JP2011163229A - Device for determining air-fuel ratio imbalance between cylinders of multi-cylinder internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately determine the air-fuel ratio imbalance between cylinders of a multi-cylinder internal combustion engine. <P>SOLUTION: The multi-cylinder internal combustion engine includes a plurality of combustion chambers 21, fuel injection valves 25 arranged corresponding to the combustion chambers, respectively, an emission control catalyst 43, an upstream side air-fuel ratio sensor 55 arranged upstream of the emission control catalyst, and a downstream side air-fuel ratio sensor 56 arranged downstream of the emission control catalyst, wherein richer air-fuel ratio control for determining the malfunction of the downstream side air-fuel ratio sensor is executed to control the air-fuel ratio of mixture formed in each combustion chamber to be a richer air-fuel ratio than a theoretical air-fuel ratio when required for determining whether the downstream side air-fuel ratio sensor is malfunctioned or not. When the richer air-fuel ratio control for determining the malfunction of the downstream side air-fuel ratio sensor is executed, this device for determining the air-fuel ratio imbalance between cylinders executes determination of the air-fuel ratio imbalance between the cylinders to estimate the air-fuel ratio of the mixture formed in each combustion chamber and determine whether the deviation occurs or not between the estimated air-fuel ratios of the mixture. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an inter-cylinder air-fuel ratio imbalance determining apparatus for a multi-cylinder internal combustion engine.

  A multi-cylinder internal combustion engine having a plurality of combustion chambers, wherein a fuel injection valve is arranged corresponding to each combustion chamber, and fuel is injected from the fuel injection valve into the corresponding combustion chamber In order to reduce the exhaust emission discharged from the combustion chamber as much as possible, it is preferable that the amounts of fuel injected from the respective fuel injection valves are all controlled to be equal and optimal. That is, in the multi-cylinder internal combustion engine, for example, the amount that can reduce the exhaust emission discharged from the combustion chamber as much as possible is the amount of fuel to be injected from the fuel injection valve (that is, the optimum amount). In this case, if the amount of fuel injected from any one of the fuel injection valves is not controlled to this optimum amount, the exhaust emission discharged from the combustion chamber may increase unexpectedly.

  Therefore, in the multi-cylinder internal combustion engine, when the amount of fuel injected from any one of the fuel injection valves is not controlled to an optimum amount, that is, between the air-fuel ratios of the air-fuel mixture formed in each combustion chamber. If there is a difference, grasping this fact is very important from the viewpoint of reducing the exhaust emission discharged from the combustion chamber as much as possible.

  As described above, Patent Document 1 discloses an apparatus for detecting a difference between the air-fuel ratios of the air-fuel mixture formed in each combustion chamber (hereinafter referred to as “inter-cylinder air-fuel ratio imbalance”). Is disclosed.

  In the apparatus disclosed in Patent Document 1, an air-fuel ratio sensor that detects the air-fuel ratio of exhaust gas by detecting the oxygen concentration in the exhaust gas is disposed in the exhaust passage, and the output value of this air-fuel ratio sensor is The frequency of change is compared with a predetermined reference value related thereto, or the length of the locus of the output value of the air-fuel ratio sensor is compared with a predetermined reference value related thereto, and the frequency of change of the output value of the air-fuel ratio sensor is determined. When there is a deviation from a predetermined reference value related to this, or when the length of the locus of the output value of the air-fuel ratio sensor deviates from a predetermined reference value related to this, an inter-cylinder air-fuel ratio imbalance occurs. Determined.

US Pat. No. 7,152,594 JP 2009-13967 A

  Incidentally, in the multi-cylinder internal combustion engine disclosed in Patent Document 1, the output of the air-fuel ratio sensor is determined by the air-fuel ratio of the air-fuel mixture formed in each combustion chamber (hereinafter, this air-fuel ratio is simply referred to as “air-fuel ratio of the air-fuel mixture”). The characteristics are different. That is, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric or substantially stoichiometric air-fuel ratio, when the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, The output characteristic of the air-fuel ratio sensor is different from the case where the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio. Therefore, in each case, the determination accuracy of the inter-cylinder air-fuel ratio imbalance is different.

  Accordingly, an object of the present invention is to determine an inter-cylinder air-fuel ratio imbalance with high accuracy in a multi-cylinder internal combustion engine.

  In order to achieve the above object, a first invention of the present application includes a plurality of combustion chambers, fuel injection valves arranged corresponding to the combustion chambers, and specific components in exhaust gas discharged from the combustion chambers. An exhaust purification catalyst disposed in the exhaust passage for purifying the exhaust gas and an upstream air-fuel ratio disposed in the exhaust passage upstream of the exhaust purification catalyst for detecting the air-fuel ratio of the exhaust gas discharged from the combustion chamber A downstream air-fuel ratio sensor disposed in an exhaust passage downstream of the exhaust purification catalyst for detecting an air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst. A rich air-fuel ratio for determining an abnormality in the downstream air-fuel ratio sensor that controls the air-fuel ratio of the air-fuel mixture formed in each combustion chamber to an air-fuel ratio richer than the stoichiometric air-fuel ratio when it should be determined whether or not there is an abnormality Control is executed In a cylinder internal combustion engine, when the rich air-fuel ratio control for abnormality determination of the downstream air-fuel ratio sensor is being executed, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is estimated based on the output of the upstream air-fuel ratio sensor Thus, the inter-cylinder air-fuel ratio imbalance determination device executes an inter-cylinder air-fuel ratio imbalance determination that determines whether or not there is a deviation between the estimated air-fuel ratios of the air-fuel mixture.

  An air-fuel ratio sensor arranged in the exhaust passage for detecting the air-fuel ratio of the exhaust gas discharged from the combustion chamber, that is, a mixture formed in each combustion chamber based on the output value of the upstream air-fuel ratio sensor of the present invention When estimating the air-fuel ratio of the air-fuel mixture, the accuracy of the estimation is higher than that when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio or to an air-fuel ratio leaner than the stoichiometric air-fuel ratio. This is higher when the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio. Therefore, using the air-fuel ratio of the air-fuel mixture formed in each combustion chamber estimated based on the output value of the upstream air-fuel ratio sensor, it is determined whether or not there is a deviation between the air-fuel ratios of these air-fuel mixtures. In this case, the determination accuracy is higher than that when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio or to an air-fuel ratio leaner than the stoichiometric air-fuel ratio. It is higher when the air-fuel ratio is controlled to be richer than that.

  Here, according to the first invention of the present application, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is richer than the stoichiometric air-fuel ratio in order to determine whether an abnormality has occurred in the downstream air-fuel ratio sensor. When the air-fuel ratio is controlled, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is estimated based on the output value of the upstream air-fuel ratio sensor, and each air-fuel ratio is estimated based on the estimated air-fuel ratio of the air-fuel mixture. Since it is determined whether or not there is a deviation between the air-fuel ratios of the air-fuel mixture formed in the combustion chamber, the determination accuracy is high.

  Further, according to the first invention of the present application, when determining whether or not there is a deviation between the air-fuel ratios of the air-fuel mixture formed in each combustion chamber, in order to increase the determination accuracy, The air-fuel ratio of the air-fuel mixture formed in each combustion chamber is controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining whether or not there is a deviation between the air-fuel ratios of the air-fuel mixture formed. Instead, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is controlled to be richer than the stoichiometric air-fuel ratio in order to determine whether an abnormality has occurred in the downstream air-fuel ratio sensor. Sometimes, it is determined whether or not there is a deviation between the air-fuel ratios of the air-fuel mixtures formed in the combustion chambers. That is, according to the present invention, when it is determined whether or not there is a deviation between the air-fuel ratios of the air-fuel mixtures formed in the combustion chambers, there is a deviation between the air-fuel ratios of the air-fuel mixtures formed in the combustion chambers. Therefore, it is not necessary to control the air-fuel ratio of the air-fuel mixture formed in each combustion chamber to an air-fuel ratio richer than the stoichiometric air-fuel ratio only for the purpose of determining whether or not this occurs. For this reason, according to the present invention, it is possible to determine whether or not there is a deviation between the air-fuel ratios of the air-fuel mixtures formed in the respective combustion chambers while suppressing a decrease in fuel consumption of the internal combustion engine.

  In order to achieve the above object, the second invention of the present application is formed in each combustion chamber in the first invention when it is determined whether or not an abnormality has occurred in the upstream air-fuel ratio sensor. The upstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control for controlling the air-fuel ratio of the air-fuel mixture to be richer than the stoichiometric air-fuel ratio is executed in the multi-cylinder internal combustion engine. The inter-cylinder air-fuel ratio imbalance determination device executes the inter-cylinder air-fuel ratio imbalance determination when the rich air-fuel ratio control for side air-fuel ratio sensor abnormality determination is being executed.

  According to the second invention of this application, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio in order to determine whether or not an abnormality has occurred in the upstream air-fuel ratio sensor. In addition, since the inter-cylinder air-fuel ratio imbalance determination is executed, the execution opportunity of the inter-cylinder air-fuel ratio imbalance determination increases.

  In order to achieve the above object, according to a third aspect of the present invention, in the second aspect, the upstream air-fuel ratio is controlled when the upstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control is being executed. The inter-cylinder air-fuel ratio imbalance determination device executes the inter-cylinder air-fuel ratio imbalance determination when it is determined that no abnormality has occurred in the sensor.

  According to the third aspect of the present invention, since the determination of the air-fuel ratio imbalance among cylinders is executed based on the output value of the normal upstream air-fuel ratio sensor, the accuracy of the air-fuel ratio imbalance determination between cylinders is high.

  In order to achieve the above object, a fourth invention of the present application includes a plurality of combustion chambers, fuel injection valves respectively disposed corresponding to the combustion chambers, and exhaust gas discharged from the combustion chambers. An exhaust purification catalyst disposed in the exhaust passage for purifying specific components, and an upstream side disposed in the exhaust passage upstream of the exhaust purification catalyst for detecting the air-fuel ratio of the exhaust gas discharged from the combustion chamber An air-fuel ratio sensor, and the air-fuel ratio of the air-fuel mixture formed in each combustion chamber when determining whether or not an abnormality has occurred in the upstream air-fuel ratio sensor is richer than the stoichiometric air-fuel ratio. In the multi-cylinder internal combustion engine in which the rich air-fuel ratio control for determining upstream air-fuel ratio sensor abnormality is controlled, the upstream air-fuel ratio control is performed when the rich air-fuel ratio control for determining upstream air-fuel ratio sensor abnormality is being executed. For sensor output Cylinders for executing the inter-cylinder air-fuel ratio imbalance determination for estimating the air-fuel ratio of the air-fuel mixture formed in each combustion chamber and determining whether there is a deviation between the estimated air-fuel ratios of the air-fuel mixture It is an inter-air-fuel ratio imbalance determination device.

  An air-fuel ratio sensor arranged in the exhaust passage for detecting the air-fuel ratio of the exhaust gas discharged from the combustion chamber, that is, a mixture formed in each combustion chamber based on the output value of the upstream air-fuel ratio sensor of the present invention When estimating the air-fuel ratio of the air-fuel mixture, the accuracy of the estimation is higher than that when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio or to an air-fuel ratio leaner than the stoichiometric air-fuel ratio. This is higher when the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio. Therefore, using the air-fuel ratio of the air-fuel mixture formed in each combustion chamber estimated based on the output value of the upstream air-fuel ratio sensor, it is determined whether or not there is a deviation between the air-fuel ratios of these air-fuel mixtures. In this case, the determination accuracy is higher than that when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio or to an air-fuel ratio leaner than the stoichiometric air-fuel ratio. It is higher when the air-fuel ratio is controlled to be richer than that.

  Here, according to the fourth aspect of the present invention, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is richer than the stoichiometric air-fuel ratio in order to determine whether or not an abnormality has occurred in the upstream air-fuel ratio sensor. When the air-fuel ratio is controlled to a proper air-fuel ratio, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is estimated based on the output value of the upstream air-fuel ratio sensor, and based on the estimated air-fuel ratio of the air-fuel mixture Since it is determined whether or not there is a deviation between the air-fuel ratios of the air-fuel mixture formed in each combustion chamber, the determination accuracy is high.

  Furthermore, according to the fourth aspect of the present invention, when determining whether or not there is a deviation between the air-fuel ratios of the air-fuel mixture formed in each combustion chamber, each combustion chamber has The air-fuel ratio of the air-fuel mixture formed in each combustion chamber is controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining whether or not there is a deviation between the air-fuel ratios of the air-fuel mixture formed. Instead, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is controlled to be richer than the stoichiometric air-fuel ratio in order to determine whether or not an abnormality has occurred in the upstream air-fuel ratio sensor. Sometimes, it is determined whether or not there is a deviation between the air-fuel ratios of the air-fuel mixtures formed in the combustion chambers. That is, according to the present invention, when it is determined whether or not there is a deviation between the air-fuel ratios of the air-fuel mixtures formed in the combustion chambers, there is a deviation between the air-fuel ratios of the air-fuel mixtures formed in the combustion chambers. Therefore, it is not necessary to control the air-fuel ratio of the air-fuel mixture formed in each combustion chamber to an air-fuel ratio richer than the stoichiometric air-fuel ratio only for the purpose of determining whether or not this occurs. For this reason, according to the present invention, it is possible to determine whether or not there is a deviation between the air-fuel ratios of the air-fuel mixtures formed in the respective combustion chambers while suppressing a decrease in fuel consumption of the internal combustion engine.

  In order to achieve the above object, according to a fifth aspect of the present invention, in the fourth aspect, the upstream air-fuel ratio is controlled when the upstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control is being executed. The inter-cylinder air-fuel ratio imbalance determination device executes the inter-cylinder air-fuel ratio imbalance determination when it is determined that no abnormality has occurred in the sensor.

  According to the fifth aspect of the present application, the inter-cylinder air-fuel ratio imbalance determination is executed based on the normal output value of the upstream air-fuel ratio sensor, so the accuracy of the inter-cylinder air-fuel ratio imbalance determination is high.

  In order to achieve the above object, according to a sixth aspect of the present invention, in any one of the first to fifth aspects, the combustion chamber is formed in each combustion chamber when the operation of the multi-cylinder internal combustion engine is started. Engine rich air-fuel ratio control for controlling the air-fuel ratio of the air-fuel mixture to be richer than the stoichiometric air-fuel ratio, or after the fuel injection from the fuel injection valve is stopped, Rich air-fuel ratio control after stopping fuel injection that controls the air-fuel ratio of the air-fuel mixture formed in each combustion chamber to an air-fuel ratio richer than the stoichiometric air-fuel ratio when fuel injection is resumed, or the exhaust purification catalyst The rich air-fuel ratio control for the exhaust purification catalyst for controlling the air-fuel ratio of the air-fuel mixture formed in each combustion chamber to an air-fuel ratio richer than the stoichiometric air-fuel ratio when the temperature is higher than a predetermined allowable upper limit temperature. Performed in a cylinder internal combustion engine When the rich air-fuel ratio control at the time of starting the engine is being executed, or when the rich air-fuel ratio control after the stop of fuel injection is being executed, or the rich air-fuel ratio for the exhaust purification catalyst The inter-cylinder air-fuel ratio imbalance determination device executes the inter-cylinder air-fuel ratio imbalance determination when control is being executed.

  According to the sixth invention of this application, the fuel injection from the fuel injection valve is resumed when the operation of the multi-cylinder internal combustion engine is started or after the fuel injection from the fuel injection valve is stopped. Or when the temperature of the exhaust purification catalyst is higher than a predetermined allowable upper limit temperature, the determination of the inter-cylinder air-fuel ratio imbalance determination is executed.

FIG. 1 is an overall view of a spark ignition type multi-cylinder internal combustion engine to which an inter-cylinder air-fuel ratio imbalance determining apparatus according to the present invention is applied. FIG. 2 is a diagram showing the purification performance of the upstream catalyst and the downstream catalyst. 3A is a diagram showing the output characteristics of the upstream air-fuel ratio sensor, and FIG. 3B is a diagram showing the output characteristics of the downstream air-fuel ratio sensor. FIG. 4 is a diagram showing a map used for determining the target air-fuel ratio. FIG. 5 is a diagram showing an example of a flowchart for calculating the time for injecting fuel from the fuel injection valve. FIG. 6 is a diagram showing an example of a flowchart for calculating the air-fuel ratio correction coefficient. FIG. 7 is a diagram illustrating an example of a flowchart for calculating the skip increase value and the skip decrease value. FIG. 8A is a diagram showing the transition of the output value of the upstream air-fuel ratio sensor when all the fuel injection valves are normal when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio. FIG. 8B shows that when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio, an amount of fuel larger than the command fuel injection amount is injected into the fuel injection valve corresponding to the first cylinder # 1. FIG. 8C is a diagram showing the transition of the output value of the upstream side air-fuel ratio sensor when the remaining fuel injection valve is normal, and FIG. 8C shows the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio. The upstream air-fuel ratio when the fuel injection valve corresponding to the first cylinder # 1 has an abnormality that only an amount of fuel smaller than the command fuel injection amount is injected and the remaining fuel injection valves are normal when controlled It is the figure which showed transition of the output value of a sensor. FIG. 9A shows the transition of the output value of the upstream air-fuel ratio sensor when all the fuel injection valves are normal when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio. FIG. 9B shows the command fuel to the fuel injection valve corresponding to the first cylinder # 1 when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio. FIG. 9C is a diagram showing the transition of the output value of the upstream air-fuel ratio sensor when there is an abnormality in which an amount of fuel larger than the injection amount is injected and the remaining fuel injection valves are normal. Is that an amount of fuel smaller than the command fuel injection amount is injected into the fuel injection valve corresponding to the first cylinder # 1 when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio Output value of upstream air-fuel ratio sensor when there is remaining fuel injection valve and normal Is a diagram showing the transition. FIG. 10 is a partial schematic perspective view showing a part of the upstream air-fuel ratio sensor. FIG. 11 is a partial cross-sectional view showing a part of the upstream air-fuel ratio sensor. 12A is a cross-sectional view showing the configuration of the air-fuel ratio detection element of the upstream air-fuel ratio sensor, and FIG. 12B shows that the air-fuel ratio detection element of the upstream air-fuel ratio sensor is more than the stoichiometric air-fuel ratio. FIG. 12B is a cross-sectional view showing a situation when a lean air-fuel ratio exhaust gas has arrived. FIG. 12B shows an air-fuel ratio richer than the stoichiometric air-fuel ratio in the air-fuel ratio detection element of the upstream air-fuel ratio sensor. It is sectional drawing which showed a mode when gas came. FIG. 13 is a diagram showing the relationship between the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor and the limit current value output from the upstream air-fuel ratio sensor. FIG. 14 is a flowchart illustrating an example of a routine for performing the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state according to the first embodiment. FIG. 15 is a flowchart showing an example of a routine for executing the setting of the inter-cylinder air / fuel ratio imbalance determination execution flag indicating whether or not the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state according to the first embodiment is executed. It is a figure. FIG. 16 is a flowchart showing an example of a routine for executing the setting of the inter-cylinder air / fuel ratio imbalance determination execution flag indicating whether or not the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state according to the second embodiment is performed. It is a figure. FIG. 17 is a flowchart showing an example of a routine for executing the setting of the inter-cylinder air / fuel ratio imbalance determination execution flag indicating whether or not the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state according to the third embodiment is performed. It is a figure. FIG. 18 is a flowchart showing an example of a routine for executing the setting of the inter-cylinder air / fuel ratio imbalance determination execution flag indicating whether or not to execute the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state according to the fourth embodiment. It is a figure. FIG. 19 is a flowchart showing an example of a routine for executing the setting of the inter-cylinder air / fuel ratio imbalance determination execution flag indicating whether or not to execute the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state according to the fifth embodiment. It is a figure. FIG. 20 is a flowchart showing an example of a routine for executing the setting of the inter-cylinder air / fuel ratio imbalance determination execution flag indicating whether or not to execute the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state according to the sixth embodiment. It is a figure. FIG. 21 is a cross-sectional view showing the configuration of an air-fuel ratio detecting element of an upstream air-fuel ratio sensor that is provided with a catalyst. FIG. 22 is a diagram showing the configuration of the hybrid system.

