JP3552617B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus for an internal combustion engine that purifies exhaust gas discharged from the internal combustion engine with a catalyst provided in an engine exhaust passage.
[0002]
[Prior art]
2. Description of the Related Art A technique for providing a catalytic converter that contains an exhaust purification catalyst (hereinafter referred to as “catalyst”) in an engine exhaust passage of an internal combustion engine and purifying exhaust gas passing through the catalytic converter with a catalyst is well known.
[0003]
Generally, when the temperature of the catalyst rises and reaches a certain temperature range, the catalyst purifies harmful components in the exhaust gas. The temperature at which the catalyst is activated so that the exhaust gas purification rate by the catalyst becomes 50% or more is referred to as the 50% purification temperature of the catalyst by the oxidation reaction. The% purification temperature is referred to as “catalyst activation temperature” for convenience.
[0004]
By the way, even if the internal combustion engine in a cold state is started once, the catalyst in the exhaust passage does not immediately warm, so the catalyst temperature at that time is usually lower than the activation temperature. Therefore, in that case, even if the exhaust gas reaches the catalyst, the catalyst cannot be exhausted, and the exhaust is released into the atmosphere as it is. Therefore, when starting the engine in cold weather, it is desirable to raise the catalyst to the active temperature as soon as possible.
[0005]
[Problems to be solved by the invention]
By the way, the temperature of the catalyst contained in the catalytic converter causes a temperature difference due to the difference in the portion of the catalyst.
[0006]
FIG. 5 shows this relationship.
The vertical axis in FIG. 5 is the catalyst bed temperature, and above that is a schematic diagram of the catalyst a, and illustrates a cylindrical shape in accordance with the shape of the exhaust passage. Further, the horizontal axis indicates each part of the catalyst a in the catalyst cross section, and the central part illustrated by way of example on the horizontal axis is a part including the axial center b extending in the longitudinal direction of the catalyst a and its peripheral part. The outer peripheral part is the outer peripheral surface c of the catalyst a. The temperature of each part is indicated by a graph line G.
[0007]
Even if exhaust gas is allowed to flow through the catalyst a, the outer peripheral portion of the catalyst a is closer to the atmosphere than the central portion of the catalyst, so that the heat release amount is larger than that of the central portion. That is, the temperature of the outer peripheral portion of the catalyst a is difficult to rise, but the central portion is likely to rise.
[0008]
Further, as is generally known, the velocity of the fluid flowing in the pipe is higher at the center of the pipe than at the wall surface side of the pipe. For this reason, the amount of exhaust gas flowing in the catalyst is also larger at the center of the catalyst than at the wall surface.
[0009]
The central part of the catalyst has a small amount of heat release and a large amount of exhaust gas flowing therethrough, so the temperature is relatively high, whereas the wall side has a large amount of heat release compared to the center part and the amount of exhaust gas flowing therethrough Therefore, the temperature distribution of the catalyst is high at the center of the horizontal axis and low at both ends as shown by the graph line G in FIG.
[0010]
As described above, the catalyst a provided in the exhaust passage of the internal combustion engine does not have a uniform catalyst temperature immediately even when exhaust gas flows. Such a phenomenon is likely to occur when the engine is not sufficiently warmed up, for example, when the internal combustion engine is started in cold weather. In some cases, inside the catalyst, it is easy to be divided into a portion that has reached the activation temperature as described above and a portion that has not reached the activation temperature, and if the exhaust gas passes through the portion that has not reached the activation temperature, the exhaust gas is not purified and the atmosphere is not purified. It will be released inside.
[0011]
On the other hand, as shown in FIG. 6, the inventor's experiment is that the activation temperature of the catalyst (the “50% purification temperature of the catalyst by oxidation reaction”) varies depending on the air-fuel ratio of the exhaust gas passing through the catalyst. And know by research.
[0012]
That is, the 50% purification temperature of the catalyst by the oxidation reaction is higher as the air-fuel ratio of the exhaust gas flowing through the catalyst is richer and lower as it is leaner. In other words, even if the catalyst temperature is low, 50% purification by oxidation reaction is possible if the exhaust gas has a lean degree corresponding to the temperature. On the other hand, when the air-fuel ratio of the exhaust gas is rich, the oxidation reaction hardly occurs.
