WO2009130801A1

WO2009130801A1 - Exhaust purification apparatus for internal combustion engine

Info

Publication number: WO2009130801A1
Application number: PCT/JP2008/058509
Authority: WO
Inventors: 利岡俊祐; 浅沼孝充; 大橋伸基; 吉田耕平
Original assignee: トヨタ自動車株式会社
Priority date: 2008-04-25
Filing date: 2008-04-25
Publication date: 2009-10-29

Abstract

In an exhaust purification apparatus of an internal combustion engine, provided is a technique for improving an NOx reduction efficiency by combining a plurality of means to reduce an air-fuel ratio of an exhaust gas, and further by setting the rate of the air-fuel ratio reduction by each of the means to an adequate value. When performing NOx reduction by lowering the air-fuel ratio of the exhaust gas flowing into the occlusion and reduction type NOx catalyst (6) by combining a reduction quantity of the air-fuel ratio of a gas exhausted from a combustion chamber of the internal combustion engine (1) and a reduction quantity of the air-fuel ratio by adding fuel from a fuel adding valve (7), the reduction quantity of the air-fuel ratio by the fuel adding valve (7) is lowered in proportion to a vaporization rate of the fuel added by the fuel adding valve (7).

Description

Technical field

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

Background art

By adding a reducing agent to the NOx storage reduction catalyst (hereinafter also referred to simply as NOx catalyst) provided in the exhaust passage, the 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, and the like.

Here, when fuel is added from the fuel addition valve, it is not significantly affected by the operating state of the internal combustion engine, so that fuel can be added in a wide operating range. However, there is an operating region where the NOx reduction efficiency deteriorates due to the NOx catalyst reaching the NOx catalyst before the fuel diffuses and the fuel passing through the nox catalyst or adhering to the exhaust passage. The NOx reduction efficiency is a value obtained by dividing the amount of the reducing agent that reacts for the reduction of NOx in the supplied reducing agent by the amount of the reducing agent that is supplied.

On the other hand, when exhausting a low air-fuel ratio gas from an internal combustion engine, the oxygen concentration can be greatly reduced in the combustion chamber. In addition, since CO with high NOx reduction efficiency is produced, NOx reduction efficiency is increased. However, if the EGR gas amount is increased or the intake air amount is decreased, the combustion state becomes unstable. Therefore, it can be used only in a narrow operating region such as during low-load operation of an internal combustion engine. Therefore, there is known a technology that increases the NOx reduction efficiency by combining the above two types of means to compensate for each other's drawbacks (for example, Patent Documents).

1 (JP 2 0 0 5-2 2 6 4 6 3), Patent Document 2 (JP 2 0 0 4-3 3 2 7 1 2), Patent Document 3 (JP 2 0 0 5 — See No. 9 0 4 3 9) and Patent Document 4 (Japanese Patent Laid-Open No. 2 0 0 2-3 8 9 2 6). ). In other words, by combining the above two types of means, the air-fuel ratio to be lowered by each of the means can be small, so that the fuel can be prevented from slipping through the NOx catalyst while suppressing the deterioration of the combustion state.

However, the operating range in which low air-fuel ratio gas can be discharged from the internal combustion engine is narrow, and when fuel is added by a fuel addition valve, the fuel is difficult to diffuse. Therefore, how to determine the ratio between the amount by which the exhaust 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 is determined. N Ox reduction efficiency varies depending on whether or not.

Disclosure of the invention

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 further, the ratio of the air-fuel ratio reduction by each means The objective is to provide a technology that further improves the NOx reduction efficiency 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 is provided in an exhaust passage of the internal combustion engine, and an occlusion reduction type NOx catalyst whose purification capacity is recovered by supplying a reducing agent;

A fuel addition valve for adding fuel to the exhaust gas in the exhaust passage upstream of the NOx storage reduction catalyst;

