JP2006161647A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To avoid increase of odor due to rise of concentration of a sulfur component in the exhaust when recovering SOx poisoning in an exhaust emission control system for an internal combustion engine provided with NOx catalyst. <P>SOLUTION: This exhaust emission control system for an internal combustion engine is provided with the NOx catalyst, a SOx poisoning recovering means for recovering SOx poisoning of the NOx catalyst, a sulfur concentration estimating means for estimating or detecting concentration of a sulfur component in the exhaust flowing from the NOx catalyst when the SOx poisoning recovering means recovers SOx poisoning of the NOx catalyst, and an exhaust flow rate control means for increasing a flow rate of the exhaust flowing into the NOx catalyst as concentration of the sulfur component in the exhaust, which is estimated or detected by the sulfur concentration estimating means, becomes higher. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust purification system that performs exhaust purification of an internal combustion engine.

  A technique for reducing and purifying NOx in exhaust gas by a so-called storage reduction type NOx catalyst (hereinafter also referred to as “NOx catalyst”) provided in an exhaust passage of an internal combustion engine is known. However, this NOx catalyst also stores SOx in the exhaust as well as NOx. And when this amount of occluded SOx increases and it becomes a SOx poisoning state, the NOx reduction and purification ability by the NOx catalyst is lowered.

Therefore, a technique for executing recovery of SOx poisoning of the NOx catalyst when the internal combustion engine is in a decelerating operation state or an idle operation state is disclosed (for example, see Patent Document 1). In this technique, when the internal combustion engine is in a decelerating operation state or an idling operation state, when the exhaust gas flow rate is small, the fuel of the reducing agent is supplied to the NOx catalyst, thereby recovering from the SOx poisoning state with a relatively small amount of fuel. It becomes possible.
JP 2002-155724 A JP 2004-108176 A JP 2004-197695 A JP 2001-81237 A JP 2000-161107 A

  When recovering the SOx poisoning of the internal combustion engine by supplying the fuel of the reducing agent to the NOx catalyst when the exhaust gas flow rate is low, such as in the deceleration operation state or the idle operation state of the internal combustion engine, the reducing agent and SOx in the NOx catalyst are recovered. Therefore, the stored SOx can be efficiently released with a small amount of fuel.

  However, on the other hand, since a large amount of sulfur is discharged with a small exhaust flow rate flowing into the NOx catalyst, the concentration of sulfur in the exhaust increases, and in particular, bad odor due to hydrogen sulfide may become prominent. There is.

  In the present invention, in view of the above problems, in an exhaust gas purification system for an internal combustion engine equipped with a NOx catalyst (occlusion reduction type NOx catalyst), when the SOx poisoning is recovered, the concentration of the sulfur content in the exhaust gas increases, and an odor It aims at avoiding becoming remarkable.

  In the present invention, in order to solve the above-described problem, attention is paid to the relationship between the concentration of sulfur generated when SOx poisoning of the NOx catalyst is recovered and the exhaust gas flow rate. This is because the effect of bad odor due to the sulfur content greatly depends on the concentration of the sulfur content.

Accordingly, the present invention is an exhaust gas purification system for an internal combustion engine, which is a storage reduction type NOx catalyst (NOx catalyst) provided in an exhaust passage of the internal combustion engine, and an SOx that recovers SOx poisoning of the storage reduction type NOx catalyst. Estimating the concentration of sulfur in the exhaust gas flowing out from the NOx storage reduction catalyst when SOx poisoning recovery of the NOx storage reduction catalyst is executed by the poisoning recovery means and the SOx poisoning recovery means. Or a sulfur concentration estimating means to detect, and an exhaust gas that increases the flow rate of exhaust gas flowing into the NOx storage reduction catalyst as the concentration of sulfur content in the exhaust gas estimated or detected by the sulfur concentration estimating means increases. A flow rate adjusting means.

  In the internal combustion engine exhaust gas purification system, NOx in the exhaust gas is reduced and purified by the NOx catalyst. And SOx is occluded by the NOx catalyst as well as NOx in the exhaust. Here, when the amount of SOx stored in the NOx catalyst increases, the amount of NOx that can be stored in the NOx catalyst decreases. As a result, the NOx reduction and purification ability of the NOx catalyst decreases.

