JP2014118915A - 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 is varied immediately after the start of rich spike treatment.SOLUTION: The exhaust post-treatment device of a diesel engine comprises: means (21) for setting an air excess rate of exhaust in initial treatment to a rich-side first fundamental air excess rate when a regeneration time of a NOx trap catalyst (15) comes; means (21) for setting an air excess rate of exhaust in post-treatment to a second fundamental air excess rate in the vicinity of 1.0 when the air excess rate of the exhaust is set to the first fundamental air excess rate; means (21) for predicting a remaining oxygen amount of the NOx trap catalyst by using a change speed of the second air excess rate when the second air excess rate detected by second air excess rate detection means crosses the vicinity of 1.0 from a lean side while the air excess rate of the exhaust is set to the first fundamental air excess rate; and means (21) for controlling a supply amount of a reducer in the post-treatment on the basis of the remaining oxygen amount which is predicted by the remaining oxygen amount prediction means.

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). In this case, 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 has disappeared when the rear lambda is reversed to rich. 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; and an initial processing means for setting the excess air ratio of the exhaust during the initial processing to a rich first basic excess air ratio when the regeneration timing of the NOx trap catalyst comes. And late processing means for setting the excess air ratio of the exhaust during the later processing to the second basic excess air ratio in the vicinity of 1.0 after the excess air ratio of the exhaust gas is set to the first basic excess air ratio. There. 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 detecting means while the excess air ratio of the exhaust gas is set to the first basic excess air ratio λ0. Is predicted by the residual oxygen amount predicting means for predicting the residual oxygen amount of the NOx trap catalyst and the residual oxygen amount predicting means based on the rate of change of the second excess air ratio when crossing the vicinity of 1.0 from the lean side. Reducing agent supply amount control means for controlling the supply amount of the reducing agent during the latter stage treatment based on the residual oxygen amount.

  According to the present invention, since the first excess air ratio cannot reach the first basic excess air ratio due to load fluctuation, even if oxygen remains in the NOx trap catalyst as it is, By controlling the supply amount of the reducing agent, it is possible to consume the residual oxygen of the NOx trap catalyst, 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 characteristic view of the residual oxygen amount of the oxidation catalyst and the NOx trap catalyst with respect to the inclination of the rear lambda when crossing 1.0 from the lean side. 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 figure of the increment per control cycle of the target excess air ratio with respect to the inclination of a rear lambda when crossing 1.0 from the lean side of a 1st 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 characteristic view of the air excess ratio correction amount with respect to the inclination of the rear lambda when crossing 1.0 from the lean side 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 characteristic view of the threshold with respect to the difference of 3rd 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 the case where the front lambda did not reach 0.9 in the initial process due to load fluctuations. The reason why the front lambda cannot reach 0.9, which is the first basic excess air ratio λ0, is the load fluctuation immediately after the start of the rich spike process. 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 process cannot be reached to 0.9 and ends. 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 later than in the case of FIG. In FIG. 3, the rear lambda becomes 1.0 at the timing of t11 and the process is shifted to the later process. However, the rear lambda continues to be on the lean side after t11, and again at the timing of t12 before t13 when the latter process is finished. Crosses zero. 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 from the inclination (change rate) when the rear lambda crosses 1.0 from the lean side. Then, the transition speed from the first basic excess air ratio λ0 to the second basic excess air ratio λ1 is controlled based on the predicted residual oxygen amount.

  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 of t1 when the initial process is started, the front lambda (actual value) during the initial process is determined from the lean 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.

  Here, the first embodiment of the present invention is not limited to the case where the rear lambda crosses 1.0 from the lean side and reverses to the rich side. 1.0 for comparison with rear lambda is a threshold value. The threshold value is not limited to 1.0 and can be arbitrarily set within the range of 1.05 to 0.95. For example, when the rear lambda crosses the threshold value of 1.05 from the lean side, it is judged that the rear lambda has reversed to the rich side, or when the rear lambda crosses the threshold value of 0.95 from the lean side, it is rich. Some of them are judged to be reversed. Since the present invention is also applied to these cases, the expression “the rear lambda crosses the vicinity of 1.0 from the lean side” is used to include these cases. The reason for shifting the threshold value from 1.0 to 1.05 on the lean side and 0.95 on the rich side from 1.0 as described above is an exhaust countermeasure. Hereinafter, in the first embodiment, a case where the threshold value is 1.0 will be described.

  Next, the second basic excess air ratio λ1 is set 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 second basic excess air ratio λ1 is set to a value slightly smaller than 1.0.

