JP2013011202A - Catalyst deterioration determination system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst deterioration determination system which promptly and accurately determines deterioration of an NOstorage-reduction catalyst.SOLUTION: The catalyst deterioration determination system adjusts the amount of a reduction agent such that an exhaust gas air-fuel ratio becomes a rich air-fuel ratio and performs a first reduction agent supply for supplying a reduction agent, and subsequently performs a second reduction agent supply for enriching the exhaust air-fuel ratio more than in the first reduction agent supply, when the amount of NOadsorbed in the NOstorage-reduction catalyst is not higher than a specific amount. The catalyst deterioration determination system performs the deterioration determination in the NOstorage-reduction catalyst, based on both a detection value that is detected by an NHdetection device in the first reduction agent supply and a detection value that is detected by an NHdetection device in the second reduction agent supply.

Description

  The present invention relates to a catalyst deterioration determination system.

When the reduction control of NOx stored in the NOx storage reduction catalyst (hereinafter also simply referred to as NOx catalyst) is executed, and then the estimated value of the NOx storage amount in the NOx catalyst reaches the reference value, A technique for determining that the NOx catalyst is deteriorated when the NOx concentration detected by the NOx sensor downstream of the NOx catalyst is equal to or higher than a predetermined concentration is known (for example, see Patent Document 1).

However, it is necessary to wait until a large amount of NOx is occluded in the NOx catalyst, which increases the time required for determining the deterioration of the NOx catalyst. For this reason, when the NOx catalyst is deteriorated, there is a possibility that NOx flows out until the deterioration determination is completed.

Further, a technique is known in which the deterioration state of the NOx catalyst is determined based on the output value of the NOx sensor downstream of the NOx catalyst when the reducing atmosphere is used (see, for example, Patent Document 2). This deterioration of the NOx catalyst indicates sulfur poisoning of the NOx catalyst. Since this determination is made after waiting until the NOx concentration becomes stable, the time required for the deterioration determination becomes longer.

JP 2007-162468 A Japanese Patent Laid-Open No. 11-229849

The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a technique that can quickly and accurately determine the deterioration of the NOx storage reduction catalyst.

In order to achieve the above object, the catalyst deterioration determination system according to the present invention is:
In a catalyst deterioration determination system for determining deterioration of an NOx storage reduction catalyst that is provided in an exhaust passage of an internal combustion engine and stores NOx and reduces the stored NOx by supplying a reducing agent,
A supply device for changing the air-fuel ratio of the exhaust gas passing through the NOx storage reduction catalyst by supplying a reducing agent to the NOx storage reduction catalyst;
An NH ₃ detector for detecting NH _{3 in} the exhaust downstream of the NOx storage reduction catalyst;
A control device that adjusts the amount of the reducing agent so that the air-fuel ratio of the exhaust gas becomes a rich air-fuel ratio when the reducing agent is supplied from the supply device;
When the NOx amount occluded in the NOx storage reduction catalyst is equal to or less than a predetermined amount, the control device adjusts the reducing agent amount so that the air-fuel ratio of the exhaust gas becomes a rich air-fuel ratio, while reducing the reducing agent from the supply device. After the first reducing agent supply is performed, the second reducing agent supply is performed to make the air-fuel ratio of the exhaust gas further richer than in the first reducing agent supply, and the first reducing agent supply a detection value by the NH ₃ detection device when a detected value of the NH ₃ detection device when the second reducing agent supply determination device based on both the deterioration determination of the NOx storage reduction catalyst and ,
Is provided.

The NOx storage reduction catalyst stores NOx when the air-fuel ratio is lean, and reduces the stored NOx when a reducing agent is present. The supply device can supply the reducing agent to the NOx storage reduction catalyst. The reducing agent may be supplied into the exhaust gas flowing through the exhaust passage or may be discharged from the internal combustion engine. Then, by supplying the reducing agent, the air-fuel ratio of the exhaust is lowered.

Here, when a reducing agent is supplied to the NOx storage reduction catalyst, H ₂ or HC reacts with NO and N
H ₃ may be generated. When the NOx storage reduction catalyst deteriorates, the reduction efficiency of the NOx storage reduction catalyst decreases. For this reason, the amount of NH ₃ produced by the NOx storage reduction catalyst is also reduced. Therefore, the detection value of the NH ₃ detection device when the reducing agent is supplied with the rich air-fuel ratio as a target becomes small according to the degree of deterioration of the storage reduction type NOx catalyst.

