JP2007239491A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an air-fuel ratio control device for an internal combustion engine capable of effectively preventing the deterioration of emission by surely purifying exhaust gas flowing into a catalyst during the failure diagnosis of an air-fuel ratio sensor in relation to the air-fuel ratio control device for an internal combustion engine suitable for a device performing air-fuel ratio control at the failure diagnosis of the air-fuel ratio sensor. <P>SOLUTION: This device is provided with a catalyst arranged in an exhaust gas passage of the internal combustion engine, an exhaust gas sensor arranged in the exhaust gas passage, a means feeding an output of the exhaust gas sensor back to a fuel injection quantity to match an air-fuel ratio of exhaust gas flowing in the catalyst to control a target air-fuel ratio, a means discriminating whether it is an abnormal period during which an abnormal output of the exhaust gas sensor is detected or not, and an air-fuel ratio control means changing an air-fuel ratio of air-fuel mixture supplied to the internal combustion engine alternately to a fuel rich side and a fuel lean side based on a feed back control quantity before the abnormal period during the abnormal period. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly to an air-fuel ratio control apparatus for an internal combustion engine suitable as an apparatus for performing air-fuel ratio control when diagnosing a failure of an air-fuel ratio sensor.

  Conventionally, as disclosed in, for example, Japanese Patent Laid-Open No. 8-327586, there is known a system for detecting an abnormality of an exhaust gas sensor (hereinafter also simply referred to as “sensor”) disposed in an exhaust passage of an internal combustion engine. Yes. The exhaust gas sensor includes an exhaust-side electrode provided so as to be exposed to the exhaust gas, and an atmosphere-side electrode provided so as to be exposed to the atmosphere layer inside the sensor element.

  The atmospheric layer is a space that is isolated from the internal space of the exhaust passage by the sensor element and that conducts to the atmosphere. In the above conventional system, when a voltage (hereinafter referred to as “reverse voltage”) is applied between the exhaust side electrode as a positive electrode and the atmosphere side electrode as a negative electrode, the sensor element corresponds to the oxygen concentration in the atmospheric layer. Current flows. That is, if the sensor element is normal, a current corresponding to the oxygen concentration in the atmosphere flows. On the other hand, if the sensor element is cracked and the exhaust gas is mixed in the atmospheric layer, the oxygen concentration in the atmospheric layer decreases, so the current flowing through the sensor element is small compared to the normal current. It becomes.

  As described above, the value of the current generated by applying the reverse voltage changes depending on whether or not the sensor element is cracked. Therefore, it is possible to determine whether or not the sensor element is cracked by paying attention to the value of the current.

JP-A-8-327586 JP-A-4-27733 JP 2004-204772 A JP 2003-3881 A

Incidentally, the internal combustion engine includes an exhaust purification catalyst. The exhaust purification catalyst has a function of storing oxygen inside. Then, CO contained in the exhaust gas, oxidation of HC, perform reduction of NO _X, respectively _{CO 2,} H 2 _O, has the ability to detoxify into O 2, _{N 2.} For this reason, when the oxidation reaction and the reduction reaction are performed in a well-balanced manner, the amount of oxygen stored in the catalyst can be prevented from being biased, and the purification capacity of the catalyst can always be kept high.

  However, since a reverse voltage is applied to the sensor during failure diagnosis of the exhaust gas sensor, feedback control based on the sensor output cannot be performed, and the air-fuel ratio of the exhaust gas is changed from the stoichiometric air-fuel ratio to the fuel lean side or the fuel rich side. May fall off. For this reason, if such a situation is continued, only the oxidation or reduction reaction is frequently performed in the catalyst, and exhaust gas cannot be sufficiently purified due to insufficient oxygen or full storage, resulting in deterioration of emissions.

  The present invention has been made to solve the above-described problems, and can reliably purify exhaust gas flowing into a catalyst and effectively prevent emission deterioration during failure diagnosis of an air-fuel ratio sensor. An object of the present invention is to provide an air-fuel ratio control apparatus for an internal combustion engine that can perform the above-described operation.

In order to achieve the above object, a first invention is an air-fuel ratio control apparatus for an internal combustion engine,
A catalyst disposed in the exhaust passage of the internal combustion engine;
An exhaust gas sensor disposed in the exhaust passage;
Feedback means for feeding back the output of the exhaust gas sensor to the fuel injection amount so that the air-fuel ratio of the exhaust gas flowing into the catalyst matches the control target air-fuel ratio;
An abnormal period determining means for determining whether an abnormal period in which an abnormal output of the exhaust gas sensor is detected;
During the abnormal period, the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is changed alternately between the fuel rich side and the fuel lean side based on the feedback control amount based on the feedback means before the abnormal period. Air-fuel ratio control means;
It is characterized by providing.

