JP2012137050A - Abnormality detector for inter-cylinder air-fuel ratio dispersion in multi-cylinder internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control the influence of mixing in a supercharger turbine to improve detection accuracy, and to prevent detection error.SOLUTION: If a wastegate valve 27 is opened, abnormality of inter-cylinder air-fuel ratio dispersion is detected based on a detection value of a pre-catalyst sensor 17, and meanwhile if the wastegate valve 27 is closed, abnormality of inter-cylinder air-fuel ratio dispersion is detected based on a detection value of a post-catalyst sensor 18. In the detection based on the pre-catalyst sensor 17, exhaust gas which passes through a west gate passage 26 is measured, so that air-fuel ratio leveling due to influence from an exhaust turbine 25b of a super charger 25 is controlled. In the detection based on the post-catalyst sensor 18, if abnormality that air-fuel ratio becomes rich occurs in some cylinders, sensor output is more on the lean side than that at normal time with increase of hydrogen during exhaustion. Thereby, detection is continuously performed.

Description

  The present invention relates to an apparatus for detecting an abnormal variation in the air-fuel ratio between cylinders of a multi-cylinder internal combustion engine, and more particularly to an apparatus for detecting that the air-fuel ratio between cylinders in a multi-cylinder internal combustion engine is relatively large. .

  In general, in an internal combustion engine equipped with an exhaust gas purification system using a catalyst, a mixture ratio of air and fuel in an air-fuel mixture burned in the internal combustion engine, that is, an air-fuel ratio, is used to efficiently remove harmful components in exhaust gas with a catalyst. Control is essential. In order to perform such air-fuel ratio control, an air-fuel ratio sensor is provided in the exhaust passage of the internal combustion engine, and feedback control is performed so that the air-fuel ratio detected thereby coincides with a predetermined target air-fuel ratio.

  On the other hand, in a multi-cylinder internal combustion engine, air-fuel ratio control is normally performed using the same control amount for all cylinders. Therefore, even if air-fuel ratio control is executed, the actual air-fuel ratio may vary between cylinders. If the degree of variation is small at this time, it can be absorbed by air-fuel ratio feedback control, and harmful components in the exhaust gas can be purified by the catalyst, so that exhaust emissions are not affected and there is no particular problem.

  However, for example, if the fuel injection system of some cylinders breaks down and the air-fuel ratio between the cylinders varies greatly, exhaust emission deteriorates, causing a problem. It is desirable to detect such a large air-fuel ratio variation that deteriorates the exhaust emission as an abnormality. In particular, in the case of an internal combustion engine for automobiles, in order to prevent the traveling of a vehicle whose exhaust emission has deteriorated, it is required to detect an abnormal variation in air-fuel ratio between cylinders in an on-board state. There is also a movement to regulate the law.

  For example, in the apparatus described in Patent Document 1, an imbalance ratio value, which is a degree of variation in air-fuel ratio between cylinders, is obtained from a locus length or locus area of an output of an air-fuel ratio sensor using a predetermined map or function. ing.

JP 2009-209747 A JP 2008-208740 A

  However, in an internal combustion engine having a supercharger, when an air-fuel ratio sensor is arranged downstream from the turbine of the supercharger, the exhaust discharged from each cylinder passes through the turbine so that the exhaust is agitated. And imbalance cannot be detected with high accuracy.

  In the apparatus disclosed in Patent Document 2, the exhaust gas is stirred while the wastegate valve of the supercharger is closed, so that the air-fuel ratio is made uniform even when the air-fuel ratio varies among the cylinders. However, the idea of utilizing the air-fuel ratio sensor output when the wastegate valve is opened is not disclosed or suggested.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an inter-cylinder air-fuel ratio variation abnormality detection device for a multi-cylinder internal combustion engine that can improve detection accuracy and prevent erroneous detection.