Further, when the reducing agent is supplied so that the air-fuel ratio of the exhaust gas becomes a rich air-fuel ratio in a state where NOx is not stored in the NOx storage reduction catalyst, the NOx flowing into the NOx storage reduction catalyst reacts with the reducing agent. Thus, NH ₃ is generated. The amount of NH ₃ produced varies depending on the degree of deterioration of the NOx storage reduction catalyst. That is, when the NOx storage reduction catalyst is normal, NOx flowing into the NOx storage reduction catalyst reacts with the reducing agent to produce a large amount of NH _3, but the NOx storage reduction catalyst deteriorates. If so, the amount of NH ₃ produced decreases according to the degree of deterioration.

Therefore, the determination device determines the deterioration of the NOx catalyst when the amount of NOx stored in the NOx catalyst is equal to or less than a predetermined amount. Here, the predetermined amount is the NOx occlusion amount when NOx is not released from the NOx catalyst even when the reducing agent is supplied or is equivalent to not being released. Further, although NOx is released, it may be the NOx occlusion amount when the influence on the deterioration determination is within an allowable range. Further, the NOx occlusion amount may be zero. Note that the amount of NOx stored in the NOx catalyst can be equal to or less than a predetermined amount immediately after supplying the reducing agent to the NOx catalyst and reducing NOx. Therefore, the deterioration determination may be performed after supplying a reducing agent to the NOx catalyst and reducing NOx.

Thus, if the amount of NOx stored in the NOx catalyst is equal to or less than the predetermined amount, it is possible to suppress the release of NOx from the NOx catalyst and the generation of NH ₃ when the reducing agent is supplied. That is, only NOx flowing into the NOx catalyst can be reacted with the reducing agent, so that the influence of NOx stored in the NOx catalyst can be eliminated.

  For the deterioration determination, the first reducing agent supply that is rich but the air-fuel ratio is relatively high (the degree of richness is low) and the air-fuel ratio is lower (the degree of richness is lower than the first reducing agent supply). (High) second reductant supply.

Here, when the NOx storage reduction catalyst is normal, the NOx reduction efficiency is high even if the richness of the air-fuel ratio of the exhaust is increased. Therefore, if NOx is present in the exhaust, NOx and the reducing agent Reacts, so the amount of NH ₃ produced increases. On the other hand, when the NOx storage reduction catalyst is deteriorated, the NOx reduction efficiency decreases, and the amount of NOx that can be reacted decreases. That is, even if NOx and a reducing agent are present in the exhaust gas, NOx cannot be reduced in the catalyst. For this reason, when the NOx storage reduction catalyst is deteriorated, if the richness of the air-fuel ratio of the exhaust is increased to some extent, the amount of NOx to be reacted reaches the limit, and even if the reducing agent is increased, the NH ₃ The production will not increase.

That is, in the case storage reduction type NOx catalyst is normal, in the case where has deteriorated, detection by NH ₃ detection device of the detection value and the second reducing agent supply by NH ₃ detection device in the first reducing agent supply Since the values are different, it is possible to determine the deterioration of the NOx storage reduction catalyst based on both values.

Note that the detection value obtained by the NH ₃ detection device may be a detection value during which the influence of the reducing agent appears in the detection value after supplying the reducing agent. The present invention may also be detected value when the exhaust gas supplied to the reducing agent passes through the NH ₃ detection device.

Thus, the determination accuracy can be increased by performing the deterioration determination when the NOx occlusion amount is equal to or less than the predetermined amount. In addition, since it is not necessary to wait for NOx to be stored in the NOx storage reduction catalyst, it is possible to quickly determine deterioration.

Further, in the present invention, the determination device has a detection value by the NH ₃ detection device at the time of supplying the first reducing agent equal to or more than a first predetermined value and the NH at the time of supplying the second reducing agent. _{When the} value detected by the _three detectors is equal to or greater than the second predetermined value, it can be determined that the NOx storage reduction catalyst is normal.

If the detected value by the NH ₃ detector is large both when the first reducing agent is supplied and when the second reducing agent is supplied, the NOx reduction efficiency is sufficiently high, and therefore the NOx storage reduction catalyst is normal. Can be determined. As described above, the higher the degree of deterioration of the NOx storage reduction catalyst, the smaller the amount of NH ₃ produced when the reducing agent is supplied. For this reason, the detection value of the NH ₃ detection device decreases with the progress of deterioration. A first predetermined value and a second predetermined value are set as threshold values for the detected value of the NH ₃ detector. The first predetermined value and the second predetermined value may be a lower limit value of a detected value when the NOx storage reduction catalyst is normal. The first predetermined value and the second predetermined value may be determined with some allowance in consideration of errors and the like. The first predetermined value and the second predetermined value may be acceptable detection values. The first predetermined value and the second predetermined value may be the same value. Further, when the NOx storage reduction catalyst is normal, the amount of NH ₃ generated increases as the rich degree increases, so the second predetermined value may be made larger than the first predetermined value.

