JP2000034946A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately detect the deterioration of a lean NOx catalyst by providing an estimation means which estimates the NOx storing capacity of the lean NOx catalyst based on the size of the output value of an oxygen concentration sensor in rich combustion and a deterioration detecting means which detects the deterioration of the catalyst based on the NOx storing capacity of the estimated lean NOx catalyst. SOLUTION: NOx catalyst 14 is arranged in an engine exhaust pipe 12, an A/F sensor 26 is arranged in its upstream, and an O2 sensor 27 is arranged in the downstream. A CPU 31 in an ECU 30 implements lean combustion in an air fuel ratio lean region, NOx in the exhaust gas exhausted in the lean combustion is stored by NOx catalyst 14, the air fuel ratio is temporarily controlled to become rich, and the stored NOx is emitted from the NOx catalyst 14. The CPU 31 finds the peak value of the output of the O2 sensor 27 in the rich combustion, and estimates the NOx storage capacity of the NOx catalyst 14 based on the peak value. The deterioration of the catalyst 14 is thus detected based on the estimated NOx storage capacity.

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 system 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 purifying exhaust gas 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. This lean NOx catalyst is known as a NOx storage reduction catalyst, and stores NOx when the air-fuel ratio of the exhaust gas is lean, and stores the NOx when the oxygen concentration of the exhaust gas is reduced, that is, when the exhaust gas is enriched. Reduce and release.

[0003] In the "exhaust gas purifying apparatus for an internal combustion engine" disclosed in Japanese Patent No. 2,692,380, an air-fuel ratio sensor is arranged in an exhaust passage downstream of a lean NOx catalyst (NOx absorbent). After the NOx releasing action of the NOx catalyst is started, when the air-fuel ratio detected by the air-fuel ratio sensor switches from lean to rich, the NOx
It is determined that the x release action has been completed. In this case, the decrease in the amount of NOx that can be stored by the NOx catalyst means that the NOx catalyst has deteriorated. Therefore, the decrease in the NOx storage amount, that is, the deterioration of the NOx catalyst is determined based on the reaction time required for NOx release. Could be detected.

[0004]

The output of the air-fuel ratio sensor (O2 sensor) installed downstream of the NOx catalyst changes abruptly at the stoichiometric air-fuel ratio (λ = 1). Even if it changes, the sensor output changes following the change.

[0005] For example, platinum Pt and barium Ba on a carrier
When the air-fuel ratio on the upstream side of the NOx catalyst is switched from lean to rich, theoretically, Ba (NO3) 2 + HC, CO → Ba + N2 + H2 O +
It becomes CO2. Therefore, the air-fuel ratio on the downstream side of the catalyst stays at the stoichiometric air-fuel ratio until the reaction between the stored NOx and the rich components (HC, CO) in the NOx catalyst is completed. It is thought to shift to the rich side. However, actually, by enriching the air-fuel ratio, a small amount of rich component flows downstream of the catalyst before the stored NOx disappears, and the sensor output changes to the rich side due to the rich component.

Therefore, the time when the stored NOx disappears and the behavior of the sensor output may not correspond to each other, and the above-mentioned conventional publication cannot accurately detect the deterioration of the NOx catalyst. In addition, since the reaction time of the sensor changes depending on how rich the exhaust gas supplied to the NOx catalyst actually becomes, or how much the degree of the richness becomes, the conventional publication described above accurately describes the decrease in the NOx storage amount. It was difficult to detect.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide an exhaust gas purifying apparatus for an internal combustion engine capable of accurately detecting deterioration of a lean NOx catalyst. It is.

[0008]

The exhaust gas purifying apparatus according to the present invention is based on the premise that it has a lean NOx catalyst provided in an engine exhaust system, performs lean combustion in an air-fuel ratio lean region, and emits during lean combustion. NOx in the exhaust gas to be absorbed is stored by a lean NOx catalyst, and the air-fuel ratio is temporarily controlled to be rich to release the stored NOx from the lean NOx catalyst.

According to the first aspect of the present invention, an oxygen concentration sensor disposed downstream of the lean NOx catalyst for detecting an oxygen concentration in exhaust gas, and an output value of the oxygen concentration sensor during rich combustion Estimating means for estimating the NOx storage capacity of the lean NOx catalyst based on the size of the lean NOx catalyst, and deterioration detecting means for detecting deterioration of the lean NOx catalyst based on the estimated NOx storage capacity of the lean NOx catalyst.

According to the above configuration, since the NOx storage capacity is estimated based on the magnitude of the sensor output value on the downstream side of the catalyst, how rich the exhaust gas supplied to the NOx catalyst actually becomes, or how rich the exhaust gas is, becomes The NOx occlusion ability can be accurately determined while reflecting the degree of the NOx occlusion. In this case, the magnitude of the sensor output value can be known from the peak value of the output value, the time integrated value (area) of the output change, the locus of the output change, and the like, and the stored NOx disappears with the enrichment of the air-fuel ratio. Even if a small amount of the rich component flows downstream of the catalyst before the process, the sensor output value changes to the rich side, and appropriate sensor output information corresponding to the state of catalyst deterioration at that time can be obtained. As a result, the deterioration of the lean NOx catalyst can be accurately detected.

According to the present invention, sulfur in the fuel or lubricating oil adheres to the NOx catalyst, so that the NOx storage capacity is reduced due to so-called sulfur poisoning or the like.
A decrease in Ox storage capacity can be detected. Incidentally, the thermal degradation refers to a state in which, for example, when the NOx catalyst is exposed to high heat and platinum agglomerates, the reaction area decreases, and the amount of exhaust gas discharged without purification increases.

In the first aspect of the present invention, it is preferable to detect that the greater the output value of the oxygen concentration sensor, the greater the degree of deterioration of the lean NOx catalyst.

According to the third aspect of the present invention, the rich gas amount at the time of rich combustion is predicted, and only when the rich gas amount becomes equal to or less than a predetermined value, NOx is estimated by the estimating means.
Estimate the storage capacity. That is, when the rich gas amount exceeds a predetermined value, the output of the oxygen concentration sensor is considered to increase uniformly regardless of whether the lean NOx catalyst has deteriorated. For example, an electromotive force output type O2
When a sensor is used, if the amount of rich gas is large, the sensor output value reaches the maximum value on the rich side regardless of the presence or absence of catalyst deterioration. Therefore, when the rich gas amount exceeds the predetermined value, the reliability of the deterioration detection is low, and when the rich gas amount is equal to or less than the predetermined value, the deterioration detection is performed, whereby the reliability can be improved.

According to the present invention, when estimating the NOx storage capacity by the estimating means, the amount of rich gas during rich combustion is limited to a predetermined value or less. According to the above configuration, there is a clear difference in the output of the oxygen concentration sensor between when there is no catalyst deterioration and when there is deterioration, and as a result, highly reliable detection of catalyst deterioration can be realized.

