JP2009128237A - Diagnosing system of nox sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diagnosing system of an NOx sensor for extending the battery life and the service life of the NOx sensor. <P>SOLUTION: This diagnosing system of the NOx sensor 100 is arranged in an exhaust passage of an internal combustion engine and detects a NOx concentration in exhaust gas. The diagnosing system of the NOx sensor 100 is characterized by comprising a first diagnosing means 170 for diagnosing whether the NOx sensor 100 is abnormal during the operation of the internal combustion engine, and a second diagnosing means 170 for diagnosing whether the NOx sensor 100 is abnormal during the stop of the internal combustion engine when the first diagnosing means 170 diagnoses that the NOx sensor 100 is abnormal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a NOx sensor diagnostic system for detecting NOx concentration in exhaust gas of an internal combustion engine.

There are known techniques for arranging a NOx sensor in an exhaust passage of an internal combustion engine and performing various controls based on the NOx concentration detected by the NOx sensor. For example, a NOx sensor is arranged on the downstream side of the NOx catalyst attached to the exhaust passage, and a technique for controlling the NOx catalyst by adjusting the air-fuel ratio of the exhaust based on the detected NOx concentration, or on the upstream side of the NOx catalyst. A technique for controlling the amount of urea added to the NOx catalyst based on the NOx concentration detected by the installed NOx sensor is known.

By the way, NOx in the exhaust gas has a low concentration, and a NOx sensor that detects the low concentration of NOx is very sensitive. Therefore, deterioration of the NOx sensor (drift of sensor output) tends to be a problem. Therefore, a technique for diagnosing a NOx sensor during after-motion after the internal combustion engine has stopped has been proposed (see Patent Document 1).
JP 2006-105965 A

  However, when diagnosing the NOx sensor during after-operation, it is necessary to energize the NOx sensor in order to maintain the active state of the NOx sensor while the internal combustion engine is stopped. It will be shorter. Moreover, since the energization time of the NOx sensor is extended, the life of the NOx sensor is also shortened.

  The present invention has been made in view of the above points, and an object thereof is to provide a NOx sensor diagnostic system capable of extending the battery life and the life of the NOx sensor.

(1) The invention of claim 1
When the internal combustion engine is operating, it is diagnosed whether the NOx sensor is abnormal. If it is determined that the internal combustion engine is abnormal, it is diagnosed whether the NOx sensor is abnormal when the internal combustion engine is stopped. For this reason, the battery life for energizing the NOx sensor becomes longer and the energization time for the NOx sensor becomes shorter than when diagnosing abnormality of the NOx sensor every time after the internal combustion engine is stopped. become longer.

In the present invention, when it is determined that the NOx sensor is abnormal during operation of the internal combustion engine, it is diagnosed again whether the NOx sensor is abnormal after the internal combustion engine is stopped. The diagnosis after the stop of the internal combustion engine has less disturbance than the diagnosis during the operation of the internal combustion engine, so that the diagnosis can be performed with high accuracy.
(2) In the invention of claim 2,
A NOx sensor detects a NOx concentration from a pump cell for exhausting or pumping oxygen in the exhaust gas introduced into the chamber, a NOx concentration from the gas after passing through the pump cell, and a residual oxygen concentration in the chamber A limit cell type NOx sensor that measures the sensor cell current and the monitor cell current when a voltage is applied to the pump cell and detects the NOx concentration from the sensor cell current at that time.

In the present invention, whether the NOx sensor is abnormal or not is diagnosed by the means described in claim 2, so that early diagnosis is possible. As a result, the life of the NOx sensor is extended, and the NOx sensor is energized. Battery life will be longer.
(3) In the invention of claim 3,
When it is diagnosed as a specific abnormality set in advance during operation of the internal combustion engine, the diagnosis after the internal combustion engine is stopped is not performed. As a result, when an obvious abnormality (for example, disconnection) is detected during operation of the internal combustion engine, it is not necessary to perform a wasteful diagnosis after the internal combustion engine is stopped.