[0013]
The present invention has been invented based on such knowledge, and the problem to be solved is that there is a temperature difference for each part in the catalyst and the air-fuel ratio of the exhaust gas passing through the catalyst, The aim is to improve the exhaust gas purification rate of the entire catalyst by making good use of the two phenomena of different catalyst activation temperatures.
[0014]
[Means for Solving the Problems]
The present invention employs the following means in order to solve the above problems.
(1) The present invention provides an exhaust purification catalyst that is provided in an exhaust passage of an internal combustion engine having a plurality of cylinders and purifies exhaust gas discharged from the cylinders in the exhaust passage, and some of the plurality of cylinders. The rich air-fuel ratio cylinder group that operates at a rich air-fuel ratio and the rest as a lean air-fuel ratio cylinder group that operates at a lean air-fuel ratio, and the rich exhaust gas that the rich air-fuel ratio cylinder group exhausts into the exhaust passage and the lean air-fuel ratio A cylinder-by-cylinder air-fuel ratio control that promotes an oxidation reaction of unburned fuel components in the exhaust gas and increases the temperature of the catalyst by introducing lean exhaust gas discharged from the cylinder group into the exhaust passage into the catalyst. An exhaust gas purification apparatus for an internal combustion engine, wherein the rich air-fuel ratio cylinder is directed toward a central portion when the catalyst is viewed in a cross section when the catalyst temperature rise is performed by the cylinder-by-cylinder air-fuel ratio control means. Allowed to flow into the exhaust gas into the catalyst, flowing the exhaust gas of the lean air-fuel ratio cylinder group towards catalytic within the outer portion when viewed the catalyst in cross section in said catalyst As a result, rich exhaust gas mainly flows in the central portion when viewed in the cross section of the catalyst, and lean exhaust gas flows in the outer portion inside the catalyst. It was characterized by that.
[0015]
In the present invention having such a configuration, when the temperature of the catalyst is raised by the cylinder-by-cylinder air-fuel ratio control means, the exhaust gas of the rich air-fuel ratio cylinder group flows into the catalyst toward the center when the catalyst is viewed in a cross section. The exhaust gas of the lean air-fuel ratio cylinder group flows into the catalyst toward the outer portion inside the catalyst when the catalyst is seen in a cross section. Rich exhaust gas flows in the central portion along the gas flow direction and including the catalyst axis.
[0016]
In addition, lean exhaust gas flows in an outer portion inside the catalyst, that is, in an outer portion of the inside of the catalyst where the rich exhaust gas flows than the central portion of the catalyst.
As described above, the richer the air-fuel ratio of the exhaust gas, the higher the 50% purification temperature of the catalyst, and the lower the leaner, the lower the 50% purification temperature.
[0017]
Therefore, for example, when the engine is started in cold weather, the heat of the exhaust gas is likely to be released even if the exhaust gas flows through the outside portion inside the catalyst close to the outside air.
[0018]
However, in the present invention, since lean exhaust gas is allowed to flow through this outer portion, the degree of leanness is adjusted in accordance with the external temperature of the catalyst, so that the exhaust gas purification rate is increased by 50% by promoting the oxidation reaction even at low temperatures. More than that.
[0019]
On the other hand, since a relatively large amount of exhaust gas flows at a higher temperature than the outer portion in the catalyst central portion, a rich amount of exhaust gas flows, so by adjusting the degree of richness according to the temperature of the catalyst central portion, The exhaust gas purification rate in the catalyst central portion can be 50% or more.
[0020]
In this way, the air-fuel ratio of the exhaust gas in the central part inside the catalyst and the air-fuel ratio of the exhaust gas in the outside part inside the catalyst can be used separately for the rich air-fuel ratio and the lean air-fuel ratio, respectively, so that both can be made 50% purification temperature. Even if there is a temperature difference for each part in the catalyst, the exhaust gas purification rate does not become uneven due to the temperature difference. Therefore, the exhaust gas purification rate can be increased as a whole catalyst.
[0021]
Even if the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas are separated and flowed into the catalyst as described above, the inside of the catalyst is not completely stratified into the rich exhaust gas and the lean exhaust gas. .
[0022]
The direction of the main flow of rich exhaust gas is toward the central portion, the direction of the main flow of lean exhaust gas is toward the outside, and the two gases are not mixed. It is extremely important to finally mix the two in order to suitably perform exhaust purification.
