JP2008303759A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately estimate an NH3 adsorption retaining amount of a NOx selective reduction catalyst in an exhaust emission control device of an internal combustion engine that suppresses the emission of NOx by arranging the NOx selective reduction catalyst at downstream of a catalyst having a NOx emission control function. <P>SOLUTION: In an exhaust passage 12 of the internal combustion engine 10, a three-way catalyst 14, an NSR 16, and an SCR 18 are arranged in this order. An ECU 30 includes a memory region for storing a calculation value of the NH3 adsorption retaining amount of the SCR 18. The NH3 adsorption amount adsorbed by the SCR 18 in exhaust gas is calculated. The NH3 consumption amount consumed by the SCR 18 in the exhaust gas upon reducing NOx is calculated. The NH3 purge amount purged from the SCR 18 is calculated. The calculation value of the NH3 adsorption retaining amount is updated based on the NH3 adsorption amount, NH3 consumption, and NH3 purge amount. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus for an internal combustion engine that suppresses NOx emission by disposing a NOx selective reduction catalyst downstream of a catalyst having a NOx purification function.

  Conventionally, as disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-293737, a system including a NOx selective reduction catalyst in an exhaust passage of an internal combustion engine is known. The NOx selective reduction catalyst has a function of adsorbing ammonia NH3, and NOx in the exhaust gas can be reduced (purified) using NH3 as a reducing material.

  More specifically, the conventional system includes a NOx selective reduction catalyst in the exhaust passage of the internal combustion engine, and a reducing material supply mechanism for supplying NH3 or urea water to the NOx selective reduction catalyst. When the internal combustion engine is operated at a lean air-fuel ratio, exhaust gas containing NOx is exhausted. At this time, the NOx selective reduction catalyst releases NH3 stored in advance and reduces NOx in the exhaust gas (purifies to N2 and H2O).

  In the conventional system described above, if the amount of NH3 supplied to the NOx selective reduction catalyst is excessive, a situation occurs in which exhaust gas containing NH3 is discharged downstream. Further, if the amount of NH3 supplied to the NOx selective reduction catalyst is too small, a sufficient reducing material (NH3) cannot be obtained, so that a situation in which NOx cannot be sufficiently purified occurs. Therefore, in order to obtain good emission characteristics in this system, it is important to supply an appropriate amount of NH3 to the NOx selective reduction catalyst.

  The conventional system estimates the amount of NH3 (NH3 adsorption / retention amount) occluded in the NOx selective reduction catalyst in order to realize an appropriate amount of NH3 supply. Specifically, the amount of NH3 adsorbed and retained therein is estimated by subtracting the amount of NH3 consumed by the NOx selective reduction catalyst (NH3 consumption) from the amount of NH3 supplied to the NOx selective reduction catalyst.

  The NOx selective reduction catalyst consumes NH3 when reducing NOx in the exhaust gas. The amount of NOx reduced inside the NOx selective reduction catalyst can be obtained by multiplying the NOx amount flowing into the NOx selective reduction catalyst from the internal combustion engine (NOx inflow amount) by the NOx purification rate of the NOx selective reduction catalyst. . Therefore, NH3 consumption can be calculated based on the multiplication value.

  The conventional system calculates the NH3 adsorption / retention amount by the above method, and controls the supply amount of NH3 so that the value becomes an appropriate value. According to such a method, the NH3 adsorption / retention amount inside the NOx selective reduction catalyst can always be set to an appropriate value, and good emission characteristics can be realized.

JP 2003-293737 A JP 2003-314256 A

  However, in the NOx selective reduction catalyst, the occluded NH3 may be purged as the exhaust gas flows. Therefore, in order to accurately estimate the NH3 adsorption / holding amount in the catalyst, it is necessary to consider the NH3 purge amount in addition to the NH3 adsorption / holding amount and the NH3 consumption amount. In the conventional system, the NH3 adsorption / retention amount is estimated without considering the NH3 purge amount by the exhaust gas. For this reason, the conventional system described above leaves room for further improvement in terms of the estimation accuracy of the NH3 adsorption retention amount.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an exhaust purification device for an internal combustion engine that can accurately estimate the amount of NH3 adsorbed and retained by the NOx selective reduction catalyst. .

In order to achieve the above object, a first invention is an exhaust purification device for an internal combustion engine,
A first catalyst having a function of purifying NOx in exhaust gas;
A NOx selective reduction catalyst disposed downstream of the first catalyst;
NH3 adsorption / retention amount storage means for storing the calculated value of NH3 adsorption / retention amount in the NOx selective reduction catalyst;
NH3 adsorption amount calculating means for calculating the NH3 adsorption amount adsorbed from the exhaust gas by the NOx selective reduction catalyst;
NH3 consumption calculation means for calculating NH3 consumption consumed as the NOx selective reduction catalyst reduces NOx in the exhaust gas;
NH3 purge amount calculating means for calculating the NH3 purge amount purged by the gas flowing through the NOx selective reduction catalyst;
NH3 adsorption / retention amount update means for updating the calculated value of NH3 adsorption / retention amount stored in the NH3 adsorption / retention amount storage means based on the NH3 adsorption amount, the NH3 consumption amount, and the NH3 purge amount;
It is characterized by providing.

The second invention is the first invention, wherein
The NH3 adsorption amount calculating means is
NH3 inflow amount calculation means for calculating the NH3 inflow amount flowing into the NOx selective reduction catalyst, and NH3 adsorption rate calculation means for calculating the NH3 adsorption rate of the NOx selective reduction catalyst, the NH3 inflow amount and the NH3 Calculate the NH3 adsorption amount based on the multiplication value with the adsorption rate,
The NH3 consumption calculation means is:
NOx inflow amount calculating means for calculating the NOx inflow amount flowing into the NOx selective reduction catalyst, and NOx purification rate calculating means for calculating the NOx purification rate of the NH3 selective reduction catalyst, the NOx inflow amount and the NOx Calculate the NH3 consumption based on the product of the purification rate,
The NH3 purge amount calculation means
Gas flow rate detecting means for detecting a gas flow rate flowing through the NOx selective reduction catalyst, and NH3 purge rate calculating means for calculating a purge rate for purging NH3 with the gas flow by the NOx selective reduction catalyst, The NH3 purge amount is calculated on the basis of a calculated value of the NH3 adsorption retention amount, the purge rate, and the gas flow rate.

The third invention is the second invention, wherein
The purge rate calculating means includes
Oxygen concentration detection means for detecting the oxygen concentration at the inlet of the NOx selective reduction catalyst;
A purge rate rule storage unit that stores a purge rate rule that defines the relationship between the calculated value of the NH3 adsorption retention amount and the oxygen concentration, and the purge rate, and
The purge rate is calculated according to the purge rate rule.

According to a fourth invention, in any one of the first to third inventions,
Comparing means for comparing the calculated value of the NH3 adsorption retention amount with its target value;
When the calculated value of the NH3 adsorption retention amount is excessive with respect to the target value, at least one of the air-fuel ratio of the internal combustion engine, the temperature of the first catalyst, and the NOx amount in the first catalyst is the NH3 When the operating state of the internal combustion engine is changed so as to change the amount of adsorption, and on the other hand, the calculated value of the NH3 adsorption retention amount is too small relative to the target value, the air-fuel ratio of the internal combustion engine, the first Control means for changing the operating state of the internal combustion engine so that at least one of the temperature of one catalyst and the amount of NOx in the first catalyst changes in a direction to increase the amount of adsorption of NH3;
It is characterized by providing.

According to a fifth invention, in any one of the first to fourth inventions,
Comparing means for comparing the calculated value of the NH3 adsorption retention amount with its target value;
When the calculated value of the NH3 adsorption retention amount is excessive with respect to the target value, at least one of the NOx inflow amount flowing into the NOx selective reduction catalyst and the NOx purification rate of the NH3 selective reduction catalyst is the NH3 When the operating state of the internal combustion engine is changed so as to increase the consumption amount, and the calculated value of the NH3 adsorption / holding amount is too small with respect to the target value, the NOx inflow amount and the NOx purification Control means for changing the operating state of the internal combustion engine so that at least one of the rates changes in a direction to reduce the NH3 consumption;
It is characterized by providing.

According to a sixth invention, in any one of the first to fifth inventions,
Comparing means for comparing the calculated value of the NH3 adsorption retention amount with its target value;
A direction in which at least one of the air-fuel ratio of the internal combustion engine and the gas flow rate flowing through the NOx selective reduction catalyst increases the NH3 purge amount when the calculated value of the NH3 adsorption retention amount is excessive with respect to the target value. When the operating state of the internal combustion engine is changed so that the calculated value of the NH3 adsorption retention amount is excessively smaller than the target value, at least one of the air-fuel ratio of the internal combustion engine and the gas flow rate is changed. One of the control means for changing the operating state of the internal combustion engine to change in a direction to reduce the NH3 purge amount,
It is characterized by providing.

