JP2005106005A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device of an internal combustion engine capable of reducing the NOx storage amount of an NOx storage catalyst while suppressing the occurrence of NOx slip when the NOx stored amount is increased. <P>SOLUTION: This exhaust emission control device performs a second rich operation by dividing an operation time into a plurality of parts or the second rich operation is performed by gradually increasing the reduction density of exhaust gas flowing into the NOx storage catalyst to a target reduction density when an operation cannot be moved to a normal intermittent first rich operation, NOx storage amount stored in the NOx storage catalyst 33 is increased more than normal, and then the operation can be moved to the rich operation. Thus, since the stored NOx is gradually discharged, the NOx storage amount can be reduced while suppressing the occurrence of NOx slip to which reducing reaction cannot follow up. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine that purifies exhaust gas using a NOx storage catalyst.

  In an engine (internal combustion engine) mounted on an automobile, as an exhaust gas countermeasure, an exhaust gas purification device using a NOx storage catalyst is equipped to purify NOx contained in the exhaust gas of the engine. In particular, the NOx storage catalyst stores NOx when the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst is lean (lean from the stoichiometric air-fuel ratio), and when the air-fuel ratio of the exhaust gas is rich (excessive concentration including the stoichiometric air-fuel ratio). In general, it is often used in diesel engines, lean burn gasoline engines, and the like, in which the exhaust gas has a high tendency to operate lean, because it has a characteristic of releasing and reducing NOx stored in the exhaust gas.

  The NOx occlusion catalyst of the exhaust purification device occludes NOx in the exhaust when the engine is in a normal lean operation state, but the NOx occlusion catalyst has a limited occlusion capacity, so the NOx occluded at a certain time is limited. It is necessary to regenerate the NOx storage catalyst by performing a rich operation in order to release and reduce the NOx (see, for example, Patent Document 1). Many regenerations are performed periodically during lean operation, specifically, when the accumulated lean operation time reaches a certain time, the engine is switched to rich operation for a predetermined time, and NOx is released from the NOx storage catalyst. It is reduced.

  By the way, the NOx storage catalyst does not function when the catalyst temperature is extremely low. Therefore, even if the rich operation is performed at a low temperature, the effect of releasing / reducing stored NOx cannot be expected, and the fuel consumption deteriorates due to the useless rich operation. Therefore, the rich operation is not shifted to such a temperature state. Further, when the catalyst temperature is extremely high, the NOx storage catalyst may be damaged due to the temperature rise caused by the reduction action. Therefore, in such a temperature state, the rich operation is not shifted.

  However, for example, when the non-rich state where the catalyst temperature is lower than the temperature at which the catalyst shifts to the rich operation as shown in FIG. 12 is prolonged (for example, when a diesel vehicle travels in a congested urban area), the A part in FIG. As shown in FIG. 5, the NOx occlusion amount that should be released and reduced by intermittent normal rich operation is accumulated with time, and NOx is occluded to an occlusion amount that greatly exceeds the occlusion amount during normal regeneration.

In such a case, as shown in part B of FIG. 13, conventionally, a rich operation having a deeper degree of richness, for example, a longer operation period is performed than the normal rich operation performed intermittently. The large amount of NOx occlusion shown in the section is reduced at once (for example, see Patent Document 2).
JP 11-148337 A Japanese Patent Laid-Open No. 06-272540

  As is well known, the regeneration of the NOx storage catalyst is performed by releasing NOx from the storage agent by a reducing agent (CO, HC, etc.) contained in the exhaust gas, and this released NOx is exhausted on the noble metal (Pt, etc.). It is carried out by reacting with the reducing agent and reducing it to harmless nitrogen.

  However, the regeneration when the NOx occlusion amount increases causes NOx to be released from the occlusion agent all at once, as shown in part E in FIG. For this reason, when the NOx occlusion amount increases, a phenomenon occurs in which the reduction reaction cannot catch up because too much NOx is released. This phenomenon is called NOx slip. When NOx slip occurs, a large amount of NOx that cannot be reduced is temporarily discharged in a large amount, thereby deteriorating the NOx purification rate.

  Accordingly, an object of the present invention is to provide an exhaust gas purification device for an internal combustion engine that can reduce the NOx occlusion amount of a NOx occlusion catalyst while suppressing the occurrence of NOx slip when the NOx occlusion amount increases.

  According to the first aspect of the present invention, in order to achieve the above object, when the intermittent normal first rich operation cannot be shifted, the NOx occlusion amount increases from the normal time, and then the rich shift becomes possible. In order to suppress the NOx slip, a configuration in which the second rich operation is executed by dividing the operation period into a plurality of times is adopted.

  In addition to the above-mentioned object, the invention according to claim 2 models the NOx occlusion amount stored in the NOx catalyst by modeling NOx occlusion and NOx reduction in the NOx occlusion catalyst so that further optimal second rich operation is performed. When the NOx occlusion amount estimated by the NOx occlusion estimation means is increased to a NOx slip occurrence threshold value or more that regulates the occurrence of NOx slip using the estimated NOx occlusion estimation means, The second rich operation is performed so that the estimated NOx occlusion amount is gradually released.

  In the invention according to claim 3, in addition to the above object, when the second rich operation is executed while being divided so that the NOx occluded can be reduced more quickly, the NOx occlusion estimation means is configured to perform NOx occlusion estimation. When it is estimated that the NOx occlusion amount has decreased until it falls below the slip generation threshold, after that, one rich operation is executed to reduce the remaining NOx occlusion amount all at once.