An 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, compared to when not recovering the purification capacity; 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 are combined and flow into the NOx storage reduction catalyst When recovering the purification capacity by reducing the air-fuel ratio of the exhaust gas to the target air-fuel ratio, the lower the vaporization rate of the fuel added from the fuel addition valve, the lower the air-fuel ratio by the fuel addition valve. A ratio changing means for reducing

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. x Reducing agent is supplied to the 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, the fuel supplied into the cylinder is exposed to the high-temperature combustion gas, so that it is mostly discharged from the internal combustion engine in a vaporized state or a reformed state. For this reason, 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 having a temperature lower than that in the cylinder, it is less likely to vaporize compared to the fuel supplied to the cylinder. Therefore, there is a risk of reaching the NOx storage reduction catalyst in the liquid state. In addition, since the diffusion does not proceed even after vaporization, the exhaust gas in a state where the air-fuel ratio is locally low may reach the NOx storage reduction catalyst. In such a case, the NOx reduction efficiency is low even if fuel is added from the fuel addition valve.

For this reason, the ratio 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. In other words, when the exhaust air-fuel ratio is lowered to the target air-fuel ratio, the fuel vaporization rate is lower. However, the amount by which the air-fuel ratio is lowered by the exhaust air-fuel ratio lowering means is increased, and the amount by which fuel is added from the fuel addition valve is decreased by the amount added. The ratio at this time may be changed step by step or continuously. The vaporization rate may be the ratio of fuel vaporized in the fuel added from the fuel addition valve. Also, instead of the evaporation rate, how much fuel added from the fuel addition valve is diffused in the exhaust, or how much fuel is added to the storage reduction type fuel from the fuel addition valve. The amount by which the air-fuel ratio is reduced by the fuel addition valve may be reduced based on whether the NO x catalyst is reached.

By doing so, the lower the NO x 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 NO x reduction efficiency can be improved. it can.

Also, a decrease in fuel vaporization rate is often seen 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.

In the present invention, the ratio 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 temperature of the NOx storage reduction catalyst, and from the fuel addition valve to the fuel By adding, the ratio of reducing the air-fuel ratio can be changed.

In other words, the vaporization rate of the fuel added from the fuel addition valve is judged by the exhaust temperature or the temperature of the NOx storage reduction catalyst. Here, the lower the temperature of the exhaust, the lower the vaporization rate of the fuel added from the fuel addition valve. As a result, it becomes 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 the fuel reacts with the NOx storage reduction catalyst. As a result, the NO x reduction efficiency decreases. On the other hand, for example, exhaust temperature or occlusion The lower the temperature of the reduced NO x catalyst, the lower the air fuel ratio is reduced by the fuel addition valve, thereby improving the NO x reduction efficiency.

Further, in the present invention, the ratio changing unit is configured to reduce the air-fuel ratio by the exhaust air-fuel ratio lowering unit according to the flow rate of the exhaust, and to add air from the fuel addition valve. It is possible to change the ratio of the amount by which the fuel ratio is decreased.

In other words, the vaporization rate of the fuel added from the fuel addition valve is determined by the exhaust flow rate. Here, the lower the flow rate of the exhaust, the more difficult it is to diffuse when the fuel is vaporized. As a result, there are places where the air-fuel ratio in the exhaust is higher and lower than the target air-fuel ratio, and the N O x 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 air / fuel ratio is reduced by the fuel addition valve, thereby improving NOx reduction efficiency. Can do.

Further, in the present invention, the ratio changing means reduces the air / fuel ratio by the exhaust air / fuel ratio reducing means in accordance with the ratio of the amount of fuel supplied from the fuel addition valve to the wall surface of the exhaust passage. The ratio of 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.

In other words, the vaporization rate of the fuel added from the fuel addition valve is judged by the ratio of the amount of fuel supplied from the fuel addition valve to the wall 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 of the exhaust passage gradually evaporates, NO x is hardly reduced unless the air-fuel ratio of the exhaust gas decreases to the value required for NO x reduction. Reduction efficiency decreases. 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 fuel addition valve lowers the air-fuel ratio. By doing so, NOX reduction efficiency can be improved.