Further, in the present invention, the determination device has a detection value by the NH ₃ detection device at the time of supplying the first reducing agent that is less than a third predetermined value and the NH at the time of supplying the second reducing agent. _{When the} value detected by the _three detectors is less than the fourth predetermined value, it can be determined that the NOx storage reduction catalyst has deteriorated.

That is, if the value detected by the NH ₃ detector is small both when the first reducing agent is supplied and when the second reducing agent is supplied, the NOx reduction efficiency is considered to be low, so the NOx storage reduction catalyst deteriorates. Can be determined. As described above, the higher the degree of deterioration of the NOx storage reduction catalyst, the smaller the amount of NH ₃ produced when the reducing agent is supplied. Therefore, the detection value by the NH ₃ detection device is reduced. A third predetermined value and a fourth predetermined value are set as threshold values at which the detection value of the NH ₃ detector becomes unacceptable. The third predetermined value and the fourth predetermined value may be detected values when the NOx storage reduction catalyst is at the boundary. Further, the third predetermined value and the fourth predetermined value may be detected values that cannot be allowed. The third predetermined value and the fourth predetermined value may be determined with some allowance in consideration of errors and the like. The third predetermined value and the fourth predetermined value may be the same value. Further, the fourth predetermined value may be larger than the third predetermined value. Further, the first predetermined value and the third predetermined value may be the same value. Further, the third predetermined value may be smaller than the first predetermined value. Further, the second predetermined value and the fourth predetermined value may be the same value. Further, the fourth predetermined value may be made smaller than the second predetermined value.

In the present invention, the determining device includes a detection value by the NH ₃ detection device when the second reducing agent supply, the detection value by the NH ₃ detection device when the first reducing agent supply, When the difference is less than the threshold value, it can be determined that the NOx storage reduction catalyst is deteriorated.

Here, when the NOx storage reduction catalyst is normal, when the air-fuel ratio of the exhaust is changed,
Since the NH ₃ in accordance with the amount of reducing agent is generated, as the degree of richness is high, it is more the amount of NH _3. Therefore, the detection value by the NH ₃ detection device at the time of supplying the second reducing agent is larger than the detection value by the NH ₃ detection device at the time of supplying the first reducing agent, and the difference between both detection values becomes relatively large. . On the other hand, when the NOx storage reduction catalyst is deteriorated, the reaction amount of NOx is limited, so that the amount of NH ₃ produced does not change even if the rich degree changes. That is, the detection value of the NH ₃ detection device when the second reducing agent supply, the detection value of the NH ₃ detection device when the first reducing agent supply, the difference is relatively small for. Therefore, it is possible to determine the deterioration of the NOx storage reduction catalyst based on the difference between the detected values. If the lower limit value of the detected value difference when the NOx storage reduction catalyst is normal is set as the threshold value, the deterioration determination can be performed by comparing the detected value difference with the threshold value. The threshold value may be a difference between detection values when the NOx storage reduction catalyst is at the boundary between normal and deteriorated.

In the present invention, the detection value may be a maximum value of NH ₃ concentration.

Here, the higher the degree of deterioration of the NOx catalyst, the more NH ₃ is produced.
Then, the maximum value of the detected value of the NH ₃ detection device is increased. Therefore, it is possible to determine the deterioration based on the maximum detected value. As described above, the deterioration determination can be easily and accurately performed by performing the deterioration determination using the maximum value of the detection value of the NH ₃ detection device correlated with the degree of deterioration of the NOx catalyst.

In the present invention, the detection value may be an integrated value of the NH ₃ concentration.

Like the maximum value of the detected value of the NH ₃ detection device decreases in accordance with the degree of the integrated value of the detected value deterioration of the NOx storage reduction catalyst of the NH ₃ detection device. Therefore, it is possible to determine the deterioration based on the integrated value of the detected values. The integrated value is obtained, for example, by adding the detection value of the NH ₃ detection device every predetermined time. As described above, the deterioration determination can be easily and accurately performed by performing the deterioration determination using the integrated value of the detection values of the NH ₃ detection device correlated with the deterioration degree of the NOx storage reduction catalyst.

