JP3374780B2 - 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 inflow gas is lean and releases the absorbed SOx as SO ₂ when the air-fuel ratio of the inflow gas is the stoichiometric air-fuel ratio or rich. However, since the SOx absorption capacity of this SOx absorbent is also limited, it is necessary to perform a process of releasing SOx from the SOx absorbent before the SOx absorbent is saturated with SOx, that is, a regeneration process.

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 set the air-fuel ratio of the inflowing exhaust gas to the stoichiometric air-fuel ratio or the rich air-fuel ratio.
It is said that the higher the temperature of the SOx absorbent, the easier the SOx is released.

Further, in the regeneration treatment technique disclosed in this publication, when SOx is released from the SOx absorbent, the released SOx is prevented from being absorbed by the NOx catalyst arranged downstream. In addition, a bypass passage that branches from the exhaust pipe connecting the SOx absorbent and the NOx catalyst to bypass the NOx catalyst is provided, and an exhaust switching valve that selectively switches the exhaust gas to either the NOx catalyst or the bypass passage is provided. When the regeneration process for releasing SOx from the SOx absorbent is performed, the exhaust switching valve causes the exhaust gas to flow to the bypass passage so that it does not flow to the NOx catalyst.
When the regeneration process is not executed, the exhaust gas is made to flow to the NOx catalyst by the exhaust switching valve so that it does not flow to the bypass passage. With this configuration, during the regeneration process, the SOx released from the SOx absorbent does not flow into the NOx catalyst.
Can prevent poisoning.

[0010]

According to the regeneration processing technique disclosed in the above publication, as described above, when the SOx absorbent is regenerated, the exhaust gas flows through the bypass passage, but the NOx catalyst does not. The exhaust switching valve is switched so that it does not flow.

However, the exhaust switching valve used here has an incomplete sealing property, and exhaust gas slightly leaks to the NOx catalyst side even if the valve body is positioned so as to close the NOx catalyst side. By the way, the degree of leakage of the exhaust switching valve currently used is about 1 to 10%.

As described above, when the exhaust gas leaking from the exhaust switching valve flows into the NOx catalyst during the regeneration process of the SOx absorbent, this exhaust gas is the regenerated exhaust of the SOx absorbent and SO ₂
Since the concentration is high, the inflowing gas flow rate is very small, but N
The Ox catalyst is poisoned by SOx.

Further, since the SOx absorbent is exposed to high temperature exhaust gas, thermal deterioration occurs over time, and if this thermal deterioration progresses, even if a stoichiometric or rich air-fuel ratio exhaust gas flows for a short time, the SOx absorbent will be discharged from the SOx absorbent. There is a phenomenon that SOx is easily desorbed. Therefore, when the thermal deterioration of the SOx absorbent progresses considerably, the exhaust gas is flowing through the NOx catalyst during the non-regeneration process of the SOx absorbent, and
When exhaust gas with a stoichiometric air-fuel ratio or rich air-fuel ratio is passed to release / reduce Ox, SOx is desorbed from the SOx absorbent, which then flows into the NOx catalyst to drive the NOx catalyst to SO.
x May be poisoned.

As described above, even in the exhaust gas purification system having the SOx absorbent and the bypass passage, the Nx of the NOx catalyst is reduced.
To maintain high Ox purification capacity for a long period of time, SOx
It is necessary to perform SOx release processing on the NOx catalyst downstream of the absorbent. To release SOx from the NOx catalyst,
As in the case of the SOx absorbent regeneration process, the exhaust gas having a stoichiometric or rich air-fuel ratio is passed through the NOx catalyst. However, when the SOx releasing process is performed for the NOx catalyst in this way, the stoichiometric or rich air-fuel ratio Exhaust gas is N
Since it passes through the SOx absorbent before flowing into the Ox catalyst, the SOx absorbed in the SOx absorbent is desorbed at that time and the S
Ox may flow into the NOx catalyst and re-poison the NOx catalyst with SOx.

The present invention has been made in view of the above problems of the conventional technique, and an object of the present invention is to prevent SOx poisoning of the NOx absorbent.

[0016]

The present invention adopts the following means in order to solve the above problems. (1) An exhaust gas purification apparatus for an internal combustion engine according to the present invention,
(A) is placed in the exhaust passage of an internal combustion engine capable of lean burn;
An SOx absorbent that 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; and (b) the SOx absorbent that is downstream of the SOx absorbent. A NOx absorbent that is arranged in the exhaust passage, 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, and (c) the SOx absorption Agent and the NO
a bypass passage branched from the exhaust passage between the x-absorbent and the NOx absorbent to flow the exhaust gas, and (d)
Exhaust path switching means for selectively switching which of the NOx absorbent and the bypass passage the exhaust gas flowing out of the SOx absorbent is flown; and (e) SOx absorbent to SOx
In the regeneration process for discharging the exhaust gas, the air-fuel ratio of the exhaust gas is controlled to the stoichiometric air-fuel ratio or the rich air-fuel ratio, and the exhaust path switching means is controlled to guide the exhaust gas flowing out from the SOx absorbent to the bypass passage. SOx absorbent regenerating means, and (f) during the SOx release processing in which the SOx absorbed in the NOx absorbent should be released from the NOx absorbent, while controlling the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or the rich air-fuel ratio. A NOx absorbent SOx releasing means for controlling the exhaust path switching means so as to guide the exhaust gas flowing out from the SOx absorbent to the NOx absorbent,
The SOx release processing from the NOx absorbent by the absorbent SOx release means is executed after the SOx absorbent regeneration processing is completed by the SOx absorbent regeneration means.

In the exhaust gas purification apparatus for an internal combustion engine described in (1) above, the SOx absorbent is arranged upstream of the NOx absorbent, and when the SOx absorbent is regenerated, the regenerated exhaust gas is passed through the bypass passage. Since SOx is made to flow and does not flow to the NOx absorbent, SOx should not normally flow into the NOx absorbent. However, due to reasons such as leakage of exhaust gas from the exhaust path switching means, N
SOx flows into the Ox absorbent, and the NOx catalyst absorbs SOx.

Therefore, when the amount of SOx absorbed in the NOx absorbent increases, the NOx purification ability decreases, so the NOx absorbent SOx releasing means executes the SOx releasing process on the NOx absorbent at a predetermined time. SOx for NOx absorbent
Since the release process is executed after the regeneration process of the SOx absorbent is completed, even if the exhaust gas whose stoichiometric air-fuel ratio or rich air-fuel ratio is controlled flows into the SOx absorbent during the SOx release process for the NOx absorbent. , SOx is not released from the SOx absorbent, and the NOx absorbent is not poisoned by SOx. Then, when the exhaust gas having a stoichiometric or rich air-fuel ratio flows into the NOx absorbent, SOx is released from the NOx absorbent.

