JP2002097978A - Fuel control device of spark ignition engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure the exhaust purifying performance and the heat resisting reliability of catalyst 32 and 34, preventing deterioration of the driving feel by elaborately controlling the condition of the air/fuel ratio of exhaust when an engine 1 with a three-way catalyst 32 and lean NOx catalyst 34 disposed in an exhaust pathway 28 is moved from a stratified charge combustion region (a) or a uniformed lean region (c) to a λ=1 region (b) between them in controlling the exhaust by changing the region so that the internal cylinder air/fuel ratio is lean in the stratified charge combustion region (a) on the low rotation/ low load side and in the uniformed lean region (c) on the higher rotation side, or so that the internal cylinder air/fuel ratio is nearly the theoretical air/fuel ratio in the λ=1 region (b) between the regions (a) and (c). SOLUTION: When the engine 1 is moved from the stratified charge combustion region (a) to the λ=1 region (b) (SA14), the quantity of fuel injected by injector 12 is increased/corrected to enrich the internal cylinder air/fuel ratio, so that the condition of the air/fuel ratio of exhaust is enriched. When the engine 1 is moved from the uniformed lean region (c) to the λ=1 region (b), the enriching correction mentioned above will not be executed basically.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an exhaust gas passage in which a catalyst is disposed in an exhaust passage, and an average air-fuel ratio (hereinafter, referred to as an in-cylinder air-fuel ratio) in an in-cylinder combustion chamber before ignition is substantially a stoichiometric air-fuel ratio or less. More particularly, the present invention relates to control of the air-fuel ratio state of exhaust gas when the engine shifts from a lean air-fuel ratio operating state to a substantially stoichiometric air-fuel ratio operating state.

[0002]

2. Description of the Related Art Conventionally, as a spark ignition type engine of this type, a so-called three-way catalyst is disposed in an exhaust passage of an engine, as disclosed in Japanese Patent Application Laid-Open No. 11-200853, for example. NO adjacent to the side
A NOx absorption-reduction type catalyst having an x-absorbing material is provided, and the in-cylinder air-fuel ratio is set to lean in a predetermined operation region where the load is not so large, and is set to a stoichiometric air-fuel ratio in other operation regions. There is known one that is controlled to be richer than that.

As the NOx absorbent of the catalyst, alkaline earth metals such as barium are mainly used. This is because the air-fuel ratio state of the exhaust gas is lean, that is, the oxygen concentration in the exhaust gas is, for example, In a 4% or more oxygen-excess atmosphere, NOx in the exhaust gas is oxidized and absorbed as nitrate. On the other hand, when the oxygen concentration decreases, the absorbed nitrate is replaced with CO in the exhaust gas to absorb the CO as a carbonate. While releasing NOx.

[0004] When the engine is operated with the lean air-fuel ratio in the cylinder as described above, the air-fuel ratio of the exhaust gas also becomes lean, and the NOx in the exhaust gas is absorbed by the NOx absorbent and the exhaust gas is exhausted. Is purified. On the other hand, when the in-cylinder air-fuel ratio becomes substantially the stoichiometric air-fuel ratio, the air-fuel ratio state of the exhaust gas becomes a state corresponding to the substantially stoichiometric air-fuel ratio, that is, the oxygen concentration becomes about 0.5 to 1% or less. By the purification function, H
As most of C, CO and NOx are purified, NOx released from the NOx absorbent reacts with HC and CO to be reduced and purified.

[0005] Further, since the NOx absorbing material has a property that its absorption performance decreases as the amount of NOx absorbed increases, the conventional example has a problem that the engine is substantially deviated from the lean operating state of the air-fuel ratio. When the operating state is shifted to the air-fuel ratio, the in-cylinder air-fuel ratio is enriched in a short time and significantly, that is, in a spike-like manner to promote the release of NOx (hereinafter also referred to as NOx purge).

By the way, in addition to the so-called three-way catalyst,
In general, an oxygen absorbing material such as ceria (CeO2) is used for a catalyst that exhibits a three-way purifying function when the air-fuel ratio state of the exhaust gas substantially corresponds to the stoichiometric air-fuel ratio, such as the NOx absorption reduction type catalyst. It is included. This oxygen absorbing material absorbs oxygen in exhaust gas when the oxygen concentration is above a certain level (for example, 0.5%), and when the oxygen concentration falls below that,
It has a property of releasing oxygen, and can appropriately adjust the fluctuation state of the oxygen concentration in the exhaust gas to enhance the three-way purification function of the catalyst.

However, when the three-way catalyst and the NOx absorption-reduction catalyst are arranged in series in the exhaust passage of the engine as in the above-mentioned conventional example, the state of the air-fuel ratio of the exhaust changes from lean to a state substantially corresponding to the stoichiometric air-fuel ratio. When the change occurs, oxygen is released from the oxygen absorbing material contained in the catalyst, and the air-fuel ratio state locally shifts to the lean side near the two catalysts, thereby impairing the three-way purification function of both the catalysts. May be done. Also,
Since HC and CO in the exhaust gas are consumed by the reaction with the released oxygen, there is a possibility that the NOx purging of the NOx absorption reduction catalyst located downstream cannot be performed efficiently.

In this regard, in the above-mentioned conventional example,
Considering the operation history of the engine and the oxygen absorption capacity of the three-way catalyst, etc., the amount of oxygen released from the three-way catalyst when the in-cylinder air-fuel ratio of the engine switches from lean to approximately the stoichiometric air-fuel ratio is estimated. In anticipation of the consumption of HC and CO by the reaction with oxygen, the three-way catalyst and the downstream NOx absorption-reduction type catalyst still exhibit a sufficient three-way purification function, and the NOx absorption-reduction type HC sufficient for the catalyst
The degree of enrichment of the in-cylinder air-fuel ratio is determined so that CO and CO are supplied.

[0009]

Generally, in a spark ignition type engine, the temperature of the exhaust gas becomes highest when the in-cylinder air-fuel ratio is substantially equal to the stoichiometric air-fuel ratio. When the air becomes rich, the heat capacity of the fuel and the latent heat of vaporization with respect to the air cause the temperature of the exhaust to decrease, while when the air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio, the fuel It is known that the heat capacity of the excess air reduces the temperature of the exhaust.

Therefore, in order to suppress a rise in exhaust gas temperature while ensuring a sufficient output when the engine is in an operating region on a high rotation speed or a high load side, the in-cylinder air-fuel ratio is usually set higher than the stoichiometric air-fuel ratio. Control is performed so as to be rich (hereinafter, this area is referred to as an enriched area).
This is the same even when the operation is performed in a lean state of the in-cylinder air-fuel ratio in the low-rotation low-load operation region as in the conventional example (hereinafter, this region is referred to as a low-rotation side lean region). Area).

On the other hand, even when the engine is in a relatively high-speed or high-load operation region, the in-cylinder air-fuel ratio is set to be lower than the stoichiometric air-fuel ratio in a medium load region where the demand for output is not so high, for example. It is considered that if the control is performed so as to achieve a lean state, it is possible to significantly reduce fuel consumption while preventing overheating of the catalyst (hereinafter, a region in which such control is performed is referred to as a high-speed lean region). .

In this case, when looking at the entire operation range of the engine, a low-speed lean operation region is provided on the low-speed low-load side, and adjacent to the high-speed or high-load side thereof,
An operating region where the in-cylinder air-fuel ratio is substantially the stoichiometric air-fuel ratio (hereinafter, λ =
1 region), and a high rotation side lean region is set at least on the high rotation side of the λ = 1 region.

When the engine shifts from the high-speed lean region to the λ = 1 region, oxygen is removed from the catalyst similarly to the shift from the low-speed lean region to the λ = 1 region in the conventional example described above. Will be released,
In order to solve the problem caused by this oxygen release, it is conceivable to temporarily make the cylinder air-fuel ratio significantly richer.

However, since the temperature state of the exhaust gas is considerably higher in the high rotation side lean region than in the low rotation side lean region, the in-cylinder air-fuel ratio is changed when the high rotation side lean region shifts to the λ = 1 region. When the enrichment is performed, CO and HC in the exhaust gas, which increase with the enrichment, react with oxygen released from the catalyst, and the reaction heat may overheat the catalyst, which may cause a decrease in reliability.

Further, the fact that the engine shifts from the high rotation side lean region to the λ = 1 region means that the load state or the rotation speed of the engine becomes low.
This corresponds to, for example, a case where the driver returns the accelerator while the vehicle is running at high speed. Therefore, if the in-cylinder air-fuel ratio is enriched and a torque fluctuation occurs at this time, the driver tends to feel a sense of discomfort.

The present invention has been made in view of the foregoing, and an object of the present invention is to provide an exhaust passage of a spark ignition type engine in which at least an exhaust air-fuel ratio state substantially corresponds to a stoichiometric air-fuel ratio. When the engine is operated while switching between the in-cylinder air-fuel ratio lean state and the approximate stoichiometric air-fuel ratio state while providing a catalyst that exhibits a three-way purification function, the exhaust air-fuel ratio state Paying attention to the fact that oxygen is released from the catalyst when the state changes to a state corresponding to the stoichiometric air-fuel ratio, and by devising control of the air-fuel ratio state of the exhaust at this time, the exhaust purification performance of the catalyst is improved. Another object of the present invention is to prevent a decrease in the driving feel while securing the heat resistance and heat resistance.

