JP5668737B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

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

  As is well known, there is a so-called urea SCR (Selective Catalytic Reduction) as a technique for purifying NOx (nitrogen oxide) discharged from an internal combustion engine (for example, a diesel engine). In the urea SCR system, a reducing agent (NH3 (ammonia)) is supplied to the catalyst (SCR catalyst) by adding an aqueous urea solution (urea water) to the exhaust gas, and the catalyst causes a reduction reaction to convert NOx into nitrogen. Decompose and purify into water.

  Various proposals have been made regarding these technologies. Japanese Patent Application Laid-Open No. 2004-228561 discloses a technology that can appropriately control the amount of urea or NH3 added to the NOx catalyst in a configuration including a NOx catalyst that adds urea or NH3 as a reducing agent.

Japanese Patent No. 4305543

  SCR catalysts are known to store NH3 and to purify NOx with the stored NH3. As illustrated in FIG. 6, the maximum possible storage amount (storable amount) of NH 3 in the SCR catalyst has a property that it usually decreases as the temperature of the SCR catalyst increases. The target storage amount of NH3 in the SCR catalyst to be considered when determining the addition amount of urea water is set within a range not exceeding this storable amount. Therefore, in the prior art, for example, as shown in FIG. 7, the target storage amount of NH 3 in the SCR catalyst is set as a numerical value obtained by subtracting the storable amount by a certain margin (margin). Here, the margin is set in order to cope with the temperature change.

  When the target storage amount is set as described above, for example, when the vehicle suddenly accelerates, the exhaust temperature rapidly rises, and the SCR catalyst temperature also rises due to the sudden rise. Therefore, as shown in FIG. 8, even if a certain amount of margin is set, the current storage amount may exceed the storable amount at the rapidly rising catalyst temperature. In this case, NH3 (ammonia) that cannot be stored by the SCR catalyst flows downstream, and so-called NH3 slip occurs.

  It is not desirable from the viewpoint of exhaust purification that NH3 released by NH3 slip is discharged outside the vehicle. In the prior art, the oxidation catalyst disposed downstream of the SCR catalyst plays a role in purifying NH3. However, in general, a catalyst functions properly at a temperature higher than the activation temperature inherent to the catalyst, but does not function properly at a temperature lower than the activation temperature (or the function is greatly reduced).

  Therefore, even if an oxidation catalyst is installed downstream of the SCR catalyst, if NH3 slip occurs at a temperature lower than the activation temperature of the oxidation catalyst, the oxidation catalyst cannot purify NH3, so NH3 deteriorates emissions. . Therefore, an exhaust purification method in which NH 3 slip hardly occurs at a temperature lower than the activation temperature of the oxidation catalyst is desired. However, in the related art including the above-mentioned Patent Document 1, the above-mentioned problem relating to the activation temperature of the oxidation catalyst is not recognized even when measures against NH3 slip are taken into consideration.

  Therefore, in view of the above, the problem to be solved by the present invention is that the oxidation catalyst is lower than the activation temperature in the configuration in which the oxidation catalyst for purifying the reducing agent is disposed downstream of the catalyst for storing the reducing agent and purifying NOx. An object of the present invention is to provide an exhaust purification device for an internal combustion engine that appropriately copes with the fact that it does not function effectively at temperature.

To achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention includes a first catalyst (4) provided in an exhaust passage of the internal combustion engine and having a function of purifying NOx, and the first catalyst. And a supply means (6) provided upstream of the exhaust passage for supplying a reducing agent for NOx purification to the first catalyst, and disposed downstream of the first catalyst from the exhaust passage. A target value of the storage amount of the first catalyst in the first catalyst of the reducing agent supplied by the supply means, the target value and the maximum storable amount in the first catalyst Is set so that the margin, which is the difference between the two, is relatively large in a region where the temperature of the first catalyst is lower than the activation temperature of the second catalyst and relatively small in a region where the temperature is higher than the activation temperature. Means (7, S50, S60); Based on the premise that with. Thereby, when the second catalyst is lower than the activation temperature, it is possible to prevent the reducing agent from being released downstream from the first catalyst even if a sudden temperature rise or the like occurs. Therefore, the deterioration of the emission due to the release of the reducing agent is appropriately suppressed.