[0023]
This is because if the two exhaust gases are completely stratified and not mixed, the rich exhaust gas is in a reducing atmosphere with a lot of unburned hydrocarbons (hereinafter referred to as “unburned HC”), and the lean exhaust gas is oxygen. Therefore, the former exhaust gas is discharged without purification of hydrocarbons (hereinafter referred to as “HC”) and carbon monoxide (hereinafter referred to as “CO”). This is because nitrogen oxides (hereinafter referred to as “NOx”) are discharged without being purified even if HC and CO are purified because they are in an oxidizing atmosphere.
[0024]
For this reason, it is basically desirable to mix lean exhaust gas and rich exhaust gas. However, even if both are mixed, since the outer peripheral portion of the catalyst is at a low temperature, the catalyst is hardly activated at the outer peripheral portion. Therefore, although the lean exhaust gas and the rich exhaust gas are basically mixed, the lean exhaust gas tends to flow toward the outside of the catalyst and the rich exhaust gas flows toward the center of the catalyst. It is.
[0025]
(2) An exhaust emission control device for an internal combustion engine according to the present invention includes an engine block in which the plurality of cylinders are arranged in series, and the number of cylinders arranged in the engine block is set to four. The air-fuel ratio cylinder group can be formed by the second and third cylinders, and the lean air-fuel ratio cylinder group can be formed by the first and fourth cylinders.
[0026]
The second and third cylinders are in the center of the engine block, and the first and fourth cylinders are respectively at both ends of the engine block. In this relationship, the second and third cylinders easily flow the exhaust gas toward the center of the catalyst, and the first and fourth cylinders flow toward the outside of the catalyst. Therefore, it can be said that the second and third cylinders and the catalyst central portion and the first and fourth cylinders and the outside of the catalyst are in a positional relationship. For this reason, it becomes easy to introduce exhaust gas discharged from the second and third cylinders and the first and fourth cylinders toward the catalyst center and the outside of the catalyst, respectively.
[0027]
(3) A plurality of exhaust pipes connected to the exhaust ports of the cylinders constituting the rich air-fuel ratio cylinder group and the exhaust ports of the cylinders constituting the lean air-fuel ratio cylinder group, respectively. An exhaust collecting pipe formed collectively is provided, and an exhaust pipe connected to the rich air-fuel ratio cylinder group is arranged at the center of the exhaust collecting pipe, and is connected to the lean air-fuel ratio cylinder group around the exhaust pipe. An exhaust pipe may be provided to connect the exhaust collecting pipe and the catalyst.
[0028]
(4) Simply performing the cylinder-by-cylinder air-fuel ratio control makes the output of each cylinder non-uniform and the driving feeling deteriorates. However, in the present invention, the ignition timing of the rich air-fuel ratio cylinder group is retarded and the ignition timing of the lean air-fuel ratio cylinder group is advanced, so that the output of each cylinder becomes uniform and it is possible to prevent deterioration in driving feeling. .
[0029]
(5) A communication hole that communicates between the exhaust pipe related to the rich air-fuel ratio cylinder group arranged in the center of the exhaust collecting pipe and the exhaust pipe related to the lean air-fuel ratio cylinder group arranged around the exhaust pipe. You may make it have.
[0030]
By having the communication holes in this way, as described above, it becomes easy to mix lean exhaust gas and rich exhaust gas. As for the number and size of the communication holes, it is desirable to find an optimum one through experiments.
[0031]
(6) The total air-fuel ratio of the rich exhaust gas discharged from the rich air-fuel ratio cylinder group and the lean exhaust gas discharged from the lean air-fuel ratio cylinder group is a relatively lean air-fuel ratio. Is preferred.
[0032]
This is because the oxidation reaction is promoted when the air-fuel ratio of the exhaust gas is slightly closer to the lean air-fuel ratio than when it is stoichiometric. This is because the purification rate of the catalyst increases when the air-fuel ratio of the exhaust gas is lean as a whole.
[0033]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described below with reference to FIGS.
[0034]
FIG. 1 shows a schematic configuration of an engine A to which an exhaust emission control device of the present invention is applied.
FIG. 1 suggests an engine body (engine block) 1 that omits the intake system and shows the relationship with the exhaust system.
