value as the threshold value and slightly smaller than the maximum oxygen storage amount. Side target value.

  On the other hand, the air-fuel ratio control amount calculating means is configured such that the output of the oxygen concentration sensor is smaller than a median value that is a substantially intermediate value between the maximum output value and the minimum output value of the oxygen concentration sensor and slightly less than the minimum output value. The target value is set to the lower side between the same median value and the lower threshold value during the period from the time when the target value is smaller than the larger lower threshold value to the time when it is larger than the same median value and slightly larger than the same maximum output value. It is preferable to be configured to set the target value.

  As is clear from the above description, if the catalyst inflow gas is well purified by the catalyst during the period from when the output of the oxygen concentration sensor becomes smaller than the low threshold value to when it becomes larger than the high threshold value, The output of the density sensor becomes a value in the vicinity of the low side target value. Therefore, if the target value of the oxygen concentration sensor in such a period is set to the low target value as in the above configuration, the period during which the catalyst inflow gas is effectively purified by the catalyst can be lengthened, and the catalyst purification is performed. The ability can be used effectively.

  In this case, the air-fuel ratio control amount calculation means includes an oxygen concentration sensor element temperature acquisition means for acquiring the element temperature of the oxygen concentration sensor, and the lower target value as the element temperature of the acquired oxygen concentration sensor increases. And low-side target value setting means for changing the value to a large value.

  As shown in FIG. 13, the oxygen concentration sensor arranged downstream of the catalyst has the same oxygen concentration during a period from when the output of the oxygen concentration sensor becomes smaller than the low threshold Lohan until it becomes larger than the high threshold Hihan. When the air-fuel ratio of the gas reaching the concentration sensor is the stoichiometric air-fuel ratio, it has been found that the higher the element temperature, the larger the value.

  Therefore, with the above configuration, if the lower target value is changed to a larger value as the element temperature of the acquired oxygen concentration sensor is higher, the target value is reduced to the oxygen concentration sensor. Since it can be more accurately matched with the value output from the oxygen concentration sensor when it has reached, it is possible to maintain the state of purifying nitrogen oxides and unburned components in the catalyst as much as possible.

  On the other hand, the air-fuel ratio control amount calculating means has an output of the oxygen concentration sensor that is larger than a median value that is a substantially intermediate value between the maximum output value and the minimum output value of the oxygen concentration sensor and slightly less than the maximum output value. The target value is set to the high side between the same median value and the same high side threshold value until it becomes smaller than the low side threshold value which is smaller than the same median value and slightly larger than the same minimum output value after becoming larger than the small high side threshold value. It is preferable to be configured to set the target value.

  If the catalyst inflow gas is well purified by the catalyst during the period from when the output of the oxygen concentration sensor becomes larger than the high side threshold value until it becomes smaller than the low side threshold value, the output of the oxygen concentration sensor becomes the high target value. It is a nearby value. Therefore, if the target value of the oxygen concentration sensor in such a period is set to the high target value as in the above configuration, the period during which the catalyst inflow gas is effectively purified by the catalyst can be lengthened, and the catalyst purification is performed. The ability can be used effectively.

  In this case, the air-fuel ratio control amount calculation means includes an oxygen concentration sensor element temperature acquisition means for acquiring the element temperature of the oxygen concentration sensor, and the higher the target value, the higher the element temperature of the acquired oxygen concentration sensor. It is preferable to include high side target value setting means for changing to a small value.

  As shown in FIG. 13, the oxygen concentration sensor arranged downstream of the catalyst has the same oxygen concentration sensor during a period from when the output of the oxygen concentration sensor becomes larger than the high threshold value until it becomes smaller than the low threshold value. It was found that when the air-fuel ratio of the gas that reaches 1 is the stoichiometric air-fuel ratio, the higher the element temperature, the smaller the value.

  Therefore, with the above configuration, if the higher target value is changed to a smaller value as the element temperature of the acquired oxygen concentration sensor is higher, the target value is changed to the (high side) target value by the stoichiometric air-fuel ratio gas to the oxygen concentration sensor. Since it can be more accurately matched with the value output from the oxygen concentration sensor when it has reached, it is possible to maintain the state of purifying nitrogen oxides and unburned components in the catalyst as much as possible.

  Hereinafter, embodiments of an air-fuel ratio control apparatus for an internal combustion engine according to the present invention will be described with reference to the drawings. This air-fuel ratio control device is also a fuel injection amount control device that controls the fuel injection amount of the engine.

(First embodiment)
FIG. 3 shows a schematic configuration of a system in which the air-fuel ratio control apparatus according to the first embodiment is applied to a four-cycle spark ignition type multi-cylinder internal combustion engine 10. FIG. 1 shows only a cross section of a specific cylinder, but the other cylinders have the same configuration.

  The internal combustion engine 10 includes a cylinder block portion 20 including a cylinder block, a cylinder block lower case, an oil pan, and the like, a cylinder head portion 30 fixed on the cylinder block portion 20, and a gasoline mixture to the cylinder block portion 20. An intake system 40 for supplying and an exhaust system 50 for releasing exhaust gas from the cylinder block 20 to the outside are included.

  The cylinder block unit 20 includes a cylinder 21, a piston 22, a connecting rod 23, and a crankshaft 24. The piston 22 reciprocates in the cylinder 21, and the reciprocating motion of the piston 22 is transmitted to the crankshaft 24 through the connecting rod 23, whereby the crankshaft 24 rotates. The heads of the cylinder 21 and the piston 22 form a combustion chamber 25 together with the cylinder head portion 30.

  The cylinder head portion 30 includes an intake port 31 communicating with the combustion chamber 25, an intake valve 32 that opens and closes the intake port 31, an intake camshaft that drives the intake valve 32, and a phase angle and lift amount of the intake camshaft are continuously provided. Variable intake timing device 33 to be changed, actuator 33a of variable intake timing device 33, exhaust port 34 communicating with combustion chamber 25, exhaust valve 35 for opening and closing exhaust port 34, exhaust camshaft 36 for driving exhaust valve 35, An ignition plug 37, an igniter 38 including an ignition coil that generates a high voltage to be applied to the ignition plug 37, and an injector (fuel injection means) 39 for injecting fuel into the intake port 31 are provided.

  The intake system 40 is provided in an intake pipe 41 including an intake manifold that communicates with the intake port 31 and forms an intake passage together with the intake port 31, an air filter 42 provided at an end of the intake pipe 41, and the intake pipe 41. A throttle valve 43 and a swirl control valve (hereinafter referred to as “SCV”) 44 that change the opening cross-sectional area of the intake passage are provided.

  The throttle valve 43 is rotationally driven in the intake pipe 41 by a throttle valve actuator 43a made of a DC motor. The SCV 44 is rotationally driven by an SCV actuator 44a made of a DC motor.

  The exhaust system 50 includes an exhaust manifold 51 communicating with the exhaust port 34, an exhaust pipe 52 connected to the exhaust manifold 51, an upstream catalyst 53 disposed in the exhaust pipe 52, and an exhaust pipe 52 downstream of the upstream catalyst 53. Is provided with a downstream catalyst 54. The exhaust port 34, the exhaust manifold 51, and the exhaust pipe 52 constitute an exhaust passage.

Each of the upstream catalyst 53 and the downstream catalyst 54 is a three-way catalyst device that supports an active component made of a noble metal such as platinum. Each catalyst has a function of oxidizing unburned components such as HC and CO and reducing nitrogen oxides (NOx) when the catalyst inflow gas has a substantially stoichiometric air-fuel ratio. Furthermore, each catalyst has an oxygen storage function for storing (storing) oxygen, and even if the air-fuel ratio shifts from the stoichiometric air-fuel ratio by this oxygen storage function, unburned components and nitrogen oxides can be purified. . This oxygen storage function is provided by ceria (CeO ₂ ) supported on the catalyst.

  On the other hand, this system includes a hot-wire air flow meter 61, an intake air temperature sensor 62, a throttle position sensor 63, a cam position sensor 64, a crank position sensor 65, a water temperature sensor 66, an air-fuel ratio sensor 67, an oxygen concentration sensor 68, and a catalyst temperature sensor 69. And an accelerator opening sensor 70.

  The air flow meter 61 outputs a signal corresponding to the mass flow rate Ga of the intake air flowing through the intake pipe 41. The intake air temperature sensor 62 detects the temperature of the intake air and outputs a signal representing the intake air temperature THA. The throttle position sensor 63 detects the opening (throttle valve opening) of the throttle valve 43 and outputs a signal representing the throttle valve opening TA.

  The cam position sensor 64 generates a signal (G2 signal) having one pulse every time the intake camshaft rotates 90 ° (that is, every time the crankshaft 24 rotates 180 °). The crank position sensor 65 outputs a signal having a narrow pulse every time the crankshaft 24 rotates 10 ° and a wide pulse every time the crankshaft 24 rotates 360 °. This signal represents the engine speed NE. The water temperature sensor 66 detects the temperature of the cooling water of the internal combustion engine 10 and outputs a signal representing the cooling water temperature THW.

  The air-fuel ratio sensor 67 is disposed in the exhaust passage and upstream of the upstream catalyst 53. The air-fuel ratio sensor 67 is a so-called “limit current type oxygen concentration sensor”, detects the air-fuel ratio in the exhaust gas flowing into the upstream side catalyst 53, and corresponds to the detected air-fuel ratio as shown in FIG. The signal Vabyfs is output.

  The oxygen concentration sensor 68 is disposed in the exhaust passage, downstream of the upstream catalyst 53 and upstream of the downstream catalyst 54. The oxygen concentration sensor 68 is a solid electrolyte type sensor (a well-known concentration cell type oxygen sensor using stabilized zirconia as a main material constituting the element), and a reference electrode made of platinum and a detection electrode made of platinum on both sides of the zirconia. And an electrode. The detection electrode is covered with a porous ceramic. The reference electrode is exposed to the atmosphere, and the detection electrode covered with the porous ceramic is exposed to gas flowing out from the upstream catalyst 53 (gas in the exhaust passage between the upstream catalyst 53 and the downstream catalyst 54). Yes.

  Such a static characteristic of the output Voxs of the oxygen concentration sensor is a characteristic that changes based on a difference in oxygen concentration (oxygen partial pressure) between the detection electrode and the reference electrode. Specifically, as shown in FIG. 1, the concentration cell type oxygen concentration sensor generally has a maximum output value max and a minimum output when the air-fuel ratio of the gas to be detected is richer and leaner than the stoichiometric air-fuel ratio. Each value min is output, and when the air-fuel ratio of the gas to be detected is the stoichiometric air-fuel ratio, a value (median value) a approximately in the middle between the maximum output value max and the minimum output value min is output.

  The accelerator opening sensor 70 outputs a signal representing the operation amount Accp of the accelerator pedal 71 operated by the driver.

  The electric control device 80 includes a CPU 81 connected to each other by a bus, a ROM 82 in which programs executed by the CPU 81, tables (maps, functions), constants, and the like are stored in advance, a RAM 83 in which the CPU 81 temporarily stores data as necessary, The microcomputer includes a backup RAM 84 that stores data while the power is turned on and holds the stored data even while the power is shut off, and an interface 85 including an AD converter.

  The interface 85 is connected to the sensors 61 to 70, supplies signals from the sensors 61 to 70 to the CPU 81, and in response to an instruction from the CPU 81, the actuator 33a, the igniter 38, the injector 39, and the throttle valve of the variable intake timing device 33. Drive signals are sent to the actuator 43a and the SCV actuator 44a.

<Outline of air-fuel ratio feedback control>
Next, an outline of the air-fuel ratio feedback control by the air-fuel ratio control apparatus configured as described above will be described with reference to FIG. This air-fuel ratio control device is also referred to as an air-fuel ratio (hereinafter referred to as “engine air-fuel ratio”) of the air-fuel mixture supplied to the engine so that the air-fuel ratio of the gas flowing out from the upstream catalyst 53 becomes the stoichiometric air-fuel ratio. Specifically, the air-fuel ratio of the engine is feedback-controlled so that the output of the oxygen concentration sensor 68 disposed downstream of the upstream catalyst 53 becomes a value corresponding to the stoichiometric air-fuel ratio.

