JP5626030B2 - Catalyst deterioration judgment system - Google Patents

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.

JP 2007-162468 A

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;
A NOx sensor that detects the NOx and NH ₃ in the exhaust downstream of than the NOx storage reduction catalyst,
A control device that adjusts the amount of reducing agent so that the air-fuel ratio of the exhaust is close to the theoretical air-fuel ratio when supplying the reducing agent from the supply device;
When NOx is occluded in the NOx storage reduction catalyst, the control device supplies the reducing agent from the supply device while adjusting the amount of reducing agent so that the air-fuel ratio of the exhaust gas is close to the theoretical air-fuel ratio. Determination that the NOx storage reduction catalyst is deteriorated when the detected value when the detected value of the NOx sensor first changes from rising to falling within a predetermined time immediately after the start is over a threshold value Equipment,
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. The NOx sensor also detects NH _{3 in the} same manner as NOx. For this reason, whether the detected value of the NOx sensor is, for example, the concentration of NO ₂ or NH ₃
Cannot be determined. However, when the reducing agent is supplied so as to be close to the theoretical air-fuel ratio, two peaks appear immediately after that in the detected value of the NOx sensor, and are detected at each peak depending on whether or not the NOx storage reduction catalyst is deteriorated. The ingredients are found to be different.

That is, when the NOx storage reduction catalyst is normal, most of the detected components are NOx in the first peak, and most of the detected components are NH in the second peak.
₃ . On the other hand, when the NOx storage reduction catalyst is deteriorated,
Most of the detected components are NOx, and in the second peak, most of the detected components are NO.

Note that the first peak of the detected value of the NOx sensor indicates that NO ₂ desorbed from the NOx storage reduction catalyst is N ₂ whether the NOx storage reduction catalyst is normal or deteriorated. believed to be due to NOx flowing out while the NOx catalyst of NO ₂ without being reduced to. This NOx increases as the reduction capacity of the NOx storage reduction catalyst decreases. That is, as the degree of deterioration increases, the NOx that flows out as NO ₂ increases. Therefore, as the degree of deterioration increases, the first maximum value of the detected value of the NOx sensor increases.

Then, when the NOx storage reduction catalyst is normal, if the reducing agent is supplied so that it is close to the stoichiometric air-fuel ratio, NO ₂ is first desorbed, and after desorption of NO ₂ is almost completed, NH ₃ is desorbed. Begins. This NH ₃ forms a second peak in the detected value of the NOx sensor. When the NOx storage reduction catalyst is normal, the amount of NOx stored is large, and therefore the amount of NOx desorbed when it is in the vicinity of the theoretical air-fuel ratio increases. For this reason, the amount of NH ₃ produced also increases. That is, the amount of NH ₃ generated when the reducing agent is supplied in the vicinity of the theoretical air-fuel ratio decreases according to the degree of deterioration of the NOx storage reduction catalyst.

On the other hand, when the NOx storage reduction catalyst is deteriorated, the second peak of the detected value of the NOx sensor is formed by NO. At this time, NH ₃ hardly flows downstream from the NOx storage reduction catalyst. Here, when the reducing agent is supplied, the reducing agent takes heat from the exhaust gas or the NOx storage reduction catalyst. If the NOx storage reduction catalyst is normal, the temperature of the NOx storage reduction catalyst is raised by the heat generated when the reducing agent reacts. However, if the NOx storage reduction catalyst is deteriorated, the heat generated by the reaction of the reducing agent is reduced.
The temperature rise of the catalyst becomes slow. That is, at the time of supplying the reducing agent, the greater the degree of deterioration of the NOx storage reduction catalyst, the lower the temperature of the catalyst. For this reason, when the reducing agent is supplied to the deteriorated storage reduction type NOx catalyst with the target near the stoichiometric air-fuel ratio, the temperature is low.
It is considered that H ₃ is adsorbed on the NOx storage reduction catalyst. Then, NH ₃ is difficult to desorb from the NOx storage reduction catalyst.

