JP2011157852A - Control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately and stably detect variation of an air-fuel ratio between cylinders without being affected by detecting environment of the individual cylinder. <P>SOLUTION: An ECU 60 calculates a combustion proportion MFB of the respective cylinders based on cylinder pressures and calculates the maximum change rate CS of the combustion proportion in a condition that the respective cylinders are equalized in ignition timing under ignition timing uniforming control. The ECU further calculates the maximum change rate CS<SB>one</SB>of the specific cylinder which is most different in maximum change rate CS from the other cylinders among all cylinders, and an average maximum change rate CS<SB>ave</SB>which averages the maximum change rates CS of the other cylinders. When a parameter K, which is a ratio (CS<SB>one</SB>/CS<SB>ave</SB>) of them, is larger than a predetermined threshold value α, A/F imbalance is determined to be abnormal. With this, even when there is variation in detection sensitivity and detection environment of sensors between the cylinders, the A/F imbalance can be determined accurately. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an internal combustion engine control device, and more particularly to an internal combustion engine control device configured to detect air-fuel ratio variation between cylinders.

  As a conventional technique, for example, as disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2004-36473), a control device for an internal combustion engine configured to detect air-fuel ratio variation between cylinders based on a specific control target model. It has been known. This controlled object model is defined based on the relationship between a specific correction coefficient that defines the fuel supply amount of each cylinder and the exhaust air / fuel ratio detected by the air / fuel ratio sensor. In the prior art, the air-fuel ratio variation of each cylinder is calculated by this controlled object model, and it is determined for each cylinder whether or not the air-fuel ratio variation is within an allowable range.

JP 2004-36473 A

  By the way, the above-described conventional technology is configured to detect air-fuel ratio variation (air-fuel ratio imbalance) between cylinders based on the exhaust air-fuel ratio or the like. However, the detection accuracy of the air-fuel ratio variation by this method is easily affected by the detection environment. That is, for example, there is a problem that the detection accuracy of the air-fuel ratio variation decreases depending on the arrangement of the air-fuel ratio sensor, the exhaust gas flow rate, the contact state between the sensor and gas, the fluctuation range of the air-fuel ratio, and the like.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to accurately and stably vary the air-fuel ratio between cylinders without being affected by the detection environment of each cylinder. An object of the present invention is to provide a control device for an internal combustion engine that can be detected.

A first aspect of the present invention is an in-cylinder pressure sensor that is mounted in each of a plurality of cylinders of an internal combustion engine and detects an in-cylinder pressure of each cylinder;
Ignition timing control means for aligning the ignition timing of each cylinder to the same timing;
A combustion rate calculating means for calculating a combustion rate of each cylinder based on the in-cylinder pressure, and further calculating a maximum rate of change for each cylinder when the combustion rate changes according to a crank angle;
Based on the maximum change rate of the specific cylinder having the maximum change rate of the combustion ratio that is the most different from the other cylinders among the cylinders and the average maximum change rate that averages the maximum change rate of the other cylinders, Air-fuel ratio variation determining means for determining whether the air-fuel ratio variation is within an allowable range;
It is characterized by providing.

  According to the second invention, the ignition timing control means selects, as the reference ignition timing, the ignition timing of the cylinder in which the air-fuel ratio is relatively lean among the cylinders, and sets the ignition timing of the other cylinders as the ignition timing. It is configured to match the reference ignition timing.

A third invention includes an in-cylinder pressure sensor mounted on each of a plurality of cylinders of an internal combustion engine for detecting an in-cylinder pressure of each cylinder;
Ignition timing control means for controlling the ignition timing of each cylinder so that the combustion gravity center position of each cylinder calculated based on the in-cylinder pressure is aligned at the same position;
Based on the ignition timing of the specific cylinder having the ignition timing most different from that of the other cylinders among the cylinders and the average ignition timing obtained by averaging the ignition timings of the other cylinders, variation in the air-fuel ratio between the cylinders is within an allowable range. Air-fuel ratio variation determining means for determining whether there is,
It is characterized by providing.

A fourth invention is an EGR means for recirculating exhaust gas of an internal combustion engine to an intake system;
Determination prohibiting means for prohibiting determination by the air-fuel ratio variation determining means when the EGR means is operating;
It is set as the structure provided with.

