JP2009180150A - Abnormality determination device of nox sensor used for exhaust emission control system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an abnormality determination device of a NOx sensor used for an exhaust emission control system, more accurately determining normality or abnormality of the NOx sensor. <P>SOLUTION: This abnormality determination device of the NOx sensor 52, 53 used for the exhaust emission control system for an internal combustion engine and outputting different output values according to NOx concentration, includes: a NOx concentration increase/decrease changing means changing to temporarily increase and decrease the NOx concentration in exhaust gas reaching the NOx sensor during an abnormality determination mode in a predetermined operating condition of the internal combustion engine; an elapsed time measuring means measuring elapsed time from the output condition of the predetermined first output value of the NOx sensor to the output of the predetermined second output value by the NOx sensor when the NOx concentration is changed to be increase/decreased by the NOx concentration increase/decrease changing means; and a determination means determining the normality or abnormality of the NOx sensor based on the elapsed time measured by the elapsed time measuring means and reference elapsed time. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an abnormality determination device for a NOx sensor used in an exhaust gas purification system for an internal combustion engine, particularly a selective reduction type NOx catalyst, and an exhaust gas having urea water supply means for supplying urea water to an exhaust passage upstream thereof. The present invention relates to an abnormality determination device for a NOx sensor used in a purification system.

  In general, a selective reduction type NOx catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen is known as an exhaust purification device disposed in an exhaust system of an internal combustion engine such as a diesel engine. This is because a necessary amount of a reducing agent is added upstream of the selective reduction type NOx catalyst, and the reducing agent is subjected to a reduction reaction with NOx (nitrogen oxide) in the exhaust gas on the catalyst, thereby discharging NOx. The concentration can be reduced.

  In the case of automobiles, it is difficult to ensure safety with respect to traveling with ammonia itself as a reducing agent, and therefore it has been proposed to use non-toxic urea water as a reducing agent. That is, if urea water is added to the exhaust gas upstream of the selective reduction type NOx catalyst, the urea water is thermally decomposed into ammonia and carbon dioxide gas in the exhaust gas, and the NOx in the exhaust gas is converted to the selective reduction type NOx catalyst. This is because ammonia is reduced and purified well by ammonia.

  By the way, in such an exhaust purification device that reduces and purifies NOx using urea water as a reducing agent, a NOx sensor is used to detect the NOx concentration in the exhaust gas. In order to know whether or not the NOx concentration detected by the NOx sensor is accurate, it is necessary to detect an abnormality in the NOx sensor. For example, Patent Document 1 discloses an abnormality detection device for a NOx sensor. It is disclosed.

  In the abnormality detection device for a NOx sensor disclosed in Patent Document 1, an output value of a different value is output according to the NOx concentration in the exhaust gas discharged from the combustion chamber of the internal combustion engine, and the exhaust value is determined from the output value. In the NOx sensor abnormality detecting device for detecting an abnormality of the NOx sensor capable of detecting the NOx concentration of NOx, the NOx concentration in the exhaust gas reaching the NOx sensor is forcibly changed, and the NOx sensor outputs at this time When the fluctuation of the output value to be deviated from the fluctuation that can be taken when the NOx sensor is normal, it is determined that the NOx sensor is abnormal.

JP 2003-120399 A

  By the way, in the NOx sensor abnormality detection device described in Patent Document 1, regardless of whether the NOx sensor is normal or abnormal, the NOx sensor forces the NOx concentration in the exhaust gas reaching the NOx sensor. It is assumed that output values of different values corresponding to the NOx concentration are output in synchronization with such fluctuations.

  However, as a result of intensive studies by the present inventors, the actual behavior when the NOx sensor is output is output in synchronization with the forced fluctuation of the NOx concentration in the exhaust gas as described above when the NOx sensor is in a new state. However, it has been found that the abnormality changes without synchronizing with the forced fluctuation of the NOx concentration when the NOx sensor deteriorates abnormally. That is, when the NOx sensor has deteriorated, when the NOx concentration changes to the higher side, an output value synchronized with this change is output as in the normal NOx sensor, but the NOx concentration changes to the lower side. When this is done, it has become impossible to synchronize with this change, and it has been found that the output is delayed as compared with a normal NOx sensor, resulting in a deviation in output value. The reason why such a phenomenon occurs is that the deterioration of the NOx sensor cathode-side pump electrode for NOx detection, which is formed from a material having a strong reducing property with respect to NOx generally used in NOx sensors, deteriorates the NOx sensor. It is considered as a factor.

  As a result of the response delay when the NOx sensor deteriorates, the NOx sensor abnormality detection device disclosed in Patent Document 1 may not be able to accurately detect the output abnormality of the NOx sensor. In order to make a decision, there was room for further improvement.

  Accordingly, an object of the present invention is to provide an abnormality determination device for a NOx sensor used in an exhaust gas purification system that can more accurately determine the normality or abnormality of a NOx sensor by positively utilizing such a response delay. It is in.

  An embodiment of an abnormality determination device for a NOx sensor used in an exhaust gas purification system according to the present invention for achieving the above object is used in an exhaust gas purification system of an internal combustion engine and outputs different output values depending on the NOx concentration. NOx sensor increase / decrease change means for temporarily increasing and decreasing the NOx concentration in the exhaust gas reaching the NOx sensor in an abnormality determination mode in a predetermined operating state of the internal combustion engine And when the NOx concentration is increased or decreased by the NOx concentration increase / decrease changing means, from the output state of the predetermined first output value of the NOx sensor until the NOx sensor outputs the predetermined second output value. Based on the elapsed time measuring means for measuring time, the elapsed time measured by the elapsed time measuring means and the reference elapsed time, the N Characterized in that it comprises a determination means for determining normality or abnormality of the x sensor.

