JP2007211609A - Device for controlling air-fuel ratio per cylinder of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To properly correct an air-fuel ratio detection timing (sample timing of output from air-fuel ratio sensor) so that the air-fuel ratio of each cylinder can be accurately estimated based on the values detected by an air-fuel ratio sensor installed at the exhaust manifold part of an internal combustion engine. <P>SOLUTION: When the air-fuel ratio is stoichiometric air-fuel ratio (theoretical air-fuel ratio), the deviation of the air-fuel ratio from the appropriate value of an air-fuel ratio detection timing is zero. When the air-fuel ratio is lean, it is deviated in the direction that the appropriate value of the air-fuel ratio detection timing is retarded (response of the air-fuel ratio sensor becomes slow). When the air-fuel ratio is rich, it is deviated in the direction that the appropriate value of the air-fuel ratio detection timing is advanced (response of the air-fuel ratio sensor becomes fast). With these characteristics taken into consideration, when a target air-fuel ratio is lean, the air-fuel ratio detection timing is corrected to be slower than when it is stoichiometric air-fuel ratio. When the target air-fuel ratio is rich, the air-fuel ratio detection timing is corrected to be faster than when it is stoichiometric air-fuel ratio. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a cylinder-by-cylinder air-fuel ratio control device for an internal combustion engine having a function of estimating the air-fuel ratio of each cylinder based on a detection value of one air-fuel ratio sensor installed in an exhaust gas collecting portion of the internal combustion engine.

In recent years, in order to reduce air-fuel ratio variation between cylinders of an internal combustion engine and improve air-fuel ratio control accuracy, as described in Patent Document 1 (Japanese Patent No. 3217680), the behavior of the exhaust system of the internal combustion engine is described. By setting the model to describe, the detection value of one air-fuel ratio sensor installed in the exhaust collecting part (the air-fuel ratio of the exhaust gas flowing through the exhaust collecting part) is input to the model, and the observer observes its internal state The air-fuel ratio of each cylinder is estimated, and the fuel injection amount of each cylinder is corrected according to the deviation between the estimated air-fuel ratio of each cylinder and the target value, so that the air-fuel ratio of each cylinder matches the target value. There is something. Further, in Patent Document 1, a delay until the exhaust gas discharged from each cylinder reaches the vicinity of the air-fuel ratio sensor and the air-fuel ratio is detected (hereinafter referred to as “exhaust system response delay”) is an engine operating state. In the engine design and manufacturing process, a map that defines the relationship between the exhaust system response delay and the engine operating state is created, and the sample timing of the air-fuel ratio sensor output ( The air-fuel ratio detection timing of each cylinder) is changed by the map according to the engine operating state.
Japanese Patent No. 3217680 (see paragraphs [0008], [0013], [0162] to [0170], etc.)

  By the way, in Patent Document 1 described above, when changing the air-fuel ratio detection timing according to the engine operating state, the air-fuel ratio detection timing is taken into account in addition to the engine speed, intake pressure, and valve timing. The changing technology is described. In paragraph [0170] of Patent Document 1, regarding the relationship between the responsiveness (reaction time) of the air-fuel ratio sensor and the air-fuel ratio, “the reaction time of the air-fuel ratio sensor (LAF sensor) indicates that the air-fuel ratio to be detected is lean. Therefore, when the air-fuel ratio to be detected is lean, it is desirable to detect the air-fuel ratio at an earlier crank angle (that is, to advance the air-fuel ratio detection timing). However, according to recent research results of the present inventor, as shown in FIG. 3, the change characteristic of the deviation of the air-fuel ratio detection timing due to the air-fuel ratio is completely opposite to the description in Patent Document 1 above. Turned out to be. Therefore, as in Patent Document 1, if the air-fuel ratio detection timing is advanced when the air-fuel ratio is lean, the air-fuel ratio detection timing is changed in the wrong direction. As a result, when the air-fuel ratio detection timing of each cylinder deviates from an appropriate value, the estimation accuracy of the air-fuel ratio of each cylinder deteriorates, and the state of cylinder-by-cylinder air-fuel ratio control deteriorates.

  The present invention has been made in view of such circumstances. Therefore, the object of the present invention is to estimate the air-fuel ratio of each cylinder based on the detection value of one air-fuel ratio sensor installed in the exhaust collection part of the internal combustion engine. In the engine equipped with the function, the air-fuel ratio detection timing of each cylinder can be corrected in an appropriate direction according to the air-fuel ratio, and the air-fuel ratio estimation accuracy of each cylinder can be improved. An object of the present invention is to provide a fuel ratio control device.

  In order to achieve the above object, the invention according to claim 1 is provided with an air-fuel ratio sensor for detecting an air-fuel ratio of the exhaust gas at an exhaust gas collecting portion where the exhaust gas of each cylinder of the internal combustion engine collects and flows. Cylinder air-fuel ratio estimation means for estimating the air-fuel ratio of each cylinder based on the detection value of the air-fuel ratio sensor at each air-fuel ratio detection timing of each cylinder, and the air-fuel ratio of each cylinder based on the estimated air-fuel ratio of each cylinder In a cylinder-by-cylinder air-fuel ratio control device for an internal combustion engine that includes a cylinder-by-cylinder air-fuel ratio control unit that performs control so as to match a target air-fuel ratio (hereinafter referred to as “cylinder-by-cylinder air-fuel ratio control”). An air-fuel ratio detection timing correction unit that corrects the air-fuel ratio or the detected air-fuel ratio according to the detected air-fuel ratio, and the air-fuel ratio detection timing correction unit corrects the air-fuel ratio detection timing when the target air-fuel ratio or the detected air-fuel ratio is lean. Remote corrected to slow one in which the target air-fuel ratio or a detected air-fuel ratio and correcting to advance than when stoichiometric the air-fuel ratio detecting timing when the rich. In this way, the air-fuel ratio detection timing of each cylinder can be corrected in an appropriate direction according to the air-fuel ratio (target air-fuel ratio or detected air-fuel ratio), and the air-fuel ratio estimation accuracy of each cylinder can be improved. Can do.

  In this case, for example, the relationship between the air-fuel ratio and the deviation from the appropriate value of the air-fuel ratio detection timing (correction amount) is measured in the adaptation process, and a correction amount table using the air-fuel ratio as a parameter based on the measurement result is obtained. It may be created and stored in the nonvolatile memory of the internal combustion engine control computer, and the correction amount corresponding to the target air-fuel ratio or the detected air-fuel ratio may be set with reference to the table during operation of the internal combustion engine. Thereby, the correction amount of the air-fuel ratio detection timing can be set appropriately according to the air-fuel ratio (target air-fuel ratio or detected air-fuel ratio).

  Further, the correction amount of the air-fuel ratio detection timing set according to the target air-fuel ratio or the detected air-fuel ratio may be corrected according to the responsiveness of the air-fuel ratio sensor. That is, the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing changes according to the response of the air-fuel ratio sensor, and the response of the air-fuel ratio sensor changes due to changes over time and manufacturing variations. If the correction amount of the air-fuel ratio detection timing set according to the air-fuel ratio (target air-fuel ratio or detected air-fuel ratio) is corrected according to the responsiveness of the air-fuel ratio sensor, the correction of the air-fuel ratio detection timing set according to the air-fuel ratio The amount can be corrected appropriately according to changes in the responsiveness of the air-fuel ratio sensor over time and manufacturing variations, and it is possible to prevent deterioration in the correction accuracy of the air-fuel ratio detection timing due to changes in the responsiveness of the air-fuel ratio sensors over time and manufacturing variations Can do.

  According to a third aspect of the present invention, there is provided learning means for learning a deviation (correction amount) from an appropriate value of the air-fuel ratio detection timing during operation of the internal combustion engine and updating and storing the learned value in a rewritable nonvolatile memory. It is also possible to correct the air-fuel ratio detection timing based on the learning value of the learning means. In this way, the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing during the operation of the internal combustion engine includes not only the deviation due to the air-fuel ratio but also the aging of the responsiveness of the air-fuel ratio sensor and deviation due to manufacturing variations. Therefore, even in a system that cannot ignore the effects of time-dependent changes in responsiveness of the air-fuel ratio sensor and manufacturing variations, the correction amount of the air-fuel ratio detection timing is determined not only by the deviation due to the air-fuel ratio but also by the responsiveness of the air-fuel ratio sensor. Appropriate settings can be made, including deviations due to changes over time and manufacturing variations.

