JP5035688B2 - Air-fuel ratio sensor abnormality diagnosis device - Google Patents

Description

  The present invention relates to an abnormality diagnosis device for an air-fuel ratio sensor, and more particularly to an abnormality diagnosis device for an air-fuel ratio sensor disposed in an exhaust passage of a multi-cylinder internal combustion engine.

  In an internal combustion engine equipped with an exhaust gas purification system using a catalyst, in order to effectively remove harmful components of exhaust gas by the catalyst, the mixture ratio of air and fuel in the air-fuel mixture burned in the internal combustion engine, that is, the air-fuel ratio is reduced. Control is essential. In order to perform such control of the air-fuel ratio, an air-fuel ratio sensor for detecting the air-fuel ratio is provided in the exhaust passage of the internal combustion engine based on the concentration of a specific component of the exhaust gas, and the detected air-fuel ratio is set to a predetermined target air-fuel ratio. Feedback control is carried out so that it can approach.

  By the way, if an abnormality such as deterioration or failure occurs in the air-fuel ratio sensor, accurate air-fuel ratio feedback control cannot be performed, and exhaust gas emission deteriorates. Therefore, it has been conventionally performed to diagnose abnormality of the air-fuel ratio sensor. In particular, in the case of an engine mounted on an automobile, it is requested by the laws and regulations of each country to detect an abnormality in the air-fuel ratio sensor in an on-board state (onboard) in order to prevent traveling in a state where exhaust gas has deteriorated. ing.

  For example, Patent Document 1 discloses an apparatus for diagnosing an abnormality of an air-fuel ratio sensor based on a comparison result between a change amount of an air-fuel ratio correction coefficient and a change amount of a target air-fuel ratio when the target air-fuel ratio changes suddenly.

JP-A-8-270482

  By the way, in a multi-cylinder internal combustion engine, the actual air-fuel ratio may shift or vary between cylinders. For example, the fuel injection amount of some cylinders may be reduced due to a failure in the fuel injection system of some cylinders. There is a case where the fuel injection amount deviates greatly. In this case, the output of the air-fuel ratio sensor is also affected by the variation in the air-fuel ratio between the cylinders, and although the air-fuel ratio sensor is normal, a normal output cannot be obtained and the air-fuel ratio sensor is erroneously diagnosed as abnormal. There is a possibility that. Even in the apparatus described in Patent Document 1, if the air-fuel ratio variation between cylinders occurs when the air-fuel ratio sensor is normal, a normal change amount of the air-fuel ratio correction coefficient cannot be obtained due to this variation, and a misdiagnosis may occur. There is sex.

  Accordingly, the present invention has been made in view of such circumstances, and an object of the present invention is to provide an abnormality diagnosis device for an air-fuel ratio sensor capable of preventing erroneous diagnosis by eliminating the influence of air-fuel ratio variation between cylinders. is there.

According to one aspect of the invention,
An abnormality diagnosis apparatus for an air-fuel ratio sensor disposed in an exhaust passage of a multi-cylinder internal combustion engine,
Diagnosing means for diagnosing abnormality of the air-fuel ratio sensor based on at least the output of the air-fuel ratio sensor;
Detecting means for detecting an imbalance parameter which is a parameter relating to air-fuel ratio variation between cylinders;
An abnormality diagnosis device for an air-fuel ratio sensor is provided, comprising: output correction means for correcting the output of the air-fuel ratio sensor based on the imbalance parameter detected by the detection means.

  According to this, based on the detected imbalance parameter, the output of the air-fuel ratio sensor can be corrected to a value that can be obtained when there is no inter-cylinder air-fuel ratio variation. As a result, the influence of the variation in the air-fuel ratio between cylinders can be eliminated, and the original air-fuel ratio sensor output can be obtained to prevent erroneous diagnosis.

  Preferably, the output correction unit corrects the output by executing a smoothing process or a filtering process on the output, and performs the correction according to the imbalance parameter detected by the detection unit. The smoothing rate in the mashing process is set, or the cutoff frequency in the filtering process is set.

  When the cylinder-to-cylinder air-fuel ratio variation occurs, fluctuations in the air-fuel ratio sensor output increase, and high-frequency fluctuations are superimposed on the output waveform, but if the smoothing or filtering process is performed on the output, High frequency fluctuation components can be suppressed or eliminated. In particular, since the output fluctuation tends to increase as the degree of variation in the air-fuel ratio between the cylinders increases, it is preferable to set the smoothing rate or the cut-off frequency according to the imbalance parameter.

  Preferably, the diagnosis means determines a parameter in the first-order lag element based on an input to a model obtained by modeling a system from a fuel injection valve to the air-fuel ratio sensor with a first-order lag element and an output of the air-fuel ratio sensor. An identifying means for identifying and an abnormality determining means for judging an abnormality of a predetermined characteristic of the air-fuel ratio sensor based on the parameters identified by the identifying means.

  By using such an identification method using a first-order lag model, a parameter in the first-order lag element is identified, and an abnormality of a predetermined characteristic of the air-fuel ratio sensor can be diagnosed based on the identified parameter. An abnormality of a predetermined characteristic of the air-fuel ratio sensor can be diagnosed instead of a simple abnormality of the air-fuel ratio sensor, so that the abnormality diagnosis can be executed more precisely.

According to another aspect of the invention,
An air-fuel ratio sensor abnormality diagnosis device for detecting an air-fuel ratio of exhaust gas of a multi-cylinder internal combustion engine,
Identification means for identifying a parameter in the first-order lag element based on an input to a model obtained by modeling a system from a fuel injection valve to the air-fuel ratio sensor with a first-order lag element, and an output of the air-fuel ratio sensor;
An abnormality determining means for determining an abnormality of a predetermined characteristic of the air-fuel ratio sensor based on the parameter identified by the identifying means;
Detecting means for detecting an imbalance parameter which is a parameter relating to air-fuel ratio variation between cylinders;
An abnormality diagnosis apparatus for an air-fuel ratio sensor is provided, comprising: identification parameter correction means for correcting the identified parameter based on the imbalance parameter detected by the detection means.

  When the air-fuel ratio variation between cylinders occurs, the fluctuation of the air-fuel ratio sensor output becomes large, and the fluctuation of the high frequency is superimposed on the output waveform. The parameter is reduced as compared with the case where there is no variation in the air-fuel ratio between cylinders. According to another aspect of the present invention, based on the detected imbalance parameter, the identified parameter can be corrected to a value that can be obtained when there is no inter-cylinder air-fuel ratio variation. As a result, the influence of variations in the air-fuel ratio between cylinders can be eliminated, and the original identification parameter can be obtained to prevent erroneous diagnosis.

  Preferably, the parameter in the first-order lag element includes at least a gain and a time constant, and the predetermined characteristic of the air-fuel ratio sensor includes at least an output corresponding to the gain and responsiveness corresponding to the time constant.

  As a result, the output and responsiveness, which are typical characteristics of the air-fuel ratio sensor, can be diagnosed individually and simultaneously, and a more detailed and detailed abnormality diagnosis can be executed.

  Preferably, the detection means calculates the imbalance parameter based on a locus length or locus area per predetermined time of the output of the air-fuel ratio sensor.

  As the variation in the air-fuel ratio between the cylinders increases, the variation and amplitude of the air-fuel ratio sensor output increase, and the locus length or locus area per predetermined time of the air-fuel ratio sensor output increases. Therefore, the imbalance parameter is calculated based on the trajectory length or trajectory area of the air-fuel ratio sensor output using this correlation.

Alternatively, the detection means comprises
A catalyst that is disposed in the exhaust passage downstream of the air-fuel ratio sensor and oxidizes and purifies at least hydrogen contained in the exhaust;
A post-catalyst air-fuel ratio sensor disposed in the exhaust passage downstream of the catalyst;
A main air-fuel ratio control that matches the air-fuel ratio detected by the air-fuel ratio sensor with a predetermined target air-fuel ratio and an auxiliary air-fuel ratio that matches the air-fuel ratio detected by the post-catalyst air-fuel ratio sensor with the target air-fuel ratio. Air-fuel ratio control means for performing fuel ratio control, calculating air-fuel ratio control means for calculating the control amount for the auxiliary air-fuel ratio control based on the output of the post-catalyst air-fuel ratio sensor;
It is preferable to comprise means for calculating the imbalance parameter based on the calculated control amount.

