JP2009185631A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine capable of accurately detecting the degree of deterioration of a NOx catalyst without being affected by NOx emission amounts. <P>SOLUTION: In responses 100, 101, 102 indicating increase in a NOx storage amount with respect to time, NOx storage amounts A0, A1, A2 at points 200, 201, 202 where an integrated value of NOx emission amounts from the internal combustion engine becomes a predetermined first emission amount are calculated. In addition, NOx storage amounts B0, B1, B2 at points 203, 204, 205 where an integrated value of NOx emission amounts from the internal combustion engine becomes a predetermined second emission amount are calculated. Storage amount difference values Δ0, Δ1, Δ2 which are differences between B0, B1, B2 and A0, A1, A2 are calculated. When the storage amount difference value decreases in order of Δ2, Δ0, Δ1, it is determined that the degree of deterioration of the NOx catalyst is high in order of responses 102, 100, 101. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  For example, in an internal combustion engine that performs lean combustion at an air-fuel ratio higher than the stoichiometric air-fuel ratio, such as a diesel engine or a lean burn gasoline engine, an ordinary three-way catalyst cannot be used. An NOx storage reduction catalyst (NOx catalyst, Lean NOx Trap, LNT) is often used for purification.

  In LNT, NOx is stored in the storage agent in a lean atmosphere. Then, by switching to a rich atmosphere in which the fuel is greater than the stoichiometric air-fuel ratio, an operation called rich purge that reduces and discharges the stored NOx is performed to purify the exhaust. In the rich purge, the air-fuel ratio is a rich atmosphere and a predetermined temperature condition (for example, 300 degrees Celsius or higher for the function of the catalyst. In the following, temperatures are all used in Celsius) is occluded. NOx is reduced to harmless nitrogen.

  The use of LNT requires countermeasures against the problem of poisoning due to sulfur components (sulfur poisoning or S poisoning) and deterioration due to heat (heat deterioration). Sulfur poisoning is a phenomenon in which a catalyst (or occluding agent) binds to sulfur in the fuel, and as a result, the catalyst can no longer function for exhaust purification under certain conditions. It is possible to return to the state before S poisoning. On the other hand, thermal degradation is a reduction in surface area due to the catalyst being exposed to a temperature higher than a predetermined temperature (for example, 700 ° C. or higher), and a catalyst such as platinum is combined with the surrounding catalyst. This is a phenomenon in which the catalyst cannot perform the function for exhaust purification, and it cannot be returned to the state before the heat deterioration. In the following, LNT degradation refers to both S poisoning and thermal degradation.

  In order to regenerate the catalyst from sulfur poisoning (S poisoning) (S poisoning regeneration or S regeneration), it is necessary to have a rich atmosphere and satisfy a predetermined temperature condition (for example, 650 degrees or more). For this purpose, for example, a post-injection is performed after the main injection, or a fuel is added to the exhaust pipe from the fuel addition valve. Each time it is considered that S poisoning has progressed in the NOx catalyst, such S regeneration is usually performed to keep the function of the catalyst.

  If S regeneration is not performed in spite of the progress of S poisoning of the catalyst, the NOx purification function of LNT is reduced. Conversely, if the S regeneration is frequently performed at an unnecessary frequency, fuel is used for the S regeneration. Further, in the case of thermal degradation of the catalyst, the function of the catalyst cannot be recovered, so that it must be warned that the NOx purification function is degraded. Therefore, it is necessary to detect the degree of deterioration of the LNT and perform S regeneration at an appropriate time or to discriminate between S poisoning and thermal deterioration.

  For example, Patent Document 1 below discloses a technique for estimating the amount of NOx stored in the LNT from the time from the start to the end of the reduction of NOx stored in the LNT. Then, the shorter the time taken for NOx reduction, the smaller the stored amount of NOx, so it is determined that the deterioration of LNT has progressed. In Patent Document 1, the end of the NOx reduction reaction is detected by changing the exhaust gas discharged from the LNT from lean to rich.

Japanese Patent No. 2692380

  However, the storage amount of NOx is affected by the amount of NOx discharged from the engine. In general, the NOx emission amount from the engine may not be a target value due to factors such as characteristics of individual devices such as the engine and delays in the control system. The technique disclosed in Patent Document 1 is an effective technique in a situation where the NOx emission amount from the engine is a preset value.

