JP3514152B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to nitrogen oxide (NO) from exhaust gas discharged from an internal combustion engine capable of lean combustion.
x) is related to an exhaust gas purification device.

[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 burn, there is a NOx absorbent represented by a storage reduction type NOx catalyst. N
The Ox absorbent has a lean air-fuel ratio (that is,
It absorbs NOx in the oxygen excess atmosphere) and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is reduced. The NOx storage-reduction type catalyst, which is a kind of this NOx absorbent, It is a catalyst that absorbs NOx when the air-fuel ratio of the exhaust gas is lean (that is, in an oxygen-excess atmosphere), releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas decreases, and reduces it to N ₂ .

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

By the way, generally, the fuel of an internal combustion engine contains a sulfur content, and when the fuel is burned in the internal combustion engine, the sulfur content in the fuel is burned and sulfur oxides (SOx) such as SO ₂ and SO ₃ are burned. ) Occurs. The storage reduction type NOx catalyst is N
Since SOx in the exhaust gas is absorbed by the same mechanism that absorbs Ox, when this NOx catalyst is arranged in the exhaust passage of the internal combustion engine, not only NOx but also SOx is absorbed by this NOx catalyst.

However, the S absorbed by the NOx catalyst is
Ox forms a stable sulfate with the passage of time,
Under the same conditions as when NOx is released and reduced from the NOx catalyst, it is difficult to decompose and release, and tends to accumulate in the catalyst. When the amount of SOx accumulated in the NOx catalyst increases,
Since the NOx absorption capacity of the catalyst decreases, NOx in the exhaust gas cannot be sufficiently removed, and the NOx purification efficiency decreases. This is the so-called SOx poisoning.

Therefore, in order to maintain the NOx purifying ability of the NOx storage reduction catalyst high over a long period of time, an SOx absorbent that mainly absorbs SOx in the exhaust gas is arranged upstream of the NOx catalyst. An exhaust emission control device has been developed that prevents SOx poisoning by preventing SOx from flowing into the catalyst.

The SOx absorbent absorbs SOx when the air-fuel ratio of the inflowing gas is lean, and releases the absorbed SOx as SO ₂ when the oxygen concentration of the inflowing gas is low. Since the SOx absorption capacity of the agent is also limited, it is necessary to perform a process of releasing SOx from the SOx absorbent, that is, a regeneration process before the SOx absorbent is saturated with SOx.

Regarding the technology for regenerating SOx absorbent,
For example, it is disclosed in the patent publication of Japanese Patent No. 2605580. According to this publication, in order to release the SOx absorbed by the SOx absorbent, it is necessary to reduce the oxygen concentration by making the air-fuel ratio of the inflowing exhaust gas stoichiometric or rich, and the temperature of the SOx absorbent is high. It is said that SOx is more easily released.

Although the SOx concentration of the regenerated exhaust of the SOx absorbent naturally becomes high, but since this regenerated exhaust is stoichiometric or rich at high temperature, even if the regenerated exhaust is passed through the NOx catalyst, the SOx in the regenerated exhaust becomes the NOx catalyst. It is difficult to be absorbed and will be discharged as it is.

In the regeneration treatment technology disclosed in the above publication, a bypass passage is provided which branches from the exhaust pipe connecting the SOx absorbent and the NOx catalyst and bypasses the NOx catalyst, and exhaust gas is bypassed with the NOx catalyst. An exhaust switching valve is provided to selectively switch the passage to the SOx
During the regeneration process for releasing SOx from the absorbent, the exhaust switching valve causes the exhaust gas to flow to the bypass passage so that it does not flow to the NOx catalyst, and when the regeneration process is not being performed, the exhaust gas is exhausted by the exhaust switching valve. By allowing the NOx catalyst to flow so that the bypass passage does not flow, the NOx catalyst is reliably prevented from absorbing SOx.

[0011]

Conventionally, the regeneration timing of the SOx absorbent is such that the SOx amount discharged from the internal combustion engine is integrated and the integrated value reaches a set value, or the operating time of the internal combustion engine is changed. In any case, whether or not the regeneration time has been reached is determined by integrating the operating state of the internal combustion engine, such as when the set time has been reached.

However, since the SOx absorbent is always exposed to the heat of the exhaust gas, thermal deterioration occurs over time, and as the thermal deterioration progresses, the SOx absorbing capacity of the SOx absorbent decreases. Therefore, when the thermal deterioration progresses, it is determined that the regeneration time has not been reached even though the SOx absorbent should actually be regenerated,
There was a possibility that the regeneration of the SOx absorbent would be delayed.

The applicant has confirmed from the following events that SOx is likely to be released from the SOx absorbent when the thermal deterioration of the SOx absorbent progresses. When the SOx absorbent has hardly deteriorated due to heat, the air-fuel ratio of the exhaust gas is stoichiometric or rich for a short time (in a spike manner) in order to release NOx from the NOx catalyst (hereinafter referred to as rich spike). Even if the exhaust gas flows to the SOx absorbent at this time, the SOx is not released from the SOx absorbent because it is extremely short. However, when the thermal deterioration of the SOx absorbent progresses, SOx is released from the SOx absorbent even when the exhaust gas of the rich spike flows into the SOx absorbent even for a short time. As a result, SOx flows into the NOx catalyst and becomes SOx.
There was a risk of poisoning.

The present invention has been made in view of the above problems of the prior art. The problem to be solved by the present invention is to regenerate the SOx absorbent at an appropriate time,
The purpose is to prevent SOx poisoning of the NOx adsorbent.

[0015]

The present invention adopts the following means in order to solve the above problems. The present invention is (a)
An SOx absorbent that is arranged in the exhaust passage of an internal combustion engine capable of lean burn, absorbs SOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed SOx when the oxygen concentration of the inflowing exhaust gas is low, (B) NOx is absorbed when the air-fuel ratio of the inflowing exhaust gas, which is arranged in the exhaust passage downstream of the SOx absorbent, is lean, and the absorbed NOx is released when the oxygen concentration of the inflowing exhaust gas is low. NOx absorbent, and (c) absorption of NOx in the NOx absorbent
Exhaust air-fuel ratio control means for controlling the exhaust gas air-fuel ratio to control the emission, and (d) Regeneration means for reducing the oxygen concentration of the exhaust gas so as to release SOx from the SOx absorbent and regenerate the SOx absorbent. And (e) detecting the SOx concentration of the exhaust gas downstream of the SOx absorbent when the air-fuel ratio of the exhaust gas is stoichiometrically or richly controlled by the exhaust air-fuel ratio control means. SOx concentration detecting means, (f) deterioration determining means for determining the state of deterioration of the SOx absorbent based on the SOx concentration of the exhaust gas detected by the SOx concentration detecting means, and (g) the deterioration determining means. Regeneration frequency changing means for changing the regeneration frequency of the SOx absorbent by the regeneration means according to the determined state of deterioration of the SOx absorbent.

