JP2015001206A - FAILURE DETERMINATION DEVICE AND FAILURE DETERMINATION METHOD FOR NOx SENSOR - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a failure determination device and a failure determination method for a NOx sensor capable of shortening the period, for which the supply of a reducer is stopped.SOLUTION: An ECU 44 decides it on the basis of a DPF temperature input from a DPF temperature sensor 26 whether or not a DPF 24 is in a regeneration processing. The ECU 44 outputs a stop signal to an injector 30 during the reproduction of the DPF 24 thereby to stop the supply of urea water. The ECU 44 decides the presence or absence of the trouble of a downstream NOx sensor 42 on the basis of the detection value of an upstream NOx sensor 28 and the detection value of the downstream NOx sensor 42, if it decides that a predetermined quantity of NOx has flown after the output of the stop signal to a selective reduction type catalyst 36.

Description

  The technology of the present disclosure relates to a failure determination device and failure determination method for a NOx sensor that determines whether or not there is a failure in a NOx sensor disposed downstream of a selective reduction catalyst.

  An exhaust gas purification device that purifies nitrogen oxides (hereinafter referred to as NOx) in exhaust gas includes a urea water supply unit that supplies urea water to exhaust gas, and a selective reduction catalyst that promotes reduction of exhaust gas by urea water. Yes. The supply amount of urea water supplied by the urea water supply unit is determined based on the detection value of the downstream NOx sensor attached downstream of the selective catalytic reduction catalyst.

  The determination of the failure of the downstream NOx sensor is important for suppressing the occurrence of an error in the urea water supply amount. In the technique described in Patent Document 1, the detection value of the downstream NOx sensor is detected after a predetermined time has elapsed since the supply of the reducing agent is stopped, and the responsiveness of the downstream NOx sensor is determined. Further, in the configuration in which NOx sensors are attached upstream and downstream of the selective catalytic reduction catalyst, whether or not the downstream NOx sensor is normal based on the difference between the detected value of the downstream NOx sensor and the detected value of the upstream NOx sensor. Has been determined.

JP 2010-174695 A

  By the way, when ammonia is adsorbed on the selective catalytic reduction catalyst, the difference between the detected value of the downstream NOx sensor and the detected value of the upstream NOx sensor varies depending on the amount of adsorbed ammonia. Therefore, the failure determination of the downstream NOx sensor based on the difference between the detection value of the downstream NOx sensor and the detection value of the upstream NOx sensor is usually performed after a predetermined amount of NOx flows after the urea water supply is stopped. .

  On the other hand, in a state where ammonia is not adsorbed on the selective catalytic reduction catalyst, most of NOx contained in the exhaust gas is discharged into the atmosphere. Therefore, in the failure determination of the downstream NOx sensor based on the difference between the detection value of the upstream NOx sensor and the detection value of the downstream NOx sensor, it is desirable to shorten the period during which the supply of the reducing agent is stopped.

  An object of the technology of the present disclosure is to provide a NOx sensor failure determination apparatus and failure determination method capable of shortening the period during which the supply of the reducing agent is stopped.

  A NOx sensor failure determination apparatus that solves the above problems includes a supply unit that supplies a reducing agent to the selective catalytic reduction catalyst from upstream of the selective catalytic reduction catalyst, and an upstream NOx sensor that is positioned upstream of the selective catalytic reduction catalyst. A determination unit that determines whether there is a failure in which a difference between a detected value of a downstream NOx sensor located downstream of the selective catalytic reduction catalyst and the reference value is larger than a predetermined range using the value as a reference value The determination unit supplies the reducing agent to the supply unit when the exhaust gas flowing into the filter is heated to regenerate the filter located upstream of the upstream NOx sensor. When the amount of NOx flowing into the selective catalytic reduction catalyst reaches a predetermined amount after the supply is stopped, the detected value of the upstream NOx sensor and the downstream NOx concentration are reduced. To get the detected value of the service.

  A NOx sensor failure determination method that solves the above problem is applied to an exhaust purification device that includes an upstream NOx sensor positioned upstream of a selective catalytic reduction catalyst and a downstream NOx sensor positioned downstream of the selective catalytic reduction catalyst, A NOx sensor failure determination method for determining whether or not a downstream NOx sensor has failed, wherein a filter located upstream of the upstream NOx sensor is regenerated by increasing the temperature of exhaust gas flowing into the filter. And the step of stopping the supply of the reducing agent to the selective catalytic reduction catalyst located downstream of the filter, and the amount of NOx that has flowed into the selective catalytic reduction catalyst from the stop of the supply of the reducing agent. Acquiring the detection value of the upstream NOx sensor and the detection value of the downstream NOx sensor when a predetermined amount is reached. , As a reference value to the detected value of the upstream NOx sensor, and a step of determining whether there is a greater failure than deviation is a predetermined range between the detection value and the reference value of the downstream NOx sensor.

