JP2009074388A - Cylinder air-fuel ratio variation abnormality detecting device for multi-cylinder internal combustion engine - Google Patents

Abstract

Dispersion abnormality of an air-fuel ratio between cylinders is preferably detected.
A first air-fuel ratio sensor and a second air-fuel ratio sensor are provided on the upstream and downstream sides of a catalyst element capable of purifying hydrogen, respectively, and main air-fuel ratio control is executed based on the output of the first air-fuel ratio sensor. The auxiliary air-fuel ratio control is executed based on the second air-fuel ratio sensor output. When the control amount ΔVrg for the auxiliary air-fuel ratio control is updated at a predetermined speed and the control amount reaches a predetermined abnormality determination value ΔVrgs, it is determined that an abnormality in the air-fuel ratio variation between cylinders has occurred. When an abnormality is detected, the guard range ΔVrgH is expanded to include the abnormality determination value, and the update amount of the control amount is increased. As a result, interference between the guard range and the abnormality determination value can be prevented and the detection time can be prevented from prolonging.
[Selection] Figure 11

Description

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

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

  On the other hand, in a multi-cylinder internal combustion engine, air-fuel ratio control is normally performed using the same control amount for all cylinders. Therefore, even if air-fuel ratio control is executed, the actual air-fuel ratio may vary between cylinders. If the degree of variation is small at this time, it can be absorbed by air-fuel ratio feedback control, and harmful components in the exhaust gas can be purified by the catalyst, so that exhaust emissions are not affected and there is no particular problem. However, for example, if the fuel injection system of some cylinders breaks down and the air-fuel ratio between the cylinders varies greatly, exhaust emission deteriorates, which becomes a problem. It is desirable to detect such a large air-fuel ratio variation that deteriorates the exhaust emission as an abnormality. In particular, in the case of an internal combustion engine for automobiles, in order to prevent a vehicle running with deteriorated exhaust emissions from occurring, it is required to detect an abnormal variation in air-fuel ratio between cylinders in an on-board state. There is also a movement to regulate the law.

  Patent Document 1 discloses an apparatus for diagnosing a failure of a fuel supply system when an air-fuel ratio feedback correction coefficient in air-fuel ratio feedback control is a predetermined value or more.

JP-A-4-318250

  However, in the apparatus described in Patent Document 1, it is possible to diagnose that there is any failure in the entire fuel supply system, but there is a variation abnormality in which any one of the cylinders causes an air-fuel ratio shift with respect to the other cylinders. Cannot be detected.

  Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to provide an inter-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine that can suitably detect an abnormality in variation between cylinders. It is to provide.

According to the present invention,
A catalytic element that is disposed in an exhaust passage of a multi-cylinder internal combustion engine and oxidizes and purifies at least hydrogen contained in the exhaust;
A first air-fuel ratio sensor that is disposed in an exhaust passage upstream of the catalyst element and detects a first exhaust air-fuel ratio that is an air-fuel ratio of exhaust that does not pass through the catalyst element;
A second air-fuel ratio sensor that is disposed in an exhaust passage downstream of the catalyst element and detects a second exhaust air-fuel ratio that is an air-fuel ratio of exhaust that has passed through the catalyst element;
The main air-fuel ratio control for matching the detected value of the first exhaust air-fuel ratio with a predetermined first target air-fuel ratio and the detected value of the second exhaust air-fuel ratio match with a predetermined second target air-fuel ratio Air-fuel ratio control means for executing such auxiliary air-fuel ratio control, wherein the control amount for the auxiliary air-fuel ratio control is updated at a predetermined update speed based on the output of the second air-fuel ratio sensor Means,
A variation abnormality detecting means for detecting the occurrence of an inter-cylinder air-fuel ratio variation abnormality when the control amount reaches a predetermined abnormality determination value that corrects the second exhaust air-fuel ratio to a richer side;
The air-fuel ratio control means executes the auxiliary air-fuel ratio control while keeping the control amount within a predetermined guard range, and the detection of the abnormality in the air-fuel ratio variation between cylinders by the variation abnormality detection means is performed. An apparatus for detecting an abnormality in an air-fuel ratio variation between cylinders of a multi-cylinder internal combustion engine, wherein at least one of expanding a range so as to include at least the abnormality determination value and increasing an update speed of the control amount is executed. Is provided.

  When the air-fuel ratio shifts to the rich side in some cylinders, the amount of hydrogen in the exhaust tends to increase extremely. On the other hand, when exhaust gas containing hydrogen passes through the catalyst element, hydrogen is oxidized and purified. Therefore, the detected value of the first exhaust air / fuel ratio of the exhaust gas that does not pass through the catalyst element and therefore hydrogen is not purified is the detection value of the second exhaust air / fuel ratio of the exhaust gas that has passed through the catalyst element and purified hydrogen. It shifts to the rich side due to the influence of hydrogen. In other words, the second exhaust air / fuel ratio detected value is shifted to the lean side due to the influence of hydrogen than the first exhaust air / fuel ratio detected value. Therefore, an abnormal variation in the air-fuel ratio between cylinders is detected on the basis of the leaning (divergence) state. The amount of shift to the lean side is more conspicuous when only the air-fuel ratio of some cylinders is shifted to the rich side than when all the cylinders are equally shifted. This is because the former has more hydrogen in the exhaust than the latter. Therefore, by monitoring the lean state, it is possible to detect an abnormality in the air-fuel ratio variation between the cylinders, as distinguished from when all the cylinders are uniformly displaced. The air-fuel ratio sensor does not require high responsiveness, so that it is very practical and can detect variation abnormality with high accuracy and suitably.

  When an abnormality in the air-fuel ratio variation between cylinders occurs due to an injector failure or the like in some cylinders, the second air-fuel ratio sensor continuously detects a lean value. The value is corrected to the rich side to eliminate the shift. Therefore, using this, when the control amount becomes a value greater than or equal to a predetermined value that corrects the second exhaust air-fuel ratio to a richer side, it is detected that an abnormality in the air-fuel ratio variation between cylinders has occurred. To do.

  By the way, when the guard range of the control amount is determined, the guard range may interfere with the abnormality determination value that is the comparison target of the control amount, and the control amount may not reach the abnormality determination value. On the other hand, since the control amount is gradually updated, there is a problem that it takes time to actually reach the abnormality determination value. In order to solve these problems, at least one of expanding the guard range to include at least the abnormality determination value and increasing the update rate of the control amount is executed. As a result, it is possible to suitably detect an abnormality in the air-fuel ratio variation.

Preferably, before the detection of the variation abnormality in the air-fuel ratio between the cylinders by the variation abnormality detection means, a preliminary detection means for preliminarily detecting that there is a possibility of occurrence of an abnormality in the variation in the air-fuel ratio between the cylinders,
The variation abnormality detecting means detects the abnormality between the cylinder air-fuel ratios when it is detected by the preliminary detection means that there is a possibility of occurrence of the abnormality in the air-fuel ratio variation between the cylinders.

  Preferably, the preliminary detection means has an integrated value obtained by integrating a difference between an output of the first air-fuel ratio sensor and a sensor output corresponding to the first target air-fuel ratio for a predetermined time exceeding a predetermined value. When the second exhaust air-fuel ratio leaner than the first target air-fuel ratio is detected for a predetermined time or more by the second air-fuel ratio sensor, and between the stored oxygen amount and the released oxygen amount of the catalyst element When at least one of the ratio or difference is greater than a predetermined value, it is detected that there is a possibility of occurrence of abnormal variation in air-fuel ratio between cylinders.

  According to the present invention, an excellent effect is exhibited that it is possible to suitably detect abnormality in variation in the air-fuel ratio between cylinders.