The second invention is the first invention, wherein
The feedback control amount is calculated based on an output value of the exhaust gas sensor immediately before the abnormal period.

The third invention is the first or second invention, wherein
The abnormal period is a period from when a diagnosis voltage used for failure diagnosis of the exhaust gas sensor is applied to the exhaust gas sensor until the output of the exhaust gas sensor is stabilized.

The fourth invention is the first to third inventions,
The air-fuel ratio control means alternately changes between a fuel rich side and a fuel lean side every predetermined time.

The fifth invention is the first to third inventions,
The air-fuel ratio control means is characterized in that it fluctuates alternately between a fuel rich side and a fuel lean side every predetermined number of fuel injections.

  According to the first invention, the air-fuel ratio feedback control during an abnormal period in which an abnormal output of the exhaust gas sensor is detected, the air-fuel mixture supplied to the internal combustion engine based on the feedback control amount based on the feedback control before the abnormal period Can be alternately changed between the fuel rich side and the fuel lean side. The internal combustion engine causes exhaust gas generated by combustion of the air-fuel mixture to flow into the catalyst. For this reason, according to the present invention, fuel-rich or fuel-lean exhaust gas can alternately flow into the catalyst, and the exhaust purification capacity of the catalyst can be enhanced.

  According to the second invention, the feedback control amount can be calculated based on the output value of the exhaust gas sensor immediately before the abnormal output of the exhaust gas sensor is detected. The correct sensor output value is detected immediately before the abnormal period. Therefore, according to the present invention, the feedback control amount can be calculated with high accuracy based on the output value.

  According to the third invention, the abnormal period is a period from when a diagnosis voltage used for failure diagnosis is applied to the exhaust gas sensor to when the output of the exhaust gas sensor is stabilized in failure diagnosis of the exhaust gas sensor. be able to.

  According to the fourth aspect of the invention, the air-fuel mixture can be alternately changed between the fuel rich side and the fuel lean side every predetermined time with reference to the feedback control amount. For this reason, according to the present invention, the combustion of the fuel lean or fuel rich mixture can be reliably switched at the same time.

  According to the fifth aspect of the present invention, the air-fuel mixture can be alternately changed between the fuel rich side and the fuel lean side every predetermined number of times of fuel injection based on the feedback control amount. Therefore, according to the present invention, a desired amount of fuel lean or fuel-rich exhaust gas can be surely and alternately flow into the catalyst.

  Several embodiments of the present invention will be described below with reference to the drawings. In addition, the same code | symbol is attached | subjected to the element which is common in each figure, and the overlapping description is abbreviate | omitted. The present invention is not limited to the following embodiments.

Embodiment 1 FIG.
[Configuration of Embodiment 1]
FIG. 1 is a diagram for explaining a hardware configuration according to the first embodiment of the present invention. As shown in FIG. 1, the system of this embodiment includes an internal combustion engine 10. An intake passage 12 and an exhaust passage 14 communicate with the internal combustion engine 10. An air flow meter 16 that detects the amount of intake air is disposed in the intake passage 12. A throttle valve 18 is disposed downstream of the air flow meter 16. In the vicinity of the throttle valve 18, a throttle sensor 20 for detecting the throttle opening degree TA is disposed.

  Each cylinder of the internal combustion engine 10 is provided with an injector 22, an intake valve 24, an ignition plug 26, and an exhaust valve 28 for injecting fuel into the intake port.

  A crank angle sensor 32 for detecting the rotation angle of the crankshaft 30 is attached in the vicinity of the crankshaft 30 of the internal combustion engine. According to the output of the crank angle sensor, the rotational position of the crankshaft 30, the engine speed NE, and the like can be detected.

An exhaust purification catalyst (hereinafter simply referred to as “catalyst”) 34 is disposed in the exhaust passage 14 of the internal combustion engine 10. The catalyst 34 selectively holds (stores) NO _X in the exhaust by adsorbing, absorbing, or both when the inflowing exhaust air-fuel ratio is lean, and the inflowing exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich air-fuel. When the fuel ratio is reached, the stored NO _X is reduced and purified using the reducing components (HC, CO) in the exhaust. In other words, the catalyst 34 is oxidized by holding (storing) oxygen contained in the gas flowing through the exhaust passage 14, and when the reducing component is contained in the exhaust, the catalyst 34 is released into the reduced state by releasing oxygen. It is what is done.