  According to the above configuration, the cumulative amount of NOx required for acquiring the detection value of the sensor starts to be measured from the time of filter regeneration. The amount of NOx during filter regeneration is usually higher than when the filter is not regenerated. Therefore, if the NOx amount is measured from the regeneration of the filter, the time for the accumulated NOx amount to reach the predetermined amount is naturally shortened. As a result, the time required for determining the failure of the downstream NOx sensor is shortened.

  In the NOx sensor failure determination apparatus, the determination unit calculates the amount of NOx flowing into the selective reduction catalyst based on an exhaust flow rate that is the flow rate of the exhaust gas and a detection value of the upstream NOx sensor. Is preferred.

  According to the above configuration, the amount of NOx flowing into the selective catalytic reduction catalyst is, for example, a simple method and high accuracy in the amount of NOx compared to a configuration estimated from a plurality of parameters indicating the operating state of the engine. Desired.

  In the NOx sensor failure determination device, the detected value of the upstream NOx sensor is an average value of a plurality of output values from the upstream NOx sensor output at each of a plurality of different timings, and the downstream NOx sensor The detection value is preferably an average value of a plurality of output values from the downstream NOx sensor output at each of the plurality of different timings.

  According to the above configuration, the detection value of the upstream NOx sensor is a value reflecting a plurality of output values output from the upstream NOx sensor, and the detection value of the downstream NOx sensor is a plurality of outputs output from the downstream NOx sensor. The value reflects the value. Therefore, even when the output value of the upstream NOx sensor and the output value of the downstream NOx sensor include an error due to a sudden external cause, the reliability of the determination result is prevented from being lost due to such an error. It is done.

  In the NOx sensor failure determination apparatus, the determination unit is configured on the condition that a detection value of a temperature sensor for detecting the temperature of the filter is equal to or greater than a predetermined value indicating that regeneration of the filter is in progress. It is preferable to stop the supply of the reducing agent to the supply unit.

  According to the above configuration, the timing for stopping the supply of the reducing agent is determined based on the state of the filter itself. Therefore, for example, the timing at which the supply of the reducing agent is stopped is set to a more appropriate timing as compared with the configuration in which the supply of the reducing agent is stopped immediately after the heated exhaust gas is supplied to the filter.

The schematic block diagram which shows schematic structure of the exhaust gas purification apparatus which comprised one Embodiment of the failure determination apparatus of the NOx sensor in the technique of this indication. The flowchart which shows the process sequence of a determination process.

Hereinafter, with reference to FIG.1 and FIG.2, one Embodiment of the failure determination apparatus and failure determination method of a NOx sensor in this indication is described.
As shown in FIG. 1, an exhaust gas purification device 20 that purifies exhaust gas is disposed in the exhaust passage 12 of the diesel engine 10. The exhaust gas flowing into the exhaust gas purification device 20 flows into a pre-stage oxidation catalyst 22 (DOC: Diesel Oxidation Catalyst).

  The pre-stage oxidation catalyst 22 is obtained by supporting a metal such as platinum or palladium, a metal oxide, or the like on a support made of alumina, silica, zeolite, or the like. The pre-stage oxidation catalyst 22 oxidizes hydrocarbons (HC), carbon monoxide (CO), and nitrogen monoxide (NO) contained in the exhaust, and converts them into water, carbon dioxide, nitrogen dioxide, and the like.

  The exhaust gas that has passed through the front-stage oxidation catalyst 22 flows into a DPF 24 (DPF: Diesel particulate filter). The DPF 24 is composed of ceramics or a metal porous body, and collects particulate matter (PM) in the exhaust gas. In the regeneration process of the DPF 24, in order to raise the temperature of the exhaust gas flowing into the DPF 24, for example, combustion gas is supplied from a burner (not shown) to the exhaust passage 12 upstream of the upstream oxidation catalyst 22 or from a fuel injection valve (not shown). Fuel is injected.

