JP2000027677A - Exhaust emission control device for lean burn internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent variation in air-fuel ratio from delaying due to an O2 storage function of catalyst. SOLUTION: Start catalysts(SC) 5a, 5b each having an O2 storage function are located in exhaust passages 2a, 2b of an engine 1, and an NOx occlusion and reduction catalyst 7 is located on a downstream side merging exhaust passage 2. During lean mixture operation of an engine, the catalyst 7 is allowed to absorb NOX from exhaust gas, and the engine is operated with a rich mixture when the NOX is emitted so as to increase the air-fuel ratio of exhaust gas flowing into the start catalysts 5a, 5b and the catalyst 7. An ECU 30 carries out secondary fuel injection which does not contribute combustion, by means of cylinder fuel injection values 111 to 114 during expansion or exhaust stroke of each engine cylinder so as to increase the air-fuel ratio of exhaust gas flowing into the start catalysts 5a, 5b in order to emit oxygen occluded in the start catalysts 5a, 5b when the ECU 30 changes over the operation from the rich mixture operation into the lean mixture operation. Thereby, it is possible to prevent variation in air-fuel ratio from a lean mixture into a rich mixture from delaying upon change-over of air-fuel ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to an exhaust gas purifying apparatus for an internal combustion engine provided with an exhaust gas purifying catalyst having an O ₂ storage function.

[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
By performing the driving rich spike operation, NO_X
NO absorbed from the storage reduction catalyst_XAnd release
And the released NO_XHas been reduced and purified. That is,
Exhaust air-fuel ratio becomes stoichiometric or rich air-fuel ratio
And the oxygen concentration in the exhaust is
As well as drastically decreasing, unburned HC and CO
The amount increases rapidly. For this reason, rich spike operation
Engine operating air-fuel ratio to stoichiometric air-fuel ratio or rich air-fuel ratio
If switched, NO_XExhaust gas flowing into the storage reduction catalyst
The air-fuel ratio is calculated from the lean air-fuel ratio to the theoretical air-fuel ratio or rich air
The fuel ratio changes and NO decreases due to a decrease in the oxygen concentration in the exhaust gas._XSucking
NO from storage reduction catalyst_XIs released. Also as above
Relatively high in stoichiometric or rich air-fuel ratio exhaust
NO unburned HC and CO components_XOcclusion return
NO released from the source catalyst_XIs the unburned HC and CO in the exhaust
Reacts with components and is reduced.

[0004]

The above-mentioned Patent Registration No. 260
According to the exhaust gas purifying apparatus described in No. 0492, it is possible to efficiently purify NO _X generated during the operation of the engine lean air-fuel ratio. However, the above-mentioned patent registration No. 2600492
When a three-way catalyst having an O ₂ storage function is added as a start catalyst to the device of No. ₂ , a problem may occur.

The main purpose of the start catalyst is to remove HC and CO components emitted 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 it is added to the exhaust gas purifying apparatus of the above-mentioned Patent Registration No. 2600492, the start catalyst is disposed in the exhaust passage on the upstream side of the NO _X storage reduction catalyst.

[0006] However, when placed in the upstream side exhaust passage of the NO _X occluding and reducing catalyst a catalyst having an O ₂ storage function as start catalyst is in a lean air-fuel ratio operation of the NO _X from _the NO _X storage reduction catalyst release, when the rich spike operation for reducing and purifying be problems rich spike operation early in the nO _X is released without being purified from _the nO _X storage reduction catalyst occurs it has been found.

[0007] This problem occurs when the rich spike operation is performed.
Changes from a lean air-fuel ratio of the exhaust gas flowing into the _X occluding and reducing catalyst to rich is considered to produce a delayed because of the O ₂ storage operation of the start catalyst. That is, the air-fuel ratio of the exhaust gas from the rich spike operation is performed engine changes abruptly from lean to rich, but because the start catalyst has the O ₂ storage function, the exhaust start catalysts of the rich air-fuel ratio When the gas flows in, the absorbed oxygen is released from the start catalyst, and the air-fuel ratio of the exhaust gas flowing out of the start catalyst is maintained near the stoichiometric air-fuel ratio. For this reason, even if the rich spike operation is started, the air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst does not become a sufficient rich air-fuel ratio until the start catalyst finishes releasing the entire amount of oxygen absorbed, and the stoichiometric air-fuel ratio is not obtained. In some cases, a lean air-fuel ratio close to the fuel ratio is maintained.

[0008] When the air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst changes from the lean air-fuel ratio to a lean air-fuel ratio close to the stoichiometric air-fuel ratio, the oxygen concentration near the surface of the NO _X storage reduction catalyst rapidly decreases. As will be described later, the NO _X storage reduction catalyst holds NO _X in the form of nitrate ions bonded to alkaline earth (eg, Ba) or the like. However, when the oxygen concentration near the catalyst surface decreases, the NO _X storage reduction catalyst becomes NO. NO _X held near the surface of the _X storage reduction catalyst is simultaneously released from the catalyst surface. In this case, if the exhaust gas flowing into the NO _X storage reduction catalyst is maintained at a lean air-fuel ratio close to the stoichiometric air-fuel ratio, a sufficient amount of HC to reduce the total amount of released NO _X is contained in the exhaust gas. since CO is not included, a portion of the released nO _X becomes as it flows out to the nO _X occluding and reducing catalyst downstream without being reduced. Therefore, the O ₂ storage function of the start catalyst, unpurified from the rich-spike operation during the NO _X occluding the exhaust air-fuel ratio flowing into the reduction catalyst is sufficiently reaching the richer is delayed _the NO _X storage reduction catalyst of the NO _X is considered to be to flow out.

[0009] As described above, since the total amount of oxygen absorbed by the start catalyst is the same rich air fuel ratio also start catalyst upstream of the exhaust gas when released start catalyst downstream, NO _X Exhaust gas with a sufficiently rich air-fuel ratio is supplied to the storage reduction catalyst. Therefore, NO _X released After some time from the start of the rich spike operation from _the NO _X storage reduction catalyst is as its whole amount on _the NO _X storage reduction catalyst is purified, _the NO _X storage reduction catalyst Unpurified NO from
_X spill stops. However, if unpurified NO _X flows out of the NO _X storage-reduction catalyst every time the rich spike operation is performed as described above, there is a problem that the overall NO _X purification rate decreases.

Further, in an engine in which the operating air-fuel ratio of the engine is switched from a lean air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio in accordance with the operating state of the engine, the exhaust air from the engine is not required even if a rich spike operation is not performed. ratio is sometimes switched from lean to the stoichiometric air-fuel ratio or rich air-fuel ratio, but flows for this case the O ₂ storage capability of the exhaust gas purifying catalyst, the NO _X occluding and reducing catalyst during the operating air-fuel ratio switching If a period occurs in which the exhaust air-fuel ratio is temporarily maintained at a lean air-fuel ratio close to the stoichiometric air-fuel ratio, unpurified NO _X is released as in the above case, causing a problem that the exhaust properties deteriorate.

The present invention has been made in view of the above problems, and has been described with reference to a lean air-fuel ratio, a stoichiometric air-fuel ratio or a rich air-fuel ratio of an exhaust air-fuel ratio downstream of an exhaust purification catalyst when an exhaust purification catalyst having an O ₂ storage function is disposed in an exhaust passage. The purpose is to prevent the delay of the change to.

[0012]

According to the first aspect of the present invention, the internal combustion engine switches the 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. An exhaust purification catalyst having an O ₂ storage function disposed in an engine exhaust passage, and contributing to combustion when switching the engine from a lean air-fuel ratio operation to a stoichiometric air-fuel ratio or a rich air-fuel ratio operation. Storage reducing means for reducing the amount of oxygen stored in the exhaust gas purification catalyst by supplying fuel not to the engine and making the exhaust air-fuel ratio flowing into the exhaust gas purification catalyst richer than the engine operation air-fuel ratio. An engine exhaust purification device is provided.