  There is a characteristic that the control amount in the auxiliary air-fuel ratio control changes to an increasing value that corrects the air-fuel ratio to a richer side as the variation in the air-fuel ratio between the cylinders becomes larger. Therefore, using this characteristic, an imbalance parameter is calculated based on the control amount.

Alternatively, the detection means comprises
A catalyst disposed in the exhaust passage downstream of the air-fuel ratio sensor;
Means for detecting the amount of oxygen stored and released from the catalyst;
It is preferable to comprise means for calculating the imbalance parameter based on the ratio or difference between the detected stored oxygen amount and the released oxygen amount.

  When the cylinder-to-cylinder air-fuel ratio variation occurs, the symmetrical relationship between the stored oxygen amount and the released oxygen amount is broken, and the other becomes larger than the other. Therefore, using this, the imbalance parameter is calculated based on the ratio or difference between the stored oxygen amount and the released oxygen amount.

  Preferably, the imbalance parameter is an imbalance ratio, and the imbalance ratio is expressed by the formula: IB = (Q−Qs) / Qs (where IB is the imbalance ratio, Q is the fuel injection amount of the imbalance cylinder, Qs Is expressed by the fuel injection amount of the balance cylinder).

  This imbalance ratio is particularly suitable as a parameter relating to variations in the air-fuel ratio between cylinders, that is, an imbalance parameter.

  According to the present invention, an excellent effect is exhibited in that erroneous diagnosis can be prevented by eliminating the influence of the variation in 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. Catalysts 11 and 19 made of a three-way catalyst are respectively attached in series on the upstream side and the downstream side of the exhaust pipe 6. 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 upstream 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 upstream catalyst 11, and detect the air-fuel ratio based on the oxygen concentration in the exhaust. In this way, the single pre-catalyst sensor 17 is installed at the exhaust merging portion on the upstream side of the upstream catalyst 11.

  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 pre-catalyst sensor 17 is a so-called wide-range air-fuel ratio sensor, and can continuously detect an air-fuel ratio over a relatively wide range. FIG. 2 shows the output characteristics of the pre-catalyst sensor 17. As shown in the figure, the pre-catalyst sensor 17 outputs a voltage signal Vf having a magnitude proportional to the detected exhaust air-fuel ratio (pre-catalyst air-fuel ratio A / Ff). The output voltage when the exhaust air-fuel ratio is stoichiometric 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 when the exhaust air-fuel ratio (post-catalyst air-fuel ratio A / Fr) is leaner than the stoichiometric, a low voltage of about 0.1 V is obtained. When the exhaust air-fuel ratio 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.

The upstream catalyst 11 and the downstream catalyst 19 are NOx, which are harmful components in the exhaust when the air-fuel ratio A / F of the exhaust gas flowing into each of them is close to the stoichiometric air-fuel ratio (stoichiometric, for example, A / F = 14.6). Purify HC and CO simultaneously. The air-fuel ratio width (window) that can simultaneously purify these three with high efficiency is relatively narrow. In addition, the upstream catalyst 11 and the downstream catalyst 19 also purify by oxidizing (combusting) hydrogen H ₂ present in the exhaust gas.

  In particular, the ECU 20 executes air-fuel ratio control so that the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 11 is controlled near the stoichiometric range. This air-fuel ratio control is detected by a main air-fuel ratio control (main air-fuel ratio feedback control) that makes the exhaust air-fuel ratio detected by the pre-catalyst sensor 17 coincide with a stoichiometry that is a predetermined target air-fuel ratio, and detected by the post-catalyst sensor 18. The auxiliary air-fuel ratio control (auxiliary air-fuel ratio feedback control) is performed so that the exhaust air-fuel ratio thus made coincides with the stoichiometry.

[Basic contents of air-fuel ratio sensor abnormality diagnosis]
Next, abnormality diagnosis of the air-fuel ratio sensor in the present embodiment will be described. The object of diagnosis in this embodiment is an air-fuel ratio sensor installed on the upstream side of the upstream catalyst 11, that is, a pre-catalyst sensor 17.

  In the abnormality diagnosis, the system from the injector 12 to the pre-catalyst sensor 17 is modeled by a first-order lag element, and the parameters of the first-order lag element are identified based on the input to this model and the output of the pre-catalyst sensor 17 ( Presumed. Then, based on the identified parameter, an abnormality of a predetermined characteristic of the pre-catalyst sensor 17 is determined.

  As an input, a ratio Ga / Q between the fuel injection amount Q calculated based on the energization time of the injector 12 and the intake air amount Ga calculated based on the output of the air flow meter 5, that is, the input air-fuel ratio is used. Hereinafter, the input or the input air-fuel ratio is represented by u (t) (u (t) = Ga / Q). On the other hand, as the output of the pre-catalyst sensor 17, the output air-fuel ratio, that is, the pre-catalyst air-fuel ratio A / Ff converted from the output voltage Vr of the pre-catalyst sensor 17 is used. Hereinafter, the output or the output air-fuel ratio is represented by y (t) (y (t) = A / Ff). A parameter in the first-order lag element is identified from how the output air-fuel ratio y (t) is given when the input air-fuel ratio u (t) is given to the model, and predetermined characteristics of the pre-catalyst sensor 17 are determined based on the identified parameter. Is determined to be abnormal.

  As shown in FIG. 4, in the present embodiment, active control (identification active control) for forcibly oscillating the air-fuel ratio is executed during parameter identification. In this active control, the target air-fuel ratio A / Ft, that is, the input air-fuel ratio u (t) is oscillated at a constant cycle so that the target air-fuel ratio u (t) fluctuates by the same amplitude on the lean side and the rich side with respect to the predetermined center air-fuel ratio A / Fc. It is done. The amplitude of the vibration is larger than that in the normal air-fuel ratio control, for example, 0.5 at the air-fuel ratio. The center air-fuel ratio A / Fc is made equal to the stoichiometric air-fuel ratio.

  The reason for executing this active control is that the active control is executed during the steady operation of the engine, so that each control amount and each detected value are stabilized, and diagnostic accuracy is improved. This is also because when the input air-fuel ratio u (t) is deliberately changed, the quality (especially output and responsiveness) of each sensor can be suitably diagnosed. However, abnormality diagnosis may be executed during normal air-fuel ratio control.

  As shown in the figure, the input air-fuel ratio u (t) has a substantially stepped waveform, whereas the output air-fuel ratio y (t) has a waveform with a first-order lag. In the figure, L is a dead time based on a transport delay from the input air-fuel ratio u (t) to the output air-fuel ratio y (t). That is, the dead time L corresponds to a time difference from the time of fuel injection in the injector 12 until the exhaust gas resulting from the fuel injection reaches the pre-catalyst sensor 17.

  Assuming that the dead time L is zero for simplification, the first-order lag element is represented by G (s) = k / (1 + Ts). Here, k is the gain of the pre-catalyst sensor 17, and T represents the time constant of the pre-catalyst sensor 17. The gain k is a value related to output among the characteristics of the pre-catalyst sensor 17, while the time constant T is a value related to responsiveness among the characteristics of the pre-catalyst sensor 17. In FIG. 4, a solid line representing the output air-fuel ratio y (t) indicates a case where the pre-catalyst sensor 17 is normal. On the other hand, when an abnormality occurs in the output characteristics of the pre-catalyst sensor 17, the gain k becomes larger than normal, and the sensor output increases (expands) as indicated by a, or the gain k becomes smaller than normal. As shown by b, the sensor output decreases (reduces). Therefore, an increase abnormality or a decrease abnormality of the sensor output can be specified by comparing the identified gain k with a predetermined value. On the other hand, if an abnormality occurs in the responsiveness of the pre-catalyst sensor 17, in most cases, the time constant T becomes larger than normal, and the sensor output comes out with a delay as shown by c. Therefore, the responsiveness abnormality of the sensor can be specified by comparing the identified time constant T with a predetermined value.

  Next, a method for identifying the gain k and time constant T executed by the ECU 20 will be described.