  Hereinafter, an embodiment of an inter-cylinder air-fuel ratio imbalance determining apparatus for a multi-cylinder internal combustion engine of the present invention will be described with reference to the drawings. FIG. 1 is an overall view of a spark ignition type multi-cylinder internal combustion engine to which an inter-cylinder air-fuel ratio imbalance determining apparatus according to the present invention is applied. The spark ignition type multi-cylinder internal combustion engine described below is a so-called four-cycle internal combustion engine that sequentially repeats four strokes of an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke.

  In FIG. 1, reference numeral 10 indicates a spark ignition type multi-cylinder internal combustion engine (hereinafter simply referred to as “internal combustion engine”). The internal combustion engine 10 has a main body (hereinafter referred to as “engine main body”) 20. The engine body 20 includes a cylinder block and a cylinder head. The engine body 20 has four combustion chambers 21 formed by an inner wall surface of a cylinder bore formed in the cylinder block, a top surface of a piston disposed in the cylinder bore, and a lower wall surface of the cylinder head.

  In FIG. 1, # 1 indicates the combustion chamber (hereinafter, this combustion chamber is also referred to as “first cylinder”) 21 shown on the lowermost side, and # 2 indicates the first cylinder # 1. A combustion chamber (hereinafter, this combustion chamber is also referred to as “second cylinder”) 21 shown immediately above is shown, and # 3 is a combustion chamber (hereinafter referred to as “combustion chamber”) shown immediately above the second cylinder # 2. This combustion chamber is also referred to as a “third cylinder”) 21, and # 4 is a combustion chamber illustrated immediately above the third cylinder # 3 (hereinafter, this combustion chamber is also referred to as a “fourth cylinder”). ) 21.

  In addition, an intake port 22 communicating with each combustion chamber 21 is formed in the cylinder head. Air is sucked into the combustion chamber 21 through the intake port 22. The intake port 22 is opened and closed by an intake valve (not shown). Further, an exhaust port 23 communicating with each combustion chamber 21 is formed in the cylinder head. Exhaust gas is discharged from the combustion chamber 21 to the exhaust port 23. The exhaust port 23 is opened and closed by an exhaust valve (not shown).

  The cylinder head is provided with spark plugs 24 corresponding to the combustion chambers 21. Each spark plug 24 is disposed in the cylinder head so as to be exposed in the combustion chamber 21 so that the mixture of fuel and air formed in the combustion chamber 21 can be ignited. Further, a fuel injection valve 25 is disposed in the cylinder head corresponding to each intake port 22. Each fuel injection valve 25 is disposed in the cylinder head so as to be exposed in the intake port 22 so that fuel can be injected into the intake port 22.

  An intake branch pipe 31 is connected to the intake port 22. The exhaust branch pipe 31 has branch portions respectively connected to the intake port 22 and a surge tank portion in which these branch portions are gathered. An intake pipe 32 is connected to the surge tank portion of the intake branch pipe 31. In the present embodiment, an intake passage 30 is formed by the intake port 22, the intake branch pipe 31, and the intake pipe 32. An air filter 33 is disposed in the intake pipe 32. Further, a throttle valve 34 is rotatably disposed in the intake pipe 32 between the air filter 33 and the intake branch pipe 31. An actuator 34 a that drives the throttle valve 34 is connected to the throttle valve 34. When the throttle 34 is rotated by the actuator 34a, the flow passage area inside the intake pipe 31 is changed, and thereby the amount of air taken into the combustion chamber 21 is controlled.

  On the other hand, an exhaust branch pipe 41 is connected to the exhaust port 23. The exhaust branch pipe 41 includes a branch portion 41a connected to the exhaust port 23 and an exhaust collection portion 41b in which these branch portions are gathered. An exhaust pipe 42 is connected to the exhaust collecting portion 41 b of the exhaust branch pipe 41. In the present embodiment, an exhaust passage 40 is formed by the exhaust port 23, the exhaust branch pipe 41, and the exhaust pipe 42. The exhaust pipe 42 is provided with an exhaust purification catalyst (hereinafter, this exhaust purification catalyst is referred to as an “upstream catalyst”) 43 that purifies a specific component in the exhaust gas. Further, an exhaust purification catalyst 44 that purifies specific components in the exhaust gas (hereinafter, this exhaust purification catalyst is referred to as a “downstream catalyst”) 44 is disposed in the exhaust pipe 42 downstream of the upstream catalyst 43.

  The upstream catalyst 43 is a so-called three-way catalyst. As shown in FIG. 2, the temperature is higher than a certain temperature (that is, so-called activation temperature), and the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 43 is higher. Nitrogen oxides in the exhaust gas (hereinafter referred to as “NOx”) and carbon monoxide (hereinafter referred to as “CO”) when the air-fuel ratio is in the region X near the theoretical air-fuel ratio. Hydrocarbons (hereinafter referred to as “HC”) can be simultaneously purified with a high purification rate. On the other hand, the upstream catalyst 43 occludes oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into it is leaner than the stoichiometric air-fuel ratio, and the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 43 It has the ability to release oxygen stored therein when the air-fuel ratio is richer than the stoichiometric air-fuel ratio (hereinafter, this ability is referred to as “oxygen storage / release ability”). Therefore, as long as the oxygen storage / release capability functions normally, even if the air-fuel ratio of the exhaust gas flowing into the upstream side catalyst 43 is leaner than the stoichiometric air-fuel ratio, it is richer than the stoichiometric air-fuel ratio. Even at the air-fuel ratio, the internal atmosphere of the upstream catalyst 43 is maintained in the vicinity of the theoretical air-fuel ratio, so NOx, CO, and HC in the exhaust gas are simultaneously purified at a high purification rate in the upstream catalyst 43. The

  The downstream side catalyst 44 is also a so-called three-way catalyst, and like the upstream side catalyst 43, NOx, CO, and HC can be simultaneously purified with a high purification rate and have oxygen storage / release capability.

  An air flow meter 51 for detecting the amount of air flowing through the intake pipe 32, that is, the amount of air taken into the combustion chamber 21 (hereinafter, this amount of air is referred to as “intake amount”) is provided in the intake pipe 32. Is arranged.

  The engine body 20 is provided with a crank position sensor 53 that detects the rotational phase of a crankshaft (not shown). The crank position sensor 53 outputs a narrow pulse every time the crankshaft rotates 10 °, and outputs a wide pulse every time the crankshaft rotates 360 °. Based on these pulses, the rotational speed of the crankshaft, that is, the engine rotational speed is calculated. The accelerator opening sensor 57 detects the amount of depression of the accelerator pedal AP.

  An air-fuel ratio sensor (hereinafter referred to as “upstream air-fuel ratio sensor”) 55 for detecting the air-fuel ratio of the exhaust gas is disposed in the exhaust pipe 42 upstream of the upstream catalyst 43. Further, an exhaust pipe 42 downstream of the upstream catalyst 43 and upstream of the downstream catalyst 44 is also provided with an air-fuel ratio sensor (hereinafter referred to as “downstream side”) for detecting the air-fuel ratio of the exhaust gas. 56) (referred to as “air-fuel ratio sensor”).

  As shown in FIG. 3A, the upstream air-fuel ratio sensor 55 outputs a smaller output value I as the air-fuel ratio of the detected exhaust gas becomes richer, and the air-fuel ratio of the detected exhaust gas. This is a so-called limiting current type oxygen concentration sensor that outputs an output value I that is larger as the gas is leaner.

  On the other hand, as shown in FIG. 3B, the downstream air-fuel ratio sensor 56 has a relatively large constant value when the detected air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio. The output value Vg is output, and when the detected air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, a relatively small constant output value Vs is output, and the detected air-fuel ratio of the exhaust gas is This is a so-called electromotive force type oxygen concentration sensor that outputs an intermediate output value Vm between the relatively large constant output value Vg and the relatively small constant output value Vs when the stoichiometric air-fuel ratio is obtained.

  An electric control unit (ECU) 60 shown in FIG. 1 is composed of a microcomputer, and a CPU (microprocessor) 61, a ROM (read only memory) 62, and a RAM (random) connected to each other by a bidirectional bus. Access memory) 63, a backup RAM 64, and an interface 65 including an AD converter. The interface 65 is connected to the ignition plug 24, the fuel injection valve 25, and the actuator 34 a for the throttle valve 34. An air flow meter 51, a crank position sensor 53, an upstream air-fuel ratio sensor 55, a downstream air-fuel ratio sensor 56, and an accelerator opening sensor 57 are also connected to the interface 65.

  By the way, in the present embodiment, an air-fuel mixture formed in the combustion chamber 21 according to the operating state of the internal combustion engine 10, in particular, the engine speed and the engine load (hereinafter, the air-fuel mixture formed in the combustion chamber is simply “mixed”). The air / fuel ratio to be targeted as the air / fuel ratio (hereinafter referred to as “target air / fuel ratio”) TA / F is expressed by the relationship between the engine speed N and the engine load L as shown in FIG. It is stored in advance in the electronic control unit 60 in the form of a function map. During operation of the internal combustion engine (hereinafter referred to as “engine operation”), the target air-fuel ratio TA / F corresponding to the engine speed N and the engine load L is read from the map of FIG. The amount of fuel injected from each fuel injection valve 25 (hereinafter, this amount is referred to as “fuel injection amount”) is controlled in accordance with the intake air amount detected by the air flow meter 51 so as to reach the target air-fuel ratio. The intake air amount is controlled by controlling the opening of the throttle valve 34 in accordance with the output required for the internal combustion engine.

  Here, control of the fuel injection amount when the target air-fuel ratio is the stoichiometric air-fuel ratio and the air-fuel ratio of the air-fuel mixture is controlled to this stoichiometric air-fuel ratio will be described.

  When the upstream air-fuel ratio sensor 55 detects that the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio of the mixture is leaner than the stoichiometric air-fuel ratio. It will be. Therefore, at this time, in this embodiment, the fuel injection amount is gradually increased so that the air-fuel ratio of the air-fuel mixture approaches the stoichiometric air-fuel ratio. On the other hand, when the upstream air-fuel ratio sensor 55 detects that the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, the air-fuel ratio of the mixture becomes richer than the stoichiometric air-fuel ratio. Will be. Therefore, at this time, in the present embodiment, the fuel injection amount is gradually decreased so that the air-fuel ratio of the air-fuel mixture approaches the stoichiometric air-fuel ratio. By controlling the fuel injection amount in this way, the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio as a whole.

  By the way, when the fuel injection amount is controlled as described above, the air-fuel ratio of the air-fuel mixture becomes richer than the stoichiometric air-fuel ratio across the stoichiometric air-fuel ratio or leaner than the stoichiometric air-fuel ratio. It becomes. In other words, the air-fuel ratio of the air-fuel mixture swings across the stoichiometric air-fuel ratio. Here, from the viewpoint of controlling the air-fuel ratio of the air-fuel mixture to the stoichiometric air-fuel ratio, it is desirable that the amplitude of the air-fuel ratio of the air-fuel mixture across the stoichiometric air-fuel ratio is small. That is, when the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio of the air-fuel mixture is brought close to the stoichiometric air-fuel ratio as quickly as possible, and the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio. When the air-fuel ratio is low, it is desirable to bring the air-fuel ratio of the mixture close to the stoichiometric air-fuel ratio as quickly as possible.

  Therefore, in the present embodiment, when the upstream air-fuel ratio sensor 55 detects that the air-fuel ratio of the air-fuel mixture is reversed from an air-fuel ratio leaner than the stoichiometric air-fuel ratio to a rich air-fuel ratio, the fuel injection amount is skipped. The weight can be reduced relatively. According to this, when the air-fuel ratio of the air-fuel mixture is reversed from a lean air-fuel ratio to a rich air-fuel ratio than the stoichiometric air-fuel ratio, the air-fuel ratio of the air-fuel mixture becomes relatively large and approaches the stoichiometric air-fuel ratio. On the other hand, when the upstream air-fuel ratio sensor 55 detects that the air-fuel ratio of the air-fuel mixture has been reversed from the air-fuel ratio richer than the stoichiometric air-fuel ratio to the lean air-fuel ratio, the fuel injection amount is increased relatively in a skipping manner. To be harassed. According to this, when the air-fuel ratio of the air-fuel mixture is inverted from the air-fuel ratio richer than the stoichiometric air-fuel ratio to the lean air-fuel ratio, the air-fuel ratio of the air-fuel mixture is relatively large and can be brought close to the stoichiometric air-fuel ratio. Thus, the air-fuel ratio amplitude of the air-fuel mixture across the stoichiometric air-fuel ratio becomes small.

  By the way, in order to bring the air-fuel ratio of the air-fuel mixture closer to the stoichiometric air-fuel ratio more quickly, the fuel injection amount is skipped when the air-fuel ratio of the air-fuel mixture is reversed from a lean air-fuel ratio to a rich air-fuel ratio. The amount of the air-fuel ratio when the air-fuel ratio of the air-fuel mixture is reversed from the air-fuel ratio leaner than the stoichiometric air-fuel ratio to the rich air-fuel ratio (hereinafter referred to as “skip reduction value”) The larger the difference from the stoichiometric air-fuel ratio, the greater the difference, and the amount by which the fuel injection amount is increased in a skipping manner when the air-fuel ratio of the mixture is reversed from an air-fuel ratio richer than the stoichiometric air-fuel ratio to a lean air-fuel ratio. The difference between the air-fuel ratio of the air-fuel mixture and the stoichiometric air-fuel ratio is large when the air-fuel ratio of the air-fuel mixture is reversed from a rich air-fuel ratio to a lean air-fuel ratio. Desirably large

  Therefore, in this embodiment, these skip reduction value and skip increase value are controlled as follows.

  That is, the longer the period in which the downstream air-fuel ratio sensor 56 detects an air-fuel ratio leaner than the stoichiometric air-fuel ratio (hereinafter, this period is referred to as “lean period”), the greater the air-fuel ratio of the air-fuel mixture becomes. It can be said that the air-fuel ratio is leaner than that. That is, as described above, the air-fuel ratio of the exhaust gas flowing out from the upstream catalyst 43 should be the stoichiometric air-fuel ratio due to the oxygen storage / release capability of the upstream catalyst 43. However, the case where the lean period is long still means that a large amount of oxygen flows into the upstream catalyst 43 so that the upstream catalyst 43 cannot occlude, that is, the air-fuel ratio of the air-fuel mixture is greatly increased. It can be said that the air-fuel ratio is leaner than the air-fuel ratio. Therefore, in the present embodiment, when the upstream air-fuel ratio sensor 55 detects that the air-fuel ratio of the air-fuel mixture has been reversed from an air-fuel ratio richer than the stoichiometric air-fuel ratio to a lean air-fuel ratio, the lean period becomes longer The skip increase value is increased.

  On the other hand, the longer the period in which the downstream air-fuel ratio sensor 56 detects an air-fuel ratio that is richer than the stoichiometric air-fuel ratio (hereinafter, this period is referred to as “rich period”), the air-fuel ratio of the air-fuel mixture greatly increases. It can be said that the air-fuel ratio is richer than that. That is, as described above, the air-fuel ratio of the exhaust gas flowing out from the upstream catalyst 43 should be the stoichiometric air-fuel ratio due to the oxygen storage / release capability of the upstream catalyst 43. However, the case where the rich period is still long means that the amount of oxygen flowing into the upstream catalyst 43 is so small that all the oxygen stored in the upstream catalyst 43 is released, that is, the mixture gas It can be said that this is a case where the air-fuel ratio is substantially richer than the stoichiometric air-fuel ratio. Therefore, in the present embodiment, when the upstream air-fuel ratio sensor 55 detects that the air-fuel ratio of the air-fuel mixture has been reversed from an air-fuel ratio leaner than the stoichiometric air-fuel ratio to a rich air-fuel ratio, the rich period becomes longer. The skip weight loss value is increased.

  By controlling the fuel injection amount in this way, the air-fuel ratio of the air-fuel mixture is accurately controlled to the stoichiometric air-fuel ratio as a whole.

  Next, an example of a flowchart for executing control of the fuel injection amount according to the present embodiment will be described. The flowcharts shown in FIGS. 5 to 7 are used as flowcharts for executing control of the fuel injection amount according to the present embodiment.

  FIG. 5 is a flowchart for calculating the time for injecting fuel from the fuel injection valve 25. When the routine of FIG. 5 is started, first, at step 10, the ratio Ga / N of the intake air amount Ga to the engine speed N is calculated. Next, at step 11, a value Ga / N · α obtained by multiplying the ratio Ga / N calculated at step 10 by a constant α is input to the basic fuel injection time TAUP. Next, at step 12, an air-fuel ratio correction coefficient (this correction coefficient is a coefficient calculated by the routine of FIG. 6 and will be described in detail later) at the basic fuel injection time TAUP calculated at step 11, FAF and the internal combustion engine A value TAUP · FAF · β · γ multiplied by a constant β and a constant γ determined according to the operating state is input to the fuel injection time TAU, and the routine ends. In the present embodiment, fuel is injected from the fuel injection valve 25 for the fuel injection time TAU calculated in step 12.

  FIG. 6 is a flowchart for calculating the air-fuel ratio correction coefficient FAF used in step 12 of FIG. When the routine of FIG. 6 is started, first, in step 20, the air-fuel ratio A / F of the exhaust gas detected by the upstream air-fuel ratio sensor 55 is larger than the theoretical air-fuel ratio A / Fst (A / F> A / Fst), that is, whether or not the air-fuel ratio of the exhaust gas discharged from the combustion chamber 21 is leaner than the stoichiometric air-fuel ratio. If it is determined that A / F> A / Fst, the routine proceeds to step 21 and the subsequent steps. On the other hand, when it is determined that A / F ≦ A / Fst, the routine proceeds to step 25 and subsequent steps.

  In step 20, it is determined that A / F> A / Fst, that is, it is determined that the air-fuel ratio of the exhaust gas discharged from the combustion chamber 21 is leaner than the stoichiometric air-fuel ratio, and the routine is step 21. When the routine proceeds to, it is determined whether or not the air-fuel ratio of the exhaust gas detected by the upstream side air-fuel ratio sensor 55 is immediately after the air-fuel ratio richer than the stoichiometric air-fuel ratio is reversed to a lean air-fuel ratio. Here, when it is determined that the air-fuel ratio of the exhaust gas has just been reversed from the air-fuel ratio richer than the stoichiometric air-fuel ratio to the lean air-fuel ratio, the routine proceeds to step 22, and the routine of FIG. A value FAF + RSR obtained by adding RSR to the air-fuel ratio correction coefficient FAF calculated when executed (the skip increase value is a value calculated by the routine of FIG. 7 and will be described in detail later) is a new value. The air-fuel ratio correction coefficient FAF is used. Next, in step 23, the air-fuel ratio correction coefficient FAF calculated in step 22 is guarded to be a value within the allowable range, and the routine ends. On the other hand, when it is determined in step 21 that the air-fuel ratio of the exhaust gas is not immediately after the air-fuel ratio richer than the stoichiometric air-fuel ratio is reversed to the lean air-fuel ratio, the routine proceeds to step 24, and the previous FIG. A value FAF + KIR obtained by adding the constant value KIR to the air-fuel ratio correction coefficient FAF calculated when this routine is executed is set as a new air-fuel ratio correction coefficient FAF. Next, at step 23, the air-fuel ratio correction coefficient FAF calculated at step 24 is guarded to a value within an allowable range, and the routine ends.