When a specific abnormality set in advance is diagnosed during operation of the internal combustion engine, for example, if an error signal is output, it is possible to notify the driver that the NOx sensor is abnormal at an early stage.
(4) In the invention of claim 4,
When it is diagnosed as a specific abnormality set in advance during operation of the internal combustion engine, use of the NOx concentration detected by the NOx sensor is stopped. This can prevent a secondary failure that may occur due to an abnormality in the NOx sensor (which may occur by using an incorrect NOx value).
(5) In the invention of claim 5,
When a specific abnormality is diagnosed after the internal combustion engine is stopped, use of the NOx concentration detected by the NOx sensor is stopped. This can prevent a secondary failure that may occur due to an abnormality in the NOx sensor.

Examples of the method of stopping the use of the NOx concentration include a method of stopping the control of the NOx sensor so as not to detect the NOx concentration and a method of not using the detected NOx concentration for various controls.
(6) In the invention of claim 6,
If the internal combustion engine starts when diagnosing the NOx sensor after the internal combustion engine is stopped, the diagnosis is stopped. By doing so, the diagnosis of the NOx sensor does not become inaccurate due to the start of the internal combustion engine and the accompanying inflow of exhaust gas. Further, the state of the NOx sensor can be returned to the state of detecting the NOx concentration at an early stage.

1. Configuration of NOx Sensor Diagnosis System The overall configuration of the NOx sensor diagnosis system will be described with reference to FIGS. 1 and 2. The NOx sensor diagnostic system is a system for diagnosing a NOx sensor that detects NOx concentration in exhaust gas of an engine mounted on a vehicle. As shown in FIG. 1, the NOx sensor 100, a sensor control circuit 170, and an engine ECU 180 are used. Consists of

  The NOx sensor 100 is a sensor that is disposed in the exhaust passage 200 (see FIG. 2) of the engine and detects the NOx concentration in the exhaust gas. The configuration and operation of the NOx sensor 100 will be described in detail later.

  The sensor control circuit 170 supplies a voltage applied to the NOx sensor 100 and heater power using a vehicle battery (not shown), and acquires the sensor output of the NOx sensor 100. In addition, the sensor control circuit 170 executes processing for diagnosis, abnormality determination, and correction of the NOx sensor 100 described later.

  The engine ECU 180 acquires engine information (water temperature, speed, rotation speed, etc.), and based on the engine information, makes various requests (sensor drive start request, zero detection request, sensor diagnosis request, sensor stop) to the sensor control circuit 170. Request). The engine ECU 180 calculates the urea addition amount to the exhaust gas based on the output value of the NOx sensor 100 acquired from the sensor control circuit 170, and the urea addition valve so that the urea addition amount becomes the calculated value. 190 (see FIG. 2) is driven. Further, engine ECU 180 receives information on the presence or absence of abnormality of NOx sensor 100 from sensor control circuit 170, and when there is an abnormality, turns on a diagnostic (not shown).

  Next, the arrangement of the NOx sensor 100 and the urea addition valve 190 in the exhaust passage of the engine will be described with reference to FIG. As shown in FIG. 2A, an addition valve 190, a catalyst 210, the NOx sensor 100, and a DOC 220 are attached in series in the exhaust passage 200 of the engine in order from the upstream side. The catalyst 210 is a NOx catalyst, and more specifically, an SCR (Selective catalytic reduction) catalyst. 2B, the NOx sensor 100 may be attached upstream of the catalyst 210.

  Next, the configuration of the NOx sensor 100 will be described in detail based on FIG. The NOx sensor 100 has a three-cell structure including a pump cell, a monitor cell, and a sensor cell, and is embodied as a so-called limit current type combined gas sensor that can simultaneously detect the oxygen concentration and the NOx concentration in the exhaust gas.

  In the NOx sensor 100, solid electrolytes (solid electrolyte elements) 141 and 142 made of an oxygen ion conductive material have a plate shape, and are spaced apart from each other by a predetermined interval through a spacer 143 made of an insulating material such as alumina. Are stacked. Among these, a pinhole 141a is formed in the upper solid electrolyte 141 of FIG. 3, and exhaust gas around the sensor is introduced into the first chamber 144 through the pinhole 141a. The first chamber 144 communicates with the second chamber 146 via a constriction 145 as a diffusion passage. Reference numeral 147 denotes a porous diffusion layer.