Then, after the supply of the reducing agent is completed, the air-fuel ratio of the exhaust gas shifts from the air-fuel ratio near the stoichiometric air-fuel ratio to the lean air-fuel ratio. At this time, it is considered that NH ₃ adsorbed on the NOx storage reduction catalyst is oxidized by oxygen in the exhaust to generate NO and H ₂ O. This NO is measured by a NOx sensor. Here, if the NOx storage reduction catalyst is deteriorated, the amount of NH ₃ produced is small because the reducing ability is low. For this reason, since NO generated by the oxidation of NH ₃ is also reduced, the second maximum value of the detected value of the NOx sensor is relatively small. That is, if the NOx storage reduction catalyst is normal, a large amount of NH ₃ is desorbed, so that the second maximum value is relatively large. If the NOx catalyst is deteriorated, a little NH ₃ is oxidized and NO is reduced. Is generated, the second maximum value becomes relatively small.

Then, the first maximum value of the detected value of the NOx sensor, that is, the detected value when the detected value of the NOx sensor first changes from rising to lowering is formed by NOx released by reducing the reducing ability. Therefore, it can be said that the greater the maximum value, the higher the degree of deterioration. Therefore, it can be determined that the NOx storage reduction catalyst has deteriorated when the detected value when the detected value of the NOx sensor first changes from rising to falling is equal to or greater than the threshold value. Note that the threshold value here is the first maximum value of the detected value of the NOx sensor when the NOx storage reduction catalyst is at the boundary.

  This phenomenon appears in a relatively short time after the supply of the reducing agent is started. Therefore, the deterioration determination can be performed within a predetermined time immediately after the supply of the reducing agent is started. Note that the predetermined time referred to here can be a time during which two peaks are formed by supplying the reducing agent.

Thus, by performing the deterioration determination of the NOx storage reduction catalyst based on the first maximum value detected by the NOx sensor, it is possible to perform the deterioration determination without being affected by NH ₃ . For this reason, the determination accuracy can be increased. Further, since the deterioration determination can be performed immediately after the supply of the reducing agent is started, it is possible to quickly determine the deterioration.

In addition, in order to achieve the above-described problem, the catalyst deterioration determination system according to the present invention includes:
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;
A NOx sensor that detects the NOx and NH ₃ in the exhaust downstream of than the NOx storage reduction catalyst,
A control device that adjusts the amount of reducing agent so that the air-fuel ratio of the exhaust is close to the theoretical air-fuel ratio when supplying the reducing agent from the supply device;
When NOx is occluded in the NOx storage reduction catalyst, the control device supplies the reducing agent from the supply device while adjusting the amount of reducing agent so that the air-fuel ratio of the exhaust gas is close to the theoretical air-fuel ratio. When the detected value of the NOx sensor changes from rising to falling for the second time within a predetermined time immediately after starting the NOx, it is determined that the NOx storage reduction catalyst is deteriorated when the detected value is equal to or less than a threshold value. A determination device;
Is provided.

The second maximum value of the detected value of the NOx sensor, that is, the detected value when the detected value of the NOx sensor changes from rising to falling for the second time is as described above when the NOx storage reduction catalyst is normal. Is large, and small when it is degraded.

Then, it can be determined that the NOx storage reduction catalyst is deteriorated when the detected value when the detected value of the NOx sensor changes from rising to falling for the second time is equal to or less than the threshold value. In addition,
The threshold value here is the second maximum value of the detected value of the NOx sensor when the NOx storage reduction catalyst is at the boundary of whether or not it is deteriorated.

  This phenomenon also appears in a short time after the supply of the reducing agent is started. Therefore, the deterioration determination can be performed within a predetermined time immediately after the supply of the reducing agent is started. Note that the predetermined time referred to here can be a time during which two peaks are formed by supplying the reducing agent.

Further, in the present invention, the determination device is configured to reduce the amount of reducing agent when the NOx is stored in the NOx storage reduction catalyst so that the air-fuel ratio of the exhaust is close to the stoichiometric air-fuel ratio by the control device. Within a predetermined time immediately after starting the supply of the reducing agent from the supply device while adjusting the control value, the detected value when the detected value of the NOx sensor first changes from rising to falling is equal to or greater than the threshold value, or the second time It can be determined that the NOx storage reduction catalyst has deteriorated when the detected value when the change from rising to falling is below the threshold value.

That is, since it can be determined that the NOx catalyst has deteriorated from either the first maximum value or the second maximum value detected by the NOx sensor, the accuracy of the NOx catalyst deterioration determination can be improved. Can do.
In the present invention, the predetermined time may be 10 seconds.

That is, when 10 seconds have elapsed since the start of the supply of the reducing agent, since two peaks are formed, it is possible to determine the deterioration of the NOx storage reduction catalyst. And since degradation determination can be performed in a short period of 10 seconds, rapid degradation determination is possible.