  According to the embodiment of the abnormality determination device for the NOx sensor used in the exhaust gas purification system according to the present invention, the exhaust gas that reaches the NOx sensor by the NOx concentration increase / decrease changing means during the abnormality determination mode in a predetermined operation state of the internal combustion engine. The NOx concentration in the gas is once increased and decreased. When the NOx concentration is increased or decreased, the elapsed time from the output state of the predetermined first output value of the NOx sensor to the output of the predetermined second output value is measured by the elapsed time measuring means. Is done. Based on the elapsed time measured by the elapsed time measuring means and the reference elapsed time, the determination means determines whether the NOx sensor is normal or abnormal.

  Here, the “reference elapsed time” means that the NOx sensor outputs a predetermined second output value from the output state of the predetermined first output value of the NOx sensor when the NOx sensor is in a normal state such as in a new state. This is the elapsed time until it is done, for example, obtained in advance through experiments and stored as data. In order to compensate for individual differences among NOx sensors mounted on the vehicle, the reference elapsed time required when a new vehicle is mounted or replaced with the NOx sensor may be measured and stored as data. Therefore, when the elapsed time measured by the elapsed time measuring means is substantially equal to the reference elapsed time, it means that the NOx sensor is functioning normally, and therefore the NOx sensor is determined to be normal. The On the other hand, if there is a predetermined difference between the elapsed time measured by the elapsed time measuring means and the reference elapsed time, it means that there is a response delay of the NOx sensor, so the NOx sensor has deteriorated. That is, it is determined as abnormal (failure).

  Therefore, according to this embodiment, when the NOx concentration is increased or decreased, the process from the output state of the predetermined first output value of the NOx sensor until the NOx sensor outputs the predetermined second output value. By measuring the time and comparing it with the reference elapsed time, it is possible to determine whether the NOx sensor is normal or abnormal accurately and in a short time.

  Here, the exhaust gas purification system has a selective reduction type NOx catalyst and urea water supply means for supplying urea water to an exhaust passage upstream thereof, and the NOx sensor is disposed downstream of the selective reduction type NOx catalyst. The NOx concentration increase / decrease changing means may change the urea water supply amount from the urea water supply means.

  According to this aspect, in the abnormality determination mode in the predetermined operating state of the internal combustion engine, the urea water supply amount from the urea water supply means is reduced to the equivalent ratio or less by the NOx concentration increase / decrease changing means, and then the selective reduction is performed. The NOx concentration downstream of the type NOx catalyst is increased or decreased. Then, when the NOx concentration is increased or decreased, the elapsed time from the output state of the predetermined first output value of the NOx sensor by the elapsed time measuring means until the NOx sensor outputs the predetermined second output value. Is measured, and the determination means determines whether the NOx sensor is normal or abnormal based on the measured elapsed time and the reference elapsed time.

  Therefore, according to this embodiment, it is possible to accurately determine the normality or abnormality of the NOx sensor including the urea water supply means and disposed downstream of the selective reduction type NOx catalyst in a short time.

  The exhaust gas purification system has a selective reduction type NOx catalyst and urea water supply means for supplying urea water to an exhaust passage upstream thereof, and the NOx sensor is disposed upstream of the urea water supply means. The NOx concentration increase / decrease changing means may change the EGR amount.

  According to this aspect, during the abnormality determination mode in a predetermined operating state of the internal combustion engine, the NOx concentration increase / decrease changing means increases the amount of EGR and then increases the NOx concentration upstream of the selective reduction type NOx catalyst. The Then, when the NOx concentration is increased or decreased, the elapsed time from the output state of the predetermined first output value of the NOx sensor by the elapsed time measuring means until the NOx sensor outputs the predetermined second output value. Is measured, and the determination means determines whether the NOx sensor is normal or abnormal based on the measured elapsed time and the reference elapsed time.

  Therefore, according to this embodiment, the normality or abnormality of the NOx sensor including the urea water supply means and disposed upstream of the selective reduction type NOx catalyst can be determined accurately and in a short time.

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic system diagram of an internal combustion engine according to an embodiment of the present invention. In the figure, 10 is a compression ignition type internal combustion engine or diesel engine for automobiles, 11 is an intake manifold communicated with an intake port, 12 is an exhaust manifold communicated with an exhaust port, and 13 is a combustion chamber. . In the present embodiment, fuel supplied from a fuel tank (not shown) to the high pressure pump 17 is pumped to the common rail 18 by the high pressure pump 17 and accumulated in a high pressure state, and the high pressure fuel in the common rail 18 is transferred from the injector 14 to the combustion chamber. 13 is directly injected and supplied. Exhaust gas from the engine 10 passes from the exhaust manifold 12 through the turbocharger 19 and then flows into the exhaust passage 15 downstream thereof. After being purified as described later, the exhaust gas is discharged to the atmosphere. In addition, as a form of a diesel engine, it is not restricted to the thing provided with such a common rail type fuel injection device. It is also optional to include other exhaust purification devices such as EGR devices.

  On the other hand, the intake air introduced from the air cleaner 20 into the intake passage 21 passes through the air flow meter 22, the turbocharger 19, the intercooler 23, and the throttle valve 24 in order to reach the intake manifold 11. The air flow meter 22 is a sensor for detecting the intake air amount, and specifically outputs a signal corresponding to the flow rate of the intake air. The throttle valve 24 is an electronically controlled type.

