JP5067663B2 - NOx sensor abnormality diagnosis device - Google Patents

Description

  The present invention relates to an NOx sensor abnormality diagnosis device, and more particularly to an NOx sensor abnormality diagnosis device provided in an exhaust passage of an internal combustion engine.

  A NOx sensor is known as a means for detecting the concentration of nitrogen oxide (NOx) in exhaust gas discharged from an 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.

  By the way, if the NOx sensor becomes abnormal due to a failure or thermal deterioration, an accurate NOx concentration cannot be detected, resulting in problems such as deterioration of exhaust emission. Actually, in the field of automobiles, in order to prevent traveling in a state where exhaust gas has deteriorated, it is becoming legal to detect abnormalities in various exhaust gas-related components in an on-board state (onboard).

  Such abnormality diagnosis of the NOx sensor is disclosed in Patent Document 1, for example. In this case, when the oxygen concentration in the exhaust gas supplied to the NOx sensor is known, a specific voltage is applied to the oxygen pump cell of the NOx sensor, and the output value output from the oxygen pump cell at this time is the oxygen value. When the pump cell deviates from a value that can be assumed when the pump cell is normal, it is diagnosed that the NOx sensor has failed.

JP 2003-270194 A

  By the way, in the case of a NOx sensor disposed on the downstream side of the NOx catalyst, it is generally difficult to diagnose abnormality 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. Further, the NOx sensor has a complicated structure, and it is difficult to select parameters or characteristics that correlate with a decrease in the NOx concentration detection performance of the sensor.

  Under such circumstances, the present inventor newly found a parameter or characteristic correlated with a decrease in NOx concentration detection performance after extensive research. An object of the present invention is to provide a NOx sensor abnormality diagnosis device capable of diagnosing abnormality of a NOx sensor using such novel parameters or characteristics.

According to the present invention,
An abnormality diagnosis device for a NOx sensor provided in an exhaust passage of an internal combustion engine,
Measuring means for measuring the oxygen adsorption capacity in the sensor cell of the NOx sensor;
An NOx sensor abnormality diagnosis device is provided, comprising: a determination unit that determines abnormality of the NOx sensor based on the oxygen adsorption capacity measured by the measurement unit.

  As a result of earnest research, the present inventor has found that the sensor cell that emits the sensor output of the NOx sensor has an oxygen adsorption capacity, and that this oxygen adsorption capacity decreases as the NOx sensor deteriorates. Therefore, in the present invention, the abnormality diagnosis of the NOx sensor is performed by utilizing the correlation between the oxygen adsorption capacity of the sensor cell and the degree of deterioration of the NOx sensor as such a new parameter or characteristic.

  Preferably, the measuring means turns on the heater of the NOx sensor before starting the internal combustion engine, and measures the oxygen adsorption capacity based on the output of the NOx sensor while the heater is turned on.

  When the NOx sensor heater is turned on before starting the internal combustion engine and the warming-up of the NOx sensor is started, the internal combustion engine is not started and the exhaust gas and the NOx contained therein are not present for a while. Thus, the sensor output rises temporarily and behaves as if the NOx concentration has temporarily increased. This is because oxygen adsorbed on the electrode of the sensor cell while the sensor is left is decomposed on the electrode, and an oxygen ion current flows in the sensor cell. At this time, since the rising of the sensor output differs depending on the oxygen adsorption amount in the sensor cell, the oxygen adsorption ability of the NOx sensor is measured using this.

  Preferably, the measuring means executes any one of decrease, stop, and reverse rotation of the applied voltage for a predetermined time to the oxygen pump cell of the NOx sensor during the fuel cut of the internal combustion engine, and then applies the applied voltage to the original predetermined voltage. Then, the oxygen adsorption capacity is measured based on the NOx sensor output after the return.

