JP2008115838A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique to further improve a NOx reducing efficiency by combining a plurality of means to reduce an exhaust air-fuel ratio and setting a rate of a reducing the air-fuel ratio by each means to an appropriate value, in an exhaust emission control device for an internal combustion engine. <P>SOLUTION: A rate of reducing the air-fuel ratio by a fuel adding valve 7 is made lower as according an evaporation rate of fuel added from the fuel adding valve 7 is lower, when NOx is reduced by combining a rate of reducing the air-fuel ratio of gas discharged from the combustion chamber of the internal combustion engine 1 and a rate of reducing the air-fuel ratio by adding fuel from the fuel adding valve 7 and reducing the air-fuel ratio of exhaust gas led to flow into a storage-reduction type NOx catalyst 6 to a target air-fuel ratio. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  By adding a reducing agent to the NOx storage reduction catalyst (hereinafter also simply referred to as NOx catalyst) provided in the exhaust passage, NOx stored in the NOx catalyst can be reduced. The reducing agent can be added by providing a fuel addition valve in the exhaust passage and injecting fuel from the fuel addition valve, or discharging a gas containing the reducing agent from the internal combustion engine. For example, the reducing agent can be supplied while lowering the air-fuel ratio of the exhaust gas by increasing the EGR gas amount, reducing the intake air amount by the intake throttle valve, sub-injection into the cylinder, or the like.

Here, when fuel is added from the fuel addition valve, it is not greatly affected by the operating state of the internal combustion engine, so that fuel addition in a wide operating range is possible. However, there is an operating region where the NOx reduction efficiency deteriorates because the fuel reaches the NOx catalyst before the fuel diffuses and the fuel passes through the NOx catalyst or the fuel adheres to the exhaust passage. The NOx reduction efficiency is a value obtained by dividing the amount of reducing agent that reacts for NOx reduction in the supplied reducing agent by the amount of reducing agent that is supplied.

  On the other hand, when exhausting a low air-fuel ratio gas from the internal combustion engine, the oxygen concentration can be greatly reduced in the combustion chamber, and CO with high NOx reduction efficiency is generated, so that the NOx reduction efficiency increases. . However, if the amount of EGR gas is increased or the amount of intake air is reduced, the combustion state becomes unstable, so that it can be used only in a narrow operating region such as during low-load operation of the internal combustion engine.

Thus, a technique is known in which the above two types of means are combined to compensate for each other's disadvantages and increase NOx reduction efficiency (see, for example, Patent Document 1). That is, by combining the above two types of means, the air-fuel ratio to be reduced by each means can be small, so that fuel can be prevented from passing through the NOx catalyst while suppressing deterioration of the combustion state.
JP 2005-226463 A JP 2004-332712 A JP-A-2005-90439 JP 2002-38926 A

  However, the operating range in which the low air-fuel ratio gas can be discharged from the internal combustion engine is narrow, and when the fuel is added by the fuel addition valve, the fuel is difficult to diffuse, so the air-fuel ratio is reduced by the exhaust air-fuel ratio reducing means. The NOx reduction efficiency changes depending on how the ratio of the amount to be decreased and the amount to decrease the air-fuel ratio by adding fuel from the fuel addition valve are determined.

The present invention has been made in view of the above-described problems, and in an exhaust gas purification apparatus for an internal combustion engine, a plurality of means for reducing the air-fuel ratio of exhaust gas are combined, and the ratio of the air-fuel ratio reduction by each means is set appropriately. It aims at providing the technique which improves NOx reduction | restoration efficiency more by setting to a proper value.

In order to achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention comprises:
An NOx storage reduction catalyst that is provided in the exhaust passage of the internal combustion engine and whose purification capacity is restored by supplying a reducing agent;
A fuel addition valve for adding fuel into the exhaust in the exhaust passage upstream of the NOx storage reduction catalyst;
Exhaust air / fuel ratio lowering means for lowering the air / fuel ratio of the gas discharged from the combustion chamber of the internal combustion engine when recovering the purification capacity of the NOx storage reduction catalyst as compared to when not recovering the purification capacity;
The air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is combined with the amount by which the air-fuel ratio is lowered by the exhaust air-fuel ratio lowering means and the amount by which the air-fuel ratio is lowered by adding fuel from the fuel addition valve When the purification capacity is recovered by lowering the air-fuel ratio to the target air-fuel ratio, the air-fuel ratio lowering ratio is such that the lower the vaporization rate of the fuel added from the fuel addition valve, the smaller the air-fuel ratio is lowered by the fuel addition valve Change means,
It is characterized by comprising.

