JP2009174921A - Gas concentration detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas concentration detector for early and highly accurately executing deterioration determination on an oxygen pump cell or a NOx sensor cell. <P>SOLUTION: Based on an amount ΔN(t) of change in a NOx sensor output N(t), an inflection point of the sensor output is determined (step 110) to perform activity determination on a NOx sensor at the determined inflection point (step 112). If the time T from the starting of NOx sensor warm-up to the inflection point is longer than a reference value Tth, the oxygen pump cell is determined to be deteriorated (step 124). If a NOx sensor output N(t+ΔT) is larger than a reference value Nth after the lapse of prescribed time from the inflection point, the NOx sensor cell is determined to be deteriorated (step 130). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a gas concentration detection device that detects the concentration of a specific gas component in exhaust gas discharged from an engine, and more particularly to deterioration determination of an oxygen pump cell or a gas concentration detection cell.

  An apparatus that performs heater energization control so that the element resistance of the pump cell matches the target element resistance is known (for example, see Patent Document 1). According to this device, it is determined that the sensor has deteriorated when the element resistance of the sensor cell is out of the predetermined range during heater energization control.

  An apparatus that performs sensor deterioration determination based on the ratio of pump cell current values before and after the oxygen partial pressure change is known (see, for example, Patent Document 2).

JP 2003-166967 A JP 2007-147383 A JP 2007-147386 A JP 2004-177179 A

However, in addition to the element resistance (impedance) used for sensor deterioration determination in Patent Document 1, the power supplied to the heater and the heater resistance have individual sensor differences. Therefore, since it is necessary to consider the influence of individual sensor differences, it may not be possible to accurately determine sensor deterioration.
In order to activate the sensor, it is necessary to perform heater energization control, but it is necessary to excessively raise the element temperature in consideration of the influence of individual sensor differences. As a result, there is a possibility that the sensor deterioration cannot be determined at an early stage. Furthermore, if the element temperature is raised excessively, the impedance changes, which may further reduce the accuracy of deterioration determination.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a gas concentration detection device that can perform deterioration determination of an oxygen pump cell or a NOx sensor cell early and with high accuracy. To do.

In order to achieve the above-mentioned object, the first invention provides an oxygen pump cell that discharges surplus oxygen in a gas to be measured with application of a voltage,
A gas concentration detection cell that detects the concentration of the specific gas component from the gas after excess oxygen is discharged by the oxygen pump cell, and outputs a current value according to the concentration;
Inflection point acquisition means for acquiring a time when an inflection point appears in the output of the gas concentration detection cell during warm-up of the oxygen pump cell and the gas concentration detection cell and during discharge of surplus oxygen;
A gas concentration detection device comprising an activity determination means for determining the time when the inflection point appears as the activation time of the gas concentration detection cell,
Element temperature acquisition means for acquiring element temperature of the oxygen pump cell or the gas concentration detection cell;
Deterioration determination means for determining that the oxygen pump cell or the gas concentration detection cell is deteriorated based on the element temperature acquired at the time when the inflection point appears is provided.

Further, the second invention is an oxygen pump cell that discharges surplus oxygen in the gas to be measured in accordance with voltage application,
A gas concentration detection cell that detects the concentration of the specific gas component from the gas after excess oxygen is discharged by the oxygen pump cell, and outputs a current value according to the concentration;
Inflection point acquisition means for acquiring a time when an inflection point appears in the output of the gas concentration detection cell during warm-up of the oxygen pump cell and the gas concentration detection cell and during discharge of surplus oxygen;
A gas concentration detection device comprising an activity determination means for determining the time when the inflection point appears as the activation time of the gas concentration detection cell,
Deterioration determination means for determining that the oxygen pump cell has deteriorated when the time from the warm-up start time to the time when the inflection point appears is longer than a reference value.

The third invention is the second invention, wherein
The oxygen pump cell outputs a current value corresponding to the amount of surplus oxygen to be discharged,
The inflection point acquisition means changes the output of the gas concentration detection cell based on the change of the output of the oxygen pump cell in consideration of the correlation between the output of the oxygen pump cell and the output of the gas concentration detection cell. Estimate when the music point appears,
The deterioration determination means determines that the oxygen pump cell is deteriorated when the time from the warm-up start time to the time when the inflection point estimated by the inflection point acquisition means appears is longer than a reference value. It is characterized by doing.

Moreover, 4th invention is 2nd invention.
An air-fuel ratio detection cell that outputs a current value corresponding to the air-fuel ratio of the gas to be measured;
The inflection point acquisition means takes into account the correlation between the output of the air-fuel ratio detection cell and the output of the gas concentration detection cell, and based on the change in the output of the air-fuel ratio detection cell, the gas concentration detection cell Estimate when the inflection point appears in the output of
The deterioration determination means determines that the oxygen pump cell is deteriorated when the time from the warm-up start time to the time when the inflection point estimated by the inflection point acquisition means appears is longer than a reference value. It is characterized by doing.

The fifth invention is the invention according to any one of the second to fourth inventions,
The deterioration determination means determines that the gas concentration detection cell is deteriorated when an output of the gas concentration detection cell after a predetermined time has elapsed from a time when the inflection point appears is a reference value or more. Features.

  In the present invention, the time when the inflection point appears in the output of the gas concentration detection cell during the warm-up of the oxygen pump cell and the gas concentration detection cell and during the discharge of excess oxygen is determined as the activation time of the gas concentration detection cell. . Here, when oxygen existing before warm-up is removed, an inflection point appears in the output of the gas concentration detection cell. According to the present invention, unlike the general determination of sensor activity (main activity), the gas concentration detection cell has a timing when the concentration of the specific gas component starts to be detected by the gas concentration detection cell without being affected by residual oxygen. The active state is determined.

  Furthermore, in the first invention, it is determined that the gas concentration detection cell or the gas concentration detection cell is deteriorated based on the element temperature acquired at the time when the inflection point appears. The time when such an inflection point appears is a time when the gas concentration detection cell starts detecting the concentration of the specific gas component without being affected by the residual oxygen, and the influence of the individual sensor difference is small. Therefore, according to the first aspect, since it is possible to determine the deterioration of the cell in a state where the influence of the individual sensor difference is reduced, it is possible to accurately determine the deterioration of the oxygen pump cell or the gas concentration detection cell. In addition, since it is not necessary to excessively increase the element temperature in consideration of individual sensor differences, it is possible to determine deterioration of the oxygen pump cell or the gas concentration detection cell at an early stage.

  In the second invention, when the time from the warm-up start time to the time when the inflection point appears is longer than the reference value, it is determined that the oxygen pump cell has deteriorated. The time when the inflection point appears is the time when the concentration of the specific gas component starts to be detected by the gas concentration detection cell without being affected by the residual oxygen, and the influence of the individual sensor difference is small. Therefore, according to the second invention, it is possible to accurately determine the deterioration of the oxygen pump cell in a state where the influence of the individual sensor difference is reduced. In addition, since it is not necessary to excessively increase the element temperature by performing heater energization control in consideration of individual sensor differences, it is possible to determine the deterioration of the oxygen pump cell at an early stage.

