JP2005061340A - Exhaust-emission control system of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust-emission control system of an internal combustion engine in which NOx-purification function of a storage-type NOx-catalyst can be effectively improved by optimizing HC-supply amount in purging the NOx. <P>SOLUTION: A reducing-agent-supply means supplies intermittently dividedly in a plurality of times a reducing agent, i.e. a fuel (light oil, HC) in the range of total supply amount of the reducing agent, when NOx-storage amount has reached a prescribed value (B12). As temperature of the storage-type NOx-catalyst detected by a catalyst-temperature detection means (18) is lower, so the supply amount of the reducing agent at one time is restrained by increasing the number of divided supplies, and further as the temperature is higher, so the supply amount of the reducing agent at one time is increased by reducing the number of divided supplies of the agent (B10). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to a NOx purification technology for a storage type NOx catalyst.

  In recent years, vehicles in which an NOx purification NOx catalyst is disposed in an exhaust passage of an internal combustion engine (engine) have been put into practical use, and recently, a storage type NOx catalyst capable of purifying NOx even in an oxygen-excess atmosphere has been developed. . In particular, in a diesel engine, combustion is performed under a lean air-fuel ratio, so the amount of oxygen in the exhaust gas is large, and the three-way catalyst that has been put to practical use in a gasoline engine does not function, and an occluded NOx catalyst in the exhaust passage It is effective to provide

This NOx storage catalyst stores NOx in the exhaust as nitrate X—NO ₃ in an oxygen excess state (oxidation atmosphere), and releases the stored NOx in a CO (carbon monoxide) excess state (reduction atmosphere). It is configured as a catalyst having the property of being reduced to ₂ (nitrogen) (at the same time, carbonate X-CO ₃ is produced). For example, when the storage type NOx catalyst is provided in the exhaust passage of a gasoline engine, it is periodically switched to a rich air-fuel ratio operation to generate a reducing atmosphere with a large amount of CO, thereby purifying and reducing the stored NOx (NOx purge). Thus, the storage type NOx catalyst is regenerated.

  On the other hand, in a diesel engine, since combustion is performed under a lean air-fuel ratio as described above, it is difficult to perform a rich air-fuel ratio operation, and a post that resupplys fuel (light oil) after the expansion stroke. By performing injection or adding fuel (light oil) into the exhaust, HC (hydrocarbon) is supplied into the exhaust to create a reducing atmosphere, and the HC is transformed into CO in the exhaust passage and in the catalyst. And NOx purge is performed.

However, depending on the fuel supply method, etc., the fuel may be excessively supplied, and the fuel may not function as a reducing agent and slip through the storage NOx catalyst in the HC state and slip to the downstream side of the catalyst. In such a case, there is a problem that exhaust emission deteriorates.
In view of this, for example, it has been considered to suppress the excessive fuel supply by dividing and supplying the fuel to prevent the fuel slip in the storage-type NOx catalyst (see Patent Document 1).
JP 2002-106332 A (paragraph 0058 etc.)

By the way, the oxidation reaction of HC includes a complete oxidation reaction that oxidizes to CO ₂ and an incomplete oxidation reaction that stops the reaction at CO. In order to perform NOx purge, an incomplete oxidation reaction is mainly caused. Therefore, a large amount of CO needs to be present in the storage type NOx catalyst.
However, it has been confirmed that each of the above reactions depends on the exhaust gas temperature or the temperature of the NOx storage catalyst, that is, the catalyst temperature and the amount of supplied fuel. easily insufficient CO to CO ₂ in progress, while the slip of HC does not proceed oxidation reaction itself and often supplied fuel quantity low catalyst temperature is liable to occur. That is, the amount of CO varies depending on the catalyst temperature and the amount of supplied fuel, and the NOx purification performance changes accordingly.

This problem occurs in the same manner for each divided supply even when the fuel is divided and supplied as described above. The technique disclosed in Patent Document 1 is a slip of HC in the storage type NOx catalyst. Can be prevented, but the amount of CO produced is not taken into consideration, and is not effective in terms of improving the NOx purification performance.
The present invention has been made to solve such problems. The object of the present invention is to optimize the amount of HC supplied during the NOx purge and effectively improve the NOx purification performance of the storage type NOx catalyst. An object of the present invention is to provide an exhaust purification device for an internal combustion engine that can be improved.