When NOx stored in the NOx storage reduction catalyst is reduced or when sulfur poisoning recovery is performed, the NOx storage reduction is performed while reducing the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst.
A reducing agent is supplied to the Ox catalyst. In order to supply the reducing agent to the NOx storage reduction catalyst, the reducing agent is added from the fuel addition valve, or the air-fuel ratio of the gas discharged from the internal combustion engine is lowered. The fuel addition from the fuel addition valve and the reduction of the air-fuel ratio of the gas discharged from the internal combustion engine can be performed simultaneously, or only one of them can be performed.

  Here, since the fuel supplied into the cylinder is exposed to high-temperature combustion gas, most of the fuel is discharged from the internal combustion engine in a vaporized state or a reformed state. Therefore, the fuel discharged from the internal combustion engine is likely to react with the NOx storage reduction catalyst.

However, since the fuel added from the fuel addition valve is added to the exhaust gas whose temperature is lower than that in the cylinder, it is less likely to vaporize compared to the fuel supplied into the cylinder. Therefore, it may reach the NOx storage reduction catalyst as it is in the liquid state, or since the diffusion has not progressed even when vaporized, the exhaust gas with a low air-fuel ratio locally reaches the NOx storage reduction catalyst. . In such a case,
Even if fuel is added from the fuel addition valve, the NOx reduction efficiency is low.

Therefore, the air-fuel ratio reduction rate changing means decreases the amount of fuel added from the fuel addition valve as the vaporization rate of the fuel added from the fuel addition valve is lower. That is, when lowering the air-fuel ratio of the exhaust gas to the target air-fuel ratio, the lower the fuel vaporization rate, the larger the amount by which the air-fuel ratio is lowered by the exhaust air-fuel ratio lowering means, and adding the fuel from the fuel addition valve This reduces the amount by which the air-fuel ratio is lowered. The ratio at this time may be changed stepwise or may be changed continuously. The vaporization rate may be the ratio of fuel vaporized in the fuel added from the fuel addition valve. Also, instead of the vaporization rate, how much the fuel added from the fuel addition valve is diffused in the exhaust gas, or how much of the fuel added from the fuel addition valve is the NOx storage reduction type. The amount by which the air-fuel ratio is reduced by the fuel addition valve may be reduced based on whether it reaches the catalyst.

By doing so, the lower the NOx reduction efficiency by the fuel supplied from the fuel addition valve, the smaller the amount of fuel supplied from the fuel addition valve, so the overall NOx reduction efficiency can be improved.

  Further, the fuel vaporization rate is often lowered when the internal combustion engine is operated at a relatively low load. In such an operating state, even if the air-fuel ratio of the gas discharged from the combustion chamber of the internal combustion engine is lowered, the combustion state is hardly deteriorated. Therefore, it is possible to increase the amount by which the air-fuel ratio is lowered by the exhaust air-fuel ratio lowering means. it can.

In the present invention, the air-fuel ratio reduction rate changing means is configured to reduce the air-fuel ratio by the exhaust air-fuel ratio lowering means according to the temperature of the exhaust gas or the NOx storage reduction catalyst, and from the fuel addition valve. The proportion of the fuel / air ratio can be changed by adding the fuel.

That is, the vaporization rate of the fuel added from the fuel addition valve is determined by the temperature of the exhaust or the temperature of the NOx storage reduction catalyst. Here, the lower the exhaust gas temperature, the lower the vaporization rate of the fuel added from the fuel addition valve. This makes it difficult to uniformly reduce the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst. Further, the lower the temperature of the NOx storage reduction catalyst, the more difficult it is for the fuel to react with the NOx storage reduction catalyst. As a result, the NOx reduction efficiency decreases. On the other hand, for example, as the temperature of the exhaust gas or the NOx storage reduction catalyst decreases, the amount by which the air-fuel ratio is reduced by the fuel addition valve is reduced, so that the NOx reduction efficiency can be improved.