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 effect | action of NOx in a NOx catalyst. It is the figure which showed the state of this NOx catalyst when a NOx catalyst is normal. It is the figure which showed the state of this NOx catalyst when a NOx catalyst has deteriorated. It is the figure which showed transition of the detected value of the downstream NOx sensor when rich spike control is performed when the NOx catalyst is normal. It is the figure which showed transition of the detected value of a downstream NOx sensor at the time of performing rich spike control when the NOx catalyst has deteriorated. FIG. 5 is a diagram showing a reaction flow in the NOx catalyst when the deterioration determination rich spike control targeting the vicinity of the theoretical air-fuel ratio is performed when the NOx catalyst is deteriorated. FIG. 5 is a diagram showing a flow of reaction in the NOx catalyst when the NOx catalyst is deteriorated and the rich spike control for deterioration determination targeting the vicinity of the theoretical air / fuel ratio is performed and then the lean air / fuel ratio is shifted. It is a time chart which showed transition of the air fuel ratio of exhaust at the time of rich spike control concerning an example, and NOx concentration detected by a downstream NOx sensor. 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.

  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, an upstream side N for measuring the NOx concentration in the exhaust is provided in the exhaust passage 2 upstream from the injection valve 5.
An Ox sensor 7 is attached. A downstream NOx sensor 8 that measures the NOx concentration in the exhaust and a temperature sensor 9 that measures the temperature of the exhaust are attached to the exhaust passage 2 downstream of the NOx catalyst 4. In the present embodiment, the downstream NOx sensor 8 corresponds to the NOx sensor in the present invention.

  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 with the target in the vicinity of the stoichiometric air-fuel ratio (may be slightly richer than the stoichiometric air-fuel ratio), and based on the NOx concentration detected by the downstream NOx sensor 8 at this time, the NOx catalyst. 4. Degradation judgment of 4 is performed. The target air-fuel ratio at this time is an air-fuel ratio that is set to determine the deterioration of the NOx catalyst 4, and is a larger air-fuel ratio than during rich spike control that is normally performed to reduce NOx. Hereinafter, the rich spike control performed for determining the deterioration of the NOx catalyst 4 is referred to as “deterioration determination rich spike control”.

Here, NOx and NH ₃ are detected as NOx by the downstream NOx sensor 8. For this reason, it is difficult to determine whether NH ₃ has been detected by the downstream NOx sensor 8 or whether NOx has been detected. However, by setting the air-fuel ratio of the exhaust to be close to the theoretical air-fuel ratio, two peaks appear in the detection value of the downstream side NOx sensor 8. And each local maximum is
It changes according to the degree of deterioration of the NOx catalyst 4.

  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. FIG. 4 is a view showing a state of the NOx catalyst 4 when the NOx catalyst 4 is normal. FIG. 5 is a view showing the state of the NOx catalyst 4 when the NOx catalyst 4 is deteriorated.

NOx catalyst 4 is, the NO when the air-fuel ratio of the exhaust gas is lean is oxidized with _{O 2} on Pt, Ba
Occluded as Ba (NO ₃ ) ₂ (see FIG. 2). On the other hand, when the reducing agent is supplied and the air-fuel ratio of the exhaust is made rich near the stoichiometric air-fuel ratio, Ba (NO ₃ ) ₂ is released as NO ₂ and further reduced to N ₂ on Pt (FIG. 3). reference).

However, when the NOx catalyst 4 is deteriorated (sintered), the surface area of Pt becomes smaller than normal (see FIGS. 4 and 5). For this reason, it becomes difficult for the released NO ₂ to be reduced to N ₂ . Then, for example, part of NO ₂ desorbed from Ba flows downstream from the NOx catalyst 4. The amount of NO ₂ flowing downstream from the NOx catalyst 4 increases according to the degree of deterioration of the NOx catalyst 4. The NO ₂ flowing out downstream of the NOx catalyst 4 in this way is detected by the downstream NOx sensor 8.

Further, if the exhaust air-fuel ratio is a rich air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio, the NOx catalyst 4
O and H ₂ or HC react to produce NH ₃ .

