JP4221125B2 - Exhaust gas purification device for lean combustion internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for a lean combustion internal combustion engine, and more particularly to hydrogen sulfide (H₂The present invention relates to an exhaust emission control device capable of suppressing emission of S).
[0002]
[Related background]
During engine operation at a lean air-fuel ratio, the sulfur component (S component) contained in the fuel and thus in the exhaust gas reacts with oxygen in the exhaust purification device to produce sulfur oxide, which is converted into a catalyst, for example, as a sulfate. It is known to adhere to. This adhering substance is desorbed and released as sulfur dioxide during engine operation in which the catalyst is exposed to high-temperature and rich air-fuel ratio exhaust gas. Hydrogen sulfide (H with sulfur₂S) is generated. H₂Since S has a strange odor, H to the atmosphere₂A problem arises when the discharge amount of S increases. In this connection, the catalyst is H such as nickel.₂Add S release inhibitor and add H₂A technique for suppressing S release is known. For example, in an exhaust purification system using a three-way catalyst, H produced by the S component adhering to the three-way catalyst₂H for S₂Some S release inhibitors add nickel oxide. In this case, about 5 grams are usually added per liter of catalyst capacity.
[0003]
In an internal combustion engine, particularly a lean combustion internal combustion engine, lean combustion operation is performed in the widest possible engine operating range in order to improve fuel consumption and exhaust characteristics. Nitrogen oxides emitted from the internal combustion engine during this lean combustion operation Since the three-way catalyst cannot sufficiently purify the exhaust gas, an exhaust purification device for a lean combustion internal combustion engine is equipped with an occlusion-type NOx catalyst. Even in an internal combustion engine having this type of exhaust purification device, H₂It is desirable to suppress the amount of S released.
[0004]
[Problems to be solved by the invention]
Therefore, in view of the above known technique, H₂It is conceivable to arrange a three-way catalyst to which nickel oxide is added in order to suppress the S release, on the downstream side of the storage type NOx catalyst. Quantity or H₂H from an exhaust purification device having an occlusion-type NOx catalyst in which the amount of S produced is greater than that of a three-way catalyst₂It is difficult to sufficiently suppress the amount of S released.
[0005]
Further, an exhaust purification device in which nickel is added to a NOx occlusion reduction catalyst arranged downstream of a three-way catalyst has been proposed in Japanese Patent Laid-Open No. 10-317946. However, like this proposed device, an NOx occlusion agent and When nickel is added together, the NOx purification ability may be reduced.
The present invention relates to H from a lean burn internal combustion engine equipped with an exhaust purification device having a NOx storage catalyst.₂An object of the present invention is to provide an exhaust emission control device that can sufficiently suppress the S release amount without impairing the purification performance of the NOx storage catalyst.
[0008]
[Means for Solving the Problems]
  In the exhaust emission control device according to the first aspect of the present invention, H is disposed downstream of the NOx storage catalyst.₂S(Hydrogen sulfide)Equipped with a release suppression catalyst, NOx occlusion touchIn the medium30 to 300% H in molar ratio to the added occluding agent₂S release inhibitor H₂It is added to the S release inhibiting catalyst. Preferably, for example, nickel oxide with respect to the occluding agent is 50 to 100% H in molar ratio.₂Add S release inhibitor.
[0009]
  According to the first aspect of the present invention, a large amount of H is produced from the S component released from the NOx storage catalyst.₂Even when S is generated, H into the atmosphere₂S release is prevented. That is, H₂The maximum value of the amount of S produced is determined mainly according to the amount of NOx storage agent added to the NOx storage catalyst corresponding to the sulfur storage capacity of the NOx storage catalyst. In the present invention, however, the maximum value of H is determined according to the amount of NOx storage agent added.₂S release control touchTo mediumH₂The lower limit of the addition amount of S release inhibitor is set, H₂S release control touchMediumLarge amount of H₂S can be occluded. On the other hand, H₂The upper limit of the amount of S release inhibitor added is set, H₂S release control touchMediatorBody H₂Purification performance other than the S emission suppression function is also ensured.
