JP2021038713A - Exhaust emission control device - Google Patents

Abstract

To provide an exhaust emission control device for enabling an initial value to be set for feedback control of the addition amount of urea water based on a NOx sensor detection value in consideration of the possibility that the NOx sensor detection value includes a detection error based on NH3.SOLUTION: An exhaust emission control device 100 includes a reduction catalyst, an addition valve 9e for adding urea water, a downstream NOx sensor 25, and an ECU 10 for calculating a correction value for the addition amount of the urea water on the basis of a detection result of the downstream NOx sensor 25 to perform feedback control. The ECU 10 calculates an initial value for the correction value at a power source ON timing and a difference between the initial value and the correction value in a period from the power source ON timing to a power source OFF timing to set the next initial value to be used at the next power source ON timing in a first period on the basis of a comparison result between a first difference calculated in the first period as a period from the most recent power source ON timing and a second difference calculated in a second period from the power source ON timing before the most recent one.SELECTED DRAWING: Figure 1

Description

The present invention relates to an exhaust gas purification device.

Conventionally, as an exhaust purification device for purifying nitrogen oxides (NOx) contained in exhaust gas, an addition valve that adds urea water to the exhaust gas, a selective reduction catalyst located downstream of the addition valve, and a selective reduction catalyst An exhaust gas purification device having a NOx sensor located downstream of the above is known.

Japanese Unexamined Patent Publication No. 2010-90796

_{In the exhaust purification device as described above, NH 3} may be contained in the exhaust gas that has passed through the selective reduction catalyst. In general, a NOx sensor has a property of reacting not only to NOx but also to NH ₃ , and the detection value of the NOx sensor may include a detection value based on NOx and a detection error based on _{NH 3.} ..

In the present invention, the initial value of feedback control of the amount of urea water added based on the detection value of the NOx sensor is set in consideration of the possibility that _{the detection value of the NOx sensor includes a detection error based on NH 3.} It is an object of the present invention to provide an exhaust gas purification device capable of the above.

The exhaust purification device according to one aspect of the present invention is provided in the exhaust passage of the internal combustion engine, has a reduction catalyst for reducing NOx contained in the exhaust gas of the internal combustion engine, and is provided on the upstream side of the reduction catalyst. Based on the detection results of the addition valve to which water is added, the NOx sensor provided on the downstream side of the reduction catalyst, and the NOx sensor, the correction value of the amount of urea water added by the addition valve is calculated and feedback control is performed. A control unit is provided, and the control unit obtains an initial value of a correction value at the power ON timing of the control unit and a difference between the initial value and the correction value in the period from the power ON timing to the power OFF timing of the control unit. The first difference calculated in the first period, which is the period from the latest power ON timing in the past, and the second difference calculated in the second period, which is the period from the power ON timing earlier than the latest in the past. Based on the comparison result of, the next initial value to be used at the next power ON timing of the first period is set.

In the exhaust gas purification device according to one aspect of the present invention, the control unit performs feedback control based on the detection result of the NOx sensor, and calculates the correction value of the amount of urea water added by the addition valve. The control unit sets the next initial value to be used at the next power ON timing in the first period based on the comparison result between the first difference and the second difference. The first difference is calculated in the first period, which is the period from the power ON timing of the latest past, and the second difference is calculated in the second period, which is the period from the power ON timing earlier than the latest in the past. ing. Result of comparing the first difference and the second difference, for example, when the relationship between the initial value and the correction value is changed greatly between the first period and the second period is detected based on the NH ₃ in the detection value of the NOx sensor It can be inferred that the possibility that an error is included has changed significantly between the first period and the second period. Therefore, based on the comparison result between the first difference and the second difference, the amount of urea water added based on the detection value of the NOx sensor is compared with the case where it is not based on the comparison result between the first difference and the second difference. The initial value of the feedback control can be set in consideration of the possibility that _{the detection value of the NOx sensor includes a detection error based on NH 3.}

In one embodiment, when the first difference and the second difference have the same sign, the control unit sets the next initial value using the correction value in the first period, and the first difference and the second difference have different signs. In the case of, the next initial value may be set by using the initial value at the power ON timing most recently in the past. As a result of comparing the first difference and the second difference in this way, when the first difference and the second difference have different signs, the detection value of the NOx sensor is NH _{3 in the first period and the second period.} It can be inferred that the change in the possibility that the detection error based on the above is included is larger than the case where the first difference and the second difference have the same sign.

