JP2015101988A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine, which includes a SCR catalyst, a reductant solution addition part and a NOx sensor installed in the exhaust passage, for suppressing a reduction in the NOx eliminating rate of the SCR catalyst by adequately controlling the addition of reductant solution while suppressing the water crack of the NOx sensor.SOLUTION: When an exhaust gas temperature is within a temperature range where the sensor may be exposed to water and the estimated adsorption amount of reductant adsorbed to the SCR catalyst is not smaller than a predetermined amount, the exhaust emission control device stops the energization of the sensor, and then controls the addition part on the basis of the estimated exhaust amount of NOx to be exhausted from the internal combustion engine. When the exhaust gas temperature is within the temperature range and the estimated adsorption amount is smaller than the predetermined amount, the exhaust emission control device stops the energization of the sensor, and then increases the exhaust gas temperature up to a temperature exceeding the temperature range and controls the addition part on the basis of a detection value obtained by starting the energization of the sensor after increasing the exhaust gas temperature.

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine including a selective reduction catalyst that selectively reduces NOx in exhaust gas using a reducing agent.

  2. Description of the Related Art Conventionally, there has been known an exhaust purification device that includes a selective reduction catalyst (SCR catalyst) that selectively reduces nitrogen oxide (NOx) in exhaust gas using a reducing agent such as ammonia in an exhaust passage of an internal combustion engine. In the exhaust purification apparatus, a solution in which a reducing agent or a reducing agent precursor is dissolved (for example, urea water; hereinafter also referred to as “reducing agent solution”) is added to the exhaust gas flowing into the SCR catalyst. Generally, a NOx sensor that detects the amount of NOx in the exhaust is provided in the exhaust passage, and the addition of the reducing agent solution is controlled based on the detection value of the NOx sensor.

  Here, in order for the detection element in the NOx sensor to be in an active state, the temperature needs to be maintained at a predetermined temperature (for example, about 700 ° C.). Therefore, a general NOx sensor includes an electric heater, and the temperature of the detection element is maintained in the vicinity of the predetermined temperature when energized. Here, depending on the exhaust temperature, an event called “water exposure” may occur in which condensed water or the like in which the water vapor in the exhaust is condensed adheres to the detection element of the NOx sensor. If the detection element is exposed to water while being heated to the vicinity of the predetermined temperature described above, there is a possibility that an event referred to as so-called "water cracking" occurs in which the detection element is damaged by thermal shock.

  In order to deal with such a problem, for example, in Patent Document 1, the amount of condensed water in the exhaust passage is estimated, and when it is determined that all of the estimated amount of condensed water has evaporated, the NOx sensor A technique for starting energization of a battery is disclosed. Further, for example, in Patent Document 2, when it is determined that condensed water can be generated in the exhaust passage, heating of the exhaust passage using a heating means provided separately prevents the generation of condensed water. Technology is disclosed. In any of the techniques, the energization is started after the state in which the generation of condensed water is suppressed is achieved, so that the detection element of the NOx sensor is also referred to as “water cracking of the NOx sensor”. ) Is suppressed.

JP 2010-174657 A JP 2004-316594 A

  By the way, under an actual operating condition of the internal combustion engine, an operating state in which the exhaust temperature does not reach the temperature at which the generation of condensed water is suppressed may continue for a relatively long time. In this case, in the technique described in the above-mentioned Patent Document 1, since the period during which the energization to the NOx sensor is stopped is prolonged, the addition of the reducing agent solution itself cannot be performed or even if it is performed, the addition control is not performed. There is a risk that the accuracy may decrease. As a result, it becomes difficult to supply an appropriate amount of reducing agent solution, so the NOx purification rate of the SCR catalyst (the ratio of the amount of NOx purified by the SCR catalyst to the amount of NOx flowing into the SCR catalyst) decreases. There is a risk of doing.

  Although the technique described in Patent Document 2 described above can suppress the moisture of the NOx sensor by heating the exhaust passage, it is necessary to provide a separate combustion burner as a heating means. Is not preferable because fuel is additionally consumed.

  The present invention has been made in view of such circumstances, and in an exhaust gas purification apparatus for an internal combustion engine in which an SCR catalyst, a reducing agent solution addition section, and a NOx sensor are installed in an exhaust passage, the NOx sensor It aims at suppressing the fall of the NOx purification rate of a SCR catalyst by controlling addition of a reducing agent solution appropriately, suppressing the cracking of water.