  According to the first invention, the ignition timing control means can align the ignition timing of each cylinder when determining the air-fuel ratio variation between the cylinders. Thereby, it is possible to prevent the difference in the ignition timing of each cylinder from affecting the combustion speed, and to accurately reflect the variation in the air-fuel ratio in the individual cylinders in the combustion speed (combustion ratio). Therefore, even when the cylinder-by-cylinder ignition timing control is normally performed, it is possible to prevent the detection resolution from being lowered when determining the air-fuel ratio variation. As a result, the air-fuel ratio variation determination means can accurately and stably determine the air-fuel ratio variation based on the maximum change rate of the combustion ratio of each cylinder, and is highly robust against variations in the detection environment between cylinders. Sex can be obtained.

  According to the second aspect of the invention, the ignition timing control means sets the ignition timing of the other cylinders to the lean cylinder with reference to the ignition timing of the cylinder (lean cylinder) in which the air-fuel ratio is relatively lean among the cylinders. Can be matched. In this case, when the ignition timing of the rich cylinder is used as a reference, the ignition timing is retarded in the lean cylinder having a thin mixture, and the combustion state may be deteriorated. Therefore, according to the ignition timing control means, it is possible to smoothly align the ignition timing of each cylinder while maintaining a good combustion state.

  According to the third aspect of the invention, the ignition timing control means can reflect the variation in the air-fuel ratio in the ignition timing by aligning the combustion gravity center positions of the cylinders. As a result, the air-fuel ratio variation determining means can accurately determine the air-fuel ratio variation based on the difference in the ignition timing of each cylinder even when the detection environment varies among the cylinders. In addition, according to the third aspect of the invention, the air-fuel ratio variation can be stably determined while optimizing the ignition timing of each cylinder by the ignition timing control means, and both drivability and determination accuracy can be achieved. Further, since the ignition timing is controlled for each cylinder, knock control based on the ignition timing can be easily performed, and the occurrence of knocking can be suppressed.

  According to the fourth aspect of the invention, during the execution of the EGR control, the combustion ratio of each cylinder is likely to fluctuate due to the influence, and the determination accuracy of the air-fuel ratio variation is lowered. For this reason, the determination prohibiting means can prohibit the determination by the air-fuel ratio variation determining means during the operation of the EGR means. As a result, even in an internal combustion engine that performs EGR control, the A / F imbalance can be accurately determined.

It is a whole block diagram for demonstrating the system configuration | structure of Embodiment 1 of this invention. It is a characteristic diagram which shows the relationship between the emitted-heat amount in a cylinder, and a crank angle. It is a characteristic diagram which shows the relationship between a combustion ratio and a crank angle. It is a characteristic diagram which shows the relationship between the change rate of a combustion rate, and a crank angle. It is a characteristic diagram which shows the relationship between the change rate of a combustion ratio, and A / F imbalance. It is a characteristic diagram which shows the relationship between the determination parameter K and an A / F imbalance rate. It is explanatory drawing for demonstrating the influence of the ignition timing control classified by cylinder with respect to A / F imbalance. FIG. 6 is a characteristic diagram showing a relationship between an A / F imbalance rate and a determination parameter for each of execution and non-execution of unified ignition timing control. In Embodiment 1 of this invention, it is a flowchart of the control performed by ECU. In Embodiment 2 of this invention, it is a flowchart of the control performed by ECU.

Embodiment 1 FIG.
[Configuration of Embodiment 1]
The first embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is an overall configuration diagram for explaining a system configuration according to the first embodiment of the present invention. The system of the present embodiment includes an internal combustion engine 10 composed of a multi-cylinder engine. FIG. 1 illustrates one cylinder among a plurality of cylinders mounted on the internal combustion engine 10. Each cylinder 12 of the internal combustion engine 10 is provided with a combustion chamber 16 that expands and contracts by reciprocating movement of the piston 14. The piston 14 is connected to a crankshaft 18 that is an output shaft of the internal combustion engine 10. The internal combustion engine 10 also includes an intake passage 20 that sucks intake air into each cylinder 12 and an exhaust passage 22 that discharges exhaust gas from each cylinder 12.