[0017]

In order to achieve the above object, according to the present invention, there is provided an engine in which the engine is shifted from a low-rotation side lean region (first setting region) where a catalyst temperature is relatively low.
1 region (second setting region), or when the engine is in an accelerating operation state, or when the catalyst is NOx
When the catalyst is of the absorption type, when it is necessary to perform NOx purging of the catalyst, enrichment correction for correcting at least the air-fuel ratio state of the exhaust gas to be richer than the state corresponding to the stoichiometric air-fuel ratio is performed. On the other hand, the high-rotation side lean region where the engine has a relatively high catalyst temperature (third setting region)
, The shift to the λ = 1 region is not performed.

Specifically, according to the first aspect of the present invention, FIG.
As schematically shown in FIG. 2, at least when the air-fuel ratio state of the exhaust gas substantially corresponds to the stoichiometric air-fuel ratio, the catalyst a that exhibits the three-way purifying function, and the average air before ignition in the combustion chamber b in the cylinder. The in-cylinder air-fuel ratio, which is the fuel ratio, is set to be leaner than the stoichiometric air-fuel ratio when the engine c is in the first setting region on the side of low rotation and low load, and the engine c is higher than the first setting region. In the second setting region on the rotation or high load side or in the acceleration operation state, the stoichiometric air-fuel ratio becomes substantially the same.
A fuel control device for a spark ignition type engine including an air-fuel ratio control means d for switching and controlling is premised. The air-fuel ratio control means d is controlled so that the in-cylinder air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio when the engine c is in the third setting region on the higher rotation or higher load side than the second setting region. A shift determining unit e for determining that the engine c has shifted from one of the first and third setting areas to the second setting area, or that the engine c has shifted to the acceleration operation state. And when the shift determining means e determines that the engine c has shifted from the first setting area to the second setting area or has shifted to the accelerating operation state of the engine c, at least the stoichiometric air-fuel ratio state of the exhaust gas is determined. While the enrichment correction is performed so as to be richer than the state corresponding to, the enrichment correction is not performed when the transition of the engine c from the third setting area to the second setting area is determined. A structure comprising an exhaust air-fuel ratio condition correcting means f.

With the above configuration, first, when the engine c is in the first or third set range, the air-fuel ratio control means d controls the in-cylinder air-fuel ratio of the engine c to be leaner than the stoichiometric air-fuel ratio. As a result, fuel efficiency is reduced. Also,
At this time, the exhaust gas is lean, that is, has a high oxygen concentration, and a part of the oxygen in the exhaust gas is gradually absorbed by the catalyst a.

When the engine c shifts from the first setting range to the second setting range, or when the engine c is in an acceleration operation state, oxygen is released from the catalyst a. The enrichment correction control is performed by the exhaust air-fuel ratio state correction means, and the air-fuel ratio state of the exhaust gas becomes richer than the state corresponding to the stoichiometric air-fuel ratio, that is, the state in which the oxygen concentration is lower. Even if oxygen is released from the catalyst a, the local air-fuel ratio state of the exhaust near the catalyst a can be set to an appropriate state substantially corresponding to the stoichiometric air-fuel ratio. The purification function can be exhibited.

Although the exhaust gas temperature rises with the enrichment correction, the temperature state of the catalyst a is kept relatively low in the first setting region. a does not overheat.

On the other hand, when the engine c is in the third setting area, the temperature state of the exhaust gas is high and the temperature state of the catalyst is relatively high. When the process shifts to, overheating of the catalyst a can be prevented by not performing the exhaust air-fuel ratio enrichment correction, whereby the reliability of the catalyst a can be ensured. In addition, it is possible to avoid the occurrence of torque fluctuation due to the enrichment of the air-fuel ratio and to prevent the driver from feeling uncomfortable.

When the engine c is in the third setting region, the in-cylinder air-fuel ratio is usually controlled to be richer than that in the first setting region. Since the gas flow rate is lower and the rotation speed is higher than the first setting region, the exhaust flow velocity becomes relatively high, and as a result, the amount of oxygen absorbed by the oxygen absorbing material of the catalyst a does not become very large. For this reason, the amount of oxygen released from the catalyst a when the operation state is switched to a substantially stoichiometric air-fuel ratio is reduced, and the influence of this is small.

Next, in the invention of claim 2, as schematically shown in FIG. 1 (b), while the NOx in the exhaust gas in the oxygen-excess atmosphere is absorbed, the absorbed N2 is reduced due to the decrease in the oxygen concentration.
When the in-cylinder air-fuel ratio, which is the average air-fuel ratio before ignition in the in-cylinder combustion chamber b and the NOx absorption type catalyst a that releases Ox, the engine c is in the first setting region on the low rotation speed and low load side. To be leaner than the stoichiometric air-fuel ratio,
In a second setting region at a higher rotation speed or a higher load side than the first setting region, a fuel control device for a spark ignition type engine provided with air-fuel ratio control means d for switching and controlling so as to become substantially the stoichiometric air-fuel ratio is assumed. And And the air-fuel ratio control means d
When the engine c is in the third setting region on the higher rotation or higher load side than the second setting region, the in-cylinder air-fuel ratio is controlled so as to be leaner than the stoichiometric air-fuel ratio. NOx absorption state determination means e for determining that the NOx absorption amount in the catalyst a has become equal to or greater than a predetermined amount.
A shift determining means f for determining that the engine c has shifted from one of the first or third setting area to the second setting area; and the shift determining means f determines whether the engine c has shifted from the first setting area to the second setting area. When the shift to the second setting region is determined, or when the NOx absorption state determination means e determines that the NOx absorption amount is equal to or more than the predetermined amount, at least the air-fuel ratio state of the exhaust corresponds to the stoichiometric air-fuel ratio. While enrichment correction to make it richer than engine c
When the transition from the third setting region to the second setting region is determined, the exhaust air-fuel ratio state correcting means g that does not perform the enrichment correction is provided.

With this configuration, similarly to the first aspect of the present invention, when the engine c is in the first or third set region, the in-cylinder air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio, thereby reducing fuel consumption. NOx in exhaust gas in an oxygen-excess atmosphere
Is gradually absorbed and purified by the NOx absorption type catalyst a. Further, the engine c is moved from the first setting area to the second setting area.
When the process shifts to the set range or when the NOx absorption amount by the catalyst a becomes equal to or more than a predetermined amount, and there is a concern that the NOx absorption capability of the catalyst a is reduced, the exhaust air-fuel ratio state correcting means corrects the enrichment. The control is performed, and the air-fuel ratio state of the exhaust gas is set to a state substantially corresponding to the stoichiometric air-fuel ratio or richer than that, so that the NOx is released from the catalyst a and the reduction purification is performed efficiently. At this time, the exhaust gas temperature rises with the enrichment correction, but in the first setting region, the temperature state of the catalyst a is maintained at a relatively low state. Temperature is NOx
It stays in a moderate temperature range with high purification performance.

On the other hand, when the engine c is in the third setting region, the temperature condition of the catalyst a is relatively high because the temperature condition of the exhaust gas is high. When shifting to the region, the overheating of the catalyst a can be prevented by not performing the exhaust air-fuel ratio enrichment correction, thereby ensuring the reliability of the catalyst a. In addition, it is possible to avoid the occurrence of torque fluctuation due to the enrichment of the air-fuel ratio and to prevent the driver from feeling uncomfortable.

According to the third aspect of the present invention, the exhaust air-fuel ratio state correcting means performs the enrichment correction by increasing the amount of fuel supplied to the in-cylinder combustion chamber so that the in-cylinder air-fuel ratio becomes richer than the stoichiometric air-fuel ratio. Further, when the exhaust air-fuel ratio state correcting means performs the enrichment correction, the exhaust air-fuel ratio state correcting means is provided with an ignition timing correcting means for correcting the ignition timing of the cylinder to the retard side.

In this configuration, the enrichment correction control is performed by the exhaust air-fuel ratio state correction means, and the amount of fuel supplied to the in-cylinder combustion chamber is increased, so that the exhaust air-fuel ratio state is reliably enriched. You. In addition, the increase in engine torque accompanying this is canceled by the correction of the ignition timing to the retard side, and the torque fluctuation is reduced.

According to the fourth aspect of the invention, a fuel injection valve for directly injecting fuel into the in-cylinder combustion chamber, and fuel is injected by the fuel injection valve during the compression stroke of the cylinder when the engine is in the first setting region. On the other hand, in the second and third setting regions, fuel injection control means for injecting fuel during the intake stroke of the cylinder by the fuel injection valve is provided, and further, the exhaust air-fuel ratio state correction means is provided as enrichment correction. It is assumed that fuel is additionally injected by the fuel injection valve in an expansion stroke or an exhaust stroke of a cylinder.

As a result, the enrichment correction control is performed by the exhaust air / fuel ratio state correction means, and additional fuel injection is performed by the fuel injection valve during the cylinder expansion stroke or the exhaust stroke. , The air-fuel ratio state of the exhaust gas can be reliably enriched.

According to the fifth aspect of the present invention, the air-fuel ratio control means includes:
When the engine is in the third setting region and in the accelerating operation state, a correction control unit for correcting the in-cylinder air-fuel ratio to be richer than the stoichiometric air-fuel ratio is provided. By doing so, when the engine is in the acceleration operation state, the in-cylinder air-fuel ratio becomes richer than the stoichiometric air-fuel ratio, and a sufficient output corresponding to the acceleration operation is obtained.

In the invention of claim 6, the catalyst is of a NOx absorption type that absorbs NOx in exhaust gas in an oxygen-excess atmosphere and releases the absorbed NOx by lowering the oxygen concentration. Sulfur poisoning determining means for determining a predetermined sulfur poisoning state in which performance deteriorates is provided. When the determination by the sulfur poisoning determination unit is performed, the air-fuel ratio control unit sets the in-cylinder air-fuel ratio to be richer than the stoichiometric air-fuel ratio so as to promote the desorption of sulfur from the catalyst. A correction control unit for correcting the error is provided.

