JP5394982B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

  The present invention relates to an apparatus for controlling an air-fuel ratio of an internal combustion engine.

  In an internal combustion engine having a plurality of cylinders, if the air-fuel ratio of the air-fuel mixture varies between the cylinders, the purification rate of the catalyst that purifies the exhaust gas is lowered, and the emission may be deteriorated. Therefore, it has been proposed to control the air-fuel ratio for each cylinder.

  In the following Patent Document 1, in an internal combustion engine that performs cylinder-by-cylinder air-fuel ratio control, the learning value for each cylinder at the time of purge execution is calculated based on the cylinder-by-cylinder correction amount at the time of purge execution, and the cylinder-by-cylinder correction amount at the time of purge stop is calculated. It is described that the learning value for each cylinder at purge stop is calculated based on this, and the distribution ratio for each cylinder of purge fuel is calculated based on these learning values, and the purge amount is controlled based on this.

JP 2005-207405 A

  When the air-fuel ratio is controlled for each cylinder, the air-fuel ratio fluctuates due to the purge. Therefore, it is desirable to perform the control considering the purge.

  In the above-described conventional technique, learning is performed when purge is performed and when purge is stopped. Therefore, it is necessary to perform learning whenever the purge execution state changes. Therefore, it takes time to reflect these learning values in the air-fuel ratio control, the emission performance may be deteriorated, and the calculation load may be increased by performing learning a plurality of times.

  Therefore, an object of the present invention is to provide a technique for performing cylinder-by-cylinder air-fuel ratio control in consideration of purge more efficiently.

  According to one aspect of the present invention, an air-fuel ratio control apparatus for an internal combustion engine having a plurality of cylinders is disposed in an exhaust merging portion where exhaust gases of a plurality of cylinders of the internal combustion engine merge, and the air-fuel ratio of the exhaust gas is reduced. An air-fuel ratio detecting means (16) for detecting, a purge flow rate controlling means (28) for controlling a purge flow rate for supplying evaporated fuel to the intake passage of the internal combustion engine, and when the purge flow rate is a predetermined value (S2 is No), and cylinder-by-cylinder air-fuel ratio correction amount calculation means (S4) for calculating an air-fuel ratio correction amount (KAF (i)) for each cylinder so as to suppress variations in the air-fuel ratio among the plurality of cylinders, The cylinder-by-cylinder air-fuel ratio correction amount calculation means calculates the cylinder-by-cylinder air-fuel ratio correction amount when the purge flow rate (QPGCBASE) is greater than the predetermined value (Yes in S2). Average amount For larger cylinders, the cylinder-by-cylinder air-fuel ratio correction amount is decreased, and for cylinders with the cylinder-by-cylinder air-fuel ratio correction amount smaller than the average value, the cylinder-by-cylinder air-fuel ratio correction amount is increased (S12, KAFPurge (i)). ).

  According to the present invention, in an internal combustion engine that performs cylinder-by-cylinder air-fuel ratio control, fluctuations in cylinder-by-cylinder air-fuel ratio due to an increase in the purge amount can be suitably suppressed, and variations in torque between cylinders can be suppressed. Can do. Therefore, it is possible to improve emission performance and merchantability. Further, according to the present invention, the correction amount when the purge flow rate increases from the predetermined value is increased or decreased using the cylinder-by-cylinder air-fuel ratio correction amount when the purge flow rate is a predetermined value. And quick correction can be realized. Therefore, it is possible to avoid a reduction in emission due to the time required for performing the correction.

  According to one embodiment of the present invention, the cylinder-by-cylinder purge that estimates the fuel amount (TPURGE (i)) in the purge gas supplied to each cylinder from the fuel amount (QPURGE) in the purge gas supplied to the intake passage. Fuel amount estimation means (equation (8)) is provided, and when the purge flow rate becomes larger than the predetermined value, the cylinder specific air-fuel ratio correction amount calculation means decreases or increases the cylinder specific air-fuel ratio correction amount. The amount is calculated from the amount of fuel in the purge gas of the corresponding cylinder (formula (11)).

  According to the present invention, the amount by which the cylinder air-fuel ratio correction amount increases or decreases when the purge flow rate increases is determined based on the amount of fuel contained in the purge supplied to each cylinder, and is thus supplied to the combustion chamber. Correction according to the amount of fuel can be performed, and thus the emission performance can be improved.

Other features and advantages of the present invention will be apparent from the detailed description that follows.

The figure which shows the whole structure of an engine and its control apparatus according to one Embodiment of this invention. The figure for demonstrating arrangement | positioning of the air fuel ratio sensor according to one Embodiment of this invention. The figure which shows the variable valve mechanism according to one Embodiment of this invention. The figure which shows the operating characteristic of the variable valve mechanism according to one Embodiment of this invention. The figure for demonstrating the influence which purge has on the air-fuel ratio according to cylinder according to one Embodiment of this invention. 1 is a flowchart of a fuel control process according to an embodiment of the present invention. The flowchart of the calculation process of the air-fuel ratio correction amount according to cylinder which considered purge according to one Embodiment of this invention. The flowchart of the calculation process of the air-fuel ratio correction amount according to cylinder which considered purge according to other embodiment of this invention. The figure which shows the map which prescribes | regulates the correction coefficient of the cylinder-by-cylinder air-fuel ratio correction amount with respect to the purge flow rate according to another embodiment of the present invention. The flowchart of the learning process of the air-fuel ratio correction amount according to cylinder according to one Embodiment of this invention. The figure which shows the behavior of the air fuel ratio deviation signal and the reference signal of each cylinder according to one Embodiment of this invention. The figure which shows a mode that the cylinder specific air-fuel-ratio correction amount is determined through control of a correlation function according to one Embodiment of this invention. The figure which shows notionally the map of the reference value of the cylinder-by-cylinder air-fuel ratio correction amount according to the operating state of the internal combustion engine according to one embodiment of the present invention. The figure which shows the map which prescribes | regulates the reference value of the cylinder-by-cylinder air-fuel ratio correction amount with respect to the lift amount according to one embodiment of the present invention.

  Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an overall configuration diagram of an internal combustion engine (hereinafter referred to as an engine) and its control device according to an embodiment of the present invention.

  An electronic control unit (hereinafter referred to as “ECU”) 1 is a computer including a central processing unit (CPU) and a memory. The memory can store a computer program for realizing various controls of the vehicle and data (including a map) necessary for executing the program. The ECU 1 receives a signal from each part of the vehicle and performs an operation according to data and a program stored in the memory to generate a control signal for controlling each part of the vehicle.

  The engine 2 is an engine having a plurality of cylinders (four cylinders in this embodiment). The engine 2 is provided with a continuously variable valve mechanism 31 that can change the valve timing of the intake valve, and in this embodiment, includes a variable lift mechanism and a variable phase mechanism. The variable lift mechanism is a mechanism that can continuously change the lift amount of the intake valve of each cylinder in accordance with a control signal from the ECU 1. An example of the variable lift mechanism will be described later. The variable phase mechanism is a mechanism that can continuously change the phase of the intake valve of each cylinder in accordance with a control signal from the ECU 1. The variable phase mechanism can be realized by any known method. For example, a method of electromagnetically controlling the phase of the intake valve to advance or retard has been proposed (see, for example, Japanese Patent Laid-Open No. 2000-227033).