[0015] On the other hand, in the invention according to claim 5, the NOx inflow amount flowing into the lean NOx catalyst at the time of lean combustion,
NOx purification rate calculating means for calculating the NOx purification rate by the lean NOx catalyst from the ratio of the amount of rich gas required for NOx purification by the lean NOx catalyst during rich combustion, and deterioration of the lean NOx catalyst based on the calculated NOx purification rate. And a deterioration detecting means for detecting

That is, the lean NOx catalyst is degraded and NO
When the x storage capacity decreases, the amount of rich gas required for NOx purification in the catalyst decreases. Therefore, by determining the NOx purification rate using the amount of rich gas required for NOx purification as one element and detecting catalyst deterioration from the NOx purification rate, the deterioration of the lean NOx catalyst can be accurately determined as in the first aspect of the present invention. Can be detected.

According to the fifth aspect of the present invention, in the sixth aspect,
As described in the above, NO in the lean NOx catalyst
It is preferable to detect that the degree of deterioration of the lean NOx catalyst increases as the amount of rich gas required for x purification decreases and the NOx purification rate decreases.

According to the present invention, the NOx inflow amount A flowing into the lean NOx catalyst during lean combustion is calculated based on the detection result of the upstream sensor, and based on the detection result of the upstream sensor. The rich gas inflow amount B flowing into the lean NOx catalyst during rich combustion is calculated. Also, based on the detection result of the downstream sensor, the lean NO
x Excess gas amount C discharged from the catalyst is calculated. The NOx purification rate calculating means calculates (BC) / A based on the calculated NOx inflow amount A during lean combustion, the rich gas inflow amount B during rich combustion, and the surplus gas amount C. The NOx purification rate is calculated from the result.

In this case, when the deterioration of the lean NOx catalyst proceeds, the surplus gas amount C discharged from the catalyst becomes larger than the rich gas inflow amount B during the rich combustion, and as a result, the NOx purification rate decreases. And N
When the Ox purification rate decreases, it is detected that the catalyst has deteriorated.

According to the fifth and seventh aspects of the present invention, when calculating the NOx purification rate as a deterioration determination parameter, the "NOx inflow amount (A)" flowing into the lean NOx catalyst and the "NOx purification amount" in the lean NOx catalyst are calculated. The amount of rich gas (rich gas inflow amount B-excess gas amount C) required for the combustion is obtained, but the "NOx inflow amount (A)" includes information on lean combustion such as lean degree and lean time.
The "rich gas amount (B-C)" includes information related to rich combustion such as a rich degree and a rich time. Therefore, when it is considered that the “rich gas amount required for NOx purification” changes as a reflection of catalyst deterioration, the rich gas amount is appropriately corrected according to various conditions during lean combustion or rich combustion. Become. Therefore, catalyst deterioration can always be detected accurately regardless of changes in combustion conditions. Further, in this case, the disadvantage that the execution of the deterioration detection is restricted by the combustion condition is avoided.

For example, if the lean combustion is extended, N
Although the Ox inflow amount A increases, a large amount of NOx is stored in the NOx catalyst at the same time, and the rich gas amount (BC) required for NOx purification naturally increases. Therefore, although the degree of catalyst deterioration remains unchanged, NOx
There is no inconvenience that the purification rate is changed and catalyst deterioration is erroneously detected. When the rich combustion is extended, the rich gas inflow amount B during the rich combustion increases, but at the same time, the surplus gas amount C during the rich combustion also increases. Therefore, there is no inconvenience that the NOx purification rate is changed due to the change of the combustion state and catalyst deterioration is erroneously detected.

[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 NOx storage reduction catalyst (hereinafter, referred to as a NOx catalyst) is provided in the middle of an exhaust system passage of the internal combustion engine, and a limiting current type air-fuel ratio sensor (A / A) is provided upstream of the NOx catalyst. F sensor) and an oxygen sensor (O2 sensor) on the downstream side. 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 combustion passes through an exhaust manifold 11 and an exhaust pipe 12, passes through a NOx catalyst 14 provided in the exhaust pipe 12, and is then discharged to the atmosphere. The NOx catalyst 14 mainly stores NOx in exhaust gas during combustion at a lean air-fuel ratio, and converts the stored NOx into rich components (CO, H) during combustion at a rich air-fuel ratio.
C) and release.

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 NOx catalyst 14 in the exhaust pipe 12, and the A / F sensor 26 detects the oxygen concentration of the exhaust gas discharged from the engine 1 ( Alternatively, a wide range and linear air-fuel ratio signal (AF) in proportion to (CO concentration in unburned gas)
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.
OX2) 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. FIG. 2 is a flowchart showing a fuel injection control routine executed by the CPU 31. This routine is executed every fuel injection of each cylinder (in this embodiment, every 180 ° CA).

Now, when the routine of FIG. 2 starts,
First, in step 101, the CPU 31 detects the sensor detection result indicating the engine operating state (the engine speed Ne, the intake pressure P
M, cooling water temperature Tw, etc.) and the following step 102
Calculates the basic injection amount Tp according to the engine speed Ne and the intake pressure PM at that time using the basic injection map stored in the ROM 32 in advance. Also, the CPU 31
Sets a target air-fuel ratio AFTG in step 200 according to a routine shown in FIG.

Thereafter, the CPU 31 determines in step 103 the air-fuel ratio correction coefficient FAF based on the deviation between the actual air-fuel ratio AF (sensor measurement value) at that time and the target air-fuel ratio AFTG.
Set. In the present embodiment, air-fuel ratio F / B control based on modern control theory is performed.
The FAF value is set according to the setting procedure disclosed in JP-A-53-53. However, the detailed description is omitted.

After setting the FAF value, the CPU 31 uses the following equation at step 104 to calculate the basic injection amount Tp, the air-fuel ratio correction coefficient FAF, and other correction coefficients FALL (water temperature,
A final fuel injection amount TAU is calculated from various correction coefficients such as an air conditioner load.

TAU = Tp ・ FAF ・ FALL After calculating the fuel injection amount TAU, the CPU 31
A control signal corresponding to the value is output to the fuel injection valve 7, and the present routine ends once.

The F / B control is performed in such a manner that the cooling water temperature Tw is equal to or higher than a predetermined temperature, that the cooling water temperature is not high and the load is not high, and that the A / F sensor 26 is in an active state.
This is executed when the condition is satisfied, and when the F / B condition is not satisfied, the air-fuel ratio open control is executed (FAF = 1.0).

Next, the procedure for setting the target air-fuel ratio AFTG (the process of step 200) will be described with reference to the flowchart of FIG. In this process, the target air-fuel ratio AFTG is appropriately set so that rich combustion is temporarily performed during execution of lean combustion. That is, in the present embodiment, the lean time TL and the rich time TR are set so as to have a predetermined time ratio based on the value of the cycle counter counted for each fuel injection. Thus, lean combustion and rich combustion are performed alternately.

The processing of FIG. 3 will be described step by step. CPU
First, at step 201, it is determined whether or not the current cycle counter is "0". If the cycle counter is 0, then at step 202, based on the engine speed Ne and the intake pressure PM, a lean operation is performed. Time TL and rich time T
Set R. If step 201 is NO (if the cycle counter is $ 0), the CPU 31 skips the processing of step 202.

Here, the lean time TL and the rich time TR
Are equivalent to the number of fuel injections at a lean air-fuel ratio and the number of fuel injections at a rich air-fuel ratio, respectively, and are basically set to larger values as the engine speed Ne is higher or the intake pressure PM is higher. Is done. In the present embodiment, FIG.
The rich time TR is obtained by a map search based on the relationship. On the other hand, the lean time TL is obtained from the rich time TR and a predetermined coefficient α as TL = TR · α. The coefficient α may be a fixed value of about “50”, but may be variably set according to the engine operating state such as Ne and PM.

