JP2010007581A - Air fuel ratio control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To execute quickly the lean limit control of an air-fuel ratio while inhibiting the deterioration of drivability as much as possible. <P>SOLUTION: A temporary throttle opening determination means B21 determines a temporary throttle opening TAtmp based on an acceleration operation quantity PA acquired by an accelerator operation quantity acquisition means B1. A temporary intake air quantity corresponding value determination means B22 determines a value KLtmp equivalent to a cylinder intake air quantity when an actual throttle opening is adjusted to TAtmp. A fuel injection quantity calculation means B24 determines a fuel injection quantity TAU based on KLtmp. A combustion state index number acquisition means B4 acquires a combustion rate MFB30. A target throttle opening determination means B5 determines a target throttle opening TAtgt necessary for making MFB 30 coincide with a target combustion rate MFB30tgt while using a feed-forward learning value A/Fff acquired by a temporary lean limit value acquisition means B6. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an air-fuel ratio control apparatus that controls an air-fuel ratio of an internal combustion engine toward a lean limit (a lean-side limit air-fuel ratio that can be normally combusted).

  This type of device is configured to improve fuel efficiency and reduce NOx (nitrogen oxide) by feedback control of the air-fuel ratio of the fuel mixture supplied into the cylinder (combustion chamber) to the lean side as much as possible. Has been. As this type of apparatus, for example, those disclosed in JP-A-5-321726, JP-A-2004-156505, JP-A-2007-32531, and the like are known.

  As disclosed in Japanese Patent Application Laid-Open No. 5-321726, etc., most devices of this type obtain a torque fluctuation amount, and based on the torque fluctuation amount, lean the air-fuel ratio of the fuel mixture. Feedback control is performed on the side. Specifically, the apparatus determines the fuel injection amount based on the acquired intake air amount, and changes the fuel injection amount so that the above-described torque fluctuation amount matches a predetermined determination value. The air-fuel ratio is shifted to the lean side as much as possible.

Further, the apparatus disclosed in Japanese Patent Application Laid-Open No. 2007-32531 performs lean limit control based on the in-cylinder pressure obtained from the output of the in-cylinder pressure sensor, instead of the torque fluctuation amount in the above-described configuration. It has become. Specifically, in such a device, when the crank angle at which the maximum value of the in-cylinder pressure occurs is within a predetermined range, it is determined that the lean limit has not yet been reached, and in this case, the air-fuel ratio further becomes lean. Thus, the fuel injection amount is controlled.
JP-A-5-321726 JP 2004-156505 A JP 2007-32531 A (paragraphs [0093] to [0099] and FIG. 14)

  However, in the conventional apparatus of this type, the lean air-fuel ratio is reduced by reducing the fuel injection amount determined based on the acquired intake air amount as described above during the lean limit control. Yes. Therefore, in the conventional apparatus, even when the accelerator operation amount is constant, the amount of fuel contributing to combustion is reduced when the air-fuel ratio is made lean. As a result, the engine-generated torque is relatively reduced, and there is a problem that the driver feels uncomfortable.

  The present invention has been made to cope with the above problems. An object of the present invention is to provide an air-fuel ratio control apparatus for an internal combustion engine that can quickly perform lean limit control of the air-fuel ratio while suppressing the loss of drivability as much as possible.

  An air-fuel ratio control apparatus of the present invention is configured to control an air-fuel ratio of an internal combustion engine toward a lean limit, and includes an accelerator operation amount acquisition means, a fuel injection amount determination means, a fuel injection means, a combustion state Index value acquisition means, provisional lean limit value acquisition means, target throttle opening determination means, and throttle valve control means are provided. Each of these means operates as follows.

  The accelerator operation amount acquisition means acquires an accelerator operation amount (an operation amount of an accelerator pedal by a driver). The fuel injection amount determination means determines a fuel injection amount based on the accelerator operation amount acquired by the accelerator operation amount acquisition means and a predetermined reference air-fuel ratio. The fuel injection unit causes a fuel injector to inject the fuel of the fuel injection amount determined by the fuel injection amount determination unit.

  The fuel injection amount determining means may include provisional throttle opening degree determining means, air amount estimating means, and fuel injection amount calculating means.

  The temporary throttle opening determining means determines the temporary throttle opening based on the accelerator operation amount acquired by the accelerator operation amount acquiring means. The temporary throttle opening is determined so as to increase as the accelerator operation amount increases.

  The air amount estimation means estimates a provisional intake air amount correspondence value based on the provisional throttle opening. This provisional intake air amount corresponding value is a value corresponding to the amount of air sucked into the cylinder of the internal combustion engine, assuming that the actual opening of the throttle valve is the provisional throttle opening. . The provisional intake air amount corresponding value may be the intake air amount itself or a value proportional to the intake air amount (for example, a value obtained by dividing the intake air amount of each cylinder by the maximum volume of each cylinder). Good.

  The fuel injection amount calculating means is formed in the cylinder when it is assumed that an amount of air corresponding to the provisional intake air amount corresponding value estimated by the air amount estimating means is sucked into the cylinder. The fuel injection amount is calculated so that the air-fuel ratio of the fuel mixture coincides with the reference air-fuel ratio.

  The combustion state index value acquisition unit acquires a combustion state index value. The combustion state index value is a value indicating the combustion state (combustion progress state) of the fuel mixture formed in the cylinder by the fuel injection of the fuel injection amount in the fuel injector. For example, as the combustion state index value, the combustion ratio of the fuel in the fuel mixture at a predetermined crank angle can be used. This fuel ratio is acquired based on the pressure in the cylinder. In this case, the combustion state index value acquisition unit may also be referred to as a combustion ratio acquisition unit.

  The target throttle opening determining means determines a target throttle opening that is a target value of the throttle valve opening. The target throttle opening is determined so that the combustion state index acquisition value, which is the acquired value of the combustion state index value by the combustion state index value acquisition means, matches a predetermined combustion state index target value. And the combustion state index target value. The throttle valve control means controls the throttle valve so that the opening of the throttle valve coincides with the target throttle opening.

  The target throttle opening degree determining means may comprise air-fuel ratio corresponding value calculating means and target throttle opening degree calculating means.

  The air-fuel ratio corresponding value calculating means calculates an air-fuel ratio corresponding value based on the combustion state index acquisition value and the combustion state index target value. The air-fuel ratio corresponding value is a value corresponding to the air-fuel ratio of the fuel mixture to be formed in the cylinder in order to make the combustion state index acquisition value coincide with the combustion state index target value. The air-fuel ratio correspondence value may be the air-fuel ratio (target air-fuel ratio) itself for making the combustion state index acquired value coincide with the combustion state index target value. It may be a correction value for the air-fuel ratio.

