JP2009257888A - Deterioration determination control device and deterioration restoration control device for nox sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a deterioration determination control device and a deterioration restoration control device for a NOx sensor for easily determining or diagnosing and restoring deterioration. <P>SOLUTION: The deterioration determination control device includes the NOx sensor 50 wherein each of a pump cell 240, a monitor cell 260 and a sensor cell 250 includes a pair of electrodes, and the pump cell and the monitor cell use respectively electrodes containing an Au material, and the sensor cell uses electrodes containing an Rh material; and a deterioration determination means for determining deterioration of the NOx sensor based on an output from the NOx sensor, when detecting for a prescribed time, a prescribed operation state under an atmosphere wherein the NOx concentration in exhaust gas is below a prescribed value, or NOx does not exist in the exhaust gas and the O<SB>2</SB>concentration is constant. The deterioration restoration control device includes a voltage application stopping means for stopping voltage application to the electrodes of the pump cell 240 for a prescribed time, when determined to be in the deteriorated state. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a NOx sensor deterioration determination control device and a deterioration recovery control device for detecting the concentration of NOx (nitrogen oxide) in a gas to be detected, and in particular, NOx contained in exhaust gas of an internal combustion engine. The present invention relates to a deterioration determination control device and a deterioration recovery control device for a NOx sensor for detecting the concentration of water.

  A NOx sensor is known for detecting the NOx concentration in the gas to be detected, particularly the NOx concentration in the exhaust gas discharged from the internal combustion engine. Such an NOx sensor is disposed, for example, on the downstream side of the NOx catalyst in an exhaust purification system (so-called urea SCR system) using a selective reduction type NOx catalyst in a diesel engine, and the detected NOx concentration is added to the reducing agent with respect to the NOx catalyst. Used for quantity control.

  Usually, the NOx sensor has a three-cell structure including a pump cell, a monitor cell, and a sensor cell, and the pump cell discharges or pumps oxygen in the exhaust gas introduced into the sensor chamber. The monitor cell detects the residual oxygen concentration in the sensor chamber after passing through the pump cell, and the sensor cell detects the NOx concentration from the gas after passing through the pump cell (see Patent Document 1).

  The pump cell is composed of a solid electrolyte body and a pair of electrodes opposed to the upper and lower surfaces thereof, and the electrode on the sensor chamber side of the pair of electrodes is Au that is inactive with respect to NOx reductive decomposition. A Pt electrode is used as the electrode containing the material and the electrode on the atmosphere passage side. The monitor cell is composed of a solid electrolyte body and a pair of electrodes opposed to the upper and lower surfaces thereof, and in the same manner as the pump cell, the electrode on the sensor chamber side of the pair of electrodes is inactive against the reductive decomposition of NOx. A Pt electrode is used as an electrode including an Au material, which is a simple electrode, and an electrode on the atmosphere passage side. On the other hand, the sensor cell is provided adjacent to the monitor cell, and the electrode on the air passage side of the pair of electrodes disposed opposite to the upper and lower surfaces of the solid electrolyte body is a Pt electrode that is separate or common with the monitor cell. As the electrode on the sensor chamber side, an electrode containing an Rh material that is an active electrode for NOx reductive decomposition is used.

  By the way, when the NOx sensor deteriorates, it becomes impossible to detect an accurate NOx concentration, resulting in problems such as deterioration of exhaust emission. In fact, in the field of automobiles, in order to prevent traveling in a state where exhaust gas has deteriorated, diagnosis of deterioration of various exhaust gas-related parts (OBD; On BOard DiagnOsis) is being legalized. .

Japanese Patent Laid-Open No. 9-288084

  However, in the case of a NOx sensor, particularly a NOx sensor disposed downstream of the NOx catalyst, it is generally difficult to diagnose the deterioration of the NOx sensor. This is because the exhaust gas supplied to the NOx sensor is a gas after passing through the NOx catalyst in addition to the variation in the operating state of the engine, and it is difficult to estimate the true NOx concentration to be compared with the sensor detected concentration. Because.

  Accordingly, an object of the present invention is to provide a NOx sensor deterioration determination control device and deterioration recovery that can easily determine or diagnose and recover deterioration even in the case of a NOx sensor disposed downstream of a NOx catalyst. It is to provide a control device.

  Here, as a result of diligent research by the present inventors, each of the pump cell, the monitor cell, and the sensor cell includes a pair of electrodes, the pump cell and the monitor cell include an electrode containing Au material, and the sensor cell includes an electrode containing Rh material. In the NOx sensor used, the Au material scatters from the electrode containing the Au material used in the pump cell (and the monitor cell) during the use process, and this adheres to the electrode containing the Rh material of the sensor cell. , Hereinafter also referred to as Au poisoning). As a result of this Au poisoning, Rh in the sensor cell electrode moves to the inside of the electrode, the reaction area between the gas and Rh decreases, the reduction capacity of the sensor cell decreases, and the output of the NOx sensor gradually decreases. Presumed.