Equation (20) is a function of values at the current sample time t and the previous sample time t−1, and this equation means that b ₁ and b ₂ are based on the current value and the previous value, That is, T and k are updated every time. Thus, the time constant T and the gain k are sequentially identified by the sequential least square method. In this sequential identification method, a large number of sample data is acquired and temporarily stored, and the calculation load can be reduced as compared with the method of performing identification, and the capacity of the buffer for temporarily storing data can be reduced. It is suitable for mounting on an ECU (particularly an automotive ECU).

  The sensor characteristic abnormality determination method executed by the ECU 20 is as follows. First, when the identified time constant T is larger than a predetermined time constant abnormality determination value Ts, a response delay occurs, and it is determined that the pre-catalyst sensor 17 is responsive abnormality. On the other hand, when the identified time constant T is equal to or less than the time constant abnormality determination value Ts, the pre-catalyst sensor 17 is determined to be normal with respect to responsiveness.

  If the identified gain k is greater than the predetermined gain increase abnormality determination value ks1, the pre-catalyst sensor 17 is determined to have an output increase abnormality, and the identified gain k is the gain reduction abnormality determination value ks2 (<ks1). If it is smaller, it is determined that the pre-catalyst sensor 17 has an output decrease abnormality. When the identified gain k is not less than the gain reduction abnormality determination value ks2 and not more than the gain increase abnormality determination value ks1, it is determined that the pre-catalyst sensor 17 is normal with respect to the output.

  As described above, according to the abnormality diagnosis according to the present invention, the abnormality of the predetermined characteristic of the air-fuel ratio sensor is determined instead of simply determining the abnormality of the air-fuel ratio sensor itself. Based on the two identification parameters T and k, abnormalities in the two sensor characteristics of responsiveness and output are determined particularly simultaneously and individually. Therefore, it is possible to realize a very precise and preferable one for abnormality diagnosis of the air-fuel ratio sensor.

  5 and 6 show the results of sequentially identifying the time constant T and the gain k by the sequential least square method in the case of the normal pre-catalyst sensor 17 and the abnormal case of the pre-catalyst sensor 17. FIG. 5 shows the case of the normal pre-catalyst sensor 17, and FIG. 6 shows the case of the abnormal pre-catalyst sensor 17. FIG. 5A and FIG. 6A show the vibration of the input air-fuel ratio (broken line) and the output air-fuel ratio (solid line).

  FIG. 5B and FIG. 5B show transitions of the time constant T (broken line) and the gain k (solid line) from the start of active control. The time constant T and the gain k are updated every sampling time and gradually converge to a constant value. The time constant T and the gain k are acquired at a time point (determination time) t1 after a predetermined time (for example, 5 seconds) has elapsed from when the active control starts (at the start of identification) t0, and the values almost converge. The obtained time constant T and gain k are compared with the abnormality determination values Ts1, ks1, and ks2, and abnormality determination of responsiveness and output is made.

  When a test was performed using an abnormal pre-catalyst sensor 17 having a response almost equal to that of the normal pre-catalyst sensor 17 and an output of ½, the time constant T at the determination time t1 is It was almost the same as 0.18 for the normal sensor and 0.17 for the abnormal sensor. On the other hand, the gain k at the determination time t1 is 1 for the normal sensor and 0.5 for the abnormal sensor. As a result, it was confirmed that the same result as the actual sensor could be obtained.

  By the way, an actual engine has various disturbances such as load fluctuations, and the identification accuracy and robustness cannot be improved unless these are properly considered. For this reason, in the abnormality diagnosis according to the present embodiment, various corrections are performed on the following input / output data.

  FIG. 7 is a block diagram of the entire system for identifying model parameters. Such a system is built in the ECU 20. In order to identify the parameters T and k as described above in the identification unit (identification unit) 50, an input calculation unit 52, a bias correction unit (bias correction unit) 54, and a dead time correction unit (dead time correction unit) 56 are provided. It is done. Note that an active control flag output unit 58 is also provided because abnormality diagnosis is performed during active control.

  The input calculation unit 52 calculates the input air-fuel ratio u (t). In the above example, the input air-fuel ratio u (t) is a ratio Ga / Q between the fuel injection amount Q calculated based on the energization time of the injector 12 and the intake air amount Ga calculated based on the output of the air flow meter 5. Met. However, here, the fuel injection amount Q calculated based on the injector energization time is corrected based on the fuel wall adhesion amount and the evaporation amount, and the corrected fuel injection amount Q ′ is used to input the air / fuel ratio u (t ) Is calculated. u (t) = Ga / Q ', and as a result, the input air-fuel ratio u (t) is corrected based on the amount of fuel adhering to the wall and the amount of evaporation.

  When fuel is injected from the injector 12, most of the fuel is sucked into the in-cylinder combustion chamber 3, but the remaining portion adheres to the inner wall surface of the intake port and does not enter the combustion chamber 3. Therefore, if the fuel amount injected from the injector 12 is fi, and the fuel adhesion rate for all cylinders is R (<1), the amount of the injected fuel amount fi that adheres to the wall surface of the intake port is R · fi, The amount entering the combustion chamber 3 is represented by (1-R) · fi.

  On the other hand, some of the fuel adhering to the wall surface of the intake port evaporates and enters the combustion chamber 3 in the next intake stroke, but the rest remains and remains attached. Therefore, if the fuel amount adhering to the wall surface of the intake port is fw and the fuel residual ratio for all cylinders is P (<1), the amount of fuel adhering to the wall surface of the wall surface adhering fuel amount fw is P · fw, The amount that enters the combustion chamber 3 is represented by (1-P) · fw.

The intake, compression, expansion, and exhaust strokes of the four-cycle engine are completed once to make one cycle (that is, one cycle = 720 ° crank angle), the current cycle is ks, and the next cycle is ks + 1. Further, when the fuel amount entering the in-cylinder combustion chamber 3 is fc, the following relationship is established.
fw (ks + 1) = P · fw (ks) + R · fi (ks) (21)
fc (ks) = (1-P) .fw (ks) + (1-R) .fi (ks) (22)

  The expression (21) means that the wall-attached fuel amount fw (ks + 1) of the next cycle is the residual amount P · fw (ks) of the wall-attached fuel amount fw (ks) of the current cycle and the injected fuel of the current cycle. That is, it is expressed as the sum of the amount fi (ks) and the wall surface adhesion R · fi (ks). In addition, the expression (22) means that the inflow fuel amount fc (ks) flowing into the combustion chamber 3 in the current cycle is an evaporation amount (1-) of the wall surface attached fuel amount fw (ks) in the current cycle. P) · fw (ks) and the sum (1-R) · fi (ks) of the injected fuel amount fi (ks) of this cycle that flows directly into the combustion chamber 3 without adhering to the wall surface. That is.

  Thus, when calculating the input air-fuel ratio u (t), the value of the inflow fuel amount fc is used as the value of the fuel injection amount Q ′. This inflow fuel amount fc is nothing but a correction of the fuel injection amount calculated based on the energization time of the injector 12 based on the amount of fuel adhering to the wall and the evaporation amount. Therefore, by using the value of the inflow fuel amount fc for calculating the input air-fuel ratio u (t), the value of the input air-fuel ratio can be made a more accurate value close to the actual situation, and the parameter identification accuracy can be improved. Is possible.

  Note that the higher the engine temperature and the intake air temperature, the more the fuel vaporization is promoted, so the fuel adhesion amount decreases and the fuel evaporation amount increases. Therefore, the fuel residual rate P and the fuel adhesion rate R are preferably functions of at least one of the engine temperature (or water temperature) and the intake air temperature. The correction based on the fuel wall adhesion amount and the evaporation amount as described herein is referred to as “fuel dynamics correction”.

  FIG. 8 shows the test results obtained by examining the difference in the input air-fuel ratio u (t) during active control when there is no fuel dynamics correction (broken line) and when there is a fuel dynamics correction (solid line). As shown in the circle in the figure, the waveform of the input air-fuel ratio u (t) is slightly rounded immediately after the input air-fuel ratio u (t) is reversed, compared with the case where there is no fuel dynamics correction. It is in.

  Next, the bias correction unit 54 will be described. In the bias correction unit 54, both the input air-fuel ratio u (t) and the output air-fuel ratio y (t) are removed so as to remove the bias between the input air-fuel ratio u (t) and the output air-fuel ratio y (t). Is shift-corrected.

