Control apparatus for internal combustion engine

Abstract

Invention suppresses deterioration of emission if there is air-fuel ratio imbalance among cylinders. Apparatus (100) controlling an engine including first and second air-fuel ratio sensors respectively disposed on upstream and downstream of catalyst, has: first determining device determining first F/B controlled variable according to deviation between output value and target value of first air-fuel ratio sensor; second determining device determining second F/B controlled variable according to deviation between output value and target value of second air-fuel ratio sensor; controlling device controlling fuel injection amount based on first and second F/B controlled variables; detecting device detecting air-fuel ratio imbalance among cylinders; and correcting device correcting second F/B controlled variable in direction in which there is hardly change of fuel injection amount to lean air-fuel ratio side, according to output deviation between first and second air-fuel ratio sensors, if air-fuel ratio imbalance is detected.

Claims

1. A control apparatus for an internal combustion engine, configured to control the internal combustion engine, the internal combustion engine comprising: an exhaust gas purifying catalyst disposed in an exhaust passage; a first air-fuel ratio sensor disposed on an upstream side of the catalyst and configured to output a first output value according to an air-fuel ratio of a catalyst inflow gas; and a second air-fuel ratio sensor disposed on a downstream side of the catalyst and configured to output a second output value according to an air-fuel ratio of a catalyst emission gas, the control apparatus comprising a controller, the controller being configured to: determine a first F/B controlled variable for making the first output value converge on a first target value, according to a first deviation which is a deviation between the first output value and the first target value; determine a second F/B controlled variable for making the second output value converge on a second target value, according to a second deviation which is a deviation between the second output value and the second target value; control a fuel injection amount of the internal combustion engine on the basis of the determined first F/B controlled variable and the determined second F/B controlled variable; detect air-fuel ratio imbalance among a plurality of cylinders of the internal combustion engine; and correct the second F/B controlled variable so that the fuel injection amount is less likely to change to a lean air-fuel ratio side by decreasing the second F/B controlled variable compared to a case where the air-fuel ratio imbalance is not detected, according to an output deviation between the first air-fuel ratio sensor and the second air-fuel ratio sensor, if the air-fuel ratio imbalance is detected.

2. The control apparatus for the internal combustion engine according to claim 1, wherein the controller is configured to correct the second F/B controlled variable such that the fuel injection amount further increases in comparison with a case where the correction is not performed.

3. The control apparatus for the internal combustion engine according to claim 1, wherein the controller is configured to detect the air-fuel ratio imbalance on the basis of the output deviation.

4. The control apparatus for the internal combustion engine according to claim 1, wherein the controller is configured to correct the second F/B controlled variable by correcting an element value which constitutes the second F/B controlled variable, the element value is stored on a standard map and a correction map each of which is associated with the second deviation, the standard map corresponding to a case where the first output value does not deviate to a rich air-fuel ratio side with respect to an actual air-fuel ratio, the correction map corresponding to a case where the first output value deviates to the rich air-fuel ratio side with respect to the actual air-fuel ratio, the controller is configured to determine the second F/B controlled variable by selecting the element value corresponding to the second deviation from the standard map, and the controller is configured to correct the second F/B controlled variable by selecting the element value corresponding to the second deviation from the correction map.

5. The control apparatus for the internal combustion engine according to claim 4, wherein in the standard map, the element value in the case where the second deviation is in a rich air-fuel ratio side region with respect to a reference value and the element value in the case where the second deviation is in a lean air-fuel ratio side region with respect to the reference value have a symmetric relation in which the element values have different signs, and the correction map is a map in which the element value in the case where the second deviation is in the rich air-fuel ratio side region with respect to the reference value and the element value in the case where the second deviation is in the lean air-fuel ratio side region with respect to the reference value have an asymmetric relation, by changing, in the standard map, the element value in the rich air-fuel ratio side region with respect to the reference value in a direction in which sensitivity to the second deviation decreases.

6. The control apparatus for the internal combustion engine according to claim 1, wherein each of the first and second target values is a value corresponding to a theoretical air-fuel ratio.

7. The control apparatus for the internal combustion engine according to claim 1, wherein the controller is configured to correct the second F/B controlled variable in a direction of suppressing the change of the fuel injection amount to the lean air-fuel ratio side if the output deviation indicates that the first output value is on a rich air-fuel ratio side by a predetermined value or more with respect to the second output value.

8. The control apparatus for the internal combustion engine according to claim 1, wherein the output deviation includes any one of (1) a deviation between the first output valued and the second output value, (2) a deviation between a peak value of the first output value and a peak value of the second output value, (3) a deviation between an average value of the first output value and an average value of the second output value, and (4) a deviation between a response speed of the first air-fuel ratio sensor and a response speed of the second air-fuel ratio sensor.

