JP2012057480A - Device for determining inter-cylinder imbalance of air-fuel ratio in multi-cylinder internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably and accurately determine whether or not an inter-cylinder imbalance state in air-fuel ratio has occurred regardless of responsiveness of an air-fuel ratio sensor.SOLUTION: It is assumed that an air-fuel ratio of a mixed exhaust gas passing through a collecting exhaust passage coincides with a target air-fuel ratio (a theoretical air-fuel ratio) through feedback control based on a detected air-fuel ratio from an air-fuel ratio sensor arranged in the collecting exhaust passage in which exhaust passages extending from a plurality of cylinders are collected. When taking as "a unit combustion cycle period" a period in which a crank angle passes and requiring for completing one combustion stroke by each of the plurality of cylinders, an imbalance state is determined to have occurred based on the detection of "a condition in which there exists only one minimal value of a detected air-fuel ratio having a value richer than the target air-fuel ratio in the unit combustion cycle period" or "a condition in which there exists only one maximal value of a detected air-fuel ratio having a value leaner than the target air-fuel ratio in the unit combustion cycle period".

Description

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

  Conventionally, the air-fuel ratio of the gas (mixed exhaust gas) passing through the collective exhaust passage is determined based on the output value of the air-fuel ratio sensor disposed in the collective exhaust passage formed by collecting the exhaust passages extending from the plurality of cylinders. Air-fuel ratio control apparatuses that perform feedback control are widely known. The mixed exhaust gas is an exhaust gas obtained by mixing exhaust gases discharged from each cylinder. More specifically, in this air-fuel ratio control device, the air-fuel ratio feedback amount common to the plurality of cylinders is based on the output value of the air-fuel ratio sensor so that the air-fuel ratio of the mixed exhaust gas matches the stoichiometric air-fuel ratio. Calculated. The air-fuel ratio of the mixed exhaust gas is feedback-controlled by adjusting the amount of fuel injected to each of the plurality of cylinders based on the air-fuel ratio feedback amount.

  By the way, in a multi-cylinder internal combustion engine, a plurality of cylinders having an EGR gas amount recirculated to an intake system by an EGR mechanism, a variation among cylinders of an injection amount from a fuel injection valve, a variation between cylinders of a maximum lift amount of an intake valve, Variations in distribution to the device may occur. When the characteristic variation between the cylinders occurs, the air-fuel ratio variation (air-fuel ratio imbalance among cylinders, air-fuel ratio variation among cylinders, air-fuel ratio non-uniformity between cylinders) may occur between the cylinders.

  When the air-fuel ratio imbalance state between cylinders occurs, even if the air-fuel ratio of the mixed exhaust gas (that is, the average of the air-fuel ratios of all cylinders) matches the stoichiometric air-fuel ratio by the above-described air-fuel ratio feedback control. However, a cylinder (rich cylinder) that becomes rich (rather than the theoretical air-fuel ratio) and a cylinder (lean cylinder) in which the air-fuel ratio becomes lean (rather than the theoretical air-fuel ratio) always occur. Although details will be described later, normally, when the air-fuel ratio of the mixed exhaust gas matches the stoichiometric air-fuel ratio, only one cylinder is a rich cylinder and all the remaining cylinders are lean cylinders (rich imbalance state) Alternatively, a state (lean imbalance state) occurs in which only one cylinder becomes a lean cylinder and all the remaining cylinders become rich cylinders.

  As described above, when the air-fuel ratio imbalance state between the cylinders occurs, the combustion state of the air-fuel mixture in each cylinder becomes a combustion state different from complete combustion. As a result, although the air-fuel ratio of the mixed exhaust gas (the average of the air-fuel ratios of all cylinders) matches the stoichiometric air-fuel ratio, the amount of emissions discharged from each cylinder (the amount of unburned matter and nitrogen oxides) Increase). Therefore, it is important to detect the occurrence of an imbalance state between the air-fuel ratios and to take some measures to suppress an increase in the amount of emissions.

  One conventional apparatus for determining whether or not such an air-fuel ratio imbalance state between cylinders has occurred is to calculate the locus length of the output value (output signal) of the air-fuel ratio sensor disposed in the collective exhaust passage. And the trajectory length is compared with the “reference value that changes according to the engine speed”, and it is determined whether or not an air-fuel ratio imbalance among cylinders has occurred based on the comparison result. (For example, refer to Patent Document 1).

  However, the trajectory length largely depends on the responsiveness of the air-fuel ratio sensor. Therefore, in the determination method described in the above document, the determination result of whether or not the air-fuel ratio imbalance among cylinders has occurred is easily influenced by the responsiveness of the air-fuel ratio sensor. As a result, the determination accuracy may be lowered depending on the responsiveness of the air-fuel ratio sensor.

US Pat. No. 7,152,594

  The present invention has been made to address the above-described problems, and its object is to stably and accurately determine whether or not an air-fuel ratio imbalance state has occurred regardless of the responsiveness of the air-fuel ratio sensor. An object of the present invention is to provide an air-fuel ratio imbalance among cylinders determination apparatus.

  In order to achieve the above object, an air-fuel ratio imbalance among cylinders determination apparatus (this determination apparatus) of a multi-cylinder internal combustion engine according to the present invention is applied to a multi-cylinder internal combustion engine having a plurality of cylinders. This determination apparatus includes an air-fuel ratio sensor, a plurality of fuel injection valves, a feedback amount calculation unit, a feedback control unit, and a determination unit.

