JPH0821282A - Abnormality detecting device for air-fuel ratio sensor - Google Patents

Abstract

PURPOSE:To prevent the inside and the outside of a detecting part from being decided to be normal even when the inside and the outside of the detecting part are temporarily brought into the same state as that when they are normal by a method wherein gas flowing through an exhaust passage is switched from only air to exhaust gas occasioned by combustion when the detecting part of an air-fuel ratio sensor is defected and the inside and the outside thereof are intercommunicated. CONSTITUTION:An oxygen sensor 29 is formed in a tubular shape and has a detecting part filled with air and based on a difference in density between the inside and the outside of the detecting part, an air-fuel ratio is detected. An electronic control unit (ECU) 35 measures a lean time in which an air-fuel ratio is rendered rich after it is rendered more lean than a theoretical air-fuel ratio, and decides that the oxygen sensor 29 is abnormal when the measurements exceeds a preset value. When during measurement of a lean time, gas flowing through an exhaust passage 8 is switched from air to exhaust gas occasioned by combustion, during a time in which a predetermined time starting from this point of time elapses, the ECU 35 forcibly continues measurement of a lean time even when an air-fuel ratio by the oxygen sensor 29 is switched from lean to rich.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for detecting an abnormality of an air-fuel ratio sensor for detecting an air-fuel ratio which is a ratio of air and fuel in a mixture.

[0002]

2. Description of the Related Art In a general internal combustion engine, a mixture of air and fuel is introduced into a combustion chamber through an intake passage and burned. The exhaust gas generated by the combustion is discharged to the outside of the internal combustion engine through the exhaust passage. An air-fuel ratio sensor that detects an air-fuel ratio, which is a ratio of air and fuel in the air-fuel mixture, is provided in the exhaust passage. Then, the fuel amount is feedback-controlled so that the air-fuel ratio measured by the air-fuel ratio sensor becomes a predetermined air-fuel ratio (usually the theoretical air-fuel ratio). The stoichiometric air-fuel ratio is the air-fuel ratio of the air-fuel mixture containing just enough oxygen to completely burn the fuel.

The air-fuel ratio sensor has a tubular detecting portion that projects into the exhaust passage and is filled with air.
The detection part is configured by coating the inner and outer surfaces of the zirconia element with platinum electrodes. The zirconia element generates an electromotive force according to the difference in oxygen concentration inside and outside, and the air-fuel ratio sensor outputs a voltage according to the electromotive force.

In the internal combustion engine, since the fuel amount is controlled based on the air-fuel ratio by the air-fuel ratio sensor, it is desirable that the output voltage of the air-fuel ratio sensor be a highly reliable value. Then, it is required to detect the abnormality of the air-fuel ratio sensor quickly and accurately. Therefore, various techniques for detecting abnormality of the air-fuel ratio sensor have been proposed. As one of them, during the feedback control of the fuel amount for making the air-fuel ratio by the air-fuel ratio sensor a predetermined air-fuel ratio, the lean time from when the air-fuel ratio becomes leaner than the predetermined air-fuel ratio to when it becomes rich is measured. There is a technique for determining that the air-fuel ratio sensor is abnormal when the measured value becomes larger than a predetermined value. Further, as a technique related to this, for example, Japanese Patent Laid-Open No.
There is a "method for detecting abnormality of air-fuel ratio sensor" in Japanese Patent Application Laid-Open No. 0-173332.

[0005]

By the way, as one of the abnormalities of the air-fuel ratio sensor, there is a phenomenon in which the detecting portion is defective and the inside and outside thereof communicate with each other. When this phenomenon occurs, if the exhaust gas accompanying combustion always flows through the exhaust passage, both the gas inside and outside the detection unit become exhaust gas, the oxygen concentration difference becomes zero (“0”), and electromotive force is generated. do not do. For this reason,
The air-fuel ratio measured by the air-fuel ratio sensor becomes lean, and the lean time becomes larger than a predetermined value, so that it is determined to be abnormal.

However, when the gas flowing through the exhaust passage is switched from only air to exhaust gas accompanying combustion during measurement of the lean time, for example, after fuel injection is once stopped and then restarted, the detection unit The inside and outside are temporarily in the same state as normal.

That is, when only the air flows through the exhaust passage due to the stop of the fuel injection, both the gas inside and outside the detecting portion becomes the air. From this state, when the exhaust gas accompanying the combustion flows through the exhaust passage due to the restart of fuel injection, the gas inside the detection unit remains air, and the gas around the detection unit temporarily becomes the exhaust gas. Then, a difference in oxygen concentration occurs inside and outside the detection unit, and the air-fuel ratio measured by the air-fuel ratio sensor becomes temporarily rich.

In the above-mentioned conventional technique, the measurement of the lean time is stopped according to the detection result, and the determination is made using an incorrect measurement value. As a result, there is a possibility that the air-fuel ratio sensor may be erroneously determined to be normal even though it is abnormal.

The present invention has been made in view of the above-mentioned circumstances, and an object thereof is that, when the detection portion of the air-fuel ratio sensor is defective and the inside and outside thereof communicate with each other, the gas flowing through the exhaust passage is burned from only air to combustion. It is an object of the present invention to provide an abnormality detection device for an air-fuel ratio sensor that can prevent the determination of normality even when the inside / outside of the detection unit is temporarily in the same state as normal when the exhaust gas is switched to.

