JP2013221483A - Control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To more appropriately control an exhaust air-fuel ratio even if imbalance occurs in an air-fuel ratio between cylinders, in an internal combustion engine including a plurality of cylinders.SOLUTION: Main feedback control over an air-fuel ratio and sub-feedback control for complementing the main feedback control are performed, respectively, based on output of a main exhaust gas sensor 42 arranged at an upstream side of a catalyst 40 in an exhaust passage 38 and an output of a sub-exhaust gas sensor 44 arranged on a downstream side of the catalyst in the exhaust passage, and also sub-feedback learning correction is performed by calculating a sub-feedback learning value based on a sub-feedback correction value calculated in the sub-feedback control when a load and engine speed of an internal combustion engine 10 are set within a predetermined area. When imbalance in an air-fuel ratio between cylinders is detected, the predetermined area in sub-feedback leaning is changed.

Description

  The present invention relates to an internal combustion engine that performs air-fuel ratio control based on the output of a main exhaust gas sensor provided upstream of an exhaust purification catalyst in an exhaust passage and the output of a sub exhaust gas sensor provided downstream of the catalyst. The present invention relates to a control device.

  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 follows a predetermined target air-fuel ratio.

  For example, Patent Document 1 includes a catalyst disposed in an exhaust passage of an internal combustion engine, a main air-fuel ratio sensor disposed upstream of the catalyst, and a sub O2 sensor disposed downstream of the catalyst. A system for controlling a fuel injection amount, that is, an air-fuel ratio based on an output is disclosed. This system calculates a feedback correction basic charge for setting the air / fuel ratio upstream of the catalyst to the target air / fuel ratio based on the output of the main air / fuel ratio sensor, and calculates the sub feedback correction amount based on the output of the sub O2 sensor. And means for calculating a sub-feedback learning amount based on the sub-feedback correction amount.

  On the other hand, in a so-called multi-cylinder internal combustion engine having a plurality of cylinders, 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 remains between cylinders. May vary. 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 air-fuel ratio between the cylinders varies greatly due to failure of the fuel injection system of some cylinders or the valve mechanism of the intake valve, exhaust emissions become worse, which becomes a problem. Such a large air-fuel ratio variation that deteriorates the exhaust emission can be detected as an abnormality, for example.

  For example, in the internal combustion engine disclosed in Patent Document 2, it is first determined that the air-fuel ratio between the cylinders of the internal combustion engine is in an imbalance state based on the calculated value of the air-fuel ratio feedback control. In the internal combustion engine, main air-fuel ratio feedback control is executed based on the detection result of the A / F sensor provided on the upstream side of the purification catalyst in the exhaust passage, and the detection of the O2 sensor provided on the downstream side of the purification catalyst. Sub air-fuel ratio feedback control is executed based on the result. When the average value of the calculated values of the sub air / fuel ratio feedback control exceeds the normal value, it is determined that the air / fuel ratio between the cylinders is in an imbalance state. Further, in the internal combustion engine of Patent Document 2, when it is determined that there is an abnormality in the air-fuel ratio between the cylinders, a process for shortening the fuel injection time to each cylinder by a predetermined time is executed. Based on the output signal from the sensor, the misfired cylinder is identified, and the misfired cylinder is identified as the cylinder in which the air-fuel ratio imbalance occurs.

  Further, it is possible to detect the occurrence of an air-fuel ratio imbalance between the cylinders by comparing the detected air-fuel ratio change rate based on the output of the air-fuel ratio sensor provided in the exhaust passage with a determination threshold value (for example, (See Patent Document 3).

JP 2005-351250 A JP 2010-112244 A International Publication No. 2011-070688

  By the way, the sub-feedback learning in the air-fuel ratio feedback control is generally executed so that the exhaust air-fuel ratio is brought close to the stoichiometry, and is executed when the detected engine operating state is in a predetermined region. It is not executed in the predetermined operating state.

  On the other hand, when the degree of air-to-cylinder air ratio imbalance is large, especially when it is difficult to achieve the desired output torque of the engine, the same degree of torque is produced compared to the case where there is almost no air-fuel ratio imbalance between cylinders. To gain, the driver depresses the accelerator pedal strongly. As a result, when the degree of the air-to-cylinder air ratio imbalance is large, the detected load or the engine speed tends to increase as compared with the case where the imbalance is not so, and the predetermined state in which the operating state executes sub-feedback learning. The frequency of deviating from the area can be increased. This substantially hinders the execution of sub-feedback learning, and may cause a reduction in exhaust air / fuel ratio control performance.

  Therefore, the present invention was created in view of the above circumstances, and an object of the present invention is to more appropriately control the exhaust air-fuel ratio in an internal combustion engine having a plurality of cylinders even when an air-to-cylinder air ratio imbalance occurs. An object of the present invention is to provide a control device for an internal combustion engine that makes it possible.

  According to one aspect of the present invention, a main exhaust gas sensor disposed upstream of a catalyst in an exhaust passage of an internal combustion engine having a plurality of cylinders, a sub exhaust gas sensor disposed downstream of the catalyst, and the main exhaust gas sensor Main feedback means for performing main feedback control of the air-fuel ratio based on the output; sub-feedback means for performing sub-feedback control for complementing the main feedback control based on the output of the sub exhaust gas sensor; and the internal combustion engine Based on a sub-feedback correction value calculated in the sub-feedback control when an output of one or more sensors for detecting at least one of a load and a rotation speed of the motor or a detection value based on the output is in a predetermined region To calculate the sub-feedback learning value. An internal combustion engine comprising: a feedback learning unit; a detection unit that detects an air-fuel ratio imbalance among cylinders; and a changing unit that changes the predetermined region in the sub-feedback learning unit according to a detection result of the detection unit. A control device is provided.

