JP2014125975A - Exhaust post-treatment device of diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device which can suppress the deterioration of a NOx purification rate even if a load variation arises immediately after the start of rich-spike treatment.SOLUTION: The exhaust post-treatment device of a diesel engine comprises: means (21) for enriching an excess air rate of exhaust in initial treatment when a regeneration time of a NOx trap catalyst (15) has come; means (21) for setting a target excess air rate of exhaust in post-treatment to a second fundamental excess air rate in the vicinity of 1.0 after the excess air rate of the exhaust is enriched; means (21) for predicting a remaining oxygen amount of the NOx trap catalyst (15) by using a relative value of the second excess air rate and the first excess air rate in a period that the excess air rate of the exhaust is enriched; and means (21) for controlling a degree of the enrichment on the basis of the predicted remaining oxygen amount.

Description

  The present invention relates to an exhaust aftertreatment device for a diesel engine.

  An oxidation catalyst and a NOx trap catalyst are provided in the exhaust passage, and the rich spike process is divided into an initial process and a late process (see Patent Document 1). With this, the front lambda is set to the first basic excess air ratio on the rich side in the initial processing, and it is determined that all of the oxygen stored in the oxidation catalyst and the NOx trap catalyst is lost when the rear lambda is reversed from the lean side to the rich side. To do. In the latter stage process, the front lambda is controlled to the second basic excess air ratio in the vicinity of 1.0. Here, the front lambda is the excess air ratio of exhaust flowing upstream of the oxidation catalyst, and the rear lambda is the excess air ratio of exhaust flowing downstream of the NOx trap catalyst.

JP 2009-197708 A

  By the way, the front lambda may not be able to reach the first basic excess air ratio on the rich side in the initial process due to a load change immediately after the start of the rich spike process. At this time, the oxygen stored in the oxidation catalyst and the NOx trap catalyst cannot be completely consumed in the initial treatment period, and the latter period in which the excess air ratio of the exhaust gas is controlled to the second basic excess air ratio in the vicinity of 1.0. Even after shifting to the treatment, oxygen remains in the oxidation catalyst and the NOx trap catalyst. For this reason, since the reducing agent supplied in the latter stage treatment is consumed by the oxygen remaining in the oxidation catalyst and the NOx trap catalyst and is not used for the reduction of NOx deposited on the NOx trap catalyst, the NOx purification rate deteriorates. .

  Therefore, an object of the present invention is to provide an apparatus that can suppress the deterioration of the NOx purification rate even if there is a load fluctuation immediately after the start of the rich spike processing.

  The exhaust aftertreatment device for a diesel engine of the present invention traps NOx in exhaust in an oxygen atmosphere, desorbs the trapped NOx in a reducing atmosphere, and reduces and purifies using HC in the exhaust as a reducing agent. A trap catalyst, a first air excess ratio detecting means for detecting a first excess air ratio that is an excess air ratio of the exhaust upstream of the NOx trap catalyst, and a second air that is an excess air ratio of the exhaust downstream of the NOx trap catalyst. A second excess air ratio detecting means for detecting an excess ratio; an initial processing means for enriching an excess air ratio of exhaust during initial processing when the NOx trap catalyst regeneration timing is reached; and an excess air ratio of the exhaust And late processing means for setting the target excess air ratio of the exhaust during the later processing to the second basic air excess ratio in the vicinity of 1.0. In the exhaust aftertreatment device for a diesel engine according to the present invention, the second excess air ratio detected by the second excess air ratio detection means and the first excess air ratio while the excess air ratio of the exhaust gas is further enriched. Based on the residual oxygen amount prediction means for predicting the residual oxygen amount of the NOx trap catalyst based on the relative value of the first excess air ratio detected by the detection means, and the residual oxygen amount predicted by the residual oxygen amount prediction means. And a rich degree / rich period control means for controlling the degree of enrichment during the initial process or the period of enrichment during the initial process.

  According to the present invention, even if oxygen remains in the NOx trap catalyst as it is because the first excess air ratio cannot reach the target excess air ratio on the rich side in the initial process due to load fluctuations. The remaining oxygen of the NOx trap catalyst can be consumed during the initial process by controlling the degree of enrichment during the initial process, and the deterioration of the NOx purification rate can be suppressed.

1 is a schematic configuration diagram of an exhaust aftertreatment device for a diesel engine according to a first embodiment of the present invention. It is a timing chart which shows the change of the front lambda and the rear lambda at the time of rich spike processing of the conventional device when there is no load fluctuation. It is a timing chart which shows the change of the front lambda and the rear lambda at the time of rich spike processing of the conventional device when there is load change. It is a timing chart which shows the change of the target excess air ratio of exhaust at the time of rich spike processing of a 1st embodiment, and the actual excess air ratio of exhaust. It is a flowchart in order to demonstrate calculation of the target excess air ratio of exhaust at the time of rich spike processing of a 1st embodiment. It is a flowchart in order to demonstrate calculation of the target excess air ratio of exhaust at the time of rich spike processing of a 1st embodiment. It is a characteristic view of the air excess ratio correction amount basic value of the first embodiment. It is a characteristic view of the interval correction amount of the first embodiment. It is a characteristic view of the catalyst temperature correction amount of the first embodiment. It is a characteristic view of the space velocity correction amount of the exhaust gas of the first embodiment. It is a timing chart which shows the change of the target excess air ratio at the time of rich spike processing of a 2nd embodiment, and the actual excess air excess ratio. It is a flowchart for demonstrating calculation of the target excess air ratio of the exhaust_gas | exhaustion at the time of the rich spike process of 2nd Embodiment. It is a flowchart for demonstrating calculation of the target excess air ratio of the exhaust_gas | exhaustion at the time of the rich spike process of 2nd Embodiment. It is a timing chart which shows the change of the target excess air ratio and the exhaust actual exhaust air excess ratio at the time of rich spike processing of a 3rd embodiment. It is a flowchart for demonstrating calculation of the target excess air ratio of exhaust at the time of the rich spike process of 3rd Embodiment, and target initial process time. It is a flowchart for demonstrating calculation of the target excess air ratio of exhaust at the time of the rich spike process of 3rd Embodiment, and target initial process time. It is a characteristic view of the initial processing period correction amount basic value of the third embodiment. It is a characteristic view of the interval correction amount of the third embodiment. It is a characteristic view of the catalyst temperature correction amount of the third embodiment. It is a characteristic view of the space velocity correction amount of the exhaust gas of the third embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a schematic configuration diagram showing an exhaust aftertreatment device for a diesel engine according to a first embodiment of the present invention. In FIG. 1, an intake passage 2 of a diesel engine 1 is provided with an intake compressor of a variable nozzle type turbocharger 3. The intake air is supercharged by the intake compressor, cooled by the intercooler 4, passes through the normally open throttle valve 5, and then flows into the cylinder of each cylinder through the collector 6. The fuel is increased in pressure by the common rail type fuel injection device, that is, by the high pressure fuel pump 7, sent to the common rail 8, and directly injected from the fuel injection valve 9 of each cylinder into the cylinder. The air flowing into the cylinder and the injected fuel are combusted by compression ignition here, and the exhaust gas flows out to the exhaust passage 10.

  Part of the exhaust gas flowing into the exhaust passage 10 is recirculated to the intake side via the EGR valve 12 through the EGR passage 11 as EGR gas. The remainder of the exhaust passes through the exhaust turbine of the variable nozzle type turbocharger 3 and drives the exhaust turbine.

  The EGR passage 11 is provided with an EGR cooler 31. The EGR cooler 31 cools the EGR gas using cooling water or cooling air. Further, a bypass passage 32 that bypasses the EGR cooler 31 and a flow path switching valve 33 that is located at a branch portion of the bypass passage 32 and that can switch the flow path of the EGR gas are provided. For example, the flow path switching valve 33 shuts off the bypass passage 32 when not energized to flow EGR gas to the EGR cooler 31, and shuts off the passage where the EGR cooler 31 is present when energized to flow EGR gas to the bypass passage 32. . The reason why the bypass passage 32 and the flow path switching valve 33 are provided is an HC countermeasure at a low temperature.

