JPH11311142A - Air-fuel ratio controller for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform satisfactory exhaust emission control at all times, irrespective of the deteriorative state of a catalyst to be situated at the upstream side of a nitrogen oxide controlling catalyzer. SOLUTION: An engine exhaust pipe 12 is provided with a three way catalyst 13 and a nitrogen oxide catalyzer 14 in series, and an air-fuel sensor 26 is installed at the upstream side of the three way catalyst 13. A central processing unit 31 in an electronic control unit 30 performs a lean combustion in an air-fuel lean range, while nitrogen oxide in the exhaust gas discharged during the lean combustion is occluded by the NOx catalyzer 14, and further the air-fuel ratio is temporarily controlled to a rich side, thereby having the occuluded nitrogen oxide discharged out of the NOx catalyzer 14. In addition, the central processing unit 31 detects the degree of deterioration of the three way catalyst 13, and on the basis of the degree of deterioration in the three way catalyst 13, rich control over the air-fuel ratio is carried out. In fact, a reference rich value is set up on the basis of the degree of deterioration in the three way catalyst 13, while a rich integrated value in air-fuel ratio rich control is calculated by an output of the air-fuel sensor 26, and when the rich integrated value reaches the reference rich value, the air-fuel ratio rich control is terminated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to an air-fuel ratio control device for an internal combustion engine that performs lean combustion in an air-fuel ratio lean region, and purifies nitrogen oxides (NOx) in exhaust gas generated during lean combustion. For controlling the air-fuel ratio of an internal combustion engine having a NOx storage-reduction type catalyst for the purpose.

[0002]

2. Description of the Related Art In recent years, in an air-fuel ratio control apparatus for an internal combustion engine, a technique of performing so-called lean burn control, in which fuel is burned on a lean side from a stoichiometric air-fuel ratio, in order to improve fuel efficiency, is being used frequently. When performing such lean combustion, NOx is contained in exhaust gas discharged from the internal combustion engine.
NO for purifying this NOx
x catalyst is required.

For example, Japanese Patent No. 2600492 discloses that NOx is absorbed when the air-fuel ratio of exhaust gas is lean, and is released when the oxygen concentration of the exhaust gas is reduced, that is, when the exhaust gas is enriched. N
An Ox absorbent (NOx storage-reduction catalyst) and an exhaust gas purification device provided with the NOx absorbent are disclosed. Further, the apparatus disclosed in the above publication discloses a configuration in which a three-way catalyst is installed upstream of an engine exhaust passage and a NOx catalyst is installed downstream.

On the other hand, NOx generated during lean combustion is reduced to N
In a system that absorbs with an Ox catalyst, the NOx catalyst
When x becomes saturated, the NOx purification capacity reaches the limit.
Therefore, there is known a technique in which rich combustion is temporarily performed in order to recover the purification ability of the NOx catalyst and suppress the emission of NOx. For example, JP-A-8-2610
Japanese Patent Publication No. 41 discloses an apparatus that detects the degree of deterioration of a NOx catalyst (NOx absorbent) and extends the air-fuel ratio rich time in accordance with the detected degree of deterioration of the NOx catalyst.

[0005]

In an apparatus in which a three-way catalyst is disposed upstream of an engine exhaust passage and a NOx catalyst is disposed downstream thereof, oxygen and rich components are temporarily stored in the three-way catalyst (see FIG. 1). By performing the storage, the exhaust gas component supplied to the NOx catalyst on the downstream side fluctuates. That is, even if the lean combustion is switched to the rich combustion in order to reduce and release the stored NOx of the NOx catalyst, the air-fuel ratio of the exhaust gas downstream of the three-way catalyst (the air-fuel ratio of the exhaust gas supplied to the NOx catalyst) is immediately increased to the lean state. After the reaction with the stored oxygen in the three-way catalyst is completed, the fuel does not shift to rich. Therefore, in order to reliably reduce and release the stored NOx of the NOx catalyst, a rich time for rich combustion is set in consideration of the above-described reaction time with the stored oxygen.

[0006] Here, when the three-way catalyst is compared, for example, when it is new and when it is deteriorated, the new catalyst has a higher oxygen storage capacity.
Stores relatively large amounts of oxygen during lean combustion. In other words, as the three-way catalyst deteriorates, the oxygen storage capacity decreases. Therefore, when the three-way catalyst is new and deteriorated,
When the same rich combustion control is performed, N
The rich components (HC, CO) supplied to the Ox catalyst side become excessive, and the rich components may be discharged without being purified. This will be briefly described with reference to FIG.

FIG. 14 is a time chart showing changes in the air-fuel ratio and exhaust gas components when the three-way catalyst is deteriorated. In FIG. 14, at time t31, the control air-fuel ratio is switched from lean to rich, and the air-fuel ratio in front of and behind the three-way catalyst shifts to rich. At time t32, if the air-fuel ratio in front of and behind the three-way catalyst reaches the stoichiometric air-fuel ratio (λ = 1), and the three-way catalyst originally has the same oxygen storage capacity as when new, the three-way catalyst rearward The air-fuel ratio is maintained at the stoichiometric air-fuel ratio for a predetermined time (reaction time with stored oxygen), but since the three-way catalyst has deteriorated and the amount of stored oxygen is small, immediately after time t33
, The air-fuel ratio shifts to the rich side. After time t33, since the rich component is supplied to the NOx catalyst side, the stored NOx of the catalyst is reduced and released, and thereafter, at time t34,
The control air-fuel ratio is returned to the original lean state.

In this case, the period during which the control air-fuel ratio is controlled to be rich (the rich time from time t31 to time t34) is set in advance in consideration of the stored oxygen amount of the three-way catalyst based on when the three-way catalyst is new. . However, when the three-way catalyst is deteriorated as described above, the stored oxygen amount becomes smaller than the initially expected amount, and the amount of the rich component supplied to the NOx catalyst becomes excessive by the reduced amount. Therefore, the rich component is supplied even after the end of the reduction of the stored NOx, and the exhaust gas components such as HC and CO are discharged without being purified.

Conversely, if the rich time is set on the basis of the deterioration of the three-way catalyst, the rich time becomes
Insufficient rich components required for reduction and release of stored NOx,
There is a possibility that NOx is discharged without being purified.

[0010] The above-mentioned problem is caused, for example, in Japanese Patent No. 2600.
No. 492. Also, Japanese Patent Application Laid-Open
In the device disclosed in Japanese Patent Application No. -261041, the degree of deterioration of the NOx catalyst is reflected in the air-fuel ratio rich control.
There is no description of a three-way catalyst, and the problem described with reference to FIG. 14 cannot be solved. By the way,
The cause and the speed of progress of the deterioration of the NOx catalyst are different from those of the three-way catalyst. Deterioration of NOx catalyst is caused by sulfate BaSO4
It is thought that the main cause is the generation of, while the deterioration of the three-way catalyst is mainly caused by heat damage.

The present invention has been made in view of the above-mentioned problem, and an object thereof is to always perform good exhaust gas purification irrespective of the deterioration state of a catalyst located on the upstream side of the NOx catalyst. It is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine that can perform the above-mentioned operations.