  In the third invention, in consideration of the correlation between the oxygen pump cell output and the gas concentration detection cell output, the time when the inflection point appears in the gas concentration detection cell output is estimated based on the change in the oxygen pump cell output. When the time from the warm-up start time to the time when the inflection point estimated by the inflection point acquisition means appears is longer than the reference value, it is determined that the oxygen pump cell has deteriorated. Therefore, according to the third aspect, as in the second aspect, it is possible to accurately determine the deterioration of the oxygen pump cell while reducing the influence of the individual sensor difference.

  In the fourth aspect of the invention, the time at which an inflection point appears in the gas concentration detection cell output is estimated based on the change in the air / fuel ratio detection cell output in consideration of the correlation between the air / fuel ratio detection cell output and the gas concentration detection cell output. Is done. Then, when the time from the warm-up start time to the time when the inflection point estimated by the inflection point acquisition means appears is longer than the reference value, it is determined that the gas concentration detection cell has deteriorated. Therefore, according to the fourth aspect, as in the second aspect, it is possible to accurately determine the deterioration of the oxygen pump cell in a state where the influence of individual sensor differences is reduced.

  In the fifth aspect of the invention, it is determined that the gas concentration detection cell has deteriorated when the gas concentration detection cell output after the elapse of a predetermined time from the time when the inflection point appears is equal to or greater than the reference value. Therefore, it is possible to accurately determine the deterioration of the NOx sensor cell in a state where the influence of individual sensor differences is reduced.

  Embodiments of the present invention will be described below with reference to the drawings. In addition, the same code | symbol is attached | subjected to the element which is common in each figure, and the overlapping description is abbreviate | omitted.

Embodiment 1 FIG.
FIG. 1 is a diagram for explaining the configuration of a gas concentration detection apparatus 10 according to Embodiment 1 of the present invention. A gas concentration detection device 10 shown in FIG. 1 is a NOx concentration detection device that detects the concentration of nitrogen oxides (hereinafter referred to as “NOx”) in exhaust gas discharged from an engine, for example.

  The gas concentration detection device 10 has a NOx sensor 1. The NOx sensor 1 is formed by sequentially stacking a spacer 3, a NOx sensor cell 4, a spacer 5, and a heater 6 below the oxygen pump cell 2.

The oxygen pump cell 2 has only a function of removing surplus oxygen in the gas to be measured, and has a solid electrolyte body 21 and a pair of pump electrodes 22 and 23. The solid electrolyte body 21 as an element has oxygen ion conductivity, and is, for example, ZrO ₂ , HfO ₂ , ThO ₂ , BiO ₃ or the like formed into a sheet shape. The pump electrodes 22 and 23 sandwiching the solid electrolyte body 21 from above and below can be formed by a method such as screen printing, for example.

  The first pump electrode 22 formed on the surface of the solid electrolyte body 21 is exposed in the space where the exhaust gas that is the gas to be measured exists, that is, in the exhaust passage of the engine. For example, a porous cermet electrode containing a noble metal such as Pt can be used as the first pump electrode 22.

  On the other hand, the second pump electrode 23 formed on the back surface of the solid electrolyte body 21 so as to face the first pump electrode 22 is exposed in a first internal space 31 described later. As the second pump electrode 23, an electrode inert to NOx gas, for example, a porous cermet electrode containing a Pt—Au alloy and ceramics such as zirconia and alumina can be used.

  In the oxygen pump cell 2, a pinhole 24 is formed as an introduction hole penetrating the solid electrolyte body 21 and the pump electrodes 22 and 23. The hole diameter of the pinhole 24 is designed so that the diffusion speed of the exhaust gas introduced into the first internal space 31 (described later) through the pinhole 24 becomes a predetermined speed. The first internal space 31 communicates with the space where the gas to be measured exists via the pinhole 24 and the protective layer 7.

  In addition, the porous protective layer 7 is formed so as to cover the surface of the first pump electrode 22 including the pinhole 24 and the solid electrolyte body 21 in the vicinity thereof. The porous protective layer 7 can be formed of, for example, porous alumina. The porous protective layer 7 can prevent the first pump electrode 22 from being poisoned, and can prevent the pinhole 24 from being clogged with soot contained in the exhaust gas.

  The spacer 3 is formed with the first internal space 31 and the second internal space 32 described above. The spacer 3 can be formed of alumina or the like, for example. The two internal spaces 31 and 32 communicate with each other through the communication hole 33. The internal spaces 31 and 32 and the communication hole 33 can be formed by providing a punched hole in the spacer 3.

  The NOx sensor cell 4 detects the NOx concentration from the amount of oxygen generated by NOx reductive decomposition. The NOx sensor cell 4 includes a solid electrolyte body 41 and a pair of detection electrodes 42 and 43 that sandwich the solid electrolyte body 41 from above and below. These detection electrodes 42 and 43 can be formed by a method such as screen printing, for example.

  The first detection electrode 42 formed on the surface of the solid electrolyte body 41 is exposed in the second internal space 32. As the first detection electrode 42, for example, a porous cermet electrode containing a Pt—Rh alloy and ceramics such as zirconia or alumina can be used.

  On the other hand, the second detection electrode 43 formed on the back surface of the solid electrolyte body 41 so as to face the first detection electrode 42 is exposed to the atmospheric duct 51 formed in the spacer 5. Air is introduced into the air duct 51. As the second detection electrode 43, for example, a porous cermet electrode containing a noble metal such as Pt can be used. The air duct 51 can be formed by providing a cutout in the spacer 5.

  The heater 6 includes sheet-like insulating layers 62 and 63 and a heater electrode 61 embedded between the insulating layers 62 and 63. The insulating layers 62 and 63 are made of ceramics such as alumina, for example. The heater electrode 61 is made of, for example, cermet of Pt and ceramics such as alumina.

  The gas concentration detection device 10 according to the first embodiment includes an ECU (Electronic Control Unit) 8 as a control device. The ECU 8 has a pump cell control means 81, a sensor cell control means 82, and a heater control means 83. The ECU 8 may be configured separately from the engine control ECU, or may be configured as a part of the engine control ECU.

  The pump cell control means 81 is connected to the first and second pump electrodes 22 and 23 of the oxygen pump cell 2. The pump cell control means 81 applies a voltage to the first and second pump electrodes 22 and 23 and detects a current value flowing through the oxygen pump cell 2 as an “oxygen pump cell output”.

The sensor cell control means 82 is connected to the first and second detection electrodes 42 and 43 of the NOx sensor cell 4. The sensor cell control means 82 applies a voltage to the first and second detection electrodes 42 and 43 and detects a current value flowing through the NOx sensor cell 4 as “NOx sensor cell output”.
The heater control means 83 is connected to the heater electrode 61. The heater control unit 83 supplies power to the heater electrode 61.

Next, the operation of the gas concentration detection device 10 will be described.
In the space above the protective layer 7, there is exhaust gas as a gas to be measured flowing through the engine exhaust passage. This exhaust gas contains oxygen, NOx, CO ₂ , H ₂ O and the like. The exhaust gas is introduced into the first internal space 31 through the protective layer 7 and the pinhole 24. The amount of exhaust gas introduced into the first internal space 31 is determined by the diffusion resistance of the protective layer 7 and the pinhole 24.

