JP5157672B2 - Multi-cylinder engine air-fuel ratio control method - Google Patents

Description

  The present invention relates to a method for controlling the air-fuel ratio of a multi-cylinder engine having a variable valve mechanism that changes the opening / closing characteristics of an intake valve.

  Conventionally, as shown in the following Patent Document 1, a fuel injection valve provided corresponding to each cylinder of a multi-cylinder engine (multi-cylinder internal combustion engine), and a fuel that sets a fuel injection amount according to the operating state of the engine In the fuel supply device for a multi-cylinder engine provided with the injection amount setting means, the fuel injection amount set by the fuel injection amount setting means is corrected according to the variation in the air-fuel ratio for each cylinder, and the corrected amount The fuel is supplied to each cylinder by the fuel injection valve.

More specifically, in the fuel supply device disclosed in Patent Document 1, a fuel injection amount correction coefficient corresponding to each cylinder is stored in advance in an engine control unit, and all cylinders are used by using this correction coefficient. On the other hand, by individually correcting the fuel injection amount calculated in common, an appropriate fuel injection amount is supplied to each cylinder so as to suppress variations in the air-fuel ratio.
Japanese Utility Model Publication No. 1-58744

  By the way, recent engines are often equipped with a variable valve mechanism that changes the opening / closing characteristics of the intake and exhaust valves (valve lift amount and opening / closing timing). In an engine equipped with such a variable valve mechanism, There is a concern that variations in intake air amount may occur between cylinders due to manufacturing errors of the variable valve mechanism, aging deterioration, and the like. However, the variation in the intake air amount due to such a manufacturing error of the variable valve mechanism is considered to be different for each engine and should change with time. Therefore, in the method of Patent Document 1 in which the correction coefficient is determined for each cylinder in advance, it is difficult to suppress the variation in the air-fuel ratio due to the intake air amount error as described above, and the combustion state is between the cylinders. There is a possibility that the emission property may be deteriorated due to the difference.

  The present invention has been made in view of the above circumstances, and an object thereof is to effectively suppress variation in air-fuel ratio between cylinders that may occur in a multi-cylinder engine equipped with a variable valve mechanism.

In order to solve the above-mentioned problems, the present invention is a method for controlling the air-fuel ratio of a multi-cylinder engine having a variable valve mechanism for changing the opening / closing characteristics of an intake valve, the exhaust gas discharged from each cylinder The air-fuel ratio detection process for individually detecting the air-fuel ratio for each cylinder based on the air-fuel ratio, and the air-fuel ratio of each cylinder detected by the air-fuel ratio detection process is provided for each cylinder so that the air-fuel ratio becomes a desired air-fuel ratio . by controlling the amount of fuel injected from the fuel injection valve, seen including a decrease makes injection amount control process of the variation in air-fuel ratio between the cylinders, as the variable valve mechanism, the intake valve in accordance with the operating condition of the engine When a variable valve lift mechanism that changes the lift amount is used and the variable valve lift mechanism is in a low lift region where the lift amount of the intake valve is set to be relatively small, The air-fuel ratio detection process is executed to detect the air-fuel ratio for each cylinder, and when the engine operating state is in a region other than the region corresponding to the low lift region, each detected in the low lift region Based on the amount of deviation between the air-fuel ratio of the cylinder and the desired air-fuel ratio, the amount of deviation of the air-fuel ratio corresponding to the current operating state is estimated, and the amount of deviation is reduced so that the amount of deviation decreases. The fuel injection amount is determined (claim 1).

According to the present invention, the air-fuel ratio is individually detected based on the exhaust gas discharged from each cylinder, and the fuel injection amount is controlled so that the air-fuel ratio of each cylinder becomes a desired air-fuel ratio. Even if the intake air amount to each cylinder varies due to differences in intake valve opening / closing characteristics due to manufacturing errors or aging deterioration of the valve mechanism, the fuel injection amount to each cylinder is individually adjusted accordingly. By adjusting and maintaining the air-fuel ratio of each cylinder at an optimum value in the design of the engine, variation in the air-fuel ratio among the cylinders can be effectively suppressed, and emission characteristics are improved by uniformizing the combustion state, etc. Can be achieved.

In addition, when the variable valve lift mechanism is in a low lift region where the lift amount of the intake valve is set to be relatively small, the air-fuel ratio detection process is executed to detect the air-fuel ratio for each cylinder. The amount of intake air is determined by detecting the air-fuel ratio of each cylinder under the condition that the lift amount of the cylinder is small and the intake air amount to each cylinder is likely to vary, and controlling the fuel injection amount to each cylinder based on the detected air-fuel ratio. Therefore, even under a situation where there is a tendency to vary, an appropriate amount of fuel corresponding to the variation can be supplied, and there is an advantage that variation in the air-fuel ratio between the cylinders can be more effectively suppressed.