  The air-fuel ratio sensor is disposed in the collective exhaust passage and generates an output value corresponding to the air-fuel ratio of the mixed exhaust gas.

  The plurality of fuel injection valves are disposed corresponding to each of the plurality of cylinders. The plurality of fuel injection valves respectively inject fuel contained in the air-fuel mixture supplied to the combustion chambers of the plurality of cylinders. That is, one or more fuel injection valves are provided for one cylinder. Each fuel injection valve injects fuel into the cylinder corresponding to the fuel injection valve.

  The feedback amount calculating means calculates a common air-fuel ratio feedback amount for the plurality of cylinders. The air-fuel ratio feedback amount is calculated based on the output value of the air-fuel ratio sensor so that the air-fuel ratio of the mixed exhaust gas matches the target air-fuel ratio (specifically, the theoretical air-fuel ratio).

  The feedback control means adjusts the amount of fuel injected from each of the plurality of fuel injection valves based on the air-fuel ratio feedback amount. Thereby, the air-fuel ratio of the mixed exhaust gas is feedback controlled. Hereinafter, for convenience of explanation, a period in which the crank angle required for completing one combustion stroke in each of the plurality of cylinders is referred to as a “unit combustion cycle period” and is represented by an output value of the air-fuel ratio sensor. The air-fuel ratio is called “detected air-fuel ratio”.

  In general, the determination means determines whether or not the air-fuel ratio imbalance among cylinders has occurred based on the number of detected extreme values of the air-fuel ratio that exist within a unit combustion cycle period. Specifically, the determination is made based on the number of “minimum value of the detected air-fuel ratio that is richer than the target air-fuel ratio” existing within the unit combustion cycle period, or the “target air-fuel ratio” existing within the unit combustion cycle period. This is based on the number of “maximum value of detected air-fuel ratio that is leaner than the fuel ratio”.

  More specifically, a state where only one “minimum value of the detected air-fuel ratio that is richer than the target air-fuel ratio” exists within the unit combustion cycle period, or “from the target air-fuel ratio within the unit combustion cycle period” It can be determined that the air-fuel ratio imbalance among cylinders has occurred based on the detection of a state in which only one “maximum value of the detected air-fuel ratio that is the lean value” exists. As will be described in detail later, according to this, it can be determined stably and accurately that “the air-fuel ratio imbalance state between cylinders has occurred” regardless of the responsiveness of the air-fuel ratio sensor.

  Further, as will be described in detail later, when the response of the air-fuel ratio sensor is within a normal range, the “minimum value of the detected air-fuel ratio that is richer than the target air-fuel ratio” is 1 within the unit combustion cycle period. Is based on the fact that there is only one and “the maximum value of the detected air-fuel ratio that is leaner than the target air-fuel ratio” is detected, “the air-fuel ratio imbalance state between cylinders has occurred” Can be determined with sufficient accuracy.

1 is a diagram showing a schematic configuration of a multi-cylinder internal combustion engine to which a determination device according to an embodiment of the present invention is applied. FIG. 2 is a view showing a state in which a catalyst, an upstream air-fuel ratio sensor, and a downstream air-fuel ratio sensor shown in FIG. 1 are arranged in a collective exhaust passage. 2 is a graph showing the relationship between the output of the upstream air-fuel ratio sensor shown in FIG. 3 is a graph showing the relationship between the output of the downstream air-fuel ratio sensor shown in FIG. 1 and the air-fuel ratio. It is the figure which showed an example of the air fuel ratio of each cylinder in case the imbalance state has generate | occur | produced and the air fuel ratio of mixed exhaust gas corresponds with a theoretical air fuel ratio. It is the graph which showed an example of transition of the detection air fuel ratio when the imbalance state has not occurred. 6 is a graph showing an example of transition of a detected air-fuel ratio when an upstream air-fuel ratio sensor has extremely high responsiveness and a rich imbalance state occurs. It is the graph which showed an example of transition of the detection air fuel ratio in case an upstream air fuel ratio sensor has extremely high responsiveness, and the lean imbalance state has occurred. It is the graph which showed an example of transition of the detection air fuel ratio in case an upstream air fuel ratio sensor has normal responsiveness, and imbalanced state has occurred. It is a figure for demonstrating the various values used when calculating the air fuel ratio correction amount for compensating the shift | offset | difference of the detected air fuel ratio resulting from the diffusion speed of the hydrogen molecule discharged | emitted from a rich cylinder being quick.

  Hereinafter, an air-fuel ratio imbalance among cylinders determination apparatus (hereinafter also simply referred to as “this apparatus”) of a multi-cylinder internal combustion engine according to an embodiment of the present invention will be described with reference to the drawings.

(Constitution)
FIG. 1 shows a schematic configuration of a system in which the present apparatus is applied to a 4-cycle, spark ignition type, multi-cylinder (in-line 4-cylinder) internal combustion engine 10. FIG. 1 shows only a cross section of a specific cylinder, but the other cylinders have the same configuration.

  The internal combustion engine 10 includes a cylinder block portion 20 including a cylinder block, a cylinder block lower case, an oil pan, and the like, a cylinder head portion 30 fixed on the cylinder block portion 20, and a gasoline mixture to the cylinder block portion 20. An intake system 40 for supplying and an exhaust system 50 for releasing exhaust gas from the cylinder block unit 20 to the outside are included.

