JP5521290B2 - Diesel engine exhaust purification system - Google Patents

Description

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

  In a diesel engine exhaust purification system in which three catalysts, a three-way catalyst, a NOx trap catalyst, and a three-way catalyst, are arranged in this order from the upstream side to the exhaust passage, the amount of ceria carried by the upstream three-way catalyst is defined as A, NOx When the amount of ceria supported by the trap catalyst is B and the amount of ceria supported by the downstream three-way catalyst is C, NOx is such that (A + B) / C is in the range of approximately 0.2 to 1.2. There is one that adjusts the amount of ceria per unit volume in the trap catalyst and the upstream three-way catalyst (see Patent Document 1).

According to this, the oxygen storage capacity in the most upstream three-way catalyst and NOx trap catalyst is reduced due to the decrease in the ceria amount A + B, and the oxygen that is desorbed even when the lean combustion is switched to the stoichiometric combustion (or rich combustion). Since the amount is small, the atmosphere in the NOx trap catalyst and the most upstream three-way catalyst is enriched relatively quickly to form a reducing atmosphere, and NOx is reduced efficiently.
JP-A-11-002114

  However, the technique of Patent Document 1 does not sufficiently consider the point of oxidizing (purifying) reducing gas components (HC, CO) released downstream without being used in the NOx trap catalyst during the rich spike process. . That is, a sufficient oxygen storage capacity is indispensable for oxidizing (purifying) the reducing gas component released downstream without being used by the NOx trap catalyst during the rich spike processing, as in the technique of Patent Document 1 above. If the amount of ceria A + B is reduced and the oxygen storage capacity of the most downstream three-way catalyst remains the same as the conventional device, the reducing gas component released downstream without being used in the NOx trap catalyst during the rich spike process is reduced. Oxygen for purification is insufficient, and the reducing gas component released downstream without being used by the NOx trap catalyst during the rich spike process cannot be sufficiently purified. In conclusion, at the level of the oxygen storage capacity of the three-way catalyst used in the conventional apparatus including the technology of Patent Document 1 above, the reducing gas component released downstream without being used in the NOx trap catalyst during the rich spike process Cannot be purified.

  In a typical three-way catalyst, ceria, a component that contributes to oxygen storage capacity, stores oxygen when the exhaust air-fuel ratio is lean, and desorbs oxygen when the exhaust air-fuel ratio is rich. There is also a problem that oxygen stored in the ceria cannot be used efficiently unless the air-fuel ratio at the most downstream three-way catalyst inlet is controlled to the lean side during the spike processing.

  Accordingly, an object of the present invention is to provide a device that can use oxygen stored in ceria even when the exhaust air-fuel ratio is rich.

  The present invention solves the above problems by the following means. In addition, in order to make an understanding easy, although the code | symbol corresponding to embodiment of this invention is attached | subjected, it is not limited to this.

The present invention traps NOx in the exhaust when the exhaust air-fuel ratio is lean in the exhaust passage, and removes the trapped NOx when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich and performs a reduction process. trap catalyst (12), a catalyst supported the ceria is a component contributing to the noble metal and oxygen storage capacity, the oxygen storage catalyst is a catalyst in which the ceria supported per unit of volume and 198 g / L ( 13) at least.

According to the present invention, NOx in the exhaust is trapped in the exhaust passage when the exhaust air-fuel ratio is lean, and the trapped NOx is desorbed and reduced when the exhaust air-fuel ratio is rich or stoichiometric. a NOx trap catalyst, a catalyst supported the ceria is a component contributing to the noble metal and oxygen storage capacity and oxygen storage catalyst is a catalyst in which the ceria supported per unit of volume and 198 g / L Arranged at least. The oxygen storage catalyst, the ceria supported per unit of volume that was 198 g / L, or Nii with the ability to mass storage of oxygen in the exhaust, even when the exhaust air-fuel ratio is rich, the catalyst surface is lean Thus, HC and CO, which are reducing gas components, can be sufficiently purified.

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

  FIG. 1 is a schematic configuration diagram of an exhaust emission control device for an engine according to the present invention. The engine 1 is a diesel engine, and fuel is supplied to the engine 1 from a fuel injection valve 2.

