JP2001070754A - Exhaust gas treatment apparatus for lean burn internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress H2S emission amount from a lean burn internal occlusion engine without deteriorating the treatment capability of a NOx-occlusion type catalyst. SOLUTION: In this exhaust gas treatment apparatus, nickel oxide is added in 10-35 g per 1 liter of a three-way catalyst (30b) installed as a H2S emission suppressing catalytic apparatus in the downstream side of a NOx occlusion type catalyst (30a) or a nickel oxide together with other H2S emission suppressing agent is added to the three-way catalyst (30b) in 30-300% by mole ratio to Ba added as an occluding agent to the NOx catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for purifying exhaust gas of a lean burn internal combustion engine, and more particularly to a device for purifying hydrogen sulfide (H) into the atmosphere.
_{The present} invention relates to an exhaust gas purification device capable of suppressing the emission of 2S).

[0002]

[Related Background Art] During operation of an engine at a lean air-fuel ratio, a sulfur component (S component) contained in fuel and eventually exhaust gas reacts with oxygen in an exhaust gas purification device to generate sulfur oxides.
It is known that this sulfur oxide adheres to the catalyst, for example, as a sulfate. This adhering substance is released and released as sulfur dioxide during engine operation in which the catalyst is exposed to exhaust gas having a high temperature and a rich air-fuel ratio. In this case, particularly when the degree of richness of the exhaust air-fuel ratio is large, carbon dioxide is emitted. Hydrogen sulfide (H ₂ S) is produced together with sulfur. Since H ₂ S has an unpleasant odor, a problem arises when the amount of H ₂ S discharged into the atmosphere increases. In this connection, H ₂ S such as nickel is added to the catalyst.
A technique for suppressing the release of H ₂ S by adding a release inhibitor is known. For example, in an exhaust gas purification system using a three-way catalyst, H ₂ S generated by the S component attached to the three-way catalyst is used.
In some cases, nickel oxide is added as an H ₂ S release inhibitor, and in this case, usually about 5 grams per liter of catalyst capacity is added.

[0003] In an internal combustion engine, particularly in a lean-burn internal combustion engine, a lean combustion operation is performed in an engine operating range as wide as possible in order to improve fuel efficiency and exhaust characteristics. Since nitrogen oxides cannot be sufficiently purified by the three-way catalyst, some exhaust purification devices of lean-burn internal combustion engines are equipped with a storage-type NOx catalyst. It is desirable to suppress the amount of ₂ S release.

[0004]

Therefore, according to the above-mentioned prior art, a three-way catalyst comprising nickel oxide added to suppress H ₂ S emission is disposed downstream of the storage NOx catalyst. However, simply arranging this type of three-way catalyst of the known art releases H ₂ S from an exhaust purification device having a storage type NOx catalyst in which the amount of S occlusion, that is, the amount of H ₂ S generated is larger than that of the three-way catalyst. It is difficult to sufficiently control the amount.

[0005] An exhaust gas purifying apparatus in which nickel is added to a NOx storage reduction catalyst disposed downstream of a three-way catalyst has been proposed in Japanese Patent Laid-Open Publication No. Hei 10-317946. When nickel is added together with the occluding agent, the NOx purification ability may decrease. The present invention relates to a method for measuring the amount of H ₂ S released from a lean-burn internal combustion engine equipped with an exhaust purification device having a NOx storage catalyst by using NO.
An object of the present invention is to provide an exhaust gas purifying apparatus capable of sufficiently suppressing the purifying ability of the x storage catalyst without impairing it.

[0006]

According to a first aspect of the present invention, there is provided an exhaust gas purifying apparatus comprising: an H ₂ S emission suppressing catalyst device provided downstream of a NOx storage catalytic device;
It is characterized by adding 0 to 35 grams of nickel oxide. According to the first aspect of the invention, when the lean burn internal combustion engine is in, for example, a lean combustion operation state, NOx in the exhaust gas is stored in the NOx storage catalyst device, and a sulfur component (S component) in the exhaust gas is also reduced. It is stored in the NOx storage catalyst device. The storage amount of the S component is larger than that of the three-way catalyst. When the internal combustion engine is operated, for example, in a high load range and the NOx storage catalyst is exposed to a rich air-fuel ratio and a high-temperature atmosphere, a large amount of the S component stored in the NOx storage catalyst is released to generate H ₂ S. Is done. NO
Since the H ₂ S emission suppression catalyst device provided downstream of the x storage catalyst device contains at least about twice as much nickel oxide as the conventional three-way catalyst, the NOx storage catalyst device has no NOx storage device. H ₂ S generated from the S component released from the catalyst is occluded in the H ₂ S release suppressing catalyst device. Thus, the exhaust gas purifying apparatus of the present invention has a H ₂ S release control catalyst device is higher H ₂ S release suppression capability as compared to the exhaust gas purification system using the conventional three-way catalyst, the exhaust gas purifying device Thus, even when a large amount of H ₂ S is generated, the emission of H ₂ S into the atmosphere is suppressed, and the problem of the generation of odor due to the emission of H ₂ S is eliminated or greatly reduced. Also, unlike the conventional device in which nickel is added to the NOx storage catalyst device itself, the exhaust gas purification device of the present invention adds nickel oxide to the H ₂ S emission suppression catalyst device arranged downstream of the NOx storage catalyst device. Therefore, there is no possibility that the purification ability of the NOx storage catalyst is reduced. Furthermore, H ₂ and a suitable upper limit is set for the amount of nickel oxide in the S release control catalyst device, also H ₂ S release control catalyst device purifying capability other than H ₂ S release control function of itself Secured.