  During the fuel cut that stops the fuel injection of the engine, air is supplied to the NOx sensor and its sensor cell. In addition, the oxygen pump cell of the NOx sensor decreases, stops, and reverses the applied voltage for a predetermined time. When either one is executed, oxygen can be more actively supplied to the electrode of the sensor cell to adsorb oxygen. Thereafter, by returning the applied voltage of the oxygen pump cell to the original predetermined state while the fuel cut is continued, the sensor output behavior corresponding to the oxygen adsorption amount of the sensor cell is obtained as described above. The oxygen adsorption capacity of is measured.

  Preferably, the abnormality diagnosis device for the NOx sensor corrects the output of the NOx sensor based on at least the oxygen adsorption capacity measured by the measuring unit when the determining unit determines that the NOx sensor is normal. Means.

  As described above, there is a correlation between the oxygen adsorption capacity of the sensor cell and the degree of NOx sensor deterioration. Therefore, based on the measured oxygen adsorption capacity, the output of the NOx sensor is corrected so as to keep the degree of deterioration of the NOx sensor constant. As a result, various controls using the NOx sensor output can be performed accurately and stably.

  According to the present invention, an excellent effect is achieved that abnormality diagnosis of a NOx sensor can be performed using a new parameter or characteristic correlated with a decrease in NOx concentration detection performance.

  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 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 the exhaust gas, and a DPR (Diesel Particulate Reduction) catalyst that captures particulate matter (PM) in the exhaust gas and removes it by combustion. 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 reduction type NOx catalyst (SCR: Selective Catalytic Reduction) 34 has a base material such as zeolite or alumina supporting a noble metal such as Pt, or a transition metal such as Cu supported 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 produced 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 the structure of the sensor portion of the NOx sensor 50. The sensor part of the NOx sensor 50 is composed of six oxygen ion conductive solid electrolyte layers such as zirconia oxide laminated on each other. 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 limiting member 62 and a second diffusion 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. A 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 control member 62 is in contact with the exhaust gas as the atmospheric gas. Accordingly, the exhaust gas flows 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.

  As described above, in the present embodiment, the cathode-side first pump electrode 67 and the anode-side first pump electrode 68 face each other of the first oxygen pump cell for discharging oxygen from the exhaust gas in the first chamber 64. Two electrode plates are configured.

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, a voltage (electromotive force) V ₀ indicated by reference numeral 11 is generated between the reference electrode 70 and the cathode-side first pump electrode 67. 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, the first voltage V ₀ matches the voltage generated when the oxygen concentration in the first chamber 64 is 1 ppm. The voltage of the 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.

  Thus, in the present embodiment, the reference electrode 70 and the cathode-side first pump electrode 67 face each other in the first oxygen concentration monitoring cell for monitoring the oxygen concentration in the exhaust gas in the first chamber 64. Two electrode plates are formed.

The cathode-side first pump electrode 67 is made of a material having a low reducing property with respect to NOx, for example, an alloy of gold (Au) and platinum (Pt). Therefore, in the first chamber 64, NO ₂ contained in the exhaust gas may be reduced to NO, but NO is not reduced any more. 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.

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.

  In other words, in the present embodiment, the cathode-side second pump electrode 72 and the anode-side first pump electrode 68 are respectively opposed to the second oxygen pump cell for discharging oxygen from the exhaust gas in the second chamber 65. One electrode plate is constructed.

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 detecting the NOx concentration in the exhaust gas, this voltage V ₁ matches the voltage generated when the oxygen concentration in the second chamber 65 is 0.01 ppm. The voltage of the second pump voltage source 73 is feedback controlled. 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.

  In other words, in the present embodiment, the reference electrode 70 and the cathode-side second pump electrode 72 are two facing each other of the second oxygen concentration monitoring cell for monitoring the oxygen concentration in the exhaust gas in the second chamber 65. Configure the electrode plate.

  The cathode-side second pump electrode 72 is also formed of a material having a low reducing property with respect to NOx, for example, an alloy of gold (Au) and platinum (Pt). Therefore, NOx contained in the exhaust gas is not reduced even when it contacts the cathode-side second pump electrode 72.

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 is made of a material having a strong reducing property with respect to NOx, for example, an alloy of rhodium (Rh) and platinum (Pt). 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 It becomes oxygen ions and moves in the third layer L ₃ toward the reference electrode 70. 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.