  In an intake passage 3 for introducing air into the engine 1, an air flow meter 4 that detects the intake air amount of the engine 1, an electronically controlled throttle valve 5 that adjusts the intake air amount, and a throttle valve that detects the opening of the throttle valve 5 An opening sensor 6 is provided.

  A three-way catalyst 11 and a NOx trap catalyst 12 are provided in the exhaust passage 8 for exhausting the exhaust from the engine 1 from the upstream side.

  The three-way catalyst 11 is a catalyst that purifies HC, CO, and NOx at a stoichiometric air-fuel ratio, and is disposed immediately below the exhaust manifold for the purpose of purifying exhaust during a cold start. The NOx trap catalyst 12 traps NOx in the exhaust when the air-fuel ratio of the inflowing exhaust is lean, and desorbs the NOx trapped when the air-fuel ratio of the inflowing exhaust is the stoichiometric or rich. This is a catalyst for reducing and purifying the desorbed NOx using HC and CO in the exhaust as a reducing agent.

  The NOx trap catalyst 12 and the three-way catalyst 11 all carry barium Ba, but the NOx trap catalyst 12 uses the barium Ba as the NOx trapping agent with respect to the amount of barium Ba carried on the three-way catalyst 11. Thus, approximately twice as much barium Ba is supported per unit volume, and a NOx trap catalyst and a normal three-way catalyst are distinguished by the amount of barium Ba supported. However, the NOx trapping agent is not limited to barium Ba.

  Various detection signals such as an accelerator operation amount from an accelerator operation amount sensor (not shown) and an engine rotation speed from an engine rotation speed sensor are input to the engine controller 21. The engine controller 21 includes one or more arithmetic units, a memory, an input / output interface, and the like. Based on an input signal and an internal parameter, the engine controller 21 controls the intake air amount via the throttle valve 5 and the fuel injection valve. 2 to control the fuel injection amount and the fuel injection timing.

  Further, the NOx trap catalyst 12 has an upper limit on the amount of NOx that can be trapped, and when the NOx trap amount increases and approaches the upper limit, the NOx trap ability of the NOx trap catalyst 12 decreases and the NOx cannot be trapped sufficiently. The controller 21 determines that the NOx trap capability is reduced when the NOx trap amount of the NOx trap catalyst 12 exceeds a predetermined amount, and a rich spike process for temporarily setting the air-fuel ratio of the engine 1 to the stoichiometric air-fuel ratio or rich. , The NOx trap catalyst 12 is regenerated, and the NOx trap ability of the NOx trap catalyst 12 is restored.

  In the rich spike process, the NOx trap catalyst 12 can be regenerated by supplying a reducing gas component larger than the amount of the reducing gas component commensurate with the NOx trap amount. The reducing gas component (that is, HC, CO) that has not been used for the reduction of the gas is directly released downstream.

  In this case, there is a case where a normal three-way catalyst having an oxygen storage capability is provided downstream of the NOx trap catalyst 12.

  The oxygen storage capacity of the three-way catalyst varies mainly depending on the amount of ceria Ce supported on the three-way catalyst. Conventionally, the amount of ceria supported per unit volume is the same for each catalyst (three-way catalyst 11, NOx trap catalyst, ordinary three-way catalyst provided on the most downstream side), and the amount of ceria in each catalyst is determined by the catalyst capacity. Therefore, it is not set in consideration of the amount of HC and CO released from the NOx trap catalyst 12 during the rich spike processing.