  In addition to the above object, the invention as set forth in claim 4 further provides the NOx catalyst based on the accumulation of the lean operation time of the internal combustion engine so that the optimum second rich operation is performed as in the case of claim 2. Using the means for determining the rich transition time, the accumulated lean operation time is increased to a time corresponding to the NOx slip occurrence threshold that defines the occurrence of NOx slip, and then stored when the rich transition becomes possible. The second rich operation is performed so that NOx is gradually released.

  According to the fifth aspect of the present invention, in addition to the above-mentioned object, the necessary number of divisions is set for the second rich operation based on the accumulated lean operation time so that the unnecessary second rich operation is not executed. Executed according to the number of divisions.

  In the invention according to claim 6, in addition to the above object, the second rich operation repeats the normal first rich operation performed intermittently a plurality of times so that the second rich operation is executed with simpler control. Or it was made to perform by the operation which repeats the operation which weakened the richness from the 1st rich operation a plurality of times.

  In order to achieve the above object by a method different from that of the first aspect of the invention, the intermittent normal first rich operation cannot be shifted, and the NOx occlusion amount increases from the normal time. Then, when the rich transition becomes possible, the second rich operation in which the exhaust gas flowing into the NOx occlusion catalyst in the internal combustion engine is made to be the target reduction concentration while gradually increasing the reduction concentration is adopted to suppress the NOx slip. .

  In addition to the above-described object, the invention according to claim 8 models the NOx occlusion amount stored in the NOx catalyst by modeling NOx occlusion and NOx reduction in the NOx occlusion catalyst so that further optimal second rich operation is performed. When the NOx occlusion amount estimated by the NOx occlusion amount estimation means increases to a NOx slip occurrence threshold value or more that prescribes the occurrence of NOx slip, and then the rich transition becomes possible, using the estimated NOx occlusion amount estimation means, The second rich operation is performed so that the estimated storage NOx amount is gradually released.

  The invention according to claim 9 similarly uses the means for determining the rich transition timing of the NOx catalyst based on the accumulation of the lean operation time of the internal combustion engine, and the accumulated lean operation time defines the occurrence of NOx slip. The second rich operation is executed so that the stored NOx is gradually released when it increases to the time corresponding to the NOx slip occurrence threshold to be reached and then rich transition becomes possible.

  According to the tenth aspect of the present invention, in addition to the above object, the NOx storage estimating means estimates the degree of the second rich operation that gradually increases the reduction concentration so that the second rich operation is more optimally performed. The inclination is set according to the amount of occlusion or the inclination according to the accumulated time of the lean operation time, and the second rich operation is executed in accordance with the inclination.

  According to the eleventh aspect of the present invention, in addition to the above object, when the second rich operation in which the reduction concentration is gradually increased is executed so that the NOx stored can be rapidly reduced, the NOx storage catalyst When the temperature rises to a predetermined temperature value, after that, an operation for increasing the reduction concentration at a stretch to the target reduction concentration is performed.

  In the invention according to claim 12, in addition to the above object, the second rich operation is performed by changing the excess air ratio of the exhaust gas flowing into the NOx storage catalyst so that the second rich operation is executed with simple control. The exhaust gas was configured to operate at the target reduction concentration while gradually increasing the reduction concentration.

  According to the first and seventh aspects of the invention, the regeneration when the NOx occlusion amount increases is caused by the second rich operation so that the occluded NOx is not released at once but gradually released. Therefore, it is possible to suppress the occurrence of NOx slip where the reduction reaction cannot catch up.

  Therefore, it is possible to reduce the NOx occlusion amount of the NOx occlusion catalyst while suppressing the occurrence of NOx slip, and to prevent NOx from being temporarily discharged in large quantities.

  According to the second, fourth, eighth, and ninth aspects of the present invention, the second optimally stored in accordance with the NOx occlusion state when the NOx occlusion amount increases is further determined based on the estimated NOx occlusion amount. Rich operation can be executed.

  According to the third aspect of the present invention, the NOx amount is reduced (released) at a stretch from the same situation as in the normal state where no occurrence of NOx slip is observed. It becomes possible to make it.

  According to the fifth aspect of the present invention, the second rich operation is performed without waste by the execution of the second rich operation according to the required number of divisions determined by the estimated NOx occlusion amount, and the deterioration of fuel consumption is suppressed. Can be achieved.

  According to the sixth aspect of the present invention, the second rich operation can be executed with simple control.

  According to the invention described in claim 10, the second rich operation for gradually increasing the reduction concentration can be optimally executed in accordance with the state of the NOx occlusion amount.

  According to the eleventh aspect of the present invention, the second rich operation in which the reduction concentration is gradually increased can be executed in accordance with the state in which the NOx storage catalyst functions sufficiently, and the stored NOx is rapidly reduced. It becomes possible to make it.

  According to the twelfth aspect of the present invention, the second rich operation in which the reduction concentration is gradually increased can be executed with simple control.

[First Embodiment]
Hereinafter, the present invention will be described based on a first embodiment shown in FIGS.