  In order to achieve the above-described object, in the exhaust gas purification apparatus for an internal combustion engine according to claim 1, provided in the exhaust passage of the internal combustion engine, NOx in the exhaust gas is occluded in an oxidizing atmosphere, and the occluded gas is stored in a reducing atmosphere. A NOx storage catalyst having a function of releasing and reducing NOx and an oxygen scavenging function of capturing oxygen in an oxidizing atmosphere, and an upstream side of the storage NOx catalyst are provided on the exhaust side of the NOx catalyst. Reducing agent supplying means for supplying HC as a reducing agent, catalyst temperature detecting means for detecting the temperature of the storage type NOx catalyst, NOx storage amount detecting means for detecting the NOx storage amount of the storage type NOx catalyst, and the NOx Reducing agent supply amount setting means for setting a reducing agent supply amount in accordance with the NOx storage amount detected by the storage amount detection means, wherein the reducing agent supply means is N detected by the NOx storage amount detection means. When the occlusion amount reaches a predetermined amount, the reducing agent is intermittently supplied by being divided into a plurality of times within the range of the reducing agent supply amount set by the reducing agent supply amount setting means, The lower the temperature of the occlusion-type NOx catalyst detected by the catalyst temperature detection means, the more the number of times of the division is reduced to suppress the reducing agent supply amount once, while the higher the temperature is, the higher the division is. The number of times is reduced to increase the supply amount of the reducing agent at one time.

Therefore, since the storage-type NOx catalyst has an oxygen scavenging function, the storage-type NOx catalyst is appropriately supplemented with oxygen by intermittently supplying the reducing agent in a plurality of times by the reducing agent supply means. However, the trapped oxygen can be effectively used for the oxidation of the reducing agent, that is, the generation of CO, and the NOx purge can be efficiently performed.
The exhaust gas purification apparatus for an internal combustion engine according to claim 2 is further provided on the exhaust downstream side of the storage type NOx catalyst, and detects exhaust air / fuel ratio detection means on the exhaust downstream side of the storage type NOx catalyst. And the reducing agent supply means determines the reducing agent supply amount at one time so that the exhaust air-fuel ratio detected by the exhaust air-fuel ratio detection means does not become closer to the rich air-fuel ratio than a predetermined value. It is characterized in that the agent is dividedly supplied.

  In this way, the reducing agent supply amount is adjusted so that the exhaust air-fuel ratio detected by the exhaust air-fuel ratio detection means does not become closer to the rich air-fuel ratio than the predetermined value, and the reducing agent is supplied in a divided manner. , While reducing the rich air-fuel ratio on the exhaust downstream side of the NOx storage catalyst, that is, preventing the slip of the reducing agent (HC) in the NOx storage catalyst, the reducing agent supply amount can be optimized once. The purification performance is more effectively improved.

  According to the exhaust gas purification apparatus for an internal combustion engine according to claim 1 of the present invention, the trapped oxygen is effectively oxidized while the replenishment agent is divided and the trapped oxygen is replenished appropriately, that is, the oxidation of the reducing agent, that is, the amount of CO. When the NOx storage amount reaches a predetermined amount and the NOx purge of the storage type NOx catalyst is to be performed, the reducing agent is set according to the NOx storage amount. Within the range of supply amount, for example, under the same engine condition (engine speed, etc.), the lower the temperature of the storage-type NOx catalyst detected by the catalyst temperature detection means, the more the number of divisions is increased. While reducing the supply amount of the reducing agent, the higher the temperature, the smaller the number of divisions, and the reduced agent supply amount is increased so that the reducing agent supply amount is increased. Optimization of the reductant supply amount The amount of CO produced can be made appropriate, and the NOx purification performance can be effectively improved regardless of the catalyst temperature.

  According to the exhaust gas purification apparatus for an internal combustion engine according to claim 2, the reducing agent supply amount is adjusted once so that the exhaust air-fuel ratio detected by the exhaust air-fuel ratio detection means does not become closer to the rich air-fuel ratio than a predetermined value. Since the reducing agent is dividedly supplied, the rich air-fuel ratio on the exhaust downstream side of the storage-type NOx catalyst, that is, the slip of the reducing agent (HC) in the storage-type NOx catalyst is surely prevented, and one time By optimizing the reducing agent supply amount, the NOx purification performance can be more effectively improved.