[0007] In the present invention, preferably, the H ₂ S emission suppressing catalyst device is a three-way catalyst device, and the three-way catalyst device preferably has 15 to 25 grams of nickel per liter of capacity. An oxide is added. According to this preferred embodiment, the amount of nickel oxide added to the three-way catalyst unit is at least about 3 in the prior art.
Since it is twice or more, even when a large amount of H ₂ S is generated at the time of release of the S component from the NOx storage catalyst device, the three-way catalyst device reliably stores H ₂ S. In addition, the upper limit of the amount of nickel oxide added is set to a value that ensures the three-way function of the three-way catalyst device more reliably, and the three-way catalyst device sufficiently purifies harmful components in exhaust gas. Done.

[0008] The exhaust gas purifying apparatus according to the second aspect of the present invention.
Is H downstream of the NOx storage catalyst device. _TwoS release control catalyst
The NOx storage catalyst device is equipped with a storage
30 to 300% H_TwoS release inhibitor
H_TwoIt is characterized in that it is added to the S release suppression catalyst device.
Preferably, the storage agent is, for example, nickel oxide.
50 to 100% H by mole ratio_TwoS release inhibitor
Added.

According to the invention described in claim 2, the release of H ₂ S into the atmosphere is prevented even when the S component released from the NOx storage catalyst amounts of H ₂ S generated. That is, the maximum value of the generation amount of H ₂ S mainly depends on the amount of N 2 to the NOx storage catalyst corresponding to the sulfur content storage capacity of the NOx storage catalyst.
It depends on the amount of the Ox storage agent added, but in the present invention, NO
the lower limit of the amount of H ₂ S release inhibitor to H ₂ S release control catalyst device in accordance with the amount of x absorbent is set,
The H ₂ S release suppressing catalyst device can store a large amount of H ₂ S. On the other hand, the upper limit of the added amount of the H ₂ S release inhibitor is set, and the purification performance other than the H ₂ S release suppression function of the H ₂ S release suppression catalyst device itself is ensured.

[0010] In the invention according to claim 1 or 2,
Preferably, the exhaust gas purification device includes a sulfur component release control (forcible S) that promotes the release of the sulfur component from the NOx storage catalyst device when the amount of storage of the sulfur component by the NOx storage catalyst device exceeds or is likely to exceed a predetermined amount. (Purge). According to this preferred aspect, when the purification performance of the NOx storage catalyst device is reduced due to the poisoning by the sulfur component, the purification performance can be regenerated by executing the sulfur component release control.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A lean-burn internal combustion engine equipped with an exhaust gas purifying apparatus according to a first embodiment of the present invention will be described below. The lean-burn internal combustion engine according to the present embodiment is an in-cylinder injection type spark ignition type in-line four-cylinder gasoline engine that can perform fuel injection during a compression stroke and an expansion stroke as needed in addition to fuel injection during an intake stroke. It is configured. The fuel injection mode of this in-cylinder injection engine varies in accordance with the change in the engine operating range, and the air-fuel ratio of the air-fuel mixture changes from the super-lean air-fuel ratio to the rich air-fuel ratio. Fuel efficiency and exhaust characteristics are improved while generating engine output. This type of in-cylinder injection engine is conventionally known, and will be briefly described below.

As shown in FIG. 1, an electromagnetic fuel injection valve 6 is mounted on a cylinder head 2 of an engine 1 together with a spark plug 4 for each cylinder. The fuel injection valve 6
A fuel supply device (not shown) having a fuel tank, a low-pressure fuel pump, and a high-pressure fuel pump is connected via a fuel pipe, and fuel in the fuel tank is supplied from a fuel injection valve 6 to a combustion chamber 8.
The fuel can be directly injected into the inside at a desired fuel pressure.