Further, when the vehicle is in a region other than the low lift region, the amount of deviation of the air-fuel ratio corresponding to the current operating state is determined based on the amount of deviation between the air-fuel ratio of each cylinder detected in the low lift region and the desired air-fuel ratio. The fuel injection amount is determined so that the amount of deviation is reduced, and therefore the estimated value is estimated under a situation where the intake valve lift amount is somewhat large and the variation in the intake air amount is expected to be small. By controlling the fuel injection amount by a relatively simple method based on this, there is an advantage that the air-fuel ratio can be made uniform efficiently with the required accuracy according to the situation.

Operating range of the engine corresponding to the low lift region is preferably a low rotation and low load region (claim 2).

  In this way, when the engine output is not so much required, the intake valve lift amount is reduced to reduce the intake air amount, and the fuel consumption is improved by reducing the pumping loss. There is an advantage that the variation of the air-fuel ratio can be suppressed by properly controlling the air-fuel ratio.

  As described above, according to the air-fuel ratio control method for a multi-cylinder engine of the present invention, variation in air-fuel ratio between cylinders that can occur in a multi-cylinder engine equipped with a variable valve mechanism can be effectively suppressed.

  1 and 2 are schematic views showing an example of an engine to which an air-fuel ratio control method for a multi-cylinder engine according to the present invention is applied. The engine shown in this figure is an in-line four-cylinder engine, and the engine body 1 is provided with four cylinders 11 to 14 arranged in a line. Each cylinder 11 to 14 is loaded with a piston 3, and a combustion chamber 5 is formed above the piston 3. In FIG. 2, reference numeral 6 denotes a crankshaft that rotates according to the vertical movement of the piston 3.

  Each of the cylinders 11 to 14 of the engine body 1 is provided with two intake ports 7 and two exhaust ports 8 opened near the top of the combustion chamber 5 for each of the cylinders 11 to 14. End portions of an intake passage 23 and an exhaust passage 24 are connected to the intake port 7 and the exhaust port 8, respectively. The intake and exhaust passages 23 and 24 and the combustion chamber 5 of each cylinder 11 to 14 are connected to each other. The intake port 7 and the exhaust port 8 communicate with each other. In FIG. 1, only the exhaust passage 24 is illustrated, and the intake passage 23 is not illustrated.

  Each of the cylinders 11 to 14 is provided with an intake valve 15 and an exhaust valve 16 for opening and closing the intake port 7 and the exhaust port 8, respectively. These intake / exhaust valves 15 and 16 are driven to open and close by a valve operating mechanism including camshafts 17 and 18 disposed on the upper portion of the engine body 1.

  Of the camshafts 17 and 18, intake cams 17a and exhaust cams 18a are attached to a plurality of axial positions corresponding to the positions of the intake valves 15 and the exhaust valves 16, respectively. The intake and exhaust valves 15 and 16 are driven to reciprocate in the opening and closing direction by swinging or rotating about the camshafts 17 and 18. The camshafts 17 and 18 are linked and connected to the crankshaft 6 via a cam chain or the like (not shown), and are rotationally driven with respect to the crankshaft 6 at a half speed.

  Of the valve operating mechanisms for opening and closing the intake and exhaust valves 15 and 16, the valve operating mechanism for the intake valve 15 employs a variable valve mechanism 20 for changing the opening / closing characteristics thereof. Specifically, in the engine of this embodiment, the opening / closing timing (phase angle) of the intake valve 15 is changed according to the operating state of the engine by changing the rotational phase of the camshaft 17 with respect to the crankshaft 6. The lift amount of the intake valve 15 is changed by changing the swing trajectory of the VCT 21 as a variable valve timing (Variable Camshaft Timing) mechanism and the intake cam 17a attached to the camshaft 17 according to the operation of various links. VVE22 as a variable valve lift (Variable Valve Event) mechanism that changes (valve opening amount) steplessly according to the operating state of the engine is equipped. The variable valve mechanism 20 is configured by these VCT21 and VVE22. Has been. In addition, conventionally well-known thing is applicable to said VCT21 and VVE22, For example, what was disclosed by Unexamined-Japanese-Patent No. 2006-97647 is mentioned, for example.

  FIG. 3 is a diagram showing the opening / closing characteristics of the intake valve 15 that is opened / closed by the valve operating mechanism employing the variable valve mechanism 20. Specifically, FIG. 3 shows the relationship between the crank angle and the lift amount of the intake valve 15, where “TDC” on the horizontal axis indicates the crank angle when the piston 3 is at top dead center, and “BDC” The crank angle when the piston 3 is at the bottom dead center is shown.

