JP2001003782A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform control of an executing timing of SOx desorption treatment of an occlusion reduction type NOx catalyst to an optimum value. SOLUTION: This control device is provided in the exhaust pipe 16 of a lean combustible internal combustion engine with an occlusion reduction type NOx catalyst 17. In this case, an incoming gas SOx sensor 23 is provided upper stream from the NOx catalyst 17 and an outgoing gas SOx sensor 24 is provided downstream therefrom. Based on incoming gas SOx concentration detected by the incoming gas SOx sensor 23 and outgoing gas SOx concentration detected by the outgoing gas SOx sensor 24, an SOx storage amount of the NOx catalyst 17 is calculated, and when an SOx storage amount exceeds a given value, SOx desorption treatment is executed and when an SOx storage amount is reduced to a value lower than a given value during SOx desorption treatment, SOx desorption treatment is completed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing nitrogen oxides (NO) from exhaust gas discharged from an internal combustion engine capable of lean combustion.
The present invention relates to an exhaust gas purification device capable of purifying x).

[0002]

2. Description of the Related Art As an exhaust gas purifying apparatus for purifying NOx from exhaust gas discharged from an internal combustion engine capable of lean combustion, there is a NOx absorbent represented by a NOx storage reduction catalyst. N
The Ox absorbent has an air-fuel ratio of the inflowing exhaust gas that is lean (ie,
NOx is absorbed during an oxygen-excess atmosphere, and the absorbed NOx is released when the oxygen concentration of the inflowing exhaust gas decreases. The NOx storage reduction catalyst, which is a type of NOx absorber, air-fuel ratio of the exhaust gas is lean (i.e., under oxygen-rich atmosphere) to absorb NOx when the oxygen concentration of the inflowing exhaust gas is the catalyst for reducing the absorbed NOx release and N ₂ when dropped.

When this storage-reduction type NOx catalyst (hereinafter sometimes simply referred to as a catalyst or NOx catalyst) is disposed in an exhaust passage of an internal combustion engine capable of lean combustion, when exhaust gas having a lean air-fuel ratio flows, the exhaust gas contains NOx is absorbed by the catalyst, and NO is absorbed by the catalyst when exhaust gas having a stoichiometric (stoichiometric air-fuel ratio) or rich air-fuel ratio flows.
x is released as NO ₂ , and HC and C
It is reduced to N ₂ by a reducing component such as O, that is, NOx is purified.

In general, the fuel of an internal combustion engine contains sulfur, and when the fuel is burned in the internal combustion engine, the sulfur in the fuel burns and sulfur oxides such as SO ₂ and SO ₃ (SOx ) Occurs. The storage reduction type NOx catalyst includes N 2
Since SOx in the exhaust gas is absorbed by the same mechanism as that for absorbing Ox, NOx is contained in the exhaust passage of the internal combustion engine.
When a catalyst is arranged, this NOx catalyst absorbs not only NOx but also SOx.

However, the sulfur absorbed by the NOx catalyst
Ox forms stable sulfate over time,
It is difficult to be decomposed and released, and tends to accumulate in the catalyst. When the accumulated amount of SOx in the NOx catalyst increases, the N
Since the Ox absorption capacity decreases, the NOx purification rate decreases. This is so-called SOx poisoning. NO of storage reduction type NOx catalyst
In order to maintain the x purification performance high over a long period of time, the SOx desorption process is performed on the NOx catalyst, and the absorbed SOx
Must be desorbed, and the execution time of the SOx desorption process is very important.

[0006]

Here, as one of the methods for determining the execution time of the SOx desorption process, there is an idea that a predetermined amount of SOx is accumulated in the NOx catalyst. In this case, conventionally, as disclosed in the patent publication No. 2,745,985 or the like, the amount of SOx accumulated in the NOx catalyst is determined by the travel distance of the vehicle or the NOx at the inlet and outlet of the NOx catalyst. The estimation is based on the concentration difference, or the temperature difference between the inlet and the outlet of the NOx catalyst. That is, in the related art, the execution time of the SOx desorption process has not been determined based on the direct data on the SOx.

Therefore, the amount of sulfur absorbed by the NOx catalyst
There is a possibility that the Ox accumulation amount is insufficiently grasped and the execution time of the SOx desorption process becomes inappropriate. The present invention has been made in view of such problems of the conventional technology, and an object of the present invention is to solve the SOx desorption based on the SOx concentration of the exhaust gas at the outlet of the NOx storage reduction catalyst. By controlling the treatment, the NOx of the NOx storage reduction catalyst can be
The purpose is to maintain high Ox purification capacity.

[0008]

The present invention has the following features to attain the object mentioned above. The exhaust gas purifying apparatus for an internal combustion engine according to the present invention is provided in the exhaust passage of the internal combustion engine capable of lean combustion, absorbs NOx when the air-fuel ratio of the exhaust gas is lean, and oxygen of the exhaust gas which flows therein. (B) a NOx absorbent that releases NOx absorbed when the concentration is low, and (b) SOx concentration detecting means provided in an exhaust passage downstream of the NOx absorbent to detect the SOx concentration of exhaust gas. Exhaust air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio when desorbing the SOx absorbed by the NOx absorbent, and the NOx detected by the SOx concentration detecting means. The exhaust air-fuel ratio control means is operated based on the SOx concentration downstream of the absorbent.

Since the SOx concentration downstream of the NOx absorbent is detected by the SOx concentration detecting means, the SOx concentration of the NOx absorbent is
x It is possible to accurately grasp the progress of poisoning. Since the exhaust air-fuel ratio control means is operated based on the SOx concentration detected by the SOx concentration detection means, NO
An optimal SOx desorption process can be performed on the x absorbent.

As the internal combustion engine capable of lean combustion in the present invention, a direct-injection lean-burn gasoline engine or a diesel engine can be exemplified. The air-fuel ratio of the exhaust gas refers to the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the NOx absorbent.

When the internal combustion engine is a lean burn gasoline engine, the exhaust air-fuel ratio control means can be executed by means for controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber. When the internal combustion engine is a diesel engine,
The exhaust air-fuel ratio control means controls so-called sub-injection for injecting fuel in an intake stroke, an expansion stroke, or an exhaust stroke;
Alternatively, it can be realized by means for controlling the supply of the reducing agent into the exhaust passage upstream of the NOx absorbent.