  The slope of the rear lambda dλ / dt (change rate of the second excess air ratio) when crossing 1.0 from the lean side at the end of the initial process is a value for predicting the residual oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15. It becomes. For example, as shown in FIG. 2, when there is no load variation, the rear lambda inclination dλ / dt when crossing 1.0 from the lean side is relatively large, and when there is a load variation as shown in FIG. The inclination of the rear lambda dλ / dt when crossing 1.0 is relatively small. This is represented by a model in the second row of FIG. According to the rear lambda when there is no load fluctuation indicated by the solid line in the second row in FIG. 4, the slope dλ / dt of the rear lambda when crossing 1.0 from the lean side is relatively large. On the other hand, according to the rear lambda when there is a load fluctuation indicated by a broken line in the second stage of FIG. 4, the inclination dλ / dt of the rear lambda when crossing 1.0 from the lean side is relatively small. Here, since the slope dλ / dt of the rear lambda when crossing 1.0 from the lean side is negative and difficult to handle, it is handled as an absolute value.

  FIG. 2, FIG. 3, and FIG. 4 summarize the relationship between the slope dλ / dt of the rear lambda when crossing 1.0 from the lean side and the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15. 5. That is, as shown in FIG. 5, when the rear lambda inclination dλ / dt when crossing 1.0 from the lean side is relatively large, it is predicted that the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15 are relatively small. it can. Further, when the rear lambda inclination dλ / dt crossing 1.0 from the lean side is relatively small, it can be predicted that the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15 are relatively large.

  Here, the slope of the rear lambda dλ / dt when crossing 1.0 from the lean side can be calculated from the data of the rear lambda sampled at a predetermined period and the sampling value.

  As described above, when the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15 are predicted by dλ / dt, the oxidation catalyst 14 is larger when dλ / dt is relatively smaller than when dλ / dt is relatively large. And the amount of residual oxygen in the NOx trap catalyst 15 is large. Therefore, when there is a load fluctuation (dλ / dt is relatively small), the first basic excess air ratio λ0 changes to the second basic excess air ratio λ2 than when there is no load fluctuation (dλ / dt is relatively large). What is necessary is just to control so that the transition speed of may become slow.

  For example, as shown by the solid line in the first stage of FIG. 4, when there is no load variation (dλ / dt is relatively large), the transition from the first basic excess air ratio λ0 to the second basic excess air ratio λ1. The speed is set relatively fast. On the other hand, when the load fluctuates (dλ / dt is relatively small) as shown by the dashed line in the first row in FIG. 4, the first basic excess air ratio λ0 changes to the second basic excess air ratio λ1. The transition speed is set to be relatively slow. As a result, an area difference indicated by hatching is generated between the target excess air ratio when there is a load fluctuation and the excess air ratio when there is no load fluctuation (see the first row in FIG. 4). In other words, when there is a load change, it is possible to supply the reducing agent by the amount corresponding to the area difference during the latter process, compared to when there is no load change. By increasing the supply amount of the reducing agent during the latter stage treatment, the residual oxygen of the oxidation catalyst and the NOx trap catalyst can be consumed.

  The third to sixth stages in FIG. 4 show the movements of the initial process flag and the late process flag introduced in the flowchart of FIG. 6 to be described later. Here, the third stage and the fourth stage are the initial processing flag and the latter stage processing flag when there is no load fluctuation, and the fifth stage and the sixth stage are the initial processing flag and the latter stage processing when there is a load fluctuation. Each movement of the flag.

  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. The wide area air-fuel ratio sensor detects the air-fuel ratio of exhaust gas linearly.

  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 flowchart of FIG.

  The flow in FIG. 6 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 → lag (initially set to zero when the engine is started) is observed. Here, it is assumed that the late process flag = 0, and the process proceeds to step 3. In step 3, an initial processing flag (initially set to zero when the engine is started) is checked. 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 first 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 first basic excess air ratio λ0 may be given as a map value corresponding to, for example, operating conditions (engine load and rotation speed). In order to consume the residual oxygen of the oxidation catalyst and the NOx trap catalyst, the first basic air excess ratio λ0 is set to a value smaller than 1.0. For simplicity, a constant value (for example, 0.9) may be used. In step 6, the initial processing flag = 1 is set and the current processing is terminated.

