JP2018003702A - Abnormality determination device of air-fuel ratio sensor - Google Patents

Abstract

Provided is an abnormality determination device for an air-fuel ratio sensor, which is capable of specifying a sensor output switching point with higher accuracy regardless of an intake air flow rate. A dead time corresponding to a switching delay time of an output of an air-fuel ratio sensor is acquired during active air-fuel ratio control in which an air-fuel ratio of an engine is stepwise changed between a rich air-fuel ratio and a lean air-fuel ratio. . At that time, the determination device obtains the detected air-fuel ratio obtained by converting the detection value of the air-fuel ratio sensor into the air-fuel ratio as dispersion calculation data every predetermined time, and obtains the variance of the plurality of dispersion calculation data. . Then, the point in time when the variance changes from less than the predetermined threshold to the threshold or more is specified as the sensor output switching point, and the dead time is acquired from the sensor output switching time. When determining the variance, the determination device increases the certain time as the flow rate of the air taken into the engine is smaller. [Selection diagram] FIG.

Description

  The present invention relates to an abnormality determination device for an air-fuel ratio sensor that outputs a value corresponding to an air-fuel ratio of exhaust gas of an engine that is disposed in an exhaust passage of an internal combustion engine and that flows through the disposed portion.

  An air-fuel ratio sensor disposed in an exhaust passage of an internal combustion engine may be called, for example, an air-fuel ratio of an air-fuel mixture supplied to the engine (hereinafter referred to as “engine air-fuel ratio” or “control air-fuel ratio”). ) Changes from a rich air-fuel ratio (an air-fuel ratio smaller than the stoichiometric air-fuel ratio) to a lean air-fuel ratio (an air-fuel ratio larger than the stoichiometric air-fuel ratio), or changes from a lean air-fuel ratio to a rich air-fuel ratio From this time, it starts to change so as to output a value corresponding to the changed control air-fuel ratio with a delay of a time called a dead time (see time Ls in FIG. 3). If this dead time is significantly longer than the expected time, the air-fuel ratio feedback using the value output by the air-fuel ratio sensor (hereinafter referred to as “sensor value of the air-fuel ratio sensor” or “sensor value”). It is difficult to control properly.

Accordingly, one of conventionally known “air-fuel ratio sensor abnormality determination devices” (hereinafter referred to as “conventional device”) is as follows.
(1) Obtain the sensor value of the air-fuel ratio sensor or the variance of the detected air-fuel ratio by converting the sensor value to the air-fuel ratio,
(2) Based on this dispersion, “the time point at which the sensor value begins to change so as to output a value corresponding to the changed control air-fuel ratio (hereinafter also referred to as“ the sensor output switching time point ”) is specified. And
(3) The time from when the control air-fuel ratio is switched to when the sensor output is switched is acquired as wasted time,
(4) Abnormality determination of the air-fuel ratio sensor is performed using this dead time, or the gain of the air-fuel ratio feedback control is changed (for example, see Patent Document 1).

Japanese Unexamined Patent Application Publication No. 2008-267883 (see paragraphs 0004, 0086 to 0090 and FIG. 12)

  The reason why the conventional device uses the variance to identify the time point when the sensor output is switched is to specify the time point when the sensor output is switched more accurately even when noise is superimposed on the sensor value of the air-fuel ratio sensor. is there.

  However, when the flow rate of air supplied to the internal combustion engine (hereinafter, sometimes simply referred to as “intake air flow rate”) is small, compared to when the intake air flow rate is large, the control air-fuel ratio is changed after the switching time. A change amount (that is, a change speed) per unit time of the sensor value of the air-fuel ratio sensor becomes small. Therefore, since the amount of change with respect to the unit time of the dispersion is small, when it is intended to specify the time point when the dispersion becomes larger than the threshold value as the sensor output switching time point, the threshold value must be set to a small value. Therefore, if noise is superimposed on the sensor value of the air-fuel ratio sensor, the variance exceeds the small threshold value, so that an incorrect time point may be specified as the sensor output switching time point.

  The present invention has been made to address the above-described problems. That is, one of the objects of the present invention is to specify the time point at which the sensor output is switched using the variance of the sensor value of the air-fuel ratio sensor or the variance of the detected air-fuel ratio obtained by converting the sensor value of the air-fuel ratio sensor to the air-fuel ratio. Another object of the present invention is to provide an abnormality determination device for an air-fuel ratio sensor, which can more accurately specify the time point at which the sensor output is switched regardless of the intake air flow rate.

The air-fuel ratio sensor abnormality determination device of the present invention (hereinafter also referred to as “the device of the present invention”)
An abnormality determination device for an air-fuel ratio sensor (56) that outputs a value corresponding to an air-fuel ratio of exhaust gas of the engine that is disposed in an exhaust passage of the internal combustion engine (10) and that flows through the disposed portion,
Engine air-fuel ratio changing means (step 1040) for stepwise changing the air-fuel ratio of the air-fuel mixture supplied to the engine between a predetermined rich air-fuel ratio and a predetermined lean air-fuel ratio;
A sensor value (Vabyfs) that is a value output by the air-fuel ratio sensor or a detected air-fuel ratio (AFS) obtained by converting the sensor value into an air-fuel ratio is acquired as dispersion calculation data every time a predetermined time period elapses, Dispersion calculating means (step 610, step 710, step 810 and step 810) for calculating the variance (σ4, σ8, σ12, σ16) of the plurality of data based on a plurality of data temporally continuous among the data for dispersion calculation 910),
The time point when the variance changes from less than a predetermined threshold value to the threshold value or more is specified as the sensor output switching time point (steps 625 and 630, step 725 and step 730, (steps 825 and step 830, step 925 and step 930). ), From the control air-fuel ratio switching time (for example, tRL) when the air-fuel ratio of the air-fuel mixture supplied to the engine is changed stepwise between the rich air-fuel ratio and the lean air-fuel ratio. The dead time that is the time until the output switching time is acquired (step 635, step 735, step 835, step 935), and the time based on the dead time (for example, the dead time itself or an average value of a plurality of dead times) ) Is equal to or longer than a predetermined threshold time, an output reversal delay abnormality of the air-fuel ratio sensor occurs. And abnormality determining means determines that there (step 640 through step 655, step 740 to step 755, step 840 to step 855, step 940 to step 955),
In the air fuel ratio sensor abnormality determination device (70) comprising:
The variance calculating means includes
The certain time is configured to be longer as the flow rate of air sucked into the engine is smaller (step 615, step 715, step 815, step 915).

  According to this configuration, the sampling period of the dispersion calculation data (the predetermined time) becomes longer as the flow rate of air taken into the engine is smaller. Therefore, since the apparent change speed of the “sensor value and detected air-fuel ratio of the air-fuel ratio sensor” is small when the intake air flow rate is small, the dispersion change speed when the sensor output is switched also increases. Therefore, even when the intake air flow rate is small and noise is superimposed on the sensor value of the air-fuel ratio sensor, it is possible to specify the sensor output switching time point with high accuracy. As a result, it is possible to reduce the possibility that the air-fuel ratio sensor in which the output inversion delay abnormality has not occurred is erroneously determined as the air-fuel ratio sensor in which the output inversion delay abnormality has occurred.