  If the NOx emission amount from the engine is small, the stored amount of NOx decreases even if the LNT is not deteriorated. Therefore, the method of Patent Document 1 erroneously determines that the LNT is deteriorated. On the other hand, even if the LNT is deteriorated, if the NOx emission amount from the engine is large, the NOx occlusion amount of the LNT also becomes large. Therefore, the method of Patent Document 1 erroneously determines that the LNT is not deteriorated.

  Therefore, there is a need for a method for determining the degree of deterioration of LNT that is not erroneously determined due to the influence of NOx emission from the engine. However, such a determination method has not been proposed in the prior art.

  In view of the above problems, the problem to be solved by the present invention is to provide an exhaust purification device for an internal combustion engine that can detect the degree of deterioration of the NOx catalyst without being affected by the increase or decrease of the NOx emission amount from the internal combustion engine. That is.

Means for Solving the Problems and Effects of the Invention

  In order to solve the above-described problems, an exhaust gas purification apparatus for an internal combustion engine according to the present invention is an exhaust gas purification apparatus for an internal combustion engine provided with an NOx catalyst in the exhaust passage for storing NOx in a lean atmosphere and reducing NOx in a rich atmosphere. , An emission amount calculating means for acquiring the NOx emission amount from the internal combustion engine, and an occlusion amount calculating means for acquiring the NOx occlusion amount in the NOx catalyst, and the NOx calculated by the emission amount acquiring means. The NOx occlusion amount calculated by the occlusion amount calculation means when the integrated value of the emission amount becomes a predetermined first emission amount is set as the first occlusion amount, and the NOx calculated by the emission amount acquisition means. The NOx occlusion amount calculated by the occlusion amount calculating means when the integrated value of the emission amount of the gas becomes the predetermined second emission amount is set as the second occlusion amount, and the second emission amount is the first emission amount. A determination means for determining that a difference between the second storage amount and the first storage amount is smaller than a predetermined threshold, and the second storage amount and the Regeneration means for regenerating the NOx catalyst when it is determined that the difference from the first occlusion amount is smaller than the threshold value.

  Thus, in the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the NOx catalyst is regenerated when the difference between the two NOx occlusion amounts with respect to the two accumulated NOx emissions from the internal combustion engine is smaller than a predetermined threshold value. Therefore, it is possible to accurately detect the degree of deterioration of the NOx catalyst without causing erroneous determination due to the amount of NOx emission as in the prior art, and to regenerate the NOx catalyst. Therefore, since the degree of deterioration of the NOx catalyst can be accurately detected, the catalyst regeneration process can be performed at an appropriate timing, so that the performance of the catalyst can be maintained at a high level, and it is also possible to avoid deterioration of fuel consumption due to unnecessary frequent regeneration.

  Further, a second discriminating means for discriminating that a difference between the second occlusion amount and the first occlusion amount is larger than a second threshold value larger than the threshold value, and the second discriminating means by the second discriminating means. There may be provided adjusting means for increasing the interval between the reduction processes of the NOx catalyst when it is determined that the difference between the storage amount and the first storage amount is larger than the second threshold value.

  Thereby, when the difference of the NOx occlusion amount with respect to two different NOx emission integrated values from the internal combustion engine is larger than the predetermined second threshold, the interval of the reduction process of the NOx catalyst is widened. Therefore, when the difference in the NOx occlusion amount is larger than the predetermined second threshold, it is accurately detected that the degree of deterioration of the NOx catalyst is small. When the degree of deterioration is small, the reduction processing interval of the NOx catalyst is increased, Deterioration of fuel consumption due to the reduction process can be suppressed. Therefore, combined with the above effects, the catalyst regeneration process is performed at an appropriate timing to maintain the catalyst performance high, and the unnecessary fuel consumption is prevented from deteriorating due to frequent regeneration, and unnecessary frequent NOx. It is also possible to avoid deterioration of fuel consumption due to reduction.

  The ratio of the NOx amount occluded in the NOx catalyst and the integrated amount of the NOx emission amount is used as a purification rate, and the integrated value of the NOx emission amount becomes the first emission amount when the NOx catalyst is integrated. The first emission amount is set so that the purification rate becomes a predetermined target purification rate, and the average purification rate until the integrated value of the NOx emission amount becomes the second emission amount is the predetermined purification rate. The second discharge amount may be set so as to achieve the target purification rate.