As the NOx absorbent, a storage reduction type NOx catalyst can be exemplified. The storage reduction NOx catalyst is
Air-fuel ratio of the inflowing exhaust gas absorbs NOx when the lean, the oxygen concentration in the exhaust gas flowing to release NOx absorbed to decrease a catalyst for reducing the N _2. This storage-reduction NOx catalyst uses, for example, alumina as a carrier, and on the carrier, for example, alkali metals such as potassium K, sodium Na, lithium Li, cesium Cs, alkaline earths such as barium Ba, calcium Ca, and lanthanum La. And at least one selected from rare earths such as yttrium Y and a noble metal such as platinum Pt.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the SOx concentration upstream of the NOx absorbent and the SOx concentration
Based on the concentration difference of the SOx concentration downstream of the NOx absorbent detected by the concentration detecting means, S absorbed by the NOx absorbent is
An SOx accumulation amount calculating means for calculating the Ox amount;
The air-fuel ratio control by the exhaust air-fuel ratio control means can be started when the SOx storage amount calculated by the x-storage amount calculation means reaches a predetermined amount.

When the SOx concentration upstream of the NOx absorbent is higher than the SOx concentration downstream of the NOx absorbent, it can be considered that the difference in the concentration is absorbed by the NOx absorbent. Therefore, the SOx amount (SOx accumulation amount) absorbed by the NOx absorbent can be calculated by multiplying the concentration difference by the exhaust gas amount.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means is increasing, and the SOx concentration downstream of the NOx absorbent is increased. The air-fuel ratio control by the exhaust air-fuel ratio control means can be started when the upstream SOx concentration approaches a predetermined value. NOx
This is because the SOx concentration downstream of the NOx absorbent approaches the SOx concentration upstream of the NOx absorbent as the SOx accumulation amount of the absorbent approaches a saturated state.
The progress of SOx poisoning can be grasped without calculating the Ox accumulation amount.

In this case, the amount of SOx absorbed in the NOx absorbent is determined based on the concentration difference between the SOx concentration upstream of the NOx absorbent and the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means. It is preferable that a SOx accumulation amount calculating means for calculating is provided, and when the SOx accumulation amount calculated by the SOx accumulation amount calculating means is equal to or less than a predetermined amount, the air-fuel ratio control by the exhaust air-fuel ratio control means is preferably prohibited.
This is because even if the exhaust air-fuel ratio control means is operated in a state where the SOx accumulation amount of the NOx absorbent is small, the SOx cannot be efficiently desorbed from the NOx absorbent and the reducing agent is wasted.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the air-fuel ratio control by the exhaust air-fuel ratio control means is started when the SOx concentration downstream of the NOx absorbent approaches the SOx concentration upstream of the NOx absorbent to a predetermined value. In this case, the SOx absorbed by the NOx absorbent is determined based on the concentration difference between the SOx concentration upstream of the NOx absorbent and the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means.
An SOx accumulation amount calculating means for calculating the amount may be provided, and the air-fuel ratio control condition of the exhaust air-fuel ratio control means may be corrected according to the magnitude of the SOx accumulation amount calculated by the SOx accumulation amount calculating means. Optimum S depending on the magnitude of SOx accumulation
This is because there is an Ox desorption condition. Here, the air-fuel ratio control condition is a rich degree of the air-fuel ratio, a rich air-fuel ratio continuation time, or the like.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means is falling, and the SOx concentration downstream of the NOx absorbent is lower than the NOx absorbent. When the upstream SOx concentration approaches a predetermined value, the air-fuel ratio control by the exhaust air-fuel ratio control means can be terminated. N
When SOx is desorbed from the Ox absorbent, even if the exhaust air-fuel ratio is switched from rich to lean before SOx desorption is completely completed, SOx will be released from the NOx absorbent for a while after switching to lean. This is because they are desorbed. As a result, the amount of the reducing agent used for SOx desorption can be reduced.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the SOx concentration upstream of the NOx absorbent can be detected by SOx concentration detecting means provided in an exhaust passage upstream of the NOx absorbent. It is also possible to estimate from the operating state of. By considering the SOx concentration upstream of the NOx absorbent, the progress of SOx poisoning can be grasped more accurately.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to FIGS.

[First Embodiment] FIG. 1 is a diagram showing a schematic configuration in the case where the present invention is applied to a gasoline engine for a vehicle capable of lean combustion. In this figure, reference numeral 1 denotes an engine body, reference numeral 2 denotes a piston, reference numeral 3 denotes a combustion chamber, reference numeral 4 denotes a spark plug, reference numeral 5 denotes an intake valve, reference numeral 6 denotes an intake port, reference numeral 7 denotes an exhaust valve, reference numeral 8 denotes an exhaust port. Are shown respectively.

The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and a fuel injection valve 11 for injecting fuel toward the intake port 6 is attached to each branch pipe 9. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12 and an air flow meter 21, and a throttle valve 14 is arranged in the intake duct 12.

On the other hand, the exhaust port 8 is connected to the exhaust manifold 15.
And a NOx storage reduction catalyst (NOx
The casing 18 is connected to a muffler (not shown) via an exhaust pipe 19. In the following description, the storage reduction type N
The Ox catalyst 17 is abbreviated as the NOx catalyst 17.

An electronic control unit (ECU) 30 for engine control is composed of a digital computer, and is connected to each other by a bidirectional bus 31 such as a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, and a CPU (Central Processor). Unit) 3
4, an input port 35 and an output port 36 are provided. The air flow meter 21 generates an output voltage proportional to the amount of intake air, and the output voltage is input to an input port 35 via an AD converter 38.

The exhaust pipe 16 upstream of the casing 18 has
An incoming gas SOx sensor (SOx concentration detecting means upstream of the NOx absorbent) 23 for generating an output voltage proportional to the SOx concentration of the exhaust gas flowing into the NOx catalyst 17 is provided. An output gas SOx sensor (SOx concentration detection means downstream of the NOx absorbent) 24 that generates an output voltage proportional to the SOx concentration of the exhaust gas flowing out of the NOx catalyst 17 is provided. The output voltages of these SOx sensors 23, 24 are
8 to the input port 35.