  In the present invention, the air-fuel ratio reduction rate changing means reduces the air-fuel ratio by adding the fuel from the fuel addition valve by the amount by which the exhaust air-fuel ratio reducing means reduces the air-fuel ratio according to the flow rate of the exhaust. The ratio of the amount to be reduced can be changed.

That is, the vaporization rate of the fuel added from the fuel addition valve is determined by the exhaust gas flow rate. Here, as the flow rate of the exhaust gas decreases, it becomes difficult to diffuse the fuel into a wide range when the fuel is vaporized, so that a location where the air-fuel ratio in the exhaust is higher and lower than the target air-fuel ratio is created, and the NOx reduction efficiency Decreases. On the other hand, for example, as the flow rate of exhaust gas decreases, the amount of fuel that is difficult to vaporize can be reduced by reducing the amount by which the fuel addition valve lowers the air-fuel ratio, so that the NOx reduction efficiency can be improved. it can.

  In the present invention, the air-fuel ratio reduction rate changing means adjusts the air-fuel ratio by the exhaust air-fuel ratio reduction means according to the ratio of the amount of fuel supplied from the fuel addition valve that adheres to the wall surface of the exhaust passage. The ratio between the amount to be reduced and the amount to decrease the air-fuel ratio by adding fuel from the fuel addition valve can be changed.

That is, the vaporization rate of the fuel added from the fuel addition valve is determined by the ratio of the amount of the fuel supplied from the fuel addition valve that adheres to the wall surface of the exhaust passage. Here, if a large amount of fuel added from the fuel addition valve adheres to the wall surface of the exhaust passage, it becomes difficult to set the air-fuel ratio of the exhaust to the target air-fuel ratio. For example, even if the fuel adhering to the wall surface of the exhaust passage gradually evaporates, NOx is hardly reduced unless the air-fuel ratio of the exhaust gas is reduced to a value necessary for NOx reduction. . On the other hand, for example, the higher the proportion of the amount of fuel supplied from the fuel addition valve that adheres to the wall surface of the exhaust passage, the smaller the amount by which the air / fuel ratio is reduced by the fuel addition valve, thereby reducing the NOx reduction efficiency. Can be improved.

In the present invention, the air-fuel ratio reduction rate changing means adds the fuel from the fuel addition valve by the amount by which the exhaust air-fuel ratio lowering means lowers the air-fuel ratio according to the temperature distribution of the NOx storage reduction catalyst. By doing so, the ratio of the amount by which the air-fuel ratio is lowered can be changed.

That is, the vaporization rate of the fuel added from the fuel addition valve is determined by the temperature distribution of the NOx storage reduction catalyst. Here, if fuel is supplied from the fuel addition valve to the NOx storage reduction catalyst, the temperature in the NOx storage reduction catalyst may become uneven. That is, since the fuel added from the fuel addition valve is difficult to diffuse depending on the conditions, the air-fuel ratio of the exhaust is locally lowered. Therefore, the temperature may be locally increased even in the NOx storage reduction catalyst. That is, there may be a low temperature portion and a high temperature portion in the NOx storage reduction catalyst, and in this case, it can be determined that the fuel vaporization rate is reduced. In such cases, NOx
Reduction efficiency is reduced. In addition, if the temperature in the NOx storage reduction catalyst becomes non-uniform, there is a risk that a part of the NOx catalyst will become below the activation temperature. On the other hand, as the degree of temperature non-uniformity in the NOx storage reduction catalyst increases, the amount of fuel that has a low vaporization rate can be reduced by reducing the amount by which the fuel addition valve lowers the air-fuel ratio. Therefore, NOx reduction efficiency can be improved.

  According to the present invention, in an exhaust gas purification apparatus for an internal combustion engine, NOx reduction efficiency is further improved by combining a plurality of means for reducing the air-fuel ratio of exhaust gas and further setting the ratio of air-fuel ratio reduction by each means to an appropriate value. Let

  Hereinafter, specific embodiments of an exhaust emission control device for an internal combustion engine 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 1 to which an exhaust gas purification apparatus for an internal combustion engine according to this embodiment is applied and an exhaust system thereof. The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine.