  The cylinder block unit 20 includes a cylinder 21, a piston 22, a connecting rod 23, and a crankshaft 24. The piston 22 reciprocates in the cylinder 21, and the reciprocating motion of the piston 22 is transmitted to the crankshaft 24 through the connecting rod 23, whereby the crankshaft 24 rotates. The wall surface of the cylinder 21 and the upper surface of the piston 22 form a combustion chamber 25 together with the lower surface of the cylinder head portion 30.

  The cylinder head portion 30 includes an intake port 31 communicating with the combustion chamber 25, an intake valve 32 that opens and closes the intake port 31, an intake camshaft that drives the intake valve 32, and continuously changes the phase angle of the intake camshaft. A variable intake timing control device 33, an actuator 33 a of the variable intake timing control device 33, an exhaust port 34 communicating with the combustion chamber 25, an exhaust valve 35 that opens and closes the exhaust port 34, and an exhaust camshaft that drives the exhaust valve 35. A variable exhaust timing control device 36 that continuously changes the phase angle of the exhaust camshaft, an actuator 36a of the variable exhaust timing control device 36, a spark plug 37, and an igniter 38 that includes an ignition coil that generates a high voltage applied to the spark plug 37. And the fuel intake port A fuel injection valve for injecting a preparative 31 (fuel injection means, fuel supply means) 39.

  One fuel injection valve 39 is provided for each combustion chamber 25. The fuel injection valve 39 is provided in the intake port 31. Thus, each of the plurality of cylinders includes the fuel injection valve 39 that supplies fuel independently of the other cylinders.

  The intake system 40 includes an intake manifold 41, an intake pipe 42, and a throttle valve 43. The intake manifold 41 includes a plurality of branch portions 41a and a surge tank 41b. One end of each of the plurality of branch portions 41 a is connected to each of the plurality of intake ports 31. The other ends of the plurality of branch portions 41a are connected to the surge tank 41b. One end of the intake pipe 42 is connected to the surge tank 41b. The intake port 31, the intake manifold 41, and the intake pipe 42 constitute an intake passage.

  The throttle valve 43 is provided in the intake pipe 42 so that the opening cross-sectional area (throttle valve opening) of the intake passage is variable. The throttle valve 44 is rotationally driven in the intake pipe 42 by a throttle valve actuator 44a (a part of the throttle valve driving means).

  The exhaust system 50 includes an exhaust manifold 51 including a plurality of branches connected at one end to the exhaust port 34 of each cylinder, and the other ends of the plurality of branches of the exhaust manifold 51 and all the branches are gathered. The exhaust pipe 52 connected to the collecting portion (the exhaust collecting portion of the exhaust manifold 51), the upstream catalyst (three-way catalyst) 53 disposed in the exhaust pipe 52, and the exhaust downstream of the upstream catalyst 53 A downstream catalyst (three-way catalyst) (not shown) disposed in the pipe 52 is provided. The exhaust port 34, the exhaust manifold 51, and the exhaust pipe 52 constitute an exhaust passage. Thus, the upstream catalyst 53 is disposed in a collective exhaust passage formed by collecting exhaust passages extending from a plurality of cylinders.

  Furthermore, the internal combustion engine 10 includes an exhaust gas recirculation pipe 54 that constitutes an external EGR passage, and an EGR valve 55. One end of the exhaust gas recirculation pipe 54 is connected to a collecting portion of the exhaust manifold 51. The other end of the exhaust gas recirculation pipe 54 is connected to the surge tank 41b. The EGR valve 55 is disposed in the exhaust gas recirculation pipe 54. The EGR valve 55 changes the opening cross-sectional area of the exhaust gas recirculation pipe 54.

  On the other hand, the internal combustion engine 10 includes an air flow meter 71, a throttle position sensor 72, a water temperature sensor 73, a crank position sensor 74, an intake cam position sensor 75, an exhaust cam position sensor 76, an upstream air-fuel ratio sensor 77, and a downstream air-fuel ratio sensor 78. , And an accelerator opening sensor 79.

  The air flow meter 71 detects a mass flow rate (intake air flow rate) Ga of intake air flowing through the intake pipe 42. The throttle position sensor 72 detects the opening degree of the throttle valve 43 (throttle valve opening degree). The water temperature sensor 73 detects the temperature of the cooling water of the internal combustion engine 10. The crank position sensor 74 detects the phase (change) of the rotation angle of the crankshaft 24. This detection result represents the engine speed NE.

  The intake cam position sensor 75 detects the phase (change) of the rotation angle of the intake camshaft. Based on the signals from the crank position sensor 74 and the intake cam position sensor 75, the absolute crank angle CA with reference to the compression top dead center of the reference cylinder (for example, the first cylinder) is acquired. This absolute crank angle CA is set to “0 ° crank angle” at the compression top dead center of the reference cylinder, and increases to 720 ° crank angle according to the rotation angle of the crank angle. Set to The exhaust cam position sensor 76 detects the phase (change) of the rotation angle of the exhaust camshaft.