  FIG. 1 shows a main part of an internal combustion engine mounted on an automobile (vehicle), for example, a four-cycle diesel engine. In FIG. 1, reference numeral 1 denotes an engine main body composed of a cylinder block 2 and a cylinder head 3. 4 is a cylinder formed in the cylinder block 2, 5 is a piston provided in a reciprocating manner in the cylinder 4, 8 and 9 are intake / exhaust ports provided in the cylinder head 3, and 6 and 7 are intake / exhaust ports. The intake / exhaust valves 10 for opening and closing the ports 8 and 9 are injectors provided in the cylinder head 3. Among these, the intake port 8 is connected to the discharge portion of the compressor 14 of the turbocharger 13 via the first intake passage 12 extending from the intake port 8. In addition, the suction part of the compressor 14 is connected to the 2nd intake passage 15 which goes to an air cleaner (not shown). However, 17 is an intercooler interposed in the first intake passage 12. The exhaust port 9 is connected to an inlet portion of the turbine 19 of the turbocharger 13 via a first exhaust passage 18 extending from the exhaust port 9. A second exhaust passage 20 that is open to the atmosphere is connected to the outlet of the turbine 19. Moreover, the injector 10 is connected to ECU21 which comprises a control part. The injection operation of the injector 10 is controlled in accordance with the injection timing and the fuel injection amount that are set in the ECU 21 in advance according to the operating state of the engine, and the engine performs a predetermined cycle (suction, compression, expansion, exhaust) by this control. 4 cycles).

  Incidentally, between the downstream side of the first intake passage 12 and the first exhaust passage portion, each device constituting the EGR device 22, for example, an EGR passage 24 in which an EGR cooler 23 is interposed, and the EGR passage 24 are opened and closed. The EGR valve 25 and the electric fresh air amount control valve 26 for controlling the fresh air intake amount upstream from the outlet of the EGR passage 24 are provided.

  An exhaust gas purification device 30 is assembled in the exhaust system of such a diesel engine. The exhaust gas purification device 30 uses a configuration in which a NOx storage catalyst 33 with a built-in casing 32, a reducing agent addition unit 34 that supplies a reducing agent, and a control system 35 that performs regeneration control using the ECU 21, for example, are used. .

  Specifically, the NOx storage catalyst 33 is interposed in the middle of the second exhaust passage 20. As the structure, for example, a structure in which a carrier is supported with a noble metal such as platinum (Pt) and barium (Ba) as an occlusion agent is used. With this structure, when the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst 33 is lean (lean than the stoichiometric air-fuel ratio), NOx in the exhaust is oxygenated on platinum (Pt) as shown in FIG. When the air-fuel ratio of the exhaust gas that is absorbed by barium (Ba) in the form of nitrate ions and flows into the NOx storage catalyst 33 is rich (overconcentration including theoretical air fuel), the reaction is reversed in FIG. ) The nitrate ion in barium (Ba) is released in the form of NOx as shown in Fig. 3), and the released NOx reacts with unburned HC, CO, etc. in the exhaust gas on platinum (Pt) and is reduced to nitrogen Has brought.

  In the reducing agent adding section 34, for example, a reducing agent injector 36 that supplies, for example, fuel (for example, light oil) as a reducing agent, which is installed in an exhaust passage portion upstream of the NOx storage catalyst 33, and the previous EGR device 22, A structure using the new air amount control valve 26 in combination is used. This makes it possible to perform unburned HC, CO when necessary, using the rich operation, that is, the fuel from the injector 36 and the recirculation of the EGR gas in which the amount of fresh air is suppressed by the EGR device 22 and the new amount control valve 26. Rich exhaust gas containing a large amount of gas is allowed to flow into the NOx storage catalyst 33. Note that the degree of richness (reduction concentration) can be varied by adjusting the injection amount of the injector 36 and the amount and concentration of EGR gas.

  The control system 35 employs control using a function for estimating the NOx occlusion amount of the NOx occlusion catalyst 33. As disclosed in Japanese Patent Application No. 2003-059007 filed earlier by the present applicant, the estimation function is, for example, an exhaust gas flow rate from an exhaust gas flow rate sensor 38 installed in an exhaust portion upstream of the NOx storage catalyst 33. Similarly, by receiving the exhaust gas temperature from the exhaust gas temperature sensor 39, the NOx occlusion amount during lean operation and the NOx emission amount during rich operation are independently calculated, and from these NOx occlusion amount and NOx release amount, The NOx occlusion amount in the NOx occlusion catalyst 33 is estimated. Specifically, the gas flow rate of the exhaust gas flowing into the NOx storage catalyst 33, the NOx concentration upstream of the NOx storage catalyst 33, the reducing agent concentration upstream of the NOx storage catalyst 33, the NOx storage catalyst 33 obtained by the calculation. Based on the downstream NOx concentration and the reducing agent concentration on the downstream side of the NOx storage catalyst 33 obtained by the calculation, the NOx storage amount of the NOx storage catalyst 33 is estimated by the following equation (1). This equation (1) is set in the ECU 21. Note that the NOx concentration output from the NOx sensor 39 installed on the downstream side of the NOx storage catalyst 33 is used to correct the initial value of the NOx storage amount. Here, only the point where the NOx occlusion amount can be estimated is shown, and the detailed method for estimating the NOx occlusion amount of the NOx occlusion catalyst 33 is omitted.

NOx occlusion amount = α1Σ gas flow rate × {(upstream NOx concentration−downstream NOx concentration) − (upstream reducing agent concentration−downstream reducing agent concentration)} (1)
However, α1 is a constant, and the upstream NOx concentration and the upstream reduction concentration are obtained from the state of engine operation.