(2) Further, the exhaust gas purifying apparatus for an internal combustion engine according to the present invention is (a) disposed in an exhaust passage of an internal combustion engine capable of lean combustion, and emits SOx when the air-fuel ratio of the inflowing exhaust gas is lean. A first SOx absorbent that releases SOx absorbed 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 is lean, and NOx is absorbed when the oxygen concentration of the inflowing exhaust gas is low, which is arranged in the exhaust passage downstream of the first SOx absorbent. NOx absorbent that releases NOx, and (c)
A bypass passage branching from the exhaust passage between the first SOx absorbent and the NOx absorbent to bypass the NOx absorbent to flow exhaust gas; and (d) from the first SOx absorbent. An exhaust path switching means for selectively switching which one of the NOx absorbent and the bypass passage the exhausted exhaust gas flows, and (e) is arranged downstream of the exhaust path switching means and upstream of the NOx absorbent, A second SOx absorbent that absorbs SOx when the air-fuel ratio of the exhaust gas is lean and does not easily release the absorbed SOx even if the oxygen concentration of the inflowing exhaust gas becomes low; and (f) the first SOx absorption During the regeneration process for releasing SOx from the agent, the air-fuel ratio of the exhaust gas is controlled to the stoichiometric air-fuel ratio or the rich air-fuel ratio, and the exhaust gas flowing out from the first SOx absorbent is guided to the bypass passage. exhaust Characterized by comprising a SOx absorbent reproducing means for controlling the road switching means.

In the exhaust gas purification device for an internal combustion engine described in (2) above, the first SOx absorbent is arranged upstream of the NOx absorbent, and when the first SOx absorbent is regenerated. Regenerated exhaust flows through the bypass passage, NOx
Since it does not flow into the absorbent, it would normally be N
No SOx should flow into the Ox absorbent. However, the exhaust gas containing SOx may flow from the first SOx absorbent toward the NOx absorbent due to leakage of exhaust gas from the exhaust path switching means. However, in this exhaust emission control device, the second SOx absorbent provided upstream of the NOx absorbent absorbs these SOx, so that SOx does not flow into the NOx absorbent. As a result, SOx poisoning of the NOx absorbent can be reliably prevented.

In the exhaust gas purification apparatus for an internal combustion engine according to the present invention described in (1) or (2) above, examples of the lean burn internal combustion engine include a direct injection lean burn gasoline engine and a diesel engine. can do. In the case of a lean burn gasoline engine, air-fuel ratio control of exhaust gas can be realized by air-fuel ratio control of air-fuel mixture supplied to the combustion chamber. In the case of a diesel engine, air-fuel ratio control of exhaust gas is performed by so-called sub-injection in which fuel is injected in an intake stroke, an expansion stroke, or an exhaust stroke, or an exhaust gas upstream of an SOx absorbent or a first SOx absorbent. It can be realized by supplying a reducing agent into the passage. Here, the air-fuel ratio of the exhaust gas means the air and fuel (hydrocarbons) supplied into the exhaust passage upstream of the engine intake passage and the SOx absorbent, the first SOx absorbent, or the NOx absorbent. Says the ratio.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention described in (1) or (2), 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, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases, and N ₂
It is a catalyst that reduces to. This occlusion reduction type NOx catalyst is
For example, alumina is used as a carrier, and potassium K, sodium Na, lithium Li, alkali metals such as cesium Cs, alkaline earths such as barium Ba and calcium Ca, and rare earths such as lanthanum La and yttrium Y are used on the carrier. At least one selected and platinum Pt
Examples thereof include those carrying a noble metal such as

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention described in (1) or (2), the exhaust path switching means is composed of a single switching valve provided at a branch portion of the bypass passage. Can be done, or N rather than branching
A first opening / closing valve is provided in the exhaust passage near the Ox absorbent, a second opening / closing valve is provided in the bypass passage, and the other opening / closing valve is closed when one opening / closing valve is opened. You can also

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention described in (1) above, the SOx absorbent may be composed of a NOx storage reduction catalyst, or a catalyst having no storage agent (for example, alumina). It can also be composed of a catalyst consisting of only).

In the exhaust gas purification device for an internal combustion engine according to the present invention described in (2) above, the first SOx absorbent can be composed of a NOx storage reduction catalyst. Further, the second SOx absorbent is preferably one having a strong SOx storage power of the storage agent and a large SOx storage amount, and further, in order to suppress the SOx release property, the support amount of a noble metal such as Pt is lowered. It is more preferable to use a base metal instead of the noble metal, or to reduce the dispersibility of the noble metal.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments 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.

[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 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 therein, 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. Is exhaust pipe 2
It is connected via 2 to a muffler (not shown). Hereinafter, the NOx storage reduction catalyst 20 will be referred to as the NOx catalyst 20. The SOx absorbent 17 and the NOx catalyst 20 will be described in detail later.

The inlet pipe portion 21a of the casing 21 and the exhaust pipe 22 are also connected by a bypass passage 26 that bypasses the NOx catalyst 20. An exhaust gas switching valve (exhaust flow switching means) 28 whose valve body is operated by an actuator 27 is provided at an inlet portion 21a of the casing 21 which is a branch portion of the bypass pipe 26. This exhaust switching valve 28 closes the inlet of the bypass pipe 26 by the actuator 27 as shown by the solid line in FIG.
Either the bypass closed position in which the inlet to 0 is fully opened, or the bypass open position in which the inlet to the NOx catalyst 20 is closed and the inlet of the bypass pipe 26 is fully opened as shown by the broken line in FIG. One position can be selected and 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
A temperature sensor 23 that generates an output voltage proportional to the temperature of the exhaust gas that has passed through the SOx absorbent 17 is attached in the unit 9. The output voltage of the temperature sensor 23 is an AD converter 38.
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, the lean air-fuel ratio, and K
When> 1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, the rich air-fuel ratio.

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 at a constant speed operation of 120 km / h or more, the value of the correction coefficient K is set to 1.0 and theoretical air-fuel ratio control is performed, and in the engine full load operation region, the value of the correction coefficient K is larger than 1.0. It is set so that the rich air-fuel ratio control is performed.

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 of the inflowing exhaust gas decreases.
It absorbs and releases.

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 of the air-fuel mixture supplied to the combustion chamber 3 decreases.

The detailed mechanism of the action of NOx absorption and release by the NOx catalyst 20 is not clear in some parts. 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 a carrier, but other noble metals,
The same mechanism can be achieved by using alkali metals, alkaline earths and rare earths.

That is, when the air-fuel ratio of the inflowing exhaust gas becomes considerably lean, the oxygen concentration of the inflowing exhaust gas greatly increases, and as shown in FIG. 4 (A), oxygen O ₂ becomes O ₂ ⁻ or O ^{2 −.} It adheres to the surface of platinum Pt in the form of. 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 of the inflowing exhaust gas is high, platinum P
As long as NO ₂ is generated on the surface of t and the NOx absorption capacity of the NOx catalyst 20 is not saturated, NO ₂ is absorbed in the NOx catalyst 20 and nitrate ion NO ₃ ⁻ is generated.

On the other hand, when the oxygen concentration of the inflowing exhaust gas decreases and the amount of NO ₂ produced decreases, the reaction proceeds in the opposite direction (N
O ₃ ^- → NO ₂₎ proceeds to nitrate ions N of the NOx catalyst 20
O ₃ ⁻ is released from the main catalyst 20 in the form of NO ₂ or NO. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx catalyst 20.
As shown in FIG. 3, when the lean degree of the inflowing exhaust gas is low, the oxygen concentration of the inflowing exhaust gas is low, and therefore, when the leaning degree of the inflow exhaust gas is low, NOx is reduced.
NOx is released from the catalyst 20.