  The continuously variable valve mechanism 31 is provided with a sensor 32 for detecting the lift amount of the intake valve of each cylinder and a sensor 33 for detecting the phase of the intake valve. The result is sent to the ECU 1.

  In this embodiment, the lift amount and the phase can be changed continuously. However, the present invention is not limited to this, and the mechanism can change the lift amount and the phase stepwise. In addition, the present invention is applicable.

  An intake passage 3 and an exhaust passage 4 are connected to the engine 2. A throttle valve 5 is provided in the intake passage 3. The opening degree of the throttle valve 5 is controlled in accordance with a control signal from the ECU 1. A throttle valve opening (θTH) sensor 6 for detecting the opening of the throttle valve is connected to the throttle valve 5, and this detected value is sent to the ECU 1.

  In this embodiment, the intake air amount to each cylinder of the engine 2 not only controls the opening degree of the throttle valve 5, but also controls the lift amount of the intake valve of each cylinder via the continuously variable valve mechanism 31. It is realized by doing.

  A fuel injection valve 7 is provided for each cylinder between the engine 2 and the throttle valve 5 and slightly upstream of the intake valve (not shown) of the engine 2. The fuel injection valve 7 is connected to the fuel tank 29 and injects fuel from the fuel tank 29. The fuel injection timing and the fuel injection amount of the fuel injection valve 7 are changed according to a control signal from the ECU 1.

  An air flow meter (AFM) 8 that detects the amount of air flowing through the intake passage 3 is provided upstream of the throttle valve 5.

  An absolute pressure (PB) sensor 9 is provided downstream of the throttle valve 5 and detects the pressure PB in the intake passage 3. An intake air temperature (TA) sensor 10 is provided downstream of the absolute pressure sensor 9 and detects the temperature in the intake passage 3. These detected values are sent to the ECU 1. The engine 2 is provided with an engine water temperature sensor 11 for detecting the engine water temperature TW, and the detected value of the sensor is sent to the ECU 1. Further, an atmospheric pressure sensor 12 for detecting the atmospheric pressure PA is installed at an arbitrary position outside the engine, and the detected value of the sensor is sent to the ECU 1.

  A crank angle sensor 13 for detecting the rotation angle of the crankshaft of the engine 2 is connected to the ECU 1, and the detected value of the sensor is supplied to the ECU 1. The crank angle sensor 13 generates one pulse (CRK pulse) every predetermined crank angle (for example, 30 degrees), and can specify the rotational angle position of the crankshaft by the pulse. The ECU 1 calculates the engine speed NE based on the CRK pulse. The crank angle sensor 13 outputs a TDC signal to the ECU 1 at a crank angle related to the top dead center (TDC) position of the piston.

  The fuel tank 29 is connected to the canister 21 through the charge passage 20. The canister 21 incorporates an adsorbent 22 that adsorbs the evaporated fuel generated in the fuel tank 29 and has an outside air intake port 23.

  The canister 21 is connected to the downstream side of the throttle valve 5 in the intake passage 3 via the purge passage 27. A purge control valve 28 is provided in the purge passage 27. The purge control valve 28 is duty controlled in accordance with a control signal from the ECU 1. The valve opening amount of the purge control valve 28 can be continuously controlled by changing the ratio (duty ratio) of the valve opening (on) time and the valve closing (off) time represented by the control signal, that is, the duty signal. it can. Thus, the purge control valve 28 controls the flow rate (purge flow rate) of the purge gas flowing through the purge passage 27 toward the intake passage 3 and supplies the evaporated fuel in the purge gas to the intake passage 3.

  As described above, the mixture of the fuel from the fuel injection valve 7, the evaporated fuel from the purge passage 27, and the air from the intake passage 3 burns in the combustion chamber of the engine 2. As a result, exhaust gas from each cylinder flows out into the exhaust passage 4. The exhaust passage 4 is provided with a catalyst device (CAT) 15 that can be realized by various catalysts, for example, and purifies the exhaust gas flowing out into the exhaust passage and releases it to the atmosphere.

  An air-fuel ratio (LAF) sensor 16 for detecting the air-fuel ratio of the exhaust gas is provided upstream of the catalyst device 15. The air-fuel ratio sensor 16 linearly detects the air-fuel ratio in the region ranging from lean to rich of the air-fuel mixture and sends it to the ECU 1. In this embodiment, the detected equivalent ratio KACT is detected from the output of the air-fuel ratio sensor 16. The detected equivalent ratio KACT is a signal indicating the air-fuel ratio, and is calculated by “theoretical air-fuel ratio / air-fuel ratio”. If the value of the detected equivalent ratio KACT is smaller than 1, it indicates that the air-fuel ratio is lean, and if it is larger than 1, it indicates that it is rich.

  An exhaust gas (O 2) sensor 17 is provided downstream of the catalyst device 15. The exhaust gas sensor 17 is a binary exhaust gas concentration sensor. The exhaust gas sensor 17 outputs a high-level signal when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, and outputs a low-level signal when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. The output signal is sent to the ECU 1.

  The ECU 1 detects the operating state of the engine 2 in accordance with programs and data (including a map) stored in the memory in accordance with input signals from the various sensors, and controls the throttle valve 5, the fuel injection valve 7, and the purge control. A control signal for controlling the valve 28, the variable valve mechanism 31 and the like is generated.

  FIG. 2 is a schematic diagram for explaining the arrangement of the air-fuel ratio (LAF) sensor 16 when the engine 2 of FIG. 1 is an in-line four-cylinder engine. In this engine, four cylinders 2a to 2d are provided, and intake pipes 3a to 3d branched at a collecting portion 35 of the intake passage 3 are connected to each cylinder, and exhaust pipes 4a to 4b of each cylinder are connected to a collecting portion 36. Are connected to the exhaust passage 4. The air-fuel ratio sensor 16 is provided downstream of the collecting portion 36 on the exhaust passage 4.

  FIG. 3 is a view for explaining an example of a variable lift mechanism mounted on the variable valve mechanism 31 of FIG. As shown in (a), the variable lift mechanism swings the cam shaft 61 provided with the cam 62, the control arm 65 supported by the cam holder so as to be swingable around the support portion 65a, and the control arm 65. A control shaft (control shaft) 66 provided with a control cam 67 to be driven, a sub cam 63 that is swingably supported by the control arm 65 via a sub cam shaft 63b, and that swings in accordance with the cam 62; A rocker arm 64 that is driven by the sub cam 63 and drives the intake valve 14 is provided. The rocker arm 64 is supported in the control arm 65 so as to be swingable by a rocker shaft 68.

  The sub cam 63 has a roller 63 a that abuts the cam 62, and swings about the sub cam shaft 63 b as the cam shaft 61 rotates. The rocker arm 64 has a roller 64a that contacts the sub cam 63, and the movement of the sub cam 63 is transmitted to the rocker arm 64 via the roller 64a. The control arm 65 has a roller 65b that abuts on the control cam 67, and swings about the support portion 65a as the control shaft 66 rotates.