Thereafter, the CPU 31 increments the cycle counter by "1" at step 203, and determines at step 204 whether or not the value of the cycle counter has reached a value corresponding to the lean time TL. Period counter <TL
In the case of, the CPU 31 proceeds to step 205 and sets the target air-fuel ratio AFTG as a lean control value based on the engine speed Ne and the intake pressure PM at that time. After setting the AFTG value, the CPU 31 ends this routine and returns to the original state shown in FIG.
Return to the routine.

The AFTG value is obtained, for example, by searching a target air-fuel ratio map shown in FIG.
A value corresponding to / F = 20 to 23 is set (however, if the execution condition of the lean combustion is not satisfied, such as not during the steady operation, the AFTG value is set near the stoichiometric state). In such a case, the air-fuel ratio is controlled lean based on the AFTG value set in step 205.

If the period counter ≧ TL, the CPU
The process proceeds to step 206, where the target air-fuel ratio AFTG is set as a rich control value. The AFTG value may be a fixed value in a rich region, or may be variably set by searching a map based on the engine speed Ne and the intake pressure PM. When performing the map search, the AFTG value is set such that the richer the engine speed Ne or the intake pressure PM becomes, the stronger the richness becomes.

Thereafter, the CPU 31 determines in step 207 that the value of the cycle counter is equal to the total time "TL + TR"
It is determined whether or not the value has reached a value corresponding to "TR". If the cycle counter <TL + TR, the routine is terminated and the process returns to the original routine of FIG. In such a case, the air-fuel ratio is richly controlled by the AFTG value set in step 206.

On the other hand, when the cycle counter ≧ TL + TR and the result of the determination in step 207 is affirmative, the CPU 31
In step 208, the cycle counter is cleared to "0", and thereafter, the present routine is ended and the process returns to the original routine of FIG.
At the time of the next process following the clearing of the cycle counter, step 201 is affirmatively determined, and the lean time TL and the rich time TR are newly set. And the lean time TL
Again, based on the rich time TR, the above-described lean control and rich control are performed.

FIG. 6 is a time chart for explaining the air-fuel ratio control operation according to the routines of FIGS. 2 and 3. In FIG. 6, during the period from time t1 to time t2 (period of counter = 0 to TL), the air-fuel ratio is controlled lean,
NOx in the exhaust gas is stored in the NOx catalyst 14. Further, a period from time t2 to t3 (cycle counter = TL to TL
(+ TR period), the air-fuel ratio is controlled to be rich, and the unburned gas components (HC, CO) in the exhaust gas cause NOx catalyst 14
Is reduced and released. Thus, the lean control and the rich control of the air-fuel ratio are repeatedly performed according to the lean time TL and the rich time TR.

FIG. 7 is a flowchart showing a routine for detecting the deterioration of the NOx catalyst 14. This routine is executed by the CPU 31 for each fuel injection of each cylinder. In the present embodiment, during rich control, the output VOX2 of the O2 sensor 27 on the downstream side of the NOx catalyst (for convenience, this is referred to as "rear O2 output") is monitored, and the NOx storage capacity of the NOx catalyst 14 is determined based on the peak value. Is estimated. And
The degree of catalyst deterioration is detected based on the estimated NOx storage capacity.

That is, as shown in FIGS. 9A and 9B, if the degree of catalyst deterioration is different, the rear O2 output VOX
2, the peak value is different and the peak value is larger than that in (a) in FIG. 4B, so that it can be determined that the catalyst deterioration is progressing. 10 (a) and 10 (b) show the case where the rich time is extended. In this case, the rear O2 output VOX2 reaches the rich-side maximum value (about 1 V) regardless of the presence or absence of catalyst deterioration. Therefore, it becomes difficult to detect deterioration by the peak value of the rear O2 output.

Hereinafter, the processing of FIG. 7 will be described in detail. CP
U31 first determines in step 301 whether or not the conditions for performing the deterioration detection are satisfied. Deterioration detection conditions include:
This includes that the rich time is shorter than a predetermined value. For example, in the state of FIG. 9, the condition is satisfied because the peak value of the rear O2 output VOX2 can be determined. In the state of FIG. 10, the condition is not satisfied because the peak value of the rear O2 output VOX2 cannot be determined. In addition, ・ The degree of richness is within a predetermined range which is predetermined, ・ The lean time or rich time during lean combustion is within a predetermined range, ・ Steady operation in which the catalyst temperature becomes around 350 ° C. The state of being in a state may be included in the implementation conditions. Then, if the above-mentioned execution conditions are satisfied, the CPU 31 proceeds to step 302,
If the execution condition is not satisfied, this routine is once ended as it is.

Thereafter, the CPU 31 determines in step 302 whether or not the counter CCATDT is "0".
Step C303, provided that CCATDT = 0.
Proceed to. Then, the CPU 31 determines in step 303 whether or not it is time to start rich control. If step 303 is YES, CPU 31 proceeds to step 3
04, the counter CCATDT stores a predetermined value
TDT "is set. The predetermined value KCCATDT may be about three times as long as the rich time TR.

For example, at time t2 in FIG.
03 is YES, and at this time t2, the predetermined value KCCAT
DT is set. If step 303 is NO
The CPU 31 ends this routine.

When the predetermined value KCCATDT is set at the beginning of the rich control as described above, step 302 becomes NO from the next time, and the CPU 31 decrements the counter CCATDT by "1" in step 305, and thereafter proceeds to step 306.

Then, the CPU 31 determines in a step 306 whether or not the counter CCATDT is "0". If CCATDT ≠ 0, the CPU 31 proceeds to step 307 to determine whether the rear O2 output VOX2 is larger than the previous maximum value Vmax. VOX2>
If it is Vmax, the CPU 31 proceeds to step 308 and outputs the maximum value Vmax by the rear O2 output VOX2 at that time.
Is updated, and if VOX2 ≦ Vmax, this routine is terminated as it is. In other words, the peak value of the rear O2 output VOX2 is obtained by repeatedly performing steps 307 and 308.

On the other hand, if CCATDT = 0 and the affirmative determination is made in step 306, the CPU 31 proceeds to step 3
09, the calculated maximum value Vma of the rear O2 output
x (rear O2 output peak value) based on the NOx catalyst 14
Is estimated. At this time, it is estimated that the larger the maximum value Vmax of the rear O2 output, the smaller the NOx storage amount.

Thereafter, the CPU 31 proceeds to step 3
At 10, the degree of deterioration of the NOx catalyst 14 is determined based on the estimated NOx storage amount using the relationship of FIG. 8. In FIG. 8, the larger the estimated NOx storage amount (the smaller the rear O2 output peak value), the smaller the degree of catalyst deterioration, and conversely, the smaller the NOx storage amount (the larger the rear O2 output peak value), the more the catalyst. The relationship is such that the degree of deterioration is increased. In this case, if it is in the hatched area 8 in FIG.

If it is determined in step 310 that deterioration has occurred, the CPU 31 turns on an abnormality warning light (MIL: Malfunction Indicator Light) in step 311 to warn the driver that an abnormality has occurred, and has a NOx storage capacity. The reproduction process for restoring is performed. Finally, C
In step 312, the PU 31 determines the maximum value V of the rear O2 output.
max is cleared to "0", and then this routine is terminated.

