JP4983536B2 - Exhaust gas purification system for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine.

As a technique for reducing the amount of nitrogen oxide (NOx) contained in exhaust gas from an internal combustion engine to the atmosphere, an exhaust purification system having a selective reduction type NOx catalyst (SCR) in the middle of an exhaust passage is known. Yes. In an exhaust purification system equipped with an SCR, ammonia (NH ₃ ) is supplied to the SCR as a reducing agent. In the SCR, NH ₃ and NOx undergo a redox reaction, whereby NOx is reduced to N ₂ . A related technique is described in Patent Document 1.
Japanese Patent Laid-Open No. 10-47041 Japanese Patent Laid-Open No. 2000-18026 JP 2000-265828 A JP-A-8-189388

In an exhaust purification system equipped with an SCR, a part of NH ₃ supplied to the SCR may flow out of the SCR without reacting with the SCR. In this case, toxic NH ₃ may be discharged into the atmosphere, which is not preferable. In contrast, a technique has been proposed in which an oxidation catalyst is disposed in the exhaust passage downstream of the SCR, and NH ₃ flowing out of the SCR is oxidized by the oxidation catalyst, thereby suppressing the emission of NH ₃ into the atmosphere. .

However, in this technique, NOx is generated when NH ₃ is oxidized in the oxidation catalyst, so that it is possible to suppress the emission of NH ₃ into the atmosphere, but NOx emission, which is the original purpose of the exhaust purification system, can be suppressed. There was a problem that the effect of reducing the amount would be diminished.

The present invention has been made in view of the above problems, and in an exhaust purification system equipped with an SCR that reduces NOx using NH ₃ as a reducing agent, it is possible to suitably suppress the discharge of NH ₃ into the atmosphere. Aims to provide a new technology.

In order to achieve the above object, an exhaust purification system for an internal combustion engine of the present invention comprises:
An SCR disposed in an exhaust passage of the internal combustion engine for selectively reducing NOx using NH ₃ as a reducing agent;
NH ₃ supply means for supplying NH ₃ to the SCR;
NH ₃ detection means for detecting NH _{3 in} the exhaust gas downstream from the SCR;
Exhaust temperature raising means for raising the temperature of the exhaust;
With
When the NH ₃ is supplied to the SCR by the NH ₃ supply means, the NH ₃ when a predetermined allowable amount or more of NH ₃ in the exhaust gas flowing in the downstream of the SCR is detected by the detecting means, the exhaust gas temperature increasing The temperature of the exhaust gas is raised by the means.

Here, the NH ₃ supply means may be any means as long as it can supply NH ₃ to the SCR. For example, in the case of a system in which an NOx storage reduction catalyst (NSR) or a three-way catalyst (TWC) is arranged in the exhaust passage upstream from the SCR, hydrocarbon (HC), hydrogen (H ₂ ), etc. by supplying the rich component, since the NOx desorbed from the NOx and NSR in exhaust those rich component is NH ₃ reacts generated, NH ₃ in the SCR that is disposed downstream of these catalysts Can be supplied. Also, by supplying urea water into the exhaust gas upstream from the SCR, a reaction in which the urea water is decomposed and NH ₃ is generated in the high-temperature exhaust gas proceeds, so NH ₃ can be supplied to the SCR. it can.

Further, the NH ₃ detecting means may be any means as long as NH ₃ in the exhaust gas can be detected. For example, the NH ₃ detection means can be a sensor that directly measures NH ₃ in the exhaust. Further, the amount of NH _{3 in} the exhaust downstream from the SCR is estimated based on a map or function obtained in advance by experiments, simulations, or the like regarding the relationship between the operating state of the internal combustion engine and the amount of NH ₃ flowing downstream from the SCR. It can be a means. It is also possible to detect the NH ₃ in the exhaust gas based on the output of the H ₂ sensor for detecting of H ₂ of the NOx sensor and the exhaust gas to detect the NOx in the exhaust gas.