  On the other hand, it is determined in step 20 that A / F ≦ A / Fst, that is, it is determined that the air-fuel ratio of the exhaust gas discharged from the combustion chamber 21 is richer than the stoichiometric air-fuel ratio, and the routine is executed. When the routine proceeds to step 25, it is determined whether or not the air-fuel ratio of the exhaust gas detected by the upstream air-fuel ratio sensor 55 is immediately after the air-fuel ratio leaner than the stoichiometric air-fuel ratio is inverted to a rich air-fuel ratio. Here, when it is determined that the air-fuel ratio of the exhaust gas is immediately after the air-fuel ratio leaner than the stoichiometric air-fuel ratio is reversed to the rich air-fuel ratio, the routine proceeds to step 26, and the routine of FIG. A value FAF-RSL obtained by subtracting RSL from the air-fuel ratio correction coefficient FAF calculated at the time of execution (this skip reduction value is a value calculated by the routine of FIG. 7 and will be described in detail later). A new air-fuel ratio correction coefficient FAF is used. Next, at step 23, the air-fuel ratio correction coefficient FAF calculated at step 26 is guarded to a value within an allowable range, and the routine is terminated. On the other hand, if it is determined in step 25 that the air-fuel ratio of the exhaust gas is not immediately after reversing from an air-fuel ratio leaner than the stoichiometric air-fuel ratio to a rich air-fuel ratio, the routine proceeds to step 27 and the previous FIG. The value FAF-KIL obtained by subtracting the constant value KIL from the air-fuel ratio correction coefficient FAF calculated when this routine is executed is set as a new air-fuel ratio correction coefficient. Next, at step 23, the air-fuel ratio correction coefficient FAF calculated at step 27 is guarded to be a value within the allowable range, and the routine ends.

  FIG. 7 is a flowchart for calculating the skip increase value RSR used in step 22 of FIG. 6 and the skip increase value RSL used in step 26 of FIG. When the routine of FIG. 7 is started, first, at step 40, the air-fuel ratio A / F of the exhaust gas detected by the downstream air-fuel ratio sensor 56 is larger than the theoretical air-fuel ratio A / Fst (A / F> A / Fst), that is, whether or not the air-fuel ratio of the exhaust gas flowing out from the upstream side catalyst 43 is leaner than the stoichiometric air-fuel ratio. Here, when it is determined that A / F> A / Fst, the routine proceeds to step 41. On the other hand, when it is determined that A / F ≦ A / Fst, the routine proceeds to step 44.

  In step 40, it is determined that A / F> A / Fst, that is, it is determined that the air-fuel ratio of the exhaust gas flowing out from the upstream side catalyst 43 is leaner than the stoichiometric air-fuel ratio, and the routine is step 41. When proceeding to, a value RSR + ΔRS obtained by adding a predetermined amount ΔRS to the skip increase value RSR calculated when the routine of FIG. 7 was executed last time is set as a new skip increase value RSR. Next, in step 42, the skip increase value RSR calculated in step 41 is guarded so as to be a value within the allowable range. Next, at step 43, the value obtained by subtracting the skip increase value RSR guarded at step 42 from the constant R is set as a new skip decrease value RSL, and the routine is terminated.

  On the other hand, it is determined in step 40 that A / F ≧ A / Fst, that is, it is determined that the air-fuel ratio of the exhaust gas flowing out from the upstream catalyst 43 is richer than the stoichiometric air-fuel ratio, and the routine is executed. When the processing proceeds to step 44, a value RSR-ΔRS obtained by subtracting the predetermined amount ΔRS from the skip increase value RLR calculated when the routine of FIG. 7 was executed last time is set as a new skip increase value RSR. Next, in step 42, guard is performed so that the skip increase value RSR calculated in step 44 becomes a value within the allowable range. Next, in step 43, a value obtained by subtracting the skip increase value RSR guarded in step 42 from the constant R is set as a new skip increase value RSL, and the routine is ended.

  By the way, the internal combustion engine 10 has four fuel injection valves 25. Of these fuel injection valves 25, for example, when one fuel injection valve 25 is defective, the following phenomenon occurs.

  That is, in the present embodiment, the amount of fuel injected from each fuel injection valve 25 so that the air-fuel ratio of the air-fuel mixture becomes the target air-fuel ratio based on the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensors 55 and 56. Is controlled. That is, when it is determined that the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio based on the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensors 55, 56, the fuel injection valve 25 performs fuel injection. When the injection amount is increased and it is determined that the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio based on the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensors 55, 56, each fuel injection valve 25 The fuel injection amount is reduced. In other words, in the present embodiment, the air-fuel ratio sensors 55 and 56 are not arranged for each combustion chamber 21 but are arranged in common for each combustion chamber 25, so that the air-fuel mixture is empty. When it is determined that the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, it is determined that the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio in all the combustion chambers 21. When it is determined that the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio, it is determined that the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio in all the combustion chambers 21. Will be. For this reason, when it is determined that the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio, the fuel injection amount is increased in all the fuel injection valves 25, and the air-fuel ratio of the air-fuel mixture becomes higher than the stoichiometric air-fuel ratio. When it is determined that the air-fuel ratio is also rich, the fuel injection amount is reduced in all the fuel injection valves 25.

  Here, for example, when a command is issued from the electronic control unit 60 to each fuel injection valve 25 so that the same amount of fuel is injected from all the fuel injection valves 25, the amount commanded from the electronic control unit 60 If there is a problem with one fuel injection valve 25 (hereinafter referred to as “command fuel injection amount”) that is larger than the amount of fuel (hereinafter referred to as “abnormal fuel injection valve”). A certain fuel injection valve) and the remaining fuel injection valves (hereinafter these fuel injection valves are referred to as “normal fuel injection valves”) 25 are injected with a command fuel injection amount of fuel and formed in the corresponding combustion chambers 21. Even if the air-fuel ratio of the air-fuel mixture is the stoichiometric air-fuel ratio, the air-fuel ratio of the air-fuel mixture formed in the combustion chamber 21 corresponding to the abnormal fuel injection valve 25 is richer than the stoichiometric air-fuel ratio. turn into. Therefore, at this time, emission of exhaust gas discharged from the combustion chamber 21 corresponding to the abnormal fuel injection valve 25 is deteriorated.

  When the exhaust gas discharged from the combustion chamber 21 corresponding to the abnormal fuel injection valve 25 reaches the upstream air-fuel ratio sensor 55, the air-fuel ratio of the mixture becomes richer than the stoichiometric air-fuel ratio. Since the fuel injection amount is reduced in all the fuel injection valves 25, the air-fuel ratio of the air-fuel mixture formed in the combustion chamber 21 corresponding to the normal fuel injection valve 25 is leaner than the stoichiometric air-fuel ratio. The air / fuel ratio will be low. Therefore, at this time, emission of exhaust gas discharged from the combustion chamber 21 corresponding to the normal fuel injection valve 25 is also deteriorated.

  Of course, the air-fuel ratio of the air-fuel mixture formed in the combustion chamber 21 corresponding to the abnormal fuel injection valve 25 becomes richer than the stoichiometric air-fuel ratio, or the combustion chamber 21 corresponding to the normal fuel injection valve 25. Even if the air-fuel ratio of the air-fuel mixture formed in the engine becomes an air-fuel ratio leaner than the stoichiometric air-fuel ratio, according to the air-fuel ratio control of the present embodiment, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber 21 Since the fuel injection amount in each fuel injection valve 25 is controlled so that becomes the stoichiometric air-fuel ratio, it cannot be said that the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio as a whole. . However, even if it can be said that the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio as a whole, when the air-fuel ratio of the air-fuel mixture formed in each combustion chamber 21 is viewed individually, the present embodiment While the air-fuel ratio control is being executed, the air-fuel ratio of the air-fuel mixture is significantly richer than the stoichiometric air-fuel ratio or significantly leaner than the stoichiometric air-fuel ratio. In any case, the emission of exhaust gas discharged from each combustion chamber 21 is deteriorated.

  On the other hand, when a command is issued from the electronic control unit 60 to each fuel injection valve 25 so that the same amount of fuel is injected from all the fuel injection valves 25, the command fuel injection amount commanded from the electronic control unit 60 If there is a problem in which only a smaller amount of fuel is injected than one fuel in one fuel injection valve 25 (hereinafter, this defective fuel injection valve is also referred to as “abnormal fuel injection valve”), the remaining normal fuel injection valves Even if the air-fuel ratio of the air-fuel mixture formed in the corresponding combustion chamber 21 is the stoichiometric air-fuel ratio when the command fuel injection amount of fuel is injected at 25, the combustion chamber 21 corresponding to the abnormal fuel injection valve 25 The air-fuel ratio of the air-fuel mixture formed in the air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio. Therefore, at this time, emission of exhaust gas discharged from the combustion chamber 21 corresponding to the abnormal fuel injection valve 25 is deteriorated.

  When the exhaust gas discharged from the combustion chamber 21 corresponding to the abnormal fuel injection valve 25 reaches the upstream air-fuel ratio sensor 55, the air-fuel ratio of the air-fuel mixture is an air-fuel ratio leaner than the stoichiometric air-fuel ratio. Since the fuel injection amount is increased in all the fuel injection valves 25, the air-fuel ratio of the air-fuel mixture formed in the combustion chamber 21 corresponding to the normal fuel injection valve 25 is richer than the stoichiometric air-fuel ratio. Become. Therefore, at this time, emission of exhaust gas discharged from the combustion chamber 21 corresponding to the normal fuel injection valve 25 is also deteriorated.

  Of course, the air-fuel ratio of the air-fuel mixture formed in the combustion chamber 21 corresponding to the abnormal fuel injection valve 25 becomes leaner than the stoichiometric air-fuel ratio, or the combustion chamber 21 corresponding to the normal fuel injection valve 25. Even if the air-fuel ratio of the air-fuel mixture formed in the engine becomes richer than the stoichiometric air-fuel ratio, according to the air-fuel ratio control of the present embodiment, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber 21 Since the fuel injection amount in each fuel injection valve 25 is controlled so that becomes the stoichiometric air-fuel ratio, it cannot be said that the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio as a whole. . However, even if it can be said that the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio as a whole, when the air-fuel ratio of the air-fuel mixture formed in each combustion chamber 21 is viewed individually, the present embodiment While the air-fuel ratio control is being executed, the air-fuel ratio of the air-fuel mixture is significantly leaner than the stoichiometric air-fuel ratio or significantly richer than the stoichiometric air-fuel ratio. In any case, the emission of exhaust gas discharged from each combustion chamber 21 is deteriorated.

  In this way, even when there is a problem that a certain amount of fuel injection valve 25 is injecting a larger amount of fuel than the command fuel injection amount, only a smaller amount of fuel than the command fuel injection amount is injected. Even when there is a problem in a specific fuel injection valve 25, the emission of exhaust gas discharged from the combustion chamber 21 is deteriorated.

  In view of these circumstances, there is a problem with a specific fuel injection valve 25, and a state in which an amount of fuel larger than the command fuel injection amount is injected in the fuel injection valve 25 or an amount less than the command fuel injection amount. A state in which only fuel is injected, that is, a state in which there is a variation between the air-fuel ratios of the air-fuel mixture formed in each combustion chamber 21 (hereinafter this state is referred to as an “inter-cylinder air-fuel ratio imbalance state”) has occurred. It is extremely important to know the state of the exhaust gas emission and to take measures to improve the deterioration of the exhaust gas emission.

  Therefore, in the present embodiment, based on the following knowledge, it is determined whether or not an inter-cylinder air / fuel ratio imbalance state has occurred, that is, whether or not there is an inter-cylinder air / fuel ratio imbalance state.

  That is, when the rotation angle of the crankshaft is referred to as a crank angle, in the internal combustion engine 10, the first cylinder # 1, the fourth cylinder # 4, and the third cylinder # # are shifted at a timing shifted by 180 ° in each combustion chamber 21. 3. The exhaust stroke is sequentially performed in the order of the second cylinder # 2. Therefore, exhaust gases are sequentially discharged from the combustion chambers 21 with a crank angle shifted by 180 °, and these exhaust gases reach the upstream air-fuel ratio sensor 55 sequentially. Therefore, the upstream air-fuel ratio sensor 55 is generally discharged from the first cylinder # 1, the air-fuel ratio of the exhaust gas discharged from the fourth cylinder # 4, the air-fuel ratio of the exhaust gas discharged from the fourth cylinder # 4, and the third cylinder # 3. The air-fuel ratio of the exhaust gas and the air-fuel ratio of the exhaust gas discharged from the second cylinder # 2 are sequentially detected.

  Here, when all the fuel injection valves 25 are normal, the output value output from the upstream air-fuel ratio sensor 55 corresponding to the air-fuel ratio of the exhaust gas that has reached the upstream air-fuel ratio sensor 55 (hereinafter, this output value is referred to as “output value”). “Upstream side air-fuel ratio sensor output value”) changes as shown in FIG. That is, as described above, according to the air-fuel ratio control of the present embodiment, when the air-fuel ratio of the air-fuel mixture formed in each combustion chamber 21 is to be controlled to the stoichiometric air-fuel ratio, it is formed in each combustion chamber 21. The air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio as a whole by making the air-fuel ratio richer than the stoichiometric air-fuel ratio or making the air-fuel ratio leaner than the stoichiometric air-fuel ratio. When the upstream air-fuel ratio sensor 55 detects that the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio of the air-fuel mixture reaches the stoichiometric air-fuel ratio as quickly as possible. Is set to the fuel injection amount in each fuel injection valve 25, and when the upstream air-fuel ratio sensor 55 detects that the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio, it is as quickly as possible. A device is devised in which a reduction value for the fuel injection amount in each fuel injection valve 25 is set so that the air-fuel ratio of the mixture reaches the stoichiometric air-fuel ratio. For this reason, if all the fuel injection valves 25 are normal, the upstream air-fuel ratio sensor output value corresponds to the theoretical air-fuel ratio output value as shown in FIG. The up and down movement is repeated with a relatively small width across the board.

  On the other hand, there is a problem that a larger amount of fuel than the command fuel injection amount is injected into the fuel injection valve 25 corresponding to the first cylinder # 1, and the fuel injection valves 25 corresponding to the remaining cylinders # 2 to # 4. Is normal, the upstream air-fuel ratio sensor output value changes as shown in FIG. That is, since the air-fuel ratio of the air-fuel mixture formed in the first cylinder # 1 corresponding to the abnormal fuel injection valve 25 is significantly richer than the stoichiometric air-fuel ratio, the first cylinder # 1 The air-fuel ratio of the exhaust gas discharged from No. 1 is also richer than the stoichiometric air-fuel ratio. For this reason, when the exhaust gas discharged from the first cylinder # 1 reaches the upstream air-fuel ratio sensor 55, the upstream air-fuel ratio sensor output value is the air-fuel ratio of the exhaust gas discharged from the first cylinder # 1, In other words, the output value decreases rapidly toward the output value corresponding to the air-fuel ratio that is significantly richer than the stoichiometric air-fuel ratio. According to the air-fuel ratio control of the present embodiment, when the upstream air-fuel ratio sensor output value becomes an output value corresponding to an air-fuel ratio that is significantly richer than the theoretical air-fuel ratio, that is, the upstream air-fuel ratio sensor 55. When the air-fuel ratio is significantly richer than the stoichiometric air-fuel ratio, the fuel injection amounts in all the fuel injection valves 25 are greatly reduced, and the fourth cylinder # 4, the third cylinder # 3, and the second cylinder The air-fuel ratio of the air-fuel mixture formed in cylinder # 2 becomes an air-fuel ratio that is significantly leaner than the stoichiometric air-fuel ratio. For this reason, when the exhaust gas discharged from the fourth cylinder # 4 to the second cylinder # 2 reaches the upstream air-fuel ratio sensor 55, the upstream air-fuel ratio sensor output value is output from the cylinders # 4 to # 2. The air-fuel ratio of the exhaust gas exhausted, that is, the output value corresponding to the air-fuel ratio that is significantly leaner than the stoichiometric air-fuel ratio increases at a stretch. According to the air-fuel ratio control of the present embodiment, when the upstream air-fuel ratio sensor output value becomes an output value corresponding to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, that is, the upstream air-fuel ratio sensor 55 When an air-fuel ratio leaner than the air-fuel ratio is detected, the fuel injection amount in all the fuel injection valves 25 is increased, and again, the air-fuel ratio of the air-fuel mixture formed in the first cylinder # 1 is higher than the stoichiometric air-fuel ratio. The air / fuel ratio becomes significantly richer. For this reason, when there is a problem that a certain amount of fuel is injected into a specific fuel injection valve 25 more than the commanded fuel injection amount, as shown in FIG. The output value repeats vertical movement with a relatively large width across the output value corresponding to the theoretical air-fuel ratio.

  On the other hand, there is a problem that the fuel injection valve 25 corresponding to the first cylinder # 1 only injects an amount of fuel smaller than the command fuel injection amount, and the fuel injection valves 25 corresponding to the remaining cylinders # 2 to # 4 are normal. In this case, the upstream air-fuel ratio sensor output value changes as shown in FIG. That is, since the air-fuel ratio of the air-fuel mixture formed in the first cylinder # 1 corresponding to the abnormal fuel injection valve 25 is an air-fuel ratio that is significantly leaner than the stoichiometric air-fuel ratio, the first cylinder # 1 The air-fuel ratio of the exhaust gas discharged from No. 1 is also an air-fuel ratio that is significantly leaner than the stoichiometric air-fuel ratio. For this reason, when the exhaust gas discharged from the first cylinder # 1 reaches the upstream air-fuel ratio sensor 55, the upstream air-fuel ratio sensor output value is the air-fuel ratio of the exhaust gas discharged from the first cylinder # 1, In other words, the output value increases at a stretch toward an output value corresponding to an air-fuel ratio that is significantly leaner than the stoichiometric air-fuel ratio. Then, according to the air-fuel ratio control of the present embodiment, when the upstream air-fuel ratio sensor output value becomes an output value corresponding to an air-fuel ratio that is significantly leaner than the theoretical air-fuel ratio, that is, the upstream air-fuel ratio sensor 55. When the air-fuel ratio is significantly leaner than the stoichiometric air-fuel ratio, the fuel injection amounts in all the fuel injection valves 25 are greatly increased, and the fourth cylinder # 4, the third cylinder # 3, and the second cylinder The air-fuel ratio of the air-fuel mixture formed in cylinder # 2 becomes an air-fuel ratio that is significantly richer than the stoichiometric air-fuel ratio. For this reason, when the exhaust gas discharged from the fourth cylinder # 4 to the second cylinder # 2 reaches the upstream air-fuel ratio sensor 55, the upstream air-fuel ratio sensor output value is output from the cylinders # 4 to # 2. The air-fuel ratio of the discharged exhaust gas, that is, the output value corresponding to the air-fuel ratio that is significantly richer than the stoichiometric air-fuel ratio, decreases at a stretch. According to the air-fuel ratio control of this embodiment, when the upstream air-fuel ratio sensor output value becomes an output value corresponding to an air-fuel ratio richer than the stoichiometric air-fuel ratio, that is, the upstream air-fuel ratio sensor 55 When an air-fuel ratio richer than the air-fuel ratio is detected, the fuel injection amounts in all the fuel injection valves 25 are reduced, and again, the air-fuel ratio of the air-fuel mixture formed in the first cylinder # 1 is greater than the stoichiometric air-fuel ratio. A significantly leaner air-fuel ratio. For this reason, when there is a problem that a certain amount of fuel is injected into a specific fuel injection valve 25, the upstream side air-fuel ratio sensor as shown in FIG. 8C. The output value repeats vertical movement with a relatively large width across the output value corresponding to the theoretical air-fuel ratio.

  Thus, the transition of the upstream air-fuel ratio sensor output value when there is an abnormality in a specific fuel injection valve 25 is the transition of the upstream air-fuel ratio sensor output value when all the fuel injection valves 25 are normal. Is very different.

  In particular, when all the fuel injection valves 25 are normal, as shown in FIG. 8A, the air-fuel ratio of the exhaust gas that reaches the upstream air-fuel ratio sensor 55 changes toward the rich side. Accordingly, when the upstream air-fuel ratio sensor output value decreases, the average slope of the line followed by the upstream air-fuel ratio sensor output value (hereinafter, this average slope is simply referred to as “slope”) is a relatively small slope α1. It is. On the other hand, when the upstream air-fuel ratio sensor output value increases as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 changes toward the lean side, the upstream air-fuel ratio sensor output value follows. The average slope of the line (hereinafter, this average slope is also simply referred to as “slope”) is a relatively small slope α2. In this case, the absolute value of the inclination α1 and the absolute value of the inclination α2 are substantially equal.