In order to solve the above-described problem, an exhaust gas purification apparatus for an internal combustion engine according to the present invention includes:
A selective reduction catalyst that is provided in an exhaust passage of the internal combustion engine, adsorbs a reducing agent, and selectively reduces nitrogen oxide in exhaust using the adsorbed reducing agent;
An addition unit that is provided upstream of the selective reduction catalyst in the exhaust passage and adds a solution in which a reducing agent or a precursor of the reducing agent is dissolved in exhaust that flows into the selective reduction catalyst;
A sensor that is provided in the exhaust passage and detects the amount of nitrogen oxide in the exhaust flowing into the selective catalytic reduction catalyst when energized;
An adsorption amount estimation unit for estimating an adsorption amount of the reducing agent adsorbed on the selective catalytic reduction catalyst;
An emission amount estimation unit for estimating an emission amount of nitrogen oxides discharged from the internal combustion engine;
If the exhaust temperature is within a predetermined temperature range that the sensor can be exposed to water and the estimated adsorption amount is equal to or greater than a predetermined amount, the estimated current amount is stopped after the power supply to the sensor is stopped. The addition unit is controlled based on a discharge amount, and the sensor is energized when the exhaust temperature is within the predetermined temperature range and the estimated adsorption amount is less than the predetermined amount. After stopping, the control unit controls the addition unit based on a detection value obtained by increasing the exhaust temperature to a temperature exceeding the predetermined temperature range and starting energization of the sensor after the exhaust temperature rises And so on.

  The selective reduction catalyst (SCR catalyst) adsorbs a reducing agent such as ammonia, and purifies nitrogen oxide (NOx) in the exhaust gas by a selective reduction reaction using the adsorbed reducing agent. The adding unit adds a solution (reducing agent solution) in which the reducing agent or the reducing agent precursor is dissolved, such as urea water, to the exhaust gas. Further, as the above-described sensor, for example, a sensor including a detection element that detects NOx in exhaust gas and an electric heater that heats the detection element is assumed. When this type of sensor is energized, it is heated by an electric heater to a predetermined temperature range (approximately 700 ° C. or higher) in which the detection element is activated. By using the sensor, the amount of NOx in the exhaust gas flowing into the SCR catalyst can be grasped with high accuracy. The sensor may be provided on the upstream side of the SCR catalyst.

  For example, the adsorption amount estimation unit estimates the adsorption amount of the reducing agent adsorbed on the SCR catalyst from the history of the addition amount of the reducing agent solution by the addition unit and the history of the operating state of the internal combustion engine. The estimated adsorption amount is referred to as “estimated adsorption amount”. In addition, the above-described emission amount estimation unit estimates, for example, the amount of NOx emitted from the internal combustion engine based on the operating state of the internal combustion engine. The estimated emission amount is referred to as “estimated emission amount”.

  By the way, depending on the temperature of the exhaust gas flowing through the exhaust passage, the water vapor in the exhaust gas may condense, and the sensor may get wet due to the attachment of condensed water. For example, in a configuration in which a sensor is installed in the vicinity of the addition unit, depending on the exhaust temperature, the reducing agent solution added to the exhaust may adhere to the sensor without being sufficiently dispersed. Sensor moisture may be generated. As described above, since there is a correlation between the exhaust gas temperature and the generation of sensor water, a predetermined temperature range can be determined as the exhaust gas temperature range that the sensor can receive water. The predetermined temperature range (hereinafter, also referred to as “water temperature range”) can be determined by, for example, experiments or simulations.

Therefore, in the exhaust emission control device according to the present invention, when the exhaust temperature is within the water temperature range, heating of the detection element is stopped by stopping energization of the sensor, thereby causing water cracking of the sensor. To avoid. However, when the energization of the sensor is stopped, the addition of the reducing agent solution by the adding unit cannot be controlled based on the detection value of the sensor, and thus the accuracy of the control may be reduced. In this case, since it may be difficult to add a sufficient amount of the reducing agent solution, the NOx purification rate of the SCR catalyst may be reduced due to a shortage of the reducing agent.

  Here, as described above, since the SCR catalyst adsorbs the reducing agent, a certain amount of the reducing agent may exist in the SCR catalyst even after the energization of the sensor is stopped. By using this reducing agent adsorbed, even if the supply of the reducing agent to the SCR catalyst is insufficient, the shortage can be compensated. That is, if the reducing agent is adsorbed on the SCR catalyst, a reduction in accuracy of the reducing agent solution addition control can be tolerated to some extent.