A temperature sensor 25 for generating an output voltage proportional to the temperature of the exhaust gas is mounted in the exhaust pipe 19 downstream of the casing 18, and the output voltage of the temperature sensor 25 is used to control the corresponding AD converter 38. Input port 3 through
5 is input. The input port 35 is connected to a rotation speed sensor 26 that generates an output pulse representing the engine rotation speed. The output ports 36 are connected to the ignition plug 4 and the fuel injection valve 11 via corresponding drive circuits 39, respectively.

In this gasoline engine, the fuel injection time TAU is calculated based on, for example, the following equation. TAU = TP · K Here, TP indicates a basic fuel injection time, and K indicates a correction coefficient. The basic fuel injection time TP indicates a fuel injection time required for setting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained in advance by an experiment, and the engine load Q / N
As a function of (intake air amount Q / engine speed N) and engine speed N, the ROM 3 is previously stored in the form of a map as shown in FIG.
2 is stored. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder. If K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, if K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, lean, and K> 1.
When it becomes 0, the air-fuel ratio of the mixture supplied to the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

In the gasoline engine according to the present embodiment, the lean air-fuel ratio control is performed in the low engine load operation region in the low engine load operation region, and the lean air-fuel ratio control is performed in the low engine operation region. During warm-up operation, acceleration, and high-speed constant-speed operation at the time of engine start, the value of the correction coefficient K is set to 1.0 and stoichiometric control is performed. The value is set to a value larger than 1.0 to perform the rich air-fuel ratio control.

In an internal combustion engine, a low-medium load operation is usually performed most frequently. Therefore, during most of the operation period, the value of the correction coefficient K is made smaller than 1.0, and the lean mixture is burned. become.

FIG. 3 schematically shows the concentration of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from this figure, the concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes richer. The concentration of oxygen O _{2 in} the discharged exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NOx contained in casing 18
The catalyst (storage-reduction type NOx catalyst) 17 uses, for example, alumina as a carrier, and on the carrier, for example, an alkali metal such as potassium K, sodium Na, lithium Li, and cesium Cs, and an alkaline earth such as barium Ba and calcium Ca. , Lanthanum La, at least one selected from rare earths such as yttrium Y, and a noble metal such as platinum Pt.

When the NOx catalyst 17 is disposed in the exhaust passage of the engine, the NOx catalyst 17 absorbs NOx when the air-fuel ratio of the inflowing exhaust gas (hereinafter also referred to as the exhaust air-fuel ratio) is lean, and the NOx catalyst 17 When the oxygen concentration in the gas decreases, the NOx absorption / release operation is performed to release the absorbed NOx. Here, the exhaust air-fuel ratio refers to the engine intake passage and N
The ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the Ox catalyst 17.

When no fuel (hydrocarbon) or air is supplied into the exhaust passage upstream of the NOx catalyst 17, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3. Therefore, in this case, the NOx catalyst 1
Numeral 7 absorbs NOx when the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean, and releases the absorbed NOx when the oxygen concentration in the mixture supplied to the combustion chamber 3 decreases. .

It is considered that the NOx absorption / release operation of the NOx catalyst 17 is performed by the mechanism shown in FIG. In the following, this mechanism will be described by using platinum P
The case where t and barium Ba are supported will be described as an example, but other noble metals, alkali metals, alkaline earths,
The same mechanism is obtained even when rare earth elements are used.

First, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), oxygen O ₂ adheres to the surface of platinum Pt in the form of O ₂ ⁻ or O ²⁻ . On the other hand, NO contained in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ).

Next, a part of the generated NO ₂ is absorbed in the NOx catalyst 17 while being oxidized on the platinum Pt and combined with the barium oxide BaO, and as shown in FIG. NO ₃ ^- diffuses into the NOx catalyst 17 in the form of. In this way, NOx is absorbed in the NOx catalyst 17.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is generated on the surface of the platinum Pt,
As long as the absorption capacity is not saturated, NO ₂ is absorbed in the NOx catalyst 17 and nitrate ions NO ₃ ⁻ are generated.

On the other hand, the oxygen concentration in the inflowing exhaust gas
Decreases and NO_TwoThe reaction goes in the reverse direction when the amount of
(NO_Three ^-→ NO_Two), And nitrate ion in the NOx catalyst 17
NO _Three ^-Is NO_TwoAlternatively, the NOx catalyst 17 is discharged in the form of NO.
Will be issued. That is, the oxygen concentration in the inflow exhaust gas decreases.
Then, NOx is released from the NOx catalyst 17.
As shown in FIG. 3, the degree of lean of the incoming exhaust gas
The oxygen concentration in the incoming exhaust gas decreased
If the degree of leanness of the inflow exhaust gas is reduced, NO
NOx is released from the x catalyst 17.

On the other hand, at this time, if the mixture supplied to the combustion chamber 3 has a stoichiometric or rich air-fuel ratio, a large amount of unburned HC and CO is discharged from the engine as shown in FIG. retardant HC, CO is oxygen on the platinum Pt O ₂ ^-
Or, it is oxidized by reacting with O ²⁻ .

When the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, the oxygen concentration in the inflowing exhaust gas is extremely reduced, so that NO ₂ or NO is released from the NOx catalyst 17, and this NO ₂ or NO As shown in FIG. 4 (B), it reacts with unburned HC and CO to be reduced to N ₂
Becomes

[0041] That is, HC in the inflowing exhaust gas, CO, first oxygen O ₂ on the platinum Pt ^- or O ^2- immediately be reacted with oxidized, then the platinum Pt on the oxygen O ₂ ^- or O ^2- If HC and CO still remain after consumption, this H
C, NOx in the released NOx and the inflow exhaust gas is made to reduction to N ₂ from the NOx catalyst 17 by the CO.

In this way, NO _{2 is} deposited on the surface of platinum Pt.
Alternatively, when NO is no longer present, NO ₂ or NO is released from the NOx catalyst 17 one after another, and is further reduced to N ₂ . Therefore, when the exhaust air-fuel ratio is set to the stoichiometric air-fuel ratio or rich, NOx is released from the NOx catalyst 17 within a short time.

As described above, when the exhaust air-fuel ratio becomes lean, NOx is absorbed by the NOx catalyst 17, and when the exhaust air-fuel ratio is made stoichiometric or rich, NOx is released from the NOx catalyst 17 in a short time, and N ₂ Is reduced to Therefore, emission of NOx into the atmosphere can be prevented.