  The engine controller 21 receives signals of an accelerator opening degree (accelerator pedal depression amount) ACC from the accelerator sensor 22 and an engine rotational speed Ne from the crank angle sensor 23. Then, the engine controller 21 calculates the fuel injection timing and the fuel injection amount of the main injection based on the engine load (accelerator opening degree and the like) and the engine speed Ne, and sends a valve opening command signal corresponding to them to the fuel injection valve 9. Output to. Further, the engine controller 21 performs EGR control and supercharging pressure control in a coordinated manner so that the target EGR rate and the target intake air amount can be obtained. The engine controller 21 is constituted by a microcomputer having a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and an input / output interface (I / O interface).

  A filter (Diesel Particulate Filter) 13 that collects particulates in the exhaust is disposed downstream of the exhaust turbine in the exhaust passage 10. When the particulate accumulation amount of the filter 13 reaches a predetermined value (threshold value), the engine controller 21 performs the regeneration process of the filter 13. For example, by performing post-injection in the expansion stroke or exhaust stroke immediately after the main injection, the exhaust is raised to the temperature at which the particulates burn, the filter 13 is regenerated, and the particulates deposited on the filter 13 are removed by combustion. Then, the filter 13 is regenerated. The post injection amount and the post injection timing are determined in advance according to the engine load and rotation speed (operating conditions) so as to obtain the target regeneration temperature, and the post according to the engine load and rotation speed at that time. Post injection is performed so that the injection amount and the post injection timing can be obtained.

  In order to perform complete regeneration in which all of the particulates accumulated on the filter 13 are burned and removed, it is necessary to raise the particulate combustion temperature as much as possible within a range not exceeding the allowable temperature of the filter 13 during regeneration processing. It becomes. For this reason, in this embodiment, an oxidation catalyst (noble metal) 14 is disposed upstream of the filter 13. The oxidation catalyst 14 burns exhaust components (HC, CO) flowing in by post-injection for regeneration processing of the filter 13 to raise the temperature of the exhaust gas and promote the combustion of particulates in the filter 13. The carrier constituting the filter 13 may be coated with an oxidation catalyst. At this time, the oxidation reaction at the time of burning the particulates can be promoted, the bed temperature of the filter 13 can be substantially increased, and the burning of the particulates in the filter 13 can be promoted.

  The catalyst is not limited to the oxidation catalyst 14. Any catalyst having an oxidation function can be replaced with an oxidation catalyst. FIG. 1 shows a case where a three-way catalyst (TWC) is employed as the oxidation catalyst 14.

  Between the oxidation catalyst 14 and the filter 13, NOx (nitrogen oxide) in the exhaust is trapped (for example, adsorbed) in an oxygen atmosphere, and NOx trapped in the reducing atmosphere is desorbed to reduce HC in the exhaust. A NOx trap catalyst (LNT) 15 used for reducing and purifying as an agent is provided. The oxygen atmosphere is obtained when the exhaust air excess ratio is larger than 1.0 (a value corresponding to the theoretical air-fuel ratio). On the other hand, the reducing atmosphere is obtained when the exhaust air excess ratio is 1.0 or less.

  For this reason, when the NOx accumulation amount of the NOx trap catalyst 15 reaches a predetermined value (threshold), it is necessary to perform rich spike processing in order to switch the exhaust gas flowing through the NOx trap catalyst 15 from the oxygen atmosphere to the reducing atmosphere. In this rich spike processing, post-injection is performed in the expansion stroke or exhaust stroke immediately after the main injection, the amount of unburned HC discharged to the exhaust passage 10 is increased, and this HC is used as a reducing agent to the NOx trap catalyst 15. Is to supply.

  Since the diesel engine 1 is operated at an excess air ratio (air-fuel ratio leaner than the theoretical air-fuel ratio) larger than 1.0 (value corresponding to the theoretical air-fuel ratio) during normal operation, the addition of post-injection alone The excess air ratio of the exhaust cannot be switched to 1.0. For this reason, the throttle valve 5 in the fully open position during normal operation is closed during the rich spike process to reduce the intake air amount (cylinder intake air amount) Qac flowing into the cylinder, thereby reducing the excess air ratio of the exhaust gas to 1. Switch to 0 or less. That is, the post injection amount and the throttle valve opening (intake air) so that the excess air ratio determined by the total fuel injection amount Qfuel of the main injection amount and the post injection amount and the cylinder intake air amount Qac is 1.0 or less. Quantity). Here, if the throttle valve opening during the rich spike process is determined, the post injection amount is uniquely determined.

  Further, the NOx amount per predetermined time trapped by the NOx trap catalyst 15 is calculated every predetermined time (fixed period), and the NOx amount per predetermined time is added (integrated) to be accumulated on the NOx trap catalyst 15. The amount of NOx deposited is calculated. This NOx accumulation amount is compared with a predetermined threshold value, and when the NOx accumulation amount becomes equal to or greater than the threshold value (when the regeneration timing of the NOx trap catalyst 15 is reached), post injection (rich spike processing) is executed. .

  In this way, when the NOx accumulation amount becomes equal to or greater than the threshold during normal operation, the throttle valve opening is switched from the fully open state to the predetermined throttle valve opening (throttle throttling is performed) and post injection is started. Then, when a certain period has elapsed, the post injection is terminated and the throttle valve 5 is returned to the fully open position.

  By the way, immediately after switching from the oxygen atmosphere to the reducing atmosphere, oxygen stored in the oxygen atmosphere remains in the oxidation catalyst 14 and the NOx trap catalyst 15. Therefore, immediately after switching from the oxygen atmosphere to the reducing atmosphere, even if HC and CO as NOx reducing agents are supplied, the oxygen stored in the NOx trap catalyst 15 is desorbed before NOx. These reducing agents are oxidized (consumed). Similarly, HC and CO as NOx reducing agents are oxidized (consumed) by oxygen stored in the oxidation catalyst upstream of the NOx trap catalyst 15. That is, the NOx deposited on the NOx trap catalyst 15 cannot be reduced unless all the oxygen stored in the oxidation catalyst 14 and the NOx trap catalyst 15 is consumed.

  For this reason, there is a conventional apparatus that separates the rich spike processing into two stages of initial processing and late processing. This will be described with reference to FIG. 2. FIG. 2 shows how the front lambda (first excess air ratio) and the rear lambda (second excess air ratio) change during the rich spike process. Here, the front lambda is the excess air ratio of the exhaust gas flowing upstream of the oxidation catalyst 14. The rear lambda is an excess air ratio of exhaust flowing downstream of the NOx trap catalyst 15.

  As shown in FIG. 2, in the conventional apparatus, the front lambda is set to a first basic excess air ratio λ0 of about 0.9 (rich) smaller than 1.0 (theoretical air-fuel ratio) in the initial processing from the timing t1, Oxygen stored in the oxidation catalyst 14 and the NOx trap catalyst 15 is consumed. In this state, it waits for the rear lambda to reach 1.0, and at the timing t2 when the rear lambda becomes 1.0, it is determined that all of the oxygen stored in the oxidation catalyst 14 and the NOx trap catalyst 15 has disappeared, and the initial processing is performed. finish.

  In the latter process from the timing t2, the reducing agent is supplied to the NOx trap catalyst 15 by depositing the NOx trap catalyst 15 by controlling the front lambda to the second basic excess air ratio λ1 near 1.0. NOx is reduced and purified.

  It is determined that all the NOx accumulated in the NOx trap catalyst 15 has been reduced and purified at a timing t3 when a predetermined time elapses from the timing t1, and the latter process and therefore the rich spike process are terminated.

  However, it has been found that there is room for improvement in the conventional apparatus. This will be described with reference to FIG. 3. FIG. 3 also shows how the front lambda and the rear lambda change during the rich spike processing. However, unlike FIG. 2, this is a case where the front lambda does not reach the first basic excess air ratio λ0 of 0.9 in the initial process due to the load fluctuation. Thus, the reason why the front lambda cannot reach the first basic excess air ratio λ0 of 0.9 is the load fluctuation immediately after the start of the rich spike processing. For example, when the accelerator pedal is depressed and the vehicle is accelerated, if the NOx accumulation amount is equal to or greater than the threshold value as described above, the rich spike processing is started with the first basic excess air ratio λ0 being 0.9. In this case, the accelerator pedal may be returned immediately after starting the initial process. At this time, the fuel injection amount (the above-mentioned post injection amount) is reduced as compared with the case where the accelerator pedal is not returned, and the front lambda during the initial processing can reach the first basic excess air ratio λ0 of 0.9. It ends without. If the rapid decrease of the engine load immediately after the start of the rich spike process is defined as “load fluctuation immediately after the start of the rich spike process”, the front lambda during the initial process is reduced to 0 by the load fluctuation as shown in FIG. .9 cannot be reached. The load fluctuation immediately after the start of the rich spike process is hereinafter simply referred to as “load fluctuation”.