In the regeneration process of step 311, for example, a process for recovering sulfur poisoning which is a main cause of catalyst deterioration is executed. Since the regeneration process is not the gist of the present invention, a detailed description thereof will be omitted. However, if the summary is briefly described, the temperature (catalyst temperature) of the NOx catalyst 14 is increased by increasing the ratio of rich combustion during lean combustion. At the same time, stoichiometric control or weak rich control at the air-fuel ratio λ = 1 is performed. When rich components (HC, CO) are supplied to the NOx catalyst 14 at a high temperature, the sulfate BaSO4 generated by sulfur poisoning is reduced to release sulfur. Thus, the NOx catalyst 14 is regenerated.

Further, despite the execution of the catalyst regeneration treatment,
If the deterioration state of the NOx catalyst 14 is continuously detected, the NOx catalyst 14 is considered to be in a state in which it cannot be regenerated, and it is finally determined that an abnormality has occurred. If it is finally determined that an abnormality has occurred, the lean control thereafter is prohibited and, for example, λ =
1, the stoichiometric control is performed. Further, the abnormality warning lamp may be turned on after it is finally determined that an abnormality has occurred.

In this embodiment, steps 307 to 309 in FIG. 7 correspond to the estimating means, and step 310 corresponds to the deterioration detecting means. According to the present embodiment described in detail above, the following effects can be obtained.

In this embodiment, the NOx storage capacity of the NOx catalyst 14 is estimated based on the peak value of the rear O2 output VOX2 during rich combustion, and the deterioration of the catalyst 14 is detected based on the estimated NOx storage capacity. I did it. According to this configuration, it is possible to accurately determine the NOx storage capacity while reflecting how rich the exhaust gas supplied to the NOx catalyst 14 is actually or how rich the exhaust gas is. In this case, even if a small amount of rich component flows downstream of the catalyst and the sensor output value changes to the rich side before the stored NOx disappears due to the enrichment of the air-fuel ratio, it depends on the state of the catalyst deterioration at that time. Thus, appropriate sensor output information can be obtained. As a result, the NOx catalyst 1
4 can be accurately detected.

Further, a condition for performing the deterioration detection is set, and the NOx storage capacity is estimated only when, for example, the rich time is shorter than a predetermined value. In this case, by performing the deterioration detection only when the rich gas amount is equal to or less than the predetermined value,
Its reliability can be increased.

Next, second to fifth embodiments of the present invention will be described. However, in the configurations of the following embodiments, the same components as those of the above-described first embodiment are denoted by the same reference numerals in the drawings, and the description is simplified. The following description focuses on differences from the first embodiment.

(Second Embodiment) In the second embodiment, the amount of NOx purified by the NOx catalyst 14 and the amount of NOx
From the ratio with the NOx inflow amount to the NOx purification rate (= NOx
Purification amount / NOx inflow amount) ", and the deterioration of the NOx catalyst 14 is detected in accordance with the NOx purification rate.

Here, the “NOx purification amount” can be obtained as the actual rich gas amount required for NOx purification. In such a case, the air-fuel ratio before and after the NOx catalyst during rich combustion is monitored to determine the difference between the rich gas inflow amount and the surplus gas amount, and the NOx purification amount is calculated from the difference between the rich gas inflow amount and the surplus gas amount. Actually, during rich combustion, the output AF of the A / F sensor 26 on the upstream side of the catalyst is integrated, and the rich gas integrated value AFA as the rich gas inflow amount is calculated.
In addition to calculating D, the output VOX2 of the O2 sensor 27 on the downstream side of the catalyst during the rich combustion is integrated to calculate a rear O2 output integrated value VOX2AD as the surplus gas amount. Then, the rich gas integrated value AFAD and the rear O2
The difference from the output integrated value VOX2AD is defined as the NOx purification amount (NOx purification amount = AFAD-VOX2AD).

The “NOx inflow amount” is determined by the NOx catalyst 14
Can be obtained as the amount of NOx supplied to the fuel cell. In practice, the engine operating state (Ne,
Based on PM, A / F), the NOx integrated amount CNOXAD as the NOx inflow amount is calculated.

Then, the calculation result of (AFAD-VOX2AD) / CNOXAD is defined as “NOx purification rate”, and the deterioration of the NOx catalyst 14 is detected using the NOx purification rate as a deterioration determination parameter.

The control operation of the CPU 31 in the present embodiment will be described with reference to the flowcharts of FIGS. 11 and 13 to 16. FIG. 11 shows a procedure for estimating the NOx integrated amount of the NOx catalyst 14, and FIGS. 13 and 14 show a procedure for detecting catalyst deterioration. The processing of FIGS. 13 and 14 is performed instead of the processing of FIG.

In FIG. 11, the CPU 31 first determines in step 401 whether the current output AF (air-fuel ratio on the upstream side of the catalyst) of the A / F sensor 26 is a lean value.
On condition that the step is determined to be affirmative, step 4
Go to 02. In step 402, the CPU 31 estimates the NOx amount CNOX (mol) contained in the exhaust gas based on the engine operating state. In estimating the CNOX value, for example, a basic NOx amount according to the engine speed Ne and the intake pressure PM at each time is obtained using the map of FIG. 12A and the relationship is obtained by using the relationship of FIG. An A / F correction value corresponding to the air-fuel ratio at each time is obtained. Then, the NOx basic amount is multiplied by the A / F correction value, and the product is set as a CNOX value (CNOX = NOx basic amount / A / F correction value).

In FIG. 12 (a), the higher the engine speed Ne or the higher the intake pressure PM, the higher the NOx
The base amount is set to a large value. In FIG. 12B, the A / F correction value = 1.0 is set at the stoichiometric air-fuel ratio (λ = 1), and the A / F value of “1.0” or more is set on the lean side.
The correction value is set. However, when the air-fuel ratio is leaner than a certain level (for example, A / F> 16), the combustion temperature drops, so that further correction on the increasing side becomes unnecessary, and the A / F correction value converges to a predetermined value.

Thereafter, the CPU 31 calculates the NOx integrated amount CNOXAD in step 403. At this time, the CNOX value calculated in step 402 is added to the previous value of the CNOXAD value, and the sum is set as the current value of the CNOXAD value (CNOXAD = CNOXAD + CNOX).

On the other hand, in the catalyst deterioration detection routine of FIG.
It is determined whether ATDT is "0" or not, and CCATDT
The process proceeds to step 502 on condition that = 0. Then, the CPU 31 determines in step 502 whether or not it is the timing to start the rich control.

If step 502 is NO, the CPU 3
1 proceeds to step 503 to determine whether or not the lean control is currently being performed. When lean control is being performed, the CPU 31
Calculates the smoothed O2 output value VOX2SM from the rear O2 output VOX2 in step 504. That is, VOX2SM = (31/32) VOX2SM + (1/3)
1) An O2 output smoothed value VOX2SM is calculated by using an arithmetic expression VOX2.

If step 502 is YES, the CPU 31 proceeds to step 505, where the counter CC
A predetermined value “KCCATDT” is set in ATDT. The predetermined value KCCATDT may be about three times as long as the rich time TR (same as step 304 in FIG. 7). When the predetermined value KCCATDT is set, step 501 becomes NO from the next time, and the CPU 31 decrements the counter CCATDT by “1” in step 506, and thereafter proceeds to step 600.