Accordingly, one form of the NOx sensor deterioration determination control device according to the present invention that achieves the above object is a NOx sensor that is provided in an exhaust system of an engine and detects the NOx concentration of exhaust gas, and includes a pump cell, a monitor cell, and a sensor cell. A NOx sensor having a pair of electrodes, the pump cell and the monitor cell using an Au material, and the sensor cell using an electrode containing an Rh material; and the NOx concentration in the exhaust gas is a predetermined value or less An operating state detecting means for detecting a predetermined operating state in which NOx is not present in the gas and the O ₂ concentration is constant, and when the predetermined operating state is detected for a predetermined time by the operating state detecting means Deterioration determining means for determining deterioration of the NOx sensor based on the output of the NOx sensor. To do.

According to this aspect, the operating state detection means causes the NOx concentration in the exhaust gas to be equal to or lower than the predetermined value or the NOx is not present in the exhaust gas and the predetermined operating state in which the O ₂ concentration is constant is maintained for a predetermined time. when it is detected, in other words, when the NOx concentration is absent predetermined value or less or NOx, the exhaust gas O ₂ concentration is under constant atmosphere reaches the monitor cell and the sensor cell of the NOx sensor, the deterioration determination means, The deterioration of the NOx sensor is determined based on the output of the NOx sensor. That, NOx concentration is not present a predetermined value or less or NOx, since if the O ₂ concentration exhaust gas under certain atmosphere reaches the monitor cell is a O ₂ concentration is constant, it corresponds to the concentration A constant current value is obtained. In contrast, NOx concentration is not present a predetermined value or less or NOx, if the O ₂ concentration exhaust gas under certain atmosphere reaches the sensor cell, when deterioration of the NOx sensor is not a normal, O ₂ concentration Because it is constant, the same constant current value as the monitor cell corresponding to this concentration can be obtained. On the other hand, when the NOx sensor is deteriorated, the reaction surface of Rh is reduced as a result of Au poisoning. Only current values below the constant current value can be obtained. Therefore, the deterioration of the NOx sensor can be determined by comparing the current values of these monitor cells and sensor cells.

  Here, it is preferable that the predetermined operation state is a fuel cut operation state of the engine.

According to this embodiment, it is possible to easily obtain exhaust gas in an atmosphere in which NOx does not exist and the O ₂ concentration is constant.

  Further, the determination means may determine that the NOx sensor is in a deteriorated state when the output of the NOx sensor is negative.

  As described above, when the current values of the monitor cell and the sensor cell are compared, the process can be simplified by simply subtracting the current value of the sensor cell from the current value of the monitor cell.

  An embodiment of the NOx sensor deterioration recovery control device according to the present invention that achieves the above object is a NOx sensor that is provided in an exhaust system of an engine and detects the NOx concentration of exhaust gas, and includes a pump cell, a monitor cell and a sensor cell. A NOx sensor each comprising a pair of electrodes, the pump cell and the monitor cell using an Au material, and the sensor cell using an electrode containing an Rh material; and a determining means for determining deterioration of the NOx sensor; And a voltage application stopping unit that stops the voltage application to the electrode of the pump cell for a predetermined time when it is determined by the determining unit that the battery is in a deteriorated state.

According to this aspect, when the NOx sensor is determined to be in the deteriorated state by the determination means, the voltage application to the electrode of the pump cell is stopped for a predetermined time by the voltage application stop means. Then, since the discharge or pumping of oxygen in the pump cell in which the voltage application is stopped is stopped, this oxygen is sent to the sensor cell, and the sensor chamber is in an oxidizing atmosphere. Under this oxidizing atmosphere, Rh that has moved to the inside of the electrode is oxidized to Rh ₂ O ₃ and moves to the surface of the electrode. After the predetermined time elapses, the voltage application stop is released, so that the sensor chamber of the sensor cell is in a reducing atmosphere, Rh ₂ O ₃ is decomposed into Rh and O _2, and the Rh reaction area of the sensor cell is based on As a result, the deterioration of the NOx sensor is recovered.

  Here, a deterioration degree calculating means for calculating the deterioration degree of the NOx sensor, and a predetermined time for voltage application to be stopped by the voltage application stopping means is determined according to the deterioration degree of the NOx sensor calculated by the calculating means. It is preferable to further comprise a voltage application stop time determining means.

  According to this aspect, since the voltage application stop time determining means determines the predetermined time during which voltage application is stopped, that is, the stop time, according to the degree of deterioration of the NOx sensor, the deterioration of the NOx sensor is efficiently recovered. be able to.

  According to the present invention, even in the case of a NOx sensor disposed on the downstream side of a NOx catalyst, an excellent effect is exhibited that the deterioration can be easily determined or diagnosed and recovered.

