JP2013007278A - Inter-cylinder air-fuel ratio dispersion abnormality detection device of multi-cylinder internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inter-cylinder air-fuel ratio dispersion abnormality detection device capable of controlling degradation of exhaust emission in a multi-cylinder internal combustion engine.SOLUTION: The inter-cylinder air-fuel ratio dispersion abnormality detection device includes: an air-fuel ratio feedback control means for controlling the air-fuel ratio to the predetermined target air-fuel ratio based on the output of an air-fuel ratio detection means; a learning control means for performing the learning control while performing the feedback control; a fuel injection amount change control means for performing the fuel injection amount change control for forcibly changing the fuel injection amount of the predetermined object cylinder by the predetermined amount; and a detection means for detecting the inter-cylinder air-fuel ratio dispersion abnormality based on the rotational fluctuation of the predetermined object cylinder before and after the change when the fuel injection amount change control is performed to the predetermined object cylinder. When the fuel injection amount change control is performed by the fuel injection amount change control means, the learning control by the learning control means is stopped. Further, the target air-fuel ratio in the air-fuel ratio feedback control is changed from the predetermined target air-fuel ratio by the amount corresponding to the change amount in the fuel injection amount of the predetermined object cylinder in the fuel injection amount change control.

Description

  The present invention relates to an apparatus for detecting a variation abnormality in an air-fuel ratio between cylinders of a multi-cylinder internal combustion engine.

  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. In order to improve the accuracy of this feedback control, air-fuel ratio learning control is performed.

  On the other hand, in a multi-cylinder internal combustion engine, since air-fuel ratio feedback control is normally performed using the same control amount for all cylinders, even if air-fuel ratio feedback control is executed, the actual air-fuel ratio may vary between cylinders. is there. 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. 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, it is required to detect an abnormal variation in air-fuel ratio between cylinders in an in-vehicle state in order to prevent the vehicle from running with deteriorated exhaust emissions (so-called OBD; On-Board Diagnostics). Recently, there is a movement to make it legally regulated.

For example, in the internal combustion engine disclosed in Patent Document 1, 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 O ₂ sensor provided on the downstream side of the purification catalyst. Based on this detection result, the sub air-fuel ratio feedback control is executed. 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. Furthermore, in the internal combustion engine of Patent Document 1, when it is determined that there is an air-fuel ratio abnormality between the cylinders, a process for reducing the fuel injection time to each cylinder by a predetermined time is executed, thereby causing a misfire. It is specified that an air-fuel ratio imbalance has occurred in the cylinder in which this occurs.

JP 2010-112244 A

  By the way, when an air-fuel ratio abnormality has occurred in any of the cylinders, if the amount of fuel injection to the cylinder is forcibly increased or decreased and changed and controlled, the rotational fluctuation of the cylinder becomes significantly large. Therefore, by detecting such an increase in rotational fluctuation, it is possible to detect an abnormality in the air-fuel ratio variation between cylinders, that is, that the air-fuel ratio between cylinders is in an imbalance state. On the other hand, when the air-fuel ratio feedback control or the air-fuel ratio learning control is executed to cause the air-fuel ratio to follow a predetermined target air-fuel ratio such as the stoichiometric air-fuel ratio while performing such fuel injection amount change control, By executing the air-fuel ratio feedback control and the air-fuel ratio learning control, the fuel injection amount value different from the normal value changed by the fuel injection amount change control is learned. As a result, when the fuel injection amount change control is finished and the routine returns to the normal air-fuel ratio feedback control or the air-fuel ratio learning control, the learning value different from the normal is reflected in the air-fuel ratio feedback control, and the exhaust emission deteriorates. May occur.

  Accordingly, the present invention has been made in view of the above circumstances, and an object thereof is to provide an inter-cylinder air-fuel ratio variation abnormality detecting device capable of suppressing deterioration of exhaust emission in a multi-cylinder internal combustion engine.

  In order to achieve the above object, an inter-cylinder air-fuel ratio variation abnormality detecting device according to an embodiment of the present invention controls an air-fuel ratio to a predetermined target air-fuel ratio based on an output of air-fuel ratio detecting means provided in an exhaust passage. Air-fuel ratio feedback control means, learning control means for learning control during execution of this feedback control, and fuel injection amount change control for forcibly changing the fuel injection quantity of a predetermined target cylinder including one or more cylinders And when the fuel injection amount change control is executed for the predetermined target cylinder, the air-fuel ratio between cylinders is based on at least the rotational fluctuation of the predetermined target cylinder after the change. In a multi-cylinder internal combustion engine having a detecting means for detecting a variation abnormality, fuel injection amount change control is executed by the fuel injection amount change control means during air-fuel ratio feedback control The time, characterized by stopping the learning control by the learning control means.

  According to this aspect, when the fuel injection amount change control by the fuel injection amount change control means is executed during the air-fuel ratio feedback control, the learning control by the learning control means is stopped, so that the fuel injection amount change control ends. Then, when returning to normal air-fuel ratio feedback control or air-fuel ratio learning control, a learning value different from normal is not reflected in the air-fuel ratio feedback control. Therefore, the deterioration of exhaust emission is suppressed.

  Here, in the above aspect, when the fuel injection amount change control by the fuel injection amount change control means is executed, the amount corresponding to the change amount of the fuel injection amount of the predetermined target cylinder in the fuel injection amount change control. Preferably, it further includes target air-fuel ratio changing means for changing the target air-fuel ratio in the air-fuel ratio feedback control from the predetermined target air-fuel ratio to the post-change target air-fuel ratio.

  According to this aspect, when the fuel injection amount change control by the fuel injection amount change control unit is executed, the target air-fuel ratio in the air-fuel ratio feedback control is set to the fuel injection amount of the predetermined target cylinder by the target air-fuel ratio change unit. The predetermined target air-fuel ratio is changed from the predetermined target air-fuel ratio to the changed target air-fuel ratio by an amount corresponding to the change amount. Therefore, it is possible to prevent air-fuel ratio fluctuations due to disturbances and the like, and as a result, it is possible to prevent erroneous detection of abnormal variation in the air-fuel ratio between cylinders and suppress deterioration in drivability.