  The input air-fuel ratio u (t) and the output air-fuel ratio y (t) are biased to the lean side or the rich side with respect to one, due to factors such as load fluctuation, learning deviation, and sensor value deviation (deviation). There is a case. FIG. 9 is a test result showing the state of the bias. In the figure, u (t) c and y (t) c respectively represent values obtained by passing the input air-fuel ratio u (t) and the output air-fuel ratio y (t) through a low-pass filter, or their moving averages. Since the air-fuel ratio detected by the pre-catalyst sensor 17 is controlled to be close to the theoretical air-fuel ratio (A / F = 14.6), the output air-fuel ratio y (t), which is the detected value of the pre-catalyst sensor 17, is controlled. Fluctuates around the stoichiometric air-fuel ratio, and the value passed through the low-pass filter or the moving average y (t) c is also kept near the stoichiometric air-fuel ratio. On the other hand, the input air-fuel ratio u (t) is biased to the lean side in the illustrated example for the reason described above.

  Since it is not preferable to perform identification in such a bias state, a correction that removes the bias is performed. Specifically, as shown in FIG. 10, the data of the input air-fuel ratio u (t) and the output air-fuel ratio y (t) are passed through the low-pass filter, or the moving average is calculated, and the bias value u (t) c and y (t) c are calculated sequentially. Then, sequentially, the difference Δu (t) (= u (t) −u (t) c) between the input air-fuel ratio u (t) and its bias value u (t) c and the output air-fuel ratio y (t) And a difference Δy (t) (= y (t) −y (t) c) between the bias value y (t) c and the bias value y (t) c, and the differences Δu (t) and Δy (t) are set to zero reference values. Replaced. These differences Δu (t) and Δy (t) are collectively displayed as ΔA / F (the same applies to FIGS. 5A and 6A).

  Thus, the bias is removed, and the input / output air-fuel ratio values Δu (t) and Δy (t) after the bias removal are changed to zero reference values as shown in FIG. That is, the center of both fluctuations is set to zero, and influences such as load fluctuations and learning deviation can be eliminated. As a result, robustness against load fluctuation, learning deviation, and the like can be improved.

  In this example, the method of correcting both the input / output air-fuel ratio and removing the bias by adjusting the fluctuation center of the input-output air-fuel ratio to zero is adopted, but other methods can also be adopted. For example, it is possible to correct only the input air-fuel ratio and align the fluctuation center with the fluctuation center of the output air-fuel ratio, or vice versa. The correction target may be at least one of the input and output air-fuel ratios.

  Next, the dead time correction unit 56 will be described. As described above, there is a dead time L due to transport delay between the input air-fuel ratio u (t) and the output air-fuel ratio y (t). However, in order to accurately identify the model parameter, it is preferable to perform correction so as to remove this dead time L. Therefore, such a correction is performed by the dead time correction unit 56. Specifically, the dead time L is calculated by a method described later, and the input air-fuel ratio u (t) is delayed so as to approach the output air-fuel ratio y (t) by this dead time L.

  FIG. 12 shows the input air-fuel ratio (dashed line) before the dead time correction, the input air-fuel ratio (solid line) and the output air-fuel ratio (one-dot chain line) after the dead time correction. Note that values after bias correction are used as the input air-fuel ratio and the output air-fuel ratio. When the input air-fuel ratio is delayed by the dead time L, the oscillation of the input air-fuel ratio and the oscillation of the output air-fuel ratio come to synchronize without time difference, thereby improving the accuracy of model parameter identification.

  As a first aspect of the dead time calculation, there is a method of calculating the dead time according to a predetermined map or function based on at least one parameter relating to the engine operating state. FIG. 13 shows an example of such a dead time calculation map. In this map, the dead time L is calculated based on the detected value of the engine speed Ne.

  There is the following as a second mode of dead time calculation. First, the dispersion values of the input air-fuel ratio and the output air-fuel ratio during active control are sequentially obtained by the following equations.

η is a value of the input air-fuel ratio or the output air-fuel ratio, and η _avg is a moving average of M times, that is, an average value of data from (t) to (M−1) times before (t− (M−1)). It is. For example, M is 5 or the like. The dispersion value increases as the change in the input air-fuel ratio or the output air-fuel ratio increases.

  FIG. 14 shows the test results regarding the dead time correction, and shows the case of a normal sensor. The upper graph shows a: the input air-fuel ratio before the dead time correction, b: the input air-fuel ratio after the dead time correction, and c: the output air-fuel ratio. Note that the input air-fuel ratios indicated by a and b are values after the fuel dynamics correction and the bias correction are performed, and the output air-fuel ratios indicated by c are values after the bias correction is performed. The middle graph shows the dispersion value of the input air-fuel ratio before the dead time correction indicated by d: a and the dispersion value of the output air-fuel ratio indicated by e: c. In the lower graph, the sawtooth waveform f is the dead time counter value, the horizontal line g at the high position is the dead time calculated as described later, and the horizontal line h at the lower position is the dead time g ¼. Each of the values is shown.

  FIG. 15 shows only a, c, d and e in FIG. 12 in a simplified manner. As can be seen from FIG. 15, the dispersion value d of the input air-fuel ratio and the dispersion value e of the output air-fuel ratio have the peaks dp and ep of the dispersion values d and e at the timing when the input air-fuel ratio a and the output air-fuel ratio c are reversed. Arise. Therefore, the time difference (ep−dp) between these peaks is calculated as the dead time g. Returning to FIG. 14, when the dispersion value peak dp of the input air-fuel ratio occurs, the dead time counter f starts counting time from the time when the dispersion value peak dp occurs. Then, when the dispersion value peak ep of the output air-fuel ratio occurs, the count is stopped and the count value is held as a dead time g. The dead time g is updated every time the input air-fuel ratio is reversed, and the dead time h after the annealing is calculated for each inversion. The reason for calculating the dead time h after annealing is to remove the influence of noise. The value of the dead time h after annealing will eventually converge to a certain value. Therefore, the dead time h after the annealing is acquired at the time after the predetermined time when the dead time h after the annealing converges to a substantially constant value from the start of the active control, and the obtained value Is determined as the final dead time L.

  By the way, although the above calculation method demonstrated by FIG.14 and FIG.15 is a case of a normal sensor, when it is the case of an abnormal sensor, it is not necessarily appropriate to employ | adopt the same method. That is, as shown in FIGS. 16 and 17, for example, in the case of an abnormal sensor in which a response delay occurs, a sufficiently large value cannot be obtained as the dispersion value e of the output air-fuel ratio b, and its peak ep appears. The error also increases with respect to timing.

  Therefore, the dispersion value peak ep of the output air-fuel ratio is compared with a predetermined threshold value eps, and when the dispersion value peak ep is larger than the threshold value eps as shown in FIGS. 14 and 15, as described above. A time difference g between the dispersion value peaks dp and ep of the input / output air-fuel ratio (ep−dp) is defined as a dead time g. On the other hand, as shown in FIGS. 16 and 17, when the dispersion value peak ep of the output air-fuel ratio is less than or equal to the threshold value eps, the time difference between the extreme values ap and cp of the input / output air-fuel ratios a and c itself (cp− ap) is the dead time g. Thereby, even in the case of an abnormal sensor, the dead time can be calculated accurately.

  Now, it is possible to calculate the dead time using only one of these first and second modes and correct the dead time of the input air-fuel ratio. In the third mode below, The dead time is calculated using both the embodiment and the second embodiment. In the third aspect, the first dead time calculated from the map according to the first aspect is compared with the second dead time calculated from the dispersion value or the extreme value according to the second aspect, and both are approaching. If so, the first dead time calculated from the map is used. On the other hand, if the two are separated, the second dead time calculated from the dispersion value or the extreme value is used, and the map data is updated using the second dead time. Originally, the optimum time delay value does not deviate greatly from the map value, but the optimum time delay value may deviate greatly from the map value for some reason. On the other hand, the second dead time calculated from the variance value or the extreme value can be said to be an actual value reflecting the current situation. Therefore, by performing such a map update, it becomes possible to cope with unforeseen situations, and it is possible to always maintain the map data at an optimum value in accordance with the current situation.