An intake port is formed in the cylinder head 2 in a substantially upright direction for each cylinder, and each intake port communicates with one end of an intake manifold 10. Intake manifold 1
The throttle valve 11 provided on the other end side of the throttle valve 0 is provided with a throttle sensor 11a for detecting a throttle opening degree θth. An exhaust port is formed in the cylinder head 2 in a substantially horizontal direction for each cylinder, and each exhaust port communicates with one end of the exhaust manifold 12.

The exhaust manifold 12 has an exhaust pipe (exhaust passage).
A muffler (not shown) is connected to the
The exhaust pipe 14 has a high temperature sensor 16 for detecting the exhaust gas temperature.
Is provided. The exhaust gas purification device of the present embodiment
A small proximity device arranged in the exhaust pipe 14 close to the gin 1
Three-way catalyst 20 in the exhaust pipe 14
And an exhaust purification catalyst device 30 arranged downstream of the
You. The exhaust purification catalyst device 30 includes a storage NOx catalyst (NO
x storage catalyst device) 30a and a three-way contact arranged downstream thereof
Medium (H _TwoSb catalyst 30b).
Reference numeral 32 denotes NOx which is disposed downstream of the three-way catalyst 30b.
This represents a NOx sensor 32 for detecting a concentration.

The storage type NOx catalyst 30a is made of platinum (P
t), a catalyst species comprising a noble metal such as rhodium (Rh) and a NOx storage agent comprising an alkali metal or an alkaline earth metal such as barium (Ba) or potassium (K), wherein NOx is contained in an oxidizing atmosphere. Once with nitrate X-NO
₃ and a function of reducing NOx to N ₂ (nitrogen) or the like mainly in a reducing atmosphere where CO is present. This storage type NOx catalyst 30a has NO
x as well as the oxide S of the sulfur component contained in the exhaust gas
Ox is also occluded as the sulphate X-SO _4, such as barium sulfate BaSO _4. As shown in the following reaction formula, this sulfate X-SO ₄ becomes SO ₂ when the storage NOx catalyst 30a is exposed to a reducing atmosphere, and at this time, hydrogen sulfide (H ₂
S) is generated.

BaSO ₄ + CO → BaCO ₃ + SO ₂ SO ₂ + H ₂ → H ₂ S + O _{2 If} sulfate adheres to the NOx catalyst 30a, the NOx is removed.
Since the purification efficiency is lowered, so-called forced S purge for increasing the NOx catalyst temperature and enriching the air-fuel mixture is generally performed periodically in an attempt to remove sulfate.
At this time, H ₂ S emitting an off-flavor is generated. Further, when the engine 1 is operated in a high load range such as when the vehicle is traveling on an uphill road or during an acceleration operation, the engine 1 is operated at a rich air-fuel ratio and the NOx catalyst temperature rises. ₂ is desorbed and released, and H ₂ S is generated accordingly (natural S purge).

As is apparent from the above reaction formula, H ₂ S
Is increased as the amount of sulfate (S component) attached to the NOx catalyst 30a increases. Adhesion amount i.e. storage amount of S components in the NOx catalyst, a ternary number than in the case of the catalyst, a large amount of H ₂ in the exhaust gas purification apparatus provided with a NOx catalyst
S tends to be generated. In the present embodiment, nickel oxide as an H ₂ S release inhibitor is added to the three-way catalyst 30b in order to suppress the release of H ₂ S into the atmosphere during the forced S purge or the natural S purge. . As shown in FIG. 2 and the following reaction formula, this nickel oxide
The x-catalyst 30a acts to convert H ₂ S released from the catalyst 30a to nickel sulfide, and thus the three-way catalyst 30b to which nickel oxide is added has a function of storing H ₂ S. The occluded H ₂ S reacts with oxygen in an oxidizing atmosphere to be converted into SO ₂ with low odor and is released.

H ₂ S + NiO → NiS + O ₂ NiS + 3/2 * O ₂ → NiO + SO ₂ As is apparent from the above reaction formula, the larger the amount of nickel oxide added to the three-way catalyst 30b, the higher the H of the three-way catalyst 30b.
_{Although the} 2S storage capacity increases, the three-way function of the three-way catalyst 30b tends to decrease.

In this embodiment, the NOx catalyst 30a sends H
Even when _{the 2} S is large amount released in order to ensure the H ₂ S adsorption capacity such as to allow occlude H ₂ S, the lower limit of the addition amount of the nickel oxide, volume per liter of the three-way catalyst 30b It is set to a value of 10 grams or more, preferably 15 grams or more. Also, in order to keep the three-way function of the three-way catalyst 30b from deteriorating, the upper limit of the amount of nickel oxide added is set to 35 g or less, preferably 25 g or less. That is, the amount of nickel oxide added to the three-way catalyst 30b is 10 to 35 grams, preferably 15 to 2 grams per liter of the capacity of the three-way catalyst 30b.
Set to a value within the range of 5 grams. Note that nickel may be added in place of nickel oxide. In this case, the upper and lower limits of the amount of nickel added can be obtained by converting the upper and lower limits of the amount of nickel oxide added.