  If the front lambda during the initial processing does not reach 0.9, all of the oxygen stored in the oxidation catalyst 14 and the NOx trap catalyst 15 cannot be consumed, and the oxygen that could not be consumed in the oxidation catalyst 14 and the NOx trap catalyst 15 Remains. As a result, the rear lambda crosses 1.0 from the lean side later than in the case of FIG. In FIG. 3, the rear lambda reaches 1.0 from the lean side at the timing of t11, and the process moves to the later stage. However, the rear lambda continues to be on the lean side after t11, and at the timing of t12 before t13 when the later stage process ends. Crossing 1.0 again. This means that the reducing agent supplied for NOx purification in the period from t11 to t12 is used for consumption of oxygen remaining in the oxidation catalyst 14 and the NOx trap catalyst 15. Since the reducing agent is used for the consumption of residual oxygen, all of the NOx deposited on the catalyst 15 cannot be reduced, and the NOx purification rate becomes worse.

  Therefore, in the first embodiment of the present invention, the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15 are predicted based on the relative values of the rear lambda and the front lambda during the initial process. Since the first basic excess air ratio λ0 is calculated at the start of the initial process, the correction amount (the degree of enrichment during the initial process) of the first basic excess air ratio λ0 is controlled based on the predicted residual oxygen amount. Here, the relative value between the rear lambda and the front lambda is the difference between the rear lambda and the front lambda or the ratio between the rear lambda and the front lambda. In the following description, the difference between the rear lambda and the front lambda will be described.

  Specifically, referring to FIG. 4, FIG. 4 is a model diagram showing how the front lambda and the rear lambda, which are the exhaust target air excess ratio and the exhaust actual air excess ratio, change during the rich spike processing. It is. Here, in the first stage of FIG. 4, the change in the target excess air ratio when there is no load fluctuation is indicated by a solid line, and the change in the target excess air ratio when there is a load fluctuation is superimposed by a broken line. In the second row of FIG. 4, the change of the front lambda and the rear lambda when there is no load change is shown by a solid line, and the change of the rear lambda when there is a load change is shown by a broken line. The timings of t1, t2, and t3 on the horizontal axis are matched to those in FIG.

  In FIG. 4, when the first basic excess air ratio λ0 is set to 0.9 at the timing t1 when the initial process is started, the front lambda (actual value) during the initial process is less than the lean side value if there is no load fluctuation. It drops significantly to 0.9 and then settles to 0.9.

  By setting the first basic air excess ratio λ0 to 0.9 on the rich side, the oxygen remaining in the oxidation catalyst 14 and the NOx trap catalyst 15 is consumed, and the rear lambda becomes lean at the timing t2 when all the oxygen is consumed. Cross 1.0 from the side. The initial process is terminated at the timing t2 when the rear lambda crosses 1.0 from the lean side, and the process shifts to the latter process.

  In the latter process, the target excess air ratio of exhaust is set to the second basic excess air ratio λ1 in the vicinity of 1.0. As a result, the reducing agent is supplied to the NOx trap catalyst 15, and the NOx deposited on the NOx trap catalyst 15 is reduced and purified. It is known in advance how long the late treatment can be continued to reduce all the NOx deposited on the NOx trap catalyst 15. Accordingly, the late process is terminated at the timing t3 when the period elapses, that is, the timing t3 when the predetermined time Δt2 has elapsed from t1, and the normal operation is resumed.

  The second basic excess air ratio λ1 is set to a value slightly smaller than 1.0 (a value on the rich side). This is due to the following reason. That is, in a diesel engine, the utilization rate of oxygen is poor, and combustion is always performed with excess oxygen. For this reason, even if the second basic excess air ratio λ1 is set to a value slightly smaller than 1.0, oxygen is substantially present in the exhaust gas. In order to oxidize HC and CO generated in the vicinity of 1.0 with oxygen present in the exhaust gas, the target excess air ratio of the exhaust gas during the later processing is set to a value slightly smaller than 1.0. .

  Now, the difference Δλ between the rear lambda and the front lambda at the time of the initial processing is a value for predicting the residual oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15. For example, the difference Δλ is relatively large when there is no load fluctuation as shown in FIG. 2, and the difference Δλ is relatively small when there is a load fluctuation as shown in FIG. That is, when the difference Δλ is relatively large, the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15 are relatively small. When the difference Δλ is relatively small, the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15 are It can be predicted to be relatively large.

  Here, immediately after the start of the initial processing, the rear lambda and the front lambda as the actual values change significantly following the first basic excess air ratio λ0. Therefore, the timing of t21 when each change has settled to some extent, that is, a fixed time from t1. The difference Δλ at the timing when Δt1 has passed is adopted. The timing of t21 is determined by conformance.

  Thus, if the residual oxygen amount is predicted by the difference Δλ, the residual oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15 is larger when the difference Δλ is relatively small than when the difference Δλ is relatively large. Represents. Therefore, if the first basic excess air ratio λ0 calculated at the start of the initial process is controlled to the rich side using the excess air ratio correction amount at the timing when it is determined that there is load fluctuation (the difference Δλ is relatively small). Good. For example, as indicated by the solid line in the first row of FIG. 4, when there is no load fluctuation (the difference Δλ is relatively large), the target excess air ratio of the exhaust during the initial process is changed to the first basic excess air ratio λ0. It is set. On the other hand, when it is found that there is a load fluctuation (the difference Δλ is relatively small) at t21 as shown by the dashed line in the first row in FIG. 4, the excess air ratio is higher than the first basic excess air ratio λ0. A value that is smaller by the correction amount is set as the target excess air ratio mλ. As a result, an area difference indicated by hatching is generated between the target excess air ratio during initial processing when there is a load fluctuation and the target excess air ratio during initial processing when there is no load fluctuation (FIG. 4 first). (See the steps). In other words, when there is a load change, the reducing agent can be supplied during the initial process by an amount corresponding to the area difference more than when there is no load change. By increasing the supply amount of the reducing agent during the initial treatment, the residual oxygen of the oxidation catalyst and the NOx trap catalyst can be consumed.

  The third to fifth stages in FIG. 4 show the movement of the initial process flag, correction flag, and late process flag introduced in the flowcharts of FIGS. 5A and 5B described later. Here, the third to fifth stages are movements of the initial process flag, the correction flag, and the late process flag when there is no load fluctuation.

  The front lambda is a front wide air-fuel ratio sensor 24 (see FIG. 1) provided upstream of the oxidation catalyst 14, and the rear lambda is a rear wide air-fuel ratio sensor 25 (see FIG. 1) provided downstream of the NOx trap catalyst 15. To detect. Each wide area air-fuel ratio sensor linearly detects the air-fuel ratio of the exhaust.

  Since the first embodiment includes the oxidation catalyst 14, it is necessary to consider not only the oxygen remaining in the NOx trap catalyst 15 but also the oxygen remaining in the oxidation catalyst 14, but the oxidation catalyst 14 is not included. In some cases, the present invention also has application. In this case, only oxygen remaining in the NOx trap catalyst 15 needs to be considered.

  The rich spike processing of the present embodiment performed by the engine controller 21 will be described in detail with reference to the flowcharts of FIGS. 5A and 5B.

  The flow in FIGS. 5A and 5B is for calculating the target excess air ratio mλ of exhaust during the rich spike process, and is executed at regular intervals (for example, every 10 ms).

  In step 1, it is determined whether or not it is time to regenerate the NOx trap catalyst 15. For example, the regeneration permission flag is switched from zero to 1 when the NOx accumulation amount becomes equal to or greater than a threshold value. If the regeneration permission flag = 0, the regeneration permission flag = 0, the regeneration timing of the NOx trap catalyst 15 is not reached. If the regeneration permission flag = 1, the regeneration timing of the NOx trap catalyst 15 is determined. That's fine. If the regeneration permission flag = 0 and the regeneration timing of the NOx trap catalyst 15 is not reached, the current process is terminated.