Then, in step 600, the CPU 31 follows the routine of FIG.
OX2AD is calculated. In addition, the CPU 31 calculates a rich gas integrated value AFAD according to a routine of FIG.

Thereafter, the CPU 31 proceeds to step 5 in FIG.
In 07, it is determined whether or not the counter CCATDT is "0". If CCATDT ≠ 0, CPU 31
Terminates this routine. When CCATDT becomes 0 following the countdown in step 506, the CPU 31 proceeds to step 508, and NOXCONV = CNOXAD / (AFAD-VOX2
AD) is used to calculate the deterioration determination value NOXCONV.

Thereafter, the CPU 31 uses the relationship of FIG.
The purification rate is calculated, and the NOx
The degree of catalyst deterioration is determined based on the purification rate. In FIG. 18, a relationship is given such that the higher the NOx purification rate, the smaller the degree of catalyst deterioration, and conversely, the lower the NOx purification rate, the greater the degree of catalyst deterioration. In this case, if it is in the shaded area of FIG. 18, it is determined that "deterioration exists".

If it is determined in step 510 that there is deterioration, the CPU 31 turns on an abnormality warning lamp in step 511 to warn the driver that an abnormality has occurred,
A regeneration process for restoring the Ox storage capacity is performed (same as step 311 in FIG. 7). Finally, the CPU
31 is CNOXAD, VOX2A in step 512
The values of D and AFAD are cleared to "0", and then this routine is terminated.

Next, the calculation procedure of the rear O2 output integrated value VOX2AD (the process of step 600) will be described with reference to FIG.
This will be described with reference to the flowchart of FIG. In this process, the CPU 31 firstly proceeds to step 601 to change the O2 output smoothed value VOX2SM from the rear O2 output VOX2 at that time.
(The calculated value in step 504 in FIG. 13 described above), and the difference is defined as the O2 output deviation VOX2DV (VOX2DV).
= VOX2-VOX2SM). Further, the CPU 31 determines in a subsequent step 602 whether or not the absolute value of the O2 output deviation VOX2DV is 0.02 V or more, that is, the rear O2 at that time.
It is determined whether or not the output VOX2 has changed to the rich side by "0.02 V" or more with respect to the O2 output smoothed value VOX2SM measured during the lean combustion.

If | VOX2DV | <0.02 V (NO in step 602), the CPU 31 terminates this routine and returns to the original routine of FIGS. If | VOX2DV | ≧ 0.02 V (step 6
02 is YES), the CPU 31 calculates a “VOX2DV1 value” from the product of the O2 output deviation VOX2DV and the intake air amount QA in step 603 (VOX2DV1 =
VOX2DV · QA). The intake air amount QA is calculated based on the engine speed Ne and the intake pressure PM at that time.

Further, the CPU 31 calculates the rear O2 output integrated value VOX2AD in step 604, and thereafter terminates this routine and returns to the original routine of FIGS.
In step 604, the calculated VOX2DV1 value is added to the previous value of the VOX2AD value, and the sum is calculated as VOX2A.
The present value of the D value is set (VOX2AD = VOX2AD + V
OX2DV1).

The procedure for calculating the rich gas integrated value AFAD (the process of step 700) will be described with reference to the flowchart of FIG. In the process, the CPU 31
First, in step 701, the output A of the A / F sensor 26 is calculated from the air-fuel ratio reference value AFSD (for example, stoichiometric air-fuel ratio 1.0).
F (actual air-fuel ratio) is subtracted and the difference is referred to as the rich deviation AFDV
(AFDV = AFSD-AF). CPU3
1 is “AFDV> 0” in the following step 702, that is, if the actual air-fuel ratio AF at that time is equal to the air-fuel ratio reference value AF
It is determined whether or not it is richer than SD.

When AFDV ≦ 0 (Step 702: N
O), the CPU 31 terminates this routine and returns to the original routine of FIGS. If AFDV> 0 (YES in step 702), the CPU 31 calculates a rich gas amount AFDV1 by a product of the rich deviation AFDV and the intake air amount QA in step 703 (AFD).
V1 = AFDV · QA). Further, the CPU 31 calculates the rich gas integrated value AFAD in step 704, and thereafter ends this routine and returns to the original routine of FIGS. At step 704, the calculated AFDV1 value is added to the previous value of the AFAD value, and the sum is set as the current value of the AFAD value (AFAD = AFAD + AFDV1).

FIG. 19 is a time chart showing a series of operations for detecting catalyst deterioration. Before the time t11 in FIG. 19, the air-fuel ratio lean control is performed, and the rear O2
The O2 output smoothed value VOX2SM is calculated from the output VOX2 (step 504 in FIG. 13).

At time t11, the air-fuel ratio rich control is started, and a predetermined value KCCATDT is set in a counter CCATDT. Further, the NOx integrated amount CNOXAD is determined during a period (time t) until the air-fuel ratio on the upstream side of the catalyst becomes a rich value.
12 (the period up to 12) (the process of FIG. 11).

From time t11 to time t13 when the counter CCATDT becomes "0", the rich gas integrated value AFAD corresponding to the part S1 in the figure and the rear O2 output integrated value VOX2AD corresponding to the part S2 in the figure are calculated. (Steps 700 and 600 in FIG. 13). Then, at time t13
When CCATDT = 0, CNOXAD value, AFA
Deterioration determination value NOXCONV from D value and VOX2AD value
Is calculated, and deterioration detection is performed according to the NOXCONV value (steps 508 and 509 in FIG. 14). After time t13, the O2 output smoothed value VOX2SM is calculated again.

In FIG. 19, if the deterioration of the NOx catalyst 14 progresses, the NOx storage capacity of the catalyst 14 decreases, so the rich gas integrated value AFAD (S1 part in the figure)
To the rear O2 output integrated value VOX2AD (S2 in the figure)
), And as a result, the NOx purification rate decreases. Then, it is detected that the catalyst has deteriorated due to the decrease in the NOx purification rate.

In this embodiment, steps 508 and 509 in FIG. 14 correspond to the NOx purification rate calculating means, and steps 509 and 510 correspond to the deterioration detecting means.

As described above, according to the second embodiment, the NOx inflow amount (NOx
The NOx purification rate is calculated from the ratio of the integrated amount CNOXAD) to the rich gas amount (rich gas integrated value AFAD-rear O2 output integrated value VOX2AD) required for NOx purification by the same NOx catalyst 14 during rich combustion. , The deterioration of the NOx catalyst 14 is detected.
In such a case, similarly to the first embodiment, the deterioration of the NOx catalyst 14 can be accurately detected.

In this case, “NOx integrated amount CNOXA
"D" includes information on lean combustion such as lean degree and lean time, and "rich gas integrated value AFAD-rear O2 output integrated value VOX2AD" includes information on rich combustion such as rich degree and rich time. Therefore, when it is considered that the “rich gas amount required for NOx purification” changes as a reflection of catalyst deterioration, the rich gas amount is appropriately corrected according to various conditions during lean combustion or rich combustion. Become. Therefore, catalyst deterioration can always be detected accurately regardless of changes in combustion conditions. Further, in this case, it is possible to avoid the inconvenience that the deterioration detection is restricted by the combustion conditions, for example, the deterioration detection can be performed only when the rich time is shorter than a predetermined value.