That is, in general, the NOx absorption type catalyst adsorbs the sulfur component in the exhaust gas by the same mechanism as that for absorbing NOx, and when the adsorption amount of the sulfur component increases to a certain extent, the NOx absorption performance decreases. (Sulfur poisoning). Therefore, in the present invention, a sulfur poisoning determining means for determining that the catalyst is in a predetermined sulfur poisoning state in which the NOx absorption performance is reduced is provided, and when the sulfur poisoning determining means makes a determination, the in-cylinder air-fuel ratio is reduced. By correcting sulfur to be richer than the stoichiometric air-fuel ratio and promoting the desorption of sulfur from the catalyst, even when a NOx absorption type catalyst is used, the exhaust gas purification performance of the catalyst can be stably maintained. Can be.

Next, according to a second solution of the present invention, in an engine provided with a NOx absorption type catalyst in the exhaust passage of the engine, the NOx purification performance by this catalyst changes according to the NOx absorption amount. It is noted that the engine has a remarkable temperature dependency. As in the case of the first solution, the engine is shifted from the low rotation side lean region (first setting region) where the catalyst temperature is relatively low to the λ = 1 region (first setting region). 2 setting region), while performing the enrichment correction of the in-cylinder air-fuel ratio,
When the engine shifts from the high rotation side lean region (third setting region) where the catalyst temperature is relatively high to the λ = 1 region,
The enrichment correction is performed only when the NOx absorption amount in the catalyst is large.

Specifically, according to the invention of claim 6, FIG.
As schematically shown in FIG.
x, and a NOx-absorbing-type catalyst a that releases the absorbed NOx due to a decrease in oxygen concentration, and an in-cylinder air-fuel ratio that is an average air-fuel ratio before ignition in the in-cylinder combustion chamber b, Is higher than the stoichiometric air-fuel ratio when it is in the first setting region on the low rotation and low load side, and the second rotation on the high rotation or high load side from the first setting region.
In the setting region, it is assumed that a fuel control device for a spark ignition type engine including an air-fuel ratio control means d for switching and controlling so as to become substantially the stoichiometric air-fuel ratio. The air-fuel ratio control means d is controlled so that the in-cylinder air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio when the engine c is in the third setting region on the higher rotation or higher load side than the second setting region. NOx absorption state determination means e for determining that the NOx absorption amount in the catalyst a has become equal to or greater than a set amount.
A shift determining means f for determining that the engine c has shifted from one of the first or third setting area to the second setting area; and the shift determining means f determines whether the engine c has shifted from the first setting area to the second setting area. When the transition to the second setting region is determined, at least the air-fuel ratio state of the exhaust gas is enriched so as to be richer than the state corresponding to the stoichiometric air-fuel ratio. An exhaust air-fuel ratio state correction unit g that performs the enrichment correction only when the NOx absorption state determination unit e determines when the shift to the 2 setting region is determined.

With this configuration, similarly to the first aspect of the present invention, when the engine c is in the first or third set region, the in-cylinder air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio, thereby reducing fuel consumption. NOx in exhaust gas in an oxygen-excess atmosphere
Is gradually absorbed and purified by the NOx absorption type catalyst a. Further, the engine c is moved from the first setting area to the second setting area.
When shifting to the set region, the enrichment correction control is performed by the exhaust air-fuel ratio state correction means, and the air-fuel ratio state of the exhaust is made substantially corresponding to the stoichiometric air-fuel ratio or richer than that, so that the catalyst a NOx emission and reduction purification are performed efficiently. At this time, the exhaust gas temperature rises with the enrichment correction, but in the first setting region, the temperature state of the catalyst a is maintained at a relatively low state. Temperature stays in an appropriate temperature range with high NOx purification performance.

On the other hand, when the engine c is in the third setting area, the temperature state of the catalyst a is relatively high because the temperature state of the exhaust gas is high. When the operation shifts to the region, basically, the enrichment correction of the exhaust air-fuel ratio is not performed, so that the temperature state of the catalyst a can be suppressed from further increasing. However, at this time, if it is determined by the NOx absorption state determination means e that the NOx absorption amount in the catalyst a has become equal to or greater than the set amount, there is a concern that the NOx absorption performance may be reduced. Make the correction
By promoting the release of NOx from the catalyst a and reduction purification,
The NOx purification performance of the catalyst a can be ensured.

[0038]

(Embodiment 1) FIG. 2 shows a fuel control apparatus A for a spark ignition engine according to Embodiment 1 of the present invention.
1 is a multi-cylinder engine mounted on a vehicle.
The engine 1 has a cylinder block 3 in which a plurality of cylinders 2, 2,... (Only one is shown) are provided in series, and a cylinder head 4 arranged on the cylinder block 3. A piston 5 is inserted into the cylinder 2 so as to be able to reciprocate up and down in the figure, and a combustion chamber 6 is defined in the cylinder 2 between the top surface of the piston 5 and the bottom surface of the cylinder head 4. On the other hand, a crankshaft 7 is rotatably supported in the cylinder block 3.
And the piston 5 are connected by a connecting rod. An electromagnetic crank angle sensor 8 for detecting a rotation angle of the crank shaft 7 is provided at one end of the crank shaft 7. The electromagnetic crank angle sensor 8 faces a water jacket of the cylinder block 3 and detects a cooling water temperature (engine water temperature). A water temperature sensor 9 for detecting is provided.

The cylinder head 4 for each cylinder 2 has
An ignition plug 11 connected to an ignition circuit 10 is mounted so as to face the upper part of the combustion chamber 6, and an injector 12 is provided at a peripheral portion of the combustion chamber 6 so as to inject and supply fuel directly toward the center of the cylinder. (Fuel injection valve) is installed. That is, although not shown in detail, the combustion chamber 6 is of a pent roof type having a shape like a roof in which two inclined surfaces of a ceiling part are put on each other,
Two intake and exhaust ports 13, 14 are respectively opened on each of the inclined surfaces, and intake and exhaust valves 15, 15,... Are arranged so as to open and close the respective open ends.

The injector 12 is disposed below the two intake ports 13 and 13 so as to be sandwiched between the two intake ports 13 and 13.
5 faces the peripheral portion of the combustion chamber 6 in proximity to the umbrella portion 5. On the other hand, the injector 12 is connected to a high-pressure fuel pump 18 via a fuel supply passage 17 common to all the cylinders 2, 2,..., And an appropriate amount of fuel is supplied by the high-pressure fuel pump 18 and a high-pressure pressure regulator (not shown). The pressure is adjusted and supplied to the injector 12. The fuel supply passage 17 is provided with a fuel pressure sensor 19 for detecting the pressure state (fuel pressure) of the fuel inside.

When fuel is injected by the injector 12 after the middle stage of the compression stroke of the cylinder 2, the fuel spray is trapped in an elliptical cavity 5 a formed on the top surface of the piston 5, and the ignition plug A layer of a relatively rich mixture is formed near 11. On the other hand, when fuel is injected by the injector 12 in the intake stroke of the cylinder 2,
The fuel spray diffuses into the combustion chamber 6 and is sufficiently mixed with the intake air to form a substantially uniform mixture in the combustion chamber 6 by the time of ignition.

As shown in the drawing, one side surface (left side surface in the figure) of the engine 1 has an intake port 13 for each cylinder 2.
The intake passage 20 is connected so as to communicate with each other. The intake passage 20 supplies the intake air filtered by an air cleaner (not shown) to the combustion chamber 6 of the engine 1, and reduces the amount of intake air taken into the engine 1 in order from the upstream side to the downstream side. A hot wire type air flow sensor 21 for detecting, a throttle valve 22 composed of a butterfly valve for restricting the intake passage 20, and a surge tank 23 are provided. The throttle valve 22 is not mechanically connected to an accelerator pedal (not shown), but is of an electric type whose valve shaft is opened and closed by being rotated by an electric motor. Further, a throttle opening sensor 24 for detecting an opening of the throttle valve 22 and an intake pressure sensor 25 for detecting a pressure state of intake air downstream of the throttle valve 22 are provided.

The intake passage 20 downstream of the surge tank 23 is an independent passage branching for each cylinder 2. The downstream end of each independent passage is further branched into two and the intake passages are respectively branched. It communicates with ports 8,8. A swirl control valve 26 is provided on one of the branches, and when the swirl control valve 26 is closed, most of the intake air flows into the combustion chamber 6 from the other branch.
As a result, a strong swirl is generated in the combustion chamber 6. On the other hand, when the swirl control valve 26 is opened, the intake air is sucked from both the branch passages, whereby the tumble component of the intake air increases and the swirl component decreases.

On the other hand, an exhaust passage 2 for discharging burned gas from the combustion chamber 6 is provided on the other side of the engine 1 (the right side in the figure).
8 are connected. The upstream end of the exhaust passage 28
An exhaust manifold 29 branched for each cylinder 2 and communicating with the exhaust port 14 is provided. The downstream end of the exhaust manifold 29 is gathered into one, and a first oxygen for detecting the oxygen concentration in the exhaust is provided at the gathering portion. A density sensor 30 is provided. The first oxygen concentration sensor 30 is a so-called λO2 sensor whose output is inverted in a stepwise manner at the stoichiometric air-fuel ratio. In addition, an exhaust pipe 3 is provided at a collecting portion of the exhaust manifold 29.
1 is connected to the downstream end of the exhaust pipe 31, while a three-way catalyst 32 and a lean NOx catalyst 34 (N
Ox absorption type catalyst), and a second oxygen concentration sensor 33 composed of a λO2 sensor like the first oxygen concentration sensor 30 is disposed in the exhaust passage 28 between the two catalysts 32 and 34. ing.