  The target throttle opening calculating means calculates the target throttle opening based on the provisional intake air amount corresponding value estimated by the air amount estimating means and the air-fuel ratio corresponding value.

  The provisional lean limit value acquisition means acquires a provisional lean limit value. The provisional lean limit value is a value corresponding to an air-fuel ratio estimated to be close to the lean limit (preferably slightly richer than the lean limit). In the present invention, the target throttle opening is set based on the provisional lean limit value, the combustion state index acquisition value, and the combustion state index target value. That is, for the feedback control of the opening degree of the throttle valve based on the combustion state index acquisition value and the combustion state index target value by the target throttle opening degree determining means, the provisional lean limit value is feedforward Functions as a learning value.

  For example, the provisional lean limit value acquisition unit acquires the provisional lean limit value by acquiring the state of the fuel mixture corresponding to a plurality of air-fuel ratios. Specifically, the provisional lean limit value acquisition means acquires the specific heat ratio at least in the expansion stroke (preferably the compression stroke and the expansion stroke) based on the pressure in the cylinder at a plurality of air-fuel ratios, A change mode of the specific heat ratio in the same stroke accompanying the change in the fuel ratio is estimated, and the provisional lean limit value is acquired based on the estimation result. The provisional lean limit value can be acquired, for example, after the internal combustion engine is started and before a predetermined lean limit control condition is established. The acquisition of the provisional lean limit value is based on the following knowledge.

That is, in the expansion stroke, as the air-fuel ratio becomes leaner, the combustion becomes slower and the gas with a large heat capacity such as NOx and CO ₂ decreases and the ratio of air increases. Therefore, as the air-fuel ratio becomes leaner, the specific heat ratio of the gas in the cylinder (hereinafter referred to as “in-cylinder gas”) approaches the specific heat ratio of air (about 1.403) in a predetermined manner. Here, misfiring or the like occurs in a predetermined region where the air-fuel ratio becomes too lean. For this reason, the aspect of the change in the specific heat ratio of the in-cylinder gas accompanying the change in the air-fuel ratio differs between the region and the other regions. For example, when misfire occurs, the specific heat ratio of the in-cylinder gas in the expansion stroke becomes equal to the specific heat ratio of the unburned fuel mixture in the compression stroke. Therefore, for example, by obtaining the specific heat ratio of the in-cylinder gas in the expansion stroke and the compression stroke while setting the air-fuel ratio of the fuel mixture to plural by changing the reference air-fuel ratio, etc. A change mode of the specific heat ratio of the in-cylinder gas is estimated (by function approximation or the like), and the provisional lean limit value is acquired based on the estimation result.

  When the internal combustion engine is provided with a plurality of cylinders, the provisional lean limit value acquisition unit acquires the provisional lean limit value for each of the plurality of cylinders, and the target throttle opening determination unit includes: It is preferable to determine the target throttle opening based on the lowest value among the plurality of provisional lean limit values corresponding to the plurality of cylinders.

  In the air-fuel ratio control apparatus of the present invention having the above-described configuration, an amount of fuel corresponding to the accelerator operation amount is injected, and the throttle valve based on the combustion state index acquisition value and the combustion state index target value The lean limit control is performed by the feedback control of the opening.

  Therefore, according to the present invention, lean burn (lean limit) driving can be performed while suppressing the driver's uncomfortable feeling as much as possible. Even when the intake air amount cannot be accurately estimated during transient operation or the like, the air-fuel ratio can be made lean while suppressing fluctuations in engine-generated torque.

  Here, in the present invention, the responsiveness of the lean limit control is improved by using the provisional lean limit value (as a feedforward value or a learning value). Therefore, the fuel efficiency improvement effect by lean limit control can be obtained even better than the above-described conventional technology.

  Further, when the plurality of cylinders are provided in the internal combustion engine, lean limit control is performed based on the lowest value among the plurality of provisional lean limit values corresponding to each cylinder. That is, among the plurality of cylinders, the slowest combustion is set as a representative cylinder, and lean limit control is performed based on the representative cylinder. Thereby, the control load (calculation processing load) is reduced.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, an embodiment of the present invention (an embodiment which is considered best by the applicant at the time of filing of the present application) will be described with reference to the drawings.

  In addition, the description about the following embodiment is specific to the extent possible, merely an example of the embodiment of the present invention in order to satisfy the description requirement (description requirement / practicability requirement) of the specification required by law. It is only what is described in. Therefore, as will be described later, it is quite natural that the present invention is not limited to the specific configurations of the embodiments described below. Modifications to the embodiments are listed together at the end, as insertions during the description of the embodiment impede understanding of the description of the consistent embodiment.

<System configuration>
FIG. 1 shows a piston reciprocating spark-ignition multi-cylinder (four-cylinder) four-cycle engine 1 (hereinafter simply referred to as “engine 1”), and an air-fuel ratio control apparatus 2 (one embodiment of the present invention). FIG. 1 is a diagram showing a schematic configuration of a system S (vehicle) including “hereinafter referred to simply as“ control device 2 ”. FIG. 1 shows a cross-sectional view of a specific cylinder of the engine 1 by a plane orthogonal to the cylinder arrangement direction (the configurations in other cylinders are also the same).

<< Engine >>
The engine 1 includes a cylinder block 11 and a cylinder head 12. An intake passage 13 and an exhaust passage 14 are connected to the engine 1.

  The cylinder block 11 is formed with a cylinder bore 111 that is a substantially cylindrical through hole. As described above, the cylinder block 11 has a plurality of cylinder bores 111 arranged in a line along the cylinder arrangement direction. A piston 112 is accommodated inside each cylinder bore 111 so as to be capable of reciprocating along a central axis of the cylinder bore 111 (hereinafter referred to as “cylinder central axis”). A cylinder head 12 is joined to one end portion (upper end portion in the figure) of the cylinder block 11. The cylinder block 11 and the cylinder head 12 are fixed to each other by bolts or the like (not shown).

  A crankshaft 113 is rotatably supported in the cylinder block 11 while being arranged in parallel with the cylinder arrangement direction. The crankshaft 113 is coupled to the piston 112 via a connecting rod 114 so as to be rotationally driven based on reciprocal movement along the cylinder central axis of the piston 112.

