JP2003013779A - Air-fuel ratio control device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrict deterioration of air-fuel ratio control performance when characteristics of an oxygen sensor is changed in an air-fuel ratio control device to feedback-control an air-fuel ratio using the oxygen sensor of which output has linearity around a stoichiometric air-fuel ratio. SOLUTION: In a range where the sensor output has linearity, an actual air-fuel ratio is determined based on the sensor output, and the air-fuel ratio is feedback-controlled based on deviation between the actual air-fuel ratio and a target air-fuel ratio. When correlation between the sensor output and the air-fuel ratio in the linearity range is changed in such a way that the sensor output is quickly changed around the stoichiometric air-fuel ratio (when sensitivity is abnormal), if the sensor output is rich or lean to the stoichiometric air-fuel ratio is only determined, and air-fuel ratio feedback control is continued.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine in which an air-fuel ratio is feedback-controlled by using an oxygen sensor having a substantially linear relationship with the air-fuel ratio when the output value is within a predetermined range including the stoichiometric air-fuel ratio equivalent value. Relates to the air-fuel ratio control device.

[0002]

2. Description of the Related Art Conventionally, an air-fuel ratio feedback control based on the output of an oxygen sensor is stopped when an abnormality of the oxygen sensor is judged based on element resistance, output characteristics, etc., and when an abnormality is diagnosed. A fuel ratio control device is known (see Japanese Patent Application Laid-Open No. 8-2714775).

In addition, normally, the rich
The output characteristic of the oxygen sensor used to detect only lean is set to have substantially linearity with respect to the air-fuel ratio near the stoichiometric air-fuel ratio, and the actual air-fuel ratio is obtained using the linearity, A device for feedback controlling the air-fuel ratio is known (see Japanese Patent Laid-Open No. 07-127505).

[0004]

By the way, in the case of an oxygen sensor which is set to have substantially linearity in the vicinity of the stoichiometric air-fuel ratio, the output in the region set to have the linearity as a failure mode is In some cases, such as an oxygen sensor used to detect only rich / lean, the air-fuel ratio changes suddenly around the theoretical air-fuel ratio, and it may not be possible to obtain the actual air-fuel ratio from the output value.

However, even if the output characteristics change as described above, it is possible to detect only rich lean, so that the air-fuel ratio feedback control is performed based on the fact that the original output cannot be obtained. If it is completely stopped, there is a problem that the exhaust property is unnecessarily deteriorated because control that can effectively function is not performed.

The present invention has been made in view of the above problems, and even when the output characteristic of an oxygen sensor set to have substantially linearity near the stoichiometric air-fuel ratio changes,
It is an object of the present invention to provide an air-fuel ratio control device for an engine, which can make the most of the detection function of the oxygen sensor to reduce the deterioration of exhaust properties.

[0007]

Therefore, the invention according to claim 1 is an oxygen sensor in which the output value changes in response to the oxygen concentration in the engine exhaust, and the sensor is within a predetermined range including the theoretical air-fuel ratio equivalent value. In an air-fuel ratio control device for an engine, the output value of which includes an oxygen sensor having substantially linearity with respect to the air-fuel ratio, and the feedback control of the air-fuel ratio by obtaining the actual air-fuel ratio from the output value of the predetermined range, A change in the correlation between the output value and the air-fuel ratio is discriminated, and when the correlation changes, only the rich / lean with respect to the theoretical air-fuel ratio is discriminated based on the output value, and the air-fuel ratio is feedback controlled.

According to this structure, the oxygen sensor is set so that the output characteristic of the oxygen sensor which detects only rich lean with respect to the normal stoichiometric air-fuel ratio has substantially linearity with respect to the air-fuel ratio near the stoichiometric air-fuel ratio. By doing so, it is possible to detect the actual air-fuel ratio, but when the output characteristic in the linear region changes, the actual air-fuel ratio cannot be detected. Only this is detected and the air-fuel ratio feedback control is continued.

