JP2018131993A - Diagnostic device of exhaust sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration in emission and furthermore to exactly diagnose the responsiveness of an exhaust sensor.SOLUTION: A diagnostic device of an exhaust sensor comprises: a prohibition part for prohibiting fuel addition from a fuel addition valve in a first period being a period in which an absolute change rate after a start of the execution of fuel cut processing reaches a maximum absolute change rate, and in a second period being a prescribed period in the execution of the fuel cut processing after a lapse of a reference period after the start of the execution of the fuel cut processing; a calculation part for calculating a responsiveness index value by dividing the maximum absolute change rate by a value of an output signal when the absolute change rate reaches the maximum absolute change rate, and a convergence value of the output signal based on a value of an output signal in the second period; and a diagnosis part for diagnosing the responsiveness of the exhaust sensor on the basis of the responsiveness index value.SELECTED DRAWING: Figure 7A

Description

  The present invention relates to an exhaust sensor diagnostic apparatus.

  In a limit current type exhaust sensor (for example, an air-fuel ratio sensor or NOx sensor), the air-fuel ratio is detected in the air-fuel ratio sensor and the NOx concentration in the exhaust gas is detected in the NOx sensor by detecting the magnitude of the limit current value. be able to.

  Patent Document 1 discloses a technique for diagnosing an output abnormality of an air-fuel ratio sensor during execution of a fuel cut process.

  In addition, in order to maintain the function of the exhaust purification catalyst of the internal combustion engine, a technique for adding fuel from a fuel addition valve provided in the exhaust passage is known.

Japanese Patent Laying-Open No. 2015-075082 JP 2011-117462 A JP 2008-291762 A JP 2003-214245 A

  When the fuel cut process is executed, the oxygen concentration in the exhaust gas changes relatively greatly and the oxygen concentration converges to a predetermined concentration. Therefore, during the fuel cut process, an output corresponding to the oxygen concentration in the exhaust gas is output. It becomes easy to diagnose the output abnormality of the exhaust sensor to be performed.

  Here, in the limit current type exhaust sensor, although the responsiveness of the sensor has not decreased, the rate of change of the sensor output may decrease (for example, the period until the sensor output converges is normal) However, this corresponds to a state in which the magnitude of the limit current value has decreased). Therefore, even if the fuel cut process is being performed in which it is easy to diagnose the output abnormality of the exhaust sensor, if the sensor responsiveness is diagnosed based only on the sensor output change rate, the sensor responsiveness may not be accurately diagnosed. There is.

  On the other hand, in order to maintain the function of the exhaust purification catalyst even during execution of the fuel cut process, fuel addition from the fuel addition valve provided in the exhaust passage can be performed (the fuel cut process refers to the in-cylinder operation). This represents a process of stopping the supply of fuel, and the fuel addition from the fuel addition valve provided in the exhaust passage can be executed regardless of the fuel cut process). When such fuel addition is performed during execution of the fuel cut process, the oxygen concentration in the exhaust gas can vary according to the fuel addition. In this case, although the function of the exhaust purification catalyst can be maintained, it becomes difficult to diagnose an output abnormality of the exhaust sensor. That is, in this case, it becomes difficult to diagnose the responsiveness of the sensor even during the fuel cut process.

  The present invention has been made in view of the above problems, and an object thereof is to accurately diagnose the responsiveness of an exhaust sensor while suppressing deterioration of emissions.

  In order to solve the above-described problems, an exhaust sensor diagnostic apparatus according to the present invention is a detection unit that detects an oxygen concentration in exhaust gas, and includes an exhaust-side electrode that is exposed to exhaust and an atmospheric-side electrode that is exposed to the atmosphere. A detection unit comprising: a pair of electrodes configured by: a porous diffusion rate-controlling layer that introduces exhaust into the exhaust-side electrode and controls the speed of exhaust reaching the exhaust-side electrode; and An exhaust sensor that is provided so as to cover and has a plurality of vent holes, is provided in an exhaust passage of the internal combustion engine, and outputs an output signal corresponding to the oxygen concentration in the exhaust, and the exhaust sensor A fuel addition valve provided in the upstream exhaust passage, and fuel cut processing means for executing fuel cut processing for stopping the supply of fuel into the cylinder of the internal combustion engine during operation of the internal combustion engine. The internal combustion engine An exhaust sensor diagnostic apparatus, wherein a period from the start of execution of the fuel cut process to the convergence of the output signal that changes with the execution of the fuel cut process is defined as a reference period, and the output in the reference period When the maximum absolute change rate, which is the absolute value of the signal change rate, is defined as the maximum absolute change rate, this is a period from when the fuel cut process starts until the absolute change rate reaches the maximum absolute change rate. Prohibition of fuel addition from the fuel addition valve for one period and a second period during the fuel cut process and a predetermined period after the reference period has elapsed since the start of the fuel cut process And a calculation unit for calculating a responsiveness index value having a correlation with the responsiveness of the exhaust sensor, wherein the output when the absolute change rate becomes the maximum absolute change rate is the maximum absolute change rate. A calculation unit for calculating the responsiveness index value by dividing by the difference between the value of the signal and the convergence value of the output signal based on the value of the output signal in the second period; and the responsiveness index value And a diagnostic unit for diagnosing the responsiveness of the exhaust sensor based on.

  According to the present invention, it is possible to accurately diagnose the responsiveness of the exhaust sensor while suppressing deterioration of emissions.

1 is a diagram illustrating a schematic configuration of an internal combustion engine and an intake / exhaust system thereof according to a first embodiment. It is a typical expanded sectional view of the air-fuel ratio sensor vicinity in FIG. FIG. 3 is a cross-sectional view taken along line A-A ′ in FIG. 2. It is a figure which shows the time transition of the fuel cut process execution flag, the output of an air fuel ratio sensor, the absolute change rate, and the maximum absolute change rate when the fuel cut process is executed. It is a figure which shows the time transition of the output of a fuel addition process execution flag, a fuel cut process execution flag, and an air fuel ratio sensor when a fuel addition process and a fuel cut process are performed. FIG. 6 is a diagram showing a time transition of a fuel addition process execution flag, a fuel cut process execution flag, an output of an air-fuel ratio sensor, and a maximum absolute change rate when a fuel addition process, a fuel cut process, and a fuel addition prohibition process are executed. is there. It is a 1st flowchart which shows the control flow performed in the diagnostic apparatus of the exhaust sensor which concerns on 1st embodiment. It is a 2nd flowchart which shows the control flow performed in the diagnostic apparatus of the exhaust sensor which concerns on 1st embodiment. It is a figure which shows schematic structure of the internal combustion engine which concerns on 2nd embodiment, and its intake / exhaust system. It is a typical expanded sectional view of the NOx sensor vicinity in FIG. FIG. 10 is a sectional view taken along line B-B ′ in FIG. 9.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be exemplarily described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the component parts described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified.

