JP2010156243A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately determine the degree of deterioration of a catalyst having an oxidizing function. <P>SOLUTION: This exhaust emission control device for an internal combustion engine includes: a catalyst 3 provided in the exhaust passage of the internal combustion engine and having an oxidizing function; a NOx sensor 6 for measuring the concentration of NOx included in exhaust; a pH sensor 25 for measuring the pH on a downstream side of the catalyst 3; and an estimating means for estimating the ratio of NO<SB>2</SB>in NOx based on the concentration of NOx obtained by the NOx sensor 6 and the pH obtained by the pH sensor 25. The concentration of NO<SB>2</SB>or the quantity of NO<SB>2</SB>are computed based on the pH. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

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

Deterioration of oxidation catalyst provided upstream from NOx catalyst, NOx provided downstream from NOx catalyst
A technique for making a determination based on the NOx concentration obtained by a sensor is known (for example, see Patent Document 1). Since this technology is affected by the NOx reduction reaction in the NOx catalyst, if the NOx catalyst is deteriorated or there is an abnormality in the reducing agent supply device, there is a possibility that the deterioration determination of the oxidation catalyst is erroneous. That is, it is necessary to determine the deterioration of the oxidation catalyst under the condition that the NOx catalyst has not deteriorated and the reducing agent supply device has no abnormality.

Also, the pH downstream of the catalyst is estimated from the NOx concentration upstream of the catalyst, and the estimated pH is compared with the pH obtained from the pH sensor downstream of the catalyst to determine the degree of activity of the catalyst. The technique which performs is known (for example, refer patent document 2). However, the pH estimated from the NOx concentration upstream of the catalyst may deviate from the actual value depending on the conditions.
Therefore, there is a possibility that the determination accuracy of the activity level is lowered.
Special table 2008-523305 gazette JP 2008-138553 A JP 2007-247589 A JP 2008-180200 A

  The present invention has been made in view of the above-described problems, and an object thereof is to more accurately determine the degree of deterioration of a catalyst having an oxidation function.

In order to achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention employs the following means. That is, the exhaust gas purification apparatus for an internal combustion engine according to the present invention is
A catalyst provided in the exhaust passage of the internal combustion engine and having an oxidation function;
A NOx sensor for measuring the NOx concentration in the exhaust;
A pH sensor for measuring the pH downstream of the catalyst;
Estimating means for estimating the ratio of NO ₂ in NOx based on the NOx concentration obtained by the NOx sensor and the pH obtained by the pH sensor;
It is characterized by providing.

Catalyst having an oxidation function, to oxidize NO in the exhaust to NO _2. As the degree of deterioration of the catalyst increases, the ability to oxidize NO decreases. In other words, when the deterioration of the catalyst proceeds, the ratio of NO on the downstream side of the catalyst becomes higher and the ratio of NO ₂ becomes lower.

The NOx sensor measures the NOx concentration in the exhaust gas, and the measurement location may be upstream or downstream of the catalyst. This is because NOx passes through the catalyst so that NO becomes NO ₂
However, since NO and NO ₂ are both NOx, the NOx concentration itself does not change.

Here, pH is measured by pH sensor will vary with NO ₂ amount or concentration of NO ₂ in the exhaust gas. That is, there is a correlation between pH and the amount of NO ₂ or the concentration of NO ₂ . For example, considering the amount of NO ₂ in the same volume, the pH decreases as the amount of NO ₂ increases.

The ratio of NO ₂ in NOx can be calculated from the NOx amount and NO ₂ amount in the exhaust gas. Further, the ratio of NO ₂ in NOx can be calculated from the NOx concentration and NO ₂ concentration in exhaust gas. The NOx amount may be calculated from the NOx concentration obtained from the NOx sensor. It is also possible to calculate the NO ₂ concentration from NO ₂ amount obtained from the pH sensor.

Since the ratio of NO ₂ calculated in this way is based on the actual measurement value measured by the sensor, the accuracy is high. Incidentally, NOx is When made of NO ₂ and NO, may also be calculated the ratio of NO from a ratio of NO _2.