  As described above, the concentration cell type oxygen concentration sensor generally outputs the maximum output value max and the minimum output value min when the air-fuel ratio of the detection target gas is richer and leaner than the theoretical air-fuel ratio, and the detection target When the air-fuel ratio of this gas is the stoichiometric air-fuel ratio, a value (median value) a that is substantially intermediate between the maximum output value max and the minimum output value min should be output.

  However, since the oxygen concentration sensor 68 detects the air-fuel ratio of the gas flowing out from the upstream side catalyst 53, even if the air-fuel ratio of the gas reaching the oxygen concentration sensor is the stoichiometric air-fuel ratio, the oxygen concentration sensor The output Voxs 68 has different values depending on the state of the gas that has reached the oxygen concentration sensor 68 in the past (air-fuel ratio or concentration of oxygen or unburned components).

  More specifically, when the upstream catalyst 53 is unable to store oxygen inside (that is, when the oxygen storage amount of the upstream catalyst 53 reaches the maximum oxygen storage amount Cmax), the upstream catalyst 53 When an air / fuel ratio leaner than the stoichiometric air / fuel ratio flows into the side catalyst 53, a large amount (high concentration) of oxygen flows out from the upstream side catalyst 53. As a result, since the difference in oxygen concentration (oxygen partial pressure) between the reference electrode and the detection electrode of the oxygen concentration sensor 68 becomes small, the output Voxs of the oxygen concentration sensor 68 is minimized as shown before time t1 in FIG. It becomes a value near the output value min.

  In this state, when an air-fuel ratio richer than the stoichiometric air-fuel ratio flows into the upstream catalyst 53, unburned components contained in the gas are oxidized by oxygen stored in the upstream catalyst 53, Almost no discharge is made from the upstream catalyst 53.

  At this time, a large amount of oxygen has arrived around the detection electrode of the oxygen concentration sensor 68 at a time before this time, and the oxygen remains. This remaining oxygen does not disappear with a very small amount (low concentration) of unburned components discharged from the upstream catalyst 53. As a result, since the oxygen concentration (oxygen partial pressure) difference between the reference electrode and the detection electrode of the oxygen concentration sensor 68 remains small, the output of the oxygen concentration sensor 68 is output as shown at times t1 to t2 in FIG. Voxs is a value slightly larger than the minimum output value min.

  Thereafter, when an air-fuel ratio richer than the stoichiometric air-fuel ratio flows into the upstream catalyst 53 continuously, unburned components begin to leak from the upstream catalyst 53. As a result, as shown at times t2 to t3 in FIG. 5, the output Voxs of the oxygen concentration sensor 68 increases rapidly toward the maximum output value max.

  On the other hand, when all the oxygen stored in the upstream catalyst 53 has been consumed (that is, when the oxygen storage amount of the upstream catalyst 53 is “0”), When an air-fuel ratio richer than the stoichiometric air-fuel ratio flows into the side catalyst 53, a large amount (high concentration) of unburned components flows out from the upstream catalyst 53. As a result, oxygen around the detection electrode of the oxygen concentration sensor 68 disappears, and the difference in oxygen concentration (oxygen partial pressure) between the reference electrode and the detection electrode increases, so as shown at times t3 to t4 in FIG. The output Voxs of the oxygen concentration sensor 68 is a value near the maximum output value max.

  In this state, when an air-fuel ratio leaner than the stoichiometric air-fuel ratio flows into the upstream catalyst 53, excess oxygen contained in this gas is occluded in the upstream catalyst 53, so that the upstream catalyst 53 Almost no discharge.

  As a result, oxygen hardly reaches the vicinity of the detection electrode of the oxygen concentration sensor 68 (only low concentration oxygen reaches), so that the difference in oxygen concentration (oxygen partial pressure) between the reference electrode and the detection electrode of the oxygen concentration sensor 68 is reached. Will remain large. Therefore, as shown at times t4 to t5 in FIG. 5, the output Voxs of the oxygen concentration sensor 68 is slightly smaller than the maximum output value max but larger than the median value a.

  Thereafter, when an air-fuel ratio leaner than the stoichiometric air-fuel ratio continuously flows into the upstream catalyst 53, oxygen begins to leak from the upstream catalyst 53. As a result, as shown at times t5 to t6 in FIG. 5, the output Voxs of the oxygen concentration sensor 68 decreases rapidly toward the minimum output value min.

  When oxygen cannot be stored in the upstream catalyst 53, a large amount (high concentration) of oxygen flows out from the upstream catalyst 53. As a result, the difference in oxygen concentration (oxygen partial pressure) between the reference electrode and the detection electrode of the oxygen concentration sensor 68 is reduced, so that the output Voxs of the oxygen concentration sensor 68 is again as shown after time t6 in FIG. It becomes a value near the minimum output value min.

  From the above, this air-fuel ratio control apparatus is able to perform the “period in which a large amount of oxygen is flowing out from the upstream catalyst 53 (before time t1 and after time t6)” and “a large amount of unburned components from the upstream catalyst 53. In the “outflow period (time t3 to t4)”, the air-fuel ratio of the air-fuel mixture supplied to the engine is feedback-controlled so that the output Voxs of the oxygen concentration sensor 68 becomes the median value a. This is because, during these periods, if the stoichiometric air-fuel ratio gas reaches the oxygen concentration sensor 68, the output Voxs of the oxygen concentration sensor 68 should become the median value a.

  In this case, the present air-fuel ratio control apparatus sets the output Voxs as “the period during which a large amount of oxygen is flowing out from the upstream catalyst 53 (the period in which the output Voxs of the oxygen concentration sensor 68 is substantially equal to the minimum output value min)”. Is recognized as a period shorter than the low threshold Lohan, which is slightly larger than the minimum output value min. This low threshold Lohan is naturally smaller than the median a.

  Similarly, the present air-fuel ratio control apparatus determines that “a period during which a large amount of unburned components are flowing out from the upstream catalyst 53 (a period during which the output Voxs of the oxygen concentration sensor 68 is substantially equal to the maximum output value max)” This is recognized as a period in which the output Voxs is greater than the high threshold Hihan, which is slightly smaller than the maximum output value max. This high threshold Hihan is naturally larger than the median a.

  On the other hand, the present air-fuel ratio control device states that “after a large amount of oxygen flows out from the upstream catalyst 53, rich air-fuel ratio gas flows into the upstream catalyst 53. During the period during which the stoichiometric air-fuel ratio gas is flowing out (time t1 to t3), the output Voxs of the oxygen concentration sensor 68 is greater than the lower threshold Lohan and smaller than the median a. The air-fuel ratio of the air-fuel mixture supplied to the engine is feedback controlled so that the value Lotgt is obtained.

  In addition, the air-fuel ratio control apparatus performs such a period (time t1 to t3) after the output Voxs of the oxygen concentration sensor 68 becomes substantially equal to the minimum output value min in the past (specifically, the output Voxs is After being smaller than the low threshold Lohan), the output Voxs is recognized as a period that is a value between the low threshold Lohan and the high threshold Hihan.

  On the other hand, the present air-fuel ratio control device states that “after a large amount of unburned components have flowed out of the upstream catalyst 53, lean air-fuel ratio gas is flowing into the upstream catalyst 53, and Is a period in which a gas with a substantially stoichiometric air-fuel ratio is flowing out (time t4 to t6) ”, the output Voxs of the oxygen concentration sensor 68 is smaller than the high-side threshold value Hihan and higher than the median value a. The air-fuel ratio of the air-fuel mixture supplied to the engine is feedback-controlled so that the side target value Hitgt is obtained.

  In addition, the air-fuel ratio control apparatus performs such a period (time t4 to t6) after the output Voxs of the oxygen concentration sensor 68 becomes substantially equal to the maximum output value max in the past (specifically, the output Voxs is After becoming larger than the high threshold Hihan), the output Voxs is recognized as a period that is a value between the high threshold Hihan and the low threshold Lohan.

  As described above, the air-fuel ratio control apparatus sets the target value Vosxref of the output Voxs of the oxygen concentration sensor 68 to the oxygen concentration sensor 68 in the past (immediately before the current time) and / or at the present time. The air-fuel ratio of the air-fuel mixture supplied to the engine is controlled so that the actual output Voxs of the oxygen concentration sensor 68 coincides with the target value Voxsref by changing according to the state of the gas that has reached. Thereby, from the upstream side catalyst 53, a gas with a substantially stoichiometric air-fuel ratio (nitrogen oxide and unburned components are extremely small amounts, and the ratio thereof is completely purified by the downstream side catalyst 54. Gas) flows out. Therefore, the amount of nitrogen oxides and unburned components released into the atmosphere can be further reduced.

<Actual operation>
Next, the actual operation of the air-fuel ratio control apparatus configured as described above will be described with reference to FIGS. 6 to 9 showing routines (programs) executed by the CPU 81 of the electric control apparatus 80 in flowcharts. The flowchart shown in FIG. 6 corresponds to the air-fuel ratio control means for controlling the air-fuel ratio of the gas flowing into the upstream side catalyst 53 based on the calculated air-fuel ratio control amount, and is shown in FIGS. The flowchart corresponds to air-fuel ratio control amount calculation means for calculating the air-fuel ratio control amount for making the current output Voxs of the oxygen concentration sensor 68 coincide with a predetermined target value Voxsref corresponding to the theoretical air-fuel ratio.

  The CPU 81 performs the routine for calculating the final fuel injection amount Fi and instructing the fuel injection shown in FIG. 6, and the crank angle of a predetermined cylinder becomes a predetermined crank angle before the intake top dead center (for example, BTDC 90 ° CA). Each time it is executed repeatedly. Therefore, when the crank angle of the predetermined cylinder reaches the predetermined crank angle, the CPU 81 starts the process from step 600 and proceeds to step 605, where the CPU 81 proceeds based on the intake air amount Ga measured by the air flow meter 61 and the engine speed NE. Thus, the basic fuel injection amount Fbase for setting the air-fuel ratio of the engine to the stoichiometric air-fuel ratio is obtained from the map.

  Next, the CPU 81 proceeds to step 610 and sets a value obtained by adding an air-fuel ratio feedback correction amount DFi described later to the basic fuel injection amount Fbase as the final fuel injection amount Fi. Next, the CPU 81 proceeds to step 615 to give an instruction for injecting the fuel of the final fuel injection amount Fi to the injector 39 corresponding to the predetermined cylinder, proceeds to step 695, and once ends this routine. As described above, the fuel of the final fuel injection amount Fi that has been feedback-corrected is injected into the cylinder that reaches the intake stroke. This routine is executed for the other cylinders in the same manner as described above.

  Next, calculation of the air-fuel ratio feedback correction amount DFi will be described. The CPU 81 repeatedly executes the routine shown in FIG. 7 every elapse of a predetermined time. Accordingly, when the predetermined timing is reached, the CPU 81 starts the process from step 700 and proceeds to step 705 to determine whether or not the air-fuel ratio feedback control condition is satisfied. The air-fuel ratio feedback control condition is, for example, that the engine coolant temperature THW is equal to or higher than a first predetermined temperature, the intake air amount (load) per one rotation of the engine is equal to or lower than a predetermined value, and the air-fuel ratio sensor 67 is normal. It is established when

  If the description continues assuming that the air-fuel ratio feedback control condition is satisfied, the CPU 81 makes a “Yes” determination at step 705 to proceed to step 710 where the current output vabyfs of the air-fuel ratio sensor 67 and a sub-state to be described later. Based on the sum of the feedback control amount vafsfb (vabyfs + vafsfb) and the map shown in FIG. This air-fuel ratio is an apparent air-fuel ratio upstream of the upstream catalyst 53 and is hereinafter referred to as “upstream control air-fuel ratio abyfs”.