  An intake passage 2 and an exhaust passage 3 are connected to the internal combustion engine 1. A throttle 4 is provided in the middle of the intake passage 2. The throttle 4 is opened and closed by an electric actuator. An air flow meter 5 that outputs a signal corresponding to the flow rate of the intake air flowing through the intake passage 2 is provided in the intake passage 2 upstream of the throttle 4. The air flow meter 5 measures the amount of fresh intake air in the internal combustion engine 1.

  On the other hand, an NOx storage reduction catalyst 6 (hereinafter referred to as NOx catalyst 6) is provided in the middle of the exhaust passage 3. The NOx catalyst 6 has a function of storing NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and reducing the stored NOx when the oxygen concentration of the inflowing exhaust gas is reduced and a reducing agent is present. Have. The NOx catalyst 6 may be supported on a particulate filter that collects particulate matter in the exhaust gas.

Further, in this embodiment, a fuel addition valve 7 for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust passage 3 upstream from the NOx catalyst 6 is provided. Here, the fuel addition valve 7 is opened by a signal from the ECU 20 described later to inject fuel. The fuel injected from the fuel addition valve 7 into the exhaust passage 3 enriches the air-fuel ratio of the exhaust flowing from the upstream of the exhaust passage 3 and reduces the NOx stored in the NOx catalyst 6. .

  The internal combustion engine 1 is provided with an EGR device 8 that recirculates part of the exhaust gas flowing through the exhaust passage 3 to the intake passage 2. The EGR device 8 includes an EGR passage 81 and an EGR valve 82.

The EGR passage 81 connects the exhaust passage 3 upstream of the NOx catalyst 6 and the intake passage 2 downstream of the throttle 4. The exhaust gas is recirculated as the exhaust gas flows through the EGR passage 81. The EGR valve 82 adjusts the amount of EGR gas flowing through the EGR passage 81 by adjusting the passage sectional area of the EGR passage 81.

Further, a temperature sensor 9 that outputs a signal corresponding to the temperature of the exhaust gas flowing in the exhaust passage 3 is provided in the exhaust passage 3 downstream of the NOx catalyst 6 and the air-fuel ratio of the exhaust gas flowing in the exhaust passage 3. And an air-fuel ratio sensor 10 for outputting a signal. Based on the output signal of the temperature sensor 9, the temperature of the NOx catalyst 6 is detected.

  Further, the internal combustion engine 1 is provided with a fuel injection valve 11 for supplying fuel into the cylinder of the internal combustion engine 1.

  The internal combustion engine 1 configured as described above is provided with an ECU 20 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 20 is a unit that controls 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.

  In addition to the above sensors, the ECU 20 outputs an electric signal corresponding to the amount by which the driver depresses the accelerator pedal 12 to detect an engine load, and a crank position for detecting the engine speed. Sensors 14 are connected via electrical wiring, and output signals from these various sensors are input to the ECU 20. On the other hand, the fuel injection valve 11 and the fuel addition valve 7 are connected to the ECU 20 via electric wiring, and the ECU 20 controls the opening and closing timing of the fuel injection valve 11 and the fuel addition valve 7.

In this embodiment, when NOx stored in the NOx catalyst 6 is reduced, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 6 is spiked in a relatively short cycle toward the target air-fuel ratio (
The so-called rich spike control is executed to make it rich in a short time). This rich spike control is performed not only when NOx is reduced by the NOx catalyst 6, but also when the sulfur poisoning of the NOx catalyst 6 is recovered, or when the NOx catalyst 6 is supported by the particulate filter. It is also done when raising. The air-fuel ratio of the exhaust can be lowered by injecting fuel from the fuel addition valve 7 or by reducing the air-fuel ratio of the gas discharged from the internal combustion engine 1. That is, by supplying a reducing agent to the NOx catalyst 6 by these means, NOx occluded in the NOx catalyst 6 can be reduced. In this embodiment, the fuel addition valve 7 corresponds to the reducing agent supply means in the present invention.

