JP2007032438A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the worsening of emission even when the output property of a downstream side air-fuel ratio detecting means is changed with the deterioration of a catalyst. <P>SOLUTION: The air-fuel ratio control device for the internal combustion engine controls the air-fuel ratio of the engine in accordance with the outputs of air-fuel ratio detecting means arranged on the upstream and downstream sides of the exhaust gas purifying catalyst for generating the outputs corresponding to an exhaust air-fuel ratio. It comprises a changing means for changing a predetermined reference value VDREF for the output VD of the downstream side air-furl ratio detecting means depending on the deteriorating condition of the catalyst. Over a period when the new product of the catalyst deteriorates, the air-fuel ratio is always controlled in an adequate manner by using the reference value VDREF equivalent to the predetermined air-fuel ratio. The worsening of emission is prevented even when the output property of the downstream side air-fuel ratio detecting means is changed with the deterioration of the catalyst. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and in particular, an engine based on the output of an air-fuel ratio detection means that is disposed on each of an upstream side and a downstream side of an exhaust purification catalyst and generates an output corresponding to the exhaust air-fuel ratio. The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that controls the air-fuel ratio of the engine.

  A catalyst is disposed in the engine exhaust passage, an upstream air-fuel ratio sensor is disposed in the exhaust passage upstream of the catalyst, and a downstream air-fuel ratio sensor is disposed in the exhaust passage downstream of the catalyst, so that the output of the upstream air-fuel ratio sensor Based on this, the parameter for controlling the air-fuel ratio is calculated, the parameter is corrected by the correction coefficient calculated based on the output of the downstream air-fuel ratio sensor, and the air-fuel ratio becomes the target air-fuel ratio by the corrected parameter. An air-fuel ratio control apparatus for an internal combustion engine that controls the air-fuel ratio is well known. For example, in the air-fuel ratio control apparatus disclosed in Patent Document 1, the deviation (proportional term) between the output value of the downstream air-fuel ratio sensor and the target air-fuel ratio equivalent value by PID control, and the integral value (integral term) of this deviation. ) And the differential value (derivative term) of the deviation, the correction coefficient is calculated. The air-fuel ratio control apparatus disclosed in Patent Document 2 calculates a smoothing value that changes with a change rate smaller than the output value of the downstream air-fuel ratio sensor based on the output of the downstream air-fuel ratio sensor. The correction coefficient is calculated based on the deviation between the normalized value and the output value of the downstream air-fuel ratio sensor.

JP-A-7-197837 JP-A-10-306742 JP 2003-314334 A JP-A-6-288279

  In the air-fuel ratio control apparatus as described above, since the output value of the downstream air-fuel ratio sensor is also used for air-fuel ratio control, the output characteristic of the downstream air-fuel ratio sensor (air-fuel ratio versus output value characteristic, so-called λ characteristic) Changes affect emissions. On the other hand, it has been found that when the catalyst deteriorates, the output characteristics of the downstream air-fuel ratio sensor change, which causes a problem that the emission deteriorates. In particular, while emission regulations are being strengthened, it is important to take some measures against the problem of emission deterioration due to such catalyst deterioration.

  Accordingly, the present invention has been made in view of such circumstances, and an object thereof is an internal combustion engine capable of preventing deterioration of emissions even when the output characteristics of the downstream air-fuel ratio detection means change due to catalyst deterioration. An air-fuel ratio control apparatus is provided.

  In order to achieve the above object, an embodiment of the present invention is arranged on the upstream side and the downstream side of an exhaust purification catalyst, respectively, and based on the output of an air-fuel ratio detection means that generates an output corresponding to the exhaust air-fuel ratio. An air-fuel ratio control apparatus for an internal combustion engine for controlling an air-fuel ratio is characterized by comprising a changing means for changing a predetermined reference value for the output of the downstream air-fuel ratio detection means in accordance with the deterioration state of the catalyst. .

  According to this aspect of the present invention, since the predetermined reference value for the output of the downstream air-fuel ratio detecting means is changed according to the deterioration state of the catalyst, the predetermined air-fuel ratio is always maintained from when the catalyst is new to when it is deteriorated. Air-fuel ratio control can be performed using a considerable reference value, and control can be performed appropriately. And even if the output characteristic of the downstream air-fuel ratio detecting means changes due to the deterioration of the catalyst, it is possible to prevent the emission from deteriorating.

  Here, it is preferable that the changing means changes the reference value to a rich value as the catalyst deteriorates.

  According to the inventor's earnest research, when the catalyst deteriorates from a new state, the output characteristic of the downstream air-fuel ratio detecting means shifts to the whole lean side, and the downstream air-fuel ratio detecting means corresponding to the predetermined air-fuel ratio is detected. It has been found that the output value shifts to a richer value from the value when the catalyst is new. Therefore, by changing the reference value to a rich value as the catalyst deteriorates in this way, the reference value can always be equivalent to the predetermined air-fuel ratio from when the catalyst is new to when it is deteriorated.

  Further, the catalyst is a catalyst having an oxygen storage capacity, and the changing means calculates an oxygen storage capacity of the catalyst corresponding to a deteriorated state of the catalyst, and the reference according to the calculated oxygen storage capacity It is preferable to change the value.

  In a catalyst having oxygen storage capacity, the amount of oxygen that can be stored by the catalyst decreases as the catalyst deteriorates. Therefore, the deterioration state of the catalyst can be detected by calculating the oxygen storage capacity of the catalyst. According to this preferred embodiment, since the reference value is changed according to the oxygen storage capacity, it is possible to change the reference value according to the deterioration state of the catalyst.

  In this case, the changing means performs an annealing process using the oxygen storage capacity calculated at a predetermined time and the oxygen storage capacity calculated at a time before the predetermined time, and after the annealing process, The reference value may be changed according to the oxygen storage capacity.

  Thereby, the change for every change of the reference value becomes slow.

  According to the present invention, even if the output value characteristic of the downstream side air-fuel ratio detecting means changes due to catalyst deterioration, an excellent effect that emission deterioration can be prevented is exhibited.