On the upstream side of the exhaust pipe 31, a part of the exhaust gas flowing through the exhaust passage 28 is recirculated to the intake system.
The upstream end of the GR passage 35 is branched and connected.
The downstream end of the R passage 35 is connected to the intake passage 20 between the throttle valve 22 and the surge tank 23, and in the vicinity thereof, an electric EGR whose opening is adjusted by an electric motor is provided.
A valve 36 is provided.

Although not shown in detail, the upstream three-way catalyst 32 is formed by forming two catalyst layers, an inner catalyst layer and an outer catalyst layer, on the wall surface of a cordierite honeycomb carrier. The inner catalyst layer supports a noble metal such as palladium Pd using alumina and ceria as support materials, while the outer catalyst layer supports platinum and rhodium as ceria support materials. Here, ceria (CeO2) has a valence of cerium atom Ce of 3 or more.
The crystal lattice changes by changing between tetravalent and has a function as an oxygen absorbing material that absorbs or releases oxygen in accordance with the change. Conventionally, fluctuation of oxygen concentration in exhaust gas is reduced. Are used to improve the action of the catalyst.

The downstream lean NOx catalyst 34
Absorbs NOx in the exhaust gas in an oxygen-excess atmosphere where the oxygen concentration in the exhaust gas is high (for example, in a state where the oxygen concentration is 4% or more), and releases the absorbed NOx when the oxygen concentration becomes less than 1-2%, for example. It is of the NOx absorption reduction type that performs reduction and purification. The catalyst 34 also has a two-layer structure similar to the three-way catalyst 32, and platinum and NOx are formed on the inner catalyst layer.
Barium, which is an absorbent, is supported on alumina and ceria as support materials, while platinum, rhodium, and barium are supported on zeolite as a support material in the outer catalyst layer.

The NOx purification performance of the lean NOx catalyst 34 has, for example, a temperature dependency as shown in FIG. That is, the temperature of the lean NOx catalyst 34 is 20
When the temperature is 0 ° C. or lower, the catalyst is inactive and has a low NOx purification performance.
Sufficiently high purification performance is obtained in the range of about 0 to 400 ° C. Then, in an oxygen-excess atmosphere, as shown by the solid line in the figure, when the temperature is 400 ° C. or higher, the NOx purification performance is reduced again. On the other hand, if the exhaust gas is in a state substantially corresponding to the stoichiometric air-fuel ratio, the lean NOx catalyst 3
4 has the same function as the three-way catalyst 32, and exhibits extremely high NOx purification performance in a temperature state of about 250 ° C. or more, as indicated by a broken line in the figure.

With the arrangement of the two catalysts 32 and 34 as described above, when the engine 1 is operated near the stoichiometric air-fuel ratio, the two catalysts 32 and 34 exhibit a three-way purification function,
HC, CO and NOx in the exhaust react and are almost completely purified. On the other hand, when the engine 1 is operated with a lean air-fuel ratio, HC and CO in the exhaust gas are purified by the two catalysts 32 and 34, and the NO in the exhaust gas is reduced.
x is absorbed by the lean NOx catalyst 34 and removed.

The operation of the ignition circuit 10, the injector 12, the motor of the throttle valve 22, the actuator of the swirl control valve 26, and the actuator of the EGR valve 36 are controlled by a control unit 40 (hereinafter referred to as ECU). On the other hand, this ECU 40 has at least
The crank angle sensor 8, water temperature sensor 9, air flow sensor 21, throttle opening sensor 24, intake pressure sensor 2
5 and the output signals of the oxygen concentration sensors 30 and 33, and in addition, an output signal of an accelerator opening sensor 37 for detecting the opening of an accelerator pedal, and an intake temperature sensor (not shown) for detecting an intake air temperature. Each output signal of an atmospheric pressure sensor or the like for detecting the atmospheric pressure is input.

(Outline of Engine Control) The ECU 40
Are control parameters related to the engine output, such as the fuel injection amount and injection timing by the injector 12, the intake air amount adjusted by the throttle valve 22, the intake swirl intensity adjusted by the swirl control valve 26, and the EGR valve 3.
The exhaust gas recirculation ratio and the like adjusted by the control unit 6 are controlled according to the operating state of the engine 1. And
When the engine 1 has been warmed up, the fuel injection mode by the injector 12 is switched according to the operating state thereof,
The engine is operated in either a stratified combustion state or a uniform combustion state.

More specifically, as shown in FIG. 4 which shows an example of a control map after the engine is warmed up, in the entire operation range of the engine 1 defined by the engine load and the engine speed, the low-speed low-load side Is set to a stratified combustion region (a) (first setting region). That is, for example, a net average effective pressure obtained from the output value of the airflow sensor 21 and the engine speed is used as the engine load, the load is up to about half of the full load, and the engine speed is about the allowable maximum speed. If not more than ２, it is determined that the engine 1 is in the stratified combustion region (A).

In the stratified combustion region (a), as shown schematically in FIG. 5 (a), the middle of the compression stroke of the cylinder 2 by the injector 12 after the injector 12, that is, for example, is indicated by an arrow in the drawing.
Fuel is injected at a time during a crank angle period of BTDC 120 ° CA to BTDC 35 ° CA to achieve a stratified combustion state in which the mixture is burned in a stratified state in which the air-fuel mixture is unevenly distributed in the vicinity of the ignition plug 11. Note that, as shown by a virtual line in the figure, a part of the fuel may be injected during the intake or compression stroke before the crank angle period.

On the other hand, the regions (b), (c), and (d) on the higher rotation or higher load side than the stratified combustion region (a) are all uniform combustion regions, as shown in FIGS. As shown in c), the fuel is injected by the injector 12 in the intake stroke of the cylinder 2 and sufficiently mixed with the intake air to form a uniform air-fuel mixture in the combustion chamber 6, and a uniform combustion state is established. Specifically, in the λ = 1 region (b) (second setting region) adjacent to the high load or high rotation side of the stratified combustion region (a), the cylinder is the average air-fuel ratio in the combustion chamber 6 before ignition. In order for the internal air-fuel ratio to be approximately the stoichiometric air-fuel ratio (A / F ≒ 14.7),
The amount of fuel injection by the injector 12 is feedback-corrected based on a signal from the first oxygen concentration sensor 30.

In the uniform lean area (c) (third setting area) adjacent to the high rotation side of the λ = 1 area (b),
The ratio of the fuel injection amount to the intake amount is made relatively smaller than in the λ = 1 region (b), and the in-cylinder air-fuel ratio is set to A / F =
By setting it to about 18 to 20, the fuel efficiency at the time of high-speed operation of the engine 1 is improved. Further, the λ
= 1 region (b) or the enrichment region (d) adjacent to the high rotation or high load side of the uniform lean region (c), the ratio of the fuel injection amount to the intake air amount is relatively smaller than in the λ = 1 region (b). And the in-cylinder air-fuel ratio is set to, for example, A / F = 12 to
By setting it to about 13, a large output corresponding to a high load is obtained.

Further, in addition to the basic control as described above, in this embodiment, the lean NOx catalyst 34
When the engine 1 is in a predetermined sulfur poisoning state in which the Ox absorption performance decreases or the engine 1 is in a predetermined acceleration operation state, even if the engine 1 is in the stratified combustion region (a) or the uniform lean region (c), As described above, the in-cylinder air-fuel ratio is made rich.

The control of the throttle valve 22 is as follows.
Basically, the throttle opening is adjusted based on the accelerator opening and the engine rotation speed so as to obtain the required torque characteristics. Specifically, when the engine 1 is set to the stratified combustion state, In order to reduce the pump loss, the throttle valve 22 is opened relatively widely, and the air-fuel ratio in the cylinder at this time is, for example, A / F = about 30 to 1
It is 40 and extremely lean. When the engine 1 is in a uniform combustion state, the opening of the throttle valve 22 is controlled to be relatively small.

Although not shown, when the engine 1 is in the stratified combustion region (A), or when the engine 1 is on the relatively low rotation and low load side in the λ = 1 region (B), the EGR valve 36 is turned off. When opened, a part of the exhaust gas is recirculated to the intake passage 20 through the EGR passage 35. Further, in a non-warm-up state in which the fuel is difficult to vaporize and atomize, the engine 1 is made to be in a uniform combustion state in all operating regions in order to secure combustion stability.

The operation control of the injector 12 and the throttle valve 22 as described above is performed by the ROM of the ECU 40.
This is realized by the CPU executing a control program stored electronically in the CPU. That is, according to the procedure of the operation control of the injector 12, when the engine 1 is in the stratified combustion region (A), the fuel is injected by the injector 12 in the compression stroke of the cylinder 2, while the λ = 1 region (B) In the uniform lean region (C), the fuel injection control unit 40a that injects fuel in the intake stroke of the cylinder 2 is configured by software.

Further, according to the control procedure of the throttle valve 22, based on the load state and the rotation speed of the engine 1,
Intake air amount control unit 40 for controlling the amount of intake air to cylinder 2
b is configured by software. The fuel injection control unit 40a and the intake air amount control unit 40b determine the in-cylinder air-fuel ratio of the engine 1 when the engine 1 is in the stratified combustion region (a) or the uniform lean region (c). The air-fuel ratio control means is configured to switch and control the air-fuel ratio so that the air-fuel ratio becomes leaner and the stoichiometric air-fuel ratio becomes approximately in the λ = 1 region (b).

