JP2013199913A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To determine deterioration of a catalyst having oxidizing capabilities more accurately.SOLUTION: An exhaust emission control device for an internal combustion engine comprises a catalyst provided in an exhaust passage of the internal combustion engine and having oxidizing capabilities and a NOx sensor provided downstream of the catalyst and detecting a NOx concentration in exhaust, wherein the exhaust emission control device comprises a determination means for determining that the catalyst is deteriorated when a length of a locus of a detection value of the NOx sensor in a predetermined period is equal to or more than a threshold.

Description

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

  Patent Document 1 describes that NOx sensors are provided on the upstream side and the downstream side of the catalyst, and the deterioration of the catalyst is determined based on the detected value of the NOx sensor.

Patent Document 2 describes that the NO ₂ ratio is estimated based on the temperature of the oxidation catalyst and the sensor value is corrected.

  However, if NOx sensors are provided upstream and downstream of the catalyst, the cost increases. In addition, since the NOx sensor may fail, it is necessary to determine whether the catalyst has deteriorated or the NOx sensor has failed.

Further, when the oxidation catalyst is provided upstream of the NOx sensor and the selective reduction type NOx catalyst (hereinafter also referred to as SCR catalyst) is provided downstream of the NOx sensor, the degree of deterioration of the oxidation catalyst. As a result, the ratio of NO ₂ in NOx flowing into the SCR catalyst changes. Then, according to the ratio of NO ₂ in NOx flowing into the SCR catalyst, since the change rate of reaction of NOx in the SCR catalyst, the purification rate of NOx changes.

For example, in the SCR catalyst, when NO and NO ₂ are present, they are reduced to N ₂ by a reaction such as “NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O”. This reaction rate is relatively fast. On the other hand, when NO ₂ does not exist, it is reduced to N ₂ by a reaction such as “4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O”. This reaction rate is relatively slow. Accordingly, when NO ₂ is present in the exhaust gas, the rate at which NOx is reduced is faster than when NO ₂ is not, so the NOx purification rate is increased.

For this reason, the amount of reducing agent supplied to the SCR catalyst, the amount of EGR gas supplied to the internal combustion engine, the threshold value at the time of determining the failure of the SCR catalyst, and the like may be changed according to the NO ₂ ratio. Therefore, there is a demand for accurately determine the ratio of NO ₂ in NOx flowing into the SCR catalyst. Here, NO is oxidized to NO ₂ when the exhaust gas passes through the oxidation catalyst, but this ratio varies depending on the degree of deterioration of the oxidation catalyst. Since the ratio of NO ₂ in NOx discharged from the internal combustion engine can be obtained in advance by experiments or the like, if the degree of deterioration of the oxidation catalyst upstream of the SCR catalyst can be obtained accurately, the SCR catalyst The ratio of NO ₂ in NOx flowing into the gas can be obtained. Conversely, if the ratio of NO ₂ in NOx flowing into the SCR catalyst (which may be the ratio of NO ₂ in NOx flowing out from the oxidation catalyst) is known, the degree of deterioration of the oxidation catalyst can also be known.

Although it is conceivable to determine the deterioration of the oxidation catalyst based on the purification rate of HC, it appears earlier that the ability to oxidize NO to NO ₂ is reduced rather than the reduction rate of HC. If we want to find the ratio of ₂ accurately, it is not enough.

JP 2011-058415 A JP 2010-031761 A

  The present invention has been made in view of the above-described problems, and an object of the present invention is to determine deterioration of a catalyst having oxidation ability with higher accuracy.

In order to achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention comprises:
In an exhaust emission control device for an internal combustion engine, comprising: a catalyst provided in an exhaust passage of the internal combustion engine and having an oxidizing ability; and a NOx sensor provided downstream of the catalyst and detecting NOx concentration in the exhaust gas.
A determination unit is provided that determines that the catalyst has deteriorated when the length of the locus of the detection value of the NOx sensor in a predetermined period is equal to or greater than a threshold value.

  The length of the detected value locus of the NOx sensor is, for example, the length of a line indicating the transition of the detected value when the horizontal axis is time and the vertical axis is the detected value, and the transition of the detected value is illustrated in the figure. It is. When the NOx sensor detects NOx over time, a value obtained by integrating the length of the line connecting the previous detection value and the current detection value every time becomes the length of the locus of the detection value.