  According to the one aspect of the present invention, since the above-described configuration is provided, the predetermined region in the sub-feedback learning unit is changed according to the detection result of the detection unit that detects the air-fuel ratio imbalance among cylinders. Therefore, even if an air-fuel ratio imbalance among cylinders occurs, it is possible to secure a certain degree of opportunity for performing sub-feedback learning, and thus it is possible to more appropriately control the exhaust air-fuel ratio.

  For example, the changing means may enlarge the predetermined area when the air-fuel ratio imbalance among cylinders is detected by the detecting means. In general, when the air-fuel ratio imbalance between cylinders occurs, the driver tends to step on the accelerator pedal strongly in order to obtain the same level of output as when the imbalance does not occur. This leads to an increase in the load or the engine rotation speed, and increases a tendency that an output or a detection value corresponding to at least one of the load and the engine rotation speed exceeds a predetermined region in which the sub feedback learning is performed. On the other hand, when the air-fuel ratio imbalance between cylinders is detected, the predetermined area is expanded, so that the output or detection value corresponding to at least one of the load and the engine rotation speed is expanded and changed to the predetermined area. Since the possibility of entering increases, the execution opportunity of sub-feedback learning can be increased.

  More preferably, the changing means may change the predetermined area so that the predetermined area expands as the degree of imbalance between cylinders increases. In this way, it is possible to prevent the predetermined area from being enlarged and changed, and to appropriately execute sub-feedback learning for causing the exhaust air-fuel ratio to follow the target air-fuel ratio, and to secure an execution opportunity for sub-feedback learning. Can be compatible.

1 is a schematic view of an internal combustion engine according to an 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 showing the example of a waveform in air-fuel ratio feedback control, (A) is a waveform example of the output of the post-catalyst sensor, (B) is a change example of the integral term component in the sub-feedback correction value, (C) is ( It is a graph showing the example of a change of the subfeedback learning value corresponding to A) and (B). It is a graph which shows the example of a fluctuation | variation of the exhaust air fuel ratio according to the grade of the air-fuel ratio imbalance between cylinders. FIG. 5 is an enlarged schematic view corresponding to a V portion in FIG. 4. It is a graph which shows the relationship between an imbalance ratio and an output fluctuation parameter. It is a flowchart of one embodiment. It is a graph showing the predetermined area | region which performs subfeedback learning. FIG. 9 is a chart showing an example of a correction value for enlarging the region of FIG. 8 in relation to an imbalance rate.

  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 (engine) 10 according to an embodiment of the present invention. As shown in the figure, the engine 10 generates power by burning a mixture of fuel and air in a combustion chamber 14 formed in the engine 10 including the cylinder block 12 and reciprocating the piston in the cylinder. . The engine 10 is a one-cycle four-stroke engine. The engine 10 is a multi-cylinder internal combustion engine for automobiles, and more specifically, an in-line four-cylinder spark ignition internal combustion engine, that is, a gasoline engine. The engine 10 is mounted on a vehicle. 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 having two or more cylinders.

  Although not shown, an intake valve that opens and closes an intake port and an exhaust valve that opens and closes an exhaust port are provided for each cylinder in the cylinder head of the engine 10. Each intake valve and each exhaust valve are opened and closed by a camshaft. A spark plug 16 for igniting the air-fuel mixture or fuel in the combustion chamber 14 is attached to the top of the cylinder head for each cylinder. The firing order is the order of # 1, # 3, # 4, and # 2 cylinders.

  The intake port of each cylinder is connected to a surge tank 20 which is an intake air collecting chamber via a branch pipe 18 for each cylinder. An intake pipe 22 is connected to the upstream side of the surge tank 20, and an air cleaner 24 is provided at the upstream end of the intake pipe 22. An air flow meter 26 for detecting the intake air amount and an electronically controlled throttle valve 28 are incorporated in the intake pipe 22 in order from the upstream side. The intake port, the branch pipe 18, the surge tank 20, and the intake pipe 22 each define a part of the intake passage 30.

  A fuel injection valve (injector) 32 that injects fuel into the intake passage, particularly into the intake port, is provided for each cylinder. The fuel injected from the injector 32 is mixed with intake air to form an air-fuel mixture, which is sucked into the combustion chamber 14 when the intake valve is opened, compressed by the piston, and ignited and burned by the spark plug 16. The engine 10 is not limited to the port injection type, and may be a type in which fuel is directly injected into the cylinder.