  As shown in FIG. 2, the upstream air-fuel ratio sensor 77 (air-fuel ratio sensor in the present invention) includes an upstream catalyst 53 in the collective exhaust passage downstream of the collective portion HK (exhaust collective portion) of the exhaust manifold 51. It is arranged further upstream. The upstream side air-fuel ratio sensor 77 is disclosed in, for example, “Limit current type wide area sky provided with diffusion resistance layer” disclosed in JP-A-11-72473, JP-A-2000-65782, JP-A-2004-69547, and the like. "Fuel ratio sensor".

  Hereinafter, the exhaust gas passing through the collective exhaust passage is referred to as “mixed exhaust gas”. The mixed exhaust gas is an exhaust gas obtained by mixing exhaust gases discharged from each cylinder. The upstream air-fuel ratio sensor 77 generates an output value Vabyfs (V) corresponding to the air-fuel ratio of the mixed exhaust gas flowing into the upstream catalyst 53. This output value Vabyfs is converted into an air-fuel ratio (hereinafter referred to as “detected air-fuel ratio”) abyfs represented by the output value Vabyfs using the air-fuel ratio conversion table (map) Mapabyfs shown in FIG. .

  Referring to FIGS. 1 and 2 again, the downstream air-fuel ratio sensor 78 is disposed downstream of the upstream catalyst 53 and upstream of the downstream catalyst in the collective exhaust passage. The downstream air-fuel ratio sensor 78 is a known electromotive force type oxygen concentration sensor (a well-known concentration cell type oxygen concentration sensor using stabilized zirconia). The downstream air-fuel ratio sensor 78 generates an output value Voxs (V) corresponding to the air-fuel ratio of the mixed exhaust gas flowing out from the upstream catalyst 53 (and hence the temporal average value of the air-fuel ratio of the air-fuel mixture supplied to the engine). To do.

  As shown in FIG. 4, the output value Voxs becomes the maximum output value max (for example, about 0.9 V) when the air-fuel ratio is richer than the stoichiometric air-fuel ratio, and when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. The minimum output value min (for example, about 0.1 V), and when the air-fuel ratio is the stoichiometric air-fuel ratio, it becomes a voltage Vst (for example, about 0.5 V) approximately halfway between the maximum output value max and the minimum output value min. Further, the output value Voxs suddenly changes from the maximum output value max to the minimum output value min when the air-fuel ratio changes from an air-fuel ratio richer than the stoichiometric air-fuel ratio to a lean air-fuel ratio. When the air-fuel ratio leaner than the fuel ratio changes to a rich air-fuel ratio, it suddenly changes from the minimum output value min to the maximum output value max.

  Referring again to FIG. 1, accelerator opening sensor 79 detects an operation amount (accelerator pedal operation amount) of accelerator pedal AP operated by the driver.

  The electrical control device 90 is connected to each other by a bus “a CPU 91, a ROM 92 that stores programs executed by the CPU 91, various tables (maps, functions), constants, and the like in advance, and the CPU 91 temporarily stores data as necessary. It is a well-known microcomputer comprising a RAM 93, a backup RAM 94, and an interface 95 including an AD converter.

  The interface 95 is connected to the sensors 71 to 79 and supplies a signal from these sensors to the CPU 91. Further, the interface 95 is driven to the actuator 33a, the actuator 36a, the igniter 38 of each cylinder, the fuel injection valve 39 provided for each cylinder, the throttle valve actuator 43a, the EGR valve 55, etc. according to the instruction of the CPU 91. A signal (instruction signal) is transmitted.

(Air-fuel ratio feedback control)
Next, the outline of the air-fuel ratio feedback control by this apparatus will be described. In the present apparatus, based on the output value Vabyfs of the upstream air-fuel ratio sensor 77 and the output value Vaxs of the downstream air-fuel ratio sensor 78, the air-fuel ratio of the mixed exhaust gas is made to coincide with the target air-fuel ratio (= theoretical air-fuel ratio). Feedback control.

  Examples of this feedback control include the following. That is, the feedback correction value (sub feedback correction amount) is obtained by performing PID processing on the deviation between the output value Voxs of the downstream air-fuel ratio sensor 78 and the target value Vst corresponding to the theoretical air-fuel ratio. A value obtained by correcting the output value Vabyfs of the upstream air-fuel ratio sensor 77 by this sub-feedback correction amount is applied to the air-fuel ratio conversion table Mapabyfs shown in FIG. 3 to obtain the apparent air-fuel ratio. A PID process is performed on the difference between the apparent air-fuel ratio and the stoichiometric air-fuel ratio to obtain an air-fuel ratio feedback amount. This air-fuel ratio feedback amount is a value common to all cylinders.

  With this air-fuel ratio feedback amount, an amount of fuel obtained by correcting the “basic fuel injection amount obtained based on the engine speed NE, the intake air flow rate Ga, and the target air-fuel ratio” is the fuel injection valve 39 of each cylinder. Each is injected from. In this manner, the air-fuel ratio of the mixed exhaust gas is feedback-controlled by adjusting the amount of fuel injected from each fuel injection valve 39 based on the air-fuel ratio feedback amount common to all cylinders.

(Air-fuel ratio imbalance between cylinders)
Next, an air-fuel ratio imbalance state between cylinders (hereinafter also simply referred to as “an imbalance state”) will be described. “Air-fuel ratio imbalance among cylinders” refers to variations in air-fuel ratio among cylinders. The air-fuel ratio imbalance among cylinders is, for example, variation between cylinders in the actual injection amount from the fuel injection valve 39, variation between cylinders in the actual maximum lift amount of the intake valve 32, and recirculation to the intake passage by the EGR system. This may be caused by variations in the distribution of the amount of exhaust gas to each cylinder.