  Thereby, the purification rate at the time when the integrated value of the NOx emission amount becomes the first emission amount becomes the predetermined target purification rate, so that the average purification rate up to that point can be equal to or higher than the target purification rate. Furthermore, since the average purification rate until the integrated value of the NOx emission amount becomes the second emission amount becomes the predetermined target purification rate, the average purification until the first and second emission amounts are reached. The rate can be higher than the target purification rate. Therefore, it is possible to accurately detect the degree of deterioration of the NOx catalyst, set an interval between catalyst regeneration and NOx reduction processing at an appropriate timing, and achieve a NOx purification rate that is equal to or higher than the target purification rate.

  Embodiments of an exhaust emission control device for an internal combustion engine according to the present invention will be described below with reference to the accompanying drawings. First, FIG. 1 is a schematic diagram of a device configuration of an exhaust gas purification device 1 for an internal combustion engine according to an embodiment of the present invention. The exhaust purification device 1 in FIG. 1 is configured for a four-cylinder diesel engine 2 (hereinafter simply referred to as an engine). 1 mainly includes an engine 2, an intake pipe 3, an exhaust pipe 4, an exhaust recirculation pipe 5 (EGR pipe), and an electronic control unit 7 (ECU).

  Air (fresh air, intake air) to the engine 2 is supplied through the intake pipe 3. An intake throttle 31 is disposed in the intake pipe 3. The amount of intake air supplied to the engine 2 increases or decreases as the opening of the intake throttle 31 is adjusted.

  The engine 2 is equipped with an injector 21 and an engine speed sensor 22. Fuel is supplied into the cylinder by injection from the injector 21. The engine speed sensor 22 measures the speed of the engine 2 (per unit time). The engine speed sensor 22 may be, for example, a crank angle sensor that measures the rotation angle of a crank connected from the engine 2. The detected value of the crank angle sensor may be sent to the ECU 7 to calculate the engine speed.

  The EGR pipe 5 performs exhaust gas recirculation (ExGR) from the exhaust pipe 5 to the intake pipe 3. The EGR pipe 5 is equipped with an EGR valve 51. For example, during a rich purge, the EGR valve 51 is opened and the intake air throttle 31 is throttled to reduce the amount of fresh air. Also, the combustion temperature in the engine can be lowered by opening the EGR valve 51 to recirculate the exhaust gas, so that the amount of NOx discharged from the engine 2 can be reduced.

  Exhaust gas from the engine 2 is discharged to the exhaust pipe 4. On the exhaust pipe 4, LNT6 (NOx catalyst) is equipped. The LNT 6 may have a structure in which, for example, a carrier layer is formed on a ceramic substrate, and a storage agent and a catalyst are supported on the carrier. As the carrier, for example, gamma alumina is suitable because it can carry a large amount of storage agent and catalyst due to its large surface area due to surface irregularities. Further, for example, barium, lithium, potassium or the like may be used as the occlusion agent, and platinum or the like may be used as the catalyst.

  In LNT6, the fuel is leaner than the stoichiometric air-fuel ratio (usually, the A / F value (air-fuel ratio value) is 17 or more), and NOx in the exhaust is occluded by the occlusion agent in a lean atmosphere. Then, when the air-fuel ratio is adjusted to a rich atmosphere in which the fuel is more than the stoichiometric air-fuel ratio (usually the A / F value is 14.5 or less) and the predetermined temperature condition is satisfied as described above, the fuel is stored in the storage agent. The NOx that has been reduced is reduced by the reducing agent generated from the components in the fuel and discharged as harmless nitrogen. In order to form a rich atmosphere, for example, a technique such as rich combustion in which fuel is injected at a short interval after main injection from the injector 21 or post injection at a long interval is used.

  A / F sensors 61 and 62 are arranged on the inlet side and the outlet side of the LNT 6. The air / fuel ratio is measured by the A / F sensors 61 and 62. In addition, an accelerator opening sensor 81 is arranged to measure information on accelerator depression by the driver.

  The measured values of the engine speed sensor 22, the A / F sensors 61 and 62, and the accelerator opening sensor 81 described above are sent to the ECU 7. Further, the ECU 7 adjusts the timing and amount of fuel injection to the engine 2 by the injector 21 and the opening between the intake throttle 31 and the EGR valve 51. The ECU 7 may have a structure including a CPU that performs various calculations and a memory 71 that stores various types of information.