  In this case, taking into account that the deviation (correction amount) of the air-fuel ratio detection timing from the appropriate value changes in accordance with the air-fuel ratio, the deviation from the appropriate value of the air-fuel ratio detection timing (see FIG. 4). Correction amount) may be learned for each air-fuel ratio. Thereby, the correction amount of the air-fuel ratio detection timing can be learned with high accuracy.

  As a result of studying the relationship between the air-fuel ratio and the deviation (correction amount) of the air-fuel ratio detection timing from the appropriate value as a result of experiments, the inventor found that the deviation from the appropriate value of the air-fuel ratio detection timing is as shown in FIG. It turned out that it changes to Z shape according to an air fuel ratio. That is, when the air-fuel ratio is stoichiometric (theoretical air-fuel ratio), the deviation from the appropriate value of the air-fuel ratio detection timing is 0, and in the region where the air-fuel ratio is relatively close to the stoichiometric value, the leaner the appropriate value of the air-fuel ratio detection timing becomes. It shifts in the direction of slowing down (the responsiveness of the air-fuel ratio sensor is slowed down), and the richer the shift is in the direction of increasing the appropriate value of the air-fuel ratio detection timing (the responsiveness of the air-fuel ratio sensor becomes faster). However, in a region where the air-fuel ratio is some distance away from the stoichiometry (the air-fuel ratio region richer than λrich and the air-fuel ratio region leaner than λleen in FIG. 3), the air-fuel ratio further changes to the rich / lean side. However, the amount of deviation from the appropriate value of the air-fuel ratio detection timing hardly changes.

  In consideration of the Z-shaped change characteristic of the deviation of the air-fuel ratio detection timing due to the air-fuel ratio, as in claim 5, the predetermined air-fuel ratio range including stoichiometric (range of λrich to λlean in FIG. 3) In the rich-side air-fuel ratio region and the lean-side air-fuel ratio region (that is, the air-fuel ratio region in which the response of the air-fuel ratio sensor hardly changes even when the air-fuel ratio changes), the deviation of the air-fuel ratio detection timing from the appropriate value ( The correction amount) may be learned one by one. In this way, the learning process becomes simpler than the case where the correction amount is finely learned for each air-fuel ratio in all air-fuel ratio regions, and there is an advantage that the calculation load of the learning process can be reduced. .

  In this case, the correction of the air-fuel ratio detection timing is performed within a predetermined air-fuel ratio range (within the range of λrich to λlean in FIG. 3) in which the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing changes according to the air-fuel ratio. The amount may be finely learned for each air-fuel ratio, but the rich side learned by the learning means when the target air-fuel ratio or the detected air-fuel ratio is within a predetermined air-fuel ratio range including stoichiometry, as in claim 6. The correction amount of the air-fuel ratio detection timing may be set by interpolation correction between the learned value in the air-fuel ratio region and the learned value in the lean air-fuel ratio region. In this way, within the predetermined air-fuel ratio range including stoichiometry, the correction amount can be set by interpolation correction of the two learned values learned in the air-fuel ratio regions on both sides without learning. Compared to the case where the correction amount is finely learned for each air-fuel ratio in the air-fuel ratio region, there is an advantage that the learning process becomes simple and the calculation load of the learning process can be reduced. Here, the interpolation correction may use linear interpolation (linear interpolation) approximating with a straight line whose correction amount is 0 at the time of stoichiometry, or using curve interpolation (spline interpolation) approximating with a cross-shaped curve. Also good.

  Further, as in claim 7, it is preferable to prohibit the control for changing the air-fuel ratio during learning of the deviation of the air-fuel ratio detection timing. Thereby, it is possible to accurately learn the deviation (correction amount) in the air-fuel ratio detection timing while maintaining the air-fuel ratio constant.

  As shown in FIG. 3, in the region where the air-fuel ratio is somewhat away from the stoichiometry, even if the air-fuel ratio further changes to the rich / lean side, the deviation in the air-fuel ratio detection timing hardly changes. When learning the deviation of the air-fuel ratio detection timing in the region, the change of the air-fuel ratio within this air-fuel ratio region may be allowed and only the change of the air-fuel ratio outside this air-fuel ratio region may be prohibited.

  Further, as in the eighth aspect, it is preferable to perform learning at a predetermined interval in order to update the learning value in accordance with the change with time of the responsiveness of the air-fuel ratio sensor. For example, it is learned during the first run after the learning value is cleared by replacing the in-vehicle battery, and thereafter, every elapse of a predetermined period, every predetermined total travel distance, every predetermined number of travels, or It is preferable to perform learning every predetermined number of times of refueling. As a result, the learning value can be updated at any time according to the change over time in the responsiveness of the air-fuel ratio sensor.

  Further, when the internal combustion engine is operated at a lean air-fuel ratio or a rich air-fuel ratio, the learning execution condition may be satisfied to learn the deviation of the air-fuel ratio detection timing. It may be changed to the rich side to learn the deviation of the air-fuel ratio detection timing. In this case, in the state where the oxygen storage amount (oxygen storage amount) of the exhaust gas purification catalyst is increased to near the saturation level, the catalyst's ability to purify lean components such as NOx is reduced. If learning is performed by forcibly changing the fuel ratio to the lean side, the emission amount of lean components such as NOx that cannot be completely purified by the catalyst will increase. In addition, in the state where the amount of oxygen storage of the catalyst is small, the rich component purification ability of the catalyst such as HC, CO, etc. is reduced. In this state, if the learning is performed by forcibly changing the air-fuel ratio to the rich side. In addition, the discharge amount of rich components such as HC and CO that cannot be purified by the catalyst increases.

  As a countermeasure against this, it is preferable to determine the timing for performing learning by changing the air-fuel ratio to the lean side or the rich side based on the state of the exhaust gas purifying catalyst. In this way, for example, the air-fuel ratio is forcibly changed to the rich side when the state of the catalyst is high in the ability to purify rich components such as HC and CO (when the oxygen storage amount is large). Therefore, it is possible to learn the deviation of the air-fuel ratio detection timing without deteriorating the emission in the rich-side air-fuel ratio region, and the state of the catalyst becomes high in the purification of lean components such as NOx. When the air-fuel ratio is low (when the oxygen storage amount is low), the air-fuel ratio is forcibly changed to the lean side and learning is performed. The deviation can be learned, and the problem of emission deterioration during learning can be solved.

  Several embodiments embodying the best mode for carrying out the present invention will be described below.

  A first embodiment of the present invention will be described with reference to FIGS. First, a schematic configuration of the entire engine control system will be described with reference to FIG. An air cleaner 13 is provided at the most upstream portion of the intake pipe 12 of an in-line four-cylinder engine 11 that is an internal combustion engine, for example, and an air flow meter 14 that detects the intake air amount is provided downstream of the air cleaner 13. . On the downstream side of the air flow meter 14, a throttle valve 15 whose opening is adjusted by a motor or the like and a throttle opening sensor 16 for detecting the throttle opening are provided.

  Further, a surge tank 17 is provided on the downstream side of the throttle valve 15, and an intake pipe pressure sensor 18 for detecting the intake pipe pressure is provided in the surge tank 17. The surge tank 17 is provided with an intake manifold 19 for introducing air into each cylinder of the engine 11, and a fuel injection valve 20 for injecting fuel is attached in the vicinity of the intake port of the intake manifold 19 of each cylinder. Yes. During engine operation, the fuel in the fuel tank 21 is sent to the delivery pipe 23 by the fuel pump 22 and fuel is injected from the fuel injection valve 20 of each cylinder at each injection timing of each cylinder. A fuel pressure sensor 24 that detects fuel pressure (fuel pressure) is attached to the delivery pipe 23.