[0035]
A water jacket (not shown) is formed in the engine body 1, and engine cooling water is circulated between the engine body 1 and a radiator (not shown) to cool the engine. The engine coolant temperature is detected by a temperature sensor (not shown).
[0036]
The intake system is provided with an air flow meter (not shown) for detecting the intake air amount.
The engine body 1 includes a first cylinder # 1, a second cylinder # 2, a third cylinder # 3, and a fourth cylinder # 4.
[0037]
Each cylinder # 1-4 is provided with a spark plug 2 and a fuel injection valve 3, respectively. Then, fuel is directly injected into each cylinder from the fuel injection valve 3.
The cylinders # 1 to # 4 are divided into two cylinder groups. That is, a cylinder group #L composed of the first cylinder # 1 and the fourth cylinder # 4 and a cylinder group #R composed of the second cylinder # 2 and the third cylinder # 3.
[0038]
Further, the order of the exhaust stroke of the engine body 1 is set as No. 1 cylinder # 1 → No. 3 cylinder # 3 → No. 4 cylinder # 4 → No. 2 cylinder # 2, and the exhaust stroke of each cylinder is continuous with each other. Not divided into cylinders.
[0039]
The cylinders # 1 to # 4 are connected to a casing 6 containing a start-up catalyst 5 as an exhaust purification catalyst via an exhaust manifold 4 as an exhaust collecting pipe constituting a part of the exhaust passage.
[0040]
A three-way catalyst is applied to the starting catalyst 5. The casing 6 is connected to a casing 9 containing an NOx storage reduction catalyst (hereinafter referred to as “NOx catalyst”) 8 via an exhaust pipe 7, and the casing 9 is connected to a muffler (not shown) via an exhaust pipe 10. .
[0041]
The start-up catalyst 5 formed of a three-way catalyst oxidizes HC and CO and reduces NOx to the maximum when the air-fuel ratio of the exhaust gas is stoichiometric.
The NOx catalyst 8 absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean (oxidizing atmosphere), and has absorbed until then in the reducing atmosphere where the oxygen concentration in the exhaust gas is reduced and HC and CO are high. Release NOx and nitrogen N ₂ It is a catalyst that reduces to
[0042]
An electronic control unit (ECU) 20 for engine control includes a central processing controller CPU, a read-only memory ROM, a random access memory RAM, an input interface circuit, and an output interface circuit, which are connected to each other by a bidirectional bus (not shown). Etc.
[0043]
The input interface circuit is electrically connected to various sensors (not shown) such as a temperature sensor and an air flow meter. When output signals of these various sensors enter the ECU 20 from the input interface circuit, these parameters are temporarily stored. Are stored in random access memory RAM. Then, when the CPU performs necessary arithmetic processing based on these parameters, the parameters stored in the random access memory RAM are called up as necessary through the bidirectional bus.
[0044]
The output interface circuit is electrically connected to various devices that operate based on output signals of the various sensors, such as the spark plug 2 and the fuel injection valve 3, and these various devices are appropriately connected based on the calculation results of the CPU. Operate.
[0045]
The ignition timing, the fuel injection timing period, and the fuel injection period of the spark plug 2 and the fuel injection valve 3 are controlled by the CPU. In addition, since CPU belongs to ECU20, control of CPU is synonymous with control of ECU20.
[0046]
The ECU 20 controls the fuel injection valve 3 according to the operating state of the engine to adjust the amount of fuel combusted in the cylinder in order to obtain the engine output. By this adjustment, the exhaust gas is discharged to the lean air-fuel ratio. Gas or rich air-fuel ratio exhaust gas.
[0047]
Further, in the engine A, the cylinder group #R that operates # 2 and # 3 of the four cylinders with a rich air-fuel ratio becomes a rich air-fuel ratio cylinder group, and the remaining # 1 and # 4 The amount of fuel injected from the fuel injection valve 3 is controlled so that the cylinder group #L that operates at a lean air-fuel ratio becomes a lean air-fuel ratio cylinder group.
[0048]
By introducing the rich exhaust gas and the lean exhaust gas discharged by the rich air-fuel ratio cylinder group #R and the lean air-fuel ratio cylinder group #L to the start-up catalyst 5 in the casing 6, unburned in the exhaust gas The cylinder-by-cylinder air-fuel ratio control that promotes the oxidation reaction of the fuel component to increase the temperature of the starting catalyst 5 is executed according to the operating state of the engine A.