(First embodiment)
<Configuration of internal combustion engine and its intake / exhaust system>
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine and its intake / exhaust system according to the present embodiment. An internal combustion engine 1 shown in FIG. 1 is a compression ignition type internal combustion engine (diesel engine). However, the present invention can also be applied to a spark ignition type lean burn internal combustion engine using gasoline or the like as fuel.

  The internal combustion engine 1 includes a fuel injection valve 3 that injects fuel into the cylinder. When the internal combustion engine 1 is a spark ignition type internal combustion engine, the fuel injection valve 3 may be configured to inject fuel into the intake port.

  The internal combustion engine 1 is connected to the intake passage 4. An air flow meter 40 and a throttle valve 41 are provided in the intake passage 4. The air flow meter 40 outputs an electrical signal corresponding to the amount (mass) of intake air (air) flowing through the intake passage 4. The throttle valve 41 is disposed downstream of the air flow meter 40 in the intake passage 4. The throttle valve 41 adjusts the intake air amount of the internal combustion engine 1 by changing the passage cross-sectional area in the intake passage 4.

  The internal combustion engine 1 is connected to the exhaust passage 5. In the exhaust passage 5, the fuel addition valve 2, the first air-fuel ratio sensor 6a, the oxidation catalyst 51, the second air-fuel ratio sensor 6b, and the selective reduction type NOx catalyst 52 (hereinafter referred to as “SCR catalyst 52”) are sequentially arranged in the exhaust flow. In some cases, a temperature sensor 53 and a third air-fuel ratio sensor 6c are provided. Here, the temperature sensor 53 outputs an electrical signal corresponding to the temperature of the exhaust. The first air-fuel ratio sensor 6a, the second air-fuel ratio sensor 6b, and the third air-fuel ratio sensor 6c output an electric signal corresponding to the air-fuel ratio of the exhaust, and details will be described later. In the present embodiment, these air-fuel ratio sensors correspond to the exhaust sensors in the present invention.

  The fuel addition valve 2 adds fuel to the exhaust gas flowing through the exhaust passage 5. The oxidation catalyst 51 oxidizes HC, CO, etc. flowing into the catalyst. The SCR catalyst 52 reduces NOx flowing into the catalyst. More specifically, ammonia generated by hydrolysis of urea supplied by a urea water addition valve (not shown) is adsorbed on the SCR catalyst 52, and NOx in the exhaust is reduced using the adsorbed ammonia as a reducing agent. . Here, when the temperature of the SCR catalyst 52 is lower than the activation temperature of the catalyst, the NOx purification ability of the catalyst is lowered. Therefore, in the present embodiment, fuel addition from the fuel addition valve 2 is executed in order to maintain the temperature of the SCR catalyst 52 higher than the activation temperature of the catalyst. Details will be described later.

  The internal combustion engine 1 is provided with an electronic control unit (ECU) 20. The ECU 20 is a unit that controls the operating state and the like of the internal combustion engine 1. In addition to the air flow meter 40, the first air-fuel ratio sensor 6a, the second air-fuel ratio sensor 6b, and the third air-fuel ratio sensor 6c, the ECU 20 is electrically connected with various sensors such as an accelerator opening sensor 7 and a crank position sensor 8. Connected. The accelerator opening sensor 7 is a sensor that outputs an electrical signal correlated with an operation amount (accelerator opening) of an accelerator pedal (not shown). The crank position sensor 8 is a sensor that outputs an electrical signal correlated with the rotational position of the engine output shaft (crankshaft) of the internal combustion engine 1. Then, the output signals of these sensors are input to the ECU 20. The ECU 20 derives the engine load of the internal combustion engine 1 based on the output signal of the accelerator opening sensor 7 and derives the engine rotation speed of the internal combustion engine 1 based on the output signal of the crank position sensor 8. Further, the ECU 20 estimates the flow rate of exhaust discharged from the internal combustion engine 1 (hereinafter also referred to as “exhaust flow rate”) based on the output value of the air flow meter 40, and sets the output value of the temperature sensor 53. Based on this, the temperature of the SCR catalyst 52 (hereinafter sometimes referred to as “SCR catalyst temperature”) is estimated.

  Various devices such as the fuel addition valve 2, the fuel injection valve 3, and the throttle valve 41 are electrically connected to the ECU 20. These various devices are controlled by the ECU 20. For example, when the ECU 20 detects that the accelerator is turned off by the output from the accelerator opening sensor 7, the ECU 20 stops the fuel injection from the fuel injection valve 3 during the operation of the internal combustion engine 1, that is, the supply of fuel into the cylinder. A fuel cut process is executed to stop the operation. In addition, when ECU20 performs a fuel cut process, it functions as a fuel cut process means which concerns on this invention.

  Here, as described above, in order to maintain the temperature of the SCR catalyst 52 higher than the activation temperature of the catalyst, fuel addition from the fuel addition valve 2 is executed. Specifically, the ECU 20 adds fuel to the exhaust gas flowing through the exhaust passage 5 using the fuel addition valve 2 based on the estimated SCR catalyst temperature. When fuel is added in this way, the fuel flows into the oxidation catalyst 51 provided in the exhaust passage 5 downstream of the fuel addition valve 2. As a result, the fuel is oxidized in the oxidation catalyst 51, and heat is generated accordingly. The generated heat is supplied to the SCR catalyst 52 provided in the exhaust passage 5 downstream of the oxidation catalyst 51, whereby the temperature of the SCR catalyst 52 is maintained higher than the activation temperature of the catalyst.

  Note that the ECU 20 can also perform fuel addition from the fuel addition valve 2 in order to maintain the temperature of the SCR catalyst 52 higher than the activation temperature of the catalyst even during execution of the fuel cut processing. If such fuel addition is performed during the fuel cut process, the oxygen concentration in the exhaust gas is likely to fluctuate according to the fuel addition. In particular, the oxygen concentration in the exhaust after the oxidation catalyst 51 is likely to fluctuate. This is because, as described above, since the fuel is oxidized in the oxidation catalyst 51, if such fuel addition is performed during the fuel cut process, oxygen is consumed in the oxidation catalyst 51 as the fuel is oxidized. This is because that.

<Structure of air-fuel ratio sensor>
Next, the structure of the first air-fuel ratio sensor 6a, the second air-fuel ratio sensor 6b, and the third air-fuel ratio sensor 6c will be briefly described with reference to FIGS. Here, since these air-fuel ratio sensors have the same structure, they will be simply referred to as “air-fuel ratio sensors” in the following description of these air-fuel ratio sensors. FIG. 2 is a schematic enlarged sectional view of the vicinity of the air-fuel ratio sensor in FIG. 3 is a cross-sectional view taken along line AA ′ in FIG.

  In FIG. 2, the air-fuel ratio sensor includes a sensor main body 10 to be described later and a protective cover 9 that is a heat-resistant housing member that covers the sensor main body 10 and a part of which is exposed to the exhaust passage 5. The The mechanical strength of the sensor body 10 is ensured by being covered by the protective cover 9.