Referring to FIG. 1, an exhaust gas purification apparatus for an internal combustion engine according to the present invention is schematically shown.
The engine 1 is composed of, for example, a diesel engine, and an air flow sensor 4 for detecting an intake air amount Qa is provided in the intake passage 2 of the engine 1.
On the other hand, an occlusion-type NOx catalyst 10 that mainly purifies NOx in the exhaust is interposed in the exhaust passage 6 of the engine 1.

As described above, the storage-type NOx catalyst 10 stores NOx in the exhaust as nitrate X—NO ₃ in an oxygen excess state (oxidation atmosphere), and releases the stored NOx in a CO excess state (reduction atmosphere). _It has the property of reducing to ₂ . The occlusion-type NOx catalyst 10 contains ceria (Ce) and has a function of capturing oxygen (O ₂ ) in an oxygen-excess state (oxidizing atmosphere) (O ₂ storage function). .

An injection valve (reducing agent supply means) 12 that supplies light oil, that is, fuel (HC) as a reducing agent to the exhaust passage 6 is provided on the exhaust upstream side of the storage NOx catalyst 10. The injection valve 12 is connected to a light oil tank 13 through a pipe line 12a.
Than this, the gas oil (HC) from the injection valve 12 is supplied to the exhaust passage 6, it is oxidized HC in the exhaust passage within 6 by O ₂ trapped in O ₂ and occlusion-type NOx catalyst 10 in the exhaust CO The NOx occluded in the occlusion-type NOx catalyst 10 is favorably released by the CO and reduced (NOx purge).

A NOx sensor 14 for detecting the NOx concentration in the exhaust discharged from the combustion chamber of the engine 1 is provided on the exhaust upstream side of the injection valve 12, and the storage downstream of the storage NOx catalyst 10 stores the NOx. There is provided an O ₂ sensor (exhaust air / fuel ratio detecting means) 16 for detecting the oxygen concentration in the exhaust gas exhausted through the type NOx catalyst 10 and thus the exhaust air / fuel ratio.
Further, the storage type NOx catalyst 10 is provided with a temperature sensor (catalyst temperature detection means) 18 for detecting the temperature of the storage type NOx catalyst 10, that is, the catalyst temperature Tc.

The ECU 20 is a control device for performing comprehensive control of the exhaust gas purification apparatus for an internal combustion engine according to the present invention including the engine 1, and includes a CPU, a memory, a timer counter, and the like.
Various sensors such as the air flow sensor 4, NOx sensor 14, O ₂ sensor 16 and temperature sensor 18 are connected to the input side of the ECU 20, and various devices such as the injection valve 12 are connected to the output side. It is connected.

Hereinafter, the NOx purge control according to the present invention of the exhaust purification apparatus configured as described above will be described. Specifically, in the present invention, the light oil is divided from the injection valve 12 and injected intermittently (pulsed) into the exhaust passage 6, and the NOx purge control by the divided injection will be described.
In order to generate CO from HC in light oil, O ₂ is indispensable for split injection of light oil, and in order to generate CO appropriately and effectively, as well as in exhaust, the above-mentioned storage type NOx Where it is necessary to effectively use O ₂ trapped by the catalyst 10, there is a limit to the amount of O ₂ trapped by the storage-type NOx catalyst 10, and when the trapped O ₂ disappears, the production of CO rapidly decreases. This is because it is required to supplement the storage NOx catalyst 10 with O ₂ in the exhaust gas by interrupting the addition of light oil.

That is, by dividing light oil from the injection valve 12 and intermittently (pulsingly) adding it to the occlusion type NOx catalyst 10, O ₂ in the exhaust gas is well supplemented to the occlusion type NOx catalyst 10 during the injection interval. As a result, CO can be generated more efficiently than when diesel oil is continuously added, and NOx purification efficiency can be improved while preventing wasteful supply of diesel oil.

Referring to FIG. 2, the control routine of the NOx purge control according to the present invention is shown in a flowchart, and FIG. 3 is a control block diagram of the NOx purge control. Hereinafter, referring to the control block diagram, FIG. However, the NOx purge control according to the present invention will be described in detail along the flowchart (reducing agent supply means).
First, in step S10, the amount of NOx occluded in the occlusion-type NOx catalyst 10, that is, the occlusion NOx amount Qnox is calculated (NOx occlusion amount detection means). Here, for example, the NOx mass flow rate is calculated based on the exhaust flow rate, and the NOx mass flow rate is regarded as the occluded NOx amount Qnox. Specifically, since the exhaust flow rate is considered to be substantially equal to the intake air amount Qa, the NOx concentration on the upstream side of the catalyst detected by the NOx sensor 14 using the intake air amount information Qa from the airflow sensor 4 is used here. The NOx mass flow rate and thus the stored NOx amount Qnox are obtained from the information and the intake air amount information Qa.

