JP2005171809A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress degradation of emission when a temperature of an exhaust purification catalyst is increased to a catalytic activity temperature, the temperature of the exhaust purification catalyst is estimated, and then fuel is supplied to the exhaust purification catalyst based on the estimated temperature. <P>SOLUTION: An exhaust emission control device for an internal combustion engine comprises: a first catalyst temperature estimation means for estimating the temperature of the catalyst based on an inflow exhaust temperature, an amount of reducer in the exhaust, and heat capacity in the catalyst; and a second catalyst temperature estimation means for estimating a temperature of a predetermined part of the catalyst based on the inflow exhaust temperature and the heat capacity in the catalyst after excluding temperature increase of the catalyst based on the reducer. When a temperature of the catalyst estimated by first catalyst temperature estimation means exceeds a temporary catalyst activity temperature, the reducer is added to the exhaust (S102, S103), and the addition of the reducer to the exhaust is prohibited based on the temperature of the predetermined part of the catalyst estimated by the second catalyst temperature estimation means (S105, S107). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification apparatus that purifies exhaust gas from an internal combustion engine.

  As an exhaust gas purification catalyst for purifying exhaust gas discharged from an internal combustion engine, a so-called storage reduction type NOx catalyst, a three-way catalyst, an oxidation catalyst, or the like is used. These exhaust purification catalysts need to reach the activation temperature in order to exhibit their catalytic functions. Further, when the exhaust gas purification catalyst is deteriorated, the catalyst function may not be sufficiently exhibited.

  Therefore, whether or not catalyst deterioration has occurred in the exhaust purification catalyst from the temperature difference between the temperature of the exhaust flowing into the exhaust purification catalyst when the reducing agent is added to the exhaust flowing into the exhaust purification catalyst and the estimated temperature of the exhaust purification catalyst A technique for judging the above is disclosed (for example, see Patent Document 1). That is, the degree of catalyst deterioration of the exhaust purification catalyst is determined using the decrease in the oxidizing ability of the exhaust purification catalyst due to catalyst deterioration.

Further, when supplying a reducing agent to the exhaust purification catalyst in order to set the temperature of the exhaust purification catalyst to the activation temperature, the actual temperature of the exhaust purification catalyst and the temperature of the exhaust purification catalyst estimated from the amount of reducing agent in the exhaust, etc. A technique for prohibiting the supply of the reducing agent when the temperature difference exceeds the allowable range is disclosed (for example, see Patent Document 2). That is, the fuel supplied to the exhaust purification catalyst by not reaching the exhaust purification catalyst temperature (the estimated temperature of the exhaust purification catalyst described above), which is the temperature at which the exhaust purification catalyst should be originally, by supplying the reducing agent. Is released to the outside air.
JP 2003-214153 A JP 2001-227325 A

  In order to efficiently exhibit the catalytic function of the exhaust purification catalyst of the internal combustion engine, it is preferable to quickly bring the temperature of the exhaust purification catalyst to the catalyst activation temperature. Therefore, in order to raise the temperature of the exhaust purification catalyst, in addition to using the temperature of the exhaust gas flowing into the exhaust purification catalyst, a reducing agent is supplied to the exhaust purification catalyst to reduce the reaction heat between the reducing agent and the exhaust purification catalyst. By using this, the temperature of the exhaust purification catalyst is increased.

  Here, for the reaction with the reducing agent in the exhaust purification catalyst, the temperature of the exhaust purification catalyst needs to reach a temperature at which the reaction can occur (hereinafter referred to as “provisional catalyst activation temperature”). is there. The temporary catalyst activation temperature is lower than the above-described catalyst activation temperature. However, when estimating the temperature of the exhaust purification catalyst and supplying the reducing agent to the exhaust purification catalyst based on the estimated temperature, if the accuracy of the estimated temperature is poor, the actual temperature of the exhaust purification catalyst becomes the temporary catalyst. Fuel may be supplied to the exhaust purification catalyst when the estimated temperature is not achieved. As a result, the supplied reducing agent is not subjected to the reaction with the oxidation catalyst, and is released to the outside air, which may deteriorate the emission.

  In the present invention, in view of the above problems, when the temperature of the exhaust purification catalyst is raised to the catalyst activation temperature, the temperature of the exhaust purification catalyst is estimated and then the fuel is supplied to the exhaust purification catalyst based on the estimated temperature. The purpose is to suppress the deterioration of emissions.

  In order to solve the above-described problems, the present invention firstly estimates the temperature of the exhaust purification catalyst, supplies fuel to the exhaust purification catalyst, and reacts the exhaust purification catalyst with the supplied reducing agent. Appropriateness of fuel supply to the exhaust purification catalyst based on the estimated temperature of the exhaust purification catalyst estimated by excluding temperature rise due to heat, that is, whether the temperature of the exhaust purification catalyst has reached the temporary catalyst activation temperature at the time of fuel supply Judgment was made on whether or not. By excluding the temperature rise due to reaction heat, the temperature of the exhaust purification catalyst is estimated to the low temperature side with relatively high accuracy. Therefore, by using the estimated temperature as a reference, it is possible to stably determine whether or not the temperature of the exhaust purification catalyst has reached the temporary catalyst activation temperature.

  Accordingly, the present invention provides an exhaust purification device for an internal combustion engine, provided in an exhaust passage of the internal combustion engine, having an oxidizing ability, and a reducing agent adding means for adding a reducing agent to the exhaust gas flowing into the exhaust purification catalyst. And first catalyst temperature estimating means for estimating the temperature of the exhaust purification catalyst based on at least the temperature of the exhaust flowing into the exhaust purification catalyst, the amount of reducing agent contained in the exhaust, and the heat capacity in the exhaust purification catalyst; In addition to excluding the temperature increase of the exhaust purification catalyst based on the reducing agent contained in the exhaust gas flowing into the exhaust purification catalyst, at least based on the temperature of the exhaust gas flowing into the exhaust purification catalyst and the heat capacity in the exhaust purification catalyst Second catalyst temperature estimating means for estimating the temperature of the predetermined portion of the exhaust purification catalyst, and the temperature of the exhaust purification catalyst estimated by the first catalyst temperature estimating means The temperature of the exhaust purification catalyst estimated by the first catalyst temperature estimating means is the provisional catalyst activation temperature, the reducing agent addition control means for adding the reducing agent to the exhaust gas by the reducing agent addition means, When the catalyst activation determining means for determining that the exhaust purification catalyst is in an activated state when a higher catalyst activation temperature is exceeded, and when the reducing agent is added to the exhaust gas by the reducing agent addition control means, Reducing agent addition prohibiting means for prohibiting the addition of the reducing agent to the exhaust gas by the reducing agent addition control means based on the temperature of the predetermined portion of the exhaust purification catalyst estimated by the second catalyst temperature estimating means.