  As shown in FIG. 3, a plurality of ventilation holes 9a are formed on the surface of the protective cover 9, and the inside and outside of the protective cover 9 communicate with each other. That is, the air-fuel ratio sensor is configured such that the exhaust gas flowing through the exhaust passage 5 passes through the vent hole 9a of the protective cover 9 and reaches the sensor body 10. In FIG. 3, the protective cover 9 has a single structure, but the protective cover 9 may have a double structure in which a vent 9a is formed.

  Next, a schematic configuration of the sensor body 10 will be described. The sensor body 10 includes a sensor element 11 made of an oxygen ion conductive solid electrolyte. The sensor element 11 is made of, for example, zirconium oxide (zirconia). An exhaust side electrode 12 is formed on one side surface of the sensor element 11, and an atmosphere side electrode 13 is formed on the other side surface. The exhaust side electrode 12 and the atmosphere side electrode 13 are made of a metal material having high catalytic activity such as platinum. By forming the exhaust side electrode 12 and the atmosphere side electrode 13 in this way, the sensor element 11 is sandwiched between the pair of electrodes.

  A diffusion rate-determining layer 14 is laminated on the side surface of the exhaust side electrode 12 opposite to the side surface on the sensor element 11 side. The diffusion control layer 14 is laminated so as to cover the exhaust side electrode 12 and one side surface of the sensor element 11 on which the exhaust side electrode 12 is formed. The diffusion control layer 14 is a member made of a porous material such as ceramics, and has a function of controlling the diffusion of exhaust gas. That is, the diffusion-controlling layer 14 is made fine and dense so that various components in the exhaust can diffuse at an appropriate diffusion rate. A protective layer 16 is laminated on the side surface of the diffusion rate limiting layer 14 opposite to the side surface on the sensor element 11 side. A gas chamber 15 is formed between the sensor element 11 and the diffusion control layer 14. Exhaust gas is introduced into the gas chamber 15 via the diffusion-controlling layer 14. Here, since the exhaust-side electrode 12 is disposed in the gas chamber 15, the exhaust-side electrode 12 is exposed to the exhaust through the diffusion rate controlling layer 14. Note that the gas chamber 15 is not necessarily provided, and may be configured such that the diffusion-controlling layer 14 is in direct contact with the surface of the exhaust-side electrode 12.

  A heater layer 17 is laminated on the other side surface of the sensor element 11. A heater 18 is embedded in the heater layer 17, and the heater 18 heats the sensor body 10 to a desired activation temperature (for example, 700 ° C.) by receiving power supply from an external electric circuit (not shown). Can do. This electric circuit is electrically connected to the ECU 20, and the electric power supplied to the heater 18 is controlled by the ECU 20. An atmospheric chamber 19 is formed between the sensor element 11 and the heater layer 17. The atmosphere chamber 19 communicates with the atmosphere through an air hole (not shown), and the atmosphere-side electrode 13 is kept exposed to the atmosphere even when the air-fuel ratio sensor is disposed in the exhaust passage 5. Is done. In the present embodiment, the sensor body 10 described above corresponds to the detection unit in the present invention.

Here, the principle of detecting the air-fuel ratio of the exhaust by the air-fuel ratio sensor will be described. Exhaust gas introduced into the protective cover 9 from the air holes 9 a flows into the diffusion-controlling layer 14 and proceeds while diffusing toward the exhaust-side electrode 12. The exhaust contains a reducing agent such as CO and HC and an oxidizing agent such as O ₂ and NOx. These components react with each other until reaching the equilibrium state in the process of reaching the surface of the exhaust-side electrode 12 or after reaching the exhaust-side electrode 12. When the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio, both the oxidizing agent and the reducing agent disappear. On the other hand, when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, the reducing agent remains, and when it is the lean air-fuel ratio, the oxidant remains.

  Here, a region composed of the exhaust side electrode 12, the atmosphere side electrode 13, and the sensor element 11 sandwiched therebetween is referred to as “cell 25”. In the present embodiment, a predetermined applied voltage is applied between the exhaust side electrode 12 and the atmosphere side electrode 13 via a power supply line (not shown). When oxidant remains in the exhaust gas reaching the surface of the exhaust-side electrode 12 with an applied voltage applied between the electrodes (when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio) In this case, oxygen in the exhaust gas becomes oxygen ions and propagates from the exhaust-side electrode 12 to the atmosphere-side electrode 13 via the sensor element 11, whereby a current flows through the cell 25. On the other hand, when the reducing agent remains on the exhaust side electrode 12 side (when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio), oxygen in the atmosphere becomes oxygen ions and the sensor element 11 Then, the electric current propagates from the atmosphere side electrode 13 to the exhaust side electrode 12 and reacts with the reducing agent, whereby a current in the direction opposite to the above flows in the cell 25. Note that when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio, in principle, oxygen ions do not propagate between the electrodes via the sensor element 11, so that no current flows through the cell 25.

When the diffusion rate of the exhaust gas is controlled by the diffusion-controlling layer 14, a region where the current value is saturated uniformly even when the applied voltage is increased is generated. This current value is referred to as a limiting current value. Here, when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, since the diffusion of the oxidant to the exhaust-side electrode 12 is rate-controlled by the diffusion rate-limiting layer, the current is saturated to the limit current value, When the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, the current is saturated to the limit current value because the diffusion of the reducing agent to the exhaust-side electrode 12 is limited by the diffusion rate-limiting layer. By detecting these limit current values, the air-fuel ratio of the exhaust can be detected.

In such an air-fuel ratio sensor, if PM, an oil component, or the like adheres to the vent hole 9 a of the protective cover 9, the amount of exhaust flowing into the protective cover 9 per unit time tends to decrease. When the inflow amount of exhaust gas per unit time into the protective cover 9 decreases, the concentration of O ₂ , HC, CO, NOx, etc. in the exhaust gas changes according to the operating state of the internal combustion engine 1. However, it takes a certain amount of time for these concentrations in the exhaust gas in the protective cover 9 to change to concentrations corresponding to the operating state of the internal combustion engine 1. Therefore, until the concentration of O ₂ , HC, CO, NOx, etc. in the exhaust in the protective cover 9 changes to a concentration corresponding to the operating state of the internal combustion engine 1, the output of the air-fuel ratio sensor is the operating state of the internal combustion engine 1. Therefore, the output corresponding to the density corresponding to is not obtained. That is, the responsiveness of the air-fuel ratio sensor is lowered.

  Further, in the above air-fuel ratio sensor, when PM, an oil component, or the like adheres to the diffusion control layer 14, the porosity of the diffusion control layer 14 is likely to be lowered. On the other hand, when the diffusion control layer 14 is deteriorated, the porosity of the diffusion control layer 14 tends to increase. Here, when the porosity of the diffusion control layer 14 decreases, the resistance when exhaust passes through the diffusion control layer 14 becomes larger than when the porosity is high. Therefore, the lower the porosity of the diffusion control layer 14, the more easily the current flowing through the cell 25 is saturated with a small current. That is, the lower the porosity of the diffusion rate controlling layer 14, the easier it is to reduce the magnitude of the limiting current value.