  In the characteristic diagram of FIG. 3, the lift curve L1 shows the minimum lift control state when the lift amount of the intake valve 15 is the smallest, and the lift curve L2 is the maximum when the lift amount of the intake valve 15 is the largest. The lift control state is shown. The variable valve mechanism 20 composed of the VCT 21 and the VVE 22 continuously changes the opening / closing characteristics of the intake valve 15 between the two lift curves L1, L2 in accordance with the operating state of the engine. That is, the VVE 22 changes the lift amount of the intake valve 15 and the VCT 21 changes the opening / closing timing of the intake valve 15, so that the opening / closing characteristics of the intake valve 15 continuously change within the range shown in FIG. It has become. In the example of FIG. 3, the lift peak position is controlled to gradually shift toward the advance side as the valve lift amount decreases.

  In FIG. 3, a region indicated by a symbol S represents a low lift region where the lift amount of the intake valve 15 is set to be relatively small. FIG. 4 is a diagram for explaining the correspondence between the engine operating state and various control modes. According to these figures, the opening / closing characteristic of the intake valve 15 is set to the low lift region S when the engine is in a low rotation and low load region. In other words, by setting the opening / closing characteristics of the intake valve 15 to the low lift region S in the low engine speed / low load region, the intake air amount is reduced to reduce the pumping loss, thereby improving the fuel consumption. .

  Returning to FIGS. 1 and 2 again, the overall configuration of the engine will be described. In the engine body 1, a fuel injection valve 25 that directly injects fuel from the side of the combustion chamber 5 and a mixture of the fuel injected from the fuel injection valve 25 and the intake air are contained in the combustion chamber 5. A spark plug 26 for igniting (see FIG. 2 for both) is provided for each of the cylinders 11-14. The fuel injection amount from the combustion injection valve 25 is individually controlled by an ECU 50 described later, and the fuel injection amount can be appropriately increased or decreased for each of the cylinders 11 to 14.

  The engine body 1 is provided with a crank angle sensor 28 for detecting the rotation angle of the crankshaft 6 and a cam angle sensor 29 for detecting the rotation angle of the camshaft 17 on the intake side. . Each of the sensors 28 and 29 is configured to, for example, electromagnetically read the passage of teeth of a pulsar (a disk-shaped member having teeth on the outer periphery) that rotates integrally with the crankshaft 6 or the camshaft 17. 6 or an electromagnetic angle sensor that detects the rotation angle of the camshaft 17. The engine main body 1 is further provided with a water temperature sensor 30 for detecting the temperature of engine coolant.

  The intake passage 23 has an air flow sensor 31 for detecting the flow rate of intake air (intake air amount) introduced into the combustion chamber 5 through the inside thereof, and a throttle valve 33 for adjusting the intake air amount. Are provided in order from the upstream side. 1 and 2, reference numeral 35 denotes a surge tank for reducing intake air pulsation.

A catalytic converter 37 containing a conventionally known three-way catalyst is provided in the middle of the exhaust passage 24, and harmful components (HC, CO and NOx) in the exhaust gas led out through the exhaust passage 24 are provided. Is purified by the three-way catalyst in the catalytic converter 37. Further, an upstream side of the catalytic converter 37 is provided with an O ₂ sensor 39 for detecting the oxygen concentration in the exhaust gas in order to calculate the air-fuel ratio of the engine. The O ₂ sensor 39 is configured as, for example, a so-called linear oxygen sensor that detects a change in oxygen concentration as a change in voltage value and outputs the voltage signal to an ECU 50 described later.

Next, an engine control system will be described. The engine of this embodiment is provided with an ECU 50 including a conventionally known CPU, various memories, and the like, and the operation of the engine is comprehensively controlled by the ECU 50. Specifically, the ECU 50 receives detection signals from various sensors such as the crank angle sensor 28, the cam angle sensor 29, the air flow sensor 31, and the O ₂ sensor 39, and based on various control information obtained thereby. Various control operations such as control of the fuel injection amount injected from the fuel injection valve 25, control of the ignition timing of the spark plug 26, control of the opening / closing timing of the intake valve 15 and the lift amount, and the like are executed. .

  The ECU 50 includes an operation state determination unit 51, a valve control unit 52, a cylinder-by-cylinder air-fuel ratio detection unit 53, and a fuel injection amount control unit 54 as functional elements.

  The operation state determination unit 51 operates the engine based on the detected value of the intake air amount input from the air flow sensor 31, the engine rotation speed (rpm) calculated based on the input signal from the crank angle sensor 28, and the like. The state is determined.

  The variable valve mechanism 20 is configured so that the valve control unit 52 operates the intake valve 15 based on the operating state of the engine determined by the operating state determining unit 51 with an appropriate opening / closing characteristic according to the operating state. Is to control. For example, when the engine operating state is in a low rotation / low load range, the lift amount of the intake valve 15 is reduced and the lift peak position is shifted to the advance side to the VCT 21 and VVE 22 of the variable valve mechanism 20. By performing such control, the opening / closing characteristics of the intake valve 15 are set in the range of the low lift region S in FIG.