That is, according to the first aspect of the invention, when the operating air-fuel ratio of the engine is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio, fuel that does not contribute to combustion is supplied to the engine. This fuel becomes unburned HC components without being burned, and is discharged from the engine together with the exhaust gas. For this reason,
Exhaust gas having an air-fuel ratio richer than the engine operating air-fuel ratio and containing a large amount of unburned HC flows into the exhaust purification catalyst. in this case,
Oxygen is released from the exhaust purification catalyst by the O ₂ storage function of the exhaust purification catalyst. However, since there is a limit to the rate of release of oxygen by the O ₂ storage, if a large amount of unburned HC components are contained in the inflowing exhaust gas, the released oxygen must oxidize the entire amount of unburned HC components in the exhaust gas. And the air-fuel ratio of the exhaust gas downstream of the exhaust purification catalyst becomes an air-fuel ratio richer than the stoichiometric air-fuel ratio. In other words, the oxygen stored in the exhaust purification catalyst is released and immediately consumed,
The exhaust air-fuel ratio downstream of the exhaust purification catalyst also immediately changes to the rich air-fuel ratio. Therefore, the O _{2 of the} exhaust purification catalyst
The delay of the change in the air-fuel ratio due to the storage function is prevented.
The supply of fuel that does not contribute to combustion is stopped when the amount of oxygen stored in the exhaust purification catalyst is sufficiently reduced (that is, when the release of oxygen from the exhaust purification catalyst is reduced to a level that does not cause a practical problem). You. Further, in the case of an engine having an in-cylinder fuel injection valve for directly injecting fuel into a cylinder as the storage reducing means, even in a case where fuel is injected into a cylinder during an expansion stroke or an exhaust stroke of each cylinder. Alternatively, in an engine having an exhaust port fuel injection valve that injects fuel into each cylinder exhaust port, fuel may be injected into the exhaust port. The supply of fuel that does not contribute to combustion by the O ₂ storage reducing means may be performed during lean air-fuel ratio operation immediately before switching of the engine operating air-fuel ratio, or during operation of the stoichiometric air-fuel ratio or rich air-fuel ratio immediately after switching. You may go to

According to the invention described in claim 2, further in an exhaust passage of the exhaust gas purifying catalyst downstream air-fuel ratio of the exhaust flowing flows to absorb NO _X in the exhaust gas when a lean air-fuel ratio air-fuel ratio of the exhaust gas is provided with _the NO _X storage reduction catalyst to release the NO _X absorbed when it becomes a rich air-fuel ratio, the storage decreasing means further time to emit NO _X absorbed from _the NO _X storage reduction catalyst The exhaust gas purifying apparatus according to claim 1, wherein the amount of oxygen stored in the exhaust gas purifying catalyst is reduced.

According to the invention described in claim 3, rich air and the operating air-fuel ratio of the short time the engine when it should emit NO _X absorbed from the _the NO _X storage reduction catalyst during the lean air-fuel ratio operation of the engine 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, further comprising means for performing a rich spike operation for switching to a fuel ratio, wherein the amount of oxygen stored in the exhaust gas purifying catalyst is reduced by the storage reducing means during the rich spike operation. .

[0016] That is, according to claim 2 and invention are arranged _the NO _X storage reduction catalyst downstream of the exhaust purification catalyst having an O ₂ storage function in the claim 3, NO _X absorbed from _the NO _X storage reduction catalyst When the gas is to be released, the O ₂ storage function of the exhaust gas purification by the storage reducing means is reduced. Therefore, not only when the engine operating air-fuel ratio by changes in operating conditions are switched from a lean air-fuel ratio to the stoichiometric air-fuel ratio or rich air-fuel ratio, _the NO _X storage reduction catalyst when releasing NO _X from _the NO _X storage reduction catalyst change sufficiently rich air-fuel ratio of the exhaust gas flowing from the air-fuel ratio immediately lean air-fuel ratio of the exhaust gas also flowing to the NO _X occluding and reducing catalyst when changing from a lean air-fuel ratio to the stoichiometric air-fuel ratio or a rich air-fuel ratio This prevents unpurified NO _X from flowing out of the NO _X storage reduction catalyst.

According to the fourth aspect of the present invention, there is provided an exhaust gas purifying apparatus for an internal combustion engine which performs a lean air-fuel ratio operation as required, wherein the exhaust gas purifying catalyst is provided in an engine exhaust passage and has an O ₂ storage function. when the disposed in an exhaust passage of the exhaust gas purifying catalyst downstream air-fuel ratio of the exhaust flowing the air-fuel ratio of the exhaust gas absorbed flowing NO _X in the exhaust gas when a lean air-fuel ratio becomes a rich air-fuel ratio rich and _the NO _X storage reduction catalyst releases the absorbed NO _X, the operating air-fuel ratio for a short time the engine when it should emit NO _X absorbed from the _the NO _X storage reduction catalyst during the lean air-fuel ratio operation of the engine when Means for performing a rich spike operation for switching to an air-fuel ratio;
Storage reducing means for reducing the amount of oxygen stored in the exhaust purification catalyst by making the exhaust air-fuel ratio flowing into the exhaust purification catalyst more rich than the air-fuel ratio during the rich spike operation for a predetermined period immediately after the start of the rich spike operation. When,
An exhaust gas purification device for an internal combustion engine provided with:

[0018] That is, in the invention of claim 4, when performing the rich spike operation for releasing the reduction purification of the NO _X from _the NO _X storage reduction catalyst, and flows into the predetermined period exhaust purifying catalyst immediately after the rich spike start The exhaust air-fuel ratio is kept richer than the exhaust air-fuel ratio during the subsequent rich spike operation. Thereby, even while oxygen is being released from the exhaust purification catalyst by the O ₂ storage function, the exhaust gas contains unburned HC and CO components in an amount sufficient to consume the entire amount of released oxygen. Therefore, the exhaust air-fuel ratio on the downstream side of the exhaust purification catalyst becomes a sufficient rich air-fuel ratio even while oxygen is being released from the exhaust purification catalyst. Therefore, the NO _X occluding and reducing catalyst of the exhaust purification catalyst downstream looks like the exhaust of sufficient richer from the time of the rich spike operation start is supplied, unpurified from _the NO _X storage reduction catalyst NO _X
Outflow is prevented. The air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst immediately after the rich spike start is released from sufficient quantity and downstream of the NO _X occluding and reducing catalyst to consume the entire amount of oxygen released from the exhaust purifying catalyst Unburned HC and C in an amount that is the sum of an amount sufficient to purify the entire amount of NO _X and unburned HC
It is set to include the O component. Further, the storage reducing means does not contribute to combustion in the engine as in the first to third aspects, in which fuel is injected into the cylinder during expansion or exhaust stroke of the cylinder or in which fuel is injected into the exhaust port. In addition to the fuel supply, the air-fuel ratio of the engine may be further made richer during a rich spike operation for a predetermined period after the start of the rich spike operation. Further, the predetermined period is set to a time sufficient for releasing the entire amount of oxygen absorbed from the exhaust purification catalyst.

According to the fifth aspect of the present invention, the storage reducing means includes storage estimating means for estimating the amount of oxygen stored in the exhaust gas purification catalyst based on the operating state of the engine. The exhaust gas purifying apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the oxygen amount reducing operation is performed according to the oxygen amount. That is,
In the invention according to claim 5, the storage reducing unit estimates the amount of oxygen stored in the exhaust gas purification catalyst and performs an oxygen amount reducing operation according to the estimated amount of oxygen. For example, the storage reducing unit reduces the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst (that is, increases the degree of richness) or lengthens the time to continue the oxygen amount reducing operation as the stored oxygen amount increases. As a result, the oxygen amount reducing operation is accurately performed, and the delay in the change in the exhaust air-fuel ratio on the downstream side of the exhaust purification catalyst is reliably prevented. The estimation of the stored oxygen amount of the exhaust purification catalyst by the storage reducing means is performed based on the engine operating state such as the catalyst temperature, the exhaust flow rate, and the history of the change in the air-fuel ratio of the engine (the duration of the lean air-fuel ratio operation and the rich air-fuel ratio operation). It is performed based on.

According to the present invention, the storage estimating means estimates the amount of oxygen stored in the exhaust purification catalyst based on the operating state of the engine and the deterioration state of the exhaust purification catalyst. An exhaust emission control device for an internal combustion engine according to claim 5 is provided. In the invention according to claim 6, the storage reduction unit estimates the stored oxygen amount based on the deterioration state of the exhaust gas purification catalyst in addition to the engine operation state. The O ₂ storage function decreases with the deterioration of the exhaust gas purification catalyst, and the storable oxygen amount (saturated oxygen amount) of the exhaust gas purification catalyst decreases with the deterioration of the exhaust gas purification catalyst. That is, the exhaust purification catalyst does not store oxygen in an amount equal to or more than the saturated oxygen amount. Therefore,
By taking into account the state of deterioration of the exhaust purification catalyst, the stored oxygen amount of the exhaust purification catalyst can be more accurately estimated, and the operation of reducing the amount of oxygen can be performed more accurately.