In connection with the above-mentioned setting of the amount of nickel oxide added, the present inventors have determined that a ternary catalyst obtained by adding 0, 5, 10, 15, and 20 grams of nickel per liter of catalyst volume in terms of nickel oxide. NOx for each of the catalysts
An experiment was conducted in which the temperature of the catalyst was raised to release H ₂ S from the NOx catalyst while measuring the H ₂ S concentration in the exhaust gas downstream of the three-way catalyst. FIG. 3 shows a change in NOx catalyst temperature and a change in H ₂ S concentration over time for each of the three-way catalysts having different nickel addition amounts. In addition, a graph of FIG. 4 showing the relationship between the nickel oxide addition amount and the peak value of the H ₂ S concentration was created based on the experimental data of FIG.

As can be seen from FIG. 3, in the above nickel addition amount region, the larger the nickel addition amount, the higher the H ₂ content.
The S concentration decreases. Further, by adding 20 g of nickel oxide per liter of catalyst capacity to the three-way catalyst, the H ₂ S concentration peak value can be reduced to about one-fourth as compared with the case where the amount of nickel oxide added is 5 g per liter. It can be seen from FIG. 4 that it can be reduced.

According to the exhaust gas purifying apparatus of this embodiment configured as described above, even when the engine 1 is cold started, the close three-way catalyst 20 is quickly activated to purify the exhaust gas. , N during lean burn operation of the engine 1
NOx is stored by the Ox catalyst 30a. Further, at the time of natural S purge accompanying engine operation in a high load region, NO
H ₂ S released from the x catalyst 30a is occluded in the form of NiS by the NiO-added three-way catalyst 30b, and H ₂ S is released into the atmosphere.
₂ S emission is suppressed.

The exhaust gas purifying apparatus according to the present embodiment is configured so that the forced Sx is maintained in order to maintain the required NOx purifying efficiency of the NOx catalyst 30a.
Purging (more generally, sulfur component release control for promoting release of the S component from the NOx catalyst) can be performed. This forced S purge is performed under the control of the control means of the exhaust gas purification device. In the present embodiment, an electronic control unit (ECU) 40 that controls the operation of the engine 1 has the function of the control means.

The ECU 40 includes an input / output device and a storage device (R
OM, RAM, nonvolatile RAM, etc.), central processing unit (C
PU), a timer counter, and the like. Various sensors such as a throttle sensor 11a, a crank angle sensor 13, a high temperature sensor 16, and a NOx sensor 32 are connected to the input side, and the ignition plug 4 and the fuel injection are connected to the output side. The valve 6 and the like are connected.

In connection with engine operation control, the ECU 40
Is designed to select a fuel injection mode based on detection information input from various sensors and calculate a fuel injection amount, an ignition timing, and the like. For example, a target in-cylinder pressure (a target average effective pressure Pe) corresponding to the engine load based on the throttle opening information θth from the throttle sensor 11a and the engine rotation speed information Ne detected based on the crank angle information from the crank angle sensor 13 ) Is determined, and the fuel injection mode is set according to the target average effective pressure Pe and the engine rotation speed information Ne. Then, the fuel injection amount is determined based on a target air-fuel ratio (target A / F) set from the target average effective pressure Pe and the engine rotation speed Ne.

In connection with the forced S purge, the ECU 40
The forced S purge control routine shown in FIG. 5 is performed. Note that air-fuel ratio control for NOx purging is performed under the control of the ECU 40. Such air-fuel ratio control is conventionally known, and a description thereof will be omitted. In the forced S purge control routine, it is determined whether an S purge condition is satisfied (step S1). The S purge condition can be set in various ways. For example, it is possible to determine whether the S purge condition is satisfied when the total execution time of the lean combustion operation from the time when the previous forced S purge is completed reaches a predetermined time. In the present embodiment, the estimated amount Qs of SOx stored in the NOx catalyst 30a
Is determined when the S purge condition is satisfied.

In determining the S purge condition, the estimated SO
The x storage amount Qs is obtained, for example, from the following equation (1). Qs = Qs (n−1) + ΔQf · K−Rs (1) K = K1 · K2 · K3 (2) Rs = α · R1 · R2 · dT (3) where Qs (n−1) Is the previous value of the estimated SOx occlusion amount, ΔQf
Indicates an integrated fuel injection amount per execution cycle of the control routine.