  On the other hand, when there is an abnormality in which a certain amount of fuel is injected into a specific fuel injection valve 25, the upstream side air-fuel ratio sensor 55 as shown in FIG. 8B. When the upstream air-fuel ratio sensor output value decreases as the air-fuel ratio of the exhaust gas that reaches the exhaust gas changes toward the rich side, the slope of the line that the upstream air-fuel ratio sensor output value follows is relatively large The inclination is α3. On the other hand, when the upstream air-fuel ratio sensor output value increases as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 changes toward the lean side, the upstream air-fuel ratio sensor output value follows. The inclination of the line is a relatively large inclination α4. In this case, the absolute value of the slope α3 of the line that the upstream air-fuel ratio sensor output value follows when the upstream air-fuel ratio sensor output value decreases becomes the upstream side air-fuel ratio sensor output value when the upstream air-fuel ratio sensor output value increases. The output value of the fuel ratio sensor is slightly larger than the absolute value of the line inclination α4.

  On the other hand, when there is an abnormality in which only a smaller amount of fuel than the commanded fuel injection amount is injected into a specific fuel injection valve 25, the upstream air-fuel ratio sensor 55 is reached as shown in FIG. When the upstream air-fuel ratio sensor output value increases as the air-fuel ratio of the exhaust gas that changes becomes leaner, the slope of the line followed by the upstream air-fuel ratio sensor output value is a relatively large value α5. It is. On the other hand, when the upstream air-fuel ratio sensor output value becomes smaller as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 changes toward the rich side, the upstream air-fuel ratio sensor output value follows. The slope of the line is a relatively large slope α6. In this case, the absolute value of the slope α5 of the line that the upstream air-fuel ratio sensor output value follows when the upstream air-fuel ratio sensor output value increases is equal to the upstream air-fuel ratio sensor output value when the upstream air-fuel ratio sensor output value decreases. The output value of the fuel ratio sensor is slightly larger than the absolute value of the line inclination α6.

  In this way, the absolute value of the slope of the line followed by the upstream air-fuel ratio sensor output value is the amount of fuel larger than the command fuel injection amount for a specific fuel injection valve 25 when all the fuel injection valves 25 are normal. When there is an abnormality that injects fuel, and when there is an abnormality that causes the specific fuel injection valve 25 to inject less fuel than the fuel of the command fuel injection amount, a specific value is taken. Therefore, if the absolute value of this slope is used, it is possible to determine whether or not there is an inter-cylinder air-fuel ratio imbalance state. That is, the absolute value of the slope of the line followed by the upstream air-fuel ratio sensor output value when there is an abnormality in a specific fuel injection valve 25 is basically the upstream side when all the fuel injection valves 25 are normal. The air-fuel ratio sensor output value is larger than the absolute value of the slope of the line followed. Therefore, when all the fuel injection valves 25 are normal, the absolute value of the slope that the line followed by the upstream air-fuel ratio sensor output value can be set as the threshold value, or a value larger than the absolute value of the slope is set as the threshold value. When the absolute value of the slope of the line followed by the upstream air-fuel ratio sensor output value is larger than this threshold during engine operation, it is determined that an inter-cylinder air-fuel ratio imbalance condition has occurred. it can.

  Incidentally, the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state described with reference to FIGS. 8A to 8C is performed when the air-fuel ratio of the air-fuel mixture is to be controlled to the stoichiometric air-fuel ratio (that is, The air-fuel ratio of the exhaust gas arriving at the upstream-side air-fuel ratio sensor 55 is the stoichiometric air-fuel ratio). This also applies when the air-fuel ratio is to be controlled to be rich (that is, when the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is richer than the stoichiometric air-fuel ratio).

  That is, when all of the fuel injection valves 25 are normal when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, the exhaust gas air that has reached the upstream air-fuel ratio sensor 55 is exhausted. Corresponding to the air-fuel ratio of the fuel ratio, the upstream air-fuel ratio sensor output value (that is, the output value of the upstream air-fuel ratio sensor 55) changes as shown in FIG. That is, the transition of the upstream air-fuel ratio sensor output value at this time is generally richer than the transition of the upstream air-fuel ratio sensor output value shown in FIG. Repeated up and down movement with a relatively small width in a shifted form. However, when the upstream air-fuel ratio sensor output value decreases as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 changes toward the rich side, it is shown in FIG. The average slope of the line followed by the upstream air-fuel ratio sensor output value is a relatively small slope α1 similar to the average slope of the line followed by the upstream air-fuel ratio sensor output value shown in FIG. is there. Similarly, when the upstream air-fuel ratio sensor output value increases as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 changes toward the lean side, it is shown in FIG. The average slope of the line followed by the upstream air-fuel ratio sensor output value is relatively small, like the average slope of the line followed by the upstream air-fuel ratio sensor output value shown in FIG. It is.

  On the other hand, when the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio richer than the stoichiometric air-fuel ratio, the command fuel injection amount (that is, from the electronic control unit 60) to the fuel injection valve 25 corresponding to the first cylinder # 1. There is a problem that a larger amount of fuel than the amount of fuel to be injected from each fuel injection valve 25 commanded to each fuel injection valve 25 is injected, and the fuel corresponding to the remaining cylinders # 2 to # 4 When the injection valve 25 is normal, the upstream air-fuel ratio sensor output value changes as shown in FIG. That is, the transition of the upstream air-fuel ratio sensor output value at this time is generally richer than the transition of the upstream air-fuel ratio sensor output value shown in FIG. Repeated up and down movement with a relatively large width in a shifted state. Then, when the upstream air-fuel ratio sensor output value becomes smaller as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 changes toward the rich side, it is shown in FIG. 9B. The average slope of the line followed by the upstream air-fuel ratio sensor output value is a relatively large slope α3, similar to the average slope of the line followed by the upstream air-fuel ratio sensor output value shown in FIG. is there. Similarly, the average slope of the line followed by the upstream air-fuel ratio sensor output value as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 changes toward the lean side is shown in FIG. As with the average slope of the line followed by the upstream air-fuel ratio sensor output value shown in FIG.

  On the other hand, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, only a smaller amount of fuel than the command fuel injection amount is injected into the fuel injection valve 25 corresponding to the first cylinder # 1. When the fuel injection valves 25 corresponding to the remaining cylinders # 2 to # 4 are normal due to a malfunction that is not performed, the upstream side air-fuel ratio sensor output value changes as shown in FIG. 9C. . That is, the transition of the upstream air-fuel ratio sensor output value at this time is generally richer than the transition of the upstream air-fuel ratio sensor output value shown in FIG. Repeated up and down movement with a relatively large width in a shifted state. When the upstream air-fuel ratio sensor output value increases as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 changes toward the lean side, it is shown in FIG. The average slope of the line followed by the upstream air-fuel ratio sensor output value is a relatively large value α5, similar to the average slope of the line followed by the upstream air-fuel ratio sensor output value shown in FIG. is there. Similarly, when the upstream air-fuel ratio sensor output value becomes smaller as the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 changes toward the rich side, it is shown in FIG. The average slope of the line that the upstream air-fuel ratio sensor output value follows is a relatively large value α6, similar to the average slope of the line that the upstream air-fuel ratio sensor output value follows in FIG. It is.

  As described above, even when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, a specific fuel is used in the same manner as when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio. When there is an abnormality in the injection valve 25 (that is, when an inter-cylinder air-fuel ratio imbalance state has occurred), the absolute value of the slope of the line followed by the upstream air-fuel ratio sensor output value is all the fuel injection valves 25. Is normal (that is, when the inter-cylinder air-fuel ratio imbalance state has not occurred), the upstream air-fuel ratio sensor output value is larger than the absolute value of the slope of the line followed. Therefore, when the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio richer than the stoichiometric air-fuel ratio, the slope that can be taken by the line that the upstream air-fuel ratio sensor output value follows when all the fuel injection valves 25 are normal Is set as a threshold value, or a value larger than the absolute value of the slope is set as a threshold value, and the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio. When the absolute value of the slope of the line followed by the upstream air-fuel ratio sensor output value is larger than this threshold value, it can be determined that an inter-cylinder air-fuel ratio imbalance state has occurred.

  Further, the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state described with reference to FIGS. 8A to 8C is performed when the air-fuel ratio of the air-fuel mixture is to be controlled to the stoichiometric air-fuel ratio (that is, The air-fuel ratio of the exhaust gas arriving at the upstream side air-fuel ratio sensor 55 is the stoichiometric air-fuel ratio). As in the case of trying to control to an air-fuel ratio, the determination of the presence or absence of this air-fuel ratio imbalance state is made when the air-fuel ratio of the mixture is to be controlled to an air-fuel ratio leaner than the stoichiometric air-fuel ratio ( That is, this also applies to the case where the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is leaner than the stoichiometric air-fuel ratio.

  As described above, whether or not the above-described inter-cylinder air-fuel ratio imbalance state is present is determined even when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio. Even when the air-fuel ratio is controlled to be rich or even when the air-fuel ratio of the air-fuel mixture is controlled to be leaner than the stoichiometric air-fuel ratio, it can be used.

  However, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio, and when the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the cylinder is based on the above-described concept. When the presence or absence of the inter-air-fuel ratio imbalance state is determined, the determination accuracy is lower than when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio. Therefore, hereinafter, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio and when the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the inter-cylinder air-fuel ratio imbalance state is reached. Explains why the determination accuracy is low when the presence / absence is determined, and also determines whether there is an inter-cylinder air / fuel ratio imbalance state when the air / fuel ratio of the mixture is controlled to an air / fuel ratio richer than the stoichiometric air / fuel ratio The reason why the determination accuracy becomes high will be described.

  First, the reason why the determination accuracy is lowered when it is determined whether there is an inter-cylinder air-fuel ratio imbalance state when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio will be described.

  As described above, the upstream air-fuel ratio sensor 55 outputs an output value in accordance with the characteristics shown in FIG. 3A in accordance with the air-fuel ratio of the exhaust gas arriving there. The mechanism by which the upstream air-fuel ratio sensor 55 outputs such an output value is as follows.

  That is, the upstream air-fuel ratio sensor 55 includes an air-fuel ratio detection element 55a, an outer protective cover 55b, and an inner protective cover 55c, as shown in FIGS. The protective covers 55b and 55c accommodate the air-fuel ratio detection element 55a therein so as to cover the air-fuel ratio detection element 55a. The protective covers 55b and 55c are provided with inflow holes 55b1 and 55c1 through which exhaust gas that has reached the upstream air-fuel ratio sensor 55 flows into the air through the exhaust pipe 42 and reaches the air-fuel ratio detection element 55a. Outflow holes 55b2 and 55c2 for allowing the exhaust gas flowing into the exhaust pipe 42 to flow out to the exhaust pipe 42 are provided.

  The upstream air-fuel ratio sensor 55 is disposed in the exhaust pipe 42 so that the protective covers 55b and 55c are exposed in the exhaust pipe 42. Therefore, the exhaust gas EX flowing through the exhaust pipe 42 passes through the inflow hole 55b1 of the outer protective cover 55b and flows between the outer protective cover 55b and the inner protective cover 55c as shown by the arrow Ar1 in FIGS. Flows into the space between. Next, as indicated by an arrow Ar2, the exhaust gas flows into the inner space of the inner protective cover 55c through the inflow hole 55c1 of the inner protective cover 55c, and reaches the air-fuel ratio detecting element 55a. Thereafter, as shown by the arrow Ar3, the exhaust gas flows out to the exhaust pipe 42 through the outflow hole 55c2 of the inner protective cover 55c and the outflow hole 55b2 of the outer protective cover 55b. Since the exhaust gas that has reached the upstream air-fuel ratio sensor 55 flows in the upstream air-fuel ratio sensor 55 in this way, the exhaust gas that has reached the upstream air-fuel ratio sensor 55 passes through the vicinity of the outflow hole 55b2 of the outer protective cover 55b. The exhaust gas flowing in is sucked into the inflow hole 55b1 of the outer protective cover 55b.

  As shown in FIG. 12A, the air-fuel ratio detection element 55a includes a solid electrolyte layer 551, an exhaust gas side electrode layer 552, an atmosphere side electrode layer 553, and a diffusion resistance layer (or diffusion rate limiting layer). 554, an exhaust gas side wall 555, an atmospheric side wall 556, and a heater 557.

The solid electrolyte layer 551 is a sintered body of an oxygen ion conductive oxide, for example, a stabilized zirconia element in which CaO is dissolved as a stabilizer in ZrO ₂ (zirconia). The solid electrolyte layer 551 exhibits oxygen cell characteristics and oxygen pump characteristics, which will be described later, when the temperature is higher than a certain temperature (that is, so-called activation temperature).

  The exhaust gas side electrode layer 552 is made of a noble metal having high catalytic activity, for example, Pt (platinum). Further, the exhaust gas side electrode layer 552 is disposed on one surface of the solid electrolyte layer 551. Further, the exhaust gas side electrode layer 552 is formed to have sufficient permeability by chemical plating, for example, to be porous.

  On the other hand, the atmosphere side electrode layer 553 is made of a noble metal having high catalytic activity, for example, Pt (platinum). The atmosphere-side electrode layer 553 is disposed on the surface of the solid electrolyte layer 551 opposite to the surface of the solid electrolyte layer 551 on which the exhaust gas-side electrode layer 552 is disposed. That is, the solid electrolyte layer 551 is sandwiched between the exhaust gas side electrode layer 552 and the atmosphere side electrode layer 553. The atmosphere-side electrode layer 553 is formed to have sufficient permeability by chemical plating, that is, in a porous shape.

  A power source 679 is connected to the exhaust gas side electrode layer 552 and the atmosphere side electrode layer 553. A voltage is applied to the exhaust gas side electrode layer 552 and the atmosphere side electrode layer 553 from the power source 679 so that the potential of the atmosphere side electrode layer 553 is higher than that of the exhaust gas side electrode layer 552.

  The diffusion resistance layer 554 is made of a porous ceramic that is a heat-resistant inorganic substance. Further, the diffusion resistance layer 554 is disposed by, for example, a plasma spraying method so as to cover the surface of the exhaust gas side electrode layer 552 excluding the surface of the exhaust gas side electrode layer 552 in contact with the surface of the solid electrolyte layer 551. ing.

  The exhaust gas side wall 555 is made of alumina ceramic that is dense and does not allow exhaust gas to pass therethrough. Further, the exhaust gas side wall 555 is disposed so as to cover the diffusion resistance layer 554 except for a part of the diffusion resistance layer 554 (particularly, corner portions of the diffusion resistance layer 554). That is, the exhaust gas side wall 555 has a through hole 558 that exposes a part of the diffusion resistance layer 554 to the outside.

  The atmospheric side wall 556 is made of alumina ceramic that is dense and does not allow exhaust gas to pass therethrough. The atmosphere side wall 556 is disposed so as to form a space (hereinafter referred to as “atmosphere chamber”) 559 surrounding the atmosphere side electrode layer 553 therein. The atmosphere is introduced into the atmosphere chamber 559.

  The heater 557 is embedded in the atmospheric side wall 556. When electric power is supplied to the heater 557, the solid electrolyte layer 551, the exhaust gas side electrode layer 552, and the atmosphere side electrode layer 553 are heated by the heat generated by the heater 557.

  Here, when exhaust gas having an air / fuel ratio leaner than the stoichiometric air / fuel ratio arrives at the air / fuel ratio detecting element 55a, the air / fuel ratio detecting element 55a functions as shown in FIG. 12 (B). That is, since the concentration of oxygen in the exhaust gas around the exhaust gas side electrode layer 552 is relatively high, oxygen moves from the exhaust gas side electrode layer 552 to the atmosphere side electrode layer 553 through the solid electrolyte layer 551. Specifically, the exhaust gas that has reached the air-fuel ratio detection element 55 a flows into the diffusion resistance layer 554 through the through hole 558. When the exhaust gas reaches the exhaust gas side electrode layer 552 through the diffusion resistance layer 554, oxygen in the exhaust gas is ionized by the exhaust gas side electrode layer 552. This ionized oxygen, that is, oxygen ions, reaches the atmosphere-side electrode layer 553 through the solid electrolyte layer 551. The oxygen ions that have reached the atmosphere-side electrode layer 553 emit electrons to the atmosphere-side electrode layer 553 and become oxygen, and flow into the atmosphere chamber 559. With such an action, the current I flows from the positive electrode of the power source 560 toward the negative electrode. As shown in FIG. 13, the magnitude of the current I flowing at this time is the oxygen in the exhaust gas that has reached the exhaust gas side electrode layer 552 if the voltage of the power source 560 is set to a predetermined value Vp. It becomes a constant value proportional to the concentration of. In the present embodiment, when the voltage of the power source 560 is set to a predetermined value Vp and the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the power source 560 The air-fuel ratio of the exhaust gas is grasped based on the current I flowing from the positive electrode toward the negative electrode (that is, the limit current Ip).

On the other hand, when exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio arrives at the air-fuel ratio detection element 55a, the air-fuel ratio detection element 55a functions as shown in FIG. That is, since the concentration of oxygen in the exhaust gas around the exhaust gas side electrode layer 552 is relatively low, oxygen moves from the atmosphere side electrode layer 553 to the exhaust gas side electrode layer 552 through the solid electrolyte layer 551. Specifically, oxygen in the atmosphere introduced into the atmosphere chamber 559 is ionized by the atmosphere-side electrode layer 553. This ionized oxygen, that is, oxygen ions reaches the exhaust gas side electrode layer 553 through the solid electrolyte layer 551. Oxygen ions that have reached the exhaust gas side electrode layer 552 emit electrons to the exhaust gas side electrode layer 552, while unburned matter in the exhaust gas around the exhaust gas side electrode layer 552, such as hydrocarbon (HC), Carbon monoxide (CO) and hydrogen (H ₂ ) are oxidized. With such an action, the current I flows from the negative electrode of the power source 560 toward the positive electrode. As shown in FIG. 13, the magnitude of the current I flowing at this time is also the concentration of oxygen in the exhaust gas around the exhaust gas side electrode layer 552 if the voltage of the power source 560 is set to a predetermined value Vp. It becomes a constant value proportional to. In the present embodiment, when the voltage of the power source 560 is set to a predetermined value Vp and the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is an air-fuel ratio richer than the stoichiometric air-fuel ratio, the power source 560 The air-fuel ratio of the exhaust gas is grasped based on the current I flowing from the negative electrode toward the positive electrode (that is, the limit current Ip).

  As described above, the upstream air-fuel ratio sensor 55 outputs an output value corresponding to the air-fuel ratio of the exhaust gas arriving there. That is, when the air-fuel ratio of the exhaust gas arriving there is an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the upstream side air-fuel ratio sensor 55 increases as the lean degree of the air-fuel ratio of the exhaust gas arriving there increases. When the air-fuel ratio of the exhaust gas arriving there is an air-fuel ratio richer than the stoichiometric air-fuel ratio, the larger the richness of the air-fuel ratio of the exhaust gas arriving there, the larger the negative current value Is output. Therefore, when the air-fuel ratio of the exhaust gas flowing into the upstream air-fuel ratio sensor 55 is the stoichiometric air-fuel ratio, the current value output from the upstream air-fuel ratio sensor 55 is zero.

