JP3832550B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to hydrogen sulfide (H in exhaust gas).₂The present invention relates to an exhaust emission control device capable of regenerating a NOx catalyst device while suppressing odor due to S).
[0002]
[Related background]
An occlusion type NOx catalyst has been put to practical use to purify NOx in the exhaust gas of a lean combustion internal combustion engine. With this type of catalyst, NOx in the exhaust gas is temporarily occluded on the catalyst during lean operation of the internal combustion engine. At this time, since the sulfur component (S component) in the exhaust gas is also occluded on the catalyst, there arises a problem that the NOx purification performance of the catalyst is deteriorated (S poisoning).
[0003]
Therefore, it is known that by increasing the temperature of the S-poisoned storage-type NOx catalyst and placing the catalyst in a reducing atmosphere, the S component stored in the catalyst is released from the catalyst and its purification efficiency is restored. Yes. However, according to such a technique, when the S component is released from the storage-type NOx catalyst, H₂A new problem arises that S is generated and odor is generated.
[0004]
In order to solve this problem, in the technique described in Japanese Patent Laid-Open No. 8-294618, a downstream catalytic converter is disposed downstream of the NOx storage catalyst, and the air-fuel ratio is set in the rich region and the lean region based on the theoretical air-fuel ratio. The air-fuel ratio perturbation is performed so that the air-fuel ratio fluctuates alternately.₂Capture and oxidize S to produce H₂S release to the atmosphere is suppressed.
[0005]
[Problems to be solved by the invention]
However, from the NOx storage type catalyst, H₂H when S is released₂The S release rate varies greatly depending on the storage amount of the S component of the NOx catalyst and the engine operating state. Therefore, when air-fuel ratio perturbation is executed regardless of the engine operating state, the downstream catalytic converter generates H₂S is not captured and oxidized sufficiently, and a large amount of H₂S may be released into the atmosphere and odor may be generated.
[0006]
The present invention relates to hydrogen sulfide (H₂An object of the present invention is to provide an exhaust emission control device capable of regenerating the NOx occlusion catalyst device from S poisoning while reliably suppressing the release of S) into the atmosphere and preventing the generation of odor.
[0007]
[Means for Solving the Problems]
  The exhaust emission control device according to the first aspect of the present invention is the hydrogen sulfide release rate from the NOx storage catalyst device estimated based on the engine operating state.Is larger than a predetermined value set according to the vehicle speedThe air-fuel ratio changing means is operated under the control of the control means, and this operation changes the exhaust air-fuel ratio between the rich side and the lean side with reference to the theoretical air-fuel ratio.
[0008]
  In the exhaust emission control device according to the first aspect of the present invention, when the exhaust air-fuel ratio is lean, the sulfur component (S component) in the exhaust gas stored together with NOx in the NOx storage catalyst device has an exhaust air-fuel ratio of stoichiometric or When it is rich, it is released from the NOx storage catalyst device, and from this released S component, hydrogen sulfide (H₂S) is generated, but when hydrogen sulfide is generated and released, the hydrogen sulfide release rate estimated based on the engine operating conditionWhen is greater than the predetermined valueThe air-fuel ratio changing means can be operated under the control of the control means. Then, the operation of the air-fuel ratio changing means causes the exhaust air-fuel ratio to fluctuate alternately between the rich side and the lean side with reference to the theoretical air-fuel ratio.
[0009]
During operation of the air-fuel ratio changing means, hydrogen sulfide is released while the exhaust air-fuel ratio is rich, and hydrogen sulfide is not released when the exhaust air-fuel ratio becomes lean. That is, hydrogen sulfide is intermittently released, and the average value of the release rate is compared to the case where continuous release of hydrogen sulfide is allowed without changing the exhaust air-fuel ratio between the rich side and the lean side. It decreases considerably. Here, the hydrogen sulfide release rate is closely related to the hydrogen sulfide concentration in the exhaust gas, and the hydrogen sulfide concentration is closely related to the degree of odor accompanying the discharge of hydrogen sulfide. Thus, the hydrogen sulfide release rate changes depending on the engine operating state and corresponds to the degree of odor. In the present invention, the exhaust air-fuel ratio is increased when the hydrogen sulfide release rate is increased and the odor may be increased. By alternately changing the value between the rich side and the lean side, the hydrogen sulfide release rate can be suppressed and the odor can be weakened, and the problem of the generation of odor accompanying the discharge of hydrogen sulfide can be eliminated or greatly reduced. On the other hand, since the exhaust air-fuel ratio is maintained at a value close to the theoretical air-fuel ratio on average, the purification ability of the NOx storage catalyst device can be regenerated. That is, in the present invention, the air-fuel ratio fluctuation means operates based on the hydrogen sulfide release rate that changes depending on the engine operating state, and the regeneration of the NOx storage catalyst device is performed while preventing the generation of odor due to the release of hydrogen sulfide. become.