  In the exhaust gas purification apparatus for an internal combustion engine described above, the exhaust gas is purified by the exhaust gas purification catalyst provided in the exhaust passage. Here, in order to efficiently perform exhaust purification, for example, removal of NOx and particulate matter in the exhaust, the temperature of the exhaust purification catalyst needs to reach the active catalyst temperature. Therefore, when the temperature of the exhaust purification catalyst is low, it is preferable to quickly raise the temperature to the active catalyst temperature.

  Therefore, when the temperature of the exhaust purification catalyst is relatively low, the temperature of the exhaust purification catalyst is raised by the temperature of the exhaust discharged from the internal combustion engine. When the temperature of the exhaust purification catalyst reaches a temperature at which the reducing agent in the exhaust can be oxidized, that is, the temporary catalyst activation temperature, the reducing agent is added to the exhaust flowing into the exhaust purification catalyst, thereby A reducing agent is supplied to the catalyst, and heat of reaction due to oxidation of the reducing agent is generated between the reducing agent and the exhaust purification catalyst, thereby raising the temperature of the exhaust purification catalyst. As a result, the temperature of the exhaust purification catalyst can be quickly increased.

  At this time, the determination of the start of supply of the reducing agent to the exhaust purification catalyst by the fuel addition means is based on the catalyst temperature estimated by the first catalyst temperature estimation means. The first catalyst temperature estimating means estimates the catalyst temperature based on at least the temperature of the inflowing exhaust, the amount of reducing agent in the exhaust including the supplied reducing agent, and the heat capacity of the exhaust purification catalyst as described above. That is, it is assumed that at least the thermal energy of the inflowing exhaust gas and the oxidizing thermal energy generated by the oxidation of the reducing agent contribute to the temperature increase of the exhaust purification catalyst. Furthermore, by considering the heat capacity of the exhaust purification catalyst, the influence of fluctuations in the inflow exhaust temperature can be mitigated, and a relatively stable catalyst temperature can be estimated.

  Before the reducing agent is supplied by the fuel addition means, most of the reducing agent contained in the exhaust is a reducing agent such as unburned fuel contained in the exhaust from the internal combustion engine 1. Since the temperature of the exhaust purification catalyst is lower than the temporary catalyst activation temperature, the contribution of the reducing agent in the exhaust to the temperature increase of the exhaust purification catalyst may be handled very small or ignored.

  However, since the accuracy of the estimated temperature by the first catalyst temperature estimating means is reduced, the reduction to the exhaust purification catalyst by the reducing agent addition control means is performed even though the temperature of the exhaust purification catalyst has not reached the temporary catalyst activation temperature. When the agent is supplied, the reducing agent is not oxidized but is released to the outside air, which may deteriorate the emission. Therefore, the reducing agent addition prohibiting means determines whether or not the reducing agent addition control means supplies the reducing agent to the exhaust purification catalyst appropriately based on the estimated temperature by the second catalyst temperature estimating means, and the reducing agent to the exhaust purification catalyst is determined. When it is determined that the supply is not appropriate, the supply of the reducing agent is prohibited.

  Here, the second catalyst temperature estimating means estimates a temperature of a predetermined portion of the exhaust purification catalyst, that is, a more preferable portion of the exhaust purification catalyst for determining whether or not the reducing agent supply to the exhaust purification catalyst is appropriate. In estimating the temperature, the contribution of the oxidation heat energy generated by the oxidation of the reducing agent supplied to the exhaust purification catalyst to the temperature increase of the exhaust purification catalyst is not considered. By estimating the temperature of the exhaust purification catalyst by excluding the temperature rise due to the heat of oxidation reaction, the factor considered for the temperature estimation is reduced, the estimated temperature becomes relatively accurate, and the estimated temperature becomes the low temperature side temperature . Therefore, it is possible to more stably determine whether or not the temperature of the exhaust purification catalyst has reached the provisional catalyst activation temperature with reference to the estimated temperature of the predetermined portion that is estimated relatively accurately as the low temperature side temperature. It becomes.

  Thereby, even when the reducing agent is supplied to the exhaust purification catalyst and the temperature of the exhaust purification catalyst is increased, when the reducing agent is not oxidized and the reducing agent may be released to the outside air, The reducing agent addition prohibiting means prohibits the supply of the reducing agent to the exhaust purification catalyst, thereby suppressing the deterioration of the emission.

  Here, in the exhaust gas purification apparatus for an internal combustion engine described above, the second catalyst temperature estimating means estimates the temperature of the front end part, which is a part where exhaust flows into the exhaust purification catalyst, and the reducing agent addition prohibiting means is When the reducing agent is added to the exhaust gas by the reducing agent addition control means, the temperature of the front end portion of the exhaust purification catalyst estimated by the second catalyst temperature estimating means is equal to or lower than the temporary catalyst activation temperature. In this case, addition of the reducing agent to the exhaust by the reducing agent addition control means may be prohibited.

  That is, the predetermined portion of the exhaust purification catalyst whose temperature is estimated by the second catalyst temperature estimation means is the front end portion where the exhaust flows into the exhaust purification catalyst. Since the front end part is the part where exhaust gas flows first in the exhaust purification catalyst, it is strongly subject to fluctuations in the exhaust temperature. In particular, at a time immediately after the first catalyst temperature estimating means determines that the catalyst temperature has exceeded the provisional catalyst activation temperature, if the exhaust temperature decreases due to the operating state of the internal combustion engine, the catalyst temperature at the preceding stage also decreases, The catalyst temperature may be lower than the temporary catalyst activation temperature.

  At this time, since the catalyst temperature at the front part estimated by the second catalyst temperature estimation means is estimated by excluding the heat of oxidation due to oxidation of the reducing agent, the catalyst temperature at the front part is equal to the lower limit temperature in the exhaust purification catalyst. Can be considered. Therefore, when the catalyst temperature at the front stage estimated by the second catalyst temperature estimation means is equal to or lower than the provisional catalyst activation temperature, the supplied additive is not sufficiently oxidized and is likely to be released to the outside air. The supply of the reducing agent by the reducing agent addition control means is prohibited. In addition, in the temperature estimation by the second catalyst temperature estimation means, the heat capacity of the exhaust purification catalyst is taken into account, so that the influence due to the steep fluctuation of the inflowing exhaust gas temperature is mitigated, and a relatively stable catalyst temperature can be estimated. .