  Embodiments of the present invention will be described below with reference to the drawings. In addition, the same code | symbol is attached | subjected to the element which is common in each figure, and the overlapping description is abbreviate | omitted.

  FIG. 1 shows an air-fuel ratio control apparatus for an internal combustion engine according to this embodiment. The internal combustion engine 10 is a spark ignition type internal combustion engine, and is a multi-cylinder gasoline engine in this embodiment (only one cylinder is shown). The internal combustion engine 10 is provided with an intake passage 12 and an exhaust passage 14. The intake passage 12 includes an air filter 16 at an upstream end. The air filter 16 is assembled with an intake air temperature sensor 18 for detecting the intake air temperature THA (that is, the outside air temperature).

  An air flow meter 20 is disposed downstream of the air filter 16. The air flow meter 20 is a sensor that detects the flow rate of air flowing through the intake passage 12. A throttle valve 22 is provided downstream of the air flow meter 20. A throttle sensor 24 that detects the throttle opening degree ODT and an idle switch 26 that is turned on when the throttle valve 22 is fully closed are disposed in the vicinity of the throttle valve 22.

  A surge tank 28 is provided downstream of the throttle valve 22. Further, a fuel injection valve 30 for injecting fuel into the intake port of the internal combustion engine 10 is disposed further downstream of the surge tank.

  A catalyst 32 is disposed in the exhaust passage 14. The catalyst 32 is a three-way catalyst and is a catalyst capable of storing oxygen. When unburned components such as HC and CO are contained in the exhaust gas, the oxygen stored in itself is released. In the case where the exhaust gas contains an oxidizing component such as NOx, they can be reduced and the oxygen released from the oxidizing component can be occluded. Thus, the exhaust gas discharged from the internal combustion engine 10 is treated and purified inside the catalyst 32.

In the exhaust passage 14, an upstream air-fuel ratio sensor 36 serving as upstream air-fuel ratio detection means is provided upstream of the catalyst 32, and a downstream O ₂ sensor 38 serving as downstream air-fuel ratio detection means is provided downstream of the catalyst 32. Is arranged. The upstream air-fuel ratio sensor 36 is formed of a so-called global air-fuel ratio sensor that generates an output voltage linearly over a wide air-fuel ratio region. The output characteristic of the upstream air-fuel ratio sensor 36 is shown in FIG. 2, and the upstream air-fuel ratio sensor 36 generates a higher output voltage VU as the detected exhaust air-fuel ratio becomes higher (lean side). Yes. Thus, a sensor that generates an output linearly over a wide air-fuel ratio region is called a linear output type sensor.

On the other hand, the target air-fuel ratio may be determined in any way, but in this embodiment, the target air-fuel ratio is the stoichiometric air-fuel ratio. When the target air-fuel ratio is the stoichiometric air-fuel ratio, the downstream O ₂ sensor 38 is formed of a so-called Z-characteristic oxygen concentration sensor whose output value changes stepwise in the vicinity of the theoretical air-fuel ratio in accordance with the oxygen concentration in the exhaust gas. The As shown by the solid line in FIG. 3, the downstream O ₂ sensor 38 generates an output voltage VDREF (VD0) when the detected exhaust air-fuel ratio is the stoichiometric air-fuel ratio. When the exhaust air-fuel ratio deviates from the stoichiometric air-fuel ratio to the rich side, the output voltage VD of the downstream O ₂ sensor 38 suddenly increases from VDREF, and when the exhaust air-fuel ratio deviates from the stoichiometric air-fuel ratio to the lean side, the downstream side The output voltage VD of the O ₂ sensor 38 suddenly decreases from VDREF. Thus, a sensor whose output changes suddenly at a predetermined air-fuel ratio is called a switching output type sensor. The air-fuel ratio detection means referred to in the present invention includes both a linear output type sensor and a switching output type sensor. The output voltage VDREF when the air-fuel ratio is the target air-fuel ratio (theoretical air-fuel ratio) becomes a reference value (hereinafter also referred to as “reference voltage”) for the output of the downstream O ₂ sensor 38. This reference value VDREF is a control target value corresponding to the target air-fuel ratio in the air-fuel ratio control described below.

As will be described in detail later, the output characteristics of the downstream O ₂ sensor 38 change according to the deterioration state of the catalyst 32. The solid line in FIG. 3 shows the characteristics when the catalyst 32 is new. At this time, the reference voltage VDREF has a value of VD0. Hereinafter, the description proceeds under the assumption that the catalyst 32 is new.

  In the present embodiment, an electronic control unit (hereinafter referred to as an ECU (Electronic Control Unit)) 42 as a control means is provided. In addition to the various sensors and the fuel injection valve 30 described above, the ECU 42 is connected with a water temperature sensor 44 that detects the cooling water temperature THW of the internal combustion engine 10.

Next, an air-fuel ratio control method for the internal combustion engine will be described. First, the fuel injection amount QF is calculated based on the following equation.
QF = QFTGT · C1 + FBM + C2

  Here, QFTGT represents a target fuel injection amount, FBM represents a first feedback correction coefficient, and C1 and C2 represent correction coefficients determined according to the operating state. C1 is determined according to, for example, the engine coolant temperature or the intake air temperature, and C2 is determined in consideration of the amount of fuel attached to the wall surface of the intake passage.

The target fuel injection amount QFTGT is a fuel injection amount required to make the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 4 the stoichiometric air-fuel ratio, and is calculated based on the following equation.
QFTGT = QA / AFS

  Here, QA represents the actual intake air amount, and AFS represents the theoretical air-fuel ratio. The intake air amount QA is calculated based on the detected value of the air flow meter 20.