[0049]
Further, when the cylinder-by-cylinder air-fuel ratio control is executed, the start-up catalyst 5 uses the exhaust gas from the rich air-fuel ratio cylinder group #R to the catalyst 5 toward the center when the catalyst 5 is viewed in a cross section. The exhaust gas from the lean air-fuel ratio cylinder group #L is caused to flow into the catalyst 5 toward the outer portion inside the catalyst when the catalyst 5 is viewed in a cross section.
[0050]
Therefore, for example, the shape of the exhaust manifold 4 is as follows. This will be described with reference to FIGS.
That is, the exhaust manifold 4 corresponds to the exhaust ports of cylinders # 2 and # 3 constituting the rich air-fuel ratio cylinder group #R and the exhaust ports of cylinders # 1 and # 4 constituting the lean air-fuel ratio cylinder group #L, respectively. Connected exhaust branch pipes 4 ₁ , 4 ₂ , 4 ₃ , 4 ₄ Have The exhaust branch pipe 4 related to the rich air-fuel ratio cylinder group #R located at the center of the exhaust manifold 4 ₂ , 4 ₃ Are joined together to form a single joining pipe 25, and the exhaust branch pipe 4 associated with the lean air-fuel ratio cylinder group #L is formed around the joining pipe 25. ₁ , 4 ₄ It is in the form of a go that wraps around.
[0051]
Therefore, the exhaust branch pipe 4 ₁ , 4 ₂ , 4 ₃ , 4 ₄ The start-up catalyst 5 in the casing 6 connected to the center is mainly rich in the central portion, in other words, in the central portion of the catalyst 5 along the direction in which the exhaust gas flows and including the axis of the catalyst 5. Exhaust gas is exhaust branch 4 ₂ , 4 ₃ The lean exhaust gas is introduced into the exhaust branch pipe 4 at an outer portion inside the catalyst 5, that is, outside the center of the catalyst 5 in the catalyst 5. ₁ , 4 ₄ It is introduced from.
[0052]
In this way, the exhaust branch pipe 4 ₁ , 4 ₂ , 4 ₃ , 4 ₄ Thus, the cylinder-by-cylinder air-fuel ratio control is realized by introducing exhaust gas into the start-up catalyst 5.
By the way, simply performing the cylinder-by-cylinder air-fuel ratio control makes the output of each cylinder non-uniform and the driving feeling deteriorates. However, by retarding the ignition timing of the rich air-fuel ratio cylinder group #R and advancing the ignition timing of the lean air-fuel ratio cylinder group #L, the output of each cylinder becomes uniform, and deterioration of driving feeling is prevented. It is supposed to be.
[0053]
Further, as shown in FIG. 3, the exhaust branch pipe 4 is provided around the merging pipe 25. ₁ , 4 ₄ By providing a plurality of communication holes 30 that communicate with each other, rich exhaust gas and lean exhaust gas may pass through the communication holes 30 to increase their mixing ratio.
[0054]
Next, a flowchart for explaining a cylinder group-specific air-fuel ratio control execution routine will be described with reference to FIG.
The cylinder group-specific air-fuel ratio control execution routine includes six steps from S101 to S106. All of these steps are performed by the CPU belonging to the ECU 20.
[0055]
In S101, it is determined based on the parameter detected by the temperature sensor whether the engine coolant temperature of the engine body 1 is equal to or lower than a predetermined value. The predetermined value referred to here is a value that serves as one guideline for determining whether the warm-up is still insufficient to perform the cylinder group air-fuel ratio control.
[0056]
If the engine coolant temperature is already sufficiently high, for example, 70 ° C. or higher, the catalyst has already been sufficiently activated at that time, so that the start-up catalyst 5 can sufficiently perform the exhaust purification function without performing cylinder-by-cylinder air-fuel ratio control. it is conceivable that. For this reason, cylinder-by-cylinder air-fuel ratio control is performed only when the water temperature is equal to or lower than a predetermined value. When the water temperature is higher than the predetermined value, a negative determination is made and normal operation is performed without performing cylinder-by-cylinder air-fuel ratio control. .