  Further, the exhaust temperature raising means may be any means as long as it is a means for raising the temperature of the exhaust downstream from at least the SCR. For example, it can be a means for increasing the temperature of the exhaust gas flowing into the SCR or a means for increasing the temperature of the exhaust gas discharged from the internal combustion engine. Further, the exhaust gas temperature may be raised by direct means such as a heater, fuel injection timing control, fuel injection control such as sub-injection, ignition timing control, and EGR control.

The predetermined allowable amount is an upper limit value that is allowable as the amount of NH ₃ discharged from the vehicle to the atmosphere, and is determined in advance.

According to the present invention, when NH ₃ exceeding a predetermined allowable amount is detected in the exhaust downstream of the SCR, the temperature of the exhaust is raised. Thereby, the temperature of the exhaust becomes high. Accordingly, the progress of the NH ₃ synthesis reaction (H ₂ + N ₂ → NH ₃ , exothermic reaction) is suppressed in the exhaust gas, and the progress of the NH ₃ decomposition reaction that is the reverse reaction of this NH ₃ synthesis reaction is promoted. Therefore, even when a part of NH ₃ supplied to the SCR flows out into the exhaust passage downstream of the SCR, the NH ₃ is decomposed by the NH ₃ decomposition reaction, so that the NH ₃ is discharged into the atmosphere. Can be suppressed. Further, since NOx is not generated by this NH ₃ decomposition reaction, it can be suppressed that the NOx reduction effect by the exhaust purification system is reduced.

In the present invention,
The exhaust temperature raising means can be a means for raising the temperature of the exhaust gas by oscillating the air-fuel ratio of the exhaust gas.

Here, “vibrating the air-fuel ratio” means that the air-fuel ratio is repeatedly changed at a relatively short time interval with a relatively small fluctuation range above and below the air-fuel ratio with a certain air-fuel ratio as the center. . The fluctuation range may be symmetric or asymmetric with respect to the air-fuel ratio that is the center of fluctuation. In addition, the repetition of the fluctuation may be periodic or may have no periodicity. The air-fuel ratio that is the center of fluctuation can be set to a basic air-fuel ratio according to the operating conditions of the internal combustion engine, an air-fuel ratio in the vicinity of stoichiometry, or the like according to the purpose of control. By executing such air-fuel ratio control, the exhaust on the lean side and the exhaust on the rich side from the air-fuel ratio at the center of vibration are mixed. The exhaust on the lean side from the vibration center contains a relatively large amount of O ₂ , and the exhaust on the rich side contains a relatively large amount of rich components such as HC and CO. Therefore, when these exhaust gases coexist, a reaction in which the rich component is oxidized by O ₂ proceeds, and the temperature of the exhaust gas rises due to the oxidation reaction heat.

In particular, in a configuration in which the TWC is disposed in the exhaust passage, it is preferable to oscillate the air-fuel ratio around a predetermined air-fuel ratio near the stoichiometry. The predetermined air-fuel ratio in the vicinity of the stoichiometry is an air-fuel ratio between the air-fuel ratio at which the HC purification rate at the TWC is maximized and the air-fuel ratio at which the CO purification rate at the TWC is maximized. Thereby, since the reaction activities of both the HC oxidation reaction and the CO oxidation reaction in TWC can be enhanced, the temperature of the exhaust gas can be further increased by the reaction heat of these oxidation reactions.

Here, it is preferable that the range of vibration of the air-fuel ratio by perturbation control be within the air-fuel ratio region on the lean side from 14.1. By doing so, an increase in the concentration of H _{2 in} the exhaust gas can be suitably suppressed, so that the NH ₃ decomposition reaction easily proceeds.
In the present invention,
In the case of a configuration further comprising means for performing a rich spike that temporarily makes the air-fuel ratio of the exhaust rich during lean operation of the internal combustion engine,
Wherein when the rich spike execution, the NH ₃ when a predetermined allowable amount or more of NH ₃ in the exhaust gas flowing in the downstream of the SCR is detected by the detecting means, the rich near stoichiometric air-fuel ratio of the exhaust after execution spike The temperature of the exhaust gas may be raised by oscillating around the air-fuel ratio.