One aspect of the present invention is:
A turbocharger installed in connection with a multi-cylinder internal combustion engine;
A catalytic converter installed in an exhaust passage of the multi-cylinder internal combustion engine and having a function of purifying exhaust emission by oxidation-reduction reaction;
A wastegate passage that bypasses the turbocharger turbine;
A wastegate valve that opens and closes the wastegate passageway;
A first air-fuel ratio sensor installed in the exhaust passage downstream from the outlet of the waste gate passage and upstream from the catalytic converter;
First abnormality detection means for detecting a variation in the air-fuel ratio between cylinders by comparing a value of a parameter correlated with the degree of fluctuation in the output of the first air-fuel ratio sensor with a predetermined first threshold value;
In a cylinder-to-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine comprising:
A second air-fuel ratio sensor installed in the exhaust passage downstream of the catalytic converter;
A second abnormality detection means for detecting a variation in air-fuel ratio between cylinders by comparing a parameter value correlated with an output of the second air-fuel ratio sensor with a predetermined second threshold value;
Selecting means for selecting the first abnormality detecting means when the waste gate valve is in an open state; and selecting the second abnormality detecting means when the waste gate valve is in a closed state;
The inter-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine is further provided.

  In this aspect, the selection means selects the first abnormality detection means when the wastegate valve is in the open state, and selects the second abnormality detection means when the wastegate valve is in the closed state. Therefore, when the wastegate valve is in the open state, leveling of the air-fuel ratio due to the influence of the turbocharger turbine is suppressed, and detection accuracy can be improved. Further, since the catalytic converter has a function of generating hydrogen from the hydrocarbons in the exhaust gas, when an abnormality occurs in which the air-fuel ratio becomes rich in some cylinders, the second air-fuel ratio increases with an increase in the amount of hydrogen in the exhaust gas. The output of the sensor becomes leaner than when it is normal, so that an abnormality can be detected. Therefore, even when the wastegate valve is in the closed state, the abnormality can be detected by the second abnormality detecting means, so that the detection can be continuously performed.

  According to the present invention, it is possible to improve the detection accuracy by suppressing the influence of the exhaust agitation by the turbine and to prevent an erroneous detection.

1 is a schematic view of an internal combustion engine according to a first embodiment of the present invention. It is a graph which shows the output characteristic of a pre-catalyst sensor and a post-catalyst sensor. It is a graph which shows the fluctuation | variation of the air fuel ratio sensor output according to atmospheric pressure. FIG. 4 is an enlarged view corresponding to a part IV in FIG. 3. It is a flowchart which shows the routine for the abnormality detection of the air-fuel ratio variation between cylinders in 1st Embodiment. It is a flowchart which shows the routine for the air-fuel ratio variation abnormality detection between cylinders in a reference example.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic view of an internal combustion engine according to the present embodiment. As shown in the figure, an internal combustion engine (engine) 1 is powered by burning a mixture of fuel and air inside a combustion chamber 3 formed in a cylinder block 2 and reciprocating a piston in the combustion chamber 3. Is generated. The internal combustion engine 1 of the present embodiment is a multi-cylinder internal combustion engine mounted on an automobile, more specifically, an in-line 4-cylinder spark ignition internal combustion engine, that is, a gasoline engine. However, the internal combustion engine to which the present invention is applicable is not limited to this, and the number of cylinders, the type, and the like are not particularly limited as long as it is a multi-cylinder internal combustion engine.

  Although not shown, the cylinder head of the internal combustion engine 1 is provided with an intake valve that opens and closes an intake port and an exhaust valve that opens and closes an exhaust port for each cylinder. Each intake valve and each exhaust valve is a camshaft. Or it is opened and closed by a solenoid actuator. A spark plug for igniting the air-fuel mixture in the combustion chamber 3 is attached to the top of the cylinder head for each cylinder.