In one aspect of the device of the present invention,
The abnormality determining means includes
It is determined whether or not a time difference obtained by subtracting a reference dead time that changes based on the flow rate of air sucked into the engine from the dead time is equal to or greater than a predetermined value, and the time difference is the predetermined value. When it is determined as above, the dead time is determined to be equal to or longer than the predetermined threshold time (steps 640 to 655, steps 740 to 755, steps 840 to 855, and steps 940 to 940). Step 955).

  Even in a normal air-fuel ratio sensor in which an output reversal delay abnormality has not occurred, the detected value inevitably includes a dead time, and the dead time becomes smaller as the intake air flow rate increases. The dead time exhibited by a normal air-fuel ratio sensor is referred to as a reference dead time. Therefore, the time difference obtained by subtracting this reference dead time from the “acquired dead time” is a value that accurately represents the inversion delay time of the output of the air-fuel ratio sensor that is going to be determined to be abnormal regardless of the intake air flow rate. . Therefore, as in the above aspect, it is possible to accurately determine whether or not an output inversion delay abnormality has occurred by determining whether or not this time difference is greater than or equal to a predetermined value.

  In the above description, in order to help understanding of the present invention, names and / or symbols used in the embodiment are attached to the configuration of the invention corresponding to the embodiment described later in parentheses. However, each component of the present invention is not limited to the embodiment defined by the reference numerals. Other objects, other features and attendant advantages of the present invention will be readily understood from the description of the embodiments of the present invention described with reference to the following drawings.

FIG. 1 is a schematic diagram of an internal combustion engine to which an “air-fuel ratio sensor abnormality determination device” according to an embodiment of the present invention is applied. FIG. 2 is a graph showing the relationship between the air-fuel ratio of exhaust gas and the output value of the air-fuel ratio sensor. FIG. 3 is a time chart showing the control air-fuel ratio and the detected air-fuel ratio during active air-fuel ratio control. FIG. 4 is a time chart illustrating the detected air-fuel ratio and dispersion. FIG. 5 is a diagram for explaining various dispersions. FIG. 6 is a flowchart showing a routine executed by the CPU shown in FIG. FIG. 7 is a flowchart showing a routine executed by the CPU shown in FIG. FIG. 8 is a flowchart showing a routine executed by the CPU shown in FIG. FIG. 9 is a flowchart showing a routine executed by the CPU shown in FIG. FIG. 10 is a flowchart showing a routine executed by the CPU shown in FIG.

  Hereinafter, an air-fuel ratio sensor abnormality determination device according to an embodiment of the present invention (hereinafter also simply referred to as “the determination device”) will be described with reference to the drawings.

(Constitution)
FIG. 1 is a schematic configuration diagram of an internal combustion engine 10 to which the present determination apparatus is applied. The engine 10 is a four-cycle / spark ignition / multi-cylinder (in-line four-cylinder) / fuel injection engine, and includes an engine body 20, an intake system 30, and an exhaust system 40.

  The engine body portion 20 includes a cylinder block portion and a cylinder head portion. The engine body 20 includes a plurality of cylinders (combustion chambers) 21. Each cylinder communicates with an “intake port and exhaust port” (not shown). A communicating portion between the intake port and the combustion chamber 21 is opened and closed by an intake valve (not shown). A communicating portion between the exhaust port and the combustion chamber 21 is opened and closed by an exhaust valve (not shown). Each combustion chamber 21 is provided with a spark plug (not shown).

  The intake system 30 includes an intake manifold 31, an intake pipe 32, a plurality of fuel injection valves 33, and a throttle valve 34.

  The intake manifold 31 includes a plurality of branch portions 31a and a surge tank 31b. One end of each of the plurality of branch portions 31a is connected to each of the plurality of intake ports. The other ends of the plurality of branch portions 31a are connected to the surge tank 31b.

  One end of the intake pipe 32 is connected to the surge tank 31b. An air filter (not shown) is disposed at the other end of the intake pipe 32.

  One fuel injection valve 33 is provided for each cylinder (combustion chamber) 21. The fuel injection valve 33 is provided at the intake port. In response to the injection instruction signal, the fuel injection valve 33 injects fuel of the indicated fuel injection amount included in the injection instruction signal into the intake port (and hence the cylinder corresponding to the fuel injection valve 33). .

  The throttle valve 34 is rotatably disposed in the intake pipe 32. The throttle valve 34 has a variable opening cross-sectional area of the intake passage. The throttle valve 34 is rotationally driven in the intake pipe 32 by a throttle valve actuator (not shown).

  The exhaust system 40 includes an exhaust manifold 41, an exhaust pipe 42, an upstream catalyst 43 disposed in the exhaust pipe 42, and a “downstream catalyst (not shown) disposed in the exhaust pipe 42 downstream of the upstream catalyst 43. Is provided.

  The exhaust manifold 41 includes a plurality of branch portions 41a and a collecting portion 41b. One end of each of the plurality of branch portions 41a is connected to each of the plurality of exhaust ports. The other ends of the plurality of branch portions 41a are gathered in the gathering portion 41b. The collecting portion 41b is also referred to as an exhaust collecting portion HK because exhaust gas discharged from a plurality of (two or more, four in this example) cylinders gathers.

  The exhaust pipe 42 is connected to the collecting portion 41b. The exhaust port, the exhaust manifold 41 and the exhaust pipe 42 constitute an exhaust passage.

  Each of the upstream side catalyst 43 and the downstream side catalyst is a so-called three-way catalyst device (exhaust purification catalyst) carrying an active component made of a noble metal (catalyst substance) such as platinum, rhodium and palladium.

  This determination apparatus includes a hot-wire air flow meter 51, a throttle position sensor 52, a water temperature sensor 53, a crank position sensor 54, an intake cam position sensor 55, an upstream air-fuel ratio sensor 56, a downstream air-fuel ratio sensor 57, and an accelerator opening sensor 58. It has.

The air flow meter 51 outputs a signal corresponding to the mass flow rate (intake air flow rate) Ga of intake air flowing through the intake pipe 32. That is, the intake air flow rate Ga represents the intake air amount sucked into the engine 10 per unit time.
The throttle position sensor 52 detects the opening (throttle valve opening) of the throttle valve 34 and outputs a signal representing the throttle valve opening TA.
The water temperature sensor 53 detects the temperature of the cooling water of the internal combustion engine 10 and outputs a signal representing the cooling water temperature THW.

  The crank position sensor 54 outputs a signal having a narrow pulse every time the crankshaft rotates 10 ° and a wide pulse every time the crankshaft rotates 360 °. This signal is converted into an engine speed NE by an electric control device 70 described later.