9. The control apparatus for the internal combustion engine according to claim 1, wherein the controller is configured to correct a gain by which the second deviation is to be multiplied, or a learning value of the second controlled variable.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic configuration diagram conceptually illustrating a configuration of an engine system in an embodiment of the present invention.

(2) FIG. 2 is a block diagram illustrating an ECU when air-fuel ratio F/B control is performed.

(3) FIG. 3 is a flowchart illustrating the air-fuel ratio F/B control in FIG. 2.

(4) FIG. 4 is a conceptual diagram illustrating a standard map referred to in the air-fuel ratio F/B control in FIG. 2.

(5) FIG. 5 is a conceptual diagram illustrating a correction map to referred to in the air-fuel ratio F/B control in FIG. 2.

DESCRIPTION OF EMBODIMENT

Embodiment of the Invention

(6) Hereinafter, with reference to the drawings, an embodiment of the present invention will be explained.

(7) <Configuration of Embodiment>

(8) Firstly, with reference to FIG. 1, a configuration of an engine system 10 in the embodiment of the present invention will be explained. FIG. 1 is a schematic configuration diagram conceptually illustrating the configuration of the engine system 10.

(9) In FIG. 1, the engine system 10 is mounted on a not-illustrated vehicle, and is provided with an ECU 100 and an engine 200.

(10) The ECU 100 is provided with a CPU, a ROM, a RAM and the like, and is an electronic control unit configured to control the operation of the engine system 10. The ECU 100 is one example of the control apparatus for the internal combustion engine of the present invention. The ECU 100 is configured to perform air-fuel ratio F/B control described later, in accordance with a control program stored in the ROM.

(11) The ECU 100 is an integrated electronic control unit configured to function as one example of each of the first determining device, the second determining device, the controlling device, the detecting device, and the correcting device of the present invention. The physical, mechanical and electrical configurations of each of the devices of the present invention, however, are not limited to this example, and each of the devices may be also configured as various computer systems or the like such as, for example, a plurality of ECUs, various processing units, various controllers, or micro computer apparatuses.

(12) The engine 200 is a multi-cylinder gasoline engine, which is one example of the internal combustion engine of the present invention.

(13) In FIG. 1, the engine 200 is provided with a plurality of cylinders 201 contained in a cylinder block CB. In FIG. 1, the cylinders 201 are arranged in a depth direction of the paper, and only one cylinder 201 is illustrated in FIG. 1.

(14) In the engine 200, a combustion chamber formed inside the cylinder 201 is provided with a piston 202 which produces a reciprocating motion in a vertical direction in the drawing according to explosive power caused by the combustion of an air-fuel mixture. The reciprocating motion of the piston 202 is converted into a rotational motion of a crankshaft 204 via a connecting rod 203, and is used as the power of the vehicle on which the engine 200 is mounted.

(15) In the vicinity of the crankshaft 204, there is disposed a crank position sensor 205 configured to detect a rotational position of the crankshaft 204 (i.e. a crank angle). The crank position sensor 205 is electrically connected to the ECU 100. The detected crank angle is referred to by the ECU 100 with a regular or irregular period, and is used, for example, for the calculation of the engine's rotation number NE and for the other control.

(16) In the engine 200, an air flowing from the exterior (or intake air) is purified by a not-illustrated cleaner and is then supplied to an intake tube 206 which is common to the cylinders. In the intake tube 206, there is disposed a throttle valve 207 configured to adjust an intake air amount which is the amount of the intake air. The throttle valve 207 is configured as a type of electronically controlled throttle valve whose driving state is controlled by a not-illustrated throttle valve motor which is electrically connected to the ECU 100.

(17) The ECU 100 performs drive control of the throttle valve motor, basically to obtain a throttle opening degree according to an accelerator opening degree Ta detected by a not-illustrated accelerator position sensor. The ECU 100 can also adjust the throttle opening degree without a driver's intention via motion control of the throttle valve motor.

(18) The intake air adjusted by the throttle valve 207 as occasion demands is supplied through an intake port 208 corresponding to each cylinder 201 to the inside of the cylinder 201 upon opening of an intake valve 209. The intake valve 209 is configured such that the opening/closing timing thereof is determined according to the cam profile of a cam 210 having a cross-sectionally substantially oval shape as illustrated.