[0010]

In order to achieve the above object, a first aspect of the present invention is to provide an intake passage for introducing a mixture of air and fuel into a combustion chamber, and a mixture in the combustion chamber. An exhaust passage that allows the exhaust gas generated by the combustion of air to flow, and a tubular detector that projects into the exhaust passage and is filled with air inside, and the difference in oxygen concentration inside and outside the detector. Based on the above, an air-fuel ratio sensor for detecting an air-fuel ratio which is a ratio of air and fuel in the air-fuel mixture is provided, and the fuel amount is controlled so that the air-fuel ratio by the air-fuel ratio sensor becomes a predetermined air-fuel ratio set in advance. Used in the internal combustion engine,
When the output of the air-fuel ratio sensor does not correspond to a change in the actual air-fuel ratio, the determination means that determines that the air-fuel ratio sensor is abnormal, and the air-fuel ratio by the air-fuel ratio sensor has shifted from the lean side to the rich side. It is provided with a determination prohibition unit that prohibits the determination by the determination unit during the subsequent predetermined period.

According to a second aspect of the present invention, in addition to the configuration of the first aspect of the present invention, the determination means is configured so that when the air-fuel ratio measured by the air-fuel ratio sensor becomes leaner than the predetermined air-fuel ratio, the air-fuel ratio is increased. Is to measure the lean time until it becomes richer than a predetermined air-fuel ratio, and the air-fuel ratio sensor is determined to be abnormal when the measured value becomes larger than a predetermined value, the determination prohibiting means, When the gas flowing through the exhaust passage is switched from air to exhaust gas accompanying combustion during measurement of the lean time by the determination means, the air-fuel ratio measured by the air-fuel ratio sensor will change from lean until the predetermined time elapses from that point. Even if it shifts to rich, the measurement of lean time is forced to continue.

[0012]

In the internal combustion engine, the fuel amount is controlled so that the air-fuel ratio measured by the air-fuel ratio sensor becomes a predetermined air-fuel ratio.
At this time, if the air-fuel ratio sensor operates normally and its output corresponds to the actual change of the air-fuel ratio, the air-fuel ratio by the air-fuel ratio sensor should repeat rich and lean. When the output of the air-fuel ratio sensor does not correspond to the actual change in the air-fuel ratio, the determination means of the first aspect of the invention determines that the air-fuel ratio sensor is abnormal.

By the way, as one of the abnormalities of the air-fuel ratio sensor, there is a phenomenon that the detecting portion is defective and the inside and outside thereof communicate with each other. If this phenomenon occurs and the gas flowing through the exhaust passage switches from only air to exhaust gas accompanying combustion during the determination by the determination means, the inside and outside of the detection unit temporarily become in the same state as normal.

More specifically, when only air flows through the exhaust passage, the air comes into contact with the outer surface of the detection unit and enters the detection unit. From this state, when the exhaust gas accompanying the combustion flows through the exhaust passage, the exhaust gas is temporarily in contact with the outer surface of the detection unit while the air remains in the detection unit. Then, a difference in oxygen concentration occurs inside and outside the detection unit, and the air-fuel ratio measured by the air-fuel ratio sensor becomes temporarily rich. what if,
If the abnormality determination is performed according to this detection result, the determination unit may erroneously determine that the air-fuel ratio sensor is normal although it is abnormal.

On the other hand, in the first aspect, when the air-fuel ratio measured by the air-fuel ratio sensor shifts from the lean side to the rich side, the processing content of the abnormality determination is changed. In making this change,
The fact that the air-fuel ratio sensor erroneously detects rich when the detector is defective is based on the fact that the exhaust gas around the detector is short in time until it enters the detector. Therefore, even if the rich is detected for a short time as described above, the abnormality determination based on the signal may not be performed.

From such a point of view, when the air-fuel ratio measured by the air-fuel ratio sensor shifts from the lean side to the rich side, the judgment prohibiting means prohibits the judgment by the judging means for a predetermined period from that point. Therefore, even if the air-fuel ratio sensor, which should normally detect lean, temporarily detects rich due to the lack of the detecting portion, the determination based on the detection result is not performed.

Particularly in the second invention, the judging means is
When the air-fuel ratio measured by the air-fuel ratio sensor becomes leaner than the predetermined air-fuel ratio, the lean time until the air-fuel ratio becomes richer than the predetermined air-fuel ratio is measured. Then, when the measured value becomes larger than a predetermined value, the determination means determines that the output of the air-fuel ratio sensor does not correspond to the actual change of the air-fuel ratio and the air-fuel ratio sensor is abnormal.

When the gas flowing through the exhaust passage is switched from the air to the exhaust gas accompanying combustion during the measurement of the lean time by the judging means, the content of the abnormality judgment processing is changed. The determination prohibiting means forcibly continues the lean time measurement until the predetermined time elapses from the time when the gas type is switched, even if the air-fuel ratio by the air-fuel ratio sensor changes from lean to rich.

Therefore, even if the air-fuel ratio sensor, which should normally detect lean, temporarily detects rich due to the lack of the detecting portion, the measurement of the lean time is not stopped. The air-fuel ratio sensor is not determined to be normal based on the measurement value obtained by the measurement stop.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment embodying the first and second inventions will be described below with reference to the drawings. FIG. 1 shows a schematic configuration of a gasoline engine (hereinafter, simply referred to as an engine) 1 as an internal combustion engine mounted on a vehicle. In a cylinder block 1a of the engine 1, a plurality of cylinders (cylinders, only one is shown in the figure) 2 are arranged in parallel. Each cylinder 2
A piston 3 is housed therein so as to be vertically reciprocable. Each piston 3 is connected to a crankshaft 5 by a connecting rod 4.