When rich spike is performed, rich components such as hydrocarbons (HC) and hydrogen (H ₂ ) in the exhaust gas are emitted from NOx in the exhaust gas, and in the case where the NSR is provided in the exhaust passage. It reacts with the released NOx to generate NH ₃ . This NH ₃ is supplied to the SCR and acts as a reducing agent that reduces NOx in the SCR. At this time, a part of NH ₃ supplied to the SCR may pass through the SCR and flow out to the exhaust passage downstream of the SCR.

According to the present invention, when the rich spike execution, when a predetermined allowable amount or more of NH ₃ is detected in the exhaust gas flowing through the downstream SCR by NH ₃ detection means, the air-fuel ratio of the exhaust gas after the rich spike execution It is oscillated around the air-fuel ratio in the vicinity of stoichi.

By causing the air-fuel ratio of the exhaust to oscillate around the air-fuel ratio near the stoichiometric ratio, the temperature of the exhaust increases as described above. In addition, since the increase in the H ₂ concentration in the exhaust gas is suppressed, the NH ₃ synthesis reaction (H ₂ + N ₂ → NH ₃ ) does not easily proceed in the exhaust gas, and the NH ₃ decomposition reaction (NH ₃ → H ₂ + N _2). ) Is promoted. Therefore, even if a part of NH ₃ supplied by rich spike control flows into the exhaust downstream of the SCR, the NH ₃ is decomposed by the NH ₃ decomposition reaction, so that NH ₃ is released into the atmosphere. It can suppress being discharged. Further, since NOx is not generated by this NH ₃ decomposition reaction, it can be suppressed that the NOx reduction effect by the exhaust purification system is reduced.

Further, in the configuration in which the TWC is disposed in the exhaust passage, as described above, centering on the air-fuel ratio between the air-fuel ratio at which the HC purification rate in the TWC becomes maximum and the air-fuel ratio at which the CO purification rate becomes maximum. By oscillating the air-fuel ratio, the reaction activities of both the HC oxidation reaction and the CO oxidation reaction in the TWC can be increased. Therefore, the temperature of the exhaust gas can be further increased by the reaction heat of these oxidation reactions. . The NH ₃ synthesis reaction is an exothermic reaction. Therefore, when the temperature of the exhaust gas rises and becomes a high temperature environment, the progress of the NH ₃ synthesis reaction can be more reliably suppressed, and the progress of the NH ₃ decomposition reaction can be further promoted. Therefore, the discharge of NH ₃ into the atmosphere can be more reliably suppressed.

Further, it is preferable that the range of vibration of the air-fuel ratio be within the range of the air-fuel ratio leaner than 14.1. By doing so, an increase in the concentration of H _{2 in} the exhaust gas can be suitably suppressed, so that the NH ₃ decomposition reaction easily proceeds.

According to the present invention, in an exhaust purification system including an SCR that reduces NOx using NH ₃ as a reducing agent, it is possible to suitably suppress the discharge of NH ₃ into the atmosphere.

  A specific embodiment of an exhaust gas purification system for an internal combustion engine according to the present invention will be described below 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. An exhaust passage 5 leading to the combustion chamber is connected to the internal combustion engine 1. This exhaust passage 5 communicates with the atmosphere downstream.

In the middle of the exhaust passage 5, a three-way catalyst (TWC) 4, an occlusion reduction type NOx catalyst (NSR) 2, and an ammonia selective reduction type NOx catalyst (SCR) 3 are arranged in this order from the internal combustion engine 1 side. NSR2 has a function of storing NOx in exhaust when the air-fuel ratio of the inflowing exhaust gas is lean, and reducing NOx stored in the presence of a reducing agent when the air-fuel ratio of the inflowing exhaust gas is stoichiometric or rich. . The SCR ₃ has a function of selectively reducing NOx in the exhaust gas using NH ₃ as a reducing agent. For example, nitric oxide (NO) is reduced to N ₂ by a reaction of 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O. Nitrogen dioxide (NO ₂ ) is reduced to N ₂ by a reaction of 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O. The TWC 4 has a function of reducing NOx in the exhaust and oxidizing HC and CO in the exhaust when the air-fuel ratio of the inflowing exhaust is within a predetermined air-fuel ratio region near the stoichiometry. In addition, a filter for collecting particulate matter in the exhaust may be provided in the exhaust passage 5.