(19) On the other hand, the cam 210 is fixed to an intake cam shaft (whose reference numeral is omitted) coupled with the crankshaft 204 via a power transmitting device such as, for example, a cam sprocket and a timing chain. Therefore, the opening/closing phase of the intake valve 209 has an unambiguous relation, in one fixed state, with the rotational phase of the crankshaft 204 (i.e. the crank angle).

(20) Here, the fixed state between the intake cam 210 and the intake cam shaft varies depending on the hydraulic pressure of control oil supplied by a hydraulic pressure driving apparatus 211. More specifically, the intake cam 210 is coupled with the intake cam shaft via a wing-shaped member referred to as a vane, and the rotational phase of the vane and the intake cam shaft is configured to vary depending on the hydraulic pressure applied to a hydraulic chamber of the hydraulic pressure driving apparatus 211. Therefore, the rotational phase of the intake cam 210 fixed to the vane and the intake cam shaft also varies depending on the hydraulic pressure. The hydraulic pressure driving apparatus 211 is electrically connected to the ECU 100, and the ECU 100 can change the opening/closing timing of the intake valve 209 through the control of the hydraulic pressure driving apparatus 211.

(21) The intake air supplied to the intake port 208 is mixed with fuel (gasoline in the embodiment) injected from an intake port injector 212 in which an injection valve is partially exposed to the intake port 208, to make the aforementioned air-fuel mixture. The gasoline as the fuel is stored in a not-illustrated fuel tank, and is supplied to the intake port injector 212 via a not-illustrated delivery pipe by the action of a not-illustrated low pressure feed pump. In the intake port injector 212, a not-illustrated driving apparatus which drives the injection valve is electrically connected to the ECU 100. Due to that the ECU 100 controls a valve opening period of the injection valve via this driving apparatus, the intake port injector 212 can supply the intake port 208 with an amount of fuel spray according to the valve opening period.

(22) In the combustion chamber of the engine 200, there is partially exposed a spark plug (whose reference numeral is omitted) of an ignition apparatus 213, which is a spark ignition apparatus. The air-fuel mixture compressed in a compression stroke of the engine 200 is ignited and burned by an ignition operation of the spark plug. The ignition apparatus 213 is electrically connected to the ECU 100, and the ignition timing of the ignition apparatus 213 is controlled by the ECU 100.

(23) On the other hand, the air-fuel mixture which causes the combustion reaction in the combustion chamber flows out to an exhaust port 216 upon opening of the exhaust valve 215, when the exhaust valve 215, which is subject to opening/closing drive in accordance with opening/closing timing determined according to the cam profile of an exhaust cam 214 which is indirectly coupled with the crankshaft 204, is opened in an exhaust stroke subsequent to a combustion stroke.

(24) An exhaust tube 217 is coupled with the exhaust port 216 in each cylinder via a not-illustrated exhaust manifold. The exhaust tube 217 is one example of the exhaust passage of the present invention.

(25) In the exhaust tube 217, there is disposed a three-way catalyst 218 which is one example of the exhaust gas purifying catalyst of the present invention. The three-way catalyst 218 is a known catalyst apparatus in which noble metal such as platinum is carried on a catalyst support, and is configured to purify the exhaust gas by allowing the oxidation combustion of HC and CO and the reduction of nitrogen oxide NOx to proceed at substantially the same time.

(26) On the upstream side of the three-way catalyst 218 in the exhaust tube 217, there is disposed a first air-fuel ratio sensor 219 configured to detect an upstream side air-fuel ratio A/Fin which is the air-fuel ratio of a catalyst inflow gas which flows into the three-way catalyst 218. The first air-fuel ratio sensor 219 is, for example, a wide range air-fuel ratio sensor of a limiting current type provided with a diffusion resistance layer, and is one example of the first air-fuel ratio sensor of the present invention.

(27) The first air-fuel ratio sensor 219 is a sensor configured to output an output voltage value Vf (i.e. one example of the first output value of the present invention) according to the upstream side air-fuel ratio A/Fin. In other words, the first air-fuel ratio sensor 219 is configured to indirectly detect the input side air-fuel ratio A/Fin from a voltage value having an unambiguous relation with the upstream-side air-fuel ratio A/Fin.

(28) The output voltage value Vf is equal to a reference output voltage value Vst when the upstream side air-fuel ratio A/Fin is the theoretical air-fuel ratio. The output voltage value Vf is lower than the reference output voltage value Vst if the upstream side air-fuel ratio A/Fin is on the rich air-fuel ratio side, and is higher than the reference output voltage value Vst if the upstream side air-fuel ratio A/Fin is on a lean air-fuel ratio side. In other words, the output voltage value Vf continuously changes with respect to a change in the upstream side air-fuel ratio A/Fin. The first air-fuel ratio sensor 219 is electrically connected to the ECU 100, and the detected output voltage value Vf is referred to by the ECU 100 with a regular or irregular period.