  On the other hand, the exhaust port of each cylinder is connected to the exhaust manifold 34. The exhaust manifold 34 includes a branch pipe 34a for each cylinder forming an upstream portion thereof, and an exhaust collecting portion 34b forming a downstream portion thereof. An exhaust pipe 36 is connected to the downstream side of the exhaust collecting portion 34b. The exhaust port, the exhaust manifold 34 and the exhaust pipe 36 each define a part of the exhaust passage 38. A catalytic converter 40 including a three-way catalyst as an exhaust purification catalyst is attached to the exhaust pipe 36. The catalytic converter 40 constitutes an exhaust purification device. Note that the catalytic converter 40 has NOx, which is a harmful component in the exhaust, when the air-fuel ratio (exhaust air-fuel ratio) A / F of the inflowing exhaust is near the stoichiometric air-fuel ratio (stoichiometric, for example, A / F = 14.6). It functions to purify HC and CO simultaneously.

  First and second air-fuel ratio sensors for detecting the exhaust air-fuel ratio, that is, a pre-catalyst sensor 42 and a post-catalyst sensor 44 are installed on the upstream side and the downstream side of the catalytic converter 40, respectively. The pre-catalyst sensor 42 and the post-catalyst sensor 44 are installed in the exhaust passage at positions immediately before and immediately after the catalytic converter 40.

  The spark plug 16, the throttle valve 28, the injector 32, and the like described above are electrically connected to an electronic control unit (ECU) 50. The ECU 50 is configured to substantially perform various functions as various control means (control device) and various detection means (detection unit) in the engine 10. The ECU 50 includes a CPU (not shown), a storage device including a ROM and a RAM, an input / output port, and the like. In addition to the air flow meter 26, the pre-catalyst sensor 42, and the post-catalyst sensor 44, the ECU 50 detects a crank angle sensor 52 for detecting the crank angle of the internal combustion engine 10 and an accelerator opening as shown in the figure. An accelerator opening sensor 54 for performing the operation, a water temperature sensor 56 for detecting the engine cooling water temperature, and other various sensors are electrically connected via an A / D converter (not shown). The ECU 50 controls the ignition plug 16, the throttle valve 28, the injector 32, etc. so as to obtain a desired output based on outputs from various sensors and / or detected values, etc., and the ignition timing, throttle opening, fuel injection Control the amount, fuel injection timing, etc.

  As described above, the ECU 50 substantially functions as an ignition control unit, a fuel injection control unit, an intake air amount control unit, and the like. As will be understood from the description to be described later, the ECU 50 includes an air-fuel ratio control unit including a main feedback unit, a sub-feedback unit, and a sub-feedback learning unit, a detection unit that detects an air-fuel ratio imbalance among cylinders, Each function of the changing means for changing the predetermined area in the sub-feedback learning means is substantially assumed.

  The throttle valve 28 is provided with a throttle opening sensor (not shown), and an output signal from the throttle opening sensor is sent to the ECU 50. The ECU 50 normally feedback-controls the opening (throttle opening) of the throttle valve 28 to an opening determined according to the accelerator opening.

  Further, the ECU 50 detects the amount of intake air per unit time, that is, the amount of intake air based on the output signal from the air flow meter 26. The ECU 50 detects the load of the engine 10 based on at least one of the detected accelerator opening, throttle opening, and intake air amount.

  The ECU 50 detects the crank angle itself and the rotational speed of the engine 10 based on the crank pulse signal from the crank angle sensor 52. Here, “the number of rotations” means the number of rotations per unit time and is synonymous with the rotation speed. In the present embodiment, the rotation speed refers to the rotation speed rpm per minute.

  The ECU 50 normally sets the fuel injection amount (or fuel injection time) using data stored in advance in the storage device based on the intake air amount and the engine rotation speed, that is, the engine operating state. Based on the fuel injection amount, fuel injection from the injector 32 is controlled.

  Incidentally, the pre-catalyst sensor 42, which is an air-fuel ratio sensor, 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 42. As shown in the drawing, the pre-catalyst sensor 42 outputs a voltage signal Vf having a magnitude proportional to the detected exhaust air-fuel ratio (pre-catalyst air-fuel ratio). The output voltage when the exhaust air-fuel ratio is stoichiometric (theoretical air-fuel ratio) is Vref (for example, about 3.3 V).

  On the other hand, the post-catalyst sensor 44, which is an air-fuel ratio sensor, comprises a so-called O2 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 44. As shown in the drawing, the output voltage when the exhaust air-fuel ratio (post-catalyst air-fuel ratio) is stoichiometric, that is, the stoichiometric equivalent value is Vrefr (for example, 0.45 V). The output voltage of the post-catalyst sensor 44 changes within a predetermined range (for example, 0 to 1 V). Generally, when the exhaust air-fuel ratio is leaner than stoichiometric, the output voltage Vr of the post-catalyst sensor is lower than the stoichiometric equivalent value Vrefr, and when the exhaust air-fuel ratio is richer than the stoichiometric, the output voltage Vr of the post-catalyst sensor is higher than the stoichiometric equivalent value Vrefr. Get higher.

  The catalytic converter 40 includes a three-way catalyst, and as described above, the function of simultaneously purifying NOx, HC and CO, which are harmful components in the exhaust gas, when the air-fuel ratio A / F of the exhaust gas flowing into the catalytic converter 40 is in the vicinity of stoichiometry. Have However, the width (window) of the air-fuel ratio that can simultaneously purify these three with high efficiency is relatively narrow.

  Therefore, during normal operation of the engine 10, the ECU 50 executes air-fuel ratio control (stoichiometric control) for controlling the air-fuel ratio of the exhaust gas flowing into the catalytic converter 40 in the vicinity of the stoichiometric.