However, when the NOx catalyst 4 deteriorates (sintering), the Pt sintering causes N
Ba that can occlude Ox decreases (see FIG. 5). For this reason, the storage capacity of the NOx catalyst 4 is lowered. That is, when the NOx catalyst 4 deteriorates, the amount of NOx stored as Ba (NO ₃ ) ₂ decreases. By surface area of Pt is reduced, the amount of NH ₃ that is generated when the supply of the reducing agent also decreases. NH ₃ reacts with O ₂ in the downstream NOx sensor 8 to become NO, and is detected as NOx.

Here, FIG. 6 is a diagram showing the transition of the detected value of the downstream side NOx sensor 8 when the rich spike control is performed when the NOx catalyst 4 is normal. (A), (B), and (C) in FIG.
Each has a different air-fuel ratio (which may be a target air-fuel ratio or an actual air-fuel ratio) at the time of reducing agent injection. 6A shows when the air-fuel ratio is 14 (that is, when the air-fuel ratio is rich), and FIG. 6B shows when the air-fuel ratio is 14.8 (that is, near the stoichiometric air-fuel ratio). FIG. 6C shows a case where the air-fuel ratio is 16 (that is, a lean air-fuel ratio). FIG. 7 is a graph showing the transition of the detection value of the downstream NOx sensor 8 when rich spike control is performed when the NOx catalyst 4 is deteriorated. 7A, 7 </ b> B, and 7 </ b> C have different air-fuel ratios at the time of reducing agent injection (which may be a target air-fuel ratio or an actual air-fuel ratio). 7A shows when the air-fuel ratio is 14 (that is, when the air-fuel ratio is rich), and FIG. 7B shows when the air-fuel ratio is 14.8 (that is, when the air-fuel ratio is close to the theoretical air-fuel ratio). FIG. 7C shows a case where the air-fuel ratio is 16 (that is, a lean air-fuel ratio). Note that A in the NOx concentrations in FIGS. 6 and 7 all indicate the same value.

When the air-fuel ratio is 14, both the normal and deteriorated NOx catalyst 4 reacts with H ₂ , HC and NO in the NOx catalyst 4 to generate NH _3. Most of what is detected by the sensor 8 is NH ₃ . When the NOx catalyst 4 is deteriorated, there exists a maximum value that is considered to be due to NOx release in the initial stage (FIG. 7A).
) (See the area surrounded by the broken line)), but this maximum value is not seen when it is normal.

When the air-fuel ratio is 14.8, there are two peaks in the detected value of the downstream NOx sensor 8 both when the NOx catalyst 4 is normal and when it is deteriorated. When the component investigation was attempted at each of these two peaks, when the NOx catalyst 4 was normal, most of the components were at the time of the first peak (see the portion surrounded by the broken line in FIG. 6B). Was NO ₂ , and it was found that most of the components were NH ₃ at the time of the second peak (see the portion surrounded by the one-dot chain line in FIG. 6B). When the NOx catalyst 4 is deteriorated, most of the components are NO ₂ at the first peak (see the portion surrounded by the broken line in FIG. 7B), and the second It was found that most of the components were NO at the peak (see the portion surrounded by the alternate long and short dash line in FIG. 7B). This NO is considered to be obtained by oxidizing NH ₃ .

Further, when the air-fuel ratio is 16, there is one peak in the NOx concentration both when the NOx catalyst 4 is normal and when it is deteriorated. Most of the components at this time are NO ₂ released because the reduction reaction was not completed when NOx was reduced. For this reason, NOx
The detected value of the downstream NOx sensor 8 is larger when the catalyst 4 is deteriorated than when it is normal.

As described above, the downstream NOx sensor 8 also detects NH _{3 in the} same manner as NOx. However, NOx and NH ₃ are detected separately by making the target air-fuel ratio close to the theoretical air-fuel ratio when the reducing agent is supplied. can do. That is, the first peak of the detected value detects the concentration of NOx, and the second peak detects the concentration of NH ₃ . By utilizing this phenomenon, the deterioration of the NOx catalyst 4 can be determined.

Here, FIG. 8 is a diagram showing a reaction flow in the NOx catalyst 4 when the rich spike control for deterioration determination targeting the vicinity of the theoretical air-fuel ratio is performed when the NOx catalyst 4 is deteriorated. . FIG. 9 shows the flow of the reaction in the NOx catalyst 4 when the NOx catalyst 4 is deteriorated and after the rich spike control for deterioration determination targeting the vicinity of the theoretical air-fuel ratio is performed and then the lean air-fuel ratio is shifted. FIG.