  The lower solid electrolyte 142 in FIG. 3 is provided with a pump cell 110 and a monitor cell 120. The pump cell 110 functions to exhaust or pump oxygen in the exhaust gas introduced into the first chamber 144. At the same time, the oxygen concentration in the exhaust gas is detected. The monitor cell 120 generates a current output when an electromotive force or a voltage is applied depending on the oxygen concentration in the second chamber 146. Here, the pump cell 110 has a pair of upper and lower electrodes 111 and 112 with a solid electrolyte 142 interposed therebetween, and in particular, the electrode 111 on the first chamber 144 side is a NOx inert electrode (an electrode that is difficult to decompose NOx gas). . Similarly, the monitor cell 120 has a pair of upper and lower electrodes 121 and 122 with the solid electrolyte 142 interposed therebetween, and the electrode 121 on the second chamber 146 side in particular is a NOx inert electrode (an electrode that hardly decomposes NOx gas). is there. The pump cell 110 and the monitor cell 120 decompose oxygen present in the chambers 144 and 146 and discharge it from the electrodes 112 and 122 to the atmosphere passage 150 side.

  The sensor cell 130 is provided to face the monitor cell 120 and has a pair of upper and lower electrodes 131 and 132 with a solid electrolyte 141 interposed therebetween. The sensor cell 130 detects the NOx concentration from the gas after passing through the pump cell 110, and discharges oxygen generated when NOx is decomposed in the second chamber 146 from the electrode 132 to the atmosphere passage 148 side.

  An insulating layer 149 is provided on the lower surface of the solid electrolyte 142 in FIG. 3, and an atmospheric passage 150 is formed by the insulating layer 149. In addition, a heater 151 for heating the entire sensor is embedded in the insulating layer 149.

  Next, the principle of NOx measurement by the NOx sensor 100 will be described. In the NOx sensor 100 having the above configuration, the exhaust gas is introduced into the first chamber 144 through the porous diffusion layer 147 and the pinhole 141a. When the exhaust gas passes through the vicinity of the pump cell 110, a decomposition reaction occurs by applying a voltage between the electrodes 111 and 112 of the pump cell 110, and the exhaust gas passes through the pump cell 110 according to the oxygen concentration in the first chamber 144. Oxygen is taken in and out. At this time, since the electrode 111 on the first chamber 144 side is a NOx inert electrode, NOx in the exhaust gas is not decomposed in the pump cell 110, but only oxygen is decomposed and discharged to the atmospheric passage 150.

  Thereafter, the exhaust gas that has passed in the vicinity of the pump cell 110 flows into the second chamber 146, and the monitor cell 120 generates an output corresponding to the residual oxygen concentration in the gas. The output of the monitor cell 120 is detected as a monitor cell current by applying a predetermined voltage between the electrodes 121 and 122 of the monitor cell 120. Further, by applying a predetermined voltage between the electrodes 131 and 132 of the sensor cell 130, NOx in the gas is reduced and decomposed, and oxygen generated at that time is discharged to the atmospheric passage 148. At that time, the current flowing through the sensor cell 130 is detected as the NOx concentration contained in the exhaust gas.

  The sensor control circuit 170 includes a well-known logical operation circuit including a CPU, a memory, an A / D, a D / A converter, and the like. Each of the pump cell 110, the monitor cell 120, and the sensor cell 130 has a power supply circuit. In each power supply circuit, the pump cell current Ip is measured by the current detector 171 and the monitor cell current Im is measured by the current detector 172. Is is measured by the current detector 173. The current values measured by the current detectors 171 to 173 are taken into the sensor control circuit 170, respectively.

  The sensor control circuit 170 detects the oxygen concentration in the exhaust gas based on the pump cell current Ip measured by the current detector 171 and sets the pump cell voltage Vp as needed according to the pump cell current Ip using a predetermined applied voltage characteristic. . The sensor control circuit 170 detects the NOx concentration in the exhaust gas based on the sensor cell current Is measured by the current detector 173.