  An air-fuel ratio sensor (A / F sensor) 36 is disposed in the exhaust passage 14 upstream of the catalyst 34. The air-fuel ratio sensor 36 is a sensor that linearly detects the oxygen concentration in the exhaust gas, and is based on the oxygen concentration in the exhaust gas flowing into the catalyst 34, and the air-fuel ratio of the air-fuel mixture that has been combusted in the internal combustion engine 10. Is detected.

  The air-fuel ratio control apparatus according to the present embodiment includes an ECU (Electronic Control Unit) 50. The ECU 50 is connected to the various sensors described above, the injector 22, and the like. The ECU 50 can control the operating state of the internal combustion engine 10 based on those sensor outputs.

[Air-fuel ratio control in Embodiment 1]
Air-fuel ratio feedback control:
The apparatus of the present embodiment executes air-fuel ratio feedback control based on the output of the air-fuel ratio sensor 36 and the like. In this air-fuel ratio feedback control, more specifically, based on the air-fuel ratio sensor output, the air-fuel ratio of exhaust gas discharged from the internal combustion engine (hereinafter referred to as “control A / F”) is set to the control target A / F. A process (hereinafter referred to as “main feedback control”) for controlling the fuel injection amount so as to coincide with (usually the theoretical air-fuel ratio) is performed.

The exhaust gas for which the main feedback control has been executed flows into the catalyst 34 disposed in the exhaust passage 14. As described above, the catalyst 34 has a function of storing oxygen therein. Then, CO contained in the exhaust gas, oxidation of HC, perform reduction of NO _X, respectively _{CO 2,} H 2 _O, has the ability to detoxify into O 2, _{N 2.} For this reason, when the oxidation reaction and the reduction reaction are performed in a well-balanced manner, the amount of oxygen stored in the catalyst can be prevented from being biased, and the purification capacity of the catalyst can always be kept high.

Air-fuel ratio sensor failure diagnosis:
In the apparatus of the present embodiment, failure diagnosis determination of the air-fuel ratio sensor 36 is periodically executed. The air-fuel ratio sensor 36 includes an electrolyte layer made of zirconia or the like, an exhaust side electrode provided so as to be exposed to the exhaust gas, and an air side electrode provided so as to be exposed to the air layer. The air-fuel ratio sensor 36 is selectively applied with a positive voltage for detecting the sensor output and a reverse voltage for performing failure diagnosis. When a positive voltage is applied, a sensor current corresponding to the oxygen excess / deficiency in the exhaust gas, that is, a sensor current corresponding to the air-fuel ratio of the exhaust gas flows between the atmosphere side electrode and the exhaust side electrode. Therefore, the exhaust air / fuel ratio can be detected by detecting the sensor current.

  On the other hand, when a reverse voltage is applied, oxygen in contact with the surface of the atmosphere side electrode is ionized and pumped toward the exhaust side electrode. As a result, a negative current (reverse current) having a correlation with the oxygen concentration in the atmosphere layer flows between the exhaust side electrode and the atmosphere side electrode. That is, if the sensor element is normal, a current corresponding to the oxygen concentration in the atmosphere flows, the sensor element is cracked, and if the exhaust gas is mixed in the atmosphere layer, Since the oxygen concentration is lowered, the current flowing through the sensor element is smaller than the normal current.

  Thus, the value of the current generated by applying a voltage between the exhaust-side electrode and the atmosphere-side electrode varies depending on whether or not the sensor element is cracked. Therefore, it can be determined whether or not the sensor element is cracked based on the fluctuation of the current value.

Air-fuel ratio control during failure diagnosis:
Since the reverse voltage is applied to the sensor during the above-described sensor failure diagnosis, the sensor output accompanying the positive voltage application cannot be detected. Moreover, immediately after application of the reverse voltage, an accurate sensor output cannot be detected due to the influence of oxygen ions pumped to the exhaust-side electrode during application of the reverse voltage. For this reason, in addition to the reverse voltage application period, a period until a stable sensor output signal is obtained (hereinafter referred to as “failure diagnosis period”), the main feedback control cannot be performed, and the control A / There is a possibility that F deviates from the stoichiometric air-fuel ratio to the fuel lean side or the fuel rich side.