  A plurality of concave portions are provided at positions corresponding to one end portions (upper end portions in the figure) of the respective cylinder bores 111 at the end portions (lower end portions in the figure) of the cylinder head 12 on the cylinder block 11 side. That is, in the state where the cylinder head 12 is joined and fixed to the cylinder block 11, the space inside the cylinder bore 111 on the cylinder head 12 side (upper side in the drawing) from the top surface of the piston 112 and the space inside the above-described recess. Thus, the combustion chamber CC is formed. An intake port 121 and an exhaust port 122 are formed in the cylinder head 12 so as to communicate with the combustion chamber CC.

  The cylinder head 12 is provided with an intake valve 123, an exhaust valve 124, an intake valve control device 125, an exhaust camshaft 126, a spark plug 127, an igniter 128, and an injector 129.

  The intake valve 123 is a valve for opening and closing the intake port 121 (controlling the communication state between the intake port 121 and the combustion chamber CC). The exhaust valve 124 is a valve for opening and closing the exhaust port 122 (controlling the communication state between the exhaust port 122 and the combustion chamber CC).

  The intake valve control device 125 includes an intake cam, an intake camshaft (both not shown), and a mechanism for adjusting and controlling the relative rotation angle (phase angle) of both with an oil pressure. The valve opening timing (intake valve opening timing) VT can be changed while the period (valve opening crank angle width) is constant. Since the specific configuration of the intake valve control device 125 is well known, the description thereof is omitted. The exhaust camshaft 126 is configured to drive the exhaust valve 124.

  The spark plug 127 is provided so that the spark generating electrode at the tip thereof is exposed to the combustion chamber CC. The igniter 128 includes an ignition coil that generates a high voltage to be applied to the spark plug 127. An injector 129 corresponding to the fuel injector of the present invention is configured to be able to inject fuel to be supplied into the combustion chamber CC in the intake port 121.

<<< Intake and exhaust passage >>>
An intake passage 13 including an intake manifold, a surge tank, and the like is connected to the intake port 121. The exhaust port 122 is connected to an exhaust passage 14 including an exhaust manifold.

  A throttle valve 132 for changing the opening cross-sectional area of the intake passage 13 is interposed at a position in the intake passage 13 between the air filter 131 and the intake port 121. The throttle valve 132 is configured to be rotationally driven by a throttle valve actuator 133 formed of a DC motor. The throttle valve actuator 133 is configured to make the actual opening (throttle opening TA) of the throttle valve 132 coincide with the target throttle opening TAtgt when a drive signal representing the target throttle opening TAtgt is given.

  The exhaust passage 14 is a passage for exhaust gas discharged from the combustion chamber CC via the exhaust port 122. An upstream side catalytic converter 141 and a downstream side catalytic converter 142 are interposed in the exhaust passage 14. The upstream catalytic converter 141 and the downstream catalytic converter 142 are internally provided with a three-way catalyst having an oxygen storage function, and are configured to purify HC, CO, and NOx in the exhaust gas.

<Control device>
The air-fuel ratio control device of the present invention and the control device 2 corresponding to each means included therein are provided with an electronic control device 200. The electronic control device 200 is a so-called microcomputer, and includes a CPU 201, a ROM 202, a RAM 203, a backup RAM 204, an interface 205, and a bus 206. The CPU 201, ROM 202, RAM 203, backup RAM 204, and interface 205 are connected to each other via a bus 206.

  The ROM 202 stores in advance a routine (program) executed by the CPU 201, tables (lookup tables, maps), parameters, and the like that are referred to when the routine is executed. The RAM 203 is configured to temporarily store data as necessary when the CPU 201 executes a routine. The backup RAM 204 is configured so that data is stored when the CPU 201 executes a routine with the power turned on, and the stored data can be retained even after the power is shut off.

  The interface 205 is electrically connected to various sensors to be described later, and is configured to be able to transmit signals from these sensors to the CPU 201. The interface 205 is electrically connected to operation parts such as the intake valve control device 125, the igniter 128, the injector 129, the throttle valve actuator 133, and the like, and an operation signal for operating these operation parts is received from the CPU 201. It is comprised so that it can transmit to these operation | movement parts.

  That is, the control device 2 receives signals from the above-described various sensors via the interface 205 and sends the above-described operation signals to each operation unit based on the calculation result of the CPU 201 corresponding to the signals. It is configured as follows.

<< Various sensors >>
The control device 2 includes a coolant temperature sensor 211, an in-cylinder pressure sensor 212, a cam position sensor 213, a crank position sensor 214, an air flow meter 215, an upstream air-fuel ratio sensor 216a, a downstream air-fuel ratio sensor 216b, a throttle position sensor 217, an accelerator. Various sensors such as an opening sensor 218 are provided.

  The coolant temperature sensor 211 and the in-cylinder pressure sensor 212 are attached to the cylinder block 11. The coolant temperature sensor 211 is configured to output a signal corresponding to the coolant temperature Tw in the cylinder block 11. The in-cylinder pressure sensor 212 is configured to output a signal corresponding to the pressure in the combustion chamber CC (in-cylinder pressure Pc).

  The cam position sensor 213 is attached to the cylinder head 12. This cam position sensor 213 has a waveform signal (G2) having a pulse corresponding to the rotation angle of the above-described unillustrated intake camshaft (included in the intake valve control device 125) for reciprocating the intake valve 123. Signal).

  The crank position sensor 214 is attached to the cylinder block 11. The crank position sensor 214 is configured to output a waveform signal having a pulse corresponding to the rotation angle of the crankshaft 113. That is, the crank position sensor 214 outputs a signal corresponding to the engine speed Ne. Further, based on signals from the cam position sensor 213 and the crank position sensor 214, the crank angle θ based on the compression top dead center of each cylinder is obtained.

  The air flow meter 215 is attached to the intake passage 13. The air flow meter 215 is configured to output a signal corresponding to an intake air flow rate Ga that is a mass flow rate per unit time of intake air flowing through the intake passage 13.

  The upstream air-fuel ratio sensor 216 a and the downstream air-fuel ratio sensor 216 b are interposed in the exhaust passage 14. The upstream air-fuel ratio sensor 216a is disposed upstream of the upstream catalytic converter 141 in the exhaust gas flow direction. The downstream air-fuel ratio sensor 216b is disposed at a position between the upstream catalytic converter 141 and the downstream catalytic converter 142. The upstream air-fuel ratio sensor 216a and the downstream air-fuel ratio sensor 216b are oxygen concentration sensors, and are configured to output signals corresponding to the oxygen concentration (air-fuel ratio) of exhaust gas passing therethrough.

  The throttle position sensor 217 is disposed at a position corresponding to the throttle valve 132. The throttle position sensor 217 is configured to output a signal corresponding to the actual rotational phase of the throttle valve 132 (that is, the throttle opening degree TA).