In addition, since the catalyst temperature at the front part estimated by the second catalyst temperature estimation means is equal to or lower than the provisional catalyst activation temperature, the temperature at the front part again becomes lower after the supply of the reducing agent to the exhaust purification catalyst is prohibited. When the temporary catalyst activation temperature is exceeded, the supply of the reducing agent to the exhaust purification catalyst may be resumed.

  Further, in the above-described exhaust gas purification apparatus for an internal combustion engine, the exhaust gas purification apparatus further includes an exhaust gas temperature detection means that is provided on the downstream side of the exhaust gas purification catalyst and detects the temperature of the exhaust gas flowing out from the exhaust gas purification catalyst. In this case, the second catalyst temperature estimating means estimates the temperature of the rear end portion, which is a portion where exhaust flows out from the exhaust purification catalyst, and the reducing agent addition prohibiting means is connected to the exhaust gas by the reducing agent addition control means. The temperature difference between the temperature of the rear end portion of the exhaust purification catalyst estimated by the second catalyst temperature estimation means and the exhaust temperature detected by the exhaust temperature detection means when the reducing agent is added is When the temperature falls below a predetermined temperature, addition of the reducing agent to the exhaust by the reducing agent addition control means may be prohibited.

  That is, the predetermined part of the exhaust purification catalyst whose temperature is estimated by the second catalyst temperature estimation means is the rear end part from which the exhaust flows out of the exhaust purification catalyst. Here, the exhaust gas temperature detection means is provided downstream of the exhaust gas purification catalyst, and the exhaust gas temperature detected by the exhaust gas temperature detection means can be regarded substantially the same as the temperature of the exhaust gas purification catalyst. The catalyst temperature at the rear stage estimated by the second catalyst temperature estimation means is estimated by excluding the heat of oxidation due to the oxidation of the supplied reducing agent. Therefore, when oxidation of the reducing agent supplied in the exhaust purification catalyst is properly performed, the detected value of the exhaust temperature by the exhaust temperature detecting means and the estimated value of the catalyst temperature at the subsequent stage by the second catalyst temperature estimating means And the temperature difference exceeds the predetermined temperature.

  Therefore, based on the temperature difference, it is possible to determine whether or not the reducing agent addition control means is properly supplying the reducing agent to the exhaust purification catalyst, and the temperature difference is not more than a predetermined temperature. In this case, since the supplied additive is not sufficiently oxidized and is likely to be released to the outside air, the supply of the reducing agent by the reducing agent addition control means is prohibited.

  Further, since the temperature difference between the detected value of the exhaust temperature by the exhaust temperature detecting means and the estimated value of the catalyst temperature at the subsequent stage by the second catalyst temperature estimating means is equal to or less than a predetermined temperature, the reducing agent is not supplied to the exhaust purification catalyst. After the prohibition, when the temperature difference again exceeds the predetermined temperature, the supply of the reducing agent to the exhaust purification catalyst may be resumed.

  Here, in order to solve the above-described problems, the present invention focuses on the temperature transition of the exhaust gas flowing out from the exhaust purification catalyst over a relatively long period. Based on the transition of the exhaust temperature over a relatively long period of time, even if the exhaust temperature flowing out from the exhaust purification catalyst varies with the variation of the operating state of the internal combustion engine, the trend of the transition of the exhaust temperature relatively accurately, that is, the exhaust It is possible to determine whether the temperature is in the falling state or in the rising state.

  Accordingly, the present invention provides an exhaust purification device for an internal combustion engine, provided in an exhaust passage of the internal combustion engine, having an oxidizing ability, and a reducing agent adding means for adding a reducing agent to the exhaust gas flowing into the exhaust purification catalyst. Catalyst temperature estimating means for estimating the temperature of the exhaust purification catalyst based on at least the temperature of the exhaust gas flowing into the exhaust purification catalyst, the amount of reducing agent contained in the exhaust gas, and the heat capacity in the exhaust purification catalyst; An exhaust temperature detecting means provided on the downstream side of the exhaust purification catalyst for detecting the temperature of the exhaust gas flowing out from the exhaust purification catalyst, and the temperature of the exhaust purification catalyst estimated by the catalyst temperature estimating means is a temporary catalyst activation temperature. A reducing agent addition control means for adding a reducing agent to the exhaust gas by the reducing agent addition means, and a temperature of the exhaust purification catalyst estimated by the catalyst temperature estimating means. When the catalyst activation temperature higher than the temporary catalyst activation temperature is exceeded, the catalyst is judged to be in an active state, and the reducing agent is added to the exhaust gas by the reducing agent addition control means. When the exhaust gas temperature decrease amount detected by the exhaust gas temperature detection means exceeds a permissible exhaust gas temperature decrease allowable range determined based on the operating state of the internal combustion engine during a predetermined period, the reducing agent addition control means And a reducing agent addition prohibiting means for prohibiting the addition of the reducing agent to the exhaust gas due to the exhaust gas.

  As described above, the predetermined period is a period necessary to determine the temperature transition tendency of the exhaust gas flowing out from the exhaust purification catalyst. In other words, when it is determined that the catalyst temperature has exceeded the provisional catalyst activation temperature in the exhaust purification catalyst and the reducing agent is being supplied to the exhaust purification catalyst, the reducing agent is oxidized to increase the temperature of the exhaust purification catalyst. This is a detection period during which it is possible to detect a tendency of fluctuations in the exhaust gas temperature depending on the operating state of the internal combustion engine in determining whether or not it contributes. The exhaust temperature decrease allowable range is an allowable range of exhaust temperature decrease caused by the operating state of the internal combustion engine.

  That is, when the reducing agent is supplied and the catalyst temperature is in the rising state, the exhaust temperature flowing out from the exhaust purification catalyst is also in the rising state. However, the exhaust temperature may also decrease depending on the operating state of the internal combustion engine, but when the amount of decrease is a temperature decrease exceeding the allowable exhaust temperature decrease range, the reducing agent supplied in the exhaust purification catalyst is oxidized. There is a high probability of being released to the outside air. Therefore, in such a case, the supply of the reducing agent by the reducing agent addition control means is prohibited to suppress the deterioration of the emission.