According to a seventh invention, in any one of the first to sixth inventions,
The NH3 purge amount calculating means includes a gas flow rate detecting means for detecting a gas flow rate flowing through the NOx selective reduction catalyst, and an NH3 purge for calculating a purge rate at which the NOx selective reduction catalyst purges NH3 with a gas flow. Rate calculating means, and calculating the NH3 purge amount based on a calculated value of the NH3 adsorption retention amount, the purge rate, and the gas flow rate,
NH3 concentration prediction means for calculating an NH3 concentration prediction value downstream of the NOx selective reduction catalyst based on the gas flow rate and the NH3 purge amount;
An NH3 sensor that measures the NH3 concentration downstream of the NOx selective reduction catalyst;
Calculation method correction means for correcting the NH3 adsorption retention amount calculation method so that the difference between the NH3 concentration predicted value concentration and the measured value of NH3 concentration by the NH3 sensor is small,
It is characterized by providing.

Further, an eighth invention is any one of the first to seventh inventions,
NOx inflow amount calculating means for calculating the NOx inflow amount flowing into the NOx selective reduction catalyst;
NOx purification rate calculating means for calculating the NOx purification rate of the NH3 selective reduction catalyst;
NOx emission calculation means for calculating NOx emission downstream of the NOx selective reduction catalyst by subtracting the product of the NOx inflow amount and the NOx purification rate from the NOx inflow amount;
It is characterized by providing.

According to a ninth invention, in any one of the first to eighth inventions,
An emission comparison means for comparing the calculated value of the NOx emission and the emission target value;
Control means for changing the operating state of the internal combustion engine so that the emission calculation value matches the emission target value;
It is characterized by providing.

  According to the first invention, the first catalyst generates NH3 along with the purification of NOx. The produced NH3 flows into the NOx selective reduction catalyst. In the present invention, the calculated value of the NH3 adsorption holding amount is updated based on the NH3 adsorption amount, the NH3 consumption amount, and the NH3 purge amount. Therefore, according to the present invention, it is possible to estimate the NH3 adsorption / retention amount inside the NOx selective reduction catalyst with extremely high accuracy.

  According to the second invention, the NH3 adsorption amount can be calculated easily and with high accuracy based on the multiplication value of the NH3 inflow amount flowing into the NOx selective reduction catalyst and the NH3 adsorption rate of the NOx selective reduction catalyst. it can. Further, the NH3 consumption can be calculated easily and with high accuracy based on the product of the NOx inflow amount flowing into the NOx selective reduction catalyst and the NOx purification rate of the NH3 selective reduction catalyst. Furthermore, the NH3 purge amount can be easily and accurately calculated based on the product of the calculated value of the NH3 adsorption retention amount, the gas flow rate through the NOx selective reduction catalyst, and the NH3 purge rate inside the NOx selective reduction catalyst. It can be calculated.

  According to the third aspect, the oxygen concentration at the inlet of the NOx selective reduction catalyst can be detected. Further, the purge rate of the NOx selective reduction catalyst can be calculated easily and with high accuracy in accordance with the purge rate rule, based on the oxygen concentration thus detected and the calculated value of the NH3 adsorption retention amount. .

  According to the fourth aspect of the invention, when the calculated value of the NH3 adsorption holding amount is excessive with respect to the target value, the operating state of the internal combustion engine is changed so that the NH3 adsorption amount decreases. Further, when the calculated value of the NH3 adsorption holding amount is too small with respect to the target value, the operating state of the internal combustion engine is changed so that the NH3 adsorption amount increases. Therefore, according to the present invention, the NH3 adsorption / retention amount of the NOx selective reduction catalyst can be brought close to the target value.

  According to the fifth aspect of the invention, when the calculated value of the adsorption amount of NH3 is excessive with respect to the target value, the operating state of the internal combustion engine is changed so that the NH3 consumption amount increases. When the calculated value of the NH3 adsorption / holding amount is too small with respect to the target value, the operating state of the internal combustion engine is changed so that the NH3 consumption is reduced. Therefore, according to the present invention, the NH3 adsorption / retention amount of the NOx selective reduction catalyst can be brought close to the target value.

  According to the sixth aspect of the invention, when the calculated value of the NH3 adsorption / holding amount is excessive with respect to the target value, the operating state of the internal combustion engine is changed so that the NH3 purge amount increases. Further, when the calculated value of the NH3 adsorption / holding amount is too small with respect to the target value, the operating state of the internal combustion engine is changed so that the NH3 purge amount is reduced. Therefore, according to the present invention, the NH3 adsorption / retention amount of the NOx selective reduction catalyst can be brought close to the target value.

  According to the seventh aspect of the invention, the calculation method of the NH3 adsorption / retention amount can be modified so that the difference between the predicted value and the actual measurement value of the NH3 concentration becomes small. The error in the predicted NH3 concentration is due to the error in the calculated NH3 adsorption amount. For this reason, if the error of the predicted value of the NH3 concentration is reduced, the error of the calculated value of the NH3 adsorption retention amount is also reduced. Therefore, according to the present invention, the NH3 adsorption / retention amount of the NOx selective reduction catalyst can be calculated with high accuracy.

  According to the eighth aspect of the invention, the NOx amount reduced by the NOx selective reduction catalyst can be obtained as a product of the NOx inflow amount and the NOx purification rate. Further, by subtracting the multiplied value from the NOx inflow amount, the NOx emission downstream of the NOx selective reduction catalyst can be calculated with high accuracy.

  According to the ninth aspect, the calculated value of NOx emission can be obtained, and the operating state of the internal combustion engine can be changed so that the calculated value matches the emission target value. For this reason, according to the present invention, it is possible to make both the motion characteristics and the emission characteristics of the internal combustion engine compatible at a high level by setting the NOx emission to an appropriate level.

Embodiment 1 FIG.
[Configuration of Embodiment 1]
FIG. 1 is a diagram for explaining the configuration of the first embodiment of the present invention. As shown in FIG. 1, the system of the present embodiment includes an internal combustion engine 10. An exhaust passage 12 communicates with the internal combustion engine 10. In the exhaust passage, a three-way catalyst 14, an NSR (NOx storage reduction catalyst) 16, and an SCR (NOx selective reduction catalyst) 18 are arranged in this order.

  The internal combustion engine 10 easily discharges HC and CO when the air-fuel ratio is rich, and easily discharges NOx when the air-fuel ratio is lean. The three-way catalyst 14 reduces NOx (purifies to N2) while adsorbing oxygen in a lean atmosphere. On the other hand, in a rich atmosphere, HC and CO are oxidized (purified to H2O and CO2) while releasing oxygen. In a rich atmosphere, ammonia NH3 is generated by the reaction of nitrogen contained in the exhaust gas with hydrogen.

  The NSR 14 adsorbs NOx contained in the exhaust gas in a lean atmosphere. Further, the NSR 14 releases NOx stored in a rich atmosphere. NOx released under a rich atmosphere is reduced by HC and CO. At this time, NH 3 is also generated in the NSR 14 as in the case of the three-way catalyst 14.

  The SCR 16 has a function in which the three-way catalyst 14 and the NSR 16 occlude NH3 produced in a rich atmosphere, and in a lean atmosphere, NH3 is used as a reducing material and NOx in the exhaust gas is selectively reduced. . According to the SCR 18, it is possible to effectively prevent NH3 and NOx that have blown through the subsequent stage of the NSR 16 from being released into the atmosphere.

  The system shown in FIG. 1 includes an air-fuel ratio (A / F) sensor 20 on the upstream side of the three-way catalyst 14. According to the A / F sensor 20, the exhaust air-fuel ratio of the internal combustion engine 10 can be measured. Further, the system shown in FIG. 1 includes an NH 3 sensor 22 on the upstream side of the SCR 18. The NH3 sensor 22 reacts with NOx and NH3 in the exhaust gas and generates a signal corresponding to their concentration. Therefore, according to the NH3 sensor 22, the NH3 concentration at the inlet of the SCR 18 can be detected under a rich atmosphere, and the NOx concentration at the inlet of the SCR 18 can be detected under a lean atmosphere.

  The system shown in FIG. 1 further includes an air flow meter (AFM) 24 that detects the intake air amount Ga of the internal combustion engine 10. Since the gas flow rate flowing through the exhaust passage 12 is the same as the intake air amount Ga, the AFM 24 can detect the gas flow rate in the exhaust passage 12.

  The A / F sensor 20, the NH 3 sensor 22, and the AFM 24 are connected to an ECU (Electronic Control Unit) 30. The ECU 30 can control the state of the system shown in FIG. 1 based on outputs supplied from these sensors.