In the exhaust gas purifying apparatus for an internal combustion engine of the present invention having the above-described structure, NOx in the exhaust gas is Nx when the exhaust air-fuel ratio control means controls the air-fuel ratio of the exhaust gas to be lean.
The NOx absorbed by the NOx absorbent is released when the oxygen concentration of the exhaust gas is reduced by being absorbed by the Ox absorbent and the exhaust air-fuel ratio control means controlling the air-fuel ratio of the exhaust gas to stoichiometric or rich. To be done. Then, when the exhaust air-fuel ratio control means controls the air-fuel ratio of the exhaust gas to stoichiometric or rich, the SOx concentration detection means detects the SOx concentration of the exhaust gas downstream of the SOx absorbent, and the detected SOx concentration Based on this, the deterioration determining means determines the deterioration state (deterioration degree) of the SOx absorbent. Further, the regeneration frequency changing means changes the regeneration frequency of the SOx absorbent according to the determination result of the deterioration determining means, and the regeneration means changes the SOx absorbent at the changed regeneration frequency.
Regenerate the absorbent. As a result, even if the SOx absorption capacity of the SOx absorbent is reduced due to deterioration, the SOx absorbent will be regenerated at an appropriate time and the NOx absorption capacity will be reduced.
It is possible to prevent the absorbent from being poisoned by SOx.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, examples of the lean burn internal combustion engine include a direct injection type lean burn gasoline engine and a diesel engine.

In the present invention, the air-fuel ratio of exhaust gas is
The ratio of air and fuel (hydrocarbons) supplied into the exhaust passage upstream of the engine intake passage and the SOx absorbent.

In an exhaust gas purification apparatus for an internal combustion engine according to the present invention
The SOx absorbent is on a carrier made of alumina.
Like copper Cu, iron Fe, manganese Mn, nickel Ni
Transition metals, sodium Na, titanium Ti and Lithium
Example of carrying at least one selected from Mu Li
Can be shown. Also, SOx is sulfate ion SO_Four ^2-
In order to facilitate absorption in the SOx absorbent in the form of
Platinum Pt, Palladium Pd, Rhodium on the absorbent carrier
It is preferable to carry one of the Rh.

In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the NOx absorbent may be a NOx storage reduction catalyst. The NOx storage reduction catalyst absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean,
N absorbed when the oxygen concentration in the inflowing exhaust gas decreases
It is a catalyst that releases Ox and reduces it to N ₂ . This occlusion reduction type NOx catalyst uses, for example, alumina as a carrier, and potassium K, sodium Na, lithium L
i, alkali metal such as cesium Cs, barium B
a, alkaline earth such as calcium Ca, lanthanum L
Examples thereof include those in which at least one selected from rare earths such as a and yttrium Y and a noble metal such as platinum Pt are supported.

In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, when the internal combustion engine is a gasoline engine, the exhaust air-fuel ratio control means and the regeneration means are means for controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber. It is feasible. Further, when the internal combustion engine is a diesel engine, the exhaust air-fuel ratio control means and the regeneration means have a function of controlling so-called auxiliary injection that injects fuel in the intake stroke, the expansion stroke, or the exhaust stroke, or more than SOx absorbent. It can be realized by means of controlling the supply of the reducing agent into the upstream exhaust passage.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the deterioration determining means determines the deterioration state of the SOx absorbent. The higher the SOx concentration detected by the SOx concentration detecting means, the more the deterioration of the SOx absorbent progresses. Determine that
Since the amount of SOx released from the SOx absorbent also relates to the temperature of the SOx absorbent, it is preferable that the temperature of the SOx absorbent does not cause a determination error. For that purpose, for example, the SOx concentration, which is a criterion for determining the deterioration state of the SOx absorbent, may be set in advance according to the temperature of the SOx absorbent. Alternatively, the SOx concentration, which is a criterion for determining the deterioration state when the SOx absorbent is at a predetermined reference temperature, is set in advance, and the SOx concentration detected by the SOx concentration detecting means is detected.
The deterioration state may be determined after correcting the concentration to the SOx concentration at the reference temperature.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the change of the regeneration frequency of the SOx absorbent by the regeneration frequency changing means is, for example, a SOx which serves as a criterion for determining whether or not the SOx absorbent has reached the regeneration time. This can be achieved by changing the absorption capacity and engine operating time. The regeneration frequency changing means changes the regeneration frequency of the SOx absorbent to increase as the deterioration of the SOx absorbent increases.

In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the deterioration determination means determines the deterioration of the SOx absorbent by
It is preferable to carry out at a time immediately after the completion of the regeneration of the SOx absorbent. This is for the following reason. Even if the deterioration states of the SOx absorbent are the same, the larger the amount of SOx absorbed by the SOx absorbent, the easier the SOx is released. Therefore, the determination accuracy is higher when the deterioration determination of the SOx absorbent is executed when the SOx amount absorbed by the SOx absorbent is about the same, and the SOx amount absorbed immediately after the regeneration of the SOx absorbent is performed. Since SOx should be less likely to be released because it is extremely small, it is possible to more strictly determine the deterioration by executing the deterioration determination of the SOx absorbent at a time closer to this.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the exhaust passage is branched from between the SOx absorbent and the NOx absorbent to bypass the NOx absorbent and to pass the exhaust gas. Exhaust flow switching means is provided for selectively flowing either the NOx catalyst or the bypass passage, and SO
x During regeneration of the absorbent, exhaust gas is caused to flow through the bypass passage, and S
It is also possible to let the exhaust gas flow through the NOx absorbent when the Ox absorbent is not being regenerated. By doing so, it is possible to prevent the regeneration exhaust of the SOx absorbent from flowing into the NOx absorbent. However, the present invention can be realized without providing the bypass passage and the exhaust flow switching means in this way.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described below with reference to the drawings of FIGS.