[0012]

An air-fuel ratio control apparatus according to the present invention is provided at an upstream side of an engine exhaust passage and has at least an upstream catalyst having an oxidizing effect, and is provided at a downstream side of the engine exhaust passage. The present invention is applied to an internal combustion engine having a downstream catalyst having a reducing action, performs lean combustion in an air-fuel ratio lean region, and stores NOx in exhaust gas discharged at the time of lean combustion by the downstream catalyst. It is assumed that the fuel ratio is temporarily controlled to be rich to release the stored NOx from the downstream catalyst.

According to the first aspect of the present invention, the deterioration detecting means for detecting the degree of deterioration of the upstream catalyst and the rich control for performing the rich control of the air-fuel ratio based on the detected degree of deterioration of the upstream catalyst. Control means.

In short, for example, an upstream catalyst realized by a three-way catalyst or an oxidation catalyst has an oxygen storage capacity, and the amount of stored oxygen changes according to the degree of deterioration of the catalyst. In this case, when the upstream catalyst deteriorates, the oxygen storage capacity decreases, and the change in the air-fuel ratio of the exhaust gas before the upstream catalyst immediately appears in the change in the air-fuel ratio after the catalyst. In the case of the present invention,
The degree of deterioration of the upstream catalyst is detected, and the air-fuel ratio rich control is changed according to the degree of deterioration. Therefore, when the upstream side catalyst deteriorates as in the conventional apparatus, the excess rich component (HC,
CO, H2, etc.) are supplied, and as a result, a problem that HC and CO are discharged in large amounts is eliminated.

As a result, good exhaust gas purification can always be performed regardless of the state of deterioration of the catalyst located upstream of the NOx catalyst. The above-described invention is also effective when individual oxygen storage capacities differ depending on individual differences and operating temperatures of the three-way catalyst, and in such a case, it becomes possible to implement good exhaust gas purification.

Actually, as described in claim 2, the rich control means obtains a reference rich amount at the time of air-fuel ratio rich control based on the degree of deterioration of the upstream side catalyst, and according to the obtained reference rich amount. To perform rich control. More specifically, as described in claim 3, the rich control means calculates a rich amount integrated value at the time of air-fuel ratio rich control, and calculates the rich amount integrated value and the reference rich amount. And a means for terminating the air-fuel ratio rich control when the former value (rich amount integral value) reaches the latter value (reference rich amount).

According to the second and third aspects of the present invention, the air-fuel ratio rich control is performed by the reference rich amount corresponding to the degree of deterioration of the upstream side catalyst. Thus, rich control without excess or deficiency can be performed even when the catalyst is deteriorated.

[0018] In the invention described in claim 4, the rich control means determines that the degree of deterioration of the upstream side catalyst is greater,
Set the reference rich amount to a small value. That is, as described above, as the deterioration of the upstream catalyst progresses, the oxygen storage capacity of the catalyst decreases. Therefore, as the degree of catalyst deterioration increases, the rich component that reacts with the stored oxygen of the upstream catalyst during the air-fuel ratio rich control decreases, and the rich time is shortened by the reduced amount.

According to a fifth aspect of the present invention, the deterioration detecting means includes means for calculating an amount of a gas component that is not purified within the catalyst after the internal combustion engine is started until the upstream catalyst reaches a predetermined temperature. Means for detecting the degree of deterioration of the upstream-side catalyst based on the calculated unpurified gas component amount.

That is, when comparing the upstream catalyst before deterioration (when it is new) and after deterioration, the amount of gas components not purified in the catalyst (the amount of unpurified gas components) is different. Focusing on this, for example, the amount of the unpurified gas component is calculated from the time the engine is started until the catalyst is warmed up, and the deterioration of the catalyst is detected based on the amount of the unpurified gas component. As a result, it is possible to accurately detect catalyst deterioration in consideration of an increase in emissions before the catalyst is activated. Note that, before the catalyst is warmed up, the difference in the purification rate due to the difference in the degree of catalyst deterioration is large, and it is possible to easily and accurately detect catalyst deterioration.

In this case, as described in claim 6, the integral value of the air-fuel ratio fluctuation amount every time after passing through the upstream side catalyst is determined as the unpurified gas component amount, and the unpurified gas component amount is determined. It is preferable to detect that the larger the value, the greater the degree of deterioration.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) A first embodiment of the present invention will be described below with reference to the drawings. In the air-fuel ratio control system according to the present embodiment,
The target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is set to be leaner than the stoichiometric air-fuel ratio, and so-called lean burn control for performing lean combustion based on the target air-fuel ratio is performed. As a main configuration of the system, a three-way catalyst serving as an upstream catalyst and a NOx storage reduction catalyst (hereinafter referred to as a NOx catalyst) serving as a downstream catalyst are provided in the middle of the exhaust system passage of the internal combustion engine. A limiting current type air-fuel ratio sensor (A / F sensor) is provided upstream of the source catalyst, and an oxygen sensor (O2 sensor) is provided downstream of the NOx catalyst. An electronic control unit (hereinafter, referred to as an ECU) mainly including a microcomputer takes in the detection results of the A / F sensor and the O2 sensor, and performs feedback control of the air-fuel ratio based on the detection results. Hereinafter, the detailed configuration will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an air-fuel ratio control system according to the present embodiment. As shown in FIG.
The internal combustion engine is configured as a four-cylinder, four-cycle spark ignition engine (hereinafter, referred to as engine 1). The intake air is supplied from upstream to an air cleaner 2, an intake pipe 3, a throttle valve 4, a surge tank 5, and an intake manifold 6.
And is mixed with fuel injected from the fuel injection valve 7 for each cylinder in the intake manifold 6. Then, the mixture is supplied to each cylinder as a mixture having a predetermined air-fuel ratio.

A high voltage supplied from an ignition circuit 9 is distributed and supplied to a spark plug 8 provided in each cylinder of the engine 1 through a distributor 10, and the ignition plug 8 controls the mixture of each cylinder at a predetermined timing. To ignite. Exhaust gas discharged from each cylinder after the combustion passes through an exhaust manifold 11 and an exhaust pipe 12 to reach HC,
Three-way catalyst 13 for purifying three components of CO and NOx
And a NOx catalyst 14 for purifying NOx in exhaust gas
After passing through, it is discharged to the atmosphere.

Here, the NOx catalyst 14 stores NOx during combustion at a lean air-fuel ratio, and converts the stored NOx into a rich component (C) during combustion at a rich air-fuel ratio.
O, HC, etc.) and release. The three-way catalyst 13
The NOx catalyst has a smaller capacity than the NOx catalyst 14, and is activated early after the low-temperature start of the engine 1 to purify harmful gases.