[Basic operation of the first embodiment]
FIG. 2 is a timing chart for explaining the basic operation of the system of this embodiment. Specifically, FIG. 2A shows an example of control of the exhaust air / fuel ratio of the internal combustion engine 10. FIG. 2B shows a period in which a relatively large amount of NOx is generated in the exhaust gas. FIG. 2C shows a change in the NOx occlusion holding amount in the NSR 16. FIG. 2D shows the amount of NH3 at the SCR 18 inlet.

  In the present embodiment, the ECU 30 normally operates the internal combustion engine 10 at a lean air-fuel ratio (lean operation) (see FIG. 2A). The internal combustion engine 10 discharges a relatively large amount of NOx during the lean operation (see FIG. 2B). As a result, during lean operation, NOx tends to flow into the NSR 16, and the NOx occlusion retention amount in the NSR increases with time (see FIG. 2C).

  If the exhaust gas containing a large amount of NOx continues to flow after the NSR 16 has occluded NOx to its full capacity, a large amount of NOx will blow through the subsequent stage of the NSR 16 and exhaust emissions will deteriorate. For this reason, when the NOx occlusion holding amount of the NSR 16 approaches the occlusion capacity, the ECU 30 performs rich spike control (see FIG. 2A).

  In the rich spike control, the exhaust air-fuel ratio of the internal combustion engine 10 is temporarily made rich. During this time, the internal combustion engine 10 discharges exhaust gas containing a large amount of HC and CO (see FIG. 2B). In this case, in the three-way catalyst 14, HC and CO are purified and NH3 is generated. Further, the NSR 16 exhausts the stored NOx when the exhaust gas is enriched. Accompanying this discharge, NH3 is also produced in NSR16. As a result, a large amount of NH3 is generated at the inlet of the SCR 18 (see FIG. 2D).

  The SCR 18 adsorbs NH3 generated during rich spike control, thereby increasing the amount of NH3 adsorbed and retained therein. When NOx flows into the SCR 18 during lean operation, the stored NH3 is released, and NH3 becomes a reducing material to purify NOx in the exhaust gas. As a result, NOx blow-through to the rear stage of the SCR 18 is prevented. In the system of the present embodiment, excellent emission characteristics are realized by repeating the above processing.

[Estimation of NH3 adsorption retention in SCR]
Next, a method in which the system of the present embodiment estimates the NH3 adsorption / retention amount of the SCR 18 will be described. As described above, the SCR 18 adsorbs NH3 generated by the three-way catalyst 14 and the NSR 16 during the rich spike control. Further, the SCR 18 consumes the occluded NH3 when purifying NOx flowing in during the lean operation. Furthermore, NH3 occluded in the SCR 18 is purged by a gas flowing through the inside thereof, particularly oxygen, and is also reduced by the influence thereof. Therefore, the NH3 adsorption / holding amount of the SCR 18 can be expressed by the following equation.

NH3 adsorption retention (t)
= NH3 adsorption retention (t-1) + NH3 adsorption (t)-NH3 consumption (t)-NH3 purge (t) (1)
However, t is a time and t-1 is a time one sampling period before the time t.

  FIG. 3 is a diagram for explaining a method of estimating the NH3 adsorption amount corresponding to the second term on the right side of the equation (1). The NH3 adsorption amount 40 per unit time in the SCR 18 is a multiplication value of the NH3 inflow amount 42 into the SCR 18 and the NH3 adsorption rate 44 of the SCR 18 (see reference numeral 46).

  The NH3 inflow 42 into the SCR 18 can be calculated by multiplying the NH3 concentration 48 at the inlet of the SCR 18 by the exhaust gas flow rate 50 (see reference numeral 52). In the system shown in FIG. 1, the NH 3 sensor 22 can detect the NH 3 concentration 48 at the inlet of the SCR 18. Further, since the exhaust gas flow rate is the same as the intake air amount, it can be detected by the AFM 24. Therefore, according to this system, the NH3 inflow amount 42 to the SCR 18 can be calculated.

  The NH3 adsorption rate 44 of the SCR 18 increases as the NH3 concentration in the gas flowing into the SCR 18 increases, and decreases as the NH3 adsorption retention amount of the SCR 18 increases. The ECU 30 stores a map that defines these relationships, that is, the relationship between the NH3 adsorption rate, the NH3 concentration, and the NH3 adsorption retention amount C_AM (see the frame indicated by reference numeral 44). Therefore, the ECU 30 can detect the NH3 adsorption rate 44 if the NH3 concentration 54 and the NH3 adsorption retention amount C_AM56 are known.

  In the system shown in FIG. 1, the NH3 sensor 22 can identify the NH3 concentration at the inlet of the SCR 18. Further, the ECU 30 calculates the NH3 adsorption / retention amount in the SCR 18 according to the above equation (1) at each sampling time. Therefore, the ECU 30 can specify the NH3 adsorption rate 44 of the SCR 18 by setting the value calculated one cycle before as the NH3 adsorption retention amount 56.

  As described above, the ECU 30 can calculate both the NH 3 inflow amount 42 into the SCR 18 and the NH 3 adsorption rate 44 in the SCR 18. Therefore, by multiplying them, the ECU 30 can calculate the NH3 adsorption amount 40 for each sampling period.

  FIG. 4 is a diagram for explaining a method of estimating the NH3 consumption corresponding to the third term on the right side of equation (1). The NH3 consumption 60 in the SCR 18 is proportional to the amount of NOx reduced in the SCR 18. Therefore, the NH3 consumption 60 can be calculated based on the product of the NOx inflow 62 into the SCR 18 and the NOx purification rate 64 in the SCR 18 (see reference numeral 66).

  The NOx inflow amount 62 into the SCR 18 can be calculated by multiplying the NOx concentration 68 at the inlet of the SCR 18 by the exhaust gas flow rate 50 (see reference numeral 72). As described above, the NH3 sensor 22 generates an output corresponding to the NH3 concentration in the rich atmosphere, and generates an output corresponding to the NOx concentration in the lean atmosphere. For this reason, in the system of the present embodiment, the NOx concentration 68 at the inlet of the SCR 18 can be detected by the NH3 sensor 22. Further, the exhaust gas flow rate can be detected by the AFM 24. Therefore, according to this system, the NOx inflow amount 62 into the SCR 18 can be calculated.

  The NOx purification rate 64 of the SCR 18 becomes higher as the NOx concentration in the gas flowing into the SCR 18 is higher, and as the NH3 adsorption / retention amount of the SCR 18 is larger. The ECU 30 stores a map that defines these relationships as shown in a frame denoted by reference numeral 64. Therefore, the ECU 30 can predict the NOx purification rate 64 of the SCR 18 if the NOx concentration 74 at the inlet of the SCR 18 and the NH3 adsorption / retention amount C_AM76 in the SCR 18 are known.

  The NOx concentration 74 at the inlet of the SCR 18 can be detected by the NH3 sensor 22 as described above. Further, the NH3 adsorption / retention amount 76 in the SCR 18 is calculated for each sampling time by the equation (1). For this reason, the ECU 30 can predict the NOx purification rate 64 of the SCR 18 for each predetermined sampling period.

  As described above, the ECU 30 can calculate both the NOx inflow amount 62 into the SCR 18 and the NOx purification rate 64 in the SCR 18. Therefore, the ECU 30 can calculate the NH3 consumption 60 for each sampling period by multiplying them.

  FIG. 5 is a diagram for explaining a method of estimating the NH3 purge amount corresponding to the fourth term on the right side of the equation (1). The NH3 amount 80 purged from the SCR 18 by the flow of exhaust gas can be calculated by multiplying the NH3 adsorption / retention amount 82 in the SCR 18 by the purge rate PR84 per unit gas flow rate and the exhaust gas flow rate 86 (see reference numeral 88). .

  The NH 3 adsorption / retention amount 82 in the SCR can be detected by the calculation of the equation (1) performed every sampling cycle. Further, the exhaust gas flow rate 86 can be detected by the AFM 24. Further, the purge rate PR can be predicted by the method described below.

  The purge rate PR is a ratio between the amount of NH3 purged by the unit gas flow rate and the amount of NH3 stored in the SCR 18. The more NH3 is occluded in the SCR 18, the easier it is to be purged. For this reason, the purge rate PR becomes larger as the NH3 adsorption / holding amount increases. Further, NH3 stored in the SCR 18 is purged mainly by oxygen gas. For this reason, the purge rate PR increases as the oxygen concentration in the exhaust gas increases.

  The ECU 30 stores a map in which the purge rate PR is defined by the relationship between the oxygen concentration in the exhaust gas and the NH3 adsorption / retention amount C_AM in the SCR 18 as shown in a frame denoted by reference numeral 84. For this reason, the ECU 30 can specify the purge rate PR if the NH3 adsorption / retention amount 90 of the SCR 18 and the oxygen concentration 92 at the inlet of the SCR 18 are known.