In the present embodiment, air-fuel ratio feedback that combines main feedback control based on the output of the pre-catalyst sensor 42 as the main exhaust gas sensor and sub-feedback control based on the output of the post-catalyst sensor 44 as the sub exhaust gas sensor. Control is executed. More specifically, in the present embodiment, the ECU 50 calculates the corrected A / F output CE represented by the equation (1) based on the output E1 of the pre-catalyst sensor 42 and the output of the post-catalyst sensor 44, A process of controlling the fuel injection amount is executed so that the corrected A / F output CE becomes a value corresponding to the target air-fuel ratio.
CE = E1 + E2 + E3 (1)
In this equation (1), “E1” is the output voltage of the pre-catalyst sensor 42. “E2” is a sub-feedback correction value calculated based on the output of the post-catalyst sensor 44. “E3” is a sub-feedback learning value.

  In the present embodiment, the corrected A / F output CE is calculated according to the above equation (1), and further, the main feedback control is performed to bring the corrected A / F output CE close to a value corresponding to the target air-fuel ratio. Is done. Specifically, in the main feedback control, a process of converting the corrected A / F output CE into an air-fuel ratio, a process of calculating a difference ΔA / F between the air-fuel ratio obtained as a result and the target air-fuel ratio, and A process of reflecting the difference ΔA / F in the correction of the fuel injection amount with a predetermined gain is executed.

  When the pre-catalyst sensor 42 exhibits ideal characteristics, the output E1 and the pre-catalyst air-fuel ratio have a unique relationship. In this case, if the main feedback is executed so that the output E1 of the pre-catalyst sensor 42 becomes a value corresponding to the stoichiometric air-fuel ratio, the exhaust gas flowing into the catalytic converter 40 has an air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio. Thus, only the purified exhaust gas flows out downstream of the catalytic converter 40.

  However, in reality, the pre-catalyst sensor 42 does not necessarily exhibit ideal output characteristics due to individual differences or aging of the pre-catalyst sensor 42 and the signal transmission system, or changes in the operating state of the engine 10. It is not a thing. For this reason, even when the main feedback control is being executed, rich or lean exhaust gas may blow through downstream of the catalytic converter 40. Such a situation can be detected by the post-catalyst sensor 44 which is a sub O2 sensor. Therefore, if the post-catalyst sensor 44 detects that rich exhaust gas has blown downstream of the catalytic converter 40, it can be determined that the pre-catalyst air-fuel ratio has shifted to the rich side as a whole. In this case, if the output E1 of the pre-catalyst sensor 42 is corrected so that the fuel injection amount is calculated to be smaller than the current amount, the pre-catalyst air-fuel ratio obtained as a result of the main feedback can be brought close to the stoichiometric air-fuel ratio. Is possible.

  On the other hand, if it is detected by the post-catalyst sensor 44 that lean exhaust gas has blown downstream of the catalytic converter 40, it can be determined that the pre-catalyst air-fuel ratio has shifted to the lean side as a whole. In this case, if the output E1 of the pre-catalyst sensor 42 is corrected so that the fuel injection amount is calculated to be larger than the current amount, the pre-catalyst air-fuel ratio obtained as a result of the main feedback can be brought close to the stoichiometric air-fuel ratio. Is possible. The sub feedback correction value E2 included in the above equation (1) is a correction value for realizing such a function. That is, the sub feedback control has a function that complements the main feedback control.

  Specifically, the ECU 50 calculates a sub-feedback correction value E2 by performing a predetermined calculation on the difference or deviation between the output of the post-catalyst sensor 44 and its target value (an output value corresponding to stoichiometry). More specifically, in the present embodiment, the sub feedback correction value E2 is calculated by PID control. That is, the sub feedback correction value E2 is calculated as the sum of a proportional term, an integral term, and a derivative term based on the difference between the output of the post-catalyst sensor 44 and its target value.

  The component of the integral term included in the sub-feedback correction value E2 can be grasped as an error that is permanently present in the main feedback. The sub-feedback learning value E3 in the above equation (1) is a value calculated by transferring the integral term component from the sub-feedback correction value E2 at a predetermined update timing, and constantly corrects the inherent error component. Is to do.

  According to such processing, the error component that is permanently present in the main feedback control is absorbed in the sub-feedback learning value E3, and only the fluctuation component of the error component included in the main feedback control is absorbed in the sub-feedback correction value E2. Can be made. As learning progresses, the sub-feedback learning value E3 converges to a value that appropriately reflects the above-described constant error component, and takes a stable value.

  FIG. 3A shows an example of a waveform of the output of the post-catalyst sensor 44. 3B represents a change example of the integral term component of the sub-feedback correction value E2 corresponding to FIG. 3A, and FIG. 3C is shown in FIGS. 3A and 3B. The change example of the corresponding sub feedback learning value E3 is shown.

  As shown in FIGS. 3B and 3C, the ECU 50 transfers the sub-feedback learning value E3 by transferring the integral term component in the sub-feedback correction value E2 to the sub-feedback learning value E3 at a predetermined update cycle. Update. The broken line in FIG. 3B indicates the integral term component of the sub feedback correction value E2 when the integral term component is not transferred to the sub feedback learned value E3 as shown in FIG. An example of a waveform is shown.