The exhaust passage 5 upstream of the NSR 2 is provided with an addition valve 9 for adding and supplying HC (light oil) as a reducing agent into the exhaust gas flowing through the exhaust passage 5. The addition valve 9 is opened by a signal from the ECU 8 described later and injects HC into the exhaust gas. Due to the HC injected from the addition valve 9, the air-fuel ratio of the exhaust flowing from the upstream of the exhaust passage 5 is lowered to the rich side. When processing to reduce NOx occluded in NSR2, rich spike control is performed in which the air-fuel ratio of the exhaust gas flowing into NSR2 is spiked to a rich air-fuel ratio for a relatively short period of time by adding HC from addition valve 9 To do. One rich spike may be formed by adding HC multiple times by the addition valve 9. An NH ₃ sensor 6 for detecting NH ₃ in the exhaust is attached to the exhaust passage 5 downstream of the SCR ₃ . In this embodiment, the NH ₃ sensor 6 corresponds to the NH ₃ detection means in the present invention. The NH ₃ detection means may be implemented by means other than the NH ₃ sensor 6. For example, it is possible to attach a NOx sensor or an H ₂ sensor and detect NH ₃ in the exhaust based on the output of those sensors. It is also possible to detect the NH ₃ in the exhaust gas on the basis of those mapping or function of seeking advance by experiment or simulation or the like the correlation between the NH ₃ concentration in the exhaust and the operation state of the internal combustion engine 1.

The internal combustion engine 1 configured as described above is provided with an ECU 8 that is an electronic control computer for controlling the internal combustion engine 1. The ECU 8 has a function of controlling the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the request of the driver. Various sensors other than the NH ₃ sensor 6 are connected to the ECU 8 through electric wiring, and output signals of these sensors are input to the ECU 8. In addition to the addition valve 9, various devices are connected to the ECU 8 through electrical wiring, and the operation of these devices is controlled by the ECU 8.

When the amount of NOx stored in the NSR2 exceeds a predetermined threshold, rich spike control is executed to reduce the NOx stored in the NSR2. This threshold value is determined in advance by experiments, simulations, or the like. When rich spike control is executed, the concentration of rich components such as HC and H ₂ in the exhaust gas increases, and these rich components react with NOx in the exhaust gas and NOx released from NSR 2 to react with NH _3. appear. In addition, NH ₃ may be generated from the rich component H ₂ and N ₂ in the exhaust gas by an NH ₃ synthesis reaction (H ₂ + N ₂ → NH ₃ : exothermic reaction). These NH ₃ flows into the SCR ₃ disposed downstream of the NSR 2 and functions as a reducing agent for reducing NOx in the SCR 3. Further, the rich component supplied into the exhaust by the rich spike control may react with NOx in the exhaust in the TWC ₄ to generate NH ₃ . This NH ₃ also functions as a reducing agent in the downstream SCR ₃ . That is, it can be said that the NSR 2, TWC 4, ECU 8 that performs rich spike control, and the addition valve 9 in this embodiment correspond to NH ₃ supply means in the present invention.

By the way, not all NH ₃ supplied to the SCR ₃ is consumed for the reduction reaction of NOx, and some NH ₃ may flow out of the SCR ₃ as it is. Since NH ₃ has toxicity, it is not preferable that NH ₃ flowing out from SCR ₃ is discharged into the atmosphere. Therefore, in the exhaust purification system of the present embodiment, when the NH ₃ sensor 6 detects that NH ₃ exceeding the predetermined allowable amount is present in the exhaust flowing downstream from the SCR ₃ , after the rich spike control is executed, The air-fuel ratio of the exhaust gas is oscillated around the air-fuel ratio in the vicinity of a predetermined stoichiometry.