1 is a schematic diagram of a system configuration in an embodiment of an exhaust purification system according to the present invention. The flowchart which shows the example of the process sequence of an exhaust gas purification system. The figure which shows the 1st example of the setting of the target storage amount of a reducing agent. The figure which shows the 2nd example of the setting of the target storage amount of a reducing agent. The figure which shows the 3rd example of the setting of the target storage amount of a reducing agent. The figure which shows the example of the relationship between SCR catalyst temperature and NH3 storable amount. The figure which shows the example of the setting of the target storage amount in a prior art. The figure which shows the generation | occurrence | production cause of NH3 slip.

  Embodiments of the present invention will be described below with reference to the drawings. First, FIG. 1 is a schematic diagram of an apparatus configuration in an embodiment of an exhaust purification system 1 (hereinafter, system) according to the present invention.

  The system 1 includes, for example, a urea SCR catalyst 4 (SCR catalyst) and an oxidation catalyst 5 in order from the upstream side in an exhaust pipe 3 of an engine 2 (for example, a diesel engine) mounted on an automobile. An addition valve 6 is provided upstream of the SCR catalyst 4. The addition valve 6 includes a urea tank 60 in which urea water is stored, and urea water supplied from the urea tank 60 is added (injected) into the exhaust pipe 3 by the addition valve 6.

  The SCR catalyst 4 has a well-known structure as a catalyst for urea SCR, that is, a structure in which a metal catalyst is supported on the surface of a suitable base material made of, for example, zeolite. The urea water added by the addition valve 6 is hydrolyzed by, for example, the heat of the exhaust gas to generate NH3, which is adsorbed (stored) on the SCR catalyst 4. NH3 stored in the SCR catalyst 4 reduces and purifies NOx in the exhaust gas to nitrogen and water.

  The oxidation catalyst 4 is a catalyst having an oxidation function, and has a function of burning and purifying components such as CO (carbon monoxide) and HC in the exhaust gas, but has flowed from the SCR catalyst 4 in the present invention. Has the function of purifying the reducing agent. Further, an exhaust temperature sensor 30 and a NOx sensor 31 are disposed upstream and downstream of the SCR catalyst 4 in the exhaust pipe 3. The exhaust temperature sensor 30 and the NOx sensor 31 detect the exhaust temperature and NOx in the exhaust at the respective arrangement positions.

  The system 1 includes an ECU 7 (Electronic Control Unit) according to the present invention. The ECU 7 has a structure similar to that of a normal computer, that is, a CPU that performs information processing such as various operations, a RAM as a work area of the CPU, a non-volatile memory 70 that stores various necessary programs and data, and the like. The ECU 7 acquires detection values of the exhaust temperature sensor 30 and the NOx sensor 31, and controls the fuel injection amount in the engine 2, the urea water addition amount from the addition valve 6, and the like.

  Under the above configuration, the system 1 executes processing related to NOx purification. The processing procedure is shown in FIG. The processing procedure of FIG. 2 may be programmed in advance and stored in the memory 70, and the ECU 7 may be automatically called and executed.

  In the procedure of FIG. 2, first, the ECU 7 takes in the exhaust gas temperature in S10. Specifically, the measured value of the exhaust temperature sensor 30 is acquired. Next, in S20, the ECU 7 calculates the (internal) temperature Tscr of the SCR catalyst 4. Specifically, a mathematical model for calculating the (internal) temperature of the SCR catalyst from the exhaust temperature may be stored in the memory 70 and calculated from the model and the actual value of the exhaust temperature acquired in S10.