  (A), (b), and (c) show the high lift state, the middle lift state, and the low lift state of the intake valve 14, respectively, as indicated by arrows. By changing the position of the control arm 65 via the control shaft 66, it is possible to continuously transition between the high lift state as shown in (a) and the low lift state as shown in (c). In the state shown in (a), the movement of the sub cam 63 is transmitted to the intake valve 14 via the rocker arm 64, and the intake valve 14 opens with the maximum lift amount. In the state shown in (b), the amount of transmission of the movement of the sub cam 63 to the intake valve 14 via the rocker arm 64 is less than (a), and therefore the valve is opened with a lift amount smaller than (a). In the state shown in (c), since the movement of the sub cam 63 is hardly transmitted to the rocker arm 64, the intake valve 14 is in a low lift state.

  An actuator motor (not shown) is connected to the control shaft 66. By rotating the control shaft 66 by the motor, the lift amount of the intake valve 14 is continuously changed, and the combustion chamber (cylinder) The intake air amount to the inside can be changed continuously. In this embodiment, the above-described sensor 32 (FIG. 1) for detecting the lift amount is provided so as to detect the rotational angle position of the control shaft 66. The detected rotation angle position CSA of the control shaft 66 is used as a parameter indicating the lift amount.

  Referring to FIG. 4, the operation characteristics of the exhaust valve of each cylinder and the operation characteristics of the intake valve 14 controlled by the continuously variable valve mechanism 31 are shown as an example. The vertical axis represents the lift amount, and the horizontal axis represents the crank angle in one combustion cycle.

  EX indicates the operating characteristic of the exhaust valve. IN indicates the operating characteristic of the intake valve 14, and as indicated by a plurality of solid lines, the operating characteristic IN is continuously changed according to the operating state of the engine, that is, the phase and lift amount of the intake valve are continuously changed. Can be made. Thus, in this embodiment, the lift amount of the intake valve 14 can be continuously changed from a low lift to a high lift according to the operating state of the engine. Note that “lift amount” in the following description represents a peak in each operating characteristic, that is, a maximum lift amount (amount of opening the intake valve (mm)).

  Here, it will be considered how the air-fuel ratio between the cylinders is affected by the purge.

The air-fuel ratio AF (i) of each cylinder is determined by the intake air amount Gair (i) (g / TDC) and the fuel amount Fuel (i) (g / TDC) as shown in the equation (1). Is done. Here, i is a number for identifying a cylinder.

The fuel amount Fuel (i) to each cylinder includes a fuel amount Fuel _INJ supplied by the fuel injection valve 7 and a fuel amount Fuel _Purge included in the purge gas. When the average value of the intake air amount to all cylinders is represented by Gair _ave and the ratio of the purge gas contained in the intake air amount Gair (i) to each cylinder is represented by K, Fuel (i) is represented as follows. be able to. 14.7 is the average air-fuel ratio of all cylinders, and here, the stoichiometric air-fuel ratio (stoichiometric) is used as a value realized by air-fuel ratio feedback control.

From the above, equation (1) is expressed as equation (3), which indicates the air-fuel ratio of each cylinder when purging is performed.

Of course, when purging is not performed, the air-fuel ratio of each cylinder is expressed as follows.

  As shown in the equations (3) and (4), the variation in the air-fuel ratio between the cylinders occurs according to the difference in the intake air amount Gair (i) between the cylinders. A difference is generated according to the amount of purge gas to each cylinder (Gair (i) × K).

  Here, referring to (a1) and (a2) in FIG. 5, the lift amount of the intake valve between the cylinders when the air-fuel ratio feedback control is performed but the cylinder-by-cylinder air-fuel ratio control is not performed. The influence of the purge component (fuel component in the purge gas) due to the variation is shown. Here, in the air-fuel ratio feedback control, the average value of the air-fuel ratios of all the cylinders calculated based on the detection result of the air-fuel ratio sensor 16 is used as a predetermined target air-fuel ratio (in this embodiment, the stoichiometric air-fuel ratio (stoichiometric)). ) Is a control for convergence. On the other hand, the cylinder-by-cylinder air-fuel ratio control is a control for eliminating variations in the air-fuel ratio between cylinders.

  (A1) shows a case where the lift amount of the intake valve is controlled to a low lift value (for example, 1 mm), and (a2) is a case where the lift amount of the intake valve is controlled to a high lift value (for example, 9 mm). Indicates the case. Here, the engine has four cylinders, the lift amount is shifted to the valve opening side for the first and second cylinders, and the valve closing side for the third and fourth cylinders. Assume that the lift amount is off. The deviation of the lift amount indicates that the actual lift amount slightly deviates from the target lift amount for some reason even though the valve is controlled according to the target lift amount.

  In (a1) and (a2), the air-fuel ratio before purge introduction is shown in the bar graph on the left side of each cylinder. Since the air-fuel ratio feedback control is performed, the average value of the air-fuel ratios of the four cylinders is almost the stoichiometric air-fuel ratio. However, since the cylinder-by-cylinder air-fuel ratio control is not performed, the air-fuel ratio varies between cylinders. This variation is caused by the shift in the lift amount as described above, and is large in (a1) and small in (a2). This is because when the lift amount is low, the sensitivity (relative ratio) to the intake air amount due to the shift of the lift amount is larger than when the lift amount is high.

  The air-fuel ratio after introducing the purge when such a variation in the air-fuel ratio occurs is shown in the bar graph on the right side of each cylinder. When the lift amount of the valve is shifted to the open side, air easily enters the cylinder, and as shown in the above equation (3), the evaporated fuel due to the purge also easily enters the cylinder. Therefore, the air-fuel ratio changes in the rich direction as indicated by the arrow 101 in accordance with the introduction of the purge. On the other hand, when the lift amount of the valve is deviated to the closed side, air is difficult to enter the cylinder, and evaporated fuel is also difficult to enter the cylinder. Accordingly, the air-fuel ratio changes to lean as indicated by the arrow 103 in accordance with the introduction of the purge.

  As described above, since the sensitivity to the intake air amount due to the variation in the lift amount is large in the low lift state, the variation in the air-fuel ratio due to the purge introduction in (a1) is larger than the variation in the air-fuel ratio due to the purge introduction in (a2). . However, it can be seen that the variation in the air-fuel ratio between the cylinders is reduced by the fluctuation. That is, when the cylinder-by-cylinder air-fuel ratio control is not being executed, the variation in the cylinder-to-cylinder air-fuel ratio can be alleviated by introducing a purge.

  (B) shows the same graph when not only the air-fuel ratio feedback control but also the cylinder-by-cylinder air-fuel ratio control is performed, and the lift amount of the intake valve is maintained at a low lift as in (a1). Similarly to (a1), it is assumed that the intake valves of the first and second cylinders are shifted to the open side, and the intake valves of the third and fourth cylinders are shifted to the closed side.