  Therefore, in the control device according to the present invention, when the exhaust gas temperature is within the above-described wet water temperature range and the estimated adsorption amount of the reducing agent is equal to or greater than a predetermined amount, after energization of the sensor is stopped, The addition control of the reducing agent solution is performed based on the estimated NOx emission amount. Here, since the accuracy of the addition control can be lower than the accuracy of the addition control of the reducing agent solution based on the detection value of the sensor, the supply of the reducing agent can be insufficient due to insufficient addition of the reducing agent solution. However, since the supply shortage of the reducing agent can be compensated for by the adsorbed reducing agent, a decrease in the NOx purification rate of the SCR catalyst is suppressed. Note that the predetermined amount related to the estimated amount of adsorption may be an estimated amount of adsorption when it is determined that the NOx purification rate can be prevented from decreasing below the allowable value due to insufficient supply of the reducing agent. The predetermined amount and the allowable value of the NOx purification rate can be determined in advance by experiments or the like. Thereby, it is possible to prevent the NOx purification rate of the SCR catalyst from falling below the allowable value while avoiding water cracking of the sensor. In particular, even when the operation state in which the exhaust temperature does not rise continues, as long as the estimated adsorption amount is a predetermined amount or more, the NOx purification rate of the SCR catalyst is suitably maintained by performing the addition control. It becomes possible to do.

  In the control device according to the present invention, when the exhaust gas temperature is within the above-described wet water temperature range and the estimated adsorption amount of the reducing agent is less than the predetermined amount, the energization to the sensor is stopped, The exhaust temperature is raised to a temperature exceeding the wet temperature range. Then, energization of the sensor is started after the exhaust temperature rises, and the addition of the reducing agent solution by the adding unit is controlled based on the detection value obtained thereafter. As a result, it is possible to suitably maintain the NOx purification rate of the SCR catalyst by highly accurate addition control while suppressing water cracking of the sensor. Further, according to the present invention, the increase in the exhaust gas temperature is limited when the estimated adsorption amount is less than the predetermined amount. As a result, the frequency of exhaust gas temperature increasing operation is reduced, so that a reduction in fuel consumption of the internal combustion engine is suppressed.

  According to the present invention, in an exhaust gas purification apparatus for an internal combustion engine in which an SCR catalyst, a reducing agent solution addition portion, and a NOx sensor are installed in an exhaust passage, the reducing agent is suppressed while preventing water cracking of the NOx sensor. By appropriately controlling the addition of the solution, it is possible to suppress a decrease in the NOx purification rate of the SCR catalyst.

It is a figure which shows schematic structure of the exhaust gas purification apparatus of the internal combustion engine which concerns on an Example. It is a graph which shows the relationship between the ammonia adsorption amount of the SCR catalyst which concerns on an Example, and a NOx purification rate. It is a flowchart which shows the routine of urea water addition control which concerns on an Example.

Hereinafter, specific embodiments of the present invention will be described with reference to the drawings. The dimensions, materials, shapes, relative arrangements, and the like of the components described in the present embodiment are not intended to limit the technical scope of the invention to those unless otherwise specified.

[Example]
Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine to which the present invention is applied and its exhaust purification device. An internal combustion engine 1 shown in FIG. 1 is an automobile diesel engine having a plurality of cylinders. However, the internal combustion engine to which the exhaust gas purification apparatus according to the present invention can be applied is not limited to a diesel engine, and may be a gasoline engine or another type of internal combustion engine.

  An exhaust passage 2 is connected to the internal combustion engine 1. Inside the casing 3 provided in the exhaust passage 2, an oxidation catalyst 4, a mixer 5 and an SCRF 6 are installed in this order from the upstream side. Further, an exhaust pressure sensor upstream of the urea water addition valve 7, the first NOx sensor 8, the temperature sensor 9, and the differential pressure sensor 10 is installed in the mixer 5. An exhaust pressure sensor, an A / F sensor 11 and a second NOx sensor 12 on the downstream side of the differential pressure sensor 10 are installed downstream of the casing 3 in the exhaust passage 2. The internal combustion engine 1 is also provided with an ECU 100 that is an electronic control unit for controlling the internal combustion engine 1. The urea water addition valve 7 is electrically connected to the ECU 100 and is controlled by the ECU 100. Sensors such as the first NOx sensor 8 are electrically connected to the ECU 100, and output signals from these are input to the ECU 100.