  When the regeneration timing of the NOx trap catalyst 15 is reached in step 1 from the regeneration permission flag = 1, the process proceeds to step 2 where a late processing flag (initially set to zero when the engine is started) is observed. Here, it is assumed that the late process flag = 0, the process proceeds to step 3, and the initial process flag (initially set to zero when the engine is started) is seen. Here, it is assumed that the initial processing flag = 0, and the process proceeds to steps 4 and 5.

  In step 4, the first basic excess air ratio λ0 is calculated, and in step 5, this is entered into the exhaust target air excess ratio mλ. The basic excess air ratio λ0 is a value for consuming all the oxygen stored in the oxidation catalyst 14 and the NOx trap catalyst 15 when there is no load fluctuation. The basic excess air ratio λ0 may be given as a map value corresponding to the operating conditions (engine load and rotation speed), for example. In order to consume residual oxygen, the basic excess air ratio λ0 is a value smaller than 1.0, and may be simply a constant value (for example, 0.9). In step 6, the initial processing flag = 1 is set and the current processing is terminated.

  Since the initial processing flag is set to 1 in step 6, the process proceeds from step 3 to step 7 and subsequent steps next time. In step 7, the correction flag (initially set to zero when the engine is started) is checked. Here, assuming that the correction flag = 0, the process proceeds to step 8 to compare the elapsed time from the initial processing start timing with the fixed time Δt1. Here, the fixed time Δt1 is a value that determines the timing for calculating the difference Δλ after the start of the initial process, and is determined in advance. When the elapsed time from the initial processing start timing is less than the predetermined time Δt1, the process proceeds to step 9, where the value of “mλ (previous)” that is the previous target exhaust air excess ratio is used as it is for the current exhaust target air excess ratio mλ. The basic exhaust air excess ratio λ0 calculated at the start of the initial process is maintained.

  While the elapsed time from the initial process start timing is less than the predetermined time Δt1, the process proceeds from step 8 to step 9, and the operation of step 9 is repeated. Eventually, when the elapsed time from the initial process start timing reaches a certain time Δt1, it is determined that it is time to calculate the difference Δλ after the initial process starts. At this time, the process proceeds from step 8 to step 10 and thereafter.

  Steps 10 to 16 predict the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15, and use the first basic target excess air ratio λ 0 calculated at the start of the initial processing using a correction amount corresponding to the predicted residual oxygen amount. This is the part that controls to the smaller side (rich side). Here, the target excess air ratio mλ is calculated by the following equation.

mλ = λ0−λHOS (1)
Where λ0 is the first basic excess air ratio,
λHOS: excess air ratio correction amount,
That is, a value obtained by subtracting the air excess ratio correction amount λHOS from the first basic air excess ratio λ0 to reduce the first basic air excess ratio λ0 to the smaller side (rich side) is set as the target exhaust air excess ratio mλ.

  The excess air ratio correction amount λHOS on the right side of the above equation (1) is calculated by the following equation.

λHOS = λHOS0 + IntHOS + TlnntHOS + SVHOS
... (2)
However, λHOS0: air excess rate correction amount basic value,
IntHOS: interval correction amount,
TlntHOS: catalyst temperature correction amount,
SVHOS: Space velocity correction amount,
Hereinafter, each correction amount will be specifically described. First, in step 10, the difference Δλ between the front lambda detected by the front wide area air-fuel ratio sensor 24 and the rear lambda detected by the rear wide area air-fuel ratio sensor 25 is calculated. The values of the front lambda and the rear lambda at this time are those when a predetermined time Δt1 has elapsed from the initial processing start timing. Here, since the value of the rear lambda is larger than the value of the front lambda, a value obtained by subtracting the front lambda from the rear lambda is defined as a difference Δλ. The difference Δλ is a value for predicting the residual oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15. When the difference Δλ is relatively small, it is predicted that the residual oxygen amount in the oxidation catalyst 14 and the NOx trap catalyst 15 is larger than when the difference Δλ is relatively large.

  In step 11, the excess air ratio correction amount basic value λHOS 0 is calculated by searching a table having the contents shown in FIG. 6 from the difference Δλ. As shown in FIG. 6, the excess air ratio correction amount basic value λHOS0 is zero when the difference Δλ exceeds the predetermined value Δλ0, and increases as the difference Δλ decreases when the difference Δλ is equal to or smaller than the predetermined value Δλ0. is there. The difference Δλ, which is a value for predicting the remaining oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15, is relatively large when this value is relatively small in the region where the difference Δλ is equal to or less than the predetermined value Δλ0. It represents that there is relatively more residual oxygen. The reason why λHOS0 is set larger than when there is no load variation (Δλ is relatively large) when there is load variation (Δλ is relatively small) is as follows. That is, when there is a load variation (Δλ is relatively small), there is more residual oxygen in the catalysts 14 and 15 than when there is no load variation (Δλ is relatively large), and the more residual oxygen is on the smaller side (rich) This is because it is necessary to increase the correction amount.

  Further, the predetermined value Δλ0 on the horizontal axis is a difference Δλ when there is no load fluctuation and is obtained in advance by adaptation. When the difference Δλ at that time is equal to or greater than the predetermined value Δλ0, unlike the case where the difference Δλ0 is less than the predetermined value Δλ0 (there is load fluctuation), no residual oxygen is generated, and therefore the first basic excess air ratio λ0 is reduced. There is no need to correct to the side (rich side). Therefore, in the region where the difference Δλ is equal to or larger than the predetermined value Δλ0, the excess air ratio correction amount basic value λHOS0 is set to zero.

  In step 12, an interval correction amount IntHOS is calculated by searching a table having the contents shown in FIG. 7 from the interval Int of the rich spike process. Here, the “rich spike processing interval” is a period from the end of one rich spike processing to the start of the next rich spike processing. The predetermined value Int0 on the horizontal axis is the interval Int when the excess air ratio correction amount basic value λHOS0 in FIG. 6 is adapted. As shown in FIG. 7, the interval correction amount IntHOS has a positive value when the interval Int is larger than the predetermined value Int0, and has a negative value when the interval Int is smaller than the predetermined value Int0. This is because it is considered that the residual oxygen is larger in the region where the interval Int of the rich spike process is larger than the predetermined value Int0 than that at the time of adaptation, and the residual oxygen is smaller in the region where the interval Int is smaller than the predetermined value Int0 than that at the time of adaptation.

  In step 13, the catalyst temperature correction amount TlntHOS is calculated by searching a table having the contents shown in FIG. 8 from the catalyst temperature Tlnt of the NOx trap catalyst 15. The catalyst temperature Tlnt is detected by a catalyst temperature sensor 26 (see FIG. 1). The predetermined value Tlnt0 on the horizontal axis is the catalyst temperature when the excess air ratio correction amount basic value λHOS0 in FIG. 6 is adapted. As shown in FIG. 8, the catalyst temperature correction amount TlntHOS has a positive value in a region where the catalyst temperature Tlnt is smaller than the predetermined value Tlnt0, and has a negative value in a region where the catalyst temperature Tlnt is larger than the predetermined value Tlnt0. This is because it is considered that the residual oxygen is more desorbed in the region where the catalyst temperature Tlnt is higher than the predetermined value Tlnt0 than when it is matched, and the residual oxygen is desorbed in the region smaller than the predetermined value Tlnt0 than when it is matched.

  In step 14, the space velocity correction amount SVHOS is calculated by searching a table having the contents shown in FIG. 9 from the exhaust space velocity SV. The exhaust space velocity SV [1 / hour] is calculated by dividing the exhaust volume flow rate Qexh [l / hour] by the catalyst volume [l] of the NOx trap catalyst 15. A method for calculating the volume flow rate Qexh of the exhaust gas is known. As shown in FIG. 9, the space velocity correction amount SVHOS is a positive value when the exhaust space velocity SV is larger than the predetermined value SV0, and is a negative value when the exhaust space velocity SV is smaller than the predetermined value SV0. Become. This is because it is considered that the residual oxygen is larger in the region where the exhaust space velocity SV is larger than SV0 than when it is adapted, and the residual oxygen is smaller in the region where the exhaust space velocity SV is smaller than the predetermined value SV0 than when adapted. The reason why the residual oxygen is smaller in the region where the space velocity SV of the exhaust gas is smaller than the predetermined value SV0 is smaller than that at the time of adaptation because the oxygen consumption is faster when the space velocity SV of the exhaust gas is smaller.