[0021]

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.

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,
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, 21a is # 1, # 4
The exhaust manifold 21b connects the exhaust ports of the cylinder group consisting of cylinders to the individual exhaust passages 2a, and the exhaust manifold 21b connects the exhaust ports of the cylinder group consisting of # 2 and # 4 cylinders to the individual exhaust passage 2b. In the present embodiment, start catalysts (hereinafter, referred to as "SC") 5a and 5b made of a three-way catalyst are arranged on the individual exhaust passages 2a and 2b, respectively. 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 the air-fuel ratio sensors 3 and 3 arranged upstream of the start catalysts 5a and 5b of the individual exhaust passages 2a and 2b.
Reference numeral 1 denotes an air-fuel ratio sensor disposed at the outlet of the NO _X storage reduction catalyst 7 in the exhaust passage 2. Air-fuel ratio sensor 29
Reference numerals a, 29b, and 31 denote so-called linear air-fuel ratio sensors that output voltage signals corresponding to the exhaust air-fuel ratio in a wide air-fuel ratio range. Further, in FIG.
Electronic control unit (ECU). The ECU 30
In the present embodiment, the microcomputer 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, in addition to performing the above-described basic control, the ECU 30 changes the fuel injection mode of the in-cylinder injection valves 111 to 114 according to the engine operating state to change the operating air-fuel ratio of the engine, as described later. Control and further NO _X
In order to release the NO _X absorbed from the storage reduction catalyst 7, a rich spike operation for switching the short-time operation air-fuel ratio to the rich air-fuel ratio during the lean air-fuel ratio operation of the engine is performed. Further, the ECU 30 controls the SC5 when changing the engine operating air-fuel ratio from lean to rich or during a rich spike operation.
The stored oxygen amount reducing operation for reducing the amount of oxygen stored in a and 5b is performed.

The input ports of the ECU 30 are provided with start catalysts 5a, 5b from the air-fuel ratio sensors 29a, 29b.
A signal representing the exhaust air-fuel ratio at the inlet and an air-fuel ratio sensor 3
Signal representative of the exhaust air-fuel ratio in _the NO _X storage reduction catalyst 7 exit from 1, also, in addition to a signal corresponding to the intake pressure of the engine from the intake pressure sensor 33 provided in the engine intake manifold (not shown) are respectively input, A signal corresponding to the engine speed is input from a speed sensor 35 arranged near the engine crankshaft (not shown). Further, in the present embodiment, a signal representing the accelerator pedal depression amount (accelerator opening) of the driver is input to an input port of the ECU 30 from an accelerator opening sensor 37 arranged near an accelerator pedal (not shown) of the engine 1. Have been. ECU3
The 0 output port 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 this 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.

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).

If 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 enters a high-load operation range, the amount of fuel is further increased from the state described above, and the above-described 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 the present embodiment, the optimal operation mode (from the above) is set in advance based on experiments and the like according to 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 determines 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. If the above mode (stoichiometric air-fuel ratio or rich air-fuel ratio homogeneous mixture combustion) is selected, EC
U30 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 converts the fuel injection amount calculated above into 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 and 29b. As described 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 the present 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 arranged 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 the start of 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 oxygen is being absorbed by cerium, the exhaust air-fuel ratio at the catalyst outlet is close to the stoichiometric air-fuel ratio. Also, 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 changes to the same lean air-fuel ratio as the exhaust air-fuel ratio at the catalyst inlet. Similarly, when the cerium has sufficiently absorbed oxygen, when the air-fuel ratio of the exhaust gas flowing into the catalyst changes from the lean air-fuel ratio to the rich air-fuel ratio, oxygen is released from the cerium, and the oxygen concentration in the exhaust increases. Thus, the air-fuel ratio at the catalyst outlet becomes close to the stoichiometric air-fuel ratio. In this case as well, 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 is the same as the rich air-fuel ratio at the catalyst inlet. Become. 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 are provided with an O ₂ storage function, when the operating air-fuel ratio of the engine changes from lean to rich, the change of the exhaust air-fuel ratio downstream of the SCs 5a and 5b is delayed and temporarily changed. There will be a period during which the air-fuel ratio is 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 NO _X in the exhaust (NO _2, NO) nitrate ions NO ₃ ^- absorbed absorbed in the form of, the inflow exhaust gas becomes rich The effect of absorbing and releasing NO _X that releases the released NO _X is performed.

The mechanism of the absorption and release will be described below by taking 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, the inflow NO ₂ and NO ₂ produced by the above exhaust platinum Pt on the further absorbed into the absorbent while being oxidized barium oxide BaO and bound with nitrate ions NO ₃ ^- in the form of absorbent Spread. For this reason,
So NO _X in the exhaust gas is absorbed in the form of nitrates into _the NO _X absorbent in the 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 absorbent in the form of
Will be done. In this case, reduction products such as CO
Minutes, HC, CO_TwoWhen such components are present,
NO due to these components _TwoIs reduced.

In this embodiment, a lean air-fuel ratio operation is possible.
Engine 1 is used and engine 1 operates at a lean air-fuel ratio
NO_XThe storage reduction catalyst
NO in the air_XAbsorb. The engine 1 has a rich air-fuel ratio
NO when driving_XThe storage reduction catalyst 7 absorbs the absorbed NO.
_XTo release and purify. In the present embodiment, the lean air-fuel
NO during specific operation_XNO absorbed by storage reduction catalyst 7_Xamount
Increases, the engine air-fuel ratio is briefly reset from the lean air-fuel ratio.
Rich spike operation to switch to the
O_XNO from storage reduction catalyst_XRelease and reduction purification (NO
_X(Regeneration of the storage reduction catalyst). Real truth
In the embodiment, the ECU 30 determines NO_XIncrease or decrease the counter value
NO _XThe storage reduction catalyst 7 holds the absorption.
NO_XEstimate the amount. NO_XUnit time for storage reduction catalyst 7
NO absorbed per hit_XNO_XFor storage reduction catalyst
NO in exhaust gas flowing per unit time_XQuantity, ie
NO generated per unit time in engine 1_XProportional to the amount
ing. On the other hand, NO generated by the engine per unit time_X
Depends on the amount of fuel supplied to the engine, air-fuel ratio, exhaust flow rate, etc.
NO if engine operating conditions are determined_XOcclusion reduction
NO absorbed by the catalyst_XYou can know the quantity. This implementation
In the form, the engine operating conditions (accelerator opening, engine rotation
Number, intake air amount, intake pressure, air-fuel ratio, fuel supply amount, etc.)
NO generated by the engine per unit time _XReal amount
Measure, NO_XAbsorbed per unit time by the storage reduction catalyst 7
NO_XThe amount is, for example, the engine load (fuel injection amount) and the engine
ROM of the ECU 30 in the form of a numerical map using the rotational speed
Is stored in The ECU 30 operates at regular intervals (the above unit
Engine load (fuel injection amount) and engine speed
NO per unit time using this map_XOcclusion reduction
NO absorbed in the medium_XCalculate the quantity, NO_XHold the counter
NO_XIncrease by the amount absorbed. This makes NO_XCow
Is always NO_XNO absorbed by storage reduction catalyst 7
_XTo represent the amount of The ECU 30 is a lean engine.
During the air-fuel ratio operation, the above NO_XCounter value is greater than or equal to a specified value
When it increases to a short time (for example, about 0.5 to 1 second)
Degree) engine in the above-mentioned or other modes (stoichiometric air-fuel ratio or
Is a rich spa operated with rich air-fuel ratio homogeneous mixture combustion)
Perform an operation. Thereby, NO_XOcclusion reduction catalyst
NO absorbed from_XIs released and reduced and purified. In addition,
How long does the rich spike keep the exhaust air-fuel ratio rich?
NO_XBased on the type and capacity of the storage reduction catalyst
Determined by experiments. Also, perform rich spikes
Go and NO_XNO from the storage reduction catalyst_XRelease, reduction purification
NO after_XThe value of the counter is reset to zero.
Thus, NO_XNO of the storage reduction catalyst 7_XDepending on the amount of absorption
By performing a rich spike,_XOcclusion return
The source catalyst 7 is properly regenerated and NO_XAbsorption reduction catalyst absorbs
NO_XTo prevent saturation.