[0057]
In S102, it is determined whether the estimated catalyst temperature of the starting catalyst 5 is equal to or higher than a predetermined value. Here, the estimated catalyst temperature is a temperature obtained by estimating the average temperature of the entire starting catalyst 5, and the predetermined value is, for example, the lowest temperature at which the starting catalyst 5 can perform the exhaust purification function. If the exhaust purification function cannot be performed, unburned fuel components such as unburned HC may be released into the atmosphere as they are, so the minimum range that allows the release of unburned fuel components into the atmosphere is limited. This is to ensure.
[0058]
The temperature of the starting catalyst 5 may be measured by a temperature sensor, but the catalyst temperature is generally estimated from the integrated value of the intake air amount detected by the air flow meter. If a positive determination is made in S102, the process proceeds to S103, and if a negative determination is made, normal operation is performed.
[0059]
In S103, it is determined whether the intake air amount is equal to or less than a predetermined value. If the amount of intake air is too large and the amount of oxygen in the exhaust gas is large, the unburned fuel component will burn too much, resulting in an unfavorable condition such as abnormally high temperature of the start-up catalyst 5 and thermal degradation. It is because there is a possibility of becoming. If a positive determination is made in S103, the process proceeds to S104, and if a negative determination is made, normal operation is performed.
[0060]
In S104, it is determined whether or not a predetermined time has elapsed after starting. This predetermined time is the time required for the temperature of the starting catalyst 5 to secure the upper limit of the temperature range suitable for performing the cylinder-by-cylinder air-fuel ratio control, and varies depending on the type of the internal combustion engine and the vehicle type.
[0061]
If a negative determination is made in S104, it is determined that the catalyst temperature is within a temperature range suitable for performing the cylinder-by-cylinder air-fuel ratio control, the process proceeds to S105, and the cylinder-by-cylinder air-fuel ratio control is performed. Normal operation is performed assuming that the catalyst temperature is outside the temperature range suitable for the operation.
[0062]
The program that constitutes the cylinder-by-cylinder air-fuel ratio control execution routine is stored in the ROM of the ECU 20, and since the attribute of the ROM is in the ECU 20, the ECU 20 to which the program that performs cylinder-by-cylinder air-fuel ratio control belongs is designated It will be called control means.
[0063]
What consists of the structure described above is the exhaust gas purification apparatus for an internal combustion engine according to the embodiment of the present invention.
Next, the function and effect of the engine A to which the exhaust gas purification apparatus for an internal combustion engine having such a configuration is applied will be described.
[0064]
In the engine A, the exhaust gas of the rich air-fuel ratio cylinder group #R flows into the start-up catalyst 5 toward the center when the start-up catalyst 5 is seen in a cross section, and the start-up catalyst 5 is seen in a cross section. In this case, the exhaust gas of the lean air-fuel ratio cylinder group #L is caused to flow into the start-up catalyst 5 toward the outside.
[0065]
For this reason, first, rich exhaust gas flows through the central portion of the starting catalyst 5, in other words, the central portion of the inside of the starting catalyst 5 along the direction in which the exhaust gas flows and includes the axis of the starting catalyst 5. Further, lean exhaust gas flows in an outer portion in the start-up catalyst 5, that is, in an outer portion of the start-up catalyst 5 in which the rich exhaust gas flows than the central portion of the start-up catalyst 5.
[0066]
As described above, as a characteristic of the catalyst, that is, as a characteristic of the start-up catalyst 5, the richer the air-fuel ratio of the exhaust gas, the higher the 50% purification temperature, and the leaner, the lower the 50% purification temperature. .
[0067]
Accordingly, when the engine 5 is not warmed up yet, for example, when the engine is started in cold weather, the heat of the exhaust gas even if the exhaust gas is allowed to flow to the outside portion inside the startup catalyst 5 on the side close to the outside air. Is likely to be released, and thus the outer portion is difficult to increase in temperature.
[0068]
However, in the starting catalyst 5, lean exhaust gas flows through this outer portion, so that the exhaust gas purification rate is 50% even at low temperatures by adjusting the degree of lean according to the temperature of the outer portion of the starting catalyst 5. More than that.
[0069]
In contrast, since a relatively large amount of rich exhaust gas flows at a higher temperature than the outside portion in the start-up catalyst 5 in the center of the start-up catalyst 5, the degree of richness is determined in the center of the start-up catalyst 5. By adjusting according to this temperature, the purification rate of the exhaust gas can be made 50% or more even at the center of the catalyst 5 at the time of starting.