  Next, output characteristics of the NOx sensor 100 will be described with reference to FIG. 4A shows the characteristics (Vp-Ip characteristics) of the pump cell current Ip with respect to the pump cell voltage Vp, and FIG. 4B shows the characteristics of the monitor cell current Im and sensor cell current Is with respect to the pump cell voltage Vp (Vp-Im). Characteristic, Vp-Is characteristic). 4A and 4B show the characteristics under constant oxygen concentration and NOx concentration.

  As shown in FIG. 4A, the pump cell current Ip flowing through the pump cell 110 has a limit current characteristic with respect to the pump cell voltage Vp. The limit current region is composed of a straight line portion slightly rising to the right with respect to the V axis, and the region shifts in a direction in which the pump cell current Ip increases as the oxygen concentration increases (lean). The Vp side lower than the limit current region is a resistance dominant region, and the slope of the resistance dominant region substantially matches the element impedance Rip of the pump cell 110.

  An applied voltage straight line LX1 is defined as an applied voltage characteristic of the pump cell 110, and the pump cell voltage Vp is variably controlled on the applied voltage straight line LX1 according to the current pump cell current Ip. Incidentally, the electrode 111 (electrode on the first chamber 144 side) of the pump cell 110 is a NOx inactive electrode. However, if the pump cell voltage Vp is too large, NOx is decomposed, so the applied voltage straight line LX1 is the NOx in the pump cell 110. It is set on condition that the gas is not decomposed. Actually, the applied voltage straight line LX1 is set so as to maintain the inside of the first chamber 144 at a predetermined low oxygen concentration (near the stoichiometry). For example, a slight residual oxygen (surplus oxygen) of about several ppm to several tens ppm is generated. The pump cell voltage Vp is controlled so as to remain in the first chamber 144.

  Further, in FIG. 4B, when the characteristics of the monitor cell current Im with respect to the pump cell voltage Vp (Vp-Im characteristics) are seen, the monitor cell current Im rapidly increases in a region where the pump cell voltage Vp is low, and the pump cell voltage Vp becomes high to some extent. Then, the monitor cell current Im becomes almost constant. That is, as can be seen from the characteristics of FIG. 4A, the pump cell current Ip is small in the low Vp region (resistance-dominated region), and the residual oxygen amount in the first chamber 144 is increased. Further, since the pump cell current Ip is substantially constant in the limit current region of the pump cell characteristic, the residual oxygen amount in the first chamber 144 is substantially constant. Therefore, the monitor cell current Im changes as illustrated with respect to the pump cell voltage Vp. In this case, there is an inflection point (A in FIG. 4) where the monitor cell current Im changes suddenly. Note that where the inflection point is considered in terms of the characteristics of the monitor cell current Im may be determined by appropriately setting a change rate (slope) of the reference monitor cell current Im and satisfying the reference change rate. .

  Further, looking at the characteristics (Vp-Is characteristics) of the sensor cell current Is with respect to the pump cell voltage Vp, there is a flat current region (flat region) with respect to the pump cell voltage Vp, and the sensor cell current Is is stabilized in the flat region. Therefore, if the pump cell voltage Vp is controlled at the control point B in FIG. 4, the NOx concentration in the exhaust gas can be detected with high accuracy. In this case, the inflection point A of the monitor cell current Im exists slightly away from the flat region of the sensor cell current Is. Further, the inflection point A and the control point B are separated by “offset value”, and this offset value is a numerical value unique to each sensor model. However, the offset value varies depending on what criterion the inflection point of the monitor cell current Im is set.

  Next, the output characteristics of the NOx sensor 100 will be described based on FIG. 6 when the element impedance Rip of the pump cell 110 is increased or decreased due to individual differences of the NOx sensor 100, changes with time (deterioration), or the like. In FIG. 6, the characteristic indicated by the solid line is the basic characteristic described with reference to FIG. 4, and the characteristic indicated by the alternate long and short dash line is the characteristic when the impedance is increased.

  When the element impedance Rip of the pump cell 110 is increased, the pump cell characteristic is inclined as shown in FIG. 6A, so that the pump cell current Ip is decreased. Then, since the residual oxygen amount in the first chamber 144 increases, the monitor cell current Im and the sensor cell current Is change as indicated by a one-dot chain line as shown in FIG. In this case, the value of the sensor cell current Is increases and the detection accuracy of the sensor cell current Is decreases.