  The accelerator opening sensor 218 is configured to output a signal corresponding to the operation amount (accelerator operation amount PA) of the accelerator pedal 220 by the driver.

<Overview of control system configuration>
FIG. 2 is a functional block diagram of the control device 2 shown in FIG. Hereinafter, the outline of the configuration of the control system for the fuel injection amount and the throttle opening in this embodiment will be described with reference to FIGS. 1 and 2. In the present specification, each block shown in the functional block diagram of FIG. 2 is realized by the CPU 201 executing a predetermined program (not shown).

  The control device 2 of this embodiment determines the fuel injection amount based on the accelerator operation amount, and performs lean burn so that the air-fuel ratio of the fuel mixture formed by the fuel injection of the fuel injection amount becomes the lean limit. By controlling the throttle opening during operation, the air-fuel ratio of the engine 1 is controlled toward the lean limit.

  More specifically, as conceptually shown in FIG. 2 which is a functional block diagram, the control device 2 of the present embodiment includes an accelerator operation amount acquisition unit B1, a fuel injection amount determination unit B2, a fuel injection unit B3, Combustion state index value acquisition means B4, target throttle opening degree determination means B5, provisional lean limit value acquisition means B6, and throttle valve control means B7 are provided.

  The accelerator operation amount acquisition means B1 acquires the accelerator operation amount PA based on a signal from the accelerator opening sensor 218.

  The fuel injection amount determination means B2 includes provisional throttle opening degree determination means B21, provisional intake air amount corresponding value determination means B22, reference air-fuel ratio setting means B23, and fuel injection amount calculation means B24. The fuel injection amount determination means B2 determines the fuel injection amount TAU based on the accelerator operation amount PA acquired by the accelerator operation amount acquisition means B1.

  Temporary throttle opening determination means B21 is based on the accelerator operation amount PA and the throttle opening table (see MAPTAtmp (PA) in FIG. 2), and the temporary throttle opening amount is determined so as to increase as the accelerator operation amount PA increases. The opening degree TAtmp is determined.

  The provisional intake air amount corresponding value determination means B22 as the air amount estimation means of the present invention estimates the provisional intake air amount correspondence value KLtmp based on the provisional throttle opening degree TAtmp. This provisional intake air amount correspondence value KLtmp is the intake air amount (temporary in-cylinder intake air) sucked into the cylinder bore 111 (combustion chamber CC) when the actual throttle opening degree TA is assumed to be the temporary throttle opening degree TAtmp. The value corresponding to the quantity Mctmp). Here, the provisional intake air amount correspondence value KLtmp is a value obtained by dividing the provisional in-cylinder intake air amount Mctmp by the maximum volume k of the cylinder when the actual throttle opening degree TA is assumed to be the provisional throttle opening degree TAtmp. Yes (ie KLtmp = Mctmp / k). Hereinafter, the value KLtmp is also referred to as “provisional load factor KLtmp”.

  The reference air / fuel ratio setting means B23 sets the reference air / fuel ratio AFr based on the operating state. That is, the reference air-fuel ratio setting means B23 sets (1) the reference air-fuel ratio AFr to be richer than the stoichiometric air-fuel ratio stoich when the provisional load factor KLtmp is high, that is, in the case of high load operation (during rapid acceleration, etc.). (2) When a feedforward learning value A / Fff, which will be described later, is acquired after the predetermined air-fuel ratio feedback condition is satisfied and before the predetermined lean limit control condition is satisfied, the reference air-fuel ratio AFr is centered on the stoichiometric air-fuel ratio stoich. (3) In other cases, the reference air-fuel ratio AFr is basically set to the stoichiometric air-fuel ratio stoich.

  The fuel injection amount calculation means B24 is formed in the cylinder when it is assumed that an amount of air corresponding to the provisional intake air amount correspondence value KLtmp (that is, the provisional cylinder intake air amount Mctmp) is sucked into the cylinder. The fuel injection amount TAU is calculated so that the air-fuel ratio of the fuel mixture to be made coincides with the reference air-fuel ratio AFr. Accordingly, the fuel injection amount calculation means B24 divides the amount of air corresponding to the provisional intake air amount correspondence value KLtmp (that is, the provisional cylinder intake air amount Mctmp = k · KLtmp) by the reference air-fuel ratio AFr, thereby fuel injection. The amount TAU is calculated (that is, TAU = k · KLtmp / AFr).

  The fuel injection means B3 injects an amount of fuel corresponding to the fuel injection amount TAU determined as described above into the injector 129 corresponding to the cylinder that reaches the current intake stroke. As described above, the control device 2 according to the present embodiment determines the fuel injection amount TAU based on the accelerator operation amount PA, and supplies the fuel of the fuel injection amount TAU to the engine 1.

  The combustion state index value acquisition means B4 is configured to acquire a combustion ratio as a combustion state index value (combustion state index acquisition value) based on signals from the cam position sensor 213 and the crank position sensor 214, and a cylinder angle θ. It is acquired based on the in-cylinder pressure Pc acquired from the internal pressure sensor 212.

  Here, FIG. 3A is a time chart showing how the combustion ratio changes with respect to the crank angle. In FIG. 3A, the combustion ratio at the crank angle θ is expressed as MFBθ. The combustion ratio MFBθ changes as shown in FIG. 3A as the crank angle θ of the cylinder in the combustion stroke elapses. As understood from FIG. 3A, the combustion ratio MFBθ is an index value indicating the combustion state (the degree of progress of combustion) of the fuel mixture formed in the combustion chamber CC.

The combustion ratio MFBθ can be obtained relatively easily from the in-cylinder pressure Pc according to the following equation (1). Note that details of the method for obtaining the combustion ratio are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2006-144645 and 2007-32531.

This equation (1) is based on the knowledge that the change pattern of the accumulated amount Q of heat that contributes to work on the piston 112 out of the generated heat substantially matches the change pattern of Pc (θ) V (θ) ^κ . . Pc (θ) is the in-cylinder pressure at the crank angle θ, V (θ) is the volume of the combustion chamber CC at the crank angle θ, and κ is an average / typical specific heat ratio (eg, 1.32) of the fuel mixture. The denominator of equation (1) is a value corresponding to 100% of MFB. In this equation (1), the crank angle θ is 0 at the compression top dead center, and takes a negative value that increases in absolute value as it advances from the compression top dead center toward the compression top dead center. It is defined to take a positive value in which the absolute value increases as the angle is retarded from the top dead center toward the compression top dead center.

