JP4400382B2 - Exhaust gas purification system for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust purification system that purifies NOx contained in exhaust gas from an internal combustion engine.

  In order to purify NOx generated by the combustion of fuel in an internal combustion engine, a so-called NOx occlusion reduction type catalyst (hereinafter referred to as “NOx catalyst”) is used. The NOx catalyst has the property of storing NOx in the exhaust into the catalyst when the air-fuel ratio of the exhaust is in a lean state, and releasing the stored NOx when the air-fuel ratio of the exhaust becomes stoichiometric or rich. Yes. Therefore, when the oxygen concentration of the exhaust gas is lean, the NOx in the exhaust gas is occluded, and then the fuel as a reducing agent is supplied into the exhaust gas to reduce the oxygen concentration in the exhaust gas and release the stored NOx. NOx is reduced and purified by the fuel.

Here, the NOx catalyst is provided in parallel in the exhaust passage of the internal combustion engine, and the exhaust gas flowing into the NOx catalyst is shut off one by one, the reducing agent is supplied to the NOx catalyst, and the NOx occluded in the NOx catalyst is reduced. A technique for purifying is disclosed (for example, see Patent Document 1). According to this technology, during the reduction and purification of NOx stored in one NOx catalyst, NOx can be reduced and purified at any time regardless of the operating conditions of the internal combustion engine by guiding the exhaust to the other NOx catalyst. It becomes.
Japanese Patent No. 2727906 US Pat. No. 5,956,949 JP-A-8-121153

  In an internal combustion engine provided with two NOx catalysts in parallel with the exhaust passage, while reducing and purifying the stored NOx by reducing the exhaust flow rate flowing into one NOx catalyst and supplying a reducing agent, By guiding the exhaust to the other NOx catalyst, it is possible to occlude NOx in the exhaust. As a result, NOx can be reduced and purified at any time regardless of the operating conditions of the internal combustion engine.

  However, if the exhaust flow rate flowing into the NOx catalyst is reduced in order to reduce or purify NOx stored in one NOx catalyst, the exhaust flow rate flowing into the other NOx catalyst increases, the back pressure in the exhaust passage increases, and the internal combustion engine There is a risk that the load of Further, when there are two exhaust passages due to the arrangement of the cylinders of the internal combustion engine and the like, if two NOx catalysts are provided in each exhaust passage, there is a possibility that the mounting property on the vehicle is deteriorated.

  In the present invention, in view of the above-described problems, in an internal combustion engine provided with a plurality of NOx catalysts in parallel with an exhaust passage, the increase in the back pressure of the exhaust passage is suppressed as much as possible during storage and reduction of NOx stored in the NOx catalyst. In addition, an object of the present invention is to provide an exhaust purification system that can be mounted on a vehicle.

In order to solve the above-described problems, the present invention reduces the number of NOx catalysts provided in the exhaust passage to three when the cylinders of the internal combustion engine are divided into two common cylinder groups. We focused on how to connect each NOx catalyst and the two cylinder groups through the exhaust passage. As a result, the total number of NOx catalysts can be reduced as compared with the case where two NOx catalysts are provided in the exhaust passage connected to the two cylinder groups, and the reduction of stored NOx in one NOx catalyst,
This is because it is possible to suppress fluctuations in the back pressure in the exhaust passage during purification as much as possible.

  Therefore, first, the present invention relates to an internal combustion engine in which the cylinders of the internal combustion engine are divided into two cylinder groups having common exhaust, and the NOx storage reduction catalyst is provided in two exhaust passages connected to each cylinder group. In the exhaust purification system, the three NOx storage reduction catalysts are provided in parallel with the exhaust passage, and each of the two exhaust passages is provided so that the exhaust can be divided and flow into the three NOx storage reduction catalysts. Is branched and connected to the three NOx storage reduction catalysts, and the exhaust flow rate that flows into each of the NOx storage reduction catalysts provided in each of the two exhaust passages and one exhaust passage is adjusted. Exhaust flow rate adjusting means, and fuel supply means provided in an exhaust passage upstream of the three NOx storage reduction catalysts and supplying fuel to the respective NOx storage reduction catalysts In the three NOx storage reduction catalysts, when the NOx storage amount of one NOx storage reduction catalyst is equal to or less than the predetermined storage amount, exhaust flows into the three NOx storage reduction passages from the two exhaust passages. When the NOx storage amount of the one NOx storage reduction catalyst exceeds the predetermined storage amount, the exhaust flow rate adjusting means reduces the exhaust flow rate flowing into the one storage reduction NOx catalyst, and the fuel supply And NOx purification means for supplying fuel to the one NOx storage reduction catalyst by means.

  In the internal combustion engine exhaust gas purification system, NOx contained in the exhaust gas is purified mainly by the NOx storage reduction catalyst (NOx catalyst). The NOx purification by the NOx catalyst is performed by storing NOx in the exhaust into the catalyst when the air-fuel ratio of the exhaust is in a lean state, and releasing the stored NOx when the air-fuel ratio of the exhaust becomes stoichiometric or rich. This is done by reducing NOx with a reducing agent (fuel component) in the exhaust.

  Here, the cylinders of the internal combustion engine are divided into two cylinder groups that share the exhaust. This is because the exhaust from all cylinders is not concentrated in one exhaust passage due to the structure of the internal combustion engine, etc., but is divided into two exhaust passages, thereby ensuring piping space and avoiding exhaust interference. Can be achieved. As an example, the internal combustion engine may be a V-type engine or a horizontally opposed engine having two banks, and a cylinder group in each of the two banks may be connected to each of the two exhaust passages. Good. The internal combustion engine is an in-line multi-cylinder engine, and a plurality of cylinders constituting the in-line multi-cylinder engine are divided into two cylinder groups that do not cause exhaust interference, and each of the two cylinder groups is the two cylinder groups. It may be connected to each of the exhaust passages.

