JP2008223679A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To objectively evaluate the degree of deterioration of a NOx catalyst by a value unambiguously corresponding to the exhaust emission conversion performance of the NOx catalyst, in an exhaust emission control device for an internal combustion engine. <P>SOLUTION: The NOx catalyst is divided into a plurality of sections in an exhaust emission flowing direction, and a catalyst temperature is estimated in each section. For the plurality of sections, each degradation index showing the degradation state of the catalyst is calculated based on a temperature history of each section, and further the degradation index of each section is corrected according to the degree of contribution of each section to the exhaust emission conversion performance as the whole NOx catalyst. The degree of contribution is so set that the degree of contribution of the section upstream in a gas flow direction is larger than the degree of contribution of the section downstream therein. The degradation index as the whole NOx catalyst is calculated from the corrected degradation index of each section. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus provided with a NOx catalyst.

  An internal combustion engine, particularly an internal combustion engine that performs a lean combustion operation, includes a NOx catalyst for purifying NOx contained in exhaust gas. The NOx catalyst has a function of absorbing NOx in a lean atmosphere, but it is known that the NOx absorption state is not uniform throughout the NOx catalyst, and there is a difference between parts. Therefore, conventionally, a technique has been proposed in which the NOx catalyst is divided into a plurality of cells and the NOx absorption state of the catalyst is determined for each cell.

For example, Patent Document 1 describes that the NOx absorption amount is calculated for each of the divided cells, and the NOx absorption amount in the entire NOx catalyst is calculated by summing the NOx absorption amount of each cell. Yes. Patent Document 2 describes that the catalyst temperature is measured or estimated for each of the divided cells, and the NOx adsorption capacity of each cell is determined based on the cell temperature. Patent Document 2 also describes a technique in which the operation mode of the internal combustion engine is determined according to the temperature of at least one cell, paying attention to the fact that the catalyst temperature is related to the desorption of NOx and SOx. .
JP 2003-120274 A Special table 2003-503622 gazette Japanese Patent Laid-Open No. 2001-50086

  By the way, although the NOx catalyst is exposed to a high temperature atmosphere, if the temperature exceeds the heat resistant temperature, deterioration due to sintering occurs. Since the exhaust gas purification performance of the NOx catalyst varies depending on the degree of deterioration, it is important to accurately grasp the deterioration state of the NOx catalyst at the present time in order not to increase NOx emission into the atmosphere.

  As a method for evaluating the deterioration state of the NOx catalyst, a method of measuring or estimating the temperature of the NOx catalyst (overall representative temperature) and calculating a deterioration index indicating the deterioration state based on the temperature history is known. Specifically, the amount exceeding the heat-resistant temperature of the catalyst is integrated over time, and the integrated value is used as the deterioration index. According to this, it becomes possible to evaluate the deterioration state of the NOx catalyst by objective numerical values.

  However, since the temperature in the NOx catalyst is not uniform, the deterioration state is not uniform in the entire NOx catalyst, and it is considered that there is a difference in each part. Moreover, it is considered that the degree of deterioration of the exhaust purification performance due to deterioration varies depending on which part in the NOx catalyst has deteriorated. This is because there are a portion that contributes to the purification of exhaust gas and a portion that contributes little to the purification of exhaust gas depending on the position in the NOx catalyst.

  From the above, it is difficult to say that the method of calculating the deterioration index based on the temperature history of the entire NOx catalyst can accurately grasp the actual deterioration state of the NOx catalyst, that is, the degree of deterioration of the exhaust purification performance. This is because the deterioration index obtained by calculation and the actual exhaust purification performance of the NOx catalyst do not necessarily correspond uniquely. However, objectively indicating the deterioration state of the NOx catalyst is meaningful in taking measures against the deterioration of the NOx catalyst such as determination of the replacement timing of the NOx catalyst.

  Accordingly, the present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and its first object is to make it possible to objectively evaluate the degree of deterioration of the NOx catalyst by a numerical value that uniquely corresponds to the exhaust gas purification performance of the NOx catalyst. And

  Further, the exhaust gas purification performance of the NOx catalyst not only decreases due to deterioration of the catalyst itself, but also decreases due to poisoning by sulfur. Sulfur poisoning is a state in which sulfur contained in the exhaust gas is stored as a sulfate in the NOx catalyst, and the storage capacity of NOx decreases due to the storage of sulfur. However, unlike the deterioration of the catalyst itself, the exhaust purification performance reduced by sulfur poisoning can be recovered by poisoning recovery processing. In the poisoning recovery process, sulfur is desorbed by raising the temperature of the NOx catalyst to a predetermined temperature or higher while making the air-fuel ratio of the exhaust gas flowing into the NOx catalyst rich or stoichiometric.