According to the second aspect of the invention, the target air-fuel ratio is
The change is made within a predetermined range including the stoichiometric air-fuel ratio, and the change in the correlation between the output value and the air-fuel ratio is judged depending on whether the output value at that time is within the reference range. According to such a configuration, the target air-fuel ratio is varied around the stoichiometric air-fuel ratio, and the output range at that time indicates whether or not the oxygen sensor has a value commensurate with the variation in the target air-fuel ratio on the initial characteristics. Determine changes in output characteristics in the linear region.

According to the third aspect of the invention, the air-fuel ratio feedback control based on the actual air-fuel ratio obtained from the output value is
Proportional control based on the deviation between the actual air-fuel ratio and the target air-fuel ratio, and integral control based on rich / lean discrimination with respect to the target air-fuel ratio are performed, and air-fuel ratio feedback control when the correlation changes is performed with respect to the theoretical air-fuel ratio. The configuration is such that only integration control based on rich / lean discrimination is performed.

According to this structure, when the oxygen sensor has the initial characteristic, proportional control based on the deviation between the actual air-fuel ratio obtained from the sensor output and the target air-fuel ratio and integral control based on rich / lean discrimination with respect to the target air-fuel ratio are performed. The feedback control of the air-fuel ratio causes the output characteristic of the oxygen sensor to change, making it impossible to detect the actual air-fuel ratio.The proportional control based on the deviation is stopped, and the air-fuel ratio is fed back only through integral control based on rich / lean discrimination. To control.

According to a fourth aspect of the present invention, the air-fuel ratio feedback control based on the output value of the oxygen sensor is prohibited when the output value sticks to a predetermined maximum value or minimum value for a predetermined time or more. . According to this configuration,
When the output value of the oxygen sensor sticks to the maximum or minimum value and does not move for a predetermined time or more, it is determined that the sensor output does not correspond to the actual air-fuel ratio and rich lean detection cannot be performed. However, the air-fuel ratio feedback control based on the output value of the oxygen sensor is prohibited.

[0013]

According to the invention described in claim 1, even if the output characteristic of the oxygen sensor changes and the actual air-fuel ratio cannot be detected, the air-fuel ratio feedback control based on the rich / lean determination is performed. Further, there is an effect that a large decrease in air-fuel ratio controllability can be suppressed, and as a result, deterioration of exhaust properties can be suppressed.

According to the second aspect of the present invention, the inclination of the sensor output with respect to the air-fuel ratio in the region where the sensor output shows substantially linearity can be determined, and the state in which only rich / lean determination can be performed can be accurately determined. is there. According to the third aspect of the present invention, when the oxygen sensor exhibits the initial characteristic, the target air-fuel ratio can be controlled with good response and convergence stability by the proportional control according to the air-fuel ratio deviation and the integral control based on rich / lean discrimination. , When rich lean only can be identified from the output of the oxygen sensor,
There is an effect that the integral control can stably control the vicinity of the target air-fuel ratio.

According to the invention described in claim 4, it is possible to judge the state where the oxygen sensor cannot judge rich / lean,
It is possible to prevent the feedback control of the air-fuel ratio based on the erroneous detection result.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. FIG. 1 is a system configuration diagram of an engine in the embodiment. In FIG. 1, air is sucked into a combustion chamber of each cylinder of an engine 1 mounted on a vehicle through an air cleaner 2, an intake passage 3, and an electronically controlled throttle valve 4 which is driven to open and close by a motor.

An electromagnetic fuel injection valve 5 for directly injecting fuel (gasoline) is provided in the combustion chamber of each cylinder, and the fuel injected by the fuel injection valve 5 and the sucked air are introduced into the combustion chamber. A mixture is formed. Fuel injection valve 5
Is energized by the injection pulse signal output from the control unit 20 to open the valve, and injects the fuel whose pressure is adjusted to a predetermined pressure.

The air-fuel mixture formed in the combustion chamber is a spark plug 6.
Is ignited and burned. However, the engine 1 is not limited to the above direct injection gasoline engine, and may be an engine configured to inject fuel into the intake port.
Exhaust gas from the engine 1 is discharged from an exhaust passage 7, and an exhaust purification catalyst 8 is provided in the exhaust passage 7.