According to the present invention, the initial value of the feedback control of the addition amount of urea water based on the detection value of the NOx sensor is set in consideration of the possibility that _{the detection value of the NOx sensor includes a detection error based on NH 3.} can do.

It is a schematic block diagram of the engine system provided with the exhaust gas purification device of embodiment. It is a figure which illustrates the internal structure of the catalytic converter of FIG. It is a timing chart which shows the operation example of the initial value setting by the ECU. It is a flowchart which illustrates the process of the ECU of FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or equivalent elements are designated by the same reference numerals, and duplicate description will be omitted.

[Engine system configuration]
FIG. 1 is a schematic configuration diagram of an engine system including the exhaust gas purification device of the embodiment. As shown in FIG. 1, the exhaust gas purification device 100 is mounted on a vehicle such as a truck and purifies the exhaust gas discharged from a diesel engine (hereinafter, simply referred to as an engine) 1 which is an internal combustion engine. The engine 1 has a plurality of injectors 2 that inject fuel into a plurality of cylinders. The vehicle on which the exhaust gas purification device 100 is mounted may be a large vehicle such as a tractor or a bus, a medium-sized vehicle, a small vehicle, and a light vehicle, in addition to a truck.

The intake passage 3 of the engine 1 is provided with an air cleaner 4, a turbocharger compressor 5, and an intercooler 6 in this order from the upstream side in the flow direction of the intake air. The engine 1 includes an EGR unit 7 that recirculates a part of the exhaust gas as EGR (exhaust gas recirculation) gas. An exhaust passage 8 for exhausting exhaust gas is connected to the engine 1. A catalytic converter 9 is provided in the exhaust passage 8.

FIG. 2 is a diagram illustrating the internal configuration of the catalytic converter 9 of FIG. As shown in FIGS. 1 and 2, in the catalyst converter 9, the oxidation catalyst [DOC: Diesel Oxidation Catalyst] 9a and the diesel exhaust particulate filter [DPF] are sequentially arranged from the upstream side to the downstream side in the exhaust gas flow direction. : Diesel Particulate Filter] 9b, selective reduction catalyst [SCR: Selective Catalytic Reduction] (reduction catalyst) 9c, and NH ₃ slip catalyst 9d are arranged.

DOC9a oxidizes and purifies HC, CO, etc. contained in the exhaust gas. DPF9b removes PM from the exhaust gas by collecting particulate matter [PM: Particulate Matter] contained in the exhaust gas. SCR9c reduces and purifies NOx contained in the exhaust gas. The NH ₃ slip catalyst 9d oxidizes and purifies _{NH 3} contained in the exhaust gas that has passed through the SCR 9c.

The exhaust gas purification device 100 includes an addition valve 9e arranged on the upstream side of the SCR9c, specifically, between the DPF9b and the SCR9c in the catalytic converter 9. The addition valve 9e is connected to the urea water tank via a supply pipe (not shown), and urea water is added to the SCR9c. When urea water is added to SCR9c by the addition valve 9e, the urea water becomes NH ₃ and is adsorbed on SCR 9c, and the NH ₃ reacts with NOx in the exhaust gas to reduce NOx.

The exhaust purification device 100 includes an air flow sensor 21, an engine state sensor 22, an upstream NOx sensor 23, an exhaust temperature sensor 24, a downstream NOx sensor (NOx sensor) 25, and an ECU [Electronic Control Unit] 10. And have. The sensors 21 to 25 and the addition valve 9e are connected to the ECU 10.

The air flow sensor 21 is, for example, a detector provided in the intake passage 3 of the engine 1 to detect the intake air amount of the engine 1. The air flow sensor 21 transmits a detection signal of the detected intake air amount to the ECU 10.