  Further, the engine 11 is provided with variable valve timing mechanisms 27 and 28 for changing the opening and closing timings of the intake valve 25 and the exhaust valve 26, respectively. Further, the engine 11 is provided with an intake cam angle sensor 31 and an exhaust cam angle sensor 32 that output a cam angle signal in synchronization with the rotation of the intake cam shaft 29 and the exhaust cam shaft 30, and the rotation of the crank shaft of the engine 11. Is provided with a crank angle sensor 33 for outputting a pulse of a crank angle signal at every predetermined crank angle (for example, every 30 ° C. A).

  On the other hand, an air-fuel ratio sensor 37 for detecting the air-fuel ratio of the exhaust gas is installed in the exhaust collecting portion 36 where the exhaust manifold 35 of each cylinder of the engine 11 gathers. A catalyst 38 such as a three-way catalyst for purifying CO, HC, NOx and the like is provided.

  Outputs of various sensors such as the air-fuel ratio sensor 37 are input to an engine control circuit (hereinafter referred to as “ECU”) 40. The ECU 40 is mainly composed of a microcomputer, and executes various engine control programs stored in a built-in ROM (storage medium), so that the fuel of the fuel injection valve 20 of each cylinder according to the engine operating state. Control injection quantity and ignition timing.

  In the first embodiment, the ECU 40 executes the routines for cylinder-by-cylinder air-fuel ratio control shown in FIGS. 4 to 7 described later, thereby detecting values detected by the air-fuel ratio sensor 37 using a cylinder-by-cylinder air-fuel ratio estimation model described later. The air-fuel ratio of each cylinder is estimated based on (the actual air-fuel ratio of the exhaust gas flowing through the exhaust collecting portion 36), the average value of the estimated air-fuel ratio of all cylinders is calculated, and the average value is used as the reference air-fuel ratio (all cylinders). And the deviation between the estimated air-fuel ratio of each cylinder and the reference air-fuel ratio is calculated for each cylinder, and the fuel correction amount (fuel injection amount of each cylinder is set so that the deviation becomes small). Correction amount), and correcting the fuel injection amount of each cylinder based on the calculation result, the air-fuel ratio of the air-fuel mixture supplied to each cylinder is corrected for each cylinder, and the variation in air-fuel ratio among the cylinders is corrected. (Hereinafter, this control is referred to as “air-fuel ratio for each cylinder”). That your ").

  Here, a specific example of a model (hereinafter referred to as “cylinder-specific air-fuel ratio estimation model”) for estimating the air-fuel ratio of each cylinder based on the detection value of the air-fuel ratio sensor 37 (the actual air-fuel ratio of the exhaust gas flowing through the exhaust collecting portion 36). An example will be described.

  Focusing on the gas exchange in the exhaust collecting section 36, the detected value of the air-fuel ratio sensor 37 is set to a predetermined weight for the estimated air-fuel ratio history of each cylinder and the detected value history of the air-fuel ratio sensor 37 in the exhaust collecting section 36, respectively. The model is obtained by multiplying and adding, and the air-fuel ratio of each cylinder is estimated using the model. At this time, a Kalman filter is used as an observer.

More specifically, a gas exchange model in the exhaust collecting portion 36 is approximated by the following equation (1).
ys (t) = k1 * u (t-1) + k2 * u (t-2) -k3 * ys (t-1) -k4 * ys (t-2)
...... (1)
Here, yS is a detected value of the air-fuel ratio sensor 37, u is an air-fuel ratio of the gas flowing into the exhaust collecting portion 36, and k1 to k4 are constants.

  In the exhaust system, there are a primary delay element of gas inflow and mixing in the exhaust collecting portion 36 and a primary delay element due to a response delay of the air-fuel ratio sensor 37. Therefore, in the above equation (1), the history for the past two times is referred to in consideration of these first order lag elements.

When the above equation (1) is converted into a state space model, the following equations (2a) and (2b) are derived.
X (t + 1) = A.X (t) + B.u (t) + W (t) (2a)
Y (t) = C · X (t) + D · u (t) (2b)
Here, A, B, C, and D are model parameters, Y is a detected value of the air-fuel ratio sensor 37, X is an estimated air-fuel ratio of each cylinder as a state variable, and W is noise.

Further, when the Kalman filter is designed by the above equations (2a) and (2b), the following equation (3) is obtained.
X ^ (k + 1 | k) = A.X ^ (k | k-1) + K {Y (k) -C.A.X ^ (k | k-1)} (3) where X ^ (X hat) is the estimated air-fuel ratio of each cylinder, and K is the Kalman gain. The meaning of X ^ (k + 1 | k) represents that the estimated value of the next time (k + 1) is obtained from the estimated value of time (k).

  As described above, by configuring the cylinder-by-cylinder air-fuel ratio estimation model using the Kalman filter type observer, it is possible to sequentially estimate the air-fuel ratio of each cylinder as the combustion cycle progresses.

  Next, a method for setting the air-fuel ratio detection timing of each cylinder (sample timing of the output of the air-fuel ratio sensor 37) will be described. In the first embodiment, the delay until exhaust gas discharged from each cylinder reaches the vicinity of the air-fuel ratio sensor 37 and the air-fuel ratio is detected (hereinafter referred to as “response delay of the exhaust system”) depends on the engine operating state. In consideration of the change, the air-fuel ratio detection reference timing of each cylinder is set by a map or the like according to the engine operating state (for example, engine load, engine speed, etc.). In general, as the engine load and the engine speed decrease, the response delay of the exhaust system increases, so the air-fuel ratio detection reference timing of each cylinder is shifted to the retard side as the engine load and the engine speed decrease. Is set to This air-fuel ratio detection reference timing corresponds to an appropriate air-fuel ratio detection timing when the target air-fuel ratio is stoichiometric (the excess air ratio λ = 1.0).

  FIG. 2 is a time chart showing an example of the behavior of the output amplitude of the air-fuel ratio sensor 37 when only one cylinder is rich. In this case, an appropriate air-fuel ratio detection timing is a timing at which the output amplitude of the air-fuel ratio sensor 37 reaches a peak. As a result of experimentally examining the relationship between the air-fuel ratio and the deviation from the appropriate value of the air-fuel ratio detection timing, as shown in FIG. 3, the deviation from the appropriate value of the air-fuel ratio detection timing becomes a Z-shape according to the air-fuel ratio. It turns out to change. That is, when the air-fuel ratio is stoichiometric (λ = 1.0), the deviation from the appropriate value of the air-fuel ratio detection timing is 0, and in the region where the air-fuel ratio is relatively close to stoichiometric, the leaner the air-fuel ratio detection timing is, the more appropriate The value deviates in the direction of slowing down (the responsiveness of the air-fuel ratio sensor becomes slow), and the richer the value, the more appropriate the value of the air-fuel ratio detection timing becomes faster (the responsiveness of the air-fuel ratio sensor becomes faster). However, in the region where the air-fuel ratio is far away from the stoichiometry, even if the air-fuel ratio further changes to the rich / lean side, the deviation from the appropriate value of the air-fuel ratio detection timing (responsiveness of the air-fuel ratio sensor) changes almost. do not do.

  In consideration of the change characteristic of the deviation of the air-fuel ratio detection timing due to such an air-fuel ratio, in the first embodiment, for example, the difference between the air-fuel ratio and the appropriate value of the air-fuel ratio detection timing (correction amount) in the adaptation process is described. Measure the relationship, create a table of deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing using the air-fuel ratio as a parameter based on the measurement result, and store it in a nonvolatile memory such as the ROM of the ECU 40 The correction amount of the air-fuel ratio detection timing corresponding to the target air-fuel ratio or the detected air-fuel ratio is set with reference to the table during engine operation. The correction amount of the air-fuel ratio detection timing becomes a negative value (correction in a direction to advance the air-fuel ratio detection timing) at the rich side air-fuel ratio, and a positive value (correction in a direction to delay the air-fuel ratio detection timing) at the lean side air-fuel ratio. )

  In this case, the correction amount table of the air-fuel ratio detection timing may be a table in which the correction amount is finely set for each air-fuel ratio in all the air-fuel ratio regions. However, as shown in FIG. Since the deviation (correction amount) from the appropriate value changes in a Z-shape according to the air-fuel ratio, the air-fuel ratio is a certain distance from the stoichiometric range (the air-fuel ratio region on the rich side from λrich in FIG. 3 and the leaner than λlean) In the air-fuel ratio region), even if the air-fuel ratio further changes to the rich / lean side, the amount of deviation from the appropriate value of the air-fuel ratio detection timing hardly changes. In consideration of such a Z-shaped change characteristic of the deviation of the air-fuel ratio detection timing due to the air-fuel ratio, each of the air-fuel ratio region richer than λrich and the air-fuel ratio region leaner than λlean in FIG. You may make it set the correction amount of an air fuel ratio detection timing one by one.