  In the equation (1), the crank angle θs (θs <0) is determined so that both the intake valve 123 and the exhaust valve 124 are closed in the compression stroke toward the target combustion stroke (expansion stroke) and is higher than the ignition timing. The time is sufficiently advanced (for example, θs = −30 °, that is, BTDC 30 ° CA). The crank angle θe (θe> 0) is a predetermined timing later than the latest timing at which combustion in the target combustion stroke is substantially finished and a timing advanced from the exhaust valve opening timing (for example, θe = 60). °, ie ATDC 60 ° CA).

  FIG. 3B is a graph showing changes in the combustion ratio at various air-fuel ratios. In FIG. 3B, the solid line indicates the lean limit (the air / fuel ratio is leaner than the stoichiometric air / fuel ratio, and the combustion becomes unstable when the air / fuel ratio becomes more lean). When the air-fuel ratio is richer than that, the broken line indicates the combustion ratio MFBθ when the air-fuel ratio is further leaner than the lean limit.

  As understood from the points P1, P2, and P3 in FIG. 3B, the combustion ratio (hereinafter referred to as θ = ATDC 30 ° CA (30 ° crank angle after compression top dead center)) corresponding to the latter stage of combustion of the fuel mixture. The “combustion rate MFB30”) becomes smaller as the air-fuel ratio becomes leaner (as the air-fuel ratio becomes larger). According to experiments, combustion becomes unstable in a region where the air-fuel ratio is excessively lean such that the combustion ratio MFB30 is equal to or less than the value indicated by the point P2 (about 90%).

Accordingly, the combustion state index value acquisition means B4 includes an in-cylinder pressure Pc acquired from the in-cylinder pressure sensor 212, a crank angle θ acquired based on signals from the cam position sensor 213 and the crank position sensor 214, and the following (2 ) And the combustion ratio MFB30 is obtained.

  Then, the control device 2 of the present embodiment acquires the actual combustion rate MFB30 by the combustion state index value acquisition means B4, and uses the acquired combustion rate MFB30 as the target combustion rate MFB30tgt (combustion state index target) corresponding to the lean limit. The air-fuel ratio is feedback-controlled by target throttle opening degree determining means B5 and throttle valve control means B7, which will be described later.

  The target throttle opening degree determining means B5 adjusts the opening degree of the throttle valve 132 so that the combustion ratio MFB30 as the combustion state index acquisition value coincides with the target combustion ratio MFB30tgt as the combustion state index target value. To be adjusted. Specifically, the target throttle opening degree determining means B5 is based on the combustion ratio MFB30 and the target combustion ratio MFB30tgt (actually based on the deviation ΔMFB30 = (MFB30−MFB30tgt)), and the combustion ratio MFB30 and the target combustion. The final target value (target throttle opening degree TAtgt) of the throttle valve 132 for obtaining the air-fuel ratio necessary for setting the difference (ΔMFB30) from the ratio MFB30tgt to “0” is determined. Yes.

  More specifically, the target throttle opening degree determining means B5 includes an air-fuel ratio corresponding value calculating means B51 and a target throttle opening degree calculating means B52.

  The air-fuel ratio corresponding value calculating means B51 calculates the air-fuel ratio corresponding value AFL based on the combustion ratio MFB30 and the target combustion ratio MFB30tgt. This air-fuel ratio corresponding value AFL is a value corresponding to the air-fuel ratio of the fuel mixture to be formed in the combustion chamber CC in order to make the combustion ratio MFB30 coincide with the target combustion ratio MFB30tgt.

  The target throttle opening calculation means B52 is based on the provisional load factor KLtmp corresponding to the fuel injection amount TAU calculated by the fuel injection amount calculation means B24 and the above-mentioned air-fuel ratio corresponding value AFL. The target throttle opening degree TAtgt necessary for obtaining the air-fuel ratio of the fuel mixture corresponding to the air-fuel ratio corresponding value AFL with respect to TAU is calculated.

  Here, in the present embodiment, the target throttle opening degree determining means B5 includes the provisional lean limit value acquisition means B6 in addition to the combustion ratio MFB30, the target combustion ratio MFB30tgt, and the provisional load ratio KLtmp. Based on the feedforward learning value A / Fff as the limit value, the target throttle opening degree TAtgt is calculated.

  The provisional lean limit value acquisition means B6 is the specific heat of the gas in the combustion chamber CC (in-cylinder gas) when the reference air-fuel ratio setting means B23 changes the reference air-fuel ratio AFr within a predetermined range around the theoretical air-fuel ratio stoich. The feedforward learning value A / Fff is acquired based on the change mode of the ratio.

  FIG. 4 is a graph showing a change in the specific heat ratio of the in-cylinder gas accompanying the change in the air-fuel ratio. As shown in the first graph of FIG. 4, in the compression stroke, the ratio of air in the fuel mixture increases as the air-fuel ratio becomes leaner. Therefore, the specific heat ratio of the in-cylinder gas (unburned fuel mixture in the combustion chamber CC) in the compression stroke increases as the air-fuel ratio becomes leaner and gradually approaches the specific heat ratio of air (about 1.403).

Also in the expansion stroke after ignition, the proportion of air in the fuel mixture increases as the air-fuel ratio becomes leaner. Further, in the expansion stroke, combustion becomes slow as the air-fuel ratio becomes leaner, and a gas having a large heat capacity such as CO ₂ (specific heat ratio κ = 1.30) or NOx (NO specific heat ratio κ = 1.400) is generated. Decrease. For this reason, as indicated by the two-dot chain line in the second graph of FIG. 4, the specific heat ratio of the in-cylinder gas becomes the specific heat ratio of air (about 1) as the air-fuel ratio becomes lean even in the expansion stroke. .403) (in this graph, the vertical axis only indicates that the specific heat ratio of the in-cylinder gas gradually approaches that of air as the air-fuel ratio becomes leaner. It is not shown that the specific heat ratio becomes higher as it goes up.

  Here, when the air-fuel ratio shifts further to the lean side than the lean limit, misfire or the like occurs. Therefore, in the expansion stroke after ignition, the mode of change in the measured value of the specific heat ratio (black circle in the figure) accompanying the change in the air-fuel ratio differs between the rich side and the lean side from the lean limit. For example, when misfire occurs, the specific heat ratio in the expansion stroke approaches the specific heat ratio in the compression stroke (in some cases, it becomes equal).