Therefore, in the present invention, the catalyst 13 is loaded with ceria Ce, which is a component that contributes to the oxygen storage capacity, and the amount of ceria Ce per unit volume is at least twice that of a conventional three-way catalyst. The catalyst 13 as described above is provided on the most downstream side of the exhaust passage (that is, downstream of the NOx trap catalyst 12). The amount of ceria supported by an ordinary three-way catalyst is 24 g / L to 85 g / L, whereas the amount of ceria supported by the catalyst 13 of this embodiment is 198 g / L, for example. Therefore, the amount of ceria supported by the catalyst 13 is about 2.3 to 8.3 times the amount of ceria supported by the ordinary three-way catalyst. As a result, the oxygen storage capacity of the catalyst 13 (the amount of oxygen that can actually be stored when the catalyst 13 is disposed in the exhaust passage 8) exhibits at least 30% or more of the capacity of a normal three-way catalyst. More specifically, as shown in FIG. 10, for example, an ordinary three-way catalyst includes alumina (Al ₂ O ₃ ) containing noble metals (Pt, Rh, etc.) after supporting ceria Ce on a support. On the other hand, as shown in FIG. 11, for example, the catalyst 13 supports ceria Ce containing a noble metal (Pt or the like), and then has a noble metal (Rh or the like) and iron on the surface. It is configured by supporting Fe (thin coating).

  This newly configured catalyst 13 is a catalyst in which only the oxygen storage capacity is greatly expanded as compared with a normal three-way catalyst. In order to distinguish this catalyst 13 from a three-way catalyst, it is hereinafter referred to as “oxygen storage catalyst”. Here, the above-mentioned “ordinary three-way catalyst” refers to a three-way catalyst configured for simultaneous purification of HC, CO, and NOx. The noble metal functions as a catalyst when ceria Ce desorbs oxygen and oxidizes HC and CO with the desorbed oxygen. However, the present invention is not limited to the configuration in which ceria Ce is supported as a component contributing to the oxygen storage capacity.

  FIG. 2 shows the characteristics of the newly configured oxygen storage catalyst 13. FIG. 2 shows the relationship between the air-fuel ratio and the HC conversion rate (ηHC) for the oxygen storage catalyst 13. FIG. 2 shows that the HC conversion rate is higher as the air-fuel ratio becomes richer. This is because the oxygen storage catalyst 13 has a capacity to store a larger amount of oxygen in the exhaust than a normal three-way catalyst, so even when the exhaust air-fuel ratio is rich, the catalyst surface is in a state close to lean and stored. This is because HC can be oxidized (purified) using oxygen.

  FIG. 3 shows the engine outlet, the NOx trap catalyst 12 inlet, and the oxygen storage catalyst 13 outlet at the time of rich spike processing when the three-way catalyst 11, the NOx trap catalyst 12, and the oxygen storage catalyst 13 are arranged in this order from the upstream side of the exhaust passage 8. It shows how each excess air ratio λ (which is a value obtained by dividing the air-fuel ratio by the stoichiometric air-fuel ratio) changes. The change in the excess air ratio at the engine outlet indicates the change in the excess air ratio corresponding to the rich spike process, and the excess air ratio at the engine outlet decreases stepwise across the line of λ = 1 from the value immediately before the rich spike process. After that, the vehicle runs horizontally and increases stepwise across the line of λ = 1 again to return to the value immediately before the rich spike processing.

  Next, when the excess air ratio at the inlet of the NOx trap catalyst 12 is reached, the amount of decrease from the line of λ = 1 is smaller than in the case of the excess air ratio at the engine outlet. Although not shown, when the excess air ratio at the inlet of the oxygen storage catalyst 13 is reached, the amount of decrease from the line of λ = 1 is smaller than in the case of the excess air ratio at the inlet of the NOx trap catalyst 12. In this way, the amount of decrease from the line of λ = 1 decreases each time the three-way catalyst 11 and the NOx trap catalyst 12 pass through. This means that the oxygen in the three-way catalyst 11 and the NOx trap catalyst 12 is constant by a certain amount. It means being consumed. That is, the air-fuel ratio during the rich spike process becomes lean every time it passes through the three-way catalyst 11 and the NOx trap catalyst 12. In this way, the amount of decrease from the line of λ = 1 decreases as the three-way catalyst 11 and the NOx trap catalyst 12 pass through the catalyst. This means that oxygen is added to the three-way catalyst 11 and the NOx trap catalyst 12. It means that a certain amount is consumed. That is, the air-fuel ratio during the rich spike process becomes lean every time it passes through the three-way catalyst 11 and the NOx trap catalyst 12. Further, the timing at which the excess air ratio changes is delayed each time the three-way catalyst 11 and the NOx trap catalyst 12 pass through.