  In the air-fuel ratio sensor described above, the influence on the sensor output due to the change in the response of the air-fuel ratio sensor (change in the period until the output of the air-fuel ratio sensor converges) and the magnitude of the limit current value The effect on the sensor output due to the change is combined with the output of the air-fuel ratio sensor.

<Response diagnosis of air-fuel ratio sensor>
Next, a method for diagnosing the responsiveness of the air-fuel ratio sensor will be described. The ECU 20, which is an exhaust sensor diagnostic apparatus according to the present invention, correlates the sensor response with the sensor response in order to eliminate the influence on the sensor output due to a change in the magnitude of the limit current value. A responsiveness index value is calculated. Based on this responsiveness index value, the responsiveness of the sensor is diagnosed. Hereinafter, a method for calculating the responsiveness index value will be described.

  FIG. 4 shows time transitions of the fuel cut process execution flag and the output of the air-fuel ratio sensor when the fuel cut process is executed in the internal combustion engine 1. Here, the period from the start of execution of the fuel cut process to the convergence of the output of the air-fuel ratio sensor that changes with the execution of the fuel cut process is defined as a reference period. In FIG. 4, the absolute value of the change rate of the output of the air-fuel ratio sensor in the reference period (hereinafter referred to as “absolute change rate”) along with the time transition of the fuel cut processing execution flag and the output of the air-fuel ratio sensor may be referred to. )) And the time transition of the maximum value of the absolute change rate (hereinafter also referred to as “maximum absolute change rate”). Here, the output of the air-fuel ratio sensor represents the output of the second air-fuel ratio sensor 6b in FIG. 1 as an example, and the absolute change rate and the maximum absolute change rate are values based on the output of the second air-fuel ratio sensor 6b. It shall be expressed as an example.

In the control shown in FIG. 4, the internal combustion engine 1 is performing a normal operation from time t0 to time t1 (at this time, fuel cut processing and fuel addition processing described later are not executed). At this time, the air-fuel ratio of the exhaust gas becomes the air-fuel ratio corresponding to the normal operation state of the internal combustion engine 1, and in FIG. 4, the output value of the air-fuel ratio sensor is substantially constant at C0. When the fuel cut process execution flag changes from 0 to 1 at time t1 and the ECU 20 executes the fuel cut process, the air-fuel ratio of the exhaust gas changes to a leaner side than before after a certain delay time has elapsed. Then, when this exhaust gas passes through the vent hole 9a of the protective cover 9 of the air-fuel ratio sensor and reaches the sensor body 10, the output value of the air-fuel ratio sensor starts to increase from C0.

  In the process of increasing the output of the air-fuel ratio sensor, the output increase becomes relatively slow immediately after the start of the output increase, and the output increase tends to increase after a certain period of time has elapsed since the start of the output increase. Therefore, the output value of the air-fuel ratio sensor begins to increase from C0 at time t2, and then the absolute change rate increases, and at time t3, the absolute change rate becomes R1. After time t3, the absolute change rate decreases with time, and the output value of the air-fuel ratio sensor converges to C1 at time t4 when the absolute change rate becomes substantially zero.

  Here, the ECU 20 acquires the output value of the air-fuel ratio sensor in the process of increasing the output of the air-fuel ratio sensor, and calculates the absolute change rate. Then, the maximum absolute change rate is calculated based on the absolute change rate. In FIG. 4, the maximum absolute change rate is R1. Further, the ECU 20 determines whether the output of the air-fuel ratio sensor has converged, for example, according to a method described later. A predetermined period after the reference period has elapsed since the fuel cut process was started (in other words, after the time t4 when the fuel cut process was being performed and the output of the air-fuel ratio sensor converged). Based on the output value of the air-fuel ratio sensor during a predetermined period), the convergence value of the output of the air-fuel ratio sensor is calculated. Then, the ECU 20 divides the maximum absolute change rate by the difference between the output value of the air-fuel ratio sensor when the absolute change rate becomes the maximum absolute change rate and the convergence value of the output of the air-fuel ratio sensor. Responsiveness index value is calculated.

  By calculating the responsiveness index value in this way, it is possible to eliminate the influence on the sensor output due to the change in the magnitude of the limit current value. Therefore, by diagnosing the responsiveness of the sensor based on this responsiveness index value, the responsiveness of the sensor can be accurately diagnosed.

  However, when fuel is added from the fuel addition valve 2 in order to maintain the temperature of the SCR catalyst 52 higher than the activation temperature of the catalyst during execution of the fuel cut processing, the responsiveness index value is set to It becomes difficult to calculate. This will be described with reference to FIG. The process of adding fuel to the exhaust flowing through the exhaust passage 5 using the fuel addition valve 2 performed by the ECU 20 is referred to as “fuel addition process”.

  FIG. 5 is a diagram showing time transitions of the fuel addition process execution flag, the fuel cut process execution flag, and the output of the air-fuel ratio sensor when the fuel addition process and the fuel cut process are executed in the internal combustion engine 1. In the time transition of the output of the air-fuel ratio sensor in FIG. 5, a line L1 represented by a solid line represents a time transition when the fuel addition process is executed, and a line L2 represented by a broken line represents a fuel addition process. FIG. 4 shows the time transition when the control is not executed (the same as the time transition of the output of the air-fuel ratio sensor shown in FIG. 4 above) for reference. Further, the output of the air-fuel ratio sensor represents the output of the second air-fuel ratio sensor 6b in FIG. 1 as an example.

In the control shown in FIG. 5, the internal combustion engine 1 performs normal operation (at this time, the fuel cut process and the fuel addition process are not executed) from time t0 to time t11. When the fuel addition process execution flag changes from 0 to 1 at time t11 and the ECU 20 executes the fuel addition process, the air-fuel ratio of the exhaust in the exhaust passage 5 downstream of the fuel addition valve 2 becomes richer than before. Change. As a result, the output value of the air-fuel ratio sensor starts to decrease from C0. In particular, in the second air-fuel ratio sensor 6b provided in the exhaust passage 5 downstream of the oxidation catalyst 51, oxygen is consumed as the fuel is oxidized in the oxidation catalyst 51, so the output value of the air-fuel ratio sensor is compared. Easy to change. Here, the fuel addition process is intermittently executed at a predetermined cycle. Therefore, the ECU 20 temporarily stops the fuel addition after adding the fuel from the fuel addition valve 2 for a certain period from time t11. Once the fuel addition is stopped, the output value of the air-fuel ratio sensor, which has decreased from C0, starts to increase. Further, in the control shown in FIG. 5, the fuel cut process execution flag is changed from 0 to 1 at time t1 after time t11, and the fuel cut process is executed. The output value of the sensor is larger than C0.

  When the fuel cut processing is executed, the output value of the air-fuel ratio sensor increases from C0 in this way, but at the timing when the next fuel addition is performed in the process of this increase, the output of the air-fuel ratio sensor The value temporarily decreases than before. That is, the output value of the air-fuel ratio sensor increases while fluctuating according to the fuel addition process that is executed intermittently at a predetermined cycle. Therefore, when the fuel addition process is executed, the absolute change rate does not become the maximum at time t3 as shown in FIG. 4 above, and the maximum absolute change rate in this case is affected by the fuel addition process. End up. In addition, when the fuel addition process is executed even after time t4 when the output of the air-fuel ratio sensor represented by the line L2 in FIG. 5 converges, the output value of the air-fuel ratio sensor varies according to the fuel addition process. Keep doing. Therefore, it becomes difficult to calculate the convergence value of the output of the air-fuel ratio sensor.