  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 showing an internal combustion engine to which a NOx sensor deterioration determination and recovery control apparatus according to the present invention is applied. In the figure, 10 is a compression ignition type internal combustion engine or diesel engine mounted on an automobile, 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. It is. 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.

  The exhaust passage 15 is provided with a NOx catalyst, particularly a selective reduction type NOx catalyst 34, for reducing and purifying NOx in the exhaust gas. An oxidation catalyst that oxidizes and purifies unburned components (especially HC) in exhaust gas, and a DPR (Diesel Particulate ReductiOn) catalyst that collects particulate matter (PM) in exhaust gas and removes it by combustion is added. May be provided. Further, a urea adding device 48 for adding urea water as a reducing agent to the NOx catalyst 34 is provided. Specifically, a urea addition valve 40 for injecting urea water is provided in the exhaust passage 15 upstream of the NOx catalyst 34. Urea water is supplied to the urea addition valve 40 from a urea supply pump 42 through a supply line 41, and the urea supply pump 42 sucks and discharges urea water stored in the urea tank 44.

  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 CPU, a ROM, a RAM, an input / output port, a storage device, and the like. 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 detection values of various sensors. Further, the ECU 100 controls the urea addition valve 40 and the urea supply pump 42 in order to control the urea addition amount. As sensors connected to the ECU 100, in addition to the air flow meter 22, the NOx sensor 50 provided on the downstream side of the NOx catalyst 34, the pre-catalyst exhaust temperature provided on the upstream side and the downstream side of the NOx catalyst 34, respectively. A sensor 52 and a post-catalyst exhaust temperature sensor 54 are included. The NOx sensor 50 is a so-called limit current type NOx sensor that generates an output signal having a magnitude proportional to the NOx concentration of the exhaust gas. The structure will be described in detail later.

  As other sensors, a crank angle sensor 26, an accelerator opening sensor 27, and an engine switch 28 are connected to the ECU 100. The crank angle sensor 26 outputs a crank pulse signal to the ECU 100 when the crank angle 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 27 outputs a signal corresponding to the accelerator pedal opening (accelerator opening) operated by the user to the ECU 100. The engine switch 28 is turned on by the user when the engine is started and turned off when the engine is stopped.

Selective catalytic NOx catalyst (SCR: Selective Catalytic ReductiOn) 34 is supported by supporting a noble metal such as Pt on the surface of a base material such as zeolite or alumina, or by supporting a transition metal such as Cu on the surface of the base material by ion exchange. Examples thereof include those obtained by carrying a titania / vanadium catalyst (V ₂ O ₅ / WO ₃ / TiO ₂ ) on the surface of the substrate. The selective reduction type NOx catalyst 34 reduces and purifies NOx when the catalyst temperature is in the active temperature range and urea as a reducing agent is added. When urea is added to the catalyst, ammonia is generated on the catalyst, and this ammonia reacts with NOx to reduce NOx.

  The temperature of the NOx catalyst 34 can be directly detected by a temperature sensor embedded in the catalyst, but in the present embodiment, this is estimated. Specifically, the ECU 100 estimates the catalyst temperature based on the pre-catalyst exhaust temperature and the post-catalyst exhaust temperature detected by the pre-catalyst exhaust temperature sensor 52 and the post-catalyst exhaust temperature sensor 54, respectively. Note that the estimation method is not limited to such an example.

The amount of urea added to the NOx catalyst 34 is controlled based on the NOx concentration detected by the NOx sensor 50. Specifically, the urea injection amount from the urea addition valve 40 is controlled so that the value of the detected NOx concentration is always zero. In this case, the urea injection amount may be set based only on the value of the detected NOx concentration, or the NOx concentration may be set to zero based on the engine operating state (for example, engine speed and accelerator opening). The urea injection amount may be set, and the basic urea injection amount may be feedback-corrected so that the detected NOx concentration value becomes zero. Since the NOx catalyst 34 can reduce NOx only when urea is added, urea is basically added constantly during engine operation and when fuel injection is performed. In addition, control is performed so that urea is added only in the minimum amount necessary for NOx reduction. This is because when urea is added excessively, ammonia is discharged downstream of the catalyst (so-called NH ₃ slip), which causes a strange odor and the like.

  Next, details of the NOx sensor 50 will be described. FIG. 2 shows an enlarged structure of the sensor element 200 of the NOx sensor 50. As described below, the sensor element 200 of the NOx sensor 50 includes an oxygen ion conductive solid electrolyte layer such as zirconia oxide, an insulating material layer such as alumina, a porous diffusion layer, and the like, which are laminated with each other.

