JP2000054824A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately estimate an NOx occulsion amount of an NOx occulsion/reduction catalyst. SOLUTION: Exhaust emission controlling catalysts (SC) 5a, 5b having O2 storage function are arranged on exhaust passages 2a, 2b of an internal combustion engine 1. An NOx occulsion and reduction catalyst 7 is arranged on a downstream side joined exhaust passage 2. A value of an NOx counter is increased by a constant rate during lean air-fuel ratio operation of the engine by means of an electronic control unit(ECU) 30. The value is decreased by a constant rate during rich air-fuel ratio driving of the engine. When the air-fuel ratio is shifted from the lean to the rich state, and the exhaust air-fuel ratio at an inlet of the occulsion/reduction catalyst is kept near a theoretical value due to the O2 storage function of the SCs 5a, 5b, the variation of the value of the NOx counter is prevented by ECU 30. It is thus possible to accurately reflect the value of the NOx upon the NOx occulsion amount.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to purification of exhaust gas from an internal combustion engine.
For the device, specifically, in the engine exhaust passage,_Twostorage
The exhaust purification catalyst with the function and the inflow exhaust air-fuel ratio
NO during exhaust_XAbsorbs and flows into the exhaust
NO absorbed when the oxygen concentration of _XEmit
NO_XExhaust purification device for an internal combustion engine, comprising: a storage reduction catalyst.
About the installation.

[0002]

2. Description of the Related Art An exhaust gas purifying three-way catalyst such as a three-way catalyst having an O ₂ storage function is disposed in an exhaust passage of an engine which is operated near a stoichiometric air-fuel ratio to form three components of HC, CO, and NO _{X in} exhaust gas. There are known techniques for purifying water. Oxygen is O ₂ storage function of the three-way catalyst, the air-fuel ratio of the inflowing exhaust gas is absorbed oxygen component in the exhaust gas in the catalyst during the lean, that holds the air-fuel ratio of the exhaust gas flowing is absorbed when the rich Release function. As is well known, HC in the exhaust gas when the exhaust air-fuel ratio the three-way catalyst flows are in the narrow range near the stoichiometric air-fuel ratio, CO, can be simultaneously purify three components of NO _X, the exhaust air-fuel ratio Has a property that it is not possible to purify the above three components at the same time if it deviates from the stoichiometric air-fuel ratio. On the other hand, if the O ₂ storage function is added to the three-way catalyst, the excess oxygen in the exhaust is absorbed by the catalyst when the exhaust gas flowing into the three-way catalyst becomes leaner than the stoichiometric air-fuel ratio, and when the exhaust gas becomes rich, the oxygen from the catalyst becomes smaller. Is released, and even when the air-fuel ratio of the exhaust gas flowing into the catalyst deviates from the stoichiometric air-fuel ratio, the atmosphere of the three-way catalyst can be maintained near the stoichiometric air-fuel ratio. Therefore, by purifying exhaust gas of an engine operated at an air-fuel ratio near the stoichiometric air-fuel ratio using a three-way catalyst having an O ₂ storage function, HC, C
O, it is possible to satisfactorily purify three components of NO _X.

On the other hand, when the inflowing exhaust air-fuel ratio is lean
NO in exhaust_X(Nitrogen oxides)
NO absorbed when oxygen concentration in the air decreases_XEmit
NO _XStorage reduction catalysts are known. NO_XOcclusion reduction
An example of an exhaust gas purification device using a medium is, for example, a patent registration.
No. 2600492. The above features
Exhaust gas purifiers are designed to operate in lean air-fuel ratio engines.
NO in air passage_XArrange the storage reduction catalyst and make the engine lean
NO during fuel ratio operation_XNO in the exhaust gas on the storage reduction catalyst_XSuck
NO_XNO of storage reduction catalyst_XAbsorption increased
When the engine is operated for a short time at stoichiometric air-fuel ratio or rich air-fuel ratio
NO by performing rich spike operation_XSucking
NO absorbed from the storage reduction catalyst_XAnd release
NO released_XHas been reduced and purified. That is, exhaust
When the air-fuel ratio of the engine reaches the stoichiometric air-fuel ratio or the rich air-fuel ratio,
The oxygen concentration in the exhaust is sharply lower than that of
And the amount of unburned HC and CO components in the exhaust
Increases drastically. For this reason, rich spike operation
Seki operation air-fuel ratio switches to stoichiometric air-fuel ratio or rich air-fuel ratio
NO_XAir fuel of exhaust gas flowing into the storage reduction catalyst
The ratio is changed from lean air-fuel ratio to stoichiometric air-fuel ratio or rich air-fuel ratio
Changes and the NO concentration in the exhaust_XOcclusion reduction
NO from catalyst_XIs released. Also the theory as above
Relatively large amounts of unexposed air-fuel ratio or rich air-fuel ratio
NO since fuel HC and CO components are contained_XStorage reduction catalyst
NO released from_XRepresents the unburned HC and CO components in the exhaust
It reacts and is reduced.

[0004] The exhaust gas purifying apparatus described in the above-mentioned Patent Registration No. 2600492 has NO that is increased or decreased according to the operating state of the engine.
NO _X of the NO _X occluding and reducing catalyst based on the value of the _X counter
It determines storage amount, by performing the rich spike operation when the value of the NO _X counter reaches a predetermined value, N
O _X occluding and reducing catalyst is prevented from being saturated with absorbed NO _X. When the engine is operated at a lean air-fuel ratio, the NO _X counter of the above patent increases the NO _X counter at regular time intervals by an absorbed NO _X amount set according to the operating state of the engine, and the engine operates at a rich air-fuel ratio. When the operation is performed, the NO _X storage amount is estimated by performing an operation of decreasing the NO _X counter at regular time intervals by the released NO _X amount set according to the operating state of the engine. That is, NO emitted from the engine per unit time during lean air-fuel ratio operation
_X amount is engine operating condition (engine load, air-fuel ratio, exhaust flow rate, etc.)
The NO _X storage reduction catalyst absorbs this amount of NO _X per unit time. Therefore, N per unit time
The NO _X storage amount of the O _X storage reduction catalyst is proportional to the NO _X amount discharged from the engine per unit time. In the above patent,
Have set up _the amount of NO _X absorbed in the NO _X occluding and reducing catalyst per advance unit time in accordance with the engine operating conditions as the absorption amount of NO _X, according to the engine operating conditions for each lean air-fuel ratio during the operation a certain time of the engine To calculate the above absorbed NO _X amount,
The value of the _X counter is increasing. In addition, during the engine rich air-fuel ratio operation, the NO _X amount released from the NO _X storage reduction catalyst per unit time is similarly determined by the engine operating state (air-fuel ratio,
Exhaust flow rate). Therefore, NO _X occluded amount of NO _X discharged from the reducing catalyst may be set as the released amount of NO _X, the engine rich air-fuel ratio operation of the isochronous rich spike per advance unit time in accordance with the engine operating condition is in the above patent Sometimes, the value of the NO _X counter is decreased by the above-mentioned released NO _X amount at regular time intervals.

[0005]

The above-mentioned Patent Registration No. 260
In the exhaust purifying apparatus according to 0492 No. is determined timing of release of the NO _X from _the NO _X storage reduction catalyst by estimating _the NO _X storage amount of the NO _X occluding and reducing catalyst by using a NO _X counter . However, a problem may occur when a three-way catalyst having an O ₂ storage function is added as a start catalyst to the device of the above patent.

The main purpose of the start catalyst is to remove HC and CO components released from the engine in large quantities at the time of starting the engine, so that the temperature rises shortly after the engine starts and reaches the catalyst activation temperature. It is necessary to arrange the exhaust passage as close to the engine as possible. For this reason, when the start catalyst is added to the exhaust purification device of the above patent, the start catalyst is disposed in the exhaust passage on the upstream side of the NO _X storage reduction catalyst.

[0007] However, O ₂ when the exhaust gas purifying catalyst having a storage function is arranged on the upstream side exhaust passage of the NO _X occluding and reducing catalyst, the value of the NO _X counter is accurate and _the NO _X storage amount of the NO _X occluding and reducing catalyst It has been found that there may be a case where it is no longer possible to respond. This problem is believed to occur because the change in the exhaust air-fuel ratio for the O ₂ storage function is delayed by the exhaust purification catalyst outlet when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst changes.

That is, in the apparatus of the above patent, the engine is
NO if operating at air-fuel ratio_XIncrement counter value
When the engine air-fuel ratio is switched to rich, NO_XMosquito
The counter value is decreasing. However, NO_XOcclusion return
O upstream of the source catalyst_TwoExhaust purification catalyst with storage
When it is located, the engine operation air-fuel ratio is changed from lean to rich
Even if the exhaust air-fuel ratio becomes rich_TwoS
The total amount of oxygen stored in the exhaust purification catalyst by the storage function
Exhaust air-fuel passing through the exhaust purification catalyst until the amount is released
The ratio does not change to a rich air-fuel ratio. For this reason, exhaust purification
NO on the downstream side of the medium _XThe exhaust air-fuel ratio flowing into the storage reduction catalyst is
Even if the engine air-fuel ratio changes to a rich air-fuel ratio,
Until the oxygen release is completed, it is maintained near the stoichiometric air-fuel ratio.
No_XNO from the storage reduction catalyst_XIs not released. Subordinate
As described in the above-mentioned Patent Registration No. 2600492,
N from when the operating air-fuel ratio was changed from lean to rich
O _XWhen an operation for reducing the storage reduction catalyst is performed, NO_X
The counter value is the actual NO _XIt ’s smaller than the storage capacity
I will. Therefore, when the rich spike operation is performed, NO_XCow
NO when the counter decreases to a predetermined value ($ 0)_XRelease
When it is determined that the operation has been completed and the lean air-fuel ratio
NO_XThe storage reduction catalyst still stores NO_XRemains
NO_XWill resume absorption. Also,
NO from this state_XAs you increase the value of the counter,
NO_XNO for the storage reduction catalyst_XCorresponding to the counter value
NO more than amount_XIs occluded. others
No _XRich spike operation based on counter value
NO when the start and end of_XAnticipated for storage reduction catalyst
NO in excess of_XMay be occluded, and NO_X
The absorption efficiency of the storage reduction catalyst decreases or NO_XOcclusion reduction
NO absorbed by the medium_XBecause it can be saturated
is there.

