JP2017145704A - Exhaust gas purification system for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a filter and a NOx catalyst which are efficiently regenerated while suppressing an excessive temperature rise of the filter. An exhaust gas purification system includes an NSC having an oxidizing function and a function of purifying NOx in an exhaust, a filter for collecting PM in the exhaust, and a post-injection prohibiting means for permitting or prohibiting the execution of post-injection. By supplying exhaust gas with a high temperature and oxidizing atmosphere to the filter, PM accumulated on the filter is burned and removed, and post-injection is performed to raise the temperature of the exhaust gas within the period permitted by the post-injection prohibition means. It is provided with a regeneration control means for alternately performing an operation and a DeSOx operation for desorbing SOx by supplying exhaust gas having a high temperature and a reducing atmosphere to the NSC. The post-injection prohibition means prohibits the execution of post-injection until the post-injection prohibition time set according to the execution time of the immediately preceding DeSOx operation elapses after switching from the DeSOx operation to the filter regeneration operation. [Selection diagram] FIG. 8

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine. More specifically, the present invention relates to an exhaust gas purification system for an internal combustion engine that combines a NOx catalyst that purifies NOx in exhaust gas and a filter that collects particulate matter in the exhaust gas.

  Conventionally, an NOx catalyst for purifying NOx and a filter for collecting particulate matter (hereinafter also referred to as “PM (particulate matter)”) are provided in series along the exhaust passage of the internal combustion engine, thereby providing an internal combustion engine. An exhaust purification system for an internal combustion engine that purifies both NOx and PM contained in the exhaust of the engine has been proposed (see, for example, Patent Document 1).

  When the amount of collected PM in the filter increases, the pressure loss before and after the filter increases, and the fuel consumption may deteriorate. For this reason, when a certain amount of PM is collected in the filter, a filter regeneration operation is performed in which PM collected in the filter is burned and removed by supplying exhaust gas in a high temperature and oxidizing atmosphere to the filter.

  On the other hand, the NOx catalyst has a main function of purifying NOx in exhaust gas, but also has a function of capturing SOx contained in fuel and oil. When the amount of SOx trapped in the NOx catalyst increases, the NOx purification performance may be reduced. For this reason, when a certain amount of SOx is trapped in the NOx catalyst, SOx trapped in the NOx catalyst is desorbed by supplying exhaust gas in a reducing atmosphere at a temperature as high as that in the filter regeneration operation to the NOx catalyst. A NOx catalyst regeneration operation (hereinafter also referred to as “DeSOx operation”) that removes and restores the function of the NOx catalyst is performed. As described above, in the exhaust gas purification system that combines the filter and the NOx catalyst, it is necessary to execute both the filter regeneration operation and the DeSOx operation at appropriate timings in order to maintain the original functions. is there.

  In the exhaust purification system of Patent Document 1, when it is determined that both regeneration of the filter and regeneration of the NOx catalyst are necessary, the exhaust gas flowing into the NOx catalyst is started while starting the filter regeneration operation for raising the temperature of the exhaust gas. The DeSOx operation in which the atmosphere is reduced is repeatedly performed in a pulse manner. As described above, in order to desorb SOx trapped by the NOx catalyst, a reducing atmosphere and exhaust gas must be heated to a high temperature. According to the exhaust gas purification system of Patent Document 1, the temperature of the NOx catalyst can be raised together with the filter by performing the filter regeneration operation. Therefore, the NOx catalyst can be heated using the energy for raising the temperature of the filter, so that the filter and NOx catalyst are regenerated compared to the case where the filter regeneration operation and the DeSOx operation are performed at different timings. Deterioration of fuel consumption can be suppressed.

JP 2005-155374 A

  In order to burn and remove PM accumulated on the filter, it is necessary to forcibly raise the temperature of the filter, and so-called post-injection is often employed as the temperature raising means used at that time. Post injection refers to fuel injection after a predetermined period after main injection. Most of the fuel supplied by performing post injection does not contribute to the combustion in the combustion chamber, but is supplied to the exhaust passage as it is, and burns with the catalyst provided in the exhaust passage, contributing to the temperature rise of the filter. . In the filter regeneration operation, the temperature of PM accumulated on the filter is increased by performing post injection, and PM is burned and removed by performing lean control and supplying exhaust gas in an oxidizing atmosphere containing a large amount of oxygen.

  On the other hand, in the DeSOx operation, when the temperature of the NOx catalyst is increased by performing the filter regeneration operation, the lean control is switched to the rich control, and the exhaust in the reducing atmosphere containing a large amount of reducing components such as unburned HC and CO is reduced to the NOx catalyst. The SOx trapped by the NOx catalyst is desorbed by supplying to the catalyst. However, when the DeSOx operation is performed and exhaust gas in a reducing atmosphere is continuously supplied to the NOx catalyst, a part of the unburned HC in the exhaust gas adheres to the PM deposited on the filter without burning. For this reason, as in the invention of Patent Document 1 described above, after performing DeSOx operation, switching from rich control to lean control to perform filter regeneration operation, and further performing post injection, exhaust gas in an oxidizing atmosphere is supplied to the filter. The unburned HC adhering to the PM burns rapidly in a short time, and the filter temperature may rise rapidly.