  In addition, although FIG. 5 demonstrated the case where the output of the said 2nd air fuel ratio sensor 6b in FIG. 1 was used as an output of an air fuel ratio sensor, the situation where it becomes difficult to calculate the responsiveness index value as mentioned above is as follows. The same applies not only when the output of the second air-fuel ratio sensor 6b is used but also when the output of the first air-fuel ratio sensor 6a or the third air-fuel ratio sensor 6c is used.

<Fuel addition prohibition processing>
Therefore, the ECU 20 is executing the period from the start of execution of the fuel cut process until the absolute change rate reaches the maximum absolute change rate (hereinafter also referred to as “first period”), and the fuel cut process. Fuel addition from the fuel addition valve 2 is prohibited during a predetermined period after the reference period has elapsed since the start of execution of the fuel cut processing (hereinafter, also referred to as “second period”). Such processing performed by the ECU 20 is referred to as “fuel addition prohibition processing”. The ECU 20 executes the fuel addition prohibiting process, thereby functioning as a prohibiting unit according to the present invention.

  FIG. 6 shows the fuel addition process execution flag, the fuel cut process execution flag, the output of the air-fuel ratio sensor, and the maximum absolute change when the fuel addition process, the fuel cut process, and the fuel addition prohibition process are executed in the internal combustion engine 1. It is a figure which shows the time transition of a rate. In the time transition of the output of the air-fuel ratio sensor in FIG. 6, a line L3 represented by a solid line represents a time transition when the fuel addition prohibiting process is executed, and a line L4 represented by a broken line represents a fuel addition A time transition when the prohibition process is not executed (same as the line L1 shown in FIG. 5 above) is shown for reference. Further, the output of the air-fuel ratio sensor represents the output of the second air-fuel ratio sensor 6b in FIG. 1 as an example, and the maximum absolute change rate represents the value based on the output of the second air-fuel ratio sensor 6b as an example. To do.

  In the control shown in FIG. 6, as in the control shown in FIG. 5, the fuel addition process execution flag is changed from 0 to 1 at time t11, and the ECU 20 executes the fuel addition process. Then, at time t1 after time t11, the fuel cut processing execution flag is changed from 0 to 1, and when the fuel cut processing is executed, the fuel addition processing execution flag is changed from 1 to 0, and the fuel addition from the fuel addition valve 2 is performed. Is stopped. Thereafter, the output value of the air-fuel ratio sensor increases and the absolute change rate reaches the maximum value at time t3. At time t3, the fuel addition process execution flag is changed from 0 to 1, and the ECU 20 restarts the fuel addition process. That is, the ECU 20 adds the fuel that prohibits the fuel addition from the fuel addition valve 2 in the first period from the start of execution of the fuel cut process (time t1) until the absolute change rate reaches the maximum absolute change rate (time t3). Prohibition processing is being executed.

As described above, when the fuel addition prohibition process is executed in the first period, the value may vary in accordance with the fuel addition from the fuel addition valve 2 in the process of increasing the output value of the air-fuel ratio sensor. Disappear. Then, the maximum absolute change rate is calculated in the first period in which the fuel addition prohibition process is being executed. Therefore, the maximum absolute change rate can be calculated without being affected by the fuel addition process. This is not limited to the case where the output of the second air-fuel ratio sensor 6b is used as the output of the air-fuel ratio sensor, and the same applies to the case where the output of the first air-fuel ratio sensor 6a or the third air-fuel ratio sensor 6c is used.

  Further, in the control shown in FIG. 6, the fuel addition process execution flag is changed from 1 to 0 at time t41 after time t4, which is the time when the output of the air-fuel ratio sensor converges when the fuel addition process is not executed. Fuel addition from the fuel addition valve 2 is stopped. Then, at time t42 when a predetermined period Δt4 has elapsed from time t41, the fuel addition processing execution flag is changed from 0 to 1, and the fuel addition processing is resumed. That is, the ECU 20 performs the fuel from the fuel addition valve 2 during the second period, which is a predetermined period (Δt4) after the reference period has elapsed (time t4) during execution of the fuel cut process and during the execution of the fuel cut process. A fuel addition prohibition process for prohibiting the addition is performed.

  As described above, when the fuel addition prohibition process is executed in the second period, the output value of the air-fuel ratio sensor does not fluctuate according to the fuel addition from the fuel addition valve 2. Therefore, it becomes easy to calculate the convergence value of the output of the air-fuel ratio sensor. This is not limited to the case where the output of the second air-fuel ratio sensor 6b is used as the output of the air-fuel ratio sensor, and the same applies to the case where the output of the first air-fuel ratio sensor 6a or the third air-fuel ratio sensor 6c is used.

  In the control shown in FIG. 6, the fuel addition from the fuel addition valve 2 is performed by the fuel addition prohibition process only in the first period for calculating the maximum absolute change rate and the second period for calculating the convergence value of the output of the air-fuel ratio sensor. Since the fuel addition from the fuel addition valve 2 is performed during other periods, the temperature of the SCR catalyst 52 can be made as high as possible. As a result, emission deterioration is suppressed.

  Here, a control flow executed by the ECU 20, which is an exhaust sensor diagnostic apparatus according to the present invention, will be described with reference to FIGS. 7A and 7B. 7A and 7B are flowcharts showing a control flow according to the present embodiment. In the present embodiment, the ECU 20 repeatedly executes this flow at a predetermined calculation cycle while the internal combustion engine 1 is in operation. In the present embodiment, this flow is executed at a predetermined execution interval, and the predetermined execution interval is defined as a period Δt. In the description of this flow, the first air-fuel ratio sensor 6a, the second air-fuel ratio sensor 6b, and the third air-fuel ratio sensor 6c are simply referred to as “air-fuel ratio sensors”.

  In this flow, first, in S101, it is determined whether or not an execution condition for executing a response diagnosis of the air-fuel ratio sensor is satisfied. In S101, after diagnosing the responsiveness of the air-fuel ratio sensor last time, for example, when a vehicle on which the internal combustion engine 1 is mounted travels a predetermined distance, or when the internal combustion engine 1 operates for a certain time, or the internal combustion engine 1 When the engine is stopped and then restarted, an affirmative determination is made. Note that the above is an example, and in S101, it is possible to determine whether or not an execution condition for executing the responsiveness diagnosis of the air-fuel ratio sensor is established based on a known technique. If an affirmative determination is made in S101, the ECU 20 proceeds to the process of S102. If a negative determination is made in S101, the execution of this flow is terminated.