The cylinder-by-cylinder air-fuel ratio detection unit 53 individually detects the air-fuel ratio for each of the cylinders 11 to 14 based on detection signals input from the crank angle sensor 28, the cam angle sensor 29, and the O ₂ sensor 39. Is. Specifically, the cylinder-by-cylinder air-fuel ratio detection unit 53 receives the reference signal input from the crank angle sensor 28 every time the crankshaft 6 makes one revolution (that is, every half stroke), and 1/2 of the crankshaft 6. Each of the cylinders 11 to 14 is based on the timing relationship with the reference signal input from the cam angle sensor 29 every time the camshaft 17 rotating at a speed of 1 makes one rotation (that is, every other stroke). Determine if the second cylinder is in the exhaust stroke. When it is determined that a specific cylinder is in the exhaust stroke (more precisely, when a predetermined delay time has elapsed since the start of the exhaust stroke), oxygen input from the O ₂ sensor 39 is input. The air-fuel ratio is calculated based on the concentration value, and the air-fuel ratio at this time is stored as the air-fuel ratio of the specific cylinder.

  The fuel injection amount control unit 54 individually controls the amount of fuel injected from the fuel injection valve 25 into the combustion chamber 5 of each cylinder 11-14 for each cylinder 11-14. Specifically, the fuel injection amount control unit 54 examines the variation in the air-fuel ratio of each cylinder 11-14 based on the air-fuel ratio of each cylinder 11-14 detected by the cylinder-by-cylinder air-fuel ratio detection unit 53, and determines the variation. If there is, the amount of fuel injected next from the fuel injection valve 25 to each of the cylinders 11 to 14 is corrected so as to reduce the variation in the air-fuel ratio. In addition, although mentioned later for details, in this embodiment, each said cylinder is adjusted by adjusting the fuel injection quantity from the said fuel injection valve 25 so that the air fuel ratio of each cylinder 11-14 may respectively correspond with a theoretical air fuel ratio. Variations in the air-fuel ratio of 11 to 14 are reduced.

  However, in the present embodiment, the fuel injection amount correction control as described above is executed only when the operating state of the engine is in the operating region T excluding the vicinity of the full load line AL as shown in FIG. The That is, in the vicinity of the full load line AL (that is, the region outside the operation region T), priority is given to gaining the output of the engine, and more fuel is injected and the air-fuel ratio is set closer to rich. It is not necessary to perform the above-described control of correcting the fuel injection amount so that the air-fuel ratio of each cylinder 11-14 matches the stoichiometric air-fuel ratio. Therefore, the fuel injection amount correction control as described above is executed only in the operation region T excluding the vicinity of the full load line AL of the engine. However, even when the operating state of the engine is in the operating region T, for example, when the engine is cold, the air-fuel ratio is set closer to rich, so the fuel injection amount correction control is not executed. Hereinafter, the operation region T in which the control for correcting the fuel injection amount with the theoretical air-fuel ratio as the target is executed as described above is referred to as a theoretical air-fuel ratio region T.

  Next, more specific contents of the fuel injection amount correction control will be described. In this embodiment, when determining the fuel injection amount, two types of control methods corresponding to the operating state of the engine are used properly. That is, even when the engine operating state is within the range of the theoretical air-fuel ratio region T, it depends on whether or not the lift amount of the intake valve 15 is within the range of the low lift region S where the lift amount of the intake valve 15 is small (that is, the engine is operated at a low speed. Depending on whether or not the load is in a low load range, the specific control method differs as in the following (A) and (B).

(A) In the Low Lift Region S First, the control contents when the engine operating state is in the region corresponding to the low lift region S where the lift amount of the intake valve 15 is small will be described. In this case, the fuel injection amount control unit 54 determines the fuel injection amount from the fuel injection valve 25 based on the air-fuel ratios of the cylinders 11 to 14 actually detected by the cylinder-by-cylinder air-fuel ratio detection unit 53. For example, when the engine rotation speed is a low rotation speed corresponding to the low lift region S, the air-fuel ratios of the cylinders 11 to 14 detected by the cylinder-by-cylinder air-fuel ratio detection unit 53 are shown in FIG. It is assumed that there are variations as shown (numbers # 1 to # 4 on the horizontal axis represent cylinder numbers). Then, the fuel injection amount control unit 54 determines the amount of fuel to be injected into each of the cylinders 11 to 14 based on the variation in the air-fuel ratio of each of the cylinders 11 to 14 thus obtained. By correcting, the air-fuel ratio of each cylinder 11-14 is controlled to be constant in the vicinity of the theoretical air-fuel ratio. Hereinafter, as the correspondence between cylinder numbers # 1 to # 4 in FIG. 5 and cylinders 11 to 14 shown in FIGS. 1 and 2, cylinder 11 is the first cylinder (# 1) and cylinder 12 is The second cylinder (# 2), the cylinder 13 is the third cylinder (# 3), and the cylinder 14 is the fourth cylinder (# 4).