The engine state sensor 22 is a sensor for acquiring the engine state. The engine state sensor 22 includes, for example, a detector that detects the rotation speed of the engine 1 (engine rotation speed), the load of the engine 1, and the like. The engine state sensor 22 transmits a detection signal regarding the detected engine state to the ECU 10.

The upstream NOx sensor 23 is a detector provided between the DPF 9b and the addition valve 9e in the catalytic converter 9 and detects the amount of upstream NOx. The upstream NOx amount is the NOx amount (NOx concentration) contained in the exhaust gas upstream of the SCR9c. The upstream NOx sensor 23 transmits a detection signal of the detected upstream NOx amount to the ECU 10.

The exhaust temperature sensor 24 is a detector that detects the temperature of the exhaust gas between the DPF 9b and the SCR 9c in the catalytic converter 9. The exhaust temperature sensor 24 transmits the detected exhaust temperature detection signal to the ECU 10.

The downstream NOx sensor 25 is _{a detector provided downstream of the NH 3} slip catalyst 9d in the catalyst converter 9 (that is, on the downstream side of the SCR 9c) and detects the amount of downstream NOx. The amount of downstream NOx is the amount of NOx (NOx concentration) contained in the exhaust gas downstream of the _{NH 3 slip catalyst 9d.} The downstream NOx sensor 25 has a property of reacting not only with NOx but also with NH _3. Therefore, the amount of downstream NOx may include a detection error corresponding to the amount of _{NH 3} slipped downstream of the _{NH 3 slip catalyst 9d.} The downstream NOx sensor 25 transmits a detection signal of the detected downstream NOx amount to the ECU 10.

The ECU 10 is an electronic control unit having a CPU [Central Processing Unit], a ROM [Read Only Memory], a RAM [Random Access Memory], a CAN [Controller Area Network] communication circuit, and the like. The ECU 10 realizes various functions by loading the program stored in the ROM into the RAM and executing the program loaded in the RAM in the CPU. The ECU 10 may be composed of a plurality of electronic control units.

The ECU 10 has an engine state acquisition unit 11, a reducing agent control unit 12, and an initial value setting unit 13 as functional configurations.

The engine state acquisition unit 11 acquires various engine states based on the detection signals of the airflow sensor 21, the engine state sensor 22, the upstream NOx sensor 23, the exhaust temperature sensor 24, and the downstream NOx sensor 25.

The engine state acquisition unit 11 acquires the temperature of the exhaust gas between the DPF 9b and the SCR 9c in the catalytic converter 9 as the temperature of the SCR 9c based on the detection signal of the exhaust temperature sensor 24. Alternatively, the engine state acquisition unit 11 may calculate the fuel injection amount from the engine speed and the load, and acquire the estimated temperature of the SCR 9c from the fuel injection amount and the intake air amount. In this case, the exhaust temperature sensor 24 may be omitted.

The engine state acquisition unit 11 acquires the amount of upstream NOx based on the detection signal of the upstream NOx sensor 23. Alternatively, the engine state acquisition unit 11 may calculate the fuel injection amount from the engine speed and the load, and acquire an estimated value of the upstream NOx amount from the fuel injection amount and the intake air amount. In this case, the upstream NOx sensor 23 may be omitted.

The engine state acquisition unit 11 acquires the amount of downstream NOx based on the detection signal of the downstream NOx sensor 25.

The reducing agent control unit 12 calculates the amount of urea water added by the addition valve 9e based on the amount of upstream NOx. The reducing agent control unit 12 acquires, for example, the amount of urea water added to purify the NOx contained in the exhaust gas upstream of the SCR9c according to the amount of upstream NOx. The reducing agent control unit 12 adds urea water to the addition valve 9e at a predetermined addition timing with the calculated addition amount of urea water.