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. For this reason, even if exhaust gas with a rich air-fuel ratio flows from the engine into the SCs 5a and 5b during the rich spike, NO
_During the time when oxygen is released in the SCs 5a and 5b, exhaust gas having a lean air-fuel ratio near the stoichiometric air-fuel ratio may flow into the _X storage reduction catalyst 7, and the NOx storage reduction catalyst 7 is not purified immediately after the rich spike starts. there is a possibility that of the NO _X flows out.
Similarly, the operating air-fuel ratio of the engine is changed to a lean air-fuel ratio by the change of the operating condition of the engine 1 (the operating mode described above).
Even when the air conditioner is switched to the stoichiometric air-fuel ratio or the rich air-fuel ratio (the above-mentioned or the operation mode), there is a possibility that the unpurified NO _X flows out of the NO _X storage reduction catalyst 7 immediately after the change.

Therefore, in the embodiment described below, when the engine air-fuel ratio is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio for the purpose of rich spike operation or operation mode switching, the exhaust gas which flows into the SCs 5a and 5b in advance. by the air-fuel ratio to the rich solves the problems caused by the SCs 5a, 5b of the O ₂ storage. SC
By setting the exhaust air-fuel ratio flowing into the 5a, 5b to the rich air-fuel ratio, exhaust containing a large amount of HC and CO components flows into the SCs 5a, 5b. For this reason, the oxygen stored in the catalyst by the O ₂ storage is consumed to oxidize the HC and CO components in the exhaust gas, and the release of oxygen from the catalyst is completed in a short time. Further, by setting the HC and CO component amounts to be larger than the amount that consumes the entire amount of oxygen released from the catalyst, the exhaust gas on the downstream side of the SCs 5a and 5b can be discharged even while oxygen is being released from the catalyst by the O ₂ storage function. The rich air-fuel ratio is maintained. This prevents the unpurified NO _X from flowing out of the NO _X storage reduction catalyst 7.

The stored oxygen reduction operation for setting the exhaust air-fuel ratio flowing into the SCs 5a and 5b to a rich air-fuel ratio when the engine air-fuel ratio is switched includes, for example, (A) a cylinder injection operation for each cylinder during the cylinder expansion stroke or the exhaust stroke. (B) A fuel injection valve for injecting fuel into an exhaust port of each cylinder is provided to inject fuel into an engine exhaust port (hereinafter, referred to as “secondary fuel injection”). Port injection "
And (C) temporarily make the engine combustion air-fuel ratio significantly rich when switching the engine air-fuel ratio. the above
In the methods (A) and (B), the fuel injected into the expansion, the exhaust stroke, and the exhaust port of the cylinder is vaporized without burning and generates a large amount of HC and CO components in the exhaust gas. That is, since these fuels do not contribute to combustion, there is an advantage that even when a relatively large amount of fuel is supplied, fluctuations in engine output do not occur. On the other hand, since these fuels do not contribute to combustion, a relatively large amount of oxygen remains in the exhaust when the engine is operated at a lean air-fuel ratio. That is, when the fuel that does not contribute to combustion is supplied to the engine as described above, the air-fuel ratio of the exhaust gas becomes a rich air-fuel ratio as a whole, but unreacted oxygen and HC and CO components are separated in the exhaust gas. Will exist. Therefore, these oxygen and HC and CO components are SC
5a and 5b, and depending on the operating conditions, SC5
The temperatures of a and 5b may rise excessively.

In the above method (C), the combustion air-fuel ratio itself of the engine temporarily temporarily becomes a large rich air-fuel ratio.
Although the problem of overheating of a and 5b does not occur, the combustion torque of the engine increases due to the combustion of a large amount of fuel, and the output torque may fluctuate depending on the operation state of the engine. Therefore, it is preferable to select one of the methods (A) to (C) according to the characteristics of the engine and the operating state.

When the method (B) (exhaust port fuel injection) is applied, it is almost the same as the case (A) (secondary fuel injection). )
The case where the methods (C) and (C) are applied will be described as an example. (1) First Embodiment FIG. 2 shows SCs 5a and 5b according to a first embodiment of the present invention.
It is a flowchart explaining the stored oxygen amount reduction operation of FIG. This operation is executed by the ECU 30 at predetermined intervals (for example, at every constant rotation angle of the crankshaft).

[0043] In the operation of FIG. 2, when the operating air-fuel ratio switching from the lean air-fuel ratio operation by the driver condition changes in the engine to the stoichiometric air-fuel ratio or a rich air-fuel ratio operation, and because of the NO _X release from _the NO _X storage reduction catalyst 7 During the rich spike operation, the amount of stored oxygen in the SCs 5a and 5b is reduced by injecting fuel from the in-cylinder fuel injection valve during the expansion or exhaust stroke of each cylinder immediately before switching the operating air-fuel ratio of the engine.
That is, in this embodiment SCs 5a, oxygen release is to switch the operating air-fuel ratio of the engine from the end by the O ₂ storage function from 5b.

[0044] Thus, since the operating air-fuel ratio of the engine SC5a when being switched from lean to rich (or stoichiometric air-fuel ratio), oxygen release from 5b no, NO _X
Air-fuel ratio of the exhaust gas flowing into the occlusion-reduction catalyst 7 is as changed immediately to a rich air-fuel ratio (or theoretical air-fuel ratio) from the lean air-fuel ratio, the unpurified from _the NO _X storage reduction catalyst 7 N
O _X release is prevented.

When the operation starts in FIG. 2, in step 201, the accelerator opening degree (the amount of depression of the accelerator pedal by the driver) ACC from the accelerator opening degree sensor 37 described above.
P is the engine speed NE calculated based on the output of the speed sensor 35, and the stored oxygen amount OS of the SCs 5a and 5b.
And C are read respectively. Note that SC5a, 5b
The calculation of the stored oxygen amount OSC will be described later in detail.

[0046] Then, based on the accelerator opening ACCP and the engine rotational speed NE read by the step 203, the optimal operation mode M ₁ is selected from among the operation modes from the foregoing. In this embodiment, the optimal operation mode for each accelerator opening and engine speed is stored in the ROM of the ECU 30 as a numerical table using the accelerator opening ACCP and the engine speed NE as parameters. ACC read by
An optimum operation mode (と) is selected from this numerical table based on P and NE. The value of M ₁ (M ₁ = 〜) in step 203 depends on the optimal operation mode (ie, step 2
Reference numeral 23 denotes a switching operation target operation mode).

Next, at step 205, it is determined whether or not the current operation mode M ₀ is a lean air-fuel ratio operation (any of the above modes). M ₀ is a parameter indicating in which operation mode the engine is operating from the present (M ₀ = 〜). If not currently performed lean air-fuel ratio operation in step 205, that is, if it is performed currently stoichiometric or rich air-fuel ratio operation, a lean air-fuel ratio be either the target drive mode M ₁ is from Since the air-fuel ratio does not switch from the air-fuel ratio to the rich air-fuel ratio (or the stoichiometric air-fuel ratio), and there is no possibility that unpurified NO _X flows out of the NO _X storage reduction catalyst 7, step 223 is immediately executed and the operation of the engine is started. mode is switched to the target operation mode M ₁ (the current mode is continued when it is operated at the current target operating mode). Then, after the switching is completed, in step 225, the current operation mode M ₀ is changed to the operation mode after switching (M ₁ ).
Is updated to a value according to.

On the other hand, if the engine has been operated in the current operation mode in step 205,
At 07, it is determined based on the value of the rich spike flag FR whether or not execution of a rich spike operation for releasing NO _X from the NO _X storage reduction catalyst 7 is currently requested. As described above, in the present embodiment, the ECU 30 accumulates the value of the NO _X counter CNOX representing the amount of NO _X absorbed by the NO _X storage reduction catalyst 7 based on the engine operating state by a separately executed routine (not shown). When the value of the counter CNOX increases beyond a predetermined value, the value of the rich spike flag FR is set to 1. If a rich spike operation is currently requested in step 207, it is necessary to execute a stored oxygen amount reduction operation (steps 213 to 217), which will be described later. Further, in next step 209 if the current rich spike operation is not required, the target drive mode M ₁ whether rich air-fuel ratio or stoichiometric air-fuel ratio operation (mode or) is determined. If the target operation mode M1 is not _one of the modes, the switching from the lean air-fuel ratio operation to the rich air-fuel ratio operation does not occur in this case as well, so the process proceeds to step 223, where the operation mode is switched to the target mode. You.

