JP4367011B2 - Exhaust gas purification system for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust purification system that performs NOx purification of an internal combustion engine.
[0002]
[Prior art]
An NOx storage reduction catalyst (hereinafter referred to as “NOx”) that stores NOx and reduces the stored NOx in the presence of a reducing agent in order to purify NOx in exhaust gas discharged from an internal combustion engine, particularly an internal combustion engine that performs lean combustion. NOx catalyst ") is used. However, the NOx purification capacity of the NOx catalyst decreases as the amount of NOx stored in the NOx catalyst (hereinafter referred to as “NOx storage amount”) increases. Therefore, in order to reduce the NOx occluded in the NOx catalyst to reduce the NOx occlusion amount and restore the NOx purification capacity, the exhaust pipe provided with the NOx catalyst is warmed with exhaust heat to promote the vaporization of the reducing agent. The technique of making it public is disclosed (for example, refer patent document 1).
[0003]
Further, the exhaust gas flowing into the NOx catalyst is reduced in order to reduce the total amount of NOx stored in the NOx catalyst during the previous engine stop after the internal combustion engine is started until the first lean air-fuel ratio operation is performed. A technique for increasing the amount of fuel added as a reducing agent to be added is disclosed (for example, see Patent Document 2). Similarly, a technology has been disclosed in which the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is slightly rich when the internal combustion engine is started (see, for example, Patent Document 3).
[0004]
When the degree of purification efficiency by the NOx catalyst is deteriorated, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is set to a rich air-fuel ratio to recover the NOx purification capacity, and the temperature of the NOx catalyst is higher than a certain temperature. When it is low, a technique for prohibiting the exhaust from being rich is disclosed (for example, see Patent Document 4).
[0005]
Furthermore, as the amount of NOx occluded in the NOx catalyst increases, the technology for reducing the NOx occluded more reliably by making the air-fuel ratio of the exhaust gas flowing into the NOx catalyst a richer air-fuel ratio is disclosed. (For example, see Patent Document 5).
[0006]
[Patent Document 1]
JP-A-6-272539
[Patent Document 2]
International Publication WO98 / 51919
[Patent Document 3]
JP 2002-89249 A
[Patent Document 4]
JP 11-107811 A
[Patent Document 5]
JP-A-6-272540
[0007]
[Problems to be solved by the invention]
By the way, in the NOx catalyst, as the NOx occlusion amount in the NOx catalyst increases, the NOx purification capacity of the NOx catalyst decreases. Therefore, in the prior art, even if the stored NOx is reduced by adding fuel as a reducing agent to the exhaust flowing into the NOx catalyst, the fuel is added directly to the exhaust, so the molecular weight of the fuel Is relatively large, and it is difficult to efficiently reduce the stored NOx. Therefore, it takes time to recover the NOx purification capability of the NOx catalyst, and the state in which the NOx purification capability is reduced on average continues. Furthermore, as a result, the amount of fuel used before the recovery of the NOx purification capacity increases, and the fuel efficiency may deteriorate.
[0008]
Further, in order to reduce the stored NOx by adding fuel to the exhaust, the state of the NOx catalyst and the operating state of the internal combustion engine must be in a state in which the reduction reaction is performed efficiently. If the fuel is added to the exhaust gas when it is not in such a state, a part of the added fuel is not subjected to the reduction reaction with the NOx catalyst, and as a result, the fuel consumption may be deteriorated.
[0009]
The present invention has been made in view of the above problems, and includes a NOx catalyst that occludes and reduces NOx contained in exhaust from an internal combustion engine, and adds fuel to the exhaust flowing into the NOx catalyst. An exhaust purification system that purifies exhaust gas by reducing NOx stored in the NOx catalyst, and quickly reduces NOx stored in the NOx catalyst to maintain the NOx catalyst's NOx purification capacity on average high. In addition, an object of the present invention is to provide an exhaust purification system for an internal combustion engine that suppresses deterioration of fuel consumption.
[0010]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention focuses on the amount of fuel added per unit time of fuel added to exhaust gas for NOx purification capability and the timing of fuel addition. This depends on the progress of recovery of the NOx purification capacity of the NOx catalyst and the amount of fuel required for the NOx purification capacity, that is, the fuel consumption, depends on the fuel addition amount per unit time of the added fuel and the timing of fuel addition. .