  The two exhaust passages are branched and connected to the three NOx catalysts, so that the exhaust from the internal combustion engine is purified by the three NOx catalysts. However, if the NOx occlusion amount of each NOx catalyst increases, the NOx occlusion capacity decreases, so it is necessary to reduce and purify the stored NOx. This reduction and purification of NOx reduces the flow rate of exhaust flowing into the NOx catalyst and supplies the NOx catalyst as fuel as a reducing agent in a so-called low SV state, so that more efficient storage of NOx can be achieved with less fuel. Reduction and purification become possible.

  More specifically, NOx purification in the exhaust gas is performed by the NOx purification means. First, when none of the three NOx catalysts needs to reduce and purify the stored NOx, the exhaust from the two exhaust passages is poured into the three NOx catalysts to purify the exhaust. Preferably, the exhaust gas flow rate adjusting means equalizes the exhaust gas flow rates flowing into the three NOx catalysts so that the increase in the stored NOx amount in the NOx catalyst is not biased.

The determination of the necessity for reduction and purification of the stored NOx is made based on whether or not each stored amount of NOx exceeds a predetermined stored amount that is a reference value. The predetermined storage amount may be determined from the transition of NOx storage efficiency in the NOx catalyst measured through experiments or the like. Further, the NOx occlusion amount in the NOx catalyst may be estimated based on parameters related to the NOx amount in the exhaust, such as the operating state of the internal combustion engine, the exhaust flow rate to each NOx catalyst, and the like.

  Next, when the NOx occlusion amount of one NOx catalyst exceeds the predetermined occlusion amount, the exhaust flow rate flowing into the one NOx catalyst is reduced by the exhaust flow rate adjusting means to make it a low SV state. In this state, the NOx catalyst is supplied with fuel to reduce and purify the stored NOx. At this time, the exhaust gas from the internal combustion engine mainly flows into the remaining two NOx catalysts excluding the one NOx catalyst via the two exhaust passages.

  Here, in the conventional exhaust purification system, when there are two exhaust passages of the internal combustion engine, the NOx stored can be reduced regardless of the operating state of the internal combustion engine by providing two NOx catalysts in parallel in each exhaust passage. Create a low SV condition suitable for purification. For this reason, four NOx catalysts are required as a whole, and the mountability on a vehicle is deteriorated. However, in the exhaust purification system according to the present invention, the number of NOx catalysts is three, and the low SV suitable for reducing and purifying the stored NOx by the NOx purification means regardless of the operating state of the internal combustion engine. It is possible to create a state. Therefore, the exhaust purification system according to the present invention is superior in mountability to a vehicle.

  In the exhaust purification system according to the present invention, the number of catalysts through which exhaust flows in the entire exhaust passage varies from three to two during the reduction and purification of the stored NOx. On the other hand, in the above-described conventional exhaust purification system, each exhaust passage includes two NOx catalysts, and therefore the number of catalysts through which exhaust flows varies from two to one during the reduction and purification of the stored NOx. As a result, the exhaust purification system according to the present invention can suppress the increase in the back pressure of the exhaust passage as much as possible during the reduction and purification of the stored NOx.

  Here, in the exhaust gas purification system for an internal combustion engine, the exhaust flow rate adjusting means includes two upstream side flow path adjustment valves provided at respective branch points of the two exhaust passages, or the two exhaust gas control systems. When the NOx storage reduction catalyst in which each of the passages is branched and connected is connected to one exhaust passage again on the exhaust outlet side, the two downstream side flows provided in the respective connecting portions The fuel supply means may be three fuel addition valves located in the exhaust passages on the upstream side of the three storage reduction type NOx catalysts.

  That is, the inflow of exhaust gas to the NOx catalyst connected to each of the exhaust passages branched by the two upstream flow path regulating valves is controlled. In addition, the flow of exhaust gas that passes through the NOx catalyst can be controlled by the two downstream side flow path regulating valves, and thus the flow rate of the exhaust gas flowing into the NOx catalyst can be controlled. Preferably, the upstream side flow path adjustment valve and the downstream side flow path adjustment valve are respectively provided in one exhaust passage. In addition, the fuel addition valve is provided at a position corresponding to the three NOx catalysts so that fuel can be supplied to each NOx catalyst.

  Here, in the exhaust gas purification system for an internal combustion engine, the exhaust flow rate adjusting means is two upstream side flow path adjusting valves provided at respective branch points of the two exhaust passages, and the fuel supply The means may be two fuel addition valves located in the upstream exhaust passage of each of the two upstream flow path regulating valves.

  That is, the inflow of exhaust gas to the NOx catalyst connected to each of the exhaust passages branched by the two upstream flow path regulating valves is controlled. In addition, the fuel addition valve enables fuel supply to the three NOx catalysts with the opening / closing operation of the upstream side flow regulating valve.

Here, regarding NOx storage reduction, purification, and supply of fuel to the NOx catalyst by the fuel addition valve, the NOx purification means is one of the NOx storage reduction catalysts connected to the one exhaust passage. When fuel is supplied to the NOx catalyst, the upstream side flow regulating valve provided in the one exhaust passage blocks the flow of exhaust to the remaining NOx storage reduction catalyst and enters the one exhaust passage. Fuel may be added to the exhaust gas by the fuel addition valve provided, and the flow of exhaust gas to the one NOx storage reduction catalyst may be shut off by the upstream flow path adjustment valve after a predetermined time has elapsed. .

  The predetermined time is the time required for the added fuel to reach the NOx catalyst where the stored NOx is reduced and purified after being added to the exhaust. For example, the predetermined time is added to the exhaust by a fuel addition valve. This is the time required to pass the point where the flow path adjustment valve is provided after the added fuel is added. Therefore, after the predetermined time has elapsed, the upstream flow path regulating valve causes the exhaust flow to the one NOx storage reduction catalyst to flow into the low SV state for the NOx catalyst where the NOx storage is reduced and purified. Blocked.