(29) On the downstream side of the three-way catalyst 218 in the exhaust tube 217, there is disposed a second air-fuel ratio sensor 220 configured to detect a downstream side air-fuel ratio A/Fout which is the air-fuel ratio of a catalyst emission gas which flows out from the three-way catalyst 218. The second air-fuel ratio sensor 220 is, for example, a wide range air-fuel ratio sensor of a limiting current type provided with a diffusion resistance layer, and is one example of the second air-fuel ratio sensor of the present invention.

(30) The second air-fuel ratio sensor 220 is a sensor configured to output an output voltage value Vr (i.e. one example of the second output value of the present invention) according to the downstream side air-fuel ratio A/Fout. In other words, the second air-fuel ratio sensor 220 is configured to indirectly detect the downstream side air-fuel ratio A/Fout from a voltage value having an unambiguous relation with the downstream-side air-fuel ratio A/Fout.

(31) The output voltage value Vr is equal to the reference output voltage value Vst when the downstream side air-fuel ratio A/Fout is the theoretical air-fuel ratio. The output voltage value Vr is lower than the reference output voltage value Vst if the downstream side air-fuel ratio A/Fout is on the rich air-fuel ratio side, and is higher than the reference output voltage value Vst if the downstream side air-fuel ratio A/Fout is on the lean air-fuel ratio side. In other words, the output voltage value Vr continuously changes with respect to a change in the downstream side air-fuel ratio A/Fout. The second air-fuel ratio sensor 220 is electrically connected to the ECU 100, and the detected output voltage value Vr is referred to by the ECU 100 with a regular or irregular period.

(32) In the engine 200, in a water jacket disposed to surround the cylinder block CB, there is disposed a water temperature sensor 221 configured to detect a coolant temperature Tw which is the temperature of a coolant (LLC) circulated and supplied to cool the engine 200. The water temperature sensor 221 is electrically connected to the ECU 100, and the detected coolant temperature Tw is referred to by the ECU 100 with a regular or irregular detection period.

(33) In the engine 200, moreover, in the intake tube 206, there is disposed an airflow meter 222 configured to detect an intake air amount Ga. The airflow meter 222 is electrically connected to the ECU 100, and the detected intake air amount Ga is referred to by the ECU 100 with a regular or irregular detection period.

(34) The engine 200 in the embodiment is a non-supercharged engine which uses gasoline as fuel; however, the internal combustion engine of the present invention is not limited to the engine 200 and may have various configurations. For example, in the internal combustion engine of the present invention, the number of cylinders, cylinder arrangement, fuel types, fuel injection aspects, intake/exhaust system configurations, valve train or system, combustion methods, presence or absence of a supercharger, supercharging aspects and the like may be different from those of the engine 200.

(35) <Operation of Embodiment>

(36) <Outline of Air-Fuel Ratio F/B Control>

(37) In the engine 200, a fuel injection amount Qpfi of the intake port injector 212 is controlled by the ECU 100 in the air-fuel ratio F/B control performed all the time in an operating period of the engine 200.

(38) Now, with reference to FIG. 2, a logical configuration of the air-fuel ratio F/B control will be explained. FIG. 2 is a block diagram illustrating the ECU 100 when the air-fuel ratio F/B control is performed. In FIG. 2, a duplicate portion of FIG. 1 will carry the same reference numeral, and the explanation thereof will be omitted.

(39) In FIG. 2, the ECU 100 is provided with control blocks, which are an upstream side target A/F determination unit 101, a basic injection amount determination unit 102, an adder 103, a downstream target A/F determination unit 104, a sub F/B arithmetic unit 105, an adder 106, and a main F/B arithmetic unit 107.

(40) The upstream side target A/F determination unit 101 is a control block which determines an upstream side target air-fuel ratio A/Fintg which is a target air-fuel ratio on the upstream side of the three-way catalyst 218. The upstream side target air-fuel ratio A/Fintg is basically the theoretical air-fuel ratio (14, 6) except for transient operation conditions or the like. From the upstream side target A/F determination unit 101, an upstream side target voltage value Vfref corresponding to the upstream side target air-fuel ratio A/Fintg is outputted. The upstream side target voltage value Vfref is one example of the first target value of the present invention.