[0011]
Therefore, in an exhaust gas purification system for an internal combustion engine, a NOx catalyst for storing and reducing NOx contained in the exhaust gas from the internal combustion engine, a fuel addition means for adding fuel to the exhaust gas flowing into the NOx catalyst, and the fuel addition means Determines whether or not a condition for supplying fuel to the NOx catalyst is satisfied, and determines whether or not a condition for supplying fuel to the NOx catalyst is satisfied by the fuel addition determination means. When the fuel is added, the fuel is added to the exhaust gas by the fuel addition means to make the air-fuel ratio of the exhaust gas flowing into the NOx catalyst a rich air-fuel ratio, thereby supplying the fuel to the NOx catalyst, and storing the NOx in the NOx catalyst. It is determined that the condition for supplying fuel to the NOx catalyst is not established by the normal reduction control means for reducing the amount and the fuel addition availability determination means. When the state shifts from a state to a state in which it is determined that the condition for supplying fuel to the NOx catalyst is established by the fuel addition availability determination unit, the fuel to the exhaust gas that is performed through the normal reduction unit for a predetermined period of time Is added to the NOx catalyst by changing the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to a rich rich air-fuel ratio on the rich side. Centralized reduction control means for supplying fuel and reducing the NOx occlusion amount of the NOx catalyst; An exhaust NOx sensor for detecting the concentration of NOx contained in the exhaust gas flowing out from the NOx catalyst; With When the NOx occlusion amount of the NOx catalyst is reduced by the concentrated reduction control means, if it is determined that the concentration of NOx detected by the exhaust NOx sensor is lower than a predetermined threshold value, the concentrated reduction control means The reduction of the NOx occlusion amount of the NOx catalyst due to is stopped and the reduction of the NOx occlusion amount of the NOx catalyst by the normal reduction control means is started. The
[0012]
The NOx catalyst has a NOx purification capability for reducing the stored NOx by storing NOx and reducing the oxygen concentration in the atmosphere of the NOx catalyst and supplying fuel as a reducing agent. This NOx purification capacity decreases as the NOx storage amount of the NOx catalyst increases. Therefore, if the NOx occlusion amount is large, the ratio of the NOx concentration contained in the exhaust gas flowing out from the NOx catalyst to the NOx concentration contained in the exhaust gas flowing into the NOx catalyst (hereinafter referred to as “NOx purification rate”) will decrease. . Therefore, in order to maintain the NOx purification rate at a high level, it is preferable to make the NOx occlusion amount of the NOx catalyst as small as possible.
[0013]
Therefore, fuel as a reducing agent is supplied to the NOx catalyst by adding fuel to the exhaust gas flowing into the NOx catalyst by the fuel addition means. Then, the NOx occlusion amount is reduced by the NOx reduction reaction in the NOx catalyst, so that the NOx purification rate is maintained at a high level.
[0014]
However, the fuel addition by the fuel addition means is not always feasible, and can be performed only under conditions where the NOx reduction reaction in the NOx catalyst can be performed efficiently. If fuel addition is performed under conditions other than the above conditions, the NOx reduction reaction is not efficiently performed, so the amount of fuel required for fuel addition increases and fuel consumption deteriorates, or the fuel is a NOx catalyst or NOx catalyst. There is a risk that the amount of adhesion to an exhaust system or the like of an internal combustion engine provided with an increase. Accordingly, the fuel addition propriety determination means determines whether or not a condition for performing fuel addition by the fuel addition means to supply fuel to the NOx catalyst is satisfied, and determines that the condition is satisfied. Sometimes, fuel is added to the exhaust by the fuel addition means.
[0015]
Here, the exhaust gas purification system for the internal combustion engine includes the normal reduction control means and the concentrated reduction control means as fuel addition means for reducing the NOx occlusion amount of the NOx catalyst. In both means, the fuel addition is performed when it is determined by the fuel addition possibility determination means that the condition for supplying the fuel to the NOx catalyst is satisfied. Here, the difference in fuel addition by each means is the amount of fuel added to exhaust gas per unit time and the timing of fuel addition to exhaust gas.
[0016]
That is, the concentrated reduction control means performs intensive fuel addition in which the amount of fuel added to the exhaust gas per unit time is higher than that of the normal reduction control means. As a result, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst when the fuel addition by the concentrated reduction control unit is performed is the air-fuel ratio of the exhaust gas flowing into the NOx catalyst when the fuel addition by the normal reduction control unit is performed. As a result, the rich air-fuel ratio, which is the richer air-fuel ratio, is obtained. Therefore, the reduction reaction in the NOx catalyst is promoted, and the NOx occlusion amount of the NOx catalyst is reduced more rapidly. As a result, the NOx purification rate indicating the NOx purification capacity of the NOx catalyst is restored to a high level more quickly. On the other hand, in the fuel addition by the centralized reduction control means, the amount of fuel provided for the reduction reaction is increased, so that the fuel consumption is deteriorated.