  Further, in the present embodiment, an exhaust gas recirculation device 25 that communicates the exhaust manifold 12 and the intake passage 21 downstream of the throttle valve 24 is provided, and an EGR valve 26 that controls the exhaust gas recirculation amount (EGR amount) is interposed. .

  The exhaust passage 15 includes, in order from the upstream side, an oxidation catalyst 30 that oxidizes unburned components contained in the exhaust gas, a DPF catalyst 32 that captures and treats particulates (PM), and is contained in the exhaust gas in the passage. A NOx catalyst 34 for reducing and purifying NOx and an ammonia oxidation catalyst 36 are provided. The NOx catalyst 34 of the present embodiment is a selective reduction type NOx catalyst, and can reduce NOx continuously and purify it when a reducing agent is added.

  An addition valve 40 for selectively adding urea as a reducing agent to the selective reduction NOx catalyst 34 is provided in the exhaust passage 15 downstream of the DPF catalyst 32 and upstream of the selective reduction NOx catalyst 34. . Urea is used in the form of a urea aqueous solution, and is injected and supplied into the exhaust passage 15 from the addition valve 40 toward the selective reduction type NOx catalyst 34 on the downstream side. A urea dispersion plate 41 is provided downstream of the addition valve 40 and upstream of the selective reduction type NOx catalyst 34 to uniformly distribute the injected urea aqueous solution to the selective reduction type NOx catalyst 34. Further, a supply device 42 for supplying a urea aqueous solution is connected to the addition valve 40, and a tank 44 for storing the urea aqueous solution is connected to the supply device 42.

Further, an electronic control unit (hereinafter referred to as ECU) 100 is provided as a control means for controlling the entire engine. The ECU 100 includes a digital computer and includes a ROM (Read Only Memory), a RAM (Random Access Memory), a CPU (Central Processor Unit), an input port, an output port, a storage device, and the like that are connected to each other via a bidirectional bus. . The ECU 100 controls the injector 14, the high pressure pump 17, the throttle valve 24, and the like so that desired engine control is executed based on output values of various sensors. The ECU 100 also controls the addition valve 40 and the supply device 42 to control the urea addition amount. Further, the ECU 100 is configured to execute a failure determination process described later by executing a program stored in the ROM, and an abnormality of the NOx sensor used in the exhaust gas purification system according to the present invention. It is comprised so that it may function also as an example of a determination apparatus.

  As sensors connected to the ECU 100, in addition to the air flow meter 22 described above, in the present embodiment, a NOx sensor 52 provided on the downstream side of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36, and an addition valve are also used. 40, a pre-catalyst exhaust temperature sensor 54 and a post-catalyst exhaust temperature sensor 56 provided on the upstream side and the downstream side of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36, respectively. The NOx sensor 52, the pre-catalyst exhaust temperature sensor 54, and the post-catalyst exhaust temperature sensor 56 output a signal corresponding to the NOx concentration and temperature of the exhaust gas at the installation position to the ECU 100, respectively. In FIG. 1, in addition to the NOx sensor 52 provided on the downstream side of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36, a NOx sensor 53 disposed on the upstream side of the addition valve 40 is also shown. Although shown, since this NOx sensor 53 is additional, in the following description, unless it is pointed out as a matter specific to the NOx sensor 53 arranged on the upstream side of the addition valve 40, NOx including both is included. The sensor 52 will be described.

Here, a thermistor or the like is used as the exhaust temperature sensor, and a sensor using stabilized zirconia as a solid electrolyte may be used as the NOx sensor 52. As described later, the NOx sensor 52 using the stabilized zirconia as a solid electrolyte decomposes NOx in the exhaust gas into O ₂ and N ₂ to detect the O ₂ partial pressure, and thereby detects the O ₂ partial pressure in the exhaust gas. The NOx concentration is detected. Further, if NH ₃ is present in the exhaust gas, this NH ₃ is oxidized into NO in the solid electrolyte made of stabilized zirconia of the NOx sensor 52, and NO resulting from the oxidation of NH ₃ is also detected as NOx. The output value of the NOx sensor 52 is the sum of both the NOx concentration and the NH ₃ concentration in the exhaust gas.

  As other sensors, a crank angle sensor 27 and an accelerator opening sensor 28 are connected to the ECU 100. The crank angle sensor 27 outputs a crank pulse signal to the ECU 100 when the crank rotates, and the ECU 100 detects the crank angle of the engine 10 based on the crank pulse signal and calculates the rotational speed of the engine 10. The accelerator opening sensor 28 outputs a signal according to the accelerator pedal opening (accelerator opening) operated by the user to the ECU 100.

The selective reduction type NOx catalyst (SCR: Selective Catalytic Reduction) 34 ion-exchanges a transition metal such as copper Cu on the surface of the base material such as platinum or platinum supported on a base material such as zeolite or alumina. And those having a titania / vanadium catalyst (V ₂ O ₅ / WO ₃ / TiO ₂ ) supported on the surface of the substrate. The selective reduction type NOx catalyst 34 reduces and purifies NOx when the catalyst temperature (catalyst bed temperature) is in the active temperature range and urea as a reducing agent is added. When urea water is added to the catalyst, ammonia (NH ₃ ) is generated on the selective reduction type NOx catalyst as typically shown in the following chemical reaction formula and the like, and this ammonia reacts with NOx to react with NOx. Are reduced to produce nitrogen (N ₂ ) and water (H ₂ O).
・ CO (NH ₂ ) ₂ + H ₂ O → 2NH ₃ + CO ₂
・ 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O
・ 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O