  The present invention provides an exhaust gas purification system for an internal combustion engine that purifies exhaust gas by combining a filter and a NOx catalyst, and efficiently regenerates the filter and the NOx catalyst while suppressing excessive temperature rise of the filter. It is intended.

  (1) An exhaust purification system (for example, an exhaust purification system 2 to be described later) of an internal combustion engine (for example, an engine 1 to be described later) is provided in an exhaust passage (for example, an exhaust pipe 11 to be described later) of the internal combustion engine, and has an oxidation function. And a NOx catalyst having a function of purifying NOx in the exhaust (for example, NSC of a catalytic converter 31 described later) and a particulate matter in the exhaust is provided on the downstream side of the NOx catalyst in the exhaust passage. A filter (for example, a filter 32 to be described later), post injection prohibiting means (for example, an ECU 7 to be described later) for permitting or prohibiting execution of post injection that is fuel injection after a predetermined period after the main injection, and a high temperature in the filter. In addition, the particulate matter accumulated on the filter is burned and removed by supplying exhaust gas in an oxidizing atmosphere, and allowed by the post-injection prohibiting means. A filter regeneration operation (for example, a filter regeneration operation of S23 in FIG. 4 described later), in which the post injection is performed within the period and the exhaust temperature is increased by burning unburned fuel using the oxidation function; Regeneration control means for alternately performing NOx catalyst regeneration operation for desorbing SOx in the NOx catalyst by supplying exhaust gas of high temperature and reducing atmosphere to the NOx catalyst (for example, DeSOx operation of S24 in FIG. 4 described later). (For example, ECU 7 described later). The post-injection prohibiting means includes at least one of an execution time of the immediately preceding NOx catalyst regeneration operation and an air-fuel ratio of exhaust at the time of execution of the NOx catalyst regeneration operation after switching from the NOx catalyst regeneration operation to the filter regeneration operation. The execution of the post-injection is prohibited until a prohibition time (for example, a post-injection prohibition time in S37 in FIG. 6 described later) elapses.

  In the present invention, the air-fuel ratio of exhaust refers to the ratio of oxygen to reducing components such as HC and CO in the exhaust. In the following, the term “air excess ratio” obtained by dividing the air-fuel ratio by the stoichiometric air-fuel ratio may be used instead of the term “air-fuel ratio”. In the present invention, the oxidizing atmosphere means a state in which the oxygen concentration in the exhaust gas in the filter or the NOx catalyst is relatively high with respect to the concentration of the reducing component. Specifically, for example, the combustion air-fuel ratio in the internal combustion engine is stoichiometric. Realized by making it leaner. In the present invention, the reducing atmosphere refers to a state in which the oxygen concentration in the exhaust gas in the filter or NOx catalyst is relatively low with respect to the concentration of the reducing component. Specifically, this reducing atmosphere is realized, for example, by making the combustion air-fuel ratio richer than stoichiometric by performing after injection or the like.

  The fuel injection performed after the main injection is classified into after injection and post injection performed after this after injection. In the present invention, after main injection, for example, fuel injection within 45 to 65 [degATDC] is defined as after injection, and fuel injection after 65 [degATDC] is defined as post injection.

  (2) In this case, it is preferable that the post injection prohibiting unit prohibits the execution of the post injection during the execution of the NOx catalyst regeneration operation.

  (3) In this case, it is preferable that the post-injection prohibiting unit sets the prohibition time longer as the execution time of the immediately preceding NOx catalyst regeneration operation is longer.

  (4) In this case, it is preferable that the post-injection prohibiting unit sets the prohibition time longer as the exhaust air-fuel ratio becomes smaller when the immediately preceding NOx catalyst regeneration operation is executed.

  (5) In this case, it is preferable that the post-injection prohibiting unit sets the prohibition time longer as the temperature of the filter is higher when the NOx catalyst regeneration operation is switched to the filter regeneration operation.

  (6) In this case, the exhaust purification system includes a collection amount estimation means (for example, a differential pressure sensor 34 and an ECU 7 described later) for estimating the amount of particulate matter collected by the filter, and the NOx catalyst. SOx amount estimating means (for example, ECU 7 described later) for estimating the trapped SOx amount, and the regeneration control means has a predetermined amount in which the particulate matter amount is set based on the SOx amount. It is preferable to start the filter regeneration operation and the NOx catalyst regeneration operation alternately in response to exceeding a threshold value (for example, a regeneration start threshold value in FIG. 2 described later).

  (7) In this case, it is preferable that the regeneration control unit sets the threshold value to a smaller value as the SOx amount increases.