  Therefore, the control device 2 of the present embodiment measures the specific heat ratio of the in-cylinder gas in the expansion stroke and the compression stroke at a plurality of air-fuel ratios by changing the reference air-fuel ratio AFr by the reference air-fuel ratio setting means B23 (FIG. Based on this measured value, the provisional lean limit value acquisition means B6 estimates the change mode of the specific heat ratio of the in-cylinder gas accompanying the change in the air-fuel ratio (the two-dot chain line and the solid curve in the figure) Thus, the feedforward learning value A / Fff is acquired. Then, the target throttle opening determining means B5, the provisional lean limit value acquiring means B6, and the throttle valve control means B7 are feedforward control using the feedforward learning value A / Fff for the feedback control of the throttle opening TA. By applying, the responsiveness of lean limit control is improved.

  For example, an approximate function fκexp (A / F) is obtained by the least square method or the like based on a measured value (black circle in the figure) of the specific heat ratio κexp of the expansion stroke in the second graph of FIG. Based on (A / F), the feedforward learning value A / Fff can be acquired.

  Specifically, as shown in the second graph of FIG. 4, the sky corresponding to the intersection of the approximate function fκexp (A / F) and the horizontal line indicating the specific heat ratio κair = 1.403 of air. The feedforward learning value A / Fff can be acquired by obtaining the fuel ratio A / F or by applying a predetermined process (a process of multiplying a safety factor of less than 1 or subtracting a predetermined number). .

  Alternatively, as shown in the first graph of FIG. 4, the approximate function fκcomp (A / F) is obtained by the least square method or the like based on the measured value of the specific heat ratio κcomp of the compression stroke (white circle in the figure). Assuming that the approximate function fcomp (A / F) and the approximate function fκexp (A / F) change in a similar shape, the measured value of the specific heat ratio κexp of the expansion stroke at the rich air-fuel ratio ( The approximate function fκexp (A / F) is obtained based on the approximate function fκcomp (A / F) and the approximate function fκcomp (A / F) (the second graph in the second graph of FIG. 4). (See dotted line), obtaining an air-fuel ratio A / F corresponding to the time point when the deviation between the approximate function fκexp (A / F) and the measured value of the specific heat ratio κexp of the expansion stroke exceeds a predetermined range, or Is subjected to the predetermined processing as described above, so that the feedforward learning value A Fff may be obtained.

  Alternatively, the approximate function fκcomp (A / F) based on the measured value of the specific heat ratio κcomp in the compression stroke (white circle in the figure) and the approximate function fκexp (A / F) based on the measured value of the specific heat ratio κexp in the expansion stroke (black circle in the figure) The feedforward learning value A / Fff can be obtained by obtaining the air-fuel ratio A / F corresponding to the intersection point with F) or by performing the predetermined processing as described above.

In the third graph of FIG. 4, the reason why the specific heat ratio κexp of the expansion stroke shows a value equal to or higher than the specific heat ratio κair of air = 1.403 is as follows. That is, CO ₂ and NOx having a large heat capacity are generated by the combustion of fuel. Moreover, the temperature in the combustion chamber CC rises due to the thermal energy of combustion, and the movement speed of molecules increases.

  Here, the details of the method for obtaining the specific heat ratio κ are as follows (for details, refer to JP-A-2005-76613).

In the case of the compression stroke, the in-cylinder pressure P _{1 at} a predetermined crank angle θ ₁ immediately after the intake valve 123 is closed and the in-cylinder pressure P _{2 at} a predetermined crank angle θ ₂ immediately before ignition are measured. Further, the in-cylinder volumes V ₁ and V _{2 at} the respective crank angles θ ₁ and θ ₂ are uniquely determined mechanically based on the relationship between the crank angle and the piston position. Assuming that the compression stroke is an adiabatic stroke, the relationship that the value of PV ^κ becomes constant is established. Therefore, the measured value κcomp of the specific heat ratio of the compression stroke is calculated by the following equation (3).
κcomp = log (P ₂ / P ₁ ) / log (V ₂ / V ₁ ) (3)

The measured value κexp of the specific heat ratio of the expansion stroke is also the in-cylinder pressure P _{3 at} the predetermined crank angle θ ₃ after ignition, the in-cylinder volume V ₃ , and the in-cylinder pressure P at the predetermined crank angle θ ₄ just before the exhaust valve 124 is opened. ₄ , based on the in-cylinder volume V ₄ , it is calculated by the following equation (4) similar to the above equation (3).
κexp = log (P ₄ / P ₃ ) / log (V ₄ / V ₃ ) (4)

  The throttle valve control means B7 uses the throttle valve actuator 133 to control the throttle valve 132 so that the actual opening (throttle opening TA) of the throttle valve 132 coincides with the determined target throttle opening TAtgt. It is supposed to be.

<Specific example of operation>
Next, a specific example of the operation executed by the control device 2 of the present embodiment will be described using a flowchart.

  FIG. 5 is a flowchart showing an example of a routine 500 for obtaining the feedforward learning value A / Fff in a certain cylinder. In the following description, “step” is abbreviated as “S” (in the drawings, “step” is also abbreviated as “S”). The CPU 201 executes the feedforward learning value acquisition routine 500 shown in FIG. 5 at every predetermined timing.

  When this routine is started, first, at S510, it is determined whether or not a predetermined air-fuel ratio feedback condition is satisfied. The air-fuel ratio feedback conditions are, for example, (1) the cooling water temperature Tw is 40 ° C. or higher, (2) the upstream air-fuel ratio sensor 216a and the downstream air-fuel ratio sensor 216b are in an active state, and the like. If the air-fuel ratio feedback condition is not satisfied (S510 = No), all subsequent processing is skipped, and this routine is temporarily terminated.

  When the air-fuel ratio feedback condition is satisfied (S510 = Yes), the process proceeds to S520, and it is determined whether or not a predetermined lean limit control condition is satisfied. The lean limit control conditions are, for example, (1) the cooling water temperature Tw is 70 ° C. or higher, (2) the high load operation as described above is not performed, and the like.

  When the air-fuel ratio feedback condition is first established after the engine 1 is started, the determination in S520 is Yes, and the process proceeds to S530 (note that the acquired feedforward learning value is obtained after the lean limit control condition is established). (Because lean limit control is performed based on A / Fff, the determination in S520 is No, all subsequent processing is skipped, and this routine is temporarily terminated.)

  If the predetermined lean limit control condition is not established (S520 = Yes), the process proceeds to S530, and acquisition of measured values of κcomp and κexp (see FIG. 4) at a plurality of predetermined air-fuel ratios is completed. It is determined whether or not there is. When there is a predetermined plurality of air-fuel ratios for which measurement values of κcomp and κexp have not been acquired (S530 = No), all subsequent processing is skipped, and this routine is temporarily ended.