K is a correction coefficient calculated from the above equation (2), and includes three S representing the degree of poisoning corresponding to the air-fuel ratio A / F, the S content in the fuel, and the catalyst temperature Tcat. It is expressed by the product of the poisoning coefficients K1, K2 and K3. Rs indicates the released S amount per control routine execution cycle, and is obtained from the above equation (3). In equation (3), α is a release rate (set value) per unit time, dT indicates an execution cycle of a fuel injection control routine, and R1 and R2 are catalyst temperatures Tcat.
And a release capacity coefficient corresponding to the air-fuel ratio A / F.

The catalyst temperature Tcat is obtained by correcting the exhaust gas temperature detected by the high temperature sensor 16 with the temperature difference read from a temperature difference map (not shown). The temperature difference map is set in advance by an experiment or the like. In this temperature difference map, the difference between the catalyst temperature Tcat and the exhaust gas temperature is given as a function of the target average effective pressure Pe and the engine speed information Ne.

If it is determined in step S1 that the S purge condition is satisfied, the S purge mode is set, and the timer for measuring the time elapsed from when the S purge mode is set is reset and then started (step S2). . Then, it is determined whether the H ₂ S release rate, that is, the degree of increase of the H ₂ S concentration with respect to time exceeds a predetermined value (step S3).

Although the H ₂ S release speed may be detected from the actual H ₂ S release speed information detected by the sensor, in the present embodiment, the engine speed Ne and the target average effective pressure P
e: An intake air amount A / N per intake stroke, vehicle speed, catalyst temperature Tcat, exhaust temperature, engine cooling water temperature, and the like are obtained from a map (not shown) according to an engine operating state represented by a function such as a function. . For this reason, an experiment for obtaining the H ₂ S release speed in various engine operating states is performed, and a map is created in advance based on the experimental results.

The predetermined value for determining the H ₂ S release rate is:
Value corresponding to the lower limit of the concentration of H ₂ S, such as feel the odor of H ₂ S to those on near the outlet of the exhaust pipe 14 (hereinafter,
Limit value). This limit value changes in particular according to the vehicle speed. That is, even when H ₂ S is released from the exhaust pipe 14 during traveling of the vehicle, H ₂ S is quickly diffused into the atmosphere, and the limit value at which odor is sensed is relatively high. On the other hand, when the vehicle stops running, H ₂ S is hardly diffused,
The limits will be small. Therefore, the predetermined value of the H ₂ S release speed is preferably set according to the vehicle speed. In the present embodiment, the limit value expressed as a function of the vehicle speed is mapped in advance, and the limit value determined according to the vehicle speed is determined. It reads from the map. However, it is not essential to variably set the predetermined value of the H ₂ S release rate, and a fixed value may be used.

Generally, immediately after the S purge mode is set, the H ₂ S release speed does not exceed the predetermined value, and the flow proceeds to step S3.
Is negative (No). In this case, the S purge operation is performed (Step S4). In the S purge operation, in order to expose the storage NOx catalyst 30a to a reducing atmosphere,
The target A / F is set to a predetermined rich air-fuel ratio (for example, 12) to perform the rich air-fuel ratio operation, and the exhaust air-fuel ratio is set to the rich air-fuel ratio. Further, the ignition timing is retarded to increase the exhaust gas temperature, thereby raising the temperature of the NOx catalyst 30a. When the engine is operated at such a rich air-fuel ratio, incomplete combustion of the fuel occurs, and a large amount of carbon monoxide hydrocarbon required for removing the sulfur oxide SOx is generated.
0a, the S purge is promoted in combination with a rise in the NOx catalyst temperature accompanying a rise in exhaust gas temperature due to ignition timing retard control.

When the release of SOx proceeds due to the progress of the S purge, H ₂ S is generated under a rich air-fuel ratio.
The nickel oxide added to the three-way catalyst 30b provided downstream of the x catalyst 30a converts H ₂ S into nickel sulfide (see FIG. 2). In this way, H ₂ S is converted to three-way catalyst 3
Since the ozone is stored in Ob, the H ₂ S concentration in the exhaust gas released into the atmosphere is reduced.

Subsequent to step S4 relating to the S purge operation, it is determined whether or not the S purge mode has been maintained for a predetermined time by referring to the elapsed time of the timer started in step S2 (step S5). If the S purge maintaining time does not reach the predetermined time, it is determined that the S component has not been sufficiently removed from the NOx catalyst 30b, and the process returns to step S3. Note that the determination of the end of the S purge operation may be performed on the condition that the estimated SOx storage amount Qs is equal to or less than a predetermined amount.

As described above, H ₂ S generated as a result of the reduction of the S component in the NOx catalyst 30a is stored in the three-way catalyst 30b, but the amount of the S component stored in the NOx catalyst 30a is large. For this reason, the H ₂ S release rate may temporarily increase during the S purge operation. In this case, if it is determined in step S3 that the H ₂ S release speed has exceeded the predetermined speed, switching from the S purge operation to the S purge suppression operation is performed to reduce the H ₂ S release speed (step S3). S
6).