  When raising the temperature of the exhaust purification catalyst to the catalyst activation temperature, it is possible to estimate the temperature of the exhaust purification catalyst and suppress emission deterioration when fuel is supplied to the exhaust purification catalyst based on the estimated temperature It becomes.

  Here, an embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described based on the drawings.

  FIG. 1 is a block diagram showing a schematic configuration of an intake and exhaust system of an internal combustion engine to which the present invention is applied. Here, the internal combustion engine 1 is a compression ignition type internal combustion engine. An intake passage 2 is connected to the combustion chamber of the internal combustion engine 1. Further, exhaust gas generated by combustion in the internal combustion engine 1 is discharged from the internal combustion engine 1 to the exhaust passage 3. In the middle of the exhaust passage 3, there are provided an oxidation catalyst 4 having oxidation ability and a filter 5 (hereinafter simply referred to as “filter”) carrying a so-called storage reduction type NOx catalyst on the downstream side of the oxidation catalyst 4. Since the NOx storage reduction catalyst contains platinum as a component, the filter 5 acts as a catalyst having oxidation ability. A fuel addition valve 6 is provided in the exhaust passage 3 upstream of the oxidation catalyst 4 to add fuel from the internal combustion engine 1 to the exhaust gas flowing through the exhaust passage 3. The fuel added to the exhaust gas from the fuel addition valve 6 is supplied to the oxidation catalyst 4 and the filter 5, acts as a reducing agent for these catalysts, and is oxidized by the oxidizing ability of these catalysts, so that the heat of oxidation is increased. Occur.

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

  Various sensors for detecting the operating state of the internal combustion engine 1 such as a crank position sensor 9 and an accelerator opening sensor 10 are connected to the ECU 20 via electric wiring, and their output signals are input to the ECU 20. ing. Further, an exhaust gas temperature sensor 7 and an exhaust air / fuel ratio sensor 8 provided on the downstream side of the filter 5 are electrically connected to the ECU 20. The temperature of the exhaust gas flowing out from the filter 7 is detected by the exhaust temperature sensor, and the air-fuel ratio of the exhaust gas is detected by the exhaust air-fuel ratio sensor 8.

On the other hand, the fuel addition valve 6 is connected to the ECU 20 via an electrical wiring, and the amount of fuel supplied from the fuel addition valve 6 to the exhaust gas flowing through the exhaust passage 3 is controlled according to a command from the ECU 20. Although not shown in FIG. 1, the fuel injection valve provided in the internal combustion engine 1 is also electrically connected to the ECU 20, and the timing and amount of fuel injection from the fuel injection valve are controlled in accordance with a command from the ECU 20. Is done.

  In the exhaust system of the internal combustion engine 1 configured as described above, particulate matter contained in the exhaust is collected by the filter 5 and then oxidized and removed, or NOx in the exhaust is stored in the filter 5 as a NOx storage reduction catalyst. The exhaust gas is purified by being reduced. Further, the oxidation catalyst 4 oxidizes the fuel in the exhaust gas to generate oxidation heat and raise the temperature of the exhaust gas flowing into the filter 5. As a result, the temperature of the NOx storage reduction catalyst in the filter 5 rises, and the exhaust purification ability by the catalytic function is exhibited.

  However, in order for the catalytic functions of the oxidation catalyst 4 and the filter 5 to be exerted, the temperature of each catalyst needs to reach an activation temperature (hereinafter referred to as “catalytic activation temperature”). Therefore, in order to quickly raise the catalyst temperature of the oxidation catalyst 4 and the filter 5 to the catalyst activation temperature, the thermal energy of the exhaust from the internal combustion engine 1 is used, and the fuel is added to the exhaust from the fuel addition valve 6. The oxidation heat generated when the fuel is oxidized by the oxidation ability of the oxidation catalyst 4 and the filter 5 is used. At this time, the addition of the fuel from the fuel addition valve 6 reaches the temperature at which the catalyst temperature of the oxidation catalyst 4 or the filter 5 can exhibit the oxidation ability of each of them (hereinafter referred to as “provisional catalyst activation temperature”). Need to be. The temporary catalyst activation temperature is a temperature lower than the above-described catalyst activation temperature at which the catalyst function can be sufficiently exhibited.

  When fuel is added from the fuel addition valve 6 when the catalyst temperatures of the oxidation catalyst 4 and the filter 5 have not reached the temporary catalyst activation temperature, the fuel is not oxidized by the oxidation catalyst 4 or the filter 5 and is released to the outside air. Released and worsens emissions. Therefore, fuel addition from the fuel addition valve 6 is necessary to quickly raise the temperature of the oxidation catalyst 4 and the filter 5 to the catalyst activation temperature. On the other hand, these catalyst temperatures reach the temporary catalyst activation temperature. If not, it is not preferable that the fuel addition from the fuel addition valve 6 is performed in order to suppress the deterioration of the emission. Therefore, in order to execute fuel addition from the fuel addition valve 6, it is necessary to accurately grasp the catalyst temperatures of the oxidation catalyst 4 and the filter 5.

  Therefore, the following describes the grasp of the catalyst temperature of the oxidation catalyst 4 and the filter 5 and the addition of fuel from the fuel addition valve 6 in order to increase the catalyst temperature of the oxidation catalyst 4 and the filter 5 and suppress the deterioration of the emission. .

  Here, the oxidation catalyst 4 is divided into two in the flow direction of the exhaust gas, and the upstream portion thereof is referred to as a front end portion 4a and the downstream portion thereof is referred to as a rear portion 4b. Further, the filter 5 is divided into three along the flow direction of the exhaust gas, the most upstream portion is the front end portion 5a, the most downstream portion is the rear end portion 5c, the front end portion 5a and the rear end portion 5c. The part between is set as the intermediate part 5b. Moreover, in estimating the catalyst temperature of each site | part, an initial value is given to each. For example, when the internal combustion engine 1 is started, the temperatures of the oxidation catalyst 4 and the filter 5 are considered to be substantially the same as the outside air temperature, and the same value as the outside air temperature is set as the initial value of the catalyst temperature of each part. .

  Part of the fuel added from the fuel addition valve 6 directly reaches the oxidation catalyst 4, and the rest adheres to the wall surface of the exhaust passage 3. Further, part of the fuel that has already adhered to the wall surface of the exhaust passage 3 evaporates and reaches the oxidation catalyst 4 again. Further, the fuel remaining as an unburned component in the exhaust gas without being burned in the combustion of the fuel in the internal combustion engine 1 reaches the oxidation catalyst 4. The amount of fuel reaching the oxidation catalyst 4 is calculated based on the operating state of the internal combustion engine 1 such as the engine rotational speed and the engine load, the temperature of the exhaust gas immediately after being discharged from the internal combustion engine 1 estimated from the operating state, and the like. To do.