Further, the engine operating air-fuel ratio changes from rich to lean.
A similar problem arises when switching to. Engine luck
Exhaust gas purification by switching the air-fuel ratio from rich to lean
When the exhaust air-fuel ratio flowing into the catalyst becomes lean,
Purification catalyst is O_TwoAbsorbs oxygen in exhaust gas for storage function
Take it. As a result, excess oxygen in the exhaust gas
And the exhaust purification catalyst absorbs oxygen.
During the operation, NO on the downstream side of the exhaust purification catalyst_XFor storage reduction catalyst
The inflowing exhaust gas does not have a lean air-fuel ratio, _XOcclusion return
Source catalyst is NO_XDoes not absorb. Exhaust purification catalyst is maximum oxygen
Absorbs oxygen up to the storage amount and further absorbs oxygen in exhaust
If it is no longer possible, NO on the downstream side of the exhaust purification catalyst_XOcclusion reduction
The air-fuel ratio of the exhaust gas flowing into the medium becomes lean and NO_XOcclusion return
Source catalyst is NO_XBegins to absorb oxygen, but the exhaust purification catalyst
NO while absorbing_XWhen the value of the counter is increased
Actually NO_XNO stored in the storage reduction catalyst_XThe amount is NO
_XIt becomes smaller than the value of the counter, and the actual NO_X
Storage amount and NO_XThe value of the counter no longer corresponds
Problems arise.

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides an exhaust purification system in which a NO _X storage reduction catalyst is disposed downstream of an exhaust purification catalyst having an O ₂ storage function, and accurately estimates the NO _X storage amount of the NO _X storage reduction catalyst. The purpose is to provide a means that allows the user to do this.

[0011]

According to the first aspect of the present invention, an internal combustion engine that switches the operating air-fuel ratio between a lean air-fuel ratio operation and a stoichiometric or rich air-fuel ratio operation as required. An exhaust purification device having an O ₂ storage function disposed in an engine exhaust passage, and an inflowing exhaust air-fuel ratio disposed downstream of the exhaust purification catalyst in the exhaust passage. NO in exhaust
Absorbs _X, and _the NO _X storage reduction catalyst the oxygen concentration in the inflowing exhaust gas to release NO _X absorbed when lowered, NO _X amount occluded in the _the NO _X storage reduction catalyst based on the engine operating condition and _the NO _X storage amount estimating means for estimating a, when the engine operating air-fuel ratio is changed, the _the NO _X storage amount estimated by the _the NO _X storage amount estimating means to the amount of oxygen stored in the exhaust purification in the catalysts exhaust gas purification apparatus for an internal combustion engine and a _the NO _X storage amount correcting means for correcting is provided based.

Namely, the operating air-fuel ratio of the NO _X storage amount estimating means agencies in the invention of claim 1, the exhaust flow rate, of the NO _X occluding and reducing catalyst based on the engine operating state such as the fuel injection NO _X
Estimate the amount of occlusion. _The NO _X storage amount correcting means correcting the _the NO _X storage amount estimated by the above oxygen stored amount of the exhaust purification catalyst. For example, even if the more oxygen stored amount of the exhaust purification catalyst the engine operating conditions (e.g., operating air-fuel ratio) is changed, the storage is until appears as change in state of the change is actually flowing to the NO _X occluding and reducing catalyst exhaust A delay time occurs according to the amount of oxygen. Therefore, when estimating the NO _X storage amount based on the engine operation state, the NO _X storage amount is corrected according to the stored oxygen amount (the delay time with respect to the change in the engine operation state), thereby improving the estimation accuracy of the NO _X storage amount. I do.

According to the second aspect of the present invention, the NO
_X storage amount estimating means calculates based absorption amount of NO _X absorbed in the NO _X occluding and reducing catalyst is discharged from the engine, and a discharge amount of NO _X released from _the NO _X storage reduction catalyst on the engine operating state, NO when engine operation air-fuel ratio is lean air-fuel ratio
_By increasing the estimated NO _X storage amount of the _X storage reduction catalyst by the absorbed NO _X amount and decreasing the estimated NO _X storage amount by the released NO _X amount when the engine operating air-fuel ratio is a rich air-fuel ratio, NO an exhaust purification system of an internal combustion engine according to claim 1 to estimate _the NO _X storage amount of _X occluding and reducing catalyst.

[0014] That is, in the invention of claim 2, _the NO _X storage amount estimating means estimates the _the NO _X storage amount using the NO _X counter. As described above, the engine operating air-fuel ratio from lean to rich or when switching from rich to lean, the change of the exhaust air-fuel ratio actually flowing to the NO _X occluding and reducing catalyst time corresponding to the storage amount of oxygen of the exhaust gas purifying catalyst Only cause a delay. Therefore, even when estimating the _the NO _X storage amount of the NO _X occluding and reducing catalyst based the NO _X counter, the value of the NO _X counter by correcting the value of the NO _X counter in accordance with the oxygen stored amount of the exhaust gas purifying catalyst Accurately corresponds to the NO _x storage amount.

According to the third aspect of the present invention, when the engine operating air-fuel ratio changes from a lean air-fuel ratio to a rich air-fuel ratio, the correcting means may change the engine operating air-fuel ratio to a rich air-fuel ratio. 3. The exhaust gas of an internal combustion engine according to claim 2, wherein the operation of reducing the estimated NO _X amount by the NO _X storage amount estimation means is prohibited until the entire amount of oxygen stored in the exhaust gas purification catalyst is released from the exhaust gas purification catalyst. A purification device is provided.

That is, according to the third aspect of the present invention, the engine operating idle
When the fuel ratio changes from lean air-fuel ratio to rich air-fuel ratio
Is the oxygen stored in the exhaust purification catalyst even after the rich air-fuel ratio change.
NO until the total amount of_XInhibit counter decrement operation
Stop. While oxygen is being released from the exhaust purification catalyst,
Even if the engine operating air-fuel ratio becomes a rich air-fuel ratio, _X
The exhaust air-fuel ratio flowing into the storage reduction catalyst does not become rich,
NO_XNO from the storage reduction catalyst_XIs not released. Follow
NO during this period_XWhen the counter is decremented,
NO_XCounter value and actual NO_XError between occlusion amount
Occurs. In the present invention, oxygen is released from the exhaust purification catalyst.
NO while_XProhibit counter decrement operation
NO_XThe above error occurs in the counter value
No, NO_XCounter value is exactly NO_XVs. storage
To respond.

According to the fourth aspect of the present invention, when the engine operating air-fuel ratio changes from the rich air-fuel ratio to the lean air-fuel ratio, the correcting means sets the engine operating air-fuel ratio after the engine operating air-fuel ratio changes to the lean air-fuel ratio. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein the operation of increasing the estimated NO _X amount by the NO _X storage amount estimating means is inhibited until oxygen is absorbed by the exhaust gas purifying catalyst up to the maximum storage amount. You.

[0018] That is, when the engine operating air-fuel ratio in the invention of claim 4 is changed from rich to lean, the increase operation of the NO _X counter to exhaust air-fuel ratio flowing into the NO _X occluding and reducing catalyst is actually lean Ban. Also the engine operating air-fuel ratio becomes lean, the exhaust air-fuel ratio during the flowing actually _the NO _X storage reduction catalyst oxygen in the exhaust gas purifying catalyst is absorbed not become lean, the _the NO _X storage reduction catalyst NO
_X is not absorbed. Therefore, if the NO _X counter is increased during this period, an error occurs between the value of the NO _X counter and the actual NO _X storage amount. In the present invention, the engine operating air-fuel ratio without increasing the NO _X counter immediately after change to a lean exhaust gas purification catalyst is to wait until a state saturated with oxygen by absorbing oxygen up storage amount counter Start increasing. When the exhaust purification catalyst absorbs oxygen up to the maximum storage amount, it cannot absorb oxygen in the exhaust any more.
Exhaust gas flowing into the O _X occluding and reducing catalyst actually becomes lean. For this reason, by prohibiting the increase operation of the NO _X counter while oxygen is being absorbed by the exhaust purification catalyst, the above-described error is prevented from occurring, and the value of the NO _X counter is accurately set to N.
O _X will correspond to the storage amount.

According to the fifth aspect of the present invention, the correction means estimates the amount of oxygen stored in the exhaust purification catalyst based on the exhaust air-fuel ratio flowing into the exhaust purification catalyst. An exhaust gas purification apparatus for an internal combustion engine according to claim 1, further comprising estimating means. That is, in the invention of claim 5, the correcting means includes the stored oxygen amount estimating means.
The stored oxygen amount estimating means estimates the amount of oxygen stored in the exhaust gas purification catalyst based on the exhaust air-fuel ratio flowing into the exhaust gas purification catalyst. Air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst when a lean is oxygen absorbed into the exhaust purification catalyst by O ₂ storage function, the oxygen storage amount of the catalyst is increased. Also,
When the exhaust air-fuel ratio flowing into the exhaust purification catalyst is rich, the stored oxygen is released from the exhaust purification catalyst, so that the stored oxygen amount of the catalyst decreases. The stored oxygen amount estimating means of the present invention, for example, increases the estimated stored oxygen amount by a predetermined amount at regular intervals when the inflowing exhaust air-fuel ratio is lean, and increases the estimated storage oxygen amount at regular intervals when the inflowing exhaust air-fuel ratio is rich. By reducing the estimated stored oxygen amount by a predetermined amount, the stored oxygen amount of the exhaust purification catalyst is estimated.

According to the sixth aspect of the present invention, when the engine operating air-fuel ratio changes, the correcting means controls the amount of oxygen stored in the exhaust purification catalyst based on the exhaust air-fuel ratio after passing through the exhaust purification catalyst. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, further comprising a stored oxygen amount estimating unit for estimating the stored oxygen amount.
That is, in the invention of claim 6, the correcting means includes the stored oxygen amount estimating means. The stored oxygen amount estimating means estimates the amount of oxygen stored in the exhaust purification catalyst based on the exhaust air-fuel ratio after passing through the exhaust purification catalyst when the engine operating air-fuel ratio changes.
When the operating air-fuel ratio changed from lean to rich or from rich to lean, the exhaust air-fuel ratio after passing through the exhaust purification catalyst did not immediately change to rich or lean, and the entire amount of oxygen stored in the exhaust purification catalyst was released. After that (in the case of a change from lean to rich) or after being saturated with the oxygen absorbed by the exhaust purification catalyst (in the case of a change from rich to lean), the air-fuel ratio changes to the same as the exhaust gas that flows. That is, the change in the exhaust air-fuel ratio after passing through the exhaust purification catalyst is delayed as compared with the change in the exhaust air-fuel ratio flowing into the catalyst, and the delay time increases as the oxygen storage amount of the catalyst increases. In the present invention, for example, the engine air-fuel ratio (the exhaust air-fuel ratio flowing into the exhaust purification catalyst)
When changes between lean and rich, the time from when the engine operating air-fuel ratio changes to when the exhaust air-fuel ratio after passing through the catalyst changes following the change in the engine operating air-fuel ratio is measured, Based on this time, the catalyst storage oxygen amount is estimated.