Here, the predetermined allowable amount is an upper limit value that is allowable as the amount of NH ₃ discharged from the vehicle into the atmosphere, and is determined in advance. Further, the air-fuel ratio in the vicinity of a predetermined stoichiometry is an air-fuel ratio that can reduce NOx occluded in NSR2 and that can suppress an increase in H ₂ concentration in the exhaust gas. Determined. In order to suppress an increase in the H ₂ concentration in the exhaust gas, it is preferable that the air / fuel ratio is leaner than A / F14.1. In the present embodiment, the air-fuel ratio in the vicinity of the predetermined stoichiometric is set to A / F 14.6, and the air-fuel ratio is vibrated at a relatively short cycle within a range of ± 0.3 around the air-fuel ratio. The control for oscillating the air-fuel ratio is hereinafter referred to as air-fuel ratio perturbation control.

  FIG. 2 is a time chart showing an example of the time change of the air-fuel ratio in the case where the normal rich spike control is executed and the case where the air-fuel ratio perturbation control is executed after the rich spike control is executed. As shown in FIG. 2, in the internal combustion engine of the present embodiment, the air-fuel ratio is controlled to be lean (A / F 25) during normal lean operation. When normal rich spike control is executed, the air-fuel ratio is controlled to be rich (A / F12) in a spike for a relatively short period.

As shown in FIG. 2, normal rich spike control is performed at times t1 and t2. This shows the air-fuel ratio control when NH ₃ exceeding the allowable amount is not detected in the exhaust gas downstream from the SCR 3 when the rich spike is executed. In this case, when the rich spike control is finished, the air-fuel ratio immediately returns to the lean air-fuel ratio (A / F25).

On the other hand, at time t3, air-fuel ratio perturbation control is executed from time t4 to t5 following normal rich spike control. In the air-fuel ratio perturbation control, the air-fuel ratio is controlled to the air-fuel ratio (A / F 14.6) near the stoichiometric ratio, and the air-fuel ratio (A / F 14.6) near the stoichiometric ratio is compared between the rich side and the lean side. The air-fuel ratio is vibrated with a short period. This shows the air-fuel ratio control when NH ₃ exceeding the allowable amount is detected in the exhaust gas downstream from the SCR 3 when the rich spike is executed. As shown in FIG. 2, in this embodiment, control is performed so that the lower limit value of the oscillating air-fuel ratio does not become richer than A / F14.1. After execution of the air-fuel ratio perturbation control, the air-fuel ratio returns to the normal lean operation air-fuel ratio (A / F25).

By executing the air-fuel ratio perturbation control, the reaction activities of both the HC oxidation reaction and the CO oxidation reaction in the TWC 4 increase, and the temperature of the exhaust gas rises due to the reaction heat of these oxidation reactions. In addition, the air-fuel ratio at the vibration center of the air-fuel ratio perturbation control is set to the air-fuel ratio A / F 14.6 near the stoichiometry, and the lower limit value of the vibration range of the air-fuel ratio is controlled not to be richer than A / F 14.1. Therefore, an increase in the H ₂ concentration in the exhaust is suppressed. That is, it can be said that the TWC 4, the ECU 8 that performs air-fuel ratio perturbation control, and the addition valve 9 in this embodiment correspond to the exhaust gas temperature raising means of the present invention. The air-fuel ratio control may be performed not only by the addition of exhaust fuel by the addition valve 9 but also by engine fuel injection control, intake air amount control, EGR control, etc., or by combining these with exhaust fuel addition, air-fuel ratio control is performed. You can go.

By executing the air-fuel ratio perturbation control, the temperature of the exhaust gas becomes high and an increase in H _{2 in} the exhaust gas is suppressed. Therefore, the reverse reaction of the NH ₃ synthesis reaction in the exhaust gas (NH ₃ decomposition reaction: NH ₃ → H ₂ + N ₂ ) is promoted. This is because the NH ₃ synthesis reaction is an exothermic reaction.