A combustion chamber 6 is formed above each piston 3, and an intake passage 7 and an exhaust passage 8 are in communication therewith. The cylinder head 1 is provided between each combustion chamber 6 and the intake passage 7.
An intake valve 9 attached to b selectively connects and disconnects. Further, the combustion chamber 6 and the exhaust passage 8 are selectively opened and closed by an exhaust valve 10 attached to the cylinder head 1b.

In the intake passage 7, an air cleaner 11, a surge tank 12, and an intake manifold 13 are arranged in this order from the upstream side to the combustion chamber 6, through which the air outside the engine 1 is passed. Is taken into. A throttle valve 14 is rotatably mounted in the intake passage 7. Air (intake air) flowing through the intake passage 7 changes in flow passage area according to the rotation angle of the throttle valve 14.
The amount of is adjusted.

An electromagnetic fuel injection valve 15 for supplying fuel to each combustion chamber 6 is attached to the intake manifold 13. The injection valve 15 includes a needle valve, a solenoid coil, and the like. When the solenoid coil is energized, the needle valve moves and the injection port is opened. High-pressure fuel is injected with the opening of the injection port.
Then, a mixture of the injected fuel and air is introduced into the combustion chamber 6.

To ignite the air-fuel mixture, the engine 1 is provided with an igniter 16, an ignition coil 17, a distributor 18, and a spark plug 19 for each cylinder. The igniter 16 is external (electronic control device described later)
Based on the ignition signal from, the energization of the primary coil of the ignition coil 17 is selectively permitted and cut off. When the power supply to the primary coil is cut off, a high voltage secondary voltage is generated in the secondary coil of the ignition coil 17. This secondary voltage is distributed to each spark plug 19 by the distributor 18. Then, a current flows between the electrodes of the spark plug 19 (discharge occurs), and a spark is generated.

The spark burns the air-fuel mixture in the combustion chamber 6, causing the piston 3 to reciprocate. This reciprocating motion is converted into rotational motion by the connecting rod 4, and the crankshaft 5 is rotationally driven. Gas (exhaust gas) in the combustion chamber 6 generated by the combustion is led to the exhaust passage 8.

In the exhaust passage 8, an exhaust manifold 20 and a catalytic converter 21 are arranged in this order from the combustion chamber 6 toward the downstream side, and exhaust gas passes through the exhaust manifold 20 and the catalytic converter 21.
Is discharged to the outside. The catalytic converter 21 is a device that purifies harmful three components (carbon monoxide, hydrocarbons, and nitrogen oxides) in the exhaust gas by the action of the catalyst 22. The catalytic converter 21 purifies the above three components most efficiently when the air-fuel ratio of the air-fuel mixture (the ratio of air to fuel) becomes the stoichiometric air-fuel ratio. The stoichiometric air-fuel ratio is the air-fuel ratio of an air-fuel mixture containing approximately the amount of oxygen required to completely burn the fuel (approximately 1
4.7).

Each part of the engine 1 has an air flow meter 23 and an intake air temperature sensor 2 for detecting its operating condition.
4, a throttle sensor 25, a water temperature sensor 26, a rotation speed sensor 27, a reference position sensor 28, an oxygen sensor 29 as an air-fuel ratio sensor, and a vehicle speed sensor 30 are provided.

The air flow meter 23 detects the amount of air flowing through the intake passage 7 (intake air amount), and the intake air temperature sensor 24 detects a temperature change of the air (intake air temperature). The throttle sensor 25 detects the rotation angle (throttle opening) of the throttle valve 14, and the water temperature sensor 26 detects the temperature of the cooling water of the engine 1 (cooling water temperature). Rotation speed sensor 27
And the reference position sensor 28 are both the crankshaft 5
It is provided in the distributor 18 that has a built-in rotor that rotates in synchronization with. The rotation speed sensor 27 detects the rotation speed of the crankshaft 5 (engine rotation speed) from the rotation of the rotor, and the reference position sensor 28 detects the rotation angle of the crankshaft 5 (crank angle) in a specific cylinder from the rotation of the rotor. To detect. Vehicle speed sensor 30
Is provided in a transmission (not shown) and detects the traveling speed (vehicle speed) of the vehicle.

The oxygen sensor 29 is for detecting the air-fuel ratio of the air-fuel mixture, and is attached to the exhaust passage 8.
As shown in FIG. 2A, the oxygen sensor 29 includes a detection unit 31 having a substantially circular tube shape having a closed tip (lower end in the figure). Most of the detector 31 is a zirconia element (ZrO ₂ ).
The inner surface and the outer surface thereof are coated with platinum, respectively. The detection unit 31 includes a pair of protective covers 32 having a large number of holes 32a and 33a on the outer circumference.
Covered by 33.

The detection unit 31 and both protective covers 32 and 33 project into the exhaust passage 8, and the gas (exhaust gas generated by combustion, air, etc.) flowing through the passage 8 is detected by the detection unit 3.
It comes in contact with the outer surface of 1. In addition, the detection unit 3
The inside of 1 is filled with air. The zirconia element has a property of generating an electromotive force when there is a large difference in oxygen concentration between the inner and outer surfaces at high temperature. The oxygen sensor 29 outputs a voltage having a magnitude corresponding to this electromotive force. In addition, when the inside and outside of the detection unit 31 are communicated with each other due to a defect, the exhaust gas enters the inside, and air contacts the outer surface of the detection unit 31,
Negative electromotive force is generated. In this case, the oxygen sensor 2
No. 9 outputs no voltage (the output voltage becomes "0" volt).