  The NH3 adsorption / holding amount 90 in the SCR can be obtained by the calculation of the equation (1). The oxygen concentration 92 at the inlet of the SCR 18 can be predicted if the exhaust gas flow rate 94 and the exhaust air-fuel ratio 96 are known. In the system shown in FIG. 1, the exhaust gas flow rate 94 can be detected by the AFM 24, and the exhaust air-fuel ratio 96 can be detected by the A / F sensor 20. Therefore, the ECU 30 can predict the purge rate PR84 of the SCR 18 after predicting the oxygen concentration at the inlet of the SCR 18.

  As described above, the ECU 30 can detect all of the NH 3 adsorption / retention amount 82 in the SCR 18, the purge rate PR of the SCR 18, and the exhaust gas flow rate 86. Therefore, the ECU 30 can calculate the NH3 purge amount 80 for each sampling period by multiplying them.

  As described with reference to FIGS. 3 to 5, the ECU 30 determines the NH3 adsorption / retention amount for each sampling period based on the NH3 adsorption / retention amount as long as the NH3 adsorption / retention amount at a certain time is determined. And the NH3 purge amount can be calculated. Therefore, if the initial value of the NH3 adsorption / holding amount is read when the internal combustion engine 10 is started, the latest NH3 adsorption / holding amount can be predicted for each sampling period in accordance with the above equation (1). .

[Specific Processing in Embodiment 1]
FIG. 6 is a flowchart of a routine executed by the ECU 30 in order to predict the NH3 adsorption / retention amount using the above method. The routine shown in FIG. 6 is started when the internal combustion engine 10 is started, and thereafter repeatedly executed every predetermined sampling period.

  In the routine shown in FIG. 6, first, the NH 3 adsorption / holding amount (t−1) calculated in the previous processing cycle is read (step 100). The ECU 30 stores the latest predicted value of the NH3 adsorption holding amount when the internal combustion engine 10 is stopped. When this step 100 is executed for the first time after the internal combustion engine 10 is started, the NH3 adsorption / holding amount stored at the time of stoppage is read as an initial value.

  Next, the NH3 adsorption amount (t) (see FIG. 3) in the SCR 18 is calculated (step 102). Specifically, the NH3 adsorption amount (t) is obtained by the process shown in FIG. 3 based on the detection value of the AFM 24, the detection value of the NH3 sensor 22, and the NH3 adsorption retention amount (t-1) read in Step 100. Is calculated.

  Next, NH3 consumption (t) (see FIG. 4) in the SCR 18 is calculated (step 104). Specifically, based on the detected value of the AFM 24, the detected value of the NH3 sensor (NOx sensor) 22, and the NH3 adsorption / retention amount (t-1) read in step 100, NH3 consumption is performed by the process shown in FIG. A quantity (t) is calculated.

  Next, the NH3 purge amount (t) (see FIG. 5) in the SCR 18 is calculated (step 106). Specifically, based on the detected value of the AFM 24, the detected value of the A / F sensor 20, and the NH3 adsorption retention amount (t-1) read in step 100, the NH3 purge amount ( t) is calculated.

  Next, the latest value of the NH3 adsorption holding amount is calculated (step 108). Specifically, the calculation according to the above (1) is performed on the basis of NH3 (t-1) read by the processing of steps 100 to 106, the calculated NH3 adsorption amount (t), and the like. As a result, the NH 3 adsorption retention amount (t) at the current time t is calculated.

  Next, it is determined whether or not the calculated NH3 adsorption / holding amount (t) exceeds the maximum storable amount of the SCR 18 (step 110). Since the actual NH3 adsorption / retention amount does not exceed the maximum storable amount, if the above condition is satisfied, the current NH3 adsorption / retention amount (t) is corrected to the maximum storable amount (step 112). . On the other hand, if the condition of step 110 is not satisfied, the current processing cycle is terminated while the NH3 adsorption / holding amount (t) calculated in step 108 is maintained.

  As described above, according to the routine shown in FIG. 6, it is possible to calculate the NH3 adsorption / retention amount in consideration of the NH3 purge amount by using the hardware configuration shown in FIG. For this reason, according to the system of the present embodiment, the NH3 adsorption / holding amount in the SCR 18 can be accurately predicted during the operation of the internal combustion engine 10.

  By the way, in the system of the first embodiment described above, the SCR 18 is arranged on the downstream side of the NSR 16, but the present invention is not limited to this. That is, as shown in FIG. 7, the SCR 18 may be disposed upstream of the NSR 16, that is, immediately below the three-way catalyst 14. Further, the NSR 16 and the SCR 18 may be replaced with a catalyst having a two-layer coat structure in which both are integrated. Further, the NSR 16 may be omitted, and only the three-way catalyst 14 and the SCR 18 may be disposed in the exhaust passage 12.

  In the first embodiment described above, the three-way catalyst 14 and the NSR 16 correspond to the “first catalyst” in the first invention, and the SCR 18 corresponds to the “NOx selective reduction catalyst” in the first invention. ing. Further, the “NH3 adsorption / holding amount storage means” in the first aspect of the present invention is realized by the ECU 30 storing the NH3 adsorption / holding amount calculated according to the equation (1). Further, the “NH 3 adsorption amount calculating means” in the first aspect of the present invention is realized by the ECU 30 executing the processing of step 102 described above. Further, the “NH 3 consumption calculating means” in the first aspect of the present invention is realized by the ECU 30 executing the processing of step 104 described above. Further, the “NH 3 purge amount calculating means” in the first aspect of the present invention is realized by the ECU 30 executing the processing of step 106 described above. Further, the “NH 3 adsorption / retention amount updating means” in the first aspect of the present invention is realized by the ECU 30 executing the processing of step 108.

  Further, in the first embodiment described above, the ECU 30 calculates the NH3 inflow amount 42 shown in FIG. 3 so that the “NH3 inflow amount calculating means” in the second aspect of the invention is the NH3 adsorption rate 44 shown in FIG. By calculating the above, the “NH 3 adsorption rate calculating means” in the second invention is realized. Further, the ECU 30 calculates the NOx inflow amount 62 shown in FIG. 4 so that the “NOx inflow amount calculating means” in the second invention calculates the NOx purification rate 64, thereby calculating “NOx in the second invention”. "Purification rate calculating means" is realized respectively. In addition, the ECU 30 detects the exhaust gas flow rate 86 shown in FIG. "Rate calculating means" is realized respectively.

  Further, in the first embodiment described above, the ECU 30 detects the O2 concentration 92 shown in FIG. 5 so that the “oxygen concentration detecting means” in the third invention is described in a frame denoted by reference numeral 84. The “purge rate rule storage means” according to the third aspect of the present invention is realized by storing the purge rate map.

Embodiment 2. FIG.
[Configuration of Embodiment 2]
Next, a second embodiment of the present invention will be described with reference to FIGS. FIG. 8 shows the hardware configuration of the system of this embodiment. The configuration shown in FIG. 8 is the same as the configuration shown in FIG. 1 except that a throttle opening (TA) sensor 32 and a rotation speed sensor 34 are added. However, in this system, the NH3 sensor 22 arranged upstream of the SCR 18 is used only as a NOx sensor.

  That is, in the system of the first embodiment described above, the NH3 sensor 48 detects the NH3 concentrations 48 and 54 necessary for calculating the NH3 adsorption amount 40 (see FIG. 3). The system of this embodiment is characterized in that the NH3 concentration is estimated based on the operating state of the internal combustion engine 10.

[Estimation of NH3 concentration at SCR inlet]
FIG. 9 is a diagram for explaining a method of estimating the NH3 concentration at the inlet of the SCR 18, that is, the NH3 concentrations 48 and 54 in FIG. The NH3 concentrations 48 and 54 at the SCR inlet are a ratio of the amount 42 of NH3 flowing into the SCR 18 and the exhaust gas flow rate 50 (see reference numeral 116). Of these two parameters, the exhaust gas flow rate 50 can be detected by the AFM 24. The other parameter, that is, the NH3 inflow amount 42 into the SCR 18 is obtained by adding the NH3 amount generated by the three-way catalyst 14 and the NH3 amount generated by the NSR 16 in the system of this embodiment. be able to.