The intake pipe 3 is provided with an intake air temperature sensor 21 and an intake air pressure sensor 22. The intake air temperature sensor 21 detects the temperature of the intake air (intake air temperature Tam), and the intake air pressure sensor 22 is located downstream of the throttle valve 4. Each of the intake pipe negative pressures (intake pressure PM) is detected. The throttle valve 4 is provided with a throttle sensor 23 for detecting the opening of the valve 4 (throttle opening TH). The throttle sensor 23 outputs an analog signal corresponding to the throttle opening TH. The throttle sensor 23 also has a built-in idle switch, and outputs a detection signal indicating that the throttle valve 4 is substantially fully closed.

A water temperature sensor 24 is provided on a cylinder block of the engine 1.
The temperature of the cooling water circulating in the inside (cooling water temperature Thw) is detected. The distributor 10 is provided with a rotation speed sensor 25 for detecting the rotation speed of the engine 1 (engine rotation speed Ne).
, Ie, 24 pulse signals are output at equal intervals every 720 ° CA.

Further, an A / F sensor 26 of a limiting current type is disposed upstream of the three-way catalyst 13 in the exhaust pipe 12, and the A / F sensor 26 detects the oxygen concentration of the exhaust gas discharged from the engine 1. (Or CO concentration in unburned gas)
, A wide-area and linear air-fuel ratio signal (AFm) is output. Further, an O2 sensor 27 is disposed downstream of the NOx catalyst 14, and the O2 sensor 27 outputs an electromotive force signal (V) depending on whether the exhaust gas has a rich or lean air-fuel ratio.
s) is output.

The ECU 30 includes a CPU 31, a ROM 32,
The RAM 33, the backup RAM 34, and the like are configured as a logical operation circuit, and are connected via a bus 37 to an input port 35 for inputting a detection signal of each sensor and an output port 36 for outputting a control signal to each actuator and the like. I have. The ECU 30 detects the detection signals (intake temperature Tam, intake pressure PM, throttle opening T
H, cooling water temperature Thw, engine speed Ne, air-fuel ratio signal, etc.) are input through the input port 35. Then, control signals such as the fuel injection amount TAU and the ignition timing Ig are calculated based on these values, and the control signals are output to the fuel injection valve 7 and the ignition circuit 9 via the output port 36, respectively.

Next, the operation of the air-fuel ratio control system configured as described above will be described. As a basic configuration of the present control system, F / B control of the air-fuel ratio is performed in the air-fuel ratio region leaner than the stoichiometric air-fuel ratio, and the air-fuel ratio rich control is temporarily performed during the air-fuel ratio lean control. . Particularly, in the present embodiment, in addition to the so-called "main F / B control" based on the detection result of the A / F sensor 26 on the upstream side of the three-way catalyst 13, the output voltage of the O2 sensor 27 on the downstream side of the NOx catalyst 14 is used. That is, so-called “sub F / B control” is performed.

That is, in the main F / B control, a feedback process is performed in accordance with the PI control procedure according to the deviation between the sensor output (actual air-fuel ratio) AFm of the A / F sensor 26 and the target air-fuel ratio MAF. In the sub F / B control, O2
In order to feed back the output voltage (actual voltage) Vs of the sensor 27 to a predetermined target voltage MVs (for example, a value corresponding to the stoichiometric air-fuel ratio), the main F is determined based on the integral value of the deviation between the actual voltage Vs and the target voltage MVs. / B control target air-fuel ratio MAF
Is corrected. Hereinafter, details of the air-fuel ratio control will be described with reference to FIG.

FIG. 2 is a flowchart showing an air-fuel ratio control routine executed by the CPU 31. The routine is executed every time fuel is injected into each cylinder (in this embodiment, 180 ° C.).
A).

In FIG. 2, the CPU 31 first determines whether or not the air-fuel ratio F / B execution condition is satisfied in step 101. If not, the CPU 31 opens the air-fuel ratio in step 102, and then executes this routine. Stop once. Here, the air-fuel ratio F / B execution conditions include: that the engine cooling water temperature Thw is equal to or higher than a predetermined temperature; that the A / F sensor 26 and the O2 sensor 27 are sufficiently activated; When all of these conditions are satisfied, the air-fuel ratio F / B execution condition is satisfied. When the F / B execution condition is satisfied, the CPU 31 proceeds to the processing after step 103.

The CPU 31 determines in step 103 that the sub F /
The target voltage MVs for the B control is set. Target voltage M
Vs is a target value of the output of the O2 sensor 27 on the downstream side of the NOx catalyst 14, and is calculated by a two-dimensional map (not shown) preset according to the engine speed Ne and the intake pressure PM. In this case, for example, the higher the rotation speed Ne, the larger the target voltage MVs.

The CPU 31 proceeds to step 104
To determine the voltage deviation ΔVs between the output voltage (actual voltage Vs) of the O2 sensor 27 and the target voltage MVs (ΔVs = Vs−MV)
s) In the subsequent step 105, the integral value VsSUM (i) of the voltage deviation ΔVs is calculated by the following equation.

VsSUM (i) = VsSUM (i−1) + ΔVs Here, the suffix “i” indicates the current value, and “i−1” indicates the previous value (the same notation is used below).

Thereafter, the CPU 31 determines in step 106 the voltage deviation ΔVs calculated in step 106 and its integral value VsSUM
Using (i), the sub F / B correction amount ΔFs is calculated by the following equation.

ΔFs = KPs · ΔVs + KIs · VsSUM (i) where KPs is a proportional coefficient and KIs is an integral coefficient.
Further, the CPU 31 converts the sub F / B correction amount ΔFs calculated in step 107 into a correction amount ΔMAF for correcting the target air-fuel ratio MAF of the main F / B control. For example, when the sub F / B correction amount ΔFs is rich (Δ
Fs <0), the target air-fuel ratio MA of the main F / B control
A correction amount ΔMAF is calculated as an amount for correcting F to the lean side (ΔMAF> 0). Conversely, the sub F / B correction amount ΔFs
Is lean (if ΔFs ≧ 0), a correction amount ΔMAF is calculated as an amount for correcting the target air-fuel ratio MAF of the main F / B control to the rich side (ΔMAF ≦ 0).

In step 200, the CPU 31 sets a target air-fuel ratio MAF for main F / B control. At this time, the target air-fuel ratio MAF is set such that the air-fuel ratio rich control is temporarily performed during the air-fuel ratio lean control according to a routine of FIG. 3 described later.

Thereafter, the CPU 31 corrects the target air-fuel ratio MAF of the main F / B control using the target air-fuel ratio correction amount ΔMAF calculated in step 108, and calculates a new target air-fuel ratio MAFm (MAFm = MAF + ΔMA).
F). Further, in step 109, the CPU 31 calculates a deviation ΔAFm between the output AFm of the A / F sensor 26 on the upstream side of the three-way catalyst 13 and the target air-fuel ratio MAFm (ΔAFm = M
AFm-AFm), and in the following step 110, this deviation ΔA
The integral value AFmSUM of Fm is calculated by the following equation.

AFmSUM (i) = AFmSUM (i-
1) + ΔAFm Subsequently, the CPU 31 uses the deviation ΔAFm of the target air-fuel ratio MAFm calculated in step 111 and the integral value AFmSUM thereof to calculate the correction amount ΔF of the main F / B control.
m is calculated by the following equation.