  The intake passage 20 is provided with an air cleaner 24, a surge tank 26, and an electronically controlled throttle valve 28 for adjusting the amount of intake air. On the other hand, the exhaust passage 22 is provided with a catalyst 30 for purifying exhaust gas. Each cylinder 12 has a fuel injection valve 32 for injecting fuel into the combustion chamber 16 (in the cylinder), an ignition plug 34 for igniting the air-fuel mixture in the cylinder, and an intake passage 20 with respect to the combustion chamber 16. An intake valve 36 that opens and closes and an exhaust valve 38 that opens and closes the exhaust passage 22 with respect to the combustion chamber 16 are provided. The internal combustion engine 10 also includes an EGR passage 40 that recirculates the exhaust gas flowing through the exhaust passage 22 to the intake passage 20 and an electromagnetically driven EGR valve 42 that adjusts the flow rate of the exhaust gas flowing through the EGR passage 40. These constitute the EGR means.

  Furthermore, the system of the present embodiment includes a sensor system including a crank angle sensor 44, an intake pressure sensor 46, an in-cylinder pressure sensor 48, a knock sensor 50, and the like, and an ECU (Electronic Control Unit) that controls the operating state of the internal combustion engine 10. 60. The crank angle sensor 44 outputs a signal synchronized with the rotation of the crankshaft 18, and the ECU 60 can detect the engine rotational speed and the crank angle based on this output. The intake pressure sensor 46 detects intake pressure in the surge tank 26, for example, and the ECU 60 can calculate the intake air amount of the internal combustion engine based on the intake pressure. The in-cylinder pressure sensor 48 is constituted by a general pressure sensor using a piezoelectric element, a strain gauge, or the like, and detects the pressure in the combustion chamber 16 (in-cylinder pressure). Further, the knock sensor 50 is constituted by a vibration sensor, a pressure sensor, or the like provided in each cylinder, and detects vibration generated due to knocking.

  The sensor system includes various sensors necessary for controlling the vehicle and the internal combustion engine (for example, a water temperature sensor for detecting the cooling water temperature of the internal combustion engine, an intake air temperature sensor for detecting the temperature of the intake air) in addition to the sensors 44 to 50 described above. , An accelerator opening sensor that detects the accelerator opening, an air-fuel ratio sensor that detects the exhaust air-fuel ratio, and the like. These sensors are connected to the input side of the ECU 60. Various actuators including a throttle valve 28, a fuel injection valve 32, a spark plug 34, an EGR valve 42, and the like are connected to the output side of the ECU 60.

  Then, the ECU 60 detects the state of the internal combustion engine using a sensor system, and performs operation control by driving each actuator based on the detection result. Specifically, the engine speed and the crank angle are detected based on the output of the crank angle sensor 44, and the intake air amount is calculated based on the intake pressure detected by the intake pressure sensor 46. Further, the load factor of the internal combustion engine is calculated based on the intake air amount and the engine speed, and the fuel injection timing, ignition timing, etc. are determined based on the detected value of the crank angle. Then, the fuel injection amount is calculated based on the intake air amount, the load factor, etc., and the fuel injection valve 32 is driven and the spark plug 34 is driven. Further, the ECU 60 executes EGR control for controlling the amount of exhaust gas recirculated to the intake system via the EGR passage 40 by driving the EGR valve 42 based on the operating state of the internal combustion engine.

  Further, the ECU 60 detects the in-cylinder pressure P of each cylinder at an arbitrary crank angle by the in-cylinder pressure sensor 48, stores the detection result as time-series data, and selects an arbitrary crank based on map data stored in advance. And a function of calculating the in-cylinder volume V of each cylinder at a corner. Further, the ECU 60 executes determination control for determining the air-fuel ratio variation among the cylinders as described below.

[Features of Embodiment 1]
In general, when an air-fuel ratio variation (A / F imbalance) between cylinders is detected based on the exhaust air-fuel ratio, the detection accuracy may be lowered depending on the arrangement of the air-fuel ratio sensor, the engine operating state, and the like. Therefore, in the present embodiment, the A / F imbalance is determined based on the change rate of the combustion ratio MFB (Mass Fraction Burned) of each cylinder without using the exhaust air-fuel ratio. The specific method will be described below.