By the way, in this embodiment, as described above, the mixture supplied to the combustion chamber 3 is made rich at the time of full load operation, and the mixture is made the stoichiometric air-fuel ratio at the time of high load operation or the like. During low-medium load operation, the air-fuel mixture becomes lean, so that NOx in the exhaust gas becomes NOx during low-medium load operation.
The NOx is absorbed by the catalyst 17, and is released and reduced from the NOx catalyst 17 during full load operation and high load operation. However, if the frequency of full load operation or high load operation is low, and the frequency of low / medium load operation is high and the operation time is long, the release and reduction of NOx cannot be made in time, and the NOx absorption capacity of the NOx catalyst 17 becomes saturated. NOx
Can not be absorbed.

Therefore, in this embodiment, when the lean air-fuel mixture is being burned, that is, when the medium-to-low load operation is being performed, the stoichiometric or rich stoichiometric or rich operation is performed in a relatively short cycle. The air-fuel ratio of the air-fuel mixture is controlled so that the air-fuel mixture is burned, and NOx is released and reduced in a short cycle. As described above, the exhaust air-fuel ratio (in this embodiment, the air-fuel ratio of the air-fuel mixture) is used to absorb and release NOx.
Is alternately repeated in a relatively short cycle with "lean" and "spike-like stoichiometric air-fuel ratio or rich air-fuel ratio (rich spike)" is referred to as lean-rich spike control. In this application,
The lean-rich spike control is included in the lean air-fuel ratio control.

On the other hand, the fuel contains sulfur (S), and when the sulfur in the fuel burns, sulfur oxides (SOx) such as SO ₂ and SO ₃ are generated, and the NOx catalyst 17 These SOx also absorb. It is considered that the SOx absorption mechanism of the NOx catalyst 17 is the same as the NOx absorption mechanism. That is, assuming that platinum Pt and barium Ba are supported on the carrier in the same manner as when the NOx absorption mechanism is described, as described above, when the exhaust air-fuel ratio is lean, oxygen O ₂ is reduced. O ₂ ^- or O ^2-
The SOx (for example, SO ₂ ) in the inflowing exhaust gas becomes platinum Pt in the form of
Is oxidized to SO ₃ on the surface.

Thereafter, the generated SO ₃ is further oxidized on the surface of the platinum Pt, is absorbed in the NOx catalyst 17 and combines with the barium oxide BaO, and enters the NOx catalyst 17 in the form of sulfate ion SO ₄ ^2−. diffused to form the sulfate BaSO _4. Since this BaSO ₄ crystal tends to be coarse and relatively stable, it is difficult to decompose and desorb once formed.
When the amount of BaSO ₄ generated in the NOx catalyst 17 increases, the amount of BaO that can participate in the absorption of the NOx catalyst 17 decreases, and the NOx absorption capacity decreases. This is SOx poisoning. Therefore, the NOx of the NOx catalyst 17
In order to maintain a high absorption capacity, it is necessary to execute a SOx desorption process for desorbing the SOx absorbed by the NOx catalyst 17 at an appropriate timing.

In order to desorb SOx from the NOx catalyst 17, it is necessary to set the air-fuel ratio of the inflowing exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio. We know that it is easy to release.

In this embodiment, even when the air-fuel ratio of the exhaust gas is set to the stoichiometric air-fuel ratio or the rich air-fuel ratio for the SOx desorption process, the ECU 30 controls the amount of fuel injected from the fuel injection valve 11. This is performed by controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 to a stoichiometric air-fuel ratio or a rich air-fuel ratio. Therefore, the ECU 30 and the fuel injection valve 11 constitute exhaust air-fuel ratio control means.

FIG. 5 shows a case where the lean-rich spike control is performed for the NOx absorption / reduction / reduction processing,
An example of a change over time of the exhaust air-fuel ratio, the SOx accumulation amount of the NOx catalyst 17, and the SOx concentration upstream and downstream of the NOx catalyst 17 when performing stoichiometric or rich air-fuel ratio control for Ox desorption processing is shown. I have. Note that, in this figure, the exhaust air-fuel ratio at the time of NOx absorption / reduction / reduction processing is shown in a lean manner without rich spikes.
The rich display in the exhaust air-fuel ratio during the SOx desorption process is a concept including the stoichiometric air-fuel ratio. Hereinafter, referring to FIG.
The change with time of the x accumulation amount and the SOx concentration will be described.

(1) t1 to t2 When the exhaust gas having a lean air-fuel ratio flows through the NOx catalyst 17 when the SOx accumulation amount of the NOx catalyst 17 is small, the NOx catalyst 17 absorbs the SOx in the exhaust gas. The SOx accumulation amount of the catalyst 17 increases with time. Further, while SOx in the exhaust gas is being absorbed by the NOx catalyst 17,
The SOx concentration of the exhaust gas downstream of the NOx catalyst 17 (hereinafter, referred to as catalyst exit gas) is lower than the SOx concentration of the exhaust gas upstream of the NOx catalyst 17 (hereinafter, referred to as catalyst entrance gas).

(2) t2 to t3 When the SOx storage capacity of the NOx catalyst 17 increases and the SOx absorption capacity decreases, the SOx concentration of the catalyst outgas gradually approaches the SOx concentration of the catalyst inlet gas. . This means that the SOx passing through without being absorbed by the NOx catalyst 17 gradually increases, and as a result, the SOx of the NOx catalyst 17 increases.
The degree of increase in the accumulation amount becomes slow.

(3) t3 to t4 At time t3, when high-temperature / rich air-fuel ratio control (constant air-fuel ratio) is started to desorb SOx from the NOx catalyst 17, SOx desorbed from the NOx catalyst 17 is converted to NOx catalyst 17
Of the NOx catalyst 17, the SOx concentration of the catalyst outgas becomes higher than the SOx concentration of the catalyst inflow gas.
The Ox accumulation amount decreases with time. SOx of catalyst output gas
The concentration reaches a peak at a predetermined time after the start of the high temperature / rich air-fuel ratio control, and thereafter gradually decreases.

(4) t4 to t5 However, the desorption of SOx from the NOx catalyst 17 continues for a while after the high-temperature / rich air-fuel ratio control is terminated at t4 and the control is shifted to the lean / rich spike control.
Although the reason why SOx desorption continues even after the end of the high temperature / rich air-fuel ratio control is not clear, it is clear from many experimental results that this phenomenon has reproducibility. And t5
In this case, when SOx is not desorbed from the NOx catalyst 17, the SOx concentration of the catalyst outgas and the SOx concentration of the catalyst incoming gas become equal, and at this time, the SOx accumulation amount of the NOx catalyst 17 becomes minimum.