FIG. 1 is a diagram showing a schematic configuration when the present invention is applied to a gasoline engine for a vehicle capable of lean combustion. In this figure, reference numeral 1 is an engine body, reference numeral 2 is a piston, reference numeral 3 is a combustion chamber, reference numeral 4 is an ignition plug, reference numeral 5 is an intake valve, reference numeral 6 is an intake port, reference numeral 7 is an exhaust valve, and reference numeral 8 is an exhaust port. Are shown respectively.

The intake port 6 is connected to the surge tank 10 via a corresponding branch pipe 9, and each branch pipe 9 is provided with a fuel injection valve 11 for injecting fuel into the intake port 6. The surge tank 10 is connected to an air cleaner 14 via an intake duct 12 and an air flow meter 13, and a throttle valve 15 is arranged in the intake duct 12.

On the other hand, the exhaust port 8 is connected to the exhaust manifold 16
Is connected to a casing 18 containing a SOx absorbent 17, and an outlet portion of the casing 18 is connected via an exhaust pipe 19 to a casing 21 containing a NOx storage reduction catalyst (NOx absorbent) 20. Below, storage reduction type N
The Ox catalyst 20 is abbreviated as the NOx catalyst 20. This casing 2
1 is connected to a muffler (not shown) via an exhaust pipe 22.

Inlet 21a of casing 21 and exhaust pipe 2
2 is connected by a bypass pipe 26 that bypasses the NOx catalyst 20, and an exhaust switching valve (exhaust flow) in which a valve element is operated by an actuator 27 is connected to an inlet portion 21a of a casing 21 that is a branch portion of the bypass pipe 26. A switching means) 28 is provided. The exhaust switching valve 28 is closed by the actuator 27 by the actuator 27, as shown by the solid line in FIG. 1, to close the inlet portion of the bypass pipe 26 and fully open the inlet portion to the NOx catalyst 20. As shown, either one of the bypass open positions that closes the inlet portion to the NOx catalyst 20 and fully opens the inlet portion of the bypass pipe 26 is activated.

An electronic control unit (ECU) 30 for engine control is composed of a digital computer, and has a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Central Processor) which are mutually connected by a bidirectional bus 31. Unit) 3
4, an input port 35, and an output port 36. The air flow meter 13 generates an output voltage proportional to the intake air amount, and this output voltage is input to the input port 35 via the AD converter 37.

On the other hand, the exhaust pipe 1 downstream of the SOx absorbent 17
Reference numeral 9 denotes a temperature sensor 23 that generates an output voltage proportional to the temperature of the exhaust gas that has exited the SOx absorbent 17, and an SOx sensor that generates an output voltage that is proportional to the SOx concentration of the exhaust gas that has exited the SOx absorbent 17. (SOx concentration detection means) 24 is attached, and the output voltage of the temperature sensor 23 and SOx
The output voltages of the sensor 24 are AD converters 38 and 40, respectively.
Is input to the input port 35 via. Further, the input port 35 is connected to a rotation speed sensor 41 that generates an output pulse representing the engine rotation speed. The output port 36 is connected to the spark plug 4, the fuel injection valve 11, and the actuator 27 via a corresponding drive circuit 39, respectively.

In this gasoline engine, the fuel injection time TAU is calculated based on the following equation, for example. TAU = TP · K Here, TP indicates the basic fuel injection time, and K indicates the correction coefficient. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained by an experiment in advance, 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.
It is stored in 2. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder, and if K = 1.0, the air-fuel mixture supplied to the engine cylinder has a stoichiometric air-fuel ratio. On the other hand, when 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, becomes lean, and K> 1.
When it becomes 0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, it becomes rich.

In the gasoline engine of this embodiment,
In the engine low / medium load operation region, the value of the correction coefficient K is set to a value smaller than 1.0 to perform lean air-fuel ratio control, and the engine high load operation region, warm-up operation at engine start, acceleration,
And the constant coefficient operation of 120 km / h or more, the value of the correction coefficient K is set to 1.0 and stoichiometric control is performed, and the value of the correction coefficient K is set to a value larger than 1.0 in the engine full load operation region. It is set to perform rich air-fuel ratio control.

In the internal combustion engine, the frequency of low-medium load operation is usually highest, and therefore, the value of the correction coefficient K is made smaller than 1.0 in most of the operating period to burn the lean air-fuel mixture. become.

FIG. 3 schematically shows the concentrations of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from this figure, the concentrations of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increase 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 exhaust gas discharged increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NOx contained in the casing 21
The catalyst 20 uses, for example, alumina as a carrier, and potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, an alkaline earth such as barium Ba, calcium Ca, lanthanum La, and yttrium Y are supported on the carrier. At least one selected from such rare earths and a noble metal such as platinum Pt are supported. The ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the NOx catalyst 20 is referred to as the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 20 (hereinafter abbreviated as exhaust air-fuel ratio). The NOx catalyst 20 absorbs NOx when the exhaust air-fuel ratio is lean, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases.
It acts to absorb and release Ox.

When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx catalyst 20, 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 2
0 absorbs NOx when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 is lean, and releases the absorbed NOx when the oxygen concentration in the air-fuel mixture supplied to the combustion chamber 3 decreases. .

If the above-mentioned NOx catalyst 20 is arranged in the engine exhaust passage, the NOx catalyst 20 actually performs the NOx absorption / release action, but the detailed mechanism of this absorption / release action is not clear. However, it is considered that this absorbing and releasing action is performed by the mechanism shown in FIG. Next, this mechanism will be described by taking as an example the case where platinum Pt and barium Ba are supported on the carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, 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 platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ).

Next, a part of the produced NO ₂ is absorbed on the NOx catalyst 20 while being oxidized on the platinum Pt and is bonded with the barium oxide BaO, and as shown in FIG. It diffuses in the NOx catalyst 20 in the form of NO ₃ ⁻ . In this way, NOx is absorbed in the NOx catalyst 20.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is produced on the surface of platinum Pt, and NOx of the NOx catalyst 20 is generated.
As long as the absorption capacity is not saturated, NO ₂ is absorbed in the NOx catalyst 20 and nitrate ion NO ₃ ⁻ is produced.

On the other hand, the oxygen concentration in the inflowing exhaust gas
Is lowered and NO₂When the amount of produced
(NO₃ ^-→ NO₂), The nitrate ion in the NOx catalyst 20
No ₃ ^-Is NO₂Or released from the NOx catalyst 20 in the form of NO
Will be issued. That is, the oxygen concentration in the inflowing exhaust gas decreases
Then, NOx is released from the NOx catalyst 20.
As shown in FIG. 3, the degree of leanness of the inflowing exhaust gas
The lower the value, the lower the oxygen concentration in the inflowing exhaust gas,
If the leanness of the inflowing exhaust gas is lowered, NO
NOx will be released from the x catalyst 20.