  When the purge is not introduced, as shown by the bar graph on the left side of each cylinder, the cylinder-to-cylinder air-fuel ratio control substantially eliminates the variation among the cylinders, and all the cylinders are almost at the stoichiometric air-fuel ratio. . When purge is introduced in this state, the air-fuel ratio becomes as shown by the bar graph on the right side of each cylinder. In the cylinder in which the valve is shifted to the open side, the air-fuel ratio becomes rich as shown by the arrow 105, and in the cylinder in which the valve is shifted to the closed side, the air-fuel ratio becomes lean as shown by the arrow 107. Thus, in the case of an engine that performs cylinder-by-cylinder air-fuel ratio control, if purge is performed, the air-fuel ratio between cylinders varies, and the smoothing effect by cylinder-by-cylinder air-fuel ratio control may be impaired. In particular, when realizing a low lift state in an internal combustion engine provided with the variable valve mechanism as described above, the amount of the variation may increase.

  Therefore, in the present invention, in the engine that executes the cylinder-by-cylinder air-fuel ratio control, even when the purge is executed, the cylinder considering the purge so as to prevent the air-fuel ratio variation between the cylinders as in (b) can be prevented. We propose a method for realizing separate air-fuel ratio control. This specific method will be described below.

  FIG. 6 is a flowchart showing a fuel control process executed by the ECU 1 (more specifically, the CPU of the EUC 1). This process can be performed at predetermined time intervals.

  In step S1, information necessary for determining a predetermined condition for executing the purge is acquired. For example, the predetermined condition may include 1) the engine water temperature is a predetermined value (for example, 65 degrees or more), 2) the air-fuel ratio feedback control is being performed, and 3) the fuel is not being cut. . Therefore, in step S1, the detection value of the engine water temperature sensor 11 is acquired, information indicating whether the air-fuel ratio feedback control is being executed is acquired, and information indicating whether the fuel cut is being executed are acquired.

  In step S2, based on the information acquired as described above, it is determined whether or not all of the predetermined conditions are satisfied. If any of the conditions is not satisfied, it is determined that the purge execution condition is not satisfied, and the process proceeds to step S3. If all the conditions are satisfied, it is determined that the purge execution condition is satisfied, and the process proceeds to step S6.

  When the purge is not executed, the purge flow rate QPGCBASE is set to zero in step S3. The purge flow rate indicates the amount of purge gas flowing through the intake passage 3 of the engine 2. Based on the purge flow rate QPGCBASE, the duty ratio of the purge control valve 28 is controlled (substantially closed) so that the purge gas does not flow.

  On the other hand, when purging is executed, a predetermined value or a value determined according to the operating state of the engine is set in the purge flow rate QPGCBASE in step S6. Based on the purge flow rate QPGCBASE, the duty ratio of the purge control valve 28 is controlled so that the purge gas of the flow rate QPGCBASE flows.

  Alternatively, the duty ratio of the purge control valve 28 may be controlled by determining the purge execution condition in another control process. In this case, in this process, the current duty ratio of the purge control valve 28 is acquired, and if the purge control valve 28 is closed, the process proceeds to step S3, where the purge flow rate QPGCBASE is set to zero, and the purge control valve 28 If the valve 28 is open, the process proceeds to step S6, and the flow rate corresponding to the duty ratio may be set to QPGCBASE.

  When the purge is not executed, the learning process of the cylinder-by-cylinder air-fuel ratio is executed in step S4, and the learning value KREF (i) of the cylinder-by-cylinder air-fuel ratio correction amount and the learning value KREF (i) are The cylinder-by-cylinder air-fuel ratio correction amount KAF (i), which is corrected (corrected) so as to be adapted to the operating state, is calculated. As described above, i is a cylinder identification number, and in this embodiment, there are 4 cylinders, so i takes a value of 1 to 4. Here, both KREF (i) and KAF (i) are correction amounts (correction coefficients) of the fuel injection amount of each cylinder for eliminating the variation in the air-fuel ratio between the cylinders when the purge flow rate is zero. Show. However, the former is learned under a predetermined operating condition, and the latter is different in that the learned value KREF (i) is corrected to match the current operating condition. Therefore, the former is used to calculate the learning value KAF (i) suitable for the operation state after the increase when the purge flow rate is increased thereafter (S2 is Yes), and the latter is used in the next step S5. This is used to correct the fuel injection amount. The learning process including the calculation of both will be described later with reference to FIG.

In step S5, the fuel injection amount Tout (i) of each cylinder is calculated according to the following equation using the cylinder-by-cylinder air-fuel ratio correction amount KAF (i) calculated in step S4.
Tout (i) = TIM × KCMD × KAF × KAF (i) (5)

  Here, TIM indicates the basic fuel injection amount calculated in step S7 described later. KCMD indicates a target air-fuel ratio (expressed by an equivalence ratio) calculated in step S8 described later. KAF is an air-fuel ratio correction amount calculated in step S9 described later, and is calculated by the above-described air-fuel ratio feedback control. The values (latest values) calculated in the previous steps S7 to S9 can be used for TIM, KCMD, and KAF in equation (5).

  As shown in the above equation (5), since KAF (i) is multiplied, the fuel injection amount Tout (i) to each cylinder is calculated so that the variation in the air-fuel ratio among the cylinders is eliminated. can do. The fuel injection valve 7 of each cylinder is controlled according to Tout (i) thus calculated.

  When purging is performed, a basic fuel injection amount TIM is calculated in step S7. This can be calculated by any appropriate method. For example, the basic fuel injection amount TIM can be determined according to the intake air amount detected by the AFM 8.

  In step S8, a target air-fuel ratio KCMD is calculated. This can be calculated or determined by any suitable technique. For example, the target air-fuel ratio KCMD can be set to a predetermined value indicating the theoretical air-fuel ratio. For example, in a predetermined operating range (for example, a throttle full open range (WOT range)), the target air-fuel ratio KCMD can be set to a value indicating a predetermined rich air-fuel ratio.

  In step S9, an air-fuel ratio correction amount KAF is calculated through air-fuel ratio feedback control. As described above, by executing the air-fuel ratio feedback control, the actual air-fuel ratio KACT (representing the average value of the air-fuel ratios of all the cylinders calculated based on the detection result of the air-fuel ratio sensor 16 as described above). A correction amount KAF for converging to the target air-fuel ratio KCMD is calculated. The feedback control can be realized by any appropriate method, for example, PI control can be used.

  In step S10, it is determined whether or not to perform cylinder-by-cylinder air-fuel ratio control. For example, a predetermined condition can be set in advance based on the operating state of the engine, and the cylinder-by-cylinder air-fuel ratio control can be performed when the predetermined condition is satisfied. For example, as the predetermined condition, the engine speed NE detected via the CRK sensor 13 is within a predetermined range, and the fluctuation of the predetermined time is within the predetermined range, and the intake pipe pressure sensor 9 and the large It can include that the magnitude of the gauge pressure (absolute pressure−atmospheric pressure) of the intake pipe detected via the atmospheric pressure sensor 12 is equal to or greater than a predetermined value, and that fluctuations within a predetermined time are within a predetermined range. .