The first feedback correction coefficient FBM is for making the amount of fuel actually supplied into the combustion chamber 4 the target fuel amount QFTGT based on the output of the upstream air-fuel ratio sensor 36. Calculated based on the formula.
FBM = KMP / FBMP + KMI / FBMI

  Here, FBMP represents a deviation between the fuel amount actually supplied into the combustion chamber 4 and the target fuel amount QFTGT, FBMI represents an integral value of FBMP, KMP represents a proportional term gain, and KMI represents an integral term gain. When the air-fuel ratio of the air-fuel mixture actually supplied into the combustion chamber 4 is expressed by AFACT, the amount of fuel actually supplied into the combustion chamber 4 is expressed by QA / AFACT. Therefore, FBMP is represented by QA / AFACT-QFTGT.

The actual air-fuel ratio AFACT is obtained from FIG. 2 based on VUC determined according to the output voltage VU of the upstream side air-fuel ratio sensor 36. This VUC is calculated based on the following equation.
VUC = VU + FBS

Here, FBS represents a second feedback correction coefficient calculated based on the output voltage of the downstream O ₂ sensor 38. The second feedback correction coefficient FBS is for making the output voltage VD of the downstream O ₂ sensor 38 become the reference voltage VDREF corresponding to the theoretical air-fuel ratio, and is calculated based on the following equation.
FBS = KSP / FBSP + KSI / FBSI + KSR / FBSR

Here, FBSP is a deviation between the output voltage VD of the downstream O ₂ sensor 38 and the reference voltage VDREF, FBSI is an integral value of the FBSP, FBSR is a deviation between the output voltage VD of the downstream O ₂ sensor 38 and a reference voltage described later, KSP represents a proportional term gain, KSI represents an integral term gain, and KSR represents an FBSR gain.

By the way, when the output voltage VD of the downstream O ₂ sensor 38 suddenly changes to the lean side, for example, a large amount of NOx is discharged from the catalyst 32. In this case, if the second feedback correction coefficient FBS is obtained by the proportional term and the integral term, that is, if it is obtained by KSP · FBSP + KSI · FBSI, the rapid change in the output voltage VD of the downstream O ₂ sensor 38 is good. Can not compensate.

Therefore, in this embodiment, based on the output voltage VD of the downstream O ₂ sensor 38, a gradual change voltage that changes at a smaller rate of change than the output voltage VD is calculated, and the output voltage VD of the downstream O ₂ sensor 38 is calculated. The second feedback correction coefficient FBS is calculated on the basis of the deviation FBSR between the gradual change voltage and the output voltage VD when the output voltage VD decreases when the output voltage VD decreases. In this way, the air-fuel ratio is controlled so that the output voltage VD of the downstream O ₂ sensor 38 is directed toward the gradually changing voltage. As a result, it is possible to prevent the output voltage VD of the downstream O ₂ sensor 38 from rapidly decreasing. Further, the deviation from the gradually changing voltage increases as the output voltage VD of the downstream O ₂ sensor 38 decreases rapidly, that is, the correction amount increases as the decrease rate of the output voltage VD increases. Therefore, a rapid decrease in the output voltage VD of the downstream O ₂ sensor 38 can be quickly compensated, and thus a large amount of NOx can be prevented from being discharged from the catalyst 32. In the present embodiment, the correction based on the FBSR is stopped when the output voltage VD of the downstream O ₂ sensor 38 starts to increase and becomes larger than the gradually changing voltage.

The gradually changing voltage may be determined in any way as long as the rate of change (absolute value) is smaller than the rate of change of the output voltage VD of the downstream O ₂ sensor 38. _The smoothed value VDSM obtained by smoothing the output voltage VD of the _two sensors 38 is used. FIG. 4 shows the relationship between the output voltage VD of the downstream O ₂ sensor 38 and the smoothed value VDSM. As shown in the figure, when the output voltage VD of the downstream O ₂ sensor 38 rapidly decreases, a deviation FBSR occurs between the smoothed value VDSM. A second feedback correction coefficient FBS is calculated based on the deviation FBSR.

  On the other hand, the gain KSR of the deviation FBSR may be determined in any way. However, the amount of NOx discharged from the engine per unit time increases as the engine load increases. That is, the correction amount needs to be increased as the engine load is higher. Therefore, in the present embodiment, as shown in FIG. 5, the gain KSR is determined so as to increase as the throttle opening degree ODT representing engine load increases.

FIG. 6 shows a routine for calculating the actual intake air amount QA and the target fuel injection amount QFTGT. This routine is executed by interruption every predetermined crank angle. Referring to FIG. 6, first, at step 50, the actual intake air amount QA _i is updated. That is, generally speaking, the intake air amount QA _i calculated in the i-th previous routine is set to QA _{i + 1} (i = 0, 1,... N). In the following step 51, the target fuel injection amount QFTGT _i is updated. That is, generally speaking, the target fuel injection amount QFTGT _i calculated in the i-th previous routine is set to QFTGT _{i + 1} . As described above, in this embodiment, the amount of fuel actually supplied into the combustion chamber 4 (QA / AFACT) based on the intake air amount QA in the intake passage and the air-fuel ratio AFACT detected in the exhaust passage. Is calculated. In this case, a certain amount of time is required until the intake air or air-fuel mixture reaches the upstream air-fuel ratio sensor 36 after the intake air amount is calculated. Therefore, in order to accurately determine the amount of fuel actually supplied into the combustion chamber 4, the intake air amount calculated before this required time before the upstream air-fuel ratio sensor 36 detects the air-fuel ratio is used. It is necessary to use it. Therefore, in this embodiment, a number n corresponding to this required time is introduced, and the air-fuel ratio detected by the upstream air-fuel ratio sensor 36 in the current routine and the intake air amount calculated in the routine n times before are calculated. Based on this, the amount of fuel actually supplied into the combustion chamber 4 is obtained (see step 64 described later, FIG. 7). Therefore, the intake air amount and the target fuel injection amount for the past n times are stored in the ECU 42 in the form of QA _i and QFTGT _i .

In the following step 52, the intake air amount QA ₀ in this routine is calculated. In the following step 53, the target fuel injection amount QFTGT0 in the current routine is calculated based on the following equation.
QFTGT0 = QA ₀ / AFS

  FIG. 7 shows a routine for executing the first feedback control performed based on the output of the upstream air-fuel ratio sensor 36. This routine is executed by the same interruption for each set crank angle as in the routine of FIG.