On the other hand, at this time, when the air-fuel mixture supplied to the combustion chamber 3 is made stoichiometric or rich and the exhaust air-fuel ratio becomes stoichiometric or rich, a large amount of unburned HC is discharged from the engine as shown in FIG. , CO are discharged, and these unburned HC, CO are oxidized by reacting with oxygen O ₂ ⁻ or O ^{2 −} on platinum Pt.

Further, when the exhaust air-fuel ratio becomes stoichiometric or rich, the oxygen concentration in the inflowing exhaust gas is extremely lowered, so that NO ₂ or NO is released from the NOx catalyst 20, and this NO ₂ or NO is released as shown in FIG. As shown in B), it reacts with unburned HC and CO and is reduced to N ₂ .

[0046] 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 the consumption of
The NOx discharged from the NOx catalyst 20 and the NOx discharged from the engine are reduced to N ₂ by C and CO.

In this way, NO ₂ is formed on the surface of platinum Pt.
Alternatively, when NO is no longer present, NO ₂ or NO is released from the NOx catalyst 20 one after another and further reduced to N ₂ . Therefore, when the exhaust air-fuel ratio is made stoichiometric or rich, the NOx catalyst 20 is reduced to N within a short time.
Ox will be released.

Thus, when the exhaust air-fuel ratio becomes lean, NOx is absorbed by the NOx catalyst 20, and when the exhaust air-fuel ratio is made stoichiometric or rich, NOx is released from the NOx catalyst 20 in a short time and reduced to N ₂ . To be done. Therefore, it is possible to prevent the emission of NOx into the atmosphere.

By the way, in this embodiment, as described above, the air-fuel mixture supplied into the combustion chamber 3 is rich during full-load operation, and during high-load operation, warm-up operation during engine startup, and acceleration. At the same time, and at a constant speed operation of 120 km / h or more, the air-fuel mixture becomes the stoichiometric air-fuel ratio, and at the time of low and medium load operation, the air-fuel mixture becomes lean, so that NOx in the exhaust gas becomes NOx catalyst 20 at the time of low and medium load operation. The NOx is absorbed and NOx is released from the NOx catalyst 20 during the full load operation and the high load operation and is reduced. However, if the frequency of full load operation or high load operation is low and the frequency of low and medium load operation is high and the operation time is long, NOx release / reduction will not be in time and the NOx absorption capacity of the NOx catalyst 20 will be saturated. It becomes impossible to absorb NOx.

Therefore, in this embodiment, when the lean air-fuel mixture is being burned, that is, when the medium-low load operation is being performed, the stoichiometric or rich condition is spiked (short time) at 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. In this way, the exhaust air-fuel ratio (air-fuel ratio of the air-fuel mixture in this embodiment) is used for intake and release of NOx.
Is controlled so that “lean” and “spiky stoichiometry or rich (hereinafter, referred to as rich spike)” are alternately repeated in a relatively short cycle. To call.

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 20 is contained in the exhaust gas. These SOx are also absorbed. The SOx absorption mechanism of the NOx catalyst 20 is considered to be the same as the NOx absorption mechanism. That is, when the platinum Pt and barium Ba are carried on the carrier as an example similar to the case of explaining the NOx absorption mechanism, as described above, when the exhaust air-fuel ratio is lean, the oxygen O ₂ is O ₂ ^- or O ^2-
Is adhered to the surface of platinum Pt of the NOx catalyst 20, and SOx (for example, SO ₂ ) in the inflowing exhaust gas is platinum Pt.
Is oxidized to SO ₃ on the surface of.

Thereafter, the produced SO ₃ is absorbed in the NOx catalyst 20 while being further oxidized on the surface of platinum Pt and is bonded to barium oxide BaO, and is absorbed in the NOx catalyst 20 in the form of sulfate ion SO ₄ ^2−. It diffuses to form the sulfate salt BaSO ₄ . The sulfate BaSO ₄ is stable and difficult to decompose, and remains in the NOx catalyst 20 without being decomposed even if the air-fuel ratio of the inflowing exhaust gas is made rich. Therefore, if the production amount of BaSO _{4 in} the NOx catalyst 20 increases with the lapse of time, the amount of BaO that can participate in the absorption of the NOx catalyst 20 decreases, and the NOx absorption capacity decreases. This is SOx poisoning.

Therefore, in this embodiment, SOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean and absorbed when the oxygen concentration of the inflowing exhaust gas is low so that SOx does not flow into the NOx absorbent 20. Released SOx S
The Ox absorbent 17 is arranged upstream of the NOx absorbent 20. This SOx absorbent 17 is S when the air-fuel ratio of the exhaust gas flowing into the SOx absorbent 17 is lean.
NOx is also absorbed with Ox, but when the air-fuel ratio of the inflowing exhaust gas is made stoichiometric or rich and the oxygen concentration becomes low, not only the absorbed SOx but also NOx is released.

As described above, the NOx catalyst 20 uses SOx.
Is absorbed, stable sulfate BaSO ₄ is produced,
As a result, even if the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 20 is stoichiometric or rich, the SOx will be NOx catalyst 20.
No longer emitted from. Therefore, SOx absorbent 17
In order for SOx to be released from the SOx absorbent 17 when the air-fuel ratio of the exhaust gas flowing into the engine is made stoichiometric or rich, the absorbed SOx is in the form of sulfate ions SO ₄ ^2- Or the sulfate BaSO ₄ is produced, even if the sulfate BaSO ₄ is produced.
It is necessary for aSO ₄ to exist in the SOx absorbent 17 in an unstable state. SOx that enables this
As the absorbent 17, copper C on a carrier made of alumina.
u, iron Fe, manganese Mn, transition metals such as nickel Ni, sodium Na, titanium Ti and lithium Li.
SOx absorbent 1 that carries at least one selected from
7 can be used.