  Therefore, in such a case, the SOx poisoning recovery means recovers the NOx purification ability of the NOx catalyst stored in the NOx catalyst and in the SOx poisoning state. Specifically, the temperature of the NOx catalyst is raised to a temperature suitable for SOx poisoning recovery, the air-fuel ratio flowing into the NOx catalyst is made rich, and an atmosphere in which a reducing agent exists around the NOx catalyst is formed. For example, such a state may be formed by controlling the combustion state in the internal combustion engine, or the state may be formed by adding fuel to the exhaust gas.

  Thus, when SOx poisoning recovery of the NOx catalyst is achieved by the SOx poisoning recovery means, the sulfur content stored in the NOx catalyst flows out into the exhaust gas flowing out from the NOx catalyst, so the amount of sulfur content in the exhaust gas Will increase. Therefore, the concentration of sulfur in the exhaust gas is estimated or detected by the sulfur concentration estimating means, and the exhaust flow rate flowing into the NOx catalyst is adjusted by the exhaust flow rate adjusting means based on the result.

  As a result, when SOx poisoning of the NOx catalyst is recovered, the concentration of the sulfur content in the exhaust gas that flows out of the NOx catalyst and is released into the atmosphere can be maintained at a relatively low value. It becomes possible to avoid becoming conspicuous. The exhaust flow rate adjusting means adjusts the exhaust flow rate flowing into the NOx catalyst, so that the temperature of the NOx catalyst is maintained at a higher temperature than when the fresh air flows into the NOx catalyst and the sulfur concentration is adjusted. It becomes possible.

  Here, in the exhaust gas purification system for an internal combustion engine, the sulfur concentration estimating means may estimate that the concentration of sulfur in the exhaust gas is higher as the SOx storage amount of the NOx storage reduction catalyst is larger. . That is, it is considered that the amount of sulfur released during the recovery of SOx poisoning of the NOx catalyst increases as the SOx storage amount increases.

  Further, in the exhaust gas purification system for an internal combustion engine described above, when the SOx poisoning recovery means performs SOx poisoning recovery of the NOx storage reduction catalyst during idle operation, the exhaust flow rate adjusting means includes the sulfur concentration The exhaust gas flow rate that flows into the NOx storage reduction catalyst may be increased by increasing the idle rotation speed as the concentration of the sulfur content in the exhaust gas estimated or detected by the estimating means increases.

  When the internal combustion engine is in an idle operation state, the exhaust flow rate flowing into the NOx catalyst is relatively small. Therefore, in this case, the exhaust amount flowing into the NOx catalyst is adjusted by adjusting the idle rotation speed itself.

  Here, in the exhaust gas purification system for an internal combustion engine described above, the SOx poisoning recovery means raises the temperature of the NOx storage reduction catalyst and sets the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst on the lean side. When the SOx poisoning of the NOx storage reduction catalyst is recovered by alternately switching between the air fuel ratio and the rich side air fuel ratio, the exhaust flow rate adjusting means is configured to provide an air fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst. When the lean air-fuel ratio is set, the flow rate of exhaust gas flowing into the NOx storage reduction catalyst may be increased.

When the SOx poisoning recovery means recovers the SOx poisoning state of the NOx catalyst, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is alternately switched between the lean side air-fuel ratio and the rich side air-fuel ratio. By doing so, the SOx poisoning is recovered without excessively increasing the temperature of the NOx catalyst. Here, the SOx occluded in the NOx catalyst is mainly released when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is the rich air-fuel ratio. When the air-fuel ratio of the exhaust gas is set to the lean side air-fuel ratio, it is also a time to suppress the temperature rise of the NOx catalyst, so by increasing the exhaust gas flow rate flowing into the NOx catalyst here, at the time of SOx poisoning recovery Excessive temperature rise can be prevented, and it becomes possible to reduce the concentration of sulfur in the average exhaust gas and to prevent the bad odor due to the sulfur from becoming prominent.