ΔFm = KPm · ΔAFm + KIm · AF
mSUM (i) Here, KPm is a proportional coefficient, and KIm is an integral coefficient.
Finally, the CPU 31 calculates the fuel injection amount from the basic injection amount Tp calculated from the engine speed Ne and the intake pressure PM in step 112, a correction coefficient FALL based on the intake air temperature and the like, and the calculated correction amount ΔFm for the main F / B control. TAU is calculated by the following equation, and this routine ends.

TAU = Tp · FALL · ΔFm The correction coefficient FALL includes correction coefficients such as the cooling water temperature Thw and EGR. When calculating the TAU value,
It is preferable that a manifold wet amount is also added as a correction amount at the time of transition.

Next, the procedure for setting the target air-fuel ratio MAF performed in step 200 will be described with reference to FIG. In FIG. 3, the CPU 31 firstly executes step 201
It is determined whether or not the rich control flag XREX indicating whether or not the air-fuel ratio rich control is being performed is “0”. Here, XREX = 0 indicates that rich control is not being performed, that is, lean control is being performed, and XREX = 1 indicates that rich control is being performed. When the IG key is turned on (when the power is turned on), the flag XREX is cleared to "0" by the initialization processing.

If XREX = 0, the CPU 31 proceeds to step 202 and determines whether or not the value of a lean counter representing the number of combustions at the lean air-fuel ratio is less than a predetermined value α. The predetermined value α may be, for example, a value of about “100”. If lean counter <α (YES in step 202),
In step 203, the CPU 31 sets the target air-fuel ratio MAF to "1.5", and in step 204, increments the rich counter by "1", and thereafter returns to the original routine of FIG. In such a case, the MAF value set in the above-mentioned step 203 is used for the calculation after step 108 in FIG. 2 described above, whereby the air-fuel ratio is controlled lean.
That is, if step 202 is YES, the air-fuel ratio lean control up to that point is continuously performed.

When the lean counter gradually increases and the lean counter ≧ α (NO in step 202), C
The PU 31 determines in step 205 that the rich control flag XREX
Is set to "1". Further, in the subsequent step 206, the CPU 31 reads the degree of deterioration of the three-way catalyst 13 stored in the backup RAM 34, and sets a reference rich area RAFASDS according to the degree of deterioration. Here, the degree of deterioration of the three-way catalyst 13 is detected according to a routine of FIG. 6 described later.

The reference rich area RAFADSD is NOx
This corresponds to a reference rich amount necessary for reducing and releasing NOx stored in the catalyst 14, and this reference rich amount is converted into an area as a time integral value of the air-fuel ratio rich on the upstream side of the three-way catalyst 13. . Specifically, the reference rich area RAF
The ADSD is obtained in accordance with the degree of deterioration of the three-way catalyst 13 at that time, and is set as a value corresponding to the amount of oxygen stored in the three-way catalyst 13 using, for example, the relationship shown in FIG. According to FIG. 4, as the degree of deterioration of the three-way catalyst 13 increases, the reference rich area RAFADSD is set to a smaller value.
In short, when the deterioration of the three-way catalyst 13 progresses, the catalyst 1
3 has a reduced oxygen storage capacity. Therefore, the larger the degree of catalyst deterioration, the more the three-way catalyst 1 during the air-fuel ratio rich control.
The rich component reacting with the stored oxygen of No. 3 is reduced, and the rich area (rich time) is shortened by the reduced amount.

Next, the CPU 31 sets the target air-fuel ratio MAF to "0.75" in step 207, and then returns to the original routine of FIG. In such a case, the MAF value set in step 207 described above is used for the calculation in step 108 and subsequent steps in FIG. 2 described above, whereby the air-fuel ratio is richly controlled.
That is, when the answer to step 202 is NO, the air-fuel ratio lean control up to that point is switched to the air-fuel ratio rich control.

When the air-fuel ratio control is switched from the lean control to the rich control, the CPU 31 makes a negative decision in step 201 and proceeds to step 208, where the rich deviation integrated value RAF
Calculate AD. The rich deviation integrated value RAFAD is calculated by, for example, the following equation.

RAFAD (i) = RAFAD (i-1)
+ | AFSD−AFm | · Exhaust gas flow rate correction coefficient That is, the previous value RAFAD (i−
1) is added to the absolute value of the difference between the air-fuel ratio reference value AFSD (for example, the stoichiometric air-fuel ratio) and the actual air-fuel ratio AFm multiplied by the exhaust gas flow rate correction coefficient. The value is RAFAD (i). However, "| AFS
D-AFm | · exhaust gas flow rate correction coefficient ”is only when the value is positive, that is, when the actual air-fuel ratio AFm
Only when it is on the rich side of D, it is added to the previous value RAFAD (i-1) of the rich deviation integrated value. Here, the actual air-fuel ratio AFm is determined by the A / F located on the upstream side of the three-way catalyst 13.
This is the output of the sensor 26.

The exhaust gas flow rate correction coefficient is obtained according to the relationship shown in FIG. According to FIG. 5, a larger exhaust gas flow rate correction coefficient is given as the exhaust gas flow rate increases. Note that the exhaust gas flow rate is proportional to the intake air amount obtained from the engine speed Ne and the intake pressure PM at that time. In calculating the exhaust gas flow rate correction coefficient, the exhaust gas flow rate is calculated based on the intake air amount. Has become. However, the exhaust gas flow rate is not limited to the calculation method described above, and can be directly detected by an exhaust gas flow meter provided in the exhaust pipe 12.

Thereafter, the CPU 31 determines in step 209 whether or not the calculated rich deviation integrated value RAFAD is smaller than the reference rich area RAFADSD. RA
If FAD <RAFADSD (Step 209 is YE
S), the CPU 31 proceeds to step 207 and continues the air-fuel ratio rich control up to that point.

If RAFAD ≧ RAFADSD (NO in step 209), the CPU 31 proceeds to step 21
Go to 0. Then, the CPU 31 clears the rich control flag XREX to “0” in step 210,
In the following step 211, the rich deviation integrated value RAFAD is cleared to “0”, and thereafter, the process proceeds to step 203. Thereby, the air-fuel ratio rich control is terminated, and the process returns to the air-fuel ratio lean control.

As described above, when the three-way catalyst 13 deteriorates with time, the oxygen storage capacity of the catalyst 13 decreases. That is, the oxygen saturation adsorption amount decreases. As a result, the output of the O2 sensor 27 downstream of the NOx catalyst 14 is affected, and the deviation ΔVs (= Vs−MVs) between the output voltage Vs of the O2 sensor 27 and the target voltage MVs decreases, and the integral value decreases. Become. This is a three-way catalyst 13
Before the deterioration of the catalyst, the oxygen storage capacity is high, so that the change in the air-fuel ratio downstream of the catalyst is delayed, and the voltage deviation ΔVs becomes a relatively large value. Therefore, in the present embodiment, the degree of deterioration of the three-way catalyst 13 is detected according to the integral value of the voltage deviation ΔVs.