An occlusion NOx amount Qnox may be obtained by providing an exhaust flow rate sensor in the exhaust passage 6 so as to directly detect the exhaust flow rate.
Here, the stored NOx amount Qnox is obtained based on the NOx concentration information only from the NOx sensor 14 on the upstream side of the catalyst. However, a similar NOx sensor is provided on the downstream side of the catalyst, and the NOx sensor 14 and the downstream side of the catalyst are provided. The stored NOx amount Qnox may be obtained based on the output difference from the NOx sensor, thereby improving the accuracy of the stored NOx amount Qnox.

Alternatively, the relationship between the NOx concentration information of the NOx sensor 14 and the stored NOx amount Qnox may be set in advance as a map, and the stored NOx amount Qnox may be obtained from the map. In this case, the NOx concentration on the upstream side of the catalyst may be obtained indirectly from the operating state of the engine 1. For example, the NOx flow rate may be obtained from the engine rotational speed Ne and the engine torque, and the NOx concentration upstream of the catalyst may be obtained from the NOx flow rate and the exhaust flow rate (intake air amount Qa).
In step S12, it is determined whether or not the stored NOx amount Qnox is equal to or greater than a predetermined amount X1. Here, the predetermined amount X1 is set to a value slightly smaller than the saturation amount of NOx stored in the storage-type NOx catalyst 10. If the determination result is false (No) and it is determined that the occluded NOx amount Qnox is less than the predetermined amount X1, the routine is directly exited. On the other hand, when the determination result is true (Yes) and it is determined that the stored NOx amount Qnox is equal to or greater than the predetermined amount X1, it is determined that the NOx purge is necessary, and the process proceeds to step S14.

  In step S14, the supply amount of light oil injected from the injection valve 12, that is, the total light oil addition amount Qfttl, is set based on the stored NOx amount Qnox (reducing agent supply amount setting means). Specifically, as will be described later, a single target light oil addition amount Qf is set according to the catalyst temperature Tc, and the CO amount that can be generated by one injection is determined from this target light oil addition amount Qf. The addition amount Qfttl is set to the sum of the target light oil addition amount Qf according to the catalyst temperature Tc so as to obtain CO that can reduce all of the stored NOx.

In step S16, based on the information from the temperature sensor 18, the temperature of the storage type NOx catalyst 10 is detected.
In step S18, the target light oil addition amount Qf for one time when performing divided injection, that is, pulse injection, is set according to the catalyst temperature Tc (block B10).
According to experiments, when supplying a reducing agent, that is, fuel (light oil, HC) to the storage-type NOx catalyst 10 to perform NOx purge, the complete oxidation reaction (CO ₂ generation) and the incomplete oxidation reaction (CO generation) As shown in FIG. 4, depending on the temperature of the storage-type NOx catalyst 10, that is, the catalyst temperature Tc and the amount of fuel supplied (light oil addition amount), the complete oxidation reaction proceeds when the catalyst temperature Tc is high and the amount of fuel supplied is small. As a result, CO tends to be insufficient with respect to CO _2. On the other hand, if the catalyst temperature Tc is low and the amount of supplied fuel is large, the oxidation reaction itself does not proceed and slip of the reducing agent (HC) is likely to occur. As a result, it was confirmed that CO was generated satisfactorily when the amount of supplied fuel was appropriate (the shaded area in FIG. 4).

  Therefore, when the stored NOx amount Qnox reaches the predetermined amount X1 and the NOx purge should be performed, for example, within the range of the reducing agent supply amount set according to the stored NOx amount Qnox, for example, the same engine state (engine speed Ne, etc. ), The temperature of the storage-type NOx catalyst 10 detected by the temperature sensor 18, that is, the catalyst temperature Tc is lower (for example, the temperature A side in FIG. 4), and the number of divisions is increased and the reducing agent is performed once. While suppressing the supply amount (light oil addition amount) (for example, addition amount C in FIG. 4), the higher the catalyst temperature Tc (for example, the temperature B side in FIG. 4), the smaller the number of divisions. The reducing agent is supplied so as to increase the amount of reducing agent supplied in one cycle (for example, the addition amount D in FIG. 4).