  The sensor element 200 includes a first chamber 220 and a second chamber 221 into which exhaust gas is introduced, a first atmospheric passage 230 and a second atmospheric passage 231 communicating with the atmosphere, and a pump cell 240 provided on the first chamber 220 side. The sensor cell 250 and the monitor cell 260 are provided on the second chamber 221 side. The sensor cell 250 and the monitor cell 260 are arranged adjacent to each other in the longitudinal direction of the sensor element 200. The first chamber 220 communicates with the second chamber 221 through the restriction 210, and exhaust gas is introduced into the first chamber 220 through the porous diffusion layer 209 and the pinhole 211.

  The sensor element 200 has a sheet-like structure constituting the pump cell 240 via a spacer 272 constituting the first chamber 220 and the second chamber 221 below the sheet-like solid electrolyte body 271 constituting the sensor cell 250 and the monitor cell 260. A solid electrolyte body 273 is laminated, and a spacer 274 and a sheet-like heater 212 constituting the first atmospheric passage 230 are further laminated. A spacer 275 constituting the porous diffusion layer 209 and the second atmospheric passage 231 is laminated above the solid electrolyte body 271.

  The solid electrolyte bodies 271 and 273 are made of a solid electrolyte having oxygen ion conductivity such as zirconia, and the spacers 272, 274 and 275 are made of an insulating material such as alumina. The porous diffusion layer 209 is made of porous alumina or the like.

  The pump cell 240 includes a solid electrolyte body 273 and a pair of electrodes 241 and 242 disposed opposite to the upper and lower surfaces thereof, and discharges or pumps oxygen in the exhaust gas introduced into the first chamber 220 into the first atmospheric passage 230. The oxygen concentration in the first chamber 220 is adjusted. Of the pair of electrodes, the electrode 241 on the first chamber 220 side includes, for example, a Pt—Au porous cermet electrode as an electrode containing an Au material that is inert to the NOx reductive decomposition, on the first atmospheric passage 230 side. For example, a Pt porous cermet electrode is preferably used for the electrode 242. The porous cermet electrode is formed by pasting and firing a metal component and ceramics such as zirconia and alumina.

  The monitor cell 260 includes a solid electrolyte body 271 and a pair of electrodes 261 and 262 arranged opposite to the upper and lower surfaces thereof, and residual oxygen in exhaust gas introduced into the second chamber 221 from the first chamber 220 through the throttle 210. Detect concentration. Of the pair of electrodes, the electrode 261 on the second chamber 221 side includes, for example, a Pt—Au porous cermet electrode as an electrode containing an Au material that is inactive with respect to NOx reductive decomposition, on the second atmospheric passage 231 side. As the electrode 262, for example, a Pt porous cermet electrode is used. By applying a predetermined voltage between the electrodes 261 and 262, a current output corresponding to the residual oxygen concentration can be obtained.

  The sensor cell 250 includes a solid electrolyte body 271 and a pair of electrodes 251 and 252 arranged to face the upper and lower surfaces thereof. The sensor cell 250 is provided adjacent to the monitor cell 260, and the electrode 252 on the second atmospheric passage 231 side of the pair of electrodes may be a common electrode with the electrode 262 of the monitor cell 260. The sensor cell 250 detects the NOx concentration and the residual oxygen concentration in the exhaust gas introduced into the second chamber 221, and the electrode 251 on the second chamber 221 side has Rh active against NOx reductive decomposition. For example, a Pt—Rh porous cermet electrode is used as the electrode containing the material. By applying a predetermined voltage between the electrodes 251 and 252, a current output corresponding to the NOx concentration and the residual oxygen concentration can be obtained.

  The heater 212 is formed by embedding a heater electrode in a sheet made of an insulating material such as alumina. The heater electrode generates heat by power supply from the outside, and the heater 212 heats the entire sensor element 200 to keep each of the cells 240, 250, 260 above the activation temperature (for example, about 750 to 800 ° C.). Used for.

  Here, the principle of operation of the NOx sensor 50 having the above configuration will be described. In FIG. 2, the exhaust gas that is the gas to be measured passes through the porous diffusion layer 209 and the pinhole 211 and is introduced into the first chamber 220. The amount of gas introduced is determined by the diffusion resistance of the porous diffusion layer 209 and the pinhole 211. Here, when a voltage is applied to the electrodes 241 and 242 of the pump cell 240 so that the electrode 242 on the first atmospheric passage 230 side becomes a positive electrode, oxygen in the exhaust gas is reduced on the electrode 241 on the first chamber 220 side. It is decomposed into oxygen ions and discharged to the electrode 242 side by the pumping action (in the direction of the white arrow in FIG. 2). When the direction of the applied voltage is reversed, oxygen is introduced from the first atmospheric passage 230 side to the first chamber 220 side.