In this embodiment, the urea addition amount with respect to the selective reduction type NOx catalyst 34 is a basic urea water corresponding to the NOx generation amount to be processed based on the information of the engine operating state (for example, the engine rotation speed and the accelerator opening). The amount of addition is determined by reading from a control map that is obtained and set in advance by experiments. Further, the ECU 100 may control the NOx concentration detected by the NOx sensor 52 provided downstream of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36. Specifically, the urea injection amount from the addition valve 40 is controlled so that the output value of the NOx concentration is always zero. In this case, the urea injection amount may be set based only on the output value of the post-catalyst NOx concentration, or basic urea water based on the information on the engine operating state (for example, engine speed and accelerator opening) described above. The addition amount may be read and determined from the control map, and this may be subjected to feedback correction control based on the output value of the NOx sensor 52. Since the selective reduction type NOx catalyst 34 can reduce NOx only when urea is added, urea water is usually added constantly. Further, control is performed so that urea is added only in a minimum amount necessary for reducing NOx discharged from the engine. In order to prevent the so-called NH ₃ slip that ammonia is discharged downstream of the catalyst due to excessive addition of urea, the above-described ammonia oxidation catalyst 36 is provided.

  Further, as described above, in the embodiment additionally including the NOx sensor 53 arranged on the upstream side of the addition valve 40 in addition to the NOx sensor 52, the addition of the basic urea water addition amount described above is performed. The urea water addition amount is obtained in correspondence with the NOx concentration detected by the NOx sensor 53 arranged on the upstream side of the valve 40, and this is subjected to feedback correction control based on the output value of the NOx sensor 52. Good.

Next, details of the NOx sensor 52 will be described with reference to FIG. 2 showing the structure of the sensor portion of the NOx sensor 52. The sensor part of the NOx sensor 52 is composed of six oxygen ion conductive solid electrolyte layers such as zirconia oxide laminated on each other, and these six solid electrolyte layers are hereinafter referred to as a first layer L ₁ and a second layer L _{2 in} order from the top. These are referred to as a third layer L ₃ , a fourth layer L ₄ , a fifth layer L ₅ , and a sixth layer L ₆ .

Between the first layer L ₁ and the third layer L ₃ , for example, a first diffusion rate limiting member 62 and a second diffusion rate limiting member 63 made of a porous material or a material in which pores are formed are provided. Has been placed. A first chamber 64 is formed between the diffusion control members 62 and 63. Further, the second diffusion-controlling member 63 is provided between the second layer L _2, the second chamber 65 is formed. In addition, an air chamber 66 communicating with the outside air is formed between the third layer L ₃ and the fifth layer L ₅ . On the other hand, the outer end surface of the first diffusion rate controlling member 62 is in contact with exhaust gas as a gas to be detected outside the sensor. Therefore, the exhaust gas is introduced into the first chamber 64 through the first diffusion control member 62.

On the other hand, a cathode-side first pump electrode 67 is formed on the inner peripheral surface of the first layer L ₁ facing the first chamber 64. An anode-side first pump electrode 68 is formed on the outer peripheral surface of the first layer L ₁ . A voltage is applied between the first pump electrodes 67 and 68 by a first pump voltage source 69. When a voltage is applied between the first pump electrodes 67 and 68, oxygen contained in the exhaust gas in the first chamber 64 comes into contact with the cathode-side first pump electrode 67 and becomes oxygen ions. The oxygen ions flow in the first layer L ₁ toward the anode-side first pump electrode 68. Therefore, oxygen contained in the exhaust gas in the first chamber 64 moves through the first layer L ₁ and is pumped out of the chamber. At this time, the amount of oxygen pumped out increases as the voltage of the first pump voltage source 69 increases.

On the other hand, a reference electrode 70 is formed on the inner peripheral surface of the third layer L ₃ facing the atmospheric chamber 66. By the way, in a layer made of a solid electrolyte having oxygen ion conductivity (hereinafter referred to as a solid electrolyte layer), if there is a difference in oxygen concentration on both sides of the solid electrolyte layer, the oxygen concentration is changed from the higher oxygen concentration side to the lower oxygen concentration side. Thus, oxygen ions move in the solid electrolyte layer. In the example shown in FIG. 2, the oxygen concentration in the atmospheric chamber 66 is higher than the oxygen concentration in the first chamber 64, so that oxygen in the atmospheric chamber 66 is charged by contacting with the reference electrode 70. Receives oxygen ions. The oxygen ions move in the third layer L ₃ , the second layer L _2, and the first layer L ₁ , and discharge electric charges at the cathode side first pump electrode 67. As a result, the voltage (electromotive force) V ₀ indicated by reference numeral 71 between the reference electrode 70 and the cathode side first pump electrode 67 is generated. This voltage V ₀ is proportional to the oxygen concentration difference between the atmosphere chamber 66 and the first chamber 64.

In the example shown in FIG. 2, when detecting the NOx concentration in the exhaust gas, this voltage V ₀ is a voltage generated when the oxygen concentration in the first chamber 64 is a predetermined value on the high concentration side, for example, 1 ppm. So that the voltage of the first pump voltage source 69 is feedback-controlled. That is, the oxygen in the first chamber 64 is pumped through the first layer L ₁ so that the oxygen concentration in the first chamber 64 becomes 1 ppm, and thereby the oxygen concentration in the first chamber 64 becomes 1 ppm. Maintained.