  However, since sulfate is highly stable, there is a problem that once desorbed sulfur adheres to the NOx catalyst again downstream. If the re-deposition of sulfur is simply prevented, the entire NOx catalyst may be raised to a temperature above the desorption temperature. However, keeping the NOx catalyst in a high temperature state for a long time also leads to deterioration of the NOx catalyst.

  Accordingly, the present invention relates to an exhaust emission control device for an internal combustion engine, and a second object thereof is to reliably and promptly recover the sulfur poisoning of the NOx catalyst while suppressing deterioration of the NOx catalyst due to heat.

In order to achieve the above first object, a first invention is an exhaust emission control device for an internal combustion engine,
A NOx catalyst disposed in the exhaust passage of the internal combustion engine;
Temperature estimation means for dividing the NOx catalyst into a plurality of cells in the exhaust gas flow direction, and estimating the catalyst temperature for each cell;
Cell deterioration index calculating means for calculating a deterioration index indicating the deterioration state of the catalyst for each of the plurality of cells based on the temperature history of the cell;
Contribution degree correction means for correcting the deterioration index of each cell according to the contribution degree of each cell to the exhaust gas purification performance of the entire NOx catalyst;
An overall deterioration index calculating means for calculating a deterioration index of the NOx catalyst as a whole from the corrected deterioration index of each cell;
The contribution correspondence correction means is characterized in that the deterioration index of each cell is corrected by setting the contribution of the upstream cell in the exhaust gas flow direction to be greater than the contribution of the downstream cell.

According to a second invention, in the first invention,
A poisoning index calculating means for calculating a poisoning index indicating a poisoning state by sulfur for each of the plurality of cells based on a sulfur balance history in the cell;
A poisoning influence degree calculating means for calculating the influence degree of sulfur poisoning on the exhaust gas purification performance for each of the plurality of cells based on the deterioration index of the cell;
A poisoning correction means for correcting the deterioration index of each cell according to the poisoning index and the poisoning influence degree;
The poisoning influence degree calculating means calculates the poisoning influence degree to a smaller value as the deterioration index of the cell is larger,
The overall deterioration index calculating means calculates a deterioration index for the entire NOx catalyst from the deterioration index of each cell corrected by the contribution correspondence correction means and the poisoning correspondence correction means.

In order to achieve the second object described above, a third invention is an exhaust gas purification apparatus for an internal combustion engine,
A NOx catalyst disposed in the exhaust passage of the internal combustion engine;
The NOx catalyst is divided into a plurality of cells in the exhaust gas flow direction, and a poisoning index for calculating a poisoning index indicating a poisoning state by sulfur for each of the plurality of cells based on a sulfur balance history in the cell. A calculation means;
A temperature raising means for determining a target cell according to a distribution pattern of poisoning indexes in the NOx catalyst, and raising the temperature of the target cell to a temperature higher than that of other cells and higher than a desorption temperature of sulfur;
A history update means for estimating a temperature distribution in the NOx catalyst when the temperature of the target cell is raised, and updating a sulfur balance history in each cell based on the temperature distribution;
It is characterized by having.

According to a fourth invention, in the third invention,
The temperature raising means sets a cell having the highest poison index as a target cell.

According to a fifth invention, in the third invention,
The temperature raising means is characterized in that the most upstream cell in the exhaust gas flow direction is set as a target cell among cells whose poisoning index exceeds a predetermined allowable value.

A sixth invention is the invention according to any one of the third to fifth inventions,
The temperature raising means includes air-fuel ratio control means for alternately oscillating the air-fuel ratio lean and rich, and the air-fuel ratio control means controls the oscillation cycle and / or amplitude of the air-fuel ratio according to the position of the target cell. It is a feature.

A seventh invention is the sixth invention, wherein
The air-fuel ratio control means sets the air-fuel ratio oscillation cycle to a shorter cycle as the target cell is positioned upstream in the exhaust gas flow direction, and sets the air-fuel ratio oscillation cycle to a longer cycle as it is positioned downstream. It is a feature.

In an eighth aspect based on the third aspect,
The temperature raising means includes an exhaust temperature control means for periodically changing the temperature of the exhaust gas, and the exhaust temperature control means is used when the target cell is an upstream end in the exhaust gas flow direction or a cell in the vicinity thereof. The exhaust gas temperature is periodically changed.

In a ninth aspect based on the eighth aspect,
The exhaust temperature control means periodically changes the temperature of the exhaust gas by alternately changing the ignition timing between the advance side and the retard side.