[0010]
ThisIn this case, when the hydrogen sulfide release rate exceeds a predetermined value, the air-fuel ratio changing means is operated to reduce the hydrogen sulfide release rate, thereby reducing the odor exceeding the odor level corresponding to the predetermined value of the hydrogen sulfide release rate. Can be prevented from occurring.At this time, since the limit value at which odor is sensed changes according to the vehicle speed, the predetermined value is set as the limit value according to the vehicle speed.
  According to a second aspect of the present invention, in the first aspect, the predetermined value is set to be smaller when the vehicle is stopped than when the vehicle is traveling.
  That is, the predetermined value is set to be large because the limit value for feeling odor is relatively high when the vehicle is traveling, and is set to be small because the limit value is small when the vehicle is stopped.
[0011]
GoodMore preferably, the air-fuel ratio fixing means for maintaining the exhaust air-fuel ratio at a stoichiometric air-fuel ratio or a rich air-fuel ratio over a predetermined period is further provided, and the control means fixes the air-fuel ratio when the hydrogen sulfide release rate from the NOx storage catalyst device is small. Activate the means.
  In this preferred embodiment, when the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio by the operation of the air-fuel ratio fixing means, the S component stored in the NOx storage catalyst device is released, and thereby the S coverage of the NOx storage catalyst device. The poison is reduced and its purification performance is restored. In addition, although the S component and thus hydrogen sulfide are released by the operation of the air-fuel ratio fixing means, the operation is limited to when the hydrogen sulfide release speed is low, so there is little possibility that odor is generated during the operation of the air-fuel ratio fixing means. . Further, since the theoretical air-fuel ratio operation or rich air-fuel ratio operation of the internal combustion engine by the operation of the air-fuel ratio fixing means is limited to a predetermined period, there is little possibility of deteriorating fuel consumption.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an internal combustion engine equipped with an exhaust emission control device according to an embodiment of the present invention will be described.
The internal combustion engine of the present embodiment is composed of an in-cylinder spark-ignition 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. ing. 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 engine is conventionally known, but will be briefly described below.
[0013]
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.
[0014]
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.
[0015]
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.
[0016]
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 occlusion-type NOx catalyst 30a, not only NOx but also oxide SOx of sulfur component contained in the exhaust gas contains 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.
[0017]
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.₂Desorbed and released, accompanied by H₂S is generated (natural S purge).
[0018]
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, it is H against the three-way catalyst 30b.₂Nickel oxide as an S release inhibitor is added. This nickel oxide, as shown in the following reaction formula, is H 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.
[0019]
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.
[0020]
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 storage capacity, the lower limit value 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. 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. In addition, H₂As the S release inhibitor, Pd, Mn, Fe, Zn, Co, Cu, or the like may be used. H₂The amount of S release inhibitor added depends on the type of additive.₂Since the S release suppression is different, the preferable value is made different, but a value within a range of 30% to 300% in terms of a molar ratio with respect to the added amount of the NOx storage agent is preferable.
[0021]
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.
[0022]
The exhaust purification apparatus of the present embodiment can perform forced S purge (more generally, sulfur component release control that promotes the 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.
[0023]
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.
[0024]
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.
[0025]
In the present embodiment, the H shown in FIG.₂The S release suppression control routine is executed by the ECU 40, and the forced S purge is performed in this control routine. 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.
H in FIG.₂In the S release suppression 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.
[0026]
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.
[0027]
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 as 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.
[0028]
The catalyst temperature Tcat is obtained by correcting the exhaust temperature detected by the high temperature sensor 16 using the correction coefficient and the exhaust flow velocity read from the temperature correction map (not shown). The temperature correction map is set in advance through experiments or the like. In this temperature correction map, the correction of the catalyst temperature Tcat and the exhaust gas temperature in the steady state is given as a function of the target average effective pressure Pe and the engine speed information Ne. It is done.
[0029]
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).
[0030]
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 H based on engine operating conditions expressed by functions such as cooling water temperature₂The S release rate is estimated.
[0031]
That is, 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 result and stored in the storage device of the ECU 40. Then, during engine operation under the control of the ECU 40, from this map, H corresponding to the engine operation state is obtained.₂The S release rate will be determined.