(5) t5 to t6 After t5, SOx is again absorbed by the NOx catalyst 17, and the SOx concentration of the catalyst output gas gradually becomes smaller than the SOx concentration of the catalyst input gas. At t6, the SOx concentration of the catalyst output gas equilibrates.

That is, the point C where the SOx concentration of the catalyst exit gas coincides with the SOx concentration of the catalyst entrance gas is determined by the S
This is a switching point to the Ox absorption state, and when the NOx catalyst 17 is adsorbing SOx in the exhaust gas, the SOx concentration of the catalyst output gas becomes smaller than the SOx concentration of the catalyst input gas, and the SOx is released from the NOx catalyst 17. When the catalyst is desorbed, the SOx concentration of the catalyst exit gas becomes larger than the SOx concentration of the catalyst entrance gas.

Then, during a period (t0 to t3) in which the SOx concentration of the catalyst exit gas is smaller than the SOx concentration of the catalyst input gas, the SOx amount absorbed by the NOx catalyst 17 is calculated by multiplying the SOx concentration difference by the exhaust gas amount. Calculated and S
The period during which the Ox concentration is higher than the SOx concentration of the gas entering the catalyst (t
In 3 to t5), when the SOx concentration difference is multiplied by the exhaust gas amount, the SOx amount desorbed from the NOx catalyst 17 is calculated.

Therefore, in the first embodiment, t0
, The SOx amount absorbed by the NOx catalyst 17 is calculated, and the accumulated SOx amount is calculated to calculate the SOx accumulation amount. When the calculated SOx accumulation amount reaches a predetermined upper limit value,
The rich air-fuel ratio control is started. After the start of the high-temperature / rich air-fuel ratio control, the amount of SOx desorbed from the NOx catalyst 17 is calculated, and the SOx amount is sequentially subtracted from the SOx accumulation amount.
The Ox accumulation amount is calculated, and when the SOx accumulation amount becomes equal to or less than a predetermined lower limit, the high temperature / rich air-fuel ratio control is terminated.

By doing so, the SOx of the NOx catalyst 17
The accumulated amount can be accurately grasped, SOx desorption can be started at an optimal time, and supply of exhaust gas with a rich air-fuel ratio can be ended at an optimal time. As a result, the NOx purifying ability of the NOx catalyst 17 can be kept high for a long period of time, and the deterioration of fuel efficiency due to the SOx desorption can be reduced.

Next, an SOx desorption control execution routine according to the first embodiment will be described with reference to FIG. A flowchart comprising the steps constituting this control routine is stored in the ROM 32 of the ECU 30, and this control routine is executed by the CPU 34 at regular intervals.

<Step 101> First, the ECU 30
In step 101, the SOx concentration of the catalyst input gas detected by the input gas SOx sensor 23 (hereinafter, sometimes abbreviated as the input gas SOx concentration) is read, and the SOx concentration of the catalyst output gas detected by the output gas SOx sensor 24 is read. Concentration (hereinafter,
Outgoing gas SOx concentration).

<Step 102> Next, the ECU 30
Proceeding to step 102, the incoming gas SOx concentration
It is determined whether the concentration is higher than the Ox concentration. Step 102
The affirmative judgment in the above means that the NOx catalyst 17 is absorbing SOx, and the negative judgment means that the NOx catalyst 17 is desorbing SOx.

<Step 103> If an affirmative determination is made in step 102, the ECU 30 proceeds to step 103 and adds the SOx accumulation amount. In detail, Step 1
This routine is executed this time, by subtracting the outgoing gas SOx concentration from the incoming gas SOx concentration read in 01 to obtain the SOx concentration difference, and reading the current intake air amount detected by the air flow meter 21 as the exhaust gas amount. After that, the amount of SOx absorbed by the NOx catalyst 17 is calculated between the next execution and the next time, and the absorbed SOx amount is added by the SOx counter to obtain the current SOx accumulation amount.

<Step 104> Next, the ECU 30
Proceeding to step 104, it is determined whether the SOx accumulation amount has exceeded a preset upper limit value. If a negative determination is made in step 104, it is not yet time to execute the SOx desorption process, and the process proceeds to return.

<Step 105> If an affirmative determination is made in step 104, the ECU 30 proceeds to step 105 and sets the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or the rich air-fuel ratio in order to desorb SOx from the NOx catalyst 17. Start rich air-fuel ratio control to be controlled. During the execution of the rich air-fuel ratio control, the NOx catalyst 1
7, the catalyst bed temperature of the NOx catalyst 17 is controlled to a temperature optimum for SOx desorption.

<Step 106> The execution of the rich air-fuel ratio control and the temperature rise control causes SOx to be desorbed from the NOx catalyst 17, and the outgassing SOx concentration becomes higher than the incoming gas SOx concentration. , And a negative determination is made in step 102, the ECU 30 proceeds to step 106, and decrements the SOx accumulation amount.

More specifically, the input gas SOx concentration is subtracted from the output gas SOx concentration read in step 101 to obtain SOx
The concentration difference is obtained, and the current intake air amount detected by the air flow meter 21 is read, and this is taken as the exhaust gas amount.
The amount of SOx desorbed from the NOx catalyst 17 between this execution and the next execution of this routine is calculated, and the desorbed SOx amount is subtracted by the SOx counter, and the current SOx amount is subtracted.
x Determine the amount of accumulation.

<Step 107> Next, the ECU 30
Proceeding to step 107, it is determined whether the SOx accumulation amount is equal to or less than a preset lower limit value. If a negative determination is made in step 107, the process proceeds to return because the SOx has not been sufficiently desorbed from the NOx catalyst 17, and the rich air-fuel ratio control and the temperature increase control are continued.

<Step 108> If an affirmative determination is made in step 107, the SOx has been sufficiently desorbed from the NOx catalyst 17, so the ECU 30 proceeds to step 108 and ends the rich air-fuel ratio control and the temperature increase control.

In the first embodiment, the ECU 3
In the series of signal processing based on 0, the part that executes step 103 is the part of the upstream and downstream Sx of the NOx catalyst (NOx absorbent).
This can be referred to as SOx accumulation amount calculation means for calculating the amount of SOx absorbed by the NOx catalyst (NOx absorbent) based on the Ox concentration difference.