  The ECU 21 uses the NOx occlusion amount estimated from the model equation of the NOx occlusion catalyst 33 to shift from lean operation to periodic rich operation. Therefore, a normal rich transition threshold (Th0) is set as the threshold. It is. Further, the ECU 21 determines whether or not the NOx occlusion catalyst 33 is in a sufficiently functioning state when the estimated NOx occlusion amount reaches the normal rich transition threshold (Th0), that is, the catalyst temperature is extremely low. A determination function (corresponding to rich transition determination means) is set for determining whether or not the lean operation can be shifted to the normal rich operation based on whether or not the temperature is other than a high state. In addition, when the ECU 21 determines that the shift is possible, for example, the ECU 21 has a function to execute the rich normal rich operation once to release the NOx occlusion amount up to the normal rich shift threshold (Th0) at once. Therefore, the normal rich operation is intermittently performed (normal regeneration).

  Further, the ECU 21 is set to perform control so that the rich operation is optimally performed when the NOx storage amount increases, using the NOx storage amount estimated from the model expression of the NOx storage catalyst 33. That is, the ECU 21 is set with a NOx slip occurrence threshold (Th1) as a threshold that regulates the occurrence of NOx slip (a situation in which NOx that cannot be fully reduced is temporarily discharged), and further terminates the rich operation. A rich end threshold value (Th2) is set as a prescribed threshold value (Th1> Th2). In addition, when the estimated NOx occlusion amount becomes equal to or greater than the NOx slip occurrence threshold (Th1), the ECU 21 has a function of executing a rich operation (corresponding to the second rich operation) until the rich end threshold (Th2) is reached. It is set (equivalent to the second rich operation means). For the execution of the rich operation, a control for executing the rich operation by dividing the longer operation period into a plurality of parts, instead of extending the rich operation period in accordance with the NOx occlusion amount, is adopted. Here, for example, control is set to repeatedly perform rich operation with a lower degree of richness than intermittent normal rich operation. Thus, NOx is gradually released from the NOx storage catalyst 33 in the rich operation when the NOx storage amount increases.

  Such regeneration control of the NOx storage catalyst 33 is shown in the flowchart of FIG.

  The operation of the exhaust gas purification device 30 will be described with reference to the flowchart. It is assumed that the diesel engine is normally operated (lean operation). At this time, since the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst 33 is lean, the NOx contained in the exhaust gas reacts with oxygen on platinum (Pt) as shown in FIG. It is absorbed by barium (Ba) in the form of nitrate ions. That is, NOx in the exhaust gas is stored in barium (Ba) as the storage agent.

  During such lean operation, the ECU 21 sequentially stores the NOx stored in the NOx storage catalyst 33 in consideration of the operation state of the diesel engine by the calculation using the model expression of the NOx storage catalyst 33 (the given equation (1)). The amount is estimated.

  The estimated NOx occlusion amount normally reaches the rich transition threshold value (Th0), and the NOx occlusion catalyst 33 can sufficiently perform its function (catalyst state excluding a state where the catalyst temperature is extremely low and an extremely high state). Then, the ECU 21 determines that it is time to regenerate the NOx storage catalyst 33. Then, a signal for requesting a shift from lean operation to rich operation for regeneration is output (step S1).

  Here, since the NOx occlusion amount estimated so far does not reach the occlusion amount that causes NOx slip (step S2), the ECU 21 is a rich signal for performing the normal rich operation as shown in FIG. 4A. α is output. Then, in response to the rich signal α, for example, the EGR valve 25 is “open” and the fresh air control valve 26 is “closed”, and fuel for reduction, light oil here, enters the exhaust passage 20 from the reduction injector 36. Be injected. Thereby, exhaust gas in a reducing atmosphere containing unburned HC, CO (both are reducing agents), that is, rich exhaust gas, flows into the NOx storage catalyst 33, and the normal rich operation is executed once.

  Then, in the NOx storage catalyst 33, a reaction opposite to the previous NOx storage is performed. That is, as shown in FIG. 2B, when exhaust gas is rich, nitrate ions in barium (Ba) are released in the form of NOx, and the released NOx is unburned in the exhaust gas on platinum (Pt). It reacts with a reducing agent such as HC and CO to be reduced to nitrogen. As a result of this release and reduction, the NOx occlusion amount occluded in the NOx occlusion catalyst 33 is reduced at a stretch while suppressing NOx from being discharged to the outside as shown by β and γ in FIGS. 4B and 4C. Decrease.

  In the model equation (the equation (1)) of the NOx occlusion catalyst 33, a decrease in the NOx occlusion amount caused by such NOx release and reduction is also considered.

  During the lean operation, such normal rich operation is intermittently performed as long as the state in which the function of the NOx storage catalyst 33 can be sufficiently exerted continues.

  On the other hand, a state in which the catalyst temperature is extremely low or extremely high is observed, and a state in which the normal rich operation cannot be performed (for example, when traveling in a congested urban area) is prolonged, and the estimated NOx storage catalyst 33 It is assumed that the NOx occlusion amount has increased to a value equal to or greater than the NOx slip occurrence threshold (Th1) that regulates the occurrence of NOx slip as shown in FIG.

  Thereafter, when the rich becomes possible, the ECU 21 determines that the rich signal whose richness is reduced by shortening the operation period (compared to the normal rich), for example, the NOx occlusion amount estimated by the model formula is the rich end threshold value. (Th2) It is repeatedly executed a plurality of times until it becomes the following.

  As a result, the nitrate ions in the barium Ba are not released at once in the form of NOx, but are gradually released by the divided rich operation, as shown in FIG. 4B. The released NOx reacts with the reducing components HC, CO, etc. in the exhaust gas and is reduced (nitrogen) on platinum (Pt) as shown in FIG. Thereby, as shown in FIG. 4C, the NOx occlusion amount is gradually released, that is, decreased in accordance with the reduction reaction. Of course, the NOx occlusion amount estimated by the model equation also gradually decreases (releases).