If cylinder-by-cylinder air-fuel ratio control is not performed, in step S11, the fuel injection amount Tout (i) of each cylinder is calculated as in the following equation. Since the cylinder-by-cylinder air-fuel ratio correction amount KAF (i) is not used, the same amount of fuel injection amount Tout is calculated for all cylinders.
Tout (i) = TIM × KCMD × KAF (6)

  When the cylinder-by-cylinder air-fuel ratio control is performed, the process proceeds to step S12, and the cylinders that are considered for purging for each cylinder based on the purge flow rate QPGCBASE obtained in step S6 and the cylinder-by-cylinder air-fuel ratio correction amount KAF (i). Another air-fuel ratio correction amount KAFPurge (i) is calculated. The cylinder-by-cylinder air-fuel ratio correction amount KAF (i) used here is calculated by correcting the learned value KREF (i) obtained in step S4 so as to match the current engine operating state, as described above. Is. A specific calculation method of the cylinder-by-cylinder air-fuel ratio correction amount KAFPurge (i) in consideration of the purge will be described later with reference to FIGS.

  Proceeding to step S13, the fuel injection amount Tout (i) of each cylinder is calculated using the cylinder-by-cylinder air-fuel ratio correction amount KAFPurge (i) in consideration of the purge, as in the following equation.

Tout (i) = TIM × KCMD × KAF × KAFPurge (i) (7)
As is apparent from comparison with equation (5), in equation (7), KAF Charge (i) is used instead of KAF (i). Both are correction coefficients for eliminating the variation in the air-fuel ratio between the cylinders, but the former is a correction coefficient that does not consider purge, and the latter is a correction coefficient that considers purge.

  Thus, when the purge flow rate is zero, the cylinder-by-cylinder air-fuel ratio correction amount is learned in step S4 (KREF (i)). Thereafter, when the purge flow rate increases, in step S12, the learned air-fuel ratio for each cylinder is learned. By using the correction amounts (KREF (i), KAF (i)), the cylinder-by-cylinder air-fuel ratio correction amount KAFPurge (i) considering the purge is obtained. A method for calculating the cylinder-by-cylinder air-fuel ratio correction amount KAFPurge (i) in consideration of the purge will be specifically described.

  FIG. 7 is a flowchart of a process for calculating the cylinder-by-cylinder air-fuel ratio correction amount KAFPurge (i) in consideration of the purge in step S12 of FIG. 6 according to one embodiment of the present invention.

  In step S21, the learning value KREF (i) of the cylinder-by-cylinder air-fuel ratio correction amount calculated in step S4 when the purge is not executed is read for each cylinder. For example, by storing, in a memory or the like, the value calculated in step S4 when the previous purge was not executed, this can be read in step S21. As described above, the learning value KREF (i) is obtained by learning the air-fuel ratio correction amount when purge is not being executed, but is learned under a predetermined operating state of the engine. Therefore, the learned value KREF (i) is corrected to a learned value KAF (i) corresponding to the current engine operating state in step S22. A specific method of this correction will be described later with reference to steps S47 and S48 in FIG. Thus, the learned cylinder-by-cylinder air-fuel ratio correction amount KAF (i) corresponding to the current operating state is calculated.

  In step S23, assuming that the air-fuel ratio deviation between the cylinders occurs because the distribution ratio of the intake air amount among the cylinders is not uniform, the intake air amount from the learned cylinder-by-cylinder air-fuel ratio correction amount KAF (i) The distribution ratio of the air amount between the cylinders is calculated. That is, as described with reference to FIG. 5, if there is a shift in the lift amount between the cylinders, a difference occurs in the intake air amount to each cylinder. As a result, the air-fuel ratio varies among the cylinders, and this is reflected in the cylinder-by-cylinder air-fuel ratio correction amount KAF (i). Therefore, the distribution ratio between the cylinders of the intake air amount (which can be detected by the AFM 8) is calculated from the cylinder-by-cylinder air-fuel ratio correction amount KAF (i).

As a specific example, for example, consider a case where each of the first to fourth cylinders has the following values for the cylinder-by-cylinder air-fuel ratio correction amount KAF (i). In this embodiment, each cylinder KAF (i) is calculated so that the average value of KAF (i) of all cylinders is 1.
KAF (1) = 1.2, KAF (2) = 1.2, KAF (3) = 0.8, KAF (4) = 0.8

The distribution ratio of the intake air amount can be calculated by the reciprocal of KAF (i) and is as follows.
1st cylinder = 0.83, 2nd cylinder = 0.83, 3rd cylinder = 1.25, 4th cylinder = 1.25

  Thus, the first and second cylinders having a value larger than the average value of KAF (i) (here, value 1) are cylinders that are trying to correct the air-fuel ratio to be rich. Conversely, the third and fourth cylinders having values smaller than the average value are cylinders that are trying to correct the air-fuel ratio toward lean. Therefore, the distribution of the intake air amount to the first and second cylinders is reduced as compared with the third and fourth cylinders.

In step S24, the fuel amount TPURGE (i) (cc) that flows in as a purge gas component is calculated for each cylinder. Here, since the fuel amount in the purge gas is introduced into each cylinder in the form of evaporated fuel, the distribution of the evaporated fuel amount increases as the intake air amount distribution increases. In other words, the proportion distributed to each cylinder (referred to as a purge distribution coefficient) is considered to be equal to the distribution proportion of the intake air amount. Accordingly, the amount of fuel QPURGE (cc) in the purge gas is calculated, and the amount of purge gas flowing into each cylinder is expressed as follows based on the amount of fuel QPURGE in the purge gas and the purge distribution coefficient of each cylinder. The amount of fuel TPURGE (i) (cc) can be calculated.
TPURGE (i) = QPURGE × (1 / KAF (i)) (8)

  Here, the fuel amount QPURGE in the purge gas can be calculated by multiplying the purge flow rate QPGCBASE (cc) obtained in step S6 by the purge concentration K (%) (QPURGE = QPGCBASE × K).

  The purge concentration K indicates the ratio of the amount of fuel contained in the purge flow rate to the purge flow rate, and can be learned, for example, during the previous operation cycle. For example, purging is performed by controlling the purge control valve 28 in a stable operation state of the previous operation cycle (for example, in an idling operation state and in a fully warmed-up state) (however, for each cylinder) Air-fuel ratio control is not executed). Air-fuel ratio feedback control is executed to calculate the fuel correction amount QAFFB (cc). As described above, this is the fuel correction amount necessary to converge the actual air-fuel ratio to the target air-fuel ratio. This correction amount QAFFB is considered to compensate for the air-fuel ratio that has changed due to the execution of the purge. Therefore, it can be considered that the fuel corresponding to the correction amount QAFFB was included in the purge gas. By dividing the correction amount QAFFB by the purge flow rate QPGCF ′ (cc) at that time (this may be the target purge flow rate at that time or may be detected from the duty ratio of the purge control valve 28), the purge concentration K (%) can be calculated.