  Even if the NOx concentration is the same, the length of the locus of the detected value (hereinafter also referred to as the locus length) varies depending on the state of deterioration of the catalyst. That is, as the degree of deterioration of the catalyst increases, the amount of NOx adsorbed by the catalyst decreases, so that NOx easily passes through the catalyst. For this reason, the detected value of the NOx sensor is close to the NOx concentration upstream of the catalyst. On the other hand, the closer the catalyst is to a new one, the more NOx is adsorbed and desorbed by the catalyst, so it takes time for NOx to reach the NOx sensor. Thereby, since the fluctuation | variation of NOx concentration is absorbed by the catalyst, the fluctuation | variation of NOx concentration downstream from a catalyst becomes small.

  From the above, the higher the degree of catalyst deterioration, the longer the locus of the detected value of the NOx sensor. Therefore, it is possible to determine whether or not the catalyst has deteriorated by comparing the locus length of the detected value of the NOx sensor for a predetermined period with the threshold value. That is, if the length of the locus of the detected value of the NOx sensor in the predetermined period is equal to or greater than the threshold value, it can be determined that the catalyst has deteriorated. Here, the predetermined period can be a period required until the length of the locus of the detected value of the NOx sensor becomes a size necessary for determining the deterioration of the catalyst. The threshold value also changes depending on the predetermined period. The threshold here is a lower limit value of the length of the locus of the detected value of the NOx sensor when the catalyst is deteriorated. Alternatively, it may be determined that the longer the locus of the detected value of the NOx sensor, the higher the degree of catalyst deterioration. Note that the catalyst is degraded means that the function of the catalyst is inferior to that of a new product, and the detected value of the NOx sensor is different from that of the new catalyst. Further, the catalyst may be deteriorated when the degree of deterioration of the catalyst exceeds an allowable range. Further, the catalyst may be deteriorated when the degree of deterioration of the catalyst is a predetermined value or more. This can include cases where the degree of degradation is high but within an acceptable range. For example, it may be determined that the catalyst has deteriorated before the degree of deterioration exceeds the allowable range. In addition, the new article can include not only a state where it is not deteriorated at all, but also a case where there is almost no influence on the purification performance even if it is deteriorated.

  In the present invention, the determination unit can determine that the catalyst is normal when the length of the locus of the detected value of the NOx sensor in a predetermined period is less than a threshold value.

Here, when the length of the locus of the detected value of the NOx sensor in the predetermined period is less than the threshold value, it is considered that the oxidation ability of the catalyst is high. That is, it is considered that the fluctuation of the detected value of the NOx sensor is suppressed because the catalyst has a high ability to adsorb NOx. In this case, even if the catalyst has not deteriorated or has deteriorated, it can be determined that it is within the allowable range.
That is, it can be determined that the catalyst is normal. Note that the normality of the catalyst may mean that the catalyst is in a new state or a state close to a new state.

  Further, in the present invention, the determination unit is configured such that the length of the locus of the detection value of the NOx sensor in a predetermined period is less than a threshold value and the integrated value of the detection value of the NOx sensor in the predetermined period is equal to or greater than the threshold value. It can be determined that the catalyst is normal.

Here, when the length of the locus of the detected value of the NOx sensor in the predetermined period is less than the threshold value, it is considered that the catalyst is normal, but it is also considered that the NOx sensor has failed. Therefore, it is necessary to determine whether or not the NOx sensor has failed. Here, since the amount of NOx adsorbed by the catalyst is very small, there is not much difference between the total amount of NOx flowing into the catalyst and the total amount of NOx flowing out from the catalyst when the catalyst is normal. Note that NO is oxidized to NO ₂ in the catalyst, but both NO and NO ₂ are detected as NOx.

  That is, when the NOx sensor is normal and the catalyst is normal, the integrated value of the detected values of the NOx sensor is relatively large. Therefore, if the integrated value of the detected values of the NOx sensor for a predetermined period is compared with a threshold value, it can be determined whether or not the catalyst is normal. That is, if the integrated value of the detected values of the NOx sensor in the predetermined period is equal to or greater than the threshold value, it can be determined that the catalyst has not deteriorated (is normal). The predetermined period referred to here can be a period required until the integrated value of the detected values of the NOx sensor reaches a magnitude necessary for determining deterioration of the catalyst. The threshold value also changes depending on the predetermined period. The threshold here is a lower limit value of the integrated value of the detected values of the NOx sensor when the catalyst is normal.