  Further, when the element impedance Rip of the pump cell 110 decreases, the pump cell characteristic rises as shown in the figure, so that the pump cell current Ip increases. Then, since the amount of residual oxygen in the first chamber 144 decreases, the monitor cell current Im and the sensor cell current Is change in the opposite manner to the case of FIG. In this case, the value of the sensor cell current Is decreases, and the detection accuracy of the sensor cell current Is also decreases.

  As described above, when the element impedance Rip of the pump cell 110 changes carelessly, the detection accuracy of the sensor cell current Is decreases, and consequently the NOx concentration is erroneously detected. This is because the flat area of the sensor cell current Is fluctuates as the element impedance Rip changes, and the sensor cell current Is cannot be detected correctly even when a predetermined pump cell voltage Vp is applied.

2. Diagnosis Process Performed by NOx Sensor Diagnosis System The process executed by the NOx sensor diagnosis system will be described with reference to the flowchart of FIG. This process is performed by the sensor control circuit 170 every predetermined time (for example, every few seconds). This process is performed by the sensor control circuit 170 during a “pump cell voltage diagnosis period” that is different from the gas concentration detection period for detecting the NOx concentration. That is, in the normal gas concentration detection period, the pump cell voltage Vp is controlled according to the pump cell current Ip at that time based on the applied voltage characteristics, and the NOx concentration is detected periodically (for example, every 4 msec). Further, in the pump cell voltage diagnosis period, the NOx concentration detection is temporarily interrupted, and the process described later is executed.

  In step 10, it is determined whether or not a detection condition is satisfied for the operating state of the engine. The detection condition is a state in which the fuel supply is cut during deceleration or idling. If the detection condition is satisfied, the process proceeds to step 20; otherwise, the process stays at step 10.

  In step 20, the deviation between the actual output value (NOx concentration) A of the NOx sensor 100 under the detection condition and the reference output value (NOx concentration) B preset as the output value when the detection condition is satisfied is calculated. .

  In step 30, it is determined based on the deviation calculated in step 20 whether the NOx sensor 100 is abnormal. That is, if the deviation exceeds a predetermined reference value L1, it is determined that the NOx sensor 100 is abnormal, and if the deviation does not exceed the predetermined reference value L1, it is determined that the NOx sensor 100 is normal. To do. If it is determined that the NOx sensor 100 is abnormal, the process proceeds to step 40. If it is determined that the NOx sensor 100 is normal, the process returns to step 10.

  In step 40, based on the deviation calculated in step 20, it is determined whether it is necessary to perform the main diagnosis on the NOx sensor 100. Specifically, if the deviation does not exceed the specific abnormality reference value L2 (a value larger than the above-described reference value L1), it is determined that this diagnosis is necessary, and the deviation is a predetermined specific abnormality reference value. If it exceeds L2, it is determined that this diagnosis is not necessary because it is an obvious abnormality. If it is determined that the main diagnosis is necessary, the process proceeds to step 50. If it is determined that the main diagnosis is not necessary, the process proceeds to step 90. In addition, as a case where the deviation exceeds a predetermined specific abnormality reference value L2, there is a case where the wiring of the NOx sensor 100 is disconnected.

  In step 50, it is determined whether or not the engine is stopped. If it is determined that the engine is stopped, the process proceeds to step 60. If it is determined that the engine is not stopped, the process stays at step 50.

In step 60, the NOx sensor 100 is continuously driven even after the engine is stopped. That is, the NOx sensor 100 is continuously energized by the battery.
In step 70, NOx sensor abnormality detection (main diagnosis) is performed during after-motion after the engine stops. This NOx sensor abnormality detection will be described in detail later.

  In step 80, it is determined whether or not the NOx sensor 100 is determined to be abnormal as a result of the NOx sensor abnormality detection in step 70. When it is determined that the NOx sensor 100 is abnormal, the process proceeds to step 100, and when it is determined that the NOx sensor 100 is not abnormal, this process is terminated.

  On the other hand, if it is determined in step 40 that this diagnosis is not necessary, an error signal is output in step 90. In response to the error signal, an error notification is made by lighting a not-shown diagnostic in the vehicle.