(Procedure of Air-fuel Ratio Control) The ECU 40
When the engine 1 is in the λ = 1 region (b), the feedback control of the fuel injection amount by the injector 12 is performed so that the in-cylinder air-fuel ratio of each cylinder 2 becomes substantially the stoichiometric air-fuel ratio. The in-cylinder air-fuel ratio of each cylinder 2 periodically changes to the rich side and the lean side within a predetermined range including the stoichiometric air-fuel ratio. This is because when the three-way catalyst 32 and the lean NOx catalyst 34 have exhaust gas purifying characteristics as shown by a solid line in FIG. 6 and the air-fuel ratio state of the exhaust gas is within a predetermined air-fuel ratio range near the stoichiometric air-fuel ratio. HC and CO in
And NOx are purified simultaneously and extremely effectively.

As described above, the catalysts 32 and 34 of this embodiment contain ceria, which is an oxygen absorbing material, and the engine 1 is in the stratified combustion region (a) on the low-speed low-load side. Time and high rotation side uniform lean area (c)
, Excessive oxygen in the exhaust gas is absorbed by ceria, and the amount of oxygen absorbed by ceria increases. Then, when the engine 1 shifts from the stratified combustion region (a) or the like to the λ = 1 region (b) or the enrichment region (c), oxygen is released from ceria. Even if the fuel ratio is feedback controlled so as to become the stoichiometric air-fuel ratio as described above, and the air-fuel ratio state of the exhaust substantially corresponds to the stoichiometric air-fuel ratio, the vicinity of the catalysts 32 and 34 becomes locally lean. There is a possibility that the three-way purifying function of the catalysts 32 and 34 may not be sufficiently exhibited.

In other words, when the engine 1 shifts from the lean operating range (a) (c) of the in-cylinder air-fuel ratio to the λ = 1 region (b), oxygen is released from the catalysts 32 and 34. Considering this, the relationship between the average air-fuel ratio of the combustion chamber 6 of the engine 1 and the purification rates of HC, CO, and the like at this time is apparently shifted to the rich side as shown by the broken line in FIG. It is. This is because the in-cylinder air-fuel ratio of each cylinder 2 becomes extremely lean in the stratified combustion region (a), and the oxygen concentration in the exhaust gas becomes high. It becomes a problem.

On the other hand, in this embodiment, when the engine 1 shifts from the stratified combustion region (A) to the λ = 1 region (B), the in-cylinder air-fuel ratio of each cylinder 2 becomes the stoichiometric air for a predetermined period thereafter. The fuel injection amount is subjected to feedforward control so as to be richer than the fuel ratio.

Hereinafter, the procedure of the air-fuel ratio switching control by the ECU 40 will be specifically described with reference to the flowcharts of FIGS.

First, in step SA1 after the start shown in FIG. 7, various sensor signals of the crank angle sensor 8, the air flow sensor 21, the accelerator opening sensor 37 and the like are input, and various data are read from the memory of the ECU 40. Subsequently, in step SA2, it is determined whether a sulfur poisoning flag Fs indicating that the lean NOx catalyst 34 is in a predetermined sulfur poisoning state is set, and if the determination is YES, the sulfur poisoning flag Fs is set. If (Fs = 1), the process proceeds to step SA9,
If the determination is NO, the process proceeds to Step SA3.

Subsequently, in step SA3, it is determined whether or not the engine 1 is in the stratified combustion region (A) based on the load state of the engine 1 and the engine speed. If the engine 1 is not in the stratified charge combustion region (A), the process proceeds to step SA14 shown in FIG.
In ES, if the engine 1 is in the stratified combustion region (A), the process proceeds to Step SA4, and the fuel is
Injection is performed in the compression stroke of the cylinder 2 so as to achieve a stratified combustion state (stratified compression stroke injection control). Subsequently, step SA5
In, normal ignition timing control (stratified Ig control) corresponding to the stratified combustion state is performed.

Subsequently, in step SA6, the sulfur poisoning state of the lean NOx catalyst 34 is estimated. This estimation is made based on, for example, the travel distance since the last control for promoting the desorption of the sulfur component from the catalyst 34 and the total amount of fuel consumed during the control, as described later. The amount may be determined. Then, in the next step SA7, by comparing the amount of adsorption of the sulfur component with a preset reference value (set amount), the lean NOx catalyst 34 is in a predetermined poisoning state where the NOx absorption performance is reduced. Determine whether If the determination is YES, the sulfur poisoning flag Fs is set in step SA8, and if the determination is NO, the process returns as it is.

If the sulfur poisoning flag Fs is set based on the result of estimation of the sulfur poisoning state as described above, YES is determined in step SA2 in the next and subsequent control cycles, and the process proceeds to step SA9. In step SA9, fuel is injected in the intake stroke of the cylinder 2 so that the in-cylinder air-fuel ratio of the engine 1 becomes substantially equal to or higher than the stoichiometric air-fuel ratio (λ ≦ 1 intake stroke injection control). Then, in order to suppress the torque fluctuation due to the enrichment of the in-cylinder air-fuel ratio, the ignition timing is corrected to the retard side in step SA10 (Ig retard).

Subsequently, in step SA11, the state of regeneration of the lean NOx catalyst 34 from yellow poisoning due to the enrichment of the air-fuel ratio in all cylinders is estimated. In the next step SA12, sulfur is desorbed from the lean NOx catalyst 34. , N
If it is determined that the state has returned to the state of high Ox absorption performance (regeneration is completed), the process proceeds to Step SA13 and the sulfur poisoning flag Fs
(Fs ← 0), and thereafter returns.

That is, when it is determined that the lean NOx catalyst 34 is in the sulfur poisoning state, the in-cylinder air-fuel ratio is corrected to be rich regardless of the operating state of the engine 1, and the catalyst 34
The desorption of the sulfur component from the sulfur is promoted, whereby the NOx purification performance of the lean NOx catalyst 34 in the exhaust gas can be stably maintained.

On the other hand, when it is determined in step SA3 that the engine 1 is not in the stratified combustion region (A) and the process proceeds to step SA14 shown in FIG. 8, the engine 1 is switched to the stratified combustion region (A) in step SA14. ) Is determined to be in the transition state to the λ = 1 area (b). If the determination is NO, the process proceeds to step SA20. If the determination is YES, the process proceeds to step SA15. In order to correct the in-cylinder air-fuel ratio to be slightly richer than the stoichiometric air-fuel ratio for a predetermined period, the process proceeds to step SA15. Set the transition A / F timer TA / F that measures the lapse of time to perform. Subsequently, in step SA16, a transition Ig timer TIg for measuring the lapse of time during which the Ig retard is performed in order to alleviate the torque fluctuation accompanying the enrichment of the air-fuel ratio in all cylinders is set.

Subsequently, in step SA17, fuel is injected in the intake stroke of the cylinder 2 so that the in-cylinder air-fuel ratio of the engine 1 becomes richer than the stoichiometric air-fuel ratio (λ <1 intake stroke injection control). Subsequently, in step SA18, it is determined whether or not the transition Ig timer is set,
Here, since the timer is set, it is determined to be YES and the process proceeds to step SA19 to correct the ignition timing to the retard side (Ig retard), and then returns to step SA6.

That is, when the engine 1 is in the stratified combustion region (a)
For a while after shifting from to the λ = 1 region (b), the air-fuel ratio state of the exhaust gas is made richer than the state corresponding to the stoichiometric air-fuel ratio by enrichment correction of the in-cylinder air-fuel ratio. As a result, the adverse effects caused by the release of oxygen from the catalysts 32 and 34 are eliminated, and the release of NOx from the lean NOx catalyst 34 and the reduction and purification are promoted.

In step SA14, the engine 1 is switched to the λ = 1 region (b) at step SA20, in which it is determined that the state of the engine 1 is not shifted to the λ = 1 region (b). It is determined whether or not there is, and if the determination is NO, the process proceeds to step SA24, while if the determination is YES, the process proceeds to step SA21 and the above-described transition A / A
Determine whether the F timer is set. If this determination is YES and the timer is being set, it is the period in which the in-cylinder air-fuel ratio is to be enriched and corrected, and therefore, the process proceeds to step SA17.
On the other hand, if the timer count is completed (the determination is N
O), proceeding to step SA22, the injector 12 injects fuel in the intake stroke of each cylinder 2 so that the in-cylinder air-fuel ratio becomes substantially the stoichiometric air-fuel ratio (λ = 1 intake stroke injection control). Subsequently, in step SA23, normal ignition timing control corresponding to a substantially stoichiometric air-fuel ratio uniform combustion state is performed (λ = 1 Ig control), and thereafter, the process proceeds to step SA6.

Further, in step SA20,
In step SA24, in which the engine 1 determines that the engine 1 is not in the λ = 1 area (b) and proceeds, it is determined whether the engine 1 is in the uniform lean area (c). If this determination is YES, in a succeeding step SA25, it is determined whether or not the engine 1 is in an accelerating operation state based on a change state of the accelerator opening and the engine rotation speed. Then, if this determination is NO and the engine 1 is not in the accelerating operation state, in the next step SA26, fuel is injected in the intake stroke of the cylinder 2 so that the in-cylinder air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio (λ > 1 intake stroke injection control), and in the subsequent step SA27, normal ignition timing control corresponding to a uniform combustion state with a lean air-fuel ratio in the cylinder is performed (λ> 1 Ig control), and thereafter, the routine proceeds to step SA6.

On the other hand, when the engine 1 is not in the uniform lean region (c) (NO in step SA24), that is, in the enrich region (d) or in the accelerating operation state (YES in step SA25), the process proceeds to step SA28. Then, fuel is injected in the intake stroke of the cylinder 2 so that the in-cylinder air-fuel ratio becomes richer than the stoichiometric air-fuel ratio (λ <1 intake stroke injection control). In the subsequent step SA29, the air-fuel ratio is rich and uniform. Is performed (λ <1 Ig control), and then the above-described step SA6
Proceed to.