On the other hand, at this time, when the air-fuel mixture supplied into the combustion chamber 3 is stoichiometric or rich and the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, as shown in FIG. Unburned HC and CO are discharged, and these unburned HC and CO are oxygen O ₂ ⁻ or O ²⁻ on platinum Pt.
It reacts with and is oxidized.

Further, when the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, the oxygen concentration of the inflowing exhaust gas extremely decreases, so NO ₂ or NO is released from the NOx catalyst 20, and this NO ₂ or NO is As shown in FIG. 4B, 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
NOx discharged from the NOx catalyst and 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 set to the stoichiometric air-fuel ratio or the rich air-fuel ratio, the NOx catalyst 2 is exhausted within a short time.
NOx will be released from 0.

As described above, when the exhaust air-fuel ratio becomes lean, NOx is absorbed by the NOx catalyst 20, and when the exhaust air-fuel ratio is set to the stoichiometric air-fuel ratio or the rich air-fuel ratio, NOx becomes the NOx catalyst 2.
It is released from 0 in a short time and reduced to N ₂ . Therefore, it is possible to prevent the emission of NOx into the atmosphere.

In this embodiment, as described above, the air-fuel mixture supplied to the combustion chamber 3 has a rich air-fuel ratio during full load operation, and during high load operation and warm-up operation during engine startup. During acceleration, acceleration, and constant speed operation of 120 km / h or more, the air-fuel mixture has a stoichiometric air-fuel ratio, and during low-medium load operation, the air-fuel mixture has a lean air-fuel ratio, so NOx in exhaust gas during low-medium-load operation is reduced. It is absorbed by the NOx catalyst 20, and NOx is released and reduced from the NOx catalyst 20 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 and medium load operation is high and the operation time is long, NO
The release and reduction of x cannot be done in time, and the N of the NOx catalyst 20
The absorption capacity of Ox is saturated and NOx cannot be absorbed.

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.
In the following description, the lean / rich spike control is referred to as "lean" and "spike theoretical air-fuel ratio or rich air-fuel ratio" are alternately repeated in a relatively short cycle. 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 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 . This sulfate BaSO ₄ is stable and difficult to decompose, and is not decomposed even if the air-fuel ratio of the inflowing exhaust gas is changed to the stoichiometric air-fuel ratio or the rich air-fuel ratio for a short time due to the rich spike in the lean rich spike control described above. NO
x It remains in the catalyst 20. 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
It is poisoned.

Therefore, in this exhaust emission control device, when the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio, the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio so that SOx does not flow into the NOx catalyst 20. Or when the rich air-fuel ratio is reached and the oxygen concentration drops, the absorbed SOx is released S
The Ox absorbent 17 is arranged upstream of the NOx catalyst 20. This SOx absorbent 17 is the SOx absorbent 1
When the air-fuel ratio of the exhaust gas flowing into 7 is a lean air-fuel ratio, it also absorbs NOx together with SOx, but when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio and the oxygen concentration decreases, the absorbed SOx Not only is NOx released. The SOx absorbent 17 is an NOx storage reduction catalyst,
It can be composed of a catalyst having no storage agent (for example, a catalyst composed of only alumina).

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, so that Downstream NOx catalyst 2
SOx does not flow into 0, and the NOx catalyst 20 absorbs only NOx in the exhaust gas.

On the other hand, 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 the stoichiometric air-fuel ratio or the rich air-fuel ratio and the oxygen concentration drops, the SOx absorbed in the SOx absorbent 17 is easily released from the SOx absorbent 17. .

By the way, according to the research by the present applicant, SOx
The following has been found regarding the absorbing and releasing action of the absorbent 17.
When the amount of SOx absorbed in the SOx absorbent 17 is small, the SOx absorbent 17 has a strong SOx adsorption force,
Even if a stoichiometric or rich air-fuel ratio exhaust gas is passed through the x absorbent 17 for a short time (for example, 5 seconds or less), SOx is not released from the SOx absorbent 17. With respect to this, the present applicant considers that the rich spike duration time in the lean rich spike control performed to release NOx from the NOx catalyst 20 when the SOx amount absorbed by the SOx absorbent 17 is small. It has been confirmed that SOx is not released from the SOx absorbent 17. However, SOx absorbent 17
Even when the amount of SOx absorbed in S is small, S
When exhaust gas with a stoichiometric or rich air-fuel ratio is allowed to flow through the Ox absorbent 17 for a long time, the SOx
Is released.

However, when the amount of SOx absorbed in the SOx absorbent 17 increases, the SOx adsorbing power of the SOx absorbent 17 becomes weaker, so that the exhaust gas having a stoichiometric or rich air-fuel ratio is supplied to the SOx absorbent 17 for a short time. Even if it flows, SOx will leak from the SOx absorbent 17 and NOx in the downstream will leak.
There is a risk of poisoning the catalyst 20.

Therefore, in this embodiment, the SOx absorbed in the SOx absorbent 17 is calculated from the history of the operating state of the engine.
The amount of the SOx absorbent 17 is estimated, and when the estimated SOx absorption amount reaches a predetermined amount, it is determined that the SOx absorbent 17 is to be regenerated, and a regeneration process for releasing SOx from the SOx absorbent 17 is executed. SOx
When executing the regeneration processing of the 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 substitutes the exhaust gas temperature at that time detected by the temperature sensor 23 as the temperature of the SOx absorbent 17 to determine the engine operating state. Based on the temperature of the SOx absorbent 17, the stoichiometric or rich conditions and the processing time that can release SOx most efficiently with less deterioration in fuel consumption are selected, and the exhaust gas under the selected air-fuel ratio condition is selected as the SOx absorbent 17 for the selected processing time. Run by running.

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 above a predetermined temperature (for example, 550 ° C.). 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, a large amount of SOx released from the SOx absorbent 17 is included in the exhaust gas flowing out from the SOx absorbent 17 (hereinafter referred to as regeneration exhaust gas).
Therefore, if this regenerated exhaust flows into the NOx catalyst 20, the SOx in the regenerated exhaust is absorbed by the NOx catalyst 20, and the NOx catalyst 20 is poisoned by SOx, and SO
x There is no point in providing the absorbent 17. Therefore, in this exhaust emission control device, in order to prevent the SOx released from the SOx absorbent 17 from being absorbed by the NOx catalyst 20 during the regeneration processing of the SOx absorbent 17, the exhaust switching is performed during the regeneration processing of the SOx absorbent 17. The valve 28 is held at the bypass open position so that the regeneration exhaust gas flowing out from the SOx absorbent 17 is guided into the bypass pipe 26.

If the exhaust gas switching valve 28 is held at the bypass open position while the SOx absorbent 17 is being regenerated as described above, the regenerated exhaust gas should not normally flow into the NOx catalyst 20. Since the exhaust switching valve 28 does not have a perfect sealing property, some regenerated exhaust gas actually leaks from the exhaust switching valve 28 and flows into the NOx catalyst 20. Therefore, the NOx catalyst 20 absorbs SOx in the regenerated exhaust gas.