  Here, as a method of reducing the air-fuel ratio of the gas discharged from the internal combustion engine 1, it is possible to exemplify reducing the intake air amount or increasing the EGR gas amount. The air-fuel ratio of the gas discharged from the internal combustion engine 1 can also be increased by performing sub-injection (post-injection) in which fuel is injected again during the expansion stroke or exhaust stroke after the main injection from the fuel injection valve 11. Can be reduced.

  The intake air amount can be reduced by moving the throttle 4 to the closing side. The EGR gas amount can be increased by moving the EGR valve 82 to the open side. In the present embodiment, the ECU 20, which controls the throttle 4, the EGR valve 82, or the fuel injection valve 11, corresponds to the exhaust air / fuel ratio lowering means in the present invention.

  The reduction in the air-fuel ratio of the gas discharged from the internal combustion engine 1 is hereinafter referred to as “combustion rich” or “rich by combustion”. Further, the reduction in the air-fuel ratio of the exhaust due to the fuel injection from the fuel addition valve 7 is hereinafter referred to as “exhaust addition rich”.

The reduction of NOx can be performed either by combustion rich or exhaust addition rich, or by combining these.

Combustion rich can greatly reduce the air-fuel ratio in the combustion chamber and has high NOx reduction efficiency because CO having high NOx reduction efficiency is generated in the combustion chamber. However, the rich combustion can be performed only when the load of the internal combustion engine 1 is relatively low.

On the other hand, the exhaust addition rich can be performed with almost no influence of the load of the internal combustion engine 1, and therefore can be performed in a wider range of operation than the combustion rich. However, there is an operating region in which the NOx reduction efficiency deteriorates because the fuel is difficult to diffuse or the fuel adheres to the wall surface of the exhaust passage 3.

  Therefore, in this embodiment, both the combustion rich and the exhaust addition rich are used in combination, and the ratio of the reducing agent added from each is changed according to the situation, thereby maximizing the advantages of each addition method. I use it.

  Here, FIG. 2 is a diagram showing an example of the ratio of the combustion rich and the exhaust addition rich for achieving the target air-fuel ratio when the reducing agent is added. 2A shows a case where the combustion rich ratio is relatively high, FIG. 2B shows a case where the exhaust addition rich ratio is relatively high, and FIG. 2C shows a case where only the exhaust addition rich exists. . Further, (X) indicates the amount of decrease in the air-fuel ratio due to combustion rich, and (Y) indicates the amount of decrease in the air-fuel ratio due to exhaust addition rich.

Prior to the addition of the reducing agent, the internal combustion engine 1 is operated at the base air-fuel ratio. This base air-fuel ratio is an air-fuel ratio set by the load of the internal combustion engine 1 or the like. When combustion rich is performed, the air-fuel ratio is lowered in the combustion chamber of the internal combustion engine 1, and the exhaust gas flows through the exhaust passage 3. Then, by adding fuel from the fuel addition valve 7 into the exhaust, the air-fuel ratio of the exhaust is further lowered to the target air-fuel ratio. When the exhaust gas having the target air-fuel ratio flows into the NOx catalyst 6, NOx stored in the NOx catalyst 6 is reduced.

  That is, the actual air-fuel ratio is adjusted to the target air-fuel ratio by changing the air-fuel ratio of the exhaust from the internal combustion engine 1 and adding fuel from the fuel addition valve 7 in accordance with the air-fuel ratio of the exhaust at that time. When only exhaust addition rich is performed, the air-fuel ratio of the exhaust from the internal combustion engine 1 becomes the base air-fuel ratio. Here, a value obtained by dividing the fuel added from the fuel addition valve 7 by the amount of fuel necessary to reduce the base air-fuel ratio to the target air-fuel ratio is referred to as an exhaust addition ratio. That is, the exhaust gas addition ratio in the case shown in FIG. 2 is the lowest in FIG. 2 (A) and the highest in FIG. 2 (C).

In this embodiment, the lower the exhaust gas temperature or the lower the NOx catalyst 6 temperature,
Reduce the rate of exhaust addition.