  Since the initial process flag is set to 1 in step 6, the process proceeds from step 2 to step 7 and subsequent steps next time. In step 7, the rear lambda is compared with 1.0 as a threshold value. When the rear lambda is larger than 1.0, it is determined that the rear lambda is not reversed from the lean side to the rich side, and the process proceeds to step 8. In step 8, the value of “mλ (previous)”, which is the target excess air ratio of the previous exhaust, is directly transferred to the target excess air ratio mλ of the current exhaust, thereby calculating the first basic air excess ratio calculated in step 4. Maintain λ0.

  When the rear lambda becomes 1.0 or less in step 7, it is determined that the rear lambda has reversed from the lean side to the rich side, and the process proceeds to steps 9 and 10 to proceed to the later stage processing. In steps 9 and 10, the initial process flag = 0 and the late process flag = 1.

  In step 11, as in step 8, the value of “mλ (previous)” that is the target excess air ratio of the previous exhaust is transferred to the target air excess ratio mλ of the current exhaust as it is. 1 Maintain the basic excess air ratio λ0.

  In step 12, the first basic excess air ratio λ0 is added to “mλ (previous)” which is the previous value of mλ.

  Since the initial process flag = 0 and the late process flag = 1 in steps 9 and 10, the process proceeds from step 2 to step 13 onward next time.

  In step 13, it is checked whether or not a predetermined time Δt2 has elapsed since the start of the rich spike process. If the predetermined time Δt2 has not elapsed since the start of the rich spike process, the process proceeds to step 14, and based on the data of the rear lambda detected by the wide area air-fuel ratio sensor 25, the slope of the rear lambda when crossing 1.0 from the lean side dλ / dt is calculated. Since the slope of the rear lambda dλ / dt when crossing 1.0 from the lean side is calculated as a negative value, the absolute value is taken and converted to a positive value. The rear lambda slope dλ / dt converted to a positive value indicates that the amount of residual oxygen in the oxidation catalyst 14 and the NOx trap catalyst 15 is greater when this value is relatively small than when this value is relatively large. Represent.

  In step 15, the table of FIG. 7 is searched for from the slope dλ / dt (absolute value) of the rear lambda when crossing 1.0 from the lean side calculated in this way, thereby determining the target excess air ratio. An increment Δλ per control cycle is calculated. As shown in FIG. 7, when Δλ crosses 1.0 from the lean side, the slope dλ / dt (absolute value) of the rear lambda is relatively small when 1.0 is crossed from the lean side. The rear lambda inclination dλ / dt (absolute value) is a value that is smaller than when it is relatively large.

  In step 16, the target excess air ratio mλ during the later stage process is calculated by the following equation using the increment Δλ per control cycle of the target excess air ratio.

mλ = mλ (previous) + Δλ (1)
Where mλ (previous): previous value of mλ,
“mλ (previous)”, which is the previous value of mλ, includes the first basic excess air ratio λ0 as an initial value (see step 12). In the equation (1), a value obtained by adding an increment Δλ per control period to “mλ (previous)” that is the previous value of mλ is the current mλ. That is, by repeating the calculation of equation (1) at a constant cycle, the target excess air ratio mλ during the latter process increases by an increment Δλ.

  Here, when there is no load fluctuation (dλ / dt is relatively large), the above increase Δλ is larger than when there is load fluctuation (dλ / dt is relatively small), so the target excess air ratio mλ is It approaches λ1 more quickly than λ0 (see the solid line in the first row in FIG. 4). That is, the transition speed from the first basic excess air ratio λ0 to the second basic excess air ratio λ1 becomes relatively high. On the other hand, when there is load fluctuation (dλ / dt is relatively small), the above increase Δλ is smaller than when there is no load fluctuation (dλ / dt is relatively large), so the target excess air ratio mλ is λ0. It approaches λ1 more slowly (see the broken line in the first row in FIG. 4). That is, the transition speed from the first basic excess air ratio λ0 to the second basic excess air ratio λ1 is relatively slow. Thus, the transition speed from the first basic excess air ratio λ0 to the second basic excess air ratio λ1 is controlled by the presence or absence of load fluctuation (dλ / dt).

  In step 17, the target excess air ratio mλ during the later process calculated in this way is compared with the second basic excess air ratio 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. When the target excess air ratio mλ during the latter process is less than λ1, it is determined that the target excess air ratio mλ during the latter process has not yet reached the second basic excess air ratio λ1, and the current process is terminated.

  Eventually, when the target excess air ratio mλ in the later stage processing reaches the second basic excess air ratio λ1 in Step 17, the process proceeds to Step 18 and the second fundamental excess air ratio λ1 is set to the target excess air ratio mλ in the later stage processing. In this manner, the target excess air ratio mλ during the latter process is larger than the first basic excess air ratio λ0 by an increment Δλ per control cycle, and after reaching the second basic excess air ratio λ1, the second basic excess air ratio is reached. Maintain λ1.