  The intake port of each cylinder is connected to a surge tank 8 which is an intake air collecting chamber via a branch pipe 4 for each cylinder. An intake pipe 13 is connected to the upstream side of the surge tank 8, and the intake pipe 13 is connected to the outlet of the compressor 25 a of the supercharger 25. The inlet of the compressor 25 a is connected to the air cleaner 9. The intake pipe 13 incorporates an air flow meter 5 for detecting the amount of intake air (the amount of intake air per unit time, that is, the intake flow rate), and an electronically controlled throttle valve 10. An intake passage is formed by the intake port, the branch pipe 4, the surge tank 8 and the intake pipe 13. Around the intake pipe 13, an intercooler 11 for cooling the intake air flowing through the intake pipe 13 is disposed. Engine cooling water is guided into the intercooler 11 and the intake air is cooled by the engine cooling water. An air bypass passage 20 and an electronically controlled air bypass valve (ABV) 21 that opens and closes the air bypass passage 20 are installed so as to bypass the compressor 25a of the supercharger 25. The ABV 21 prevents the pressure on the upstream side of the throttle valve 10 from rising suddenly when the throttle valve 10 is suddenly closed, thereby preventing the generation of surge noise from the supercharger 25.

  An injector (fuel injection valve) 12 that injects fuel into the intake passage, particularly into the intake port, is provided for each cylinder. The fuel injected from the injector 12 is mixed with intake air to form an air-fuel mixture. The air-fuel mixture is sucked into the combustion chamber 3 when the intake valve is opened, compressed by the piston, and ignited and burned by the spark plug 7.

  On the other hand, the exhaust port of each cylinder is connected to the exhaust manifold 14. The exhaust manifold 14 includes a branch pipe for each cylinder that forms the upstream portion thereof, and an exhaust collecting portion that forms the downstream portion thereof. The downstream side of the exhaust collecting portion is connected to the inlet of the exhaust turbine 25 b of the supercharger 25. The outlet of the exhaust turbine 25 b is connected to the exhaust pipe 6. An exhaust passage is formed by the exhaust port, the exhaust manifold 14 and the exhaust pipe 6. The exhaust pipe 6 is provided with a waste gate passage 26 and an electronically controlled waste gate valve (WGV) 27 that opens and closes the exhaust gate 25 so as to bypass the exhaust turbine 25 b of the supercharger 25. The WGV 27 is configured to drive a valve body by a motor and a gear mechanism, and the gear mechanism is provided with a WGV opening sensor 28 for detecting the opening degree of the valve body by detecting the rotational position thereof. ing. The WGV 27 may be a diaphragm type controlled by a supercharging pressure or an intake pipe pressure.

  A catalyst composed of a three-way catalyst, that is, an upstream catalyst 11 and a downstream catalyst 19 are attached to the exhaust pipe 6 in series. The upstream catalyst 11 and the downstream catalyst 19 are, for example, those in which a precious metal such as platinum (Pt), palladium (Ph), or rhodium (Rd) is supported on alumina, and carbon monoxide (CO), hydrocarbon (HC) ) And nitrogen oxides (NOx) and the like can be purified by catalytic reaction.

  A pre-catalyst sensor 17 and a post-catalyst sensor 18 for detecting the air-fuel ratio of the exhaust gas are respectively installed on the upstream side and the downstream side of the upstream catalyst 11. The pre-catalyst sensor 17 and the post-catalyst sensor 18 are installed at positions immediately before and immediately after the upstream catalyst 11, and detect the air-fuel ratio based on the oxygen concentration in the exhaust gas. The pre-catalyst sensor 17 corresponds to the “first air-fuel ratio sensor” referred to in the present invention, and is installed in the exhaust pipe 6 on the downstream side of the exit, that is, the end portion of the waste gate passage 26. The pre-catalyst sensor 18 corresponds to the “second air-fuel ratio sensor” referred to in the present invention.