  Here, in the exhaust gas purification system for an internal combustion engine as described above, when fuel is supplied to the one NOx storage reduction catalyst by the NOx purification means, the NOx storage amount stored in the other NOx storage reduction catalyst is stored. Based on the NOx occlusion amount estimated by the purification NOx occlusion amount estimation means and the NOx occlusion amount estimation means during purification, fuel is supplied to the one NOx storage reduction catalyst by the NOx purification means. And determining means for determining whether or not the NOx occlusion amount of the other NOx storage reduction catalyst exceeds the predetermined storage amount, and the NOx occlusion amount of the other NOx storage reduction catalyst is determined by the determination means. When it is determined that the predetermined storage amount is exceeded, the value of the predetermined storage amount for the one NOx storage reduction catalyst is subtracted from the initial value by a value equal to or greater than the predetermined correction amount. Or a correction means for correcting the NOx occlusion amount of the one NOx storage reduction catalyst to an NOx occlusion amount obtained by adding an amount greater than the predetermined correction amount to the initial amount. .

  In the exhaust purification system for an internal combustion engine, when the NOx purification means performs reduction and purification of the stored NOx in one NOx catalyst, the exhaust from the internal combustion engine flows into the remaining two NOx catalysts. However, if the NOx storage amount of one of the two NOx catalysts exceeds the predetermined storage amount during this period, the two NOx catalysts simultaneously enter a low SV state for reduction and purification. Exhaust gas from the exhaust gas concentrates on one NOx catalyst, and the back pressure in the exhaust passage increases, which is not preferable.

  Therefore, when the determination means determines that the NOx storage amount of the other NOx catalyst exceeds the predetermined storage amount based on the estimated value by the purification NOx storage amount estimation means, that is, two NOx catalysts are simultaneously low SV. When the state is expected, the correction means corrects the predetermined storage amount for the one NOx catalyst, or corrects the NOx storage amount known for the one NOx catalyst.

  By correcting the predetermined storage amount as described above by the correcting means, or by correcting the NOx storage amount grasped for the one NOx catalyst by the correcting means as described above, The relative relationship between the NOx occlusion amount of the NOx catalyst and the predetermined occlusion amount is corrected, and the NOx occlusion in the one NOx catalyst is reduced and purified at an earlier timing than the original timing.

As a result, while the one NOx catalyst is placed in the low SV state, the NOx occlusion amount of the other NOx catalyst does not exceed the initial predetermined occlusion amount before correction. In the other NOx catalyst, the reduction and purification of the stored NOx may be started as soon as the reduction and purification in the one NOx catalyst is completed. By doing so, it is possible to avoid that at least two of the three NOx catalysts are simultaneously placed in a low SV state for the reduction and purification of the stored NOx.

  Here, the predetermined correction amount is determined based on the NOx occlusion amount estimated by the purification-time NOx occlusion amount estimating means, and the NOx occlusion amount of the one NOx storage reduction type NOx catalyst and the NOx occlusion amount of the other NOx storage reduction type NOx catalyst. The amount obtained by subtracting the difference from the amount may be used. By setting the predetermined NOx occlusion amount or the correction amount of the NOx occlusion amount of one NOx catalyst in this way, at least two of the three NOx catalysts are simultaneously placed in a low SV state for reduction and purification of the occluded NOx. It is possible to avoid the damage more reliably.

  Secondly, in order to solve the above-described problems, the present invention relates to an NOx storage reduction catalyst in which two cylinders of an internal combustion engine are divided into two cylinder groups that share exhaust, and two exhaust passages connected to each cylinder group. In the exhaust gas purification system for an internal combustion engine, the three NOx storage reduction catalysts are provided in parallel with the exhaust passage, and the exhaust gas can flow into the three storage reduction NOx catalysts in a divided manner. Each of the two exhaust passages branches and is connected to the three NOx storage reduction catalysts, and is provided in each of the two exhaust passages, and the NOx storage reduction NOx catalyst is connected to one exhaust passage. Exhaust flow rate adjusting means for adjusting the exhaust flow rate flowing into each of the three NOx storage reduction type NOx catalysts, and an exhaust passage upstream of the three NOx storage reduction catalysts, The fuel supply means to supply, the reduction of the exhaust flow rate to the one NOx storage reduction catalyst by the exhaust flow rate adjusting means, and the fuel supply to the one NOx storage reduction catalyst by the fuel supply means, sequentially, And NOx purification means that executes in one storage reduction type NOx catalyst and flows exhaust gas from the two exhaust passages into the remaining two storage reduction type NOx catalysts.

  The relationship between the NOx catalyst and the two exhaust passages in the exhaust purification system is as described above. One of the features of the above exhaust purification system is that one of the three NOx catalysts is always placed in a low SV state by the NOx purification means, and the exhaust from the internal combustion engine flows into the remaining two NOx catalysts, The NOx catalyst placed in the low SV state is sequentially changed. As a result, regardless of whether the NOx stored in the NOx catalyst is reduced or purified, the exhaust gas from the internal combustion engine always flows into the two NOx catalysts, so that the back pressure in the exhaust passage is constantly stabilized. Further, by setting the number of NOx catalysts to three, the mounting property to the vehicle is improved as compared with the conventional exhaust purification system.

  In an internal combustion engine provided with a plurality of NOx catalysts in parallel with the exhaust passage, an increase in the back pressure of the exhaust passage is suppressed as much as possible during storage and reduction of NOx stored in the NOx catalyst, and the exhaust purification system is applied to the vehicle. Mountability can be improved.

  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 diagram showing a schematic configuration of an exhaust purification system of an internal combustion engine 1 to which the present invention is applied. The internal combustion engine 1 is a V-type engine and has two cylinder groups 2L and 2R that share exhaust. The two cylinder groups 2L and 2R are connected to the first exhaust passage 3L and the second exhaust passage 3R, respectively.