  Next, the CPU 81 proceeds to step 715, where the in-cylinder intake air amount Mc (k−N), which is the intake air amount of the cylinder that has reached the intake stroke before N strokes (N intake strokes) from the present time, is obtained. By dividing by the upstream control air-fuel ratio abyfs, the in-cylinder fuel supply amount Fc (k−N) N strokes before the present time is obtained. The value N varies depending on the displacement of the internal combustion engine, the distance from the combustion chamber 25 to the air-fuel ratio sensor 67, and the like.

  In this way, in order to obtain the in-cylinder fuel supply amount Fc (k−N) before the N stroke from the current time, the in-cylinder intake air amount Mc (k−N) before the N stroke from the current time is set to the upstream control air-fuel ratio. The reason for dividing by abyfs is that it takes time corresponding to the N stroke until the air-fuel mixture burned in the combustion chamber 25 reaches the air-fuel ratio sensor 67. The in-cylinder intake air amount Mc is obtained for each intake stroke of each cylinder based on the output Ga of the air flow meter 61 and the engine rotational speed NE at that time (for example, first order lag with respect to the output Ga of the air flow meter 61) This value is obtained by dividing the processed value by the engine rotational speed NE) and stored in the RAM 73 corresponding to each intake stroke.

  Next, the CPU 81 proceeds to step 720, where the in-cylinder intake air amount Mc (k−N) N strokes before the current time is set to the target air-fuel ratio abyfr (k−N) (in this example, the theoretical value). The target in-cylinder fuel supply amount Fcr (k−N) N strokes before the current stroke is obtained by dividing by the air / fuel ratio.

Then, the CPU 81 proceeds to step 725 and sets a value obtained by subtracting the in-cylinder fuel supply amount Fc (k−N) from the target in-cylinder fuel supply amount Fcr (k−N) as the in-cylinder fuel supply amount deviation DFc. That is, the in-cylinder fuel supply amount deviation DFc is an amount representing the excess or deficiency of the fuel supplied into the cylinder at the time point before N strokes. Next, the CPU 81 proceeds to step 730 and obtains the air-fuel ratio feedback correction amount DFi based on the equation (1).
DFi = (Gp · DFc + Gi · SDFc) · KFB (1)

  In the above equation (1), Gp is a preset proportional gain, and Gi is a preset integral gain. The coefficient KFB in the equation (1) is preferably variable depending on the engine speed NE, the in-cylinder intake air amount Mc, and the like, but is set to “1” here. The value SDFc is an integral value of the in-cylinder fuel supply amount deviation DFc, and is updated in the next step 735. That is, in step 735, the CPU 81 adds the in-cylinder fuel supply amount deviation DFc obtained in step 725 to the integral value SDFc of the in-cylinder fuel supply amount deviation DFc at that time, thereby obtaining a new in-cylinder fuel supply amount deviation. The integrated value SDFc is obtained, and this routine is once terminated in step 795.

  As described above, the air-fuel ratio feedback correction amount DFi is obtained by the proportional integral control, and this air-fuel ratio feedback correction amount DFi is reflected in the final fuel injection amount Fi by the above-described step 610 and step 615 of FIG. As a result, since the excess or deficiency of the fuel supply amount before the N stroke from the present time is compensated, the average value of the air-fuel ratio of the engine (and hence the air-fuel ratio of the gas flowing into the upstream side catalyst 53) is substantially the target air-fuel ratio abyfr. Can be matched.

  On the other hand, if the air-fuel ratio feedback control condition is not satisfied at the time of determination in step 705, the CPU 81 makes a “No” determination at step 705 to proceed to step 740, and set the value of the air-fuel ratio feedback correction amount DFi to “0”. ”And the routine proceeds to step 795 to end the present routine tentatively. Thus, when the air-fuel ratio feedback control condition is not satisfied, the air-fuel ratio feedback correction amount DFi is set to “0” and the air-fuel ratio (basic fuel injection amount Fbase) is not corrected.

  Next, air-fuel ratio feedback control based on the output Voxs of the oxygen concentration sensor 68 will be described. Such control is also called sub-feedback control. By this sub-feedback control, the above-described sub-feedback control amount vafsfb is calculated.

  The CPU 81 executes the routine shown in FIG. 8 every elapse of a predetermined time in order to obtain the sub feedback control amount vafsfb. Accordingly, when the predetermined timing is reached, the CPU 81 starts the process from step 800 and proceeds to step 805 to determine whether or not the sub feedback control condition is satisfied. The sub-feedback control condition is, for example, that the air-fuel ratio feedback control condition in step 705 described above is satisfied, the engine coolant temperature THW is equal to or higher than a second predetermined temperature higher than the first predetermined temperature, and oxygen This is established when the density sensor 68 is normal.

  Now, assuming that the sub-feedback control condition is satisfied, the CPU 81 makes a “Yes” determination at step 805 to proceed to step 810, where the oxygen concentration sensor 68 is controlled by a subroutine detailed in FIG. The target value Voxsref of the output Voxs is determined. The target value Voxsref is determined so that the purification efficiency of the upstream catalyst 53 is good (best), and here, the oxygen concentration sensor 68 outputs when the stoichiometric air-fuel ratio gas reaches the oxygen concentration sensor 68. It will be set to a value (equivalent theoretical air / fuel ratio). The operation based on the subroutine shown in FIG. 9 will be described later.

Next, the CPU 81 proceeds to step 815 to obtain the output deviation amount DVoxs by subtracting the current output Voxs of the oxygen concentration sensor 68 from the determined target value Voxsref. Next, the CPU 81 proceeds to step 820 to obtain the sub feedback control amount vafsfb based on the following equation (2).
vafsfb = Kp · DVoxs + Ki · SDVoxs (2)

  In the above equation (2), Kp is a preset proportional gain, and Ki is a preset integral gain. SDVoxs is an integral value of the output deviation amount DVoxs and is a value updated in the next step 825. That is, when the CPU 81 proceeds to step 825, it adds the output deviation amount DVoxs obtained in step 815 to the integral value SDVoxs of the output deviation amount at that time to obtain the new integrated value SDVoxs of the output deviation amount, and thereafter Then, the process proceeds to step 895 to end this routine once.

  In this way, the sub feedback control amount vafsfb is obtained, and this value is added to the actual output of the air-fuel ratio sensor 67 in step 710 of FIG. 7 described above, and the sum (vabyfs + vafsfb) is shown in FIG. The upstream control air-fuel ratio abyfs is converted based on the map. In other words, the upstream control air-fuel ratio abyfs is equivalent to the sub-feedback control amount vafsfb obtained based on the output Voxs of the oxygen concentration sensor 68 with respect to the air-fuel ratio actually detected by the air-fuel ratio sensor 67. Only different air-fuel ratios are required.

  As a result, the in-cylinder fuel supply amount Fc (k−N) calculated in step 715 of FIG. 7 changes according to the output Voxs of the oxygen concentration sensor 68, so that air-fuel ratio feedback correction is performed in steps 725 and 730. The amount DFi is changed according to the output Voxs of the oxygen concentration sensor 68. Thereby, the air-fuel ratio of the engine is controlled so that the air-fuel ratio on the downstream side of the upstream catalyst 53 matches the stoichiometric air-fuel ratio.

  For example, when the output Voxs of the oxygen concentration sensor 68 indicates a value corresponding to an air / fuel ratio that is leaner than the stoichiometric air / fuel ratio, the output deviation DVoxs obtained in step 815 becomes a positive value, and therefore in step 820. The required sub-feedback control amount vafsfb is a positive value. Therefore, abyfs obtained in step 710 is obtained as a leaner value (a larger value) than the air-fuel ratio actually detected by the air-fuel ratio sensor 67.

  Therefore, the in-cylinder fuel supply amount Fc (k−N) obtained in step 715 is a small value, and the in-cylinder fuel supply amount deviation DFc obtained in step 725 is a large value. Accordingly, the air-fuel ratio feedback correction amount DFi becomes a large positive value. As a result, the final fuel injection amount Fi obtained in step 610 in FIG. 6 is controlled to be larger than the basic fuel injection amount Fbase so that the engine air-fuel ratio becomes a rich value.

  Conversely, if the output Voxs of the oxygen concentration sensor 68 shows a value corresponding to the rich air-fuel ratio rather than the stoichiometric air-fuel ratio because the average air-fuel ratio of the engine is rich, the output deviation amount DVoxs becomes a negative value. Therefore, the sub feedback control amount vafsfb becomes a negative value. Accordingly, abyfs obtained in step 710 is obtained as a richer value (a smaller value) than the air-fuel ratio that is actually detected by the air-fuel ratio sensor 67.

  Accordingly, since the in-cylinder fuel supply amount Fc (k−N) is a large value, the in-cylinder fuel supply amount deviation DFc is obtained as a negative value. As a result, the air-fuel ratio feedback correction amount DFi becomes a negative value. Thus, the final fuel injection amount Fi is controlled to be smaller than the basic fuel injection amount Fbase so that the air-fuel ratio of the engine becomes a lean value.

  Next, the operation for determining the target value Voxsref of the output Voxs of the oxygen concentration sensor 68 will be described with reference to the subroutine shown in FIG. The CPU 81 starts the process from step 900 when executing the process of step 810 in FIG. 8 and proceeds to step 905 to determine whether or not the value of the air-fuel ratio flag XAF is “0”.

  As will be described later, the value of the air-fuel ratio flag XAF is set to “0” when the output Voxs of the oxygen concentration sensor 68 becomes smaller than the low threshold Lohan due to a large amount of oxygen flowing out from the upstream catalyst 53. When the output Voxs of the oxygen concentration sensor 68 becomes larger than the high side threshold value Hihan due to a large amount of unburned components flowing out from the upstream side catalyst 53, it is set to “1”. Further, the value of the air-fuel ratio flag XAF is set to “0” in an initial routine (not shown) that is started when an ignition key (not shown) is turned on.

  Now, assuming that a large amount of oxygen is flowing out from the upstream side catalyst 53 and the value of the air-fuel ratio flag XAF is “0”, the CPU 81 determines “Yes” in step 905 and proceeds to step 910. Then, it is determined whether or not the output Voxs of the oxygen concentration sensor 68 is smaller than the low threshold Lohan.

  In this case, since the output Voxs of the oxygen concentration sensor 68 is smaller than the lower threshold value Lohan, the CPU 81 determines “Yes” in step 910 and proceeds to step 915 to set the median value a to the target value Voxsref and then Proceed to step 995 to end the present routine tentatively. As a result, the output deviation DVoxs calculated in step 815 of FIG. 8 increases, so that the air-fuel ratio of the air-fuel mixture supplied to the engine by the above-described air-fuel ratio feedback control changes to the rich side (the air-fuel ratio becomes smaller). Smaller).

  As a result, gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio starts to flow into the upstream catalyst 53. Therefore, when a predetermined time elapses, the output Voxs of the oxygen concentration sensor 68 becomes larger than the low-side threshold Lohan. At this time, when the CPU 81 executes the routine shown in FIG. 9, the CPU 81 determines “Yes” in step 905, determines “No” in step 910, and proceeds to step 920, in which the oxygen concentration sensor 68 is set. It is determined whether the output Voxs is greater than the high threshold Hihan.

  At this time, since the output Voxs of the oxygen concentration sensor 68 is smaller than the high side threshold value Hihan, the CPU 81 makes a “No” determination at step 920 and proceeds to step 925 to set the low side target value Lotgt described above as the target value Voxsref. Set. As a result, the air-fuel ratio of the gas flowing out from the upstream side catalyst 53 is maintained at a value near the stoichiometric air-fuel ratio.