As described above, according to this embodiment, when the NH ₃ sensor 6 detects NH ₃ exceeding the allowable amount in the exhaust downstream of the TWC 4, the air-fuel ratio perturbation control is executed after the rich spike control is executed. Thus, since the conditions for promoting the progress of the NH ₃ decomposition reaction are established, NH ₃ in the exhaust gas can be decomposed into N ₂ and H ₂ . Thereby, it is possible to prevent the NH ₃ that has passed through the SCR ₃ from being discharged into the atmosphere when the rich spike control is executed. Further, since NOx is not generated in the NH ₃ decomposition reaction, it is possible to suppress the NOx reduction effect of the exhaust purification system from being reduced by executing the air-fuel ratio perturbation control.

Here, the period for executing the air-fuel ratio perturbation control (period in FIG. 2 t4 to t5) is a NH ₃ flowing out from SCR3 downstream emission to the atmosphere of the NH ₃ is below the allowable amount The time that can be decomposed to the extent is determined in advance by experiments and simulations. This period may be a variable value corresponding to the amount of NH ₃ downstream of the SCR ₃ detected by the NH ₃ sensor 6 or may be a predetermined constant value.

  Here, a specific execution procedure by the ECU 8 of the air-fuel ratio control of the present embodiment described above will be described based on FIG. FIG. 3 is a flowchart showing a routine for performing the air-fuel ratio control of the present embodiment. This routine is repeatedly executed at predetermined intervals during operation of the internal combustion engine 1.

  In step S101, the ECU 8 determines whether or not a rich spike control execution condition is satisfied. Specifically, the rich spike execution condition is satisfied when it is determined that the NOx occlusion amount in the NSR 2 exceeds the predetermined threshold described above. If an affirmative determination is made in step S101, the ECU 8 proceeds to step S102. If a negative determination is made in step S101, the ECU 8 ends the execution of this routine. As a method for determining the threshold value and determining whether the NOx occlusion amount exceeds the threshold value, a method used in a known NSR system can be used.

  In step S102, the ECU 8 executes rich spike control. Specifically, the addition valve 9 is opened to inject and supply fuel into the exhaust.

In step S103, the ECU 8 measures the amount of NH ₃ flowing out from the SCR ₃ by the NH ₃ sensor 6.

In step S104, the ECU 8 determines whether or not the NH ₃ outflow amount measured in step S103 exceeds the predetermined allowable amount described above. If an affirmative determination is made in step S104, the ECU 8 proceeds to step S105. If a negative determination is made in step S104, the ECU 8 once ends the execution of this routine.

In step S105, the ECU 8 executes air-fuel ratio perturbation control. Here, the determination result in step S104 is immediately fed back to the rich spike control, and the normal rich spike control is shifted to the air-fuel ratio perturbation control. Then, after the aforementioned predetermined period has elapsed, the ECU 8 proceeds to step S106 and returns to the normal lean operation with the lean air-fuel ratio.

  The above-described embodiment is an example for explaining the present invention, and various modifications can be made to the above-described embodiment without departing from the gist of the present invention.

  For example, in the above-described embodiment, the addition valve 9 for supplying the fuel into the exhaust gas is provided in the exhaust passage 5 in order to execute the rich spike, but the spiked rich air-fuel ratio is intermittently added to the NSR 2. The rich spike may be executed by any means as long as it can be supplied. For example, the rich spike may be executed by sub-injection control or engine air-fuel ratio control.

In the above embodiment, the example in which air-fuel ratio perturbation control is executed as a means for increasing the temperature of the exhaust gas has been described. However, as described above, as a means for increasing the temperature of the exhaust gas, NH ₃ decomposition reaction can be performed. Any means may be used as long as the condition that the progress is not hindered is satisfied. For example, the exhaust gas temperature may be increased directly by a heater or the like, or by fuel injection timing, ignition timing in a spark ignition engine, valve timing control, multi-injection control, EGR control, or the like.

In the above-described embodiment, an example in which air-fuel ratio perturbation control is continuously executed after execution of rich spike has been described. However, air-fuel ratio perturbation control may be executed at intervals after execution of rich spike. Further, when NH ₃ exceeding an allowable amount is detected during execution of a certain rich spike, air-fuel ratio perturbation control may be executed instead of the next rich spike.