In this SOx absorbent 17, the SOx absorbent 1
When the air-fuel ratio of the exhaust gas flowing into 7 is lean, SO ₂ in the exhaust gas is absorbed in the SOx absorbent 17 in the form of sulfate ion SO ₄ ²⁻ while being oxidized on the surface of the SOx absorbent 17,
Then, it is diffused in the SOx absorbent 17. In this case, S
On the carrier of the Ox absorbent 17, platinum Pt, palladium Pd,
If one of rhodium Rh is carried, SO
₂ is easily adsorbed on platinum Pt, palladium Pd, and rhodium Rh in the form of SO ₃ ²⁻ , and thus SO ₂ is easily absorbed in the SOx absorbent 17 in the form of sulfate ion SO ₄ ²⁻ .
Therefore, in order to promote the absorption of SO ₂ , it is preferable to carry any one of platinum Pt, palladium Pd, and rhodium Rh on the carrier of the SOx absorbent 17.

When this SOx absorbent 17 is arranged upstream of the NOx catalyst 20, when the air-fuel ratio of the exhaust gas flowing into the SOx absorbent 17 becomes lean, the SOx in the exhaust gas is absorbed by the SOx absorbent 17, and therefore, Downstream NOx catalyst 2
SOx does not flow into 0, and the NOx catalyst 20 absorbs only NOx in the exhaust gas.

Meanwhile, the SOx absorbed by the SOx absorbent 17 as described above sulfate ions SO ₄ ^{2 -} a form on whether you are diffused in the SOx absorbent 17, or sulfate BaSO ₄ in an unstable state ing. Therefore, SOx absorbent 1
When the air-fuel ratio of the exhaust gas flowing into 7 becomes stoichiometric or rich and the oxygen concentration decreases, the SOx absorbed in the SOx absorbent 17 is easily released from the SOx absorbent 17.

By the way, it has been confirmed that the SOx absorbent 17 is thermally deteriorated with time because it is exposed to the heat of exhaust gas, and it has been found that this thermal deterioration causes the following phenomenon. The first phenomenon is the SOx absorbent 17
This is a problem related to SOx emission. When the SOx absorbent 17 has not been thermally deteriorated or has not progressed so much (in other words, when the degree of thermal deterioration is small), S
Even if exhaust gas having a stoichiometric or rich air-fuel ratio is flown into the Ox absorbent 17 for a short time (for example, 5 seconds or less), SOx is not released from the SOx absorbent 17. In this regard, the applicant of the present invention has found that when the SOx absorbent 17 is not thermally deteriorated, the SOx absorbent is used in the rich spike duration time of the lean rich spike control for releasing NOx from the NOx catalyst 20. It has been confirmed that SOx is not released from 17.

However, as the thermal deterioration of the SOx absorbent 17 progresses (in other words, as the degree of thermal deterioration increases), the exhaust gas having the stoichiometric or rich air-fuel ratio was passed through the SOx absorbent 17 for a short time. Also in this case, SOx is released from the SOx absorbent 17.
Moreover, at this time, the S released from the SOx absorbent 17
The amount of Ox tends to increase as the thermal deterioration progresses.

The second phenomenon caused by the thermal deterioration of the SOx absorbent 17 is a problem relating to the SOx absorption capacity, and as the thermal deterioration of the SOx absorbent 17 progresses, the SOx which can be absorbed by the SOx absorbent 17 is absorbed. Quantity, ie SOx
Absorption capacity is decreasing. Therefore, SOx absorbent 17
If the regeneration time of the SOx absorbent 17 is advanced and the regeneration frequency is not increased as the thermal deterioration of
The regeneration of the absorbent 17 cannot be done in time, and the NOx catalyst 20
May be SOx poisoned.

Therefore, in this embodiment, at the time of the first rich spike immediately after the lean rich spike control is performed after the regeneration of the SOx absorbent 17, the SOx concentration of the gas emitted from the SOx absorbent 17 is measured by the SOx sensor 24. Detected by the SOx absorbent 17 based on the detected SOx concentration
The degree (degree) of heat deterioration of the SOx absorbent 17 is determined, and the regeneration frequency of the SOx absorbent 17 is changed based on the state of heat deterioration. In this embodiment, the SOx absorbent 1
The change of the regeneration frequency of No. 7 is realized by changing the setting of the SOx absorption capacity, which is the criterion for determining whether it is the regeneration time of the SOx absorbent 17.

This will be described in detail below. First, SO
The regeneration processing of the x absorbent 17 will be described. The regeneration timing of the SOx absorbent 17 is determined when the ECU 30 integrates the SOx amount absorbed by the SOx absorbent 17 from the history of engine operating conditions and the integrated value reaches a preset SOx absorption capacity. And Here, the SOx absorption capacity, which is a criterion for determining whether or not the regeneration time is reached, can be changed based on the state of thermal deterioration of the SOx absorbent 17, and is appropriately set from a thermal deterioration determination map described later.

When the ECU 30 determines that it is time to regenerate the SOx absorbent 17, it executes regeneration processing for releasing SOx from the SOx absorbent 17. When executing the regeneration process of the SOx absorbent 17, the ECU 30 determines the engine operating state at that time from the engine speed N and the engine load Q / N, and the exhaust gas temperature at that time detected by the temperature sensor 23 is SOx. Substituting as the temperature of the absorbent 17, the stoichiometric or rich condition and the processing time that can release SOx most efficiently with less fuel consumption deterioration based on the engine operating state and the temperature of the SOx absorbent 17 are selected, and the selected air-fuel ratio condition is selected. This is performed by flowing the exhaust gas of No. 3 into the SOx absorbent 17 for a selected processing time.

Further, it is known that in order to release SOx from the SOx absorbent 17, it is necessary to raise the temperature of the SOx absorbent 17 to a high temperature of a predetermined temperature (for example, 550 ° C.) or higher. During the regeneration process of the SOx absorbent 17, the temperature of the exhaust gas temperature is controlled by an appropriate means to control the temperature of the SOx absorbent 17 to the predetermined temperature (hereinafter, referred to as SOx release temperature) or higher.

When the SOx absorbent 17 is regenerated, the SOx absorbent 1
Exhaust gas (hereinafter, referred to as regeneration exhaust gas) flowing out from 7 contains a large amount of SOx released from the SOx absorbent 17. Since this regenerated exhaust gas is stoichiometric or rich at high temperature, SOx in the regenerated exhaust gas is difficult to be absorbed by the NOx catalyst 20 even if it is passed through the NOx catalyst 20, and it should pass through as it is, but it is completely absorbed by the NOx catalyst 20. There is no guarantee that it will not be done. Therefore, in this embodiment, when the SOx absorbent 17 is regenerated, the SOx absorbent 17 is regenerated.
In order to prevent the SOx released from the SOx from flowing into the NOx catalyst 20, the regeneration exhaust gas is guided into the bypass pipe 26 during the regeneration treatment of the SOx absorbent 17.