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 flowchart which shows the main routine of air-fuel ratio feedback control. It is a calculation map of the main air-fuel ratio correction amount. It is a flowchart which shows the subroutine for the setting of an auxiliary air fuel ratio correction amount. It is a flowchart which shows the sensor output difference after a catalyst, and the mode of the integration. It is a calculation map of the auxiliary air-fuel ratio correction amount. It is a time chart for demonstrating the value showing rotation fluctuation. It is a time chart for demonstrating another value showing rotation fluctuation. 3 is a graph conceptually showing a relationship between an imbalance rate of one target cylinder and a rotational fluctuation amount. FIG. 6 is a graph obtained by enlarging a part of the characteristic line in FIG. 5, and is a graph for explaining a relationship between an increase in fuel injection amount and a change in rotational fluctuation amount before and after the increase. It is a figure explaining the relationship between the target air-fuel ratio, the fuel injection amount to each cylinder, and the voltage signals output from the pre-catalyst sensor and the post-catalyst sensor at that time. In the case of the equivalent amount Qst, in the (B) row, when the fuel injection amount of only the # 1 cylinder is increased by 40% from the stoichiometric equivalent amount Qst, in the (C) row, the fuel injection amount is uniformly 10% from (B). When feedback control is performed to correct the decrease, and (D) row shows the case where the fuel injection amount is the same as (B) row, but the target air-fuel ratio is changed, and (a) column shows each cylinder (B) column shows the target air-fuel ratio and the voltage signal output from the pre-catalyst sensor, and (c) column also shows the target air-fuel ratio and the voltage signal output from the post-catalyst sensor. It is a flowchart which shows an example of the control routine of embodiment of this invention.

  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 drawing, the engine 10 burns a fuel / air mixture in a combustion chamber 14 formed in the engine 10 including the cylinder block 12, and reciprocates the piston in the combustion chamber 14 to generate power. appear. 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. Here, 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 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. An intake passage 30 is substantially formed by the intake port, the branch pipe 18, the surge tank 20 and the intake pipe 22.

  A fuel injection valve (injector) 32 for injecting fuel into the intake passage, particularly 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.

  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. An exhaust passage 38 is substantially formed by the exhaust port, the exhaust manifold 34 and the exhaust pipe 36. A catalytic converter 40 including a three-way catalyst is attached to the exhaust pipe 36. This catalytic converter 40 forms 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, in other words, a pre-catalyst sensor 42 and a post-catalyst sensor 44 are installed on the upstream side and 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, and output signals based on the oxygen concentration in the exhaust.

  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, fuel injection amount, fuel injection, etc. Control timing, throttle opening, etc. 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.

  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. Note that the fuel injection amount by the normal fuel injection control is referred to as a reference injection amount here.

  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 figure, the pre-catalyst sensor 42 outputs a voltage signal Vf having a magnitude proportional to the exhaust air / fuel ratio (A / Ff) before the catalyst. 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 44, which is an air-fuel ratio sensor, is a so-called O ₂ sensor, and has a characteristic that the output value changes suddenly at the stoichiometric boundary. As shown in FIG. 2, the output voltage when the exhaust air-fuel ratio (A / Fr) after the catalyst 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 44 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 44 is a stoichiometric equivalent value. It becomes higher than Vrefr.

  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. This air-fuel ratio control is a main air-fuel ratio control that feedback-controls the air-fuel ratio (specifically, the fuel injection amount) of the air-fuel mixture so that the exhaust air-fuel ratio detected by the pre-catalyst sensor 42 matches a predetermined target air-fuel ratio. (Main air-fuel ratio feedback control) and feedback control of the air-fuel ratio (specifically, the fuel injection amount) of the air-fuel mixture so that the exhaust air-fuel ratio detected by the post-catalyst sensor 44 matches the predetermined target air-fuel ratio. It consists of auxiliary air-fuel ratio control (auxiliary air-fuel ratio feedback control).

  Here, the stoichiometric control will be described. FIG. 3 shows a main routine for stoichiometric control. This main routine is repeatedly executed by the ECU 50 every engine cycle (= 720 ° CA).

  First, in step S301, a basic fuel injection amount, that is, a basic injection amount Qb is calculated so that the air-fuel ratio of the in-cylinder mixture is stoichiometric. The basic injection amount Qb is calculated by, for example, an expression: Qb = Ga / 14.6 based on the intake air amount Ga detected by the air flow meter.

  In step S302, the output Vf of the pre-catalyst sensor 42 is acquired. In step S303, the difference between the sensor output Vf and the stoichiometric equivalent sensor output Vref (see FIG. 2), that is, the pre-catalyst sensor output difference ΔVf = Vf−Vref is calculated.

  In step S304, based on the pre-catalyst sensor output difference ΔVf, a main air-fuel ratio correction amount (correction coefficient) Kf is calculated from a map as shown in FIG. The pre-catalyst sensor output difference ΔVf and the main air-fuel ratio correction amount Kf form a control amount for main air-fuel ratio feedback control. For example, when the gain is Pf, it is expressed by Kf = Pf × ΔVf.

  In step S305, the auxiliary air-fuel ratio correction amount Kr set in the subroutine shown in FIG. 5 is acquired. Finally, in step S306, the final fuel injection amount to be injected from the injector 2, that is, the final injection amount Qfnl is calculated by the formula: Qfnl = Kf × Qb + Kr.

  According to the map of FIG. 4, the larger the pre-catalyst sensor output Vf is greater than the stoichiometric equivalent sensor output Vref (ΔVf> 0), that is, the greater the actual pre-catalyst air-fuel ratio is away from the stoichiometric side, the greater the correction is made to 1. The amount Kf is obtained, and the basic injection amount Qb is corrected to be increased. Conversely, the smaller the pre-catalyst sensor output Vf is smaller than the stoichiometric equivalent sensor output Vreff (ΔVf <0), that is, the more the actual pre-catalyst air-fuel ratio is further away from stoichiometric, the smaller the correction amount Kf is obtained for 1. The basic injection amount Qb is corrected to decrease. In this way, main air-fuel ratio feedback control is performed so that the air-fuel ratio detected by the pre-catalyst sensor 42 matches the stoichiometry.