When a positive voltage and a negative voltage are applied from the pump cell control means 81 to the first and second pump electrodes 22 and 23, respectively, residual oxygen and exhaust gas are exposed on the second pump electrode 23 exposed in the first internal space 31. The oxygen therein is reduced to the oxygen ion O ²⁻ . This oxygen ion ² permeates the solid electrolyte body 21 by the pumping action and is discharged to the first pump electrode 22 side. At this time, the current value flowing through the oxygen pump cell 2 is detected by the pump cell control means 81 as an oxygen pump cell output. Excess oxygen is discharged by the oxygen pump cell 2, so that the oxygen concentration in the exhaust gas is lowered to a level that does not affect the NOx concentration detection by the NOx sensor cell 4.

The exhaust gas from which excess oxygen has been removed to a low oxygen concentration is introduced into the second internal space 32 through the communication hole 33. When a predetermined voltage is applied between the first detection electrode 42 and the second detection electrode 43 from the sensor cell control means 82, residual oxygen and NOx that is a specific component in the exhaust gas are decomposed on the first detection electrode 42. Oxygen ions O ²⁻ are generated. NOx is once decomposed into NO (single gasification) and then further decomposed into oxygen ions O ²⁻ . The oxygen ions O ²⁻ pass through the solid electrolyte body 41 and are discharged from the second detection electrode 43 to the air duct 51. At this time, the current value flowing through the NOx sensor cell 4 is detected by the sensor cell control means 82 as the NOx sensor cell output.

  In addition, electric power is supplied from the heater control means 83 to the heater electrode 61 in order to heat the solid electrolyte bodies 21 and 41 to the activation temperature. The temperature of the oxygen pump cell 2 is detected by the pump cell control means 81. On the other hand, the temperature of the NOx sensor cell 4 is detected by the sensor cell control means 82.

[Features of Embodiment 1]
Like the NOx sensor 1, in a NOx sensor using an element made of a solid electrolyte body, in order to obtain normal characteristics, it is necessary to heat the element temperature to a predetermined activation temperature by energizing the heater. Further, there is a demand for activating the NOx sensor 1 early in order to reduce emissions. That is, there is a request that the NOx sensor cell 4 of the NOx sensor 1 is activated early and the output of the NOx sensor cell is used for various controls. In order to realize the early activation of the NOx sensor 1, it is important to grasp the state of the NOx sensor 1 quickly and with high accuracy.

  According to the apparatus of Patent Document 4, the activity determination is performed based on the element impedance. In addition, there is known an apparatus that determines the activity of a gas concentration sensor based on power supplied to a heater, heater resistance, and the like.

  However, the element impedance, the heater supply power, the heater resistance, and the like vary among sensors (hereinafter referred to as “sensor individual differences”). For this reason, it is difficult to quickly and accurately grasp the sensor state based on the element impedance and the like. If the sensor activity determination is performed early based on the element impedance or the like, a situation may occur in which the NOx sensor cell detects the NOx concentration while being influenced by residual oxygen after the activity determination. In other words, there may be a situation where the accuracy of the NOx sensor cell output is low despite the activation determination. If so, the emission reduction effect may be insufficient. Furthermore, in the method of determining the activity of the gas concentration sensor based on the element impedance or the like, it is difficult to realize early activation to the maximum with each sensor because it is affected by individual sensor differences.

Therefore, in the first embodiment, as described below, the state of the NOx sensor 1 is accurately grasped, and the activity determination of the NOx sensor 1 is performed early and with high accuracy.
FIG. 2 is a diagram illustrating a change in oxygen pump cell output and a change in NOx sensor cell output when the NOx sensor is warmed up. A broken line Lp in FIG. 2 indicates a change in the oxygen pump cell output, and a solid line Ls indicates a change in the NOx sensor cell output.

At time t0 in FIG. 2, warming up of the NOx sensor 1 is started as the engine starts. That is, energization from the heater control means 83 to the heater electrode 61 is started. By such energization, the temperatures of the oxygen pump cell 2 and the NOx sensor cell 4, that is, the temperatures of the solid electrolyte bodies 21 and 41 are gradually increased. At the time t0, oxygen in the atmosphere remains in the first internal space 31 near the oxygen pump cell 2 and the second internal space 32 near the NOx sensor cell 4.
The NOx sensor 1 may be warmed up when returning from a fuel cut longer than a predetermined time (long time) in addition to when the engine is started. Note that the NOx sensor 1 may not be warmed up even when returning from a long fuel cut.

  Thereafter, when the temperature of the solid electrolyte body 41 of the NOx sensor cell 4 reaches a predetermined temperature at time t1, a NOx sensor cell output is obtained. After this time t1, the NOx sensor cell output increases as the activity of the NOx sensor cell 4 (solid electrolyte body 41) increases. This is because NOx introduced into the second internal space 32 in the vicinity of the NOx sensor cell 4 is not decomposed on the first detection electrode 42, but oxygen remaining in the second internal space 32 is generated on the first detection electrode 42. It is because it is decomposed. Thereafter, at time t3, the NOx sensor cell output reaches the upper limit value. That is, the upper limit value of the oxygen concentration detectable by the NOx sensor cell 4 is reached.

  Further, when the temperature of the solid electrolyte body 21 of the oxygen pump cell 2 reaches a predetermined temperature at time t2 after time t1, an oxygen pump cell output is obtained. After this time t2, as the activity of the oxygen pump cell 2 (solid electrolyte body 21) increases, the amount of oxygen remaining in the first internal space 31 near the oxygen pump cell 2 increases. For this reason, the oxygen pump cell output increases.

  As the activity of the oxygen pump cell 2 increases, the amount of oxygen discharged from the first internal space 31 increases. Furthermore, the amount of exhaust gas introduced into the first internal space 31 increases. As a result, the residual oxygen concentration in the first internal space 31 is reduced, and the amount of oxygen supplied from the first internal space 31 to the second internal space 32 is also reduced. Therefore, as the activity of the oxygen pump cell 2 increases, the residual oxygen concentration in the second internal space 32 gradually decreases. As a result, the NOx sensor cell output decreases after time t4.

  Thereafter, at time t5 when residual oxygen in the second internal space 32 is removed, an inflection point appears in the NOx sensor output. That is, the NOx sensor cell output curve changes greatly before and after time t5. The NOx sensor output before the inflection point appears is dominated by the influence of the oxygen concentration remaining in the second internal space 32, that is, the influence of the activity of the oxygen pump cell 2. On the other hand, the NOx sensor output after the inflection point appears is influenced by the NOx concentration in the second internal space 32 and the oxygen concentration adsorbed on the first detection electrode 42 of the NOx sensor cell 4, that is, the activity of the NOx sensor cell 4. Receive. From this, it is possible to grasp that the oxygen remaining in the first and second internal spaces 31, 32 before the warming-up of the NOx sensor 1 is removed at time t5 when the inflection point appears. Therefore, after the time t5 when the inflection point appears, the NOx concentration can be accurately detected by the NOx sensor cell 4 without being affected by the residual oxygen.