  On the other hand, when the integrated value of the detected values of the NOx sensor is less than the threshold value, it may be determined that the NOx sensor has failed.

  In the present invention, the determination means can set the threshold based on an estimated value or a detected value of the NOx concentration in the gas discharged from the internal combustion engine.

  The NOx concentration in the gas discharged from the internal combustion engine may be the NOx concentration in the exhaust passage between the internal combustion engine and the catalyst, or the NOx concentration in the gas flowing into the catalyst. The NOx concentration in the gas discharged from the internal combustion engine changes depending on the operating state of the internal combustion engine, and thereby the length of the detected value locus of the NOx sensor and the integrated value also change. In accordance with this change, the determination accuracy can be increased by changing the threshold value.

In order to achieve the above object, an exhaust emission control device for an internal combustion engine according to the present invention includes:
In an exhaust emission control device for an internal combustion engine, comprising: a catalyst provided in an exhaust passage of the internal combustion engine and having an oxidizing ability; and a NOx sensor provided downstream of the catalyst and detecting NOx concentration in the exhaust gas.
Ratio estimation means is provided for estimating the ratio of NO ₂ in NOx flowing out of the catalyst based on the length of the locus of the detected value of the NOx sensor in a predetermined period.

Here, the higher the degree of deterioration of the catalyst, the lower the ratio of oxidation from NO to NO _{2 in the} catalyst, so the NO ₂ ratio in NOx also decreases. There is a correlation between the length of the locus of the detected value of the NOx sensor and the degree of deterioration of the catalyst. Moreover, the degree of degradation of the catalyst, NO is correlated to the ratio to be oxidized to NO _2. Furthermore, the NO ₂ ratio in NOx discharged from the internal combustion engine can be estimated based on the operating state of the internal combustion engine. From the above, the ratio of NO oxidized by the catalyst can be estimated based on the length of the locus of the detected value of the NOx sensor. That is, the NO ₂ ratio in NOx can be estimated based on the length of the locus of the detected value of the NOx sensor. Since the NO ₂ ratio is a value corresponding to the degree of deterioration of the catalyst, it is possible to determine the deterioration of the catalyst based on the NO ₂ ratio.

Further, since the NOx purification rate varies depending on the NO ₂ ratio in the NOx, the amount of reducing agent or EGR gas may be adjusted so that the NOx purification rate becomes high. Further, the threshold value when determining a system failure or the correction value of the NOx sensor may be changed according to the NO ₂ ratio in NOx.

  According to the present invention, deterioration of a catalyst having oxidation ability can be determined with higher accuracy.

It is a figure which shows schematic structure of the internal combustion engine which concerns on an Example, and its exhaust system. It is a time chart which showed transition of the detected value of a NOx sensor. It is the time chart which showed transition of the locus length of the detected value of the NOx sensor. It is a time chart which showed transition of the integrated value of the detected value of a NOx sensor. It is the time chart which showed transition of the locus length of the detected value of the NOx sensor. It is a time chart which showed transition of the integrated value of the detected value of a NOx sensor. It is the figure which showed the relationship between the locus | trajectory length of the detected value of a NOx sensor, and an estimated locus | trajectory length. It is the figure which showed the relationship between the integrated value of the detected value of a NOx sensor, and an estimated integrated value. It is the flowchart which showed the flow of deterioration determination of the oxidation catalyst which concerns on an Example. And the temperature of the oxidation catalyst is a diagram showing the relationship between the NO ₂ ratio in NOx flowing into the SCR catalyst.

  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.

<Example 1>
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 and its exhaust system according to the present embodiment. The internal combustion engine 1 shown in FIG. 1 may be a diesel engine or a gasoline engine.

  An exhaust passage 2 is connected to the internal combustion engine 1. An oxidation catalyst 31 is provided in the exhaust passage 2. In addition, it may replace with the oxidation catalyst 31 and may provide the other catalyst (for example, three-way catalyst) provided with an oxidation capability.