FIG. 2 is a characteristic diagram showing the relationship between the amount of heat generated in the cylinder and the crank angle. This figure illustrates the case where the reference point before the start of combustion is set to BTD 60 ° CA and the reference point after the end of combustion is set to ATDC 60 ° CA. The combustion rate MFB is calculated by normalizing the heat generation amount at an arbitrary crank angle with reference to the heat generation amount before the start of combustion and the heat generation amount after the end of combustion. The amount of heat generated in the cylinder is proportional to PV ^κ calculated based on the cylinder pressure P, the cylinder volume V, and the specific heat ratio κ. Accordingly, the combustion ratio MFB _theta at any crank angle theta, based on this and PV ^kappa _theta at a crank angle, and PV ^kappa _{-60 °} prior to the start of combustion, and ^PV κ _{+ 60 °} after completion combustion, the following It is expressed as (1).

^{_{MFB θ = (PV κ θ -PV}} κ -60 °) / (PV κ + 60 ° -PV κ -60 °) ··· (1)

Based on the in-cylinder pressure P detected by the in-cylinder pressure sensor 48 of each cylinder, the in-cylinder volume V of the cylinder calculated according to the crank angle, and the specific heat ratio κ stored in advance, the ECU 60 Can calculate the combustion ratio MFB _θ . FIG. 3 is a characteristic diagram showing the relationship between the combustion ratio and the crank angle. FIG. 4 shows the rate of change of the combustion rate per unit crank angle (dMFB / dθ) based on this figure.

  FIG. 4 is a characteristic diagram showing the relationship between the change rate of the combustion ratio and the crank angle. As shown in this figure, the rate of change (gradient) of the combustion rate per unit crank angle becomes maximum at a crank angle of about 50 ° CA, for example. In the following description, the maximum value of the change rate is referred to as the maximum change rate CS. The ECU 60 can calculate the maximum change rate CS for each cylinder based on the combustion ratio MFB of each cylinder.

  FIG. 5 is a characteristic diagram showing the relationship between the change rate of the combustion ratio and the A / F imbalance. FIG. 5A shows a state in which the air-fuel ratios of all cylinders are uniform, and FIGS. 5B and 5C show a state in which the air-fuel ratio of a certain cylinder varies between the lean side and the rich side. Each is shown. As shown in FIG. 5, since the combustion speed is slow in the lean cylinder, the maximum change rate CS of the combustion ratio is reduced, and in the rich cylinder, the combustion speed is fast, so that the maximum change rate CS is increased. That is, since there is a certain correlation between the maximum change rate CS of the combustion ratio and the air-fuel ratio, the A / F imbalance can be determined based on the maximum change rate CS of each cylinder.

Next, a method for determining A / F imbalance will be described. First, the ECU 60 selects, as a specific cylinder (abnormal cylinder), a cylinder having the maximum change rate CS of the combustion ratio that is most different from other cylinders among all the cylinders. Subsequently, an average maximum change rate CS _ave is calculated by averaging the maximum change rates CS of other cylinders (normal cylinders) excluding the specific cylinder. Then, a ratio (CS _one / CS _ave ) between the maximum change rate CS _one of the specific cylinder and the average maximum change rate CS _ave of the other cylinders is calculated, and this ratio is adopted as the determination parameter K.

In the above processing, it is preferable to select the cylinder that gives the largest determination parameter K among all the cylinders as the specific cylinder. As an example of such a selection method, the ratio (CS _one / CS _ave ) is calculated for each of the cylinder having the largest maximum change rate CS and the cylinder having the smallest maximum change rate CS among all the cylinders. The cylinder with the larger value may be selected as the specific cylinder. The determination parameter K obtained in this way represents the degree of variation in air-fuel ratio (degree of A / F imbalance) of a specific cylinder with respect to other cylinders.

  FIG. 6 is a characteristic diagram showing the relationship between the determination parameter K and the A / F imbalance rate. The A / F imbalance rate is, for example, the ratio of the deviation amount of the air-fuel ratio of a specific cylinder based on the average value of the air-fuel ratios of the other cylinders, and corresponds to the degree of A / F imbalance. As shown in FIG. 6, the determination parameter K has a correlation with the A / F imbalance rate, and increases as the A / F imbalance rate increases. Therefore, in the A / F imbalance determination process, it is determined whether or not the A / F imbalance rate is within an allowable range by comparing the determination parameter K with a predetermined threshold value α.