In the pump cell 240, the oxygen concentration in the chamber can be controlled by using this oxygen pump action and adjusting the magnitude and direction of the applied voltage and taking in and out oxygen. Usually, in order to reduce the influence of oxygen at the time of NOx detection, oxygen introduced into the first chamber 220 is discharged and the inside of the second chamber 221 is maintained at a predetermined low oxygen concentration. Since the electrode 241 on the first chamber 220 side is a NOx inert electrode, NO ₂ contained in the exhaust gas may be reduced to NO in the pump cell 240, but NO is not reduced any more. Therefore, NOx is almost made into NO in the first chamber 220, and the exhaust gas containing this NOx flows into the second chamber 221 through the throttle 210.

  In the present embodiment, the pump cell 240 is controlled using an applied voltage map that is predetermined according to the pump cell current Ip measured by the current detector 281. The pump cell 240 has a limit current characteristic with respect to the oxygen concentration. In the VI characteristic diagram showing the relationship between the pump cell applied voltage Vp and the pump cell current Ip, the limit current detection area is composed of a straight line portion substantially parallel to the V axis. The voltage value increases as the oxygen concentration increases. Therefore, by variably controlling the pump cell applied voltage Vp according to the pump cell current Ip, oxygen introduced into the first chamber 220 is quickly discharged, and the inside of the first chamber 220 is controlled to a predetermined low oxygen concentration. Thereby, the influence of oxygen which becomes interference gas at the time of detecting NOx which is specific gas can be made small.

  Exhaust gas that has passed in the vicinity of the pump cell 240 flows into the second chamber 221 that communicates with the first chamber 220 through the throttle 210. When a predetermined voltage is applied between the electrodes 261 and 262 of the monitor cell 260 so that the electrode 262 on the second atmospheric passage 231 side becomes a positive electrode, the trace amount of oxygen remaining in the exhaust gas is on the second chamber 221 side. It is reduced and decomposed on the electrode 261 to become oxygen ions, which are discharged to the electrode 262 side by the pumping action (in the direction of the white arrow in FIG. 2). Since the electrode 261 is a NOx inert electrode, the monitor cell current Im measured by the current detector 283 depends on the amount of oxygen reaching the electrode 261 in the second chamber 221 and does not depend on the amount of NOx. Therefore, the residual oxygen concentration can be detected by detecting the monitor cell current Im.

  On the other hand, in the sensor cell 250, since the electrode 251 on the second chamber 221 side is a NOx active electrode, a predetermined voltage is applied between the electrodes 251 and 252 so that the electrode 252 on the second atmospheric passage 231 side becomes a positive electrode. Then, residual oxygen and NOx in the exhaust gas are reduced and decomposed on the electrode 251 on the second chamber 221 side to become oxygen ions, and are exhausted to the electrode 252 side by the pumping action (in the direction of the white arrow in FIG. 2). Therefore, the sensor cell current Is measured by the current detector 182 depends on the amount of oxygen and NOx reaching the second chamber 221. The sensor cell 250 and the monitor cell 260 are adjacent to each other in the second chamber 221, and the amount of oxygen reaching the electrodes 251 and 261 on the second chamber 221 side is substantially equal. Therefore, the monitor cell current Im (for the amount of oxygen) is calculated from the sensor cell current Is. By subtracting, the NOx concentration can be detected.

  Now, in the NOx sensor 50 having such a basic configuration, as the NOx sensor 50 deteriorates, it has NOx catalytic ability from the pump cell electrode 241 in the first chamber 220 and the monitor cell electrode 261 in the second chamber 221. Or the Au material which is a metal material having a low catalytic ability (hereinafter also referred to as a low catalytic ability metal material) scatters and gradually adheres to the surface of the sensor cell electrode 251 in the second chamber 221, It has been found that the NOx catalytic ability of Rh of the sensor cell electrode 251 is gradually reduced. As a result, the output of the NOx sensor 50 with respect to the same NOx concentration of the exhaust gas gradually decreases toward the low concentration side as the NOx sensor 50 deteriorates. The amount of Au attached to the sensor cell electrode 251 correlates with the degree of deterioration of the NOx sensor 50.

  Next, a processing procedure of deterioration determination control in the NOx sensor deterioration determination control device using the NOx sensor 50 having the above-described configuration will be described with reference to the flowchart of FIG. Note that the illustrated routine is repeatedly executed by the ECU 100 at predetermined intervals (for example, 16 msec).

  Therefore, when control is started, first, in step S301, it is determined whether or not a predetermined condition suitable for performing deterioration diagnosis or deterioration determination of the NOx sensor 50 is satisfied. The predetermined conditions for the deterioration diagnosis include, for example, 1) completion of engine warm-up (cooling water temperature is a predetermined value or higher) and 2) NOx sensor activated (heater temperature is higher than a predetermined value). 3) NOx The predetermined condition is satisfied when a condition is established such that the concentration is less than a predetermined value or exhaust gas not containing NOx can be supplied to the NOx sensor 50 (for example, during traveling).