  In the present embodiment, the degree of catalyst deterioration in the LNT 6 is determined under the above-described apparatus configuration. When it is determined that the catalyst of the LNT 6 is deteriorating, S regeneration is performed. When it is determined that the LNT is not deteriorating, an operation for increasing the interval between the NOx reduction processes is performed. The procedure for such processing is shown in FIG. The processing in FIG. 2 may be executed sequentially by the ECU 7.

  First, in step S10, NOx emissions from the engine 2 are integrated. For this process, for example, a plane showing the operating state with the engine speed and load as coordinate axes is divided into a plurality of regions, and the NOx emission amount per unit time from the engine 2 in each region is stored in the memory 71. You just have to. And in S10, the process of integrating | accumulating the NOx discharge | emission amount of a corresponding area | region may be performed according to transition of a driving | running state. Here, the engine speed may be measured by the engine speed sensor 22. The load may be a value measured by the accelerator opening sensor 81. The fuel injection amount in the engine 2 may be used instead of the load. In that case, a command value from the ECU 7 may be used as the value of the fuel injection amount.

  Next, in S20, it is determined whether or not the integrated value of the NOx emission amount obtained in S10 is larger than the first emission amount. Here, the value of the first discharge amount may be determined in advance. When the integrated value of the NOx emission amount is larger than the first emission amount (S20: YES), the process proceeds to S30. If the integrated value of the NOx emission amount is equal to or less than the first emission amount (S20: NO), the process returns to S10 and the above procedure is repeated until the integrated value exceeds the first emission amount.

  In S30, a reduction process (rich purge) of NOx occluded in LNT6 is executed. In this procedure, as described above, the normal lean atmosphere is switched to the rich atmosphere, and by achieving a predetermined temperature condition, NOx stored in the LNT 6 is reduced and discharged.

  When the NOx reduction process in S30 ends, next, in S40, the amount of NOx stored in the LNT 6 is estimated from the time taken for NOx reduction. An estimation method will be described. In general, when the air-fuel ratio is made rich to perform NOx reduction, the measured value of the upstream A / F sensor 61 immediately becomes rich, but the measured value of the downstream A / F sensor 62 is being reduced to NOx. Substantially maintains the stoichiometric air-fuel ratio, and becomes rich after the end of NOx reduction.

  Therefore, the time taken for NOx reduction is the time from when the upstream A / F sensor 61 becomes rich until the downstream A / F sensor 62 becomes rich. Since the NOx occlusion amount is reduced as the NOx occlusion amount decreases, the NOx occlusion amount can be estimated from the time taken for NOx reduction. The relationship between the time taken for NOx reduction for step S30 and the amount of NOx stored in the LNT 6 (NOx storage amount) is obtained in advance and stored in the memory 71 and used. That's fine. As the above relationship, for example, a value obtained by multiplying the time taken for NOx reduction by a predetermined coefficient may be used as the NOx occlusion amount in the LNT6.

  In S30, not the time required for NOx reduction, but the horizontal axis represents time, and the vertical axis represents the air-fuel ratio plot of the measured value of the upstream A / F sensor 61 and the downstream A / F sensor. The area between the 62 measurement plots may be used. For the procedure S30, the relationship between the same area and the NOx occlusion amount of the LNT 6 is obtained in advance and stored in the memory 71, and this may be used. As the above relationship, for example, a numerical value obtained by multiplying the area value by a predetermined coefficient may be used as the NOx occlusion amount in the LNT 6.

  Furthermore, in the relationship from the time or area as described above to the NOx occlusion amount, the exhaust gas flow rate may be multiplied by the time or area. The reason is that as the exhaust gas flow rate is larger, the amount of reducing agent in the exhaust gas is generally larger, so that it is considered that more reduction reaction has been performed. Accordingly, the NOx occlusion amount can be calculated with higher accuracy in consideration of the exhaust gas flow rate. For the exhaust gas flow rate, for example, an air flow meter may be installed in the intake pipe 3, and the intake air amount measured thereby may be regarded as the exhaust air amount.

  The downstream A / F sensor 62 may be replaced with an O2 sensor that measures the oxygen concentration. In the case of the O2 sensor, there is a feature that information on whether it is rich or lean can be obtained in a form closer to the on / off value than the A / F sensor, so whether or not the outlet side of the LNT6 has become richer, that is, the reduction of NOx It can be detected whether or not it has been completed.