If the ratio of NO ₂ in NOx can be obtained, the oxidation ability of the catalyst can be determined, and therefore the degree of deterioration of the catalyst can be obtained. That is, NO ₂ in NOx
It can be determined that the lower the ratio is, the higher the degree of deterioration of the catalyst is. Further, when provided with a NOx catalyst on the downstream side of the catalyst having the oxidation capacity, because you know the ratio of NO ₂ in NOx flowing into the NOx catalyst, more the deterioration determination of the NOx catalyst Can be done accurately. Furthermore, the amount of reducing agent to be supplied to the NOx catalyst can be determined more accurately.

In the present invention, comprising a condensing device for condensing exhaust,
The pH sensor can measure the pH of the condensate obtained by the condensing device.

  Thus, by providing the condensing device and measuring the pH of the condensate, the pH of the exhaust gas can be easily measured. The condensing device may condense a part of the exhaust gas flowing through the exhaust passage or may condense the whole.

In the present invention, comprising a measuring device for measuring the amount of exhaust gas condensed in the condensing device in a predetermined period,
Calculating the amount of NOx from the NOx concentration obtained by the NOx sensor during the predetermined period and the amount of exhaust;
Calculate the amount of NO ₂ from the pH obtained by measuring the condensate produced in the predetermined period with the pH sensor,
From the NO ₂ amount and the NOx amount, the ratio of NO ₂ in NOx can be calculated.

Here, by multiplying the amount of exhaust flowing into the condenser during a predetermined period by the NOx concentration,
The NOx amount (total NOx amount) in the exhaust amount can be calculated. This total NOx amount can be the amount of NOx flowing into the condenser during a predetermined period. Also, N in the amount of exhaust from pH
The O ₂ amount (total NO ₂ amount) can be calculated. The NO ₂ amount can be NO ₂ amount flowing to the condenser at a predetermined period. In other words, it is possible to calculate the NOx amount and the NO ₂ amount flowing to the condenser at a predetermined period, it is possible to calculate the ratio of NO _2. Predetermined period, and measurable period NOx amount and NO ₂ amount. This is the amount of exhaust gas flowing into the condenser, it may be time to reach a measurable amount of NOx amount and NO ₂ amount.

In the present invention, there can be provided a determination means for determining that the catalyst is deteriorated when the ratio of NO ₂ is lower than a threshold value.

Here, the ratio of NO and NO _{2 in} NOx discharged from the internal combustion engine is a substantially constant value corresponding to the operating state of the internal combustion engine. If the catalyst having an oxidation function is not deteriorated, NO is oxidized at a constant rate in the catalyst to become NO _2, and therefore, the NO on the downstream side of the catalyst is reduced.
And the ratio of NO ₂ also becomes substantially constant. On the other hand, as the deterioration of the catalyst proceeds, the ratio of NO oxidized by the catalyst becomes lower, so the ratio of NO ₂ becomes lower than when the catalyst is not deteriorated. That is, when the ratio of NO ₂ becomes lower than the allowable range, it can be determined that the catalyst has deteriorated. The threshold value may be a lower limit value of the NO ₂ ratio when the catalyst is not deteriorated.

  According to the present invention, the degree of deterioration of a catalyst having an oxidation function can be determined more accurately.

  Hereinafter, specific embodiments of an exhaust emission control device for an internal combustion engine according to 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 and its intake / exhaust system according to the present embodiment. The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine having four cylinders. In this embodiment, a urea SCR system is employed.

  An exhaust passage 2 is connected to the internal combustion engine 1. In the middle of the exhaust passage 2, an oxidation catalyst 3 and a selective reduction type NOx catalyst 4 (hereinafter referred to as NOx catalyst 4) are provided in order from the upstream side. The oxidation catalyst 3 may be another catalyst having oxidation ability (for example, a three-way catalyst). In this embodiment, the oxidation catalyst 3 corresponds to the catalyst having an oxidation function in the present invention.

An injection valve 5 that injects urea water into the exhaust is attached to the exhaust passage 2 downstream of the oxidation catalyst 3 and upstream of the NOx catalyst 4. The injection valve 5 is opened by a signal from the ECU 10 described later and injects urea water into the exhaust gas. The urea water injected from the injection valve 5 is hydrolyzed by the heat of the exhaust to become ammonia (NH ₃ ) and is adsorbed on the NOx catalyst 4. This N
H ₃ reduces NOx.