[0017]
Therefore, fuel addition by the centralized reduction control means is limited to a certain timing. That is, the fuel addition by the centralized reduction control means is performed for a predetermined period only when the fuel addition possibility determination means shifts from a state in which fuel supply to the NOx catalyst by the fuel addition means is not possible to a possible state. After the fuel addition by the centralized reduction control unit is performed for a predetermined period, the normal reduction is performed as long as it is determined by the fuel addition possibility determination unit that the condition for supplying the fuel to the NOx catalyst by the fuel addition unit is satisfied. The fuel is added by the control means. Here, the predetermined period during which the concentrated reduction control means is performed is a period until it is determined that the NOx purification capacity of the NOx catalyst has recovered to a sufficiently high level. For example, the NOx occlusion amount of the NOx catalyst is reduced until the NOx purification rate of the NOx catalyst is recovered to a sufficiently high level or until the NOx concentration of the exhaust gas flowing out from the NOx catalyst is sufficiently reduced. The period required for Thus, by limiting the timing of fuel addition by the centralized reduction control means, it becomes possible to suppress deterioration of fuel consumption.
[0018]
By performing such reduction control by addition of fuel, fuel cannot be supplied to the NOx catalyst, and after NOx is stored in the NOx catalyst, the fuel is supplied to the NOx catalyst. If it is determined that the fuel can be discharged, the fuel is first added by the concentrated reduction control means. As a result, NOx stored in the NOx catalyst is intensively reduced, and the NOx purification rate by the NOx catalyst is quickly recovered. After that, by adding fuel by the normal reduction control means, the NOx catalyst occludes as long as it is determined by the fuel addition availability determination means that the conditions for fuel supply to the NOx catalyst by the fuel addition means are satisfied. The amount of NOx being maintained is always kept low. Accordingly, the NOx purification rate indicating the NOx purification ability of the NOx catalyst can be maintained at a high level.
[0019]
Furthermore, when it is determined by the fuel addition possibility determination means that fuel supply to the NOx catalyst by the fuel addition means is not possible, the fuel addition by the normal reduction control means is not performed, but again the fuel addition possibility determination means When it is determined that fuel can be supplied to the NOx catalyst by the fuel addition means, the fuel is added by the centralized reduction control means.
[0020]
Here, the fuel addition propriety judging means for judging whether or not the condition for supplying the fuel to the NOx catalyst by the fuel adding means is satisfied can make the judgment based on the temperature of the NOx catalyst. That is, the fuel addition propriety determining means determines that a condition for supplying fuel to the NOx catalyst by the fuel adding means is satisfied when the temperature of the NOx catalyst is higher than a first predetermined temperature, On the other hand, when the temperature of the NOx catalyst is equal to or lower than the first predetermined temperature, it is determined that the condition for supplying fuel to the NOx catalyst by the fuel addition means is not satisfied.
[0021]
This is because the NOx catalyst needs to be equal to or higher than the first predetermined temperature, which is the activation temperature of the NOx catalyst, in order to efficiently contribute to the NOx reduction reaction. Therefore, when the temperature of the NOx catalyst does not reach the activation temperature, the reduction reaction of the stored NOx due to the fuel supply to the NOx catalyst does not occur efficiently, so the conditions for supplying fuel to the NOx catalyst are not satisfied. It is determined.
[0022]
Further, when the temperature of the NOx catalyst rises excessively, the composition of the NOx catalyst changes, thereby reducing the efficiency of the NOx reduction reaction by the NOx catalyst. Therefore, the fuel addition propriety determination means has a condition for supplying fuel to the NOx catalyst by the fuel addition means when the temperature of the NOx catalyst is lower than a second predetermined temperature which is higher than the first predetermined temperature. On the other hand, if the temperature of the NOx catalyst is equal to or higher than the second predetermined temperature, it is determined that the condition for supplying fuel to the NOx catalyst by the fuel addition means is not satisfied.
[0023]
Here, the second predetermined temperature is a threshold value for determining that the efficiency of the NOx reduction reaction by the NOx catalyst is reduced by excessively increasing the temperature of the NOx catalyst. Therefore, when the temperature of the NOx catalyst is excessively raised, the reduction reaction of the stored NOx due to the fuel supply to the NOx catalyst does not occur efficiently, so the conditions for supplying fuel to the NOx catalyst are not satisfied. It is determined.
[0024]
Next, the fuel addition enable / disable determining unit that determines whether or not the condition for supplying the fuel to the NOx catalyst by the fuel adding unit is satisfied can make the determination based on the engine load of the internal combustion engine. . That is, it is determined that the condition for supplying fuel to the NOx catalyst by the fuel addition means is satisfied when the engine load of the internal combustion engine belongs to a load region where the load is lower than a high load region, When the engine load of the internal combustion engine belongs to the high load region, it is determined that the condition for supplying fuel to the NOx catalyst by the fuel addition means is not satisfied.
[0025]
This is because when the engine load of the internal combustion engine belongs to the high load region, the amount of exhaust gas flowing into the NOx catalyst increases, and the fuel added in the NOx catalyst is not efficiently used for the NOx reduction reaction, and the NOx catalyst. , And as a result, fuel consumption deteriorates. Therefore, when the engine load of the internal combustion engine belongs to the high load region, the reduction reaction of the stored NOx due to the fuel supply to the NOx catalyst does not occur efficiently, so the condition for supplying the fuel to the NOx catalyst is satisfied. It is determined that there is no.