The catalyst 8 is a three-way catalyst that oxidizes harmful three components of exhaust gas, carbon monoxide CO and hydrocarbons HC, and reduces nitrogen oxide NOx to produce harmless carbon dioxide and water vapor. And nitrogen. Further, an evaporative fuel processing device for combusting the evaporative fuel generated in the fuel tank 9 is provided.

The canister 10 is an airtight container filled with an adsorbent 11 such as activated carbon, and an evaporative fuel introduction pipe 12 extending from the fuel tank 9 is connected to the canister 10. Therefore, the evaporated fuel generated in the fuel tank 9 is guided to the canister 10 through the evaporated fuel introducing pipe 12 and adsorbed and collected. Further, the canister 10 has a fresh air introduction port 13
And the purge pipe 14 is led out, and a purge control valve 15 whose opening and closing is controlled by a control signal from the control unit 20 is interposed in the purge pipe 14.

In the above structure, when the purge control valve 15 is controlled to be opened, the suction negative pressure of the engine 1 is changed to the canister 10.
As a result, the evaporated fuel adsorbed by the adsorbent 11 of the canister 10 is purged by the air introduced from the fresh air introduction port 13, and the purge air passes through the purge pipe 14 to the downstream side of the throttle valve 4 of the intake passage 3. It is taken in, and then burned in the combustion chamber of the engine 1.

The control unit 20 includes a CPU and R
A microcomputer including an OM, a RAM, an A / D converter, an input / output interface, etc.,
Input signals from various sensors are received, arithmetic processing is performed based on these signals, and the throttle valve 4, fuel injection valve 5, spark plug 6
And controlling the operation of the purge control valve 15 and the like. As the various sensors, a crank angle sensor 21 for detecting a crank angle of the engine 1 and a cam sensor 22 for extracting a cylinder discrimination signal from a cam shaft are provided, and the rotation speed of the engine 1 is determined based on the signal from the crank angle sensor 21. It is calculated.

In addition, an air flow meter 23 for detecting the intake air flow rate Qa upstream of the throttle valve 4 in the intake passage 3,
Accelerator sensor 24 for detecting the accelerator pedal depression amount (accelerator opening) APS, throttle valve 4 opening TV
A throttle sensor 25 that detects O, a water temperature sensor 26 that detects the cooling water temperature Tw of the engine 1, an oxygen sensor 27 that changes its output value in response to the oxygen concentration in exhaust gas, a vehicle speed VS
A vehicle speed sensor 28 for detecting P is provided.

The oxygen sensor 27 is the same as that disclosed in JP-A-11-32.
As disclosed in Japanese Patent No. 6266, a zirconia tube is provided so as to project into an exhaust pipe, and the zirconia tube has a ratio of the oxygen concentration in the exhaust outside the zirconia tube to the oxygen concentration in the atmosphere inside. It is an oxygen concentration cell type oxygen sensor that generates electromotive force. As shown in FIG. 2, the output value Es (electromotive force) of the oxygen sensor 27 changes around the theoretical air-fuel ratio, and the electromotive force is high on the rich side of the theoretical air-fuel ratio and lean on the lean side of the theoretical air-fuel ratio. Has a characteristic of lowering the electromotive force, but has a predetermined range (0.3 (V) ≦ output value Es including the theoretical air-fuel ratio equivalent value (0.6 V in the present embodiment).
The output characteristic is set so that the output value Es is substantially linear with respect to the air-fuel ratio when ≦ 0.8 (V).

The oxygen sensor of the oxygen concentration battery type is usually set to a characteristic that the output value Es suddenly changes near the stoichiometric air-fuel ratio, and is used as a sensor for judging rich / lean of the actual air-fuel ratio with respect to the theoretical air-fuel ratio. The oxygen sensor 27 in the present embodiment has linearity with respect to the air-fuel ratio within a predetermined range near the theoretical air-fuel ratio as shown in FIG. 2 by adjusting the characteristics of the electrodes, the zirconia tube, the protective layer and the like. It is as an output characteristic.

