JP5999008B2 - Inter-cylinder air-fuel ratio imbalance detector for multi-cylinder internal combustion engine - Google Patents

Description

  The present invention relates to an apparatus for detecting an imbalance (abnormal variation) in an air-fuel ratio between cylinders of a multi-cylinder internal combustion engine. In particular, the multi-cylinder internal combustion engine has a relatively large variation in air-fuel ratio between cylinders. The present invention relates to a detection device.

  In general, in an internal combustion engine equipped with an exhaust gas purification system using a catalyst, in order to purify the harmful components in the exhaust gas with a catalyst with high efficiency, the mixture ratio of the air-fuel mixture burned in the internal combustion engine, that is, the empty The fuel ratio is controlled. The air-fuel ratio is feedback controlled so that the air-fuel ratio detected by the air-fuel ratio sensor provided in the exhaust passage of the internal combustion engine matches the predetermined target air-fuel ratio.

  On the other hand, in a multi-cylinder internal combustion engine, since air-fuel ratio control is normally performed using the same control amount for all cylinders, even if air-fuel ratio control is executed, the actual air-fuel ratio may vary between cylinders. When the degree of variation is small, this can be absorbed by air-fuel ratio feedback control, and the exhaust gas harmful components can be purified by the catalyst, so that the exhaust emission is not affected and there is no 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 variation in the air-fuel ratio 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 with deteriorated exhaust emissions, it is required to detect the inter-cylinder air-fuel ratio imbalance in an on-board state (in-board). There is also a movement to regulate the law.

  Therefore, for example, in the apparatus described in Patent Document 1, an air-fuel ratio sensor is provided in an exhaust passage downstream of the exhaust collecting portion and upstream of the catalyst, and based on the output of the air-fuel ratio sensor, each cylinder is emptied. The fuel ratio is estimated individually.

  In the apparatus described in Patent Document 2, in order to accurately detect the air-fuel ratio in the exhaust system including the turbocharger and the waste gate valve, the waste gate valve is opened when detecting the air-fuel ratio imbalance. The exhaust gas discharged from the bypass passage is directly applied to the air-fuel ratio sensor.

JP-A-11-009038 JP 2012-180793 A

  However, in order to apply a system such as Patent Document 1 to an exhaust system including a turbocharger, a bypass passage that bypasses the turbine of the turbocharger, and a wastegate valve that opens and closes the bypass passage, the downstream side of the exhaust turbine In the case where the air-fuel ratio sensor is provided, the accuracy of detection of the air-fuel ratio of each cylinder may deteriorate due to the influence of the exhaust gas agitation by the exhaust turbine.

  Further, according to the configuration of Patent Document 2, since the exhaust gas discharged from the bypass passage is directly applied to the air-fuel ratio sensor, it can be expected that the detection accuracy of the air-fuel ratio is improved to some extent. However, since the flow rate of the exhaust gas is not always constant, there is a risk that the exhaust gas hits the air-fuel ratio sensor depending on the flow rate, and it is desirable to further improve this.

  Accordingly, the present invention was created in view of the above circumstances, and its purpose is to further suppress detection errors by suppressing changes in the degree of exhaust gas hitting the air-fuel ratio sensor accompanying changes in the exhaust flow rate, and to detect errors. An object of the present invention is to provide an inter-cylinder air-fuel ratio imbalance detection device for a multi-cylinder internal combustion engine that can suppress the above-described problem.

One aspect of the present invention is:
A turbocharger installed in connection with a multi-cylinder internal combustion engine;
A bypass passage provided with an exhaust passage of the multi-cylinder internal combustion engine so as to bypass the turbine of the supercharger;
A waste gate valve for opening and closing the bypass passage;
An air-fuel ratio sensor installed in a portion on the downstream side of the confluence of the downstream side of the turbine and the downstream side of the bypass passage in the exhaust passage;
Waste gate valve control means for controlling the waste gate valve;
Determination means for comparing the degree of fluctuation of the output of the air-fuel ratio sensor or the value of a parameter correlated therewith with a predetermined threshold value to execute an inter-cylinder air-fuel ratio imbalance determination;
In the inter-cylinder air-fuel ratio imbalance detection device of a multi-cylinder internal combustion engine equipped with
The waste gate valve control means opens the waste gate valve and controls the opening degree of the waste gate valve based on an exhaust flow rate when performing the air-fuel ratio imbalance determination between the cylinders. An inter-cylinder air-fuel ratio imbalance detection device for a multi-cylinder internal combustion engine.