  Therefore, when the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 changes from an air-fuel ratio leaner than the stoichiometric air-fuel ratio to an air-fuel ratio richer than the stoichiometric air-fuel ratio, the output value of the upstream air-fuel ratio sensor 55 Changes from a positive current value to a negative current value. In this case, oxygen ions flowing from the exhaust gas side electrode layer 552 of the upstream side air-fuel ratio sensor 55 to the atmosphere side electrode layer 553 via the solid electrolyte layer 551 are solid from the atmosphere side electrode layer 553 to the exhaust gas side electrode layer 552. It flows through the electrolyte layer 551. That is, the direction of oxygen ions flowing in the solid electrolyte layer 551 is reversed. However, when the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 changes from an air-fuel ratio leaner than the stoichiometric air-fuel ratio to an air-fuel ratio richer than the stoichiometric air-fuel ratio, the exhaust gas-side electrode layer 552 until then. Since the oxygen ions flowing in the solid electrolyte layer 551 toward the atmosphere side electrode layer 553 must change the direction of the flow, the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is the theoretical sky. When the air-fuel ratio leaner than the fuel ratio is changed to an air-fuel ratio richer than the stoichiometric air-fuel ratio, the direction of oxygen ion flow in the solid electrolyte layer 551 is not immediately reversed.

  On the other hand, when the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 changes from an air-fuel ratio richer than the stoichiometric air-fuel ratio to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the output value of the upstream air-fuel ratio sensor 55 Changes from a negative current value to a positive current value. In this case, oxygen ions that have flowed from the atmosphere-side electrode layer 553 of the upstream air-fuel ratio sensor 55 to the exhaust gas-side electrode layer 552 via the solid electrolyte layer 551 are solidified from the exhaust gas-side electrode layer 552 to the atmosphere-side electrode layer 553. It flows through the electrolyte layer 551. That is, the direction of oxygen ions flowing in the solid electrolyte layer 551 is reversed. However, when the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 changes from an air-fuel ratio richer than the stoichiometric air-fuel ratio to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the exhaust gas from the atmosphere-side electrode layer 553 is exhausted until then. Since the direction of the oxygen ions flowing in the solid electrolyte layer 551 toward the gas side electrode layer 552 has to be changed, the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is higher than the stoichiometric air-fuel ratio. When the air-fuel ratio changes from a rich air-fuel ratio to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, the direction of oxygen ion flow in the solid electrolyte layer 551 does not immediately reverse.

  Here, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio, as described above, the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. The air-fuel ratio richer than the stoichiometric air-fuel ratio is repeated. Therefore, in this case, in order for the upstream air-fuel ratio sensor 55 to output an output value that indicates the air-fuel ratio of the exhaust gas arriving there very accurately, the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is When the air-fuel ratio leaner than the stoichiometric air-fuel ratio and the air-fuel ratio richer than the stoichiometric air-fuel ratio are changed, the direction of the flow of oxygen ions flowing in the solid electrolyte layer 551 of the upstream air-fuel ratio sensor 55 is instantaneous. Must be reversed. However, as described above, the direction of the flow of oxygen ions flowing in the solid electrolyte layer 551 is not immediately reversed. Therefore, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio, the output value of the upstream air-fuel ratio sensor 55 is the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 (ie, the air-fuel ratio of the air-fuel mixture). (Fuel ratio) will not be shown very accurately.

  In order to accurately determine the presence / absence of the inter-cylinder air-fuel ratio imbalance state based on the output value of the upstream air-fuel ratio sensor 55, the exhaust value from which the output value of the upstream air-fuel ratio sensor 55 arrives at the upstream air-fuel ratio sensor 55 It is desirable to show the air / fuel ratio of the gas (that is, the air / fuel ratio of the air / fuel mixture) very accurately. In other words, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio, the output value of the upstream air-fuel ratio sensor 55 does not indicate the air-fuel ratio of the exhaust gas that arrives at the upstream air-fuel ratio sensor 55 very accurately. Therefore, based on the output value of the upstream air-fuel ratio sensor 55, the presence or absence of the inter-cylinder air-fuel ratio imbalance state cannot be accurately determined.

  For these reasons, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio, the determination accuracy decreases when it is determined whether there is an inter-cylinder air-fuel ratio imbalance state.

  Next, the reason why the determination accuracy is lowered when the air-fuel ratio imbalance state between cylinders is determined when the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio leaner than the stoichiometric air-fuel ratio will be described.

  When the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, combustion in the combustion chamber 21 becomes unstable, and in some cases, misfiring may occur in the combustion chamber 21. In this case, the air-fuel ratio of the exhaust gas discharged from each combustion chamber 21 does not reflect the air-fuel ratio of the air-fuel mixture that should be originally controlled.

  As described above, in order to accurately determine the presence / absence of the inter-cylinder air-fuel ratio imbalance state based on the output value of the upstream air-fuel ratio sensor 55, the output value of the upstream air-fuel ratio sensor 55 is the air-fuel ratio of the mixture. Is desired to be shown very accurately. However, when the air-fuel ratio of the air-fuel mixture is controlled to be leaner than the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas discharged from each combustion chamber 21 reflects the air-fuel ratio of the air-fuel mixture that should be controlled originally. Therefore, the presence or absence of the inter-cylinder air-fuel ratio imbalance state cannot be accurately determined based on the output value of the upstream air-fuel ratio sensor 55.

  For these reasons, when the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, the determination accuracy is lowered when it is determined whether there is an inter-cylinder air-fuel ratio imbalance state.

  Next, the reason why the determination accuracy becomes high when the presence / absence of the inter-cylinder air-fuel ratio imbalance state is determined when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio will be described.

  When the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is also richer than the stoichiometric air-fuel ratio. Therefore, in this case, in order for the upstream air-fuel ratio sensor 55 to output an output value that accurately indicates the air-fuel ratio of the exhaust gas arriving there, oxygen ions flowing in the diffusion resistance layer 551 of the upstream air-fuel ratio sensor 55 The direction of flow need not be reversed. That is, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, the output value of the upstream air-fuel ratio sensor 55 is the air-fuel ratio of the exhaust gas arriving there (that is, the air-fuel ratio of the air-fuel mixture). The air / fuel ratio) is accurately indicated, and therefore the presence / absence of the inter-cylinder air / fuel ratio imbalance state can be accurately determined based on the output value of the upstream air / fuel ratio sensor 55.

  In addition, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, combustion in the combustion chambers 21 is stable, and the air-fuel ratio of the exhaust gas discharged from each combustion chamber 21 is This reflects the air-fuel ratio of the air-fuel mixture that should be controlled originally. Therefore, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, the output value of the upstream air-fuel ratio sensor 55 indicates the air-fuel ratio of the air-fuel mixture very accurately. Therefore, the presence / absence of the inter-cylinder air / fuel ratio imbalance state can be accurately determined based on the output value of the upstream air / fuel ratio sensor 55.

  For these reasons, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, the determination accuracy becomes high when it is determined whether there is an inter-cylinder air-fuel ratio imbalance state.

  By the way, as described above, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, the determination accuracy is high when it is determined whether there is an inter-cylinder air-fuel ratio imbalance state. Therefore, when it should be determined whether or not there is an inter-cylinder air-fuel ratio imbalance state, if the air-fuel ratio of the air-fuel mixture is forcibly made richer than the stoichiometric air-fuel ratio, the presence or absence of the inter-cylinder air-fuel ratio imbalance state is It is possible to accurately determine whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  However, if the air-fuel ratio of the air-fuel mixture is made richer than the stoichiometric air-fuel ratio only for the purpose of determining whether there is an inter-cylinder air-fuel ratio imbalance state, the fuel efficiency of the internal combustion engine 10 will deteriorate. On the other hand, in the internal combustion engine 10 of the present embodiment, the air-fuel ratio of the air-fuel mixture may be richer than the stoichiometric air-fuel ratio for purposes other than the purpose of determining whether or not there is an inter-cylinder air-fuel ratio imbalance state. That is, in the internal combustion engine 10, the air-fuel ratio of the air-fuel mixture may be made richer than the stoichiometric air-fuel ratio for the purpose of determining whether there is an abnormality in the output of the downstream air-fuel ratio sensor 56. The determination of whether there is an abnormality in the output of the downstream air-fuel ratio sensor 56 is performed as follows.

  That is, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio is discharged from each combustion chamber 21. Then, the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio flows into the upstream side catalyst 43. Here, since the upstream side catalyst 43 has oxygen storage / release capability, oxygen stored in the upstream side catalyst 43 is discharged from the upstream side catalyst 43. Exhaust gas flows out. When the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio continues to flow into the upstream side catalyst 43 and the oxygen stored in the upstream-side catalyst 43 is exhausted, the upstream side catalyst 43 is richer than the stoichiometric air-fuel ratio. Air-fuel ratio exhaust gas flows out. At this time, if there is no abnormality in the output of the downstream air-fuel ratio sensor 56, that is, if the downstream air-fuel ratio sensor 56 is normal, the output value of the downstream air-fuel ratio sensor 56 (hereinafter referred to as "downstream"). The "side air-fuel ratio sensor output value") accurately corresponds to the air-fuel ratio of the exhaust gas flowing out from the upstream side catalyst 43, that is, the air-fuel ratio of the air-fuel mixture that is controlled to be richer than the stoichiometric air-fuel ratio. Output value. Therefore, when the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio is flowing out from the upstream catalyst 43, the downstream air-fuel ratio sensor output value is an output value that accurately corresponds to the air-fuel ratio of the air-fuel mixture. In other words, the output of the downstream air-fuel ratio sensor 56 is not abnormal, and it can be said that the downstream air-fuel ratio sensor 56 is normal. On the other hand, when exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio flows out from the upstream catalyst 43, the downstream air-fuel ratio sensor output value does not correspond to the air-fuel ratio of the air-fuel mixture. In other words, it can be said that an abnormality in the output of the downstream air-fuel ratio sensor 56 has occurred.

  Therefore, in the internal combustion engine 10, when it is determined whether or not the output of the downstream air-fuel ratio sensor 56 is abnormal, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio over a predetermined period. Is done. Here, the predetermined period is sufficient to eliminate the oxygen stored in the upstream catalyst 43 when exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio continues to flow into the upstream catalyst 43. Set to time. Then, when the predetermined period has elapsed after the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, the downstream air-fuel ratio sensor output value is richer than the stoichiometric air-fuel ratio. It is determined whether or not the air-fuel ratio of the exhaust gas corresponding to the air-fuel ratio of the air-fuel mixture controlled to the air-fuel ratio is indicated. When it is determined that the downstream air-fuel ratio sensor output value at this time indicates the air-fuel ratio of the exhaust gas corresponding to the air-fuel ratio of the air-fuel mixture that is controlled to be richer than the stoichiometric air-fuel ratio, An abnormality in the output of the downstream air-fuel ratio sensor 56 has not occurred, and it is determined that the downstream air-fuel ratio sensor 56 is normal. On the other hand, when it is determined that the downstream air-fuel ratio sensor output value at this time does not indicate the air-fuel ratio of the exhaust gas corresponding to the air-fuel ratio of the air-fuel mixture that is controlled to be richer than the stoichiometric air-fuel ratio, It is determined that an abnormality in the output of the downstream air-fuel ratio sensor 56 has occurred.

  In the present embodiment (hereinafter referred to as “first embodiment”), as described above, the air-fuel ratio of the air-fuel mixture is set to be higher than the stoichiometric air-fuel ratio for the purpose of determining whether or not the output of the downstream air-fuel ratio sensor 56 is abnormal. When the air-fuel ratio is controlled to be rich, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  According to this, since the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of the inter-cylinder air-fuel ratio imbalance state, the inter-cylinder air-fuel ratio imbalance state Compared with the case where the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of fuel, the fuel efficiency of the internal combustion engine 10 is improved.

  Next, an example of a routine for determining whether or not there is an inter-cylinder air-fuel ratio imbalance state according to the first embodiment will be described. The flowchart shown in FIG. 14 and FIG. 15 is used for determining whether or not there is an inter-cylinder air-fuel ratio imbalance state in the first embodiment. The routine of FIG. 14 and the routine of FIG. 15 are executed at predetermined time intervals.

  When the routine of FIG. 14 is started, first, in step 100, a flag indicating whether or not execution of determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is permitted (hereinafter referred to as “inter-cylinder air-fuel ratio”). It is determined whether or not F1 (referred to as an “imbalance determination execution flag”) is set (F1 = 1). Here, when the inter-cylinder air-fuel ratio imbalance determination execution flag F1 is set (F1 = 1), execution of the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state is permitted, and the inter-cylinder air-fuel ratio imbalance is permitted. When the determination execution flag F1 is reset (F1 = 0), execution of the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is not permitted. Further, the inter-cylinder air-fuel ratio imbalance determination execution flag F1 is a flag that is set, for example, according to the routine shown in FIG.

  When it is determined in step 100 that F1 = 1, it is permitted to execute the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state. To execute, the routine proceeds to step 101. On the other hand, if it is determined in step 100 that F1 = 0, the execution of the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is not permitted, so the routine ends as it is.

  When it is determined in step 100 that F1 = 1 and the routine proceeds to step 101, the rate of change of the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 based on the output value of the upstream air-fuel ratio sensor 55 is determined. The absolute value | ΔA / F | is calculated. Next, at step 102, the counter C indicating the number of times of calculating the absolute value | ΔA / F | of the change rate of the air-fuel ratio of the exhaust gas at step 101 is incremented.

  Next, at step 103, it is judged if the counter C incremented at step 102 is equal to a predetermined threshold Cth (C = Cth). Here, if it is determined that C = Cth, the routine proceeds to step 104. On the other hand, when it is determined that C ≠ Cth, the routine returns to step 100.

  Steps 102 and 103 are provided in order to obtain more data on the change rate of the exhaust gas air-fuel ratio calculated in step 101 in order to increase the determination accuracy of the presence or absence of the inter-cylinder air-fuel ratio imbalance state. Step. Therefore, even if there is only one data of the change rate of the air-fuel ratio of the exhaust gas calculated in step 101, if the accuracy of determining whether there is an inter-cylinder air-fuel ratio imbalance state is sufficiently high, these data The step may be omitted.

  When it is determined in step 103 that C ≠ Cth and the routine returns to step 100, it is once again determined whether or not the inter-cylinder air-fuel ratio imbalance determination execution flag F1 is set (F1 = 1). . Here, when it is determined that F1 = 1, the routine proceeds to step 101 and subsequent steps. On the other hand, when it is determined that F1 = 0, the routine ends.

  On the other hand, when it is determined in step 103 that C = Cth and the routine proceeds to step 104, the average value ΔA of the absolute value | ΔA / Fth | / Fave is calculated.

  Next, at step 105, it is judged if the average value ΔA / Fave calculated at step 104 is larger than a predetermined threshold value ΔA / Faveth (ΔA / Fave> ΔA / Faveth). Here, when it is determined that ΔA / Fave> ΔA / Faveth, an inter-cylinder air-fuel ratio imbalance state has occurred, so the routine proceeds to step 106 where the inter-cylinder air-fuel ratio imbalance state is determined. An alarm is activated to indicate that it has occurred.

  Since the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance has been completed, in step 107, the data of the absolute value | ΔA / Fave | of the change rate of the exhaust gas air / fuel ratio calculated in step 101 is cleared. Then, at step 108, the counter C incremented at step 102 is cleared, and the routine ends.

  On the other hand, when it is determined in step 105 that ΔA / Fave ≦ ΔA / Faveth, that is, it is determined that the average value ΔA / Fave calculated in step 104 is equal to or smaller than a predetermined threshold value ΔA / Faveth. Sometimes, since the inter-cylinder air-fuel ratio imbalance state has not occurred, the routine ends as it is.

  Next, the flowchart shown in FIG. 15, which is an example of a routine for executing the setting of the inter-cylinder air-fuel ratio imbalance determination execution flag of the first embodiment used in step 100 of the routine of FIG. 14, will be described.

  When the routine of FIG. 15 is started, first, in step 200, a flag (hereinafter referred to as this flag) indicating whether or not a precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is satisfied. It is determined whether or not F2 is set (F2 = 1) (the flag is referred to as “inter-cylinder air-fuel ratio imbalance determination precondition flag”). Here, when the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is set (F2 = 1), a precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is satisfied. When the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is reset (F2 = 0), the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is not satisfied. .

  If it is determined in step 200 that F2 = 1, the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is established, and the routine proceeds to step 201. On the other hand, when it is determined that F2 = 0, the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is not satisfied, and the routine proceeds to step 203.

  Note that the precondition for executing the determination of whether or not there is an inter-cylinder air-fuel ratio imbalance state is the temperature of cooling water for cooling the internal combustion engine 10 (this temperature represents the temperature of the internal combustion engine 10). Is above a predetermined temperature (for example, 75 ° C.), the engine speed is within a predetermined range (for example, 1200 rpm to 2000 rpm), and the intake air amount is within a predetermined range (for example, 10 g / sec). ˜20 g / sec), the temperature of the upstream air-fuel ratio sensor 55 is equal to or higher than the activation temperature, and the atmospheric pressure is equal to or higher than a predetermined value (for example, 75 kPa).

  When it is determined in step 200 that F2 = 1, and the routine proceeds to step 201, the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio for the purpose of determining an abnormality in the output of the downstream air-fuel ratio sensor 56. It is determined whether or not a flag F3 is set (F3 = 1) (hereinafter, this flag is referred to as a "downstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control flag"). . Here, when the rich air-fuel ratio control flag F3 for determining the downstream air-fuel ratio sensor abnormality is set (F3 = 1), the air-fuel mixture is empty for the purpose of determining whether or not the output of the downstream air-fuel ratio sensor 55 is abnormal. When the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio and the downstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control flag F3 is reset (F3 = 0), the downstream air-fuel ratio sensor 55 The air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio for the purpose of determining output abnormality.

  If it is determined in step 201 that F3 = 1, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio for the purpose of determining whether there is an abnormality in the output of the downstream air-fuel ratio sensor 56. Therefore, the routine proceeds to step 202, where “1” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 201, it is determined that F2 = 1 in step 200. Therefore, when it is determined that F3 = 1 in step 201, the inter-cylinder air-fuel ratio imbalance state is reached. The preconditions for executing the presence / absence determination are satisfied, and the air / fuel ratio of the air / fuel mixture is controlled to be richer than the stoichiometric air / fuel ratio. Therefore, the routine proceeds to step 202, where “1” is input to the inter-cylinder air / fuel ratio imbalance determination execution flag F1. In this case, in step 100 of the routine of FIG. = 1 and it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  On the other hand, when it is determined in step 201 that F3 = 0, the air-fuel ratio of the air-fuel mixture is made richer than the stoichiometric air-fuel ratio for the purpose of determining whether the output of the downstream air-fuel ratio sensor 56 is abnormal. Since it is not controlled, the routine proceeds to step 203, where “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 201, it is determined in step 200 that F2 = 1, and the precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is satisfied. When it is determined in step 201 that F3 = 0, the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio, and it is accurately determined whether there is an inter-cylinder air-fuel ratio imbalance state. In this state, the routine proceeds to step 203, where “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, F1 = 0 in step 100 of the routine of FIG. Therefore, it is determined that there is an inter-cylinder air-fuel ratio imbalance state.

  If it is determined in step 200 that F2 = 0, and the routine proceeds to step 203, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when it is determined in step 200 that F2 = 0, the condition that is a precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is not satisfied, and the routine proceeds to step 203. Then, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, it is determined that F1 = 0 in step 100 of the routine of FIG. Judgment will not be made.

  Incidentally, in the internal combustion engine 10, there is a case where the air-fuel ratio of the air-fuel mixture is made richer than the stoichiometric air-fuel ratio for the purpose of determining whether or not the oxygen storage / release capability of the upstream catalyst 43 has deteriorated. The determination as to whether or not the oxygen storage / release capability of the upstream catalyst 43 has deteriorated is made as follows.