  Specifically, for example, the relationship between the operating state of the internal combustion engine 1 and the amount of direct arrival at the oxidation catalyst 4 due to the addition of fuel from the fuel addition valve 6 is measured in advance by experiments or the like, so that the operating state of the internal combustion engine 1 is determined. Then, a map for calculating the amount of direct arrival at the oxidation catalyst 4 using the addition amount from the fuel addition valve 6 as a parameter is created, and the amount of fuel reaching the oxidation catalyst 4 is calculated by accessing the map. When the fuel addition from the fuel addition valve 6 is not executed, the amount of fuel reaching the oxidation catalyst 4 is the amount remaining in the exhaust gas as an unburned component in the combustion of the internal combustion engine 1.

  Next, the ratio of the fuel that has reached the oxidation catalyst 4 that is oxidized by the front end portion 4a is calculated. The degree of fuel oxidation (hereinafter referred to as “oxidation rate”) in the catalyst having oxidation ability is determined by the flow rate of exhaust gas flowing into the catalyst and the temperature of the catalyst itself. That is, the oxidation rate of the fuel decreases as the flow rate of the exhaust gas flowing into the catalyst increases, and the oxidation rate of the fuel increases as the temperature of the catalyst itself increases. Therefore, the oxidation rate of the fuel is calculated based on the exhaust gas flow rate estimated from the operating state of the internal combustion engine 1 and the current temperature of the front end portion 4a. And the fuel which is not oxidized is supplied to the rear-end part 4b located in the downstream of the front-end part 4a. Hereinafter, similarly, the oxidation rate of the fuel at the rear end portion 4b is calculated.

  Next, the sum total of the thermal energy which exists in the front-end part 4a is calculated. In the front end part 4a, the thermal energy of the exhaust gas flowing into the front end part 4a, the thermal energy of oxidation heat generated by the oxidation of the fuel in the front end part 4a, and the heat that the front end part 4a currently has There is energy. Then, the sum of these thermal energies is equally distributed to the front end portion 4a and the exhaust existing there, and the temperature of the new front end portion 4a is calculated assuming that the front end portion 4a and the exhaust fluctuate to the same value. To do. Specifically, the sum of the thermal energy present in the front end part 4a is divided by the total heat capacity of the exhaust present in the front end part 4a and the front end part 4a, whereby the temperature of the exhaust present in the front end part 4a and the front end part 4a. Is calculated.

  Thereafter, the exhaust gas existing in the front end part 4a moves to the rear end part 4b located on the downstream side of the front end part 4a. Similarly, the temperature of the rear end portion 4b is calculated based on the heat energy of the exhaust, the heat energy of the oxidation heat of the fuel, and the heat energy of the rear end portion 4b itself.

  Next, calculation of the catalyst temperature of each part in the filter 5 will be described. Similarly to the calculation of the catalyst temperature of each part in the oxidation catalyst 4 described above, the calculation of the catalyst temperature of each part in the filter 5 is performed by the thermal energy of the exhaust gas flowing into each part of the filter 5, the oxidation thermal energy of the fuel, and each part. Calculated based on its own thermal energy. Here, the amount of fuel supplied to the front end portion 5a of the filter 5 is the amount remaining without being oxidized in each portion of the oxidation catalyst 4, and the exhaust gas flowing into the front end portion 5a of the filter 5 is the oxidation catalyst. 4 is exhaust gas having a temperature calculated as described above at the rear end portion 4b.

  As described above, the temperature of each part of the oxidation catalyst 4 and the filter 5 is calculated, and when the catalyst temperature of the front end part 5a of the filter 5 exceeds the temporary catalyst activation temperature, the fuel addition from the fuel addition valve 6 is executed. Further, when the temperature of each part of the filter 5 exceeds the catalyst activation temperature, that is, when the detected value of the exhaust temperature sensor 7 exceeds the catalyst activation temperature, the exhaust purification ability of the NOx storage reduction catalyst in the filter 5 is sufficiently exhibited. Will be.

  However, when the calculated values of the catalyst temperature at each part of the oxidation catalyst 4 and the filter 5 are different from the actual catalyst temperature, especially when the calculated value is higher than the actual catalyst temperature, the oxidation of the added fuel is performed. May not be sufficiently performed. In such a case, it is preferable not to add fuel from the fuel addition valve 6. Therefore, control of fuel addition from the fuel addition valve 6 (hereinafter referred to as “fuel addition control”) will be described with reference to FIG. The fuel addition control is a routine executed when raising the catalyst temperature of the oxidation catalyst and the filter 5.

  In S101, thdp1, which is the catalyst temperature of the front end portion 5a of the filter 5, is calculated. Regarding the calculation of thdp1, as described above, the thermal energy of the exhaust gas flowing into the front end portion 5a, the oxidation thermal energy of the fuel that reaches the front end portion 5a and is oxidized among the fuel supplied from the internal combustion engine 1, the front end portion 5a It is calculated based on its own thermal energy. At this time, fuel addition from the fuel addition valve 6 is not performed. When the process of S101 ends, the process proceeds to S102.

  In S102, it is determined whether thdpl calculated in S101 exceeds the provisional catalyst activation temperature Tp_thav. That is, it is determined whether or not the fuel in the exhaust can be oxidized when the catalyst temperature of the front end portion 5a reaches the temporary catalyst activation temperature Tp_thav. If it is determined that thdpl exceeds the provisional catalyst activation temperature Tp_thav, the process proceeds to S103. On the other hand, if it is determined that thdpl does not exceed the provisional catalyst activation temperature Tp_thav, the process of S101 is performed again.

  In S103, fuel addition from the fuel addition valve 6 is started. That is, at this time, since it is determined in S102 that thdpl exceeds the provisional catalyst activation temperature Tp_thav, the added fuel is oxidized by the oxidation ability of the NOx storage reduction catalyst in the filter 5, and the temperature of the NOx storage reduction catalyst In order to increase, fuel addition from the fuel addition valve 6 is started. When the process of S103 ends, the process proceeds to S104.