  Here, the threshold value α is set corresponding to the upper limit value (boundary on the rich side of the allowable range) of the A / F imbalance rate. In the present embodiment, as shown in FIG. The threshold value α is set corresponding to a state where the / F imbalance rate is + 20%. When the determination parameter K exceeds the threshold value α, it can be determined that the air-fuel ratio is excessively shifted to the rich side in any cylinder and the degree of A / F imbalance is out of the allowable range. In the above description, the case where the A / F imbalance rate shifts to the rich side is illustrated, but the present invention is not limited to this, and the determination is made when the A / F imbalance rate shifts to the lean side. Also good. In this case, for example, a threshold value corresponding to the lower limit value of the A / F imbalance ratio (boundary boundary of the allowable range) is set, and it is determined whether or not the determination parameter K is smaller than this threshold value. Good.

  According to the above determination method, even when there are variations in the detection sensitivity and detection environment of the in-cylinder pressure sensor 48 between the cylinders, these variations are suppressed from affecting the determination parameter K, and the determination parameter K is accurately set. Can be calculated. That is, the combustion ratio MFB used for calculating the determination parameter K includes the in-cylinder pressure P in the denominator and numerator of the equation as shown in the equation (1). Therefore, even if an error occurs in the detected value of the in-cylinder pressure P, this error can be canceled when calculating the combustion ratio, and the degree of A / F imbalance can be accurately reflected in the determination parameter K. Thereby, high robustness can be obtained with respect to variations in the detection environment between the cylinders, and the determination accuracy of the A / F imbalance can be improved.

  Incidentally, the A / F imbalance determination control described above may be executed together with cylinder-by-cylinder ignition timing control (for example, cylinder-by-cylinder MBT control) for optimizing the combustion center-of-gravity position of each cylinder. The combustion center-of-gravity position is a crank angle at which the heat generation amount reaches a predetermined ratio (for example, 50%) based on the total heat generation amount generated in one combustion cycle of the cylinder. According to the cylinder specific ignition timing control, for example, the combustion state can be improved by controlling the ignition timing so that the combustion gravity center position becomes a predetermined target crank angle. However, when the cylinder-by-cylinder ignition timing control is executed when an A / F imbalance occurs between the cylinders, there is a problem that the determination accuracy of the A / F imbalance is lowered as described below.

  FIG. 7 is a characteristic diagram for explaining the influence of the cylinder specific ignition timing control on the A / F imbalance. First, FIG. 7A shows the rate of change in the combustion ratio of each cylinder in a state where an A / F imbalance between the cylinders occurs and the ignition timing control for each cylinder is not executed. The peak value of the waveform shown in this figure corresponds to the maximum change rate CS of the combustion rate. As shown in FIG. 7A, for example, the difference in the maximum change rate CS generated between a cylinder (lean cylinder) in which the air-fuel ratio is shifted to the lean side and other cylinders is used to detect the A / F imbalance. It is big enough.

  On the other hand, FIG. 7B shows a case where the cylinder specific ignition timing control is executed under the same conditions as FIG. 7A. The cylinder specific ignition timing control advances the ignition timing of a lean cylinder having a low combustion speed. As a result, the combustion speed of the lean cylinder becomes faster than before the ignition timing control is executed, so that the difference in the maximum change rate CS between the lean cylinder and the other cylinders decreases as shown in FIG. 7B. However, the detection resolution of A / F imbalance is reduced. Further, since the combustion speed is high in the cylinder (rich cylinder) in which the air-fuel ratio is shifted to the rich side, the ignition timing is retarded by the cylinder-specific ignition timing control. As a result, the combustion speed of the rich cylinder becomes slower than that before execution of the ignition timing control, so that the difference in the maximum change rate CS from the other cylinders is reduced as in the case of the lean cylinder. That is, when the cylinder specific ignition timing control is executed, the ignition timing of each cylinder is controlled so that the combustion speeds of the rich cylinder and the lean cylinder are close to each other, so that the A / F imbalance detection resolution is lowered. .