  Here, the value of the final injection amount Qfnl obtained in step S306 is uniformly used for all cylinders to be controlled. That is, during one engine cycle, an amount of fuel equal to the final injection amount Qfnl is sequentially injected from the injector 2 of each cylinder, and in the next engine cycle, the fuel of the newly calculated final injection amount Qfnl is injected into the injector 2 of each cylinder. It is injected sequentially.

  As is well known, other corrections (water temperature correction, battery voltage correction, etc.) can be added when calculating the final injection amount Qfnl.

  FIG. 5 shows a subroutine for setting the auxiliary air-fuel ratio correction amount (learning control). This subroutine is repeatedly executed by the ECU 50 at a predetermined calculation cycle.

  First, in step S501, a timer provided in the ECU 50 is counted, and in step S502, the output Vr of the post-catalyst sensor 44 is acquired. In step S503, a difference between the sensor output Vr and the stoichiometric equivalent sensor output Vrefr (see FIG. 2), that is, a post-catalyst sensor output difference ΔVr = Vrefr−Vr is calculated, and the post-catalyst sensor output difference ΔVr is integrated last time. Is integrated into the value. FIG. 6 shows the post-catalyst sensor output difference ΔVr and how it is integrated.

  In step S504, it is determined whether the timer value has exceeded a predetermined value ts. If it does not exceed the predetermined value ts, the routine is once terminated.

  If it is determined in step S504 that the timer value exceeds the predetermined value ts, the process proceeds to step S505, where the post-catalyst sensor output difference integrated value ΣΔVr is updated and stored as the post-catalyst sensor learning value ΔVrg. Is done. In step S506, based on the post-catalyst sensor learning value ΔVrg, the auxiliary air-fuel ratio correction amount Kr is calculated from the map as shown in FIG. 7, and the auxiliary air-fuel ratio correction amount Kr is updated, stored, or learned. The post-catalyst sensor learning value ΔVrg and the auxiliary air-fuel ratio correction amount Kr form a control amount for auxiliary air-fuel ratio feedback control. For example, when the gain is Pr, it is expressed by Kr = Pr × ΔVrg. Finally, in step S507, the post-catalyst sensor output difference integrated value ΣΔVr and the timer are reset.

  The reason why the post-catalyst sensor output difference ΔVr is integrated for a predetermined time ts is to detect a time-average shift amount of the post-catalyst sensor output Vr with respect to the stoichiometric equivalent sensor output Vrefr. The predetermined value ts that defines the integration time is much longer than one engine cycle. Therefore, the post-catalyst sensor learning value ΔVrg and the auxiliary air-fuel ratio correction amount Kr are updated, that is, learning is performed at a cycle longer than one engine cycle. Is called.

  According to the map of FIG. 7, the post-catalyst sensor output Vr becomes zero as the time average of the post-catalyst sensor output Vrfr is smaller than the stoichiometric equivalent sensor output Vrefr (ΔVrg> 0), that is, the actual post-catalyst air-fuel ratio is further away from the stoichiometric side. On the other hand, a larger correction amount Kr is obtained, and the basic injection amount Qb is increased and corrected when the final injection amount is calculated. On the contrary, as the post-catalyst sensor output Vr is larger than the stoichiometric equivalent sensor output Vrefr on a time average (ΔVrg <0), that is, as the actual post-catalyst air-fuel ratio moves away from the stoichiometric side, the correction amount becomes smaller. Kr is obtained, and the basic injection amount Qb is corrected to decrease. Thus, the auxiliary air-fuel ratio feedback control is performed so that the post-catalyst air-fuel ratio detected by the post-catalyst sensor 44 matches the stoichiometry.

  Even if the main air-fuel ratio feedback control is executed due to reasons such as individual variations in the engine and vehicle and deterioration of the pre-catalyst sensor 42, the result may deviate from stoichiometry. Fuel ratio feedback control is executed.

  In this example, every time a new learning value ΔVrg and correction amount Kr are calculated, these values are updated by themselves. However, an averaging process such as annealing is performed to delay the update rate. Also good.

  For example, in some cylinders (especially one cylinder) of all cylinders, a failure of the injector 32 or the like may occur, and variations in air-fuel ratio (imbalance) may occur between the cylinders. For example, due to a poor valve closing of the injector 32, the fuel injection amount of the # 1 cylinder becomes larger than the fuel injection amounts of the other # 2, # 3, and # 4 cylinders, and the air-fuel ratio of the # 1 cylinder becomes the other # 2, # 3. , Is larger than the air-fuel ratio of the # 4 cylinder and shifts to the rich side.

  Even at this time, if a relatively large correction amount is given by the above-described air-fuel ratio feedback control, the air-fuel ratio of the total gas (exhaust gas after merging) supplied to the pre-catalyst sensor 42 may sometimes be stoichiometrically controlled. However, looking at each cylinder, # 1 cylinder is richer than stoichiometric and # 2, # 3, # 4 cylinders are leaner than stoichiometric, and the overall balance is only stoichiometric. Obviously, this is not preferred. In view of this, the present embodiment is equipped with a device that detects such a variation in air-fuel ratio between cylinders.

  Here, a value that is an imbalance rate is used as an index value that represents the degree of variation in the air-fuel ratio between cylinders. The imbalance rate is the amount of fuel injection in a cylinder (imbalance cylinder) causing the fuel injection amount deviation when only one of the plurality of cylinders is causing the fuel injection amount deviation. Is a value indicating whether or not the fuel injection amount of the cylinder (balance cylinder) that has not caused the fuel injection amount deviation, that is, the reference injection amount. When the imbalance 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 injection amount is Qs, IB = (Qib−Qs) / Qs × 100. The greater the imbalance rate IB, the greater the fuel injection amount deviation between the imbalance cylinder and the balance cylinder, and the greater the air-fuel ratio variation.