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic view of an internal combustion engine according to the present embodiment. As shown in the figure, the internal combustion engine 1 generates power by burning a mixture of fuel and air in a combustion chamber 3 formed in a cylinder block 2 and reciprocating a piston in the combustion chamber 3. . The internal combustion engine 1 of the present embodiment is a multi-cylinder internal combustion engine for automobiles, more specifically, a parallel 4-cylinder spark ignition internal combustion engine, that is, a gasoline engine. However, the internal combustion engine to which the present invention is applicable is not limited to this, and the number of cylinders, the type, and the like are not particularly limited as long as it is a multi-cylinder internal combustion engine.

  Although not shown, the cylinder head of the internal combustion engine 1 is provided with an intake valve for opening and closing the intake port and an exhaust valve for opening and closing the exhaust port for each cylinder. Each intake valve and each exhaust valve is provided by a camshaft. Can be opened and closed. A spark plug 7 for igniting the air-fuel mixture in the combustion chamber 3 is attached to the top of the cylinder head for each cylinder.

  The intake port of each cylinder is connected to a surge tank 8 which is an intake air collecting chamber via a branch pipe 4 for each cylinder. An intake pipe 13 is connected to the upstream side of the surge tank 8, and an air cleaner 9 is provided at the upstream end of the intake pipe 13. An air flow meter 5 for detecting the intake air amount and an electronically controlled throttle valve 10 are incorporated in the intake pipe 13 in order from the upstream side. An intake passage is formed by the intake port, the branch pipe, the surge tank 8 and the intake pipe 13.

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

  On the other hand, the exhaust port of each cylinder is connected to the exhaust manifold 14. The exhaust manifold 14 includes a branch pipe 14a for each cylinder forming an upstream portion thereof and an exhaust collecting portion 14b forming a downstream portion thereof. An exhaust pipe 6 is connected to the downstream side of the exhaust collecting portion 14b. An exhaust passage is formed by the exhaust port, the exhaust manifold 14 and the exhaust pipe 6. A catalyst 11 made of a three-way catalyst is attached to the exhaust pipe 6. This catalyst 11 constitutes the catalyst element referred to in the present invention. First and second air-fuel ratio sensors for detecting the air-fuel ratio of the exhaust gas, that is, a pre-catalyst sensor 17 and a post-catalyst sensor 18 are installed on the upstream side and the downstream side of the catalyst 11, respectively. The pre-catalyst sensor 17 and the post-catalyst sensor 18 are installed in the exhaust passage immediately before and after the catalyst 11 and detect the air-fuel ratio based on the oxygen concentration in the exhaust gas. In this way, the single pre-catalyst sensor 17 is installed in the exhaust passage on the upstream side of the catalyst, which becomes the exhaust merging portion.

  The spark plug 7, the throttle valve 10, the injector 12, and the like described above are electrically connected to an electronic control unit (hereinafter referred to as ECU) 20 as control means. The ECU 20 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like, all not shown. In addition to the air flow meter 5, the pre-catalyst sensor 17, and the post-catalyst sensor 18, the ECU 20 includes a crank angle sensor 16 that detects the crank angle of the internal combustion engine 1 and an accelerator that detects the accelerator opening, as shown in the figure. The opening sensor 15 and other various sensors are electrically connected via an A / D converter or the like (not shown). The ECU 20 controls the ignition plug 7, the throttle valve 10, the injector 12, etc. so as to obtain a desired output based on the detection values of various sensors, etc., and the ignition timing, fuel injection amount, fuel injection timing, throttle opening. Control the degree etc. The throttle opening is normally controlled to an opening corresponding to the accelerator opening.

The catalyst 11 simultaneously purifies 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 catalyst 11 is near the stoichiometric air fuel ratio (stoichiometric, for example, A / F = 14.6). To do. The air-fuel ratio width (window) that can simultaneously purify these three with high efficiency is relatively narrow. In addition, the catalyst 11 oxidizes (combusts) and purifies hydrogen H ₂ mixed in the exhaust gas.

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

On the other hand, the post-catalyst sensor 18 is a so-called O ₂ sensor, and has a characteristic that the output value changes suddenly with the stoichiometric boundary. FIG. 3 shows the output characteristics of the post-catalyst sensor 18. As shown in the figure, the output voltage Vr of the post-catalyst sensor 18 changes transiently at the stoichiometric boundary, and shows a low voltage of about 0.1 V when the detected exhaust air-fuel ratio is leaner than stoichiometric. When is richer than stoichiometric, it shows a high voltage of about 0.9V. A substantially intermediate voltage Vrefr = 0.45V is set as a stoichiometric equivalent value. When the sensor output voltage is higher than Vrefr, the exhaust air-fuel ratio is richer than stoichiometric, and when the sensor output voltage is lower than Vrefr, the exhaust air-fuel ratio is leaner than stoichiometric. In addition, the exhaust air-fuel ratio is detected.

  When the exhaust gas discharged from the combustion chamber 3 contains hydrogen, the air-fuel ratio of the exhaust gas containing hydrogen before passing through the catalyst 11, that is, the first exhaust air-fuel ratio is the first air-fuel ratio. It is detected by the pre-catalyst sensor 17 which is a sensor. On the other hand, when the exhaust gas containing hydrogen passes through the catalyst 11, the hydrogen in the exhaust is purified by the catalyst 11. After passing through the catalyst 11, the air-fuel ratio of the exhaust gas from which hydrogen has been purified, that is, the second exhaust air-fuel ratio, is detected by a post-catalyst sensor 18, which is a second air-fuel ratio sensor.

  The sensor element of the post-catalyst sensor 18 is provided with a catalyst, and this catalyst, that is, the sensor catalyst can also purify hydrogen in the exhaust gas. Therefore, the sensor catalyst also constitutes a part of the catalyst element referred to in the present invention. If there is unpurified hydrogen in the catalyst 11, unpurified hydrogen is purified by this sensor catalyst, and the exhaust air / fuel ratio after hydrogen purification is reduced after the catalyst. It can be detected by the sensor 18. However, the catalyst of the post-catalyst sensor 18 is arbitrary and can be omitted. The sensor catalyst is not provided in the pre-catalyst sensor 17.

  In the present embodiment, the ECU 20 performs the following air-fuel ratio control so that the air-fuel ratio of the exhaust gas flowing into the catalyst 11 is controlled near the stoichiometric range. In this air-fuel ratio control, the main air-fuel ratio control for matching the exhaust air-fuel ratio detected by the pre-catalyst sensor 17 with a predetermined first target air-fuel ratio and the exhaust air-fuel ratio detected by the post-catalyst sensor 18 are predetermined. The auxiliary air-fuel ratio control is made to coincide with the second target air-fuel ratio. The first target air-fuel ratio and the second target air-fuel ratio are set equal to the theoretical air-fuel ratio.

  FIG. 4 shows an air-fuel ratio control routine. This routine is repeatedly executed by the ECU 20 every engine cycle (= 720 ° crank angle).

  First, in step S101, a basic fuel injection amount, that is, a basic injection amount Qb is calculated so that the air-fuel ratio of the mixture in the combustion chamber 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 S102, the output Vf of the pre-catalyst sensor 17 is acquired. In step S103, a difference between the sensor output Vf and the stoichiometric equivalent sensor output Vref (see FIG. 2), that is, a pre-catalyst sensor output difference ΔVf = Vf−Vref is calculated.

  In step S104, based on the pre-catalyst sensor output difference ΔVf, the main air-fuel ratio correction amount (correction coefficient) Kf is calculated from a map (which may be a function or the same below) as shown in FIG. The pre-catalyst sensor output difference ΔVf and the main air-fuel ratio correction amount Kf form control amounts for main air-fuel ratio control. For example, when the gain is Pf, it is expressed by Kf = Pf × ΔVf. In step S105, the value of the auxiliary air-fuel ratio correction amount Kr set by another routine shown in FIG. 6 is acquired. Finally, in step S106, the final fuel injection amount to be injected from the injector 12, that is, the final injection amount Qfnl is calculated by the formula: Qfnl = Kf × Qb + Kr.