  In FIG. 5, for example, in the first cylinder 11 (# 1), the air-fuel ratio is shifted to a richer side than the stoichiometric air-fuel ratio, so the fuel injection amount control unit 54 next injects fuel into the first cylinder 11. In doing so, the air-fuel ratio of the cylinder 11 is controlled to coincide with the stoichiometric air-fuel ratio by reducing the injection amount by the amount of deviation. On the other hand, for example, in the third cylinder 13 (# 3), the air-fuel ratio is shifted to a leaner side than the stoichiometric air-fuel ratio. Therefore, when the fuel is injected into the cylinder 13 next time, The injection amount is increased by that amount. Then, by correcting the fuel injection amount for each of the cylinders 11 to 14 in this way, from the next combustion, the air-fuel ratio of each of the cylinders 11 to 14 becomes closer to the stoichiometric air-fuel ratio, and the cylinders 11 to 11 are corrected. The variation in the air-fuel ratio among the 14 will be reduced.

(B) When other than the low lift region S Next, the engine operating state is a region other than the region corresponding to the low lift region S (more specifically, a region where the low lift region S is excluded from the theoretical air-fuel ratio region T). Will be described, that is, the control content when the lift amount of the intake valve 15 is in an operating region where the lift amount is set to a medium or high level. In this case, the fuel injection amount control unit 54 determines the air flow corresponding to the current operation state based on the amount of deviation between the air-fuel ratio and the stoichiometric air-fuel ratio of each cylinder 11-14 detected in the low lift region S. The amount of deviation in the fuel ratio is estimated, and the amount of fuel injected from the fuel injection valve 25 is controlled so that the estimated amount of deviation is reduced.

  FIG. 6 is a diagram for explaining how the air-fuel ratio is estimated. In this figure, the horizontal axis indicates the engine speed, and the vertical axis indicates the amount of deviation between the air-fuel ratio and the stoichiometric air-fuel ratio of each cylinder 11-14. In addition, a dashed-dotted line X in the figure indicates the upper limit of the low lift region S, and a plurality of plots attached to the low lift region S on the left side (low rotational speed side) of the line X are as follows. The data of the deviation amount between the air-fuel ratio of each cylinder 11-14 and the stoichiometric air-fuel ratio actually detected by the cylinder-by-cylinder air-fuel ratio detection unit 53 is shown.

  When the engine operating state is in a region other than the region corresponding to the low lift region S (that is, the region on the right side of the line X), the fuel injection amount control unit 54 detects each detected in the low lift region S. From the data of the deviation amount between the air-fuel ratio of the cylinders 11 to 14 and the theoretical air-fuel ratio, the change tendency of the deviation amount of the air-fuel ratio with respect to the engine speed (that is, the change tendency of the broken line connecting the plots in the low lift region S) Based on this tendency, as shown by a broken line in the figure, the deviation amount of the air-fuel ratio at a specific rotation speed is estimated for each cylinder 11-14. Based on the estimated value of the deviation amount of the air-fuel ratio of each of the cylinders 11 to 14 thus obtained, the amount of injection to be injected into each of the cylinders 11 to 14 at the next fuel injection is adjusted. As a result, from the next combustion, the air-fuel ratios of the cylinders 11 to 14 become closer to the stoichiometric air-fuel ratio, and variations in the air-fuel ratio among the cylinders 11 to 14 are reduced.

  Next, the specific contents of the control operation performed by the ECU 50 configured as described above will be described based on the flowchart of FIG. Here, only the control operation for determining the fuel injection amount to each of the cylinders 11 to 14 will be described as a part related to the air-fuel ratio control method of the present invention.

  When the processing shown in the flowchart of FIG. 7 starts, the ECU 50 first determines the engine speed, the intake air amount, and the cooling based on detection signals input from the crank angle sensor 28, the air flow sensor 31, the water temperature sensor 30, and the like. Processing for reading various state quantities such as water temperature is executed (step S1).

  Next, the ECU 50 executes processing for determining the opening / closing characteristics of the intake valve 15 based on the various state quantities read in step S1 (step S3). Specifically, based on the engine speed, the engine load based on the intake air amount, and the cooling water temperature of the engine, the current operating state of the engine is grasped, and the intake valve 15 suitable for the operating state is determined. Determine the opening and closing characteristics.