  The intake cam position sensor 55 outputs one pulse every time the intake cam shaft rotates 90 degrees, 90 degrees, and 180 degrees from a predetermined angle. The electric control device 70 described later acquires an absolute crank angle CA based on the compression top dead center of the reference cylinder (for example, the first cylinder) based on signals from the crank position sensor 54 and the intake cam position sensor 55. It has become.

  The upstream air-fuel ratio sensor 56 is disposed in “any one of the exhaust manifold 41 and the exhaust pipe 42” at a position between the collecting portion 41 b (exhaust collecting portion HK) of the exhaust manifold 41 and the upstream catalyst 43. . The upstream air-fuel ratio sensor 56 corresponds to the air-fuel ratio sensor in the present invention.

  The upstream air-fuel ratio sensor 56 is disclosed in, for example, “Limit current type wide area air-fuel ratio including diffusion resistance layer” disclosed in JP-A-11-72473, JP-A-2000-65782, JP-A-2004-69547, and the like. Sensor ".

  As shown in FIG. 2, the upstream air-fuel ratio sensor 56 is a value corresponding to “the partial pressure of oxygen (oxygen concentration, oxygen amount) and the partial pressure of unburned material (unburned material concentration, unburned material amount)”. Is output. In other words, the sensor value Vabyfs, which is a value output from the upstream air-fuel ratio sensor 56, increases as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 56 becomes leaner.

  The electric control device 70 to be described later stores the air-fuel ratio conversion table (map) MapAFS (Vabyfs) shown in FIG. The electric control device 70 detects the actual upstream air-fuel ratio abyfs by applying the sensor value Vabyfs to the air-fuel ratio conversion table MapAFS (Vabyfs). The air-fuel ratio detected in this way is referred to as “detected air-fuel ratio AFS” for convenience in this specification.

  Referring to FIG. 1 again, the downstream air-fuel ratio sensor 57 is disposed in the exhaust pipe 42. The downstream air-fuel ratio sensor 57 is disposed downstream of the upstream catalyst 43 and upstream of the downstream catalyst. The downstream air-fuel ratio sensor 57 is a known electromotive force type oxygen concentration sensor.

  The accelerator opening sensor 58 shown in FIG. 1 outputs a signal representing the operation amount Accp (accelerator pedal operation amount, accelerator pedal AP opening amount) of the accelerator pedal AP operated by the driver. The accelerator pedal operation amount Accp increases as the operation amount of the accelerator pedal AP increases.

  The electric control device 70 is also referred to as an ECU 70. The ECU is an abbreviation for an electric control unit, and is an electronic control circuit having a known microcomputer including a CPU, a ROM, a RAM, a backup RAM, an interface including an AD converter, and the like as main components. The CPU realizes various functions to be described later by executing instructions (routines, programs) stored in a memory (ROM).

  The backup RAM is supplied with electric power from a battery mounted on the vehicle regardless of the position of an ignition key switch (not shown) of the vehicle on which the engine 10 is mounted (any one of an off position, a start position, an on position, etc.). It is like that. When receiving power from the battery, the backup RAM stores data according to an instruction from the CPU (data is written) and holds (stores) the data so that the data can be read. Therefore, the backup RAM can hold data even when the operation of the engine 10 is stopped.

  The electric control device 70 is connected to the above-described sensors and the like, and supplies signals from these sensors to the CPU. Further, the electric control device 70 is responsive to an instruction from the CPU to provide a spark plug (actually an igniter) provided for each cylinder, a fuel injection valve 33 provided for each cylinder, and a throttle. A drive signal (instruction signal) is sent to a valve actuator or the like.

  The electric control device 70 sends an instruction signal to the throttle valve actuator so that the throttle valve opening TA increases as the acquired accelerator pedal operation amount Accp increases.

(Outline of air-fuel ratio sensor abnormality determination method)
Next, the “air-fuel ratio sensor abnormality determination method” employed by the determination apparatus will be described.

  As is well known, the sensor value Vabyfs, which is the value output from the air-fuel ratio sensor (in this example, the upstream air-fuel ratio sensor 56), is converted to the detected air-fuel ratio AFS as described above, and is supplied to the engine 10 It is used for feedback control of the air air-fuel ratio (hereinafter referred to as “engine air-fuel ratio”). Therefore, the output characteristic of the air-fuel ratio sensor is required to be normal. In other words, when the output characteristics of the air-fuel ratio sensor become abnormal, it is necessary to detect that fact and turn on the warning lamp, or to control the air-fuel ratio of the engine without using the detected air-fuel ratio AFS.

  As one of the parameters representing the output characteristics of the air-fuel ratio sensor 56, a time called “dead time” is known. As shown in FIG. 3, the dead time is obtained when the control air-fuel ratio (engine air-fuel ratio) changes stepwise from the rich air-fuel ratio to the lean air-fuel ratio. This is a delay time until the air-fuel ratio AFS) starts to change from a value indicating a rich air-fuel ratio to a value indicating a lean air-fuel ratio. Similarly, when the control air-fuel ratio changes stepwise from the lean air-fuel ratio to the rich, the dead time is the time when the sensor value Vabyfs of the air-fuel ratio sensor 56 (and thus the detected air-fuel ratio AFS) is rich from the value indicating the lean air-fuel ratio. It is also a delay time until the air-fuel ratio starts to change to a value.

  More specifically, as shown in FIG. 3A, when the control air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio at the rich / lean switching time tRL, the solid line C1 in FIG. As shown, at time t1 after time tRL, the air-fuel ratio sensor 56 starts to show a value that has reacted to the air-fuel ratio of the exhaust gas. The time point corresponding to this time t1 is also referred to as “sensor output switching time point” for convenience. Further, the rich / lean switching time tRL is also referred to as “control air-fuel ratio switching time” for convenience. The dead time is the time from the control air-fuel ratio switching time (tRL) to the sensor output switching time (t1).

  The dead time is inevitable time. This is because a predetermined time is required until the exhaust gas generated by the combustion of the air-fuel mixture whose air-fuel ratio has changed from the lean air-fuel ratio to the rich air-fuel ratio is discharged from the combustion chamber 21 and reaches the air-fuel ratio sensor 56. This is also because a predetermined time is required until the element of the air-fuel ratio sensor 56 starts to react to “the exhaust gas whose air-fuel ratio has changed from the lean air-fuel ratio to the rich air-fuel ratio”. However, these times are shorter as the intake air flow rate Ga is larger. Therefore, if the air-fuel ratio sensor 56 is normal, the dead time is substantially equal to the reference dead time Ls set in advance based on the intake air flow rate Ga.