According to the present invention, it is possible to quickly and accurately determine the deterioration of the NOx storage reduction catalyst.

It is a figure which shows schematic structure of the internal combustion engine which concerns on an Example, and its exhaust system. It is a figure for demonstrating the NOx occlusion effect | action in a NOx catalyst. It is a figure for demonstrating the reduction | restoration effect | action of NOx in a NOx catalyst. NOx is a diagram showing the relationship between the duration of the rich spike control when performing rich spike control to normal NOx catalysts that have not been occluded, the NH ₃ concentration on the downstream side. It is the figure which showed the relationship between the continuation time of rich spike control when rich spike control is performed with respect to the deteriorated NOx catalyst in which NOx is not occluded, and the downstream NH ₃ concentration. Is a timing chart illustrating the changes of the air-fuel ratio and the NH ₃ concentration during degradation determination according to the embodiment. It is the flowchart which showed the flow of the deterioration judgment of the NOx catalyst.

Hereinafter, specific embodiments of the catalyst deterioration determination system according to the present invention will be described with reference to the drawings.

<Example 1>
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine and its exhaust system according to the present embodiment. The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine having four cylinders.

  An exhaust passage 2 is connected to the internal combustion engine 1. An occlusion reduction type NOx catalyst 4 (hereinafter referred to as NOx catalyst 4) is provided in the middle of the exhaust passage 2.

The NOx catalyst 4 is constituted by, for example, using alumina (Al ₂ O ₃ ) as a carrier, and carrying, for example, barium (Ba) and platinum (Pt) on the carrier.

This NOx catalyst 4 stores NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and reduces the stored NOx when the oxygen concentration of the inflowing exhaust gas decreases and a reducing agent is present. Have

An injection valve 5 that injects a reducing agent into the exhaust gas in the exhaust passage 2 upstream of the NOx catalyst 4.
Is attached. The injection valve 5 is opened by a signal from the ECU 10 described later, and injects the reducing agent into the exhaust. For example, the fuel (light oil) of the internal combustion engine 1 is used as the reducing agent, but the reducing agent is not limited thereto.

The fuel injected from the injection valve 5 into the exhaust passage 2 lowers the air-fuel ratio of the exhaust flowing from the upstream side of the exhaust passage 2. When NOx stored in the NOx catalyst 4 is reduced, so-called rich spike control is performed in which the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 4 is decreased in a relatively short cycle by injecting fuel from the injection valve 5. Run. The amount of reducing agent injected from the injection valve 5 is determined based on, for example, the operating state of the internal combustion engine 1 (engine speed and fuel injection amount). The relationship among the amount of reducing agent, engine speed, and engine load can be mapped in advance. Further, an air-fuel ratio sensor may be attached to the exhaust passage 2 and the amount of reducing agent may be feedback controlled so that the air-fuel ratio detected by the air-fuel ratio sensor becomes a target value.

  In this embodiment, the injection valve 5 corresponds to the supply device in the present invention. The reducing agent can also be supplied by discharging unburned fuel from the internal combustion engine 1. That is, an in-cylinder injection valve for injecting fuel into the cylinder is provided, and sub-injection (post-injection) for injecting fuel again during the expansion stroke or exhaust stroke after performing main injection from the in-cylinder injection valve is performed. Alternatively, by delaying the fuel injection timing from the in-cylinder injection valve, the gas containing a large amount of reducing agent can be discharged from the internal combustion engine 1.

Further, in the exhaust passage 2 downstream of the NOx catalyst 4, NH ₃ concentration for measuring the NH ₃ concentration in the exhaust gas is measured.
_Three sensors 8 and a temperature sensor 9 for measuring the temperature of the exhaust are attached. In the present embodiment, the NH ₃ sensor 8 corresponds to the NH ₃ detector in the present invention. The NH ₃ sensor 8 may be a NOx sensor that detects NOx. NOx sensor detects the NOx and NH ₃ as a NOx.

  The internal combustion engine 1 configured as described above is provided with an ECU 10 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 10 controls the operation state of the internal combustion engine 1 according to the operation conditions of the internal combustion engine 1 and the request of the driver.

In addition to the above sensors, the ECU 10 outputs an electric signal corresponding to the amount of depression of the accelerator pedal 11 by the driver to detect the engine load, and an accelerator position sensor 12 for detecting the engine speed. 13 are connected via electric wiring, and the output signals of these various sensors are input to the ECU 10.