[0026]
Here, as the fuel addition means, a fuel addition valve provided in the exhaust system of the internal combustion engine and upstream of the NOx catalyst can be considered. In this case, the concentrated fuel addition by the concentrated reduction control means shortens the fuel addition interval to the exhaust gas by the fuel addition valve or the fuel addition valve compared to the fuel addition by the normal reduction control means. This is performed by at least one of increasing the valve opening time per time.
[0027]
By adding fuel with such a fuel addition valve, the fuel addition by the centralized reduction control means increases the amount of fuel added per unit time compared to the fuel addition by the normal reduction control means, and flows into the NOx catalyst. It becomes possible to make the air-fuel ratio of the exhaust gas to be a strong rich air-fuel ratio.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
<First Embodiment>
Here, an embodiment of an exhaust gas purification system for an internal combustion engine according to the present invention will be described based on the drawings. FIG. 1 is a block diagram showing a schematic configuration of an exhaust purification system to which the present invention is applied, an internal combustion engine 1 including the exhaust purification system, and a control system thereof.
[0029]
The internal combustion engine 1 is an 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 accumulation chamber 4 that accumulates fuel at a predetermined pressure. The pressure accumulating chamber 4 communicates with the fuel pump 6 through the fuel supply pipe 5.
[0030]
Next, an intake branch pipe 7 is connected to the internal combustion engine 1, and each branch pipe of the intake branch pipe 7 communicates with a combustion chamber of the cylinder 2 via an intake port. Here, the communication between the combustion chamber of the cylinder 2 and the intake port is performed by opening and closing the intake valve. The intake branch pipe 7 is connected to the intake pipe 8.
[0031]
On the other hand, an exhaust branch pipe 9 is connected to the internal combustion engine 1, and each branch pipe of the exhaust branch pipe 9 communicates with the combustion chamber of the cylinder 2 through an exhaust port. Here, the communication between the combustion chamber of the cylinder 2 and the exhaust port is performed by opening and closing the exhaust valve. The exhaust branch pipe 9 is provided with a fuel addition valve 21 for adding fuel to the exhaust gas flowing through the exhaust branch pipe 9.
[0032]
Further, the exhaust branch pipe 9 is connected to an exhaust pipe 10, and the exhaust pipe 10 is connected to a muffler (not shown) downstream. Further, in the middle of the exhaust pipe 10, there is provided a NOx catalyst 11 for storing and reducing NOx in the exhaust discharged from the internal combustion engine to purify NOx in the exhaust. Therefore, the exhaust gas to which fuel is added by the above-described fuel addition valve 21 flows into the NOx catalyst 11.
[0033]
Here, the fuel injection valve 3 and the fuel addition valve 21 are opened and closed by a control signal from an electronic control unit (hereinafter referred to as an ECU: Electronic Control Unit) 20. That is, in accordance with a command from the ECU 20, the fuel injection timing and the fuel injection amount in the fuel injection valve 3 and the fuel addition valve 21 are controlled for each valve.
[0034]
Further, an accelerator opening sensor 25 is electrically connected to the ECU 20, and the ECU 20 receives a signal corresponding to the accelerator opening and calculates an engine torque required for the internal combustion engine 1 based on the signal. Further, the crank position sensor 24 is electrically connected to the ECU 20, and the ECU 20 receives a signal corresponding to the rotation angle of the output shaft of the internal combustion engine 1, and calculates the engine rotational speed and the like of the internal combustion engine 1. Then, the ECU 20 determines the load region where the internal combustion engine 1 is placed from the required engine torque, the engine rotational speed and the like detected from the accelerator opening sensor 25 and the crank position sensor 24.
[0035]
The exhaust pipe 10 upstream of the NOx catalyst 11 is provided with an exhaust temperature sensor 22 that detects the temperature of the exhaust gas flowing into the NOx catalyst 11 and is electrically connected to the ECU 20. Thus, the ECU 20 detects the exhaust temperature flowing into the NOx catalyst 11 and estimates the temperature of the NOx catalyst 11 from the exhaust temperature. Further, the exhaust pipe 10 downstream of the NOx catalyst 11 is provided with an exhaust NOx sensor 23 for detecting the concentration of NOx contained in the exhaust gas flowing out from the NOx catalyst 11 (hereinafter referred to as “exhaust NOx concentration”). Electrically connected.
[0036]
The exhaust gas purification system including these sensors, the NOx catalyst 11, the fuel addition valve 21 and the like purifies NOx contained in the exhaust gas discharged from the internal combustion engine 1. Here, the NOx purification capacity of the NOx catalyst 11 will be described with reference to FIG.