  When acquisition of κcomp and κexp is completed (S530 = Yes), the process proceeds to S540, and an approximate function fcomp (A / F) or fκexp (A / F) is acquired based on the measured value. . Subsequently, the process proceeds to S550, where the feedforward learning value A / Fff is acquired / updated as described above, and this routine is once ended.

  FIG. 6 is a flowchart showing an example of a routine 600 for controlling the throttle opening degree TA. The CPU 201 executes a throttle control routine 600 shown in FIG. 6 at every predetermined timing.

  When this routine is started, first, at S610, it is determined whether the above-described lean limit control condition is satisfied. When the lean limit control condition is not satisfied (S610 = No), the process proceeds to S620, the normal (not lean limit control) throttle opening degree TA is controlled, and this routine is temporarily ended.

  When the lean limit control condition is satisfied (S610 = Yes), the process proceeds to S630, and it is determined whether or not the feedforward learning value A / Fff in all the cylinders has been acquired / updated. Before the feedforward learning value A / Fff is acquired / updated for all the cylinders (S630 = No), the process proceeds to S620, and the normal (non-lean limit) throttle opening degree TA is controlled. The routine ends once. On the other hand, when the feedforward learning value A / Fff has been acquired and updated for all the cylinders (S630 = Yes), the process proceeds to S640, and among the feedforward learning values A / Fff corresponding to a plurality of cylinders. Based on the minimum value, lean limit control is performed, and this routine is temporarily terminated.

<Effects of Configuration of Embodiment>
As described above, in the control device 2 of the present embodiment, when the air-fuel ratio is changed to leaner for lean burn (lean limit) operation, the amount of fuel contributing to combustion is not reduced. Therefore, it is possible to avoid the problem that the driver feels uncomfortable due to a relatively large decrease in engine generated torque.

  In addition, even when the cylinder intake air amount Mc cannot be accurately estimated during transient operation or the like, the throttle opening degree TA so that the combustion state index acquisition value (MFB30) matches the combustion state index target value (MFB30tgt). Is controlled. Therefore, the air-fuel ratio is made lean while reducing the fluctuation of the engine generated torque.

  Further, in the control device 2 of the present embodiment, the feedforward learning value A / Fff acquired based on the change mode of the specific heat ratio accompanying the change in the air-fuel ratio is used in the lean limit control by the feedback control of the throttle opening TA. Feedforward control is performed. Therefore, as shown in FIG. 7, the control of the present embodiment (see the solid line in the figure) is more responsive than the case where feedforward is not performed (see the two-dot chain line in the figure). .

  Furthermore, in the control device 2 of the present embodiment, lean limit control is performed based on the lowest value among the plurality of feedforward learning values A / Fff corresponding to each cylinder. That is, of the plurality of cylinders, the slowest combustion is set as a representative cylinder, and lean limit control is performed based on the representative cylinder. Thereby, the control load (calculation processing load) is reduced.

<List of examples of modification>
Note that, as described above, the above-described embodiment is merely an example of a specific configuration of the present invention considered to be the best by the applicant at the time of filing of the present application. It should not be limited at all by the embodiment. Therefore, it goes without saying that various modifications can be made to the specific configurations shown in the above-described embodiments within a range that does not change the essential part of the present invention.

  Hereinafter, some modifications will be exemplified. Here, in the following description of the modified example, components having the same configurations and functions as the components in the above-described embodiment are given the same name and the same reference numerals in the modified example. And And about description of the said component, description in the above-mentioned embodiment shall be used suitably in the range which is not inconsistent.

  However, it goes without saying that the modified examples are not limited to the following. The limited interpretation of the present invention based on the description of the above-described embodiment and the following modifications unfairly harms the interests of the applicant (especially rushing the application under the principle of prior application), but improperly imitates the imitator. It is beneficial and not allowed.

  Further, it goes without saying that the configuration of the above-described embodiment and the configuration described in each of the following modifications can be applied in an appropriate combination within a technically consistent range.

  (1) The present invention can be applied to gasoline engines, diesel engines, methanol engines, bioethanol engines, and any other types of internal combustion engines. The number of cylinders, cylinder arrangement system (series, V type, horizontally opposed), and fuel injection system (port injection, in-cylinder direct injection) are not particularly limited. In addition, the present invention is not limited to the specific mechanical configuration shown in the above-described embodiment.

  (2) The present invention is not limited to the specific control system and control operation shown in the above-described embodiment.

  For example, the present invention does not mean that the fuel injection amount TAU determined based on the accelerator operation amount PA should not be reduced at all in the lean (limit) control of the air-fuel ratio. For example, even if the fuel injection amount is reduced within a range that does not give the driver a sense of incongruity (a minimum required torque can be obtained), it does not depart from the scope of the present invention.

  The target combustion ratio MFB30tgt can be set to various values (for example, a value within the range of 85 to 97%) depending on the structure of the engine 1.

  The air-fuel ratio corresponding value AFL in the above-described embodiment is a value corresponding to the air-fuel ratio of the fuel mixture supplied to the engine 1. However, the present invention is not limited to this. For example, the air-fuel ratio correspondence value may be a ratio to the theoretical air-fuel ratio.

  The predetermined crank angle θ for determining the combustion rate is not limited to ATDC 30 ° CA. The combustion ratio may be obtained by an expression other than the expression (1), such as an expression based on a combustion model called a Wiebe function (see, for example, JP-A-2006-9720).

  The change in the air-fuel ratio when acquiring the measured values of the specific heat ratio κcomp in the compression stroke and the specific heat ratio κexp in the expansion stroke at a plurality of air-fuel ratios is not limited to forced air-fuel ratio change (so-called air-fuel ratio active control). For example, an air-fuel ratio that is appropriately controlled according to the operating state of the engine 1 after the air-fuel ratio feedback condition is satisfied and before the lean limit control condition is satisfied (for example, for early warm-up of the upstream catalytic converter 141, etc.) It is possible to obtain measured values of the specific heat ratio κcomp of the compression stroke and the specific heat ratio κexp of the expansion stroke corresponding to (shift from the theoretical air-fuel ratio). Thereby, the fuel consumption reduction effect further increases.

  Without using the measured value of the specific heat ratio κcomp of the compression stroke and the approximate function fκcomp (A / F) based on the measured value, the measured value of the specific heat ratio κexp of the expansion stroke and the approximate function fκexp (A / F) based on the measured value When the learning value A / Fff is acquired, the measurement value of the specific heat ratio κcomp of the compression stroke and the acquisition of the approximate function fκcomp (A / F) based thereon can be omitted.