The first exhaust passage 3L branches in the middle to form a first upstream passage 4L and a second upstream passage 5L. The second exhaust passage 3R branches in the middle to form a third upstream passage 4R and a fourth upstream passage 5R. Further, the first upstream passage 4L is connected to the first NOx catalyst 16, the second upstream passage 5L and the fourth upstream passage 5R are joined and connected to the second NOx catalyst 15, and the third upstream passage 4R is connected to the third NOx catalyst 14. The first NOx catalyst 16, the second NOx catalyst 15, and the third NOx catalyst 14 are so-called storage reduction type NOx catalysts.

  Next, the exhaust passage piping on the downstream side of each NOx catalyst will be described. A first downstream passage 6L extends from the first NOx catalyst 16, a second downstream passage 7L and a fourth downstream passage 7R extend from the second NOx catalyst 15, and from the third NOx catalyst 14. A third downstream passage 6R extends. The first downstream passage 6L and the second downstream passage 7L are combined to form a third exhaust passage 8L, and the third downstream passage 6R and the fourth downstream passage 7R are combined to form the fourth exhaust passage 8R. Form.

  Here, at the branch point of the first exhaust passage 3L, in the exhaust flowing through the first exhaust passage 3L, an upstream flow that adjusts the ratio of the exhaust amount flowing through the first upstream passage 4L and the second upstream passage 5L. A passage adjusting valve 9L is provided, and the ratio of the amount of exhaust flowing through the third upstream passage 4R and the fourth upstream passage 5R in the exhaust flowing through the second exhaust passage 3R is determined at the branch point of the second exhaust passage 3R. An upstream flow path adjustment valve 9R to be adjusted is provided. Further, a downstream-side passage adjustment valve 17L that adjusts the ratio of the exhaust amount that flows through each passage and joins the third exhaust passage 8L is provided at the coupling portion between the first downstream-side passage 6L and the second downstream-side passage 7L. In addition, a downstream-side flow regulating valve 17R that adjusts the ratio of the exhaust amount that flows through each passage and joins the fourth exhaust passage 8R is provided at the joint between the third downstream-side passage 6R and the fourth downstream-side passage 7R. It has been.

  A first fuel addition valve 13 is provided in the middle of the first upstream passage 4L to add fuel to the exhaust gas flowing through the passage, and the passage is provided near the joint between the second upstream passage 5L and the fourth upstream passage. A second fuel addition valve 12 for adding fuel to the flowing exhaust gas is provided, and a third fuel addition valve 11 for adding fuel to the exhaust gas flowing through the passage is provided in the middle of the third upstream side passage 4R.

  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 and its exhaust purification system. The ECU 20 includes a CPU, a ROM, a RAM, and the like that store various programs and maps to be described later, and is a unit that controls the operating state of the internal combustion engine 1, an exhaust purification system, and the like.

  Here, the upstream side flow path adjustment valves 9L and 9R and the downstream side flow path adjustment valves 17L and 17R are electrically connected to the ECU 20, and the opening degree of these flow path adjustment valves is controlled by a control signal from the ECU 20. Is adjusted. As a result, the exhaust flow in the exhaust purification system of the internal combustion engine 1 is controlled. The first fuel addition valve 13, the second fuel addition valve 12, and the third fuel addition valve 11 are electrically connected to the ECU 20, and these fuel addition valves are controlled by a control signal from the ECU 20, The fuel is added to the exhaust gas flowing through.

  In the exhaust purification system of the internal combustion engine 1 configured as shown in FIG. 1, when the air-fuel ratio of the exhaust flowing into the first NOx catalyst 16, the second NOx catalyst 15, and the third NOx catalyst 14 is in a lean state, NOx in the exhaust is occluded into the catalyst by the NOx catalyst. Further, when the air-fuel ratio of the exhaust gas becomes stoichiometric or rich, the NOx stored in each NOx catalyst is released, the fuel in the exhaust gas acts as a reducing agent, and NOx is reduced and purified.

  Here, in the exhaust purification system, when the NOx occlusion amount of each NOx catalyst is relatively small, i.e., when each NOx catalyst has sufficient ability to occlude NOx in the exhaust, two cylinders are left. The opening degree of the upstream flow path adjustment valves 9L and 9R and the downstream flow path adjustment valves 17L and 17R is adjusted by a command from the ECU 20 so that the exhaust discharged from the groups 2L and 2R flows evenly into each NOx catalyst. Is done. Thereby, the increase degree of the NOx occlusion amount in each NOx catalyst becomes substantially constant.

  However, in each NOx catalyst, if the NOx occlusion amount exceeds the predetermined occlusion amount D0, it becomes difficult to sufficiently store NOx in the exhaust gas, and a large amount of NOx can be released into the atmosphere. Therefore, in such a case, it is necessary to reduce and purify the stored NOx of each NOx catalyst. In the exhaust gas purification system of the internal combustion engine 1 according to the present embodiment, the exhaust gas flowing into the NOx catalyst to be reduced and purified is reduced to a low SV state, and then fuel as a reducing agent is supplied to the NOx catalyst. Is done.