The relationship between the output voltage of the oxygen sensor 29 and the air-fuel ratio is shown in FIG. That is, the output voltage is about "1" when the air-fuel ratio is smaller (rich) than the theoretical air-fuel ratio.
It becomes a volt, and becomes about “0” volt when the air-fuel ratio is larger (lean) than the theoretical air-fuel ratio. Also, the output voltage is
It changes greatly when the air-fuel ratio is near the stoichiometric air-fuel ratio.

The characteristics of the oxygen sensor 29 are those under relatively high temperature conditions, and change greatly at low temperatures. Therefore, the heater 3 formed of ceramics or the like in a tubular shape
4 is arranged in the detection unit 31. The heater 34 keeps the detector 31 in a high temperature state, and a voltage having a characteristic close to that of FIG. 3 is always output.

The vehicle has a fuel injection valve 15 based on the detection signals of the air flow meter 23 and the sensors 24 to 30.
An electronic control unit (hereinafter referred to as ECU) 35 is provided to control each operation of the igniter 16 and to detect an abnormality of the oxygen sensor 29.

As shown in FIG. 4, the ECU 35 includes a central processing unit (hereinafter referred to as CPU) 36, a read-only memory (hereinafter referred to as ROM) 37, a random access memory (hereinafter referred to as RAM) 38, a backup RAM 39,
An external input circuit 40 and an external output circuit 41 are provided, which are connected to each other by a bus 42. ROM37
Stores in advance a control program such as an "abnormality detection routine" described later, a map defining the basic injection time and the basic ignition timing, initial data, and the like. The CPU 36 executes various arithmetic processes according to these control programs and the like. R
The AM 38 temporarily stores the calculation result by the CPU 36. The backup RAM 39 is backed up by a battery (not shown) to retain various data in the RAM 38 even after the power supply to the ECU 35 is stopped.

An intake air amount signal from the air flow meter 23, an intake air temperature signal from the intake air temperature sensor 24, a throttle opening signal from the throttle sensor 25, and a water temperature sensor 26.
The cooling water temperature signal from the engine, the rotational speed signal from the rotational speed sensor 27, the reference position signal from the reference position sensor 28, the oxygen concentration signal from the oxygen sensor 29, and the vehicle speed signal from the vehicle speed sensor 30 are input to the external input circuit 40. It CPU3
6 is the intake air amount Q and the intake air temperature TH based on these signals.
A, throttle opening TA, cooling water temperature THW, engine speed NE, crank angle reference position, oxygen concentration, and vehicle speed SPD are detected.

On the other hand, the CPU 36 uses the following equation (1) to
The amount of fuel injected from the fuel injection valve 15 (injection amount) is calculated. Here, the injection amount is determined by the time during which the needle valve of the fuel injection valve 15 is open (injection time), that is, the energization time to the solenoid coil. Therefore, in the formula (1), the injection time TAU is calculated as the injection amount.

TAU = TP × f (1) In the equation (1), TP is a basic injection time for making the air-fuel ratio the stoichiometric air-fuel ratio, and is obtained by referring to the map stored in the ROM 37. The map shows the engine speed NE
And the basic injection time TP with the intake air amount Q as a parameter
Is specified.

Further, f is a correction coefficient relating to intake air temperature, warm-up increase, post-start increase, air-fuel ratio feedback control, and the like. The coefficient relating to the intake air temperature is for correcting the deviation of the air-fuel ratio caused by the difference in the intake air density due to the intake air temperature. The coefficient relating to the warm-up increase amount is for increasing the basic injection time TP for the purpose of improving the driving performance during cold. The coefficient for increasing the amount of fuel after starting is
Is for stabilizing the engine speed NE immediately after the start of the engine.

A feedback correction coefficient FAF is used as a coefficient for feedback control of the air-fuel ratio.
The correction coefficient FAF is for correcting the basic injection time TP so that the air-fuel ratio of the air-fuel mixture converges to the stoichiometric air-fuel ratio, and is obtained as follows.

As shown in FIG. 5, the CPU 36 compares the output voltage of the oxygen sensor 29 with the reference value Vr corresponding to the stoichiometric air-fuel ratio. If the output voltage is higher than the reference value Vr, it is judged to be rich, and the reference value is set. If it is lower than Vr, it is judged to be lean. In the case of rich, it is compared with the previous detection result and it is determined whether or not the lean is reversed to the rich. If there is an inversion,
FAF-RS (RS is the amount of skip) is a new correction factor F
If AF is set and there is no inversion, FAF-KI (KI
Is an integration amount, and RS ≧ KI) is a new correction coefficient FAF.