  In a state where the target excess air ratio mλ during the second stage process maintains the second basic excess air ratio λ1, the predetermined time Δt2 eventually elapses from the start of the rich spike process. At this time, the process proceeds from step 13 to steps 19, 20, and 21 to end the rich spike processing. In step 19, a value (lean) greater 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 20 and 21, the late process flag = 0 and the regeneration permission flag = 0. Since the reproduction permission flag is set to 0 in step 21, it is not possible to proceed from step 1 to step 2 onward next time.

  In a 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 of FIG. 6 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 When the time comes, initial processing means for setting the exhaust excess air ratio during the initial processing to the first basic excess air ratio λ0 on the rich side (see steps 1 to 6, steps 1 to 3, 7, and 8 in FIG. 6). Then, after the exhaust air excess ratio is changed to the first basic air excess ratio λ0, the exhaust air excess ratio during the latter processing is changed to the second basic air excess ratio λ1 in the vicinity of 1.0 (step of FIG. 6). 2, , 7, 9-12, steps 2, 13, 14-18) and the rear lambda when the rear lambda crosses 1.0 from the lean side while the exhaust air excess ratio is set to the first basic excess air ratio λ0. By the residual oxygen amount predicting means (see steps 2, 13, and 14 in FIG. 6) and the residual oxygen amount predicting means for predicting the residual oxygen amount of the NOx trap catalyst 15 based on the slope of (the change rate of the second excess air ratio). Reducing agent supply amount control means (see Steps 2, 13, 14, and 15 to 18 in FIG. 6) for controlling the supply amount of the reducing agent during the later stage treatment based on the predicted residual oxygen amount. According to the present embodiment, since the front lambda cannot reach the first basic excess air ratio λ0 due to load fluctuations, even if oxygen remains in the NOx trap catalyst 15 during the latter stage treatment, By controlling the supply amount of the reducing agent during the later stage treatment, it becomes possible to increase the supply amount of the reducing agent during the later stage treatment, and to consume the residual oxygen of the NOx trap catalyst 15 with this increased reducing agent, thereby deteriorating the NOx purification rate. Can be suppressed.

  According to this embodiment, the reducing agent supply amount control means is based on the rear lambda inclination dλ / dt (the residual oxygen amount predicted by the residual oxygen amount prediction means) when crossing 1.0 from the lean side. Since the transition speed control means controls the transition speed from the first basic excess air ratio λ0 to the second basic excess air ratio λ1 (see steps 2, 13, 14, 15-18 in FIG. 6), the first basic air excess By controlling the transition speed from the rate λ0 to the second basic excess air rate λ1, it is possible to increase the supply amount of the reducing agent during the latter stage treatment and consume the residual oxygen of the NOx trap catalyst 15, thereby deteriorating the NOx purification rate. Can be suppressed.

  According to this embodiment, the residual oxygen amount predicting means has a relative dram / dt (second air excess rate) gradient dλ / dt (change rate of the second air excess rate) relative to 1.0 from the lean side. Therefore, it is predicted that the amount of residual oxygen in the NOx trap catalyst 15 is larger than when the slope of the rear lambda dλ / dt (change rate of the second excess air ratio) when crossing 1.0 from the lean side is relatively large. Therefore (see FIG. 7), the residual oxygen amount of the NOx trap catalyst 15 can be easily predicted.

  According to this embodiment, the transition speed control means is 1.0 from the lean side when the slope dλ / dt of the rear lambda (second excess air ratio) when crossing the vicinity of 1.0 from the lean side is relatively small. Since the transition speed is slower than when the slope dλ / dt of the rear lambda when crossing the neighborhood is relatively large (see steps 14, 15, 16, and FIG. 7 in FIG. 6), there is a load fluctuation, and NOx during the initial processing Even in the case where oxygen remains in the trap catalyst 15, the supply amount of the reducing agent during the latter stage treatment is increased due to the delay of the transition speed, so that the residual oxygen in the NOx trap catalyst 15 is consumed. Thus, a reducing atmosphere necessary for NOx purification can be obtained.

(Second Embodiment)
FIG. 8 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 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. 8, 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. 8, 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 transition speed from the first basic excess air ratio λ0 to the second basic excess air ratio λ1 is controlled based on the rear lambda inclination dλ / dt when the rear lambda crosses 1.0 from the lean side. On the other hand, in the second embodiment, the target excess air ratio of the exhaust gas during the latter-stage process is controlled based on the slope dλ / dt of the rear lambda when the rear lambda crosses 1.0 from the lean side.