  The NOx purification rate by the NOx catalyst increases as NOx is concentrated and stored in the upstream portion in the exhaust gas flow direction in the NOx catalyst. This is because the NOx catalyst has an oxygen storage capacity. NOx occluded in the NOx catalyst can be reduced by a reducing agent (CO or the like) in the exhaust gas by enriching the air-fuel ratio of the exhaust gas. However, since the reducing agent in the exhaust gas is also consumed by the reaction with the stored oxygen in the NOx catalyst, the stored oxygen in each part of the NOx catalyst is consumed in the upstream part of that part. No reduction will be made after it has been done. For this reason, the purification rate of NOx occluded in the downstream portion of the exhaust gas flow direction is lower than the purification rate of NOx occluded in the upstream portion. This means that the upstream portion has a higher contribution than the downstream portion in the exhaust gas flow direction from the viewpoint of the contribution of each portion to the exhaust purification performance of the entire NOx catalyst.

  In the first invention, the NOx catalyst is divided into a plurality of cells in the flow direction of the exhaust gas, and the deterioration index (cell deterioration index) for each cell in consideration of the contribution degree of each cell to the exhaust purification performance of the NOx catalyst as a whole. And the deterioration index (total deterioration index) of the NOx catalyst as a whole is calculated from the cell deterioration index of each cell. By considering the contribution of each cell in the calculation of the cell deterioration index, the overall deterioration index finally obtained is a numerical value that uniquely corresponds to the exhaust purification performance of the NOx catalyst. Therefore, according to the first invention, the degree of deterioration of the NOx catalyst can be accurately evaluated based on the overall deterioration index.

  The exhaust purification performance of the NOx catalyst is reduced not only by the original deterioration (thermal deterioration) of the NOx catalyst but also by poisoning by sulfur. Therefore, when evaluating the degree of deterioration of the entire NOx catalyst, comprehensive evaluation may be performed in consideration of not only the influence of thermal deterioration but also the influence of sulfur poisoning. When the NOx catalyst is divided into a plurality of cells, a poisoning index indicating a sulfur poisoning state can be calculated for each cell based on the sulfur balance history. The reduction in exhaust gas purification performance due to sulfur poisoning increases as the poisoning index in the cell increases. On the other hand, in cells where thermal deterioration has progressed, the degree of influence of sulfur poisoning on the deterioration of exhaust purification performance is reduced.

  According to the second invention, the cell is considered according to the poisoning index indicating the poisoning state by sulfur and the degree of influence of the sulfur poisoning on the exhaust gas purification performance while considering the contribution degree of each cell to the calculation of the cell deterioration index. Since the deterioration index is further corrected, the degree of deterioration of the entire NOx catalyst including the influence of sulfur poisoning can be comprehensively evaluated based on the finally obtained overall deterioration index.

  According to the third invention, sulfur can be reliably desorbed from the target cell without unnecessarily heating cells other than the target cell. Furthermore, when the temperature of the target cell is raised, the sulfur balance history in each cell is updated from the temperature distribution in the NOx catalyst at that time, and the distribution pattern of the poisoning index in the NOx catalyst is updated accordingly. The next target cell can then be determined. That is, according to the third aspect of the invention, it is possible to perform the optimum poisoning recovery process according to the poisoning situation while constantly grasping the poisoning situation in the NOx catalyst.

  According to the fourth invention, by setting the cell having the highest poisoning index, that is, the cell having the highest sulfur poisoning amount, as the target cell, the sulfur desorption rate as a whole of the NOx catalyst is improved. The poisoning recovery process can be completed in a short time. If the poisoning recovery process is completed in a short time, not only can the energy used for raising the temperature of the catalyst be saved, but also the risk of thermal degradation of the NOx catalyst is reduced by reducing the time during which the NOx catalyst is exposed to high temperatures.

  According to the fifth aspect of the present invention, the most upstream cell in the exhaust gas flow direction among the cells whose poisoning index exceeds a predetermined allowable value, that is, the foremost cell in which sulfur is accumulated is used as the target cell. By setting, sulfur can be sequentially desorbed from the upstream portion toward the downstream portion. According to this, re-poisoning by sulfur flowing from the upstream after the desorption process can be suppressed, and an efficient and effective poison recovery process can be performed. As a result, the poisoning recovery process can be completed in a short time.

  According to the sixth invention, the temperature distribution can be generated in the NOx catalyst by alternately oscillating the air-fuel ratio lean and rich. This is because the NOx catalyst has an oxygen storage capacity. When the air-fuel ratio changes from lean to rich, the stored oxygen in the NOx catalyst reacts with the reducing agent in the exhaust gas to generate heat. Further, when the air-fuel ratio changes from rich to lean, an exothermic reaction occurs when oxygen in the exhaust gas is stored in the NOx catalyst. And after these reaction is complete | finished, the cooling effect by exhaust gas lower than catalyst temperature arises. The temperature of each part in the NOx catalyst is determined by the sum of the heating effect by the exothermic reaction and the cooling effect by the exhaust gas. Then, an exothermic reaction occurs and the cell having the least cooling effect has the highest temperature.