  In the present embodiment, the fuel injection amount of a predetermined target cylinder is actively increased, that is, forcibly increased or decreased, and at least based on the rotation fluctuation as the output fluctuation of the target cylinder after the fuel injection quantity is increased or decreased, Detect variation anomalies.

  Here, this rotational fluctuation will be described. The rotational fluctuation refers to a change in engine rotational speed or crankshaft rotational speed. FIG. 8 shows a time chart as an example for explaining the rotation fluctuation. The illustrated example is an example of an in-line four-cylinder engine, and the ignition order is the order of # 1, # 3, # 4, and # 2 cylinders. In FIG. 8, (A) shows the crank angle (° CA) of the engine. One engine cycle is 720 (° CA), and the crank angle for a plurality of cycles detected sequentially is shown in a sawtooth shape in the figure. FIG. 8B shows the time required for the crankshaft to rotate by a predetermined angle, that is, the rotation time T (s). Here, the predetermined angle is 30 (° CA), but may be another value (for example, 10 (° CA)). The longer the rotation time T (upward in the figure), the slower the engine rotation speed. Conversely, the shorter the rotation time T, the faster the engine rotation speed. The rotation time T is detected by the ECU 50 based on the output of the crank angle sensor 52. FIG. 8C shows a rotation time difference ΔT described later. In the figure, “normal” indicates a normal case in which no air-fuel ratio shift has occurred in any cylinder, and “lean shift abnormality” indicates that, for example, the imbalance rate IB = −30% is applied only to the # 1 cylinder. An abnormal case in which a lean shift occurs is shown. The lean deviation abnormality can be caused by, for example, clogging of an injector nozzle hole or a poor valve opening.

  First, the rotation time T at the same timing of each cylinder is detected by the ECU. Here, the rotation time T at the timing of compression top dead center (TDC) of each cylinder is detected. The timing at which the rotation time T is detected is referred to as detection timing.

  Next, at each detection timing, the ECU calculates a difference (T2−T1) between the rotation time T2 at the detection timing and the rotation time T1 at the immediately preceding detection timing. This difference is the rotation time difference ΔT shown in FIG. 8C, and ΔT = T2−T1.

  Usually, in the combustion stroke after the crank angle exceeds TDC, the rotational speed increases, so the rotational time T decreases. In the subsequent compression stroke, the rotational speed decreases, and the rotational time T increases.

However, as shown in FIG. 8B, when the # 1 cylinder has an abnormal lean shift, even if the # 1 cylinder is ignited, a sufficient torque (output) cannot be obtained and the rotational speed is difficult to increase. Thus, the rotation time T in the # 3 cylinder TDC is long. Therefore, the rotation time difference ΔT in the # 3 cylinder TDC is a large positive value as shown in FIG. The rotation time and rotation time difference in the # 3 cylinder TDC are defined as the rotation time and rotation time difference of the # 1 cylinder, respectively, and are represented by T ₁ and ΔT ₁ , respectively. The same applies to the other cylinders.

Next, since the # 3 cylinder is normal, when the # 3 cylinder is ignited, the rotational speed increases sharply. As a result, at the timing of the next # 4 cylinder TDC, the rotation time T is only slightly reduced compared to that of the # 3 cylinder TDC. Therefore, the rotation time difference ΔT ₃ of the # 3 cylinder detected in the # 4 cylinder TDC is a small negative value as shown in FIG. Thus, the rotation time difference ΔT of a certain cylinder is detected for each ignition cylinder TDC.

In the subsequent # 2 cylinder TDC and # 1 cylinder TDC, the same tendency as in the case of the # 4 cylinder TDC is observed, and the rotation time difference ΔT ₄ of the # ₄ cylinder and the rotation time difference ΔT ₂ of the # 2 cylinder detected at both timings. Both are small negative values. The above characteristics are repeated every engine cycle.

  Thus, it can be seen that the rotation time difference ΔT of each cylinder is a value representing the rotation fluctuation of each cylinder, and is a value correlated with the air-fuel ratio deviation amount of each cylinder. Therefore, the rotation time difference ΔT of each cylinder can be used as an index value of the rotation fluctuation of each cylinder, that is, the rotation fluctuation amount. As the air-fuel ratio deviation amount of each cylinder increases, the rotational fluctuation of each cylinder increases and the rotation time difference ΔT of each cylinder increases. On the other hand, as shown in FIG. 8C, in the normal case, the rotation time difference ΔT is always near zero.

  In the example of FIG. 8, the case of the lean deviation abnormality is shown, but the same tendency also occurs when the reverse rich deviation abnormality, that is, when a large rich deviation occurs in only one cylinder. This is because when a large rich shift occurs, combustion is insufficient due to excessive fuel even when ignited, and sufficient torque cannot be obtained, resulting in large rotational fluctuations.

  Next, an example of another value representing the rotation fluctuation, that is, another rotation fluctuation amount will be described with reference to FIG. FIG. 9 (A) shows the crank angle (° CA) of the engine as in FIG. 8 (A). FIG. 9B shows an angular velocity ω (rad / s) that is the reciprocal of the rotation time T. ω = 1 / T. As a matter of course, the larger the angular velocity ω, the faster the engine rotational speed, and the smaller the angular velocity ω, the slower the engine rotational speed. The waveform of the angular velocity ω has a shape obtained by vertically inverting the waveform of the rotation time T. FIG. 9C shows an angular velocity difference Δω that is a difference in angular velocity ω, similarly to the rotation time difference ΔT. The waveform of the angular velocity difference Δω also has a shape obtained by vertically inverting the waveform of the rotation time difference ΔT. “Normal” and “lean deviation abnormality” in the figure are the same as those in FIG.

  First, the angular velocity ω at the same timing of each cylinder is detected by the ECU. Again, the angular velocity ω at the timing of compression top dead center (TDC) of each cylinder is detected. The angular velocity ω is calculated by dividing 1 by the rotation time T.

  Next, at each detection timing, the ECU calculates a difference (ω2−ω1) between the angular velocity ω2 at the detection timing and the angular velocity ω1 at the immediately preceding detection timing. This difference is an angular velocity difference Δω shown in FIG. 9C, and Δω = ω2−ω1.