On the other hand, if FR = 1 (rich spike request) in step 207 and if the target operation mode is or in step 209, that is, the engine operating air-fuel ratio is changed from the lean air-fuel ratio to the rich air-fuel ratio ( Or stoichiometric air-fuel ratio).
Proceeding to 11, it is determined whether the current oxygen reduction operation has been completed. If the reduction operation has not been completed, the secondary fuel injection (in-cylinder fuel injection in the expansion or exhaust stroke) for reducing the oxygen amount in steps 213 to 217 is executed, and the fuel not contributing to the combustion is supplied to the cylinder. Supply.

That is, in step 213, step 2
01 The current stored oxygen amount O of SC5a, 5b read in 01
From the SC, the total fuel amount (HC amount) required to consume the entire amount of oxygen stored in the SCs 5a and 5b and to maintain the exhaust gas downstream of the SCs 5a and 5b richer than the stoichiometric air-fuel ratio.
Is calculated, and the total fuel amount is divided by a predetermined number of executions of secondary fuel injection (described later) to calculate a secondary fuel injection amount required for each injection. Then, in step 215, it is determined whether or not it is time to set the secondary fuel injection amount of any of the cylinders. If it is the set timing, the calculated secondary fuel injection amount is sent to the fuel injection circuit in step 217. Set. This allows
When the secondary fuel injection timing (expansion or exhaust stroke) comes, the secondary fuel injection is executed in each cylinder. When the secondary fuel injection has been performed a predetermined number of times (the number of cylinders), it is determined in step 211 that the oxygen amount reducing operation has been completed, and the steps from step 219 are performed.

That is, the stored oxygen amount reducing operation is completed and
When oxygen release from C5a and 5b is completed, step 2
19, a rich spike operation is currently required (F
It is determined whether or not R = 1). If the rich spike operation has been requested, the rich spike operation is executed in step 221. If the rich spike operation has not been requested, the target operation mode M ₁ (in this case, rich Alternatively, switching to the stoichiometric air-fuel ratio operation mode) is performed.

In the rich spike operation of step 221, the engine is operated in the mode rich air-fuel ratio homogeneous mixture combustion until the value of the NO _X counter CNOX becomes 0, and the NO absorbed in the NO _X storage reduction catalyst 7 is operated. The entire amount of _X is released and reduced and purified. Next, the number of times of secondary fuel injection in the present embodiment will be described. In the present embodiment, the secondary fuel injection is performed once for each of the cylinder groups # 1 and # 4 and the cylinder groups # 2 and # 3, or once for all the cylinders # 1 to # 4. That is, if the capacity of the SCs 5a and 5b is relatively small, and the amount of stored oxygen in the SC can be reduced by one secondary fuel injection for each of the SCs 5a and 5b, the secondary fuel only once for each cylinder group. Injection is performed, and the capacity of the SCs 5a and 5b is relatively large,
If the amount of stored oxygen cannot be sufficiently reduced by one secondary fuel injection, two secondary fuel injections are performed for each of the SCs 5a and 5b (that is, once for all the cylinders # 1 to # 4). You. Which secondary fuel injection is executed is determined in advance according to the capacity of the SCs 5a and 5b. Since each cylinder group is composed of cylinders in which the ignition order is not continuous, if the secondary fuel injection is performed once for each cylinder group, the two-cylinders in the continuous ignition order are executed in step 211 described above. (For example, # 1 and # 3, or # 3 and # 2
Etc.) When the secondary fuel injection is performed once each, it is determined that the stored oxygen amount reduction operation has been completed.

As described above, in the present embodiment, the engine is kept in the lean air-fuel ratio operation mode (from) while the stored oxygen amount reducing operation is being performed. (2) Second Embodiment Next, a second embodiment of the present invention will be described. In the first embodiment, SC5 is performed only by the secondary fuel injection.
The storage oxygen amount reduction operations a and 5b were performed, and the operation mode was not switched to the rich air-fuel ratio operation until the storage oxygen amount reduction operation (secondary fuel injection) was completed. In the present embodiment, in an engine that requires secondary fuel injection once for all cylinders, the operation mode is switched (intake stroke fuel injection) at the timing when a command to switch from the lean air-fuel ratio operation mode to the rich air-fuel ratio operation mode is issued. Is switched to the intake stroke fuel injection and the intake stroke fuel injection amount is increased by an amount corresponding to the secondary fuel injection amount. For the cylinders that cannot make the transition to the intake stroke fuel injection in time, Perform fuel injection.

FIG. 3 shows the secondary fuel injection (FIG.
FIG. 4 shows an example in which secondary fuel injection is performed from the end of the expansion stroke to the early stage of the exhaust stroke) and the timing of increasing the fuel injection in the intake stroke. FIG. 3 shows the timing of switching from the mode (lean stratified combustion (compression stroke single injection) to the mode (stoichiometric air-fuel ratio homogeneous mixture combustion (intake stroke injection)), but also switching between other modes. 3
Is the same as

FIG. 3 shows the fuel injection timing of each of the # 1 to # 4 cylinders and the fuel injection amount setting timing for performing the fuel injection. In FIG. 3, CS
ET is the compression stroke fuel injection amount set timing, CINJ
Indicates a compression stroke fuel injection execution timing, EXSET indicates a secondary fuel injection amount set timing, EXINJ indicates a secondary fuel injection execution timing, ISET indicates an intake stroke fuel injection amount set timing, and IINJ indicates an intake stroke fuel injection execution timing. . In FIG. 3, CH indicates the start timing of the stored oxygen reduction operation for switching the operation mode. In addition, in FIG.
"Pressure", "inflation" and "exhaust" are the intake strokes of each cylinder,
It represents a compression stroke, an expansion stroke, and an exhaust stroke. As shown in FIG. 3, in the present embodiment, the secondary fuel injection amount is set at the end of the compression stroke (EXSET), and the intake stroke fuel injection amount is set at the beginning of the exhaust stroke (ISET).

Now, assuming that the operation for reducing the stored oxygen amount of the catalyst for switching the operation mode is started at the timing of FIG. 3 CH, since CH corresponds to the middle stage of the compression stroke in the # 1 cylinder, the fuel injection timing of the intake stroke ( IINJ) has been completed, and the setting of the compression stroke fuel injection amount has been completed at timing CSET. Therefore, since the operation mode cannot be switched immediately in the # 1 cylinder, the compression stroke fuel injection IINJ is executed as it is,
The secondary fuel injection amount (EXINJ) is executed by setting the secondary fuel injection amount by XSET.

On the other hand, in the # 3 cylinder, the timing of CH corresponds to the middle stage of the intake stroke. At this timing, however, the intake stroke fuel injection amount setting timing has already passed, so that it is not possible to immediately shift to the intake stroke fuel injection. . Therefore, in the # 3 cylinder, the compression stroke fuel injection is performed as it is, and the compression stroke fuel injection amount is set by CSET, and the secondary fuel injection amount is set at the secondary fuel injection amount set timing (EXSET) at the end of the subsequent compression stroke. The amount is set and the secondary fuel injection is executed.

Similarly, in the # 4 cylinder, the timing of CH corresponds to the middle stage of the exhaust stroke. In this case, too, since the intake stroke fuel injection amount set timing (IINJ) has passed, the injection stroke fuel injection is immediately started. Cannot be migrated. Therefore, the secondary fuel injection (EXIN) is performed while the compression stroke fuel injection (CINJ) is continued as in the case of the # 3 cylinder.
J) is executed.

On the other hand, in the # 2 cylinder, since the timing of CH corresponds to the middle stage of the expansion stroke, the fuel injection amount set timing (ISET) of the intake stroke has not yet been reached, and it is possible to shift to the intake stroke fuel injection. . Therefore, in the # 2 cylinder, the operation mode is switched to perform the fuel injection during the intake stroke, and the fuel injection amount set by ISET is increased by an amount corresponding to the secondary fuel injection amount. That is, for the # 2 cylinder, the operation mode is switched without performing the secondary fuel injection, and instead, the fuel injection amount is increased by adding an amount corresponding to the secondary fuel injection amount to the intake stroke fuel injection amount immediately after the operation mode switching. Set the amount.