In this S purge suppression operation, the target A / F is modulated with the stoichiometric air-fuel ratio as a boundary. That is, as shown in FIG. 6, the target A / F is set to the rich first target A / F or the lean second target A / F every predetermined cycle Tcycle.
Are set alternately. Here, the first and second target A / F
Is set to a value that sets the average value of the target A / F during the S purge suppression operation to a value that can promote the S purge (for example, about 14.3 which is a slight rich or about 12 on the rich side). .

This S purge suppression operation is performed by the three-way catalyst 30b.
The purpose of the present invention is to make the most of the H ₂ S storage / release function of nickel oxide added to steel. That is,
Before the H ₂ S occlusion action (H ₂ S + NiO → NiS + O ₂ ) by nickel oxide in the rich air-fuel ratio operation (reducing atmosphere) is saturated, the operation mode is switched from the rich air-fuel ratio operation to the lean air-fuel ratio operation (oxidizing atmosphere), and the three-way catalyst is used. 30b converts H ₂ S stored in the form of nickel sulfide into SO ₂ with low odor (NiS + 3/2 * O ₂ → NiO + SO ₂ ).

Note that, instead of the above-described A / F modulation in which the air-fuel ratio is switched at a predetermined cycle between the rich air-fuel ratio and the lean air-fuel ratio, the rich air-fuel ratio operation over a predetermined rich time and the lean operation over a predetermined lean time are performed. The air-fuel ratio operation may be performed alternately. In this case, the three-way catalyst 30b
Time at which the S component trapping action of the nickel oxide added to the metal is saturated, and the amount of sulfur trapped in the form of nickel sulfide
Considering that the release time required to release the component as SO ₂ varies depending on the engine operating conditions, etc.,
It is preferable that each of the enrichment time and the leaning time is set to a value equal to the saturation time and the release time determined according to the engine operating state and the like, but even if the enrichment time and the leaning time are set to fixed values, H ₂ S suppression effect can be obtained.

FIG. 7 shows NOx by ignition timing retard control.
The change in the concentration of H ₂ S in the exhaust gas downstream of the three-way catalyst 30b due to the temperature rise of the catalyst 30a is measured when the air-fuel ratio is maintained at a constant value of 14.3 and when the average air-fuel ratio becomes a value of 14.3. The case where / F modulation is performed will be described. As can be seen from FIG. 7, compared to the case of engine operation at a constant air-fuel ratio,
By performing A / F modulation that periodically switches the air-fuel ratio between the rich side and the lean side during engine operation, the emission of H ₂ S is suppressed despite the air-fuel ratio being equal. I understand.

[0041] When the S purge suppression operation at step S6 is performed, reduces the concentration of H ₂ S in the flue gas, therefore, H ₂
The S release rate also decreases. Step S following step S6
In 5, it is determined whether or not a predetermined time has elapsed since the setting of the S purge mode. If the determination result is negative, the process returns to step S3. In this manner, the S purge operation or the S purge suppression operation is selectively performed until the predetermined time elapses from the setting of the S purge mode, depending on whether the H ₂ S release speed exceeds the predetermined value. You.

If it is determined in step S5 that a predetermined time has elapsed from the time when the S purge mode was set, N
It is determined that the release of the S component stored in the Ox catalyst 30a, that is, the regeneration capability of the NOx catalyst 30a has been sufficiently performed, and the S purge mode is released (step S).
7), the forced S purge control ends. Hereinafter, an exhaust emission control device according to a second embodiment of the present invention will be described.

The basic structure of this exhaust gas purification apparatus is the same as that of the first embodiment, but the amount of nickel oxide (H ₂ S release inhibitor) added to the three-way catalyst 30b is reduced.
Compared to the first embodiment determined according to the capacity of b,
The amount of the H ₂ S release inhibitor added to the three-way catalyst 30b was
The difference is that it is determined according to the addition amount of the NOx storage agent to the x catalyst 30a.

As is clear from the description of the first embodiment, the amount of S component occluded by the NOx catalyst 30a (H ₂ S
Is determined mainly by the amount of the NOx storage agent (eg, Ba, K) added to the NOx catalyst 30a, and the H ₂ S storage capacity of the three-way catalyst 30b is
It is determined by the amount of the H ₂ S release inhibitor (for example, nickel oxide) added to the three-way catalyst 30b. In other words, the lower limit of the amount of the H ₂ S release inhibitor added to the three-way catalyst 30b is N
It can be determined according to the amount of the NOx storage agent added to the Ox catalyst 30a.