[0010]
  In the invention according to claim 1,By sulfur component release control means,The NOx storage catalyst is subjected to sulfur component release control (forced S purge) that promotes the release of the sulfur component from the NOx storage catalyst when the storage amount of the sulfur component exceeds or exceeds a predetermined amount.
  further,Sulfur component release control meansIsDetected by hydrogen sulfide release rate detection meansWhen the increase degree of the hydrogen sulfide concentration in the exhaust gas exceeds a predetermined value variably set according to the vehicle speed, the release of the sulfur component of the NOx catalyst by the forced S purge is suppressed.
  According to this preferred embodiment, when the purification performance deteriorates due to poisoning by the sulfur component of the NOx storage catalyst device, the purification performance is regenerated by performing forced S purge, while the concentration of hydrogen sulfide in the exhaust gas increases. When the degree is high, the forced S purge is suppressed, so that the hydrogen sulfide release inhibitor (nickel oxide) added to the hydrogen sulfide release suppression catalyst is H.₂S occlusion / release function can be fully utilized.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a lean combustion internal combustion engine equipped with an exhaust emission control device according to a first embodiment of the present invention will be described.
The lean combustion internal combustion engine of the present embodiment is an in-cylinder injection spark ignition type in-line four-cylinder gasoline engine that can perform fuel injection in a compression stroke and an expansion stroke as needed in addition to fuel injection in an intake stroke. It is configured. The fuel injection mode in this in-cylinder injection engine changes variously according to changes in the engine operating range, and accordingly, the air-fuel ratio of the air-fuel mixture changes from the super-lean air-fuel ratio to the rich air-fuel ratio. The fuel consumption and the exhaust characteristics are improved while generating the engine output. This type of in-cylinder injection type engine is conventionally known, and will be briefly described below.
[0012]
As shown in FIG. 1, an electromagnetic fuel injection valve 6 is attached to the cylinder head 2 of the engine 1 together with a spark plug 4 for each cylinder. The fuel injection valve 6 is connected to a fuel supply device (not shown) having a fuel tank, a low-pressure fuel pump and a high-pressure fuel pump via a fuel pipe, and the fuel in the fuel tank is transferred from the fuel injection valve 6 to the combustion chamber 8. Can be directly injected at a desired fuel pressure.
[0013]
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 the intake manifold 10. A throttle valve 11 provided on the other end side of the intake manifold 10 is provided with a throttle sensor 11a for detecting a throttle opening θth. Further, 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.
[0014]
A muffler (not shown) is connected to the exhaust manifold 12 via an exhaust pipe (exhaust passage) 14, and the exhaust pipe 14 is provided with a high temperature sensor 16 for detecting the exhaust temperature.
The exhaust purification apparatus of this embodiment includes a small proximity three-way catalyst 20 disposed in the exhaust pipe 14 in the vicinity of the engine 1 and an exhaust purification disposed in the exhaust pipe 14 downstream of the proximity three-way catalyst 20. And a catalytic device 30. The exhaust purification catalyst device 30 includes a storage-type NOx catalyst (NOx storage catalyst device) 30a and a three-way catalyst (H₂S release suppression catalyst device) 30b. Reference numeral 32 represents a NOx sensor 32 that is arranged downstream of the three-way catalyst 30b and detects the NOx concentration.
[0015]
The storage-type NOx catalyst 30a includes a catalyst species made of a noble metal such as platinum (Pt) or rhodium (Rh), and a NOx storage agent made of an alkali metal or alkaline earth metal such as barium (Ba) or potassium (K). NOx is once converted to nitrate X-NO in an oxidizing atmosphere._ThreeNOx can be stored as N in a reducing atmosphere mainly containing CO.₂It has the function of reducing to (nitrogen) etc. In this NOx storage catalyst 30a, not only NOx but also oxide SOx of sulfur component contained in exhaust gas is barium sulfate BaSO._FourSulfate X-SO_FourAs occluded. As shown in the following reaction formula, this sulfate X-SO_FourIs the SO when the NOx storage catalyst 30a is exposed to a reducing atmosphere.₂In this case, hydrogen sulfide (H₂S) is generated.