  Referring to FIG. 7, first, at step 60, it is judged if a condition for executing the first feedback control is satisfied. In this embodiment, the condition is satisfied when the upstream air-fuel ratio sensor 36 is activated, the engine warm-up operation is completed, and a set time has elapsed since the fuel increase control or fuel cut is completed. Is determined to be true. When it is determined that the condition is not satisfied, the routine proceeds to step 61 where the first feedback correction coefficient FBM is set to zero and the processing cycle is ended. That is, in this case, the correction using the first feedback correction coefficient FBM is not performed.

On the other hand, when it is determined in step 60 that the condition is satisfied, the routine proceeds to step 62, where the output voltage VU of the upstream air-fuel ratio sensor 36 is set to the second feedback correction coefficient FBS calculated in the routine of FIG. By adding, VUC is calculated. In the following step 63, the air-fuel ratio AFACT corresponding to the VCU is calculated using FIG. In the following step 64, the actual supplied fuel is calculated based on the following equation based on the intake air amount QA _n and the target fuel injection amount QFTGT _n calculated in the routine n times before and the air-fuel ratio AFACT calculated in step 63. A deviation FBMP between the amount and the target fuel amount QFTGT is calculated.
FBMP = QA _n / AFACT−QFTGT _n

In the following step 65, an integral value FBMI of the deviation FBMP is calculated based on the following equation.
FBMI = FBMI + FBMP

In the following step 66, the first feedback correction coefficient FMB is calculated based on the following equation.
FBM = KMP / FBMP + KMI / FBMI

FIG. 8 shows a routine for executing the second feedback control performed based on the output of the downstream O ₂ sensor 38. This routine is executed by interruption every predetermined time.

Referring to FIG. 8, first, at step 70, it is judged if the output voltage VD of the downstream O ₂ sensor 38 is smaller than the smoothed value VDSM. When VD <VDSM, the routine proceeds to step 71 where the smoothed value VDSM is calculated based on the following equation, and then the routine proceeds to step 73.
VDSM = VDSM + (VD−VDSM) / 128

On the other hand, when VD ≧ VDSM, the routine proceeds to step 72 where the output voltage VD is made the smoothed value VDSM. Next, the routine proceeds to step 73. In step 73, it is determined whether or not a condition for executing the second feedback control is satisfied. In the present embodiment, the downstream O ₂ sensor 38 is activated, the engine warm-up operation is completed, and the set time has elapsed since the fuel increase control or the fuel cut has been completed. It is determined that the condition is satisfied when the feedback control 1 is being executed. When it is determined that the condition is not satisfied, the routine proceeds to step 74 where the second feedback correction coefficient FBS is set to zero and the processing cycle is ended. That is, in this case, the correction using the second feedback correction coefficient FBS is not performed.

On the other hand, when it is determined in step 73 that the condition is established, the routine proceeds to step 75, where a deviation FBSP between the output voltage VD of the downstream O ₂ sensor 38 and the reference voltage VDREF is calculated based on the following equation. The
FBSP = VDREF−VD

In the following step 76, a deviation FBSR between the output voltage VD of the downstream O ₂ sensor 38 and the smoothed value VDSM is calculated based on the following equation.
FBSR = VDSM-VD

In this case, when the smoothed value VDSM is set to the output voltage VD in step 72, the deviation FBSR becomes zero and no correction based on the deviation FBSR is performed. In the following step 77, an integral value FBSI of the deviation FBSP is calculated based on the following equation.
FBSI = FBSI + FBSP

In the following step 78, the second feedback correction coefficient FBS is calculated based on the following equation.
FBS = KSP / FBSP + KSI / FBSI + KSR / FBSR

  FIG. 9 shows a routine for calculating the fuel injection amount. This routine is executed by interruption every predetermined crank angle.

Referring to FIG. 9, first, at step 80, correction coefficients C1 and C2 are calculated. In the following step 81, the fuel injection amount QF is calculated based on the following equation.
QF = QFTGT · C1 + FBM + C2

  Each fuel injection valve 30 performs fuel injection for a time corresponding to the fuel injection amount QF.

As described above, the air-fuel ratio control apparatus for an internal combustion engine according to the present embodiment has parameters (for example, the first feedback correction coefficient) for controlling the air-fuel ratio based on the output (VU) of the upstream air-fuel ratio sensor (36). FBM) is calculated and a predetermined correction coefficient (second feedback correction coefficient FBS) is calculated based on the output (VD) of the downstream air-fuel ratio sensor (downstream O ₂ sensor 38), and this correction is performed. Parameter correction means for correcting the parameter by a coefficient, and air-fuel ratio control means for controlling the air-fuel ratio to be a predetermined target air-fuel ratio by the corrected parameter. These means are comprised by ECU42 in this embodiment. Then, the air-fuel ratio of the engine is feedback controlled according to the output of the downstream air-fuel ratio sensor so that the output of the downstream air-fuel ratio sensor becomes a reference value (reference voltage VDREF) corresponding to the target air-fuel ratio (theoretical air-fuel ratio). Is done. The air-fuel ratio of the engine is feedback-controlled according to the output of the upstream air-fuel ratio sensor.

However, this air-fuel ratio control apparatus has the following problems as described above. That is, in this air-fuel ratio control apparatus, since the output value VD of the downstream O ₂ sensor 38 is also used for air-fuel ratio control, if the output characteristic of the downstream O ₂ sensor 38 changes, this change affects the emission. . On the other hand, it has been found that when the catalyst 32 deteriorates, the output characteristic of the downstream O ₂ sensor 38 changes, which causes a problem that the emission deteriorates. In particular, at the present time when emission regulations are being tightened, there is something to deal with the change in the output value characteristic of the downstream O ₂ sensor 38 due to the deterioration of the catalyst 32 and the deterioration of the emission due to this. It is important to take these measures.