  In the above-described embodiment, the configuration including the addition valve for adding fuel to the exhaust gas is described as the air-fuel ratio control means. However, the air-fuel ratio of the exhaust gas can be controlled by other means. For example, means for performing sub-injection or controlling the engine air-fuel ratio can be used.

Further, in the above embodiment, in the exhaust purification system provided with NSR2, NH ₃ as a reducing agent is generated by rich spike control performed to reduce NOx stored in NSR2, and the generated NH ₃ is Although the configuration of being supplied to the SCR ₃ and acting as a reducing agent has been described, the present invention can be applied to all systems having a configuration of supplying NH ₃ as a reducing agent to the SCR.

For example, the present invention may be applied to a system in which NH ₃ obtained by injecting and supplying urea water into exhaust gas upstream from the SCR and decomposing the urea water in high-temperature exhaust gas is used as a reducing agent for SCR. it can. In that case, when urea water injection is performed to perform NOx reduction in the SCR, NH _{3 in} the exhaust downstream from the SCR is detected, and when NH ₃ exceeding a predetermined allowable amount is detected, as described above. Exhaust temperature raising control (for example, air-fuel ratio perturbation control) may be performed.

In the above embodiment, the configuration for executing the air-fuel ratio perturbation control for oscillating the air-fuel ratio around the stoichiometric air-fuel ratio has been described, but even when such air-fuel ratio perturbation control is not executed, By making the air-fuel ratio stoichiometric or lean, the concentration of H _{2 in} the exhaust gas can be suppressed, so that the progress of the NH ₃ decomposition reaction can be promoted. Therefore, when the exhaust gas temperature is raised by means other than air-fuel ratio perturbation control, the NH ₃ decomposition reaction can be promoted more reliably by making the air-fuel ratio stoichiometric or lean.

It is a figure which shows schematic structure of the internal combustion engine and its exhaust system in an Example. It is a time chart which shows an example of the change of an air fuel ratio when performing normal rich spike control in an Example, and when perturbation control is performed after rich spike control execution. It is a flowchart which shows the routine for performing the air fuel ratio control in an Example.

Explanation of symbols

1 Internal combustion engine 2 NOx storage reduction catalyst (NSR)
3 Ammonia selective reduction type NOx catalyst (SCR)
4 Three-way catalyst (TWC)
5 Exhaust passage 6 NH ₃ sensor 8 ECU
9 Additive valve

Claims (3)

A selective reduction type NOx catalyst that is disposed in an exhaust passage of the internal combustion engine and selectively reduces NOx using NH ₃ as a reducing agent;
And NH ₃ supply means for supplying NH ₃ to the selective reduction type NOx catalyst,
NH ₃ detection means for detecting NH _{3 in} the exhaust downstream from the selective reduction NOx catalyst;
Exhaust temperature raising means for raising the temperature of the exhaust;
With
When the NH ₃ is supplied to the NOx selective reduction catalyst by the NH ₃ supply means, the NH ₃ detection means by the selective reduction type NOx catalyst than a predetermined in exhaust gas flowing through the downstream above the allowable level NH ₃ detection If so, an exhaust gas purification system for an internal combustion engine, wherein the exhaust gas temperature is raised by the exhaust gas temperature raising means. In claim 1,
An exhaust gas purification system for an internal combustion engine, wherein the exhaust temperature raising means raises the temperature of the exhaust gas by oscillating the air-fuel ratio of the exhaust gas. In claim 1 or 2,
Means for performing a rich spike that temporarily makes the air-fuel ratio of the exhaust rich during lean operation of the internal combustion engine;
During execution of the rich spike, the NH ₃ when a predetermined allowable amount or more of NH ₃ in the exhaust gas flowing in the downstream of the selective reduction type NOx catalyst is detected by the detection means, the air-fuel ratio of the exhaust gas after the execution of the rich spike An exhaust gas purification system for an internal combustion engine, wherein the temperature of exhaust gas is raised by oscillating the air-fuel ratio in the vicinity of the stoichiometry.

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