A NOx sensor 6 for measuring the NOx concentration in the exhaust is attached to the exhaust passage 2 downstream of the oxidation catalyst 3 and upstream of the injection valve 5. The NOx sensor 6 may be attached to the exhaust passage 2 upstream from the oxidation catalyst 3. A pH sensor 25 for measuring the pH of exhaust gas is attached to the exhaust passage 2 downstream of the oxidation catalyst 3 and upstream of the injection valve 5.

  An intake passage 30 is connected to the internal combustion engine 1. An air flow meter 31 that outputs a signal corresponding to the flow rate of the intake air flowing through the intake passage 30 is provided in the middle of the intake passage 30.

  The internal combustion engine 1 configured as described above is provided with an ECU 10 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 10 is a unit that controls the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the request of the driver.

  In addition to the above sensors, the ECU 10 outputs an electric signal corresponding to the amount of depression of the accelerator pedal 11 by the driver, and an accelerator opening sensor 12 that can detect the engine load, and a crank position that detects the engine speed. Sensors 13 are connected via electrical wiring, and output signals from these various sensors are input to the ECU 10.

  On the other hand, the injection valve 5 is connected to the ECU 10 via electric wiring, and the ECU 10 controls the opening and closing timing of the injection valve 5.

By the way, it is considered that the following reaction occurs in the NOx catalyst 4.
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O
This is a purification reaction between NO and NO ₂ that occurs in the entire temperature range. According to this reaction, NO and NO ₂ are purified by an equal number.

From this relationship, it is considered that the purification rate becomes the highest when the ratio of NO ₂ is 50%. However, the ratio of NO in NOx discharged from the internal combustion engine 1 is, for example, about 70%.
Therefore, NO is oxidized by the oxidation catalyst 3 to make NO ₂ , and the ratio of NO to NO _{2 in} NOx is made closer.

However, when the oxidation catalyst 3 deteriorates, the amount of NO that is oxidized decreases, so that the NOx purification rate in the NOx catalyst 4 can decrease. Therefore, in this embodiment, the deterioration of the oxidation catalyst 3, that is, the deterioration of the oxidation function is detected based on the pH obtained by the pH sensor 25.

Here, FIG. 2 is a diagram showing the ratio of NO ₂ in the upstream side and the downstream side of the NOx concentration and in NOx than the oxidation catalyst 3. The solid line is the NO ₂ ratio when the oxidation catalyst 3 is not deteriorated, the broken line is the NO ₂ ratio when the oxidation catalyst 3 is deteriorated, and the alternate long and short dash line is the NO x when the oxidation catalyst 3 is not deteriorated. This is the concentration, and the two-dot chain line is the NOx concentration when the oxidation catalyst 3 is deteriorated.

As shown in FIG. 2, the NOx concentration hardly changes depending on whether or not the oxidation catalyst 3 is deteriorated. That is, since NO and NO ₂ are measured as NOx, they are not affected by the deterioration of the oxidation catalyst 3. However, the NO ₂ ratio varies depending on whether the oxidation catalyst 3 is deteriorated. This is due to the characteristics of palladium (Pd) and platinum (Pt) contained in the oxidation catalyst 3 according to this embodiment. That is, if Pd is contained in the oxidation catalyst 3, NO _{2 in} the exhaust gas reacts with HC and CO to generate H ₂ O, CO ₂ , and NO. That, NO is generated from the NO _2. On the other hand, when Pt is contained in the oxidation catalyst 3, HC, CO, and NO in the exhaust are oxidized. That is, NO ₂ is generated from NO.

Here, when Pd and Pt are compared, Pt deteriorates at a lower temperature. Further, Pd hardly deteriorates unless it becomes a high temperature and rich atmosphere. That is, Pt is more likely to deteriorate. Then, as the deterioration of Pt proceeds, the amount of NO that is oxidized decreases. Meanwhile, Pd because hardly deteriorated, certain NO results from NO _2. That is, the deterioration of the oxidation catalyst 3 progresses, than the amount of NO which is oxidized (increase in NO ₂₎ by Pt, (the amount of decrease in NO ₂₎ NO amount generated by Pd for more increases, the overall NO ₂ The ratio decreases.

Meanwhile, NOx sensor 6, although it is possible to measure the NOx concentration, it is not possible to measure the ratio between NO and NO _2. That is, it is difficult to determine the deterioration of the oxidation catalyst 3 because the NO ₂ ratio cannot be measured only with the NO x sensor 6.