  Therefore, in the first embodiment, the activation determination of the NOx sensor 1 is performed at time t5 when the inflection point appears in the NOx sensor cell output. Here, the activity determination of the NOx sensor 1 in the present invention is different from general sensor activity (main activity). In the present invention, when the NOx sensor output that is not affected by the residual oxygen starts to be detected, that is, when the NOx sensor cell output can be used for various controls without being influenced by the residual oxygen, (Hereinafter the same). Thus, the NOx sensor 1 activation determination is performed at the time t5 when the NOx sensor cell 4 starts to detect the NOx concentration without being affected by the residual oxygen, thereby satisfying the requirement for the early activation of the NOx sensor 1 to the maximum. be able to.

Next, a method for identifying the inflection point will be described with reference to FIG.
FIG. 3 is a diagram for explaining a method of specifying the inflection point of the NOx sensor cell output.
First, the NOx sensor cell output N is acquired at predetermined intervals, and the change amount ΔN of the NOx sensor cell output is calculated at each time. Here, the change amount ΔN (t) at time t is calculated according to the following equation (1). When the calculated change amount ΔN (t) becomes smaller than a predetermined reference value ΔNth, the NOx sensor cell output N (t) at the time t is specified as the inflection point.
ΔN (t) = N (t-1) -N (t) (1)
In the example shown in FIG. 3, the NOx sensor cell output N decreases from time t10 to time t14. Therefore, the change amounts ΔN (t11) to ΔN (t14) calculated by the above equation (1) at each time t11 to time t14 all take a positive value. The change amounts ΔN (t11) to ΔN (t13) are equal to or greater than a predetermined reference value ΔNth, but the change amount ΔN (t14) is smaller than the reference value ΔNth. For this reason, the NOx sensor cell output N (t14) at time t14 is specified as the inflection point. Therefore, the activation determination of the NOx sensor 1 is performed at time t14 when an inflection point appears in the NOx sensor cell output.

  By the way, it is known that the NOx sensor 1 deteriorates due to poisoning or changes over time. Specifically, the pump electrodes 22 and 23 of the oxygen pump cell 2 and the detection electrodes 42 and 43 of the NOx sensor cell 4 are deteriorated. When the NOx sensor 1 is deteriorated, it is necessary to perform output correction, poisoning recovery control, and the like. Therefore, it is necessary to determine the deterioration of the NOx sensor 1 early and accurately.

  According to the apparatus of Patent Document 1, when performing heater energization control so that the pump cell element resistance matches the target element resistance, it is determined that the sensor has deteriorated when the sensor cell element resistance is out of the predetermined range. Is done. FIG. 4 is a diagram showing a correlation between element temperature and impedance when the sensor is deteriorated and when the sensor is not deteriorated. As shown in FIG. 4, the correlation between the element temperature and the impedance changes as the sensor deteriorates.

  However, in addition to the element resistance (impedance), the heater supply power and the heater resistance have individual sensor differences as described above. Therefore, it is necessary to determine the deterioration in consideration of the influence of individual sensor differences. As a result, there is a possibility that the sensor deterioration cannot be accurately determined only by the element resistance or the like. Furthermore, when performing heater energization control for activating the sensor, it is necessary to excessively raise the element temperature in consideration of the influence of individual sensor differences. For this reason, there is a possibility that sensor deterioration cannot be determined early. Furthermore, since the impedance changes as the element temperature rises excessively, there is a possibility of further degrading the accuracy of deterioration determination.

Therefore, in the first embodiment, the element temperature at the time when the inflection point appears in the NOx sensor cell output is acquired, and the deterioration determination of the NOx sensor 1 is performed based on the acquired element temperature. Specifically, when the element temperature at the time when the inflection point appears is higher than the reference value, it is determined that the NOx sensor 1 has deteriorated.
Here, as described above, the time when the inflection point appears in the NOx sensor cell output is the time when the NOx sensor cell 4 starts to detect the NOx concentration without being affected by the residual oxygen, and the sensor individual difference with respect to the element temperature. The impact is small. Therefore, the deterioration determination of the NOx sensor 1 can be accurately performed based on the element temperature at the inflection point.

[Specific Processing in Embodiment 1]
FIG. 5 is a flowchart showing a routine executed by the ECU 8 in the first embodiment. This routine is started at predetermined intervals. This predetermined interval corresponds to, for example, the interval from time t10 to time t11, the interval from time t11 to time t12, and the like shown in FIG.

  According to the routine shown in FIG. 5, first, it is determined whether or not the NOx sensor 1 is warming up (step 100). In this step 100, it is determined whether or not the NOx sensor 1 is warmed up when the engine is started or when returning from a long-time fuel cut. If it is determined in step 100 that the NOx sensor 1 is not warming up, the NOx sensor cell output as shown in FIG. 2 cannot be obtained, so this routine is temporarily terminated.

  If it is determined in step 100 that the NOx sensor 1 is warming up, the NOx sensor cell output N (t) is acquired (step 102). Then, using the NOx sensor cell output N (t) acquired in step 102, a change amount ΔN (t) is calculated according to the above equation (1) (step 104). Thereafter, it is determined whether or not the change amount ΔN (t) calculated in step 104 is larger than zero (that is, whether the change amount ΔN (t) is a positive value) (step 106). In this step 106, it is determined whether or not the current NOx sensor cell output N (t) is smaller than the previous NOx sensor cell output N (t-1).

  If it is determined in step 106 that the change amount ΔN (t) is less than or equal to zero, it is determined that the current NOx sensor cell output N (t) is greater than or equal to the previous NOx sensor cell output N (t−1). Is done. That is, it is determined that the NOx sensor output has not decreased. In this case, this routine is temporarily terminated.

  On the other hand, if it is determined in step 106 that the change amount ΔN (t) is greater than zero, the current NOx sensor cell output N (t) is smaller than the previous NOx sensor cell output N (t−1). It is judged. In this case, it is subsequently determined whether or not the change amount ΔN (t) is smaller than the reference value ΔNth (step 108). If it is determined in step 108 that the change amount ΔN (t) is greater than or equal to the reference value ΔNth, it is determined that an inflection point has not yet appeared in the NOx sensor cell output N, and this routine is temporarily terminated.

  If it is determined in step 108 that the change amount ΔN (t) is smaller than the reference value ΔNth, the NOx sensor cell output N (t) is specified as the inflection point (step 110). In the example shown in FIG. 3, since the change amount ΔN (t14) is smaller than the reference value ΔNth, the NOx sensor cell output N (t14) at time t14 is specified as the inflection point. Then, the time when the inflection point appears (for example, time t14 in FIG. 3) is determined to be the active time of the NOx sensor cell 4 (step 112). In step 112, the activation determination of the NOx sensor 1 is performed.

  Thereafter, the element temperature at the time t at which the inflection point is specified is detected (step 114). In this step 114, the element temperature of the oxygen pump cell 2 is detected by the pump cell control means 81, or the element temperature of the NOx sensor cell 4 is detected by the sensor cell control means 82. Then, it is determined whether or not the element temperature detected in step 114 is higher than a reference value (step 116). This reference value is a threshold value for determining whether or not the NOx sensor 1 has deteriorated, and can be stored in the ECU 8 in advance.