  In this aspect, the waste gate valve control means opens the waste gate valve and controls the opening degree of the waste gate valve based on the exhaust gas flow rate when the inter-cylinder air-fuel ratio imbalance determination is executed. For this reason, according to the exhaust gas flow rate, it is possible to set the opening degree of the waste gate valve so that the contact state of the exhaust gas with the air-fuel ratio sensor is suitable for detection. Therefore, it is possible to further suppress the detection accuracy by suppressing the change in the degree of exhaust gas hitting the air-fuel ratio sensor accompanying the change in the exhaust flow rate, thereby further improving the detection accuracy.

  Preferably, the waste gate valve control means ensures that the position of the main flow of exhaust gas that has passed through the waste gate valve and the position of the air-fuel ratio sensor always coincide with each other within a predetermined range suitable for detecting the air-fuel ratio. The opening degree of the waste gate valve is controlled.

  More preferably, the waste gate valve control means controls the opening degree of the waste gate valve so that the main flow of the exhaust gas that has passed through the waste gate valve hits the air-fuel ratio sensor. According to these aspects, it is possible to improve detection accuracy by suppressing a change in the degree of exhaust gas hitting the air-fuel ratio sensor accompanying a change in the exhaust flow rate.

In another aspect of the present invention, in each of the above aspects,
When the inter-cylinder air-fuel ratio imbalance determination is executed, the low-rotation height for controlling the multi-cylinder internal combustion engine and the automatic transmission connected to the multi-cylinder internal combustion engine so that the engine speed is higher and the load is higher than during normal control. An inter-cylinder air-fuel ratio imbalance detection apparatus for a multi-cylinder internal combustion engine, further comprising load control means.

  In general, the determination of the air-fuel ratio imbalance between cylinders can be performed with high accuracy because the noise component is relatively reduced and the resolution is increased as the engine speed is lower and the load is higher. Therefore, according to this aspect, when the imbalance determination is performed, the multi-cylinder internal combustion engine and the automatic transmission are controlled so that the engine speed and the load are lower than those during normal control. Can be executed with high accuracy.

  According to the present invention, there is an excellent effect that it is possible to further suppress detection errors by suppressing a change in the degree of exhaust gas hitting the air-fuel ratio sensor accompanying a change in the exhaust gas flow rate, further improving detection accuracy. Demonstrated.

1 is a schematic view of an internal combustion engine according to an embodiment of the present invention. It is a principal part enlarged view which shows the exhaust passage of the vicinity of a turbine, a waste gate valve, and a pre-catalyst sensor. It is a graph which shows the example of a setting of a WGV opening degree map. 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 when the air-fuel ratio variation between cylinders does not arise (line a) and when it occurs (diagram b). FIG. 6 is an enlarged view corresponding to a VI part in FIG. 5. It is a flowchart which shows the routine for the air-fuel ratio imbalance detection between cylinders.

  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 the internal combustion engine has a plurality of cylinders. An output shaft (not shown) of the internal combustion engine 1 is connected to a torque converter (not shown), an automatic transmission 30 and a differential gear assembly (not shown), and drives drive wheels. The automatic transmission 30 is stepped, but may be stepless.

  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. Opened and closed by a shaft or 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 bypass passage 26 and an electronically controlled waste gate valve (WGV) 27 that opens and closes the bypass passage 26 so as to bypass the exhaust turbine 25 b of the supercharger 25. The WGV 27 is configured to drive the valve body by a motor and a gear mechanism. The gear mechanism includes, for example, a worm gear and a helical gear, and a WGV opening sensor 28 is provided for detecting the opening of the valve body by detecting the rotational position of the helical gear. 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 noble metal such as platinum (Pt), palladium (Ph), or rhodium (Rd) is supported on alumina, and carbon monoxide (CO), hydrocarbon (HC) ) And nitrogen oxides (NOx) 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 installed on the upstream side and the downstream side of the upstream catalyst 11, respectively. 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 an air-fuel ratio sensor according to the present invention, and is installed in the exhaust pipe 6 on the downstream side of the junction point between the downstream side of the exhaust turbine 25b and the downstream side of the bypass passage 26 in the exhaust passage.