(41) The basic injection amount determination unit 102 is a control block which determines a basic injection amount Qbase which is the base of the fuel injection amount Qpfi. The basic injection amount Qbase is determined on the basis of the upstream target air-fuel ratio A/Fintg (which may be converted from the upstream side target voltage value Vfref or may be obtained directly from the upstream side target air-fuel ratio determination unit 101) and the intake air amount Ga detected by the airflow meter 222. The determined basic injection amount Qbase is a basic injection amount at a time point at which the intake air whose intake air amount Ga is detected by the airflow meter 222 arrives at the intake port 208. The arrival timing is known on the basis of the crank angle of the engine 200.

(42) Here, the basic injection amount Qbase is corrected by main F/B control and sub F/B control. Specifically, the main F/B control is correction control for the basic injection amount Qbase performed such that the upstream side air-fuel ratio A/Fin detected by the first air-fuel ratio sensor 219 converges on the upstream side target air-fuel ratio A/Fintg. The sub F/B control is correction control for the basic injection amount Qbase performed such that the downstream side air-fuel ratio A/Fout detected by the second air-fuel ratio sensor 220 converges on a downstream side target air-fuel ratio A/Fouttg. The practical aspect of this type of F/B control is ambiguous, and the control in the embodiment described later is merely one example.

(43) Firstly, the sub F/B control will be explained. The sub F/B control is established by the downstream side target air-fuel ratio determination unit 104, the sub F/B arithmetic unit 105 and the adder 106.

(44) The downstream side target air-fuel ratio determination unit 104 is a control block which determines the downstream side target air-fuel ratio A/Fouttg which is a target value of the air-fuel ratio of the gas on the downstream side of the three-way catalyst 218, namely, the catalyst emission gas. The downstream side target air-fuel ratio A/Fouttg is assumed to be basically the theoretical air-fuel ratio (14, 6). The downstream side target air-fuel ratio determination unit 104 outputs a downstream side target voltage value Vrref corresponding to the downstream side target air-fuel ratio A/Fouttg. The downstream side target voltage value Vrref is one example of the second target value of the present invention.

(45) The sub F/B arithmetic unit 105 is a control block which calculates a sub F/B controlled variable Vfcor which is a controlled variable for correcting the output voltage value Vf of the first air-fuel ratio sensor 219. The sub F/B controlled variable Vfcor is one example of the second F/B controlled variable of the present invention.

(46) The sub F/B controlled variable Vfcor is a value obtained by multiplying the absolute value |Vr| of a downstream side voltage variation Vr (Vr=VrVrref), which is a deviation between the output voltage value Vr of the second air-fuel ratio sensor 220 and the downstream side target voltage value Vrref, by a sub F/B gain Gfbr (Gfbr>0) and a sub F/B correction amount Kr1. The sub F/B gain Gfbr is one example of the element value of the present invention.

(47) The sub F/B correction amount Kr1 has a negative value if the downstream side voltage deviation Vr has a negative value (i.e. the downstream side air-fuel ratio A/Fout is on the rich side with respect to the target), and has a positive value if the downstream side voltage deviation Vr has a positive value (i.e. the downstream side air-fuel ratio A/Fout is on the lean side with respect to the target).

(48) The sub F/B controlled variable Vfcor outputted from the sub F/B arithmetic unit 105 is added to the output voltage value Vf of the first air-fuel ratio sensor 219 on the adder 106, and is outputted to the main F/B arithmetic unit 107 as an upstream side correction output voltage value Vf.

(49) Next, the main F/B control will be explained. The main F/B control is established by the upstream side target air-fuel ratio determination unit 101 and the main F/B arithmetic unit 107.

(50) The main F/B arithmetic unit 107 is a control block which calculates a main F/B controlled variable Qcor which is a controlled variable for correcting the basic fuel injection amount Qbase. The main F/B controlled variable Qcor is one example of the first F/B controlled variable of the present invention.

(51) The main F/B controlled variable Qcor is a value obtained by multiplying the absolute value |Vf| of a upstream side voltage variation Vf (Vf=VfVfref), which is a deviation between the upstream side correction output voltage value Vf outputted from the adder 106 and the upstream side target voltage value Vfref, by a main F/B gain Gfbf (Gfbf>0) and a main F/B correction amount Kf1.

(52) According to the main F/B control, if the correction output voltage value Vf is on the rich side with respect to the target, the main F/B controlled variable Qcor has a negative value and the basic injection amount Qbase is corrected to the decreasing side. As a result, the air-fuel ratio of the catalyst inflow gas (the upstream side air-fuel ratio A/Fin) is corrected to the lean side. On the other hand, if the correction output voltage value Vf is on the lean side with respect to the target, the main F/B controlled variable Qcor has a positive value and the basic injection amount Qbase is corrected to the increasing side. As a result, the air-fuel ratio of the catalyst inflow gas (the upstream side air-fuel ratio A/Fin) is corrected to the rich side.