  That is, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio is discharged from each combustion chamber 21. Then, the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio flows into the upstream side catalyst 43. Here, since the upstream side catalyst 43 has oxygen storage / release capability, oxygen stored in the upstream side catalyst 43 is discharged from the upstream side catalyst 43. Exhaust gas flows out. When the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio continues to flow into the upstream side catalyst 43 and the oxygen stored in the upstream-side catalyst 43 is exhausted, the upstream side catalyst 43 is richer than the stoichiometric air-fuel ratio. Air-fuel ratio exhaust gas flows out. When the downstream air-fuel ratio sensor 56 detects that the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio has started to flow out from the upstream catalyst 43, the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio. Air-fuel ratio. Then, exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio is discharged from each combustion chamber 21. Then, the exhaust gas having an air / fuel ratio leaner than the stoichiometric air / fuel ratio flows into the upstream side catalyst 43. Here, since the upstream catalyst 43 has an oxygen storage / release capability, oxygen in the exhaust gas flowing into the upstream catalyst 43 is stored in the upstream catalyst 43, so The exhaust gas with the fuel ratio flows out. Then, the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio continues to flow into the upstream catalyst 43, and the amount of oxygen stored in the upstream catalyst 43 is the maximum amount of oxygen that can be stored in the upstream catalyst 43. When the value exceeds the upper limit, the upstream catalyst 43 can no longer store oxygen in the exhaust gas, so that an air / fuel ratio leaner than the stoichiometric air / fuel ratio flows out from the upstream catalyst 43. Therefore, after the air-fuel ratio of the air-fuel mixture starts to be controlled to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, and the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio starts to flow into the upstream catalyst 43, the upstream catalyst 43 The time required until the downstream air-fuel ratio sensor 56 detects that the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio has started to flow out, and the air-fuel ratio leaner than the stoichiometric air-fuel ratio. The amount of oxygen occluded in the upstream catalyst 43 based on the leanness of the air-fuel ratio of the air-fuel mixture at the time, that is, the amount of oxygen that can be occluded to the maximum by the upstream catalyst 43 (hereinafter this amount is referred to as “maximum occlusion”). "Available oxygen amount"). If the maximum storable amount is larger than a predetermined threshold value, it can be said that the upstream side catalyst 43 is normal without deterioration of the oxygen storage / release capability of the upstream side catalyst 43. On the other hand, if the maximum storable amount is equal to or less than the predetermined threshold value, it can be said that the oxygen storage / release capacity of the upstream catalyst 43 has deteriorated.

  Therefore, in the internal combustion engine 10, when it is determined whether or not the oxygen storage / release capability of the upstream catalyst 43 has deteriorated, the air-fuel ratio of the air-fuel mixture is first controlled to be richer than the stoichiometric air-fuel ratio. When the downstream air-fuel ratio sensor 56 detects that the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio has started to flow out from the upstream catalyst 43, the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio. The air / fuel ratio is controlled. When the downstream air-fuel ratio sensor 56 detects that the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio has started to flow out from the upstream catalyst 43, the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio. When the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio starts to flow into the upstream catalyst 43 and starts to be controlled to a higher air-fuel ratio, the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio starts from the upstream catalyst 43 Based on the time it took for the downstream air-fuel ratio sensor 56 to detect that the air flow started to flow out, and the leanness of the air-fuel ratio of the air-fuel mixture when the air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio Thus, the amount of oxygen stored in the upstream catalyst 43, that is, the amount of oxygen that can be stored to the maximum by the upstream catalyst 43, that is, the maximum oxygen storage capacity is calculated. Then, it is determined whether or not the calculated maximum oxygen storage capacity is greater than a predetermined threshold value. Here, the predetermined threshold value is set to a maximum oxygen storage capacity that can be said that the oxygen storage / release capability of the upstream catalyst 43 is not deteriorated. When it is determined that the calculated maximum oxygen storage capacity is greater than a predetermined threshold value, it is determined that the oxygen storage / release capability of the upstream catalyst 43 has not deteriorated. On the other hand, when it is determined that the calculated maximum oxygen storage capacity is equal to or less than the predetermined threshold value, it is determined that the oxygen storage / release capability of the upstream catalyst 43 has deteriorated.

  In the present embodiment (hereinafter referred to as “second embodiment”), as described above, the air-fuel ratio of the air-fuel mixture is made higher than the stoichiometric air-fuel ratio for the purpose of determining whether or not the oxygen storage / release capability of the upstream catalyst 43 has deteriorated. When the air-fuel ratio is also controlled to be rich, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  According to this, since the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of the inter-cylinder air-fuel ratio imbalance state, the inter-cylinder air-fuel ratio imbalance state Compared with the case where the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of fuel, the fuel efficiency of the internal combustion engine 10 is improved.

  Next, an example of a routine for performing the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state according to the second embodiment will be described. The flowchart shown in FIG. 14 and FIG. 16 is used for determining whether or not there is an inter-cylinder air-fuel ratio imbalance state in the second embodiment. The routine of FIG. 14 and the routine of FIG. 16 are executed at predetermined time intervals. Since the routine of FIG. 14 has already been described, the description of the routine of FIG. 14 is omitted.

  When the routine of FIG. 16 is started, first, in step 300, a flag F2 indicating whether or not a precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is satisfied is set. It is determined whether or not (F2 = 1). Here, the flag F2 is the same as the inter-cylinder air-fuel ratio imbalance determination precondition flag F2 used in step 200 of FIG. Therefore, when the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is set (F2 = 1), a precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance is established. When the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is reset (F2 = 0), the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is not satisfied.

  When it is determined in step 300 that F2 = 1, the precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is established, and the routine proceeds to step 301. On the other hand, when it is determined in step 300 that F2 = 0, the precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance condition is not satisfied, and the routine proceeds to step 303. move on.

  When it is determined in step 300 that F2 = 1, and the routine proceeds to step 301, the air-fuel ratio of the air-fuel mixture is greater than the stoichiometric air-fuel ratio for the purpose of determining whether or not the oxygen storage / release capacity of the upstream catalyst 43 has deteriorated. It is determined whether or not a flag (hereinafter referred to as “upstream catalyst deterioration determination rich air-fuel ratio control flag”) F4 is set (F4 = 1) indicating whether or not the air-fuel ratio is controlled to be rich. Is done. Here, when the rich air-fuel ratio control flag F4 for upstream side catalyst deterioration determination is set (F4 = 1), the air-fuel mixture is empty for the purpose of determining whether or not the oxygen storage / release capacity of the upstream side catalyst 43 has deteriorated. When the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio and the upstream side catalyst deterioration determination rich air-fuel ratio control flag F4 is reset (F4 = 0), the upstream side catalyst 43 stores and releases oxygen. The air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio for the purpose of determining the presence or absence of performance deterioration.

  When it is determined in step 301 that F4 = 1, the air-fuel ratio of the air-fuel mixture is made richer than the stoichiometric air-fuel ratio for the purpose of determining whether or not the oxygen storage / release capability of the upstream catalyst 43 has deteriorated. Since it is controlled, the routine proceeds to step 302, where “1” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 301, it is determined that F2 = 1 in step 300. Therefore, when it is determined that F4 = 1 in step 301, the inter-cylinder air-fuel ratio imbalance state is reached. The preconditions for determining the presence / absence of the air / fuel ratio are satisfied and the air / fuel ratio of the air / fuel mixture is controlled to be richer than the stoichiometric air / fuel ratio. Therefore, the routine proceeds to step 302 where “1” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, F1 = 1 in step 100 of the routine of FIG. Thus, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  On the other hand, when it is determined in step 301 that F4 = 0, the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio for the purpose of determining whether or not the oxygen storage / release capacity of the upstream catalyst 43 has deteriorated. Since the fuel ratio is not controlled, the routine proceeds to step 303, where “0” is input to the inter-cylinder air / fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 301, it is determined in step 300 that F2 = 1, and the precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is satisfied. When it is determined in step 301 that F4 = 0, the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio, and it is accurately determined whether there is an inter-cylinder air-fuel ratio imbalance state. In this state, the routine proceeds to step 303 where “0” is input to the inter-cylinder air / fuel ratio imbalance determination execution flag F1, and in this case, F1 = 0 in step 100 of the routine of FIG. Therefore, it is determined that there is an inter-cylinder air-fuel ratio imbalance state.

  When it is determined in step 300 that F2 = 0, and the routine proceeds to step 303, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when it is determined in step 300 that F2 = 0, the condition that is a precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is not satisfied, and the routine proceeds to step 303. Then, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, it is determined that F1 = 0 in step 100 of the routine of FIG. Judgment will not be made.

  By the way, in the internal combustion engine 10, there is a case where the air-fuel ratio of the air-fuel mixture is made richer than the stoichiometric air-fuel ratio for the purpose of determining whether or not the responsiveness of the upstream air-fuel ratio sensor 55 is abnormal. Whether the upstream air-fuel ratio sensor 55 is abnormal or not is determined as follows.

  That is, when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio is discharged from each combustion chamber 21. Then, the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio arrives at the upstream air-fuel ratio sensor 55. On the other hand, when the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio is discharged from each combustion chamber 21. Then, the exhaust gas having an air / fuel ratio leaner than the stoichiometric air / fuel ratio arrives at the upstream air / fuel ratio sensor 55. Therefore, when the air-fuel ratio of the air-fuel mixture is changed from an air-fuel ratio richer than the stoichiometric air-fuel ratio to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 becomes the stoichiometric air-fuel ratio. The air-fuel ratio changes from a richer air-fuel ratio to a leaner air-fuel ratio than the stoichiometric air-fuel ratio. In this case, the upstream air-fuel ratio sensor output value (that is, the output value of the upstream air-fuel ratio sensor 55) is a negative value corresponding to the air-fuel ratio richer than the stoichiometric air-fuel ratio, and the air-fuel ratio leaner than the stoichiometric air-fuel ratio. It increases toward a positive value corresponding to the fuel ratio. At this time, if the upstream air-fuel ratio sensor 55 is normal, the upstream air-fuel ratio sensor output value increases with a relatively large increase rate. Therefore, when the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 changes from an air-fuel ratio richer than the stoichiometric air-fuel ratio to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the upstream air-fuel ratio sensor output value increases. If the rate is larger than a predetermined threshold value, it can be said that the upstream air-fuel ratio sensor 55 is normal without any abnormality in the responsiveness of the upstream air-fuel ratio sensor 55. On the other hand, when the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 changes from an air-fuel ratio richer than the stoichiometric air-fuel ratio to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the upstream air-fuel ratio sensor output value increases. If the rate is equal to or less than the predetermined threshold, it can be said that an abnormality in the responsiveness of the upstream air-fuel ratio sensor 55 has occurred.

  Therefore, in the internal combustion engine 10, when it is determined whether or not the responsiveness of the upstream air-fuel ratio sensor 55 is abnormal, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio for a predetermined period. Thereafter, the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. The air-fuel ratio of the air-fuel mixture changes from an air-fuel ratio richer than the stoichiometric air-fuel ratio to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, and the air-fuel ratio of the exhaust gas that arrives at the upstream air-fuel ratio sensor 55 is greater than the stoichiometric air-fuel ratio. When the rich air-fuel ratio changes to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, it is determined whether or not the rate of increase of the upstream air-fuel ratio sensor output value is greater than a predetermined threshold value. Here, the predetermined threshold value is set to an increase rate of the upstream air-fuel ratio sensor output value to such an extent that it can be said that the responsiveness abnormality of the upstream air-fuel ratio sensor 55 has not occurred. When it is determined that the rate of increase of the upstream air-fuel ratio sensor output value is greater than a predetermined threshold value, there is no abnormality in the responsiveness of the upstream air-fuel ratio sensor 55, and the upstream air-fuel ratio sensor 55 Is determined to be normal. On the other hand, when it is determined that the rate of increase in the upstream air-fuel ratio sensor output value is equal to or less than the predetermined threshold value, it is determined that an abnormality in the responsiveness of the upstream air-fuel ratio sensor 55 has occurred.

  In the present embodiment (hereinafter referred to as “third embodiment”), as described above, the air-fuel ratio of the air-fuel mixture is made higher than the stoichiometric air-fuel ratio for the purpose of determining the presence / absence of abnormality in the responsiveness of the upstream air-fuel ratio sensor 55. When the air-fuel ratio is also controlled to be rich, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  According to this, since the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of the inter-cylinder air-fuel ratio imbalance state, the inter-cylinder air-fuel ratio imbalance state Compared with the case where the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of fuel, the fuel efficiency of the internal combustion engine 10 is improved.

  In the internal combustion engine 10, when determining whether or not the upstream air-fuel ratio sensor 55 is abnormal in response, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio for a predetermined period. After that, a series of air-fuel ratio control in which the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio leaner than the stoichiometric air-fuel ratio is executed a plurality of times, and finally the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio. Every time the air-fuel ratio of the exhaust gas that is controlled to the fuel ratio and arrives at the upstream air-fuel ratio sensor 55 changes between an air-fuel ratio richer than the stoichiometric air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio, When an increase rate of the output value of the fuel ratio sensor is obtained, and the average value of the obtained increase rates (or the minimum value of the obtained increase rates) is larger than the predetermined threshold value, the upstream air-fuel ratio sensor 55 responsive abnormalities occur The upstream air-fuel ratio sensor 55 is determined to be normal, and the average value of the calculated increase rates (or the minimum value of the calculated increase rates) is equal to or less than the predetermined threshold value. Alternatively, it may be determined that an abnormality in the responsiveness of the upstream air-fuel ratio sensor 55 has occurred.

  In this case, the abnormality of the responsiveness of the upstream air-fuel ratio sensor 55 has not occurred, and the air-fuel ratio of the air-fuel mixture finally becomes the stoichiometric airspace only when it is determined that the upstream air-fuel ratio sensor 55 is normal. When the air-fuel ratio is controlled to be richer than the fuel ratio, the presence or absence of the inter-cylinder air-fuel ratio imbalance state may be determined.

  According to this, since the presence or absence of the inter-cylinder air-fuel ratio imbalance state is determined based on the normal output value of the upstream air-fuel ratio sensor 55, it can be said that the determination accuracy is high.

  Next, an example of a routine for performing the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state according to the third embodiment will be described. The flowchart shown in FIG. 14 and FIG. 17 is used for determining whether or not there is an inter-cylinder air-fuel ratio imbalance state in the third embodiment. The routine of FIG. 14 and the routine of FIG. 17 are executed at predetermined time intervals. Since the routine of FIG. 14 has already been described, the description of the routine of FIG. 14 is omitted.

  When the routine of FIG. 17 is started, first, in step 400, a flag F2 indicating whether or not a precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is satisfied is set. It is determined whether or not (F2 = 1). Here, the flag F2 is the same as the inter-cylinder air-fuel ratio imbalance determination precondition flag F2 used in step 200 of FIG. Therefore, when the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is set (F2 = 1), a precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance is established. When the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is reset (F2 = 0), the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is not satisfied.

  If it is determined in step 400 that F2 = 1, a precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance condition is established, and the routine proceeds to step 401. On the other hand, when it is determined in step 400 that F2 = 0, the precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance condition is not satisfied, and the routine proceeds to step 403. move on.

  When it is determined in step 400 that F2 = 1, and the routine proceeds to step 401, the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio for the purpose of determining whether or not the upstream air-fuel ratio sensor 55 is abnormal. Whether or not a flag indicating whether or not the air-fuel ratio is controlled (hereinafter, this flag is referred to as an “upstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control flag”) F5 is set (F5 = 1). Determined. Here, when the rich air-fuel ratio control flag F5 for upstream air-fuel ratio sensor abnormality determination is set (F5 = 1), the air-fuel ratio of the air-fuel mixture is determined for the purpose of determining whether or not the responsiveness of the upstream air-fuel ratio sensor 55 is abnormal. When the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio and the upstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control flag F5 is reset (F5 = 0), the upstream air-fuel ratio sensor 55 The air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio for the purpose of determining the presence or absence of the responsiveness abnormality.

  If it is determined in step 401 that F5 = 1, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio for the purpose of determining whether the upstream air-fuel ratio sensor 55 is abnormal. Therefore, the routine proceeds to step 402, where “1” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 401, it is determined that F2 = 1 in step 400. Therefore, when it is determined that F5 = 1 in step 401, the inter-cylinder air-fuel ratio imbalance state is reached. The preconditions for determining the presence / absence of the air / fuel ratio are satisfied and the air / fuel ratio of the air / fuel mixture is controlled to be richer than the stoichiometric air / fuel ratio. Therefore, the routine proceeds to step 402 where “1” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1. In this case, F1 = 1 in step 100 of the routine of FIG. Thus, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  On the other hand, when it is determined in step 401 that F5 = 0, the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio for the purpose of determining whether the upstream air-fuel ratio sensor 55 is abnormal. Therefore, the routine proceeds to step 403, where “0” is input to the inter-cylinder air / fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 401, it is determined in step 400 that F2 = 1, and the precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is established. When it is determined in step 401 that F5 = 0, the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio, and it is accurately determined whether there is an inter-cylinder air-fuel ratio imbalance state. In this state, the routine proceeds to step 403, where “0” is input to the inter-cylinder air / fuel ratio imbalance determination execution flag F1, and in this case, F1 = 0 in step 100 of the routine of FIG. Therefore, it is determined that there is an inter-cylinder air-fuel ratio imbalance state.

  If it is determined in step 400 that F2 = 0, and the routine proceeds to step 403, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, if it is determined in step 400 that F2 = 0, the condition that is a prerequisite for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is not satisfied, and the routine proceeds to step 403. Then, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, it is determined that F1 = 0 in step 100 of the routine of FIG. Judgment will not be made.

  By the way, in the internal combustion engine 10, the air-fuel ratio of the air-fuel mixture is made richer than the stoichiometric air-fuel ratio for the purpose of raising the temperature of the internal combustion engine 10 early when the internal combustion engine 10 is started (that is, so-called warming up). There may be a fuel ratio.

  That is, immediately after the internal combustion engine 10 is started, the temperature of the internal combustion engine 10 is relatively low, and the fuel supplied to each combustion chamber 21 is difficult to burn. Therefore, when the internal combustion engine 10 is started, the air-fuel ratio of the air-fuel mixture is made richer than the stoichiometric air-fuel ratio over a certain period. According to this, the amount of fuel supplied to each combustion chamber 21 increases, and the amount of heat generated by the combustion of fuel in each combustion chamber 21 increases, so that the temperature of the internal combustion engine 10 is raised relatively early. Even if the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio or to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, the fuel will burn well in each combustion chamber 21.

  In this embodiment (hereinafter referred to as “fourth embodiment”), as described above, the air-fuel ratio of the air-fuel mixture is set to increase the temperature of the internal combustion engine 10 early when the internal combustion engine 10 is started. When the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  According to this, since the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of the inter-cylinder air-fuel ratio imbalance state, the inter-cylinder air-fuel ratio imbalance state Compared with the case where the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of fuel, the fuel efficiency of the internal combustion engine 10 is improved.

  Next, an example of a routine for determining whether or not there is an inter-cylinder air-fuel ratio imbalance state according to the fourth embodiment will be described. The flowchart shown in FIG. 14 and FIG. 18 is used to determine whether or not there is an inter-cylinder air-fuel ratio imbalance state in the fourth embodiment. The routine of FIG. 14 and the routine of FIG. 18 are executed when the internal combustion engine 10 is started. Since the routine of FIG. 14 has already been described, the description of the routine of FIG. 14 is omitted.

  When the routine of FIG. 18 is started, first, in step 500, a flag F2 indicating whether or not a precondition for executing the determination of whether or not there is an inter-cylinder air-fuel ratio imbalance state is established. It is determined whether or not (F2 = 1). Here, the flag F2 is the same as the inter-cylinder air-fuel ratio imbalance determination precondition flag F2 used in step 200 of FIG. Therefore, when the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is set (F2 = 1), a precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance is established. When the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is reset (F2 = 0), the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is not satisfied.

  When it is determined in step 500 that F2 = 1, the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is established, and the routine proceeds to step 501. On the other hand, when it is determined in step 500 that F2 = 0, the precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance condition is not satisfied, and the routine proceeds to step 503. move on.