Further, when the SOx absorbent 17 is not regenerated, the exhaust switching valve 28 is held at the bypass closed position to shut off the bypass pipe 26, so that the lean rich spike controlled exhaust gas is discharged from the SOx absorbent 17. NOx catalyst 2
Flow to 0. At this time, since SOx in the exhaust gas is absorbed by the SOx absorbent 17, the exhaust gas from which the SOx has been removed flows into the NOx catalyst 20, and the NOx is absorbed, released, and reduced and purified.

However, since the SOx absorbent 17 is exposed to the high temperature of the exhaust gas, thermal deterioration occurs over time. As this heat deterioration progresses, the SOx adsorption force decreases, and SOx is desorbed from the SOx absorbent 17 even when exhaust gas of the rich spike during lean / rich spike control flows, and SOx is desorbed as such. The separated SOx is also absorbed by the NOx catalyst 20.

Therefore, SOx is provided upstream of the NOx catalyst 20.
The absorbent 17 is provided, and the regeneration exhaust of the SOx absorbent 17 is caused to flow through the bypass pipe 26 to regenerate the NOx catalyst 20.
Even if it is prevented from flowing into the
It is difficult to completely prevent Ox poisoning.

Therefore, in this exhaust emission control device, the NOx catalyst 20 is also subjected to SOx release processing, thereby reducing NOx.
The NOx purification capacity of the catalyst 20 is kept high for a long period of time. In the present embodiment, the ECU 30 determines the SOx release processing execution timing for the NOx catalyst 20, the engine operating state (engine speed, engine load, rich spike frequency, etc.), the S concentration of fuel, and the exhaust gas of the exhaust switching valve 28. The SOx amount absorbed by the NOx catalyst 20 is estimated and calculated from the gas leakage amount (leak rate), and S
It is assumed that the amount of Ox reaches a predetermined amount set in advance. still,
The SOx release process of the NOx catalyst 20 is performed by keeping the temperature of the NOx catalyst 20 at a high temperature equal to or higher than a predetermined temperature and continuously flowing the exhaust gas having a stoichiometric or rich air-fuel ratio to the NOx catalyst 20 for a predetermined time.

Here, when exhaust gas having a stoichiometric or rich air-fuel ratio is passed through the NOx catalyst 20 to perform SOx release processing on the NOx catalyst 20, the SOx absorbent 17 is provided upstream of the NOx catalyst 20. Therefore, the exhaust gas having the stoichiometric or rich air-fuel ratio flows through the SOx absorbent 17 before flowing into the NOx catalyst 20. Therefore, when SOx is absorbed in the SOx absorbent 17, N
When the SOx releasing process of the Ox catalyst 20 is performed, the SOx absorbent 1
There is a risk that SOx will be released from 7, the SOx will re-poison the NOx catalyst 20, and the SOx release of the NOx catalyst 20 cannot be executed effectively.

Therefore, in this exhaust emission control device, before the SOx releasing process is performed on the NOx catalyst 20, the regeneration process is performed on the SOx absorbent 17 before the SOx releasing process is performed.
After releasing SOx from the Ox absorbent 17, the NOx catalyst 2
The SOx release process of 0 was executed. With this configuration, when exhaust gas having a stoichiometric or rich air-fuel ratio for the SOx release process to the NOx catalyst 20 is flowed, even if the exhaust gas passes through the SOx absorbent 17, the SOx absorbent 17 is exhausted.
The SOx is not released from the absorbent 17, and the exhaust gas having a stoichiometric or rich air-fuel ratio that has passed through the SOx absorbent 17 is NO.
It flows into the x-catalyst 20, is released from the NOx catalyst 20, and is effectively used for reducing to SO ₂ .

Next, during the NOx purification process, the SOx absorbent 17
The flow of the exhaust gas at the time of the regeneration process and the process of SOx release of the NOx catalyst 20 will be described. <NOx Purifying Process> First, the NOx purifying process will be described. At this time, the NOx in the exhaust gas is removed by the NOx catalyst 2
In order to perform intake / release at 0 and reduction purification, lean / rich spike control of the air-fuel ratio is executed, and the exhaust gas switching valve 28 is held at the bypass closed position as shown by the solid line in FIG.
Therefore, at this time, the exhaust gas flowing out from the SOx absorbent 17 flows into the NOx catalyst 20. Then, SOx in the exhaust gas is absorbed by the SOx absorbent 17, the exhaust gas from which the SOx has been removed flows into the NOx catalyst 20, and the NOx in the exhaust gas is absorbed and released by the NOx catalyst 20 and is reduced and purified. It will be.

<Regeneration Treatment of SOx Absorber 17> Next, S
When SOx should be released from the Ox absorbent 17, that is, SOx
The process of regenerating the absorbent 17 will be described. At this time, the temperature control of the exhaust gas is executed, the temperature of the SOx absorbent 17 is controlled to be equal to or higher than the SOx release temperature, the air-fuel ratio is switched from lean / rich spike control to stoichiometric air-fuel ratio or rich air-fuel ratio control, and at the same time the exhaust gas is switched. The valve 28 is switched and held from the bypass closed position to the bypass open position shown by the broken line in FIG. When exhaust gas having a stoichiometric or rich air-fuel ratio flows into the SOx absorbent 17, SOx is released from the SOx absorbent 17, but the regenerated exhaust that has flowed out of the SOx absorbent 17 at this time flows into the bypass pipe 26 and the NOx catalyst. Almost no inflow into 20. Therefore, the NOx catalyst 20 is converted to SOx by the SOx in the regenerated exhaust gas.
There is almost no poisoning. The SOx released from the SOx absorbent 17 is reduced by unburned HC and CO in the exhaust gas and released as SO ₂ .

Here, the regeneration process of the SOx absorbent 17 is as follows.
Basically, it is executed when the ECU 30 determines that the SOx amount absorbed by the SOx absorbent 17 has reached a predetermined amount, but the ECU 30 determines that the SOx release process needs to be performed on the NOx catalyst 20. Even when this is done, the SOx absorbent 17 regeneration processing is executed prior to the SOx release processing of the NOx catalyst 20.

Next, when the regeneration process of the SOx absorbent 17 should be stopped and returned to the NOx purification process again, the exhaust gas temperature control for the regeneration process is stopped and the air-fuel ratio is changed from the stoichiometric air-fuel ratio or the rich air-fuel ratio control to the lean air-fuel ratio. The switching to the rich spike control is performed, and 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. 1.

<SOx Release Process of NOx Catalyst 20> Next,
The SOx release processing for the NOx catalyst 20 will be described. At this time, exhaust gas temperature control is executed and NO
The temperature of the x catalyst 20 is controlled to a predetermined temperature or higher, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or the rich air-fuel ratio, and at the same time, the exhaust gas switching valve 28 is held in the bypass closed position shown by the solid line in FIG. The exhaust gas having a stoichiometric or rich air-fuel ratio passes through the SOx absorbent 17 and flows into the NOx catalyst 20. As a result, the SOx absorbed in the NOx catalyst 20 is released, and the released SOx is reduced by unburned HC and CO in the exhaust gas and released as SO ₂ .