  On the other hand, the injection valve 5 is connected to the ECU 10 via electric wiring, and the ECU 10 controls the opening and closing timing of the injection valve 5. In this embodiment, the ECU 10 that adjusts the amount of reduction supplied from the injection valve 5 corresponds to the control device in the present invention.

The ECU 10 performs rich spike control twice, and determines the deterioration of the NOx catalyst 4 based on the NH ₃ concentration detected by the NH ₃ sensor 8 at this time. In the rich spike control at the time of deterioration determination, first, the reducing agent is injected from the injection valve 5 within a range where the air-fuel ratio of the exhaust gas becomes rich, and then the reducing agent is injected from the injection valve 5 so that the air-fuel ratio further decreases. . The first rich spike control is referred to as the first reducing agent supply, and the second rich spike control is referred to as the second reducing agent supply. Then, the NH ₃ concentration is detected by the NH ₃ sensor 8 in each of the first reducing agent supply and the second reducing agent supply.

  Here, FIG. 2 is a view for explaining the NOx occlusion action in the NOx catalyst 4. FIG. 3 is a view for explaining the NOx reduction action in the NOx catalyst 4.

The NOx catalyst 4 oxidizes NO with O ₂ on Pt when the air-fuel ratio of the exhaust gas is lean, and Ba
Occluded as Ba (NO ₃ ) ₂ . On the other hand, when a reducing agent is supplied to make the exhaust air-fuel ratio rich, Ba (NO ₃ ) ₂ is released as NO ₂ and further reduced to N ₂ on Pt. At this time, in the NOx catalyst 4, NO and H ₂ react to generate NH ₃ and H ₂ O. Further, HC and NO react to generate NH ₃ , H ₂ O, and CO ₂ . Note that the NOx flowing into the NOx catalyst 4 reacts similarly to the NOx released from the NOx catalyst 4. The NH ₃ thus generated is detected by the NH ₃ sensor 8.

By the way, if the reducing agent is supplied when NOx is not stored in the NOx catalyst 4, NOx
Although NOx from the catalyst 4 is not released, NOx in the exhaust gas is NH ₃ is produced by reacting with a reducing agent. The amount of NH ₃ produced varies depending on the degree of deterioration of the NOx catalyst 4.

Here, FIG. 4 is a diagram showing the relationship between the rich spike control duration when the rich spike control is performed on the normal NOx catalyst 4 in which NOx is not occluded and the NH ₃ concentration on the downstream side. It is. FIG. 5 shows the relationship between the rich spike control duration when the rich spike control is performed on the deteriorated NOx catalyst 4 in which NOx is not occluded and the downstream NH ₃ concentration. FIG. A triangle mark (solid line) indicates when the air-fuel ratio is 14, and a square mark (dashed line) indicates when the air-fuel ratio is 13.

The normal NOx catalyst 4 is, for example, the NOx catalyst 4 that has not deteriorated at all, or the NOx catalyst 4 that has deteriorated but has a degree of deterioration within an allowable range. Further, the deteriorated NOx catalyst 4 is a NOx catalyst 4 whose degree of deterioration exceeds an allowable range.

In the normal NOx catalyst 4, even if the NOx occlusion amount is 0, when NOx flows into the NOx catalyst 4, NOx and the reducing agent react to generate a relatively large amount of NH ₃ . In addition, the higher the degree of richness, the more NH ₃ is generated.

On the other hand, in the deteriorated NOx catalyst 4, even if NOx flows into the NOx catalyst 4, the reduction efficiency is low, so the reaction between NOx and the reducing agent on Pt becomes slow. For this reason, the amount of NH ₃ produced is less than that of the normal NOx catalyst 4. Further, since the deteriorated NOx catalyst 4 reaches the limit of the catalytic reaction even if the air-fuel ratio of the exhaust gas is made richer, the amount of NH ₃ produced does not change much.

By detecting this phenomenon with the NH ₃ sensor 8, the degree of deterioration of the NOx catalyst 4 can be determined.

FIG. 6 is a time chart showing the transition of the air-fuel ratio and the NH ₃ concentration at the time of deterioration determination according to the present embodiment. In the NH ₃ concentration, the solid line indicates the deteriorated NOx catalyst 4, and the alternate long and short dash line indicates the normal NOx catalyst 4.

  In this embodiment, as the first reducing agent supply, the reducing agent is supplied so that the air-fuel ratio of the exhaust gas becomes 14. Further, as the second reducing agent supply, the reducing agent is supplied so that the air-fuel ratio of the exhaust gas becomes 13.