Further, in the present invention, the ratio changing means is configured to reduce the air / fuel ratio by the exhaust air / fuel ratio lowering means in accordance with the temperature distribution of the NOx storage reduction catalyst, and fuel from the fuel addition valve. It is possible to change the ratio of the amount by which the air-fuel ratio is decreased by adding.

In other words, the vaporization rate of the fuel added from the fuel addition valve is judged by the temperature distribution of the NOx storage reduction catalyst. Here, when 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. In other words, the fuel added from the fuel addition valve is difficult to diffuse depending on the conditions, so the air-fuel ratio of the exhaust is locally lowered. Therefore, the temperature may be locally increased even in the NOx storage reduction catalyst. In other words, in the storage reduction type NOx catalyst, there may be a place where the temperature is low and a place where the temperature is high. In this case, it can be determined that the fuel vaporization rate is lowered. In such a case, the NOx reduction efficiency is reduced. In addition, if the temperature in the NOx storage reduction catalyst becomes non-uniform, there is a risk that the temperature will be partially below the activation temperature. On the other hand, the higher the degree of temperature non-uniformity in the NOx storage reduction catalyst, the smaller the amount by which the fuel addition valve lowers the air-fuel ratio, thereby reducing the amount of fuel supplied with a low vaporization rate. NOx reduction efficiency can be improved.

As described above, according to the present invention, in the exhaust gas purification apparatus for an internal combustion engine, a plurality of means for reducing the air-fuel ratio of the exhaust gas are combined, and the ratio of the air-fuel ratio reduction by each means is set to an appropriate value. As a result, the NO x reduction efficiency can be further improved.

Brief Description of Drawings

FIG. 1 is a diagram illustrating a schematic configuration of an internal combustion engine to which an exhaust gas purification apparatus for an internal combustion engine according to an embodiment is applied, and an intake system and an exhaust system thereof.

FIG. 2 is a diagram showing an example of the ratio of the combustion rich and exhaust rich for achieving the target air-fuel ratio when reducing agent is added. FIG. 3 is a diagram illustrating the relationship between the exhaust gas temperature or NOX catalyst temperature and the exhaust gas addition ratio.

Fig. 4 is a diagram illustrating the relationship between the exhaust gas flow rate and the exhaust gas addition ratio. Fig. 5 is a diagram illustrating the relationship between the wall surface deposition rate and the exhaust gas addition ratio. FIG. 4 is a diagram illustrating the relationship between the catalyst temperature non-uniformity and the exhaust gas addition ratio.

BEST MODE FOR CARRYING OUT THE INVENTION

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.

(Example 1)

FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 to which the exhaust gas purification apparatus for an internal combustion engine according to this embodiment is applied, and its intake system and exhaust system. 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. This slot 4 is opened and closed by the electric actuator overnight. 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 occlusion reduction type NOx catalyst 6 (hereinafter referred to as NOx catalyst 6) is provided in the middle of the exhaust passage 3. The NO x catalyst 6 occludes NO x in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and stores the NO x that has been occluded when the oxygen concentration of the inflowing exhaust gas decreases and a reducing agent is present. Has the function of reducing. The N O x catalyst 6 may be carried on a particulate filter that collects particulate matter in the exhaust gas.

Further, in this embodiment, the exhaust passage 3 upstream from the NO x catalyst 6 is circulated. It has a fuel addition valve 7 that adds fuel (light oil) as a reducing agent to the exhaust gas. 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 makes the air-fuel ratio of the exhaust flowing from the upstream of the exhaust passage 3 rich, and is stored in the N Ox catalyst 6. Reduce Ox. The internal combustion engine 1 is provided with an EGR device 8 that recirculates a part of the exhaust gas flowing through the exhaust passage 3 to the intake passage 2. The EGR device 8 includes an EGR passage 8 1 and an EGR valve 8 2.

The EGR passage 8 1 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 by the exhaust gas flowing through the EGR passage 8 1. Further, the EGR valve 8 2 adjusts the amount of EGR gas flowing through the EGR passage 8 1 by adjusting the passage sectional area of the EGR passage 8 1.