  The oxidation catalyst 4 oxidizes unburned fuel, carbon monoxide and the like in the exhaust discharged from the internal combustion engine 1. The mixer 5 is composed of a plurality of dispersion plates, and uniformly disperses the urea water added from the urea water addition valve 7 in the exhaust gas flowing into the SCRF 6. SCRF6 is a SCR catalyst supported (coated) on a wall flow type filter that collects PM in exhaust gas. The urea water addition valve 7 adds urea water in which urea as a precursor of ammonia is dissolved to the exhaust gas flowing into the SCRF 6. Here, urea in urea water generates ammonia by being hydrolyzed in SCRF6. The SCRF 6 adsorbs the ammonia thus generated, and purifies NOx in the exhaust by a selective reduction reaction using the adsorbed ammonia as a reducing agent. In this embodiment, the SCRF 6 and the urea water addition valve 7 correspond to the selective reduction catalyst and the addition unit according to the present invention, respectively. Further, instead of the SCRF 6, a filter (DPF) for collecting PM in the exhaust and the SCR catalyst may be provided separately. In this case, it is preferable to dispose the DPF directly under the oxidation catalyst 4 and dispose the SCR catalyst directly under the mixer 5. Further, an SCR catalyst may be further installed downstream of the casing 3 in the exhaust passage 2. The urea water addition valve 7 may add ammonia water in which ammonia is dissolved instead of urea water. Both solutions are pumped from a tank (not shown) to the urea water addition valve 7.

  The first NOx sensor 8 detects the amount of NOx in the exhaust gas flowing into the SCRF 6. The first NOx sensor 8 includes therein a detection element and an electric heater that heats the detection element to a predetermined activation temperature (about 700 ° C.). When power is supplied from the ECU 100, the temperature of the detection element is maintained at the activation temperature by the electric heater, so the first NOx sensor 8 can detect the amount of NOx in the exhaust. In the present embodiment, the first NOx sensor 8 corresponds to a sensor according to the present invention.

The temperature sensor 9 detects the temperature of the exhaust gas flowing into the SCRF 6. The differential pressure sensor 10 detects the differential pressure across the SCRF 6 using upstream and downstream sensors. The A / F sensor 11 detects the air / fuel ratio of the exhaust gas flowing out from the SCRF 6. The second NOx sensor 12 has the same configuration as the first NOx sensor 8 and detects the amount of NOx in the exhaust gas flowing out from the SCRF 6. In addition, you may change suitably the installation position and installation number of various sensors. The ECU 100 is electrically connected to a crank position sensor 13 that detects the rotational position of the crankshaft of the internal combustion engine 1 and an accelerator opening sensor 14 that detects the opening of an accelerator pedal provided in the vehicle. Output signals from these are input to the ECU 100. The ECU 100 grasps the operating state of the internal combustion engine 1 based on the output signals from the sensors, and controls the fuel injection amount.

  In the exhaust gas purification apparatus configured as described above, the ECU 100 in principle is in the exhaust gas flowing into the SCRF 6 so that the NOx purification rate of the SCRF 6 becomes a desired value based on the detection value of the first NOx sensor 8. An amount of urea water corresponding to the amount of NOx is added from the urea water addition valve 7. However, depending on the exhaust gas temperature, the wetness of the first NOx sensor 8 may occur due to the adhesion of urea water added from the urea water addition valve 7. In particular, in the configuration in which the first NOx sensor 8 is installed in the vicinity of the urea water addition valve 7 as in the exhaust gas purification device according to the present embodiment, the sensor tends to be exposed to water. Further, depending on the exhaust temperature, the first NOx sensor 8 may get wet with condensed water obtained by condensing water vapor in the exhaust. Here, when the first NOx sensor 8 is wetted while the internal detection element is heated to the above-described predetermined activation temperature, there is a risk of water cracking that damages the detection element due to thermal shock. Therefore, in this embodiment, in order to avoid the moisture cracking of the first NOx sensor 8, when the exhaust temperature is low enough to cause the first NOx sensor 8 to be wetted, the first NOx sensor 8 is energized. Stop and stop heating the sensor. However, since the output from the sensor cannot be obtained when the energization is stopped, the addition of urea water by the urea water addition valve 7 may not be controlled with high accuracy. In this case, the ammonia supplied to the SCRF 6 may be insufficient due to the lack of added urea water.