  Further, within the predetermined air-fuel ratio range (within the range of λrich to λlean in FIG. 3) in which the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing changes according to the air-fuel ratio, the correction amount of the air-fuel ratio detection timing May be set finely as table data for each air-fuel ratio, but table data for the correction amount in the air-fuel ratio region richer than λrich and table data for the correction amount in the air-fuel ratio region leaner than λlean The correction amount of the air-fuel ratio detection timing may be set by this interpolation correction. Here, the interpolation correction may use linear interpolation (linear interpolation) approximating with a straight line in which the correction amount of the air-fuel ratio detection timing becomes 0 when stoichiometric (λ = 1.0), or a cross-shaped curve Curve interpolation (spline interpolation) approximated by may be used.

  The setting of the air-fuel ratio detection timing and the cylinder-by-cylinder air-fuel ratio control of the first embodiment described above are executed by the ECU 40 according to the routines shown in FIGS. The processing contents of each routine will be described below.

[Air-fuel ratio control routine for each cylinder]
The cylinder-by-cylinder air-fuel ratio control main routine of FIG. 4 is started at every predetermined crank angle (for example, every 30 ° C. A) in synchronization with the output pulse of the crank angle sensor 33. When this routine is started, first, in step 101, a cylinder-by-cylinder air-fuel ratio control execution condition determination routine of FIG. 5 described later is executed to determine whether or not the execution conditions for cylinder-by-cylinder air-fuel ratio control are satisfied. . Thereafter, the routine proceeds to step 102, where the execution condition of the cylinder-by-cylinder air-fuel ratio control is satisfied depending on whether or not the cylinder-by-cylinder air-fuel ratio control execution flag set in the cylinder-by-cylinder air-fuel ratio control execution condition determination routine is ON. It is determined whether or not. As a result, when it is determined that the cylinder-by-cylinder air-fuel ratio control execution flag is OFF (execution condition is not established), this routine is terminated without performing the subsequent processing.

  On the other hand, when it is determined that the cylinder-by-cylinder air-fuel ratio control execution flag is ON (execution condition is satisfied), the routine proceeds to step 103, where an air-fuel ratio detection timing calculation routine of FIG. An air-fuel ratio detection timing (sample timing of the output of the air-fuel ratio sensor 16) corresponding to the engine operating state such as the rotational speed and the target air-fuel ratio is set. Thereafter, the routine proceeds to step 104 where it is determined whether or not the current crank angle is the air-fuel ratio detection timing of each cylinder. If it is not the air-fuel ratio detection timing, this routine is terminated without performing the subsequent processing. .

  On the other hand, if the current crank angle is the air-fuel ratio detection timing, the routine proceeds to step 105, where the cylinder-by-cylinder air-fuel ratio control execution routine of FIG.

[Cylinder-specific air-fuel ratio control execution condition determination routine]
The cylinder-by-cylinder air-fuel ratio control execution condition determination routine of FIG. 5 is a subroutine that is executed in step 101 of the cylinder-by-cylinder air-fuel ratio control main routine of FIG. When this routine is started, first, at step 201, it is determined whether or not the air-fuel ratio sensor 37 is in a usable state. Here, the usable state is, for example, a state in which the air-fuel ratio sensor 37 is in an active state and has not failed. If the air-fuel ratio sensor 37 is not in a usable state, the cylinder-by-cylinder air-fuel ratio control execution condition is not satisfied, the routine proceeds to step 205, the cylinder-by-cylinder air-fuel ratio control execution flag is set to OFF, and this routine ends.

  On the other hand, if the air-fuel ratio sensor 37 is in a usable state, the process proceeds to step 202, where it is determined whether or not the coolant temperature is equal to or higher than a predetermined temperature (the engine 11 is warmed up). The cylinder-by-cylinder air-fuel ratio control execution condition is not satisfied, and the routine proceeds to step 205, where the cylinder-by-cylinder air-fuel ratio control execution flag is set to OFF, and this routine ends.

  If the cooling water temperature is equal to or higher than the predetermined temperature, the process proceeds to step 203, and the current engine operation region is classified by cylinder with reference to the operation region map using the engine speed and the engine load (for example, intake pipe pressure) as parameters. It is determined whether or not it is an execution range of air-fuel ratio control. For example, the estimation accuracy of the cylinder-by-cylinder air-fuel ratio is deteriorated in the high rotation range and the low load range, and therefore the cylinder-by-cylinder air-fuel ratio control is prohibited.

  If the current engine operating range is not the execution range for cylinder-by-cylinder air-fuel ratio control, the cylinder-by-cylinder air-fuel ratio control execution condition is not satisfied, the process proceeds to step 205, the cylinder-by-cylinder air-fuel ratio control execution flag is set to OFF, and this routine is executed. Exit. On the other hand, if the current engine operation region is the execution region of the cylinder-by-cylinder air-fuel ratio control, the cylinder-by-cylinder air-fuel ratio control execution condition is satisfied, the process proceeds to step 204, and the cylinder-by-cylinder air-fuel ratio control execution flag is turned ON. Set and end this routine.

[Air-fuel ratio detection timing calculation routine]
The air-fuel ratio detection timing calculation routine of FIG. 6 is a subroutine executed in step 103 of the cylinder-by-cylinder air-fuel ratio control main routine of FIG. When this routine is started, first, at step 301, the air-fuel ratio detection reference timing of each cylinder is calculated by a map or the like according to the current engine operating state (for example, engine load, engine speed, etc.). This air-fuel ratio detection reference timing corresponds to an appropriate air-fuel ratio detection timing when the target air-fuel ratio is stoichiometric (the excess air ratio λ = 1.0).

  Thereafter, the process proceeds to step 302, and a correction amount table of the air-fuel ratio detection timing set in consideration of the Z-shaped change characteristic (see FIG. 3) of the deviation of the air-fuel ratio detection timing due to the air-fuel ratio is referred to. A correction amount of the air-fuel ratio detection timing according to the current target air-fuel ratio (or its average value) is calculated. The correction amount of the air-fuel ratio detection timing becomes a negative value (correction in a direction to advance the air-fuel ratio detection timing) at the rich side air-fuel ratio, and a positive value (correction in a direction to delay the air-fuel ratio detection timing) at the lean side air-fuel ratio. )

  In this case, the correction amount table of the air-fuel ratio detection timing may be a table in which the correction amount is finely set for each air-fuel ratio in all air-fuel ratio regions, but the Z-shaped deviation of the air-fuel ratio detection timing due to the air-fuel ratio. In consideration of the change characteristics of the mold, one correction amount for the air-fuel ratio detection timing is set for each of the air-fuel ratio region richer than λrich and the air-fuel ratio region leaner than λleen in FIG. Also good.

  Further, within a predetermined air-fuel ratio range (within the range of λrich to λlean in FIG. 3) in which the responsiveness of the air-fuel ratio sensor 37 changes according to the air-fuel ratio, the correction amount of the air-fuel ratio detection timing is finely set for each air-fuel ratio. Although it may be set as data, the air-fuel ratio detection is performed by interpolating between the correction amount table data of the air-fuel ratio region richer than λrich and the correction amount table data of the air-fuel ratio region leaner than λlean. A timing correction amount may be set. Here, the interpolation correction may be performed by approximating with a straight line or a character-like curve in which the correction amount of the air-fuel ratio detection timing becomes 0 when stoichiometric (λ = 1.0).

  The correction amount data of the air-fuel ratio detection timing may be tabulated using the current detected air-fuel ratio (or its average value) as a parameter instead of the current target air-fuel ratio (or its average value).