When the control A / F deviates to the fuel lean side, fuel lean exhaust gas flows into the catalyst. The fuel lean exhaust gas contains a large amount of NO _X. For this reason, NO _X reduction reaction is frequently performed inside the catalyst, and a large amount of oxygen is stored inside the catalyst. If such a situation continues, the oxygen storage amount of the catalyst reaches a limit (full storage), NO _X that cannot undergo a reduction reaction is discharged as it is, and the emission deteriorates.

  On the other hand, when the control A / F deviates to the fuel rich side, the fuel rich exhaust gas flows into the catalyst. The fuel-rich exhaust gas contains a large amount of HC and CO. For this reason, oxidation reactions of HC and CO are frequently performed inside the catalyst, and oxygen stored in the catalyst is discharged. If such a situation continues, the oxygen stored in the catalyst will be insufficient, and HC and CO that cannot be oxidized will be discharged as they are, which will cause the emission to deteriorate.

  Therefore, in the present embodiment, during the failure diagnosis period of the air-fuel ratio sensor 36, the fuel / air ratio A / F of the fuel gas supplied to the internal combustion engine 10 is determined based on the main feedback control amount immediately before the failure diagnosis period. It is assumed that the rich side and the fuel lean side are changed alternately. When the fuel-rich exhaust gas and the fuel-lean exhaust gas alternately flow into the catalyst, the oxidation reaction and the reduction reaction at the catalyst are executed alternately. For this reason, the storage and discharge of oxygen in the catalyst are repeatedly performed in a balanced manner, and the bias of the oxygen storage amount can be effectively prevented and the exhaust gas can be reliably purified. Therefore, it is possible to effectively prevent the deterioration of the emission even in the failure diagnosis period.

[Specific Processing in Embodiment 1]
FIG. 2 is a flowchart of a routine executed by the ECU 50 in order to realize the above-described main feedback control during the failure diagnosis period, and is executed every predetermined time (for example, every 16 msec). According to the routine shown in FIG. 2, first, it is determined whether or not it is a failure diagnosis period of the air-fuel ratio sensor 36 (step 100). The failure diagnosis period is a period specified based on the characteristics of the sensor, the applied voltage of the reverse voltage, the applied time, and the like. Specifically, it is determined whether or not the specified fault diagnosis period is met.

  If it is determined in step 100 that it is not the failure diagnosis period, normal main feedback control processing is executed (step 102). Specifically, for example, when calculating the main feedback correction value by PI control, first, the control A / F is specified based on the sensor signal of the air-fuel ratio sensor 36. The ECU 50 stores a map that defines the relationship between the sensor signal (voltage) and the control A / F. Here, the control A / F corresponding to the sensor signal is specified according to the map. Then, the deviation (ΔA / F) between the control A / F and the control target A / F is calculated, and the main feedback correction amount is calculated as the sum of the proportional term and the integral term based on ΔA / F.

  On the other hand, if it is determined in step 100 that the failure diagnosis period has elapsed, it is determined in the previous routine whether or not rich control shown in step 106 described later has been executed (step 104). Here, specifically, the executed control is specified based on the control history selected in the previous routine.

  If it is determined in step 104 that the previous rich control has not been executed, that is, if the normal main feedback control shown in step 102 or the lean control shown in step 108 described later is performed in the previous routine. Then, rich control is executed (step 106). Here, specifically, first, the same processing as the normal main feedback control executed in step 102 described above is executed. The main feedback correction amount calculated here is a fuel injection correction amount for setting the control A / F to the control target A / F (theoretical air-fuel ratio). Therefore, in order to realize rich control, next, a process of multiplying the correction amount by the rich correction coefficient β (β ≧ 1.0) is executed. The rich correction coefficient β is a value of 1 or more (for example, β = 1.1), and is set within a range in which a rapid torque fluctuation does not occur. For this reason, the rich control correction amount is larger than the feedback correction amount for realizing the stoichiometric air-fuel ratio, so that fuel-rich exhaust gas flows into the catalyst.