  When this deterioration diagnosis condition is not satisfied, this routine is once ended. When it is determined that the deterioration diagnosis condition is satisfied, the routine proceeds to the next step S302. In step S302, it is determined whether or not the engine 10 is in a fuel cut operation state in which the fuel injection operation from the injector 14 into the combustion chamber 13 is stopped for a predetermined time. Whether or not the fuel cut operation state has a predetermined time is determined based on the rotational speed of the engine 10 and the accelerator opening from the accelerator opening sensor 27, or the drive signal to the injector 14 is a predetermined time. This can be done based on whether it is stopped.

In this fuel cut operation state, combustion in the combustion chamber 13 is not performed, so NOx does not exist in the exhaust gas discharged from the combustion chamber, and exhaust gas in an atmosphere with a constant O ₂ concentration exists. The NOx sensor 50 is reached.

  Accordingly, the process proceeds to the next step S303, where it is determined whether or not the output of the NOx sensor 50 is negative (<0). Specifically, when the output current value Im from the monitor cell 260 of the NOx sensor 50 and the output current value Is from the sensor cell 250 are acquired and the output current value Is of the sensor cell 250 is subtracted from the output current value Im of the monitor cell. It is determined whether or not it is negative.

  Here, referring to the graph of FIG. 4, the relationship between the output current value Is from the sensor cell 250 and the output current value Im from the monitor cell 260 in each of the cases where the NOx sensor 50 is normal and deteriorated. The relationship of the output of the NOx sensor 50 will be described. 4A shows when the NOx sensor 50 is normal, and FIG. 4B shows when the NOx sensor 50 has deteriorated. The upper side shows the sensor current and the lower side shows the NOx sensor output.

First, when exhaust gas in which NOx does not exist in the exhaust gas and the O ₂ concentration is constant reaches the monitor cell 260 of the NOx sensor 50, the output current value Im of the monitor cell 260 has an O ₂ concentration of Therefore, regardless of whether the NOx sensor 50 is normal or deteriorated, a constant output current value Im corresponding to this concentration (shown as = A in FIGS. 4A and 4B) is obtained.

On the other hand, similarly, when NOx is not present in the exhaust gas and the exhaust gas in an atmosphere having a constant O ₂ concentration reaches the sensor cell 250, the output current value Is of the sensor cell 250 is equal to that of the NOx sensor 50. Since the O ₂ concentration is constant when there is no deterioration and it is normal, a constant output current value Is equal to the output current value Im of the monitor cell 260 corresponding to this concentration (shown as = A in FIG. 4A). Is obtained). On the other hand, when the NOx sensor is deteriorated, the output current value Is of the sensor cell 250 is smaller than the reaction area of Rh at the electrode 251 of the sensor cell 250 as a result of Au poisoning. It is lower than the current value Im. As a result, when the NOx sensor 50 is normal, the output current value Im of the monitor cell 260 and the output current value Is of the sensor cell 250 are equal, so the output of the NOx sensor 50 as a result of the subtraction is “0” (FIG. 4 (A) lower reference), on the other hand, when the NOx sensor 50 is deteriorated, the negative value is smaller than “0” (refer to FIG. 4B lower reference).

  Returning to FIG. 3, if the determination in step S <b> 303 is that the output of the NOx sensor 50 is not negative, the process proceeds to step S <b> 304, the NOx sensor 50 is determined to be normal, and the output of the NOx sensor 50 is negative. Proceeding to S305, it is determined that the NOx sensor 50 is in a deteriorated state.

In the above-described embodiment, the fuel cut operation state of the engine 10 is selected in order to obtain exhaust gas in which NOx is not present in the exhaust gas discharged from the combustion chamber and the O ₂ concentration is constant. However, the present invention is not necessarily limited to this, and as a similar state, an operation state in which the exhaust gas in which the NOx concentration in the exhaust gas is equal to or lower than a predetermined value or NOx does not exist in the exhaust gas is obtained in advance by experiment, May be selected. This is because if the NOx concentration in the exhaust gas is equal to or lower than the predetermined value, a decrease in the output current value Is of the sensor cell 250 is recognized as in the case where NOx does not exist.

  Next, the processing procedure of the deterioration recovery control after the NOx sensor 50 having the above-described configuration is determined to be deteriorated will be described with reference to the flowchart of FIG. Note that the illustrated routine is repeatedly executed by the ECU 100 at predetermined intervals (for example, 16 msec).

  Therefore, when a deterioration recovery control instruction for the NOx sensor 50 is given in step S501, the process proceeds to the next step S502, and voltage application to the electrodes 241 and 242 of the pump cell 240 is stopped. Then, in the next step S503, it is determined whether or not a predetermined time has elapsed after stopping the voltage application. If not, the process returns to step S502. In other words, voltage application to the electrodes 241 and 242 of the pump cell 240 is stopped for a predetermined time. Then, after this predetermined time has elapsed, this routine ends, that is, the stop of voltage application is released.