More specifically, the NOx in the exhaust gas is absorbed and released by the NOx catalyst 20 to reduce and purify it.
When the rich spike control is being executed, the exhaust switching valve 28 is held in the bypass closed position as shown by the solid line in FIG. 1, and therefore, at this time, the SOx absorbent 1
The exhaust gas flowing out of 7 flows into the NOx catalyst 20.
Then, SOx in the exhaust gas is absorbed by the SOx absorbent 17, and only NOx in the exhaust gas is absorbed and released by the NOx catalyst 20 to be reduced and purified.

Next, when SOx should be released from the SOx absorbent 17, that is, when the regeneration processing of the SOx absorbent 17 is executed, the air-fuel ratio control is switched from the lean rich spike control to the stoichiometric or rich control, and at the same time, the exhaust gas is exhausted. The switching valve 28 is switched and held from the bypass closed position to the bypass open position shown by the broken line in FIG. As a result, the regenerated exhaust flowing out of the SOx absorbent 17 does not flow into the NOx catalyst 20 but flows into the bypass pipe 26.
Therefore, it is possible to reliably prevent the NOx catalyst 20 from being poisoned by SOx by SOx in the regenerated exhaust gas.
Note that SOx in the exhaust gas (regenerated exhaust gas) is reduced by unburned HC and CO in the exhaust gas and released as SO ₂ .

Next, when the regeneration processing of the SOx absorbent 17 is to be stopped, the air-fuel ratio control is switched from stoichiometric or rich control to lean / rich spike control,
At the same time, the exhaust gas switching valve 28 is switched from the bypass open position to the bypass closed position shown by the solid line in FIG.

While the SOx absorbent 17 is being regenerated, unburned HC, CO and NOx are discharged from the engine body 1. However, since the SOx absorbent 17 has ternary activity, these unburned HC, CO and NOx are considerably purified in the SOx absorbent 17. Therefore, these unburned H
There is no risk that C, CO and NOx will be released into the atmosphere.

Next, the procedure for determining the state of thermal deterioration of the SOx absorbent 17 will be described. FIG. 5 shows SOx absorbent 17
Air-fuel ratio of the exhaust gas flowing into the exhaust gas (hereinafter referred to as "inlet gas air-fuel ratio") and SOx of the exhaust gas flowing out from the SOx absorbent 17
It is a figure which shows an example of a density | concentration (henceforth the outgoing gas SOx density | concentration).

In this figure, the input gas air-fuel ratio is
NOx is absorbed and released in the NOx catalyst 20 during the rich spike control, and the SOx absorbent 17 is regenerated during the high temperature stoichiometric control of the incoming gas air-fuel ratio. In this example, in the lean / rich spike control, the lean operation duration is 40 seconds and the stoichiometric operation duration as the rich spike is about 2 seconds during constant speed running at 60 km / h, and this is alternately repeated. .
On the other hand, during the regeneration process of the SOx absorbent 17, the air-fuel ratio is set to stoichiometric control, and the duration thereof is set to a time sufficiently longer than the rich spike duration time of the lean rich spike control, for example, about 1 hour.

When the SOx absorbent 17 is not thermally deteriorated, even if the rich spike exhaust gas flows into the SOx absorbent 17 by performing lean rich spike control on the incoming gas air-fuel ratio, Therefore, SOx absorbent 17
Therefore, SOx is not released from the exhaust gas SOx, and the SOx concentration of the exhaust gas remains substantially zero until the next regeneration of the SOx absorbent 17.

However, when the thermal deterioration of the SOx absorbent 17 gradually progresses, when the rich-spike exhaust gas flows into the SOx absorbent 17 under lean / rich spike control of the incoming gas air-fuel ratio, SOx is released from the SOx absorbent 17, and as a result, the exhaust gas containing SOx comes to flow out from the SOx absorbent 17 in synchronization with the rich spike.

Here, the outgassing SOx concentration tends to become higher as the SOx absorbent 17 is thermally deteriorated, and the peak value of the outgassing SOx concentration at each rich spike is the rich spike. As the number of times increases, it tends to increase gradually. Therefore, in this embodiment, the state of thermal deterioration of the SOx absorbent 17 (thermal deterioration) is determined from the peak value of the outgassing SOx concentration detected by the SOx sensor 24 during the first rich spike immediately after the SOx absorbent 17 is regenerated. It was decided to judge the degree of).

The reason for determining the heat deterioration state of the SOx absorbent 17 as "the first rich spike immediately after the regeneration of the SOx absorbent 17" is as follows. S
Immediately after the regeneration of the Ox absorbent 17, the SOx amount absorbed by the SOx absorbent 17 is extremely small, and the SOx should be hardly released. Therefore, the most suitable condition for strict determination of the thermal deterioration of the SOx absorbent 17 is immediately after regeneration, and the determination of the thermal deterioration state at the same time each time improves the determination accuracy.

However, the timing for determining the state of thermal deterioration of the SOx absorbent 17 is preferably near the time immediately after the regeneration of the SOx absorbent 17, and it is not always necessary to say "immediately after the regeneration of the SOx absorbent 17". It is not limited to the “first rich spike time”. Therefore, for example, the determination time can be set to “the nth rich spike time (n is a natural number) counted immediately after the SOx absorbent 17 is regenerated”. Also, SOx absorbent 1
It is also possible to average the peak values of the outgassing SOx concentration at the n-th to (n + α) th rich spike counting from immediately after the regeneration of No. 7, and determine the state of thermal deterioration based on the average value. is there.

Based on the peak value of the outgassing SOx concentration obtained as described above, the ECU 30 preliminarily stores the ROM 32 in the ROM 32.
Refer to the thermal deterioration determination map stored in
The state of heat deterioration of the absorbent 17 is determined.

FIG. 6 shows an example of the heat deterioration determination map. In this example, when the peak value dp is 0 ≦ dp <d1, it is judged that the heat deterioration state is level 1 (L1),
When the peak value dp is d1≤dp <d2, it is determined that the state of thermal deterioration is level 2 (L2), and the peak value dp is d2≤.
When dp <d3, it is determined that the state of thermal deterioration is level 3 (L3), and when the peak value dp is dp≥d3, it is determined that the state of thermal deterioration is level 4 (L4). Here, the peak value d is d1 <d2 <d3 <d4.