The reducing agent control unit 12 calculates a correction value for the amount of urea water added by the addition valve 9e based on the detection result of the downstream NOx sensor 25, and performs feedback control. More specifically, the reducing agent control unit 12 performs feedback control so that the amount of downstream NOx becomes a predetermined target amount of downstream NOx. The reducing agent control unit 12 sets, for example, a downstream NOx amount preset as suitable for the engine state as a target downstream NOx amount based on the detection results of the sensors 21 to 25.

The reducing agent control unit 12 calculates the basic addition amount of urea water so that the amount of downstream NOx becomes the target amount of downstream NOx. The reducing agent control unit 12 calculates the amount of urea water required to reduce the amount of upstream NOx acquired based on the detection signal of the upstream NOx sensor 23 as the basic addition amount. The reducing agent control unit 12 may use an estimated value of the upstream NOx amount obtained from the fuel injection amount and the intake air amount as the upstream NOx amount.

Reducing agent control unit 12 uses the previously stored estimation model, to calculate an estimated value of the NH ₃ amount of exhaust gas passing through the SCR9c and NH ₃ slip catalyst 9d. The estimation model is, for example, a model derived from the results of experiments or simulations performed in advance. The estimation model is configured to include the exhaust flow rate, catalyst temperature, target addition amount, and the like as parameters.

The reducing agent control unit 12 calculates the deviation between the amount of downstream NOx and the target amount of downstream NOx. The reducing agent control unit 12 calculates a correction value for feedback-controlling the amount of urea water added based on the deviation between the amount of downstream NOx and the target amount of downstream NOx. Reducing agent control unit 12, for example, the deviation, the target downstream NOx amount downstream NOx amount, and, based on the estimated value of the amount of NH _3, to calculate a correction value. The correction value may include, for example, a proportional term for performing proportional control based on deviation, an integral term for performing integral control based on deviation, a differential term for performing differential control based on deviation, and the like. ..

The reducing agent control unit 12 calculates a value obtained by correcting the basic addition amount with a correction value as a target addition amount for adding urea water to the addition valve 9e. As an example, the reducing agent control unit 12 calculates the target addition amount by multiplying the basic addition amount by the correction value.

The initial value setting unit 13 sets the initial value of the correction value of the feedback control by the reducing agent control unit 12. The initial value may be, for example, an integral term used at the start timing of feedback control. The feedback control start timing can be, for example, the power ON timing of the ECU 10.

The power ON timing means the timing when the key switch of the vehicle is turned from OFF to ON (the key is turned ON) and the power is turned on to the ECU 10. The timing when the power is turned on to the ECU 10 may be simply the timing when the power is turned on to the ECU 10, or after the power is turned on to the ECU 10, a predetermined start-up process is completed and the feedback control is started. It may be the timing when it becomes possible. The key switch may be a push button type switch.

The power OFF timing means, for example, the timing at which the key switch of the vehicle is turned from ON to OFF (the key is turned from ON to OFF) and the power cutoff process of the ECU 10 is started. The power OFF timing is not limited to the timing at which the power cutoff process is started, and may be the timing at which the feedback control is completed before the power supply cutoff process is started, or the key switch is changed from ON to OFF. It may be a timing at which the probability of being driven becomes high (for example, a timing at which a certain time has elapsed after the running vehicle stopped and the parking brake was activated).

FIG. 3 is a timing chart showing an operation example of initial value setting by the ECU. In FIG. 3, the horizontal axis is time, and the times t1 and t3 correspond to the power ON timing, and the times t2 and t4 correspond to the power OFF timing. In FIG. 3, the vertical axis represents the correction value, and is represented by a coefficient as an example.

The initial value setting unit 13 calculates the initial value of the correction value at the power ON timing of the ECU 10. For example, when the power OFF timing of the ECU 10 is reached, the initial value setting unit 13 calculates the initial value (next initial value) of the correction value used at the next power ON timing. In the example of FIG. 3, when the power OFF timing is reached at time t2, the initial value setting unit 13 calculates the next initial value to be used at the next power ON timing after time t3. When the power OFF timing at time t4 is reached, the initial value setting unit 13 calculates the next initial value to be used at the next power ON timing (not shown).