The three-way catalyst 14 generates NH3 by reducing NOx present in the catalyst. For this reason, the amount of NH3 produced by the three-way catalyst 14 is a value obtained by multiplying the NOx amount 120 in the three-way catalyst 14 by the NH3 production rate 122 of the three-way catalyst 14. Similarly, the amount of NH3 produced by the NSR 16 is a value obtained by multiplying the NOx amount 124 in the NSR 16 by the NH3 production rate 126 of the NSR 16. Accordingly, the NH3 amount 42 flowing into the SCR 18 can be obtained by the following equation (see reference numeral 128).
NH3 inflow = (NOx amount in the three-way catalyst) x (NH3 production rate of the three-way catalyst)
+ (NOx amount in NSR) x (NH3 production rate of NSR) (2)

(Calculation of the first term on the right side)
The three-way catalyst 14 does not occlude NOx. Therefore, the NOx amount 120 in the three-way catalyst 14 can be regarded as the same amount as the NOx amount 130 flowing into the three-way catalyst 14. Since the NOx amount 130 flowing into the three-way catalyst 14 is the NOx amount exhausted by the internal combustion engine 10, the operation state 132 (engine speed NE, engine load KL, throttle opening degree TA, etc.) of the internal combustion engine 10 is reached. Based on this, it can be estimated by a known method. Therefore, in the present embodiment, the ECU 30 can estimate the NOx amount 120 in the three-way catalyst 14 based on outputs from the AFM 24, the throttle opening sensor 32, the rotation speed sensor 34, and the like.

(Calculation of the second term on the right side)
The NH3 production rate of the three-way catalyst 14 is mainly determined by the amount of NOx present therein, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 14, and the temperature of the three-way catalyst 14. The ECU 30 stores a map in which the NH3 generation rate is determined in relation to these three parameters, as shown in the frame denoted by reference numeral 122. Therefore, the ECU 30 can calculate the NH3 production rate of the three-way catalyst 14 if the NOx amount 134 in the three-way catalyst 14, the temperature 136 of the three-way catalyst 14, and the exhaust air-fuel ratio 138 are known (reference numeral 122). , 139).

  The NOx amount 134 in the three-way catalyst 14 is the same as the NOx amount 130 flowing into the three-way catalyst 14 as described above, and can be calculated based on the operating state 132 of the internal combustion engine 10. Further, the temperature 136 of the three-way catalyst 14 can also be obtained from the operating state 132 of the internal combustion engine 10 by a known method. Further, the exhaust air / fuel ratio 138 can be detected by the A / F sensor 20. Therefore, according to the configuration of the present embodiment, the ECU 30 can predict the NH3 production rate 122 of the three-way catalyst 14 by referring to the map shown in the frame denoted by reference numeral 122.

(Calculation of the third term on the right side)
NSR 16 stores NOx. For this reason, the NOx amount 124 in the NSR 16 is the sum of the NOx amount 140 flowing into the NSR 16 and the NOx amount 142 stored in the NSR 16 (see reference numeral 144 in FIG. 9). The NOx amount 140 flowing into the NSR 14 is an amount obtained by subtracting the NOx amount purified in the three-way catalyst 14 from the NOx amount 130 flowing into the three-way catalyst 14. The amount of NOx purified in the three-way catalyst 14 can be obtained by multiplying the NOx amount 130 flowing into the three-way catalyst 14 by the NOx purification rate 146 of the three-way catalyst 14. Accordingly, the NOx amount 140 flowing into the NSR 16 can be calculated based on the NOx inflow amount 130 into the three-way catalyst 14 and the NOx purification rate 146 of the three-way catalyst 14 (see reference numeral 148).

  The NOx amount 142 stored in the NSR 16 can be obtained by integrating the NOx amount that has flowed into the NSR 16 after the end of the rich spike (see reference numeral 150). As described above, the NOx inflow amount to the NSR 16 can be calculated based on the NOx inflow amount 130 to the three-way catalyst 14 and the NOx purification rate 146 of the three-way catalyst 14. Therefore, the ECU 30 can specify both the NOx inflow amount 140 to the NSR 16 and the NOx occlusion holding amount 142 in the NSR 16 and add them to calculate the NOx amount 124 in the NSR 16.

(Calculation of the fourth term on the right side)
The NH3 production rate of the NSR 16 is mainly determined by the amount of NOx existing therein, the air-fuel ratio of the exhaust gas flowing into the NSR 16, and the temperature of the NSR 16. As shown in the frame denoted by reference numeral 126, the ECU 30 stores a map that determines the NH3 generation rate in relation to these three parameters. Therefore, the ECU 30 can calculate the NH3 production rate of the NSR 16 if the NOx amount 152 in the NSR 16, the temperature 154 of the NSR 16 and the exhaust air-fuel ratio 156 are known.

  As described above, the NOx amount 152 in the NSR 16 can be obtained as the sum of the NOx amount 140 flowing into the NSR 16 and the NOx occlusion holding amount 142 in the NSR 142. Similarly to the case of the three-way catalyst 16, the temperature 154 of the NSR 16 can be obtained by a known method based on the operating state 132 of the internal combustion engine 10. The exhaust air / fuel ratio 156 can be detected by the A / F sensor 20. Therefore, according to the configuration of the present embodiment, the ECU 30 can predict the NH3 generation rate 126 of the NSR 16 by referring to the map shown in the frame denoted by reference numeral 126 (see reference numerals 126 and 158). .

  As described above, the ECU 30 can calculate all of the first term to the fourth term on the right side of the equation (2) based on the operating state 132 of the internal combustion engine 10 and the output of the A / F sensor 20. it can. For this reason, the system of this embodiment can calculate the NOx amount 120 flowing into the SCR 16 without using an NH3 sensor (see reference numeral 128). When the NH3 inflow amount 120 into the SCR 16 is known, the NH3 concentration 48, 50 at the inlet of the SCR 18 can be calculated by dividing the value by the exhaust gas flow rate 50.

  As described with reference to FIG. 3, the system according to the first embodiment detects the NH3 concentrations 48 and 50 at the inlet of the SCR 18 using the NH3 sensor 22 in order to calculate the NH3 adsorption amount 40. In the system of this embodiment, as described above, the NH3 concentrations 48 and 50 at the inlet of the SCR 18 can be obtained by calculation without using an NH3 sensor. For this reason, in the system of this embodiment, the NH3 adsorption amount 40 can be calculated without using the NH3 sensor.

  As described with reference to FIG. 4, in calculating the NH3 consumption 60, it is not necessary to detect the NH3 concentration at the inlet of the SCR 18 (it is sufficient if the NOx concentration can be detected). Further, as described with reference to FIG. 5, it is not necessary to detect the NH3 concentration in calculating the NH3 purge amount 80 from the SCR 18. For this reason, according to the system of this embodiment, it is possible to realize a highly accurate NH3 adsorption / retention amount prediction considering the NH3 purge amount without arranging an NH3 sensor upstream of the SCR 18.

  By the way, in Embodiment 2 mentioned above, NH3 inflow 48 calculated by the method shown in FIG. 9 is substituted for NH3 inflow 48 shown in FIG. 3, and NH3 amount 42 flowing into the SCR is calculated. However, the present invention is not limited to this. That is, according to the method shown in FIG. 9, the NH3 amount 42 flowing into the SCR 18 can be calculated before the NH3 concentration 48 is calculated. Therefore, the system of the present embodiment may calculate the NH3 adsorption amount 40 by substituting the NH3 inflow amount 42 shown in FIG. 9 into the NH3 inflow amount 42 shown in FIG. 3.

  In the second embodiment described above, the NH3 sensor 22 is used to detect the NOx 68 and 74 (see FIG. 4) at the inlet of the SCR 18, but the present invention is not limited to this. That is, the NOx concentrations 68 and 74 at the inlet of the SCR 18 can be estimated by a known method based on the operating state of the internal combustion engine 10. For this reason, the NH3 sensor 22 may be excluded, and the NH3 consumption 60 (see FIG. 4) may be obtained by estimation.

  In the second embodiment described above, the ECU 30 executes the process of step 102 shown in FIG. 6 using the NH3 concentrations 48 and 54 calculated by the method shown in FIG. "NH3 adsorption amount calculation means" has been realized. Further, the ECU 30 calculates the NH3 inflow amount 42 by dividing the NH3 concentration 48 calculated by the method shown in FIG. 9 by the exhaust gas flow rate 50, or calculates the NH3 inflow amount 42 by the method shown in FIG. Thus, the “NH3 inflow amount calculating means” in the second invention is realized.

Embodiment 3 FIG.
Next, Embodiment 3 of the present invention will be described with reference to FIGS. The system of the present embodiment can be realized by causing the ECU 30 to execute a routine shown in FIG. 11 described later in the system of the first or second embodiment.

  As in the case of the first or second embodiment, the system of the present embodiment can accurately estimate the NH3 adsorption / retention amount in the SCR 18 in real time. FIG. 10 is a diagram showing the relationship between the operating state of the internal combustion engine 10 and the estimated NH3 adsorption / holding amount. More specifically, FIG. 10A shows a change in the exhaust air-fuel ratio that occurs as the lean operation and the rich spike are repeated in the internal combustion engine 10. FIG. 10B shows a change in the NH3 adsorption / retention amount estimated by the ECU 30. Further, FIG. 10C shows the change in NH3 concentration at the SCR 18 inlet.