[0016]
BaSO_Four+ CO → BaCO_Three+ SO₂
SO₂+ H₂→ H₂S + O₂
If sulfate is attached to the NOx catalyst 30a, the NOx purification efficiency is lowered. Therefore, the removal of the sulfate is intended, and generally the so-called forced S purge that raises the NOx catalyst temperature and enriches the air-fuel mixture. Is carried out periodically, but at this time it gives off a strange odor H₂S is generated. In addition, when the engine 1 is operated in a high load region, such as when the vehicle is traveling on an uphill road or during acceleration operation, the engine 1 is operated at a rich air-fuel ratio and the NOx catalyst temperature rises.₂Is desorbed and released, and with this, H₂S is generated (natural S purge).
[0017]
As is clear from the above reaction equation, H₂The amount of S generated increases as the amount of sulfate (S component) attached to the NOx catalyst 30a increases. The amount of S component adhering to the NOx catalyst, that is, the amount of occlusion, is larger than in the case of the three-way catalyst.₂S tends to be generated.
In this embodiment, H to the atmosphere during forced S purge or natural S purge₂In order to suppress the release of S, the three-way catalyst 30b has H₂Nickel oxide as an S release inhibitor is added. This nickel oxide, as shown in FIG. 2 and the following reaction formula, is released from the NOx catalyst 30a.₂The three-way catalyst 30b, which acts to convert S to nickel sulfide, and thus nickel oxide is added, is H₂The function of occluding S is exhibited. So occluded H₂S reacts with oxygen in an oxidizing atmosphere to reduce SO₂To be released.
[0018]
H₂S + NiO → NiS + O₂
NiS + 3/2 * O₂→ NiO + SO₂
As is clear from the above reaction formula, as the amount of nickel oxide added to the three-way catalyst 30b increases, the H of the three-way catalyst 30b increases.₂Although the S occlusion capacity is increased, on the other hand, the three-way function of the three-way catalyst 30b tends to be lowered.
[0019]
In the present embodiment, the NOx catalyst 30a to H₂Even when a large amount of S is released, H₂H that can store S₂In order to ensure the S occlusion capacity, the lower limit of the amount of nickel oxide added is set to a value of 10 grams or more, preferably 15 grams or more per liter of the capacity of the three-way catalyst 30b. Further, in order to keep the decrease in the three-way function of the three-way catalyst 30b to an acceptable level, the upper limit value of the addition amount of nickel oxide is set to a value of 35 grams or less, preferably 25 grams or less. That is, the amount of nickel oxide added to the three-way catalyst 30b is set to a value within the range of 10 to 35 grams, preferably 15 to 25 grams per liter of the three-way catalyst 30b. Note that nickel may be added instead 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.
[0020]
In connection with the setting of the above nickel oxide addition amount, the present inventors have respectively added three, five, three, and three-way catalysts each having 0, 5, 10, 15, and 20 grams of nickel per liter of catalyst capacity in terms of nickel oxide. The temperature of the NOx catalyst is increased so that the NOx catalyst₂H in the exhaust gas downstream of the three-way catalyst while releasing S₂An experiment for measuring the S concentration was conducted. FIG. 3 shows changes in NOx catalyst temperature over time and H₂The change in the S concentration is shown for each of the three-way catalysts having different nickel addition amounts. Further, based on the experimental data of FIG.₂The graph of FIG. 4 showing the relationship with the S concentration peak value was created.
[0021]
As can be seen from FIG. 3, in the nickel addition amount region, the larger the nickel addition amount, the higher the H₂S concentration decreases. In addition, by adding 20 grams of nickel oxide to a three-way catalyst per liter of catalyst capacity, the amount of nickel oxide added is 5 grams compared to 5 grams per liter.₂It can be seen from FIG. 4 that the S concentration peak value can be reduced to about 1/4.