  That is, in this embodiment, the cathode pump electrode 75 and the reference electrode 70 constitute two electrode plates facing each other of the oxygen generation cell for decomposing NOx in the exhaust gas and newly generating oxygen. . Further, the cathode pump electrode 75 and the reference electrode 70 also constitute two electrode plates facing each other of the sensor cell for detecting the oxygen concentration generated by the oxygen generation cell.

As described above, NOx is almost converted into NO in the first chamber 64, and oxygen hardly exists in the second chamber 65. Therefore, the current I ₁ is proportional to the NOx concentration contained in the exhaust gas, and the NOx concentration in the exhaust gas can be detected from the current I ₁ .

An electric heater 79 for heating the sensor part of the NOx sensor 50 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 50.

  Next, abnormality diagnosis of the NOx sensor 50 will be described.

  As a result of intensive research, the present inventor has that the sensor cell including the cathode pump electrode 75 and the reference electrode 70 has an oxygen adsorption capacity, and that the oxygen adsorption capacity decreases as the NOx sensor 50 deteriorates. I found.

Among the sensor cells, the cathode-side pump electrode 75 located in the second chamber 65 is more specifically a porous cermet formed from an alloy of rhodium (Rh) and platinum (Pt) and zirconia oxide (ZrO ₂ ) as a ceramic. It is composed of. Of these, rhodium is a main component that gives the cathode pump electrode 75 a relatively strong catalytic ability capable of reducing NO as well as an oxygen adsorption ability such as adsorption of oxygen.

  Of the above-described cells, the sensor cell has the greatest influence on the NOx detection performance due to deterioration. The reason is that the cathode pump electrode 75 of the sensor cell finally decomposes NO and directly detects NOx, and the electrode 75 has a catalytic ability to decompose NO. This is because the catalytic performance is reduced with deterioration, and the stability is lacking compared to other Pt—Au electrodes.

  On the other hand, according to the research result of the present inventor, it has been found that as the NOx sensor, and thus the cathode pump electrode 75, deteriorates, the oxygen adsorption capacity of the cathode pump electrode 75 decreases. The reason is considered to be that the rhodium and platinum particles constituting the cathode pump electrode 75 are gradually aggregated due to the influence of heat, resulting in a decrease in gas diffusibility within the electrode and a decrease in active sites. .

  Therefore, the oxygen adsorption capacity of the sensor cell or the cathode-side pump electrode 75 correlates with the deterioration degree of the NOx sensor 50, and in this embodiment, the abnormality diagnosis of the NOx sensor 50 is performed using this fact.

The oxygen adsorption capacity of the sensor cell is measured, for example, as follows. FIG. 3 shows test results showing changes in the sensor output I ₁ during the warm-up of the NOx sensor 50. Here, when the engine is stopped, the NOx sensor 50 is left in the air at room temperature for a predetermined time, oxygen is adsorbed on the cathode pump electrode 75, and then the heater 79 of the NOx sensor 50 is turned on without starting the engine. At the same time, the NOx sensor 50 is turned on (voltage is applied to the first and second oxygen pump cells and the sensor cell), and the NOx sensor 50 is warmed up only by the heater 79. In the figure, a solid line indicates a new sensor, and a broken line indicates a deteriorated sensor after an endurance test. It is known that oxygen adsorption at the cathode-side pump electrode 75 is more likely to occur as the temperature of the electrode is lower, that it occurs reliably at about room temperature, and that it does not occur after the NOx sensor 50 is warmed up. For example, when the heater 79 of the NOx sensor 50 is controlled to a constant temperature in the range of about 750 to 800 ° C., oxygen adsorption does not occur.