Since SCRF 6 adsorbs ammonia, a certain amount of ammonia may exist in SCRF 6 even after energization of first NOx sensor 8 is stopped. Here, regarding the relationship between the ammonia adsorption amount of the SCRF 6 and the NOx purification rate when the exhaust temperature in the vicinity of the first NOx sensor 8 is within the temperature range (within the water temperature range) that the sensor can be wet with, This will be described with reference to FIG. FIG. 2 is a graph showing the relationship, in which the horizontal axis indicates the ammonia adsorption amount (NH ₃ adsorption amount), and the vertical axis indicates the NOx purification rate. Further, the temperature (bed temperature) of SCRF 6 is equal to or higher than the activation temperature of the supported SCR catalyst.

  As shown in FIG. 2, the NOx purification rate of SCRF6 depends on the ammonia adsorption amount. When the ammonia adsorption amount is relatively large, the NOx purification rate of SCRF6 is relatively high. This is because when the exhaust gas temperature is within the water temperature range, the temperature of the exhaust gas flowing into the SCRF 6 is also lowered to the same extent, so that the bed temperature of the SCR catalyst carried by the SCRF 6 is not sufficiently increased. Due to that. That is, in this case, although the SCR catalyst is active, the degree of the SCR catalyst can be relatively low, so that the NOx purification rate of SCRF 6 becomes more dependent on the ammonia adsorption amount. Therefore, when the state of SCRF 6 is, for example, in state 1 where the ammonia adsorption amount is relatively small, if the supply of ammonia is insufficient due to insufficient addition of urea water, the NOx purification rate is relatively reduced due to the decrease in ammonia adsorption amount. It can be greatly reduced. That is, in this case, since it is highly necessary to control the addition of urea water with high accuracy, energization to the first NOx sensor 8 is resumed early, and the addition control based on the detection value of the sensor is performed. Is desirable.

  On the other hand, when the state of SCRF 6 is, for example, in the state 2 where the ammonia adsorption amount is relatively large, even if the supply of ammonia is insufficient due to insufficient addition of urea water, the supply shortage due to the adsorbed ammonia. Therefore, the NOx purification rate of SCRF6 can be maintained. That is, as long as a sufficient amount of ammonia is adsorbed on the SCRF 6, a decrease in the accuracy of the urea water addition control can be allowed.

Therefore, in this embodiment, after the energization of the first NOx sensor 8 is stopped, the SCRF
An ammonia adsorption amount of 6 is estimated. When the estimated adsorption amount is sufficiently large, the urea water addition by the urea water addition valve 7 is controlled based on the estimated NOx discharge amount on the assumption that the supply shortage is compensated by the adsorbed ammonia. Is done. Hereinafter, the execution procedure of the urea water addition control in this embodiment will be described with reference to FIG. FIG. 3 is a flowchart showing an addition control routine executed by the ECU 100 as the control unit according to the present invention. This routine is stored in the ECU 100 and is periodically executed. This routine is based on the premise that the bed temperature of the SCR catalyst carried on the SCRF 6 is equal to or higher than its activation temperature.

  First, in step S101, the ECU 100 determines whether or not the temperature of the exhaust gas in the vicinity of the first NOx sensor 8 is within the above-described wet water temperature range. As described above, since there is a correlation between the exhaust gas temperature and the generation of water in the first NOx sensor 8, this water temperature range can be determined in advance by experiments, simulations, or the like. . Further, the detected value of the temperature sensor 9 may be used as the exhaust temperature in the vicinity of the first NOx sensor 8.

  If an affirmative determination is made in step S101, the ECU 100 proceeds to step S102 and stops energization of the first NOx sensor 8. As a result, it is possible to avoid water cracking of the sensor.

  Next, the ECU 100 proceeds to step S103, and determines whether or not the adsorption amount of ammonia in the SCRF 6 is a predetermined amount or more. Here, the ECU 100 estimates the adsorption amount of ammonia adsorbed on the SCRF 6 from the history of urea water addition performed previously and the history of the operating state of the internal combustion engine 1, and further, the estimated adsorption amount is determined. It is determined whether or not it is a fixed amount or more. This predetermined amount is an estimated amount of adsorption when it is determined that the NOx purification rate of SCRF 6 can be suppressed from falling below an allowable value by compensating for the shortage of ammonia supply using ammonia adsorbed on SCRF 6. It is. The predetermined amount and the allowable value of the NOx purification rate may be determined in advance through experiments or the like. If an affirmative determination is made in this step, the ECU 100 proceeds to step S104. The ECU 100 that estimates the estimated adsorption amount of ammonia in this step corresponds to the adsorption amount estimation unit according to the present invention.