  Therefore, it is possible to reduce the NOx occlusion amount of the NOx occlusion catalyst 33 while suppressing the occurrence of NOx slip where the reduction reaction cannot catch up. As shown in FIG. 4B, a large amount of NOx is temporarily discharged. I'm sorry.

  In particular, the NOx occlusion amount of the NOx occlusion catalyst 33 is modeled to estimate the NOx occlusion amount to be occluded in the NOx occlusion catalyst 33, and the NOx occlusion amount is increased so that the estimated NOx occlusion amount is gradually decreased. Since the rich operation at the time is executed, the optimal rich operation can be performed in accordance with the NOx occlusion state when the NOx occlusion amount increases, and the fuel consumption is not lost unnecessarily. In addition, the rich operation when the NOx occlusion amount is increased can be simply controlled because the rich operation with a lower degree of richness than usual is repeated a plurality of times.

[Second Embodiment]
5 and 6 show a second embodiment of the present invention.

  The second embodiment is a modification of the first embodiment. As shown in the diagram of FIG. 5 and the flowchart of FIG. 6, when the NOx occlusion amount is increased, the rich operation is executed while being divided. When the estimated NOx occlusion amount is estimated to be lower than the NOx slip occurrence threshold (Th1), the remaining NOx occlusion amount is reduced at a stretch in one rich operation since it is the normal rich operation range thereafter. It is what I did.

  If it does in this way, the rich operation at the time of NOx occlusion amount increase can be advanced rapidly.

  5 and 6, the same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.

[Third Embodiment]
FIG. 7 shows a flowchart as the main part of the third embodiment of the present invention.

  In the third embodiment, the NOx storage catalyst is modeled as in the first embodiment, and the NOx storage amount is not estimated, but the rich transition timing of the NOx storage catalyst is based on the integration of the lean operation time of the engine. When the lean operation time is increased, the stored NOx is gradually released (decreased) when the lean operation time is increased.

  Specifically, a function of detecting the rich transition timing of the NOx storage catalyst based on the accumulation of the lean operation time of the engine is used instead of the model formula. Specifically, as shown in the flowchart of FIG. 7, first, from the time when the previous rich operation is completed (step S21), the lean operation time is started to be accumulated by the lean time timer (step S22). Then, the accumulated time of the lean operation time normally reaches the rich transition threshold value, and the NOx storage catalyst 33 can sufficiently perform its function (catalyst state excluding a state where the catalyst temperature is extremely low and a state where it is extremely high). Then, a signal for requesting a shift from lean operation to rich operation for regeneration is output (step S23).

  Here, since the accumulated lean operation time does not reach the lean operation time that causes the NOx slip (step S24), the normal rich operation is executed (step S3) as in the first and second embodiments. Similarly, the NOx occlusion amount occluded in the NOx occlusion catalyst 33 decreases at a stretch.

  On the other hand, a state in which normal rich operation cannot be executed (for example, when the catalyst temperature is extremely low or high due to running in a congested urban area) is prolonged, and the lean operation time accumulated by the lean time timer corresponds to the NOx slip occurrence threshold. It is assumed that the time has increased to the time (t1) or longer (step S24). Thereafter, when the rich operation becomes possible, the required number of divisions of the rich operation with a reduced rich degree is set based on the accumulated lean operation time (step S25). Subsequently, the weak rich operation is executed a plurality of times according to the set number of divisions (step S26). After the execution, a flag indicating that the regeneration of the NOx storage catalyst is completed is attached, and the operation returns to the lean operation.

  Even when a method for estimating the NOx occlusion amount from the accumulation of the lean operation time is used, the optimal rich operation can be performed by the divided rich operation as in the case of using the model formula. In particular, the required number of divisions is obtained from the integration of the lean operation time, and the rich operation at the time when the NOx storage amount increases is executed by the same number of times, so that an optimal rich operation without waste according to the NOx storage state can be performed.

  In the first to third embodiments described above, the rich operation when the NOx occlusion amount is increased is performed by repeatedly executing the operation with reduced richness, but the normal rich operation is repeatedly performed. May be implemented. Even when such normal rich operation is repeated, NOx is gradually released from the NOx occlusion catalyst. Therefore, as in the first to third embodiments described above, the generation of NOx slip can be suppressed with simple control. However, the NOx occlusion amount can be reduced.

[Fourth Embodiment]
8 to 10 show a fourth embodiment of the present invention.

  In the present embodiment, instead of performing the chopped rich operation as in the first to third embodiments, performing the rich operation of gradually increasing the reduction concentration of the exhaust gas flowing into the NOx storage catalyst to the target reduction concentration, The NOx occluded in the NOx occlusion catalyst is gradually released. In addition, since the engine periphery and the exhaust gas purifying apparatus of the present embodiment are the same as those of the first embodiment, the drawings showing the new configuration are omitted, and here, description will be given by using FIG. 1, FIG. 2, and FIG. To do.

  In order to perform this control, the ECU 21 of the regeneration control system becomes a main part, a. Using the method for estimating the NOx occlusion amount of the NOx occlusion catalyst as described in the first to third embodiments, it is determined that the shift to the normal rich operation is impossible, and the NOx occlusion amount increases from the normal time. Thereafter, when the shift to the rich operation becomes possible, the reduction concentration is gradually reduced to the target reduction concentration by the method of gradually reducing the excess air ratio λ by the operation of the EGR device of the first embodiment and the reduction injector. A function of increasing, and b. A function of setting the degree of richness that gradually increases the reduction concentration according to the NOx occlusion amount, c. A function to increase the reduction concentration at a stretch from the middle is set.