Then, in advance, for each level of thermal deterioration, SO
The SOx absorption capacity C, which is a criterion for determining whether or not the regeneration time of the x absorbent 17 has been reached, is set. In this example, the SOx absorption capacity of level 1 (L1) is C1, the SOx absorption capacity of level 2 (L2) is C2, the SOx absorption capacity of level 3 (L3) is C3, and the SOx absorption capacity of level 4 (L4) is It is C4. Here, the SOx absorption capacity C is C1>C2> C3
> C4, and the SOx absorption capacity C is set smaller as the level L of thermal deterioration increases.

Then, the level of heat deterioration determined after the regeneration of the current SOx absorbent and the level of heat degradation determined after the previous regeneration of the SOx absorbent are compared to determine whether the level has changed, When the deterioration level has not changed, the SOx absorption capacity is not changed, and when the thermal deterioration level has changed, the SOx absorption capacity is changed to the SOx absorption capacity corresponding to the currently determined thermal deterioration level. That is, the level of thermal deterioration L
When is increased, the SOx absorption capacity is reduced.

The exhaust gas of the rich spike is SOx.
It is found that the amount of SOx released from the SOx absorbent 17 when flowing into the absorbent 17 is related to the temperature of the SOx absorbent 17, and that SOx is released more easily when the temperature of the SOx absorbent 17 is higher. ing. So SOx absorbent 1
In order not to cause an error in the thermal deterioration level determination depending on the temperature of 7, the temperature of the SOx absorbent 17 is divided into predetermined temperature ranges, and the thermal deterioration determination map corresponding to FIG. Is created and stored in the ROM 32.

In this case, the SOx absorption capacities C1, C2, C3,
For C4, the same value is used for all heat deterioration judgment maps,
Peak value d, which is a threshold value that determines the level of thermal deterioration
Only the values of 1, d2, d3, and d4 are made different for each thermal deterioration determination map in each temperature range. For example, d2 in the thermal deterioration judgment map in the temperature range including 450 ° C is set to 1 ppm,
D in the thermal deterioration determination map in the temperature range including 550 ° C
2 is set to 2 ppm, and d2 in the thermal deterioration determination map in the temperature range including 700 ° C is set to 5 ppm.

Next, referring to FIG. 7, the SOx absorbent regeneration processing execution routine in this embodiment will be described.
A flow chart including each step constituting this routine is stored in the ROM 32 of the ECU 30, and the processes in each step of the flow chart are all executed by the CPU 34 of the ECU 30. It should be noted that this reproduction processing execution routine is executed every predetermined time.

<Step 101> First, the ECU 30
In step 101, the SOx amount absorbed by the SOx absorbent 17 is calculated from the current operating state of the engine 1, and S
The SOx absorption amount absorbed by the SOx absorbent 17 from the time of the Ox absorbent regeneration processing to the present is integrated.

<Step 102> Next, the ECU 30
Proceeding to step 102, it is determined whether it is the regeneration time of the SOx absorbent 17. That is, the SO calculated in step 102
When the x absorption amount has reached the SOx absorption capacity, which is the criterion, the ECU 30 determines that it is the regeneration time of the SOx absorbent 17, proceeds to step 103, and step 101
When the SOx absorption amount calculated in step 1 has not reached the SOx absorption capacity, it is determined that it is not the regeneration time and the process proceeds to return.

<Step 103> In step 103, the ECU 30 executes SOx absorbent regeneration control.
That is, the ECU 30 executes temperature control so that the exhaust gas temperature becomes equal to or higher than the SOx release temperature, executes air-fuel ratio control so that the air-fuel ratio of the exhaust gas becomes a predetermined stoichiometric or rich condition, and regenerated exhaust gas bypasses. The switching control of the exhaust switching valve 28 is executed so that the exhaust switching valve 28 flows to the pipe 26.

Then, the ECU 30 terminates the SOx absorbent regeneration control when the SOx absorbent regeneration control is executed for a predetermined time, shifts the air-fuel ratio control of the exhaust gas to the lean rich spike control, and NOx catalyst 2
The exhaust switching valve 28 is switched so as to flow to 0, and the routine proceeds to step 104.

<Step 104> In step 104, the ECU 30 causes the SOx sensor 24 to output the SO gas at the first rich spike immediately after the SOx absorbent has been regenerated.
The temperature sensor 23 detects the peak value of x concentration.
The SOx absorbent temperature is detected by.

<Step 105> Next, the ECU 30
In step 105, it is determined whether or not the SOx absorption capacity, which serves as a criterion for determining whether or not the SOx absorbent 17 should be regenerated, needs to be changed. More specifically, the ECU 30 refers to the thermal deterioration determination map in the temperature range corresponding to the SOx absorbent temperature detected in step 104 and refers to the current SOx concentration based on the peak value of the outgas SOx concentration detected in step 104. It is determined what level the heat deterioration state of the absorbent 17 is. Further, it is determined whether or not the current level of heat deterioration of the SOx absorbent 17 has changed from the level of heat deterioration determined after the previous SOx absorbent regeneration.

Then, if the level of thermal deterioration has not changed between this time and the previous time, in step 105, SO
x Judge that there is no need to change the absorption capacity and proceed to return.

<Step 106> On the other hand, when the level of the thermal deterioration determined this time is different from the level of the thermal deterioration determined the previous time, it is determined that the SOx absorption capacity needs to be changed, and the process proceeds to Step 106. The ECU 30 proceeds to S
The Ox absorption capacity is changed to the SOx absorption capacity corresponding to the level of the thermal deterioration determined this time, and the process returns. That is, when the level of thermal deterioration increases, the SOx absorption capacity is reduced. As a result, when the present routine is executed next time, it is determined in step 102 whether or not it is the regeneration time of the SOx absorbent 17 with the changed SOx absorption capacity as the criterion. As a result, when the state of thermal deterioration of the SOx absorbent 17 progresses, the frequency of regeneration of the SOx absorbent 17 increases.

In this embodiment, the part of the series of signal processing by the ECU 30 that executes step 105 can be referred to as deterioration determining means for determining the state of deterioration of the SOx absorbent 17, and the part that executes step 106 is , SOx absorbent 17 can be said to be a regeneration frequency changing means for changing the regeneration frequency.

Thus, according to this embodiment, S
Depending on the state of heat deterioration of the Ox absorbent 17, the SOx absorbent 17
Since the regeneration frequency of NOx is changed, the SOx absorbent 17 is regenerated at an appropriate time, and the NOx catalyst 20 can be reliably prevented from being poisoned by SOx. As a result, the NOx purification rate of the NOx catalyst 20 can always be maintained in a high state.