When the air-fuel ratio is lean, the CPU 36
Compares with the previous detection result and determines whether or not the state is reversed from rich to lean. If there is inversion, FAF + RS is set as a new correction coefficient FAF, and if there is no inversion, FAF
Let F + KI be the new correction coefficient FAF. Therefore, when the air-fuel ratio is reversed between rich and lean, the correction factor FA
When F changes stepwise (skips) and the air-fuel ratio is rich or lean, the correction coefficient FAF gradually increases or decreases. In this way, the CPU 36 obtains the feedback correction coefficient FAF based on the detection value of the oxygen sensor 29 and the injection time TAU.
It is used to calculate

The air-fuel ratio feedback control using the correction coefficient FAF is performed when the following stop condition is not satisfied. The stop condition is, for example, that the engine 1 is started, that the amount of injected fuel is being increased after the start,
The cooling water temperature THW is lower than a predetermined value, the vehicle is running under a high load, and the fuel is cut off.

The CPU 36 controls the injection time T as described above.
When the AU is calculated, a drive signal corresponding to the calculated value is output to the fuel injection valve 15 via the external output circuit 41. Further, the CPU 36 obtains a basic ignition timing corresponding to the engine rotation speed NE and the intake air amount Q at that time from a map, and corrects it by a signal from each sensor to determine the ignition timing. The CPU 36 outputs an ignition signal to the igniter 16 according to the ignition timing.

Next, the operation and effect of this embodiment constructed as described above will be described. The flowchart of FIG. 6 shows a routine for detecting an abnormality of the oxygen sensor 29 among the respective processes executed by the CPU 36, which is executed every predetermined time. Each process of this abnormality detection routine is executed based on the counter C for lean time measurement.

The timing chart of FIG. 7 shows the correspondence between the output voltage of the oxygen sensor 29 and the value of the counter C. This figure shows a case where fuel injection is performed after timing t1, fuel injection is stopped at timing t5, and fuel injection is performed again at timing t6. Further, the same drawing shows a case where the tip of the detection unit 31 of the oxygen sensor 29 is missing at the timing t3 and the inside and outside of the detection unit 31 are communicated with each other.

First, the case where the abnormality detection routine is started at the timing t1 will be described. At this timing, the detection unit 31 is not missing and the oxygen sensor 29 is operating normally. Further, it is assumed that a sufficient time has elapsed since the last fuel injection stop (fuel cut) was completed. Further, fuel is injected from the fuel injection valve 15,
Immediately after the output voltage of the oxygen sensor 29 becomes lower than the reference value Vr and the air-fuel ratio switches from rich to lean. The value of the counter C is “0”.

The CPU 36 executes steps 101 to 10 in FIG.
In 5, it is determined whether or not the preconditions for detecting the abnormality of the oxygen sensor 29 are satisfied. Step 101
Then, it is determined whether or not the condition for performing the feedback control of the air-fuel ratio is satisfied, in other words, whether or not the above-described stop condition is not satisfied. In step 102, it is determined whether the engine rotation speed NE detected by the rotation speed sensor 27 is equal to or higher than a predetermined value α (for example, 1500 rpm).

In step 103, it is determined whether the vehicle speed SPD by the vehicle speed sensor 30 is less than a predetermined value β (for example, 100 km / h). In step 104, it is determined whether or not the abnormality of the vehicle speed sensor 30 and the water temperature sensor 26 has never been detected in the past. In step 105, it is determined whether the cooling water temperature THW is equal to or higher than a predetermined value γ (for example, 70 ° C.).

If all of the judgment conditions of these steps 101 to 105 are satisfied, the CPU 36 judges that the above-mentioned preconditions are satisfied, and the oxygen sensor 2 is judged in step 106.
It is determined whether the air-fuel ratio of 9 is rich. As described above, the timing t1 is immediately after the air-fuel ratio by the oxygen sensor 29 is switched from rich to lean, so the CPU 36 makes a negative determination in step 106, and increments the value of the counter C by “1” in step 109. In this case, the value of the counter C is "0" to "1".
become.

In step 110, the CPU 36 sets the value of the counter C to a predetermined value ε (for example, a value corresponding to 90 seconds).
It is determined whether or not this is the case. Since this value is "1" at the timing t1, the CPU 36 makes a negative determination in step 110, and immediately ends this routine.

In the control cycle from the next time onward, if the air-fuel ratio by the oxygen sensor 29 is lean, steps 101 to 106, 109 and 110 are repeated as described above. Each time the processing of step 109 is performed, the value of the counter C is incremented by "1".

At timing t2, the oxygen sensor 29
When the output voltage becomes higher than the reference value Vr and the air-fuel ratio becomes rich, the CPU 36 executes the processes of steps 101 to 105. The CPU 36 makes an affirmative decision in step 106, and decides in step 107 whether or not a predetermined time δ or more has elapsed after the fuel cut is completed. This predetermined time δ
Is set to a time (value) slightly longer than the time when the oxygen sensor 29 detects the air-fuel ratio rich when the fuel injection is restarted after the fuel cut in the state where the detection unit 31 is missing. . Here, since fuel cut has been performed in the past and sufficient time has elapsed since the last fuel cut was completed, the CPU 36 makes an affirmative decision in step 107 and then moves on to step 108 where the counter C Reset the value to "0". This value “0” is smaller than the predetermined value ε. For this reason, the CPU 36 makes a negative determination in step 110, and immediately ends this routine.

After the timing t2, if the air-fuel ratio measured by the oxygen sensor 29 is rich, the value of the counter C continues to be held at "0". Therefore, the CPU 36 proceeds to step 101.
~ 108, 110 processing is repeated, the oxygen sensor 29.
Is not abnormal.