Therefore, in this embodiment, NO _{2 in} NOx is based on the pH obtained by the pH sensor 25.
Find the ratio. Here, the pH of the exhaust gas (H ⁺ molar concentration in the aqueous solution) is proportional to the NO ₂ concentration or the NO ₂ amount in the exhaust gas. That is, both are in a correlation. This is according to the following equation.
NO ₂ + H ₂ O → HNO ₃ + H ⁺

That is, the NO ₂ concentration in the exhaust gas can be obtained based on the pH of the exhaust gas. Then, the NO ₂ ratio (NO ₂ / NOx) in NOx can be calculated from the NOx concentration and NO ₂ concentration in the exhaust gas. The relationship between pH and NO ₂ concentration may be obtained in advance and stored in the ECU 10.

  FIG. 3 is a flowchart showing a flow for determining deterioration of the oxidation catalyst 3 according to this embodiment. This routine is executed every predetermined time.

In step S101, it is determined whether or not the temperature of the oxidation catalyst 3 is equal to or higher than a predetermined value. The predetermined value is a temperature at which the oxidation catalyst 3 is at a temperature after light-off and the NO ₂ ratio is constant. This may be after the oxidation catalyst 3 is completely warmed up. That is, it is a temperature at which the oxidation function of the oxidation catalyst 3 is sufficiently exhibited. If it is above such temperature, since the ratio of NO ₂ in NOx becomes constant when the oxidation catalyst 3 is not deteriorated, the deterioration determination can be easily performed.

  If an affirmative determination is made in step S101, the process proceeds to step S102, and if a negative determination is made, the routine is temporarily terminated because an accurate determination cannot be made.

In step S103, the NO ₂ concentration is estimated. That is, the NO ₂ concentration is calculated from the pH obtained by the pH sensor 25.

  In step S103, the NOx concentration is read. That is, the NOx concentration obtained by the NOx sensor 6 is read.

In step S104, the NO ₂ ratio is calculated. That is, the NO ₂ ratio is calculated from the NOx concentration and the NO ₂ concentration. In this embodiment, the ECU 10 that processes step S104 corresponds to the estimation means in the present invention.

In step S105, it is determined whether or not the NO ₂ ratio is greater than or equal to a threshold value. This threshold is a lower limit value of the NO ₂ ratio when the oxidation catalyst 3 is normal. This threshold value is obtained in advance by experiments or the like in association with the operating state of the internal combustion engine 1. If an affirmative determination is made in step S105, the process proceeds to step S106, and the oxidation catalyst 3 is stored as normal. If a negative determination is made in step S105, the process proceeds to step S107, and it is stored that the oxidation catalyst 3 has deteriorated. In this embodiment, the ECU 10 that processes step S107 corresponds to the determination means in the present invention.

In this routine, it is determined whether or not the temperature of the oxidation catalyst 3 is equal to or higher than a predetermined value in step S101, and the deterioration determination of the oxidation catalyst 3 is performed only when an affirmative determination is made. The deterioration determination may be performed according to the temperature of the oxidation catalyst 3. That is, before the temperature of the oxidation catalyst 3 becomes sufficiently high (before full activation), the NO ₂ ratio varies depending on the temperature of the oxidation catalyst 3, so even if the threshold value in step S105 is changed according to the temperature of the oxidation catalyst 3 good.

In this way, the deterioration of the oxidation catalyst 3 can be determined. Further, if the NO ₂ ratio is known, it is possible to determine the deterioration of the NOx catalyst 4. Furthermore, the amount of urea water injected from the injection valve 5 can be determined.

As described above, according to the present embodiment, the NO ₂ ratio in NOx can be calculated based on the pH. Thereby, the deterioration determination of the oxidation catalyst 3 can be performed. Further, it is possible to determine the deterioration of the NOx catalyst 4 and determine the injection amount of urea water. NO in this example
_{Since the 2} ratio is calculated based on actual measurement by the sensor, a more accurate value can be obtained. In addition, since the pH is not affected by the deterioration of the NOx catalyst 4 or the abnormality of the injection valve 5, an accurate NO ₂ ratio can be obtained regardless of the state of the NOx catalyst 4 or the injection valve 5.