FIG. 6 is a flowchart showing a routine for detecting deterioration of the three-way catalyst 13. The routine is also executed by the CPU for each fuel injection of each cylinder (in this embodiment, every 180 ° CA).
31 is executed.

Now, when FIG. 6 starts, the CPU 31
Is the integral value DV of the voltage deviation ΔVs in step 301.
sSUM (i) is calculated by a 1/8 smoothing operation using the following equation.

DVsSUM (i) = DVsSUM (i-
1) + | ΔVs / 8 | In the above equation, the smoothing operation is a process for removing disturbance such as noise, and the smoothing constant is 1/8 in addition to the above 1/8.
/ 16, 1/4, 1/2, etc.

Thereafter, the CPU 31 starts to integrate the voltage deviation ΔVs in step 302, and then, for a predetermined time (for example, 1
Minutes) has elapsed. Then, on condition that a predetermined time has elapsed, the CPU 31 proceeds to step 30.
3 and the three-way catalyst 1 according to the DVsSUM value at that time.
3 is detected. At this time, for example, according to the relationship in FIG. 7, it is detected that the smaller the DVsSUM value, the greater the degree of deterioration. The detection result of the degree of deterioration is stored in the backup RAM 34 each time.

Further, the CPU 31 clears the integral value DVsSUM (i) of the voltage deviation to "0" in step 304, and thereafter ends this routine. Step 303 above
The detection result of the deterioration of the three-way catalyst 13 obtained in the above is used for setting the reference rich area RAFADSD in step 206 of FIG.

The above-described technique for detecting the deterioration of the three-way catalyst 13 is disclosed in Japanese Patent Application Laid-Open No. 8-338286 by the present applicant.
This is disclosed in detail in "Exhaust System Failure Diagnosis Apparatus for Internal Combustion Engine" in Japanese Patent Publication No.

Next, the control operation will be described more specifically with reference to the time chart of FIG. Here, FIG.
8 shows the behavior of the air-fuel ratio and the exhaust gas component when the three-way catalyst 13 is new, and FIG. 8B shows the behavior of the air-fuel ratio and the exhaust gas component when the three-way catalyst 13 deteriorates. FIG.
In (a) and (b), the air-fuel ratio rich control is temporarily performed during the air-fuel ratio lean control, and the air-fuel ratio before and after the three-way catalyst 13 is controlled by controlling the control air-fuel ratio to the rich side. It has shifted to the rich side. However, in actuality, the air-fuel ratio behind the three-way catalyst changes with a delay of the exhaust gas transport from the front air-fuel ratio, but in FIG. 8, for convenience, both air-fuel ratios change in synchronization. It is described as one.

As shown in FIG. 8A, at time t11
Previously, the air-fuel ratio lean control has been performed, and at this time, a lean counter (not shown) is counted up for each combustion in each cylinder (step 204 in FIG. 3). Then, the time t1 when the value of the lean counter reaches the predetermined value α
At 1, the control air-fuel ratio is switched from lean to rich (NO at step 202 in FIG. 3). At time t11, the reference rich area RAFADSD is calculated based on the degree of deterioration of the three-way catalyst 13 (step 206 in FIG. 3).

At time t12, the air-fuel ratio in front of and behind the three-way catalyst 13 reaches the stoichiometric air-fuel ratio (λ = 1). At this time, although the air-fuel ratio in front of the three-way catalyst 13 immediately changes to the rich side from the stoichiometric air-fuel ratio, since the three-way catalyst 13 stores oxygen, the stored oxygen and the rich component (HC , CO, etc.), and the air-fuel ratio behind the three-way catalyst 13 is temporarily maintained at the stoichiometric air-fuel ratio. Then, when the reaction between the stored oxygen and the rich component ends, the air-fuel ratio behind the three-way catalyst 13 shifts to the rich side (time t13). After time t13, since the rich component is supplied to the NOx catalyst 14, the NOx stored in the catalyst 14 is reduced and released.

After switching to the air-fuel ratio rich control (at time t
11), the air-fuel ratio in front of the three-way catalyst 13, that is, A /
With the detection value of the F sensor 26 being richer than the stoichiometric air-fuel ratio, the rich deviation integrated value RAFAD is calculated (step 208 in FIG. 3). Then, the rich deviation integrated value RA
Time t when FAD reaches reference rich area RAFADSD
At 14, the control air-fuel ratio is returned to the lean value (NO at step 209 in FIG. 3).

Thereafter, the air-fuel ratio behind the three-way catalyst 13 is determined by the lean component in the exhaust gas fed from the upstream side and the same
For a predetermined period (time t
After the stoichiometric air-fuel ratio is maintained for 15 to t16), the control returns to the lean control value. According to FIG. 8A, HC and CO components in the exhaust gas during the air-fuel ratio rich control are also suppressed to a very small amount.

On the other hand, when the three-way catalyst 13 is deteriorated, FIG.
As shown in (b), at time t21, the control air-fuel ratio is switched from lean to rich, and the three-way catalyst 13
The reference rich area RAFADSD is calculated based on the degree of deterioration (step 206 in FIG. 3). In this case, since the catalyst is deteriorating, a relatively small reference rich area RAFADSD is given (see FIG. 4).

Thereafter, at time t22, the air-fuel ratio in front of and behind the three-way catalyst 13 reaches the stoichiometric air-fuel ratio (λ = 1).
At this time, the air-fuel ratio behind the three-way catalyst 13 is once held at the stoichiometric air-fuel ratio. However, since the three-way catalyst 13 has deteriorated, the amount of oxygen stored in the catalyst is small, and in the case of FIG. The air-fuel ratio shifts to the rich side in a shorter time (time t2).
3). That is, the time during which the stored oxygen of the three-way catalyst 13 reacts with the rich component in the exhaust gas, that is, the time t22 in FIG.
-T23 is shorter than the times t12-t13 in FIG. After time t23, the rich component is NOx catalyst 1
4 side, so that the N stored in the catalyst 14
Ox is reduced and released.

At time t24 when the rich deviation integrated value RAFAD reaches the reference rich area RAFADSD,
The control air-fuel ratio is returned to the lean value (step 20 in FIG. 3).
9 is NO). According to FIG. 8B, FIG.
As in (a), the HC and CO components in the exhaust gas during the air-fuel ratio rich control are also suppressed to a very small amount.

In this embodiment, the routine of FIG. 6 corresponds to the deterioration detecting means described in the claims, and the routine of FIG. 3 corresponds to the rich control means. Steps 103 to 108 in FIG. 2 correspond to an air-fuel ratio correction unit.

According to the present embodiment described in detail above, the following effects can be obtained. In the present embodiment, the degree of deterioration of the three-way catalyst 13 located on the upstream side of the NOx catalyst 14 is detected, and rich control of the air-fuel ratio is performed based on the degree of deterioration of the three-way catalyst 13. As a result, regardless of the state of deterioration of the three-way catalyst 13, good exhaust gas purification can always be performed. That is, excessive rich components (HC, CO, H
2) is supplied, and as a result, a problem that HC and CO are discharged in large amounts is eliminated. The present embodiment is also effective when individual oxygen storage capacities are different due to individual differences of the three-way catalyst 13 and use temperature, and in such a case, it becomes possible to implement good exhaust gas purification. .