At the timing t3, when the tip of the detecting portion 31 of the oxygen sensor 29 is lost and the inside and outside thereof are communicated with each other, the fuel is injected from the fuel injection valve 15, so that the combustion chamber 6
The exhaust gas generated by the combustion in the exhaust gas flows through the exhaust passage 8. This exhaust gas comes into contact with the outer surface of the detection unit 31, and as shown by the arrow in FIG.
After passing through the inside of the heater 34 from the above, it enters the space S between the outer surface of the heater 34 and the inner surface of the detection unit 31. Therefore, the gas inside and outside the detection unit 31 is exhaust gas, the oxygen concentration difference is "0", and no electromotive force is generated. Oxygen sensor 2
The output voltage of 9 is about "0" volt, and the sensor 29 detects that the air-fuel ratio is lean.

After executing the processes of steps 101 to 105, the CPU 36 makes a negative decision in step 106 and increments the value of the counter C for measuring the lean time by "1" in step 109. In this case, since the value of the counter C becomes "1", the CPU 36 makes a negative determination in step 110, and immediately ends this routine.

In the control cycle from the next time onward, each processing of steps 101 to 106, 109 and 110 is repeated as described above. Each time the processing of step 109 is performed, the value of the counter C is incremented by "1".

By this addition, when the value of the counter C reaches the predetermined value ε at the timing t4, the CPU 36 executes the step 1
It is determined that the determination condition 10 is satisfied, and it is determined in step 111 that the oxygen sensor 29 is abnormal. Then, the abnormality determination flag is set to, for example, "1" and stored in the backup RAM 39, and this routine is once ended.

When the fuel cut is started at the timing t5, the combustion in the combustion chamber 6 is not performed, and only the air flows through the exhaust passage 8. This air is detected by the detection unit 31 of the oxygen sensor 29.
The outer surface of the heater
After passing through the inside, it enters the space S in the detection unit 31. At this time, the gas inside and outside the detection unit 31 does not switch to air at the same time, and strictly speaking, after the surroundings of the detection unit 31 become air, the inside of the detection unit 31 switches from exhaust gas to air with a slight delay. Replace Therefore, the inside of the detection unit 31 is instantaneously filled with exhaust gas and the outside is surrounded by air.
Then, the oxygen concentration outside the detection unit 31 becomes higher than that inside, and a negative electromotive force is generated. The output voltage of the oxygen sensor 29 is "0" volt.

After that, both the gas inside and outside the detector 31 become air, the oxygen concentration difference becomes "0", and the electromotive force is not generated. The output voltage of the oxygen sensor 29 is about "0" volt. Therefore, while the fuel cut is being performed, the oxygen sensor 29 detects that the air-fuel ratio is lean.

Since the lean air-fuel ratio is detected from the start of the fuel cut to the end thereof, the CPU 36 executes steps 101 to 10 after the timing t5.
6, 109 to 111 are repeated.

When the fuel cut is completed at the timing t6, the fuel is again injected from the fuel injection valve 15, the mixture of the fuel and air is burned in the combustion chamber 6, and the exhaust gas generated by the combustion is exhausted. Flow through passage 8. The exhaust gas comes into contact with the outer surface of the detection portion 31 of the oxygen sensor 29, passes through the inside of the heater 34 from the defective portion A, and then enters the space S inside the detection portion 31. At this time, the gas inside and outside the detection unit 31 does not switch to the exhaust gas at the same time, and strictly speaking, after the surroundings of the detection unit 31 become the exhaust gas, the inside of the detection unit 31 is exhausted from the air gas with a slight delay. Switch to gas.

Therefore, in a moment, a state occurs in which the inside of the detection unit 31 is air and the surroundings of the detection unit 31 are exhaust gas. This state is the same as the state when the detection unit 31 is not defective. The oxygen concentration outside the detection unit 31 becomes smaller than that inside, and a positive electromotive force is generated. The output voltage of the oxygen sensor 29, which has been "0" volt until then, becomes about "1" volt, and the air-fuel ratio by the sensor 29 switches from lean to rich.

However, the oxygen sensor 29 detects the air-fuel ratio rich for a short period of time. After the fuel injection is restarted, when the exhaust gas after combustion around the oxygen sensor 29 enters the space S inside the detection unit 31, both the gas inside and outside the detection unit 31 become exhaust gas, and the oxygen concentration difference becomes “0”. ,
No electromotive force is generated. The output voltage of the oxygen sensor 29 is about “0” volt, and the sensor 29 detects the lean air-fuel ratio.

As described above, when the fuel cut is stopped and the fuel injection is restarted, the air-fuel ratio by the oxygen sensor 29 becomes rich for a moment immediately after the fuel injection is restarted, and then becomes lean thereafter.

Therefore, at timing t6, CP
After executing each processing of steps 101 to 105, U36 determines that the determination condition of step 106 is satisfied, and determines whether or not a predetermined time δ or more has elapsed since the fuel cut was completed in step 107. .

The CPU 36 determines that the determination condition of step 107 is not satisfied, and increments the value of the counter C by "1" in step 109. That is, even when the air-fuel ratio rich is detected by the oxygen sensor 29, the lean time measurement is continued. For this reason, the state in which the value of the counter C is equal to or greater than the predetermined value ε continues, and the CPU 36 determines that the determination condition of step 110 is satisfied, and determines the abnormality of the oxygen sensor 29 in step 111. End the routine once.