Thereafter, the process proceeds to step 303, and the correction amount set according to the current target air-fuel ratio (or its average value) is added to the air-fuel ratio detection reference timing set according to the current engine operating state, The final air-fuel ratio detection timing is obtained.
Air-fuel ratio detection timing = Air-fuel ratio detection reference timing + Correction amount

In short, the air-fuel ratio detection reference timing (appropriate air-fuel ratio detection timing at the time of stoichiometry) set according to the current engine operating state is corrected according to the current target air-fuel ratio (or its average value). The final air-fuel ratio detection timing is determined by correcting the amount. In this case, the correction amount of the air-fuel ratio detection timing becomes a negative value (correction in a direction for advancing the air-fuel ratio detection timing) at the rich-side air-fuel ratio, and a positive value (a direction for delaying the air-fuel ratio detection timing) at the lean-side air-fuel ratio. Therefore, when the current target air-fuel ratio (or its average value) is lean, the air-fuel ratio detection timing is corrected to be later than that during stoichiometric, and when it is rich, the air-fuel ratio detection timing is stoichiometric. It will be corrected so as to be earlier than the time of.
The processing in steps 302 and 303 plays a role corresponding to air-fuel ratio detection timing correction means in the claims.

[Cylinder-specific air-fuel ratio control execution routine]
The cylinder-by-cylinder air-fuel ratio control execution routine of FIG. 7 is a subroutine executed in step 105 of the cylinder-by-cylinder air-fuel ratio control main routine of FIG. 4, and plays a role as cylinder-by-cylinder air-fuel ratio control means in the claims. .

  When this routine is started, first, in step 401, the output (air-fuel ratio detection value) of the air-fuel ratio sensor 37 is read, and in the next step 402, the current air-fuel ratio estimation target is calculated using the cylinder-by-cylinder air-fuel ratio estimation model. The air-fuel ratio of the cylinder to be estimated is estimated based on the detection value of the air-fuel ratio sensor 37. At this time, the cylinder-by-cylinder air-fuel ratio is estimated in consideration of the phase shift of the air-fuel ratio of each cylinder due to unequal interval combustion. The processing in step 402 serves as cylinder-by-cylinder air-fuel ratio estimating means in the claims. Thereafter, the process proceeds to step 403, where an average value of estimated air-fuel ratios of all cylinders is calculated, and the average value is set as a reference air-fuel ratio (target air-fuel ratio of all cylinders).

  Thereafter, the process proceeds to step 404, where the deviation between the estimated air-fuel ratio of each cylinder and the reference air-fuel ratio is calculated, and the cylinder-specific correction amount is calculated so that the deviation becomes small, and then the process proceeds to step 405, where the cylinder-specific correction is performed. By correcting the fuel injection amount for each cylinder based on the amount, the air-fuel ratio of the air-fuel mixture supplied to each cylinder is corrected for each cylinder so as to reduce the variation in air-fuel ratio among the cylinders.

  According to the first embodiment described above, when the target air-fuel ratio is lean, the air-fuel ratio detection timing is corrected so as to be later than when the target air-fuel ratio is rich, and when the target air-fuel ratio is rich, the air-fuel ratio detection timing is corrected. Therefore, the air-fuel ratio detection timing of each cylinder can be corrected in an appropriate direction according to the air-fuel ratio, and the air-fuel ratio estimation accuracy of each cylinder can be improved.

  Incidentally, the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing changes according to the responsiveness of the air-fuel ratio sensor 37, and the responsiveness of the air-fuel ratio sensor 37 changes due to changes over time and manufacturing variations. As shown in FIG. 8, the slower the responsiveness of the air-fuel ratio sensor 37, the greater the deviation (correction amount) of the air-fuel ratio detection timing from the appropriate value. Therefore, in the correction method of the air-fuel ratio detection timing of the first embodiment ignoring the time-dependent change and the manufacturing variation of the air-fuel ratio sensor 37, if the time-dependent change of the air-fuel ratio sensor 37 and the manufacturing variation increase, A decrease in the correction accuracy of the fuel ratio detection timing, and thus a decrease in the air-fuel ratio estimation accuracy of each cylinder is inevitable.

  As a countermeasure against this, in the second embodiment of the present invention, the air-fuel ratio detection timing correction routine set in accordance with the target air-fuel ratio (or detected air-fuel ratio) is executed by executing the air-fuel ratio detection timing calculation routine of FIG. Correction is made according to the response of the fuel ratio sensor 37 (step 302a).

  In this case, the time-dependent change in the responsiveness of the air-fuel ratio sensor 37 is estimated by, for example, the integrated travel distance, the number of travels, etc., and the correction amount of the air-fuel ratio detection timing set according to the target air-fuel ratio (or the detected air-fuel ratio) is empty. You may make it correct | amend according to the time-dependent change of the responsiveness of the fuel ratio sensor 37. FIG.

  In addition, the relationship that the degree of variation in the estimated air-fuel ratio between the cylinders during the cylinder-by-cylinder air-fuel ratio control increases as the deviation in the air-fuel ratio detection timing caused by the time-dependent change in the responsiveness of the air-fuel ratio sensor 37 or manufacturing variations increases. Then, during the air-fuel ratio control for each cylinder, the degree of variation in the estimated air-fuel ratio between the cylinders is detected as information on the deviation in the air-fuel ratio detection timing, and the air-fuel ratio detection set according to the target air-fuel ratio (or detected air-fuel ratio) The timing correction amount may be corrected according to the degree of variation in the estimated air-fuel ratio between the cylinders.

  In this way, even when the deviation of the air-fuel ratio detection timing occurs due to a change in the responsiveness of the air-fuel ratio sensor 37 over time or manufacturing variations, the air-fuel ratio detection timing set according to the target air-fuel ratio (or detected air-fuel ratio) is increased. The correction amount can be appropriately corrected according to the time-dependent change of the response of the air-fuel ratio sensor 37 and manufacturing variations, and the correction accuracy of the air-fuel ratio detection timing is reduced due to the time-dependent change of the response of the air-fuel ratio sensor 37 and manufacturing variations. Can be prevented.

  In the first and second embodiments, the correction amount of the air-fuel ratio detection timing is set according to the target air-fuel ratio (or the detected air-fuel ratio) with reference to the table of preset correction amounts of the air-fuel ratio detection timing. However, in the third embodiment of the present invention, the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing is learned during engine operation by executing the routines of FIGS. Is updated and stored in a rewritable nonvolatile memory such as a backup RAM of the ECU 40.

  In this case, the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing in all the air-fuel ratio regions may be learned in detail for each air-fuel ratio, but from the appropriate values of the air-fuel ratio and the air-fuel ratio detection timing. As shown in FIG. 3, the relationship with the deviation (correction amount) has a characteristic that the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing changes in a Z shape according to the air-fuel ratio. In a region that is somewhat distant from the stoichiometric range (an air-fuel ratio region richer than λrich and an air-fuel ratio region leaner than λlean in FIG. 3), even if the air-fuel ratio further changes to the rich / lean side, the air-fuel ratio The amount of deviation of the detection timing from the appropriate value hardly changes. In consideration of such a Z-shaped change characteristic of the deviation of the air-fuel ratio detection timing due to the air-fuel ratio, the air-fuel ratio region richer than λrich and the air-fuel ratio region leaner than λlean in FIG. The correction amount of the fuel ratio detection timing may be learned one by one. In this way, the learning process becomes simpler than the case where the correction amount is finely learned for each air-fuel ratio in all air-fuel ratio regions, and there is an advantage that the calculation load of the learning process can be reduced. .