  (1) In the present invention, the filter regeneration operation for burning and removing particulate matter in the filter and the NOx catalyst regeneration operation for desorbing SOx in the NOx catalyst are alternately performed. Further, in the filter regeneration operation, the post injection is executed within the period permitted by the post injection prohibiting means, and the fuel supplied by the post injection is burned by using the oxidation function in the NOx catalyst, so that the filter and the NOx catalyst are burned. Increase temperature. The post-injection prohibiting unit prohibits the execution of post-injection until a predetermined prohibition time has elapsed after switching from the NOx catalyst regeneration operation to the filter regeneration operation. As a result, the PM accumulated on the filter together with the unburned HC adhering to the particulate matter of the filter during the NOx catalyst regeneration operation is suddenly changed from the NOx catalyst regeneration operation to the filter regeneration operation. Since combustion can be prevented, overheating of the filter can be prevented. In the present invention, the length of the prohibition time is set according to at least one of the execution time of the immediately preceding NOx catalyst regeneration operation and the air-fuel ratio of the exhaust gas at the time of execution. Since the execution time and the air-fuel ratio of the exhaust are correlated with the amount of unburned HC adhering to the particulate matter deposited on the filter when the NOx catalyst regeneration operation is switched to the filter regeneration operation, By setting the length of the prohibition time according to the parameter, it is possible to more reliably prevent overheating of the filter. Further, in the present invention, the unburned HC supplied by the immediately preceding NOx catalyst regeneration operation is used instead of not performing the post-injection until the prohibition time elapses after switching from the NOx catalyst regeneration operation to the filter regeneration operation. Since the temperature of the filter can be maintained at a high temperature, it is possible to suppress consumption of fuel used for post injection while avoiding excessive temperature rise of the filter.

  (2) In the present invention, since the filter regeneration operation and the NOx catalyst regeneration operation are alternately performed, the SOx is desorbed at the temperature of the NOx catalyst without performing post injection for a while after the filter regeneration operation. The temperature is maintained at such a level. Therefore, in the present invention, not only immediately after switching from the NOx catalyst regeneration operation to the filter regeneration operation, but also post injection during execution of the NOx catalyst regeneration operation is prohibited. Thereby, since the period which prohibits post-injection can be made longer, consumption of the fuel utilized for post-injection can further be suppressed.

  (3) The amount of unburned HC adhering to the particulate matter at the start of the filter regeneration operation increases as the execution time of the immediately preceding NOx catalyst regeneration operation becomes longer. Therefore, the longer the execution time of the immediately preceding NOx catalyst regeneration operation, the longer the time required for the unburned HC adhering to the particulate matter to be gently burned under the exhaust of the oxidizing atmosphere. In the present invention, the post-injection prohibition time is set longer as the execution time of the immediately preceding NOx catalyst regeneration operation is longer. As a result, the post-injection prohibition time can be set to an appropriate length without excess or deficiency in accordance with the execution time of the immediately preceding NOx catalyst regeneration operation.

  (4) The amount of unburned HC adhering to the particulate matter at the start of the filter regeneration operation becomes smaller as the air-fuel ratio of the exhaust gas at the time of execution of the immediately preceding NOx catalyst regeneration operation becomes smaller (that is, as it becomes deeper toward the rich side). ) Become more. Therefore, the smaller the air-fuel ratio of the exhaust gas in the immediately preceding NOx catalyst regeneration operation, the longer the time required for gently burning the unburned HC adhering to the particulate matter under the oxidizing atmosphere exhaust gas. In the present invention, the post-injection prohibition time is set longer as the exhaust air-fuel ratio in the immediately preceding NOx catalyst regeneration operation is smaller. As a result, the post-injection inhibition time can be set to an appropriate length without excess or deficiency in accordance with the air-fuel ratio of the exhaust gas in the immediately preceding NOx catalyst regeneration operation.

  (5) The higher the temperature of the filter at the start of the filter regeneration operation, the easier the particulate matter deposited on the filter burns rapidly with the unburned HC adhering to the immediately preceding NOx catalyst regeneration operation. In the present invention, the higher the temperature of the filter at the start of the filter regeneration operation, the longer the prohibition time is set, and the longer the period for decreasing the temperature of the filter or suppressing the temperature from rising further. Thus, the post injection prohibited time can be set to an appropriate length without excess or deficiency in accordance with the temperature of the filter at the start of the filter regeneration operation.

  (6) Since extra fuel is required to raise the temperature in the filter regeneration operation, the filter regeneration operation has a threshold value with a certain amount of particulate matter collected in the filter in order to suppress fuel consumption as much as possible. Start over. Here, when the filter regeneration operation and the NOx catalyst regeneration operation are alternately performed within the same period as in the present invention, if the threshold value is set to a large value in accordance with the permissible capacity of the particulate matter of the filter, the fuel is correspondingly increased. However, the NOx catalyst tends to maintain the trapped state of SOx, and there is a possibility that sufficient NOx purification performance cannot be exhibited. In the present invention, the amount of SOx trapped in the NOx catalyst is estimated, and a threshold value for starting the filter regeneration operation is set based on the amount of SOx. Thereby, the filter regeneration operation and the NOx catalyst regeneration operation can be executed at an appropriate timing so that the NOx purification performance of the NOx catalyst does not deteriorate too much.

  (7) In the present invention, the filter regeneration operation threshold is set to a smaller value as the amount of SOx in the NOx catalyst increases. As a result, when the amount of SOx in the NOx catalyst is large, the filter regeneration operation and the NOx catalyst regeneration operation can be executed at a faster timing than when the amount of SOx is small. .