  A DPF temperature sensor 26 is disposed in the DPF 24. The DPF temperature sensor 26 detects the DPF temperature Td that is the temperature of the DPF 24 at a predetermined control cycle, and outputs a signal indicating the detected DPF temperature Td to the ECU 44.

  An upstream NOx sensor 28 is disposed downstream of the DPF 24 and downstream of the selective reduction catalyst 36 described later. The upstream NOx sensor 28 detects the NOx concentration Cnx1 of the exhaust gas that has passed through the DPF 24 at a predetermined control cycle, and outputs a signal indicating the detected NOx concentration Cnx1 to the ECU 44.

  An electronically controlled injector 30 that supplies urea water as a reducing agent to the exhaust passage 12 is disposed downstream of the upstream NOx sensor 28. A predetermined concentration of urea water stored in a tank 32 is pumped to the injector 30 by a pump pump 34. The pressure pump 34 incorporates a relief valve (not shown), and pumps urea water in the tank 32 to the injector 30 at a predetermined pressure. The injector 30 is controlled to open and close by the ECU 44. The urea water supplied to the exhaust is hydrolyzed into ammonia by the heat of the exhaust. The injector 30, the tank 32, the pressure feed pump 34, and the urea water are included in the supply unit.

  A selective reduction catalyst 36 is disposed downstream of the injector 30. The selective reduction catalyst 36 performs selective catalytic reduction that reduces NOx with ammonia. The selective reduction catalyst 36 is obtained by, for example, supporting zeolite or zirconia having high adsorptivity on a support made of a honeycomb-shaped ceramic. NOx in the exhaust gas reacts with ammonia by the catalytic action of the selective reduction catalyst 36 and is reduced to nitrogen and water.

  The selective reduction catalyst 36 is provided with a catalyst temperature sensor 38. The catalyst temperature sensor 38 detects the catalyst temperature Ts, which is the temperature of the selective reduction catalyst 36, in a predetermined control cycle, and outputs a signal indicating the detected catalyst temperature Ts to the ECU 44.

  Exhaust gas that has passed through the selective catalytic reduction catalyst 36 flows into a post-stage oxidation catalyst 40 (ASC: Ammonia Slip Catalyst). The post-stage oxidation catalyst 40 is obtained by, for example, supporting a metal such as platinum or palladium, a metal oxide, or the like on a carrier made of alumina, silica, zeolite, or the like. The post-stage oxidation catalyst 40 decomposes ammonia that has not been consumed in the reduction reaction in the selective reduction catalyst 36.

  A downstream NOx sensor 42 is disposed downstream of the post-stage oxidation catalyst 40. The downstream NOx sensor 42 detects the NOx concentration Cnx2 of the exhaust gas that has passed through the post-stage oxidation catalyst 40 at a predetermined control cycle, and outputs a signal indicating the detected NOx concentration Cnx2 to the ECU 44.

  The ECU 44 is a microcomputer that includes a CPU, a RAM, a ROM, and the like. The ECU 44 acquires the DPF temperature Td, the catalyst temperature Ts, the NOx concentration Cnx1, and the NOx concentration Cnx2 that are output values of various sensors. In addition, a signal indicating the intake air amount Ga is input from the air flow meter 46 to the EGR at a predetermined control cycle. The ECU 44 also handles the intake air amount Ga as an exhaust flow rate that is the flow rate of the exhaust gas flowing through the exhaust passage 12. That is, the ECU 44 functions as an acquisition unit that acquires the DPF temperature Td, the catalyst temperature Ts, the NOx concentration Cnx1, the NOx concentration Cnx2, and the intake air amount Ga.

  The ECU 44 executes various calculations and processes based on information input from the various sensors, control programs and various data stored in advance in the ROM, and these. The ECU 44 executes urea water supply processing through opening / closing control of the injector 30. The ECU 44 as a determination unit executes a determination process for determining whether or not the downstream NOx sensor 42 has a failure. When the downstream NOx sensor 42 is determined to be “abnormal” in the determination process, the ECU 44 operates the alarm device 50 to notify the determination result to the driver.