In step S25, a detection value (g / TDC) of AFM 8 is acquired, and a value obtained by dividing the detection value by the number of cylinders is defined as GAIRAFM (g). In this embodiment, since there are four cylinders, a value obtained by dividing the detection value by a factor of one is GAIRAFM (g). Based on the GAIRAFM and the distribution ratio of the intake air amount, the charge air amount (g) of each cylinder is calculated, and as shown in the equation (9), the fuel amount TO (i) ( cc) is calculated. That is, the intake air amount detected by the AFM 8 is distributed to the four cylinders, but the distribution to each cylinder is performed according to the calculated distribution ratio (1 / KAF (i)), and the distributed intake air amount is distributed. The amount of fuel corresponding to is required for the cylinder. The following “unit conversion coefficient” is a coefficient for converting the air amount into the fuel amount.
TO (i) = GAIRAFM × (1 / KAF (i)) × unit conversion coefficient (cc / g) (9)

In step S26, the fuel injection amount TBASE (cc) to each cylinder of the engine when the cylinder-by-cylinder air-fuel ratio control is not performed is calculated. That is, when cylinder-by-cylinder air-fuel ratio control is not performed, the fuel amount TBASE commensurate with GAIRAFM is supplied to each cylinder.
TBASE = GAIRAFFM × unit conversion coefficient (cc / g) (10)

In step S27, the results of steps S24 to S26 are used to calculate the cylinder-by-cylinder air-fuel ratio correction amount KAFPurge (i) in consideration of purge for each cylinder. Here, the fuel amount to be actually supplied to the cylinder is determined by subtracting the fuel amount from the purge gas to the cylinder from the fuel amount TO corresponding to the intake air amount to each cylinder. Therefore, the correction amount KAFPurge can be calculated by the following equation.
KAFPurge (i) = (TO (i) −TPURGE (i)) / TBASE (11)

  Thus, KAFPcharge (i) is calculated for each cylinder. Here, considering the above specific example again, TO (i) and TPURGE (i) in Expression (11) are affected by 1 / KAF (i). KAF (i) is larger than the average value 1 (1.2) for the first and second cylinders, and smaller than the average value 1 (0.8) for the third and fourth cylinders. Therefore, 1 / KAF (i) is smaller than the average value 1 (0.83) in the first and second cylinders, and larger than the average value 1 (1.25) in the third and fourth cylinders.

  Therefore, the magnitude of the cylinder-by-cylinder air-fuel ratio correction amount (represented by KAF (i) when purging is not considered, and represented by KAFPurge (i) when purging is considered) is considered in purging. In this case, the correction amount is decreased in the first and second cylinders, and the correction amount is increased in the third and fourth cylinders, compared with the case where the purge is not considered. In other words, for a cylinder in which the air-fuel ratio correction amount when purge is not executed is larger than the average value, when the purge is executed (that is, when the purge flow rate is increased), the correction amount is decreased. For cylinders whose air-fuel ratio correction amount is smaller than the average value when purging is not performed, the correction amount is increased when purging is performed (that is, when the purge flow rate is increased).

  As described above, the air-fuel ratio correction amount KAF (i) is larger than 1 in the first and second cylinders, and therefore, it is going to be corrected to rich. In other words, the first and second cylinders are considered to have their valves shifted to the open side, and thus the fuel vapor evaporated by the purge tends to flow. Therefore, the influence (fuel amount) due to the purge is reduced as compared with the third and fourth cylinders. On the other hand, the third and fourth cylinders are considered that the air-fuel ratio correction amount KAF (i) is smaller than 1, and the valve is considered to be shifted to the closing side, so that the purge is difficult to flow. Therefore, the influence (fuel amount) of the purge is increased as compared with the first and second cylinders. In this way, even when the cylinder-by-cylinder air-fuel ratio control is being executed, it is possible to prevent the air-fuel ratio from varying between the cylinders due to the introduction of the purge.

  FIG. 8 shows an alternative approach of step S12 of FIG. 6 according to one embodiment of the present invention. This is a simplified method compared to FIG.

  Steps S31 and S32 are the same as steps S21 and S22 of FIG. 7, and for each cylinder, the learned value KREF (i) calculated in step S4 is read, and the learned value is determined according to the current operating state of the engine. The learned air-fuel ratio correction amount KAF (i) for each cylinder is calculated by correcting KREF (i).

  In step S33, based on the purge flow rate QPGCBASE obtained in step S6 of FIG. 6, a map as shown in FIG. 9 is referred to and a corresponding correction coefficient Kp is obtained. The correction coefficient Kp is set to decrease as the purge flow rate increases. That is, as the purge flow rate increases, the amount of fuel in the purge gas increases, so that the air-fuel ratio correction amount is reduced accordingly.

In step S34, using the obtained correction coefficient Kp, an air-fuel ratio correction amount KAFPurge (i) that considers purge is calculated for each cylinder as in the following equation.
KAFPurge (i) = (KAF (i) −1) × Kp + 1 (12)

  Here, the value 1 indicates the average value of the cylinder-by-cylinder air-fuel ratio correction amount of all cylinders. This equation is calculated so that KAF Charge (i) is smaller than KAF (i) when KAF (i) is larger than the average value 1, and when KAF (i) is smaller than the average value, It is configured so that KAF Charge (i) is calculated to be larger than KAF (i).

  As a specific example, for example, when KAF (i) = 1.2 and Kp = 0.8, KAFPurge (i) is 1.16, and the cylinder-by-cylinder air-fuel ratio correction considering purge is performed. The amount KPurge (i) is smaller than the cylinder-by-cylinder air-fuel ratio correction amount KAF (i) that does not consider purge. Thus, in the case of a cylinder that is to be corrected richly, as described above, the correction amount is decreased to reduce the influence of the purge. On the other hand, when KAF (i) = 0.8 and Kp = 0.8, KAFPurge (i) is 0.84. The cylinder-by-cylinder air-fuel ratio correction amount KAFPurge (i) considering purge is larger than the cylinder-by-cylinder air-fuel ratio correction amount KAF (i) not considering purge. Thus, in the case of a cylinder that is to be corrected lean, the correction amount is increased as described above to increase the influence of the purge.

  Next, the cylinder-by-cylinder air-fuel ratio learning process executed in step S4 of FIG. 6 will be described. A flowchart illustrating an example of the process is shown in FIG.

  In step S41, it is determined whether a learning condition for the cylinder-by-cylinder air-fuel ratio correction amount is satisfied. For example, it is possible to set the learning condition that the engine is idling and is completely warmed up (when the engine water temperature is, for example, 80 degrees or more). If this judgment is No, it will not learn (S42) and will progress to step S47. If this judgment is Yes, it will progress to Step S43. When learning after step S43 is performed, although not shown in the figure, it is preferable that the lift amount of the intake valve of each cylinder be controlled to a predetermined low value LT1. As described above, when the lift amount is low, the sensitivity (relative ratio) to the intake air amount due to the shift of the lift amount is larger than when the lift amount is high.

  In step S43, a signal representing the air-fuel ratio KACT (expressed in equivalent ratio) is acquired from the air-fuel ratio sensor 16.

  In step S44, an air-fuel ratio deviation DKCMD signal representing a deviation of the air-fuel ratio KACT from the target air-fuel ratio KCMD is generated based on the signal representing the air-fuel ratio KACT, and the signal of the air-fuel ratio deviation DKCMD is set for each cylinder. The correlation function Cr # i is calculated for each cylinder by multiplying the predetermined reference signal Fcr # i. Here, the calculation method of the correlation function will be described with reference to FIG.