Whether the NOx catalyst 4 is normal or deteriorated, the NH ₃ concentration increases immediately after the supply of the reducing agent. The degree of increase is higher in the normal NOx catalyst 4 than in the deteriorated NOx catalyst 4.

Here, when the air-fuel ratio of the exhaust gas becomes a rich air-fuel ratio, NH ₃ flows out from the NOx catalyst 4 downstream. When the NOx catalyst 4 is deteriorated, the surface area of Pt is reduced, so that the reduction efficiency in the NOx catalyst 4 is reduced, so that the amount of NH ₃ generation described with reference to FIG. 3 is also reduced. Therefore, the amount of NH ₃ flowing downstream from the NOx catalyst 4 decreases according to the degree of deterioration of the NOx catalyst 4. Therefore, if NH ₃ flowing out from the NOx catalyst 4 is detected by the NH ₃ sensor 8, it is possible to determine the deterioration of the NOx catalyst 4.

That is, the maximum value of the NH ₃ concentration at the time of supplying the first reducing agent and the second reducing agent is higher in the normal NOx catalyst 4. In the normal NOx catalyst 4, the NH ₃ concentration is higher when the second reducing agent is supplied than when the first reducing agent is supplied. However, in the deteriorated NOx catalyst 4, the first reducing agent is supplied. The NH ₃ concentration is almost the same between the time when the second reducing agent is supplied. If the NOx catalyst 4 is normal, the amount of NH ₃ generated increases as the air-fuel ratio becomes rich until the air-fuel ratio becomes approximately 13.

Therefore, the ECU 10 determines that the maximum NH ₃ concentration when the first reducing agent is supplied is equal to or greater than the first predetermined value, and the maximum NH ₃ concentration when the second reducing agent is supplied is equal to or greater than the second predetermined value. Sometimes, it can be determined that the NOx catalyst 4 is normal. The first predetermined value and the second predetermined value may be set as the same threshold value (normal threshold value). Further, the ECU 10 determines that the maximum NH ₃ concentration at the time of supplying the first reducing agent is less than the third predetermined value, and the maximum value of the NH ₃ concentration at the time of supplying the second reducing agent is less than the fourth predetermined value. Sometimes, it can be determined that the NOx catalyst 4 has deteriorated.
The third predetermined value and the fourth predetermined value may be set as the same value threshold (deterioration threshold). Further, the normal threshold value and the deterioration threshold value may be the same value. Further, ECU 10 includes a maximum value of the NH ₃ concentration at the second reducing agent supply, when the difference between the maximum value of the NH ₃ concentration at the first reducing agent supply, is less than the threshold value, NOx catalyst 4 It can be determined that it has deteriorated. The first predetermined value, the second predetermined value, the third predetermined value, the fourth predetermined value, and the threshold value can be obtained in advance by experiments or the like.

Instead of the maximum value of the NH ₃ concentration can be used an integrated value of NH ₃ concentrations. That is, as the degree of deterioration of the NOx catalyst 4 increases, the maximum value of the NH ₃ concentration decreases and the integrated value of the NH ₃ concentration decreases.

In addition, before determining whether the NOx catalyst 4 is deteriorated, rich spike control (also referred to as normal rich spike control) for releasing NOx stored in the NOx catalyst 4 is performed and stored in the NOx catalyst 4. NOx that is present may be reduced. If it does so, it can control that NOx is released from NOx catalyst 4 at the time of degradation judgment of NOx catalyst 4.

FIG. 7 is a flowchart showing a flow for determining the deterioration of the NOx catalyst 4. This routine is executed every predetermined period.

In step S101, it is determined whether or not a precondition for determining deterioration of the NOx catalyst 4 is satisfied. For example, it is determined that the precondition is satisfied when the NH ₃ sensor 8 is normal and the temperature of the NOx catalyst 4 is a temperature suitable for NOx reduction. Whether or not the NH ₃ sensor 8 is normal can be determined by a known technique. The temperature suitable for NOx reduction is, for example, the temperature at which the NOx catalyst 4 is activated. The temperature of the NOx catalyst 4 is detected by a temperature sensor 9. Note that it may be determined whether a condition for performing rich spike control is satisfied.

  If an affirmative determination is made in step S101, the process proceeds to step S102, and if a negative determination is made, this routine is terminated.