  As shown in FIG. 2, the bypass passage 26 and the exhaust passage that passes through the exhaust turbine 25 b merge at a junction point downstream of the exhaust turbine 25 b. The WGV 27 opens and closes the bypass passage 26 at the junction. The opening degree of the WGV 27 is determined in accordance with an operating condition based on a detection value of a crank angle sensor 16 or a supercharging pressure sensor (not shown). For example, when the engine 1 is started or idling, the WGV 27 is opened to maintain the bed temperature of the catalyst 11, while the WGV 27 is closed when a large supercharging pressure is required, such as during acceleration.

  In the present embodiment, the WGV 27 rotates between 0 degrees (fully closed) and 90 degrees (fully opened) around the fulcrum, thereby controlling the flow rate of the exhaust gas passing through the exhaust turbine 25b and the bypass passage. The discharge direction discharged from the nozzle 26 is controlled. In particular, the flow rate of the exhaust gas passing through the exhaust turbine 25b is adjusted between 0 degree and 45 degrees or less, and when the opening degree of the WGV 27 is 45 degrees or more, the exhaust gas is separated from the bypass passage 26 regardless of the opening degree. A constant flow rate flows through each of the exhaust turbines 25b. That is, when the opening degree of the WGV 27 is equal to or greater than a predetermined value, the rotational speed of the exhaust turbine 25b is constant regardless of the opening degree of the WGV 27.

  The pre-catalyst sensor 17 described above is provided downstream of the discharge port of the bypass passage 26 and in the extending direction of the bypass passage 26. The pre-catalyst sensor 17 is provided so as to protrude into the exhaust passage from the pipe wall of the exhaust pipe 6 on the side where the fulcrum of the WGV 27 is provided.

  By the way, the axis 26a at least in the vicinity of the outlet of the bypass passage 26 is not parallel to the axis 6a of the exhaust pipe 6 and is deflected in a direction closer to the base of the pre-catalyst sensor 17 than the axis 6a of the exhaust pipe 6. . On the other hand, the exhaust gas exiting from the bypass passage 26 is more likely to go straight as the exhaust gas flow rate increases. As a result, even when the opening degree of the WGV 27 is the same, the main flow path of the exhaust gas changes according to the exhaust gas flow rate.

  Specifically, when the WGV 27 is at a certain opening shown by the solid line in FIG. 2, the main flow of the exhaust gas is that of the exhaust pipe 6 as indicated by an arrow f1 when the exhaust flow rate is relatively large. When the exhaust flow rate is relatively small, passing through a position closer to the base of the pre-catalyst sensor 17 than the axis 6a, it passes through a position closer to the axis 6a of the exhaust pipe 6 than the arrow f1 as indicated by the arrow f2. .

  Such a change in the main flow position according to the exhaust gas flow rate is compensated or suppressed by using the WGV 27, and the exhaust gas main flow hits the pre-catalyst sensor 17 (that is, the relative position between the main flow and the pre-catalyst sensor 17). In this embodiment, when the determination of the air-fuel ratio imbalance between cylinders is performed, the opening degree of the WGV 27 is set based on the exhaust gas flow rate. Trying to control.

  That is, in the present embodiment, when the air-fuel ratio imbalance determination between cylinders is performed, if the exhaust gas flow rate is relatively small, the opening degree of the WGV 27 is relatively set as indicated by a two-dot chain line 27a in FIG. When the exhaust gas flow rate is relatively large, the opening of the WGV 27 is controlled to a relatively small opening as shown by the solid line in FIG. The position of the arrow f1 is always passed, and the degree of contact of the exhaust gas with the pre-catalyst sensor 17 is made constant within a predetermined range in a state suitable for detecting the air-fuel ratio. That is, in the present embodiment, the opening degree of the WGV 27 is controlled so that the position of the main flow of exhaust gas that has passed through the WGV 27 and the position of the pre-catalyst sensor 17 always coincide with each other within a predetermined range suitable for air-fuel ratio detection. It is what is done.

  Referring again to FIG. 1, the spark plug 7, the throttle valve 10, the injector 12, the ABV 21 and the WGV 27, etc. 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 arranged on the downstream side of the intercooler 11 in the intake pipe 13, the pressure on the upstream side of the throttle valve 10 (supercharging) A supercharging pressure sensor 29 for detecting the pressure) and other various sensors are electrically connected via an A / D converter or the like (not shown).