  A NOx sensor 4 that detects the NOx concentration in the exhaust gas is provided in the exhaust passage 2 downstream of the oxidation catalyst 31. An SCR catalyst 32 that selectively reduces NOx may be provided downstream of the NOx sensor 4. However, if only the deterioration of the oxidation catalyst 31 is determined, the SCR catalyst 32 need not be provided. In addition, the oxidation catalyst 31 may be carried on a particulate filter, and a particulate filter may be provided between the oxidation catalyst 31 and the NOx sensor 4. A plurality of oxidation catalysts may be provided upstream of the NOx sensor 4.

  The internal combustion engine 1 is provided with an ECU 5 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 5 controls the internal combustion engine 1 in accordance with the operating conditions of the internal combustion engine 1 and the driver's request.

  In addition to the NOx sensor 4, the ECU 5 outputs an electric signal corresponding to the amount of depression of the accelerator pedal 6 by the driver, and an accelerator opening sensor 7 that detects the engine load, and a crank position that detects the engine speed. Sensors 8 are connected via electric wiring, and output signals from these various sensors are input to the ECU 5.

  Then, the ECU 5 determines the deterioration of the oxidation catalyst 31. Here, the detected value of the NOx sensor 4 is affected by the degree of deterioration of the oxidation catalyst 31.

  FIG. 2 is a time chart showing the transition of the detected value of the NOx sensor 4. The new catalyst is an oxidation catalyst 31 in a new state or a state close to a new state, and is a catalyst that has not deteriorated. Moreover, the deterioration catalyst is the oxidation catalyst 31 having a relatively high degree of deterioration. A deterioration catalyst is good also as the oxidation catalyst 31 when the degree of deterioration is more than a threshold value. Further, the deteriorated catalyst may be a catalyst within a range that can be said to be normal, although the oxidation ability is inferior to that of a new catalyst. The vehicle speed is the speed of the vehicle on which the internal combustion engine 1 is mounted. The detected value of the NOx sensor 4 in the deteriorated catalyst and the new catalyst is a detected value at the same vehicle speed.

  When the oxidation catalyst 31 is a new catalyst, more NOx is adsorbed on the oxidation catalyst 31 and then desorbed. For this reason, the maximum value of the detected value of the NOx sensor 4 becomes relatively small. That is, the time from when NOx is discharged from the internal combustion engine 1 until it reaches the NOx sensor 4 becomes longer (the responsiveness becomes lower), and the NOx concentration in the exhaust downstream of the oxidation catalyst 31 approaches the average. . Thereby, when the oxidation catalyst 31 is a new catalyst, the fluctuation range of the detected value of the NOx sensor 4 is small. For this reason, the NOx concentration tends to change when passing through the oxidation catalyst 31.

  On the other hand, when the oxidation catalyst 31 is a deteriorated catalyst, the amount of NOx adsorbed to the oxidation catalyst 31 is reduced, so the time from when NOx is exhausted from the internal combustion engine 1 until it reaches the NOx sensor 4 is shortened ( Responsiveness increases). For this reason, the NOx concentration in the exhaust downstream of the oxidation catalyst 31 becomes close to the NOx concentration in the gas discharged from the internal combustion engine 1, and the maximum value of the detected value also increases. For this reason, the fluctuation range of the detected value of the NOx sensor 4 is large. Thus, when the oxidation catalyst 31 is a deteriorated catalyst, the NOx concentration hardly changes when the exhaust gas passes through the oxidation catalyst 31.

  Here, FIG. 3 is a time chart showing the transition of the locus length of the detection value of the NOx sensor 4. FIG. 4 is a time chart showing the transition of the integrated value of the detected value of the NOx sensor 4. The locus length of the detected value of the NOx sensor 4 is the length of a line indicating the transition of the detected value of the NOx sensor 4 as shown in FIG. Further, the integrated value of the detected value of the NOx sensor 4 is a value obtained by integrating the detected value of the NOx sensor 4 for every predetermined time, for example, and shows the transition of the detected value of the NOx sensor 4 as shown in FIG. The area below the line.

  In the case of a new catalyst, the fluctuation range of the detected value of the NOx sensor 4 is smaller than in the case of a deteriorated catalyst. For this reason, the trajectory length of the new catalyst is shorter than the trajectory length of the deteriorated catalyst. The ratio of the trajectory length of the new catalyst to the trajectory length of the deteriorated catalyst is, for example, about 40%.