  On the other hand, when the air-fuel ratio sensor 56 is in the “output reverse delay abnormal state”, as shown by the broken line C2 in FIG. 3, the dead time is “a time Lm that is significantly longer than the reference dead time Ls”. Therefore, the determination device acquires the dead time, and when the time Ldet obtained by subtracting the reference dead time Ls from the dead time is equal to or greater than a predetermined value, the air-fuel ratio sensor 56 is judged to be in the “output reverse delay abnormal state”. . The time Ldet is also referred to as “delay time for determination” for convenience.

  By the way, in order to acquire the dead time with high accuracy, it is necessary to specify the sensor output switching time point as accurately as possible. However, for example, the sensor value Vabyfs of the air-fuel ratio sensor is acquired every time a predetermined time elapses, the detected air-fuel ratio AFS is acquired based on the sensor value Vabyfs, and the value AFSold acquired a predetermined time before the value AFSnow acquired this time If the time when the sign of the difference Δ obtained by subtracting is changed from negative to positive or from positive to negative is set as the sensor output switching time, there is a high possibility that the erroneous time is specified as the sensor output switching time. This is because noise frequently superimposes on the sensor value Vabyfs of the air-fuel ratio sensor.

Therefore, as shown in FIG.
(1) The sensor value Vabyfs of the air-fuel ratio sensor 56 is acquired every time the fixed time ts elapses.
(2) The detected air-fuel ratio AFS is acquired based on the sensor value Vabyfs,
(3) The variance σ of the detected air-fuel ratio AFS is calculated every time a fixed time ts elapses based on the following equation (1):
(4) The time when the variance σ changes from less than the predetermined threshold σth to the threshold σth or more is specified as the sensor output switching time.

  In the above formula (1), the value M is an integer of 1 or more, for example, 5. AFS (t−i) is a value AFS acquired at the sampling timing i times before the present time, and AFSave (t) is an average value of the values AFS acquired at the past M sampling timings including the current time. is there.

  However, when the intake air flow rate is small, the change rate of the sensor value Vabyfs of the air-fuel ratio sensor 56 is smaller than when the intake air flow rate is large. Therefore, the smaller the intake air flow rate, the smaller the change rate of the dispersion σ that occurs after the control air-fuel ratio switching time. Therefore, when it is intended to specify the time point when the variance is smaller than the threshold value σth as the sensor output switching time point when the variance becomes larger than the threshold value σth, the threshold value σth must be set to a small value when the intake air flow rate is small. For this reason, when noise is superimposed on the sensor value Vabyfs of the air-fuel ratio sensor 56, a situation in which the variance σ exceeds the threshold σth due to the noise occurs. As a result, a time point that is not correct as the sensor output switching time point may be specified as the sensor output switching time point.

  Therefore, as shown by a black circle in FIG. 5, the determination device uses the sampling time of the dispersion calculation data (the sensor value Vabyfs of the air-fuel ratio sensor 56 and the detected air-fuel ratio AFS) which is data used for the dispersion σ as the intake air. Apparently, the smaller the flow rate Ga, the longer. More specifically, the determination apparatus acquires the sensor value Vabyfs of the air-fuel ratio sensor 56 every time a constant sampling time ts (for example, 4 ms) elapses, and the sensor value Vabyfs is a table shown in FIG. Is used to obtain the detected air-fuel ratio AFS.

  Then, when the intake air flow rate Ga is very large (that is, Ga = Ga4), this determination apparatus detects the detection sky obtained at every elapse of the sampling time ts, as indicated by a black circle in FIG. The variance σ is calculated based on the fuel ratio AFS. This variance σ is referred to as variance σ4 for convenience because the sampling time ts is 4 ms. When the intake air flow rate Ga is very large, the determination device identifies the time when the variance σ4 changes from a value smaller than the threshold σth to a larger value as the “sensor output value switching time”.

  When the intake air flow rate Ga is relatively large (i.e., Ga = Ga3 <Ga4), the determination device has a time 2 · ts that is twice the sampling time ts, as indicated by a black circle in FIG. The variance σ is calculated based on the detected air-fuel ratio AFS obtained every time. Since this substantial distribution σ can be regarded as a substantial sampling time of 8 ms, it is referred to as the dispersion σ8 for convenience. When the intake air flow rate Ga is relatively large, the determination device identifies the time point at which the variance σ8 changes from a value smaller than the threshold value σth to a larger value as a “sensor output value switching time point”.

  When the intake air flow rate Ga is relatively small (i.e., Ga = Ga2 <Ga3 <Ga4), the determination device 3 is a time 3 · 3 times the sampling time ts, as indicated by a black circle in FIG. The variance σ is calculated based on the detected air-fuel ratio AFS obtained every time ts elapses. This variance σ can be regarded as a substantial sampling time of 12 ms, and is referred to as variance σ12 for convenience. When the intake air flow rate Ga is relatively small, the determination device identifies the time point at which the variance σ12 changes from a value smaller than the threshold value σth to a larger value as a “sensor output value switching time point”.

  Furthermore, this determination device, when the intake air flow rate Ga is very small (ie, Ga = Ga1 <Ga2 <Ga3 <Ga4), as indicated by a black circle in FIG. 5E, is four times the sampling time ts. The variance σ is calculated based on the detected air-fuel ratio AFS obtained every time 4 · ts. This variance σ can be regarded as a substantial sampling time of 16 ms, and is referred to as variance σ16 for convenience. When the intake air flow rate Ga is very small, the determination device identifies the time when the variance σ16 changes from a value smaller than the threshold σth to a larger value as the “sensor output value switching time”.

  As a result, the apparent change rate of the detected air-fuel ratio AFS (or the sensor value Vabyfs of the air-fuel ratio sensor 56) when the intake air flow rate is small (in terms of data processing) increases, and the detected air-fuel ratio when the intake air flow rate is large. It approaches the changing speed of AFS (or the sensor value Vabyfs of the air-fuel ratio sensor). As a result, when the intake air flow rate is small, there is no need to reduce the threshold value σth used when specifying the sensor output switching time point, so that noise is superimposed on the sensor value Vabyfs of the air-fuel ratio sensor 56. In addition, the sensor output switching time can be accurately identified.

(Specific operation)
The CPU of the electric control device 70 executes the “4 ms routine” shown by the flowchart in FIG. 6 every time a predetermined time (4 ms) elapses. Accordingly, when the predetermined timing comes, the CPU starts processing from step 600 in FIG. 6 and proceeds to step 605 to determine whether or not the value of the flag Xactive is “1”.

  The value of the flag Xactive is set to “0” in an initial routine executed by the CPU when an ignition key switch (not shown) of the vehicle on which the engine 10 is mounted is changed from the off position to the on position. It has become. Further, the value of the flag Xactive is set to “1” when a condition for executing the abnormality determination of the air-fuel ratio sensor 56 (air-fuel ratio sensor abnormality determination execution condition) is satisfied (steps 1010 and 1020 in FIG. 10 described later). The value is set to “0” when a sufficient time has elapsed since the value was set to “1” (see step 1050 and step 1060 in FIG. 10 described later). Further, the value of the flag Xactive is also set to “0” by processing such as step 660 described later.