According to the seventh aspect of the present invention, the stored oxygen amount estimating means includes a stored oxygen amount correcting means for correcting the stored oxygen amount estimated value based on the degree of deterioration of the exhaust purification catalyst. 6. An exhaust gas purifying apparatus for an internal combustion engine according to item 6. That is, in the invention of claim 7, in the invention of claim 6, the estimated value of the stored oxygen amount is corrected based on the degree of deterioration of the exhaust purification catalyst. O ₂ when the exhaust purification catalyst deteriorates
Since the storage function also decreases, the maximum oxygen amount (saturation amount) that can be stored by the exhaust purification catalyst decreases. For example, if the lean air-fuel ratio operation of the engine continues for a long time, oxygen is absorbed up to the maximum stored oxygen amount, and no more oxygen can be absorbed. Therefore, when the estimated stored oxygen amount reaches the predetermined maximum stored oxygen amount, the stored oxygen amount estimation means according to claim 6 does not further increase the estimated stored oxygen amount even if the engine is operating at the lean air-fuel ratio. In the present invention, for example, the estimated value of the stored oxygen amount is corrected by setting the maximum stored oxygen amount according to the degree of deterioration of the catalyst. Thus, the stored oxygen amount of the catalyst is accurately estimated regardless of the deterioration of the catalyst.

According to the invention described in claim 8, the NO
_XThe storage amount estimating means is configured to determine_XDeterioration of storage reduction catalyst
Determination means for determining the degree of engine operation,_XSucking
NO based on the degree of deterioration of the storage reduction catalyst_XEstimate storage capacity
The exhaust gas purification of an internal combustion engine according to claim 1 or claim 2.
An apparatus is provided. That is, in the invention of claim 8,
NO in claim 1 or 2_XStorage amount estimation means
Is NO_XDeterioration of storage reduction catalyst and engine operating condition
NO based on _XNO of storage reduction catalyst_XEstimate storage capacity
You. NO_XThe storage reduction catalyst is the maximum that can be stored with deterioration
NO_XThe amount decreases. In the invention of claim 1 or 2,
NO estimated from Seki operation state_XMaximum storage amount is NO_XSucking
Estimated NO when storage reaches_XIncrease the storage capacity further
Absent. In the present invention, for example, NO_XDeterioration of storage reduction catalyst
NO depending on the degree_XBy setting the maximum storage amount,
NO_XThe amount of occlusion can be accurately estimated.

[0023]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing a schematic configuration of an embodiment when the present invention is applied to an internal combustion engine for a vehicle. In FIG. 1, reference numeral 1 denotes an automobile internal combustion engine. In the present embodiment, the engine 1 is a four-cylinder gasoline engine having four cylinders # 1 to # 4, and fuel injection valves 111 to 114 for directly injecting fuel into the cylinders are provided in the # 1 to # 4 cylinders. Is provided. As will be described later, the internal combustion engine 1 of the present embodiment is a lean burn engine that can operate at a higher (lean) air-fuel ratio than the stoichiometric air-fuel ratio.

Further, in this embodiment, the cylinders # 1 to # 4 are grouped into two cylinder groups each including two cylinders whose ignition timings are not continuous with each other. (For example, in the embodiment of FIG. 1, the cylinder ignition order is 1-3-4-2, and #
The cylinders # 1 and # 4 and the cylinders # 2 and # 3 each constitute a cylinder group. The exhaust port of each cylinder is connected to an exhaust manifold for each cylinder group, and is connected to an exhaust passage for each cylinder group. In FIG. 1, reference numeral 21a denotes an exhaust manifold for connecting the exhaust ports of the cylinder group consisting of # 1 and # 4 cylinders to the individual exhaust passage 2a, and reference numeral 21b denotes the exhaust ports of the cylinder group consisting of # 2 and # 4 cylinders for the individual exhaust passage 2b Exhaust manifold connected to In the present embodiment, the individual exhaust passage 2
Start catalysts (hereinafter, referred to as "SC") 5a and 5b each composed of a three-way catalyst are arranged on a and 2b. The individual exhaust passages 2a and 2b join the common exhaust passage 2 downstream of the SC.

On the common exhaust passage 2, a NO _X storage reduction catalyst 7 described later is arranged. FIG. 1 shows 29a and 29b.
Indicate SC5a, 5b of the individual exhaust passages 2a, 2b.
An upstream air-fuel ratio sensor 31 arranged upstream is a downstream air-fuel ratio sensor arranged upstream of the NO _X storage reduction catalyst 7 in the exhaust passage 2. Air-fuel ratio sensor 29a,
29b and 31 are so-called linear air-fuel ratio sensors that output a voltage signal corresponding to the exhaust air-fuel ratio in a wide air-fuel ratio range.

Further, reference numeral 30 in FIG. 1 denotes an electronic control unit (ECU) of the engine 1. In the present embodiment, the ECU 30 is a microcomputer having a known configuration including a RAM, a ROM, and a CPU, and performs basic control such as ignition timing control and fuel injection control of the engine 1. In the present embodiment, the ECU 30 performs the basic control described above,
As described later, the in-cylinder injection valve 111 depends on the engine operating state.
To 114 to change the fuel injection mode to change the operating air-fuel ratio of the engine. In the present embodiment, EC
U30 determines NO _X based on the engine operating state in a manner described later.
With estimating the _the NO _X storage amount of storage reduction catalyst, the estimated _the NO _X storage amount is absorbed increases to a predetermined amount NO
_In addition to performing a rich spike operation to switch the air-fuel ratio to the rich air-fuel ratio for a short time during the lean air-fuel ratio operation of the engine in order to release _X , it also stores NO _X when the engine air-fuel ratio changes between rich and lean. Perform the operation to correct the amount.

The input port of the ECU 30 receives signals representing the exhaust air-fuel ratio on the upstream side from the upstream air-fuel ratio sensors 29a and 29b to the SCs 5a and 5b and the signals from the air-fuel ratio sensor 31 to S
A signal representing the exhaust air-fuel ratio downstream of C5a, 5b is
A signal corresponding to the intake pressure of the engine is input from an intake pressure sensor 33 provided in an engine intake manifold (not shown). A signal corresponding to the engine speed is input. Further, in the present embodiment, E
A signal representing the accelerator pedal depression amount (accelerator opening) of the driver is input to an input port of the CU 30 from an accelerator opening sensor 37 arranged near an accelerator pedal (not shown) of the engine 1. The output port of the ECU 30 is connected to the fuel injection valves 111 to 114 of each cylinder via a fuel injection circuit (not shown) in order to control the amount and timing of fuel injection into each cylinder.

In the present embodiment, the ECU 30 operates the engine 1 in the following five combustion modes according to the operating state of the engine. Lean air-fuel ratio stratified combustion (compression stroke single injection) Lean air-fuel ratio homogeneous mixture / stratified combustion (intake stroke / compression stroke twice injection) Lean air-fuel ratio homogeneous mixture combustion (intake stroke single injection) Theoretical air-fuel ratio homogeneous mixing Air Combustion (Single-Injection Injection Injection) Rich Air-Fuel Ratio Homogeneous Mixture Combustion (Single-Intake Injection Injection) That is, in the light-load operation region of the engine 1, the lean air-fuel ratio stratified combustion is performed. In this state, in-cylinder fuel injection is performed only once in the latter half of the compression stroke of each cylinder, and the injected fuel forms a combustible air-fuel mixture layer near the cylinder ignition plug. Further, the fuel injection amount in this operating state is extremely small, and the overall air-fuel ratio in the cylinder is about 25 to 30.

Further, when the load increases from the above-described state and the load becomes a low-load operation range, the lean air-fuel ratio homogeneous mixture / stratified combustion is performed. As the engine load increases, the amount of fuel injected into the cylinder increases. However, in the above-described stratified combustion, since the fuel injection is performed in the latter half of the compression stroke, the injection time is limited and the amount of fuel that can be stratified is limited. There is. Therefore, in this load region, a target amount of fuel is supplied to the cylinder by injecting in advance the amount of fuel that is insufficient only by fuel injection in the latter half of the compression stroke into the first half of the intake stroke. The fuel injected into the cylinder in the first half of the intake stroke produces an extremely lean homogeneous mixture by the time of ignition. In the latter half of the compression stroke, fuel is further injected into this extremely lean homogeneous mixture, and a layer of ignitable combustible mixture is generated near the ignition plug. At the time of ignition, the combustible air-fuel mixture layer starts burning, and the flame propagates to the surrounding lean air-fuel mixture layer, so that stable combustion is performed. In this state, the amount of fuel supplied by the injection in the intake stroke and the compression stroke is further increased, but the overall air-fuel ratio becomes slightly lower (for example, about 20 to 30 in air-fuel ratio).

When the engine load further increases, the engine 1 performs the above-described lean air-fuel ratio homogeneous mixture combustion. In this state, fuel injection is performed only once in the first half of the intake stroke,
The fuel injection amount is further increased from the above. In this state, the homogeneous mixture generated in the cylinder has a lean air-fuel ratio relatively close to the stoichiometric air-fuel ratio (for example, an air-fuel ratio of about 15 to 25).

When the engine load further increases and the engine becomes in the high-load operation range, the amount of fuel is further increased from the state described above, and the above stoichiometric air-fuel ratio homogeneous mixture operation is performed. In this state, a homogeneous air-fuel mixture having a stoichiometric air-fuel ratio is generated in the cylinder, and the engine output increases. Further, when the engine load further increases and the engine becomes full load operation, the rich air-fuel ratio homogeneous mixture operation in which the fuel injection amount is further increased from the state described above. In this state, the air-fuel ratio of the homogeneous mixture generated in the cylinder is rich (for example, 12 to 1 in air-fuel ratio).
About 4).

In this embodiment, the optimal operation mode (from the above) is set in advance based on experiments and the like in accordance with the accelerator opening (the amount of depression of the accelerator pedal by the driver) and the engine speed. The map is stored in the ROM using the accelerator opening and the engine speed. During the operation of the engine 1, the ECU 30 determines which of the above operation modes should be currently selected based on the accelerator opening detected by the accelerator opening sensor 37 and the engine speed, and determines the fuel according to each mode. The injection amount, fuel injection timing and number of times are determined.

That is, when the above mode (lean air-fuel ratio combustion) is selected, the ECU 30 calculates the fuel based on the accelerator opening and the engine speed based on the maps prepared in advance for each of the above modes. Determine the injection amount. When the above mode (stoichiometric air-fuel ratio or rich air-fuel ratio homogeneous mixture combustion) is selected, E
The CU 30 sets the fuel injection amount based on the intake pressure detected by the intake pressure sensor 33 and the engine speed based on a map prepared in advance for each of the above modes.