The control unit 20 controls the fuel injection amount so that the actual air-fuel ratio detected from the output value Es of the oxygen sensor 27 matches the target air-fuel ratio when a predetermined air-fuel ratio feedback control condition is satisfied. The air-fuel ratio feedback correction coefficient for correcting the above is calculated. The flowchart of FIG. 3 shows a failure diagnosis of the oxygen sensor 27 used for the air-fuel ratio feedback control. Here, when it is determined that the air-fuel ratio feedback control can be performed based on the output value of the oxygen sensor 27. Only, the air-fuel ratio feedback control is executed according to the flowchart described later.

In the flowchart of FIG. 3, in step S1, it is determined whether or not the failure diagnosis condition is satisfied. As the failure diagnosis condition, it is determined that there is no failure in the air-fuel ratio control parts other than the oxygen sensor 27 such as the fuel injection valve 5 and the air flow meter 23, and that the air-fuel ratio feedback control is in a predetermined operation range.

If the diagnostic condition is satisfied, the process proceeds to step S2. In step S2, it is determined whether the output value Es of the oxygen sensor 27 exceeds a predetermined value (1) or falls below a predetermined value (2). The predetermined value (1) is
It is a value of about 0.95, and it is determined whether the output value Es is included in the maximum value region, and the predetermined value (2) is
It is determined whether the output value Es is a value of about 0.02 and is included in the minimum value region.

The output value Es is a predetermined value (2) ≦ output value Es
If ≦ predetermined value (1) and the value is within the normal output range, the process proceeds to step S3. In step S3, the target air-fuel ratio is sequentially switched between an air-fuel ratio slightly richer than the stoichiometric air-fuel ratio and an air-fuel ratio slightly leaner than the stoichiometric air-fuel ratio. Fluctuate.

The air-fuel ratio that is slightly richer and leaner than the stoichiometric air-fuel ratio is, for example, 0.97, 1.03 in terms of excess air ratio.
The air-fuel ratio is about the same. In the next step S4, it is determined whether the output value Es output along with the switching of the target air-fuel ratio exceeds a predetermined value (3) or falls below a predetermined value (4). In other words, it is determined whether or not the output value Es output along with the switching of the target air-fuel ratio is a predetermined value (4) ≦ output value Es ≦ a predetermined value (3).

The predetermined values (3) and (4) are values set according to the target air-fuel ratio switched in step S3, and in the present embodiment, the predetermined value (3).
Is 0.7V, and the predetermined value (4) is 0.5V. In the initial state, as shown in FIG. 2, the output characteristic of the oxygen sensor 27 is such that the output value Es near the stoichiometric air-fuel ratio changes comparatively gently with respect to the change in the air-fuel ratio, but the output changes due to deterioration over time. May be changed more rapidly to have a characteristic close to the output of the rich / lean sensor in which the output suddenly changes at the stoichiometric air-fuel ratio (the characteristic when the sensitivity is abnormal in FIG. 2).

When the change of the output characteristic as described above occurs, the fluctuation range of the output value Es corresponding to the fluctuation of the target air-fuel ratio becomes larger than that in the initial state, and the fluctuation range is increased by a predetermined value ( 4) .ltoreq.output value Es.ltoreq.predetermined value (3) is determined as occurrence of an output value Es that does not satisfy. Therefore, in step S4, when it is determined that the output value Es output along with the switching of the target air-fuel ratio exceeds the predetermined value (3) or falls below the predetermined value (4), near the stoichiometric air-fuel ratio. Output value Es for air-fuel ratio change at
It is determined that the change in is changing more rapidly than in the initial state, and the process proceeds to step S5.

In step S5, a failure determination of the oxygen sensor 27 is performed. In the failure mode, particularly, a sensitivity abnormality determination indicating a state in which the output change near the stoichiometric air-fuel ratio is sharper is performed. Here, when the sensitivity abnormality occurs, the output value Es changes abruptly with a slight change in the air-fuel ratio in the vicinity of the theoretical air-fuel ratio, so that the actual air-fuel ratio cannot be obtained from the output value Es even in the vicinity of the theoretical air-fuel ratio. But,
Since the output characteristic at that time is close to the characteristic of the oxygen sensor that detects only the rich lean with respect to the stoichiometric air-fuel ratio, it can be used for the determination of rich lean.