  In this case, the correction of the air-fuel ratio detection timing is performed within a predetermined air-fuel ratio range (within the range of λrich to λlean in FIG. 3) in which the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing changes according to the air-fuel ratio. The amount may be finely learned for each air-fuel ratio, but when the target air-fuel ratio (or detected air-fuel ratio) is within a predetermined air-fuel ratio range including stoichiometry, the learned value in the rich air-fuel ratio region and the lean side The correction amount of the air-fuel ratio detection timing may be set by interpolation correction with the learning value in the air-fuel ratio region. In this way, within the predetermined air-fuel ratio range including stoichiometry, the correction amount can be set by interpolation correction of the two learned values learned in the air-fuel ratio regions on both sides without learning. Compared to the case where the correction amount is finely learned for each air-fuel ratio in the air-fuel ratio region, there is an advantage that the learning process becomes simple and the calculation load of the learning process can be reduced. Here, the interpolation correction may use linear interpolation (linear interpolation) approximating with a straight line whose correction amount is 0 at the time of stoichiometry, or using curve interpolation (spline interpolation) approximating with a cross-shaped curve. Also good.

  Further, during the learning of the deviation (correction amount) in the air-fuel ratio detection timing, it is preferable to prohibit the control for changing the air-fuel ratio (for example, sub feedback control, catalyst neutralization control, etc.). Thereby, it is possible to accurately learn the deviation (correction amount) in the air-fuel ratio detection timing while maintaining the air-fuel ratio constant.

  As shown in FIG. 3, in the region where the air-fuel ratio is somewhat away from the stoichiometry, even if the air-fuel ratio further changes to the rich / lean side, the deviation in the air-fuel ratio detection timing hardly changes. When learning the deviation of the air-fuel ratio detection timing in the region, the change of the air-fuel ratio within this air-fuel ratio region may be allowed and only the change of the air-fuel ratio outside this air-fuel ratio region may be prohibited.

  Further, it is preferable to perform learning at a predetermined interval in order to update the learning value in accordance with a change in the responsiveness of the air-fuel ratio sensor 37 with time. For example, it is learned during the first run after the learning value is cleared by replacing the in-vehicle battery, and thereafter, every elapse of a predetermined period, every predetermined total travel distance, every predetermined number of travels, or It is preferable to perform learning every predetermined number of times of refueling. Thereby, the learning value can be updated at any time according to the temporal change of the responsiveness of the air-fuel ratio sensor 37.

  Further, when the engine 11 is operated at a lean air-fuel ratio or a rich air-fuel ratio, the learning execution condition may be satisfied to learn the deviation in the air-fuel ratio detection timing, or the target air-fuel ratio may be forcibly set to the lean side during learning. Alternatively, it may be changed to the rich side to learn the deviation of the air-fuel ratio detection timing. In this case, in the state where the oxygen storage amount (oxygen storage amount) of the exhaust gas purifying catalyst 38 is increased to near the saturation level, the lean component purifying ability of the catalyst 38 such as NOx is lowered. If the learning is performed by forcibly changing the air-fuel ratio to the lean side, the discharge amount of lean components such as NOx that cannot be purified by the catalyst 38 will increase. Further, in the state where the oxygen storage amount of the catalyst 38 is small, the capability of purifying rich components such as HC and CO of the catalyst 38 is lowered. In this state, the target air-fuel ratio is forcibly changed to the rich side. As a result, the discharge amount of rich components such as HC and CO that cannot be purified by the catalyst 38 increases.

  As a countermeasure, it is preferable to determine the timing for executing learning by changing the target air-fuel ratio to the lean side or the rich side based on the state of the catalyst 38. In this way, for example, when the state of the catalyst 38 is high in the ability to purify rich components such as HC and CO (when the oxygen storage amount is large), the target air-fuel ratio is forcibly set to the rich side. Therefore, it is possible to learn the deviation of the air-fuel ratio detection timing without deteriorating the emission in the rich-side air-fuel ratio region, and the state of the catalyst 38 has the ability to purify the lean component such as NOx. When the air-fuel ratio is high (when the oxygen storage amount is low), the target air-fuel ratio is forcibly changed to the lean side and learning is performed. It is possible to learn the deviation in the fuel ratio detection timing, and to solve the problem of emission deterioration during learning.

  The learning correction of the air-fuel ratio detection timing and the cylinder-by-cylinder air-fuel ratio control according to the third embodiment described above are executed by the ECU 40 according to the routines shown in FIGS. The processing contents of each routine will be described below.

[Air-fuel ratio control routine for each cylinder]
The cylinder-by-cylinder air-fuel ratio control main routine of FIG. 10 is started at every predetermined crank angle (for example, every 30 ° C. A) in synchronization with the output pulse of the crank angle sensor 33. When this routine is started, first, in step 501, the cylinder-by-cylinder air-fuel ratio control execution condition determination routine of FIG. 5 described in the first embodiment is executed, and the execution condition of the cylinder-by-cylinder air-fuel ratio control is satisfied. It is determined whether or not. Thereafter, the routine proceeds to step 502, where the execution condition of the cylinder-by-cylinder air-fuel ratio control is satisfied depending on whether or not the cylinder-by-cylinder air-fuel ratio control execution flag set in the cylinder-by-cylinder air-fuel ratio control execution condition determination routine is ON. It is determined whether or not. As a result, when it is determined that the cylinder-by-cylinder air-fuel ratio control execution flag is OFF (execution condition is not established), this routine is terminated without performing the subsequent processing.

  On the other hand, if it is determined that the cylinder-by-cylinder air-fuel ratio control execution flag is ON (execution condition is satisfied), the routine proceeds to step 503, where an air-fuel ratio detection timing calculation routine of FIG. The air-fuel ratio detection timing is set according to the engine operating state such as the rotational speed and the target air-fuel ratio. Thereafter, the routine proceeds to step 504, where a correction amount learning execution condition determination routine of FIG. 12 described later is executed to determine whether or not a learning execution condition for the correction amount of the air-fuel ratio detection timing according to the target air-fuel ratio is satisfied. judge.

  Thereafter, the process proceeds to step 505, where whether or not the correction amount learning execution condition is satisfied is determined by whether or not the correction amount learning execution flag set in the correction amount learning execution condition determination routine of FIG. judge. As a result, when it is determined that the correction amount learning execution flag is OFF (execution condition is not satisfied), the process proceeds to step 506 to determine whether or not the current crank angle is the air-fuel ratio detection timing of each cylinder. If it is not the fuel ratio detection timing, this routine is terminated without performing the subsequent processing.

  On the other hand, if the current crank angle is the air-fuel ratio detection timing, the routine proceeds to step 507, where the cylinder-by-cylinder air-fuel ratio control execution routine of FIG. Execute.

  On the other hand, if it is determined in step 505 that the correction amount learning execution flag is ON (execution condition is satisfied), the process proceeds to step 508 to execute a correction amount learning routine shown in FIGS. The correction amount of the air-fuel ratio detection timing according to the air-fuel ratio is learned.

[Air-fuel ratio detection timing calculation routine]
The air-fuel ratio detection timing calculation routine of FIG. 11 is a subroutine executed in step 503 of the cylinder-by-cylinder air-fuel ratio control main routine of FIG. When this routine is started, first, in step 601, the air-fuel ratio detection reference timing of each cylinder is calculated from a map or the like according to the current engine operating state (for example, engine load, engine speed, etc.). This air-fuel ratio detection reference timing corresponds to an appropriate air-fuel ratio detection timing when the target air-fuel ratio is stoichiometric (the excess air ratio λ = 1.0).

  Thereafter, the routine proceeds to step 302, where the current target air-fuel ratio (or detected air-fuel ratio) is determined with reference to the correction amount learning value table of the air-fuel ratio detection timing learned in the later-described correction amount learning routine of FIGS. ) Is read (or interpolated). The correction amount learning value of the air-fuel ratio detection timing becomes a negative value (correction in a direction that accelerates the air-fuel ratio detection timing) at the rich-side air-fuel ratio, and a positive value (a direction that delays the air-fuel ratio detection timing) at the lean-side air-fuel ratio. Correction).

  Thereafter, the process proceeds to step 603, where the correction amount learning value according to the current target air-fuel ratio (or detected air-fuel ratio) is added to the air-fuel ratio detection reference timing set according to the current engine operating state, A typical air-fuel ratio detection timing is obtained.

Air-fuel ratio detection timing = Air-fuel ratio detection reference timing + correction amount learned value In short, the air-fuel ratio detection reference timing (appropriate air-fuel ratio detection timing at the time of stoichiometry) set according to the current engine operating state is set to the current target. The final air-fuel ratio detection timing is determined by correcting with a correction amount learning value corresponding to the air-fuel ratio (or detected air-fuel ratio).