  Specifically, when the NOx catalyst in which the storage NOx amount exceeds the predetermined storage amount D0 is the first NOx catalyst 16, the exhaust from the first exhaust passage 3L is caused to flow from the first upstream side by the upstream side flow path adjustment valve 9L. Fuel is added into the exhaust gas from the first fuel addition valve 13 in a state where it does not flow into the passage 4L. In addition, when the NOx catalyst whose stored NOx amount exceeds the predetermined stored amount D0 is the second NOx catalyst 15, the exhaust from the first exhaust passage 3L is transferred to the second upstream passage 5L by the upstream flow passage adjustment valve 9L. In a state in which the exhaust from the second exhaust passage 3R does not flow into the fourth upstream passage 5R by the upstream flow path adjustment valve 9R so as not to flow in, fuel is supplied from the second fuel addition valve 12 into the exhaust. Added. When the NOx catalyst whose stored NOx amount exceeds the predetermined stored amount D0 is the third NOx catalyst 14, the exhaust from the second exhaust passage 3R is transferred to the third upstream passage 4R by the upstream flow passage adjustment valve 9R. The fuel is added into the exhaust gas from the third fuel addition valve 11 in a state where it does not flow.

  The exhaust gas purification system of the internal combustion engine 1 configured as described above can purify exhaust gas discharged from the two cylinder groups 2L and 2R with a total of three NOx catalysts. Accordingly, the number of NOx catalysts can be reduced as compared with the case where the exhaust from each cylinder group is purified by two NOx catalysts as in the conventional case, so that the internal combustion engine 1 can be mounted on a vehicle. improves.

  Further, by reducing and purifying the stored NOx with respect to the NOx catalyst placed in the low SV state in this way, it is possible to efficiently reduce and purify with less fuel. Further, regarding the increase in the back pressure of the first exhaust passage 3L and the second exhaust passage 3R caused by placing the NOx catalyst in the low SV state, in this embodiment, the fluctuation range of the number of NOx catalysts into which the exhaust flows is from three. On the other hand, since the fluctuation range is two to one in the conventional case described above, the exhaust purification system according to this embodiment can suppress the increase in the back pressure in the exhaust passage as compared with the conventional case. It becomes possible.

  Here, when the NOx occlusion amounts of the NOx catalysts are close to each other, the NOx occlusion is simultaneously reduced and purified in the two NOx catalysts, and there is a possibility that the NOx occlusion amount is placed in a low SV state. In such a case, the exhaust pressure from the internal combustion engine 1 collects in one NOx catalyst, so that the back pressure in the exhaust passage may increase significantly. Therefore, NOx purification control that makes it possible to avoid two NOx catalysts being simultaneously placed in a low SV state will be described with reference to FIG. The NOx purification control in the present embodiment is a routine that is repeatedly executed by the ECU 20 at a constant cycle. The NOx purification control is started when no NOx catalyst is placed in the low SV state.

  In S101, the NOx occlusion amounts of the first NOx catalyst 16, the second NOx catalyst 15, and the third NOx catalyst 14 are estimated. Specifically, for example, the NOx occlusion amount of the NOx catalyst 14 is determined based on the operation history such as the engine load and the engine speed of the internal combustion engine 1, the opening degree of the upstream side flow regulating valves 9L and 9R, and the like. Estimated based on the exhaust flow rate to the catalyst. When the process of S101 ends, the process proceeds to S102.

In S102, based on the processing result of S101, the NOx occlusion between the NOx occlusion maximum catalyst having the largest NOx occlusion amount and the NOx occlusion amount second catalyst having the next largest NOx occlusion amount is shown. The NOx occlusion amount difference ΔD, which is the amount difference, is calculated. When the process of S102 ends, the process proceeds to S103.

  Among the three NOx catalysts, the catalyst with the largest storage amount reaches the predetermined storage amount D0 the earliest, and then the NOx storage amount reaches the predetermined storage amount. It is. Therefore, in S103, when the stored NOx is reduced and purified in the catalyst with the largest storage amount, the increase amount DP of the NOx storage amount in the second storage catalyst is estimated. In the estimation, it is considered that the exhaust gas flow rate flowing into the second storage amount catalyst is increased by placing the most storage amount catalyst in the low SV state. When the process of S103 ends, the process proceeds to S104.

  In S104, it is determined whether or not the NOx occlusion amount difference ΔD calculated in S102 is equal to or smaller than the NOx occlusion amount increase DP estimated in S103. When the NOx occlusion amount difference ΔD exceeds the NOx occlusion amount increase DP, the NOx occlusion amount is also stored in the second occlusion amount catalyst while the NOx occlusion is being reduced and purified in the most occlusion amount catalyst. The stored NOx is reduced and purified beyond the amount D0. As a result, the two NOx catalysts are simultaneously brought into the low SV state, and there is a possibility that the back pressure in the exhaust passage increases. That is, the process of S104 predicts that two NOx catalysts simultaneously enter a low SV state based on the relationship between the NOx occlusion amount difference ΔD and the NOx occlusion amount increase DP. In S104, when the NOx occlusion amount difference ΔD calculated in S102 is equal to or smaller than the NOx occlusion amount increase DP estimated in S103, the process proceeds to S105. On the other hand, in S104, if the NOx occlusion amount difference ΔD calculated in S102 is not less than or equal to the increase amount DP of the NOx occlusion amount estimated in S103, the process proceeds to S106.

  In S105, the value of the predetermined storage amount D0, which is a criterion for determining whether or not to reduce and purify the stored NOx, is corrected in the most storage amount catalyst. In the present embodiment, a value obtained by subtracting the NOx storage amount increase DP in the second storage amount catalyst estimated in S103 from the initial predetermined storage amount D0 of the most storage amount catalyst. As a result, in the catalyst having the largest storage amount, the relative relationship between the NOx storage amount and the predetermined storage amount D0 changes, and the reduction and purification of the stored NOx is executed earlier than the initial time. When the process of S105 ends, the process proceeds to S107.

  In S106, the value of the predetermined storage amount D0 is set to the initial value in the catalyst having the largest storage amount. Therefore, in this case, in the catalyst having the largest storage amount, the time of reduction and purification of the stored NOx is the initial time. When the process of S106 ends, the process proceeds to S107.