  Next, in steps S50 to S80, the same procedure as in steps S10 to S40 is repeated. However, the first discharge amount is replaced with a second discharge amount having a larger value.

  As described above, in the present embodiment, the timing for executing the rich purge is determined by the integrated value of the NOx emission amount from the engine 2 becoming a predetermined value (or exceeding it). The first and second emission amounts described above may be emission amounts uniquely set in order to determine the degree of deterioration of the LNT 6. Alternatively, one of the first and second discharge amounts may be a discharge amount that determines the timing for executing the normally used rich purge.

  Next, the difference value of the amount of occlusion is calculated in S90. Here, the difference value of the storage amount is a value of a difference between the NOx storage amount obtained in S80 and the NOx storage amount obtained in S40. Next, in S100, a reference storage amount difference value is calculated. Here, the reference occlusion amount difference value is obtained when the integrated NOx emission amount is the second emission amount under the condition that the deterioration level of the LNT 6 and the NOx emission amount from the engine 2 set in advance are set as the reference. This is a difference value between the NOx occlusion amount in the LNT6 and the NOx occlusion amount in the LNT6 when the NOx emission amount integrated value is the first emission amount. The reference storage amount difference value may be calculated in advance and stored in the memory 71, and only it may be called in S100.

  Next, a first threshold value and a second threshold value are calculated in S110. As will be described below, the first threshold is a threshold at which the degree of deterioration of the LNT 6 is high and the S regeneration is determined to be necessary if the occlusion amount difference value is smaller than that value. For that purpose, the first threshold value may be set to a value smaller by a predetermined value than the reference storage amount difference value obtained in S100.

  Further, the second threshold value is a threshold value for performing the process of widening the interval between the rich purges because the deterioration degree of the LNT 6 is low if the occlusion amount difference value is larger than that value. For this purpose, the second threshold value may be set to a value that is larger by a predetermined value than the reference storage amount difference value obtained in S100. The predetermined value for setting the first threshold may be a value different from the predetermined value for setting the second threshold.

  Next, in S120 and S130, the magnitude relationship between the storage amount difference value obtained in S90 and the first threshold value and the second threshold value set in S110 is determined. As shown in FIG. 2, if the occlusion amount difference value is smaller than the first threshold (S120: YES), it is determined that the degree of deterioration of LNT6 is large, so the routine proceeds to S140, where S poisoning regeneration is executed. If the occlusion amount difference value is not less than the first threshold value (S120: NO) and not more than the second threshold value (S130: NO), it is determined that the degree of deterioration is moderate, and this series of processing is terminated.

  If the occlusion amount difference value is larger than the second threshold value (S120: NO and S130: YES), it is determined that the degree of deterioration of LNT6 is low, and the operation proceeds to S150 to increase the interval between the NOx reduction processes. Do it. The operation of increasing the interval between the NOx reduction processes here may be, for example, increasing the NOx emission integrated value that determines the timing for executing the NOx reduction process.

  Hereinafter, the effects of the processing from S120 to S150 will be described with reference to FIG. FIG. 3 shows three response examples when the horizontal axis represents time and the vertical axis represents the NOx occlusion amount. The origin in FIG. 3 corresponds to a state where the LNT 6 is new or a rich purge is executed and the NOx occlusion amount becomes zero. Responses 100, 101, and 102 in FIG. 3 indicate how the NOx occlusion amount in the LNT 6 increases as the engine 2 operates and continues to discharge NOx.

  The response 100 is a reference characteristic, and is a response when the degree of deterioration of the LNT 6 is medium and the NOx emission amount from the engine 2 is also medium. The response 101 is a response when the degree of deterioration of the LNT 6 is small and the NOx emission amount from the engine 2 is also small. The response 102 is a response when the degree of deterioration of the LNT 6 is large and the NOx emission amount from the engine 2 is also large.

  As the degree of deterioration increases, that is, in the order of the response 101, the response 100, and the response 102, the NOx occlusion amount approaches the saturated state at an earlier time, and the response curve becomes closer to the horizontal. The reason for this is illustrated by FIG. FIG. 4 shows the storage characteristics of the LNT 6 when the horizontal axis represents the NOx storage amount and the vertical axis represents the NOx purification rate. While the NOx occlusion amount is small, a high NOx purification rate is maintained. However, as the NOx occlusion amount increases, the occlusion performance decreases, and the NOx purification rate decreases from a certain point. As shown in FIG. 4, such a reduction in the occlusion performance appears as the NOx occlusion amount decreases as the degradation of the LNT 6 progresses. Therefore, the greater the degree of deterioration of LNT6, the higher the NOx occlusion amount reaches its peak at a faster stage.