Therefore, here, FIG. 4 is used as a map, and from the map, the target light oil addition amount Qf is set so that the incomplete oxidation reaction effectively proceeds and CO is generated satisfactorily. Specifically, the target light oil addition amount Qf is set so as to enter the incomplete oxidation reaction region (shown by diagonal lines) according to the catalyst temperature Tc. Here, in order to reduce COx most appropriately and effectively, for example, when the catalyst temperature Tc is the temperature A on the normal low temperature side, the target light oil addition amount Qf is the addition amount C, and the temperature B on the normal high temperature side In this case, the target light oil addition amount Qf is set as the addition amount D. Thereby, the target light oil addition amount Qf is optimized, and the addition of excess light oil is suitably prevented.
In step S20, light oil is injected in pulses from the injection valve 12 based on the target light oil addition amount Qf set in this way (block B12). Specifically, here, the injection time is variably controlled so that the target light oil addition amount Qf is obtained while the injection pressure is constant. That is, when the catalyst temperature Tc is the temperature A, the injection time is shortened, and when the catalyst temperature Tc is the temperature B, the injection time is lengthened.

In step S22, it is determined whether or not one injection based on the target light oil addition amount Qf has been completed. If the determination result is true (Yes) and it is determined that the injection has been completed by the target light oil addition amount Qf, the process proceeds to step S24, and the light oil injection is interrupted.
That is, when it is determined that one injection based on the target light oil addition amount Qf has been completed, as described above, O ₂ trapped by the storage-type NOx catalyst 10 is used to generate CO and decreases. Therefore, an injection interval is provided, and O ₂ in the exhaust gas is replenished to the storage-type NOx catalyst 10 during this interval.

By the way, when the determination result of step S22 is false (No) and it is determined that the light oil is still being injected, the process proceeds to step S26.
In step S26, based on the air-fuel ratio information from the O ₂ sensor 16, it is determined whether or not the exhaust air-fuel ratio on the downstream side of the NOx storage catalyst 10, that is, the excess air ratio λ on the downstream side of the catalyst is equal to or less than a predetermined value λ1. (Block B20). Here, the predetermined value λ1 is set to a value slightly closer to the lean air-fuel ratio than the theoretical air-fuel ratio (λ = 1.0). If the determination result is false (No) and it is determined that the excess air ratio λ is greater than the predetermined value λ1, the process returns to step S20 and the light oil injection is continued. On the other hand, when the determination result is true (Yes) and it is determined that the excess air ratio λ is equal to or less than the predetermined value λ1, the process proceeds to step S24, even if one injection based on the target light oil addition amount Qf is not completed. Discontinue light oil injection.

  That is, when the excess air ratio λ is determined to be equal to or less than the predetermined value λ1, the target light oil addition amount Qf becomes excessive for some reason, and the HC in the added light oil passes through the storage-type NOx catalyst 10 to the downstream side of the catalyst. It can be judged that it is slipping. Therefore, when it is determined that the HC is slipping in this way, the injection of light oil is interrupted even if one injection is not completed. This reliably prevents HC from slipping to the downstream side of the catalyst.

In step S28, it is determined whether or not the total amount ΣQfn (n is the number of injections) of light oil injected based on the target light oil addition amount Qf has reached the total light oil addition amount Qfttl, that is, whether or not the NOx purge has been completed (step S28). Block B14). Since the total light oil addition amount Qfttl has not yet been reached in the first divided injection, the determination result is false (No), and the process proceeds to step S30.
In step S30, it is determined whether or not the time change of the excess air ratio λ, that is, the change rate dλ / dt of the excess air ratio λ is positive (≧ 0) (block B20). That is, when interrupting the injection of diesel fuel, exhaust air-fuel ratio becomes, the exhaust less HC is the lean air-fuel ratio side, that is, the excess air factor λ is become to migrate to the larger side, here, O ₂ sensor Based on the air-fuel ratio information from 16, it is determined whether or not the excess air ratio λ has started to shift to the larger side.