  In the supply process, the ECU 44 calculates the supply amount of urea water every predetermined control cycle. The ECU 44 calculates the supply amount of urea water on the assumption that the concentration of urea water is a desired concentration (for example, 32.5% wt). The ECU 44 calculates a basic supply amount of urea water necessary for purifying NOx in the exhaust based on the intake air amount Ga, the NOx concentration Cnx1, and the catalyst temperature Ts. When the downstream NOx sensor 42 is normal, the ECU 44 calculates the supply amount of urea water by correcting the basic supply amount based on the NOx purification rate η (= Cnx1 / Cnx2) based on the NOx concentration Cnx1 and the NOx concentration Cnx2. To do. On the other hand, when the downstream NOx sensor 42 is abnormal, the ECU 44 calculates the basic supply amount as the supply amount without correcting the basic supply amount. Then, the ECU 44 outputs a control signal to the injector 30 so that urea water corresponding to the calculated supply amount is supplied to the exhaust gas.

  With reference to FIG. 2, the process procedure of the determination process which ECU44 performs is demonstrated. The determination process is repeatedly executed. Further, it is assumed that the upstream NOx sensor 28 is determined to be normal by another determination process.

  As shown in FIG. 2, the ECU 44 acquires the DPF temperature Td from the DPF temperature sensor 26 in the first step S11, and determines whether or not the DPF temperature Td is equal to or higher than the regeneration temperature Tt in the next step S12. To do. The regeneration temperature Tt is a value defined in advance in the various data, and is a temperature that is always reached when the regeneration process of the DPF 24 is executed. The regeneration temperature Tt of the present embodiment is a target temperature of the DPF temperature Td during the regeneration process. When the DPF temperature Td is lower than the regeneration temperature Tt (step S12: NO), the ECU 44 once ends the determination process.

  On the other hand, when the DPF temperature Td is equal to or higher than the regeneration temperature Tt (step S12: YES), the ECU 44 outputs a stop signal for stopping the supply of urea water to the injector 30 in order to interrupt the supply process. (Step S13).

  In the next step S14, the ECU 44 calculates the NOx amount based on the newly acquired intake air amount Ga and NOx concentration Cnx1, and uses the calculated NOx amount to flow into the selective catalytic reduction catalyst 36 after outputting the stop signal. An integrated value Snx, which is the integrated NOx amount, is calculated. The ECU 44 compares the integrated value Snx and the threshold value St, and repeats the process of step S14 until the integrated value Snx exceeds the threshold value St. That is, the ECU 44 proceeds to the next step S15 on condition that the integrated value Snx exceeds the threshold value St. The threshold value St is a value defined in advance in the various data described above, and ammonia adsorbed on the selective catalytic reduction catalyst 36 when the stop signal is output, that is, when the temperature of the DPF 24 reaches the regeneration temperature Tt is consumed. It is assumed that

  In the next step S15, the ECU 44 newly acquires each of the intake air amount Ga, the NOx concentration Cnx1, and the NOx concentration Cnx2 by a predetermined sampling number. The ECU 44 calculates the NOx amount in each control cycle based on the intake air amount Ga and the NOx concentration Cnx1, and calculates a reference value Gnx1 that is an average value of the NOx amount. Further, the ECU 44 calculates the NOx amount in each control cycle based on the intake air amount Ga and the NOx concentration Cnx2, and calculates a comparison value Gnx2 that is an average value of the NOx amounts.

  In the next step S16, the ECU 44 calculates a deviation R that is a ratio of the comparison value Gnx2 to the reference value Gnx1, and the deviation R is equal to or higher than a lower limit normal value Rmin that is a normal range defined in advance in the various data and is an upper limit normal. It is determined whether or not it is included in the range of the value Rmax or less. When the deviation R is included in the normal range (step S16: YES), the ECU 44 determines that the downstream NOx sensor 42 is “normal” (step S17) and once ends the determination process. On the other hand, when the deviation R is not included in the normal range (step S16: NO), the ECU 44 determines that the downstream NOx sensor 42 is “abnormal” (step S18) and once ends the determination process. After completion of the determination process, the ECU 44 performs a supply process according to the determination result, and activates the alarm device 50 when the downstream NOx sensor 42 is determined to be “abnormal”.

Next, the effect | action of the determination process mentioned above is demonstrated.
The exhaust gas flowing out from the DPF 24 during the regeneration process of the DPF 24 contains more NOx than during the regeneration of the DPF 24 due to combustion of fuel for regenerating the DPF 24 or combustion of particulate matter itself. Therefore, by stopping the supply of urea water during the regeneration process of the DPF 24, the NOx concentration Cnx1 detected by the upstream NOx sensor 28 is increased, and the time until the integrated value Snx reaches the threshold value St in step S14 is shortened. Become. As a result, the time required for the determination process, that is, the period during which the supply of urea water is stopped is shortened.