  When it is determined in step 500 that F2 = 1 and the routine proceeds to step 501, the air-fuel ratio of the air-fuel mixture is the stoichiometric air-fuel ratio for the purpose of raising the temperature of the internal combustion engine 10 early when the internal combustion engine 10 is started. It is determined whether or not a flag (hereinafter referred to as a “warm-up rich air-fuel ratio control flag”) F6 is set (F6 = 1) indicating whether or not the air-fuel ratio is controlled to be richer. The Here, when the warm-up rich air-fuel ratio control flag F6 is set (F6 = 1), the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio for the purpose of raising the temperature of the internal combustion engine 10 early. When the rich air-fuel ratio control flag F6 for upstream air-fuel ratio sensor abnormality determination is reset (F6 = 0), the air-fuel ratio of the air-fuel mixture is increased for the purpose of raising the temperature of the internal combustion engine 10 early. However, the air-fuel ratio is not controlled to be richer than the stoichiometric air-fuel ratio.

  When it is determined in step 501 that F6 = 1, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio for the purpose of raising the temperature of the internal combustion engine 10 early. The routine proceeds to step 502, where “1” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 501, it is determined that F2 = 1 in step 500. Therefore, when it is determined that F6 = 1 in step 501, the inter-cylinder air-fuel ratio imbalance state is reached. The preconditions for determining the presence / absence of the air / fuel ratio are satisfied and the air / fuel ratio of the air / fuel mixture is controlled to be richer than the stoichiometric air / fuel ratio. Therefore, the routine proceeds to step 502 where “1” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, F1 = 1 in step 100 of the routine of FIG. Thus, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  On the other hand, when it is determined in step 501 that F6 = 0, the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio for the purpose of raising the temperature of the internal combustion engine 10 early. The routine then proceeds to step 503, where "0" is input to the inter-cylinder air / fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 501, it is determined that F2 = 1 in step 500, and the precondition for executing the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state is satisfied. When it is determined in step 501 that F6 = 0, the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio, and it is accurately determined whether there is an inter-cylinder air-fuel ratio imbalance state. In this state, the routine proceeds to step 503, where “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, F1 = 0 in step 100 of the routine of FIG. Therefore, it is determined that there is an inter-cylinder air-fuel ratio imbalance state.

  When it is determined in step 500 that F2 = 0, and the routine proceeds to step 503, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when it is determined in step 500 that F2 = 0, the condition that is a prerequisite for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is not satisfied, and the routine proceeds to step 503. Then, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, it is determined that F1 = 0 in step 100 of the routine of FIG. Judgment will not be made.

  By the way, in the internal combustion engine 10, when the output required for the internal combustion engine 10 is extremely small, particularly when it is zero, control for stopping the fuel injection from each fuel injection valve 25 to each combustion chamber 21 (that is, So-called fuel cut control) is executed. And in the internal combustion engine 10, when the output requested | required of the internal combustion engine 10 becomes large, fuel cut control is stopped.

  By the way, when the fuel cut control is being executed, the air-fuel ratio of the air-fuel mixture is a lean air-fuel ratio that is larger than the stoichiometric air-fuel ratio. For this reason, exhaust gas having an air-fuel ratio larger than the stoichiometric air-fuel ratio and leaner than the stoichiometric air-fuel ratio is discharged from each combustion chamber 21, and exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio flows into the upstream catalyst 43. Will do. That is, while the fuel cut control is being performed, since the exhaust gas having an air-fuel ratio larger than the stoichiometric air-fuel ratio flows into the upstream catalyst 43, the upstream catalyst 43 exhausts into the exhaust gas flowing there. There is a high possibility that the oxygen in the gas is stored in a large amount and the amount of oxygen stored in the upstream catalyst 43 reaches the maximum storable oxygen amount (that is, the maximum amount that the upstream catalyst 43 can store). . In this case, when the air-fuel ratio leaner than the stoichiometric air-fuel ratio flows into the upstream side catalyst 43 after the fuel cut control is stopped, the upstream side catalyst 43 stores the oxygen in the exhaust gas flowing there. You can't do that. That is, the oxygen storage / release capability of the upstream catalyst 43 is not exhibited. On the other hand, as described above, if the air-fuel ratio richer than the stoichiometric air-fuel ratio flows into the upstream catalyst 43, the upstream catalyst 43 releases the oxygen stored therein. Even if the amount of oxygen occluded reaches the maximum storable oxygen amount, if the air-fuel ratio richer than the stoichiometric air-fuel ratio is caused to flow into the upstream catalyst 43, the upstream catalyst 43 will occlude there. Even if the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio flows into the upstream catalyst 43 after that, the upstream catalyst 43 stores the oxygen in the exhaust gas that has flowed there. Will be able to.

  Therefore, in the internal combustion engine 10, the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio for the purpose of releasing the oxygen stored in the upstream catalyst 43 from the upstream catalyst 43 immediately after the fuel cut control is stopped. Let the air-fuel ratio.

  In the present embodiment (hereinafter referred to as “fifth embodiment”), as described above, oxygen stored in the upstream catalyst 43 after the fuel cut control is stopped is released from the upstream catalyst 43. When the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio richer than the stoichiometric air-fuel ratio, it is determined whether there is an inter-cylinder air-fuel ratio imbalance state.

  According to this, since the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of the inter-cylinder air-fuel ratio imbalance state, the inter-cylinder air-fuel ratio imbalance state Compared with the case where the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of fuel, the fuel efficiency of the internal combustion engine 10 is improved.

  Next, an example of a routine for performing the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state according to the fifth embodiment will be described. The flowchart shown in FIG. 14 and FIG. 19 is used for determining whether or not there is an inter-cylinder air-fuel ratio imbalance state in the fifth embodiment. The routine of FIG. 14 and the routine of FIG. 19 are executed when the fuel cut control is stopped. Since the routine of FIG. 14 has already been described, the description of the routine of FIG. 14 is omitted.

  When the routine of FIG. 19 is started, first, in step 600, a flag F2 indicating whether or not a precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is established. It is determined whether or not (F2 = 1). Here, the flag F2 is the same as the inter-cylinder air-fuel ratio imbalance determination precondition flag F2 used in step 200 of FIG. Therefore, when the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is set (F2 = 1), a precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance is established. When the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is reset (F2 = 0), the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is not satisfied.

  If it is determined in step 600 that F2 = 1, the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance condition is established, and the routine proceeds to step 601. On the other hand, when it is determined in step 600 that F2 = 0, the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance condition is not satisfied, and the routine proceeds to step 603. move on.

  When it is determined in step 600 that F2 = 1, and the routine proceeds to step 601, the fuel cut control is stopped and the mixture stored in the upstream catalyst 43 is released for the purpose of releasing the oxygen stored in the upstream catalyst 43. A flag F7 indicating whether or not the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio (hereinafter referred to as "rich air-fuel ratio control flag for storing oxygen release") F7 is set (F7 = 1) ) Is determined. Here, when the rich oxygen-fuel ratio control flag F7 for releasing stored oxygen is set (F7 = 1), the air-fuel ratio of the air-fuel mixture for the purpose of releasing the oxygen stored in the upstream catalyst 43 from the upstream catalyst 43. Is controlled to be richer than the stoichiometric air-fuel ratio, and when the rich oxygen-fuel ratio control flag F7 for releasing stored oxygen is reset (F7 = 0), the oxygen stored in the upstream catalyst 43 is upstream. For the purpose of releasing from the side catalyst 43, the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio.

  If it is determined in step 601 that F7 = 1, the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio for the purpose of releasing the oxygen stored in the upstream catalyst 43 from the upstream catalyst 43. Therefore, the routine proceeds to step 602, where “1” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 601, it is determined that F2 = 1 in step 600. Therefore, when it is determined that F7 = 1 in step 601, the inter-cylinder air-fuel ratio imbalance state is reached. The preconditions for determining the presence / absence of the air / fuel ratio are satisfied and the air / fuel ratio of the air / fuel mixture is controlled to be richer than the stoichiometric air / fuel ratio. Therefore, the routine proceeds to step 602, where “1” is input to the inter-cylinder air / fuel ratio imbalance determination execution flag F1, and in this case, F1 = 1 in step 100 of the routine of FIG. Thus, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  On the other hand, when it is determined in step 601 that F7 = 0, the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio for the purpose of releasing oxygen stored in the upstream catalyst 43 from the upstream catalyst 43. Since the air-fuel ratio is not controlled, the routine proceeds to step 603, where “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 601, it is determined in step 600 that F2 = 1, and the precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is satisfied. When it is determined in step 601 that F7 = 0, the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio, and it is accurately determined whether there is an inter-cylinder air-fuel ratio imbalance state. In this state, the routine proceeds to step 603 where “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, F1 = 0 in step 100 of the routine of FIG. Therefore, it is determined that there is an inter-cylinder air-fuel ratio imbalance state.

  If it is determined in step 600 that F2 = 0, and the routine proceeds to step 603, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, if it is determined in step 600 that F2 = 0, the condition that is a prerequisite for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is not satisfied, and the routine proceeds to step 603. Then, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, it is determined that F1 = 0 in step 100 of the routine of FIG. No judgment is made.

  By the way, when the output required for the internal combustion engine 10 is extremely large, the temperature of the exhaust gas discharged from each combustion chamber 21 becomes very high. Thus, if the exhaust gas having a very high temperature continues to flow into the upstream catalyst 43, the temperature of the upstream catalyst 43 becomes very high, and the upstream catalyst 43 may be thermally deteriorated. On the other hand, when the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio flows into the upstream catalyst 43, when the fuel in the exhaust gas volatilizes in the upstream catalyst 43, the upstream catalyst 43 is deprived of heat, and the upstream catalyst The temperature of 43 falls.

  Therefore, in the internal combustion engine 10, when the output required for the internal combustion engine 10 is extremely large, the air-fuel ratio of the air-fuel mixture may be made richer than the stoichiometric air-fuel ratio for the purpose of lowering the temperature of the upstream catalyst 43. is there.

In this embodiment (hereinafter referred to as “sixth embodiment”), as described above, when the output required for the internal combustion engine 10 is extremely large, the air-fuel mixture is emptied for the purpose of lowering the temperature of the upstream catalyst 43. When the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  According to this, since the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of the inter-cylinder air-fuel ratio imbalance state, the inter-cylinder air-fuel ratio imbalance state Compared with the case where the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio only for the purpose of determining the presence or absence of fuel, the fuel efficiency of the internal combustion engine 10 is improved.

  Next, an example of a routine for determining whether or not there is an inter-cylinder air-fuel ratio imbalance state according to the sixth embodiment will be described. The flowchart shown in FIG. 14 and FIG. 20 is used to determine whether or not there is an inter-cylinder air-fuel ratio imbalance state in the fifth embodiment. The routine of FIG. 14 and the routine of FIG. 20 are executed when the fuel cut control is stopped. Since the routine of FIG. 14 has already been described, the description of the routine of FIG. 14 is omitted.

  When the routine of FIG. 20 is started, first, in step 700, a flag F2 indicating whether or not a precondition for executing the determination of whether or not there is an inter-cylinder air-fuel ratio imbalance state is set. It is determined whether or not (F2 = 1). Here, the flag F2 is the same as the inter-cylinder air-fuel ratio imbalance determination precondition flag F2 used in step 200 of FIG. Therefore, when the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is set (F2 = 1), a precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance is established. When the inter-cylinder air / fuel ratio imbalance determination precondition flag F2 is reset (F2 = 0), the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is not satisfied.

  If it is determined in step 700 that F2 = 1, the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance condition is satisfied, and the routine proceeds to step 701. On the other hand, when it is determined in step 700 that F2 = 0, the precondition for executing the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance condition is not satisfied, and the routine proceeds to step 703. move on.

  If it is determined in step 700 that F2 = 1, and the routine proceeds to step 701, the output required for the internal combustion engine 10 is extremely large, so that the air-fuel ratio of the air-fuel mixture is decreased for the purpose of lowering the temperature of the upstream catalyst 43. Whether a flag F8 indicating whether or not the air-fuel ratio is richer than the stoichiometric air-fuel ratio (hereinafter referred to as "rich air-fuel ratio control flag for reducing catalyst temperature") F8 is set (F8 = 1) Is determined. When the rich oxygen-fuel ratio control flag F8 for releasing stored oxygen is set (F8 = 1), the air-fuel ratio of the air-fuel mixture is richer than the stoichiometric air-fuel ratio for the purpose of lowering the temperature of the upstream catalyst 43. When the rich air-fuel ratio control flag F8 for releasing stored oxygen is reset (F8 = 0), the air-fuel ratio of the air-fuel mixture is greater than the stoichiometric air-fuel ratio for the purpose of lowering the temperature of the upstream catalyst 43. Even the rich air-fuel ratio is not controlled.

  If it is determined in step 701 that F8 = 1, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio for the purpose of lowering the temperature of the upstream catalyst 43. Advances to step 702, "1" is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 701, it is determined that F2 = 1 in step 700. Therefore, when F7 = 1 is determined in step 701, the inter-cylinder air-fuel ratio imbalance state is reached. The preconditions for determining the presence / absence of the air / fuel ratio are satisfied and the air / fuel ratio of the air / fuel mixture is controlled to be richer than the stoichiometric air / fuel ratio. Therefore, the routine proceeds to step 702, where “1” is input to the inter-cylinder air / fuel ratio imbalance determination execution flag F1, and in this case, F1 = 1 in step 100 of the routine of FIG. Thus, it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state.

  On the other hand, when it is determined in step 701 that F8 = 0, the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio for the purpose of lowering the temperature of the upstream catalyst 43. The routine proceeds to step 703, where “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, when the routine proceeds to step 701, it is determined that F2 = 1 in step 700, and the precondition for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is satisfied. When it is determined in step 701 that F8 = 0, the air-fuel ratio of the air-fuel mixture is not controlled to be richer than the stoichiometric air-fuel ratio, and it is accurately determined whether there is an inter-cylinder air-fuel ratio imbalance state. In this state, the routine proceeds to step 703, where “0” is input to the inter-cylinder air / fuel ratio imbalance determination execution flag F1, and in this case, F1 = 0 in step 100 of the routine of FIG. Therefore, it is determined that there is an inter-cylinder air-fuel ratio imbalance state.

  If it is determined in step 700 that F2 = 0, and the routine proceeds to step 703, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and the routine ends. That is, if it is determined in step 700 that F2 = 0, the condition that is a prerequisite for executing the determination of the presence / absence of the inter-cylinder air-fuel ratio imbalance state is not satisfied, and the routine proceeds to step 703. Then, “0” is input to the inter-cylinder air-fuel ratio imbalance determination execution flag F1, and in this case, it is determined that F1 = 0 in step 100 of the routine of FIG. No judgment is made.

By the way, when the air-fuel mixture burns in the combustion chamber 21, hydrogen (H ₂ ) is generated. The amount of hydrogen generated by the combustion of the air-fuel mixture is greater when the air-fuel mixture richer than the stoichiometric air-fuel ratio burns than when the air-fuel mixture leaner than the stoichiometric air-fuel ratio burns. There are more. Therefore, the amount of hydrogen in the exhaust gas discharged from the combustion chamber 21 is such that the air-fuel mixture richer than the stoichiometric air-fuel ratio is larger than when the air-fuel mixture leaner than the stoichiometric air-fuel ratio burns. More when burned. That is, the amount of hydrogen in the exhaust gas is larger in the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio than in the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio.

On the other hand, hydrogen is a smaller molecule than oxygen (O ₂ ), carbon monoxide (CO), and hydrocarbon (HC). Therefore, the hydrogen in the exhaust gas that has arrived at the upstream air-fuel ratio sensor 55 diffuses in the diffusion resistance layer 559 at a faster rate than the oxygen, carbon monoxide, and hydrocarbons contained in the exhaust gas. . Hydrogen affects the amount of oxygen ions flowing in the solid electrolyte layer 551 of the upstream air-fuel ratio sensor 55. That is, when the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio has arrived at the upstream air-fuel ratio sensor 55, and when the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio has arrived at the upstream air-fuel ratio sensor 55 , The amount of oxygen ions flowing in the solid electrolyte layer 551 of the upstream air-fuel ratio sensor 55 even if the richness and leanness of these exhaust gases are the same as the stoichiometric air-fuel ratio. Indicates that the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is higher than the stoichiometric air-fuel ratio than when the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is leaner than the stoichiometric air-fuel ratio. The richer air / fuel ratio is more. In other words, the output characteristic of the upstream air-fuel ratio sensor 55 is greatly increased in accordance with the amount of hydrogen in the exhaust gas arriving at the upstream air-fuel ratio sensor 55, that is, the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55. It will be different.

  As described above, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio, the upstream air-fuel ratio sensor 55 has an air-fuel ratio richer than the stoichiometric air-fuel ratio and the stoichiometric air-fuel ratio. Since the lean air-fuel ratio exhaust gas alternately arrives, if the output characteristics of the upstream air-fuel ratio sensor 55 greatly differ depending on the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55. Even if the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio based on the output value of the upstream air-fuel ratio sensor 55, the air-fuel ratio of the air-fuel mixture cannot be accurately controlled to the stoichiometric air-fuel ratio.

  In order to eliminate the fact that the output characteristics of the upstream air-fuel ratio sensor 55 differ depending on the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55, as shown in FIG. Some of the air-fuel ratio detection elements 55a shown have a catalyst 561 in the through hole 558. The catalyst 561 is disposed in the through hole 558 so as to close the through hole 558. Further, the catalyst 561 is a porous body and, like the upstream catalyst 43, carries a catalyst substance that promotes a redox reaction and an oxygen storage material that exhibits oxygen storage / release capability.

  21, the exhaust gas that has arrived at the upstream air-fuel ratio sensor 55 flows into the diffusion resistance layer 554 through the catalyst 561. When the exhaust gas passes through the catalyst 561, the catalyst 561 oxidizes hydrogen in the exhaust gas and reduces the amount of oxygen in the exhaust gas. According to this, when an air-fuel ratio richer than the stoichiometric air-fuel ratio arrives at the upstream air-fuel ratio sensor 55, the amount of hydrogen in the exhaust gas is small before the exhaust gas flows into the diffusion resistance layer 554. As a result, the fact that the output characteristics of the upstream air-fuel ratio sensor 55 differ depending on the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is eliminated.

  The function of the air-fuel ratio detection element 55a shown in FIG. 21 is the same as that of the air-fuel ratio detection element 55a shown in FIG. The function is the same as

  By the way, when the upstream air-fuel ratio sensor 55 has an air-fuel ratio detection element 55a provided with the catalyst 561 shown in FIG. 21, the air-fuel ratio of the mixture is controlled to be richer than the stoichiometric air-fuel ratio. Sometimes determining whether or not there is an inter-cylinder air-fuel ratio imbalance condition is compared to determining whether or not there is an inter-cylinder air-fuel ratio imbalance condition when the air-fuel ratio of the mixture is controlled to the stoichiometric air-fuel ratio. There is a further effect.

  That is, as described above, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio, the upstream air-fuel ratio sensor 55 has an air-fuel ratio richer than the stoichiometric air-fuel ratio and the stoichiometric air-fuel ratio. Lean air-fuel ratio exhaust gas alternately arrives. In this case, in order for the upstream air-fuel ratio sensor 55 to output an output value that corresponds very accurately to the air-fuel ratio of the exhaust gas arriving there, the air-fuel ratio of the exhaust gas arriving at the upstream air-fuel ratio sensor 55 is theoretically The flow of oxygen ions flowing through the solid electrolyte layer 551 of the air-fuel ratio detection element 55a of the upstream air-fuel ratio sensor 55 when it changes between an air-fuel ratio richer than the air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio. The direction needs to be reversed immediately. However, as described above, the direction of the flow of oxygen ions flowing in the solid electrolyte layer 551 is not immediately reversed. When the air-fuel ratio detection element 55a of the upstream air-fuel ratio sensor 55 has the catalyst 561 in the through hole 558, the direction of oxygen ions flowing through the solid electrolyte layer 551 is increasingly reversed. It becomes difficult. Therefore, when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio, the output value of the upstream side air-fuel ratio sensor 55 becomes more and more the air-fuel ratio of the exhaust gas arriving at the upstream side air-fuel ratio sensor 55 (ie, the mixture The air-fuel ratio) is not shown very accurately.