  If it is determined in step 116 that the element temperature is higher than the reference value, it is determined that the NOx sensor 1 has deteriorated (step 118). Thereafter, the present routine is terminated, and correction control for NOx sensor cell output and poisoning recovery control are performed by another routine. On the other hand, if it is determined in step 116 that the element temperature is equal to or lower than the reference value, it is determined that the NOx sensor 1 has not deteriorated, and this routine is immediately terminated.

  As described above, the NOx sensor 1 of the first embodiment has a configuration in which the NOx concentration is detected by the NOx sensor cell 4 after the residual oxygen is discharged by the oxygen pump cell 2. For this reason, during the warm-up of the NOx sensor 1, a change in the NOx sensor cell output as shown in FIG. 2 is obtained. In other words, the change in the NOx sensor cell output as shown in FIG. 2 is obtained by the difference in activity (activity difference) between the oxygen pump cell 2 and the NOx sensor cell 4. The inflection point appearing in the NOx sensor cell output indicates that the NOx sensor cell 4 can detect the NOx concentration without being affected by the residual oxygen. In the first embodiment, unlike the general sensor activity (main activity) determination, it is determined that the NOx sensor 1 is in an active state at the time when this inflection point appears. That is, the activity determination of the NOx sensor 1 is accurately performed based on the inflection point appearing in the NOx sensor cell output regardless of the element impedance or the like having the individual sensor difference, regardless of the individual sensor difference. Therefore, the early activation of the NOx sensor 1 can be realized to the maximum, and the demand for emission reduction can be sufficiently satisfied by using the highly accurate NOx sensor cell output for various controls.

  Further, in the first embodiment, the element temperature at the time when the inflection point appears in the NOx sensor cell output is acquired, and when the acquired element temperature is higher than the reference value, the NOx sensor 1 is deteriorated. Determined. At the time when the inflection point appears in the NOx sensor output, that is, the time when the NOx sensor cell 4 can detect the NOx concentration without being affected by the residual oxygen, the individual sensor temperature difference of the element temperature is small. Therefore, since the deterioration determination of the NOx sensor 1 can be performed in a state where the influence of the individual sensor difference is reduced, the deterioration determination can be performed with high accuracy. In addition, since it is not necessary to perform heater energization control in consideration of individual sensor differences and excessively increase the element temperature, it is possible to determine the deterioration of the NOx sensor 1 at an early stage. As a result, NOx sensor output correction, poisoning recovery control, and the like can be performed early.

(Modification)
By the way, in the first embodiment, the deterioration determination is performed based on the element temperature at the inflection point. However, a physical property value having a correlation with the element temperature is obtained at the inflection point, and the deterioration is performed based on the physical property value. A determination may be made.
FIG. 6 is a flowchart showing a routine executed by the ECU 8 in the modification of the first embodiment. According to the routine shown in FIG. 6, the processing up to step 112 is executed as in the routine shown in FIG.
Thereafter, the physical property value at the inflection point is acquired (step 115). In step 115, impedance, heater resistance, or heater power, which is a physical property value correlated with the element temperature, is acquired. This impedance can be acquired by the pump cell control means 81 or the sensor cell control means 82.

Thereafter, it is determined whether or not the physical property value acquired in step 115 is larger than a reference value (step 117). If it is determined in step 117 that the physical property value is larger than the reference value, it is determined that the NOx sensor 1 has deteriorated (step 118). Thereafter, the present routine is terminated, and correction control for NOx sensor cell output and poisoning recovery control are performed by another routine.
On the other hand, if it is determined in step 117 that the physical property value is equal to or less than the reference value, it is determined that the NOx sensor 1 has not deteriorated, and this routine is immediately terminated.

  In the first embodiment, the oxygen pump cell 2 is the “oxygen pump cell” in the first and second inventions, and the NOx sensor cell 4 is the “gas concentration detection cell” in the first and second inventions. The means 81 and the sensor cell control means 82 correspond to the “element temperature acquisition means” in the first invention, respectively. Further, in the first embodiment, the ECU 8 executes the process of step 110, so that the “inflection point acquisition means” in the first invention executes the process of step 112. When the “activity determination unit” executes the process of step 114, the “element temperature acquisition unit” in the first invention executes the processes of steps 116 and 118, and the “deterioration determination unit” in the first invention executes. Are realized.

Embodiment 2. FIG.
Next, a second embodiment of the present invention will be described with reference to FIGS.
The gas concentration detection apparatus according to the second embodiment can be realized by causing the ECU 8 to execute a routine shown in FIG. 9 to be described later using the hardware configuration shown in FIG.

[Features of Embodiment 2]
The deterioration of the NOx sensor 1 includes a case where the oxygen pump cell 2 deteriorates, a case where the NOx sensor cell 4 deteriorates, and a case where both the oxygen pump cell 2 and the NOx sensor cell 4 deteriorate.

  For example, when the oxygen pump cell 2 deteriorates, the waveform of the NOx sensor cell output changes as shown in FIG. FIG. 7 is a diagram showing a change in the NOx sensor cell output waveform accompanying the deterioration of the oxygen pump cell 2 in the second embodiment. Specifically, FIG. 7 is a diagram showing a change in the NOx sensor output waveform when the pump electrodes 22 and 23 of the oxygen pump cell 2 are deteriorated due to poisoning or aged use change. A solid line Ls1 in FIG. 7 indicates the NOx sensor cell output when the oxygen pump cell 2 is not deteriorated. A broken line Ls2 in FIG. 7 indicates the NOx sensor output when the oxygen pump cell 2 deteriorates.

  When the oxygen pump cell 2 is deteriorated, the oxygen removing ability of the oxygen pump cell 2 is lowered. For this reason, when the oxygen pump cell 2 deteriorates, as shown in FIG. 7, the fall of the NOx sensor cell output before the inflection point becomes gradual as compared with the case where the oxygen pump cell 2 does not deteriorate. Therefore, when the oxygen pump cell 2 is deteriorated, the time at which the inflection point appears is later than when the oxygen pump cell 2 is not deteriorated.

  Therefore, in the second embodiment, as shown in FIG. 7, the times Ta and Tb from the warm-up start time t0 of the NOx sensor 1 to the times ta and tb at which the inflection points appear are calculated. Then, when the calculated times Ta and Tb are longer than the reference value Tth, it is determined that the oxygen pump cell 2 has deteriorated. The reference value Tth can be obtained in advance by experiments or the like.

  On the other hand, when the NOx sensor cell 4 deteriorates, the waveform of the NOx sensor cell output changes as shown in FIG. FIG. 8 is a diagram showing a change in the NOx sensor output waveform accompanying the deterioration of the NOx sensor cell 4 in the second embodiment. Specifically, FIG. 8 is a diagram showing a change in the NOx sensor output waveform when the detection electrodes 42 and 43 of the NOx sensor cell 4 are deteriorated due to poisoning or a change in use over time. A solid line Ls1 in FIG. 8 indicates the NOx sensor cell output when the NOx sensor cell 4 is not deteriorated. A broken line Ls3 in FIG. 8 indicates the NOx sensor output when the NOx sensor cell 4 deteriorates.