  In step 15, the excess air ratio correction amount λHOS is calculated from the air excess ratio correction amount basic value λHOS 0, the interval correction amount IntHOS, the catalyst temperature correction amount TlntHOS, and the space velocity correction amount SVHOS calculated by the following equation.

λHOS = λHOS0 + IntHOS + TlnntHOS + SVHOS
... (3)
Equation (3) is the same as equation (2) above.

  In step 16, the exhaust target air excess ratio mλ is calculated from the excess air ratio correction amount λHOS and the first basic excess air ratio λ0 by the following equation.

mλ = λ0−λHOS (4)
Equation (4) is the same as equation (1) above. In step 17, the correction flag = 1 is set and the current process is terminated.

  Since the correction flag is set to 1 in step 17, the process proceeds from step 7 to step 18 in FIG.

  In FIG. 5B, in step 18, it is checked whether or not the rear lambda detected by the rear wide area air-fuel ratio sensor 25 has reversed 1.0 (theoretical air-fuel ratio) from the lean side to the rich side. Before the rear lambda crosses 1.0 from the lean side, the routine proceeds to step 19, and the value of “mλ (previous)”, which is the target excess air ratio of the previous exhaust, is directly transferred to the target air excess ratio mλ of the current exhaust. Thus, the target excess air ratio mλ of the exhaust gas calculated in step 15 is maintained.

  Eventually, when the rear lambda crosses 1.0 from the lean side and reverses to the rich side in step 18, the process proceeds to step 20 in order to shift from the initial process to the later process, and the second basic air excess ratio is set to the target exhaust air excess ratio mλ. Set λ1. The second basic excess air ratio λ1 is a value for reducing and purifying all NOx accumulated in the NOx trap catalyst 15 when there is no load fluctuation. A value in the vicinity of 1.0 is set as the second basic excess air ratio λ1. Actually, a value smaller than 1.0 is set instead of 1.0. This is because HC and CO generated near 1.0 are oxidized with oxygen present in the exhaust.

  In step 21, the initial process flag is set to 0 to end the initial process. In step 22, the late process flag = 1 is set and the current process is terminated.

  Since the initial process flag = 0 and the late process flag = 1 in steps 21 and 22, the process proceeds from step 2 in FIG. 5A to step 23 and subsequent steps in FIG. 5B next time.

  In FIG. 5B, in step 23, it is determined whether or not a predetermined time Δt2 has elapsed since the start of the rich spike processing. If the predetermined time Δt2 has not elapsed since the start of the rich spike process, the process proceeds to step 24, and the value of “mλ (previous)”, which is the target excess air ratio of the previous exhaust, is directly used as the target air excess ratio mλ of the current exhaust. By shifting, the second basic excess air ratio λ1 set in step 20 is maintained.

  Eventually, when a predetermined time Δt2 has elapsed from the start of the rich spike process in step 23, the process proceeds to steps 25, 26, 27, and 28 to end the rich spike process. In step 25, a value (lean) larger than 1.0 is entered in the target excess air ratio of the exhaust. This is the target excess air ratio when the engine is operated lean.

  In steps 26, 27, and 28, the correction flag = 0, the late process flag = 0, and the regeneration permission flag = 0.

  Since the reproduction permission flag = 0 is set at step 28 in FIG. 5B, it is not possible to proceed to step 2 and subsequent steps from step 1 in FIG. 5A next time.

  In the flow (not shown), the throttle valve opening and the post injection amount are controlled so that the target excess air ratio mλ calculated by the flow in FIGS. 5A and 5B is obtained.

  Here, the effect of this embodiment is demonstrated.

  In this embodiment, the front wide air-fuel ratio sensor 24 (first excess air ratio detection means) that detects the NOx trap catalyst 15 and the front lambda (first excess air ratio that is the excess air ratio of the exhaust gas upstream of the NOx trap catalyst 15). ), Rear lambda (second excess air ratio that is the excess air ratio of the exhaust downstream of the NOx trap catalyst 15), a rear wide air-fuel ratio sensor 25 (second excess air ratio detection means), and regeneration of the NOx trap catalyst 15 Initial processing means (see steps 1-6, steps 1-3, 7-9 in FIG. 5A) for enriching the excess air ratio of exhaust during initial processing, and the exhaust air excess ratio being rich when the time comes Late processing means (see Steps 18, 20 to 22, Steps 23 and 24 in FIG. 5B) that sets the excess air ratio of the exhaust during the latter processing after the conversion to the second basic air excess ratio in the vicinity of 1.0; Remaining oxygen amount prediction means for predicting the residual oxygen amount of the NOx trap catalyst 15 based on the relative values of the rear lambda and the front lambda while the exhaust air excess ratio is being enriched (see steps 3, 7, 8, and 10 in FIG. 5A) ) And an excess air ratio correction amount basic value λHOS0 (the degree of enrichment during the initial process) based on the residual oxygen amount predicted by the residual oxygen amount prediction means (step 3 in FIG. 5A). , 7, 8, 10, 11, 15, 16, 17, and steps 18 and 19 in FIG. 5B). According to the present embodiment, when the front lambda cannot reach the first basic excess air ratio λ0 calculated at the start of the initial process due to load fluctuations, oxygen remains in the NOx trap catalyst 15 as it is. Even in this case, the residual oxygen in the NOx trap catalyst 15 can be consumed during the initial process by controlling the excess air ratio correction amount basic value λHOS0 (the degree of enrichment during the initial process), and the NOx purification rate is deteriorated. Can be suppressed.

  According to the present embodiment, the relative value of the rear lambda (the detected second excess air ratio) and the front lambda (the detected first excess air ratio) is the difference Δλ between them, and the difference Δλ is in accordance with the difference Δλ. Since it is predicted that the smaller the smaller the amount of remaining oxygen in the NOx trap catalyst 15, the remaining amount of oxygen in the NOx trap catalyst 15 can be easily predicted.

  According to the present embodiment, the enrichment degree control means increases the excess air ratio correction amount basic value λHOS0 (the degree of enrichment during the initial process) as the difference Δλ is smaller (see FIG. 6), so the NOx trap catalyst. Regardless of the amount of residual oxygen of 15, the residual oxygen of the NOx trap catalyst 15 can be consumed.

  According to the present embodiment, the excess air ratio correction amount basic value λHOS0 (the degree of enrichment during the initial process) is set to at least one of the interval Int of the rich spike process, the catalyst temperature Tlnt of the NOx catalyst 15, and the exhaust space velocity SV. (See Steps 12 to 15 in FIG. 5A), the interval Int of the rich spike process, the catalyst temperature Tlnt of the NOx trap catalyst 15, and the exhaust space velocity SV are adapted to the excess air ratio correction amount basic value λHOS0. The excess air ratio correction amount λHOS (the degree of enrichment during the initial process) is optimally provided even when there is a difference from the interval Int of the rich spike process, the catalyst temperature Tlnt of the NOx trap catalyst 15, and the exhaust space velocity SV. Can do.

(Second Embodiment)
FIG. 10 is a model diagram of the second embodiment showing how the front lambda and the rear lambda, which are the exhaust target air excess ratio and the exhaust exhaust air excess ratio, change during the rich spike processing. It replaces FIG. The same parts as those in FIG. 4 of the first embodiment are described in the same manner. Here, in the first stage of FIG. 10, the change in the target excess air ratio when there is no load fluctuation is indicated by a solid line, and the change in the target excess air ratio when there is a load fluctuation is indicated by an alternate long and short dash line. In the second row of FIG. 10, the change of the front lambda and the rear lambda when there is no load change is shown by a solid line, and the change of the rear lambda when there is a load change is shown by a broken line.

  In the first embodiment, it is determined that all the oxygen has disappeared from the oxidation catalyst 14 and the NOx trap catalyst 15 at the timing t2 when the rear lambda during the initial process is reversed from the lean side to the rich side, and the process is shifted to the later stage process. . Actually, the rear lambda sensor 25 only detects the oxygen concentration of the exhaust gas downstream of the NOx trap catalyst 15, and does not measure the residual oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15. Therefore, in the second embodiment, the difference Δλ between the rear lambda and the front lambda is integrated during the initial processing, and when the integrated value exceeds the threshold value, all the oxygen has disappeared from the oxidation catalyst 14 and the NOx trap catalyst 15. Judge and switch to later processing.