  In general, if the NOx emission amount from the engine 2 is the same, the greater the degree of deterioration of the LNT 6, the smaller the NOx occlusion amount. However, if the NOx emission amount is large, the NOx occlusion amount may become large even if the degree of deterioration of the LNT 6 is large. As described above, in the responses 100, 101, and 102 in FIG. 3, the NOx emission amount increases in the order of the responses 101, 100, and 102. As a result, in the initial time zone in FIG. 3, the NOx occlusion amount increases in the order of the responses 102, 100, and 101 with the higher deterioration level, contrary to what is imagined from the deterioration degree. The above is the reason why the response curve as shown in FIG. 3 is obtained.

  Points 200, 201, and 202 indicate points in time when the integrated value of the NOx emission amount reaches the first emission amount. As shown in FIG. 3, the NOx emissions at points 200, 201, and 202 are A0, A1, and A2, respectively. In step S40 in FIG. 2, A0, A1, and A2 are calculated. Points 203, 204, and 205 indicate the time when the integrated value of the NOx emission amount reaches the second emission amount. As shown in FIG. 3, the NOx emissions at points 203, 204, and 205 are B0, B1, and B2, respectively. In step S80 in FIG. 2, B0, B1, and B2 are calculated.

  Δ0, Δ1, and Δ2 in FIG. 3 are occlusion amount difference values calculated in S90 in FIG. As shown in FIG. 3, Δ0, Δ1, and Δ2 are values of B0-A0, B1-A1, and B2-A2, respectively. Δ0 may be a reference storage amount difference value obtained (or called) in S100.

  As described above, when the points 200, 201, and 202 are compared, the NOx occlusion amount is larger in the order of the points 202, 200, and 201 in which the degree of deterioration of the LNT 6 is larger due to the influence of the NOx emission amount. In the prior art, it has been determined that the smaller the NOx occlusion amount, the more the LNT 6 is degraded. Therefore, for example, when determining the deterioration level by looking only at points 200, 201, and 202, the NOx occlusion amount is small in the order of A1, A0, and A2, so the deterioration level of LNT6 is in the order of responses 101, 100, and 102. It is misjudged as high.

  However, according to the present invention, the occlusion amount difference values Δ0, Δ1, and Δ2 are calculated as described above, and it is determined in S120 of FIG. In FIG. 3, the values become smaller in the order of Δ2, Δ0, and Δ1. This is because, as explained above, the NOx occlusion amount approaches saturation as soon as the degree of deterioration increases. Therefore, the occlusion amount difference value decreases in the order of Δ2, Δ0, and Δ1 in the order of responses 102, 100, and 101 in which the degree of deterioration of LNT6 is large. Therefore, the degree of deterioration of the LNT 6 can be accurately detected from the small amount of occlusion amount difference. This is the reason why the degradation degree of LNT 6 can be accurately detected by the method of the present invention.

  In S140 of FIG. 2, since the occlusion amount difference value is smaller than a first threshold value, it is determined that the degree of deterioration of the LNT 6 is large, and S poisoning regeneration is executed. In S150, it is determined that the degree of deterioration of the LNT 6 is small because the storage amount difference value is larger than a second threshold value. When the degree of deterioration of the LNT 6 is small, more NOx can be occluded, so that a process of increasing the NOx occlusion time or increasing the interval of the NOx reduction process is performed. Thereby, deterioration of fuel consumption due to execution of rich purge can be suppressed. The above is the effect of the processing from S120 to S150.

  The first discharge amount and the second discharge amount may be determined from the target purification rate as follows. Here, the purification rate (or NOx purification rate) is the ratio between the amount of NOx stored in the LNT 6 and the amount of NOx discharged from the engine. The target purification rate is a purification rate required for exhaust purification. For example, the first emission amount is determined so that the purification rate at the time when the integrated value of the NOx emission amount becomes the first emission amount becomes a predetermined target purification rate. Further, the second emission amount is determined, for example, so that the average purification rate up to the time when the integrated value of the NOx emission amount becomes the second emission amount becomes the predetermined target purification rate.