  On the other hand, when the excess air ratio at the outlet of the oxygen storage catalyst 13 is reached, the amount of decrease from the line of λ = 1 is smaller than the case of the excess air ratio at the inlet of the NOx trap catalyst 12 at the beginning of the rich spike process, but is reversed. Further, it increases across the excess air ratio at the inlet of the NOx trap catalyst 12. This is because the oxygen storage catalyst 13 has the ability to store a large amount of oxygen in the exhaust gas as compared with the ordinary three-way catalyst as described above, so that even if the exhaust air-fuel ratio is rich, the catalyst surface is close to lean. This means that HC and CO can be purified.

  In order to purify HC and CO released from the NOx trap catalyst 12 during the rich spike process, judging from these newly found characteristics of FIGS. 2 and 3, the inlet of the oxygen storage catalyst 13 during the rich spike process is removed. It is only necessary to set the target excess air ratio λm during the rich spike process so that the air-fuel ratio becomes rich. This is because if the air-fuel ratio at the inlet of the oxygen storage catalyst 13 during the rich spike process is made rich, the air-fuel ratio at the inlet of the NOx trap catalyst 12 becomes richer than the air-fuel ratio at the inlet of the oxygen storage catalyst 13, and the NOx trap catalyst. The NOx detachment from the NOx 12 and the reduction and purification using HC and CO as reducing gas components can be performed, and the HC and CO released from the NOx trap catalyst 12 are converted into the oxygen storage catalyst 13 by the oxygen storage catalyst 13. This is because it can be oxidized (purified) with a large amount of stored oxygen.

  Next, the target excess air ratio λm during the rich spike process is set according to the following parameters.

<1> Oxygen storage amount of oxygen storage catalyst 13 <2> Catalyst temperature of oxygen storage catalyst 13 <3> In-cylinder temperature <4> NOx trap amount of NOx trap catalyst 12 For example, as shown in FIG. Comparing the case where the oxygen storage amount of 13 is large and the case where the oxygen storage amount is small, when the oxygen storage amount is large, the target air excess ratio during the rich spike process becomes smaller (rich side) than when the oxygen storage amount is small. Set. This is because the air-fuel ratio during the rich spike process can be made richer when the oxygen storage amount of the oxygen storage catalyst 13 is larger.

  Further, as shown in FIGS. 5 and 6, comparing the case where the catalyst temperature and in-cylinder temperature of the oxygen storage catalyst 13 are high and the case where the temperature is low, the oxygen temperature is higher when the catalyst temperature and the cylinder temperature of the oxygen storage catalyst 13 are higher. The target excess air ratio during the rich spike process is set to be larger (lean side) than when the storage catalyst 13 has a low catalyst temperature or in-cylinder temperature. This is because the desorption rate of oxygen from the oxygen storage catalyst 13 is higher when the temperature is higher than when the temperature is lower (a large amount of oxygen is desorbed), and the air-fuel ratio during the rich spike process is not made so rich. This is because it can be done.

  Further, as shown in FIG. 7, when comparing the case where the NOx trap amount of the NOx trap catalyst 12 is large and the case where the NOx trap amount is small, the target during the rich spike process when the NOx trap amount is larger than when the NOx trap amount is small. Set to the side where the excess air ratio becomes smaller (rich side). This is because when the amount of NOx trap is larger, the HC and CO conversion rates of the oxygen storage catalyst 13 cannot be increased unless the air-fuel ratio during the rich spike process is made richer.

  FIG. 8 is a flowchart showing the contents of the processing performed by the engine controller 21, which is repeatedly executed every predetermined time, for example, every 10 msec.

  However, here, the case of <2> above, that is, the case where the target excess air ratio during the rich spike process is set according to the catalyst temperature of the oxygen storage catalyst 13 will be described.

  Step 1 looks at the rich spike flag. Since this rich spike flag is initially set to zero when the engine is started, the routine proceeds to step 2 where the NOx trap amount ΣNOx of the NOx trap catalyst 12 is compared with the reference value ΣNOxc. The reference value ΣNOxc is set by conducting an experiment in advance.