Here, a mechanism by which the deterioration recovery of the NOx sensor 50 can be performed by stopping the voltage application to the electrodes 241 and 242 of the pump cell 240 will be described with reference to FIG. FIG. 6 is an explanatory view schematically showing a cross-sectional structure of the electrode 251 on the second chamber 221 side of the sensor cell 250 in the NOx sensor 50. FIG. 6A shows a state in which the NOx sensor 50 is in a new state, and FIG. When in a poisoned state, (C) is in an oxidizing atmosphere, (D) is the state of movement of Rh ₂ O ₃ by the oxidizing atmosphere, and (E) is the Rh ₂ O ₃ Rh in a reducing atmosphere. And the state of decomposition into O ₂ are shown. In the electrode 251 of the sensor cell 250 composed of Pt-Rh porous cermet, in the new state of the NOx sensor 50 shown in FIG. 6A, Pt and Rh are evenly distributed and arranged. In the Au poisoning state shown in FIG. 6 (B), it is estimated that Rh moves to the inside of the electrode and the Rh reaction area with NOx gas is reduced. Therefore, when the voltage application to the electrodes 241 and 242 of the pump cell 240 is stopped, the discharge or pumping of oxygen in the pump cell 240 in which the voltage application has been stopped is stopped, and this oxygen is sent to the sensor cell 250. The sensor chamber is in an oxidizing atmosphere shown in FIG. Under this oxidizing atmosphere, Rh that has moved to the inside of the electrode 251 is oxidized to Rh ₂ O ₃ and moves to the surface of the electrode 251 as shown in FIG. Then, since the voltage application stop to the pump cell 240 is released after the lapse of a predetermined time, the sensor chamber of the sensor cell 250 is in a reducing atmosphere, and Rh ₂ O ₃ is decomposed into Rh and O _2, and FIG. ), The deterioration of the NOx sensor 50 is recovered as a result of the Rh reaction area of the sensor cell 250 returning to the original state.

  The above-mentioned predetermined time is a fixed value that is obtained and set in advance through experiments or the like. However, as described below as another embodiment, the predetermined time may be a variable value that varies depending on the degree of deterioration of the NOx sensor 50. Good. In this case, the stop time T corresponding to the deterioration degree R of the NOx sensor 50 is obtained by, for example, the processing procedure shown in the flowchart of FIG. That is, in step S701, it is determined whether the NOx sensor 50 has been activated, and when activated, the process proceeds to step S702. The determination of whether or not the NOx sensor 50 is activated may be based on whether or not the engine 10 has been started. In step S702, the sensor output convergence time Ts of the NOx sensor 50 is measured. In the next step S703, the deterioration degree R of the NOx sensor 50 is calculated based on the sensor output convergence time Ts and the reference convergence time Tb.

  Here, the relationship between the sensor output convergence time Ts and the reference convergence time Tb will be described with reference to the graph of FIG. The graph of FIG. 8 shows how the sensor output changes immediately after activation of the NOx sensor 50, with the sensor output on the vertical axis and time on the horizontal axis. In the graph of FIG. 8, a solid line a indicates a deterioration sensor, and a broken line b indicates a normal sensor. According to experiments, it has been found that the NOx sensor 50 outputs a certain current value at the same time as activation, and then the output gradually converges. And the convergence time is shortened as the deterioration degree R of the NOx sensor increases. That is, in the case of a normal sensor, the convergence time is Tb (this is referred to as a reference convergence time Tb), whereas in the case of a deteriorated sensor, the convergence time is Ts (Ts <Tb).

  Therefore, in step S703 of the flowchart of FIG. 7, the deterioration degree R of the NOx sensor 50 is obtained as R = Ts / Tb. Then, in the next step S704, the stop time T corresponding to the calculated deterioration degree R is obtained from the map and determined. This map is obtained in advance by experiments or the like and stored in the ECU 100.

  Next, referring to the flowchart of FIG. 9, a processing procedure in the case where deterioration recovery control is performed by varying the stop time T according to the deterioration degree R after the NOx sensor 50 having the above-described configuration is determined to deteriorate. I will explain. This routine is also repeatedly executed by the ECU 100 at predetermined intervals (for example, 16 msec).

  Therefore, when an instruction to control deterioration recovery of the NOx sensor 50 is made in step S901, the process proceeds to the next step S902, and a stop time T corresponding to the deterioration degree R of the NOx sensor 50 is set. In the next step S903, voltage application to the electrodes 241 and 242 of the pump cell 240 is stopped. Further, in the next step S904, it is determined whether or not the voltage application stop time T has elapsed. If not, the process returns to step S903. In other words, voltage application to the electrodes 241 and 242 of the pump cell 240 is stopped for the stop time T. Then, after the stop time T has elapsed, this routine ends, that is, the stop of voltage application is released.