When the mode (stoichiometric air-fuel ratio homogeneous mixture combustion) is selected, the ECU 30 further uses the fuel injection amount calculated above as an air-fuel ratio sensor so that the engine exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio. Feedback correction is performed based on the outputs of 29a, 29b and 31. As mentioned above,
In the engine 1 of the present embodiment, the fuel injection amount is increased as the engine load increases, and the operation mode is changed according to the fuel injection amount.

Next, the start catalysts 5a and 5b and the NO _X storage reduction catalyst of this embodiment will be described.
The start catalysts (SC) 5a and 5b use a honeycomb-shaped carrier such as cordierite to form a thin coating of alumina on the surface of the carrier, and form platinum Pt, palladium Pd, rhodium Rh, etc. on the alumina layer. As a three-way catalyst supporting the noble metal catalyst component. Three-way catalyst for purifying HC at the stoichiometric air-fuel ratio near, CO, three components of the NO _X at a high efficiency. Three-way catalyst, the air-fuel ratio of the inflowing exhaust gas because the reducing capacity of the higher becomes the NO _X than the stoichiometric air-fuel ratio decreases, to sufficiently purify NO _X in the exhaust gas when the engine 1 is a lean air-fuel ratio operation It is not possible.

The SCs 5a and 5b are disposed in portions of the exhaust passages 2a and 2b close to the engine 1 so as to reach the activation temperature of the catalyst in a short time after starting the engine and start the catalytic action. It is of a relatively small capacity in order to reduce the noise. Next, SCs 5a, the O ₂ storage function of 5b will be described. It is generally known that when a metal component such as cerium (Ce) is supported on an exhaust purification catalyst such as a three-way catalyst in addition to a catalyst component, the exhaust purification catalyst exhibits an oxygen storage function (O ₂ storage function). I have.
That is, cerium supported on the catalyst as an additive is:
When the air-fuel ratio of the exhaust flowing into the catalyst is higher than the stoichiometric air-fuel ratio (when the exhaust air-fuel ratio is lean), it combines with oxygen in the exhaust to form ceria (cerium oxide) and stores oxygen.
When the air-fuel ratio of the inflowing exhaust gas is equal to or lower than the stoichiometric air-fuel ratio (when the exhaust air-fuel ratio is rich), ceria releases oxygen and returns to metallic cerium, so that oxygen is released. O ₂
In an exhaust purification catalyst having a storage function, even when the exhaust air-fuel ratio flowing into the catalyst changes from a rich air-fuel ratio to a lean air-fuel ratio, oxygen in the exhaust gas is absorbed by cerium, so that the oxygen concentration in the inflowing exhaust gas decreases. Therefore, while the oxygen in the exhaust gas is being absorbed by the cerium, the exhaust air-fuel ratio at the catalyst outlet is close to the stoichiometric air-fuel ratio. Further, when the total amount of cerium supported by the catalyst is combined with oxygen (that is, the catalyst is saturated with oxygen) and cannot absorb oxygen any more, the exhaust air-fuel ratio at the exhaust purification outlet becomes smaller than the exhaust air at the catalyst inlet. The air-fuel ratio changes to the same lean air-fuel ratio. Similarly, in a state where cerium has sufficiently absorbed oxygen, when the air-fuel ratio of exhaust flowing into the catalyst changes from a lean air-fuel ratio to a rich air-fuel ratio, oxygen is released from cerium,
As the oxygen concentration in the exhaust gas increases, the air-fuel ratio at the catalyst outlet becomes close to the stoichiometric air-fuel ratio. Also in this case, after the entire amount of oxygen combined with cerium is released, no more oxygen is released from the catalyst, so the exhaust air-fuel ratio at the catalyst outlet becomes a rich air-fuel ratio like the air-fuel ratio at the catalyst inlet. . That is, when the exhaust purification catalyst has the O ₂ storage function, the change of the exhaust air-fuel ratio on the downstream side of the catalyst from lean to rich or from rich to lean causes a delay as compared with the upstream side of the catalyst.

Since the SCs 5a and 5b of the present embodiment have an O ₂ storage function, when the operating air-fuel ratio of the engine changes from lean to rich or from rich to lean, S 5
The change of the exhaust air-fuel ratio on the downstream side of C5a, 5b is delayed, and a period occurs in which the air-fuel ratio is temporarily maintained near the stoichiometric air-fuel ratio. Next, the NO _X storage reduction catalyst 7 of the present embodiment will be described. The NO _X storage-reduction catalyst 7 of this embodiment uses, for example, alumina as a carrier, and on this carrier, for example, an alkali metal such as potassium K, sodium Na, lithium Li, or cesium Cs, or an alkaline earth such as barium Ba or calcium Ca. And at least one component selected from rare earths such as lanthanum La, cerium Ce and yttrium Y, and a noble metal such as platinum Pt. When the air-fuel ratio of the exhaust gas _the NO _X storage reduction catalyst is flowing is lean, the exhaust NO _X (N
The O _2, NO) nitrate ions NO ₃ ^- is absorbed in the form of inflow exhaust gas to release NO _X absorbed and becomes rich N
Absorption and desorption of O _X out perform the action.

The mechanism of the absorption and desorption will be described below using platinum Pt and barium Ba as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths. When the oxygen concentration in the inflowing exhaust gas increases (that is, when the air-fuel ratio of the exhaust gas becomes a lean air-fuel ratio), these oxygens become
₂ ^- or deposited at O ^2- form, NO _X in the exhaust gas platinum P
O ₂ on t ^- or react with O ^2-, thereby NO ₂ is produced. Further, NO ₂ and NO ₂ produced by the above in the inflowing exhaust gas is further oxidized nitrate ions NO ₃ while being absorbed in the catalyst bonding with the barium oxide BaO while on the platinum Pt ^- diffuses in the catalyst in the form of . Therefore, so NO _X in the exhaust gas is absorbed in the form of nitrates in the catalyst under a lean atmosphere.

Further, the oxygen concentration in the inflow exhaust gas is greatly reduced.
Then (that is, the air-fuel ratio of the exhaust gas becomes
NO on platinum Pt)_TwoGeneration amount
As the reaction decreases, the reaction proceeds in the opposite direction,
Nitrate ion NO_Three ^-Is NO _TwoReleased from the catalyst in the form of
Become so. In this case, reducing components such as CO
HC, CO_TwoThe presence of these components on platinum Pt
NO_TwoIs reduced.

In the present embodiment, the engine 1 capable of operating at a lean air-fuel ratio is used. When the engine 1 is operated at a lean air-fuel ratio, the NO _X storage reduction catalyst absorbs NO _X in the inflowing exhaust gas. I do. Further, when the engine 1 is operated at a rich air-fuel ratio, NO _X occluding and reducing catalyst 7 absorbs NO
_Releases and purifies _X. In the present embodiment, when the amount of NO _X absorbed in the NO _X occluding and reducing catalyst 7 during the lean air-fuel ratio operation is increased, performs the rich spike operation for switching a short time the engine air-fuel ratio from a lean air-fuel ratio to a rich air-fuel ratio, N
Release of NO _X from the O _X storage reduction catalyst and reduction purification (NO
(Regeneration of the _X storage reduction catalyst).

Next, a method of estimating the NO _X storage amount of the NO _X storage reduction catalyst 7 in this embodiment will be described. In the present embodiment, ECU 30 is NO _X of the NO _X occluding and reducing catalyst 7 by increasing or decreasing the value of the NO _X counter CNOX
Estimate the amount of occlusion. _The NO _X storage amount of the reduction catalyst 7 is absorbed per unit time in the NO _X is _the amount of NO _X in the exhaust gas flowing per unit time in the NO _X occluding and reducing catalyst, i.e. generated per unit time in the engine 1 NO It is proportional to the amount of _X. On the other hand, the fuel supply amount to NO _X amounts engine generated per unit time by the engine, the air-fuel ratio, because determined by the exhaust flow rate, etc., the engine operating condition is absorbed in the NO _X occluding and reducing catalyst if Sadamare NO _X You can know the quantity. In the present embodiment, engine operating conditions (accelerator opening, engine speed, intake air amount, intake pressure, air-fuel ratio, fuel supply amount, etc.) are set in advance.
Actually measuring the amount of NO _X engine generated per unit time by changing the amount obtained by multiplying a constant factor in NO _X amount of NO _X absorbed per unit time storage reduction catalyst 7 (e.g. engine NO _X generation amount ) Is stored in the ROM of the ECU 30 in the form of a numerical map using, for example, the engine load (fuel injection amount) and the engine speed. The ECU 30 calculates the absorbed NO _X amount of the NO _X storage reduction catalyst per unit time from the engine load (fuel injection amount) and the engine speed at regular time intervals (each of the above unit time periods) using the map. the NO _X the _X counter
Increase by the amount absorbed.

When the engine is operated at the rich air-fuel ratio, N
O_XNO from the storage reduction catalyst 7_XIs released and reduction purification
Is done. NO_XReleased from the storage reduction catalyst 7 per unit time
NO issued_XThe quantity is NO_XFlows into the storage reduction catalyst 7
It is determined by the air-fuel ratio and the flow rate of the exhaust gas. NO_X
The exhaust air-fuel ratio and the flow rate flowing into the storage reduction catalyst 7 are SC5
a, 5b O_TwoIf the storage function is ignored, engine operating conditions
Determined by the matter. Therefore, in this embodiment, the engine operation is performed in advance.
Conditions (accelerator opening, engine speed, intake air amount, intake pressure
Power, air-fuel ratio, fuel supply, etc.)
Sometimes NO per unit time_XRelease from storage reduction catalyst 7
NO issued_XMeasure the amount and release NO per unit time
_XFor example, the engine load (fuel injection amount) and the engine speed
Stored in the ROM of the ECU 30 in the form of a numerical map using numbers
I have. The ECU 30 determines that the engine has a rich air-fuel ratio.
During operation, the engine load (fuel injection amount) and engine
This map is used to calculate N per unit time
O_XCalculate the release amount and set NO _XThe value of the counter is set to this release N
O_XDecrease by the amount.