  In S104, based on the thermal energy of the exhaust gas flowing into the front end portion 5a and the own thermal energy of the front end portion 5a, excluding the increase in the catalyst temperature of the portion due to the oxidation heat energy of the fuel supplied to the portion. Thus, the fuel oxidation heat removal filter front end part temperature thdp1_off which is the catalyst temperature of the front end part 5a of the filter 5 is calculated. That is, in the above-described method for calculating the catalyst temperature, the sum of the thermal energy of the exhaust flowing into the front end portion 5a and the thermal energy of the front end portion 5a is the total heat capacity of the exhaust existing in the front end portion 5a and the front end portion 5a. By dividing, thdp1_off is calculated. When the process of S104 ends, the process proceeds to S105.

  In S105, it is determined whether thdp1_off calculated in S104 is equal to or lower than the temporary catalyst activation temperature Tp_thav. Since the front end portion 5a is a portion into which exhaust gas flows in the filter 5, the exhaust temperature is strongly affected by the operating state of the internal combustion engine. Furthermore, since thdp1_off is calculated by excluding oxidation heat energy due to fuel oxidation, thdp1_off can be regarded as the lower limit temperature of the catalyst temperature in the filter 5. Therefore, whether or not the fuel addition from the fuel addition valve 6 is executed is determined based on whether or not thdp1_off is equal to or lower than the temporary catalyst activation temperature Tp_thav.

  Therefore, if it is determined in S105 that thdp1_off is equal to or lower than the provisional catalyst activation temperature Tp_thav, it means that there is a high probability that the fuel in the exhaust gas is not sufficiently oxidized at the front end portion 5a, and the process proceeds to S107. Fuel addition from the valve 6 is prohibited. On the other hand, if it is determined that thdp1_off exceeds the provisional catalyst activation temperature Tp_thav, this means that the fuel in the exhaust gas can be sufficiently oxidized at the front end portion 5a, and the process proceeds to S108, where the fuel from the fuel addition valve 6 is fueled. When the addition is prohibited, the fuel addition is resumed, and when the fuel addition from the fuel addition valve 6 is being performed, the fuel addition is continuously performed. When the process of S106 or S107 ends, the process proceeds to S108.

In S108, it is determined whether or not the exhaust temperature flowing out from the filter 5, which is a value detected by the exhaust temperature sensor 7, exceeds the catalyst activation temperature thav. Since the exhaust gas temperature sensor 7 is disposed on the downstream side of the filter 5, the exhaust gas temperature detected by the exhaust gas temperature sensor 7 can be almost identical to the actual catalyst temperature of the rear end portion 5 c of the filter 5. Therefore, when the actual catalyst temperature at the rear end portion 5c exceeds the catalyst activation temperature thav, it is determined that the NOx storage reduction catalyst in the filter 5 has reached the active state. When it is determined that the detected value by the exhaust temperature sensor 7 exceeds the catalyst activation temperature thav, this control is terminated. On the other hand, if it is determined that the detected value by the exhaust temperature sensor 7 is equal to or lower than the catalyst activation temperature thav, the processes after S104 are performed again.

  According to this control, the temperature increase of the NOx storage reduction catalyst in the filter 5 is quickly performed by using the fuel addition by the fuel addition valve 6, and the fuel to which the catalyst temperature is added at the time of the fuel addition is increased. When the temperature is not properly oxidized, the fuel addition is prohibited. As a result, it is possible to suppress the deterioration of emission due to the added fuel.

  Another embodiment of the fuel addition control in the exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described with reference to FIG. The exhaust gas purifying apparatus for the internal combustion engine in which the control flow shown in FIG. 3 is performed is the same as that shown in FIG. Also, in the control flow shown in FIG. 3, the same processes as those in the control flow shown in FIG. 2 are denoted by the same reference numerals, and the description thereof is omitted.

  In the control flow shown in FIG. 3, when the process of S103 is completed, the process proceeds to S201. In S201, excluding the increase in the catalyst temperature of the part due to the oxidation thermal energy of the fuel supplied to the part, the thermal energy of the exhaust gas flowing into the rear end part 5a and the own thermal energy of the rear end part 5c Based on this, the fuel oxidation heat removal filter rear end portion temperature thdp2_off, which is the catalyst temperature of the rear end portion 5c of the filter 5, is calculated. That is, in the above-described method for calculating the catalyst temperature, the sum of the thermal energy of the exhaust gas flowing into the rear end portion 5c and the own thermal energy of the rear end portion 5c is calculated as the exhaust gas existing in the rear end portion 5c and the rear end portion 5c. By dividing by the total heat capacity, thdp2_off is calculated. When the process of S201 ends, the process proceeds to S202.

  In S202, Δth, which is a temperature difference between thdp2_off calculated in S201 and the detected value by the exhaust temperature sensor 7, is calculated. Since the exhaust gas temperature sensor 7 is disposed on the downstream side of the filter 5, the exhaust gas temperature detected by the exhaust gas temperature sensor 7 can be almost identical to the actual catalyst temperature of the rear end portion 5 c of the filter 5. Therefore, Δth means the difference between the actual catalyst temperature of the rear end portion 5c and the catalyst temperature when it is assumed that there is no oxidation of the fuel. In other words, Δth is the rear end portion 5c due to the oxidation heat energy of the fuel. It is a parameter that represents the degree of contribution to the catalyst temperature rise. That is, when the catalyst temperature at the rear end portion 5c actually reaches the provisional catalyst activation temperature and the fuel is appropriately oxidized at the rear end portion 5c, the actual catalyst temperature rises, so that the value of Δth increases. . On the other hand, if the catalyst temperature at the rear end portion 5c does not actually reach the provisional catalyst activation temperature and the fuel is not properly oxidized at the rear end portion 5c, the fuel is supplied to the rear end portion 5c. However, since the actual catalyst temperature does not increase, the value of Δth does not increase, or the amount of increase is relatively small.

Therefore, in S203, it is determined whether or not the added fuel from the fuel addition valve 6 is oxidized based on Δth calculated in S202. Specifically, it is determined whether Δth is equal to or lower than a predetermined temperature Δth0. Δth0 is a threshold value of a temperature difference between thdp2_off and a value detected by the exhaust temperature sensor 7 for determining whether or not the added fuel from the fuel addition valve 6 is oxidized. If it is determined that Δth is equal to or smaller than Δth0, it means that there is a high probability that the fuel in the exhaust gas is not sufficiently oxidized at the rear end portion 5c, and the process proceeds to S107, where fuel addition from the fuel addition valve 6 is performed. prohibited. On the other hand, if it is determined that Δth exceeds Δth0, it means that the fuel in the exhaust gas can be sufficiently oxidized at the rear end portion 5c, and S1.
When the fuel addition from the fuel addition valve 6 is prohibited, the fuel addition is resumed. When the fuel addition from the fuel addition valve 6 is being performed, the fuel addition is continued. When the process of S106 or S107 ends, the process proceeds to S108.