  By the way, the sub-feedback learning for calculating the sub-feedback learning value E3 is executed when the engine operating state is in a predetermined region (predetermined learning region). The predetermined region is determined by the engine rotational speed and the engine load. Here, the predetermined region is a region in which sub-feedback learning is executed so that the exhaust air-fuel ratio approaches the stoichiometry. When the engine load is high, the H2 component in the exhaust increases, which affects the output of the pre-catalyst sensor, and the detected exhaust air / fuel ratio may shift to a richer side than the actual exhaust air / fuel ratio. Therefore, it is preferable to execute sub-feedback learning when the engine load is high or when the engine rotational speed correlated with the load is high, and use the result for air-fuel ratio control in a light load operation state where the engine load is low. I can't say that. Therefore, the area where sub-feedback learning is performed is limited as such.

  Thus, since the area | region where subfeedback learning is performed is limited, subfeedback learning is difficult to be appropriately performed, so that the time which an engine driving | running state deviates from a predetermined area | region becomes long. For example, when the driver depresses the accelerator pedal strongly, the driving state determined by the engine load and the engine speed can be out of a predetermined region.

  On the other hand, for example, in some cylinders (particularly one cylinder) of all the cylinders, a failure of the injector 32 or the like may occur, and variations in the air-fuel ratio (imbalance) may occur between the cylinders. . For example, the fuel injection amount of the # 1 cylinder becomes smaller than the fuel injection amounts of the other # 2, # 3, and # 4 cylinders due to the poor valve opening of the injector 32, and the air-fuel ratio of the # 1 cylinder becomes the other # 2, # 3. , # 4 cylinder is larger than the air-fuel ratio and shifts to the lean side. At this time, the output torque in the # 1 cylinder is lower than the output torque of the other cylinders. As a result, at the same accelerator opening, that is, at the same detection load, when such an air-to-cylinder air ratio imbalance occurs, the output of the entire engine is greater than when no air-fuel ratio imbalance among cylinders occurs. Can be reduced. Therefore, in the same way as when the cylinder-to-cylinder air ratio imbalance does not occur, the driver makes the relative depression of the accelerator pedal to obtain the same desired output when the cylinder-to-cylinder air ratio imbalance occurs. If the number is increased, the engine operating state is out of the predetermined region, and the opportunity for performing sub-feedback learning can be reduced. Such a decrease in the execution opportunity of the sub-feedback learning delays the convergence of the correction value in the air-fuel ratio control, and as a result, may cause a decrease in the exhaust air-fuel ratio control performance that brings the exhaust air-fuel ratio closer to stoichiometry.

  Therefore, even when the air-fuel ratio imbalance between cylinders occurs, the predetermined region is changed so as to ensure a certain degree of sub feedback learning execution opportunities. Hereinafter, detection of a predetermined region of sub-feedback learning in the detection of inter-cylinder air-fuel ratio imbalance and air-fuel ratio feedback control will be described.

  The engine of this embodiment is equipped with a device for detecting such an inter-cylinder air-fuel ratio imbalance (an inter-cylinder air-fuel ratio imbalance detection device).

  As shown in FIG. 4, when an inter-cylinder air-fuel ratio imbalance occurs, the fluctuation of the exhaust air-fuel ratio during one engine cycle (= 720 ° CA) increases. The air-fuel ratio diagrams a, b, and c in (B) show no imbalance among cylinders, and one cylinder has a rich shift with an imbalance ratio of 20%, and only one cylinder has a rich shift with an imbalance ratio of 50%. In this case, an example of the air-fuel ratio A / F detected by the pre-catalyst sensor 42 is shown. As can be seen from FIG. 4, the amplitude of the air-fuel ratio fluctuation increases as the degree or degree of the air-fuel ratio imbalance between cylinders increases.

  Here, the imbalance rate (%) is one parameter representing the degree of variation in the air-fuel ratio between cylinders, that is, the degree of imbalance. In other words, the imbalance rate is the amount of fuel injection in a cylinder (imbalance cylinder) in which the fuel injection amount deviation occurs when only one of the cylinders has a fuel injection amount deviation. It is a value indicating whether the fuel injection amount is deviated from the fuel injection amount of the cylinder (balance cylinder) that has not caused the fuel injection amount deviation. When the imbalance rate 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 fuel injection amount is Qs, IB = (Qib−Qs) / Qs × 100. The larger the absolute value of the imbalance rate IB, the greater the difference in fuel injection amount between the imbalance cylinder and the balance cylinder, and the greater the degree of air-fuel ratio imbalance between cylinders.

  As can be understood from FIG. 4, the output fluctuation of the pre-catalyst sensor 42 increases as the imbalance rate increases, that is, as the degree of inter-cylinder air-fuel ratio imbalance increases.

  Therefore, using this characteristic, in this embodiment, the output fluctuation parameter X representing the output fluctuation degree of the pre-catalyst sensor 42 is used as a parameter representing the degree or degree of the inter-cylinder air-fuel ratio imbalance, and the output fluctuation parameter X is used. To detect.

  A method for detecting the output fluctuation parameter X will be described below. FIG. 5 is an enlarged view corresponding to the V portion in FIG. 4, and particularly shows a fluctuation of the sensor output before the catalyst within one engine cycle. As the pre-catalyst sensor output, a value obtained by converting the output voltage Vf of the pre-catalyst sensor 42 into an air-fuel ratio A / F is used. However, the output voltage Vf of the pre-catalyst sensor 42 can also be used directly.