  Normally, the rotational speed increases in the combustion stroke after the crank angle exceeds TDC, so the angular speed ω increases. In the subsequent compression stroke, the rotational speed decreases, and the angular speed ω decreases.

However, as shown in FIG. 9B, when the # 1 cylinder has an abnormal lean shift, even if the # 1 cylinder is ignited, a sufficient torque cannot be obtained and the rotational speed is difficult to increase. The angular velocity ω in the cylinder TDC is small. Therefore, the angular velocity difference Δω in the # 3 cylinder TDC is a large negative value as shown in FIG. The angular velocity and the angular velocity difference in the # 3 cylinder TDC are the angular velocity and the angular velocity difference of the # 1 cylinder, respectively, and are represented by ω ₁ and Δω ₁ , respectively. The same applies to the other cylinders.

Next, since the # 3 cylinder is normal, when the # 3 cylinder is ignited, the rotational speed increases sharply. As a result, at the timing of the next # 4 cylinder TDC, the angular velocity ω is only slightly increased compared to that of the # 3 cylinder TDC. Therefore, the angular velocity difference Δω ₃ of the # 3 cylinder detected in the # 4 cylinder TDC is a small positive value as shown in FIG. 9C. Thus, the angular velocity difference Δω of a certain cylinder is detected for each subsequent ignition cylinder TDC.

Subsequent # 2 cylinder TDC and # 1 cylinder also seen the same tendency as in the case of the fourth cylinder TDC at TDC, the angular velocity difference [Delta] [omega ₄ and # 2 cylinder of the detected # 4 cylinder in both timing angular difference [Delta] [omega ₂ Both are small positive values. The above characteristics are repeated every engine cycle.

  Thus, it can be seen that the angular velocity difference Δω of each cylinder is a value representing the rotational fluctuation of each cylinder and is a value correlated with the air-fuel ratio deviation amount of each cylinder. Therefore, the angular velocity difference Δω of each cylinder can be used as an index value of the rotation fluctuation of each cylinder. As the air-fuel ratio deviation amount of each cylinder increases, the rotational fluctuation of each cylinder increases, and the angular velocity difference Δω of each cylinder decreases (increases in the minus direction).

  On the other hand, as shown in FIG. 9C, when normal, the angular velocity difference Δω is always near zero. As described above, there is a similar tendency in the case of reverse rich shift abnormality.

  Next, referring to the conceptual diagram of FIG. 10, the change in the rotational fluctuation amount when the fuel injection amount of a certain cylinder is actively increased, that is, forcibly increased or decreased to change the air-fuel ratio in the cylinder. explain. However, in this case, when the fuel injection amount is actively increased or decreased, the operation of the throttle valve 28 and the like is controlled so that the intake air amount does not change.

  In FIG. 10, the horizontal axis represents the imbalance rate IB, and the vertical axis represents the amount of rotational fluctuation. Here, the imbalance rate IB of only one cylinder among all four cylinders is changed by increasing or decreasing the fuel injection amount. At this time, the imbalance rate IB of the one cylinder and the rotational fluctuation amount of the one cylinder are changed. Is shown according to line L1. The one cylinder is referred to as an active target cylinder. All the other cylinders are balance cylinders, and the stoichiometric equivalent amount is injected as the reference injection amount Qs.

  In FIG. 10, the imbalance rate IB increases in the positive direction as the imbalance rate corresponding to the stoichiometric equivalent amount of fuel injection moves to the left from the line S of 0%. The amount is excessive, that is, a rich state. On the contrary, in FIG. 10, the imbalance rate IB increases (decreases) in the negative direction as the imbalance rate IB moves to the right side from the 0% line S, and the fuel injection amount is too small, that is, in a lean state. Become. Also, the amount of fluctuation in rotation increases as the position moves upward in FIG.

  As can be understood from the characteristic line L1, even if the imbalance rate IB of the active target cylinder increases from 0% in the positive direction or increases in the negative direction, the rotational fluctuation amount of the active target cylinder tends to increase. . As the imbalance rate IB is further away from 0%, the slope of the characteristic line L1 becomes steeper, and the change amount or change rate of the rotational fluctuation amount with respect to the change amount or change rate of the imbalance rate IB tends to increase.

  Here, FIG. 11 is an enlarged view of a part of the range where the imbalance rate IB is positive in FIG. Note that the line L2 in FIG. 11 corresponds to a part of the line L1 in FIG.

  In FIG. 11, examples of two imbalance rates IB in the active target cylinder are represented by lines A and B. The imbalance rate IBa in the line A is an example of a value that is deviated in the plus direction from the 0% imbalance rate (see the line S in FIG. 10), which is a stoichiometric equivalent value, but within an allowable range. On the other hand, the imbalance rate IBb in the line B is an example of a value outside the allowable range that is shifted in a direction where the fuel injection amount is larger than the imbalance rate IBa in the line A.

  Here, a case where the state of the active target cylinder when the stoichiometric control is performed during the normal operation is a state on the line A will be considered. At this time, it is assumed that the fuel injection amount of the active target cylinder is forcibly increased by a predetermined amount Δf1, as indicated by an arrow F1. Although the predetermined amount Δf1 can be arbitrarily set, for example, the fuel injection amount is increased by about 45% as an imbalance rate. In the vicinity of IB = 0% (the right end side in FIG. 11), the slope of the characteristic line L2 is gentle. Therefore, when the state of the active target cylinder during the stoichiometric control is the state on the line A, the fuel The rotational fluctuation amount Va ′ in the state on the line A ′ when the injection amount is increased is not significantly different from the rotational fluctuation amount Va before the increase.