  The above-described spark plug 7, throttle valve 10, injector 12, ABV 21 and WGV 27 are electrically connected to an electronic control unit (hereinafter referred to as ECU) 22 as a controller. The ECU 22 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like, all not shown. In addition to the air flow meter 5, the pre-catalyst sensor 17, the post-catalyst sensor 18, and the WGV opening sensor 28, the ECU 22 includes a crank angle sensor 16 that detects the crank angle of the internal combustion engine 1, an accelerator, as shown in the figure. An accelerator opening sensor 15 for detecting the opening degree, a water temperature sensor 23 for detecting the temperature of the cooling water of the internal combustion engine 1, and other various sensors are electrically connected via an A / D converter (not shown) or the like. . The ECU 22 controls the ignition plug 7, the throttle valve 10, the injector 12, etc. so as to obtain a desired output based on the detection values of various sensors, etc., and the ignition timing, throttle opening, fuel injection amount, fuel injection Control time etc. The throttle opening is normally controlled to an opening corresponding to the accelerator opening. The ECU 22 also controls the ABV 21 and the WGV 27 to bypass intake air and exhaust gas as necessary.

  The pre-catalyst sensor 17 is a so-called wide-range air-fuel ratio sensor, and can continuously detect an air-fuel ratio over a relatively wide range. FIG. 2 shows the output characteristics of the pre-catalyst sensor 17. As shown in the figure, the pre-catalyst sensor 17 outputs a voltage signal Vf having a magnitude proportional to the detected exhaust air-fuel ratio (pre-catalyst air-fuel ratio A / Ff). The output voltage when the exhaust air-fuel ratio is stoichiometric (theoretical air-fuel ratio, for example, A / F = 14.6) is Vreff (for example, about 3.3 V).

On the other hand, the post-catalyst sensor 18 is a so-called O ₂ sensor, and has a characteristic that the output value changes suddenly with the stoichiometric boundary. FIG. 2 shows the output characteristics of the post-catalyst sensor 18. As shown in the figure, the output voltage when the exhaust air-fuel ratio (post-catalyst air-fuel ratio A / Fr) is stoichiometric, that is, the stoichiometric equivalent value is Vrefr (for example, 0.45 V). The output voltage of the post-catalyst sensor 18 changes within a predetermined range (for example, 0 to 1 (V)). When the exhaust air-fuel ratio is leaner than stoichiometric, the output voltage of the post-catalyst sensor becomes lower than the stoichiometric equivalent value Vrefr, and when the exhaust air-fuel ratio is richer than stoichiometric, the output voltage of the post-catalyst sensor becomes higher than the stoichiometric equivalent value Vrefr.

  The upstream catalyst 11 and the downstream catalyst 19 simultaneously purify NOx, HC, and CO when the air-fuel ratio A / F of the exhaust gas flowing into each of them is close to the stoichiometry, but the air-fuel ratio that can simultaneously purify these three with high efficiency The width (window) is relatively narrow.

  Air-fuel ratio control (stoichiometric control) is executed by the ECU 22 so that the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 11 is controlled in the vicinity of stoichiometric. This air-fuel ratio control is detected by a main air-fuel ratio control (main air-fuel ratio feedback control) that makes the exhaust air-fuel ratio detected by the pre-catalyst sensor 17 coincide with a stoichiometry that is a predetermined target air-fuel ratio, and detected by the post-catalyst sensor 18. The auxiliary air-fuel ratio control (auxiliary air-fuel ratio feedback control) is performed so that the exhaust air-fuel ratio thus made coincides with the stoichiometry.

  Now, for example, it is assumed that the injectors 12 of some cylinders out of all the cylinders have failed and air-fuel ratio variations (imbalance) occur between the cylinders. For example, the # 1 cylinder has a larger fuel injection amount than the other # 2, # 3, and # 4 cylinders, and its air-fuel ratio is greatly shifted to the rich side. Even at this time, if a relatively large correction amount is given by the above-described main air-fuel ratio feedback control, the air-fuel ratio of the total gas supplied to the pre-catalyst sensor 17 may sometimes be stoichiometrically controlled. However, looking at each cylinder, # 1 cylinder is larger and richer than stoichiometric, and # 2, # 3 and # 4 cylinders are leaner than stoichiometric. Is clear. In view of this, the present embodiment is equipped with a device that detects such a variation in air-fuel ratio between cylinders.