  In an exhaust gas purification system for an internal combustion engine equipped with a NOx catalyst (occlusion reduction type NOx catalyst), it is possible to prevent the concentration of sulfur in the exhaust gas from increasing when the SOx poisoning is recovered, and the odor from becoming prominent. It becomes possible.

  Now, an embodiment of an exhaust gas purification system for an internal combustion engine according to the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a schematic configuration of an internal combustion engine 1 to which an exhaust purification system according to the present invention is applied and its control system. The internal combustion engine 1 is a compression ignition type internal combustion engine having four cylinders 2. Further, a fuel injection valve 3 for directly injecting fuel into the combustion chamber of the cylinder 2 is provided. The fuel injection valve 3 is connected to a pressure accumulating chamber 4 that stores fuel pressurized to a predetermined pressure. An intake branch pipe 7 is connected to the internal combustion engine 1, and each branch pipe of the intake branch pipe 7 is connected to a combustion chamber via an intake port. Similarly, an exhaust branch pipe 12 is connected to the internal combustion engine 1, and each branch pipe of the exhaust branch pipe 12 is connected to a combustion chamber via an exhaust port. Here, the intake port and the exhaust port are provided with an intake valve and an exhaust valve, respectively.

  The intake branch pipe 7 is connected to the intake pipe 8. An air flow meter 9 for detecting the amount of intake air flowing through the intake pipe 8 is provided upstream of the intake pipe 8, and an intake throttle valve 10 for adjusting the flow rate of intake air flowing through the intake pipe 8 is provided further downstream thereof. It has been. The intake throttle valve 10 is provided with an intake throttle actuator 11 that is configured by a step motor or the like and that opens and closes the intake throttle valve 10.

  The intake pipe 8 upstream of the intake throttle valve 10 is provided with a compressor side of a supercharger 16 that operates using exhaust energy as a drive source, and the exhaust branch pipe 12 is provided with a turbine side of the supercharger 16. . The supercharger 16 is a so-called variable capacity supercharger. The supercharger 16 has a movable nozzle vane therein, and the supercharging pressure by the supercharger 16 is controlled by adjusting the opening degree of the nozzle vane. . An intercooler 15 is provided in the intake pipe 8 downstream of the supercharger 16 and upstream of the intake throttle valve 10 for cooling the intake air that has been pressurized by the supercharger 16 and has reached a high temperature. . The intercooler 15 is a heat exchange device to which cooling water for cooling is supplied and the cooling capacity of the intercooler 15 is controlled by adjusting the supply amount.

  Further, the turbine side of the supercharger 16 is connected to an exhaust pipe 13, and the exhaust pipe 13 is connected to a muffler downstream. In the middle of the exhaust pipe 13, a NOx catalyst 14 of a so-called storage reduction type NOx catalyst is provided. The exhaust pipe 13 upstream of the NOx catalyst 14 is provided with a fuel addition valve 17 that adds fuel to the exhaust.

Further, the internal combustion engine 1 is provided with an EGR device 21. The EGR device 21 recirculates a part of the exhaust gas flowing through the exhaust branch pipe 12 to the intake branch pipe 7. The EGR device 21 includes an EGR passage 22 extending from the exhaust branch pipe 12 (upstream side) to the intake branch pipe 7 (downstream side), and an EGR passage 22.
It is composed of an EGR cooler 23 for cooling EGR gas provided in order from the upstream side, and an EGR valve 24 for adjusting the flow rate of EGR gas.

  The internal combustion engine 1 is also provided with an electronic control unit (hereinafter referred to as “ECU”) 20 for controlling the internal combustion engine 1. The ECU 20 includes a CPU, a ROM, a RAM, and the like for storing various programs and maps to be described later, and controls the operating conditions of the internal combustion engine 1 according to the operating conditions of the internal combustion engine 1 and the driver's request. Unit.

  Here, the fuel injection valve 3 performs an opening / closing operation by a control signal from the ECU 20. That is, according to a command from the ECU 20, the fuel injection timing and the fuel injection amount from the fuel injection valve 3 are controlled for each injection valve in accordance with the operation state such as the engine load and engine speed of the internal combustion engine 1. The fuel addition valve 17 is also controlled in accordance with a command from the ECU 20.