The higher the degree of deterioration of the NOx catalyst 4, the lower the temperature of the NOx catalyst 4 because the amount of reaction heat generated is reduced even if the reducing agent is supplied. When the temperature of the NOx catalyst 4 is lowered, NH ₃ is adsorbed on the NOx catalyst 4. For this reason, it is considered that NH ₃ is not included in the component flowing out from the deteriorated NOx catalyst 4. However, since the NOx catalyst 4 has deteriorated, NH ₃
Therefore, the amount of NH ₃ adsorbed on the NOx catalyst 4 is small.

Then, after the supply of the reducing agent is completed, the air-fuel ratio of the exhaust gas shifts from the vicinity of the theoretical air-fuel ratio to the lean air-fuel ratio. The lean air-fuel ratio can be the air-fuel ratio of the exhaust discharged from the internal combustion engine 1 or the air-fuel ratio of the exhaust when the reducing agent is being injected with the lean air-fuel ratio as a target. At this time, it is considered that NH ₃ adsorbed on the NOx storage reduction catalyst is oxidized by oxygen in the exhaust to generate NO and H ₂ O. This NO is measured by a NOx sensor.
However, since the NOx storage reduction catalyst is deteriorated, the amount of NH ₃ adsorbed is not large.
The amount of O generated is not large, and the second maximum value detected by the downstream NOx sensor 8 is relatively small.

From the above, the downstream side NOx sensor 8 when the reducing agent is supplied targeting the vicinity of the theoretical air-fuel ratio.
Since the first maximum value of the detected value is due to NO ₂ , it becomes a larger value as the degree of deterioration of the NOx catalyst 4 is higher. Therefore, if a threshold value for determining deterioration is set in advance, it can be determined that the NOx catalyst 4 has deteriorated when the first maximum value of the detected value of the downstream NOx sensor 8 is equal to or greater than the threshold value. .

Further, the second maximum value of the detected value of the downstream side NOx sensor 8 when the reducing agent is supplied near the theoretical air-fuel ratio is due to NO or NH ₃ , but the degree of deterioration of the NOx catalyst 4 The lower the value, the larger the value, and the higher the degree of deterioration, the smaller the value. Therefore, if a threshold value for determining deterioration is set in advance, it can be determined that the NOx catalyst 4 is deteriorated when the second maximum value of the detected value of the downstream NOx sensor 8 is equal to or less than the threshold value. .

Further, it may be determined that the NOx catalyst 4 has deteriorated when the first maximum value is equal to or greater than the threshold value or the second maximum value is equal to or less than the threshold value. Thus, by performing the deterioration determination of the NOx catalyst 4 based on the two maximum values, the accuracy of the deterioration determination can be further increased.

Since the first maximum value and the second maximum value are affected by the amount of NOx stored in the NOx catalyst 4, the accuracy of the deterioration determination can be improved by performing the deterioration determination after aligning the conditions. Can be increased. For this reason, in this embodiment, before the deterioration determination, rich spike control for NOx reduction (hereinafter also referred to as normal rich spike control) in which the reducing agent is supplied with the target air-fuel ratio set to 14.1 is performed. Then, all the NOx stored in the NOx catalyst 4 is reduced. Thereafter, NOx is occluded, and if the NOx catalyst 4 is normal, a deterioration determination is performed at a time when it is determined that a predetermined amount of NOx is occluded.

FIG. 10 is a time chart showing the transition of the air-fuel ratio of the exhaust and the NOx concentration detected by the downstream NOx sensor 8 during the rich spike control according to this embodiment. Here, as the period during which the reducing agent is injected from the injection valve 5 is lengthened, the amount of reducing agent supplied increases and the amount of decrease in the air-fuel ratio increases. For this reason, the air-fuel ratio of the exhaust gas can be adjusted by adjusting the injection period of the reducing agent.

Here, during the period indicated by A in FIG. 10, normal rich spike control is performed. This normal rich spike control is a control for reducing the NOx stored in the NOx catalyst 4, and the air-fuel ratio is 14.1 by making the opening time per injection valve 5 relatively long. Rich air-fuel ratio. Then, since NH ₃ and NOx flow out from the NOx catalyst 4, these components are detected by the downstream NOx sensor 8.

Further, during the period indicated by B in FIG. 10, NOx reduction or NOx catalyst 4 deterioration determination is not performed. That is, during the period indicated by B, the NOx catalyst 4 is made to occlude an amount of NOx necessary for determining the deterioration of the NOx catalyst 4.