[Second Embodiment] In the above-described first embodiment, the SOx accumulation amount of the NOx catalyst 17 is calculated from the difference between the upstream and downstream SOx concentrations of the NOx catalyst 17, and the calculated S
The start time and the end time of the high-temperature / rich air-fuel ratio control for the SOx desorption process were determined based on the Ox accumulation amount.
In the second embodiment, without calculating the SOx accumulation amount, N
The start time and the end time of the high temperature / rich air-fuel ratio control are determined based on the comparison value of the SOx concentration upstream and downstream of the Ox catalyst 17.

As described above, when the SOx accumulation amount of the NOx catalyst 17 increases and approaches the saturated state, the SOx of the catalyst output gas becomes smaller.
The x concentration approaches the SOx concentration of the gas entering the catalyst (between t2 and t3 in FIG. 5). Therefore, depending on how close the outgassing SOx concentration approaches the incoming gas SOx concentration, the NOx
The degree of SOx accumulation in the catalyst 17 can be grasped. Therefore, in the second embodiment, the ratio between the outgassing SOx concentration and the incoming gas SOx concentration is a predetermined ratio (for example, 1: 2).
(At A in FIG. 5), the high-temperature / rich air-fuel ratio control is started.

When SOx is desorbed from the NOx catalyst 17, even if the exhaust air-fuel ratio is switched from rich to lean before the SOx is completely desorbed, NOx is maintained for a while after switching to lean. SOx is desorbed from the catalyst 17 (between t4 and t5 in FIG. 5). Therefore, when the rich air-fuel ratio control is being executed, the outgassing S
Before the Ox concentration matches the incoming gas SOx concentration, the outgassing SO
The time when the x concentration approaches the input gas SOx concentration to a predetermined value can be set as the end time of the high temperature / rich air-fuel ratio control, and by doing so, the amount of the reducing agent used can be reduced. Thus, in the second embodiment, when the ratio between the outgassing SOx concentration and the incoming gas SOx concentration reaches a predetermined ratio (for example, 2: 1) (part B in FIG. 5), the high-temperature / rich air The fuel ratio control is ended.

By doing so, the SOx of the NOx catalyst 17
The degree of accumulation can be accurately grasped, and SOx desorption can be started at an optimal time. Further, the supply of the exhaust gas having the rich air-fuel ratio can be terminated at an optimal time. As a result, the NOx purifying ability of the NOx catalyst 17 can be kept high for a long period of time, and the deterioration of fuel efficiency due to the SOx desorption can be reduced.

Next, an SOx desorption control execution routine in the second embodiment will be described with reference to FIG. A flowchart comprising the steps constituting this control routine is stored in the ROM 32 of the ECU 30, and this control routine is executed by the CPU 34 at regular intervals.

<Step 201> First, the ECU 30
In step 201, the SOx concentration of the catalyst input gas detected by the input gas SOx sensor 23 is read, and the output gas S
The SOx concentration of the catalyst output gas detected by the Ox sensor 24 is read.

<Step 202> Next, the ECU 30
Proceeding to step 202, the input gas SOx concentration
It is determined whether the concentration is higher than the Ox concentration. Step 202
The affirmative judgment in the above means that the NOx catalyst 17 is absorbing SOx, and the negative judgment means that the NOx catalyst 17 is desorbing SOx.

<Step 203> If an affirmative determination is made in step 202, the ECU 30 proceeds to step 203 and determines whether or not the output gas SOx concentration is increasing. As shown in FIG. 5, when the NOx catalyst 17
Immediately after switching to the absorption state, the output gas SOx concentration decreases (t0 to t1), and as the SOx accumulation amount of the NOx catalyst 17 approaches saturation, the output gas SOx concentration increases (t2 to t3). In step 203, the NOx catalyst 17
Is determined in which of these states. Step 20
If a negative determination is made in 3, the ECU 30 proceeds to return because it is not yet the start time of the SOx desorption process.

<Step 204> If an affirmative determination is made in step 203, the ECU 30 proceeds to step 20.
Then, the process proceeds to Step 4 to calculate the concentration ratio α between the outgassing SOx concentration and the incoming gas SOx concentration. α = (outgas SOx concentration) / (incoming gas SOx concentration)

<Step 205> Next, the ECU 30
Proceeding to step 205, it is determined whether or not the density ratio α calculated in step 204 is larger than an upper limit value (for example, 0.5). If a negative determination is made in step 205, it is not yet time to execute the SOx desorption process, so E
The CU 30 proceeds to return.

<Step 206> If an affirmative determination is made in step 205, the ECU 30 proceeds to step 206 and sets the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or the rich air-fuel ratio in order to desorb SOx from the NOx catalyst 17. Start rich air-fuel ratio control to be controlled. During the execution of the rich air-fuel ratio control, the NOx catalyst 1
7, the catalyst bed temperature of the NOx catalyst 17 is controlled to a temperature optimum for SOx desorption.

<Step 207> Since the execution of the rich air-fuel ratio control and the temperature rise control causes SOx to be desorbed from the NOx catalyst 17, and the outgassing SOx concentration becomes higher than the incoming gas SOx concentration, the next time this routine is executed , A negative determination is made in step 202, and the ECU 30 proceeds to step 207.

In step 207, the ECU 30
It is determined whether the output gas SOx concentration is falling. As shown in FIG. 5, for a while after the start of the rich air-fuel ratio control, the outgassing SOx concentration increases, and eventually reaches a peak value.
Thereafter, the output gas SOx concentration decreases (from t3 to t3).
Four). In step 207, it is determined which state the NOx catalyst 17 is in.

If a negative determination is made in step 207, the rich air-fuel ratio control should not be terminated yet, so the ECU 30 proceeds to return and continues the rich air-fuel ratio control and the temperature increase control.

<Step 208> If an affirmative determination is made in step 207, the ECU 30 proceeds to step 20.
Proceeding to 8, the concentration ratio α between the outgassing SOx concentration and the incoming gas SOx concentration is calculated. α = (outgas SOx concentration) / (incoming gas SOx concentration)

<Step 209> Next, the ECU 30
Proceeding to step 209, it is determined whether the density ratio α calculated in step 208 is smaller than a lower limit (for example, 2). If a negative determination is made in step 209,
Since the rich air-fuel ratio control should not be terminated, the ECU 3
The value 0 advances to the return, and the rich air-fuel ratio control and the temperature increase control are continued.