  The condition for executing the abnormality determination of the air-fuel ratio sensor 56 is, for example, that the abnormality determination of the air-fuel ratio sensor 56 has not been executed once after the engine 10 is started, the cooling water temperature THW is equal to or higher than the threshold water temperature, and the engine This is established when the rotational speed NE is within a predetermined range and the throttle valve opening degree TA is within the predetermined range.

  By the way, the CPU performs a so-called “active air-fuel ratio control (forced air-fuel ratio control)” when the value of the flag Xactive is “1” by executing a routine shown in FIG. 10 described later. More specifically, in the active air-fuel ratio control, the CPU maintains the control air-fuel ratio (engine air-fuel ratio) at a predetermined rich air-fuel ratio for a predetermined time TR as shown in FIG. Thereafter, the air-fuel ratio control for maintaining the control air-fuel ratio at a predetermined lean air-fuel ratio for a predetermined time TL is repeatedly performed. When the value of the flag Xactive is “0”, the CPU stops the active air-fuel ratio control and performs the normal air-fuel ratio control.

  If the value of the flag Xactive is “0”, the CPU makes a “No” determination at step 605 to directly proceed to step 695 to end the present routine tentatively. On the other hand, if the value of the flag Active is “1”, the CPU makes a “Yes” determination at step 605 to proceed to step 610 to calculate the above-described variance σ4. In the expression described in step 610, the value M is “5” (or an integer of 1 or more), and AFS4 (ti) is the case where the apparent sampling time is ts (= 4 ms). The detected air-fuel ratio AFS acquired at the sampling timing i times before the present time, and AFSave (t) is the average value of the values AFS4 acquired at the past M sampling timings including the current time.

  Next, the CPU proceeds to step 615 to determine whether or not the current intake air flow rate Ga is an intake air flow rate at which the dispersion σ4 should be used. More specifically, the CPU determines whether or not the current intake air flow rate Ga is equal to or greater than the first threshold flow rate GL. The first threshold flow rate GL is set to correspond to the intake air flow rate when the load on the engine 10 is relatively large. If the current intake air flow rate Ga is not equal to or greater than the first threshold flow rate GL, the CPU makes a “No” determination at step 615 to proceed to step 608 and set the variance σ4 calculated at step 610 to “the previous value of the variance σ4”. Remember as. Thereafter, the CPU proceeds directly to step 695 to end the present routine tentatively.

  If the current intake air flow rate Ga is greater than or equal to the first threshold flow rate GL, the CPU makes a “Yes” determination at step 615 to proceed to step 620, where the variance σ4 calculated at step 610 is set to “current value of variance σ4”. "Is memorized. Next, the CPU proceeds to step 625 to determine whether or not the previous value of the variance σ4 is smaller than the threshold value σth. If the previous value of the variance σ4 is larger than the threshold σth, the CPU makes a “No” determination at step 625 to proceed to step 695 via step 608 to end the present routine tentatively.

  On the other hand, if the previous value of the variance σ4 is smaller than the threshold σth, the CPU makes a “Yes” determination at step 625 to proceed to step 630 to determine whether or not the current value of the variance σ4 is greater than or equal to the threshold σth. judge. If the current value of the variance σ4 is smaller than the threshold σth, the CPU makes a “No” determination at step 630 to proceed to step 695 via step 608 to end the present routine tentatively.

  On the other hand, if the current value of the variance σ4 is equal to or greater than the threshold σth, the CPU makes a “Yes” determination at step 630 to proceed to step 632, where switching immediately before the control air-fuel ratio changes from the rich air-fuel ratio to the lean air-fuel ratio. It is determined whether or not the switch has been made. When switching immediately before the control air-fuel ratio is not switching from rich air-fuel ratio to lean air-fuel ratio (that is, when switching immediately before the control air-fuel ratio is switching from lean air-fuel ratio to rich air-fuel ratio), CPU Determines “No” at step 632, proceeds to step 695 via step 608, and ends this routine once.

  On the other hand, if the switching immediately before the control air-fuel ratio is the switching from the rich air-fuel ratio to the lean air-fuel ratio, it is determined as “Yes” in the CPU step 632, and the processing from step 635 to step 645 described below is performed. Go to step 650 in order.

Step 635: The CPU calculates a dead time L4 by subtracting the previous rich / lean switching time tRL from the current time tnow.
Step 640: The CPU acquires the reference dead time Lseijo by applying the intake air flow rate Ga to the lookup table MapLseijo (Ga). According to this table MapLseijo (Ga), the reference dead time Lseijo is acquired as a value that becomes shorter as the intake air flow rate Ga is larger. As described above, the reference dead time Lseijo is a dead time indicated by a normal air-fuel ratio sensor.
Step 645: The CPU obtains the determination dead time delay Ldet by subtracting the reference dead time Lseijo from the dead time L4.

  Next, the CPU proceeds to step 650 to determine whether or not the determination dead time delay Ldet is equal to or greater than a predetermined abnormality determination threshold L4th. If the determination dead time delay Ldet is not equal to or greater than the predetermined abnormality determination threshold L4th, the CPU makes a “No” determination at step 650 to proceed directly to step 660.

  On the other hand, if the determination dead time delay Ldet is equal to or greater than the predetermined abnormality determination threshold L4th, the CPU makes a “Yes” determination at step 650 to proceed to step 655, where the air-fuel ratio sensor 56 It is determined that the state is abnormal. More specifically, the CPU sets the value of a flag (abnormality occurrence flag) Xijo indicating that the air-fuel ratio sensor 56 is in the “output reverse delay abnormal state” to “1”. The value of the abnormality occurrence flag Xijo is stored in the backup RAM. At this time, the CPU may turn on a warning lamp (not shown). Thereafter, the CPU proceeds to step 660.

  In step 660, the CPU sets the value of the flag Xactive to “0”, proceeds to step 695, and once ends this routine.

  Further, the CPU executes the “8 ms routine” shown by the flowchart in FIG. 7 every time a predetermined time (8 ms) elapses. Accordingly, when the predetermined timing comes, the CPU starts processing from step 700 in FIG. 7 and proceeds to step 705 to determine whether or not the value of the flag Xactive is “1”.

  If the value of the flag Xactive is “0”, the CPU makes a “No” determination at step 705 to directly proceed to step 795 to end the present routine tentatively. On the other hand, if the value of the flag Active is “1”, the CPU makes a “Yes” determination at step 705 to proceed to step 710 to calculate the above-described variance σ8. In the expression described in step 710, the value M is “5” (or an integer of 1 or more), and AFS8 (ti) has an apparent sampling time of 2 · ts (= 8 ms). Is the detected air-fuel ratio AFS acquired at the sampling timing i times before the current time, and AFSave (t) is the average value of the values AFS8 acquired at the past M sampling timings including the current time.