  Next, in S30, the ECU 7 calculates the activation temperature Tdoc of the oxidation catalyst 5. The numerical value calculated here may be the activation temperature of the oxidation catalyst 5 itself, but more preferably, the internal temperature of the SCR catalyst 4 when the internal temperature of the oxidation catalyst 5 is the activation temperature may be calculated. . In the latter case, specifically, a normal value (or an average value) of the temperature difference between the internal temperature of the SCR catalyst 4 and the internal temperature of the oxidation catalyst 5 is obtained, and the numerical value of the temperature difference is calculated. What is necessary is just to add to the activation temperature of the oxidation catalyst 5. Such Tdoc and the activation temperature of the oxidation catalyst 5 may be stored in advance in the memory 70 and recalled to the RAM in S30.

  As is well known, since the catalyst has a property that changes whether or not the catalyst function is exerted almost continuously depending on whether the temperature is higher or lower than a certain temperature, the boundary temperature is defined as the activation temperature (or activation temperature). do it. Alternatively, in this embodiment, the activation temperature (activation temperature) of the oxidation catalyst 5 is more strictly defined as the catalyst temperature at which the NH3 purification rate becomes a predetermined percentage (for example, 50%, 80%, 90%, etc.). May be. Usually, the activation temperature of the oxidation catalyst 5 is in the range of 200 to 250 degrees Celsius.

  Next, in S40, the ECU 7 compares the Tscr calculated in S20 with the Tdoc calculated (called) in S30. When Tscr is greater than Tdoc (S40: Yes), the process proceeds to S60, and when Tscr is equal to or less than Tdoc (S40: No), the process proceeds to S50.

  In S50 and S60, the ECU 7 sets a target storage amount of NH3 in the SCR catalyst 4. When the process proceeds to S50, a target storage amount of NH3 at a temperature at which the oxidation catalyst 5 is not activated is set. Since the oxidation catalyst 5 is not activated, as described above, when NH3 slip occurs in the SCR catalyst 4 due to a rapid temperature increase for some reason, for example, sudden acceleration, the NH3 purification in the oxidation catalyst 5 is performed. Is not considered. Therefore, in S50, the margin from the storable amount of the target storage amount of NH3 in the SCR catalyst 4 is increased.

  On the other hand, when it progresses to S60, the target storage amount of NH3 in the temperature which the oxidation catalyst 5 has activated is set. In this case, since the oxidation catalyst 5 is activated, as described above, even if NH3 slip occurs in the SCR catalyst 4, it can be regarded that the purification of NH3 in the oxidation catalyst 5 is performed. Accordingly, in S60, the aim is to store as much NH3 as possible in order to reduce the margin from the storable amount of the target storage amount of NH3 in the SCR catalyst 4 and increase the NOx purification rate. A specific example of the target storage amount setting in S50 and S60 will be described later.

  Subsequently, in S70 to S90, the ECU 7 performs a series of processes for calculating the urea water addition amount from the addition valve 6. First, in S70, the ECU 7 estimates the current value of the NH3 storage amount in the SCR catalyst 4. Specifically, starting from a time point when the stored amount is regarded as zero, (a) NOx emission amount from the engine 2, (b) NOx outflow amount downstream of the SCR catalyst 4, (c) SCR catalyst 4 Each amount of NOx purification amount at (d), (d) NH3 consumption amount at the SCR catalyst 4 and (e) NH3 supply amount to the SCR catalyst 4 is repeatedly calculated and integrated, for example, at a predetermined period.

  The calculation of each amount will be specifically described. (A) The NOx emission amount from the engine 2 is calculated from the operating state (load, rotation speed) of the engine 2 by, for example, a map. A map showing the relationship between the operating state (load, rotation speed) of the engine 2 and the NOx emission amount may be obtained in advance and stored in the memory 70. (B) The NOx outflow amount to the downstream of the SCR catalyst 4 may be obtained by measuring the NOx sensor 31.