On the other hand, a cathode-side second pump electrode 72 is formed on the inner peripheral surface of the first layer L ₁ facing the second chamber 65. A voltage is applied by the second pump voltage source 73 between the cathode side second pump electrode 72 and the anode side first pump electrode 68. When a voltage is applied between the pump electrodes 72 and 68, oxygen contained in the exhaust gas in the second chamber 65 comes into contact with the cathode-side second pump electrode 72 and becomes oxygen ions. The oxygen ions flow in the first layer L ₁ toward the anode-side first pump electrode 68. Accordingly, oxygen contained in the exhaust gas in the second chamber 65 moves through the first layer L ₁ and is pumped out of the chamber. At this time, the amount of oxygen pumped out of the room increases as the voltage of the second pump voltage source 73 increases.

On the other hand, as described above, if there is a difference in oxygen concentration on both sides of the solid electrolyte layer, oxygen ions move in the solid electrolyte layer from the higher oxygen concentration side toward the lower oxygen concentration side. In the example shown in FIG. 2, since the oxygen concentration in the atmospheric chamber 66 is higher than the oxygen concentration in the second chamber 65, the oxygen in the atmospheric chamber 66 receives charges by contacting the reference electrode 70. It becomes oxygen ion. The oxygen ions move in the third layer L ₃ , the second layer L _2, and the first layer L ₁ , and discharge electric charges at the cathode side second pump electrode 72. As a result, a voltage (electromotive force) V ₁ indicated by reference numeral 74 is generated between the reference electrode 70 and the cathode-side second pump electrode 72. This voltage V ₁ is proportional to the oxygen concentration difference between the atmosphere chamber 66 and the second chamber 65.

In the example shown in FIG. 2, when the NOx concentration in the exhaust gas is detected, this voltage V ₁ is when the oxygen concentration in the second chamber 65 is a predetermined value on the low concentration side, for example, 0.01 ppm. The voltage of the second pump voltage source 73 is feedback-controlled so as to coincide with the voltage generated at the time. That is, the oxygen in the second chamber 65 is pumped through the first layer L ₁ so that the oxygen concentration in the second chamber 65 is 0.01 ppm, whereby the oxygen concentration in the second chamber 65 is 0. .01 ppm.

On the other hand, a cathode pump electrode 75 for NOx detection is formed on the inner peripheral surface of the third layer L ₃ facing the second chamber 65. The cathode pump electrode 75 has a strong reducing property with respect to NOx. Therefore, NOx in the second chamber 65, which actually occupies most of the NOx, is decomposed into N ₂ and O ₂ on the cathode side pump electrode 75. As shown in FIG. 2, a constant voltage 76 is applied between the cathode side pump electrode 75 and the reference electrode 70, so that O ₂ decomposed and generated on the cathode side pump electrode 75 is Then, oxygen ions are moved in the third layer L ₃ toward the reference electrode 70 and pumped out. At this time, a current I ₁ indicated by reference numeral 77 proportional to the amount of oxygen ions flows between the cathode pump electrode 75 and the reference electrode 70. Since the decomposed and generated oxygen amount and the nitrogen amount are proportional to each other, the current I ₁ eventually becomes the output of the NOx sensor 52 indicating the NOx concentration in the exhaust gas.

The cathode-side first pump electrode 67 in the first chamber 64 in contact with the exhaust gas and the cathode-side second pump electrode 72 in the second chamber 65 do not have a catalytic ability capable of reducing NOx or the catalytic ability thereof. Is composed of low material. These electrodes 67 and 72 are made of, for example, cermet of gold Au, platinum Pt, and ceramics. On the other hand, the cathode pump electrode 75 in the second chamber 65 includes a material having a catalytic ability capable of reducing NOx or having a high catalytic ability. The cathode-side pump electrode 75 is composed of, for example, a porous cermet made of rhodium Rh, platinum Pt, and zirconia ZrO ₂ as ceramics. Among these, rhodium Rh has a high NOx catalytic ability capable of reducing NOx, particularly NO. .

In the first chamber 64, the oxygen in the first chamber 64 is discharged so that the oxygen concentration in the first chamber 64 becomes 1 ppm by the action of the first pump cell and the monitor cell. At this time, NO ₂ contained in the exhaust gas may be reduced to NO, but NO is not further reduced. Therefore, NOx is substantially made into NO in the first chamber 64, and the exhaust gas containing this NOx flows into the second chamber 65 through the second diffusion control member 63.

In the second chamber 65, the oxygen in the second chamber 65 is discharged so that the oxygen concentration in the second chamber 65 becomes 0.01 ppm by the action of the second pump cell and the monitor cell. In addition, NOx (mostly NO) in the second chamber 65 is decomposed into N ₂ and O ₂ on the cathode-side pump electrode 75 by the action of the sensor cell, and the oxygen O ₂ decomposed is discharged. At the same time, an output current I ₁ corresponding to the amount of nitrogen generated by decomposition is generated.

An electric heater 79 for heating the sensor part of the NOx sensor 52 is disposed between the fifth layer L ₅ and the sixth layer L ₆ . The heater 79 is controlled to be heated to about 750 to 800 ° C. when normal NOx is detected by the NOx sensor 52.