[0037]
FIG. 2 is a graph showing the transition of the NOx purification capacity by the fuel addition of the fuel addition valve 21 when the NOx storage amount of the NOx catalyst 11 is different. The horizontal axis of the graph indicates elapsed time, and the vertical axis of the graph indicates NOx concentration. A line L3 in FIG. 2 represents the NOx concentration of the exhaust gas flowing into the NOx catalyst 11. Here, the lines L1 and L2 in FIG. 2 indicate that the exhaust gas having the NOx concentration indicated by the line L3 flows into the NOx catalyst 11 when the NOx storage amount of the NOx catalyst 11 is relatively high and low, respectively. The graph shows the transition of the NOx concentration in the exhaust discharged from the NOx catalyst 11 when fuel addition is performed from the fuel addition valve 21 to the exhaust at regular intervals and with the same addition amount per unit time. Therefore, the difference in the transition of the NOx concentration represented by the line L1 and the line L2 indicates that the NOx purification capacity of the NOx catalyst 11 is greatly influenced by the NOx occlusion amount.
[0038]
If the NOx storage amount of the NOx catalyst 11 is large (corresponding to the case represented by the line L1 in FIG. 2), even if the fuel is added to the exhaust by the fuel addition valve 21, the NOx stored in the NOx catalyst 11 is reduced. Since the reduction reaction is not performed efficiently, the recovery of the NOx purification capacity becomes gradual. As a result, the average NOx purification rate by the NOx catalyst 11 decreases, and the average value of the NOx concentration of the exhaust gas flowing out from the NOx catalyst 11 increases. On the other hand, when the NOx occlusion amount of the NOx catalyst 11 is small (corresponding to the case represented by the line L2 in FIG. 2), the NOx purification capacity is quickly recovered, and the average value of the NOx purification rate by the NOx catalyst 11 is restored. And the average value of the NOx concentration of the exhaust gas flowing out from the NOx catalyst 11 can be suppressed from increasing.
[0039]
Therefore, by maintaining the NOx occlusion amount of the NOx catalyst 11 as low as possible, the NOx purification capacity of the NOx catalyst 11 can be maintained, and the NOx purification rate of the NOx catalyst 11 can be maintained at a high level. It becomes. However, fuel addition to the exhaust gas by the fuel addition valve 21 for NOx purification by the NOx catalyst 11 may not be executed depending on the temperature of the NOx catalyst 11 or the engine load of the internal combustion engine. The NOx occlusion amount of the NOx catalyst 11 increases, and the NOx purification rate by the NOx catalyst 11 gradually decreases.
[0040]
Therefore, control for maintaining the NOx purification capacity of the NOx catalyst 11 and maintaining the NOx purification rate of the NOx catalyst 11 at a high level will be described with reference to FIGS. 3 and 4. FIG. 3 is a flowchart of NOx reduction control when NOx purification by the NOx catalyst 11 is performed. The control is executed by the ECU 20.
[0041]
FIG. 4 is a graph showing an example of the transition of the NOx purification rate by the NOx catalyst 11 when the ECU 20 executes the NOx reduction control shown in FIG. 3 when the internal combustion engine 1 is started. The horizontal axis of the graph represents elapsed time, and the vertical axis of the graph represents the NOx purification rate by the NOx catalyst 11 and the vehicle speed of the vehicle including the internal combustion engine 1. Here, the line L4 in FIG. 4 represents the transition of the NOx purification rate by the NOx catalyst 11 when the present NOx reduction control is performed. A line L5 in FIG. 4 represents the transition of the NOx purification rate by the NOx catalyst 11 when NOx reduction is performed only by addition of normal fuel, which will be described later. A line L6 indicates a change in the vehicle speed of the vehicle. Hereinafter, the NOx reduction control in FIG. 3 will be described together with the transition of the NOx purification rate by the NOx catalyst 11 shown in FIG.
[0042]
First, in S101, it is determined whether or not a condition for performing NOx purification by the NOx reduction reaction in the NOx catalyst 11 is satisfied by adding fuel to the exhaust gas by the fuel addition valve 21 and supplying the fuel to the NOx catalyst 11. To do. Specifically, this determination is performed based on the temperature of the NOx catalyst 11 and the engine load of the internal combustion engine 1.
[0043]
The determination in S101 based on the temperature of the NOx catalyst 11 will be described based on FIG. FIG. 5 is a graph showing the transition of the NOx purification rate by the NOx catalyst 11 with respect to the temperature of the NOx catalyst 11. The horizontal axis of the graph represents the temperature of the NOx catalyst 11, and the vertical axis of the graph represents the NOx purification rate by the NOx catalyst 11. A line L7 in FIG. 5 shows the transition of the NOx purification rate.