  According to this control, the temperature increase of the NOx storage reduction catalyst in the filter 5 is quickly performed by using the fuel addition by the fuel addition valve 6, and the fuel to which the catalyst temperature is added at the time of the fuel addition is increased. When the temperature is not properly oxidized, the fuel addition is prohibited. As a result, it is possible to suppress the deterioration of emission due to the added fuel.

  Another embodiment of the fuel addition control in the exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described with reference to FIG. The exhaust gas purification apparatus for the internal combustion engine in which the control flow shown in FIG. 4 is performed is the same as that shown in FIG. Also, in the control flow shown in FIG. 4, the same processes as those in the control flow shown in FIG. 2 are denoted by the same reference numerals, and the description thereof is omitted.

  In the control flow shown in FIG. 4, when the process of S103 is completed, the process proceeds to S301. In S301, the exhaust gas temperature is acquired by the exhaust gas temperature sensor 7 at intervals of 10 seconds after shifting to S301.

  Here, the relationship between the exhaust temperature acquired by the exhaust temperature sensor 7 and the catalyst temperature of the NOx storage reduction catalyst in the filter 5 will be described with reference to FIG. FIG. 5 is a graph showing the temperature transition of the exhaust gas flowing out from the filter 5. The horizontal axis of the graph represents time, and the vertical axis represents the exhaust temperature. Also, the transition of the exhaust gas temperature is represented by the graph middle lines L1 and L2. In the five times from T1 to T5 on the horizontal axis in FIGS. 5A and 5B, the time interval is 10 seconds corresponding to the exhaust temperature acquisition interval in S301. That is, the exhaust gas temperature flowing out of the filter 5 is detected by the exhaust gas temperature sensor 7 at intervals of 10 seconds.

  FIG. 5A shows the transition of the exhaust gas temperature when the temperature of the NOx storage reduction catalyst of the filter 5 actually reaches the temporary catalyst activation temperature. In this case, unless the internal combustion engine 1 is in an operating state in which the exhaust gas temperature discharged from the internal combustion engine 1 is reduced, such as in a deceleration operation, the oxidation of the fuel added from the fuel addition valve 6 is represented by time T1 to T2. The exhaust gas temperature flowing out from the filter 5 gradually increases due to thermal energy or the like. Further, when the internal combustion engine 1 is in an operation state in which the exhaust gas temperature discharged from the internal combustion engine 1 is reduced, such as in a deceleration operation, the exhaust gas temperature is reduced as indicated by times T2 to T4. The amount of decrease is Δthsc1. However, since the oxidation heat energy of the fuel is generated, the amount of decrease Δthsc1 is relatively small, and the time during which the exhaust temperature decreases is also relatively short, after which the exhaust temperature starts to increase.

  On the other hand, the transition of the exhaust gas temperature shown in FIG. 5 (b) shows the fuel from the fuel addition valve 6 even though the actual temperature of the NOx storage reduction catalyst of the filter 5 has not reached the temporary catalyst activation temperature. It is transition of the exhaust temperature when addition is performed. Since the temperature of the NOx storage reduction catalyst of the filter 5 does not actually reach the temporary catalyst activation temperature, no oxidation heat energy of the fuel is generated. Accordingly, when the internal combustion engine 1 is in an operating state in which the exhaust temperature discharged from the internal combustion engine 1 is reduced, such as in a deceleration operation, the exhaust temperature is reduced as shown by the times T1 to T4. The amount of decrease is Δthsc2. As described above, since no oxidizing heat energy is generated in the fuel, the exhaust temperature decrease amount Δthsc2 is larger than the decrease amount Δthsc1 in FIG. 5A, and the exhaust temperature decrease state continues for a long time. .

Therefore, at the time of fuel addition from the fuel addition valve 6, it is possible to determine whether the temperature of the NOx storage reduction catalyst actually reaches the temporary catalyst activation temperature based on the amount of decrease in the exhaust temperature. Become. At this time, the amount of decrease in the exhaust temperature that is the criterion for the determination needs to be the amount of decrease in the exhaust temperature over a relatively long time. This is because if the interval for calculating the amount of decrease in the exhaust temperature is lengthened, whether the tendency of fluctuations in the exhaust temperature is in a descending state, or the exhaust temperature varies temporarily, although the exhaust temperature temporarily decreases. This is because it can be determined whether or not the vehicle is in the rising state. Then, as shown in FIG. 5B, when the fluctuation of the exhaust gas temperature is in a descending state, it means that the temperature of the NOx storage reduction catalyst has not actually reached the temporary catalyst activation temperature.

  Therefore, after the processing of S301, in S302, it is determined whether or not the exhaust temperature detected by the exhaust temperature sensor 7 is in a decreasing state. Specifically, as shown in FIGS. 5A and 5B, the value of Δthsc1 or Δthsc2, which is the amount of decrease in the exhaust temperature over a relatively long period, is determined based on the fluctuation of the operating state of the internal combustion engine. When the exhaust gas temperature drop amount exceeds the amount of decrease in the exhaust gas temperature, it can be determined that the catalyst temperature of the NOx storage reduction catalyst of the filter 5 does not actually reach the provisional catalyst activation temperature. For example, when the exhaust temperature decreases by 20 degrees because the operating state of the internal combustion engine becomes a deceleration state, the amount of decrease in the exhaust temperature of Δthsc1 is 10 degrees, and the amount of decrease in the exhaust temperature of Δthsc2 is 30 degrees, The catalyst temperature of the NOx storage reduction catalyst of the filter 5 in which a decrease in the exhaust temperature of Δthsc1 appears is determined to actually reach the temporary catalyst activation temperature, while the exhaust temperature of the filter 5 in which the decrease of the exhaust temperature of Δthsc2 appears. It is determined that the catalyst temperature of the NOx storage reduction catalyst does not actually reach the provisional catalyst activation temperature.

  If it is determined that the exhaust gas temperature detected by the exhaust gas temperature sensor 7 is in a lowered state, it means that there is a high probability that the filter 5 will not sufficiently oxidize the fuel in the exhaust gas. Fuel addition from the valve 6 is prohibited. On the other hand, if it is determined that the exhaust gas temperature detected by the exhaust gas temperature sensor 7 is not in the lowered state, it means that the filter 5 can sufficiently oxidize the fuel in the exhaust gas, and the process proceeds to S108, where fuel addition is performed. When the fuel addition from the valve 6 is prohibited, the fuel addition is resumed. When the fuel addition from the fuel addition valve 6 is being performed, the fuel addition is continuously performed. When the process of S106 or S107 ends, the process proceeds to S108.