  According to the map of FIG. 5, the larger the pre-catalyst sensor output Vf is larger 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 larger 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 pre-catalyst air-fuel ratio detected by the pre-catalyst sensor 17 matches the stoichiometry.

  The value of the final injection amount Qfnl obtained in step S106 is uniformly used for all cylinders. That is, during one engine cycle, an amount of fuel equal to the final injection amount Qfnl is sequentially injected from the injector 12 of each cylinder, and in the next engine cycle, the fuel of the newly calculated final injection amount Qfnl is injected into the injector 12 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. 6 shows a routine for setting the auxiliary air-fuel ratio correction amount. This routine is repeatedly executed by the ECU 20 at a predetermined calculation cycle.

  First, in step S201, the timer provided in the ECU 20 is counted, and in step S202, the output Vr of the post-catalyst sensor 17 is acquired. In step S203, a difference between the sensor output Vr and the stoichiometric equivalent sensor output Vrefr (see FIG. 3), that is, a post-catalyst sensor output difference ΔVr = Vrefr−Vr is calculated, and the post-catalyst sensor output difference ΔVr is set to the previous integrated value. Accumulated. FIG. 7 shows the post-catalyst sensor output difference ΔVr and its integration.

  In step S204, it is determined whether or not the timer value exceeds a predetermined value ts. If the predetermined value ts is not exceeded, the routine is terminated.

  If the timer value exceeds the predetermined value ts, the post-catalyst sensor output difference integrated value ΣΔVr at this time is updated and stored as the post-catalyst sensor learning value ΔVrg in step S205. In step S206, the auxiliary air-fuel ratio correction amount Kr is calculated from the map as shown in FIG. 8 based on the post-catalyst sensor learning value ΔVrg, and the auxiliary air-fuel ratio correction amount Kr is updated and stored. 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 control. For example, when the gain is Pr, it is expressed by Kr = Pr × ΔVrg. Finally, in step S207, 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 in a period much longer than one engine cycle. .

  According to the map of FIG. 8, the post-catalyst sensor output Vr becomes zero as the time average of the post-catalyst sensor output Vrr 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. In this way, auxiliary air-fuel ratio feedback control is performed so that the post-catalyst air-fuel ratio detected by the post-catalyst sensor 18 matches the stoichiometry. Even if the main air-fuel ratio feedback control is executed for reasons such as deterioration of the pre-catalyst sensor 17, the result may deviate from the stoichiometric condition. Therefore, the auxiliary air-fuel ratio feedback control is executed for the purpose of correcting this deviation.

  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.

  Next, the abnormality detection of the variation in air-fuel ratio between cylinders in this embodiment will be described.

  As disclosed in Patent Document 1, if an abnormality that affects all cylinders occurs in the fuel supply system such as an injector or the air system such as an air flow meter, the absolute value of the feedback correction amount in the main air-fuel ratio control Since the value increases, the abnormality can be detected and diagnosed by monitoring this with the ECU. For example, if the fuel injection amount is totally deviated by 5% from the stoichiometric amount (that is, the fuel injection amount is deviated by 5% from the stoichiometric amount in all cylinders), the feedback correction amount in the main air-fuel ratio control Is a value for correcting the 5% deviation, that is, a correction amount corresponding to -5%, and it is possible to detect that the fuel supply system or the air system is shifted by 5%. When the feedback correction amount becomes a relatively large predetermined value or more, it can be detected that the fuel supply system or the air system is abnormal as a whole. Also in this embodiment, such an abnormality detection means, that is, another abnormality detection means based on the main air-fuel ratio correction amount Kf or the pre-catalyst sensor output difference ΔVf is provided.

  On the other hand, let us consider a case where the fuel supply system and the air system are not displaced as a whole, but variations (imbalance) occur between the cylinders. FIG. 9 shows a case where only one cylinder (# 1 cylinder) is shifted to the air-fuel ratio rich side from the other three cylinders (# 2 to # 4 cylinders). For example, an abnormality has occurred in the injector of the # 1 cylinder, and the fuel injection amount of the # 1 cylinder is greatly shifted by 20% from the stoichiometric equivalent, while the # 2 to # 4 cylinders are normal and the fuel injection amount is equivalent to the stoichiometric. Suppose there is. At this time, the total shift is 20% (20 + 0 + 0 + 0 = 20), which should be the same as when all cylinders are shifted by 5% (5 + 5 + 5 + 5 = 20).

  However, the amount of hydrogen generated from the combustion chamber is greater when only one cylinder is significantly shifted to the rich side than when it is shifted to the rich side evenly for all the cylinders. Since the oxygen concentration in the exhaust gas decreases as the amount of hydrogen increases, the output Vf of the pre-catalyst sensor 17 is shifted when only one cylinder is shifted than when all cylinders are shifted equally. It will shift to the rich side.

  FIG. 10 shows the relationship between the rich air-fuel ratio deviation amount (horizontal axis) with respect to the stoichiometric mixture of one cylinder and the hydrogen amount (vertical axis) generated in the combustion chamber. As shown in the figure, the amount of generated hydrogen increases in a quadratic function as the air-fuel ratio rich shift amount increases. Accordingly, when only one cylinder is shifted to the rich side by 20%, the amount of generated hydrogen is larger than when all the cylinders are shifted by 5%, and the pre-catalyst sensor output Vf shows a richer value. .

  Even when the total deviation is the same, the emission is worse when the air-fuel ratio varies between the cylinders than when the whole is displaced. For example, in the latter case, when all the cylinders are deviated by 5%, for example, if the correction of -5% is performed by the auxiliary air-fuel ratio feedback control, the deviation of all the cylinders can be eliminated uniformly. However, if only one cylinder is displaced by 20% in the former, # 1 cylinder = 15%, # 2 cylinder = −5%, # 3 cylinder even if the auxiliary air-fuel ratio feedback control is corrected by −5% = -5%, # 4 cylinder = -5%, and the total deviation seems to be eliminated (15 + (-5) + (-5) + (-5) = 0), but by cylinder If it sees, it will have shifted | deviated, Therefore, an emission worsens per cylinder.

  On the other hand, in the main air-fuel ratio feedback control, control is performed so that the pre-catalyst air-fuel ratio as a total is detected and stoichiometrically controlled. Therefore, variation in the air-fuel ratio between cylinders occurs from the correction amount of the main air-fuel ratio feedback control. It cannot be detected. In other words, even if there is a variation in the air-fuel ratio between cylinders, if the total deviation is zero, the correction amount will be zero, and it appears that the main air-fuel ratio feedback control is normally performed without any problems. End up.

  Therefore, in the present embodiment, when there is a variation in the air-fuel ratio between the cylinders, the amount of hydrogen is larger than that when the whole is shifted, and the characteristic that the pre-catalyst sensor output Vf shifts to the rich side is used as follows. Thus, an abnormality in variation in the air-fuel ratio between cylinders is detected.

  When hydrogen is contained in the exhaust, a catalyst is allowed to act on the exhaust, whereby the hydrogen in the exhaust can be oxidized (combusted) and purified. Then, the air-fuel ratio of the exhaust gas that has not passed through the catalyst and hydrogen has not been purified, that is, the first exhaust air-fuel ratio is detected by the first air-fuel ratio sensor, and the air-fuel ratio of the exhaust gas that has passed through the catalyst and has been purified of hydrogen, ie, the first exhaust air-fuel ratio. The second air-fuel ratio is detected by a second air-fuel ratio sensor. The first exhaust air / fuel ratio detection value is shifted to the rich side due to the influence of hydrogen than the second exhaust air / fuel ratio detection value. In other words, the second exhaust air / fuel ratio detected value is shifted to the lean side due to the influence of hydrogen than the first exhaust air / fuel ratio detected value. Therefore, an abnormal variation in the air-fuel ratio between cylinders is detected on the basis of the leaning (divergence) state.