If an affirmative determination is made in S101, it is then determined in S102 whether or not a fuel cut process is being executed. When the fuel cut process is executed, the oxygen concentration in the exhaust gas fluctuates relatively greatly, and the oxygen concentration converges to a predetermined concentration. Therefore, it becomes possible to calculate a response index value Valin described later. If an affirmative determination is made in S102, the ECU 20 proceeds to the process of S103. If a negative determination is made in S102, the ECU 20
Advances to the process of S127.

  If an affirmative determination is made in S102, it is then determined in S103 whether or not a calculation completion flag nflagr, which is a flag indicating whether or not the calculation of the maximum absolute change rate has been completed, is zero. The calculation completion flag nflagr is a flag that is set to 1 when the calculation of the maximum absolute change rate is completed. The value of the calculation completion flag nflagr is set to 1 by the process of S112 described later, or is initialized to 0 by the processes of S126 and S129 described later and stored in the ROM of the ECU 20. If an affirmative determination is made in S103, the calculation of the maximum absolute change rate has not been completed. In this case, the ECU 20 proceeds to the process of S104. On the other hand, when a negative determination is made in S103, the calculation of the maximum absolute change rate has been completed. In this case, the ECU 20 proceeds to the process of S114.

  If an affirmative determination is made in S103, then fuel addition from the fuel addition valve 2 is prohibited in S104. Here, when the fuel addition process is performed before the process of S104, the fuel addition from the fuel addition valve 2 is stopped by the process of S104. The fuel addition process is executed according to a known flow different from this flow. In the control flow of the fuel addition process, if the fuel addition process execution condition is satisfied (for example, whether or not the fuel addition process needs to be executed based on the SCR catalyst temperature estimated by the temperature sensor 53). Is determined), a flag is set (based on the flag, it can be determined whether or not the fuel addition process is being performed). When the fuel addition from the fuel addition valve 2 is prohibited in S104, the prohibition of fuel addition is continued during the subsequent execution of this flow. However, this is not the case when the process for restarting fuel addition from the fuel addition valve 2 is executed in S113, S121, or S128.

  Next, in S105, the current value of the output value of the air-fuel ratio sensor (hereinafter also referred to as “current output value”) Crtnow is acquired. In S105, the current value of the limit current value corresponding to the air-fuel ratio of the exhaust that reaches the sensor body 10 of the air-fuel ratio sensor is acquired as the output current value Crtnow.

  Next, in S106, the current value Rcnow of the absolute change rate is calculated. In S106, the absolute value of the value obtained by subtracting the past value of the output value of the air-fuel ratio sensor (hereinafter also referred to as “output past value”) Crold from the current output value Crtnow acquired in S105 is divided by the period Δt. As a result, the current value Rcnow of the absolute change rate is calculated. Here, the output past value Crold is updated by the processing of S110 described later, or initialized by the processing of S126 and S129 described later, and stored in the ROM of the ECU 20.

  Next, in S107, it is determined whether or not the current value Rcnow of the absolute change rate calculated in S106 is larger than the maximum value Rcmax of the absolute change rate. Here, the maximum value Rcmax of the absolute change rate is updated by the process of S108 described later, or the value is initialized by the processes of S126 and S129 described later, and stored in the ROM of the ECU 20. If an affirmative determination is made in S107, the ECU 20 proceeds to the process of S108, and if a negative determination is made in S107, the ECU 20 proceeds to the process of S110.

When an affirmative determination is made in S107, next, the maximum value Rcmax of the absolute change rate is updated in S108. In S108, the maximum value Rcmax of the absolute change rate is updated to the current value Rcnow of the absolute change rate calculated in S106. Next, in S109, the output value (hereinafter also referred to as “first output value”) Crt1 of the air-fuel ratio sensor when the absolute change rate becomes the maximum value is acquired. In S109, the current output value Crtnow at this time is acquired as the first output value Crt1.

  Next, in S110, the output past value Crold is updated. In S110, the output past value Crtold is updated to the output current value Crtnow acquired in S105.

  Next, in S111, it is determined whether or not the calculation of the maximum absolute change rate has been completed. As shown in FIG. 4 above, the absolute change rate starts to increase after a certain amount of time has elapsed from the start of execution of the fuel cut process, and decreases after the value reaches the maximum absolute change rate. Therefore, in S111, for example, the maximum value Rcmax of the absolute change rate is updated in the previous execution of the flow, and the maximum value Rcmax of the absolute change rate is not updated in the execution of the current flow (that is, a negative determination is made in S107). The maximum absolute change rate when the absolute change rate maximum value Rcmax is not updated after the absolute change rate maximum value Rcmax is updated (ie, a negative determination is made in S107). It is determined that the calculation has been completed. At this time, the maximum absolute change rate Rcmax stored in the ROM of the ECU 20 becomes the maximum absolute change rate. If an affirmative determination is made in S111, the ECU 20 proceeds to the process of S112, and if a negative determination is made in S111, the ECU 20 proceeds to the process of S114.

  If an affirmative determination is made in S111, the calculation completion flag nflagr is set to 1 in S112. In S113, when the execution condition of the fuel addition process is satisfied (in this case, the execution flag of the fuel addition process is set), the fuel addition from the fuel addition valve 2 is resumed. .

  Next, in S114, a convergence determination flag nflagc for determining the convergence of the output of the air-fuel ratio sensor is read. The convergence determination flag nflagc is a flag that is set to 1 when the output of the air-fuel ratio sensor is estimated to converge, and is set according to a known flow different from this flow, and stored in the ROM of the ECU 20. Is done. In S114, the convergence determination flag nflagc stored in the ROM of the ECU 20 is read. For example, the convergence determination flag nflagc is set as follows. First, an integrated value of the amount of air that has passed through the cylinder since the start of the fuel cut process is calculated. When the integrated value is equal to or greater than a predetermined threshold value, it is determined that scavenging in the cylinder has been completed. Then, the elapsed time after completion of scavenging in the cylinder is counted, and when the elapsed time exceeds a predetermined time, the convergence determination flag nflagc is set to 1. On the other hand, when scavenging in the cylinder is not completed, or when the elapsed time is less than the predetermined time, the convergence determination flag nflagc is set to 0.

  Next, in S115, it is determined whether or not the convergence determination flag nflagc read in S114 is 1. If the determination in step S115 is affirmative, the output of the air-fuel ratio sensor is estimated to converge. In this case, the ECU 20 proceeds to the process of step S116. On the other hand, when a negative determination is made in S115, it is estimated that the output of the air-fuel ratio sensor does not converge. In this case, the execution of this flow is terminated.

  If an affirmative determination is made in S115, then fuel addition from the fuel addition valve 2 is prohibited in S116. The process of S116 is substantially the same as the process of S104 described above.

  Next, in S117, the current output value Crtnow is acquired. The process of S117 is substantially the same as the process of S105 described above.

Next, in S118, the integrated value Crtsum of the current output value is calculated. In S118, the integrated value Crtsum of the current output value is calculated by integrating the output current value Crtnow acquired in S117. In S119, 1 is added to the integration counter nct indicating the number of integration of the current output value. Then, in S120, it is determined whether or not the value of the integration counter nct is a threshold value nctth. If an affirmative determination is made in S120, the ECU 20 proceeds to the process of S121. If a negative determination is made in S120, the execution of this flow is terminated.