  Next, the CPU proceeds to step 715 to determine whether or not the current intake air flow rate Ga is an intake air flow rate at which the dispersion σ8 should be used. More specifically, the CPU determines whether or not the current intake air flow rate Ga is greater than or equal to the second threshold flow rate GM and less than the first threshold flow rate GL. The second threshold flow rate GM is smaller than the first threshold flow rate GL, and is set to correspond to the intake air flow rate when the load of the engine 10 is medium.

  If the current intake air flow rate Ga is equal to or greater than the second threshold flow rate GM and not less than the first threshold flow rate GL, the CPU makes a “No” determination at step 715 to proceed to step 708 and calculate the variance calculated at step 710 σ8 is stored as “previous value of variance σ8”. Thereafter, the CPU proceeds directly to step 795 to end the present routine tentatively.

  If the current intake air flow rate Ga is greater than or equal to the second threshold flow rate GM and less than the first threshold flow rate GL, the CPU makes a “Yes” determination at step 715 to proceed to step 720 and calculate at step 710. The variance σ8 is stored as “current value of variance σ8”. Next, the CPU proceeds to step 725 to determine whether or not the previous value of the variance σ8 is smaller than the threshold value σth. When the previous value of the variance σ8 is larger than the threshold value σth, the CPU makes a “No” determination at step 725 to proceed to step 795 via step 708 to end the present routine tentatively.

  On the other hand, if the previous value of the variance σ8 is smaller than the threshold σth, the CPU makes a “Yes” determination at step 725 to proceed to step 730 to determine whether or not the current value of the variance σ8 is greater than or equal to the threshold σth. judge. If the current value of the variance σ8 is smaller than the threshold σth, the CPU makes a “No” determination at step 730 to proceed to step 795 via step 708 to end the present routine tentatively.

  On the other hand, if the current value of the variance σ8 is equal to or greater than the threshold σth, the CPU makes a “Yes” determination at step 730 to proceed to step 732, where switching immediately before the control air-fuel ratio changes from the rich air-fuel ratio to the lean air-fuel ratio. It is determined whether or not the switch has been made. When switching immediately before the control air-fuel ratio is not switching from rich air-fuel ratio to lean air-fuel ratio (that is, when switching immediately before the control air-fuel ratio is switching from lean air-fuel ratio to rich air-fuel ratio), CPU Determines “No” in step 732, proceeds to step 795 via step 708, and ends this routine once.

  On the other hand, when the switching immediately before the control air-fuel ratio is switching from the rich air-fuel ratio to the lean air-fuel ratio, it is determined as “Yes” in the CPU step 732, and the processing of steps 735 to 745 described below is performed. Go to step 750 in order.

Step 735: The CPU calculates a dead time L8 by subtracting the previous rich / lean switching time tRL from the current time tnow.
Step 740: The CPU obtains the reference dead time Lseijo by applying the intake air flow rate Ga to the lookup table MapLseijo (Ga).
Step 745: The CPU obtains the determination dead time delay Ldet by subtracting the reference dead time Lseijo from the dead time L8.

  Next, the CPU proceeds to step 750 to determine whether or not the dead time delay for determination Ldet is equal to or greater than a predetermined abnormality determination threshold L8th. If the determination dead time delay Ldet is not equal to or greater than the predetermined abnormality determination threshold L8th, the CPU makes a “No” determination at step 750 to directly proceed to step 760.

  In contrast, if the dead time delay for determination Ldet is equal to or greater than the predetermined abnormality determination threshold L8th, the CPU makes a “Yes” determination at step 750 to proceed to step 755, where the air-fuel ratio sensor 56 It is determined that the state is “abnormal state”, and the value of the abnormality occurrence flag Xijo is set to “1”. At this time, the CPU may turn on a warning lamp (not shown). Thereafter, the CPU proceeds to step 760.

  In step 760, the CPU sets the value of the flag Xactive to “0”, proceeds to step 795, and once ends this routine.

  Further, the CPU executes a “12 ms routine” shown by a flowchart in FIG. 8 every time a predetermined time (12 ms) elapses. Therefore, when the predetermined timing comes, the CPU starts the process from step 800 in FIG. 8 and proceeds to step 805 to determine whether or not the value of the flag Xactive is “1”.

  If the value of the flag Xactive is “0”, the CPU makes a “No” determination at step 805 to directly proceed to step 895 to end the present routine tentatively. On the other hand, if the value of the flag Active is “1”, the CPU makes a “Yes” determination at step 805 to proceed to step 810 to calculate the variance σ12 described above. In the expression described in Step 810, the value M is “5” (or an integer of 1 or more), and AFS12 (ti) has an apparent sampling time of 3 · ts (= 12 ms). The detected air-fuel ratio AFS obtained at the sampling timing i times before the present time, and AFSave (t) is an average value of the values AFS12 obtained at the past M sampling timings including the current time.

  Next, the CPU proceeds to step 815 to determine whether or not the current intake air flow rate Ga is an intake air flow rate at which the dispersion σ12 should be used. More specifically, the CPU determines whether or not the current intake air flow rate Ga is greater than or equal to the third threshold flow rate GS and less than the second threshold flow rate GM. The third threshold flow rate GS is smaller than the second threshold flow rate GM, and is set to correspond to the intake air flow rate when the load of the engine 10 is a light load.

  If the current intake air flow rate Ga is greater than or equal to the third threshold flow rate GS and not less than the second threshold flow rate GM, the CPU makes a “No” determination at step 815 to proceed to step 808, and the variance calculated at step 810 σ12 is stored as “previous value of variance σ12”. Thereafter, the CPU proceeds directly to step 895 to end the present routine tentatively.

  If the current intake air flow rate Ga is greater than or equal to the third threshold flow rate GS and less than the second threshold flow rate GM, the CPU makes a “Yes” determination at step 815 to proceed to step 820 and calculate at step 810. The variance σ12 is stored as “current value of variance σ12”. Next, the CPU proceeds to step 825 to determine whether or not the previous value of the variance σ12 is smaller than the threshold value σth. If the previous value of the variance σ12 is larger than the threshold σth, the CPU makes a “No” determination at step 825 to proceed to step 895 via step 808 to end the present routine tentatively.

  On the other hand, if the previous value of the variance σ12 is smaller than the threshold σth, the CPU makes a “Yes” determination at step 825 to proceed to step 830 to determine whether or not the current value of the variance σ12 is greater than or equal to the threshold σth. judge. If the current value of the variance σ12 is smaller than the threshold σth, the CPU makes a “No” determination at step 830 to proceed to step 895 via step 808 to end the present routine tentatively.

  On the other hand, if the current value of the variance σ12 is equal to or greater than the threshold σth, the CPU makes a “Yes” determination at step 830 to proceed to step 832 where switching immediately before the control air-fuel ratio changes from the rich air-fuel ratio to the lean air-fuel ratio. It is determined whether or not the switch has been made. When switching immediately before the control air-fuel ratio is not switching from rich air-fuel ratio to lean air-fuel ratio (that is, when switching immediately before the control air-fuel ratio is switching from lean air-fuel ratio to rich air-fuel ratio), CPU Determines “No” in step 832, proceeds to step 895 via step 808, and ends this routine once.