When heating by the heater 79 is started from time t0, the sensor output I ₁ is a value corresponding to a NOx concentration of zero for a while, even though the internal combustion engine has not been started and there is no exhaust gas and NOx contained therein. It rises from I ₁₀ and behaves as if the NOx concentration has increased. This is because oxygen O ₂ adsorbed on the cathode-side pump electrode 75 while the sensor is left is decomposed on the cathode-side pump electrode 75 and moves toward the reference electrode 70 as oxygen ions. This is because a current I ₁ proportional to the amount of oxygen ions flows between 75 and the reference electrode 70. The rising sensor output I ₁ eventually returns to a value I ₁₀ equivalent to zero NOx concentration along with decomposition and desorption of adsorbed oxygen on the cathode pump electrode 75.

At this time, as can be seen from the figure, as the NOx sensor 50 deteriorates, the amount of adsorbed oxygen decreases and the behavior of the sensor output I ₁ decreases. That is, the peak value I ₁ P of the sensor output I ₁ is reduced, and the time during which the sensor output I ₁ based on the adsorbed oxygen appears is also reduced.

Therefore, in the present embodiment, the oxygen adsorption capacity of the sensor cell, more specifically, the oxygen adsorption capacity parameter as the index value is measured using the behavior of the sensor output I ₁ .

The first example of oxygen adsorption capacity measurement is to measure the oxygen adsorption capacity of the sensor cell based on the peak value I ₁ P of the sensor output I ₁ . That is, the ECU 100 acquires the peak value I ₁ P when the sensor output I ₁ rises, and uses the acquired value as the oxygen adsorption capacity of the sensor cell.

The second example of the oxygen adsorption capacity measurement is to measure the oxygen adsorption capacity of the sensor cell based on the integrated value of the sensor output I ₁ . That is, the ECU 100 sequentially integrates the sensor output I _1n for each predetermined sampling period while the sensor output I ₁ rises, and uses the final integrated value ΣI _1n as the oxygen adsorption capacity of the sensor cell. Here, the sensor output I _1n is a difference obtained by subtracting a value I ₁₀ corresponding to zero NOx concentration from the actual sensor output I ₁ .

The third example of oxygen adsorption capacity measurement is to measure the oxygen adsorption capacity of the sensor cell based on the rise time of the sensor output I ₁ . That is, the ECU 100 measures the time Δt during which the sensor output I ₁ is equal to or greater than the predetermined value I ₁ s, which is slightly larger than the value I ₁₀ equivalent to zero NOx concentration, and determines the rise time Δt as the oxygen adsorption capacity of the sensor cell. To do.

In any of these measurement methods, the parameter values I ₁ P, ΣI _1n , and Δt become smaller as the NOx sensor 50 deteriorates. Therefore, when these parameter values are equal to or less than a predetermined abnormality determination value, it is determined that the NOx sensor 50 is abnormal (deteriorated).

4 to 6, the sensor output peak value I ₁ P, the sensor output integrated value ΣI _1n, and the sensor output rise time Δt are used as the oxygen adsorption capacity parameters of the sensor cell. The graph showing the correlation of is shown. As shown in the figure, each parameter value gradually decreases as the NOx sensor deterioration degree increases. And each parameter value, the corresponding abnormality determination value I ₁ Ps, ΣI _1n s, is compared to .DELTA.ts, NOx sensor 50 if the following abnormality determination value is determined to be abnormal. Simultaneously with this abnormality determination, a warning device such as a check lamp is turned on, and the user is prompted to replace the NOx sensor 50.

By the way, using the correlation between each parameter value and the NOx sensor deterioration degree as illustrated, in this embodiment, when the NOx sensor 50 is determined to be normal, the NOx sensor output I ₁ is corrected. . Specifically, the detected NOx concentration C obtained based on the actual NOx sensor output I ₁ is the NOx concentration obtained when the NOx sensor has a certain degree of deterioration, that is, a new NOx sensor 50 in this embodiment. Thus, the detected NOx concentration C is corrected. FIG. 7 shows an example of the correction map. Based on the actually obtained NOx sensor output I ₁ and the parameter value, the NOx concentration correction amount ΔC is calculated from the map, and this NOx concentration correction amount ΔC is calculated as the actual NOx concentration. The corrected NOx concentration C ′ obtained by a new NOx sensor is obtained by adding to the detected NOx concentration C converted from the sensor output I ₁ . This corrected NOx concentration C ′ is used for various controls such as control of the amount of reducing agent added to the NOx catalyst as described above. The NOx concentration correction amount ΔC is set to be larger as the actual NOx sensor output I ₁ is larger and the parameter value is smaller. If the parameter value is a large value equivalent to a new NOx sensor, the NOx concentration correction amount ΔC is naturally zero. The correction method is not limited to such a method of adding correction amounts, and for example, a method of multiplying correction amounts is also possible.