This will be described in more detail. The catalyst 32 deteriorates with time due to the heat of the exhaust gas through its use, and the noble metal supported on the catalyst 32 is poisoned by the exhaust gas. According to the earnest study of the present inventor, when the catalyst 32 deteriorates from a new state, the output characteristic of the downstream O ₂ sensor 38 as shown in FIG. It has been found that the whole shifts to the lean side as indicated by the alternate long and short dash line. As a result, the output value of the downstream O ₂ sensor 38 corresponding to the theoretical air fuel ratio (target air fuel ratio), that is, the reference value VDREF is also shifted from the value VD0 when the catalyst is new to a higher value VDh on the rich side.

  The reason is considered as follows. That is, a new catalyst has a wide air-fuel ratio range (referred to as an optimal purification window) indicating an optimal purification rate, and the catalyst purification rate is also high. Therefore, the concentration of harmful components (HC, CO, NOx) in the exhaust gas (referred to as catalyst downstream gas) after passing through the catalyst is low. On the contrary, in the deteriorated catalyst, the optimum purification window is narrow and the purification rate of the catalyst is also lowered. Therefore, even under the same operating conditions, the concentration of harmful components in the catalyst downstream gas is higher in the case of a deteriorated catalyst than in the case of a new catalyst.

Here, in the case of a deteriorated catalyst, even when the exhaust gas passing therethrough is in the vicinity of the theoretical air-fuel ratio, the downstream O ₂ sensor 38 converts the rich gas such as hydrogen (H ₂ ) or methane (CH ₄ ) in the exhaust gas. It has been found that it reacts to a large extent, making it easier to produce rich output than when the catalyst is new. That is, these hydrogen and methane have smaller molecular weights and higher diffusion rates than oxygen (O ₂ ). Thus also the exhaust gas is in the vicinity of the stoichiometric air-fuel ratio, the diffusion speed difference affects the output characteristics of the O ₂ sensor, O ₂ sensor can easily put the output of the rich side. When the catalyst deteriorates, even if the exhaust gas itself is in the vicinity of the stoichiometric air-fuel ratio, harmful components in the exhaust gas increase as a whole, and rich gases such as hydrogen and methane get on the sensor electrodes and the sensor output tends to be rich. When the catalyst is new, its purification performance is high and there are few harmful components in the exhaust gas, so this is not a problem. However, when the catalyst is deteriorated, there are many harmful components in the exhaust gas, and the O ₂ sensor is strongly influenced by rich gases such as hydrogen and methane. As a result, as described above, the output value corresponding to the stoichiometric air-fuel ratio of the O ₂ sensor 38 is shifted to the richer side than when the catalyst is new.

  When air-fuel ratio control is performed using the same reference value VD0 as when the catalyst is new even when the catalyst is deteriorated, naturally, an air-fuel ratio different from the planned air-fuel ratio (specifically, an air-fuel ratio leaner than the stoichiometric air-fuel ratio). The air-fuel ratio control is performed based on the (fuel ratio), which is not preferable.

In particular, in the current situation where emission regulations are stricter, air-fuel ratio control is performed in a range where the concentration of harmful components is relatively low. In this case, the deviation of the O ₂ sensor output as described above has a large effect on the emission deterioration. To come.

As disclosed in Patent Document 2 and also adopted in this embodiment, there is a method of making the output of the downstream O ₂ sensor, but this is not possible to control at the optimal control point it is obvious. Patent Document 3 discloses that the control gain in the air-fuel ratio feedback control is changed in accordance with the maximum oxygen storage amount of the catalyst. However, such a change in the control gain causes a shift in the O ₂ sensor output. There is no compensation. Further, Patent Document 4 discloses correcting the output characteristics of the O ₂ sensor with respect to the temperature and the air-fuel ratio, but the target value correction based on the sensor element temperature is an optimum considering the new to deteriorated catalyst. Cannot be controlled with a clean window.

Therefore, in order to solve the above-described problem, in the air-fuel ratio control apparatus according to the present embodiment, the reference value for the output of the downstream O ₂ sensor 38 is changed according to the deterioration state of the catalyst 32. . That is, as shown in FIG. 3, the reference value which was VD0 when the catalyst is new is changed to a richer value such as VDh when the catalyst is deteriorated. As a result, air-fuel ratio control can always be performed using a reference value equivalent to the target air-fuel ratio (theoretical air-fuel ratio) from when the catalyst is new to when it is deteriorated, and control can be performed appropriately. And even if the output characteristics of the downstream O ₂ sensor change due to catalyst deterioration, it is possible to prevent the emission from deteriorating.

This will be described in detail below. FIG. 10 shows a routine for changing the downstream O ₂ sensor output reference value. This routine is executed by interruption every predetermined crank angle set by the ECU 42.

First, in step 90, it is determined whether or not a condition for changing the reference value is satisfied. In the present embodiment, it is determined that the condition is satisfied when the downstream O ₂ sensor 38 is activated and the engine warm-up operation is completed. When it is determined that the condition is not satisfied, the routine is terminated. That is, in this case, the reference value is not changed.

  On the other hand, when it is determined in step 90 that the condition is satisfied, the routine proceeds to step 91, where it is determined whether or not a command for calculating the oxygen storage capacity OSC of the catalyst 32 has been issued. Here, the reason for calculating the oxygen storage capacity OSC of the catalyst 32 is to detect the deterioration state of the catalyst 32. That is, as the catalyst 32 deteriorates, the amount of oxygen that can be stored by the catalyst 32 decreases. Therefore, the deterioration state of the catalyst 32 can be detected by calculating the oxygen storage capacity OSC of the catalyst 32. The oxygen storage capacity OSC of the catalyst 32 refers to the maximum amount of oxygen that can be stored by the catalyst 32. A command for calculating the oxygen storage capacity OSC of the catalyst 32 is generated in a separate routine within the ECU 42. The generation cycle can be, for example, every operation of the engine.

  If it is determined that a command for calculating the oxygen storage capacity OSC has not been issued, the routine is terminated. On the other hand, if it is determined that a command for calculating the oxygen storage capacity OSC has been issued, the routine proceeds to step 92 where the calculation of the oxygen storage capacity OSC is executed. This calculation is performed by another routine shown in FIGS. This point will be described in detail later.