  As shown in FIG. 3, the exhaust air / fuel ratio A / F detected by the pre-catalyst sensor 17 tends to fluctuate periodically with one engine cycle (= 720 ° CA) as one cycle. When the variation in the air-fuel ratio between cylinders occurs, the fluctuation within one engine cycle increases. The air-fuel ratio diagram a in (B) shows the case where the WGV 27 is closed, and b shows the case where the WGV 27 is opened. As can be seen, when the WGV 27 is closed, the amplitude of the air-fuel ratio fluctuation becomes relatively small. Note that FIG. 3 is schematically shown for easy understanding.

  Here, the imbalance ratio (%) is a parameter representing the degree of variation in the air-fuel ratio between cylinders. In other words, the imbalance ratio is the amount of fuel injection in a cylinder (imbalance cylinder) causing the fuel injection amount deviation when only one of the cylinders has caused the fuel injection amount deviation. The ratio is a value indicating whether the fuel injection amount is not deviated from the fuel injection amount of the cylinder (balance cylinder) that has not caused the fuel injection amount deviation, that is, the reference injection amount. When the imbalance ratio is IB, the fuel injection amount of the imbalance cylinder is Qib, and the fuel injection amount of the balance cylinder, that is, the reference injection amount is Qs, IB = (Qib−Qs) / Qs. The greater the imbalance ratio IB, the greater the fuel injection amount deviation between the imbalance cylinder and the balance cylinder, and the greater the air-fuel ratio variation.

[Cylinder air-fuel ratio variation abnormality detection]
As can be understood from the above description, when the air-fuel ratio variation abnormality occurs, the fluctuation of the sensor output before the catalyst becomes large. Therefore, it is possible to detect an abnormality in the air-fuel ratio variation by monitoring the degree of variation. In the present embodiment, a variation parameter that is a parameter correlated with the variation degree of the pre-catalyst sensor output is calculated, and the variation parameter is compared with a predetermined abnormality determination value to detect a variation abnormality.

  Here, a method for calculating the variation parameter will be described. FIG. 4 is an enlarged view corresponding to the IV part of FIG. 3, and particularly shows fluctuations in the sensor output before the catalyst within one engine cycle. Here, as the pre-catalyst sensor output, a value obtained by converting the output voltage Vf of the pre-catalyst sensor 17 into an air-fuel ratio A / F is used. However, the output voltage Vf of the pre-catalyst sensor 17 can be directly used.

(B) As shown in the figure, the ECU 22 acquires the value of the pre-catalyst sensor output A / F at every predetermined sample period τ (unit time, for example, 4 ms) within one engine cycle. The value A / F _n obtained in this timing (second timing), the difference .DELTA.A / F _n between the value A / F _n-1 obtained at the previous timing (first timing), the following equation Obtained by (1). This difference ΔA / F _n can be rephrased as a differential value or inclination at the current timing.

Most simply, this difference ΔA / F _n represents the fluctuation of the sensor output before the catalyst. This is because as the degree of fluctuation increases, the slope of the air-fuel ratio diagram increases in absolute value, and the difference ΔA / F _n increases in absolute value. Therefore, the value of the difference ΔA / F _{n at} a predetermined timing can be used as a variation parameter.

However, in this embodiment, in order to improve accuracy, an average value of a plurality of differences ΔA / F _n is used as a variation parameter. In the present embodiment, the difference ΔA / F _n is integrated at each timing within one engine cycle, and the final integrated value is divided by the number of samples N to obtain the average value of the differences ΔA / F _n within one engine cycle. . Further, the average value of the difference ΔA / F _n is integrated for M engine cycles (for example, M = 100), the final integrated value is divided by the number of cycles M, and the average value of the difference ΔA / F _n within the M engine cycle Ask for.