  On the other hand, if it is determined in step 104 that the rich control shown in step 106 has been executed in the previous routine, the lean control is executed (step 108). Here, specifically, first, the same processing as the normal main feedback control executed in step 102 described above is executed. The main feedback correction amount calculated here is a fuel injection correction amount for setting the control A / F to the control target A / F (theoretical air-fuel ratio) as described above. Therefore, in this step, next, a process of multiplying the correction amount by the lean correction coefficient α (α ≦ 1.0) is executed. The lean correction coefficient α is a value equal to or less than 1 (for example, α = 0.9), and is set within a range in which a rapid torque fluctuation does not occur, similarly to the rich correction coefficient β. For this reason, the lean control correction amount is smaller than the feedback correction amount for realizing the stoichiometric air-fuel ratio, so that fuel-lean exhaust gas flows into the catalyst.

  As described above, this routine is repeatedly executed every predetermined time (for example, every 16 msec). For this reason, in the failure diagnosis period, the rich control shown in step 106 and the lean control shown in step 108 are executed alternately. Therefore, the fuel gas corresponding to these fuel injection correction amounts is sequentially supplied to the internal combustion engine 10, whereby the fuel-rich exhaust gas and the fuel-lean exhaust gas can alternately flow into the catalyst 34. As a result, the oxidation reaction and the reduction reaction in the catalyst are executed in a well-balanced manner, and the exhaust gas is always reliably purified without unevenness in the amount of oxygen stored in the catalyst, and the deterioration of the emission can be prevented even during the failure diagnosis period. .

  In the first embodiment described above, the present embodiment is executed in an internal combustion engine in which the air-fuel ratio sensor 36 is arranged in the exhaust passage 14, but the system configuration is not limited to this. That is, in an internal combustion engine provided with two exhaust passages typified by a V type and the like and an air-fuel ratio sensor disposed in each exhaust passage, air-fuel ratio control may be performed based on the respective sensor signals. This also applies to other embodiments described below.

  In the first embodiment described above, the main feedback control is performed based on the P control and the I control, but the control method is not limited to this. That is, it is good also as performing only P control or I control, and good also as main feedback control which added D control (differential control) to these. This also applies to other embodiments described below.

  In the first embodiment described above, the air-fuel ratio sensor 36 is disposed upstream of the catalyst 34 disposed in the exhaust passage 14, and main feedback control is performed based on the output signal of the sensor. The air-fuel ratio control method is not limited to this. That is, a sub-oxygen sensor may be disposed downstream of the catalyst 34, and air-fuel ratio control may be executed in combination with sub-feedback control that performs air-fuel ratio control based on the output signal of the sensor. This also applies to other embodiments described below.

  In the first embodiment described above, regarding the feedback control in the failure diagnosis period, the fuel is controlled so that the exhaust air-fuel ratio varies alternately between the fuel rich side and the fuel lean with the feedback control amount immediately before the failure diagnosis as a reference value. Although the injection amount is corrected to increase or decrease, the reference value is not limited to this. That is, the air-fuel ratio control during failure diagnosis may be executed using the learning value calculated based on the feedback control amount history before the failure diagnosis period as a reference value. This also applies to other embodiments described below.

  In the first embodiment described above, the exhaust air-fuel ratio is alternately changed based on the constant rich control amount or the lean control amount calculated by each correction coefficient for the feedback control in the failure diagnosis period. However, the air-fuel ratio control method is not limited to this. In other words, in order to prevent torque fluctuation due to a sudden change in the air-fuel ratio, the control amount during rich or lean control is changed stepwise, and the air-fuel ratio of the exhaust gas flowing into the catalyst is changed stepwise. Also good.

  In the first embodiment described above, the air-fuel ratio sensor 36 corresponds to the “exhaust gas sensor” in the first invention, and the failure diagnosis period corresponds to the “abnormal period” in the first invention. When the ECU 50 executes the process of step 100, the “abnormal period determination means” in the first invention performs the process of step 102, and the “feedback means” in the first invention results in the above-mentioned step 102. In the processing of step 104, the “air-fuel ratio control means” in the first invention is realized by alternately selecting and executing step 106 and step 108, respectively.

Embodiment 2. FIG.
[Configuration of Embodiment 2]
Next, a second embodiment of the present invention will be described with reference to FIGS. The system of the present embodiment can be realized by causing the ECU 50 to execute a routine shown in FIG. 4 to be described later using the same hardware configuration as that of the first embodiment.