As can be seen from the timing chart of FIG. 3, in the present embodiment, after the operation of reducing the stored catalyst oxygen amount for switching the operation mode is started, the secondary fuel injection amount set timing is earlier than the intake stroke fuel injection amount set timing. (In the case of FIG. 3, # 1, # 3, and # 4 cylinders) perform the secondary fuel injection while continuing the compression stroke fuel injection (that is, without switching the operation mode).
For the cylinder (# 2 cylinder) in which the intake stroke fuel injection amount set timing is earlier than the secondary fuel injection amount set timing, the operation mode is switched to perform the intake stroke fuel injection,
During the fuel injection in the intake stroke, the amount of fuel corresponding to the secondary fuel injection amount of the other cylinders is increased. In this case as well, when the secondary fuel injection or the intake stroke fuel injection amount is increased once in all cylinders, the operation of reducing the stored oxygen amount of the catalyst ends, and thereafter the operation mode is switched in all cylinders. .

That is, in this embodiment, the operation of reducing the amount of stored catalyst is started before the operation mode is switched (# 1, # 1).
After the switching of the operation mode (# 2 cylinder), the process ends. Thereby, the operation mode switching time can be shortened. FIG. 4 is a flowchart illustrating the stored oxygen amount reducing operation of the present embodiment. 4 is performed as a routine executed by the ECU 30 at predetermined intervals. The flowchart of FIG. 4 corresponds to steps 213 to 217 of the flowchart of FIG.
Is different from the flowchart of FIG. 2 only in that Therefore, only the differences will be described here.

In step 413, step 213 in FIG.
Similarly, the amount of one secondary fuel injection is set from the stored oxygen amount OSC of SC5a, 5b, and step 415
Now, the setting timing of the current intake stroke fuel injection amount (FIG. 3
In the case of (ISET), in step 419, an amount increased by adding the secondary fuel injection amount calculated in step 413 to the intake stroke fuel injection amount after switching the operation mode is set as the intake stroke fuel injection amount. If the timing is not at the intake stroke fuel injection amount setting timing, step 41
7. In step 421, the secondary fuel injection amount is set. As a result, in the cylinder in time for the intake stroke fuel injection amount set timing (ISET), the operation mode is switched and the fuel injection amount is increased instead of the secondary fuel injection.

In the present embodiment, the operation of reducing the stored oxygen amount is performed by executing the secondary fuel injection in the cylinders other than the cylinder in time for the set timing of the fuel injection during the intake stroke. Without the above, the above-mentioned #
As in the case of a two-cylinder engine, the intake stroke fuel injection amount may be increased by an amount corresponding to the secondary fuel injection amount. In this case, the operation of reducing the stored oxygen amount of the catalyst is performed in each cylinder immediately after switching the operation mode.

(3) Third Embodiment Next, a third embodiment of the present invention will be described. In the first embodiment, when switching the engine operation mode,
The operation mode is switched after the operation of reducing the stored oxygen amount of the catalyst is completed. In the second embodiment, the reduction operation is performed in some or all cylinders immediately after the operation mode is switched. On the other hand, in the present embodiment, the stored oxygen amount reduction operation is performed independently of the switching of the operation mode. That is,
The switching of the operation mode is performed in each cylinder as usual, and the secondary fuel injection is executed regardless of the operation mode until the switching of the operation mode of each cylinder is completed. In actual operation, for example, the mode (lean air-fuel ratio stratified combustion (compression stroke 1
From a very lean combustion state to rapid acceleration, etc. (rich air-fuel ratio homogeneous mixture combustion (intake stroke single injection))
It may be necessary to shift to rich air-fuel ratio combustion, but in such a case, if the operation mode is switched directly, a sudden change in the output torque may occur due to a sudden change in the combustion air-fuel ratio. is there. Therefore, in such a case, the operation mode is not directly switched from the operation mode to the operation mode, but once the operation mode is changed from the mode (lean air-fuel ratio homogeneous mixture / stratified combustion (intake stroke / compression stroke twice injection)) to (lean air-fuel ratio homogeneous mixture). There is a case where the operation mode is switched via combustion (single injection in the intake stroke).

In this embodiment, when the operation mode is switched, the above switching operation such as →→→ is executed independently, and at the same time, the secondary fuel injection is executed until the mode switching is completed. That is, in the present embodiment, the switching of the operation mode and the operation of reducing the stored oxygen amount of the catalyst are performed in parallel. This prevents the operation mode switching time from being affected by the execution of the stored oxygen amount reduction operation.

FIG. 5 is a flowchart for explaining the operation of reducing the stored oxygen amount of the catalyst according to the present embodiment. This operation is E
It is performed as a routine executed by the CU 30 at predetermined intervals. 5, steps 501 to 509 indicate the same operation as steps 201 to 209 in FIG.
In this embodiment, corresponding to the target operation mode M ₁ and the current operation mode M ₀ proceeds immediately to step 523 if there is no switching of the operation mode to the rich air-fuel ratio from a lean air-fuel ratio in steps 501 509 Mode A switching operation is performed.

On the other hand, if it is necessary to switch the operation mode from the lean air-fuel ratio operation to the rich air-fuel ratio operation in steps 501 to 509, the process proceeds to step 511,
From the target operation mode M ₁ and the current operation mode M ₀ , M
₀ → on the basis of the number of cycles required to shift to M _1, the calculated secondary fuel injection several times executable, second per time from the secondary fuel injection execution times and the current catalyst storing oxygen amount OSC The next fuel injection amount is calculated. The secondary fuel injection amount is S
It is calculated as an amount that consumes the entire amount of oxygen released from C5a and 5b and generates HC enough to maintain the exhaust gas downstream of SC5a and 5b at a rich air-fuel ratio.

After calculating the secondary fuel injection amount, in steps 513 to 521, the secondary fuel injection is executed until the operation mode switching is completed. Further, in this case step 521 and 523, proceeds (step 521) and proceeds operation corresponding to the current operation mode M ₀ and the target drive mode M ₁ to parallel the rich spike operation and the secondary fuel injection is performed . Then, step 513 or step 5
Upon completion of the shift operation 21, it is determined in step 513 that the mode switching has been completed, and the secondary fuel injection is stopped.

Next, the stored oxygen amount OS of the SCs 5a and 5b used for calculating the secondary fuel injection amount in each of the above embodiments.
A method for estimating C will be described. In the present embodiment, SC
Air-fuel ratio sensors 29a, 29b arranged at the entrances of 5a, 5b
From the exhaust air-fuel ratio AF at the inlet of the SCs 5a and 5b and the engine intake air weight flow rate (gram / sec) GA
The stored oxygen amounts OSC of a and 5b are calculated.

[0070] As described above, the O ₂ storage function of the catalyst, SCs 5a, exhaust air-fuel ratio flowing into 5b is surplus oxygen in the exhaust gas when the lean of the stoichiometric air-fuel ratio SC5
a and 5b, and when richer than the stoichiometric air-fuel ratio, the absorbed oxygen is released from the SCs 5a and 5b. Therefore, the amount of oxygen absorbed by the SCs 5a and 5b or released from the SCs 5a and 5b corresponds to the amount of oxygen required to make the exhaust air having the air-fuel ratio AF to be the stoichiometric air-fuel ratio.

Now, the fuel of a certain amount F is burned and the air-fuel ratio AF
Assuming that the weight of air required to generate the exhaust gas is GA, GA = AF × F. Further, assuming that the weight of air required to burn the same amount of fuel F to generate exhaust gas having the stoichiometric air-fuel ratio ST is GA ′, GA ′ = ST × F. On the other hand, when the oxygen concentration in the air and AO _2, 'is in the air, AO ₂ × GA and AO ₂ × GA respective' weight GA and GA becomes. That is, the amount of oxygen required to burn a certain amount of fuel F to generate exhaust gas having the stoichiometric air-fuel ratio ST is AO ₂ × GA ′ = AO ₂ × ST × F. On the other hand, when the same fuel is burned to generate exhaust gas having the air-fuel ratio AF, the amount of oxygen is AO ₂ × GA = AO ₂ × AF × F.
Therefore, the amount of oxygen required to make the exhaust air with the air-fuel ratio AF to be the stoichiometric air-fuel ratio, that is, the amount of oxygen absorbed by the SCs 5a and 5b when AF> ST is (AO ₂ × GA)
− (AO ₂ × GA ′) = AO ₂ × F × (AF-ST) Since F = GA / AF, the amount of released / absorbed oxygen is AO ₂ × GA × (AF-ST) / AF =
AO ₂ × GA × (ΔAF / AF). Where ΔA
F = (AF-ST). Also, since GA is the air flow rate per unit time (second), if AF> ST, the catalyst will have AO ₂ × GA ×
(ΔAF / AF) oxygen is absorbed, and the stored oxygen amount OSC of the catalyst increases by AO ₂ × GA × (ΔAF / AF). (If AF <ST, ΔAF becomes minus and the stored oxygen amount OSC of the catalyst decreases).