As described above, the oxygen sensor 29 detects the rich air-fuel ratio for a short period. The oxygen sensor 29 detects the lean air-fuel ratio at a timing before a predetermined time δ has elapsed after the end of the fuel cut. Therefore, at the timing t7 when the period elapses,
When the air-fuel ratio by the oxygen sensor 29 is switched from rich to lean, the CPU 36 performs steps 101 to 106, 10
After each processing of 9 and 110 is executed, this routine is once ended.

Although not shown in the timing chart of FIG. 7, if any of the determination conditions of steps 101 to 105 of FIG. 6 is not satisfied, the CPU 36 determines the abnormality of the oxygen sensor 29. It is determined that the precondition is not satisfied, and the value of the counter C is cleared in step 108. Then, since the condition of step 110 is not satisfied,
The CPU 36 once terminates this routine without judging the oxygen sensor 29 to be abnormal.

CPU 36 in the abnormality determination routine
The processing of steps 106, 109, 110, and 111 according to step 1 constitutes the judgment means, and the processing of both steps 107 and 109 constitutes the judgment prohibition means.

As described above in detail, in the state where the detecting portion 31 of the oxygen sensor 29 is defective and the inside and outside thereof are communicated with each other, during measurement of the lean time, the gas flowing through the exhaust passage 8 is accompanied by combustion from only air. When switching to exhaust gas, the detection unit 3
The inside and outside of 1 are temporarily in the same state as normal. Detection unit 3
A difference in oxygen concentration occurs inside and outside 1, and the air-fuel ratio measured by the oxygen sensor 29 becomes temporarily rich. At this time, the process of step 107 in FIG. 6 is omitted, the measurement of the lean time is stopped as shown by the chain double-dashed line in FIG. 7, and the value of the counter C is cleared (corresponding to the conventional technique). Although 29 is abnormal, there is a possibility that it may be erroneously determined to be normal.

On the other hand, in the present embodiment, when the gas flowing through the exhaust passage 8 is switched from air to exhaust gas accompanying combustion during the measurement of the lean time, a predetermined time δ elapses from the switching time. Even if the air-fuel ratio measured by the oxygen sensor 29 changes from lean to rich, the lean time measurement is forcibly continued. In other words, when the detection unit 31 is missing, the oxygen sensor 29 erroneously detects richness is a short time until the exhaust gas around the detection unit 31 enters the air in the detection unit 31. From this point of view, when the rich is detected for a short time after the fuel injection is restarted, the abnormality determination based on the signal is not performed (prohibited).

Therefore, even if the oxygen sensor 29, which should normally detect lean, temporarily detects rich due to the lack of the detecting portion 31, the measurement of lean time is not stopped. Unlike the prior art, it is possible to prevent the air-fuel ratio sensor from being erroneously determined to be normal based on the measurement value erroneously obtained by stopping the measurement.

In particular, in this embodiment, when the fuel injection from the fuel injection valve 15 is once stopped and then restarted, the gas in the exhaust passage 8 is switched from air to exhaust gas accompanying combustion. As a result, the time measurement is forced to continue. Therefore, switching from air to exhaust gas can be indirectly detected by stopping / restarting fuel injection, and a sensor for directly detecting the type of gas flowing through the exhaust passage 8 is not required. I'm sorry.

Further, if the abnormality diagnosis of the oxygen sensor 29 is performed according to the abnormality detection routine in which the process of step 107 is omitted and the fuel injection control is performed based on the diagnosis result, the feedback correction coefficient FAF is shown in FIG. Changes as indicated by the chain double-dashed line. Timing t6
Since the oxygen sensor 29 is determined to be abnormal before that, the correction coefficient FAF is "0", and the air-fuel ratio feedback control is not performed.

At timing t6, the output voltage of the oxygen sensor 29 becomes about "1" volt, and it is detected that the air-fuel ratio is rich. Then, the CPU 36 cancels the abnormality diagnosis and tries to perform normal feedback control. The correction coefficient FAF once becomes a negative value according to the detection result of the oxygen sensor 29, and returns to “0” (timing t).
7). After that, since the air-fuel ratio measured by the oxygen sensor 29 remains lean, the CPU 36 increases the correction coefficient FAF so that the air-fuel ratio becomes the stoichiometric air-fuel ratio. When the correction coefficient FAF reaches the upper limit guard due to this increase (timing t8), it is held at that value.

The correction coefficient FAF is the injection time T
When reflected in AU, the air-fuel ratio becomes excessively rich, which may cause problems such as engine stall and rough idle. Engine stall is the engine speed N
This is a phenomenon in which E decreases and the operation of the engine 1 stops.
Rough idle is engine speed N when idling.
This is a phenomenon in which E greatly fluctuates or the engine 1 vibrates greatly.

On the other hand, in the present embodiment, even if the oxygen sensor 29 temporarily detects rich due to the lack of the detecting portion 31 as described above, it is not judged to be normal. Therefore, the feedback correction coefficient FAF is set at the timing t6.
After that, it is held at "0" as shown by the solid line in FIG. Therefore, the feedback control of the air-fuel ratio using the correction coefficient FAF is not performed, an excessive rich air-fuel ratio can be avoided, and problems such as engine stall and rough idle can be prevented.

The present invention can be embodied in another embodiment shown below. (1) The abnormality detection device according to the first and second aspects of the invention can be applied to a case where an abnormality is detected in the air-fuel ratio sensor having no heater.