  Next, specific abnormality diagnosis processing will be described with reference to FIG. The illustrated routine is repeatedly executed by the ECU 100 at predetermined intervals (for example, 16 msec).

  In the first step S101, it is determined whether or not a predetermined condition suitable for performing an abnormality diagnosis, in particular, a predetermined condition suitable for turning on the heater 79 of the NOx sensor 50 before starting the engine is determined. Note that such heater-on before starting the engine is referred to as preheating. For example, in the case of a vehicle in which the door is opened because an occupant enters the vehicle with the engine stopped, the door switch is turned on simultaneously with the opening of the door, and preheating is started, the door switch is turned on. one of. It is also preferable that one of the conditions is that the battery voltage is sufficiently high to withstand preheating. In addition, one condition may be that the engine switch 28 is in the accessory-on position before the cell starter-on position.

  If it is determined that the predetermined condition is not satisfied, this routine is terminated. On the other hand, if it is determined that the predetermined condition is satisfied, the heater 79 is turned on in step S102, and the NOx sensor 50 is warmed up. As soon as the heater is turned on, voltage application to each cell is started, and the NOx sensor 50 itself starts operating.

In the next step S103, it is determined whether or not the engine has been started, specifically, whether or not the cell starter has been turned on. When the engine is started, this routine is terminated. On the other hand, if the engine has not been started yet, the process proceeds to step S104, and any of the parameters indicating the oxygen adsorption capacity of the sensor cell, that is, the sensor output peak value I ₁ P, the sensor output integrated value ΣI _1n, and the sensor output rise time Δt. Is measured.

  Next, in step S105, it is determined whether or not measurement of the parameter has been completed. If the measurement has not been finished yet, this routine is finished. On the other hand, if the measurement is finished, the routine proceeds to step S106, where the abnormality determination of the NOx sensor 50 is executed. That is, the measured parameter is compared with the deterioration determination value corresponding to the measured parameter. When the parameter is larger than the deterioration determination value, the NOx sensor 50 is normal, and when the parameter is equal to or less than the deterioration determination value, the NOx sensor 50 is determined to be abnormal. Is done.

Next, in step S107, it is determined whether the determination result is abnormal. If the determination result is abnormal, a warning device such as a check lamp is turned on in step S108, a warning is given to the user, and this routine is terminated. On the other hand, if the determination result is normal, the process proceeds to step S109, where the measured value of the parameter is updated and stored, and this routine is terminated. As described above, this parameter measurement value is used when calculating a correction amount for correcting the NOx sensor output I ₁ at the subsequent NOx concentration detection.

  Next, another aspect of abnormality diagnosis will be described. One aspect of the abnormality diagnosis described above is to diagnose the abnormality by measuring the oxygen adsorption capacity based on the NOx sensor output during the preheating of the heater before starting the engine. On the other hand, in this other aspect, during the fuel cut after the engine is started, the oxygen pump cell of the NOx sensor is subjected to any one of decrease, stop and reverse of the applied voltage for a predetermined time, and then the applied voltage is restored to the original. It returns to a predetermined state, measures the oxygen adsorption capacity based on the NOx sensor output after the return, and diagnoses an abnormality.