1 is a diagram showing a configuration of an exhaust gas purification system for an internal combustion engine according to an embodiment of the present invention. It is a flowchart which determines the timing which reproduces | regenerates a filter and NSC. It is an example of the map which prescribed | regulated the relationship between the SOx trapping amount and the reproduction | regeneration start threshold value according to this. It is a flowchart which shows the specific procedure of an alternating regeneration driving | operation. It is a time chart which shows an example of an alternating regeneration driving | operation. It is a flowchart which shows the specific procedure of a post injection prohibition process. It is a figure which shows an example of the post injection prohibition time determination table. It is a time chart which shows an example of the change of the post injection quantity at the time of applying the post injection prohibition process of FIG. 6 during alternate regeneration operation.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and its exhaust purification system 2 according to the present embodiment. The exhaust purification system 2 includes a catalyst purification device 3 provided in an exhaust pipe 11 extending from an exhaust port of the engine 1, an electronic control unit (hereinafter referred to as “ECU”) 7 that controls the engine 1 and the catalyst purification device 3, Is provided.

  The engine 1 is based on so-called lean combustion in which the combustion air-fuel ratio is leaner than stoichiometric, more specifically, for example, a diesel engine or a lean burn gasoline engine, but is not limited thereto. The engine 1 is provided with a fuel injection valve 17 that injects fuel into each cylinder. The actuator that drives the fuel injection valve 17 is electromagnetically connected in accordance with a control signal from the ECU 7. The ECU 7 determines the fuel injection amount and fuel injection timing from the fuel injection valve 17 under fuel injection control (not shown), and drives the fuel injection valve 17 so that this is realized.

  The catalyst purification device 3 includes a catalytic converter 31 provided in the exhaust pipe 11, a filter 32, a pre-filter temperature sensor 33, a differential pressure sensor 34, and an air-fuel ratio sensor 35.

  The catalytic converter 31 includes a flow-through honeycomb structure as a base material, and a NOx occlusion reduction type catalyst (hereinafter abbreviated as “NSC”) is supported on the base material. NSC stores NOx in exhaust under an oxidizing atmosphere, and reduces and purifies NOx stored in a reducing atmosphere, and HC, CO in exhaust, and post-injection described later under an oxidizing atmosphere. And an oxidation function for burning supplied unburned fuel and the like. There is an upper limit to the amount of NOx that can be stored by NSC. Further, when the NOx storage amount of NSC increases, the NOx purification performance by NSC decreases. Therefore, the ECU 7 performs a DeNOx operation for reducing and purifying NOx stored in the NSC by supplying exhaust gas in a reducing atmosphere to the NSC at a predetermined timing, and maintains the NOx purification performance of the NSC at a high level.

  The NSC also has a function of capturing SOx contained in the fuel and oil of the engine 1, and the NOx purification performance by the NSC decreases when the SOx trapping amount increases. Therefore, the ECU 7 performs a DeSOx operation that promotes the desorption of SOx trapped in the NSC by supplying exhaust gas of high temperature and reducing atmosphere to the NSC at a predetermined timing, and maintains the NOx purification performance of the NSC at a high level. Specific procedures, execution timings, and the like of this DeSOx operation will be described later in detail with reference to FIGS.

  The filter 32 is provided in the exhaust pipe 11 on the downstream side of the catalytic converter 31. The filter 32 includes a wall flow type honeycomb structure having a plurality of cells partitioned by a porous wall, and plugs provided alternately on the upstream side and the downstream side with respect to each cell. PM such as soot and SOF contained in the exhaust discharged from the engine 1 is collected in the process of passing through the pores of the porous wall of the filter 32. If an excessive amount of PM accumulates on the filter 32, the pressure drop before and after the filter 32 increases, which increases the fuel injection amount in the engine 1, and as a result, the fuel consumption may deteriorate. Further, when an excessive amount of PM is deposited, an abnormal temperature rise occurs and the filter 32 may be melted. Therefore, the ECU 7 performs a filter regeneration operation that promotes combustion removal of PM accumulated on the filter 32 by supplying exhaust gas in a high temperature and oxidizing atmosphere to the filter 32, and the filter 32 has an excessive amount of PM. To prevent deposition. The specific procedure and execution timing of the filter regeneration operation will be described later in detail with reference to FIGS.

  The pre-filter temperature sensor 33 is provided on the upstream side of the filter 32 in the exhaust pipe 11. The pre-filter temperature sensor 33 detects the temperature of the exhaust gas flowing into the filter 32 and transmits a signal corresponding to the detected value to the ECU 7. The ECU 7 calculates the temperature of the filter 32 (hereinafter also simply referred to as “filter temperature”) based on the output signal of the pre-filter temperature sensor 33.

  The differential pressure sensor 34 detects a differential pressure (pressure drop) generated before and after the filter 32 and transmits a signal corresponding to the detected value to the ECU 7. In the ECU 7, the PM accumulation amount of the filter 32 is calculated based on the output signal of the differential pressure sensor 34.

  The air-fuel ratio sensor 35 is provided, for example, on the upstream side of the catalytic converter 31 in the exhaust pipe 11. The air-fuel ratio sensor 35 detects the air-fuel ratio of the exhaust gas flowing into the catalytic converter 31, and transmits a signal corresponding to the detected value to the ECU 7.

  The ECU 7 is an I / O interface that performs A / D conversion of the sensor detection signal, a CPU that executes processing in accordance with flowcharts shown in FIGS. 2, 4, and 6 to be described later, and the mode determined under this processing. The microcomputer includes a drive circuit that drives various devices, and a RAM, a ROM, and the like that store various data such as a map and a prohibited time determination table in FIG.