  Now, in the NOx sensor 52 having such a basic configuration, as the NOx sensor 52 deteriorates, the cathode-side first pump electrode 67 in the first chamber 64 and the cathode-side second pump electrode in the second chamber 65. 72, a metal material having no NOx catalytic ability or a low catalytic ability (hereinafter also referred to as a low catalytic ability metal material) scatters and gradually spreads on the surface of the cathode pump electrode 75 in the second chamber 65. It has been found that the NOx catalytic ability and the oxygen exhausting function of the cathode pump electrode 75 are gradually reduced. In the case of this embodiment, the low catalytic ability metal material is gold Au. As a result of the gold Au being scattered and adhering to the cathode pump electrode 75, the NOx catalytic ability of rhodium Rh of the cathode pump electrode 75 is increased. It will decrease. As a result of the deterioration of the function of the cathode pump electrode 75, the output of the NOx sensor 52 when the NOx concentration of the exhaust gas changes becomes higher. However, the NOx sensor 52 deteriorates as the NOx sensor 52 deteriorates. It cannot synchronize with the change when the concentration changes to the lower side, resulting in a response delay output. That is, the degree of functional deterioration of the cathode pump electrode 75 correlates with the degree of deterioration of the NOx sensor 52.

  Next, referring to the time chart of FIG. 4 together with the flowchart of FIG. 3, an example of a processing procedure for executing normality or abnormality determination by the NOx sensor abnormality determination device used in the exhaust gas purification system of the present embodiment will be described. However, it will be explained. The illustrated determination routine is executed at a predetermined cycle when a predetermined condition is satisfied by the ECU 100.

  Therefore, before executing the normal or abnormal determination routine, it is determined by a normal control routine by the ECU 100 whether or not a predetermined condition is satisfied. Specifically, the bed temperature of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 is within a predetermined temperature range as described later, whether the NOx sensor 52 has been activated, whether urea water is added It is determined whether control is in progress and whether normal or abnormal determination is incomplete as will be described later. When any one of these predetermined conditions is not satisfied, this routine is not executed. This is because correct determination cannot be made even if the determination is executed when the predetermined condition is not satisfied.

  Here, a method for obtaining the bed temperature Tc of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 will be described. The temperatures of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 can be directly detected by a temperature sensor embedded in the catalyst, but in the present embodiment, they are estimated. Specifically, the ECU 100 estimates the bed temperature based on the pre-catalyst exhaust temperature and the post-catalyst exhaust temperature detected by the pre-catalyst exhaust temperature sensor 54 and the post-catalyst exhaust temperature sensor 56, respectively.

  Now, assume that the temperature of the exhaust gas upstream of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 is Tf (° C.), and the amount of the exhaust gas is Ga (g / s). Here, since the amount of exhaust gas can be considered to be equal to the amount of air taken into the engine, the amount of intake air Ga is defined as the amount of exhaust gas. This amount of exhaust gas is the amount of exhaust gas flowing into the catalyst per unit time (here 1 second). On the other hand, the bed temperature of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 is Tc (° C.), the weight of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 is Mc (g), the selective reduction type NOx catalyst 34 and the ammonia oxidation. The temperature of the exhaust gas on the downstream side of the catalyst discharged from the catalyst 36 is defined as Tr (° C.).

Further, Ef is the thermal energy of the exhaust gas upstream of the catalyst, and Ec is the thermal energy of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36. These thermal energies Ef and Ec can be expressed by the following equations (1) and (2). However, Cg is the specific heat ratio of the exhaust gas, Cc is the specific heat ratio of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36, and both are constant values.
Ef = Ga × Tf × Cg (J / s) (1)
Ec = Mc × Tc × Cc (J) (2)

By the way, considering the thermal equilibrium when the exhaust gas having the thermal energy Ef is supplied to the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 having the thermal energy Ec, a unit time has elapsed since the start of the exhaust gas supply. When it is considered that the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 and the exhaust gas are in a thermal equilibrium state later, and the temperature of both after the thermal equilibrium is Tm, the thermal equilibrium formula is expressed by the following formula (3).
Ef + Ec = Ga * Tm * Cg + Mc * Tm * Cc (3)

  This temperature Tm is a basic value of the estimated temperatures of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36. However, in reality, the exhaust gas and the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 do not reach a complete thermal equilibrium state, and the temperature is set downstream of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36. The exhaust gas of Tr is exhausted and the heat energy escapes. Therefore, the thermal energy Er that has escaped downstream is calculated based on the temperature Tr, and thereby the basic estimated temperature Tm of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36 is feedback-corrected, and the final estimated bed temperature Tc is calculated. Calculated.

  As understood from the above description, in this embodiment, the pre-catalyst exhaust temperature Tf, which is the exhaust gas temperature upstream of the selective reduction type NOx catalyst 34, is detected by the pre-catalyst exhaust temperature sensor 54, and the selective reduction type NOx catalyst. The post-catalyst exhaust temperature Tr, which is the exhaust gas temperature downstream of the gas catalyst 34 and the ammonia oxidation catalyst 36, is detected by the post-catalyst exhaust temperature sensor 56. Then, an intake air amount Ga that can be regarded as equivalent to the exhaust gas amount is detected by the air flow meter 22. Based on these output values, the ECU 100 estimates the bed temperature Tc of the selective reduction type NOx catalyst 34 and the ammonia oxidation catalyst 36.

  Then, before execution of the determination routine, it is determined whether or not the estimated bed temperature Tc is within a predetermined temperature range (for example, 200 to 400 ° C.). At the same time, whether or not the NOx sensor 52 has been activated, for example, whether or not the detected impedance has reached a predetermined value corresponding to the activation temperature of the NOx sensor 52, further controlling the addition of urea water. Is determined, for example, by the presence or absence of a valve opening operation control signal from the ECU 100 to the addition valve 40. As a result of these determinations, the determination routine is executed when a predetermined condition is satisfied.