[0022]
According to the exhaust purification apparatus of the present embodiment configured as described above, the proximity three-way catalyst 20 is quickly activated even when the engine 1 is cold-started, and exhaust purification is performed. During the lean combustion operation, NOx is occluded by the NOx catalyst 30a. Further, at the time of natural S purge accompanying engine operation in a high load region, H released from the NOx catalyst 30a.₂S is occluded in the form of NiS by the three-way catalyst 30b to which NiO is added, and H into the atmosphere₂S emission is suppressed.
[0023]
The exhaust purification apparatus of the present embodiment can perform forced S purge (more generally, sulfur component release control that promotes release of S component from the NOx catalyst) in order to maintain the required NOx purification efficiency of the NOx catalyst 30a. It has become. This forced S purge is performed under the control of the control means of the exhaust emission control device.
In the present embodiment, an electronic control unit (ECU) 40 that controls the operation of the engine 1 has the function of this control means.
[0024]
The ECU 40 includes an input / output device, a storage device (ROM, RAM, non-volatile RAM, etc.), a central processing unit (CPU), a timer counter, etc., and a throttle sensor 11a, a crank angle sensor 13, and a high temperature sensor 16 on its input side. Various sensors such as the NOx sensor 32 are connected, and the spark plug 4 and the fuel injection valve 6 are connected to the output side thereof.
[0025]
In connection with engine operation control, the ECU 40 selects a fuel injection mode based on detection information input from various sensors and calculates a fuel injection amount, an ignition timing, and the like. For example, the target in-cylinder pressure (target average effective pressure Pe) corresponding to the engine load based on the throttle opening degree information θth from the throttle sensor 11a and the engine rotational 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 rotational speed information Ne. Then, the fuel injection amount is determined based on the target air-fuel ratio (target A / F) set from the target average effective pressure Pe and the engine speed Ne.
[0026]
In connection with the forced S purge, the ECU 40 executes a forced S purge control routine shown in FIG. Note that air-fuel ratio control for NOx purging is performed under the control of the ECU 40, but such air-fuel ratio control is conventionally known and will not be described.
In the forced S purge control routine, it is determined whether or not 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 establishment of the S purge condition is determined when the estimated amount Qs of SOx stored in the NOx catalyst 30a reaches a predetermined amount.
[0027]
In determining the S purge condition, the estimated SOx occlusion amount Qs is obtained from the following equation (1), for example.
Qs = Qs (n-1) +. DELTA.Qf.K-Rs (1)
K = K1, K2, K3 (2)
Rs = α ・ R1 ・ R2 ・ dT (3)
Here, Qs (n−1) is the previous value of the estimated SOx occlusion amount, and ΔQf is the fuel injection integrated amount per execution cycle of this control routine.
[0028]
K is a correction coefficient calculated from the above equation (2), and three S poison coefficients representing the degree of poisoning corresponding to the air-fuel ratio A / F, the S content in the fuel, and the catalyst temperature Tcat, respectively. Expressed by the product of K1, K2 and K3.
Rs indicates the amount of released S per control routine execution cycle, and is obtained from the above equation (3). In equation (3), α is the release rate (set value) per unit time, dT indicates the execution period of the fuel injection control routine, and R1 and R2 are the catalyst temperature Tcat and the air-fuel ratio A / F, respectively. The corresponding release capacity factor is shown.
[0029]
The catalyst temperature Tcat is obtained by correcting the exhaust temperature detected by the high temperature sensor 16 with the temperature difference read from the temperature difference map (not shown). The temperature difference map is set in advance by experiments or the like. In this temperature difference map, the difference between the catalyst temperature Tcat and the exhaust temperature is given as a function of the target average effective pressure Pe and the engine rotational speed information Ne.
[0030]
If it is determined in step S1 that the S purge condition is satisfied, the S purge mode is set and a timer for measuring the elapsed time from the S purge mode setting time is reset (step S2). And H₂S release rate, ie H against time₂It is determined whether or not the increase degree of the S concentration exceeds a predetermined value (step S3).