In step S102, it is determined whether the NOx occlusion amount of the NOx catalyst 4 is equal to or less than a predetermined amount. Here, the predetermined amount is set as a value equal to NOx not stored in the NOx catalyst 4. In this step, it may be determined whether or not NOx is occluded in the NOx catalyst 4. In this step, it is determined whether or not it is necessary to perform rich spike control for releasing NOx from the NOx catalyst 4 (may be normal rich spike control). NOx
The occlusion amount can be estimated from, for example, the NOx occlusion amount estimated based on the operating state (engine speed and engine load) of the internal combustion engine 1 and the ratio of NOx occluded in the NOx catalyst 4. Further, when rich spike control is performed, the NOx occlusion amount that decreases due to the rich spike control is subtracted. The reduced NOx occlusion amount by rich spike control can be obtained in advance by experiments or the like. These may be obtained on the assumption that the NOx catalyst 4 is normal.

  If an affirmative determination is made in step S102, the process proceeds to step S105, and if a negative determination is made, the process proceeds to step S103.

In step S103, normal rich spike control is performed. That is, rich spike control for releasing NOx stored in the NOx catalyst 4 is performed. The air-fuel ratio of the exhaust at this time is an air-fuel ratio suitable for NOx release / reduction.

  In step S104, it is determined whether or not the NOx occlusion amount of the NOx catalyst 4 is equal to or less than a predetermined amount. In this step, processing similar to that in step S102 is performed. If an affirmative determination is made in step S104, the process proceeds to step S105, and if a negative determination is made, the process returns to step S103. That is, normal rich spike control is performed until the amount of NOx stored in the NOx catalyst 4 becomes equal to or less than a predetermined amount.

  In step S105, rich spike control is performed with the air-fuel ratio 14 as a target. That is, the first reducing agent is supplied. In this embodiment, the ECU 10 that processes step S105 and subsequent steps corresponds to the determination device according to the present invention.

In step S106, the detection value of the NH ₃ sensor 8 is stored. Note that the maximum value or integrated value of the detection values of the NH ₃ sensor 8 may be stored. In this step, the detection value of the NH ₃ sensor 8 when the first reducing agent is supplied is stored. The integrated value may be an integrated value while NH ₃ is detected by the NH ₃ sensor 8, may be an integrated value during rich spike control, or may be an integrated value for a predetermined time. Good. The integrated value may be obtained, for example, by sequentially adding detection values of the NH ₃ sensor 8 read at a predetermined cycle. Further, the integrated value may be a time integrated value of NH ₃ concentration. Further, the integrated value may be an area below the line indicating the NH ₃ concentration in FIG.

  In step S107, rich spike control is performed with the air-fuel ratio 13 as a target. That is, the second reducing agent is supplied.

In step S108, the detection value of the NH ₃ sensor 8 is stored. The detection value is stored in the same manner as in step S106. In this step, the detection value of the NH ₃ sensor 8 when the second reducing agent is supplied is stored.

In step S109, the detected value of NH ₃ sensor 8 stored in step S106 is greater than or equal to a threshold value (which may be a normal threshold or a first predetermined value), and the detected value of NH ₃ sensor 8 stored in step S108. Is greater than or equal to a threshold value (which may be a normal threshold value or a second predetermined value). For example, it is determined whether or not the maximum value of the detected value of the NH ₃ sensor 8 when each reducing agent is supplied is equal to or greater than the normal threshold. Alternatively, it is determined whether or not the integrated value of the detected values of the NH ₃ sensor 8 at the time of supplying each reducing agent is equal to or greater than a normal threshold value. The normal threshold is set as a lower limit value of a detection value when the NOx catalyst 4 is normal, or a detection value when the NOx catalyst 4 is at the boundary between normal and deterioration. The two normal thresholds may be the same value or different values. If an affirmative determination is made in step S109, the process proceeds to step S110, where it is determined that the NOx catalyst 4 is normal. On the other hand, if a negative determination is made in step S109, the process proceeds to step S111, and it is determined that the NOx catalyst 4 has deteriorated. The normal threshold values (first predetermined value, second predetermined value) are obtained in advance through experiments or the like and stored in the ECU 10.

In step S109, the detected value of the NH ₃ sensor 8 stored in step S106 is less than a threshold value (deterioration threshold or a third predetermined value), and the NH ₃ sensor 8 stored in step S108 is stored. When the detected value is less than a threshold value (may be a deterioration threshold value or a fourth predetermined value), it may be determined that the NOx catalyst 4 has deteriorated. The deterioration threshold is set as a lower limit value of a detection value when the NOx catalyst 4 is normal or a detection value when the NOx catalyst 4 is at the boundary between normal and deterioration. The two deterioration threshold values may be the same value or different values. The deterioration threshold may be smaller than the normal threshold. Further, when the detected value is equal to or higher than the deterioration threshold and lower than the normal threshold, the deterioration determination may be performed again for confirmation. The deterioration threshold values (third predetermined value and fourth predetermined value) are obtained in advance through experiments or the like and stored in the ECU 10.