<Step 210> If an affirmative determination is made in step 209, the ECU 30 proceeds to step 210 and ends the rich air-fuel ratio control and the temperature increase control for the SOx desorption process.

[Third Embodiment] In the above-described second embodiment, the start timing and the start of the high temperature / rich air-fuel ratio control are determined based on the concentration ratio of the SOx concentration upstream and downstream of the NOx catalyst 17. However, when the SOx concentration of the exhaust gas flowing into the NOx catalyst 17 is low, the SOx accumulation amount of the NOx catalyst 17 may be small even if the SOx concentration ratio satisfies a predetermined condition. Thus, the SOx of the NOx catalyst 17
Even if the high temperature / rich air-fuel ratio control is executed in a state where the accumulated amount is small, SOx is not efficiently desorbed from the NOx catalyst 17,
The reducing agent is wasted.

Therefore, in the third embodiment, NO
If the SOx accumulation amount of the x catalyst 17 has not reached the predetermined amount, the execution of the SOx desorption process is prohibited, and the SOx accumulation amount is equal to or more than the predetermined amount, and the SOx upstream and downstream of the NOx catalyst 17
The SOx desorption process is executed only when the Ox concentration ratio satisfies a predetermined condition.

Next, an SOx desorption control execution routine according to the third embodiment will be described with reference to FIG. A flowchart comprising the steps constituting this control routine is stored in the ROM 32 of the ECU 30, and this control routine is executed by the CPU 34 at regular intervals.

<Steps 301 to 302> Step 30
Steps 1 and 302 are the same as steps 201 and 202 in the second embodiment, respectively, and a description thereof will be omitted.

If the determination in step 302 is affirmative,
The ECU 30 proceeds to step 303 and adds the SOx accumulation amount. More specifically, the SOx concentration difference is obtained by subtracting the outgoing gas SOx concentration from the incoming gas SOx concentration read in step 101, while the current intake air amount detected by the air flow meter 21 is read and this is used as the exhaust gas amount. N between this execution and the next execution of this routine
The amount of SOx absorbed by the Ox catalyst 17 is calculated, and this absorption S
The Ox amount is added by the SOx counter, and the current SOx
Obtain the accumulated amount.

<Step 304> Next, the ECU 30
Proceeding to step 304, the SOx accumulation amount is set to S
It is determined whether or not the Ox desorption execution lower limit is exceeded. If a negative determination is made in step 304, it is not yet time to execute the SOx desorption process, and the process proceeds to return.

<Steps 305 to 311> If an affirmative determination is made in step 304, the ECU 30 proceeds to step 305. Step 305 to step 31
1 is the same as step 204 to step 210 in the second embodiment, and thus the description is omitted.

It should be noted that the process corresponding to step 203 of the control routine of the second embodiment is not included in the control routine of the third embodiment. When it is more than
The period during which the output gas SOx concentration falls (from t0 to t in FIG. 5)
This is because 1) has already passed.

<Step 312> After ending the rich air-fuel ratio control and the temperature raising control in step 311, the ECU 30 proceeds to step 312, resets the SOx counter, and ends this routine.

In the third embodiment, the ECU 3
In the series of signal processing based on 0, the part that executes step 303 is the part of the upstream and downstream Sx of the NOx catalyst (NOx absorbent).
This can be referred to as SOx accumulation amount calculation means for calculating the amount of SOx absorbed by the NOx catalyst (NOx absorbent) based on the Ox concentration difference.

[Other Embodiments] In the above-described second and third embodiments, the determination of the end time of the rich air-fuel ratio control and the temperature increase control is made by determining the ratio α between the output gas SOx concentration and the input gas SOx concentration.
Is determined based on whether or not a predetermined condition (α <2) is satisfied. Alternatively, the end is determined based on whether an elapsed time from the start of the rich air-fuel ratio control has reached a predetermined time. The timing may be determined.

When the end time of the rich air-fuel ratio control is determined based on the elapsed time as described above, the SOx accumulation amount before the start of the SOx desorption process is calculated, and the SOx accumulation amount is calculated according to the magnitude of the SOx accumulation amount. It is also possible to correct the SOx desorption processing conditions (air-fuel ratio control conditions of the exhaust air-fuel ratio control means) such as the degree of richness and the rich air-fuel ratio continuation time to execute the rich air-fuel ratio control for the SOx desorption processing. It is possible.

In each of the above embodiments, the NOx catalyst 17
The incoming gas SOx sensor 23 is provided upstream of
Although the SOx concentration of the exhaust gas flowing into the NOx catalyst 17 is detected by the Ox sensor 23, the SOx concentration of the exhaust gas flowing into the NOx catalyst 17 depends on the fuel amount and the exhaust gas amount. Injection amount, air-fuel ratio, intake air amount, engine speed, etc.). Therefore, instead of providing the input gas SOx sensor 23, the ECU 30 may calculate and estimate the SOx concentration of the catalyst input gas from the engine operating state.

In each of the embodiments described above, the present invention is applied to a gasoline engine. However, it is needless to say that the present invention can be applied to a diesel engine. In the case of a diesel engine, the combustion in the combustion chamber is performed in a much leaner region than the stoichiometric region, so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 17 is very lean under a normal engine operating condition, and NOx and Although SOx is absorbed, NOx and SOx are hardly released.

In the case of a gasoline engine, the air-fuel ratio of the exhaust gas is made stoichiometric or rich by making the air-fuel mixture supplied to the combustion chamber 3 stoichiometric or rich, as described above, thereby reducing the oxygen concentration in the exhaust gas. And N
Although NOx and SOx absorbed in the Ox catalyst 17 can be released, in the case of a diesel engine, if the air-fuel mixture supplied to the combustion chamber is made stoichiometric or rich, soot is generated during combustion. And cannot be adopted.

Therefore, when the present invention is applied to a diesel engine, in order to make the exhaust air-fuel ratio stoichiometric or rich, separately from burning fuel to obtain engine output, a reducing agent (for example, fuel) is used. (Light oil) must be supplied to the exhaust gas. The supply of the reducing agent to the exhaust gas can be performed by sub-injecting the fuel into the cylinder during the intake stroke, the expansion stroke, or the exhaust stroke, or the reducing agent can be supplied into the exhaust passage upstream of the NOx catalyst 17. It is also possible by supplying.