Next, when the SOx release process of the NOx catalyst 20 should be stopped and returned to the NOx purification process again, the exhaust gas temperature control for the SOx release process is stopped and the air-fuel ratio is changed from the stoichiometric air-fuel ratio or the rich air-fuel ratio control. Switch to lean / rich spike control. When the air-fuel ratio of the exhaust gas becomes lean rich spike control, the NOx absorbent 20
Release of SOx from is stopped.

Next, with reference to FIG. 5, the SOx release processing execution routine for the NOx catalyst 20 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.

<Step 101> First, the ECU 30
In step 101, it is determined whether the NOx catalyst 20 needs SOx release processing. That is, the ECU 30 determines the NOx catalyst 2 from the operating state of the engine as described above.
The SOx amount absorbed to 0 is estimated and calculated, and when the SOx amount reaches a predetermined amount, it is determined that the SOx releasing process is necessary,
When it has not reached, it is determined that the SOx release process is not necessary.

<Step 102> When the affirmative determination is made in Step 101, that is, when it is determined that the SOx releasing process needs to be performed on the NOx catalyst 20, the ECU 30
Proceeds to step 102 and S for the NOx catalyst 20
Prior to the Ox releasing process, the SOx absorbent 17 is regenerated. At this time, the regeneration process of the SOx absorbent 17 is executed regardless of the amount of SOx absorbed in the SOx absorbent 17. At this time, since the exhaust gas switching valve 28 is held at the bypass open position, the regenerated exhaust gas flowing out from the SOx absorbent 17 flows into the bypass pipe 26. In the flow chart of FIG. 5, the SOx absorbent 17 is set to "S trap".
Is called.

<Step 103> Next, the ECU 30
In step 103, it is determined whether the SOx absorbent 17 regeneration process has ended. Here, the method of determining whether or not the regeneration process has ended depends on the type of the SOx absorbent 17. For example, when the SOx absorbent 17 absorbs SOx but has no occlusion performance (in other words, does not have an occlusion agent), this type of SOx absorbent is irrespective of the amount of SOx absorbed. Since the SOx is completely released when the regeneration process is performed for a certain period of time, it is possible to determine whether or not the regeneration is completed depending on whether or not the regeneration process is continuously executed for a predetermined time. Further, in the case of an SOx absorbent whose regeneration end cannot be determined by the duration of the regeneration process, an SOx sensor is provided immediately downstream of the SOx absorber 17, and the regeneration end time is set based on the detection value of the SOx sensor. Can be determined. When a negative determination is made in step 103, the ECU 30 returns to step 102 and S
The regeneration process of the Ox absorbent 17 is continued.

<Step 104> When the affirmative determination is made in step 103, the ECU 30 proceeds to step 104, and executes the SOx release processing of the NOx catalyst 20. At this time, the exhaust switching valve 28 is switched from the bypass open position to the bypass closed position and held, and the exhaust gas having the stoichiometric or rich air-fuel ratio passes through the SOx absorbent 17 and then NOx.
It flows into the catalyst 20. In this case, since the regeneration process of the SOx absorbent 17 has just been completed, the exhaust gas is SO
Even if the SOx absorbent 17 passes through, the SOx absorbent 17 does not release SOx. Then, the exhaust gas having a stoichiometric or rich air-fuel ratio that has passed through the SOx absorbent 17 becomes NO.
It is effectively used to release SOx from the x catalyst 20 and reduce it to SO ₂ .

<Step 105> Next, the ECU 30
In step 105, it is determined whether or not the SOx releasing process of the NOx catalyst 20 has been completed. In this embodiment,
S when SOx release processing is continuously executed for a predetermined time
It is determined that the Ox releasing process has been completed.

If a negative decision is made in step 105,
The ECU 30 returns to step 104 and continues the SOx release processing of the NOx catalyst 20. When a positive determination is made in step 105, the ECU 30 proceeds to return and returns to the NOx purification processing. Also, when the negative determination is made in step 101, the ECU 30 proceeds to return and returns to the NOx purification processing.

As described above, according to the exhaust purification system of this embodiment, since the SOx absorbent 17 is regenerated immediately before the SOx releasing process of the NOx catalyst 20, the SOx of the NOx catalyst 20 is regenerated. SOx absorbent 1 during release treatment
No SOx is released from 7, therefore S
It is possible to prevent the NOx catalyst 20 from being poisoned by SOx during the Ox releasing process. Further, SOx can be released from the NOx catalyst 20 to restore the NOx purification capacity.

In the first embodiment, E
Of the series of signal processing by the CU 30, step 102
The part that executes step can be called SOx absorbent regeneration means, and the part that executes step 104 is the NOx absorbent SOx.
It can be called a discharge means.

[Second Embodiment] Next, a second embodiment of the exhaust purification system for an internal combustion engine according to the present invention will be described. In the above-described first embodiment, when a small amount of exhaust gas containing SOx is allowed to flow into the NOx catalyst 20, and when the NOx catalyst 20 absorbs a predetermined amount of SOx,
SOx release processing is executed for the NOx catalyst 20 to obtain SOx.
Is emitted to recover the NOx catalyst 20 from SOx poisoning. However, in the second embodiment, the NOx catalyst 20 is recovered.
The SOx of the NOx catalyst 20 is prevented by injecting SOx into the
It completely prevents Ox poisoning. Therefore, in the second embodiment, the SOx releasing process is not performed on the NOx catalyst 20.

Next, an exhaust purification system of the second embodiment will be described with reference to FIG. In the second embodiment, the SOx absorbent 24 is housed inside the casing 21 also on the upstream side of the NOx catalyst (NOx absorbent) 20. That is, the SOx absorbent 24 is arranged downstream of the exhaust switching valve (exhaust path switching means) 28 and upstream of the NOx catalyst (NOx absorbent) 20. Where this second
In the embodiment, the SOx absorbents 17 and 24 will be referred to as the first SOx absorbent 17 and the second SOx absorbent 24, respectively.

The first SOx absorbent 17 is the same as the SOx absorbent 17 of the first embodiment, and when the air-fuel ratio of the inflowing exhaust gas is lean, it absorbs SOx and inflows the exhaust gas. It is a SOx absorbent that releases SOx absorbed when the oxygen concentration is low. Therefore, by the reproduction processing under the same conditions as in the first embodiment, the first
The SOx absorbed in the SOx absorbent 17 can be released.

On the other hand, the second SOx absorbent 24
Is SO when the air-fuel ratio of the inflowing exhaust gas is lean.
SOx that absorbs x but does not release the absorbed SOx even if the oxygen concentration of the inflowing exhaust gas becomes low
Absorbent is used.

The main purpose of providing the second SOx absorbent 24 is to reduce S during the regeneration process for the first SOx absorbent 17.
Regenerated exhaust gas with a high Ox concentration leaks through the exhaust gas switching valve 28 and becomes N
SOx in the regeneration exhaust when flowing toward the Ox catalyst 20
Is stored so that SOx does not flow into the NOx catalyst 20.