  Furthermore, according to the sixth aspect of the invention, the temperature distribution in the NOx catalyst can be arbitrarily changed by changing the oscillation period and / or amplitude of the air-fuel ratio. Higher and higher than the desorption temperature. This is because the aforementioned exothermic reaction occurs from the upstream end of the NOx catalyst toward the downstream, but the region where the exothermic reaction occurs can be changed by the oscillation cycle and / or amplitude of the air-fuel ratio.

  According to the seventh aspect, by changing the vibration period of the air-fuel ratio, the temperature distribution in the NOx catalyst can be controlled more finely than when the amplitude is changed. If the oscillation cycle of the air-fuel ratio is shortened, the region where the exothermic reaction occurs is limited to the upstream portion of the NOx catalyst, and the cell having the highest temperature is also the upstream cell of the NOx catalyst. As the air-fuel ratio oscillation period is lengthened, the region where the above-described exothermic reaction occurs is expanded downstream of the NOx catalyst, and the cell having the highest temperature also moves downstream of the NOx catalyst.

  According to the eighth invention, when the target cell is an upstream end in the exhaust gas flow direction or a cell in the vicinity thereof, the temperature of the target cell can be efficiently raised to the desorption temperature or higher. it can. In this case, since the average temperature of the exhaust gas is not changed, it is possible to suppress the temperature rise in the cell downstream of the target cell.

  According to the ninth aspect, the temperature of the exhaust gas can be clearly and periodically changed by changing the ignition timing. The amplitude of the exhaust gas temperature can be controlled by the amount of advance and retard of the ignition timing.

Embodiment 1.
Embodiment 1 of the present invention will be described.

  FIG. 1 is a diagram showing the configuration of an exhaust emission control device for an internal combustion engine to which the present invention is applied. As shown in this figure, an exhaust passage 4 is connected to the internal combustion engine 2. A NOx catalyst 6 for purifying NOx in the exhaust gas is disposed in the exhaust passage 4. The operation of the internal combustion engine 2 is controlled by an ECU (Electronic Control Unit) 10. The ECU 10 has a function of determining deterioration of the NOx catalyst 6. Hereinafter, a method for determining the deterioration of the NOx catalyst 6 executed in the present embodiment will be described.

  FIG. 2 is a model of the NOx catalyst 6 used for deterioration determination. As shown in this figure, in the model, the NOx catalyst 6 is divided into a plurality of cells in the exhaust gas flow direction. Here, it is assumed that the cell is divided into five cells from a cell P1 corresponding to the inlet of the NOx catalyst 6 to a cell P5 corresponding to the outlet. However, the number of divisions of the NOx catalyst 6 in the model is not limited, and it is of course possible to divide into more cells.

In the deterioration determination using the model of FIG. 2, a deterioration index (hereinafter referred to as a cell deterioration index) indicating the deterioration state of the catalyst is calculated for each of the cells P1 to P5. The cell deterioration index is calculated based on the catalyst temperature history of the cell, as shown in the following equation (1). Equation (1) is an example of a cell deterioration index calculation formula, and other calculation formulas can be used. For example, the cell temperature may be integrated over time for the cell temperature exceeding the heat resistant temperature, and the integrated value may be used as the cell deterioration index.
Cell degradation index = Σcell temperature × time (1)

  A known method can be used for estimating the catalyst temperature of each of the cells P1 to P5. For example, a method of attaching one or a plurality of temperature sensors to the NOx catalyst 6 and estimating the catalyst temperature of each of the cells P1 to P5 from the measurement result can be used.

  FIG. 3 shows some examples of calculation results of the cell deterioration index. In FIG. 3, the area of the shaded portion in each cell P1 to P5 indicates the cell deterioration index of that cell. In the example (A), the cell P1 at the entrance has the highest cell degradation index, and the cell degradation index decreases as it approaches the exit. In the example (B), the cell deterioration index is the highest in the center cell P3, and the cell deterioration index decreases as the distance from the exit approaches the entrance. In the example (C), the cell P5 at the exit has the highest cell degradation index, and the cell degradation index decreases as it approaches the entrance.

  Comparing the three examples of FIG. 3, the total area of the shaded portions is equal. When the cell deterioration indexes of the cells P1 to P5 are simply summed, the total value is consistent among the above three examples. Therefore, if the total value of the cell deterioration indexes of the cells P1 to P5 is the deterioration index of the NOx catalyst 6 as a whole, the above three examples show the same degree of deterioration.