  FIG. 11 shows an air-fuel ratio indicating a deviation of the air-fuel ratio KACT with respect to the target air-fuel ratio KCMD (in this embodiment, the stoichiometric air-fuel ratio) in one combustion cycle (in this embodiment, a period of a crank angle of 720 degrees). An example of the transition of the signal of the deviation DKCMD is shown. In this embodiment, in one combustion cycle, combustion is performed in the order of the first cylinder, the third cylinder, the fourth cylinder, and the second cylinder every crank angle period of 180 degrees. Therefore, in the first period, the air-fuel ratio of the exhaust gas resulting from the combustion of the first cylinder is detected, and in the example of the figure, it is shown that the air-fuel ratio is richer than the target air-fuel ratio (DKCMD is a positive value). In the second period, the air-fuel ratio of the exhaust gas resulting from the combustion of the third cylinder is detected, and in the example shown in the figure, it is shown that the air-fuel ratio is leaner than the target air-fuel ratio (DKCMD is a negative value). Similarly, in the third period, the air-fuel ratio of the exhaust gas of the fourth cylinder is detected, which is rich, and in the fourth period, the air-fuel ratio of the exhaust gas of the second cylinder is detected, This is lean.

  Further, the drawing shows a reference signal Fcr # 1 for the first cylinder, a reference signal Fcr # 3 for the third cylinder, a reference signal Fcr # 4 for the fourth cylinder, and a reference signal for the second cylinder. The transition of Fcr # 2 is shown. The reference signal is predetermined for each cylinder, and is generated so as to correspond to the exhaust gas emission behavior of each cylinder. In this example, the reference signal is generated so as to have a triangular wave signal having a peak value of 1 only in the crank angle range of the exhaust timing of each cylinder. For example, Fcr # 1 has a triangular wave signal in the first period in which the exhaust gas of the first cylinder is exhausted. The shape of the reference signal is not limited to a triangular wave, and may be a sine wave or a square wave.

The correlation function Cr # i for each cylinder is calculated by multiplying the DKCMD signal and the reference signal Fcr # i for the cylinder, as shown in the following equation. Here, N is the number of pulses of the CRK signal per combustion cycle. If the CRK signal is emitted every 30 degrees, N = 24. K is a time step.

  For example, when the DKCMD signal is multiplied by the reference signal Fcr # 1 for the first cylinder, the first reference signal Fcr # 1 has a value only in the first period, and thus the correlation function Cr # of the first cylinder. 1 is calculated so that only the section corresponding to the first period has a value reflecting the value of the air-fuel ratio deviation DKCMD, that is, a value reflecting the value of the air-fuel ratio KACT. The richer the value of the air-fuel ratio KACT, the larger the correlation function Cr, and the smaller the value, the smaller the value. Thus, a signal indicating the magnitude of the air-fuel ratio of each cylinder is calculated as the correlation function Cr # i.

  The behavior of the exhaust gas in each cylinder, more specifically, the timing at which the exhaust gas from each cylinder reaches the air-fuel ratio sensor 16 may vary depending on the engine speed and the load. When this timing fluctuates, the peak position of the air-fuel ratio KACT signal in each period (in the example shown, the crank angle position is the maximum value in the first and third periods, and the minimum value is in the second and fourth periods. The corresponding crank angle position) may fluctuate and the peak position of the corresponding reference signal may shift. Therefore, in order to increase the accuracy, the peak position can be defined in advance and stored in a map or the like according to the engine speed NE and the load. As a parameter indicating the load, an intake air amount can be used. In step S44, the map is referenced based on the engine speed NE and the intake air amount detected via the crank angle sensor 13 and the AFM 8, the corresponding peak position is obtained, and a triangular wave that peaks at the obtained position. Can be used to calculate the correlation function Cr # i.

  For each cylinder, when the air-fuel ratio is the target air-fuel ratio KCMD, the value of the deviation DKCMD is zero, so the value of the correlation function Cr # i is also zero. Therefore, in order to set the air-fuel ratio of each cylinder to the target air-fuel ratio KCMD, a correction amount (correction amount for correcting the fuel injection amount of the cylinder) KAFX is set so that the correlation function Cr # i converges to zero. What is necessary is just to calculate. This is shown in step S45 of FIG. 10, and in this step, a correction amount KAFX (i) for converging the correlation function Cr # i to zero is calculated for each cylinder. For example, the correction amount KAFX (i) can be calculated by using feedback control such as PI control.

  Here, FIG. 12 will be referred to in order to explain the effect of the correction amount KAFX (i). This figure shows an example of changes in the correction amount KAFX (i), the correlation function Cr # i, and the air-fuel ratio KACT (i) for the first and fourth cylinders (second and second cylinders). 3 cylinders are omitted). In the period up to time t1, the cylinder-by-cylinder air-fuel ratio control is not performed so that the cylinder-by-cylinder air-fuel ratio correction flag is set to zero. Therefore, as is apparent from comparing the air-fuel ratio KACT (1) of the first cylinder and the air-fuel ratio KACT (4) of the fourth cylinder, the air-fuel ratio remains varied between the cylinders. Control for converging the correlation functions Cr # 1 and Cr # 4 is not performed, and the correction amount KAFX (i) is maintained at 1. However, since the air-fuel ratio feedback control is executed, the average air-fuel ratio KACT of all the cylinders converges toward the target air-fuel ratio KCMD.

  At time t1, the cylinder-by-cylinder air-fuel ratio correction execution flag is set to 1, whereby the cylinder-by-cylinder air-fuel ratio control is executed. Specifically, at a predetermined time interval, a correction amount KAFX (i) for calculating the correlation function Cr # i of each cylinder to zero is calculated, and the fuel corrected based on the correction amount KAFX (i). A quantity is injected into the corresponding cylinder. As a result, as shown in the figure, the correlation functions Cr # 1 and Cr # 4 converge toward zero, and the air-fuel ratio KACT (i) of each cylinder converges to the target air-fuel ratio KCMD. Thus, the average value of the air-fuel ratios of all the cylinders converges to the target air-fuel ratio, and the air-fuel ratios of the respective cylinders can also converge to the target air-fuel ratio.

  Returning to FIG. 10, in step S46, the cylinder-by-cylinder air-fuel ratio correction amount KAFX (i) calculated in step S45 is stored as a learning value KREF (i). For example, the learned value KREF (i) can be stored in a memory or the like that retains the contents even when the vehicle ignition is turned off, and can be used in the next driving cycle. The learning value KREF (i) stored in this way is read in step S21 of FIG. 7 or step S31 of FIG. 8 in order to calculate the above-described KAFPurge (i).

  Next, in steps S47 and S48, for each cylinder, the learned value KREF (i) is corrected to a learned value corresponding to the current operating state and the current lift deviation amount, that is, the cylinder-specific air-fuel ratio correction amount KAF (i) ( to correct. Steps S47 and S48 are performed even when learning is performed, but are also performed when learning is not performed (S42). Since the learned value KREF (i) is calculated under a stable predetermined operating state, the learned value KREF (i) is used so as to correspond to the current operating state in order to be used in the current operating state. Need to be corrected. Further, since it is unclear how much the lift amount actually deviates regardless of whether or not learning is performed, it is necessary to correct the learning value KREF (i) so as to adapt to the current lift deviation amount. Processing for that is performed in steps S47 and S48.