  If the NOx sensor 100 is abnormal in step 80, the control of the NOx sensor 100 is stopped in step 100. That is, the detection of the NOx concentration by the NOx sensor 100 is not performed after this point.

In step 110, an error signal is output. Then, according to the error signal, an error notification is made by lighting a not-shown diagnostic in the vehicle.
Next, based on the flowchart of FIG. 7, the NOx sensor abnormality detection process at the time of stop performed in the said step 70 is demonstrated in detail. This processing pays attention to the fact that the correlation between the inflection point of the monitor cell current Im and the flat region of the sensor cell current Is is invariant regardless of the change of the element impedance Rip, and according to the position of the inflection point of the monitor cell current Im. To make a diagnosis.

  In FIG. 7, first, in step 210, the pump cell voltage Vp corresponding to the pump cell current Ip is initialized (map calculation) using the applied voltage straight line LX1 (applied voltage characteristic) shown in FIG. Then, the pump cell voltage Vp is applied to the pump cell 110. In step 220, the pump cell voltage Vp is swept to both the positive and negative sides with a predetermined amplitude ΔV. In step 230, the change amount ΔIm of the monitor cell current is measured each time. In this case, for example, the pump cell voltage Vp is swept at a cycle of 200 msec or less (frequency 5 Hz or more), and more preferably at a cycle of 100 msec or less (frequency 10 Hz or more).

  In step 240, if the change amount ΔIm of the monitor cell current measured in step 230 exceeds a predetermined reference value, it is determined that the NOx sensor 100 is abnormal, and if it is less than the reference value, it is normal. judge.

  Next, the operation procedure of the process of FIG. 7 will be described more specifically. As shown in FIG. 8, first, the voltage value V1 is initially set as the pump cell voltage Vp based on the applied voltage characteristics, and the pump cell voltage Vp is temporarily oscillated on both the positive and negative sides based on the voltage value V1 at that time. That is, a short sweep waveform is added to the pump cell voltage Vp. At this time, if the characteristics of the monitor cell current Im change due to individual sensor differences or changes over time, the inflection point of the monitor cell current Im approaches the voltage value V1, and the monitor cell current Im changes greatly with the Vp sweep.

  For example, when there is no inflection point near the voltage value V1 (when the inflection point is not close to the voltage value V1), as shown in FIG. 9A, the monitor cell current Im and the sensor cell current accompany the Vp sweep. Is slightly changes. In this case, it is determined that the NOx sensor 100 is normal.

  On the other hand, when an inflection point exists near the voltage value V1 (when the inflection point approaches the voltage value V1), as shown in FIG. 9B, the monitor cell current Im and the sensor cell accompany the Vp sweep. The change of the current Is becomes non-uniform on both the positive and negative sides. In this case, it is determined that the NOx sensor 100 is abnormal.

  Note that after step 240, the output of the NOx sensor 100 may be corrected based on the change amount ΔIm of the monitor cell current measured in step 230. This correction can be performed as follows.

  That is, the voltage correction value KV is calculated according to the change amount ΔIm of the monitor cell current measured in step 230. At this time, for example, the voltage correction value KV is calculated according to the change amount ΔIm of the monitor cell current using the relationship of FIG. Next, the applied voltage characteristic is corrected using the voltage correction value KV. In this case, the voltage correction value KV may be stored and held in a backup memory such as a backup RAM or a flash ROM, or the applied voltage characteristic itself may be updated with the voltage correction value KV.

  The result corrected or updated as described above is applied when the NOx concentration is detected thereafter. In other words, when controlling the pump cell voltage during the gas concentration detection period, the sensor control circuit 170 corrects the pump cell voltage Vp using the voltage correction value KV stored in the backup RAM or the like. Alternatively, the sensor control circuit 170 uses the applied voltage characteristics whose characteristics have been updated, and controls the pump cell voltage Vp according to the pump cell current Ip at that time.