  When the NOx sensor cell 4 is deteriorated, the NOx resolution by the NOx sensor cell 4 is lowered. For this reason, when the NOx sensor cell 4 is deteriorated, as shown in FIG. 8, the falling of the NOx sensor cell output after the inflection point becomes gentler than when the NOx sensor cell 4 is not deteriorated. Therefore, when the NOx sensor cell 4 is deteriorated, the NOx sensor cell output after the inflection point is higher than when the NOx sensor cell 4 is not deteriorated.

  Therefore, in the third embodiment, as shown in FIG. 8, NOx sensor cell outputs N (ta + ΔT) and N (tc + ΔT) after a predetermined time ΔT has elapsed from the times ta and tc at which the inflection points appear. Then, when the acquired NOx sensor cell outputs N (ta + ΔT) and N (tc + ΔT) are larger than the reference value Nth, it is determined that the NOx sensor cell 4 is deteriorated. The reference value Nth can be obtained in advance by experiments or the like.

[Specific Processing in Second Embodiment]
FIG. 9 is a flowchart showing a routine executed by the ECU 8 in the second embodiment. This routine is started at predetermined intervals.
According to the routine shown in FIG. 9, first, the processing up to step 112 is executed as in the routine shown in FIG.

  Thereafter, a time T from the start of warm-up to the inflection point is calculated (step 120). In the example shown in FIG. 7, a time Ta from time t0 to time ta or a time Tb from time t0 to time tb is calculated.

  Then, it is determined whether or not the time T calculated in step 120 is longer than the reference value Tth (step 122). The reference value Tth is a threshold value used for determining the deterioration of the oxygen pump cell 2, and can be obtained in advance by experiments or the like and stored in the ECU 8.

  If it is determined in step 122 that the time T is longer than the reference value Tth, it is determined that the oxygen pump cell 2 has deteriorated (step 124). Thereafter, the present routine is terminated, and correction control for NOx sensor cell output and poisoning recovery control are performed by another routine.

  On the other hand, if it is determined in step 122 that the time T is shorter than the reference value Tth, it is determined that the oxygen pump cell 2 has not deteriorated. In this case, the NOx sensor cell output N (t + ΔT) after the elapse of the predetermined time ΔT from the time t when the inflection point appears is acquired (step 126).

  Thereafter, it is determined whether or not the NOx sensor cell output N (t + ΔT) acquired in step 126 is larger than a reference value Nth (step 128). This reference value Nth is a threshold value used for determining the deterioration of the NOx sensor cell 4, and can be obtained in advance by experiments or the like and stored in the ECU 8.

  If it is determined in step 128 that the sensor cell output N (t + ΔT) is greater than the reference value Nth, it is determined that the NOx sensor cell 4 has deteriorated (step 130). Thereafter, the present routine is terminated, and correction control for NOx sensor cell output and poisoning recovery control are performed by another routine.

  On the other hand, if it is determined in step 128 that the sensor cell output N (t + ΔT) is equal to or less than the reference value Nth, it is determined that the NOx sensor cell 4 has not deteriorated. In this case, this routine is finished as it is.

  As described above, in the second embodiment, the deterioration determination of the oxygen pump cell 2 and the NOx sensor cell 4 is performed based on the waveform of the NOx sensor output N. Thereby, the deterioration of the oxygen pump cell 2 and the deterioration of the NOx sensor cell 4 can be distinguished with high accuracy.

  Specifically, the deterioration determination of the oxygen pump cell 2 is performed based on the waveform of the NOx sensor output N before the inflection point. When the time T from the start of warming up to the inflection point is longer than the reference value Tth, it can be determined that the oxygen pumping capacity of the oxygen pump cell 2 is reduced, and therefore it is determined that the oxygen pump cell 2 has deteriorated. be able to. At the time when the inflection point appears, the influence of individual sensor differences is small. Therefore, the deterioration determination of the oxygen pump cell 2 can be accurately performed in a state where the influence of the individual sensor difference is reduced. Furthermore, since it is not necessary to perform heater energization control in consideration of individual sensor differences to excessively increase the element temperature, it is possible to determine the deterioration of the oxygen pump cell 2 at an early stage.

  Further, the deterioration determination of the NOx sensor cell 4 is performed based on the waveform of the NOx sensor output N after the inflection point. If the NOx sensor output after the lapse of the predetermined time ΔT from the time t at which the inflection point appears is larger than the reference value Nth, it can be determined that the NOx resolution of the NOx sensor cell 4 is lowered, and therefore the NOx sensor cell 4 deteriorates. Can be determined. As described above, the influence of the individual sensor difference is small at the time when the inflection point appears. Therefore, it is possible to accurately determine the deterioration of the NOx sensor cell 4 in a state where the influence of individual sensor differences is reduced. Furthermore, since it is not necessary to perform heater energization control in consideration of individual sensor differences to excessively increase the element temperature, it is possible to determine the deterioration of the NOx sensor cell 4 at an early stage.

  In the second embodiment, the reference value Tth is used for determining the deterioration of the oxygen pump cell 2. However, the reference value Tth may be corrected using the engine water temperature, the intake air temperature, the intake air amount, etc. as parameters. That is, the reference value Tth may be corrected in consideration of the warm-up condition of the NOx sensor 1. Further, the reference value Tth may be derived with reference to a map in which the reference value Tth is defined in relation to these parameters.

  In the second embodiment, when the ECU 8 executes the process of step 110, the “inflection point acquisition means” in the second invention executes the process of step 112, thereby executing the process of step 112. The “activity determination means” executes the processing of steps 120, 122, and 124, and the “degradation determination means” in the second invention executes the processing of steps 126, 128, and 130, thereby executing the processing of the fifth invention. “Deterioration determination means” is realized.

Embodiment 3 FIG.
Next, Embodiment 3 of the present invention will be described with reference to FIGS.
The gas concentration detection apparatus according to the third embodiment can be realized by causing the ECU 8 to execute a routine shown in FIG. 12 described later using the hardware configuration shown in FIG.

[Features of Embodiment 3]
In the first and second embodiments, the inflection point is specified based on the comparison result between the change amount ΔN (t) of the NOx sensor cell output and the reference value ΔNth, and the NOx sensor 1 at the time when the inflection point appears. Activity determination was performed.

By the way, the oxygen pump cell 2 and the NOx sensor cell 4 have the same configuration, and both output a current value when oxygen ions O ²⁻ flow in the cell. Therefore, there is a correlation between the oxygen pump cell output and the NOx sensor cell output.
Therefore, in the present third embodiment, this correlation is used to specify the inflection point of the NOx sensor cell output.

  FIG. 10 is a diagram showing a correlation between the oxygen pump cell output and the NOx sensor cell output when the NOx sensor is warmed up. A broken line Lp in FIG. 10 indicates a change in the oxygen pump cell output, and a solid line Ls indicates a change in the NOx sensor cell output.