When the calculation of the oxygen storage capacity OSC is completed, the process proceeds to step 93, where the reference value VDREF is calculated based on the calculated oxygen storage capacity OSC. For this calculation, a map shown in FIG. 11 stored in the ECU 42 is used. In this map, the relationship between the oxygen storage capacity OSC and the reference value VDREF is input. In other words, the oxygen storage capacity OSC and the stoichiometric air-fuel ratio corresponding to the deterioration state of the catalyst having this oxygen storage capacity OSC. The relationship with the output voltage VD of the downstream O ₂ sensor 38 is input. As shown, when the catalyst is a new catalyst, the oxygen storage capacity is OSC0 and the output voltage VD0. As the catalyst deteriorates, the oxygen storage capacity decreases as OSC1, OSC2, OSC3, and the output voltage becomes VD1, VD2 and VD3 become higher values, that is, richer values.

When the calculation of the reference value is finished in this way, in step 94, a smoothing process is executed using the calculated reference value to determine a final reference value. Here, using the reference value VDREF calculated in step 93 of the current cycle and the final reference value VDREF _n−1 calculated in the previous cycle, an annealing process is executed according to the following formula, A final reference value VDREF _n is calculated.
VDREF _n = (m × VDREF _n−1 + VDREF) / (m + 1)

m is a constant that determines how much the influence of the final reference value VDREF _n−1 of the previous cycle is reflected in the current value, and is 3 in this embodiment. As the constant m increases, the change of the reference value at each change (update) time becomes slower. The final reference value VDREF _n determined in this way is learned by the ECU 42 and used for air-fuel ratio control until the next relearning is performed. Note that the annealing process may be executed using the final reference value of the cycle before the previous cycle. This routine is completed as described above.

  Next, the calculation of the oxygen storage capacity OSC relating to step 92 will be described with reference to FIGS. In summary, when calculating the oxygen storage capacity OSC, the feedback control for bringing the air-fuel ratio closer to the stoichiometric air-fuel ratio is temporarily stopped, and the air-fuel ratio is forcedly vibrated to be rich or lean. To be controlled. When the air-fuel ratio is controlled to be rich, unburned components such as HC and CO are contained in the exhaust gas, and these unburned components are oxidized by oxygen stored in the catalyst, and oxygen is released from the catalyst. The When substantially all oxygen is released from the catalyst, the amount of oxygen stored in the catalyst at that time becomes the minimum stored oxygen amount O2SUMmin. On the other hand, when the air-fuel ratio is controlled to be lean, the exhaust gas contains an oxidizing component such as NOx, and oxygen released from this oxidizing component is stored in the catalyst. When oxygen is completely stored in the catalyst, the amount of oxygen stored in the catalyst at that time becomes the maximum stored oxygen amount O2SUMmax. The oxygen storage capacity OSC is calculated by subtracting the minimum stored oxygen amount O2SUMmin from the maximum stored oxygen amount O2SUMmax.

  FIG. 12 shows a control routine for causing forced oscillation of the air-fuel ratio. This routine is executed by interruption every predetermined crank angle set by the ECU 42.

First, at step 80, it is judged if the lean flag Xlean has been switched from OFF to ON. The lean flag Xlean is a flag that is ON while the downstream O ₂ sensor 38 generates an output that is lower than a predetermined lean determination value Vl (hereinafter referred to as “lean output”) (see FIG. 13). . Therefore, the condition of step 80 is established when the output of the downstream O ₂ sensor 38 changes from a value equal to or higher than the lean determination value to a value lower than the determination value in the current processing cycle. If this condition is satisfied, next, in step 81, control is performed to fix the air-fuel ratio of the air-fuel mixture to a predetermined value on the rich side.

On the other hand, if it is determined in step 80 that the lean flag Xlean has not been switched from OFF to ON, then in step 82, it is determined whether or not the rich flag Xrich has been switched from OFF to ON. The rich flag Xrich is a flag that is ON while the downstream O ₂ sensor 38 generates an output exceeding the predetermined rich determination value Vr (hereinafter referred to as “rich output”) (see FIG. 13). . Therefore, the condition of step 82 is established when the output of the downstream O ₂ sensor 38 changes from a value equal to or less than the rich determination value to a value exceeding the determination value in the current processing cycle. If this condition is satisfied, then in step 83, control is performed to fix the air-fuel ratio of the air-fuel mixture to a predetermined value on the lean side.

  On the other hand, if it is determined in step 82 that the rich flag Xrich has not been switched from OFF to ON, the rich fixed control or lean fixed control similar to the previous cycle is maintained.

FIG. 13 is a timing chart for explaining an operation realized by executing the routine of FIG. FIG. 4A shows the output change of the downstream O ₂ sensor 38, and FIG. 4B shows the output change of the upstream air-fuel ratio sensor.

  According to the routine of FIG. 12, simultaneously with the start of calculation of the oxygen storage capacity OSC, the air-fuel ratio of the air-fuel mixture is fixed to a predetermined value on the rich side or lean side by the process of step 84. FIG. 13 shows a case where the air-fuel ratio of the air-fuel mixture is fixed to a predetermined value on the rich side until time t0. While the air-fuel ratio of the air-fuel mixture is fixed rich, the output of the upstream air-fuel ratio sensor 36 becomes a value biased toward the rich side as shown in FIG. During this time, the catalyst 32 releases the stored oxygen into the exhaust gas and oxidizes unburned components such as HC and CO.

When all the oxygen stored in the catalyst 32 is released, the exhaust gas is not purified inside the catalyst 32, and the oxygen-deficient exhaust gas containing HC and CO begins to flow downstream. When the oxygen-deficient exhaust gas starts to flow out downstream of the catalyst 32, the output of the downstream O ₂ sensor 38 indicates that the exhaust gas is rich in fuel, as shown in FIG. A larger value. Therefore, if the output of the downstream O ₂ sensor 38 is monitored, it is possible to detect the timing when the oxygen-deficient exhaust gas starts to flow downstream of the catalyst 32, that is, the timing when the oxygen in the catalyst 32 is exhausted. Can do. In FIG. 13, time t0 corresponds to that time.