  While the lean operation is being performed in the internal combustion engine 10 (see FIG. 10A), the exhaust gas containing NOx flows into the SCR 18. At this time, the SCR 18 purifies NOx using NH3 as a reducing material. As a result, the NOx occlusion retention amount of the SCR 18 tends to decrease with time (see FIG. 10B).

  During execution of the rich spike control (see FIG. 10A), exhaust gas containing HC and CO is discharged from the internal combustion engine 10. At this time, NH 3 is generated in the interior of the three-way catalyst 14 and in the interior of the NSR 16 by reacting nitrogen in the exhaust gas with unburned hydrogen. As a result, a large amount of NH3 is temporarily generated at the entrance of the SCR 18 in synchronization with the execution of the rich spike (see FIG. 10C). The generated NH 3 is adsorbed by the SCR 18. For this reason, the NH3 adsorption / holding amount of the SCR 18 shows an increasing tendency during execution of the rich spike control (see FIG. 10B).

  The value indicated by the alternate long and short dash line in FIG. 10B indicates the target value of the NH3 adsorption / retention amount at the end of the rich spike control. If the NH3 adsorption / holding amount of the SCR 18 becomes too close to the storage capacity, a situation occurs in which NH3 contained in the exhaust gas blows through downstream. On the other hand, if a sufficient NH3 adsorption / holding amount is not secured at the end of the rich spike control, the amount of NH3 that can be released during lean operation becomes small. For this reason, there is a target value for the NH3 adsorption retention amount.

  The chart shown in FIG. 10 (B) shows that the NH3 adsorption retention amount has exceeded the target value at the end of the first rich spike control (time t1). Since the system of this embodiment can detect the NH3 adsorption / retention amount of the SCR 18 in real time, it can immediately detect that NH3 is excessive with respect to the target value at time t1. In this case, the ECU 30 changes the operating state of the internal combustion engine 10 so that the NH3 adsorption / holding amount of the SCR 18 decreases.

  Specifically, if the NH3 adsorption / holding amount is excessive at the end of the rich spike control, the ECU 30 corrects the target air-fuel ratio at the next rich spike control to the lean side. In the chart shown in FIG. 10A, since the NH3 adsorption retention amount was excessive at the end of the first rich spike control, the target air-fuel ratio becomes the first target value (one-dot chain line during the second rich spike control. It is shown that the leaning is performed as compared to the value indicated by.

  The NH3 concentration at the inlet of the SCR 18 becomes lower as the exhaust air-fuel ratio becomes leaner. On the other hand, as described with reference to FIG. 3, the amount of NH3 adsorbed on the SCR 18 (NH3 adsorption amount 40) becomes smaller as the NH3 concentrations 48 and 54 at the inlet of the SCR 18 are lower. For this reason, when the target air-fuel ratio is made lean, the amount of NH3 adsorbed on the SCR 18 during the rich spike control decreases. Therefore, according to the above processing, the NH3 adsorption amount by the second rich spike control can be reduced, and the NH3 adsorption retention amount at the end (time t2) can be brought close to the target value.

  On the other hand, the system of the present embodiment enriches the target air-fuel ratio at the time of rich spike control when the NH3 adsorption retention amount of the SCR 18 is insufficient with respect to the target value at the end of the rich spike control. According to such processing, the amount of NH3 adsorbed on the SCR 18 can be increased with rich spike control. For this reason, according to the system of the present embodiment, the NH3 adsorption / holding amount at the end of the rich spike control can always be controlled in the vicinity of the target value.

[Specific Processing in Embodiment 3]
FIG. 11 is a flowchart of a routine executed by the ECU 30 in the present embodiment. In the routine shown in FIG. 11, it is first determined whether or not the current processing cycle is the end of the rich spike control (step 160). If it is determined that the rich spike control is not finished, the current cycle is finished.

  On the other hand, if it is determined that the rich spike control is finished, the NH3 adsorption / retention amount at that time is read (step 162). Next, it is determined whether or not the read NH3 adsorption / retention amount substantially matches the target value (is within an allowable range centered on the target value) (step 164). As a result, when it is determined that the NH3 adsorption retention amount substantially matches the target value, it is determined that it is not necessary to correct the current operation state, and the current processing cycle is ended as it is.

  On the other hand, if it is determined in step 164 that the NH3 adsorption / retention amount does not substantially match the target value, it is determined whether or not the NH3 adsorption / retention amount is larger than the target value (step 166). ).

  If NH3 adsorption / holding amount> target value holds, it can be determined that the NH3 adsorption / holding amount is excessive. In this case, the target air-fuel ratio AF_RS in the rich spike control is corrected to a value that is larger by a predetermined value (0.01), that is, in the lean direction (step 168). When this process is executed, the exhaust air-fuel ratio is made leaner in the next rich spike control than in the current rich spike control. As a result, the NH3 production amount decreases, the NH3 adsorption amount decreases, and the NH3 adsorption retention amount decreases toward the target value.

  On the other hand, if it is determined in step 166 that NH3 adsorption / holding amount> target value is not satisfied, it can be determined that the NH3 adsorption / holding amount of the SCR 18 is insufficient. In this case, the target air-fuel ratio AF_RS in the rich spike control is corrected to a value that is smaller by a predetermined value (0.01), that is, in the rich direction (step 170). When this process is executed, the exhaust air-fuel ratio is enriched in the next rich spike control as compared with the current rich spike control. As a result, the NH3 production amount increases, the NH3 adsorption amount increases, and the NH3 adsorption retention amount increases toward the target value.

  As described above, according to the routine shown in FIG. 11, the target air-fuel ratio AF_RS at the time of rich spike control can be appropriately increased or decreased based on the relationship between the predicted value of the NH 3 adsorption retention amount in the SCR 18 and the target value. . Therefore, according to the system of the present embodiment, the NH3 adsorption / holding amount of the SCR 18 at the end of the rich spike control can be appropriately managed in the vicinity of the target value.

  By the way, in the first embodiment described above, when it is determined that the NH3 adsorption holding amount is excessive or insufficient, the NH3 adsorption amount is controlled by controlling the target air-fuel ratio AF_RS at the time of rich spike control. The control target for bringing the amount close to the target value is not limited to AF_RS. That is, the NH3 adsorption / holding amount of the SCR 18 changes as at least one of the NH3 adsorption amount, the NH3 consumption amount, and the NH3 purge amount changes, as is apparent from the above equation (1). Therefore, if the operating state of the internal combustion engine 10 is changed so that a desired change occurs in at least one of the NH3 adsorption amount, NH3 consumption amount, and NH3 purge amount, the NH3 adsorption retention amount can be brought close to the target value. It is.

  Specifically, the NH3 adsorption amount 40 is a value corresponding to the NH3 amount 42 flowing into the SCR 18, as shown in FIG. As shown in FIG. 9, the NH3 inflow amount 42 into the SCR 18 is the sum of the NH3 generation amount of the three-way catalyst 14 and the NH3 generation amount of the NSR 16 (see reference numeral 128). The amount of NH3 generated varies depending on the temperature of the catalysts 14 and 16, the amount of NOx in the catalysts 14 and 16, and the exhaust air-fuel ratio. For this reason, when it is determined that the NH3 adsorption / retention amount is excessive or insufficient, at least one of the temperature of the catalysts 14 and 16, the amount of NOx in the catalysts 14 and 16, and the exhaust air / fuel ratio is set so as to eliminate the excess and deficiency. It is also possible to execute rich spike control by changing one of them.

  As shown in FIG. 4, the NH3 consumption 60 is determined by the NOx amount 62 flowing into the SCR 18 and the NOx purification rate 64 of the SCR 18. Further, the NOx purification rate 64 of the SCR 18 varies depending on the NOx concentration at the SCR 18 inlet. For this reason, when it is determined that the NH3 adsorption / retention amount is excessive or insufficient, the amount of NOx flowing into the SCR 18 or the NOx concentration at the inlet of the SCR 18 is changed in the direction in which the excess or shortage is eliminated. The operating conditions may be changed.

  Further, as shown in FIG. 5, the NH 3 purge amount 80 changes according to the NH 3 purge rate 84 of the SCR 18 and the exhaust gas flow rate 86. The NH3 purge rate 84 of the SCR 18 shows a change according to the oxygen concentration at the inlet of the SCR 18. Therefore, when it is determined that the NH3 adsorption / retention amount is excessive or insufficient, the lean operation is performed so that at least the oxygen concentration at the inlet of the SCR 18 and the exhaust gas flow rate change in a direction to eliminate the excess / deficiency. It is good also as changing conditions.