The greater the degree of fluctuation of the pre-catalyst sensor output, the larger the average value of the differences ΔA / F _n within the M engine cycle becomes in absolute value. Therefore, if the average value is an absolute value or more than a predetermined abnormality determination value, it is determined that there is a variation abnormality, and if the average value is smaller than the abnormality determination value, it is determined that there is no variation abnormality, that is, normal.

Since the pre-catalyst sensor output A / F may increase or decrease, the difference ΔA / F _n or the average value thereof may be obtained for only one of these cases and used as a variation parameter. it can. Especially when only one cylinder has a rich shift, when the pre-catalyst sensor receives the exhaust gas corresponding to that one cylinder, its output rapidly changes to the rich side (that is, rapidly decreases). (Rich imbalance determination). In this case, only the lower right region in the graph of FIG. 4B is used for rich shift detection. In general, the transition from lean to rich is often performed more steeply than the transition from rich to lean. Therefore, according to this method, it can be expected to detect a rich shift with high accuracy. However, the present invention is not limited thereto, it is used only the value of the increase side, or by integrating the absolute value of using both the value of the increase side and a decrease side (difference .DELTA.A / F _n, threshold the integrated value Can also be compared).

  In addition, any value that correlates with the degree of fluctuation in the pre-catalyst sensor output can be used as the fluctuation parameter. For example, the variation parameter can be calculated based on the difference between the maximum value and the minimum value of the sensor output before the catalyst within one engine cycle (so-called peak to peak). This is because the difference increases as the degree of fluctuation of the pre-catalyst sensor output increases.

  By the way, in an internal combustion engine having a supercharger, when an air-fuel ratio sensor is disposed downstream of the turbine of the supercharger, the exhaust discharged from each cylinder passes through the turbine so that the exhaust is agitated. And imbalance cannot be detected with high accuracy. For example, as described above, as shown in FIG. 3, when measurement is performed with the WGV 27 open (curve b), even when the engine has a significant variation in A / F values, measurement is performed with the WGV 27 closed ( In the case of the curve a), the variation in the A / F value is not significant. For this reason, if an abnormality in the air-fuel ratio variation is detected regardless of the operating state of the WGV 27, the detection accuracy may be reduced and erroneous detection may occur. In consideration of such a phenomenon, in the present embodiment, a decrease in detection accuracy is suppressed by the following abnormality detection routine.

[Inter-cylinder air-fuel ratio variation abnormality detection routine]
Next, an inter-cylinder air-fuel ratio variation abnormality detection routine will be described with reference to FIG.

First, in step S101, it is determined whether a predetermined precondition suitable for performing abnormality detection is satisfied. This precondition is satisfied when the following conditions are satisfied.
(1) The engine has been warmed up. For example, the warm-up is terminated when the water temperature detected by the water temperature sensor 23 is equal to or higher than a predetermined value.
(2) At least the pre-catalyst sensor 17 is activated.
(3) The engine is in steady operation.
(4) The stoichiometric control is in progress.
(5) The engine is operating in the detection region.
(6) The output A / F of the pre-catalyst sensor 17 is decreasing.

  Of these, (6) indicates that this routine is based on the above-described rich imbalance determination (a method in which only the value on the decrease side is used for detecting the rich shift). If the precondition is not satisfied, the routine is terminated.

  On the other hand, when the precondition is satisfied, the open / close state of the WGV 27 is detected based on a signal from the WGV opening sensor 28 (S102). Then, based on the detection result, it is determined whether the WGV 27 is in an open state (S103).

When the determination in step S103 is affirmative, that is, when the WGV 27 is in the open state, the exhaust gas that does not pass through the exhaust turbine 25b and flows through the waste gate passage 26 is supplied to the pre-catalyst sensor 17. In this case, the air-fuel ratio fluctuation is then detected based on the output of the pre-catalyst sensor 17 (S104). Here, the output A / F _{n of the} pre-catalyst sensor 17 (first air-fuel ratio sensor) at the current timing is acquired, and the output difference ΔA / F _n at the current timing is calculated and stored from the previous equation (1). The These processes are repeatedly executed until M cycles (M is an arbitrary integer) are completed. When the M cycle is completed, the average value ΔA / F _{AV of} the output difference ΔA / F _n calculated so far is, for example, the integrated value of the difference ΔA / F _{n as} the number of samples N and the number of engine cycles M as described above. It is calculated by dividing. This average value ΔA / F _AV represents the air-fuel ratio fluctuation.