  As a calculation method in that case, for example, a functional relationship between the NOx emission integrated value and the NOx purification rate in the reference characteristic (response 100 in FIG. 3) is obtained. In the functional relationship, the NOx emission amount integrated value at the point where the NOx purification rate is the predetermined target purification rate is the first emission amount. Further, in the same functional relationship, when the average value of the NOx purification rate from a certain point to the origin is a predetermined target purification rate, the NOx emission integrated value at that point is the second emission amount.

  At this time, it is clear from FIG. 4 and the like that the purification rate at all times up to the first emission amount is higher than the target purification rate. Therefore, the average purification rate up to the first emission amount is higher than the target purification rate. In the above calculation method, the second discharge amount is always larger than the first discharge amount. As described above, by determining the first and second emission amounts using the target purification rate, the purification rate up to the first and second emission amounts can be made higher than the target purification rate. The above is the embodiment of the present invention.

  In the above embodiment, the procedure of S10 and S50 constitutes an emission amount calculating means. The procedures of S40 and S80 constitute the occlusion amount calculation means. The value of A0 or A1 or A2 constitutes the first storage amount. The value of B0 or B1 or B2 constitutes the second occlusion amount. The procedure of S120 constitutes a determination means. The first threshold functions as a threshold. The procedure of S140 constitutes playback means. The procedure of S130 constitutes a second determination unit. The procedure of S150 constitutes the adjusting means. In the above embodiment, the internal combustion engine is not a diesel engine, but may be another engine that performs lean combustion, such as a lean burn gasoline engine.

1 is a schematic diagram of an internal combustion engine and an exhaust purification device thereof as an embodiment of the present invention. The flowchart of a catalyst deterioration degree determination process. The figure which shows the time response of NOx occlusion amount. The figure which shows the relationship between NOx purification rate and NOx occlusion amount.

Explanation of symbols

1 Exhaust gas purification device 2 Diesel engine (internal combustion engine)
3 Intake pipe 4 Exhaust pipe (exhaust passage)
5 EGR pipe 6 NOx catalyst (LNT)
7 ECU
22 Engine speed sensor 31 Intake throttle 51 EGR valve 61, 62 A / F sensor 81 Accelerator opening sensor

Claims (3)

An exhaust gas purification apparatus for an internal combustion engine comprising a NOx catalyst in the exhaust passage for storing NOx in a lean atmosphere and reducing NOx in a rich atmosphere,
An emission amount calculating means for acquiring an NOx emission amount from the internal combustion engine;
A storage amount calculating means for acquiring a storage amount of NOx in the NOx catalyst,
The NOx occlusion amount calculated by the occlusion amount calculating means when the integrated value of the NOx emission amount calculated by the emission amount obtaining means becomes a predetermined first emission amount is defined as the first occlusion amount. ,
The NOx occlusion amount calculated by the occlusion amount calculating means when the integrated value of the NOx emission amount calculated by the emission amount obtaining means becomes a predetermined second emission amount is set as the second occlusion amount. ,
The second emission amount is larger than the first emission amount,
Discrimination means for discriminating that a difference between the second occlusion amount and the first occlusion amount is smaller than a predetermined threshold;
Regenerating means for regenerating the NOx catalyst when it is determined by the determining means that the difference between the second stored amount and the first stored amount is smaller than the threshold value. An exhaust purification device for an internal combustion engine. Second discriminating means for discriminating that a difference between the second occlusion amount and the first occlusion amount is larger than a second threshold value which is larger than the threshold value;
Adjustment means for widening the interval between the NOx catalyst reduction processes when the second determination means determines that the difference between the second storage amount and the first storage amount is greater than the second threshold value. An exhaust emission control device for an internal combustion engine according to claim 1, comprising: The ratio of the amount of NOx occluded in the NOx catalyst and the integrated amount of the NOx emission amount is used as a purification rate.
The first emission amount is set so that the purification rate when the integrated value of the NOx emission amount becomes the first emission amount becomes a predetermined target purification rate,
2. The second emission amount is set such that an average of the purification rate up to a time point when the integrated value of the NOx emission amount becomes the second emission amount becomes the predetermined target purification rate. 2. An exhaust emission control device for an internal combustion engine according to 2.

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