  As shown in FIG. 5B, the ECU 50 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 (= A / Fn−A) 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). / Fn-1). This difference ΔA / Fn can be rephrased as a differential value or inclination at the current timing.

  Most simply, the difference ΔA / Fn or its magnitude (absolute value) represents the fluctuation of the sensor output before the catalyst. This is because the gradient of the air-fuel ratio diagram increases as the degree of fluctuation increases, and the absolute value of the difference ΔA / Fn increases. Therefore, the difference ΔA / Fn at a predetermined timing can be used as an output fluctuation parameter.

  However, in this embodiment, in order to improve accuracy, an average value of a plurality of differences ΔA / Fn is used as an output fluctuation parameter. In the present embodiment, the output of the pre-catalyst sensor 42 is acquired during one or a plurality of (for example, 100) engine cycles, the difference ΔA / Fn at each timing is calculated, and the difference ΔA / F is expressed as a positive sign. The absolute value of the difference ΔA / F between the same signs is integrated into those having a negative sign and those having a negative sign, each final integrated value is divided by the number of samples, and the positive sign in a predetermined engine cycle is The average value of the absolute value of the difference ΔA / Fn and the average value of the absolute value of the difference ΔA / F having a negative sign are obtained. The final two average values thus obtained are set as output fluctuation parameters X (+) and X (−). The output fluctuation parameter X (+) is a positive parameter X (+) related to the average value of the difference ΔA / F, that is, a positive value difference ΔA / F only when the pre-catalyst sensor output increases. Further, the output fluctuation parameter X (−) is a negative parameter X (−) regarding the difference ΔA / F, which is an average value of the difference ΔA / F, that is, a negative value only when the pre-catalyst sensor output decreases. . As the degree of fluctuation of the pre-catalyst sensor output increases, the output fluctuation parameter X increases at least one of the parameter X (+) and the parameter X (−).

  If only one cylinder is richly deviated, the pre-catalyst sensor output rapidly changes to the rich side (ie, suddenly decreases) when the pre-catalyst sensor 42 receives exhaust gas corresponding to the one cylinder. It is possible to use a value only on the decrease side for detecting a rich shift. Further, the lean shift can be detected by using only the increasing value.

  FIG. 6 shows the relationship between the imbalance rate IB (%) and the output fluctuation parameter X. As shown in the figure, there is a strong correlation between the imbalance rate IB and the output fluctuation parameter X, and as the absolute value of the imbalance rate IB increases (as the degree of inter-cylinder air-fuel ratio imbalance increases). The parameter X (X (+), X (−)) also increases.

  Therefore, it is possible to detect that an inter-cylinder air-fuel ratio imbalance has occurred based on the output fluctuation parameter X and whether the imbalance is due to lean deviation or rich deviation. Further, the degree of the air-fuel ratio imbalance among cylinders can be detected based on the output fluctuation parameter X (can be known).

  Here, detection of the air-fuel ratio imbalance between cylinders and correction correction of a predetermined region in sub-feedback learning at the time of detection will be described based on the flowchart of FIG. Note that the routine of FIG. 7 is executed only once after the engine is started by the ECU 50. However, the processing described below may be appropriately executed or repeated.

First, in step S701, it is determined whether or not a predetermined precondition suitable for detecting an air-fuel ratio imbalance among cylinders is satisfied. This precondition is satisfied, for example, when the following all conditions (1) to (5) are satisfied. Note that the precondition in step S701 may be, for example, only the following condition (5), specifically, only that the stoichiometric control is being executed.
Condition (1) The warm-up of the engine has been completed. The ECU 50 determines that the warm-up has ended when the water temperature detected using the water temperature sensor 56 exceeds a predetermined value (for example, 75 ° C.). Further, an oil temperature sensor for detecting the temperature of the engine oil may be provided, and it may be determined that the warm-up has ended when the detected oil temperature exceeds a predetermined value.
Condition (2) The pre-catalyst sensor 42 and the post-catalyst sensor 44 are activated. The ECU 50 determines that both sensors are activated when the impedance of both sensors is a value corresponding to a predetermined activation temperature.
Condition (3) The catalyst of the catalytic converter 40 is activated. The ECU 50 determines that the catalyst is activated when the temperature of the catalyst of the catalytic converter 40 estimated based on the engine operating state is a value corresponding to a predetermined activation temperature.
Condition (4) The engine is in steady operation. The ECU 50 determines that the engine is in steady operation when the fluctuation range of the engine speed and load within a predetermined time is within a predetermined value.
Condition (5) The normal air-fuel ratio feedback control is being executed.

  If the precondition is not satisfied, a negative determination is made in step S701, and the process returns to step S701. On the other hand, if the precondition is satisfied, an affirmative determination is made in step S701, and the pre-catalyst sensor output A / Fn is acquired in step S703. Here, the pre-catalyst sensor output A / Fn is a value obtained by converting the output voltage Vf of the pre-catalyst sensor 42 into an air-fuel ratio.

  Next, in step S705, the output variation parameter X (X (+), X (-)) is obtained as described above. In step S707, the negative parameter X (−) regarding the average value of the difference ΔA / F, that is, the negative value difference ΔA / F only when the pre-catalyst sensor output is decreased, and the pre-catalyst sensor output are The average value of the absolute value of the difference ΔA / F only when increasing, that is, the positive parameter X (+) regarding the difference ΔA / F, which is a positive value, is compared.