  In S107, it is determined whether or not the NOx occlusion amount exceeds the predetermined occlusion amount D0 corrected in S105 or the initial predetermined occlusion amount D0 set in S106 in the most occlusion amount catalyst. When the NOx storage amount exceeds the predetermined storage amount D0, the process proceeds to S108, and when the NOx storage amount does not exceed the predetermined storage amount D0, the process of S107 is performed again.

  In S108, control of the opening degree of the flow path adjustment valve and addition of exhaust fuel from the fuel addition valve are performed in order to reduce and purify the storage NOx in the catalyst having the largest storage amount. Specific control of the flow path adjustment valve and the fuel addition valve is as described above. After the process of S108, the present NOx purification control is terminated.

  Here, FIG. 3 shows the state of the NOx occlusion amounts of the most storage amount catalyst and the second storage amount catalyst when the NOx purification control shown in FIG. 2 is performed. FIG. 3A shows the state of the NOx occlusion amount in the maximum occlusion amount catalyst and the second occlusion amount catalyst at the stage where the process of S103 shown in FIG. 2 is performed. The percentage in the figure represents the NOx occlusion amount when the NOx occlusion amount is the predetermined occlusion amount D0 as 100%.

  Therefore, in FIG. 3 (a), the NOx occlusion amount D1 of the catalyst with the largest occlusion amount is 85%, and the NOx occlusion amount D2 of the second occlusion amount catalyst is 80%. Therefore, the NOx occlusion amount difference ΔD in the NOx purification control shown in FIG. 2 is 5%. Further, in this embodiment, the increase amount DP of the NOx occlusion amount in the second occlusion amount catalyst estimated in S103 is 10%.

  Here, when the NOx occlusion amount of the catalyst with the largest occlusion amount reaches 100%, the NOx occlusion is reduced and purified, but the NOx occlusion amount of the second storage amount catalyst reaches 95% at the start of the reduction. . Therefore, there is a possibility that the reduction and purification of the second storage amount catalyst will be performed while the reduction and purification of the catalyst with the largest storage amount is being performed. Therefore, in the state of the maximum storage amount catalyst and the second storage amount catalyst shown in FIG. 3A, the process of S105 is performed according to the determination in S104.

  Next, FIG. 3B shows the state of the NOx occlusion amounts of the most occlusion amount catalyst and the second occlusion amount catalyst when the processes of S105, S107, and S108 in the NOx purification control are performed. By the process of S105, the value of the predetermined storage amount D0 is corrected from the initial 100% to 90% in the most storage amount catalyst. Then, when the NOx occlusion amount D1 of the most occlusion amount catalyst becomes 90%, the processing of S108 is executed through the determination of S107. At this time, the NOx occlusion amount D2 of the second occlusion amount catalyst is 85%, and the NOx occlusion amount D2 reaches only 95% even when the reduction and purification with the most occlusion amount catalyst is completed. Therefore, simultaneous reduction and purification of the stored NOx by the most stored amount catalyst and the second stored amount catalyst are avoided.

  Further, as shown in FIG. 3C, the predetermined storage amount D0 is corrected in order to avoid the simultaneous reduction and purification of the stored NOx in the maximum storage amount catalyst and the second storage amount catalyst. Instead, the NOx occlusion amount of the catalyst with the largest occlusion amount may be raised by an amount corresponding to the increase amount DP of the NOx occlusion amount. Even in this way, in the catalyst with the largest storage amount, the reduction and purification of the stored NOx is performed at an earlier stage than the initial start time, and thus the simultaneous execution of the reduction and purification of the stored NOx can be avoided.

  In the above-described embodiment, the correction amount of the predetermined storage amount or the correction amount of the NOx storage amount of the most storage amount catalyst is the increase amount DP of the NOx storage amount, but this correction amount is the NOx storage amount. If the increase amount DP is equal to or greater than the amount obtained by subtracting the NOx occlusion amount difference ΔD, the start timing of the reduction and purification of the NOx occlusion in the catalyst with the largest occlusion amount is made earlier than the initial start timing, and the NOx occlusion in the two NOx catalysts is increased. It is possible to avoid simultaneous execution of reduction and purification.

  Further, when the exhaust purification system of the internal combustion engine 1 shown in FIG. 1 is mounted on a vehicle, when the mounting space is limited, the exhaust purification system may be configured in a vertical row as shown in FIG. FIG. 4 is a diagram schematically showing the exhaust purification system of the internal combustion engine 1. The same components as those of the exhaust purification system shown in FIG.

  In the exhaust gas purification system of the internal combustion engine 1 shown in FIG. 4, the exhaust discharged from the internal combustion engine passes through the upstream exhaust passage 24 and sequentially reaches the first NOx catalyst 16, the second NOx catalyst 15, and the third NOx catalyst 14. Distributed. The distribution of the exhaust gas is performed by a first exhaust flow rate adjustment valve 21, a second exhaust flow rate adjustment valve 22, and a third exhaust flow rate adjustment valve 23 provided on the upstream side of each NOx catalyst. The fuel addition valve is not shown in FIG. 4, but is provided in the exhaust passage on the upstream side of each NOx catalyst. Further, the exhaust from each NOx catalyst is collected in the downstream exhaust passage 25.

In the exhaust gas purification system of the internal combustion engine 1 shown in FIG. 4, the back pressure in the upstream exhaust passage 24 decreases as the downstream NOx catalyst is used. Accordingly, since the ease of entering the exhaust gas flowing into each NOx catalyst is different, the adjustment of the opening degree of the first exhaust flow rate adjustment valve 21, the second exhaust flow rate adjustment valve 22, and the third exhaust flow rate adjustment valve 23 is performed on the upstream side exhaust gas. It is preferable to consider the back pressure in the passage 24.

  A second embodiment of the exhaust gas purification system for an internal combustion engine according to the present invention will be described with reference to FIG. FIG. 5 is a diagram schematically showing the exhaust purification system of the internal combustion engine 1 according to the present invention. The same components as those of the exhaust purification system shown in FIG.