  In the first embodiment described above, the routine shown in FIG. 2 is executed every predetermined time, so that the rich control and the lean control in the failure diagnosis period are alternately executed. However, the routine shown in FIG. 2 is executed in time synchronization, whereas the combustion cycle of the internal combustion engine 10 is executed in synchronization with the crank angle. For this reason, the fuel injection timing by the injector 22 is also executed in synchronization with the rotation angle of the normal crankshaft 30. For this reason, depending on the fuel injection timing, it may be assumed that combustion based on the rich control and the lean control is not reliably performed alternately.

  Therefore, in the present embodiment, the rotation angle of the crankshaft and the correction of the main feedback control are synchronized. This makes it possible to reliably inject fuel-rich fuel gas and fuel-lean fuel gas alternately. Therefore, it is possible to prevent a bias in the oxygen storage amount of the catalyst, always reliably purify the exhaust gas, and prevent the emission from deteriorating even during the failure diagnosis period.

[Characteristic Operation in Embodiment 2]
FIG. 3 is a timing chart showing the operation of each cylinder of the internal combustion engine 10. Here, the internal combustion engine 10 is a four-cycle four-cylinder engine. As shown in this figure, in the internal combustion engine 10, fuel is injected for each cylinder. For example, in the first cylinder (# 1), a mixture of fuel and intake air injected from the injector 22 is sucked into the combustion chamber (intake stroke) and compressed by the piston ascending (compression stroke). The air-fuel mixture is combusted by ignition by the spark plug 26 (combustion stroke), and exhaust gas resulting from the combustion is exhausted from the exhaust valve 28 to the exhaust passage 14 (exhaust stroke). During this one cycle stroke, the crankshaft 30 rotates 720 °. In the combustion cycle of each cylinder, the combustion stroke in the internal combustion engine 10 is sequentially executed by providing a phase difference of 180 ° in the crank angle in the order of # 1 → # 3 → # 4 → # 2.

  Here, as shown in FIG. 3, the fuel injection amounts of the first cylinder (# 1) and the fourth cylinder (# 4) are defined as fuel lean, and the fuel of the third cylinder (# 3) and the second cylinder (# 2). If the injection amount is fuel-rich, the fuel-lean exhaust gas and the fuel-rich exhaust gas are alternately discharged. For this reason, by performing such fuel injection amount correction in the failure diagnosis period, the oxidation reaction and the reduction reaction in the catalyst can be executed in a well-balanced manner, and the uneven oxygen storage amount of the catalyst can be prevented. Therefore, exhaust gas can always be reliably purified even during a failure diagnosis period, and deterioration of emissions can be effectively prevented.

[Specific Processing in Embodiment 2]
FIG. 4 is a flowchart of a routine executed by the ECU 50 in order to realize the above-described main feedback control during the failure diagnosis period, and is executed at every predetermined crank angle (for example, every 180 °). According to the routine shown in FIG. 4, first, it is determined whether or not it is a failure diagnosis period of the air-fuel ratio sensor 36 (step 200). Here, specifically, the same processing as step 100 shown in FIG. 2 is executed.

  If it is determined in step 200 that it is not the failure diagnosis period, the same processing as in normal main feedback control is executed (step 202). Here, specifically, the same processing as step 102 shown in FIG. 2 is executed, and the main feedback correction amount is calculated.

  On the other hand, when it is determined in step 200 that the failure diagnosis period has elapsed, it is determined whether or not it is the timing of fuel injection of the first cylinder or the fourth cylinder (step 204). Specifically, first, the current crank position is specified based on the output signal of the crank angle sensor 32. The fuel injection timing is synchronized with the crank angle. For this reason, it is possible to determine the fuel injection timing of which cylinder based on the specified crank angle.

  If the result in step 204 is negative, that is, if it is determined that the fuel injection timing of the third cylinder or the second cylinder is reached, rich control is executed (step 206). Here, specifically, the same processing as step 106 shown in FIG. 2 is executed. As a result, the rich control correction amount becomes larger than the feedback correction amount for realizing the theoretical air-fuel ratio, and the fuel-rich exhaust gas flows into the catalyst.

  On the other hand, if it is determined in step 204 that the fuel injection timing is for one cylinder or four cylinders, lean control is executed (step 208). Here, specifically, the same processing as step 108 shown in FIG. 2 is executed. As a result, the lean control correction amount becomes smaller than the feedback correction amount for realizing the stoichiometric air-fuel ratio, and fuel-lean exhaust gas flows into the catalyst.