For example, if the lean combustion is extended, N
While the Ox integrated amount CNOXAD increases, a large amount of NOx is simultaneously stored in the NOx catalyst 14, and the rich gas amount (AFAD-VOX2AD) required for NOx purification naturally increases. Therefore, there is no inconvenience that the NOx purification rate is changed due to the change of the combustion condition and the catalyst deterioration is erroneously detected, even though the degree of catalyst deterioration is unchanged. When the rich combustion is extended, the rich gas integrated value AFAD during the rich combustion increases, but at the same time, the rear O2 output integrated value VOX2AD during the rich combustion also increases. Therefore, there is no inconvenience that the NOx purification rate is changed due to the change of the combustion state and catalyst deterioration is erroneously detected.

(Third Embodiment) FIG. 20 shows an outline of an air-fuel ratio control system according to a third embodiment.
As shown in FIG. 20, in the present embodiment, a three-way catalyst 15 serving as a start catalyst is disposed upstream of the NOx catalyst 14. That is, the three-way catalyst 1
Numeral 5 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. An A / F sensor 26 is provided upstream of the three-way catalyst 15, and O 2 F 2 is provided downstream of the NOx catalyst 14.
A sensor 27 is provided.

In this case, the upstream three-way catalyst 15 temporarily stores oxygen in the exhaust gas during lean combustion. Therefore, during rich combustion, the rich components (HC, CO) react with the stored oxygen in the three-way catalyst 15, and after the reaction is completed, the rich components become NOx catalyst 1
4 will be sent. Further, it is known that the oxygen storage capacity of the three-way catalyst 15 changes according to the degree of deterioration of the three-way catalyst 15, and, for example, as the catalyst deteriorates, the oxygen storage capacity decreases.

Therefore, in the present embodiment, the degree of deterioration of the three-way catalyst 15 is detected, and the air-fuel ratio rich control is performed according to the degree of deterioration of the catalyst. In such a case, the CPU 31 determines the rich control amount according to the degree of catalyst deterioration at each time, for example, using the relationship in FIG. In FIG. 21, if the degree of catalyst deterioration is small, a relatively large rich control amount is set because the oxygen storage capacity of the three-way catalyst 15 is large. That is, the duration of the rich control is set relatively long. If the degree of catalyst deterioration is large, a relatively small rich control amount is set because the oxygen storage capacity of the three-way catalyst 15 is small. That is, the duration of the rich control is set relatively short.

When the rich control amount (rich time) is set according to the degree of deterioration of the three-way catalyst 15 as described above, NOx
It is possible to always supply a constant rich gas amount to the catalyst 14. Therefore, the deterioration of the catalyst 14 can be detected based on the size of the rear O2 output VOX2. In this case, using the catalyst deterioration detection procedure of FIG. 7 described above, NO is determined according to the peak value of the rear O2 output VOX2 during rich combustion.
The degree of deterioration of the x catalyst 14 is detected.

As a method of detecting the degree of deterioration of the three-way catalyst 15, for example, Japanese Patent Application Laid-Open No.
The method disclosed in “Exhaust System Failure Diagnosis Apparatus for Internal Combustion Engine” of JP-A-8286 can be applied. The method for detecting the catalyst deterioration will be described briefly. That is, the CPU 31 performs the sub-feedback control so that the rear O2 output VOX2 (the output of the O2 sensor 27 on the downstream side of the catalyst) matches the target value, and obtains the integral value of the deviation of the rear O2 output VOX2. Then, the degree of deterioration of the three-way catalyst 15 is detected based on the integrated value of the VOX2 deviation. At this time, VOX2
It is detected that the smaller the integrated value of the deviation, the greater the degree of catalyst deterioration.

The operation of the present embodiment will be described with reference to the time chart of FIG. FIGS. 22A and 22B show the behavior of the air-fuel ratio and the like when the three-way catalyst 15 is new and when the three-way catalyst 15 is deteriorated, respectively. At time t in FIG.
At 21, the duration of the rich control is set based on the degree of deterioration of the three-way catalyst 15 at that time, and the rich control is started in accordance with the duration.

Thereafter, at time t22, the air-fuel ratio before and after the three-way catalyst 15 reaches the stoichiometric air-fuel ratio (λ = 1). At this time, although the air-fuel ratio in front of the three-way catalyst 15 immediately changes to a richer side than the stoichiometric air-fuel ratio, the oxygen stored in the three-way catalyst 15 at the time of the lean control exists. The components (HC, CO, etc.) react, and the air-fuel ratio behind the three-way catalyst 15 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 15 shifts to the rich side (time t23). After time t23, the rich component is NOx
Since it is supplied to the catalyst 14, the NOx stored in the catalyst 14 is reduced and released.

At time t24, the lean control is resumed, and the air-fuel ratio behind the three-way catalyst 15 is adjusted to a predetermined value at which the lean component in the exhaust gas fed from the upstream side and the rich component stored in the catalyst 15 react. After maintaining the stoichiometric air-fuel ratio for the period (time t25 to t26), the control returns to the lean control value.

On the other hand, when the three-way catalyst 15 is deteriorated,
As shown in (b), at time t31, the control air-fuel ratio is switched from lean to rich, and the three-way catalyst 15
Is set based on the degree of deterioration of the rich control. In this case, since the deterioration of the three-way catalyst 15 is progressing, a relatively small rich control amount is given (FIG. 21).
reference).

At time t32 when the air-fuel ratio before and after the three-way catalyst 15 reaches the stoichiometric air-fuel ratio (λ = 1), the air-fuel ratio behind the three-way catalyst 15 is once maintained at the stoichiometric air-fuel ratio.
5 is deteriorated, the amount of oxygen stored in the catalyst is small, and the air-fuel ratio shifts to the rich side in a shorter time than in the case of FIG. 22A (time t33). That is, the time from t32 to t33 in FIG. 22B, which is the time when the stored oxygen of the three-way catalyst 15 reacts with the rich component in the exhaust gas, is shown in FIG.
It becomes shorter than the time t22 to t23 of 2 (a). After time t33, since the rich component is supplied to the NOx catalyst 14, the NOx stored in the catalyst 14 is reduced and
Released. Thereafter, at time t34, the control air-fuel ratio is returned to the lean value.

According to FIGS. 22A and 22B, the rich time is controlled according to the degree of deterioration of the three-way catalyst 15. Thus, during rich control, a necessary amount of rich gas is always supplied regardless of whether the three-way catalyst 15 has deteriorated,
In addition, the amount of rich gas downstream of the NOx catalyst is regulated by a value at which deterioration of the catalyst can be detected.

As described above, in the third embodiment, the NOx catalyst 1
Although the three-way catalyst 15 is provided on the upstream side of the NOx catalyst 4, the NOx catalyst 14 is also used in this case as in the above-described embodiments.
Can be accurately detected.

In this embodiment, the A / F sensor 26 is provided upstream of the three-way catalyst 15 so that the engine 1
The distance between the sensor and the sensor 26 is shortened, and the response time from when the air-fuel ratio changes to when the sensor output changes is shortened.
Therefore, the sensor detection accuracy during the transient operation is improved.