  However, the degree of deterioration of the NOx catalyst 6 varies depending on how much each cell is deteriorated. This is because there are cells that contribute to purification of exhaust gas and cells that contribute little to purification of exhaust gas depending on the position of the cell in the NOx catalyst 6. The NOx occluded in each of the cells P1 to P5 can be reduced by the reducing agent in the exhaust gas by making the air-fuel ratio of the exhaust gas rich. However, since the reducing agent in the exhaust gas is also consumed by the reaction with the stored oxygen in each cell P1 to P5, NOx stored in each cell P1 to P5 is stored in the cell upstream of that cell. There is no reduction after oxygen has been consumed. For this reason, the purification rate of NOx occluded in the cell near the outlet is lower than the purification rate of NOx occluded in the cell near the inlet. Therefore, from the viewpoint of the contribution of each cell P1 to P5 to the exhaust purification performance of the NOx catalyst 6 as a whole, the contribution of the cell closer to the inlet is higher than that of the cell closer to the outlet.

Therefore, in the present embodiment, as shown in the following equation (2), coefficients C1 to C5 indicating the contributions of the cells P1 to P5 to the exhaust purification performance of the NOx catalyst 6 as a whole are set, and this coefficient C1 The deterioration index of the entire NOx catalyst 6 (hereinafter referred to as the overall deterioration index) is calculated from the cell deterioration indexes of the cells P1 to P5 corrected by .about.C5. In the formula (2), the cell deterioration indexes of the cells P1 to P5 are expressed as X1 to X5. Each value of the coefficients C1 to C5 is set so that the magnitude relationship between the cells close to the entrance is higher than the cells close to the exit so that C1>C2>C3>C4> C5. Yes.
Overall degradation index = C1 * X1 + C2 * X2 + C3 * X3 + C4 * X4 + C5 * X5 (2)

  When the total deterioration index calculated by the above equation (2) is compared for the three examples in FIG. 3, the total deterioration index in Example (A) is calculated to be the largest, and the total deterioration index in Example (C) is calculated to be the smallest. The That is, according to the above equation (2), it is determined that the degree of deterioration of the NOx catalyst 6 is higher in the order of the examples (A), (B), and (C), and this determination result is the measurement result of the actual exhaust purification performance. It also matches. That is, by calculating the overall deterioration index using the above equation (2), a numerical value uniquely corresponding to the exhaust purification performance of the NOx catalyst 6 can be obtained, and the degree of deterioration of the NOx catalyst is accurately evaluated by the numerical value. be able to.

Embodiment 2.
Next, a second embodiment of the present invention will be described.

  The present embodiment is characterized in the deterioration determination of the NOx catalyst 6 as in the first embodiment. As in the first embodiment, the model shown in FIG. 2 is used to determine the deterioration of the NOx catalyst 6. The difference from the deterioration determination executed in the first embodiment is that in this embodiment, the degree of deterioration of the NOx catalyst 6 is comprehensively determined in consideration of the influence of sulfur poisoning. This is because the exhaust gas purification performance of the NOx catalyst 6 is reduced not only by the original deterioration (thermal deterioration) of the NOx catalyst but also by poisoning by sulfur.

Therefore, in the present embodiment, the overall deterioration index of the entire NOx catalyst 6 is calculated by the following equation (3). In formula (3), D1 to D5 are sulfur poisoning influence coefficients of the cells P1 to P5, and Y1 to Y5 are sulfur poisoning indexes of the cells P1 to P5. In the equation (3), the cell deterioration indexes X1 to X5 of the cells P1 to P5 are corrected with the contribution factors C1 to C5, and further corrected with the sulfur poisoning indexes Y1 to Y5 and the sulfur poisoning influence factors D1 to D5. is doing.
Overall degradation index = C1 * X1 * D1 * Y1 + C2 * X2 * D2 * Y2 + C3 * X3 * D3 * Y3 + C4 * X4 * D4 * Y4 + C5 * X5 * D5 * Y5 (3)

  In the above equation (3), the sulfur poisoning indices Y1 to Y5 are numerical values indicating the poisoning states of the cells P1 to P5 due to sulfur. The larger the sulfur poisoning index value, the greater the reduction in exhaust purification performance due to sulfur poisoning. The sulfur poisoning index is calculated from the estimated value of the sulfur poisoning amount of each cell P1 to P5. The sulfur poisoning amount of each cell P1 to P5 can be estimated by calculating the sulfur balance history for each cell. Since the method for estimating the sulfur poisoning amount is known, its description is omitted here.

  In the above equation (3), the sulfur poisoning influence coefficients D1 to D5 are coefficients indicating the degree of influence that the sulfur poisoning of the cells P1 to P5 has on the exhaust purification performance. The degree of influence of sulfur poisoning on the exhaust gas purification performance is greater in cells that are not thermally degraded, and the effect of sulfur poisoning is less in cells that are thermally degraded. FIG. 4 is a diagram showing the relationship between the sulfur poisoning influence coefficient and the cell deterioration index. The larger the cell deterioration index is, the smaller the sulfur poisoning influence coefficient of the cell is set.