[Other Embodiments] In the above-described embodiments, the example in which the present invention is applied to a gasoline engine has been described, but it goes without saying that the present invention can be applied to a diesel engine. In the case of a diesel engine, since combustion in the combustion chamber is performed in a much leaner range than stoichiometry, the air-fuel ratio of the exhaust gas flowing into the SOx absorbent 17 and the NOx catalyst 20 is extremely lean under normal engine operating conditions. Therefore, although SOx and NOx are absorbed, SOx and NOx are hardly released.

Further, in the case of a gasoline engine, as described above, the air-fuel mixture supplied to the combustion chamber 3 is made stoichiometric or rich so that the SOx absorbent 17 and the NOx are exhausted.
Although the air-fuel ratio of the exhaust gas flowing into the catalyst 20 can be made stoichiometric or rich, SOx and NOx absorbed in the SOx absorbent 17 and NOx catalyst 20 can be released.
In the case of a diesel engine, if the air-fuel mixture supplied to the combustion chamber is stoichiometric or rich, there is a problem that soot is generated during combustion, and therefore it cannot be adopted.

Therefore, when the present invention is applied to a diesel engine, in order to make the air-fuel ratio of the inflowing exhaust gas stoichiometric or rich, in addition to burning the fuel to obtain the engine output, the reducing agent ( For example, it is necessary to supply light oil (fuel) to the exhaust gas. The reducing agent can be supplied to the exhaust gas by sub-injecting fuel into the cylinder during the intake stroke, the expansion stroke, or the exhaust stroke, or the reducing agent can be introduced into the exhaust passage upstream of the SOx absorbent 17. It is also possible to supply.

Even if the diesel engine is equipped with an exhaust gas recirculation device (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 stoichiometric or rich.

[0098]

According to the exhaust gas purification apparatus for an internal combustion engine of the present invention, (a) SO is arranged when the air-fuel ratio of the exhaust gas flowing into the exhaust passage of the internal combustion engine capable of lean combustion is lean.
a SOx absorbent that absorbs x and releases the absorbed SOx when the oxygen concentration of the inflowing exhaust gas is low;
an NOx absorbent which is arranged in the exhaust passage downstream of the x absorbent and which absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is low. (C) Exhaust air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas so as to control the absorption and release of NOx in the NOx absorbent, and (d) SOx is released from the SOx absorbent to remove the SOx absorbent. Regenerating means for reducing the oxygen concentration of the exhaust gas for regeneration, and (e) SOx of the exhaust gas downstream of the SOx absorbent when the air-fuel ratio of the exhaust gas is controlled to be stoichiometric or rich by the exhaust air-fuel ratio control means. SOx concentration detecting means for detecting the concentration, and (f) deterioration determining means for determining the state of deterioration of the SOx absorbent based on the SOx concentration of the exhaust gas detected by the SOx concentration detecting means, G) Regeneration of the SOx absorbent by providing regeneration frequency changing means for changing the regeneration frequency of the SOx absorbent by the regeneration means according to the deterioration state of the SOx absorbent determined by the deterioration determining means. It can be executed at an appropriate time, and as a result, the excellent effect that the NOx absorbent can be surely prevented from being poisoned by SOx is exhibited.

Further, in the exhaust gas purification apparatus for an internal combustion engine according to the present invention, when the deterioration determination means determines the deterioration of the SOx absorbent at a time immediately after the end of regeneration of the SOx absorbent, Thus, there is an effect that it is possible to improve the accuracy of determining the deterioration state of the SOx absorbent.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of an exhaust emission control device for an internal combustion engine according to the present invention.

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

FIG. 3 shows unburned HC in exhaust gas discharged from the engine,
It is a diagram which shows the concentration of CO and oxygen roughly.

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

FIG. 5 is a diagram showing an example of changes in SOx absorbent output gas SOx concentration in the embodiment.

FIG. 6 is a diagram showing an example of a heat deterioration determination map in the embodiment.

FIG. 7 is a SOx absorbent regeneration processing execution routine of the above embodiment.

[Explanation of symbols]

1 Engine body (internal combustion engine) 3 Combustion chamber 4 Spark plug 11 Fuel injection valve 16,22 Exhaust pipe (exhaust passage) 17 SOx absorbent 20 NOx storage reduction catalyst (NOx absorbent) 23 Temperature sensor 24 SOx sensor (SOx concentration detection means) 26 Bypass pipe 28 Exhaust switching valve 30 ECU

Continuation of front page (51) Int.Cl. ⁷ identification code FI F02D 41/14 330 330 F02D 41/14 330Z (56) References JP-A-6-336914 (JP, A) JP-A-7-208151 (JP, A) Japanese Patent Laid-Open No. 8-261041 (JP, A) Patent 2605580 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) F01N 3/08-3/36 F02D 41/14

Claims (2)

(57) [Claims] (A) SOx absorbed when the air-fuel ratio of the inflowing exhaust gas is lean and arranged in the exhaust passage of an internal combustion engine capable of lean combustion, and absorbed when the oxygen concentration of the inflowing exhaust gas is low. And (b) the oxygen concentration of the inflowing exhaust gas is low when NOx is absorbed when the air-fuel ratio of the inflowing exhaust gas, which is arranged in the exhaust passage downstream of the SOx absorbent, is lean. Sometimes absorbed N
NOx absorbent that releases Ox, (c) exhaust air-fuel ratio control means that controls the air-fuel ratio of exhaust gas to control the absorption and release of NOx in the NOx absorbent, and (d) the SO
An exhaust gas purification apparatus for an internal combustion engine, comprising: a regenerating unit that reduces the oxygen concentration of the exhaust gas so as to regenerate the SOx absorbent by releasing SOx from the x absorbent, and (e) the exhaust gas by the exhaust air-fuel ratio control unit. S for detecting the SOx concentration of the exhaust gas downstream of the SOx absorbent when the air-fuel ratio of S is controlled to stoichiometric or rich S
Ox concentration detection means, (f) deterioration determination means for determining the state of deterioration of the SOx absorbent based on the SOx concentration of the exhaust gas detected by the SOx concentration detection means, and (g) determination by the deterioration determination means An exhaust emission control device for an internal combustion engine, comprising: a regeneration frequency changing means for changing the regeneration frequency of the SOx absorbent by the regeneration means according to the deteriorated state of the SOx absorbent. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the deterioration determination unit determines whether or not the SOx absorbent is deteriorated at a time immediately after the regeneration of the SOx absorbent is completed.