(53) Now, the correction output voltage value Vf will be briefly explained.

(54) If the downstream side air-fuel ratio A/Fout is on the rich side with respect to the target, the sub F/B correction amount Kr1 has a negative value, and the sub F/B controlled variable Vfcor thus has a negative value. Therefore, the correction output voltage value Vf is corrected to the richer side than the output voltage value Vf of the first air-fuel ratio sensor 219. This results in strong correction to the lean side by the main F/B controlled variable Qcor in the main F/B control described above.

(55) On the other hand, if the downstream side air-fuel ratio A/Fout is on the lean side with respect to the target, the sub F/B correction amount Kr1 has a positive value, and the sub F/B controlled variable Vfcor thus has a positive value. Therefore, the correction output voltage value Vf is corrected to the leaner side than the output voltage value Vf of the first air-fuel ratio sensor 219. This results in strong correction to the rich side by the main F/B controlled variable Qcor in the main F/B control described above.

(56) In other words, the sub F/B control in the embodiment is control for correcting the output voltage value of the first air-fuel ratio sensor 219 in order to make the air-fuel ratio of the catalyst emission gas (i.e. the downstream side air-fuel ratio A/Fout) converge on the downstream side target air-fuel ratio A/Fouttg. To put it differently, the sub F/B control is incorporated as a portion of the main F/B control.

(57) The practical aspect of the main F/B control and the sub F/B control is ambiguous as described above. For example, the sub F/B control may not be the control for correcting the output voltage value Vf of the first air-fuel ratio sensor 219 as described above but may be control for correcting the upstream side target air-fuel ratio A/Fintg, or may be control for directly correcting the basic injection amount Qbase. In any case, good controllability is given to the air-fuel ratio of the catalyst emission gas by disposing the second air-fuel ratio sensor 220 configured to linearly detect the downstream side air-fuel ratio A/Fout, on the downstream side of the three-way catalyst 218

(58) <Details of Air-Fuel Ratio F/B Control>

(59) Next, with reference to FIG. 3, the details of the air-fuel ratio F/B control will be explained. FIG. 3 is a flowchart illustrating the air-fuel ratio F/B control.

(60) In FIG. 3, the air-fuel ratio F/B control is performed as one sub routine of fuel injection control performed by the ECU 100 on an upper stream.

(61) In the air-fuel ratio F/B control, firstly, it is determined whether or not a stoichiometric F/B condition is satisfied (step S101). The stoichiometric F/B condition is a condition in which each of the upstream side target air-fuel ratio A/Fintg and the downstream side target air-fuel ratio A/Fouttg is the theoretical air-fuel ratio. The condition as described above is determined in advance according to operating conditions of the engine 200 or the vehicle on which the engine 200 is mounted.

(62) If the stoichiometric F/B condition is not satisfied (the step S101: NO), the ECU 100 moves the processing to a step S103 and performs another control. Another control is a general term of the sub routine that is different from the air-fuel ratio F/B control, and is not mentioned here.

(63) If the stoichiometric F/B condition is satisfied (the step S101: YES), the ECU 100 performs the stoichiometric F/B control (step S102). The stoichiometric F/B control is the air-fuel ratio F/B control whose control blocks are exemplified in FIG. 2. In the stoichiometric F/B control, the sub F/B correction amount described above is set to Kr1.

(64) In the step S102, a standard map which is one of control maps stored in the ROM is used, and the sub F/B correction amount Kr1 is set. Now, with reference to FIG. 4, the standard map will be explained. FIG. 4 is a conceptual diagram illustrating the standard map.

(65) In FIG. 4, the standard map describes that the sub F/B correction amount Kr1 has a relation of characteristic L_Kr1 (solid line)

(66) Specifically, if the downstream side voltage deviation Vr (i.e. one example of the output deviation of the present invention) is on the horizontal axis and the sub F/B correction amount Kr1 is on the vertical axis, the sub F/B correction amount Kr1 has a negative value in a negative value region (i.e. a rich air-fuel ratio region) on the left side of the origin (i.e. a state in which the downstream side air-fuel ratio A/Fout is the theoretical air-fuel ratio), and has a positive value in a positive value region (i.e. a lean air-fuel ratio region) on the right of the origin. The absolute value of the sub F/B correction amount Kr1 has a linear relation with the absolute value of the downstream side voltage deviation Vr, and the sub F/B correction amount Kr1 is symmetric on the rich air-fuel ratio side and the lean air-fuel ratio side.