  More specifically, as indicated by the solid line in the first stage of FIG. 8, when there is no load fluctuation (dλ / dt is relatively large), the second basic air is set as the target excess air ratio mλ of the exhaust during the latter process. An excess rate λ1 is set. On the other hand, when the load fluctuates (dλ / dt is relatively small) as shown by the one-dot chain line in the first row in FIG. 8, the second basic excess air ratio λ1 becomes smaller (rich side). The value corrected to is set as the target excess air ratio mλ of the exhaust gas during the latter process. As a result, an area difference indicated by hatching occurs between the target excess air ratio when there is load fluctuation and the target excess air ratio when there is no load fluctuation (see the first row in FIG. 8). 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 FIG. 9 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 number is attached | subjected to the part same as FIG. 6 of 1st Embodiment.

  The difference from FIG. 6 of the first embodiment is the position of step 14 and steps 31 to 33. The difference from the first embodiment will be mainly described. When the rear lambda becomes 1.0 or less in step 7 in FIG. 9, it is determined that the rear lambda has crossed 1.0 from the lean side to the rich side, Proceed to steps 14, 31, 32, 9, and 10 to proceed to later processing.

  In step 14, the rear lambda inclination dλ / dt when the rear lambda crosses 1.0 from the lean side is calculated based on the rear lambda data detected by the wide area air-fuel ratio sensor 25. Since the slope of the rear lambda dλ / dt when crossing 1.0 from the lean side is calculated as a negative value, the absolute value is taken and converted to a positive value. The slope dλ / dt converted to a positive value indicates that the residual oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15 is larger when the value is relatively small than when the value is relatively large.

  In step 31, the air excess rate correction amount is obtained by searching a table having the contents shown in FIG. 10 from the slope dλ / dt (absolute value) of the rear lambda when crossing 1.0 from the lean side thus calculated. λHOS is calculated. As shown in FIG. 10, the excess air ratio correction amount λHOS is larger when dλ / dt (absolute value) is relatively smaller than when dλ / dt (absolute value) is relatively large.

  In step 32, the target excess air ratio mλ during the latter process is calculated by the following equation using the excess air ratio correction amount λHOS.

mλ = λ1-λHOS (2)
In the equation (2), a value (rich side value) smaller than the second basic excess air ratio λ1 by the excess air ratio correction amount λHOS is set as the target excess air ratio mλ of the exhaust gas in the later stage processing.

  When there is no load fluctuation (dλ / dt is relatively large), the above-described excess air ratio correction amount λHOS is smaller and almost zero than when there is load fluctuation (dλ / dt is relatively small). At this time, the target excess air ratio mλ during the latter process is a value close to λ1 (see the solid line in the first stage in FIG. 8). On the other hand, when there is a load fluctuation (dλ / dt is relatively small), the excess air ratio correction amount λHOS is larger than when there is no load fluctuation (dλ / dt is relatively large). At this time, the target excess air ratio mλ during the latter process takes a value smaller than λ1 (see the one-dot chain line in the first row in FIG. 8). In this manner, the target excess air ratio during the later stage processing is controlled by the presence or absence of load fluctuation (dλ / dt).

  In Steps 9 and 10, since the process is advanced to the later process, the initial process flag = 0 and the later process flag = 1.

  Since the initial process flag = 0 and the late process flag = 1 in steps 9 and 10, the process proceeds from step 2 to step 13 onward next time. In step 13, it is checked whether or not a predetermined time Δt2 has elapsed since the start of the rich spike process. If the predetermined time Δt2 has not elapsed since the start of the rich spike processing, the routine proceeds to step 33. In step 33, as in step 8, 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, thereby calculating the exhaust calculated in step 32. The target excess air ratio mλ is maintained.

  When a predetermined time Δt2 has elapsed from the start of the rich spike process in step 13, the process proceeds to steps 19, 20, and 21 to end the rich spike process, and these operations are executed as in the first embodiment.

  Thus, also by 2nd Embodiment, there exists an effect similar to 1st Embodiment. Furthermore, according to the second embodiment, the reducing agent supply amount control means is based on the rear lambda slope dλ / dt (the residual oxygen amount predicted by the residual oxygen amount prediction means) when crossing 1.0 from the lean side. Therefore, the excess air ratio control means for controlling the excess air ratio of the exhaust gas during the late process (see Steps 2, 7, 14, 31, 32 in FIG. 9). It is possible to increase the supply amount of the reducing agent during the latter stage processing and consume the residual oxygen of the NOx trap catalyst 15, and to suppress the deterioration of the NOx purification rate.