Further, in the exhaust passage 3 downstream of the NOx catalyst 6, a temperature sensor 9 that outputs a signal corresponding to the temperature of the exhaust flowing in the exhaust passage 3, and the air-fuel ratio of the exhaust flowing in the exhaust passage 3 are set. Air-fuel ratio sensors 10 and, which output corresponding signals, are attached. 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 ECU 20 as an electronic control unit for controlling the internal combustion engine 1. This E C U 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 demands of the driver.

In addition to the above sensor, the ECU 20 outputs an electric signal corresponding to the amount of depression of the accelerator pedal 12 by the driver, and an accelerator opening sensor 13 that can detect the engine load, and the engine speed Crank position sensor 14 is connected via electrical wiring, and the output signals of these various sensors Input to 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 set at a relatively short cycle toward the target air-fuel ratio. The so-called rich spike control is executed to make the spike rich (short time). This rich spike control is performed not only when NOx is reduced by the NOx catalyst 6 but also when sulfur poisoning of the NOx catalyst 6 is recovered. In addition, when the NOx catalyst 6 is supported on the particulate filter, this is also performed when the temperature of the filter is raised. In order to reduce the air-fuel ratio of the exhaust, either 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 Can do. That is, by supplying a reducing agent to the N Ox catalyst 6 by these means, N Ox occluded in the N Ox 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 for lowering 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. Gas discharged from the internal combustion engine 1 can also be obtained by performing sub-injection (boss soot injection) during the expansion stroke or exhaust stroke after the main injection from the fuel injection valve 1 1. It is possible to reduce the air-fuel ratio.

The amount of intake air 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 this embodiment, the ECU that controls the throttle 4, the EGR valve 82, or the fuel injection valve 11 is used. 20 corresponds to the exhaust air-fuel ratio lowering means in the present invention.

In addition, the decrease in the air-fuel ratio of the gas discharged from the internal combustion engine 1 is

It is called “combustion rich” or “rich by combustion”. In addition, the reduction in the air-fuel ratio of the exhaust due to the injection of fuel from the fuel addition valve 7 is hereinafter referred to as “r exhaust addition rich”.

The reduction of NO x can be performed either by combustion rich or exhaust addition rich, and can also be performed in combination. Combustion rich can significantly reduce the air-fuel ratio in the combustion chamber. In addition, CO with high NO x reduction efficiency is generated in the combustion chamber, so NO x reduction efficiency is high. 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 gas 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 latch. However, there is an operating region in which the NOx reduction efficiency deteriorates because the fuel hardly diffuses or the fuel adheres to the wall surface of the exhaust passage 3.

Therefore, in this embodiment, the advantages of each addition method can be maximized by using both combustion rich and exhaust addition rich in combination, and changing the ratio of the reducing agent added from each in accordance with the situation. It is used as much as possible. Here, FIG. 2 is a diagram showing an example of the ratio of the combustion rich and the exhaust addition ridge to achieve the target air-fuel ratio when the reducing agent is added. (A) shows a comparatively high ratio of combustion rich, (B) shows a comparatively high ratio of exhaust addition rich, and (C) shows a case of exhaust rich only. In addition, (X) indicates the air fuel ratio decrease due to combustion rich, and (Y) indicates the air fuel ratio decrease due to the 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 the 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 to the exhaust gas, the air-fuel ratio of the exhaust gas 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 gas 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 gas at that time. Note that when only the exhaust gas addition rich is performed, the air-fuel ratio of the exhaust gas from the internal combustion engine 1 becomes the base air-fuel ratio. Here, the value obtained by dividing the fuel added from the fuel addition valve 7 by the amount of fuel required to lower the base air-fuel ratio to the target air-fuel ratio is referred to as the exhaust addition ratio. That is, in the case shown in FIG. 2, the ratio of exhaust addition is the lowest in (A) and the highest in (C).

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

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 increase stepwise as the exhaust gas temperature or NO x catalyst 6 temperature rises. . This relationship can be obtained beforehand through experiments or the like.