[0031]
H₂The S release rate is the actual H detected by the sensor.₂In this embodiment, the engine speed Ne, the target average effective pressure Pe, the intake air amount A / N per intake stroke, the vehicle speed, the catalyst temperature Tcat, the exhaust temperature, the engine It is determined from a map (not shown) according to the engine operating state represented by a function such as cooling water temperature. For this reason, H in various engine operating states₂An experiment for determining the S release rate is performed, and a map is created in advance based on the experimental results.
[0032]
H₂The predetermined value for the determination of the S release rate is such that H near the exit of the exhaust pipe 14 is H₂H that makes you feel the odor of S₂A value corresponding to the lower limit value of the S concentration (hereinafter referred to as a limit value) is set. This limit value varies depending on the vehicle speed. That is, even when the vehicle is running, the exhaust pipe 14₂Even if S is released, H₂S is quickly diffused into the atmosphere, and the limit value for feeling odor is relatively high. On the other hand, H₂S is difficult to diffuse and the limit value is small. Therefore, H₂The predetermined value of the S release speed is preferably set according to the vehicle speed. In this 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 read from the map. I have to. However, H₂It is not essential to variably set the predetermined value of the S release rate, and a fixed value may be used.
[0033]
Generally, immediately after setting the S purge mode,₂The S release rate does not exceed a predetermined value, and the determination result in 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 the reducing atmosphere, the target A / F is set to a predetermined rich air-fuel ratio (for example, 12), the rich air-fuel ratio operation is performed, and the exhaust air-fuel ratio is set to the rich air-fuel ratio. And Further, the ignition timing is retarded to raise the exhaust gas temperature, thereby raising the temperature of the NOx catalyst 30a. By operating the engine at such a rich air-fuel ratio, incomplete combustion of the fuel occurs, a large amount of carbon monoxide hydrocarbons necessary for the removal of sulfur oxide SOx is generated and supplied to the NOx catalyst 30a, and the ignition timing is reduced. The S purge is promoted in combination with the increase of the NOx catalyst temperature accompanying the increase of the exhaust gas temperature by the retard angle control.
[0034]
When SOx release progresses as the S purge progresses, the H₂S is produced, but nickel oxide added to the three-way catalyst 30b provided downstream of the NOx catalyst 30a causes H to₂S is converted to nickel sulfide (see FIG. 2). Like this, H₂Since S is occluded in the three-way catalyst 30b, H in the exhaust gas released to the atmosphere₂S concentration is reduced.
[0035]
Following step S4 related to the S purge operation, it is determined whether or not the S purge mode is maintained for a predetermined time with reference to the elapsed time by the timer activated in step S2 (step S5). If the S purge maintenance time is less than the predetermined time, it is determined that the S component removal from the NOx catalyst 30b is not sufficiently performed, and the process returns to step S3. Note that the end of the S purge operation may be determined on condition that the estimated SOx occlusion amount Qs is equal to or less than a predetermined amount.
[0036]
As described above, H generated as the S component is reduced in the NOx catalyst 30a.₂S is occluded in the three-way catalyst 30b, but because of the large amount of the S component occluded in the NOx catalyst 30a, the H is increased during the S purge operation.₂The S release rate may temporarily increase. In this case, H₂When it is determined in step S3 that the S release rate exceeds the predetermined rate, H₂In order to reduce the S release speed, switching from the S purge operation to the S purge suppression operation is performed (step S6).
[0037]
In this S purge suppression operation, the target A / F is modulated with the theoretical air-fuel ratio as a boundary. That is, as shown in FIG. 6, the target A / F is alternately set to the first target A / F on the rich side or the second target A / F on the lean side every predetermined cycle Tcycle. Here, the first and second target A / F are average values of the target A / F during the S purge suppression operation, which are values that can promote S purge (for example, about 14.3 that is slightly rich or about about the rich side). 12) is set to each value.