  In other words, it can be said that the second exhaust air-fuel ratio detection value after hydrogen purification is the true exhaust air-fuel ratio, and the first exhaust air-fuel ratio detection value before hydrogen purification is equal to the true exhaust air-fuel ratio. The exhaust air / fuel ratio is apparently richly offset by adding the minute. In other words, the first air-fuel ratio sensor is deceived. As the air-fuel ratio rich shift amount of the remaining cylinders with respect to the remaining cylinders increases, the hydrogen content increases in a quadratic function. Therefore, when the first exhaust air-fuel ratio detection value is greatly deviated to the rich side from the second exhaust air-fuel ratio detection value, that is, the second exhaust air-fuel ratio detection value is leaner than the first exhaust air-fuel ratio detection value. When there is a large deviation, it can be considered that the variation in air-fuel ratio between cylinders has occurred.

  Hereinafter, an aspect of detecting an abnormality in the air-fuel ratio variation between cylinders according to this principle will be described.

  As shown in FIG. 9, for example, it is assumed that an abnormality occurs in the injector only in the # 1 cylinder, and the air-fuel ratio of the # 1 cylinder deviates more to the rich side than the air-fuel ratios of the other # 2- # 4 cylinders. Since the main air-fuel ratio feedback control is executed at this time, the air-fuel ratio of the total exhaust gas after the exhaust gases of all the cylinders merge is controlled in the vicinity of the stoichiometry as shown in FIG. That is, the pre-catalyst sensor output Vf is in the vicinity of the stoichiometric equivalent sensor output Vreff. However, the air-fuel ratio of the # 1 cylinder is larger and richer than stoichiometric, and the air-fuel ratio of the # 2 to # 4 cylinders is leaner than stoichiometric, and as a whole balance is only near the stoichiometric. Moreover, as a result of the large amount of hydrogen generated from the # 1 cylinder, the output Vf of the pre-catalyst sensor 17 erroneously displays the air-fuel ratio shifted to the rich side from the true air-fuel ratio as a stoichiometric.

  On the other hand, when the exhaust gas containing hydrogen passes through the catalyst 11, the hydrogen is purified and its influence is removed. Therefore, as shown in FIG. 9B, the output Vr of the post-catalyst sensor 18 displays a true air-fuel ratio, that is, an air-fuel ratio that is leaner than stoichiometry. That is, the post-catalyst sensor output Vr is a lower value on the lean side than the stoichiometric equivalent sensor output Vrefr.

  From another viewpoint, for example, to correct the rich deviation of the pre-catalyst air-fuel ratio detection value of 25, the lean correction of -25 is performed in the main air-fuel ratio feedback control, and the rich deviation of the pre-catalyst air-fuel ratio detection value is set to zero. . However, 5 out of 25 is not a pure air-fuel ratio shift but is caused by the influence of hydrogen, and the main air-fuel ratio feedback control is overcorrected by 5 on the lean side. Therefore, the post-catalyst air-fuel ratio results in a shift of 5 by lean.

  Therefore, although the pre-catalyst air-fuel ratio is controlled to be stoichiometric by the main air-fuel ratio feedback control, the post-catalyst sensor 18 continuously detects the lean post-catalyst air-fuel ratio from the stoichiometric ( That is, the post-catalyst sensor output sticks lean). Such a difference in the air-fuel ratio before and after the catalyst is because a significant amount of hydrogen is generated due to failure of injectors of some cylinders.

  When the post-catalyst sensor 18 detects an exhaust air-fuel ratio that is leaner than stoichiometric, rich correction is performed by auxiliary air-fuel ratio feedback control, and the fuel injection amount is uniformly increased for all cylinders. Then, the rich deviation of the pre-catalyst air-fuel ratio detection value is further increased, and the post-catalyst air-fuel ratio is maintained lean. Eventually, the main air-fuel ratio correction amount and the auxiliary air-fuel ratio correction amount converge to the degree of variation abnormality.

  Incidentally, as described with reference to FIGS. 6 to 8, in the auxiliary air-fuel ratio feedback control, the post-catalyst sensor learning value ΔVrg and the auxiliary air-fuel ratio correction amount Kr are determined at predetermined time intervals (that is, at a predetermined update speed). Is learned or updated. Here, if the variation in air-fuel ratio between cylinders occurs due to the failure of an injector of some cylinders, the post-catalyst sensor output Vr continuously becomes a lean value, so the post-catalyst sensor learning value ΔVrg and the auxiliary air-fuel ratio correction amount Kr Is a large positive value that returns a large lean shift to stoichiometry.

  This is shown in FIG. FIG. 11 shows the relationship between the ratio of deviation when the fuel injection amount of only one cylinder out of all cylinders deviates from the stoichiometric amount, that is, the imbalance ratio (%), and the post-catalyst sensor learning value ΔVrg. It is a test result. The imbalance ratio is positive when there is a rich shift and negative when there is a lean shift. As shown in the figure, the post-catalyst sensor learning value ΔVrg becomes a larger value, that is, a value that corrects the air-fuel ratio to the rich side as the imbalance ratio increases in the rich shift direction.

  Therefore, in this aspect, when the post-catalyst sensor learning value ΔVrg is equal to or greater than a predetermined abnormality determination value ΔVrgs, it is determined that an inter-cylinder air-fuel ratio variation abnormality has occurred. Alternatively, when the auxiliary air-fuel ratio correction amount Kr calculated based on the post-catalyst sensor learning value ΔVrg becomes equal to or greater than the predetermined value Krs, it is determined that the inter-cylinder air-fuel ratio variation abnormality has occurred.

  According to this aspect, the air-fuel ratio sensor is not required to have high responsiveness, and even a sensor that has deteriorated to some extent and has decreased responsiveness can be used sufficiently. There is no need for high-speed data samples or ECUs with high processing capabilities. In addition, it is resistant to disturbances and has high robustness, and there are no restrictions on engine operating conditions and sensor installation positions. Therefore, it is very practical and can detect abnormalities with high accuracy.

  Here, as shown in FIG. 11, the imbalance ratio corresponding to the abnormality determination value ΔVrgs is IBs. The imbalance ratio IBs is the minimum value of the imbalance ratio having an unacceptable size from the viewpoint of emissions and the like. FIG. 11 shows the gradient of the post-catalyst sensor learning value ΔVrg when only one cylinder is deviated (ie, when an imbalance failure occurs), but when all the cylinders are equally displaced (ie, when a balance failure occurs). As indicated by the imaginary line Z, the gradient becomes much gentler. The reason for this is that when the whole is deviated, the main air-fuel ratio control can easily make a uniform correction, so the influence on the auxiliary air-fuel ratio correction amount is small. However, since the correction amount of the main air-fuel ratio control becomes large when the whole is greatly deviated, an abnormality is detected prior to the abnormality detection of this embodiment by another abnormality detection means based on the main air-fuel ratio correction amount touched before. Will be detected.

  Some cylinders may deviate from the remaining cylinders. In this case, the post-catalyst sensor learning value ΔVrg is as shown in the negative imbalance ratio region in FIG. The slope of this area is gentler than that of the positive imbalance ratio area. Here, the lean deviation means that the fuel injection amount becomes smaller than a prescribed amount. When a large lean deviation occurs in a certain cylinder, the cylinder usually falls into a misfire. Therefore, the variation abnormality due to the lean deviation will be detected first by another misfire detection means. The abnormality detection of the present embodiment is particularly advantageous for rich deviation abnormality.