Although illustration is omitted, in the map, generally, as the engine speed Ne, the engine load (target average effective pressure Pe, intake air amount A / N) increase, and the catalyst temperature Tcat and the exhaust temperature increase, the H₂The engine operating state and H so that the S release rate increases.₂S release rate is related.
[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, limit values expressed as a function of the vehicle speed are mapped in advance, and limit values determined according to the vehicle speed are mapped (not shown). ). 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). H like this₂When the requirement that the S release rate is low is satisfied, an 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. Due to the engine operation at such a rich air-fuel ratio, incomplete combustion of the fuel occurs, a large amount of carbon monoxide and hydrocarbons necessary for the removal of sulfur oxide SOx are generated and supplied to the NOx catalyst 30a, and the ignition timing The S purge is promoted in combination with the increase in the NOx catalyst temperature accompanying the increase in the exhaust gas temperature by the retard angle control.
[0034]
In the present embodiment, the fuel injection amount is determined by the ECU 40 based on the target A / F set as described above, and the fuel injection valve 6 is driven to open and close according to the drive signal corresponding to the fuel injection amount. The amount of intake air is adjusted accordingly. Therefore, the air-fuel ratio is fixed so that the exhaust air-fuel ratio is maintained at the stoichiometric air-fuel ratio or the rich air-fuel ratio for a predetermined period by the ECU 40, the fuel injection valve 6, a control valve element (not shown) for adjusting the intake air amount, and a timer built in the ECU 40. Means will be constructed.
[0035]
Now, when the release of SOx proceeds 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. Like this, H₂Since S is occluded in the three-way catalyst 30b, H in the exhaust gas released to the atmosphere₂S concentration is reduced.
[0036]
Subsequent to step S4 related to the S purge operation, it is determined whether or not the S purge mode is maintained for a predetermined time (predetermined period) 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.
[0037]
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 rate, from the S purge operation to H₂Switching to the S emission suppression operation is performed (step S6).
[0038]
This H₂In the S emission suppression operation, the ECU 40 modulates the target A / F with the theoretical air-fuel ratio as a boundary. That is, as shown in FIG. 3, 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 H₂The average value of the target A / F during the S release suppression operation is set to a value that can promote S purge (for example, about 14.3 that is a rich rich or about 12 on the rich side). The fuel injection amount is determined by the ECU 40 based on the target A / F set in this way, the fuel injection valve 6 is driven to open and close according to the fuel injection amount data, and the intake air amount is changed as necessary. Adjusted. Therefore, the ECU 40, together with the fuel injection valve 6 and the control valve element (not shown) for adjusting the intake air amount, substantially varies the exhaust air-fuel ratio between the rich side and the lean side based on the theoretical air-fuel ratio. It constitutes a variable means.
[0039]
Above H₂In the S release suppression operation, H of nickel oxide added to the three-way catalyst 30b is used.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).
[0040]
It should be noted that, instead of the above-described A / F modulation for switching 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 saturation time until the S component trapping action by the nickel oxide added to the three-way catalyst 30b is saturated, and the S component trapped in the form of nickel sulfide as SO.₂It is preferable to set the enrichment time and the leaning time in consideration of the fact that the release time required for release changes as a function of the engine operating condition. That is, preferably, each of the enrichment time and the leaning time is set to a value equal to a saturation time and a discharge time determined according to an engine operating state or the like. However, even if the enrichment time and leaning time are set to fixed values, H₂S suppression effect can be obtained.
[0041]
FIG. 4 shows 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. 4, 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.
[0042]
H in step S6₂When S emission suppression operation is performed, H in the exhaust gas₂S concentration decreases, and therefore H₂The S release rate also decreases. This H₂As the S release rate decreases, H₂The degree of odor accompanying S release is reduced.
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.
[0043]
In this way, until a predetermined time elapses after the S purge mode is set,₂Depending on whether the S release rate exceeds a predetermined value, the S purge operation or H₂S emission suppression operation is selectively performed.
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 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.
[0044]
If it is determined in step S1 that the S purge condition is not satisfied, H₂It is determined whether or not the S release rate exceeds a predetermined value (step S8). Since the determination in step S8 is performed in the same manner as the determination in step S3, the description thereof is omitted. And H₂If it is determined in step S8 that the S release rate exceeds the predetermined value, H₂S emission suppression operation is performed (step S9). H in step S9₂The S emission suppression operation is the same as that in step S6, and the description thereof is omitted. As described above, even when the S purge operation is not being performed, for example, natural S purge causes H₂When S release rate increases, H₂S release suppression operation is carried out, H₂Odor generation associated with the release of S is prevented.