On the other hand, in step S4, the fluctuation of the output value Es due to the switching of the target air-fuel ratio is the predetermined value (3), (4).
When it is determined that the oxygen sensor 27 is generated within the range between the two, the output characteristic of the oxygen sensor 27 maintains the output characteristic in the initial state where the output gradually changes comparatively near the stoichiometric air-fuel ratio as shown in FIG. It is judged that it is doing, step S
Then, the process proceeds to 6 to determine the normal state of the oxygen sensor 27.

If it is determined in step S2 that the output value Es of the oxygen sensor 27 exceeds the predetermined value (1) or is less than the predetermined value (2), the process proceeds to step S7. In step S7, a state of exceeding a predetermined value (1),
Alternatively, it is determined whether or not the state of falling below the predetermined value (2) has continued for a predetermined time or more.

When the state of exceeding the predetermined value (1) or the state of falling below the predetermined value (2) continues for a predetermined time or more, in other words, the output of the oxygen sensor 27 is set to the maximum value or the minimum value. When the oxygen sensor 27 sticks for a time or more, it can be determined that the oxygen sensor 27 is no longer sensitive to the actual air-fuel ratio. Therefore, if it is determined in step S7 that the state of exceeding the predetermined value (1) or the state of falling below the predetermined value (2) continues for a predetermined time or more, the process proceeds to step S8 and the oxygen sensor 27 fails. A determination is made, but here, a disconnection / short circuit failure is determined as a failure mode insensitive to the actual air-fuel ratio.

The flow chart of FIG. 4 shows a routine for performing the air-fuel ratio feedback control in response to the result of the above-mentioned failure diagnosis. In step S11, it is determined whether the air-fuel ratio feedback control condition is satisfied, excluding the failure diagnosis result of the oxygen sensor 27. When it is determined in step S11 that the air-fuel ratio feedback control condition is satisfied, the process proceeds to step S12, and it is determined whether or not a failure of the oxygen sensor 27 is diagnosed.

When it is determined that the oxygen sensor 27 is normal, the routine proceeds to step S13, where normal air-fuel ratio feedback control is performed. In the normal air-fuel ratio feedback control, the actual air-fuel ratio is obtained based on the output value Es of the oxygen sensor 27, proportional control based on the deviation between the actual air-fuel ratio and the target air-fuel ratio, and integral control based on rich / lean determination. It is supposed to be done by
A description will be given according to the flowchart of FIG.

In the flow chart of FIG. 5, in step S51, the output value Es of the oxygen sensor 27 is within a predetermined range (0.3 (V)) which is substantially linear with respect to the air-fuel ratio.
It is determined whether or not ≦ output value Es ≦ 0.8 (V)). When it is determined in step S51 that the output value Es of the oxygen sensor 27 is within a predetermined range having substantially linearity with respect to the air-fuel ratio, the process proceeds to step S52, and the output value Es of the oxygen sensor 27 is set to A process of converting to air-fuel ratio data is performed.

The conversion may be performed on the basis of a table showing the correlation between the output value Es and the air-fuel ratio, but in order to further improve the resolution, the output value Es is differently calculated based on a preset formula. After substituting the variables, the data of the air-fuel ratio may be obtained from the variables. In the next step S53, a deviation err (deviation err = actual air-fuel ratio-target air-fuel ratio) between the actual air-fuel ratio obtained from the output value Es and the target air-fuel ratio (theoretical air-fuel ratio) is obtained.

In step S54, the deviation err is multiplied by a proportional constant Kp to calculate a proportional manipulated variable P (P = err × Kp). In step S55, the target air-fuel ratio of the actual air-fuel ratio (theoretical air-fuel ratio) is determined by determining whether the deviation err is positive or negative.
Determine rich lean against. Step S55
If it is determined that the deviation err is negative and the actual air-fuel ratio is richer than the target air-fuel ratio (theoretical air-fuel ratio), step S
Proceed to 56.