(Fourth Embodiment) In the fourth embodiment, a three-way catalyst 15 as a start catalyst is provided upstream of the NOx catalyst 14, as in the third embodiment. The air-fuel ratio control system is configured as shown in FIG. 23 is different from FIG. 20 in that the A / F sensor 26 is connected to the three-way catalyst 15 in FIG.
Downstream (between the catalysts 14 and 15).

In this embodiment, the degree of deterioration of the NOx catalyst 14 is detected based on the NOx purification rate of the NOx catalyst 14, as described in the second embodiment. That is,
According to the procedure of FIG. 11, N flowing into the NOx catalyst 14
The Ox integrated amount CNOXAD is calculated. FIG.
According to procedures 3 and 14, the actual rich gas amount (rich gas integrated value AFA) required for NOx purification
D-rear O2 output integrated value VOX2AD) and set "AFAD-VOX2AD / CNOXAD" to NO.
NOx catalyst 1 according to the NOx purification rate
4 is detected.

When the NOx purification rate is used as a deterioration determination parameter, catalyst deterioration can be detected without restricting the amount of rich gas flowing into the NOx catalyst 14 to a predetermined value. Therefore, as described in the third embodiment, the three-way catalyst 1
It is not necessary to perform the deterioration detection of No. 5 and adjust the rich control amount according to the detection result.

In this configuration, as described above, the three-way catalyst 15
Even if the oxygen storage capacity fluctuates according to the degree of deterioration of NOx, the NOx purification rate can be accurately determined from the actual rich gas amount required for NOx purification and the NOx inflow amount during lean combustion, regardless of the magnitude of the oxygen storage capacity. Can be In other words, NO is not affected by the difference in the degree of deterioration of the three-way catalyst 15.
The deterioration of the x catalyst 14 can be detected accurately.

(Fifth Embodiment) In the third and fourth embodiments, the three-way catalyst 15 having oxygen storage capacity is
Although arranged upstream of the Ox catalyst 14, in the fifth embodiment, the three-way catalyst is changed to one having no oxygen storage capacity or one having a small oxygen storage capacity. That is, in the present embodiment, the three-way catalyst is configured by supporting only a noble metal (platinum Pt) having no oxygen storage capacity on the carrier. Specifically, a carrier made of ceramic such as stainless steel or cordierite is coated with a catalyst layer constituted by supporting only platinum Pt on the surface of porous alumina Al2 O3.

In this case, the oxygen stored in the three-way catalyst 15 and the rich components (HC, CO) in the exhaust gas react with each other, and the supply amount of the rich component to the downstream side does not decrease by that much. The behaviors of the air-fuel ratio before and after the source catalyst 15 substantially match. Therefore, there is no need to perform a process of variably setting the rich control amount in accordance with the degree of deterioration of the three-way catalyst 15 as in the third embodiment. Note that NOx
As a method for detecting the deterioration of the catalyst 14, any of the methods shown in FIG. 7 or the methods shown in FIGS.

The embodiments of the present invention can be embodied in the following forms other than the above. In the first embodiment, the condition for performing the deterioration detection is satisfied only when the lean time TL and the rich time TR are relatively short, and the deterioration detection using the rear O2 output VOX2 is performed only when the condition is satisfied. However, this configuration is changed. For example, the deterioration detection is performed at a predetermined time period, and when the deterioration detection is performed, the lean time TL and the rich time TR are forcibly shortened. That is, when estimating the NOx storage capacity and detecting catalyst deterioration based on the estimated value, the rich time or rich degree during rich combustion is limited to a predetermined value or less. According to this configuration, in the case without catalyst deterioration and in the case with catalyst deterioration,
There is a clear difference in the rear O2 output VOX2, and as a result, highly reliable detection of catalyst deterioration can be realized.

In the first embodiment, the NOx storage capacity of the NOx catalyst 14 is estimated according to the peak value of the rear O2 output VOX2, and the deterioration of the catalyst 14 is detected based on the NOx storage capacity. The following (1),
Change as shown in (2). (1) The NOx storage capacity is estimated from the time integral value (area) of the VOX2 change, and catalyst deterioration is detected based on the NOx storage capacity. Specifically, based on the rear O2 output VOX2 during the rich control, the rear O2 output integrated value VOX
2AD is calculated (for example, in the second embodiment, FIG. 15
), The rear O2 output integrated value VOX2A
The NOx storage capacity is estimated according to D. In this case, VO
It can be considered that the larger the X2AD value is, the lower the NOx storage capacity of the NOx catalyst 14 is, and the deterioration of the NOx catalyst 14 is progressing. (2) During the rich control, the amount of change in the rear O2 output VOX2 per unit time is integrated, and the locus of the output value is obtained. Then, the NOx storage capacity is estimated from the locus of VOX2, and catalyst deterioration is detected based on the NOx storage capacity. In this case, the larger the locus of VOX2, the more NO
It can be considered that the NOx storage capacity of the x catalyst 14 has decreased and the catalyst 14 has been deteriorated.

In each of the above embodiments, the O2 sensor 27 is disposed downstream of the NOx catalyst 14, and the deterioration of the NOx catalyst 14 is detected using the output of the sensor 27 (rear O2 output VOX2). The sensor 27 is limited current type A / F
It is changed to a sensor, and the catalyst deterioration is detected as shown in the following (a) and (b) using the output of the A / F sensor. (A) The deterioration of the catalyst is detected from the peak value of the output of the A / F sensor disposed downstream of the NOx catalyst or the time integrated value (area) of the output. This may be performed basically in accordance with the processing of FIG. 7, and the rear O2 output VOX2 seen in steps 307 to 309 of FIG. 7 may be changed to "rear A / F sensor output". (B) In the processes of FIGS. 13 and 14, the output integrated value of the A / F sensor on the downstream side of the catalyst is calculated in place of the rear O2 output integrated value VOX2AD. That is, the output integrated value of the A / F sensor is calculated as the surplus gas amount on the downstream side of the catalyst.
In this case, the NOx purification amount in the NOx catalyst 14 is determined from the difference between the integrated output value of the A / F sensor on the upstream side of the catalyst during rich combustion and the integrated output value of the A / F sensor on the downstream side of the catalyst during rich combustion. (Amount of rich gas required for NOx purification) is calculated. Then, catalyst deterioration is detected in accordance with the NOx purification amount.

The output of the O2 sensor or A / F sensor is converted into a physical quantity and used. For example, using the relationship of FIG.
The sensor output is converted into a rich excess amount (mol), and deterioration detection of the NOx catalyst 14 is performed using any one of the peak value, the time integral value (area), and the locus of the rich excess amount. Alternatively, the output of the A / F sensor is converted into a rich excess amount (mole) using the relationship of FIG. 25, and the NOx catalyst is used using any of the peak value, time integral value (area), and locus of the rich excess amount. 14 is performed.