3. Effects of the NOx sensor diagnostic system The effects of the NOx sensor diagnostic system will be described.
(1) The NOx sensor diagnostic system diagnoses whether or not the NOx sensor 100 is abnormal during engine operation (steps 10 to 40 in FIG. 5), and only after determining that it is abnormal, NOx sensor abnormality detection (this diagnosis, step 70 in FIG. 5) is performed. Therefore, the life of the battery energized to the NOx sensor 100 becomes longer and the energization time of the NOx sensor 100 becomes shorter than when NOx sensor abnormality is detected every time after the engine is stopped. Also gets longer.
(2) When the NOx sensor diagnosis system determines that the NOx sensor 100 is abnormal during engine operation, the NOx sensor abnormality detection is performed again after the engine is stopped. Since the NOx sensor abnormality detection after the engine is stopped has less disturbance than the diagnosis during engine operation, the diagnosis can be performed with high accuracy.
(3) The NOx sensor diagnosis system performs NOx sensor abnormality detection by the method shown in the flowchart of FIG. As a result, early diagnosis is possible, and as a result, the life of the NOx sensor 100 is prolonged, and the life of the battery energizing the NOx sensor 100 is lengthened.
(4) In the NOx sensor diagnosis system, the deviation between the actual output value A of the NOx sensor 100 and the reference output value B set in advance as the output value when the detection condition is satisfied is determined as the specific abnormality reference value L2. If it exceeds, NOx sensor abnormality detection after engine stop is not performed and an error signal is output (steps 40 and 90 in FIG. 5). In other words, if it is determined that the abnormality is apparent from the diagnosis result while the engine is operating, the NOx sensor abnormality detection after the engine is stopped is not performed, and an error signal is output immediately. As a result, the driver can be notified early that the NOx sensor 100 is abnormal.
(5) When the NOx sensor diagnosis system determines that the NOx sensor 100 is abnormal by detecting the abnormality of the NOx sensor after the engine is stopped, the NOx sensor diagnosis system stops the control of the NOx sensor 100 (steps 80 and 100 in FIG. 5). . This can prevent a secondary failure that may occur due to an abnormality in the NOx sensor 100.

4). Modification 1
The NOx sensor diagnosis system may execute the process shown in the flowchart of FIG. 11 instead of the process shown in the flowchart of FIG. 5.

  In the flowchart of FIG. 11, the processing of steps 310 to 380 is the same as that of steps 10 to 80 in the flowchart of FIG. 5. In the flowchart of FIG. 11, the processes in steps 400 to 420 are the same as steps 90 to 110 in the flowchart of FIG. 5.

  In the flowchart shown in FIG. 11, when it is determined in step 340 that the main diagnosis of the NOx sensor 100 is necessary, the control of the NOx sensor 100 is stopped. That is, the NOx concentration is not detected by the NOx sensor 100 after this point. As described above, when an obvious abnormality is detected during engine operation, the control of the NOx sensor 100 is stopped, and the NOx concentration is not detected by the NOx sensor 100 after this point. A secondary failure that may occur can be prevented early.

5). Modification 2
The NOx sensor diagnosis system may execute the process shown in the flowchart of FIG. 12 instead of the process shown in the flowchart of FIG. 5.

  In the flowchart of FIG. 12, the processing of steps 510 to 560 is the same as that of steps 10 to 60 in the flowchart of FIG. In the flowchart of FIG. 12, the processes in steps 600 to 630 are the same as those in steps 80 to 110 in the flowchart of FIG. 5.

  In the flowchart shown in FIG. 12, when NOx sensor abnormality detection (step 580) is being executed, it is determined whether or not the NOx sensor abnormality detection is completed every predetermined time (step 590). For example, it is determined whether or not the engine has been started. If the engine has been started, the NOx sensor abnormality detection is terminated even if it is in progress (step 570). By doing so, the NOx sensor abnormality detection does not become inaccurate due to the start of the engine and the accompanying inflow of exhaust gas during the NOx sensor abnormality detection. In addition, after the engine is started, the state of the NOx sensor 100 can be returned to a state in which the NOx concentration is detected at an early stage.

  Needless to say, the present invention is not limited to the above-described embodiments, and can be implemented in various modes without departing from the scope of the present invention.