  FIG. 2 is a flowchart for determining the timing for reproducing the filter and the NSC. More specifically, FIG. 2 is a flowchart showing a procedure for updating the values of the alternate regeneration flag and the DeSOx permission flag that define the timing for executing the filter regeneration operation and the DeSOx operation for regenerating the NSC. It is repeatedly executed at a predetermined cycle. Here, the alternating regeneration flag is a flag that clearly indicates that the alternate execution of the filter regeneration operation and the DeSOx operation (hereinafter also referred to as “alternate regeneration operation”) is permitted. The DeSOx permission flag is a flag that clearly indicates that execution of the DeSOx operation is permitted. As will be described later with reference to FIG. 3, the ECU performs the filter regeneration operation when only the alternate regeneration flag is set to 1, and both the alternate regeneration flag and the DeSOx permission flag are set to 1. If so, the DeSOx operation is performed.

  First, in S1, the ECU estimates the current PM accumulation amount of the filter, and proceeds to S2. The PM accumulation amount can be directly acquired using, for example, the output of the differential pressure sensor, or can be estimated based on the history of the operating state of the engine such as the fuel injection amount and the exhaust temperature. In S2, the ECU determines whether or not the alternate regeneration flag is 1. When the determination of S2 is NO, the process after S3 is executed to determine whether or not the time for starting the alternate regeneration operation has been reached, and when the determination of S2 is YES, the alternate regeneration operation being performed is performed. The processing after S11 is executed to determine whether or not it is time to finish.

  In S3, the ECU estimates the SOx trapping amount that is the amount of SOx trapped by the current NSC, and proceeds to S4. The amount of SOx trapped by the NSC can be estimated based on, for example, the integrated value of the fuel injection amount or the travel distance of the vehicle. Since the NOx purification rate in NSC decreases according to the SOx trapping amount, the SOx trapping amount can be indirectly acquired using the NSC NOx purification rate.

  In S4, the ECU sets a regeneration start threshold, which is a threshold set for the PM accumulation amount, in order to determine the timing for starting the alternate regeneration operation based on the SOx trapping amount acquired in S3. In S4, for example, by using a map (for example, see FIG. 3) that defines the relationship between the SOx capture amount and the regeneration start threshold corresponding thereto, the regeneration start threshold corresponding to the SOx capture amount acquired in S3 is set. To do. Here, as exemplified in FIG. 3, it is preferable to set the regeneration start threshold value to a smaller value as the SOx trapping amount increases. As a result, the alternating regeneration operation starts at a faster timing as more SOx is captured by the NSC, so that the NOx purification performance of the NSC can be stabilized as compared with the case where the regeneration start threshold is fixed. In particular, when alternating regeneration operation is performed as in this embodiment, if the regeneration start threshold value is fixed to a large value in accordance with the PM allowable capacity of the filter, fuel consumption can be suppressed by that amount, but SOx is captured by NSC. The maintained state tends to be maintained, and there is a risk that sufficient NOx purification performance cannot be exhibited. In the present embodiment, by setting the regeneration start threshold value to a smaller value as the SOx trapping amount increases, the alternate regeneration operation can be performed at an appropriate timing so that the NOx purification performance of the NSC does not deteriorate too much.

  In S5, the ECU determines whether or not the PM accumulation amount acquired in S1 exceeds the regeneration start threshold set in S4. If the determination in S5 is YES, the ECU determines that it is time to start the alternate regeneration operation, sets the alternate regeneration flag from 0 to 1 (see S6), and ends this process. Here, by setting the alternate regeneration flag to 1, the alternate regeneration operation of the filter regeneration operation and the DeSOx operation starts (see S21 in FIG. 4 described later). If the determination in S5 is NO, the process ends with both the alternate regeneration flag and the DeSOx permission flag maintained at 0.

  In S11, the ECU determines whether or not almost all PM accumulated on the filter has been burned and removed, more specifically, the predetermined regeneration in which the PM accumulation amount acquired in S1 is set to a value slightly larger than 0. It is determined whether or not the end threshold value is reached. If the determination in S11 is YES, the ECU determines that it is time to end the alternate regeneration operation, resets the alternate regeneration flag to 0 (see S12), and further resets the DeSOx permission flag to 0 (S13). This process is terminated. If the determination in S11 is NO, that is, if the PM deposited on the filter has not been sufficiently removed, the process proceeds to S14.

  In S14, the ECU determines whether or not the DeSOx permission flag is 1. If the determination in S14 is NO, that is, if the filter regeneration operation is being performed, the ECU proceeds to S15 to determine whether it is time to start the DeSOx operation. If the determination in S14 is YES, the ECU proceeds to S18 to determine whether or not it is time to end the currently executing DeSOx operation.

  In S15, the ECU determines whether or not a predetermined DeSOx operating condition is satisfied. Here, the DeSOx operation condition defines a condition for starting the DeSOx operation, and includes, for example, the following five conditions (A) to (E).

(A) A predetermined minimum interval time has elapsed since the last DeSOx operation was completed.
(B) The PM accumulation amount is not more than a predetermined value smaller than the regeneration start threshold value.
(C) The operating state of the engine is within a predetermined region excluding at least the idle operation region.
(D) The temperature of the exhaust gas upstream of the filter is within a predetermined DeSOx allowable temperature range including at least the NSC SOx desorption temperature.
(E) A predetermined amount or more of SOx to be desorbed is captured by the NSC.