  According to the overall deterioration index calculated by the above equation (3), the degree of overall deterioration of the NOx catalyst 6 including the influence of sulfur poisoning can be comprehensively evaluated.

Embodiment 3.
Next, a third embodiment of the present invention will be described.

  As described in the second embodiment, the exhaust purification performance of the NOx catalyst 6 is also reduced by sulfur poisoning. Sulfur poisoning refers to a state in which sulfur contained in the exhaust gas is stored as a sulfate in the NOx catalyst 6. By storing sulfur, the NOx storage capacity of the NOx catalyst 6 decreases, but unlike the heat deterioration, the exhaust purification performance decreased by sulfur poisoning can be recovered by poisoning recovery processing. The present embodiment is characterized by this poisoning recovery process. Hereinafter, the sulfur poisoning recovery process of the NOx catalyst 6 executed in the present embodiment will be described.

  The model shown in FIG. 2 is also used in this embodiment. A sulfur poisoning index is calculated for each of the cells P1 to P5 in the model. As described in the second embodiment, the sulfur poisoning index is calculated based on the sulfur balance history in each of the cells P1 to P5. If the sulfur poisoning index of each cell P1-P5 is calculated, a target cell will be determined from the distribution pattern. Then, the temperature of the target cell is raised to a temperature higher than that of other cells and higher than the desorption temperature of sulfur.

  In the sulfur poisoning recovery process according to the present embodiment, as a method of raising the temperature of the target cell, a method of alternately oscillating the air-fuel ratio of the exhaust gas in a lean and rich manner around the stoichiometric as shown in FIG. 5 is used. . Since the NOx catalyst 6 has an oxygen storage capacity, when the air-fuel ratio of the inflowing exhaust gas changes from lean to rich, the stored oxygen in the NOx catalyst 6 reacts with the reducing agent in the exhaust gas to generate heat. Further, when the air-fuel ratio changes from rich to lean, an exothermic reaction occurs when oxygen in the exhaust gas is stored in the NOx catalyst. And after these reaction is complete | finished, the cooling effect by exhaust gas lower than catalyst temperature arises. Since the temperature of each part in the NOx catalyst 6 is determined by the sum of the heating effect due to the exothermic reaction and the cooling effect due to the exhaust gas, a temperature distribution as shown in FIG.

  The temperature distribution in the NOx catalyst 6 can be finely controlled by changing the oscillation cycle of the air-fuel ratio. FIG. 7 shows three examples. First, it is assumed that a temperature distribution as shown in the example (B) is realized in a certain vibration period. In that case, if the oscillation cycle of the air-fuel ratio is shortened as shown in the example (A), the region where the exothermic reaction occurs is limited to the upstream portion of the NOx catalyst 6, and the cell having the highest temperature is also upstream of the NOx catalyst 6. Cell. On the contrary, as shown in the example (C), if the oscillation cycle of the air-fuel ratio is lengthened, the region where the exothermic reaction occurs is expanded to the downstream side of the NOx catalyst, and the cell having the highest temperature is also downstream of the NOx catalyst 6. Move to. Therefore, once the target cell is determined, the temperature in the target cell can be raised higher than the other cells and higher than the desorption temperature by controlling the oscillation cycle of the air-fuel ratio according to the position of the target cell. .

  The target cell is determined so that the poisoning recovery process can be performed efficiently. In the present embodiment, the cell having the highest sulfur poisoning index, that is, the cell having the largest sulfur poisoning amount is set as the target cell. In FIG. 8, in each cell P1 to P5, the area of the shaded portion indicates the sulfur poisoning index of the cell, and the cell with an asterisk is the target cell.

  Sulfur is more likely to be stored in cells closer to the inlet. For this reason, in the initial stage of the poisoning recovery process, the sulfur poisoning index of the cell close to the inlet (cell P1 in FIG. 8) is the highest. Therefore, when the cell close to the inlet is set as the target cell and the air-fuel ratio is oscillated, the desorption of sulfur from the target cell and the neighboring cells proceeds. However, since sulfate, which is a storage form of sulfur in the NOx catalyst, has high stability, sulfur once desorbed adheres to the NOx catalyst again downstream, and eventually the cell in the center (cell P3 in FIG. 8). Has the highest sulfur poisoning index. Therefore, in the middle stage of the poisoning recovery process, the center cell is set as the target cell to vibrate the air-fuel ratio. As a result, the desorption of sulfur from the central cell and the cells in the vicinity thereof proceeds, and the peak of the sulfur poisoning index moves to the cell P5) in FIG. At the end of the poisoning recovery process, the cell close to the outlet is set as the target cell and the air-fuel ratio is vibrated. Thereby, the desorption of sulfur from the NOx catalyst 6 is completed.