  By the way, in relation to the other abnormality detecting means described above, in the main air-fuel ratio feedback control and the auxiliary air-fuel ratio feedback control, they are executed while keeping the control amount within a predetermined guard range. As shown in FIG. 5, the pre-catalyst sensor output difference ΔVf of the main air-fuel ratio feedback control can take only values within the range of the upper and lower guard values ΔVfH and ΔVfL for control (ΔVfL ≦ ΔVf ≦ ΔVfH), Correspondingly, the main air-fuel ratio correction amount Kf can only take values within the upper and lower guard values KfH and KfL (KfL ≦ Kf ≦ KfH). For example, when the calculated pre-catalyst sensor output difference ΔVf is equal to or greater than the upper guard value ΔVfH, the pre-catalyst sensor output difference ΔVf is considered to have reached the upper guard value ΔVfH. The upper guard value ΔVfH is fixed. At the same time, it is detected that the fuel supply system or the air system is abnormal as a whole (a balance failure has occurred). Thereby, it is possible to prevent the main air-fuel ratio control from being performed using an abnormally large control amount even though the fuel supply system or the air system is abnormal as a whole.

  Similarly, as shown in FIG. 8, also in the auxiliary air-fuel ratio feedback control, the post-catalyst sensor learning value ΔVrg can take only values within the range of the upper and lower guard values ΔVrgH and ΔVrgL (ΔVrgL). (.DELTA.Vrg.ltoreq..DELTA.VrgH). Correspondingly, the auxiliary air-fuel ratio correction amount Kr can only take a value within the range of the upper and lower guard values KrH, KrL (KrL.ltoreq.Kr.ltoreq.KrH). For example, when the calculated post-catalyst sensor learning value ΔVrg is equal to or higher than the upper limit guard value ΔVrgH, the post-catalyst sensor learning value ΔVrg is considered to have reached the upper limit guard value ΔVrgH, and for control purposes, the post-catalyst sensor learning value ΔVrg is The upper guard value ΔVrgH is fixed. At the same time, it is detected that the fuel supply system or the air system is abnormal as a whole (a balance failure has occurred). Thereby, it is possible to prevent the auxiliary air-fuel ratio control from being performed using an abnormally large control amount even though the fuel supply system or the air system is abnormal as a whole.

  By the way, when the guard range is determined in this way, the following problem arises in detecting an abnormality in the air-fuel ratio variation between cylinders. As shown in FIG. 11, in the case of a balance failure, the relationship between the imbalance ratio and the post-catalyst sensor learning value ΔVrg is as indicated by a virtual line Z. Corresponding to this relationship, the guard range, in particular, the upper guard value ΔVrgH is set. That is, in the case of a balance failure, the post-catalyst sensor learning value ΔVrg increases along the virtual line Z, and when the post-catalyst sensor learning value ΔVrg exceeds the upper limit guard value ΔVrgH, it is considered that a balance failure has occurred.

  However, the abnormality determination value ΔVrgs suitable for determining an imbalance failure is often higher than the upper limit guard value ΔVrgH suitable for determining a balance failure. For this reason, the problem that an imbalance failure cannot be detected by interference of both arises. Specifically, even if the actual post-catalyst sensor learning value ΔVrg increases due to an imbalance failure, the upper limit guard value ΔVrgH is reached prior to the abnormality determination value ΔVrgs, and is regarded as a balance failure. It is.

  Therefore, in order to solve this problem, in the present embodiment, when detecting the abnormality in the air-fuel ratio variation between cylinders, the guard range in the auxiliary air-fuel ratio feedback control is expanded to include at least the abnormality determination value ΔVrgs. The Specifically, the upper limit guard value ΔVrgH is changed to a value equal to or greater than the abnormality determination value ΔVrgs (in this embodiment, a value equal to the abnormality determination value ΔVrgs). As a result, the post-catalyst sensor learning value ΔVrg becomes variable beyond the predetermined guard range, that is, it becomes possible to take a value larger than the predetermined upper limit guard value ΔVrgH in terms of control, and the abnormality determination value ΔVrgs can be reached. Thus, it is possible to detect the variation in air-fuel ratio between cylinders without any problem.

  Alternatively, the guard range of the auxiliary air-fuel ratio correction amount Kr may be expanded. Specifically, the upper limit guard value KrH of the auxiliary air-fuel ratio correction amount Kr is set to the auxiliary air-fuel ratio corresponding to the abnormality determination value ΔVrgs. You may change into the value more than correction amount. When expanding these guard ranges, the lower limit guard value may not be changed, or may be changed to a smaller value (enlargement side).

  On the other hand, as shown in FIG. 7, the post-catalyst sensor learning value ΔVrg is updated every predetermined time ts, so that even if an abnormality in the air-fuel ratio variation between cylinders occurs, the post-catalyst sensor learning value ΔVrg. Takes time to actually reach the abnormality determination value ΔVrgs. If the arrival time is too long, there is a problem that the time required for detection becomes long.

  Therefore, in order to solve this problem, in the present embodiment, the update speed of the post-catalyst sensor learning value ΔVrg is increased from the predetermined speed when detecting the variation in the air-fuel ratio between cylinders. That is, the update time ts is shortened from the specified value. As a result, the post-catalyst sensor learning value ΔVrg can be updated quickly, and the detection time can be shortened. Note that, by increasing the update speed of the post-catalyst sensor learning value ΔVrg, the update speed of the auxiliary air-fuel ratio correction amount Kr also follows.

  In the present embodiment, both the above-described guard range expansion and update speed increase are performed. However, only one of them may be performed. For example, regarding the guard range expansion, when the abnormality determination value ΔVrgs in the default state is set within the guard range, the guard range need not be expanded.

  FIG. 12 shows the abnormality detection routine of this aspect. The routine is repeatedly executed by the ECU 20 every predetermined calculation cycle.

  First, in step S301, it is determined whether a precondition for abnormality detection is satisfied. This precondition includes, for example, that the engine has been warmed up and that the catalyst has reached the activation temperature.

  If the precondition is not satisfied, the routine is terminated. On the other hand, if the precondition is satisfied, it is determined in step S302 whether the execution conditions for the main air-fuel ratio and the auxiliary air-fuel ratio feedback control are satisfied. This condition is, for example, that the pre-catalyst sensor 17 and the post-catalyst sensor 18 are activated. Specifically, the element impedances of both sensors detected by the ECU 20 correspond to the minimum activation temperature of the sensor. It is lower than the predetermined value.

  If the execution condition is not satisfied, the routine is terminated. On the other hand, if the execution condition is satisfied, the process proceeds to step S303, where the guard range of the post-catalyst sensor learning value ΔVrg is expanded and the update speed of the post-catalyst sensor learning value ΔVrg is increased. That is, as described above, the upper limit guard value ΔVrgH of the post-catalyst sensor learning value ΔVrg is changed to a value equal to the abnormality determination value ΔVrgs higher than the predetermined value, and the update time ts of the post-catalyst sensor learning value ΔVrg is greater than the specified value. Is also shortened.

  If the guard range expansion and the update speed increase are performed in this way, then in step S304, main air-fuel ratio and auxiliary air-fuel ratio feedback control (stoichiometric F / B control) with the stoichiometric target air-fuel ratio is executed.

  Next, in step S305, whether or not a predetermined time has elapsed since the start of stoichiometric F / B control, that is, the post-catalyst sensor learning value ΔVrg and the auxiliary air-fuel ratio correction amount Kr are updated to values corresponding to the air-fuel ratio variation state. It is determined whether sufficient time has passed to be processed. Here, since the update speed is increased in step S303, the predetermined time can be set to a relatively short time, thereby shortening the detection time.