[0070]
As described above, the air-fuel ratio of the exhaust gas is selectively used as the rich air-fuel ratio and the lean air-fuel ratio when the central portion and the outer portion inside the start-up catalyst 5 are each 50% purification temperature. Even if there is a temperature difference for each part in 5, the exhaust gas purification rate does not become uneven due to that. Therefore, the exhaust gas purification rate can be increased as a whole catalyst.
[0071]
Even if the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas are separated and flowed into the catalyst as described above, the rich exhaust gas and the lean exhaust gas are not completely stratified.
[0072]
The trend of rich exhaust gas flow is mainly through the center of the start-up catalyst 5, and the trend of lean exhaust gas flow is mainly through the outside of the start-up catalyst 5, which is rich with lean exhaust gas. It does not mean that the exhaust gas does not mix, and it is important to mix the two more efficiently, for example, through the communication hole 30 in order to finally perform exhaust purification appropriately.
[0073]
This is because if the exhaust gases are not completely layered and mixed, the rich exhaust gas has a lot of unburned HC and is in a reducing atmosphere, and the lean exhaust gas has a lot of oxygen and is in an oxidizing atmosphere. This is because the CO and CO are discharged without being purified, and the latter is in an oxidizing atmosphere, so that HC and CO are discharged without being purified with NOx even though they can be purified.
[0074]
Basically, it is desirable to mix lean exhaust gas and rich exhaust gas. However, when mixed, since the outer peripheral portion of the start-up catalyst 5 is low in temperature, the start-up catalyst 5 is less likely to be activated in that portion than the catalyst center portion.
[0075]
For this reason, in order to finally perform exhaust purification appropriately, it is important to mix lean exhaust gas and rich exhaust gas as described above. As a result, rich exhaust gas is allowed to flow toward the center of the catalyst so that the entire start-up catalyst 5 is warmed up as uniformly as possible to easily reach the 50% purification temperature.
[0076]
Since cylinders # 2 and # 3 are in the center of engine body 1 which is an engine block, and cylinders # 1 and # 4 are at both ends of engine body 1, respectively, the first two cylinders # 2 and # 3 are started. The latter two cylinders # 1 and # 4 flow through the catalyst center of the hour catalyst 5 toward the outside of the catalyst 5 at the start.
[0077]
For this reason, the cylinders # 2 and # 3 and the central portion of the starting catalyst 5 and the cylinders # 1 and # 4 and the outside of the catalyst are in a positional relationship, and the cylinders # 2, # 3 and the cylinders # 1, # 4 It is easy to introduce the exhaust gas discharged from the engine toward the center of the starting catalyst 5 and the outside of the starting catalyst 5.
[0078]
In addition, the exhaust manifold 4 includes an exhaust branch pipe 4 related to the rich air-fuel ratio cylinder group #R located in the center thereof. ₂ , 4 ₃ Are joined together to form a single joining pipe 25, and the exhaust branch pipe 4 associated with the lean air-fuel ratio cylinder group #L is formed around the joining pipe 25. ₁ , 4 ₄ And the exhaust branch pipe 4 is formed around the merging pipe 25. ₁ , 4 ₄ Since a plurality of communication holes 30 communicating with each other are provided, it becomes easier to mix lean exhaust gas and rich exhaust gas.
[0079]
Further, the total air-fuel ratio of the rich exhaust gas discharged from the rich air-fuel ratio cylinder group #R and the lean exhaust gas discharged from the lean air-fuel ratio cylinder group #L is an air-fuel ratio that is relatively lean. It is preferable.
[0080]
This is because the oxidation reaction is promoted when the air-fuel ratio of the exhaust gas is slightly closer to the lean air-fuel ratio than when it is stoichiometric. This is because the purification rate of the catalyst increases when the air-fuel ratio of the exhaust gas is lean as a whole.
[0081]
【The invention's effect】
According to the present invention, even if there is a temperature difference for each part in the catalyst, the exhaust gas purification rate does not become uneven due to that, and therefore the exhaust gas purification rate is increased as a whole catalyst. It is done.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an exhaust emission control device for an internal combustion engine according to the present invention.
FIG. 2 is a cross-sectional view taken along the line II-II in FIG.
FIG. 3 is a diagram illustrating an application example of FIG. 2;
FIG. 4 is a flowchart for explaining a cylinder-by-cylinder air-fuel ratio control execution routine.