  According to the second embodiment, the excess air ratio control means has a relative dram / dt (second air excess ratio change rate) slope of rear lambda (second excess air ratio) when crossing 1.0 from the lean side. When it is small, the rear air lambda inclination dλ / dt (the rate of change of the second excess air ratio) crossing 1.0 from the lean side is relatively smaller than when the exhaust air excess ratio mλ during the latter-stage treatment is smaller than when the rear lambda gradient dλ / dt is relatively large. (Refer to steps 14, 31, 32, and FIG. 10 in FIG. 9), the load fluctuates, and oxygen remains in the NOx trap catalyst 15 during the initial processing. Even in this case, since the supply amount of the reducing agent during the later stage treatment is increased by the enrichment of the excess air ratio during the later stage treatment, the remaining oxygen in the NOx trap catalyst 15 is consumed and the reducing atmosphere necessary for NOx purification is consumed. To do It can be.

(Third embodiment)
FIG. 11 is a model diagram of the third embodiment showing how the front lambda and the rear lambda, which are the exhaust target air excess 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. 11, 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 two-dot chain line. The second row of FIG. 11 shows the change of the front lambda and the rear lambda when the catalyst is new and there is no load fluctuation as a solid line, and the change of the rear lambda when the catalyst is new and there is a load fluctuation is shown by a broken line. ing.

  In the first embodiment, the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15 are predicted based on the inclination of the rear lambda dλ / dt when the rear lambda crosses 1.0 from the lean side. On the other hand, in the third embodiment, the slope dλ / dt of the rear lambda is calculated during the initial process, and the residual oxygen amounts of the oxidation catalyst 14 and the NOx trap catalyst 15 are predicted based on the slope of the rear lambda dλ / dt. Then, the threshold value for comparison with the rear lambda is controlled based on the predicted residual oxygen amount.

  More specifically, as shown by the one-dot chain line in the second row of FIG. 11, when there is no load fluctuation (dλ / dt during the initial processing is relatively large), the threshold value for comparison with the rear lambda is, for example, Set to 1.05. At this time, since the rear lambda is compared with 1.05, it is determined that the rear lambda has reversed from the lean side to the rich side at the timing of t31 crossing 1.05 from the lean side. Then, as indicated by the solid line in the first stage of FIG. 11, the target excess air ratio mλ is changed from the first basic excess air ratio λ0 to the second basic excess air ratio at the timing t31 when the rear lambda crosses 1.05 from the lean side. It is switched to λ1.

  On the other hand, when there is a load variation (dλ / dt during the initial processing is relatively small) as indicated by the two-dot chain line in the second row in FIG. 11, the threshold value for comparison with the rear lambda is set. For example, 1.0 is corrected to a smaller side (rich side) than when there is no load fluctuation. At this time, since the rear lambda is compared with 1.0, it is determined that the rear lambda has reversed from the lean side to the rich side at the timing of t32 when crossing 1.0 from the lean side. Here, t32 is a timing delayed from the timing of t31. Then, as indicated by the two-dot chain line in the first row in FIG. 11, the target excess air ratio mλ is changed from the first basic excess air ratio λ0 to the second at the timing t32 when the rear lambda crosses 1.0 from the lean side. It is switched to the basic excess air ratio λ1. As a result, an area difference indicated by hatching is generated between the target excess air ratio when there is a load fluctuation and the excess air ratio when there is no load fluctuation (see the first row in FIG. 11). 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.

  In FIG. 11, the case where the threshold value is switched from 1.05 (lean side value) to 1.0 (theoretical air-fuel ratio) has been described. However, the present invention is not limited to this case. For example, when the threshold is switched from 1.0 to 0.95 (value on the rich side) or when the threshold is switched from 1.05 (value on the lean side) to 0.95 (value on the rich side) It does not matter. In short, when there is a load change, the threshold value may be switched to a smaller side (rich side) than when there is no load change.

  The third to eighth stages in FIG. 11 show the movement of the initial process flag and the late process flag introduced in the flowchart of FIG. 12 to be described later. Here, the third stage, the fourth stage, and the fifth stage are the initial process flag and the late stage process flag when there is no load fluctuation, and the sixth stage, the seventh stage, and the eighth stage are the load fluctuation. These are the movements of the initial processing flag and the late processing flag when there is.

  The flow of FIG. 12 is for calculating the target excess air ratio mλ of exhaust during the rich spike processing of the third embodiment, and is executed at regular intervals (for example, every 10 ms). The same step number is attached | subjected to the part same as FIG. 9 of 2nd Embodiment.