  As shown in FIG. 5, when an imbalance condition occurs, a cylinder (rich cylinder) in which the air-fuel ratio becomes richer than the stoichiometric air-fuel ratio even if the air-fuel ratio of the mixed exhaust gas matches the stoichiometric air-fuel ratio, A cylinder (lean cylinder) in which the air-fuel ratio is leaner than the stoichiometric air-fuel ratio is inevitably generated. FIG. 5 shows, as an example, a case where the fourth cylinder is a “rich cylinder” and the first to third cylinders are “lean cylinders”.

  The imbalance state shown in FIG. 5 is a normal state in which the fuel injection valves 39 of the first to third cylinders “inject an amount of fuel equal to the instructed fuel injection amount”, for example, and the fuel of the fourth cylinder This may occur when only the injection valve 39 is in an abnormal state of “injecting an amount of fuel that is larger than the instructed fuel injection amount”. That is, in this case, only the air-fuel ratio of the fourth cylinder greatly changes to the rich side. Thereby, the air-fuel ratio of the mixed exhaust gas passing through the collective exhaust passage (the average of the air-fuel ratios of all cylinders) becomes richer than the stoichiometric air-fuel ratio. Therefore, the air-fuel ratio of the fourth cylinder is changed to the lean side so as to approach the stoichiometric air-fuel ratio by the above-described “air-fuel ratio feedback amount” common to all the cylinders, and at the same time, the air-fuel ratio of the other three cylinders The fuel ratio is changed to the lean side so as to be away from the stoichiometric air-fuel ratio. As a result, the air-fuel ratio of the mixed exhaust gas that passes through the collective exhaust passage is substantially matched with the stoichiometric air-fuel ratio.

  However, the air-fuel ratio of the fourth cylinder is still kept richer than the stoichiometric air-fuel ratio, and the air-fuel ratios of the other three cylinders are kept leaner than the stoichiometric air-fuel ratio. From the above, the fourth cylinder is a “rich cylinder” and the first to third cylinders are “lean cylinders”. In this way, when the air-fuel ratio of the mixed exhaust gas matches the stoichiometric air-fuel ratio, a state where only one cylinder is a rich cylinder and all the remaining cylinders are lean cylinders, particularly a “rich imbalance state” Call.

  On the other hand, the fuel injection valves 39 of the first to third cylinders are in a normal state of “injecting an amount of fuel equal to the instructed fuel injection amount”, and only the fuel injection valves 39 of the fourth cylinder are “instructed” In the abnormal state of “injecting an amount of fuel that is less than the fuel injection amount”, the fourth cylinder becomes “lean cylinder” and the first to third cylinders become “rich cylinder” for the same reason as above. Become. In this way, when the air-fuel ratio of the mixed exhaust gas matches the stoichiometric air-fuel ratio, a state in which only one cylinder is a lean cylinder and all the remaining cylinders are rich cylinders is particularly referred to as a “lean imbalance state”. .

  Thus, when an imbalance state (rich imbalance state or lean imbalance state) occurs, the combustion state of the air-fuel mixture in each cylinder becomes a combustion state different from complete combustion. As a result, the amount of emissions discharged from each cylinder (the amount of unburned matter and the amount of nitrogen oxides) increases. For this reason, even if the air-fuel ratio (average of the air-fuel ratios of all cylinders) of the mixed exhaust gas that passes through the collective exhaust passage is the stoichiometric air-fuel ratio, the three-way catalyst cannot fully purify the increased emission, resulting in the exhaust gas as a result. There is a risk that the amount of emissions included will increase.

  Therefore, in order to take some measures when an imbalance condition (rich imbalance condition or lean imbalance condition) occurs, determining whether an imbalance condition has occurred suppresses an increase in the amount of emissions. Is important for. From the above, this apparatus determines whether or not an imbalance state has occurred.

(Imbalance state detection method using this device)
Hereinafter, the detection method of the imbalance state by this apparatus is demonstrated, referring FIGS. 6 to 9 show the detected air-fuel ratio abyfs when the multi-cylinder internal combustion engine is in a steady operation state and the air-fuel ratio of the mixed exhaust gas matches the target air-fuel ratio AFr (= theoretical air-fuel ratio). An example of transition of the air-fuel ratio obtained by applying the output value Vabyfs of the upstream air-fuel ratio sensor 77 to the air-fuel ratio conversion table Mapabyfs shown in FIG. 3 is shown.

  Hereinafter, for convenience of explanation, the period during which the crank angle required for completing each one combustion stroke in all the cylinders (all the cylinders that exhaust the exhaust gas reaching the upstream air-fuel ratio sensor 77) elapses is expressed as “ This is called “unit combustion cycle period”. In the case of a 4-cylinder, 4-cycle engine, the unit combustion cycle period is 720 ° crank angle.

  The exhaust gas from each cylinder reaches the upstream air-fuel ratio sensor 77 in the ignition order (accordingly, the exhaust order). Therefore, the transition of the detected air-fuel ratio abyfs is greatly different between when the imbalance state occurs and when it does not occur.