  If an affirmative determination is made in S120, then, in S121, if the fuel addition process execution condition is satisfied (in this case, the fuel addition process execution flag is set), the fuel addition valve. Fuel addition from 2 is resumed.

  Next, in S122, an output convergence value (hereinafter also referred to as “second output value”) Crt2 of the air-fuel ratio sensor is calculated. In S122, the second output value Crt2 is calculated by dividing the integrated value Crtsum of the current output value calculated in S118 by the value of the integration counter nct. Here, since the fuel addition from the fuel addition valve 2 is prohibited when the current output value Crtnow is acquired in S117, the integrated value Crtsum of the current output value calculated in S118 is influenced by the fuel addition process. I do not receive it. Therefore, the second output value Crt2 can be calculated relatively accurately by using the integrated value Crtsum of the current output value. Next, in S123, a difference Crtdif between the first output value Crt1 and the second output value Crt2 is calculated.

  Next, in S124, the responsiveness index value Valin is calculated. In S124, the responsiveness index value Valin is calculated by dividing the absolute value Rcmax of the absolute change rate (this represents the maximum absolute change rate) by the difference Crtdif calculated in S123. Next, in S125, the responsiveness of the air-fuel ratio sensor is diagnosed based on the responsiveness index value Valin. In S125, as a diagnosis of the responsiveness of the air-fuel ratio sensor, for example, when the responsiveness index value Valin is large, the responsiveness of the air-fuel ratio sensor is estimated so that the responsiveness of the air-fuel ratio sensor becomes higher than when it is small. be able to. Further, for example, an abnormality in the responsiveness of the air-fuel ratio sensor can be diagnosed based on the responsiveness index value Valin. Note that the ECU 20 functions as a calculation unit according to the present invention by calculating the responsiveness index value Valin, and the ECU 20 functions as a diagnosis unit according to the present invention by diagnosing the responsiveness of the air-fuel ratio sensor.

  Next, in S126, the output past value Crold, the absolute change rate maximum value Rcmax, the calculation completion flag nflagr, the output current value integrated value Crtsum, and the integration counter nct are initialized. In S126, the past output value Crold is initialized to a limit current value corresponding to the air-fuel ratio of the exhaust during normal operation of the internal combustion engine 1 (at this time, fuel cut processing and fuel addition processing are not executed). Further, the maximum value Rcmax of the absolute change rate, the calculation completion flag nflagr, the integrated value Crtsum of the current output value, and the integrated counter nct are initialized to 0. Then, after the process of S126, the execution of this flow is terminated.

  If a negative determination is made in S102, it is then determined in S127 whether fuel addition from the fuel addition valve 2 is prohibited. If an affirmative determination is made in S127, then in S128, if the execution condition for the fuel addition process is satisfied at this time (in this case, the execution flag for the fuel addition process is set). The fuel addition from the fuel addition valve 2 is resumed. On the other hand, if a negative determination is made in S127, the ECU 20 proceeds to the process of S129.

Next, in S129, the output past value Crold, the absolute change rate maximum value Rcmax, the calculation completion flag nflagr, the output current value integrated value Crtsum, and the integration counter nct are initialized. As a result, these values are initialized even if the fuel cut processing is completed before the diagnosis of the responsiveness of the air-fuel ratio sensor by this flow is completed. The process of S129 is substantially the same as the process of S126 described above. Then, after the process of S129, the execution of this flow is terminated.

  The exhaust sensor diagnostic apparatus according to the present invention calculates the responsiveness index value Valin as described above, and diagnoses the responsiveness of the air-fuel ratio sensor based on the responsiveness index value Valin, thereby deteriorating emissions. It is possible to accurately diagnose the responsiveness of the air-fuel ratio sensor while suppressing this.

(Second embodiment)
<Configuration of internal combustion engine and its intake / exhaust system>
FIG. 8 is a diagram showing a schematic configuration of the internal combustion engine and its intake / exhaust system according to the present embodiment. An internal combustion engine 1 shown in FIG. 8 is a compression ignition type internal combustion engine (diesel engine), and includes a fuel injection valve 3 that injects fuel into a cylinder. In the present embodiment, detailed description of substantially the same configuration and substantially the same control processing as those of the above-described first embodiment will be omitted.

  The internal combustion engine 1 is connected to the exhaust passage 5. In the exhaust passage 5, the fuel addition valve 2, the first NOx sensor 60 a, the NOx storage reduction catalyst 54 (hereinafter also referred to as “NSR catalyst 54”), and the second NOx sensor 60 b are sequentially arranged in accordance with the exhaust flow. Is provided. Details of the first NOx sensor 60a and the second NOx sensor 60b will be described later. In the present embodiment, these NOx sensors correspond to the exhaust sensor in the present invention.

  The NSR catalyst 54 chemically stores or physically adsorbs NOx contained in the exhaust when the air-fuel ratio of the exhaust is high, and releases the NOx while releasing NOx when the air-fuel ratio of the exhaust is low. The reaction with the reducing components (for example, hydrocarbon (HC), carbon monoxide (CO), etc.) in the exhaust is promoted. Here, when the amount of NOx chemically occluded or physically adsorbed by the NSR catalyst 54 increases, the NOx occlusion capacity of the NSR catalyst 54 is saturated, and the amount of NOx discharged into the atmosphere may increase. . Therefore, in the present embodiment, fuel addition from the fuel addition valve 2 is executed in order to purge NOx chemically occluded or physically adsorbed by the NSR catalyst 54.

<Structure of NOx sensor>
Next, the structure of the first NOx sensor 60a and the second NOx sensor 60b will be briefly described with reference to FIGS. Here, since these NOx sensors have the same structure, they are simply referred to as “NOx sensors” in the following description of these NOx sensors. FIG. 9 is a schematic enlarged sectional view of the vicinity of the NOx sensor in FIG. FIG. 10 is a cross-sectional view taken along line BB ′ in FIG.

  The NOx sensor according to the present embodiment includes a sensor main body 100 and a protective cover 90 as in the air-fuel ratio sensor of the first embodiment. A plurality of ventilation holes 90 a are formed on the surface of the protective cover 90. In FIG. 10, the protective cover 90 has a single structure, but the protective cover 90 may have a double structure in which a vent hole 90a is formed.

Next, a schematic configuration of the sensor main body 100 will be described. Similar to the air-fuel ratio sensor of the first embodiment, the sensor main body 100 of the NOx sensor according to this embodiment includes exhaust-side electrodes 120a, 120b, 120c that are exposed to exhaust, and an atmospheric-side electrode 130a that is exposed to the atmosphere. , 130b, 130c, sensor elements 110, 210 sandwiched between the exhaust-side electrode and the atmosphere-side electrode, and a porous diffusion-controlling layer 140 that determines the speed of exhaust reaching the exhaust-side electrode. And as shown in FIG. 10, the area | region which consists of an exhaust side electrode, an atmosphere side electrode, and the sensor element pinched | interposed among these is provided. These regions are referred to as “pump cell 250a”, “monitor cell 250b”, and “sensor cell 250c”.