  The difference from FIG. 9 of the second embodiment is steps 41 to 44 in FIG. The difference from the second embodiment will be mainly described. In FIG. 12, when the initial flag = 1 in step 3, the process proceeds to step 41, and the rear lambda data is detected based on the data of the rear lambda detected by the wide area air-fuel ratio sensor 25. The slope dλ / dt is calculated. Since the rear lambda slope dλ / dt is calculated as a negative value, the absolute value is taken and converted to a positive value. The slope dλ / dt converted to a positive value indicates that the residual oxygen amount of the oxidation catalyst 14 and the NOx trap catalyst 15 is larger when the value is relatively small than when the value is relatively large.

  In step 41, a threshold value for comparison with the rear lambda is calculated by searching a table having the contents shown in FIG. 13 from the slope of the rear lambda dλ / dt. As shown in FIG. 13, the threshold value is a characteristic toward the smaller side (rich side) when the rear lambda inclination dλ / dt is relatively small than when the rear lambda inclination dλ / dt is relatively large. is there.

  When there is no load fluctuation (dλ / dt is relatively large), the above threshold is larger than when there is load fluctuation (dλ / dt is relatively small), so the rear lambda sets the threshold from the lean side. The timing of crossing is relatively early (see the one-dot chain line in the second row in FIG. 11). When there is a load fluctuation (dλ / dt is relatively small), the threshold value is smaller than when there is no load fluctuation (dλ / dt is relatively large), so the rear lambda sets the threshold value from the lean side. The timing of crossing is relatively late (see the two-dot chain line in the second row in FIG. 11). In this way, the threshold value is controlled by the presence or absence of load fluctuation (dλ / dt during the initial process).

  In step 43, the rear lambda detected by the rear wide area air-fuel ratio sensor 25 is compared with the threshold value. When the rear lambda exceeds the threshold value, the routine proceeds to step 8, where the value of “mλ (previous)” that is the target excess air ratio of the previous exhaust is transferred to the target air excess ratio mλ of the current exhaust as it is. 1 Maintain the basic excess air ratio λ0.

  Eventually, when the rear lambda falls below the threshold value in step 43, it is determined that the rear lambda has reversed from the lean side to the rich side, and the process proceeds to the later stage process from the initial process to proceed to steps 9, 10, 44, where the initial flag = 0, Late processing flag = 1.

  In step 44, the second basic excess air ratio λ1 is set to the target excess air ratio mλ of exhaust. The second basic excess air ratio λ1 is a value for reducing and purifying all NOx accumulated in the NOx trap catalyst 15 when the oxidation catalyst 14 and the NOx trap catalyst 15 are new and there is no load fluctuation. . λ1 is a value in the vicinity of 1.0. Specifically, a value smaller than 1.0 is set as λ1 instead of 1.0. This is because HC and CO generated near 1.0 are oxidized with oxygen present in the exhaust.

  Since the initial flag = 0 and the late process flag = 1 in steps 9 and 10, the process proceeds from step 2 to step 13 onward next time. In step 13, it is checked whether or not a predetermined time Δt2 has elapsed since the start of the rich spike process. If the predetermined time Δt2 has not elapsed since the start of the rich spike process, the process proceeds to step 33, 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 moving, the second basic excess air ratio λ1 is maintained.

  Eventually, when a predetermined time Δt2 has elapsed since the start of the rich spike process in step 13, the process proceeds to steps 19, 20, and 21 to end the rich spike process, and the operations of steps 19 to 21 are executed as in the second embodiment. .

  Thus, also by 3rd Embodiment, there exists an effect similar to 1st Embodiment. Further, according to the third embodiment, the reducing agent supply amount control means is configured to perform rear lambda (during the second excess air ratio detection means) during the initial processing (while the excess air ratio of the exhaust is set to the first basic excess air ratio). A second residual oxygen amount predicting means (see steps 2, 3, and 41 in FIG. 12) for predicting the residual oxygen amount of the NOx trap catalyst 15 based on the change rate of the detected second excess air ratio) is provided. When the second excess air ratio detected by the second excess air ratio detecting means is reversed to the rich side across the threshold value from the lean side, the excess air ratio of the exhaust is changed from the first basic excess air ratio λ0 to the second basic value. Threshold control means for controlling the threshold value when switching to the excess air ratio λ1 based on the rear lambda change speed dλ / dt (the residual oxygen amount predicted by the second residual oxygen amount prediction means). So ( 12 step 2, 3, 41, 42, 43), it becomes possible to increase the supply amount of the reducing agent during the later stage treatment by controlling the threshold value, and to consume the residual oxygen of the NOx trap catalyst 15, NOx The deterioration of the purification rate can be suppressed.