  Next, the ECU 50 controls the fuel injection amount so that the air-fuel ratio of each of the cylinders 11 to 14 matches the stoichiometric air-fuel ratio based on the current operating state specified from the various state quantities (theoretical air-fuel ratio). Processing for determining whether or not (control condition) is satisfied is executed (step S5). Specifically, the current operating state of the engine is an operating state corresponding to the stoichiometric air-fuel ratio region T shown in FIG. It is determined that the theoretical air-fuel ratio control condition is satisfied.

  When it is determined YES in step S5 and it is confirmed that the theoretical air-fuel ratio control condition is satisfied, the ECU 50 proceeds to the next step S7 and opens / closes the intake valve 15 determined in step S3. It is determined whether or not the characteristic is within the range of the low lift region S where the valve lift amount is small, that is, whether or not the current operation state is in the low rotation / low load region corresponding to the low lift region S. Execute the process.

When it is determined as YES in step S7 and it is confirmed that it is within the range of the low lift region S, the ECU 50 proceeds to the next step S9 to individually set the air-fuel ratio of each cylinder 11-14. Execute the process to detect. That is, the ECU 50 specifies the number of the cylinders 11 to 14 in the exhaust stroke based on the input signals from the crank angle sensor 28 and the cam angle sensor 29, and at that time the O The air-fuel ratio of the cylinder in the exhaust stroke is calculated based on the detection value of the _two sensors 39 (that is, the oxygen concentration of the exhaust gas). And the same process is performed also about another cylinder, and the air-fuel ratio of all the cylinders 11-14 is detected separately, respectively.

  Next, the ECU 50 compares the air-fuel ratios of the cylinders 11 to 14 detected in step S9 with the stoichiometric air-fuel ratio to obtain the difference between them, as shown in FIG. A deviation amount from the fuel ratio is calculated for each of the cylinders 11 to 14, and based on this deviation amount, a feedback correction value that is taken into account when determining the fuel injection amount in step S 15 described later is calculated and stored. Processing is executed (step S11).

  Further, the ECU 50 samples the feedback correction value for the latest plural times, calculates a learning correction value for the injection amount based on the average value, and executes a process for storing this (step S13). Since the feedback correction value is calculated individually for each of the cylinders 11 to 14, this learning correction value is also calculated as an independent value for each of the cylinders 11 to 14. The learning correction value is calculated and updated sequentially every time a predetermined number of feedback correction values are sampled.

  When the calculation of the feedback correction value and the learning correction value is completed as described above, the ECU 50 proceeds to the next step S15 to calculate the amount of fuel to be injected to each of the cylinders 11 to 14, and the corresponding amount. The process which injects a fuel from the fuel injection valve 25 of each cylinder 11-14 is performed. Specifically, the feedback correction value calculated in steps S11 and S13 is set to the basic injection amount obtained from a predetermined control map in accordance with the intake air amount, engine speed, cooling water temperature, etc. acquired in step S1. Or a value obtained by adding a learning correction value or the like is calculated as the amount of fuel to be injected from the fuel injection valve 25 into the combustion chambers 5 of the cylinders 11 to 14, and the corresponding amount of fuel is calculated as the next fuel. The fuel is injected from the fuel injection valve 25 at the time of injection. Since the injection amount at this time reflects the deviation of the air-fuel ratio of each of the cylinders 11 to 14 detected so far, in the next fuel injection, the deviation of the air-fuel ratio is corrected. Variations in the air-fuel ratio among the cylinders 11-14 will be reduced.

  On the other hand, it is determined as NO in step S7, and the current operation state is outside the low lift region S (that is, the region where the low lift region S is excluded from the theoretical air-fuel ratio region T shown in FIG. 4). 6 is confirmed, the ECU 50 detects the deviation amount of the air-fuel ratio of each of the cylinders 11 to 14 detected in the low lift region S (the deviation between the actually measured value of the air-fuel ratio and the theoretical air-fuel ratio) as shown in FIG. The amount of deviation of the air-fuel ratio of each of the cylinders 11 to 14 corresponding to the current operating state, that is, the basic injection amount determined from the current operating state, of each cylinder 11 to 14 is injected. A process of estimating how much the air-fuel ratio deviates from the stoichiometric air-fuel ratio is executed (step S17).

  In this step S17, the deviation amount of the air-fuel ratio of each of the cylinders 11 to 14 in the low lift region S is read, for example, from the learning correction value stored in step S13 for each condition such as the engine speed. Can be obtained. That is, the learning correction value is a value calculated according to the deviation amount between the air-fuel ratio and the theoretical air-fuel ratio of each of the cylinders 11 to 14 detected so far. Based on the value, the deviation amount of the air-fuel ratio of each of the cylinders 11 to 14 in the low lift region S can be obtained by back calculation. Then, based on the air-fuel ratio shift amount in the low lift region S obtained in this way, the air-fuel ratio shift amount corresponding to the current operating state deviating from the low lift region S is shown in FIG. Estimated by the method as described.