When the output of the downstream O ₂ sensor 38 becomes larger than the rich determination value Vr, the rich flag Xrich is turned ON at that time, and the process of step 83 shown in FIG. 12 is executed. As a result, the air-fuel ratio of the air-fuel mixture is forcibly fixed to a predetermined value on the lean side. When the air-fuel ratio of the air-fuel mixture is fixed at a predetermined value on the lean side, the output of the upstream air-fuel ratio sensor 36 eventually becomes a value that is biased toward the lean side. The waveform shown in FIG. 13B shows a state in which the output is inverted to a value biased to the lean side at time t1.

  While the output of the air-fuel ratio sensor 36 is biased toward the fuel lean side, that is, while the oxygen-rich exhaust gas containing an oxidizing component such as NOx flows into the catalyst 32, the catalyst 32 is in the exhaust gas. Purify it by storing excess oxygen. If this state continues, the catalyst 32 will eventually store the oxygen of the oxygen storage capacity OSC and the catalyst 32 will not be able to purify the exhaust gas.

When this situation occurs, the exhaust gas containing excessive oxygen including NOx begins to flow out downstream of the catalyst 32. When exhaust gas having excessive oxygen begins to flow out downstream of the catalyst 32, the output of the downstream O ₂ sensor 38 becomes a value smaller than the lean determination value V1 indicating that the exhaust gas is fuel lean. For this reason, if the output of the downstream O ₂ sensor 38 is monitored, the timing when the exhaust gas with excess oxygen starts to flow downstream of the catalyst 32, that is, the timing when the oxygen having the oxygen storage capacity OSC full in the catalyst 32 is stored. Can be detected. In FIG. 13A, time t2 corresponds to that time.

When the output of the downstream O ₂ sensor 38 becomes smaller than the lean determination value Vl, the lean flag Xlean is turned ON at that time, and the process of step 81 shown in FIG. 12 is executed. As a result, the air-fuel ratio of the air-fuel mixture is forcibly fixed to a predetermined value on the rich side. When the air-fuel ratio of the air-fuel mixture is fixed to a predetermined value on the rich side, the output of the upstream air-fuel ratio sensor 36 will eventually become a value biased to the rich side. The waveform shown in FIG. 13B shows a state in which the output is inverted to a value biased to the lean side at time t3.

Thereafter, the air-fuel ratio of the air-fuel mixture is kept rich in fuel until the output of the downstream O ₂ sensor 38 becomes larger than the rich determination value Vr again. When the output of the downstream O ₂ sensor 38 becomes larger than Vr (time t4), the above-described processing after time t0 is repeatedly executed. As a result, a state in which the catalyst 32 has completely released oxygen (minimum oxygen storage state) and a state in which the catalyst 32 has stored oxygen to the full oxygen storage capacity OSC (maximum oxygen storage state) are repeatedly realized.

  The amount of oxygen stored by the catalyst 32 per unit time or the amount of oxygen released by the catalyst 32 per unit time can be determined based on the air-fuel ratio of the exhaust gas and the intake air amount. Hereinafter, the case where oxygen is occluded is positive, and the case where oxygen is released is negative, and these amounts are both referred to as oxygen occlusion amount O2AD. In the present embodiment, the oxygen storage capacity OSC is calculated by integrating the oxygen storage amount O2AD in the process of shifting from the minimum oxygen storage state to the maximum oxygen storage state or vice versa.

  FIG. 14 shows a routine for obtaining the oxygen storage capacity OSC. This routine is a scheduled interruption routine that is repeatedly executed every predetermined time.

In this routine, first, at step 100, the air-fuel ratio deviation amount ΔA / F is calculated. The air-fuel ratio deviation amount ΔA / F is the air-fuel ratio detected by the upstream air-fuel ratio sensor 36, that is, the air-fuel ratio AFU calculated from the output voltage VU of the upstream air-fuel ratio sensor 36 and the map of FIG. This is the difference from the air-fuel ratio AFS and is calculated by the following equation.
ΔA / F = AFU−AFS

  Next, in step 102, the intake air amount QA is calculated based on the output of the air flow meter 20.

  Next, in step 104, the amount of oxygen stored in or released from the catalyst 32 per unit time based on the air-fuel ratio deviation amount ΔA / F and the intake air amount QA, that is, the oxygen storage amount. O2AD is required. The oxygen storage amount O2AD is calculated according to a map stored in the ECU 42 or an arithmetic expression. The value of the oxygen storage amount O2AD is a positive value when the air-fuel ratio of the exhaust gas flowing into the catalyst 32 is lean (AFU> AFS, that is, ΔA / F> 0), while the catalyst 32 When the air-fuel ratio of the exhaust gas flowing into the engine is rich (when AFU <AFS, that is, when ΔA / F <0), it becomes a negative value.

Next, at step 106, it is determined whether or not the condition that the lean flag Xlean = ON and the air-fuel ratio deviation amount ΔA / F> 0 is satisfied. As described above, the lean flag Xlean is ON when the downstream O ₂ sensor 38 generates a lean output. Therefore, in this step 106, it is determined whether the exhaust gas is lean (excess oxygen) both upstream and downstream of the catalyst 32.

  The condition of step 106 is established, for example, between times t2 and t3 shown in FIG. That is, the condition is established under the condition that the catalyst 32 stores the oxygen of the oxygen storage capacity OSC full and the storage amount does not change. When this condition is satisfied, the processing after step 112 is executed.

On the other hand, if it is determined that the condition of step 106 is not satisfied, then in step 108, it is determined whether or not the condition that the rich flag Xrich = ON and the air-fuel ratio deviation amount ΔA / F <0 is satisfied. Is done. As described above, the rich flag Xrich is ON when the downstream O ₂ sensor 38 emits a rich output. Therefore, in this step 108, it is determined whether the exhaust gas is rich both upstream and downstream of the catalyst 32.