(67) Here, the sub F/B correction amount Kr1 has a relation of linearly changing with respect to the downstream side voltage deviation Vr, and F/B is stronger as the downstream side air-fuel ratio A/Fout deviates more from the target; however, this is one example. For example, the sub F/B correction amount Kr1 may have a relation of changing in stages with respect to the downstream side voltage deviation Vr, or may have a constant fixed value.

(68) Back in FIG. 3, in the process in which the stoichiometric F/B control is performed, the ECU 100 determines whether or not a deviation between the upstream side output voltage value Vf and the downstream side output voltage value Vr has a negative value, i.e. whether or not the catalyst inflow gas has a relatively rich air-fuel ratio in comparison with the catalyst emission gas (step S104). If the catalyst emission gas has a richer air-fuel ratio, or if the catalyst inflow gas has an air-fuel ratio equal to that of the catalyst emission gas (the step S104: NO), the ECU 100 resets a counter C1 (step S106) and ends the air-fuel ratio F/B control. As described above, the air-fuel ratio F/B control is a type of sub routine. Thus, even if the air-fuel ratio F/B control is ended once, the air-fuel ratio F/B control is performed again from the step S101 if an execution condition is satisfied in a not-illustrated main routine.

(69) If the catalyst inflow gas has a relatively rich air-fuel ratio (the step S104: YES), the ECU 100 increments the counter C1 (step S105), and determines whether or not the counter C1 is greater than or equal to an imbalance determination value C0 (step S107). The imbalance determination value C0 is a value adapted experimentally in advance. If the counter C1 is less than the imbalance determination value C0 (the step S107: NO), the ECU 100 ends the air-fuel ratio F/B control.

(70) On the other hand, during the continued situation that the upstream side air-fuel ratio A/Fin is less than the downstream side air-fuel ratio A/Fout (i.e. the catalyst inflow gas has a relatively rich air-fuel ratio), if the counter C1 which is incremented as occasion demands becomes greater than or equal to the imbalance determination value C0 (the step S107: YES), the ECU 100 determines that there is air-fuel ratio imbalance among the plurality of cylinders of the engine 200 (step S108). In other words, in this case, the ECU 100 functions as one example of the detecting device of the present invention.

(71) If it is determined that there is the air-fuel ratio imbalance, the ECU 100 changes the sub F/B correction amount described above from Kr1 to Kr2 and corrects the sub F/B controlled variable Vfcor, under the determination that there is a rich shift in the first air-fuel ratio sensor 219 (step S109). The sub F/B correction amount Kr2 is described in a correction map stored in the ROM. The ECU 100 changes a map for selecting the sub F/B correction amount from the previous standard map to the correction map, and selects the sub F/B correction amount Kr2. If the sub F/B correction amount is changed, the air-fuel ratio F/B control is ended.

(72) Now, with reference to FIG. 5, the correction map will be explained. FIG. 5 is a conceptual diagram illustrating the correction map. In FIG. 5, a duplicate portion of FIG. 4 will carry the same reference numeral, and the explanation thereof will be omitted.

(73) In FIG. 5, the correction map describes that the sub F/B correction amount Kr2 has a relation of characteristic L_Kr2 (solid line).

(74) Specifically, if the downstream side voltage deviation Vr is on the horizontal axis and the sub F/B correction amount Kr2 is on the vertical axis, the sub F/B correction amount Kr2 has a negative value in a negative value region (i.e. a rich air-fuel ratio region) on the left side of the origin (i.e. a state in which the downstream side air-fuel ratio A/Fout is the theoretical air-fuel ratio), and has a positive value in a positive value region (i.e. a lean air-fuel ratio region) on the right of the origin. The absolute value of the sub F/B correction amount Kr2 has a linear relation with the absolute value of the downstream side voltage deviation Vr. These points are the same as those in the standard map illustrated in FIG. 4.

(75) On the other hand, in the correction map, the sub F/B correction amount Kr2 is asymmetric on the rich air-fuel ratio side and the lean air-fuel ratio side (refer to L_Kr1 (dashed line) which is symmetric). In other words, the sub F/B correction amount Kr2 in the rich air-fuel ratio region on the left side of the origin has a smaller slope with respect to the downstream side voltage deviation Vr than the sub F/B correction amount Kr2 in the lean air-fuel ratio region on the right side of the origin. To put it differently, sensitivity to the downstream side voltage deviation Vr is low.