  Here, as a method for calculating the NOx trap amount, as described in, for example, Japanese Patent Application Laid-Open No. 2005-163591, for example, the NOx trap amount is discharged downstream from the NOx trap catalyst from the integrated value of the NOx amount discharged per predetermined time from the engine. There is a method of obtaining by deducting the integrated value of the NOx amount per predetermined time to be desorbed. The amount of NOx discharged from the engine per predetermined time is prepared in advance by experimentation on a map that defines the relationship among the engine speed, the intake air amount, and the amount of NOx discharged per predetermined time from the engine 1. Can be determined from this map. Further, the NOx amount per predetermined time desorbed downstream from the NOx trap catalyst is the exhaust flow rate (≈the intake air amount of the engine 1) flowing into the NOx trap catalyst, the air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst, A map that defines the relationship between the amount of NOx per predetermined time desorbed downstream from the NOx trap catalyst can be created in advance by experiments, and can be obtained with reference to this map.

  When the NOx trap amount ΣNOx of the NOx trap catalyst 12 is equal to or smaller than the reference value ΣNOxc, the rich spike flag = 0 is set at step 3 and the current process is terminated.

  If the NOx trap amount ΣNOx of the NOx trap catalyst 12 exceeds the reference value ΣNOxc in step 2, the process proceeds to step 4 to perform rich spike processing, and the rich spike flag = 1 is set.

  In step 5, the catalyst temperature of the oxygen storage catalyst 13 detected by the temperature sensor 22 is read. In step 6, the target air excess ratio λm during the rich spike process is obtained by searching a table having the contents shown in FIG. 5 from this catalyst temperature. calculate.

  In step 7, a timer is started (timer value tm = 0). This timer is for measuring the elapsed time since the start of rich spike processing.

  Due to the rich spike flag = 1 in step 4, next time, the process proceeds from step 1 to step 8, and the timer value tm is compared with the predetermined value tm1. The predetermined value tm1 is a value for determining the time for executing the rich spike processing, and is determined in advance by an experiment. While the timer value tm is less than the predetermined value tm1, the current process is terminated. Eventually, if the timer value tm becomes equal to or greater than the predetermined value tm1, it is determined that the regeneration of the NOx trap catalyst 12 has been completed. It is assumed that ΣNOx = 0.

  Due to the rich spike flag = 0, the process proceeds to steps 1 and 2 next time, and the NOx trap amount ΣNOx is again compared with the reference value ΣNOxc.

  The rich spike processing is executed so that the target excess air ratio during the rich spike processing can be obtained in the flow (not shown) by the rich spike flag = 1 set in this way.

  Here, the effect of this embodiment is demonstrated.

  According to this embodiment (the invention described in claim 1), when the exhaust air-fuel ratio is lean, the exhaust passage 8 traps NOx in the exhaust, and the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich. NOx trap catalyst 12 that desorbs the trapped NOx and performs a reduction process, and a catalyst that supports a noble metal and ceria Ce (a component that contributes to oxygen storage capacity), and the amount of ceria Ce per unit volume is At least an oxygen storage catalyst 13 that is a catalyst that is at least twice (for example, 2.0 to 8.3 times) that of a normal three-way catalyst is disposed. The oxygen storage catalyst 13 has an ability to store a large amount of oxygen in the exhaust gas because the amount of ceria Ce per unit volume is at least twice that of a normal three-way catalyst, and the exhaust air-fuel ratio is rich. Even in this case, the catalyst surface was in a state close to lean, and the reducing gas components HC and CO could be sufficiently purified.

  When the exhaust air-fuel ratio is rich, the ordinary three-way catalyst lacks oxygen for oxidizing the reducing gas component. According to this embodiment (the invention according to claim 2), the exhaust air-fuel ratio is Since the reducing gas component discharged when it is rich is oxidized using oxygen desorbed from the oxygen storage catalyst 13, there is no shortage of oxygen for oxidizing the reducing gas component.