  If the DeSOx operation is performed continuously over a long period of time or if the DeSOx operation is performed with a large amount of PM deposited on the filter, unburned HC will adhere to many PMs, so the DeSOx operation will change to the filter regeneration operation. When switching, the filter may reach an excessive temperature rise. In the process of S15, the conditions (A) and (B) are imposed to prevent the filter from being overheated for such a reason. In addition, when switching from DeSOx operation to filter regeneration operation, for example, if the engine is in an idle state, exhaust gas with a low flow rate and high oxygen concentration flows into the filter, so the filter may reach an excessive temperature rise. The sex becomes even higher. Therefore, in the process of S15, a condition (C) is further imposed so that the DeSOx operation can be started when the operating state of the engine is within a predetermined region. In addition, in the DeSOx operation to be performed, in order to efficiently desorb SOx from the NSC, conditions (D) and (E) are further imposed in the process of S15.

  When all of the above five conditions (A) to (E) are satisfied, the ECU determines that the DeSOx operation is permitted, sets the DeSOx permission flag from 0 to 1 (see S16), and further A target rich time corresponding to a time during which the DeSOx operation is continuously executed is set, and this process ends. If any of the conditions (A) to (E) is not satisfied, the ECU determines that it is not time to start the DeSOx operation, and maintains the DeSOx permission flag at 0. This process ends.

  In S18, the ECU determines whether or not the target rich time set in S17 has elapsed since the DeSOx operation was started. If the determination in S18 is YES, the ECU resets the DeSOx permission flag from 1 to 0 in order to switch from the DeSOx operation to the filter regeneration operation (see S13), and ends this process. If the determination in S18 is NO, the ECU ends this process while maintaining the DeSOx permission flag at 1 to continue the DeSOx operation.

  FIG. 4 is a flowchart showing a specific procedure of the alternating regeneration operation. The processing in FIG. 4 is repeatedly executed at a predetermined cycle in the ECU while referring to the alternate regeneration flag and the DeSOx permission flag updated in the processing in FIG. 2, the post injection prohibition flag updated in FIG. The

  In S21, the ECU determines whether or not the alternate regeneration flag is 1. If the determination in S21 is NO, the ECU immediately ends this process without performing the filter regeneration operation and the DeSOx operation. If the determination in S21 is YES, the ECU proceeds to S22.

  In S22, the ECU determines whether or not the DeSOx permission flag is 1. When the determination of S22 is NO, that is, when only the alternate regeneration flag is 1, the ECU burns the PM accumulated on the filter by supplying exhaust gas having an oxidizing atmosphere higher than the combustion temperature of PM to the filter. A filter regeneration operation that promotes removal is executed (see S23). More specifically, in the filter regeneration operation, lean control is performed so that the combustion air-fuel ratio becomes leaner than stoichiometric, and post-injection is executed during a period in which the post-injection prohibition flag described later is 0, and the NSC oxidation function is performed. The exhaust gas is heated by burning unburned fuel supplied by post injection. The post injection amount is controlled using, for example, the output of the pre-filter temperature sensor so that the filter temperature is maintained at the PM combustion temperature. Here, the post-injection prohibition flag is a flag that clearly indicates that the execution of post-injection is prohibited, and the value thereof is used to prevent overheating of the filter in the process of FIG. 6 described later. It is updated at an appropriate timing.

  If the determination in S22 is YES, that is, if both the alternate regeneration flag and the DeSOx permission flag are 1, the ECU supplies the NSC with exhaust gas having a temperature higher than the desorption temperature of SOx in NSC and reducing atmosphere. Thus, a DeSOx operation that promotes SOx desorption in the NSC is executed (see S24). More specifically, in the DeSOx operation, by using the output of the air-fuel ratio sensor, the combustion air-fuel ratio is set so that the air-fuel ratio of the exhaust gas flowing into the NSC becomes a predetermined target rich air-fuel ratio set richer than the stoichiometry. By performing rich control that makes the gas richer than stoichiometric, exhaust gas in a high temperature and reducing atmosphere as described above is supplied to the NSC, and SOx is desorbed.

  FIG. 5 is a time chart showing an example of the alternating regeneration operation of FIGS. The upper part of FIG. 5 shows the PM accumulation amount (see line 51 in FIG. 5), the NSC temperature (see line 52 in FIG. 5), and the vehicle speed (see line 53 in FIG. 5). The excess ratio λ is shown, and the SOx trapping amount is shown in the lower part. In FIG. 5, the period during which the filter regeneration operation is performed is indicated by hatching, and the period during which the DeSOx operation is performed is indicated by gray.

  First, at time t1, the alternate regeneration flag becomes 1 (see S3 to S6 in FIG. 2) in response to the PM accumulation amount exceeding the regeneration start threshold set in accordance with the SOx trapping amount. The alternate regeneration operation in which the regeneration operation and the DeSOx operation are alternately performed is started (see S21 in FIG. 4). Initially, the filter regeneration operation is executed (see S23 in FIG. 4). In the filter regeneration operation, exhaust gas in a high-temperature and oxidizing atmosphere is supplied to the filter by performing post-injection only within a period permitted by post-injection prohibition processing in FIG. As a result, the temperature of the filter rises, and the accumulated PM is gradually burned and removed, and the amount of accumulated PM begins to decrease. Further, in the filter regeneration operation, the unburned fuel supplied by the post injection is burned by using the oxidation function of the NSC to raise the temperature of the exhaust gas. Therefore, the temperature of the NSC rises as the temperature of the filter rises.