In view of the above, in the present embodiment, the lower limit of the amount of nickel oxide added to the three-way catalyst 30b is set to NOx
The molar ratio (molar amount of H ₂ S release inhibitor / molar amount of total NOx storage agent) is 30% or more, preferably 50%, based on the total amount of the NOx storage agent, ie, Ba and K, added to the catalyst 30a. It is set to the above value. In addition, three-way catalyst 3
If the amount of nickel oxide added to the three-way catalyst 30b is excessive, the three-way function of the three-way catalyst 30b is impaired. Therefore, the upper limit of the amount of nickel oxide added to the three-way catalyst 30b is set to the value of Ba to the NOx catalyst 30a. The molar ratio is set so as to be 300% or less, preferably 100% or less, based on the total amount of K and K added.

The configuration and operation of the exhaust gas purifying apparatus according to the present embodiment are substantially the same as those of the first embodiment, so that the description thereof will be omitted. In this embodiment, nickel oxide is used as the H ₂ S release inhibitor, but Pd, Mn, Fe,
Zn, Co, Cu, or the like may be used. Suitable amount of H ₂ S release inhibitors, H ₂ depending on the type of additive
Since the amount of S release is different, the preferred amount of addition is also different.
0% to 200% is preferable, and in the case of Fe, Zn and the like, 150% to 300% is preferable.

The present invention is not limited to the above embodiment, but can be variously modified. That is, in the first and second embodiments, the exhaust gas purification device is constituted by the close three-way catalyst and the exhaust gas purification catalyst device arranged downstream thereof, and the three-way catalyst is disposed downstream of the NOx catalyst of the exhaust gas purification catalyst device. Is provided as a H ₂ S emission suppression catalyst, but the present invention may be any as long as it includes a NOx storage catalyst device and an H ₂ S emission suppression catalyst device provided downstream of the NOx storage catalyst device. A three-way catalyst may be provided on each of the downstream sides, or a three-way catalyst may be provided on the downstream side of the two NOx storage catalysts. In the latter case, the ratio of the capacity of the upstream NOx catalyst to the capacity of the downstream NOx catalyst falls within the range of 1.2: 1 to 1.8: 1, that is, the downstream NOx catalyst is the upstream one. By making the capacity smaller than that of
Although the exhaust heat is deprived by the x catalyst, the temperature of the downstream NOx catalyst hardly rises. However, since the downstream NOx catalyst has a small capacity, the temperature easily rises even with a small amount of exhaust heat, and this disadvantage can be solved. . Further, the H ₂ S release suppressing catalyst can be constituted by a catalyst other than the three-way catalyst, and a catalyst containing an H ₂ S release suppressing agent such as nickel or its oxide as a main catalyst species is used as the H ₂ S release suppressing catalyst. It is possible.

In the exhaust gas purifying apparatuses of the first and second embodiments, the sulfur component release control (forced S purge) for promoting the release of the sulfur component from the NOx catalyst 30a is performed. Is H ₂ S by natural S purge
It is not essential to carry out the forced S purge in the present invention, since the release of methane can be suppressed. In the embodiment, when the H ₂ S release speed exceeds a predetermined value during the forced S purge, the S purge suppression operation is performed to suppress the release of H ₂ S. Is not essential in the present invention. That is, according to H ₂ S release control catalyst the H ₂ S release inhibitor was added in large amounts to the present invention formed by providing downstream of the NOx catalyst, H in force S purge without performing the S purge suppression operation ₂ S emission can be reduced.

In the present invention, sulfur component release control (forced S
When performing the purge, the control means for the forced S purge includes, as in the embodiment, an air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the lean burn internal combustion engine to a rich air-fuel ratio, and a NOx storage catalyst. It can be configured with a temperature raising control means for raising the temperature of the apparatus, but is not limited to this, and a control means including one of the air-fuel ratio control means and the temperature raising control means can be configured.

The temperature raising control means may be constituted by an ignition timing control means for retarding the ignition timing as in the embodiment, but is not limited to this. For example, in the case of an in-cylinder injection lean-burn internal combustion engine, the temperature increase control means can be constituted by fuel injection control means for performing additional fuel injection in the expansion stroke in addition to main fuel injection. Further, the temperature rise control means determines that the temperature of the NOx storage catalyst device is higher than its activation temperature and equal to or higher than a temperature suitable for desorbing the sulfur component stored in the NOx storage catalyst device or a lower temperature than the set temperature. In such a case, the temperature of the NOx storage catalyst device may be further increased in an assisted manner.