  According to this control, the temperature increase of the NOx storage reduction catalyst in the filter 5 is quickly performed by using the fuel addition by the fuel addition valve 6, and the fuel to which the catalyst temperature is added at the time of the fuel addition is increased. When the temperature is not properly oxidized, the fuel addition is prohibited. As a result, it is possible to suppress the deterioration of emission due to the added fuel. In the present embodiment, the exhaust temperature acquisition interval by the exhaust temperature sensor 7 is 10 seconds, but this is an example and can be changed as appropriate.

1 is a block diagram illustrating a schematic configuration of an exhaust emission control device for an internal combustion engine according to an embodiment of the present invention. 4 is a first flowchart regarding fuel addition control from a fuel addition valve to exhaust in the exhaust gas purification apparatus for an internal combustion engine according to the embodiment of the present invention. 6 is a second flowchart regarding control of fuel addition from the fuel addition valve to the exhaust in the exhaust gas purification apparatus for an internal combustion engine according to the embodiment of the present invention. 7 is a third flowchart regarding fuel addition control from a fuel addition valve to exhaust in the exhaust gas purification apparatus for an internal combustion engine according to the embodiment of the present invention. 4 is a graph showing a temperature transition of exhaust gas flowing out from a filter in the exhaust gas purification apparatus for an internal combustion engine according to the embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ...... Internal combustion engine 3 ... Exhaust passage 4 ... Oxidation catalyst 4a ... Front end part 4b ... Rear end part 5 ... Filter 5a ... Front end part 5c .... Rear end part 6 ... Fuel addition valve 7 ... Exhaust temperature sensor 20 ... ECU

Claims (4)

An exhaust purification catalyst provided in the exhaust passage of the internal combustion engine and having oxidizing ability;
Reducing agent addition means for adding a reducing agent to the exhaust gas flowing into the exhaust purification catalyst;
First catalyst temperature estimation means for estimating the temperature of the exhaust purification catalyst based on at least the temperature of the exhaust flowing into the exhaust purification catalyst, the amount of reducing agent contained in the exhaust, and the heat capacity in the exhaust purification catalyst;
Excluding the temperature increase of the exhaust purification catalyst based on the reducing agent contained in the exhaust gas flowing into the exhaust purification catalyst, based on at least the temperature of the exhaust gas flowing into the exhaust purification catalyst and the heat capacity in the exhaust purification catalyst Second catalyst temperature estimating means for estimating the temperature of a predetermined portion of the exhaust purification catalyst;
A reducing agent addition control means for adding a reducing agent to the exhaust by the reducing agent addition means when the temperature of the exhaust purification catalyst estimated by the first catalyst temperature estimation means exceeds a temporary catalyst activation temperature;
Catalyst activity determination means for determining that the exhaust purification catalyst is in an active state when the temperature of the exhaust purification catalyst estimated by the first catalyst temperature estimation means exceeds a catalyst activation temperature higher than the temporary catalyst activation temperature;
When the reducing agent is added to the exhaust gas by the reducing agent addition control means, the reducing agent addition is performed based on the temperature of the predetermined portion of the exhaust purification catalyst estimated by the second catalyst temperature estimating means. Reducing agent addition prohibiting means for prohibiting addition of reducing agent to exhaust by the control means;
An exhaust gas purification apparatus for an internal combustion engine. The second catalyst temperature estimating means estimates a temperature of a front end portion that is a portion where exhaust flows into the exhaust purification catalyst,
The reducing agent addition prohibiting means has a temperature of the front end portion of the exhaust purification catalyst estimated by the second catalyst temperature estimating means when the reducing agent addition control means is adding the reducing agent to the exhaust gas. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein when the temperature is lower than the temporary catalyst activation temperature, addition of the reducing agent to the exhaust gas by the reducing agent addition control means is prohibited. Exhaust temperature detection means provided on the downstream side of the exhaust purification catalyst for detecting the temperature of the exhaust gas flowing out from the exhaust purification catalyst, further comprising
The second catalyst temperature estimation means estimates a temperature of a rear end portion that is a portion from which exhaust flows out of the exhaust purification catalyst,
The reducing agent addition prohibiting means is a temperature of a rear end portion of the exhaust purification catalyst estimated by the second catalyst temperature estimating means when the reducing agent is added to the exhaust gas by the reducing agent addition control means. The addition of the reducing agent to the exhaust gas by the reducing agent addition control means is prohibited when the temperature difference between the temperature of the exhaust gas detected by the exhaust gas temperature detecting means and the temperature of the exhaust gas becomes a predetermined temperature or less. 2. An exhaust gas purification apparatus for an internal combustion engine according to 1. An exhaust purification catalyst provided in the exhaust passage of the internal combustion engine and having oxidizing ability;
Reducing agent addition means for adding a reducing agent to the exhaust gas flowing into the exhaust purification catalyst;
Catalyst temperature estimating means for estimating the temperature of the exhaust purification catalyst based on at least the temperature of the exhaust flowing into the exhaust purification catalyst, the amount of reducing agent contained in the exhaust, and the heat capacity in the exhaust purification catalyst;
An exhaust gas temperature detecting means provided on the downstream side of the exhaust gas purification catalyst for detecting the temperature of the exhaust gas flowing out from the exhaust gas purification catalyst;
A reducing agent addition control means for adding a reducing agent to the exhaust gas by the reducing agent addition means when the temperature of the exhaust purification catalyst estimated by the catalyst temperature estimation means exceeds a temporary catalyst activation temperature;
Catalyst activity determination means for determining that the exhaust purification catalyst is in an active state when the temperature of the exhaust purification catalyst estimated by the catalyst temperature estimation means exceeds a catalyst activation temperature higher than the temporary catalyst activation temperature;
When the reducing agent addition control means is adding the reducing agent to the exhaust, the amount of decrease in the exhaust temperature detected by the exhaust temperature detecting means in a predetermined period is determined based on the operating state of the internal combustion engine. A reducing agent addition prohibiting means for prohibiting addition of the reducing agent to the exhaust by the reducing agent addition control means when the exhaust gas temperature drop allowable range is exceeded,
An exhaust gas purification apparatus for an internal combustion engine.

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