  During a fuel cut that stops engine fuel injection, air is supplied to the NOx sensor 50, and this air enters the first chamber 64 and the second chamber 65. In addition to this, when any one of the decrease, stop and reverse of the applied voltage for a predetermined time is executed for the oxygen pump cell of the NOx sensor, the first chamber 64 and the second chamber 65 by the oxygen pump cell in the case of a decrease in the applied voltage. When the applied voltage is stopped, the amount of oxygen pumped out from the first chamber 64 and the second chamber 65 is zero, and when the applied voltage is reversed, Oxygen is pumped into the first chamber 64 and the second chamber 65 by the pump cell. In either case, oxygen is actively supplied to the cathode pump electrode 75 and oxygen is adsorbed to the cathode pump electrode 75. To be able to. In particular, the oxygen pump cell applied voltage is reduced, stopped, or reversed in combination with the fuel cut, so that even when the fuel cut time is short, the cathode pump electrode 75 can actively adsorb oxygen. it can. Thereafter, by returning the applied voltage of the oxygen pump cell to the original predetermined state while the fuel cut is continued, the sensor output behavior as shown in FIG. The oxygen adsorption capacity of the sensor 50 can be measured and abnormality diagnosis of the NOx sensor 50 can be executed.

  In the present embodiment, the oxygen pump cell referred to here means a first oxygen pump cell including a cathode-side first pump electrode 67 and an anode-side first pump electrode 68, a cathode-side second pump electrode 72, and an anode-side. It means at least one of the second oxygen pump cell including the first pump electrode 68. Therefore, either the decrease, stop, or reverse of the applied voltage may be executed for both oxygen pump cells, or any of the decrease, stop, or reverse of the applied voltage may be executed for either one of the oxygen pump cells. Also good. The method of applying the applied voltage may be changed between one and the other such that the applied voltage is reversed for one oxygen pump cell and the applied voltage is stopped for the other oxygen pump cell. Of course, in order to adsorb as much oxygen as possible to the cathode pump electrode 75, it is most preferable to reverse the applied voltages of both oxygen pump cells.

  A specific abnormality diagnosis process according to another aspect will be described with reference to FIG. The illustrated routine is repeatedly executed by the ECU 100 at predetermined intervals (for example, 16 msec).

  In first step S201, it is determined whether or not a predetermined condition suitable for performing abnormality diagnosis is satisfied. Here, an essential condition for satisfying the predetermined condition is that the engine is in a fuel cut. When the accelerator opening detected by the accelerator opening sensor 27 is substantially fully closed and the engine speed calculated based on the output of the crank angle sensor 16 is equal to or higher than a predetermined speed slightly higher than the idle, the fuel Cut (deceleration fuel cut) is executed. In addition, it is preferable to include that the engine is warmed up and that the NOx sensor 50 has been activated.

  If it is determined that the predetermined condition is not satisfied, this routine is terminated. On the other hand, if it is determined that the predetermined condition is satisfied, the process proceeds to step S202, and the applied voltage of the oxygen pump cell is decreased, stopped, or reversed in order to actively adsorb oxygen to the cathode pump electrode 75. In order to more actively adsorb oxygen at this time, it is preferable to reduce or stop the heating of the heater 79 of the NOx sensor 50 in order to lower the temperature of the cathode pump electrode 75.

  Next, in step S203, it is determined whether or not a predetermined time has elapsed since the decrease, stop, or reverse rotation start of the applied voltage of the oxygen pump cell. This predetermined time is set as a time such that a necessary amount of oxygen is adsorbed to the cathode pump electrode 75.

  If the predetermined time has not yet elapsed, this routine is terminated. On the other hand, if the predetermined time has elapsed, the process proceeds to step S204, where the applied voltage of the oxygen pump cell is restored to the original predetermined state.

  Thereafter, steps S205 to S210 similar to steps S104 to S109 are executed, that is, a parameter indicating the oxygen adsorption capacity of the sensor cell is measured, and abnormality determination of the NOx sensor 50 is performed based on the measured parameter. Executed.