  On the other hand, in the integrated value of the detected value of the NOx sensor 4 shown in FIG. 4, the difference between the new catalyst and the deteriorated catalyst is small. Here, since the amount of NOx adsorbed by the oxidation catalyst 31 is very small, the amount of NOx reaching the NOx sensor 4 is not so different between the new catalyst and the deteriorated catalyst. The ratio of the integrated value of the new catalyst to the integrated value of the deteriorated catalyst is, for example, about 90%.

  As described above, the difference between the locus length of the NOx concentration in the gas discharged from the internal combustion engine 1 and the locus length of the detected value of the NOx sensor 4 is large, and the NOx concentration in the gas discharged from the internal combustion engine 1 is large. When the difference between the integrated value of NO and the integrated value of the detected value of the NOx sensor 4 is small, it is considered that the oxidation catalyst 31 is a new catalyst and the NOx sensor 4 is normal. It is also possible to determine whether or not the oxidation catalyst 31 is a new catalyst by comparing the locus length of the detected value of the NOx sensor 4 in a predetermined period with a threshold value. Further, it is possible to determine whether or not the oxidation catalyst 31 is a new catalyst by comparing the integrated value of the detected values of the NOx sensor 4 during a predetermined period with a threshold value.

  Here, as the failure of the NOx sensor 4, a gain deviation or an offset deviation can be considered. When the gain deviation occurs, the ratio (slope) of the sensor output to the NOx concentration is different from that of the normal NOx sensor 4. When only a gain deviation occurs, if the actual NOx concentration becomes zero, the detection value of the NOx sensor 4 also becomes zero.

  On the other hand, when an offset deviation occurs, the ratio (inclination) of the sensor detection value to the NOx concentration is the same as that of the normal NOx sensor 4, but a certain amount of deviation from the detection value of the normal NOx sensor 4 occurs. Occurs. When only the offset deviation occurs, the detected value of the NOx sensor 4 does not become 0 even if the actual NOx concentration becomes 0. Even if an offset deviation occurs, the locus length of the detected value of the NOx sensor 4 does not change.

  Here, FIG. 5 is a time chart showing the transition of the locus length of the detection value of the NOx sensor 4. FIG. 6 is a time chart showing the transition of the integrated value of the detected value of the NOx sensor 4. “Gain 50% sensor” refers to a case where the oxidation catalyst 31 is a deteriorated catalyst and a gain deviation occurs in the NOx sensor 4 and the detected value of the NOx sensor 4 is 50% of the actual NOx concentration. Is shown.

  5 and 6, in the case of the “gain 50% sensor”, the locus length and integrated value of the detected value of the NOx sensor 4 are smaller than those of the deteriorated catalyst. That is, when a gain deviation occurs in the NOx sensor 4, the trajectory length and the integrated value change.

  In this embodiment, the deterioration of the oxidation catalyst 31 is determined by comparing the trajectory length and the integrated value with these estimated values or threshold values.

  The estimated value of the locus length of the detected value of the NOx sensor 4 (hereinafter referred to as the estimated locus length) is the locus of the NOx concentration in the gas upstream of the oxidation catalyst 31 estimated from the operating state of the internal combustion engine 1. It is long. This estimated trajectory length is a case where it is assumed that the oxidation catalyst 31 is not provided, and may be a trajectory length of a detected value of the NOx sensor 4 when the NOx sensor 4 is normal, and is discharged from the internal combustion engine 1. Alternatively, the NOx concentration locus length when the NOx concentration in the detected gas is detected or estimated may be used.

  The integrated value of the detected value of the NOx sensor 4 (hereinafter referred to as an estimated integrated value) is an integrated value of the NOx concentration in the gas upstream of the oxidation catalyst 31 estimated from the operating state of the internal combustion engine 1. . This estimated integrated value is a case where it is assumed that the oxidation catalyst 31 is not provided, and may be an integrated value of the detected value of the NOx sensor 4 when the NOx sensor 4 is normal, and is discharged from the internal combustion engine 1. The integrated value of the NOx concentration when the NOx concentration in the detected gas is detected or estimated may be used. Hereinafter, unless otherwise specified, the trajectory length, integrated value, estimated trajectory length, and estimated integrated value are values in a predetermined period.

When the locus length of the detected value of the NOx sensor 4 is approximately equal to the estimated locus length and the integrated value of the detected value of the NOx sensor 4 is approximately equal to the estimated integrated value, the NOx adsorption ability and the NO oxidizing ability Therefore, it can be determined that the degree of deterioration of the oxidation catalyst 31 is large. In such a case, the ratio of NO ₂ in NOx becomes low.