  However, if the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, and it is determined whether or not there is an inter-cylinder air-fuel ratio imbalance state, the upstream air-fuel ratio sensor 55 will receive the stoichiometric air-fuel ratio sensor 55. Since only the exhaust gas having an air-fuel ratio richer than the fuel ratio arrives, the direction of the flow of oxygen ions flowing in the solid electrolyte layer 554 of the upstream air-fuel ratio sensor 55 during the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state There is no need to reverse. For this reason, when the upstream air-fuel ratio sensor 55 has the air-fuel ratio detection element 55a provided with the catalyst 561, the air-fuel ratio between the cylinders is controlled when the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio. Since the determination of the presence or absence of the imbalance state has a further effect than the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state when the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio. is there.

  In the above-described embodiment, the determination of the presence / absence of the inter-cylinder air / fuel ratio imbalance state is made by determining the average slope of the line followed by the upstream air / fuel ratio sensor output value (that is, the output value of the upstream air / fuel ratio sensor 55). The absolute value (that is, the absolute value of the time change rate of the upstream air-fuel ratio sensor output value) is used.

  However, instead of the absolute value of the time change rate of the upstream air-fuel ratio sensor output value, for example, the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state is performed by, for example, the first-order time differential value of the upstream air-fuel ratio sensor output value. The maximum value among the absolute values may be used, or the maximum value among the absolute values of the second-order time differential values of the upstream air-fuel ratio sensor output value may be used, or at a predetermined time interval. Alternatively, the length of the trajectory between two different upstream air-fuel ratio sensor output values may be used.

  Here, in determining whether there is an inter-cylinder air-fuel ratio imbalance state, the maximum value of the absolute value of the first-order time differential value of the upstream air-fuel ratio sensor output value (hereinafter this maximum value is referred to as “upstream air-fuel ratio sensor output”). Is used), the first-order time-derivative maximum value of the upstream air-fuel ratio sensor output value is greater than that when the inter-cylinder air-fuel ratio imbalance state is not occurring. It becomes larger when the inter-air-fuel ratio imbalance condition occurs. Therefore, when the first-order time differential maximum value of the upstream air-fuel ratio sensor output value is used to determine whether or not there is an inter-cylinder air-fuel ratio imbalance state, it is determined that the inter-cylinder air-fuel ratio imbalance state has not occurred. The maximum value of the family time differential maximum value of the upstream side air-fuel ratio sensor output value at the time of power should be determined in advance as a threshold value, and the first-order time differential maximum value of the upstream side air-fuel ratio sensor output value from this predetermined threshold value Is determined that an inter-cylinder air-fuel ratio imbalance condition has occurred, and the first-order time self-maximum value of the upstream air-fuel ratio sensor output value is equal to or less than this predetermined threshold value. It is determined that no balance state has occurred.

  Here, in determining whether or not there is an inter-cylinder air-fuel ratio imbalance state, the maximum value of the absolute value of the second-order time differential value of the upstream air-fuel ratio sensor output value (hereinafter this maximum value is referred to as “upstream air-fuel ratio sensor output value”). Is used), the second-order time differential maximum value of the upstream side air-fuel ratio sensor output value is the inter-cylinder air-fuel ratio compared to when the inter-cylinder air-fuel ratio imbalance is not occurring. It becomes larger when an imbalance condition occurs. Therefore, when the second-order time differential maximum value of the upstream side air-fuel ratio sensor output value is used to determine the presence or absence of the inter-cylinder air-fuel ratio imbalance state, it should be determined that the inter-cylinder air-fuel ratio imbalance state has not occurred. The maximum value of the second-order time differential maximum value of the upstream air-fuel ratio sensor output value at the time is predetermined as a threshold value, and the second-order time differential maximum value of the upstream air-fuel ratio sensor output value is greater than this predetermined threshold value When it is determined that an inter-cylinder air-fuel ratio imbalance condition has occurred, and the second-order time differential maximum value of the upstream air-fuel ratio sensor output value is less than or equal to this predetermined threshold value, the inter-cylinder air-fuel ratio imbalance condition is It is determined that it has not occurred.

  Further, in determining whether or not there is an inter-cylinder air-fuel ratio imbalance state, the length of the trajectory between two different upstream air-fuel ratio sensor output values (hereinafter, this length is referred to as “trajectory length of the upstream air-fuel ratio sensor output value”). ) Is used, the trajectory length of the upstream side air-fuel ratio sensor output value is longer when the inter-cylinder air-fuel ratio imbalance condition occurs than when the inter-cylinder air-fuel ratio imbalance condition does not occur. Become. Therefore, when the trajectory length of the upstream air-fuel ratio sensor output value is used to determine whether or not there is an inter-cylinder air-fuel ratio imbalance state, the upstream when it is determined that the inter-cylinder air-fuel ratio imbalance state has not occurred. The maximum value of the trajectory length of the side air-fuel ratio sensor output value is predetermined as a threshold value, and the inter-cylinder air-fuel ratio imbalance state when the trajectory length of the upstream air-fuel ratio sensor output value is longer than the predetermined threshold value Is determined, and it is determined that the inter-cylinder air-fuel ratio imbalance state has not occurred when the locus length of the upstream air-fuel ratio sensor output value is equal to or smaller than the predetermined threshold value.

  In addition, the determination of the presence or absence of the inter-cylinder air-fuel ratio imbalance state in the first to sixth embodiments described above may be combined as appropriate. Accordingly, the present invention broadly relates to the purpose of determining whether or not the output of the downstream air-fuel ratio sensor 56 is abnormal, the purpose of determining whether or not the oxygen storage / release capacity of the upstream catalyst 43 has deteriorated, The purpose of determining whether the responsiveness of the fuel ratio sensor 55 is abnormal, the purpose of raising the temperature of the internal combustion engine 10 early when the internal combustion engine 10 is started, and the upstream catalyst 43 after the fuel cut control is stopped. For the purpose of releasing the stored oxygen from the upstream side catalyst 43 and the purpose of lowering the temperature of the upstream side catalyst 43 when the output required for the internal combustion engine 10 is extremely large, the air-fuel mixture is emptied. In an internal combustion engine in which the fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio for one of the above purposes. It can be said is to determine whether the air-fuel ratio imbalance state among cylinders when.

  Further, in the internal combustion engine 10, the purpose of determining whether or not the output of the downstream air-fuel ratio sensor 56 is abnormal, the purpose of determining whether or not the oxygen storage / release capability of the upstream catalyst 43 has deteriorated, The purpose of determining the presence or absence of responsive abnormality, the purpose of increasing the temperature of the internal combustion engine 10 early when the internal combustion engine 10 is started, and the oxygen stored in the upstream side catalyst 43 after the fuel cut control is stopped For purposes other than the purpose of releasing the gas from the upstream side catalyst 43 and the purpose of lowering the temperature of the upstream side catalyst 43 when the output required for the internal combustion engine 10 is extremely large. When the air-fuel ratio of the air-fuel mixture is sometimes controlled to be richer than the stoichiometric air-fuel ratio for purposes other than the purpose of determining the presence or absence, the air-fuel ratio of the air-fuel mixture is Whether the air-fuel ratio imbalance state among cylinders may be determined when the well is controlled to a rich air-fuel ratio. Therefore, the present invention more broadly relates to an internal combustion engine in which the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio for purposes other than the purpose of determining whether or not there is an inter-cylinder air-fuel ratio imbalance state. When the air-fuel ratio of the air-fuel mixture is controlled to an air-fuel ratio richer than the stoichiometric air-fuel ratio for purposes other than the purpose of determining whether there is an inter-cylinder air-fuel ratio imbalance state. It can be said that

  In particular, in the internal combustion engine 10, when determining whether or not the output of the downstream air-fuel ratio sensor 56 is abnormal, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio over a predetermined period. The On the other hand, in the internal combustion engine 10, when determining whether or not the oxygen storage / release capability of the upstream catalyst 43 has deteriorated, first, the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio, and thereafter When the downstream air-fuel ratio sensor 56 detects that the exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio has started to flow out from the upstream catalyst 43, the air-fuel ratio of the air-fuel mixture is less than the stoichiometric air-fuel ratio. The fuel ratio is controlled. That is, the air-fuel ratio of the air-fuel mixture is equal to the theoretical sky when determining whether the output of the downstream air-fuel ratio sensor 56 is abnormal or when determining whether the oxygen storage / release capacity of the upstream catalyst 43 has deteriorated. The air-fuel ratio is controlled to be richer than the fuel ratio. Therefore, it is efficient to determine whether there is an abnormality in the output of the downstream air-fuel ratio sensor 56 and whether the oxygen storage / release capability of the upstream catalyst 43 has deteriorated by a series of processes.

  Therefore, in the internal combustion engine 10, whether or not there is an abnormality in the output of the downstream side air-fuel ratio sensor 56 and whether or not the oxygen storage / release capability of the upstream side catalyst 43 has deteriorated may be determined as follows. That is, in the internal combustion engine 10, when it should be determined whether there is an abnormality in the output of the downstream side air-fuel ratio sensor 56 and whether the oxygen storage / release capability of the upstream side catalyst 43 has deteriorated, first, during a predetermined period of time. The air / fuel ratio of the air / fuel mixture is controlled to be richer than the stoichiometric air / fuel ratio. Here, the predetermined period is sufficient to eliminate the oxygen stored in the upstream catalyst 43 when exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio continues to flow into the upstream catalyst 43. Set to time. The output value of the downstream air-fuel ratio sensor 56 is richer than the stoichiometric air-fuel ratio when the predetermined period has elapsed after the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio. It is determined whether or not the air-fuel ratio of the exhaust gas corresponding to the air-fuel ratio of the air-fuel mixture that is controlled to a different air-fuel ratio is indicated. At this time, the exhaust gas corresponding to the air-fuel ratio of the air-fuel mixture in which the downstream air-fuel ratio sensor output value (that is, the output value of the downstream air-fuel ratio sensor 56) is controlled to be richer than the stoichiometric air-fuel ratio. When it is determined that the air-fuel ratio of the downstream air-fuel ratio is indicated, no abnormality in the output of the downstream air-fuel ratio sensor 56 has occurred, and it is determined that the downstream air-fuel ratio sensor 56 is normal. On the other hand, when it is determined that the downstream air-fuel ratio sensor output value at this time does not indicate the air-fuel ratio of the exhaust gas corresponding to the air-fuel ratio of the air-fuel mixture that is controlled to be richer than the stoichiometric air-fuel ratio, It is determined that the output of the downstream air-fuel ratio sensor 56 is abnormal. On the other hand, the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio when the predetermined period has elapsed since the air-fuel ratio of the air-fuel mixture began to be controlled to be richer than the stoichiometric air-fuel ratio. Controlled. When the downstream air-fuel ratio sensor 56 detects that the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio has started to flow out from the upstream catalyst 43, the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric air-fuel ratio. When the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio starts to flow into the upstream catalyst 43 and starts to be controlled to a higher air-fuel ratio, the exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio starts from the upstream catalyst 43 Based on the time it took for the downstream air-fuel ratio sensor 56 to detect that the air flow started to flow out, and the leanness of the air-fuel ratio of the air-fuel mixture when the air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio Thus, the amount of oxygen stored in the upstream catalyst 43, that is, the amount of oxygen that can be stored to the maximum by the upstream catalyst 43, that is, the maximum oxygen storage capacity is calculated. Then, it is determined whether or not the calculated maximum oxygen storage capacity is greater than a predetermined threshold value. Here, the predetermined threshold value is set to a maximum oxygen storage capacity that can be said that the oxygen storage / release capability of the upstream catalyst 43 is not deteriorated. When it is determined that the calculated maximum oxygen storage capacity is greater than the predetermined threshold value, it is determined that the oxygen storage / release capability of the upstream catalyst 43 has not deteriorated. On the other hand, when it is determined that the calculated maximum oxygen storage capacity is equal to or less than the predetermined threshold value, it is determined that the oxygen storage / release capability of the upstream catalyst 43 has deteriorated.

  Incidentally, as shown in FIG. 22, a so-called hybrid system including an electric motor M in addition to the above-described internal combustion engine 10 for outputting a driving force for driving a vehicle is known. The hybrid system shown in FIG. 22 has a driving force switching mechanism P for switching a driving force transmission path for driving the vehicle 70 according to the traveling state of the vehicle 70, and a transmission from the driving force switching mechanism P. And a transmission TM for applying a voltage to the driving force transmission system of the front wheels 71 of the vehicle 70.

  The electric motor M is an AC synchronous motor, and is drive-controlled by AC power supplied from an inverter I that converts DC power supplied from the battery B into predetermined AC power. Further, the driving force switching mechanism P establishes a driving force transmission path only between the electric motor M and the transmission TM, and a driving force transmission path only between the internal combustion engine 10 and the transmission TM. The driving force transmission path can be switched between the mode and the mode in which the driving force transmission path is established between the electric motor M and the transmission TM and between the internal combustion engine 10 and the transmission TM.

  In such a hybrid system, as in the first to sixth embodiments described above, in the internal combustion engine 10, the purpose of determining whether there is an abnormality in the output of the downstream air-fuel ratio sensor 56, or the upstream catalyst The purpose of determining whether or not the oxygen storage / release capacity 43 has deteriorated, or the purpose of determining whether or not the upstream air-fuel ratio sensor 55 is abnormal, or when the internal combustion engine 10 is started. For the purpose of increasing the temperature of the engine early, or for releasing the oxygen stored in the upstream catalyst 43 after the fuel cut control is stopped, or for the output required for the internal combustion engine 10. When the air-fuel ratio of the air-fuel mixture is controlled to be richer than the stoichiometric air-fuel ratio for the purpose of lowering the temperature of the upstream catalyst 43 when it is extremely large, the air-fuel ratio between cylinders The determination of whether or not the engine is in an imbalance state is performed only for the purpose of determining the presence or absence of an inter-cylinder air / fuel ratio imbalance state. Compared with determining whether or not there is an imbalanced state, there is a further effect.

  That is, as described above, in the hybrid system, there is a case where a transmission path for driving force is established only between the electric motor M and the transmission TM. In this case, the operation of the internal combustion engine 10 is stopped. Therefore, in the hybrid system, the internal combustion engine 10 has fewer opportunities to be operated, and as a result, the internal combustion engine 10 can control the air-fuel ratio of the air-fuel mixture to be richer than the stoichiometric air-fuel ratio. It can be said that there will be less. Here, in the internal combustion engine 10, it is determined whether there is an abnormality in the output of the downstream air-fuel ratio sensor 56 to be executed, whether the oxygen storage / release capability of the upstream catalyst 43 is deteriorated, Determination of the presence or absence of an abnormality in the response of the fuel ratio sensor 55, or an early rise in the temperature of the internal combustion engine 10 when the internal combustion engine 10 is started, or occlusion in the upstream catalyst 43 after the fuel cut control is stopped. The air-fuel ratio of the air-fuel mixture is less than the stoichiometric air-fuel ratio for the purpose of reducing the temperature of the upstream-side catalyst 43 when the generated oxygen is released from the upstream-side catalyst 43 or when the output required for the internal combustion engine 10 is extremely large. When the air-fuel ratio is controlled to be rich, if the presence / absence of the inter-cylinder air / fuel ratio imbalance state is determined at the same time, the opportunity for determining the presence / absence of the inter-cylinder air / fuel ratio imbalance state is increased accordingly. The effect is obtained that that.

  DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 21 ... Combustion chamber, 25 ... Fuel injection valve, 40 ... Exhaust passage, 43 ... Upstream catalyst, 55 ... Upstream air-fuel ratio sensor, 56 ... Downstream air-fuel ratio sensor

Claims (6)

  A plurality of combustion chambers, a fuel injection valve disposed corresponding to each of the combustion chambers, an exhaust purification catalyst disposed in an exhaust passage for purifying a specific component in exhaust gas discharged from the combustion chamber, In order to detect the air-fuel ratio of the exhaust gas discharged from the combustion chamber, the upstream air-fuel ratio sensor disposed in the exhaust passage upstream of the exhaust purification catalyst, and the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst A downstream air-fuel ratio sensor disposed in an exhaust passage downstream of the exhaust purification catalyst for detection, and each combustion is to be determined when it is determined whether or not an abnormality has occurred in the downstream air-fuel ratio sensor In a multi-cylinder internal combustion engine in which rich air-fuel ratio control for downstream air-fuel ratio sensor abnormality determination is performed to control the air-fuel ratio of the air-fuel mixture formed in the chamber to an air-fuel ratio richer than the stoichiometric air-fuel ratio, the downstream air-fuel ratio Sensor difference When the rich air-fuel ratio control for determination is executed, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is estimated based on the output of the upstream air-fuel ratio sensor, and the air-fuel ratio between the estimated air-fuel ratios is estimated. A cylinder-to-cylinder air-fuel ratio imbalance determination device that executes an inter-cylinder air-fuel ratio imbalance determination that determines whether or not there is a deviation.   An upstream air-fuel ratio sensor for controlling the air-fuel ratio of the air-fuel mixture formed in each combustion chamber to an air-fuel ratio richer than the stoichiometric air-fuel ratio when it is determined whether or not an abnormality has occurred in the upstream air-fuel ratio sensor Abnormality determination rich air-fuel ratio control is executed in the multi-cylinder internal combustion engine, and when the upstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control is executed, the inter-cylinder air-fuel ratio imbalance is performed. The inter-cylinder air-fuel ratio imbalance determination device according to claim 1, wherein the determination is performed.   The inter-cylinder air-fuel ratio imbalance determination is executed when it is determined that no abnormality has occurred in the upstream air-fuel ratio sensor when the upstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control is being executed. Item 3. The inter-cylinder air-fuel ratio imbalance determining device according to Item 2.   A plurality of combustion chambers, a fuel injection valve disposed corresponding to each of the combustion chambers, an exhaust purification catalyst disposed in an exhaust passage for purifying a specific component in exhaust gas discharged from the combustion chamber, An upstream air-fuel ratio sensor disposed in an exhaust passage upstream of the exhaust purification catalyst for detecting the air-fuel ratio of the exhaust gas discharged from the combustion chamber, and an abnormality occurs in the upstream air-fuel ratio sensor When the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is controlled to be richer than the stoichiometric air-fuel ratio, the upstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control is executed. In the multi-cylinder internal combustion engine, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber based on the output of the upstream air-fuel ratio sensor when the rich air-fuel ratio control for abnormality determination of the upstream air-fuel ratio sensor is being executed Estimate the Inter-cylinder air-fuel ratio imbalance determination apparatus for performing the air-fuel ratio imbalance determination among judges cylinders whether deviation occurs between the air-fuel ratio of the constant air-fuel mixture.   The inter-cylinder air-fuel ratio imbalance determination is executed when it is determined that no abnormality has occurred in the upstream air-fuel ratio sensor when the upstream air-fuel ratio sensor abnormality determination rich air-fuel ratio control is being executed. Item 5. The inter-cylinder air-fuel ratio imbalance determination device according to Item 4.   Rich air-fuel ratio control at engine start for controlling the air-fuel ratio of the air-fuel mixture formed in each combustion chamber to an air-fuel ratio richer than the stoichiometric air-fuel ratio when the operation of the multi-cylinder internal combustion engine is started, or the fuel When the fuel injection from the fuel injection valve is resumed after the fuel injection from the injection valve is stopped, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is made richer than the stoichiometric air-fuel ratio. Rich air-fuel ratio control after stopping fuel injection to be controlled, or when the temperature of the exhaust purification catalyst is higher than a predetermined allowable upper limit temperature, the air-fuel ratio of the air-fuel mixture formed in each combustion chamber is less than the stoichiometric air-fuel ratio Rich air-fuel ratio control for an exhaust purification catalyst that controls to a rich air-fuel ratio is executed in the multi-cylinder internal combustion engine, and when the rich air-fuel ratio control at the time of engine start is executed, or the fuel Injection stop 6. The inter-cylinder air-fuel ratio imbalance determination is executed when post-rich air-fuel ratio control is being executed or when the exhaust purification catalyst rich air-fuel ratio control is being executed. The inter-cylinder air-fuel ratio imbalance determination device according to claim 1.

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