  The contents of this control are shown in the flowchart of FIG. 8, and the behavior of the exhaust gas and the NOx storage catalyst by this control is shown in FIGS.

  The operation of the exhaust gas purification device will be described with reference to the flowchart of FIG. 8. Now, it is assumed that the diesel engine is normally operated (lean operation). At this time, since the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst 33 is lean, NOx contained in the exhaust gas reacts with oxygen on platinum (Pt) as shown in FIG. Is stored in barium (Ba).

  During this lean operation, the ECU 21 sequentially stores NOx in consideration of the operating state of the diesel engine by calculation using the model equation (given equation (1)) of the NOx storage catalyst 33 shown in the first embodiment. The amount of NOx stored in the catalyst 33 is estimated.

  The estimated NOx occlusion amount normally reaches the rich transition threshold value (Th0), and the NOx occlusion catalyst 33 can sufficiently perform its function (catalyst state excluding a state where the catalyst temperature is extremely low and an extremely high state). Then, the ECU 21 determines that it is time to regenerate the NOx storage catalyst 33. Then, a signal for requesting a shift from lean operation to rich operation for regeneration is output (step S1).

  Here, since the estimated NOx occlusion amount does not reach the occlusion amount that causes the NOx slip (step S2), the ECU 21 outputs a rich signal for performing the normal rich operation once. Then, in response to the rich signal, the reduction concentration of the exhaust gas flowing into the NOx storage catalyst 33 is rapidly increased to the target reduction concentration. Specifically, by controlling the EGR valve 25, the fresh air control valve 26, and the reduction injector 36, the excess air ratio λ of the exhaust gas is increased to the rich target λ (excess air ratio) at a stroke as shown in H part of FIG. Reduce. As a result, exhaust gas in a reducing atmosphere containing unburned HC, CO (both of which is a reducing agent), that is, exhaust gas having a reduction concentration optimum for reduction, flows into the NOx storage catalyst 33, and the normal rich operation is executed once. (Step S3).

  Then, in the NOx storage catalyst 33, a reaction opposite to the previous NOx storage is performed. That is, when exhaust gas is rich as shown in FIG. 2B, nitrate ions in barium (Ba) are released in the form of NOx, and the released NOx is unburned HC in the exhaust gas on platinum (Pt), React with a reducing agent such as CO to reduce to nitrogen. By such release and reduction, the NOx occlusion amount of the NOx occlusion catalyst 33 decreases at a stretch.

  In the model equation (the equation (1)) of the NOx occlusion catalyst 33, a decrease in the NOx occlusion amount caused by such NOx release and reduction is also considered.

  During the lean operation, such normal rich operation is intermittently performed as long as the state in which the function of the NOx storage catalyst 33 can be sufficiently exerted continues.

  On the other hand, a state in which the catalyst temperature is extremely low or extremely high is observed, and a state in which the normal rich operation cannot be performed (for example, when traveling in a congested urban area) is prolonged, and the estimated NOx storage catalyst 33 It is assumed that the NOx occlusion amount has increased to the NOx slip occurrence threshold (Th1) or more as shown in FIG.

  Thereafter, when the rich operation becomes possible, the ECU 21 performs the rich operation for reducing the increased NOx storage amount. At this time, rich operation is performed in which the excess air ratio λ is gradually decreased to gradually reach the target reduction concentration.

  In the rich operation when NOx increases, the rich degree of the rich operation is first determined. This degree is set from a map set in advance by the ECU 21. Specifically, from the map showing the inclination ratio of the excess air ratio λ formed by the estimated NOx occlusion amount and the excess air ratio λ as shown in the diagram of step S30, the degree of decrease of the excess air ratio λ according to the estimated NOx occlusion amount. That is, the inclination in the direction in which the excess air ratio λ decreases is determined (step S30). Next, the excess air ratio λ of the exhaust gas is decreased with the slope of λ determined as shown in FIG. 10B until the rich target λ is reached (steps S32 and S33). When the decrease in the excess air ratio λ reaches the rich target λ, the decrease in the inclination of λ is terminated (step S34). As a result, as shown in the portion I in FIG. 10, the rich operation at the time of NOx increase is started (rushing) while performing a rich shift that gradually increases to the target reduction concentration suitable for NOx release.

  At this time, the amount of NOx released from the barium (Ba) is released while gradually increasing according to the reducing concentration in which the concentration of the reducing agent gradually increases. Therefore, most of the released NOx is reduced, that is, FIG. As shown in (b), it is reduced on nitrogen (Pt) by reacting with HC, CO, etc., which are reducing components in the exhaust gas.

  On the other hand, during NOx release when the NOx occlusion amount increases, the catalyst temperature rises due to the reaction heat of reduction as shown in FIG. 10A, and a predetermined temperature value, for example, the lower limit temperature value of the catalyst temperature at which NOx slip does not occur. It rises until it exceeds (T1) (step S31), and it is assumed that a sufficient reduction reaction can be expected. At this time, the excess air ratio λ of the exhaust gas is controlled so that the excess air ratio λ of the exhaust gas is reduced to the target rich λ at a time while the excess air ratio λ gradually decreases as shown by a portion J in FIG. S35). Then, in accordance with the reducing ability of the NOx storage catalyst 33, the amount of NOx released increases at once, and the NOx reduction reaction proceeds.