(2) In the above embodiment, whether or not the gas flowing through the exhaust passage 8 is switched from air to exhaust gas due to combustion is indirectly determined by whether or not the fuel cut is completed. Instead of this, a sensor capable of detecting the type of gas flowing through the exhaust passage 8 may be provided in the same passage 8.

(3) The first invention is not limited to the above embodiment,
For example, when the air-fuel ratio does not change from lean to rich when returning from fuel cut (when fuel injection is restarted), it is determined as abnormal, and when oxygen sensor 29 does not output rich for a predetermined period, it is determined as abnormal. It can also be applied to anomaly detection devices such as the type.

Although the respective embodiments of the present invention have been described above, technical ideas other than the claims which can be understood from the respective embodiments will be described below together with their effects. (A) In the abnormality detection device according to claim 2, the determination prohibiting means exhausts the fuel when the fuel injection from the fuel injection valve is once stopped (fuel cut is performed) and the fuel injection is restarted. An abnormality detection device for an air-fuel ratio sensor that forcibly continues time measurement when the gas in the passage switches from air to exhaust gas accompanying combustion.

With such a configuration, the switching from air to exhaust gas can be indirectly detected by stopping / restarting fuel injection. Therefore, it is not necessary to provide a special sensor for directly detecting the type of gas flowing through the exhaust passage.

[0083]

As described in detail above, in the first invention,
When it is determined that the air-fuel ratio sensor is abnormal when the output of the air-fuel ratio sensor does not correspond to the actual change in the air-fuel ratio, the predetermined period after the air-fuel ratio measured by the air-fuel ratio sensor changes from lean to rich side. Prohibits the determination of abnormality. In particular, in the second aspect, when the gas flowing through the exhaust passage switches from air to exhaust gas accompanying combustion during measurement of the lean time, the air-fuel ratio sensor is used until a predetermined time elapses from that time. Even if the air-fuel ratio changes from lean to rich, the lean time measurement is forced to continue. Therefore, when the detection part of the air-fuel ratio sensor is missing and the inside and outside communicate with each other, the gas flowing through the exhaust passage is switched from only air to exhaust gas accompanying combustion, and the inside and outside of the detection part are temporarily normal. Even if the same state is obtained, it is possible to prevent the erroneous determination as normal.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an engine equipped with an abnormality detection device for an air-fuel ratio sensor and its peripheral parts in an embodiment embodying the first and second inventions.

2A is a partially enlarged cross-sectional view of the oxygen sensor in FIG. 1, and FIG. 2B is a partially enlarged cross-sectional view showing a state in which a tip of a detection portion of the oxygen sensor of FIG.

FIG. 3 is a characteristic diagram showing output characteristics of the oxygen sensor in FIG.

FIG. 4 is a block diagram showing an internal configuration and the like of an ECU (electronic control unit) in FIG.

FIG. 5 is a timing chart showing a correspondence relationship between an output voltage of an oxygen sensor and a feedback correction coefficient.

FIG. 6 is a flowchart illustrating an abnormality detection routine executed by the CPU in FIG.

FIG. 7 is a timing chart showing the correspondence relationship between the output voltage of the oxygen sensor, the value of the counter, and the presence / absence of fuel injection.

FIG. 8 is a timing chart showing the correspondence relationship between the output voltage of the oxygen sensor, the feedback correction coefficient, and the presence or absence of fuel injection.

[Explanation of symbols]

1 ... a gasoline engine as an internal combustion engine, 6 ... a combustion chamber,
7 ... Intake passage, 8 ... Exhaust passage, 15 ... Fuel injection valve, 29
... Oxygen sensor as air-fuel ratio sensor, 31 ... Detector, 3
6 ... CPU configuring judgment means and judgment prohibition means, δ ...
Predetermined time, ε ... Predetermined value.

Claims (2)

[Claims] 1. An intake passage for introducing a mixture of air and fuel into a combustion chamber, an exhaust passage for allowing an exhaust gas generated by combustion of the mixture in the combustion chamber to flow, and an exhaust passage protruding into the exhaust passage. An air-fuel ratio sensor for detecting the air-fuel ratio, which is the ratio of air to fuel in the air-fuel mixture, based on the difference in oxygen concentration inside and outside the detector, which has a tubular detector filled with air. Which is used in an internal combustion engine configured to control the amount of fuel in order to make the air-fuel ratio by the air-fuel ratio sensor a predetermined air-fuel ratio, and the output of the air-fuel ratio sensor is the actual air-fuel ratio. When it does not correspond to the change, the determination unit that determines that the air-fuel ratio sensor is abnormal, and the determination by the determination unit is prohibited for a predetermined period after the air-fuel ratio measured by the air-fuel ratio sensor shifts from lean to rich side. The judgment prohibition means Abnormality detection device for an air-fuel ratio sensor was e. 2. The determining means, when the air-fuel ratio measured by the air-fuel ratio sensor becomes leaner than the predetermined air-fuel ratio, measures the lean time until the air-fuel ratio becomes richer than the predetermined air-fuel ratio, and measures it. When the value becomes larger than a predetermined value, the air-fuel ratio sensor is determined to be abnormal, the determination prohibiting means, during the measurement of the lean time by the determining means, the gas flowing through the exhaust passage burns from air. When the exhaust gas is switched to the exhaust gas, the measurement of the lean time is forcibly continued until the predetermined time elapses from that time, even if the air-fuel ratio by the air-fuel ratio sensor changes from lean to rich. The air-fuel ratio sensor abnormality detection device according to claim 1.