  In the third embodiment described above, the NH3 adsorption / holding amount at the end of the rich spike control is made to coincide with the target value, but the present invention is not limited to this. That is, an object of the present invention is to cause the SCR 18 to store an appropriate amount of NH3 in an environment where lean operation and rich spike control are repeated. Therefore, the NH3 adsorption / holding amount at a specific timing other than at the end of the rich spike control, such as at the start of the rich spike control, may be made to coincide with the target value at that timing. Or it is good also as making the average value of NH3 adsorption | suction retention amount in a sufficiently long period correspond with the target value of an average value.

  In the third embodiment described above, the “comparison means” according to any one of the fourth to sixth inventions is realized by the ECU 30 executing the processing of steps 164 and 166 described above. Further, the “control means” according to the fourth aspect of the present invention is realized by the ECU 30 executing the processing of steps 168 and 170 described above.

  Further, when the NH3 adsorption / retention amount of the SCR 18 is found to be excessive or insufficient, by changing the operating conditions of the internal combustion engine so that the NH3 consumption changes in a direction in which the excess or deficiency is eliminated, The “control means” in the fifth aspect of the invention can be realized.

  Further, when an excess or deficiency in the NH3 adsorption / retention amount of the SCR 18 is recognized, the ECU 30 is caused to change the operating conditions of the internal combustion engine so that the NH3 purge amount changes in a direction to eliminate the excess or deficiency. The “control means” according to the sixth aspect of the present invention can be realized.

Embodiment 4 FIG.
Next, a fourth embodiment of the present invention will be described with reference to FIG. FIG. 12 is a diagram for explaining the system configuration of the present invention. The configuration shown in FIG. 12 is the same as the configuration shown in FIG. 1 except that the second NH 3 sensor 36 is provided on the downstream side of the SCR 18.

  According to the second NH3 sensor 36, the NH3 concentration downstream of the SCR 18 can be measured. On the other hand, the system of the present embodiment can predict the NH3 purge amount 80 from the SCR 18 in the same manner as in the first embodiment (see FIG. 4). Further, since the NH3 concentration downstream of the SCR 18 is a ratio of the NH3 purge amount 80 and the exhaust gas amount 86, the system of this embodiment can predict the NH3 concentration downstream of the SCR 18 by calculation.

  The system of this embodiment compares the measured value of NH3 concentration with the predicted value during the lean operation of the internal combustion engine 10. If there is a difference between the two, it can be determined that there is an error in the predicted value of the NH3 concentration, that is, the predicted NH3 purge amount 80. As shown in FIG. 5, the NH 3 purge amount 80 is calculated based on the NH 3 adsorption retention amount 82 and the NH 3 purge rate 84. The NH3 purge rate 84 increases as the NH3 adsorption / holding amount of the SCR 18 increases. For this reason, if the predicted value of NH3 concentration is excessive, it can be judged that the predicted value of NH3 adsorption retention amount was excessive. On the other hand, if the predicted value of NH3 concentration is excessively small, NH3 adsorption retention amount It can be determined that the predicted value of was too small.

  When the system of the present embodiment determines that the predicted value of the NH3 adsorption retention amount is excessive by the above processing, the content of the prediction calculation is corrected so that the value decreases. On the other hand, when it is determined that the predicted value of the NH3 adsorption retention amount is excessively small, the content of the prediction calculation is corrected so that the value becomes large.

  The predicted value of the NH3 adsorption / retention amount can be increased or decreased by changing at least one of the NH3 adsorption amount 40 shown in FIG. 3, the NH3 consumption amount 60 shown in FIG. 4, and the NH3 purge amount 80 shown in FIG. . The NH3 adsorption amount 40 shown in FIG. 3 can be changed by correcting the NH3 adsorption rate 44 map. Similarly, the NH3 consumption amount 60 shown in FIG. 4 is changed by correcting the map of the NOx purification rate 64, and the NH3 purge amount 80 shown in FIG. 5 is changed by correcting the map of the NH3 purge rate 84. Can be made.

  In the present embodiment, the ECU 30 corrects at least one of the above-described three maps so that the excess or deficiency is recognized when the predicted value of the NH3 concentration is excess or deficiency. According to such correction, the error included in the calculation can be reduced, and the prediction accuracy of the NH3 adsorption retention amount can be increased. Therefore, according to the system of the present embodiment, it is possible to predict the NH3 adsorption / retention amount of the SCR 18 with higher accuracy than in the case of the first embodiment.

  By the way, in the system of the fourth embodiment described above, the SCR 18 is arranged on the downstream side of the NSR 16, but the present invention is not limited to this. That is, as shown in FIG. 13, the SCR 18 may be arranged upstream of the NSR 16, that is, immediately below the three-way catalyst 14. Further, the NSR 16 and the SCR 18 may be replaced with a catalyst having a two-layer coat structure in which both are integrated. Further, the NSR 16 may be omitted, and only the three-way catalyst 14 and the SCR 18 may be disposed in the exhaust passage 12.

  In the fourth embodiment described above, the “NH 3 purge amount calculating means” in the seventh aspect of the present invention is realized by the ECU 30 calculating the NH 3 purge amount by the method shown in FIG. Further, the “NH3 concentration predicting means” in the seventh aspect of the present invention is realized by predicting the NH3 concentration based on the NH3 purge amount 80 and the exhaust gas amount. The ECU 30 corrects at least one of the map of the NH3 adsorption rate 44, the map of the NOx purification rate 64, and the map of the NH3 purge rate 84, thereby realizing the “calculation method correcting means” in the seventh invention. ing.

Embodiment 5. FIG.
Next, a fifth embodiment of the present invention will be described with reference to FIGS. The system of the present embodiment can be realized by causing the ECU 30 to execute a routine shown in FIG. 15 described later in the system of the first embodiment.

The system of the present embodiment can predict the NOx emission amount that blows down downstream of the SCR 18, that is, the NOx tail pipe emission. FIG. 14 is a diagram for explaining a method of calculating the NOx tail pipe emission 180. As shown in FIG. 14, the NOx tail pipe emission 180 can be calculated by the following arithmetic expression.
NOx tailpipe emissions = NOx inflow 62 to the SCR 18 x (1-x NOx purification rate 64 in the SCR 18) (3)

  The NOx inflow amount 62 into the SCR 18 and the NOx purification rate 64 in the SCR 18 can be calculated by the same method as in the first embodiment (see FIG. 4). For this reason, in the present embodiment, the ECU 30 can calculate the NOx tail pipe emission by the above equation (3).

  The amount of NOx in the exhaust gas increases and decreases according to the air-fuel ratio of the internal combustion engine 10, the ignition timing, the amount of EGR (Exhaust Gas Recirculation), and the like. Therefore, it is necessary to control the air-fuel ratio, ignition timing, EGR amount, etc. so that the NOx emission amount does not exceed the allowable emission value. On the other hand, the fuel consumption characteristics of the internal combustion engine 10 become better as the control range of these parameters becomes wider. For this reason, it is desirable to control the air-fuel ratio, ignition timing, EGR amount, and the like of the internal combustion engine 10 so that the NOx tail pipe emission matches the target emission value.

  FIG. 15 is a flowchart of a routine executed by the ECU 30 to satisfy the above request. In the routine shown in FIG. 15, first, the calculated value of NOx tail pipe emission is read (step 190). The ECU 30 performs the calculation shown in FIG. 4 at a predetermined cycle during the operation of the internal combustion engine 10. Here, the latest NOx tailpipe emission 180 obtained by the calculation is read.

  Next, it is determined whether or not the average of the calculated values of NOx tail pipe emission over a predetermined period is smaller than a predetermined standard value (allowable limit) (step 192).

  If the above determination is affirmative, it can be determined that there is a margin in the NOx emission amount, in other words, that the air-fuel ratio of the internal combustion engine 10 still has a margin for improving fuel efficiency. In this case, the ECU 30 corrects the lean operation cycle in the extending direction. If the period of lean operation is extended, the number of executions of rich spikes is reduced, so that the fuel consumption characteristics of the internal combustion engine 10 can be improved.

  On the other hand, if it is determined that the condition of step 192 is not satisfied, it can be determined that the NOx emission amount needs to be reduced. In this case, the ECU 30 corrects the lean operation cycle in the shortening direction. When the lean operation cycle is shortened, the margin of NSR 16 and SCR 18 related to NOx purification increases, and the amount of NOx emissions can be suppressed.

  When the above processing is repeated, the lean operation period converges to a time for matching the NOx tailpipe emission to the standard value. For this reason, according to the system of the present embodiment, excellent fuel efficiency characteristics can be imparted to the internal combustion engine 10 while suppressing the NOx emission amount to be below the allowable limit.