  The selective reduction catalyst 36 has a characteristic that the amount of adsorption of ammonia decreases as the catalyst temperature Ts increases. During the regeneration process of the DPF 24, the temperature of the exhaust gas flowing out from the DPF 24 increases, so that the catalyst temperature Ts also increases. Therefore, in the above determination process, the supply of urea water is stopped in a state where the amount of adsorption of ammonia in the selective catalytic reduction catalyst 36 is small, and the NOx contained in the exhaust gas is increased and adsorbed on the selective catalytic reduction catalyst 36. Ammonia is also easily consumed. For this reason, it is possible to set the threshold value St of the integrated value Snx to a lower value than when the determination process is performed when the regeneration process of the DPF 24 is not performed.

As described above, according to the suitability determination device and suitability determination method of the NOx sensor of the above embodiment, the effects listed below can be obtained.
(1) By stopping the supply of urea water during the regeneration process of the DPF 24, the time until the integrated value Snx reaches the threshold value St is shortened. As a result, the time required for the determination process, that is, the period during which the supply of urea water is stopped is shortened. In addition, the degree of freedom regarding the threshold value St is expanded.

  (2) The integrated value Snx, which is the amount of NOx flowing into the selective catalytic reduction catalyst 36, is the calculated value of the amount of NOx based on the intake air amount Ga and the NOx concentration Cnx1. Therefore, compared with the case where the amount of NOx flowing into the selective catalytic reduction catalyst 36 is calculated based on, for example, the operating state of the engine 10, the amount of NOx is obtained by a simple method and with high accuracy.

  (3) Based on the reference value Gnx1 based on the plurality of NOx concentrations Cnx1 and the comparison value Gnx2 based on the plurality of NOx concentrations Cnx2, whether or not the downstream NOx sensor 42 has failed is determined. Therefore, the reliability of the determination result is increased as compared with the case where the presence or absence of abnormality of the downstream NOx sensor 42 is determined based on one NOx concentration Cnx1 and one NOx concentration Cnx2. Further, even if an error due to a sudden external cause occurs in the NOx concentration, it is possible to suppress the reliability of the determination result from being lost due to such an error.

  (4) Since it is determined whether the DPF 24 is in the regeneration process based on the DPF temperature Td, the stop signal is output at an appropriate timing, that is, during the regeneration process of the DPF 24.

  (5) The regeneration temperature Tt is set to the target temperature for the regeneration process of the DPF 24. That is, the regeneration temperature Tt is the highest temperature among the DPF temperatures Td that are always reached during the regeneration process of the DPF 24. Therefore, the supply of urea water is stopped in a state where the adsorption amount of ammonia in the selective catalytic reduction catalyst 36 is small as compared with the case where the regeneration temperature Tt is lower than the target temperature. As a result, the degree of freedom regarding the threshold value St of the integrated value Snx is also improved.

In addition, the said embodiment can also be suitably changed and implemented as follows.
The ECU 44 determines whether or not the DPF 24 is in the regeneration process, for example, depending on whether or not the conditions for executing the regeneration process of the DPF 24 such as the exhaust flow rate and the pressure difference between the upstream and downstream of the DPF 24 are satisfied. You may judge. Further, for example, the ECU 44 may determine whether or not the DPF 24 is in the regeneration process by receiving a notification from the execution unit that executes the regeneration process of the DPF 24.

  The ECU 44 may determine whether or not the downstream NOx sensor 42 has failed based on the plurality of NOx concentrations Cnx1 and the plurality of NOx concentrations Cnx2. For example, the ECU 44 compares the NOx concentration Cnx1 and NOx concentration Cnx2 acquired in the same control cycle to determine suitability for each control cycle, and the downstream NOx sensor 42 fails according to the ratio determined to be normal. You may determine the presence or absence of.

The number of samplings for obtaining the reference value Gnx1 and the comparison value Gnx2 may be different from each other.
Each of the lower limit normal value Rmin and the upper limit normal value Rmax may be a value that varies depending on the operating state of the engine 10 during the calculation period of the reference value Gnx1 and the comparison value Gnx2, for example. At this time, the ECU 44 stores a map in which the lower limit normal value Rmin and the upper limit normal value Rmax are defined for each driving state.

  The exhaust flow rate is not limited to the intake air amount Ga of the air flow meter 46, and may be a measured value of an exhaust flow meter disposed in the exhaust passage 12, for example, or a value calculated by various calculations Good.