  Thereafter, when a gas containing a large amount of unburned components (rich gas) flows into the upstream catalyst 53 due to a sudden change in the operating state, a large amount of unburned components flow out of the upstream catalyst 53. As a result, the output Voxs of the oxygen concentration sensor 68 becomes larger than the high threshold value Hihan. At this time, when the CPU 81 executes the routine shown in FIG. 9, the CPU 81 determines “Yes” in step 905, “No” in step 910, determines “Yes” in the subsequent step 920, and step 930. Then, the value of the air-fuel ratio flag XAF is set to “1”.

  Next, the CPU 81 proceeds to step 915, sets the median value a to the target value Voxsref, proceeds to step 995, and once ends this routine. As a result, the output deviation amount DVoxs (= a−Voxs) calculated in step 815 of FIG. 8 becomes a negative value having a large absolute value, so that the air-fuel ratio supplied to the engine by the above-described air-fuel ratio feedback control is reduced. The air-fuel ratio changes to the lean side.

  In this state, when the CPU 81 executes the routine shown in FIG. 9, the CPU 81 makes a “No” determination at step 905 to proceed to step 935, where the output Voxs of the oxygen concentration sensor 68 is greater than the high-side threshold value Hihan. It is determined whether or not. Since the present time is immediately after the output Voxs of the oxygen concentration sensor 68 becomes larger than the high-side threshold value Hihan, the output Voxs of the oxygen concentration sensor 68 is larger than the high-side threshold value Hihan. Therefore, the CPU 81 makes a “Yes” determination at step 935 to proceed to step 940, sets the median value a to the target value Voxsref, and then proceeds to step 995 to end the present routine tentatively.

  If such a state continues, the air-fuel ratio of the gas flowing out from the upstream side catalyst 53 becomes the stoichiometric air-fuel ratio, and when a predetermined time elapses, the output Voxs of the oxygen concentration sensor 68 becomes smaller than the high-side threshold value Hihan. At this time, when the CPU 81 executes the routine shown in FIG. 9, the CPU 81 makes a “No” determination at step 905 and step 935 to proceed to step 945, where the output Voxs of the oxygen concentration sensor 68 is smaller than the low threshold Lohan. It is determined whether or not.

  At this time, since the output Voxs of the oxygen concentration sensor 68 is larger than the low-side threshold value Lohan, the CPU 81 makes a “No” determination at step 945 and proceeds to step 950 to set the above-described high-side target value Hitgt to the target value Voxsref. Then, the routine proceeds to step 995 to end the present routine tentatively.

  Thereafter, when a gas containing excess oxygen (lean gas) flows into the upstream catalyst 53 due to a sudden change in the operating state, a large amount of oxygen flows out from the upstream catalyst 53. As a result, the output Voxs of the oxygen concentration sensor 68 becomes smaller than the low threshold Lohan. At this time, when the CPU 81 executes the routine shown in FIG. 9, the CPU 81 determines “No” in both steps 905 and 935, determines “Yes” in subsequent step 945, and proceeds to step 955. Then, the value of the air-fuel ratio flag XAF is set to “0”. Then, the CPU 81 proceeds to step 940, sets the median value a to the target value Voxsref, proceeds to step 995, and once ends this routine. Such an operation is repeated, and air-fuel ratio feedback control is executed.

  As described above, the air-fuel ratio control apparatus according to the first embodiment determines the current and / or past gas state reaching the oxygen concentration sensor 68 as the state of the output Voxs of the oxygen concentration sensor 68 (air-fuel ratio flag). The target value Voxsref is determined according to the state of the gas. In other words, the CPU 81 determines that the stoichiometric air-fuel ratio gas has reached the oxygen concentration sensor 68 based on the current and / or past output Voxs of the oxygen concentration sensor 68 (that is, the history of the output Voxs of the oxygen concentration sensor 68). Then, a value that the oxygen concentration sensor 68 will output is determined, and the determined value is set as a target value Voxsref for air-fuel ratio feedback (sub-feedback) control.

  As a result, the present air-fuel ratio control apparatus can maintain the air-fuel ratio of the gas flowing out from the upstream side catalyst 53 at a value close to the theoretical air-fuel ratio as much as possible. Therefore, the emission amount of nitrogen oxides and unburned components can be reduced. Can be reduced.

(Second Embodiment)
Next, an air-fuel ratio control apparatus according to a second embodiment of the present invention will be described. When the temperature Tempccro of the upstream catalyst 53 detected by the catalyst temperature sensor 69 is lower than a predetermined threshold temperature Tth, the air-fuel ratio control device converts the target value Voxsref to the current and / or past output Voxs of the oxygen concentration sensor 68. It is different from the air-fuel ratio control apparatus according to the first embodiment only in that it always maintains the median value a. Accordingly, the following description will focus on such differences.

  The CPU 81 of this air-fuel ratio control apparatus executes the routine shown in FIG. 10 instead of FIG. In FIG. 10, the same steps as those shown in FIG. 9 are denoted by the same reference numerals, and detailed description thereof is omitted.

  Therefore, when executing the process of step 810 in FIG. 8, the CPU 81 starts the process from step 1000, proceeds to step 1005, and the temperature Tempccro of the upstream catalyst 53 detected by the catalyst temperature sensor 69 is a predetermined threshold temperature. Judge whether it is greater than Tth. The threshold temperature Tth is preferably set to a temperature corresponding to the activation temperature of the upstream catalyst 53.

  Now, if the explanation is continued after the engine is started and the upstream side catalyst 53 has not been sufficiently warmed up (that is, the temperature Tempccro is smaller than the predetermined threshold temperature Tth), the CPU 81 “ The process proceeds to step 1010, where the target value Voxsref of the output Voxs of the oxygen concentration sensor 68 is set to the median value a. Next, the CPU 81 proceeds to step 1015 to set the value of the air-fuel ratio flag XAF to “0”, proceeds to step 1095, and once ends this routine.

  Thereafter, when the upstream catalyst 53 is warmed up and the temperature Tempccro becomes higher than the predetermined threshold temperature Tth, the CPU 81 makes a “Yes” determination at step 1005 to perform the same steps 905 to 950 as in the routine shown in FIG. The routine consisting of will be executed. As a result, when the temperature Tempccro is higher than the predetermined threshold temperature Tth, the target value Voxsref is determined according to the current and / or past gas state reaching the oxygen concentration sensor 68.

  If the upstream catalyst 53 is not warmed up, the upstream catalyst 53 can hardly exhibit the oxygen storage function. Therefore, when lean and rich gases flow into the upstream catalyst 53, either a large amount (high concentration) of oxygen or a large amount (high concentration) of unburned components flows out from the upstream catalyst 53, respectively. Accordingly, in such a state, when the stoichiometric air-fuel ratio gas reaches the oxygen concentration sensor 68, the output Voxs of the oxygen concentration sensor 68 becomes equal to the median value a.

  From such a viewpoint, the air-fuel ratio control apparatus according to the second embodiment described above maintains the target value Voxsref at the median value a when the temperature Tempccro of the upstream catalyst 53 is lower than the predetermined threshold temperature Tth. As a result, the air-fuel ratio of the air-fuel mixture supplied to the engine can be controlled to a value near the stoichiometric air-fuel ratio before the upstream catalyst 53 is warmed up, and the upstream catalyst 53 after the upstream catalyst 53 is warmed up. Since the air-fuel ratio of the gas flowing out from the engine can be controlled to a value close to the stoichiometric air-fuel ratio, the amount of nitrogen oxides and unburned components released into the atmosphere can be reduced.

  In the above embodiment, the threshold temperature Tth is a fixed value. However, the threshold temperature Tth is a value indicating the degree of deterioration of the upstream catalyst 53 (for example, the maximum oxygen storage amount Cmax of the upstream catalyst 53 described later, the upstream catalyst 53 is attached). The travel distance of the vehicle, the integrated value of the gas passage amount of the upstream side catalyst 53, the locus ratio of the output of the air-fuel ratio sensor 67 and the output of the oxygen concentration sensor 68, etc.) may be changed. In this case, the catalyst activation temperature (the temperature at which a sufficient oxygen storage function is exerted) increases as the catalyst deterioration progresses. Therefore, the deterioration of the upstream catalyst 53 progresses (for example, the maximum oxygen storage amount Cmax). It is preferable to increase the threshold temperature Tth as the value decreases.

(Third embodiment)
Next, an air-fuel ratio control apparatus according to a third embodiment of the present invention will be described. In the air-fuel ratio control apparatus, when the maximum oxygen storage amount Cmax of the upstream catalyst 53 is smaller than a predetermined oxygen storage amount (threshold) Cth, the target value Voxsref depends on the current and / or past output Voxs of the oxygen concentration sensor 68. It is different from the air-fuel ratio control apparatus according to the first embodiment only in that it is always maintained at the median value a. Accordingly, the following description will focus on such differences.

  The CPU 81 of this air-fuel ratio control apparatus executes the routine shown in FIG. 11 instead of FIG. In FIG. 11, the same steps as those shown in FIG. 9 are denoted by the same reference numerals, and detailed description thereof is omitted.

  Therefore, when executing the process of step 810 in FIG. 8, the CPU 81 starts the process from step 1100 and proceeds to step 1105 to check whether the maximum oxygen storage amount Cmax of the upstream catalyst 53 is larger than the predetermined oxygen storage amount Cth. Determine whether or not.

  The maximum oxygen storage amount Cmax is estimated separately by a routine (not shown) as follows. First, the CPU 81 completes warming up of the upstream catalyst 53 (that is, the temperature Tempccro of the upstream catalyst 53 is greater than a predetermined threshold temperature Tth), and the engine operating state is in a steady operating state (for example, When the time variation of the throttle valve opening TA is small and the time variation of the engine speed NE is small), the air-fuel ratio of the air-fuel mixture supplied to the engine is less than the stoichiometric air-fuel ratio. Is also kept lean, and it is monitored whether or not the output Voxs of the oxygen concentration sensor 68 has reached the minimum output value min (or whether or not it has become smaller than the low-side threshold value Lohan).

  Then, from the time when it is confirmed that the output Voxs of the oxygen concentration sensor 68 is the minimum output value min, the CPU 81 keeps the air-fuel ratio of the air-fuel mixture supplied to the engine richer than the stoichiometric air-fuel ratio, and oxygen It is monitored whether or not the output Voxs of the density sensor 68 has reached the maximum output value max (or whether or not it has become larger than the high-side threshold value Hihan).

  During this time, the oxygen stored in the upstream catalyst 53 is consumed. When the oxygen storage amount becomes “0” and a large amount of unburned components start to flow out from the upstream catalyst 53, the output Voxs of the oxygen concentration sensor 68 becomes the maximum output value max. This time point is referred to as time ts for convenience of explanation. The CPU 81 maintains the air-fuel ratio of the air-fuel mixture supplied to the engine from time ts at a lean air-fuel ratio than the stoichiometric air-fuel ratio. At the same time, the CPU 81 monitors whether or not the output Voxs of the oxygen concentration sensor 68 has reached the minimum output value min.

  During this time, oxygen is stored in the upstream catalyst 53. When the oxygen storage amount becomes the maximum oxygen storage amount Cmax, when a large amount of oxygen starts to flow out from the upstream catalyst 53, the output Voxs of the oxygen concentration sensor 68 becomes the minimum output value min. This time point is referred to as time tf for convenience of explanation.

The CPU 81 calculates the oxygen storage amount change ΔO2 based on the following formula (3) and the following formula (4) between these times ts to tf and integrates them, and the oxygen storage amount at time tf. Is calculated as the maximum oxygen storage amount Cmax.
ΔO2 = 0.23 · mfr · | stoich − abyfs | (3)
Cmax = ΣΔO2 (section t = ts to tf) (4)

  In the above equation (3), the value “0.23” is the weight ratio of oxygen contained in the atmosphere. mfr is the total amount of the fuel injection amount Fi within the predetermined time tsample, and stoich is the stoichiometric air-fuel ratio (for example, 14.7). abyfs is an average value of the air-fuel ratio A / F detected by the air-fuel ratio sensor 67 at a predetermined time tsample.