  Incidentally, in the routine shown in FIG. 15, the lean operation cycle is changed based on the comparison between the NOx tail pipe emission and the standard value, but the present invention is not limited to this. That is, the air-fuel ratio, ignition timing, EGR amount, etc. during lean operation or rich spike control may be changed to match the NOx tailpipe emission with the standard value.

  In the fifth embodiment described above, the “NOx inflow amount calculating means” in the eighth aspect of the present invention is realized by the ECU 30 calculating the NOx inflow amount 62 shown in FIG. Further, the “NOx purification rate calculating means” according to the eighth aspect of the present invention is realized by the ECU 30 calculating the NOx purification rate 64 shown in FIG. Furthermore, the “NOx emission calculation means” in the eighth aspect of the present invention is realized by the ECU 30 executing the calculation in the frame indicated by the reference numeral 182 in FIG.

  In the fifth embodiment described above, the “emission comparison means” according to the ninth aspect of the present invention is implemented when the ECU 30 executes the process of step 192. Further, the “control means” according to the ninth aspect of the present invention is realized by the ECU 30 executing the processing of steps 194 and 196.

It is a figure for demonstrating the structure of Embodiment 1 of this invention. It is a timing chart for demonstrating the basic operation | movement of the system of Embodiment 1 of this invention. (1) It is a figure for demonstrating the method of estimating NH3 adsorption amount which is the 2nd term | claim on the right side of Formula. (1) It is a figure for demonstrating the method of estimating NH3 consumption which is the 3rd term | claim on the right side of Formula. (1) It is a figure for demonstrating the method of estimating the NH3 purge amount which is 4th term | claim on the right side of Formula. It is a flowchart of the routine performed in Embodiment 1 of the present invention. It is a figure for demonstrating the structure of the modification of the system of Embodiment 1 of this invention. It is a figure for demonstrating the structure of Embodiment 2 of this invention. It is a figure for demonstrating the method of estimating NH3 density | concentration in SCR entrance. It is a figure which shows the relationship between the driving | running state of an internal combustion engine, and NH3 adsorption | suction retention amount by estimation. It is a flowchart of the routine performed in Embodiment 3 of the present invention. It is a figure for demonstrating the structure of Embodiment 4 of this invention. It is a figure for demonstrating the structure of the modification of the system of Embodiment 4 of this invention. It is a figure for demonstrating the method in which the system of Embodiment 5 of this invention calculates NOx tail pipe emission. It is a flowchart of the routine performed in Embodiment 5 of this invention.

Explanation of symbols

10 Internal combustion engine 14 Three-way catalyst 16 NSR (NOx storage reduction catalyst)
18 SCR (NOx selective reduction catalyst)
20 Air-fuel ratio sensor 22 NH3 sensor 24 Air flow meter (AFM)
30 ECU (Electronic Control Unit)

Claims (9)

A first catalyst having a function of purifying NOx in exhaust gas;
A NOx selective reduction catalyst disposed downstream of the first catalyst;
NH3 adsorption / retention amount storage means for storing the calculated value of NH3 adsorption / retention amount in the NOx selective reduction catalyst;
NH3 adsorption amount calculating means for calculating the NH3 adsorption amount adsorbed from the exhaust gas by the NOx selective reduction catalyst;
NH3 consumption calculation means for calculating NH3 consumption consumed as the NOx selective reduction catalyst reduces NOx in the exhaust gas;
NH3 purge amount calculating means for calculating the NH3 purge amount purged by the gas flowing through the NOx selective reduction catalyst;
NH3 adsorption / retention amount update means for updating the calculated value of NH3 adsorption / retention amount stored in the NH3 adsorption / retention amount storage means based on the NH3 adsorption amount, the NH3 consumption amount, and the NH3 purge amount;
An exhaust emission control device for an internal combustion engine, comprising: The NH3 adsorption amount calculating means is
NH3 inflow amount calculation means for calculating the NH3 inflow amount flowing into the NOx selective reduction catalyst, and NH3 adsorption rate calculation means for calculating the NH3 adsorption rate of the NOx selective reduction catalyst, the NH3 inflow amount and the NH3 Calculate the NH3 adsorption amount based on the multiplication value with the adsorption rate,
The NH3 consumption calculation means is:
NOx inflow amount calculating means for calculating the NOx inflow amount flowing into the NOx selective reduction catalyst, and NOx purification rate calculating means for calculating the NOx purification rate of the NH3 selective reduction catalyst, the NOx inflow amount and the NOx Calculate the NH3 consumption based on the product of the purification rate,
The NH3 purge amount calculation means
Gas flow rate detecting means for detecting a gas flow rate flowing through the NOx selective reduction catalyst, and NH3 purge rate calculating means for calculating a purge rate for purging NH3 with the gas flow by the NOx selective reduction catalyst, 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the NH3 purge amount is calculated based on a multiplication value of the NH3 adsorption / holding amount, the purge rate, and the gas flow rate. The purge rate calculating means includes
Oxygen concentration detection means for detecting the oxygen concentration at the inlet of the NOx selective reduction catalyst;
A purge rate rule storage unit that stores a purge rate rule that defines the relationship between the calculated value of the NH3 adsorption retention amount and the oxygen concentration, and the purge rate, and
3. The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the purge rate is calculated according to the purge rate rule. Comparing means for comparing the calculated value of the NH3 adsorption retention amount with its target value;
When the calculated value of the NH3 adsorption retention amount is excessive with respect to the target value, at least one of the air-fuel ratio of the internal combustion engine, the temperature of the first catalyst, and the NOx amount in the first catalyst is the NH3 When the operating state of the internal combustion engine is changed so as to change the amount of adsorption, and on the other hand, the calculated value of the NH3 adsorption retention amount is too small relative to the target value, the air-fuel ratio of the internal combustion engine, the first Control means for changing the operating state of the internal combustion engine so that at least one of the temperature of one catalyst and the amount of NOx in the first catalyst changes in a direction to increase the amount of adsorption of NH3;
The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3, further comprising: Comparing means for comparing the calculated value of the NH3 adsorption retention amount with its target value;
When the calculated value of the NH3 adsorption retention amount is excessive with respect to the target value, at least one of the NOx inflow amount flowing into the NOx selective reduction catalyst and the NOx purification rate of the NH3 selective reduction catalyst is the NH3 When the operating state of the internal combustion engine is changed so as to increase the consumption amount, and the calculated value of the NH3 adsorption / holding amount is too small with respect to the target value, the NOx inflow amount and the NOx purification Control means for changing the operating state of the internal combustion engine so that at least one of the rates changes in a direction to reduce the NH3 consumption;
The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 4, further comprising: Comparing means for comparing the calculated value of the NH3 adsorption retention amount with its target value;
A direction in which at least one of the air-fuel ratio of the internal combustion engine and the gas flow rate flowing through the NOx selective reduction catalyst increases the NH3 purge amount when the calculated value of the NH3 adsorption retention amount is excessive with respect to the target value. When the operating state of the internal combustion engine is changed so that the calculated value of the NH3 adsorption retention amount is excessively smaller than the target value, at least one of the air-fuel ratio of the internal combustion engine and the gas flow rate is changed. One of the control means for changing the operating state of the internal combustion engine to change in a direction to reduce the NH3 purge amount,
The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 5, further comprising: The NH3 purge amount calculating means includes a gas flow rate detecting means for detecting a gas flow rate flowing through the NOx selective reduction catalyst, and an NH3 purge for calculating a purge rate at which the NOx selective reduction catalyst purges NH3 with a gas flow. Rate calculating means, and calculating the NH3 purge amount based on a calculated value of the NH3 adsorption retention amount, the purge rate, and the gas flow rate,
NH3 concentration prediction means for calculating an NH3 concentration prediction value downstream of the NOx selective reduction catalyst based on the gas flow rate and the NH3 purge amount;
An NH3 sensor that measures the NH3 concentration downstream of the NOx selective reduction catalyst;
Calculation method correction means for correcting the NH3 adsorption retention amount calculation method so that the difference between the NH3 concentration predicted value concentration and the measured value of NH3 concentration by the NH3 sensor is small,
The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 6, further comprising: NOx inflow amount calculating means for calculating the NOx inflow amount flowing into the NOx selective reduction catalyst;
NOx purification rate calculating means for calculating the NOx purification rate of the NH3 selective reduction catalyst;
NOx emission calculation means for calculating NOx emission downstream of the NOx selective reduction catalyst by subtracting the product of the NOx inflow amount and the NOx purification rate from the NOx inflow amount;
The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 7, further comprising: An emission comparison means for comparing the calculated value of the NOx emission and the emission target value;
Control means for changing the operating state of the internal combustion engine so that the emission calculation value matches the emission target value;
An exhaust purification device for an internal combustion engine according to any one of claims 1 to 8, further comprising:

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