Therefore, when the exhaust air-fuel ratio is originally AF and the intake air weight flow rate is GA, the change amount of the stored oxygen amount OSC per time Δt of the SCs 5a and 5b is AO ₂ × GA × (Δ
AF / AF) × Δt, but the actual amount of change in OSC is affected by the oxygen absorption / desorption speed of the catalyst.
The SC change amount is AO ₂ × GA × (ΔAF / AF) × Δt
× K (K is a correction coefficient based on the oxygen absorption / desorption rate). Also, the rate of oxygen absorption and desorption is actually affected by the catalyst temperature, and increases as the catalyst temperature increases. Furthermore, the rates of absorption and release of oxygen are different, with the rate of oxygen absorption being higher than the rate of release. Therefore, in the present embodiment, the change amount of the OSC per time Δt is absorbed (AF ≧ ST) and released (AF
<ST) and is expressed by the following equation.

Absorption (AF ≧ ST): AO ₂ × GA × (Δ
AF / AF) × Δt × A release (AF <ST): AO ₂ × GA × (ΔAF / AF)
× Δt × B Here, A and B are correction coefficients determined by the rate of oxygen absorption and release and the catalyst temperature. FIG. 6 shows the SC in the present embodiment.
It is a flowchart explaining the stored oxygen amount calculation operation of 5a, 5b. This operation is performed by a routine executed by the ECU 30 at regular time intervals corresponding to Δt. In this operation, the change amount of the stored oxygen amount OSC per time Δt of the SCs 5a and 5b is calculated using the above equation, and this change amount is integrated from the start of the engine to obtain the current S value.
The stored oxygen amount OSC of C5a, 5b is estimated.

In the operation of FIG. 6, first, in step 601, S
The exhaust air-fuel ratio AF at the inlet of C5a, 5b, the intake air mass flow rate GA of the engine, and the SC5a, 5b temperature TCAT are read. In the present embodiment, the exhaust air-fuel ratio AF is SC5a,
It is obtained as an average value of the exhaust air-fuel ratio detected by the air-fuel ratio sensors 29a and 29b at the inlet of the 5b. The intake air weight flow rate GA is calculated as the product of the amount of fuel (fuel injection amount) supplied to the engine per unit time and the exhaust air-fuel ratio AF. The temperature TCAT of the SCs 5a and 5b may be measured by arranging a temperature sensor on the catalyst bed, or the relationship between the engine load (fuel injection amount), the number of revolutions, and the exhaust temperature may be obtained in advance, and the engine fuel injection may be performed. The exhaust temperature is calculated on the basis of the amount (engine load) and the number of revolutions, and this exhaust temperature is approximated by TCAT.
You may use as.

As described above, AF, GA and TCAT are read.
After that, in step 603, AF ≧ ST (ST is
(AF / ST) is calculated.
The gas purification catalyst is absorbing oxygen, and the stored oxygen amount OSC is
Since it is increasing, in step 605, SC5a, 5b
Calculate correction coefficient A from oxygen absorption rate and catalyst temperature TCAT
Put out. Then, in step 607, the stored oxygen amount OSC
Is (AO_Two× GA × (ΔAF / AF) × Δt ×
A) is increased. Then, in step 609, increase
The value of the subsequent OSC is the maximum value OSC_MAXO if exceeds
SC value is OSC _MAXIs set to OSC_MAXIs S
The maximum storable oxygen amount (saturated amount) of C5a, 5b
You.

On the other hand, if AF <ST in step 603, since the SCs 5a and 5b are currently releasing oxygen, the oxygen release rate and the catalyst temperature TCA are determined in step 613.
The correction coefficient B is calculated based on T and the value of OSC is calculated as (AO ₂ × GA × (ΔAF / AF) ×
Δt × B) (in this case, since ΔAF <0, the OSC decreases). Then, step 617,
In step 619, the value of OSC is limited to the minimum value 0, and the current operation ends. When the engine is started, steps 607 and 6
The initial value of OSC at 15 is set to OSC _MAX . This is because when the engine is stopped, the air atmosphere (lean air-fuel ratio) of the SCs 5a and 5b is maintained, and the SCs 5a and 5b are saturated with oxygen.

In each of the above-described embodiments, an accurate stored oxygen amount is obtained by calculating the fuel amount required for the stored oxygen amount reduction operation of the SCs 5a and 5b using the stored oxygen amount OSC of the catalyst estimated by the operation of FIG. The reduction operation is performed to prevent the unpurified NO _X from flowing out of the NO _X storage reduction catalyst 7 when the engine operation mode is switched from the lean air-fuel ratio to the rich air-fuel ratio.

Next, the correction of the saturated oxygen amount OSC _MAX of the SCs 5a and 5b used in the operation of FIG. 6 will be described with reference to FIGS. 7 to 9. In the operation of FIG.
_{Although the} stored oxygen amount OSC may be calculated with _MAX as an appropriate constant value, it is more preferable to correct the value of OSC _MAX according to the deterioration of the catalyst. O _{2 of} catalyst
The storage function decreases as the catalyst deteriorates, and the maximum oxygen amount (saturated amount) OSC _MAX that can be stored by the catalyst also decreases. Therefore, in this embodiment, the deterioration state of the catalyst is determined,
The value of OSC _MAX is corrected according to the deterioration state.

First, a method of determining the deterioration state of the catalyst will be described. In the present embodiment, the trajectory lengths 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 NO
The deterioration state of the catalyst is determined based on the locus length of the output signal curve of the air-fuel ratio sensor 31 downstream of the _X storage reduction catalyst 7. FIG. 7 shows an air-fuel ratio sensor output VOM provided upstream of the exhaust purification catalyst and an air-fuel ratio sensor output V provided downstream of the catalyst when the engine air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio.
3 shows a general waveform of an OS. In FIG. 7, (A) shows the waveform when the O ₂ storage function of the exhaust purification catalyst is high,
FIG. 7B shows waveforms when the O ₂ storage function is reduced.

As shown in FIGS. 7A and 7B, 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
In the case of an engine having two upstream air-fuel ratio sensors 29a and 29b, the trajectory length LVOM is calculated by using the average value of the outputs of the two upstream air-fuel ratio sensors 29a and 29b as the upstream air-fuel ratio sensor output VOM. Alternatively, the output trajectory length may be calculated for each of the air-fuel ratio sensors 29a and 29b, and the average of both trajectory lengths may be used as the upstream-side air-fuel ratio sensor output trajectory length LVOM. FIG. 8 shows the maximum stored oxygen amount OS considering the deterioration of the SCs 5a and 5b according to the present embodiment.
9 is a flowchart illustrating a calculation operation of C _MAX . This operation is performed as a routine executed by the ECU 30 at regular intervals.

When the operation starts in FIG. 8, it is determined in step 801 whether or not the deterioration parameter calculation execution condition is satisfied. In the present embodiment, step 8
The condition of 01 is that the engine is operated in the mode (stoichiometric air-fuel ratio homogeneous mixture combustion (single-injection stroke injection)) and the air-fuel ratio feedback control based on the air-fuel ratio sensors 29a and 29b is performed. Is done. As described in FIG. 7, in order to use the locus length ratio LR as a parameter representing the O ₂ storage function of the catalyst is calculated in a state in which the locus length ratio LR is the engine air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio This is because it is necessary to do so.