By the way, even if the regeneration exhaust gas leaks through the exhaust gas switching valve 28 during the regeneration process, the amount of regeneration exhaust gas that leaks is extremely small because the leakage rate of the exhaust gas switching valve 28 is about several percent. The exhaust gas flows through the second SOx absorbent 24 at an extremely low space velocity (hereinafter, abbreviated as SV). Further, during the regeneration process, the second SOx absorbent 24 is cooled by heat dissipation. Therefore, the second SOx absorbent 24 is required to have a function of storing a high concentration of S in the rich region of the air-fuel ratio when the temperature is lowered at a low SV.

In order to meet such a request, the second S
As the Ox absorbent 24, the SOx storage power of the storage agent is strong and S
Ox storage amount is large, and in order to suppress SOx release property, the amount of precious metal such as Pt supported is reduced, the dispersibility of this precious metal is reduced, and a base metal is used instead of the precious metal. . The second SO configured in this way
With the x absorbent 24, SOx that has been occluded once becomes extremely difficult to be desorbed, so no regeneration treatment is performed. Even if the regeneration process is not performed, the amount of SOx due to leakage of the regeneration exhaust gas is very small, so that even if the capacity of the second SOx absorbent 24 is small, its function can be sufficiently exerted for a long period of time.

As described above, according to the exhaust purification system of the second embodiment, the exhaust gas flowing into the NOx catalyst 20 always flows through the second SOx absorbent 24 before it, so that the exhaust gas Is absorbed by the second SOx absorbent 24, and the SOx once absorbed by the second SOx absorbent 24 does not desorb even if exhaust gas with a stoichiometric or rich air-fuel ratio flows in. Therefore, SOx does not flow into the NOx catalyst 20. Therefore, SOx poisoning of the NOx catalyst 20 can be completely prevented.

Since the other structure is the same as that of the exhaust gas purifying apparatus of the first embodiment, the same mode parts are designated by the same reference numerals in FIG. 6 and their explanations are omitted. The operation of the exhaust switching valve 28, the flow of exhaust gas, the action of the first SOx absorbent 17, and the action of the NOx catalyst 20 during the NOx purification process and the regeneration process of the first SOx absorbent 17 are also the first. Since they are the same as those in the embodiment, their description will be omitted. However, as described above, this second
In the embodiment, the SOx releasing process is not performed on the NOx catalyst 20.

[Third Embodiment] Next, a third embodiment of the exhaust emission control system for an internal combustion engine according to the present invention will be described with reference to the drawings of FIGS. 7 to 9. The third embodiment is a modification of the above-described second embodiment.

In the exhaust purification system of the third embodiment, the first SOx absorbent 17 and the NOx catalyst 20 are connected via the exhaust switching valve (exhaust path switching means) 50 having four ports. Therefore, by switching the valve body position of the exhaust switching valve 50 to switch the flow of exhaust gas,
The exhaust gas can be made to flow to the NOx catalyst 20 in a forward flow or a reverse flow, or can be made to flow by bypassing the NOx catalyst 20.

More specifically, the casing 18 containing the first SOx absorbent 17 is connected to the exhaust manifold 16 connected to the exhaust port of the engine, and the casing 18 switches the exhaust gas via the exhaust pipe 51. First of valve 50
Connected to a port. A second port of the exhaust switching valve 50 is connected to an exhaust pipe 52 that discharges exhaust gas to the atmosphere, and a third port of the exhaust switching valve 50 is an inlet of a casing 21 containing a NOx catalyst 20 and a second SOx absorbent 24. 21
a through the exhaust pipe 53, and the fourth port of the exhaust switching valve 50 is connected to the outlet 2 of the casing 21 through the exhaust pipe 54.
It is connected to 1b. The second SOx absorbent 24 is incorporated in the casing 21 on the inlet 21a side, and the NOx catalyst 20
Is built in on the outlet 21b side of the casing 21. Here, the first SOx absorbent 17, the NOx catalyst 20, and the second SOx absorbent 24 are exactly the same as those in the second embodiment described above.

The exhaust switching valve 50 is an actuator 55.
The valve body position is switched by driving the actuator 55, and the actuator 55 is controlled by the ECU. When the valve body of the exhaust switching valve 50 is positioned as shown in FIG. 7, the exhaust switching valve 50 includes the exhaust pipe 51 and the exhaust pipe 53.
And the exhaust pipe 52 and the exhaust pipe 54 are connected. At this time, the exhaust gas is discharged from the exhaust manifold 16 to the first exhaust gas.
SOx absorbent 17 → exhaust pipe 51 → exhaust pipe 53 → second SOx absorbent 24 → NOx catalyst 20 → exhaust pipe 54 → exhaust pipe 52, and is discharged to the atmosphere. The flow of the exhaust gas flowing from the inlet 21a to the outlet 21b of the casing 21 in this manner is referred to as "forward flow" in the following description, and the valve body position of the exhaust switching valve 50 at this time is referred to as "forward flow position".

Further, when the valve body of the exhaust switching valve 50 is positioned as shown in FIG.
1 and the exhaust pipe 54 and the exhaust pipe 52 and the exhaust pipe 53 are connected. At this time, the exhaust gas is exhaust manifold 16 → first SOx absorbent 17 → exhaust pipe 51 → exhaust pipe 54 → NOx. It flows in the order of catalyst 20 → second SOx absorbent 24 → exhaust pipe 53 → exhaust pipe 52 and is released to the atmosphere.
In this way, the outlet 21b of the casing 21 to the inlet 21a
In the following description, the flow of the exhaust gas flowing toward is referred to as "backflow", and the valve body position of the exhaust switching valve 50 at this time is referred to as "backflow position".

As described above, when the flow direction of the exhaust gas in the NOx catalyst 20 can be switched to the forward flow and the reverse flow, various usages thereof are conceivable. One of them is from the exhaust switching valve 50 to the NOx catalyst 20. There is one that utilizes the difference in the magnitude of the temperature drop of the exhaust gas due to the difference in the flow path length. The NOx catalyst 20 has a temperature range suitable for NOx purification, and the NOx purification rate deteriorates if the temperature of the NOx catalyst 20 is too low or too high. On the other hand, the flow path length until the exhaust gas flows into the NOx catalyst 20 is longer when the exhaust switching valve 50 is in the reverse flow position than when it is in the forward flow position. The temperature drop of the exhaust gas until it flows into the NOx catalyst 20 is larger when it is set to be higher than when it is set to the forward flow position. Therefore, when the exhaust gas temperature is high, the exhaust gas is caused to flow back through the NOx catalyst 20, and when the exhaust gas temperature is low,
By flowing the Ox catalyst 20 in a forward flow, the NOx catalyst 20 is maintained in a temperature range suitable for NOx purification.

In this exhaust emission control device, the first S
During the regeneration process for the Ox absorbent 17, the exhaust switching valve 5
The 0 valve body is positioned as shown in FIG. The valve body position of the exhaust switching valve 50 at this time is referred to as a "bypass position" in the following description. In this way, the exhaust switching valve 50 short-circuits the exhaust pipe 51 and the exhaust pipe 52, so that the regenerated exhaust flowing out from the first SOx absorbent 17 is exhausted from the exhaust pipe 51 to the exhaust pipe 5.
It flows to 2 and is released to the atmosphere. That is, the exhaust gas is N
It will flow around the Ox catalyst 20.