  Then, until the estimated NOx occlusion amount becomes equal to or less than the rich end threshold (Th2) that defines the end of the rich operation (Step S5), the rich operation in which the ratio of the reducing agent concentration is changed (Step S31, S2). (S32, S33, S34, S5) and the rich operation (steps S31, S35, S5) in which the concentration of the reducing agent is changed from the middle to the target rich λ all at once, the rich operation is continued with the target λ ( Step S37), the rich operation is finished (Step S38).

  Even in such control that gradually reduces the exhaust gas reduction concentration to the target reduction concentration at the time of the rich operation rush (when NOx increases), NOx occlusion of the NOx occlusion catalyst 33 is suppressed while suppressing the occurrence of NOx slip as in the first embodiment. The amount can be reduced, and similarly, a large amount of NOx can be prevented from being temporarily discharged. Of course, as in the first embodiment, the rich operation when the NOx occlusion amount increases uses a method of estimating the NOx occlusion amount occluded in the NOx occlusion catalyst 33 by modeling the NOx occlusion and NOx reduction of the NOx occlusion catalyst 33. Thus, the estimated NOx occlusion amount is gradually released (decreased), so that an optimal rich operation can be performed. In particular, the rich operation can be performed more optimally by setting the degree of increasing the reduction concentration of the rich operation when the NOx storage amount is increased according to the inclination according to the estimated NOx storage amount. Moreover, during the rich operation when the NOx occlusion amount increases, when the catalyst temperature rises above a predetermined temperature value due to reaction heat, the exhaust gas reduction concentration is increased to the target reduction concentration all at once. The rich operation can be rapidly advanced by utilizing the increase in the catalyst performance caused by. In addition, there is an advantage that changing the reduction concentration is simple because the excess air ratio λ of the exhaust gas is controlled.

[Fifth Embodiment]
FIG. 11 is a flowchart showing the main part of the fifth embodiment of the present invention.

  In the fifth embodiment, the NOx occlusion catalyst as in the fourth embodiment is modeled and the NOx occlusion amount is not estimated, but the rich transition timing of the NOx occlusion catalyst is based on the integration of the lean operation time of the engine. When the lean operation time is increased, the exhaust gas flowing into the NOx storage catalyst 33 is set to the target reduction concentration while gradually increasing the reduction concentration.

  Specifically, the flowchart of FIG. 11 includes the content of detecting the rich transition timing of the NOx storage catalyst based on the accumulation of the lean operation time of the engine shown in the flowchart of FIG. 7 (third embodiment), and FIG. Content of gradually increasing the reducing agent concentration by inclining the reduction concentration of the exhaust gas to the rich side at the excess air ratio λ when the NOx occlusion amount is increased (= when the lean operation time is increased) shown in the flowchart of the middle (fourth embodiment) And a routine (steps S40 and S41) for setting a rich operation continuation time when the NOx occlusion amount is increased (= when the lean operation time is increased) from the operation time obtained by newly integrating the lean operation time. Is adopted.

  By using such a method of integrating the lean operation time, similarly to the fourth embodiment, the reduction concentration of the rich operation when the NOx occlusion amount increases (= when the lean operation time increases) may be gradually changed. As in the case of using the model formula of the fourth embodiment, the optimum rich operation can be performed.

  However, in the flowchart of FIG. 11, only a routine for setting the rich operation continuation time is added, and the rest is the same as the flowchart of the third embodiment and the flowchart of the fourth embodiment. The same parts as those in the flowchart of the embodiment (FIG. 7) and the flowchart of the fourth embodiment (FIG. 8) are denoted by the same reference numerals and omitted.

  In addition, this invention is not limited to each embodiment mentioned above, You may implement in various changes within the range which does not deviate from the main point of this invention. For example, in the above-described embodiment, the present invention is applied to a diesel engine. However, the present invention is not limited to this, and the present invention may be applied to other engines such as a gasoline engine. Further, as a means for rich operation, the injector for introducing the reducing agent is arranged in the exhaust pipe, but it may be implemented by a method using post injection in a cylinder or the like.

The figure which shows together schematic structure of the exhaust gas purification apparatus which concerns on the 1st Embodiment of this invention. The figure for demonstrating the absorption-and-release function of a NOx storage catalyst. The flowchart which shows the control which discharge | releases occluded NOx of a NOx occlusion catalyst by a rich operation of chopping. The figure for demonstrating the condition of the storage catalyst at the time of the discharge | release. The figure for demonstrating the condition where discharge | release of NOx advances at a stretch in the middle of the rich operation at the time of NOx occlusion amount increase used as the principal part of the 2nd Embodiment of this invention. The flowchart which shows the control which advances NOx discharge | release in the same air. The flowchart which shows the control which makes the principal part of the 3rd Embodiment of this invention discharge | release occluded NOx by the method of integrating | accumulating lean operation time. The flowchart which shows the control which discharge | releases occluded NOx of a NOx occlusion catalyst by the control which changes the reduction | restoration density | concentration of waste gas gradually used as the principal part of the 4th Embodiment of this invention. The diagram which shows transition of the exhaust gas reduction concentration at the time of normal rich operation. The diagram which shows transition of the exhaust gas reduction concentration at the time of NOx occlusion amount increase rich operation. The flowchart which shows the control which discharge | releases occluded NOx by the method of integrating | accumulating the lean operation time which becomes the principal part of the 5th Embodiment of this invention. The diagram for demonstrating the condition where NOx is occluded by the NOx occlusion catalyst without the opportunity of regeneration being obtained. The diagram for demonstrating the NOx slip which generate | occur | produces in the case of discharge | release of the occluded NOx.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Engine main body, 20 ... 2nd exhaust passage (exhaust passage), 21 ... ECU (rich transition determination means, 2nd rich operation means, NOx occlusion amount estimation means).