In practice, the reference rich amount (reference rich area RAFA before the three-way catalyst) is determined based on the degree of deterioration of the three-way catalyst 13.
DSD) is set, and the air-fuel ratio rich control is terminated when the rich amount integration value (rich deviation integrated value RAFAD before the three-way catalyst) during the air-fuel ratio rich control reaches the reference rich area RAFADSD. Thereby, even when the three-way catalyst 13 is deteriorated, rich control without excess or deficiency can be performed.

Further, an integral value DVsSUM of a deviation between the output voltage Vs of the O2 sensor 27 downstream of the NOx catalyst 14 and its target voltage MVs is obtained, and the degree of deterioration of the three-way catalyst 13 is detected based on the integral value DVsSUM. I did it.
In this case, the deterioration of the three-way catalyst 13 can be detected with high accuracy, and a highly reliable air-fuel ratio control can be realized.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIGS.
However, in the configuration of the second embodiment, the first
The same reference numerals are given to the same components in the drawings and the description is simplified. The following description focuses on differences from the first embodiment.

The second embodiment is different from the third embodiment in that an O2 sensor 27 is provided between the three-way catalyst 13 and the NOx catalyst 14, as shown in FIG. Electromotive force signal V depending on whether the air-fuel ratio is rich or lean
OX2 is output. The intake pipe 3 is provided with an air flow meter 29 for measuring the intake flow rate Q.

Then, the ECU 30 (CPU 31)
The degree of deterioration of the three-way catalyst 13 is detected based on the detection results of the F sensor 26 and the O2 sensor 27. In particular, in the present embodiment, the degree of deterioration of the three-way catalyst 13 is detected according to the amount of the gas component purified in the three-way catalyst 13 during the period from the start of the engine until the three-way catalyst 13 reaches the activation temperature. I have to do that. In the air-fuel ratio control by the CPU 31, the sub-F / B control is not performed as in the first embodiment, but the F / B control by the air-fuel ratio signal AF by the A / F sensor 26 is performed.

FIG. 10 is a flowchart showing the deterioration detection processing in the present embodiment. This processing is executed in place of the processing in FIG. This processing is executed by the CPU 31 at a predetermined time cycle (for example, a 64 msec cycle).

Now, when FIG. 10 starts, the CPU 3
First, at step 401, it is determined whether or not a deterioration detection flag XCAT indicating that the deterioration detection of the three-way catalyst 13 has been performed is "0". Here, XCAT = 0
Indicates that the deterioration has not been detected, and XCAT = 1 indicates that the deterioration has been detected.

On the condition that XCAT = 0, the CPU
31 proceeds to step 500 to estimate the catalyst temperature TCAT according to the processing of FIG. However, if XCAT = 1, the CPU 31 immediately ends this routine. That is, if the deterioration detection of the three-way catalyst 13 has been performed, the processing after step 500 is not performed.

Here, the catalyst temperature TCAT will be described with reference to FIG.
Will be described. In FIG. 11, the CPU 31
First, at step 501, it is determined whether or not the engine has been started. For example, after the IG is turned on, if the engine speed Ne has not reached the predetermined starting speed, step 5
01 is determined negatively. In other words, before the start is completed,
The CPU 31 proceeds to step 502, where the catalyst temperature TCAT
Is set as the intake air temperature Tam (= outside air temperature), and then the process returns to the original routine of FIG.

If the engine has been started, the CPU 3
1 proceeds to step 503 to estimate the exhaust gas temperature TEX. In this case, depending on whether the fuel is being cut or not, R
The exhaust gas temperature TEX is estimated by properly using two types of maps stored in the OM 32 in advance. When the fuel cut is not being performed, the exhaust gas temperature TEX is estimated according to the engine speed Ne and the intake air flow rate Q at that time. This estimation method utilizes the characteristic that the exhaust gas temperature TEX increases as the engine load (Ne, Q) increases. -During the fuel cut, the combustion heat of the fuel is lost, and the exhaust gas temperature TEX drops sharply. Therefore, instead of estimating the exhaust gas temperature TEX from the engine speed Ne and the intake air flow rate Q, the exhaust gas temperature TEX is estimated from the catalyst temperature TCAT (estimated value) at the start of the fuel cut. This estimation method is based on the assumption that as the catalyst temperature TCAT increases,
This utilizes the characteristic that the exhaust gas temperature TEX is increased by the heat radiation of the three-way catalyst 13.

Thereafter, the CPU 31 compares the previous estimated value TCAT (i-1) of the catalyst temperature with the exhaust gas temperature TEX in step 504, and according to the comparison result, determines the catalyst temperature TC
It is determined whether the AT is in a downward trend or an upward trend. If TCAT (i-1)> TEX, CPU
31 is assumed that the catalyst temperature TCAT is in a downward trend, and in step 505, the present value TCAT of the catalyst temperature is calculated by the following equation.
(I) is calculated.

TCAT (i) = TCAT (i-1) -K
1 · | TCAT (i−1) −TEX | Here, K1 is a coefficient stored in the ROM 32 in advance, and is set according to, for example, a variation value of the intake air flow rate Q or the engine speed Ne.

On the other hand, if TCAT (i−1) ≦ TEX, the CPU 31 determines that the catalyst temperature TCAT is on the rise, and calculates the current value TCAT (i) of the catalyst temperature in step 506 by the following equation.

TCAT (i) = TCAT (i-1) + K
2 · | TCAT (i−1) −TEX | Here, K2 is a coefficient stored in the ROM 32 in advance, and is set according to, for example, the intake air flow rate Q. In addition,
At the time of fuel cut, the coefficients K1 and K2 may be fixed to constant values.

After estimating the catalyst temperature TCAT as described above, the CPU 31 returns to the original routine of FIG. 10, and determines in step 402 whether or not the catalyst temperature TCAT has exceeded the temperature at which deterioration is detected (for example, 150 ° C.). , TCAT ≦ 1
If it is 50 ° C., without performing the subsequent deterioration detection processing,
This routine ends immediately. This is the catalyst temperature TCA
If T does not reach the temperature at which deterioration is detected, the temperature of the O2 sensor 27 is low, and the sensor output VOX2 is not stable. In such a case, the deterioration detection process is prohibited to prevent deterioration in the detection accuracy of catalyst deterioration. is there.

When TCAT> 150 ° C., C
The PU 31 proceeds to step 403, and increments the time counter 1. In the next step 404, the CPU 31 sets the data “ΔV” reflecting the unpurified gas component amount.
1 "(the locus of the output voltage fluctuation of the O2 sensor 27) is calculated by the following equation.

ΣV1 = ΣV1 + | VOX2 (i) -VO
X2 (i-1) | Here, the subscript "1" of "@V" represents the current value. That is, in the above equation, the trajectory of the output voltage fluctuation of the O2 sensor 27 is obtained by integrating the variation width of the output voltage VOX2 of the O2 sensor 27 at a predetermined sampling cycle (for example, 64 msec), The purpose is to evaluate the amount of unpurified gas components.