(76) If the sub F/B correction amount Kr2 is used for the sub F/B control, the correction of the fuel injection amount to the lean side becomes weaker than in the case where the sub F/B correction amount Kr1 is used, in a situation in which the downstream side air-fuel ratio A/Fout indicates a richer side value than the target.

(77) Here, if there is the air-fuel ratio imbalance among the cylinders, hydrogen is generated from the cylinder(s) having the rich air-fuel ratio. The hydrogen has small particles and a high diffusion rate, and the detection terminal of the first air-fuel ratio sensor 219 is thus easily covered with the hydrogen. As a result, the upstream side air-fuel ratio A/Fin detected by the first air-fuel ratio sensor 219 tends to be shifted to the rich side with respect to an average air-fuel ratio of the catalyst inflow gas. In other words, the rich shift easily occurs in the first air-fuel ratio sensor 219.

(78) If there is the rich shift, the upstream side output voltage value Vf in FIG. 2 is excessively deflected to the rich side. Thus, if no measures are taken, the main F/B controlled variable Qcor outputted from the main F/B arithmetic unit 107 becomes an excessively lean-side controlled variable, and the air-fuel ratio of the catalyst inflow gas likely stays on the lean side with respect to the upstream side target air-fuel ratio A/Fintg to deteriorate the emission.

(79) Thus, in the embodiment, if the catalyst emission gas has the rich air-fuel ratio (i.e. if the downstream side output voltage value Vr is less than the downstream side target voltage value Vrref), the sub F/B controlled variable Vfcor which is to be added to the upstream side output voltage value Vf is corrected by the sub F/B correction amount Kr2, so that it makes difficult to correct the fuel injection amount to the lean air-fuel ratio side. As a result, an output change due to the rich shift is canceled by an output change due to the change of the sub F/B correction amount Kr2, by which the emission deterioration can be suppressed.

(80) In the embodiment, the deviation between the upstream side output voltage value Vf and the downstream side output voltage value Vr is used as the output deviation of the present invention; however, an aspect which can be adopted by the output deviation of the present invention is not limited to this example.

(81) For example, a deviation between the peak value of the upstream side output voltage value Vf in a certain period and the peak value of the downstream side output voltage value Vr in a certain period may be used. Alternatively, a deviation between the average value of the upstream side output voltage value Vf in a certain period and the average value of the downstream side output voltage value Vr in a certain period may be used. If the average value is used, it is possible to perform more accurate and stable imbalance determination. Moreover, instead of the air-fuel ratio equivalent value of each gas as described above, a difference in response speed between the first air-fuel ratio sensor 219 and the second air-fuel ratio sensor 220 may be used. The hydrogen caused by the air-fuel ratio imbalance disappears due to catalyst reaction when passing through the three-way catalyst 218, and its influence appears only on the first air-fuel ratio sensor 219. Therefore, consequently, there is a detectable difference in the response speed between the two sensors.

(82) The embodiment exemplifies that the sub F/B correction amount, which is the correction coefficient of the sub F/B gain Gfbr, is corrected from Kr1 to Kr2 as the action of the correcting device of the present invention; however, this is one example of the action of the correcting device of the present invention.

(83) For example, when the sub F/B arithmetic unit 105 calculates the sub F/B controlled variable Vfcor, various known learning processes can be preferably performed. The learning process is, for example, a process of storing the steady component of the sub F/B controlled variable as a leaning value while the steady component is updated as occasion demands. The learning value is a value which is reflected in the sub F/B controlled variable as one example of the element value of the present invention. If the learning value of the sub F/B controlled variable is corrected to the decreasing side in cases where there is the air-fuel ratio imbalance, or in cases where there is the rich shift in the first air-fuel ratio sensor 219 and the downstream side voltage deviation Vr is shifted to the rich side, then, it is possible to avoid the excessive correction of the fuel injection amount to the lean air-fuel ratio side in the same manner as described above.

(84) The present invention is not limited to the aforementioned embodiment, but various changes may be made, if desired, without departing from the essence or spirit of the invention which can be read from the claims and the entire specification. A control apparatus for an internal combustion engine, which involves such changes, is also intended to be within the technical scope of the present invention.

INDUSTRIAL APPLICABILITY

(85) The present invention can be applied to the control of the fuel injection amount in the internal combustion engine.

DESCRIPTION OF REFERENCE NUMERALS AND LETTERS

(86) 10 engine system 100 ECU 200 engine CB cylinder block 201 cylinder 212 intake port injector 217 exhaust tube 218 three-way catalyst 219 first air-fuel ratio sensor 222 second air-fuel ratio sensor