  FIG. 18 schematically shows the procedure of the third aspect. First, in step S101, according to the first mode, a first dead time L1 is calculated from a map as shown in FIG. Next, in step S102, the second dead time L2 is calculated from the dispersion value or extreme value of the input / output air-fuel ratio according to the second mode. Thereafter, in step S103, a dead time deviation amount L12 which is an absolute value of the difference between the first and second dead times L1 and L2 is calculated, and the dead time deviation amount L12 is compared with a predetermined value L12s. If the dead time deviation amount L12 is equal to or smaller than the predetermined value L12s, the first dead time L1 calculated from the map is determined as the final dead time L in step S104, assuming that the deviation between the two is small.

  On the other hand, if the dead time deviation amount L12 is larger than the predetermined value L12s, the second dead time L2 calculated from the dispersion value or extreme value of the input / output air-fuel ratio is determined as the final value in step S105, assuming that the deviation between the two is large. The dead time L is determined. In step S106, the first dead time L1 in the map corresponding to the second dead time L2 is replaced with the second dead time L2, and the map data is updated.

  In the above-described dead time correction, the input air-fuel ratio is delayed by the dead time to make the timing coincide with the output air-fuel ratio. However, other methods can be employed. For example, if you do not perform sequential identification, for example, if you acquire a lot of sample data and store it temporarily, then perform identification, the output air-fuel ratio may be advanced by the dead time to match the timing with the input air-fuel ratio. The input air-fuel ratio can be delayed and the output air-fuel ratio can be advanced to make the timings coincide. The correction target may be at least one of the input and output air-fuel ratios.

  Next, an air-fuel ratio sensor abnormality diagnosis procedure including all the above-described corrections will be described with reference to FIG. First, in step S201, active control for forcibly oscillating the air-fuel ratio is executed. In step S202, the value of the input air-fuel ratio u (t) after fuel dynamics correction is calculated. In step S203, FIG. As shown in FIG. 11, the values of the input air-fuel ratio u (t) and the output air-fuel ratio y (t) are shift-corrected so that there is no bias between the input and output air-fuel ratios.

  In subsequent step S204, the dead time L is calculated, and the value of the input air-fuel ratio u (t) after bias correction is shift-corrected by the dead time L so that the dead time L is eliminated in step S205. In the next step S206, model parameters are derived from the relationship between the input air-fuel ratio u (t) after the dead time correction obtained in step S205 and the output air-fuel ratio y (t) after the bias correction obtained in step S203. A time constant T and a gain k are identified. In step S207, the identified parameters T and k are compared with each abnormality determination value (time constant abnormality determination value Ts, gain increase abnormality determination value ks1 and gain reduction abnormality determination value ks2), and an air-fuel ratio sensor (catalyst) The response of the front sensor 17) and the normality / abnormality of the output are determined.

Various modifications can be considered for the above-described sensor abnormality diagnosis. For example, in the case of a direct injection engine that directly injects fuel into the combustion chamber 3, it is not necessary to consider fuel adhesion to the wall surface of the intake passage, so that fuel dynamics correction is omitted. Although the above diagnosis is an application example to a so-called wide-range air-fuel ratio sensor, an application example to a so-called O ₂ sensor such as the post-catalyst sensor 18 is also possible. A sensor for detecting the air-fuel ratio of exhaust gas including such an O ₂ sensor is referred to as an air-fuel ratio sensor according to the present invention. In the above diagnosis, abnormality of two characteristics of response and output among the characteristics of the air-fuel ratio sensor is diagnosed, but not limited to this, abnormality of one or more characteristics may be diagnosed. Similarly, only one of the time constant T and the gain k, or another parameter in addition to the time constant T and the gain k, may be identified as a parameter in the first-order lag element. In the above diagnosis, the two parameters T and k in the first-order lag element are identified at the same time, and abnormalities in the two characteristics of the air-fuel ratio sensor are judged at the same time. However, the present invention is not limited to this, and at least two parameters are identified with a time difference. Alternatively, abnormality determination of at least two characteristics may be performed with a time difference.

  By the way, in the above-described abnormality diagnosis of the air-fuel ratio sensor (pre-catalyst sensor 17), if the actual air-fuel ratio shifts or varies between cylinders due to, for example, an injector failure in a certain cylinder, the influence will affect the output of the air-fuel ratio sensor. It has been found that even if the air-fuel ratio sensor is normal, it may be erroneously diagnosed as abnormal.

  FIG. 20 shows the output of the pre-catalyst sensor 17 (pre-catalyst air-fuel ratio A / Ff) when there is no such variation in the air-fuel ratio (dotted line) and when there is a solid line (solid line). Note that the waveform when there is air-fuel ratio variation is a waveform when an imbalance ratio described later is + 30%. As shown in the figure, when the air-fuel ratio variation occurs, the air-fuel ratio fluctuates during one engine cycle (= 720 ° crank angle). As a result, the sensor output fluctuates greatly, and there is no air-fuel ratio variation. High-frequency fluctuations are superimposed on the waveform. Due to the influence of the high frequency fluctuation, the finally obtained time constant T and gain k are smaller than originally obtained values.

  FIG. 21 shows the relationship between the air-fuel ratio variation, the time constant T, and the gain k in the case of a normal air-fuel ratio sensor. On the horizontal axis, an imbalance ratio (%) is taken as an imbalance parameter which is a parameter related to the variation (imbalance) of the air-fuel ratio between cylinders. The imbalance ratio is the ratio of the fuel injection amount of the cylinder (imbalance cylinder) causing the fuel injection amount deviation when only one of the cylinders is causing the fuel injection amount deviation. This is a value indicating whether there is a deviation from the fuel injection amount of the cylinder (balance cylinder) that has not caused the fuel injection amount deviation, that is, the reference injection amount. When the imbalance ratio is IB, the fuel injection amount of the imbalance cylinder is Q, and the fuel injection amount of the balance cylinder, that is, the reference injection amount is Qs, IB = (Q−Qs) / Qs. The greater the imbalance ratio, the greater the fuel injection amount deviation between the imbalance cylinder and the balance cylinder, and the greater the air-fuel ratio variation.

  On the other hand, the vertical axis represents the base ratio of the identification values of the time constant T and the gain k. That is, the base ratio is a value obtained by dividing the value of the time constant T and the gain k actually identified by the base value when the imbalance ratio is 0%. In the figure, “L → R time constant” is the time constant identified when the input air-fuel ratio is switched from lean to rich, and “R → L time constant” is the input air-fuel ratio from rich to lean. Each time constant identified when switching to is shown.

  As shown in the figure, as the imbalance ratio increases, the identification values of the time constant T and the gain k become smaller, and may be erroneously determined as abnormal with respect to a normal sensor below the above-described abnormality determination values Ts and ks2. There is.

  Therefore, in the present embodiment, the following correction is performed in order to prevent erroneous diagnosis. First, the actual imbalance ratio IB is detected. Then, based on the value of the detected imbalance ratio IB, the output air-fuel ratio used for identification is corrected (first mode of correction), or the time constant T and gain k that are the identified parameters are corrected. (Second mode of correction).

[Detection of imbalance ratio]
First, the first aspect of imbalance ratio detection will be described. As shown in FIG. 20, when the air-fuel ratio variation between cylinders occurs, high-frequency fluctuations are superimposed on the air-fuel ratio sensor output. Therefore, the imbalance ratio is detected based on the locus length or locus area per predetermined time of the air-fuel ratio sensor output. The trajectory length per predetermined time of the air-fuel ratio sensor output is a value obtained by integrating the amount of change in the air-fuel ratio sensor output during the sampling interval Δ for a predetermined time. The trajectory area per predetermined time of the air-fuel ratio sensor output is a value obtained by integrating the difference between the predetermined reference value and the actual air-fuel ratio sensor output for each sampling interval Δ for a predetermined time. As the variation in the air-fuel ratio between the cylinders increases, the amplitude in the high-frequency fluctuation increases, and the locus length or locus area per predetermined time of the air-fuel ratio sensor output increases. Therefore, the trajectory length or trajectory area is measured, and the imbalance ratio value is obtained using a predetermined map or function.

  Next, a second aspect of imbalance ratio detection will be described. In this aspect, the control amount in the auxiliary air-fuel ratio control is updated to an increasing value that corrects the air-fuel ratio to a richer side as the variation in the air-fuel ratio between the cylinders becomes larger. The imbalance ratio value is detected based on the above.

  When acquiring the control amount in the auxiliary air-fuel ratio control, the main air-fuel ratio control and the auxiliary air-fuel ratio control in which the stoichiometric value is the target air-fuel ratio are executed. Here, the air-fuel ratio control will be described.