The deviation R is not limited to the ratio of the comparison value Gnx2 to the reference value Gnx1, but may be, for example, the difference between the comparison value Gnx2 and the reference value Gnx1.
The integrated value Snx is not limited to a value obtained by integrating the NOx amount based on the intake air amount Ga and the NOx concentration Cnx1, but depends on the operation state of the diesel engine 10 such as the combustion injection amount, the engine rotational speed, the accelerator opening, etc. It may be an estimated value.

  The ECU 44 may store the deviation R in the past determination process. According to such a configuration, it is possible to grasp the degree and tendency regarding the deterioration of the downstream NOx sensor. In addition, the ECU 44 can improve the accuracy related to the supply amount of the reducing agent by correcting the NOx concentration Cnx2 detected after the end of the determination process based on the deviation R.

  The reducing agent is not limited to urea water as long as NOx in the exhaust is reduced by the selective reduction catalyst 36, and may be, for example, ammonia gas obtained by reforming urea water, or the ammonia gas itself. Or other substances.

The engine may be equipped with an EGR device that supplies a part of the exhaust to the intake air.
The engine is not limited to a diesel engine and may be a gasoline engine as long as it has a filter that collects particulate matter.

  DESCRIPTION OF SYMBOLS 10 ... Engine, 12 ... Exhaust passage, 20 ... Exhaust gas purification device, 22 ... Pre-stage oxidation catalyst, 24 ... DPF, 26 ... DPF temperature sensor, 28 ... Upstream NOx sensor, 30 ... Injector, 32 ... Tank, 34 ... Pressure feed pump, 36: selective reduction type catalyst, 38 ... catalyst temperature sensor, 40 ... post-stage oxidation catalyst, 42 ... downstream NOx sensor, 44 ... ECU, 46 ... air flow meter, 50 ... alarm device.

Claims (5)

A supply unit for supplying a reducing agent to the selective catalytic reduction catalyst from upstream of the selective catalytic reduction catalyst;
Using the detection value of the upstream NOx sensor located upstream of the selective catalytic reduction catalyst as a reference value, the difference between the detection value of the downstream NOx sensor located downstream of the selective catalytic reduction catalyst and the reference value is larger than a predetermined range A determination unit for determining whether or not there is a failure,
The determination unit
For the filter located upstream of the upstream NOx sensor, when the exhaust gas flowing into the filter is heated to regenerate the filter, the supply of the reducing agent is stopped at the supply unit, and the selection is performed. A NOx sensor failure determination device that acquires a detection value of the upstream NOx sensor and a detection value of the downstream NOx sensor when the amount of NOx flowing into the reduction catalyst reaches a predetermined amount after the supply is stopped. The determination unit
2. The NOx sensor failure determination device according to claim 1, wherein an NOx amount that has flowed into the selective catalytic reduction catalyst is calculated based on an exhaust flow rate that is the flow rate of the exhaust gas and a detection value of the upstream NOx sensor. The detection value of the upstream NOx sensor is an average value of a plurality of output values from the upstream NOx sensor output at each of a plurality of different timings.
3. The NOx sensor failure determination device according to claim 1, wherein the detection value of the downstream NOx sensor is an average value of a plurality of output values from the downstream NOx sensor output at each of the plurality of different timings. . The determination unit
The supply of the reducing agent is stopped by the supply unit on the condition that a detection value of a temperature sensor for detecting the temperature of the filter is equal to or greater than a predetermined value indicating that regeneration of the filter is in progress. The NOx sensor failure determination device according to any one of claims 1 to 3. A NOx sensor that is applied to an exhaust gas purification device that includes an upstream NOx sensor positioned upstream of a selective catalytic reduction catalyst and a downstream NOx sensor positioned downstream of the selective catalytic reduction catalyst, and that determines whether the downstream NOx sensor is faulty. A failure determination method,
For the filter located upstream of the upstream NOx sensor, when the filter is regenerated by raising the temperature of the exhaust gas flowing into the filter, the selective reduction catalyst located downstream of the filter, A step of stopping the supply of the reducing agent;
Obtaining a detection value of the upstream NOx sensor and a detection value of the downstream NOx sensor when the amount of NOx flowing into the selective reduction catalyst reaches a predetermined amount after the supply of the reducing agent is stopped; ,
Determining whether there is a failure in which the difference between the detected value of the downstream NOx sensor and the reference value is larger than a predetermined range using the detected value of the upstream NOx sensor as a reference value;
NOx sensor failure determination method comprising:

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