[0038]
This S purge suppression operation is performed by using H of nickel oxide added to the three-way catalyst 30b.₂It is intended to make the best use of the S occlusion / release function. That is, H by nickel oxide in rich air-fuel ratio operation (reducing atmosphere)₂S occlusion (H₂S + NiO → NiS + O₂) Is switched from rich air-fuel ratio operation to lean air-fuel ratio operation (oxidation atmosphere) before saturation, and the three-way catalyst 30b occluded in the form of nickel sulfide₂SO with less odor₂(NiS + 3/2 * O₂→ NiO + SO₂).
[0039]
It should be noted that, instead of the above-described A / F modulation that switches the air-fuel ratio between the rich air-fuel ratio and the lean air-fuel ratio in a predetermined cycle, the rich air-fuel ratio operation over a predetermined rich time and the lean air-fuel ratio operation over a predetermined lean time May be performed alternately. In this case, the S component captured by the nickel oxide added to the three-way catalyst 30b is saturated, and the S component captured in the form of nickel sulfide is SO 2.₂In consideration of the fact that the release time required for release changes depending on the engine operating condition, etc., the rich time and lean time are equal to the saturation time and release time determined according to the engine operating condition, etc. Although it is preferable to set to a value, even if the enrichment time and the leaning time are set to fixed values, H₂S suppression effect can be obtained.
[0040]
FIG. 7 shows the H in the exhaust gas downstream of the three-way catalyst 30b as the temperature of the NOx catalyst 30a is increased by retarding the ignition timing.₂Changes in the S concentration are shown for the case where the air-fuel ratio is maintained at a constant value of 14.3 and for the case where A / F modulation is performed so that the average air-fuel ratio becomes the value of 14.3. As can be seen from FIG. 7, by performing A / F modulation that periodically switches the air-fuel ratio between the rich side and the lean side during engine operation, compared to the case of engine operation at a constant air-fuel ratio, Despite the same air-fuel ratio, H₂It can be seen that the release of S is suppressed.
[0041]
When the S purge suppression operation in step S6 is performed, H in the exhaust gas₂S concentration decreases, and therefore H₂The S release rate also decreases. In step S5 following step S6, it is determined whether or not a predetermined time has elapsed since the S purge mode setting point. If the determination result is negative, the process returns to step S3.
In this way, until a predetermined time elapses after the S purge mode is set,₂The S purge operation or the S purge suppression operation is selectively performed depending on whether or not the S release rate exceeds a predetermined value.
[0042]
When it is determined in step S5 that the predetermined time has elapsed from the time point when the S purge mode is set, the S component stored in the NOx catalyst 30a is sufficiently released, that is, the purification capacity of the NOx catalyst 30a is sufficiently regenerated. S purge mode is canceled (step S7), and forced S purge control is terminated.
Hereinafter, an exhaust emission control device according to a second embodiment of the present invention will be described.
[0043]
The basic configuration of the exhaust gas purification apparatus is the same as that of the first embodiment, but nickel oxide (H₂S release inhibitor) is added to the three-way catalyst 30b as compared with that of the first embodiment in which the addition amount is determined according to the capacity of the three-way catalyst 30b.₂The difference is that the addition amount of the S release inhibitor is determined according to the addition amount of the NOx storage agent to the NOx catalyst 30a.
[0044]
As is apparent from the description of the first embodiment, the S component occlusion amount (H₂The maximum value of the amount of S generated is mainly determined by the amount of NOx storage agent (for example, Ba, K) added to the NOx catalyst 30a, and is also determined by the three-way catalyst 30b.₂S storage capacity is H to three-way catalyst 30b.₂It depends on the amount of S release inhibitor (for example, nickel oxide) added. In other words, H to the three-way catalyst 30b₂The lower limit value of the addition amount of the S release inhibitor can be determined according to the addition amount of the NOx storage agent to the NOx catalyst 30a.