  For example, when only one cylinder has a rich shift, when the pre-catalyst sensor 42 receives exhaust gas corresponding to that one cylinder, the output rapidly changes to the rich side, that is, rapidly decreases, so the negative parameter X (−) This value strongly reflects the influence. Similarly, when only one cylinder is deviated lean, when the pre-catalyst sensor 42 receives exhaust gas corresponding to the one cylinder, its output rapidly changes to the lean side, that is, rapidly increases, so the positive parameter X (+) is lean. This value strongly reflects the effect of deviation.

  Therefore, since the negative parameter X (−) is larger than the positive parameter X (+), when an affirmative determination is made in step S707, the process proceeds to step S709. Then, assuming that the air-fuel ratio of the imbalance cylinder, that is, the abnormal cylinder is shifted to the rich side from the stoichiometry, the negative parameter X (−) is used for the data with the positive imbalance rate as shown in FIG. The imbalance rate IB (R) related to the rich shift is calculated by performing a search or the like.

  Conversely, if the negative parameter X (−) is not greater than the positive parameter X (+), a negative determination is made in step S707, the process proceeds to step S711. Then, assuming that the air-fuel ratio of the imbalance cylinder is shifted to the lean side with respect to the stoichiometry, the data with the negative imbalance rate as shown in FIG. 6 is searched using the positive parameter X (+), etc. By doing so, the imbalance rate IB (L) regarding the lean deviation is calculated.

  Here, when the negative parameter X (−) and the positive parameter X (+) are the same, the process is performed assuming that the air-fuel ratio of the imbalance cylinder is shifted to the lean side from the stoichiometry. Processing may be performed on the assumption that the air-fuel ratio of the balance cylinder is shifted to the rich side from the stoichiometry. However, when the air-fuel ratio imbalance between cylinders occurs, it is practically impossible that the negative parameter X (−) and the positive parameter X (+) are the same. If the negative parameter X (−) and the positive parameter X (+) are the same, they can be eliminated, and in this case, other processing that can be conceived by another person skilled in the art may be performed.

  After step S709 or S711, it is determined in the next step S713 whether or not the magnitude of the imbalance rate IB is greater than or equal to a determination threshold value (predetermined value). This determination corresponds to detecting that an air-to-cylinder air ratio imbalance has occurred, and corresponds to detecting that the degree is equal to or higher than a predetermined level. Here, an affirmative determination is made when the imbalance rate IB is -10% or less or + 10% or more. However, the predetermined value may be set to a value other than 10%, and may have a percentage different from the predetermined value for IB (L) relating to lean deviation and the predetermined value for IB (R) relating to rich deviation. Good. If a positive determination is made in step S713, the process proceeds to step S715, and if a negative determination is made, the routine ends.

  In step S715, a predetermined region for sub-feedback learning is corrected or changed. This correction or change of the predetermined area is made to relax the precondition for executing the sub-feedback learning. As described above, the predetermined area is set in advance as the specified value area, and is specifically set based on the engine speed and the load. Here, the predetermined area is changed by obtaining a correction value according to the imbalance rate IB obtained in either step S709 or step S711 and correcting the predetermined area with the obtained correction value. The correction value is determined in advance based on experiments. As a result, the air-fuel ratio feedback control is executed in the engine 10 while the sub feedback learning is executed using the predetermined region after correction.

  Here, based on FIG. 8 and FIG. 9, the correction | amendment of the predetermined area | region of sub feedback learning is further demonstrated.

  FIG. 8 schematically shows a predetermined region (basic predetermined region) A in the sub-feedback learning, and a change region S of the engine operating state when the air-fuel ratio imbalance between the cylinders exceeding a predetermined level does not occur. An example of is superimposed. In FIG. 8, x1 on the horizontal axis (engine rotation speed) corresponds to 3600 rpm, x2 corresponds to 2000 rpm, y1 on the vertical axis (load; intake charge rate of each cylinder) corresponds to 60%, and y2 Corresponds to 20%. As can be understood from FIG. 8, when the vehicle is traveling in the normal urban traveling mode when the air-fuel ratio imbalance between cylinders of a predetermined level or higher does not occur, the engine operating state is approximately the predetermined region A. It changes within. Therefore, the sub-feedback learning is sufficiently performed, and the air-fuel ratio can be suitably controlled.

  However, for example, an air-fuel ratio imbalance between cylinders of a predetermined level or higher has occurred, so that the driver can obtain the desired output as compared with the case where the air-fuel ratio imbalance between cylinders does not occur. When the amount of depression of the pedal is increased, as a result, the change region of the engine operating state can shift to the region S ′ on the higher load and higher rotation side than the region S. In this case, the number of times when the engine operating state is outside the predetermined region A increases, thereby reducing the substantial execution opportunity of sub-feedback learning and reducing the air-fuel ratio control performance. Therefore, in such a case, the process reaches step S715 as described above, and the predetermined area A is corrected and changed.