In step S56, the result of subtracting the predetermined value ΔI from the previous value of the integrated operation amount I is set as the integrated operation amount I of this time. When it is determined in step S55 that the deviation err is positive and the actual air-fuel ratio is leaner than the target air-fuel ratio (theoretical air-fuel ratio), the process proceeds to step S57, where the previous value of the integrated operation amount I is set to a predetermined value. The result of adding ΔI is set as the current integrated operation amount I.

Further, in step S55, the deviation err is about 0.
When it is determined that the actual air-fuel ratio is substantially equal to the target air-fuel ratio (theoretical air-fuel ratio) in step S56, step S56.
By bypassing 57 and proceeding to step S62, the integral operation amount I is held at the previous value. In step S62, the air-fuel ratio feedback correction coefficient α is calculated as α = P + I + 1.0.

On the other hand, in step S51, the output value Es of the oxygen sensor 27 is out of the linear range (0.3 (V)> E).
If it is determined that s or Es> 0.8 (V)), step S
Proceed to 58. In step S58, the oxygen sensor 27
The actual air-fuel ratio is higher than the stoichiometric air-fuel ratio by determining whether or not the output value Es of is out of the range higher than the predetermined range, specifically, whether Es> 0.8 (V). Is also rich or not.

In step S58, when it is determined that Es> 0.8 (V) and the actual air-fuel ratio is richer than the stoichiometric air-fuel ratio, the process proceeds to step S59, and a predetermined value ΔI is obtained from the previous value of the integrated operation amount I. The result of the subtraction is set as the current integrated operation amount I. On the other hand, Es> 0.8 (V) in step S58
If not, it is determined that the actual air-fuel ratio is leaner than the stoichiometric air-fuel ratio, since 0.3 (V)> Es is satisfied and the process proceeds from step S51 to step S58.

In this case, the process proceeds to step 60, and the result of adding the predetermined value ΔI to the previous value of the integrated operation amount I is set as the current integrated operation amount I. Steps S59 and S60
When the integral operation amount I is set in step S1, the proportional operation amount P is set to 0 in the next step S61. The output value Es of the oxygen sensor 27 is within the predetermined range (0.3 (V) ≦ Es ≦ 0.8
(V)), the output value Es has substantially linearity with respect to the air-fuel ratio, and therefore the output value Es can be accurately converted into the air-fuel ratio data. However, if the output value Es is out of the predetermined range, the air-fuel ratio becomes correct. I can't ask.

However, since the rich / lean determination with respect to the stoichiometric air-fuel ratio can be performed even in a region outside the predetermined range, the integration based on the rich / lean determination is performed as when the output value Es is within the predetermined range. Control. Therefore, when the output value Es is out of the predetermined range, step S
When the process proceeds to 62, the air-fuel ratio feedback correction coefficient α is substantially calculated with α = I + 1.0.

The air-fuel ratio feedback correction coefficient α is
The final fuel injection amount Ti is obtained by multiplying the basic fuel injection amount Tp corresponding to the target air-fuel ratio, and an injection pulse signal having a pulse width corresponding to this fuel injection amount Ti is supplied to the fuel injection valve 5 of each cylinder. Is output. When it is determined in step S12 of the flowchart in FIG. 4 that a failure of the oxygen sensor 27 is diagnosed, the process proceeds to step S14, and it is determined whether the failure determination is abnormal sensitivity or disconnection / short circuit.

When a failure determination of abnormal sensitivity is made, the actual air-fuel ratio cannot be obtained from the output value Es, but the output characteristic at that time is the characteristic of the oxygen sensor that detects only rich / lean with respect to the theoretical air-fuel ratio. Since the characteristic is close to, and the rich / lean determination can be made, the process proceeds to step S15 and thereafter, and the air-fuel ratio feedback control is performed only based on the rich / lean determination.

In step S15, it is determined whether or not the output value Es of the oxygen sensor 27 is larger than the stoichiometric air-fuel ratio equivalent value (eg, 0.6 (V)), so that the actual air-fuel ratio is richer than the stoichiometric air-fuel ratio. It is determined whether or not there is. When it is determined that the output value Es is larger than the theoretical air-fuel ratio equivalent value and is in the rich state, the process proceeds to step S16, and the result obtained by subtracting the predetermined value ΔI from the previous value of the integral operation amount I is set as the current integration operation amount I. To do.