Then, it is determined whether the absolute value of the average value ΔA / F _AV of the differences ΔA / F _n is larger than a predetermined abnormality threshold value α (S105). When the absolute value of the average value ΔA / F _AV is smaller than the abnormal threshold value α, the process proceeds to step S109, where it is determined that there is no variation abnormality, that is, normal, and the routine is terminated.

On the other hand, if the absolute value of the average value ΔA / F _AV is greater than or equal to the abnormal threshold value α, the process proceeds to step S106, where it is determined that there is a variation abnormality, that is, abnormal, and the routine is terminated. Note that a check lamp or the like is used to notify the user of the abnormality when the abnormality determination is performed simultaneously or when the abnormality determination is continuously issued for two trips (that is, one trip from the engine start to the stop twice). Preferably, the warning device is activated and the abnormality information is stored in a predetermined diagnosis memory in a manner that can be called by a maintenance worker.

On the other hand, when the determination in step S103 is negative, that is, when the WGV 27 is in the closed state, the exhaust gas stirred by passing through the exhaust turbine 25b is supplied to the pre-catalyst sensor 17. Abnormality determination based on is not performed. Instead, in this case, abnormality determination based on the output of the post-catalyst sensor 18 (second air-fuel ratio sensor) is performed. Here, the output A / F _n of the post-catalyst sensor 18 is repeatedly acquired over M cycles (M is an arbitrary integer), and when the M cycle ends, the average value A / of the output A / F _n calculated so far. F _AV is calculated, for example, by dividing the integrated value of A / F _{n by} the number of samples N and the number of engine cycles M (S107). This average value A / F _AV represents the air-fuel ratio.

Then, it is determined whether the average value A / F _{AV of} the air-fuel ratio A / F _n is greater than a predetermined abnormal threshold value β, that is, lean (S108). When the average value A / F _AV is smaller than the abnormal threshold value β, the process proceeds to step S109, where it is determined that there is no variation abnormality, that is, normal, and the routine is terminated.

On the other hand, when the average value A / F _AV is equal to or greater than the abnormal threshold β (that is, when it is lean), the process proceeds to step S106, and the same processing as in the determination by the pre-catalyst sensor 17 described above is performed. It is determined that there is a variation abnormality, that is, an abnormality, and the routine is terminated.

  As a result of such a series of processes, in the present embodiment, when the WGV 27 is in the open state, abnormality detection based on the output of the pre-catalyst sensor 17 (first air-fuel ratio sensor) is selected, and when the WGV 27 is in the closed state, the catalyst Abnormality detection based on the output of the rear sensor 18 (second air-fuel ratio sensor) is selected. Therefore, when the WGV 27 is in the open state, leveling of the air-fuel ratio due to the influence of the exhaust turbine 25b of the supercharger 25 is suppressed, and detection accuracy can be improved.

  Further, when an abnormality occurs in which the air-fuel ratio becomes rich in some cylinders, the output of the post-catalyst sensor 18 (second air-fuel ratio sensor) becomes leaner than normal when the amount of hydrogen in the exhaust gas increases, This makes it possible to detect an abnormality. Therefore, even when the WGV 27 is in the closed state, the abnormality can be detected by the abnormality detection based on the output of the post-catalyst sensor 18, so that the detection can be continuously performed.

  Next, reference examples related to the present invention will be described. In the reference example shown in FIG. 6, in order to improve detection accuracy and prevent erroneous detection, the detection of the variation in air-fuel ratio between cylinders is detected when the ratio of exhaust gas passing through the wastegate passage 26 is equal to or greater than a predetermined value. It is only executed. Since the mechanical configuration of this reference example is the same as that of the first embodiment, detailed description thereof is omitted.