[0045]
From the above viewpoint, in the present embodiment, the lower limit value of the amount of nickel oxide added to the three-way catalyst 30b is set to a molar ratio (NOx storage agent to the NOx catalyst 30a, that is, the total addition amount of Ba and K is a molar ratio ( H₂The molar amount of S release inhibitor / the total molar amount of NOx storage agent) is set to 30% or more, preferably 50% or more. Further, since the three-way function of the three-way catalyst 30b is impaired when the amount of nickel oxide added to the three-way catalyst 30b is excessive, the upper limit value of the amount of nickel oxide added to the three-way catalyst 30b is set to NOx. The molar ratio is set to 300% or less, preferably 100% or less, with respect to the total amount of Ba and K added to the catalyst 30a.
[0046]
Since the configuration and operation of the exhaust emission control device of this embodiment are substantially the same as those of the first embodiment, description thereof is omitted. In the present embodiment, H₂Although nickel oxide is used as the S release inhibitor, Pd, Mn, Fe, Zn, Co, Cu, or the like may be used. H₂Suitable addition amount of S release inhibitor is H depending on the type of additive.₂For example, in the case of Mn, the molar ratio with respect to the added amount of NOx storage agent is preferably 100% to 200%, and in the case of Fe, Zn, etc., 150% to 300%. Is preferred.
[0047]
The present invention is not limited to the above embodiment, and can be variously modified.
That is, in the first and second embodiments, the exhaust purification device is configured by the proximity three-way catalyst and the exhaust purification catalyst device arranged downstream thereof, and the three-way catalyst is provided downstream of the NOx catalyst of the exhaust purification catalyst device. H₂Although provided as an S release suppression catalyst, the present invention is a NOx storage catalyst device and a H provided downstream thereof.₂S three-way catalyst may be disposed upstream and downstream of the NOx storage catalyst, for example, and a three-way catalyst may be provided downstream of the two NOx storage catalysts. Also good. In the latter case, the ratio of the capacity of the upstream NOx catalyst to the capacity of the downstream NOx catalyst is in the range of 1.2: 1 to 1.8: 1, that is, the downstream NOx catalyst is the upstream one. However, the exhaust NOx catalyst is usually taken away by the upstream NOx catalyst and the temperature of the downstream NOx catalyst is difficult to rise, but the downstream NOx catalyst has a small capacity. The temperature easily rises even with a small amount of exhaust heat, and this difficulty can be solved. H₂The S release suppression catalyst can be composed of a catalyst other than a three-way catalyst.₂A catalyst containing an S release inhibitor as a main catalyst species is H₂It can be used as a catalyst for suppressing S release.
[0048]
In the exhaust purification apparatuses of the first and second embodiments, the sulfur component release control (forced S purge) that promotes the release of the sulfur component from the NOx catalyst 30a is performed. H by purge₂Since the release of S can be suppressed, it is not essential to perform the forced S purge in the present invention.
In the embodiment, the H during the forced S purge₂When the S release rate exceeds a predetermined value, the S purge suppression operation is performed and the H₂Although the release of S is suppressed, the execution of the S purge suppression operation is not essential in the present invention. That is, H₂H with a large amount of S release inhibitor added₂According to the present invention in which the S release suppression catalyst is provided downstream of the NOx catalyst, the H during the forced S purge can be performed without performing the S purge suppression operation.₂S emission can be reduced.
[0049]
When performing sulfur component release control (forced S purge) in the present invention, the control means for forced S purge is set to change the air-fuel ratio of the air-fuel mixture supplied to the lean burn internal combustion engine to a rich air-fuel ratio as in the embodiment. The air-fuel ratio control means for controlling and the temperature rise control means for raising the temperature of the NOx storage catalyst device can be configured, but the present invention is not limited to this, and the control means comprising either the air-fuel ratio control means or the temperature rise control means is constituted. it can.
[0050]
The temperature increase control means may be configured by ignition timing control means for retarding the ignition timing as in the embodiment, but is not limited thereto. For example, in the case of an in-cylinder lean combustion 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 is configured such that the temperature of the NOx occlusion catalyst device is higher than the activation temperature and is equal to or higher than a temperature suitable for desorption of sulfur components occluded in the NOx occlusion catalyst device or a lower set temperature. In this case, the temperature of the NOx occlusion catalyst device may be further increased in an assistive manner.