Here, as the temperature of the exhaust gas is lower, the fuel added from the fuel addition valve 7 is more difficult to atomize, so the fuel vaporization rate becomes lower. As a result, the lower the temperature of the exhaust gas, the more difficult it becomes to uniformly reduce the air-fuel ratio of the exhaust gas even if fuel is added from the fuel addition valve 7. That is, the fuel may reach the NO x catalyst 6 while being in a liquid state. Moreover, even if it is vaporized, it reaches the NO x catalyst 6 with little diffusion. As a result, there are places where the air-fuel ratio of the exhaust is high and low. Even if this exhaust reaches NO x catalyst 6, the target air combustion NOX is reduced at a part of the ratio, and N 0 X is not reduced much at locations where the target air-fuel ratio has not been reached, or where the target air-fuel ratio is more critical. Therefore, the NO x reduction efficiency decreases.

Further, the lower the temperature of the N O x catalyst 6, the more difficult the fuel reacts with the N O x catalyst 6. In other words, the lower the temperature of the N O x catalyst 6, the more difficult it is to reduce N O x with the fuel added from the fuel addition valve 7. Therefore, N O x reduction efficiency decreases.

In this respect, the lowering of the NOx reduction efficiency can be suppressed by lowering the exhaust gas addition ratio as the temperature of the exhaust gas becomes lower or as the temperature of the NOx catalyst 6 becomes lower. In other words, by increasing the ratio of combustion rich, it is possible to supply more NO H x catalyst 6 to the fuel that has been vaporized from the beginning, or more highly reactive H 2 C or C 0. Combustion rich 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. In other words, by changing the combustion rich ratio according to the temperature of the exhaust gas or the temperature of the NO x catalyst 6, it is possible to suppress an increase in the ratio of the combustion rich in an operating state where the combustion rich cannot be performed. .

In this way, the NOx reduction efficiency can be improved while suppressing the deterioration of the combustion state. Therefore, it is possible to improve fuel efficiency while improving driver spirit.

In this embodiment, E C U 20 that changes the exhaust gas addition ratio corresponds to the ratio changing means in the present invention.

(Example 2)

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

FIG. 4 is a diagram illustrating the relationship between the exhaust flow rate and the exhaust gas addition ratio. Although the relationship shown in FIG. 4 is represented by a straight line, it may be a curve, and the exhaust gas addition ratio becomes higher stepwise as the exhaust gas flow rate increases. Also good. This relationship can be obtained in advance through 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 vaporization 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. In other words, by increasing the ratio of combustion rich, it is possible to supply the N O x catalyst 6 with more fuel that has been vaporized from the beginning and more highly reactive HC or CO. In addition, the combustion rich can be performed when the load of the internal combustion engine 1 is relatively low, but in such an operation state, the flow rate of the exhaust gas is reduced. In other words, by changing the combustion rich ratio in accordance with the flow rate of the exhaust gas, it is possible to suppress the combustion rich ratio from increasing during an operation state in which combustion rich cannot be performed.

(Example 3)

In this embodiment, among the fuel added from the fuel addition valve 7, the higher the rate of attachment to the exhaust passage 3 (hereinafter referred to as the wall surface deposition rate), the lower the exhaust addition rate. The other devices are the same as those in the previous embodiment, so 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 it may be a curved line. It is also possible that the exhaust gas addition ratio gradually decreases with increasing wall surface adhesion rate. This relationship can be obtained in advance through experiments or the like. The wall surface adhesion rate can be determined 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, the higher the wall surface adhesion rate, the lower the NOx reduction efficiency.

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 increases. That is, by increasing the ratio of the combustion rich, the reducing agent vaporized from the beginning can be supplied to the N O x catalyst 6. In addition, combustion rich can be performed when the load of the internal combustion engine 1 is relatively low, but in such an operating state, the wall surface adhesion rate becomes high. In other words, by changing the combustion rich ratio according to the wall surface adhesion rate, it is possible to suppress the combustion rich ratio from being increased in an operating state where combustion rich cannot be performed.