Further, in the present embodiment, the ECU 30
When the value of O _X counter CNOX is increased to a predetermined value alpha,
A rich spike operation for operating the engine for a short time in the aforementioned mode (rich air-fuel ratio homogeneous mixture combustion) is performed. Thus, NO _X absorbed from _the NO _X storage reduction catalyst is released and reduced and purified. During the rich spike operation, the value of the NO _X counter is reduced by the above-mentioned released NO _X amount in accordance with the engine load and the number of revolutions, and the value of CNOX is reduced to a predetermined value β.
When it decreases to (β ≒ 0), the rich spike operation is stopped. The above-described determination value α for starting the rich spike operation during the lean air-fuel ratio operation is α = CNOX _MAX
It is calculated as × K. Here, K is a constant of K <1 (for example, K = about 0.7). Thus, by estimating _the NO _X storage amount of the NO _X occluding and reducing catalyst 7 using the NO _X counter, NO from _the NO _X storage reduction catalyst 7
_{The X} release operation is appropriately performed, and the NO _X storage reduction catalyst is prevented from being saturated with the absorbed NO _X.

However, as described above, in this embodiment, N
SCs 5a and 5b having an O ₂ storage function are provided in an exhaust passage on the upstream side of the O _X storage reduction catalyst 7. Therefore, exhaust SCs 5a rich air-fuel ratio from the engine when the operating air-fuel ratio of the engine changes from lean to rich, SCs 5a also flow into 5b, the 5b downstream of the NO _X occluding and reducing catalyst 7 SCs 5a, at 5b Exhaust near the stoichiometric air-fuel ratio flows in while oxygen is released. In the prior art, in this case, even if the air-fuel ratio of the exhaust gas that actually flows into the NO _X storage reduction catalyst is maintained near the stoichiometric air-fuel ratio, the value of the NO _X counter CNOX is maintained as long as the engine operating air-fuel ratio is rich. Will be subtracted by the amount of released NO _X. on the other hand,
Since theory very small release rates of the NO _X from _the NO _X storage reduction catalyst air-fuel ratio near the fact the NO _X storage amount of the NO _X occluding and reducing catalyst when the exhaust air-fuel ratio is maintained near the stoichiometric air-fuel ratio Hardly decreases. For this reason, when the engine operating air-fuel ratio changes from lean to rich, the value of the NO _X counter becomes smaller than the actual NO _X storage amount. The difference between the actual NO _X storage amount and the value of the NO _X counter increases as the stored oxygen amount of the SCs 5a and 5b increases.

Further, when the engine is switched from the rich air-fuel ratio to the lean air-fuel ratio,
A similar problem occurs when returning to the ratio. In this case
Indicates that the exhaust air-fuel ratio flowing into the SCs 5a and 5b is lean.
Even though the oxygen in the exhaust was absorbed by SC5a, 5b
Therefore, until the SCs 5a and 5b are saturated with the absorbed oxygen,
The exhaust air-fuel ratio after passing through C5a and 5b is near the stoichiometric air-fuel ratio
Will be maintained. Therefore, NO_XStorage reduction catalyst
Actual NO_XNO even though the storage amount does not increase
_XOnly the value of the counter will be incremented, in this case
NO_XThe counter value is the actual NO_XLarger than storage capacity
It will get worse. NO_XCounter value and actual N
O_XThe difference between the storage amount and the maximum storage oxygen amount of SC5a and 5b is
The larger, the larger. Thus, NO_XCounter C
NOx value and actual NO_XThere is a difference between the storage amount
And NO_XNO from storage reduction catalyst 7_XDischarge operation tie
It may not be possible to properly set the timing. example
NO during rich spike operation_XCounter value is actual
NO_XIf it becomes smaller than the storage amount, during rich spikes
Actually still NO_XNO in storage reduction catalyst 7_XRemains
NO_XThe counter value decreases to the judgment value β
It's a little and the rich spike is stopped. this
In case, NO_XThe storage-reduction catalyst 7 sufficiently turns the storage capacity.
NO if not restored_XWill resume absorption, and N
O_XIt is possible to make full use of the storage capacity of the storage reduction catalyst.
Will not be able to. Also, NO during lean air-fuel ratio operation _XCoun
Value is the actual NO_XWhen it becomes larger than the storage capacity,
Is still NO_XNO of the storage reduction catalyst 7_XThe storage amount is not so much
NO despite not increasing_XOnly counter is up
Unnecessary rich spy
May be started.

In the present invention, the difference between the value of the NO _X counter and the actual NO _X storage amount is prevented by the method described in the following embodiment. Hereinafter, the NO of the present invention
An embodiment of the _X storage amount estimation operation will be described. (1) First Embodiment FIG. 2 is a flowchart illustrating a first embodiment of the NO _X storage amount estimation operation of the present invention. This operation is performed by the ECU
30 is performed by a routine executed at regular intervals.

When the operation of FIG. 2 starts, step 2
At 01, the engine speed NE and the fuel injection amount GI are read. Next, at step 203, it is determined based on the output VOM of the upstream air-fuel ratio sensors 29a and 29b whether the engine exhaust is currently at a lean air-fuel ratio, that is, whether the engine 1 is currently operating at a lean air-fuel ratio. In addition,
In this embodiment, since two upstream air-fuel ratio sensors VOM are provided, the average value of the outputs of the sensors 29a and 29b is expressed by V
Used as OM.

If it is determined in step 203 that the lean air-fuel ratio operation is currently being performed, then the routine proceeds to step 205, where the current engine operation state is performed for a unit time (this operation execution interval).
_The amount of NO _X ANOX of _the NO _X storage reduction catalyst 7 absorbs every
Is calculated. In the present embodiment, as described above, the absorption NO _X amount ANOX per unit time is actually measured based on an experiment, and stored in the ROM of the ECU 30 as a numerical map using the engine fuel injection amount GI and the rotational speed NE. is there. In step 205, based on the rotational speed NE and the fuel injection amount GI read at step 201 to calculate the absorption amount of NO _X ANOX.

Next, at step 207, SC5a, SC5
It is determined whether or not the output VOS of the air-fuel ratio sensor 31 on the downstream side b is currently the output corresponding to the rich air-fuel ratio. If the VOS is not at the lean air-fuel ratio equivalent output in step 207, that is, since the SC5a and 5b are still absorbing oxygen in the exhaust gas after the air-fuel ratio is changed from rich to lean, the SC5a It can be determined that the exhaust air-fuel ratio flowing into the NO _X storage reduction catalyst 7 downstream of the SCs 5a, 5b has not changed to lean even though the exhaust air-fuel ratio has become lean on the upstream side of the NO. In this case, even if the engine operating air-fuel ratio is lean, the exhaust air-fuel ratio flowing into the NO _X storage reduction catalyst 7 is near the stoichiometric air-fuel ratio,
_Since NO _X is not absorbed by the _X storage reduction catalyst 7, the process directly proceeds to the step 211 without executing the step 209. On the other hand, if the VOS has reached the lean air-fuel ratio equivalent output in step 207, the SCs 5a and 5b have already been saturated with the absorbed oxygen, and the absorption of oxygen has been completed and flows into the NO _X storage reduction catalyst 7. This means that the exhaust air-fuel ratio is lean. In this case, NO _X for occluding and reducing the catalyst 7 actually NO _X is absorbed, the absorption amount of NO _X ANOX is C calculated in step 205 advances to step 209
It is added to NOX.

That is, in steps 207 and 209, S
NO until oxygen is absorbed up to the maximum storage amount of C5a, 5b
_By inhibiting the increase of the _X counter CNOX, NO _X
The counter value is being corrected. Next, in step 211, it is determined whether the value of CNOX calculated as described above has reached the upper limit value α. Here, as described above, α is α
= CNOX _MAX × K In addition,
CNOX _MAX is the maximum NO _X storage amount of the NO _X storage reduction catalyst 7, and K is a constant smaller than 1.

If CNOX ≧ α in step 211
If no_XNO of the storage reduction catalyst 7_XThe storage capacity increases
And NO_XNO from the storage reduction catalyst 7_XRelease
Execution of rich spike in step 213
The value of the flag XR is set to 1, and steps 215 and 21
7, the value of CNOX is maximum NO_XStorage amount CNOX_MAXThan
Terminate the operation after restricting it from growing. Re
Switch spike execution flag XR is set to 1.
And an operation (not shown) executed by the ECU 30
Therefore, the operating air-fuel ratio of the engine 1 is switched to a predetermined rich air-fuel ratio.
In other words, a rich spike operation is performed. Steps
In steps 215 and 217, the value of CNOX is set to CNOX. _MAXLimit with
This is because even if the flag XR is set to 1, the engine operating conditions
In some cases, rich spike operation cannot be performed.
No_XNO of the storage reduction catalyst 7_XStorage capacity reaches maximum value
In such a case, the CNO
X value and actual NO_XMake sure that the storage amount matches
It is.

On the other hand, if the VOM is the output corresponding to the rich air-fuel ratio in step 203, that is, if the engine 1 is currently performing the rich air-fuel ratio operation due to the rich spike operation or the change of the operating condition, Step 219
Thus, the NO _X amount BNOX released from the NO _X storage reduction catalyst 7 per unit time is calculated. Release _the amount of NO _X BN
The value of OX is created in advance as a numerical map using the fuel injection amount GI and the rotational speed NE based on the experimental results, and stored in the ROM of the ECU 30 in the same manner as in the above-described absorbed NO _X amount ANOX, and stored in the ROM of the ECU 30. Then, the released NO _X amount BNOX is calculated from this map based on the NE and GI read in step 201.

Next, at step 221, it is determined whether or not the output of the downstream air-fuel ratio sensor 31 is currently rich, that is, after the air-fuel ratio has changed from lean to rich, the SC5a, SC5, SC5, SC5, SC5, SC5
It is determined whether or not all of the stored oxygen has been released from b. If the VOS is not in the rich output at step 221, SCs 5a, and still followed by oxygen released from 5b, exhaust air-fuel ratio flowing into the NO _X occluding and reducing catalyst 7 can be determined not to become rich. In this case, since NO _X has not actually been released from the NO _X storage reduction catalyst 7, the process proceeds to Step 225 without executing Step 223.

[0054] In step 221, already SCs 5a, 5b oxygen of the total amount is released from the case where exhaust gas of a rich air-fuel ratio is flowing in the NO _X occluding and reducing catalyst 7, and then proceeds to step 223, NO _X counter The value of CNOX is reduced by the released NO _X amount BNOX calculated in step 219. That is, in steps 221 and 223, SC5a,
NO _X counter CN by 5b prohibits the reduction operation of the NO _X counter until releasing the entire amount of oxygen stored
The value of OX is corrected.