  Processing in the ECU 22 of this reference example will be described. In FIG. 6, the ECU 22 first detects the open / closed state of the WGV 27 and the exhaust flow rate based on signals from the WGV opening sensor 28 and the air flow meter 5 (S201). Next, the ECU 22 calculates a ratio kwg of the passage flow rate of the waste gate passage 26 to the passage flow rate of the exhaust turbine 25b based on the detected opening / closing state of the WGV 27 and the exhaust flow rate (S202).

  Next, the ECU 22 compares the calculated ratio kwg with a predetermined reference value and determines whether it is equal to or greater than the reference value (S203). If the determination is negative, that is, if the flow rate ratio kwg of the waste flow through the wastegate passage 26 is low, this routine ends.

  If the determination in step S203 is affirmative, that is, if the ratio kwg of the passage flow rate through the wastegate passage 26 is greater than or equal to the reference value, the ECU 22 detects the air-fuel ratio fluctuation based on the output of the pre-catalyst sensor 17 (S204). Is compared with the abnormal threshold value α (S205). If the determination is affirmative, the abnormality determination is selected (S207), and if the determination is negative, the normal determination is selected (S206). The processes in steps S205, S206, and S207 are the same as the processes in steps S105, S106, and S109 in the first embodiment.

  As a result of such a series of processes, in the present embodiment, leveling of the air-fuel ratio due to the influence of the turbine is suppressed and detection accuracy can be improved, as in the first embodiment.

  The present invention is not limited to the above-described embodiments, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

For example, in the first embodiment, the average value A / F _AV of the output difference ΔA / F _n is used in the abnormality detection based on the output of the pre-catalyst sensor 17. A value may be used. In the abnormality detection based on the output of the post-catalyst sensor 18, the average value A / F _AV of the air-fuel ratio A / F _n is used. However, other values may be used as long as they are parameters correlated with the output.

  In each of the above embodiments, the rich deviation abnormality is detected by using the air-fuel ratio sensor output only when decreasing (when changing to the rich side). However, a mode of using the air-fuel ratio sensor output only at the time of increase (when changing to the lean side) or a mode of using both the air-fuel ratio sensor output at the time of decrease and increase is possible. Further, it is possible to detect not only the rich deviation abnormality but also the lean deviation abnormality, and the air-fuel ratio variation abnormality may be widely detected without distinguishing between the rich deviation and the lean deviation.

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 3 Combustion chamber 5 Air flow meter 6 Exhaust pipe 11 Upstream catalyst 12 Injector 14 Exhaust manifold 17 Pre-catalyst sensor 18 Post-catalyst sensor 22 Electronic control unit (ECU)
26 Wastegate passage 27 Wastegate valve (WGV)

Claims (1)

A turbocharger installed in connection with a multi-cylinder internal combustion engine;
A catalytic converter installed in an exhaust passage of the multi-cylinder internal combustion engine and having a function of purifying exhaust emission by oxidation-reduction reaction;
A wastegate passage that bypasses the turbocharger turbine;
A wastegate valve that opens and closes the wastegate passageway;
A first air-fuel ratio sensor installed in the exhaust passage downstream from the outlet of the waste gate passage and upstream from the catalytic converter;
First abnormality detection means for detecting a variation in the air-fuel ratio between cylinders by comparing a value of a parameter correlated with the degree of fluctuation in the output of the first air-fuel ratio sensor with a predetermined first threshold value;
In a cylinder-to-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine comprising:
A second air-fuel ratio sensor installed in the exhaust passage downstream of the catalytic converter;
A second abnormality detection means for detecting a variation in air-fuel ratio between cylinders by comparing a parameter value correlated with an output of the second air-fuel ratio sensor with a predetermined second threshold value;
Selecting means for selecting the first abnormality detecting means when the waste gate valve is in an open state; and selecting the second abnormality detecting means when the waste gate valve is in a closed state;
The inter-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine, further comprising:

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