That is, if the engine 1 is not in the accelerating operation state, the cylinder air-fuel ratio is controlled to be substantially the stoichiometric air-fuel ratio or leaner or richer depending on the operation state of the engine 1. When the engine 1 is in the acceleration operation state, even in the uniform lean region (C), by correcting the in-cylinder air-fuel ratio to be rich, a sufficient output corresponding to the acceleration operation can be obtained. I have to.

Step SA4 of the flow shown in FIG. 7 and steps SA22 and SA26 of the flow shown in FIG. 8 are performed when the engine 1 is in the stratified combustion region (a) by the injector 12 in the compression stroke of the cylinder 2. When the engine 1 is in the λ = 1 region (b) or in the uniform lean region (c), the fuel injection control unit 40a injects fuel in the intake stroke of the cylinder 2 when the engine 1 is in the λ = 1 region (b) or the uniform lean region (c).

In steps SA6 to SA8, the sulfur poisoning determining means 4 for determining that the lean NOx catalyst 34 is in a predetermined sulfur poisoning state in which the NOx absorption performance decreases.
0c is configured. Then, in step SA9, even when the engine 1 is in the λ = 1 region (b), in order to promote the desorption of sulfur from the lean NOx catalyst 34 at the time of the determination by the sulfur poisoning determination means 40c, the cylinder interior air is reduced. Correction control unit 40d of the air-fuel ratio control means for enriching the fuel ratio
The correction control unit 40d performs a correction procedure for enriching the in-cylinder air-fuel ratio even when the engine 1 is in the accelerating operation state by a control procedure that proceeds from step SA25 to step SA28. .

Further, a transition judging means 40e for judging that the engine 1 has shifted from the stratified combustion region (A) to the λ = 1 region (B) by step SA14. Then, in steps SA15 to SA17, when the transition determination means 40e determines that the transition from the stratified combustion region (a) to the λ = 1 region (b) of the engine 1 has occurred, at least the stoichiometric air-fuel ratio state of the exhaust gas is determined. The exhaust air-fuel ratio state correction means 40f is configured to increase the fuel injection amount by the injector 12 to make the in-cylinder air-fuel ratio rich so as to be richer than the state corresponding to the exhaust air-fuel ratio state. The correction means 40f is provided for the engine 1
Is shifted from the uniform lean region (c) to the λ = 1 region (b) so that the enrichment correction is not performed.

Further, an ignition timing correcting means 40g for correcting the ignition timing of the cylinder 2 to the retard side when the exhaust air-fuel ratio state correcting means 40d performs the enrichment correction in step SA19.

Therefore, according to the fuel control system A for the spark ignition engine according to this embodiment, first, the in-cylinder air-fuel ratio is extremely high in the wide operating region (a) on the low-speed and low-load side where the engine 1 is commonly used. The lean stratified combustion state and the lean, uniform combustion state with the in-cylinder air-fuel ratio in the uniform lean region (c) on the relatively high speed and high load side significantly reduce the fuel consumption rate. Can be On this occasion,
The exhaust gas is leaner, that is, has a higher oxygen concentration than the state corresponding to the stoichiometric air-fuel ratio. NOx in the exhaust gas is absorbed and removed by the lean NOx catalyst 34, and a part of the oxygen in the exhaust gas is removed. Three-way catalyst 32 and lean NO
The x catalyst 34 absorbs the ceria.

Then, for example, when the engine 1 is in the stratified combustion region (A), the accelerator pedal is depressed by the driver, and as shown by the white arrow in FIG. ) To the λ = 1 region (b), the combustion state of the engine 1 is switched from stratified combustion to uniform combustion, and the fuel injection amount by the injectors 12 of each cylinder 2 is corrected to increase. Thereby, as schematically shown in FIG. 10, the in-cylinder air-fuel ratio is changed from an extremely lean state (for example, A / F = 30 to 140) to a state richer than the stoichiometric air-fuel ratio (for example, A / F = 12). ~
13) (t = t1).

As a result, the air-fuel ratio state of the exhaust gas becomes richer, that is, the oxygen concentration is lower than the state corresponding to the stoichiometric air-fuel ratio. Therefore, even if oxygen is released from the ceria of the catalysts 32 and 34, this catalyst 32 , And 34, the air-fuel ratio state of the local exhaust gas substantially corresponds to the stoichiometric air-fuel ratio or becomes a state richer than the stoichiometric air-fuel ratio. Furthermore, by supplying appropriate amounts of HC and CO to the downstream lean NOx catalyst 34, it is possible to sufficiently promote the release of NOx from the catalyst 34 and the reduction purification.

At this time, HC and CO in the exhaust gas increase due to the correction of the enrichment of the in-cylinder air-fuel ratio as described above.
And CO react with the oxygen released from the catalysts 32 and 34 to increase the temperature state of the catalysts 32 and 34. However, originally in the stratified combustion region (a), the temperature state of the catalysts 32 and 34 is Since the temperature is kept relatively low, even if the temperature rises as described above, the reliability is not impaired due to overheating of the catalysts 32 and 34.

Further, since the ignition timing is corrected to the retard side in addition to the correction of the enrichment of the in-cylinder air-fuel ratio, the increase in the engine torque due to the increase in the fuel injection amount is canceled to reduce the torque fluctuation. Can be eased. In particular, when the driver depresses the accelerator pedal, the driver does not feel uncomfortable due to small torque fluctuation.

On the other hand, when the engine 1 is in the uniform lean region (C), the in-cylinder air-fuel ratio of each cylinder 2 is richer than that in the stratified combustion region (A) (for example, A / F).
= About 20), and at this time, the oxygen concentration in the exhaust becomes relatively low. In addition, since the exhaust flow velocity is relatively high in the uniform lean region (c), the amount of oxygen absorbed by the ceria of the catalysts 32 and 34 is much smaller than that in the stratified combustion region (a). On the other hand, in the uniform lean region (c), the temperature of the exhaust gas is higher and the exhaust gas flow rate is higher than in the stratified combustion region (a), so that the temperatures of the catalysts 32 and 34 are relatively higher.

Therefore, for example, when the engine 1 is in the uniform lean region (C), the driver releases the accelerator pedal, and as shown by the black arrow in FIG. ), In this embodiment, the above-described in-cylinder air-fuel ratio enrichment correction is not performed, and the normal feedback control in the λ = 1 region (b) is immediately started as shown by the broken line in FIG. I have to.

As a result, even when the temperature of the catalysts 32 and 34 is relatively high, the catalysts 32 and 34 are overheated due to the enrichment correction when shifting to the λ = 1 region (b). Can be reliably prevented, and as a result, the three-way catalyst 3
2, while maintaining the reliability of the lean NOx 34, the lean NOx 34 can be maintained in an appropriate temperature state with high NOx purification performance. Further, since the torque does not fluctuate due to the enrichment of the air-fuel ratio, the driver does not feel uncomfortable.

As described above, since the amount of oxygen absorbed by the catalysts 32 and 34 does not increase so much in the uniform lean region (C), the uniform lean region (C) is used.
In the transition from the range to the λ = 1 region (B), almost no trouble occurs even if the in-cylinder air-fuel ratio enrichment correction is not performed.

(Embodiment 2) FIGS. 11 and 12 show the procedure of air-fuel ratio control in a fuel control device A according to Embodiment 2 of the present invention. In this Embodiment 2, as in Embodiment 1 described above. , The engine 1 is shifted from the stratified combustion region (a) by λ
= 1 region (b), the in-cylinder air-fuel ratio is corrected for enrichment. On the other hand, when the engine 1 moves from the uniform lean region (c) to the λ = 1 region (b), the lean NOx catalyst The enrichment correction is performed only when the NOx absorption amount at 34 is large.

The fuel control apparatus A according to the second embodiment
Is the same as that of the first embodiment (see FIG. 2), the same reference numerals are given to the same components as in the first embodiment, and the description thereof will be omitted.

Specifically, in the flow of FIG. 11, in step SB3, it is determined whether or not the engine 1 is in the stratified combustion region (a) as in step SA2 of the first embodiment. If it is in the stratified combustion region (a), in steps SB3 and SB4, as in the first embodiment, the stratified compression stroke injection control and the normal Ig control are performed. Proceeding to SB5, it is determined whether or not the engine 1 is in the uniform lean region (c). If this determination is NO, the process proceeds to step SB11 shown in FIG.
If YES in steps SB6 and SB7, λ> 1 intake stroke injection control and normal Ig control are performed as in the first embodiment.

Then, in step SB8 following step SB4 to step SB7, the lean NOx
An estimation calculation of the NOx absorption amount in the catalyst 34 is performed. This estimation calculation may be performed, for example, by integrating the travel distance of the vehicle and the total injection amount of fuel in the meantime, and based on the integrated value. Alternatively, the operating time of the engine 1 and the total injection amount of fuel during that time are integrated, the integrated value is corrected based on the operating state of the engine 1, and the NOx absorption amount is determined based on the corrected integrated value. May be estimated.

Subsequently, at step SB9, the N
The estimated amount of the Ox absorption amount is compared with a preset reference value (set amount). If the estimated NOx absorption amount is equal to or more than the set amount, a NOx purge flag indicating that the NOx purge is performed when the operating range of the engine 1 is shifted. Set Fnox (Fnox ←
1) Return after a while.