In step 225, it is determined whether or not the value of CNOX reduced as described above has become equal to or less than a predetermined value β (β ≒ 0). If CNOX ≦ β, it is considered that almost all of the NO _X was released from the NO _X storage-reduction catalyst 7 and reduced and purified by the rich spike operation (or the engine rich air-fuel ratio operation due to a change in the operating conditions).
In step 227, the value of the rich spike execution flag XR is set to 0. Then, steps 229, 231
After limiting the value of CNOX so that it does not become 0 or less, the operation ends. In steps 229 and 231, the value of CNOX is limited so as not to become 0 or less, for example, when the rich air-fuel ratio operation is continued due to the engine operating condition, the value of the rich spike execution flag XR is set to 0. This is because the exhaust gas having the rich air-fuel ratio continues to flow into the NO _X storage reduction catalyst even if the exhaust gas is exhausted.

[0056] As described above, SCs 5a by O ₂ storage function according to this embodiment, by which is adapted to prohibit the increase or decrease of the NO _X counter CNOX if absorbing and releasing 5b oxygen is being performed, The value of the NO _X counter always coincides with the actual NO _X storage amount of the NO _X storage reduction catalyst 7. In this embodiment, the SCs 5a, 5b
Oxygen while the absorption and release is taking place is always prohibited decrease of the NO _X counter, but in fact somewhat be near stoichiometric air-fuel ratio exhaust gas air-fuel ratio is flowing into the NO _X occluding and reducing catalyst NO NO _X absorption and release of _X storage reduction catalyst occurs. For this reason, the increase / decrease of the NO _X counter may be performed by changing the increase / decrease amount of the NO _X counter to a small value instead of prohibiting the increase / decrease of the NO _X counter even during the oxygen absorption / release of the SCs 5a and 5b. .

In step 203, SC5a, SC5
b Although the engine operation air-fuel ratio is determined to be lean or rich based on the air-fuel ratio sensor output VOM on the upstream side, the engine operation state (accelerator opening ACCP, engine speed NE) is determined without using the air-fuel ratio sensor output VOM. It is also possible to determine whether the engine operating air-fuel ratio is lean or rich on the basis of the operating mode determined from the above.

(2) Second Embodiment Next, a second embodiment of the NO _X storage amount estimating operation of the present invention will be described. In the above-described first embodiment, SC5
a, based on the air-fuel ratio sensor 31 outputs VOS of 5b downstream, SCs 5a, had determined the completion of the oxygen absorption and release based on 5b of the O ₂ storage function, in this embodiment, the downstream air-fuel ratio sensor output VOS Without using SC5a, 5
The same determination is made using the stored oxygen amount counter OSC of b.

As described above, the SCs 5a and 5b absorb and hold oxygen when the inflowing air-fuel ratio is lean, and release oxygen when the inflowing air-fuel ratio is rich. The amount of oxygen absorbed and released per unit time is determined by the exhaust air-fuel ratio (more precisely, the difference between the exhaust air-fuel ratio and the stoichiometric air-fuel ratio) and the exhaust flow rate. Therefore, in this embodiment, SC5a, SC5a,
5b, the amount of oxygen AOSC absorbed per unit time and the amount of oxygen BOSC released from the SCs 5a, 5b during the rich air-fuel ratio operation are actually measured, and the engine fuel injection amount GI and the rotational speed N are measured.
The map using E is stored in the ROM of the ECU 30. The ECU 30 performs the same operation as the increase / decrease of the NO _X counter to determine the absorbed oxygen amount AOSC based on the engine operating state.
And the released oxygen amount BOSC are calculated using this map,
By increasing or decreasing the value of the stored oxygen counter OSC,
The stored oxygen amount of C5a, 5b is estimated. When the engine operating air-fuel ratio changes between lean and rich, the SC5a, SC5a, SC5a, SC5a,
It is determined that the absorption or release of oxygen from b has been completed.

FIG. 3 is a flowchart for explaining the NO _X storage amount estimating operation of the present embodiment. This operation is performed by the ECU 30
Is performed as a routine executed at regular intervals. When the operation of FIG. 3 is started, in step 301, the engine fuel injection amount GI and the engine speed NE are read, and in step 303, the upstream side air-fuel ratio sensor output VOM is output.
, It is determined whether the engine exhaust is currently at a lean air-fuel ratio, that is, whether the engine 1 is currently operating at a lean air-fuel ratio.

At step 303, the current engine is operated at the lean air-fuel ratio.
If the engine has been operated, then in step 305, the engine
On the basis of the fuel injection amount GI and the rotational speed NE, E
Units using numerical maps stored in ROM of CU30
NO per hour_XAbsorption NO of storage reduction catalyst 7_XOcclusion amount
ANOX and absorbed oxygen amount AOSC to SCs 5a and 5b
Is calculated. Then, in step 307, the stored oxygen amount
The value of the counter OSC is increased by the absorbed oxygen amount AOSC.
You. In step 309, the storage increased by the above is performed.
The value of the stored oxygen amount counter OSC is the maximum oxygen storage amount OSC
_MAXIs determined. OSC ≧ OSC
_MAXThe engine operating air-fuel ratio changes from rich to
Has changed to the maximum stored oxygen already in SC5a and 5b.
Quantity OSC_MAX(Saturated amount)
As the oxygen in the exhaust gas cannot be absorbed,
In step 311, the value of OSC is set to the maximum value OSC._MAXSet to
You. In this case, the oxygen absorption of SCs 5a and 5b is terminated.
Has been completed, so NO on the downstream side of SC5a, 5b_XOcclusion reduction
The exhaust air-fuel ratio flowing into the catalyst 7 is also a lean air-fuel ratio.
You. Therefore, NO_XThe storage reduction catalyst 7 detects NO in the exhaust gas._XTo
NO because absorbed_XThe value of the counter CNOX is
NO calculated in step 305_XIs increased by the amount ANOX
You. On the other hand, in step 309, OSC <OSC_MAXSo
SC5a and 5b are not yet saturated with oxygen
Since oxygen in the exhaust is currently being absorbed, NO _XOcclusion reduction
The exhaust air-fuel ratio flowing into the catalyst 7 is not lean.
Therefore, NO in step 313_XCounter CNOX
No increase operation is performed.

Next, in steps 315 to 321, NO_X
Rich when the value of the counter CNOX reaches a predetermined value
Setting of spike execution flag XR and maximum value of CNOX
NO _XStorage amount CNOX_MAXAnd restrictions are imposed.
The operations of steps 315 to 321 are performed in step 21 of FIG.
The operations are the same as operations 1 to 217. On the other hand, step 30
If the engine operating air-fuel ratio is rich at 3
In step 323, based on the fuel injection amount GI and the rotational speed NE
From the numerical map stored in the ROM of the ECU 30,
NO per unit time_XNO released from storage reduction catalyst 7
_XBNOX and the amount of oxygen BO released from the SCs 5a and 5b
SC is calculated. Then, in step 325, storage
The oxygen counter OSC decreases by the released oxygen BOSC.
It is. Step 327 is a procedure for storing the acid stored by the SCs 5a and 5b.
This is to determine whether or not the entire amount of the element has been released. S
When the release of oxygen from C5a and 5b has been completed (O
(SC ≦ 0), the engine operating air-fuel ratio
Already changed to_XFlow to storage reduction catalyst 7
The incoming exhaust air-fuel ratio is rich,_XOcclusion return
NO from source catalyst 7_XHas been released, so step 3
After setting the value of OSC to 0 at 29, at step 331
NO_XWhen the value of the counter CNOX is NO_XIt ’s BNOX
Reduced. Then, from step 333 to step 3
At 39, the rich spike end timer is determined based on the value of CNOX.
Imming is determined, and the value of CNOX is set to 0 if necessary.
Limited. Steps 333 to 339
Is the same as the operation of steps 225 to 231 in FIG.
You.

As described above, in this embodiment, the engine operating idle
Fuel ratio changes from lean to rich or rich to lean
When done, based on the oxygen storage amount of SC5a, 5b
NO _XNO of the storage reduction catalyst 7_XOcclusion amount estimated value CNOX
Correction (from step 309 to 313, from step 327
331) NO_XNO of the storage reduction catalyst 7_XSucking
It is possible to accurately estimate the amount of storage.

(3) Third Embodiment Next, another embodiment of the present invention will be described. The second
In the embodiment of the present invention, the SC
When calculating the stored oxygen amounts of 5a and 5b, the maximum of OSC
The value is the maximum stored oxygen amount (saturated oxygen amount) of SC5a, 5b.
OSC_MAX(Steps 309 and 31 in FIG. 3)
1). In the operation of FIG. 3, the saturated oxygen amount OSC _MAXA suitable one
The stored oxygen amount OSC may be calculated as a constant value.
However, more precisely, the OSC depends on the deterioration of the catalyst._MAXThe value of the
Is preferably corrected. O of catalyst_TwoStorage function
Decreases with deterioration of the catalyst, and the maximum acid that can be stored by the catalyst
Elemental (saturated oxygen) OSC_MAXAlso goes down. There
In this embodiment, the deterioration state of the catalyst is determined, and the deterioration state is determined.
OSC according to_MAXBy correcting the value of
The stored oxygen amounts of the SCs 5a and 5b are definitely estimated.

First, a method of determining the deterioration state of the catalyst will be described. In the present embodiment, the deterioration state of the catalyst is determined based on the trajectory length of the output signal curves of the air-fuel ratio sensors 29a and 29b on the upstream side of the SCs 5a and 5b and the trajectory length of the output signal curves of the air-fuel ratio sensor 31 on the downstream side. FIG. 4 shows general waveforms of the air-fuel ratio sensor output VOM provided upstream of the exhaust purification catalyst and the air-fuel ratio sensor output VOS provided downstream of the catalyst when the engine air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio. ing. 4A shows a waveform when the O ₂ storage function of the exhaust purification catalyst is high, and FIG. 4B shows a waveform when the O ₂ storage function is reduced.