By the way, when the valve body of the exhaust switching valve 50 is set to the bypass position, the exhaust pipes 51 and 52 are also communicated with the exhaust pipe 53 and the exhaust pipe 54, but the exhaust pipe 53 and the exhaust pipe 54 are in the second position. Since the SOx absorbent 24 and the NOx catalyst 20 are connected to each other by the casing 21, the gas flowing through the exhaust pipes 53 and 54 has a large resistance, and most of the regenerated exhaust flows toward the exhaust pipe 52 having a low resistance. Should be. However, although a small amount of regenerated exhaust gas may flow into the exhaust pipes 53 and 54 which are in communication with the exhaust pipe 51, when regenerated exhaust gas flows in, the exhaust pipes having a short flow path to the casing 21 are provided. Easy to flow to 53. Therefore, the casing 2 connected to the exhaust pipe 53
The second SOx absorbent 24 is provided on the side of the inlet 21a of No. 1 so that even when the regenerated exhaust flows in, the SOx in the regenerated exhaust is absorbed by the second SOx absorbent 24, and the NOx catalyst 20 receives the SOx.
NOx catalyst 20
Completely prevent SOx poisoning. In addition, in the third embodiment, the exhaust pipe 52 constitutes a bypass passage.

[Fourth Embodiment] Next, a fourth embodiment of the exhaust emission control system for an internal combustion engine according to the present invention will be described with reference to the drawing of FIG. The structural difference between the fourth embodiment and the above-described third embodiment is as follows.

A casing 60 containing a third SOx absorbent 61 and an HC adsorbent 62 is provided in the middle of the exhaust pipe 54, and the exhaust pipe 54 serves as the outlet 2 of the casing 21.
Exhaust pipe 5 that connects 1b to the inlet 60a of the casing 60
4A, the outlet 60b of the casing 60, and the exhaust switching valve 50
The exhaust pipe 54B is connected to the fourth port of the. The third SOx absorbent 61 is the outlet 6 of the casing 60.
0b side, and the HC adsorbent 62 is contained on the inlet 60a side of the casing 60.

The third SOx absorbent 61 is made of the same SOx absorbent as the second SOx absorbent 24. HC adsorbent 62
Is a hydrocarbon (HC) when the temperature of the HC adsorbent 62 is equal to or lower than a predetermined temperature (hereinafter, this temperature is referred to as the HC desorption temperature).
And has a property of desorbing the adsorbed HC when the temperature exceeds the HC desorption temperature, and is composed of a porous body such as zeolite.

Generally, when the engine is started, the exhaust gas temperature is low and the NOx catalyst 20 has not reached the activation temperature. Therefore, even if the exhaust gas is passed through the NOx catalyst 20, it is not purified,
Therefore, HC in the exhaust gas may be released to the atmosphere. Therefore, in this exhaust purification system, the NOx catalyst 20
Until the temperature reaches the activation temperature, HC in the exhaust gas is adsorbed by the HC adsorbent 62 and HC release to the atmosphere is prevented.

For this reason, in the fourth embodiment, the valve body of the exhaust gas switching valve 50 is switched and controlled as follows.
When the exhaust gas temperature is equal to or lower than the HC desorption temperature, the valve body of the exhaust switching valve 50 is set to the forward flow position as shown in FIG. 10, and the exhaust gas is exhausted from the exhaust manifold 16 to the first SOx absorbent 1
7 → exhaust pipe 51 → exhaust pipe 53 → second SOx absorbent 24
→ NOx catalyst 20 → exhaust pipe 54A → HC adsorbent 62 → third SOx absorbent 61 → exhaust pipe 54B → exhaust pipe 52. At this time, since the NOx catalyst 20 has not reached the activation temperature, the exhaust gas passes through the NOx catalyst 20 without being purified. After that, the exhaust gas passes through the HC adsorbent 62, but at that time, the HC in the exhaust gas is absorbed by the HC adsorbent 6
Therefore, it is possible to prevent HC in the exhaust gas from being released to the atmosphere.

Next, when the exhaust gas temperature exceeds the HC desorption temperature, the valve body of the exhaust switching valve 50 is set to the reverse flow position, and the exhaust gas is exhausted from the exhaust manifold 16 to the first SOx absorbent 17 to the exhaust pipe. 51 → exhaust pipe 54B → third SOx absorbent 61 → HC adsorbent 62 → exhaust pipe 54A → NOx catalyst 20
The second SOx absorbent 24, the exhaust pipe 53, and the exhaust pipe 52 are flowed in this order. At this time, HC is desorbed from the HC adsorbent 61 when the exhaust gas flows through the HC adsorbent 61, and the desorbed H
C flows into the NOx catalyst 20 together with the exhaust gas. Then, at this time, the NOx catalyst 20 has also reached the activation temperature, and the HC desorbed from the HC adsorbent 61 also contains N in the exhaust gas.
It is purified by the NOx catalyst 20 together with Ox.

Also in this exhaust gas purification apparatus, during the regeneration process for the first SOx absorbent 17, the valve body of the exhaust gas switching valve 50 is set to the bypass position as in the case of the third embodiment. 51 and the exhaust pipe 52 are short-circuited, and the regeneration exhaust flowing out from the first SOx absorbent 17 is exhausted by the exhaust pipe 5
It flows from 1 to the exhaust pipe 52 and is released to the atmosphere. That is,
Exhaust gas will flow around the NOx catalyst 20.

As described above, when the valve body of the exhaust switching valve 50 is set to the bypass position, the exhaust pipes 51 and 52 will not be connected to the exhaust pipe 53.
And the exhaust pipe 54, and the exhaust pipe 53,
Although there is a small amount in 54 as well, there is a possibility that regeneration exhaust gas may flow.
In the fourth embodiment, the flow path length from the exhaust gas switching valve 50 to the inlet 21a of the casing 20 and the flow path length from the exhaust gas switching valve 50 to the outlet 60b of the casing 60 are almost the same. The regeneration exhaust may flow into both casings 21 and 60. Therefore, in the fourth embodiment, not only is the second SOx absorbent 24 provided on the inlet 21a side of the casing 21, but also the exhaust pipe 5
The third SOx absorbent 61 is also provided on the outlet 60b side of the casing 60 connected to the 4B so that the SOx in the regenerated exhaust also flows into the second SOx absorbent 24 or the third SOx even when the regenerated exhaust flows in. Of the NOx catalyst 20 and the HC adsorbent 62 by preventing the SOx from flowing into the NOx catalyst 20 and the HC adsorbent 62.
Completely prevent SOx poisoning. In the fourth embodiment as well, the exhaust pipe 52 constitutes a bypass passage.

[Other Embodiments] In each of the above-described embodiments, an 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 lean range farther than the stoichiometric air-fuel ratio, the SOx absorbent 17 is not used under normal engine operating conditions.
(Including the first SOx absorbent 17, the same applies hereinafter) and N
The air-fuel ratio of the exhaust gas flowing into the Ox catalyst 20 is extremely lean, and although SOx and NOx are absorbed,
SOx and NOx are rarely released.