  On the other hand, if the switching immediately before the control air-fuel ratio is a switching from the rich air-fuel ratio to the lean air-fuel ratio, it is determined as “Yes” in the CPU step 832, and the processing of steps 835 to 845 described below is performed. Go to step 850 in order.

Step 835: The CPU calculates a dead time L12 by subtracting the previous rich / lean switching time tRL from the current time tnow.
Step 840: The CPU acquires the reference dead time Lseijo by applying the intake air flow rate Ga to the look-up table MapLseijo (Ga).
Step 845: The CPU obtains the determination dead time delay Ldet by subtracting the reference dead time Lseijo from the dead time L12.

  Next, the CPU proceeds to step 850 to determine whether or not the dead time delay for determination Ldet is equal to or greater than a predetermined abnormality determination threshold L12th. If the determination dead time delay Ldet is not equal to or greater than the predetermined abnormality determination threshold L12th, the CPU makes a “No” determination at step 850 to directly proceed to step 860.

  On the other hand, if the determination dead time delay Ldet is equal to or greater than the predetermined abnormality determination threshold L12th, the CPU makes a “Yes” determination at step 850 to proceed to step 855, where the air-fuel ratio sensor 56 It is determined that the state is “abnormal state”, and the value of the abnormality occurrence flag Xijo is set to “1”. At this time, the CPU may turn on a warning lamp (not shown). Thereafter, the CPU proceeds to step 860.

  In step 860, the CPU sets the value of the flag Xactive to “0”, proceeds to step 895, and once ends this routine.

  Further, the CPU executes the “16 ms routine” shown by the flowchart in FIG. 9 every time a predetermined time (16 ms) elapses. Therefore, at the predetermined timing, the CPU starts the process from step 900 in FIG. 9 and proceeds to step 905 to determine whether or not the value of the flag Xactive is “1”.

  If the value of the flag Xactive is “0”, the CPU makes a “No” determination at step 905 to directly proceed to step 995 to end the present routine tentatively. On the other hand, if the value of the flag Active is “1”, the CPU makes a “Yes” determination at step 905 to proceed to step 910 to calculate the above-described variance σ16. In the expression described in step 910, the value M is “5” (or an integer of 1 or more), and AFS16 (ti) has an apparent sampling time of 4 · ts (= 16 ms). The detected air-fuel ratio AFS obtained at the sampling timing i times before the present time, and AFSave (t) is the average value of the values AFS16 obtained at the past M sampling timings including the current time.

  Next, the CPU proceeds to step 915 to determine whether or not the current intake air flow rate Ga is an intake air flow rate at which the dispersion σ16 should be used. More specifically, the CPU determines whether or not the current intake air flow rate Ga is less than the third threshold flow rate GS.

  If the current intake air flow rate Ga is not less than the third threshold flow rate GS, the CPU makes a “No” determination at step 915 to proceed to step 908 to set the variance σ16 calculated at step 910 as “the previous value of the variance σ16”. Remember as. Thereafter, the CPU proceeds directly to step 995 to end the present routine tentatively.

  If the current intake air flow rate Ga is less than the third threshold flow rate GS, the CPU makes a “Yes” determination at step 915 to proceed to step 920, where the variance σ16 calculated at step 910 is set to “current value of variance σ16”. "Is memorized. Next, the CPU proceeds to step 925 to determine whether or not the previous value of the variance σ16 is smaller than the threshold value σth. If the previous value of the variance σ16 is greater than the threshold σth, the CPU makes a “No” determination at step 925 to proceed to step 995 via step 908 to end the present routine tentatively.

  On the other hand, if the previous value of the variance σ16 is smaller than the threshold σth, the CPU makes a “Yes” determination at step 925 to proceed to step 930 to determine whether or not the current value of the variance σ16 is greater than or equal to the threshold σth. judge. If the current value of the variance σ16 is smaller than the threshold σth, the CPU makes a “No” determination at step 930 to proceed to step 995 via step 908 to end the present routine tentatively.

  On the other hand, if the current value of the variance σ16 is equal to or greater than the threshold σth, the CPU makes a “Yes” determination at step 930 to proceed to step 932, where switching immediately before the control air-fuel ratio changes from the rich air-fuel ratio to the lean air-fuel ratio. It is determined whether or not the switch has been made. When switching immediately before the control air-fuel ratio is not switching from rich air-fuel ratio to lean air-fuel ratio (that is, when switching immediately before the control air-fuel ratio is switching from lean air-fuel ratio to rich air-fuel ratio), CPU Determines “No” in step 932, proceeds to step 995 via step 908, and ends this routine once.

  On the other hand, if the switching immediately before the control air-fuel ratio is the switching from the rich air-fuel ratio to the lean air-fuel ratio, it is determined as “Yes” in the CPU step 932, and the processing from step 935 to step 945 described below is performed. Go to step 950 in order.

Step 935: The CPU calculates a dead time L16 by subtracting the previous rich / lean switching time tRL from the current time tnow.
Step 940: The CPU acquires the reference dead time Lseijo by applying the intake air flow rate Ga to the lookup table MapLseijo (Ga).
Step 945: The CPU obtains the determination dead time delay Ldet by subtracting the reference dead time Lseijo from the dead time L16.

  Next, the CPU proceeds to step 950 to determine whether or not the determination dead time delay Ldet is equal to or greater than a predetermined abnormality determination threshold L16th. If the determination dead time delay Ldet is not equal to or greater than the predetermined abnormality determination threshold L16th, the CPU makes a “No” determination at step 950 to proceed directly to step 960.

  On the other hand, when the dead time delay for determination Ldet is equal to or greater than the predetermined abnormality determination threshold L16th, the CPU makes a “Yes” determination at step 950 to proceed to step 955, where the air-fuel ratio sensor 56 It is determined that the state is “abnormal state”, and the value of the abnormality occurrence flag Xijo is set to “1”. At this time, the CPU may turn on a warning lamp (not shown). Thereafter, the CPU proceeds to step 960.

  In step 960, the CPU sets the value of the flag Xactive to “0”, proceeds to step 995, and once ends this routine.

  Further, the CPU executes the “air-fuel ratio control routine” shown by the flowchart in FIG. 10 every time a predetermined time elapses. Accordingly, when the predetermined timing comes, the CPU starts processing from step 1000 in FIG. 10 and proceeds to step 1010 to determine whether or not it is immediately after the aforementioned air-fuel ratio sensor abnormality determination execution condition is satisfied.