Further, during the period indicated by C in FIG. 10, rich spike control is performed so that the air-fuel ratio of the exhaust is close to the theoretical air-fuel ratio. That is, the valve opening time of the injection valve 5 is made relatively short so that the air-fuel ratio of the exhaust gas becomes 14.8. Then, NOx and NH ₃ are detected according to the degree of deterioration of the NOx catalyst 4. The deterioration of the NOx catalyst 4 can be determined based on the detection value of the downstream side NOx sensor 8 at this time. For example, if the first maximum value of the NOx concentration detected by the downstream side NOx sensor 8 is equal to or greater than a threshold value during a predetermined period (for example, 10 seconds) from the start of the period indicated by C, the NOx catalyst 4 deteriorates. It is determined that Further, for example, if the second maximum value of the NOx concentration detected by the downstream side NOx sensor 8 is not more than a threshold value during a predetermined period (for example, 10 seconds) from the start of the period indicated by C, the NOx catalyst 4 is You may determine with having deteriorated. Further, for example, in a predetermined period (for example, 10 seconds) from the start of the period indicated by C, the first maximum value of the NOx concentration detected by the downstream NOx sensor 8 is equal to or greater than a threshold value or the second maximum value. May be determined that the NOx catalyst 4 has deteriorated.

In this way, it is possible to perform the deterioration determination of the NOx catalyst 4 without component detected by the downstream side NOx sensor 8 is aware of whether it is for or NH ₃ is NOx.

FIG. 11 is a flowchart showing a flow for determining deterioration of the NOx catalyst 4. This routine is executed by the ECU 10 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 downstream NOx sensor 8 is normal. This determination can be performed by a known technique.

  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 a rich spike execution condition is satisfied. This condition is a condition for performing the deterioration determination rich spike control. For example, it is determined that the rich spike execution condition is satisfied when NOx of a predetermined amount or more is occluded in the NOx catalyst 4 and the temperature of the NOx catalyst 4 is a temperature suitable for NOx reduction. . The amount of NOx stored in the NOx catalyst 4 is calculated based on the NOx concentration detected by the upstream NOx sensor 7. The predetermined amount referred to here is obtained in advance by experiments or the like as a value at which NO or NH ₃ is generated such that deterioration can be determined when the reducing agent is supplied. That is, if NOx is not occluded in the NOx catalyst 4, even if the NOx catalyst 4 is deteriorated, N
Ox does not flow out. Since this makes it difficult to determine deterioration, it is a condition that a predetermined amount or more of NOx is occluded in the NOx catalyst 4. Further, the temperature of the NOx catalyst 4 is detected by a temperature sensor 9. Further, it may be determined that the rich spike execution condition is satisfied when the engine speed or the intake air amount of the internal combustion engine 1 is equal to or greater than a predetermined value. That is, NOx
When the NO ₂ released from the catalyst 4 is quickly detected by the downstream NOx sensor 8, it may be determined that the rich spike execution condition is satisfied. For example, a low flow speed of the exhaust gas, NOx and NH ₃ ends up diffused before reaching the downstream side NOx sensor 8, there is a risk that the maximum value decreases. If it does so, there exists a possibility that the precision of the deterioration determination based on local maximum may become low.

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

  In step S103, deterioration determination rich spike control is performed. That is, rich spike control is performed so as to be close to the theoretical air-fuel ratio.

In step S104, it is determined whether or not the first maximum value of the NOx concentration detected by the downstream side NOx sensor 8 is greater than or equal to a threshold value. This threshold value is set in advance as a lower limit value of the first maximum value of the NOx concentration detected when the NOx catalyst 4 is deteriorated. This 1
The maximum value for the first time is the first maximum value within 10 seconds after the rich spike control is started. In this step, the downstream NOx in a predetermined period before and after the first maximum value.
You may determine whether the integrated value of the detected value of the sensor 8 is more than a threshold value. This integrated value may be an integrated value during a period in which the NOx concentration is equal to or higher than a predetermined value before and after the first maximum value. This predetermined value may be a minimum value between two peaks. The integrated value is obtained, for example, by sequentially adding the detection values of the downstream NOx sensor 8 read at a predetermined period.

If an affirmative determination is made in step S104, the process proceeds to step S105, where it is determined that the NOx catalyst 4 has deteriorated. On the other hand, if a negative determination is made in step S104, the process proceeds to step S106.