  More specifically, in FIG. 10, the area between the rear lambda and the front lambda at the time of the initial processing represents the residual oxygen amount at which the oxidation catalyst 14 and the NOx trap catalyst 15 disappear. However, if the calculation of the area between the rear lambda and the front lambda is started immediately after the start of the initial process while the front lambda is rapidly decreasing, the calculated value of the area between the rear lambda and the front lambda is not stable. For this reason, the calculation of the area between the rear lambda and the front lambda starts at the timing when the front lambda falls behind 1.0 from the lean side and the front lambda becomes a certain stable value. The calculated value can be stabilized.

  Here, whether or not the front lambda has become a stable value after the start of the initial process is determined based on the differential value of the front lambda. In other words, the front lambda crosses 1.0 from the lean side and reverses to the rich side, and the front lambda differential value (handled as an absolute value) is equal to or less than the threshold value (handled as a positive value). Judge that the value is stable. The threshold value for comparison with the differential value of the front lambda is, for example, a tangent to the front lambda as shown in the second row of FIG. At this time, the timing at which the differential value of the front lambda (handled as an absolute value) is less than or equal to this threshold value (handled as a positive value) is t31. The timing of t31 is finally determined by conformity.

  Further, starting from the timing of t31, the area between the rear lambda and the front lambda when all the oxygen disappears from the oxidation catalyst 14 and the NOx trap catalyst 15 can be known in advance through experiments and simulations. For this reason, starting from the timing of t31, the area between the rear lambda and the front lambda when all the oxygen disappears from the oxidation catalyst 14 and the NOx trap catalyst 15 is determined in advance as a threshold value. On the other hand, if the difference Δλ between the rear lambda and the front lambda is integrated at a constant period from the timing t31, the integrated value SMΔλ corresponds to the area between the rear lambda and the front lambda. Therefore, the integrated value SMΔλ of the difference Δλ that is an area equivalent value is compared with a predetermined threshold value, and at a timing t32 when the integrated value SMΔλ of the difference Δλ reaches the threshold value, the oxidation catalyst 14 and the NOx trap catalyst. From FIG. 15, it can be determined that all oxygen has disappeared.

  The flow of FIGS. 11A and 11B is for calculating the target excess air ratio mλ of exhaust during the rich spike processing of the second embodiment, and is executed at regular intervals (for example, every 10 ms). The same step numbers are attached to the same parts as those in FIGS. 5A and 5B of the first embodiment.

  5A and 5B of the first embodiment are steps 31 to 36 in FIG. 11B. To explain mainly the parts different from the first embodiment, steps 31 and 32 in FIG. 11B are parts for determining whether or not it is time to calculate the area between the rear lambda and the front lambda.

  In step 31, the front lambda is compared with 1.0, and in step 32, the differential value Dλf of the front lambda is compared with the threshold value. Here, since the front lambda is a value that becomes smaller as the initial process starts as shown in FIG. 10, the differential value Dλf of the front lambda at the time of the initial process becomes a negative value. However, negative values are difficult to handle, so handle them as absolute values. Also, a positive value is adopted as a threshold value to be compared with the front lambda differential value Dλf. If the front lambda is not less than 1.0 or the front lambda is less than 1.0, but the differential value Dλf of the front lambda is not less than the threshold value, the area between the rear lambda and the front lambda is still calculated. Judge that it is not time to do. At this time, the routine proceeds to step 19 where the value of “mλ (previous)”, which is the target excess air ratio of the previous exhaust gas, is directly transferred to the target air excess ratio mλ of the current exhaust gas, and is calculated in step 15 of FIG. 11A. Maintain the target excess air ratio for exhaust.

  On the other hand, when the front lambda is 1.0 or less and the front lambda differential value Dλf is less than or equal to the threshold value in steps 31 and 32, the value of the front lambda is stable, so the area between the rear lambda and the front lambda is reduced. It is determined that it is time to calculate. At this time, the routine proceeds to step 33, where the difference Δλ between the front lambda detected by the front wide area air-fuel ratio sensor 24 and the rear lambda detected by the rear wide area air-fuel ratio sensor 25 is calculated in the same manner as in step 9 of FIG. 11A. Again, since the value of the rear lambda is larger than the value of the front lambda, the value obtained by subtracting the front lambda from the rear lambda is taken as the difference Δλ.

  In step 34, an integrated value SMΔλ of the difference Δλ is calculated by the following equation.

SMΔλ = SMΔλ (previous) + Δλ (5)
Where SMΔλ (previous): previous value of SMΔλ,
(5) Zero is put in the initial value of SMΔλ (previous) which is the previous value of the integrated value on the right side of the equation.

  In step 35, the integrated value SMΔλ of the difference Δλ is compared with the threshold value. Here, the threshold value is the area between the rear lambda and the front lambda when all the oxygen disappears from the oxidation catalyst 14 and the NOx trap catalyst 15 starting from the timing of calculating the area between the rear lambda and the front lambda. In advance. When the integrated value SMΔλ of the difference Δλ is equal to or smaller than the threshold value, it is determined that all oxygen has not yet disappeared from the oxidation catalyst 14 and the NOx trap catalyst 15, and the process proceeds to step 19. In step 19, the value of λ (previous), which is the target excess air ratio of the previous exhaust gas, is directly transferred to the target air excess ratio mλ of the current exhaust gas, so that the target air excess air amount calculated in step 15 of FIG. 11A is obtained. Maintain rate.

  Thereafter, the integration value SMΔλ of the difference Δλ increases by continuing the integration of the difference Δλ in steps 33 and 34 (see the third row in FIG. 10). When the integrated value SMΔλ of the difference Δλ eventually exceeds the threshold value, it is determined that all the oxygen has disappeared from the oxidation catalyst 14 and the NOx trap catalyst 15, and the process proceeds to the later stage processing after step 20.

  When the predetermined time Δt2 has elapsed from the start of the rich spike process in step 23, the rich spike process is terminated, and the operations of steps 25, 26, 27, and 28 are performed in the same manner as in the first embodiment, and then the difference Δλ is performed in step 36. The integrated value SMΔλ = 0.

  Thus, also by 2nd Embodiment, there exists an effect similar to 1st Embodiment. Furthermore, according to the second embodiment, the late processing means integrates (integrates) the difference Δλ between the rear lambda (the detected second excess air ratio) and the front lambda (the detected first excess air ratio), When the integrated value SMΔλ (integrated value) of the difference Δλ exceeds the threshold value, the exhaust target air excess ratio is set to the second basic excess air ratio λ1 (steps 33, 34, 35, 20 to 20 in FIG. 11B). 22), the amount of residual oxygen in the NOx trap catalyst 15 can be calculated correctly, so that the accuracy in determining whether or not all the residual oxygen in the NOx trap catalyst 15 has disappeared is improved.

  When the oxygen in the exhaust is exhausted, the rear lambda detected by the rear wide area air-fuel ratio sensor 25 is reversed. However, the rear wide air-fuel ratio sensor 25 cannot detect a situation in which oxygen remains in the NOx trap catalyst 15 and oxygen is gradually desorbed from the NOx trap catalyst 15. Such a situation can only be evaluated by integrating (integrating) the difference Δλ.

  According to the second embodiment, the integration (integration) of the difference Δλ is such that the front lambda (the detected first excess air ratio) is 1.0 or less and the front lambda (the detected first excess air ratio). Since the differential value Dλf starts after it becomes equal to or less than a predetermined threshold (see steps 31 and 32 in FIG. 11B), a stable integrated value SMΔλ (integrated value) is obtained, and the NOx trap catalyst 15 The accuracy of determining whether or not all residual oxygen has disappeared is improved.

(Third embodiment)
FIG. 12 is a model diagram of the third embodiment showing how the front lambda and the rear lambda, which are the target exhaust excess air ratio and the actual exhaust air excess ratio, change during the rich spike processing. It replaces FIG. The same parts as those in FIG. 4 of the first embodiment are described in the same manner. Here, in the first stage of FIG. 12, the change in the target excess air ratio when there is no load fluctuation is indicated by a solid line, and the change in the target excess air ratio when there is a load fluctuation is indicated by a dashed line. In the second row of FIG. 12, the change in the front lambda and the rear lambda when there is no load change is shown by a solid line, and the change in the rear lambda when there is a load change is shown by a broken line.