[0044]
Here, when the temperature of the NOx catalyst 11 is equal to or lower than the activation temperature TN1, the NOx reduction reaction is not efficiently performed in the NOx catalyst 11. Therefore, in this case, in S101, the condition for performing NOx purification by the NOx reduction reaction in the NOx catalyst 11 is established by adding fuel to the exhaust gas by the fuel addition valve 21 and supplying the fuel to the NOx catalyst 11. Judge that it is not. On the other hand, when the temperature of the NOx catalyst 11 is equal to or higher than the upper limit temperature TN2, the efficiency of the NOx reduction reaction by the NOx catalyst 11 is reduced by changing the composition of the NOx catalyst 11. Therefore, in this case, in S101, the condition for performing NOx purification by the NOx reduction reaction in the NOx catalyst 11 is established by adding fuel to the exhaust gas by the fuel addition valve 21 and supplying the fuel to the NOx catalyst 11. Judge that it is not.
[0045]
Next, the determination in S101 based on the engine load of the internal combustion engine 1 will be described based on FIG. FIG. 6 is a graph showing the engine load region of the internal combustion engine 1. The horizontal axis of the graph indicates the engine speed of the internal combustion engine 1, and the vertical axis of the graph indicates the engine torque by the internal combustion engine 1. A line L8 in FIG. 6 shows the transition of the maximum engine torque with respect to the engine speed of the internal combustion engine 1. Here, a region R0 defined by a line L9 in FIG. 6 is a region where the engine load of the internal combustion engine 1 is a low load region or a medium load region. A region R1 between the line L9 and the line L8 in the drawing is a region where the engine load of the internal combustion engine 1 is a high load region.
[0046]
Here, when the engine load of the internal combustion engine 1 belongs to the high load region R1, the flow rate of the exhaust gas flowing into the NOx catalyst 11 becomes excessive, so even if the fuel is added to the exhaust gas by the fuel addition valve 21. The reduction reaction of NOx is not performed efficiently. Accordingly, in this case, in S101, the condition for performing NOx purification by the NOx reduction reaction in the NOx catalyst 11 is established by adding fuel to the exhaust gas by the fuel addition valve 21 and supplying the fuel to the NOx catalyst 11. Judge that it is not.
[0047]
In S101, when it is determined that the condition for performing NOx purification by the NOx reduction reaction in the NOx catalyst 11 is satisfied by adding fuel to the exhaust gas by the fuel addition valve 21 and supplying the fuel to the NOx catalyst 11, Proceed to S102. On the other hand, if it is determined in S101 that the condition is not satisfied, the process proceeds to S108.
[0048]
In S102, it is determined whether or not the addition flag is 0. Here, the addition flag is a flag for determining whether or not the fuel addition is possible from the state where the fuel addition is not possible when the fuel is added by the fuel injection valve 21. Accordingly, when it is determined in S102 that the addition flag is 0, it means that the fuel addition is possible from the state where the fuel addition is not possible. Further, when the addition flag is 1 in S102, it means that the fuel addition is already possible.
[0049]
Here, the transition from the state in which fuel addition is not possible to the state in which fuel addition is possible means that the temperature of the NOx catalyst 11 becomes higher than the activation temperature TN1 or the temperature of the NOx catalyst 11 exceeds the upper limit temperature TN2. This is when the engine load of the internal combustion engine 1 shifts from a state belonging to the region R1 to a state belonging to the region R0. That is, when the state of the NOx catalyst 11 or the internal combustion engine 1 changes as indicated by the arrows in FIG. 5 and FIG. 6, the state in which fuel addition is not possible shifts to a state in which fuel addition is possible. It is determined when.
[0050]
If it is determined in S102 that the addition flag is 0, the transition from the state in which fuel addition is not possible to the state in which fuel addition is possible, fuel addition cannot be performed until that time, and the NOx occlusion amount of the NOx catalyst 11 increases. Means that Therefore, the process proceeds to S103 in order to quickly reduce the increased NOx occlusion amount. On the other hand, if it is determined in S102 that the addition flag is not 0, the state in which fuel addition is possible continues and NOx reduction by normal fuel addition in S106 described later is performed until that point. Therefore, this means that the NOx occlusion amount of the NOx catalyst 11 is kept low. Therefore, in order to maintain the NOx occlusion amount at a low level, the process proceeds to S107, and further NOx reduction by normal fuel addition is continued.
[0051]
In S103, the stored NOx is reduced by starting concentrated fuel addition by the fuel addition valve 21. Here, the concentrated fuel addition and the normal fuel addition performed in S107 will be described. Both fuel additions are fuel additions to the exhaust gas performed by the fuel addition valve 21, but concentrated fuel additions increase the amount of fuel added per unit time compared to normal fuel additions. Specifically, the amount of fuel added per unit time is increased by shortening the fuel addition interval by the fuel addition valve 21 or by increasing the valve opening time per time of the fuel addition valve 21. . Therefore, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 11 when fuel collection is added is further richer than the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 11 when normal fuel addition is performed. It becomes an air fuel ratio. Thus, the reduction reaction of NOx stored in the NOx catalyst 11 is further promoted, while the amount of fuel required for fuel addition increases. When the process of S103 ends, the process proceeds to S104.