[Correction amount learning execution condition judgment routine]
The correction amount learning execution condition determination routine of FIG. 12 is a subroutine executed in step 504 of the cylinder-by-cylinder air-fuel ratio control main routine of FIG. When this routine is started, first, in step 701, whether or not it is the first travel after the learning value of the correction amount is cleared by replacing the vehicle-mounted battery, or a predetermined period (predetermined accumulated travel distance) from the previous learning. Or the predetermined number of times of travel) has elapsed, and if the determination result is “No”, the correction amount learning execution condition is not satisfied, the process proceeds to step 705, and the correction amount learning execution flag is set. Is set to OFF and this routine is terminated.

  On the other hand, if “Yes” is determined in step 701, the process proceeds to step 702, and in addition to the air-fuel ratio control for each cylinder, control for changing the air-fuel ratio (for example, sub-feedback control, catalyst neutralization control, etc.) ) Is stopped (prohibited), and if the control for changing the air-fuel ratio is not stopped, the correction amount learning execution condition is not satisfied, the process proceeds to step 705, and the correction amount learning execution flag is turned OFF. Set and end this routine.

  If the control for changing the air-fuel ratio is stopped in step 702, the process proceeds to step 703, and even if the correction amount learning is executed (that is, the target air-fuel ratio is changed for learning the correction amount). Also, it is determined whether or not the state of the catalyst 38 is such that the emission does not deteriorate. If the correction amount learning is executed and the catalyst 38 is in such a state that the emission deteriorates, the correction amount learning execution condition is not satisfied, and the routine proceeds to step 705, where the correction amount learning execution flag is set to OFF, This routine ends. On the other hand, if the state of the catalyst 38 is such that the emission does not deteriorate even if the correction amount learning is executed, the correction amount learning execution condition is satisfied, and the routine proceeds to step 704, where the correction amount learning execution flag is set. Set to ON and end this routine.

[Correction amount learning routine]
First, a method for learning the correction amount of the air-fuel ratio detection timing will be described with reference to FIGS. FIGS. 15 and 16 are diagrams for explaining the effect of fuel injection amount correction (fuel correction) when the air-fuel ratio detection timing is different from the proper timing. When the air-fuel ratio detection timing is appropriate, the air-fuel ratio of each cylinder can be accurately estimated. Therefore, when the fuel injection amount of a predetermined cylinder is corrected by a predetermined amount, the air-fuel ratio of that cylinder changes by an amount corresponding to the fuel correction amount. It should be. Focusing on this characteristic, in the third embodiment, by executing the correction amount learning routine of FIGS. 13 and 14, the air-fuel ratio detection timing of each cylinder is changed, and each cylinder at the air-fuel ratio detection timing is changed. The air-fuel ratio before and after fuel correction is estimated, and the air-fuel ratio detection timing at which the amount of change in the estimated air-fuel ratio before and after fuel correction is equivalent to the fuel correction amount is determined as the appropriate air-fuel ratio detection timing. The deviation (correction amount) from the value is learned.

  The correction amount learning routine of FIGS. 13 and 14 is a subroutine executed in step 508 of the cylinder-by-cylinder air-fuel ratio control main routine of FIG. 10, and serves as learning means in the claims. When this routine is started, first, in step 801, it is determined whether or not the air-fuel ratio detection timing is shifted based on the degree of variation in estimated air-fuel ratio among cylinders during cylinder-by-cylinder air-fuel ratio control. In this case, for example, the determination is made based on one or both of the following conditions (A1) and (A2).

(A1) Whether the variation in estimated air-fuel ratio between cylinders is large depending on whether the deviation between the maximum estimated air-fuel ratio and the minimum estimated air-fuel ratio among the estimated air-fuel ratios of each cylinder is a predetermined value or more Determine whether.
(A2) It is determined whether or not the degree of variation in the estimated air-fuel ratio among the cylinders is large depending on whether the standard deviation of the estimated air-fuel ratio of all the cylinders is equal to or greater than a predetermined value.
If the degree of variation in estimated air-fuel ratio between cylinders is large, it is determined that the air-fuel ratio detection timing has shifted. If the degree of variation in estimated air-fuel ratio between cylinders is small, it is determined that the air-fuel ratio detection timing has not shifted. To do.

If it is determined that the air-fuel ratio detection timing is not deviated, it is not necessary to learn the correction amount, and thus this routine is terminated without performing the subsequent learning process.
On the other hand, if it is determined that the air-fuel ratio detection timing has deviated, the process proceeds to step 802, where it is determined whether or not the number of times of retard correction (the number of times the air-fuel ratio detection timing has been corrected to the retard side) is less than the specified number. If the number of retard corrections is less than the specified number, the process proceeds to step 803 to correct the air-fuel ratio detection timing to the retard side by a predetermined crank angle. Thereafter, the process proceeds to step 804, where the air-fuel ratio before and after fuel correction of the cylinder at the air-fuel ratio detection timing after retardation correction is estimated, and the change amount of the estimated air-fuel ratio before and after fuel correction is calculated.

  Thereafter, the process proceeds to step 805, and it is determined whether or not the change amount of the estimated air-fuel ratio substantially matches the fuel correction amount equivalent, thereby determining whether or not the current air-fuel ratio detection timing is an appropriate air-fuel ratio detection timing. Determine whether. As a result, when it is determined that the current air-fuel ratio detection timing is not an appropriate air-fuel ratio detection timing, the process returns to step 802 to repeat the above-described retardation correction of the air-fuel ratio detection timing.

If it is determined that the amount of change in the estimated air-fuel ratio substantially coincides with the fuel correction amount equivalent and the air-fuel ratio detection timing has reached an appropriate air-fuel ratio detection timing before the number of retardation corrections reaches the specified number, Proceeding to step 806, the air-fuel ratio detection timing at that time is determined as an appropriate air-fuel ratio detection timing, and the deviation (deviation) between the appropriate air-fuel ratio detection timing and the air-fuel ratio detection timing before delay correction is detected. Learning is performed as a timing correction amount, and the learning value is updated and stored in a learning value storage area of a rewritable nonvolatile memory such as a backup RAM of the ECU 40. At this time, the correction amount learning value is updated and stored for each air-fuel ratio in consideration that the correction amount of the air-fuel ratio detection timing changes according to the air-fuel ratio. Alternatively, in consideration of the Z-shaped change characteristic of the deviation of the air-fuel ratio detection timing due to the air-fuel ratio, the air-fuel ratio region richer than λrich and the air-fuel ratio region leaner than λlean in FIG. The correction amount of the fuel ratio detection timing may be learned one by one.
Thereafter, the process proceeds to step 807, the count value of the number of retard corrections is reset, and this routine is terminated.

  On the other hand, if the air-fuel ratio detection timing does not reach an appropriate air-fuel ratio detection timing even if the retardation correction of the air-fuel ratio detection timing is repeated a specified number of times, the process proceeds to step 808 in FIG. It is determined whether or not the number of times (the number of times the air-fuel ratio detection timing has been corrected to the advance side) is less than the specified number. If the number of advance angle corrections is less than the specified number, the process proceeds to step 809 to set the air-fuel ratio detection timing. A predetermined crank angle is corrected from the initial position before the retard correction to the advance side. Thereafter, the routine proceeds to step 310, where the air-fuel ratio before and after fuel correction of the cylinder at the air-fuel ratio detection timing after advance angle correction is estimated, and the change amount of the estimated air-fuel ratio before and after fuel correction is calculated.

  Thereafter, the process proceeds to step 311 to determine whether or not the current air-fuel ratio detection timing is an appropriate air-fuel ratio detection timing by determining whether or not the amount of change in the estimated air-fuel ratio substantially matches the fuel correction amount equivalent. Determine whether. As a result, when it is determined that the current air-fuel ratio detection timing is not an appropriate air-fuel ratio detection timing, the process returns to step 808, and the advance correction of the air-fuel ratio detection timing described above is repeated.