  At time t2, when it is determined that the DeSOx operation in S15 of FIG. 2 is satisfied, the DeSOx permission flag is set to 1 (see S15 and S16 of FIG. 2), thereby starting the DeSOx operation (S24 of FIG. 4). reference). In the DeSOx operation, the NSC is supplied with high-temperature and reducing atmosphere exhaust. As a result, the SOx trapped in the NSC is desorbed, and the amount of SOx trapped decreases. When the DeSOx operation is performed for the target rich time set in S17 of FIG. 2, the DeSOx permission flag is reset to 0 (see S18 and S13 of FIG. 2), and the filter regeneration operation is started again. Thereafter, every time the DeSOx operation condition of S15 in FIG. 2 is satisfied, the DeSOx operation is executed, and thereby, at time t3, the filter regeneration operation and the DeSOx operation are executed alternately until the PM accumulation amount falls below the regeneration end threshold. Here, since the minimum interval time is provided in the DeSOx operation condition, the interval of each DeSOx operation is at least longer than the minimum interval time.

  FIG. 6 is a flowchart showing a specific procedure of the post-injection prohibiting process for permitting or prohibiting the execution of post-injection, and is repeatedly executed in the ECU at a predetermined cycle.

  In S31, the ECU determines whether or not the alternate regeneration flag is 1. If the determination in S31 is NO, the ECU sets a post-injection prohibition flag to 0 to permit execution of post-injection (see S32), and ends this process. If the determination in S31 is YES, the ECU determines whether or not the DeSOx permission flag is 1 (see S33).

  If the determination in S33 is YES, that is, if the DeSOx operation is currently being performed, the ECU sets the post-injection prohibition flag to 1 to prohibit the execution of post-injection in order to prevent overheating of the filter. Set (see S34), and the process is terminated.

  If the determination in S33 is NO, the ECU determines whether the DeSOx permission flag has been switched from 1 to 0 from the previous time to this time, in other words, whether the DeSOx operation has been switched from the previous time to the current time to the filter regeneration operation. Determine (see S35). If the determination in S35 is YES, the ECU proceeds to S36, acquires the execution time of the DeSOx operation that has been performed until immediately before and the current filter temperature, and proceeds to S37. In S37, the ECU sets the length of the post-injection prohibition time described later according to the execution time and filter temperature acquired in S36, and then proceeds to S34 to set the post-injection prohibition flag to 1, and this process Exit.

  In S37, using the execution time of the DeSOx operation and the filter temperature, the ECU sets the length of the post injection prohibition time by searching a post injection prohibition time determination table as shown in FIG. 7, for example. As shown in the table of FIG. 7, it is preferable to set the post-injection prohibition time longer as the filter temperature is higher and to be longer as the execution time of the immediately preceding DeSOx operation is longer.

  If the determination in S35 is NO, the ECU determines whether or not the post-injection prohibition time for which the length has been set in S37 has elapsed after switching from the DeSOx operation to the filter regeneration operation (see S38). The ECU sets the post-injection prohibition flag to 1 until the post-injection prohibition time has elapsed after switching from the DeSOx operation to the filter regeneration operation (that is, while the determination in S38 is NO) (see S34). ) After the post-injection prohibition time has elapsed (that is, after the determination in S38 is YES), the post-injection prohibition flag is set to 0 (see S32).

  According to the post-injection prohibition process as described above, the post-injection prohibition time set in S37 elapses after the DeSOx operation is being executed and the filter regeneration operation is being performed and the DeSOx operation is switched to the filter regeneration operation. In the meantime, execution of post injection is prohibited, and execution of post injection is permitted after the post injection prohibition time has elapsed during the filter regeneration operation.

  FIG. 8 is a time chart showing an example of a change in the post injection amount when the post injection prohibiting process of FIG. 6 is applied during the alternating regeneration operation. 8 shows the filter temperature (see line 81 in FIG. 8), the excess air ratio (see line 82 in FIG. 8), and the DeSOx permission flag (see line 83 in FIG. 8). Indicates the post injection amount. In FIG. 8, the change of the post injection amount in the case where the conventional post injection amount, that is, the above-described post injection prohibiting process is not applied is shown by a broken line for comparison. In FIG. 8, the DeSOx operation is performed during the period indicated in light gray, that is, between time t1 and t2, between time t4 and t5, and between time t7 and t8. An example in the case of performing the regeneration operation is shown.

  First, during execution of the DeSOx operation, the execution of post injection is prohibited (see S33 and S34 in FIG. 6). For this reason, as shown in FIG. 8, the post-injection amount becomes 0 at times t1 to t2, t4 to t5, and t7 to t8 when the DeSOx operation is executed. During the DeSOx operation, the NSC is supplied with exhaust gas in a reducing atmosphere having a high temperature and an excess air ratio of less than 1 (see S24 in FIG. 4), so the PM accumulated on the filter on the downstream side of the NSC To which unburned HC in the exhaust adheres.