If the condition is satisfied in step 801, in step 803, the upstream air-fuel ratio sensors 29 a and 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 805-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 last execution of this operation, respectively, and are updated in step 807 each time 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 809 and 811 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 811, the period T
Has elapsed, at step 813, 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 815, the value of the trajectory length ratio LR (O ₂ storage function parameter) is set based on the relationship set in advance based on the relationship.
A correction coefficient RD for SC _MAX is obtained. Then, in step 819, the current maximum stored oxygen amount OSC _MAX of the SCs 5a and 5b is calculated as OSC _MAX = OSC _MAX0 × RD. Here, OSC _MAX0 is the maximum value of the stored oxygen amount in a new state where the SCs 5a and 5b are not deteriorated at all.

FIG. 10 is a graph showing the relationship between the trajectory length ratio LR and the correction coefficient RD used for obtaining the correction coefficient RD in step 817 of FIG. As shown in FIG. 10, the value of the correction coefficient RD is set to 1.0 when the catalyst has not deteriorated at all (LR≪1.0), and as the catalyst deteriorates (the value of LR approaches 1). ) Are set to be smaller.

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. 10, the accuracy of estimating the storage oxygen amount OSC of the SCs 5a and 5b in each of the above-described embodiments is improved. Therefore, in each of the above-described embodiments, it is possible to more accurately perform the stored oxygen amount reduction operation.

[0086]

According to the invention described in each of the claims, when an exhaust purification catalyst having an O ₂ storage function is disposed in the exhaust passage, the exhaust air-fuel ratio on the downstream side of the catalyst is calculated from the lean air-fuel ratio or the stoichiometric air-fuel ratio. There is a common effect that it is possible to prevent a delay in the change to the rich air-fuel ratio.

[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 an operation for reducing the stored oxygen amount of the exhaust purification catalyst of the present invention.

FIG. 3 is a timing chart for explaining a second embodiment of the operation for reducing the stored oxygen amount of the exhaust purification catalyst of the present invention.

FIG. 4 is a flowchart illustrating a second embodiment of the operation for reducing the stored oxygen amount of the exhaust purification catalyst of the present invention.

FIG. 5 is a flowchart illustrating a third embodiment of the operation of reducing the stored oxygen amount of the exhaust purification catalyst of the present invention.

FIG. 6 is a flowchart illustrating an operation for estimating the stored oxygen amount of the exhaust purification catalyst used in the first to third embodiments.

FIG. 7 is a diagram illustrating a change in the output of the upstream air-fuel ratio sensor and the output of the downstream air-fuel ratio sensor due to deterioration of the exhaust purification catalyst.

FIG. 8 is a flowchart illustrating an operation for estimating a stored oxygen amount in consideration of exhaust gas purification catalyst deterioration.

9 is a diagram illustrating a method of calculating an output trajectory length of an air-fuel ratio sensor used in the operation of FIG. 8;

FIG. 10 is a diagram illustrating a relationship between a correction coefficient of an O ₂ storage function of an exhaust purification catalyst and a trajectory length ratio.

[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)

────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] July 5, 1999 (1999.7.5)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0061

[Correction method] Change

[Correction contents]

That is, in this embodiment, the operation of reducing the amount of stored catalyst is started before the operation mode is switched (# 1, # 1).
After the switching of the operation mode (# 2 cylinder), the process ends. Thereby, the operation mode switching time can be shortened. FIG. 4 is a flowchart illustrating the stored oxygen amount reducing operation of the present embodiment. 4 is performed as a routine executed by the ECU 30 at predetermined intervals. The flowchart of FIG. 4 corresponds to steps 213 to 217 of the flowchart of FIG.
To steps 413 , 414, 415, 416 and 417
2 is different from the flowchart of FIG. Therefore, only the differences will be described here.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0062

[Correction method] Change

[Correction contents]

In step 413, step 213 in FIG.
Like the SCs 5a, the amount of secondary fuel injection once from the storage amount of oxygen OSC of 5b is set, and step 41 4
Now, the setting timing of the current intake stroke fuel injection amount (FIG. 3
Sets the amount was increased by plus the secondary fuel injection amount calculated in step 413 to the intake stroke fuel injection amount after the operation mode is switched in step 41 6 as an intake stroke fuel injection amount when in the ISET). If the timing is not at the intake stroke fuel injection amount setting timing, step 41
5. In step 417 , the secondary fuel injection amount is set. As a result, in the cylinder in time for the intake stroke fuel injection amount set timing (ISET), the operation mode is switched and the fuel injection amount is increased instead of the secondary fuel injection.

[Procedure amendment 3]

[Document name to be amended] Drawing

[Correction target item name] Fig. 4

[Correction method] Change

[Correction contents]

FIG. 4

Continued on the front page F-term (reference) 3G091 AA02 AA12 AA13 AA24 AA28 AB03 AB05 AB06 AB09 BA14 BA21 CA18 CB02 CB03 DA01 DA02 DA06 DB01 DB04 DB06 DB10 DC03 DC06 EA01 EA05 EA06 EA07 EA08 EA18 EA14 FB02 FA00 FC04 GB02W GB04W GB04Y GB06W GB07W GB17X HA08 HA19 3G301 HA01 HA06 HA16 HA18 JA14 JA25 KA08 KA09 KA13 LB04 MA01 MA11 MA19 MA23 MA26 NA01 NA03 NA04 NA06 NA08 NB02 NB06 NB11 NC02 ND02 NE00 NE01 NE13 NE14 PD03 PD07 PDZ PE01Z PF03Z

Claims (6)

[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 purification catalyst having an O ₂ storage function; and an exhaust gas that flows into the exhaust purification catalyst by supplying fuel that does not contribute to combustion to the engine when the engine is switched from a lean air-fuel ratio operation to a stoichiometric air-fuel ratio or a rich air-fuel ratio operation. An exhaust purification device for an internal combustion engine, comprising: storage reducing means for reducing the amount of oxygen stored in the exhaust purification catalyst by making the air-fuel ratio richer than the engine operation air-fuel ratio. 2. A further absorbed and the exhaust passage of the exhaust gas purifying catalyst downstream air-fuel ratio of the exhaust flowing the oxygen concentration in the exhaust gas to absorb flowing the NO _X in the exhaust gas when the lean air-fuel ratio decreases comprising a _the NO _X storage reduction catalyst that releases the NO _X, the storage decreasing means further claims to reduce the amount of oxygen stored in the exhaust purification catalyst when it should emit NO _X absorbed from _the NO _X storage reduction catalyst Item 2. An exhaust gas purification device according to Item 1. 3. The engine according to claim 1, wherein said engine is operating at a lean air-fuel ratio.
The O _X occluding operating air-fuel ratio for a short time the engine when it should be released the absorbed NO _X from the reducing catalyst comprises a means for performing rich spike operation of switching to a rich air-fuel ratio, the exhaust gas purifying by said storage decreasing means when the rich spike operation The exhaust gas purification device for an internal combustion engine according to claim 2, wherein the amount of oxygen stored in the catalyst is reduced. 4. An exhaust purification system for an internal combustion engine that performs a lean air-fuel ratio operation as required, comprising: an exhaust purification catalyst having an O ₂ storage function disposed in an engine exhaust passage; When the air-fuel ratio of the inflowing exhaust gas, which is arranged in the exhaust passage, is lean, the NO
The oxygen concentration in the inflowing exhaust gas to absorb _X is to release and _the NO _X storage reduction catalyst to release the NO _X absorbed and reduced, the NO _X absorbed from the _the NO _X storage reduction catalyst during the lean air-fuel ratio operation of the engine Means for performing a rich spike operation for switching the operating air-fuel ratio of the engine to the rich air-fuel ratio for a short period of time; and setting the exhaust air-fuel ratio flowing into the exhaust purification catalyst for a predetermined period immediately after the start of the rich spike operation to the air during the rich spike operation. An exhaust purification device for an internal combustion engine, comprising: a storage reducing unit that reduces the amount of oxygen stored in the exhaust purification catalyst by making the fuel ratio richer than the fuel ratio. 5. The storage reducing means includes storage estimating means for estimating the amount of oxygen stored in the exhaust gas purification catalyst based on an operating state of the engine, and reducing the amount of oxygen in accordance with the estimated amount of stored oxygen. The exhaust gas purification device for an internal combustion engine according to any one of claims 1 to 4, wherein the operation is performed. 6. The storage estimation unit estimates the amount of oxygen stored in the exhaust purification catalyst based on a deterioration state of the exhaust purification catalyst in addition to an operation state of the engine.
An exhaust gas purifying apparatus for an internal combustion engine according to claim 1.