  On the other hand, a case where the state of the active target cylinder when performing the stoichiometric control is already on the line B will be considered. At this time, a rich shift exceeding the allowable range has already occurred in the active target cylinder, and the imbalance rate IBb is a relatively large positive value. For example, the imbalance rate IBb on the line B corresponds to a rich shift of about 60% in terms of the imbalance rate. As indicated by an arrow F2 from this state, if the fuel injection amount of the active target cylinder is forcibly increased by the same predetermined amount Δf1, the characteristic line in the region including the line B ′ when the fuel injection amount is increased is changed. Since the inclination of L2 is steep, the rotation fluctuation amount Vb ′ after the increase is considerably larger than the rotation fluctuation amount Vb before the increase, and the difference (Vb′−Vb) between the rotation fluctuation amounts before and after the increase is large. That is, the increase in the fuel injection amount increases the rotational fluctuation of the active target cylinder sufficiently.

  Accordingly, it is possible to detect a variation abnormality based on at least the rotational fluctuation amount of the active target cylinder after the increase when the fuel injection amount of the active target cylinder is forcibly changed by a predetermined amount. For example, when the magnitude of the rotational fluctuation amount after the increase (for example, | Vb ′ |) is larger than a predetermined amount, it can be determined that there is a variation abnormality. Furthermore, whether there is an abnormality in the air-to-cylinder air-fuel ratio variation by comparing the average value of the rotation fluctuation amount obtained for the active target cylinder for a plurality of cycles or the value obtained by statistical processing with a predetermined amount as the rotation fluctuation amount. It may be determined whether or not. As described above, when there is an abnormality in the air-fuel ratio variation between cylinders due to the increase in the fuel injection amount, it is reflected in the combustion state of the fuel in the combustion chamber, that is, the air-fuel mixture, and the result is detected as the rotational fluctuation amount. Thus, it is possible to detect an abnormality in variation in the air-fuel ratio between cylinders based on the rotation fluctuation amount.

  In the above description, control for forcibly changing the fuel injection amount by a predetermined amount (fuel injection amount increase control) is performed to detect an abnormality in the air-fuel ratio variation between cylinders. This is effective when the imbalance cylinder is shifted to the side where the fuel injection amount is large.

  On the other hand, when the fuel injection amount is shifted to the small side in the imbalance cylinder, control is performed to forcibly change the fuel injection amount by a predetermined amount Δf2 (fuel injection amount reduction control) to detect variation abnormality. It is effective to do. The case where forced reduction is performed in a region where the imbalance rate is negative can also be understood from the above case, and thus the description thereof is omitted. However, the amount of reduction (size) Δf2 in the fuel injection amount reduction control is preferably smaller than the amount of increase (size) Δf1 in the fuel injection amount increase control. This is because if too much weight reduction is performed on the lean deviation abnormal cylinder, there is a risk of misfire. However, the present invention does not exclude detecting a variation abnormality based on output fluctuation at that time by causing a misfire by reducing (or increasing) the fuel injection amount. Although the predetermined amount Δf2 can be set arbitrarily, for example, the fuel injection amount can be reduced by an imbalance rate equivalent to about 15%. The above-mentioned predetermined value, which is a threshold value for detecting an abnormality in variation in air-fuel ratio between cylinders by executing fuel injection amount increase control, and detecting an abnormality in air-fuel ratio variation between cylinders by executing fuel injection amount reduction control. The predetermined value, which is a threshold value, may be the same or different.

  The fuel injection amount increase control and the decrease control can be applied to all cylinders uniformly and simultaneously. In this case, the predetermined target cylinder is all cylinders. However, the fuel injection amount change control is applied to only a predetermined target cylinder that is a part of the cylinders at a time, and even if the target cylinder to which the fuel injection amount change control is applied is sequentially transferred to another cylinder. Good.

  As described above, in order to detect an abnormality in the air-fuel ratio variation between cylinders, as described above, the control for forcibly changing the fuel injection amount or changing the fuel injection amount, that is, the fuel injection amount change control is performed to change the rotational fluctuation amount. It is effective to increase. On the other hand, when the air-fuel ratio feedback control or the air-fuel ratio learning control for causing the air-fuel ratio to follow a predetermined target air-fuel ratio such as stoichiometry is executed simultaneously with the fuel injection amount change control, the fuel injection amount change control is changed. The air-fuel ratio feedback control including the fuel injection amount is performed, and the value is learned. As a result, as will be described later, it has been found that a variation abnormality cannot be correctly detected. Further, when the fuel injection amount change control is finished and the routine returns to the normal air-fuel ratio feedback control or the air-fuel ratio learning control, the learned value learned during the fuel injection amount change control different from the normal time is the normal air-fuel ratio. It has been found that exhaust emission is reflected in feedback control.

  Hereinafter, these points will be described with reference to FIG. FIG. 12 is a diagram illustrating the relationship among the target air-fuel ratio, the fuel injection amount to each cylinder, and the voltage signals output from the pre-catalyst sensor 42 and the post-catalyst sensor 44. In FIG. 12, (A) row shows the case where the fuel injection amount of all the cylinders is the stoichiometric equivalent amount Qst, and (B) row shows the case where the fuel injection amount is changed by 40% from the stoichiometric equivalent amount Qst only for the # 1 cylinder. Line (C) is feedback controlled so that the fuel injection amount is uniformly reduced by 10% from (B), and line (D) is the same as line (B), but the target empty (B) column is the target air-fuel ratio and the voltage signal output from the pre-catalyst sensor 42, and (c) column is the target empty. The fuel ratio and the voltage signal output from the post-catalyst sensor 44 are shown. Then, in columns (b) and (c) of FIG. 12, the target air-fuel ratio is indicated by a broken line, and the voltage signal is indicated by a solid line.

  Here, (A) and (A) in FIG. 12 show the fuel injection amount of each cylinder when the fuel injection amount of each cylinder is normal before the occurrence of abnormality. In this case, the target air-fuel ratio indicated by the broken line is stoichiometric. As a result of the stoichiometric control, the fuel injection amount of each cylinder is the stoichiometric equivalent amount Qst. This stoichiometric amount Qst is the above-described reference injection amount Qs, and the imbalance rate of the # 1, # 2, # 3, and # 4 cylinders is ± 0% as shown in the figure. In this case, the output voltage Vf of the pre-catalyst sensor 42 is substantially equal to the stoichiometric equivalent value Vreff, and the output voltage Vr of the post-catalyst sensor 44 is substantially equal to the stoichiometric equivalent value Vrefr.