In this embodiment, condensed water is obtained by condensing exhaust gas. And the pH of this condensed water is measured. Here, most of NO ₂ dissolves in water, but NO hardly dissolves in water. That is, the pH of the condensed water varies depending on the amount of NO ₂ . For example, the greater the amount of NO ₂ dissolved in the condensed water, the lower the pH of the condensed water.

  FIG. 4 is a diagram showing a schematic configuration of the internal combustion engine and its exhaust system according to the present embodiment. The internal combustion engine 1 shown in FIG. 4 is a water-cooled four-cycle diesel engine having four cylinders. In the present embodiment, differences from FIG. 1 will be mainly described. Also, the same members as those in FIG.

A branch passage 21 is connected to the exhaust passage downstream of the oxidation catalyst 3 and upstream of the NOx catalyst 4. The branch passage 21 is provided with a shut-off valve 22, an exhaust amount sensor 23, a cooling device 24, and a pH sensor 25 in order from the upstream side.

  The cooling device 24 is a heat exchanger, and cools the exhaust by, for example, heat exchange between air and exhaust. The cooling device 24 cools the exhaust gas until moisture or the like in the exhaust gas is condensed. In the present embodiment, the cooling device 24 corresponds to the condensing device in the present invention.

  Then, when the exhaust gas is condensed by the cooling device 24, the condensate flows downstream from the cooling device 24. A pH sensor 25 is provided at a location where the condensate flows. The pH sensor 25 measures the pH of the condensate. When measuring pH, the shut-off valve 22 is opened. The exhaust amount sensor 23 measures the amount of exhaust flowing through the branch passage 21. In this embodiment, the displacement sensor 23 corresponds to the measuring device according to the present invention.

Next, a flow for obtaining the NO ₂ ratio in NOx according to the present embodiment will be described. FIG. 5 is a flowchart showing a flow for obtaining the NO ₂ ratio in NOx according to the present embodiment.
This routine is executed every predetermined time. In this routine, the NO ₂ ratio is calculated after obtaining the NO x amount and the NO ₂ amount flowing into the cooling device 24 during a predetermined period. In this embodiment, the ECU 10 that processes the flow shown in FIG. 5 corresponds to the estimation means in the present invention.

  In step S201, it is determined whether a detection condition is satisfied. For example, it is determined whether or not the operating state of the internal combustion engine 1 is in a state suitable for pH measurement. It may be determined whether the oxidation catalyst 3 is in an active state. If an affirmative determination is made in step S201, the process proceeds to step S202, and if a negative determination is made, this routine is terminated.

  In step S202, the shutoff valve 22 is opened. That is, exhaust gas is introduced into the cooling device 24.

In step S203, the amount of exhaust gas that has passed through the branch passage 21 since the shut-off valve 22 is opened is integrated. That is, the total amount of exhaust flowing through the branch passage 21 is calculated by integrating the amount of exhaust obtained by the exhaust amount sensor 23. This may be the total amount of exhaust flowing into the cooling device 24. In this embodiment, to calculate the NOx amount and NO ₂ amount contained in the total amount of the exhaust.

In step S204, the time elapsed since the shutoff valve 22 was opened is counted. In the shut-off valve 22 only opens a short time, because the error of the NOx amount and the NO ₂ amount is calculated is increased, to introduce a certain amount of time, the exhaust to the cooling device 24. The time for this is counted.

In step S205, the amount of NOx flowing through the branch passage 21 after the shut-off valve 22 is opened is integrated. That is, the total amount of NOx flowing through the branch passage 21 is calculated. The total amount of NOx is calculated from the NOx concentration obtained by the NOx sensor 6 and the total amount of exhaust gas calculated in step S203.

In step S206, it is determined whether or not the amount of exhaust gas calculated in step S203 is greater than or equal to a predetermined value. In this step, it is determined whether or not a sufficient amount of exhaust gas for calculating the NO ₂ ratio has been introduced into the cooling device 24. In addition, you may make it count until the time when the quantity of exhaust_gas | exhaustion becomes more than predetermined value by step S204. If an affirmative determination is made in step S206, the process proceeds to step S207, and if a negative determination is made, this routine is temporarily terminated.