  As shown in FIG. 10, an inflection point appears not only at the inflection point of the NOx sensor cell output but also at the oxygen pump cell output. This inflection point of the oxygen pump cell output appears when oxygen remaining in the first internal space 31 is exhausted. The inventor has found that there is a correlation between a change in oxygen pump cell output and a change in NOx sensor cell output. In other words, it has been found that there is a correlation between the time 21 at which the inflection point of the oxygen pump cell output appears and the time t22 at which the inflection point of the NOx sensor cell output appears.

  Here, the difference Δtn between the times t21 and t22 can be obtained in advance by experiments or the like and stored in the ECU 8. Therefore, if the time when the inflection point appears in the oxygen pump cell output can be specified by the method described later, the difference Δtn obtained in advance is added to the time of the specified inflection point, thereby changing the inflection point to the NOx sensor cell output. The time when the point appears can be estimated.

  Next, a method for specifying the inflection point of the oxygen pump cell output will be described with reference to FIG. FIG. 11 is a diagram for explaining a method of specifying the inflection point of the oxygen pump cell output. As a method for identifying the inflection point of the oxygen pump cell output, a part of the method for identifying the inflection point of the NOx sensor cell output described in the first embodiment can be applied.

First, the oxygen pump cell output P is acquired at predetermined intervals, and the change amount ΔP of the oxygen pump cell output is calculated at each time. Here, the amount of change ΔP (t) at time t is calculated according to the following equation (2). When the calculated change amount ΔP (t) becomes smaller than a predetermined reference value ΔPth, the oxygen pump cell output P (t) at the time t is specified as the inflection point. In the following equation (2), the output P (t−1) at time (t−1) is subtracted from the output P (t) at time t so that the change amount ΔP (t) takes a positive value. ing.
ΔP (t) = P (t) -P (t-1) (2)

  In the example shown in FIG. 11, the oxygen pump cell output P increases from time t30 to time t34. Therefore, the change amounts ΔP (t31) to ΔP (t34) calculated by the above equation (2) at each time t31 to time t34 all take positive values. The change amounts ΔP (t31) to ΔP (t33) are equal to or greater than a predetermined reference value ΔPth, but ΔP (t34) is smaller than the reference value ΔPth. For this reason, the oxygen pump cell output P (t34) at time t34 is specified as the inflection point.

  Therefore, it can be estimated that the inflection point appears in the NOx sensor cell output N at the time (t34 + Δtn) obtained by adding the above Δtn to the time t34 at which the inflection point appears in the oxygen pump cell output. Therefore, the activation determination of the NOx sensor 1 can be performed at the time (t34 + Δtn).

Thereafter, similarly to the first embodiment, the element temperature at the estimated inflection point can be acquired, and deterioration determination of the NOx sensor 1 can be performed based on the acquired element temperature. Further, similarly to the modification of the first embodiment, the physical property value at the estimated inflection point can be acquired, and the deterioration determination of the NOx sensor 1 can be performed based on the acquired physical property value.
Further, as will be described below with reference to FIG. 12, it is possible to determine the deterioration of the oxygen pump cell 2 and the NOx sensor cell 4 using the estimated inflection point.

[Specific Processing in Embodiment 3]
FIG. 12 is a flowchart showing a routine executed by the ECU 8 in the third embodiment. This routine is started at predetermined intervals. This predetermined interval corresponds to, for example, the interval between time t30 and time t31 in FIG.

  According to the routine shown in FIG. 12, first, similarly to the routine shown in FIG. 5, it is determined whether or not the NOx sensor 1 is warming up (step 100). If it is determined in step 100 that the NOx sensor 1 is not warming up, since the change in the oxygen pump cell output as shown in FIG. 11 cannot be obtained, this routine is temporarily terminated.

  If it is determined in step 100 that the NOx sensor 1 is warming up, the oxygen pump cell output P (t) is acquired (step 140). Then, using the oxygen pump cell output P (t) acquired in step 140, a change amount ΔP (t) is calculated according to the above equation (2) (step 142).

  Thereafter, it is determined whether or not the change amount ΔP (t) calculated in step 142 is smaller than a reference value ΔPth (step 144). If it is determined in step 144 that the change amount ΔP (t) is greater than or equal to the reference value ΔPth, it is determined that an inflection point has not yet appeared in the oxygen pump cell output P, and this routine is temporarily terminated. On the other hand, if it is determined in step 144 that the change amount ΔP (t) is smaller than the reference value ΔPth, the oxygen pump cell output P (t) at time t is specified as the inflection point (step 146).

  Next, using the inflection point specified in step 146, the inflection point of the NOx sensor cell output is estimated in consideration of the correlation between the oxygen pump cell output P and the NOx sensor cell output N (step 148). Here, as shown in FIG. 10, a time difference Δtn between a time t21 at which the inflection point appears in the oxygen pump cell output and a time t22 at which the inflection point appears in the NOx sensor cell output is obtained in advance and stored in the ECU 8. . In step 148, it is estimated that an inflection point appears in the NOx sensor cell output at a time (t + Δtn) obtained by adding the time difference Δtn to the time t when the inflection point appears in the oxygen pump cell output. That is, the NOx sensor cell output N (t + Δtn) at time (t + Δtn) is estimated to be an inflection point.

  Thereafter, it is determined whether or not the current time has reached the time (t + Δtn) at which the inflection point appears in the NOx sensor cell output (step 150). The process of step 150 is repeated until time (t + Δtn) is reached. If it is determined in step 150 that the time (t + Δtn) has been reached, it is estimated that an inflection point has appeared in the NOx sensor cell output. In this case, the activation determination of the NOx sensor 1 is performed in the same manner as in the routine shown in FIG. 5 (step 112).

  Thereafter, similarly to the routine shown in FIG. 9, the processing of steps 120 to 130 is executed. Here, “t” in steps 126 and 128 is replaced with “t + Δtn”.

As described above, in the third embodiment, the time when the inflection point appears in the oxygen pump cell output P is specified, and the NOx sensor cell output N is considered in consideration of the correlation between the oxygen pump cell output P and the NOx sensor cell output N. The time when the inflection point appears is estimated. Therefore, it is possible to accurately estimate when the NOx sensor cell 4 starts to detect the NOx concentration with high accuracy. Therefore, as in the first and second embodiments, the early activation of the NOx sensor 1 can be realized to the maximum and the requirement for emission reduction can be sufficiently satisfied.
Furthermore, in the third embodiment, it is possible to determine the deterioration of the NOx sensor 1 using the estimated time of the inflection point. Therefore, according to the third embodiment, the same effect as in the first and second embodiments can be obtained.

(Modification)
Hereinafter, a modified example of the third embodiment will be described with reference to FIGS.
In the third embodiment, the time when the inflection point appears in the NOx sensor cell output P is estimated in consideration of the correlation between the oxygen pump cell output P and the NOx sensor cell output N.
FIG. 13 is a block diagram for explaining a main part of a gas concentration detection apparatus according to a modification of the third embodiment. The gas concentration detection apparatus shown in FIG. 13 has a NOx sensor 1A. This NOx sensor 1A further includes an air-fuel ratio sensor cell 9 in the NOx sensor 1 shown in FIG. The air-fuel ratio sensor cell 9 has a solid electrolyte body (not shown) and outputs a current value when oxygen ions O ²⁻ flow in the cell. The output of the air-fuel ratio sensor cell 9 is detected by the air-fuel ratio sensor cell control means 84 of the ECU 8A. Since the configuration of the other gas concentration detection apparatus is the same as that of the gas concentration detection apparatus 10 shown in FIG. 1, illustration and description thereof are omitted.