<When an imbalance condition has not occurred>
When the imbalance state has not occurred, the air-fuel ratio of the exhaust gas discharged from each cylinder is substantially the same (substantially the theoretical air-fuel ratio). Due to this, the detected air-fuel ratio abyfs is “a minimum value of the detected air-fuel ratio abyfr that is richer than the target air-fuel ratio AFr” (hereinafter referred to as “rich minimum value”) within an arbitrary unit combustion cycle period. ”And“ maximum value of the detected air-fuel ratio abyfr that is leaner than the target air-fuel ratio AFr ”(hereinafter, referred to as“ lean maximum value ”) are changed so that there are only a few cylinders.

  Specifically, in the case of a four-cylinder / four-cycle engine, as shown in FIG. 6, the rich minimum value (point b) and the lean maximum value within an arbitrary unit combustion cycle period (= 720 ° crank angle). A state in which only four (points a) exist is obtained.

  When the imbalance state has not occurred, the state that “there are only a few rich minimum values and lean maximum values within an arbitrary unit combustion cycle period” means that the upstream air-fuel ratio sensor 77 has responsiveness. It can be obtained regardless of the height (whether the response is extremely high or extremely low).

<When an imbalance condition has occurred>
On the other hand, when the rich imbalance state occurs, the air-fuel ratio of exhaust gas discharged from one rich cylinder becomes richer than the theoretical air-fuel ratio, and the exhaust gas discharged from all remaining cylinders (all lean cylinders). Each of the air-fuel ratios is leaner than the stoichiometric air-fuel ratio. Due to this, when the rich imbalance state occurs, the detected air-fuel ratio abyfs greatly varies with the unit combustion cycle period as one cycle.

  Hereinafter, first, it is assumed that the upstream air-fuel ratio sensor 77 has extremely high responsiveness. “When the upstream air-fuel ratio sensor 77 has extremely high responsiveness” represents the responsiveness of the upstream air-fuel ratio sensor 77 (that is, a limiting current type wide-area air-fuel ratio sensor having a diffusion resistance layer). This refers to the case where the time constant (time constant for the first-order lag response to the step input) is extremely shorter than the unit combustion cycle period. In this case, when the rich imbalance state occurs, the detected air-fuel ratio abyfs has only one rich minimum value and (lean total number of cylinders -1) pieces of the lean maximum value within an arbitrary unit combustion cycle period. Can only transition to exist.

  Specifically, in the case of a 4-cylinder, 4-cycle engine, as shown in FIG. 7, only one rich minimum value (point b) exists within an arbitrary unit combustion cycle period (= 720 ° crank angle). In addition, a state where only three lean maximum values (point a) exist is obtained.

  For the same reason, when the lean imbalance state occurs when the upstream side air-fuel ratio sensor 77 has extremely high responsiveness, the detected air-fuel ratio abyfs has a lean maximum within an arbitrary unit combustion cycle period. It can be changed so that there is only one value and there are only (the number of all cylinders-1) rich minimum values.

  Specifically, in the case of a 4-cylinder, 4-cycle engine, as shown in FIG. 8, only one lean maximum value (point a) exists within an arbitrary unit combustion cycle period (= 720 ° crank angle). In addition, a state in which only three rich minimum values (point b) exist is obtained.

  The case where the imbalance state (rich imbalance state or lean imbalance state) occurs when the upstream air-fuel ratio sensor 77 has extremely high responsiveness has been described above. However, in practice, the time constant representing the response of the upstream air-fuel ratio sensor 77 (the time constant for the first-order lag response to the step input) is longer than the unit combustion cycle period.

  Specifically, at present, the time constant representing the responsiveness of the limiting current type wide area air-fuel ratio sensor is about 300 msec or more. In contrast, when the 4-cylinder, 4-cycle engine is in the “unit combustion cycle period is the longest” state, that is, in the idling state (for example, NE = 600 rpm), the unit combustion cycle period is 200 msec. is there.

  As described above, when the time constant representing the response of the upstream air-fuel ratio sensor is longer than the unit combustion cycle period (that is, when the upstream air-fuel ratio sensor has normal response), the imbalance state (rich imbalance). 9), the detected air-fuel ratio abyfs has only one lean maximum value in any unit combustion cycle period regardless of the total number of cylinders, as shown in FIG. And it changes so that only one rich minimum exists.

  Organize the above contents. It is assumed that the multi-cylinder internal combustion engine is in a steady operation state and the air-fuel ratio of the mixed exhaust gas matches the target air-fuel ratio (= theoretical air-fuel ratio). When the imbalance state has not occurred, the rich minimum value in any unit combustion cycle period, regardless of whether the upstream air-fuel ratio sensor 77 has extremely high response or normal response. In addition, a state in which several cylinders (ie, two or more) exist in each of the local maximum values is obtained (see FIG. 6).

  On the other hand, when an imbalance state (rich imbalance state or lean imbalance state) occurs, even if the upstream air-fuel ratio sensor 77 has extremely high responsiveness, it has normal responsiveness. However, there is only one rich minimum value in any unit combustion cycle period (see point b in FIGS. 7 and 9), or one lean maximum value in any unit combustion cycle period. Only exists (see point a in FIGS. 8 and 9).

  The imbalance state detection method by this apparatus is based on such knowledge. That is, this device determines whether or not an imbalance condition has occurred based on the number of rich minimum values that exist within a unit combustion cycle period or the number of lean maximum values that exist within a unit combustion cycle period. To do.