  If it is determined that the amount of change in the estimated air-fuel ratio substantially matches the fuel correction amount and the air-fuel ratio detection timing has reached an appropriate air-fuel ratio detection timing before the advance angle correction count reaches the specified number of times, The flow proceeds to 312, and the air-fuel ratio detection timing at that time is determined as an appropriate air-fuel ratio detection timing, and the deviation (deviation) between the appropriate air-fuel ratio detection timing and the air-fuel ratio detection timing before the advance angle correction is detected. Learning is performed as a timing correction amount, and the correction amount learning value is updated and stored in a learning value storage area of a rewritable nonvolatile memory such as a backup RAM of the ECU 40. Thereafter, the process proceeds to step 313, the count value of the advance angle correction count is reset, and this routine is terminated.

  The correction amount learning map created by this routine is used as a map for reading the correction amount learning value of the air-fuel ratio detection timing in step 602 of the air-fuel ratio detection timing calculation routine of FIG.

  If the advance correction of the air-fuel ratio detection timing is repeated a specified number of times, but the air-fuel ratio detection timing does not reach the proper air-fuel ratio detection timing (that is, the proper air-fuel ratio detection timing cannot be learned) In Step 808, “No” is determined, and this routine is finished.

  In the third embodiment described above, the deviation from the appropriate value of the air-fuel ratio detection timing is learned as the correction amount of the air-fuel ratio detection timing during engine operation, and the correction amount learning value is updated to a rewritable nonvolatile memory. Since it is stored, the deviation (correction amount) from the appropriate value of the air-fuel ratio detection timing during engine operation includes not only the deviation due to the air-fuel ratio but also the time-dependent change in response of the air-fuel ratio sensor 37 and deviation due to manufacturing variations. Can learn. For this reason, even in a system in which the influence of the responsiveness of the air-fuel ratio sensor 37 over time and manufacturing variations cannot be ignored, the correction amount of the air-fuel ratio detection timing is not limited to the deviation due to the air-fuel ratio, It is possible to set appropriately including deviation due to variation, and it is possible to improve the air-fuel ratio estimation accuracy of each cylinder.

  Needless to say, the present invention is not limited to the intake port injection engine illustrated in FIG. 1 and can be implemented by being applied to an in-cylinder injection engine without departing from the scope of the invention. .

It is a schematic block diagram of the whole engine control system in Example 1 of this invention. It is a time chart which shows an example of the output amplitude behavior of the air fuel ratio sensor when only one cylinder is rich. FIG. 5 is an air-fuel ratio sensor characteristic diagram obtained by measuring a relationship between a target air-fuel ratio (target λ) and a deviation from an appropriate value of air-fuel ratio detection timing. 4 is a flowchart showing a process flow of a cylinder-by-cylinder air-fuel ratio control main routine according to the first embodiment. 4 is a flowchart showing a process flow of a cylinder-by-cylinder air-fuel ratio control execution condition determination routine according to the first embodiment. 6 is a flowchart illustrating a flow of processing of an air-fuel ratio detection timing calculation routine according to the first embodiment. 4 is a flowchart showing a flow of processing of a cylinder-by-cylinder air-fuel ratio control execution routine according to the first embodiment. FIG. 6 is an air-fuel ratio sensor characteristic diagram obtained by measuring the relationship between a deviation from an appropriate value of the air-fuel ratio detection timing, the response of the air-fuel ratio sensor, and the target air-fuel ratio (target λ). 6 is a flowchart showing a process flow of an air-fuel ratio detection timing calculation routine according to a second embodiment. FIG. 10 is a flowchart showing a process flow of a cylinder-by-cylinder air-fuel ratio control main routine according to a third embodiment. 12 is a flowchart showing a flow of processing of an air-fuel ratio detection timing calculation routine of Embodiment 3. 12 is a flowchart illustrating a flow of processing of a correction amount learning execution condition determination routine according to a third embodiment. It is a flowchart which shows the flow of a process of the corrected amount learning routine of Example 3 (the 1). It is a flowchart which shows the flow of a process of the corrected amount learning routine of Example 3 (the 2). 10 is a time chart illustrating a method for detecting a deviation from an appropriate value of the air-fuel ratio detection timing according to the third embodiment. 12 is a time chart for explaining a learning method of the correction amount of the air-fuel ratio detection timing according to the third embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Engine (internal combustion engine), 12 ... Intake pipe, 14 ... Air flow meter, 15 ... Throttle valve, 19 ... Intake manifold, 20 ... Fuel injection valve, 22 ... Fuel pump, 24 ... Fuel pressure sensor, 27, 28 ... Variable valve Timing mechanism 35 ... Exhaust manifold 36 ... Exhaust collecting part 37 ... Air-fuel ratio sensor 38 ... Catalyst 40 ... ECU (Each cylinder air-fuel ratio estimation means, Cylinder air-fuel ratio control means, Air-fuel ratio detection timing correction means, learning means)

Claims (9)

An air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas is installed in an exhaust gas collecting portion where the exhaust gas of each cylinder of the internal combustion engine collects and flows, and the detected value of the air-fuel ratio sensor at each air-fuel ratio detection timing of each cylinder The cylinder-by-cylinder air-fuel ratio estimating means that estimates the air-fuel ratio of each cylinder based on In a cylinder-by-cylinder air-fuel ratio control device having an air-fuel ratio control unit by cylinder)
Air-fuel ratio detection timing correction means for correcting the air-fuel ratio detection timing according to a target air-fuel ratio or a detected air-fuel ratio,
The air-fuel ratio detection timing correction means corrects the air-fuel ratio detection timing so as to be later than that at the time of stoichiometry when the target air-fuel ratio or the detected air-fuel ratio is lean, and when the target air-fuel ratio or the detected air-fuel ratio is rich A cylinder-by-cylinder air-fuel ratio control apparatus for an internal combustion engine, wherein the air-fuel ratio detection timing is corrected so as to be earlier than when stoichiometric.   2. The air-fuel ratio detection timing correction unit corrects the correction amount of the air-fuel ratio detection timing set according to the target air-fuel ratio or the detected air-fuel ratio according to the responsiveness of the air-fuel ratio sensor. The cylinder-by-cylinder air-fuel ratio control apparatus according to claim 1. Learning means for learning a deviation from an appropriate value of the air-fuel ratio detection timing during operation of the internal combustion engine and updating and storing the learned value in a rewritable nonvolatile memory;
2. The cylinder-by-cylinder air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio detection timing correction means corrects the air-fuel ratio detection timing based on a learning value of the learning means.   The said learning means learns the deviation from the appropriate value of the said air-fuel ratio detection timing for every air-fuel ratio, The cylinder-by-cylinder air-fuel ratio control apparatus of the internal combustion engine of Claim 3 characterized by the above-mentioned.   The learning means learns a deviation from an appropriate value of the air-fuel ratio detection timing one by one in an air-fuel ratio region richer than a predetermined air-fuel ratio range including stoichiometry and an air-fuel ratio region lean. The air-fuel ratio control apparatus for each cylinder of the internal combustion engine according to claim 3.   The air-fuel ratio detection timing correction unit is configured to detect the learning value in the rich-side air-fuel ratio region learned by the learning unit and the lean side when the target air-fuel ratio or the detected air-fuel ratio is within a predetermined air-fuel ratio range including the stoichiometry. 6. The cylinder-by-cylinder air-fuel ratio control apparatus for an internal combustion engine according to claim 5, wherein a correction amount of the air-fuel ratio detection timing is set by interpolation correction with a learned value in the air-fuel ratio region.   The said learning means is provided with a means to prohibit the control which changes an air fuel ratio during the learning of the deviation | shift from the appropriate value of the said air fuel ratio detection timing. An air-fuel ratio control device for each cylinder of an internal combustion engine.   The internal combustion engine according to any one of claims 3 to 7, wherein the learning means performs learning at a predetermined interval in order to update a learning value in accordance with a change with time of the responsiveness of the air-fuel ratio sensor. Air-fuel ratio control device for each cylinder.   9. The learning unit according to claim 3, wherein the learning unit determines a timing for performing learning by changing the air-fuel ratio to a lean side or a rich side based on a state of a catalyst for exhaust gas purification. The cylinder-by-cylinder air-fuel ratio control apparatus according to claim.

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