On the other hand, when the locus length of the detected value of the NOx sensor 4 is shorter than the estimated locus length and the integrated value of the detected value of the NOx sensor 4 is substantially equal to the estimated integrated value, the NOx adsorption capacity of the oxidation catalyst 31 is increased. Since it is high and the oxidation ability of NO is high, it can be determined that the degree of deterioration of the oxidation catalyst 31 is small or has not deteriorated. That is, it can be determined that the oxidation catalyst 31 is a new catalyst (may be normal). In such a case, the ratio of NO ₂ in NOx increases.

  Further, if the trajectory length of the detected value of the NOx sensor 4 is significantly different from the estimated trajectory length, and the integrated value of the detected value of the NOx sensor 4 is greatly different from the estimated integrated value, it can be determined that the NOx sensor 4 has failed.

  Here, FIG. 7 is a diagram showing the relationship between the trajectory length of the detected value of the NOx sensor 4 and the estimated trajectory length. FIG. 8 is a diagram showing the relationship between the integrated value of the detected value of the NOx sensor 4 and the estimated integrated value.

  “Exhaust NOx” indicates a value when it is assumed that the detected value of the NOx sensor 4 is equal to the NOx concentration exhausted from the internal combustion engine 1. That is, in FIG. 7, the locus length of the detected value of the NOx sensor 4 is equal to the estimated locus length, and in FIG. 8, the accumulated value of the detected value of the NOx sensor 4 is equal to the estimated accumulated value. “Sensor failure” indicates a case where the NOx sensor 4 has failed.

  “Threshold” in FIG. 7 is a lower limit value of the trajectory length in the deteriorated catalyst. This threshold value may be a boundary of a range that can be substantially equal to the value of “exhaust NOx” or close to the value of “exhaust NOx”. That is, if the locus length of the detected value of the NOx sensor 4 is equal to or longer than the threshold value (region above the threshold value in FIG. 7), it can be determined that the catalyst is a deteriorated catalyst. This threshold value may be a value that is a predetermined ratio to the estimated trajectory length. Further, the threshold value may be set based on the estimated trajectory length.

  Further, the “threshold value” in FIG. 8 is a lower limit value of the integrated value in the new catalyst. This threshold value may be a boundary of a range that can be substantially equal to the value of “exhaust NOx” or close to the value of “exhaust NOx”. That is, if the integrated value of the detected values of the NOx sensor 4 is equal to or greater than the threshold value (region above the threshold value), it can be determined that the NOx sensor 4 is normal because it is a new catalyst or a deteriorated catalyst. This threshold value may be a value that is a predetermined ratio with respect to the estimated integrated value. Further, the threshold value may be set based on the estimated integrated value.

  That is, as shown in FIG. 7, it can be understood that the locus length of the detected value of the NOx sensor 4 is close to the estimated locus length in the case of the deteriorated catalyst. Further, as shown in FIG. 8, it can be understood that the integrated value of the detected value of the NOx sensor 4 is close to the estimated integrated value in the case of a deteriorated catalyst and a new catalyst. Then, it is understood that the locus length is not close to the estimated locus length and the integrated value is not close to the estimated integrated value is a case where the NOx sensor 4 is out of order.

  FIG. 9 is a flowchart showing a flow for determining deterioration of the oxidation catalyst 31 according to the present embodiment. This routine is repeatedly executed by the ECU 5 every predetermined time. In this embodiment, the ECU 5 that processes the routine shown in FIG. 9 corresponds to the determination means in the present invention.

In step S101, it is determined whether a precondition for determining the deterioration of the oxidation catalyst 31 is satisfied. For example, it is determined that the precondition is satisfied when the coolant temperature of the internal combustion engine 1 is a temperature suitable for determining the deterioration of the oxidation catalyst 31 and the NOx sensor 4 is in the active state. Furthermore, it may be determined that the precondition is satisfied when the estimated trajectory length is equal to or greater than a predetermined value required for the deterioration determination.

  If an affirmative determination is made in step S101, the process proceeds to step S102. On the other hand, if a negative determination is made, the deterioration determination cannot be performed, and thus this routine is terminated.