[0052]
In S104, the exhaust NOx sensor 23 detects the NOx concentration Dn of the exhaust flowing out from the NOx catalyst 11. When the process of S104 ends, the process proceeds to S105.
[0053]
In S105, it is determined whether or not the exhaust NOx concentration Dn detected in S104 is lower than a predetermined exhaust NOx concentration D0. Here, the predetermined exhaust NOx concentration D0 is determined in order to determine that the NOx catalyst 11 has sufficiently recovered the NOx purification capacity, that is, the NOx occlusion amount of the NOx catalyst 11 is reduced, and the NOx purification rate by the NOx catalyst 11 is high. It is a threshold value for determining that the current state has been reached. Therefore, if it is determined in S105 that the exhaust NOx concentration Dn is equal to or higher than the predetermined exhaust NOx concentration D0, it means that the NOx purification capacity of the NOx catalyst 11 has not fully recovered, and NOx reduction by concentrated fuel addition Is continued. If it is determined in S105 that the exhaust NOx concentration Dn is smaller than the predetermined exhaust NOx concentration D0, the process proceeds to S106.
[0054]
In S106, the value of the addition flag is changed from 0 to 1. When the process of S106 ends, the process proceeds to S107.
[0055]
In S107, when proceeding from S106 to S107, the reduction of the stored NOx by the concentrated fuel addition started in S103 is stopped, and the reduction of the stored NOx by the normal fuel addition is started instead. On the other hand, when the process proceeds from S102 to S107, the reduction of the stored NOx by the normal fuel addition that is currently performed is continued. When the process of S107 ends, the processes after S101 are repeated again.
[0056]
Further, when the process proceeds from S101 to S107, in S108, the fuel addition by the fuel addition valve 21 is stopped because it is determined in S101 that fuel addition to the exhaust by the fuel addition valve 21 is not possible. When the process of S108 ends, the process proceeds to S109. In S109, the addition flag is set to 0. When the process of S109 ends, the processes after S101 are repeated again.
[0057]
Here, when the above-described NOx reduction control is performed, the period T1 in FIG. 4 is a state in which fuel addition to the exhaust by the fuel addition valve 21 is not possible, and the processes S101, S108, and S109 of this control are repeated. Will be done. Therefore, as the internal combustion engine 1 starts, the temperature of the NOx catalyst 11 rises and approaches the activation temperature, so that the NOx purification rate by the NOx catalyst 11 increases. However, the NOx purification rate gradually decreases as NOx is continuously stored in the NOx catalyst 11.
[0058]
Next, in the period T2, since the temperature of the NOx catalyst 11 has reached the activation temperature TN1, the processes after S102 of this control are performed. Thereby, concentrated fuel addition is performed by the fuel addition valve 21, the NOx purification capacity of the NOx catalyst 11 is quickly recovered, and the NOx purification rate by the NOx catalyst 11 is rapidly increased. When the NOx concentration of the exhaust gas flowing out from the NOx catalyst 11 becomes smaller than D0 as the NOx purification capacity of the NOx catalyst 11 recovers, normal fuel addition is performed instead of concentrated fuel addition. At this point, the process proceeds from S105 to S106 in this control. After this point, the period T3 is entered. After the period T3, normal fuel addition is performed as long as it is determined that fuel addition to the exhaust gas by the fuel addition valve 21 is possible.
[0059]
The recovery of the NOx purification capacity of the NOx catalyst 11 by this control will be described based on the graph shown in FIG. The horizontal axis of the graph indicates the elapsed time, and the vertical axis of the graph indicates the NOx concentration of the exhaust gas flowing out from the NOx catalyst 11. A line L10 in FIG. 7 shows the transition of the NOx concentration of the exhaust when this control is performed, and a line L11 in FIG. 7 shows a normal state when the temperature of the NOx catalyst 11 reaches the activation temperature TN1. The graph shows the transition of the NOx concentration in the exhaust when the fuel is added to the exhaust by only adding the fuel. Further, a line L12 in FIG. 7 indicates the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 11. Further, the period T2 and the period T3 in FIG. 7 correspond to the period T2 and the period T3 in FIG.
[0060]
As described above, according to this control, the NOx occlusion amount is sufficiently reduced quickly and the NOx purification rate by the NOx catalyst 11 is at a high level as compared with the case of adding fuel to the exhaust gas only by normal fuel addition. Become. Thereafter, the concentrated fuel addition is changed to the normal fuel addition, and the NOx purification rate by the NOx catalyst 11 is maintained at a high level. Furthermore, by performing concentrated fuel addition only under certain conditions and switching to normal fuel addition after concentrated fuel addition, it becomes possible to suppress deterioration in fuel consumption due to fuel addition.