In the case where a diesel engine is provided with an exhaust gas recirculation device (a so-called EGR device),
By introducing a large amount of exhaust gas recirculation gas into the combustion chamber, it is possible to make the air-fuel ratio of the exhaust gas a stoichiometric air-fuel ratio or a rich air-fuel ratio.

[0105]

According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, (a) a NOx absorbent provided in an exhaust passage of an internal combustion engine capable of lean combustion; and (b) a downstream portion of the NOx absorbent. SOx concentration detecting means provided in the exhaust passage;
Exhaust air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio when desorbing the SOx absorbed by the NOx absorbent, and detecting the SOx concentration by the SOx concentration detecting means. By operating the exhaust air-fuel ratio control means based on the concentration of SOx downstream of the NOx absorbent, the progress of SOx poisoning of the NOx absorbent can be accurately grasped. An optimum SOx desorption process can be executed.

When the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means is falling, and the SOx concentration downstream of the NOx absorbent approaches the SOx concentration upstream of the NOx absorbent to a predetermined value, the exhaust air space is reduced. When the air-fuel ratio control by the fuel ratio control means is terminated, the amount of the reducing agent used for SOx desorption can be reduced, and as a result, S
Fuel economy deterioration due to the Ox desorption process can be reduced.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention.
It is a schematic structure figure of an embodiment.

FIG. 2 is a diagram showing an example of a map of a basic fuel injection time.

FIG. 3 Unburned HC in exhaust gas discharged from the engine,
FIG. 3 is a diagram schematically showing the concentrations of CO and oxygen.

FIG. 4 is a diagram for explaining the NOx absorbing / releasing action of a NOx storage reduction catalyst.

FIG. 5 is a graph showing changes over time in the exhaust air-fuel ratio, the amount of SOx accumulated in the NOx catalyst, and the concentration of SOx upstream and downstream of the NOx catalyst.

FIG. 6 is a SOx desorption control execution routine in the first embodiment.

FIG. 7 shows a second embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention.
9 is a SOx desorption control execution routine in the embodiment.

FIG. 8 shows a third embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention.
9 is a SOx desorption control execution routine in the embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Engine main body (internal combustion engine) 3 Combustion chamber 4 Spark plug 11 Fuel injection valve (exhaust air-fuel ratio control means) 16 Exhaust pipe (exhaust passage) 17 Storage-reduction type NOx catalyst (NOx absorbent) 18 Casing 19 Exhaust pipe (exhaust passage) 23) Incoming gas SOx sensor (SOx concentration detecting means upstream of NOx absorbent) 24 Outgoing gas SOx sensor (SOx concentration detecting means downstream of NOx absorbent) 30 ECU (exhaust air-fuel ratio controlling means)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) F01N 3/28 301 F01N 3/28 301C F term (reference) 3G091 AA12 AA18 AA24 AB05 BA14 CB02 EA01 EA33 FB07 GB02W GB03W GB05W GB06W GB10X HA36 3G301 HA02 HA04 HA15 JA25 LB04 MA01 MA11 NE13 PA18Z PD01Z PE01Z

Claims (8)

[Claims] (1) The exhaust gas is provided in an exhaust passage of an internal combustion engine capable of lean combustion and absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and absorbs when the oxygen concentration of the inflowing exhaust gas is low. A NOx absorbent that releases NOx, and (b)
(C) SOx concentration detecting means provided in the exhaust passage downstream of the NOx absorbent and detecting the SOx concentration of the exhaust gas;
Exhaust air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio when desorbing the SOx absorbed by the NOx absorbent, wherein the exhaust air-fuel ratio is detected by the SOx concentration detecting means. NOx
An exhaust gas purifying apparatus for an internal combustion engine, wherein the exhaust air-fuel ratio control means is operated based on the concentration of SOx downstream of the absorbent. 2. An SOx concentration upstream of the NOx absorbent and a SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means.
An SOx storage amount calculating means for calculating an SOx amount absorbed by the NOx absorbent based on a concentration difference between the concentrations; and when the SOx storage amount calculated by the SOx storage amount calculating means reaches a predetermined amount. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein air-fuel ratio control by said exhaust air-fuel ratio control means is started. 3. The method according to claim 1, wherein the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means is increasing.
The air-fuel ratio control by the exhaust air-fuel ratio control means is started when the SOx concentration downstream of the Ox absorbent approaches the SOx concentration upstream of the NOx absorbent to a predetermined value.
An exhaust gas purifying apparatus for an internal combustion engine according to claim 1. 4. An SOx concentration upstream of the NOx absorbent and a SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means.
SOx accumulation amount calculating means for calculating the amount of SOx absorbed in the NOx absorbent based on the concentration difference between the concentrations. When the SOx accumulation amount calculated by the SOx accumulation amount calculating means is equal to or less than a predetermined amount, The exhaust gas purifying apparatus for an internal combustion engine according to claim 3, wherein the air-fuel ratio control by the exhaust air-fuel ratio control means is prohibited. 5. The SOx concentration upstream of the NOx absorbent and the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means.
SOx accumulation amount calculating means for calculating the SOx amount absorbed by the NOx absorbent based on the concentration difference of the concentration, and the SOx accumulation amount calculated by the SOx accumulation amount calculating means according to the magnitude of the SOx accumulation amount. 4. The exhaust gas purifying apparatus for an internal combustion engine according to claim 3, wherein an air-fuel ratio control condition of the exhaust air-fuel ratio control means is corrected. 6. The method according to claim 6, wherein the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means is decreasing.
The air-fuel ratio control by the exhaust air-fuel ratio control means is terminated when the SOx concentration downstream of the Ox absorbent approaches a predetermined value to the SOx concentration upstream of the NOx absorbent.
4. An exhaust gas purifying apparatus for an internal combustion engine according to claim 3. 7. The SOx concentration upstream of the NOx absorbent is N
The exhaust gas purifying apparatus for an internal combustion engine according to any one of claims 2 to 6, wherein the exhaust gas is detected by SOx concentration detecting means provided in an exhaust passage upstream of the Ox absorbent. 8. The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the SOx concentration upstream of the NOx absorbent is estimated from an operation state of the internal combustion engine.