  Further, the crank position sensor 30 is electrically connected to the ECU 20. The ECU 20 receives a signal corresponding to the rotation angle of the output shaft of the internal combustion engine 1, and the engine rotational speed of the internal combustion engine 1 and the piston position in each cylinder 2. Etc. are detected.

  In the internal combustion engine 1 configured as described above, exhaust gas purification is mainly performed by the NOx catalyst 14. In the NOx catalyst 14, when the air-fuel ratio of the exhaust is in a lean state, NOx in the exhaust is occluded therein, the exhaust air-fuel ratio becomes rich, and the reducing agent (fuel added from the fuel addition valve 17) is supplied. In the existing state, it functions to reduce and purify the stored NOx. Here, SOx in the exhaust gas is also stored in the NOx catalyst 14 in the same manner as NOx. When the SOx occlusion amount increases and the SOx poisoning state is reached, the NOx purification ability of the NOx catalyst 14 decreases, so the stored SOx is released from the NOx catalyst 14 to restore the NOx purification ability of the NOx catalyst 14. There is a need.

  However, when recovering from this SOx poisoning state, the stored SOx may react with the reducing agent to generate hydrogen sulfide, which may cause a bad odor. Therefore, the exhaust gas flow rate control shown in FIG. 2 is performed in order to suppress the odor from becoming prominent during recovery from the SOx poisoning state. The exhaust flow rate control in the present embodiment is a routine that is repeatedly executed by the ECU 20 at a constant cycle when the internal combustion engine 1 is in an idle operation state.

  In S101, SOx poisoning recovery control is started to eliminate the SOx poisoning state of the NOx catalyst 14. Specifically, the fuel injection amount and fuel injection timing of the fuel injection valve 3 are adjusted to raise the exhaust gas temperature, and the fuel is added from the fuel addition valve 17 into the exhaust gas. As a result, the SOx stored in the NOx catalyst 14 is released and the SOx is reduced to start generating hydrogen sulfide. When the process of S101 ends, the process proceeds to S102.

  In S102, the idle rotation speed when the SOx poisoning recovery control of the NOx catalyst 14 is being performed is calculated. Here, as shown in FIG. 3, as the amount of SOx stored in the NOx catalyst 14 increases, the amount of SOx released during the SOx poisoning recovery control increases, so the concentration of sulfur in the exhaust is high. Become. As the sulfur concentration in the exhaust gas increases, the amount of hydrogen sulfide generated increases and the odor becomes more prominent.

  Therefore, in S102, the idle rotation speed is calculated according to the amount of SOx stored in the NOx catalyst 14. As the idle rotation speed increases, the exhaust flow rate flowing into the NOx catalyst 14 increases. Therefore, as the amount of SOx stored in the NOx catalyst 14 increases, the idle rotation speed when the SOx poisoning recovery control is performed is set as the idle rotation speed. A higher rotation speed is calculated. By doing in this way, it becomes possible to suppress that the density | concentration of the sulfur content in exhaust_gas | exhaustion raises.

  The calculation of the SOx occlusion amount takes into account the amount of fuel consumed in the internal combustion engine 1 and also the amount of SOx released by the SOx poisoning recovery control started in S101. When the process of S102 ends, the process proceeds to S103.

  In S103, the fuel injection amount from the fuel injection valve 3 is adjusted to change the idle rotation speed so that the engine rotation speed of the internal combustion engine 1 is as much as possible to the idle rotation speed in the SOx poisoning recovery control calculated in S102. When the process of S103 ends, the process proceeds to S104.

  In S104, it is determined whether the SOx poisoning recovery of the NOx catalyst 14 is completed. Specifically, when the amount of SOx stored in the NOx catalyst 14 is less than a predetermined amount, it is determined that the SOx poisoning recovery is completed. If it is determined that the SOx poisoning recovery is completed, the process proceeds to S105, and if it is determined that the SOx poisoning recovery is not completed, the processes after S102 are performed again.