  FIG. 22 shows an air-fuel ratio control routine. This routine is repeatedly executed by the ECU 20 every engine cycle (= 720 ° crank angle) or every predetermined sampling interval.

  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 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 S302, the output (output voltage) Vf of the pre-catalyst sensor 17 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, the 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 control amounts for main air-fuel ratio control. For example, when the gain is Pf, it is expressed by Kf = Pf × ΔVf. In step S305, the value of the auxiliary air-fuel ratio correction amount Kr set in another routine as shown in FIG. 24 is acquired. Finally, in step S306, the final fuel injection amount to be injected from the injector 12 of each cylinder, that is, the final injection amount Qfnl is calculated by the formula: Qfnl = Kf × Qb + Kr.

  According to the map of FIG. 23, the larger the pre-catalyst sensor output Vf is than the stoichiometric equivalent sensor output Vref (ΔVf> 0), that is, the greater the actual pre-catalyst air-fuel ratio is on the lean side, the larger the correction 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 S306 is uniformly used for all cylinders. That is, during one engine cycle or a predetermined sampling interval, an amount of fuel equal to the final injection amount Qfnl is sequentially injected from the injector 12 of each cylinder, and after the next engine cycle or a predetermined sampling interval, a newly calculated final value is obtained. The fuel of the injection amount Qfnl is sequentially injected from the injector 12 of each cylinder.

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

  FIG. 24 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 (for example, 16 milliseconds).

  First, in step S401, the timer provided in the ECU 20 is counted, and in step S402, the output (output voltage) Vr of the post-catalyst sensor 17 is acquired. In step S403, 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. 25 shows the post-catalyst sensor output difference ΔVr and how it is integrated.

  In step S404, 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 S405. In step S406, the auxiliary air-fuel ratio correction amount Kr is calculated from the map as shown in FIG. 26 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 S407, 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. 26, the post-catalyst sensor output Vr becomes 0 as the post-catalyst sensor output Vr is smaller than the stoichiometric equivalent sensor output Vrefr (ΔVrg> 0), that is, as the actual post-catalyst air-fuel ratio moves 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.

  By the way, when an abnormality that affects all cylinders occurs in the fuel supply system such as the injector or the air system such as the air flow meter, the absolute value of the feedback correction amount in the main air-fuel ratio control becomes large. The abnormality can be detected and diagnosed by monitoring. For example, when the fuel injection amount is entirely deviated by + 5% from the stoichiometric equivalent amount (that is, the fuel injection amount is deviated by 5% from the stoichiometric equivalent amount in all cylinders), the feedback correction amount in the main air-fuel ratio control Becomes 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 absolute value of the feedback correction amount becomes equal to or greater than a relatively large predetermined value, it can be detected that the fuel supply system or the air system is abnormal as a whole.

  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 occur between the cylinders. FIG. 27 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 amount, while the # 2 to # 4 cylinders are normal and the fuel injection amount is equivalent to the stoichiometric amount. Suppose that it is a quantity. 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 larger when only one cylinder is greatly 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. 28 shows the relationship between the imbalance ratio (%) in one cylinder when the stoichiometric equivalent amount is set as the reference injection amount Qs and the hydrogen amount (g) generated in the combustion chamber of the one cylinder. As shown in the figure, the amount of generated hydrogen increases in a quadratic function as the imbalance ratio increases. Therefore, when only one cylinder is shifted to the rich side by 20%, the total 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. become.

  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%, the deviation of 5% can be eliminated uniformly by correcting the -5% in the main air-fuel ratio feedback control, for example. However, if only one cylinder is shifted by 20% in the former, # 1 cylinder = 15%, # 2 cylinder = -5%, # 3 cylinder even if correction of -5% is performed by the main air-fuel ratio feedback control = -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, the imbalance ratio is calculated as follows by utilizing the characteristic that the amount of hydrogen increases when the air-fuel ratio variation between cylinders is larger than when the whole is shifted and the pre-catalyst sensor output Vf shifts to the rich side. Is going to be 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 (pre-catalyst air-fuel ratio A / Ff) of the exhaust gas that does not pass through the catalyst and hydrogen is not purified is detected by the pre-catalyst sensor 17, and the air-fuel ratio (catalyst of the exhaust gas that has passed through the catalyst and purified hydrogen) The post-catalyst sensor 18 detects the post-air-fuel ratio A / Fr). The air-fuel ratio detected by the pre-catalyst sensor 17 is shifted to the rich side due to the influence of hydrogen from the air-fuel ratio detected by the post-catalyst sensor 18. In other words, the air-fuel ratio detected by the post-catalyst sensor 18 is shifted to the lean side due to the influence of hydrogen than the air-fuel ratio detected by the pre-catalyst sensor 17. Therefore, the imbalance ratio is detected based on the leaning (divergence) state.

  In other words, it can be said that the air-fuel ratio detected by the post-catalyst sensor 18 is a true exhaust air-fuel ratio. The air-fuel ratio detected by the pre-catalyst sensor 17 is obtained by adding a hydrogen component to the true exhaust air-fuel ratio. The exhaust air-fuel ratio is apparently rich. In other words, the pre-catalyst sensor 17 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, the deviation of the detected air-fuel ratio of the pre-catalyst sensor 17 to the rich side with respect to the detected air-fuel ratio of the post-catalyst sensor 18, that is, the deviation of the detected air-fuel ratio of the post-catalyst sensor 18 to the lean side of the detected air-fuel ratio of the pre-catalyst sensor 17 Based on the amount, the imbalance ratio can be detected.

  As shown in FIG. 27, 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 is greatly shifted to the rich side from the air-fuel ratios of the other # 2 to # 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. 27B, 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).

  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. In this way, the main air-fuel ratio correction amount and the auxiliary air-fuel ratio correction amount that are commensurate with the degree of variation are eventually converged.

  By the way, as described with reference to FIGS. 24 to 26, 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 every predetermined time (that is, at a predetermined update speed). Is learned or updated. Here, if the variation in the air-fuel ratio between cylinders occurs due to an injector failure in 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 are It becomes a large positive value that returns a large lean shift to stoichiometry.

  This is shown in FIG. FIG. 29 shows test results obtained by examining the relationship between the imbalance ratio and the post-catalyst sensor learning value ΔVrg. 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, the value of the post-catalyst sensor learning value ΔVrg after the post-catalyst sensor learning value ΔVrg converges to a constant value commensurate with the degree of variation is obtained, and the imbalance ratio is calculated from this value and a predetermined map or function. The value of is obtained.

  Alternatively, the imbalance ratio value may be obtained based on the auxiliary air-fuel ratio correction amount Kr calculated based on the post-catalyst sensor learning value ΔVrg.

  According to this imbalance ratio detection method, high responsiveness is not required for the air-fuel ratio sensor, and a high-speed data sample and an ECU with high processing capability are not required. 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 enables highly accurate detection.

  Next, a third aspect of imbalance ratio detection will be described. In the present embodiment, a three-way catalyst having an oxygen storage capacity is used as the upstream catalyst 11 (and the downstream catalyst 19). In this case, when the air-fuel ratio of the exhaust gas flowing into the catalyst 11 (pre-catalyst air-fuel ratio A / Ff) is leaner than stoichiometric, the catalyst 11 occludes oxygen in the exhaust gas, and the air-fuel ratio of the exhaust gas is richer than stoichiometric. Sometimes the catalyst 11 releases the oxygen already stored. 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 amount of stored oxygen and the amount of released oxygen 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 voltage Vr. (C) shows the integrated value of the amount of oxygen released from the catalyst 11, 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. However, 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 the 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, the imbalance ratio is detected 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, and the imbalance ratio is calculated from the ratio R and a predetermined map or function. A value is determined. As the ratio R increases, the imbalance ratio also increases.

  In addition to the first to third aspects, alternatively, the hydrogen concentration in the exhaust gas upstream of the catalyst may be detected by a hydrogen concentration sensor, and the imbalance ratio may be detected based on the detected value. . This is because when the air-fuel ratio variation between cylinders occurs, the hydrogen concentration in the exhaust gas increases rapidly, and a correlation is seen between the imbalance ratio and the hydrogen concentration in the exhaust gas.