  More specifically, the present apparatus has a “state in which only one rich minimum value exists within a unit combustion cycle period” or a “state in which only one lean maximum value exists within a unit combustion cycle period”. Based on the detection, it is determined that “an imbalance condition has occurred” (an imbalance condition is detected). Accordingly, it is possible to stably and accurately determine that an “imbalance state has occurred” regardless of the responsiveness of the upstream air-fuel ratio sensor.

  Further, when the time constant representing the responsiveness of the upstream air-fuel ratio sensor is longer than the unit combustion cycle period (that is, when the upstream air-fuel ratio sensor has normal responsiveness), the present apparatus determines that “unit combustion Based on the detection of “a state in which only one rich minimum value exists and only one lean maximum value exists” within the cycle period, it can also be determined that “an imbalance state has occurred”.

  The number of extreme values of the detected air-fuel ratio abyfs is actually detected as follows. That is, data of the detected air-fuel ratio abyfs is acquired for each minute crank angle (or minute time). An arbitrary unit combustion cycle period is extracted from the acquired data group. By numerically analyzing the extracted data group, the number of extreme values existing during an arbitrary unit combustion cycle is counted. As a result, the number of rich minimum values or lean maximum values existing during an arbitrary unit combustion cycle can be counted.

(Correction of detected air-fuel ratio or target air-fuel ratio)
As described above, when the imbalance state occurs, even if the air-fuel ratio of the mixed exhaust gas matches the stoichiometric air-fuel ratio, a rich cylinder and a lean cylinder always occur. A lot of unburned matter (HC, CO) and hydrogen molecules are discharged from the rich cylinder. A large amount of nitrogen oxides (NOx) and oxygen molecules are discharged from the lean cylinder.

  Thus, the various gas components discharged from each cylinder then pass through the diffusion resistance layer of the upstream air-fuel ratio sensor 77 (that is, the limiting current type wide-area air-fuel ratio sensor including the diffusion resistance layer). The speed (diffusion speed) of each gas component at that time is different for each gas component. In particular, the diffusion speed of hydrogen molecules is higher than the diffusion speed of other gas components, and is about 3 times.

  This means that the sensitivity of the upstream air-fuel ratio sensor to hydrogen molecules is about three times higher than the sensitivity of the upstream air-fuel ratio sensor to other gas components. Hydrogen molecules are reducing gases. From the above, when the imbalance state occurs, the phenomenon that the detected air-fuel ratio abyfs of the upstream air-fuel ratio sensor shifts to the rich side with respect to the true value occurs due to the high diffusion speed of hydrogen molecules. obtain.

  Therefore, in order to compensate for this deviation, it is preferable to calculate an appropriate air-fuel ratio correction amount and correct the detected air-fuel ratio abyfs to the lean side or the target air-fuel ratio to the rich side by the calculated air-fuel ratio correction amount. it is conceivable that. Hereinafter, a method for calculating the “appropriate air-fuel ratio correction amount” will be described.

  FIG. 10 is a graph corresponding to FIG. 9. When the upstream air-fuel ratio sensor has normal response, an imbalance state occurs and the air-fuel ratio of the mixed exhaust gas is the target air-fuel ratio (= theoretical air-fuel ratio). Shows an example of transition of the detected air-fuel ratio abyfs in the case of

  In FIG. 10, M is the average value (air-fuel ratio average value) of the detected air-fuel ratio abyfs within the unit combustion cycle period. The air / fuel ratio average value M is richer than the stoichiometric air / fuel ratio due to the above-described “deviation of the detected air / fuel ratio”. Sr is a value obtained by integrating the “deviation of the detected air-fuel ratio abyfs from the average air-fuel ratio M” over a period in which the detected air-fuel ratio abyfs is richer than the average air-fuel ratio M within the unit combustion cycle period ( Rich air-fuel ratio area, which corresponds to the area of the region indicated by dots in FIG. S1 is a value obtained by integrating the “deviation of the detected air-fuel ratio abyfs from the average air-fuel ratio M” over a period in which the detected air-fuel ratio abyfs is leaner than the average air-fuel ratio M within the unit combustion cycle period ( The lean air-fuel ratio area corresponds to the area of the hatched area in FIG.

  Note that the deviation of the average air-fuel ratio M from the target air-fuel ratio AFr is obtained by subjecting the sub-feedback correction amount (the difference between the output value Voxs of the downstream air-fuel ratio sensor 78 and the target value Vst corresponding to the theoretical air-fuel ratio to PID processing). It can be compensated by the integral term (I term) of the (correction amount obtained). Therefore, even if the average value M of the detected air-fuel ratio abyfs is shifted to the rich side from the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas discharged from the upstream side catalyst 53 can be maintained at a substantially stoichiometric air-fuel ratio.

  The air-fuel ratio correction amount can be easily obtained by the equation ((S1-Sr / 3)) / T. T is the unit combustion cycle period. The reason why the rich air-fuel ratio area Sr is divided by “3” in the above equation is based on the fact that the diffusion rate of hydrogen molecules is about three times the diffusion rate of other gas components as described above.

  By correcting the detected air-fuel ratio abyfs to the lean side by the air-fuel ratio correction amount calculated in this way or correcting the target air-fuel ratio AFr to the rich side, the "detected air-fuel ratio" "Abyfs rich side shift" can be compensated.