  The ECU 22 controls the ignition plug 7, the throttle valve 10, the injector 12, the automatic transmission 30, etc. so as to obtain a desired output based on detection values of various sensors, etc., and the ignition timing, throttle opening, fuel Controls the injection amount, fuel injection timing, gear ratio, and the like. The throttle opening is usually 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.

  Further, in the present embodiment, in order to control the WGV 27 based on the exhaust flow rate, a WGV target opening map that associates the exhaust flow rate with the target opening of the WGV 27 is created in advance and stored in the ROM of the ECU 22. This WGV target opening degree map is determined so that the main flow of the exhaust gas that has passed through the WGV 27 always hits the pre-catalyst sensor 17. In the WGV target opening degree map, for example, as shown in FIG. 3, the opening degree of the WGV 27 is determined to increase as the exhaust flow rate decreases.

  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. 4 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 O2 sensor, and has a characteristic that the output value changes suddenly with the stoichiometric boundary. FIG. 4 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. 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 richer than stoichiometric, and # 2, # 3 and # 4 cylinders are leaner than stoichiometric, and are only stoichiometric as a whole balance. Is clear. Therefore, in this embodiment, a device for detecting such an inter-cylinder air-fuel ratio imbalance is provided.

  As shown in FIG. 5, 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. In FIG. 5B, the air-fuel ratio diagram “a” indicates a case where no variation in the air-fuel ratio between cylinders occurs, and b indicates a case where the variation in air-fuel ratio between cylinders occurs. Note that FIG. 5 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 of the cylinder (balance cylinder) not causing the fuel injection amount deviation, that is, the reference injection amount is deviated. 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 imbalance detection]
As understood from the above explanation, when the air-fuel ratio imbalance occurs, the fluctuation of the sensor output before the catalyst becomes large. Therefore, it is possible to detect the air-fuel ratio imbalance by monitoring the degree of fluctuation. In the present embodiment, a fluctuation parameter that is a parameter correlated with the degree of fluctuation in the pre-catalyst sensor output is calculated, and the fluctuation parameter is compared with a predetermined abnormality determination value to detect imbalance.

  Here, a method for calculating the variation parameter will be described. FIG. 6 is an enlarged view corresponding to the VI portion of FIG. 5, 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.

  As shown in part (B) of FIG. 6, the ECU 22 acquires the value of the pre-catalyst sensor output A / F every predetermined sample period τ (unit time, for example, 4 ms) within one engine cycle. The difference ΔA / Fn between the value A / Fn acquired at the current timing (second timing) and the value A / Fn−1 acquired at the previous timing (first timing) is expressed by the following equation (1). Ask for. This difference ΔA / Fn can be rephrased as a differential value or inclination at the current timing.

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

  However, in this embodiment, in order to improve accuracy, an average value of a plurality of differences ΔA / Fn is used as a variation parameter. In this embodiment, the difference ΔA / Fn is integrated at each timing within one engine cycle, the final integrated value is divided by the number of samples N, and the average value of the differences ΔA / Fn within one engine cycle is obtained. Further, the average value of the difference ΔA / Fn 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 / Fn within the M engine cycle is obtained. .

  The greater the degree of variation in the sensor output before the catalyst, the larger the average value of the differences ΔA / Fn 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 an imbalance, and if the average value is smaller than the abnormality determination value, it is determined that there is no imbalance, that is, normal.

  Since the pre-catalyst sensor output A / F may increase or decrease, the difference ΔA / Fn or the average value thereof can be obtained for only one of these cases and used as a variation parameter. . In particular, when only one cylinder has a rich shift, when the pre-catalyst sensor receives exhaust gas corresponding to that one cylinder, its output rapidly changes to the rich side (that is, suddenly decreases). (Rich imbalance determination). In this case, only the downward-sloping area in the graph in part (B) of FIG. 6 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 to this. Only the value on the increase side is used, or the value on both the decrease side and the increase side is used (the absolute value of the difference ΔA / Fn is integrated, and this integrated value is used as the threshold value. (By comparison).

  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.

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

First, in step S101, it is determined whether a predetermined precondition suitable for detecting the imbalance among cylinders is satisfied. This precondition is satisfied when the following conditions are satisfied.
(1) Warm-up of the internal combustion engine 1 has been completed. 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 internal combustion engine 1 is in steady operation.
(4) The stoichiometric control is in progress.
(5) The internal combustion engine 1 is operating within 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 ECU 22 next controls the engine 1 and the automatic transmission 30 to select the operation region of “low engine speed and high load” compared with the normal control. The control is changed as follows (S102).