The initial value setting unit 13 calculates the difference between the initial value and the correction value in the period from the power ON timing of the ECU 10 to the power OFF timing of the ECU 10 (hereinafter, simply referred to as “period”). The initial value setting unit 13 may repeatedly calculate, for example, the difference between the initial value and the correction value in the period in a predetermined calculation cycle.

The initial value setting unit 13 calculates the difference between the initial value and the correction value in the first period, which is the period from the latest power ON timing in the past, as the first difference. The "past power ON timing" is a power source that is past and closest to the time when the initial value setting unit 13 is functioning (the time when feedback control is being performed). It means that it is ON timing. That is, the latest power ON timing in the past corresponds to the timing when the feedback control currently being executed is started, and the first period is a period including the present time when the feedback control is being executed. Here, the initial value setting unit 13 calculates, for example, the difference between the initial value set when the power OFF timing of the ECU 10 most recently in the past is reached and the correction value calculated by the reducing agent control unit 12 during the period. , Calculated as the first difference. The "past latest power-off timing" means that the power-off timing is in the past and is closest to the past-most recent power-on timing.

The initial value setting unit 13 calculates the difference between the initial value and the correction value in the second period, which is the period from the power ON timing earlier than the latest in the past, as the second difference. The power ON timing earlier than the latest power ON timing is a power ON timing earlier than the latest power ON timing in the past, and the power is turned ON in any one of the periods in which feedback control was performed in the past excluding the first period. Means timing. Therefore, the second period is any one of the periods in which feedback control was performed in the past excluding the first period. The calculated second difference is stored in, for example, the non-volatile memory of the ECU 10.

The initial value setting unit 13 sets the next initial value to be used at the next power ON timing of the first period based on the comparison result between the first difference and the second difference. The next power ON timing in the first period is the power ON timing of the next ECU 10 after the power OFF timing of the ECU 10 at which the feedback control currently being executed is terminated.

The initial value setting unit 13 compares the first difference with the second difference at, for example, the power OFF timing of the ECU 10 in which the key switch of the vehicle is turned from ON to OFF (the key is turned from ON to OFF). As the first difference used for comparison, the first difference at the power OFF timing of the ECU 10 at which the feedback control in the first period is completed can be used. In addition, the first difference at a predetermined timing during the implementation of the feedback control in the first period may be used. The second difference used for comparison is read from the non-volatile memory of the ECU 10.

As a result of comparison between the first difference and the second difference, the initial value setting unit 13 sets the next initial value using the correction value in the first period when the first difference and the second difference have the same sign. The first difference and the second difference have the same sign as the first change in the possibility that the _{detection value of the downstream NOx sensor 25 includes a detection error based on NH 3 between the first period and the second period.} This means that the difference and the second difference are smaller than in the case of different signs, and it can be considered that the risk of erroneous detection of the downstream NOx sensor 25 is small. Therefore, since the detection result of the downstream NOx sensor 25 in the first period has a certain degree of reliability, it is permissible to set the next initial value using the correction value in the first period.

As the correction value in the first period, for example, a correction value at the power OFF timing of the ECU 10 at which the feedback control in the first period is terminated can be used. In addition, the correction value at a predetermined timing during the execution of the feedback control in the first period may be used as the correction value in the first period. The predetermined timing may be, for example, the timing at which the magnitude of the first difference becomes maximum in the first period.

In the example of FIG. 3, when the period from the time t1 to the time t2 corresponds to the first period, the first difference d12 between the correction value and the initial value k1 in the first period is calculated. As a result of comparison between the first difference d12 and the second difference (not shown), an example is shown in which the first difference d12 and the second difference have the same sign, and the correction value in the first period (here, the power is turned off). The next initial value k2 is set using the correction value k2) at the timing time t2.