  The exhaust purification system shown in FIG. 5 differs from the exhaust purification system shown in FIG. 1 in that there are no downstream flow path adjustment valves 17L and 17R, and that the fuel addition valves are the first fuel addition valve 13 and the third fuel addition valve. 11, each of which is provided in the exhaust passage on the upstream side of the upstream side flow regulating valves 9L and 9R.

  In the exhaust purification system configured as described above, when the NOx occlusion amount of each NOx catalyst is relatively small, the exhaust discharged from the two cylinder groups 2L and 2R flows evenly into each NOx catalyst. The opening degree of the upstream side flow path adjustment valves 9L and 9R and the downstream side flow path adjustment valves 17L and 17R is adjusted by a command from the ECU 20. Thereby, the increase degree of the NOx occlusion amount in each NOx catalyst becomes substantially constant.

  On the other hand, when reducing and purifying the stored NOx of each NOx catalyst, the fuel as a reducing agent is supplied to the NOx catalyst after setting each NOx catalyst to a low SV state. Specifically, when the NOx catalyst whose stored NOx amount exceeds the predetermined stored amount D0 is the first NOx catalyst 16, first, the exhaust from the first exhaust passage 3L is first exhausted by the upstream flow path adjustment valve 9L. The fuel is introduced into the side passage 4L, and fuel is added to the exhaust from the first fuel addition valve 13 in this state. Thereafter, after a certain period of time has elapsed since the addition of fuel, that is, when it is estimated that the added fuel has flowed into the first upstream passage 4L, the upstream flow passage adjustment valve 9L causes the exhaust from the first exhaust passage 3L to Introduced into the second upstream passage 5L, the first NOx catalyst 16 is placed in a low SV state. In other NOx catalysts, when the stored NOx amount exceeds the predetermined stored amount D0, similarly, the degree of opening of the upstream side flow regulating valves 9L and 9R is controlled and fuel is added from each fuel addition valve to the exhaust gas. .

  In the exhaust purification system of the internal combustion engine 1 configured as described above, the number of fuel addition valves can be further reduced as compared with the exhaust purification system of the internal combustion engine shown in FIG. 1, and the cost required for the configuration of the exhaust purification system Can be reduced.

  As an internal combustion engine to which the exhaust gas purification system for an internal combustion engine up to the above is applied, a V-type engine is mentioned in FIG. 1 or FIG. 5, but an exhaust according to the present invention is also applied to an in-line multi-cylinder engine as shown in FIG. The application of the purification system is possible. The internal combustion engine shown in FIG. 6 is an in-line engine having four cylinders 30. In the figure, # 1, # 2, # 3, and # 4 indicate the cylinder 30 numbers. The firing order of the cylinders in the in-line engine is # 1 → # 3 → # 4 → # 2.

  The exhaust purification system of the in-line engine mainly includes an upstream exhaust passage 34 connecting each cylinder 30 and the three NOx catalysts 31, 32, 33, and a downstream exhaust passage 35 through which exhaust gas flowing out from each NOx catalyst flows. Composed. In the in-line engine, the second cylinder 30 # 2 and the third cylinder 30 # 3 are set as one cylinder group, and the first cylinder 30 # 1 and the fourth cylinder 30 # 4 are set as one cylinder group. In consideration of the firing order of the cylinders in the in-line engine, the four cylinders are divided into two cylinder groups so that the exhaust from each cylinder does not interfere.

The upstream side exhaust passage 34 connects the second cylinder 30 # 2 and the third cylinder 30 # 3 to the NOx catalyst 31, and connects the first cylinder 30 # 1 and the fourth cylinder 30 # 4 to the NOx catalyst 33. Further, the exhaust passage between the third cylinder 30 # 3 and the NOx catalyst 31 is branched and connected to the NOx catalyst 32, and the exhaust passage between the first cylinder 30 # 1 and the NOx catalyst 33 is branched to form the NOx. Connect to catalyst 32. Although not shown in FIG. 6, the exhaust passage connecting each cylinder and each NOx catalyst is provided with an exhaust flow rate adjusting valve for adjusting the exhaust flow rate and a fuel addition valve for adding fuel to the exhaust. Yes.

  By configuring the exhaust purification system of the in-line engine in this way, it is possible to suppress the increase in the back pressure in the exhaust passage and improve the mountability of the exhaust purification system on the vehicle.

It is a figure showing the schematic structure of the exhaust gas purification system of the internal combustion engine which concerns on 1st Example of this invention. 3 is a flowchart relating to NOx purification control for reducing and purifying NOx in an exhaust gas purification system for an internal combustion engine according to an embodiment of the present invention. It is a figure which shows the mode of the NOx occlusion amount in a NOx catalyst in the exhaust gas purification system of the internal combustion engine which concerns on the Example of this invention. It is the figure which showed the example of mounting to the vehicle of the exhaust gas purification system of the internal combustion engine which concerns on the Example of this invention. It is a figure showing schematic structure of the exhaust gas purification system of the internal combustion engine which concerns on the 2nd Example of this invention. It is a figure showing schematic structure of the exhaust gas purification system of the internal combustion engine which concerns on the 3rd Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 2L, 2R ... Group of cylinders 3L ... First exhaust passage 3R ... Second exhaust passage 4L ... First upstream passage 4R ... First Three upstream side passages 5L ··· Second upstream side passage 5R · · · Fourth upstream side passage 9L · · · Upstream flow path adjustment valve 9R · · · Downstream flow passage adjustment valve 11 ··· Third fuel addition valve 12 ... Second fuel addition valve 13 ... First fuel addition valve 14 ... Third NOx catalyst 15 ... Second NOx catalyst 16 ... One NOx catalyst 20 ... ECU

Claims (9)