That is, when the engine 1 is operating in one of the lean regions (a) and (c) of the in-cylinder air-fuel ratio, the amount of NOx absorbed by the lean NOx catalyst 34 is estimated, and this estimated value is set. If the amount becomes larger than the amount, the NOx purge flag Fnox is set to perform the NOx purge of the catalyst 34 when the combustion state of the engine 1 is switched.

Then, step SB1 of the flow of FIG.
As shown in 1 to SB16, when it is determined that the engine 1 has shifted from the stratified combustion region (A) to the λ = 1 region (B), the same as steps SA14 to SA19 in the air-fuel ratio control flow of the first embodiment. Then, the in-cylinder air-fuel ratio is enriched. When it is determined in step SB17 that the engine 1 has shifted from the uniform lean region (c) to the λ = 1 region (b), it is determined in subsequent step SB18 whether or not the NOx purge flag Fnox is set. If YES, step SB1 is performed.
The program proceeds to steps SB16 to SB16 to perform the enrichment correction of the in-cylinder air-fuel ratio.

On the other hand, when the engine 1 is in the uniform lean region (C)
Even when the state shifts to λ = 1 region (b) from
If the x purge flag Fnox is not set (NO in step SB18), then, in step SB1,
9 to SB24, the same air-fuel ratio control as in the first embodiment is performed.

That is, in the second embodiment, the engine 1
Shifts from the stratified combustion region (a) to the λ = 1 region (b), while performing the enrichment correction of the in-cylinder air-fuel ratio for a predetermined period, while shifting from the uniform lean region (c) to the λ = 1 region (b) Then, the enrichment correction is performed for a predetermined period only when the NOx purge flag Fnox is set.

Step SB of the flow shown in FIG.
3, SB6 and step SB2 of the flow shown in FIG.
Reference numerals 1 and 23 correspond to the fuel injection control unit 40a. The steps SB8 and SB9 constitute NOx absorption state determination means 40h that determines that the NOx absorption amount in the lean NOx catalyst 34 has become equal to or greater than a set amount.

Further, in steps SB11 and SB17, the transition determination means 40e for determining that the engine 1 has transitioned from either the stratified combustion region (a) or the uniform lean region (c) to the λ = 1 region (b). Is configured.

When it is determined in step SB14 that the engine 1 has shifted from the stratified combustion region (A) to the λ = 1 region (B), the air-fuel ratio of the exhaust gas is changed from the state corresponding to the stoichiometric air-fuel ratio. The fuel injection amount by the injector 12 is increased so that the in-cylinder air-fuel ratio is corrected to be rich so that the engine 1 becomes rich, while the transition from the uniform lean region (c) of the engine 1 to the λ = 1 region (b) is performed. When the determination is made, the exhaust air-fuel ratio state correcting means 40f for performing the enrichment correction is configured only at the time of the determination by the NOx absorption state determining means 40h.

Therefore, according to the exhaust gas purifying apparatus A of the second embodiment, the engine 1 is shifted from the stratified combustion region (a) where the temperature of the catalysts 32 and 34 is low to the λ = 1 region (b).
When the shift to is made, the in-cylinder air-fuel ratio is corrected for enrichment in the same manner as in the first embodiment, thereby ensuring the exhaust purification performance of the catalysts 32 and 34 while maintaining the lean NO on the downstream side.
NOx purging of the x catalyst 34 can be sufficiently promoted.

The engine 1 is moved from the uniform lean region (c) where the temperature of the catalysts 32 and 34 is high to the λ = 1 region (b).
When the routine shifts to, basically, the above-mentioned in-cylinder air-fuel ratio enrichment correction is not performed, and the temperature rise of the catalysts 32 and 34 is suppressed, so that the NOx purification performance particularly by the lean NOx catalyst 34 is ensured. Can be.

At this time, the NOx purge flag Fno
x is set and N by the lean NOx catalyst 34
If there is a concern that the Ox absorption performance may be reduced, then at this time, the enrichment correction is performed and the lean NOx catalyst 34
By performing the Ox purging, it is possible to prevent a decrease in NOx purification performance.

That is, in the second embodiment, the engine 1
Changes from the lean operating state of the in-cylinder air-fuel ratio to the operating state of the substantially stoichiometric air-fuel ratio, the purification performance changes due to the temperature state of the catalysts 32 and 34, and the purification performance decreases with an increase in the NOx absorption amount. Are compared and the correction control of the in-cylinder air-fuel ratio is performed so that the exhaust gas purification performance of the catalysts 32 and 34 can be maintained at a maximum.

(Other Embodiments) The configuration of the present invention is not limited to the first and second embodiments, but also includes other various configurations. That is, in each of the first and second embodiments, the normal fuel injection amount by the injector 12 is increased to enrich the in-cylinder air-fuel ratio of each cylinder 2, thereby enriching the air-fuel ratio state of the exhaust gas. However, the present invention is not limited to this, and the air-fuel ratio state of the exhaust gas may be enriched by additionally injecting fuel by the injector 12 during the expansion stroke or the exhaust stroke of the cylinder 2. In this way, it is possible to reliably enrich the air-fuel ratio state of the exhaust while suppressing fluctuations in the output torque of the engine 1.

In each of the above-described embodiments, the three-way catalyst 32 is disposed upstream of the exhaust passage 28 of the engine 1 and the lean NOx catalyst 34 is disposed downstream thereof. However, the present invention is not limited to this. A lean NOx catalyst may be arranged on the side, and a three-way catalyst may be arranged on the downstream side. Alternatively, only one of the three-way catalyst and the lean NOx catalyst may be provided. Also, lean N
The Ox catalyst 34 is not limited to the NOx absorption-reduction type as in the above embodiment, but may be a NOx having a NOx absorbent.
Any absorption type may be used.

In each of the above embodiments, the exhaust gas purifying apparatus according to the present invention is used as the exhaust gas purifying apparatus A of the direct injection engine 1. However, the present invention is not limited to this. That is,
The present invention provides a so-called port injection type engine in which an injector is arranged so as to inject fuel into an intake port of an engine, by setting a lean burn region in which an air-fuel ratio is lean on a low-speed low-load side of the engine, It is also applicable to those in which lean burn operation is performed in a uniform combustion state at an air-fuel ratio A / F = about 18 to 24.

Further, in each of the above embodiments, the amount of NOx absorbed in the lean NOx catalyst 34 during the operation of the engine 1 is estimated, and when the estimated amount of NOx absorbed becomes equal to or larger than a predetermined amount, regardless of the operating state of the engine 1. Alternatively, the air-fuel ratio state of the exhaust gas may be made rich to promote the NOx purging of the catalyst 34 (hereinafter, referred to as forcible NOx purging). In this case, the predetermined amount of the NOx absorption amount is larger than the set amount described in each of the above embodiments, and the NOx absorption performance of the catalyst 34 is significantly reduced by the increase in the NOx absorption amount. Is the amount that will be.

When the forced NOx purging is performed as described above, in step SA14 of the flow of FIG. 8 in the first embodiment, the engine 1 moves from the stratified combustion region (A) to the λ = 1 region. It is determined whether or not the state is shifted to (b), and the forcible NOx is determined.
It is determined whether or not the purge has been started, and if any of these determination results is YES, the process proceeds to the next step SA15 to enrich the exhaust air-fuel ratio state. Similarly, in the case of the second embodiment, the start of the forced NOx purge may be determined in step SB11 of the flow of FIG.

Further, in step SA14 of the flow of FIG. 8 or step SB11 of the flow of FIG. 12, it is determined that the engine 1 is in the acceleration operation state. The fuel ratio state may be made rich.

[0114]

As described above, according to the fuel control system for a spark ignition type engine according to the first or second aspect of the present invention, the engine is moved from the first set range where the catalyst temperature is relatively low to the second range. At the time of shifting to the set region, when the engine is in an accelerated operation state, or when promoting NOx purging of the catalyst, at least the exhaust air-fuel ratio state correcting means is configured to make at least the exhaust air-fuel ratio state rich. By performing the enrichment correction, even if oxygen is released from the catalyst, the local exhaust air-fuel ratio state near this catalyst is maintained in an appropriate state, and the catalyst has a sufficient three-way purification function. Can be demonstrated. On the other hand, when the engine shifts from the third setting region where the catalyst temperature is relatively high to the λ = 1 region, the overheating of the catalyst is prevented by ensuring that the enrichment correction is not performed, and the reliability is ensured. Thus, it is possible to prevent the driver from feeling uncomfortable by suppressing fluctuations in the engine output.

According to the third aspect of the invention, the increase in the engine torque due to the air-fuel ratio enrichment correction can be canceled by the ignition timing retard correction, and the torque fluctuation can be reduced.

According to the fourth aspect of the present invention, additional fuel injection is performed by the fuel injection valve during the expansion stroke or the exhaust stroke of the cylinder, so that the fluctuation of the output torque of the engine is suppressed and the air-fuel ratio state of the exhaust gas is ensured. Can be enriched.

According to the fifth aspect of the present invention, when the engine is in the acceleration operation state, the in-cylinder air-fuel ratio is corrected by the correction control unit of the air-fuel ratio control means to be rich, so that the engine can sufficiently cope with the acceleration operation. Output can be obtained.

According to the invention of claim 6, when the catalyst is in a predetermined sulfur poisoning state, the in-cylinder air-fuel ratio is corrected by the correction control unit of the air-fuel ratio control means so as to be rich, and the catalyst is controlled. By promoting the desorption of sulfur from sulfur, even when a NOx absorption type catalyst is used, the exhaust gas purification performance of the catalyst can be stably maintained.