It is a block diagram showing the structure of the vehicle to which the diagnostic system of a NOx sensor is applied. It is explanatory drawing showing arrangement | positioning of the NOx sensor 100 in an exhaust passage. 1 is a configuration diagram showing an outline of a NOx sensor 100. FIG. 4 is an explanatory diagram showing output characteristics of a NOx sensor 100. FIG. It is a flowchart showing the process which the diagnostic system of a NOx sensor performs. 4 is an explanatory diagram showing output characteristics of a NOx sensor 100. FIG. It is a flowchart showing the process which the diagnostic system of a NOx sensor performs. It is a characteristic view which shows the change of the monitor cell current with respect to a pump cell voltage. It is a time chart which shows the change of the monitor cell current and sensor cell current accompanying the change of pump cell voltage. It is a figure for setting the voltage correction value KV. It is a flowchart showing the process which the diagnostic system of a NOx sensor performs. It is a flowchart showing the process which the diagnostic system of a NOx sensor performs.

Explanation of symbols

100 ... NOx sensor, 110 ... pump cell, 111, 112 ... electrode,
120 ... monitor cell, 121 ... electrode, 130 ... sensor cell,
131, 132 ... electrodes, 141 ... solid electrolyte, 141a ... pinholes,
142 ... solid electrolyte, 143 ... spacer, 144 ... first chamber,
145 ... throttle part, 146 ... second chamber, 147 ... sign,
148, 150 ... atmospheric passage, 149 ... insulating layer, 151 ... heater,
170 ... sensor control circuit, 171, 172, 173 ... current detector,
190 ... addition valve, 200 ... exhaust passage, 210 ... catalyst, 220 ... DOC,
180 ... Engine ECU

Claims (6)

A NOx sensor diagnostic system for detecting NOx concentration in exhaust gas from an internal combustion engine,
First diagnostic means for diagnosing whether or not the NOx sensor is abnormal during operation of the internal combustion engine;
Second diagnostic means for diagnosing whether or not the NOx sensor is abnormal when the internal combustion engine is stopped when the first diagnostic means diagnoses the NOx sensor as abnormal;
A diagnostic system for a NOx sensor, comprising: The NOx sensor is
A pump cell for exhausting or pumping oxygen in the exhaust gas introduced into the chamber, a sensor cell for detecting the NOx concentration from the gas after passing through the pump cell, and a monitor cell for detecting the residual oxygen concentration in the chamber A NOx sensor of a limiting current type that measures a sensor cell current and a monitor cell current when a voltage is applied to the pump cell and detects a NOx concentration from the sensor cell current at that time,
The first diagnosis means includes
Whether or not the NOx concentration detected by the NOx sensor is within a reference NOx concentration range that is preset according to the preset operating state when the internal combustion engine is in a preset operating state. Based on whether the NOx sensor is abnormal or not,
The second diagnostic means includes
Means for initially setting the pump cell voltage according to the pump cell current each time using a predetermined applied voltage characteristic, and temporarily changing the initially set pump cell voltage to at least either positive or negative, and the change in the monitor cell current at that time The NOx sensor according to claim 1, further comprising: means for detecting an amount; and means for determining whether or not the NOx sensor is abnormal based on a change amount of the monitor cell current. Diagnostic system. The first diagnosis means diagnoses whether the NOx sensor is abnormal, diagnoses whether the abnormality is a preset specific abnormality,
3. The NOx sensor diagnosis system according to claim 1, wherein the second diagnosis unit does not perform a diagnosis when the first diagnosis unit diagnoses the NOx sensor as the specific abnormality. 4. The first diagnosis means diagnoses whether the NOx sensor is abnormal, diagnoses whether the abnormality is a preset specific abnormality,
3. The NOx according to claim 1, further comprising a first stop unit that stops use of the NOx concentration detected by the NOx sensor when the first diagnosis unit diagnoses the NOx sensor as the specific abnormality. Sensor diagnostic system.   The said 2nd diagnostic means is equipped with the 2nd stop means to stop use of the NOx density | concentration detected by the said NOx sensor when the said NOx sensor is diagnosed as abnormality, The any one of Claims 1-4 characterized by the above-mentioned. The NOx sensor diagnostic system described.   6. The NOx sensor diagnosis system according to claim 1, wherein the second diagnosis unit stops diagnosis when the internal combustion engine is started during diagnosis.

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