  In general, the inter-cylinder air-fuel ratio imbalance detection can be performed with high accuracy because the noise component is relatively decreased and the resolution is increased as the “low engine speed and high load”. In the present embodiment, considering this, the internal combustion engine 1 and the automatic transmission 30 connected to the internal combustion engine 1 and the automatic transmission 30 connected thereto are controlled during imbalance detection. The control is changed so as to select the operation region. The normal control here refers to a control state in the case where such a control change is not performed. The engine speed and the gear stage or gear ratio in the normal control are basically the vehicle speed and the required load. Is determined by a predetermined shift speed map. This change in control is performed, for example, by selecting a gear ratio map (high gear position) that is higher than that during normal operation in a gear map that defines the gear speed to be selected according to the vehicle speed or the engine speed and the required load. (I.e., changing the so-called shift line so that the upshift is performed at a lower vehicle speed or a lower rotation speed than in normal operation).

  Next, with the control changed in this way, the WGV opening is controlled according to the WGV opening map described above (S103). In this control, the exhaust flow rate is calculated based on the detection values of the air flow meter 5 and the crank angle sensor 16, for example, and the target value of the WGV opening is calculated based on the exhaust flow rate according to the WGV opening map. Then, a control output is performed on the WGV 27 so that the opening of the WGV 27 detected by the signal from the WGV opening sensor 28 matches the target value.

  When the control of the WGV opening degree based on the exhaust flow rate is finished and the opening degree of the WGV 27 matches the target value determined based on the exhaust flow rate, the air-fuel ratio is then determined based on the output of the pre-catalyst sensor 17. A variation is detected (S104). Here, the output A / Fn of the pre-catalyst sensor 17 (first air-fuel ratio sensor) at the current timing is acquired, and the output difference ΔA / Fn at the current timing is calculated from the previous equation (1) and stored. These processes are repeatedly executed until M cycles (M is an arbitrary natural number) are completed. When the M cycle is finished, the average value ΔA / FAV of the output difference ΔA / Fn calculated so far is obtained by dividing the integrated value of the difference ΔA / Fn by the number of samples N and the number of engine cycles M as described above, for example. Is calculated by This average value ΔA / FAV represents the air-fuel ratio fluctuation.

  Based on the detected air-fuel ratio fluctuation, imbalance determination is executed (S105). Specifically, it is determined whether the absolute value of the average value ΔA / FAV of the differences ΔA / Fn is larger than a predetermined abnormality threshold value α. When the absolute value of the average value ΔA / FAV is smaller than the abnormal threshold α, it is determined that there is no imbalance, that is, normal, and when it is greater than or equal to the abnormal threshold α, it is determined that there is imbalance, that is, abnormal. 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.

  As a result of such a series of processes, in the present embodiment, when executing the determination of the air-fuel ratio imbalance between cylinders, the WGV 27 is opened, and the opening degree of the WGV 27 is based on the exhaust gas flow rate so that the exhaust gas flow rate is small. It is controlled so as to become larger. For this reason, according to the exhaust gas flow rate, the degree of contact of the exhaust gas with the air-fuel ratio sensor is guided to a more suitable state for detection, and compared with the case where the control of the WGV 27 is not performed, The change in the main flow position is compensated or suppressed so that the change in the main flow position becomes smaller. That is, in this embodiment, the position of the main flow of exhaust gas and the position of the pre-catalyst sensor 17 are always matched within a predetermined range suitable for detection of the air-fuel ratio by the control of the WGV 27 based on the exhaust flow rate, and The main flow of the exhaust gas that has passed through the WGV 27 is maintained so as to hit the pre-catalyst sensor 17. For this reason, in the present embodiment, it is possible to suppress the change in the degree of exhaust gas hitting the air-fuel ratio sensor 17 accompanying the change in the exhaust flow rate, further improve the detection accuracy, and suppress erroneous detection.

  In the present embodiment, when imbalance detection is performed, the engine 1 and the automatic transmission 30 are controlled to select an operation region of “low engine speed and high load” compared to normal control. Since the control is changed to (S102), imbalance detection can be executed with high accuracy.

  It should be noted that the control change in step S102, that is, the control change that selects the operation region of “low engine speed and high load” as compared with the normal control can be performed by another mode. The present invention can also be applied to a vehicle having a mechanical structure of a drive system different from the embodiment. For example, in a hybrid vehicle that uses an internal combustion engine and a motor generator for traveling, the operating range of “low engine speed and high load” can be selected compared to during normal control by increasing the amount of regeneration by the motor generator. Thus, the processing of this embodiment can be applied as it is.