  In addition, this invention is not limited to the said embodiment, A various modification can be employ | adopted within the scope of the present invention. For example, in the above embodiment, a four-cylinder four-cycle / spark-ignition internal combustion engine is employed, but a four-cycle / spark-ignition / multi-cylinder internal combustion engine other than the four-cylinder may be employed. Also, a two-cycle / spark ignition / multi-cylinder internal combustion engine may be employed.

  In the above embodiment, the air-fuel ratio of the mixed exhaust gas matches the target air-fuel ratio (= theoretical air-fuel ratio) based on the output value Vabyfs of the upstream air-fuel ratio sensor 77 and the output value Voxs of the downstream air-fuel ratio sensor 78. However, even if feedback control is performed so that the air-fuel ratio of the mixed exhaust gas coincides with the target air-fuel ratio (= theoretical air-fuel ratio) based only on the output value Vabyfs of the upstream air-fuel ratio sensor 77. Good.

  DESCRIPTION OF SYMBOLS 39 ... Fuel injection valve, 53 ... Upstream catalyst, 77 ... Upstream air-fuel ratio sensor, 78 ... Downstream air-fuel ratio sensor, 90 ... Electric control apparatus, 91 ... CPU

Claims (4)

An air-fuel ratio imbalance determining apparatus applied to a multi-cylinder internal combustion engine having a plurality of cylinders,
An air-fuel ratio sensor that is disposed in a collective exhaust passage formed by collecting exhaust passages extending from the plurality of cylinders and generates an output value corresponding to an air-fuel ratio of mixed exhaust gas that is exhaust gas that passes through the collective exhaust passage. When,
A plurality of fuel injection valves that are arranged corresponding to each of the plurality of cylinders and inject fuel contained in the air-fuel mixture supplied to the respective combustion chambers of the plurality of cylinders;
Feedback amount calculation means for calculating an air-fuel ratio feedback amount common to the plurality of cylinders based on an output value of the air-fuel ratio sensor so that an air-fuel ratio of the mixed exhaust gas matches a target air-fuel ratio;
Feedback control means for feedback-controlling the air-fuel ratio of the mixed exhaust gas by adjusting the amount of fuel injected from each of the plurality of fuel injection valves based on the air-fuel ratio feedback amount;
Detected air that is an air-fuel ratio represented by an output value of the air-fuel ratio sensor existing within a unit combustion cycle period in which a crank angle required for completing each one combustion stroke in the plurality of cylinders elapses. The minimum value of the fuel ratio and richer than the target air-fuel ratio, or the maximum value of the detected air-fuel ratio existing within the unit combustion cycle period and the value leaner than the target air-fuel ratio. Determining means for determining whether an imbalance state between the air-fuel ratios of the air-fuel ratio of the air-fuel mixture supplied to the respective combustion chambers of the plurality of cylinders has occurred based on the number;
An air-fuel ratio imbalance among cylinders determination apparatus for a multi-cylinder internal combustion engine. In the multi-cylinder internal combustion engine air-fuel ratio imbalance determination apparatus according to claim 1,
The determination means includes
A state in which only one minimum value of the detected air-fuel ratio that is richer than the target air-fuel ratio exists within the unit combustion cycle period, or a value that is leaner than the target air-fuel ratio within the unit combustion cycle period An air-fuel ratio of a multi-cylinder internal combustion engine configured to determine that the air-fuel ratio imbalance among cylinders has occurred based on detection of a state in which only one maximum value of the detected air-fuel ratio exists. Fuel ratio imbalance determination apparatus between cylinders. In the multi-cylinder internal combustion engine air-fuel ratio imbalance determination apparatus according to claim 2,
The determination means includes
Within the unit combustion cycle period, there is only one minimum value of the detected air-fuel ratio that is richer than the target air-fuel ratio, and the maximum value of the detected air-fuel ratio that is leaner than the target air-fuel ratio is An air-fuel ratio imbalance among cylinders determination apparatus for a multi-cylinder internal combustion engine configured to determine that the air-fuel ratio imbalance among cylinders has occurred based on detection of a state in which only one exists. An air-fuel ratio imbalance among cylinders determination apparatus for a multi-cylinder internal combustion engine according to claim 3,
When it is determined by the determining means that the air-fuel ratio imbalance among cylinders has occurred, an air-fuel ratio average value that is an average value of the detected air-fuel ratio in the unit combustion cycle period, and a unit combustion cycle period A rich air-fuel ratio area which is a value obtained by integrating a deviation of the detected air-fuel ratio from the air-fuel ratio average value over a period in which the detected air-fuel ratio is richer than the air-fuel ratio average value; and the unit combustion cycle A lean air-fuel ratio area that is a value obtained by integrating a deviation of the detected air-fuel ratio from the average air-fuel ratio over a period in which the detected air-fuel ratio is leaner than the average air-fuel ratio within a period; A calculating means for calculating;
When the unit combustion cycle period is T, the rich air-fuel ratio area is Sr, and the lean air-fuel ratio area is Sl, the detection is performed by the air-fuel ratio correction amount obtained by the equation ((S1-Sr / 3)) / T. Correction means for correcting the air-fuel ratio to the lean side, or correcting the target air-fuel ratio to the rich side by the air-fuel ratio correction amount;
An air-fuel ratio imbalance among cylinders determination apparatus for a multi-cylinder internal combustion engine.

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