  In step S102, it is determined whether or not the difference between the estimated trajectory length and the trajectory length of the detected value of the NOx sensor 4 is large.

  In this step, it is determined whether the oxidation catalyst 31 is not a deteriorated catalyst. If the absolute value of the difference between the estimated trajectory length and the trajectory length is equal to or greater than the threshold value, it is determined that the difference between the estimated trajectory length and the trajectory length is large. This threshold value is the lower limit value of the absolute value of the difference between the estimated trajectory length when the oxidation catalyst 31 is a deteriorated catalyst and the trajectory length. Further, in this step, it may be determined whether the trajectory length of the detected value of the NOx sensor 4 is less than a threshold value. This threshold is the lower limit value of the locus length when the oxidation catalyst 31 is a deteriorated catalyst.

  Note that the estimated trajectory length may be the length of a line indicating the transition of the actual NOx concentration upstream of the oxidation catalyst 31. The estimated trajectory length may be the trajectory length of the detected value of the NOx sensor by attaching a NOx sensor upstream of the oxidation catalyst 31. Further, since there is a correlation between the operating state (engine speed and engine load) of the internal combustion engine 1 and the NOx concentration, the relationship between the operating state of the internal combustion engine 1 and the NOx concentration is obtained by experimentation and mapped. You may keep it. Then, the NOx concentration may be calculated from this map and the detected operating state of the internal combustion engine 1, and the length of the line indicating the transition of this value may be used as the estimated trajectory length.

  If an affirmative determination is made in step S102, the process proceeds to step S103, whereas if a negative determination is made, the process proceeds to step S106.

  Next, in step S103, it is determined whether or not the difference between the estimated integrated value and the integrated value of the detected value of the NOx sensor 4 is large.

  In this step, it is determined whether or not the NOx sensor 4 has failed. If the absolute value of the difference between the estimated integrated value and the integrated value is greater than or equal to the threshold value, it is determined that the difference between the estimated integrated value and the integrated value is large. This threshold value is the lower limit value of the absolute value of the difference between the estimated integrated value when the NOx sensor 4 is malfunctioning and the integrated value. Further, in this step, it may be determined whether or not the integrated value of the detected values of the NOx sensor 4 is less than a threshold value. This threshold is a lower limit value of the integrated value when the oxidation catalyst 31 is a new catalyst.

  The estimated integrated value may be an integrated value of the actual NOx concentration upstream of the oxidation catalyst 31. The estimated integrated value may be an integrated value of a value detected by the NOx sensor attached to the upstream side of the oxidation catalyst 31. Further, the relationship between the operating state of the internal combustion engine 1 and the NOx concentration is obtained by experimentation or the like and mapped, and the NOx concentration is calculated from this map and the detected operating state of the internal combustion engine 1, and this value is calculated. You may accumulate.

Here, when an affirmative determination is made in step S102, it is considered that the oxidation catalyst 31 is a new catalyst or the NOx sensor 4 has failed as shown in FIG. On the other hand, according to FIG. 8, if the NOx sensor 4 is normal and the oxidation catalyst 31 is a new catalyst, the difference between the estimated integrated value and the integrated value of the detected value of the NOx sensor 4 becomes small. Therefore, by comparing the estimated integrated value and the integrated value of the detected value of the NOx sensor 4, it can be determined whether the oxidation catalyst 31 is a new catalyst or whether the NOx sensor 4 has failed.

  If an affirmative determination is made in step S103, the process proceeds to step S104, where it is determined that the NOx sensor 4 has failed. On the other hand, if a negative determination is made in step S103, the process proceeds to step S105, where it is determined that the oxidation catalyst 31 is a new catalyst (normal catalyst). If it can be determined that the NOx sensor 4 has not failed by other known techniques, the process may proceed to step S105 when an affirmative determination is made in step S102. That is, step S103 and step S104 are not necessary.

  On the other hand, if a negative determination is made in step S102, the process proceeds to step S106, where it is determined that the oxidation catalyst 31 is a deteriorated catalyst. That is, when a negative determination is made in step S102, it is considered that the oxidation catalyst 31 is a deterioration catalyst as shown in FIG.