  Next, FIGS. 12B and 12A show the fuel injection amounts of the respective cylinders when the fuel injection amount is forcibly changed by a predetermined amount to the target cylinder. In this case, the fuel injection amount of the # 1 cylinder is 40% larger than the stoichiometric equivalent amount Qst, and the fuel injection amounts of the other # 2, # 3, and # 4 cylinders are the stoichiometric equivalent amount Qst. For example, as shown in the figure, the imbalance rate of the # 1 cylinder is + 40%, and the imbalance rates of the # 2, # 3, and # 4 cylinders are ± 0%.

  In this state, since the air-fuel ratio of the total gas (exhaust gas for all cylinders) supplied to the pre-catalyst sensor 42 is in a rich state, the output voltage Vf of the pre-catalyst sensor 42 is 10% richer than the stoichiometric equivalent value Vref. Similarly, the output voltage Vr of the post-catalyst sensor 44 is also richer than the stoichiometric equivalent value Vrefr (see Vrefr in FIGS. 12B and 12C). Reference). That is, even if a rich displacement of 40% occurs only in the # 1 cylinder, the influence on the pre-catalyst sensor 42 is 10% obtained by dividing 40% by the total number of cylinders 4, and the rich displacement of 10% from the pre-catalyst sensor 42 Information that it has occurred is obtained. Based on this information, the ECU 50 performs feedback control so that the fuel injection amounts of all the cylinders are uniformly reduced by 10% by stoichiometric control.

  When a certain amount of time has elapsed since the start of the reduction correction, the fuel injection amount of each cylinder changes as shown in FIGS. 12C and 12A, and the fuel injection amount of the # 1 cylinder is compared to the stoichiometric equivalent amount Qst. The fuel injection amount of the other cylinders # 2, # 3, and # 4 is in a lean state that is substantially 10% less than the stoichiometric equivalent amount Qst. In other words, as shown in the figure, the imbalance rate of the # 1 cylinder is + 30%, and the imbalance rates of the # 2, # 3, and # 4 cylinders are -10%. The output voltage Vf of the pre-catalyst sensor 42 returns to the stoichiometric equivalent value Vreff, and the output voltage Vr of the post-catalyst sensor 44 returns to the stoichiometric equivalent value Vrefr (Vf and Vr in FIGS. 12C, 12B, and 12C). reference).

  As described above, in order to detect the variation in the air-fuel ratio between the cylinders, the air-fuel ratio feedback control with the target air-fuel ratio as stoichiometric is performed even if the target cylinder is controlled to forcibly change the fuel injection amount by a predetermined amount. As long as it continues, the fuel injection amount for the other cylinders including the target cylinder is not maintained as desired (that is, not maintained in the states (B) and (b) in FIG. 12), and as a result It is not possible to correctly detect abnormality in air-fuel ratio variation between cylinders.

  Therefore, in the air-fuel ratio feedback control at this time, the amount corresponding to the change amount of the fuel injection amount of the predetermined target cylinder in the fuel injection amount change control from the predetermined target air-fuel ratio, which is stoichiometric here, is shown in FIG. As shown in (b), the target air-fuel ratio is changed. That is, the target air-fuel ratio is made 10% richer than stoichiometric. As a result, the fuel injection amount can be maintained in a desired state.

  By the way, even if the target air-fuel ratio is changed, the fuel injection amounts in FIGS. 12D and 12A are the same as those in FIGS. 12B and 12A, including the target cylinder and other cylinders. Yes, as described above, the output voltage Vf of the pre-catalyst sensor 42 is 10% richer than the stoichiometric equivalent value Vref, and similarly, the output voltage Vr of the post-catalyst sensor 44 is 10% richer than the stoichiometric equivalent value Vrefr. (See Vf and Vr in FIGS. 12D, 12B, and 12C).

  As a result, post-catalyst sensor output difference ΔVr (= Vrefr−Vr) information indicating that a 10% rich shift has occurred is obtained from the post-catalyst sensor 44, and the ECU 50 is obtained based on the integrated value as described above. The post-catalyst sensor learning value ΔVrg is updated and stored, that is, learned. Therefore, when the fuel injection amount change control is completed and the routine returns to the normal air-fuel ratio feedback control or the air-fuel ratio learning control, such a learning value different from the normal time is reflected in the air-fuel ratio feedback control, and the exhaust gas Deterioration of emissions may occur.

  Therefore, in the embodiment described below, it is possible to perform forced increase or decrease control of the fuel injection amount (hereinafter referred to as active fuel injection control) in order to detect the above-described abnormal variation in the air-fuel ratio between cylinders. Thus, the learning control is stopped or prohibited so that the exhaust emission does not deteriorate when returning to the normal control.

  Here, an example of the control routine will be described with reference to the flowchart of FIG. This control routine is repeatedly executed by the ECU 50 at predetermined intervals. That is, when the control is started, it is determined in step S1301 whether or not a predetermined condition for executing the active fuel injection control is satisfied. Here, as the predetermined condition, a predetermined operation state after the engine is started, for example, the engine coolant temperature is not less than a predetermined temperature (for example, 70 ° C.), and the load (for example, intake air amount) is in a predetermined range (for example, 15 It may be within a range of ˜50 g / s) or satisfy all of that the engine rotation speed is in a predetermined speed range (for example, 1500 rpm to 2000 rpm).

  If the result of determination in step S1301 is that the active fuel injection control condition is not satisfied, this control routine is temporarily terminated. When the condition is satisfied (YES), the process proceeds to step S1302, and it is determined whether or not the active fuel injection control is being executed. When the active fuel injection control is not being executed, the process proceeds to step S1303, and when it is being executed, the process proceeds to step S1304.

  In step S1303, which proceeds when the active fuel injection control is not being executed, normally, in order to perform exhaust gas purification in the catalytic converter 40 more appropriately as described above, the stoichiometry is performed so that the exhaust air-fuel ratio follows the stoichiometry. As the target air-fuel ratio, air-fuel ratio feedback control is executed, and the above-described air-fuel ratio learning control is executed.