FIG. 3 is a diagram illustrating the relationship between the exhaust gas temperature or the temperature of the NOx catalyst 6 and the exhaust gas addition ratio. The relationship shown in FIG. 3 is represented by a straight line, but it may be a curve, and the exhaust gas addition ratio may be increased stepwise as the exhaust gas temperature or the temperature of the NOx catalyst 6 increases. . This relationship can be obtained in advance by experiments or the like.

Here, as the temperature of the exhaust gas is lower, the fuel added from the fuel addition valve 7 is less likely to be atomized, so the fuel vaporization rate is lower. Thereby, as the temperature of the exhaust gas becomes lower, it becomes more difficult to uniformly reduce the air-fuel ratio of the exhaust gas even if fuel is added from the fuel addition valve 7. That is, since the fuel reaches the NOx catalyst 6 in a liquid state or reaches the NOx catalyst 6 without being diffused much even if it is vaporized, there are places where the air-fuel ratio of the exhaust is high and low. Even if this exhaust gas reaches the NOx catalyst 6, NOx is reduced at a portion where the target air-fuel ratio is reached, and at locations where the target air-fuel ratio is not reached or where the target air-fuel ratio is richer. , NOx
Is not reduced much. Therefore, NOx reduction efficiency is reduced.

Further, the lower the temperature of the NOx catalyst 6, the more difficult the fuel reacts with the NOx catalyst 6. That is, as the temperature of the NOx catalyst 6 becomes lower, it becomes more difficult for the fuel added from the fuel addition valve 7 to reduce NOx. Therefore, NOx reduction efficiency is reduced.

In that respect, the lower the exhaust gas temperature or the lower the temperature of the NOx catalyst 6, the lower the exhaust gas addition ratio, thereby suppressing the reduction in NOx reduction efficiency. That means
By increasing the combustion rich ratio, more NOx catalyst 6 can be supplied with fuel that has been vaporized from the beginning, and more reactive HC or CO. Also, combustion rich
Although it can be performed when the load of the internal combustion engine 1 is relatively low, in such an operating state, the temperature of the exhaust gas and the temperature of the NOx catalyst 6 are lowered. That is, by changing the combustion rich ratio in accordance with the temperature of the exhaust gas or the temperature of the NOx catalyst 6, it is possible to suppress the combustion rich ratio from being increased in an operating state where combustion rich cannot be performed.

In this way, NOx reduction efficiency can be improved while suppressing deterioration of the combustion state. Therefore, fuel efficiency can be improved while improving drivability.

  In the present embodiment, the ECU 20 that changes the exhaust gas addition ratio corresponds to the air-fuel ratio reduction ratio changing means in the present invention.

  In this embodiment, the exhaust gas addition ratio is lowered as the exhaust gas flow rate is smaller. The other devices are the same as those in the above embodiment, and the description is omitted.

  FIG. 4 is a diagram illustrating the relationship between the exhaust gas flow rate and the exhaust gas addition ratio. The relationship shown in FIG. 4 is represented by a straight line, but may be a curve, and the exhaust gas addition ratio may be increased stepwise as the exhaust gas flow rate increases. This relationship can be obtained in advance by experiments or the like. Further, the flow rate of the exhaust gas can be obtained from the intake air amount measured by the air flow meter 5.

  Here, as the flow rate of the exhaust gas decreases, the fuel added from the fuel addition valve 7 becomes difficult to atomize, so the fuel evaporation rate decreases.

In that respect, the lower the exhaust gas flow rate, the lower the NOx reduction efficiency can be suppressed by lowering the exhaust gas addition ratio. That is, by increasing the ratio of combustion rich, the NOx catalyst 6 is increased in the amount of fuel that has been vaporized from the beginning and more highly reactive HC or CO.
Can be supplied to. Further, the combustion rich can be performed when the load of the internal combustion engine 1 is relatively low, but in such an operating state, the flow rate of the exhaust gas decreases. That is, by changing the combustion rich ratio according to the flow rate of the exhaust gas, it is also possible to suppress the combustion rich ratio from being increased in an operating state where combustion rich cannot be performed.