On the other hand, when the output value Es is equal to or less than the stoichiometric air-fuel ratio equivalent value in step S15 and it is judged that the engine is in the lean state, the routine proceeds to step 17, where the predetermined value ΔI is added to the previous value of the integrated manipulated variable I. Let the result be the integrated operation amount I of this time.
When the integral operation amount I is set in steps S16 and S17, the proportional operation amount P is set to 0 in the next step S18.
Set.

Then, in step S19, the air-fuel ratio feedback correction coefficient α is calculated as α = P + I + 1.0. The air-fuel ratio feedback control by the processing of steps S15 to 19 is performed in step S13.
Compared with the normal control in (flowchart of FIG. 5),
Since the proportional control based on the air-fuel ratio deviation is not performed, the convergence to the target air-fuel ratio is reduced.
Since the air-fuel ratio feedback control can be continued even if the characteristic change of No. 7 occurs, the air-fuel ratio controllability can be maintained and the deterioration of the exhaust property can be suppressed.

Further, in step S14, the oxygen sensor 27
When the failure diagnosis of is determined to be a disconnection / short circuit,
When the output value Es of the oxygen sensor 27 is insensitive to the actual air-fuel ratio, not only the actual air-fuel ratio is detected but also the rich
Since it is not possible to discriminate the lean state, the routine proceeds to step S20 and the air-fuel ratio feedback control is stopped. When the oxygen sensor 27 is provided with a heater and the air-fuel ratio feedback control is stopped based on the diagnosis result of disconnection / short circuit, the heater energization is stopped.

[Brief description of drawings]

FIG. 1 is a system configuration diagram of an engine according to an embodiment.

FIG. 2 is an output characteristic diagram of the oxygen sensor according to the embodiment.

FIG. 3 is a flowchart showing failure diagnosis of the oxygen sensor in the embodiment.

FIG. 4 is a flowchart showing air-fuel ratio feedback control for each failure mode in the embodiment.

FIG. 5 is a flowchart showing air-fuel ratio feedback control when the oxygen sensor is normal in the embodiment.

[Explanation of symbols]

1 ... engine 4 ... Throttle valve 5 ... Fuel injection valve 6 ... Spark plug 8 ... Catalyst 20 ... Control unit 21 ... Crank angle sensor 23 ... Air flow meter 27 ... Oxygen sensor

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Claims (4)

[Claims] 1. An oxygen sensor, the output value of which changes in response to the oxygen concentration in engine exhaust, wherein the sensor output value is substantially linear with respect to the air-fuel ratio in a predetermined range including the theoretical air-fuel ratio equivalent value. An engine air-fuel ratio control device that includes an oxygen sensor and determines the actual air-fuel ratio from the output value of the predetermined range by feedback control of the air-fuel ratio, and determines the change in the correlation between the output value and the air-fuel ratio within the predetermined range. An air-fuel ratio control device for an engine, wherein when the correlation changes, only rich / lean with respect to the stoichiometric air-fuel ratio is determined based on the output value, and the air-fuel ratio is feedback-controlled. 2. A target air-fuel ratio is varied within a predetermined range including a stoichiometric air-fuel ratio, and a change in the correlation between the output value and the air-fuel ratio is judged by whether or not the output value at that time is within a reference range. The air-fuel ratio control device for an engine according to claim 1, wherein: 3. An air-fuel ratio feedback control based on the actual air-fuel ratio obtained from the output value is proportional control based on a deviation between the actual air-fuel ratio and a target air-fuel ratio, and an integration based on rich / lean discrimination with respect to the target air-fuel ratio. 3. The air-fuel ratio of the engine according to claim 1, wherein the air-fuel ratio feedback control when the correlation is changed is performed only by integral control based on rich / lean discrimination with respect to the theoretical air-fuel ratio. Control device. 4. The air-fuel ratio feedback control based on the output value of the oxygen sensor is prohibited when the output value sticks to a predetermined maximum value or minimum value for a predetermined time or more. 2. The engine air-fuel ratio control device according to 2.