Further, in the case of a gasoline engine, the exhaust gas flowing into the SOx absorbent 17 and the NOx catalyst 20 is changed by adjusting the air-fuel mixture supplied to the combustion chamber 3 to the stoichiometric air-fuel ratio or the rich air-fuel ratio as described above. The air-fuel ratio can be set to the stoichiometric air-fuel ratio or the rich air-fuel ratio to release the SOx and NOx absorbed by the SOx absorbent 17 and the NOx catalyst 20, but in the case of a diesel engine,
When the air-fuel mixture supplied to the combustion chamber has a stoichiometric air-fuel ratio or a rich air-fuel ratio, there is a problem that soot is generated during combustion and 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 the stoichiometric air-fuel ratio or the rich air-fuel ratio, in addition to burning the fuel to obtain the engine output, It is necessary to supply a reducing agent (for example, light oil as fuel) into 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 the stoichiometric air-fuel ratio or the rich air-fuel ratio.

[0112]

According to the exhaust gas purification apparatus for an internal combustion engine of the first invention of the present application, a SOx absorbent, a NOx absorbent,
The bypass passage, the exhaust path switching means, the SOx absorbent regenerating means, and the NOx absorbent SOx releasing means are provided, and S from the NOx absorbent by the NOx absorbent SOx releasing means is provided.
The Ox releasing process is performed by the SOx absorbent regenerating means.
Since the processing is executed after the regeneration processing of the absorbent, the SOx is not desorbed from the SOx absorbent during the SOx release processing of the NOx absorbent, and the NOx catalyst is not poisoned by SOx. Then, the SOx release processing of the NOx absorbent becomes effective.

According to the exhaust purification system of the internal combustion engine of the second invention of the present application, the first SOx absorbent, the NOx absorbent, the bypass passage, the exhaust path switching means, and the second SOx.
By providing the x absorbent and the SOx absorbent regenerating means, it is possible to reliably prevent the NOx absorbent from being poisoned by SOx.

[Brief description of drawings]

1 is a first embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention.
It is a schematic block diagram of the embodiment of.

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 SOx release processing execution routine for the NOx absorbent according to the first embodiment.

FIG. 6 is a second part of the exhaust purification system for an internal combustion engine according to the present invention.
It is a schematic block diagram in the embodiment of.

FIG. 7 is a third exhaust purification system for an internal combustion engine according to the present invention.
FIG. 6 is a configuration diagram of a main part at the time of forward flow in the embodiment of.

FIG. 8 is a third exhaust purification system for an internal combustion engine according to the present invention.
FIG. 6 is a configuration diagram of a main part at the time of backflow in the embodiment of FIG.

FIG. 9 is a third exhaust purification system for an internal combustion engine according to the present invention.
FIG. 6 is a configuration diagram of a main part at the time of bypass in the embodiment of FIG.

FIG. 10 is a configuration diagram of a main portion at the time of forward flow in the fourth embodiment of the exhaust emission control system for an internal combustion engine according to the present invention.

[Explanation of symbols]

1 Engine Main Body (Internal Combustion Engine) 3 Combustion Chamber 4 Spark Plug 11 Fuel Injection Valves 16, 19, 22, 51, 52, 53, 54, 54A,
54B Exhaust pipe (exhaust passage) 17 SOx absorbent, first SOx absorbent 20 NOx catalyst (NOx absorbent) 24 Second SOx absorbent 26 Bypass pipe (bypass passage) 28, 50 Exhaust switching valve (exhaust path switching) Means) 30 ECU 52 Exhaust pipe (bypass passage)

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI F01N 3/24 F01N 3/24 R 3/28 301 3/28 301C F02D 9/04 F02D 9/04 E 41/04 305 41 / 04 305A 41/14 310 41/14 310A (56) Reference JP-A-11-44211 (JP, A) JP-A-6-229230 (JP, A) JP-A-11-81987 (JP, A) Patent 2605580 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) F01N 3/08-3/28 F02D 9/04 F02D 41/04 F02D 41/14

Claims (2)

(57) [Claims] (A) SOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and SOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and it is absorbed when the oxygen concentration of the inflowing exhaust gas is low. SOx absorbent that releases SOx, (b)
NOx is arranged in the exhaust passage downstream of the SOx absorbent, and when the air-fuel ratio of the inflowing exhaust gas is lean.
A NOx absorbent that absorbs NOx and releases the NOx absorbed when the oxygen concentration of the inflowing exhaust gas is low;
A bypass passage that branches from the exhaust passage between the absorbent and the NOx absorbent to bypass the NOx absorbent to flow the exhaust gas; and (d) absorb the exhaust gas flowing out from the SOx absorbent into the NOx absorption. And an exhaust path switching means for selectively switching between the agent and the bypass passage, and (e) during the regeneration process in which SOx should be released from the SOx absorbent, the air-fuel ratio of the exhaust gas is changed to the stoichiometric air-fuel ratio or the rich air-fuel ratio. SOx absorbent regenerating means for controlling the exhaust path switching means so as to guide the exhaust gas flowing out of the SOx absorbent to the bypass passage, and (f) SOx absorbed in the NOx absorbent. SOx to be released from absorbent
NOx absorption that controls the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or the rich air-fuel ratio at the time of the release process, and controls the exhaust path switching means so as to guide the exhaust gas flowing out from the SOx absorbent to the NOx absorbent. SOx releasing means for releasing SOx from the NOx absorbent by the NOx absorbent SOx releasing means is executed after completion of regeneration processing of the SOx absorbent by the SOx absorbent regenerating means. Exhaust gas purification device for internal combustion engine. (A) SOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and SOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and it is absorbed when the oxygen concentration of the inflowing exhaust gas is low. A first SOx absorbent that releases SOx,
(B) NOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and NOx is absorbed when the oxygen concentration of the inflowing exhaust gas is low, which is arranged in the exhaust passage downstream of the first SOx absorbent. NOx absorbent that releases NOx, and (c)
A bypass passage branching from the exhaust passage between the first SOx absorbent and the NOx absorbent to bypass the NOx absorbent to flow exhaust gas; and (d) from the first SOx absorbent. An exhaust path switching means for selectively switching which one of the NOx absorbent and the bypass passage the exhausted exhaust gas flows, and (e) is arranged downstream of the exhaust path switching means and upstream of the NOx absorbent, A second SOx absorbent that absorbs SOx when the air-fuel ratio of the exhaust gas is lean and does not easily release the absorbed SOx even if the oxygen concentration of the inflowing exhaust gas becomes low; and (f) the first SOx absorption During the regeneration process for releasing SOx from the agent, the air-fuel ratio of the exhaust gas is controlled to the stoichiometric air-fuel ratio or the rich air-fuel ratio, and the exhaust gas flowing out from the first SOx absorbent is guided to the bypass passage. exhaust Exhaust purifying apparatus for an internal combustion engine characterized by comprising a SOx absorbent reproducing means for controlling the road switching means.