In the third embodiment, the method of detecting the deterioration of the three-way catalyst 15 is changed. For example, the method disclosed in “Exhaust gas purification catalyst deterioration detection device” in Japanese Patent Application Laid-Open No. 9-31612 by the present applicant is applied. In this method, the amount of gas components to be purified in the three-way catalyst from when the engine is started until the catalyst is warmed up (data reflecting the amount of unpurified gas components) is calculated. The degree of deterioration of the three-way catalyst is detected based on the amount. In this case, catalyst deterioration can be accurately detected while taking into account the increase in emissions before the catalyst is activated. Before the warm-up of the three-way catalyst, 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 the fifth embodiment, the following configuration can be applied as the three-way catalyst 15 having a small oxygen storage capacity. -A three-way catalyst is constituted by not supporting the cocatalyst having a large oxygen storage capacity on the carrier or reducing the amount of the cocatalyst supported. In this case, a co-catalyst having a large oxygen storage capacity is ceria C
eO2, barium Ba, lanthanum La and the like are known. -A three-way catalyst is constituted by reducing the amount of the noble metal (Rh, Pd) having an oxygen storage capacity. In particular, it is preferable that the carrying amount is 0.2 g / liter or less for rhodium Rh and 2.5 g / liter or less for palladium Pd.

In each of the above embodiments, the calculation using the modern control theory was performed in the F / B control of the air-fuel ratio, but instead, the calculation using the PID, PI control, etc. may be performed. . Further, the air-fuel ratio may be open-controlled during lean combustion.

[Brief description of the drawings]

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

FIG. 2 is a flowchart showing a fuel injection control routine.

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

FIG. 4 is a map for performing a rich time according to an engine operating state.

FIG. 5 is a map for setting a lean target air-fuel ratio according to an engine operating state.

FIG. 6 is a time chart showing the behavior of air-fuel ratio control.

FIG. 7 is a flowchart illustrating a catalyst deterioration detection routine.

FIG. 8 is a graph showing the relationship between the NOx storage amount and the degree of catalyst deterioration.

FIG. 9 is a diagram showing sensor output waveforms before and after catalyst deterioration.

FIG. 10 is a diagram showing sensor output waveforms before and after catalyst deterioration.

FIG. 11 is a flowchart showing a NOx amount estimation routine.

FIG. 12 is a relationship diagram for use in NOx amount calculation.

FIG. 13 is a flowchart showing a catalyst deterioration detection routine.

FIG. 14 is a flowchart showing a catalyst deterioration detection routine continued from FIG. 13;

FIG. 15 is a flowchart showing a routine for calculating a rear O2 output integrated value VOX2AD.

FIG. 16 is a flowchart illustrating a calculation routine of a rich gas integrated value AFAD.

FIG. 17 is a diagram showing a relationship between a deterioration determination value NOXCONV and a NOx purification rate.

FIG. 18 is a diagram showing the relationship between the NOx purification rate and the degree of catalyst deterioration.

FIG. 19 is a time chart for explaining an operation in the second embodiment.

FIG. 20 is a configuration diagram showing an outline of a control system in a third embodiment.

FIG. 21 is a diagram showing a relationship between a three-way catalyst deterioration degree and a rich control amount.

FIG. 22 is a time chart for explaining an operation in the third embodiment.

FIG. 23 is a configuration diagram showing an outline of a control system in a fourth embodiment.

FIG. 24 is a diagram for converting an O2 sensor output into a rich excess amount.

FIG. 25 is a diagram for converting an A / F sensor output into a rich excess amount.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine (internal combustion engine), 3 ... Exhaust pipe, 14 ... NOx
Catalyst (NOx storage reduction catalyst), 26: A / F sensor as oxygen concentration sensor (upstream sensor), 27 ... O2 sensor as oxygen concentration sensor (downstream sensor), 30 ...
ECU, 31 ... CPU as estimating means, deterioration detecting means, NOx purification rate calculating means.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 F01N 3/28 301C F02D 41/04 ZAB F02D 41/04 ZAB 305 305A F Term (reference) 3G084 AA04 BA09 BA13 BA24 DA27 EA11 EB08 EB22 EC04 FA26 FA28 FA30 3G091 AA12 AB03 AB06 BA33 CB02 DA04 DB06 DB10 DB13 EA01 EA06 EA07 EA16 EA30 EA33 EA34 FA04 FC01 HA08 HA10 HA36 HA37 HA04 3G301 NC04 NE23 PA07Z PA10Z PA11Z PD01Z PD09Z PE01Z PE08Z

Claims (7)

[Claims] 1. A lean NOx catalyst provided in an engine exhaust system is provided to perform lean combustion in an air-fuel ratio lean region and to store NOx in exhaust gas discharged during lean combustion by the lean NOx catalyst. An exhaust gas purifying apparatus for an internal combustion engine in which the air-fuel ratio is temporarily controlled to be rich to release the stored NOx from a lean NOx catalyst. An oxygen concentration sensor that detects the NOx storage capacity of the lean NOx catalyst based on the magnitude of the output value of the oxygen concentration sensor during rich combustion; and an estimated NOx storage capacity of the lean NOx catalyst. An exhaust gas purifying apparatus for an internal combustion engine, comprising: a deterioration detecting unit configured to detect deterioration of the catalyst based on the detection result. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein it is detected that the greater the output value of the oxygen concentration sensor, the greater the degree of deterioration of the lean NOx catalyst. 3. A method for predicting a rich gas amount during rich combustion, and only when the rich gas amount is equal to or less than a predetermined value,
2. The NOx storage capacity is estimated by the estimation means.
Alternatively, the exhaust gas purifying apparatus for an internal combustion engine according to claim 2. 4. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein when estimating the NOx storage capacity by the estimating means, the amount of rich gas during rich combustion is limited to a predetermined value or less. 5. A lean NOx catalyst provided in an engine exhaust system for performing lean combustion in an air-fuel ratio lean region, and storing NOx in exhaust gas discharged during lean combustion by the lean NOx catalyst. In an exhaust gas purifying apparatus for an internal combustion engine in which the air-fuel ratio is temporarily controlled to be rich to release the stored NOx from the lean NOx catalyst, the amount of NOx flowing into the lean NOx catalyst during lean combustion and the amount of NOx inflow during rich combustion From the ratio with the rich gas amount required for NOx purification by the lean NOx catalyst, N
An exhaust gas purifying apparatus for an internal combustion engine, comprising: a NOx purification rate calculating means for calculating an Ox purification rate; and a deterioration detecting means for detecting deterioration of a lean NOx catalyst based on the calculated NOx purification rate. 6. The internal combustion engine according to claim 5, wherein the lean NOx catalyst detects a degree of deterioration of the lean NOx catalyst as the amount of rich gas required for NOx purification decreases and the NOx purification rate decreases. Exhaust gas purification device. 7. An upstream sensor disposed upstream of the lean NOx catalyst for detecting oxygen concentration in exhaust gas, and an upstream sensor disposed downstream of the lean NOx catalyst for detecting oxygen concentration in exhaust gas. A downstream sensor; means for calculating a NOx inflow amount A flowing into the lean NOx catalyst at the time of lean combustion based on the detection result of the upstream sensor; and Means for calculating a rich gas inflow amount B flowing into the NOx catalyst; and means for calculating a surplus gas amount C discharged from the lean NOx catalyst during rich combustion based on the detection result of the downstream sensor. The purification rate calculation means calculates (B−C) / A based on the calculated NOx inflow amount A during lean combustion, the rich gas inflow amount B during surplus combustion, and the surplus gas amount C. The exhaust gas purifying apparatus for an internal combustion engine according to claim 5, wherein the NOx purifying rate is calculated from the result.