As shown in FIGS. 4A and 4B, when the stoichiometric air-fuel ratio is feedback-controlled, the engine air-fuel ratio (exhaust air-fuel ratio) is rich in a relatively small range around the stoichiometric air-fuel ratio. And fluctuate lean. For this reason, the output VOM of the upstream air-fuel ratio sensor also periodically changes around the stoichiometric air-fuel ratio. In this case, if the O ₂ storage function of the catalyst is sufficiently high, the exhaust air-fuel ratio at the catalyst outlet is maintained near the stoichiometric air-fuel ratio even if the exhaust air-fuel ratio flowing into the catalyst fluctuates somewhat around the stoichiometric air-fuel ratio. For this reason, with a catalyst having a sufficiently high O ₂ storage function, the downstream air-fuel ratio sensor output V
The OS does not change much as shown in FIG. Therefore, the length along the locus of the output VOS is relatively small for LOVS. However, the catalyst is deteriorated O ₂ storage function is to vary according to the air-fuel ratio also varies the air-fuel ratio of the upstream side of the downstream side to lower the oxygen storage volume of the catalyst when reduced. Therefore, the downstream air-fuel ratio sensor output V
The trajectory length LVOS of the OS increases as the O ₂ storage function decreases and becomes equal to the trajectory length LVOM of the air-fuel ratio sensor output VOM on the upstream side when the O ₂ storage function is completely lost as shown in FIG. turn into. That is, the ratio LR of the locus length LVOS of the downstream air-fuel ratio sensor output VOS and the locus length LVOM of the upstream air-fuel ratio sensor output VOM during the air-fuel ratio feedback control (LR = LVOS /
Taking LVOM), if the O ₂ storage function is high enough LR becomes much smaller than 1, the O ₂ storage function is closer to 1 and increases as drops. In the present embodiment, the outputs of the upstream air-fuel ratio sensors 29a and 29b and the downstream air-fuel ratio
Using the ratio LR of the trajectory length of the output SCs 5a, as a parameter representing the O ₂ storage function decrease in 5b. Note that the two exhaust purification catalysts 5a, 5b
And two upstream air-fuel ratio sensors 29a, 29b in the case of an engine having two upstream air-fuel ratio sensors 29a, 29b.
May be used as the upstream air-fuel ratio sensor output VOM to calculate the trajectory length LVOM, or the output trajectory length is calculated for each of the air-fuel ratio sensors 29a and 29b, and both trajectory lengths are averaged. This may be used as the upstream-side air-fuel ratio sensor output trajectory length LVOM.

FIG. 5 is a flow chart for explaining the operation of calculating the stored oxygen amount maximum value OSC _MAX in consideration of the deterioration of the SCs 5a and 5b according to the present embodiment. This operation is performed by the ECU 30
Is performed as a routine executed at regular intervals. When the operation starts in FIG.
At 1, it is determined whether or not the deterioration parameter calculation execution condition is satisfied. In the present embodiment, the condition of step 501 is that the engine is operated in the mode (stoichiometric air / fuel ratio homogeneous mixture combustion (injection of the intake stroke once)) and the air / fuel ratio feedback control based on the air / fuel ratio sensors 29a and 29b is performed. It has been implemented. As explained in FIG.
To use the locus length ratio LR as a parameter representing the O ₂ storage function of the catalyst, because it is necessary to calculate the trajectory length ratio LR in the state where the engine air-fuel ratio is feedback controlled.

If the condition is satisfied in step 501, in step 503 the upstream air-fuel ratio sensors 29a, 29
The output voltage VOM of b and the output voltage VOS of the downstream air-fuel ratio sensor 31 are read. In this embodiment, the average value of the output voltages of the sensors 29a and 29b is used as VOM. Then the locus length LVOS of trajectory length LVOM and the downstream-side air-fuel ratio sensor output VOS of the upstream step 505-side air-fuel ratio sensor output VOM _{is, LVOM = LVOM + | VOM-} VOM i-1 | LVOS = LVOS + | VOS-VOS i- Calculated as ₁ | Here, VOM _i-1 and VOS _i-1 are
These are the values of VOM and VOS at the time of the previous execution of this operation, respectively, and are updated in step 507 each time the LVOM and LVOS are calculated. That is, in the present embodiment, as shown in FIG.
Approximate calculation is performed using the integrated value of | VOM−VOM _i−1 | and | VOS−VOS _i−1 | as LVOM and LVOS, respectively.

Steps 509 and 511 are operations for determining the calculation period of the trajectory length. In the present embodiment, the above L
The integration of VOM and LVOS is performed until the value of the counter CT, which is increased by one each time the operation is performed, reaches a predetermined value T. The predetermined value T is set so that the total of the integration periods is about several tens of seconds. In step 511, the period T
Has elapsed, at step 513, the trajectory length ratio LR is calculated from the values of LVOM and LVOS integrated within the period by L
It is calculated as R = LVOS / LVOM. In step 515, the OSC _MAX correction coefficient RD is obtained from the value of the trajectory length ratio LR based on a preset relationship. In step 519, the current SCs 5a, 5b
Is the maximum value of the stored oxygen amount OSC _MAX of OSC _MAX = OSC
It is calculated as _MAX0 × RD. Here, OSC _MAX0 is S
C5a and C5b are the maximum values of the stored oxygen amount in a new state in which no deterioration has occurred.

FIG. 7 shows the correction coefficient R in step 517 in FIG.
The trajectory length ratio LR and the correction coefficient R used to determine D
It is a graph which shows the relationship with D. As shown in FIG. 7, the value of the correction coefficient RD is determined when the catalyst is not deteriorated at all (LR).
In (≪1.0), it is set to 1.0, and is set to be smaller as the deterioration of the catalyst progresses (as the value of LR approaches 1).

By setting the storage oxygen maximum value OSC _MAX of the SCs 5a and 5b according to the degree of deterioration of the catalyst according to FIG. 7, the accuracy of estimating the storage oxygen amount OSC of the SCs 5a and 5b is improved. For this reason, using the stored oxygen amount maximum value OSC _MAX set as described above, the N 2 of the second embodiment described above is used.
By performing the O _X counter setting operation, it is possible to further improve the accuracy of estimating the NO _X storage amount of the NO _X storage reduction catalyst 7 by the NO _X counter CNOX.

(4) Fourth Embodiment Next, a fourth embodiment of the present invention will be described. Above
In the first embodiment, NO_XNO of the storage reduction catalyst 7_X
NO at absorption_XWhen the value of the counter CNOX is the maximum NO_XOcclusion
Quantity CNOX_MAXValue of CNOX so that it does not increase beyond
(Steps 215 and 217 in FIG. 2, and steps
Steps 319, 321). For this reason, CNOX_MAXThe value of the
Is NO_XIt will drop significantly due to the deterioration of the storage reduction catalyst 7.
If no, NO_XPossible inaccurate estimation of occlusion
There is. For example, maximum NO_XStorage amount CNOX_MAXIs low
Lower and CNOX_MAX'(CNOX_MAX> CNO
X_MAX'), It is actually NO_XSucking
NO of the storage reduction catalyst 7_XThe storage amount is CNOX_MAX'
NO without increasing_XThe value of the counter CNOX is CNO
X_{MA X}'Beyond CNOX_MAXIf it increases to
Occurs. 2 and 3, the rich spike operation is performed.
NO to start_XThe judgment value α of the counter is CNOX_MAX× K
, But in practice CNOX_MAXDecreases
CNOX despite being_MAXRiches as a constant
NO if the pike operation timing has been determined _XOcclusion return
Source catalyst NO_XNO increased due to increased storage_XLow purification efficiency
Could be dropped. Therefore, in the present embodiment, N
O_XDeterioration of the storage reduction catalyst 7 is determined, and according to the degree of deterioration,
Maximum NO_XStorage amount CNOX_MAXBy correcting
The above problem has been solved.

Hereinafter, NO_XThe degree of deterioration of the storage reduction catalyst 7
The determination method will be described. NO_XPoor storage storage catalyst 7
Although there are various methods for determining
Is SO for sulfur component in fuel_XJudgment of deterioration due to poisoning
Another will be described. A small amount of sulfur component in engine fuel
And this sulfur component burns with the fuel and
O_XBecomes SO during lean air-fuel ratio exhaust_XExists
And NO_XNO for the storage reduction catalyst 7_XSame mechanics as absorption of
SO_XAnd retain it inside in the form of sulfate
I will be. However, NO_XRetained in storage reduction catalyst
Sulfate is more stable than nitrate, NO_XSucking
NO from storage reduction catalyst_XIs easily released under the temperature conditions
NO_XNot released from the storage reduction catalyst. Therefore, NO
_XSulfate gradually accumulates in the storage reduction catalyst, and the sulfate accumulates.
NO only for the amount_XMaximum NO of storage reduction catalyst_XOcclusion amount
CNOX_MAXGoes down. Therefore, in this embodiment,
Is NO_XSO of the storage reduction catalyst 7_XNO according to occlusion amount
_XMaximum NO of storage reduction catalyst 7_XStorage amount CNOX_MAXComplement
Correct. As mentioned above, SO_XPoisoning occurs in institutions
SO_XNO_XBy the absorption and reduction catalyst 7 being absorbed,
Occur. On the other hand, SO generated in the engine_XQuantity is institutional
It is proportional to the amount of fuel supplied per hour. For this reason,
NO_XS absorbed by the storage reduction catalyst 7 per unit time
O_XASO_XIs ASO_X= GI x NE x L
Is done. Here, GI is the fuel injection amount of the engine, and NE is the rotation.
Number, L is SO of fuel_XIt is a coefficient proportional to the concentration. You
In other words, the absorption amount ASOX is the amount of fuel burned per unit time.
It is proportional to the amount of sulfur components in it. In the present embodiment, NO_XMosquito
SO similar to Unta CNOX_XUsing counter CSOX
While the engine 1 is operating at the lean air-fuel ratio, the engine fuel
From the above formula based on the injection amount GI and the rotational speed NE,
NO per unit time_XAbsorption SO of storage reduction catalyst 7_Xamount
Calculate ASOX and calculate SO_XIncrease the counter value of CSOX
I try to make it bigger.

[0074] Further, as described above, SO _X absorbed in the NO _X occluding and reducing catalyst 7, the temperature T _D (exhaust gas temperature of normal rich-spike with the exhaust temperature T _EX during the rich air-fuel ratio operation of the engine If the temperature becomes higher than (higher temperature), it is released from the NO _X storage reduction catalyst. At this time, NO _X per unit time
The SO _X amount BSOX released from the storage reduction catalyst 7 is a function of the exhaust air-fuel ratio and the temperature. In the present embodiment, the engine 1 is operated at a rich air-fuel ratio while changing the engine operating conditions by an experiment in advance, the released SO _X amount BSOX is actually measured, and a numerical value using the engine fuel injection amount, the rotational speed NE and the exhaust gas temperature TE is used. The map is stored in the ROM of the ECU 30. The engine is operated at a rich air-fuel ratio, yet when the exhaust gas temperature becomes equal to or higher than T _D is the engine fuel injection amount GI, rotational speed N
E: The released SO _X amount BNOX is calculated using the exhaust gas temperature T _EX, and the value of the SO _X counter CSOX is set to B per unit time.
NOx is reduced. Thereby, S
The value of O _X counter CSOX is as accurately represent the SO _X amount absorbed in the NO _X occluding and reducing catalyst 7. NO
_Since the maximum NO _X storage amount CNOX _MAX of the _X storage reduction catalyst 7 decreases as the SO _X storage amount of the catalyst 7 increases,
The maximum NO _X storage amount CNOX _MAX is CNOX _MAX = CN
It can be expressed as OX _MAX0 -CSOX. CNOX
_MAX0 is the maximum NO _X storage amount of the new NO _X storage reduction catalyst 7 that has not absorbed SO _X at all.