  The present invention is not limited to the above-described embodiment, 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 imbalance detection in the above embodiment, the average value A / FAV of the output difference ΔA / Fn is used. However, other values may be used as long as they are parameters that correlate with the output fluctuation degree.

  In the above embodiment, the WGV opening sensor 28 for detecting the opening degree of the WGV 27 is provided. However, the opening degree of the WGV 27 is not limited to other control amounts such as a control amount to a motor or actuator for driving the valve body of the WGV 27. You may calculate or estimate based on a parameter.

  Further, in the above embodiment, the rich deviation abnormality is detected 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 imbalance may be widely detected without distinguishing between the rich deviation and the lean deviation.

  Further, in the above embodiment, the opening degree of the WGV 27 is controlled so that the opening degree of the WGV 27 increases as the exhaust flow rate decreases. However, the relationship between the exhaust flow rate and the opening degree of the WGV in the present invention is as follows. Depending on the geometrical positional relationship between the gate valve, the turbine, the air-fuel ratio sensor, and the exhaust passage connecting them, it can be designed in various various modes either experimentally or by simulation. In any case, according to the exhaust gas flow rate, the exhaust gas flow rate to the air-fuel ratio sensor is induced to a more suitable state for detection, and the change in the exhaust gas flow rate is compared with the case where the WGV control is not performed. The relationship between the exhaust flow rate and the opening of the WGV can be designed so as to compensate or suppress the change in the main flow position so that the change in the main flow position according to the Any of the configurations belongs to the category of the present invention as long as the change in the degree of exhaust gas hitting the air-fuel ratio sensor accompanying the change in the exhaust flow rate is suppressed.

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 Bypass passage 27 Waste gate valve (WGV)
28 WGV opening sensor

Claims (4)

A turbocharger installed in connection with a multi-cylinder internal combustion engine;
A bypass passage provided with an exhaust passage of the multi-cylinder internal combustion engine so as to bypass the turbine of the supercharger;
A waste gate valve for opening and closing the bypass passage;
An air-fuel ratio sensor installed in a portion on the downstream side of the confluence of the downstream side of the turbine and the downstream side of the bypass passage in the exhaust passage;
Waste gate valve control means for controlling the waste gate valve;
Determination means for comparing the degree of fluctuation of the output of the air-fuel ratio sensor or the value of a parameter correlated therewith with a predetermined threshold value to execute an inter-cylinder air-fuel ratio imbalance determination;
In the inter-cylinder air-fuel ratio imbalance detection device of a multi-cylinder internal combustion engine equipped with
The waste gate valve control means opens the waste gate valve and controls the opening degree of the waste gate valve based on an exhaust flow rate when performing the air-fuel ratio imbalance determination between the cylinders. An inter-cylinder air-fuel ratio imbalance detection device for a multi-cylinder internal combustion engine. An inter-cylinder air-fuel ratio imbalance detection device for a multi-cylinder internal combustion engine according to claim 1,
The waste gate valve control means controls the waste gate valve so that the position of the main flow of exhaust gas that has passed through the waste gate valve and the position of the air-fuel ratio sensor always coincide with each other within a predetermined range suitable for air-fuel ratio detection. An inter-cylinder air-fuel ratio imbalance detection apparatus for a multi-cylinder internal combustion engine, wherein the opening degree of a gate valve is controlled. An inter-cylinder air-fuel ratio imbalance detection device for a multi-cylinder internal combustion engine according to claim 1,
The waste gate valve control means controls an opening of the waste gate valve so that a main flow of exhaust gas that has passed through the waste gate valve hits the air-fuel ratio sensor. Fuel ratio imbalance detection device. An inter-cylinder air-fuel ratio imbalance detection device for a multi-cylinder internal combustion engine according to any one of claims 1 to 3,
When the inter-cylinder air-fuel ratio imbalance determination is executed, the low-rotation height for controlling the multi-cylinder internal combustion engine and the automatic transmission connected to the multi-cylinder internal combustion engine so that the engine speed is higher and the load is higher than during normal control. An inter-cylinder air-fuel ratio imbalance detection apparatus for a multi-cylinder internal combustion engine, further comprising load control means.

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