[First mode of correction (correction of output air-fuel ratio)]
If the imbalance ratio is detected as described above, the output air-fuel ratio y (t) used for identifying the time constant T and the gain k as identification parameters is corrected for the pre-catalyst sensor 17 next. Is done. In other words, the output air-fuel ratio y (t) is corrected before parameter identification, and parameter identification is performed based on the corrected output air-fuel ratio y ′ (t).

  In this correction, the output air-fuel ratio y (t) is corrected by executing a smoothing process or a filtering process on the output air-fuel ratio y (t). As described with reference to FIG. 20, if there is a variation in the air-fuel ratio between cylinders, high-frequency fluctuations are superimposed on the output of the pre-catalyst sensor 17. Is done. Then, according to the detected imbalance ratio, an annealing rate in the annealing process is set, or a cutoff frequency in the filtering process is set.

  The annealing process is executed every predetermined sampling period Δ, and the current output air-fuel ratio is y (t), the output air-fuel ratio after the current annealing is y ′ (t), the output air-fuel ratio after the previous annealing. Is y ′ (t−1) and the annealing rate is M, the output air-fuel ratio y ′ (t) after the current annealing is calculated by the following equation.

  Then, the annealing rate M corresponding to the detected imbalance ratio is acquired from a predetermined map or function, and the acquired annealing rate M is the output air-fuel ratio y ′ (t) after the current annealing. Used for calculation. FIG. 31 shows an example of a map in which the relationship between the imbalance ratio and the annealing ratio M is determined in advance. As shown in the figure, when the imbalance ratio is 0 (%), the annealing rate M is 1, and no annealing is substantially performed. However, as the imbalance ratio increases from 0 (%) to the plus side or the minus side, the annealing rate M tends to increase from 1 (2, 4 in the illustrated example), and the degree of annealing increases. To go. This corresponds to the fact that the amplitude of the high frequency fluctuation increases as the imbalance ratio increases. In this way, the smoothing process according to the imbalance ratio can be executed, the influence of the variation in the air-fuel ratio between the cylinders can be removed from the output of the pre-catalyst sensor 17, and the abnormality diagnosis of the pre-catalyst sensor 17 can be performed accurately, thereby preventing erroneous diagnosis.

  As for the filtering process, the signal of the output air-fuel ratio y (t) is passed through the low-pass filter, and only the signal lower than the predetermined cutoff frequency fc is passed through the low-pass filter. Then, a cutoff frequency fc corresponding to the detected imbalance ratio is acquired from a predetermined map or function, and a filtering process is executed based on the acquired cutoff frequency fc. Although not shown, when the imbalance ratio is 0 (%), the cut-off frequency fc is a predetermined default value, and the cut-off frequency fc increases as the imbalance ratio deviates from 0 (%) to the plus side or the minus side. It tends to decrease from the default value. This is because high frequency fluctuations increase as the imbalance ratio increases, and it is necessary to suppress this. Thus, the filtering process according to the imbalance ratio can be executed, and the abnormality diagnosis of the pre-catalyst sensor 17 can be performed accurately by removing the influence of the variation in the air-fuel ratio between the cylinders from the output of the pre-catalyst sensor 17, thereby preventing erroneous diagnosis.

  FIG. 32 shows a procedure of air-fuel ratio sensor abnormality diagnosis processing including the first mode. The process is executed by the ECU 20.

  First, in step S501, it is determined whether a precondition for abnormality diagnosis is satisfied. This precondition includes, for example, that the engine has been warmed up (for example, the cooling water temperature is 75 ° C. or higher), the pre-catalyst sensor 17 and the post-catalyst sensor 18 have been activated, and the catalysts 11 and 19 are It is already done.

  If the precondition is not satisfied, step S501 is repeated to enter a standby state. On the other hand, if the precondition is satisfied, in step S502, the value of the imbalance ratio IB is detected and acquired according to any of the first to third aspects described above.

  Next, the process proceeds to step S503, and the smoothing rate M or the cutoff frequency fc corresponding to the detected imbalance ratio value is calculated and acquired. In step S504, the output of the pre-catalyst sensor 17, that is, the output air-fuel ratio y (t) is continuously processed or filtered to correct the output air-fuel ratio y (t).

  Next, in step S505, it is determined whether a condition (identification condition) suitable for executing the identification of the gain k and the time constant T as identification parameters is satisfied. The identification condition is, for example, that the engine is in a steady operation state.

  If the identification condition is not satisfied, step S505 is repeated to enter a standby state. On the other hand, if the identification condition is satisfied, the identification of the gain k and the time constant T using the first-order lag model described above is executed in step S506.

  If the identification values of the gain k and the time constant T are thus obtained, in step S507, each identification value and the corresponding abnormality determination value (gain increase abnormality determination value ks1, gain reduction abnormality determination value ks2, and time constant abnormality) are obtained. The determination value Ts) is compared. If the identified gain k is in the range of ks1 ≦ k ≦ ks2 and the identified time constant T is in the range of T ≦ Ts, it is determined in step S508 that the pre-catalyst sensor 17 is normal. The On the other hand, if at least one of the identified gain k and time constant T is not within each range, it is determined in step S509 that the pre-catalyst sensor 17 is abnormal. The process ends here. When it is determined that the pre-catalyst sensor 17 is abnormal, abnormality of each characteristic is individually determined according to a comparison result between each identification value and each abnormality determination value. Also, a warning device such as a check lamp is activated to inform the user of the fact of the abnormality.

[Second mode of correction (correction of gain and time constant)]
Next, the second mode of correction will be described. In this aspect, the identified gain k and time constant T are corrected based on the detected imbalance ratio IB. As shown in FIG. 21, the value of the gain k and the time constant T decreases as the degree of variation in the air-fuel ratio between cylinders increases and the imbalance ratio increases. Therefore, the gain k and the time constant T are corrected to the increasing side according to the detected imbalance ratio so that the gain k and the time constant T become values when the imbalance ratio = 0 (%). As a result, the gain k and the time constant T excluding the influence of the variation in the air-fuel ratio between the cylinders can be compared with each abnormality determination value, that is, the abnormality can be determined, and erroneous diagnosis due to the influence of the variation in the air-fuel ratio between the cylinders can be prevented. .

  FIG. 33 shows an example of a map in which the relationship between the imbalance ratio, the gain correction coefficient Hk, and the time constant correction coefficient HT is determined in advance. The gain correction coefficient Hk and the time constant correction coefficient HT are values multiplied by the identified gain k and time constant T, respectively. As shown in the figure, when the imbalance ratio is 0 (%), the correction coefficients Hk and HT are 1, and no correction is substantially performed. However, the correction coefficients Hk and HT tend to increase from 1 (1.1, 1.2, 1 in the illustrated example) as the imbalance ratio increases from 0 (%) to the plus side or minus side. .3) The degree of correction increases. This corresponds to the fact that the identification value of the gain k and the time constant T decreases as the imbalance ratio increases. By this correction, the identification values of the gain k and the time constant T can be corrected to values when there is no inter-cylinder air-fuel ratio variation.

  FIG. 34 shows the procedure of abnormality diagnosis processing including the second mode. The process is executed by the ECU 20.

  First, in step S601, as in step S501, it is determined whether a precondition for abnormality diagnosis is satisfied. If the precondition is not satisfied, step S601 is repeated to enter a standby state. On the other hand, when the precondition is satisfied, the value of the imbalance ratio is detected and acquired in step S602 as in step S502.

  In step S603, the gain correction coefficient Hk and the time constant correction coefficient HT corresponding to the detected imbalance ratio value are calculated and acquired from a map as shown in FIG.

  Next, in step S604, as in step S505, it is determined whether the identification condition is satisfied. If the identification condition is not satisfied, step S604 is repeated to enter a standby state. On the other hand, if the identification condition is satisfied, the identification of the gain k and the time constant T is executed in step S605 as in step S506.

  When the identification values of the gain k and the time constant T are thus obtained, in step S606, the identification values are multiplied by the gain correction coefficient Hk and the time constant correction coefficient HT acquired in step S603, respectively, and the gain k The identification value of the time constant T is corrected to k ′ and T ′.