  According to the third embodiment, the threshold control means is detected by the rear lambda (the second air excess ratio detection means) during the initial process (while the exhaust air excess ratio is set to the first basic air excess ratio). When the change rate dλ / dt of the second excess air ratio is relatively small, the threshold value is made smaller (rich) than when the change rate of the rear lambda during the initial processing is relatively large. Therefore (see steps 41 and 42 in FIG. 12, and FIG. 13), even if there is a load fluctuation and oxygen remains in the NOx trap catalyst 15 during the initial processing, Since the supply amount of the reducing agent during the latter stage treatment is increased, the residual oxygen in the NOx trap catalyst 15 can be consumed to make a reducing atmosphere necessary for NOx purification.

  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.

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 setting the exhaust excess air ratio during the initial processing to the first basic excess air ratio on the rich side when the regeneration timing of the NOx trap catalyst is reached;
Late processing means for setting the excess air ratio of the exhaust during the later processing after the exhaust air excess ratio to the first basic excess air ratio to a second basic air excess ratio in the vicinity of 1.0;
A second time when the second excess air ratio detected by the second excess air ratio detection means crosses the vicinity of 1.0 from the lean side while the excess air ratio of the exhaust gas is set to the first excess air ratio. A residual oxygen amount predicting means for predicting a residual oxygen amount of the NOx trap catalyst according to a change rate of the excess air ratio;
An exhaust aftertreatment device for a diesel engine, comprising: a reducing agent supply amount control means for controlling a supply amount of the reducing agent during the latter process based on the residual oxygen amount predicted by the residual oxygen amount prediction means. . The reducing agent supply amount control means includes:
A transition speed control means for controlling a transition speed from the first basic excess air ratio to the second basic excess air ratio based on the residual oxygen quantity predicted by the residual oxygen quantity prediction means;
An excess air ratio control means for controlling an excess air ratio of the exhaust gas during the latter process based on the remaining oxygen amount predicted by the remaining oxygen amount prediction means;
The residual oxygen amount of the NOx trap catalyst is determined by the change rate of the second excess air ratio detected by the second excess air ratio detecting means while the excess air ratio of the exhaust gas is set to the first basic excess air ratio. A second residual oxygen amount predicting means for predicting, and when the second excess air ratio detected by the second excess air ratio detecting means is reversed from the lean side to the rich side across the threshold value, the exhaust air The threshold value when the excess rate is switched from the first basic excess air rate to the second basic excess air rate is controlled based on the residual oxygen amount predicted by the second residual oxygen amount predicting means. The exhaust aftertreatment device for a diesel engine according to claim 1, wherein the exhaust aftertreatment device is a diesel engine.   The residual oxygen amount predicting means is configured to provide a second excess of air when the rate of change of the second excess air rate when crossing the vicinity of 1.0 from the lean side is relatively small when the rate of change of the second excess air rate is near 1.0 from the lean side. 3. The exhaust aftertreatment device for a diesel engine according to claim 1, wherein the remaining oxygen amount of the NOx trap catalyst is predicted to be larger than when the rate of change of the rate is relatively large. The transition speed control means is configured to change the second excess air ratio (rear lambda) when crossing the vicinity of 1.0 from the lean side when the change speed of the second excess air ratio (rear lambda) is relatively small. The exhaust aftertreatment device for a diesel engine according to claim 2, wherein the transition speed is made slower than when the rate of change of the excess air ratio is relatively large.   The excess air ratio control means is configured to provide a second excess air amount when the second excess air ratio is crossed near 1.0 from the lean side when a change rate of the second excess air ratio is relatively small when crossing near 1.0 from the lean side. 3. The exhaust aftertreatment device for a diesel engine according to claim 2, wherein the excess air ratio of the exhaust during the latter process is made smaller than when the rate of change of the rate is relatively large.   The threshold control means is configured so that the change rate of the second excess air ratio detected by the second excess air ratio detection means while the excess air ratio of the exhaust gas is set to the first basic excess air ratio is relative. Is smaller than when the change rate of the second excess air ratio detected by the second excess air ratio detecting means is relatively large while the excess air ratio of the exhaust gas is set to the first basic excess air ratio. The exhaust aftertreatment device for a diesel engine according to claim 2, wherein the threshold value is set to a smaller side.