  Therefore, in the present embodiment, the ignition timing unified control is executed before the A / F imbalance determination control is performed. In the ignition timing unified control, the cylinder-by-cylinder ignition timing control is stopped, and the ignition timings of the respective cylinders are set to the same timing. Specifically, the ignition timing of the cylinder in which the air-fuel ratio is relatively lean among the cylinders is selected as the reference ignition timing, and the ignition timings of the other cylinders are matched with the reference ignition timing.

  FIG. 8 is a characteristic diagram showing the relationship between the A / F imbalance ratio and the determination parameter for each of the execution and non-execution of the unified ignition timing control. As shown in this figure, when the ignition timing unified control is not executed (when the cylinder-by-cylinder ignition timing control is executed), the change in the determination parameter K with respect to the change in the A / F imbalance rate (the characteristic line shown in FIG. 8). ) Is relatively small, the accuracy of A / F imbalance determination is low. On the other hand, when the ignition timing unified control is executed, the gradient of the characteristic line is increased and the S / N ratio is improved as compared with the non-execution control. Therefore, a minute difference in A / F imbalance can be accurately determined based on the determination parameter K.

  Thus, according to the unified ignition timing control, when determining the A / F imbalance, the ignition timing of each cylinder can be aligned to satisfy the determination prerequisite. Thereby, it is possible to prevent the difference in the ignition timing of each cylinder from affecting the combustion speed, and to accurately reflect the variation in the air-fuel ratio in the individual cylinders in the combustion speed (combustion ratio). Therefore, even when the cylinder-specific ignition timing control is normally performed, the detection resolution is prevented from being lowered when determining the A / F imbalance, and the A / F imbalance is stably determined with high accuracy. Can do.

  Moreover, in the present embodiment, since the ignition timing of the lean cylinder is used as a reference, it is possible to smoothly align the ignition timing of each cylinder while maintaining a good combustion state. That is, when the ignition timing of the rich cylinder is used as a reference, the ignition timing of the other cylinders is retarded in order to match the ignition timing of the rich cylinder retarded by the cylinder-specific ignition timing control. As a result, in a lean cylinder with a thin air-fuel mixture, the combustion state may deteriorate and torque fluctuation may increase. In this embodiment, such a situation can be avoided.

  In addition, since the ignition timing unified control is based on the ignition timing of the lean cylinder, the ignition timing of the other cylinders including the rich cylinder is excessively advanced beyond the MBT (Minimum spark advance for Best Torque), and knocking occurs. There is a case. In this case, the ECU 60 identifies the cylinder in which knocking has occurred based on the output of the knock sensor 50, and executes knock control that retards the ignition timing of the cylinder. Thereby, even when the ignition timing of the lean cylinder is used as a reference, the combustion state of each cylinder can be maintained satisfactorily.

  On the other hand, the maximum change rate CS of the combustion ratio used for the determination of the A / F imbalance is a parameter having a correlation with the combustion speed, and thus is influenced not only by the change of the air-fuel ratio but also by the EGR control. That is, since the EGR control slows the combustion speed by recirculating exhaust gas into the intake air, if the EGR control is executed, the combustion ratio of each cylinder is likely to fluctuate due to the influence. Imbalance determination accuracy decreases. Further, variation among cylinders in the amount of exhaust gas recirculated to each cylinder also causes a decrease in determination accuracy. For this reason, in this embodiment, when EGR control is being executed, determination of A / F imbalance is prohibited, and the ignition timing unified control associated therewith is not executed. As a result, even in an internal combustion engine that performs EGR control, the A / F imbalance can be accurately determined.

[Specific Processing for Realizing Embodiment 1]
FIG. 9 is a flowchart of the control executed by the ECU in the first embodiment of the present invention. The routine shown in this figure is repeatedly executed during the operation of the internal combustion engine. In the routine shown in FIG. 9, first, various operation information (water temperature, intake air temperature, etc.) detected by the sensor system is read (step 100). Further, it is determined whether or not EGR control is being executed. If it is being executed, the routine is ended as it is (step 102).

  When the EGR control is not executed, the aforementioned ignition timing unification control is executed to determine the A / F imbalance (step 104). Then, the combustion ratio MFB of each cylinder is calculated by the above-described method, and the maximum change rate (maximum gradient) CS of the combustion ratio is calculated (steps 106 and 108). Then, the cylinder having the largest deviation of the maximum change rate CS among all the cylinders is detected, and the determination parameter K is calculated with this cylinder as the specific cylinder (steps 110 and 112).