  As shown in the equation (3), by multiplying the total amount mfr of the injection amount within the predetermined time tsample by the deviation | stoich−abyfs | from the theoretical air / fuel ratio of the detected air / fuel ratio A / F, An excess amount of air at the same predetermined time tsample is obtained, and an oxygen storage amount change amount (an increase amount of stored oxygen) ΔO2 at the same predetermined time tsample is obtained by multiplying the excess air amount by the weight ratio of oxygen. Then, as shown in the equation (4), by integrating the oxygen storage amount change amount ΔO2 over the times ts to tf, the upstream catalyst 53 has changed from a state where the upstream catalyst 53 does not store any oxygen. The amount of oxygen increase until the oxygen is stored to the limit, that is, the maximum oxygen storage amount Cmax is estimated and calculated.

  The air-fuel ratio of the air-fuel mixture supplied to the engine from time tf is maintained at an air-fuel ratio richer than the stoichiometric air-fuel ratio until the output Voxs of the oxygen concentration sensor 68 reaches the maximum output value max (3 ) And (4) may be used to determine the maximum oxygen storage amount Cmax.

  Now, if the upstream catalyst 53 is deteriorated and the explanation is continued assuming that the maximum oxygen storage amount Cmax of the upstream catalyst 53 is smaller than the predetermined oxygen storage amount Cth, the CPU 81 makes a “No” determination at step 1105. In step 1010, the target value Voxsref of the output Voxs of the oxygen concentration sensor 68 is set to the median value a. Next, the CPU 81 proceeds to step 1015 to set the value of the air-fuel ratio flag XAF to “0”, proceeds to step 1195, and once ends this routine. As described above, when the maximum oxygen storage amount Cmax of the upstream catalyst 53 is smaller than the oxygen storage amount Cth, the median value a is the target value regardless of the current and / or past gas states that have reached the oxygen concentration sensor 68. Set to Voxsref.

  On the other hand, if the upstream catalyst 53 is not deteriorated and the maximum oxygen storage amount Cmax of the upstream catalyst 53 is larger than the oxygen storage amount Cth, the CPU 81 makes a “Yes” determination at step 1105, as shown in FIG. A routine consisting of the same steps 905 to 950 as the routine is executed. As described above, when the maximum oxygen storage amount Cmax of the upstream catalyst 53 is larger than the oxygen storage amount Cth, the target value Voxsref is determined according to the current and / or past gas state reaching the oxygen concentration sensor 68. .

  When the maximum oxygen storage amount Cmax of the upstream catalyst 53 is smaller than the predetermined oxygen storage amount Cth, the upstream catalyst 53 can hardly exhibit the oxygen storage function. Therefore, when lean and rich gases flow into the upstream catalyst 53, either a large amount (high concentration) of oxygen or a large amount (high concentration) of unburned components flows out from the upstream catalyst 53, respectively. Accordingly, in such a state, when the stoichiometric air-fuel ratio gas reaches the oxygen concentration sensor 68, the output Voxs of the oxygen concentration sensor 68 becomes equal to the median value a.

  From such a viewpoint, the air-fuel ratio control apparatus according to the third embodiment described above maintains the target value Voxsref at the median value a when the maximum oxygen storage amount Cmax of the upstream catalyst 53 is smaller than the oxygen storage amount Cth. Thus, when the maximum oxygen storage amount Cmax of the upstream catalyst 53 is smaller than the oxygen storage amount Cth, the air-fuel ratio of the air-fuel mixture supplied to the engine can be controlled to a value close to the theoretical air-fuel ratio, and the maximum of the upstream catalyst 53 When the oxygen storage amount Cmax is larger than the oxygen storage amount Cth, the air-fuel ratio of the gas flowing out from the upstream catalyst 53 can be controlled to a value near the stoichiometric air-fuel ratio, so that nitrogen oxides released into the atmosphere or unburned The amount of ingredients can be reduced.

(Fourth embodiment)
Next, an air-fuel ratio control apparatus according to a fourth embodiment of the present invention will be described. In this air-fuel ratio control apparatus, the air-fuel ratio control apparatus according to the first embodiment determines and changes the target value Voxsref of the air-fuel ratio feedback control based on the current and / or past output Voxs of the oxygen concentration sensor 68. On the other hand, it differs from the air-fuel ratio control apparatus according to the first embodiment only in that the target value Voxsref is determined and changed based on the oxygen storage amount OSA of the upstream catalyst 53.

  In other words, the air-fuel ratio control apparatus according to the fourth embodiment is based on the oxygen storage amount OSA of the upstream catalyst 53 (the current and / or past value of the oxygen storage amount OSA, that is, the history of the oxygen storage amount OSA). When the stoichiometric air-fuel ratio gas reaches the oxygen concentration sensor 68, a value that the oxygen concentration sensor 68 will output is determined, and the determined value is set as a target value Voxsref for air-fuel ratio feedback (sub-feedback) control. To do. Hereinafter, the fourth embodiment will be described focusing on differences from the first embodiment.

  The CPU 81 of this air-fuel ratio control device executes the routine shown in FIG. 12 instead of FIG. In FIG. 12, the same steps as those shown in FIG. 9 are denoted by the same reference numerals, and detailed description thereof is omitted.

  Therefore, when executing the process of step 810 in FIG. 8, the CPU 81 starts the process from step 1200 and proceeds to step 905 to determine whether or not the value of the air-fuel ratio flag XAF is “0”.

  Now, the oxygen storage amount OSA of the upstream catalyst 53 reaches the substantially maximum oxygen storage amount Cmax, a large amount of oxygen flows out from the upstream catalyst 53, and the value of the air-fuel ratio flag XAF is “0”. As described above, the CPU 81 determines “Yes” in step 905 and proceeds to step 1205, where the value of the oxygen storage amount OSA is smaller than the maximum oxygen storage amount Cmax by a small amount β (high-side oxygen storage amount threshold value). ) It is determined whether it is larger. In other words, the CPU 81 determines in step 1205 whether or not the oxygen storage amount OSA has substantially reached the maximum oxygen storage amount Cmax. The method for estimating the oxygen storage amount OSA will be described later. The high-side oxygen storage amount threshold value is naturally larger than a half value of the maximum oxygen storage amount Cmax (hereinafter referred to as “half value”).

  In this case, since the value of the oxygen storage amount OSA is larger than the maximum oxygen storage amount Cmax by a small amount β (high side oxygen storage amount threshold), the CPU 81 determines “Yes” in step 1205 and proceeds to step 915. Proceed to set the median value a to the target value Voxsref, and then proceed to Step 995 to end the present routine tentatively. At this time, since a large amount of oxygen has reached the oxygen concentration sensor 68, the output Voxs is a value in the vicinity of the minimum output value min. Accordingly, since the output deviation amount DVoxs calculated in step 815 in FIG. 8 becomes large, the air-fuel ratio of the air-fuel mixture supplied to the engine by the above-described air-fuel ratio feedback (sub-feedback) control changes to the rich side.

  As a result, gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio starts to flow into the upstream catalyst 53. Therefore, when a predetermined time elapses, the oxygen storage amount OSA decreases and a slight amount β from the maximum oxygen storage amount Cmax. Only smaller than the smaller value (high-side oxygen storage amount threshold). At this time, when the CPU 81 executes the routine shown in FIG. 12, the CPU 81 makes a “Yes” determination in step 905 and makes a “No” determination in step 1205 to proceed to step 1210, where the oxygen storage amount OSA is It is determined whether or not it is smaller than a minute amount β (low-side oxygen storage amount threshold). In other words, the CPU 81 determines in step 1210 whether or not the oxygen storage amount OSA is substantially “0”. Further, the low-side oxygen storage amount threshold is naturally smaller than the half value of the maximum oxygen storage amount Cmax.

  At this time, since the oxygen storage amount OSA is not substantially “0”, the CPU 81 makes a “No” determination at step 1210 to proceed to step 925 to set the above-described low-side target value Lotgt as the target value Voxsref. .

  Thereafter, when a gas containing a large amount of unburned components (rich gas) flows into the upstream side catalyst 53 due to reasons such as a sudden change in the operating state, the oxygen storage amount OSA decreases and a small amount β (low side) Oxygen storage amount threshold). At this time, when the CPU 81 executes the routine shown in FIG. 12, the CPU 81 determines “Yes” in step 905, “No” in step 1205, and determines “Yes” in step 1210, thereby determining step 930. Then, the value of the air-fuel ratio flag XAF is set to “1”.

  Next, the CPU 81 proceeds to step 915, sets the median value a to the target value Voxsref, proceeds to step 1295, and once ends this routine. At this time, since a large amount of unburned components have reached the oxygen concentration sensor 68, the output Voxs is a value near the maximum output value max. Accordingly, since the output deviation amount DVoxs calculated in step 815 in FIG. 8 is a negative value having a large absolute value, the air-fuel ratio of the air-fuel mixture supplied to the engine by the above-described air-fuel ratio feedback (sub-feedback) control is lean. Change to the side.

  In such a state, when the CPU 81 executes the routine shown in FIG. 12, the CPU 81 makes a “No” determination at step 905 to proceed to step 1215, where the oxygen storage amount OSA is a very small amount β (low-side oxygen storage amount). It is determined whether or not the threshold value is smaller. In other words, the CPU 81 determines in step 1210 whether or not the oxygen storage amount OSA is substantially “0”.

  Since the present time is immediately after the oxygen storage amount OSA becomes smaller than the very small amount β (low-side oxygen storage amount threshold value), the CPU 81 makes a “Yes” determination at step 1215 and proceeds to step 940 to proceed to the target value Voxsref. Is set to the median value a, and the routine proceeds to step 1295 to end the present routine tentatively.

  When such a state continues, a lean gas containing excess oxygen flows into the upstream catalyst 53, and the excess oxygen is stored in the upstream catalyst 53. Therefore, the oxygen storage amount OSA increases and becomes greater than a small amount β (low-side oxygen storage amount threshold) after a predetermined time. Further, the air-fuel ratio of the gas flowing out from the upstream catalyst 53 becomes the stoichiometric air-fuel ratio.

  At this time, when the CPU 81 executes the routine shown in FIG. 12, the CPU 81 makes a “No” determination at both steps 905 and 1215, proceeds to step 1220, and the value of the oxygen storage amount OSA is the maximum oxygen storage amount. It is determined whether or not it is larger than a value (high-side oxygen storage amount threshold) that is smaller than Cmax by a small amount β. At this time, the oxygen storage amount OSA is not a value near the maximum oxygen storage amount Cmax. Accordingly, the CPU 81 makes a “No” determination at step 1220 to proceed to step 950, sets the above-described high-side target value Hitgt as the target value Voxsref, and proceeds to step 1295 to end the present routine tentatively.

  Thereafter, when a gas containing a large amount of oxygen (lean gas) flows into the upstream catalyst 53 due to a sudden change in the operating state, the oxygen storage amount OSA becomes substantially equal to the maximum oxygen storage amount Cmax. . At this time, when the CPU 81 executes the routine shown in FIG. 12, the CPU 81 determines “No” in both steps 905 and 1215, determines “Yes” in step 1220, and proceeds to step 955. Then, the value of the air-fuel ratio flag XAF is set to “0”. Then, the CPU 81 proceeds to step 940 to set the median value “a” to the target value Voxsref, proceeds to step 1295, and once ends this routine. Such an operation is repeated, and air-fuel ratio feedback control is executed.