Both the air-fuel ratio sensor cell 9 and the NOx sensor cell 4 output current values when oxygen ions O ²⁻ flow through the cells. Therefore, there is a correlation between the air-fuel ratio sensor cell output and the NOx sensor cell output. In this modification, in order to identify the inflection point of the NOx sensor cell output, this correlation is used.

  FIG. 14 is a diagram showing the correlation between the air-fuel ratio sensor cell output and the NOx sensor cell output when the NOx sensor is warmed up. In FIG. 14, an alternate long and short dash line La indicates a change in the air-fuel ratio sensor cell output, and a solid line Ls indicates a change in the NOx sensor cell output. Further, in FIG. 14, a change in the oxygen pump cell output for reference is shown by a broken line Lp.

  As shown in FIG. 14, not only the inflection point of the NOx sensor cell output but also the inflection point appears in the air-fuel ratio sensor cell output. The inflection point can be, for example, the air-fuel ratio sensor cell output when the change amount of the air-fuel ratio sensor cell output changes from positive to negative. There is a correlation between the time t20 when the inflection point appears in the air-fuel ratio sensor cell output and the time t22 when the inflection point appears in the NOx sensor cell output. The difference Δta between the two times t20 and t22 can be obtained in advance by experiments or the like and stored in the ECU 8. Therefore, if the inflection point of the air-fuel ratio sensor cell output can be specified, the timing of the inflection point of the NOx sensor cell output can be estimated. Therefore, according to this modification, as in the second embodiment, it is possible to accurately estimate the time when the NOx sensor cell 4 starts to detect the actual NOx concentration with high accuracy. Judgment can be made.

  In the modification of the third embodiment, the air-fuel ratio sensor cell 9 corresponds to the “air-fuel ratio detection cell” in the fourth invention. In the third embodiment, the “acquiring means” according to the third aspect of the present invention is realized by the ECU 8 executing the processes of steps 142 to 148.

It is a figure for demonstrating the structure of the gas concentration detection apparatus 10 by Embodiment 1 of this invention. It is a figure which shows the change of the oxygen pump cell output at the time of NOx sensor warming-up, and the change of NOx sensor cell output. It is a figure for demonstrating the method of specifying the inflection point of a NOx sensor cell output. It is a figure which shows the correlation of element temperature and an impedance at the time of sensor degradation, and sensor non-degradation. 4 is a flowchart illustrating a routine that the ECU 8 executes in Embodiment 1 of the present invention. 7 is a flowchart showing a routine that the ECU 8 executes in a modification of the first embodiment of the present invention. In Embodiment 2 of this invention, it is a figure which shows the change of the NOx sensor cell output waveform accompanying deterioration of the oxygen pump cell 2. FIG. In Embodiment 2 of this invention, it is a figure which shows the change of the NOx sensor output waveform accompanying deterioration of the NOx sensor cell 4. FIG. In Embodiment 2 of this invention, it is a flowchart which shows the routine which ECU8 performs. It is a figure which shows the correlation of the oxygen pump cell output at the time of NOx sensor warming-up, and a NOx sensor cell output. It is a figure for demonstrating the method of specifying the inflection point of an oxygen pump cell output. In Embodiment 3 of this invention, it is a flowchart which shows the routine which ECU8 performs. It is a block diagram for demonstrating the principal part of the gas concentration detection apparatus by the modification of Embodiment 3 of this invention. It is a figure which shows the correlation of the air fuel ratio sensor cell output at the time of NOx sensor warming-up, and a NOx sensor cell output.

Explanation of symbols

1 NOx sensor 2 Oxygen pump cell 4 NOx sensor cell 6 Heater 8 ECU
9 Air-fuel ratio sensor cell 21, 41 Solid electrolyte body

Claims (5)

An oxygen pump cell that discharges excess oxygen in the gas to be measured with voltage application;
A gas concentration detection cell that detects the concentration of the specific gas component from the gas after excess oxygen is discharged by the oxygen pump cell, and outputs a current value according to the concentration;
Inflection point acquisition means for acquiring a time when an inflection point appears in the output of the gas concentration detection cell during warm-up of the oxygen pump cell and the gas concentration detection cell and during discharge of surplus oxygen;
A gas concentration detection device comprising an activity determination means for determining the time when the inflection point appears as the activation time of the gas concentration detection cell,
Element temperature acquisition means for acquiring element temperature of the oxygen pump cell or the gas concentration detection cell;
Deterioration determination means for determining that the oxygen pump cell or the gas concentration detection cell is deteriorated based on the element temperature acquired at the time when the inflection point appears. apparatus. An oxygen pump cell that discharges excess oxygen in the gas to be measured with voltage application;
A gas concentration detection cell that detects the concentration of the specific gas component from the gas after excess oxygen is discharged by the oxygen pump cell, and outputs a current value according to the concentration;
Inflection point acquisition means for acquiring a time when an inflection point appears in the output of the gas concentration detection cell during warm-up of the oxygen pump cell and the gas concentration detection cell and during discharge of surplus oxygen;
A gas concentration detection device comprising an activity determination means for determining the time when the inflection point appears as the activation time of the gas concentration detection cell,
A gas concentration detection device comprising deterioration determining means for determining that the oxygen pump cell has deteriorated when a time from a warm-up start time to a time when the inflection point appears is longer than a reference value. . The gas concentration detection apparatus according to claim 2,
The oxygen pump cell outputs a current value corresponding to the amount of surplus oxygen to be discharged,
The inflection point acquisition means changes the output of the gas concentration detection cell based on the change of the output of the oxygen pump cell in consideration of the correlation between the output of the oxygen pump cell and the output of the gas concentration detection cell. Estimate when the music point appears,
The deterioration determination means determines that the oxygen pump cell is deteriorated when the time from the warm-up start time to the time when the inflection point estimated by the inflection point acquisition means appears is longer than a reference value. A gas concentration detection device characterized by: The gas concentration detection apparatus according to claim 2,
An air-fuel ratio detection cell that outputs a current value corresponding to the air-fuel ratio of the gas to be measured;
The inflection point acquisition means takes into account the correlation between the output of the air-fuel ratio detection cell and the output of the gas concentration detection cell, and based on the change in the output of the air-fuel ratio detection cell, the gas concentration detection cell Estimate when the inflection point appears in the output of
The deterioration determination means determines that the oxygen pump cell is deteriorated when the time from the warm-up start time to the time when the inflection point estimated by the inflection point acquisition means appears is longer than a reference value. A gas concentration detection device characterized by: The gas concentration detection device according to any one of claims 2 to 4,
The deterioration determination means determines that the gas concentration detection cell is deteriorated when an output of the gas concentration detection cell after a predetermined time has elapsed from a time when the inflection point appears is a reference value or more. A gas concentration detection device.

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