  The abnormality diagnosis described above is not affected by the NOx catalyst 34 located upstream of the NOx sensor 50 in any aspect. That is, in the case of the abnormality diagnosis performed during the preheating of the heater 79 of the NOx sensor 50, the exhaust gas is discharged from the combustion chamber of the engine in order to measure the oxygen adsorption capacity before starting the engine, that is, while the engine is stopped. The measurement is performed when the atmospheric gas of the NOx sensor 50 is air. Further, in the case of another aspect of abnormality diagnosis performed during engine fuel cut, the oxygen adsorption capacity is measured when the air exhausted from the combustion chamber of the engine and passing through the NOx catalyst 34 is supplied to the NOx sensor 50. Done. Therefore, there is an advantage that diagnosis in an engine mounted state or in-vehicle state that is generally difficult can be suitably performed.

  As mentioned above, although embodiment of this invention was described, this invention can also take other embodiment. For example, the present invention can be applied to an arbitrary internal combustion engine, and can be applied to, for example, a spark ignition internal combustion engine, particularly a direct injection lean burn gasoline engine, in addition to a compression ignition internal combustion engine. In addition to the urea SCR system, the present invention can be applied to any form of exhaust purification system as an exhaust purification system. The present invention can also be applied to a NOx sensor other than the structure shown in FIG.

  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.

1 is a schematic system diagram of an internal combustion engine according to an embodiment of the present invention. It is sectional drawing which shows the structure of the sensor part of a NOx sensor. It is a graph which shows the change of the sensor output in warming up of a NOx sensor. It is a graph which shows the correlation with the oxygen adsorption capacity parameter of a sensor cell, and a NOx sensor deterioration degree, and is a case where a sensor output peak value is used as a parameter. It is a graph which shows the correlation with the oxygen adsorption capacity parameter of a sensor cell, and a NOx sensor deterioration degree, and is a case where a sensor output integrated value is used as a parameter. It is a graph which shows the correlation with the oxygen adsorption capacity parameter of a sensor cell, and a NOx sensor degradation degree, and is a case where sensor output rise time is used as a parameter. It is a figure which shows an example of a correction map. It is a flowchart which shows the abnormality diagnosis process which concerns on the one aspect | mode of abnormality diagnosis. It is a flowchart which shows the abnormality diagnosis process which concerns on another aspect of abnormality diagnosis.

Explanation of symbols

10 Engine 15 Exhaust passage 26 Crank angle sensor 27 Accelerator opening sensor 34 Selective reduction type NOx catalyst 40 Urea addition valve 50 NOx sensor 67 Cathode side first pump electrode 68 Anode side first pump electrode 70 Reference electrode 72 Cathode side second pump Electrode 75 Cathode side pump electrode 79 Heater 100 Electronic control unit (ECU)
I ₁ NOx sensor output I ₁ P NOx sensor output peak value ΣI _1n NOx sensor output integrated value Δt NOx sensor output rise time

Claims (3)

An abnormality diagnosis device for a NOx sensor provided in an exhaust passage of an internal combustion engine,
Measuring means for measuring the oxygen adsorption capacity in the sensor cell of the NOx sensor;
Determination means for determining abnormality of the NOx sensor based on the oxygen adsorption capacity measured by the measurement means ,
The measuring means turns on the heater of the NOx sensor before starting the internal combustion engine, and measures the oxygen adsorption capacity based on the output of the NOx sensor while the heater is turned on.
An abnormality diagnosis apparatus for a NOx sensor.   An abnormality diagnosis device for a NOx sensor provided in an exhaust passage of an internal combustion engine,
  Measuring means for measuring the oxygen adsorption capacity in the sensor cell of the NOx sensor;
  Determination means for determining abnormality of the NOx sensor based on the oxygen adsorption capacity measured by the measurement means;
  With
  The measuring means executes any one of decrease, stop, and reverse rotation of the applied voltage for a predetermined time to the oxygen pump cell of the NOx sensor during the fuel cut of the internal combustion engine, and then restores the applied voltage to the original predetermined state. The oxygen adsorption capacity is measured based on the NOx sensor output after the return.
  An abnormality diagnosis apparatus for a NOx sensor.   When the determination means determines that the NOx sensor is normal, the correction means includes a correction means for correcting the output of the NOx sensor based on at least the oxygen adsorption capacity measured by the measurement means.
  The abnormality diagnosis apparatus for a NOx sensor according to claim 1 or 2.

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