FIG. 8 is a flowchart for explaining the setting operation of the maximum NO _X storage amount CNOX _MAX based on the SO _X absorption amount described above. This operation is executed by the ECU 30 at regular intervals. When the operation in FIG. 8 starts, the engine fuel injection amount GI and the rotational speed NE are read in step 801, and the engine exhaust temperature T _EX is read in step 803. The exhaust gas temperature T _EX may be directly detected by using an exhaust gas temperature sensor provided on the exhaust passage. The relationship between the air-fuel ratio, the fuel supply amount, etc.) and the exhaust temperature is obtained, and the exhaust temperature is calculated based on the engine operating state.

Next, at step 805, whether or not the current engine operating air-fuel ratio is lean is determined by the upstream air-fuel ratio sensor output VOM.
If the engine is currently operating at a lean air-fuel ratio, the routine proceeds to step 809. Step 809
Then, the absorbed SO _x of the NO _X storage reduction catalyst per unit time
_{The X} amount ASOX is calculated as ASOX = GI × NE × L. Then, in step 811, the value of the SO _X counter CSOX is increased by the absorbed SO _X amount ASOX.

Step 813 is for judging the execution timing of the SO _X poisoning recovery operation. In the present embodiment, NO _X
When the SO _X storage amount CSOX of the storage reduction catalyst reaches the predetermined value γ, the exhaust temperature of the engine is reduced to T _D to recover SO _X poisoning.
Switching to rich air-fuel ratio operation as described above
SO _X is released from the _X storage reduction catalyst. That is, when was CSOX ≧ γ at step 813, the value of the SO _X poisoning recovery operation execution flag XS is set to 1 in step 815. When the value of the flag XS is set to 1, the engine is operated at a rich air-fuel ratio at which the exhaust gas temperature becomes equal to or higher than T _D by an operation (not shown) executed by the ECU 30 separately.

On the other hand, at step 805, the engine
If the relative operation has been performed, the process proceeds to step 817, where the
Exhaust gas temperature T obtained in step 803_EXIs SO_XRelease temperature T
_DIt is determined whether or not this is the case. T_EX<T_Dso
If there is, the air-fuel ratio is rich and NO_XOcclusion reduction
SO for catalyst_XIs not absorbed, but because of the low temperature, N
O_XSO from storage reduction catalyst_XIs not released. others
In step 817, T _EX<T_DIf, S
O_XWithout changing the value of the counter CSOX,
Proceed to 829.

[0079] When was T _EX ≧ T _D in step 817, then the engine fuel injection amount at step 819 GI and the rotational speed NE, based on the exhaust temperature T _EX, the ECU 30 RO
NO _X per unit time from the numerical map stored in M
Released SO _X amount BSOX from occluding and reducing catalyst 7 is calculated, the value of the SO _X counter CSOX step 821 is reduced by BSOX. In this case, in steps 823 827, CSOX is limited so as not to negative value (step 825), if the CSOX becomes 0, that is, SO _X release of the NO _X occluding and reducing catalyst 7 is completed Therefore, the value of the SO _X poisoning recovery execution flag XS is reset to 0 in step 827. As a result, if it is being executed, the SO _X poisoning recovery operation ends.

As described above, SO_XCounter CSOX value
Is set, NO in step 829 _XOf the storage reduction catalyst 7
Maximum NO_XStorage amount CNOX_MAXIs the maximum N when new
O_XStorage amount CNOX_MAX0Using CNOX_MAX= CN
OX_MAX0-Calculated as CSOX. In this embodiment
Is NO as described above_XConsidering the deterioration of the storage reduction catalyst 7,
NO_XMaximum NO of storage reduction catalyst_XStorage amount CNOX_MAX
Set this CNOX_MAXThe first or the above using
NO of the second embodiment_XPerform counter setting operation.
Thereby, NO_XThe value of the counter CNOX is more accurate
NO_XNO of storage reduction catalyst _XIt indicates the amount of occlusion.

In this embodiment, the deterioration of the NO _X storage reduction catalyst is determined by using the sulfur component amount in the fuel.
The method for determining the deterioration of the NO _X storage reduction catalyst is not limited to this, and any other method can be used as long as the method can accurately calculate the decrease in the maximum NO _X storage amount due to the deterioration.

[0082]

According to the invention described in each claim, O_TwoS
NO on the downstream side of the exhaust purification catalyst having a storage function_XOcclusion
When a reduction catalyst is placed, NO_XStorage reduction catalyst
NO _XA common effect that makes it possible to estimate the amount of occlusion
To play.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of an embodiment when the present invention is applied to an internal combustion engine for a vehicle.

FIG. 2 is a flowchart illustrating a first embodiment of a NO _X storage amount estimating operation of the NO _X storage reduction catalyst of the present invention.

FIG. 3 is a flowchart illustrating a second embodiment of the NO _X storage amount estimating operation of the NO _X storage reduction catalyst of the present invention.

FIG. 4 is a diagram illustrating a change in the output of an upstream air-fuel ratio sensor and the output of a downstream air-fuel ratio sensor due to deterioration of an exhaust purification catalyst.

FIG. 5 is a flowchart illustrating an operation for setting the maximum stored oxygen amount of the exhaust purification catalyst in consideration of the exhaust purification catalyst deterioration.

6 is a diagram for explaining a method of calculating an air-fuel ratio sensor output trajectory length used in the operation of FIG. 5;

FIG. 7 is a diagram illustrating a relationship between a maximum stored oxygen amount correction coefficient of an exhaust purification catalyst and a trajectory length ratio.

8 is a flowchart for explaining the maximum _the NO _X storage amount setting operation of the NO _X occluding and reducing catalyst in consideration of deterioration of the NO _X occluding and reducing catalyst.

[Explanation of symbols]

1 ... internal combustion engine 2 ... exhaust passage 5a, 5b ... the start catalyst (SC) 7 ... NO _X occluding and reducing catalyst 29a, 29 b, 31 ... air-fuel ratio sensor 30 ... electronic control unit (ECU)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/24 F01N 3/24 CR 3/28 301 3/28 301C F02D 41/04 305 F02D 41/04 305Z F-term (reference) 3G091 AA02 AA12 AA17 AA24 AA28 AB03 AB06 BA01 BA07 BA11 BA14 BA15 BA19 BA32 BA33 CA18 CB02 CB03 CB05 DA04 DA10 DB06 DB11 DB13 DC01 EA01 EA06 EA07 EA17 EA30 EA31 GB02 FB04 GB05W GB06W GB07W GB10X GB17X HA03 HA08 HA12 HA36 HA37 HB02 3G301 HA01 HA04 HA06 HA15 JA25 JA26 JB09 LB04 LB11 MA01 MA19 MA20 MA23 NA06 NA09 NB14 ND01 NE02 NE13 NE14 NE15 PA07A PA11A PD02A PD08A PD09A PD11A0111

Claims (8)

[Claims] 1. An exhaust gas purifying apparatus for an internal combustion engine that switches an operating air-fuel ratio between a lean air-fuel ratio operation and a stoichiometric air-fuel ratio or a rich air-fuel ratio operation as required, and is disposed in an engine exhaust passage. An exhaust gas purification catalyst having an O ₂ storage function; and
Air-fuel ratio of the exhaust gas flowing into absorbs NO _X in the exhaust gas when the lean and _the NO _X storage reduction catalyst the oxygen concentration in the inflowing exhaust gas to release NO _X absorbed when reduced, based on the engine operating condition wherein _the NO _X storage reduction and _the NO _X storage amount estimating means for estimating the occluded amount of NO _X in the catalyst, when the engine operating air-fuel ratio is changed, the _the NO _X storage amount estimated by the estimating means _the NO _X storage Te NO _X for correcting the amount based on the amount of oxygen stored in the exhaust purification catalyst
An exhaust gas purification device for an internal combustion engine, comprising: a storage amount correction unit. 2. The NO_XThe storage amount estimating means
NO discharged_XAbsorbed NO absorbed by the storage reduction catalyst_Xamount
And NO_XNO released from the storage reduction catalyst_XQuantity and
Is calculated based on the engine operating state, and the engine operating air-fuel ratio is
NO when the air-fuel ratio is_XEstimated NO for storage reduction catalyst_XSucking
Absorb NO_XEngine operating air-fuel ratio
Is the rich air-fuel ratio and the estimated NO_XRelease the stored amount
Out NO _XNO by reducing the amount_XOcclusion reduction
NO of catalyst_XThe internal combustion engine according to claim 1, wherein the storage amount is estimated.
Seki exhaust purification device. 3. When the engine operating air-fuel ratio changes from a lean air-fuel ratio to a rich air-fuel ratio, the correction means detects the amount of oxygen stored in the exhaust purification catalyst after the engine operating air-fuel ratio changes to the rich air-fuel ratio. Until the total amount is released from the exhaust purification catalyst, the estimated NO _X by the NO _X storage amount estimation means
The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein the operation of reducing the amount is prohibited. 4. When the engine operating air-fuel ratio changes from a rich air-fuel ratio to a lean air-fuel ratio, the correcting means changes the engine operating air-fuel ratio to a lean air-fuel ratio and then stores the oxygen in the exhaust purification catalyst up to a maximum storage amount. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein the operation of increasing the estimated NO _X amount by the NO _X storage amount estimating unit is prohibited until the NO _X is absorbed. 5. A storage oxygen amount estimating means for estimating an amount of oxygen stored in the exhaust gas purifying catalyst based on an exhaust air-fuel ratio flowing into the exhaust gas purifying catalyst. An exhaust gas purifying apparatus for an internal combustion engine according to claim 1. 6. The storage oxygen amount estimating means for estimating the amount of oxygen stored in the exhaust purification catalyst based on the exhaust air-fuel ratio after passing through the exhaust purification catalyst when the engine operating air-fuel ratio changes. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, further comprising: 7. The exhaust gas purification of an internal combustion engine according to claim 6, wherein said stored oxygen amount estimating means includes a stored oxygen amount correcting means for correcting a stored oxygen amount estimated value based on a degree of deterioration of said exhaust gas purifying catalyst. apparatus. 8. The NO _x storage amount estimating means includes:
_X determining means for determining the degree of deterioration of the storage reduction catalyst,
An exhaust purification system of an internal combustion engine according to claim 1 or claim 2 for estimating the _the NO _X storage amount based on the deterioration degree of the engine operating condition and _the NO _X storage reduction catalyst.