  For the air-fuel ratio corresponding to the singular point (intersection etc.) of the change in the specific heat ratio κexp of the expansion stroke accompanying the air-fuel ratio change, the above-mentioned predetermined processing (multiplying a safety factor of less than 1 or subtracting the predetermined number) The feedforward learning value A / Fff is set near the lean limit and slightly richer than this. Thereby, good lean limit control can be performed such that misfire or the like does not occur while improving responsiveness by feedforward.

  (3) Other modifications not specifically mentioned are naturally included in the technical scope of the present invention within the scope not changing the essential part of the present invention. In addition, in each element constituting the means for solving the problems of the present invention, the elements expressed in terms of operation and function are the specific structures disclosed in the above-described embodiments and modifications, It includes any structure that can realize this action / function.

1 is a diagram showing a schematic configuration of a system (vehicle) including a piston reciprocating spark ignition type multiple cylinder (four cylinders) four-cycle engine and an air-fuel ratio control apparatus according to an embodiment of the present invention. It is a functional block diagram of the control apparatus shown by FIG. It is a time chart which showed the mode of change to the crank angle of a combustion rate. It is the graph which showed the mode of the change of the combustion ratio in various air fuel ratios. It is a graph which shows the change of the specific heat ratio of the cylinder gas (gas in the combustion chamber CC) accompanying an air fuel ratio change. 3 is a flowchart showing an example of a feedforward learning value acquisition routine executed by the control device shown in FIG. 2. 3 is a flowchart illustrating an example of a throttle control routine that is executed by the control device illustrated in FIG. 2. It is a time chart which shows the effect by embodiment of this invention.

Explanation of symbols

S ... System 1 ... Engine 11 ... Cylinder block 111 ... Cylinder bore 112 ... Piston 113 ... Crankshaft 12 ... Cylinder head 129 ... Injector 13 ... Intake passage 14 ... Exhaust passage 132 ... Throttle valve 33 ... Throttle valve actuator 2 ... Air-fuel ratio control device 200 ... Electronic control unit 201 ... CPU 212 ... In-cylinder pressure sensor 214 ... Crank position sensor 217 ... Throttle position sensor 218 ... Accelerator opening sensor 220 ... Accelerator pedal B1 ... Accelerator operation amount acquisition means B2 ... Fuel injection amount determination means B21 ... Temporary Throttle opening determination means B22 ... Temporary intake air amount corresponding value determination means B23 ... Reference air / fuel ratio setting means B24 ... Fuel injection amount calculation means B3 ... Fuel injection means B4 ... Combustion state index value acquisition means B5 ... Target throttle Le opening determining means B51 ... air corresponding value calculating means B52 ... target throttle opening degree calculation unit B6 ... provisional lean limit value acquiring unit B7 ... Throttle valve control means

Claims (6)

An air-fuel ratio control device for controlling an air-fuel ratio of an internal combustion engine toward a lean limit,
An accelerator operation amount acquisition means for acquiring an accelerator operation amount;
Fuel injection amount determination means for determining a fuel injection amount based on the accelerator operation amount acquired by the accelerator operation amount acquisition means and a predetermined reference air-fuel ratio;
Fuel injection means for causing a fuel injector to inject fuel of the fuel injection amount determined by the fuel injection amount determination means;
Combustion state index value acquisition means for acquiring a combustion state index value indicating a combustion state of a fuel mixture formed in a cylinder of the internal combustion engine by the fuel injection of the fuel injection amount in the fuel injector;
A provisional lean limit value obtaining means for obtaining a provisional lean limit value estimated to be in the vicinity of the lean limit;
The provisional lean limit value and the combustion state index acquisition value are set so that a combustion state index acquisition value that is an acquired value of the combustion state index value by the combustion state index value acquisition unit matches a predetermined combustion state index target value. A target throttle opening determination means for determining a target throttle opening that is a target value of the opening of the throttle valve based on the combustion state index target value;
Throttle valve control means for controlling the throttle valve so that the opening of the throttle valve matches the target throttle opening;
An air-fuel ratio control apparatus comprising: The air-fuel ratio control apparatus according to claim 1,
The combustion state index value acquisition means is combustion ratio acquisition means for acquiring a combustion ratio at a predetermined crank angle of the fuel in the fuel mixture based on the pressure in the cylinder. apparatus. The air-fuel ratio control apparatus according to claim 1 or 2,
The provisional lean limit value acquisition means acquires the provisional lean limit value by acquiring the state of the fuel mixture corresponding to a plurality of air-fuel ratios. The air-fuel ratio control apparatus according to claim 3,
The provisional lean limit value acquisition means acquires a specific heat ratio in the compression stroke and the expansion stroke based on the pressure in the cylinder at a plurality of air-fuel ratios, and thereby in the compression stroke and the expansion stroke accompanying the change in the air-fuel ratio. An air-fuel ratio control apparatus that estimates a change mode of a specific heat ratio and acquires the provisional lean limit value based on the estimation result. The air-fuel ratio control apparatus according to any one of claims 1 to 4,
The fuel injection amount determining means includes
A provisional throttle opening determining means for determining a provisional throttle opening that increases as the accelerator operation amount increases based on the accelerator operation amount acquired by the accelerator operation amount acquisition means;
When it is assumed that the actual opening of the throttle valve is the provisional throttle opening, a provisional intake air amount corresponding value that is a value corresponding to the amount of air sucked into the cylinder is obtained as the provisional throttle opening degree. An air amount estimating means for estimating based on
When it is assumed that an amount of air corresponding to the provisional intake air amount corresponding value estimated by the air amount estimation means is sucked into the cylinder, the air-fuel ratio of the fuel mixture formed in the cylinder is Fuel injection amount calculating means for calculating the fuel injection amount so as to coincide with a reference air-fuel ratio;
Including
The target throttle opening determining means is
An air-fuel ratio corresponding value that is a value corresponding to an air-fuel ratio of the fuel mixture to be formed in the cylinder in order to make the combustion state index acquired value coincide with the combustion state index target value is set to the combustion state index acquired value. And an air-fuel ratio corresponding value calculating means for calculating based on the combustion state index target value;
Target throttle opening calculation means for calculating the target throttle opening based on the provisional intake air amount correspondence value estimated by the air amount estimation means and the air-fuel ratio correspondence value;
An air-fuel ratio control apparatus comprising: The air-fuel ratio control apparatus according to any one of claims 1 to 5,
The internal combustion engine is provided with a plurality of the cylinders,
The provisional lean limit value acquisition means acquires the provisional lean limit value for each of the plurality of cylinders,
The target throttle opening determining means determines the target throttle opening based on the lowest value among the plurality of provisional lean limit values corresponding to the plurality of cylinders. .

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