  If the predetermined time has not elapsed, the routine is terminated. On the other hand, if the predetermined time has elapsed, in step S306, the current post-catalyst sensor learning value ΔVrg is acquired.

  In step S307, it is determined whether or not the acquired post-catalyst sensor learning value ΔVrg is greater than or equal to the abnormality determination value ΔVrgs. Since the upper guard value ΔVrgH is enlarged and changed to the abnormality determination value ΔVrgs in step S303, the post-catalyst sensor learning value ΔVrg can be increased to the abnormality determination value ΔVrgs.

  If the post-catalyst sensor learning value ΔVrg is equal to or greater than the abnormality determination value ΔVrgs (that is, equal to the abnormality determination value ΔVrgs), it is determined in step S308 that an abnormality in the cylinder air-fuel ratio variation has occurred, and the routine is terminated. After this abnormality determination, it is preferable to activate a warning device such as a check lamp to notify the user of the abnormality.

  On the other hand, if the after-catalyst sensor learning value ΔVrg is less than the abnormality determination value ΔVrgs, it is considered that the cylinder-to-cylinder air-fuel ratio variation abnormality has not occurred, and the process proceeds to step S309 to update the guard range and update of the after-catalyst sensor learning value ΔVrg. The speed is returned to the default state and the routine is terminated.

  Here, the occurrence of the variation in air-fuel ratio between cylinders is detected by comparing the post-catalyst sensor learning value ΔVrg with a predetermined value, but naturally, the inter-cylinder air-fuel ratio is detected by comparing with the predetermined value of the auxiliary air-fuel ratio correction amount Kr. The occurrence of a variation abnormality may be detected.

  Next, another mode of detecting an abnormality in the air-fuel ratio variation between cylinders will be described.

  In this another aspect, before the detection of the abnormality in the variation in the air-fuel ratio between the cylinders as described above, it is preliminarily detected that there is a possibility of the abnormality in the variation in the air-fuel ratio between the cylinders. Hereinafter, the former detection is referred to as main detection, and the latter detection is referred to as preliminary detection. The main detection is performed when it is detected by the preliminary detection that there is a possibility that the variation in the air-fuel ratio between the cylinders is abnormal. Thus, by performing the preliminary detection before the main detection, a so-called double check becomes possible, and the accuracy and reliability of the detection can be increased.

  A first example of preliminary detection will now be described. As shown in FIG. 13, when the variation in air-fuel ratio between cylinders occurs, the variation in the exhaust air-fuel ratio during one engine cycle (= 720 ° CA) increases. The air-fuel ratio diagrams a, b, and c in (B) are not varied, and the pre-catalyst air in the case of a rich shift with an imbalance ratio of 20% for only one cylinder and a rich shift with an imbalance ratio of 50% for only one cylinder. Indicates the detected value of the fuel ratio. As can be seen, the greater the degree of variation, the greater the amplitude of air-fuel ratio fluctuation and the greater the frequency.

  Therefore, when the amplitude or frequency of the air-fuel ratio fluctuation becomes larger than a predetermined value, it can be detected that there is a possibility that the variation in air-fuel ratio between cylinders may occur. In the present embodiment, preliminary detection is performed as follows, focusing on the amplitude. That is, the difference between the pre-catalyst sensor output Vf and the stoichiometric equivalent sensor output Vref, specifically, the absolute value of the pre-catalyst sensor output difference ΔVf, which is the difference, is integrated for a predetermined time, and the integrated value obtained thereby is a predetermined value. Is exceeded, it is determined that there is a possibility of occurrence of an abnormal variation in air-fuel ratio between cylinders. Since the absolute value of the pre-catalyst sensor output difference ΔVf increases as the fluctuation increases, it is possible to detect the possibility of occurrence of variation abnormality by utilizing this fact.

  FIG. 14 shows an abnormality detection routine including the first embodiment of preliminary detection. The routine is repeatedly executed by the ECU 20 every predetermined calculation cycle.

  Steps S401 and S402 are the same as steps S301 and S302. In step S403, the stoichiometric F / B control is executed as in step S304. However, at this stage, the guard range expansion and the update speed increase of the post-catalyst sensor learning value ΔVrg are not yet executed.

  In the following step S404, the absolute value of the current pre-catalyst sensor output difference ΔVf is calculated, and this value is added to the previous integrated value, whereby the pre-catalyst sensor output difference ΔVf is integrated. Next, in step S405, it is determined whether or not a predetermined integration time has elapsed since the start of integration (that is, the start of stoichiometric F / B control in step S403). If the predetermined integration time has not elapsed, the routine is terminated. On the other hand, if the predetermined integration time has elapsed, in step S406, a final integrated value of the pre-catalyst sensor output difference ΔVf is acquired, and this final integrated value is compared with a predetermined preliminary abnormality determination value. .

  If the final integrated value is less than or equal to the preliminary abnormality determination value, it is determined that there is no possibility of occurrence of an abnormal variation in the air-fuel ratio between cylinders, and the routine is terminated. On the other hand, if the final integrated value exceeds the preliminary abnormality determination value, it is determined that there is a possibility that the variation in air-fuel ratio between cylinders may occur, and the process proceeds to step S407.

  Thereafter, in steps S407 to S412, the processes related to the main detection as described above are performed. That is, in step S407, as in step S303, the guard range of the post-catalyst sensor learned value ΔVrg is expanded and the update speed of the post-catalyst sensor learned value ΔVrg is increased. In this state, in step S408, it is determined whether or not a predetermined time (which is longer than the integration time) has elapsed since the start of the stoichiometric F / B control. If the predetermined time has not elapsed, the routine is terminated. On the other hand, if the predetermined time has elapsed, in steps S409 to S412, the same processing as in steps S306 to S309 is performed, and an abnormality in the air-fuel ratio variation between cylinders is appropriately detected.

  Next, a second example of preliminary detection will be described. As shown in FIG. 9, when the variation in air-fuel ratio between cylinders occurs, the pre-catalyst air-fuel ratio in the vicinity of the stoichiometry is continuously detected from the post-catalyst sensor 18 by the main air-fuel ratio feedback control. The post-catalyst sensor 18 continuously detects the lean post-catalyst air-fuel ratio from stoichiometry (that is, the post-catalyst sensor output sticks lean). Therefore, using this fact, in the second embodiment of the preliminary detection, the post-catalyst air-fuel ratio is leaner than the stoichiometry by the post-catalyst sensor 18 even though the pre-catalyst air-fuel ratio is controlled to stoichiometric by the main air-fuel ratio feedback control. When the fuel ratio is detected for a predetermined time or more, it is determined that there is a possibility that the variation in air-fuel ratio between cylinders is abnormal.

  FIG. 15 shows an abnormality detection routine including the second embodiment of preliminary detection. The routine is repeatedly executed by the ECU 20 every predetermined calculation cycle.

  Steps S501 and S502 are the same as steps S301 and S302. In step S503, the stoichiometric F / B control is executed as in step S304. However, at this stage, the guard range expansion and the update speed increase of the post-catalyst sensor learning value ΔVrg are not yet executed.

  In subsequent step S504, the output Vr of the post-catalyst sensor 18 is acquired, and whether the acquired post-catalyst sensor output Vr is lower than the stoichiometric equivalent value Vrefr, that is, the post-catalyst empty detected by the post-catalyst sensor 18. It is determined whether the fuel ratio is leaner than stoichiometric. When the post-catalyst sensor output Vr is lower than the stoichiometric equivalent value Vrefr, in step S505, the count value Cl of the lean continuation counter equipped in the ECU 20 is counted up, and the process proceeds to step S507. The lean continuation counter is for counting the time during which the detected value of the post-catalyst air-fuel ratio is leaner than the stoichiometric value. On the other hand, if the post-catalyst sensor output Vr is greater than or equal to the stoichiometric equivalent value Vrefr, the lean continuation counter is cleared in step S506, and the process proceeds to step S507.