  (C) The NOx purification amount in the SCR catalyst 4 can be calculated by subtracting the (b) NOx outflow amount downstream of the SCR catalyst 4 from the (a) NOx emission amount from the engine 2 obtained above. (D) The NH3 consumption amount in the SCR catalyst 4 is calculated from the (c) NOx purification amount in the SCR catalyst 4 obtained above. What is necessary is just to memorize | store in the memory 70 beforehand the numerical formula, map, etc. which show the relationship between the NOx purification amount in the SCR catalyst 4, and NH3 consumption.

  (E) In the calculation of the NH 3 supply amount to the SCR catalyst 4, the NH 3 amount generated and supplied to the SCR catalyst 4 is calculated from the urea water addition amount from the addition valve 6. The mathematical formula for calculating the amount of NH 3 generated from the urea water addition amount may be stored in the memory 70 in advance. Finally, (d) NH3 storage amount in the SCR catalyst 4 (estimated value) is obtained by subtracting the NH3 consumption amount in the SCR catalyst 4 from the (e) NH3 supply amount to the SCR catalyst 4 obtained as described above. ) Is calculated.

  Next, in S80, the ECU 7 calculates a deviation between the target storage amount in the SCR catalyst 4 and the current NH3 storage amount (estimated value thereof). That is, the current storage amount (estimated value) calculated in S70 is subtracted from the target storage amount set in S50 or S60.

  Subsequently, in S90, the ECU 7 calculates the urea water addition amount for storing the deviation calculated in S80 in the SCR catalyst 4 (in addition to the storage amount at the current time point). Specifically, similarly to (e) in S70 above, a mathematical expression indicating the relationship between the urea water addition amount and the NH3 amount generated therefrom is stored in the memory 70 in advance, and calculation is performed using the relationship. do it.

  Finally, in S100, the ECU 7 instructs the addition valve 6 to actually add the urea water addition amount calculated in S90. The above is the processing procedure of FIG.

  3 to 5 show specific examples of setting the NH3 target storage amount in the SCR catalyst 4 in S50 and S60 of FIG. In either case, the horizontal axis represents the SCR catalyst temperature Tscr, and the vertical axis represents the NH3 storage amount (target storage amount or storable amount). The oxidation catalyst activation temperature shown in the horizontal axis is the above-mentioned Tdoc. Therefore, in FIG. 3 to FIG. 5, the portion where the temperature is lower than the oxidation catalyst activation temperature (the left portion in the drawing) is the region set in S50, and the portion where the temperature is higher than the oxidation catalyst activation temperature (the right side in the drawing). Is a region set in S60.

  In any of FIGS. 3 to 5, as described in S50 and S60, in the region where the temperature is lower than the oxidation catalyst activation temperature, there is a margin from the storable amount of the NH3 target storage amount. In the region where the temperature is higher than the oxidation catalyst activation temperature, the NH3 slip can be considered to be purified by the oxidation catalyst 5 in the region where the temperature is higher than the oxidation catalyst activation temperature. Aiming at higher NOx purification by the amount of stored NH3, the margin is made relatively small.

  Based on such a basic concept, in the setting example of FIG. 3, the margin is set to be larger as the temperature is lower in the region where the SCR catalyst temperature is lower than the oxidation catalyst activation temperature. (The margin (minute) may be a numerical value obtained by subtracting the target storage amount from the storable amount, or a numerical value expressed as a percentage (%).) By setting as shown in FIG. The possibility of NH3 slip occurring in the low-temperature region can be reduced by a large margin in the low-temperature region below the activation temperature (compared to the case where the margin is constant regardless of the temperature).

  In FIG. 3, the margin is continuously increased, whereas in the setting example of FIG. 4, the margin is discontinuous from the region where the SCR catalyst temperature is higher to the region where the SCR catalyst temperature is lower than the oxidation catalyst activation temperature. Has increased. As a result, the margin can be further increased in the low temperature region below the oxidation catalyst activation temperature, and the possibility of NH3 slip occurring in the low temperature region can be further reduced.