In an exhaust gas purification system for an internal combustion engine in which the cylinders of the internal combustion engine are divided into two cylinder groups having common exhaust, and the NOx storage reduction catalyst is provided in two exhaust passages connected to each cylinder group,
Three NOx storage reduction catalysts are provided in parallel with the exhaust passage, and each of the two exhaust passages branches to allow the exhaust to separate into the three NOx storage reduction catalysts. Connected to three NOx storage reduction catalysts,
An exhaust flow rate adjusting means that adjusts an exhaust flow rate that flows into each of the NOx storage reduction catalyst that is provided in each of the two exhaust passages and that is connected to one exhaust passage;
Fuel supply means provided in an exhaust passage on the upstream side of the three NOx storage reduction catalysts, and for supplying fuel to each of the NOx storage reduction catalysts;
In the three NOx storage reduction catalysts, when the NOx storage amount of one NOx storage reduction catalyst is equal to or less than the predetermined storage amount, exhaust flows into the three NOx storage reduction passages from the two exhaust passages. When the NOx storage amount of the one NOx storage reduction catalyst exceeds the predetermined storage amount, the exhaust flow rate adjusting means reduces the exhaust flow rate flowing into the one storage reduction NOx catalyst, and the fuel supply NOx purification means for supplying fuel to the one NOx storage reduction catalyst by means;
An exhaust gas purification system for an internal combustion engine, comprising: The exhaust flow rate adjusting means includes two upstream flow path adjustment valves provided at respective branch points of the two exhaust passages, or an occlusion reduction type in which each of the two exhaust passages is branched and connected. When the NOx catalyst is coupled to one exhaust passage again on the exhaust outlet side, it is at least one of the two downstream flow path regulating valves provided in each coupling section,
The exhaust purification of an internal combustion engine according to claim 1, wherein the fuel supply means is three fuel addition valves located in an exhaust passage on the upstream side of each of the three NOx storage reduction catalysts. system. The exhaust flow rate adjusting means is two upstream flow path adjusting valves provided at respective branch points of the two exhaust passages,
2. The exhaust of the internal combustion engine according to claim 1, wherein the fuel supply means is two fuel addition valves positioned in an upstream exhaust passage of each of the two upstream flow path regulating valves. Purification system.   The NOx purification means adjusts the upstream-side flow path provided in the one exhaust passage when supplying fuel to one NOx storage reduction catalyst among the NOx storage reduction catalysts connected to the one exhaust passage. The fuel is added to the exhaust gas by the fuel addition valve provided in the one exhaust passage in a state where the flow of the exhaust gas to the remaining NOx storage reduction catalyst is blocked by the valve, and after the predetermined time has passed, the upstream flow 4. The exhaust gas purification system for an internal combustion engine according to claim 3, wherein the flow of exhaust gas to the one NOx storage reduction catalyst is blocked by a path regulating valve.   The internal combustion engine is a V-type engine or a horizontally opposed engine having two banks, and a cylinder group in each of the two banks is connected to each of the two exhaust passages. The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 4.   The internal combustion engine is an in-line multi-cylinder engine, and a plurality of cylinders constituting the in-line multi-cylinder engine are divided into two cylinder groups that do not cause exhaust interference, and each of the two cylinder groups has the two exhaust passages. The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 4, wherein the exhaust gas purification system is connected to each of the engine. A purification-time NOx occlusion amount estimation means for estimating an NOx occlusion amount occluded in another NOx storage reduction type NOx catalyst when fuel is supplied to the one NOx storage reduction type NOx catalyst by the NOx purification means;
Based on the NOx occlusion amount estimated by the purification NOx occlusion amount estimation means, when fuel is supplied to the one NOx occlusion reduction NOx catalyst by the NOx purification means, the other NOx occlusion reduction NOx catalyst Determination means for determining whether or not the NOx occlusion amount exceeds the predetermined occlusion amount;
When the determination means determines that the NOx storage amount of the other NOx storage reduction catalyst exceeds the predetermined storage amount, the value of the predetermined storage amount for the one NOx storage reduction catalyst is set to an initial value. Is corrected to a value obtained by subtracting a value equal to or greater than a predetermined correction amount, or the NOx occlusion amount of the one NOx storage reduction catalyst is corrected to an NOx occlusion amount obtained by adding an amount equal to or greater than the predetermined correction amount to the initial amount. Means,
The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 6, further comprising:   The predetermined correction amount is calculated from the NOx occlusion amount estimated by the purification NOx occlusion amount estimator and the NOx occlusion amount of the first NOx storage reduction catalyst and the NOx occlusion amount of the other NOx storage reduction catalyst. The exhaust gas purification system for an internal combustion engine according to claim 7, wherein the exhaust gas purification system is an amount obtained by subtracting the difference. In an exhaust gas purification system for an internal combustion engine in which the cylinders of the internal combustion engine are divided into two cylinder groups having common exhaust, and the NOx storage reduction catalyst is provided in two exhaust passages connected to each cylinder group,
Three NOx storage reduction catalysts are provided in parallel with the exhaust passage, and each of the two exhaust passages branches to allow the exhaust to separate into the three NOx storage reduction catalysts. Connected to three NOx storage reduction catalysts,
An exhaust flow rate adjusting means that adjusts an exhaust flow rate that flows into each of the NOx storage reduction catalyst that is provided in each of the two exhaust passages and that is connected to one exhaust passage;
Fuel supply means provided in an exhaust passage on the upstream side of the three NOx storage reduction catalysts, and for supplying fuel to each of the NOx storage reduction catalysts;
The reduction of the exhaust flow rate to the one storage reduction type NOx catalyst by the exhaust flow rate adjusting means and the supply of fuel to the one storage reduction type NOx catalyst by the fuel supply means are sequentially performed. And NOx purification means that flows in the exhaust gas from the two exhaust passages into the remaining two NOx storage reduction catalysts,
An exhaust gas purification system for an internal combustion engine, comprising:

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