  According to the present embodiment (the invention described in claim 3), focusing on the fact that the balance between oxygen storage and desorption of the oxygen storage catalyst 13 depends on the oxygen partial pressure in the exhaust, the exhaust air-fuel ratio is made lean. In this case, oxygen stored in the oxygen storage catalyst 13 is desorbed from the oxygen storage catalyst 13 when the exhaust air-fuel ratio is rich, and the reducing gas component is oxidized, so that necessary oxygen can be supplied.

  Corresponding to the decrease in the oxygen concentration in the exhaust gas downstream of the exhaust passage 8, according to the present embodiment (the invention according to claim 7), the oxygen storage catalyst 13 is disposed in the exhaust passage 8. Since it is arranged on the most downstream side, the oxygen storage catalyst 13 is in a condition that makes it easier to desorb oxygen, and the oxidation efficiency of the reducing gas component can be improved.

  FIG. 9 shows a change in the excess air ratio during the rich spike processing of the second embodiment.

  In the second embodiment, the oxygen storage catalyst 13 is made to store oxygen quickly immediately after the rich spike process, so the balance (equilibrium state) between the storage and desorption of oxygen in the oxygen storage catalyst 13 is intentionally shifted to the storage side once. That is, the target excess air ratio immediately after the rich spike process is temporarily set high as shown in the upper part of FIG. As a result, as shown in the lower part of FIG. 9, the amount of CO emission can be reduced as compared with the first embodiment.

The schematic block diagram of the diesel engine provided with the fuel-injection control apparatus of this invention. The characteristic figure of HC conversion rate to air fuel ratio. The timing chart which shows the behavior of the excess air ratio at the time of rich spike processing. The characteristic figure of the target excess rate during the rich spike process with respect to oxygen storage amount. The characteristic figure of the target excess rate during the rich spike process with respect to the catalyst temperature of an oxygen storage catalyst. The characteristic figure of the target excess rate in the rich spike process with respect to in-cylinder temperature. The characteristic figure of the target excess rate in the rich spike process with respect to the amount of NOx traps. The flowchart for demonstrating the setting of a rich spike flag. The timing chart which shows the behavior of the excess air ratio at the time of the rich spike process of 2nd Embodiment. The schematic block diagram of a normal three-way catalyst. The schematic block diagram of an oxygen storage catalyst.

Explanation of symbols

12 NOx trap catalyst 13 Oxygen storage catalyst 21 Engine controller

Claims (7)

In the exhaust passage,
A NOx trap catalyst that traps NOx in exhaust when the exhaust air-fuel ratio is lean, and desorbs and reduces the trapped NOx when the exhaust air-fuel ratio is rich, or
A ceria component that contributes to the noble metal and oxygen storage capacity a catalyst supported, characterized in that the oxygen storage catalyst is a catalyst in which the ceria supported per unit of volume and 198 g / L and at least placed Diesel engine exhaust purification system.   The exhaust gas purification apparatus for a diesel engine according to claim 1, wherein the reducing gas component discharged when the exhaust air-fuel ratio is rich is oxidized using oxygen desorbed from the oxygen storage catalyst.   The oxygen stored in the oxygen storage catalyst when the exhaust air-fuel ratio is lean is desorbed from the oxygen storage catalyst when the exhaust air-fuel ratio is rich to oxidize the reducing gas component. 2. An exhaust emission control device for a diesel engine according to 2.   The exhaust purification device for a diesel engine according to claim 3, wherein the oxygen storage catalyst is made rich at an inlet air-fuel ratio to desorb oxygen from the oxygen storage catalyst.   4. The exhaust purification device for a diesel engine according to claim 3, wherein the combustion air-fuel ratio of the engine is made rich to desorb oxygen from the oxygen storage catalyst.   The exhaust purification device for a diesel engine according to claim 5, wherein a target air-fuel ratio of the engine is determined so that oxygen is desorbed from the oxygen storage catalyst.   The exhaust purification device for a diesel engine according to claim 1, wherein the oxygen storage catalyst is arranged at the most downstream side of the catalyst arranged in the exhaust passage.

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