That is, as described above, during the injection interval in which the light oil injection is interrupted, O ₂ in the exhaust gas is captured by the storage-type NOx catalyst 10, and during this time, the excess air ratio λ does not change and the change rate dλ. / Dt is held substantially constant, but in step S30, O ₂ is sufficiently captured by the storage-type NOx catalyst 10, and excess O ₂ flows out downstream of the storage-type NOx catalyst 10. It is determined whether or not it has started.

If the determination result of step S30 is false (No), the rate of change dλ / dt is not yet positive, and it is determined that the excess air ratio λ has not started to shift to the large side, then the storage type NOx catalyst 10 is applied. It can be determined that the capture of O ₂ is not yet sufficient, and in this case, the process returns to step S24 and the injection of light oil continues to be interrupted. On the other hand, if the determination result is true (Yes) and the rate of change dλ / dt is determined to be positive, it can be determined that the O ₂ capture to the storage NOx catalyst 10 is sufficient, and in this case, the above step S20 is performed. The process proceeds and restarts split injection of light oil based on the target light oil addition amount Qf.

  Thereafter, similarly, the determination result of step S28 becomes true (Yes), and the light oil division is repeatedly performed until the total light oil amount ΣQfn reaches the total light oil addition amount Qfttl. When the determination result of step S30 becomes true (Yes) and the total amount of light oil ΣQfn reaches the total light oil addition amount Qfttl, the addition of light oil is terminated in step S32, thereby completing the NOx purge (block B16). .

  By the way, when the target light oil addition amount Qf is set in this way and split injection of light oil is performed until the sum of the target light oil addition amount Qf reaches the total light oil addition amount Qfttl, as described above, the total light oil addition amount Qfttl Is originally set to the sum of the target light oil addition amount Qf, and by dividing the total light oil addition amount Qfttl by the one target light oil addition amount Qf, the number of injection divisions is inevitably determined. It will be.

Actually, the number of divisions varies depending on the NOx storage capacity of the storage-type NOx catalyst 10 and the like, but in this embodiment, the number of divisions is, for example, that the storage-type NOx catalyst 10 has a temperature A in FIG. 5 times in the vicinity, 3 times in the vicinity of the temperature B, and 4 times in the vicinity of the middle of the temperatures A and B.
Here, referring to FIG. 5, the relationship between the number of light oil injections in the storage-type NOx catalyst 10 and the NOx purification rate is an experimental result when the storage-type NOx catalyst 10 is at a low temperature (for example, temperature A in FIG. 4). (Broken line) and high temperature (for example, temperature B in FIG. 4) are shown separately (solid line), but as shown in the figure, when the storage-type NOx catalyst 10 is at low temperature, When the number of light oil injections is five, the NOx purification rate is the highest and good, and when the temperature is high, the number of light oil injections is three and the NOx purification rate is the highest and good.

In reality, there are many cases where the total light oil addition amount Qfttl is not completely divided by one target light oil addition amount Qf, and a fraction is often obtained. In this case, λ discrimination is executed in step S26 as described above. Therefore, even if the injection is not completed for the fraction, the interruption is satisfactorily performed, and step S28 and step S32 are executed, and the NOx purge is completed satisfactorily.
As described above, according to the storage type NOx catalyst 10 according to the present invention, the NOx purge is efficiently performed by dividing and supplying the light oil as the reducing agent, but the divided NOx catalyst is further divided once according to the catalyst temperature Tc. The target gas oil addition amount Qf is optimized, so that CO is properly generated and NOx is always reduced and removed satisfactorily, and NOx purification performance can be achieved without adding extra light oil regardless of the catalyst temperature Tc. It will improve effectively.

That is, referring to FIG. 6, the time of the light oil injection amount (light oil injection pressure × injection time) (a) and the actual light oil addition amount Qfr (b) corresponding to this when the NOx purge control according to the present invention is performed. The change is shown as an experimental result separately when the storage NOx catalyst 10 is at a low temperature (for example, temperature A in FIG. 4) (broken line) and at a high temperature (for example, temperature B in FIG. 4) (solid line). However, when the target light oil addition amount Qf is set according to the catalyst temperature Tc and purge control is performed, as shown in FIG. 5B, the occlusion-type NOx catalyst 10 is at a low temperature ( In both cases (dashed line) and high temperature (solid line), the actual light oil addition amount Qfr is a value within the incomplete combustion region where CO is generated (for example, addition amounts C and D in FIG. 4). That is, the maximum value of the actual light oil addition amount Qfr is controlled without entering the HC slip region (see FIG. 4), the HC is prevented from slipping downstream of the catalyst, and the complete combustion region where CO ₂ is generated. (Refer to FIG. 4) is controlled to sufficiently exceed, and the amount of CO generated is an appropriate amount without shortage.