  In S105, when the SOx poisoning recovery control of the NOx catalyst 14 is completed, the generation of hydrogen sulfide is interrupted, so that the idle rotation speed of the internal combustion engine 1 is changed to the rotation before the SOx poisoning recovery control is started. Reset to normal idle speed, which is the speed. After the process of S105, this control is terminated.

  According to this control, the exhaust gas flow rate is adjusted via the idle rotation speed in accordance with the sulfur content generated during the SOx poisoning recovery control of the NOx catalyst 14. As a result, the concentration of sulfur content (hydrogen sulfide concentration) in the exhaust gas flowing out from the NOx catalyst 14 is kept at a relatively low value, and it is avoided that the bad odor becomes prominent.

  A second embodiment of the exhaust gas flow rate control for avoiding a noticeable bad odor at the time of recovery of SOx poisoning of the NOx catalyst 14 will be described based on FIG. 4 and FIG. FIG. 4 shows a flowchart regarding the exhaust flow rate control. FIG. 5 shows changes in the exhaust flow rate (shown in FIG. 5A) and changes in the sulfur concentration in the exhaust (shown in FIG. 5B) when the exhaust flow control of FIG. 4 is performed. , The transition of the SOx occlusion amount of the NOx catalyst 14 (shown in FIG. 5C) and the transition of the exhaust air-fuel ratio (shown in FIG. 5D) are shown. This exhaust flow rate control is a control performed by the ECU 20.

  In S201, SOx poisoning recovery control is started to eliminate the SOx poisoning state of the NOx catalyst 14. Specifically, the fuel injection amount and fuel injection timing of the fuel injection valve 3 are adjusted to raise the exhaust gas temperature, and the fuel is added from the fuel addition valve 17 into the exhaust gas. At that time, in order to avoid an excessive temperature rise of the NOx catalyst 14, the amount of addition from the fuel addition valve 17 is adjusted, and the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 14 is set to the rich-side air-fuel ratio and the lean-side air-fuel ratio. So-called rich spike control, which alternates with the air-fuel ratio, is performed. The transition of the air-fuel ratio of the exhaust by rich spike control is shown in FIG.

  When the air-fuel ratio of the exhaust gas is set to the rich-side air-fuel ratio, SOx stored mainly in the NOx catalyst 14 is released and SOx is reduced and hydrogen sulfide begins to be generated. Accordingly, as shown in FIG. 5B, the sulfur concentration in the exhaust gas flowing out from the NOx catalyst 14 gradually increases when the air-fuel ratio of the exhaust gas is made rich. At the same time, as shown in FIG. 5C, when the air-fuel ratio of the exhaust gas is set to the rich air-fuel ratio, the amount of SOx stored in the NOx catalyst 14 decreases. When the process of S201 ends, the process proceeds to S202.

  In S202, it is determined in the rich spike control of the SOx poisoning recovery control whether or not it is a period during which the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 14 is set to the rich air-fuel ratio. If it is determined in this determination that the period is the rich side air-fuel ratio, the process proceeds to S203, and if it is determined that the period is not the rich side air-fuel ratio, the process proceeds to S205.

  In S203, so-called low-temperature combustion is performed and fuel is added from the fuel addition valve 17 so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 14 is the rich air-fuel ratio. In low-temperature combustion, the oxygen concentration in the exhaust before and after combustion is lowered by increasing the amount of EGR gas by the EGR device 21. At the same time, the fuel addition from the fuel addition valve 17 causes the air-fuel ratio of the exhaust to become the rich side air-fuel ratio. When the process of S203 ends, the process proceeds to S204.

  In S205, high-temperature, high exhaust flow combustion is performed with the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 14 as the lean air-fuel ratio. Specifically, the fuel injection timing is shifted to the retard side, and the opening of the intake throttle valve 10 is changed to the closed side. As a result, the exhaust gas temperature is raised and the opening of the EGR valve 24 is changed to the closed side to increase the exhaust gas flow rate. Therefore, as shown in FIG. 5A, the exhaust gas flow rate is increased during the high temperature and high exhaust gas flow rate combustion as compared with the case where the low temperature combustion is performed in S203. In addition, the exhaust gas flow rate at the time of high temperature and high exhaust gas flow combustion is gradually decreased in conjunction with the gradual decrease in the SOx release amount from the NOx catalyst 14. When the processing of S205 ends, the process proceeds to S204.