(Example 4)

In this embodiment, the exhaust gas addition ratio is lowered as the temperature of the N O x catalyst 6 becomes nonuniform. The other devices are the same as in the previous embodiment, so the description is omitted.

FIG. 6 is a graph illustrating the relationship between the temperature uniformity of the NO x 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 curved line, and it is stepwise with respect to an increase in temperature uniformity of the NO x catalyst 6 In addition, the exhaust addition ratio may be high. “Temperature uniformity” is a value indicating how small the temperature difference in the NO x catalyst 6 is. The value indicating how large the temperature difference in the NO x catalyst 6 is called “temperature non-uniformity”. That is, as the temperature uniformity increases, the temperature difference in the NO x catalyst 6 decreases. In addition, the greater the temperature non-uniformity, the greater the temperature difference within the NO x catalyst 6. The relationship shown in Fig. 6 can be obtained beforehand through experiments. For example, the temperature of several locations in the NO x catalyst 6 is measured or estimated, and the difference between these temperatures is obtained. The larger the temperature difference, the greater the temperature non-uniformity (that is, the temperature uniformity becomes higher). Small).

Here, if fuel is added from the fuel addition valve 7 and the fuel vaporization rate is low, the air-fuel ratio of the exhaust gas partially decreases or increases. For this reason, the temperature of the NOx catalyst 6 may rise or fall partially. In such a case, the NOx catalyst 6 may have a location that does not reach the target air-fuel ratio or a location that is richer than the target air-fuel ratio, which may reduce the NOx reduction efficiency. .

In this regard, as the temperature non-uniformity of the N O x catalyst 6 increases, the reduction in the N O x reduction efficiency can be suppressed by lowering the exhaust gas addition ratio. In other words, when the ratio of combustion rich is increased, the reducing agent diffused to some extent from the beginning can be supplied more to the N O x catalyst 6, so that the temperature of the N O x catalyst 6 can be increased as a whole. In addition, combustion rich can be performed when the load of the internal combustion engine 1 is relatively low. In such an operating state, the temperature non-uniformity of the NOx catalyst 6 increases. That is, by changing the ratio of the combustion rich according to the non-uniformity of the temperature of the NOx catalyst 6, it is possible to suppress an increase in the ratio of the combustion rich 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 is improved while improving driver spirit. Can be improved.

Claims

The scope of the claims

1. an NOx storage reduction catalyst that is provided in the exhaust passage of an internal combustion engine and whose purification capacity is restored by supplying a reducing agent;

An 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, compared with when not recovering the purification capacity;

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 are combined and flow into the NOx storage reduction catalyst When recovering the purification capacity by reducing the air-fuel ratio of the exhaust gas to the target air-fuel ratio, the lower the vaporization rate of the fuel added from the fuel addition valve, the lower the air-fuel ratio by the fuel addition valve. A ratio changing means for reducing

An exhaust emission control device for an internal combustion engine, comprising:

2. The ratio changing unit is configured to add the fuel from the fuel addition valve by reducing the air-fuel ratio by the exhaust air-fuel ratio lowering unit according to the temperature of the exhaust gas or the temperature of the NOx storage reduction catalyst. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the ratio of the amount by which the air-fuel ratio is reduced by the step is changed.

3. The ratio changing means reduces the air-fuel ratio by adding the fuel from the fuel addition valve and the amount by which the exhaust air-fuel ratio is lowered by the exhaust air-fuel ratio lowering means according to the flow rate of the exhaust gas. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the ratio of the amount to be changed is changed.

4. The 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 to the wall of the exhaust passage. The ratio of reducing the air-fuel ratio by adding fuel from the fuel addition valve and the ratio of The exhaust emission control device for an internal combustion engine according to claim 1.

5. The ratio changing unit is configured to add fuel from the fuel addition valve by reducing the air / fuel ratio by the exhaust air / fuel ratio reducing unit according to the temperature distribution 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 amount by which the air-fuel ratio is lowered is changed.