  FIG. 9 shows an example of the relationship between the imbalance rate IB and the correction value. As shown in FIG. 9, the correction value is set so that the correction value is separated from 1 as the reference value as the imbalance rate increases. Note that the correction value is only shown for a specific imbalance rate in FIG. 9, but here the correction value is related to the imbalance rate so that it linearly increases and decreases with the increase and decrease of the imbalance rate. ing. Further, FIG. 9 only shows an example when a positive imbalance rate is provided, and a correction value for the negative imbalance rate is also set separately. Such a correction value is preferably set in advance based on experiments so that the air-fuel ratio control is more suitably executed so as not to overexpand the predetermined region.

  For example, as shown in FIG. 9, when the imbalance rate is + 10%, 1.05 is set as the correction value of the engine rotation speed, and 1.1 is set as the correction value of the load. As a result, the engine speed that defines the predetermined area A is enlarged and changed by multiplying by the correction value 1.05, and the load that defines the predetermined area A is enlarged and changed by multiplying by the correction value 1.1. Specifically, x1 of FIG. 8 is changed to x3 of 3780 rpm, x2 of FIG. 8 is changed to x4 of 2100 rpm, y1 of FIG. 8 is changed to y3 of 66%, and y2 of FIG. 8 is changed to y4 of 22%. It is done. That is, as shown in FIG. 8, the predetermined area A is enlarged and changed to the predetermined area A ′ after the change.

  Thus, since the region where the sub-feedback learning is executed is increased as the imbalance rate, that is, the degree of air-fuel ratio imbalance between cylinders increases, the engine operating state change region S ′ is changed to a predetermined value after the change. The region A ′ is included. Therefore, even when the air-to-cylinder air ratio imbalance occurs, it is possible to sufficiently obtain an execution opportunity for sub-feedback learning. Therefore, it becomes possible to control the air-fuel ratio more appropriately, and it becomes possible to appropriately purify the exhaust gas.

  Since the air-fuel ratio can be controlled more appropriately even when the air-fuel ratio imbalance between cylinders occurs as described above, the detection of the air-fuel ratio imbalance between cylinders is performed by, for example, lighting an alarm. It is possible to continue to operate the vehicle and the internal combustion engine more suitably even during the period until the driver is informed and the driver repairs the vehicle or the internal combustion engine.

  Here, the case where the sub feedback learning area is enlarged and changed has been described here, but the inter-cylinder space is reduced so that the imbalance rate becomes smaller when the sub feedback learning area is already enlarged and changed. When the degree of specific imbalance changes, the predetermined area in which the sub-feedback learning control is executed may be changed to be small. When the imbalance rate is the same, the correction value when the negative imbalance rate is the same as the correction value when the positive imbalance rate is used, but preferably the positive imbalance rate is It is preferable that the correction value is larger than the correction value when it is provided. This is because when lean deviation occurs, the output is likely to decrease due to misfire or the like. Further, in the above embodiment, when the air-fuel ratio imbalance between cylinders is detected, the area where the sub-feedback learning is performed is changed from both the load and the engine speed, but the area is the load and the engine speed. It may be changed only in either one.

  Further, the predetermined area that defines the execution condition of the sub-feedback learning may be determined based on any one of them in addition to being determined based on both the engine load and the rotational speed. Further, the predetermined region in the sub-feedback learning may be determined with respect to the detected driving state or a value corresponding thereto, in other words, for detecting at least one of the engine load and the rotational speed. It can be defined for the output of one or more sensors (detectors) or a detection value based on this output.

  In the above embodiment, the detection of the air-fuel ratio imbalance between cylinders is performed based on the change in the exhaust air-fuel ratio. However, the detection of the air-fuel ratio imbalance among cylinders may be performed by other methods. For example, the fuel injection amount in each cylinder is increased or decreased by a predetermined amount, and the detection of the air-fuel ratio imbalance between cylinders based on the change in the engine output (for example, the change in the engine speed) is detected. Including) may be performed.

  The embodiment of 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.

10 Internal combustion engine
32 Injector 40 Catalytic converter 42 Sensor before catalyst (main exhaust gas sensor)
44 Post-catalyst sensor (sub exhaust gas sensor)
52 Crank angle sensor 54 Accelerator opening sensor

Claims (3)

A main exhaust gas sensor disposed upstream of the catalyst in an exhaust passage of an internal combustion engine having a plurality of cylinders;
A sub exhaust gas sensor disposed downstream of the catalyst;
Main feedback means for performing main feedback control of the air-fuel ratio based on the output of the main exhaust gas sensor;
Sub-feedback means for performing sub-feedback control for complementing the main feedback control based on the output of the sub-exhaust gas sensor;
Sub feedback correction calculated in the sub feedback control when the output of one or more sensors for detecting at least one of the load and the rotational speed of the internal combustion engine or a detection value based on the output is in a predetermined region Sub-feedback learning means for calculating a sub-feedback learning value based on the value;
Detection means for detecting an air-fuel ratio imbalance between cylinders;
A control device for an internal combustion engine, comprising: a changing unit that changes the predetermined region in the sub-feedback learning unit according to a detection result of the detecting unit.   2. The control device for an internal combustion engine according to claim 1, wherein the change unit expands the predetermined region when an air-fuel ratio imbalance among cylinders is detected by the detection unit.   3. The control device for an internal combustion engine according to claim 1, wherein the changing unit changes the predetermined area so that the predetermined area expands as the degree of the air-fuel ratio imbalance between cylinders increases.

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