  When the temperature of the target cell is increased, the sulfur balance history in each of the cells P1 to P5 is updated from the temperature distribution in the NOx catalyst 6 at that time. As the sulfur balance history is updated, the sulfur poisoning index values of the cells P1 to P5 are also updated, and the distribution pattern of the poisoning index in the NOx catalyst 6 changes. When the distribution pattern of the poisoning index changes, the next target cell is determined in accordance with the distribution pattern, thereby moving the target cell sequentially from the entrance cell P1 to the exit cell P5 as shown in FIG. Can continue.

  According to the sulfur poisoning recovery process according to the present embodiment, sulfur can be reliably desorbed from the target cell without unnecessarily heating cells other than the target cell. Furthermore, by setting the cell having the largest amount of sulfur poisoning as the target cell, it is possible to improve the sulfur desorption rate of the entire NOx catalyst 6 and complete the poisoning recovery process in a short time. If the poisoning recovery process is completed in a short time, not only can the energy used for raising the temperature of the catalyst be saved, but also the time that the NOx catalyst 6 is exposed to a high temperature is shortened, thereby reducing the risk of thermal degradation of the NOx catalyst 6. There are also advantages.

Embodiment 4.
Next, a fourth embodiment of the present invention will be described.

  The present embodiment is characterized in the sulfur poisoning recovery process of the NOx catalyst 6 as in the third embodiment. The difference from the sulfur poisoning recovery process executed in the third embodiment is the target cell determination method. In the third embodiment, the cell having the highest sulfur poisoning index is set as the target cell. However, in this embodiment, the exhaust gas flow direction of the cells in which the sulfur poisoning index exceeds a predetermined allowable value is determined. The most upstream cell is the target cell.

  FIG. 9 shows a specific example of target cell setting. In each cell P1 to P5, the area of the shaded portion indicates the sulfur poisoning index of the cell, and the cell with an asterisk is the target cell. As shown in FIG. 9, by setting the frontmost cell in which sulfur is accumulated as the target cell, sulfur can be sequentially desorbed from the upstream portion to the downstream portion of the NOx catalyst 6. According to this, after desorption processing, it is possible to suppress re-poisoning by sulfur flowing from the upstream, and efficient and effective poisoning recovery processing is possible. As a result, the poisoning recovery process can be completed in a short time.

Embodiment 5.
Finally, Embodiment 5 of the present invention will be described.

  The sulfur poisoning recovery process of the NOx catalyst 6 according to the present embodiment is an effective process when the target cell is the inlet cell P1 or a cell in the vicinity thereof. In the present embodiment, as a method of raising the temperature of the target cell, a method of periodically changing the temperature of the exhaust gas is used as shown in FIG. Such a periodic change in the exhaust temperature can be realized by alternately changing the ignition timing to the advance side and the retard side. The amplitude of the exhaust gas temperature can be controlled by the amount of advance and retard of the ignition timing.

  By periodically changing the temperature of the exhaust gas, the temperature of the target cell (inlet cell P1 or a cell in the vicinity thereof) can be efficiently raised to the desorption temperature or higher. In addition, by imparting a temperature change, the reactivity of the catalyst in the target cell can be improved to promote the sulfur elimination reaction. In this case, since the average temperature of the exhaust gas is not changed, the temperature rise in the downstream cell from the target cell can be suppressed.

  The sulfur poisoning recovery process according to the present embodiment can be combined with the sulfur poisoning recovery process according to the third or fourth embodiment. That is, when the target cell is the inlet cell P1 or a cell in the vicinity thereof, the sulfur poisoning recovery process according to the present embodiment is performed, and the target cell moves to the center or outlet side cell, the third embodiment. Or it is possible to implement the sulfur poisoning recovery process concerning 4.

Others.
Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the following modifications may be made.

  Instead of changing the air-fuel ratio oscillation period in the sulfur poisoning recovery process, the air-fuel ratio amplitude may be changed. The amplitude of the air-fuel ratio can be changed according to the fuel injection amount. By changing the amplitude of the air-fuel ratio, the same effect as when changing the oscillation cycle of the air-fuel ratio can be obtained.

  Further, instead of changing the air-fuel ratio and the exhaust temperature in the sulfur poisoning recovery process, the composition of the reducing agent contained in the exhaust gas may be changed. When the internal combustion engine 2 is a direct injection engine, the stratification degree of the in-cylinder air-fuel mixture can be controlled by the fuel injection timing, thereby changing the amount of CO in the exhaust gas.