  Immediately after the DeSOx operation is finished and the filter regeneration operation is started, the execution of the post injection is prohibited until the post injection inhibition time set in S37 of FIG. 6 has passed (FIG. 6). S38). In FIG. 8, the period during which post injection is prohibited is shown in dark gray. For this reason, the post-injection prohibition time (from time t2 to t3, t5 to t6, t8 to t9 in FIG. 8) after the switch from the DeSOx operation to the filter regeneration operation is performed even during the filter regeneration operation. Is prohibited and the post-injection amount becomes zero. Immediately after switching from the DeSOx operation to the filter regeneration operation as described above, the post-injection is prohibited during the post-injection prohibition time, so that the unburned HC adhering to the PM can be gently burned during the prohibition time. Therefore, it is possible to prevent PM from rapidly burning immediately after switching from the DeSOx operation to the filter regeneration operation and causing the filter to overheat (see broken lines 84a, 84b, 84c in FIG. 8). In addition, since the combustion of PM is promoted by using unburned HC adhering to the PM during the period in which the execution of the post injection is prohibited, the filter temperature does not immediately decrease even if the post injection is prohibited. Accordingly, the amount of fuel consumed for post injection can be suppressed (see broken lines 85a, 85b, 85c in FIG. 8). Further, by setting the length of the post-injection prohibition time according to the filter temperature and the execution time of the immediately preceding DeSOx operation (see FIG. 7), the post-injection prohibition time can be set to an appropriate length without excess or deficiency.

Although one embodiment of the present invention has been described above, the present invention is not limited to this.
For example, in the above embodiment, the length of the post-injection prohibition time is changed according to the execution time of the immediately preceding DeSOx operation (see S37 in FIG. 6), but the present invention is not limited to this. The length of the post-injection prohibition time may be changed according to the exhaust air / fuel ratio at the time of execution of the immediately preceding DeSOx operation, or may be changed according to both the execution time and the exhaust air / fuel ratio. Even if it does, it has the almost same effect. Also, when changing according to the air-fuel ratio of the exhaust, the post-injection prohibition time is set longer because the amount of unburned HC adhering to the PM increases as the air-fuel ratio of the exhaust during the immediately preceding DeSOx operation decreases. It is preferable.

1. Engine (internal combustion engine)
11 ... Exhaust pipe (exhaust passage)
2. Exhaust purification system 31 ... Catalytic converter (NOx catalyst)
32 ... Filter 34 ... Differential pressure sensor (collection amount estimation means)
7 ... ECU (post injection prohibiting means, regeneration control means, collection amount estimation means, SOx amount estimation means)

Claims (7)

A NOx catalyst provided in the exhaust passage of the internal combustion engine and having an oxidation function and a function of purifying NOx in the exhaust;
A filter that is provided downstream of the NOx catalyst in the exhaust passage and collects particulate matter in the exhaust;
Post-injection prohibiting means for permitting or prohibiting execution of post-injection which is fuel injection after a predetermined period after main injection;
The particulate matter deposited on the filter is burned and removed by supplying high-temperature and oxidizing atmosphere exhaust to the filter, and the post-injection is executed within a period permitted by the post-injection prohibiting means, and the oxidation Filter regeneration operation that raises the temperature of exhaust gas by burning unburned fuel using the function, and NOx catalyst regeneration that desorbs SOx in the NOx catalyst by supplying high-temperature and reducing atmosphere exhaust gas to the NOx catalyst And a regeneration control means for alternately driving and
The post-injection prohibiting means includes at least one of an execution time of the immediately preceding NOx catalyst regeneration operation and an air-fuel ratio of exhaust at the time of execution of the NOx catalyst regeneration operation after switching from the NOx catalyst regeneration operation to the filter regeneration operation. An exhaust purification system for an internal combustion engine, wherein execution of the post-injection is prohibited until a prohibition time set according to the time elapses.   The exhaust purification system for an internal combustion engine according to claim 1, wherein the post-injection prohibiting unit prohibits the execution of the post-injection during the execution of the NOx catalyst regeneration operation.   The exhaust purification system for an internal combustion engine according to claim 1 or 2, wherein the post-injection prohibiting unit sets the prohibition time to be longer as the execution time of the immediately preceding NOx catalyst regeneration operation is longer.   4. The internal combustion engine according to claim 1, wherein the post-injection prohibiting unit sets the prohibition time longer as the air-fuel ratio of exhaust at the time of execution of the immediately preceding NOx catalyst regeneration operation is smaller. Exhaust purification system.   The post-injection prohibiting unit sets the prohibition time longer as the temperature of the filter is higher when the NOx catalyst regeneration operation is switched to the filter regeneration operation. 2. An exhaust gas purification system for an internal combustion engine according to 1. A collection amount estimating means for estimating the amount of particulate matter collected in the filter;
SOx amount estimation means for estimating the amount of SOx trapped in the NOx catalyst,
The regeneration control means starts alternating execution of the filter regeneration operation and the NOx catalyst regeneration operation in response to the particulate matter amount exceeding a predetermined threshold set based on the SOx amount. An exhaust purification system for an internal combustion engine according to any one of claims 1 to 5, wherein:   The exhaust purification system for an internal combustion engine according to claim 6, wherein the regeneration control means sets the threshold value to a smaller value as the SOx amount increases.