  Therefore, when the above-mentioned predetermined condition is satisfied, in other words, a routine for determining whether the exhaust gas purification system is normal or abnormal is executed in the abnormality determination mode. When this determination routine is executed, first, in step S301, the amount of exhaust gas discharged from the internal combustion engine (hereinafter referred to as “exhaust gas”) is controlled to be constant or within a predetermined range, which is satisfied. It is determined whether or not. This is performed by controlling the opening of the throttle valve 24 based on the intake air amount Ga detected by the air flow meter 22 so that the intake air amount Ga is constant or within a predetermined range. Based on the intake air amount Ga detected by the meter 22, it is determined whether or not the operating condition is that the amount of exhaust gas (exhaust gas) discharged from the internal combustion engine is constant or within a predetermined range. In this way, changes in the NOx concentration can be grasped more accurately, so that normality or abnormality can be determined more accurately. When the condition is not satisfied, this determination routine is temporarily terminated.

  When the condition is satisfied, in the next step S302, control for increasing the NOx concentration in the exhaust gas reaching the NOx sensor 52 is executed. That is, the NOx concentration in the exhaust gas reaching the NOx sensor 52 is increased to a first predetermined concentration Dx (for example, 100 ppm) at time t0 in the time chart of FIG. As control for increasing the NOx concentration, for the NOx sensor 52 disposed downstream of the selective reduction type NOx catalyst 34, the urea water supply amount from the addition valve 40 as the urea water supply means is reduced below the equivalent ratio. . When the urea water supply amount is reduced by a predetermined amount below the equivalent ratio, the NOx in the exhaust gas is not completely processed. As a result, the NOx concentration in the exhaust gas reaching the NOx sensor 52 is directed toward the first predetermined concentration Dx. Will increase. Therefore, in step S303, the output value of the NOx sensor 52 corresponding to the NOx concentration in the exhaust gas is acquired, and in the next step S304, the acquired output value of the NOx sensor 52 is the NOx in the exhaust gas described above. It is determined whether or not the first output value Ix corresponding to the first predetermined density Dx has been reached. When the output value of the NOx sensor 52 becomes the first output value Ix, the process proceeds to the next step S305, and the measurement of the elapsed time is started from this time point t1. Then, in the next step S306, control for reducing the NOx concentration in the exhaust gas reaching the NOx sensor 52 is executed. That is, the urea water supply amount from the addition valve 40 so that the NOx concentration in the exhaust gas reaching the NOx sensor 52 becomes the second predetermined concentration Dy (for example, 50 ppm) at time t1 in the time chart of FIG. Is increased. Therefore, in step S307, the output value of the NOx sensor 52 corresponding to the NOx concentration in the exhaust gas is obtained again. In the next step S308, the obtained output value of the NOx sensor 52 is obtained in the above-described exhaust gas. It is determined whether or not the second output value Iy corresponding to the second predetermined concentration Dy of NOx has been reached. When the output value of the NOx sensor 52 becomes the second output value Iy, the process proceeds to the next step S309, and the measurement of the elapsed time is terminated at this time t2, and the elapsed time Tps from the time t1 to the time t2 is stored in the ECU 100. Is done.

  Then, the process proceeds to the next step S310, and this elapsed time Tps is compared with or contrasted with a reference elapsed time Tref that is obtained in advance through experiments and stored as data. As described above, the reference elapsed time Tref is output when the NOx sensor 52 outputs the first output value Ix corresponding to the first predetermined NOx concentration when the NOx sensor 52 is in a normal state such as a new state. This is the elapsed time from the point in time until the second output value Iy corresponding to the second predetermined NOx concentration is output. Therefore, in this step S310, in this embodiment, the elapsed time Tps is divided by the reference elapsed time Tref, and it is determined whether or not the ratio exceeds a predetermined value R. The predetermined value R can be arbitrarily set as a guideline for adjustment or replacement of the NOx sensor 52. As a result of the determination, if the predetermined value R is not exceeded, the process proceeds to step S311 and it is determined that the NOx sensor 52 is normal. If the predetermined value R is exceeded, the process proceeds to step S312 and the NOx sensor 52 is determined to be abnormal.

  Next, a normal or abnormal determination routine for the NOx sensor 53 disposed on the upstream side of the addition valve 40 will be described. The determination routine for the NOx sensor 53 is basically the same as the NOx concentration increase / decrease changing means in the determination routine for the NOx sensor 52 provided downstream of the selective reduction NOx catalyst 34 and the ammonia oxidation catalyst 36 described above. The processing procedure is the same. Therefore, the description of the processing procedure for the NOx sensor 52 described with reference to the flowchart of FIG. 3 is used as the description of the processing procedure for the NOx sensor 53 disposed on the upstream side of the addition valve 40. The different parts peculiar to are mainly described.

  Therefore, in step S302 in which control for increasing the NOx concentration in the exhaust gas reaching the NOx sensor 53 is executed, the NOx concentration in the exhaust gas reaching the NOx sensor 53 is changed to a first value at time t0 in the time chart of FIG. The EGR valve 26 in the exhaust gas recirculation device 25 is controlled as control that increases to a predetermined concentration Dx of 1 (for example, 100 ppm). That is, by reducing the EGR amount from the reference exhaust gas recirculation amount (EGR amount) that minimizes the generation amount of NOx in a predetermined operating state of the engine 10, as a result, the generated NOx amount per unit output gas amount is reduced. Increase the NOx concentration.