  In the next processing, it is determined whether or not the determination parameter K is larger than the threshold value α (step 114). When this determination is established, it is determined that the A / F imbalance is abnormal, and a warning light (MIL) or the like. The determination result is notified by operating the notification means (steps 116 and 118). On the other hand, if the determination in step 114 is not established, it is determined that the degree of A / F imbalance is within the allowable range, and the routine is terminated as it is.

  In the first embodiment described above, step 104 in FIG. 9 shows a specific example of the ignition timing control means in claims 1 and 2. Steps 106 and 108 show a specific example of the combustion ratio calculation means in claim 1, and steps 110, 112 and 114 show a specific example of the air-fuel ratio variation determination means in claim 1. Further, step 102 shows a specific example of the determination prohibiting means in claim 4.

Embodiment 2. FIG.
Next, a second embodiment of the present invention will be described with reference to FIG. Although the present embodiment employs the same system configuration (FIG. 1) as that of the first embodiment, the configuration described below differs from the first embodiment in the control described below. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.

[Features of Embodiment 2]
Even when the A / F imbalance is determined, it is preferable to optimally control the ignition timing of each cylinder in consideration of drivability and fuel consumption. Therefore, in the present embodiment, the A / F imbalance is determined based on the variation in the controlled ignition timing while optimally controlling the ignition timing of each cylinder by the cylinder specific ignition timing control. In the cylinder specific ignition timing control, for example, the ignition timing of each cylinder is controlled so that the combustion gravity center position of each cylinder is aligned at the same position. As a result, the ignition timing is advanced in the lean cylinder, and the ignition timing is retarded in the rich cylinder. That is, in the state where the combustion center of gravity of each cylinder is aligned by the ignition timing control for each cylinder, the variation in the air-fuel ratio of each cylinder is reflected in the ignition timing. An imbalance determination parameter L is calculated.

In the A / F imbalance determination control, first, the cylinder having the ignition timing most different from the other cylinders controlled by the cylinder specific ignition timing control is selected as a specific cylinder (abnormal cylinder). Subsequently, an average ignition timing I _ave is calculated by averaging the ignition timings of the other cylinders (normal cylinders) excluding the specific cylinder.
Then, a ratio (I _one / I _ave ) between the ignition timing I _one of the specific cylinder and the average ignition timing I _ave of the other cylinders is calculated, and this ratio is adopted as the determination parameter L. In the above processing, as in the case of the first embodiment, it is preferable to select the cylinder that gives the largest determination parameter L among all the cylinders as the specific cylinder.

  The determination parameter L obtained in this way represents the degree of A / F imbalance (A / F imbalance rate). For this reason, the value of the determination parameter L increases as the A / F imbalance ratio increases as in the case of the determination parameter K used in the first embodiment. Therefore, in the present embodiment, for example, the A / F imbalance rate is within an allowable range by comparing the predetermined threshold β set corresponding to the upper limit value of the A / F imbalance rate with the determination parameter L. It is determined whether or not. In this case, the threshold value β is obtained based on the relationship between the A / F imbalance rate and the determination parameter L.

  When the determination parameter L exceeds the threshold value β, it can be determined that the air-fuel ratio in any cylinder is excessively shifted to the rich side and the degree of A / F imbalance is out of the allowable range. In the present invention, for example, a threshold value corresponding to the lower limit value of the A / F imbalance rate is set, and by determining whether or not the determination parameter L is smaller than this threshold value, the A / F imbalance rate is determined. It is good also as a structure which performs determination when is shifted to the lean side.

  In the present embodiment configured as described above, substantially the same operational effects as those of the first embodiment can be obtained. In other words, in the present embodiment, by aligning the combustion gravity center position of each cylinder, the variation in the air-fuel ratio can be reflected in the ignition timing, and the determination parameter L can be accurately calculated based on this ignition timing. . Therefore, even when there are variations in the detection environment between the cylinders, it is possible to suppress the influence of these variations on the determination parameter L and accurately determine the A / F imbalance. Moreover, in the present embodiment, the A / F imbalance can be stably determined while optimizing the ignition timing of each cylinder by the cylinder-specific ignition timing control, and both drivability and determination accuracy can be achieved. . Further, since the cylinder specific ignition timing control is performed, the knock control based on the ignition timing can be easily performed. For example, the occurrence of knocking can be suppressed as compared with the case where the ignition timing unified control is performed.