In step S106, it is determined whether or not the second maximum value of the NOx concentration detected by the downstream side NOx sensor 8 is equal to or less than a threshold value. This threshold value is set in advance as the upper limit value of the second maximum value of the NOx concentration detected when the NOx catalyst 4 is deteriorated. This 2
The second maximum value is the second maximum value within 10 seconds after the rich spike control is started. In this step, downstream NOx in a predetermined period before and after the second maximum value.
You may determine whether the integrated value of the detected value of the sensor 8 is below a threshold value. This integrated value may be an integrated value during a period in which the NOx concentration is equal to or higher than a predetermined value before and after the second maximum value. This predetermined value may be a minimum value between two peaks.

If an affirmative determination is made in step S106, the process proceeds to step S105, where it is determined that the NOx catalyst 4 has deteriorated. On the other hand, if a negative determination is made in step S106, the process proceeds to step S107, and it is determined that the NOx catalyst 4 is normal. In this embodiment, the ECU 10 that processes steps S104 to S107 corresponds to the determination device according to the present invention.

In this way, it is possible to determine the deterioration of the NOx catalyst 4 based on the detected value of the downstream NOx sensor 8 when the reducing agent is supplied so as to be in the vicinity of the theoretical air-fuel ratio. At this time, since NO ₂ and NH ₃ can be detected separately, the accuracy of the deterioration determination is high. In addition, if the maximum value of the detected value of the downstream side NOx sensor 8 is known, it is possible to determine the deterioration immediately. Therefore, it is not necessary to wait until the detected value becomes stable or until NOx is occluded. That is, the deterioration determination can be performed quickly.

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Exhaust passage 4 Occlusion reduction type NOx catalyst 5 Injection valve 7 Upstream side NOx sensor 8 Downstream side NOx sensor 9 Temperature sensor 10 ECU
11 Accelerator pedal 12 Accelerator opening sensor 13 Crank position sensor

Claims (4)

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;
A NOx sensor that detects the NOx and NH ₃ in the exhaust downstream of than the NOx storage reduction catalyst,
A control device that adjusts the amount of reducing agent so that the air-fuel ratio of the exhaust is close to the theoretical air-fuel ratio when supplying the reducing agent from the supply device;
When NOx is occluded in the NOx storage reduction catalyst, the control device supplies the reducing agent from the supply device while adjusting the amount of reducing agent so that the air-fuel ratio of the exhaust gas is close to the theoretical air-fuel ratio. Within a predetermined time immediately after starting the operation, the detected value of the NOx sensor decreases from an increase.
If the detection value of the NOx sensor first changes from rising to falling when the detection value turns twice, it is determined that the NOx storage reduction catalyst is deteriorated when the detection value is equal to or greater than a threshold value. A determination device;
A catalyst deterioration judgment system comprising: 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;
A NOx sensor that detects the NOx and NH ₃ in the exhaust downstream of than the NOx storage reduction catalyst,
A control device that adjusts the amount of reducing agent so that the air-fuel ratio of the exhaust is close to the theoretical air-fuel ratio when supplying the reducing agent from the supply device;
When NOx is occluded in the NOx storage reduction catalyst, the control device supplies the reducing agent from the supply device while adjusting the amount of reducing agent so that the air-fuel ratio of the exhaust gas is close to the theoretical air-fuel ratio. When the detected value of the NOx sensor changes from rising to falling for the second time within a predetermined time immediately after starting the NOx, it is determined that the NOx storage reduction catalyst is deteriorated when the detected value is equal to or less than a threshold value. A determination device;
A catalyst deterioration judgment system comprising: The determination device is when the NOx is stored in the NOx storage reduction catalyst, and the supply device adjusts the amount of reducing agent so that the air-fuel ratio of the exhaust gas is close to the theoretical air-fuel ratio by the control device. When the detected value of the NOx sensor first changes from rising to falling within a predetermined time immediately after the start of supply of the reducing agent, the detected value is equal to or greater than the threshold value, or when the detected value changes from rising to falling for the second time. 3. The catalyst deterioration determination system according to claim 1, wherein the NOx storage reduction catalyst is determined to be deteriorated when the detected value is equal to or less than a threshold value. 4.   The catalyst deterioration determination system according to any one of claims 1 to 3, wherein the predetermined time is 10 seconds.

Images