[0061]
【The invention's effect】
The present invention includes a NOx catalyst that occludes and reduces NOx contained in exhaust from an internal combustion engine, and purifies exhaust by reducing NOx occluded in the NOx catalyst by adding fuel to the exhaust flowing into the NOx catalyst. In the exhaust purification system that performs the above, only when the condition for adding the fuel to the exhaust gas and supplying the fuel to the NOx catalyst to reduce the NOx occlusion of the NOx catalyst is not established, the state is satisfied. After increasing the fuel addition to the exhaust per unit time and quickly reaching a high level of the NOx purification capacity of the NOx catalyst, the fuel addition to the exhaust per unit time is decreased. In addition, the NOx purification ability of the NOx catalyst is maintained at a high level on average, and deterioration of fuel consumption is suppressed.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration of an exhaust purification system according to an embodiment of the present invention, an internal combustion engine including the exhaust purification system, and a control system thereof.
FIG. 2 is a graph showing the transition of NOx purification ability by adding fuel when the NOx storage amount of the NOx catalyst is different.
FIG. 3 is a flowchart showing NOx reduction control performed in the exhaust purification system according to the embodiment of the present invention.
4 is a graph showing an example of the transition of the NOx purification rate by the NOx catalyst when the NOx reduction control shown in FIG. 3 is executed. FIG.
FIG. 5 is a graph showing the transition of the NOx purification rate by the NOx catalyst with respect to the temperature of the NOx catalyst.
6 is a graph showing an engine load region of the internal combustion engine 1. FIG.
7 is a graph showing the transition of the NOx purification ability of the Ox catalyst when the NOx reduction control shown in FIG. 3 is executed.
[Explanation of symbols]
1 ... Internal combustion engine
3. Fuel injection valve
11 .... NOx catalyst
20 .... ECU
21... Fuel addition valve
22 ... Exhaust temperature sensor
23 ... Exhaust NOx sensor
24 ... Crank position sensor
25 .... Accelerator opening sensor

Claims (5)

A NOx catalyst for storing and reducing NOx contained in the exhaust gas from the internal combustion engine;
Fuel addition means for adding fuel to the exhaust gas flowing into the NOx catalyst;
Fuel addition availability determination means for determining whether or not a condition for supplying fuel to the NOx catalyst is satisfied by the fuel addition means;
The air-fuel ratio of the exhaust gas that is added to the exhaust gas by the fuel addition unit and flows into the NOx catalyst when the condition for supplying the fuel to the NOx catalyst is determined by the fuel addition determination unit. Normal reduction control means for reducing the NOx occlusion amount of the NOx catalyst by supplying fuel to the NOx catalyst by setting the rich air-fuel ratio to
When a condition for supplying fuel to the NOx catalyst is satisfied by the fuel addition determination unit from a state where it is determined that the condition for supplying fuel to the NOx catalyst is not satisfied by the fuel addition determination unit. When shifting to the state to be determined, the fuel addition means performs the addition of a fuel having a larger addition amount per unit time than the addition of fuel to the exhaust gas performed through the normal reduction means for a predetermined period, and the NOx Centralized reduction control means for reducing the NOx occlusion amount of the NOx catalyst by supplying fuel to the NOx catalyst by setting the air-fuel ratio of the exhaust gas flowing into the catalyst to a rich rich air-fuel ratio on the rich side;
An exhaust NOx sensor for detecting the concentration of NOx contained in the exhaust gas flowing out from the NOx catalyst;
Equipped with a,
When it is determined that the concentration of NOx detected by the exhaust NOx sensor is lower than a predetermined threshold when the NOx occlusion amount of the NOx catalyst is reduced by the concentrated reduction control unit, the concentrated reduction control unit wherein with stops the reduction of the NOx occlusion amount of the NOx catalyst, the normal exhaust gas purification system for an internal combustion engine, characterized that you initiate reduction of the NOx occlusion amount of the NOx catalyst by the reduction control means. The fuel addition propriety determining means determines that a condition for supplying fuel to the NOx catalyst by the fuel adding means is satisfied when the temperature of the NOx catalyst is higher than a first predetermined temperature. The exhaust gas purification system for an internal combustion engine according to claim 1. The fuel addition propriety determination means satisfies a condition for supplying fuel to the NOx catalyst by the fuel addition means when the temperature of the NOx catalyst is lower than a second predetermined temperature that is higher than the first predetermined temperature. The exhaust gas purification system for an internal combustion engine according to claim 1 or 2, wherein the exhaust gas purification system is determined to be. The fuel addition propriety judging means has a condition for supplying fuel to the NOx catalyst by the fuel adding means when the engine load of the internal combustion engine belongs to a load area lower than a high load area. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein The fuel addition means is a fuel addition valve provided in an exhaust system of the internal combustion engine and upstream of the NOx catalyst,
The centralized reduction control means is configured to add the fuel by at least one of shortening an addition interval of the fuel to the exhaust by the fuel addition valve or increasing a valve opening time per one time of the fuel addition valve. The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 4, wherein fuel is added by a valve.

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