  In the setting example of FIG. 5, the target storage amount is set to a constant value in the low temperature region. The essential point in the example of FIG. 5 is that the target storage amount is set to be lower than the straight line L illustrated by the one-dot chain line in the region where the SCR catalyst temperature is lower than the oxidation catalyst activation temperature. . The straight line L is a line obtained by extending the storable amount at the point where the SCR catalyst temperature is the oxidation catalyst activation temperature to the left side in the figure.

  Obviously, if the target storage amount is set so as to be below the straight line L, NH3 slip does not occur in a region lower than the oxidation catalyst activation temperature. Therefore, when NH3 slip occurs, it is limited to a temperature range higher than the oxidation catalyst activation temperature. In that case, the oxidation catalyst 5 is activated, and even if NH3 slip occurs, NH3 is purified by the oxidation catalyst 5. Is considered. Therefore, the example of FIG. 5 is an example in which it is considered that the emission does not deteriorate even if NH3 slip occurs. (However, in the case of FIG. 5, the amount of NH 3 stored is low in the low temperature region, and as a result, the NOx purification performance is also low. FIGS. 3 and 4 are superior for NOx purification at low temperatures.)

  In addition, you may combine the idea of FIG.3, FIG.4, FIG.5 arbitrarily. That is, the margin is set to be larger as the temperature is lower in the region where the SCR catalyst temperature is lower than the oxidation catalyst activation temperature, and the region where the SCR catalyst temperature is higher from the region lower than the oxidation catalyst activation temperature. When the SCR catalyst temperature is lower than the oxidation catalyst activation temperature, the target storage amount is set below the storable amount at the point where the SCR catalyst temperature is the oxidation catalyst activation temperature. Any or all of these may be combined.

  In addition, the said Example can be suitably changed in the range which does not deviate from the meaning described in the claim. For example, the engine 2 may be a lean burn gasoline engine as well as a diesel engine. FIGS. 3 to 5 are merely examples of setting the target storage amount. The target storage amount of NH3 in the SCR catalyst 4 is a margin that is the difference between the target storage amount and the maximum storable amount in the SCR catalyst 4. What is necessary is just to set so that the temperature of the SCR catalyst 4 may be relatively high in a region lower than the activation temperature of the oxidation catalyst 5 and relatively low in a region higher than the activation temperature.

3 Exhaust pipe (exhaust passage)
4 SCR catalyst (NOx catalyst, first catalyst)
5 Oxidation catalyst (second catalyst)
6 Addition valve (supply means)

Claims (4)

A first catalyst (4) provided in an exhaust passage of the internal combustion engine and having a function of purifying NOx;
A supply means (6) provided upstream of the first catalyst in the exhaust passage for supplying a reducing agent for NOx purification to the first catalyst;
A second catalyst (5) disposed downstream of the first catalyst than the first catalyst and having a function of purifying the reducing agent;
The target value of the storage amount of the reducing agent supplied by the supply means in the first catalyst is a margin that is the difference between the target value and the maximum storable amount in the first catalyst. Setting means (7, S50, S60) that is set to be relatively large in a region lower than the activation temperature of the second catalyst and relatively small in a region higher than the activation temperature;
Equipped with a,
The setting means (7, S50, S60) sets the margin discontinuously large from a temperature range higher than the activation temperature of the second catalyst to a temperature range lower than the activation temperature. An exhaust purification device for an internal combustion engine.   The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the setting means (7, S50, S60) sets the margin larger as the catalyst temperature is lower in a temperature region lower than the activation temperature of the second catalyst. . It said setting means (7, S50, S60), in a temperature region lower than the activation temperature of the second catalyst, the target value, according to claim 1 is set lower than the maximum storable amount in the activation temperature or 2 2. An exhaust gas purification apparatus for an internal combustion engine according to 1. The information on the activation temperature of the second catalyst used in the setting means (7, S50, S60) is a numerical value obtained by adding the temperature difference between the first catalyst and the second catalyst to the activation temperature of the second catalyst. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3 .

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