Further, the CPU 31 determines in step 404
"ΣΔAF
* Q1 "is calculated by the following equation. ΣΔAF · Q1 = ΣΔAF · Q1 + Q · | Target AF-A
F | Here, the intake air flow rate Q is used as data to substitute the exhaust gas flow rate, but the exhaust gas flow rate may be actually measured or may be estimated from other data, instead of using the intake air flow rate Q. . Of course, it may be estimated from the intake air flow rate Q. Note that the subscript “1” of “ΣΔAF · Q” represents the current value. | Target AF-AF | is the absolute value of the deviation between the actual air-fuel ratio (the output voltage of the A / F sensor 26) and the target air-fuel ratio (for example, the stoichiometric air-fuel ratio). The above equation is obtained by multiplying the deviation | target AF−AF | of the air-fuel ratio by the exhaust gas flow rate (= intake flow rate Q) at a predetermined sampling cycle (for example, 64 msec) and integrating the multiplied value to obtain the catalyst inflow gas component. The variation data ΣΔAF · Q1 is obtained.

Thereafter, the CPU 31 determines in step 405 whether or not the count value of the time counter 1 has exceeded a predetermined value (10 seconds in the present embodiment).
If c does not exceed c, the processing of steps 401 to 404 is repeated. Thereby, the ΣV1 value and ΣΔAF · Q1 value for 10 seconds are calculated.

When the count value of the time counter 1 is 10 seconds
c, the CPU 31 proceeds to step 406, and the data of the catalyst inflow gas component fluctuation “Σ” for 10 seconds.
It is determined whether “ΔAF · Q1” is within a predetermined range.
If it is within the predetermined range, the CPU 31 proceeds to step 4
07, the previous integrated value of the ΣV1 value ΣV is added to the current ΣV1 value.
In addition to updating the を V value by integrating one value, ΣΔAF · Q
One value of the previous integrated value ΣΔAF · Q is replaced by the current value of ΣΔAF · Q
The ΣΔAF · Q value is updated by integrating one value. Then, C
The PU 31 proceeds to step 408, where the time counter 1,
ΣV1 value and ΣΔAF · Q1 value are both cleared to “0”.

On the other hand, if it is determined in step 406 that the data “変 動 ΔAF · Q1” of the catalyst inflow gas component fluctuation is not within the predetermined range, the CPU 31 proceeds to step 408 without performing the integration processing in step 407, and proceeds to step 408. Counter 1 clears (invalidates) both the ΣV1 value and the ΣΔAF · Q1 value. This is because, when the catalyst inflow gas component fluctuation is too large or too small, the calculation accuracy of the unpurified gas component amount is reduced.
If “AF · Q1” is not within the predetermined range, the ΣV1 value and the ΣΔAF · Q1 value are both cleared and the integration process is not performed, thereby preventing the deterioration in the accuracy of the deterioration detection due to the change in the gas component flowing into the catalyst.

Thereafter, the CPU 31 determines whether or not the estimated catalyst temperature TCAT has exceeded the activation temperature (for example, 550 ° C.) of the three-way catalyst 13 in step 409. This routine is temporarily terminated without detecting deterioration. Further, the CPU 31
Step 4 when the catalyst temperature TCAT exceeds 550 ° C.
Proceeding to 10, the degree of deterioration of the three-way catalyst 13 is detected based on the data ΔV (trajectory of the output voltage fluctuation of the O2 sensor 27) reflecting the unpurified gas component amount integrated up to that time.

Here, a method for detecting catalyst deterioration will be described with reference to FIG. FIG. 12 shows data ΔV reflecting the unpurified gas component amount and data ΔΔAF ·
It is a measurement of the relationship with Q. In FIG.
The mark indicates a measured value for a new catalyst, the mark indicates a measured value for a deteriorated catalyst, and the mark indicates a measured value for a dummy catalyst (only a ceramic carrier having no catalyst layer formed on its surface). With a new catalyst (marked with ○),
ΣV value is small irrespective of the magnitude of ΔAF · Q value, but with a deteriorated catalyst (marked with □), as ΣΔAF · Q value increases,
V value tends to increase. When the catalyst deteriorates extremely and the catalytic action is lost, the state becomes the same as that of the dummy catalyst (marked with a triangle). Therefore, if the ΣΔAF · Q value is the same, it means that the larger the ΣV, the more the catalyst deteriorates.

Using this relationship, the degree of deterioration is detected from the relationship shown in FIG. According to FIG. 13, the degree of deterioration of the three-way catalyst 13 is detected according to the ΣV value and ΣΔAF · Q value at that time, and the detected degree of deterioration is stored in the backup RAM 34 as needed.

After detecting the deterioration in step 410, the CPU 31
Is "1" in the deterioration detection flag XCAT in step 411.
Is set and the routine ends. By this flag operation, the deterioration detection process is not performed from the next time. The deterioration detection result of the three-way catalyst 13 obtained in step 410 is used for setting the reference rich area RAFADSD in step 206 of FIG.

The above-described technique for detecting the deterioration of the three-way catalyst 13 shown in FIG. 10 is disclosed in Japanese Patent Application Laid-Open No. 9-31 by the present applicant.
No. 612 discloses an exhaust gas purifying catalyst deterioration detecting device in detail.

As described above, according to the second embodiment, the first
As in the embodiment, regardless of the state of deterioration of the three-way catalyst 13, good exhaust gas purification can always be performed. Further, in the present embodiment, the amount of the gas component purified in the three-way catalyst 13 from the start of the engine 1 until the three-way catalyst 13 is warmed up (data ΔV reflecting the unpurified gas component amount).
And the three-way catalyst 1 is calculated based on the unpurified gas component amount.
The deterioration degree of No. 3 is detected. As a result, it is possible to accurately detect catalyst deterioration in consideration of an increase in emissions before the catalyst is activated. Before the warm-up of the three-way catalyst 13, the difference in purification rate due to the difference in the degree of catalyst deterioration is large, and it is possible to easily and accurately detect catalyst deterioration.

In detecting the deterioration of the three-way catalyst 13, the three-way catalyst 13 is added to the data ΔV reflecting the unpurified gas component amount.
The data 変 動 ΔAF · Q of the catalyst inflow gas component fluctuation until the temperature reaches a predetermined temperature (550 ° C.) is also taken into consideration.
For this reason, it is possible to perform highly accurate catalyst deterioration detection excluding the influence of the fluctuation of the gas component flowing into the catalyst.

The embodiment of the present invention can be embodied in the following forms other than the above. When detecting the degree of deterioration of the three-way catalyst 13, the degree of deterioration is detected stepwise. In particular,
A plurality of levels (for example, about 4 to 6 levels) are determined from a state of a new article to a deteriorated state until a failure is determined, and a reference rich amount (reference rich area) at the time of air-fuel ratio rich control is determined according to the deterioration level. RAFADSD).