  As described above, this routine is executed every crank angle (for example, every 180 °). For this reason, in the failure diagnosis period, the rich control shown in step 206 and the lean control shown in step 208 are reliably executed alternately. Therefore, by performing combustion based on the fuel injection amount, fuel-rich exhaust gas and fuel-lean exhaust gas can alternately flow into the catalyst 34 during the failure diagnosis period. Thereby, the oxidation reaction and the reduction reaction in the catalyst can be executed in a well-balanced manner, the exhaust gas can be reliably purified, and the deterioration of the emission can be effectively prevented.

  By the way, in the second embodiment described above, the air-fuel ratio control is executed so that the exhaust air-fuel ratio becomes the fuel lean for the fuel injection of the one cylinder and the four cylinders. However, the air-fuel ratio control method is not limited to this. That is, if the exhaust gas is exhausted alternately on the fuel-rich side and the fuel-lean side, the air-fuel ratio control is executed so that the exhaust air-fuel ratio becomes fuel-rich for the fuel injection of the first cylinder or the fourth cylinder. It is good.

  In the second embodiment described above, the air-fuel ratio control shown in the present embodiment is executed for the internal combustion engine of the four-cylinder engine, but the type of the internal combustion engine is not limited to this. That is, a 6-cylinder engine or an 8-cylinder engine may be used as long as fuel-rich exhaust gas and fuel-lean exhaust gas alternately flow into the catalyst. Further, in an internal combustion engine having two exhaust passages typified by a V-type engine and a catalyst disposed in each exhaust passage, fuel-rich exhaust gas and fuel-lean exhaust gas alternately flow into each catalyst. If the air-fuel ratio control is executed as described above, the type of the internal combustion engine is not limited.

  In Embodiment 2 described above, the fuel injection amount is corrected for each cylinder, and fuel-rich or fuel-lean exhaust gas is alternately introduced into the catalyst. However, the air-fuel ratio control method is not limited to this. I can't. That is, if the exhaust gas is exhausted alternately on the fuel rich side and the fuel lean side, the fuel rich or the fuel lean is performed for each of a plurality of combustions, a plurality of fuel injections, or a plurality of cylinders. Air-fuel ratio control may be executed.

  In the second embodiment described above, the air-fuel ratio sensor 36 corresponds to the “exhaust gas sensor” in the first invention, and the failure diagnosis period corresponds to the “abnormal period” in the first invention. When the ECU 50 executes the process of step 200, the “abnormal period determining means” in the first invention performs the process of step 202, and the “feedback means” in the first invention results in the above-described step 200. In the process of step 204, step 206 and step 208 are alternately selected and executed to realize the “air-fuel ratio control means” in the first invention.

It is a figure for demonstrating the structure of Embodiment 1 of this invention. It is a flowchart of the routine performed in Embodiment 1 of the present invention. It is a timing chart for demonstrating the method of the air fuel ratio control in Embodiment 2 of this invention. It is a flowchart of the routine performed in Embodiment 2 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 12 Exhaust passage 14 Exhaust passage 16 Air flow meter 22 Injector 24 Intake valve 26 Spark plug 28 Exhaust valve 30 Crankshaft 32 Crank angle sensor 34 Exhaust purification catalyst 36 Air fuel ratio sensor (A / F sensor)
50 ECU (Electronic Control Unit)

Claims (5)

A catalyst disposed in the exhaust passage of the internal combustion engine;
An exhaust gas sensor disposed in the exhaust passage;
Feedback means for feeding back the output of the exhaust gas sensor to the fuel injection amount so that the air-fuel ratio of the exhaust gas flowing into the catalyst matches the control target air-fuel ratio;
An abnormal period determining means for determining whether an abnormal period in which an abnormal output of the exhaust gas sensor is detected;
During the abnormal period, the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is changed alternately between the fuel rich side and the fuel lean side based on the feedback control amount based on the feedback means before the abnormal period. Air-fuel ratio control means;
An air-fuel ratio control apparatus for an internal combustion engine, comprising:   2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the feedback control amount is calculated based on an output value of the exhaust gas sensor immediately before the abnormal period.   3. The abnormal period is a period from when a diagnostic voltage used for failure diagnosis of the exhaust gas sensor is applied to the exhaust gas sensor until the output of the exhaust gas sensor is stabilized. An air-fuel ratio control apparatus for an internal combustion engine as described.   4. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio control means causes the fuel-rich side and the fuel-lean side to change alternately every predetermined time.   4. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio control means causes the fuel-rich side and the fuel-lean side to alternately change every predetermined number of fuel injections.

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