  When the deviation amount of the air-fuel ratio of each of the cylinders 11 to 14 is estimated in this way, the ECU 50 sets the estimated value of the deviation amount of the air-fuel ratio to the basic injection amount obtained from the current operation state based on the control map or the like. The amount of fuel to be injected into each of the cylinders 11 to 14 is obtained by adding a correction value of the fuel injection amount calculated based on the fuel injection amount, and the corresponding amount of fuel is injected from the fuel injection valve 25 at the next fuel injection. Processing is executed (step S15).

  In the flowchart of FIG. 7 described above, step S9 corresponds to the air-fuel ratio detection process according to the present invention, and step S15 corresponds to the injection amount control process according to the present invention.

  As described above, in the above-described embodiment, when the air-fuel ratio of the multi-cylinder engine provided with the variable valve mechanism 20 that changes the opening / closing characteristics of the intake valve 15 is controlled, Based on the processing for individually detecting the air-fuel ratio for each of the cylinders 11 to 14 (step S9) and the detected air-fuel ratio of each of the cylinders 11 to 14, the fuel injection valve 25 provided for each of the cylinders 11 to 14 Since the process of controlling the fuel injection amount from the cylinders 11 to 14 so as to reduce the variation in the air-fuel ratio between the cylinders 11 to 14 is performed (step S15), it can occur in a multi-cylinder engine equipped with the variable valve mechanism 20. There is an advantage that variation in the air-fuel ratio between the cylinders 11 to 14 can be effectively suppressed.

  That is, in the above embodiment, the air-fuel ratio is individually detected based on the exhaust gas from each cylinder 11 to 14 (more specifically, the oxygen concentration in the exhaust gas), and each cylinder is determined based on the variation in the detected air-fuel ratio. Since the fuel injection amount to 11 to 14 is controlled, the intake air amount to each cylinder 11 to 14 is caused by a difference in opening / closing characteristics of the intake valve 15 due to a manufacturing error of the variable valve mechanism 20 or aged deterioration. Even when there is a variation, by adjusting the fuel injection amount to each of the cylinders 11 to 14 according to the variation, the variation in the air-fuel ratio between the cylinders 11 to 14 can be effectively suppressed, and the combustion Emission can be improved by making the state uniform.

  In particular, in the above embodiment, since the fuel injection amount from the fuel injection valve 25 is controlled so that the detected air-fuel ratio of each cylinder 11-14 becomes the stoichiometric air-fuel ratio, each cylinder 11-11 14, ideal combustion can be performed, and the exhaust gas from the cylinders 11 to 14 generated by the combustion can be converted into, for example, a three-way catalyst (catalyst) on the downstream side that exhibits the best performance at the stoichiometric air-fuel ratio, for example. The converter 37) can be sufficiently purified, and there is an advantage that the emission property can be improved more effectively.

  In the above embodiment, the variable valve mechanism 20 is provided with a VVE22 (variable valve lift mechanism) that changes the lift amount of the intake valve 15 in accordance with the operating state of the engine. When the lift amount is in the low lift region S that is set to be relatively small (YES in step S7), the cylinder-by-cylinder air-fuel ratio detection unit 53 detects the air-fuel ratio for each cylinder 11-14 (step S9). ) Is executed, there is an advantage that variation in the air-fuel ratio among the cylinders 11 to 14 can be more effectively suppressed.

  That is, according to the above-described method, the air-fuel ratio for each cylinder 11-14 is detected in a situation where the lift amount of the intake valve 15 is small and the intake air amount to each cylinder 11-14 tends to vary, and the detected air By controlling the fuel injection amount to each of the cylinders 11 to 14 based on the fuel ratio, it is possible to supply an appropriate amount of fuel according to the variation even under circumstances where the intake air amount is likely to vary. The variation in the air-fuel ratio can be more effectively suppressed.

  Furthermore, in the above embodiment, since the engine operating region corresponding to the low lift region S is a low rotation / low load region, the lift amount of the intake valve 15 is reduced when the engine output is not so much required. Thus, there is an advantage that the variation of the air-fuel ratio can be suppressed by appropriately controlling the fuel injection amount to each of the cylinders 11 to 14 while reducing the intake air amount and improving the fuel consumption by reducing the pumping loss.