Further, when the excess air ratio λ of the exhaust downstream of the catalyst becomes equal to or less than the predetermined value λ1 (step S26), it is determined that HC is slipping even in the middle, and the injection of light oil is interrupted (step S24). ), The slip of HC downstream of the catalyst can be reliably prevented, and the diffusion of HC into the atmosphere can be satisfactorily suppressed.
The description of the embodiment is finished as above, but the present invention is not limited to the above embodiment.

  For example, in the above embodiment, in step S20, the injection time is variably controlled so that the target light oil addition amount Qf is obtained under a constant injection pressure. For example, FIG. 7 shows another example. As described above, the injection pressure may be variably controlled. That is, when the catalyst temperature Tc is high (for example, temperature B in FIG. 4), the injection pressure is increased (a), and when the catalyst temperature Tc is low (for example, temperature A in FIG. 4), the injection pressure is decreased (b). You may do it.

  Moreover, in the said embodiment, although the diesel engine was employ | adopted as the engine 1, the engine 1 is not limited to a diesel engine, A gasoline engine, such as a lean burn engine, may be sufficient.

1 is a schematic configuration diagram of an exhaust gas purification apparatus for an internal combustion engine according to the present invention. 3 is a flowchart showing a control routine of NOx purge control according to the present invention. It is a control block diagram of NOx purge control according to the present invention. Is a diagram showing the relationship between full oxidation reaction according to the catalyst temperature Tc and the target light oil addition amount Qf (CO ₂ evolution) and the incomplete oxidation reaction (CO evolution). It is a figure which shows the relationship between the frequency | count of light oil injection in a storage type NOx catalyst, and a NOx purification rate. It is a figure which shows the time change of the light oil injection quantity (light oil injection pressure x injection time) (a) at the time of implementing NOx purge control concerning this invention, and the actual light oil addition amount Qfr (b) according to this. It is a figure which shows the other Example in a reducing agent supply means.

Explanation of symbols

1 engine (diesel engine)
4 Airflow sensor 10 Occlusion type NOx catalyst 12 Injection valve (reducing agent supply means)
14 NOx sensor 16 O ₂ sensor 18 Temperature sensor (catalyst temperature detection means)
20 ECU (Electronic Control Unit)

Claims (2)

An oxygen trap that is provided in the exhaust passage of an internal combustion engine and has a function of storing NOx in exhaust in an oxidizing atmosphere and releasing and reducing the stored NOx in a reducing atmosphere, and traps oxygen in the oxidizing atmosphere. A storage-type NOx catalyst having a function;
A reducing agent supply means provided on the exhaust upstream side of the storage-type NOx catalyst, and supplying HC as a reducing agent to the storage-type NOx catalyst;
Catalyst temperature detecting means for detecting the temperature of the storage-type NOx catalyst;
NOx occlusion amount detection means for detecting the NOx occlusion amount of the occlusion type NOx catalyst;
Reducing agent supply amount setting means for setting a reducing agent supply amount according to the NOx storage amount detected by the NOx storage amount detection means,
When the NOx occlusion amount detected by the NOx occlusion amount detection means reaches a predetermined amount, the reducing agent supply means is within the range of the reducing agent supply amount set by the reducing agent supply amount setting means. Is divided into a plurality of times and supplied intermittently, and the lower the temperature of the occlusion-type NOx catalyst detected by the catalyst temperature detecting means, the more the number of times of the division is reduced. An exhaust emission control device for an internal combustion engine, wherein the supply amount of reducing agent is increased by decreasing the number of divisions as the temperature is higher while suppressing the supply amount of the agent. Furthermore, provided with an exhaust air-fuel ratio detecting means provided on the exhaust downstream side of the storage-type NOx catalyst and detecting the exhaust air-fuel ratio on the exhaust downstream side of the storage-type NOx catalyst,
The reducing agent supply means determines the one-time reducing agent supply amount so that the exhaust air / fuel ratio detected by the exhaust air / fuel ratio detecting means does not become closer to the rich air / fuel ratio than a predetermined value, and divides the reducing agent 2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein supply is performed.

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