  In step S104, the ECU 100 controls urea water addition by the urea water addition valve 7 based on the estimated NOx emission amount. The ECU 100 estimates the estimated NOx emission amount based on the operating state of the internal combustion engine 1. As already described, even if the supply of ammonia is insufficient due to insufficient addition of urea water in the control, the supply shortage can be compensated for by the ammonia adsorbed on the SCRF 6. Therefore, the NOx purification rate of SCRF6 is maintained at an allowable value or more. When this step is executed, this routine is terminated. In this step, the ECU 100 that estimates the estimated NOx emission amount corresponds to the emission amount estimation unit according to the present invention.

  On the other hand, if a negative determination is made in step S103, the ECU 100 proceeds to step S105 to increase the exhaust gas temperature in order to start energization of the first NOx sensor 8. Specifically, the operating state of the internal combustion engine 1 is switched to the exhaust gas raising operation in which the fuel injection amount is increased. When this step is executed, this routine is terminated.

  When a negative determination is made in step S101, it means that the first NOx sensor 8 cannot be wet because the exhaust gas temperature is sufficiently high. Therefore, the ECU 100 proceeds to step S106 and permits energization of the sensor. Then, the ECU 100 proceeds to step S107 and controls the urea water addition valve 7 based on the detection value of the first NOx sensor 8. As a result, an appropriate amount of ammonia can be supplied to the SCRF 6, so that the NOx purification rate can be suitably maintained at a desired value.

  In addition, when step S105 is performed and this routine is executed again after the exhaust gas temperature has risen to a temperature exceeding the water temperature range, a negative determination is made in step S101, so step S106 is performed. Then, energization of the first NOx sensor 8 is started. That is, according to the present embodiment, when the exhaust gas temperature is within the wet water temperature range and the estimated ammonia adsorption amount is less than the predetermined amount, the exhaust gas is exhausted to a temperature at which the first NOx sensor 8 cannot be wet. After the temperature is raised, energization of the sensor is started. Therefore, according to the present embodiment, the NOx purification rate of the SCRF 6 can be suitably maintained by activating the sensor while suppressing water cracking of the first NOx sensor 8.

  As described above, according to the present embodiment, even when the exhaust temperature is low enough to cause water cracking of the first NOx sensor 8, the NOx purification rate of the SCRF 6 is reduced while avoiding water cracking of the sensor. It becomes possible to maintain above the allowable value. In particular, even when the operation state where the exhaust temperature does not rise continues, as long as the estimated adsorption amount of ammonia is equal to or more than the above-mentioned predetermined amount, the addition control based on the estimated exhaust amount of NOx is performed. The NOx purification rate of SCRF6 is suitably maintained. Further, according to this embodiment, the increase in the exhaust gas temperature for starting energization of the first NOx sensor 8 is limited to the case where the estimated adsorption amount of ammonia is less than the above predetermined amount. It becomes possible to suppress a decrease in fuel consumption.

1 Internal combustion engine 2 Exhaust passage 6 SCRF
7 Urea water addition valve 8 First NOx sensor 100 ECU

Claims (1)

A selective reduction catalyst that is provided in an exhaust passage of the internal combustion engine, adsorbs a reducing agent, and selectively reduces nitrogen oxide in exhaust using the adsorbed reducing agent;
An addition unit that is provided upstream of the selective reduction catalyst in the exhaust passage and adds a solution in which a reducing agent or a precursor of the reducing agent is dissolved in exhaust that flows into the selective reduction catalyst;
A sensor that is provided in the exhaust passage and detects the amount of nitrogen oxide in the exhaust flowing into the selective catalytic reduction catalyst when energized;
An adsorption amount estimation unit for estimating an adsorption amount of the reducing agent adsorbed on the selective catalytic reduction catalyst;
An emission amount estimation unit for estimating an emission amount of nitrogen oxides discharged from the internal combustion engine;
If the exhaust temperature is within a predetermined temperature range that the sensor can be exposed to water and the estimated adsorption amount is equal to or greater than a predetermined amount, the estimated current amount is stopped after the power supply to the sensor is stopped. The addition unit is controlled based on a discharge amount, and the sensor is energized when the exhaust temperature is within the predetermined temperature range and the estimated adsorption amount is less than the predetermined amount. After stopping, the control unit controls the addition unit based on a detection value obtained by increasing the exhaust temperature to a temperature exceeding the predetermined temperature range and starting energization of the sensor after the exhaust temperature rises When,
An exhaust gas purification apparatus for an internal combustion engine.

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