[0051]
According to the exhaust emission control device provided with the control means, the sulfur component release control is performed when the storage amount of the sulfur component (S component) by the NOx storage catalyst device is increased, and as a result, the NOx storage catalyst device becomes a rich air-fuel ratio. In addition, exposure to a high-temperature atmosphere promotes the release of the S component from the NOx storage catalyst device, and the required NOx purification efficiency is maintained.
Further, the control means may be one that responds to catalyst regeneration information or poisoning information due to sulfur components. For example, the catalyst regeneration frequency (frequency at which sulfur components are desorbed) within a predetermined period based on the catalyst regeneration information. 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 a predetermined value, the sulfur component release control is performed. The poisoning information can be obtained from the fuel consumption and operating time (vehicle travel distance) of the lean combustion internal combustion engine, or from the output of a NOx sensor that detects the NOx concentration in the exhaust gas.
[0052]
【The invention's effect】
  The exhaust emission control device of the present invention has a molar ratio of 30 to 300% H with respect to the storage agent added to the NOx storage catalyst device.₂S release inhibitor H₂H from a lean burn internal combustion engine equipped with an exhaust purification device having a NOx storage catalyst₂S release can be sufficiently suppressed without impairing the purification capacity of the NOx storage catalyst.
[Brief description of the drawings]
FIG. 1 is a schematic view of a lean combustion internal combustion engine equipped with an exhaust emission control device according to a first embodiment of the present invention.
FIG. 2 is a view showing H by nickel oxide added to the three-way catalyst of the exhaust gas purification apparatus shown in FIG.₂It is a figure which shows S release suppression effect.
FIG. 3 shows the time variation of the three-way catalyst temperature and H₂It is a figure which shows the time change of S concentration about each of the three way catalyst which makes nickel addition amount different.
[Fig. 4] Nickel oxide addition amount and H₂It is a figure which shows the relationship with S concentration peak value.
FIG. 5 is a flowchart of a forced S purge control routine that is performed by the electronic control unit shown in FIG. 1;
6 is a diagram showing a temporal change in target A / F in A / F modulation for S purge suppression operation performed in the control routine of FIG. 5; FIG.
[Fig. 7] H in exhaust gas accompanying catalyst temperature rise₂It is a figure which shows the time change of S concentration about both the engine driving | operation with a constant air fuel ratio, and the engine driving | operation with A / F modulation.
[Explanation of symbols]
1. Engine (lean combustion internal combustion engine)
14 Exhaust pipe (exhaust passage)
30 Exhaust purification catalyst device
30a NOx catalyst (NOx storage catalyst device)
30b Three-way catalyst (H₂S release suppression catalyst device)

Claims (1)

A NOx occlusion catalyst to which an occlusion agent that occludes NOx in exhaust gas is added when the exhaust air-fuel ratio is a lean air-fuel ratio, provided in an exhaust passage of a lean combustion internal combustion engine;
And the molar ratio of 30 to 300% of the hydrogen sulfide release suppression catalyst hydrogen sulfide release control agent is added to said absorbent with provided downstream of the NOx storage catalysts,
Sulfur component release control means for controlling the internal combustion engine to perform S purge for promoting the release of the sulfur component of the NOx storage catalyst when a predetermined condition set by the operating state of the internal combustion engine is satisfied;
A hydrogen sulfide release rate detection means for detecting the degree of increase in the concentration of hydrogen sulfide in the exhaust gas ,
Said sulfur ingredient release control means, the sulfur component of the NOx storage catalyst according to the S purge when hydrogen sulfide release speed detected from the hydrogen sulfide release rate detecting means exceeds a predetermined value is variably set in accordance with the vehicle speed exhaust purification system of a lean burn internal combustion engine, wherein the kite has the function of suppressing the release of.

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