In each of the above embodiments, “reference rich area RAFADSD” is set as the reference rich amount, and the air-fuel ratio rich control is performed when the rich deviation integrated value RAFAD during the air-fuel ratio rich control reaches the reference rich area RAFADSD. Finished, but change this configuration. For example, a “reference rich time” may be set as the reference rich amount, and the air-fuel ratio rich control may be ended when the actual rich time during the air-fuel ratio rich control reaches the “reference rich time”.

In the second embodiment, the catalyst temperature TCAT is estimated in accordance with the exhaust gas temperature TEX in the process of FIG. 11, but the temperature estimation method is not limited to this, and may be estimated using other methods. . For example, a temperature sensor for detecting the exhaust gas temperature or the catalyst temperature may be provided in the engine exhaust system. Even in this case, the object of the present invention can be sufficiently achieved.

The ΣV value and ΣΔAF · Q shown in FIG.
As is clear from the relationship with the value, as the catalyst deterioration progresses,
The gradient of the ΣV value “ΣV / (ΣΔAF · Q)” tends to increase. Therefore, the slope of the ΣV value “ΣV / (ΣΔA
F · Q) ”, the degree of deterioration of the three-way catalyst 13 may be detected.

In the second embodiment, the catalyst temperature T
When the CAT is in the range of 150 ° C. to 550 ° C., the amount of the unpurified gas component is calculated, and the degree of deterioration of the three-way catalyst 13 is detected based on the calculation result. I can't. In short, any configuration may be used as long as the unpurified gas component amount is calculated during a period in which the difference between the purification rates of the three-way catalyst 13 when it is new and when it is deteriorated is large.

As a gas concentration sensor provided before and after the three-way catalyst 13 (upstream catalyst) and the NOx catalyst 14 (downstream catalyst), other sensors besides the above-mentioned A / F sensor and O2 sensor may be used. Good. For example, N for measuring NOx concentration
Ox sensor, HC sensor for measuring HC concentration, O2, N
A composite sensor or the like that compositely measures the concentration of Ox, HC, or the like can be appropriately used. In short, any configuration is possible as long as deterioration of the three-way catalyst 13 can be detected. As an example, the NOx amount in the exhaust gas passing through the three-way catalyst is measured by a NOx sensor, and when switching from rich combustion to lean combustion, the three-way catalyst is measured based on the response of the measured NOx amount. The degree of deterioration is detected.

As a method for detecting deterioration of the three-way catalyst 13 (upstream side catalyst), a method other than the first and second embodiments is applied. The degree of deterioration of the three-way catalyst 13 progresses with time due to thermal effects. Therefore, for example, the deterioration of the catalyst is detected in consideration of the total mileage of the vehicle, the total use time, or the like, or the three-way catalyst 1
Alternatively, the number of times and time at which 3 reaches a predetermined high temperature range may be measured, and catalyst deterioration may be detected according to the measurement result.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing an outline of an air-fuel ratio control system for an engine according to an embodiment of the invention.

FIG. 2 is a flowchart showing an air-fuel ratio control routine.

FIG. 3 is a flowchart showing a routine for setting a target air-fuel ratio.

FIG. 4 Deterioration degree of three-way catalyst and reference rich area RAFA
The figure which shows the relationship with DSD.

FIG. 5 is a diagram showing a relationship between an exhaust gas flow rate and an exhaust gas flow rate correction coefficient.

FIG. 6 is a flowchart showing a routine for detecting deterioration of the three-way catalyst.

FIG. 7 is a diagram for obtaining a degree of deterioration of a three-way catalyst based on an integral value DVsSUM of a voltage deviation.

FIG. 8 is a time chart for explaining an operation in the embodiment.

FIG. 9 is a configuration diagram showing an outline of a control system in a second embodiment.

FIG. 10 is a flowchart showing a routine for detecting deterioration of a three-way catalyst in the second embodiment.

FIG. 11 is a flowchart showing a routine for estimating a catalyst temperature.

FIG. 12 is a diagram showing a result of actually measuring a relationship between data ΔV reflecting an unpurified gas component amount and data ΔΔAF · Q of a catalyst inflow gas component variation.

FIG. 13 is a diagram for obtaining the degree of deterioration of the three-way catalyst based on the ΣV value and the ΣΔAF · Q value.

FIG. 14 is a time chart showing changes in an air-fuel ratio and an exhaust gas component in a conventional device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine (internal combustion engine), 12 ... Exhaust pipe, 13 ... Three-way catalyst as upstream catalyst, 14 ... N as downstream catalyst
Ox catalyst (NOx storage reduction catalyst), 26: A / F sensor as upstream sensor, 27: O2 as downstream sensor
Sensor, 30: ECU (Electronic Control Unit), 31: CP as deterioration detection means, rich control means, air-fuel ratio correction means
U.

Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI F01N 3/24 F01N 3/24 RC 3/28 301 3/28 301E F02D 41/04 ZAB F02D 41/04 ZAB 305 305Z 45/00 ZAB 45 / 00 ZAB 301 301K 314 314Z

Claims (6)

[Claims] An internal combustion engine includes an upstream catalyst provided upstream of an engine exhaust passage and having an oxygen storage function, and a downstream catalyst provided downstream of the engine exhaust passage and having an NOx storage reduction action. It is applied to perform lean combustion in the air-fuel ratio lean region, store NOx in exhaust gas discharged at the time of lean combustion by the downstream side catalyst, and further temporarily control the air-fuel ratio to rich to reduce the stored NOx. In the air-fuel ratio control device configured to release from the downstream catalyst, a deterioration detecting unit that detects the degree of deterioration of the upstream catalyst, and rich control of the air-fuel ratio is performed based on the detected degree of deterioration of the upstream catalyst. An air-fuel ratio control device for an internal combustion engine, comprising: 2. The method according to claim 1, wherein the rich control means obtains a reference rich amount at the time of air-fuel ratio rich control based on the degree of deterioration of the upstream catalyst, and performs rich control in accordance with the obtained reference rich amount. Air-fuel ratio control device for an internal combustion engine. 3. The air-fuel ratio control device according to claim 2, wherein the rich control means calculates a rich integrated value during the air-fuel ratio rich control, and calculates the rich integrated value and the reference rich value. Means for comparing the amount of the air-fuel ratio and terminating the air-fuel ratio rich control when the former value reaches the latter value. 4. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein said rich control means sets the reference rich amount to a smaller value as the degree of deterioration of the upstream catalyst increases. . 5. The deterioration detecting means includes: means for calculating an amount of a gas component which is not purified within the catalyst after the internal combustion engine is started until the temperature of the upstream catalyst reaches a predetermined temperature; 5. A means for detecting the degree of deterioration of the upstream catalyst based on the gas component amount.
An air-fuel ratio control device for an internal combustion engine according to any one of the above. 6. The air-fuel ratio control device according to claim 5, wherein the deterioration detecting means obtains, as an unpurified gas component amount, an integral value of an air-fuel ratio fluctuation amount every time after passing through the upstream catalyst. An air-fuel ratio control device for an internal combustion engine which detects that the degree of deterioration increases as the amount of unpurified gas components increases.