In step S109, it may be determined whether or not a value obtained by subtracting the detection value stored in step S106 from the detection value stored in step S108 is equal to or greater than a threshold value. That is, if the NOx catalyst 4 is normal, the NH ₃ concentration becomes higher during the supply of the second reducing agent having a higher degree of richness, so that the detected value stored in step S108 is stored in step S106. The value obtained by subtracting the detection value becomes larger. This threshold value is obtained in advance through experiments or the like and stored in the ECU 10 as a value when the NOx catalyst is normal or not.

In this manner, it is possible to determine the deterioration of the NOx catalyst 4 based on the detection values of the NH ₃ sensor 8 when the first reducing agent is supplied and when the second reducing agent is supplied. At this time, NH ₃
If the maximum value or integrated value of the detected value of the sensor 8 is known, it is possible to determine the deterioration immediately. Therefore, it is not necessary to wait until the detected value is stabilized or until NOx is occluded. That is, the deterioration determination can be performed quickly.

In this embodiment, it is determined that the degree of deterioration of the NOx catalyst 4 is higher as the maximum value of the detected value of the NH ₃ sensor 8 at the time of supplying the first reducing agent and at the time of supplying the second reducing agent is smaller. Also good. Similarly, it may be determined that the degree of deterioration of the NOx catalyst 4 is higher as the integrated value of the detected values of the NH ₃ sensor 8 is smaller.

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Exhaust passage 4 Occlusion reduction type NOx catalyst 5 Injection valve 8 Downstream NOx sensor 9 Temperature sensor 10 ECU
11 Accelerator pedal 12 Accelerator opening sensor 13 Crank position sensor

Claims (6)

In a catalyst deterioration determination system for determining deterioration of an NOx storage reduction catalyst that is provided in an exhaust passage of an internal combustion engine and stores NOx and reduces the stored NOx by supplying a reducing agent,
A supply device for changing the air-fuel ratio of the exhaust gas passing through the NOx storage reduction catalyst by supplying a reducing agent to the NOx storage reduction catalyst;
An NH ₃ detector for detecting NH _{3 in} the exhaust downstream of the NOx storage reduction catalyst;
A control device that adjusts the amount of the reducing agent so that the air-fuel ratio of the exhaust gas becomes a rich air-fuel ratio when the reducing agent is supplied from the supply device;
When the NOx amount occluded in the NOx storage reduction catalyst is equal to or less than a predetermined amount, the control device adjusts the reducing agent amount so that the air-fuel ratio of the exhaust gas becomes a rich air-fuel ratio, while reducing the reducing agent from the supply device. After the first reducing agent supply is performed, the second reducing agent supply is performed to make the air-fuel ratio of the exhaust gas further richer than in the first reducing agent supply, and the first reducing agent supply a detected value of the NH ₃ detection device when a detected value of the NH ₃ detection device when the second reducing agent supply determination device based on both the deterioration determination of the NOx storage reduction catalyst and ,
A catalyst deterioration judgment system comprising: The determination device has a detection value by the NH ₃ detection device at the time of supplying the first reducing agent equal to or greater than a first predetermined value, and a detection value by the NH ₃ detection device at the time of supplying the second reducing agent is The catalyst deterioration determination system according to claim 1, wherein when the second predetermined value or more is determined, the NOx storage reduction catalyst is determined to be normal. In the determination device, the detection value by the NH ₃ detection device when the first reducing agent is supplied is less than a third predetermined value, and the detection value by the NH ₃ detection device when the second reducing agent is supplied is The catalyst deterioration determination system according to claim 1 or 2, wherein it is determined that the NOx storage reduction catalyst is deteriorated when it is less than a fourth predetermined value. The determining device includes a detection value by the NH ₃ detection device when the second reducing agent supply, the detection value by the NH ₃ detection device when the first reducing agent supply, when the difference is less than the threshold value of The catalyst deterioration determination system according to claim 1 or 2, wherein the NOx storage reduction catalyst is determined to be deteriorated. 5. The catalyst deterioration determination system according to claim 1, wherein the detected value is a maximum value of the concentration of NH ₃ . 5. The catalyst deterioration determination system according to claim 1, wherein the detected value is an integrated value of the concentration of NH ₃ .

Images