Claims (12)

An NOx storage catalyst that is provided in an exhaust passage of the internal combustion engine and that stores NOx in the exhaust when the internal combustion engine is in a lean operation state and releases and reduces the stored NOx when the internal combustion engine is in a rich operation state; ,
Rich transition determination means for determining whether the internal combustion engine can be shifted from lean operation to intermittent normal first rich operation;
When it is determined by the rich transition determination means that the transition is impossible and the NOx occlusion amount increases from the normal time and then the rich transition becomes possible, the second rich operation is performed by dividing the operation period into a plurality of times in order to suppress NOx slip. An exhaust purification device for an internal combustion engine, comprising: a second rich operation means for executing And NOx occlusion amount estimating means for modeling NOx occlusion and NOx reduction in the NOx occlusion catalyst and estimating the NOx occlusion amount occluded in the NOx catalyst.
The second rich operation means determines that the transition is impossible, and the NOx occlusion amount estimated by the NOx occlusion amount estimation means increases to a NOx slip occurrence threshold value that defines the occurrence of the NOx slip, and then rich. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the second rich operation is performed so that the estimated NOx occlusion amount is gradually released when the shift becomes possible. . When the second rich operation means is executing the second rich operation while dividing it, when the NOx occlusion estimation means estimates that the NOx occlusion amount has decreased until it falls below the NOx slip occurrence threshold, Thereafter, one rich operation is performed so as to reduce the remaining NOx occlusion amount all at once. The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein And means for determining the rich transition timing of the NOx storage catalyst based on the accumulation of the lean operation time of the internal combustion engine,
The second rich operation means is determined to be unable to shift, and the accumulated operation time of the lean operation time increases to a time corresponding to a NOx slip occurrence threshold that defines the occurrence of the NOx slip, and then rich transition is possible. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the second rich operation is performed so that the stored NOx is gradually released. The second rich operation means sets the required number of divisions of the second rich operation based on the accumulated lean operation time, and executes the second rich operation according to the set number of divisions. The exhaust emission control device for an internal combustion engine according to claim 4, wherein The second rich operation performed by the second rich operation means is performed by repeating the intermittently performed first rich operation a plurality of times, or by repeating the operation having a lower richness than the first rich operation a plurality of times. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 5, wherein: An NOx storage catalyst that is provided in an exhaust passage of the internal combustion engine and that stores NOx in the exhaust when the internal combustion engine is in a lean operation state and releases and reduces the stored NOx when the internal combustion engine is in a rich operation state; ,
Rich transition determination means for determining whether the internal combustion engine can transition from lean operation to intermittent normal first rich operation;
Exhaust gas flowing into the NOx storage catalyst in the internal combustion engine to suppress NOx slip when the rich transition determination means determines that the transition is impossible and the NOx occlusion amount increases from the normal time and then the rich transition is possible. An exhaust purification device for an internal combustion engine, comprising: a second rich operation means for executing a second rich operation to achieve a target reduction concentration while gradually increasing the reduction concentration. And NOx occlusion amount estimating means for modeling NOx occlusion and NOx reduction in the NOx occlusion catalyst and estimating the NOx occlusion amount occluded in the NOx catalyst.
The second rich operation means determines that the transition is impossible, and the NOx occlusion amount estimated by the NOx occlusion amount estimation means increases to a NOx slip occurrence threshold value that defines the occurrence of the NOx slip, and then rich. The exhaust purification device for an internal combustion engine according to claim 7, wherein the second rich operation is executed so that the estimated NOx occlusion amount is gradually released when the shift becomes possible. . And means for determining the rich transition timing of the NOx storage catalyst based on the accumulation of the lean operation time of the internal combustion engine,
The second rich operation means is determined to be unable to shift, and the accumulated operation time of the lean operation time increases to a time corresponding to a NOx slip occurrence threshold that defines the occurrence of the NOx slip, and then rich transition is possible. The exhaust purification device for an internal combustion engine according to claim 7, wherein the second rich operation is performed so that the stored NOx is gradually released when the exhaust gas becomes. In the second rich operation means, the rich degree of the second rich operation for gradually increasing the reduction concentration depends on the inclination according to the NOx occlusion amount estimated by the NOx occlusion estimation means or the accumulated time of the lean operation time. The exhaust gas purification apparatus for an internal combustion engine according to claim 8 or 9, wherein the second rich operation is executed in accordance with the inclination. When the second rich operation means is performing a second rich operation in which the reduction concentration is gradually increased according to the inclination set by the estimated NOx storage amount, the temperature of the NOx storage catalyst has increased to a predetermined temperature value. When this is the case, after that, an operation for increasing the reduction concentration at a stretch to the target reduction concentration is executed. The exhaust emission control device for an internal combustion engine according to claim 10, wherein: The second rich operation means executed by the second rich operation means changes the excess air ratio of the exhaust gas flowing into the NOx storage catalyst so as to gradually increase the reduction concentration of the exhaust gas to a target reduction concentration. The exhaust emission control device for an internal combustion engine according to any one of claims 7 to 11, wherein the exhaust gas purification device is an internal combustion engine.

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