Next, a method for estimating the oxygen storage amount OSA will be described. When the output Voxs of the oxygen concentration sensor 68 changes from a value other than the maximum output value max to the maximum output value max (or the output Voxs changes from a value smaller than the high side threshold Hihan to a value larger than the high side threshold Hihan). The oxygen storage amount OSA is set to “0”. Then, the oxygen storage amount OSA is estimated according to the following formula (5) and the following formula (6). The basis for these formulas is the same as that for formulas (3) and (4) described above.
ΔO2 = 0.23 · mfr · (abyfs − stoich) (5)
OSA = ΣΔO2 (However, 0 ≦ OSA ≦ Cmax) (6)

  As described above, the air-fuel ratio control apparatus according to the fourth embodiment determines the current and / or past gas state (in other words, the state of the catalyst) that has reached the oxygen concentration sensor 68 as the oxygen storage amount OSA. Judging from the state, the target value Voxsref is determined according to the state of the gas. That is, the CPU 81 determines the oxygen concentration sensor when the stoichiometric air-fuel ratio gas reaches the oxygen concentration sensor 68 based on the current and / or past value of the oxygen storage amount OSA (that is, the history of the oxygen storage amount OSA). The value which 68 will output is determined, and the determined value is set as a target value Voxsref for air-fuel ratio feedback (sub-feedback) control.

  As a result, the air-fuel ratio of the gas flowing out from the upstream catalyst 53 can be maintained at a value close to the stoichiometric air-fuel ratio, so that the discharge amount of nitrogen oxides and unburned components is reduced.

(Fifth embodiment)
Next, an air-fuel ratio control apparatus according to a fifth embodiment of the present invention will be described. This air-fuel ratio control device is different from the first embodiment only in that the high-side target value and the low-side target value of the first embodiment are changed according to the element temperature of the oxygen concentration sensor 68. Accordingly, the following description will focus on such differences.

  The output Voxs of the oxygen concentration sensor 68 with respect to the air-fuel ratio of the gas that reaches the oxygen concentration sensor 68 changes according to the element temperature of the oxygen concentration sensor 68 as shown in FIG. In FIG. 13, a solid curve C1 and a dashed curve C2 indicate the outputs Voxs when the element temperature of the oxygen concentration sensor 68 is relatively low and high, respectively.

  First, the case where the element temperature of the oxygen concentration sensor 68 is low will be described. When a large amount of unburned components flow out from the upstream catalyst 53 and the air-fuel ratio of the gas that has reached the oxygen concentration sensor 68 (the oxygen concentration sensor-arriving gas) is richer than the stoichiometric air-fuel ratio, the output Voxs of the oxygen concentration sensor 68 Becomes larger than the above-described high-side threshold value Hihan. Thereafter, when the air-fuel ratio of the gas flowing into the catalyst 53 gradually increases (changes toward the lean side), the output Voxs gradually decreases and becomes smaller than the high-side threshold value Hihan. When the air-fuel ratio of the gas reaching the oxygen concentration sensor becomes the stoichiometric air-fuel ratio, the output Voxs matches the value HitgtHi that is larger than the median value a and smaller than the high-side threshold value Hihan. Thereafter, when the upstream catalyst 53 can no longer store oxygen, a large amount of oxygen begins to flow out of the upstream catalyst 53. As a result, the output Voxs becomes smaller than the lower threshold Lohan.

  From this state, when the air-fuel ratio of the gas flowing into the catalyst 53 gradually decreases (changes toward the rich side), the output Voxs gradually increases and becomes greater than the low threshold Lohan. When the air-fuel ratio of the gas reaching the oxygen concentration sensor becomes the stoichiometric air-fuel ratio, the output Voxs matches the value LotgtLo that is smaller than the median value a and larger than the low-side threshold Lohan. Thereafter, when the air-fuel ratio of the gas reaching the oxygen concentration sensor is further reduced and all of the oxygen stored in the upstream catalyst 53 is consumed and a large amount of unburned components start to flow out of the upstream catalyst 53, the output Voxs becomes It becomes larger than the high threshold Hihan.

  Next, the case where the element temperature of the oxygen concentration sensor 68 is high will be described. When a large amount of unburned components flow out from the upstream side catalyst 53 and the air-fuel ratio of the gas reaching the oxygen concentration sensor is richer than the stoichiometric air-fuel ratio, the output Voxs becomes larger than the above-described high-side threshold value Hihan. Thereafter, when the air-fuel ratio of the gas flowing into the catalyst 53 gradually increases, the output Voxs gradually decreases and becomes smaller than the high-side threshold value Hihan. When the air-fuel ratio of the gas reaching the oxygen concentration sensor becomes the stoichiometric air-fuel ratio, the output Voxs matches the value HitgtLo that is larger than the median value a and smaller than the value HitgtHi. Thereafter, when the upstream catalyst 53 can no longer store oxygen, a large amount of oxygen begins to flow out of the upstream catalyst 53. As a result, the output Voxs becomes smaller than the lower threshold Lohan.

  From this state, when the air-fuel ratio of the gas flowing into the catalyst 53 gradually decreases, the output Voxs gradually increases and becomes greater than the low threshold Lohan. When the air-fuel ratio of the gas reaching the oxygen concentration sensor becomes the stoichiometric air-fuel ratio, the output Voxs matches the value LotgtHi that is larger than the value LotgtLo and smaller than the median value a. Thereafter, when the air-fuel ratio of the gas reaching the oxygen concentration sensor further decreases and a large amount of unburned components start to flow out from the upstream catalyst 53, the output Voxs becomes larger than the high threshold Hihan.

  The air-fuel ratio control apparatus according to the fifth embodiment is based on such behavior of the output Voxs and decreases after the output Voxs becomes larger than the high-side threshold Hihan and is between the high-side threshold Hihan and the low-side threshold Lohan. If the element temperature of the oxygen concentration sensor 68 is lower than the predetermined temperature during the period, the target value Voxsref is set to the value HitgtHi that is the high-side target value for low temperature, and the target value if the element temperature of the oxygen concentration sensor 68 is equal to or higher than the predetermined temperature. Voxsref is set to a value HitgtLo which is a high temperature high side target value smaller than the low temperature high side target value HitgtHi.

  Further, the air-fuel ratio control device increases the element Voxs after the output Voxs becomes smaller than the low-side threshold Lohan, and the element temperature of the oxygen concentration sensor 68 is a predetermined temperature during the period between the low-side threshold Lohan and the high-side threshold Hihan. If it is lower, the target value Voxsref is set to the above-mentioned value LotgtLo, which is the low-side target value for low temperature. If the element temperature of the oxygen concentration sensor 68 is equal to or higher than the predetermined temperature, the target value Voxsref is larger than the low-side target value LotgtLo Set to the value LotgtHi, which is the low-side target value for high temperature. Further, the air-fuel ratio control apparatus is configured such that the target value Voxsref regardless of the element temperature of the oxygen concentration sensor 68 during the period when the output Voxs is larger than the high threshold Hihan and the period when the output Voxs is smaller than the low threshold Lohan. Is set to the median a.

  Next, the configuration and actual operation of this air-fuel ratio control apparatus will be described. This air-fuel ratio control apparatus includes an element temperature sensor (oxygen concentration sensor element temperature sensor) 72 as shown in FIG. 14 in addition to the configuration of the air-fuel ratio control apparatus according to the first embodiment. The element temperature sensor 72 is disposed in the vicinity of the element of the oxygen concentration sensor 68 and outputs a signal representing the element temperature Tsens of the oxygen concentration sensor 68. The element temperature sensor 72 is connected to the interface 85 of the electric control device 80 shown in FIG. The element temperature sensor 72 is oxygen concentration sensor element temperature acquisition means for acquiring the element temperature of the oxygen concentration sensor 68.

  The CPU 81 of this air-fuel ratio control device executes the routine shown in FIGS. 6 to 8 and the routine shown in FIG. 15 instead of FIG. The routine shown in FIG. 15 is a routine in which step 925 in FIG. 9 is replaced with steps 1505 to 1520 and step 950 in FIG. 9 is replaced with steps 1525 to 1540.

  Therefore, when executing the process of step 810 in FIG. 8, the CPU 81 starts the process from step 1500 in FIG. 15 and performs the process in step 905. Now, assuming that the value of the air-fuel ratio flag XAF is “0” and the output Voxs of the oxygen concentration sensor 68 is in a state smaller than the low-side threshold value Lohan, the CPU 81 proceeds to “Yes” in step 905. And the processing of step 910 and step 915 is performed to set the target value Voxsref to the median value a, and the routine proceeds to step 1595 to end the present routine tentatively.

  As a result, since the air-fuel ratio of the air-fuel mixture supplied to the engine by the air-fuel ratio feedback control described above changes to the rich side, the amount of oxygen contained in the gas flowing out from the upstream catalyst 53 after a predetermined time has elapsed. descend. As a result, the output Voxs of the oxygen concentration sensor 68 becomes larger than the low threshold Lohan. At this time, when the CPU 81 executes the routine shown in FIG. 15, the CPU 81 proceeds to step 920 via steps 905 and 910.

  At this time, since the output Voxs of the oxygen concentration sensor 68 is smaller than the high-side threshold value Hihan, the CPU 81 makes a “No” determination at step 920 to proceed to step 1505, and based on the output of the element temperature sensor 72 68 element temperature Tsens is acquired. Next, the CPU 81 proceeds to step 1510 to determine whether or not the acquired element temperature Tsens is equal to or higher than a predetermined temperature Tsth.

  When the element temperature Tsens is equal to or higher than the predetermined temperature Tsth, the CPU 81 determines “Yes” in step 1510 and proceeds to step 1515 to set the target value Voxsref to the value LotgtHi that is the low-side target value for high temperature. . On the other hand, when the element temperature Tsens is lower than the predetermined temperature Tsth, the CPU 81 determines “No” in step 1510 and proceeds to step 1520 to set the target value Voxsref to the value LotgtLo that is the low-side target value for low temperature. To do. Thereafter, the CPU 81 proceeds to step 1595 to end the present routine tentatively. As a result, the air-fuel ratio of the gas flowing out from the upstream side catalyst 53 is maintained at a value near the stoichiometric air-fuel ratio.

  Thereafter, a gas containing a large amount of unburned components (rich gas) flows into the upstream catalyst 53 due to reasons such as a sudden change in the operating state. As a result, a large amount of unburned components from the upstream catalyst 53 When it flows out, the output Voxs of the oxygen concentration sensor 68 becomes larger than the high-side threshold value Hihan. At this time, when the CPU 81 executes the routine shown in FIG. 15, the CPU 81 proceeds to steps 905, 910, 920, 930 and step 915, sets the value of the air-fuel ratio flag XAF to “1”, and sets the target value Voxsref to The median is set to a, and this routine is once terminated in step 1595.

  When the CPU 81 executes the routine shown in FIG. 15 in such a state (immediately after the output Voxs of the oxygen concentration sensor 68 becomes larger than the high-side threshold value Hihan), the CPU 81 performs the processing of steps 905, 935 and 940. Then, after setting the median value a to the target value Voxsref, this routine is once ended in step 1595.

  If such a state continues, the air-fuel ratio of the air-fuel mixture supplied to the engine by the above-described air-fuel ratio feedback control changes to the lean side, so that the output Voxs of the oxygen concentration sensor 68 becomes high after a predetermined time has elapsed. It becomes smaller than the threshold value Hihan. At this time, when the CPU 81 executes the routine shown in FIG. 15, the CPU 81 proceeds to step 1525 via steps 905, 935 and 945, and acquires the element temperature Tsens of the oxygen concentration sensor 68 as in step 1505. Next, the CPU 81 proceeds to step 1530 and determines whether or not the acquired element temperature Tsens is equal to or higher than a predetermined temperature Tsth.

  When the element temperature Tsens is equal to or higher than the predetermined temperature Tsth, the CPU 81 determines “Yes” in step 1530 and proceeds to step 1535 to set the target value Voxsref to the value HitgtLo that is the high-side target value for high temperature. . On the other hand, when the element temperature Tsens is lower than the predetermined temperature Tsth, the CPU 81 determines “No” in step 1530 and proceeds to step 1540 to set the target value Voxsref to the value HitgtHi that is the high-temperature target value for low temperature. To do. Thereafter, the CPU 81 proceeds to step 1595 to end the present routine tentatively. As a result, the air-fuel ratio of the gas flowing out from the upstream side catalyst 53 is maintained at a value near the stoichiometric air-fuel ratio.