In step S207, the amount of NO ₂ is calculated from the pH. Here, the pH has the following relationship.
pH = −log ₁₀ [H ⁺ ] = − log ₁₀ [NO ₂ ]
That is, the amount of NO ₂ is obtained by the following equation.
NO ₂ amount = A × M (NO ₂ ) [g] = A × 10 ^(−pH) / B × 46
However, A is a correction coefficient. This is corrected because not all the water and the like introduced into the cooling device 24 condense, and not all the NO _{2 in} the exhaust is present in the condensate. The correction coefficient A is obtained in advance by experiments or the like. For example, the correction coefficient A is determined according to the elapsed time after the shut-off valve 22 is opened. That is, as the elapsed time is longer, more condensed water is generated and more NO ₂ is dissolved in the condensed water, so the correction coefficient A is decreased. B is an Avogadro constant (6 × 10 ²³ ). 46 is the molecular weight of NO _2. The amount of NO ₂ can be obtained by substituting the pH value obtained by the pH sensor 25 into this equation.

In step S208, the NO ₂ ratio is calculated. This is obtained by dividing the NO ₂ amount calculated in step S207 by the NOx amount calculated in step S205.

  In this embodiment, a part of the exhaust gas flowing through the exhaust passage 2 is introduced into the cooling device 24, but the whole exhaust gas may be introduced into the cooling device 24. In this case, the exhaust amount may be calculated based on the intake air amount obtained by the air flow meter 31 and the fuel injection amount calculated by the ECU 10.

As described above, according to the present embodiment, the NO ₂ ratio in NOx can be calculated based on the pH. Thereby, the deterioration determination of the oxidation catalyst 3 can be performed. Furthermore, it is possible to determine the deterioration of the NOx catalyst 4 and determine the injection amount of urea water. NO in this example
_{Since the 2} ratio is calculated based on actual measurement by the sensor, a more accurate value can be obtained. Further, since the pH is not affected by the deterioration of the NOx catalyst 4 or the abnormality of the injection valve 5, an accurate NO ₂ ratio can be obtained regardless of the state of the NOx catalyst 4 or the injection valve 5.

1 is a diagram illustrating a schematic configuration of an internal combustion engine and an intake / exhaust system thereof according to Embodiment 1. FIG. It is a diagram showing the ratio of NO ₂ in the upstream side and the downstream side of the NOx concentration and in NOx than the oxidation catalyst. 3 is a flowchart showing a flow of determining deterioration of an oxidation catalyst according to Embodiment 1. FIG. 3 is a diagram illustrating a schematic configuration of an internal combustion engine and an exhaust system thereof according to a second embodiment. 6 is a flowchart showing a flow for obtaining a NO ₂ ratio in NOx according to Embodiment 2.

Explanation of symbols

1 Internal combustion engine 2 Exhaust passage 3 Oxidation catalyst 4 Selective reduction type NOx catalyst 5 Injection valve 6 NOx sensor 10 ECU
11 Accelerator pedal 12 Accelerator opening sensor 13 Crank position sensor 21 Branch passage 22 Shut-off valve 23 Exhaust amount sensor 24 Cooling device 25 pH sensor 30 Intake passage 31 Air flow meter

Claims (4)

A catalyst provided in the exhaust passage of the internal combustion engine and having an oxidation function;
A NOx sensor for measuring the NOx concentration in the exhaust;
A pH sensor for measuring the pH downstream of the catalyst;
Estimating means for estimating the ratio of NO ₂ in NOx based on the NOx concentration obtained by the NOx sensor and the pH obtained by the pH sensor;
An exhaust emission control device for an internal combustion engine, comprising: With a condensing device to condense the exhaust,
The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the pH sensor measures the pH of the condensate obtained by the condensing device. A measuring device for measuring the amount of exhaust gas condensed in the condensing device in a predetermined period;
Calculating the amount of NOx from the NOx concentration obtained by the NOx sensor during the predetermined period and the amount of exhaust;
Calculate the amount of NO ₂ from the pH obtained by measuring the condensate produced in the predetermined period with the pH sensor,
The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein a ratio of NO ₂ in NOx is calculated from the NO ₂ amount and the NOx amount. 4. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, further comprising a determination unit that determines that the catalyst has deteriorated when the ratio of NO ₂ is lower than a threshold value. 5. .

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