FIG. 5 is a diagram for explaining that the temperature inside the catalyst causes a temperature difference in the exhaust gas flowing inside the catalyst between the inside and the outside.
FIG. 6 is a diagram for explaining that the activation temperature (50% purification temperature) of the catalyst varies depending on the air-fuel ratio of the exhaust gas passing through the catalyst.
[Explanation of symbols]
A engine
1 Engine body (Engine block)
# 1 1st cylinder
# 2 Cylinder 2
# 3 Cylinder 3
# 4 4th cylinder
2 Spark plug
3 Fuel injection valve
4 Exhaust manifold (exhaust collecting pipe)
4 ₁ Exhaust branch pipe (exhaust pipe)
4 ₂ Exhaust branch pipe (exhaust pipe)
4 ₃ Exhaust branch pipe (exhaust pipe)
4 ₄ Exhaust branch pipe (exhaust pipe)
5 Start-up catalyst (exhaust purification catalyst)
6 Casing
7 Exhaust pipe (exhaust passage)
8 NOx storage reduction catalyst
9 Casing
10 Exhaust pipe (exhaust passage)
20 ECU (air-fuel ratio control means for each cylinder)
25 Junction pipe
30 communication hole
#R Rich air-fuel ratio cylinder group
#L Lean air-fuel ratio cylinder group

Claims (6)

An exhaust purification catalyst that is provided in an exhaust passage of an internal combustion engine having a plurality of cylinders and purifies exhaust gas discharged from the cylinder in the exhaust passage;
Some of the plurality of cylinders are a rich air-fuel ratio cylinder group that operates at a rich air-fuel ratio, and the remaining are a lean air-fuel ratio cylinder group that operates at a lean air-fuel ratio, and the rich air-fuel ratio cylinder group is discharged into the exhaust passage. By introducing rich exhaust gas and lean exhaust gas discharged from the lean air-fuel ratio cylinder group into the exhaust passage to the catalyst, the oxidation reaction of unburned fuel components in the exhaust gas is promoted, and the catalyst A cylinder-specific air-fuel ratio control means for increasing the temperature of the cylinder;
In an exhaust gas purification apparatus for an internal combustion engine having
When the catalyst temperature rise by the cylinder-by-cylinder air-fuel ratio control means is performed, the exhaust gas of the rich air-fuel ratio cylinder group is caused to flow into the catalyst toward the center when the catalyst is seen in a cross section, and the catalyst is By flowing the exhaust gas of the lean air-fuel ratio cylinder group into the catalyst toward the outer part inside the catalyst when viewed in a cross section, the central portion when viewed in the cross section of the catalyst is mainly rich. An exhaust gas purification apparatus for an internal combustion engine, characterized in that exhaust gas flows and lean exhaust gas flows in an outer portion inside the catalyst . An engine block having the plurality of cylinders arranged in series is provided. The number of cylinders arranged in the engine block is four, and the rich air-fuel ratio cylinder group of the four cylinders is formed by the second and third cylinders. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the lean air-fuel ratio cylinder group is formed of first and fourth cylinders. A plurality of exhaust pipes connected to the exhaust ports of the cylinders constituting the rich air-fuel ratio cylinder group and the exhaust ports of the cylinders constituting the lean air-fuel ratio cylinder group, respectively, are formed together. An exhaust pipe connected to the rich air-fuel ratio cylinder group at the center of the exhaust collecting pipe, and an exhaust pipe connected to the lean air-fuel ratio cylinder group around the exhaust pipe The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein the exhaust collecting pipe and the catalyst are connected. 4. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the ignition timing of the rich air-fuel ratio cylinder group is retarded and the ignition timing of the lean air-fuel ratio cylinder group is advanced. A communication hole that communicates between the exhaust pipe related to the rich air-fuel ratio cylinder group arranged in the center of the exhaust collecting pipe and the exhaust pipe related to the lean air-fuel ratio cylinder group arranged around the exhaust pipe. 2. An exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is an internal combustion engine. The total air-fuel ratio of the rich exhaust gas discharged from the rich air-fuel ratio cylinder group and the lean exhaust gas discharged from the lean air-fuel ratio cylinder group is a relatively lean air-fuel ratio. An exhaust emission control device for an internal combustion engine according to claim 1.

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