  Thereafter, a gas containing a large amount of excess oxygen flows into the upstream catalyst 53 due to reasons such as a sudden change in the operating state. As a result, when a large amount of oxygen flows out of the upstream catalyst 53, the oxygen concentration sensor 68. Output Voxs is smaller than the lower threshold Lohan. At this time, when the CPU 81 executes the routine shown in FIG. 15, the CPU 81 proceeds to steps 905, 935, 945, 955 and 940 to set the value of the air-fuel ratio flag XAF to “0” and set the target value Voxsref to The median is set to a, and this routine is once terminated in step 1595.

  As described above, according to the air-fuel ratio control apparatus of the fifth embodiment, the target value Voxsref is changed based on the state of the gas that has reached the oxygen concentration sensor 68 and the element temperature Tsens of the oxygen concentration sensor 68. . Therefore, the target value Voxsref of the air-fuel ratio feedback control (sub-feedback control) is more accurately set to the output Voxs indicated by the oxygen concentration sensor 68 when the air-fuel ratio of the gas reaching the oxygen concentration sensor 68 is the stoichiometric air-fuel ratio. Can be matched. As a result, the opportunity for the gas having a substantially stoichiometric air-fuel ratio to flow out from the upstream side catalyst 53 increases, so that the amount of nitrogen oxides and unburned components released into the atmosphere can be further reduced.

  In the fifth embodiment, the element temperature Tsens of the oxygen concentration sensor 68 is acquired based on the output of the element temperature sensor 72. For example, the engine load such as the accelerator pedal operation amount Accp and the engine speed NE And the element temperature Tsens of the oxygen concentration sensor 68 are obtained in advance by experiments, and this relationship is stored in the ROM 82 as a map MapTsens (Accp, NE), so that the actual accelerator pedal operation amount Accp and the actual engine speed are stored. The element temperature Tsens may be acquired (estimated) based on the speed NE and the map MapTsens (Accp, NE).

  Steps 1510 to 1520 correspond to low-side target value setting means for changing to a larger value as the element temperature Tsens of the oxygen concentration sensor 68 from which the low-side target value Logtg is acquired is higher. Steps 1530 to 1540 correspond to high-side target value setting means for changing to a smaller value as the element temperature Tsens of the oxygen concentration sensor 68 from which the high-side target value Hitgt is acquired is higher. In other words, the oxygen concentration sensor 68 from which the low-side target value Logtg is acquired may be continuously changed so as to increase as the element temperature Tsens of the oxygen-concentration sensor 68 is high. The higher the element temperature Tsens of 68, the smaller the value may be continuously changed.

(Sixth embodiment)
Next, an air-fuel ratio control apparatus according to a sixth embodiment of the present invention will be described. This air-fuel ratio control device always sets the target value Voxsref to the high side without setting the target value Voxsref to the median value a during the period from when the output Voxs of the oxygen concentration sensor 68 becomes higher than the high side threshold Hihan to below the low side threshold Lohan. The target value Voxsref is always set to the low value without setting the target value Voxsref to the median a during the period from when the output Voxs of the oxygen concentration sensor 68 becomes smaller than the low threshold Lohan to when the output Voxs becomes larger than the high threshold Hihan. This is different from the fifth embodiment in that the value is set. In the fifth embodiment, the high-side target value and the low-side target value are changed according to the element temperature Tsens.

  The hardware configuration of this air-fuel ratio control device is the same as that of the fifth embodiment. On the other hand, the CPU 81 of this air-fuel ratio control apparatus executes the routine shown in FIGS. 6 to 8 and the routine shown in FIG. 16 instead of FIG.

  Therefore, when executing the process of step 810 of FIG. 8, the CPU 81 starts the process from step 1600 of FIG. 16 and proceeds to step 1605, and the element temperature Tsens of the oxygen concentration sensor 68 based on the output of the element temperature sensor 72. To get. Next, the CPU 81 proceeds to step 1610 to determine whether or not the value of the air-fuel ratio flag XAF is “0”.

  Now, assuming that a large amount of oxygen is flowing out from the upstream side catalyst 53 and the value of the air-fuel ratio flag XAF is “0”, the CPU 81 determines “Yes” in step 1610 and proceeds to step 1615. Then, it is determined whether or not the output Voxs of the oxygen concentration sensor 68 is larger than the high threshold Hihan.

  In this case, since the output Voxs of the oxygen concentration sensor 68 is smaller than the low-side threshold Lohan, the output Voxs of the oxygen concentration sensor 68 is naturally smaller than the high-side threshold Hihan. Accordingly, the CPU 81 makes a “No” determination at step 1615 to proceed to step 1625, where the relationship between the element temperature Tsens and the low-side target value Lotgt shown at step 1625 and the actual element temperature Tsens acquired at step 1605 are obtained. Based on the above, the current low-side target value Lotgt is determined. The relationship shown in step 1625 is obtained in advance by experiments and stored in the ROM 82. According to this relationship, the low-side target value Lotgt is a value between the median value a and the low-side threshold value Lohan, and is required to increase as the element temperature Tsens increases. Next, the CPU 81 proceeds to step 1630, sets the low-side target value Lotgt obtained in step 1625 to the target value Voxsref, proceeds to step 1695, and once ends this routine.

  Thereafter, when the CPU 81 executes the routine shown in FIG. 16, the CPU 81 executes the processing of steps 1605 to 1615, step 1625, and step 1630, and sets the target value Voxsref to the low-side target value Lotgt determined in step 1625. Come to set.

  By executing the air-fuel ratio feedback control described above based on the target value Voxsref determined in this way, the air-fuel ratio of the air-fuel mixture supplied to the engine changes to the rich side (the air-fuel ratio becomes smaller). . As a result, the air-fuel ratio of the gas flowing out from the upstream catalyst 53 becomes a value near the stoichiometric air-fuel ratio.

  Thereafter, when a gas containing a large amount of unburned components flows into the upstream side catalyst 53 due to reasons such as a sudden change in the operating state, a large amount of unburned components flow out from the upstream side catalyst 53. As a result, the output Voxs of the oxygen concentration sensor 68 becomes larger than the high threshold value Hihan. At this time, when the CPU 81 executes the routine shown in FIG. 16, the CPU 81 makes a “Yes” determination at step 1615 following step 1605 and step 1610 to proceed to step 1620 to set the value of the air-fuel ratio flag XAF to “1”. Set to.

  Next, the CPU 81 proceeds to step 1635, and based on the relationship between the element temperature Tsens and the high-side target value Hitgt shown in step 1635 and the actual element temperature Tsens acquired in step 1605, the current high-side target value Hitgt. To decide. The relationship shown in step 1635 is obtained in advance by experiment and is stored in the ROM 82. According to this relationship, the high-side target value Hitgt is a value between the median value a and the high-side threshold value Hihan, and is required to decrease as the element temperature Tsens increases. Next, the CPU 81 proceeds to step 1640, sets the high side target value Hitgt obtained in step 1635 to the target value Voxsref, proceeds to step 1695, and once ends this routine.

  Thereafter, when the CPU 81 starts the processing of the routine shown in FIG. 16, the CPU 81 makes a “No” determination at step 1610 following step 1605, proceeds to step 1645, and the output Voxs of the oxygen concentration sensor 68 decreases to the lower threshold value Lohan. It is determined whether it is smaller. In this case, the output Voxs is immediately greater than the high-side threshold value Hihan and is naturally larger than the low-side threshold value Lohan. Accordingly, the CPU 81 makes a “No” determination at step 1645 to proceed to step 1635 and step 1640. As a result, the high side target value Hitgt determined in step 1635 is set to the target value Voxsref.

  By executing the above-described air-fuel ratio feedback control based on the target value Voxsref thus determined, the air-fuel ratio of the air-fuel mixture supplied to the engine changes to the lean side (the air-fuel ratio increases). . As a result, the air-fuel ratio of the gas flowing out from the upstream catalyst 53 becomes a value near the stoichiometric air-fuel ratio.

  Thereafter, when a gas containing excess oxygen (lean gas) flows into the upstream catalyst 53 due to a sudden change in the operating state, a large amount of oxygen flows out from the upstream catalyst 53. As a result, the output Voxs of the oxygen concentration sensor 68 becomes smaller than the low threshold Lohan. At this time, when the CPU 81 executes the routine shown in FIG. 16, the CPU 81 makes a “Yes” determination at step 1645 following steps 1605 and 1610, proceeds to step 1650, and sets the value of the air-fuel ratio flag XAF to “0”. Set. Then, the CPU 81 proceeds to step 1625 and step 1630, and again sets the low side target value Lotgt determined in step 1625 to the target value Voxsref.

  As described above, the air-fuel ratio control apparatus according to the sixth embodiment determines that the output Voxs of the oxygen concentration sensor 68 is smaller than the low threshold Lohan (that is, the CPU 81 determines “Yes” in step 1645). Then, after the value of the air-fuel ratio flag XAF is changed to “0” in step 1650, the CPU 81 determines “Yes” in step 1615 until it becomes larger than the high threshold Hihan (step 1620). The target value Voxsref of the air-fuel ratio feedback control is set to a low-side target value Lotgt between the median value a and the low-side threshold Lohan for a period of time until the value of the air-fuel ratio flag XAF is changed to “1”. Further, the low-side target value Lotgt is changed so as to increase as the element temperature Tsens of the oxygen concentration sensor 68 increases.

  In addition, the air-fuel ratio control apparatus determines that the output Voxs of the oxygen concentration sensor 68 is larger than the high-side threshold value Hihan (that is, the CPU 81 determines “Yes” in step 1615, and the air-fuel ratio flag in step 1620. After the XAF value is changed to “1”, the CPU 81 determines “Yes” in step 1645 until the value becomes smaller than the low threshold Lohan, and in step 1650, the value of the air-fuel ratio flag XAF is changed to “ The target value Voxsref of the air-fuel ratio feedback control is set to the high-side target value Hitgt between the median value a and the high-side threshold value Hihan. Further, the high side target value Hitgt is changed so as to decrease as the element temperature Tsens of the oxygen concentration sensor 68 increases.

  As a result, regardless of the element temperature Tsens of the oxygen concentration sensor 68, the target value Voxsref of the air-fuel ratio feedback control (sub feedback control) is set to the same oxygen concentration when the stoichiometric air-fuel ratio gas reaches the oxygen concentration sensor 68. Since the value Voxs output from the sensor 68 can be substantially matched, it is possible to maintain a state where nitrogen oxides and unburned components are purified in the upstream catalyst 53 as much as possible.

  Note that step 1625 corresponds to low-side target value setting means for changing to a larger value as the element temperature Tsens of the oxygen concentration sensor 68 from which the low-side target value Logtg has been acquired is higher. Step 1635 corresponds to high-side target value setting means for changing to a smaller value as the element temperature Tsens of the oxygen concentration sensor 68 from which the high-side target value Hitgt has been acquired is higher.

  As described above, according to each embodiment of the air-fuel ratio control apparatus of the present invention, the state of the gas (oxygen or unburned component) that has reached the oxygen concentration sensor 68 that detects the oxygen concentration of the gas flowing out from the catalyst (upstream catalyst 53). Is estimated based on the current and / or past oxygen concentration sensor output Voxs or the current and / or past oxygen storage amount OSA, and the target value of the air-fuel ratio feedback control based on the estimation result However, the target value Voxsref of the output of the oxygen concentration sensor 68 is changed. Accordingly, the air-fuel ratio of the gas flowing out from the upstream catalyst 53 is easily controlled to be close to the stoichiometric air-fuel ratio, so that the upstream catalyst 53 can maintain a state of effectively purifying nitrogen oxides and unburned components. .