  Thus, the NOx concentration in the exhaust gas that reaches the NOx sensor 53 increases toward the first predetermined concentration Dx. Therefore, in step S303, the output value of the NOx sensor 53 corresponding to the NOx concentration in the exhaust gas is acquired, and in the next step S304, the acquired output value of the NOx sensor 53 is the NOx value in the exhaust gas described above. It is determined whether or not the first output value Ix corresponding to the first predetermined density Dx has been reached. When the output value of the NOx sensor 53 becomes the first output value Ix, the process proceeds to the next step S305, and the measurement of the elapsed time is started from this time point t1. In the next step S306, control for reducing the NOx concentration in the exhaust gas reaching the NOx sensor 53 is executed. That is, at time t1 in the time chart of FIG. 4, the EGR valve 26 is controlled to open so that the NOx concentration in the exhaust gas reaching the NOx sensor 53 becomes the second predetermined concentration Dy (for example, 50 ppm). The amount is increased. Therefore, in step S307, the output value of the NOx sensor 53 corresponding to the NOx concentration in the exhaust gas is acquired again. In the next step S308, the acquired output value of the NOx sensor 53 is obtained in the above-described exhaust gas. It is determined whether or not the second output value Iy corresponding to the second predetermined concentration Dy of NOx has been reached. When the output value of the NOx sensor 53 becomes the second output value Iy, the process proceeds to the next step S309, and the measurement of the elapsed time is terminated at this time t2, and the elapsed time Tps from the time t1 to the time t2 is stored in the ECU 100. Is done.

  Then, the process proceeds to the next step S310, and this elapsed time Tps is compared with or contrasted with a reference elapsed time Tref that is obtained in advance through experiments and stored as data. As described above, the reference elapsed time Tref is output when the NOx sensor 53 outputs the first output value Ix corresponding to the first predetermined NOx concentration when the NOx sensor 53 is in a new state, for example. This is the elapsed time from the point in time until the second output value Iy corresponding to the second predetermined NOx concentration is output. Therefore, in this step S310, in this embodiment, the elapsed time Tps is divided by the reference elapsed time Tref, and it is determined whether or not the ratio exceeds a predetermined value R. As a result of the determination, if the predetermined value R is not exceeded, the process proceeds to step S311 and the NOx sensor 53 is determined to be normal, and if the predetermined value R is exceeded, the process proceeds to step S312 and the NOx sensor 53 is determined to be abnormal.

  As is clear from the above description, according to the present embodiment, it is possible to determine whether the NOx sensor 52 and / or the NOx sensor 53 is abnormal. Here, when any of them is determined to be abnormal, it is preferable that a warning or the like is appropriately given to the driver by a known method.

  Although not shown in the flowchart of FIG. 3, when the normality or abnormality of the NOx sensor 52 and / or the NOx sensor 53 is determined in this routine, a flag indicating that the determination of normality or abnormality has been completed is displayed. Set. When this flag is set, in the above-described determination of whether or not the predetermined condition is satisfied, the determination is already completed and the predetermined condition is not satisfied, and the normal or abnormal determination routine after step S301 is not executed. This is because it is sufficient that the determination of normality or abnormality is executed at least once per trip (trip). Therefore, this flag is preferably reset when the engine 10 is stopped.

  The embodiment of the present invention is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

It is the schematic which shows the abnormality determination apparatus of the NOx sensor used for the exhaust gas purification system which concerns on one Embodiment of this invention. It is sectional drawing which shows the structural example of the NOx sensor used for the exhaust gas purification system which concerns on one Embodiment of this invention. It is a flowchart which shows an example of the determination processing procedure of a normal or abnormality in embodiment of this invention. It is a time chart for demonstrating the operation | movement which concerns on the abnormality determination process of a NOx sensor in embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Engine 15 Exhaust passage 25 Exhaust gas recirculation device 34 Selective reduction type NOx catalyst 36 Ammonia oxidation catalyst 40 Urea water addition valve 42 Urea water supply device 52, 53 NOx sensor 54 Pre-catalyst exhaust temperature sensor 56 Post-catalyst exhaust temperature sensor 100 Electronic control unit (ECU)

Claims (3)

In an abnormality determination device for a NOx sensor that is used in an exhaust gas purification system of an internal combustion engine and outputs different output values depending on the NOx concentration,
NOx concentration increase / decrease changing means for once increasing and decreasing the NOx concentration in the exhaust gas reaching the NOx sensor during an abnormality determination mode in a predetermined operating state of the internal combustion engine;
When the NOx concentration increase / decrease is changed by the NOx concentration increase / decrease changing means, the elapsed time from the output state of the predetermined first output value of the NOx sensor until the NOx sensor outputs the predetermined second output value is calculated. An elapsed time measuring means for measuring,
Determination means for determining normality or abnormality of the NOx sensor based on the elapsed time measured by the elapsed time measuring means and the reference elapsed time;
An abnormality determination device for a NOx sensor used for an exhaust gas purification system. The exhaust gas purification system has a selective reduction NOx catalyst and urea water supply means for supplying urea water to an exhaust passage upstream thereof, and the NOx sensor is disposed downstream of the selective reduction NOx catalyst,
2. The NOx sensor abnormality determination device for use in an exhaust gas purification system according to claim 1, wherein the NOx concentration increase / decrease changing means changes a urea water supply amount from the urea water supply means. The exhaust gas purification system has a selective reduction NOx catalyst and urea water supply means for supplying urea water to an exhaust passage upstream thereof, and the NOx sensor is disposed upstream of the urea water supply means,
The abnormality determination device for a NOx sensor used in an exhaust gas purification system according to claim 1, wherein the NOx concentration increase / decrease changing means changes an EGR amount.

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