  The condition of step 108 is established, for example, between times t0 and t1 shown in FIG. That is, the above condition is a condition that is established under the condition that the catalyst 32 has completely released oxygen and the amount of occlusion does not change. When this condition is satisfied, the processing after step 112 is executed.

  If it is determined that step 108 is not established, it is determined that the catalyst 32 is in the process of actually storing or releasing oxygen, and the amount of oxygen stored in the catalyst 32 is changing every moment. it can. In this case, in step 110, the oxygen storage integrated amount O2SUM is updated by adding the oxygen storage amount O2AD calculated in the current processing cycle to the oxygen storage integrated amount O2SUM calculated in the previous processing cycle. Is called. Thereafter, the process proceeds to step 112.

In step 112, it is determined whether the air-fuel ratio lean exhaust gas flows out downstream of the catalyst 32, or more specifically, whether the downstream O ₂ sensor 38 emits a lean output. The downstream O ₂ sensor 38 generates a lean output only when the upstream catalyst 32 is in the maximum oxygen storage state and a fuel-lean air-fuel mixture is supplied to the internal combustion engine 10.

If it is determined in step 112 that the downstream O ₂ sensor 38 is emitting a lean output, in step 114, the current oxygen storage integrated amount O2SUM is stored as the maximum stored oxygen amount O2SUMmax, and further, the lean flag Xlean Is executed, and the rich flag Xrich is turned OFF.

On the other hand, if it is determined in step 112 that the air-fuel ratio lean exhaust gas does not flow out downstream of the catalyst 32, the routine proceeds to step 116 where the exhaust gas rich in air-fuel ratio is downstream of the catalyst 32. It is determined whether gas is flowing out, that is, whether the downstream O ₂ sensor 38 is generating a rich output. The downstream O ₂ sensor 38 produces a rich output only when the catalyst 32 is in the minimum oxygen storage state and a fuel-rich air-fuel mixture is supplied to the internal combustion engine 10.

If it is determined in step 116 that the downstream O ₂ sensor 38 is producing a rich output, in step 118, the current oxygen storage integrated amount O2SUM is stored as the minimum stored oxygen amount O2SUMmin, and the lean flag Xlean is further set. Processing to turn off the rich flag Xrich is executed.

  On the other hand, if it is determined in this step 116 that the exhaust gas rich in the air-fuel ratio does not flow out downstream of the catalyst 32, the catalyst 32 is properly purifying the exhaust gas, that is, the catalyst 32 is It can be determined that neither the maximum oxygen storage state nor the minimum oxygen storage state is present. In this case, in step 120, both the lean flag Xlean and the rich flag Xrich are turned off.

  Thus, this routine is ended. When the maximum stored oxygen amount O2SUMmax and the minimum stored oxygen amount O2SUMmin are obtained in this way, the ECU 42 subtracts the minimum stored oxygen amount O2SUMmin from the maximum stored oxygen amount O2SUMmax. The oxygen storage capacity OSC of the catalyst 32 is calculated. The oxygen storage capacity OSC is used in step 93 shown in FIG. 10 to calculate a reference value VDREF.

  As can be seen from the above description, in this embodiment, the ECU 42 constitutes a changing unit and a calculating unit.

The embodiment of the present invention is not limited to the above-described embodiment. For example, the present invention is applicable to a system in which the upstream air-fuel ratio sensor is an O ₂ sensor similar to the downstream O ₂ sensor 38. The present invention can also be applied to a system in which the downstream air-fuel ratio sensor is a global air-fuel ratio sensor similar to the upstream air-fuel ratio sensor 36.

  In the present invention, all modifications, applications, and equivalents included in the concept of the present invention defined by the claims are included in the present invention. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

1 is a system diagram of an air-fuel ratio control apparatus for an internal combustion engine according to the present embodiment. It is a graph which shows the output characteristic of an upstream air-fuel ratio sensor. It is a graph showing the output characteristic of the downstream O ₂ sensor. Is a time chart showing the relationship between the output voltage and the smoothed value of the downstream O ₂ sensor. It is a graph which shows the relationship between gain KSR of deviation FBSR and throttle opening degree ODT. 6 is a flowchart for calculating an intake air amount and a target fuel injection amount. It is a flowchart for performing 1st feedback control. It is a flowchart for performing 2nd feedback control. 3 is a flowchart for calculating a fuel injection amount. It is a flowchart of a reference value change routine. It is a calculation map of a reference value. It is a flowchart of the control routine for causing the forced oscillation of the air-fuel ratio. 13 is a timing chart for explaining operations related to the routine of FIG. It is a flowchart of a stored oxygen amount calculation routine.

Explanation of symbols

Reference Signs List 10 internal combustion engine 14 exhaust passage 32 catalyst 36 upstream air-fuel ratio sensor 38 downstream O ₂ sensor 42 electronic control unit (ECU)
VD Output voltage of downstream O ₂ sensor VDREF Reference value (reference voltage)

Claims (4)

In an air-fuel ratio control apparatus for an internal combustion engine, which is arranged respectively on the upstream side and downstream side of an exhaust purification catalyst and controls the air-fuel ratio of the engine based on the output of the air-fuel ratio detection means that generates an output corresponding to the exhaust air-fuel ratio,
An air-fuel ratio control apparatus for an internal combustion engine, comprising: changing means for changing a predetermined reference value for the output of the downstream air-fuel ratio detection means in accordance with the deterioration state of the catalyst.   2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the changing means changes the reference value to a rich value as the catalyst deteriorates.   The catalyst is a catalyst having oxygen storage capacity, and the changing means calculates the oxygen storage capacity of the catalyst corresponding to the deterioration state of the catalyst, and sets the reference value according to the calculated oxygen storage capacity. 3. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the control unit is changed. The changing means executes an annealing process using the oxygen storage capacity calculated at a predetermined time and the oxygen storage capacity calculated at a time before the predetermined time, and the oxygen storage capacity after the annealing process 4. The air-fuel ratio control apparatus for an internal combustion engine according to claim 3, wherein the reference value is changed according to

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