In this way, NOx reduction efficiency can be improved while suppressing deterioration of the combustion state. Therefore, fuel efficiency can be improved while improving drivability.

  In the present embodiment, the ECU 20 that changes the exhaust gas addition ratio corresponds to the air-fuel ratio reduction ratio changing means in the present invention.

In the present embodiment, among the fuel added from the fuel addition valve 7, the exhaust addition ratio is lowered as the ratio of the fuel adhering to the exhaust passage 3 (hereinafter referred to as the wall surface adhesion ratio) increases. The other devices are the same as those in the above embodiment, and the description is omitted. The fuel adhering to the exhaust passage 3 includes fuel adhering to a member (for example, a turbocharger) between the fuel addition valve 7 and the NOx catalyst 6.

  FIG. 5 is a diagram illustrating the relationship between the wall surface adhesion rate and the exhaust gas addition ratio. The relationship shown in FIG. 5 is represented by a straight line, but may be a curve, and the exhaust addition ratio may be reduced stepwise as the wall surface adhesion rate increases. This relationship can be obtained in advance by experiments or the like. The wall surface adhesion rate can be obtained based on the exhaust temperature, the wall surface temperature of the exhaust passage 3, and the exhaust flow rate.

Here, since the air-fuel ratio of the exhaust gas that reaches the NOx catalyst 6 increases as the wall surface adhesion rate increases, more fuel must be added to reach the target air-fuel ratio. Therefore, NOx reduction efficiency is reduced.

In that respect, the lower the NOx reduction efficiency can be suppressed by lowering the exhaust gas addition ratio as the wall surface adhesion rate becomes higher. That is, by increasing the combustion rich ratio, the reducing agent vaporized from the beginning can be supplied to the NOx catalyst 6. Further, the combustion rich can be performed when the load of the internal combustion engine 1 is relatively low, but the wall surface adhesion rate becomes high in such an operation state. That is, by changing the combustion rich ratio according to the wall surface adhesion rate, it is also possible to suppress the combustion rich ratio from being increased in an operating state where combustion rich cannot be performed.

In this way, NOx reduction efficiency can be improved while suppressing deterioration of the combustion state. Therefore, fuel efficiency can be improved while improving drivability.

  In the present embodiment, the ECU 20 that changes the exhaust gas addition ratio corresponds to the air-fuel ratio reduction ratio changing means in the present invention.

In this embodiment, the exhaust gas addition ratio is lowered as the temperature of the NOx catalyst 6 becomes uneven. The other devices are the same as those in the above embodiment, and the description is omitted.

FIG. 6 is a diagram illustrating the relationship between the temperature uniformity of the NOx catalyst 6 and the exhaust gas addition ratio. Although the relationship shown in FIG. 6 is represented by a straight line, it may be a curve, and the exhaust gas addition ratio may increase stepwise as the temperature uniformity of the NOx catalyst 6 increases. . The uniformity of temperature is a value indicating how small the temperature difference in the NOx catalyst 6 is. That is, as the temperature uniformity increases, the temperature difference in the NOx catalyst 6 decreases, and as the temperature nonuniformity increases, the temperature difference in the NOx catalyst 6 increases. The relationship shown in FIG. 6 can be obtained in advance through experiments or the like. For example, the temperature of several places in the NOx catalyst 6 may be measured or estimated to obtain a difference between these temperatures, and the temperature non-uniformity may increase as the temperature difference increases.

Here, when fuel is added from the fuel addition valve 7, if the fuel vaporization rate is low, the air-fuel ratio of the exhaust gas partially decreases or increases, so the temperature of the NOx catalyst 6 increases partially. It may become lower or lower. In such a case, a portion that does not reach the target air-fuel ratio or a portion that is richer than the target air-fuel ratio is formed in the NOx catalyst 6, and the NOx reduction efficiency may be reduced.

In that respect, a decrease in NOx reduction efficiency can be suppressed by lowering the exhaust gas addition ratio as the temperature non-uniformity of the NOx catalyst 6 increases. That is, by increasing the combustion rich ratio, the reducing agent diffused to some extent from the beginning can be supplied to the NOx catalyst 6, so that the temperature of the NOx catalyst 6 can be increased as a whole. Further, the combustion rich can be performed when the load of the internal combustion engine 1 is relatively low.
The temperature non-uniformity of the catalyst 6 is increased. That is, by changing the combustion rich ratio according to the temperature non-uniformity of the NOx catalyst 6, it is possible to suppress the combustion rich ratio from being increased in an operating state where combustion rich cannot be performed.