  A first gas chamber 150a and a second gas chamber 150b are formed between the sensor element 110 and the sensor element 210. Exhaust gas is introduced into the first gas chamber 150a through the diffusion rate limiting layer 140 and the pinhole 220. In addition, an air chamber 190 a that is in communication with the atmosphere is formed between the sensor element 110 and the heater layer 170. Here, when voltage is applied to the exhaust-side electrode 120a and the atmosphere-side electrode 130a forming the pump cell 250a, oxygen in the first gas chamber 150a can be discharged to the atmosphere chamber 190a by a pumping action.

  Further, the exhaust gas that has passed in the vicinity of the pump cell 250a flows into the second gas chamber 150b that communicates with the first gas chamber 150a via the throttle 240. Here, when oxygen in the first gas chamber 150a is discharged to the atmospheric chamber 190a by the pump cell 250a, the second gas chamber 150b is brought into a predetermined low oxygen concentration state. In addition, by stacking the spacer 230 on the sensor element 210, an atmospheric chamber 190b is formed between the sensor element 210 and the spacer 230. Note that the atmosphere chamber 190b communicates with the atmosphere. The oxygen remaining in the second gas chamber 150b moves to the atmosphere chamber 190b by a pumping action by applying a voltage to the exhaust side electrode 120b and the atmosphere side electrode 130b forming the monitor cell 250b. Here, the exhaust-side electrode 120b is configured as an electrode that is inactive with respect to NOx reductive decomposition, whereby an output current corresponding to the residual oxygen concentration is obtained.

  Further, in the exhaust-side electrode 120c and the atmosphere-side electrode 130c forming the sensor cell 250c, the exhaust-side electrode 120c is configured as an electrode that is active against the reductive decomposition of NOx. Then, by applying a voltage to the exhaust side electrode 120c and the atmosphere side electrode 130c forming the sensor cell 250c, NOx and residual oxygen in the second gas chamber 150b move to the atmosphere chamber 190b by the pumping action. At this time, an output current corresponding to the NOx concentration and the residual oxygen concentration is obtained. Then, the NOx concentration can be detected by subtracting the output current of the monitor cell 250b from the output current of the sensor cell 250c.

  Thus, the NOx sensor according to this embodiment outputs a current corresponding to the oxygen concentration in the first gas chamber 150a in the pump cell 250a, and outputs a current corresponding to the oxygen concentration in the second gas chamber 150b in the monitor cell 250b. The sensor cell 250c outputs a current corresponding to the NOx concentration and oxygen concentration in the second gas chamber 150b. That is, the NOx sensor according to the present embodiment outputs a current corresponding to the oxygen concentration in the exhaust gas. And since the diffusion of the exhaust gas to the exhaust side electrodes 120a, 120b, 120c is rate-controlled by the diffusion rate controlling layer 140, there is a region where the current is constantly saturated even when the applied voltage is increased. That is, a limit current is generated.

  In such a NOx sensor, if PM, an oil component, or the like adheres to the vent hole 90a of the protective cover 90, similarly to the air-fuel ratio sensor of the first embodiment, the responsiveness of the NOx sensor may be reduced. Further, when PM, an oil component, or the like adheres to the diffusion control layer 140, the magnitude of the limit current value may be reduced. And the influence on the output of the sensor due to the change in the responsiveness of the NOx sensor (change in the period until the output of the NOx sensor converges) and the influence on the output of the sensor due to the change in the magnitude of the limit current value Together, this is the output of the NOx sensor. Therefore, if the responsiveness of the NOx sensor is diagnosed based only on the rate of change of the output of the NOx sensor, the diagnostic accuracy may be reduced.

<Response diagnosis and fuel addition prohibition processing>
Therefore, the ECU 20 calculates a responsiveness index value having a correlation with the responsiveness of the NOx sensor, as in the first embodiment. Based on this responsiveness index value, the responsiveness of the NOx sensor is diagnosed. Here, in calculating the responsiveness index value, the ECU 20 performs the fuel addition prohibition process in the same manner as in the first embodiment, thereby chemically treating the NSR catalyst 54 during the fuel cut process. Even when fuel addition from the fuel addition valve 2 is performed to purge the occluded or physically adsorbed NOx, the responsiveness index value can be suitably calculated. Thereby, it is possible to accurately diagnose the responsiveness of the NOx sensor while suppressing the deterioration of emission.

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 2 ... Fuel addition valve 3 ... Fuel injection valve 4 ... Intake passage 5 ... Exhaust passage 6a ... First air-fuel ratio sensor 6b ... 2nd air-fuel ratio sensor 6c ... 3rd air-fuel ratio sensor 9 ... protective cover 9a ... vent hole 10 ... sensor body 11 ... sensor element 12 ... exhaust side electrode 13 ... Atmosphere side electrode 14 ... diffusion-controlled layer 20 ... ECU
40 ... Air flow meter 51 ... Oxidation catalyst 52 ... SCR catalyst 54 ... NSR catalyst 60a ... First NOx sensor 60b ... Second NOx sensor

Claims (1)

A detection unit that detects an oxygen concentration in exhaust gas, and includes a pair of electrodes composed of an exhaust side electrode exposed to the exhaust and an atmosphere side electrode exposed to the atmosphere, and introduces exhaust into the exhaust side electrode And a porous diffusion rate-determining layer that determines the speed of the exhaust gas reaching the exhaust-side electrode, and a detection unit,
A protective cover provided to cover the detection unit and having a plurality of ventilation holes;
An exhaust sensor that is provided in the exhaust passage of the internal combustion engine and outputs an output signal corresponding to the oxygen concentration in the exhaust;
A fuel addition valve provided in the exhaust passage upstream of the exhaust sensor;
Fuel cut processing means for executing fuel cut processing for stopping the supply of fuel into the cylinder of the internal combustion engine during operation of the internal combustion engine;
An exhaust sensor diagnostic device for an internal combustion engine comprising:
The period from the start of execution of the fuel cut process to the convergence of the output signal that changes with the execution of the fuel cut process is defined as a reference period, and the absolute change that is the absolute value of the change rate of the output signal in the reference period When the maximum value of the rate is the maximum absolute change rate, a first period that is a period from the start of execution of the fuel cut processing until the absolute change rate becomes the maximum absolute change rate, and execution of the fuel cut processing A prohibition unit for prohibiting fuel addition from the fuel addition valve in a second period which is a predetermined period after the reference period has elapsed from the start of execution of the fuel cut processing,
A calculation unit for calculating a responsiveness index value correlated with the responsiveness of the exhaust sensor, wherein the maximum absolute change rate is a value of the output signal when the absolute change rate becomes the maximum absolute change rate; A calculation unit for calculating the responsiveness index value by dividing by the difference between the output signal convergence value based on the value of the output signal in the second period,
A diagnostic unit for diagnosing the responsiveness of the exhaust sensor based on the responsiveness index value;
An exhaust sensor diagnostic apparatus comprising:

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