  In the processing procedure shown in the flowchart of FIG. 7 described above, the deterioration degree R of the NOx sensor 50 is obtained by using the sensor output convergence time Ts at the time of starting the sensor. In the inside, calculation is made based on the magnitude of the difference between the output current value Im of the monitor cell 260 and the output current value Is of the sensor cell 250 in the fuel cut operation state, and the calculated deterioration degree R The corresponding stop time T may be determined from the map.

  Further, by comparing the deterioration degree R with a predetermined deterioration determination value Rs (for example, 0.8), when the deterioration degree R is larger than the deterioration determination value Rs (approximately 1.0), the NOx sensor 50 is determined to be normal, and when the deterioration degree R is smaller than the deterioration determination value Rs, the NOx sensor 50 is determined to be in a deterioration state, and when it is determined to be normal, deterioration recovery control is not performed. It may be.

1 is a schematic system diagram showing an internal combustion engine to which a NOx sensor deterioration determination and recovery control device according to the present invention is applied. FIG. It is sectional drawing which shows the structure of the sensor element of a NOx sensor. It is a flowchart which shows an example of the process sequence of deterioration determination control of the NOx sensor which concerns on this invention. It is a graph which shows the relationship between the output current value Is of a sensor cell and the output current value Im of a monitor cell, and the relationship of the output of a NOx sensor, (A) when the NOx sensor is normal, (B), the NOx sensor 50 is deteriorated. In each case, the upper side shows the sensor current, and the lower side shows the NOx sensor output. It is a flowchart which shows an example of the process sequence of deterioration recovery control after a NOx sensor is determined to be deteriorated. It is explanatory drawing which shows typically the cross-section of the electrode by the side of the 2nd chamber of a sensor cell in a NOx sensor, (A) is a NOx sensor in a new state, (B) is an Au poisoning state, (C) is in oxidizing atmosphere (D) shows the state of movement of Rh ₂ O _{3 in} an oxidizing atmosphere, and (E) shows the state of decomposition of Rh ₂ O ₃ into Rh and O ₂ in a reducing atmosphere. It is a flowchart which shows an example of the process sequence for determining the stop time T corresponding to the deterioration degree R of a NOx sensor. It is a graph which shows the mode of a change of the sensor output immediately after starting of a NOx sensor. It is a flowchart which shows an example of the process sequence in the case of performing degradation recovery control by changing the stop time T according to the degradation degree R.

Explanation of symbols

10 Engine 15 Exhaust passage 34 NOx catalyst 50 NOx sensor 100 Electronic control unit (ECU)
200 sensor element 212 heater 220 first chamber 221 second chamber 230 first atmospheric passage 231 second atmospheric passage 240 pump cell 241, 242 electrode 250 sensor cell 251, 252 electrode 260 monitor cell 261, 262 electrode

Claims (5)

A NOx sensor that is provided in an exhaust system of an engine and detects a NOx concentration of exhaust gas, wherein each of the pump cell, the monitor cell, and the sensor cell includes a pair of electrodes, and the pump cell and the monitor cell each include an Au material, and A NOx sensor in which the sensor cell uses an electrode containing Rh material;
NOx concentration in the exhaust gas is not present NOx in a predetermined value or less or the exhaust gas, and the operating condition detecting means for detecting a predetermined operating condition with the O ₂ concentration is under constant atmosphere,
Deterioration determining means for determining deterioration of the NOx sensor based on the output of the NOx sensor when the predetermined operating state is detected for a predetermined time by the operating state detecting means;
A deterioration determination control device for a NOx sensor, comprising:   2. The NOx sensor deterioration determination control device according to claim 1, wherein the predetermined operation state is a fuel cut operation state of the engine.   The deterioration determination control device for a NOx sensor according to claim 1 or 2, wherein the determination unit determines that the NOx sensor is in a deteriorated state when the output of the NOx sensor is negative. A NOx sensor that is provided in an exhaust system of an engine and detects a NOx concentration of exhaust gas, wherein each of the pump cell, the monitor cell, and the sensor cell includes a pair of electrodes, and the pump cell and the monitor cell each include an Au material, and A NOx sensor in which the sensor cell uses an electrode containing Rh material;
Determination means for determining deterioration of the NOx sensor;
Voltage application stopping means for stopping voltage application to the electrode of the pump cell for a predetermined time when it is determined by the determining means that the battery is in a deteriorated state;
A deterioration recovery control device for a NOx sensor, comprising: A deterioration degree calculating means for calculating a deterioration degree of the NOx sensor;
Voltage application stop time determining means for determining a predetermined time of voltage application to be stopped by the voltage application stop means according to the degree of deterioration of the NOx sensor calculated by the calculation means;
The deterioration recovery control device for a NOx sensor according to claim 4, further comprising:

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