  In step S507, it is determined whether or not the count value Cl of the lean continuation counter has reached a predetermined value Cls or more, that is, whether or not the lean continuation time of the detected value of the post-catalyst air-fuel ratio has reached a predetermined time or more. The

  If the count value Cl does not reach the predetermined value Cls or more, it is determined that there is no possibility of occurrence of an abnormal variation in the air-fuel ratio between cylinders, and the routine is terminated. On the other hand, if the count value Cl has reached the predetermined value Cls or more, it is determined that there is a possibility of occurrence of inter-cylinder air-fuel ratio variation abnormality, and the process proceeds to step S508.

  Thereafter, in steps S508 to S513, processing related to the main detection similar to steps S407 to S412 is performed.

  Next, a third embodiment of preliminary detection will be described. In this embodiment, a three-way catalyst having an oxygen storage capacity is used as the catalyst 11. In this case, when the air-fuel ratio of the exhaust gas flowing into the catalyst (pre-catalyst air-fuel ratio) is leaner than stoichiometric, the catalyst stores oxygen in the exhaust gas, and when the exhaust gas air-fuel ratio is richer than stoichiometric, the catalyst has already stored. The oxygen that had been released is released. On the other hand, a so-called Cmax method is known as a method for diagnosing deterioration of such a three-way catalyst. This is because the oxygen storage capacity of the catalyst decreases when the catalyst deteriorates, and the amount of oxygen that can be stored (or released) by the catalyst (that is, the oxygen storage capacity OSC) is measured. Is compared with a predetermined value to determine the deterioration of the catalyst. In this deterioration detection, active air-fuel ratio control for forcibly switching the air-fuel ratio to rich and lean is executed, and during the execution of this active air-fuel ratio control, the stored oxygen amount and the released oxygen amount of the catalyst are measured multiple times. The average value is compared with a predetermined value as the final oxygen storage capacity OSC.

  Here, measurement of the occluded oxygen amount and the released oxygen amount will be described with reference to FIG. (A) shows the target air-fuel ratio A / Ft (broken line) and the pre-catalyst air-fuel ratio A / Ff (solid line) detected by the pre-catalyst sensor 17. (B) shows the post-catalyst sensor output Vr. (C) shows the integrated value of the amount of oxygen released from the catalyst, that is, the released oxygen amount OSAa, and (D) shows the integrated value of the amount of oxygen stored in the catalyst, that is, the stored oxygen amount OSAb.

  As shown in the figure, by executing the active air-fuel ratio control, the air-fuel ratio of the exhaust gas flowing into the catalyst is forcibly and alternately switched between lean and rich at a predetermined timing. For example, before the time t1, the target air-fuel ratio A / Ft is set to lean (for example, 15.1) from the stoichiometry, and lean gas flows into the catalyst 11. At this time, the catalyst 11 continuously absorbs oxygen and reduces and purifies the lean component (NOx) in the exhaust gas. However, when the oxygen is absorbed to a saturated state, that is, full, oxygen can no longer be absorbed, and the lean gas becomes the catalyst 11. And flows out downstream of the catalyst 11. When this happens, the output of the post-catalyst sensor 18 is reversed to the lean side, and the output of the post-catalyst sensor 18 reaches the stoichiometric equivalent value Vrefr (time t1). At this time, the target air-fuel ratio A / Ft is switched to richer (eg, 14.1) than stoichiometric.

  This time, rich gas flows into the catalyst 11. At this time, the catalyst 11 continues to release the oxygen stored until then, and oxidizes and purifies the rich components (HC, CO) in the exhaust gas, but when all the stored oxygen is eventually released from the catalyst 11, At that time, oxygen can no longer be released, and the rich gas passes through the catalyst 11 and flows out downstream of the catalyst 11. When this happens, the post-catalyst air-fuel ratio changes to the rich side, and the output of the post-catalyst sensor 18 reaches the stoichiometric equivalent value Vrefr (time t2). At this time, the target air-fuel ratio A / Ft is switched to the lean air-fuel ratio. In this way, switching of the air-fuel ratio to rich / lean is repeatedly performed.

  As shown in (C), in the release cycle from time t1 to time t2, the released oxygen amount OSAa for each extremely short predetermined period is sequentially accumulated. More specifically, from the time t11 when the output of the pre-catalyst sensor 17 reaches the stoichiometric value, the released oxygen for each calculation cycle from the time t2 when the output of the post-catalyst sensor 18 is reversed to the lean side (has reached Vrefr). The quantity dOSA (dOSAa) is calculated by the following equation (1), and the value for each calculation period is integrated for each period. The final integrated value obtained in this way becomes the measured value of the released oxygen amount OSAa corresponding to the oxygen storage capacity of the catalyst.

  Q is the fuel injection amount. When the air-fuel ratio difference ΔA / F is multiplied by the fuel injection amount Q, the excess or insufficient air amount can be calculated. K is the proportion of oxygen contained in the air (about 0.23).

  Similarly, in the storage cycle from time t2 to t3, as shown in (D), the output of the post-catalyst sensor 18 is reversed to the rich side from the time t21 when the output of the pre-catalyst sensor 17 reaches the stoichiometric value (Vrefr). Until the time t3, the stored oxygen amount dOSA (dOSAb) for each calculation cycle is calculated by the equation (1), and the value for each calculation cycle is integrated for each cycle. The final integrated value obtained in this way becomes a measured value of the stored oxygen amount OSAb corresponding to the oxygen storage capacity of the catalyst. By repeating the release cycle and the storage cycle in this way, a plurality of released oxygen amounts OSAa and stored oxygen amounts OSAb are measured and acquired.

  By the way, in principle, the amount of oxygen that can be stored and the amount of oxygen that can be released in the catalyst are equal, and thus the amount of released oxygen OSAa and the amount of stored oxygen OSAb should be equal. In other words, they are in a symmetrical relationship. However, when the variation in air-fuel ratio between cylinders occurs, this symmetrical relationship is lost, and both become asymmetric. That is, the output of the pre-catalyst sensor 17 is a value shifted to the rich side from the true value due to the influence of hydrogen. For this reason, the air-fuel ratio of the exhaust gas actually given to the catalyst is slightly leaner than the apparent air-fuel ratio detected by the pre-catalyst sensor 17. Therefore, the measured values of the released oxygen amount OSAa and the stored oxygen amount OSAb are not equal, and the former is larger than the latter.

  Therefore, preliminary detection is performed using this fact. That is, the released oxygen amount OSAa and the stored oxygen amount OSAb are respectively measured, and the ratio R = OSAa / OSAb of these measured values is calculated. It is determined that there is a possibility. Note that when the difference between the two measured values is larger than a predetermined value, it may be determined that there is a possibility that an abnormality in the air-fuel ratio variation between the cylinders may occur.

  FIG. 17 shows an abnormality detection routine including the third embodiment of preliminary detection. The routine is repeatedly executed by the ECU 20 every predetermined calculation cycle.

  Steps S601 and S602 are the same as steps S301 and S302. In step S603, it is determined whether or not a predetermined condition suitable for executing active air-fuel ratio control is satisfied. For example, the condition is satisfied if the engine is in a steady operation state such that the fluctuation range of the detected values of the intake air amount Ga and the engine rotational speed Ne are within a predetermined range. If the condition is not satisfied, the routine is terminated. If the condition is satisfied, the process proceeds to step S604.