Further, according to the fuel control system for a spark ignition engine according to the seventh aspect of the present invention, when the engine is provided with a NOx absorption type catalyst, the engine is shifted from the first setting range where the catalyst temperature is relatively low to the second setting range. When the shift is made to the range, the enrichment correction of the in-cylinder air-fuel ratio is performed so that the catalyst can be efficiently purged of NOx while keeping the temperature state of the catalyst in an appropriate temperature range with high NOx purification performance. it can. On the other hand, when the engine shifts from the third setting range where the catalyst temperature is relatively high to the λ = 1 range, basically, the enrichment correction of the in-cylinder air-fuel ratio is not performed, thereby suppressing the catalyst temperature rise. In addition, when the NOx absorption amount is large and there is a concern that the purification performance is degraded, the enrichment correction is performed to ensure the NOx purification performance of the catalyst.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of the present invention.

FIG. 2 is an overall configuration diagram of an engine fuel control device according to an embodiment of the present invention.

FIG. 3 is a diagram showing a change characteristic of a NOx purification rate with respect to a change in a temperature state of a catalyst in an oxygen-excess atmosphere (solid line) and in a state corresponding to a substantially stoichiometric air-fuel ratio (dashed line).

FIG. 4 is a diagram illustrating an example of a control map in which a combustion region of an engine is set.

FIG. 5 is a diagram schematically showing a form of fuel injection by an injector.

FIG. 6 shows a diagram of HC, HC, and HC using a three-way catalyst and a lean NOx absorption catalyst.
FIG. 4 is a graph showing the purification characteristics of CO and NOx in association with the air-fuel ratio of the combustion chamber of the engine.

FIG. 7 is a flowchart illustrating a first half of an air-fuel ratio control process performed by an ECU;

FIG. 8 is a flowchart showing a latter half of a procedure of air-fuel ratio control by the ECU.

FIG. 9 is an explanatory diagram schematically showing transition of an operation region.

FIG. 10 is a time chart illustrating a change in the in-cylinder air-fuel ratio when the engine shifts from a stratified combustion region or a uniform lean region to a λ = 1 region.

FIG. 11 is a diagram corresponding to FIG. 7 according to the second embodiment.

FIG. 12 is a diagram corresponding to FIG. 8 according to the second embodiment.

[Explanation of symbols]

A fuel control device for spark ignition engine 1 engine 2 cylinder 6 combustion chamber 28 exhaust passage 34 lean NOx catalyst (NOx absorption type catalyst) 40 control unit (ECU) 40a fuel injection control unit (air-fuel ratio control means) 40b intake air Amount control unit (air-fuel ratio control unit) 40c Sulfur poisoning determination unit 40d In-cylinder air-fuel ratio correction control unit (correction control unit of air-fuel ratio control unit) 40e Transition determination unit 40f Exhaust air-fuel ratio state correction unit 40g Ignition timing correction unit 40h NOx absorption state determination means

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02D 41/10 330 F02D 41/10 330A 41/34 41/34 H 43/00 301 43/00 301G 301T F02P 5/15 F02P 5/15 F F term (reference) 3G022 AA07 CA04 DA02 EA02 EA06 EA07 FA06 GA01 GA05 GA06 GA07 GA08 GA09 3G084 AA04 BA13 BA17 CA03 CA04 CA09 DA05 DA10 DA12 DA38 EA11 EB05 EB10 EB12 A03A02 A AA23 AB03 AB06 BA04 BA11 BA14 BA15 BA17 BA19 BA32. JA33 KA09 KA24 KA25 LB04 MA12 MA26 NA03 NA08 NC04 ND03 ND16 NE01 NE06 NE23 PA04Z PA07Z PA10Z PA11Z PB08Z PD09Z PE 03Z PE08Z

Claims (7)

[Claims] 1. A catalyst exhibiting a three-way purifying function at least when an air-fuel ratio state of exhaust gas substantially corresponds to a stoichiometric air-fuel ratio, and a cylinder having an average air-fuel ratio before ignition in an in-cylinder combustion chamber. The internal air-fuel ratio is set so that the engine is leaner than the stoichiometric air-fuel ratio when the engine is in the first setting region on the low-speed low-load side. An air-fuel ratio control means for switching and controlling the air-fuel ratio to be substantially the stoichiometric air-fuel ratio in a second set range or in an acceleration operation state. Controls the in-cylinder air-fuel ratio to be leaner than the stoichiometric air-fuel ratio when the engine is in a third setting region on the higher rotation or higher load side than the second setting region. First or third setting A shift determining means for determining that the engine has shifted to the second setting area from one of the constant areas, or that the engine has shifted to the accelerated operation state; and a second setting from the first setting area of the engine by the shift determining means. When the transition to the region or the transition to the accelerated operation state of the engine is determined, at least the air-fuel ratio state of the exhaust gas is enriched so as to be richer than the state corresponding to the stoichiometric air-fuel ratio. A fuel control device for a spark ignition engine, comprising: an exhaust air-fuel ratio state correction unit that does not perform the enrichment correction when it is determined that the shift from the third setting region to the second setting region is performed. 2. While absorbing NOx in exhaust gas in an oxygen-excess atmosphere, the absorbed NOx is reduced due to a decrease in oxygen concentration.
And a stoichiometric air-fuel ratio when the engine is in a first setting region on the low rotation speed and low load side, which is an average air-fuel ratio before ignition in the in-cylinder combustion chamber. So that the second rotation is higher than the first setting region.
In a fuel control apparatus for a spark ignition type engine, comprising: an air-fuel ratio control unit that switches and controls the air-fuel ratio to be substantially stoichiometric in a setting region; wherein the air-fuel ratio control unit is higher than the second setting region. When the engine is in the third setting region on the rotation or high load side, the in-cylinder air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio, and it is determined that the NOx absorption amount in the catalyst has reached a predetermined amount or more. NOx absorption state judging means for judging; shift judging means for judging that the engine has shifted from one of the first or third setting area to the second setting area; Whether the transition from the setting area to the second setting area has been determined, or
When the x-absorption state determination means determines that the NOx absorption amount has become equal to or greater than the predetermined amount, at least the air-fuel ratio state of the exhaust gas is enriched and corrected to be richer than the state corresponding to the stoichiometric air-fuel ratio. A fuel control device for a spark ignition engine, comprising: an exhaust air-fuel ratio state correction unit that does not perform the enrichment correction when the transition from the third setting region to the second setting region is determined. . 3. The exhaust air-fuel ratio state correction means according to claim 1, wherein the exhaust air-fuel ratio state correction means supplies fuel to the combustion chamber such that the in-cylinder air-fuel ratio becomes richer than the stoichiometric air-fuel ratio. Spark ignition, comprising: ignition timing correction means for correcting the ignition timing of the cylinder to the retard side when the enrichment correction is performed by the exhaust air-fuel ratio state correction means. Fuel control system for a gas engine. 4. The fuel injection valve according to claim 1, wherein the fuel is injected directly into an in-cylinder combustion chamber, and the fuel is injected into the cylinder by the fuel injection valve when the engine is in a first setting region. Fuel injection control means for injecting fuel in the second and third setting regions in the intake stroke of the cylinder by the fuel injection valve, and the exhaust air-fuel ratio state correction means performs the enrichment correction. A fuel control apparatus for a spark ignition engine, wherein fuel is additionally injected by the fuel injection valve during an expansion stroke or an exhaust stroke of a cylinder. 5. The air-fuel ratio control means according to claim 1, wherein the air-fuel ratio in the cylinder is made richer than the stoichiometric air-fuel ratio when the engine is in the third setting region and in the acceleration operation state. A fuel control device for a spark ignition type engine, comprising a correction control unit for performing correction so as to perform the correction. 6. The catalyst according to claim 1, wherein the catalyst absorbs NOx in exhaust gas in an oxygen-excess atmosphere, and releases the absorbed NOx due to a decrease in oxygen concentration. And a sulfur poisoning determining means for determining that the catalyst is in a predetermined sulfur poisoning state in which the NOx absorption performance is reduced. The air-fuel ratio control means includes a sulfur poisoning determining means when the sulfur poisoning determining means makes a determination. Wherein a correction control unit for correcting the in-cylinder air-fuel ratio to be richer than the stoichiometric air-fuel ratio is provided so as to promote the desorption of sulfur from the catalyst. Fuel control device. 7. While absorbing NOx in exhaust gas in an oxygen-excess atmosphere, said absorbed NOx is reduced due to a decrease in oxygen concentration.
And a stoichiometric air-fuel ratio when the engine is in a first setting region on the low rotation speed and low load side, which is an average air-fuel ratio before ignition in the in-cylinder combustion chamber. So that the second rotation is higher than the first setting region.
In a fuel control apparatus for a spark ignition type engine, comprising: an air-fuel ratio control unit that switches and controls the air-fuel ratio to be substantially stoichiometric in a setting region; wherein the air-fuel ratio control unit is higher than the second setting region. When the engine is in the third setting region on the rotation or high load side, the in-cylinder air-fuel ratio is controlled so as to be leaner than the stoichiometric air-fuel ratio, and it is determined that the NOx absorption amount in the catalyst has exceeded the set amount. NOx absorption state judging means for judging; shift judging means for judging that the engine has shifted from one of the first or third setting area to the second setting area; When the transition from the setting region to the second setting region is determined, at least the air-fuel ratio state of the exhaust gas is enriched so as to be richer than the state corresponding to the stoichiometric air-fuel ratio. On the other hand, when the transition from the third setting region to the second setting region of the engine is determined, the exhaust air-fuel ratio state correcting means for performing the enrichment correction only at the time of the determination by the NOx absorption state determining means is provided. A fuel control device for a spark ignition engine.