  Next, in step S607, each corrected identification value is compared with each corresponding abnormality determination value ks1, ks2, Ts. If the corrected gain k ′ is in the range of ks1 ≦ k ′ ≦ ks2 and the corrected time constant T ′ is in the range of T ′ ≦ Ts, the pre-catalyst sensor 17 is normal in step S608. Determined. On the other hand, if at least one of the corrected gain k ′ and time constant T ′ is not within each range, it is determined in step S609 that the pre-catalyst sensor 17 is abnormal. The process ends here. Note that when the pre-catalyst sensor 17 is determined to be abnormal, the abnormality of each characteristic of the sensor is determined individually and the warning device is activated as described above.

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. The first mode of correction can be applied to a method other than the diagnostic method using the first-order lag model as in the above embodiment. In addition to the imbalance ratio, any parameter that is correlated with the magnitude of the variation in the air-fuel ratio between cylinders can be used as the imbalance parameter.

  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 figure which shows roughly the mode of the change of the input air fuel ratio at the time of active control, and an output air fuel ratio. It is a graph which shows the result of having identified a gain and a time constant, and is a case of a normal sensor. It is a graph which shows the result of having identified a gain and a time constant, and is a case of an abnormal sensor. It is a block diagram of an abnormality diagnosis system. It is a test result comparing the input air-fuel ratio with and without fuel dynamics correction. It is a test result which shows the mode of a change with an input air fuel ratio and an output air fuel ratio, and is the state before bias correction. It is the schematic for demonstrating the method of bias correction. It is a test result which shows the mode of a change with an input air fuel ratio and an output air fuel ratio, and is the state after bias correction | amendment. It is a test result which shows the input air fuel ratio before and behind dead time correction. It is a dead time calculation map. It is a test result for demonstrating the 2nd aspect of dead time calculation, and is a case of a normal sensor. It is the schematic corresponding to FIG. 14 for demonstrating a dead time calculation method. It is a test result for demonstrating the 2nd aspect of dead time calculation, and is a case of an abnormal sensor. It is the schematic corresponding to FIG. 16 for demonstrating a dead time calculation method. It is a flowchart which concerns on the 3rd aspect of dead time calculation. It is a flowchart which shows the procedure of an air fuel ratio sensor abnormality diagnosis roughly. It is a graph which shows the sensor output change before catalyst in the case where there is no air-fuel ratio variation, and when there is. It is a graph which shows the relationship between an imbalance ratio, a time constant, and a gain. 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 which shows the case where 1 cylinder has shifted | deviated to the air-fuel-ratio rich side rather than the other 3 cylinders. It is a graph which shows the relationship between an imbalance ratio and hydrogen amount. It is a graph which shows the relationship between an imbalance ratio and a post-catalyst sensor learning value. It is a time chart for demonstrating the measuring method of the amount of occluded oxygen and the amount of released oxygen. It is the map which prescribed | regulated the relationship between the imbalance rate and the impersonation rate. It is a flowchart which shows the procedure of the air fuel ratio sensor abnormality diagnosis process containing the 1st aspect of correction | amendment. It is the map which prescribed | regulated the relationship between an imbalance ratio, a gain correction coefficient, and a time constant correction coefficient. It is a flowchart which shows the procedure of the air fuel ratio sensor abnormality diagnosis process containing the 2nd aspect of correction | amendment.

Explanation of symbols

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

Claims (10)

An abnormality diagnosis apparatus for an air-fuel ratio sensor disposed in an exhaust passage of a multi-cylinder internal combustion engine,
Diagnosing means for diagnosing abnormality of the air-fuel ratio sensor based on at least the output of the air-fuel ratio sensor;
Detecting means for detecting an imbalance parameter which is a parameter relating to air-fuel ratio variation between cylinders;
Output correction means for correcting the output of the air-fuel ratio sensor based on the imbalance parameter detected by the detection means ,
The diagnostic means includes
Identification means for identifying a parameter in the first-order lag element based on an input to a model obtained by modeling a system from a fuel injection valve to the air-fuel ratio sensor with a first-order lag element, and an output of the air-fuel ratio sensor;
An abnormality determining means for determining an abnormality of a predetermined characteristic of the air-fuel ratio sensor based on the parameter identified by the identifying means;
With
An abnormality diagnosis apparatus for an air-fuel ratio sensor, wherein the identification means identifies a parameter in the first-order lag element based on the output of the air-fuel ratio sensor corrected by the output correction means . The output correction means corrects the output by executing an annealing process or a filtering process on the output, and according to the imbalance parameter detected by the detection means, The abnormality diagnosis device for an air-fuel ratio sensor according to claim 1, wherein an annealing rate is set, or a cutoff frequency in the filtering process is set. Whether the output correction means sets the smoothing rate such that the degree of smoothing increases as the imbalance parameter detected by the detecting means is such that the variation in air-fuel ratio between cylinders increases. The abnormality diagnosis device for an air-fuel ratio sensor according to claim 2 , wherein the cut-off frequency that is further reduced is set . An air-fuel ratio sensor abnormality diagnosis device for detecting an air-fuel ratio of exhaust gas of a multi-cylinder internal combustion engine,
Identification means for identifying a parameter in the first-order lag element based on an input to a model obtained by modeling a system from a fuel injection valve to the air-fuel ratio sensor with a first-order lag element, and an output of the air-fuel ratio sensor;
An abnormality determining means for determining an abnormality of a predetermined characteristic of the air-fuel ratio sensor based on the parameter identified by the identifying means;
Detecting means for detecting an imbalance parameter which is a parameter relating to air-fuel ratio variation between cylinders;
Identification parameter correction means for correcting the identified parameter based on the imbalance parameter detected by the detection means ,
An abnormality diagnosis apparatus for an air-fuel ratio sensor, wherein the abnormality determination means determines an abnormality of a predetermined characteristic of the air-fuel ratio sensor based on the parameter corrected by the identification parameter correction means .   The identification parameter correcting means corrects the parameter to an increasing side as the imbalance parameter detected by the detecting means is such that the variation in the air-fuel ratio between cylinders increases, and Correct to the value when there is no variation in fuel ratio
  The abnormality diagnosis apparatus for an air-fuel ratio sensor according to claim 4.   The parameter in the first-order lag element includes at least a gain and a time constant, and the predetermined characteristic of the air-fuel ratio sensor includes at least an output corresponding to the gain and a responsiveness corresponding to the time constant.
  The abnormality diagnosis apparatus for an air-fuel ratio sensor according to any one of claims 1 to 5.   The detection means calculates the imbalance parameter based on a locus length or locus area per predetermined time of the output of the air-fuel ratio sensor.
  The air-fuel ratio sensor abnormality diagnosis device according to any one of claims 1 to 6.   The detection means includes
  A catalyst that is disposed in the exhaust passage downstream of the air-fuel ratio sensor and oxidizes and purifies at least hydrogen contained in the exhaust;
  A post-catalyst air-fuel ratio sensor disposed in the exhaust passage downstream of the catalyst;
  A main air-fuel ratio control that matches the air-fuel ratio detected by the air-fuel ratio sensor with a predetermined target air-fuel ratio and an auxiliary air-fuel ratio that matches the air-fuel ratio detected by the post-catalyst air-fuel ratio sensor with the target air-fuel ratio. Air-fuel ratio control means for performing fuel ratio control, calculating air-fuel ratio control means for calculating the control amount for the auxiliary air-fuel ratio control based on the output of the post-catalyst air-fuel ratio sensor;
  Means for calculating the imbalance parameter based on the calculated control amount;
  An abnormality diagnosis apparatus for an air-fuel ratio sensor according to any one of claims 1 to 6, further comprising:   The detection means includes
  A catalyst disposed in the exhaust passage downstream of the air-fuel ratio sensor;
  Means for detecting the amount of oxygen stored and released from the catalyst;
  Means for calculating the imbalance parameter based on a ratio or difference between the detected stored oxygen amount and released oxygen amount;
  An abnormality diagnosis apparatus for an air-fuel ratio sensor according to any one of claims 1 to 6, further comprising:   The imbalance parameter comprises an imbalance ratio, and the imbalance ratio is expressed by the formula: IB = (Q−Qs) / Qs (where IB is the imbalance ratio, Q is the fuel injection amount of the imbalance cylinder, and Qs is the balance cylinder) The abnormality diagnosis device for an air-fuel ratio sensor according to claim 1, wherein the abnormality diagnosis device is an air-fuel ratio sensor.

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