  Note that in the active fuel injection control that is executed or is executed in step S1302, the fuel injection amount that is increased or decreased as described above is calculated. The calculation of the change amount is executed by searching for data for changing the fuel injection amount stored in advance in the storage device based on the engine speed and the engine load. A predetermined calculation may be performed based on a predetermined calculation formula. For example, a change amount for increase such as 40% or 45% is calculated as the change amount. It should be noted that the amount of change may be constant instead of being variable. Then, the fuel injection amount of the target cylinder is changed based on the change amount. For example, when the target cylinder is the # 1 cylinder, the calculated change amount is added to the fuel injection amount calculated for the basic control, that is, the normal control of the cylinder, and thereby the fuel injection amount change control is performed. The fuel injection amount at is determined. For example, when 40% is set as the change amount, the changed fuel injection amount is 140% of the reference injection amount. Here, the reference injection amount is a stoichiometric amount.

  In step S1304, which is advanced when active fuel injection control is being executed, the target air-fuel ratio is changed from stoichiometric, air-fuel ratio feedback control is executed, and learning control as described above is stopped or prohibited. Is done. In other words, the change of the target air-fuel ratio is performed in correspondence with the change amount of the fuel injection amount of the predetermined target cylinder in the active fuel injection control that is determined to be being executed in step S1302. For example, when the target cylinder is the # 1 cylinder and the fuel injection amount to the target cylinder is increased and changed by a predetermined ratio (40% in the present embodiment), the change ratio is increased by 40%. The control center in the air-fuel ratio feedback control, that is, the target air-fuel ratio is changed (shifted) by an average ratio (that is, 10% in this embodiment) obtained by dividing the number by the total number of cylinders (4 in this embodiment). Is done by. Specifically, assuming that the stoichiometric air-fuel ratio is 14.6, in this case, the changed target air-fuel ratio is calculated as an air-fuel ratio that is 10% richer than the stoichiometric air-fuel ratio (that is, 13.1).

  In the feedback control to the target air-fuel ratio after the change (for example, the air-fuel ratio richer than the stoichiometric air-fuel ratio), the output Vf of the pre-catalyst sensor 42 and the air-fuel ratio after the change (that is, 13.1) Is calculated, that is, the pre-catalyst sensor output difference ΔVf = Vf−Vref ′. Further, as described above, the main air-fuel ratio correction amount (correction coefficient) Kf is calculated from the map as shown in FIG. 4 based on the pre-catalyst sensor output difference ΔVf, and the final fuel to be injected from the injector 2. The injection amount, that is, the final injection amount Qfnl is obtained. However, the feedback control to the target air-fuel ratio after the change is a feedback control to the post-change target air-fuel ratio changed from the predetermined target air-fuel ratio by an amount corresponding to the change in the fuel injection amount of the predetermined target cylinder. Therefore, the fuel injection amount surely includes the change amount for the predetermined target cylinder, and the other cylinders are prevented from becoming lean as described above.

  In particular, during the feedback control to the target air-fuel ratio after this change, the learning control based on the output Vr of the post-catalyst sensor 44 as described above is prohibited. In this case, the post-catalyst sensor learning value ΔVrg and the auxiliary air-fuel ratio correction amount Kr are not updated and are maintained at the values at the start of the fuel injection amount change control. Therefore, when returning to the normal control after the fuel injection amount change control, the feedback control is started from the values of the post-catalyst sensor learning value ΔVrg and the auxiliary air-fuel ratio correction amount Kr at the start of the fuel injection amount change control, and the exhaust emission deteriorates. Is prevented. In the above embodiment, the target air-fuel ratio of the air-fuel ratio feedback control is changed (shifted) according to the change amount substantially simultaneously with the start of changing the fuel injection amount by the fuel injection amount change control. However, the target air-fuel ratio of the air-fuel ratio feedback control may be changed after a predetermined period has elapsed after starting to change the fuel injection amount by the fuel injection amount change control. This is because the fuel injection amount of each cylinder may be maintained in a desired state before the predetermined period has elapsed.

  Although the present invention has been described based on the above embodiment, the present invention can be applied to a multi-cylinder engine having two or more cylinders of various types, and not only a port injection type engine but also an in-cylinder injection. The present invention can also be applied to a type engine, an engine using gas as a fuel, and the like. Further, the learning control is not limited to learning the post-catalyst sensor learning value ΔVrg and the auxiliary air-fuel ratio correction amount Kr described in the above embodiment, and may be a learning value for feedback control widely.

10 Internal combustion engine
32 Injector 40 Catalytic converter 42 Pre-catalyst sensor 44 Post-catalyst sensor 52 Crank angle sensor 54 Accelerator opening sensor

Claims (2)

Air-fuel ratio feedback control means for controlling the air-fuel ratio to a predetermined target air-fuel ratio based on the output of the air-fuel ratio detection means provided in the exhaust passage;
Learning control means for performing learning control during execution of the air-fuel ratio feedback control;
Fuel injection amount change control means for executing fuel injection amount change control for forcibly changing the fuel injection amount of a predetermined target cylinder including one or more cylinders by a predetermined amount;
Detecting means for detecting an abnormality in the air-fuel ratio variation between cylinders based on at least the rotational fluctuation of the predetermined target cylinder after the change when the fuel injection amount change control is executed for the predetermined target cylinder;
In a multi-cylinder internal combustion engine comprising
When the fuel injection amount change control by the fuel injection amount change control means is executed during the air-fuel ratio feedback control, the learning control by the learning control means is stopped. Anomaly detection device.   When the fuel injection amount change control by the fuel injection amount change control means is executed, an amount corresponding to the amount of change in the fuel injection amount of the predetermined target cylinder in the fuel injection amount change control is the amount in the air-fuel ratio feedback control. 2. The inter-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine according to claim 1, further comprising target air-fuel ratio changing means for changing the target air-fuel ratio from the predetermined target air-fuel ratio to a post-change target air-fuel ratio. .

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