  In step S604, active air-fuel ratio control is executed. In step S605, the released oxygen amount OSAa and the stored oxygen amount OSAb are measured, and a plurality of these measured values are acquired. Next, in step S606, average values OSAaAV and OSAbAV of the measured values of the plurality of released oxygen amounts OSAa and the measured values of the plurality of stored oxygen amounts OSAb are respectively calculated, and the ratio R = OSAaAV / OSAbAV of these average values OSAaAV and OSAbAV is Calculated. Then, the ratio R is compared with the predetermined value Rs. The predetermined value Rs is set to a value larger than 1.

  When the ratio R is equal to or less than the predetermined value Rs, it is determined that there is no possibility of occurrence of an abnormal variation in air-fuel ratio between cylinders, and the routine is terminated. On the other hand, if the ratio R is greater than the predetermined value Rs, it is determined that there is a possibility that the variation in the air-fuel ratio between the cylinders may occur, and the process proceeds to step S607.

  In step S607 and subsequent steps, processing related to the main detection is performed. First, in step S607, the stoichiometric F / B control is executed as in step S403, and in step S608, the guard range expansion and the update speed increase of the post-catalyst sensor learning value ΔVrg are executed as in step S407. In subsequent steps S609 to 613, processing similar to that in steps S408 to S412 is performed.

  In addition, about 1st Example-3rd Example of preliminary | backup detection, you may combine these 2 or more.

The preferred embodiment of the present invention has been described in detail above, but various other embodiments of the present invention are conceivable. For example, the above-described internal combustion engine is an intake port (intake passage) injection type, but the present invention can also be applied to a direct injection type engine or a dual injection type engine having both injection types. In the above embodiment, the wide area air-fuel ratio sensor is used before the catalyst and the O ₂ sensor is used after the catalyst. However, for example, a wide area air-fuel ratio sensor may be used after the catalyst, or an O ₂ sensor may be used before the catalyst. A wide range of sensors for detecting the air-fuel ratio of exhaust gas, including these wide-range air-fuel ratio sensors and O ₂ sensors, are referred to as air-fuel ratio sensors in the present invention. In the above embodiment, the target air-fuel ratio is set to be equal in both the main air-fuel ratio control and the auxiliary air-fuel ratio control, but it is not always necessary to do so. It is also possible to make the target air-fuel ratios of both controls different. Further, for example, the target air-fuel ratio of the main and auxiliary air-fuel ratio control may be slightly richer than the stoichiometry at the time of starting the engine or warming up, but the present invention is also applicable to such a case.

  In the above-described embodiment, an example in which one cylinder (# 1 cylinder) of the four-cylinder engine is richly shifted with respect to the remaining three cylinders (# 2 to # 4 cylinder) is shown, but the number of rich-shifted cylinders is not limited. . The present invention can also be applied to a case where some cylinders (for example, # 1 and # 2 cylinders) are richly shifted from the remaining cylinders (for example, # 3 and # 4 cylinders). For example, when the # 1 to # 3 cylinders are richly displaced from the # 4 cylinder, the # 4 cylinders are leanly displaced from the # 1 to # 3 cylinders. Is applicable.

  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.

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 the sensor before a catalyst. It is a graph which shows the output characteristic of a post-catalyst sensor. It is a flowchart which shows an air fuel ratio control routine. It is a calculation map of the main air-fuel ratio correction amount. It is a flowchart which shows the setting routine of an auxiliary air fuel ratio correction amount. It is a graph which shows the mode of 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 figure for demonstrating the case where one cylinder has shifted | deviated to the air-fuel-ratio rich side from the other 3 cylinders, and is a figure for demonstrating the 1st aspect of abnormality detection. It is a graph which shows the relationship between the air-fuel-ratio deviation | shift amount to the rich side of 1 cylinder, and the hydrogen amount discharged | emitted from a combustion chamber. It is the test result which investigated the relationship between the imbalance ratio and the post-catalyst sensor learning value. It is a flowchart which shows the abnormality detection routine which concerns on the one aspect | mode of abnormality detection. It is a graph which shows the fluctuation | variation of the exhaust air-fuel ratio when the air-fuel ratio variation abnormality between cylinders generate | occur | produces. It is a flowchart which shows the abnormality detection routine containing 1st Example of a preliminary | backup detection. It is a flowchart which shows the abnormality detection routine containing 2nd Example of preliminary | backup detection. It is a time chart for demonstrating the measuring method of the amount of occluded oxygen and the amount of released oxygen. It is a flowchart which shows the abnormality detection routine containing 3rd Example of preliminary | backup detection.

Explanation of symbols

1 Internal combustion engine 3 Combustion chamber 6 Exhaust pipe 11 Catalyst 12 Injector 14 Exhaust manifold 17 Pre-catalyst sensor 18 Post-catalyst sensor 20 Electronic control unit (ECU)

Claims (3)

A catalytic element that is disposed in an exhaust passage of a multi-cylinder internal combustion engine and oxidizes and purifies at least hydrogen contained in the exhaust;
A first air-fuel ratio sensor that is disposed in an exhaust passage upstream of the catalyst element and detects a first exhaust air-fuel ratio that is an air-fuel ratio of exhaust that does not pass through the catalyst element;
A second air-fuel ratio sensor that is disposed in an exhaust passage downstream of the catalyst element and detects a second exhaust air-fuel ratio that is an air-fuel ratio of exhaust that has passed through the catalyst element;
The main air-fuel ratio control for matching the detected value of the first exhaust air-fuel ratio with a predetermined first target air-fuel ratio and the detected value of the second exhaust air-fuel ratio match with a predetermined second target air-fuel ratio Air-fuel ratio control means for executing such auxiliary air-fuel ratio control, wherein the control amount for the auxiliary air-fuel ratio control is updated at a predetermined update speed based on the output of the second air-fuel ratio sensor Means,
A variation abnormality detecting means for detecting the occurrence of an inter-cylinder air-fuel ratio variation abnormality when the control amount reaches a predetermined abnormality determination value that corrects the second exhaust air-fuel ratio to a richer side;
The air-fuel ratio control means executes the auxiliary air-fuel ratio control while keeping the control amount within a predetermined guard range, and the detection of the abnormality in the air-fuel ratio variation between cylinders by the variation abnormality detection means is performed. An apparatus for detecting an abnormality in an air-fuel ratio variation between cylinders of a multi-cylinder internal combustion engine, wherein at least one of expanding a range so as to include at least the abnormality determination value and increasing an update speed of the control amount is executed. . Before detecting the variation abnormality between cylinders air-fuel ratio by the variation abnormality detection means, comprising preliminary detection means for preliminarily detecting the possibility of occurrence of variation abnormality between cylinders air-fuel ratio,
The variation abnormality detecting means performs detection of an abnormality in air-fuel ratio variation between cylinders when it is detected by the preliminary detection means that there is a possibility of occurrence of an abnormality in air-fuel ratio variation between cylinders. Item 4. The inter-cylinder air-fuel ratio variation abnormality detection device for a multi-cylinder internal combustion engine according to Item 1. When the integrated value obtained by integrating the difference between the output of the first air-fuel ratio sensor and the sensor output corresponding to the first target air-fuel ratio for a predetermined time exceeds a predetermined value, the preliminary detection means The second exhaust air-fuel ratio leaner than the target air-fuel ratio of 1 is detected by the second air-fuel ratio sensor for a predetermined time or more, and the ratio or difference between the stored oxygen amount and the released oxygen amount of the catalyst element 3. The inter-cylinder air-fuel ratio of the multi-cylinder internal combustion engine according to claim 2, wherein there is a possibility of occurrence of an abnormal variation in the inter-cylinder air-fuel ratio when at least one of the two is greater than a predetermined value. Variation abnormality detection device.

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