[Specific Processing for Realizing Embodiment 2]
FIG. 10 is a flowchart of control executed by the ECU in the second embodiment of the present invention. The routine shown in this figure is repeatedly executed during the operation of the internal combustion engine. In the routine shown in FIG. 10, first, substantially the same processing as that in steps 100 and 102 in the first embodiment (FIG. 9) is executed (steps 200 and 202).

  Next, the cylinder specific ignition timing control is executed (step 204), and the ignition timing of each cylinder set by the cylinder specific ignition timing control is acquired (step 206). Then, the cylinder having the largest ignition timing deviation is detected from all the cylinders, and the determination parameter L is calculated using this cylinder as the specific cylinder (steps 208 and 210). In the next processing, it is determined whether or not the determination parameter L is larger than the threshold value β (step 212). When this determination is established, it is determined that the A / F imbalance is abnormal, and a warning light (MIL) or the like. The determination result is notified by operating the notification means (steps 214 and 216). On the other hand, if the determination in step 212 is not established, it is determined that the degree of A / F imbalance is within the allowable range, and the routine is terminated as it is.

  In the second embodiment described above, step 204 in FIG. 10 shows a specific example of the ignition timing control means in claim 3. Steps 208, 210 and 212 show a specific example of the air-fuel ratio variation determining means in claim 3, and step 202 shows a specific example of the determination prohibiting means in claim 4.

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 12 Cylinder 14 Piston 16 Combustion chamber 18 Crankshaft 20 Intake passage 22 Exhaust passage 24 Air cleaner 26 Surge tank 28 Throttle valve 30 Catalyst 32 Fuel injection valve 34 Spark plug 36 Intake valve 38 Exhaust valve 40 EGR passage (EGR means)
42 EGR valve (EGR means)
44 Crank angle sensor 46 Intake pressure sensor 48 In-cylinder pressure sensor 50 Knock sensor 60 ECU

Claims (4)

An in-cylinder pressure sensor mounted on each of the plurality of cylinders of the internal combustion engine for detecting the in-cylinder pressure of each cylinder;
Ignition timing control means for aligning the ignition timing of each cylinder to the same timing;
A combustion rate calculating means for calculating a combustion rate of each cylinder based on the in-cylinder pressure, and further calculating a maximum rate of change for each cylinder when the combustion rate changes according to a crank angle;
Based on the maximum change rate of the specific cylinder having the maximum change rate of the combustion ratio that is the most different from the other cylinders among the cylinders and the average maximum change rate that averages the maximum change rate of the other cylinders, Air-fuel ratio variation determining means for determining whether the air-fuel ratio variation is within an allowable range;
A control device for an internal combustion engine, comprising:   The ignition timing control means is configured to select an ignition timing of a cylinder in which the air-fuel ratio is relatively lean among the cylinders as a reference ignition timing, and to adjust the ignition timing of other cylinders to the reference ignition timing. The control apparatus for an internal combustion engine according to claim 1. An in-cylinder pressure sensor mounted on each of the plurality of cylinders of the internal combustion engine for detecting the in-cylinder pressure of each cylinder;
Ignition timing control means for controlling the ignition timing of each cylinder so that the combustion gravity center position of each cylinder calculated based on the in-cylinder pressure is aligned at the same position;
Based on the ignition timing of the specific cylinder having the ignition timing most different from that of the other cylinders among the cylinders and the average ignition timing obtained by averaging the ignition timings of the other cylinders, variation in the air-fuel ratio between the cylinders is within an allowable range. Air-fuel ratio variation determining means for determining whether there is,
A control device for an internal combustion engine, comprising: EGR means for recirculating the exhaust gas of the internal combustion engine to the intake system;
Determination prohibiting means for prohibiting determination by the air-fuel ratio variation determining means when the EGR means is operating;
The control apparatus for an internal combustion engine according to any one of claims 1 to 3, further comprising:

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