It is a figure which shows the structure of the exhaust gas purification apparatus of the internal combustion engine to which this invention is applied. It is a figure which shows the catalyst model used for the deterioration determination of a NOx catalyst in Embodiment 1 of this invention. It is a figure for demonstrating the determination result of the deterioration determination using a catalyst model. It is a figure for demonstrating the sulfur poisoning influence coefficient used in Embodiment 2 of this invention. It is a figure for demonstrating the temperature rising method of the NOx catalyst concerning Embodiment 3 of this invention. It is a figure for demonstrating the temperature rising effect by the vibration of an air fuel ratio. It is a figure which shows the relationship between the oscillation period of an air fuel ratio, and the temperature distribution in a NOx catalyst. It is a figure for demonstrating the sulfur poisoning recovery process concerning Embodiment 4 of this invention. It is a figure for demonstrating the sulfur poisoning recovery process concerning Embodiment 5 of this invention. It is a figure for demonstrating the temperature rising method of the NOx catalyst concerning Embodiment 6 of this invention.

Explanation of symbols

2 Internal combustion engine 4 Exhaust passage 6 NOx catalyst 10 ECU

Claims (9)

A NOx catalyst disposed in the exhaust passage of the internal combustion engine;
Temperature estimation means for dividing the NOx catalyst into a plurality of cells in the exhaust gas flow direction, and estimating the catalyst temperature for each cell;
Cell deterioration index calculating means for calculating a deterioration index indicating the deterioration state of the catalyst for each of the plurality of cells based on the temperature history of the cell;
Contribution degree correction means for correcting the deterioration index of each cell according to the contribution degree of each cell to the exhaust gas purification performance of the entire NOx catalyst;
An overall deterioration index calculating means for calculating a deterioration index of the NOx catalyst as a whole from the corrected deterioration index of each cell;
The contribution degree correction unit corrects the deterioration index of each cell by setting the contribution degree of the upstream cell in the exhaust gas flow direction to be larger than the contribution degree of the downstream cell. Exhaust purification device. A poisoning index calculating means for calculating a poisoning index indicating a poisoning state by sulfur for each of the plurality of cells based on a sulfur balance history in the cell;
A poisoning influence degree calculating means for calculating the influence degree of sulfur poisoning on the exhaust gas purification performance for each of the plurality of cells based on the deterioration index of the cell;
A poisoning correction means for correcting the deterioration index of each cell according to the poisoning index and the poisoning influence degree;
The poisoning influence degree calculating means calculates the poisoning influence degree to a smaller value as the deterioration index of the cell is larger,
2. The overall deterioration index calculating means calculates a deterioration index for the entire NOx catalyst from the deterioration index of each cell corrected by the contribution correspondence correction means and the poisoning correspondence correction means. An exhaust purification device for an internal combustion engine. A NOx catalyst disposed in the exhaust passage of the internal combustion engine;
The NOx catalyst is divided into a plurality of cells in the exhaust gas flow direction, and a poisoning index for calculating a poisoning index indicating a poisoning state by sulfur for each of the plurality of cells based on a sulfur balance history in the cell. A calculation means;
A temperature raising means for determining a target cell according to a distribution pattern of poisoning indexes in the NOx catalyst, and raising the temperature of the target cell to a temperature higher than that of other cells and higher than a desorption temperature of sulfur;
A history update means for estimating a temperature distribution in the NOx catalyst when the temperature of the target cell is raised, and updating a sulfur balance history in each cell based on the temperature distribution;
An exhaust emission control device for an internal combustion engine, comprising:   The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the temperature raising means sets a cell having the highest poison index as a target cell.   4. The internal combustion engine according to claim 3, wherein the temperature raising means sets the most upstream cell in the exhaust gas flow direction as a target cell among the cells having poisoning indexes exceeding a predetermined allowable value. Exhaust purification device.   The temperature raising means includes air-fuel ratio control means for alternately oscillating the air-fuel ratio lean and rich, and the air-fuel ratio control means controls the oscillation cycle and / or amplitude of the air-fuel ratio according to the position of the target cell. 6. An exhaust emission control device for an internal combustion engine according to claim 3, wherein the exhaust gas purification device is an internal combustion engine.   The air-fuel ratio control means sets the air-fuel ratio oscillation cycle to a shorter cycle as the target cell is positioned upstream in the exhaust gas flow direction, and sets the air-fuel ratio oscillation cycle to a longer cycle as it is positioned downstream. The exhaust gas purification apparatus for an internal combustion engine according to claim 6,   The temperature raising means includes an exhaust temperature control means for periodically changing the temperature of the exhaust gas, and the exhaust temperature control means is used when the target cell is an upstream end in the exhaust gas flow direction or a cell in the vicinity thereof. The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the temperature of the exhaust gas is periodically changed.   9. The exhaust gas purification apparatus for an internal combustion engine according to claim 8, wherein the exhaust gas temperature control means periodically changes the temperature of the exhaust gas by alternately changing the ignition timing to an advance side and a retard side. .

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