  In the first embodiment, the oxidation catalyst 14 is corrected by correcting the first basic excess air ratio λ0 calculated at the start of the initial process to the smaller side (rich side) using the excess air ratio correction amount when there is a load fluctuation. In addition, the remaining oxygen amount in the NOx trap catalyst 15 was completely lost. However, depending on the operating conditions, the first basic excess air ratio λ0 may not be corrected to the smaller side (rich side) using the excess air ratio correction amount.

  When the load fluctuates, the first basic excess air ratio λ0 is corrected to be smaller using the excess air ratio correction amount in the first embodiment because the area between the rear lambda and the front lambda during the initial process is increased. In order to eliminate all oxygen from the catalysts 14 and 15. In this case, the area between the rear lambda and the front lambda during the initial process can be increased by extending the initial process period while maintaining the first basic excess air ratio λ0 calculated at the start of the initial process.

  Therefore, in the third embodiment, at the start of the initial process, the first basic excess air ratio λ0 is calculated and the basic initial process time τ0 is calculated, and the correction amount (initial value) of the basic initial process time τ0 is calculated based on the predicted residual oxygen amount. The period of enrichment during processing). For example, as shown by the solid line in the first stage of FIG. 12, when there is no load fluctuation (the difference Δλ is relatively large), the target excess air ratio of the exhaust during the initial process is changed to the first basic excess air ratio λ0. The initial processing time is set to the basic initial processing time τ0. On the other hand, when it is found that there is a load fluctuation (difference Δλ is relatively small) at t21 as indicated by the one-dot chain line in the first row in FIG. 12, the initial processing time is corrected from the basic initial processing time τ0. A value that is longer by the amount is set as the target initial processing time mτ. As a result, an area difference indicated by hatching is generated between the target excess air ratio during initial processing when there is a load fluctuation and the target excess air ratio during initial processing when there is no load fluctuation (FIG. 12 first). (See the steps). That is, when there is a load change, it is possible to supply the reducing agent by an amount corresponding to the area difference more than when there is no load change. By increasing the supply amount of the reducing agent, the residual oxygen of the oxidation catalyst 14 and the NOx trap catalyst 15 can be consumed.

  The flow of FIGS. 13A and 13B is for calculating the target excess air ratio mλ and the target initial processing time mτ during the rich spike processing of the third embodiment, and is executed at regular intervals (for example, every 10 ms). . The same step numbers are attached to the same parts as those in FIGS. 5A and 5B of the first embodiment.

  Parts different from FIGS. 5A and 5B of the first embodiment are steps 41 to 46 in FIG. 13A. However, in the third embodiment, since the basic excess air ratio λ0 calculated at the start of the initial process is not corrected, the positions of steps 8 and 9 are interchanged.

  The difference from the first embodiment will be mainly described. In FIG. 13A, steps 41 to 46 are performed by setting the basic initial processing time τ0 calculated at the start of the initial processing to the initial processing time correction amount according to the predicted residual oxygen amount. This is the part that is corrected to the longer side. Here, the target initial processing time mτ is calculated by the following equation.

mτ = τ0 + τHOS (6)
Where τ0: basic initial processing time,
τHOS: initial processing time correction amount,
That is, a value obtained by correcting the basic initial processing time τ0 to be longer by adding the initial processing time correction amount τHOS to the basic initial processing time τ0 is set as the target initial processing time mτ.

  The initial processing time correction amount τHOS on the right side of the equation (6) is calculated by the following equation.

τHOS = τHOS0 + IntHOS2 + TlntHOS2 + SVHOS2
... (7)
However, τHOS0: Initial processing time correction amount basic value,
IntHOS2: interval correction amount,
T1ntHOS2: catalyst temperature correction amount,
SVHOS2: Space velocity correction amount,
Hereinafter, each correction amount will be specifically described. First, in step 9, the difference Δλ between the front lambda detected by the front wide area air-fuel ratio sensor 24 and the rear lambda detected by the rear wide area air-fuel ratio sensor 25 is calculated. The values of the front lambda and the rear lambda at this time are those when a predetermined time Δt1 has elapsed from the initial processing start timing. Here, since the value of the rear lambda is larger than the value of the front lambda, a value obtained by subtracting the front lambda from the rear lambda is defined as a difference Δλ. The difference Δλ is a value for predicting the residual oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15. When the difference Δλ is relatively small, it is predicted that the residual oxygen amount in the oxidation catalyst 14 and the NOx trap catalyst 15 is larger than when the difference Δλ is relatively large.

  In step 41, an initial processing time correction amount basic value τHOS0 is calculated by searching a table having the contents shown in FIG. 14 from the difference Δλ. As shown in FIG. 14, the initial processing time correction amount basic value τHOS0 is zero when the difference Δλ exceeds the predetermined value Δλ0, and increases as the difference Δλ decreases when the difference Δλ is equal to or smaller than the predetermined value Δλ0. is there. The difference Δλ, which is a value for predicting the remaining oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15, is relatively large when this value is relatively small in the region where the difference Δλ is equal to or less than the predetermined value Δλ0. It represents that there is relatively more residual oxygen. The reason why τHOS0 is made larger when there is load variation (Δλ is relatively small) than when there is no load variation (Δλ is relatively large) is as follows. That is, when there is a load variation (Δλ is relatively large), there is more residual oxygen in the catalysts 14 and 15 than when there is no load variation (relatively large Δλ), and the longer the remaining oxygen, the longer it becomes. This is because the correction amount needs to be increased.

  Further, the predetermined value Δλ0 on the horizontal axis is obtained in advance by adaptation as a difference Δλ when there is no load fluctuation. When the difference Δλ at that time is equal to or larger than the predetermined value Δλ, no residual oxygen is generated, and therefore there is no need to correct the basic initial processing time τ0 to be longer, so the difference Δλ is equal to or larger than the predetermined value Δλ0. In the region, the initial processing time correction amount basic value τHOS0 is set to zero.

  In step 42, an interval correction amount IntHOS2 is calculated by searching a table having the contents shown in FIG. 15 from the interval Int of the rich spike processing. Here, the “rich spike processing interval” is a period from the end of one rich spike processing to the start of the next rich spike processing, as described above. The predetermined value Int0 on the horizontal axis is an interval Int when the initial processing time correction amount basic value τHOS0 in FIG. 14 is adapted. As shown in FIG. 15, the interval correction amount IntHOS2 has a positive value when the interval Int is larger than the predetermined value Int0, and has a negative value when the interval Int is smaller than the predetermined value Int0. This is because it is considered that the residual oxygen is larger in the region where the interval Int of the rich spike process is larger than the predetermined value Int0 than that at the time of adaptation, and the residual oxygen is smaller in the region where the interval Int is smaller than the predetermined value Int0 than that at the time of adaptation.

  In step 43, the catalyst temperature correction amount T1ntHOS2 is calculated by searching a table having the contents shown in FIG. 16 from the catalyst temperature T1nt of the NOx trap catalyst 15. The catalyst temperature Tlnt is detected by the catalyst temperature sensor 26 as described above. The predetermined value Tlnt0 on the horizontal axis is the catalyst temperature when the initial processing time correction amount basic value τHOS0 in FIG. 14 is adapted. As shown in FIG. 16, the catalyst temperature correction amount T1ntHOS2 has a positive value when the catalyst temperature T1nt is smaller than the predetermined value T1nt0, and has a negative value when the catalyst temperature T1nt is higher than the predetermined value T1nt0. This is because it is considered that the residual oxygen is more desorbed in the region where the catalyst temperature Tlnt is higher than the predetermined value Tlnt0 than when it is matched, and the residual oxygen is desorbed in the region smaller than the predetermined value Tlnt0 than when it is matched.