  In the above embodiment, when the engine operating state is outside the region corresponding to the low lift region S (NO in step S7), each cylinder 11 detected in the low lift region S is detected. Based on the deviation amount between the air-fuel ratio of -14 and the stoichiometric air-fuel ratio, the deviation amount of the air-fuel ratio corresponding to the current operating state is estimated (step S17), and the fuel injection valve is adjusted so that the deviation amount decreases. Since the fuel injection amount from 25 is determined (step S15), a comparison based on the estimated value is performed under a situation where the variation in intake air amount is expected to be small because the lift amount of the intake valve 15 is large to some extent. By controlling the fuel injection amount by a simple and convenient method, there is an advantage that the air-fuel ratio can be made uniform efficiently with the required accuracy according to the situation.

  In particular, as in the above-described embodiment, when the low lift region S is set to the low rotation / low load region of the engine, the engine speed is high in regions other than the low lift region S. Although a considerably high-speed calculation process is required to determine the injection amount, if the estimated value is used as described above, the air-fuel ratio is detected for each of the cylinders 11 to 14 and the fuel injection amount is based on the value. There is an advantage that the processing speed can be further increased as compared with the case where the fuel injection amount is calculated, and the fuel injection amount can be appropriately calculated at a higher processing speed corresponding to the rotational speed of the engine.

  In the above embodiment, the fuel injection amount from the fuel injection valve 25 is controlled so that the air-fuel ratios of the cylinders 11 to 14 become the stoichiometric air-fuel ratios respectively. However, for example, lean combustion is performed. Thus, for an engine designed to perform combustion at an air-fuel ratio different from the stoichiometric air-fuel ratio from the beginning, such as a so-called lean burn engine aimed at improving fuel efficiency, each cylinder 11 is brought to its designed air-fuel ratio. The fuel injection amount may be controlled so that the air-fuel ratio of ˜14 matches. The exhaust gas generated by the combustion is sufficiently purified by, for example, a catalyst whose components are adjusted so as to be compatible with lean burn (for example, a catalyst including HC or a noble metal-supported zeolite that functions as a NOx storage material). Is possible.

  In the above embodiment, the case where the air-fuel ratio control method of the present invention is applied to an in-line four-cylinder engine has been described as an example. However, the present invention is not limited to other types such as a V-type six-cylinder engine. The present invention can also be suitably applied to other multi-cylinder engines.

Further, in the above embodiment, the air-fuel ratio of each cylinder 11 to 14 is detected using the single O ₂ sensor 39 provided in the exhaust passage 24, but each cylinder 11 in the exhaust passage 24 is detected. Alternatively, an O ₂ sensor may be provided in each branch pipe portion extending independently from ˜14, and the air-fuel ratio of each cylinder 11-14 may be detected using the plurality of O ₂ sensors.

1 is a schematic plan view showing an example of an engine to which an air-fuel ratio control method for a multi-cylinder engine according to the present invention is applied. It is a schematic side view of the engine. It is a figure which shows the opening-and-closing characteristic of the intake valve opened and closed by the valve operating mechanism containing a variable valve mechanism. It is a figure which shows the correspondence of the driving | running state of an engine, and various control forms. It is a figure which illustrates the dispersion | variation in the detected air fuel ratio of each cylinder. It is a figure for demonstrating the method for estimating the air fuel ratio in those other than a low lift area | region. It is a flowchart which shows the specific content of the control action by ECU.

11 to 14 cylinders 15 intake valve 20 variable valve mechanism 22 VVE (variable valve lift mechanism)
25 Fuel injection valve S Low lift area

Claims (2)

A method for controlling an air-fuel ratio of a multi-cylinder engine having a variable valve mechanism for changing an opening / closing characteristic of an intake valve,
An air-fuel ratio detection process for individually detecting the air-fuel ratio for each cylinder based on the exhaust gas discharged from each cylinder;
By controlling the fuel injection amount from the fuel injection valve provided for each cylinder so that the air-fuel ratio of each cylinder detected by this air-fuel ratio detection process becomes a desired air-fuel ratio, the air-fuel ratio between the cylinders is controlled. a reduction makes injection amount control process variations in the ratio seen including,
As the variable valve mechanism, a variable valve lift mechanism that changes the lift amount of the intake valve according to the operating state of the engine is used.
When the variable valve lift mechanism is in a low lift region where the lift amount of the intake valve is set relatively small, the air-fuel ratio detection process is executed to detect the air-fuel ratio for each cylinder,
When the operating state of the engine is in a region other than the region corresponding to the low lift region, based on the amount of deviation between the air / fuel ratio of each cylinder detected in the low lift region and the desired air / fuel ratio, An air-fuel ratio of a multi-cylinder engine characterized by estimating an air-fuel ratio deviation amount corresponding to a current operating state and determining an injection amount of fuel in the injection amount control process so as to reduce the deviation amount Control method. The air-fuel ratio control method for a multi-cylinder engine according to claim 1 ,
An air-fuel ratio control method for a multi-cylinder engine, characterized in that an engine operation region corresponding to the low lift region is a low rotation / low load region.

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