  In S204, as in S104 described above, it is determined whether or not the SOx poisoning recovery of the NOx catalyst 14 has been completed. If it is determined that the SOx poisoning recovery has been completed, the present control is terminated. If it is determined that the SOx poisoning recovery has not been completed, the processing from S202 is performed again.

  According to this control, in the SOx poisoning recovery control, when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 14 is set to the lean air-fuel ratio, that is, when the amount of sulfur released from the NOx catalyst 14 is small, The exhaust flow rate is increased. As a result, it is possible to reduce the average sulfur concentration in the exhaust gas in the entire SOx poisoning recovery control including when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 14 is set to the rich air-fuel ratio. Thus, it can be avoided that the bad odor due to the sulfur content becomes prominent.

1 is a diagram illustrating a schematic configuration of an internal combustion engine to which an exhaust gas purification system for an internal combustion engine according to an embodiment of the present invention is applied and a control system thereof. It is a flowchart of the exhaust gas flow rate control performed in the exhaust gas purification system of the internal combustion engine which concerns on Example 1 of this invention. It is a figure which shows the relationship between the SOx occlusion amount of a NOx catalyst, and the sulfur content density | concentration in exhaust_gas | exhaustion at the time of SOx poisoning recovery control being performed in the exhaust gas purification system of the internal combustion engine which concerns on the Example of this invention. It is a flowchart of the exhaust flow control performed in the exhaust gas purification system of the internal combustion engine which concerns on Example 2 of this invention. FIG. 5 is a diagram showing fluctuations in the exhaust gas flow rate, the sulfur concentration in the exhaust gas, the SOx occlusion amount of the NOx catalyst, and the air-fuel ratio of the exhaust gas when the exhaust gas flow rate control shown in FIG. 4 is performed.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 .... Internal combustion engine 3 .... Fuel injection valve 7 .... Intake branch pipe 10 .... Intake throttle valve 11 .... Intake throttle actuator 12 .... Exhaust branch pipe 13. ... Exhaust pipe 14 ... Occlusion reduction type NOx catalyst (NOx catalyst)
17 ... Fuel addition valve 20 ... ECU
21 ... EGR device 24 ... EGR valve

Claims (4)

An NOx storage reduction catalyst provided in the exhaust passage of the internal combustion engine;
SOx poisoning recovery means for recovering SOx poisoning of the NOx storage reduction catalyst;
Sulfur concentration that estimates or detects the concentration of sulfur in the exhaust gas flowing out from the NOx storage reduction catalyst when SOx poisoning recovery of the NOx storage reduction catalyst is executed by the SOx poisoning recovery means An estimation means;
An exhaust flow rate adjusting means for increasing the exhaust flow rate flowing into the NOx storage reduction catalyst as the sulfur concentration in the exhaust gas estimated or detected by the sulfur concentration estimating means increases;
An exhaust gas purification system for an internal combustion engine, comprising:   2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the sulfur concentration estimating means estimates that the concentration of sulfur in the exhaust gas is higher as the SOx storage amount of the NOx storage reduction catalyst is larger.   When SOx poisoning recovery of the NOx storage reduction catalyst by the SOx poisoning recovery means is executed during idle operation, the exhaust flow rate adjusting means is estimated or detected by the sulfur concentration estimation means in the exhaust gas. 3. The exhaust gas of an internal combustion engine according to claim 1, wherein the exhaust flow rate flowing into the NOx storage reduction catalyst is increased by increasing the idle rotation speed as the concentration of sulfur increases. 4. Purification system.   The SOx poisoning recovery means raises the temperature of the NOx storage reduction catalyst and alternately switches the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst between a lean air-fuel ratio and a rich air-fuel ratio. When the SOx poisoning of the NOx storage reduction catalyst is recovered, the exhaust flow rate adjusting means is configured to reduce the storage reduction when the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is the lean air-fuel ratio. The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 3, wherein an exhaust flow rate flowing into the NOx catalyst is increased.

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