[0045]
The present invention is not limited to the above embodiment, and can be variously modified.
For example, in the above embodiment, the sulfur component release control (forced S purge) that promotes the release of the sulfur component from the NOx catalyst 30a is performed.₂When releasing S, for example, by performing A / F modulation (step S9 in FIG. 2) of the above embodiment, H₂Since the S release rate can be suppressed, it is not essential to perform the forced S purge (fixed air-fuel ratio) in the present invention.
[0046]
Further, in the embodiment, the catalyst system of 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 placed downstream of the NOx catalyst of the exhaust purification catalyst device.₂Although provided as an S release suppression catalyst, in the present invention, such a configuration is not essential, and any catalyst may be used as long as it has a NOx storage catalyst device capable of storing and discharging NOx in the exhaust gas. And H is placed downstream of the NOx storage catalyst device.₂Even in an exhaust emission control device that does not have an S emission suppression catalyst, H from the NOx storage catalyst device₂Depending on the S release rate, for example, by performing the A / F modulation described above, the release H₂While suppressing the generation of odor due to S, it is possible to regenerate the purification capacity of the NOx storage catalyst device poisoned with S.
[0047]
When performing sulfur component release control (forced S purge) in the present invention, the control means for forced S purge controls the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to a rich air-fuel ratio as in the embodiment. The air-fuel ratio control means (air-fuel ratio fixing means) and the temperature rise control means for raising the temperature of the NOx storage catalyst device can be configured, but the invention is not limited to this, and the air-fuel ratio control means or the temperature rise control means is comprised. The forced S purge control means can be configured.
[0048]
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.
[0049]
According to the exhaust gas purification apparatus provided with the forced S purge control means, the sulfur component release control (S purge) is performed when the storage amount of the sulfur component (S component) by the NOx storage catalyst device increases. The storage catalyst device is exposed to a rich air-fuel ratio and / or high-temperature atmosphere, the release of the S component from the NOx storage catalyst device is promoted, and the required NOx purification efficiency is maintained.
[0050]
The forced S purge control means may be responsive to catalyst regeneration information or sulfur component poisoning information. For example, based on the catalyst regeneration information, the catalyst regeneration frequency (desorption of sulfur components is performed within a predetermined period). When the frequency is determined to be 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 may be 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.
[0051]
【The invention's effect】
  The exhaust emission control device of the present invention has a hydrogen sulfide release rate from a NOx storage catalyst device that is estimated based on the engine operating state.Is larger than a predetermined value set according to the vehicle speedBy operating the air-fuel ratio changing means, the exhaust air-fuel ratio fluctuates between the rich side and the lean side with reference to the theoretical air-fuel ratio, so that hydrogen sulfide (H₂It is possible to regenerate the purification capability of the NO poisoned NOx catalyst device while suppressing the generation of odor due to the release of exhaust gas containing S) into the atmosphere.
[Brief description of the drawings]
FIG. 1 is a schematic view of an internal combustion engine equipped with an exhaust emission control device according to an embodiment of the present invention.
FIG. 2 is a flowchart of a forced S purge control routine executed by the electronic control unit shown in FIG.
FIG. 3 is a diagram showing a temporal change of a target A / F in A / F modulation for S purge suppression operation performed in the control routine of FIG. 3;
[Fig. 4] 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)
6 Fuel injection valve (Air-fuel ratio fluctuation means)
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)
40 Electronic control unit (air-fuel ratio fluctuation means, control means)

Claims (2)

Provided in an exhaust passage of an internal combustion engine mounted on a vehicle, when the air-fuel ratio of exhaust gas is exhaust air-fuel ratio of the occluded NOx with occludes NOx in the exhaust gas when the lean air-fuel ratio of the stoichiometric air-fuel ratio or a rich air-fuel ratio A NOx storage catalyst device to be released;
An air-fuel ratio changing means for changing the exhaust air-fuel ratio between the rich side and the lean side based on a substantially stoichiometric air-fuel ratio;
Control means for operating the air-fuel ratio fluctuation means when the release rate of hydrogen sulfide from the NOx occlusion catalyst device estimated based on the operating state of the internal combustion engine is greater than a predetermined value ;
Wherein the predetermined value, the exhaust gas purifying apparatus for an internal combustion engine, characterized in Rukoto set according to the vehicle speed. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the predetermined value is set smaller when the vehicle is stopped than when the vehicle is traveling.

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