  In this way, the deterioration determination of the oxidation catalyst 31 can be performed. In addition, since the NOx purification rate in the SCR catalyst 32 changes depending on whether the oxidation catalyst 31 is a new catalyst or a deteriorated catalyst, the amount of reducing agent supplied to the SCR catalyst 32, the amount of EGR gas, the SCR system You may change the threshold value at the time of determining a failure, the correction value of the NOx sensor 4, and the like. For example, control corresponding to each of the new catalyst and the deteriorated catalyst may be stored in the ECU 5 in advance.

Further, the ratio of NO ₂ in NOx flowing into the SCR catalyst 32 may be estimated based on the locus length of the detected value of the NOx sensor 4.

Here, FIG. 10 is a diagram showing the relationship between the temperature of the oxidation catalyst 31 and the NO ₂ ratio in NOx flowing into the SCR catalyst 32. Thus, the NO ₂ ratio of the deteriorated catalyst is lower than that of the new catalyst. That is, since the rate of oxidation from NO to NO ₂ decreases according to the degree of deterioration of the oxidation catalyst 31, the NO ₂ ratio in NOx also decreases. There is a correlation between the locus length of the detection value of the NOx sensor 4 and the degree of deterioration of the oxidation catalyst 31. Moreover, the degree of deterioration of the oxidation catalyst 31, NO is correlated to the ratio to be oxidized to NO _2. Further, the NO ₂ ratio in NOx discharged from the internal combustion engine 1 can be estimated based on the operating state of the internal combustion engine 1. From the above, the ratio of NO ₂ in NOx flowing into the oxidation catalyst 31 can be estimated, and further, the ratio of NO oxidized by the oxidation catalyst 31 can be estimated. That is, the NO ₂ ratio in the NOx flowing into the SCR catalyst 32 can be estimated based on the locus length of the detected value of the NOx sensor 4.

Then, the NO ₂ proportion in the in NOx, since the NOx cleaning ratio is changed, so the NOx purification rate increases, may adjust the amount of reducing agent or the amount of EGR gas supplied to the SCR catalyst 32. Further, the threshold value when determining the failure of the SCR system or the correction value of the NOx sensor 4 may be changed according to the NO ₂ ratio in NOx. Further, the degree of deterioration of the oxidation catalyst 31 can be estimated based on the estimated NO ₂ ratio. That is, it can be estimated that the lower the NO ₂ ratio, the higher the degree of deterioration of the oxidation catalyst 31. In this embodiment, the ECU 5 that estimates the ratio of NO ₂ in NOx flowing out from the oxidation catalyst 31 based on the locus length of the detected value of the NOx sensor 4 in a predetermined period corresponds to the ratio estimating means in the present invention. To do.

1 Internal combustion engine 2 Exhaust passage 4 NOx sensor 5 ECU
6 Accelerator pedal 7 Accelerator opening sensor 8 Crank position sensor 31 Oxidation catalyst 32 Selective reduction type NOx catalyst (SCR catalyst)

Claims (5)

In an exhaust emission control device for an internal combustion engine, comprising: a catalyst provided in an exhaust passage of the internal combustion engine and having an oxidizing ability; and a NOx sensor provided downstream of the catalyst and detecting NOx concentration in the exhaust gas.
An exhaust emission control device for an internal combustion engine, comprising: determination means for determining that the catalyst has deteriorated when the length of the locus of the detection value of the NOx sensor in a predetermined period is equal to or greater than a threshold value.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the determination unit determines that the catalyst is normal when a length of a locus of a detection value of the NOx sensor in a predetermined period is less than a threshold value.   The determination means determines that the catalyst is normal when the length of the locus of the detected value of the NOx sensor in a predetermined period is less than the threshold and the integrated value of the detected value of the NOx sensor in the predetermined period is equal to or greater than the threshold. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is determined to be.   The exhaust gas purification of the internal combustion engine according to any one of claims 1 to 3, wherein the determination unit sets the threshold value based on an estimated value or a detected value of a NOx concentration in the gas discharged from the internal combustion engine. apparatus. In an exhaust emission control device for an internal combustion engine, comprising: a catalyst provided in an exhaust passage of the internal combustion engine and having an oxidizing ability; and a NOx sensor provided downstream of the catalyst and detecting NOx concentration in the exhaust gas.
An exhaust emission control device for an internal combustion engine, comprising ratio estimating means for estimating a ratio of NO ₂ in NOx flowing out from the catalyst based on a length of a locus of a detected value of the NOx sensor in a predetermined period.

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