On the other hand, when the first difference and the second difference have different signs as a result of comparison between the first difference and the second difference, the initial value setting unit 13 uses the initial value at the latest power ON timing in the past to be used next time. Set the initial value. The difference between the first difference and the second difference is that the _{possibility that the detection value of the downstream NOx sensor 25 includes a detection error based on NH 3} changes significantly between the first period and the second period. It can be considered that there is a possibility that the downstream NOx sensor 25 is in a different situation from the second period in the first period (for example, there is a possibility that some erroneous detection has occurred in the downstream NOx sensor 25). Therefore, by setting the next initial value using the initial value at the power ON timing most recently in the past, it is possible to avoid the influence of erroneous detection of the downstream NOx sensor 25 in the first period.

In the example of FIG. 3, when the period from time t3 to time t4 corresponds to the first period and the period from time t1 to time t2 corresponds to the second period, the correction value and the initial value k2 in the first period The first difference d34 with and from is calculated. As a result of comparison between the first difference d34 and the second difference d12, an example is shown in which the first difference d34 and the second difference d12 have different signs, and the correction value in the first period (here, the power OFF timing) The next initial value k2 is set using the initial value k2 at the time t3, which is the latest power ON timing in the past, instead of the correction value k4) at the time t4.

Next, an example of processing by the ECU 10 will be described with reference to FIG. FIG. 4 is a flowchart illustrating the processing of the ECU 10 of FIG. The processing of the flowchart of FIG. 4 is repeatedly executed at a predetermined cycle in a period from, for example, the power ON timing to the power OFF timing of the ECU 10.

As shown in FIG. 4, the ECU 10 determines in S01 whether or not the key is ON by the initial value setting unit 13. When the initial value setting unit 13 determines that the key is not ON (S01: NO), the ECU 10 ends the process of FIG.

When the initial value setting unit 13 determines that the key is ON (S01: YES), the ECU 10 acquires the initial value this time by the initial value setting unit 13 in S02.

In S03, the ECU 10 acquires the engine state (for example, the amount of upstream NOx) by the engine state acquisition unit 11. In S04, the ECU 10 acquires the amount of downstream NOx by the engine state acquisition unit 11 based on the detection result of the downstream NOx sensor 25. In S05, the ECU 10 calculates the correction value based on the downstream NOx amount and the target downstream NOx amount by the reducing agent control unit 12. The calculated correction value is temporarily stored by the ECU 10. In S06, the ECU 10 calculates the first difference based on the initial value and the correction value this time by the initial value setting unit 13.

In S07, the ECU 10 determines whether or not the key has been turned from ON to OFF by the initial value setting unit 13. When the initial value setting unit 13 determines that the key has not been turned from ON to OFF (S07: NO), the ECU 10 ends the process of FIG.

When it is determined by the initial value setting unit 13 that the key is turned from ON to OFF (S07: YES), the ECU 10 acquires the second difference by the initial value setting unit 13 in S08. The second difference is calculated by the ECU 10 in the second period and is stored in the non-volatile memory by the ECU 10.

In S09, the ECU 10 determines whether or not the first difference and the second difference have the same sign by the initial value setting unit 13. When the initial value setting unit 13 determines that the first difference and the second difference have the same sign (S09: YES), in S10, when the ECU 10 is turned from key ON to key OFF (S07: YES). Set the next initial value with the correction value at the time of OFF. After that, the ECU 10 ends the process shown in FIG.

On the other hand, when the initial value setting unit 13 determines that the first difference and the second difference do not have the same sign (different signs) (S09: NO), in S11, the ECU 10 has acquired the initial value in S02. Set the initial value next time with the value. After that, the ECU 10 ends the process shown in FIG.