  If it is immediately after the air-fuel ratio sensor abnormality determination execution condition is satisfied, the CPU makes a “Yes” determination at step 1010 to proceed to step 1020, sets the value of the flag Xactive to “1”, and then proceeds to step 1030. . On the other hand, if it is not immediately after the air-fuel ratio sensor abnormality determination execution condition is satisfied, the CPU makes a “No” determination at step 1010 to proceed directly to step 1030.

  In step 1030, the CPU determines whether or not the value of the flag Xactive is “1”. If the value of the flag Xactive is “1”, the CPU makes a “Yes” determination at step 1030 to proceed to step 1040 to execute the above-described active air-fuel ratio control. More specifically, the CPU calculates the amount of air taken by one cylinder of the engine 10 in one intake stroke from the intake air flow rate Ga and the engine speed NE, and divides the air amount by the target air-fuel ratio. The fuel corresponding to the amount is injected from the fuel injection valve 33 of the cylinder that reaches the intake stroke. In one cycle of active air-fuel ratio control, the target air-fuel ratio is set to a constant rich air-fuel ratio that is smaller than the stoichiometric air-fuel ratio for a certain time TR, and then a certain lean air-fuel ratio that is larger than the stoichiometric air-fuel ratio for a certain time TL. Set to

  Next, the CPU proceeds to step 1050 to determine whether or not the abnormality determination execution time has elapsed since the value of the flag Xactive has changed from “0” to “1”. If the abnormality determination execution time has elapsed, the CPU makes a “Yes” determination at step 1050 to proceed to step 1060 to set the value of the flag Xactive to “0”, and then proceeds to step 1095 to execute this routine. Exit once.

  On the other hand, if the abnormality determination execution time has not elapsed, the CPU makes a “No” determination at step 1050 to directly proceed to step 1095 to end the present routine tentatively.

  On the other hand, if the value of the flag Xactive is not “1” at the time when the CPU performs the process of step 1030, the CPU makes a “No” determination at step 1030 to proceed to step 1070 to perform normal air-fuel ratio control. . More specifically, the CPU sets the target air-fuel ratio to the stoichiometric air-fuel ratio, calculates the amount of air that one cylinder sucks in one intake stroke from the intake air flow rate Ga and the engine rotational speed NE, and Fuel corresponding to the amount obtained by dividing the air amount by the target air-fuel ratio is injected from the fuel injection valve 33 of the cylinder that reaches the intake stroke. At this time, if the detected air-fuel ratio AFS is smaller than the stoichiometric air-fuel ratio, the fuel injection amount is corrected to decrease, and if the detected air-fuel ratio AFS is larger than the stoichiometric air-fuel ratio, the fuel injection amount is corrected to increase.

  As described above, the determination device obtains the dispersion of the detected air-fuel ratio AFS during the active air-fuel ratio control, and specifies the sensor output switching time based on the dispersion. At this time, the determination device increases the sampling interval of the detected air-fuel ratio AFS data (dispersion calculation data) for obtaining the dispersion as the intake air flow rate decreases. As a result, the sensor output switching time can be accurately identified.

  The present invention is not limited to the above embodiment, and various modifications can be employed within the scope of the present invention. For example, the threshold values σth used in FIGS. 6 to 9 may be different from each other. That is, the threshold σth may be set to a value that changes according to the intake air flow rate.

  The determination device obtains the dispersion (σ4, σ8, σ12, and σ16) of the detected air-fuel ratio, and specifies the time point when the sensor output is switched by the dispersion, but the dispersion of the sensor value Vabyfs of the air-fuel ratio sensor 56 is determined. The time point when the sensor output is switched may be specified based on the distribution. Further, the sampling interval of the data for dispersion calculation is not limited to 4, 8, 12 and 16 ms, but may be any time, and the dispersion is also limited to four of σ4, σ8, σ12 and σ16. Not.

  Further, the determination device determines whether or not an output reversal delay abnormal state has occurred based on a dead time that appears after the switching immediately before the control air-fuel ratio is switching from the rich air-fuel ratio to the lean air-fuel ratio. However, it may be determined whether or not an output reverse delay abnormal state has occurred based on the dead time that appears after the switching immediately before the control air-fuel ratio is the switching from the lean air-fuel ratio to the rich air-fuel ratio. (See the time from time tLR to time t3 or time t4 in FIG. 3).

  Further, the present determination device obtains the dead time only once and determines whether or not the output reversal delay abnormal state has occurred based on the dead time, but the intake air flow rate Ga is the same region (i.e., Less than GS, more than GS and less than GM, more than GM and less than GL, and more than GL), the dead time is obtained a plurality of times, and based on the average value of the dead times (ie, the value based on the dead time) It may be determined whether an output reversal delay abnormal state has occurred.

  DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 33 ... Fuel injection valve, 41 ... Exhaust manifold, 42 ... Exhaust pipe, 43 ... Upstream catalyst, 56 ... Upstream air-fuel ratio sensor (air-fuel ratio sensor), 57 ... Downstream-side air-fuel ratio sensor, 70 ... Electric control device.

Claims (2)

An abnormality determination device for an air-fuel ratio sensor that outputs a value corresponding to an air-fuel ratio of exhaust gas of the engine that is disposed in an exhaust passage of the internal combustion engine and flows through the disposed position,
Engine air-fuel ratio changing means for stepwise changing the air-fuel ratio of the air-fuel mixture supplied to the engine between a predetermined rich air-fuel ratio and a predetermined lean air-fuel ratio;
A sensor value that is a value output by the air-fuel ratio sensor or a detected air-fuel ratio obtained by converting the sensor value to an air-fuel ratio is acquired as dispersion calculation data every time a predetermined time elapses, and among the dispersion calculation data, A variance calculation means for calculating a variance of the plurality of data based on a plurality of data that are temporally continuous with each other;
A time point at which the dispersion changes from less than a predetermined threshold value to a value equal to or higher than the threshold value is specified as a sensor output switching time point, and the air-fuel ratio of the air-fuel mixture supplied to the engine is between the rich air-fuel ratio and the lean air-fuel ratio. When a dead time that is a time from the control air-fuel ratio switching time point that is changed in a stepwise manner to the sensor output switching time point is acquired, and the time based on the dead time is equal to or longer than a predetermined threshold time, An abnormality determining means for determining that an output inversion delay abnormality of the air-fuel ratio sensor has occurred;
In an air-fuel ratio sensor abnormality determination device comprising:
The variance calculating means includes
The fixed time is configured to be longer as the flow rate of air sucked into the engine is smaller.
An air-fuel ratio sensor abnormality determination device. In the air-fuel ratio sensor abnormality determination device according to claim 1,
The abnormality determining means includes
It is determined whether or not a time difference obtained by subtracting a reference dead time that changes based on the flow rate of air sucked into the engine from the dead time is equal to or greater than a predetermined value, and the time difference is the predetermined value. Configured to determine that the dead time is equal to or longer than the predetermined threshold time when it is determined as above.
An air-fuel ratio sensor abnormality determination device.

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