In this way, NOx reduction efficiency can be improved while suppressing deterioration of the combustion state. Therefore, fuel efficiency can be improved while improving drivability.

  In the present embodiment, the ECU 20 that changes the exhaust gas addition ratio corresponds to the air-fuel ratio reduction ratio changing means in the present invention.

It is a figure which shows schematic structure of the internal combustion engine which applies the exhaust gas purification apparatus of the internal combustion engine which concerns on an Example, and its exhaust system. It is the figure which showed the example of the ratio of the combustion rich for setting it as a target air fuel ratio at the time of reducing agent addition, and exhaust gas addition rich. It is the figure which illustrated the relationship between the temperature of exhaust gas or the temperature of a NOx catalyst, and an exhaust gas addition ratio. It is the figure which illustrated the relationship between the flow volume of exhaust gas, and an exhaust gas addition ratio. It is the figure which illustrated the relationship between a wall surface adhesion rate and an exhaust gas addition ratio. It is the figure which illustrated the relationship between the nonuniformity of the temperature of a NOx catalyst, and an exhaust gas addition ratio.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Intake passage 3 Exhaust passage 4 Throttle 5 Air flow meter 6 Occlusion reduction type NOx catalyst 7 Fuel addition valve 8 EGR device 9 Temperature sensor 10 Air-fuel ratio sensor 11 Fuel injection valve 12 Accelerator pedal 13 Accelerator opening sensor 14 Crank position sensor 20 ECU
81 EGR passage 82 EGR valve

Claims (5)

An NOx storage reduction catalyst that is provided in the exhaust passage of the internal combustion engine and whose purification capacity is restored by supplying a reducing agent;
A fuel addition valve for adding fuel into the exhaust gas in the exhaust passage upstream of the NOx storage reduction catalyst;
Exhaust air / fuel ratio lowering means for lowering the air / fuel ratio of the gas discharged from the combustion chamber of the internal combustion engine when recovering the purification capacity of the NOx storage reduction catalyst, as compared to when not recovering the purification capacity;
The air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is combined with the amount by which the air-fuel ratio is lowered by the exhaust air-fuel ratio lowering means and the amount by which the air-fuel ratio is lowered by adding fuel from the fuel addition valve When the purification capacity is recovered by lowering the air-fuel ratio to the target air-fuel ratio, the air-fuel ratio lowering ratio is such that the lower the vaporization rate of the fuel added from the fuel addition valve, the smaller the air-fuel ratio is lowered by the fuel addition valve Change means,
An exhaust emission control device for an internal combustion engine, comprising: The air-fuel ratio reduction rate changing means adds the fuel from the fuel addition valve by the amount by which the exhaust air-fuel ratio lowering means lowers the air-fuel ratio according to the temperature of the exhaust gas or the temperature of the NOx storage reduction catalyst. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the ratio of the air fuel ratio to be reduced by the change is changed.   The air-fuel ratio lowering ratio changing means is an amount for reducing the air-fuel ratio by the exhaust air-fuel ratio lowering means according to the flow rate of exhaust, and an amount for lowering the air-fuel ratio by adding fuel from the fuel addition valve, The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the ratio is changed.   The air-fuel ratio lowering ratio changing means reduces the air-fuel ratio by the exhaust air-fuel ratio lowering means according to the ratio of the amount of reducing agent supplied from the fuel addition valve that adheres to the wall surface of the exhaust passage. 2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the ratio of the amount by which the air-fuel ratio is lowered by adding fuel from the fuel addition valve is changed. The air-fuel ratio lowering ratio changing means is configured to reduce the air-fuel ratio by the exhaust air-fuel ratio lowering means according to the temperature distribution of the NOx storage reduction catalyst, and by adding fuel from the fuel addition valve, 2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the ratio of the amount of the internal combustion engine is decreased.

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