  In step 44, a space velocity correction amount SVHOS2 is calculated by searching a table having the contents shown in FIG. 17 from the exhaust space velocity SV. Here, the exhaust space velocity SV [1 / hour] is calculated by dividing the exhaust volume flow rate Qexh [l / hour] by the catalyst volume [l] of the NOx trap catalyst 15 as described above. A method for calculating the volume flow rate Qexh of the exhaust gas is known. As shown in FIG. 17, the space velocity correction amount SVHOS2 is a positive value when the exhaust space velocity SV is larger than the predetermined value SV0, and is a negative value when the exhaust space velocity SV is smaller than the predetermined value SV0. Become. This is because it is considered that the residual oxygen is larger in the region where the exhaust space velocity SV is larger than the predetermined value SV0 than when it is adapted, and the residual oxygen is smaller than that in the region where the exhaust space velocity SV is smaller than the predetermined value SV. The reason why the residual oxygen is smaller in the region where the space velocity SV of the exhaust gas is smaller than the predetermined value SV0 is smaller than that at the time of adaptation because the oxygen consumption is faster when the space velocity SV of the exhaust gas is smaller.

  In step 45, the initial processing time correction amount τHOS is calculated from the initial processing time correction amount basic value τHOS0, the interval correction amount IntHOS2, the catalyst temperature correction amount T1ntHOS2, and the space velocity correction amount SVHOS2 calculated as described above.

τλHOS = τλHOS0 + IntHOS2 + TlnntHOS2 + SVHOS2
... (8)
Expression (8) is the same as the above expression (7).

  In step 46, the target initial processing time mτ is calculated from the initial processing time correction amount τHOS and the basic initial processing time τ0 by the following equation.

mτ = τ0−τHOS (9)
Expression (9) is the same as the above expression (6). In step 17, the correction flag = 1 is set and the current process is terminated.

  Since the correction flag is set to 1 in step 17, the process proceeds from step 7 to step 18 in FIG.

  As described above, the third embodiment also has the same effects as the first embodiment. Furthermore, in the third embodiment, the front wide air-fuel ratio sensor 24 (first excess air) that detects the NOx trap catalyst 15 and the front lambda (the first excess air ratio that is the excess air ratio of the exhaust upstream of the NOx trap catalyst 15). Rate detection means), a rear wide air-fuel ratio sensor 25 (second air excess ratio detection means) for detecting rear lambda (second excess air ratio which is the excess air ratio of exhaust gas downstream of the NOx trap catalyst 15), and a NOx trap catalyst. The initial processing means for enriching the excess air ratio of the exhaust when the regeneration time reaches 15 (see steps 1 to 6, steps 1 to 3, 7, and 9 in FIG. 13A), and enriching the exhaust air excess ratio The late processing means (see Steps 18, 20 to 22, Steps 23 and 24 in FIG. 13B) that makes the exhaust air excess ratio close to 1.0 The remaining oxygen amount predicting means for predicting the remaining oxygen amount of the NOx trap catalyst 15 based on the relative values of the rear lambda and the front lambda during the operation (see steps 3, 7, 8, and 10 in FIG. 13A), and the remaining oxygen amount prediction The enrichment period control means (steps 3, 7, 8, 10 in FIG. 13A) for controlling the initial processing time correction amount basic value τHOS0 (the enrichment period during the initial process) based on the residual oxygen amount predicted by the means. 41, 45, 46, 17 and steps 18 and 19 in FIG. 13B). According to the present embodiment, because the front lambda cannot reach the basic excess air ratio λ0 calculated at the start of the initial process due to the load fluctuation, oxygen remains in the NOx trap catalyst 15 at the later stage as it is. Even in such a case, the residual oxygen of the NOx trap catalyst 15 can be consumed by controlling the initial processing time correction amount basic value τHOS0 (the enrichment period during the initial processing), and the deterioration of the NOx purification rate is suppressed. can do.

  According to the third embodiment, the enrichment period control means lengthens the initial processing time correction amount basic value τHOS0 (the enrichment period during the initial process) as the difference Δλ is smaller (see FIG. 14), so the NOx trap. Regardless of the amount of residual oxygen in the catalyst 15, the residual oxygen in the NOx trap catalyst 15 can be consumed.

  According to the third embodiment, the initial processing time correction amount basic value τHOS0 (the enrichment period) is corrected by at least one of the rich spike process interval Int, the catalyst temperature Tlnt of the NOx catalyst 15, and the exhaust space velocity SV. Thus, the rich spike processing interval Int, the NOx trap catalyst catalyst temperature Tlnt, and the exhaust space velocity SV satisfy the initial processing time correction amount basic value τHOS0. Even if there is a difference from the spike processing interval Int, the catalyst temperature Tlnt of the NOx trap catalyst 15, and the exhaust space velocity SV, the initial processing time correction amount τHOS (the degree of enrichment during the initial processing) can be optimally provided.

  In the embodiment, the case where the oxidation catalyst and the NOx trap catalyst are separate members has been described. However, an oxidation catalyst may be included in the NOx trap catalyst.

  In the embodiment, the case where only the common rail type fuel injection device is provided has been described. However, the present invention may be applied to a device including a fuel addition device that supplies fuel to the exhaust passage upstream of the oxidation catalyst.

  In the first embodiment, the case has been described where the rear lambda crosses 1.0 and reverses to the rich side to shift from the initial process to the later process. However, the present invention is not limited to this case. For example, when the rear lambda crosses 1.05 and reverses to the rich side, the process shifts to the later process from the initial process, or when the rear lambda crosses 0.95 to the rich side, shifts to the later process from the initial process. This also applies to Thus, the reason for shifting the switching timing to 1.05 or 0.95 is that there is a request to do so.

DESCRIPTION OF SYMBOLS 1 Engine 5 Throttle valve 9 Fuel injection valve 15 NOx trap catalyst 21 Engine controller 24 Front wide area air-fuel ratio sensor (1st excess air ratio detection means)
25 Rear wide area air-fuel ratio sensor (second excess air ratio detection means)

Claims (6)

A NOx trap catalyst that traps NOx in exhaust in an oxygen atmosphere, desorbs NOx trapped in a reducing atmosphere, and reduces and purifies using HC in exhaust as a reducing agent;
First excess air ratio detecting means for detecting a first excess air ratio that is an excess air ratio of the exhaust upstream of the NOx trap catalyst;
A second excess air ratio detecting means for detecting a second excess air ratio that is an excess air ratio of exhaust downstream of the NOx trap catalyst;
Initial processing means for enriching the excess air ratio of exhaust during initial processing when the NOx trap catalyst regeneration time comes,
Late processing means for setting the target excess air ratio of exhaust during the later processing after enriching the excess air ratio of the exhaust gas to a second basic excess air ratio in the vicinity of 1.0;
The relative ratio between the second excess air ratio detected by the second excess air ratio detection means and the first excess air ratio detected by the first excess air ratio detection means while enriching the excess air ratio of the exhaust gas. A residual oxygen amount predicting means for predicting a residual oxygen amount of the NOx trap catalyst according to a value;
Enrichment degree / riching period control means for controlling the degree of enrichment during the initial process or the period of enrichment during the initial process based on the residual oxygen quantity predicted by the residual oxygen quantity predicting means. An exhaust aftertreatment device for a diesel engine.   The relative value of the detected second excess air ratio and the detected first excess air ratio is a difference between the two, and according to this difference, it is predicted that the smaller the difference, the greater the residual oxygen amount of the NOx trap catalyst. The exhaust aftertreatment device for a diesel engine according to claim 1.   The enrichment degree / enrichment period control means increases the enrichment degree during the initial process or lengthens the enrichment period during the initial process as the difference is smaller. 2. An exhaust aftertreatment device for a diesel engine according to 2.   The degree of enrichment during the initial process or the period of enrichment during the initial process is corrected by at least one of the interval of the rich spike process, the catalyst temperature of the NOx catalyst, and the space velocity of the exhaust gas. The exhaust aftertreatment device for a diesel engine according to any one of claims 1 to 3.   The late processing means integrates a difference between the detected second excess air ratio and the detected first excess air ratio, and when the integrated value of the difference exceeds a threshold value, the exhaust target The exhaust aftertreatment device for a diesel engine according to any one of claims 1 to 4, wherein the excess air ratio is set to the second basic excess air ratio.   The integration of the difference is started when the detected first excess air ratio becomes 1.0 or less and the differential value of the detected first excess air ratio becomes less than a predetermined threshold value. The exhaust aftertreatment device for a diesel engine according to claim 5,

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