[Action and effect]
As described above, according to the exhaust gas purification device 100 according to the present embodiment, the reducing agent control unit 12 performs feedback control based on the detection result of the downstream NOx sensor 25, and the correction value of the amount of urea water added by the addition valve 9e. Is calculated. The initial value setting unit 13 sets the next initial value to be used at the next power ON timing of the first period based on the comparison result between the first difference and the second difference. The first difference is calculated in the first period, which is the period from the power ON timing of the latest past, and the second difference is calculated in the second period, which is the period from the power ON timing earlier than the latest in the past. ing. Result of comparing the first difference and the second difference, if for example, the relationship between the initial value and the correction value is changed greatly between the first period and the second period, the NH ₃ in the detection value of the downstream NOx sensor 25 It can be inferred that the possibility that the detection error is included is significantly changed between the first period and the second period. Therefore, based on the comparison result between the first difference and the second difference, the urea water based on the detection value of the downstream NOx sensor 25 is compared with the case where it is not based on the comparison result between the first difference and the second difference. The initial value of the feedback control of the addition amount can be set in consideration of the possibility that _{the detection value of the downstream NOx sensor 25 includes a detection error based on NH 3.}

According to the exhaust gas purification device 100, when the first difference and the second difference have the same sign, the initial value setting unit 13 sets the next initial value using the correction value in the first period, and sets the first difference and the second difference. When the two differences have different signs, the next initial value is set using the initial value at the power ON timing most recently in the past. As a result of comparing the first difference and the second difference, when the first difference and the second difference have different signs, the detected value of the downstream NOx sensor 25 is set to NH in the first period and the second period. _{It can be inferred that the change in the possibility that the detection error based on 3} is included is larger than the case where the first difference and the second difference have the same sign.

[Modification example]
Although the embodiments according to the present invention have been described above, the present invention is not limited to the above-described embodiments.

In the above embodiment, the initial value setting unit 13 compares the first difference and the second difference from the viewpoint of whether the first difference and the second difference have the same sign or different signs. Not limited. In short, it is possible to recognize how much the possibility that the detection value of the downstream NOx sensor 25 _{includes a detection error based on NH 3} changes between the first period and the second period, and the downstream NOx sensor 25 can be recognized. The initial value setting unit 13 can compare the first difference and the second difference by any method as long as it is a comparison method capable of recognizing whether or not there is a risk of erroneous detection. For example, the initial value setting unit 13 can avoid the influence of erroneous detection of the downstream NOx sensor 25 in the first period when the deviation between the first difference and the second difference is equal to or more than a predetermined threshold value. The initial value may be set next time.

In the above embodiment, the initial value setting unit 13 sets the next initial value using the initial value at the latest power ON timing in the past in order to avoid the influence of erroneous detection of the downstream NOx sensor 25 in the first period. However, the point is that the correction value in the first period should not be used as it is.

In the above embodiment, the catalyst converter 9 in which the DOC9a, the DPF9b, the SCR9c, and the NH ₃ slip catalyst 9d are integrally arranged has been exemplified, but the present invention is not limited thereto.

In the above embodiment, the diesel engine 1 is exemplified as the internal combustion engine, but other internal combustion engines such as a gasoline engine may be used.

8 ... Exhaust passage, 9c ... SCR (reduction catalyst), 9e ... Addition valve, 10 ... ECU (control unit), 25 ... Downstream NOx sensor (NOx sensor), 100 ... Exhaust purification device.

Claims (2)

A reduction catalyst provided in the exhaust passage of the internal combustion engine to reduce NOx contained in the exhaust gas of the internal combustion engine, and
An addition valve provided on the upstream side of the reduction catalyst and adding urea water to the reduction catalyst.
A NOx sensor provided on the downstream side of the reduction catalyst and
A control unit that calculates a correction value of the amount of urea water added by the addition valve based on the detection result of the NOx sensor and performs feedback control is provided.
The control unit
The initial value of the correction value at the power ON timing of the control unit and the difference between the initial value and the correction value in the period from the power ON timing to the power OFF timing of the control unit are calculated.
The first difference calculated in the first period, which is the period from the power ON timing of the latest past, and the second difference calculated in the second period, which is the period from the power ON timing prior to the past latest. An exhaust gas purification device that sets the next initial value to be used at the power ON timing next to the first period based on the comparison result of. The control unit
When the first difference and the second difference have the same sign, the next initial value is set using the correction value in the first period.
The exhaust gas purification device according to claim 1, wherein when the first difference and the second difference have different signs, the next initial value is set using the initial value at the power ON timing most recently in the past.

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