According to the exhaust gas purifying apparatus provided with the control means,
When the amount of sulfur component (S component) stored by the NOx storage catalyst device increases, sulfur component release control is performed. As a result, the NOx storage catalyst device is exposed to a rich air-fuel ratio and / or a high-temperature atmosphere, and the NOx storage catalyst is exposed. The release of the S component from the device is promoted, and the required NOx purification efficiency is maintained.
Further, the control means may respond to the catalyst regeneration information or the poisoning information due to the sulfur component. For example, based on the catalyst regeneration information, the catalyst regeneration frequency (frequency at which the sulfur component is desorbed) within a predetermined period is determined. When it is determined that the frequency is less than the predetermined frequency, or when the poisoning amount estimated from the poisoning information is larger than the predetermined value, the sulfur component release control is performed. The poisoning information can be obtained from the fuel consumption of the lean burn internal combustion engine and the operating time (vehicle traveling distance), or from the output of a NOx sensor that detects the NOx concentration in exhaust gas.

[0052]

According to the exhaust gas purifying apparatus of the present invention, 10 to 35 grams of nickel oxide is added per liter to the H ₂ S emission suppressing catalytic device provided downstream of the NOx storing catalytic device, or NOx Since the H ₂ S release inhibitor in a molar ratio of 30 to 300% with respect to the storage agent added to the storage catalyst device was added to the H ₂ S release suppression catalyst device,
The amount of H ₂ S released from a lean burn internal combustion engine equipped with an exhaust purification device having an Ox storage catalyst can be sufficiently suppressed without impairing the purification performance of the NOx storage catalyst.

[Brief description of the drawings]

FIG. 1 is a schematic view of a lean burn internal combustion engine equipped with an exhaust gas purification device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an H ₂ S emission suppression effect of nickel oxide added to a three-way catalyst of the exhaust gas purification device shown in FIG.

FIG. 3 is a diagram showing the time change of the three-way catalyst temperature and the time change of the H ₂ S concentration for each of the three-way catalysts having different nickel addition amounts.

FIG. 4 is a graph showing the relationship between the amount of nickel oxide added and the peak value of the H ₂ S concentration.

FIG. 5 is a flowchart of a forced S purge control routine executed by the electronic control unit shown in FIG. 1;

FIG. 6 is a diagram showing a temporal change of a target A / F in A / F modulation for an S purge suppression operation performed in the control routine of FIG. 5;

FIG. 7 is a diagram showing a temporal change in H ₂ S concentration in exhaust gas with a rise in catalyst temperature for both engine operation at a constant air-fuel ratio and engine operation with A / F modulation.

[Explanation of symbols]

1 engine (a lean burn internal combustion engine) 14 exhaust pipe (exhaust passage) 30 exhaust purification catalyst device 30a NOx catalyst (NOx storage catalytic device) 30b three-way catalyst (H ₂ S release control catalyst device)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuki Tamura 5-33-8 Shiba, Minato-ku, Tokyo Inside Mitsubishi Motors Corporation (72) Inventor Hiroyuki Nakashima 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Naoto Yamada 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Keisuke Tashiro 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Automobile Industry Co., Ltd. (72) Kazuo Koga 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Co., Ltd. (72) Uniform Iwachido Uniform 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Within Industrial Co., Ltd. (72) Inventor Ken Tanabe 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Industrial Co., Ltd. F-term (reference) 3G091 AA02 AA13 AA17 AA24 AA28 AB03 AB06 AB08 AB09 AB11 BA11 BA14 BA15 BA19 BA20 BA33 BA39 CB02 CB03 CB05 CB07 DB06 DB10 DB13 EA01 EA07 EA17 EA30 EA31 EA33 EA38 EA39 FA06 FA14 FB03 FB10 FB12 FC04 FC05 FC08 GB01W GB02W GB02Y GB03W18A03 GB03AGB04A04 BA15X BA16Y BA28Y BA30X BA31Y BA33X BA36Y BA37Y BA38X BA41X CA01 CC32 CC38 CC46 EA04

Claims (2)

[Claims] 1. An exhaust gas control system that is provided in an exhaust passage of a lean burn internal combustion engine and includes NO in exhaust gas when the exhaust air-fuel ratio is a lean air-fuel ratio.
a NOx storage catalyst device for storing x, and a hydrogen sulfide release suppression catalyst device provided downstream of the NOx storage catalyst device and containing 10 to 35 grams of nickel oxide per liter. Exhaust purification device for combustion internal combustion engine. 2. A NOx storage catalyst device provided in an exhaust passage of a lean burn internal combustion engine, to which a storage agent for storing NOx in exhaust gas is added when the exhaust air-fuel ratio is a lean air-fuel ratio; An exhaust purification device for a lean-burn internal combustion engine, further comprising a hydrogen sulfide emission suppression catalyst device provided downstream and having a hydrogen sulfide emission inhibitor added in a molar ratio of 30 to 300% with respect to the occluding agent. .