  In addition, this invention is not limited to said each embodiment, A various modification can be employ | adopted within the scope of the present invention. For example, the temperature of the upstream catalyst 53 may be estimated based on parameters (for example, the intake air amount Ga and the engine rotational speed NE) representing the operating state of the engine. In the above embodiment, the fuel injection amount is changed to control the air-fuel ratio of the gas flowing into the upstream side catalyst 53. However, in addition to the control of the fuel injection amount, or instead of the control of the fuel injection amount, the upstream side By directly adding secondary air, fuel, or the like to the side catalyst 53, the air-fuel ratio of the gas flowing into the upstream catalyst 53 may be controlled.

  The air-fuel ratio sensor 67 may be an oxygen concentration sensor similar to the oxygen concentration sensor 68. In this case, the output of the upstream oxygen concentration sensor gradually increases when it indicates that it is lean, gradually decreases when it indicates that it is rich, and the output of the upstream oxygen concentration sensor decreases from lean. A feedback correction amount that decreases in a skipping manner by a predetermined amount SK1 when indicating that the state has changed to rich, and that increases in a skipping manner by a predetermined amount SKr when indicating that the state has changed from rich to lean is obtained. The final fuel injection amount Fi may be obtained by adding or multiplying to the basic fuel injection amount Fbase.

  In such feedback control, when the output Voxs of the oxygen concentration sensor 68 is smaller than the target value Voxsref, the predetermined amount SKr is gradually increased and the predetermined amount SKl is gradually decreased, and the output Voxs of the oxygen concentration sensor 68 becomes the target value Voxsref. When larger, the predetermined amount SKr may be gradually decreased and the predetermined amount SKl may be gradually increased.

  In the above embodiment, the oxygen storage amount OSA is obtained as a value indicating the state of the catalyst. However, a hydrogen concentration sensor or NOx is provided in the exhaust passage downstream of the upstream catalyst 53 and upstream of the downstream catalyst 54. Concentration sensors may be provided, and values indicating the state of the catalyst may be estimated based on the outputs of these sensors. The oxygen storage amount OSA can also be obtained based on a catalyst model describing the oxidation / reduction reaction in the catalyst. Furthermore, the low-side target value Logtg in the second to fourth embodiments is changed so as to increase as the element temperature Tsens of the acquired oxygen concentration sensor 68 increases as in the fifth and sixth embodiments. The high-side target value Hitgt in the second to fourth embodiments may be set to a smaller value as the obtained element temperature Tsens of the oxygen concentration sensor 68 is higher as in the fifth and sixth embodiments. It may be changed.

It is the graph which showed the relationship between an air fuel ratio and the output value of an oxygen concentration sensor. It is a time chart which shows the change of the output value of the oxygen concentration sensor arrange | positioned downstream of the catalyst. It is the figure which showed the outline of the air fuel ratio control apparatus (fuel injection amount control apparatus) which concerns on 1st Embodiment of this invention applied to the internal combustion engine. FIG. 4 is a graph showing the relationship between the output of the air-fuel ratio sensor shown in FIG. 3 and the air-fuel ratio. FIG. 4 is a time chart showing a change in an output value of an oxygen concentration sensor during air-fuel ratio feedback control by the air-fuel ratio control apparatus shown in FIG. 3. It is the flowchart which showed the fuel-injection control routine which CPU shown in FIG. 3 performs. 4 is a flowchart showing a routine executed by the CPU shown in FIG. 3 to calculate an air-fuel ratio feedback correction amount. FIG. 4 is a flowchart showing a routine executed by the CPU shown in FIG. 3 to calculate a sub feedback amount. It is the flowchart which showed the routine which CPU shown in FIG. 3 performs in order to determine the target value of sub feedback control. It is the flowchart which showed the routine which CPU of the air fuel ratio control device which concerns on 2nd Embodiment of this invention performs in order to determine the target value of sub feedback control. It is the flowchart which showed the routine which CPU of the air fuel ratio control device which concerns on 3rd Embodiment of this invention performs in order to determine the target value of sub feedback control. It is the flowchart which showed the routine which CPU of the air fuel ratio control device which concerns on 4th Embodiment of this invention performs in order to determine the target value of sub feedback control. A graph showing the output of the oxygen concentration sensor with respect to the air-fuel ratio of the gas reaching the oxygen concentration sensor disposed downstream of the catalyst, divided into a case where the element temperature of the oxygen concentration sensor is low and a case where the element temperature is high. is there. It is the figure which showed a part of exhaust system of the air fuel ratio control apparatus which concerns on 5th Embodiment of this invention. It is the flowchart which showed the routine which CPU of the air fuel ratio control device which concerns on 5th Embodiment of this invention performs in order to determine the target value of sub feedback control. It is the flowchart which showed the routine which CPU of the air fuel ratio control device which relates to 6th execution form of this invention executes in order to decide the target value of sub feedback control.

Explanation of symbols

25 ... Combustion chamber, 32 ... Intake valve, 35 ... Exhaust valve, 39 ... Injector, 53 ... Upstream catalyst, 67 ... Air-fuel ratio sensor, 68 ... Oxygen concentration sensor, 69 ... Catalyst temperature sensor, 80 ... Electric control device, 81 ... CPU.

Claims (14)

A catalyst disposed in an exhaust passage of the internal combustion engine;
An oxygen concentration sensor disposed downstream of the catalyst in the exhaust passage and outputting a value corresponding to the oxygen concentration in the gas flowing out from the catalyst;
Air-fuel ratio control amount calculating means for calculating an air-fuel ratio control amount for making the current output of the oxygen concentration sensor coincide with a predetermined target value corresponding to the theoretical air-fuel ratio;
Air-fuel ratio control means for controlling the air-fuel ratio of the gas flowing into the catalyst based on the calculated air-fuel ratio control amount;
In an air-fuel ratio control device comprising:
The air-fuel ratio control amount calculating means includes
An air-fuel ratio control apparatus configured to change the target value based on a state of gas reaching the oxygen concentration sensor. The air-fuel ratio control apparatus according to claim 1,
The air-fuel ratio control amount calculating means includes
An air-fuel ratio control device configured to determine the state of the gas that has reached the oxygen concentration sensor based on current and past outputs of the oxygen concentration sensor. The air-fuel ratio control apparatus according to claim 2,
The air-fuel ratio control amount calculating means includes
When the output of the oxygen concentration sensor is smaller than a median value that is a substantially intermediate value between the maximum output value and the minimum output value of the oxygen concentration sensor and smaller than a low-side threshold value slightly larger than the minimum output value. The target value is set to the same median value, and then the output of the oxygen concentration sensor increases to a value between the low threshold value and a high threshold value that is greater than the median value and slightly smaller than the maximum output value. An air-fuel ratio control device configured to set the target value to a low target value between the low threshold value and the median value when The air-fuel ratio control apparatus according to claim 2,
The air-fuel ratio control amount calculating means includes
When the output of the oxygen concentration sensor is greater than a high-side threshold value that is greater than the median value, which is a substantially intermediate value between the maximum output value and the minimum output value of the oxygen concentration sensor, and slightly smaller than the maximum output value. The target value is set to the same median value, and then the output of the oxygen concentration sensor decreases to a value between the high side threshold value and a low side threshold value that is smaller than the median value and slightly larger than the minimum output value. An air-fuel ratio control apparatus configured to set the target value to a high target value between the high threshold value and the median value when In the air-fuel ratio control device according to any one of claims 1 to 4,
The air-fuel ratio control amount calculating means includes
The temperature of the catalyst is acquired, and when the acquired temperature of the catalyst is lower than a threshold temperature, the target value is fixed to a median value that is a substantially intermediate value between the maximum output value and the minimum output value of the oxygen concentration sensor. An air-fuel ratio control device configured to do. In the air-fuel ratio control apparatus according to claim 5,
The air-fuel ratio control amount calculating means includes
An air-fuel ratio control apparatus configured to acquire a value representing the degree of deterioration of the catalyst and to change the threshold temperature based on the obtained value representing the degree of deterioration. In the air-fuel ratio control device according to any one of claims 1 to 4,
The air-fuel ratio control amount calculating means includes
When the maximum oxygen storage amount of the catalyst is acquired and the acquired maximum oxygen storage amount is smaller than a predetermined oxygen storage amount, the target value is set to be approximately between the maximum output value and the minimum output value of the oxygen concentration sensor. An air-fuel ratio control device configured to be fixed at a median value. The air-fuel ratio control apparatus according to claim 1,
The air-fuel ratio control amount calculating means includes
An air-fuel ratio control apparatus configured to estimate an oxygen storage amount of the catalyst and determine a state of a gas that has reached the oxygen concentration sensor based on the estimated oxygen storage amount. The air-fuel ratio control apparatus according to claim 8,
The air-fuel ratio control amount calculating means includes
The target when the oxygen storage amount of the catalyst is larger than a half value that is substantially half of the maximum oxygen storage amount of the catalyst and is slightly larger than a high-side oxygen storage amount threshold value slightly smaller than the maximum oxygen storage amount. The value is set to a median value that is approximately half of the maximum output value and the minimum output value of the oxygen concentration sensor, and then the oxygen storage amount is decreased to be smaller than the same value as the high-side oxygen storage amount threshold value. The target value is set to a lower target value that is smaller than the median value and larger than the minimum output value when the value is between a low-side oxygen storage amount threshold value slightly larger than 0. Air-fuel ratio control device. The air-fuel ratio control apparatus according to claim 8,
The air-fuel ratio control amount calculating means includes
The target value is determined when the oxygen storage amount of the catalyst is smaller than a half value, which is approximately half of the maximum oxygen storage amount of the catalyst, and smaller than a low oxygen storage amount threshold value slightly larger than 0. Set to a median value that is approximately half of the maximum output value and minimum output value of the concentration sensor, and then the oxygen storage amount increases and is greater than the same value as the lower oxygen storage threshold and the maximum oxygen storage amount. The target value is configured to be set to a high-side target value that is larger than the median value and smaller than the maximum output value when the value is between a high-side oxygen storage amount threshold value slightly smaller than the amount. Air-fuel ratio control device. The air-fuel ratio control apparatus according to claim 2,
The air-fuel ratio control amount calculating means includes
When the output of the oxygen concentration sensor becomes smaller than the low-side threshold value which is smaller than the median value which is a substantially intermediate value between the maximum output value and the minimum output value of the oxygen concentration sensor and slightly larger than the minimum output value, An empty space configured to set the target value to a low target value between the same median value and the same low threshold value for a period of time that is greater than the value and slightly greater than the high output threshold value. Fuel ratio control device. In the air-fuel ratio control apparatus according to claim 3 or claim 11,
The air-fuel ratio control amount calculating means includes
Oxygen concentration sensor element temperature acquisition means for acquiring an element temperature of the oxygen concentration sensor;
An air-fuel ratio control apparatus including: a low-side target value setting unit configured to change the low-side target value to a larger value as the element temperature of the acquired oxygen concentration sensor is higher. The air-fuel ratio control apparatus according to claim 2,
The air-fuel ratio control amount calculating means includes
When the output of the oxygen concentration sensor is larger than the median value which is substantially the intermediate value between the maximum output value and the minimum output value of the oxygen concentration sensor and is slightly larger than the high side threshold value which is slightly smaller than the maximum output value, The target value is set to a high-side target value between the same median value and the same high-side threshold value for a period until it becomes smaller than the low-side threshold value that is smaller than the value and slightly larger than the minimum output value. Fuel ratio control device. In the air-fuel ratio control apparatus according to claim 4 or claim 13,
The air-fuel ratio control amount calculating means includes
Oxygen concentration sensor element temperature acquisition means for acquiring an element temperature of the oxygen concentration sensor;
An air-fuel ratio control device including high-side target value setting means for changing the high-side target value to a smaller value as the element temperature of the acquired oxygen concentration sensor is higher.

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