  In step S47, a reference map corresponding to the current operating state is selected. The reference map is a map that defines how the learning value KREF changes with respect to the lift amount when the lift amount is deviated by a predetermined value. Since the change in the learning value KREF with respect to the change in the lift amount varies depending on the engine operating state, it is preferable to create a map in advance for each engine operating state and store it in a memory or the like.

  FIG. 13 is a diagram conceptually showing a map for each driving state. In this embodiment, as the operating state, the intake valve phase CAIN, the intake pipe gauge pressure PBGA, and the engine speed NE are used. For each combination of these operating parameter values, a learned value for the lift amount LIFT. KREF is defined. Therefore, in step S47, based on the phase detected from the phase sensor 33, the intake pipe gauge pressure detected from the intake pipe absolute pressure sensor 9 and the atmospheric pressure sensor 12, and the engine speed detected from the crank angle sensor 13. , Select the corresponding reference map.

  An example of the reference map thus selected is shown by reference numeral 201 in FIG. This map is a map according to the current intake valve phase, gauge pressure, and rotation speed, and is created on the assumption that the lift amount is shifted by a predetermined amount. However, it is unknown how much the lift amount actually deviates when the learning value KREF (i) is calculated. Therefore, the reference map 201 cannot be directly referred to based on the detected current lift amount. Therefore, first, a plot as indicated by reference numeral 203 is performed from the lift amount LT1 when the learning value KREF (i) is calculated and the value of the learning value KREF (i). In this embodiment, since learning is performed in the low lift state as described above, LT1 has a relatively low value.

  With reference to the reference map 201, a value X corresponding to the lift amount LT1 is obtained. Then, a ratio α of the learning value KREF (i) to the value X is calculated (α = KREF (i) / X). For example, if X = 1.4 and the current learning value KREF (i) is 1.1, it can be calculated by α = 1.1 / 1.4.

  A characteristic calculated by multiplying each point on the reference map 201 by α is indicated by reference numeral 205. Reference numeral 205 represents a map corresponding to the current shift in the lift amount. Here, α is called a reflection coefficient.

  In this example, α is calculated to be smaller than value 1, but α may be calculated to be larger than value 1 depending on the reference map and the learning value. For example, when the plot indicated by reference numeral 203 is arranged at a position larger than the X value, α has a value larger than the value 1, and the map 205 is generated on the upper side of the reference map 201.

  Thus, the map 205 corresponding to the current operating state and the current lift deviation amount is obtained. In step S48, the map 205 is referred to based on the detected current lift amount LT2, and the corresponding learning value is obtained. Find Y. The learning value Y thus obtained is the cylinder specific air-fuel ratio correction amount KAF (i).

  Alternatively, the reference value 201 is referred to based on the detected current lift amount LT2, the corresponding learning value KREF (i) is obtained, and the learning value is multiplied by the reflection coefficient α. Y may be calculated.

  In step S22 of FIG. 7 described above, the same technique as described above performed in steps S47 and S48 is executed. That is, a reference map corresponding to the current operating state is selected, the reflection coefficient α is calculated by referring to the map using the learned value KREF (i) read in step S21, and based on the reflection coefficient α. Then, the cylinder specific air-fuel ratio correction amount KAF (i) is calculated. Similar processing is performed in step S32 of FIG. As a result, a learning value suitable for the current operating state, that is, a cylinder-by-cylinder air-fuel ratio correction amount KAF (i) is calculated, and this is used to calculate a cylinder-by-cylinder air-fuel ratio correction amount KAFPurge (i) in consideration of purge.

  In the above embodiment, as shown in FIG. 6, when the purge is not executed, that is, when the purge flow rate is zero, the cylinder-by-cylinder air-fuel ratio correction amount (KREF (i)) is learned, and then the purge is performed. Is executed, that is, when the purge flow rate is increased, the correction amount AKFPcharge (i) considering the purge is calculated using the learned correction amounts (KREF (i), KAF (i)). Alternatively, the correction amount (KREF (i)) is learned when the purge flow rate is a predetermined value (which may not be zero), and then learned when the purge flow rate is increased from the predetermined value. Using the correction amounts (KREF (i), KAF (i)), the correction amount AKFPcharge (i) considering the purge may be calculated. That is, the present invention is intended to prevent variation in the air-fuel ratio between cylinders due to an increase in the purge flow rate when the purge flow rate is increased from the standard state. The purge flow rate is not necessarily limited to zero.

  However, when the purge flow rate is not zero, the correction amount KAF (i) used when the purge is not performed needs to be newly calculated. If the state where the purge flow rate is zero is used as a reference, such a new calculation becomes unnecessary, and convenience can be improved.

  Although the present invention has been described with respect to specific embodiments, the present invention is not limited to such embodiments, and can be used for both gasoline engines and diesel engines.

1 Electronic control unit (ECU)
2 Engine 16 Air-fuel ratio sensor

Claims (2)

An air-fuel ratio control apparatus for an internal combustion engine having a plurality of cylinders,
An air-fuel ratio detecting means for detecting an air-fuel ratio of the exhaust gas, disposed in an exhaust merging portion where exhaust gases of a plurality of cylinders of the internal combustion engine merge;
Purge flow rate control means for controlling a purge flow rate for supplying evaporated fuel to the intake passage of the internal combustion engine;
A cylinder-by-cylinder air-fuel ratio correction amount calculating means for calculating an air-fuel ratio correction amount for each cylinder so as to suppress variations in the air-fuel ratio among the plurality of cylinders when the purge flow rate is a predetermined value;
Cylinder purge fuel amount estimation means for estimating the fuel amount in the purge gas supplied to each cylinder from the fuel amount in the purge gas supplied to the intake passage;
With
The cylinder-by-cylinder air-fuel ratio correction amount calculating means corresponds to a cylinder-by-cylinder air-fuel ratio correction amount that is calculated when the purge flow rate is greater than the predetermined value. the decreases in accordance with the amount of fuel in the purge gas or by increasing by calculating the cylinder air-fuel ratio correction amount in consideration of the purge increase amount, the cylinder air-fuel ratio correction amount in consideration of the purge increase amount, the purge In a cylinder where the cylinder-by-cylinder air-fuel ratio correction amount calculated when the flow rate is a predetermined value is larger than the average value of the cylinder-by-cylinder air-fuel ratio correction amounts calculated when the purge flow rate is a predetermined value , the purge flow rate There the rewritable calculated so as to reduce relative cylinder air-fuel ratio correction amount calculated at the time of the predetermined value, the cylinder air-fuel ratio correction amount calculated at the time of the purge flow rate is a predetermined value than the average value In a small cylinder The purge flow rate you calculated so as to increase relative to the cylinder air-fuel ratio correction amount when the predetermined value, the air-fuel ratio control apparatus characterized by. The cylinder-by-cylinder air-fuel ratio correction amount calculated when the purge flow rate is a predetermined value, which is used when calculating the cylinder-by-cylinder air-fuel ratio correction amount when the purge flow rate is larger than the predetermined value, is used as the internal combustion engine. And a correction means for correcting according to the driving state of
The air-fuel ratio control apparatus according to claim 1.

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